Nickel catalysts supported on calcium titanate for enhanced CO methanation

Article abstract of DOI:10.1039/c2cy20542d

Nickel catalysts supported on the perovskite oxide CaTiO₃ (CTO) were prepared by an impregnation method for CO methanation to produce synthetic natural gas (SNG). X-Ray diffraction, nitrogen adsorption, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, H ₂-temperature programmed reduction and desorption, and X-Ray photoelectron spectroscopy were employed for the characterization of samples. The results revealed that the Ni/CTO catalysts showed a better performance than Ni/Al₂O₃ for CO methanation at various reaction conditions. The life time test at 600 °C and 3.0 MPa indicates that Ni/CTO is also more active, thermally stable and resistant to carbon deposition. This is because of the relatively weak Ni-CTO support interaction, highly stable CTO support, the absence of acidic sites on the surface of CTO and the proper Ni particle size of about 20-30 nm. The work is important for the development of effective methanation catalysts for SNG production. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20542d

Catalysis
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Nickel catalysts supported on calcium titanate for
enhanced CO methanation
Chunmiao Jia,ab Jiajian Gao,a Jing Li,a Fangna Gu,*a Guangwen Xu,a Ziyi Zhongc
and Fabing Su*a
Cite this: Catal. Sci. Technol., 2013,
3, 490
Nickel catalysts supported on the perovskite oxide CaTiO3 (CTO) were prepared by an impregnation
method for CO methanation to produce synthetic natural gas (SNG). X-Ray diffraction, nitrogen
adsorption, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis,
H2-temperature programmed reduction and desorption, and X-Ray photoelectron spectroscopy were
employed for the characterization of samples. The results revealed that the Ni/CTO catalysts showed a
better performance than Ni/Al2O3 for CO methanation at various reaction conditions. The life time test
at 600 1C and 3.0 MPa indicates that Ni/CTO is also more active, thermally stable and resistant to
carbon deposition. This is because of the relatively weak Ni–CTO support interaction, highly stable CTO
support, the absence of acidic sites on the surface of CTO and the proper Ni particle size of about
20–30 nm. The work is important for the development of effective methanation catalysts for SNG
production.
Received 2nd August 2012,
Accepted 24th September 2012
DOI: 10.1039/c2cy20542d
www.rsc.org/catalysis
of the Al2O3 support and the use of various synthesis methods
together with multiple promoters.10–14 Very recently, a Ni/Al2O3
1. Introduction
Recently, methanation of carbon oxides (CO and/or CO2) to
produce synthetic natural gas (SNG) has drawn intensive atten-
tion due to the increasing demand for natural gas and the
objective of reducing the emission of greenhouse gases into the
atmosphere.1 The basic reactions in the methanation process
are highly exothermic and are shown as follows: (1) CO + 3H2 -
CH4 + H2O, DH298K = ꢀ206.1 kJ molꢀ1 and (2) CO2 + 4H2 - CH4 +
catalyst with high Ni loadings (40 and 50 wt%), prepared by the
solution combustion method, was found to be stable during a
50 h test, while a catalyst with lower Ni loadings of 10–30 wt%
deactivated gradually.15 The factors leading to the deactivation
of the Ni/Al2O3 catalyst include the sintering of the Al2O3
support and the generation of the NiAl2O4 spinel as well as
severe coke formation.16,17 In other words, although the above
catalysts are promising for industrial production, their thermal
stability and resistance to carbon deposition still present a
challenge.
It is known that perovskite oxides possess some interesting
physical and chemical properties, e.g., high thermal stability,
good reactivity of lattice oxygen, low cost and rich resources. As
catalysts or supports, perovskite oxides have been widely used
in a number of high temperature reactions,18 such as steam
reforming of methane,19 CO2 methanation,20 selective CO
oxidation reactions,21 partial oxidation of CH4.22 Urasaki
et al.19 reported that in a steam reforming reaction of methane,
the Ni/perovskite catalysts exhibited higher catalytic activities
and superior coking resistances compared to the conventional
Ni/a-Al2O3 catalyst. This is because the lattice oxygen in the
perovskite-supported catalysts was able to inhibit the formation
of carbon during the reaction process. Furthermore, Takehira
and coworkers23 evaluated the perovskite oxides XTiO3 (X = Ca,
Sr, Ba), which contain a small amount of Ni in the Ti sites,
2H2O, DH298K = ꢀ165.0 kJ molꢀ1 2,3
.
Compared to noble metal
catalysts, Ni-based catalysts are very attractive and promising
due to their relatively high activity, low cost and good avail-
ability. Different materials, including Al2O3, SiC,4 SiO2 or
5
CeO2,6 have been used to support Ni particles, and among
which, Al2O3 is the most widely used to produce a methanation
catalyst.7 The major drawbacks of Ni/Al2O3 catalysts, as pointed
out previously by some researchers, include the sintering of
Al2O3 and the formation of coke on Ni during the methanation
process.8,9 To overcome these limits, great efforts have been
made to modify the Ni/Al2O3 catalysts, e.g., through modification
a State Key Laboratory of Multiphase Complex Systems, Institute of
Process Engineering, Chinese Academy of Sciences, Beijing, China 100190.
