Chemical Engineering Journal xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Thermally activated iron containing layered double hydroxides as potential
catalyst for N2O abatement q
Tatjana J. Vulic a,⇑, Andreas F.K. Reitzmann b, Károly Lázár c
a
University of Novi Sad, Faculty of Technology, bul. Cara Lazara 1, 21000 Novi Sad, Serbia
Sued-Chemie AG, Research and Development Catalysis and Energy, Waldheimerstr. 13, 83052 Bruckmühl, Germany
c
Institute of Isotopes of the Hungarian Academy of Sciences, Department of Catalysis and Tracer Studies, P.O. Box 77, Budapest H-1525, Hungary
b
h i g h l i g h t s
" Reduction behavior mainly influences catalytic activity in N2O reduction with NH3.
" Extended M(III) substitution weakens Mg–Al–Fe–O interactions.
" Extended M(III) substitution improves catalytic behavior.
" LDH matrix with M(III) near the limit for incorporation gives the best results.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Hydrotalcite like materials
Mg–Al–Fe mixed oxides
Mössbauer spectroscopy
TPR
Nitrous oxide
a b s t r a c t
Layered double hydroxides (LDHs) and derived mixed oxides with different Mg/Al/Fe contents were
investigated. Two super-saturation precipitation methods were used for the synthesis of LDHs with general formula [Mg1x M(III)x (OH)2](CO3)x/2mH2O where M(III) presents Al and/or Fe. The content of trivalent ions x = M(III)/[M(II) + M(III)], was varied between 0.15 < x < 0.7. Such a wide range of trivalent ions
was chosen with the aim to induce the formation of different multiphase mixed oxides. Iron was introduced as constituent metal in order to obtain redox properties. LDHs and their derived mixed oxides were
characterized with respect to their crystalline structure (XRD), thermal stability (TG/DTA), textural (N2
adsorption), redox (H2 TPR) and acid properties (NH3 TPD) as well as the nature of the iron species
(Mössbauer spectroscopy). Catalytic behavior was studied in two test reactions: N2O decomposition
and reduction with NH3. It has been demonstrated that extended M(III) substitution influences the structure and surface properties of Mg–Al–Fe LDHs and derived mixed oxides, weakens Mg–Al–Fe–O interactions and improves catalytic behavior correlated with the presence of Fe–O–Fe–O–Fe entities providing
possibility for facilitated extraction of oxygen with simultaneous redox Fe3+M Fe2+ conversion. The catalytic behavior is mainly determined by redox properties, nature of iron species in mixed oxides and
by structural properties of initial LDHs. The best catalytic results were obtained when the amount of
M(III) was near the limit for the incorporation into LDH matrix.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Layered double hydroxides (LDHs) and their derived mixed
oxides reached growing attention in research and engineering
because of possibility to vary a large number of synthesis parameters and in that way modify and tailor various properties. Their
application ranges from catalysts and catalyst supports in organic
synthesis, (photo) degradation of organic wastes, greenhouse gas
q
Work was carried out in Karlsruhe Institute of Technology (KIT), Institute of
Chemical Process Engineering, Fritz-Haber-Weg 2, D-76131 Karlsruhe, Germany.
⇑ Tel.: +381 21 485 3750; fax: +381 21 450 413.
E-mail addresses: [email protected] (T.J. Vulic), andreas.reitzmann@sud-chemie.
com (A.F.K. Reitzmann), [email protected] (K. Lázár).
control emission and H2 production, anion exchangers, adsorbents,
fillers (stabilizers for polymers) to medical-pharmaceutical applications by making use of their specific properties [1–6].
LDHs are anionic clay materials and also known as hydrotalcitelike materials have layered structure which consists of Brucite-like
layers with octahedrally centred M(II) ions. Partial isomorphous
substitution of M(II) with M(III) ions creates positive charge which
is compensated with different anions present in the interlayer region together with water [3,5,7]. The general formula of LDH is:
M(II)1x M(III) x(OH)2(An)x/nmH2O, where M(II) is a divalent cation, M(III) is a trivalent cation, An is anion (usually carbonate)
and x = M(III)/[(M(II)+M(III)]. It has been reported that the value
of x between 0.2 and 0.4 is optimal for the formation of single
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.152
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
2
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
LDH phase and by exceeding this range, hydroxides or other compounds may be formed [3]. The nature and amount of M(II) and
M(III) ions influences the redox properties of LDHs and derived
mixed oxides. After thermal treatment of LDHs, mixed oxides are
formed with homogeneous interdispersion of constituting elements, larger surface area, developed porous structure, less diffusion resistance than LDHs and abundant acid and basic sites,
which makes them more interesting as catalysts or catalyst precursors [3,8].
The potential of these materials has also been studied in waste
gas catalysis, namely in N2O abatement. Different transition and
noble metal containing LHD derived mixed oxides have been reported to be active in N2O and NOx decomposition with different
combinations of divalent (Mg, Co, Ni, Mn, Cu, Ca, Zn) and trivalent
(Al, La, Rh) ions [9–14]. Zeolites, exchanged with transition metal,
especially with iron, showed high activity in decomposition of N2O
[15]. Iron exchanged zeolites exhibit also noticeable efficiencies in
the selective catalytic reduction of nitrous oxide with NH3 and
with various hydrocarbon reducing agents [16–18]. Therefore, in
our investigations iron was chosen as an active component to
study its activity in N2O abetment reactions. Fe containing LDH derived mixed oxides were reported to be catalytically active in various reactions. Mg–Al–Fe and Mg–Fe mixed oxides were
successfully applied in Fischer–Tropsch synthesis [19], in catalytic
dehydrogenation of ethylbenzene [20] and in ethylbenzene dehydrogenation in presence of carbon dioxide [21,22]. In addition,
Mg–Al–Fe–LDHs themselves (without calcination to mixed oxides)
have been shown to be efficient catalyst for the reduction of 4nitrotoluene using phenylhydrazine or hydrazine hydrate as reducing agent [23] and in the reduction of aromatic nitro compound
with hydrazine hydrate [24].
For the synthesis of Mg–Al–Fe and Mg–Fe layered double
hydroxides two coprecipitation methods, low supersaturation, LS,
and high supersaturation, HS, were chosen. The content of trivalent
ions was varied in a wide range between 0.15 < x < 0.7 with the
intention to induce the formation of different LDHs and, after thermal treatment, different multiphase mixed oxides. The objective
was to study the properties of different iron containing LDHs and
derived mixed oxides in correlation to their performance in two redox processes: the decomposition of N2O, and the reduction of N2O
with ammonia.
