Effects of preparation method and Sm2O3 promoter on CO methanation by a mesoporous NiO-Sm2O3/Al2O3 catalyst

Article abstract of DOI:10.1039/c8nj02282h

In order to investigate the effects of Sm₂O₃ as a promoter on catalytic performance, ordered mesoporous NiO-Sm₂O₃-Al₂O₃ catalysts were synthesized by a facile one-pot evaporation-induced self-assembly (EISA) method and used in the CO methanation reaction to produce synthetic natural gas. Besides, the ordered mesoporous catalysts prepared by the conventional impregnation method together with unordered mesoporous catalysts were selected as references. These catalysts were characterized by scanning electron microscopy, transmission electron microscopy, N₂ adsorption, X-ray diffraction, H₂ temperature-programmed reduction, X-ray spectroscopy, and thermogravimetric analysis. The results revealed that the ordered mesoporous catalysts synthesized by the one-pot EISA method showed enhanced catalytic performance for CO methanation compared with those prepared by the impregnation method. The addition of the Sm species can further improve the catalytic activity, and the ordered mesoporous 20N6SOMA catalyst with 20 wt% NiO and 6 wt% Sm₂O₃ species was the best. Our characterization results showed that Sm₂O₃ could improve the reducibility of Ni particles and decrease the Ni particle sizes; the majority of Sm species were in the form of Sm³⁺ (Sm₂O₃) together with a small amount of Sm²⁺ (SmO) over the reduced catalysts. The Sm³⁺/Sm²⁺ redox could increase the electron cloud density of Ni, which resulted in intensified catalytic activity over the ordered mesoporous NiO-Sm₂O₃-Al₂O₃ catalyst. Moreover, the optimal ordered mesoporous 20N6SOMA catalyst showed high stability in a 101 h lifetime test at 550 °C and 0.1 MPa due to its excellent confinement effect.

Full text of DOI:10.1039/c8nj02282h

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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DOI: 10.1039/C8NJ02282H
Effects of preparation method and Sm2O3 promoter on CO
methanation of mesoporous NiO–Sm2O3/Al2O3 catalyst
Qing Liu,*a Hongyuan Yang,a Hao Dong,a Wei Zhang,a Bing Bian,a Qingkun He,*b Jing Yang,*a
Xiangbao Meng,c Zhiwei Tian,a and Guoming Zhaoa
aKey Laboratory of Low Carbon Energy and Chemical Engineering, College of Chemical and
Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590,
Shandong, China
bCollege of Material Science and Engineering, Shandong University of Science and Technology,
Qingdao 266590, Shandong, China
cCollege of Mining and safety Engineering, Shandong University of Science and Technology,
Qingdao 266590, Shandong, China
*Corresponding author. Eꢀmail addresses: qliu@sdust.edu.cn, sdsslq@163.com (Q. Liu);
qingkunhe@163.com (Q. He); kdyangjing@163.com (J. Yang)
1

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Abstract
In order to investigate the effects of Sm2O3 promoter on catalytic performance, the ordered
mesoporous NiO–Sm2O3–Al2O3 catalysts were synthesized by a facile oneꢀpot evaporationꢀinduced
selfꢀassembly (EISA) method and used in CO methanation reaction to produce synthetic natural gas.
Besides, the ordered mesoporous catalysts prepared by the conventional impregnation method
together with the unordered mesoporous catalysts were selected as the references. These catalysts
were characterized by scanning electron microscopy, transmission electron microscopy, N2
adsorption, Xꢀray diffraction, H2 temperatureꢀprogrammed reduction, Xꢀray spectroscopy, and
thermogravimetric analysis. The results revealed that the ordered mesoporous catalysts synthesized
by oneꢀpot EISA method showed enhanced catalytic performance for CO methanation compared
with those prepared by impregnation method. The addition of Sm species can further improve the
catalytic activity, and the ordered mesoporous 20N6SOMA catalyst with 20 wt% NiO and 6 wt%
Sm2O3 species was the best. Our characterization results showed that Sm2O3 could improve the
reducibility of Ni particles and decrease the Ni particle sizes; the majority Sm species were in the
form of Sm3+ (Sm2O3) together with a small amount of Sm2+ (SmO) over the reduced catalysts. The
Sm3+/Sm2+ redox could increase the electron cloud density of Ni, which resulted in the intensified
catalytic activity over the ordered mesoporous NiO–Sm2O3–Al2O3 catalyst. Moreover, the optimal
ordered mesoporous 20N6SOMA catalyst showed high stability in a 101hꢀlifetime test at 550 oC 0.1
MPa due to its excellent confinement effect.
2

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1. Introduction
Production of synthetic natural gas (SNG) via CO methanation reaction (CO + 3H2 → CH4 + H2O,
ꢀH298 K =−206.1 kJ·mol−1) is gaining great attention in regions such as China, which is rich in coal
resource but scarce in natural gas reserve.1, 2 Thermodynamically, low temperature is conducive to
the formation of methane for CO methanation. However, from the kinetic viewpoint, increasing the
reaction temperature can increase the number of activated molecules and improve the reaction rate.
Thus, this reaction is often operated at high temperatures in the adiabatic fixed bed reactors with
high production efficiency and heat recovery for economic reasons.3, 4 The commonly used
Niꢀbased catalysts often suffer from Ni sintering and coking during methanation process under
severe conditions.5 Hence, the stability of Niꢀbased catalysts should be further enhanced.