E-mail: fngu@mail.ipe.ac.cn, fbsu@mail.ipe.ac.cn; Fax: +86-10-82544851;
Tel: +86-10-82544850
b Graduate University of Chinese Academy of Sciences, Beijing, China 100049
c Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road,
Jurong Island, Singapore 627833
c
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and these catalysts showed high catalytic activities with a high spectroscopy (XPS) test conducted on a VG ESCALAB 250 spectro-
resistance to coking because of their unique structure and the meter (Thermo Electron, U.K.) with a non-monochromatized Al–Ka
high dispersion of Ni. It was also thought that it was the X-Ray source (1486 eV). The deposited carbon content was measured
perovskite structure which consequently separated NiO and using an CS-344 Infrared Analyzer (Leco, US). H2 temperature-
then Ni metal. However, perovskite oxide-supported Ni catalysts programmed reduction (H2-TPR) and desorption (H2-TPD), and
have not been explored for the CO methanation reaction.
NH3 temperature-programmed desorption (NH3-TPD) were carried
In this paper, we used CaTiO3 (CTO), one of the perovskite out on a Quantachrome Automated chemisorption analyzer
oxides, to prepare Ni catalysts (Ni/CTO) with low NiO loadings (ChemBET pulsar TPR/TPD). The H2 consumption or NH3
for CO methanation for the production of SNG and found that desorption was detected continuously as a function of increasing
Ni/CTO catalysts exhibit better catalytic activity, superior thermal temperature using a thermal conductivity detector (TCD). For the
stability and a greater resistance to coke formation compared H2-TPR test, 0.1 g of the sample was loaded into a quartz U-tube and
with Ni/Al2O3 catalysts, suggesting that CTO would be a good heated from room temperature to 400 1C at 10 1C minꢀ1 and
alternative material to the conventional Al2O3 support. Our work maintained for 1 h under Ar flow. The sample was then cooled to
is significant for the development of effective CO methanation room temperature, followed by heating to 1000 1C at 10 1C minꢀ1
catalysts for SNG production.
under a 10.0 vol% H2/Ar atmosphere with a gas flow of
30 mL minꢀ1. For the NH3-TPD test, 0.3 g of the sample was loaded
into a quartz U-tube and reduced from room temperature to 600 1C,
according to the H2-TPR procedure. The gas flow was then switched
to a He flow (30 mL minꢀ1) and the sample was cooled to 100 1C and
2. Experimental
2.1. Catalyst preparation
All the catalysts were prepared by the wet impregnation method. saturated with ammonia (10.0 vol% NH3/He). After removing the
Commercial perovskite CTO (NanTong AoXin Machineryco., physically adsorbed ammonia in a He flow for 1 h, the sample was
Ltd., Purity 4 95%) was calcined at 400 1C for 4 h before use. heated to 600 1C at 10 1C minꢀ1 under a continuous He atmosphere.
1.945 g of Ni(NO3)2ꢁ6H2O (analytical-grade, Sinopharm Chemical For the H2-TPD test, 0.3 g of the catalyst was loaded into a quartz
Reagent Co. Ltd.) was dissolved in 13 mL of deionized water and U-tube and first reduced at 600 1C for 4 h under a 10.0 vol% H2/Ar
4.50 g of the CTO support was added to form a slurry, which was atmosphere, with a gas flow of 30 mL minꢀ1, and then cooled to
kept at room temperature for 8 h under vigorous stirring. The room temperature and maintained for 2 h. After removing the
slurry was then heated to 80 1C to evaporate the liquid, dried at physically adsorbed hydrogen by flushing with Ar for 2 h at the flow
110 1C overnight and further calcined at 400 1C for 4 h in air. The rate of 30 mL minꢀ1, the sample was heated to 600 1C with a ramp
collected catalyst was denoted as 10Ni/CTO with a NiO loading of of 10 1C minꢀ1 in an Ar flow. The number of surface Ni sites per unit
10 wt%. Accordingly, 5Ni/CTO was prepared by the same method mass of catalyst was determined by means of H2-TPD with an
using 0.973 g of Ni(NO3)2ꢁ6H2O, 4.75 g of CTO and 15 mL adsorption stoichiometry of H : Ni = 1 : 1. The peak area of the
deionized water. For
a comparison, commercial g-Al2O3 H2-TPD profile was normalized by that of H2-TPR of a standard CuO
(GongYiHuaYu alumina Co. Ltd., China; Purity 4 95%) was also sample. The turnover frequency (TOF) value was calculated based on
calcined at 400 1C in air for 4 h before use and the 5Ni/Al2O3 and the formula as followed:
10Ni/Al2O3 catalysts were prepared by the same method.
