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Effect of the ceria–alumina composite supporton the Mo-based catalyst’s sulfur-resistant activityfor the synthetic natural gas process
Baowei Wang • Yuguang Shang • Guozhong Ding •
Jing Lv • Haiyang Wang • Erdong Wang •
Zhenhua Li • Xinbin Ma • Shaodong Qin • Qi Sun
Received: 14 January 2012 / Accepted: 22 April 2012 / Published online: 24 May 2012
� Akademiai Kiado, Budapest, Hungary 2012
Abstract Ceria–alumina composite supports were prepared by the co-precipita-
tion (cop), impregnation (imp) or deposition–precipitation (dp) methods. Co–Mo
catalysts supported on these composite supports were prepared by the imp method
and their catalytic activities for sulfur-resistant methanation of synthesis gas were
investigated. The catalysts were characterized by nitrogen adsorption, X-ray dif-
fraction (XRD), and hydrogen temperature-programmed reduction (TPR). It was
found that the preparation method of ceria–alumina composite support had a marked
influence on the surface area, the interaction between ceria and alumina, and the
catalytic performance for sulfur-resistant methanation. Among them, the ceria–
alumina composite support prepared by dp method achieves the best methanation
activity due to its smaller ceria particle size, better ceria dispersion, weak interaction
between ceria–alumina as suggested by XRD and TPR results.
Keywords CeO2 � Al2O3 � Composite support � Co–Mo catalyst �Sulfur-resistant methanation
Introduction
With dwindling oil resources, oil prices straight up, the advantage of alternative
chemical product synthesis from coal instead of petrochemicals has become
increasingly evident. In addition, more and more attention and efforts were paid to
environmental issues and energy conservation; the advantages of natural gas as a
B. Wang (&) � Y. Shang � G. Ding � J. Lv � H. Wang � E. Wang � Z. Li � X. Ma
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical
Engineering and Technology, Tianjin University, Tianjin 300072, China
e-mail: [email protected]
S. Qin � Q. Sun
National Institute of Clean and Low Carbon Energy, Beijing 102209, China
123
Reac Kinet Mech Cat (2012) 106:495–506
DOI 10.1007/s11144-012-0452-2
clean energy source have become increasingly prominent [1–3]. Unfortunately,
natural gas is in short supply and there are dual needs of clean environment and
economic development. As an effective way to use coal, coal-gas, which is the best
way of coal-based energy production, has the highest energy efficiency and
development prospects [4]. There are two main gas methanation process routes [5]:
The first is non-sulfur-resistant methanation process using nickel-based catalyst,
which is very active for the methanation of syngas and achieves the purpose of
hydrogenation of carbon monoxide under atmospheric pressure. However, Ni-based
catalysts used for this process are extremely sensitive to sulfur, requiring the raw
gas to be desulfurized to have sulfur compounds less than 0.1 ppm [6]. The second
route uses sulfur-resistant methanation catalysts. The uppermost advantage of this
route is that the desulfurization process can be avoided, which greatly simplifies the
process and cuts the overall investment for industrialization [7].
Sulfur-resistant methanation reactions usually use molybdenum-based catalysts
and the mechanism of the methanation reaction on molybdenum-based catalysts was
reported. Hou and Wise [8] proposed that the CO methanation reaction includes the
following two basic reactions (1) and (2), while Happel and Hnatow [9–11] thought
that the sulfur-resistant methanation reaction is carried out according to reaction (3),
which is known as direct methanation [5].
COþ 3H2 ! CH4 þ H2O ð1ÞCOþ H2O! CO2 þ H2 ð2Þ
The overall reaction: 2COþ 2H2 ! CH4 þ CO2 ð3ÞCurrently, Mo-based catalysts are widely used in the sulfur-resistant water–gas
shift process, hydro-desulfurization process and methanation process [12–14].