Encapsulating metal nanoparticles in ordered mesoporous materials6 such as SBAꢀ15,7, 8 MSN,9
KITꢀ610, 11 and FDUꢀ1212 is an effective strategy to overcome the above problems. The ordered
mesoporous frameworks can not only offer high surface areas for the high dispersion of active
metals, but also provide the steric hindrance to hamper the sintering of metal nanoparticles. These
catalysts are usually prepared by a postꢀsynthesis impregnation method, which may cause partial or
complete blockage of channels by NiO particles.7 Moreover, the axial movement of the metal
particles accommodated in the channels is still inevitable due to the relatively weak interaction
between metals and silicaꢀbased mesoporous support. The migration of Ni particles from the inside
of channels to the external surface will occur at high temperatures, resulting in the sintering of Ni
nanoparticles. Recently, we have reported that the ordered mesoporous alumina (OMA) catalysts
prepared by a facile oneꢀpot evaporationꢀinduced selfꢀassembly (EISA) method simultaneously
inhibited the Ni sintering and coke formation in methanation reaction at high temperature.13, 14 The
strong interaction between metallic Ni species and OMA is formed over the OMAꢀbased catalysts,
3

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because the Ni nanoparticles can be anchored in the OMA framework.13, 14 Although the reported
ordered mesoporous catalysts showed high stability, their catalytic activities were still not high
enough. It is hard to exhibit high stability and maintain high catalytic activity simultaneously. Thus,
the further improvement of catalytic activity of the OMAꢀbased catalyst is required.
In generally, increasing the Ni loading is an efficient strategy to improve catalytic activity,2, 15
which is the reason why the industrial catalysts usually use high metal loadings. For example,
MCRꢀ2X catalyst from Haldor Topsoe consists of 22 wt% Ni supported on the Al2O3‒ZrO2 support,
which presents highly stable performance in methanation reaction at temperatures as high as
700 °C.16 Also, the Ni content of CEGꢀLH catalyst from Davy is as high as 47 wt%.17 On the other
hand, for the OMAꢀbased catalyst prepared by the oneꢀpot EISA method, the doping species content
in is usually small (≤10 wt%),18‒20 because too many dopants will destroy the mesoporous structure
during the EISA process. Therefore, the limited Ni doping content restricts the activity of
OMAꢀbased catalysts prepared by oneꢀpot EISA method, and the improvement of dopant amount
should be explored.
Furthermore, the addition of promoter is another efficient strategy to enhance the catalytic
activity.21‒24 Sm2O3 is one of the important rare earth oxide with special electronic, optical
properties and thermal stability, which has been widely used in various fields, such as electronic
ceramic, superconductor, solid electrolyte material, permanent magnet, and catalyst.25 Recently, Xu
et al. has published a paper on samarium oxide doped Niꢀbased mesoporous catalysts for CO2
methanation.21 They found Sm2O3 could improve the metallic Ni dispersion, intensify the CO2
chemisorption, resulting in the intensified lowꢀtemperature activity. Moreover, Zhang et al.
investigated the carrier effect on CO2 reforming of methane and found that Ni/Sm2O3–CaO catalyst
showed higher catalytic activities for than those over Ni/CaO and Ni/Sm2O3.26 This was because the
4

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multiꢀinterfacial Sm2O3–CaO support could accelerate the elimination of surface carbonaceous
species (CHx) and avoid the accumulation of whisker carbon.26 Also, Duarte et al. and Taherian et al.
investigated the influence of Sm2O3 addition to Ni/SBAꢀ15 on CH4 dry reforming.8, 27 They
revealed that Sm2O3ꢀpromoted catalyst showed excellent activity, stability and coke resistance
because of the improved interaction between NiO particles and SBAꢀ15 support pores wall as well
as high NiO dispersion after addition of Sm2O3.27 Hence, it is proposed that Sm2O3 can also be an
efficient promoter of ordered mesoporous Ni/Al2O3 catalyst for CO methanation.
Following our research works on catalysis,28‒32 especially for simultaneously enhancing the
catalytic activity and stability of Niꢀbased catalysts, the ternary Ni−Sm−Al ordered mesoporous
materials with high NiO loading were fabricated and employed for CO methanation. In addition, the
ordered and unordered mesoporous NiO–Al2O3 catalysts prepared by the conventional impregnation
method were evaluated as the references. To our knowledge, the Sm2O3 promotion effect on
Ni/OMA catalyst for CO methanation has not been published. In this work, in order to understand
the effect of NiO and Sm2O3 doping on the mesoporous structure and the interaction among
NiO−Sm2O3−Al2O3 oxides, various techniques were performed to characterize the catalysts before
and after the CO methanation reaction, which will be discussed in detail in the following text.
2. Experimental
2.1. Catalyst preparation
Samarium trinitrate hexahydrate (Sm(NO3)3·6H2O), samarium oxide (Sm2O3), 67 wt% nitric acid
(HNO3), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), aluminum isopropoxide (Al(OPri)3),
anhydrous ethanol (Sinopharm Chemical Reagent Co. Ltd., China) and the commercial porous
γꢀAl2O3 (GongYi HuaYu Alumina Co. Ltd., China) were used in this work.
