Homogeneous precipitation method preparation of modified red mud supported Ni mesoporous catalysts for ammonia decomposition

Article abstract of DOI:10.1039/c3cy00519d

Red mud modified by an acid digestion and alkali reprecipitation approach was employed as support for the preparation of Ni/MRM catalysts using the homogeneous precipitation method. The textural and structural properties of the as-received red mud (RM), the modified red mud (MRM) and the as-prepared Ni/MRM catalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray fluorescence (EDXRF), thermogravimetry-differential scanning calorimetry analysis (TG-DSC), Fourier transform infrared spectra (FT-IR), transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) and N₂ sorption techniques. The analysis results revealed a mesoporous nanocatalyst system with high surface area and uniform pore-size distribution. The results of the catalytic activity measurements showed that these mesoporous nanostructured Ni/MRM catalysts were very active for ammonia decomposition. The catalyst with 15% Ni loading and calcined at 600 °C exhibited the highest catalytic activity. Due to its unique properties and the feature of resource utilization of industrial solid waste, MRM holds great promise for developing catalysts and catalyst supports for applications in various catalytic reactions including catalytic ammonia decomposition. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00519d

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Homogeneous precipitation method preparation
of modified red mud supported Ni mesoporous
catalysts for ammonia decomposition
Cite this: Catal. Sci. Technol., 2014,
4, 361
a
a
b
b
a
*
*
Jian-Liang Cao, Zhao-Li Yan, Qing-Fang Deng, Zhong-Yong Yuan, Yan Wang,
Guang Sun,a Xiao-Dong Wang,a Bala Haria and Zhan-Ying Zhanga
Red mud modified by an acid digestion and alkali reprecipitation approach was employed as support for
the preparation of Ni/MRM catalysts using the homogeneous precipitation method. The textural and
structural properties of the as-received red mud (RM), the modified red mud (MRM) and the as-prepared
Ni/MRM catalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray fluorescence
(EDXRF), thermogravimetry-differential scanning calorimetry analysis (TG-DSC), Fourier transform infrared
spectra (FT-IR), transmission electron microscopy (TEM) combined with energy-dispersive X-ray spec-
troscopy (EDX) and N2 sorption techniques. The analysis results revealed a mesoporous nanocatalyst
system with high surface area and uniform pore-size distribution. The results of the catalytic activity
measurements showed that these mesoporous nanostructured Ni/MRM catalysts were very active for
ammonia decomposition. The catalyst with 15% Ni loading and calcined at 600 °C exhibited the highest
catalytic activity. Due to its unique properties and the feature of resource utilization of industrial solid
waste, MRM holds great promise for developing catalysts and catalyst supports for applications in various
catalytic reactions including catalytic ammonia decomposition.
Received 18th July 2013,
Accepted 14th October 2013
DOI: 10.1039/c3cy00519d
www.rsc.org/catalysis
ammonia decomposition, the high cost and limited availabil-
ity discourage their extensive applications. Much attention
1. Introduction
The on-site generation of hydrogen for fuel cells is of great
important for environmental protection consideration.1 The
direct generation of clean and cheap hydrogen motivates the
investigation on catalytic ammonia decomposition because
ammonia has a high hydrogen storage density, low price,
COx-free, complete production technology and transportation
technology and other advantages. On the other hand, the cat-
alytic removal of ammonia from gasification product gas or
direct ammonia fuel cells, are highlighted as well.2,3
has been recently been paid to base metal as catalysis, espe-
cially Ni-based catalysts, for the purpose to find an alternative
catalytic component to reduce using or even replace the
noble metal.9–11 Our recent studies10 have demonstrated that
the high surface area Ce0.8Zr0.2O2 solid solutions supported
Ni catalysts display high activity for ammonia decomposition
to hydrogen. Wang et al.11 investigated the role of support
(Al2O3, MgO and TiO2) in the high-performance Ni-based cat-
alytic system for ammonia decomposition.
