Journal of Natural Gas Chemistry 18(2009) –
Review
“Intelligent” reforming catalysts: Trace noble metal-doped
Ni/Mg(Al)O derived from hydrotalcites
Katsuomi Takehira∗
Department of Chemistry and Chemical Engineering, Graduate School of Engineering, Hiroshima University,
Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan
[ Received March 31, 2009; Revised July 3, 2009; Available online September 14, 2009 ]
Abstract
Trace amounts of noble metal-doped Ni/Mg(Al)O catalysts were prepared starting from Mg-Al hydrotalcites (HTs) and tested in daily
start-up and shut-down (DSS) operation of steam reforming (SR) of
methane or partial oxidation (PO) of propane. Although Ni/Mg(Al)O
catalysts prepared from Mg(Ni)-Al HT exhibited high and stable
activity in stationary SR, PO and dry reforming of methane and
propane, the Ni/Mg(Al)O catalysts were drastically deactivated due
to Ni oxidation by steam as purge gas when they were applied in DSS
SR of methane. Such deactivation was effectively suppressed by doping trace amounts of noble metal on the catalysts by using a “memory
effect” of HTs. Moreover, the noble metal-doped Ni/Mg(Al)O catalysts exhibited “intelligent” catalytic behaviors, i.e., self-activation
and self-regenerative activity, leading to high and sustainable activity
during DSS operation. Pt was the most effective among noble metals tested. The self-activation occurred by the reduction of Ni2+ in
Mg(Ni,Al)O periclase to Ni0 assisted by hydrogen spillover from Pt
(or Pt-Ni alloy). The self-regenerative activity was accomplished by
self-redispersion of active Ni0 particles due to a reversible reductionoxidation movement of Ni between the outside and the inside of the
Mg(Al)O periclase crystal; surface Ni0 was oxidized to Ni2+ by
steam and incorporated into Mg(Ni2+ ,Al)O periclase, whereas the
Ni2+ in the periclase was reduced to Ni0 by the hydrogen spillover
and appeared as the fine Ni0 particles on the catalyst surface. Further a “green” preparation of the Pt/Ni/[Mg3.5 Al]O catalysts was accomplished starting from commercial Mg3.5 -Al HT by calcination,
followed by sequential impregnation of Ni and Pt.
Key words
hydrocarbon reforming; Pt/Ni/Mg(Al)O catalysts; hydrotalcite precursors; self-activation; self-regenerative activity
1. Introduction
Hydrotalcite-like compounds (HTs) can be described by
n−
3+
x+
the general formula: [M2+
1−x Mx (OH)2 ] (Ax/n )·mH2 O, and
contain various cations (M2+ and M3+ ) and anion (An− ) [1],
∗
Dr. Katsuomi Takehira was born in 1942.
He received the M. Eng. and the D. Eng.
degrees in chemical engineering from the
Hiroshima University and the Tokyo Institute of Technology, Japan in 1967 and
1981. He worked for 10 years as a Research Engineer in the National Chemical
Laboratory for Industry (NCLI) in Tokyo,
MITI, Japan, and studied in the Institut
Français du Pétrole in Rueil Malmaison,
France for one and a half year. He became
a Director of the Department of Surface
Chemistry, NCLI in Tsukuba, Japan in 1993 and moved to Hiroshima University as a Professor of the Department of Chemistry and Chemical Engineering in 1997. Now he is a Professor Emeritus. During his stay in Tokyo,
Rueil Malmaison and Tsukuba, he mainly explored various types of catalytic
system for the oxidative conversion of hydrocarbons. Since 1995 he has been
engaged in the research work of reforming catalysts and has now focused
on the hydrogen production for PEFCs on the nano-sized supported metal
catalysts derived from hydrotalcite materials.
with the most common HT combining Mg and Al. The structure of HT is basically that of brucite, Mg(OH)2, in which
octahedral of Mg2+ ions (6-fold coordinated to OH− ) share
edges to form infinite sheets. These sheets are stacked on
top of each other and are held together by hydrogen bonding.
When Mg2+ ions are substituted by a trivalent ion having a
radius not too different from that of Mg2+ (such as Fe3+ and
Al3+ for pyroaurite and HT, respectively), a positive charge
is generated in the hydroxyl sheets. This net positive charge
is compensated for by CO2−
3 anions, which lie in the interlayer region between two brucite sheets (Figure 1) [2,3]. Water of crystallization is also located in the free space of this
interlayer region. The sheets containing cations are built as
in brucite, where the cations randomly occupy the octahedral
holes in the close-packed configuration of the OH− ions. Generally M2+ and M3+ ions can be accommodated in the holes
Corresponding author. E-mail: [email protected] or [email protected]
This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan.
Copyright©2009, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.
doi:10.1016/S1003-9953(08)60123-1
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Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
of the close-packed configuration of OH group in the brucitelike layers as far as the ionic radii and the valence states are
allowed.
The HTs have been used as catalysts mainly after calcination because the resultant oxides possess interesting properties
such as large surface area and basic properties. Calcination results in the formation of homogeneous mixtures of oxides with
very small crystal size, and stable to thermal treatments, which
further form small and thermally stable metal crystallites by
reduction [4,5]. Moreover, the calcined oxides show a unique
property, i.e, “memory effect ”, which allows the reconstitution of the original HT structure contacting the oxides after
calcination with aqueous solutions containing various anions.
The HT-derived oxides themselves are well known as a base
catalyst and the structure of the HT can accommodate a wide
variation of the different metals, leading to different catalytic
properties [4−8].
Figure 1. Structure of HT
Hydrogen production for polymer electrolyte fuel cells
(PEFCs) is a research area of urgent importance in addressing global warming. Steam reforming (SR) of hydrocarbons,
especially of methane (1), is the most wide spread and gener0
CH4 + H2 O → CO + 3H2, ∆H298
= +206 kJ · mol−1
as highly active and sustainable reforming catalyst [11], the
“intelligent” property could not be detected due to too small
amount of Ni dissolved in SrTiO3 . Oppositely, Ni/Mg(Al)O
obtained by replacing a part of the Mg2+ in Ni/MgO by Al3+
can dissolve much amount of Ni2+ as seen for Ni/MgO which
forms solid solutions with all Ni/Mg molar ratios [16]. In fact,
the Ni/Mg(Al)O catalysts prepared from Mg(Ni)-Al HT exhibited high and sustainable activity during stationary SR and
partial oxidation (PO) of methane [17−19]. However, the catalysts were drastically deactivated by purging not only with
oxygen but also with steam in daily start-up and shut-down
(DSS) operation [20]. The DSS operation of reformer is indispensable for the domestic use of PEFCs, where the catalyst bed must be purged by steam for enhancing the safety. A
doping of trace amounts of noble metals, i.e., Ru, Rh and
Pt, effectively suppressed such oxidative deactivation [21].
Moreover, the author has found that such noble metal-doped
Ni/Mg(Al)O catalysts exhibited “intelligent” behaviors, i.e.,
self-activation and self-regenerative activity, during the DSS
operation [22−24]. Among the noble metals tested, Pt exhibited the best catalytic behavior [24].
To prepare the Pt/Ni/Mg(Al)O catalysts in an industrial
scale, there still exist several bottlenecks to be broken; the
amounts of waste water containing nitrate ions must be minimized to achieve a “green” catalyst preparation. Because the
Ni/Mg(Al)O catalysts were prepared by coprecipitation of the
nitrates of Ni(II), Mg(II) and Al(III) [22−24], the waste water contained a large amount of nitrate ions. Moreover, the
catalyst must be prepared by using the “memory effect” to
enhance the catalytic activity due to high dispersion of the
active metal species. Considering these restrictions, the catalysts were prepared starting from commercial Mg-Al HTs, followed by calcination and sequential impregnation with Ni(II)
and Pt(IV) nitrates. The conditions of the catalyst preparations were carefully examined and optimized to produce the
“intelligent” Pt/Ni/Mg(Al)O catalysts.
The author has focused this review on our recent progresses in the preparation of the “intelligent” Pt-Ni bimetallic
catalysts derived from Mg-Al HT for hydrocarbon reforming.
(1)
ally the most economical way to make hydrogen and most
often nickel catalysts are used due to the low costs and high
activity [9]. The author previously reported oxidative reforming of methane over Ni-supported catalysts prepared from
perovskites [10,11] and reviewed such results together with
reforming reactions over the Ni catalysts derived from HTs
[12,13]. Indeed, noble metal-supported perovskite, such as
Pd0.05 /LaFe0.95O3 , has been developed as automotive catalysts and proved to exhibit an “intelligent” property, i.e, selfregenerative activity, due to a cyclic movement of Pd between
the outside (as Pd0 nanoparticles) and the inside (as Pd2+ in
the lattice) of the perovskite crystal [14]. As the result, the
growth of Pd particles was suppressed during the entire lifetime of the vehicle. This self-regenerating function was realizable also on Pt0.05 /CaTi0.95 O3 and Rh0.05 /CaTi0.95 O3 catalysts [15]. Although the author has developed Ni/SrTiO3
2. Results and discussion
2.1. Catalyst preparation and characterization
Ni-loaded Mg(Al)O catalyst with a Mg/Ni/Al molar ratio of 2.5/0.5/1.0 was prepared by co-precipitation [17−19];
Mg2.5(Ni0.5 )-Al HT, in which a part of Mg2+ in Mg-Al HT
was replaced by Ni2+ , was prepared by co-precipitation of
Mg(II), Ni(II) and Al(III) nitrates. After the calcination of
the precipitates at 850 ◦ C for 5 h; Mg2.5(Al,Ni0.5 )O periclase was obtained as powders and used as the precursor of
Ni0.5 /Mg2.5 (Al)O catalyst. The Ni loading was found to be
16.5 wt% by inductively coupled plasma spectroscopy (ICP)
analyses after the calcination at 850 ◦ C.
Ru, Rh or Pt doping was done by adopting the “memory
effect” of Mg-Al HT [4,21−24]. When Mg25 (Al,Ni0.5 )O per-
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
iclase powders were dipped in an aqueous solution of Ru(III),
Rh(III) or Pt(IV) nitrate, the HT was reconstituted on the powders due to the “memory effect”. After the calcination of the
powders at 850 ◦ C for 5 h, Ru/, Rh/and Pt/Ni0.5 /Mg2.5 (Al)O
catalysts were obtained. The powders were pressed to the particles of 0.36∼0.60 mm in a diameter and used in the reforming reactions.
As a control, 13.5 wt% Ni/γ-Al2 O3 catalyst was prepared
by an incipient wetness (i.w.) method using γ-Al2 O3 (JRCALO8) and an aqueous solution of Ni(II) nitrate, followed
by the calcination at 850 ◦ C for 5 h. Commercial Ni and
Ru catalysts were supplied from Süd-Chemie Catalysts Japan,
Inc. and were also used as controls. Both FCR (12 wt%
Ni/α-Al2 O3 ) and RUA (2 wt% Ru/α-Al2 O3 ) catalysts as received were crushed to fine powders, pressed to the particles
of 0.36∼0.60 mm in diameter and used in the reforming reactions.
To prepare Pt/Ni/Mg(Al)O catalysts in an industrial scale,
commercial Mg-Al HTs (SASOL Germany GmbH) were used
as starting materials. Three types Mg-Al HTs of chemical
formula, Mg2m Al2 (OH)4m+4CO3 · nH2 O, with Mg/Al molar
ratios of 3.5, 1.3 and 0.5 are represented as [MgxAl] HTs
(x = 3.5, 1.3 and 0.5), respectively. The trace Pt-doped Ni
catalysts were prepared by sequential impregnation with Ni2+
and Pt4+ by using the “memory effect” of Mg-Al HT. After
the calcination of [MgxAl] HTs at 850 ◦ C for 5 h, the mixed
oxides, [Mgx Al]O (x = 3.5, 1.3 and 0.5), were obtained as
powders. The mixed oxide powders were dispersed in an
aqueous solution of Ni(II) nitrate at 80 ◦ C for 2 h, filtrated
by glass filter and the separated powders were calcined at
850 ◦ C for 5 h. The Ni-loaded powders after the calcination
were again impregnated with Pt4+ and followed by the same
treatment as the Ni2+ impregnation. Prescribed amounts of
Ni(II) and Pt(IV) nitrates were dissolved in deionized water
and used in each impregnation. The amounts of Ni2+ and
Pt4+ dissolved in the solutions after the filtrations were determined by ICP analyses. Calcination temperatures of the HT,
the Ni-loaded sample and further the Pt-doped sample were
varied between 400 ◦ C and 850 ◦ C, and moreover the dispersion temperatures in aqueous solutions of Ni(II) and Pt(IV) nitrates were varied between room temperature and 80 ◦ C. The
powders after the final calcination at 400∼850 ◦ C for 5 h were
pressed to the particles of 0.36∼0.60 mm in diameter and used
in the reforming reactions.
As control, 0.05 wt% Pt-10 wt% Ni/[Mg3.5 Al]O catalyst
was prepared by coimpregnation as follows: the [Mg3.5 Al]O
powders were dispersed in a mixed aqueous solution of Ni(II)
and Pt(IV) nitrates at 80 ◦ C for 2 h, filtrated by glass filter and
the powders separated were calcined at 850 ◦ C for 5 h.
The structures of the catalysts were studied by using powder X-ray diffraction (XRD), magic-angle spinning 27 Al nuclear magnetic resonance (MAS 27 Al NMR),
thermo-gravimetric differential thermal analyses (TG-DTA),
transmission electron microscopy (TEM), X-ray absorption
(XANES and EXAFS), ICP, temperature programmed reduction (TPR), temperature programmed oxidation (TPO), and
N2 and H2 adsorption.
3
2.2. Kinetic measurements
SR of methane was conducted in a fixed-bed flow reactor with a CH4 /H2 O/N2 (50/100/25 ml·min−1 ) gas mixture at 700 ◦ C over 50 mg of the catalyst in a stationary
or a DSS-like mode (Figure 2) [21−24]. N2 was used
as an internal standard for calculating the methane conversion and the product yields. To test the ability of selfactivation of the catalyst, the stationary operations of SR and
PO in a CH4 /H2 O/N2 (50/100/25 ml·min−1 ) and a CH4 /O2 /N2
(50/25/25 ml·min−1) gas mixture, respectively, were conducted at 700 ◦ C for 180 min, after heating the catalyst
from room temperature to 700 ◦ C in a N2 (25 ml·min−1 ) gas
flow. Autothermal steam reforming (ATSR) of methane in a
CH4 /H2 O/O2 /N2 (50/100/25/25 ml·min−1) gas mixture was
conducted at 700 ◦ C in a DSS-like mode under steam purging. The thermocouple to control the reaction temperature was
placed at the center of the catalyst bed.
