Synthesis and characterization of well dispersed nickel-incorporated SBA-15 and its high activity in syngas methanation reaction

Article abstract of DOI:10.1016/j.apcata.2016.02.019

Ni-incorporated SBA-15 materials were successfully synthesized by pH adjusting method using sodium hydroxide (NaOH) as the pH regulator and tetraethyl orthosilicate (TEOS) as the silica source. The results of transmission electron microscope (TEM) and H₂ pulse chemisorption showed that nickel particles of the Ni-S15 catalysts prepared by pH adjusting method were much finer and more uniform dispersion than those of Ni/S15 catalyst prepared by traditional impregnation method. The nickel dispersions of Ni-S15(pH8) and Ni/S15 were 3.6% and 2.5%, respectively. In addition, the pH value strongly affected the amount of nickel introduced in the silica framework. The catalytic performances were also investigated for synthetic natural gas (SNG) production via syngas methanation reaction. The Ni-S15(pH8) catalyst prepared by pH adjusting method achieved the best activity with 100% CO conversion and 98.6% CH₄ yield at 350°C, 0.3 MPa and 15000 mL/g/h. Meanwhile, the Ni-S15(pH8) catalyst possessed excellent stability and high-temperature-resistant performance due to the strong interaction between NiO and support.

Full text of DOI:10.1016/j.apcata.2016.02.019

Accepted Manuscript
Title: Synthesis and characterization of well dispersed
nickel-incorporated SBA-15 and its high activity in syngas
methanation reaction
Author: Miao Tao Xin Meng Zhong Xin Zhicheng Bian
Yuhao Lv Jia Gu
PII:
S0926-860X(16)30085-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.02.019
APCATA 15778
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Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
19-11-2015
10-2-2016
15-2-2016
Please cite this article as: Miao Tao, Xin Meng, Zhong Xin, Zhicheng Bian, Yuhao
Lv, Jia Gu, Synthesis and characterization of well dispersed nickel-incorporated SBA-
15 and its high activity in syngas methanation reaction, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.02.019
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Synthesis and characterization of well dispersed nickel-incorporated SBA-15 and its high
activity in syngas methanation reaction
Miao Tao1, Xin Meng1,2, Zhong Xin1,2*, Zhicheng Bian1,Yuhao Lv1, and Jia Gu1
1Shanghai Key Laboratory of Multiphase Materials Chemical Engineering, 2State Key
Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of
Science and Technology, Shanghai, People’s Republic of China
*Information for Corresponding author
Name: Zhong Xin
E-mail address: xzh@ecust.edu.cn(Z.Xin).
Tel: +86-021-64251005; Fax: +86-021-64240862
Address: Mailbox 545, East China University of Science and Technology, No. 130, Meilong Road,
Shanghai, 200237.
Graphical abstract
Highlights:



An original nickel-incorporated catalyst was synthesized by the pH-adjusting method.
The Ni-S15 catalyst showed high activity with 98% CH4 yield in CO methanation.
This catalyst can be used in the production of SNG from coal or dry biomass.
1

Abstract:
Ni-incorporated SBA-15 materials were successfully synthesized by pH adjusting method using
sodium hydroxide (NaOH) as the pH regulator and tetraethyl orthosilicate (TEOS) as the silica
source. The results of transmission electron microscope (TEM) and H2 pulse chemisorption showed
that nickel particles of the Ni-S15 catalysts prepared by pH adjusting method were much finer and
more uniform dispersion than those of Ni/S15 catalyst prepared by traditional impregnation method.
The nickel dispersions of Ni-S15(pH8) and Ni/S15 were 3.6% and 2.5%, respectively. In addition,
the pH value strongly affected the amount of nickel introduced in the silica framework. The catalytic
performances were also investigated for synthetic natural gas (SNG) production via syngas
methanation reaction. The Ni-S15(pH8) catalyst prepared by pH adjusting method achieved the best
activity with 100% CO conversion and 98.6% CH4 yield at 350℃, 0.3MPa and 15000mL/g/h.
Meanwhile, the Ni-S15(pH8) catalyst possessed excellent stability and high-temperature-resistant
performance due to the strong interaction between NiO and support.
Keywords：nickel-incorporated SBA-15; pH-adjusting; improved dispersion; syngas methanation
1. Introduction
Among different forms of fossil fuels, natural gas is ideal owing to its ready availability, high
energy density and conversion efficiency [1]. In recent years, due to the rise of the natural gas price,
synthetic natural gas (SNG) production from coal has attracted increasing attention [2]. Coal is firstly
converted into syngas through the processes of gasification, water-gas shift and cleaning [3]. Then
syngas reacts in a fixed-bed reactor in the presence of a catalyst to produce CH4. The methanation
2