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Ternary Ni−Sm−Al ordered mesoporous catalysts were prepared via the oneꢀpot EISA method
based on the reported literatures.33, 34 Typically, P123 (2.1 g, Sigma−Aldrich) was dissolved in
anhydrous ethanol (40.0 mL), followed with addition of nitric acid (3.2 mL, 67 wt %), Al(OPri)3
(4.08 g) and stoichiometric amounts of Ni(NO3)2·6H2O and/or Sm(NO3)3·6H2O in sequence. The
obtained mixture was vigorously stirred in a beaker covered with PE film at room temperature for at
least 5 h. Then, it was placed in an oven to undergo the slow evaporation of solvent at 60 °C. The
obtained xerogel was calcined in a tubular furnace in air flow at 550 °C for 4 h at a heating rate of
1 °C min−1. The samples were denoted as 20NxSOMA (x = 0 and 6) with the fixed NiO content of
20 wt%, and ‘x’ represented the mass percentage of Sm2O3. In addition, 6SOMA with doping of 6
wt% Sm2O3 was prepared using the same EISA method with ordered mesoporous Ni−Sm−Al
materials.
Also, the Niꢀimpregnated ordered (20NOMAꢀIMP) and unordered (20NA) mesoporous catalysts
using OMA or γꢀAl2O3 as the carrier were prepared by an incipient wetness impregnation method
following our previous literatures.13, 14 It must be noted that the calcination condition in this process
was the same with that of 20NxSOMA.
2.2. Characterization
N2 adsorption was performed on a surface area & pore size analyzer (Quantachrome, NOVA
o
3200e) at −196 C. Prior to the measurement, the sample was degassed at 300 ºC for 4 h under
vacuum. The specific surface area was determined according to the BrunauerꢀEmmettꢀTeller (BET)
method in the relative pressure range of 0.05−0.3. The pore size distribution was calculated with the
BarettꢀJoynerꢀHalenda (BJH) method using the adsorption isotherm branch. Xꢀray diffraction (XRD)
patterns were recorded on an Xꢀray diffraction apparatus (PANalytical X'Pert PRO MPD) using Cu
6

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Kα radiation (40 kV, 40 mA). The sample was checked with the card number of Joint Committee on
Powder Diffraction Standards (JCPDS). The crystal size of the sample was calculated using the
DebyeꢀScherrer equation. H2 temperatureꢀprogrammed reduction (H2ꢀTPR) was performed on a
chemisorption analyzer (Quantachrome, chemBET pulsar TPR/TPD) equipped with a thermal
conductivity detector (TCD). For H2ꢀTPR, 0.05 g sample was loaded in a quartz Uꢀtube and heated
o
o
from room temperature to 200 C at 10 C min–1 and maintain for 1 h under He flow. Then, the
o
sample was cooled to room temperature and followed by heating to 1000 C at 10 oC min–1 under a
binary gas (10 vol % H2/Ar) with a gas flow of 30 mL min–1. The Xꢀray photoelectron spectroscopy
(XPS) was conducted on a VG ESCALAB 250 spectrometer (Thermo Electron) with an Al Kα
(1486 eV) Xꢀray source. The morphology of the samples was visualized using a transmission
electron microscopy (TEM) (JEMꢀ2010F, JEOL) and emission scanning electron microscope (SEM)
(JSMꢀ6700F, JEOL) equiped with an energyꢀdispersive Xꢀray spectroscopy (EDS) detector. To
determine the total amount of deposited carbon, the thermogravimetric (TG) analysis was
conducted on a thermogravimetric analyzer (Seiko Instruments, EXSTAR TG/DTA 6300) in air
flow (200 mL min–1) from room temperature up to 1000 oC (10 oC min–1).
2.3. CO methanation
CO methanation reaction was carried out in the fixedꢀbed reactor using a quartz tube (I. D. 8 mm)
at 0.1 MPa. The influence of mass and heat transfers were examined and eliminated before the
catalytic tests. Typically, the mixture of catalyst (0.05 g, 20−40 mesh) and quartz sands (as the
diluent, 5.0 g, 20−40 mesh) was reduced at 710 oC in pure H2 (100 mL min–1) for 2 h, and cooled to
the starting reaction temperature (280 oC). After that the feed gas H2, CO and N2 (internal standard)
(H2/CO/N2 = 3/1/1, mole ratio, 150 mL min–1) were charged into the reactor at a weight hourly
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space velocity (WHSV) of 180000 L g‒1 h‒1.35 The products were analyzed online by a Micro GC
3000A (Agilent Technologies) equipped with a TCD detector. Also, a lifetime test of CO
methanation was carried put at 550 oC, 0.1 MPa, 180000 L g‒1 h‒1.
3. Results and discussion
3.1. Catalyst Characterization
3.1.1. TEM and SEM observations
TEM images of the 710oCꢀreduced catalysts are shown in Fig. 1. For 20NA, the Ni particles with
average size of 6.1 nm were dispersed over the γꢀAl2O3 support. Also, some Ni particles as large as
12.5 nm were observed indicating the occurrence of Ni sintering on the unordered mesoporous
20NA (Fig. 1a). For the ordered mesoporous samples, the uniform cylindrical pores along the (100)
direction were observed in Fig. 1b−d, which is the typical characteristics of ordered mesoporous
materials.35, 36 It is thus clear that the addition of as many as 6 wt% Sm2O3 and 20 wt% NiO does
not change the mesopore structure (Fig. 1c). The Ni particles with average sizes of 3.9, 3.7 and 4.5
nm can be seen over the reduced 20NOMA, 20N6SOMA and 20NOMAꢀIMP sample, respectively.