VCO;in ꢀ VCO;out
mcat-TPD
mcat-Reaction
ACuO-TPR
Acat-TPD mCuO-TPR
MCuO
TOFCO
¼
ꢂ
ꢂ
ꢂ
RH =Ni ꢂ Vm
2
2.2. Characterization
where VCO,in and VCO,out are the volume flow rates of CO at the inlet
The X-ray powder diffraction (XRD) pattern was recorded on a
PANalytical X’Pert PRO MPD with Ka radiation of Cu (l = 1.5418 Å)
at 40 KV and 40 mA, and the Debye–Scherrer equation was used to
calculate the crystal size. The specific surface area of the catalysts
was measured on a Quantachrome surface area & pore size analyzer
NOVA 3200e. Before the measurement, the sample was degassed at
200 1C for 3 h under vacuum. The specific surface area was
determined according to the Brunauer–Emmett–Teller (BET)
method in the relative pressure range of 0.05–0.2. The microscopic
feature of the samples was observed by field emission scanning
electron microscope (SEM) (JSM-6700F, JEOL, Tokyo, Japan) and
transmission electron microscopy (TEM) (JEM-2010F, JEOL, Tokyo,
Japan). Prior to the TEM observation, the catalyst was first reduced
and outlet of the reactor at standard temperature and pressure
(STP), mL sꢀ1; RH /Ni = 2 is the stoichiometric factor of the H2 :Ni
2
ratio in the chemisorption; Vm = 22 414 mL molꢀ1 is the molar
volume of CO gas at STP; mcat-TPD is the mass of the catalysts used in
the H2-TPD experiment, g; mcat-Reaction is the mass of the catalysts
used in the CO methanation reaction, g; ACuO-TPR is the peak area of
the H2-TPR curve of standard CuO; Acat-TPD is the peak area of the
H2-TPD curve of the catalyst; MCuO = 79.54 g molꢀ1 is the mole mass
of CuO; mCuO-TPR is the mass of the CuO used in the H2-TPR
experiment, g.
2.3. Catalytic measurement
in a continuous-flow fixed bed at 600 1C for 4 h under H2, with a The catalytic tests for CO and CO2 methanation were carried
flow rate of 50 mL minꢀ1. Thermogravimetric (TG) analysis was out in a fixed-bed reactor equipped with a quartz tube and
conducted on a Seiko Instruments EXSTAR TG/DTA 6300. 10 mg of stainless steel tube (both with a diameter of 8 mm) respectively.
the sample was used and heated under air (200 mL minꢀ1) from A thermocouple was inserted into the furnace chamber and
room temperature up to 1000 1C (10 1C minꢀ1). The surface bonded to the outside of the reactor tube in the middle of the
chemical composition was analyzed by an X-Ray photoelectron catalyst bed to control the reaction temperature. The CO
c
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Table 1 Physical properties of the supports and Ni catalysts
methanation was tested under 0.1 and 3.0 MPa from 300 to
600 1C, and CO2 methanation was performed under 0.1 MPa.
0.6 g of the catalyst (20–40 mesh), diluted with 3.0 g of quartz
sand of the same mesh, was placed in the tube reactor. The
catalyst was first reduced at 600 1C for 4 h under a pure H2 flow
(100 mL minꢀ1). The gas flow was then switched to a feed gas
with a molar ratio of H2 : CO : N2 = 3 : 1 : 1 and the total flow rate
was set to 100 mL minꢀ1, determined by mass flow controllers
corresponding to a weight hourly space velocity (WHSV) of
Mean particle
size (nm)
NiOa Nib Nic
Catalyst
sample
BET surface area Total pore volume
(m2 gꢀ1
)
(m3 gꢀ1
)
CTO
5Ni/CTO
10Ni/CTO 3.2
Al2O3
5Ni/Al2O3
10Ni/Al2O3 246.3
1.6
2.9
—
—
—
0.44
0.37
0.34
—
19
24
—
—
—
—
—
16 10–20
23 20–30
—
—
—
305.2
268.5
—
3–5
6–10
10000 mL gꢀ1 ꢀ1. On heating the mixture from 300 to 600 1C
h
a
Calculated by the XRD diffraction peak (2y = 43.51) using the
with an interval of 50 1C, the catalytic activities of the catalysts
were measured under 0.1 and 3.0 MPa, respectively. A life time
b
Debye–Scherrer equation. Calculated by the XRD diffraction peak
(2y = 44.61) using the Debye–Scherrer equation. Determined according
c
test was performed at 600 1C and 3.0 MPa. For the hydrothermal to the TEM images.
treatment, 0.5 g of the unreduced catalyst was loaded into a
quartz tube and subjected to an atmosphere of steam and
hydrogen, with a volume ratio of 10 : 1, at 800 1C for 5 hours
under 0.1 MPa, and then the catalyst was cooled to room
temperature in pure H2. For the CO2 methanation, a feed gas
with a molar ratio of H2 : CO2 : N2 = 4 : 1 : 1 was used. The inlet
and outlet gas, after cooling with a cold trap, were analyzed
online by Micro GC (3000 A; Agilent Technologies). After one
hour of steady-state operation at each temperature, the concen-
trations of the H2, N2, CH4, CO and CO2 contents in the gas
products were analyzed by a thermal conductivity detector (TCD)
with a Molecular Sieve column using N2 as an internal standard.