2. Materials and methods
2.1. Synthesis
Two different coprecipitation methods, low supersaturation, LS,
and high supersaturation, HS, were chosen for the synthesis of
LDHs. In the HS method, the solution containing magnesium, aluminium and/or iron nitrate salts was quickly added to the second
solution containing Na2CO3 and NaOH. For the LS synthesis, the
Mg–Al–Fe containing solution was added drop wise at a constant
rate into 1 dm3 of distilled water and the pH of the solution was
maintained between 9.6 and 9.9 by the simultaneous addition of
the second solution containing Na2CO3 and NaOH. In both cases,
the reaction solution was vigorously stirred, the samples were
aged, washed and filtered, dried for 24 h, at 100 °C in air, and afterwards calcined for 5 h, at 500 °C in air. A detailed explanation of
synthesis methods and thermal activation is given elswere [25].
Two series of Mg–Al–Fe LDHs with different Mg/Al ratio, wide
range of trivalent ions between 0.15 < x < 0.7 and 5 mol% of iron
was prepared. Besides that, Mg–Fe LDHs with higher iron content
(30 mol%) and without aluminum were also synthesized.
Samples were denoted according to the synthesis method used
(HS or LS) and the initial molar metal ratio. For example, LS-
Mg50Al45Fe5 is the denotation for the sample synthesized by
the LS method having following initial metal amounts: 50 mol%
of magnesium, 45 mol% of aluminium and 5 mol% of iron.
2.2. Characterization
The X-ray diffraction measurements were performed in a Siemens D500 X-ray diffractometer (Cu Ka radiation, =0.154 nm,
45 kV, 25 mA) in 2 range from 3° to 63°. Atomic absorption spectroscopy, AAS using Hitachi Z-6100 instrument was used for the
elemental chemical analysis of constituent metals (Mg, Al and
Fe) in calcined samples. A semi-quantitative chemical analysis of
calcined sample surface was performed by JEOL, JSM-460 LV
instrument equipped with energy dispersive spectroscopy, EDS,
Oxford Instruments INCA X-sight system operating at 25 kV.
TG/DTA thermal analysis of all synthesized samples was carried
out in Baehr STA503 instrument from ambient temperature to
1000 °C with the heating rate of 5 °C min1, in static air atmosphere. The BET surface areas and pore radius distributions were
determined by N2 sorption at 196 °C in a Micromeritics ASAP
2010 instrument.
Acidity of the calcined samples was determined by temperature
programmed desorption (TPD) of ammonia in a Micromeritics
AutoChem 2910 apparatus using 10 vol% NH3 in He and ca.
200 mg of calcined samples, flow rate of gas mixture of
25 cm3 min1 and heating rate of 10 °C min1 in temperature
range from 50° to 600 °C. The gases evolved during TPD measurements were analyzed using Pfeiffer Vacuum QMS422 Mass Spectrophotometer. Before TPD measurements the catalysts were
preheated in a He gas flow (25 cm3 min1) to 500 °C and kept at
that temperature for 1 h. Then the flow was switched to 10% vol
NH3 in He (25 cm3 min1) and the temperature was lowered to
50 °C (20 °C min1). The catalysts were than purged from physisorbed ammonia at 50 °C in a He gas flow (25 cm3 min1) for 1 h.
Mössbauer spectra were recorded with a KFKI spectrometer at
ambient temperature in constant acceleration mode. The values
of isomer shift, IS, are given with respect to metallic a-iron. The
estimated accuracy of positional parameters (IS, and quadrupole
splitting, QS) is c.a. ±0.03 mm s1. Relative intensity, RI, in% was
calculated as relative contribution of the given component to the
full area of the spectrum. Intensity per base line, I/BL, represents
the total area of spectrum (sum of components) related to the base
line. Mössbauer spectra were recorded for each sample in a sequence of three measurements: (i) as synthesized samples; (ii)
samples calcined at 500 °C and (iii) after in situ treatment with
CO at 340 °C for 2 h.
Temperature programmed reduction (TPR) was conducted in a
Micromeritics AutoChem 2910 apparatus using ca. 200 mg of calcined samples, flow rate of gas mixture (5% vol H2 in N2) of
20 cm3min1 and heating rate of 10 °C min1 in temperature range
from 25° to 1000 °C. Before the TPR measurements the samples
were preheated in a nitrogen gas flow (20 cm3min1) from ambient temperature to 500 °C (30 °Cmin1) and then cooled to 50 °C.
2.3. Catalytic tests
Catalytic properties were studied in a quartz fixed bed flow
reactor (ø = 8 mm, L = 19 cm). The experimental conditions were:
temperature from 300 to 500 °C, pressure 101 kPa, reactant concentrations of N2O 1000 ppm (vol.) and of NH3 1000 ppm (vol.),
1
modified space velocity, GHSVmod from 2.17 to 6:25 cm3 g1
ðcatÞ s
(NTP).
Before tests, all catalysts were first heated and held 2 h at
500 °C in a He stream. The measurements were performed at different reaction temperatures starting at 500 °C and lowering it
stepwise by 25 °C. The temperature and the gas flows were
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
maintained constant until reaching steady state (waiting period > 1 h). The concentrations were measured at each reaction temperature after reaching steady state. The concentrations of gas
mixtures N2O, NH3 and He, before and after reaction, were measured with nondispersive infrared spectroscope BINOS Leybold
Heräus.
3. Results and discussion
3.1. Structural and elemental analysis
It was reported in our previous work that all Mg–Al–Fe coprecipitation products have XRD patterns typical for LDH compounds,
that HS samples have partially disordered structure, particularly in
stacking of the layers and that extended M(III)ion substitution
(x P 0.5) leads to the formation of additional aluminum hydroxide,
Al(OH)3 – Bayerite phase [25]. Also, after thermal treatment at
500 °C layered structure was destroyed and only the presence of
Mg,M(III) mixed oxide phase was detected. The XRD patterns of
the Mg–Fe as-synthesized samples are presented in Fig. 1. Sharp
and symmetric reflections from (0 0 3), (0 0 6), (1 1 0) and (1 1 3)
planes were observed as well as broad, non-symmetric reflections
from (1 0 2), (1 0 5) and (1 0 8) planes. The lattice parameters were
calculated for a hexagonal unit cell on the basis of rhombohedral
R–3 m symmetry. Basal spacing d0 = d003 was calculated as the
thickness of one layer constituted of one brucite-like sheet and
one interlayer, cation–cation distance within the brucite-like layer
as a0 = 2 d110 and lattice parameter c0 as c0 = 3 d003. The phase
composition and lattice parameter of Mg–Fe–LDHs are given in Table 1 and are in good agreement with results published by other
authors [26–28].
