Phosphates-based catalysts for synthetic gas (syngas)
production using CO2 and CH4
Bruna Rêgo de Vasconcelos
To cite this version:
Bruna Rêgo de Vasconcelos. Phosphates-based catalysts for synthetic gas (syngas) production
using CO2 and CH4. Chemical and Process Engineering. Ecole des Mines d’Albi-Carmaux,
2016. English. ¡ NNT : 2016EMAC0004 ¿.
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Phosphates-based catalysts for synthetic gas (syngas) production using
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²DPMF EPDUPSBMF et discipline ou spécialité ED MEGEP : Génie des procédés et de l'Environnement
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Centre RAPSODEE, CNRS - UMR 5302, École des Mines d'Albi-Carmaux
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Ange NZIHOU
Patrick SHARROCK
Lionel MONTAGNE
Cuong PHAM-HUU
Jean-Michel LAVOIE
Doan PHAM MINH
Ange NZIHOU
Emmanuel MARTINS
Alain GERMEAU
Patrick SHARROCK
Jury :
Professeur, Université de Lille 1, Rapporteur
Professeur, Université de Strasbourg, Rapporteur
Professeur, Université de Sherbrooke, Président du jury
Maître de conférences, École des Mines Albi, Examinateur
Professeur, École des Mines Albi, Directeur
Responsable du projet, PRAYON, Examinateur
Chef de service R&D, PRAYON, Invité
Professeur Émérite, Université Paul Sabatier Toulouse, Invité
Remerciements
J’adresse mes remerciements aux personnes qui m’ont aidée pendant ces trois
années de thèse.
Premièrement, je remercie mon directeur de thèse, le Pr Ange Nzihou, qui m’a
guidée et aidée à avancer dans mon travail et pour ses conseils et sa confiance durant
ces trois années.
Je remercie également le Pr Patrick Sharrock pour sa motivation, sa curiosité et
sa bonne humeur.
Mes remerciements les plus sincères sont pour Dr Doan Pham Minh, pour sa
patience, sa connaissance et son aide à surmonter les problèmes du quotidien.
Je remercie aussi mes encadrants de chez PRAYON, Alain Germeau, Emmanuel
Martins et Daniel Bustin pour leur financement, leur motivation et leur implication dans
le projet et pour m’avoir montré les enjeux du monde industriel.
Mes remerciements s’adressent aussi aux membres du jury : Cuong Pham-Huu,
Lionel Montagne et Jean-Michel Lavoie pour avoir accepté d’évaluer cette thèse et
pour toutes leurs remarques et questionnements, qui ont amélioré la qualité de ce
travail.
Je tiens à remercier aussi l’Ecole des Mines d’Albi-Carmaux et le centre
RAPSODEE qui ont mis à disposition leurs moyens pour mes travaux de thèse.
Pendant ma thèse au centre RAPSODEE j’ai eu le plaisir de travailler avec des
personnes qui ont eu un rôle fondamental pour l’aboutissement de mon travail et qui
ont provoqué une ambiance agréable au travail. Un grand merci à tous les techniciens
et ingénieurs pour leur aide au quotidien et leur bonne humeur. Un merci spécial à
Mickaël Ribeiro, Denis Marty et Séverine Patry pour leur aide indispensable à la mise
en œuvre de la manip et des analyseurs. Je remercie aussi Nathalie Lyczko pour toute
son aide apportée pendant ces années au niveau professionnel et personnel. Je tiens
à remercier également les enseignants-chercheurs du centre, spécialement Fabienne
Espitalier et Maria Inês Ré pour leur soutien depuis bien longtemps. Un grand merci au
secrétariat du centre pour leur efficacité et sympathie.
Mes remerciements vont aussi à Abdoul Razac Sane et à Lulu Zhao, dont leurs
travaux de stage ont beaucoup aidé à la compréhension de certains aspects de la
thèse.
Un grand merci aux doctorants et post-doctorants pour leur bonne humeur
malgré les difficultés et qui tout au long de ces années sont devenus des amis. Merci
spécialement à Marta, Marion, Marwa, Maxime, Lucia, Hugo, Margot et Graciela, qui
m’ont montré qui l’amitié n’a pas de nationalité. Je remercie chaleureusement mes
amis brésiliens : Suênia, Nayane, Jacqueline, Paulo, Andrea qui ont ramené un peu de
la joie de notre pays. Un grand merci à Ludo, pour sa complicité, sa présence et son
affection.
Finalement, mes plus vifs remerciements vont à ma famille pour leur soutien
inconditionnel. Je vous dédie ce manuscrit.
Contents
Contents .............................................................................................................................7
Abstract ........................................................................................................................... 11
Résumé ............................................................................................................................ 13
Résumé long en français ............................................................................................... 15
General Introduction ...................................................................................................... 29
Chapter 1 ......................................................................................................................... 33
Background and Literature Review .............................................................................. 33
1.1 Introduction ............................................................................................................. 33
1.2 Dry reforming of methane ....................................................................................... 34
1.2.1 Overview of DRM ............................................................................................. 34
1.2.2 Thermodynamics ............................................................................................. 36
1.2.3 Catalytic DRM .................................................................................................. 39
1.2.4 Reaction mechanism and kinetics of DRM ...................................................... 45
1.2.5 Outlook ............................................................................................................. 48
1.3 Calcium phosphates ............................................................................................... 49
1.3.1 Apatitic calcium phosphates ............................................................................ 49
1.3.2 Hydroxyapatite properties ................................................................................ 50
1.4 Conclusion .............................................................................................................. 56
1.5 Bibliography ............................................................................................................ 58
Chapter 2 ......................................................................................................................... 65
Materials and Methods ................................................................................................... 65
2.1 Introduction ............................................................................................................. 65
2.2 Catalysts supports .................................................................................................. 65
2.2.1 Hydroxyapatite ................................................................................................. 65
2.2.2 Other phosphates-based supports .................................................................. 66
2.2.3 Alumina-based supports .................................................................................. 66
2.3 Active metal and catalyst promoters ....................................................................... 66
2.4 Catalyst synthesis methods .................................................................................... 67
2.4.1 Incipient wetness impregnation (IWI)............................................................... 67
2.4.2 Co-precipitation (CP) ....................................................................................... 68
2.4.3 Cation exchange (CE)...................................................................................... 68
2.4.4 Addition of promoters ....................................................................................... 69
2.5 Characterization techniques ................................................................................... 71
2.5.1 Chemical analyses ........................................................................................... 71
2.5.2 Physical analyses ............................................................................................ 74
2.5.3 Thermal analyses ............................................................................................. 75
2.6 Catalytic reactors for DRM ...................................................................................... 76
2.6.1 U-shaped quartz reactor .................................................................................. 76
2.6.2 Fixed-bed reactor ............................................................................................. 78
2.7 Conclusions............................................................................................................. 80
2.8 Bibliography ............................................................................................................ 82
Chapter 3 ......................................................................................................................... 85
Characterization of the prepared catalysts .................................................................. 85
3.1 Introduction ............................................................................................................. 85
3.2 Characterization of hydroxyapatite-based catalysts ............................................... 85
3.2.1 Zn and Fe-based catalysts............................................................................... 86
3.2.2 Co and Cu-based catalysts .............................................................................. 95
3.2.3 Ni-based catalysts ............................................................................................ 99
3.3 Characterization of other phosphate-based catalysts........................................... 109
3.3.1 Nitrogen adsorption-desorption isotherms ..................................................... 109
3.3.2 Specific surface area (SBET) and metal amounts ........................................... 110
3.3.3 X-ray Diffraction (XRD) .................................................................................. 110
3.3.4 ESEM analysis ............................................................................................... 111
3.4 Characterization of alumina-based catalysts ........................................................ 112
3.4.1 Nitrogen adsorption-desorption isotherms ..................................................... 112
3.4.2 Specific surface area (SBET) and metal amounts ........................................... 114
3.4.3 X-ray Diffraction (XRD) .................................................................................. 114
3.4.4 ESEM microscopy .......................................................................................... 116
3.4.5 TEM microscopy ............................................................................................ 117
3.4.6 Temperature Programmed Reduction (TPR)................................................. 117
3.5 Conclusions........................................................................................................... 119
3.6 Bibliography .......................................................................................................... 121
Chapter 4 ....................................................................................................................... 125
Investigation of the active phase ................................................................................ 125
4.1 Introduction ........................................................................................................... 125
4.2 Comparison of the impact of transition metals-based catalysts in DRM .............. 125
4.2.1 Evaluation of monometallic supported catalyst (Me/hydroxyapatite, Me = Zn,
Fe, Co, Cu, Ni) .................................................................................................................. 126
4.2.2 Effect of promoter addition on Ni supported hydroxyapatite (YNi/hydroxyapatite, Y = Ce, Re, Co) ................................................................................... 136
4.3 Conclusions........................................................................................................... 140
4.4 Bibliography .......................................................................................................... 142
Chapter 5 ....................................................................................................................... 145
Parametric study of Ni/hydroxyapatite catalysts ...................................................... 145
5.1 Introduction ........................................................................................................... 145
5.2 Influence of testing conditions on the performance of hydroxyapatite catalysts .. 145
5.2.1 Investigation of the catalytic activity of the reactor walls and supports ......... 146
5.2.2 Set-up of the experimental apparatus ............................................................ 147
5.2.3 Influence of the reaction temperature ............................................................ 148
5.2.4 Influence of the catalyst pretreatment ............................................................ 150
5.2.5 Effect of support sintering .............................................................................. 152
5.2.6 Effect of pressure ........................................................................................... 154
5.3 Comparison of the hydroxyapatite-based catalysts with other catalysts .............. 156
5.3.1 Other phosphates-based catalysts ................................................................ 156
5.3.2 Promoted hydroxyapatite catalysts ................................................................ 158
5.3.3 Alumina-based catalysts ................................................................................ 160
5.4 Conclusions........................................................................................................... 163
5.5 Bibliography .......................................................................................................... 166
Chapter 6 ....................................................................................................................... 171
Catalytic stability under simulated industrial conditions ......................................... 171
6.1 Introduction ........................................................................................................... 171
6.2 Evaluation of the catalysts performance under simulated industrial conditions ... 171
6.3 Discussions ........................................................................................................... 181
6.4 Conclusions........................................................................................................... 184
6.5 Bibliography .......................................................................................................... 185
General conclusions and prospects ........................................................................... 187
Publications related to this work ................................................................................ 193
Appendix A .................................................................................................................... 195
Characterization of the prepared catalysts
(cf. Chapter 3) .............................. 195
A.1 TG ......................................................................................................................... 195
A.2 TMA ...................................................................................................................... 197
A.3 EDX ...................................................................................................................... 197
A.3.1 Zn-based catalysts ........................................................................................ 197
A.3.2 Fe-based catalysts ........................................................................................ 198
A.3.3 Co-based catalysts ........................................................................................ 198
A.3.4 Cu-based catalysts ........................................................................................ 199
A.3.5 Ni-based catalysts ......................................................................................... 199
List of Figures ............................................................................................................... 201
List of Tables ................................................................................................................. 205
Abstract
Among the products resulting from biomass or organic waste transformation, CO 2 and
CH4 are important chemical intermediates. They also have a strong environmental impact since
they are primarily responsible for the greenhouse effect and their mitigation is a key issue.
An attractive way of valorization of such gases is the dry reforming of methane (DRM),
which converts CO2 and CH4 into syngas (mixture of hydrogen and carbon monoxide). This
mixture can be used for several applications, such as the production of methanol, dimethyl
ether, hydrogen and liquid hydrocarbons. Despite such interest, the exploitation of DRM on
industrial scale has not emerged yet. The main reason is the rapid deactivation of the catalysts
due to the severe operating conditions of the process (high temperature, carbon deposition).
This thesis focuses on the development of new catalysts based on calcium phosphate
(CaP) doped with transition metals for the valorization of CO 2 and CH4 through DRM. Actually,
CaP has advantageous properties in heterogeneous catalysis, as the simultaneous presence of
acid and basic sites, good thermal stability, and wide range of surface area...
Initially, a study on the catalyst synthesis methods and an investigation of the
performance of different transition metals (Zn, Fe, Co, Cu, Ni) were carried out in order to select
the catalyst system and the preparation method. Secondly, a fixed-bed reactor capable of
operating at high temperature and pressure and for log time on stream was built and
implemented during this work in order to properly evaluate the performance of the prepared
catalysts. Then, a detailed parametric study was conducted. The influence of parameters such
as catalyst pre-treatment, temperature (T = 400-700°C) and pressure (P = 1-25bar) of the
reaction and support (hydroxyapatite, alumina-based supports) were investigated. Finally, the
catalytic stability was studied for 300h of time on stream (TOS). The CaP catalysts showing
higher yields on syngas were compared to commercial catalysts. Our catalysts showed to be
-1
-1
competitive in the same operating conditions (T = 700°C, P = 1bar, WHSV = 12272mLh gcat ,
TOS = 300h).
This work shows the interest of CaP catalysts for high temperature process, such as dry
reforming of methane.
Keywords: Dry reforming of methane, Syngas, Heterogeneous catalysis, Calcium
phosphate, Catalytic stability, Transition metals
Résumé
Parmi les produits issus de la biomasse ou de la transformation des déchets organiques,
le CO2 et le CH4 sont des intermédiaires chimiques importants qui ont de forts impacts
environnementaux. En effet, ils sont les principaux gaz responsables de l'effet de serre et leur
atténuation est un enjeu majeur.
Une voie intéressante pour la valorisation de ces gaz est le reformage à sec du méthane
(DRM), qui convertit le CO2 et le CH4 en gaz de synthèse (mélange d'hydrogène et de
monoxyde de carbone). Ce mélange peut être utilisé pour plusieurs applications telles que la
production de methanol, d'éther diméthylique, d'hydrogène et des hydrocarbures liquides.
Malgré cet intérêt, l'exploitation du DRM à l'échelle industrielle n'a pas encore vu le jour. La
raison principale est la désactivation rapide des catalyseurs en raison des conditions sévères
de fonctionnement du procédé (température élevée, dépôt de carbone).
Cette thèse porte sur le développement de nouveaux catalyseurs à base de phosphate
de calcium (CaP) dopés avec des métaux de transition pour la valorisation du CO 2 et du CH4 en
gaz de synthèse par DRM. Les CaP sont utilisés car ils possèdent des propriétés avantageuses
en catalyse hétérogène comme la présence simultanée de sites acides et basiques, bonne
stabilité thermique, large gamme de surface spécifique ...
Dans un premier temps, des études sur les méthodes de synthèse de catalyseurs et sur
la performance de différents métaux de transition (Zn, Fe, Co, Cu, Ni) ont été effectuées dans le
but de sélectionner le catalyseur et sa méthode de préparation. Un réacteur à lit fixe capable de
fonctionner à hautes température et pression a ensuite été testé pour un long temps de réaction
afin d'évaluer correctement la performance des catalyseurs préparés. Ensuite, une étude
paramétrique détaillée a été menée. L'influence des paramètres tels que le prétraitement des
catalyseurs, la température (T = 400-700°C) et la pression (P = 1-25bar) de la réaction et les
différents supports (hydroxyapatite, alumine) ont été étudiés. Enfin, la stabilité thermique et
catalytique a été étudiée durant 300h de réaction. Les catalyseurs à base de CaP ont montré
des rendements plus élevés en gaz de synthèse en comparaison aux catalyseurs
commerciaux. Ces catalyseurs sont donc compétitifs dans les mêmes conditions opératoires (T
-1
-1
= 700°C, P = 1bar, WHSV = 12272mLh gcat , t = 300h). Ce travail a montré l'intérêt des
catalyseurs à base de CaP pour des processus à haute température, tel que le reformage à sec
du méthane.
Mots-clés : Reformage à sec du méthane, Gaz de synthèse, Catalyse hétérogène,
Phosphate de calcium, Stabilité catalytique, Métaux de transition
Résumé long en français
Introduction générale
La croissance de la population et de l'économie a entraîné une forte
augmentation des émissions anthropiques de gaz à effet de serre (GES), en particulier
du CO2 et du CH4. Les centrales électriques au charbon, par exemple, représentent
environ 33-40% du total des émissions anthropiques de dioxyde de carbone dans le
monde [1]. Des études ont montré que, entre 1750 et 2011, les concentrations de CO 2
sont passées de 280 ppm à 400 ppm. L'augmentation des émissions de GES, en
particulier du CO2 et du CH4 ont provoqué des changements climatiques critiques, qui
ont conduit à des effets négatifs sur les systèmes naturels et socio-économiques, y
compris des températures plus élevées, les mauvaises récoltes, les incendies,
l'élévation du niveau de la mer, l'acidification des océans, etc. Pour limiter les effets du
changement climatique, la Conférence des Nations Unies sur les changements
climatiques de 2015 (COP21) a fixé comme objectif la limitation du réchauffement
climatique entre 1,5 et 2°C d'ici 2100. La réduction des émissions anthropiques des
GES est donc impérative. Actuellement, la combustion est la méthode la plus courante
pour disposer les mélanges CO2/CH4. Cependant, la capture et stockage du carbone
(CSC) et la capture et utilisation du carbone (CUC) ont gagné beaucoup de notoriété.
Dans ce contexte, une solution écologique pour l’atténuation du CO2 et du CH4 est leur
valorisation en produits chimiques à haute valeur ajoutée, tels que le méthanol,
l'éthylène, le propylène, le carbonate de diméthyle (DMC), etc. Pour cela, une voie
prometteuse est la transformation du mélange de CO2 et CH4 en gaz de synthèse
(syngas) via le procédé dit de reformage à sec du méthane (DRM).
La réaction DRM (Eq. 1) consomme une quantité importante de CO2 et de CH4
pour produire un mélange équimolaire de gaz de synthèse, qui peut ensuite être utilisé
pour la production de combustibles liquides et de produits chimiques. Ainsi, cette
réaction valorise une source de carbone à bas prix et disponible et contribue à la
réduction de l'empreinte carbone. Par contre, ce procédé nécessite un catalyseur pour
atteindre une cinétique suffisante pour l’exploitation industrielle. Cependant, la
désactivation des catalyseurs par dépôt de coke pendant le DRM est un obstacle
majeur au développement de cette réaction à l'échelle industrielle. Ces dernières
années (depuis 2013) sont marquées par un nombre spectaculaire des travaux
(articles scientifiques publiés) dédiés au procédé DRM.
CH4 + CO2 → 2CO + 2H2
(Eq. 1)
L'hydroxyapatite est un matériau à base de phosphate qui a été utilisée avec
succès en tant que support de catalyseur dans de nombreuses réactions chimiques, y
compris le DRM à l’échelle laboratoire. En effet, elle a une bonne stabilité thermique.
Ses sites acido-basiques favorisent l'adsorption de CO2, nécessaire pour la réaction
catalytique. De plus, sa capacité d'échange cationique facilite le dépôt de la phase
active sur le support.
Cette thèse porte sur la valorisation du CO2 et du CH4 en syngas via le procédé
DRM en utilisant des catalyseurs à base d'hydroxyapatite. Les hydroxyapatites
utilisées dans ce travail (TCP308 et TCP908) ont été fournis par PRAYON, un
partenaire industriel spécialisé dans la production des phosphates et qui a financé ce
travail.
Ce rapport de thèse est divisé en quatre parties principales:
La première partie consiste à valider l'hydroxyapatite en tant que support
de catalyseur pour le DRM et à développer un catalyseur à base
d'hydroxyapatite.
La deuxième partie est consacrée à la construction et à la mise en œuvre
d’un réacteur à lit fixe capable de fonctionner à haute température et
pression et pendant longtemps de test.
La troisième partie porte sur l’optimisation des conditions de réaction.
Finalement, la dernière partie compare les catalyseurs à base
d'hydroxyapatite préparés dans ce travail avec des catalyseurs utilisés à
l'échelle industrielle pour d'autres procédés catalytiques.
Le premier chapitre correspond à une étude bibliographique. Tout d'abord, une
étude détaillée sur la réaction de reformage à sec du méthane, y compris les aspects
thermodynamiques et cinétiques est présentée. Le développement de catalyseurs pour
DRM et les paramètres qui peuvent influencer sur la performance des catalyseurs sont
également indiqués. Le chapitre est terminé par une présentation générale de
l'hydroxyapatite et des propriétés qui font de ce matériau un support de catalyseur
prometteur.
Ensuite le deuxième chapitre montre des caractéristiques de l'hydroxyapatite et
d'autres matériaux utilisés comme supports catalytiques et phases actives étudiées
dans ce travail. Ce chapitre expose également les différentes méthodes de préparation
16
et la caractérisation des catalyseurs. Enfin, les différents dispositifs expérimentaux
utilisés pour l'étude de la performance des catalyseurs préparés sont présentés.
Le troisième chapitre présente la caractérisation physico-chimique des différents
catalyseurs préparés dans ce travail et l'influence de différents paramètres, tels que la
composition de support, le type et la quantité de la phase active et les méthodes de
préparation sur les caractéristiques des catalyseurs.
Le quatrième chapitre présente des résultats expérimentaux sur l’étude de la
phase active. Différents métaux de transition, comprenant Zn, Fe, Co, Cu et Ni ont été
étudiés en tant que phase active. La performance de ces catalyseurs dans le DRM est
présentée. L'influence de différents promoteurs, tels que Ce, Re, Co sur l’activité des
catalyseurs est également présentée.
Le cinquième chapitre est consacré à la mise en place d’un nouveau réacteur à
lit fixe capable de fonctionner à haute température (T = T amb - 850 ° C) et pression (P =
Patm - 30bar) et pendant longtemps de réaction (t> 300h). Ce chapitre présente
également l'influence des différents paramètres tels que la température et la pression
de la réaction, le prétraitement des catalyseurs et le frittage des supports sur la
performance des catalyseurs. Ensuite, ces catalyseurs sont comparés à des
catalyseurs à base d’autres phosphates et à base d'alumine. Cette étude a été menée
dans le but de comparer les catalyseurs préparés avec ceux déjà utilisés à l'échelle
industrielle pour d'autres procédés catalytiques.
Le sixième et dernier chapitre présente des résultats d'une étude comparative
entre des catalyseurs à base d'hydroxyapatite et des catalyseurs à base d'alumine
dans des conditions industrielles simulées. Cette étude a été menée en vue de
confirmer les résultats obtenus avec les catalyseurs les plus prometteurs. Les résultats
basés sur la conversion des réactifs, la sélectivité des produits et le bilan de matière
liés à la caractérisation des catalyseurs ont permis une comparaison appropriée entre
les catalyseurs.
Le développement du DRM dépend de nombreux facteurs, outre que le
développement de catalyseurs, tels que l'optimisation des conditions de la réaction et
l'optimisation de la conception des réacteurs. Les discussions sur ces facteurs et sur le
développement d'un catalyseur pour le DRM sur la base des résultats expérimentaux
de ce travail sont également présentées.
Chapitre 1 : Etat de l’art
Le reformage à sec du méthane apparaît comme une alternative intéressante du
point de vue environnemental et économique pour la production de gaz de synthèse
(syngas, mélange de CO et H2). Ce procédé, dit de reformage à sec du méthane
17
(DRM) utilise le CH4 et le CO2 en tant que réactifs. Ces gaz sont disponibles, peu
coûteux et ont un impact environnemental élevé car ils sont les principaux gaz à effet
de serre (GES). Le syngas obtenu par DRM peut ensuite être utilisé pour la production
de carburants liquides, de l'hydrogène et de nombreux autres produits chimiques à
haute valeur ajoutée.
Le reformage à sec du méthane a besoin d'un catalyseur pour avoir une
cinétique chimique exploitable. Les phases actives pour ce procédé sont généralement
des métaux et des oxydes métalliques. Parmi eux, les métaux de transition ont attiré
l'attention grâce à leur activité, disponibilité et faible coût par rapport aux métaux
nobles. Le nickel est le métal actif le plus étudié et l’alumine est le support le plus
investigué. En effet, l’alumine est largement employée pour les autres technologies de
reformage telles que le reformage autothermique (AR) et reformage à la vapeur (SR)
d'hydrocarbures.
Malgré les nombreuses études sur le reformage à sec du méthane, ce procédé
n'a pas encore été mis en œuvre à l'échelle industrielle. Le défi technique de cette
réaction concerne la désactivation du catalyseur, en raison du dépôt de coke et du
frittage du support et de la phase active. L'analyse de la littérature a montré que des
particules de métal (phase active) bien dispersées avec la taille de l’ordre de quelques
nm et qui ont une forte interaction métal-support sont nécessaires pour limiter la
désactivation catalytique.
Dans l’objectif de développer un catalyseur industriel pour DRM, nous avons
travaillé sur les catalyseurs à base de l’hydroxyapatite comme support et différents
métaux de transition comme phase active. L’hydroxyapatite a plusieurs propriétés
thermiques et physico-chimiques intéressantes. Elle contient à la fois des sites acides
et basiques dans son réseau monocristallin. De plus, elle est thermiquement stable à
haute température et elle peut accepter divers métaux dans sa structure apatitique. Ce
solide a été utilisé avec succès comme support de catalyseur dans plusieurs réactions
chimiques telles que l'oxydation du méthane, reformage à la vapeur de glycérol,
réaction de gaz à l'eau, etc.
Différents métaux de transition ont été étudiés dans ce travail : Zn, Fe, Cu, Ni,
Co. L'utilisation de promoteurs (Ce, Re, Co) pour améliorer la performance catalytique
d'un catalyseur donné, ainsi que l'ajustement de la préparation du catalyseur a été
également étudiée. Les catalyseurs préparés ont ensuite été évalués dans le
reformage à sec du méthane et comparés aux catalyseurs préparés sur des supports à
base d'alumine. La performance des catalyseurs comprend l'activité, la sélectivité et la
stabilité pendant des temps de réaction longs.
18
Chapitre 2 : Matériels et méthodes
Le présent chapitre présente les supports, les méthodes de préparation de
catalyseurs choisis dans ce travail et aussi les méthodes de caractérisation physicochimique utilisées.
En plus de la préparation et de la caractérisation de catalyseurs, ce travail vise à
développer et à mettre en œuvre plusieurs dispositifs expérimentaux pour une bonne
évaluation et compréhension de la performance des catalyseurs. Deux réacteurs à lit
fixe et un système de sécurité ont été conçus et mis en œuvre. Un débitmètre
volumétrique pour la mesure du débit de gaz de sortie et un piège à eau pour la
quantification de l'eau formée lors de la réaction ont également été mis en œuvre.
Plusieurs types de supports ont été utilisés dans ce travail : l’hydroxyapatite et
deux supports commerciales à base d’alumina, l’Al2O3 et le PuralMG30. Ces derniers
ont été utilisés pour une étude comparative avec les catalyseurs à base
d'hydroxyapatite.
Les supports d'hydroxyapatite ont été utilisés pour disperser des métaux de
transition: Fe, Zn, Co, Cu et Ni. Ces métaux ont été choisis en raison de leur faible coût
et de leur performance catalytique prometteuse dans le DRM et dans d'autres
réactions [1]–[6]. De plus, trois promoteurs ont également été utilisés afin d'améliorer la
performance catalytique des catalyseurs à base de métaux de transition : Ce, Re et
Co.
Les catalyseurs ont été synthétisés par trois méthodes de préparation
différentes : l’imprégnation à sec (IWI), la co-précipitation (CP) et l'échange cationique
(CE).
Les catalyseurs ont été caractérisés par de nombreuses techniques : ICP-AES,
DRX, MEBE-EDX, TEM-EDX, BET, IR, TMA, TG et TPX. Ces techniques ont été
utilisées afin de déterminer les propriétés thermiques et physico-chimiques des
catalyseurs.
L'activité catalytique a été évaluée en utilisant deux réacteurs : un réacteur en
quartz en forme de U et un réacteur tubulaire. La première configuration a permis
d'évaluer rapidement la performance des catalyseurs. Une fois que le catalyseur le
plus prometteur a été choisi, le réacteur tubulaire à lit fixe a été utilisé pour une étude
plus approfondie sur sa performance catalytique.
Dans ce travail, plusieurs dispositifs expérimentaux ont été développés et mis en
œuvre afin d'évaluer correctement et comparer la performance des catalyseurs.
19
Chapitre 3 : Caractérisation des catalyseurs
Ce chapitre présente des résultats de la caractérisation des catalyseurs
préparés. En outre, il montre l'influence de la méthode de préparation, des supports,
de la phase active et des promoteurs sur les propriétés texturales et physico-chimiques
des catalyseurs, qui ont un impact sur leur activité catalytique.
Le présent chapitre est divisé en trois parties: (I) la caractérisation des
catalyseurs à base d'hydroxyapatite, (II) la caractérisation des catalyseurs à base
d’autres phosphates et (III) la caractérisation de catalyseurs à base d’alumina.
La première partie présente les catalyseurs à base d'hydroxyapatite dopés avec
différents métaux de transition (Me = Zn, Fe, Co, Cu, Ni). Il met également en évidence
l'influence de la teneur en métal actif (% masse Ni = 2,85 ; 5,7 ; 11,4%), des
promoteurs (Ce, Re et Co) et du traitement thermique des supports (TCP308 et
TCP908) sur les propriétés physico-chimique des catalyseurs.
La deuxième et la troisième parties présentent brièvement les caractérisations
des catalyseurs à base d'autres phosphates et d'alumine, qui sont ensuite utilisés dans
une étude comparative avec les catalyseurs à base d'hydroxyapatite.
Parmi les trois méthodes de synthèse utilisées (imprégnation à sec, coprécipitation et échange cationique), l’imprégnation à sec (IWI) s’est montrée la plus
efficace. Cette méthode a permis le dépôt de la quantité désirée du métal et elle n’a
pas modifié la structure du support. De plus, elle est facilement applicable à l'échelle
industrielle.
Parmi les catalyseurs à base d’hydroxyapatite avec et sans traitement thermique,
les catalyseurs à base de TCP908 ont montré des caractéristiques prometteuses. Ils
ont présenté des particules de Ni de taille plus petite (20-50nm) et plus faciles à réduire
(Tred = 390°C) que les catalyseurs à base de TCP308 (taille des particules de Ni : 100200nm, Tred = 410°C). Cela a été attribué à la surface spécifique élevée (SBET = 60
m²/g) du TCP908 engendrée par la présence de mesopores (Vp = 0,072cm3/g). Ceci
pourrait conduire à une performance catalytique élevée de ces catalyseurs. En effet,
les mesopores du TCP908 ont facilité la dispersion du nickel sur le support et en plus,
ils peuvent servir de canaux à travers lesquels les molécules de gaz peuvent diffuser.
Cela peut améliorer la vitesse de réaction du DRM et aider à limiter le dépôt de coke.
Dans le cas des catalyseurs à base de Ni, le frittage des supports n'a pas eu
d'influence sur la dispersion des métaux. Néanmoins, il a augmenté l'interaction métalsupport, conduisant à des températures de réduction des catalyseurs plus élevées.
L'influence de différentes quantités de Ni (% masse Ni = 2,85, 5,7 et 11,4%) et
des promoteurs (Ce, Re, Co) a également été étudiée. Les catalyseurs avec des
quantités élevées de Ni ont montré la formation de grandes particules de Ni (≈10020
300nm). L’addition de Re n’a pas provoqué des changements de taille et de
morphologie des particules de Ni. Cependant, l'addition de Ce conduit à la formation
de grosses particules de Ni (≈100-200nm) par rapport au catalyseur Ni/TCP908 (≈2050nm). Par contre, l’addition de Co a amélioré la dispersion des particules métalliques.
Les différences de dispersion des métaux parmi ces catalyseurs peuvent conduire à
des changements dans leur activité catalytique.
Afin de pouvoir comparer postérieurement la performance de catalyseurs à base
d'hydroxyapatite avec celle de catalyseurs à base d'autres phosphates et d’alumine,
les solides Ca2P2O7, l’Al(PO3)3, l’Al2O3 et le Pural MG30 ont également été utilisés
comme support de catalyseur. Les deux phosphates (Ca2P2O7 et Al(PO3)3) ont conduit
à la formation de grandes particules de Ni (≈100-200nm) en raison de leurs faibles
surface spécifique et volumes de pores (Ca2P2O7 : SBET = 8m² / g, Vp = nd; Al(PO3)3 :
SBET <3m²/g, Vp = nd). Cela pourrait affecter négativement le contact entre les
molécules de gaz et la phase active et réduire donc l'activité catalytique de ces
catalyseurs. Cependant, l’Al2O3 et le Pural MG30 (mélange d'Al2O3 et MgO) ont conduit
à la formation de particules de Ni bien dispersées. Cela est probablement due à leurs
surfaces spécifiques et volume des pores élevés (Al2O3 : SBET = 45m²/g, Vp = 0,42
cm3/g ; Pural MG30 : SBET = 39,7m²/g, Vp = 0,17cm3/g). De plus, la présence de
mesopores sur ces deux supports peut améliorer le contact entre les réactifs et la
phase active et augmenter ainsi la vitesse de réaction du DRM. Cela pourrait aussi
faciliter l’élimination d’un possible dépôt de coke et d'améliorer la stabilité des
catalyseurs. Le frittage du Pural MG30 n'a pas modifié la taille des particules de nickel,
par rapport au Pural MG30 initial. En outre, le catalyseur Ni/Pural MG30s est plus
facilement réduit que le catalyseur Ni/Pural MG30.
Finalement, les propriétés physiques et chimiques des supports, comme la
porosité, la surface spécifique, la basicité etc ont un grand impact sur les
caractéristiques des catalyseurs qui vont ensuite influencer leur performance. Ainsi, les
chapitres suivants présenteront les performances catalytiques dans le DRM des
catalyseurs préparés, ainsi que la corrélation entre leur performance et leurs propriétés
physico-chimiques.
Chapitre 4 : Investigation de la phase active
L'objectif de ce chapitre est de comparer l'activité catalytique de différents
catalyseurs Me/hydroxyapatite (Me = Zn, Fe, Co, Cu, Ni), afin de déterminer le
système catalytique le plus efficace. L'influence de la température de réaction (T =
400-700°C) et des promoteurs (Ce, Re, Co) sur la performance des catalyseurs sont
également étudiés.
21
Comme présenté dans le chapitre précédent, les catalyseurs synthétisés à partir
du TCP908 ont montré des particules métalliques de taille plus petites et plus
facilement réduites que les catalyseurs préparés avec le TCP308. De plus, dans les
conditions expérimentales utilisées (T = 400-700°C, P = Patm), seuls les catalyseurs à
base de Ni ont été actifs dans le DRM. Puisque les catalyseurs ont été soumis à une
calcination sous air avant les essais catalytiques, des oxydes métalliques ont été
formés. Ces oxydes ont été identifiés par diffraction des rayons X pour la plupart des
catalyseurs préparés. Aucune étape de réduction n’a été effectuée avant le test
catalytique. Pour les catalyseurs qui ont montré une activité catalytique négligeable, la
raison principale peut être liée à l'état non métallique des métaux actifs. Le mélange de
CH4 et CO2 ne devrait pas être suffisant pour réduire les oxydes métalliques en
particules métalliques. Une autre raison pourrait être l’insertion de cations métalliques
dans la structure apatitique, qui forment des liaisons chimiques rigides dans le support,
et donc ces métaux sont devenus inactifs. Donc, parmi les métaux de transition
étudiés, Ni a été choisi pour la suite de cette étude.
L'influence de la teneur en Ni sur la performance catalytique a ensuite été
étudiée. Trois catalyseurs préparés avec du TCP908 contenant différentes quantités
de nickel (% Ni masse = 2,85% ; 5,7% ; 11,4%) ont été testés. Le catalyseur contenant
5,7% en masse de Ni s’est montré le plus performant.