The CO and CO2 conversion, CH4 selectivity and yield are
defined as follows:
Fi;in ꢀ Fi;out
Xið%Þ ¼
ꢂ 100
ꢂ 100
ð1Þ
ð2Þ
ð3Þ
Fi;in
F
CH4;out
SCH ð%Þ ¼
4
Fi;in ꢀ Fi;out
Xi ꢁ S
F
CH4;out
CH4
YCH ð%Þ ¼
¼
ꢂ 100
4
100
Fi;in
Fig. 1 XRD patterns of CTO and the Ni/CTO catalyst (a), enlarged view (b), Al2O3
and catalysts Ni/Al2O3 (c), and TG/DTA curves of the CTO and Al2O3 supports (d).
Here, i represents CO or CO2, X is the conversion of CO or
CO2, S is the selectivity of CH4, Y is the CH4 yield, Fx,in and Fx,out
are the molar flow rates of species x (x = CH4, CO, or CO2) at the
inlet and outlet respectively.
Fig. 1 presents the XRD patterns and TG curves of the
calcined catalysts and the support. For the Ni/CTO samples in
Fig. 1a, the diffraction peaks at 33.3, 47.9, 59.6, and 70.01 are a
typical feature of CTO.23,24 From the enlarged view in Fig. 1b,
the peaks at 37.4, 43.5, and 63.21 can be attributed to (111),
(200), and (220) of NiO (JCPDS 00-001-1239).25 As expected,
3. Results and discussion
3.1. Catalyst characterization
Table 1 compiles the surface areas and pore volumes of the with the increase in the NiO loading from 5 to 10 wt%,
supports and catalysts. CTO has a very low surface area (1.6 m2 gꢀ1
the intensities of these peaks are also obviously increased,
)
and is almost non-porous. After NiO loading, the surface area of the indicating the formation of larger NiO particles. The sizes of
CTO supported catalysts increase slightly. This can be attributed to the NiO particles on 5Ni/CTO and 10Ni/CTO are calculated to
the loading of small NiO nanoparticles onto the surface of CTO, be 19 and 24 nm, respectively (see Table 1). The XRD patterns of
which can be seen from the SEM images below. However, for the the Al2O3 support and Ni/Al2O3 catalysts are depicted in Fig. 1c,
Al2O3 and Ni/Al2O3 catalysts, their surface areas (246–305 m2 gꢀ1
)
in which an extremely weak diffraction peak at 43.31, contri-
and total pore volumes (0.34–0.44 m3 gꢀ1) are much larger than buted to NiO, is detected, indicating the low NiO loading or
those of the CTO and Ni/CTO catalysts but become smaller after the high dispersion.26 Fig. 1d shows the TG/DTA curves of the two
NiO loading, mainly because the density of the NiO particles is supports. Clearly, CTO experiences a weight loss of less than 0.5 wt%
larger than that of the porous Al2O3 and some of the pores may be at 1000 1C, much lower than that of Al2O3 (B5 wt%), suggesting the
blocked by the NiO nanoparticles.
superior thermal stability of CTO to Al2O3.27,28
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NiO strongly interacts with Al2O3 and partly generates the
NiAl2O4 spinel structure, which is highly stable and hence
difficult to be reduced.25 In short, the metal–support interaction
in Ni/CTO is relatively weaker than that of Ni/Al2O3. The surface
acidic properties of the supports and catalysts are examined by
NH3-TPD, as shown in Fig. 2b. There is no desorption peak of
ammonia on both CTO and 10Ni/CTO in the temperature range
of 100–600 1C, indicating the lack of acidic sites on their surface.
In contrast, a broad desorption peak is observed over Al2O3 and
10Ni/Al2O3 in the range of 150–450 1C, which can be attributed to
desorbed NH3 from the acid sites.30,31 In short, the surface of
10Ni/CTO is non-acidic in nature while that of 10Ni/Al2O3 shows
relatively strong acidic properties. Fig. 2c shows the H2-TPD
curves of the reduced catalysts. We can see that the TPD profiles
show H2 desorption peaks in the range of 50–360 1C for both
catalysts, and the H2 adsorption amount for the 10Ni/Al2O3
catalyst is 14.6 mmol gcatꢀ1, much higher than that of the 10Ni/
CTO catalyst (5.0 mmol gcatꢀ1), possibly because the smaller Ni
particles are well dispersed on the Al2O3 support due to its high
surface area and intricate pore structure.
Fig. 3 presents the SEM images of the supports and the
unreduced catalysts. Fig. 3a shows that the CTO support
possesses a smooth surface. For the 5Ni/CTO (Fig. 3c) and
10Ni/CTO (Fig. 3e) catalysts, it can be seen that the NiO
particles are uniformly dispersed on the CTO surface. Further-
more, the NiO particle size in the 10Ni/CTO sample appears
slightly larger than that in the 5Ni/CTO catalyst, in agreement
with the XRD results (Fig. 1). In contrast, the Al2O3 support
Fig. 2 H2-TPR (a) and NH3-TPD curves (b) of the supports and Ni catalysts,
and H2-TPD profile of the reduced Ni catalysts (c).