Recently, many catalysts for ammonia decomposition have
been investigated, and the effects of active metals (Ru, Rh,
Pt, Pd, Ni, Fe, Co and Mo) and support materials (carbon
nanotubes, activated carbon, Al2O3, MgO, ZrO2 and TiO2) on
the catalysis of ammonia decomposition were studied.4–9
Although the precious metal catalysts have high activity for
Red mud, an aluminum industry waste, is a serious pollu-
tion hazardous material due to its high alkalinity in case of
disposal. In general, red mud is a very fine material in terms
of particle size distribution, having an average particle size
<10 μm. It is primarily composed of iron, aluminum, tita-
nium, silicon, calcium oxides and hydroxides. The treatment
and resource utilization of red mud has received widespread
attention. Due to its stability, high surface area, sintering
resistance and resistance to poisoning, red mud has been
employed as catalyst and catalyst support for various catalytic
reactions, such as hydrogenation,12–16 hydrodechlorination17–19
and exhaust gas clean-up.20–23 Ng et al.24 have employed red
mud supported Ru as catalyst for ammonia decomposition
and proved that it possesses high activity.
a State Key Laboratory Cultivation Base for Gas Geology and Gas Control, School
of Materials Science and Engineering, School of Safety Science and Engineering,
Henan Polytechnic University, Jiaozuo 454000, China.
E-mail: caojianliang@gmail.com, yanwang@hpu.edu.cn; Fax: +86 391 3986901;
Tel: +86 391 3986952
b Institute of New Catalytic Materials Science, Key Laboratory of Advanced Energy
Materials Chemistry (Ministry of Education), College of Chemistry, Nankai
University, Tianjin 300071, China
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In this work, we proposed an application of modified red
under vigorous stirring. The mixture reacted at 95 °C on a
Heating Magnetic Stirrer for 10 h. Subsequently, the solution
and the resulting precipitate was washed several times with
distilled water until pH ≈ 7, washed again with anhydrous
ethanol, dried in the oven at 110 °C for 12 h, then milled and
calcined at 600 °C for 2 h. The content of Ni was 0, 3, 6, 9,
12, 15, 18 wt.%, and the corresponding catalysts were denoted
as x%-Ni/MRM (x = 0, 3, 6, 9, 12, 15 and 18). In order to make
clear the influence of the calcination temperature on the cata-
lyst property, a series of Ni/MRM catalysts calcined at differ-
ent temperature were prepared in the similar manner.
mud (MRM) as support for Ni catalysts in catalytic ammonia
decomposition. In view of the strong basicity and structural
complexity of red mud, dilute acid digestion and alkali
reprecipitation (same as sol–gel method) were applied to the
red mud sample for modifying the surface and crystal struc-
ture, similar to the reported approach of Sushil and Batra.23
The MRM sample was employed as support for the prepara-
tion of Ni/MRM catalysts by homogeneous precipitation
method. The textural and structural properties of the red
mud and the as-prepared Ni/MRM catalysts were character-
ized by means of XRD, EDXRF, TG-DSC, FT-IR, N2 sorption
and TEM-EDX. The effect of the Ni loading amount and calci-
nation temperature on the catalytic ammonia decomposition
activities of the mesoporous Ni/MRM catalysts were investi-
gated in detail. As a result, the NH3 conversion of 97.9% with
2.2 Characterization
X-ray diffraction (XRD) analysis was performed on a Bruker-
AXS D8 Advance diffractometer, with CuKα radiation at 40 kV
and 25 mA in a scanning range of 10–80° (2θ). The diffraction
peaks of the crystalline phase were compared with those of
standard compounds reported in the JCPDS Date File. The
chemical composition of samples were carried out by means
of energy dispersive X-ray fluorescence (EDXRF) spectrometer
(Panalitical Magix, PW2403). Thermogravimetry-differential
scanning calorimetry analysis (TG-DSC) was done on a Setaram
SETSYS Evolution 24 thermogravimetric analyzer. The sample
was heated with a rate of 15 °C min−1 from 20 °C to 1000 °C in
pure argon. Fourier transform infrared spectra (FT-IR) were
measured on a Bruker V70 spectrometer, with the KBr pellet
technique, and the ranges of spectrograms were 4000–400 cm−1.
