Preparation of Ni based mesoporous Al2O3 catalyst with enhanced CO2 methanation performance

Article abstract of DOI:10.1039/c8ra10348h

A Ni based mesoporous γ-Al₂O₃ (MA) catalyst was prepared via partial hydrolysis without organic surfactants and employed in the carbon dioxide methanation reaction. The obtained catalysts were characterized by N₂ adsorption-desorption, H₂-TPR, XRD, XPS, TG, SEM and TEM-EDS techniques. CO₂ methanation was performed in a fixed-bed reactor. A high surface area of MA with excellent hydrothermal stability was obtained, which promoted the dispersion of nickel species, producing a better catalytic performance. Incorporation of more NiO species into the Ni/MA catalyst increased the amount of active metallic Ni sties, further improving the catalytic activity and CH₄ selectivity. Moreover, the monolithic skeleton of MA with fabric-like walls suppressed the aggregation of active metallic Ni sites and carbon deposition, enhancing the catalyst's stability, which provides a new insight for potential industrial applications.

Full text of DOI:10.1039/c8ra10348h

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PAPER
Preparation of Ni based mesoporous Al O catalyst
2
3
with enhanced CO methanation performance†
2
Cite this: RSC Adv., 2019, 9, 8684
a
bc
d
a
a
Jianghui Lin, ‡ Caiping Ma,‡ Jing Luo, Xianghui Kong, Yanfei Xu,
a
a
e
a
Guangyuan Ma, Jie Wang, Chenghua Zhang, * Zhengfeng Li
and Mingyue Ding*
af
A Ni based mesoporous g-Al
surfactants and employed in the carbon dioxide methanation reaction. The obtained catalysts were
characterized by N adsorption–desorption, H -TPR, XRD, XPS, TG, SEM and TEM-EDS techniques. CO
2 3
O (MA) catalyst was prepared via partial hydrolysis without organic
2
2
2
methanation was performed in a fixed-bed reactor. A high surface area of MA with excellent
hydrothermal stability was obtained, which promoted the dispersion of nickel species, producing a better
catalytic performance. Incorporation of more NiO species into the Ni/MA catalyst increased the amount
Received 17th December 2018
Accepted 26th February 2019
4
of active metallic Ni sties, further improving the catalytic activity and CH selectivity. Moreover, the
monolithic skeleton of MA with fabric-like walls suppressed the aggregation of active metallic Ni sites
and carbon deposition, enhancing the catalyst's stability, which provides a new insight for potential
industrial applications.
DOI: 10.1039/c8ra10348h
rsc.li/rsc-advances
Fe, Ru, Rh, Ni, Co, etc. are used widely, in which Ru and Rh-
based catalysts have been reported as the most effective cata-
1
Introduction
2
Synthetic natural gas (SNG), as an efficient and clean energy lysts for CO methanation, whereas higher cost and limited
7
carrier, has the advantages of high combustion efficiency and availability restrict their applications. Due to relatively high
using existing gas pipelines, attracting more and more attention methanation activity and low price, Ni based catalysts are
8–10
2
with the depletion of natural gas and rapid rising of natural gas utilized extensively in the CO methanation process, but they
1,2
prices. The emission of carbon dioxide into the atmosphere is are easily suffered from the deactivation due to carbon depo-
considered as the major cause of global warming. Many sition and nickel sintering compared to noble metal cata-
1
1,12
methods including CO
verting CO
into useful compounds etc. have been used to formation of CH
reduce CO emissions, in which the CO
methanation reaction methanation reaction is an eight-electron process with kinetic
CO + H / CH + H O, DG ¼ ꢀ113 kJ mol ) as an efficient barrier. However, complete conversion of CO
2
capture, separation, storage and con- lysts.
Low temperature is in favor of CO
2
conversion and the
according to thermodynamic equilibrium that
2
4
2
2
ꢀ
1
13
(
2
4
into CH at low
2
2
4
2
approach for converting CO directly to SNG has drawn temperature for Ni based catalysts is very difficult from
2
3
14
increasing interest.
In recent years, numerous studies on CO
been carried out for metal-based catalysts with enhanced cata-
kinetics. It is necessary to modify the Ni-based catalyst for
methanation have promoting its CO methanation performance.
Promotors (such as MgO, CaO, La
lytic activity and stability. Many group VIIIB metals such as supports (including SiO , Al , ZrO and CeO
2
2
15–17
O
2 3
and so on),
) and reaction
4–6
O
2
2
3
2
2
conditions (temperature, pressure, space velocity and so on) are
a
extensively adopted to optimize Ni-based catalysts, which play
School of Power and Mechanical Engineering, Hubei International Scientic and
Technological Cooperation Base of Sustainable Resource and Energy, Wuhan an important role in improving the performance for CO
2
18–21
University, Wuhan 430072, China. E-mail: dingmy@whu.edu.cn; zhangchh@sxicc. methanation.