Elemental chemical analysis of constituent metals of the Mg–
Al–Fe samples is also reported in our previous work [25] and the
results of the Mg–Fe samples are given in Table 1. The AAS analysis
showed good agreement between initial amounts of Mg and Fe and
the bulk metal amount. Nevertheless, the surface-enhanced
information about metal composition obtained by the EDS analysis
revealed higher presence of iron on the surface of sample
HS-Mg70Fe30 when compared with the sample LS-Mg70Fe30.
3.2. Thermal analysis
A detailed thermal analysis of Mg–Al–Fe samples is already reported [25]. The data from TG/DTA analysis of Mg–Fe samples in
comparison to the Mg–Al–Fe samples having the same M(III)ion
amount is given in Table 2. Samples have two endothermic transi-
Fig. 1. XRD patterns of the Mg–Fe as-synthesized samples.
3
tions with corresponding mass losses typical for the LDHs, the first
from the loss of physisorbed and interlayer water and the second
due to the loss of hydroxyl groups and interlayer anions. A third
endothermic transition without significant mass loss is also observed at temperatures higher that 700 °C indicating stoichiometric spinel phase and single magnesium-oxide phase formation
[25]. Both Mg–Fe samples have lower temperatures of first two
endothermic transitional stages than corresponding Mg–Al–Fe
samples, indicating lower thermal stability of Mg–Fe LDHs, and
also smaller mass losses, indicating smaller amount of physisorbed
and interlayered water in Mg–Fe samples. Nevertheless, sample
HS-Mg70Fe30 has to some extent higher thermal stability than
sample LS-Mg70Fe30 since it has higher temperatures of both
transitional stages.
3.3. Textural analysis
Nitrogen adsorption isotherms of all Mg–Al–Fe samples calcined at 500 °C are presented in Fig. 2. The isotherms of Mg–Fe
samples correspond to the isotherms of Mg–Al–Fe samples isotherms with same M(III) amount and the same synthesis method,
only the amount of nitrogen adsorbed is smaller in case of Mg–
Fe samples. In accordance to IUPAC classification [29], all HS samples have adsorption isotherms of the Type IV, with the hysteresis
loop and the plateau at high p/po, typical for mesoporous oxide
catalysts and supports. The samples HS-Mg85Al10Fe5, HSMg70Al25Fe5 and HS-Mg70Fe30 have hysteresis loop Type H1 representative of an adsorbent with a narrow distribution of relatively
uniform cylindrical mesopores. The samples HS-Mg50Al45Fe5 and
HS-Mg30Al65Fe5, have somewhat different shape of hysteresis
loop, belonging to Type H2 associated with a more complex pore
structure with narrow slit-shaped pores, probably due to the formation of additional Bayerite phase in these samples.
All LS samples have adsorption isotherms of the Type II and hysteresis loop H3 with no limiting adsorption at high relative pressure and no well-defined mesopore structure, typical for micro
and mesoporous materials with plate like aggregates and nonuniform slit-shaped pores, such as clays. The sample LS-Mg30Al65Fe5
has similar but somewhat different shape of hysteresis loop, compared to other LS samples. It has the combination of H2 and H3
hysteresis loop which could be explained with the presence of
the additional Bayerite phase similar as in case of multiphase HS
samples.
The most commonly used procedure for determination of
mesopore size distribution is the BJH method, proposed by Barrett,
Joyner and Halenda, based on the notional emptying of the pores –
desorption branch. The steep region of the desorption branch for
the hysteresis loops Type H2 and H3 (obtained for the samples
with extended M(III) substitution) is a feature depending on the
nature of adsorptive rather then the distribution of pore size and
also the reason not to take this curve region for the calculation of
mesopore size distribution [29]. In these cases adsorption branch
corresponds better to equilibrium and should be used for calculations of pore size distribution [30]. The distribution based on the
desorption data is indicative of the pore opening/mouth while
the adsorption data provides information of the actual (interior)
pore size. To enable the comparison of samples with different
M(III) content and taking into account that the samples with extended M(III) substitution have this type of hysteresis loop, the
adsorption branch was chosen for the calculation of pore size distribution using BJH method (Figs. 3 and 4).
The BET surface areas of synthesized layer double hydroxides
are between 70 and 100 m2 g1. The BET surface area values of calcined samples are given in Table 3. The calcination results in increase of BET surface area caused by the formation of small
venting holes (crater) at the crystal surface built during the ex-
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
4
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Table 1
Precipitation products phase composition and lattice parameters, initial metal amounts and the metal amounts measured by AAS and EDS of Mg–Fe samples.
Sample
HS-Mg70Fe30
LS-Mg70Fe30
XRD analysis
Initial amounts (mol%)
AAS amounts (mol%)
EDS amounts (mol%)
Phase
d0 (nm)
a0 (nm)
c0 (nm)
Mg
Fe
Mg
Fe
Mg
Fe
LDH
LDH
0.775
0.785
0.311
0.311
2.324
2.355
70
70
30
30
72.2
72.6
27.8
27.4
66.7
80.3
33.3
19.7
Table 2
Data from TG/DTA analysis: mass loss of the first m1, second m2 and third transition stage m3, total mass loss mtot, temperatures of endothermic peaks corresponding to the first
T1, the second T2 and to the third transition T3.
HS-Mg70Fe30
HS-Mg70Al25Fe5
LS-Mg70Fe30
LS-Mg70Al25Fe5
m1 (%)
m2 (%)
m3 (%)
mtot (%)
T1 (°C)
T2 and T 02 (°C)
T3 (°C)
17
19
15
18
17
20
17
21
5
4
5
4
39
43
37
43
212
236
179
213
336, 379
418
324, 373
356, 405
>1000
827
>1000
691
Fig. 2. Adsorption isotherms of Mg–Al–Fe samples after calcination.
Fig. 3. Pore size distribution of Mg–Al–Fe samples after calcinations.
haust of water and CO2 [31]. The increase in M(III) ion content increases in general the BET surface area evidenced also in pore size
distribution.
