Effect of cobalt addition on the structure and properties of Ni-MCM-41 for the partial oxidation of methane to syngas

Article abstract of DOI:10.1039/c9ra03534f

A one-stssep hydrothermal crystallization method was used to synthesize Co-Ni-MCM-41 catalysts for the partial oxidation of methane to syngas reaction. Co was added as an assistant in the synthesis process. The formation of a Ni-Co alloy decreased the dam

Full text of DOI:10.1039/c9ra03534f

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Effect of cobalt addition on the structure and
properties of Ni–MCM-41 for the partial oxidation
Cite this: RSC Adv., 2019, 9, 25508
of methane to syngas
a
Yufeng Li,a Junwen Wang,
Chuanmin Ding, a Lichao Ma,a Yanan Xue,a Jing Guo,a
*
*
Shunqiang Wang,a Yuanyuan Meng,a Kan Zhangb and Ping Liub
A one-step hydrothermal crystallization method was used to synthesize Co–Ni–MCM-41 catalysts for the
partial oxidation of methane to syngas reaction. Co was added as an assistant in the synthesis process. The
formation of a Ni–Co alloy decreased the damage of Ni ions to the framework of MCM-41. The Ni–Co alloy
introduced more Ni into the channel exposing more active sites. The properties of the synthesized catalysts
were characterized by XRD, N2 adsorption–desorption, TEM, ICP, FT-IR, H2-TPR, XPS and TGA techniques.
Co–Ni–MCM-41 catalysts showed superior catalytic performance and sintering resistance than Ni–MCM-
41 catalyst without Co. The Ni–Co alloy inhibited the formation of the NiO, thus reducing the sintering of
the catalyst. The result was attributed to higher metal dispersion and more regular pore structure of the Co–
Ni–MCM-41 catalysts. When the Co content was 1%, a conversion of 88% and selectivity of 87% was
achieved.
Received 11th May 2019
Accepted 14th July 2019
DOI: 10.1039/c9ra03534f
rsc.li/rsc-advances
effect of zeolite improved the catalytic performance and
stability of the catalyst. However, the small pores and the
1. Introduction
acidity of these zeolites are not conducive to the POM
reaction.
In recent years, methane (CH4) has attracted enormous atten-
tion as an efficient and clean energy source. The partial oxida-
tion of methane reaction (POM) provides an important
intermediate (syngas) for chemical processes.1,2 Compared with
conventional steam methane reforming (CSMR), POM is a mild
exothermic reaction with a rapid reaction rate, high CH4
conversion and requires a smaller reactor. In addition, the
suitable H2/CO ratio of 2 is benecial to methanol synthesis and
Fischer–Tropsch synthesis.3,4
Ni-based catalysts have been the subject of many POM
research studies due to their excellent performance, high
abundance and low cost.5 However, Ni-based catalysts have
the problems of deactivation due to carbon deposition and
sintering at high temperatures.6 Much effort has been made
to improve the anti-carbon and anti-sintering properties of
the Ni-based catalysts, such as increasing metal dispersion,
decreasing the metal particle size, conning the active
components within porous materials.7–9 The connement of
the zeolite or mesoporous materials can effectively control
the agglomeration and sintering of the nanoparticles. Iglesia
developed a strategy to encapsulate a series of metal clusters
(Pt, Pd, Ru, and Rh) in different aluminosilicate zeolites such
as SOD, GIS, ANA and LTA zeolites.10–12 The connement
The MCM-41 mesoporous molecular sieve has been widely
studied for its high specic surface area, regular pore structure
and high thermal stability.13 The ordered mesoporous structure
is more favorable for POM reaction compared with the micro-
porous molecular sieve. Nickel has been successfully encapsu-
lated in the MCM-41 mesoporous materials.14,15 However,
a large amount of nickel entered the framework, leading to the
destruction of the molecular sieve structure and the reduction
of the active sites.