However, Mo-based sulfur-resistant methanation activity is lower than that of Ni-
based catalysts, which is a major problem to solve. To improve the methanation
activity, Co, Ni, W, Ru or La can be potential additives [4, 15] and CeO2, ZrO2,
SiO2, Al2O3 co-carriers [16]. Co is usually the first choice as an additive of Mo-
based catalysts, which is evidenced by the fact that Co–Mo impregnated on Al2O3
was usually used in hydrotreating process [17]. Meanwhile, CeO2 is a good co-
carrier and mixed oxides containing CeO2 are effective supports for methane
reforming with CO2 [18], water–gas shift reaction [19], etc. Ceria is widely used as
a co-carrier due to its unique acid–base and redox properties. It is well known that
ceria can affect [20] the thermal and structural stability of the catalyst support; the
dispersion of supported metal; the oxidation and reduction of noble metals; the
storage and release of oxygen in ceria containing catalysts and the decrease of
carbon formation on the catalyst surface, etc. It is reported that the MoO3–CeO2
catalyst is better for methanation processes than the MoO3–Al2O3 catalyst [21].
However, the catalyst using pure CeO2 as carrier is hardly applied for practical
production due to its low surface area. Using special methods can achieve large
surface area particles as nano-CeO2, but it is easily sintered and pore collapsed
because of its poor mechanical strength. Therefore, trying to maintain higher surface
area and good mechanical strength while keeping the surface property of CeO2 may
improve the methanation activity of Mo-based catalyst.
496 B. Wang et al.
123
For this purpose, we have tried to use cerium oxide as co-carrier of Al2O3 to
obtain CeO2–Al2O3 composite support by using three different methods. Then we
prepared Co–Mo catalysts supported on these CeO2–Al2O3 supports to study their
sulfur-resistant methanation performance. The effect of preparation methods of
CeO2–Al2O3 composite on the catalytic methanation performance was investigated
in detail.
Experimental
Preparation of CeO2–Al2O3 composite support
The co-precipitation (cop), impregnation (imp) or deposition–precipitation (dp)
method has been used to obtain the CeO2–Al2O3 composite support. The
preparation procedures were given as follows:
Cop: the CeO2–Al2O3 composite supports were prepared via cop from aqueous
nitrate solutions of Ce, and Al. The 10 % (wt%) ammonia solution, which was in
10 % excess compared to the theoretical value, was added dropwise to a vigorously
stirred nitrate solutions at 40 �C. Then the precipitates were under stirring for more
than 30 min at 40 �C, and aged for another 4 h. The precipitates thus obtained were
filtered and extensively washed with deionized water until the pH of the filtrate near
neutral. The precipitates were dried overnight at 120 �C, calcined in air at 600 �C
for 4 h, the rate of temperature increasing was maintained at 5 �C/min. The sample
we got was CeO2–Al2O3 composite support prepared by cop, which is referred as
CeO2–Al2O3 (cop) in the text.
Imp: the CeO2–Al2O3 oxides were prepared by incipient wetness imp of alumina
with an aqueous solution of cerium nitrate (Ce(NO3)3�6H2O). The alumina was
added to a solution of a known amount of the salt and the mixture was stirred
regularly for 5 h at room temperature. The natural dried precursor was further dried
at 120 �C overnight and calcined at 600 �C for 4 h as described above. The sample
we got in this way was CeO2–Al2O3 composite support prepared by imp, which is
referred to as CeO2–Al2O3 (imp) in the text.
Dp: the CeO2–Al2O3 oxides of dp prepared as followings: First the Al2O3 powder
was suspended to a certain concentration of cerium nitrate, then 10 % (wt%)
ammonia solution in 10 % excess compared to theoretical value was added
dropwise to a vigorously stirred nitrate solution at 40 �C, maintaining the pH close
to 8.0 by simultaneous addition of ammonia solution. Then the precipitates were
under stirring for more than 30 min at 40 �C and aged for another 4 h. The
precipitates thus obtained was filtered and extensively washed with deionized water
until the pH of the filtrate near neutral. The precipitate was then dried overnight at
120 �C, calcined in air at 600 �C for 4 h as described above. The sample we got in
this way was CeO2–Al2O3 composite support prepared by dp, which is referred as
CeO2–Al2O3 (dp) in the text.