Figure 2. DSS-like reaction mode [21]
PO of propane was conducted using a fixed bed-flow reactor in a C3 H8 /O2 /N2 (10/18.7/71.3 ml·min−1 ) gas mixture
over 50 mg of the catalyst for testing the activity with several
types of the reaction mode, i.e, stationary mode, temperaturecycled mode, O2 purging mode and increasing temperature
mode. The detail of each reaction mode is shown later in the
results of PO of propane.
To simulate the catalyst deactivation in an actual reformer,
steaming treatment was performed with a fixed bed flow reactor in a H2 /H2 O/N2 (20/100/25 ml·min−1 ) gas mixture for
10 h at 900 ◦ C. Each 300 mg of the catalyst was steamed,
and a 50-mg catalyst sample after steaming was supplied for
testing the catalytic activity in both stationary and DSS SR of
methane.
Turnover frequency (TOF) was evaluated in SR of
methane with a small amount of catalysts [22−24]; all catalysts were crushed, and a 10 mg of the catalyst powders
(0.075∼0.180 mm in diameter) were dispersed in ca. 20 mg of
quartz wool and used in the reaction at either 500 ◦ C or 600 ◦ C
in a CH4 /H2 O/N2 (88.8/177.6/44.4 ml·min−1 ) gas mixture at
−1
a GHSV of 1.6×106 ml·g−1
cat ·h .
2.3. Activity of Ni/Mg(Al)O catalysts
Ni/Mg(Al)O catalysts were prepared by calcination followed by reduction of ternary Mg(Ni)-Al HT with various
4
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Mg/Al molar ratios [17,18,24−27] and were tested in PO
[26,27], SR [17], dry reforming [25] and autothermal reforming of methane [18]. The high and stable activity was obtained
in all reforming reactions and were attributed to stable and
well-dispersed Ni particles evolved from Mg(Ni,Al)O periclase; Ni2+ in the Mg(Ni,Al)O periclase was reduced to Ni0
and migrated to the periclase surface to form fine Ni particles.
When the Ni/Mg(Al)O catalyst was used in PO of methane,
the reaction proceeds via combustion (2), followed by SR (1)
and dry reforming (3).
0
CH4 + 2O2 → CO2 + 2H2O, ∆H298
= −802 kJ/mol
(2)
0
CH4 + CO2 → 2CO + 2H2, ∆H298
= +247 kJ/mol
(3)
Among the supported Ni catalysts prepared,
Ni0.5 /Mg2.5 (Al)O derived from the HT exhibited the highest activity and sustainability; methane conversion reached
thermodynamic equilibrium even at the enormously high
GHSV of 9×105 ml·h−1 ·g−1
cat (Figure 3(1)) [18]. In the PO,
Ni catalysts were frequently deactivated due to Ni oxidation, resulting in a dramatic decrease in methane conversion
as seen for FCR (Figure 3(8)). All Ni catalysts prepared
by impregnation of Mg(Al)O periclase with Ni(II) nitrate
were deactivated with increasing GHSV, among which the
catalyst impregnated in aqueous solution was more sustainable than the others (Figure 3(2)). Suffix “aq”, “acac” and
“ac” shows that the “16.3 wt% Ni/Mg3 (Al)O” catalyst was
prepared by impregnation in aqueous, acetylacetone and acetone solution of Ni(II) nitrate, respectively. This is due to
the surface reconstitution of Mg(Ni)-Al HT by the “memory effect”, leading to the higher dispersion of Ni particles
on 16.3 wt% Ni/Mg3 (Al)O-aq. Although FCR is an established commercial catalyst for SR process, Ni0.5 /Mg2.5 (Al)O
showed a higher sustainability than FCR in SR of methane
Figure 3. PO of methane over Ni catalysts [18]. (1) Ni0.5 /Mg2.5 (Al)O, (2)
16.3 wt% Ni/Mg3 (Al)O-aq, (3) 16.3 wt% Ni/Mg3 (Al)O-acac, (4) 16.3 wt%
Ni/Mg3 (Al)O-ac, (5) 16.3 wt% Ni/MgO, (6) 16.3 wt% Ni/γ-Al2 O3 , (7) 16.3
wt% Ni/α-Al2 O3 , (8) FCR. CH4 /O2 /N2 (2/1/1) mixed gas was used at 800 ◦ C
for 50 and 5 mg of the catalyst diluted by quartz beads at the low (dotted line)
and the high (full line) space velocities, respectively
(Figure 4) [17]; the reaction was carried in a large scale using
each 20 cc of Ni0.5 /Mg2.5 (Al)O and FCR at 800 ◦ C, with the
steam to carbon ratio (S/C) = 1.6 and at a GHSV = 2500 h−1
for 600 h. When GHSV was increased to 10000 h−1 for evaluating precisely the catalyst deactivation, the reaction temperature inevitably decreased to 660 ◦ C because the thermal conduction of the reactor wall was not high enough
to compensate heat consumed by endothermic SR reaction.
Ni0.5 /Mg2.5 (Al)O kept methane conversion at the value of
thermodynamic equilibrium, whereas FCR showed clear decrease in methane conversion, during 600 h of the reaction.
Figure 4. Catalyst life test in SR of methane over Ni0.5 /Mg2.5 (Al)O
and FCR [17]. FCR, 20 cc; Ni0.5 /Mg2.5 (Al)O), 17.3 g; S/C = 1.6; (•, ◦),
Ni0.5 /Mg2.5 (Al)O; (, ), FCR
When Ni0.5 /Mg2.5(Al)O was prepared by coprecipitation
of Ni(II), Mg(II) and Al(III) nitrates, Ni species distributed
uniformly in the catalyst particles and therefore those located deeply inside the particles could not work as the active
species. In fact, the reduction degree obtained from the H2 TPR measurements of Ni0.5 /Mg2.5(Al)O was 80%∼85% [21],
indicating that a part of Ni remained as Ni2+ in Mg(Ni,Al)O
periclase particles even after reduction with H2 . Preferential
Ni loading in the outer layer of the catalyst particles, i.e.,
eggshell-type Ni loading, on Mg(Al)O periclase particles was
preferable and has been achieved by adopting the “memory
effect” of Mg-Al HT [19]. Active Ni species were effectively
enriched in the surface layer of the catalyst particles by controlling the conditions of the impregnation of Mg(Al)O periclase in Ni(II) nitrate. In fact, the eggshell-type Ni-loaded
catalyst showed an enhanced activity per unit amount of Ni
due to the surface enrichment of active Ni species. A measurement of the effectiveness factor of the catalyst confirmed
that an intraparticle mass transfer limitation exists in SR of
methane over Ni0.5 /Mg2.5 (Al)O at 800 ◦ C (Figure 5) [19].
The eggshell-type Ni-loaded catalyst exhibited high activity
due to surface Ni enrichment not only in SR [28] but also in
ATSR [29] of methane.
In contrast to a large-scale use of reformers in the industry under stationary operating conditions, temperature is frequently varied in DSS operations to produce hydrogen for
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Figure 5. Effectiveness factor of Ni0.5 /Mg2.5 (Al)O in SR of methane
[19]. Reaction temperature, 700 ◦ C; Average particle size of the catalyst,
0.15∼0.48 mm in diameter
PEFCs in domestic use [20]. Between shut-down and startup, the catalyst bed in the reformer is purged by steam to enhance safety. Thus, the catalyst must be tolerable multiple
cycles under such unusual transient conditions without deterioration. Deactivations of Ni-loaded catalysts caused by coking, sintering and oxidation of the active metal species have
been frequently reported [20]. Ni/Mg(Al)O catalysts with
various (Mg+Ni)/Al molar ratios were tested in DSS SR of
methane by purging with three types of gases, i.e., steam, air
and spent gas, together with RUA, FCR and 13.5 wt% Ni/γAl2 O3 [20] (Figure 6). Most of Ni/Mg(Al)O catalysts were
severely deactivated in DSS SR, although these catalysts exhibited high and stable activity in stationary SR of methane
(Figures 3 and 4) [17,18]. It is remarkable that air, i.e.,
O2 , damaged most severely all supported Ni catalysts. Both
steam and spent gas completely deactivated MgO-supported
Ni catalysts, whereas they did not deactivate severely Al2 O3 supported Ni catalysts. The sustainability of Ni/Mg(Al)O
Figure 6. Sustainability of catalysts during DSS-like operations [20]. Ni
loading, 16 wt% for Ni/Mg(Al)O; Sustainability = CH4 conversion at the
fourth cycle/CH4 conversion at the first cycle
5
varied dependently of the (Mg+Ni)/Al molar ratio. The XRD
observations indicated that the deactivation occurred by the
oxidation of Ni0 to Ni2+ , followed by the incorporation into
Mg(Ni2+,Al)O periclase; the reflections of Ni metal disappeared and those of Mg(Ni,Al)O periclase were intensified after the deactivation. The Ni oxidation took place directly with
O2 or H2 O, or was possibly induced by hydration of MgO to
Mg(OH)2 brucite, followed by OH spillover to Ni metal surfaces. Mg(OH)2 is thermodynamically more stable than MgO
[30] and a great number of OH groups may be formed on the
surface of Mg(Al)O under steam atmosphere.
Ni can be incorporated also as anion into interlayer of
Mg-Al HT [31−34]; Ni2+ reacts with an anionic chelating agent of EDTA4− and produces a stable anionic species
[Ni(EDTA)]2−. Ni/Mg17 (Al2 )O catalyst prepared by coprecipitation of Mg(II) and Al(III) nitrates with the presynthesized nickel chelate was the most active in converting
methane, with a high hydrogen yield, in SR of methane [31].
This is due to the highest dispersion of the active Ni species
occurring via reconstitution of HT by the “memory effect”.
Ru, Rh and Pt were also supported on Mg(Al)O using noble metal cations prechelated with EDTA4− by adopting the
“memory effect”; Ru exhibited the highest activity and its
loading could be lowered from 2.0 to 0.1 wt% without any
decrease in catalytic activity in both PO and dry reforming
of methane [33]. Moreover, Ru-M (M = Cr, Fe, Co, Ni and
Cu) bimetallic catalysts were also prepared by repeating the
EDTA-chelating method for Mn+ and Ru3+ ; 0.10 wt% Ru/5.0
wt% Ni/Mg(Al)O catalyst exhibited the best catalytic performance in dry reforming of methane [34].
Figure 7.
XRD patterns during the preparation of 0.10 wt%
Ru/Ni0.5 /Mg2.5 (Al)O [21]. (1) Mg2.5 (Ni0.5 )-Al HT, (2) after calcination of (1) at 850 ◦ C for 5 h, (3) after dispersing (2) in aqueous solution of
Ru(III) nitrate, followed by drying, (4) after calcination of (3) at 850 ◦ C for
5 h, (5) after reduction of (4) at 900 ◦ C for 1 h (H2 /N2 = 1/5), (6) after using
(5) in the 4 cycled DSS operation followed by decreasing temperature to
200 ◦ C under N2 purge.
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Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
2.4. Improvement of Ni/Mg(Al)O catalyst by noble metal doping
Noble metals, Ru, Rh and Pt, can be doped on
Ni/Mg(Al)O catalysts simply by dispersing Mg(Ni,Al)O periclase particles in aqueous solutions of these noble metals
nitrates. After calcination, followed by reduction, the noble metal-doped catalysts were prepared and tested in DSS
SR of methane [21,35,36]. A trace amount of noble metals, Ru, Rh and Pt, effectively suppressed oxidative deactivation of Ni0.5 /Mg2.5 (Al)O. XRD patterns of the samples during preparation of 0.10 wt% Ru/Ni0.5 /Mg2.5(Al)O are shown
in Figure 7 [21]. The powders as deposited by coprecipitation
exhibited Mg(Ni)-Al HT reflection lines (Figure 7(1)), and
Mg(Ni,Al)O periclase reflections appeared after the calcination at 850 ◦ C (Figure 7(2)). After dispersing Mg(Ni,Al)O
periclase in an aqueous solution of Ru(III) nitrate, Mg(Ni)Al HT was reconstituted together with Mg(OH)2 brucite (Figure 7(3)). The formation of Mg(OH)2 brucite suggests that
the reconstitution of HT from the periclase proceeds via the
hydration of MgO. MgO is thermodynamically unstable compared with Mg(OH)2 and reacts very easily even with moisture in the air, especially at low coordination atomic site, to
form Mg(OH)2 brucite [30]. After the calcination at 850 ◦ C
for 5 h, both Mg(Ni)-Al HT and Mg(OH)2 disappeared and
the periclase phase regenerated (Figure 7(4)). After the reduction at 900 ◦ C for 1 h, Ni metal reflections appeared (Figure 7(5)). It must be noticed that Ni metal reflections were still
observed even after the fourth step-DSS operation for not only
Ru- (Figure 7(6)) [21] but also Rh- and Pt-doped Ni/Mg(Al)O
catalysts [35,36].
The results of DSS SR of methane over Ru/Ni/Mg(Al)O
are shown in Figure 8 [21]. Ni0.5 /Mg2.5(Al)O showed a clear
deactivation due to Ni oxidation just after the first steam purging. This was confirmed by XRD analyses; no Ni metal
reflection was observed and the reflections of Mg(Ni,Al)O
periclase were intensified, suggesting that Ni0 was oxidized
to Ni2+ and incorporated into the periclase. 13.5 wt% Ni/γAl2 O3 was also deactivated and NiO reflections appeared just
after the first steam purging (data are not shown). When 0.50
wt% Ru was doped on the 13.5 wt% Ni/γ-Al2 O3 , the catalytic
behavior was improved, but the catalyst was totally deactivated due to Ni oxidation after the second steam purging (Figure 8). Ru doping on Ni0.5 /Mg2.5 (Al)O was quite effective;
the high activity was sustained during the four-cycled operations even when Ru doping decreased from 0.50 wt% to 0.05
wt% (Figure 8). With 0.01 wt% Ru doping, however, the catalyst was totally deactivated after the third steam purging (data
are not shown). Usually in the catalyst preparation by the
“memory effect”, 1.0 g of Mg2.5 (Al,Ni0.5 )O periclase powders was dispersed in 5 ml of Ru(III) nitrate-aqueous solution.