process is highly exothermic and is shown as follows: CO+3H2=CH4+H2O, △H298K=－206.1kJ
/mol [4]. The catalyst is considered unanimously as a key factor in the production of methane.
Ni-based Al2O3 is the most widely used catalyst to produce methane [5, 6]. However, low metal
dispersion and nickel sintering during methanation reaction limited the wide application of Ni/Al2O3
catalyst [7, 8]. Therefore, it is still a challenge to develop methanation catalysts with good metal
dispersion and superior heat-resistance.
In addition, the chemical and texture properties of the support usually affect the metal-support
interaction and metal dispersion, which further influence the reactant conversion and product
selectivity [9, 10]. Mesoporous silicas have attracted increasing attentions due to the large surface
area and uniform mesoporous channels since the researchers at Mobil Corporation reported M41S in
the early 1990s. In our previous work [11], the nickel based MCM-41 catalyst prepared by
impregnation method showed good catalytic performance in the CO methanation, but poor thermal
stability, which was easily sintered after calcined at high temperature. We also found that the nickel
incorporated MCM-41 catalyst prepared by a hydrothermal synthesis method showed good
performance in the heat resistant test, and yet the mesoporous structures of the catalysts gradually be
destroyed when the Ni content was higher than 10wt% [12]. SBA-15 [13], another version of the
structured silica materials, has a broader application in catalysis [14-16] because of its higher surface
area, larger pore size, thicker pore wall, higher thermal and hydrothermal stabilities than MCM-41
materials. There is also no acidic site on the surface, which inhibits the formation of carbon
deposition. What’s more, there are silanol groups on the silica surface which influence the interaction
between metal and support [17]. There are extensive literatures [18-20] reported that the catalysts
using SBA-15 as the support had improved dispersion of active metals, higher effective diffusion
3

coefficient and catalytic performance.
However, it is very difficult to prepare SBA-15 containing heteroatoms in the framework
because of the strong acidic synthesis condition (pH＜1). Metals always exist in cationic form rather
than oxo species under acidic condition and they therefore cannot be introduced into the framework
of SBA-15. Many efforts have been devoted to the study on the incorporating of Al [21], Ti [22], Fe
[23], Co [24] and Sn [25] into the SBA-15 framework. Wu et al [26] reported that the Al3+ and Ti4+
could be largely introduced into SBA-15 mesoporous molecular sieves by “pH-adjusting” method for
the first time. Cui and coworkers [27] synthesized cobalt-substituted SBA-15 by a simple and
effective method designated as “pH-adjusting” and demonstrated that most of cobalt species were in
the single-site Co(Ⅱ) state. They also found that the catalyst exhibited much higher activity than
Co-MCM-41 and Co/SBA-15 in the epoxidation of styrene with O2.
So far, however, few reports [28] were presented to demonstrate the incorporation of Ni into the
SBA-15 silica framework. The effect of pH value on the physicochemical properties and catalytic
performance were also unknown. In this work, several Ni incorporated SBA-15 catalysts were
prepared by direct synthesis. Sodium hydroxide was used to adjust the pH of synthesis gels by taking
advantage of the pH control and the salt addition simultaneously. All catalysts were evaluated in
syngas methanation reaction. The physicochemical properties of the catalysts were also characterized
by X-ray diffraction (XRD), N2 adsorption-desorption, H2-temperature programmed reduction
(H2-TPR), inductively coupled plasma (ICP) elemental analysis, H2 pulse chemisorptions, scanning
electron microscope (SEM) and transmission electron microscopy (TEM).
2. Experimental
2.1 Preparation of catalysts
4