Ce(NO3)3�6H2O, Al(NO3)3�9H2O, (NH4)4Mo7O24�4H2O and Co(NO3)2�6H2O
used in the article were AR (99.0 %), from Tianjin Kermel Chemical Reagent Co.,
Ltd. Furthermore, the Al2O3 which was used in the process of imp and dp was
Effect of the ceria–alumina composite 497
123
prepared in accordance with cop of CeO2–Al2O3 (cop) in addition to no cerium
nitrate in the solution. CeO2 was also prepared to provide a reference in the
characterization. Its preparation conditions was as follows: cerium nitrate calcined
in air at 600 �C for 4 h, the rate of temperature increasing was maintained at 5 �C/
min.
Catalyst preparation
The concentration of CeO2 in the composite supports was confirmed by XRF
measurement results given in Table 1.
5 % wt% CoO–15 % wt% MoO3/CeO2–Al2O3 catalysts were prepared by
incipient wetness imp using ammonium molybdate and cobalt nitrate as precursor.
The impregnated mixture was stirred at room temperature for 5 h, then dried at
120 �C overnight and calcined at 600 �C for 4 h (heating rate of 5 �C/min). Before
catalytic evaluation, the oxidized catalysts were sulfided by 3 % H2S/H2 gas at
400 �C for 4 h.
Catalyst characterization
N2 physisorption of the prepared CeO2–Al2O3 composite supports and the cobalt–
molybdenum catalysts was performed at -196 �C using a Micromeritics Tristar
3000 system to obtain the adsorption–desorption isotherms. The samples were
degassed at 300 �C for 4 h before any measurements were collected.
The temperature-programmed reduction (TPR) profiles of the prepared catalysts
were obtained using a Micromeritics AutoChem 2910. Before the TPR measure-
ments, the CeO2–Al2O3 composite support (200 mg) was flushed with high purity
argon at 200 �C for 1 h to remove traces of water and was cooled to 60 �C. A gas
mixture of 10 % v/v of hydrogen in argon was used at a flow rate of 30 mL/min, and
the temperature was increased at a rate of 10 �C/min from 60 to 1,000 �C.
X-ray diffraction (XRD) analysis of the CeO2–Al2O3 composite support and
cobalt–molybdenum catalyst was performed using a RigakuD/max-2500 X-ray
diffractometer with a Ni-filtered Cu Ka radiation source (k = 0.154 nm). The scan
speed was 8�/min with a scanning angle that ranged from 5� to 90�. The phase
identification was determined by comparison with the Joint Committee on Powder
Diffraction Standards.
The surface CeO2 of supports was analyzed using the laser Raman spectrometer
(RENISHAW, Invia reflex, wavelength 488 nm) with high-sensitivity systems of
Table 1 Chemical composition of the supports
Supports Composition (%, w/w)
CeO2 Al2O3
Al2O3–25 % CeO2 (dp) 25.1 74.8
Al2O3–25 % CeO2 (imp) 26.6 73.2
Al2O3–25 % CeO2 (cop) 25.8 73.3
498 B. Wang et al.
123
integrated research grade microscopes as a secondary analysis to back the XRD
results. XRF of the supports was analyzed in Tsinghua University on instrument
Rigaku ZSX Primus with 3.0 g samples.
Catalytic activity test
The catalytic performance of cobalt–molybdenum catalysts was determined in a
continuous fixed bed reactor with an inner diameter of 12 mm (stainless steel). The
0.43–0.85 mm catalysts (loading 3 mL) were previously sulfided by 3 % H2S/H2
mixture gas at 400 �C for 4 h. The reaction was operated at 5,000 h-1, 3 MPa,
n(CO)/n(H2)/n(N2) = 2:2:1, at temperature ranges from 435 to 610 �C. The feed
gas has H2S with a concentration of 0.24 vol.% in order to maintain the catalyst’s
activity. The schematic representation of the reactor system is shown in Fig. 1. The
reaction gas was preheated to 200 �C before entering into the reactor. The reactor’s
temperature was controlled by three-stage temperature control (shown in Fig. 1) in
order to make sure the catalyst bed at a constant temperature zone. A thermocouple
was located inside a stainless steel sheath in direct contact with the catalyst bed to
ensure accurate temperature measurement. The hot point temperature of the catalyst
bed was controlled at 435, 535 or 610 �C during the catalytic test. The reaction’s
pressure (3 MPa) was controlled by a back pressure regulator. The flow of the feed
gas was controlled accurately by mass flowmeters. The tail gas out of the reactor
was analyzed by an on-line GC (Agilent 7890A) equipped with thermal
conductivity detector (the operating temperature is 250 �C), FID (the operating
temperature is 300 �C). The GC provides the composition of CO, CO2, N2 and CH4
used ESTD where the exhaust gas flow was provided by soap film flowmeter (shown
as h in Fig. 1). CO conversion, CH4 and CO2 selectivity is calculated by the
following formula:
Fig. 1 Schematic representation of the reactor system: (a) preheater, (b) reactor, (c) condenser, (d) gas–liquid separator, (e) desulfurization equipment, (f) to analysis system, (g) mixer, (h) to soap filmflowmeter
Effect of the ceria–alumina composite 499
123
XCO ¼nCOin
� nCOout
nCOin
� 100 %
SCH4¼ nCH4out
� nCH4in
nCOin� nCOout
� 100 %
SCO2¼ nCO2out
� nCO2in
nCOin� nCOout
� 100 %
Results and discussion
Characterization of the samples
As shown in Fig. 2, CeO2–Al2O3 composite supports prepared by different methods
showed adsorption–desorption curves typical of mesoporous materials [22].