When 0.10 wt% Ru doping was carried out by incipient wetness (i.w.) method using a 0.5 ml of Ru(III) nitrate-aqueous
solution, corresponding to the pore volume of the periclase,
the obtained catalyst exhibited a clear deactivation after the
second steam purging (data are not shown). No HT reflection
was observed in the XRD patterns during the i.w. preparation.
This indicates that the “memory effect” plays an important
role in the noble metal doping on the catalyst. In the absence
of Ni, the activity of 0.10∼0.50 wt% Ru/Mg3 (Al)O catalysts
gradually decreased during the reaction regardless of steam
purging, suggesting that the deactivation was due to gradual
surface change, sintering or oxidation, of Ru (Figure 8). It
must be noticed that Ru alone was not active enough with
such small loading, indicating that the active species of the
Ru/Ni0.5 /Mg2.5 (Al)O catalyst was Ni.
Figure 8. DSS SR of methane over Ru/Ni0.5 /Mg2.5 (Al)O under steam purging [21]. (1) Ni0.5 /Mg2.5 (Al)O, (2) 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O, (3) 0.30
wt% Ru/Ni0.5 /Mg2.5 (Al)O, (4) 0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)O, (5) 0.05 wt%
Ru/Ni0.5 /Mg2.5 (Al)O, (6) 0.50 wt% Ru/13.5 wt%Ni/γ-Al2 O3 , (7) 0.50 wt%
Ru/Mg3 (Al)O, (8) 0.30 wt% Ru/Mg3 (Al)O, (9) 0.10 wt% Ru/Mg3 (Al)O
2.5. Noble metal/Ni/Mg(Al)O catalysts for PO of propane
In contrast to PO of methane, only introductory studies
concerning PO of higher hydrocarbons have been limited to
noble metal catalysts [37−41]. An extensive work by Schmidt
and co-workers on short contact-time reactors [37,38] revealed that Rh has high activity and selectivity, superior to
those of other noble metals, and moreover Rh catalyzes the
direct PO of methane (4). Rh-impregnated alumina foams
0
CH4 + 1/2O2 → CO + 2H2, ∆H298
= −36 kJ/mol
(4)
filled in a metallic microchannel reactor were studied for production of hydrogen-rich syngas through a short contact-time
catalytic PO of propane [40,41]. However, these noble metal
catalysts do not seem acceptable for this process due to their
high costs. Only a few papers have been reported on Ni catalysts for PO of higher hydrocarbons [42,43]. Ni-supported
catalysts were modified by alkali metal or rare-earth metal
oxides [42]. Ni/Mg(Al)O catalysts were prepared from the
HT as the precursors [43]. As a bimetallic catalyst, Pt-Ni/δAl2 O3 was devoted to PO of propane and butane [44−46].
The bimetallic system showed superior catalytic performance
compared to the monometallic system due to the action as
7
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
“micro heat exchanger”; the heat generated by Pt sites during
the exothermic total oxidation was readily transferred through
the catalyst particles acting as “micro heat exchanger” to the
Ni sites, which promote endothermic SR [44−46].
The author has found an improved behavior of noble
metal/Ni0.5 /Mg2.5 (Al)O bimetallic catalysts in PO of propane
under accelerated deactivation conditions [47,48] or DSSlike conditions [49]. The reaction was carried out with
a temperature-cycled mode (Figure 9) for testing sustainability of the catalysts in oxidative reaction atmosphere.
The results of PO of propane over Ru/, Rh/, Pd/, Ir/, and
Pt/Ni0.5 /Mg2.5(Al)O are depicted in Figure 10 together with
those over Ni0.5 /Mg2.5 (Al)O, 13.5 wt% Ni/γ-Al2 O3 , RUA
and FCR as controls [47]. The conversion of propane decreased with decreasing reaction temperature from 700 ◦ C to
400 ◦ C and again increased with increasing temperature to
600 ◦ C; both the decrease and the recovery in the activity were
clearly dependent of the noble metal species (Figure 10a). In
the absence of noble metal, an activity decreased significantly
at 400 ◦ C over both Ni0.5 /Mg2.5 (Al)O and 13.5 wt% Ni/γAl2 O3 and their recoveries at 600 ◦ C were low. FCR showed a
low activity and a severe deactivation at 400 ◦ C, and moreover
the worst activity recovery at 600 ◦ C. RUA showed a considerable deactivation at 400 ◦ C, whereas the activity was well
recovered at 600 ◦ C. Among the noble metals/Ni bimetallic catalysts, both Ru/and Rh/Ni0.5 /Mg2.5 (Al)O exhibited the
highest activity and sustainability, followed by Pd/, Pt/, and
Ir/Ni0.5/Mg2.5 (Al)O catalysts. Ru doping was the most sustainable against both coking and Ni oxidation [47]. Moreover,
when Ru/, and Pt/Ni/Mg(Al)O catalysts were prepared starting from HTs and citrate precursors, those derived from HTs
exhibited superior catalytic behavior in PO of propane than
those derived from citrate [50].
Figure 9. Temperature-cycled operation mode for PO of propane [47]
Figure 10. PO of propane by temperature-cycled operation over noble metal/Ni/Mg(Al)O. (a) propane conversion, (b) selectivity to H2 O [47]; (1) 0.10 wt%
Ru/Ni0.5 /Mg2.5 (Al)O, (2) RUA, (3) 0.10 wt% Rh/Ni0.5 /Mg2.5 (Al)O, (4) 0.10 wt% Pd/Ni0.5 /Mg2.5 (Al)O, (5) 0.10 wt% Ir/Ni0.5 /Mg2.5 (Al)O, (6) 0.10 wt%
Pt/Ni0.5 /Mg2.5 (Al)O, (7) Ni0.5 /Mg2.5 (Al)O, (8) 13.5 wt% Ni/γ-Al2 O3 , (9) FCR
PO of propane over Ni catalysts proceeds via combustion
(5), followed by SR (6) and dry reforming (7). Direct PO of
propane (8) can proceed on Rh-supported catalyst as reported
0
C3 H8 + 5O2 → 3CO2 + 4H2 O, ∆H298
= −2046 kJ/mol
(5)
0
C3 H8 + 3H2O → 3CO + 7H2, ∆H298
= +497 kJ/mol
(6)
0
= +620 kJ/mol (7)
C3 H8 + 3CO2 → 6CO + 4H2, ∆H298
0
C3 H8 + 3/2O2 → 3CO + 4H2, ∆H298
= −229 kJ/mol
(8)
with short contact-time reactor [40,41]. An excess doping
of Ru on Ni0.5 /Mg2.5 (Al)O may result in an appearance of
the direct PO activity (vide infra), although the direct PO
is exceptional on the Ni catalysts. Water-gas shift reaction
(9), methanation (10), dehydrogenation of propane (11) and
0
CO + H2O → CO2 + H2 , ∆H298
= −41 kJ/mol
(9)
0
CO + 3H2 → CH4 + H2 O, ∆H298
= −205 kJ/mol
(10)
0
C3 H8 → C3 H6 + H2, ∆H298
= +124 kJ/mol
(11)
coke formation from propane (12) will play a role, depending
on reactant composition, temperature and heat transfer rate,
0
C3 H8 → 3C + 4H2, ∆H298
= +104 kJ/mol
(12)
8
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
residence time and catalytic system involved. Additional side
reactions, including cracking of propane (13) and carbon deposition from carbon monoxide (14) must be considered; the
0
∆H298
= +89 kJ · mol−1
C3 H8 → C2 H4 + CH4
2CO → C(s) + CO2
0
∆H298
= −172 kJ · mol−1
(13)
(14)
latter is particularly unwonted and generally occurs when the
O2 /C3 H8 molar ratio in the reaction mixture becomes too low.
In the present work, an excess O2 /C3 H8 molar ratio of
1.88, compared with the molar ratio of 1.5 required for completing syngas production (8), was adopted to accelerate deactivation. The selectivity to H2 O inversely well correlated
to the decrease in propane conversion (Figure 10), indicating that deactivation was accompanied by combustion reaction (5) due to Ni oxidation [47]. Assuming that combustion reaction alone proceeded under the present reaction conditions, conversion of propane is calculated as 37.6% taking
account of the molar ratio of O2 /C3 H8 = 1.88 in the reaction
(5). Ni0.5 /Mg2.5 (Al)O was severely deactivated at 400 ◦ C not
only by Ni oxidation but also by coking, judging from propane
conversion of 33% below 37.6% (Figure 10a). FCR showed
only 0.6% propane conversion and negligible small H2 production at 400 ◦ C (Figure 10a); this is certainly due to both Ni
oxidation and coking, resulting in damaging of the active sites
not only for reforming but also for combustion.
Product distributions in the first step reaction at 700 ◦ C
and coke deposition after temperature-cycled operation of
propane PO are shown in Table 1 [47].
Over both
Ni0.5 /Mg2.5 (Al)O and 13.5 wt% Ni/γ-Al2 O3 , a small amount
of C2 ∼C3 compounds, i.e., C2 H4 , C2 H6 and C3 H6 , were
formed together with H2 , CO and CO2 as main products. It
must be emphasized that significant coke formation occurred
on these supported Ni catalysts. Over 0.10 wt% Ru/Mg3 (Al)O
and RUA, the selectivity to H2 decreased whereas those to
C2 ∼C3 compounds increased, suggesting that Ru catalyzed
propane cracking although no significant coking took place.
The 0.10 wt% doping of noble metals on Ni0.5 /Mg2.5 (Al)O
remarkably suppressed both coking and C2 ∼C3 formation except Ir doping, among which the Ru doping was the most
effective as observed on 0.10 wt% Ru/Ni0.5 /Mg2.5(Al)O (Table 1). The 0.10 wt% Ru/Ni0.5 /Mg2.5(Al)O exhibited the
highest activity and sustainability in PO of propane [47−49].
A strong synergy between Ru and Ni most likely appeared
on the catalysts, leading to the most improved catalytic behavior due to the formation of bimetallic system such as
Ni-Ru alloy.
Table 1. Product distribution in partial oxidation of propanea [47]
Catalystb
Ni0.5 /Mg2.5 (Al)O
13.5 wt% Ni/γ-Al2 O3
0.1 wt% Ru/Mg3 (Al)Od
0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Rh/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Pd/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Ir/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O
0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O
0.05 wt% Ru/Ni0.5 /Mg2.5 (Al)O
0.01 wt% Ru/Ni0.5 /Mg2.5 (Al)O
FCRe
RUAf
Selectivity
to H2 (%)
78.4
83.4
39.7
84.9
85.8
82.8
76.6
84.0
86.4
85.7
82.9
69.3
47.0
CO
76.8
78.2
59.9
81.7
81.0
80.4
74.8
81.0
81.4
81.2
79.8
71.2
64.1
CO2
15.0
15.7
11.6
14.1
14.6
14.0
13.7
13.8
14.5
14.5
14.6
23.7
12.8
Selectivity (%)
CH4
C2 H4
4.6
3.2
4.1
1.6
11.4
13.6
4.2
0.0
4.3
0.1
4.8
0.7
5.2
5.1
4.7
0.5
4.1
0.0
4.3
0.0
4.5
1.0
1.4
1.3
9.9
10.0
C2 H6
0.4
0.2
2.0
0.0
0.0
0.1
0.6
0.1
0.0
0.0
0.1
0.1
1.7
C3 H6
0.0
0.2
1.5
0.0
0.0
0.0
0.6
0.0
0.0
0.0
0.0
2.3
1.5
Coke deposition
(wt%)c
40.4
80.0
1.8
6.1
18.9
13.1
11.5
24.8
4.4
21.0
63.9
11.8
0.4
a Reaction
temperature, 700 ◦ C; propane conversion, 100% (d 99.2%, e 78.3%, f 98.8%)
Metal doping was carried out by dispersing 1.0 g of Mg2.5 (Al,Ni0.5 )O periclase powders in 5 ml of aqueous solution of the nitrates of noble metals for
1 h at room temperature
c Obtained by TPO experiment of the catalysts after the reaction.
b
2.6. Physicochemical property of noble metal/Ni/Mg(Al)O
catalysts
As shown in Table 2, H2 uptake dramatically increased,
although specific surface area significantly decreased, by noble metal doping on Ni0.5 /Mg2.5(Al)O [47]. H2 uptake was
enhanced even with 0.01 wt% of Ru doping. Although 0.10
wt% Ru doping on Ni0.5 /Mg2.5 (Al)O resulted in a great increase in the H2 uptake, the contribution of Ru in the H2 uptake must be negligible, since a chemisorption stoichiometry
is H/Rus = 1/1 [51] and the molar ratio of Ru/Ni is quite small,
1/275 of total Ni amount or at least 1/220 of surface Ni amount
assuming 80% of Ni reduction degree [21,52]. Therefore, direct contribution of Ru on the H2 uptake must be small even
when all Ru species are located on the surface of Ni metal particles, and the other factor such as surface Ru-Ni alloy formation or decrease in the size of Ni particles must be considered
for explaining the prominent effect of Ru doping. Ni particle
sizes calculated from both XRD and H2 uptake decreased with
increasing noble metal doping [22,47]. The reforming activity of the noble metal/Ni/Mg(Al)O catalysts seems mainly
due to Ni metal particles. Indeed, the H2 selectivity was
remarkably low on 0.10 wt% noble metal/Mg3 (Al)O at
700 ◦ C.
TPR profiles of Ru/Ni/Mg(Al)O catalysts togetherwith the fresh catalyst are depicted in Figure 11 [48].
Ni0.5 /Mg2.5 (Al)O exhibited a single and intensive peak at
895 ◦ C due to Ni reduction (Figure 11(1)) [35,47]. When
9
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Ni0.5 /Mg2.5 (Al)O was doped with noble metals, Ru, Rh and
Pt, the peak shifted toward lower temperature with increasing the doping amounts [35], suggesting a formation of noble
metal-Ni alloy. The Ni reduction temperature, 895 ◦ C, for the
fresh Ni0.5 /Mg2.5 (Al)O decreased with increasing Ru doping
and finally reached 820 ◦ C on 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O
(Figure 11(5)). The peak observed at 895 ◦ C can be assigned
to the Ni2+ ions located deep in the Mg(Al,Ni)O lattice as
solid solutions [54,55]. For instance, NiO-MgO solid solu-
tions prepared by the calcination at the temperature above
850 ◦ C showed no reduction peak up to 1100 ◦ C [56]. In the
Mg(Al,Ni)O periclase obtained by replacing the Mg2+ sites
with Al3+ ions in the NiO-MgO solid solutions, the lattice defects were produced and caused an easy Ni reduction. Moreover, Ru assisted further the Ni reduction in the Mg(Al,Ni)O
periclase due to easy H2 dissociation on Ru, followed by
spillover of hydrogen to Ni. These resulted in the decrease
in the Ni reduction temperature.