All chemicals used were analytical grade, including poly (ethylene glycol) -block- poly
(propylene glycol) -block- poly (ethylene glycol) EO20PO70EO20 (P123, Aldrich), tetraethyl
orthosilicate (TEOS), hydrochloric acid of concentration 36%-38%, nickel(II) nitrate hexahydrate
(Ni(NO3)2·6H2O), sodium hydroxide (NaOH) and deionized water.
The nickel-incorporated SBA-15 was prepared by the pH-adjusting method. The molar ratio of
TEOS, P123, HCl and H2O is 1, 0.017, 5.88 and 136. The detailed synthesis route [29] was as
follows: 4.0g P123 triblock copolymer surfactant was dissolved in 2.4M HCl solution at 40℃ to
obtain a clear solution. Subsequently, TEOS was gradually added to the solution and stirred for 4h.
Then 1.3g Ni(NO3)2·6H2O was added to the mixture, followed by additional stirring at 40℃ for 20h.
The resulting gel was transferred into an autoclave and crystallized statically at 110℃ for 24h. After
the procedure above, the pH of the system was adjusted to a certain value by adding sodium
hydroxide aqueous solution dropwise at room temperature and the mixture was crystallized again at
110℃ for another 24h. After being cooled to room temperature, the product was isolated by filtering,
washing with water and drying at 80℃ overnight. Finally, the sample was calcined under air at
550℃ for 6h (heating ramp 1℃/min) to remove the surfactant. The obtained product is denoted as
Ni-S15(pHx), where x standing for 6, 7 and 8. On the contrary, Ni/S15 was prepared by the
traditional impregnation method and used as a reference.
2.2 Characterizations
The textural properties of the catalysts were derived from N2 adsorption/desorption
measurement using an ASAP 2000 (Micromeritics) instrument. The pore size distribution was
calculated by the Barret-Joyner-Hallender (BJH) method using the desorption branch. The specific
surface area was calculated according to the Brunauer-Emmett-Teller (BET) method during the
5

relative pressure (p/p0) range of 0.1-0.3. Prior to the test, the sample was degassed to 300℃ for 4h
under vacuum. Nickel content in each sample was analyzed with inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) on TJA Atomscan 16 spectrometer. X-ray diffraction (XRD) was
used to investigate the structure of catalysts and the crystalline phases of the Ni particles on the
catalysts. The XRD patterns were obtained with a Bruker D8 Advance X-ray diffractometer using
Ni-filtered Cu radiation at 3kW. The small and wide angle scan ranges were 0.5°-5° and 10°-80° with
0.02° steps, respectively. The reduction behavior of the catalysts was investigated by
H2-temperature-programmed reduction (H2-TPR), and the TPR profiles were measured by
Micromeritics AutoChem II 2920 adsorption instrument. Prior to H2-TPR measurement, the sample
was purged with high purity Ar at a flow rate of 30mL/min at 150℃ for 2h before cooling down to
50℃. A stream of Ar containing 10% H2 at a flow rate of 30mL/min was then switched to the
catalyst while the temperature was increased at a rate of 10℃/min to 800℃. H2 pulse chemisorption
(Micromeritics AutoChem II 2920) was carried out at 40℃ to investigate the dispersion and metallic
surface area of nickel metal in the reduced catalyst. First, the catalyst sample was reduced in a
fixed-bed reactor by H2 (50mL/min) for 2h at a certain temperature and cooled to room temperature
in N2 atmosphere. Then 0.05g sample was filled in the quartz tube and pretreated in He for 1h at
200℃ to remove the impurities and adsorbed hydrogen. After cooling to room temperature in Ar,
10%H2/Ar pulse adsorption was performed at 40℃ for 10 times. Transmission electron microscope
(TEM) images of the samples were obtained with a JEOL-2010 instrument. TEM-EDS analyses was
conducted to examine the composition of the samples. The samples were prepared by directly
suspending the powder in ethanol with ultrasonication. A copper microscope grid covered with
perforated carbon was dipped into the suspension for observation.
6

2.3 Catalytic performances tests
Syngas to methane reaction was conducted in a fixed-bed stainless steel reactor with an inner
diameter of 10mm. To prevent hot spot formation in the process, the sample was diluted with quartz
sand (3g, 50-80mesh). Before the reaction, the catalyst (0.5g, powder, sieve fraction 100 mesh) was
reduced by high purity H2 with 50mL/min at a certain temperature for 2h. Subsequently, the reactor
was cooled down to 300℃ before the gas flow (H2) was switched to syngas (H2/CO=3,
WHSV=15000ml/g/h) and the pressure was increased to 0.3MPa. The outlet gases were analyzed by
an on-line gas chromatograph (Techcomp, GC7890T). A thermal conductivity detector (TCD) and
flame ionization detector (FID) were used to analyze gaseous products (CO, CO2 and CH4). The
equations used for calculating the CO conversion and CH4 yield are shown below:
moles of CO reacted
(1)
(2)
C onversion
of CO(%)