N2 adsorption–desorption isotherms were collected at liquid
nitrogen temperature using a Quantachrome AsiQM0000-3
sorption analyzer. Before carrying out the measurement,
each sample was degassed at 180 °C for more than 6 h. The
specific surface areas (SBET) of the samples were calculated
following the multi-point BET (Brunauer–Emmett–Teller)
procedure. The pore size distributions were determined from
the adsorption branch of the isotherms using the DFT
method. Transmission electron microscopy (TEM) combined
with energy-dispersive X-ray spectroscopy (EDX) analysis
was performed on a JEOL JEM-2200FS microscope, operating
at 200 kV. The samples were dispersed in ethanol and treated
with ultrasound for 5 min, and then deposited on a copper
grid coated with preformed holey carbon film.
the H2 formation rate of 32.8 mmol min−1 gcat was achieved
−1
at 700 °C over the Ni catalyst supported on the mesoporous/
high-surface-area MRM.
2. Experimental section
2.1 Catalysts’ preparation
The red mud (RM) sample was collected from a downstream
slurry pond in Henan Zhongmei Aluminum Corporation, China
(the main chemical composition is showed in Table 1). Further-
more red mud exhibits strong alkaline property (pH = 10.26),
due to the sodium oxide.25 The sample was dried, ground and
sieved to a size of <150 μm.
In a typical process of red mud modification, 25 g of red
mud sample was taken and 100 ml distilled water was added
to it. After stirring for 5 min, 150 ml of 6 M HCl solution was
added. The sample was digested at 85 °C on a water-bath
heater for 2 h. The digested sample was subsequently pre-
pared by sol–gel method, as described below. It was precipi-
tated by adding aqueous ammonia and simultaneous stirring
until pH ≈ 8, and kept for 30 min. The solution and the
resulting precipitate was aged at 70 °C for 30 min, sonicated
for 10 min, washed several times with warm distilled water
until pH ≈ 7, and then washed again with anhydrous etha-
nol. Finally, the precipitate was separated by filtration, dried
for 12 h in air at 110 °C, but not calcined. The as-prepared
sample was modified red mud, and denoted as MRM.
2.3 Catalytic testing
The catalysts for NH3 decomposition were prepared by
homogeneous precipitation method. At room temperature,
the calculated amount of Ni(NO3)2·6H2O and an excess of
urea (the mole ratio of Ni and urea was 1 : 4) were dissolved
in 200 ml of distilled water and then mixed with 6 g of MRM
Ammonia decomposition experiments were carried out in a
continuous-flow fixed-bed microreactor. And the reactor tube
was a quartz tube with an inner diameter of 6 mm, loaded
with about 0.1 g of powder catalyst deposited between two
Table 1 The main chemical composition of RM, MRM and 600 °C-calcined 15%-Ni/MRM catalyst
Main composition (wt.%)
Samples
Al2O3
SiO2
Fe2O3
TiO2
K2O
Na2O
CaO
MgO
NiO
0.062
<0.001
19.64
RM
MRM
15%-Ni/MRM
23.25
29.97
26.55
16.94
21.9
19.41
15.05
19.4
17.23
4.27
5.48
4.87
1.82
1.77
1.56
3.72
0.12
0.1
16.94
1.93
1.78
1.93
0.73
0.69
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quartz wool plugs. Before the measurement, the prepared
catalyst sample was reduced in situ in a H2 flow for 2 h and
then purged with a flow of pure Ar. The pure NH3 was used
as the reactant at a flow rate of 50 mL min−1 at atmospheric
pressure. The conversion was measured at steady state after
15 min. The products were analyzed online by a SP-6800A6
gas chromatograph equipped with a thermal conductivity
detector and Poropak Q column, using N2 as the carrier gas.
The H2 formation rate was translated from the H2 content in
the outer gas, and the catalytic activity was described as the
conversion of ammonia.
excess of Ni species and the growth of the NiO nanoparticles
in the Ni/MRM systems. The average size of NiO particles in
the 12%-Ni/MRM catalyst calcined at 600 °C, calculated from
the NiO (2 2 0) reflection peak (2θ = 62.4°) by the Scherrer for-
mula, is 14 nm. While the average size of NiO particles in the
15 and 18%-Ni/MRM catalysts calcined at 600 °C, calculated
from the NiO (2 0 0) reflection peak (2θ = 43.3°) by the
Scherrer formula, are 17 and 23 nm, respectively.