Table 2. Physicochemical properties of noble metal/Ni catalysts [47]
Catalysta
BET surface areab (m2 ·g−1
cat )
H2 uptakec (µmol·g−1
cat )
Dispersiond (%)
Ni0.5 /Mg2.5 (Al)O
13.5 wt% Ni/γ-Al2 Of3
0.10 wt% Ru/Mg3 (Al)O
0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Rh/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Pd/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Ir/Ni0.5 /Mg2.5 (Al)O
0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O
0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O
0.05 wt% Ru/Ni0.5 /Mg2.5 (Al)O
0.01 wt% Ru/Ni0.5 /Mg2.5 (Al)O
158.0
106.3
121.5
146.7
148.4
134.9
140.0
134.9
148.0
138.3
137.7
120.7
74.4
0.56
221.9
184.0
148.8
204.2
225.3
261.4
187.2
183.5
13.1
6.5
−
24.0
19.9
16.1
22.1
24.4
28.3
20.3
19.9
Ni particle size (nm)
XRDe
H2 uptakef
6.9
7.4
10.0
14.9
−
−
5.2
4.0
5.7
4.9
5.8
6.0
5.3
4.4
5.5
4.0
5.0
3.4
5.7
4.8
5.7
4.9
a
Noble metal doping was carried out by dispersing 1.0 g of Mg2.5 (Ni0.5 ,Al)O periclase powders in 5 ml of aqueous solution of the nitrates of noble
metals for 1 h at room temperature.
b Calcined at 850 ◦ C for 5 h.
c Determined by the H pulse method.
2
d Calculated from the H uptake assuming the reduction degree of 80% for hydrotalcite derived catalyst [21,52] and 100% for impregnated catalystf .
2
e
Calculated from the full width at half maximum of the reflection of Ni (200) plane in the XRD using the Scherrer equation.
f Calculated using the equation: d = 971/D/10 where D (%) is the dispersion [53].
A weak and broad peak observed around 400 ◦ C for 0.50
wt%Ru/Ni0.5 /Mg2.5 (Al)O (Figure 11(5)) can be ascribed to
the reduction of RuO2 to Ru metal [47], because no other
stable ruthenium oxides are known to exist in the solid state
[57,58]. This indicates that a part of Ru was separated as
RuO2 from Ni-Ru binary system and existed as fine particles on the catalyst surface. The formation of RuO2 phase
in the reaction mixture (O2 /C3 H8 = 1.88), which was favored
below 450 ◦ C [58], was responsible for propane combustion to
CO2 and H2 O (5). This was supported by decrease in propane
conversion, whereas increase in H2 O selectivity, at 400 ◦ C on
0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O (Data are not shown in Figure 10). The RuO2 ↔ Ru equilibrium was shifted toward Ru
metal at the temperature above 450 ◦ C, where the reaction rate
increased considerably, and simultaneously CO and H2 were
produced as the primary reaction products by the direct PO
(8). Thus, Ru likely catalyzed direct PO of propane (8) above
500 ◦ C, whereas RuO2 catalyzed combustion of propane (5)
at 400 ◦ C.
XRD patterns of Ru/Ni0.5 /Mg2.5 (Al)O catalysts after reduction are depicted in Figure 12. Ni0.5 /Mg2.5 (Al)O showed
rather sharp and intensive reflections of Ni metal (Figure 12(1)), which were broadened and slightly shifted toward
the lower angles with increasing Ru doping as seen in the
reflection of Ni (200) at 2θ = 52.5o [47]. Basile et al. [59]
reported that Rh3+ was completely soluble in Mg(Al)O periclase, whereas Ru3+ was not soluble and remained as separated phase. Such Ru species separated from the periclase
reacted with Ni0 species released from Mg2.5 (Al,Ni0.5)O periclase during the reduction, resulting in the formation of Ru-Ni
alloy on the catalyst surface.
Figure 11. TPR of Ru/Ni/Mg(Al)O catalysts [48]. (1) Ni0.5 /Mg2.5 (Al)O, (2)
0.01 wt% Ru/Ni0.5 /Mg2.5 (Al)O, (3) 0.05 wt% Ru/Ni0.5 /Mg2.5 (Al)O, (4) 0.10
wt% Ru/Ni0.5 /Mg2.5 (Al)O, (5) 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O. Full line,
fresh sample; dotted line, sample after reduction with H2 /N2 (5/10 ml·min−1 )
at 900 ◦ C for 1 h, followed by oxidation with O2 /N2 (18.7/71.3 ml·min−1 ) at
700 ◦ C for 1 h
10
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Figure 12. XRD patterns of Ru/Ni0.5 /Mg2.5 (Al)O catalysts after reduction
[47]. (1) Ni0.5 /Mg2.5 (Al)O, (2) 0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)O, (3) 0.50
wt% Ru/Ni0.5 /Mg2.5 (Al)O
ions having square-pyramidal coordination in the outermost
layer of the Mg(Al)O periclase. This species were probably
produced during the oxidation treatment. The H2 consumption decreased with increasing Ru doping (Figure 11), suggesting that this species showed no positive contribution on
the catalytic activity. On the other hand, the peak observed at
750 ◦ C revealed increasing H2 consumption with increasing
Ru doping, well correlating with the catalytic activity in PO
of propane with the increasing temperature mode [48]. 0.10
wt% Ru/Ni0.5 /Mg2.5(Al)O after the H2 /O2 treatment exhibited clearly self-activation in PO of propane even at 600 ◦ C
[48], which was induced by spillover of hydrogen from Ru to
Ni. The increasing H2 consumption at 750 ◦ C may be due to
the increasing dispersion of Ni (and Ru) particles. Hydrogen
spillover from Ru to Ni was accelerated on such finely dispersed metal particles. The author concludes that, during the
H2 /O2 pretreatment, both Ni and Ru metals were finely dispersed and simultaneously hydrogen spillover from Ru to Ni
was accelerated, both resulting in the high activity as well as
the effective self-activation.
2.7. Self-activation of noble metal/Ni/Mg(Al)O catalysts
Ru/, Rh/, and Pt/Ni0.5 /Mg2.5(Al)O were self-activated
during stationary PO of propane without reduction pretreatment with H2 [47,48]. During propane PO at 600 ◦ C, the catalysts were interruptedly purged with O2 at 700 ◦ C (O2 purging
mode). After the O2 purging, Ni0.5 /Mg2.5 (Al)O was totally
deactivated by Ni oxidation, whereas Ru/Ni0.5 /Mg2.5 (Al)O
showed no deactivation. Moreover, both propane conversion
and H2 production rate increased on Ru/Ni0.5 /Mg2.5(Al)O and
such activity increase was enhanced with increasing Ru doping [48].
In the TPR profiles of Ru/Ni0.5 /Mg2.5 (Al)O after the
H2 /O2 treatment (dotted line in Figure 11) [48], the reduction
peaks of Ni observed between 800 and 900 ◦ C shifted toward
the lower temperature and were separated into two peaks, i.e.,
around 550 ◦ C and 750 ◦ C. H2 consumptions of the two peaks
were estimated by the peak deconvolution and are depicted in
Figure 13 together with those of the fresh samples [48]. Summation of the two H2 consumptions after the H2 /O2 treatment
was almost constant independently of the Ru doping and well
coincided with that of fresh sample. This suggests that H2 was
mainly consumed by Ni on the catalysts, consisting with the
trace amounts of Ru doping (vide supra). The H2 consumption at 550 ◦ C decreased, whereas that at 750 ◦ C increased,
with increasing Ru doping (Figure 13). Freni et al. [54] observed a peak at 530 ◦ C in the TPR of Ni/MgO and assigned
to Ni2+ ions having square-pyramidal coordination in the outermost layer of the MgO structure. Pure NiO has a reduction
peak at 385 ◦ C [56], while Ni/γ-Al2 O3 has a peak at 410 ◦ C
assigned to the reduction of separated NiO [60]. Ni/γ-Al2 O3
prepared by impregnation shows two peaks assigned to octahedral (527 ◦ C) and tetrahedral (687 ◦ C) Ni2+ sites [61]. The
author concludes that the peak at 550 ◦ C is assigned to Ni2+
Figure 13. H2 consumption in the TPR of Ru/Ni/Mg(Al)O catalysts [48].
() H2 consumption on the fresh sample between 800 ◦ C and 900 ◦ C; (◦) H2
consumption by the first peak around 550 ◦ C on the sample after the H2 /O2
treatment; (N) H2 consumption by the second peak around 750 ◦ C on the
sample after the H2 /O2 treatment; (▽) Summation of the H2 consumptions by
the first and the second peaks
All Ru/, Rh/ and Pt/Ni0.5 /Mg2.5 (Al)O were self-activated
in PO of propane [47,48]. However, when these catalysts were
tested in SR of methane, Pt was the most effective, followed
by Rh, but Ru was not effective for self-activation [22−24].
When the doping amounts of Ru, Rh and Pt were varied between 0.05 and 0.50 wt%, Pt and Rh were effective for selfactivation with the doping above 0.05 and 0.10 wt%, respectively, whereas Ru showed no reforming activity for 3 h even
with 0.50 wt% doping without prereduction (Figure 14). Thus
the ability of noble metals inducing self-activation was in the
order of Pt > Rh ≫ Ru. This possibly depends on the activity of the metal in CH4 dissociation. A theoretical study of
the C−H activation was conducted on a number of transition
metals M (Ru, Ni, Rh, Ir, Pd and Pt) by simulating the metal
M(1 1 1) surface by a cluster model [62]. Dehydrogenations
of CHx to CHx−1 are highly endothermic in the gas-phase,
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
and the calculated values of dissociation energy, i.e, D, in the
following four steps are CH3 −H, 4.85 eV; CH2 −H, 5.13 eV;
CH−H, 4.93 eV; C−H, 3.72 eV [62]. On the metal surface,
there is a significant reduction in the D values, owing to the
presence of strong metal-CHx−1 and metal-H bonds. Summation of the energies for the four discrete steps gives the total
dissociation energy, Dtot , for CH4 ,s → Cs + 4Hs and should
be a more realistic measure for the activity of the metal in
CH4 dissociation. The total dissociation is shown to be quite
exothermic on Rh (by −0.7 eV); it is slightly endothermic on
Ru (0.01 eV) and Ir (0.3 eV), and it is rather endothermic on
Pd and Pt (∼1 eV). This indicates that the total dissociation of
CH4 on Rh is thermodynamically the most favorable among
the transition metals. The Dtot values vary in the order: Rh
≈ Ni < Ru < Ir < Pt < Pd [62]. However, in the presence
of adsorbed oxygen, oxygen at metal on-top sites promotes
CH4 dissociation; oxygen promotes CH4 dissociation on Pt,
whereas no such effect is observed on the other transition metals [63]. In the presence of adsorbed oxygen, in addition to
the direct dissociation of CH4 on bare metal surfaces, we may
consider the following reactions:
CHx,s + Os → CHx−1,s + OHs (x = 4, 3, 2, 1)
(15)
Because the H atom binds more strongly with Os than
with the bare metal, the CH4 dissociation reactions in the presence of chemisorbed oxygen, i.e, Os , have lower reaction energies due to hydroxyl formation [64,65]. The Os species increases adsorption energies of H on Pt, but decreases those
on the other transition metals. Therefore, Os promotes CH4
dissociation on Pt, but does not promote it on the other transition metals. In fact, in the PO of methane over Ru/, Rh/,
and Pt/Ni0.5 /Mg2.5(Al)O, Pt doping was the most effective for
self-activation; even the material with 0.05 wt% doping exhibited reforming activity and produced hydrogen with high
selectivity without induction period [24]. In fact, we observed
the activity order of noble metals for the self-activation was
Pt > Rh ≫ Ru; the effect of “adsorbed oxygen species”
was clearly confirmed [24]. The same order was obtained
in SR of methane (Figure 14) [24]; steam produced plenty
of OHs on the MgO surface, because Mg(OH)2 is thermodynamically stable compared with MgO under steam atmosphere [30]. MgO reacted even with moisture in air, especially
at low coordination atomic sites, to form Mg(OH)2 brucite;
MgO surface was thus covered by OHs and further the OHs
species migrated from MgO to Pt metal surface and possibly
formed Pt-OH species. One may consider that such OHs can
also promote CH4 dissociation instead of Os on Pt. On the
contrary, not only Pt and Rh but also Ru combined with Ni
catalyzed PO of propane without prereduction. This is probably due to easy C−H bond cleavage for propane compared
with methane. Noble metals as co-catalysts dissociate C−H
bond of hydrocarbons dependently of own ability and produce
H atoms, which in turn migrate to Mg(Ni,Al)O periclase surface by spillover and reduce Ni2+ to Ni0 . As a result, metallic
Ni species was formed on the catalysts and catalyzed SR of
methane and PO of propane.
11
Figure 14. Self-activation of Pt/, Rh/ and Ru/Ni0.5 /Mg2.5 (Al)O with various doping amount in stationary SR of methane [24]. (1) 0.50 wt%
Pt/Ni0.5 /Mg2.5 (Al)O, (2) 0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O, (3) 0.05 wt%
Pt/Ni0.5 /Mg2.5 (Al)O, (4) 0.01 wt% Pt/Ni0.5 /Mg2.5 (Al)O, (5) 0.50 wt%
Rh/Ni0.5 /Mg2.5 (Al)O, (6) 0.10 wt% Rh/Ni0.5 /Mg2.5 (Al)O, (7) 0.05 wt%
Rh/Ni0.5 /Mg2.5 (Al)O, (8) 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O, (9) 0.10 wt%
Ru/Ni0.5 /Mg2.5 (Al)O
2.8. Self-regenerative activity of noble metal/Ni/Mg(Al)O catalysts
Ru/, Rh/, and Pt/Ni0.5 /Mg2.5 (Al)O catalysts exhibited
self-regenerative activity during steam-purged DSS SR of
methane [22−24]. Over Ru/Ni0.5 /Mg2.5 (Al)O (Figure 8),
methane conversion decreased just after each steam purging,
while gradually increased during each reaction. The decrease
in methane conversion after steam purging is likely due to the
surface oxidation of Ni particles and became more intensive
with decreasing the Ru doping. This indicates that Ni was reversibly reduced and oxidized during the DSS operation; a
part of active Ni0 was oxidized to Ni2+ by steam purging,
whereas the Ni2+ was rereduced to Ni0 by hydrogen spillover
from Ru or Ni-Ru alloy during the reaction.