Apparently, the ordered mesoporous samples show high antiꢀsintering property compared with the
unordered mesoporous one. As reported in our previous works,13, 14 the Ni particles were anchored
in the OMA framework over the samples prepared by oneꢀpot EISA method, which can
significantly enhance the interaction between metal and support resulting in its excellent
antiꢀsintering property. For the Niꢀimpregnated ordered mesopore 20NOMAꢀIMP, the Ni particle
size is a little larger than those of 20NOMA and 20N6SOMA, because the Ni particles can disperse
across both inside and outside of channels of the OMA support. It is inevitable that the sintering
will occur for the Ni particles outside of channels during the reduction process at high temperature.
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As we know, the high Ni content is important for enhancing the catalytic activity,37, 38 thus the
ordered mesoporous catalysts with 20 wt% NiO in this work may be highly active for CO
methanation. As seen in the HRTEM images of the reduced 20N6SOMA catalyst (Fig. 1e and 1f),
the observed lattice spacing of ca. 0.20 nm corresponds to the Ni (111) plane, while no lattice of
Sm2O3 can be seen due to its high dispersion and/or poor crystallization.14, 39 In addition, Ni
particles are embedded in the framework, which can improve the interaction with the OMA support
(Fig. 1f).
(a)
(c)
(b)
2
4
6
0
1
2
3
4
5
6
7
2
4
6
8
10 12
Particle Size (nm)
Particle Size (nm)
Particle Size (nm)
(f)
(e)
(d)
0
2
4
6
8
Particle Size (nm)
Fig. 1. TEM images of the reduced catalysts: (a) 20NA, (b) 20NOMA, (c) 20N6SOMA, and (d)
20NOMAꢀIMP; and HRTEM images of reduced 20N6SOMA catalyst (e and f).
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(a)
(d)
(b)
(c)
(e)
(f)
Fig. 2. EDS element layered image (a) and elemental mapping images of Al (b), O (c), Ni (d) and
Sm (e) of 20N6SOMA catalyst; the corresponding EDS spectrum of 20N6SOMA (f). (Note: Pt
spraying was used to enhance the electroconductivity of samples.)
The elemental mapping images and EDS spectrum of the asꢀsynthesized 20N6SOMA were
shown in Fig. 2. The relative Al, O, Ni, and Sm element mapping images and EDS peaks can be
clearly seen in Fig. 2, implying the successful incorporation of all the elements into the 20N6SOMA
sample. Moreover, as seen in EDS element layered image (Fig. 2a), the Sm species are
homogeneously dispersed over the catalyst; and what’s more, they are located close to the Ni
species, which is convenient for the electron transfer between them.40, 41
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800
600
400
200
0
(a)
(b)
20NOMA
20NOMA
20N6SOMA
20N6SOMA
20NOMAꢀIMP
20NA
20NOMAꢀIMP
20NA
60
2
0.0
0.2
0.4
0.6
0.8
P/P0)
1.0
10
Pore size (nm)
Relative pressure
(
Fig. 3. N2 adsorption isotherms (a) and pore size distribution curves (b) of the asꢀsynthesized
catalysts. (For clarity, the isotherm of 20NOMAꢀIMP, 20N6SOMA and 20NOMA was vertically
shifted for 70, 240 and 350 cm3 g–1, respectively.)
3.1.2. N2 adsorption analysis
The N2 adsorptionꢀdesorption isotherms and the pore size distribution curves of the
asꢀsynthesized samples are displayed in Fig. 3. Overall, all the samples show type IV
adsorptionꢀdesorption isotherms according to the IUPAC classification.42 The adsorptionꢀdesorption
hysteresis loops of 20NOMA and 20N6SOMA prepared by the oneꢀpot EISA method take place in
the P/P0 range of 0.60–0.65 and belong to the steep H1 type, indicating that the prepared samples
contain highly uniform mesoporous channels.43 Apparently, the addition of Sm2O3 species and NiO
species has no obvious effect on the mesoporous structure, which is consistent with the
aforementioned TEM results. However, the impregnated ordered mesoporous 20NOMAꢀIMP shows
H4ꢀtype hysteresis loop with lower P/P0 value of 0.40–0.45, indicating that the pore diameter and
orderliness decrease compared with those of 20NOMA.44 This is because the NiO particles disperse
across both the inside and outside of the channels, and some pores are even occupied by NiO
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particles during the impregnation process.13 As expected, the unordered mesoporous sample 20NA
shows type IV isotherm with H4ꢀtype hysteresis loop. Furthermore, the pore size distribution curves
of the oneꢀpotꢀprepared samples (20NOMA and 20N6SOMA) are relatively narrow with the peak
values at around 12.0 nm. The pore size distribution curve of 20NOMAꢀIMP shifts to low value
compared with those of oneꢀpotꢀprepared samples due to the partial blockage of pores. In addition,
for 20NA, its pore size distribution curve is flat owing to its irregular channels.