With the increase of M(III) ion content, the bimodal size distribution changes simultaneously (Figs. 3 and 4), the fraction of small
mesopores (2–3 nm) increases and the fraction of bigger mesop-
ores shifts from 30 nm to 6 nm. The changes in the pore size distribution could be related to the initial ordering of the LDH structure
prior to calcination. The XRD analysis showed that with the
increasing amount of aluminum in the LDH samples, the intensity
of characteristic XRD reflections decreases and diffraction lines
broaden, suggesting a decrease in the ordering of the LDH struc-
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
5
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Fig. 4. Pore size distribution of Mg–Al–Fe and Mg–Fe samples after calcinations having x = 0.3.
Table 3
The initial M(III) molar ratio, x, BET surface area of calcined samples, S, the temperatures of the first, T1-TPR and second TPR peak maxima, T2-TPR, the temperature of the first TPD
peak maxima T1-TPD and area of the first TPD peak A1-TPD.
Sample
x, –
S (m2 g1)
T1-TPR (°C)
T2-TPR (°C)
T1-TPD (°C)
A1-TPD (a.u. g1)
HS-Mg70Fe30
HS-Mg85Al10-Fe5
HS-Mg70Al25Fe5
HS-Mg50Al45Fe5
HS-Mg30Al65Fe5
LS-Mg70Fe30
LS-Mg85Al10-Fe5
LS-Mg70Al25Fe5
LS-Mg50Al45Fe5
LS-Mg30Al65Fe5
0.3
0.15
0.3
0.5
0.7
0.3
0.15
0.3
0.5
0.7
126
167
264
281
303
142
163
266
362
344
385
450
473
409
442
385
448
466
409
454
625
789
737
764
762
736
816
745
760
746
124
129
136
130
129
122
125
136
139
139
130
122
229
243
292
146
101
187
239
300
ture. The formation of additional Bayerite phase causes a decrease
in the amount of carbonate bounded in LDH interlayer and consequently a decrease in surface area of the calcined samples. Lower
surface area of HS samples with extended M(III)ion substitution
(x P 0.5) when compared to corresponding LS samples, which have
no Bayerite phase of smaller amount of it, also supports this statement. The increase of surface area with increasing amount of aluminum in the LDH samples eventually leads to increased amounts
of alumina after calcination, which contributes to the higher surface area compared to the Mg/M(III) mixed oxides.
The pore size distribution of Mg–Fe samples is similar to the
pore size distribution of the Mg–Al–Fe samples with the same
amount of M(III) ions samples (Fig. 4). The Mg–Fe samples have
smaller fraction of small mesopores (ca. 2.5 nm) and consequently
lower surface area, which could be explained with the smaller
amount of physisorbed and interlayer water in these samples detected by TG analysis (Table 2).
3.4. Acidic properties
Fig. 5. TPD profiles of LS-Mg–Al–Fe samples.
Ammonia TPD profile of all LS-Mg–Al–Fe samples are presented
in Fig. 5. All other samples have similar TPD profiles. Simultaneously with TPD measurement the evolved gases were analyzed
by mass spectrometry, MS. The MS analysis showed that the first
peak with temperatures between 120 and 140 °C corresponds to
the amount of ammonia desorbed and that the second one at temperatures around 590 °C corresponds to the amount of water
desorbed.
The conclusions about strength and the amount of acid sites,
presented in Table 3, were taken considering the temperature of
the first peak maxima and its area. The amount of iron in samples
decreases acidity compared to samples with aluminum. The increase of M(III) content increases also the acidity. All samples have
acid sites of similar strength, although the strength of acid sites in
LS-Mg–Al–Fe samples is slightly higher than in HS-Mg–Al–Fe samples. According to reported findings [32,33] LDHs have Lewis acid
sites with medium–high acidic strength.
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
6
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
3.5. Mössbauer spectroscopy analysis
The Mössbauer spectra of the HS and LS samples are shown in
the as synthesized, calcined and CO treated sequence in Figs. 6
and 7, the data obtained from the corresponding evaluations are
presented in Tables 4–6.
The as synthesized samples exhibit asymmetric Fe3+ doublet.
This asymmetry in the doublet of the as synthesized materials
has been reported for other LDHs as well. The asymmetry can be
correlated either with relaxation effects emerging between distant
and separated Fe3+ ions at lower iron content [34], or by distribution of quadrupole splitting at higher iron contents, as suggested in
[26,35]. The presence of asymmetric doublet is prevalent in the
x = 0.3 samples, thus fitting with regard to relaxation phenomena
was applied for them. For samples of larger extent of substitution
(x = 0.5) existence and separation of two different iron environments are assumed, in correspondence with results of previous
studies where Fe3+ components with a smaller and a larger qudrupole splittings are distinguished [35–37]. These two Fe3+ components are shown as Fe3+ (1) and Fe3+ (2) in the corresponding
tables. An additional parameter, I/BL is also displayed. This parameter is the relative intensity of the total spectrum related to the
base line (spectral area in count numbers related to that of the base
line), it carries information on the average value of the probability
of the Mössbauer effect and is strongly correlated to the bonding
strength of the iron species present in the sample [38].
After calcination at 500 °C, profound changes in structure are
observed: iron ions became more strongly bonded, as the significant increase of the I/BL parameter attests. The ca. 50% increase
is significant and it clearly demonstrates that upon calcination
the open, layered structure transforms to a close, three dimensional matrix. LDHs with similar compositions may be transformed
to fluorite-type periclase or spinel oxides, with corresponding
Mössbauer parameters different from those of LDHs [20,34]. Our
spectra were decomposed to three components Fe3+(3), Fe3+(4)
and Fe3+(5). The first of them can probably be assigned to the component of periclase structure [20], the second and third can be
attributed to the different sites in the spinel matrix [20,24,26]. It
should be emphasized here that the method of Mössbauer spectroscopy provides local information from the very vicinity of iron.
Fig. 7. Mössbauer spectra of samples LS-Mg70Fe30 (right), LS-Mg70Al25Fe5
(middle) and LS-Mg50Al45Fe5 (left) recorded: (a) as synthesized; (b) calcined at
500 °C and (c) treated with CO at 340 °C Decomposition of spectra containing Fe2+
components is also displayed in row (c) for illustration.
Thus, e.g. spinel structure may already appear in the local environment of iron at the 500 °C treatment, however the transformation
resulting in the formation of separate extended spinel phase proceeds only at 800 °C as TG and XRD studies revealed.