 100
moles of CO supplied
moles of CH formed
4
Yield of CH 4 (%)

 100
moles of CO supplied
3. Results and discussion
3.1 Physicochemical properties of catalysts
3.1.1 Effect of pH value on the textural properties
The textural properties of pure silica SBA-15 and fresh Ni-S15(pHx) (x=6, 7, 8) catalysts were
characterized by N2 adsorption-desorption measurement. Combined with the results of Fig.S1, it
suggested that the pH-adjusting method was effective in maintaining the highly ordered
mesostructure of the catalysts even under high pH conditions. Besides, the order of 2D-hexagonal
P6mm structure of Ni-S15 samples decreased as the pH increased. Corresponding textural properties
of all the samples were summarized in Tab.1. Compared with pure silica SBA-15, the introduction of
nickel made the surface area significantly decrease. Meanwhile, with the increasing of pH value, the
7

surface area gradually decreased. It was proposed that some of nickel species was incorporated into
mesoporous channels. On the other hand, pore diameter had an obvious increase when nickel was
introduced into the molecular sieves, and the parameter became larger with pH value rising. It might
be because of the longer bond length of Ni with oxygen than that of Si-O [30]. As pH value increased,
more nickel ions introduced into the framework of the mesoporous silica, which made pore diameter
further increase. For Ni/S15 catalyst, the BET surface area decreased and the average pore size
increased after addition of nickel, indicating the pore blockage by the introduced nickel [31, 32].
3.1.2 Crystalline structures
The crystalline structures of the fresh catalysts and the sizes of the Ni particles on the catalysts
were determined using XRD. The results were shown in Fig.1. For all the catalysts, the diffraction
peaks at about 1°on the small angle XRD patterns implied that the catalysts retained the ordered
hexagonal mesoporous structures of SBA-15 [33]. Moreover, the samples synthesized by
pH-adjusting method showed multiple well-resolved peaks. This indicated that the structural order of
Ni-S15 catalysts was higher than that of Ni/S15. In this block copolymer template system, it has been
well-documented that ordered structure of the mesoporous materials can be improved by the addition
of salts into the synthesis gels [34]. The presence of salt in the synthesis gel was proposed to
decrease the critical micelle concentration and therefore to facilitate the arrangement of the
mesostructures [35]. In our study, the produced NaCl was responsible for effectively improving the
ordered structure since sodium hydroxide was used to adjust pH during the synthesis. However,
further increased of pH negatively affected the structure of the samples, which was in agreement
with the results of Lou et al [24]. The mesoporous silica synthesized under acidic condition had an
8

electrically neutral framework. Under weakly alkaline conditions, hydrolysis tend to speed up due to
the presence of OH‐. On the other hand, although lower pH was favorable to highly ordered
mesostructure, it resulted in lower amount of nickel introduction. For instance, when pH was 8, 8.4
wt% of nickel was introduced into the mesoporous molecular sieves for Ni-S15(pH8), whereas there
was only 1.7 wt% of nickel content if pH decreased to 6, confirmed by ICP-AES analysis shown in
Table 1. An obvious phenomenon was that the amount of nickel species incorporated into the
SBA-15 increased with the increase of pH value. It was due to the formation of Ni(OH)2
precipitation which held the Ni2+ incorporating into the framework of support.
In the wide angle XRD patterns (Fig.1B), the wide and broad peaks at 10-30°were ascribed to
amorphous silica. The pattern of Ni/S15 catalyst prepared with impregnation method showed the
characteristic diffraction peaks of the NiO particles (2θ=37.2º, 43.2º and 62.8º) [36]. However,
there was no obviously characteristic diffraction peaks of NiO particles in the curve of Ni-S15(pH6)
catalyst. It could be due to the low nickel content. With the increase of pH value, the diffraction
peaks intensity of nickel species increased, which suggested the particle size and content of nickel
increasing. It was in line with the results of ICP-AES analysis. It was generally known that the
particle sizes increased with the increasing metal content, so the particle size increased with the
increase of pH value. Even so, the increasing nickel surface area was obtained. But it was unable to
calculate the NiO particle size due to its diffusion distribution. Compared with the Ni/S15 catalyst, it
could be found that the NiO particles of the Ni-S15 catalysts were much finer and more uniform
dispersion. It was also proved by the results of H2 pulse chemisorption (seen in Tab.1). Although the
nickel content of Ni/S15 (9.8%) was larger than that of Ni-S15(pH8) (8.4%), the nickel surface area
was smaller. This showed that the increase of nickel content was not enough to offset the adverse
9