L'effet de l'addition d’un promoteur (Ce, Re, Co) au catalyseur Ni/TCP908 a
également été étudiée. L’addition de Re s’est montrée complètement néfaste à la
performance du catalyseur. En effet, les catalyseurs au rhénium ont tendance à former
des oxydes inactifs dans les conditions de réaction [7]–[10], qui peuvent être difficiles à
réduire. De plus, ils imposent une acidité supplémentaire à la surface du catalyseur [9].
L’addition du Ce n’a pas conduit à un gain d'activité catalytique. Les catalyseurs
bimétalliques Co-Ni préparés par imprégnations successives ont conduit à la formation
de particules métalliques de petite taille (≈50nm). Ces catalyseurs ne sont pas actifs à
700°C, puisque cette température peut ne pas être suffisante pour réduire l'oxyde
métallique en particules métalliques. Cependant, ils ont été actifs à 750°C et stables
pendant 4h de réaction. D'autre part, le catalyseur bimétallique Co-Ni préparé par CP
n'a pas été actif, même à 750°C.
Même si les catalyseurs bimétalliques ont un potentiel pour le DRM,
l’investigation de ces catalyseurs n’a pas été approfondie dans ce travail. Les
catalyseurs monométalliques à base de Ni préparés par la méthode IWI ont été choisis
en raison de la simplicité de la préparation et de leur performance catalytique
prometteuse. La concentration de Ni 5,7% massique a été également choisie.
22
Chapitre 5 : Etude paramétrique des catalyseurs Ni/hydroxyapatite
Le chapitre précédent a balayé différentes phases actives pour le DRM. Parmi
les métaux de transition testés (Me = Zn, Fe, Co, Cu, Ni), le Ni s’est montré le plus
prometteur. L'influence des méthodes de préparation et des promoteurs sur la
performance des catalyseurs Ni ont également étudiés. Toutefois, il existe d'autres
variables qui peuvent également influencer les performances des catalyseurs. En fait, il
y a eu beaucoup d'efforts sur le développement des catalyseurs, mais l'optimisation
des conditions de réaction a souvent été sous-estimée [11], [12]. Donc, ce chapitre
présente d'abord une étude paramétrique afin d'obtenir des conditions de réaction
appropriées pour l’obtention d’une sélectivité élevée en syngas. Des paramètres tels
que la température et la pression de la réaction, le prétraitement des catalyseurs et le
frittage des supports sont étudiés. Ensuite, une comparaison des catalyseurs à base
d'hydroxyapatite avec des catalyseurs à base de phosphates et d'alumine est réalisée.
L'objectif est de comparer les catalyseurs développés dans cette étude avec ceux déjà
utilisés à l'échelle industrielle pour d'autres procédés industriels.
La température et la pression de la réaction devraient être soigneusement
évaluées en raison de leur importance sur les performances catalytiques des
catalyseurs et sur la viabilité globale du processus catalytique. Les études
thermodynamiques montrent que des températures élevées et des basses pressions
sont nécessaires pour l’obtention d’une production élevée de gaz de synthèse et pour
limiter le dépôt de carbone. Cependant, des températures élevées impliquent une forte
consommation d'énergie. De plus, différents procédés tels que Fisher-Tropsch, sont
alimentées en gaz de synthèse à des pressions élevées (20-25 bars) [13]. Dans ce
cas, il est préférable de produire du syngas à pression élevée [14]. Ainsi, l'influence de
la température réactionnelle et de la pression sur la performance du catalyseur a été
évaluée. Le DRM a été étudié dans la gamme de températures de 400 à 700°C en
utilisant différents catalyseurs. La température de 700°C s’est montré la plus approprié
pour l'obtention de conversions élevées des réactifs ainsi que pour limiter le dépôt de
coke. En ce qui concerne la pression réactionnelle, la pression atmosphérique a été la
plus favorable dans la plage de pression étudiée de 1 à 25 bars.
Des nanoparticules métalliques de nickel sont très sensibles à l'exposition à l'air.
En contact avec un oxydant (oxygène dans l'air), ces particules métalliques sont
violemment oxydées en oxydes de nickel. Ainsi, nous avons comparé les catalyseurs
Ni/TCP908 et Ni/TCP308 avec et sans réduction in situ. Le prétraitement a
possiblement eu un impact sur la taille des particules de métal et de la force des
interactions métal-support, ce qui affectera la performance catalytique des catalyseurs
[15], [16]. La réduction in situ a été réalisée avec un courant de 4% de H2/N2 avant la
23
réaction. La réduction in situ a été favorable pour l'activité et la stabilité des
catalyseurs. Les catalyseurs prétraités se sont montrés actifs dès 500°C, tandis que
les catalyseurs calcinés correspondants étaient actifs seulement à partir de 600°C.
Pour les catalyseurs à base de TCP308, celui prétraité était plus actif que celui calciné.
En fait, en raison de la forte interaction métal-support de ce catalyseur, l'atmosphère
du mélange réactionnel n'a pas été suffisante pour réduire complètement les particules
NiOX.
L'influence du frittage de support sur les performances catalytiques des
catalyseurs Ni/hydroxyapatite a été étudiée parce que le frittage à la fois du support et
des particules de nickel peut se produire à des températures de réaction élevées. Ceci
peut provoquer la désactivation irréversible des catalyseurs [17]–[19]. Ainsi, la
performance catalytique du Ni/TCP908 a été comparée à celle du Ni/TCP908S dont le
support a préalablement été fritté. À des températures inférieures à 700°C, le
Ni/TCP908 était légèrement plus actif que le Ni/TCP908S. Ceci a été expliqué par une
forte interaction métal-support de ce dernier, ce qui a conduit à une éventuelle
réduction partielle des particules de nickel au cours de la réduction in situ à 400°C. À
700°C, les deux catalyseurs ont présenté des performances catalytiques comparables.
Ce résultat a été expliqué par la similitude de la forme et de la taille des particules de
nickel de ces catalyseurs.
D'autres phosphates produits par PRAYON ont également été étudiés comme
supports, y compris le Ca2P2O7 (pyrophosphate) et l’Al(PO3)3 (phosphate d'aluminium).
Cependant, les catalyseurs synthétisés avec ces supports ont montré une activité
catalytique négligeable dans des conditions expérimentales similaires par rapport aux
catalyseurs Ni/hydroxyapatite. Les faibles volumes de pores et surfaces spécifiques de
ces supports (Ca2P2O7 : SBET = 8m² / g, Vp = nd; Al (PO3)3 : SBET <3 m² / g, Vp = nd) ont
conduit à la formation de grandes particules de nickel, ce que a été défavorable pour la
performance catalytique de ces catalyseurs. Donc, l’investigation de ces catalyseurs
n’a pas été poursuivie dans ce travail.
L'effet de l'addition du cérium sur l'activité et la stabilité du catalyseur Ni/TCP908
a été à nouveau vérifiée en utilisant le réacteur à lit fixe pendant 50h de réaction. Au
contraire des résultats obtenus dans le chapitre 4 avec le réacteur en quartz en forme
de U, le Ni/TCP908 a montré une activité et une stabilité plus élevée que Ni/CeTCP908, lorsque la réaction a été effectuée à des valeurs élevées de WHSV. Ce
résultat pourrait être attribué à la formation de grosses particules de nickel sur le
Ni/Ce-TCP908 (≈100-200nm) comme indiqué dans le chapitre 3.
Les catalyseurs à base d'alumine ont également été comparés aux catalyseurs à
base d'hydroxyapatite. Les supports à base d'alumine sont largement utilisés dans les
24
procédés industriels grâce à leur résistance mécanique élevée [20] et à leur haute
stabilité chimique et physique. En dépit des volumes de pores plus larges de ce
support (Vp = 0,42cm3/g) par rapport au TCP908 (Vp = 0,072cm3/g), le catalyseur
Ni/Al2O3 s’est montré moins actif que le Ni/TCP908 et a été rapidement désactivé
pendant les 50h de test. Ceci a été expliqué par l'acidité du support d’alumine, ce qui
est favorable au dépôt du carbone. Au contraire, le catalyseur Ni/Pural MG30 dont le
support contient 30% en MgO et 70% en Al2O3, a montré une très bonne performance
catalytique, même légèrement supérieure à celle du Ni/TCP908. L'addition de 30%
massique de MgO à l'alumine a probablement permis la neutralisation de l'acidité de ce
support. Par ailleurs, le Pural MG30 est un support mésoporeux (Vp = 0,17cm3/g), ce
qui a pu favoriser la dispersion du nickel et le transfert de matière à l'intérieur du
catalyseur.
Malgré les volumes de pores importants du support Pural MG30, le Ni/Pural
MG30 et le Ni/TCP908 ont présenté des performances catalytiques similaires au cours
des 50h de réaction à 700°C. Cela indique que les propriétés physiques moins
favorables de l'hydroxyapatite ont été compensées par sa composition chimique. Pour
affirmer ce résultat, des tests de plus longues durées sont nécessaires. Ainsi, le
prochain chapitre sera consacré à ces expériences.
Chapitre 6 : La stabilité catalytique dans des conditions industrielles
simulées
Les chapitres précédents ont été consacrés aux premières étapes de la mise au
point d'un catalyseur. Tout d'abord, la phase active (Ni) et la méthode de préparation
(IWI) ont été identifiés afin de produire un catalyseur performant pour DRM. Ensuite,
une étude paramétrique a été réalisée afin d'optimiser les conditions de réaction. Enfin,
une comparaison des catalyseurs à base d'hydroxyapatite avec d'autres catalyseurs
déjà utilisés pour d'autres procédés industriels a été réalisée. Pour confirmer les
résultats, des tests catalytiques avec des temps de réaction plus longs (>300h) ont été
effectués. Ce chapitre présente les résultats de ces expériences. En outre, il présente
des discussions sur le développement d'un catalyseur DRM.
Le DRM a été réalisé à 700°C pendant 300h avec des catalyseurs préparés sur
les supports frittés. Les trois catalyseurs étudiés (Ni/TCP308S, Ni/TCP908S et
Ni/PuralMG30S) ont présenté de bonnes performances catalytiques. Cependant, le
Ni/TCP908S a été le plus performant suivi par le Ni/Pural MG30S, puis par le
Ni/TCP308S. Ces résultats affirment que les catalyseurs à base d’hydroxyapatite sont
performants pour le DRM dans les conditions opérationnelles industrielles simulées.
25
La suite de ces travaux peut porter non seulement sur l'amélioration de la
synthèse de catalyseurs (réduction de la taille des particules de Ni, modification de la
basicité des supports apatitiques en ajoutant de MgO comme additif…), mais aussi sur
l'optimisation des conditions d'essais et de la conception du réacteur et des études
cinétiques et thermodynamiques. L’ensemble de ces éléments sont essentiels pour le
développement d'un catalyseur résistant au coke et d'une technologie viable pour le
DRM à l’échelle industrielle.
26
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28
General Introduction
Increase in population growth and economy has driven large increases in
anthropogenic greenhouse gas emissions, especially CO2 and CH4. Studies have
shown that between 1750 and 2011 the CO2 concentrations increased from 280ppm to
about 400ppm and about 40% of the total of these emissions remained in the
atmosphere. The rest was stored on land and in the ocean [1]. These emissions are
due to human activities, especially the combustion of fossil fuels [2]. The coal-fired
power plants, for example, account for about 33–40% of the total anthropogenic
emissions of carbon dioxide worldwide [3]. The increase in the greenhouse gases
emissions, especially CO2 and CH4 have caused severe climate changes, which have
provoked negative impacts on natural and socio-economical systems including higher
temperatures, crop failures, fire, rising sea levels, ocean acidification etc. That is the
reason why the 2015 United Nations Climate Change Conference (COP21) set a target
limiting global warming between 1.5 and 2°C by 2100.
The consequences caused by climate change have shown the need to mitigate
anthropogenic greenhouse gases emissions. Flaring is the most common method to
dispose of CO2/CH4 mixtures. Carbon capture and storage (CCS) and carbon capture
and utilization (CCU) have gained much attention. However, one environmentally
interesting solution for CO2 and CH4 mitigation is its valorization into high-value added
chemicals, such as methanol, ethylene, propylene, dimethyl carbonate (DMC), etc.
Due to the depletion of carbon-based energy resources, a promising pathway for
greenhouse gases valorization is the syngas production via dry reforming of methane
(DRM). This reaction consumes a significant amount of CO2 and CH4 to produce an
equimolar mixture of syngas, a building block that can be used for the production of
liquid fuels and chemicals. So, this reaction valorizes a cheap and available source of
carbon and contributes to the reduction of the overall carbon footprint. However, the
deactivation by coking of the catalysts used in DRM is a major obstacle to the
development of this reaction at industrial scale.
Hydroxyapatite is a phosphate material that has been successfully used as a
catalyst support in many chemical reactions, including DRM. In fact, it provides thermal
General Introduction
stability to the catalyst and its acido-basic sites favors the CO2 adsorption. Moreover,
its ion exchange capacity facilitates the deposition of the active phase over the support.
This thesis focuses on the valorization of CO2 and CH4 through DRM using
hydroxyapatite-based catalysts. The hydroxyapatites used in this work were provided
by PRAYON, an industrial partner specialized in phosphates and funder of this work.
The manuscript of this thesis report is divided in four main parts and developed
within six chapters. The four parts are developed as followed:
The first part consists in the validation of the hydroxyapatite as catalyst
support for DRM and on the development of a hydroxyapatite-based
catalyst.
The second part consists in building and implementing a fixed-bed reactor
capable of operating at high temperature and pressure and for long time
on stream.
The third part consists on the optimization of the conditions of reaction.
The last part compares the hydroxyapatite-based catalysts prepared in
this work with catalysts used at industrial scale for other catalytic
processes.
The first chapter corresponds to a literature review. Firstly, an overview on DRM
including thermodynamic and kinetic aspects is presented. The development of
catalysts for DRM and the parameters that might influence the catalysts’ performance
is also shown. Finally, a general presentation of the hydroxyapatite and the properties
that make this material a promising catalyst support is shown.
In the second chapter, the characteristics of the hydroxyapatite and of other
materials used as catalyst support in this work and the active phases investigated are
presented. This chapter also presents the various methods of preparation and
characterization of catalysts used. Finally, the different experimental devices used for
the investigation of the performance of the prepared catalysts are described.
The third chapter presents the physico-chemical characterization of the different
catalysts prepared in this work and the influence of different parameters, such as
support composition, type and amount of active phase and preparation methods on the
characteristics of the catalysts.
The forth chapter relates to the experimental results of the investigation of the
active phase. Different transition metals, such as Zn, Fe, Co, Cu and Ni were
investigated as active phase and the performance on DRM of the hydroxyapatite-based
30
General Introduction
catalysts doped with these metals is shown. The influence of different promoters, such
as Ce, Re, Co on the performance of catalysts is also presented.
The fifth chapter is devoted to the setup of the new fixed-bed reactor capable of
operating at high temperature (T = TRT - 850°C) and pressure (P = Patm - 30bar) and for
long times on stream (t>300h) and to the parametric study on the hydroxyapatite-based
catalysts with the view to optimize the conditions of reaction. The influence of different
parameters, such as temperature and pressure of the reaction, pretreatment of the
catalysts and sintering of the supports on the catalysts performance is presented.
Then, a comparison of these catalysts with other phosphates-based catalysts and
alumina-based catalysts is also shown. This study was conducted with the aim to
compare the prepared catalysts with the ones already used at industrial scale for other
industrial processes.
The sixth and last chapter presents the results of a comparative study of the
hydroxyapatite-based catalysts and alumina-based catalyst under simulated industrial
conditions with long TOS (>300h). This study was conducted with the view to affirm the
results obtained with the most promising catalysts. The results based on reactants’
conversion, products selectivity and mass balances linked with the characterization of
the catalysts allowed a properly comparison of the catalysts.
The development of a DRM technology depends on many factors besides the
catalyst development, such as optimization of the reaction conditions and optimization
of the reactor design. Discussions about these factors and about the development of a
DRM catalyst based on the experimental results of this work are also presented.
31
General Introduction
Bibliography
[1]
R. K. Pachauri, M. R. Allen, V. R. Barros, J. Broome, W. Cramer, and R. Christ,
“Climate Change 2014 Synthesis Report,” 2014.
[2]
N. Heidari and J. M. Pearce, “A review of greenhouse gas emission liabilities as
the value of renewable energy for mitigating lawsuits for climate change related
damages,” Renew. Sustain. Energy Rev., vol. 55, pp. 899–908, 2016.
[3]
L. Li, N. Zhao, W. Wei, and Y. Sun, “A review of research progress on CO2
capture, storage, and utilization in Chinese Academy of Sciences,” Fuel, vol.
108, pp. 112–130, 2013.
32
Chapter 1
Background and Literature Review
1.1 Introduction
Global warming becomes one of the most serious concerns of the 21th century.
While this manuscript is being written, the 2015 United Nations Climate Change
Conference (COP 21) takes place in Paris (November 30 to December 11, 2015). The
main objective is to set up a convention on global warming issue via the reduction of
greenhouse gas emissions to limit the global temperature increase to 2 °C by 2100 [1].
Fossil energy consumption is the main cause of the global warming. Oil and coal
each account for 40% of global warming emissions from fossil fuels worldwide [2].
However, it is still the most source of energy used worldwide. In 2014, oil was still the
world’s leading fuel, with 32.6% of global energy consumption [3]. Taking into account
the fact that world population incessantly increases together with the energy demand,
this last one is expected to rise by 37% from 2013 to 2035 with an average of 1.4% a
year [3].
The development of renewable energies is the most adapted solution to reduce
the fossil fuels consumption. The last decades are marked by the increase of
renewable energy market in different countries. Renewable energy sources – in power
generation as well as transport – increased in 2014, reaching a record of 3.0% of
overall energy consumption, up from 0.9% a decade ago. Renewable energy used in
power generation grew by 12.0% [4]. In France, for example, the government plans
that, by 2020, 27% of electricity consumed will be generated by renewable energy
sources [5].
Carbon dioxide and methane are the two main eco-destructive gases. The
valorization of these molecules has become a highly strategic societal and industrial
Background and Literature Review
target. Among various chemical processes, dry reforming of methane to generate
syngas (mixture of hydrogen and carbon monoxide) has attracted much attention in the
last few years. Syngas can be used directly in heat generator by combustion, or as
starting material for the synthesis of fuels and chemicals.
Dry reforming of methane needs specific catalyst because of low reactivity of
carbon dioxide and methane. Up-to-date, alumina-based catalysts are the most
investigated since this catalyst was already used in other reforming processes [6].
However, hydroxyapatite-based catalysts have been successfully used in several
important organic reactions [7]. In order to understand how these materials work and to
develop a performing catalyst, parameters regarding the preparation of the catalysts
and the addition of different metals and promoters need to be investigated. Moreover,
an understanding of the thermodynamics and kinetics of the process is needed.
1.2 Dry reforming of methane
1.2.1 Overview of DRM
Climate changes caused by greenhouse gases emissions, especially CO2, have
driven scientists to develop technologies for CO2 utilization. Fossil fuel combustion
systems such as coal-fired power plants are one of the major sources of CO2
emissions, which accounts for about 33–40% of the total anthropogenic emissions of
carbon worldwide [8]. In this context, key enabling technological actors are carbon
capture and storage (CCS) and carbon capture and utilization (CCU) [9]. However, the
amount of CO2 converted into other chemicals is close to two orders of magnitude
larger than CCS. In fact, CCS is still in its very initial stage (only 4-5Mt/year of CO2 is
stored) [10].
So, the valorization of CO2 into more valuable chemicals and fuels has become
an environmentally interesting option. There are several chemical processes that are
able to convert CO2 into chemicals. Some examples are indicated below [8]:
Reaction of CO2 with ethane and propane to form ethylene and
propylene, which are basic raw materials in petro-chemical industry;
CO2 hydrogenation to methanol, which is an alternative form of energy
and a starting feedstock for many chemicals;
Reaction of CO2 with methanol and ethanol to form dimethyl carbonate
(DMC) and diethyl carbonate (DEC), which can be widely used as solvent
34
Background and Literature Review
or fuel additive in industry due to its high octane number and good
volatility;
Reaction of CO2 and epoxide to form cyclic carbonate, which can be used
in many application areas, such as solvents, monomers and valuable raw
materials and intermediates in the production of pharmaceuticals and fine
chemicals;
However, increases in energy demand and depletion of carbon-based energy
resources have shown the need of substitution of fossil fuels for renewable energies
[11]. In this context, another attractive way to CO2 valorization is the methane dry
reforming reaction (Eq. 1.1 of the next section). In this reaction, CO2 reacts with
methane to form syngas, from which many other chemicals, such as methanol,
dimethyl ether, hydrogen and liquid hydrocarbons, can be produced [12].
DRM is also an interesting option for the valorization of natural gas and biogas,
which are composed mainly by carbon dioxide and methane. The economic aspect is
also propitious. Natural gas reserves, for example, are larger than petroleum reserves
[11], [13], [14]. Biogas, formed by anaerobic digestion of organic biomass and wastes,
is largely produced at different scales [15]–[17].
Currently, steam reforming is the most used process to convert methane and
produce syngas. However, this process has two main disadvantages: (1) it is highly
endothermic (ΔH°298 = 206kJ/mol) [18], which means high amount of energy is required
and (2) the H2/CO ratio is higher than the one required for GTL (gas-to-liquids) process
[14]. Also, energy lost is important during steam reforming because this process
requires high molar ratio of steam per carbon, typically around 1.7.
For these reasons, dry reforming of methane (DRM) has been intensively
investigated. This reaction produces syngas with H2/CO ratio close to 1/1, which is
ideal for Fisher-Tropsch synthesis of long-chain hydrocarbons [12]. Reforming of
methane with CO2 instead of H2O is attractive because it can be implemented in areas
where water is not abundant [19]. Furthermore, CH4 and CO2 are relatively inexpensive
and available. Power plants, for example, emit a large amount of CO2 at relatively high
temperatures. Moreover, natural gas is being flared at remote natural gas and
petroleum fields because the transport of liquids is less expensive than the transport of
gases. Landfill gases commonly consist of 50% of CH4 and 50% of CO2. Digestion of
wastewater also produces methane and carbon dioxide. So, the conversion of these
gases into higher-value compounds is very attractive [20].
35
Background and Literature Review
Despite economic and environmental benefits, no technical solution for DRM has
emerged at the industrial scale yet. The main reason is the catalytic deactivation by
carbon deposition and by catalyst sintering [21]. In addition, DRM is highly endothermic
and requires high temperature, above 600 °C, to be significantly exploitable. Supported
noble metal catalysts such as platinum (Pt), rhodium (Rh), ruthenium (Ru), palladium
(Pd) were found active and stable in this reaction [14], [22]–[27]. However, they are
expensive which represents a burden for process profitability. So, effort has been
focused on catalysts based on transition metals, which have significantly lower costs
and may be active in this reaction. Cobalt (Co), iron (Fe) and specially nickel (Ni) are
the most studied transition metals for DRM. They showed activity in the DRM but they
still present poor stability.
In the past few years, much research has been conducted to understand the
coke formation over the catalysts in order to improve the catalytic performance. For this
purpose, parameters such as the nature of metals and supports, the method of
catalysts synthesis, the nature of metal-support interaction and the size of metal
particles have been investigated [16], [28]–[34].
On this topic we will firstly communicate about the chemical reactions and the
thermodynamics involved in this process. Secondly, we will show the advances on the
preparation of catalysts and the influence of metal, support and promoters on its
stability. Finally, a brief review of reaction mechanisms and kinetics will be presented.
1.2.2 Thermodynamics
The dry reforming reaction (DRM) converts CH4 and CO2 into equimolar amounts
of H2 and CO, according to the following equation:
1.1
The chemical equilibrium calculation software FactSage 6.3.1 was used to plot
the equilibrium curves for this reaction at different temperatures, pressures and
CO2/CH4 ratios. This calculation took into account the main conditions/hypotheses as
following:
Initial CO2/CH4 ratio equal to 1/1
Liquid phase products formation was neglected
36
Background and Literature Review
Only the products present in higher amounts were considered (H2, CO, H2O
and C(s))
Figure 1.1 shows the species present at equilibrium at atmospheric pressure and
temperature varying between 100 and 900°C.
Figure 1.1. Equilibrium mole amount of reacting species versus temperature: (♦) CH4, (■)
CO2, (▲) H2, (x) CO, (*) H2O, (●) C(s) (CO2/CH4 = 1, P = 1bar)
Methane and carbon dioxide conversion as well as hydrogen and carbon
monoxide production are favored at high temperatures, especially over 600°C. On the
other hand, graphite and water are mainly formed below 600°C as undesirable byproducts. In fact, DRM is usually accompanied by side reactions. These reactions are
listed below:
Reverse water-gas shift:
1.2
Boudouard reaction:
1.3
Methane cracking:
1.4
Carbon gasification:
1.5
Table 1 shows the limiting temperatures for these reactions. The reverse watergas shift reaction (Equation 1.2) is an endothermic reaction that consumes CO2 and H2
and produce CO and H2O, decreasing the H2/CO ratio to a value less than one at a
temperature till 820°C. Although, at temperatures above 900°C, CH4 and CO2 are
completely consumed, resulting in H2/CO molar ratio equal to 1/1 [17], [19], [21], as can
be seen in Figure 1.1. This is the reason why DRM studies are conducted at high
temperatures.
37
Background and Literature Review
Table 1. Limiting temperatures for the reactions of the CO2/CH4 system [21]
Reaction
a
T (°C)
DRM
RWGS
640
820
a
Lower limit
b
b
b
a
BR
MC
700
557
Upper limit
Under stoichiometric conditions of DRM, carbon deposition occurs according to
equations 1.3 and 1.4. Boudouard reaction is favored at low temperatures (Tupper limit =
700°C). However, methane cracking occurs at high temperatures (Tlower
limit
= 557°C).
So, there is no temperature range where DRM is thermodynamically carbon free [17].
Although, according to equation 1.5, the carbon formation can be inhibited if steam
were added to the feed [35]–[37].
DRM is strongly influenced by the pressure. Figure 1.2 shows the effect of
pressure on the equilibrium composition of species calculated by FactSage for the
molar ratio of CO2/CH4 equal to 1/1 at 700°C. This is related to the large entropy
increase for DRM.
Figure 1.2. Influence of reaction pressure on the distribution of products of DRM using
FactSage software: (♦) CH4, (■) CO2, (▲) H2, (x) CO, (*) H2O, (●) C(s) (CO2/CH4 = 1/1, T =
700°C)
High pressure is not favorable for DRM. The hydrogen and carbon monoxide
production as well as the methane and carbon dioxide conversion decrease with the
increase of the reaction pressure. Conversely, coke and water formation increase
substantially at high pressures. Among coke formation reactions, only the reactions 1.3
38
Background and Literature Review
and the inverse 1.5 are favored at high pressure [38]. This implies that these reactions
are the predominant sources of coke under the operating conditions indicated.
The molar ratio of CO2/CH4 influences also on the methane and carbon dioxide
conversion as well as on the coke deposition. Figure 1.3 shows the equilibrium
obtained using FactSage software at 700°C for various molar ratios of CO2/CH4.
Figure 1.3. Influence of molar ratio of CO2/CH4 on the distribution of products of DRM
using FactSage software: (♦) CH4, (■) CO2, (▲) H2, (x) CO, (*) H2O, (●) C(s) (T = 700°C, P =
1bar)
Reaction 1.1 implies that increasing CO2 amounts shift the reaction to the right
and thus the methane conversion increases, which is in agreement with the equilibrium
curves of Figure 1.3. Although, a rapid drop in CO2 conversion is observed in Figure
1.3 after the stoichiometric feed ratio. This can be explained by the fact that the CO2
excess, unlike methane, cannot undergo appreciable chemical transformation due to
the high stability of CO2 molecule under these conditions [38].
At CO2/CH4 ratios equal or superior to 1/1, the carbon formation is not
thermodynamically favored. In fact, the excess of CO2 acts as an oxidizing agent which
gasifies coke deposition to form carbon monoxide [39].
1.2.3 Catalytic DRM
The deactivation of the catalysts is the main challenge for the development of a
DRM technology. This deactivation is mainly due to the carbon deposition over the
active sites and metal and the support sintering. So, parameters that can influence the
carbon deposition and catalyst sintering have been widely investigated in the literature
in order to prevent the catalyst deactivation. The most investigated parameters are the
39
Background and Literature Review
active metal, supports, promoters and preparation methods. So, this section will
discuss about the influence of these parameters on activity and stability of various
catalysts for this process.
1.2.3.a Influence of active metal
Due to the endothermicity of DRM, catalysts are useful in order to lower the
activation energy. The selectivity in H2 and CO can also be improved using a catalyst.
Transition and noble metals based catalysts have been extensively studied in this
reaction.
Noble metal (Rh, Pt, Pd, Ru) based catalysts have been widely investigated due
to their high activity in DRM and especially low coke deposition. Rostrup-Nielsen [40]
compared the activity of Rh, Ru, Pt, Pd and Ni supported on MgO and found the
following order of activity: Ru > Rh > Ni > Ir > Pt > Pd. They also showed that the rate
of carbon formation over the noble metals catalysts was much lower than that over Ni
catalysts, in particular at 500°C.
Coronel et al. [26] reported that Rh supported on La2O3 and La2O3-SiO2 was
highly active and stable for the DRM, thanks to the strong metal-support interaction
(SMSI). No deactivation was observed over 180 h of time-on-stream and no carbon
deposition was detected.
Ballarini et al. [14] investigated the activity of platinum-based catalysts with
platinum loadings of 0.02, 0.1 and 0.5wt% in DRM. The active phase was supported on
alkaline and alkaline earth metal-doped alumina. They observed that the higher the Pt
content, the better the methane conversion was. And after 135 min of time-on-stream,
the catalytic deactivation was significant for the catalysts containing 0.02 and 0.1wt% of
Pt, and was much lower with the catalyst doped with 0.5wt% of Pt. Temperature
programmed oxidation (TPO) showed negligible carbon contents in all spent catalysts
which means that the carbon deposition did not provoke the catalytic deactivation. It
was explained by the sintering of Pt particles. TEM images showed the presence of
large particles on the spent catalyst that were absent on the fresh catalyst. This result
is in agreement with the findings of Rostrup-Nielsen [40]. In addition, significant
amounts of H2O formed by the RWGS would lead to hydrothermal aging and reduce
the metal-support interaction.
Despite their higher activity and lower coke deposition compared to transition
metals, noble metals are expensive, which renders their utilizations difficult for
40
Background and Literature Review
application at industrial scale. Platinum, rhodium and palladium metals, for example,
cost around 20-30k€/kg. Ruthenium is cheaper, costing around 1-2k€/kg [41], [42]. This
is the reason why transition metals such as Ni, Co, Cu, Fe and Zn have been widely
investigated for DRM. For example, nickel is abundant and presents high activity in this
reaction and has affordable cost. However, the rapid deactivation by coke formation is
still the major problem of catalysts used for DRM, including transition metals-based
catalysts.
Kroll et al. [43] investigated the Ni/SiO2 catalyst in the DRM at 700°C. They
reported that the catalysts presented high activity at the beginning of the reaction but
were quickly deactivated due to carbon deposition.
Therdthianwong et al. [44] and Ruckenstein et al. [45] arrived to similar
conclusion on their studies about the activity of Ni/SiO2, Ni/Al2O3 and Ni/TiO2 catalysts.
They explained the deactivation by particles sintering, carbon deposition and metalsupport interactions modification.
Hou et al. [46] investigated the effect Rh addition to Ni/Al2O3 catalyst. The initial
Ni/Al2O3 catalyst were active but exhibited certain coke depositions. The addition of
small amounts of Rh increased the catalytic activity and no coke was detected when
the molar ratio of Rh/Ni was higher than 0.1. This high activity and stability was
attributed to the synergic effect of Rh and Ni, which led to the formation of RhNi
clusters. Moreover, Rh addition enhanced the reducibility of the catalyst, which
increased the amount of metallic nickel during the reduction and reaction processes,
enhancing the reforming activity of the catalyst. Wu et al. [47] also found that Rh
addition improved the activity and stability of Ni/Al2O3 and Ni/boron nitride catalysts and
only a slight deactivation was detected.
The effect of Pt addition to Ni/Al2O3 catalyst was studied by Garcia-Diéguez et al.
[48]. They also found that the catalytic activity of Ni/Al2O3 could be improved by
introducing a small amount of Pt. The addition of Pt favored the formation of small
metal particles, increased the reducibility of Ni and therefore decreased the carbon
formation, which allowed keeping the catalyst stability. The authors also demonstrated
that the addition of a significant amount of Ni to Pt/Al2O3 catalyst led to higher catalytic
performance compared to Ni/Al2O3 catalyst.
Work has been reported on the DRM over Ni-Co bimetallic catalysts. Estephane
et al. [49] investigated the activity of the bimetallic catalyst Ni-Co/ZSM-5. The catalyst
containing 1wt% of Ni and 2wt% of Co showed high activity and stability with only 5wt%
of carbon deposition after 12h of time-on-stream. They suggested that the cobalt would
41
Background and Literature Review
play the role of oxidation catalyst for the removal of deactivating deposited carbon
species in the proximity of catalytic sites. Xu et al. [15] reported a synergetic effect
between Ni and Co. They concluded that the formation of Ni/Co alloy on NiCo/AlLa
catalyst, the high dispersion of metals and the uniform distribution of pore diameter
would improve the activity and anti-coking property of the catalyst. Sengupta et al. [50]
added that the simultaneous presence of Ni and Co improved the reduction
characteristics of the catalysts, which is favorable to the DRM.
1.2.3.b Influence of promoters and supports
Promoters and supports have strong influence on catalyst activity and stability.
Catalyst deactivation could be overcome by the addition of alkali or alkaline dopants,
by the utilization of supports with basic characteristics or by the development of highly
dispersed metal catalysts [28].
Alipour et al. [51] investigated the influence of alkaline earth promoters (MgO,
CaO and BaO) on the activity and coke formation of Ni/Al2O3 catalyst during DRM at
700°C. They showed that the addition of these promoters decreased the carbon
formation over the catalyst. The promoters would have enhanced the basic properties
of the catalysts and, consequently, the CO2 adsorption was favored, which resulted in
coke resistance. In addition, the catalyst promoted with MgO presented higher activity
when compared to the catalysts promoted with CaO and BaO. In fact, the catalyst
promoted with MgO showed higher reducibility, which increased the amount of metallic
nickel and enhanced the reforming activity of the catalyst.
Similar results were obtained by San Jose-Alonso et al. [52] in which Co-Al2O3
modified by potassium (K) showed lower reactant conversions and coke deposition
compared to the unpromoted catalyst. This was explained by a partial coverage of the
active sites for the methane decomposition by K addition, and its ability to catalyze the
carbon gasification.
The effect of ZrO2 promoter over Ni/Al2O3 was investigated by Therdthianwong et
al. [44]. The catalyst deactivation was evidently inhibited by the ZrO2 addition. This
promoter enhanced the CO2 dissociation which formed oxygen intermediates near the
contact between ZrO2 and Ni, where the coke was gasified afterwards.
Daza et al. [53] studied the influence of different amounts of cerium (Ce)
promoter on Ni/Mg-Al catalyst. They reported that Ce had no significant effect on the
catalyst activity and selectivity but played an important role on the inhibition of carbon
42
Background and Literature Review
deposition. In fact, doping with Ce enhanced the thermal stability and the strength of
basic sites which favors the CO2 adsorption and assists the gasification of carbon
deposits. They also emphasized the importance of promoter weight percentage. The
catalyst promoted with 1wt% of Ce presented filamentous carbon deposits after the
reaction but they were absent on the catalyst promoted with 3wt% of Ce.
