CO and CO2 methanation over M (M[dbnd]Mn, Ce, Zr, Mg, K, Zn, or V)-promoted Ni/Al@Al2O3 catalysts

Article abstract of DOI:10.1016/j.cattod.2019.08.058

Effects of metal promoter on CO and CO₂ methanation were examined over Ni-M (M = Mn, Ce, Zr, Mg, K, Zn, or V)/Al@Al₂O₃ catalysts prepared by the co-impregnation method. Ni-M (M = Mn, Ce, or Zr)/γ-Al₂O₃ catalysts were also investigated for comparison. The prepared catalysts were characterized with a variety of techniques such as N₂ physisorption, CO₂ chemisorption, H₂ chemisorption, temperature-programmed reduction with H₂ (H₂-TPR), temperature-programmed desorption of CO₂ (CO₂-TPD), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Among different promoters, Mn, Ce, Mg, V, and Zr are beneficial to enhance both CO and CO₂ methanation activity due to the improvement of the Ni dispersion. The Ni-V/Al@Al₂O₃ catalyst performs the highest CO methanation activity due to the largest Ni sites. However, it is not the best one for CO₂ methanation among tested catalysts because of the much decrease in CO₂ adsorption capacity. The promotional effect of Mn is the most remarkable for both CO and CO₂ methanation. On the other hand, the negative effect of K and Zn was observed on both CO and CO₂ methanation by the small number of active Ni sites and the decrease in the amount of basic sites. The CO₂ methanation mechanism over Ni-Mn/Al@Al₂O₃ catalyst is elucidated by the transform route: adsorbed carbonate species – formate species – methane under hydrogenation process.

Full text of DOI:10.1016/j.cattod.2019.08.058

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CO and CO2 methanation over M (M]Mn, Ce, Zr, Mg, K, Zn, or V)-promoted
Ni/Al@Al2O3 catalysts
Thien An Le, Jieun Kim, Jong Kyu Kang, Eun Duck Park⁎
Department of Chemical Engineering and Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ni
Effects of metal promoter on CO and CO2 methanation were examined over Ni-M (M = Mn, Ce, Zr, Mg, K, Zn, or
V)/Al@Al2O3 catalysts prepared by the co-impregnation method. Ni-M (M = Mn, Ce, or Zr)/γ-Al2O3 catalysts
were also investigated for comparison. The prepared catalysts were characterized with a variety of techniques
such as N2 physisorption, CO2 chemisorption, H2 chemisorption, temperature-programmed reduction with H2
(H2-TPR), temperature-programmed desorption of CO2 (CO2-TPD), X-ray diffraction (XRD), high-resolution
transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and in-situ diffuse re-
flectance infrared Fourier transform spectroscopy (DRIFTS). Among different promoters, Mn, Ce, Mg, V, and Zr
are beneficial to enhance both CO and CO2 methanation activity due to the improvement of the Ni dispersion.
The Ni-V/Al@Al2O3 catalyst performs the highest CO methanation activity due to the largest Ni sites. However,
it is not the best one for CO2 methanation among tested catalysts because of the much decrease in CO2 ad-
sorption capacity. The promotional effect of Mn is the most remarkable for both CO and CO2 methanation. On
the other hand, the negative effect of K and Zn was observed on both CO and CO2 methanation by the small
number of active Ni sites and the decrease in the amount of basic sites. The CO2 methanation mechanism over
Ni-Mn/Al@Al2O3 catalyst is elucidated by the transform route: adsorbed carbonate species – formate species –
methane under hydrogenation process.
γ-Al2O3
Al@Al2O3
CO methanation
CO2 methanation
Promoter
Mn
Ce
CO2 (g) + 4H2 (g)
CH4 (g) + 2H2O (g) H2o98K
=
165 kJ/mol
1. Introduction
(2)
CO2 utilization technology is an ideal solution to cope with CO2
emission problem causing global warming effect [1]. Since CO2 is the
thermodynamically stable carbon-containing molecule, its chemical
transformation requires other highly reactive chemicals such as ep-
oxides and hydrogen [2]. Considering the CO2 emission rate, hydrogen
can be the most suitable reactant to transform CO2 into value-added
chemicals. Therefore, the power-to-gas (P2G) concept has been devel-
oped in which the captured CO2 can be transformed into synthetic
natural gas, which can be distributed through the gas grid, using hy-
drogen which can be produced via water electrolysis using renewable
energy [3]. This P2G technology is also considered to be effective to
store the surplus electricity from renewable energy with characteristics
of unstable electricity generation. Additionally, CO from the biomass or
organic waste gasifier can be used to produce synthetic natural gas.
These CO and CO2 methanation reactions are called the Sabatier reac-
tion described as follows.
Since these reactions are highly exothermic, a series of adiabatic
reactors with intermediate heat exchangers are required to achieve high
yields of methane. In order to achieve high single-pass CO and CO2
conversions, these reactions should be performed at low temperatures
using a reactor equipped with a heat exchanger, which requires a highly
active catalyst.
Until now, Ni-based catalysts have been widely used in the com-
mercial methanation process because of their relative fair activity, low
cost, and high availability compared with the noble metal catalyst
[4–15]. However, the low-temperature catalytic activity of Ni-based
catalysts should be further improved. In order to enhance the catalytic
activity, various supports [4,10–13,16–19] and a variety of preparation
methods [6–8,12,13,20–23] have been investigated to fabricate the
better Ni-based catalysts. Besides, the addition of second metal as a
promoter has also been attempted. Various promoters such as alkali
metals (Na, K [24–27]), alkali earth metals (Mg [28–30], Ca, Ba [28]),
3d transition metals (V [19], Mn [6,12,31,32], Zn [33–35]), 4d
CO (g) + 3H2 (g)
CH4 (g) + H2O (g) H2o98 K
=
206 kJ/mol
(1)
⁎ Corresponding author.
E-mail address: edpark@ajou.ac.kr (E.D. Park).
https://doi.org/10.1016/j.cattod.2019.08.058
Received 27 May 2019; Received in revised form 27 August 2019; Accepted 29 August 2019
0920-5861/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:ThienAnLe,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2019.08.058