The 15%-Ni/MRM sample was taken as being represen-
tative for the investigation of the thermal stability of the
as-prepared Ni/MRM catalysts. Fig. 2 shows the XRD patterns
of 15%-Ni/MRM catalysts calcined at different temperature
(500, 600 and 700 °C). From Fig. 2, we can see that goethite
(FeOOH) is not completely transformed into oxide in the sam-
ples of 15%-Ni/MRM catalyst calcined at 500 °C. This indi-
cated that only when the calcination temperature reached
600 °C the samples can accomplish crystal phase transition.
While, with the increase of the precalcination temperature of
the Ni/MRM catalysts from 500 to 700 °C, a progressive
increase in the relative intensity of the lines of hematite,
aluminum oxide (γ-Al2O3) and quartz (SiO2) phase is seen,
indicating an increase of the degree of crystallinity and the
growth of particle size by the thermal treatment.
Table 1 shows the main chemical composition of RM, MRM
and 600 °C-calcined 15%-Ni/MRM catalyst. From Table 1, we
can see that the content of Al2O3, SiO2, Fe2O3, TiO2 and K2O
has changed little, and the inert components (Na2O and CaO)
were mainly removed. As for NiO, its content in RM itself is
very low, as well as that in MRM. When supported Ni species
with the theoretical content of 15 wt.% (calculated as Ni), we
got the practical content of 19.64 wt.% (calculated as NiO),
which is equivalent to the theoretical value.
The TG-DSC results of RM and the uncalcined 15%-Ni/MRM
catalyst are shown in Fig. 3. The TG-DSC curves of RM show a
continuous weight loss of 13% from 20 to 1000 °C (Fig. 3a).
The weight loss of 1% from 20 to 233 °C can be attributed to
3. Results and discussion
Fig. 1 shows the XRD patterns of RM and Ni/MRM catalysts
with different Ni loading amounts calcined at 600 °C. It can
be seen clearly from Fig. 1 that RM is a complex mixture of
phases mainly comprised of hydrogarnet (3CaO·Al2O3·SiO2·4H2O),
cancrinite (Na6CaAl6Si6(CO3)O24·2H2O), hematite (Fe2O3), goethite
(FeOOH), corundum (Al2O3), gibbsite (γ-Al(OH)3), diaspore
(AlO(OH)), perovskite (CaTiO3), quartz (SiO2), rutile (TiO2) and
calcite (CaCO3). Meanwhile, the main phases in MRM-600 are
hematite (Fe2O3), aluminum oxide (γ-Al2O3) and quartz (SiO2).
After modified and calcined at 600 °C, complex group of com-
pounds in RM, such as hydrogarnet and cancrinite, are
decomposed. And, most Na and Ca have been removed. When
the Ni content is below 12%, no reflections characteristic of
NiO structure presented, which may be due to the high disper-
sion of the Ni species with too small particle sizes on the sur-
face of the support to be identified by the conventional X-ray
diffraction method. When the Ni species content increased to
12%, the weak diffraction peaks attributed to NiO crystal
phase appeared at 37.7, 43.3 and 62.4° (2θ), indicating the
Fig. 1 XRD patterns of RM (1, hydrogarnet (3CaO·Al2O3·SiO2·4H2O);
2, cancrinite (Na6CaAl6Si6(CO3)O24·2H2O); 3, hematite (Fe2O3); 4, goethite
(FeOOH); 5, corundum (Al2O3); 6, gibbsite (Al(OH)3); 7, diaspore (AlO(OH));
8, perovskite (CaTiO3); 9, quartz (SiO2); 10, rutile (TiO2); 11, calcite (CaCO3))
and Ni/MRM catalysts with different Ni loading amounts calcined at
600 °C (1, hematite (Fe2O3); 2, aluminum oxide (γ-Al2O3); 3, quartz
(SiO2); 4, bunsenite (NiO)).
Fig. 2 XRD patterns of 15%-Ni/MRM catalysts calcined at different
temperature: 1, hematite (Fe2O3); 2, aluminum oxide (γ-Al2O3); 3, quartz
(SiO2); 4, bunsenite (NiO); 5, goethite (FeOOH).
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the physical water content of RM by taking DSC curve into
account which produced an endothermic peak at 84.4 °C.