CeO2 supports have been extensively studied due to their good redox properties
and high oxygen storage capacity (OSC) which improves carbon formation resistance.
Chen et al. [54] investigated the carbon resistance of Ni/CeAlO3-Al2O3 catalyst. They
reported that the gasification of surface carbon at nickel-support boundary was
promoted by the combination of CO2 activation and Ce3+/Ce4+ cycling which would
decrease the possibility of graphene layer formation.
Similarly, Makri et al. [30] investigated the effects of chemical composition of Ce1xMxO2-γ
(M = Zr4+, Pr3+) supports on the carbon formation. They reported that these
supports possess significant amounts of labile oxygen and oxygen vacancy, which play
an important role in reducing the coke formation. Although, an improvement in the rate
of oxygen transfer from support to Ni-support interface would be more important than
the increase of oxygen storage capacity itself.
Frontera et al. [55] stated that the heterogeneity of the support strongly affects
the deposited Ni species. So, they studied the effect of highly ordered Silicalite-1, high
defective pure silica MCM-41 and pure silica delaminated zeolite ITQ-6 supports on the
preparation of catalysts and their activity. The defecti-free Silicalite-1 surface led to
large Ni particles that showed high activity in DRM but high amount of coke was
formed. Increasing the defects on the support surface, like in the case of MCM-41
support allowed the deposition of small metal particles and consequently, limited the
amount of carbon deposition. ITQ-6 support surface led to the highest dispersion of Ni
and prevented the sintering of metal particles. The Ni particles remained free from
carbon deposition since they were well dispersed and strongly linked to the support.
1.2.3.c Influence of synthesis method
The performance of a given catalyst is generally influenced by its preparation
method. The structure, the redox properties, the metal dispersion etc. depend on the
conditions of the preparation method. There are many different synthesis methods
reported in the literature. Some of them for various catalytic reactions will be discussed
below.
43
Background and Literature Review
Klaigaew et al. [56] studied the effects of preparation methods on the activity of
cobalt supported on silica fiber catalyst in Fisher-Tropsch synthesis at 240°C. They
tested four different methods: wet impregnation (WE), strong electrostatic adsorption
(SEA), deposition precipitation (DP) and hydrothermal method (HT). They reported that
the catalysts synthesized by SEA showed the best activity (80% of CO conversion)
because it promoted the formation of small cobalt particles and high catalyst
reducibility.
Jha et al. [57] compared the effect of the sol-gel, co-precipitation, solvothermal
and the incipient wetness impregnation methods on the synthesis of Ni-Cu-CeO2. The
catalysts were further tested on the water-gas shift reaction at 450°C for 25h. The
catalysts synthesized by sol-gel and co-precipitation methods displayed higher activity
(80-85% of CO conversion) than the others catalysts. They explained these results by
the formation of a mesoporous catalyst which facilitated the uninterrupted diffusion of
molecules to and from the active sites of catalysts and by the strong metal-support
interaction.
Ni-doped alumina catalysts were synthesized by equilibrium deposition filtration
technique (EDF), wet impregnation, and incipient wetness impregnation [58]. The
catalysts were further tested in the dry reforming of biogas at 750-800°C. The catalysts
revealed differences in their structure and physicochemical properties, which affected
their catalytic activity. The catalyst with low nickel loading (8wt%) synthesized by EDF
method had interesting catalytic properties in terms of specific surface area, metal
dispersion, and resistance against sintering which were responsible for its high activity
in dry reforming of biogas. Note that methane and carbon dioxide are the main
components of biogas.
Galvez et al. [59] synthesized 10wt%Ni/SBA-15 catalyst by three different
methods and tested their influence on catalysts activity and stability on dry reforming of
methane. The catalysts were prepared via incipient wetness impregnation (IM), coprecipitation (PM) and chemical reduction-precipitation method (RM). For the coprecipitation method, the SBA was suspended in distillated water at 80°C. Then, small
amounts of urea and nickel nitrate were added to the suspension, which was then
cooled to room temperature, precipitated, filtered and washed with distilled water. The
chemical reduction-precipitation method was very similar to the co-precipitation
method. The difference is that ascorbic acid was also added to the suspension of the
support after the urea addition. Physico-chemical features of the catalyst were strongly
influenced by the preparation procedure. PM and RM routes led to Ni-particles confined
44
Background and Literature Review
in the mesopores of SBA-15, whereas Ni was mostly deposited on the external surface
of the silica particles, in the case of the IM catalysts. Moreover, dispersion of this active
phase was found to be enhanced in the case of the PM and RM catalysts, in
comparison to the IM one. Such differences were also reflected in activity, selectivity
and stability during DRM. The catalyst prepared by RM route showed better catalytic
performance compared to the other two. In this case, carbon deposits were formed
mostly on the external surface of the catalyst particles, due to Ni-species confinement
inside the pores of the support. Extent of deactivation was considerably smaller than in
the case of the IM catalyst.
Pan et al. [60] synthesized Ni/SiO2 by the standard incipient wetness
impregnation (IWI) and by IWI with plasma treatment. Further characterizations
showed that the catalyst with plasma treatment had stronger interaction between Ni
particles and the support compared to the catalyst without plasma treatment. Moreover,
plasma treatment led to small Ni particles, which were homogeneously dispersed on
the support. Both catalysts presented similar activity on the dry reforming of methane.
However, the plasma treated catalyst enhanced resistance to coke deposition. This
was attributed to the stronger metal-support interactions and the homogeneous
dispersion of small Ni particles.
1.2.4 Reaction mechanism and kinetics of DRM
Kathiraser et al. [61] stated on their review of DRM over Ni-based catalysts that
kinetic studies are basically chemical steps performed in order to identify a suitable
mechanistic model. Correlation of mechanisms with experimental data would give the
fit that most closely describes the reaction rate and defines reaction steps. The reaction
kinetics can further be used for optimization of the catalyst design and chemical
systems which can lead to the development of a cost-effective DRM technology. Three
models are commonly reported for DRM reaction: Power-law model, Eley Rideal (ER)
and Langmuir Hinshelwood-Hougen Watson (LHHW).
The Power-law model supports the kinetic rate in the form: r = k(PCH4)m(PCO2)n.
The main advantage of this model is the simplicity. However, it can sufficiently explain
the different reaction mechanistic steps only in a narrow range of partial pressure data.
In ER model, one reactant is associatively adsorbed on the catalyst surface. The
rate determining step (RDS) is the reaction of the adsorbed species with the other
reactant from the gas phase leading directly to products [62]. This model is more
45
Background and Literature Review
rigorous than the Power-law model. However, there is little literature data on this model
for Ni-based catalysts, and there is a lack of experimental evidence in most of studies.
On the contrary, LHHW model is widely used for studying mechanistic aspects of
DRM and appears to be convenient for Ni-based catalysts. This model proposes that
both reactants are associatively adsorbed on the catalyst surface and the determining
step is the surface reaction of the adsorbed species to form the final products [62].
The kinetics and mechanistic aspects of DRM especially over Ni-based catalysts
have been extensively studied in the past few years and most of them are derived from
the LHHW model. It is generally accepted that the mechanism of DRM over Ni-based
catalysts involves the following steps:
(1) Dissociative adsorption of methane on active metal sites to produce hydrocarbonate species (CHx) and hydrogen. This step is generally considered as
the rate determining step (RDS);
(2) CO2 adsorption on nickel sites or basic support surface followed by direct
CO2 dissociation into CO and O. These species are combined with H to form
OH;
(3) The surface OH groups of the support react with adsorbed CHx to yield CHxO,
which decomposes into H2 and CO;
(4) CHxO can also be produced by the reaction of adsorbed CHx intermediates
with O, which can be originated from CO2 dissociation or from OH group’s
cleavage. This step is also generally considered as the rate determining step.
The first proposal of a reaction mechanism for the DRM was done by Bodrov and
Apel’baum [63] in 1967. They found that the kinetics for dry reforming over a Ni film
matched their kinetic model for the steam reforming. They proposed the following
mechanism where * denotes an active site on the catalyst surface:
First, CH4 adsorbs at an active site to yield CH2 species. This step was
considered as the rate determining step. Subsequently, CO2 is converted to CO by
RWGS reaction. Then, H2O formed by the combination of H2 and O species reacts with
46
Background and Literature Review
CH2 species to form H2 and CO. They suggested that steam reforming was faster than
the methane decomposition step, which means that the kinetics for CO2 reforming is
nearly the same as for steam reforming. This mechanism model has been used as
base for recent studies on DRM.
Despite the numerous studies, there are still some debates about the accurate
mechanism and the RDS of LHHW model. Nevertheless, CH4 dissociation and the
reaction between CHx and O have been mostly considered. The difference among the
suggested mechanisms could originate from the nature of the catalysts and the
reaction conditions used in the studies [64].
Luo et al. [65] and Osaki et al. [66] studied the reaction steps of DRM over the NiLa2O3/5A catalyst in the temperature range of 450-800°C. They suggested that CO2
would be dissociated and form CHxO. The oxygen generated by the CO2 decomposition
and the CHxO formation would assist the methane decomposition on Ni0. They
concluded that the CHxO decomposition was the rate determining step. Conversely,
Erdöhelyi et al. [67], [68] and Guo et al. [69] showed that for nickel based catalysts, the
H generated from the CH4 decomposition would improve the CO2 dissociation.
Still in the single RDS (rate determining step) view-point, Wei et al. [70] reported
that the CH4 reaction rates were limited solely by C-H bond activation step and this
step would be the only one kinetically relevant. Similarly, Wang et al. [21] reported in
their review about DRM that the methane decomposition was the rate determining step.
Other authors suggested the existence of two rate determining steps. Munera et
al. [71] studied the DRM over rhodium supported on a lanthanum-based support. Their
observations suggest the mechanism wherein the slow steps are the methane
decomposition and the surface reaction of the lanthanum oxycarbonate with carbon
originated from the methane decomposition. Similarly, Tsipouriari and Verykios [72]
concluded that both CH4 dissociation and surface reaction of carbon species with
La2O2CO3 were the rate determining steps. On the other hand, Bradford and Vannice
[73] reported the dissociative adsorption of CH4 and the decomposition of CHxO as the
rate determining steps.
According to Cui et al. [64], large temperature range used for DRM might be the
reason for so many differences in the mechanisms reported in the literature. This could
have remarkably affected the reforming mechanism and the rate determining step.
Thus, they studied the DRM over a Ni/Al2O3 catalyst in the temperature range of 550750°C. They concluded that CH4 dissociation was the rate determining step and CO
desorption also restrained the dry reforming in a temperature range 550-575°C.
47
Background and Literature Review
However, the reaction between CHx and CO2 became the rate determining step at 650750°C.
1.2.5 Outlook
For the all the foregoing reasons the design and the preparation of catalysts of
easy regeneration and resistant to coking and sintering is the key for the development
of a DRM technology.
The size, shape, structure and surface composition have significant effects on
the coke resistance of Ni catalysts, of which the size of metal particles is the most
important. The coke formation could be completely inhibited if Ni particles were kept
below several nanometers during the DRM [74]. However, the synergy of size with
morphology, structure and composition of the catalyst must not be neglected.
The use of promoters, the nature and composition of supports and the method of
catalyst preparation are the most widespread approaches to control the Ni particles
size. That is the reason why the research on DRM is focused on this direction.
Promoters and supports with basic and/or redox properties are well known for
enhancing the CO2 adsorption, the rate of surface reaction, and the nickel dispersion
which improve the coke resistance of the catalyst. Also, a strong interaction between Ni
and the support makes the catalyst more resistant to sintering and coking. So, the
combination of high nickel dispersion, small nickel particles, supports with good thermal
stability and supports and/or promoters that improve the rate of surface carbon reaction
seems to be a good approach for the development of a DRM catalyst. However, the
design, synthesis and characterization of Ni catalysts with controllable size, shape and
structure remain a challenge.
Further studies on reaction mechanisms, carbon formation and reactivity and
advances on catalyst preparation and characterization methods will be crucial to the
understanding and to the development of an active and stable catalyst for this reaction.
As stated earlier on this chapter, Ni appeared as the most studied metal for DRM
due to its activity, availability and low-cost. However, the type of supports investigated
is still very variable. The next topic will be focused on calcium phosphates and how
they seem to be suitable choices as catalyst support for this catalytic process.
48
Background and Literature Review
1.3 Calcium phosphates
Calcium phosphates exist in many different types and are characterized by their
Ca/P atomic ratios. However, for a given Ca/P ratio, different chemical formulas are
possible and the same chemical composition may correspond to different
crystallographic structures [75]. The main calcium phosphates referenced by their Ca/P
atomic ratio and their usual abbreviations are given in Table 2 below.
Table 2. Different calcium phosphates
Nom
Abbreviation
Chemical formula
Ca/P
MCP
Ca(H2PO4)2.H2O
0.5
Monocalcium phosphate anhydrous
MCPA
Ca(H2PO4)2.H2O
0.5
Dicalcium phosphate dihydrate (brushite)
DCPD
CaHPO4.2H2O
1
Dicalcium phosphate anhydrous
(monetite)
DCPA
CaHPO4
1
Ca2P2O7
1
Monocalcium phosphate hydrate
Calcium pyrophosphate
Octacalcium phosphate
OCP
Ca8H2(PO4)6.5H2O
1.33
Tricalcium phosphate (α and β)
TCP
Ca3(PO4)2
1.5
Apatitic calcium phosphate
ACP
Ca9(HPO4)(PO4)5(OH)
1.5
Amorphous calcium phosphate
ACP
Ca9(PO4)6.nH2O
1.5
Hydroxyapatite
Ca-HA
Ca10(PO4)6(OH)2
1.67
Tetracalcium phosphate monoxide
TCPM
Ca4(PO4)2O
2
1.3.1 Apatitic calcium phosphates
Among calcium phosphates, the apatites include a great number of ionic
compounds described by the general chemical formula:
Me10(XO4)6(Y)2
where Me is usually a divalent cation (Ca2+, Cd2+, Pb2+, Fe2+, Zn2+…); XO4, a
trivalent anion (PO43−, AsO43−, VO43−, MnO43−…) and Y, a monovalent anion (OH−, Cl−,
F−, Br−, I−…). The most common compound in nature is fluorapatite (Ca10(PO4)6F2) and
the most used in the field of biology is hydroxyapatite (Ca10(PO4)6(OH)2) [76]. This last
one will be the focus of this present study.
49
Background and Literature Review
1.3.2 Hydroxyapatite properties
1.3.2.a Crystal structure
The stoichiometric hydroxyapatite of chemical formula Ca10(PO4)6(OH)2 has an
Ca/P atomic ratio of 10/6. It crystallizes in the hexagonal P63/m space group with lattice
constants a and b equal to 9.432 Ǻ and c equal to 6.881 Ǻ [77], [78]. Many studies
were developed to understand the hydroxyapatite structure. These studies revealed
that phosphorous in hydroxyapatite is in tetrahedral coordination to oxygen atoms
which are coordinated to two types of calcium atoms: Ca (I) and Ca (II), where Ca (II) is
bigger than Ca (I). The crystal structure of hydroxyapatite is formed by a stack of
phosphate groups forming two types of tunnels. The first, in form of columns, is
occupied by Ca (I) atoms which are coordinated to nine oxygen atoms. The second
one, in helix form, is bordered by Ca (II) atoms which are coordinated to six oxygen
atoms and one OH [77]–[80]. Figure 1.4 and Figure 1.5 illustrate a simplified view of a
hydroxyapatite unit with both types of tunnels.
Figure 1.4. Simplified view of a Ca10(PO4)6(OH)2 unit [78]
50
Background and Literature Review
Figure 1.5. Simplified view of hydroxyapatite [75]
An interesting property of hydroxyapatite is its ability to undergo substitutions in
its crystal lattice. These substitutions can involve both cations and anions of the apatitic
structure with similar charges and size. Ca2+ ions can be substituted by other bivalent
cations such as Pb2+, Ba2+, Pb2+ and Cd2+ but also by monovalent and trivalent cations
such as K+ and Al3+. In fact, a few substitutions are possible involving replacement of
an ion of HA by another ion of different charge with appropriate charge compensation
through incorporation of an additional ion. Among the anionic substitutions, OH− can be
replaced by ions such as F−, CI−, I−, and CO32−, and PO43− by AsO43−, VO43−, and CO32−
[75], [77], [80], [81].
This property can be very valuable for catalyst supports because it allows an
easy incorporation of the active phase to the catalyst, and can enhance the metalsupport interaction.
1.3.2.b Infrared spectrum
The absorption of infrared radiation by hydroxyapatite is due to the vibrations of
phosphate and hydroxide ions. So, the IR spectrum of a hydroxyapatite includes
characteristic bands of OH− and PO34− groups. Figure 1.6 below shows the infrared
spectrum of a stoichiometric hydroxyapatite.
This spectrum shows the presence of two peaks for the OH− ions at 3600 cm−1
and 630 cm−1 assigned to OH− stretching frequencies. Two peaks groups of the PO43−
51
Background and Literature Review
ions are around 1050 and 600 cm−1 corresponding to symmetric and antisymmetric
deformations.
Figure 1.6. Infrared spectrum of a stoichiometric hydroxyapatite [75]
The characteristic bands and their intensity are summarized in Table 3.
Table 3. Positions and relative intensities of IR characteristic bands of hydroxyapatites [75]
-1
52
Wavelength (cm )
Intensity
Appearance
3571
medium
(vs) OH
1091
strong
(v3) PO4
3−
1037
very strong
(v3) PO4
3−
960
strong
(v1) PO4
3−
632
medium
(vs) OH
601
strong
(v4) PO4
3−
570
strong
(v4) PO4
3−
474
medium
(v2) PO4
3−
−
−
Background and Literature Review
1.3.2.c XRD pattern
Figure 1.7 represents the characteristic pattern of a crystalline hydroxyapatite.
The three most important peaks are at 2θ situated at 31.9°, 32.1° and 32.9°. When
HAP is synthesized at moderate temperature (below 100°C) in an aqueous solution,
HAP of low crystallinity is formed. Well-crystalline HAP is obtained after a thermal
treatment at high temperature, typically above 800°C [82].
Figure 1.7. XRD pattern of a crystalline HAP.
1.3.2.d Thermal stability
In order to eliminate remaining volatile impurities and improve the crystallization,
Ca-HA is usually thermally treated around 900°C [83]. Nevertheless, heating above
850°C under air atmosphere may provoke additional loss of water (HAP may contain till
3% of water that can be easily eliminated by heating at 600°C) from the crystal
structure by the reaction [84]:
It means that heating the hydroxyapatite under air atmosphere favors the loss of
water and moves the equilibrium to the right. The loss of OH− from the Ca-HA structure
originates the oxyapatite of formula Ca10(PO4)6O. On the contrary, heating the Ca-HA
under steam atmosphere moves the equilibrium to the left which stabilizes the Ca-HA.
At temperatures above 1200°C hydroxyapatite may still decompose into β-TCP
and tetracalcium phosphate [85].
53
Background and Literature Review
All these decompositions depend on the atmosphere. Studies [83], [84], [86]
showed that under steam atmosphere and with steam partial pressure above 100
mmHg, the hydroxyapatite conserves its structure even for temperatures above
1400°C.
The decomposition of the hydroxyapatite at temperatures far above 850°C
proves its high thermal stability. A catalyst support must be thermally stable; otherwise
the catalyst will exhibit severe losses in its activity. This infers that the hydroxyapatite
could be a good candidate for a catalyst support.
1.3.2.e Utilization of hydroxyapatite
Hydroxyapatite has various utilizations. The most common one is the application
in the field of medicine. In fact, Ca-HA constitutes the major part of the calcified tissues
of mammals, making it one of the most important biomaterials today. Because of its
biocompatibility, it is the key for implants with foreign body rejection reaction [87]. It
has great potential for pharmaceutical, dental, orthopedic, maxillofacial applications
etc. For example, it can be used as cement in facial surgery to replace autogenous
bone grafts or as a vehicle for drugs through a structure having interconnected pores
[7], [88], [89]. Nevertheless, despite its biocompatibility, this material can also be a
pathogen for the organism: body malfunctions can incite the body to produce Ca-HA
with specific morphologies that can provoke the formation of kidney stones or
calcification of cartilage which is dangerous to the body [90].
In recent years, Ca-HA has also shown great applications in the field of
environment, such as treatment of soils and wastewaters contaminated with heavy
metals [91]–[98]. Bailliez et al. [95] showed that once the hydroxyapatite is in contact
with an aqueous solution containing Pb2+ ions, it incorporates this pollutant into its
apatitic structure to form Ca10-xPbx(PO4)6(OH)2. In their study, Ca8.51Pb1.49(PO4)6(OH)2
was identified. Ion exchange between Ca2+ and Pb2+ ions has been usually reported
but other mechanisms have also been discussed such as dissolution-precipitation, or
surface complexation. HAP showed an adsorption capacity (qmax) of 320mg Pb/g HAP,
indicating its strong affinity for Pb2+ pollutant. Corami et al. [91] studied the cadmium
removal from single and multi-metal (Ca2+, Pb2+, Zn2+, Cu2+) solutions by sorption on
hydroxyapatite. In a single-metal solution, the amount of sorbed Cd ranged from 6.5 to
188.9mg Cd/g Ca-HA. However, in a multi-metal system, the amount of sorbed Cd
decreased by 63-83%.
54
Background and Literature Review
Kemiha et al. [99] developed Ca-HA-based porous accumulator for the treatment
of heavy metals in the gas phase. For abatement test, synthetic polluted air containing
evaporated CdCl2 passed through this accumulator at high temperature, before being
trapped in an acid solution. An important abatement of Cd in the range of 17-23% was
obtained, proving that the Ca-HA-based composite is active for Cd removal from the
gas phase.
HAP is also used in the stabilization of radioactive waste. Carpena et al. [100]
synthesized a composite containing radioactive iodine or cesium, minerals and calcium
phosphates coated with hydroxyapatite. This one would act as a barrier against
aqueous corrosion. Leaching tests showed that no cesium was released, proving the
efficiency of the composite.
Due to the unusual property of containing both acid and basic sites in a singlecrystal lattice, its thermal stability and its ion exchange property, Ca-HA can also be
used as catalysts supports in many different industrial processes.
Matsumura et al. [101] used a hydroxyapatite containing sodium ions in its
structure as catalyst in the selective dehydration of lactic acid into acrylic acid. The
catalyst showed a high selectivity of 87% in lactic acid. They explained this high
selectivity through the presence of acid-base pair sites of the hydroxyapatite which
could be effective in this reaction.
HAP was also used as catalyst support for gold (Au) and (Ru) in the water-gas
shift reaction (WGSR). Au/HAP catalyst was more active than Ru/HAP by a factor of 15
(on a catalyst mass basis) at reaction temperatures around 110-120°C [102].
Hydroxyapatite doped with nickel (Ni) was used as catalysts in methane oxidation
reaction. The reactions were carried out at 800°C in a fixed bed reactor. The catalyst
showed to have activity and selectivity comparable to those of the commercial catalysts
(Ni/Al2O3 and Ni/Al2O3 with MgO doping), but there was much less coke deposition on
the HAP catalysts [103].
Yaakob et al. [104] used hydroxyapatite doped with nickel in the steam reforming
of glycerol to produce hydrogen. They obtained a hydrogen yield of 82.19% at a
reaction temperature of 600°C with an amount of Ni of 3wt%. The same catalyst was
used by Boukha et al. [105] to produce syngas via dry reforming of methane. This
catalyst showed good activity in this reaction at 600°C. But no data on the catalytic
stability was reported.
55
Background and Literature Review
From the literature data and the competence of our research team about calcium
phosphates, Ca-HA was chosen as a catalyst support for the dry reforming of methane
process.
1.4 Conclusion
Dry reforming of methane appeared as an attractive alternative to the production
of syngas from both environmental and economic points of view. The process involves
CH4 and CO2 as reactants which are two available greenhouse gases, inexpensive and
have high environmental impact. They could be used to produce syngas that can be
further utilized for the production of liquid fuels, hydrogen and many other valuable
chemicals.
Dry reforming of methane needs a catalyst for an exploitable chemical kinetic.
Active phases for this process are generally metals and metal oxides. Among them,
transition metals have gained attention thanks to its activity, its availability, and its lowcost compared to noble metals. Nickel is the active metal the most studied while
alumina is the support the most investigated since this oxide is the standard for other
reforming technologies such as autothermal reforming (AR) and steam reforming (SR)
of hydrocarbons.
Despite numerous studies on the dry reforming of methane, this process has not
been implemented at industrial scale yet. The technical challenge of this reaction
relates to the deactivation of catalyst, due to the coke deposition and metal and support
sintering. The literature analysis showed that well-dispersed nickel particles with
particle size of some nm, which are regularly and homogeneously distributed on the
surface of support, and which have strong metal-support interaction are necessary to
limit and/or overcome the catalytic deactivation.
For this purpose, this work focused on the development of new performing
catalysts, using hydroxyapatite as catalyst support. Hydroxyapatite has several
interesting physico-chemical and thermal properties. It contains both acid and basic
sites in a single-crystal lattice. It is thermally stable at high temperature and it can
accept various metals into its apatitic structure. This solid has been successfully used
as catalyst support in several chemical reactions such as methane oxidation, steam
reforming of glycerol, water-gas shift reaction etc.
Different transition metals were investigated. The use of promoters for enhancing
catalytic performance of a given catalyst, as well as the adjustment on the catalyst
56
Background and Literature Review
preparation was also studied. The prepared catalysts were then evaluated in the dry
reforming of methane and compared with catalysts prepared on alumina-based
supports. The performance of the catalysts includes the activity, the selectivity and the
stability for long reaction times.
57
Background and Literature Review
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64
Chapter 2
Materials and Methods
2.1 Introduction
The present chapter presents the supports, the preparation methods of catalysts
chosen in this work and also the physico-chemical characterization methods used.
Besides the preparation and characterization of catalysts, this work intended to
develop and implement several experimental devices in order to correctly evaluate and
compare the activity and stability of the catalysts. Two fixed-bed reactors and a security
system were designed and implemented. A volumetric flowmeter for the measurement
of outlet gas flow and a water trap for the quantification of water formed during the
reaction were also implemented. The description of these reactors and devices will be
shown in the last section of this chapter.
2.2 Catalysts supports
2.2.1 Hydroxyapatite
Hydroxyapatite was chosen for catalyst support for all the reasons already
mentioned on section 1.3.2. Two different hydroxyapatites commercialized by
PRAYON company were used as reference:
TCP308 with low specific surface area (SBET = 7 m2/g)
TCP908 with high specific surface area (SBET = 60 m2/g)
Both supports were used with and without pretreatment step. They are called
thereafter TCP308 and TCP908 without pretreatment, and TCP308s and TCP908s with
pretreatment by thermal sintering. This pretreatment consists of heating the supports at
1000°C under air atmosphere and keeping at this temperature for 5h. The objective of
Materials and Methods
the sintering step was the stabilization of the supports. In fact, one of the reasons for
catalyst deactivation is the support sintering. So, the idea was to sinter the supports
prior to metal deposition, which avoids the sintering of support during catalytic test.
2.2.2 Other phosphates-based supports
In order to test the influence of other types of phosphates supports on the
catalytic
activity
of
the
prepared
catalysts,
two
different
phosphates
also
commercialized by PRAYON were used as reference:
Ca2P2O7 with low specific surface area (SBET = 8 m2/g)
Al(PO3)3 with high specific surface area (SBET < 3 m2/g)
Both supports were used without a pretreatment step.
2.2.3 Alumina-based supports
Two commercial alumina-based supports were also used in order to do a
comparative study with the hydroxyapatite-based catalysts:
γ-Al2O3, called thereafter Al2O3
Pural MG30, containing 30 and 70 wt% of MgO and Al2O3, respectively,
called thereafter Pural MG30 when it was used as received and Pural
MG30s when it was pretreated by thermal sintering.
The sintering step consisted of heating the Pural MG30 at 1200°C under air
atmosphere and keeping at this temperature for 5h.
2.3 Active metal and catalyst promoters
Five transition metals were studied: Zn, Fe, Co, Cu and Ni. Three different
promoters were also evaluated: Ce, Re and Co. The choice of the metals and
promoters was based on their price and known catalytic properties in DRM and in other
reactions [1]–[6]. The precursors of these metals are summarized in Table 2.1.
Table 2.1. Precursors of the active metals and catalyst promoters used in this work
Metal
66
Precursor
Supplier and purity (%)
Ni
Ni(NO3)2.6H2O
Fisher Scientific, >98%
Fe
Fe(NO3)3.9H2O
Fisher Scientific, >98%
Materials and Methods
Metal
Precursor
Supplier and purity (%)
Zn
Zn(NO3)2.6H2O
Fisher Scientific, >97%
Co
Co(NO3)2.6H2O
Acros Organics, >98%
Cu
Cu(NO3)2.3H2O
Acros Organics, >98%
Ce
Ce(NO3)3.6H2O
Acros Organics, >99%
Re
NH4ReO4
Sigma Aldrich, >99%
2.4 Catalyst synthesis methods
Three different synthesis methods were used to prepare the catalysts: incipient
wetness impregnation, co-precipitation and cation exchange. These methods were
chosen based on the literature review, on their simplicity to adapt to industrial scale
and/or on the possibility of combination with a hydroxyapatite property. The supports
were doped with 5.7wt% of transition metals in nitrate form. Nickel based catalysts
containing 2.85 to 11.5 wt% of Ni were also prepared.
2.4.1 Incipient wetness impregnation (IWI)
Table 2.2. Wettable volume for 10g of supports
Type of support
Support
Wettable volume (mL)
TCP308
5
TCP308s
5
Phosphates-based
TCP908
8
supports
TCP908s
7
Ca2P2O7
6
Al(PO3)3
3
Al2O3
10
Pural MG30
12
Pural MG30s
11
Alumina-based
supports
This method was chosen due to its simplicity and facility to be used at industrial
scale. The incipient wetness impregnation method consists of wetting perfectly the
67
Materials and Methods
support with an aqueous solution containing the exact desired final amount of metal, in
order to deposit homogeneous layers of metal precursor on the surface of the support.
So, in order to add the exact desired amount of metal, the wettable volume of each
support was determined. This volume is the amount of aqueous solution required to
wet perfectly the powder, without excess of the liquid phase. The wettable volume for
10g of the supports used to prepare the catalysts is shown in Table 2.2.
2.4.2 Co-precipitation (CP)
This method was only used for the preparation of hydroxyapatite-based catalysts.
The co-precipitation method (CP) is widely used in the literature for catalyst synthesis
and it is the method used by PRAYON to produce the hydroxyapatite at industrial
scale. This method (CP) consists on the simultaneous precipitation of calcium source
and active metal precursor with orthophosphate source. The idea is to simultaneously
incorporate the active phase to the apatitic structure of hydroxyapatite. For the
preparation, the metal precursor was added to concentrated phosphoric acid (75%). A
homogeneous and viscous solution was obtained. This solution and lime milk were
simultaneously added in a 300 mL glass reactor at constant flow rate under stirring.
Prior to the reaction, a foot of water (20mL) was added to the reactor in order to
improve the contact between the reactants. The synthesis lasted 1 hour. At the end of
the experiment, the mixture was filtered with 0.45 µm filter paper and dried at 50°C.
2.4.3 Cation exchange (CE)
This method was only used for the preparation of hydroxyapatite-based catalysts.
Hydroxyapatite has high cation exchange capacities for various metals, including all the
metals used in this work. Surface Ca2+ species can be replaced by metal cations. For a
given preparation, an aqueous solution of metal nitrate with predefined metal
concentration was prepared in a glass reactor. Then, hydroxyapatite powder was
added to the solution under stirring at room temperature. The pH and temperature
variations were measured over the reaction time. Samples of the suspension were
collected at different time intervals in order to evaluate the concentration of the soluble
Ca, P, and metal ions. At the end of the experiment, the mixture was filtered with 0.45
µm filter paper and dried at 50°C.
68
Materials and Methods
2.4.4 Addition of promoters
2.4.4.a Cerium addition
Cerium (Ce) was chosen due to the capacity of its oxide to promote coke
gasification and to avoid sintering of the active metal [7]. The Ni/Ce-TCP908 catalyst
was synthesized by two successive incipient wetness impregnations. First, the cerium
precursor was added to TCP908 by IWI. Then, the solid was calcined at 450°C under
air atmosphere. The resultant solid was doped with nickel precursor by IWI and
calcined at 500°C under air atmosphere.
2.4.4.b Rhenium addition
Rhenium (Re) was chosen due to its known activity in various chemical reactions
[8]–[11]. The Re/Ni-TCP908 was synthesized by IWI followed by wet impregnation.
First, the TCP908 was doped with a nickel precursor by IWI. Then, the solid was
calcined at 500°C. The resultant solid was doped with Re by wet impregnation,
according to Ly et al. [12] as follows:
1) 10g of Ni/TCP908 was added to 41.7mL of water;
2) 21mL of an aqueous solution containing 4wt% of Re was added to the
suspension of Ni/TCP908 and stirred for 5h at Tamb under 500rpm;
3) The suspension was evaporated under vacuum at 50°C in a rotary
evaporator;
4) The final catalyst was dried at 50°C for a week.
2.4.4.c Cobalt addition
Ni-Co bimetallic catalysts have been widely reported in the literature as a
performing catalyst due to the synergetic effect of the formation of a Co-Ni alloy for the
suppression of carbon deposits [13]. Moreover, Shimura et al. [14] reported that the
impregnation sequence has an influence on the catalytic performance of bimetallic NiCo
catalysts.
So,
three
different
catalysts
promoted
with
Cobalt
(Co)
were synthesized in this work: Ni/Co-TCP908, Co/Ni-TCP908, and Ni-Co/TCP908. The
objective was to test the influence of the promoter and of the impregnation sequence.
Ni/Co-TCP908 and Co/Ni-TCP908 were synthesized by two successive incipient
wetness impregnations. The difference between them is the order of the metal addition.
For Ni/Co-TCP908, TCP908 was doped with the cobalt precursor and calcined at
69
Materials and Methods
500°C. Then, the parent catalyst was doped with the nickel precursor and calcined at
500°C. As for the Co/Ni-TCP908, the support was doped first with the nickel precursor
and calcined at 500°C. Then, the parent catalyst was doped with the cobalt precursor
and calcined at 500°C.
Ni-Co/TCP908
was
synthesized
by
IWI
but
the
metals
were
added
simultaneously. A solution containing the desired amounts of each metal was prepared
and added to the TCP908. The material was then calcined at 500°C.
Table 2.3 summaries all the catalysts synthesized in this work.