Especially, the nature of the support has
ac.cn
a signicant inuence on the dispersion of active metallic sites
and interaction of metal–support. CeO (ref. 22) and/or TiO
b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
8
2
2
Academy of Sciences, Taiyuan 030001, China
(
ref. 23) supported Ni based catalysts display excellent CO2
c
University of Chinese Academy of Sciences, Beijing 100049, China
d
methanation activity and CH4 selectivity by providing oxide
vacancies and enhancing H adsorption, whereas active Ni
Xiamen Tobacco Industrial Co., Ltd., Xiamen 361022, China
e
2
Synfuels China Co. Ltd., Beijing 101407, China
f
nanoparticles undergo easily the thermal sintering due to their
Key Laboratory of Advanced Energy Materials Chemistry, Ministry of Education,
Nankai University, Tianjin 300071, China
low Tammann temperature and “hot spots” existed inside the
†
1
‡
Electronic supplementary information (ESI) available. See DOI: catalyst bed. In addition, mesoporous molecular sieves pos-
0.1039/c8ra10348h
sessing large surface area and ordered mesoporous channel
These authors contributed equally to this work.
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ꢁ
2
have been developed widely to optimize CO methanation calcined at 550 C for 5 h. In this process, the content (wt%) of
24–26
performance.
However, the silica based zeolites occur easily Ni was changed from 10 to 25%. The prepared Ni-based cata-
the collapse of mesoporous structures because of their intrinsic lysts were denoted by XNi/MA, where X denotes the content of
nature of the hydrothermal instability. As a result, designing Ni in weight percentage. In contrast, the commercial alumina
mesoporous materials with high dispersion of Ni nanoparticles (CA) modied Ni catalyst was prepared by the impregnation
and excellent thermal stability is necessary for CO
methanation.
2
method. The content of Ni was 25% (wt) in the Ni/CA catalyst,
which is denoted to 25Ni/CA.
Mesoporous alumina with large surface area and good
2
7
thermal stability is used as a potential support. The sol–gel
method is applied for the preparation of porous alumina, which
results easily in the formation of inhomogeneous porous
2
.2 Catalyst characterization
N
2
adsorption and desorption isotherms were measured by an
automatic physical adsorption analyzer (Micromeritics
ASAP2020). The sample was degassed at 150 C for 3 h to remove
28
29
structure via the cracking of gels. Xu et al. developed a one-
step evaporation induced self-assembly (EISA) strategy to
synthesize the ordered mesoporous alumina material, which
presented the strong metal–support interaction and
outstanding thermal stability compared to the traditional non-
ꢁ
physical absorbed water and impurities on the surface before
the measurement. The specic surface area was calculated by
Brunauer–Emmett–Teller (BET) method and pore size distri-
bution was analyzed by Barrett–Joyner–Halenda (BJH) method.
Powder X-ray diffraction (XRD) was performed with a Bruker
2 3
mesoporous Al O support, displaying better catalytic perfor-
mance. However, low-temperature CO2 methanation perfor-
mance is still needed to be improved. Developing a novel and
economic stable mesoporous alumina with large surface area
and narrow pore size distribution increasingly draws attention.
Recently, Shang et al. prepared a new mesoporous g-alumina
MA) material by partial hydrolysis without the adding of
organic surfactants, which showed a high surface area and
excellent thermal stability up to 900 C. However, an application
of this kind of MA supported Ni based catalyst to CO metha-
AXS-D8 Advance (Germany) diffractometer using Co Ka (g ¼
˚
1.78 A) radiation at 35 kV and 40 mA. XPS spectra of the catalysts
were checked by a VG system (MultiLab 2000), using an Al Ka X-
ray source (1486.6 eV). The C 1s as a reference signal was
adjusted to 284.6 eV.
30
(
H -TPR measurements were performed in a quartz tube
2
equipped with a thermal conductivity detector (TCD). The
samples were pretreated with high purity N at 150 C and a ow
rate of 30 mL min for 1.5 h to remove water and other
contaminants. The samples were cooled down to room
temperature, and then 5% H /N was introduced into the
2 2
system at a ow rate of 30 mL min . The TCD signal and
sample temperature were recorded while the temperature being
ramped to 900 C at a heating rate of 10 C min
Scanning electron microscopy (SEM) images were obtained
on a QUANTA F250 scanning electron microscope to charac-
terize the surface morphology of the catalysts. Energy dispersive
X-ray spectroscopy (EDX) was performed on a GENESIS EDX
detector. High resolution transmission electron microscopy
ꢁ
ꢁ
2
2
ꢀ1
nation has never been reported so far.