The third stage, the in situ treatment with CO at 340 °C, was
intended originally to demonstrate the existence of Fe3+–
O–Fe3+–O–Fe3+ chains, with the assumption that reduction of iron
with CO may proceed via extraction of oxygen from the chain with
simultaneous reduction of two neighboring Fe3+ to Fe2+. This
assumption seems to be proven in different extents. This kind of
reduction is less expressed in the samples of low M(III) substitution (x = 0.3), it reaches only 10% for the HS-Mg70–Al25–Fe5 and
23% for the LS-Mg70–Al25–Fe5 sample. The increase in aluminum
amount increases also the extent of reduction, as the results from
x = 0.5 samples measurements show. The Fe–O–Fe–O chains are
probably originated from the presence of a minor ferrihydrite
phase formed already at the synthesis of LDHs. This ferrihydrite
may probably be incorporated into the bayerite phase detected
by XRD. Its assignment by the Mössbauer method is less successful,
since its parameters are similar to those of Fe3+(1). Further, the
brownish color of the samples is a convincing indication for the
presence of an amorphous ferric oxihydroxide. The same experiences were collected and the assumptions proven by low temperature (268.8 °C) Mössbauer measurements [24,36]. The effect of
the CO treatment is not restricted only to the assumed ferrihydrite-bayerite phase, the spinel oxide is also modified. For instance,
the Fe3+(5) component fully disappears (is reduced to Fe2+), and the
quadrupole splittings of the Fe3+(3) and Fe3+(4) components are
noticeably reduced.
3.6. Redox properties
Fig. 6. Mössbauer spectra of samples HS-Mg70Fe30 (right), HS-Mg70Al25Fe5
(middle) and HS-Mg50Al45Fe5 (left) recorded: (a) as synthesized; (b) calcined at
500 °C and (c) treated with CO at 340 °C. Decomposition of spectra containing Fe2+
components is also displayed in row (c) for illustration.
The TPR measurements of all iron containing materials have
two characteristic peaks at temperatures lower than 900 °C
(Fig. 8, e.g. HS samples). For all of the samples, TPR signal did not
reach the base line until 1000 °C and presumably the complete
reduction of iron was not achieved. The hydrogen consumption
was represented per mol iron to enable the comparison of samples
with different iron content. All HS and LS samples with the same
initial chemical composition have almost the same TPR profiles
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
7
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Table 4
Mössbauer parameters and assignments of iron species recorded for samples HS-Mg70Al25Fe5 and LS-Mg70Al25Fe5 as synthesized; calcined at 500 °C and after treatment with
CO at 340 °C.
Sample
HS-Mg70Al25Fe5
Fe species
IS (mm s1)
QS (mm s1)
RI (%)
LS-Mg70Al25Fe5
LDH
Fe3+(sp)a
0.34
0.58
50
50
I/BL (a.u.)
IS (mm s1)
QS (mm s1)
RI (%)
0.34
0.67
50
50
7.36
Calcined at 500 °C
Fe3+(3)
Fe3+(4)
Fe3+(5)
0.24
0.30
0.44
0.81
1.47
0.80
44
25
31
8.17
0.23
0.29
0.43
0.79
1.45
0.76
28
45
27
13.66
Treated with CO at 340 °C
0.32
Fe3+
Fe3+
0.33
Fe2+
Fe2+
1.02
1.12
0.69
55
35
1.92
10
13.86
0.31
0.33
1.00
1.00
1.10
0.64
1.60
2.25
53
24
18
5
14.07
a
I/BL (a.u.)
14.00
Special fitting: the doublet is fitted assuming the same intensity but the same line width is not imposed (as otherwise is the common case at all the other fits).
Table 5
Mössbauer parameters and assignments of iron species recorded for samples HS-Mg50Al45Fe5 and LS-Mg50Al45Fe5 as synthesized; calcined at 500 °C and after treatment with
CO at 340 °C.
Sample
HS-Mg50Al45Fe5
Fe species
IS (mm s1)
QS (mm s1)
RI (%)
LS-Mg50Al45Fe5
LDH
Fe3+(1)
Fe3+(2)
Fe3+(5)
0.27
0.36
0.43
0.68
1.19
0.68
46
16
37
I/BL (a.u.)
IS (mm s1)
QS (mm s1)
RI (%)
0.34
0.35
0.61
1.00
58
42
10.89
Calcined at 500 °C
Fe3+(3)
0.24
Fe3+(4)
0.31
Fe3+(5)
0.43
0.85
1.47
0.84
39
32
29
8.23
0.25
0.31
0.44
0.81
1.42
0.80
42
35
23
14.69
Treated with CO at 340 °C
Fe3+
0.36
Fe3+
0.32
2+
Fe
0.82
2+
Fe
1.05
1.19
0.78
1.46
1.81
21
29
18
32
I/BL (a.u.)
11.40
0.35
0.36
0.91
0.96
1.22
0.72
1.61
2.23
30
41
20
9
14.82
12.21
Table 6
Mössbauer parameters and assignments of iron species recorded for samples HS- Mg70Fe30 and LS- Mg70Fe30 as synthesized; calcined at 500 °C and after treatment with CO at
340oC.
Sample
HS-Mg70Fe30
Fe species
IS (mm s
LDH
Fe3+(sp.)a
0.34
1
)
LS-Mg70Fe30
QS (mm s
0.54
1
)
RI (%)
I/BL (a.u.)
50
50
IS (mm s1)
QS (mm s1)
RI (%)
0.35
0.56
50
50
11.43
Calcined at 500 °C
Fe3+(3)
Fe3+(4)
Fe3+(5)
0.24
0.31
0.43
0.85
1.46
0.84
39
32
29
7.39
0.21
0.27
0.39
0.69
1.30
0.66
40
21
40
18.02
Treated with CO at 340 °C
Fe3+
0.30
Fe3+
0.32
Fe2+
0.94
Fe2+
1.02
1.01
0.63
1.20
1.82
12
14
64
10
11.15
0.32
0.35
0.94
1.05
18.21
a
I/BL (a.u.)
1.02
0.63
1.15
1.78
24
22
52
3
11.74
Special fitting: the doublet is fitted assuming the same intensity but the same line width is not imposed (as otherwise is the common case at all the other fits).