Table 2.3. Catalysts synthesized with different transition metals and supports (x: prepared; -: not
prepared)
Number
70
Catalyst
Nominal metal
content, wt%
CoPrecipitation
Cation
Exchange
Incipient
Wetness
Impregnation
1
Zn/TCP308
5.7
-
x
x
2
Zn/Ca-HA
5.7
x
-
-
3
Fe/TCP308
5.7
-
x
x
4
Fe/TCP908
5.7
-
-
x
5
Fe/Ca-HA
5.7
x
-
-
6
Co/TCP308
5.7
-
-
x
7
Co/TCP908
5.7
-
-
x
8
Co/Ca-HA
5.7
x
-
-
9
Cu/TCP308
5.7
-
-
x
10
Cu/TCP908
5.7
-
-
x
11
Cu/Ca-HA
5.7
x
-
-
12
Ni/TCP308
5.7
-
-
x
13
Ni/TCP308s
5.7
-
-
x
14
Ni/TCP908
2.85
-
-
x
15
Ni/TCP908
5.7
-
-
x
16
Ni/TCP908
11.5
-
-
x
17
Ni/TCP908s
5.7
-
-
x
18
Ni/Ce-TCP908
Ni: 5.7
Ce: 5
-
-
x
Materials and Methods
Number
Catalyst
Nominal metal
content, wt%
CoPrecipitation
Cation
Exchange
Incipient
Wetness
Impregnation
19
Ni/Co-TCP908
Ni: 2.85
Co: 2.85
-
-
x
20
Co/Ni-TCP908
Ni: 2.85
Co: 2.85
-
-
x
21
Ni-Co/TCP908
Ni: 2.85
Co: 2.85
-
-
x
22
Re-Ni/TCP908
Ni: 5.7
Re: 4
-
-
x
23
Ni/Ca2P2O7
5.7
-
-
x
24
Ni/Al(PO3)3
5.7
-
-
x
25
Ni/Al2O3
5.7
-
-
x
26
Ni/Pural MG30
5.7
-
-
x
27
Ni/Pural MG30s
5.7
-
-
x
2.5 Characterization techniques
2.5.1 Chemical analyses
2.5.1.a Inductively Coupled Plasma Atomic Emission Spectroscopy (ICPAES)
ICP-AES is widely used for elemental analysis. It is a type of emission
spectroscopy that uses the inductively coupled plasma to produce excited atoms and
ions that emit electromagnetic radiation at wavelengths characteristic of a given
element. The sample is firstly nebulized and introduced in a plasma flame. This flame
is usually created by the ionization of argon gas. At high temperature of plasma, the
sample is atomized, excited and ionized. The excited atoms and ions emit radiation.
Each element has its characteristic emission wavelength, which allows the
identification of the element. The intensity of emission allows the quantification of the
element by using a calibration curve of the analyzed element [15], [16].
In this work, ICP-AES measurements were realized with a HORIBA Jobin Yvon
Ultima 2 apparatus. Dissolution step of the solid samples in aqua regia was required
before analysis. The main advantages of this equipment are the determination of the
71
Materials and Methods
concentrations of 72 of elements at once and also their identification even at very low
concentrations (ppb).
2.5.1.b Infrared Spectroscopy (IR)
The infrared spectroscopy allows the analysis of chemical functional groups
present in a given sample. Polar chemical bonds absorb infrared radiation at specific
wavelengths which correspond to allowed vibration frequencies. Infrared spectra arise
from transitions between vibrational (and rotational) energy levels with corresponding
absorption or emission of energy at discrete frequencies corresponding to
characteristic modes of vibration of molecules or surface groups [17].
In this work, IR spectra were recorded using a SHIMADZU spectrometer - 8400S
sweeping between 4000 cm−1 and 500 cm−1.
2.5.1.c X-ray Diffraction (XRD)
The X-ray diffraction allows determining atomic planes in regular crystalline
products. XRD technique is based on the interaction between monochromatic X-rays
and a crystalline phase. The incident rays interact with the sample and produce
interference and diffraction patterns. The angles of diffracted rays are determined, and
their intensities measured [18], [19]. The identification of the samples is done by
comparing with standards of a given database (JCPDS in this work).
In the present work, this technique was used to characterize the supports and the
prepared catalysts. The X-ray diffraction patterns were obtained with a Phillips
Panalytical X'pert MPD diffractometer equipped with a radiation source of Cu (Kα =
1.543 Å). The data acquisition was performed between 10 and 75° in 2θ with a step of
0.017° (2θ).
2.5.1.d Environmental Scanning Electron Microscopy coupled to EnergyDispersive X-ray analysis (ESEM-EDX)
Environmental Scanning Electron Microscopy allows the observation and
characterization of solid materials at nanometer (nm) to micrometer (µm) scale. It
provides three-dimensional-like images of surfaces of solid materials. So, it can be
used in a wide range of scientific fields, including metallurgy, geology, biology and
medicine [20], [21].
72
Materials and Methods
This technique is based on electron-sample interactions. When accelerated
electrons with high amount of kinetic energy interact with a solid sample they
decelerate and dissipate energy as a variety of signals. These signals include
secondary electrons (SE), backscattered electrons (BSE), diffracted backscattered
electrons (EBSD that are used to determine crystal structures and orientations of
minerals), photons (characteristic X-rays that are used for elemental analysis), visible
light, and heat. Usually, ESEM images are built from signals obtained with SE or BSE
detectors.
When ESEM is coupled to energy-dispersive X-ray analysis (EDX), it provides
the elemental composition of the surface of sample (analysis depth around 1 µm) at
semi-quantitative level. An energy-dispersive detector is used to separate the
characteristic X-rays of different elements into an energy spectrum in order to
determine the abundance of specific elements [22].
In the present work, ESEM-EDX analysis was used to characterize the shape
and size of metal particles, as well as carbon deposition on the surface of spent
catalysts. The analysis was done with a Philips XL30 ESEM FEG. Solid powder was
dispersed on a carbon-based support for direct observation of sample.
2.5.1.e Transmission Electron Microscopy coupled to Energy-Dispersive Xray analysis (TEM-EDX)
TEM is another surface analysis technique with higher resolution compared to
SEM. Its resolution can reach 0.08 nm. It enables the investigation of crystal structures,
specimen orientations and chemical compositions of phases, precipitates and
contaminants through diffraction pattern, X-ray and electron-energy analysis.
Transmission electron microscopy is used to produce images from a sample by
illuminating the sample with electrons (i.e. the electron beam) within a high vacuum,
and detecting the electrons that are transmitted through the sample [23].
In this technique, a beam of electrons is transmitted through an ultra-thin sample,
interacting with the sample as it passes through. The electrons are collected from
below the sample onto a phosphorescent screen or through a camera. In the regions
where electrons do not pass through the sample the image is dark. Where electrons
are unscattered, the image is brighter, and there are a range of greys in between
depending on the way the electrons interact with and are scattered by the sample [23],
[24].
73
Materials and Methods
When TEM is coupled to energy-dispersive X-ray analysis (EDX), it can provide
quantitative and qualitative elemental analysis from features as small as 1 nm.
In this work, TEM-EDX analysis was carried out using a MET FEG JEOL JEM
2100F apparatus.
2.5.1.f Temperature-programmed Analysis (TPX)
TPX provides valuable information about physical, chemical and thermal
behaviors of catalyst. It determines physico-chemical properties such as metal
dispersion, acid or base strength, surface acidity or basicity, distribution of strength of
active sites, BET surface area, and more [25]. Analyses are named accordingly to its
objective:
Pulse chemisorption: usually used to determine the metal dispersion.
Temperature-programmed reduction (TPR): indicates the temperature of
reduction of catalysts and provides information about the number of
reducible species present on the surface of catalyst.
Temperature-programmed desorption (TPD): determines the number,
type, and strength of active sites available on the surface of a catalyst.
Temperature-programmed oxidation (TPO): examines the extent to which
a catalyst can be oxidized or the extent to which it was previously
reduced.
Titration: Determination of acid and basic sites.
Most temperature-programmed experiments follow simplified steps. Firstly, the
gas chosen for the experiment flows into the analyzer and interacts with the sample as
the temperature changes. The gas flows through a TCD detector, which collects the
data and transmits it to the computer.
In this work, these analyzes were used to characterize the reduction of the
catalysts and to determine the metal dispersion and acid and basic sites of the catalyst.
TPX experiments were performed with a Micromeritics AutoChem 2920 Analyzer.
2.5.2 Physical analyses
2.5.2.a Physical nitrogen adsorption
Physical adsorption using nitrogen as probe molecule allows determining the
specific surface area and the porosity of a solid. Standard nitrogen adsorption is
74
Materials and Methods
usually carried out at 77K, after outgassing step at around 105°C under high vacuum.
From isotherm obtained, the specific surface area can be determined using for
example BET model (SBET, m2/g), which takes into account multilayer adsorption by
external and pore surface of solid [26]. The isotherm can suggest also the type and
size of porosity present in the solid. Different models are available for the determination
of porosity distribution such as: method Horvath-Kawazoe; method Dubinin-Stoeckli;
method BJH etc. This technique is widely used in catalysis, fuel cell technology,
adsorption, absorption, sintering studies etc. In this work, isotherm of nitrogen
adsorption was used for SBET and porosity measurements. The isotherms of nitrogen
adsorption were obtained with a Micromeritics Tristar II 3020 and with a Micromeritics
3FLEX.
2.5.2.b Density analyses
True density of the catalyst carrier especially is an important consideration during
manufacture and ―post-mortem‖ investigations since it relates to the crystal phase
which impacts both physical strength and to a varying degree the manner in which the
metals bonds to the support surface [27]. Gas pycnometry is one of the most reliable
techniques for obtaining true density of materials. This technique uses the gas
displacement method to measure volume of solid materials [28]. Helium was used as
displacement medium in this work. The true density measurements were obtained with
a Micromeritics AccuPyc 1330.
2.5.3 Thermal analyses
2.5.3.a Thermogravimetric Analysis (TGA)
Thermogravimetric analysis (TGA) measures weight changes in a material as a
function of temperature and/or time under a controlled atmosphere. It is mainly used for
studying thermal behavior of a solid: thermal stability, determination of composition etc.
The result of TGA can be explained by:
Cumulative or integral curve (TG), which describes the mass loss as a
function of time or temperature.
Derivative curve (DTG) that is used to more accurately detect the
temperature at the beginning and at the end of the transformations.
75
Materials and Methods
TGA measurement can be associated with heat flux measurement. In this work,
we exploited only TGA signal to evaluate the thermal stability of the synthesized
catalysts. A SDT Q600 apparatus from TA Instruments was used. For a given analysis,
about 30 mg of solid was introduced into a platinum crucible. The sample was then
heated under air flow (100 mL/min) up to 1200°C at the heating rate of 5°C/min.
2.5.3.b Thermomechanical Analysis (TMA)
Thermomechanical Analysis (TMA) measures dimension change (dilatation or
shrinkage) of a solid, liquid or pasty material as a function of temperature and/or time
under a defined mechanical force. TMA can provide valuable insight into the
composition, structure, the sintering and production conditions or application
possibilities for various materials [29].
In this work, TMA was used for support sintering studies. For a given
measurement, the sample was submitted to a load of 10g. The sample was then
heated under air flow (3L/h) up to 1200°C at the heating rate of 5°C/min. The
equipment used was a SETARAM SETSYS Evolution TMA 1600.
2.6 Catalytic reactors for DRM
Two catalytic reactors were used in this work. In the first part of the study, a
simple u-shaped quartz reactor system was used in order to rapidly evaluate the
performance of the different transition metals catalysts. In parallel with this study, a
fixed-bed tubular reactor was also designed and fabricated. This fixed-bed reactor was
used after the selection of the most promising catalyst system.
2.6.1 U-shaped quartz reactor
The U-shaped quartz reactor system shown in Figure 2.1 and Figure 2.2 is
composed of:
U-shaped quartz reactor (internal diameter of 8 mm; length of 50 cm).
Gas cylinders (pure N2 and mixture of reactants containing 20% CO2,
20% CH4 and 60% N2; volume percentage).
Rotameters to control inlet gas flow.
Electric furnace to control the reaction temperature.
Three-way valve to collect the outlet gas.
76
Materials and Methods
Figure 2.1. Schematic representation of the U-shaped reactor system
Figure 2.2. Photo of the U-shaped reactor system
For catalytic test, 0.3g of catalysts was introduced into the reactor and was set at
the reactor bottom. Then, the quartz reactor was placed inside a vertical electrical
furnace. After heating to the desired temperature (400 to 700°C) under N2 flow (70
mL/min), the reaction was started by feeding the reactor with the reactant mixture (70
mL/min at ca. 25°C and atmospheric pressure). The reaction was kept for less than 10
h of time-on-stream at atmospheric pressure. During the reaction, gas products were
analyzed using a µ-GC (Agilent A3000).
This reactor was not convenient for testings overnight due to lab security
regulation. In order to evaluate the catalysts long time-on-stream stability, a fixed-bed
tubular reactor was designed and manufactured.
77
Materials and Methods
2.6.2 Fixed-bed reactor
The fixed-bed tubular reactor manufactured by TOP INDUSTRIE shown in Figure
2.3 and Figure 2.4 is composed of:
Reactor tube made by non-porous inert alumina (internal diameter of 8
mm; length of 25 cm). This tube was surrounded by an Inconel 625 tube
to protect against mechanical impacts.
Electrical furnace.
K-type thermocouple placed at the center of catalyst bed to control the
reaction temperature.
Mass flowmeters to control the gas flow.
Pressure regulator.
Non-return valves to ensure the gas flow in only one direction
Three-way valve to collect the outlet gas.
Silica gel tube as a water trap.
Cynlinders of various gas.
Pressure sensor.
Dispositive for monitoring the reaction temperature, pressure and inlet gas
flow rate every 10s.
Security pressure disk.
Figure 2.3. Photo of the fixed-bed reactor
78
Materials and Methods
Figure 2.4. Schematic representation of the fixed-bed reactor
For catalytic test with the fixed-bed reactor, 0.3 g of catalyst powder was firstly
diluted 2 to 10 times with a non-porous inert alumina of similar particle size, and then
introduced into the reactor tube. The non-porous alumina was also used to maintain
the catalyst bed at the center of the reactor. The pressure lost due to catalyst and inert
alumina powders was typically 0.5-0.9 bar. The reactor was heated under nitrogen flow
up to desired temperature (400 to 700°C). The reaction was then started by switching
to the mixture of reactants gas (70 to 90 mL/min). As mentioned above, the reaction
temperature was controlled by a thermocouple inserted in the center of the catalyst
bed. In parallel, the reaction pressure was controlled by a manual regulator placed at
the exit of the reactor. In this work, the reaction pressure was investigated in the range
of atmospheric pressure to 30 bar. The gas flows were controlled by mass flowmeters
and regulators. The pressure, temperature and gas flow rate were numerically
controlled by a system with tactile screen. Non-return valves were placed just after the
mass flowmeters in order to ensure good circulation of gas lines.
A volumetric flowmeter and a silica gel tube were used to measure the outlet gas
flow rate and the amount of water formed during the reaction, respectively. These two
devices were necessary to the correct evaluation of the catalytic activity. Without these
79
Materials and Methods
two devices, satisfactory mass balances calculations, reactants conversion and
products selectivity determination would not have been possible. Quantification of the
water formed during the reaction was crucial to understand the catalytic performance of
different catalysts. To the best of our knowledge, water quantification has not yet been
reported in the literature for this catalytic process. Only some research teams [1], [30]–
[33] calculated the amount of water formed by mass difference.
A security system composed by various gas detectors, electrovalves and a
security central was also used. In case of gas leak, the detectors detect the presence
of dangerous gases. If their level exceeds the security limit, the security central
activates the electrovalves for stopping gas flow. This security system is required for
catalytic test overnight in our lab.
During the reaction, gas samples were withdrawn and analyzed by µ-GC (Agilent
A3000). The conversion and the selectivity of the reaction were calculated using the
following formulas:
Methane conversion:
Carbon dioxide conversion:
Hydrogen selectivity:
Carbon monoxide selectivity:
Water selectivity:
2.7 Conclusions
Several types of catalyst supports were used as catalytic supports in this work:
hydroxyapatite and commercial alumina-based powder. This last one was used for a
comparative study with the hydroxyapatite-based catalysts.
The hydroxyapatite supports were doped with five different transition metals: Fe,
Zn, Co, Cu and Ni. These metals were chosen due to their attractive cost and
promising catalytic performance in DRM and other reactions [1]–[6]. In addition, three
different promoters were also used in order to enhance the catalytic performance of
transition metal catalysts: Ce, Re and Co.
80
Materials and Methods
The catalysts were synthesized by three different preparation methods: incipient
wetness impregnation, co-precipitation and cation exchange.
The catalysts were further characterized by many different techniques: ICP-AES,
XRD, ESEM-EDX, TEM-EDX, BET, IR, TMA, TG, and TPX. These techniques were
chosen to determine the physico-chemical, thermal and thermomechanical properties
of catalysts and later correlate their characteristics with their catalytic performance.
The catalytic activity was evaluated using two reactors: U-shaped quartz reactor
and tubular reactor. The first one was used to rapidly evaluate the performance of the
different transition metals catalysts. Once the most promising catalyst system was
selected, the fixed-bed tubular reactor was used for further in-depth study on their
catalytic performance.
In this work, several experimental devices were developed and implemented in
order to correctly evaluate and compare the activity and stability of the catalysts. This
contributed to the engineering of the catalytic reactor for DRM at lab scale.
81
Materials and Methods
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[Online]. Available: http://www.micromeritics.com/Product-Showcase/AutoChemII-2920.aspx. [Accessed: 01-Jan-2015].
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Barrett-Joyner-Halenda (BJH) Pore Size and Volume Analysis.‖ [Online].
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84
Chapter 3
Characterization of the prepared catalysts
3.1 Introduction
This chapter will present the characterization results of the prepared catalysts.
Also, it will show the influence of the preparation method, supports, active phase and
promoters on the textural and physico-chemical properties of the catalysts, which will
later impact on their catalytic activity.
The present chapter will be divided in three parts: (I) characterization of
hydroxyapatite-based catalysts, (II) characterization of other phosphates-based
catalysts and (III) characterization of alumina based-catalysts.
The first part will present the hydroxyapatite-based catalysts prepared with
different doping metals (Me = Zn, Fe, Co, Cu, Ni). It will also highlight the influence of
different amounts of active metal (Ni wt% = 2.85, 5.7, 11.4%), promoters (Ce, Re and
Co) and thermal treatment of supports (TCP308 and TCP908) on the textural and
physico-chemical properties of catalysts.
The second and the third parts will briefly present the characterizations of other
phosphates and alumina-based catalysts that will later be used in a comparative study
with the hydroxyapatite-based catalysts.
3.2 Characterization of hydroxyapatite-based catalysts
This section will present the characterization results of the first part of this work
that consisted in scanning the possible active metals for the DRM. So, firstly, it will
present the hydroxyapatite-based catalysts doped with Zn, Fe, Co, Cu and Ni. Then, as
Ni is often reported in the literature as one of the most active metals for the DRM, it
Characterization of the prepared catalysts
was chosen as reference to the investigation of other parameters that might influence
the activity of catalysts such as promoters and amount of the active metal.
This first part will present only the essential characterization results of the
catalysts that can directly affect their catalytic activity. Other characterizations, such as
TGA-DTG, TMA of supports and EDX analysis can be found on the Appendix A.
3.2.1 Zn and Fe-based catalysts
3.2.1.a Nitrogen adsorption-desorption isotherms
The nitrogen adsorption-desorption isotherms at 77K were recorded for the
determination of specific surface areas and characterization of porous texture of the
hydroxyapatites supports (TCP908 and TCP308). The shape of these isotherms
depends on the texture of the solid. Figure 3.1 and Figure 3.2 present the results
obtained with TCP908 and TCP308, respectively.
Figure 3.1. Nitrogen adsorption-desorption isotherms of TCP908
The adsorption-desorption isotherms of the two supports studied are classified
into type II according to the IUPAC classification [1]. This type of isotherm is
encountered when the adsorption occurs on nonporous powders or on powders with
pore diameters larger than micropores [2]. In fact, both supports present a limited
uptake at low relative pressures (P/P0) indicating the inexistence of micropores.
Moreover, the adsorption isotherm of TCP908 (Figure 3.2) presented a small
hysteresis loop (P/P0>0.6), indicating the presence of mesopores. However, no
86
Characterization of the prepared catalysts
hysteresis loop was observed for the TCP308 (Figure 3.1), which indicates that it is a
nonporous support.
Figure 3.2. Nitrogen adsorption-desorption isotherms of TCP308
This difference in the porosity between the TCP908 and TCP308 may affect the
catalytic properties of the catalysts prepared with these supports. The mesopores of
the TCP908 might help increasing the metal dispersion and serve as channels through
which the gas molecules can diffuse. This can improve the rate of DRM and help
eliminating a possible coke deposit [3], [4].
The measurements of pore volume and other support characteristics, such as
density and particle size are indicated in Table 3.1.
Table 3.1. Measurement of particle size, density and pore volumes of the hydroxyapatite supports
Support
D50
(µm)
ρ
3
(g/cm )
Vp
3
(cm /g)
TCP308
5.2
3
nd
TCP908
6.1
2.7
0.072
3.2.1.b Specific surface area (SBET) and metal amounts
Table 3.2 below presents the SBET and ICP measurements of the initial supports
and of the Fe and Zn-based catalysts. Doping the supports with metals led to a small
87
Characterization of the prepared catalysts
reduction of the specific surface area of the catalysts. However, Fe/TCP308 catalyst
presented higher SBET than the initial TCP308 support.
Table 3.2. SBET and ICP measurements of Fe and Zn-based catalysts
Catalyst
Preparation
method
SBET
(m²/g)
Me
theoretical
(%)
Me
measured
(%)
TCP308
-
7
-
-
TCP908
-
60
-
-
IWI
6
6
5
CE
7
6
0.7
CP
30
6
6
IWI
18
6
6
CE
113
11
21
Fe/TCP908
IWI
56
6
4
Fe/Ca-HA
CP
95
6
6
Zn/TCP308
Zn/Ca-HA
Fe/TCP308
The catalysts synthesized by IWI and CP showed a metal amount similar to the
theoretical value. So, these two methods seem to be effective for the controlled
preparation of catalysts. However, the catalysts synthesized by CE presented values
very different from the theoretical ones. High amount of Fe was detected on
Fe/TCP308. In fact, CE was carried out at low pH (pH<5) during the synthesis to avoid
iron precipitation [5], [6]. So, the hydroxyapatite structure could have been modified or
destroyed and the Fe could have precipitated in a different form. IR analysis in the next
section confirmed this change. Zn/TCP308 catalyst showed very low Zn amount. In the
literature, hydroxyapatite has high affinity for Zn2+ fixation [7]. TCP308 has low specific
surface area (7m²/g) and it is not favorable for the exchange between Zn2+ and Ca²+.
Support aging due to the conservation of this support at industrial scale can also
explain the difference compared to literature data.
3.2.1.c Infrared spectroscopy (IR)
Figure 3.3 and Figure 3.4 below show the IR results of the Zn and Fe-based
catalysts, respectively.
88
Characterization of the prepared catalysts
Figure 3.3. IR results of Zn-based catalysts
Figure 3.4. IR results of the Fe-based catalysts
In the wavelength range of 1900 to 500 cm–1, hydroxyapatite has the
characteristic IR peaks at around 1200-916 cm–1 and 603-530 cm–1 (PO43– group) and
633 cm–1 (OH– group) [8]. The catalysts prepared by IWI showed the same peaks
compared to those of the initial supports (TCP308 and TCP908). Moreover, the
TCP908 showed a peak that corresponds to the carbonates (CO32-). It may be due to
the insertion of carbonate anions into the apatitic structure during the hydroxyapatite
synthesis, or during its conservation, or simply to the presence of calcium carbonate in
the limestone used for hydroxyapatite synthesis. On the other hand, for the samples
synthesized by CP the characteristic peaks of the hydroxyapatite were not well defined.
Similarly, the Fe-based catalyst synthesized by CE (Figure 3.4) did not present these
89
Characterization of the prepared catalysts
characteristic bands, which may indicate the transformation of the crystalline apatitic
structure of the initial TCP308 support into a more amorphous solid. However, this was
not the case for the Zn/TCP308 synthesized by CE (Figure 3.3). For this catalyst, the
hydroxyapatite structure was not transformed during the doping step.
3.2.1.d X-ray Diffraction (XRD)
Figure 3.5 Figure 3.6 below present the XRD results of Zn and Fe-based
catalysts, respectively.
Figure 3.5. XRD patterns Zn-based catalysts: (♦) Ca10(PO4)6(OH)2, (▼) Ca19Zn2(PO4)14
(pattern number: JCPDS 00-048-1196), (Δ) ZnO (pattern number: COD 96-900-4181)
Figure 3.6. XRD patterns of the Fe-based catalysts: (♦) Ca10(PO4)6(OH)2 (pattern number:
COD 96-230-0274)
90
Characterization of the prepared catalysts
The XRD patterns confirm the IR results. The catalysts prepared by IWI
presented the same crystalline apatitic phase (♦) as the initial supports (TCP308 and
TCP908). Moreover, the deposition of Zn by IWI led to the formation of the crystalline
ZnO phase (Δ) at 34.4, 36.3, 47.5, 56.6 and 67.9° (Figure 3.5). However, for the Febased catalysts (Figure 3.6), no iron-containing crystalline phase was observed,
despite high iron contents of these catalysts. This confirms the presence of an
amorphous phase.
No modification of the crystalline structure of TCP308 was observed after zinc
deposition by CE. As expected, there was no peak of zinc oxide (ZnO) in the pattern of
this catalyst. In fact, the content of Zn in this catalyst was only 0.7 wt%, which was too
small for XRD analysis. But in the literature, it is well known that this preparation
method leads to the insertion of Zn2+ ions in the apatitic structure of the support [7]. For
the Fe/TCP308 catalyst synthesized by CE, the crystallinity of the apatitic phase was
strongly decreased compared to the initial support. This was in agreement with IR
results and was explained by the degradation of the support at low pH during the CE
synthesis.
The catalysts prepared by CP showed very low crystallinity. However, traces of
hydroxyapatite (♦) were detected. No evidence of iron or zinc oxides was observed for
these catalysts by XRD, which suggests the formation of amorphous zinc-phosphates.
In fact, the Zn/Ca-HA (CP) showed a Ca19Zn2(PO4)14 phase (▼).
3.2.1.e ESEM microscopy
Figure 3.7 shows the ESEM analyses of the Zn-based catalysts.
Large Zn particles (>100nm) were detected on the catalyst synthesized by IWI.
However, no Zn particles were detected on the catalyst synthesized by CE. This can be
related to the low amount of Zn (0.7wt.%) of this catalyst, and by the insertion of Zn2+
into the apatitic structure. Also, no evidence of Zn particle could be observed for the
catalyst prepared by CP, despite the high Zn content of this catalyst (6wt.%). CP
synthesis did not only distribute Zn on the surface, but also inside catalyst particles.
Because Zn precursor was initially homogenized with orthophosphoric acid, we
suppose that Zn was later homogeneously distributed inside as well as on the surface
of this catalyst.
91
Characterization of the prepared catalysts
Figure 3.7. ESEM images of Zn-based catalysts
Figure 3.8 shows the ESEM analyses of the Fe-based catalysts
Figure 3.8. ESEM images of Fe-based catalysts
92
Characterization of the prepared catalysts
None of the Fe-based catalysts showed the presence of iron oxide particles.
Nevertheless, iron was detected by EDX analysis (Appendix A, section A.3.2). So, iron
must be very well dispersed on the support surface (and inside catalyst particles for CP
preparation). The metal dispersion has a direct impact on the catalytic activity of
catalysts. Generally, the catalytic activity is higher when the metal is well dispersed.
However, this is not the only parameter that influences the catalytic activity [9]. The
availability of the surface and pores of the support to the reactants during the reaction,
for example, also influences the activity and stability of the catalysts.
3.2.1.f TEM microscopy
Further investigations on the Zn and Fe-based catalysts synthesized by IWI were
realized with TEM-EDX. Figure 3.9 shows TEM results for Zn/TCP308 synthesized by
IWI.
Figure 3.9. TEM of Zn/TCP308 synthesized by IWI
Zn particles of size equal or less than 100nm can be clearly observed in this
figure. This observation is confirmed by the cartography in Figure 3.10. A clearly
defined particle of about 100nm is shown on the cartography and identified as a Zn
particle. These results confirm the ESEM observations.
Figure 3.11 shows TEM results for Fe/TCP308 synthesized by IWI. There are two
clearly different morphologies in Figure 3.11. The region represented by ―2‖ has a welldefined rod-shaped structure, whereas the region ―1‖ has an amorphous structure. The
cartography in Figure 3.12 indicates that these rod-shaped structures are composed
mostly with calcium and phosphorus, which means that this is a region where the
hydroxyapatite was not impregnated with Iron. On the other hand, the region
93
Characterization of the prepared catalysts
represented by ―1‖ is composed by apatite and Iron. There are no isolated iron
particles. However, the regions containing this metal are amorphous. This confirms the
ESEM results about iron particles dispersion. Moreover, no pores were observed on
the TEM images of the TCP308-based catalysts. This result is in agreement with the
nitrogen adsorption-desorption isotherms presented previously in this chapter (Figure
3.2).
Figure 3.10. Cartography of Zn/TCP308 synthesized by IWI
Figure 3.11. TEM of Fe/TCP308 synthesized by IWI
94
Characterization of the prepared catalysts
Figure 3.12. Cartography of Fe/TCP308 synthesized by IWI
3.2.2 Co and Cu-based catalysts
3.2.2.a Specific surface area (SBET) and metal amounts
As the CE method was not effective on doping the hydroxyapatite supports with
the desired amount of metals, Co and Cu-based catalysts were synthesized only by
IWI and CP methods.
Table 3.3 presents the SBET and ICP measurements of the Co and Cu-based
catalysts. As expected, doping the supports with metals led to a small reduction of the
specific surface area of the catalysts. Moreover, both synthesis methods led to the
loaded metal content similar to the corresponding theoretical value.
Table 3.3. SBET and ICP measurements of Co and Cu-based catalysts
Catalyst
Preparation
method
SBET
(m²/g)
Me
theoretical
(%)
Me
measured
(%)
TCP308
-
7
-
-
TCP908
-
60
-
-
Co/TCP308
IWI
7
6
5
Co/TCP908
IWI
59
6
5
Co/Ca-HA
CP
29
6
5
Cu/TCP308
IWI
5
6
5
Cu/TCP908
IWI
50
6
5
Cu/Ca-HA
CP
31
6
5
95
Characterization of the prepared catalysts
3.2.2.b Infrared spectroscopy (IR)
Figure 3.13 shows the IR results of the supports and of the prepared catalysts.
Figure 3.13. IR results of Co-based catalysts
Figure 3.14. IR results of the Cu-based catalysts
The catalysts prepared by IWI showed peaks of the characteristic groups of the
hydroxyapatite (PO43–, OH–). Moreover, all the TCP908-based catalysts showed a peak
that corresponds to the carbonates (CO32–), which indicates the presence of calcium
carbonate over the TCP908 and/or carbonate apatites. This has been observed with
iron- and zinc-based catalysts in the previous section.
On the other hand, the solid prepared by co-precipitation process showed no
characteristic peaks of well crystallized apatite phase. Vibrations of P–O bonds from
phosphate groups were observed but did not correspond to those of hydroxyapatite,
but rather corresponded to amorphous orthophosphates.
96
Characterization of the prepared catalysts
3.2.2.c X-ray Diffraction (XRD)
Figure 3.15 and Figure 3.16 below present the XRD results of Co and Cu-based
catalysts, respectively.
Figure 3.15. XRD patterns of Co-based catalysts: (♦) Ca10(PO4)6(OH)2 (pattern number:
COD 96-230-0274), (●) Ca9.46Co0.84P6O26 (pattern number: COD 96-230-0278), (□) Co3O4
(pattern number: COD 96-900-5888)
Figure 3.16. XRD patterns of Cu-based catalysts: (♦) Ca10(PO4)6(OH)2 (pattern number:
COD 96-230-0274), (○) Cu4O4 (pattern number: COD 96-101-1195)
97
Characterization of the prepared catalysts
The XRD patterns confirm the IR results. The catalysts prepared by incipient
wetness impregnation showed a crystalline hydroxyapatite phase (♦). Also, crystalline
phases of Co3O4 (□) at 36.8 and 59.3° (Figure 3.15) and Cu4O4 (○) at 38.7, 66.5 and
68° (Figure 3.16) have been observed for the catalysts prepared by this method.
The catalysts prepared by CP showed very low crystallinity. However, for Co/CaHA catalyst, traces of a cobalt exchanged hydroxyapatite (●) were detected.
Conversely to the catalyst synthesized by IWI, no evidence of a crystalline Co 3O4
phase was detected. The Cu/Ca-HA catalyst only presented traces of a hydroxyapatite
phase (♦).
3.2.2.d ESEM-EDX analyses
Figure 3.17 and Figure 3.18 show the SEM analyses of the Co-based catalysts
and Cu-based catalysts, respectively.
Figure 3.17. ESEM of Co-based catalysts
For the catalysts prepared by IWI method, the support TCP908 led to the
formation of smaller (Co and Cu particles Co particle size ≈ 100nm; Cu particle size ≈
200nm) than TCP308 (Co particle size ≈ 200nm; Cu particle size = 400-500nm). In
fact, TCP908 has a larger specific surface area (SBET = 60m²/g) and porosity (Vp =
0.072cm3/g), which could have improved the dispersion of metal particles compared to
98
Characterization of the prepared catalysts
the TCP308 (SBET = 7m²/g, Vp=nd). This is an important characteristic of the support
that can directly impact the activity of the catalysts as discussed previously on this
chapter.
For the catalysts prepared by CP method, no metal particles were detected on
the Co/Ca-HA catalyst. However, they were detected by EDX analysis, which means
that the cobalt particles were well dispersed over the support. Copper particles could
be observed on the Cu/Ca-HA, but only at high magnifications. This indicates a good
metal dispersion for this catalyst.
Figure 3.18. ESEM of Cu-based catalysts
3.2.3 Ni-based catalysts
As Ni is known to be one of the most active metals to the DRM, it was chosen as
reference to the investigation of other parameters that might influence the activity of
catalysts such as the addition of a promoter and the amount of nickel. Moreover,
TCP908 was chosen as the reference support since it presents higher specific surface
area (SBET = 60m²/g) and pore volumes (Vp = 0.072cm3/g) than TCP308 (SBET = 7m²/g,
Vp = nd), which favors the metal dispersion and may improve the catalytic activity of
catalysts. So, this section will present the characterization of hydroxyapatite-based
catalysts doped with different amounts of Ni as well as the characterization of Ni-based
99
Characterization of the prepared catalysts
catalysts promoted with Cerium (Ce), Cobalt (Co) and Rhenium (Re). It will focus on
the influence of these parameters on the properties of catalysts, which can later
influence their catalytic activity.
The IWI method appeared as the most effective method for the catalysts
preparation among the three methods investigated. This method allowed controlling the
desired amount of metal and did not strongly modify the support structure. Moreover, it
is simple to apply at industrial scale. So, this method was chosen as reference to this
stage of the study.
3.2.3.a Specific surface area (SBET) and metal contents
Table 3.4. SBET and ICP measurements of Ni-based catalysts (-: not measured)
100
Catalyst
Preparation
method
SBET
(m²/g)
Me
theoretical
(%)
Me
measured
(%)
TCP308
-
7
-
-
TCP308S
-
<3
-
-
TCP908
-
60
-
-
TCP908S
-
<3
-
-
Ni/TCP308
IWI
8
5.7
5
Ni/TCP308s
IWI
<3
5.7
4
Ni/TCP908
IWI
55
2.85
2.9
Ni/TCP908
IWI
51
5.7
5
Ni/TCP908
IWI
42
11.4
12
Ni/TCP908s
IWI
<3
5.7
4
Ni/Ce-TCP908
IWI
53
Ni: 5.7
Ce: 5
Ni: 5
Ce: -
Ni/Co-TCP908
IWI
49
Ni: 2.85
Co: 2.85
Ni: 2.84
Co:2.60
Co/Ni-TCP908
IWI
46
Ni: 2.85
Co: 2.85
Ni: 2.36
Co:2.36
Ni-Co/Ca-HA
IWI
45
Ni: 2.85
Co: 2.85
Ni: 2.41
Co:2.57
Re-Ni/TCP908
IWI
48
Ni: 5.7
Re: 4
Ni: 5
Re: -
Characterization of the prepared catalysts
Table 3.4 presents the specific surface area and ICP measurements for the Nibased catalysts. As expected, the catalysts showed a metal amount similar to the
theoretical value. Also, doping the supports with metals led to a small reduction of the
specific surface area of the catalysts.