Previous studies have shown that the addition of additives
can effectively improve the catalytic performance of nickel-
based catalysts, prevent the loss of Ni nanoparticles and
inhibit the formation of carbon deposition.16–18 A CoNi@SiO2
catalyst was prepared by Li and applied in the POM reaction.19
Compared with Ni@SiO2 and Co@SiO2 catalysts, it showed
better catalytic activity and anti-carbon deposition performance
due to the formation of a Co–Ni alloy. The alloy improved the
reduction temperature of the catalyst thus inhibiting carbon
deposition over the catalyst at high temperatures.
The Ni–Co alloy conned within MCM-41 may improve the
dispersibility of Ni and avoid destruction of the molecular sieve
structure. At the same time, the alloy may be benecial to
enhance the stability of nickel. Herein, the MCM-41 zeolite was
used to encapsulate metallic nickel and cobalt under direct
hydrothermal conditions. The role of introduced Co on catalytic
performance and stability was investigated further.
aCollege of Chemistry & Chemical Engineering, Taiyuan University of Technology,
Taiyuan 030024, PR China. E-mail: wangjunwen@tyut.edu.cn; dingchuanmin@tyut.
edu.cn; Fax: +86 351 6014 498; Tel: +86 351 6014 498
bInstitute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
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catalyst sandwiched by two silica wool was placed in the center
of the quartz tube. A thermocouple xed in the furnace was used
to monitor and measure the temperature of the catalyst bed.
Before the reaction, the catalysts were reduced by H2 with a ow
2. Experimental section
2.1 Catalyst preparation
The Co–Ni–MCM-41 catalysts were synthesized via one-step hydro-
thermal crystallization method.14 Cetyltrimethylammonium
bromide (CTAB) was used as the structural template and tetrae-
thylorthosilicate (TEOS) was used as silicon source. Typically,
2.345 g CTAB was dissolved in the 100 mL distilled water and stirred
for 30 min at ambient temperature. Appropriate amount of
Ni(NO3)2$6H2O was dissolved in 20 mL distilled water. Ultrasonic
oscillation was conducted for 5 min until the Ni(NO3)2$6H2O was
fully dissolved and ammonia solution was added slowly to obtain
rate of 70 mL minꢁ1 at 750 C for 2 h. Then CH4 and O2 (2 : 1,
ꢀ
molar ratio) with a total ow of 150 mL minꢁ1 (GHSV ¼ 18 L gꢁ1
hꢁ1) were fed into the reactor. The catalysts were tested in the
temperature of 750 ꢀC. The catalytic activity tests of different
GHSV (10.8–32.4 L gꢁ1 hꢁ1) were operated at 750 ꢀC. The outlet
mixture products were analyzed by a GC-920 gas chromatograph
equipped with TCD and FID detectors.
In this work, methane conversion (XCH ), CO selectivity (SCO
)
4
2+
and H2 selectivity (SH ) were investigated to determine the
a complex Ni(NH3)6 solution with pH of 10. Then the two solu-
2
catalytic performance of the catalysts. All the performance data
of catalysts were calculated using the following equations:
tions were mixed and stirred for 30 min. 10 mL TEOS was dropped
into the mixture and ammonia solution was used to adjust the pH
of the solution to 10. The molar ratio of the composition was
1.0SiO2 : 0.152CTAB : 2.8NH3 : 0.1Ni : xCo : 141.2H2O (x ¼ 0.005,
0.01, 0.015). Aer continuous stirring for 6 h, the mixture was
transferred into a Teon-lined stainless steel autoclave and crys-
tallized at 110 ꢀC for 48 h. The resulting solid was ltered, washed
with deionized water until neutral, dried for 24 h at 70 ꢀC and nally
calcined at 550 ꢀC for 6 h. The obtained catalysts were denoted as
0.5Co–Ni–MCM-41, 1Co–Ni–MCM-41 and 1.5Co–Ni–MCM-41.
Additionally, Ni–MCM-41 and Co–MCM-41 was prepared by
similar method.