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
8
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
mation of solid solutions after calcination and in case of samples
with additional Bayerite phase, evidenced by both Mössbauer
and TPR analysis, extra LDH framework aluminum cations support
the reduction of iron. An increase in temperature of the first TPR
peak is followed by a decrease of the second reduction peak temperature (corresponding to the reduction of Fe2+ Fe0 and the rest
of Fe3+ Fe2+). This behavior favors the reduction of the first stage
(Fe3+ Fe2+). Since the limit for the incorporation of M(III) ions into
the LDH matrix is around x = 0.5, the samples with this M(III) ion
content have the weakest interaction among different Mg–
Al–Fe–O components and therefore the lowest temperature of
the first TPR maxima, compared to other Mg–Al–Fe containing
samples. On the contrary, the samples with x = 0.3 present the
samples with single LDH phase, the most intensive XRD peaks,
the strongest interaction among different Mg–Al–Fe–O components and in return the highest temperature of the first TPR
maxima.
3.7. Catalytic tests
The nitrous oxide molecule is quite stable at room temperature
and its simplest form of catalytic decomposition can be described
as an adsorption of N2O at the active center followed by a decomposition giving N2 and surface oxygen O (Eq. (4)). This surface
oxygen desorbs in combination with another oxygen atom or in direct reaction with another N2O (Eqs. (5) and (6)) [15]:
Fig. 8. TPR profiles of HS samples with different Mg:Al:Fe content.
with small differences in temperatures of both peak maxima listed
in Table 3.
The Mg–Fe samples with 30% of iron have two separated peaks,
first sharp and symmetrical and second broad and nonsymmetrical. This type of reduction behavior was already reported for Mg–
Fe–LDH derived mixed oxides with different iron amount between
10 and 50% [19] and with 50% of iron [39] concluding that the presence of Mg2+ cations retards the reduction of iron and suggesting
that the two reduction peaks in TPR profiles correspond to the
sequentional reduction of iron species from Fe3+ to Fe2+ in first
stage and from Fe2+ to Fe0 in second, presented with following
equations:
MgFe2 III O4 þ H2 ¼ Mg1x Fex II O þ H2 O
ð1Þ
Mg1x Fex II O þ H2 ¼ a Fe þ MgO þ H2 O
ð2Þ
On the contrary, the TPR profiles of the Mg–Al–Fe–LDH derived
mixed oxides with 5% of iron have broad and overlapping peaks
shifted to higher temperatures showing that the second reduction
stage begins before the first stage is completed. This indicates stronger interaction among different Mg–Al–Fe–O components of mixed
oxide. Similar reduction behavior was reported for Mg–Al–Fe–LDH
derived mixed oxides (with 50% of Mg and Fe content varied between 12.5% and 50%) [39] explaining the first stage as the partial
reduction of iron species from Fe3+ to Fe2+:
MgFeIII AlO4 þ H2 ¼ Mg1x FeIIx O þ H2 O þ MgðAlÞO
ð3Þ
and the second stage as the reduction from the rest of Fe3+ species
to Fe2+, proceeding according to Eq. (3), and the reduction of Fe2+
species to Fe0, proceeding according to Eq. (2).
The presence of aluminum intensifies interactions among different Mg–Al–Fe–O species, since it retards the reduction of iron.
This also agrees with the assumption derived from the thermal
analysis that the presence of aluminum stabilizes layered structure
of LDHs. It has also been reported that the presence of aluminum
cations inside LDH framework retards the reduction of iron [39],
but the reduction of complex mixed oxides is affected by the for-
N2 O þ ! N2 þ O
ð4Þ
N2 O þ O ! N2 þ O2 þ ð5Þ
2O $ O2 þ 2
ð6Þ
The experimental results of catalytic N2O decomposition are
presented in Fig. 9. Low iron containing samples (5%), independent
on Mg/Al–ratio, have very low N2O conversion. The increase of iron
content (30%) improves conversion and confirms the crucial role of
iron in the catalytic act, proved also with a very small conversion in
case of iron free sample (at 500 °C <0.04). This is in correspondence
with reduction behavior, since the decomposition of N2O is based
on the redox cycle Fe3+ Fe2+ [15]. The influence of Mg/Al ratio on
N2O decomposition for the samples with 5% iron cannot be established because no significant difference in the range of measurement accuracy was observed. These samples all have very low
conversion probably because, according to TPR results, their reduction cycle starts at temperatures above 450 °C. Better performance
of LS-Mg70Fe30 sample compared to HS-Mg70Fe30 sample can be
explained with their reduction behavior. Both samples have the
same temperature of the peak maxima in the first reduction stage
(Fe3+ Fe2+) but the temperature of the second reduction stage is for
the sample LS-Mg70Fe30 about 110 °C higher enabling better completion of the first reduction stage responsible for the catalytic
reaction. The Mössbauer analysis confirmed it also showing that
after calcination sample LS-Mg70Fe30 has higher amount (40%)
of the most easily reduced Fe3+ species Fe3+(5) component, than
HS-Mg70Fe30 sample (29%).
The experimental results of the catalytic N2O reduction with
NH3 are presented in Fig. 10. The other reaction path with reducing
agent has overall higher activity, since the presence of reducing
agent e.g. ammonia, boosts the removal of surface oxygen O⁄
[16] (Eq. (7)), which is the reaction rate determining step in catalytic decomposition of N2O [15].
2NH3 þ 3O $ N2 þ 3H2 O þ 3
ð7Þ
In this reaction path, the crucial role of iron is also confirmed,
but the activity of samples with lower iron content is generally
increased being dependent on the Mg/Al-ratio among the series.
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
9
1
Fig. 9. N2O decomposition at GHSV mod ¼ 2:17 cm3NTP g1
with all Mg–Al–Fe and Mg–Fe catalysts.
cat s
1
Fig. 10. N2O reduction with NH3 at GHSV mod ¼ 2:17 cm3NTP g1
with all Mg–Al–Fe and Mg–Fe catalysts.
cat s
Differentiation of lower iron content samples could be made in correspondence with reduction behavior supplemented with
Mössbauer analysis. Other sample properties such as BET surface,
acid properties and thermal behavior, do not influence the catalytic
behavior in both reactions significantly. The order of the activity
within the Mg–Al–Fe series follows the order obtained with TPR
analysis of the first reduction stage.