The sustainability for a long-term reaction seems mainly
owing to anti-sintering property and is important for the reforming catalysts in the PEFCs. The catalyst-life tests have
been frequently performed with an actual reformer for several
months or years. To simulate the effect of catalyst ageing or
passivation due to sintering in an actual reformer, a steaming
for a short period was sometimes applied in a laboratory scale
[65]; this treatment was meant to simulate accelerated ageing
of the catalyst under conditions representative of what the catalyst would experience during SR, i.e., high-temperatures and
humid, reducing atmospheres.
Steaming of catalyst was carried out in a H2 /H2 O/N2
(20/100/25 ml·min−1 ) for 10 h at 900 ◦ C. The results of DSS
SR of methane over 0.10 wt% Ru/Ni0.5 /Mg2.5(Al)O before
and after steaming are shown in Figure 15 together with the
12
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
other catalysts as controls. The activity was compared by
methane conversion because SR of methane always proceeded
selectively to H2 , CO, and CO2 following the thermodynamic
equilibrium under the present condition of S/C = 2/1. Before
steaming, Ni0.5 /Mg2.5 (Al)O, 13.5 wt% Ni/γ-Al2 O3 and FCR
were completely deactivated just after the first steam purging
due to the Ni oxidation [20]. The activity of RUA was stable
during the DSS operation and slightly lower than that of 0.10
wt% Ru/Ni0.5 /Mg2.5(Al)O. After the deactivation, reflection
lines of NiO appeared for both 13.5 wt% Ni/γ-Al2 O3 and
FCR, whereas reflections of Mg(Ni,Al)O periclase were intensified for Ni0.5 /Mg2.5 (Al)O [20]. This indicates that Ni
metal was oxidized to Ni2+ and incorporated into Mg(Ni,
Al)O periclase. In contrast, RUA deactivation was likely due
not to such a sudden oxidation of Ru metal, but rather to sintering or gradual surface oxidation of Ru particles. Among the
catalysts tested, 0.10 wt% Ru/Ni0.5 /Mg2.5(Al)O alone showed
stable activity during DSS SR of methane
After steaming, the deactivation was the most enhanced
for FCR, followed by Ni0.5 /Mg2.5(Al)O and 13.5 wt% Ni/γAl2 O3 catalysts, and all these supported Ni catalysts showed
a sudden deactivation just after the first steam purging. RUA
was deactivated by steaming, but showed no such sudden deactivation; the activity gradually decreased during DSS operation regardless of each steam purging. Among the catalysts tested, 0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)O alone showed a
stable activity during DSS SR of methane even after steaming. Interestingly, time course of methane conversion during the DSS operation (Figure 15) revealed that 0.10 wt%
Ru/Ni0.5 /Mg2.5 (Al)O was rather stabilized after steaming;
a sudden decrease in methane conversion observed just after each steam purging disappeared and methane conversion
became rather constant throughout the DSS operation after
steaming.
The activities of the catalysts before and after steaming were roughly compared based on CH4 conversions in
DSS SR of methane (Figure 15). However, the CH4 conversions observed over the noble metal/Ni0.5 /Mg2.5(Al)O catalysts were always close to the thermodynamic equilibrium
and could not be compared precisely. A more precise evaluation of catalyst deactivation must be done based on TOF
of the catalyst. TOF values of the catalysts were calculated based on both surface Ni amount (TOF-s) and total
Ni amount (TOF-t), and are shown in Table 3 together with
those as controls. TOF-t seems practically more reliable
compared with TOF-s for evaluating the catalyst sustainability, because TOF-s values were significantly affected by the
heavy sintering of Ni particles. For both FCR and RUA, only
TOF-t values were calculated because surface metal amount
could not be correctly determined by H2 pulse measurements.
Their TOF-t values were one unit smaller compared with
those of the Ni0.5 /Mg2.5 (Al)O-based catalysts, and moreover
heavy deactivation took place on these commercial catalysts
after steaming. Among the catalysts prepared as controls,
13.5 wt% Ni/γ-Al2 O3 was the most severely deactivated, followed by Ni0.5 /Mg2.5 (Al)O. One must notice that TOF-t values of 0.10 wt% Pt/, Rh/and Ru/Ni0.5 /Mg2.5 (Al)O showed no
remarkable decrease even after steaming, indicating that noble
metal doping on Ni/Mg(Al)O was quite effective for suppressing the deactivation due to sintering [22−24].
Figure 15. Comparison of the activity of Ni and Ru catalysts before and
after steaming at 900 ◦ C for 10 h in steam purged DSS SR of methane
[22]. Reaction conditions: CH4 /H2 O/N2 = 50/100/25 ml·min−1 ; 700 ◦ C;
catalyst, 50 mg. 0.10 wt%Ru-Ni0.5 /Mg2.5 (Al)O, before (1), after (2);
Ni0.5 /Mg2.5 (Al)O, before (3), after (4); 13.5 wt% Ni/γ-Al2 O3 , before (5),
after (6); RUA, before (7), after (8); FCR, before (9), after (10)
The sizes of Ni particles on the supported Ni catalysts before and after steaming, and after steaming followed by DSS
SR of methane were calculated based on XRD and H2 pulse
measurements (Table 4). After the passivation by steaming,
the sizes of Ni particles on all catalysts significantly increased,
indicating an occurring of heavy sintering of Ni particles. The
H2 pulse method gave a larger Ni particle size than the XRD
method, suggesting a formation of multi-crystalline Ni particles. Both Ni0.5 /Mg2.5 (Al)O and 13.5 wt% Ni/γ-Al2 O3 catalysts exhibited neither Ni metal reflection in the XRD nor
H2 uptake in the pulse measurements. This indicates that
Ni was totally oxidized during DSS SR. Both FCR and RUA
showed no significant change in the metal particle sizes during steaming followed by DSS SR of methane probably due
to the specified preparation for industrial use. The Ni metal
reflections still remained on the passivated FCR, suggesting
that only surface layer of Ni particles was oxidized. In fact,
all passivated Ni0.5 /Mg2.5 (Al)O, 13.5 wt% Ni/γ-Al2 O3 and
FCR were perfectly deactivated after the first steam purging
in DSS SR due to the Ni oxidation. One must notice that, on
0.10 wt% noble metal/Ni0.5 /Mg2.5(Al)O catalysts after steaming, the sizes of Ni particles substantially decreased after DSS
SR and closed to the original value. All these catalysts exhibited sustainable activity during the DSS SR of methane
(Table 3 and Figure 15) [22−24]. Moreover the decreased
Ni dispersions on these catalysts after steaming were recovered after DSS SR, although not completely to the original
values before steaming. Such size decrease, i.e, redispersion,
13
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
of the sintered Ni particles was clearly observed not only in
XRD and H2 pulse measurements but also in TEM images;
0.10 wt% Pt/Ni0.5 /Mg2.5(Al)O passivated by steaming exhib-
ited the maximum Ni particle size of ca. 36 nm (Figure 16(a)),
whereas the maximum size decreased to ca. 15 nm after followed by DSS SR (Figure 16(b)) [24].
Table 3. Turnover frequency of Ni catalysts before and after steaminga,b [22–24]
Before or
after
Before
After
Catalyst
Ni0.5 /Mg2.5 (Al)Of
0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)Of
0.10 wt%Rh/Ni0.5 /Mg2.5 (Al)Of
0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)Of
13.5 wt% Ni/γ-Al2 Of3
FCRg
RUAg
Ni0.5 /Mg2.5 (Al)Of
0.10 wt% Ru/Ni0.5 /Mg2.5 (Al)Of
0.10 wt%Rh/Ni0.5 /Mg2.5 (Al)Of
0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)Of
13.5 wt% Ni/γ-Al2 Of3
FCRg
RUAg
CH4 conversion (%)
500 ◦ C
600 ◦ C
14.6
43.5
12.8
37.7
12.4
33.5
13.7
38.0
9.35
29.3
3.55
29.3
7.13
25.3
5.42
25.9
10.3
37.1
8.95
29.9
7.43
30.5
3.75
11.9
0.43
0.71
1.98
8.04
H2 uptakec
(µmol·g−1
cat )
163.2
213.4
189.1
205.0
71.1
−
−
26.6
55.3
55.3
52.3
24.0
−
−
TOF-sd (s−1 )
500 ◦ C
600 ◦ C
2.95
8.81
1.99
5.84
2.17
5.84
2.20
6.12
4.35
13.6
−
−
−
−
6.73
32.1
6.14
22.2
5.34
17.9
4.70
19.3
5.16
16.3
−
−
−
−
TOF-te (s−1 )
500 ◦ C
600 ◦ C
0.35
1.06
0.31
0.93
0.30
0.81
0.33
0.92
0.27
0.84
0.03
0.08
0.54
1.90
0.13
0.63
0.25
0.90
0.22
0.73
0.18
0.74
0.09
0.29
0.003
0.005
0.149
0.605
a
Steaming was carried out at 900 ◦ C for 10 h in a H2 /H2 O/N2 (20/100/25 ml·min−1 ). The catalysts were used as powders of 0.075∼0.180 mm in
diameter.
b
−1f
SR of methane was carried out between 500∼600 ◦ C in CH4 /H2 O/N2 (88.8/177.6/44.4 ml·min−1 ) at GHSV of 1.6×106 ml·g−1
or 3.6×105
cat ·h
−1g after prereduction at 900 ◦ C for 0.5 h.
ml·g−1
cat ·h
c Determined by the H pulse method.
2
d TOF value was calculated based on surface Ni amount (total Ni amounte )
Table 4. Physicochemical properties of noble metal/Ni catalysts before and after steaming, and followed by DSS SR of methanea [22–24]
Catalyst
Ni0.5 /Mg2.5 (Al)O
0.10 Ru Ni0.5 /Mg2.5 (Al)O
0.10 Rh Ni0.5 /Mg2.5 (Al)O
0.10 Pt Ni0.5 /Mg2.5 (Al)O
13.5 wt% Ni/γ-Al2 O3
FCR
RUA
BET surface area
−1
(m2 ·gcat
)
before after after DSS
173.6 56.8
76.5
128.7 56.4
71.8
133.1 59.5
83.5
141.2 56.9
76.1
106.8 61.0
67.7
12.3 11.7
−
11.4 10.6
−
Dispersionb
(%)
before after after DSS
11.0
3.7
−
20.3
5.3
7.3
15.3
6.4
7.2
20.6
4.0
6.0
6.4
1.7
−
−
−
−
−
−
−
before
XRDc H2 pulsed
6.8
6.5
5.2
5.0
6.0
5.6
5.5
5.2
9.0
15.8
23.1
−
(23.6)
−
Ni particle size (nm)
after
XRDc H2 pulsed
18.4
26.4
16.8
18.2
16.8
15.3
16.5
24.0
21.0
58.1
24.3
−
(25.6)
−
after DSS
XRDc H2 pulsed
n.d.
n.d.
8.3
13.4
9.4
13.5
10.0
16.2
n.d.
n.d.
26.1
−
(37.0)
−
a
After steaming at 900 ◦ C, followed by the steam purged DSS SR of methane between 200 ◦ C and 700 ◦ C. The catalysts were calcined at 850 ◦ C for
5 h before catalytic tests.
b Calculated from the H uptake assuming the reduction degree of 80% for hydrotalcite derived catalyst [21,52] and 100% for impregnated catalyst.
2
c
Calculated from the full width at half maximum of the reflections of Ni (200) and Ru (101) planes in the XRD using the Scherrer equation.
d Calculated using the equation: d = 971/D/10 where D (%) is a dispersion [53]
Figure 16. TEM images of 0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O after steaming treatment (a), and followed by DSS SR of methane (b) [24]
14
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
2.9. Structure of noble metal/Ni bimetallic species
Ni K-edge XANES spectra of 0.50 wt%
Pt/Ni0.5 /Mg2.5(Al)O after calcination, after reduction, after
steaming and after followed by DSS SR of methane (data
are not shown) showed the following results [24]: the peak
shape closely resembled that of mixture of NiO and NiAl2 O4
after calcination, whereas the lattice Ni2+ in Mg(Al,Ni)O periclase was reduced to metallic Ni after reduction. However,
Ni was not fully reduced but existed partly as oxidized state
in Mg(Al,Ni)O, well coinciding with ca. 80%∼85% of reduction degree for Mg(Al,Ni)O periclase [21,53]. After steaming
followed by DSS SR, Ni remained in the metallic state. In
the Fourier transforms of k 3 -weighted Ni K-edge EXAFS
spectra, two peaks of Ni−O and Ni−Ni bonds were observed
after calcination. After reduction, both peaks disappeared
and a new peak of Ni metal appeared, indicating that Ni2+
in Mg2.5 (Ni0.5 ,Al)O periclase was reduced to Ni metal. Pt
apparently showed no effect on the spectra of Ni, because
the Pt/Ni molar ratio was too small (i.e., 1/106) on 0.50 wt%
Pt/Ni0.5 /Mg2.5(Al)O. After reduction followed by DSS SR,
Ni was partly oxidized but remained mainly in the metallic
state.