Fig. 2 shows the H2-TPR, H2-TPD, and NH3-TPD curves of the
supports and Ni catalysts. From Fig. 2a, it can be seen that there
is no obvious reduction peak for both the CTO and Al2O3
supports, implying that they are non-reducible under this
condition. The reducible NiO peaks of the catalysts can be
approximately classified into three types: an a-type (weak
interaction of the NiO and support, 290–475 1C), a b-type
(mid-interaction, 475–753 1C), and a g-type (strong interaction,
753–894 1C).9,15,29 The H2 consumption curves of the Ni cata-
lysts are fitted by Gaussian-type functions and the quantitative
results are listed in Table 2. Accordingly, we can see that the
reducible NiO peak in the 10Ni/CTO samples is located in the
range of 295–471 1C, which all belong to the reduction of a-type
NiO i.e. free Ni which interacts weakly with the CTO support,
and the 5Ni/CTO catalyst possesses the same NiO type (reducible
peak from 311 to 471 1C). In contrast, for the Ni/Al2O3 catalysts,
both the reducible peaks include three sub-peaks corresponding
to the a-type, b-type, and g-type. In particular, the b-type NiO
becomes dominant and the fraction increases with the NiO
loading (42.4% for 5Ni/Al2O3 to 53.5% for 10Ni/Al2O3 in
Table 2). The g-type NiO is also found in both catalysts, implying
Table 2 TPR quantitative data for the Ni catalysts
Tm (1C)
Fraction of total area (%)
a-type b-type g-type a-type
b-type
NiO
g-type
NiO
NiO
NiO
NiO
NiO
Catalysts
5Ni/CTO
10Ni/CTO
5Ni/Al2O3
391
384
408
—
—
604
588
—
—
743
739
100
100
24.2
12.0
—
—
42.4
53.5
—
—
33.4
34.4
Fig. 3 SEM images of the supports and the Ni catalysts before reduction:
(a) CTO, (b) Al2O3, (c) 5Ni/CTO, (d) 5Ni/Al2O3, (e) 10Ni/CTO, and (f) 10Ni/Al2O3.
10Ni/Al2O3 351
c
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itself consists of small particles (Fig. 3b). After loading with
NiO, no visible change can be seen in 5Ni/Al2O3 (Fig. 3d) and
10Ni/Al2O3 (Fig. 3f). All the SEM images indicate that the NiO
particles are well dispersed on the surface of the CTO and Al2O3
supports.
Fig. 4 shows the TEM images of the reduced Ni catalysts. The
Ni particles are well dispersed on the CTO support surface (5Ni/
CTO in Fig. 4a and 10Ni/CTO in Fig. 4c). The Ni particle sizes in
the 5Ni/CTO and 10Ni/CTO catalysts are about 10–20 nm and
20–30 nm, respectively. Fig. 4e shows that smaller Ni particles
(5–10 nm) are dispersed on the Al2O3 support. In addition,
Fig. 4b, d, and f show the HRTEM images of the Ni particles
with an exposed Ni lattice space of 0.2 nm, corresponding to
the Ni(111) space, which is the active face for methanation.11,32
Fig. 5 displays the XPS spectra of the Ni 2p and O 1s orbitals
and Table 3 lists the data for the other elements of the
supports. The XPS spectra of all the samples confirm that the
expected elements (Ni, Ca, Ti, Al and O) are present in Ni/CTO
and Ni/Al2O3. The surface of the Ni/CTO catalysts are covered
with a small portion of metal Ni (Ni 2p3/2 at around 853.0 eV)
and a larger portion of Ni oxide (Ni 2p3/2 at around 856.4 eV),
due to the partial surface oxidation of Ni during the sample
Fig. 5 Ni 2p (a) and O 1s (b) XPS spectra of the 5Ni/CTO, 10Ni/CTO and 10Ni/Al2O3
catalysts after reduction.