F
ꢁ F
CH4;in
CH4;in
CH4;out
XCH ð%Þ ¼
ꢂ 100%
ꢂ 100%
(1)
(2)
(3)
4
F
FCO;out
SCOð%Þ ¼
F
CH4;in
ꢁ F
CH4;out
F
H2;out
SH ð%Þ ¼
ꢂ 100%
2
ðFCH ;in ꢁ FCH ;outÞ ꢂ 2
4
4
In these equations, FCH ,in, FCH ,out denotes the CH4 mole of
4
4
inlet gas and CH4 mole of outlet gas; FCO,out, FH ,out denotes the
2
CO and H2 mole of outlet gas, respectively.
2.2 Catalyst characterization
The as-prepared catalyst was characterized by X-ray diffraction
using a BRUKER AXS D8 ADVANCE diffractometer wꢁit1h Cu Ka
radiation (40 kV, 40 mA) at a scanning rate of 2ꢀ min in both
small angle (2q range 0.5–10ꢀ) and wide angle (2q range 10–80ꢀ).
N2 adsorption–desorption isotherms were measured on a Micro-
metrics ASPA 2000 gas adsorption analyzer. Before test, the
sample was degassed under high vacuum at 200 ꢀC for 12 h. The
TEM images were taken over a Tecnai G2-F30 instrument. The
content of active components of the catalyst was determined by
Thermo ICAP6300 inductively coupled plasma atomic emission
spectrometer. The framework of the catalyst was measured using
a Nicolet-Impact 400 FT-IR spectrometer. Temperature pro-
grammed reduction of H2 (H2-TPR) was performed on a chemical
adsorption analyzer (TP-5676) equipped with a thermal conduc-
tive detector to study the reducibility of the catalyst. 50 mg
samples were heated to 400 ꢀC at a rate of 10 ꢀC minꢁ1 under N2
ow of 50 mL minꢁ1 and kept at this temperature for 1 h to
remove adsorbed water. Aer cooled down to room temperature,
3. Results and discussion
3.1 Characterization
3.1.1 Phase composition. Fig. 1A showed the small angle
XRD of the as-prepared catalysts. All the catalyst had a strong
diffraction peaks between 1 and 3ꢀ and two weak diffractions
between 3 and 6ꢀ which correspond to (100), (110), (200) lattice
planes of the MCM-41, respectively.20 The result showed that all
the catalysts possessed the long-range ordered hexagonal mes-
oporous framework of the MCM-41. Compared with the Ni–
MCM-41 catalysts, the diffraction peaks of the Co–Ni–MCM-41
catalysts shied to low angles, which indicated that Co
entered the inside of the molecular sieve.21 The diffraction peak
intensity of (100) lattice decreased and widened gradually as the
Co content increased. Although the molecular sieve retained the
hexagon mesoporous framework, the entry of Co ions partly
destroyed the framework structure and affected the long-range
order. In addition, compared with Ni–MCM-41, the 0.5Co–Ni–
MCM-41 showed more regular pore structure. It might be due to
the formation of Ni–Co alloy reduced the damage of Ni ions to
the framework.
the samples were switched to a 25% H2/N2 (v/v, 60 mL minꢁ1
)
mixture. The sample temperature was programmed to 900 ꢀC at
rate of 10 C minꢁ1. The surface oxidation states of Ni and Co
ꢀ
were analyzed using X-ray photoelectron spectroscopy (XPS) on
a Thermo ESCALAB 250 spectrometer. TG data was recorded on
a SETARAM thermal analyzer in a 50 mL minꢁ1 air ow frꢁo1m
Fig. 1B showed the wide angle XRD of the as-prepared
catalysts. There were no obvious diffraction peaks of metal
species from the XRD patterns, indicating that most metal ions
were inserted into the framework or highly dispersed in the
channel of the molecular sieve. It was worth noting that nickel
silicate reections of 0.5Co–Ni–MCM-41 was lower than those of
room temperature to 1000 ꢀC with heating rate of 10 ꢀC min
.