3.2.3.b X-ray diffraction (XRD)
Unpromoted Ni-based catalysts: Influence of support and Ni amount
Figure 3.19 and Figure 3.20 present the influence of support, Ni amount and
support sintering on the XRD patterns of the Ni-based catalysts without promoters,
respectively.
All the prepared catalysts on Figure 3.19 and Figure 3.20 presented a crystalline
hydroxyapatite phase (♦) and a nickel oxide phase at 37, 43 and 63° (◊), since the IWI
method consists in depositing the active phase on the support. Moreover, the TCP908based catalysts doped with different amounts of Ni (2.85; 5.7 and 11.4%) showed
similar patterns with the difference on the intensity of the nickel oxide peaks (Figure
3.19). The higher was the nickel content, the higher was the intensity of nickel oxide
peaks. As expected, the catalyst prepared from the sintered support (Figure 3.20)
showed higher crystallinity than the catalysts prepared from the support without thermal
treatment.
Figure 3.19. XRD patterns of the unpromoted Ni-based catalysts: influence of support
and Ni amount, (♦) Ca10(PO4)6(OH)2 (pattern number: COD 96-230-0274), (◊) NiO (pattern
number: JCPDS 00-47-1049)
101
Characterization of the prepared catalysts
Figure 3.20. XRD patterns of the unpromoted Ni-based catalysts: influence of support
sintering, (♦) Ca10(PO4)6(OH)2 (pattern number: COD 96-230-0274), (◊) NiO (pattern
number: JCPDS 00-47-1049)
Promoted Ni-based catalysts: influence of Ce, Re and Co
Figure 3.21 presents the XRD patterns for the Ni-based catalysts promoted with
Ce, Re and Co.
Ni/TCP908 doped with Ce (5wt%) and Re (4wt%) in Figure 3.21 presented a
crystalline hydroxyapatite phase (♦) and a nickel oxide phase (◊) at 37, 43 and 63°.
However, no crystalline phase containing rhenium and cerium was observed. This
suggests high dispersion of Re and Ce in these catalysts, or they existed in an
amorphous phase.
As described on chapter 2 (section 2.4.4c), Ni-Co/TCP908 catalyst was
synthesized by co-impregnation method. This catalyst presented a crystalline
hydroxyapatite phase (♦). Crystalline phase of cobalt oxide was also observed with
peaks at 36, 45 and 59° (□). However, no crystalline nickel phase was detected. So, for
this catalyst, Ni must be well dispersed or in an amorphous phase.
Ni/Co-TCP908 and Co/Ni-TCP908 catalysts were synthesized by successive
impregnations (Chapter 2, section 2.4.4c). They presented a crystalline phase of cobalt
exchanged hydroxyapatite (●). So, cobalt was partially inserted in the hydroxyapatite
structure. This is supported by the detection of CaO as a new crystalline phase (Δ). We
suppose that Co2+ has partially replaced Ca2+ on the surface of TCP908 particle. The
thermal treatment of the catalyst led to the formation of CaO. For Ni/Co-TCP908
catalyst (deposition of Co, followed by Ni), crystalline phase of nickel oxide was
102
Characterization of the prepared catalysts
observed with peaks at 37, 43 and 63° (◊). However, Co/Ni-TCP908 catalyst
(deposition of Ni, followed by Co) did not present a nickel oxide or a cobalt oxide
phase. So, both metals might be well dispersed on the support.
Figure 3.21. XRD patterns of Ni-based catalysts promoted with Ce, Re and Co. (♦)
Ca10(PO4)6(OH)2 (pattern number: COD 96-230-0274), (●) Ca9.46Co0.84P6O26 (pattern number:
COD 96-230-0278), (◊) NiO (pattern number: JCPDS 00-047-1049), (□) Co3O4 (pattern
number: COD 96-900-5898), (Δ) CaO (pattern number: COD 96-900-6701)
3.2.3.c ESEM-EDX and TEM-EDX analyses
Unpromoted Ni-based catalysts: Influence of support and Ni amount
Figure 3.22 shows the ESEM analyses of the Ni-based catalysts without
promoters.
Ni particles on 5.7%Ni/TCP908 were much smaller (20-50nm) and better
dispersed than on 5.7%Ni/TCP308 (100-200nm). This is probably due to the higher
specific surface area of the TCP908 (SBET = 60m²/g, Vp = 0.072cm3/g) generated by the
presence of mesopores when compared to TCP308 (SBET = 7m²/g, Vp = nd), which
enhances the metal dispersion and may improve the catalytic activity of this catalyst.
The size and dispersion of nickel particles were also different for the TCP908based catalysts with different nickel contents. The nickel particles were not visible on
the 2.85%Ni/TCP908. This might be due to the low Ni doping amount. However, nickel
was detected by EDX analysis. 11.4%Ni/TCP908 catalyst showed larger Ni particles
(100-300nm) than 5.7%Ni/TCP908 (20-50nm). Similar results have been previously
observed by Kim et al. with an alumina support [10].
103
Characterization of the prepared catalysts
Figure 3.22. ESEM of the unpromoted Ni-based catalysts
Further investigations on 5.7%Ni/TCP908 catalyst were realized with TEM-EDX.
Figure 3.23 and Figure 3.24 show the TEM and cartography results for this catalyst,
respectively.
Figure 3.23. TEM images of 5.7%Ni/TCP908
Differently from TEM images of TCP308-based catalysts presented previously
(Figure 3.9, Figure 3.11), Figure 3.23 shows the presence of pores on the TCP908
structure. This result is in agreement with the nitrogen adsorption-desorption isotherms
presented previously in this chapter (section 3.2.1.a). Figure 3.23 also shows the
104
Characterization of the prepared catalysts
presence of non-spherical Ni particles of size equal or less than 20nm dispersed on the
surface of the support TCP908. This observation was confirmed by the cartography
presented on Figure 3.24. A clearly defined particle of about 20nm is shown on the
cartography and identified as a Ni particle.
Figure 3.24. Cartography of 5.7%Ni/TCP908
As indicated in chapter 2 (section 2.2.1), nickel deposition was also carried out
on the supports previously sintered. Figure 3.25 compared ESEM images of the
catalysts prepared from the supports with and without sintering step.
Figure 3.25. ESEM of Ni-based catalysts prepared from the supports with and without
sintering step
105
Characterization of the prepared catalysts
Firstly, we observed the change of morphology of the support. Hydroxyapatite
particles were sintered according to the reduction of SBET (Table 3.4). This sintering
phenomenon has been well known for hydroxyapatite-based materials [11]–[13]. For
both TCP308 and TCP908 supports with and without sintering step, the size and shape
of Ni particles were similar. On the other hand, the density of nickel particles increased
with the sintered support. This is explained by the reduction of SBET.
Ni-based catalysts promoted with Ce, Re and Co
Figure 3.26 presents the influence of promoter addition on the textural properties
of the 5.7%Ni/TCP908 catalyst.
Figure 3.26. ESEM images of Ni-based catalysts promoted with Re and Ce
As described on the previous chapter, Re/Ni-TCP908 catalyst was synthesized
by adding Re on 5.7%Ni/TCP908 catalyst. Both Re and Ni were detected by EDX
analysis. No morphological changes were observed for nickel particles after the
Rhenium addition. The separation between Ni and Re particles is not clear. EDX
analysis in Appendix A shows that Re and Ni are usually associated on the catalyst
surface.
Figure 3.26 also presents the ESEM results for Ni/Ce-TCP908 catalyst.
Conversely to the Re/Ni-TCP908, this catalyst was prepared by adding Ni over the
106
Characterization of the prepared catalysts
5%Ce/TCP908 catalyst. In this case, Ni particles seem to be larger (100-200nm) than
for the 5.7%Ni/TCP908 (20-50nm). It might be to the higher total metal amount of the
Ni/Ce-TCP908 when compared to the 5.7%Ni/TCP908. In fact, as shown previously on
Figure 3.22, higher metal amounts led to bigger metal particles.
Figure 3.27 presents the ESEM of Ni-based catalysts promoted with cobalt.
Reminding that Ni-Co/TCP908 was obtained by co-precipitation, Ni/Co-TCP908 and
Co/Ni-TCP908 were prepared by addition of Ni on Co/TCP908 and Co on Ni/TCP908,
respectively.
Figure 3.27. ESEM of Ni-based catalysts promoted with Co
EDX analysis confirmed the presence of both Ni and Co on the three catalysts
presented on Figure 3.27. They all exhibited nanometric well-dispersed metal particles.
This observation is in agreement with the literature review. In fact, cobalt addition is
known to enhance Ni particles dispersion [14]. The Ni-Co/TCP908, which was
synthesized by the co-impregnation method, showed larger metal particles (100200nm) when compared to the other two catalysts (≈ 50nm) on the Figure 3.27 that
were synthesized by successive impregnations. This difference on the size of metal
particles can have an impact on their catalytic activity.
107
Characterization of the prepared catalysts
3.2.3.d Temperature Programmed Reduction (TPR)
The TPX analyzer used in this work was implemented in the last months of this
thesis. So, only a few selected catalysts could be analyzed. As the hydroxyapatitebased catalysts will be later compared to commercial catalysts without promoters, only
the unpromoted Ni-based catalysts were analyzed.
Unpromoted Ni-based catalysts: Influence of support
Figure 3.28 presents the TPR results for the unpromoted nickel-based catalysts,
prepared by IWI method.
Figure 3.28. TPR results of unpromoted nickel-based catalysts
The XRD patterns of these catalysts presented on Figure 3.19 showed that the Ni
was present mostly on its oxide form (NiO) on the support surface. TPR results on
Figure 3.28 confirm these observations.
In fact, pure NiO usually presents two reduction peaks due to its stepwise
reduction that proceeds as follows: NiO → Niγ+ → Ni0. These reductions take place
around 425 and 500°C and the corresponding peaks are called β and γ, respectively.
The β peak represents the first reduction step and has a higher H2 consumption than
the γ peak, which represents the second reduction step. This means that the NiO can
be reduced in a great degree (or entirely) in the first reduction step [15], [16]. Moreover,
the reduction of nickel species that are at higher temperatures (T > 600°C) are usually
108
Characterization of the prepared catalysts
related to the Ni species incorporated into the supports structure [17]–[19]. This nickel
has stronger interaction with support, and needs higher reduction temperature.
All the reduction peaks present on Figure 3.28 were between 400 and 500°C,
which means that they represent the reduction of the NiO species at the external
surface of the hydroxyapatite supports. So, the different peaks observed for a given
catalyst in Figure 3.28 indicate the presence of Ni species with different oxidation
degrees. This has been previously observed for Ca-HA supported Ni catalysts [20].
For both TCP308 and TCP908 supports, the reduction temperatures of the
catalysts synthesized from the sintered supports (Ni/TCP308s and Ni/TCP908s) were
higher than those of the catalysts synthesized from the initial supports. This means that
sintered supports generates catalysts with stronger metal-support interactions and,
consequently, more difficult to reduce to the metallic state.
3.3 Characterization of other phosphate-based catalysts
This section will briefly present the catalysts synthesized with other phosphate
supports provided by PRAYON, including Ca2P2O7 and Al(PO3)3. The objective is to
investigate the influence of phosphate structure on the catalytic performance of
prepared catalysts.
3.3.1 Nitrogen adsorption-desorption isotherms
The Ca2P2O7 and Al(PO3)3 are non-porous materials, so the nitrogen adsorptiondesorption isotherms were not measured for these two supports. The measurements of
pore volume and other support characteristics, such as density and particle size are
indicated in Table 3.5.
Table 3.5. Measurement of particle size, density and pore volumes of the phosphates supports
Support
D50
(µm)
ρ
3
(g/cm )
Vp
3
(cm /g)
Ca2P2O7
8.7
3
nd
Al(PO3)3
167
3.3
nd
109
Characterization of the prepared catalysts
3.3.2 Specific surface area (SBET) and metal amounts
Table 3.6. SBET and ICP measurements of phosphate-based catalysts
(m²/g)
Me
theoretical
(%)
Me
measured
(%)
-
<3
-
-
-
8
-
-
Ni/Al(PO3)3
IWI
<3
5.7
5
Ni/Ca2P2O7
IWI
9
5.7
4.5
Preparation
SBET
method
Al(PO3)3
Ca2P2O7
Catalyst
Table 3.6 shows the SBET and ICP measurements of the supports and of the
phosphate-based catalysts. As expected, the catalysts showed a metal amount similar
to the theoretical value. Also, doping the supports with metals did not lead to changes
in the specific surface area values of the catalysts.
3.3.3 X-ray Diffraction (XRD)
XRD patterns for the Ca2P2O7 and Al(PO3)3-based catalysts are presented in
Figure 3.29 and Figure 3.30, respectively.
Figure 3.29. XRD of Ni/Ca2P2O7 catalyst, (♦) Ca2P2O7 (pattern number: JCPDS 01-0712123), (Δ) NiO (pattern number: COD 96-432-0488)
Both the initial Al(PO3)3 (■) and Ca2P2O7 (♦) supports are well crystalline. As
expected, the deposition of nickel on these supports led to the formation of crystalline
oxide with characteristic peaks at 37, 43 and 63° (Δ). The high intensity of these peaks
110
Characterization of the prepared catalysts
suggested the formation of large nickel crystallites. No modification of the support
structures was observed after nickel addition.
Figure 3.30. XRD of Ni/Al(PO3)3 catalyst, (■) Al(PO3)3 (pattern number: COD 96-101-0267),
(Δ) NiO (pattern number: COD 96-432-0488)
3.3.4 ESEM analysis
Figure 3.31 present the ESEM analysis of the phosphate-based catalysts.
Figure 3.31. ESEM of phosphate based catalysts
Conversely to the TCP908-based catalysts presented on Figure 3.25, both
catalysts presented large Ni particles (≈200nm). This might be due to the low specific
surface area and pore volumes of the initial supports that did not favor the nickel
dispersion. ESEM observation confirmed XRD results presented above. This difference
in the nickel dispersion may lead to catalytic performances very different from the
hydroxyapatite-based catalysts.
111
Characterization of the prepared catalysts
3.4 Characterization of alumina-based catalysts
This section will briefly present the characterizations of the alumina-based
catalysts. For DRM reaction, there is no available industrial application yet. Only test at
pilot scale has been demonstrated. The literature study showed that alumina-based
supports seemed to be the most performing for DRM, using nickel as the active metal
[21]. In this patent, high CH4 and CO2 conversions were obtained for long reaction
times. So, in this work, the same alumina-based support, purchased from industrial
partners, was used as reference industrial support. It is also well known that alumina is
one of the most used support for heterogeneous catalytic application due to its
availability and mechanical strength [22].
3.4.1 Nitrogen adsorption-desorption isotherms
The nitrogen adsorption-desorption isotherms at 77K were recorded for the
determination of specific surface areas and characterization of porous texture of the
alumina-based supports (Al2O3 and Pural MG30). Figure 3.32 and Figure 3.33 present
the results obtained with Al2O3 and Pural MG30, respectively.
Figure 3.32. Nitrogen adsorption-desorption isotherms of Al2O3 support
The nitrogen adsorption-desorption isotherm of the Al2O3 support presented in
Figure 3.32 corresponds to the type-IV with H1 hysteresis loop according to the IUPAC
112
Characterization of the prepared catalysts
classification. This type of isotherm is encountered when the adsorption occurs on
mesoporous solids with cylindrical pores [23]. Moreover, this support presented a
limited uptake at low relative pressures (P/P0) indicating the inexistence of micropores.
So, this is essentially a mesoporous support.
Figure 3.33 shows the nitrogen adsorption-desorption isotherm of Pural MG30.
This isotherm is classified into type II according to the IUPAC classification [1]. This
type of isotherm is encountered when the adsorption occurs on nonporous powders or
on powders with pore diameters larger than micropores [2]. In fact, it presented a small
hysteresis loop (P/P0>0.4) indicating the presence of mesopores. It also presented a
limited uptake at low relative pressures (P/P0) indicating the inexistence of micropores.
Besides improving the metal dispersion, the presence of mesopores on these two
supports may improve the contact between the gas molecules and the active phase
during the DRM and consequently improve the rate of this reaction. This might also
help eliminating a possible coke deposit and improve the catalysts stability, as
previously explained.
Figure 3.33. Nitrogen adsorption-desorption isotherms of Pural MG30 support
The measurements of pore volume and other support characteristics, such as
density and particle size are indicated in Table 3.7.
113
Characterization of the prepared catalysts
Table 3.7. Measurement of particle size, density and pore volumes of the alumina-based supports
Support
D50
(µm)
ρ
3
(g/cm )
Vp
3
(cm /g)
45
3.3
0.42
39.7
3.3
0.17
Al2O3
Pural MG30
3.4.2 Specific surface area (SBET) and metal amounts
Table 3.8. SBET and ICP measurements of supports and alumina-based catalysts
Preparation
method
SBET
(m²/g)
Me
theoretical
(%)
Me
measured
(%)
Al2O3
-
170
-
-
Pural MG30
-
148
-
-
Pural MG30S
-
<3
-
-
Ni/Al2O3
IWI
161
5.7
5.6
Ni/Pural MG30
IWI
94
5.7
6
Ni/Pural MG30s
IWI
<3
5.7
5.7
Catalyst
Table 3.8 below presents the SBET and ICP measurements of the supports and of
alumina-based catalysts. Doping the supports with metals led to a small reduction of
the specific surface area of the catalysts, as expected. Remind that Pural MG30 is
composed of 70wt% of alumina and 30wt% of MgO.
3.4.3 X-ray Diffraction (XRD)
Figure 3.34 and Figure 3.35 present the XRD patterns of the Al2O3 catalysts of
the Pural MG30 catalysts, respectively.
For alumina catalysts (Figure 3.34), the initial support (Al2O3) was poorly
crystalline. Characteristic peaks of γ-Al2O3 (◊) at 39.7, 45.9 and 67.3° were observed.
The deposition of Ni on this support by IWI method did not modify the crystallinity of the
support. No peak of nickel-based compounds was detected which suggests the
formation of well-dispersed nickel particles.
114
Characterization of the prepared catalysts
Figure 3.34. XRD patterns of Ni/Al2O3 catalyst, (◊) γ-Al2O3 (pattern number: COD 96-1010462)
Figure 3.35. XRD patterns of Pural MG30 catalysts, (■) MgAl2O4 (pattern number: COD 96900-5475), (◊) Al2O3 (pattern numbers: JCPDS 01-073-2294; JCPDS 00-016-0394; COD 96100-0443), (▲) MgO (pattern numbers: COD 96-901-3256, COD 96-901-3226, COD 96-9013222, JCPDS 01-075-0447), (●) NiO (pattern numbers: JCPDS 00-001-1239, JCPDS 01-0780429)
The initial Pural MG30 (Figure 3.35) support presented a mixture of Al2O3 phase
(◊) with main characteristic peaks at 11.8, 35, 39 and 47°C and an MgO crystalline
phase (▲). After doping (Ni) and calcination (T = 500°C) steps, significant changes in
the crystalline structure of this support could be observed. The resultant Ni/Pural MG30
catalyst also presented an Al2O3 phase but with characteristic peaks at 35, 44.2, 44.8,
62.5 and 65.2°. This catalyst also presented a crystalline diffraction plane at 43.1° that
could be both a NiO (●) and an MgO phase (▲). In fact, NiO and MgO have the same
crystalline structure. The dimensions of their unit cells are very similar, since Ni2+ and
Mg2+ have similar ionic radii (0.07nm for Ni2+ and 0.065nm for Mg2+). So, pure MgO and
115
Characterization of the prepared catalysts
pure NiO present very similar 2θ values, which render the dissociation of these two
phases very difficult [24]–[26].
The sintering the initial Pural MG30 support at 1200°C led to the formation of new
MgAl2O4 spinel phase (■) with main characteristic peaks at 36.8, 44.8 and 65.2°. It also
led to the formation of an MgO phase at 42.8 and 62.2° (▲). Heating the initial Pural
MG30 support at different temperatures (T = 500°C, 1200°C) led to significant changes
in the support structure. It indicates that the Pural MG30 was not thermally stable, but it
evolved into spinel as stable structure. So, the Ni/PuralMG30 catalyst may suffer
changes in its catalytic performance during a catalytic test due to changes in the
support structure.
The deposition of nickel by IWI on this sintered support did not change the
crystalline phase of the support. However, a phase that could be both MgO (▲) and/or
NiO (●) was also present at 43.3 and 62.8°.
3.4.4 ESEM microscopy
Figure 3.36 presents the ESEM analyses for the alumina-based catalysts.
Figure 3.36. ESEM of alumina-based catalysts
116
Characterization of the prepared catalysts
No Ni particles were observed for Ni/Al2O3 and Ni/PuralMG30 catalysts, obtained
from the initial supports. However, EDX analysis showed the presence of Ni on the
catalyst surface. In fact, the initial supports present high specific surface area and pore
volumes (Al2O3: SBET = 170m²/g, Vp = 0.42cm3/g; Pural MG30: SBET = 148m²/g, Vp =
0.17cm3/g), which may have increased the Ni particles dispersion.
Ni/Pural MG30s catalyst presented very small (<50nm) and well dispersed nickel
particles despite the great reduction on the SBET of the support after thermal treatment.
This result is interesting and would be further studied in more detail in order to
understand how the sintered Pural MG30s with SBET < 3 m2/g was able to well disperse
nickel on its surface.
3.4.5 TEM microscopy
Figure 3.37 presents the TEM images of the sintered Pural MG30-based catalyst.
Ni particles on Ni/Pural MG30s catalyst were difficult to distinguish. The boundary
between the Ni particles and the support is not clear and their size could not be
estimated. Moreover, traces of Ni can be found in the support particles by EDX
analysis.
Figure 3.37. TEM of the Ni/Pural MG30s catalyst
3.4.6 Temperature Programmed Reduction (TPR)
The reducibility of the alumina-based catalysts was investigated by TPR under H2
atmosphere. Figure 3.38 presents the H2-TPR profiles of the alumina-based catalysts.
All the catalysts presented reduction peaks in the temperature range of 350500°C, which corresponds to the reduction of NiO species.
117
Characterization of the prepared catalysts
Ni/Al2O3 catalyst presented one broad peak at 360°C, which corresponds to the
reduction peaks of small NiO particles [27], [28]. This broad peak may indicate a
relatively strong metal-support interaction of Ni particles with Al2O3 support.
Figure 3.38. H2-TPR profiles of alumina-based catalysts
Ni/Pural MG30s presented two peaks at 420 and 435°C that corresponds most
likely to the bulk small NiO particles with different clustering degrees [15], [16], [19],
[22]. Ni/Pural MG30 showed only one broad peak at 460°C, which also corresponds to
the reduction of small NiO particles but with strong metal-support interaction [19], [27],
[29].
The MgO addition seems to have an impact on the reducibility of the catalysts.
As can be seen in Figure 3.38, the catalysts prepared with the Pural MG30 which
contains 30 wt.% of MgO as support presented larger reduction peaks at higher
temperatures (Tred > 400°C) than the catalyst synthesized with Al2O3 as support (Tred =
360°C). These peaks at higher temperatures are related to the reduction of small NiO
particles with strong metal-support interaction. This may have an impact on the
performance of these catalysts. Generally, strong metal-support interaction is
responsible for higher catalytic activity in DRM process [9], [30].
118
Characterization of the prepared catalysts
3.5 Conclusions
Different catalysts were prepared from different hydroxyapatites (TCP308 and
TCP908) as supports and different transition metals (Zn, Fe, Co, Cu and Ni) as active
metals, and using different synthesis methods. These catalysts were then analyzed
and characterized by different techniques.
Among the three synthesis methods used (IWI, CE and CP), IWI showed to be
the most efficient. This method allowed the deposition of the exact desired amount of
metal and did not transform the support structure. Moreover, it is simple to apply at
industrial scale.
Among the hydroxyapatites catalysts with and without thermal treatment,
TCP908-based catalysts showed promising characteristics. They presented smaller
(20-50nm) and easier to reduce metal particles (Tred = 390°C) when compared to the
TCP308-based catalysts (Ni particle size: 100-200nm, Tred = 410°C). This was
attributed to the higher specific surface area (SBET = 60m²/g) of the TCP908 generated
by the presence of mesopores (Vp = 0.072cm3/g). This might lead to high catalytic
performance of these catalysts. The mesopores of the TCP908 helped increasing the
nickel dispersion over the support and may serve as channels through which the gas
molecules can diffuse. This can improve the rate of DRM and help eliminating a
possible coke deposit. In the case of Ni-based catalysts, support sintering did not
influence the metal dispersion. Nevertheless, it increased the metal-support interaction,
leading to high reduction temperature.
The influence of different Ni amounts (Ni wt.% = 2.85, 5.7 and 11.4%) and
promoters (Ce, Re, Co) was also investigated. The catalysts with high amounts of Ni
showed large Ni particles (100-300nm). Re addition did not seem to have an influence
on the size and morphology of Ni particles. However, Ce addition led to the formation
of larger Ni particles (100-200nm) when compared to the unpromoted Ni catalyst (2050nm). On the other hand, Co addition enhanced the dispersion of the metal particles.
The differences of metal dispersion among these catalysts may lead to differences in
their catalytic activity.
In order to later compare the performance of hydroxyapatite-based catalysts with
other phosphates and commercial catalysts, Ca2P2O7, Al(PO3)3, Al2O3 and Pural MG30
were also used as catalyst support. Both phosphates (Ca2P2O7, Al(PO3)3) led to the
formation of large Ni particles (100-200nm) due to their low specific surface area and
pore volumes (Ca2P2O7: SBET = 8m²/g, Vp = nd; Al(PO3)3: SBET < 3m²/g, Vp = nd ). This
119
Characterization of the prepared catalysts
might negatively affect the contact between the gas molecules and the active phase
and thus the catalytic activity of these catalysts. However, Al2O3 and Pural MG30
(mixture of Al2O3 and MgO) led to the formation of well-dispersed Ni particles probably
due to the high specific surface area and pore volumes of these supports (Al2O3: SBET =
45m²/g, Vp = 0.42 cm3/g; Pural MG30: SBET = 39.7m²/g, Vp = 0.17cm3/g). Moreover,
the presence of mesopores on these two supports may improve the contact between
the reactants and the active phase and thus improve the rate of DRM. This might also
help eliminating a possible coke deposit and improve the catalysts stability. The
sintering of Pural MG30s did not affect the size of nickel particle, compared to the initial
Pural MG30. Moreover, the Ni/Pural MG30s catalyst was more easily reduced than
Ni/Pural MG30 catalyst.
In conclusion, the physical and chemical properties of the supports, such as
porosity, specific surface area, basicity etc have great impact on the catalysts
characteristics that will later influence their performance. So, the next chapters will
present the catalytic performance of the prepared catalysts in DRM as well as
correlation between their performance and their physico-chemical properties.
120
Characterization of the prepared catalysts
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H. Liu, H. Wang, J. Shen, Y. Sun, and Z. Liu, ―Preparation, characterization and
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123
Chapter 4
Investigation of the active phase
4.1 Introduction
There has been much effort to develop a catalyst for DRM that is stable under
the extreme conditions of this reaction (high temperatures, severe heat and mass
transfer limitations etc) [1], [2]. Carbon deposition, which deactivates the catalysts and
clogs the reactors, is the main reason why this reaction has not yet been used at
industrial scale. Noble metals-based catalysts showed high activity and stability in this
reaction. However, due to their high costs, transition metals have gained much
attention [3]. Nevertheless, they are more prone to carbon deposition [4], [5]. So,
research has focused on the elaboration of new supports and/or promoters that can
enhance the activity and stability of the active phase of catalysts [2], [6]–[9].
The objective of this chapter is to investigate the possible active phases for DRM
that when doped on a hydroxyapatite support can generate an active and stable
catalyst.
So,
this
chapter
will
compare
the
catalytic
activity
of
different
Me/hydroxyapatite (Me = Zn, Fe, Co, Cu, Ni) catalysts, in order to find the most
efficient one. The influence of the reaction temperature (T = 400-700°C) and of
promoters (Ce, Re, Co) on the catalysts performance will also be investigated.
4.2 Comparison of the impact of transition metals-based
catalysts in DRM
The investigation of the active phase was realized on the U-shaped quartz
reactor described in chapter 2 (section 2.6.1). In order to evaluate the catalysts
performance at different temperatures, the reaction temperature varied between 400
Investigation of the active phase
and 700°C with a heating rate of 20°C/min. The molar composition of the inlet gas
mixture was CH4/CO2/N2 = 20/20/60 and the weight hourly space velocity (WHSV) was
10500mLh−1gcat−1. The catalysts were previously calcined at different temperatures to
their stabilization. No activation step by thermal treatment under reductive atmosphere
was done prior to catalytic tests. The temperature of calcination was chosen from TGADTG results of each catalyst presented on Appendix A. TGA-DTG showed the
temperature where metal precursors were decomposed.
Blank tests with the reactor and with the hydroxyapatite supports (TCP308 and
TCP908) were realized with the view to discriminate their influence on the results of the
catalytic tests of the prepared catalysts. These results are presented in the next
section.
The main objective of this first step of the study was to compare the catalytic
performance of the different catalysts prepared with hydroxyapatite as catalyst support.
We analyzed only the composition of the gas mixture sampled from the reactor outlet.
The volume concentration of each gas present in this mixture was determined. On the
other hand, we did not calculate the conversion of CH4 or CO2, and the selectivity in
CO or H2 or other byproducts because there was a change in the gas flow rate at the
reactor outlet compared to that at the reactor inlet. This change has not been
determined with the U-shaped quartz reactor used for this comparative study. In fact,
for the DRM reaction, the conversion of 1 mole of CH4 and 1 mole of CO2 leads to the
formation of 2 moles of CO and 2 moles of H2. The presence of side reactions
complicates the determination of the outlet gas flow rate. So an experimental
measurement is necessary. This measurement was only carried out with the fixed-bed
reactor, presented in the next chapter. However, for the comparative study, the
composition of reactor outlet gas mixture is sufficient to draw conclusions.
4.2.1 Evaluation of monometallic supported catalyst
(Me/hydroxyapatite, Me = Zn, Fe, Co, Cu, Ni)
4.2.1.a Investigation of the catalytic activity of the reactor walls and
supports (TCP308 and TCP908): Blank tests
A blank test with the empty reactor was realized in order to determine its catalytic
activity on DRM. For this test, no catalyst or support was introduced in the reactor.
Figure 4.1 shows the blank test result.
126
Investigation of the active phase
Figure 4.1. Blank test with the U-shaped quartz reactor (reaction conditions:
CH4/CO2/N2=20/20/60, T=400-700°C)
As expected, there was no conversion of methane or carbon dioxide and no
production of hydrogen and carbon monoxide in the temperature range investigated.
So, the reactor walls do not influence the catalytic results obtained for the prepared
catalysts.
The results of the catalytic tests with the supports are shown in Figure 4.2 below.
Figure 4.2. Catalytic test of hydroxyapatite supports: TCP308 (left side) and TCP908 (right
-1
-1
side) (reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
The CH4 and CO2 amounts of the outlet gases were practically similar to the inlet
gases (20%vol), which means that no reaction took place. As expected, the supports
were not active for DRM in the temperature range of 400 to 700°C.
127
Investigation of the active phase
4.2.1.b Zn-based catalysts
Figure 4.3 presents the results of the catalytic tests of the Zn-based catalysts in
the temperature range of 400-700°C. The volume concentrations of CH4 and CO2
remaining, and CO and H2 generated of the samples withdrawn from the reactor outlet
are presented. The catalysts prepared by IWI and CP were investigated, while the
catalyst prepared by CE was not evaluated because of uncontrolled zinc content by
this preparation method.
Figure 4.3. Catalytic tests of Zn-based catalysts (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
None of Zn-based catalysts were active in DRM in the temperature range and
test conditions used. ESEM images of the Zn-based catalysts presented in chapter 3
(Figure 3.7) showed the formation of large metal particles (>100nm) for both catalysts.
In fact, it has been extensively reported in the literature that small metal particles and
thus high metal dispersion enhance the catalytic activity of catalysts [10]–[16]. So, the
formation of large metal particles, and probably low metal dispersion, was not favorable
128
Investigation of the active phase
for the catalytic reaction. The low specific surface area and pore volumes of the
TCP308 (SBET = 7m2/g, Vp =nd) may be responsible for the low zinc dispersion and
catalytic activity of the Ni/TCP308 catalyst. Moreover, XRD patterns (Chapter 3, Figure
3.5) showed the presence of the ZnO phase for the catalyst synthesized by IWI. The
test conditions may not have been enough to reduce the ZnO particles into metallic Zn,
which is known to be the active phase [11], [17]–[19].
Further experiments with an in-situ reduction of these catalysts will be needed for
affirming their low activity for DRM.
4.2.1.c Fe-based catalysts
Figure 4.4. Catalytic tests of the Fe-based catalysts (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
The catalytic activity of the Fe-based catalysts was tested in the temperature
range of 400-700°C. Figure 4.4 shows the catalytic activity of the catalysts synthesized
by IWI and CP. The volume concentrations of CH4 and CO2 remaining, and CO and H2
129
Investigation of the active phase
generated of the samples withdrawn from the reactor outlet at each temperature are
presented. The catalyst prepared by CE has not been tested because of its low iron
content and the transformation of apatitic structure during CE synthesis.
Despite the high dispersion of the iron particles as shown by the ESEM-EDX
analysis in chapter 3 (Figure 3.8), the three iron-based catalysts did not present
catalytic activity in the DRM under the test conditions used. In the literature, Iron has
been used as active metal and/or promoter for DRM [20], [21]. However, Iron catalysts
prepared in this work were not active at 400-700°C. This might be due to the fact that
the catalysts have not been pretreated by reduction to get metallic Iron particles as real
active phase, prior to DRM. Previous studies also indicated that, for DRM process, the
catalytic activity of a given catalyst does not only depend on its active phase, but also
on other factors such as the metal-support interaction [11], [22].
4.2.1.d Co-based catalysts
Figure 4.5 presents the results of the catalytic tests of the Co-based catalysts in
the temperature range of 400-700°C.
The characterization study in the previous chapter (Chapter 3, section 3.2.2.d)
already showed that the three Co-based catalysts had different characteristics
regarding metal dispersion, particle size and surface area. So, different catalytic
activities were expected for these catalysts. However, they were all inactive in DRM
under the experimental conditions used.
It has been reported that catalysts containing less than 6wt% of Co can be
deactivated by metal oxidation [23]. Ruckenstein et al. [24] explained that because of
the coexistence of both reductive (CH4, H2 and CO) and oxidative (CO2 and H2O)
species in DRM, the atmosphere in the reactor is both reductive and oxidative. The
reductive atmosphere stimulates the generation of metallic Co and the dissociative
adsorption of methane, while the oxidative atmosphere favors the oxidation of metallic
Co sites. The catalysts would be stable when there is a balance between the carbon
generation and its oxidative removal. This balance could be achieved by optimizing the
metal loading. In addition, because these catalysts have not been reduced prior to
RDM test, Co particles were under oxide forms, which are not favorable to initiate the
reaction.