Table 2 lists the morphological properties of CeO2, Al2O3, and CeO2–Al2O3
composite supports. The surface area of Al2O3 we prepared was 283 m2/g, which was
Fig. 2 Adsorption–desorption isotherm of CeO2–Al2O3 composite support: a CeO2–Al2O3 (cop),b CeO2–Al2O3 (imp), c CeO2–Al2O3 (dp)
500 B. Wang et al.
123
larger than the CeO2–Al2O3 composite supports. Even though, the surface area of
CeO2–Al2O3 composites was much larger than that of pure CeO2 support (58.0 m2/g).
It can be seen that the composite support prepared by the dp method has larger surface
area than the other two preparation methods, which is attributed to the high degree
CeO2 dispersion on Al2O3 support and smaller CeO2 particle size proved later by
XRD. The size of the catalysts surface area was consistent with the carrier.
The XRD results are shown in Fig. 3. It is clear to observe the characteristic
peaks of CeO2 at 2h = 28�, 33�, 47�, and 56� [23] on the CeO2 sample, which also
appeared in the CeO2–Al2O3 composite support. Even though not much difference
existed among CeO2 peaks for the CeO2–Al2O3 composite support prepared by the
three different methods, the crystallite size of CeO2 was not same value, which was
determined using the diffraction peak of the (111) CeO2 plane (2h = 28�) by the
Scherrer equation [24]:
DXRD ¼ kK=bcoshHere, k is the wavelength (Cu Ka1, 0.154 nm), h is the diffraction angle, K is a
constant (0.89), and b is the corrected half-width of the diffraction peak.
The calculated results of crystallite sizes of CeO2 are listed in Table 2. It can be
seen that the crystallite size of CeO2 prepared by the dp method is 8.50 nm, which is
smaller than the 10.6 nm prepared by the imp method and 11.4 nm by the cop
method. This means that CeO2–Al2O3 composite support prepared by the dp method
has a smaller crystallite size, so a high degree of CeO2 dispersion on Al2O3 surface.
Another difference is that the characteristic peaks of Al2O3 (2h = 14, 38�) [25] in
the dp sample are not detected in the other two samples. This result has been
repeatedly verified. The reason for that may be the good dispersion of CeO2 on the
Al2O3 (which was proved by the BET and XRD results) and the low interaction
between Ce and Al (which would be proved by the TPR).
In order to verify the above XRD results, the Raman spectrum was obtained for
the composite supports prepared by three different methods and the results are
shown in Fig. 4.