2.3 Catalyst evaluation
The catalytic activity was tested in the xed bed quartz tubular Ni–MCM-41, which suggested that there were less nickel silicate
reactor (inner diameter: 10 mm) at atmospheric pressure. 0.5 g existing in 0.5Co–Ni–MCM-41.22 This may be due to that a part
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Fig. 1 Small angle XRD patterns (A) and wide angle XRD patterns (B) of Ni–MCM-41 and Co–Ni–MCM-41 catalysts.
of Ni ions formed alloy with Co ions, which reduced the content addition of Co, it could be seen that the maxima of the metallic
of Ni ions connected to the framework. The results were in peak were shied to low angle (44.38ꢀ) while the diffraction
accord with the small angle XRD analysis.
peak of Co at 44.22ꢀ (File no. 15-806) was absent. Occurrence of
The diffraction patterns of the reduced catalysts were dis- the new peak at 44.38ꢀ indicated the formation of a Ni–Co alloy
played in Fig. 2A and B. The diffraction peaks of Ni phase could in Co–Ni–MCM-41 catalysts. Similar conclusions were reported
be observed in all the reduced catalysts. Fig. 2B presented the over different support materials in the previous studies.19,23,24
XRD analysis of the reduced catalysts in the range of 2 theta Takanabe et al.25 found that the diffraction peak at 2 theta of
between 42ꢀ and 47ꢀ. The diffraction peak at 2 theta of 44.52ꢀ in 44.22ꢀ (Co) were shied to 44.51ꢀ (Ni) as the Co : Ni increased
Ni–MCM-41 was assigned to Ni (File no. 04-0850). With the from 0 : 100 to 100 : 0 in the titania supported bimetallic
Fig. 2 Wide angle XRD patterns of reduced catalysts (A and B) and used catalysts (C).
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the vibration intensity of all the catalysts at 960 cmꢁ1 had hardly
changed, indicating that the catalyst framework still interacted
with Ni strongly.
3.1.3 N2 adsorption–desorption analysis. Fig. 4 showed the
N2 adsorption–desorption curve and pore size distribution of
the Ni–MCM-41 and the Co–Ni–MCM-41 catalysts. All the cata-
lysts exhibited the type IV isothermal curves and type H1
hysteresis loops, indicated typic characteristic of the meso-
porous materials.34 The hysteresis loop between the relative
pressure of 0.8 and 1.0 indicated that the catalysts possessed
mesoporous structure due to mesoporous stacking. Comparing
the adsorption and desorption curves of Ni–MCM-41 and Co–
Ni–MCM-41 catalysts, it could be found that with the increase of
Co content in the catalyst, the H1 type hysteresis ring gradually
became larger, which proved that more mesoporous structures
were formed. The reason was that the formation of Ni–Co alloy
inhibited the connection between Ni and the framework of
molecular, thus protecting the original well-ordered meso-
porous structure of the MCM-41. It was observed that the pore
size of the 0.5Co–Ni–MCM-41 catalyst was smaller than that of
the Ni–MCM-41 catalyst. However, the pore size increased with
the increase of Co. The reason might be that more Ni–Co alloy
entered into the molecular sieve channel, resulting in the
increase of pore size, and the increase of pore size could
somewhat indicate the damage of Co to the channel. With the
increased of Co content, the specic surface area pore volume of
the Co–Ni–MCM-41 catalysts gradually decreased (Table 1),
while the pore diameter gradually increased due to the pore
blockage caused by Ni–Co alloy entering into the channel of the
molecular sieve.
3.1.4 Microstructure of the catalysts. Fig. 5 showed the
morphologies of Ni–MCM-41 and Co–Ni–MCM-41 catalysts.
Long-range ordered channel structure was observed in Ni–
MCM-41, 0.5Co–Ni–MCM-41 and 1Co–Ni–MCM-41 catalysts
(Fig. 5B, D and F), and 0.5Co–Ni–MCM-41 presented more
regular channels than that of Ni–MCM-41. This result was
accord to the small angle XRD analysis. However, Fig. 5G and
H showed the collapse of the zeolite structure and the
agglomeration of metal particles (ꢃ200 nm). As the Co content
increase, more silicon atoms were substituted for Co atoms,
leading to the degradation of the pore system. In addition,
small metal particles (ꢃ1.5 nm) were observed in 0.5Co–Ni–
MCM-41 (Fig. 5C) while there were few larger metal particles
(12 nm) in Ni–MCM-41 catalyst. This result proved that the
addition of Co was benecial to the formation of metals in
channels rather than in the silica framework.14 Small black
spots which assigned to metal particles (<1 nm) were also
observed in 1Co–MCM-41 catalyst (Fig. 5E). It could be seen
that appropriate addition of Co could effectively increase the
dispersion of Ni while the addition of large amounts of Co
destroyed the framework of MCM-41, causing the metal
aggregation without the connement of the framework.