130
Investigation of the active phase
Figure 4.5. Catalytic tests results of Co-based catalysts (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
4.2.1.e Cu-based catalysts
Figure 4.6 presents the results of the catalytic tests of the Cu-based catalysts in
the temperature range of 400-700°C. The characterization study previously showed
that these catalysts were also very different from each other, in particular concerning
copper distribution on the surface of the support. Cu/TCP308 synthesized by IWI
showed the formation of large Cu particles of about 400-500nm (Chapter 3, Figure
3.18), which was explained by the low specific surface area (SBET = 7m²/g) and low
porosity (Vp = nd) of TCP308. On the other hand, smaller Cu particles of about 200nm
were obtained with TCP908 support by the same IWI method. So, the supports had an
impact on the dispersion of the copper particles. For CP method, no cooper particles
were visualized for the Cu/Ca-HA (CP). These differences in particle size and
dispersion of the active phase should have an impact on the catalytic activity of these
131
Investigation of the active phase
catalysts. However, they were all inactive for DRM under the experimental conditions
used.
Figure 4.6. Catalytic activity evaluation of Cu-based catalysts (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
4.2.1.f Ni-based catalysts
This section will present the results of the catalytic tests with Ni-based catalysts.
The influence of the different hydroxyapatite-based supports (TCP308 and TCP908)
and of the Ni loading (Ni wt% = 2.85; 5.7; 11.4%) will be discussed.
Influence of supports: TCP308 and TCP908
The catalytic activity of the catalysts 5.7%Ni/TCP308 and 5.7%Ni/TCP908, both
synthesized by IWI, was evaluated in the temperature range of 400-700°C. Figure 4.7
shows the volume concentrations of CH4 and CO2 remaining, and CO and H2
generated of the samples withdrawn from the reactor outlet.
132
Investigation of the active phase
Figure 4.7. Catalytic tests of Ni-based catalyst with different hydroxyapatite supports
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
At 400 and 500°C, both catalysts did not have any notable catalytic activity. At
around 600°C, Ni/TCP308 was found active. At 700°C, both catalysts showed high
activity. At this temperature, Ni/TCP908 was slightly more active than TCP308 with
amounts of H2 and CO produced by Ni/TCP908 slightly higher than those produced by
TCP308.
Despite the similarities in their catalytic activity at 700°C, Ni/TCP908 was chosen
as reference for the study of the influence of Ni amount and promoters. In fact, ESEM
(Chapter 3, Figure 3.22) and TEM results (Chapter 3, Figure 3.23) showed the
formation of well dispersed Ni particles (20-50nm) for this catalyst, which may lead to
the formation of smaller amount of coke and consequently contribute to higher stability
[25] of the prepared catalysts than the Ni/TPC308. Moreover, conversely to the
TCP308, TCP908 (Vp=0.072cm3/g) showed the presence of mesopores (Chapter 3,
Figure 3.1) that can improve the rate of DRM and help eliminating the coke deposit.
Further investigations on these two catalysts concerning stability and carbon
deposition will be presented in the next chapter.
133
Investigation of the active phase
Influence of Ni amount
This part of the study investigated the influence of the amount of Ni on the
catalytic activity of the catalysts. Figure 4.8 presents the volume concentrations of CH4
and CO2 remaining, and H2 and CO generated in the gas product mixture sampled at
the reactor outlet.
Figure 4.8. Catalytic evaluation of Ni/TCP908 with different amounts of Ni (wt.%=2.85; 5.7;
-1
-1
11.4) (reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
The catalyst 2.85%Ni/TCP908 showed no catalytic activity at 400-700°C. As
shown in chapter 3 (Figure 3.22), ESEM analysis did not reveal the presence of Ni
particles on the surface of the support. Different hypotheses might be proposed which
link with the low activity of this catalyst as following:
(i) Ni2+ cation of the metal precursor were inserted into the apatitic structure of
TCP908 support by cationic exchange with Ca2+ cations, when this support was
impregnated with aqueous solution of nickel nitrate (IWI method). These ―apatitic Ni2+‖
are embedded and become inactive species.
134
Investigation of the active phase
(ii) Nickel nitrate was simply deposited on the surface of TCP908 and the
calcination at 500°C led simply to highly dispersed NiO particles, which were not visible
under SEM observation (Chapter 3, Figure 3.22). These small particles have strong
metal-support interaction and were not reduced by the gas reactant mixture at 400700°C. Thus, Ni particles of this catalyst have not yet reached the ―activated form‖.
(iii) A combination of both phenomena. Supplementary characterization such as
XPS may help to confirm these hypotheses. A catalytic test with an in-situ reduction
prior to the reduction may be also useful.
About the catalyst 5.7%Ni/TCP908, high catalytic activity at 700°C was observed
as already shown in Figure 4.7. In this case, Ni particles were activated at the
beginning of the catalytic test at 600°C. The formation of reducers such as H2, CO from
DRM and methane cracking allowed the activation of the Ni particles.
When the Ni loading was increased to 11.4%, CH4 was practically entirely
consumed. The hydrogen concentration in the gas product sampled from the reactor
outlet was high and saturated the detector. Thus, H2 peak integration could not be
done, which prevented the quantification of H2 concentration. For this experiment, the
reactor was clogged because of the formation of high coke amount. Gas reactant
mixture could not pass through the catalyst bed after some hours at 700°C. It has been
reported that the increase in Ni loading is responsible for the increase of the average
metal particle size, and this was observed for the nickel catalysts prepared on
hydroxyapatite support by IWI method (Chapter 3, Figure 3.22). Consequently, large
nickel particles favor the coke formation [13].
The objective of using a catalyst is to soften the experimental conditions, in
particular the reaction temperature of DRM for a better global energy balance. The
temperature of 700°C seems to be a good compromise, regarding the literature reports
on this process. At this temperature, the Ni loading of 5.7% appeared as the most
appropriate because the loading of 2.85% was not performing enough and the loading
of 11.4% led to oversized catalyst bed (the catalytic performance aligned with the
thermodynamic limit of the DRM reaction). In addition, the Ni loading of 5.7% had a
better Ni dispersion than the Ni loading of 11.4%. From the economical point of view, a
high metal amount is not favorable because it directly impacts the turn-over-frequency
(TOF) of the catalysts [26]. Thus, for the next steps of this study, 5.7% nickel-based
catalysts will be used. Optimum loading of active phase has been reported on the
literature [13], [23].
135
Investigation of the active phase
4.2.2 Effect of promoter addition on Ni supported hydroxyapatite (YNi/hydroxyapatite, Y = Ce, Re, Co)
4.2.2.a Ce addition
Figure 4.9 compares the catalytic performance of Ni/Ce-TCP908 and
5.7%Ni/TCP908.
Figure 4.9. Comparison of catalytic performance between Ni/Ce-TCP908 and Ni/TCP908
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
Cerium addition has been extensively reported as an effective promoter in
various catalytic processes. It improves the catalyst performance by improving the
dispersion of the active phase and increasing the activity and stability of the catalyst
[21], [27]. Cerium oxides are also well known for their oxygen storage and transfer
properties [28]–[30]. This property is generally favorable for the catalytic reaction
involving oxygen-containing species transfer. In this work, Ni/Ce-TCP908 catalyst
showed high catalytic activity at 700°C in DRM. However, its catalytic performance was
136
Investigation of the active phase
similar to that of monometallic catalyst Ni/TCP908. In this study, nickel particles of
Ni/Ce-TCP908 catalyst (100-200nm) were found to be larger than those of Ni/TCP908
(20-50nm) (Chapter 3, Figure 3.26). So, the similarity of the catalytic performance of
these two catalysts might be due to the a compromise between the decrease of
average Ni particle size of Ni/Ce-TCP908 catalyst compared to that of Ni/TCP908
catalyst, and the beneficial effect of the cerium oxides added.
The effect of Ce addition may be important for the study of catalytic stability for
long reaction time. In fact, coke resistance is much researched for DRM reaction. CeO2
addition is known to considerably improve the catalysts resistance to carbon formation
due to its redox and oxygen storage capacities [31]. Tests of long reaction time are
needed to confirm this hypothesis.
4.2.2.b Re addition
The result of the catalytic test for the Re/Ni-TCP908 catalysts is presented in
Figure 4.10.
Figure 4.10. Catalytic evaluation of Re/Ni-TCP908 (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
Despite the small metal particles (20-50nm), the Re/Ni-TCP908 catalyst was
inactive for DRM with the test conditions used. The Re addition completely inactivated
the 5.7%Ni/TCP908 catalyst.
Claridge et al. [32] reported that rhenium catalyst was very active for DRM at
temperatures above 1000°C. However, below 900°C, it was significantly less active.
Moreover, many studies reported that rhenium catalysts can be reduction resistant due
to its tendency to form inactive oxides under reaction conditions [32]–[35]. Furthermore,
137
Investigation of the active phase
rhenium oxides species have been proposed to impose additional acidity to the catalyst
surface [34], which can favor the coke deposition. In fact, this reaction is usually
favored by basic catalysts, which are responsible for enhancing the CO2 adsorption.
4.2.2.c Co addition
The results of the catalytic tests for the Ni-TCP908 catalyst promoted with Co by
different methods are presented in Figure 4.11.
Figure 4.11. Catalytic evaluation of Ni/TCP908 catalyst promoted with Co (reaction
-1
-1
conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
Despite the good dispersion of the metal particles observed on the ESEM images
(Chapter 3, Figure 3.27), none of the three catalysts was active in the DRM under the
test conditions used. There have been many studies that report the positive effect of
doping Ni catalysts with Co. Generally, it enhances the Ni dispersion and the formation
of a Ni-Co alloy is efficient for the suppression of carbon deposits [14], [23], [28], [33]–
[35], [36]. However, it was not found in our case. XRD analysis showed the presence of
138
Investigation of the active phase
metal oxide particles. Was the reaction temperature of 700°C not enough for
―activating‖ these metallic particles? Note again that the catalyst bed was not
pretreated by a reduction step prior to the test.
In order to verify this hypothesis, catalytic tests were performed at 750°C with the
same catalysts. The results of these tests are shown in Figure 4.12.
Figure 4.12. Catalytic evaluation at 750°C of Ni-based catalysts promoted with Co
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat )
Both the catalysts synthesized by successive impregnations (Co/Ni-TCP908 and
Ni/Co-TCP908) showed low amounts of CH4 and of CO2 and high amounts of H2 and
CO (22-25%), which means they were strongly active for DRM at 750°C. In addition,
they were stable during the 4h of time on stream (TOS). These results confirmed the
hypothesis that the reaction conditions of the tests at 700°C were not enough to reduce
the metal ions. San José-Alonso et al. [23] showed that the reduction temperature of
cobalt oxide species (Co3O4) was at about 730°C for 2.5wt%Co/Al2O3, which would
explain the activity at 750°C of the catalysts in Figure 4.12.
139
Investigation of the active phase
On the other hand, the Ni-Co/TCP908 catalyst prepared by CP method was not
active even at 750°C. We suppose that the co-precipitation led to the formation of
nickel phosphates and cobalt phosphates, which were not active species for DRM.
Further characterizations are needed for confirming this hypothesis.
4.3 Conclusions
This chapter focused on the identification of the active phase for DRM. Different
transition metals (Zn, Fe, Co, Cu, Ni) doped on two different hydroxyapatites (TCP308
and TCP908) were tested in order to find the most promising combination. Catalysts
containing a promoter (Co, Re, Ce) were also studied.
In general, the catalysts synthesized with the TCP908 showed smaller metal
particles than the catalysts prepared with the TCP308. This could be explained by the
high specific surface area and pore volumes of TCP908 (SBET = 60m²/g, Vp =
0.072cm3/g) compared to that of TCP308 (SBET = 7m²/g, Vp = nd). Under the
experimental conditions used (T = 400-700°C, P = Patm), only the Ni-based catalysts
were active in DRM. As the catalysts were submitted to air calcination before the
catalytic tests, metal oxides were formed, which were identified by XRD for most of the
prepared catalysts. No further in-situ reduction was done prior to the catalytic test. For
catalysts which showed negligible catalytic activity, we suppose that the main reason
must be due to the non-metallic state of the active metals. The mixture of CH4 and CO2
must not be enough for reducing metal oxides to metallic particles. Another reason
might be the insertion of metal cations into the apatitic structure which form rigid
chemical bonds embedded in the support, and thus these metals became inactive.
Among the investigated transition metals, Ni was chosen for this study.
The influence of the Ni content on the catalytic performance was then
investigated. Three catalysts prepared with TCP908 containing different amounts of
nickel (Ni wt% = 2.85%, 5.7%, 11.4%) were tested. The catalyst containing 5.7%Ni was
found to be the most appropriate compromise.
The effect of promoter addition (Ce, Re, Co) to a catalyst Ni/TCP908 was also
investigated. Re addition deactivated completely the Ni/TCP908 catalyst. In fact,
rhenium catalysts tend to form inactive oxides under reaction conditions [32]–[35],
which can be difficult to reduce and impose additional acidity to the catalyst surface
[34]. Ce addition seemed to not lead to a gain of catalytic activity. The bimetallic Co-Ni
catalysts prepared by successive impregnations led to the formation of small metal
140
Investigation of the active phase
particles (≈50nm). They were not active at 700°C, because this temperature might be
not sufficient for reducing metal oxide to metallic particles. They were found active at
750°C and stable for 4h of TOS. On the other hand, the Co-Ni bimetallic catalyst
prepared by CP was not active, even at 750°C.
Even if bimetallic catalysts have potential for DRM, they were not further
investigated in this study. Ni monometallic catalysts prepared by IWI method were
chosen because of the simplicity in the preparation and the promising catalytic
performance. The Ni loading of 5.7% is also selected as the most appropriate
compromise.
141
Investigation of the active phase
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144
Chapter 5
Parametric study of Ni/hydroxyapatite catalysts
5.1 Introduction
The previous chapter investigated the possible active phases for DRM that could
generate an active and stable catalyst. Among the transition metals tested (Me = Zn,
Fe, Co, Cu, Ni), Ni showed to be the most promising one. It also investigated the
influence of the preparation methods and promoters on the performance of the
catalysts. However, there are other variables that can also influence the performance
of the catalysts. In fact, there has been much effort on the development of the catalysts
but less attention has been paid on the optimization of the reaction conditions [1], [2].
So, this chapter will first present a parametric study in order to obtain suitable reaction
conditions for the production of high yields of syngas. Parameters such as temperature
and pressure of the reaction, pretreatment of the catalysts and sintering of the supports
will be investigated. Then, it will present a comparison of the hydroxyapatite-based
catalysts with other phosphates-based catalysts and alumina-based catalysts. The aim
is to compare the prepared catalysts with the ones already used at industrial scale for
other industrial processes.
5.2 Influence of testing conditions on the catalytic activity of
hydroxyapatite-based catalysts
The parametric study of the Ni/hydroxyapatite catalysts was performed with the
fixed-bed reactor described in chapter 2 (section 2.6.1). The implementation of this
reactor was necessary for the parametric study and for better evaluation of the
performance of the catalysts. In fact, this experimental apparatus allows better contact
Parametric study of Ni/hydroxyapatite catalysts
of the gas-solid phases and better reaction temperature control than in the U-shaped
reactor and avoids eventual preferential passages of the gas reactive mixture. In
particular, it allows controlling the pressure of reactor and long duration tests, which
was not possible with the U-shaped reactor.
So, this section will validate the new experimental apparatus. Blank tests with the
reactor, and the supports were also realized with the view to discriminate their
influence on the results of the catalytic tests of the prepared catalysts. Finally, the
investigation of the influence of the reaction parameters on the catalytic performance of
catalysts will be presented. For this parametric study, the test conditions will be varied
in order to identify the most appropriate experimental conditions. Moreover, the amount
of remaining reactants and products formed could be quantified. In fact, the fixed-bed
reactor was equipped with mass flow-meters at the reactor inlet, and a gas volumetric
flowmeter at the reactor outlet, which are necessary for the quantification of each
gaseous product.
5.2.1 Investigation of the catalytic activity of the reactor walls and
supports: Blank tests
In the blank tests the reaction temperature varied between 400 and 700°C with a
heating rate of 20°C/min. The molar composition of the inlet gas mixture was
CH4/CO2/N2 = 20/20/60 and the weight hourly space velocity (WHSV) was
10500mLh−1gTCP908−1.
A blank test with the empty reactor was realized in order to determine its catalytic
activity on DRM. For this test, no catalyst or support was introduced in the reactor. The
results are presented in Figure 5.1.
Figure 5.1. Blank test with the fixed-bed reactor (reaction conditions: CH4/CO2/N2 =
20/20/60, T = 400-700°C, P = Patm)
146
Parametric study of Ni/hydroxyapatite catalysts
As expected, there was no notable conversion of methane or carbon dioxide and
no production of hydrogen and carbon monoxide in the temperature range investigated.
So, the reactor is inert and should not influence the catalytic results obtained with the
prepared catalysts under the experimental conditions used.
Blank test with the hydroxyapatite support and with the inert alumina was also
realized. For this test, TCP908 and the inert pure alumina were introduced in the
reactor. The results are presented in the Figure 5.2.
Figure 5.2. Blank test with the TCP908 and with the inert alumina (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat , P = Patm)
The CH4 and CO2 amounts of the outlet gases were similar to the inlet gases
(20%vol), which means that no reaction took place. As expected, neither the
hydroxyapatite nor the alumina was active for DRM in the temperature range of 400 to
700°C.
5.2.2 Set-up of the experimental apparatus
Figure 5.3 presents the catalytic tests with the 5.7%Ni/TCP908 catalyst in the Ushaped reactor and in the fixed-bed reactor. The reaction temperature varied between
400 and 700°C with a heating rate of 20°C/min. The molar composition of the inlet gas
mixture was CH4/CO2/N2 = 20/20/60 and the weight hourly space velocity (WHSV) was
12353mLh−1gcat−1. The 5.7%Ni/TCP908 catalyst was previously calcined at 500°C. No
activation step by thermal treatment under reductive atmosphere was done prior to
catalytic tests. The temperature of calcination was chosen from TGA-DTG results
presented in Appendix A. This calcination allowed transforming the nickel precursor
into nickel oxides. For the test with the fixed-bed reactor, the catalyst was diluted ten
times with inert alumina in order to avoid clogging the reactor.
147
Parametric study of Ni/hydroxyapatite catalysts
Figure 5.3. Comparison of the catalytic activity of 5.7%Ni/TCP908 in the U-shaped reactor
and in the fixed-bed reactor (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 400-700°C,
-1
-1
WHSV = 12353mLh gcat , P = Patm, catalyst dilution (test with the fixed bed reactor) = 10x)
The Ni/TCP908 catalyst was active around 600°C in the fixed-bed reactor, while
it was active only at 700°C in the U-shaped reactor. In fact, the fixed-bed reactor
provides better reaction temperature control than the U-shaped reactor. The
temperature is controlled by a thermocouple inside the reactor, while for the U-shaped
reactor the temperature is controlled by a thermocouple inside the reactor walls. This
difference in the temperature control explains the difference in the temperature of
activation of the catalysts. However, once at 700°C, the catalyst presented similar
catalytic performance with both reactors. This allows validating the catalytic results
obtained with the U-shaped quartz reactor in the previous chapter.
5.2.3 Influence of the reaction temperature
In order to evaluate the catalysts performance at different temperatures, the
reaction temperature varied between 400 and 700°C with a heating rate of 20°C/min.
The molar composition of the inlet gas mixture was CH4/CO2/N2 = 20/20/60 and the
148
Parametric study of Ni/hydroxyapatite catalysts
weight hourly space velocity (WHSV) was 12353mLh−1gcat−1. The catalysts were
previously calcined at different temperatures to their stabilization. No activation step by
thermal treatment under reductive atmosphere was done prior to catalytic tests. Figure
5.4
shows
the
catalytic
performance
of
the
hydroxyapatite-based
catalysts
(5.7%Ni/TCP908 and 5.7%Ni/TCP308) at different temperatures.
Figure 5.4. Catalytic performance of hydroxyapatite-based catalysts at different
temperatures (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 400-700°C,
-1
-1
WHSV=12353mLh gcat , P=Patm, catalyst dilution=10x)
Both catalysts were found active at around 600 and 700°C. In fact, as DRM is a
strongly endothermic reaction [3], which explains these catalytic results. In chapter 1
(Figure 1.1), thermodynamic calculations showed also that carbon deposits are
thermodynamically favored at temperatures lower than 600°C and CO2 and CH4
conversions are favored at temperatures higher than 600°C. So, in order to avoid
carbon deposition and to enhance reactants’ conversion, the reaction temperature was
fixed at 700°C for the investigation of the other parameters that will be presented in this
chapter.
Ni/TCP908 showed higher catalytic performance at 600°C and 700°C than the
Ni/TCP308. In fact, the TCP908 (SBET = 60 m²/g, Vp = 0.072cm3/g) presents higher
porosity than TCP308 (SBET = 60 m²/g, Vp ≈ 0cm3/g), which helps increasing the metal
dispersion and provides larger contact area for the reactants. Moreover, ESEM results
showed that Ni/TCP908 had smaller Ni particles (around 20-50nm) than those of
Ni/TCP308 (around 100-200nm). TPR analysis (Chapter 3, Figure 3.28) showed also
that the reduction temperature of Ni particles was lower with Ni/TCP908 (Tred = 370390°C) than with Ni/TCP308 (Tred = 370-410°C). This would explain the difference in
the catalytic performance of these two catalysts. This aspect will be more detailed in
the next topic (Influence of the catalyst pretreatment).
149
Parametric study of Ni/hydroxyapatite catalysts
5.2.4 Influence of the catalyst pretreatment
The pretreatment conditions before reaction and the activation of catalysts are
key factors which can influence the metal particle size and metal-support interactions,
and will later affect their catalytic performance [4], [5]. Different atmospheres are
effective in the reduction of catalysts. The most investigated atmospheres are H2, He,
CO and CH4 [6]–[8]. However, Liu et al. [4] reported that CH4 and CO pretreatments are
unfavorable to the DRM because they led to the formation of high amounts of carbon
species during the reaction. H2 has been reported as efficient reducer for NiOx [4], [9].
Moreover, it is a product of the DRM. So, H2/N2 was used for the in situ reduction of
nickel catalysts in this work.
The reduction of nickel oxide particles depends not only on the atmosphere but
also on other features. In fact, after the rupture of Ni—O bonds to produce Ni0 atoms,
there is a nucleation step of the Ni0 atoms into metallic clusters and then the growth of
the nickel metallic clusters into crystallites. This nucleation step is influenced by the
presence of defects in the surface of NiOx particles, which are influenced by impurities
on the catalyst, supports and preparation methods [9].
In this study, the reaction temperature varied between 400 and 700°C with a
heating rate of 20°C/min. The molar composition of the inlet gas mixture was
CH4/CO2/N2 = 20/20/60 and the weight hourly space velocity (WHSV) was
12353mLh−1gcat−1. The catalyst was diluted ten times with inert alumina in order to
avoid clogging the reactor. No calcination step was realized. Prior to the reaction, the
catalysts were in situ reduced by flowing 4%H2/N2 for 1h at 400°C.
Figure 5.5 and Figure 5.6 present the influence of the reduction step on the
catalytic
performance of
the
5.7%Ni/TCP908 and
5.7%Ni/TCP308 catalysts,
respectively. The catalytic performance of each catalyst with and without in situ
reduction is reported.
The pretreatment by in situ reduction under H2 atmosphere was found to be
favorable for both catalysts. For the Ni/TCP908 catalyst, CH4 and CO2 consumptions
were observed at 500°C after the in situ reduction step. The same catalyst was active
only at 600°C without the pretreatment step. However, at 600-700°C, the similar
catalytic performance was observed for the catalysts with and without pretreatment.
Probably, the in situ reduction under H2 atmosphere at 400°C and the reduction under
the atmosphere of the reaction led to the formation of similar reduced nickel-based
particles (Ni0). In fact, the reaction temperature of 600°C and the reducing properties of
150
Parametric study of Ni/hydroxyapatite catalysts
the atmosphere reactants’ mixture were probably enough to reduce the NiOx particles
into Ni0. This phenomenon has been reported in the literature [10] and can be
confirmed by TPR (Figure 3.28) and ESEM (Figure 3.22) results presented on chapter
3. TPR (under H2 atmosphere) curves presented two sharp peaks between 370 and
390°C, which mean that there are NiOx particles with different oxidation states but
weakly interacting with the TCP908 support. Moreover, the SEM images showed the
formation of small metal particles for this catalyst. These results indicate that the Ni
particles could have been easily reduced and activated by the reaction atmosphere.
Figure 5.5. Influence of pretreatment on 5.7%Ni/TCP908 (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=12353mLh gcat , P=Patm, catalyst dilution=10x)
For the Ni/TCP308 catalyst, CH4 and CO2 consumption were also observed at
500°C after the in situ reduction step. As expected, the catalyst without the
pretreatment was active only at 600°C. Moreover, CH4 and CO2 consumptions at 600
and 700°C were higher with the pretreated catalyst compared to the catalyst without
pretreatment. This result can be explained by the TPR curves (Figure 3.28) and ESEM
images (Figure 3.22) presented on chapter 3. TPR curves presented three large
reduction peaks between 380 and 410°C. This indicates the presence of NiOx particles
having different oxidation states and having stronger interaction with the support than
that of Ni/TCP908 catalyst. Moreover, the SEM images for this catalyst showed the
151
Parametric study of Ni/hydroxyapatite catalysts
presence of large Ni particles. So, the NiOx particles are harder to reduce compared to
the Ni/TCP908 catalyst and the sole reduction under the atmosphere of the reaction
mixture is not enough to completely reduce the NiOx particles.
Figure 5.6. Influence of pretreatment on 5.7%Ni/TCP308 (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=12353mLh gcat , P=Patm, catalyst dilution=10x)
Once again, we observe that the support TCP908 showed better results than
TCP308. The remaining concentrations of CH4 and CO2 were higher with Ni/TCP308
than those with Ni/TCP908, both catalysts being in situ reduced. In parallel, the
concentrations of H2 and CO formed were lower with Ni/TCP308 than those with
Ni/TCP908. This is again explained by higher porosity and specific surface area of the
TCP908 when compared to the TCP308 (Chapter 3, section 3.2.1a), which helps
increasing the nickel dispersion and provides larger contact area for the reactants. This
is in agreement with the ESEM observations (Chapter 3, Figure 3.22).
5.2.5 Effect of support sintering
The DRM reaction is usually performed at high temperature (T>700°C) in order to
get high equilibrium conversion to syngas and avoid carbon deposition. However,
sintering of both support and nickel particles occur at high reaction temperatures and
are responsible for irreversible catalyst deactivation [11]–[13]. So, as the catalyst
152
Parametric study of Ni/hydroxyapatite catalysts
sintering seems inevitable, the objective of this section is to investigate the influence of
the support sintering on the catalytic performance of catalysts.
In this study, the reaction temperature varied between 400 and 700°C with a
heating rate of 20°C/min. The molar composition of the inlet gas mixture was
CH4/CO2/N2 = 20/20/60 and the weight hourly space velocity (WHSV) was
12353mLh−1gcat−1. The catalyst was diluted ten times with inert alumina in order to
avoid clogging the reactor. No calcination step was realized. Prior to the reaction,
catalysts were in situ reduced by flowing 4%H2/N2 for 1h at 400°C.
Figure 5.7 compares the catalytic performance of the 5.7%Ni/TCP908 (support
without thermal treatment) and of the 5.7%Ni/TCP908S (support with thermal treatment
at 1000°C prior to nickel deposition). The conditions of preparation of these catalysts
were already described in chapter 2 (Section 2.2.1).
Figure 5.7. Effect of support sintering on the catalytic performance of Ni/TCP908
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=12353mLh gcat , P=Patm,
catalyst dilution=10x)
Reminding that the sintering step led to the reduction of SBET of TCP908 from 60
to less than 3 m2/g (Chapter 3, Table 3.4). In Figure 5.7, both catalysts with and without
support sintering had similar catalytic performance in the test conditions used, except
153
Parametric study of Ni/hydroxyapatite catalysts
slight difference below 700°C. This result might be explained taking into account the
intrinsic thermal stability of hydroxyapatite [14]. As presented previously in chapter 1
(section 1.3.2d), hydroxyapatite decomposes only at temperatures above 850°C [15]–
[17], conversely to other common catalyst support such as alumina. Despite changes in
its morphology, the sintering step must not have changed the crystalline structure of
the hydroxyapatite. This was confirmed by the XRD analysis presented on chapter 3.
Moreover, SEM images highlighted that despite the decrease of SBET after sintering
step, TCP908S support did not lead to the increase of the size or the modification of the
shape of Ni particles, compared to those on the initial TCP908 support. Only the
density of Ni particles on TCP908S increased. In fact, the catalytic activity of a catalyst
depends, among other factors, on the size and the morphology of Ni particles [18].
According to its shape and size, the Ni particles will more or less favor the carbon
fibers growth on its surface, which will later deactivate the catalyst [19], [20]. In our
case, because there was no change in the size and the shape of Ni particles, similar
catalytic performance was expected for both catalysts.
The difference of the catalytic performance below 700°C could be explained by
TPR results. Ni/TCP908S showed a broad reduction peak at 430°C while Ni/TCP908
presented two sharp peaks between 370 and 390°C. So, the reduction temperature of
400°C was enough to reduce Ni particles of the Ni/TCP908 catalyst. However, it only
partially reduced Ni particles of the Ni/TCP908S. This explains the small difference on
the consumption of CO2 and CH4 below 700°C. We suppose that at 700°C, Ni particles
of the Ni/TCP908S must be completely reduced under H2 atmosphere generated by the
DRM. Consequently Ni/TCP908S showed similar catalytic performance at 700°C
compared to Ni/TCP908.
5.2.6 Effect of pressure
DRM is usually performed at atmospheric pressure in order to minimize the coke
deposition, which is favored at high pressures [21]. Therefore, there are few studies
about DRM at high pressure [21]. However, working at high pressures can be essential
for the overall process economy. In fact, the utilization of the syngas for the FisherTropsch synthesis, for example, requires 20-25 bars [22]. So, either the syngas
production is performed at high pressure or the syngas is compressed after production.
However, compressing the syngas with high CO amount is hard. So, the production of
the syngas at high pressure is preferable [23]. Moreover, the energy consumption in
154
Parametric study of Ni/hydroxyapatite catalysts
this case is reduced [24]. So, the influence of pressure on the catalytic performance of
catalysts will be investigated in this section.
Figure 5.8 presents the effect of pressure (P = 1, 5, 15 and 25 bar) on the
catalytic performance of the 5.7%Ni/TCP908S. In this study, the reaction temperature
was kept at 700°C with a heating rate of 20°C/min. The molar composition of the inlet
gas mixture was CH4/CO2/N2 = 20/20/60 and the WHSV was 12353mLh−1gcat−1. The
catalyst was diluted ten times with inert alumina in order to avoid clogging the reactor.
No calcination step was realized. Prior to the reaction, catalysts were in situ reduced by
flowing 4%H2/N2 for 1h at 400°C.
Figure 5.8. Influence of pressure on the catalytic performance of 5.7%Ni/TCP908S
-1
-1
(reaction conditions: CH4/CO2/N2 = 20/20/60, T = 700°C, WHSV = 12353mLh gcat , P = Patm25bar, catalyst dilution = 10x)
The amounts of CO2, CH4, H2 and CO shift drastically with the increase of the
reaction pressure. CO2 amounts increase from 2% at 1bar to 9% at 25 bars and CH4
amounts increase from 2% at 1bar to 12% at 25 bars. Conversely, H2 amounts
decrease from 26% at 1bar to 10% at 25 bars and CO amounts decrease from 27% at
1 bar to 14% at 25 bars. This confirms that DRM is unfavorable at high pressures,
according to the stoichiometry of the reaction. Similar conclusions were obtained in the
study of Jafarbegloo et al. [3]. They found that increasing the pressure led to decrease
in the conversion of CH4 and CO2 and, consequently, to the decrease of H2 and CO
production. They attributed this result to the coke formation. In fact, Boudouard reaction
(Equation 1.2) and inverse carbon gasification (Equation 1.5) are the main reactions
responsible for coke formation and they are favored at high pressures.
Due to high disparities in the consumption of the reactants within the pressure
range investigated, the reaction pressure was kept close to atmospheric for the next
part of this study.
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Parametric study of Ni/hydroxyapatite catalysts
5.3 Comparison of the hydroxyapatite-based catalysts with
other catalysts
In this section, other phosphates and commercial alumina were also tested as
catalysts supports. The objective is to compare the hydroxyapatite-based catalysts with
other catalysts already in use at industrial scale for other industrial processes.
5.3.1 Other phosphates-based catalysts
In this work, other phosphates produced by PRAYON were tested as catalysts
supports. The objective was to scan the catalytic potential of other phosphate-based
supports. Ca2P2O7 (pyrophosphate) and Al(PO3)3 (aluminum phosphate) were selected.
In fact, Ca2P2O7 has been used as catalyst support in other chemical reactions, such
as partial oxidation of n-butane and synthesis of styrene via oxydehydrogenation of
ethylbenzene [25], [26]. Al(PO3)3 and modified-Al(PO3)3 have been used in reactions
such as partial oxidation of methane, DRM and dichlorodifluoromethane hydrolysis
[27], [28]. So, the catalytic performance of 5.7%Ni/Ca2P2O7 and 5.7%Ni/Al(PO3)3
catalysts, which were prepared by IWI method, was compared to the 5.7%Ni/TCP908
catalyst. Figure 5.9 presents the results of the catalytic tests.
The reaction temperature varied between 400 and 700°C with the heating rate of
20°C/min. The molar composition of the inlet gas mixture was CH4/CO2/N2 = 20/20/60
and the weight hourly space velocity (WHSV) was 12353mLh−1gcat−1. The catalyst was
diluted ten times with inert alumina in order to avoid clogging the reactor. No
calcination step was realized. Prior to the reaction, catalysts were in situ reduced by
flowing 4%H2/N2 for 1h at 400°C.
Conversely to the Ni/TCP908, Ni/Ca2P2O7 and Ni/Al(PO3)3 catalysts presented
very low catalytic activity in the test conditions used. They were also quickly
deactivated after 4h of TOS. ESEM images (Chapter 3, figure 3.31) and XRD patterns
(Chapter 3, Figure 3.29 and Figure 3.30) presented in chapter 3 showed the presence
of large Ni particles for both catalysts. Moreover, nitrogen adsorption-desorption
isotherms (Chapter 3, section 3.3.1) revealed that they are non-porous materials
(Ca2P2O7: SBET = 8m²/g, Vp = nd; Al(PO3)3: SBET <3 m²/g, Vp = nd), which did not favor
the nickel dispersion. On the other hand, Ni/TCP908 catalyst containing small Ni
particles showed high activity and stability as previously communicated. In fact, the
presence of mesopores on this catalyst was responsible for increasing the nickel
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Parametric study of Ni/hydroxyapatite catalysts
dispersion and improving the contact between the gas (reactants) and the solid phase
(catalyst). This might have increased the DRM reaction and helped removing the
carbon deposits formed.