Herein, we prepared a Ni based catalyst supported on mes-
oporous g-alumina synthesized by partial hydrolysis of
a nontoxic and inexpensive inorganic aluminum salt. The
nickel species loading, interaction of Ni–MA, and CO
nation performance were studied in details. The catalysts were
characterized by N physisorption, scanning electron micros-
copy (SEM), transmission electron microscopy (TEM), X-ray
photoelectron spectroscopy (XPS), powder X-ray diffraction
XRD), hydrogen temperature-programmed reduction (H -TPR)
and thermogravimetric analysis (TG) for obtaining the struc-
ꢀ
1
2
metha-
ꢁ
ꢁ
ꢀ1
.
2
(
2
(HRTEM) experiments were performed in a Philips CM200 high
ture–performance relationship.
resolution transmission electronic microscopy with 200 kV
accelerating voltage.
2
Experimental
The amounts of carbon deposition at the used catalysts were
determined by a TGA/DSC analyzer (METTLER TOLEDO). Aer
drying at room temperature, a certain amount of sample was
2.1 Catalyst synthesis
MA was prepared via a one-pot template-free partial hydrolysis
path, as presented in the article. Typically, 0.1 mol of
Al(NO
ꢁ
ꢁ
ꢀ1
heated to 900 C in air with a heating rate of 10 C min on an
Al crucible.
30
2 3
O
3
)
3
$9H
2
O was dissolved in 50 mL of deionized water at
ꢁ
ꢀ1
75 C, and 1 mol L (NH ) CO aqueous solution was dropped
4
2
3
2.3 Catalyst testing
slowly under vigorous magnetic stirring until the formation of
transparent gel. The gel was aged for 24 h, and then dispersed in CO
2
methanation was performed at atmospheric pressure in
ꢁ
a watch glass at 100 C for 12 h. The as-prepared solid was a vertical continuous-ow xed-bed reactor (inner diameter of
ꢁ
treated at 200 C in air for 10 h to remove ammonium nitrate, 12 mm and a length of 330 mm). The ow of reaction gases was
ꢁ
and further calcined at 500 C for 10 h.
controlled using mass ow controllers. The actual reaction
The Ni/MA catalysts with different nickel contents were temperature of the catalyst was monitored using a thermo-
prepared by impregnation method. A certain amount of couple placed in the middle of the tubular reactor. 500 mg of
Ni(NO ) $6H O was dissolved in distilled water under the stir- the catalyst mixed with 500 mg quartz sand (40–70 mesh) were
3
2
2
ring and then the MA support was added into nickel nitrate placed between two layers of quartz wool. Before the reaction,
ꢁ
solution under the stirring for 12 h at room temperature. the catalyst was reduced in situ at 500 C with a heating rate of
ꢁ
ꢁ
ꢀ1
Aerwards, the suspension was dried at 100 C for 12 h and
2 C min under a ow of H at gas hourly space velocity
2
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GHSV) of 2000 mL g
Paper
ꢀ1
ꢀ1
(
h
for 10 h. Subsequently, the
P P
Surface area (SBET), pore volume (V ) and pore diameter (D )
ꢁ
temperature was cooled to 200 C and the mixtures of H and of the MA support and Ni/MA catalysts with different nickel
2
CO (molar ratio H /CO ¼ 4/1) were introduced into the reactor loadings are presented in Table 1. BET surface area, average
2
2
2
ꢀ
1
ꢀ1
2
ꢀ1
3
at 6000 mL g
h
of GHSV. The catalytic tests were carried out pore volume and pore diameter of MA is 237.0 m g , 0.32 cm
ꢁ
ꢀ1
at different temperatures, ranging from 200 to 500 C.
g
and 3.1 nm, respectively. In contrast, BET surface area of CA
2
ꢀ1
2
ꢀ1
The gas products were analyzed using an on-line gas chro- is 166.2 m g , which is lower than that of MA (237.0 m g ),
matogram (Fuli 9790II) equipped with a TCD detector using demonstrating that preparation of MA by partial hydrolysis of
porapak Q and Molecular Sieve 5A packed column. The values inorganic aluminum salt facilitates the increase of BET surface
of CO
2
conversion (XCO2) and CH
4
selectivity (SCH4) were calcu- area compared to the traditional preparation method of CA. As
Ni species is added into the MA support, the BET surface area
lated by the following equations:
presents a slight decreasing trend. The BET surface area
decreases slowly from 158.8 m g of 10Ni/MA to 152.4 m g
of 25Ni/MA. The average pore diameter of Ni/MA catalysts is in
the range of 3.3–4.0 nm (Table 1), while the average NiO crys-
talline diameter calculated by Scherrer equation from XRD is
W
CO ;in ꢀ WCO ;out
2
2
2
ꢀ1
2
ꢀ1
X
CO2
¼
ꢂ 100%
ꢂ 100%
W
CO ;in
2
W
CH ;out
4
S
CH4 ¼ W
CH ;out þ WCO;out
4
where WCO ,in and WCO ,out are relative volume percentage of over 10 nm, indicating that Ni species are dispersed mainly on
2
2
CO
relative volume percentage of CH and CO in the effluent gas, of MA. Therefore, it is possible that partial blockage of NiO
2
in the feed and effluent gas, and WCH ,out and WCO,out are the MA surface without entering into the mesoporous structures
4
4
respectively.