Energy-dispersive spectroscopy (EDS) was used to observe the
distribution of metals in MCM-41. As shown in Fig. 5I–K, Co
element had similar distribution with Ni element. The results
turned out that Co and Ni was uniformly distributed in the
1Co–Ni–MCM-41.
Fig. 3 FT-IR spectra of the catalysts: (a) 0.5Co–Ni–MCM-41 (b) 1Co–
Ni–MCM-41 (c) 1.5Co–Ni–MCM-41 (d) Ni–MCM-41.
catalysts. XRD patterns of SBA-15 loaded with Ni and Co cata-
lysts also supported this conclusion.26 In addition, these studies
proved the Ni–Co alloy in bimetallic catalysts suppressed the
oxidation of cobalt.27,28 The formation of Ni–Co alloy was further
discussed in the following sections. The low diffraction peak of
metallic particles in the 0.5Co–Ni–MCM-41 catalyst indicated
that the smaller metal particles were well dispersed in the
catalyst. With the increasing of Co content, the diffraction peak
intensity of the metal gradually increased, accompanied by the
narrowing of the half-peak width, which indicating the growth
of the metallic particles. It might be the fact that the formation
of Ni–Co alloy decreased the Ni species entering the framework.
The interaction between metal Ni and molecular sieve was
weakened, leading to the metal agglomeration at high temper-
ature. In addition, it was found that the inuence on the size of
Ni particles was small when the content of Co was less than 1%
according to Debye–Scherrer formula.29
Fig. 2C showed the diffraction patterns of the used catalysts.
The 0.5Co–Ni–MCM-41 and 1Co–Ni–MCM-41 exhibited high
reection of metallic Ni, while most Ni existed as oxides in the
Ni–MCM-41 and 1.5Co–Ni–MCM-41. This result might be due to
the formation of Ni–Co alloy inhibited the oxidation of Ni in the
Co–Ni–MCM-41 catalysts. In addition, the NiO particle size in
1.5Co–Ni–MCM-41 catalyst was larger that of other catalysts. It
might be due to that the addition of a large number of Co ions
caused the collapse of the framework of the molecular sieve, Ni
particles were more likely to form NiO particles with oxygen
without the connement of the channel and bare NiO particles
were easily aggregated into larger particles.30
3.1.2 Skeletal structure analysis with FT-IR spectra. The FT-
IR spectra of the Co–Ni–MCM-41 and Ni–MCM-41 catalysts were
presented in Fig. 3. The absorption bands at 1080 cmꢁ1 and
806 cmꢁ1 were the asymmetric and symmetric stretch of the Si–
O–Si bridges, respectively.31 The bands at 458 cmꢁ1 were due to
Si–O bending vibration and the bands at 565 cmꢁ1 were
assigned to the symmetric stretching vibration of Si–O–Si
bridges.32 The shoulder peak at 960 cmꢁ1 was used to prove that
the metal entered the framework of the molecular sieve, thus
weakening the vibration of the Si–O bond.33 It could be seen that
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Fig. 4 Adsorption–desorption isotherms and pore size distribution of the catalysts: (a) 0.5Co–Ni–MCM-41 (b) 1Co–Ni–MCM-41 (c) 1.5Co–Ni–
MCM-41 (d) Ni–MCM-41.