Figure 5.9. Comparison of the catalytic performance of 5.7%Ni/TCP908 with other
phosphate-based catalysts (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 400-700°C,
-1
-1
WHSV = 12353mLh gcat , P = Patm, catalyst dilution = 10x)
Despite the low specific surface area and pore volumes of Ca2P2O7 and Al(PO3)3
supports, the formation of large Ni particles on their surfaces could have also be
favored by the preparation method used. The IWI method might not be the most
appropriate for the deposition of nickel on these supports. Pelletier et al. [27] compared
the activity of Ni/modified-aluminum phosphate catalysts prepared by the coprecipitation method (CP) and the incipient wetness impregnation method (IWI). The
catalyst prepared by CP was stable in DRM at 750°C for 100h of TOS (time on
stream), while the catalyst prepared by IWI was rapidly deactivated. They argued that
the catalyst prepared by CP presented small and well dispersed Ni particles with strong
metal-support interaction, which favored the activity and stability of this catalyst, which
was not the case for the catalyst prepared by IWI.
157
Parametric study of Ni/hydroxyapatite catalysts
In conclusion, among the phosphate-based supports investigated, the
hydroxyapatite appeared as the best performing one due to its capacity in dispersing
the active phase and in providing better contact between the reactants and the catalyst
than the other phosphates investigated.
5.3.2 Promoted hydroxyapatite catalysts
Ni/Ce-TCP908 catalyst was already investigated in chapter 4. This catalyst
presented similar catalytic performance to the monometallic catalyst Ni/TCP908.
However, CeO2 addition is known to considerably improve the catalysts resistance to
carbon formation due to its redox and oxygen storage capacities [13]. So, its effect
would be more pronounced in the stability of the catalysts and 4h of TOS was not
enough to test the stability of catalysts. Therefore, this topic will compare the catalytic
performance of Ni/TCP908 and Ni/Ce-TCP908 for 50h of TOS.
As observed with the previous catalytic tests, 700°C is the temperature where the
catalysts present high performance and the coke formation is less thermodynamically
favored. So, in this study, the reaction temperature was kept at 700°C with a heating
rate of 20°C/min. The WHSV, the catalyst dilution and the conditions of in situ reduction
were changed in order to better evaluate the performance of the catalysts. The molar
composition of the inlet gas mixture was CH4/CO2/N2 = 20/20/60 and the WHSV was
15882mLh−1gcat−1. The catalyst was diluted two times with inert alumina in order to
avoid clogging the reactor and preferential gas passages in the reactor. No calcination
step was realized. Prior to the reaction, catalysts were in situ reduced by flowing
4%H2/N2 for 2h at 700°C.
For this section of the study, a volumetric flowmeter was used to measure the
outlet gas flow rate. This allowed calculating the conversions of CH4 and CO2, as well
as the selectivity in CO, and H2, according to the formula presented in Chapter 2. A
silica gel tube was also used for quantifying water as a by-product of the reaction.
Thus, the selectivity in water could be determined also. Figure 5.10 compares the
catalytic performance of the studied catalysts.
Ni/TCP908 was found to be more active and more stable than Ni/Ce-TCP908.
For Ni/TCP908, the initial CH4 and CO2 conversions reached around 76 and 79%,
respectively, and were practically unchanged within 50h of TOS. The selectivity in the
main product of DRM, CO and H2, was also stable around 90% along the reaction.
Thus, the selectivity in water was low and did not exceed 2% at the end of the reaction.
158
Parametric study of Ni/hydroxyapatite catalysts
Figure 5.10. Comparison of the catalytic performance of Ni/TCP908 and Ni/Ce-TCP908
catalysts (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 700°C, P = Patm, WHSV =
-1
-1
15882mLh gcat , catalyst dilution = 2x)
On the other hand, for Ni/Ce-TCP908, the initial CH4 and CO2 conversions were
only around 62 and 65%, respectively, and drastically decreased to 22% and 36% after
50h of TOS. In parallel with the decrease of the reactants’ conversions, the selectivity
in CO and H2 gradually decreased to less than 70% at the end of the reaction, while
the selectivity in H2O increased. The formation of water was probably due to the
159
Parametric study of Ni/hydroxyapatite catalysts
reverse water-gas shift and/or the reverse carbon gasification (Chapter 1, Equation 1.2
and Equation 1.5):
Reverse water-gas shift:
Reverse carbon gasification:
Taking into account the high reaction temperature (700°C) and the standard
reaction heat, the reverse water-gas shift must be more favorable than the reverse
carbon gasification. Or the formation of water was accompanied by coke deposition.
This explained the deactivation of Ni/Ce-TCP908 with the reaction time. Further study
is needed to affirm the favorable effect of Ce-hydroxyapatite system for the formation of
water.
In the literature, Ce addition is known to prevent coke deposition due to its
oxygen storage and transfer properties [29]–[31]. So, it usually improves the catalyst
stability. It can also improve the catalyst performance by improving the dispersion of
the active phase [32], [33]. However, in our case, nickel particles of Ni/Ce-TCP908
catalyst were found to be larger than those of Ni/TCP908 (Chapter 3, Figure 3.26). This
caused the lower initial activity of Ni/Ce-TCP908 compared to that of Ni/TCP908.
The results presented in Figure 5.10 are different from the results presented in
chapter 4. In chapter 4 (Figure 4.9), were already compared the performance of Ni/CeTCP908 and Ni/TCP908 using U-shaped quartz reactor. We observed the similar
catalytic activity of these two catalysts at 700°C within 4h of TOS. The experiments in
Chapter 4 were carried out at 700°C, with WHSV equal to 10500mLh-1gcat-1. Probably,
under this low WHSV value, the CH4 and CO2 conversions were close to the
thermodynamic limits, which did not allow discriminating these two catalysts.
Unfortunately, we could not determine the CO2 and CH4 conversions from the Figure
4.9 because we did not measure the flow rate of outlet gases.
In conclusion, the addition of Ce to TCP908 support by IWI did not allow
improving the catalytic performance of Ni/TCP908. On the other hand, this modification
led to an inhibitory effect and thus, Ce addition will be not further considered.
5.3.3 Alumina-based catalysts
Chapter 3 (section 3.4) highlighted the benefits of the alumina-based supports.
They present high chemical and physical stability and high mechanical resistance [34].
Moreover, the addition of basic promoters is known to decrease the acidity of these
supports and improve the coke resistance of catalysts [35], [36]. So, this section will
160
Parametric study of Ni/hydroxyapatite catalysts
compare the performance of the hydroxyapatite-based catalysts (5.7%Ni/TCP308 and
5.7%Ni/TCP908) with two alumina-based catalysts (5.7%Ni/Al2O3 and 5.7%Ni/Pural
MG30). Pural MG30 support was composed of a mixture of Al2O3 and MgO as
presented in chapter 3 (figure 3.35). Catalytic tests were carried out with the fixed-bed
reactor and the catalytic performance was evaluated for long TOS of 50h. The reaction
temperature was kept at 700°C with a heating rate of 20°C/min. The molar composition
of the inlet gas mixture was CH4/CO2/N2 = 20/20/60 and WHSV was 15882mLh−1gcat−1.
The catalyst was diluted two times with inert alumina in order to avoid clogging the
reactor. No calcination step was realized. Prior to the reaction, catalysts were in situ
reduced by flowing 4%H2/N2 for 2h at 700°C.
Figure 5.11 compares the catalytic performance of the catalysts. All four catalysts
presented similar initial catalytic activity, except Ni/Pural MG30 which was slightly more
active than other catalysts. However, the catalytic behaviors were different from each
other within 50h of TOS.
The activity of Ni/TCP908 and Ni/Pural MG30 was mostly unchanged during 50h
of TOS. In parallel, the selectivity in CO and H2 was also stable. H2O was only formed
at trace amounts by these catalysts, which was favorable for coke elimination. ESEM
and TEM results presented in chapter 3 (Figures 3.22, 3.23, 3.34) showed that both
catalysts have small Ni particles, which prevent carbon deposition [1], [37]–[41]. The
higher activity of Ni/Pural MG30 can be explained by its strong metal-support
interaction that prevents Ni particles from sintering. Moreover, Pural MG30 (Vp =
0.17cm3/g) presented larger pore volumes when compared to TCP908 (Vp =
0.072cm3/g), which may have favored a higher dispersion of the active phase and a
more efficient contact with the gaseous reactants [42]. Despite the larger pore volumes
of the Pural MG30, Ni/Pural MG30 and Ni/TCP908 presented similar catalytic
performances. It indicates that the smaller surface area of the hydroxyapatite support is
compensated by its chemistry. Tests with longer duration or with more extreme
conditions (presence of impurity, poison or humidity…) are needed to affirm the
catalytic performance of these two catalysts.
161
Parametric study of Ni/hydroxyapatite catalysts
Figure 5.11. Comparison of the catalytic performance of the hydroxyapatite-based
catalysts with the alumina-based catalysts (reaction conditions: CH4/CO2/N2=20/20/60,
-1
-1
T=700°C, P = Patm, WHSV = 15882mLh gcat , catalyst dilution = 2x)
On the other hand, the activity of Ni/TCP308 and Ni/Al2O3 drastically decreased
during the 50h of TOS. For Ni/TCP308 catalyst, CH4 conversion decreased from 80%
to 34% and CO2 conversion decreased from 80% to 45%. For Ni/Al2O3 catalyst, CH4
conversion was reduced from 80% to 48% and CO2 conversion was reduced from 80%
to 56%. The hydrogen selectivity also decreased for both catalysts. This decrease in
162
Parametric study of Ni/hydroxyapatite catalysts
hydrogen selectivity was explained by the increase in water selectivity. No notable
changes in CO selectivity were observed. As explained above, the reverse water-gas
shift reaction (Equation 1.2) and the reserve carbon gasification (Equation 1.5) were
responsible for the changes in selectivity. These reactions favored the formation of H2O
and CO, and decreased the selectivity in H2. Also, the reverse carbon gasification led
to the coke deposition and thus, caused the deactivation of these catalysts. TCP308 is
a non-porous support, which did not favor the dispersion of the Ni particles on
Ni/TCP308 catalyst and consequently led to the formation of large Ni particles, which
are known to favor coke deposition [39]. On the other hand, Ni/Al2O3 catalyst presented
small and well dispersed Ni particles. However, despite the large pore volumes of Al2O3
support (Vp = 0.42cm3/g), it has an acidic character that promotes carbon formation.
Moreover, it can suffer phase changes at high temperatures that can also lead to
sintering and deactivation of the catalyst [35].
Ni/Pural MG30 and Ni/Al2O3 presented very different catalytic performances.
Pural MG30 contained 30 and 70 wt.% of MgO and Al2O3, respectively. The addition of
a basic promoter, MgO in this case, is known to improve the ability of the catalyst to
remove the carbon deposit [1], [35], [43], [44]. TPR results showed that MgO addition
also improved the metal-support interaction. Ni/Pural MG30 had a broad reduction
peak at 460°C, which corresponds to the reduction of small Ni particles with strong
metal-support interaction. On the other hand, Ni/Al2O3 presented a reduction peak at
360°C, which indicates weaker metal-support interaction compared to Ni/Pural MG30.
Consequently, Ni particles of Ni/Al2O3 were easier to be sintered which explains the
instability of Ni/Al2O3 catalyst [1], [45].
In conclusion, Ni/TCP908 was once again found to be more active and stable
than Ni/TCP308. Ni/Al2O3 prepared from an industrial alumina support, was less
performing than Ni/TCP908. But the addition of MgO to Al2O3 (Pural MG30 support)
strongly improved the catalytic activity and stability of alumina-based catalyst. For 50h
of TOS, Ni/TCP908 and Ni/Pural MG30 were found to be the most performing catalysts
in DRM at 700°C.
5.4 Conclusions
This chapter focused on the investigation of the influence of the reaction
conditions on the performance of nickel-based catalysts. Parameters such as the
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Parametric study of Ni/hydroxyapatite catalysts
temperature and the pressure of the reaction, the pretreatment of the catalysts and the
sintering of the supports were investigated.
Reaction temperature and pressure need to be carefully evaluated due to their
importance on the catalytic performance of the catalysts and the overall viability of the
catalytic process. Thermodynamic studies show that high temperatures and low
pressures are needed to obtain high syngas production and limit carbon deposition.
However, high temperatures imply high energy consumption. Moreover, different
processes such as Fisher-Tropsch, are fed with syngas at high pressures (20-25 bars)
[22]. In this case, the production of the syngas at high pressure is preferable [23]. So,
the influence of the reaction temperature and pressure on the catalyst performance
was evaluated. DRM was investigated in the temperature range of 400 to 700°C using
different catalysts. The temperature of 700°C seemed to be the most appropriate, for
obtaining high reactants’ conversions as well as limiting coke deposit. Concerning the
reaction pressure, the atmospheric pressure was the most favorable in the pressure
range of 1 to 25 bars investigated.
Metallic nickel nanoparticles are very sensitive to air exposure. In contact with
oxidant (oxygen in air), metallic nickel nanoparticles are violently oxidized into nickel
oxides. Thus, we have compared Ni/TCP908 and Ni/TCP308 catalysts with and without
in situ reduction as pretreatment. The pretreatment may have had an impact on the
metal particle size and on the strength of metal-support interactions, which will later
affect catalytic performance of catalysts [4], [5]. The in situ reduction was performed
with 4%H2/N2 stream prior to the reaction. The in situ reduction was favorable for the
activity and the stability of both catalysts supported on TCP908 and TCP308. The
pretreated catalysts were active at 500°C, while the corresponding calcined catalysts
were active only at 600°C. For TCP308-based catalysts, the pretreated one was more
active than the calcined one. In fact, due to the strong metal-support interaction of this
catalyst, the reduction under the atmosphere of the reaction mixture was not enough to
completely reduce the NiOx particles.
The influence of the support sintering on the catalytic performance of nickelhydroxyapatite catalysts was investigated because the sintering of both support and
nickel particles can occur at high reaction temperatures which causes the irreversible
catalytic deactivation [11]–[13]. So, the catalytic performance of Ni/TCP908 was
compared to that of Ni/TCP908S, with the support previously sintered. Below 700°C,
Ni/TCP908 was slightly more active than Ni/TCP908S. This was explained by strong
metal-support interaction of this catalyst, leading to the possible partially reduction of
164
Parametric study of Ni/hydroxyapatite catalysts
nickel particles during the in situ reduction at 400°C. At 700°C, both catalysts
presented comparable catalytic performance. This result was explained by the
similarity of the shape and the size of nickel particles of these catalysts.
Other phosphates produced by PRAYON were also investigated as catalyst
supports, including Ca2P2O7 (pyrophosphate) and Al(PO3)3 (aluminum phosphate).
However, the catalysts obtained by the deposition of nickel by IWI method on these
supports showed negligible catalytic activity under similar experimental conditions,
compared to nickel-hydroxyapatite catalysts. The small pore volumes and low specific
surface areas of these supports (Ca2P2O7: SBET = 8m²/g, Vp = nd; Al(PO3)3: SBET <3
m²/g, Vp = nd) led to the formation of large nickel particles and was unfavorable to the
catalytic performance of these catalysts. These supports were not further investigated.
The effect of Cerium addition on the activity and stability of the Ni/TCP908
catalyst was checked again using the fixed-bed reactor operating for long TOS of 50h.
In contrast to the results obtained in chapter 4 with the U-shaped quartz reactor,
Ni/TCP908 showed higher activity and stability than Ni/Ce-TCP908, when the reaction
was carried out at high value of WHSV. This result could be attributed to the formation
of large nickel particles of Ni/Ce-TCP908 (100-200nm) catalyst as shown in chapter 3
(Figure 3.26).
Alumina-based catalysts were also compared to the hydroxyapatite-based
catalysts. Alumina-based supports are widely used in industrial processes thanks to
their high chemical and physical stability and high mechanical resistance [34]. Despite
the larger pore volumes of this support (Vp = 0.42cm3/g) when compared to the
TCP908 (Vp = 0.072cm3/g), Ni/Al2O3 catalyst was less active than Ni/TCP908, and was
quickly deactivated within 50h of TOS. This was explained by the acidity of the support,
which favors the carbon deposition. On the other hand, Ni/Pural MG30 catalyst showed
very good catalytic performance, even slightly better than Ni/TCP908. The addition of
30 wt.% of MgO to alumina probably allowed the neutralization of the acidity of alumina
support. Moreover, Pural MG30 is a mesoporous support (Vp = 0.17cm3/g), which may
have favored the nickel dispersion and the mass transfer diffusions inside the catalyst
and thus improved its catalytic property.
Despite the larger pore volumes of the Pural MG30, Ni/Pural MG30 and
Ni/TCP908 presented similar catalytic performances during the 50h of TOS at 700°C. It
indicates that the smaller surface area of the hydroxyapatite support is compensated
by its chemistry. To affirm this result, tests with longer duration are needed. So, the
next chapter will be dedicated to these experiments.
165
Parametric study of Ni/hydroxyapatite catalysts
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169
Chapter 6
Catalytic stability under simulated industrial
conditions
6.1 Introduction
The previous chapters were dedicated to the initial steps of the development of a
catalyst. Firstly, the active phase (Ni) and the preparation method (IWI) were identified
in order to produce a performing catalyst that could be used further at industrial scale.
Then, a parametric study was carried out for optimizing the reaction conditions. Finally,
a comparison of the hydroxyapatite-based catalysts with other catalysts already used
for other industrial processes was realized. To affirm the results obtained on the
performance of the most promising catalysts, catalytic tests for long reaction time (>
300h of TOS) were performed. This chapter presents the results of these experiences.
Moreover, it presents some discussions about the development of a DRM catalyst.
6.2 Evaluation of the catalytic performance of the most
promising catalysts under simulated industrial conditions
Chapter
5
(section
5.3)
compared
the
catalytic
performance
of
the
hydroxyapatite-based catalysts with other catalysts already used for other industrial
processes. So, in order to confirm the results obtained on the performance of the most
promising catalysts, catalytic tests for long reaction time (300h of TOS) under
simulated industrial conditions are needed. The previous chapter showed that the
Ni/Pural MG30 and Ni/TCP908 catalysts presented similar catalytic behavior, and were
very promising for DRM process thanks to their high stability during 50h of TOS. On the
Catalyst stability under simulated industrial conditions
other hand, Ni/TCP308 and Ni/Al2O3 showed much lower catalytic performance. So,
this section only focused on Ni/Pural MG30, Ni/TCP908, and Ni/TCP308 catalysts.
Ni/Al2O3 was not further investigated for long reaction time.
The catalyst preparation and the test conditions were chosen in order to better
discriminate the catalysts and to fit the industrial requirements. As presented in chapter
5, the catalyst sintering at the DRM conditions is practically inevitable due to the high
temperatures used, which is one of the reasons for the catalyst deactivation. So, in this
study the supports were previously sintered before the Ni deposition.
The temperature of sintering was chosen based on the TMA results shown in
Figure 6.1.
Figure 6.1. TMA results of supports
The TCP908 support shrinks from 700°C to 1050°C and its final shrinkage is of
about 18%. This temperature range corresponds to the sintering temperature of the
hydroxyapatites and is well described in the literature [1]. On the other hand, the Pural
MG30 support presents a shrinking temperature range of about 400°C-1200°C and
presents a final shrinkage of about 21%. This low initial shrinking temperature and the
higher shrinkage values compared to the TCP908 could be related to the
transformation of the crystalline structure of the Pural MG30, as indicated by the XRD
diffractogrammes (Chapter 3, figure 3.33) and also to the higher porosity (Vp =
0.17cm3/g) of this support when compared to the TCP908 (Chapter 3, sections 3.2.1a
and 3.4.1). So, in order to stabilize these two supports, the hydroxyapatites-based
supports were treated at 1000°C for 5h and the Pural MG30 support was treated at
1200°C for 5h.
172
Catalyst stability under simulated industrial conditions
After support sintering, Ni deposition was realized by IWI, as reported in chapter
2 (section 2.4.1), due to its simplicity to adapt to industrial scale, as well as the
formation of nickel nanoparticles.
The reaction temperature was kept at 700°C. The molar composition of the inlet
gas mixture was CH4/CO2/N2 = 20/20/60 and WHSV was 12272mLh−1gcat−1. The
catalyst was diluted two times with inert alumina in order to avoid clogging the reactor.
No calcination step was realized. Prior to the reaction, catalysts were in situ reduced by
flowing 4%H2/N2 for 2h at 700°C. The tests were performed during 300h of TOS. The
catalyst bed was oversized by adapting the WHSV in order to better evaluate the
catalysts during the 300h.
Figure 6.2 compares the catalytic performance of the catalysts under simulated
industrial conditions of test.
The sintered Ni/TCP908S catalyst showed the best performance under the test
conditions used. The initial conversions of CH4 and CO2 were about 80%. However,
they progressively decreased during the test, and reached around 60% after 300h of
TOS. This was remarkably different from the catalytic behavior of the non-calcined
Ni/TCP908 catalyst, which did not show notable initial deactivation (Chapter 5, Figure
5.11). ESEM analysis showed that the sizes of Ni particles formed on the surface of
both TCP908 with and without sintering step were similar to each other (Chapter 3,
Figure 3.18). But the density of Ni particles’ distribution was higher on TCP908S
surface, than on TCP908 surface. So, maybe this high density made the Ni/TCP908S
easier to be sintered during the reaction, or it favored the carbon deposition. Further
analyses such as ESEM and TEM of the spent catalysts are needed for understanding
this difference. About the selectivity to syngas, CO selectivity was kept around 80%
during the test. However, H2 selectivity slightly decreased during the test. This
decrease could be explained by the small increase in water selectivity. This suggests
the occurrence of the reverse water-gas shift reaction (Chapter 1, Equation 1.2) that
consumes H2 and CO2 to produce H2O and CO. The occurrence of other side reactions
such as Boudouard (Chapter 1, Equation 1.3) and methane cracking (Chapter 1,
Equation 1.4) may have led to the deposition of coke and thus to the deactivation of the
catalyst, as observed above. ESEM images of the spent catalysts showed in Figure 6.3
confirmed this hypothesis.
173
Catalyst stability under simulated industrial conditions
Figure 6.2. Comparison of the catalytic performance of the hydroxyapatite-based
catalysts with the Pural MG30-based catalyst (reaction conditions: CH4/CO2/N2=20/20/60,
-1
-1
T=700°C, P = Patm, WHSV = 12272mLh gcat , catalyst dilution = 2x)
Figure 6.3 shows the formation of carbon fibers over the Ni/TCP908S catalyst.
Some carbon fibers embedded the Ni particles and provoked the catalyst deactivation
because the reactants could not access the active sites anymore. However, in some
cases, the Ni particles are at the top of the fiber and therefore they are still accessible
174
Catalyst stability under simulated industrial conditions
to the reactants, which explain the good activity of this catalyst even after 300h of TOS.
The formation of carbon fibers over a hydroxyapatite supported was reported in our
previous work [2].
Figure 6.3. ESEM images of the Ni/TCP908S after 300h of TOS at 700°C.
Figure 6.4. ESEM images of the Ni/PuralMG30S after 300h of TOS at 700°C
The catalytic behavior of Ni/Pural MG30S was different from Ni/TCP908S. After an
initial deactivation corresponding to a decrease of 30% in CH4 conversion and of 20%
in CO2 conversion within 50h of TOS, the catalytic activity of this catalyst was found to
be relatively stable. High selectivity to syngas was observed. H2 and CO selectivity
were stabilized around 80% during the 300h of TOS. In consequence, very low H 2O
selectivity was observed for this catalyst. The low amounts of water and the stability of
175
Catalyst stability under simulated industrial conditions
this catalyst after 50h of TOS indicate the occurrence of the carbon gasification
reaction (CG) (Chapter 1, Equation 1.5), where the carbon deposit reacts with the
water formed to produce syngas. Figure 6.4 shows the ESEM images of the spent
catalyst.
Figure 6.4 also shows the formation of carbon fibers over the Ni/Pural MG30S,
which may have provoked the initial deactivation of this catalyst. However, the Ni
particles observed for this spent catalyst had similar size (20-50nm) and shape than
the Ni particles of the fresh catalyst (Chapter 3, Figure 3.25). This indicates that no
sintering of the active phase took place, which contributed to the stability of this
catalyst.
So, both Ni/TCP908S and Ni/Pural MG30S presented excellent catalytic
performance for DRM. ESEM and TEM analyses in chapter 3 (Figures 3.25 and 3.34)
showed that both catalysts presented small Ni particles (<20nm), which could limit
carbon deposition [3]–[6] and could partially explain their stability. In addition to the Ni
particle size, the basic properties of the Pural MG30 support might also be favorable to
limit coke formation [7]. Further investigation on the influence of acidity-basicity on the
catalytic performance of Ca-HA based catalyst is needed.
The Ni/TCP308S catalyst showed the similar initial activity compared to
Ni/TCP908S and Ni/Pural MG30S. However, it showed the highest catalytic
deactivation. The CH4 conversion dropped of 80 to 30%, and the CO2 conversion fell
from 80 to 40% during the catalytic test. This deactivation must be due to the presence
of large Ni particles formed on the support of TCP308S (Chapter 3, Figure 3.18). Large
Ni particles favor coke formation and thus causes the catalyst deactivation [7]. This is
linked to the lower selectivity to H2 than to CO, and to the higher selectivity in water. As
explained above, the low selectivity in H2 was attributed to the reverse water-gas shift
reaction. Figure 6.5 shows the ESEM images of the Ni/TCP308S after test.
Figure 6.5 shows the presence of large Ni particles on the spent catalyst when
compared to the fresh catalyst, which indicates the sintering of the active phase. The
sintering of the active phase along with the carbon fibers formed may explain the high
initial deactivation of this catalyst. However, some Ni particles were at the top of the
carbon fibers and therefore were still available for the reactants, which can explain the
activity of this catalyst despite the carbon deposits and sintering of the active phase.
176
Catalyst stability under simulated industrial conditions
Figure 6.5. ESEM images of the Ni/TCP308S after 300h of TOS at 700°C
Meaningful information about the selectivity of catalysts to syngas and its further
application at industrial scale can be obtained through the evaluation of the H2/CO
molar ratio. DRM produces syngas in a H2/CO ratio close to one, which is ideal for
Fisher-Tropsch synthesis of hydrocarbons [3]. Figure 6.6 shows the H2/CO ratios
measured during the DRM at 700°C for the three catalysts investigated.
Figure 6.6. H2/CO ratios measured during the DRM experiments at 700°C
Ni/TCP908S and Ni/PURAL MG30S showed H2/CO ratios around unit at the
beginning of the reaction, and then decreased to around 0.8. This is expected for the
DRM reaction since its theoretical chemical equation leads to an equimolar mixture of
H2 and CO. The fact that the molar H2/CO ratios were lower than unit can be explained
by the formation of various by-products, in particular water and carbon deposit,
accordingly, but not limited, to the reverse water-gas shift, the reverse carbon
gasification, the methane cracking and the Boudouard reaction. The amount of water
formed indicated the competition of each side-reaction that involves water. As shown in
177
Catalyst stability under simulated industrial conditions
Figure 6.2, the selectivity in water was lower with Ni/PURAL MG30S than with
Ni/TCP908S within 150-300h of TOS. Consequently, the molar ratio of H2/CO was
higher with Ni/PURAL MG30S than with Ni/TCP908S during this reaction time. Also,
Ni/PURAL MG30S showed higher stability than Ni/TCP908S. So, the concentration of
water in the syngas appeared as an important indicator for the stability of the catalyst in
DRM. This suggests the possibility to improve the life time of DRM catalyst by injecting
some amount of water at the reactor inlet, which must in general be favorable for
limiting the formation of water by the DRM reaction.
The above observation about the relationship between the selectivity in water
and the molar ratio of H2/CO could be confirmed with Ni/TCP308S. This catalyst also
had the initial H2/CO ratio close to unit at the beginning of the reaction. However, it
quickly decreased to around 0.6. As shown Figure 6.2, the high selectivity to water
caused by the reverse water-gas shift, decreased the selectivity to H2. In parallel, this
catalyst was deactivated probably due to the coke deposition, resulting from the side
reactions, as explained above.
Figure 6.7, Figure 6.8 and Figure 6.9 present the mass balances for the catalytic
tests with Ni/TCP908S, Ni/Pural MG30S and Ni/TCP308S catalysts, respectively.
Reminding that the amounts of coke on the spent catalysts could not be measured in
this work, the amounts of carbon measured at the outlet of the reactor did not take into
account the coke formed. Because the coke formation is thermodynamically inevitable,
the mass balance on carbon in this work cannot be closed. For these results, the
following formulas were used:
Hin, Oin, and Cin: Calculated atomic flux of hydrogen, oxygen and carbon fed into
the reactor, respectively. These calculations were based on the flow rate of the
inlet gas mixture, measured by mass flow control at room temperature.
Hout, Oout, and Cout: Calculated atomic flux of hydrogen, oxygen and carbon
leaving the reactor, respectively. These calculations were based on the total
flow rate of the outlet gas, and its composition determined by µ-GC.
Hydrogen balance, Oxygen balance, and Carbon balance: Ratio of Hout to Hin,
Oout to Oin, and Cout to Cin, respectively.
178
Catalyst stability under simulated industrial conditions
Figure 6.7. Mass balance for the catalytic test with Ni/TCP908S
Figure 6.8. Mass balance for the catalytic test with Ni/Pural MG30S
179
Catalyst stability under simulated industrial conditions
Figure 6.9. Mass balance for the catalytic test with Ni/TCP308S
For Ni/TCP908S catalyst (Figure 6.7), the mass balance was nearly reached for
all three fluxes of oxygen, hydrogen and carbon during the 300h of TOS. This indicates
the formation of other light hydrocarbons [8] that were not detected by the µ-GC
analyzer used. The undetermined H and C was around 5-10 %. Moreover, the carbon
balance was slightly lower for the first 50h than for the last 250h of TOS. This was
probably linked to high amounts of coke formed at the beginning of the reaction, which
was supported by the initial deactivation of the catalyst.
Similar results were obtained for the Ni/Pural MG30S and Ni/TCP308S (Figure 6.8
and Figure 6.9). The mass balance was nearly closed for all three fluxes of oxygen,
hydrogen and carbon, which varied in the narrow range of 5 to 10%. We also observed
the pronounced deficit of carbon at the beginning of the reaction (50 first hours of TOS)
compared to the last 250h of TOS. This was also attributed to the initial strong coke
deposition.
Considering that Ni/Pural MG30S became stable after about 50h of TOS while
Ni/TCP908S was not yet stable after 300 h of TOS, further study needs to be done for
explaining this observation. Catalytic test with a TOS superior to 300h will allow
180
Catalyst stability under simulated industrial conditions
checking the stability of Ni/TCP908S. Also, SEM and/or TEM analysis can highlight the
shape and size change for Ni particles after catalytic test.
In conclusion, all three catalysts investigated presented good performance on
DRM under simulated industrial conditions; Ni/TCP908S and Ni/Pural MG30S being the
most performing catalysts. The catalytic results highlighted that the size of Ni particles
and the occurrence of side reactions play an important role on the catalysts’
performance. Small Ni particles could limit coke formation and thus could reduce the
catalytic deactivation, which was observed for the Ni/Pural MG30S catalyst. Moreover,
the occurrence of side reactions, as previously explained, may help the coke removal
and to keep the H2/CO ratio close to one, which is ideal for Fisher-Tropsch
applications.
These catalytic tests showed that the hydroxyapatites manufactured by PRAYON
could directly compete with Pural MG30, identified as the most appropriate catalytic
support for DRM from the literature data.
6.3 Discussions
The development of resistant and performing catalysts is capital to the
development of DRM at industrial scale and to the improvement of the existing
technologies for syngas production. Problems related to operational constraints and
associated cost burdens could also be overcome.
The hydroxyapatite-based catalysts developed on this work showed excellent
performance in DRM under simulated industrial conditions. Comparing these catalysts
with the catalysts for DRM reported in the literature, they seem very promising. In fact,
usually the test conditions applied in the literature favor the reactants’ conversion close
to the thermodynamic equilibrium. The reaction temperatures are usually between
600°C and 800°C, the WHSV applied are low and the TOS is not enough to evaluate
the catalysts deactivation. Alipour et al. [9], for example, studied the 5%Ni/Al2O3
catalyst promoted with MgO at 700°C for 22h of TOS and WHSV equal to 300mLhgcat-1. They obtained CH4 and CO2 conversions of 80-90%. On the other hand, similar
1
results were obtained with our hydroxyapatite-based catalysts under far more extreme
conditions (T=700°C, t=300h, WHSV=12272mLh-1gcat-1, or the residence time in our
case was 41 time smaller than that used by Alipour et al.), which indicates the
competence of our catalysts for DRM.
181
Catalyst stability under simulated industrial conditions
However, the development of these hydroxyapatite-based catalysts is still in its
initial stage. Many aspects, such as optimization of the catalysts production, conditions
of test, reactor design and kinetic comprehension are still needed in order to develop a
highly performing catalyst and a DRM technology.
With regard to the optimization of the catalysts production, size and shape of Ni
particles and surface composition of the catalysts are important parameters affecting
the carbon deposition. It has been known that Ni particles smaller than 15 nm allow
limiting coke formation. On the other hand, Ni particle greater than 20nm, which was
the case of our catalysts, is favorable for carbon deposition [10]. So, the next study
could focus on the elaboration of hydroxyapatite-based catalyst, containing Ni particles
smaller than 15 nm. This can call for the use of other synthesis methods, such as solgel (SG), or plasma treatment. The decrease of Ni content can help to reduce Ni
particles size.
The acido-basicity of a given hydroxyapatite depends on the molar ratio of Ca to
P. Previously, Tsuchida et al. [11] showed that the density of acid and basic sites of
hydroxyapatite varied strongly in a narrow range of the molar ratio of Ca/P. Moreover,
DRM was favored by superficial basic sites, which could be responsible for the
inhibition of carbon deposition on the active sites. So, the investigation of the basicity of
the hydroxyapatite supports is another factor that needs to be further realized. The
density of basic sites can be controlled by adjusting the molar ratio of Ca to P, or by
precipitating phosphate anions (PO43−) by a mixture of Ca2+ and Mg2+, inspired from the
Pural MG30 support. The density and the strength of acido-basic sites of a solid can be
determined by temperature programmed analyses, and by calorimetry.
The comprehension of the coke formation over the active sites of the catalysts is
also very important for the optimization of the catalyst production. The analysis of the
catalysts after test by different techniques such as, SEM, TEM, Raman spectroscopy
and TPO can gives information about the amount and the type of carbon species
formed over the catalyst, which can later help to develop a coke resistant catalyst.
However, these analyses could not be correctly performed in this work due to the
dilution of the catalyst with alumina, which were both in powder form and could not be
separated after catalytic test. Molding the catalyst could be a solution to dilution of the
catalyst and could allow further investigation of the carbon deposits. Moreover, it would
better approximate industrial requirements.
Finally, the elucidation of kinetic parameters and mechanism of the DRM as well
as transport diffusional limitations could help understanding how a catalyst works and
182
Catalyst stability under simulated industrial conditions
thus how to optimize it. Laboratory experiments and derivation of intrinsic kinetic
reaction rates are the initial steps to the development of performing catalysts and
industrial reactors.
The overall performance of the DRM process can be enhanced by the catalytic
aspect, but also by the experimental aspect. This last one relates to both the reaction
conditions, and the reactor technology. The main reaction conditions to be well
controlled are the temperature, the pressure, and the WHSV. Low pressure is favorable
for the reaction of DRM. About the temperature and WHSV, the increase of the
reaction temperature and the decrease of WHSV are favorable for the conversion of
CO2 and CH4. When the DRM reaction is close to the thermodynamic limit (oversized
process), the carbon deposition seems to be limited. But the choice of the reaction
pressure may be affected by the subsequent utilization of the syngas obtained. For
example, obtaining the syngas at high pressure (25-50 bar) is favorable for FisherTropsch synthesis. Also, in chapter 4, for example, we showed that an increase of the
reaction temperature from 700°C to 750°C was responsible for better catalytic
performance. Of course, it is only meaningful if this increase of temperature (which
consumes energy) leads to the increase of the global benefit of the process.