particles on the surface of MA results in the decrease of BET
32
surface area and pore volume. In addition, with the gradual
increasing of Ni species content, the NiO nanoparticles size
presents an increasing trend. The crystalline size of NiO
increases from 13.6 to 22.7 nm as the Ni content increases from
10 to 25 wt%, implying that the adding of more Ni species
facilitates the formation of larger crystallites. On the other
3
Results and discussion
3.1 Catalyst characterization
3
.1.1 adsorption analysis. The N adsorption–desorption
N
2
2
2
ꢀ1
hand, the 25Ni/CA catalyst presents 100.3 m g of BET surface
isotherms and corresponding pore size distribution curves of the
MA support and Ni/MA catalysts are shown in Fig. 1a and b,
respectively. It is apparent from Fig. 1a that the MA support
displays the IV type isotherm with an apparent hysteresis loop
according to the IUPAC classication. The type H2 hysteresis loop
2
area, which is only two-thirds of the 25Ni/MA catalyst (152.4 m
g
ꢀ1
), whereas the crystallites size (31.7 nm) of Ni species over
the 25Ni/CA is 1.4 times bigger than that of 25Ni/MA, revealing
better dispersion ability of MA compared to CA.
3.1.2
2
H -TPR results. The reduction behavior of the Ni/MA
of the MA support appears in the region of 0.4–0.7 p/p , indi-
0
31
catalysts with different nickel loadings is analyzed by H -TPR in
cating a narrow mesopore size distribution. The mesoporous
structures of MA remain unchanged aer the adding of NiO
species, as shown in Fig. 1a, suggesting that mesoporous struc-
tures of MA are not destroyed aer adding the Ni species. BJH
pore size analysis shown in Fig. 1b presents that all of the Ni/MA
catalysts exhibit narrow pore size distribution around 3.5 nm.
2
Fig. 2a. It is reported that the reduced Ni species are usually
divided into three types (a, b and g) based on the peak
33
temperature. The a-type NiO is free nickel oxide located in the
ꢁ
low temperature (about 420 C), which has a weak interaction
2 3
with the Al O support. The b-type is assigned to the NiO species
2
Fig. 1 (a) N adsorption–desorption isotherms and (b) BJH pore size distributions of the support and Ni/MA catalysts.
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Table 1 Textural properties of the as-prepared catalysts
a
2
ꢀ1
b
p
c
p
3
ꢀ1
d
Sample
S
BET /(m g
)
D
/(nm)
V
/(cm g
)
Crystalline size /(nm)
CA
MA
166.2
237.0
158.8
156.7
156.6
152.4
100.3
4.69
3.10
3.36
3.81
3.74
4.00
4.97
0.32
0.25
0.18
0.20
0.19
0.22
0.25
—
—
1
1
2
2
2
0Ni/MA
5Ni/MA
0Ni/MA
5Ni/MA
5Ni/CA
13.6
15.8
17.6
22.7
31.7
a
b
c
d
Calculated by the BET equation. BJH desorption average pore diameter. BJH desorption pore volume. Average NiO crystalline diameter
calculated by Scherrer equation from XRD.
possessing a middle interaction with the support in the mild- catalyst with higher nickel loading facilitates the reduction of
ꢁ
temperature region (about 450–650 C). The g-type belongs to nickel oxide species. As known mentioned above (Table 1), the
the less reducible NiAl
(
2
O
4
in the high-temperature region NiO nanoparticles size increases gradually with the increase in
about 650–850 C). As shown in Fig. 2a, reduction peaks of Ni species content. It is possible that a larger NiO particle size in
nickel species shi slowly towards lower temperature with the the Ni/MA catalysts weakens the metal–support interaction,
ꢁ
34
35
increasing in NiO concentration, indicating that the Ni/MA promoting the reduction of NiO species. In addition, the
2
Fig. 2 (a) H -TPR profiles of the fresh Ni/MA catalysts, (b) XRD patterns of the fresh Ni/MA catalysts, (c) XRD patterns of the spent Ni/MA catalysts
and (d) XPS spectra in Ni 2p of the fresh Ni/MA catalysts.