Table 3 The XPS spectra results from the catalyst supports
Binding energy (eV)
Ca
Ti
Al
Ca 2p3/2
Ca 2p1/2
Ti 2p3/2
Ti 2p1/2
Al 2p
Catalyst sample
5Ni/CTO
10Ni/CTO
10Ni/Al2O3
346.7
346.4
—
350
350
—
458.8
458.6
—
464.6
464.3
—
—
—
73.9
transfer before the XPS test.33,34 Moreover, the Ni 2p3/2 peak
from 10Ni/CTO shifts to a lower binding energy of 852.3 eV
compared with that from 5Ni/CTO (856.4 eV), indicating that
the interaction of the Ni particles with the CTO support
becomes weaker as the loading amount of Ni increases. For
the 10Ni/Al2O3 catalyst, two peaks at 856.4 and 874.0 eV, which
are assigned to NiO 2p3/2 and NiO 2p1/2 respectively, are
observed, although no obvious signal from metal Ni is identified.35
The absence of a reduced Ni phase in the 10Ni/Al2O3 catalyst is
likely attributed to the oxidation of metallic Ni after exposure to air
because the catalysts were not reduced again in situ during the XPS
measurement. Additionally, the formation of a highly stable
NiAl2O4 phase also accounts for this phenomenon.35 The O 1s
Fig. 4 TEM images: (a and b) reduced 5Ni/CTO and Ni particles, (c and d)
reduced 10Ni/CTO and Ni particles; (e and f) reduced 10Ni/Al2O3 and Ni particles. signals in Fig. 5b for the Ni/CTO catalysts can be fitted to two peaks
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around 530.0 and 532.0 eV, respectively. With the increase of the
NiO loading, the proportion of O in NiO (530.0 eV) increases
compared with O in CTO (532.0 eV).36,37 10Ni/Al2O3 has a broader
O 1s peak, which can be fitted with three sub-peaks corresponding
to three types O on NiO (529.8 eV), Al2O3 (531.0 eV) as well as
NiAl2O4 (532.0 eV) due to the strong interaction between Ni and
support Al2O3, respectively.35,38,39 Apart from these, the binding
energies of the other elements (Ca 2p3/2, Ca 2p1/2, Ti 2p3/2, Ti 2p1/2
,
and Al 2p) have no obvious chemical shift, implying that the
chemical environment of both supports may not be changed after
Ni loading.40
3.2. Catalyst activity
Fig. 6 shows the activities of the Ni catalysts for CO methana-
tion at 0.1 MPa. Overall, the CO conversion and CH4 yield over
the Ni/CTO catalysts present a volcano-shaped trend as the
reaction temperature changes, in agreement with the thermo-
dynamic calculation.3 The CO conversion in Fig. 6a almost
reaches 100% over the 10Ni/CTO catalyst at 400 1C, while over
the 5Ni/CTO catalyst it needs a higher reaction temperature of
450 1C to reach the same CO conversion. At the same time, the
maximum CO conversion over 10Ni/Al2O3 is only 28% at 600 1C,
and that over the 5Ni/Al2O3 catalyst is even lower (8%) at 500 1C.
The maximum CH4 selectivity for the 10Ni/CTO and 5Ni/CTO
catalysts is 85% at 400 1C and 80% at 450 1C in Fig. 6b,
respectively. In addition, the maxima for the CH4 selectivity
over 10Ni/Al2O3 and 5Ni/Al2O3 are around 80% and 88%,
respectively. From Fig. 6c, the maximum CH4 yield for the
10Ni/CTO catalyst is 84% at 400 1C, and 80% for 5Ni/CTO at 450 1C.
Meanwhile, the maximum CH4 yields for the 10Ni/Al2O3 and
5Ni/Al2O3 samples are just 24% and 6%, respectively. In order
to further confirm the superiority of the 10Ni/CTO catalyst, the
TOF values were calculated based on the H2-TPD and CO
conversion results at 400 1C. The TOFCO value over 10Ni/CTO
is 2.4 sꢀ1, 24 times that of the 10Ni/Al2O3 catalyst (0.1 sꢀ1),
indicating the catalytic efficiency of reduced 10Ni/CTO is
significantly higher than that of 10Ni/Al2O3. In addition, we
tried to completely reduce both catalysts at 800 1C for 4 h in
pure H2 and found that the CO conversion and CH4 yield over
10Ni/Al2O3 are still much lower than that for 10Ni/CTO
at 0.1 MPa and 120 000 mL gꢀ1 hꢀ1 (data not shown here).
Therefore, we can deduce that 10Ni/CTO is more active than the
10Ni/Al2O3 catalyst.
Fig. 6 Catalytic activities of the Ni catalysts at 0.1 MPa: (a) CO conversion,
(b) CH4 selectivity, and (c) CH4 yield.
Previously, a great deal of effort has been expended to
understand the interaction between the metal component
and the support and a strong metal support interaction (SMSI) nanoparticles. In order to fully reduce the NiO species in this
was often used to explain experimental phenomena.4,36,41,42 catalyst, a higher reduction temperature is needed, which will
However, the above characterization results demonstrate that adversely lead to the agglomeration of Ni particles and the
the Ni/CTO catalysts have weaker metal–support interactions support and the formation of NiAl2O4.35 These may not be
compared with the Ni/Al2O3 catalysts but show better catalytic helpful for improving the catalytic activity of Ni/Al2O3. Contrarily,
performances. Thus, the SMSI model is not appropriate to for the Ni/CTO catalysts, all the NiO particles can be reduced, even
explain the above phenomena. Therefore, we suspect that the at the low reduction temperature of 600 1C, which leads to a full
NiO particles strongly interact with the support and may not be utilization of the Ni species and minimum impact to the sintering
fully available in the catalytic reaction as some of them are of Ni and the support. We can thus conclude that a weak
difficult to be reduced to act as an ‘‘active’’ role. This is what interaction in the Ni/CTO catalysts is responsible for its good
occurred in Ni/Al2O3, although it has a high dispersion of Ni catalytic performances.