Furthermore the weight loss of 2.8% from 233 to 335 °C is
due to the decomposition of gibbsite,26–28 partial boehmite
(formed from the slight decomposition of gibbsite)28 and par-
tial goethite27,28 that was deduced from the interpretation of
the endothermic peak at 289.8 °C in DSC. Another endother-
mic peak is observed at 552.6 °C in DSC. This endothermic
peak at these temperatures from 335 to 645 °C with the total
mass loss of 5.6%, refers to the decomposition of boehmite
(formed from the slight decomposition of gibbsite),26,27
diaspore26 and goethite. An endothermic peak starting at
about 645 °C and ending at 783 °C is seen at 742 °C in DSC,
resulted from the decomposition of calcite.26,28 And this cal-
cite decomposition caused 3% of the mass loss. After 783 °C,
the endothermic peak occuring at 898.3 °C in DSC is related
to sodium aluminum carbonate silicate decomposition in the
red mud.26 Fig. 3b shows the TG-DSC curves of the uncalcined
15%-Ni/MRM catalyst, which have a continuous weight loss of
22.7% from 20 to 1000 °C. The weight loss of 8.7% from 20 to
209 °C can be attributed to the physical water and the decom-
position of partial Ni(OH)2 by taking the DSC curve into
account which produced an endothermic peak at 91.4 °C. Two
weak endothermic peaks are observed at 228.2 and 256.3 °C
in DSC. The endothermic peaks at these temperatures from
209 to 332 °C together with the mass loss of 6.6%, refer to
the decomposition of gibbsite, partial boehmite (formed from
the slight decomposition of gibbsite) and partial goethite. The
endothermic peak starting at about 332 °C and ending at
612 °C seen at 559.2 °C in the DSC resulted from the decomposi-
tion of boehmite (formed from the slight decomposition of
gibbsite), diaspore and goethite. What's more, the decomposi-
tion of goethite to hematite and the decomposition of gibbsite
to γ-Al2O3 are believed to occur (consistent with the XRD analysis
results in Fig. 1). Finally, it can be observed that the mass loss of
the uncalcined 15%-Ni/MRM catalyst has almost completed at
600 °C (consistent with the XRD analysis results in Fig. 2).
3650–3200 cm−1 corresponds to OH vibrations and appears as
a result of hydration due to the presence of H2O in RM, as
well as of higher intensities in 15%-Ni/MRM at 3451 cm−1.29
The peak at 1425 cm−1 represents the existence of CaCO3 in
RM,26 which disappears owing to its reaction with HCl after
acid treatment. The stretching vibrations of Fe–O bands of
hematite structure are detected in RM at the peaks at 1630,
540 and 476 cm−1,26 and show higher intensities at 1630 cm−1,
equal at 559 and 486 cm−1 in 15%-Ni/MRM catalyst. What's
more, for 15%-Ni/MRM catalyst, it is a remarkable fact that
compared with standard FT-IR spectra of hematite, the inten-
sity of the peak at 486 cm−1 is much higher than that of the
peak at 559 cm−1, which may be because the peak at 486 cm−1
not only represents hematite but also represents NiO. Vibra-
tion relating to goethite can be observed in RM at 756 cm−1,23
and disappears in 15%-Ni/MRM. The peak at 1003 cm−1 in
RM is for the stretching vibration of Si–O band29 and of lower
intensities in 15%-Ni/MRM at 1015 cm−1. The Al–O bands of
the corundum structure at 683 and 621 cm−1 in the spectrum
of RM disappear after modification.26,29 And the stretching
vibration relating to γ-Al2O3 can be observed in 15%-Ni/MRM
sample at the peak of 719 cm−1. This may be due to the fact
that the Al in RM is partially solubilized after acid treatment,
and translates into gibbsite after alkaline reprecipitation, sub-
sequently translates into γ-Al2O3 after calcination (consistent
with the TG-DSC results in Fig. 3b). These results are consis-
tent with the XRD analysis results of RM and 600 °C-calcined
15%-Ni/MRM catalyst.
Fig. 5 presents the N2 adsorption–desorption isotherms
and the corresponding pore size distribution curves of RM and
Ni/MRM catalysts with different Ni loading amounts calcined
at 600 °C. Their textural properties are provided in Table 2.