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Table 2 XPS calculate data of the fresh Ni/MA catalysts
3.1.4 XPS analysis. Phase compositions on the surface
layers of the Ni/MA catalysts are further analyzed by XPS. As
shown in Fig. 2d, a main Ni 2p3/2 peak around 855.8 eV with
a shoulder peak around 862.0 eV have appeared for all of the Ni/
Sample
S
T
NiO/S (%)
1
1
2
2
0Ni/MA
5Ni/MA
0Ni/MA
5Ni/MA
8.1
2
+
12.7
19.1
20.3
MA catalysts, which are attributed to the Ni peaks in the form
36
of NiAl O spinel species, indicating that the NiAl O species
2
4
2 4
is formed on the surface of the Ni/MA catalysts. In addition, a Ni
p3/2 peak around 854.0 eV also exists, corresponding to the NiO
2
37
species. The intensity of NiO peak on the surface layers pres-
ents a slightly increasing trend with the increasing Ni concen-
intensity of b-type NiO presents an increasing trend with the
increasing in nickel loading, implying that incorporation of
more NiO species in the Ni/MA catalysts is in favor of the
formation of b-type NiO species. Xu et al. prepared the mes-
oporous NiO modied Ni catalyst via one-step evaporation
induced self-assembly method, which is composed mainly of g-
tration in the Ni/MA catalysts. The ratio of SNiO to S
T
(SNiO +
S
NiAl O4) over the Ni/MA catalysts is calculated according to the
2
29
2 4
area integrals of NiO and NiAl O , and listed in Table 2. It can
be found that the ratio of SNiO to S increases from 8.1 to 20.3%
T
with the increase of Ni loading from 10 to 25 wt%, revealing the
formation of more NiO on the catalyst surface with higher
nickel contents. The increasing NiO content for the Ni/MA
catalyst with higher Ni loading promotes the formation of
2 4
type NiAl O with much strong metal–support interaction that
0
hard to be reduced to Ni active site. Therefore, the formation of
b-type NiO and a-type NiO in the present study suggests that the
MA modied Ni catalyst via partial hydrolysis without using
organic surfactants decreases the interaction of Ni–Al O ,
2 3
promoting the reduction of Ni species.
active metallic Ni sites for CO
ment with the results of XRD and H
.1.5 SEM and TEM results. The morphologies and porous
2
methanation. This is in agree-
2
-TPR.
3
3.1.3 XRD analysis. Fig. 2b shows XRD patterns of the
structures of the support and Ni/MA catalysts are characterized
by SEM. It can be seen from Fig. 3a that the MA support consists
of various types of monolithic skeletons with porous frame-
work, which may be stemmed from the nanoparticles
aggregation.
support and fresh Ni/MA catalysts with different nickel
ꢁ
ꢁ
contents. It is seen that diffraction peaks at 2q ¼ 22.5 , 43.9 ,
ꢁ ꢁ
5
3.6 , 79.2 are observed for the support, which are attributed to
alumina phase (JCPDS no. 50-0741). Combined with the BET
results, it is considered that the MA support is composed
As nickel species are added into MA, the fabric-like walls
with inside diameter about 100 nm are displayed on the surface
layers (Fig. 3b–e). These porous frameworks are well cross-
linked together to form the fabric-like walls. It is possible that
the combination of Ni species with MA during the impregnation
and calcination processes changes the surface morphologic
structures of Ni/MA. NiO spherical particles are observed inside
the fabricate walls, and the nanoparticles size increases with the
increasing nickel content in the Ni/MA catalysts. Aer reaction,
the fabric-like structures remain almost unchanged (Fig. 3f–i),
validating the excellent hydrothermal stability of Ni/MA. There
is no particle aggregation of Ni species is during reaction,
suggesting that the MA support played an important role in
restraining the sintering of Ni particles. In addition, no obvious
carbon deposition is observed for the Ni/MA catalyst in the SEM
patterns, indicating that the distinct monolithic skeleton of MA
facilitates the transfer of reaction and product molecules, and
suppresses the carbon deposition on the surface of MA.
TEM measurements are performed to characterize the
structure and distribution of Ni species over the 25Ni/MA
catalyst (Fig. 4). Two kinds of lattice spacing about 0.24 nm
and 0.21 nm are observed on the surface of MA (Fig. 4a and b),
2 3
mainly of mesoporous g-Al O . As nickel species is added into
ꢁ
ꢁ
MA, diffraction peaks of NiO phase appear at 2q ¼ 43.5 , 50.7 ,
ꢁ
7
4.5 (JCPDS no. 71-1179). In addition, broad diffraction peaks
ꢁ
ꢁ
ꢁ
at 2q ¼ 43.3 , 52.8 , 77.8 (JCPDS no. 10-0339) assigned to
NiAl are observed in the Ni/MA catalysts. This indicates that
incorporation of nickel species into MA results in the formation
of NiO and NiAl O , which is consistent with the H -TPR results.
2 4
O
2
4
2
It is apparent that the intensity of NiO diffraction peaks
increases gradually with the increase of Ni content from 10 to
2
5 wt%, which demonstrates that incorporation of more Ni
32
species into MA promotes the formation of NiO phase.