effect of increasing metal particle size.
3.1.3 Morphologies and metal distributions
The morphologies of SBA-15 and the distributions of Ni particles over the catalysts were
further investigated by SEM and TEM, and the results were shown in Fig.2. Figure (A) showed the
SEM image of SBA-15, it could be seen that the SBA-15 possessed very long rod-shaped material.
The TEM image (Fig.2B) showed the highly ordered one-dimensional channels of SBA-15. Besides,
ordered mesoporous structures of SBA-15 could be observed on the TEM images of all the catalysts,
which were consistent with the results of BET and XRD. It was observed that more uniform
distributions and smaller particles of the Ni species formed in the Ni-S15 catalysts compared with
that in the Ni/S15 catalyst. The amount of nickel particles increased with the increase of pH value,
which was in accordance with the results of ICP-AES analysis. It could be speculated that using pH
adjusting method could decrease the nickel particle sizes over SBA-15 from the results of XRD and
TEM.
TEM-EDS technology was carried out to demonstrate the incorporation of nickel species into
the SBA-15 more clearly. The curves (Fig.2) proved once again the co-existence of nickel, silicon
and oxygen elements in the samples. The peaks of Cu and C could be ascribed to the elements in the
copper microscope grid covered with perforated carbon. In addition, the co-assembly of metal
hydrate and TEOS had been proved under the direction of surfactant [37, 38]. When the alkali
(NaOH) was added to adjust the pH value, the metal hydrate would react with OH- forming Ni(OH)2
precipitation, which led to the incorporation of metal species into the framework of the support. For
better understanding the synthesis procedure, a schematic diagram was shown in Fig.3.
3.1.4 Effect of pH value on the metal-support interactions
10

The reduction behaviors of NiO and the interactions between Ni particles and supports were
characterized by H2-temperature programmed reduction (H2-TPR) method, and the results were
shown in Fig.4. According to the reduction temperature, the reducible NiO were classified into three
types: I-type, II-type and III-type NiO species. Generally, the reduction temperature of NiO reflected
the interaction between NiO and support [39]. The I-type NiO species were ascribed to free nickel
oxides possessing a weak interaction with the support, the II-type NiO species had a moderate
interaction with the support, and the III-type NiO species had the strongest interaction with the
support which were the most difficult to reduce. The TPR quantitative results of the catalysts were
listed in Tab.2.
It could be seen that there were three reduction peaks of NiO on the TPR profiles of all the
catalysts prepared by pH-adjusting method. However, there were just I-type and II-type NiO species
for Ni/S15 catalyst, which suggested that the interaction between NiO and support was relatively
weak. For Ni-S15 catalysts, the fraction of III-type NiO species increased and I-type NiO species
decreased with the increase of pH value. It was probably due to that the excess OH- would react with
Ni2+ forming Ni(OH)2 precipitation, which held the Ni2+ incorporating into the framework of
SBA-15. The amount of Ni species incorporated into the siliceous framework of Ni-S15 increased
along with the increase of pH value, which led to the stronger interaction between NiO and support.
The agglomeration of nickel particles became more difficult with the enhancement of interaction
between NiO and support, which resulting in the better anti-high-temperature performance of the
catalyst. It could be confirmed by the results of the heat-resistant performance test.
11

3.2 Catalytic performance of catalysts in CO methanation
3.2.1 Effect of reaction temperature and pH value on the activities of catalysts
Generally, the reaction temperature had great influence on catalytic performance. The influences
of reaction temperature on the CO conversion and CH4 yield over Ni-S15 and Ni/S15 catalysts were
presented in Fig.5. The carbon balance values were about 99% on the average. As illustrated in
Figure S2 and Table S1, the nickel dispersion and metallic surface area obtained the maximum,
which resulted in the best catalytic activity when the reduction temperature was 600℃. Therefore, all
the catalysts were first reduced by H2 (50mL/min) at 600℃. The CO conversion and CH4 yield over
Ni-S15(pH6) increased quickly with the increasing of reaction temperature at first. For example, CH4
yield was 0% at 300℃, quickly rose to 35.8% at 350℃, and to 62.6% at 400℃. The CO conversion
of Ni-S15(pH8) and Ni/S15 catalysts maintained 100% in the range of 300-450℃. The yield of CH4
over all catalysts first increased and then decreased as the reaction temperature increased. It was due
to thermodynamic limitation. For the Ni-S15(pH6), Ni-S15(pH7) and Ni-S15(pH8) catalysts, the
highest CH4 yield was 73.9%, 96.0% and 98.6%, respectively. This further confirmed that lower pH
led to the introduction of less nickel into the molecular sieves which brought about lower activity. In
addition, the highest CH4 yield of the Ni/S15 catalyst was only 76.2%. It was clear that pH-adjusting
method could significantly improve the catalytic activity of the Ni-based catalysts in the CO
methanation reaction. The activity of nickel based catalyst depended on the number of active sites
located on the surface. According to the results of H2 pulse chemisorption, the metal dispersions of
Ni-S15(pH8) and Ni/S15 were 3.6% and 2.5%, respectively. The Ni-S15(pH8) catalyst had better
dispersion of nickel particles which resulted in higher catalytic activity than Ni/S15 catalyst.
During methanation, some side-reactions may occur which affects the CH4 selectivity and CH4
12