Table 1. Physical and chemical properties of the asꢀsynthesized catalysts.
a
SBET
Vb
Ni crystal size (nm)c
Ni particle size (nm)d
Samples
(m2 g–1) (cm–3 g–1)
Reduced
5.1
Spent
8.5
Reduced
6.1
Spent
9.2
20NA
173
281
252
234
0.32
0.65
0.71
0.38
20NOMA
3.2
2.9
5.2
‒
3.9
3.7
4.5
‒
20N6SOMA
20NOMAꢀIMP
3.5
6.1
4.1
6.2
a Surface area, derived from BET equation;
b Pore volume, obtained from the volume of nitrogen adsorbed at the relative pressure of 0.97;
c Crystal size of Ni, derived from XRD by Debye–Scherrer equation;
d Average Ni particle size counted in the TEM images.45
The surface areas and pore volumes of all the samples are listed in Table 1. The samples prepared
by the oneꢀpot EISA method exhibit high surface areas and large pore volumes, while the
impregnated 20NOMAꢀIMP shows lower surface area and pore volume, which provide the further
evidence of partial blockage of channels after impregnation. In addition, 20NA shows the lowest
surface area and smallest pore volume due to the inherent property of the γꢀAl2O3 support as well as
the blockage of pores by NiO species.2
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3.1.3. XRD analysis
As seen in Fig. 4a, 20NOMA and 20N6SOMA show the strong (100) diffraction peak at around
0.85o on the smallꢀangle XRD patterns, indicating the successful formation of p6mm ordered
mesoporous structure.20, 36 For 20NOMAꢀIMP, the diffraction peak shifts to around 1.20o due to the
smaller pore diameter after loading of 20 wt% NiO species by impregnation method.33 For 20NA,
what is expected that no diffraction peak is observed owing to its unordered mesoporous structure.
Fig. 4b shows the wideꢀangle XRD patterns of the asꢀsynthesized catalysts. No apparent NiO,
Sm2O3 and Al2O3 diffraction peaks are observed over the wideꢀangle XRD patterns of 20NOMA
and 20N6SOMA, which is similar with the reported literatures.13, 18 In contrast, there are five
diffraction peaks at 37.3, 43.6, 63.2, 75.6, and 79.8o corresponding to NiO (JCPDS 03ꢀ065ꢀ6920)
on the wideꢀangle XRD pattern of the asꢀsynthesized 20NOMAꢀIMP. Apparently, the NiO species
are poorly dispersed over 20NOMAꢀIMP compared with those over the catalysts prepared by
oneꢀpot EISA method. In addition, the diffraction peaks of NiO, γꢀAl2O3 (JCPDS 00ꢀ034ꢀ0493) and
NiAl2O4 (JCPDS 01ꢀ077ꢀ1877) can be seen on the XRD pattern of the asꢀsynthesized 20NA. This is
because the abundant –OH groups and vacancies over the surface of γꢀAl2O3 will facilitate the
formation of NiAl2O4 at high temperature.46 The wideꢀangle XRD patterns of the reduced samples
were shown in Fig. 4c. The peaks at around 44.5 and 51.7o corresponding to (111) and (200) planes
of metallic Ni (JCPDS 01ꢀ070ꢀ1849) are observed on the XRD patterns of 20NOMA and
20N6SOMA. There are still no Sm2O3 and OMA peaks, indicating their high dispersion or poor
crystallinity after reduction process. Similarly, for 20NOMAꢀIMP, the peaks at 44.6, 51.8 and 76.6o
of metallic Ni are observed, and the OMA support also retains amorphous. For 20NA, besides the
Ni diffraction peaks at 44.5, 51.8 and 76.6o, the diffraction peaks at 37.4, 44.6, and 67.3o attribute to
γꢀAl2O3 (JCPDS 00ꢀ034ꢀ0493).47
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Furthermore, the Ni nanoparticle sizes of these samples are estimated from the WXRD patterns
by Debye–Scherrer equation and listed in Table 1. The Ni particle sizes of oneꢀpotꢀprepared
20NOMA and 20N6SOMA are 3.2 and 2.9 nm, respectively, indicating that the Ni particles can
highly disperse over the OMA support. However, the Ni particle size of 20NOMAꢀIMP is 5.2 nm,
suggesting the poorer Ni dispersion in the sample prepared by the impregnation method.
Additionally, the Ni particle size of the reduced 20NA is 5.1 nm which is similar with that of
20NOMAꢀIMP. In all, the different preparation methods have significant influence on the Ni
particles, and the catalysts prepared by oneꢀpot EISA method have small Ni nanoparticle size. Also,
the addition of Sm species in 20N6SOMA can further decrease the Ni particle size compared with
that of 20NOMA.
ꢀ
ꢁ
ꢂ
Al2O3
NiAl2O4
NiO
(b)
ꢂ
ꢁ
(a)
(c)
Al2O3
Ni
ꢂ
ꢂ
ꢂ
20NOMA
20NOMA
ꢂ
ꢀ
ꢁ
ꢂ
20N6SOMA
20NOMA
20N6SOMA
ꢂ
ꢂ
ꢂ
20N6SOMA
20NOMAꢀIMP
ꢁ
20NOMAꢀIMP
ꢁ
ꢁ
ꢁ
ꢁ
20NOMAꢀIMP
20NA
20NA
20NA
1
2
3
4
5
20 40 60 80
2 Theta (degree)
20 40 60 80
2 Theta (degree)
2 Theta (degree)
Fig. 4. Smallꢀangle (a) and wideꢀangle (b) XRD patterns of the asꢀsynthesized samples, and
wideꢀangle XRD patterns of the reduced samples (c).