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600 1C but is only 45% for the 10Ni/Al2O3 sample at 600 1C.
In addition, the thermodynamic equilibrium data3 is included as
a comparison. It can be seen that the CO conversion on the
10Ni/CTO catalyst almost reaches the thermodynamic equilibrium
value above 600 1C and there is a large difference for 10Ni/Al2O3.
For the CH4 selectivity and yield, a gap between the experimental
and thermodynamic equilibrium can be seen, especially over
10Ni/Al2O3. Therefore, the results from the high pressure test still
indicate that the 10Ni/CTO catalyst is more active than 10Ni/Al2O3.
Fig. 8 shows the life time test results of 10Ni/CTO and
10Ni/Al2O3 at 600 1C and 3.0 MPa. For the 10Ni/CTO catalyst,
Fig. 7 Catalytic activities of the Ni catalysts at 3.0 MPa: (a) CO conversion,
(b) CH4 selectivity, and (c) CH4 yield.
Fig. 7 shows the catalytic properties of CO methanation under
3.0 MPa over the 10Ni/CTO and 10Ni/Al2O3 catalysts. As we know,
this reaction is volume-reduced and normally conducted at high
pressure (2.94–3.43 MPa) in industry.43 It can be seen that the CO
conversion (Fig. 7a) over the 10Ni/CTO catalyst reaches almost 100%
and remains constant from 350 to 600 1C. However, the CO con-
version over the 10Ni/Al2O3 catalyst increases with heating and only
reaches 73% at a temperature of 600 1C. It can be seen that the CH4
selectivity (Fig. 7b) on 10Ni/CTO remains between 80 to 90% during
the whole temperature range, while the selectivity for 10Ni/Al2O3
reaches a maximum of 60% at 600 1C. In Fig. 7c, the maximum
value for the CH4 yield on 10Ni/CTO is around 88% from 450 to
Fig. 8 Catalytic performance of the Ni catalysts during a long test run at 600 1C
and 3.0 MPa: (a) CO conversion, (b) CH4 selectivity, and (c) CH4 yield.
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According to the thermodynamic analysis,3 most of the carbon
comes from the Boudouard reaction because its equilibrium con-
stant (K) is much larger than that of the other reactions. It should
be pointed out that, differing from the literature reported condi-
tions, such as those used in the removal of CO46 and other gases
with high H2 content,47,48 the feed gas (CO : H2 :N2 = 3:1:1) used
in our work is rich in carbon content and thus carbon deposition is
much easier. It has been reported that carbon deposition is closely
related to the surface chemistry of the active metal and the support,
the dispersion and size of the active metal nanoparticles and the
reaction conditions. The Al2O3 used here is highly porous and
acidic and carbon is favorably deposited in the pores.31,49 The
sintering of the Al2O3 support at high temperatures leads to the
aggregation of Ni particles, which would exacerbate the carbon
deposition.50 In contrast, CTO has less pores and basic surface
properties, thus showing a higher resistance to carbon deposition.
In addition, the migration of mobile oxygen from the perovskite
support to the metallic Ni particles in Ni/CTO19,24,31,48 to reduce
deposited carbon may be another reason.
Fig. 9 XRD patterns of the reduced and used 10Ni/Al2O3 (a) and 10Ni/CTO
(b) catalysts, and SEM images of the used 10Ni/Al2O3 (c) and 10Ni/CTO (d) catalysts.
The catalytic properties of 10Ni/CTO and 10Ni/Al2O3 after an
HT treatment are shown in Fig. 10. It can be seen that the
the CO conversion (Fig. 8a), CH4 selectivity (Fig. 8b), and CH4 maximum CO conversion and CH4 yield over 10Ni/CTO-HT is
yield (Fig. 8c) is B100, B84, and B83%, respectively, and these around 65% (Fig. 10a) and 45% (Fig. 10b) at 550 1C, respec-
remain constant during a 50 h run. In contrast, the activity of tively, much higher than those of 10Ni/Al2O3-HT, although the
the 10Ni/Al2O3 catalyst has a remarkable decline after only 20 h.
For instance, the CO conversion decreases from 70 to 45% and
the CH4 yield drops from 41to 18%, suggesting a better long
term performance for 10Ni/CTO.