yield. Besides the syngas methanation reaction (CO+H2=CH4+H2O), water-gas shift reaction
(CO+H2O=CO2+H2) can also occur. In addition, the carbon monoxide disproportionation reaction
(2CO=C+CO2) is important since carbon on the catalyst surface is considered as a necessary
intermediate during the methanation reaction [40]. The carbon balance values were about 99% on the
average by calculating the sums of unreacted carbon monoxide, carbon dioxide and methane in the
product. Therefore, the main by-product is carbon dioxide in the syngas methanation process.
3.2.2 Heat-resistant performance
CO methanation reaction is a strongly exothermic reaction [41] and the heat released in the
process of the reaction may easily lead to the catalyst sintering and the loss of the catalyst activity.
Thus, the catalytic stability of the catalyst after high temperature calcination plays a vital role in the
industry. The activities of the catalysts were tested at 400℃ under 0.3MPa with a WHSV of 15000
ml/g/h, after that the catalysts were calcined at 700℃ for 2h under 0.3MPa in reactant gases
(n(H2):n(CO) =3:1, 15000 ml/g/h), and then cooled to 400℃ for the activity evaluation under the
same conditions. The high-temperature resistance performance of the catalysts were evaluated
through comparing the activities of catalysts before and after calcination, and the catalytic activities
of the catalysts before and after calcination were shown in Tab.3.
For all the Ni-based catalysts except Ni-S15(pH8), there were significant declines of catalytic
activities after calcination at 700℃ for 2h in reactant gases. The CH4 yields of Ni-S15(pH6),
Ni-S15(pH7) and Ni/S15 decreased by 37.5%, 16.9% and 40.0%, respectively. In order to investigate
the main factor in the decrease of catalytic activities, N2 adsorption-desorption and H2 pulse
chemisorption analysis were carried out. The results were shown in Tab.1. The BET surface areas of
all the catalysts were dropped by 20-25% and the average pore sizes barely changed. This was due to
13

the partial collapse of the pore structure. It was also noticeable that the descending degree of all the
catalysts were lower than that of SBA-15 (~47%), showing that the incorporation of Ni in SBA-15
had the effect in protecting the pore structures from collapse upon calcination. The similar
phenomenon was also observed in the paper of Shih-Yuan Chen [42]. The metal dispersions of
Ni-S15(pH6), Ni-S15(pH7) and Ni/S15 catalysts dropped 0.9%, 0.6% and 1.1%, respectively.
However, the metal dispersion of Ni-S15(pH8) remained unchanged. It was suggested that the main
reason for the decline of activities during calcination in reactant gases was the nickel particles
sintering, not the structure change of support. In addition, the high-temperature-resistant performance
of Ni-S15 catalysts became better with the increase of pH value. It was because using pH-adjusting
method could strengthen the interactions between NiO and support based on the TPR results. At the
same time, the fraction of III-type NiO species increased with the increase of pH value. Thus,
Ni-S15(pH8) catalyst showed the best high-temperature-resistant performance with excellent
catalytic activity.
Considering the 100% conversion of CO at 400℃, the excellent methanation behavior of
Ni-S15(pH8) after the calcination step might be caused by its pristine methanation activity. In order
to better understand the phenomenon, the heat-resistant performance was repeated at 250℃. The
thermodynamic equilibrium was not achieved under this thermal condition. The results were listed in
Tab.3 and the same trend were obtained. The CO conversions of Ni-S15(pH6) and Ni-S15(pH7)
catalysts were almost zero, so they were not listed here. For the catalyst Ni-S15(pH8), the CO
conversion and CH4 yield didn’t decline. However, the CH4 yield of Ni/S15 decreased by 61.2%.
Since there were no other side product, the CH4 yield and CO conversion were equal. Therefore, the
Ni-S15(pH8) catalyst had not only a high catalytic activity but also an excellent thermostability.
14