3.1.4. H2ꢁTPR analysis
Fig. 5 presents the H2ꢀTPR profiles of the catalysts, and the TPR profiles of pure Sm2O3 and
14

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6SOMA are also included as the reference. There is only a weak H2 reduction peak at around 446
oC on the H2ꢀTPR profile of pure Sm2O3, attributing to the reduction of Sm3+ to Sm2+. While, no
obvious reduction peak could be observed on H2ꢀTPR profile of 6SOMA due to the low amount of
doping of Sm2O3, and thus all the hydrogen consuming peaks of the catalysts are attributed to the
NiO reduction. There is only one broad H2 reduction peak at around 690 oC on the H2ꢀTPR profile
of 20NOMA. This result suggests that the strong interaction between NiO species and OMA
support is obtained over the sample prepared by the oneꢀpot EISA method. After addition of Sm2O3
o
species, the reduction peak shifts to lower temperature (around 625 C), indicating the addition of
Sm2O3 species could improve the reducibility of Ni nanoparticles in ordered mesoporous Ni‒Sm‒
Al catalysts. Here, Sm2O3 species may play the similar role in decreasing of the reduction
temperature with V2O3 and La2O3 promoters in the literatures.13, 48 As seen from the H2 reduction
o
profile of 20NOMAꢀIMP, there are two reduction peaks at 325 and 710 C, respectively. The weak
reduction peak at 325 oC can be attributed to the reduction of NiO species with the weak interaction
o
with the support, which scatter outside of the channels;13 the strong peak at 710 C corresponds to
the reduction of NiO species in the channels with a strong interaction with the support, or a certain
amount of NiAl2O4 phase may also be formed during the calcination process.49 In addition, a broad
o
reduction peak at around 710 C appears on the H2ꢀTPR profile of 20NA, which provides the
envidence of the existence of NiAl2O4 phase. The abundant –OH groups and vacancies over the
surface of γꢀAl2O3 can facilitate the formation of NiAl2O4.49 In short, in order to completely reduce
the NiO species, the reduction process was carried out in H2 flow at 710 oC for 1 h.
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690 oC
625 oC
20NOMA
710 oC
20N6SOMA
325 oC
20NOMAꢀIMP
20NA
446 oC
6SOMA
Sm2O3
300
600
900
Temperature (oC)
Fig. 5. H2ꢀTPR profiles of the catalysts.
3.1.5. XPS analysis
In order to obtain the surface chemical composition over the catalysts, XPS measurements of the
reduced 20NOMA and 20N6SOMA were carried out and the XPS spectra of Ni2p and Sm3d were
shown in Fig. 6. In Fig. 6a, for 20NOMA, it can be seen that the peak at around 853.1 eV attributes
to Ni0; the peaks of Ni2p at around 856.1 and 861.8 eV belonging to Ni2+ can be also observed,
which originates from the quick oxidation of Ni0 in air during the sample transfer process in the ex
situ XPS measurement.28 For 20N6SOMA, there are peaks at 856.1 and 861.8 eV of Ni2+, while the
peak of Ni0 shifts to lower value at 852.7 eV after addition of Sm species. The decrement indicates
that an interaction may occur between the Ni species and Sm species in 20N6SOMA. Here, Sm
species play the similar role with the MnOx and TiO2 species in the literatures,18, 50 which can
reduce the binding energy of Ni species. In addition, the XPS spectrum of Sm 3d was shown in Fig.
6b. The 3d5/2 peak at 1083.8 eV and 3d3/2 peak at 1110.9 eV attributing to Sm3+ (Sm2O3) could be
observed;51, 52 the weak peak at around 1081.2 eV related to Sm2+ (SmO) was also seen.53 This
result indicated that the Sm species was mainly presented as Sm2O3 (Sm3+) together with a small
amount of SmO (Sm2+) species over the reduced 20N6SOMA catalyst.21 Thus, the electron transfer
16

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from the Sm3+/Sm2+ redox could increase the electron cloud density of the Ni atoms (Ni0/Niδ–),18, 50
leading to the decrease in binding energy of Ni0. Similar with the reported literatures,18, 50 this may
facilitate CO dissociation on the catalyst surface and is beneficial for the enhanced CO methanation.
(b)
Ni 2p
Sm 3d
(a)
Ni0
856.1 eV
Ni2+
1083.8 eV
852.7 eV
Sm3+
861.8 eV
Ni2+
1110.9 eV
Sm3+
20N6SOMA
20NOMA
853.1 eV
1081.2 eV
Sm2+
865
860
855
850
845
1120 1110 1100 1090 1080 1070
Binding energy (eV)
Binding energy (eV)
Fig. 6. XPS spectra of the reduced 20NOMA and 20N6SOMA: (a) Ni 2p and (b) Sm 3d.
3.2. CO methanation activity
The catalysts were evaluated for CO methanation reaction and the results are exhibited in Fig. 7.
Also, the CO thermodynamic equilibrium data calculated by the Gibbs free energy minimization
method were given in Fig. 8a as the reference.54 20NA shows the poorest CO conversion among all
the catalysts. For 20NOMAꢀIMP, the catalytic activity is close to that of 20NA, which is also very
poor. On the contrary, although their components are identical, the activity of 20NOMA has been
obviously enhanced compared with that of 20NOMAꢀIMP. This result indicates that the catalyst
prepared by the oneꢀpot EISA method has higher catalytic activity than that prepared by the
impregnation method. After the addition of Sm species, the activity of 20N6SOMA catalysts can be
o
further improved with the maximum CO conversion and CH4 yield of 95% at 420 C and 74% at
17

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o
440 C, respectively. CO conversion of 20N6SOMA can nearly reach the thermodynamics
equilibrium above 380 oC, which further proves its high activity. In order to optimize the amount of
Sm species in the ordered mesoporous Ni‒Sm‒Al catalysts, 20N2SOMA, 20N4SOMA and
20N8SOMA were also prepared using the same oneꢀpot EISA method, and the CO methanation
activity results were shown in Fig. S1. Among them, 20N2SOMA shows the poorest activity, while
the activity can be enhanced with the increase of Sm species. However, the activity of 20N8SOMA
decreased compared with that of 20N6SOMA, indicating that the excess Sm species may be adverse
for CO methanation.2 Although the mainly detected byꢀproduct is CO2, the variation tendency of
CH4 selectivity is very complicated in Fig. 7b and S1b, and the different preparation methods as
well as the addition of Sm species seem to have a very small effect on the CH4 selectivity. In
addition, 20N6SOMA has higher CO conversion than the reported Ni20Cr2ꢀOMA14 and 20%
NiꢀSi/SiO255, which have the same NiO loading and much lower WHSV at 0.1 MPa. Also,
o
20N6SOMA has higher lowꢀtemperature activity below 400 C than that of NiOꢀLa2O3/Al2O3
56
catalyst with 3 wt% La2O3. The above results can further prove that Sm2O3 is an efficient
promoter for CO methanation. After carefully checking the catalytic activity of different catalysts,
we find that there is a significant relationship between Ni particle size and the activity (Table 1 and
Fig. 7). The catalysts prepared by the oneꢀpot EISA method with the smaller Ni particle size and
higher Ni dispersion show higher catalytic activity than those prepared by the impregnation method.