3.1.5 Elemental analysis. In order to prove that the addi-
3.1.6 H2-TPR analysis. The reduction behavior of the cata-
tion of Co ions could inhibit the connection between Ni ions lysts was tested by using H2-TPR. Fig. 6 showed the reduction
and the framework of the molecular sieve, the as-prepared Co– curves of the different samples. Two reduction peaks were
Ni–MCM-41 catalyst was cleaned with ammonium acetate to observed in the low temperature and the high temperature
remove Ni in the pore channel of the molecular sieve,35,36 and regions, respectively. For Ni–MCM-41 catalysts, the reduction
the content of Ni in the catalyst before and aer cleaning (Ni– peak at 500 ꢀC were assigned to the reduction of bulk NiO and
ꢀ
NH4AC) was tested by ICP. As shown in Table 1, with the the higher temperature reduction peak at 750 C were consid-
increase of Co content, the Ni content in the skeleton gradually ered due to the strong interaction between small NiO particles
decreased from 5.74% to 5.02%, while the Ni content in the and SiO2 framework.22,37,38 However, compared with the Ni–
pore passage gradually increased from 3.18% to 4.1%. It was MCM-41 catalyst, the low temperature reduction peaks of the
further proved that the addition of Co decreased the content of Co–Ni–MCM-41 catalysts were at around 380 ꢀC, shiing to
Ni in the skeleton to some extent and protected the skeleton lower temperature area. To investigate this phenomenon, the
structure of the molecular sieve.
H2-TPR prole of Co–MCM-41 (Co content was 1.5 wt%) was
Table 1 Characteristics of the catalysts determined by N2 sorption and ICP
Catalysts
Nia (wt%)
Ni–NH4ACa (wt%)
Surface areab (m2 gꢁ1
)
Pore volumec (cm3 gꢁ1
)
Pore sized (nm)
Ni–MCM-41
8.92
9.01
8.87
9.12
5.74
5.48
5.26
5.02
594
541
472
382
0.69
0.74
0.68
0.63
4.77
4.68
4.99
5.57
0.5Co–Ni–MCM-41
1Co–Ni–MCM-41
1.5Co–Ni–MCM-41
a
b
Calculated by ICP. Calculated by the BET. c BJH desorption pore volume. d BJH desorption average pore size.
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Fig. 5 TEM image of fresh catalysts: (A and B) Ni–MCM-41 (C and D) 0.5Co–Ni–MCM-41 (E and F) 1Co–Ni–MCM-41 (G and H) 1.5Co–Ni–MCM-
41 catalysts with EDS elemental mapping of the 1Co–Ni–MCM-41 catalyst (I–K).
ꢀ
also presented in Fig. 6. A weak reduction peak around 310 C 41 catalysts conrmed the formation of the Ni–Co alloy. Xu
which correspond to cobalt oxide species was observed. The et al.23 found that the major reduction peak shied to low
single peak between those of Ni and Co oxides in Co–Ni–MCM- temperature as Co component increase, indicating the
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Fig. 8 TGA of curves of the used catalysts.
Fig. 6 H2-TPR profiles of the catalysts: (a) Ni–MCM-41, (b) 0.5Co–Ni–
MCM-41, (c) 1Co–Ni–MCM-41, (d) 1.5Co–Ni–MCM-41 and (e) Co–
MCM-41.
assigned to Ni2+ 2p3/2 and 2p1/2
. The peaks appear at around
45
782 and 796 eV in the Co 2p region were ascribed to Co 2p3/2 and
23,46
Co 2p1/2
.
However, there was no evidence of electronic
effects arising from the alloying of Ni and Co, such as electron
transfer between Ni and Co in Co–Ni–MCM-41 catalysts.27
Considering that we prepared Co–Ni–MCM-41 catalysts by one-
step hydrothermal crystallization method rather than co-
impregnation method, XPS could not fully reect the informa-
tion of the oxidation state of Ni and Co because Ni and Co might
distribute deep in the catalysts.
3.1.8 TGA analysis. Carbon deposition was an important
index to evaluate POM reaction. Nickel-based catalysts were
prone to carbon deposition at high temperature, which was
one of the important reasons for catalyst deactivation. Fig. 8
showed the thermogravimetric diagram of the Co–Ni–MCM-41
and Ni–MCM-41 catalysts aer reaction of 100 h at 750 ꢀC.