Combined with the BET results, it is considered that the NiO
particles are formed mainly on the surface of MA without
entering into the pore channel. Besides, the intensity of NiAl O
2
4
diffraction peaks presents a slightly decreasing trend with the
increasing Ni loading, implying that the Ni loading increasing
weakens the interaction of metal–support via the increase of
particles size of NiO, decreasing the amount of Ni-Al solid
solution.
ꢁ
ꢁ
Aer reaction, new diffraction peaks at 2q ¼ 52.2 , 61.0
appeared in the XRD patterns (shown in Fig. 2c) are assigned to
Ni phase (JCPDS no. 87-0712). It is found that the intensity of Ni
diffraction peaks presents an increasing trend with the
increasing in nickel species content. The highest fraction of
metallic Ni phase is observed over the 25Ni/MA catalyst, which
conrms the formation of more metallic Ni sites for the Ni/MA
catalyst with higher Ni loading. It is accepted that metallic Ni
38,39
corresponding to NiO (111) and (200) crystals, respectively.
The high-angle annular dark eld scanning transmission elec-
tron microscopy (HAADF-STEM) imaging and elemental
mapping from Fig. 4c–f show the distribution of Ni, Al and O
elements, respectively. To the marked nanoparticle (red circle
marked in Fig. 4c) anchored outside the MA surface, only Ni and
O are present while no Al is observed in the mapping shown in
Fig. 4d–f, demonstrating the only presence of NiO nanoparticles
sites play a crucial role in promoting the CO
reaction.
2
methanation
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Fig. 3 SEM images of (a) MA support, (b–e) fresh catalysts: 10/15/20/25 Ni/MA and (f–i) spent catalysts: 10/15/20/25 Ni/MA.
Fig. 4 (a, b) TEM, (c) STEM-HAADF, (d) Al map, (e) Ni map and (f) O map images of the fresh 25Ni/MA catalyst.
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Fig. 5 (a) TEM, (b) STEM-HAADF, (c) Al map, (d) Ni map, (e) O map images of the spent 25Ni/MA catalyst.
on the MA surface. This further validates that Ni species added indicating that the NiO phase on the surface of 25Ni/MA has
in the Ni/MA catalysts is mainly covered on the surface layers been converted completely to metallic Ni in the CO metha-
2
without entering into the mesoporous structures of MA. The nation reaction. The spent Ni nanoparticles size is about
NiO nanoparticle size is about 30 nm, which is comparative with 25 nm, indicating that the Ni particles remain a stable state,
the value calculated by XRD. The combination of TEM, SEM and and no sintering of Ni nanoparticles appeared during reac-
XRD provides an unambiguous illustration on the NiO particle tion. This might be attributed to the fabric-like walls of MA,
size distribution of Ni/MA.
suppressing the aggregation of Ni nanoparticles during the
TEM images and elemental mapping of the used 25Ni/MA reaction.
catalyst are shown in Fig. 5. As shown in Fig. 5a, the meso-
porous structures of 25Ni/MA remain unchanged during
3
.2 Catalyst testing
reaction, indicating that the reaction of CO2 methanation
does not destroy the mesoporous structures. The elemental
mapping shown in Fig. 5b–e presents the existing of Ni, Al and
O, respectively. Only metallic Ni element (Fig. 5d) is observed
for the marked nanoparticle (red circle in Fig. 5b) on the
surface layers, whereas no Al or O exists (Fig. 5c and e),
CO methanation performances of the Ni/MA catalysts are
2
shown in Fig. 6. As shown in Fig. 6a, the catalytic activity of all of
the catalysts increases rstly with the increasing reaction
temperature, and then reaches the maximum value at a speci-

ed temperature. Subsequently, the catalytic activity begins to
selectivity of Ni/MA catalysts, GHSV ¼ 6000 mL gꢀ
1
h
ꢀ1
, H
/CO
¼ 4, 1 atm.
Fig. 6 (a) CO
2
conversion and (b) CH
4
2
2
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decrease with further increasing temperature. A similar provides more active sites for CO
2
methanation. Especially, the
changing trend is observed for CH4 selectivity (Fig. 6b). The adding of more Ni loading increases the particles size of NiO,
maximum value (77.2% of CO conversion and 99.9% of CH
as conrmed by XRD, which facilitates the weakened interac-
2
4
ꢁ
selectivity) is displayed at 360 C over the 25Ni/MA catalyst. tion of Ni–MA, promoting the formation of active metallic Ni.
ꢁ
When the temperature is higher than 360 C, CO
2
conversion It is also considered that the particle size of NiO plays an
metha- indirect role in promoting the catalytic activity.
and CH
4
selectivity begin to drop. As we known, CO
2
nation is an exothermic reaction, which is favorable at low
temperature based on its thermodynamic equilibrium.