3.2.4 Catalytic stability test
The catalytic stability test of Ni-S15(pH8) and Ni/S15 catalysts were investigated at 400℃
under 0.3MPa with a WHSV of 15000 ml/g/h, and the results were shown in Fig.6. The CO
conversions of both the catalysts maintained at 100% in the 100h stability test. The CH4 yields of
Ni-S15(pH8) catalyst showed no obvious decrease and maintained at about 95%. However, the CH4
yields over the Ni/S15 catalyst decreased from 76% to 66% after testing for 100h. Therefore, the
Ni-S15(pH8) catalyst showed excellent stability under reaction conditions. It could be seen that the
present Ni incorporated SBA-15 catalysts had a good catalytic stability and prospective for the
practical application.
As mentioned before, there is a competitive relationship between the CO methanation reaction
(CO+3H2=CH4+H2O) with the WGS reaction (CO+H2O=CO2+H2) in the process of SNG production.
As CO methanation is a structure-insensitive reaction and the water-gas shift reaction is a
structure-sensitive reaction [43, 44], the catalytic performance of the catalyst in the methanation
process is obviously affected by the metal dispersion and particle size [45]. Zhang et al. [46] found
that the relatively larger particle size of metal nickel in the catalyst was conducive to the WGS
reaction. For the catalyst Ni/S15, the nickel particles were sintered with time in the stability test due
to the weak interaction between nickel and support. Therefore, as the nickel crystalline size increased,
CO conversion had no obvious decline but the selectivity of CH4 and the yield of CH4 declined.
According to the results of TPR, it was considered that stronger interaction between active sites and
the SBA-15 support effectively obstructed the sintering of nickel in the syngas methanation reaction.
This could ultimately lead to excellent catalytic stability. Combined with the results of H2 pulse
chemisorption, the Ni-S15(pH8) catalyst had the larger surface area and better dispersion of nickel
15

particles which resulted in more active sites and higher catalytic activity. In conclusion, the excellent
catalytic activity and stability could be obtained at the same time for the Ni-S15(pH8) catalyst.
In order to compare the catalytic performance of the Ni-S15 catalyst with the results in the
literatures, some syngas methanation catalysts were listed in Tab.4. It should be noted that it was
difficult to directly compare the performance of these catalysts because different reaction conditions
were used. However, more than 90% of CH4 selectivity was difficult to reach and some nickel
catalysts deactivated rapidly. In consequence, the Ni-S15(pH8) catalyst possessed excellent catalytic
performance with good application prospects for the production of SNG.
4. Conclusions
In summary, uniformly dispersed Ni-incorporated SBA-15 materials have been directly
synthesized by the pH-adjusting method. The pH of the system was adjusted to a certain value by
adding sodium hydroxide aqueous solution. The physicochemical properties of the catalysts were
characterized by XRD, N2 adsorption-desorption, H2-TPR, ICP elemental analysis, H2 pulse
chemisorption and TEM. The incorporation of nickel ions was strongly affected by the pH in the
synthesis gel. Comparing with the Ni/S15 catalyst, the nickel dispersion and surface area of the
catalysts prepared by pH-adjusting method were effectively improved. In addition, Ni-S15 catalyst
exhibited much higher activity than Ni/S15 catalyst in CO methanation reaction. The Ni-S15(pH8)
catalyst achieved the best activity with 100% CO conversion and about 98.6% CH4 yield at 350℃,
0.3MPa and 15000mL/g/h. According to the results of TPR analysis, it could be speculated that using
16

pH-adjusting method could strengthen the interactions between NiO and support. Therefore, Ni-S15
catalyst showed better high-temperature-resistant performance than Ni/S15 catalyst. Meanwhile, it
possessed excellent stability in the CO methanation reaction.
Acknowledgments
This research was financially supported by National Natural Science Funds of China (Grant No.
U1203293 and 91434128), the Program of Shanghai Subject Chief Scientist (Grant No.
10XD1401500) and the Program of Shanghai Leading Talents (2013).
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Figure captions
Figure 1 XRD patterns of Ni-based catalysts. (A) Small angle XRD patterns (B) wide angle XRD
patterns.
21