In the Pt L3 -edge XANES spectra of 0.50 wt%
Pt/Ni0.5 /Mg2.5(Al)O, an intensified “white line” peak (electron transition from 2p to 5d) at 11, 564 eV, along with some
peaks at higher energy, was observed after calcination (Figure 17a(1)) [24]. The oscillation and the edge position closely
resembled those of PtO2 (Figure 17a(7)), indicating that Pt
was oxidized. Pt foil showed a weak “white line” peak at
11, 562 eV and the edge position was lower than that of PtO2
(Figure 17a(6)). The absorption intensity of the white line for
Pt L3 -edge reflects the vacancy in the 5d orbital of Pt atom
and thus is related to the oxidation state [67,68]. The strong
white line intensity for the calcined sample (Figure 17a(1)),
as well as for PtO2 (Figure 17a(7)), indicated a high oxidation state for the platinum constituent (Pt4+ ), whereas for all
other samples, i.e., after reduction (Figure 17a(2)), after reduction followed by DSS SR of methane (Figure 17a(3)), after steaming (Figure 17a(4)) and after steaming followed by
DSS SR of methane (Figure 17a(5)), the intensity was almost the same as that of Pt foil, indicating that Pt existed
mainly in a reduced state. Fourier-transformed k3 -weighted
Pt L3 -edge EXAFS spectra of 0.50 wt% Pt/Ni0.5 /Mg2.5 (Al)O
are depicted in Figure 17b together with those of Pt foil and
PtO2 as controls [24]. Pt foil exhibited a peak corresponding
to Pt−Pt bond with a distance 2.64 Å (non-phase shift corrected) (Figure 17b(6)); whereas PtO2 showed several peaks,
among which a peak due to Pt−O bond appeared at 1.68 Å
(non phase shift corrected) (Figure 17b(7)) [67,68]. 0.50
wt% Pt/Ni0.5 /Mg2.5 (Al)O exhibited two peaks at 1.68 and
2.98 Å (both non-phase shift corrected) after calcination (Figure 17b(1)); the former is assigned to the Pt−O bond, whereas
the latter cannot be assigned to any of the Pt−O, Pt−Pt and
Pt−Ni bonds. No peak of Pt−Pt bond suggests that Pt species
was highly dispersed on the catalyst surface. The peak that
appeared at 2.25 Å (non-phase shift corrected) after reduction
(Figure 17b(2)) was smaller than 2.64 Å of Pt−Pt bond observed for Pt foil, and can be assigned to the Pt−Ni bond. This
peak was observed also after reduction followed by DSS SR
of methane (Figure 17b(3)); it weakened and shifted slightly
toward a shorter bond length after steaming (Figure 17b(4)),
and then it came back to the original bond length as well as the
original shape after steaming followed by DSS SR of methane
(Figure 17b(5)). This indicates that the Pt−Ni species was
stable and survived during DSS SR of methane and moreover
regenerated after steaming followed by DSS SR of methane.
Table 5 shows curve fitting results of Pt L3 -edge EXAFS
of 0.50 wt% Pt/Ni0.5 /Mg2.5 (Al)O [24]. Neither Pt−Pt nor
Pt−O bond was observed, but the Pt−Ni bond alone appeared
for all samples, indicating the Pt−Ni alloy formation. The
Figure 17. Pt L3 -edge XANES (a) and Fourier transforms of k 3 -weighted Pt L3 -edge EXAFS (b) of 0.50 wt% Pt/Ni0.5 /Mg2.5 (Al)O during preparation, before
and after steaming, followed by DSS SR of methane [24]. (1) after calcination, (2) after reduction, (3) after reduction followed by DSS SR, (4) after steaming at
900 ◦ C, (5) after steaming followed by DSS SR, (6) Pt foil, (7) PtO2
15
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Pt−Ni bond length was 2.53 Å, which is 0.04 Å longer than
the Ni-Ni bond in metallic Ni (2.49 Å) and 0.24 Å shorter than
the Pt−Pt bond in metallic Pt (2.77 Å). The coordination number of Pt−Ni shell was 6.8, far smaller than that of Pt foil (12),
suggesting that Pt is located in the surface layer of Ni particles.
In the present work, Pt was doped on Mg(Al,Ni)O periclase
particles by adopting the “memory effect” and, therefore, Pt
is possibly located on the surface of Ni particles after calcination, followed by reduction. A similar noble metal location in
the surface layer of Ni particles was observed also for Ru-Ni
[22] and Rh-Ni [23] systems. The coordination number of the
Pt−Ni shell (6.8) was larger than those observed for Ru-Ni
(3.5) and Rh-Ni (5.0), indicating that Pt was more deeply incorporated into Ni particles than Ru and Rh on 0.50 wt% Ru/,
and Rh/Ni0.5 /Mg2.5(Al)O [22,23]. Moreover, a small increase
in coordination number (7.7) after steaming followed by DSS
SR of methane indicates that Pt was even more deeply incorporated during such treatment. One may conclude that Pt was
always located mainly in the surface layer of Ni particles during the reaction.
Table 5. Curve fitting results of Pt L3 -edge EXAFS of 0.50 wt% Pt/Ni0.5 /Mg2.5 (Al)O after reduction, steaming and DSS SR of methanea [24]
Sample
Reducedb
Reduced-DSSb
Reduced-steamedb
Reduced-steamed-DSSb
PtOc2
Pt foilc
Shells
Pt−Ni
Pt−Ni
Pt−Ni
Pt−Ni
Pt−O
Pt−Pt
C.N.
6.8±1.1
6.8±1.1
6.7±1.1
7.7±1.2
6
12
R (Å)
2.53±0.01
2.53±0.01
2.52±0.01
2.54±0.01
1.99
2.77
σ (Å)
0.074±0.01
0.073±0.01
0.090±0.01
0.081±0.01
∆E0 (eV)
9.4±2.2
9.8±2.2
1.9±2.3
9.8±2.2
Rf (%)
3.4
8.5
19.5
5.4
C.N, coordination number; R, bond length (Å); ∆E0 , difference in the origin of photoelectron energy between the reference and the sample; σ,
Debye-Waller factor (Å); Rf , residual factor.
b *R = 1.78−2.57 Å, *k = 4−16.0 Å−1 .
c Data from X-ray crystallography
a
Chen et al. [69] reported that the surfaces of Pt-Ni
bimetallic catalysts prepared by sequential impregnations of
Pt and Ni on γ-Al2 O3 were always Pt-terminated with the
Pt/Ni molar ratio of 1/1, regardless of the sequential order,
i.e, first with Pt and then with Ni, or vice versa. This is due
to the segregation of Pt to the surface during the hydrogen reduction, as predicted by means of density functional theory
(DFT) modeling and, moreover, verified experimentally using Auger electron spectroscopy and high-resolution electron
energy loss spectroscopy on single crystal Ni/Pt(111) surfaces
[70]. Tomishige et al. [71] reported that sequential impregnation i.e., first Ni followed by Pt, resulted in surface enriched Pt
location in Ni particles and better catalytic performances than
co-impregnation methods in oxidative steam reforming of
methane. In the present work, Pt/Ni/Mg(Al)O catalysts were
prepared by Pt impregnation on Mg(Ni,Al)O periclase and Ni
particles were enriched with Pt in the surface layer. In fact, no
significant change was observed in either the Fourier transforms (Figure 17b(2–5)) or the curve fitting results (Table 5)
of Pt L3 -edge EXAFS of 0.50 wt% Pt/Ni0.5 /Mg2.5 (Al)O during the reaction, i.e, reduced, reduced-DSS, reduced-steamed
and reduced-steamed-DSS. Only the sample after reduction
followed by steaming exhibited a small shift and a shape alteration in Pt-Ni peak (Figure 17b(4)) and a greater Rf value
compared with the other samples (Table 5). This suggests that
the surface enriched Pt-Ni alloy structure was somewhat distorted due to heavy Ni sintering after steaming. One must
notice that the original Pt-Ni alloy structure was recovered after DSS SR of methane even after such heavy sintering (Figure 17b(5) and Table 5).
TPR curves of 0.10 wt% Pt/Ni0.5 /Mg2.5(Al)O after reduction exhibited a single peak of the Ni2+ →Ni0 reduction at
850 ◦ C (Figure 18(1)). After steaming at 900 ◦ C, the reduc-
tion peak was separated into two peaks, one at 918 ◦ C and another at 267 ◦ C (Figure 18(2)). It seems that Pt/Ni/Mg(Al)O
was reductively decomposed to large-sized Ni particles and
Mg(Al,Ni)O of lower Ni content after steaming. A part of
Ni2+ ions in Mg2.5(Ni0.5 ,Al)O periclase were first reduced to
Ni0 , exposed to steaming and finally grew to isolated largesized Ni particles. Simultaneously Mg(Al,Ni)O periclase lost
a part of the Ni2+ ions, was exposed to steaming and finally
formed a sintered structure containing hard-to-reduce Ni2+ .
The peak at 267 ◦ C can be assigned to the isolated Ni particles, whereas the peak at 918 ◦ C to the hard-to-reduce Ni2+ .
After steaming, some other peaks also appeared at 121 ◦ C and
200 ◦ C, indicating a phase separation of Ni-Pt alloy; Pt
Figure 18. TPR profiles of 0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O before and after
steaming at 900 ◦ C followed by DSS SR of methane [24]. (1) after reduction,
(2) after steaming at 900 ◦ C, (3) after steaming at 900 ◦ C followed by DSS
SR
16
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
exhibited reduction peaks at 100∼200 ◦ C, whereas Ni was reduced at 250∼400 ◦ C in the TPR of supported Ni-Pt bimetallic catalysts [72]. When 0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O after steaming was exposed to DSS SR of methane, the peak at
200 ◦ C disappeared and the peak at 267 ◦ C shifted to a higher
temperature of 290 ◦ C, whereas the peak at 918 ◦ C completely
shifted toward a lower temperature of 830 ◦ C (Figure 18(3)).
These results suggest that the separated species, i.e., Pt and
Ni, were again combined and reconstituted to the original
bimetallic phase on the catalyst; Ni particles were covered by
Ni-Pt alloy on the surface and Ni2+ was again incorporated in
the Mg(Al)O periclase phase.
alysts in DSS mode under steam purging (Figure 19) [24].
Not only 0.10 wt% but even 0.50 wt% Ru doping resulted
in a catalyst deactivation after the first steam purging, indicating that Ru was not suitable for DSS ATSR. Contrarily, 0.10
wt% Pt-doped catalyst was sustainable enough for the steampurged DSS ATSR of methane. The author concludes that
Pt/Ni/Mg(Al)O is more sustainable than Ru/Ni/Mg(Al)O not
only in air-steam-purged DSS SR but also in steam-purged
DSS ATSR of methane.
2.10. Superior catalytic behavior of Pt/Ni/Mg(Al)O
Among Rh, Ru and Pt, Ru is frequently used as a reforming catalyst for PEFCs due to the relatively low cost [9,72].
Although Pt is cheaper than Rh, Pt has been used as oxidation or combustion catalyst [73] but has not been used as reforming catalyst. Pt assisted self-activation of Pt/Ni/Mg(Al)O
probably by C−H activation in SR of methane (vide supra)
[24,63]. The author confirmed that Pt doping enhanced the
activity and, moreover, assisted the self-regeneration of active Ni sites by reversible reduction-oxidation of the Ni on/in
Mg(Ni,Al)O periclase [24].
When one considers the down stream of H2 production
for PEFCs, water-gas shift and then CO elimination steps follow SR of methane [9]. It was reported that steam condensation caused a deactivation of water-gas shift catalysts due
to heavy Cu sintering [74,75]. Therefore, purging conditions
must be chosen to avoid steam condensation. When steam
was partially replaced with air in purge gas of DSS SR of
methane over Pt/, and Ru/Ni0.5 /Mg2.5 (Al)O, the former exhibited a stable activity, whereas the latter was quickly deactivated due to Ni oxidation [24]. This indicates that Pt doping
is more effective than Ru doping for the catalyst sustainability under air-steam combined purging. ATSR with steam and
oxygen seems to be a hopeful candidate for the future PEFCs
reformers, because the exothermic oxidation can supply the
heat for endothermic steam reforming [76]. ATSR of methane
was carried out on both Pt/, and Ru/Ni0.5 /Mg2.5 (Al)O cat-
Figure 19. Comparison of the activities of Pt/Ni0.5 /Mg2.5 (Al)O and
Ru/Ni0.5 /Mg2.5 (Al)O in DSS ASTR of methane.
Table 6. Curve fitting results of Ru K-edge EXAFS of the 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O catalyst after reduction and DSS ATSR of methanea [24]
Sample
0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)Ob
0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)Oc
RuO2
Ru metal
Shells
Ru-Ni
Ru-O
Ru-O
Ru-Ru
C.N.
3.7±0.8
3.8±0.6
6
12
R (Å)
2.49±0.01
2.07±0.01
σ (Å)
0.079±0.02
0.064±0.02
∆E0 (eV)
−12.9±2.9
9.2±2.0
Rf (%)
3.0
0.8
2.66
C.N, coordination number; R, bond length (Å); ∆E0 , difference in the origin of photoelectron energy between the reference and the sample; σ,
Debye-Waller factor (Å); Rf , residual factor; b After reduction; c After DSS ATSR
a
Ru K-edge XANES spectra of 0.50 wt% Ru/Ni0.5 /
Mg2.5 (Al)O revealed that Ru was mainly reduced to metallic state after reduction, whereas Ru was considerably oxidized after followed by DSS ATSR of methane, resulting in the deactivation [24]. Such Ru oxidation after DSS
ATSR was also confirmed by the Fourier transforms of k 3 weighted Ru K-edge EXAFS spectra (Figure 20) [24]. Ru
foil as control exhibited an intensive peak at 2.27 Å (nonphase shift corrected), corresponding to the Ru–Ru bond in
metallic Ru (Figure 20(1)) [77]. After reduction, 0.50 wt%
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Ru/Ni0.5 /Mg2.5 (Al)O showed a peak at a little shorter distance, i.e, 2.08 Å (non-phase shift corrected), tentatively assigned to the Ru-Ni bond due to the formation of Ni-Ru alloy (Figure 20(2)). After reduction followed by DSS ATSR of
methane, the peak shifted toward further shorter distance, i.e.,
1.58 Å (non-phase shift corrected), due to the formation of
Ru–O bond [77] by Ru oxidation (Figure 20(3)). Hosokawa et
al. [78] observed a peak at 3.0 Å (non-phase shift corrected)
for bulk RuO2 supported on CeO2 and assigned it to Ru–Ru
bonding in the bulk RuO2 . In the present work, no such peak
was observed, suggesting that RuO2 was dispersed and isolated as monomeric state on 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O
passivated by DSS ATSR. In the curve fitting results of Ru
K-edge EXAFS of 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O (Table 6)
[24], neither Ru-Ru nor Ru-O bond was observed but Ru–Ni
bond alone appeared for the catalyst after reduction. The Ru–
Ni bond length was 2.49 Å, very close to the Ni–Ni bond in
metallic Ni (2.49 Å) and 0.17 Å shorter than the Ru-Ru bond
in metallic Ru (2.66 Å). The coordination number of Ru-Ni
shell was 3.7, far smaller than that of the Ru metal (12), suggesting that Ru is located in the surface layer of Ni particles
[22,24]. After reduction followed by DSS ATSR of methane,
the Ru-Ni shell disappeared and Ru-O shell was found at
2.07 Å, indicating that Ru was separated as RuO2 from the
Ru-Ni binary system [24]. Such phase separation is reasonably accompanied by the corruption of the active sites, resulting in the catalyst deactivation due to the Ni oxidation.