On the other hand, it is seen that the CH
range of 250–400 C increases gradually with the increasing in
4
selectivity in the
40
ꢁ
Raising the reaction temperature facilitates the reverse water- nickel loading (shown in Fig. 6b). As the reaction temperature
ꢁ
gas shi reaction, increasing the amount of CO vice-product.
is over 400 C, CH selectivity for all of the catalysts begins to
4
The effect of nickel loadings on the catalytic performances decrease with the increasing temperature on stream, which
2
6
is further studied. From Fig. 6a it can be found that CO2 may be attributed to the reverse water gas shi reaction,
conversion presents an increasing trend with the increasing Ni resulting in the conversion of CO
2
to CO, decreasing the CH
4
content. The optimal catalytic activity increases from 62 to selectivity.
7
7% as the Ni content increases from 10 to 25%, accompanied
In addition, comparison of CO methanation performance
2
with the obvious decrease of suitable reaction temperature for MA modied and CA modied Ni catalysts are shown in
ꢁ
from 420 to 360 C. It is generally accepted that metallic Ni Fig. 7. It can be found that the MA modied Ni catalyst presents
plays a unique role in providing active sites for CO metha- higher catalytic activity and CH4 selectivity compared to the
2
3
5
nation. Rahmani et al. prepared the Ni/Al O catalysts with traditional CA modied Ni catalyst. The optimal CO conversion
2
3
2
ꢁ
different Ni loadings, and found that CO2 conversion of 25Ni/CA is 73.6% at 380 C, whereas that of 25Ni/MA is 77.2%
increased gradually with the increasing nickel loading in CO
methanation. Du et al. investigated CO
ꢁ
2
at 360 C, revealing better low-temperature CO
2
methanation
2
6
2 2
methanation activity by the modication of MA (Fig. 7a). According to the N
performance of Ni-MCM-41 catalysts containing 1–3 wt% Ni adsorption–desorption analysis and XRD results mentioned
species, and found that higher selectivity and space-time yield above, the MA support prepared by partial hydrolysis method
2
ꢀ1
were obtained for the Ni-MCM-41 catalyst with higher Ni displays BET surface area of 237 m g and average pore size of
loading. In the present study, Ni species added into the MA 3.1 nm. The commercial CA presents a lower BET surface area
2
ꢀ1
support is dispersed well on the MA support without entering about 1.4 times (166 m g ) and a higher pore size about 1.5
into the mesoporous structures, conrmed by the BET, SEM times (4.7 nm). Therefore, larger surface area and narrower pore
and TEM results. In addition, the H -TPR results indicate that size of MA facilitate the dispersion of active nickel species on
2
the amount of a-type and b-type Ni species increases with the the surface, promoting the catalytic activity at lower
Ni loading increasing, implying that incorporation of more Ni temperature.
species into MA weakens the metal–support interaction,
In order to further analyze the effect of other Al
2 3
O support
promoting the reduction of NiO and increasing the amount of with different pore size on the CO
2
methanation performance,
41
active metallic Ni sites, which plays a critical role in improving the results of Rahmani et al. have been selected herein.
7
the CO methanation performance. The used Ni/MA catalyst Comparative data are listed in Table 1S (in the ESI†). From
2
with higher Ni loading presents more active metallic Ni sites Table 1S† it can be found that the Al O support is prepared by
2
3
conrmed also by both the XRD and XPS results, which further sol–gel method, which exhibits higher pore size of 8.1 nm
selectivity of 25Ni/CA and 25Ni/MA catalysts, GHSV ¼ 6000 mL gꢀ
1
h
ꢀ1
, H
2
/CO
¼ 4, 1 atm.
Fig. 7 (a) CO
2
conversion and (b) CH
4
2
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(
Fig. 1S†). From Fig. 1Sa† it can be seen that the Ni/MA catalyst
ꢁ
displays the excellent stability in 400 C, and there is no deac-
tivation appeared during reaction. In contrast, the catalytic
activity of the Ni/CA catalyst decreases gradually in the initial
stage and then remained unchanged basically with time on
4
stream. Especially, the CH selectivity begins to decrease aer
reaction for 27 h (Fig. 1Sb†). Besides, the reusability of 25Ni/MA
ꢁ
at 400 C is further tested (Fig. 2S†), and no deactivation of the
catalyst is observed during reaction for 80 h. All of these results
suggest that the CA supported Ni catalyst takes place easily the
deactivation in the CO2 methanation reaction, while the MA
support plays a critical role in improving the stability of the
catalyst.
The TEM results indicate that metallic Ni is formed on the
surface of MA. No aggregation of Ni particles and no formation
of carbon deposition are exhibited. It is possible that the MA
support with cross-linked fabricate walls restrains the aggre-
gation of metallic Ni particles, improving the catalyst stability.