Fig. 9a shows the XRD pattern of the used 10Ni/Al2O3
catalyst after a 20 h test and a freshly reduced catalyst for
comparison. For the reduced catalyst, there are no metallic Ni
peaks identified but after the long lifetime test, the Ni peaks
located at 44.8, 51.9, and 76.61 appear and the Ni particle size is
calculated to be about 26 nm, suggesting the sintering and
agglomeration of the Ni particles after long term methanation.25
However, for the 10Ni/CTO catalyst in Fig. 9b, no obvious change
is observed between the reduced and used catalyst after the 50 h
reaction, indicating its good stability. Fig. 9c shows the SEM
image of the 10Ni/Al2O3 catalyst after a 20 h test. It can be seen
that there are some filamentous carbons formed, suggesting
that carbon deposition occurred on the surface of 10Ni/Al2O3,
consistent with previous results.26 In contrast, for the 10Ni/CTO
catalyst in Fig. 9d, no such carbon filaments can be found even
after a 50 h reaction. The amount of carbon deposited on the
10Ni/CTO catlyst surface after a 50 h test and on 10Ni/Al2O3 after
a 20 h, analyzed by a CS-344 Infrared Analyzer, is 1.4 and
1.6 wt%, and the average carbon deposition rate is 0.03 and
0.08 wt% hꢀ1, respectively, suggesting a much stronger resis-
tance to carbon deposition for 10Ni/CTO. It has been pointed out
that coking and sintering are the main factors for the deactiva-
tion of metal nanoparticles and the Ni-based catalysts are more
susceptive to deposited carbon than other active components
during the methanation process.1,9,44,45 In the CO methana-
tion process, the carbon deposition mainly comes from the
Boudouard reaction (2CO 2 C + CO2), carbon monoxide reduction
Fig. 10 Catalytic activities of the Ni catalysts after hydrothermal treatment:
(CO + H2 2 C + H2O) and methane cracking (CH4 2 C + 2H2).8 (a) CO conversion and (b) CH4 yield.
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activity of both catalysts declines compared with that of the hydrogenate than CO2.3 As expected, for the CO2 methanation, the
fresh catalysts in Fig. 6. According to the XRD analysis on the Ni/CTO also exhibits better activity than Ni/Al2O3, indicating that
catalysts after hydrothermal treatment (not shown here), a the Ni/CTO catalyst is a promising candidate not only for CO but
severe phase transformation of the Al2O3 support (partly from also for CO2 methanation to produce SNG.
g-type to Z-type) occurs and the Ni peaks appear as a result of
As we know, the family of perovskite-type oxides has many
the aggregation of small Ni particles (size increased from about members, including BaTiO3, SrTiO3, MgTiO3, LaNiO3, and so
5–10 nm to about 18 nm), while the structure of 10Ni/CTO is on. Based on the above results, the use of CTO as a Ni catalyst
almost unchanged and the Ni particle size is slightly larger support may not be the best choice. Thus, more exploration is
(from 23 to 30 nm). Hence, the better stability of 10Ni/CTO, needed to seek better perovskite oxide supports for methanation
especially for the CTO support, is confirmed, which guarantees catalysts with much high activity and stability in our future work.
the superior catalytic activity and stability of the Ni/CTO
catalysts. In industrial methanation processes, steam is often
4. Conclusions
added to the reaction system for the benefit of heat transmis-
sion and the diminishment of deposited carbon.51 Thus, the
In this work, we report the preparation and characterization of
hydrothermal stability of the Ni catalysts is in high demand.52
perovskite CTO supported Ni catalysts with low NiO loadings
CO2 methanation, as another essential methanation
process, was also carried out over the 10Ni/CTO and 10Ni/Al2O3
catalysts, as shown in Fig. 11. Apparently, the maximum CO2
conversion over 10Ni/CTO reaches 75% at 400 1C, as shown in
Fig. 11a, while the maximum conversion over 10Ni/Al2O3 is only
around 5%, and it finally reaches its maximum value of 84% at
600 1C. Fig. 11b shows that the CH4 yield over 10Ni/CTO sub-
stantially exceeds that over the 10Ni/Al2O3 catalyst. Furthermore,
we can see that the CO2 conversion over the Ni catalysts is
lower than that of CO, possibly because CO is much easier to
for CO methanation. It reveals that the Ni/CTO catalysts show
better catalytic performances than Ni/Al2O3 at 0.1 MPa and
3.0 MPa, respectively. Nearly 100% CO conversion and 85%
CH4 selectivity can be achieved over the 10Ni/CTO catalyst
(NiO loading: 10 wt%) under 0.1 MPa, at a WHSV of
10 000 mL gꢀ1 hꢀ1 and a H2/CO feed ratio of 3. A life time test
at 3.0 MPa and 600 1C demonstrates that the Ni/CTO catalyst
maintains its good stability after a 50 h reaction. It is found that
the main factors which account for the better catalytic activity
of Ni/CTO than Ni/Al2O3 are the weak interaction between Ni
and CTO support, the proper Ni particle size (resistance to
carbon deposition), the superior stability and non-acidic nature
of CTO. This work demonstrates that the perovskite oxide CTO
would be an appropriate Ni catalyst support for CO and CO2
methanation process.
Acknowledgements
The authors gratefully acknowledge the support from the
Hundred Talents Program of the Chinese Academy of Sciences
(CAS), State Key Laboratory of Multiphase Complex Systems of
China (No. MPCS-2009-C-01), National Key Technology R&D
Program of China (No. 2010BAC66B01) and National Key Basic
Research Program of China (No. 2011CB200906). F. Gu is
grateful to the support of K. C. Wang Post-doctoral Fellowships
of the CAS and China Postdoctoral Science Foundation (No.
20100480026).
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