17
birth of active Ni species, i.e., redispersion of Ni particles
(Figure 21). It seems that Mg(Al)O periclase plays an important role in the self-regenerative activity of Pt/Ni/Mg(Al)O
catalyst. Ni2+ ions in Mg(Al,Ni)O periclase were reduced
by hydrogen spillover from Pt metal to form fine Ni0 particles on the surface of periclase particles. Conversely, surface
Ni0 particles were oxidized to Ni2+ by steam and incorporated into the periclase. The Ni0 oxidation was probably commenced at the periphery of Ni particles assisted by OH groups
on Mg(Al)O periclase as the support and gradually proceeded
into the bulk of the Ni particles. The Ni2+ ions formed were
incorporated into Mg(Ni,Al)O periclase and no isolated NiO
formed on both Ni/Mg(Al)O and Pt/Ni/Mg(Al)O. In contrast,
both FCR and 13.5 wt% Ni/γ-Al2 O3 produced isolated NiO
and exhibited no self-regenerative activity. On these Al2 O3 supported Ni catalysts, NiO could not be reductively redispersed to fine Ni particles even in the presence of Pt. It is most
likely that the crystal structure of Mg(Al,Ni)O periclase plays
an important role in the self-regenerative activity by reversible
reduction-oxidation between Ni0 outside and Ni2+ inside of
the periclase assisted by steam and Pt (or Ni-Pt alloy) on the
Pt/Ni/Mg(Al)O catalyst.
Figure 21. Self-activation and self-regenerative activity of Pt/Ni/Mg(Al)O
catalyst [24]
2.11. Structure of commercial Mgx -Al HTs (x = 3.5, 1.3 and
0.5)
Figure 20. Fourier transforms of k 3 -weighted Ru K-edge EXAFS spectra
of 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O before and after DSS ASTR of methane
[24]. (1) Ru foil, (2) 0.50 wt% Ru/Ni0.5 /Mg2.5 (Al)O after reduction, (3) 0.50
wt% Ru/Ni0.5 /Mg2.5 (Al)O after DSS ASTR
One must notice that trace Pt-doped Ni/Mg(Al)O exhibited sustainable activity even under oxidative conditions, i.e.,
steam-oxygen-purged SR and steam-purged ATSR. Moreover
this catalyst exhibited “intelligent” catalytic behaviors, i.e.,
self-activation and self-regenerative activity, as well as high
TOF values in DSS SR of methane. The self-activation was
accomplished by the hydrogen-spillover from Pt and, moreover, the self-regeneration was achieved by the continuous re-
Aiming the “green” preparation of the Pt/Ni/Mg(Al)O
catalysts, Pt/Ni/[Mgx Al]O catalysts were prepared starting from three commercial Mgx -Al HTs (x = 3.5, 1.3 and
0.5). The three Mgx -Al HTs samples as received exhibited the reflection lines of Mg-Al HT in the XRD; the
line intensities decreased with decreasing x and finally the
reflections of boehmite (AlOOH) were observed as impurity in Mg0.5-Al HT [79]. Mg3.5-Al HT exhibited the most
intensive HT reflections among three samples and produced
mainly Mg(Al)O periclase after the calcination above 500 ◦ C.
Mg1.3-Al HT produced Mg(Al)O periclase after the calcination at 500 ◦ C and additionally MgAl2 O4 spinel reflections
at 850 ◦ C. Mg0.5 -Al HT afforded both the periclase and the
spinel at 500 ◦ C and mainly the spinel at 850 ◦ C. For all the
three Mgx -Al HTs (x = 3.5, 1.3 and 0.5), the HT reflections
remained even at 300 ◦ C, followed by a phase transition into
Mg(Al)O periclase at 500 ◦ C. Such phase transition appeared
18
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
as two-steps weight loss process for Mg3.5 -Al HT by TG-DTA
(Figure 22). The first step around 200 ◦ C is attributed to dehydration of loosely bound water in the interlayer space and
the second one around 500 ◦ C is due to simultaneous dehydroxylation and decarbonation of the lattice OH− , strongly
bound water and CO−
3 groups [80−82]. Both weight losses
and DTA signal intensities at 200 ◦ C and 500 ◦ C decreased
with decreasing x. This well coincided with the XRD observations, i.e, the HT reflections were weakened with decreasing x.
Figure 22. DT-TGA curves of Mg3.5 -Al HTs [79]
Typical MAS 27 Al NMR spectra of Mg3.5 -Al HT during the calcination are depicted in Figure 23. In the spectra of Mgx -Al HTs (x = 3.5, 1.3 and 0.5) during calcination,
two resonance lines appeared in the regions of 69∼78 and
9∼16 ppm, which corresponded, respectively, to Al3+ cations
tetrahedrally (AlTd ) and octahedrally (AlOh ) coordinated to
oxygen [83−85]. Mgx -Al HTs as received showed an intensive peak at 9.3∼9.6 ppm and a weak peak at 76.6∼77.5 ppm.
The former peak is assigned to AlOh in Mg-Al HT and the
peak intensity was enhanced with increasing x. Moreover,
the peak of AlOh (9.0 ppm) possibly in boehmite overlapped
with increasing the intensity with decreasing x [86−88]. The
peak assigned to AlOh was weakened with raising the calcination temperature above 300 ◦ C due to the decomposition of
Mg-Al HT. The peak at 76.6∼77.5 ppm is assigned to AlTd O-Mg in Mg(Al)O periclase as solid solutions [86,87]; this
peak was slightly enhanced for Mg3.5-Al HT, a peak splitting
occurred and a new peak appeared at 69.6 ppm for Mg1.3 -Al
HT, and the peak shifted from 76.6 ppm toward 69.9 ppm for
Mg0.5 -Al HT after the calcination above 500 ◦ C. This indicates that a transition from AlOh to AlTd took place during
the calcination; the chemical shift observed at 69.6−69.9 ppm
for both Mg1.3 -Al HT and Mg0.5 -Al HT is assigned to AlTd
bonded to Al3+ (AlTd -O-Al) in MgAl2 O4 spinel [84]. Meanwhile, at a higher temperature of ambient pressure dehydration, γ-alumina or dehydrated pseudo-boehmite shows a single 6-coordinate AlO6 resonance at 14.3 ppm [87]. In fact,
all the three Mgx -Al HTs (x = 3.5, 1.3 and 0.5) calcined at
higher temperature exhibited a shoulder at 15.4−15.7 ppm
(Figure 23), suggesting that such isolated alumina was formed
during solid-state thermal conversion to α-alumina in the
[MgxAl] samples.
Figure 23. 27 Al NMR spectra of Mg3.5 -Al HT during the calcination [79].
Mg3.5 -Al HT was calcined at 200 ◦ C, 300 ◦ C and 500 ◦ C for 0.5 h, and finally
at 850 ◦ C for 5 h. Naked number represents chemical shift
Based on both the DT-TGA and the MAS 27 Al NMR observations, the author concludes that the first phase transition
from AlOh (Mg-Al HTs) to AlTd (Mg(Al)O periclase) took
place between 300 ◦ C and 500 ◦ C, followed by a splitting
of the AlTd to AlTd -O-Mg and AlTd -O-Al due to MgAl2 O4
spinel formation above 850 ◦ C [19,79]. It is assumed that
the [Mg3.5 Al]O mainly consist of periclase Mg(Al)O, the
[Mg1.3Al]O is a mixture of the periclase and the spinel, and
the [Mg0.5Al]O is mainly composed of the spinel.
2.12. “Green” preparation of “intelligent” Pt/Ni/[Mg3.5 Al]O
catalysts
When [Mgx Al]O powders after the calcination were dispersed in aqueous solution of Ni(II) nitrate, the Mg-Al HT
reconstitution was enhanced with increasing x and by calcination at 850 ◦ C, whereas the reconstitution was not significant
for [MgxAl]O with decreasing x and [Mg3.5Al]O calcined at
500 ◦ C. This is due to the higher content of Mg(Al)O periclase in [Mgx Al]O with increasing x and after the calcination
at 850 ◦ C [89]. When 0.05 wt% Pt/10 wt% Ni/[Mg3.5Al]O
was used in SR of methane at 700 ◦ C, the reaction immediately started without prereduction treatment. After SR of
methane, Ni metal reflections were observed, indicating that
the catalyst was self-activated [24].
TPR profiles of 0.05 wt% Pt/10 wt% Ni/[Mg3.5Al]O prepared by changing the dispersion time are depicted in Figure 24. A dispersion for 0.5 h exhibited a small amount of Ni
reducible (Figure 24(1)), indicating that the Ni incorporation
19
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Figure 24. TPR profiles of 0.05 wt% Pt/10 wt% Ni/[Mg3.5 Al]O
prepared by varying dispersion times at 80 ◦ C [89].
Number in
parenthesis showed the amounts of reducible Ni (%) in the catalyst
calculated based on total Ni amounts used.
(1) 10Ni/[Mg3.5 Al]O
850-(0.5 h)-850, (2) 0.05Pt/10Ni/[Mg3.5 Al]O 850-(0.5 h)-850-850, (3)
10Ni/[Mg3.5 Al]O 850-(2 h)-850, (4) 0.05Pt/10Ni/[Mg3.5 Al]O 850-(2 h)850-850, (5) 10Ni/[Mg3.5 Al]O 850-(4 h)-850, (6) 0.05Pt/10Ni/[Mg3.5 Al]O
850-(4 h)-850-850
in the catalyst was not completed. Here, the amount of reducible Ni was calculated from the peak area of H2 consumption per total amount of Ni used. In fact, ICP analyses showed
that a certain amount of Ni2+ ions still remained in the aqueous solution after dispersion followed by filtration (vide infra). When the dispersion time was extended from 0.5 h to 4 h,
the Ni2+ incorporation was enhanced (Figure 24(5)). After
dispersion in Ni(II) nitrate, Ni reduction peak was separated
into two peaks for all samples, indicating that two types of Ni
species formed on the Ni/[Mg3.5Al]O samples. The peak separation was enhanced by extending the dispersion time (Figures 24(1), (3) and (5)), whereas the separated two peaks were
combined to one single peak after 0.05 wt% Pt doping (Figures 24(2) and (4)). The peak observed around 800 ◦ C is pos-
sibly assigned to the Ni2+ ions in the outermost layer or subsurface layers of Mg(Al)O lattice [54,90], whereas that around
970 ◦ C is assigned to the completely incorporated Ni2+ in
Mg(Ni,Al)O lattice [91]. The most intensive, sharp and single Ni reduction peak appeared at 870 ◦ C, when [Mg3.5Al]O
powders were dispersed in Ni(II) nitrate for 2 h, followed by
Pt doping (Figure 24(4)). It seems that Pt assisted the combination of these two Ni2+ species into one Ni2+ species probably by forming Ni-Pt bimetallic system (Figure 24(2), (4) and
(6)).
During the catalyst preparation by sequential impregnation, the samples were calcined three times, i.e., 1st , Mgx Al HTs as received; 2nd , [MgxAl]O after Ni2+ impregnation and 3rd , Ni/[Mgx Al]O after Pt4+ doping. These calcination temperatures significantly affected the Ni reduction
property. Here the three calcination temperatures are shown
by being combined with hyphen in series. The calcination at
850−850−850 ◦ C exhibited the intensive peak at 870 ◦ C assigned to Ni2+ incorporated in Mg(Ni,Al)O lattice. Contrarily, the calcination at 500−500−500 ◦ C showed a decrease
in the amount of reducible Ni and new peaks at 450 ◦ C and
760 ◦ C, which are assigned to “free” NiO and Ni2+ ions in the
outermost layer or sub-surface layers of the Mg(Al)O lattice,
respectively [90]. All the calcinations at 850−850−850 ◦ C,
500−850−850 ◦ C and 500−500−850 ◦ C exhibited almost
similar Ni reduction peak at 860 ◦ C assigned to Ni2+ incorporated in Mg(Ni,Al)O lattice. The calcination at 850 ◦ C even
at the final stage seems effective for the growth of the periclase crystallites, resulting in an enhanced Ni2+ incorporation
in Mg(Ni,Al)O lattice.
After each step of Ni impregnation and Pt doping, the
aqueous solution was separated by filtration and the amounts
of metals in the solution were analyzed by ICP (Table 7).
Metal loadings on the catalysts were also determined by ICP
analyses. Al3+ was not detected in both solutions after Ni
impregnation and Pt doping, whereas Mg2+ was dissolved in
the solution after Ni impregnation. Ni2+ incorporation into
[MgxAl]O varied dependently of the impregnation temperature. The impregnation at room temperature resulted in an
incomplete Ni2+ incorporation and a significant Ni2+ was
Table 7. Ni loading followed by Pt doping on [Mgx Al]O prepared from commercial [Mgx Al] HTs (x = 3.5, 1.3 and 0.5)a [89]
No.
Catalysts
1
2
3
4
5
6
7
8
0.05Pt/10Ni/[Mg0.5 Al]O
0.05Pt/10Ni/[Mg1.3 Al]O
0.05Pt/10Ni/[Mg3.5 Al]O
0.05Pt/10Ni/[Mg3.5 Al]O
0.05Pt/10Ni/[Mg3.5 Al]O
0.10Pt/16Ni/[Mg3.5 Al]O
0.10Pt/16Ni/[Mg3.5 Al]O
0.05Pt-10Ni/[Mg3.5 Al]O
a
Calcination &
(dispersion)
temperatures (o C)
850-(80)-850-(80)-850
850-(80)-850-(80)-850
850-(80)-850-(80)-850
850-(80)-850-(80)-850e
500-(80)-500-(80)-500
850-(80)-850-(80)-850
850-(80)-850-(80)-850f
850-(80)-850
Amount of metals dissolved in solutionb (mmol)
First impregnation
Second impregnation
Ni
Pt (×10−3 )
Mg
Ni
Pt (×10−3 )
Mg
0.460
–
0.721
0.000
0.000
0.000
0.633
–
0.391
0.000
0.002
0.000
0.000
–
0.327
0.000
0.003
0.000
0.033
–
0.232
0.000
0.002
0.000
0.073
–
0.406
0.000
0.007
0.000
0.334
–
1.202
0.000
0.004
0.000
0.007
–
1.358
0.000
0.007
0.000
0.003
0.001
0.400
–
–
–
Metal loadingc
(wt%)
Ni
Pt
3.5
0.027
5.1
0.039
9.0
0.039
9.5
0.049
8.4
0.052
12.6
0.11
15.3
0.10
8.7
0.058
Catalysts were prepared by sequential or coimpregnation of Ni2+ and Pt4+ for 2 h (e 4 h and f 6 h) followed by filtration and calcination.