For a parametric or comparative study, it is of interest to study the
thermodynamics of DRM under different conditions. It helps not only to predict the
tendency related to the operational conditions on the performance of the catalytic
process, but to correctly understand and interpret catalytic results.
The optimization of the condition of the catalyst bed is also essential to the
development of a DRM technology. In this work, a fixed bed reactor was used as it
allows easily evaluating the catalysts and is the most used for DRM experiments at lab
scale. However, it generates steep temperature gradients, which can provoke carbon
deposition. The fluidized bed reactor (FIBR) could be a suitable candidate for dry
reforming reactions. In fact, the fluidized bed reactor provides excellent heat
dissipation, avoiding temperature gradients. Moreover, the use of small particles should
allow overcoming the intra-particle mass transfer limitation which seriously reduces
catalyst efficiency in fixed bed reactors. However, the synthesis of larger sintered and
porous catalyst supports is justified.
183
Catalyst stability under simulated industrial conditions
6.4 Conclusions
This chapter evaluated and compared the catalytic performances of the
hydroxyapatite-based catalysts and of the commercial Pural MG30-based catalyst
under simulated industrial conditions. The objective was to verify the resistance of the
prepared catalysts to the extreme conditions of DRM for long TOS. The DRM was
performed at 700°C for 300h of TOS. All three catalysts investigated presented good
performance on DRM under industrial conditions. However, Ni/TCP908S was the most
performing followed by Ni/Pural MG30S, then Ni/TCP308S.
Much progress on the development of a performing catalyst for DRM was
realized in this work. However, further work is still needed for the improvement of the
prepared catalysts and on the comprehension of the deactivation of our catalysts.
Moreover, besides the enhancement of catalysts’ production, the optimization of test
conditions, of the reactor design and kinetic and thermodynamic studies are essential
to the development of a coke-resistant catalyst and of a DRM technology.
184
Catalyst stability under simulated industrial conditions
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Catalyst stability under simulated industrial conditions
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186
General conclusions and prospects
The aim of this work was the valorization of CH4 and CO2 through DRM using
hydroxyapatite-based catalysts. The work was divided in four main parts. The first part
consisted in the development of a hydroxyapatite-based catalyst. The second part was
focused on the development of a new fixed-bed reactor. The third part was devoted to
the optimization of the conditions of reaction. Finally, the last part was dedicated to a
comparative study between the performances of the hydroxyapatite-based catalysts
and the commercial ones.
In the first part of the work, two hydroxyapatite supports, TCP308 (SBET = 7m²/g,
Vp = nd) and TCP908 (SBET = 60m²/g, Vp = 0.072cm3/g), provided by PRAYON, five
transition metals (Zn, Fe, Co, Cu and Ni) and three preparation methods (incipient
wetness impregnation, cation exchange and co-precipitation) were investigated in order
to find the most promising combination. The influence of the active phase amount (Ni
wt% = 2.85%, 5.7%, 11.4%) and promoters (Co, Re, Ce) were also investigated.
This study showed that metal particles were smaller when prepared on TCP908
(around 20-50nm) than on TCP308 (around 100-200nm), which could be explained by
the high specific surface area and porosity of TCP908. Moreover, the incipient wetness
impregnation method (IWI) showed to be the most effective method for catalyst
preparation because it allowed controlling the desired amount of metal and did not
strongly modify the support structure. For this first part of the study, the prepared
catalysts were only calcined under air atmosphere prior to DRM reaction. The
investigation of the activity of the different active metals showed that only Ni-based
catalysts (Ni/TCP308 and Ni/TCP908) prepared by IWI had significant catalytic activity
under the conditions used. The investigation of the influence of the Ni amount (Ni wt%
= 2.85%, 5.7%, 11.4%) on the catalytic performance of the Ni/TCP908 showed that
5.7%Ni was the most appropriate compromise.
The study of the effect of promoter addition (Ce, Re, Co) on the performance of
the Ni/TCP908 catalyst showed that none of the promoters tested had a notable
positive effect on the catalyst performance. Rhenium addition completely deactivated
the catalyst probably due to the formation of inactive oxides during the reaction. Cerium
General conclusions and prospects
addition seemed to not lead to a gain of catalytic activity. The bimetallic Co-Ni catalyst
was not active under the test conditions used, probably because the temperature of
reaction was not sufficient for the activation of the catalyst. So, 5.7%Ni/TCP908
prepared by IWI method was the most appropriate compromise and were chosen for
the next step of the study because of the simplicity in the preparation and the promising
catalytic performance.
The second part of the work was devoted to the development of a new fixed bed
reactor. This was an important part of the work because the set-up of this apparatus
took 18 months of this thesis work and allowed a proper evaluation of the catalysts
performance. In fact, it is capable of operating at high temperature (T = TRT - 850°C)
and pressure (P = Patm - 30bar) and for long times on stream (t>300h).
The third part of the work included a parametric study on the performance of the
selected nickel-based catalysts. Parameters such as the temperature and the pressure
of the reaction, the pretreatment of the catalysts and the sintering of the supports were
investigated.
For the investigation of the influence of the reaction conditions, DRM was
performed in the temperature range of 400-700°C and in the pressure range of 125bars. This study revealed that 700°C seemed to be the temperature the most
appropriate for obtaining high reactants’ conversions as well as for limiting coke
deposit. About the reaction pressure, despite the industrial advantages in producing
syngas at elevated pressures, the atmospheric pressure showed to be the most
favorable in the pressure range investigated (1-25 bars). These results are in
agreement with the thermodynamic predictions.
The study on the influence of the pretreatment on the catalysts’ performance
showed that the in situ reduction with 4%H2/N2 stream prior to the reaction was
favorable for the activity and the stability of both Ni-catalysts supported on TCP908 and
TCP308. The pretreated catalysts under H2 atmosphere were active at 500°C, while
the corresponding calcined catalysts were active only at 600°C and above. This
pretreatment allowed the activation of the catalysts prior to the reaction and probably
had an impact on the determination of metal particle size and on the strength of metalsupport interactions, which favored the catalytic performance of catalysts.
Since DRM is favored at high temperature, which leads to support sintering and
catalyst deactivation, the influence of the support sintering on the catalytic performance
of the catalysts needed to be investigated. Thus, TCP908S was prepared by sintering
TCP908 at 1000°C for 5h. Catalytic results revealed that despite the reduction of the
188
General conclusions and prospects
support surface area after sintering, the catalytic performance at 700°C of Ni/TCP908S
was similar to that of Ni/TCP908, catalyst prepared from the initial support. This result
was explained by the similarity of the shape and the size of nickel particles of these
catalysts. Below 700°C, Ni/TCP908 was slightly more active than Ni/TCP908S. This
was attributed to the strong metal-support interaction of Ni/TCP908S, which suggests
the possible partial reduction of nickel particles during the in situ reduction at 400°C.
The last part of the work was devoted to a comparative study. In this part, other
commercial supports were used to prepare catalysts and their performance was
compared to the hydroxyapatite-based catalysts. The investigation of PRAYON
(Ca2P2O7 and Al(PO3)3) phosphates as catalyst supports showed that the Ni-based
catalysts prepared with these supports showed negligible catalytic activity when
compared to the Ni/TCP908 catalyst. In fact, conversely to the TCP908 support,
Ca2P2O7 and Al(PO3)3 showed negligible specific surface area and pore volumes
(Ca2P2O7: SBET = 8m²/g, Vp = nd; Al(PO3)3: SBET <3 m²/g, Vp = nd). The presence of
mesopores on the Ni/TCP908 catalyst (TCP908: Vp = 0.072cm3/g) was responsible for
increasing the nickel dispersion and improving the contact between the gas (reactants)
and the solid phase (catalyst). This might have increased the DRM reaction and helped
removing the carbon deposits formed.
Commercial alumina (SBET = 170 m2/g, Vp = 0.42cm3/g), and Pural MG30 (Al2O3MgO, 30 wt.% of MgO; SBET = 148 m2/g, Vp = 0.17cm3/g) purchased from Sasol were
also used as industrial references for preparing nickel catalyst by IWI. The comparative
study of the hydroxyapatite-based catalysts with the alumina-based catalysts revealed
the promising aspects of the hydroxyapatite-based catalysts developed in this work. In
fact, the Ni/TCP908 catalyst showed better performance than Ni/Al2O3 within 50h of
TOS at 700°C and similar performance than Ni/Pural MG30 under the same conditions.
In fact, despite the large pore volumes of the Al2O3 support (Vp = 0.42cm3/g), the acidity
of this support favored the carbon deposition and led to the low catalytic performance
in DRM of the Ni/Al2O3 catalyst. The modification of Al2O3 support by adding 30wt.% of
MgO (Pural MG30) probably allowed the neutralization of the acidity of alumina support
and thus improved the catalytic property of this catalyst. Moreover, Pural MG30 is a
mesoporous support (Vp = 0.17cm3/g), which may have favored the nickel dispersion
and the mass transfer diffusions inside the catalyst and thus improved its catalytic
property. Despite the larger pore volumes of the Pural MG30, Ni/Pural MG30 and
Ni/TCP908 presented similar catalytic performances during the 50h of TOS at 700°C. It
189
General conclusions and prospects
indicates that the smaller surface area of the hydroxyapatite support is compensated
by its chemistry.
In order to affirm the results obtained with the most performing nickel-based
catalysts, catalytic tests were performed during 300h with three catalysts prepared with
sintered supports: Ni/TCP308S, Ni/TCP908S, and Ni/Pural MG30S. These catalysts
were chosen in order to evaluate the catalysts performance without the risk of
deactivation due to support sintering. Under simulated industrial conditions (T = 700°C,
TOS = 300h), all three catalysts showed high catalytic activity in DRM. Ni/TCP908S
was the most performing followed by Ni/Pural MG30S, and then Ni/TCP308S. However,
Ni/TCP908S was slowly deactivated during the 300h of TOS. The catalytic results
highlighted that the size of Ni particles and the occurrence of side reactions played
important roles on the catalytic activity and stability of the catalysts. Small Ni particles
could limit coke formation and thus could reduce the catalytic deactivation, which was
observed for the Ni/Pural MG30S catalyst. Moreover, the occurrence of side reactions
strongly influenced the formation of by-products such as coke and water, and thus
directly influenced the activity and stability of the catalysts.
This last part of the work showed that the hydroxyapatite-based catalysts are
competitive to the catalysts prepared with the commercial supports in DRM. However,
further works are still needed for the improvement of the catalyst preparation and for
the comprehension of the deactivation of these catalysts. Controlling the size of metal
Ni particles below 15 nm could be a good starting point for the preparation of cokeresistant catalysts. Also, the investigation of the basic sites of the hydroxyapatites is
needed to understand and improve their influence on the catalysts’ performance.
Moreover, molding the catalyst could be a solution to dilution of the catalyst and could
allow further investigation of the carbon deposits and of the catalysts’ deactivation.
Much effort was realized during this work on the implementation of experimental
apparatus for the evaluation of the catalysts and of characterization techniques
necessary for understanding the catalysts’ properties. The fixed-bed reactor, which
allowed a refined control of temperature, pressure and gas flow rates, and the security
system, which allowed the long time-on-stream evaluation of the catalysts, was
implemented on the last year of this work. Moreover, TPX, which provides essential
information about the catalysts properties, such as the presence of acidic and basic
sites, the metal dispersion, strength of metal-support interactions, temperature of
reduction etc. was implemented in the last months of this work. So, future work for
190
General conclusions and prospects
further comprehension and improvement of the hydroxyapatites-based catalysts will be
realized.
Besides the enhancement of catalysts production, the optimization of the reactor
design is also necessary to the development of a coke-resistant catalyst and of a DRM
technology. The use of other reactor types, such as fluidized bed reactors (FIBD), with
improved heat and mass transfer could improve the catalysts’ efficiency.
The elucidation of kinetic parameters and mechanism of the DRM as well as
transport diffusional limitations could help understanding how a catalyst works and thus
how to optimize it. The determination of the intrinsic kinetic reaction rates are the initial
steps to the development of performing catalysts and industrial reactors.
Finally, tests at pilot scale would help testing and improving the catalysts
resistance under real industrial conditions. The use of biogas as reactant in DRM is an
interesting option because it contains in addition to CH4 and CO2 other compounds
such as sulphur and chlorinated compounds that have potential to poison the catalyst.
So, it could help testing and improving the resistance of the catalysts to poisoning.
Moreover, it is a way to the valorization of these gases.
191
Publications related to this work
Publications in international journals
•
DE VASCONCELOS B. R., TRAN N. D., PHAM MINH D., NZIHOU A.,
SHARROCK
P.,
“Synthesis
of
carbon
nanotubes/hydroxyapatite
composites using catalytic methane cracking”, Composite Interfaces, 22
(2015) 673-687, (Published)
•
DE VASCONCELOS B. R., ZHAO Lulu, PHAM MINH D., NZIHOU A.,
SHARROCK P., “Catalytic transformation of carbon dioxide and methane
into syngas over ruthenium and platinum supported hydroxyapatites”,
Catalysis Today, (in preparation)
Publications in international conferences
•
DE VASCONCELOS B. R., PHAM MINH D., NZIHOU A., SHARROCK P.,
“Syngas Production over Nickel-Based Catalysts Via Dry Reforming of
Methane: Influence of Catalyst Support”, 2015 AIChE Annual Meeting,
Salt Lake City, United States of America (November 2015)
•
LYCZKO, N., PHAM MINH D., DE VASCONCELOS B., NZIHOU A.,
SHARROCK P., “Phosphates: versatile products for environmental and
energy applications”, SYMPHOS 2015, Marrakesh, Morocco
•
DE VASCONCELOS B. R., PHAM MINH D., TRAN N. D., NZIHOU A.,
SHARROCK P., “Growth of carbon nanotubes surfaces catalyzed by
metals”, 5th International Conference on Engineering for Waste and
Biomass Valorization, Rio de Janeiro, Brazil
•
PHAM MINH D., TRAN N. D., DE VASCONCELOS B. R., WEISSHORTALA E., NZIHOU A., SHARROCK P., “Calcium phosphate based
catalysts: preparation, characterization and catalytic activity in the
valorization of carbon dioxide”, XI EUROPACAT, Lyon, France
Appendix A
Characterization of the prepared catalysts
(cf. Chapter 3)
A.1 TG
Figure A.1. TG (left side) and DTG (right side) of Fe-based catalysts
Figure A.2. TG (right side) and DTG (left side) of Zn-based catalysts
Appendix A
Figure A.3. TG (right side) and DTG (left side) of Co-based catalysts
Figure A.4. TG (right side) and DTG (left side) of Cu-based catalysts
Figure A.5. TG (right side) and DTG (left side) of Ni-based catalysts
196
Appendix A
A.2 TMA
Figure A.6. TMA results of supports
A.3 EDX
A.3.1 Zn-based catalysts
Figure A.7. EDX of Zn/TCP308 (CE)
197
Appendix A
A.3.2 Fe-based catalysts
Figure A.8. EDX of Fe-based catalysts
A.3.3 Co-based catalysts
Figure A.9. EDX of Co/Ca-HA (CP)
198
Appendix A
A.3.4 Cu-based catalysts
Figure A.10. EDX of Cu/Ca-HA (CP)
A.3.5 Ni-based catalysts
Figure A.11. EDX of Ni-based catalysts
199
List of Figures
Figure 1.1. Equilibrium mole amount of reacting species versus temperature: (♦) CH4, (■) CO2,
(▲) H2, (x) CO, (*) H2O, (●) C(s) (CO2/CH4 = 1, P = 1bar) ................................................... 37
Figure 1.2. Influence of reaction pressure on the distribution of products of DRM using
FactSage software: (♦) CH4, (■) CO2, (▲) H2, (x) CO, (*) H2O, (●) C(s) (CO2/CH4 = 1/1, T =
700°C) ................................................................................................................................. 38
Figure 1.3. Influence of molar ratio of CO2/CH4 on the distribution of products of DRM using
FactSage software: (♦) CH4, (■) CO2, (▲) H2, (x) CO, (*) H2O, (●) C(s) (T = 700°C, P = 1bar)
............................................................................................................................................. 39
Figure 1.4. Simplified view of a Ca10(PO4)6(OH)2 unit [78] .......................................................... 50
Figure 1.5. Simplified view of hydroxyapatite [75] ....................................................................... 51
Figure 1.6. Infrared spectrum of a stoichiometric hydroxyapatite [75] ........................................ 52
Figure 1.7. XRD pattern of a crystalline HAP. ............................................................................. 53
Figure 2.1. Schematic representation of the U-shaped reactor system ...................................... 77
Figure 2.2. Photo of the U-shaped reactor system ..................................................................... 77
Figure 2.3. Photo of the fixed-bed reactor................................................................................... 78
Figure 2.4. Schematic representation of the fixed-bed reactor ................................................... 79
Figure 3.1. Nitrogen adsorption-desorption isotherms of TCP908 ............................................. 86
Figure 3.2. Nitrogen adsorption-desorption isotherms of TCP308 ............................................. 87
Figure 3.3. IR results of Zn-based catalysts................................................................................ 89
Figure 3.4. IR results of the Fe-based catalysts ......................................................................... 89
Figure 3.5. XRD patterns Zn-based catalysts: (♦) Ca10(PO4)6(OH)2, (▼) Ca19Zn2(PO4)14 (pattern
number: JCPDS 00-048-1196), (∆) ZnO (pattern number: COD 96-900-4181) ................. 90
Figure 3.6. XRD patterns of the Fe-based catalysts: (♦) Ca10(PO4)6(OH)2 (pattern number: COD
96-230-0274) ....................................................................................................................... 90
Figure 3.7. ESEM images of Zn-based catalysts ........................................................................ 92
Figure 3.8. ESEM images of Fe-based catalysts ........................................................................ 92
Figure 3.9. TEM of Zn/TCP308 synthesized by IWI.................................................................... 93
Figure 3.10. Cartography of Zn/TCP308 synthesized by IWI ..................................................... 94
Figure 3.11. TEM of Fe/TCP308 synthesized by IWI.................................................................. 94
Figure 3.12. Cartography of Fe/TCP308 synthesized by IWI ..................................................... 95
Figure 3.13. IR results of Co-based catalysts ............................................................................ 96
Figure 3.14. IR results of the Cu-based catalysts ....................................................................... 96
Figure 3.15. XRD patterns of Co-based catalysts: (♦) Ca10(PO4)6(OH)2 (pattern number: COD
96-230-0274), (●) Ca9.46Co0.84P6O26 (pattern number: COD 96-230-0278), (□) Co3O4
(pattern number: COD 96-900-5888) .................................................................................. 97
Figure 3.16. XRD patterns of Cu-based catalysts: (♦) Ca10(PO4)6(OH)2 (pattern number: COD
96-230-0274), (○) Cu4O4 (pattern number: COD 96-101-1195).......................................... 97
Figure 3.17. ESEM of Co-based catalysts .................................................................................. 98
Figure 3.18. ESEM of Cu-based catalysts .................................................................................. 99
Figure 3.19. XRD patterns of the unpromoted Ni-based catalysts: influence of support and Ni
amount, (♦) Ca10(PO4)6(OH)2 (pattern number: COD 96-230-0274), (◊) NiO (pattern
number: JCPDS 00-47-1049) ............................................................................................ 101
Figure 3.20. XRD patterns of the unpromoted Ni-based catalysts: influence of support sintering,
(♦) Ca10(PO4)6(OH)2 (pattern number: COD 96-230-0274), (◊) NiO (pattern number: JCPDS
00-47-1049) ....................................................................................................................... 102
Figure 3.21. XRD patterns of Ni-based catalysts promoted with Ce, Re and Co. (♦)
Ca10(PO4)6(OH)2 (pattern number: COD 96-230-0274), (●) Ca9.46Co0.84P6O26 (pattern
number: COD 96-230-0278), (◊) NiO (pattern number: JCPDS 00-047-1049), (□) Co3O4
(pattern number: COD 96-900-5898), (∆) CaO (pattern number: COD 96-900-6701) ..... 103
Figure 3.22. ESEM of the unpromoted Ni-based catalysts ....................................................... 104
Figure 3.23. TEM images of 5.7%Ni/TCP908 ........................................................................... 104
Figure 3.24. Cartography of 5.7%Ni/TCP908 ........................................................................... 105
Figure 3.25. ESEM of Ni-based catalysts prepared from the supports with and without sintering
step .................................................................................................................................... 105
Figure 3.26. ESEM images of Ni-based catalysts promoted with Re and Ce........................... 106
Figure 3.27. ESEM of Ni-based catalysts promoted with Co .................................................... 107
Figure 3.28. TPR results of unpromoted nickel-based catalysts .............................................. 108
Figure 3.29. XRD of Ni/Ca2P2O7 catalyst, (♦) Ca2P2O7 (pattern number: JCPDS 01-071-2123),
(∆) NiO (pattern number: COD 96-432-0488) ................................................................... 110
Figure 3.30. XRD of Ni/Al(PO3)3 catalyst, (■) Al(PO3)3 (pattern number: COD 96-101-0267), (∆)
NiO (pattern number: COD 96-432-0488) ......................................................................... 111
Figure 3.31. ESEM of phosphate based catalysts .................................................................... 111
Figure 3.32. Nitrogen adsorption-desorption isotherms of Al2O3 support ................................. 112
Figure 3.33. Nitrogen adsorption-desorption isotherms of Pural MG30 support ...................... 113
Figure 3.34. XRD patterns of Ni/Al2O3 catalyst, (◊) γ-Al2O3 (pattern number: COD 96-101-0462)
........................................................................................................................................... 115
Figure 3.35. XRD patterns of Pural MG30 catalysts, (■) MgAl2O4 (pattern number: COD 96-9005475), (◊) Al2O3 (pattern numbers: JCPDS 01-073-2294; JCPDS 00-016-0394; COD 96100-0443), (▲) MgO (pattern numbers: COD 96-901-3256, COD 96-901-3226, COD 96901-3222, JCPDS 01-075-0447), (●) NiO (pattern numbers: JCPDS 00-001-1239, JCPDS
01-078-0429) ..................................................................................................................... 115
Figure 3.36. ESEM of alumina-based catalysts ........................................................................ 116
Figure 3.37. TEM of the Ni/Pural MG30s catalyst ..................................................................... 117
Figure 3.38. H2-TPR profiles of alumina-based catalysts ......................................................... 118
Figure 4.1. Blank test with the U-shaped quartz reactor (reaction conditions:
CH4/CO2/N2=20/20/60, T=400-700°C) .............................................................................. 127
Figure 4.2. Catalytic test of hydroxyapatite supports: TCP308 (left side) and TCP908 (right side)
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) ... 127
Figure 4.3. Catalytic tests of Zn-based catalysts (reaction conditions: CH4/CO2/N2=20/20/60,
-1
-1
T=400-700°C, WHSV=10500mLh gcat ) .......................................................................... 128
Figure 4.4. Catalytic tests of the Fe-based catalysts (reaction conditions: CH4/CO2/N2=20/20/60,
-1
-1
T=400-700°C, WHSV=10500mLh gcat ) .......................................................................... 129
Figure 4.5. Catalytic tests results of Co-based catalysts (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) ..................................... 131
Figure 4.6. Catalytic activity evaluation of Cu-based catalysts (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) ..................................... 132
Figure 4.7. Catalytic tests of Ni-based catalyst with different hydroxyapatite supports (reaction
-1
-1
conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) .................. 133
Figure 4.8. Catalytic evaluation of Ni/TCP908 with different amounts of Ni (wt.%=2.85; 5.7; 11.4)
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) ... 134
Figure 4.9. Comparison of catalytic performance between Ni/Ce-TCP908 and Ni/TCP908
-1
-1
(reaction conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) ... 136
Figure 4.10. Catalytic evaluation of Re/Ni-TCP908 (reaction conditions: CH4/CO2/N2=20/20/60,
-1
-1
T=400-700°C, WHSV=10500mLh gcat ) .......................................................................... 137
Figure 4.11. Catalytic evaluation of Ni/TCP908 catalyst promoted with Co (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) ..................................... 138
Figure 4.12. Catalytic evaluation at 750°C of Ni-based catalysts promoted with Co (reaction
-1
-1
conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat ) .................. 139
Figure 5.1. Blank test with the fixed-bed reactor (reaction conditions: CH4/CO2/N2 = 20/20/60, T
= 400-700°C, P = Patm) ...................................................................................................... 146
Figure 5.2. Blank test with the TCP908 and with the inert alumina (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=10500mLh gcat , P = Patm) ...................... 147
Figure 5.3. Comparison of the catalytic activity of 5.7%Ni/TCP908 in the U-shaped reactor and
in the fixed-bed reactor (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 400-700°C,
-1
-1
WHSV = 12353mLh gcat , P = Patm, catalyst dilution (test with the fixed bed reactor) = 10x)
........................................................................................................................................... 148
Figure 5.4. Catalytic performance of hydroxyapatite-based catalysts at different temperatures
-1
-1
(reaction conditions: CH4/CO2/N2 = 20/20/60, T = 400-700°C, WHSV=12353mLh gcat ,
P=Patm, catalyst dilution=10x) ............................................................................................ 149
Figure 5.5. Influence of pretreatment on 5.7%Ni/TCP908 (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=12353mLh gcat , P=Patm, catalyst
dilution=10x) ...................................................................................................................... 151
Figure 5.6. Influence of pretreatment on 5.7%Ni/TCP308 (reaction conditions:
-1
-1
CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=12353mLh gcat , P=Patm, catalyst
dilution=10x) ...................................................................................................................... 152
Figure 5.7. Effect of support sintering on the catalytic performance of Ni/TCP908 (reaction
-1
-1
conditions: CH4/CO2/N2=20/20/60, T=400-700°C, WHSV=12353mLh gcat , P=Patm,
catalyst dilution=10x) ......................................................................................................... 153
Figure 5.8. Influence of pressure on the catalytic performance of 5.7%Ni/TCP908S (reaction
-1
-1
conditions: CH4/CO2/N2 = 20/20/60, T = 700°C, WHSV = 12353mLh gcat , P = Patm-25bar,
catalyst dilution = 10x) ....................................................................................................... 155
Figure 5.9. Comparison of the catalytic performance of 5.7%Ni/TCP908 with other phosphatebased catalysts (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 400-700°C, WHSV =
-1
-1
12353mLh gcat , P = Patm, catalyst dilution = 10x) ........................................................... 157
Figure 5.10. Comparison of the catalytic performance of Ni/TCP908 and Ni/Ce-TCP908
catalysts (reaction conditions: CH4/CO2/N2 = 20/20/60, T = 700°C, P = Patm, WHSV =
-1
-1
15882mLh gcat , catalyst dilution = 2x) ............................................................................ 159
Figure 5.11. Comparison of the catalytic performance of the hydroxyapatite-based catalysts with
the alumina-based catalysts (reaction conditions: CH4/CO2/N2=20/20/60, T=700°C, P =
-1
-1
Patm, WHSV = 15882mLh gcat , catalyst dilution = 2x) ..................................................... 162
Figure 6.1. TMA results of supports .......................................................................................... 172
Figure 6.2. Comparison of the catalytic performance of the hydroxyapatite-based catalysts with
the Pural MG30-based catalyst (reaction conditions: CH4/CO2/N2=20/20/60, T=700°C, P =
-1
-1
Patm, WHSV = 12272mLh gcat , catalyst dilution = 2x) ..................................................... 174
Figure 6.3. ESEM images of the Ni/TCP908S after 300h of TOS at 700°C. ............................. 175
Figure 6.4. ESEM images of the Ni/PuralMG30S after 300h of TOS at 700°C ......................... 175
Figure 6.5. ESEM images of the Ni/TCP308S after 300h of TOS at 700°C .............................. 177
Figure 6.6. H2/CO ratios measured during the DRM experiments at 700°C ............................. 177
Figure 6.7. Mass balance for the catalytic test with Ni/TCP908S .............................................. 179
Figure 6.8. Mass balance for the catalytic test with Ni/Pural MG30S ........................................ 179
Figure 6.9. Mass balance for the catalytic test with Ni/TCP308S .............................................. 180
Figure A.1. TG (left side) and DTG (right side) of Fe-based catalysts ...................................... 195
Figure A.2. TG (right side) and DTG (left side) of Zn-based catalysts ...................................... 195
Figure A.3. TG (right side) and DTG (left side) of Co-based catalysts ..................................... 196
Figure A.4. TG (right side) and DTG (left side) of Cu-based catalysts ..................................... 196
Figure A.5. TG (right side) and DTG (left side) of Ni-based catalysts ...................................... 196
Figure A.6. TMA results of supports .......................................................................................... 197
Figure A.7. EDX of Zn/TCP308 (CE) ........................................................................................ 197
Figure A.8. EDX of Fe-based catalysts ..................................................................................... 198
Figure A.9. EDX of Co/Ca-HA (CP) .......................................................................................... 198
Figure A.10. EDX of Cu/Ca-HA (CP) ........................................................................................ 199
Figure A.11. EDX of Ni-based catalysts .................................................................................... 199
List of Tables
Table 1. Limiting temperatures for the reactions of the CO2/CH4 system [21] ............................ 38
Table 2. Different calcium phosphates ........................................................................................ 49
Table 3. Positions and relative intensities of IR characteristic bands of hydroxyapatites [75].... 52
Table 2.1. Precursors of the active metals and catalyst promoters used in this work ................ 66
Table 2.2. Wettable volume for 10g of supports ......................................................................... 67
Table 2.3. Catalysts synthesized with different transition metals and supports (x: prepared; -: not
prepared) ............................................................................................................................. 70
Table 3.1. Measurement of particle size, density and pore volumes of the hydroxyapatite
supports ............................................................................................................................... 87
Table 3.2. SBET and ICP measurements of Fe and Zn-based catalysts ...................................... 88
Table 3.3. SBET and ICP measurements of Co and Cu-based catalysts ..................................... 95
Table 3.4. SBET and ICP measurements of Ni-based catalysts (-: not measured) .................... 100
Table 3.5. Measurement of particle size, density and pore volumes of the phosphates supports
........................................................................................................................................... 109
Table 3.6. SBET and ICP measurements of phosphate-based catalysts ................................... 110
Table 3.7. Measurement of particle size, density and pore volumes of the alumina-based
supports ............................................................................................................................. 114
Table 3.8. SBET and ICP measurements of supports and alumina-based catalysts ................. 114
Phosphates-based catalysts for synthetic gas (syngas) production using CO2 and CH4
Abstract: Among the products resulting from biomass or organic waste transformation, CO2 and CH4 are important
chemical intermediates. They also have a strong environmental impact since they are primarily responsible for the
greenhouse effect and their mitigation is a key issue. An attractive way of valorization of such gases is the dry
reforming of methane (DRM), which converts CO2 and CH4 into syngas (mixture of hydrogen and carbon monoxide).
This mixture can be used for several applications, such as the production of methanol, dimethyl ether, hydrogen and
liquid hydrocarbons. Despite such interest, the exploitation of DRM on industrial scale has not emerged yet. The main
reason is the rapid deactivation of the catalysts due to the severe operating conditions of the process (high
temperature, carbon deposition). This thesis focuses on the development of new catalysts based on calcium
phosphate (CaP) doped with transition metals for the valorization of CO2 and CH4 through DRM. Actually, CaP has
advantageous properties in heterogeneous catalysis, as the simultaneous presence of acid and basic sites, good
thermal stability, and wide range of surface area... Initially, a study on the catalyst synthesis methods and an
investigation of the performance of different transition metals (Zn, Fe, Co, Cu, Ni) were carried out in order to select
the catalyst system and the preparation method. Secondly, a fixed-bed reactor capable of operating at high
temperature and pressure and for log time on stream was built and implemented during this work in order to properly
evaluate the performance of the prepared catalysts. Then, a detailed parametric study was conducted. The influence
of parameters such as catalyst pre-treatment, temperature (T = 400-700°C) and pressure (P = 1-25bar) of the reaction
and support (hydroxyapatite, alumina-based supports) were investigated. Finally, the catalytic stability was studied for
300h of time on stream (TOS). The CaP catalysts showing higher yields on syngas were compared to commercial
catalysts. Our catalysts showed to be competitive in the same operating conditions (T = 700°C, P = 1bar, WHSV =
12272mLh-1gcat-1, TOS = 300h). This work shows the interest of CaP catalysts for high temperature process, such as
dry reforming of methane.
Keywords: Dry reforming of methane, Syngas, Heterogeneous catalysis, Calcium phosphate, Catalytic
stability, Transition metals
Catalyseurs à base de pour phosphates pour la production de gaz de synthèse (syngas) à partir du
dioxyde de carbone (CO2) et du méthane (CH4)
Résumé : Parmi les produits issus de la biomasse ou de la transformation des déchets organiques, le CO 2 et le CH4
sont des intermédiaires chimiques importants qui ont de forts impacts environnementaux. En effet, ils sont les
principaux gaz responsables de l'effet de serre et leur atténuation est un enjeu majeur. Une voie intéressante pour la
valorisation de ces gaz est le reformage à sec du méthane (DRM), qui convertit le CO 2 et le CH4 en gaz de synthèse
(mélange d'hydrogène et de monoxyde de carbone). Ce mélange peut être utilisé pour plusieurs applications telles
que la production de methanol, d'éther diméthylique, d'hydrogène et des hydrocarbures liquides. Malgré cet intérêt,
l'exploitation du DRM à l'échelle industrielle n'a pas encore vu le jour. La raison principale est la désactivation rapide
des catalyseurs en raison des conditions sévères de fonctionnement du procédé (température élevée, dépôt de
carbone). Cette thèse porte sur le développement de nouveaux catalyseurs à base de phosphate de calcium (CaP)
dopés avec des métaux de transition pour la valorisation du CO2 et du CH4 en gaz de synthèse par DRM. Les CaP
sont utilisés car ils possèdent des propriétés avantageuses en catalyse hétérogène comme la présence simultanée
de sites acides et basiques, bonne stabilité thermique, large gamme de surface spécifique ... Dans un premier temps,
des études sur les méthodes de synthèse de catalyseurs et sur la performance de différents métaux de transition (Zn,
Fe, Co, Cu, Ni) ont été effectuées dans le but de sélectionner le catalyseur et sa méthode de préparation. Un réacteur
à lit fixe capable de fonctionner à hautes température et pression a ensuite été testé pour un long temps de réaction
afin d'évaluer correctement la performance des catalyseurs préparés. Ensuite, une étude paramétrique détaillée a été
menée. L'influence des paramètres tels que le prétraitement des catalyseurs, la température (T = 400-700°C) et la
pression (P = 1-25bar) de la réaction et les différents supports (hydroxyapatite, alumine) ont été étudiés. Enfin, la
stabilité thermique et catalytique a été étudiée durant 300h de réaction. Les catalyseurs à base de CaP ont montré
des rendements plus élevés en gaz de synthèse en comparaison aux catalyseurs commerciaux. Ces catalyseurs sont
donc compétitifs dans les mêmes conditions opératoires (T = 700°C, P = 1bar, WHSV = 12272mLh -1gcat-1, t = 300h).
Ce travail a montré l'intérêt des catalyseurs à base de CaP pour des processus à haute température, tel que le
reformage à sec du méthane.
Mots-clés : Reformage à sec du méthane, Gaz de synthèse, Catalyse hétérogène, Phosphate de calcium,
Stabilité catalytique, Métaux de transition