Table 2 Morphological properties of the samples
Materials BET surface area
(m2/g)
Pore volume
(cm3/g)
Pore size
(nm)
CeO2 diameter
(nm)
Al2O3 283 0.31 4.05 –
Al2O3–25 % CeO2 (dp) 222 0.32 4.70 8.50
Al2O3–25 % CeO2 (imp) 188 0.18 3.29 10.6
Al2O3–25 % CeO2 (cop) 183 0.37 8.07 11.4
CeO2 58.0 0.19 11.56 12.5
CoO–MoO3/Al2O3–25 %
CeO2 (dp)
173 0.23 4.98 –
CoO–MoO3/Al2O3–25 %
CeO2 (imp)
130 0.12 3.60 –
CoO–MoO3/Al2O3–25 %
CeO2 (cop)
120 0.26 9.77 –
Effect of the ceria–alumina composite 501
123
It was reported [20] that CeAlO3 species were formed when the calcination
temperature of the carriers (CeO2–Al2O3) was above 450 �C. However, this kind of
species (CeAlO3) was not found either by XRD patterns or by Raman spectra in our
study, even though the CeO2–Al2O3 composite was calcined at 600 �C, which was the
same as Li reported [26]. Also, the intensity of the CeO2 peak (450 cm-1) in the
Raman spectra is different, as shown in Fig. 4. The decreasing order is CeO2 [CeO2–Al2O3 (dp) [ CeO2–Al2O3 (imp) [ CeO2–Al2O3 (cop), which is in agreement
with XRD results and further proved that the CeO2–Al2O3 composite support
Fig. 3 XRD of CeO2–Al2O3 composite supports: (a) CeO2–Al2O3 (cop), (b) CeO2–Al2O3 (imp),(c) CeO2–Al2O3 (dp), (d) CeO2
Fig. 4 Raman spectra of CeO2–Al2O3 composite: (a) CeO2–Al2O3 (cop), (b) CeO2–Al2O3 (imp),(c) CeO2–Al2O3 (dp), (d) CeO2
502 B. Wang et al.
123
prepared by the dp method achieved a higher degree of CeO2 dispersion and a
correspondingly smaller crystallite size.
To determine the interaction between Ce and Al in the ceria–alumina composite
supports, we introduced the TPR characterization. Fig. 5 shows the TPR profiles of
CeO2 and CeO2–Al2O3 supports prepared by different methods.
As shown in Fig. 5, the reduction temperature of pure CeO2 corresponds to
520 �C. However, after introducing CeO2 into Al2O3, the reduction temperature of
CeO2 was shifted to higher temperature due to the interaction between Al2O3 and
CeO2. The interaction of Ce–Al decreases in the order of CeO2–Al2O3
(dp) \ CeO2–Al2O3 (imp) \ CeO2–Al2O3 (cop). The reduction temperature of
CeO2–Al2O3 (dp) is 550 �C, which is a little higher than the pure CeO2, but the
reduction temperature of CeO2–Al2O3 (imp) and CeO2–Al2O3 (cop) is much higher
than CeO2–Al2O3 (dp) and reaches about 600 �C.
Catalytic activity
The above characterization results show that CeO2–Al2O3 (dp) exhibited higher
specific surface area, higher CeO2 dispersion, lower CeO2 crystallite size and
weaker interaction between CeO2 and Al2O3 than the CeO2–Al2O3 (imp) and CeO2–
Al2O3 (cop). In order to know the correlation of these factors with the methanation
activity, the 5 % CoO–15 % MoO3/25 % CeO2–Al2O3 catalysts using three
different CeO2–Al2O3 composite supports were evaluated for the reaction of
sulfur-resistant methanation activity. Catalysts were evaluated in the reaction of
sulfur-resistant methanation under chemical control conditions. In order to compare
the carbon monoxide conversions, experiments were performed at the same space
Fig. 5 TPR patterns of carriers prepared by different methods: (a) CeO2–Al2O3 (cop), (b) CeO2–Al2O3
(imp), (c) CeO2–Al2O3 (dp), (d) CeO2
Effect of the ceria–alumina composite 503
123
time and feed composition (5,000 h-1, 3 MPa, n(CO)/n(H2)/n(N2) = 2:2:1, 0.24
vol.% H2S). Fig. 6 shows the CO conversion and Fig. 7 shows CH4 and CO2
selectivity of the catalysts at different temperatures.
As seen from the Fig. 6, the CO conversion of the three catalysts increases with
increasing temperature. Compared to the Co–Mo/CeO2–Al2O3 (imp) and Co–Mo/
CeO2–Al2O3 (cop) catalysts, Co–Mo/CeO2–Al2O3 (dp) shows higher CO conver-
sion and better catalytic activity among these three catalysts at the same evaluating
conditions. The catalytic activity of Co–Mo/CeO2–Al2O3 (dp) at the relatively low
temperature of 435 �C is only 38 %, while the other two prepared by imp and cop
are lower at 30 and 26 %. The activity of the catalysts increases as the reaction
temperature increases and the dp catalyst shows higher reactivity than the other two.