From Fig. 5 and Table 2, we can see that the as-received red
mud has a very small surface area, substantially non-porous
structure. However, the surface area and pore volume of
600 °C-calcined MRM and Ni/MRM catalysts are all enlarged
noticeably compared with that of RM. The isotherms of all
the prepared samples are of classical type IV, characteris-
tic of mesoporous materials according to the IUPAC. The
The FT-IR spectra of RM and 600 °C-calcined 15%-Ni/MRM
catalyst are shown in Fig. 4. The broad peak at the region of
Fig. 3 TG-DSC curves of (a) RM and (b) the uncalcined 15%-Ni/MRM catalyst.
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loop is typical for wormhole-like mesostructures and hierarchi-
cal scaffold-like mesoporous structures formed by surfactant-
assisted nanoparticle assembly.30 The pore size distribution
curves of 600 °C-calcined MRM and Ni/MRM catalysts with
different Ni contents are determined by the DFT method from
the adsorption branch of the isotherms, exhibit a narrow peak
centered at 3.8–7.5 nm (Fig. 5), indicating the good homogene-
ity of the pores. All the samples possess high surface areas of
about 200 m2 g−1 (Table 2). With the increase of Ni content
from 3 to 18%, the pore volume increased from 0.34 to
0.77 cm3 g−1. This illustrates that the homogeneous precipita-
tion method has a positive impact on the textural properties of
the prepared catalysts.
Fig. 6 depicts the N2 adsorption–desorption isotherms and
the corresponding pore size distributions of the 15%-Ni/MRM
catalysts calcined at different temperature. The textual proper-
ties of the samples are also listed in Table 2. The pore size dis-
tributions of the 15%-Ni/MRM catalysts calcined at different
temperature are also narrow (Fig. 6). With the increase of the
calcination temperature, the surface areas of these catalysts
decreased from 259 to 180 m2 g−1 (due to the particle sintering,
Table 2). Interestingly, a high surface area of 180 m2 g−1 can
still be obtained after calcination at 700 °C for 2 h, indicating
the high thermal stability of these modified red mud supported
Ni catalysts.
Fig. 4 FT-IR spectra of RM and 600 °C-calcined 15%-Ni/MRM catalyst.
Fig. 7 shows the TEM-EDX images of the 600 °C-calcined
15%-Ni/MRM catalyst. The composition of this sample is con-
firmed by the EDX spectrum (as shown in Fig. 7b), which
reveals the presence of Fe, Al, Si, O and Ni on the surface of
the 600 °C-calcined 15%-Ni/MRM catalyst (Cu peaks from the
TEM grid). And the EDX Ni element map (Fig. 7d) demon-
strates the high dispersion of Ni species in the catalyst. The
HRTEM image (Fig. 7e) demonstrates that the 15%-Ni/MRM
Fig.
5 (Left) N2 adsorption–desorption isotherms and (right) the
corresponding pore size distribution curves of RM and Ni/MRM
catalysts with different Ni loading amounts calcined at 600 °C.
catalyst possesses
a disordered wormhole-like mesopore
structure (illustrated in combination with the H2 type of the
hysteresis loop in Fig. 5), formed by the agglomeration of the
nanoparticles. The accessible pores are connected randomly,
lacking discernible long-range order in the pore arrangement
among the small particles, being in good agreement with the
N2 adsorption–desorption isotherms. In addition, we observe
well-defined hysteresis loops with a steep desorption branch
and less steep adsorption branch belong to the H2-type, indi-
cating that the effective radii of the mesoporous bodies are het-
erogeneously distributed and the effective radii of the narrow
entrances are all of equal size.30 The H2 type of the hysteresis
Table 2 The physical properties and catalytic activities of the investigated catalysts
Calcination
temperature (°C)
Surface areaa
Vtot
DDFT-ads
(nm)
Conversionc
(%)
H2 productionc
Ea
b
Samples
(m2 g−1
)
(cm3 g−1
)
(mmol min−1 gcat
)
(kJ mol−1
)
−1
RM
MRM
MRM-600
110
110
600
600
600
600
600
600
600
500
700
8
225
190
199
205
221
214
219
217
259
180
0.08
0.39
0.39
0.34
0.49
0.61
0.67
0.76
0.77
0.81
0.66
14.6
5.0
4.8
3.8
5.7
7.5
7.5
7.5
7.5
7.5
7.5
—
—
—
—
—
—
63.4
69.3
72.0
75.9
93.2
97.9
82.0
78.8
51.5
21.2
23.2
24.1
25.4
31.2
32.8
27.5
26.4
17.2
170.7
116.7
96.4
91.4
77.1
70.3
83.2
82.3
92.6
3%-Ni/MRM
6%-Ni/MRM
9%-Ni/MRM
12%-Ni/MRM
15%-Ni/MRM
18%-Ni/MRM
15%-Ni/MRM
15%-Ni/MRM
a
b
c
Multi-point BET surface area. Maximum of DFT pore diameter as determined from the adsorption branch. The data were measured at
700 °C under pure NH3 flow of 50 ml min−1
.