In short, combined with the above results, both the preparation method and addition of Sm species
can affect the catalytic activity obviously, and 20N6SOMA is the best catalyst owing to its smallest
Ni particle size after addition of moderate amount of Sm species.57
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100
100
80
60
40
20
0
(b)
(a)
80
60
40
Thermodynamics
20NA
20NOMA
20N6SOMA
20NA
20NOMA
20N6SOMA
20NOMAꢀIMP
20
20NOMAꢀIMP
0
275
325
375
425
280 320 360 400 440
Temperature (oC)
Temperature (oC)
Fig. 7. Catalytic properties of the catalysts at 0.1 MPa, 180000 mL g–1 h–1: (a) CO conversion and
(b) CH4 selectivity.
3.3. Catalyst stability
A 101hꢀlifetime test of 20NA, 20NOMAꢀIMP and 20N6SOMA was carried out using a harsh
condition at 550 °C, 180000 mL g–1 h–1, 0.1 MPa, and the results are displayed in Fig. 8. The CO
conversion over 20NA decreases by 17.8 %, although its CH4 selectivity remains stable in the total
test. In contrast, for the ordered mesoporous catalysts 20N6SOMA and 20NOMAꢀIMP, the
catalytic activities are much higher and remain constant in the total 101hꢀlifetime test. For
20N6SOMA, the Ni particles are anchored in the OMA framework with strong interaction. For
20NOMAꢀIMP, although some Ni particles distribute outside of the channels with the relatively
weak interaction with the OMA support, the major Ni particles are still in the inside of the channel,
which provides the well confinement effect for metallic Ni particles (Fig. 1d and Fig. 5). This result
indicates that the excellent stability is closely associated with the confinement effect of the ordered
mesostructure for stabilization of the metallic Ni species.
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(a)
(b)
80
80
60
40
20
60
40
WHSV=180000 mL—gꢀ1—hꢀ1
WHSV=180000 mL—gꢀ1—hꢀ1
20NA
20NOMAꢀIMP
20N6SOMA
20NA
20NOMAꢀIMP
20N6SOMA
20
0
0
20 40 60 80 100
Time(h)
0
20 40 60 80 100
Time(h)
Fig. 8. Lifetime test of 20N6SOMA, 20NOMAꢀIMP and 20NiA at 550 oC, 0.1 MPa, 180000 mL g‒1
h‒1: (a) CO conversion and (b) CH4 selectivity.
3.4. Characterization of the spent catalysts
In order to investigate the reasons of the high stability of ordered mesoporous catalysts, the spent
catalysts were characterized by TEM, SEM, XRD and TG analysis. As seen in Fig. 9a, the obvious
o
Ni sintering can be seen over 20NAꢀST (9.2 nm) compared with the 710 C reduced one in Fig. 1a
(20NAꢀRe, 6.1 nm). For 20NOMAꢀIMP, most of the Ni particles remain their small sizes around
5.0 nm, while a large Ni particle of 21.0 nm can be also observed (Fig. 9b). After careful analysis,
the Ni particles outside of the channels are apt to agglomerate, and those inside the channels can
retain their sizes.7 Meanwhile, the Ni particles of 20N6SOMA can maintain their small sizes (4.1
nm), indicating its excellent Ni antiꢀsintering property (Fig. 9c). The average Ni nanoparticle sizes
of the spent 20NA, 20NOMAꢀIMP and 20N6SOMA derived from XRD patterns are 8.5, 6.1 and 3.5
nm, respectively. Combined with the TEM and XRD results, it is apparent that the catalyst prepared
by oneꢀpot EISA method has the highest antiꢀsintering performance due to its excellent confinement
effect.
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(a)
(b)
1
3
5
7
9 11131517192123
1
3
5
7
9 11131517192123
Particle Size (nm)
Particle Size (nm)
ꢂ
ꢁ Al2O3
Ni
ꢂ
(d)
(c)
ꢂ
ꢂ
20N6SOMAꢀST
20NOMAꢀIMPꢀST
ꢁ
ꢁ
ꢁ
20NAꢀST
20
40
60
80
2
4
6
8
Particle Size (nm)
2 Theta (degree)
Fig. 9. TEM images of spent catalysts: (a) 20NAꢀST, (b) 20NOMAꢀIMPꢀST, and (c)
20N6SOMAꢀST; XRD patterns of the catalysts after lifetime tests (d). (Note: The spent catalysts
after the lifetime test were denoted as 20NAꢀST, 20NOMAꢀIMPꢀST and 20N6SOMAꢀST,
respectively.)