When the temperature reached about 450 ꢀC, the carbon
species on the surface of the catalyst were oxidized, resulting
in the decline of the mass with carbon deposition.43 In addi-
tion, the mass of the Ni–MCM-41 and the 1.5Co–Ni–MCM-41
decreased signicantly, indicating that the catalyst had more
carbon deposition. There were some results showing the
formation of the carbon deposition was related to oxidation
species, which was accord with the XRD analysis of the used
existence of Co–Ni mixed oxide with homogeneity which was
favorable for their reduction. The simultaneous reduction of Ni
and Co was benecial to the formation of Ni–Co alloy. Similar
conclusions were also reported in other studies.39,40 In addition,
Rynkowski et al.41 reported TPR of alumina-supported Ni–Pt
samples and they found the single peak which assigned to one-
stage reduction of bimetallic catalysts illustrated the alloying of
metals in the reduction process. In our present study, one-step
hydrothermal crystallization method was also thought to
benecial for the formation of Ni–Co alloy. The formation of Ni–
Co alloy could weaken the interaction between the metal and
support, and promote the reduction process.42,43
3.1.7 Oxidation states of Ni and Co. X-ray photoelectron
spectroscopy (XPS) was used to investigate the oxidation of Ni
and Co. Although XPS is known as a surface specic technique,
the results could somewhat reect the information of nano-
clusters within the pores close to the surface of the zeolite
crystal.44 Fig. 7 showed the Ni 2p region of the as-prepared
catalysts. It could be seen that there was an absent peak
around 852.8 eV which assigned to metallic Ni. The peaks at
857.4 and 874.6 eV with satellite peaks at 863.6 and 881.1 eV was
Fig. 7 XPS spectra in the Ni 2p region and the Co 2p region of the as-prepared catalysts.
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Fig. 9 The catalytic ability comparison of the Ni–MCM-41 and Co–MCM-41 catalysts: (A) CH4 conversion, (B) H2 selectivity, (C) CO selectivity, at
750 ꢀC, GHSV ¼ 18 L gꢁ1
h
ꢁ1, atmospheric pressure.
catalysts.47 The carbon deposition rate of the catalysts was
Ni-based catalysts have been widely studied in methane
1.5Co–Ni–MCM-41 > Ni–MCM-41 > 1Co–Ni–MCM-41 z 0.5Co– reforming. Liu et al.48 prepared Ni-based bimetallic catalysts
Ni–MCM-41, which proved that the addition of appropriate supported on MCM-41 by a direct hydrothermal method, and
amount of Co could improve the anti-carbon deposition high metal dispersions were obtained with the incorporation
performance of the catalyst.
of Zr cations, while remarkably low dispersions were ob-
tained with the incorporation of Ti or Mn. Ni/ZrO2 catalyst
prepared by hydrothermal method possessed small metal
particles and higher catalytic performance (about 85% of CH4
conversion) in partial oxidation of methane.49 Habimana
et al.50 reported Ni-based SBA-15 catalysts with Cu promoter
for partial oxidation of methane to syngas, best catalytic
performance was obtained in 12.5%Ni/2.5%Cu/SBA-15 cata-
lyst (at 750 ꢀC the conversion of CH4 reached 89%). When the
Cu content was 10%, the catalytic performance was signi-
cantly affected (about 84% of CH4 conversion). In the present
study, the appropriate addition of Co in the Ni–MCM-41
catalyst could increase the dispersion of the metals and
prevent the oxidation of Ni which was contributed to the
improvement of the catalytic activity. And our catalysts
remained high active aer reaction of 100 h with little carbon
deposition.