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It is likely that a part of NiO nanoparticles assemble into the
flocculent structure on the surface of the support during the
loading process.
The catalytic activities of the prepared Ni/MRM catalysts
for NH3 decomposition as a function of reaction temperature
are presented in Fig. 8 and 9. The results show a similar
behavior that the ammonia conversions increased with the
increase of the catalytic reaction temperature measured in
the catalyst bed for all the catalysts, indicating faster decom-
position at higher temperature. The data of ammonia conver-
sion and H2 production are shown in Table 2.
Fig. 8 shows the catalytic activities of the Ni/MRM cata-
lysts with different Ni content and calcined at 600 °C. The cat-
alytic activity of pure 600 °C-calcined MRM support is also
included in the same figure for comparison purpose. It is
noted from Fig. 8 that the activity of MRM-600 is low (63.4%
of ammonia was decomposed at 700 °C), while the activities
of all the Ni/MRM catalysts are higher than that of pure MRM.
The pure MRM support also shows some catalytic activity,
which is due to the support's large surface area and pore
volume, and its main components (Fe2O3, Al2O3 and SiO2)
play an important role for ammonia decomposition. After
supported Ni species, the activity of the catalyst increased
Fig.
6 (Left) N2 adsorption–desorption isotherms and (right) the
corresponding pore size distribution curves of 15%-Ni/MRM catalysts
calcined at different temperature. The absorbed volume was shifted
by 200 and 400 units for the curves of data sets 15%-Ni/MRM-600
and 15%-Ni/MRM-700, and the dV/dD value was shifted by 0.15
and 0.3 units for the curves of data sets 15%-Ni/MRM-600 and
15%-Ni/MRM-700, respectively.
some flocculent structure in the low-resolution TEM images
(Fig. 7a). From the HRTEM image (Fig. 7e), we can see that
the flocculent structure is assembled from the nanoparticles.
Fig. 7 TEM-EDX images of 600 °C-calcined 15%-Ni/MRM catalyst: (a) TEM image, (b) EDX spectrum related to (a), (c) BF TEM image, (d) EDX Ni
element map related to (c), (e) HRTEM image.
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Paper
obviously (97.9% NH3 decomposition for the most active cata-
lyst). This highlights that there is a synergistic effect between
Ni species and the MRM support, which strongly affects the
catalytic activity in ammonia decomposition. In comparison
with the reported results of red mud support Ru catalysts for
ammonia decomposition,24 the high surface area and porosity
should respond for the high activity of the Ni/MRM catalysts.
This is in agreement with the reported results that the cata-
lytic decomposition of ammonia is sensitive to the textural
and structural property of the catalyst.10,31–33 The catalytic
activities of Ni/MRM catalysts enhanced with the increase of
Ni loadings from 3 to 15%, and the 15%-Ni/MRM catalyst
exhibited the highest catalytic activity (97.9% NH3 conversion
and 32.8 mmol min−1 gcat−1 H2 production at reaction temper-
ature of 700 °C). This catalytic activity is comparable to the
Ni/MCM-41 (TIE) catalyst,32 which showed 98.8% NH3 conver-
sion and 33.1 mmol min−1 gcat−1 H2 production at reaction tem-
perature of 700 °C. At the same time, when the Ni loadings
increased from 0 to 9%, the catalytic activities enhanced
slightly. However, once the Ni loadings reached 12%, the cata-
lytic activities soared to 93.2% NH3 conversion at reaction tem-
perature of 700 °C. A further increase of the Ni content (18%)
resulted in the significant decrease of the catalytic activity. This
indicate that the finely dispersed Ni species should be respon-
sible for the activity, whereas the excess of Ni species and the
growth of the NiO nanoparticles when the high loading maybe
have a negative effect on the catalytic activity of ammonia
decomposition, supported by the XRD results (Fig. 1).