Better performance between the samples with the same chemical composition, synthesized with different methods, having the
same temperature of the first TRP maxima was already mentioned
for LS-Mg70Fe30 and HS-Mg70Fe30 samples in N2O decomposition reaction and explained with the strength of Mg–Fe–O interactions. This can also be applied within the Mg–Al–Fe–LDH derived
mixed oxide series taking into account the strength of Mg–
Al–Fe–O interactions. In case of HS-Mg50Al45Fe5 and LSMg50Al45Fe5 samples having the same temperature of the first
TRP maxima, catalytically better performing sample, HSMg50Al45Fe5, has higher temperature of the second reduction
stage and, as Mössbauer analysis showed, higher amount of
Fe3+(5) component (29% compared to 23% in LS-Mg50Al45Fe5)
responsible for the Fe3+ Fe2+ redox cycle.
4. Conclusions
Mg–Al–Fe and Mg–Fe layered double hydroxides were successfully synthesized by two different coprecipitation methods and in a
wide range of M(III) ions. The extended M(III) substitution influences the structure and surface properties of Mg–Al–Fe–LDHs
and their derived mixed oxides, weakens Mg-Al-Fe-O interactions
and improves catalytic behavior. Thermally activated ironcontaining LDHs exhibited catalytic activity in N2O decomposition
and reduction with NH3. This feature is mainly influenced by
reduction behavior and can be correlated with the presence of
Fe–O–Fe–O–Fe entities providing possibility for facilitated extraction of oxygen with simultaneous redox Fe3+ Fe2+ conversion.
The best results are obtained when the amount M(III) substitution
is near the limit for the incorporation into the LDH matrix (x = 0.5)
and the methastabile mixed oxides are formed with the weakest
interactions between constituent metals and oxygen. The presence
of small amount of additional Bayerite phase, e.g. extra-LDHframework aluminum (and iron), in x = 0.5 samples, positively
influences catalytic behavior, but higher concentrations of
extra-LDH-framework aluminum (x = 0.7) lead to the significant
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152
10
T.J. Vulic et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
formation of Bayerite phase and decrease catalytic activity. The
high supersaturation synthesis method, HS, provides LDHs with
more structural defects, especially in stacking of layers which
facilitates metastability of derived mixed oxides. This feature is
favorable for the catalytic activity in N2O decomposition and
reduction with NH3.
Acknowledgements
The financial support received from DAAD, 38th International
Seminar for Research and Teaching in Chemical Engineering and
Physical Chemistry, Universitaet Karlsruhe, Germany and from Serbian Ministry of Education and Science (Contract No. II145008) is
gratefully acknowledged.
References
[1] P.S. Braterman, Z.P. Xu, F. Yarberry, Layered double hydroxides (LDHs), in: S.M.
Auerbach, K.A. Carrado, P.K. Dutta (Eds.), Handbook of layered materials,
Marcel Dekker, Inc., New York, 2004, pp. 420–449.
[2] F. Li, X. Duan, Applications of layered double hydroxides, Struct. Bond. 119
(2006) 193–223.
[3] Z.P. Xu, J. Zhang, M.O. Adebajo, H. Zhang, C. Zhou, Catalytic applications of
layered double hydroxides and derivatives, Appl. Clay Sci. 53 (2011) 139–150.
[4] Kostas S. Triantafyllidis, Efrosyni N. Peleka, Vasilis G. Komvokis, Paul P. Mavros,
Iron-modified hydrotalcite-like materials as highly efficient phosphate
sorbents, J. Colloid Interf. Sci. 342 (2010) 427–436.
[5] A. Vaccari, Preparation and catalytic properties of cationic and anionic clays,
Catal. Today 41 (1998) 53–71.
[6] E.M. Serwicka, K. Bahranowski, Environmental catalysis by tailored materials
derived from layered minerals, Catal. Today 90 (2004) 85–92.
[7] S.P. Newman, W. Jones, in: W. Jones, C.N.R. Rao (Eds.), Supramolecular
Organisation and Material Design, Cambridge University Press, Cambridge,
UK, 2002, pp. 295–331.
[8] H. Wang, H. Yi, P. Ning, X. Tang, L. Yu, D. He, S. Zhao, Calcined hydrotalcite-like
compounds as catalysts for hydrolysis carbonyl sulfide at low temperature,
Chem. Eng. J. 166 (2011) 99–104.
[9] J. Oi, A. Obushi, A. Ogata, G.R. Bamwenda, R. Tanak, T. Hibino, S. Kushiyama, Zn,
Al, Rh-mixed oxides derived from hydrotalcite-like compound and their
catalytic properties for N2O decomposition, Appl. Catal. B: Environ. 13 (1997)
197–203.
[10] J. Perez-Ramirez, J. Overeijnder, F. Kaptejn, J.A. Moulijn, Structural promotion
and stabilizing effect of Mg in the catalytic decomposition of nitrous oxide
over calcined hydrotalcite-like compounds, Appl. Catal. B: Environ. 23 (1999)
59–72.
[11] K.S. Chang, H. Song, Y.-S. Park, J. -W. Woo, Analysis of N2O decomposition over
fixed bed mixed metal oxide catalysts made from hydrotalcite-type
precursors, Appl.Catal.A: Gen. 273 (2004) 223–231.
[12] L. Obalova, K. Pacultova, J. Balabanova, K. Jiratova, Z. Bastlc, M. Valaskova, Z.
Lacny, F. Kovanda, Effect of Mn/Al ratio in Co–Mn–Al mixed oxide catalysts
prepared from hydrotalcite-like precursors on catalytic decomposition of N2O,
Catal. Today 119 (2007) 233–238.
[13] J.J. Yu, J. Cheng, C.Y. Ma, H.L. Wang, L.D. Li, Z.P. Hao, Z.P. Xu, NOx
decomposition, storage and reduction over novel mixed oxide catalysts
derived from hydrotalcite-like compounds, J. Colloid. Interf. Sci. 333 (2009)
423–430.
[14] K. Karásková, L. Obalová, K. Jirátová, F. Kovanda, Effect of promoters in Co–Mn–
Al mixed oxide catalyst on N2O decomposition, Chem. Eng. J. 160 (2010) 480–
487.
[15] F. Kapteijn, J. Rodriguez-Mirasol, J.A. Moulijn, Heterogeneous catalytic
decomposition of nitrous oxide, Appl. Catal. B: Environ. 9 (1996) 25–64.
[16] B. Coq, M. Mauvezin, G. Dalay, S. Kieger, Kinetics and mechanism of the N2O
reduction by NH3 on a Fe-zeolite-Beta catalyst, J. Catal. 195 (2000) 298–303.