This maybe attributed to the larger specific surface area, higher dispersion of CeO2,
smaller crystallite size of CeO2 and weaker interaction between CeO2 and Al2O3
evidenced by the characterization results of N2-adsorption, XRD and TPR.
Therefore, the dp method is better than imp method and cop method for producing
CeO2–Al2O3 composite support for high performance Mo-based methanation
catalyst. The reactivity of dp catalyst is 57 % when the reaction temperature reaches
600 �C, which is much higher than the 435 �C, which indicates it is beneficial to
improve the catalytic activity with rising the temperature. However, the selectivities
of CH4 and CO2 of the three catalysts are similar at the same temperature as seen
from Fig. 7. Meanwhile, we can see clearly that the selectivity of CO2 decreases
with increasing temperature while the selectivity of CH4 behaves in the opposite
way which corresponds to the thermodynamic equilibrium of sulfur-resistant
methanation. It is also shown that at the relatively low temperature of 435 �C, the
selectivity of CH4 is lower than the selectivity of CO2, at 535 �C, the selectivities
are nearly the same and the selectivity of CH4 is higher than the selectivity of CO2
Fig. 6 CO conversions of the catalysts at different temperatures: (a) Co–Mo/CeO2–Al2O3 (cop), (b) Co–Mo/CeO2–Al2O3 (imp), (c) Co–Mo/CeO2–Al2O3 (dp)
504 B. Wang et al.
123
at 610 �C. Therefore, the different CeO2–Al2O3 composite supports mainly
influence the catalytic activity and have no effect on the products’ selectivity.
Discussion
In the cop process, with the addition of the precipitating agent ammonia, cerium nitrate
and aluminum nitrate generate precipitation and both precipitation particles grow at
the same time from the crystallite nuclei, thus results in the strongest interaction
between CeO2 and Al2O3 among the three methods. In the imp process, cerium nitrate
was impregnated onto the inner surface of Al2O3 support. Since the Al2O3 has been
formed as Al2O3 powder before the imp of cerium nitrate, the interaction between
CeO2 and Al2O3 is weaker compared to cop method. While in the dp process, Al2O3
powder is used again but the difference lies in that during the formation of the
composite, the deposit of cerium compound onto Al2O3 pores is in the form of
precipitant rather than the molecular cerium nitrate, which results in the weaker
interaction between CeO2 and Al2O3 compared to imp method. However, there are
many factors to be optimized in the cop process such as: the addition speed of the
precipitating agent ammonia, stirring speed, pH of the solution, aging time etc. Maybe
cerium nitrate and aluminum nitrate can generate precipitation similar to the dp
process. So the optimization factors for the cop method need to be investigated further.
Conclusions
The Co–Mo catalysts impregnated on the CeO2–Al2O3 composite supports have
good activity for sulfur-resistant methanation process. The preparation method of
the CeO2–Al2O3 composite supports has a significant impact on the specific surface
Fig. 7 CH4 and CO2 selectivities of the catalyst at different temperatures: (a) Co–Mo/CeO2–Al2O3
(cop), (b) Co–Mo/CeO2–Al2O3 (imp), (c) Co–Mo/CeO2–Al2O3 (dp)
Effect of the ceria–alumina composite 505
123
area, CeO2 dispersion and the interaction between CeO2 and Al2O3, which was
proved to be correlated with the sulfur-resistant methanation activity. Compared
with imp and cop methods, CeO2–Al2O3 composite support prepared by dp method
has a larger surface area, smaller crystallite size of CeO2, and weaker interaction
between CeO2 and Al2O3, which leads to better Co–Mo/CeO2–Al2O3 catalysts for
the sulfur-resistant methanation process. The optimization for this kind of catalyst
including the loading of CoO and MoO3, the factors for CeO2–Al2O3 composite
preparation using dp or cop method need to be investigated further.
Acknowledgments The authors are grateful to the National Institute of Clean and Low Carbon Energy
for the financial support.
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