(a)
(b)
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104
(d)
(c)
102
100
98
1.4%
0.6%
7.6%
20NAꢀRe
20NAꢀST
20NOMAꢀIMPꢀRe
20NOMAꢀIMPꢀST
20N6SOMAꢀRe
20N6SOMAꢀST
96
94
300
600
900
Temperature (oC)
Fig. 10. SEM images of spent catalysts: (a) 20NAꢀST, (b) 20NOMAꢀIMPꢀST, and (c)
20N6SOMAꢀST; TG curves of the reduced and spent catalysts in air (d). (Note: The reference
o
catalysts reduced in H2 flow at 710 C for 1 h were denoted as 20NAꢀRe, 20NOMAꢀIMPꢀRe and
20N6SOMAꢀRe, respectively.)
SEM images of the spent 20NA, 20NOMAꢀIMP and 20N6SOMA catalysts are shown in Fig.
10a−c. For 20NOMAꢀIMP and 20N6SOMA, no carbon filament can be observed even after the
101hꢀlifetime test.58 On the contrary, some filamentous carbon59 can be observed on the surface of
the spent 20NA catalyst (Fig. 10a). The amount of carbon deposited on the spent catalysts is further
measured by TG analysis, and the results are presented in Fig. 10d. There were slight decrements on
the TG curves of the spent catalysts because of their small amounts of adsorption water. The weight
o
o
increase with peak value at around 420 C (except for 20NOMA at 470 C) in Fig. 10d can be
attributed to the oxidation of Ni to NiO. This is consistent with the oxidation of NiꢀYolk@Ni@SiO2
sample.60 The carbon content in the spent 20NA, 20NOMAꢀIMP and 20N6SOMA is estimated to
be 7.6, 1.4 and 0.6 wt%, respectively, suggesting the enhanced resistance to coke formation over the
ordered mesoporous catalysts. Taherian et al. found that the incorporation of Sm2O3 into the
Ni/SBAꢀ15 catalyst diminished the coke formation because oxygen vacancies on the surface of
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Sm2O3 led to reaction of active oxygen with the deposited carbon.61 Thus, the Sm2O3 species may
play the similar role in diminishing formation of coke over 20N6SOMA. Moreover, there is no new
diffraction peak corresponding to graphitic carbon in the XRD pattern of the spent 20NA (Fig. 9d),
indicating that its deposited carbon is amorphous.62 In all, compared with the catalysts prepared by
the conventional impregnation method, the ordered mesoporous 20N6SOMA catalyst clearly shows
higher antiꢀcoking and antiꢀsintering properties.
4. Conclusions
In order to investigate the effect of different preparation method and the addition of Sm species
on catalytic activities and stability for CO methanation, the ordered mesoporous NiO–Sm2O3–Al2O3
catalysts were synthesized by the oneꢀpot EISA method; also, the ordered mesoporous catalysts
prepared by the conventional impregnation method as well as the unordered mesoporous
NiO/γꢀAl2O3 catalyst prepared by impregnation method were selected as the reference. The ordered
mesoporous NiO–Al2O3 catalyst prepared by the oneꢀpot EISA method (20NOMA) showed higher
catalytic activity than that prepared by impregnation method (20NOMAꢀIMP). After addition of Sm
species, the catalytic activity of the ordered mesoporous NiO–Sm2O3–Al2O3 catalyst could be
further improved. This is because Sm2O3 can improve the reducibility of Ni particles and decrease
the Ni particle sizes and leading to the higher Ni dispersion. The Sm species were mainly in the
form of Sm3+ (Sm2O3) with a small amount of Sm2+ (SmO) over the reduced catalysts. The electron
transfer from the Sm3+/Sm2+ redox could increase the electron cloud density of Ni, which resulted in
the intensified catalytic performance over the ordered mesoporous NiO–Sm2O3–Al2O3 catalyst. The
amount of Sm species was optimized, and the ordered mesoporous 20N6SOMA catalyst is the best.
Besides, the sample could maintain ordered mesoporous structure after doping of 20 wt% NiO and
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6 wt% Sm2O3 species. Moreover, the optimal 20N6SOMA catalyst shows superior stability in a
o
101hꢀlifetime test at 550 C, 0.1 MPa because of the confinement effect of ordered mesoporous
structure. In brief, the ordered mesoporous NiO–Sm2O3–Al2O3 catalyst could be considered as a
promising catalyst candidate with enhanced catalytic activity and stability for CO methanation.
Acknowledgments
The authors gratefully acknowledge the supports from National Key Research and Development
Program of China during the 13th FiveꢀYear Plan Period (No. 2017YFC0805203), National Natural
Science Foundation of China (No. 21606146), Natural Science Foundation of Shandong Province
(Nos. ZR2016BB17 and 2016ZRB01037), Scientific Research Foundation of Shandong University
of Science and Technology for Recruited Talents (Nos. 2016RCJJ005 and 2016RCJJ006), and
China National Coal Association Science and Technology Research Program (No.
MTKJ2016ꢀ266).
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Page 29 of 29
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ02282H
Graphical Abstract
The well-dispersed Ni and Sm2O3 nanoparticles embedded in ordered mesoporous Al2O3 material
simultaneously exhibits high catalytic activity and stability for CO methanation reaction.