Fig. 9 The activities of 15%-Ni/MRM catalysts calcined at different
temperature.
of the catalyst, further reducing the surface area and total
pore volume, and then decreasing the catalyst's activity.
When the reaction temperature is below 450 °C, the catalytic
activities of the catalysts calcined at 500–700 °C are close
to each other. When the reaction temperature is increased
further, the catalyst calcined at 600 °C shows higher activity
than that of the 500 and 700 °C-calcined samples. The 600 °C-
calcined 15%-Ni/MRM catalyst, which has the high surface
area of 219 m2 g−1 and the nano-scale particle size, exhibits
the highest catalytic activity for ammonia decomposition. It
is believed that the high activity of these Ni/MRM catalysts
correspond to their high surface area and mesoporous
structure. The mesoporous frameworks provides a large sur-
face to volume ratio, and hence more active sites are available
for ammonia decomposition, which is beneficial for achieving
enhanced catalytic activity. The TG-DSC data have indicated
that after 600 °C calcination, the 15%-Ni/MRM catalyst has
reached the thermal balance. Hence, the calcination tempera-
ture of 600 °C is suitable.
Fig. 9 displays the catalytic performance for ammonia
decomposition of 15%-Ni/MRM catalysts calcined at different
temperature. It is seen that the catalytic activities of the
15%-Ni/MRM catalysts increased with the increased pre-
calcination temperature from 500 to 600 °C. However, when
the precalcination temperature increased to 700 °C, the cata-
lytic activities became particularly low (even lower than that of
the pure MRM support). This indicates that the high calcina-
tion temperature will result in the growth and agglomeration
The apparent activation energies (Ea) were obtained
from the Arrhenius relationship between the rate constant (k)
and the temperature (T), which can be described by the
equation:31 ln(k) = −Ea/RT + constant, and the results are
listed in Table 2. It is seen that the MRM support had the
highest activation energy (170.7 kJ mol−1). After supported
the active species Ni (3%), the activation energy decreased to
116.7 kJ mol−1, and correspondingly the ammonia conversion
increased from 63.4% to 69.3% at 700 °C. By increasing the
Ni contents, the catalyst of 15%-Ni/MRM showed the highest
activity and the lowest activation energy (70.3 kJ mol−1) in the
present work. This activation energy is comparable to that of
the Ni/CeO2 catalysts,10 and much lower than most of the
published Ni-based catalysts.10,32
4. Conclusions
Mesoporous nanostructured Ni/MRM catalysts with high
surface areas and narrow mesopore size distributions have
Fig. 8 The activities of the Ni/MRM catalysts calcined at 600 °C with
different Ni loading.
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been successfully prepared by the homogeneous precipitation
method. The resulting Ni/MRM catalysts are found to be
active in catalytic ammonia decomposition, and the catalytic
activity comparable to that of the previously reported Ni-based
catalysts systems. The content of Ni and the precalcination
temperature can affect the catalytic properties of the catalysts
remarkably. The 600 °C-calcined 15%-Ni/MRM catalyst has
the highest activity. The synergistic effect between Ni and
nanostructural MRM support, the highly dispersed Ni species,
and high surface area of the catalysts were responsible for
the high catalytic activity and hydrogen production. This
investigation demonstrates the feasibility of utilizing the
waste from the aluminum industry for the decomposition of
ammonia to hydrogen.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51172065), China Postdoctoral Science
Foundation funded project (2012M521394), Specialized
Research Fund for the Doctoral Program of Higher Education
(20124116120002), State Key Laboratory Cultivation Base for
Gas Geology and Gas Control (WS2013B03), Program for
Innovative Research Team in the University of Henan
Province (2012IRTSTHN007), Foundation of He'nan Scientific
and Technology Key Project (112102310425, 112102310029,
132102210251), the Education Department Natural Science
Foundation of He'nan Province (2011B150009, 13A430315).
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368 | Catal. Sci. Technol., 2014, 4, 361–368
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