The micro-structures of the spent catalyst are further analyzed.
The spent 25Ni/MA catalyst aer reaction for 100 h is charac-
terized by XRD. From Fig. 9a it is seen that phase composition
of the spent 25Ni/MA is composed mainly of metallic Ni. The Ni
particle size calculated using Scherrer equation is about
21.5 nm, which is close to the value in initial reaction, further
Fig. 8 Stability test of the 25Ni/MA catalyst at 340 ꢁC, GHSV ¼
ꢀ1
ꢀ1
h , H
6
000 mL g
2
/CO
2
¼ 4, 1 atm.
compared to the MA support in the present study. However, the
optimized CO
2
conversion for 25Ni/Al
2
O
3
reported by Rahmani
41
ꢁ
et al. is 73% at 350 C, which is lower than the value of 25Ni/
MA in this study, suggesting that the MA support with narrower
pore size by partial hydrolysis preparation method displays an
validating the stability of Ni active nanoparticles during the CO
methanation process.
2
The behavior of carbon deposition over the Ni/MA catalysts
is analyzed by TG. As shown in Fig. 9b, two weight loss steps
can be seen for all of the spent catalysts. The rst weight loss
outstanding CO
typical Ni-based catalysts for CO
From Table 2S† it is seen that the Ni/MA catalyst presents the
highest CO conversion at a low temperature, further verifying
excellent CO methanation performance of the MA support
2
methanation performance. In addition, some
2
methanation are selected.
ꢁ
in zone Ⅰ (below 290 C) is attributed to thermal desorption of
2
physical adsorbed H
2
O, and the second weight loss in zone III
2
ꢁ
(
above 420 C) is related to the removal of graphite coke that
modied Ni catalyst.
The optimized 25Ni/MA catalyst is carried out for the
stability test. As shown in Fig. 8, the 25Ni/MA catalyst presents
4
2,43
led to the catalyst deactivation.
gain in zone II (290–420 C) for the 20Ni/MA and 25Ni/MA
Besides, there is a weight
ꢁ
catalysts, which is ascribed to the oxidation of reduced Ni
4
77% of catalytic activity with almost 100% of CH selectivity,
4
4
species. It is possible that more metallic Ni sites formed over
the spent Ni/MA catalyst with higher Ni loading facilitate the
oxidation of metallic Ni. From Fig. 9b, it is seen that all cata-
lysts display excellent carbon resistance with only 2.8% carbon
which remains unchanged during the reaction for 100 h, indi-
cating that modication by the MA support improves obviously
the catalyst stability. Furthermore, higher temperature metha-
ꢁ
nation (400 C) of MA and CA supported Ni catalysts is tested
Fig. 9 (a) XRD pattern of the 25Ni/MA catalyst after 100 h reaction and (b) TG curves of the spent Ni/MA catalysts.
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deposition amount at most. The weight loss of the used 25Ni/
MA catalyst (100 h) is only 1.1%, revealing excellent anti-
9 G. Zhou, H. Liu, K. Cui, H. Xie, Z. Jiao, G. Zhang, K. Xiong
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1
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modied Ni catalyst displayed higher catalytic activity and CH
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2015, 5, 22759–22776.
selectivity as well as excellent stability compared to the tradi-
tional Ni/CA catalyst. The characterization results indicated that
the MA support was synthesized successfully by partial hydro-
lysis method, and displayed excellent hydrothermal stability.
Modication of MA decreased the metal–support interaction,
promoting the reduction of NiO species and formation of
metallic Ni active sites over the Ni/MA catalysts. The monolithic
skeleton of MA with fabric-like walls stabilized the nano-nickel
particle sizes, which restrained the sintering of metallic Ni
particles and carbon deposition on the catalyst, promoting
obviously the catalyst stability.
1
1
5 J. N. Park and E. W. McFarland, J. Catal., 2009, 266, 92–97.
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2
2
Conflicts of interest
2
2
2 S. Tada, T. Shimizu, H. Kameyama, T. Haneda and
R. Kikuchi, Int. J. Hydrogen Energy, 2012, 37, 5527–5531.
3 S. Tada, D. Minori, F. Otsuka, R. Kikuchi, K. Osada,
K. Akiyama and S. Satokawa, Fuel, 2014, 129, 219–224.
There are no conicts to declare.
Acknowledgements
24 M. C. Bacariza, I. Graça, S. S. Bebiano, J. M. Lopes and
C. Henriques, Chem. Eng. Sci., 2018, 175, 72–83.
The authors gratefully acknowledge the nancial support from
International cooperation and exchange program of the
National Natural Science Foundation of China (51861145102),
International Science and Technology cooperation project of
Xinjiang Production and Construction Corps (2017BC008), 111
project (B12015) and Fundamental Research Fund for the
Central Universities (2042017kf0173).
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