Amount of metal in the solution after filtration was determined by ICP analysis. 1.95 and 0.0017 mmol of Ni(II) and Pt(IV) nitrate, respectively, were
dissolved in distilled water and used in the impregnation for 0.05 wt% Pt/10 wt% Ni/[Mgx Al]O catalysts.
c Amount of metal loaded or doped on the catalyst was determined by ICP analysis and is shown as a loading amount (wt%)
b
20
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
dissolved in the solution for all [MgxAl]O (x = 3.5, 1.3 and
0.5) (data are not shown). At 80 ◦ C, Ni2+ incorporation
was completed in [Mg3.5Al]O (No. 3), but was not completed
in both [Mg0.5Al]O (No. 1) and [Mg1.3 Al]O (No. 2). For
[Mg3.5Al]O, the calcination at 850 ◦ C was preferable than
that at 500 ◦ C for completing Ni2+ incorporation (Nos. 3 and
5), indicating that Ni2+ incorporation was enhanced in well
grown Mg(Al)O crystallites [79,89]. After Pt doping, neither
Mg2+ nor Ni2+ was detected and Pt4+ dissolution was negligibly small compared with the total Pt4+ amounts used in all
cases. This indicates that Pt4+ impregnation was almost completed independently of the impregnation temperatures and x
in [MgxAl]O. On the 0.05 wt% Pt-10 wt% Ni/[Mg3.5 Al]O
prepared by coprecipitation, both Ni2+ and Pt4+ was com-
pletely incorporated on the catalysts calcined at 850 ◦ C and
impregnated at 80 ◦ C (No. 8).
When [Mg3.5Al]O powders were calcined at 850 ◦ C, the
Ni2+ incorporation was completed by the impregnation at
80 ◦ C for 2 h on both 0.05 wt%/10 wt% Ni/[Mg3.5 Al]O and
0.05 wt%-10 wt% Ni/[Mg3.5 Al]O catalysts. However, the
Ni2+ incorporation was not completed under such conditions
when Ni loading and Pt doping were increased from 10 and
0.05 wt% to 16 and 0.1 wt%, respectively (Nos. 6 and 7).
During dispersion in the Ni(II) nitrate, Ni2+ was incorporated
in the catalysts, whereas Mg2+ was dissolved in the solutions, suggesting that Mg2+ ions in Mg(Al)O periclase were
replaced by Ni2+ ions during the reconstitution of Mg(Ni)-Al
HT by the “memory effect”.
Table 8. Physicochemical properties of Pt/Ni/[Mg3.5 Al]O catalysts before and after steaming, and followed by DSS SR of methanea [89]
No.
Catalystsb
1
2
3
4
5
6
7
8
0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O
0.05Pt/10Ni/[Mg3.5 Al]O 850−850−850
0.05Pt/10Ni/[Mg3.5 Al]O 500−500−500
0.10Pt/16Ni/[Mg3.5 Al]O 850−850−850
0.10Pt/16Ni/[Mg3.5 Al]O 500−850−850
0.10Pt/16Ni/[Mg3.5 Al]O 850−850−850 h
0.05Rh/10Ni/[Mg3.5 Al]O 850−850−850
0.10Rh/16Ni/[Mg3.5 Al]O 850−850−850
Specific surface area
(m2 ·g−1
cat )
before after after DSSg
141.2 56.9
76.1
100.3 45.4
66.0
204.7 37.6
44.8
101.2 39.3
55.4
109.7 54.7
–
122.5 51.9
59.6
108.8 40.5
58.0
107.5 36.6
44.7
D c,d
(%)
before after
20.6
4.0
10.2
4.3
4.2
1.1
10.2
2.6
12.5
5.3
10.0
3.9
9.9
5.2
11.0
2.5
before
5.5
9.6
14.6
8.5
8.5
8.1
9.0
8.4
Ni particle size (nm)
XRDe
H2 pulsef
after after DSSg
before after
16.5
10.0
5.2
24.0
22.9
12.8
9.5
22.8
21.0
20.7
23.0
84.6
19.2
17.8
9.5
36.8
19.7
–
7.8
18.3
23.9
22.1
9.8
24.8
21.7
12.7
9.8
18.5
19.5
18.7
8.8
38.7
a
Steaming was carried out at 900 ◦ C for 10 h in H2 /H2 O/N2 (20/100/25 ml·min−1 ).
Pt or Rh/Ni/[Mg3.5 Al]O catalyst was prepared by dispersing at 80 ◦ C for 2 h (h 6 h) in Ni(II) and for 2 h in Pt(IV) or Rh(III) nitrate aqueous solution.
c Determined by H pulse method.
2
d Calculated from the H uptake assuming the reduction degree of 80% for hydrotalcite derived catalysts [21,52] and 100% for impregnated catalysts.
2
e Calculated from the full width at half maximum of Ni (2 0 0) reflections in the XRD using the Scherrer equation.
f Calculated using a equation: d = 971/D/10 where D (%) is a dispersion [53].
g After steaming at 900 ◦ C, followed by steam purged DSS SR between 200 and 700 ◦ C
b
2.13. Sustainability of Pt/Ni/[Mgx Al]O
When Ni loading was increased to 16 wt% on
[Mg3.5Al]O, weak reflections of hexagonal Ni(OH)2 were
observed together with Mg(Ni)-Al HT in the XRD patterns
after the Ni2+ impregnation [89]. This indicates that Ni2+
ions were mainly incorporated into Mg-Al HT [19], but were
partly hydrolyzed and deposited as Ni(OH)2 on the surface
of basic Mg(Al)O periclase. Specific surface area, Ni dispersion and Ni particle size of Pt/Ni/[Mg3.5 Al]O before and
after steaming, followed by DSS SR of methane, are shown
in Table 8. Ni dispersion was lower on Pt/Ni/[Mg3.5 Al]O
than on Pt/Ni/Mg(Al)O (No. 1) prepared by coprecipitation
[24], probably due to surface aggregation of Ni atoms during the Ni2+ impregnation on [Mg3.5 Al]O. Specific surface
area increased with decreasing the calcination temperature
(Nos. 2 and 3). Ni dispersions were similar on all catalysts
calcined at 850−850−850 ◦ C (No. 2), 500−850−850 ◦ C and
500−500−850 ◦ C (data are not shown), whereas that calcined
at 500−500−500 ◦ C alone showed uniquely low Ni dispersion as well as large-sized Ni particles (No. 3).
To simulate the catalyst ageing in an actual use, steaming [66] was applied for Pt/Ni[Mgx Al]O (x = 3.5, 1.3 and
0.5) catalysts. After steaming, specific surface area and
Ni dispersion decreased, indicating that sintering took place
for all Ni catalysts (Table 8). When such heavily sintered catalysts after steaming were used in DSS SR of
methane, Ni particle sizes decreased due to redispersion of
Ni particles [22−24]. The high Mg/Al molar ratio of 3.5,
the calcination at 850 ◦ C and the Ni loading of 10 wt%
were preferable for the efficient redispersion of Ni particles after DSS SR (No. 2) among Pt/Ni/[Mgx Al]O samples
tested. However, the Ni redispersion on 0.05 wt% Pt/10
wt% Ni/[Mg3.5Al]O (No. 2) was not so efficient compared
with that on 0.10 wt% Pt/Ni0.5 /Mg2.5(Al)O prepared by coprecipitation (No. 1). This may be again due to surface aggregation of Ni atoms during the Ni2+ impregnation. Use
of [Mg0.5Al]O (data are not shown), calcination at 500 ◦ C
(No. 3) and 16 wt% Ni loading (No. 4) resulted in no efficient
redispersion of Ni particles after DSS SR. Even when the
dispersion time was extended from 2 h to 6 h for 0.10 wt%
Pt/16 wt% Ni/[Mg3.5Al]O, no clear decrease in Ni particle
size was observed after DSS SR (No. 6), indicating low ability
of redispersion of Ni particles. Also for Rh/Ni/[Mg3.5 Al]O,
10 wt% Ni loading (No. 7) exhibited better Ni redispersion after DSS SR than 16 wt% Ni loading (No. 8). The
16 wt% Ni loading seems too large to incorporate completely Ni2+ in the Mg(Ni,Al)O periclase. It is concluded
Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
that the most efficient Ni redispersion was accomplished on
0.05 wt% Pt/10 wt% Ni/[Mg3.5 Al]O 850−850−850 (No. 2)
among Pt/Ni/[Mgx Al]O catalysts prepared.
The activities of Pt/Ni/[Mgx Al]O (x = 3.5, 1.3 and 0.5)
catalysts before and after steaming were compared based on
the TOF-t (Figure 25). The ratio of TOF-t after steaming
(TOF-tafter ) to TOF-t before steaming (TOF-tbefore ) was used
to evaluate the sustainability of each catalyst. Among the Ni
catalysts tested as controls, FCR was the most severely deactivated, followed by 13.5 wt% Ni/γ-Al2 O3 , Ni0.5 /Mg2.5 (Al)O
and 0.10 wt% Pt/Ni0.5 /Mg2.5 (Al)O. This indicates that Pt doping on Ni/Mg(Al)O was effective for suppressing the deactivation due to sintering [24]. Rh/and Pt/Ni/[Mg3.5 Al]O prepared by calcining at 850 ◦ C at all steps also exhibited high
and sustainable activity. 0.10 wt% Rh/16 wt% Ni/[Mg3.5 Al]O
was the most sustainable, however the TOF-t was lower compared with those of Pt-doped Ni catalysts. TOF-t was higher
with 10 wt% Ni loading than with 16 wt% Ni loading. When
Pt/Ni/[Mg3.5 Al]O was calcined at 500 ◦ C at all steps, both activity and sustainability decreased. This well coincided with
no decrease in the Ni particle size after DSS SR of methane
(Table 8, No. 3).
21
Ni and Pt was favorable for the preparation of the sustainable Pt/Ni/[Mgx Al]O catalyst. Both higher Mg/Al molar ratio and higher calcination temperature produced well crystallized Mg(Al)O periclase and the sequential impregnation of
Ni and Pt produced surface Pt-enriched Ni particles. Mg(Al)O
periclase works as Ni reservoir which reversibly incorporates
and releases Ni dependently of the atmosphere, i.e., Ni0 is
oxidatively incorporated into Mg(Ni2+,Al)O under steam atmosphere, whereas the Ni2+ is reductively released to form
Ni0 particles on the catalyst surface. It seems that steam
plays an important role in the oxidative incorporation of Ni
into Mg(Ni2+ ,Al)O periclase [23,79], whereas hydrogen dissociated from methane works for the reductive release of Ni0
from the periclase. Pt or Pt-Ni alloy on the surface of Ni
particles dissociates C−H bond of methane to produce hydrogen; hydrogen spillover from Pt reductively releases Ni0
from Mg(Ni2+,Al)O. One may conclude that both the combination of Pt-Ni bimetal as active species and Mg3.5 (Al)O
periclase as catalyst support elaborated the “intelligent” catalyst for DSS SR of methane. Thus the “green” preparation of
Pt/Ni[Mg3.5Al]O catalyst has been accomplished. Scientific
research to clarify the mechanistic details of self-regenerative
activity is under progress by in situ XAFS observation. Further catalyst developments will be continued by supporting
this catalyst on mechanically strong supports by wash-coating
or spray-drying method to form monolithic catalysts.
3. Conclusions
Figure 25. TOF-t values of Pt/ and Rh/Ni/[Mg3.5 Al]O before and after steaming, and the other catalysts as controls [89]. All Pt/ and
Rh/Ni/[Mg3.5 Al]O catalysts were prepared by dispersing [Mg3.5 Al]O in
Ni(II) nitrate for 2 h (a 6 h) and prereduced at 600 ◦ C in H2 /N2
(5/25 ml·min−1 ) for 120 min. TOF-t values were calculated based on total
amount of Ni on the catalysts. Blank bar, TOF-t before steaming; grey bar,
TOF-t after steaming; •, ratio of TOF-t before steaming to TOF-t after steaming
Use of higher Mg/Al molar ratio (Mg3.5/Al), higher calcination temperature (850 ◦ C) and sequential impregnation of
Ni/Mg(Al)O catalysts derived from the HT exhibited high
and stable activity in SR of methane due to the formation
of stable and well-dispersed Ni metal particles. However,
the catalysts were severely deactivated due to Ni oxidation
during DSS SR. Trace amounts of noble metals doping on
Ni/Mg(Al)O by using the “memory effect” was effective
for suppressing the Ni oxidation. The noble metal-doped
Ni/Mg(Al)O catalysts exhibited an “intelligent” properties,
i.e., self-activation and self-regenerative activity, leading to
the high and sustainable activity during DSS SR. Among the
noble metals, Ru, Rh and Pt, Pt was the most effective for
self-activation, that was induced by the reduction of Ni2+ in
Mg(Ni,Al)O periclase to Ni0 assisted by hydrogen spillover
from Pt (or Pt-Ni alloy). The self-regenerative activity was
accomplished by self-redispersion due to a cyclic movement
of Ni between the outside and the inside of the periclase
crystal; Ni0 is oxidized to Ni2+ by steam and incorporated
into Mg(Ni,Al)O periclase, whereas the Ni2+ in the periclase is reduced to Ni0 by hydrogen spillover from Pt and
appeared as the fine Ni particles on the catalyst surface. Finally, Pt/Ni/[Mg3.5 Al]O catalysts were prepared by “green”
process starting from commercial Mg3.5 -Al HT by calcination, followed by sequential impregnation of Ni and Pt. Further catalyst preparation in industrial scale will be accomplished by supporting the Pt/Ni/[Mg3.5 Al]O powders on mechanically strong supports by wash-coating or splay-drying to
make monolithic forms.
22
Katsuomi Takehira / Journal of Natural Gas Chemistry Vol. 18 No. 3 2009
Acknowledgements
The author would like to thank Prof. Shishido T. of Kyoto University, Japan, for helpful discussions and XAFS analyses and acknowledge Dr. Li D. and Dr. Zhan Y. as postdoctral Research Fellows, Hiroshima University, Japan (present address; Li D.: Tsukuba
University, Japan and Zhan Y.: Fuzou University, China), for their
excellent experimental works. This work was partly supported by
the New Energy and Industrial Technology Development Organization (NEDO), Japan.
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