3.2 Catalytic performance of the catalysts
3.2.1 Effect of Co and Co content on catalyst performance.
Fig. 9 showed the CH4 conversion rate and H2, CO selectivity of
the Co–Ni–MCM-41 and the Ni–MCM-41 catalysts in the POM
reaction. The CH4 conversion and H2, CO selectivity of the
0.5Co–Ni–MCM-41 and 1Co–Ni–MCM-41 catalyst were both
higher than that of the Ni–MCM-41 catalyst. The 1Co–Ni–MCM-
41 catalyst showed excellent catalytic performance and dis-
played the highest CH4 conversion of 88%. According to the
XRD analysis results aer the reaction, the introduced Co ions
were contributed to forming the Ni–Co alloy, which inhibited
the Ni particles from combining with oxygen to form NiO
particles, thus decreasing the sintering of the catalyst. It was
worth noting that the catalytic activity of 1.5Co–Ni–MCM-41
catalyst was signicantly lower than those of the other three
catalysts, because the introduction of a large number of Co ions
led to the collapse of the framework of the molecular sieve and
the sharp decline in specic surface area. Part of the active sites
in the catalyst were buried by SiO2, resulting in the reduction of
catalytic activity.
3.2.2 Catalytic stability. The stability oꢀf Ni–MCM-41 and
Co–Ni–MCM-41 catalysts were tested at 750 C with a GHSV of
18 L gꢁ1 hꢁ1. As shown in Fig. 10, the CH4 conversion of the
0.5Co–Ni–MCM-41 catalyst and 1Co–Ni–MCM-41 catalyst
reached the stability in a short time without decreasing
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4. Conclusions
In this work, the well-ordered Co–Ni–MCM-41 catalysts with the
assistant of Co were synthesized under direct hydrothermal
condition for POM reaction. The result showed that the Ni–Co
alloy formed in the channel of MCM-41 zeolite, and inhibited
the formation of the NiO. With the increase of the Co content,
the nickel species in the framework of zeolite decreased
signicantly, weakening the damage of Ni ions to the frame-
work. The Co–Ni–MCM-41 catalysts displayed superior catalytic
performance and stability than Ni–MCM-41 due to the highly
dispersed Ni–Co alloy and more exposed active sites. The Co–
Ni–MCM-41 catalyst remained high active aer reaction of 100 h
with little carbon deposition. The connement effect of the
regular pore structure and the formation of Ni–Co alloy
improved the sintering and coking resistance of the catalyst.
Moreover, such alloy encapsulated catalyst could also be
extended to apply in other eld.
Fig. 10 The catalytic stability comparison of the Ni–MCM-41 and Co–
MCM-41 catalysts, at 750 ꢀC, GHSV ¼ 18 L gꢁ1
h
ꢁ1, atmospheric
pressure.
tendency, indicating that there was no deactivation of the two
catalysts. However, the Ni–MCM-41 and 1.5Co–Ni–MCM-41
catalysts showed a slight decrease in the CH4 conversion aer
the reaction of 70 h. Combining with the XRD and TGA analysis,
the formation of carbon deposition on the catalyst surface lea-
ded to the decrease of catalytic activity.
Conflicts of interest
There are no conicts to declare.
Fig. 11 showed the inuence of GHSV on the catalytic
performance of the 1Co–Ni–MCM-41 catalyst. It was observed
that the CH4 conversion rate and the selectivity of the
product presented a slow decline trend with the rise of GHSV
from 10.8 L gꢁ1 hꢁ1 to 32.4 L gꢁ1 hꢁ1. The CH4 conversion
decreased sharply with continuous increase of GHSV, which
might be caused by insufficient contact of the feed gas at the
active site on the catalyst. The product selectivity presented
similar trend with the CH4 conversion. This result might be
explained by combustion reforming mechanism in the POM
reaction: CH4 was rstly completely oxidized to CO2 and H2O,
and then the remaining CH4 reformed with CO2 and H2O.
Under fast ow rates, CH4 had no enough time to react with
pregenerated CO2 and H2O, resulting low product
selectivity.51
Acknowledgements
This research was funded by Natural Science Foundation of
China, grant number 21706178; the Natural Science Foundation
of Shanxi Province, grant number 201801D221074,
201801D121058; China Postdoctoral Science Foundation
2019M651079. Authors also gratefully acknowledge the support
and guidance of the 602 groups of teachers from Institute of
Coal Chemistry, Chinese Academy of Sciences.
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