Figure 2 SEM of SBA-15 (A) and TEM-EDS images of Ni-based catalysts. (B) SBA-15, (C)
Ni-S15(pH6), (D) Ni-S15(pH7), (E) Ni-S15(pH8), (F) Ni/S15.
22

Figure 3 A schematic diagram for the preparation of the Ni incorporated SBA-15 catalyst.
23

Figure 4 TPR profiles of Ni-based SBA-15 catalysts.
24

Figure 5 Effect of pH value on the catalytic performance of the catalysts. (A) CO conversion,
(B)CH4 yield. Condition: n(H2):n(CO) =3:1, 15000 ml/g/h, 0.3MPa.
25

Figure 6 Catalytic stability of the Ni-S15(pH8) and Ni/S15 catalysts. (A) CO conversion, (B) CH4
yield. Condition: n(H2):n(CO) =3:1, 15000 ml/g/h, 400℃, 0.3MPa.
26

Table captions
Table1 Physicochemical properties of SBA-15 and catalysts.
BET
surface
areaa
BET
surface
areac
(m2/g)
400
Average
pore sized
(nm)
Metal
Metallic
surface
areaf(m2/g
Average
pore sizeb
(nm)
Nickel
contente
(wt%)
Metal
dispersion
g(%)
dispersion
Samples
f
(m2/g)
756
(%)
-
)
-
SBA-15
Ni-S15(pH6
5.1
7.9
3.9
7.8
-
-
502
620
600
1.7
2.0
2.8
0.8
1.6
1.1
)
Ni-S15(pH7
483
326
358
8.3
9.6
5.8
8.3
5.4
2.2
)
Ni-S15(pH8
)
422
476
9.8
6.1
8.4
9.8
3.6
2.5
2.0
1.1
3.5
1.4
Ni/S15
a Calculated by the BET equation.
b BJH desorption average pore size.
c BET surface area of the catalyst calcined in feed gas at 700℃ for 2h and calculated by the BET
equation.
d BJH desorption average pore size of the catalyst calcined in feed gas at 700℃ for 2h.
e Calculated by ICP-AES.
f Calculated by H2 pulse chemisorption.
g Metal dispersion of the catalyst calcined in feed gas at 700℃ for 2h and calculated by H2 pulse
chemisorption.
Table 2 TPR quantitative data of the catalysts.
Tm(℃)
II
Fraction of total area（%）
Catalyst
I
III
-
I
II
III
-
Ni/S15
366.7
327.3
320.4
332.5
454.6
437.5
432.7
464.5
0.68
0.34
0.19
0.16
0.32
0.34
0.48
0.34
Ni-S15(pH6)
Ni-S15(pH7)
Ni-S15(pH8)
539.0
551.2
579.1
0.32
0.33
0.50
27

Table 3 Thermostability performance of Ni-based catalysts.
Reaction
temperature/
℃
Before Calcination
After Calcination
Catalysts
CO
conversion/%
83.3
CO
conversion/%
61.6
CH4 yield /%
CH4 yield /%
Ni-S15(pH6)
Ni-S15(pH7)
400
400
400
250
400
250
62.6
96.0
97.5
64.2
76.2
39.7
39.1
79.8
97.4
63.8
45.7
15.4
100
100
96.3
100
Ni-S15(pH8)
Ni/S15
64.2
100
39.7
63.8
67.5
15.4
Table 4 Summary of the syngas methanation catalysts developed in recent years.
Catalytic performance
Active
metal
content
wt%
15
Stability
Preparatio
n methods
Referen
ce
WHSV
P/MPa /(mL·g-1 · T/℃
h-1)
Catalysts
test
XCO SCH4
/% /%
time/h
Ni-Al2O3
Ni-Al2O3
Ni-Mg-Al2
O3
CP
I
0.1
0.1
2500 h-1
240 000
400 98.2 84.7
120*
50*
[6]
10
450
300
61
64
68
58
[47]
I
40
20
0.1
0.1
36 000
4800
50
42
[48]
[49]
Silicificati
on
Si-Ni/SiO2
350
50
28
28

Ni/MCM-4
1
HT
I
10
10
0.1
3.0
12 000
10 000
350 97.9 88.2
100
50
[12]
[45]
Ni/CaTiO3
350
95
72
I: impregnation; CP: co-precipitation; HT: hydrothermal synthesis; * deactivation was observed.
29