[17] G. Delahay, M. Mouvezin, A. Guzman-Vargas, B. Coq, Effect of the reductant
nature on the catalytic removal of N2O on Fe-zeolite-b catalysts, Catal.
Commun. 3 (2002) 385–389.
[18] T. Nobukawa, M. Yoshida, S. Kameoka, S.-I. Ito, K. Tomishige, K. Kunompri, InSitu observation of reaction intermediate in the selective catalytic reduction
of N2O with CH4 over Fe ion-exchanged BEA zeolite catalyst for the
elucidation of its reaction mechanism using FTIR, J.Phys.Chem.B 108 (2004)
4071–4079.
[19] J. Shen, B. Guang, M. Tu, Y. Chen, Preparationa and characterization of Fe/MgO
catalysts obtained from hydrotalcite-like compounds, Catal. Today 30 (1996)
77–82.
[20] Y. Ohishi, T. Kawabata, T. Shishido, K. Takaki, Q. Zhang, Y. Wang, K. Nomura, K.
Takehira, Mg–Fe–Al mixed oxides with mesoporous properties prepared from
hydrotalcite
as
precursors:
catalytic
behavior
in
ethylbenzene
dehydrogenation, Appl. Catal.A: Gen. 288 (2005) 220–231.
[21] P. Kustowsi, A. Rafalska-Lasocha, D. Majda, D. Tomaszewska, R. Dzimbaj,
Preparation and characterization of new Mg–Al–Fe oxide catalyst precursors
for dehydrogenation of ethylbenzene in the presence of carbon dioxide, Solid
State Ionics 141–142 (2001) 237–242.
[22] Y. Ohishi, T. Kawabata, T. Shishido, K. Takaki, Q. Zhang, Y. Wang, K. Nomura, K.
Takehira, Mg–Fe–Al mixed oxides with mesoporous properties prepared from
hydrotalcite as precursors: catalytic behaviour in ethylbenzene
dehydrogenation, Appl. Catal. A: Gen. 288 (2005) 220–231.
[23] S.M. Auer, J.-D. Grunwaldt, R.A. Koeppel, A. Baiker, Reduction of 4-nitrotoluene
over Fe–Mg–Al lamellar double hydroxides, J. Mol. Chem. A: Chem. 139 (1999)
305–313.
[24] P.S. Kumbhar, J. Sanchez-Valente, J.M.M. Millet, F. Figueras, Mg–Fe hydrotalcite
as a catalyst for the reduction of aromatic nitro compounds with hydrazine
hydrate, J. Catal. 191 (2000) 467–473.
[25] T. Vulic, A. Reitzmann, J. Ranogajec, R. Marinkovic-Neducin, The influence of
synthesis method and Mg-Al-Fe content on the thermal stability of layered
double hydroxides, J. Therm. Anal. Calorim. (2012), http://dx.doi.org/10.1007/
s10973-012-2230-9.
[26] O.P. Ferreira, O.L. Alves, D.X. Gouveia, A.G. Souza Filho, J.A.C. de Paiva, J.
Mendes Filho, Thermal decomposition and structural reconstruction effect on
Mg–Febased hydrotalcite compounds, J. Solid State Chem. 177 (2004) 3058–
3069.
[27] J.M. Fernandez, M.A. Ulibarri, F.M. Labajos, V. Rives, The effect of iron on the
crystalline phases formed upon thermal decomposition of Mg–Al–Fe
hydrotalcites, J. Mater. Chem. 8 (1998) 2507–2514.
[28] N. Benselka-Hadj Abdelkader, A. Bentouamib, Z. Derriche, N. Bettahar, L.-C. de
Ménorval, Synthesis and characterization of Mg–Fe layer double hydroxides
and its application on adsorption of orange G from aqueous solution, Chem.
Eng. J. 169 (2011) 231–238.
[29] K.S.W. Sing, J. Rouquerol, in: G. Ertl, H. Knoezinger, J. Wetkamp (Eds.),
Handbook of Heterogeneous Catalysis, vol. 2, VCH Verlagsgesellschaft mbH,
Weinheim, 1997, pp. 427–432. ISBN: 3-527-29212-8.
[30] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press
Inc. Ltd. LCCCN: 66-29432, London, UK, 1967.
[31] W.T. Reichle, S.Y. Kang, D.S. Everhardt, The nature of the thermal
decomposition of a catalytically active anionic clay mineral, J. Catal. 101
(1986) 352–359.
[32] F. Prinetto, G. Ghiotti, R. Durand, D. Tichit, Investigation of acidbase
properties of catalysts obtained from layered double hydroxides, J. Phys.
Chem. B 104 (2000) 11117–11126.
[33] S. Casanave, H. Martinez, C. Guimon, A. Auroux, V. Hulea, E. Dimitriu, AcidBase properties of MgCuAl mixed oxides, J. Therm. Anal. Cal. 72 (2003) 191–
198.
[34] M. Blume, Magnetic relaxation and asymmetric quadrupole doublets in the
Mössbauer effect, Phys. Rev. Lett. 14 (1965) 96–98.
[35] C.B. Koch, Structures and properties of anionic clay minerals, Hyp. Interact.
117 (1998) 131–157.
[36] J. Sanchez-Valente, J.M.M. Millet, F. Figueras, L. Fournes, Mössbauer
spectroscopic study of iron containing hydrotalcite catalysts for the
reduction of aromatic nitro compounds, Hyp. Interact. 131 (2000) 43–50.
[37] P. Kustrowski, A. Wegrzyn, A. Rafalska-Lasocha, A. Pattek-Janczyk, R. Dziembaj,
Substitution of Fe3+ for Al3+ cations in layered double hydroxide [LiAl2(OH)6]2
CO3nH2O, Clays Clay Min. 53 (2005) 18–27.
[38] J.W. Niemantsverdriet, A.M. van der Kraan, W.N. Delgass, Characterization of
surface phases in bimetallic FeRhSiO2 catalysts by in situ Mössbauer
spectroscopy at cryogenic temperatures, J. Catal. 89 (1984) 138–147.
[39] X. Ge, M. Li, J. Shen, The reduction of Mg–Fe–O and Mg–Fe–Al–O complex
oxides studied by temperature-programmed reduction combined with in situ
Mössbauer spectroscopy, J. Solid State Chem. 161 (2001) 38–44.
Please cite this article in press as: T.J. Vulic et al., Thermally activated iron containing layered double hydroxides as potential catalyst for N2O abatement,
Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.152