T.A. Le, et al.
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Table 1
Physicochemical properties of Ni-based catalysts.a
Pore volumeb
Average pore diameterb
(nm)
Ni dispersionc
(%)
CASAc
H2 uptaked
(mmol/gcat.
CO2 uptakee
b
Catalyst
SBET
(m2/g)
(cm3/g)
(m2/gcat.
)
)
(μmol/gcat.)
Ni/Al@Al2O3 [41]
Ni-Mn/Al@Al2O3
Ni-Zr/Al@Al2O3
Ni-Ce/Al@Al2O3
Ni-Mg/Al@Al2O3
Ni-K/Al@Al2O3
Ni-Zn/Al@Al2O3
Ni-V/Al@Al2O3
Ni/γ-Al2O3 [8]
Ni-Mn/γ-Al2O3
Ni-Zr/γ-Al2O3
115
107
109
110
109
124
104
100
130
125
112
109
0.15
0.14
0.14
0.15
0.14
0.16
0.13
0.13
0.22
0.20
0.19
0.19
5.3
5.1
5.0
5.3
5.1
5.2
4.9
5.1
7.4
6.5
6.8
7.0
3.2
3.7
3.3
3.6
3.5
2.2
2.7
4.1
1.7
2.3
1.8
2.2
2.2
2.5
2.2
2.4
2.3
1.5
1.8
2.7
1.2
1.6
1.2
1.5
0.24
0.69
0.37
0.55
0.45
0.17
0.20
0.89
0.14
0.19
0.14
0.17
31
35
31
35
37
35
24
19
28
32
30
32
Ni-Ce/γ-Al2O3
a
b
c
d
e
All the catalysts were calcined in the air and reduced in H2 both at 500 ℃.
The specific surface area, pore volume, and average pore diameter were determined by N2 physisorption.
The Ni dispersion and catalytic active surface area (CASA) were determined based on the H2 chemisorption.
H2 uptake in the temperature range of 35–500 ℃ were determined based on the H2-TPR.
The chemisorbed CO2 uptake was measured at 35 ℃.
transition metals (Zr [28,36]) and lanthanides (La [24,26], Ce
[14,37–39]) were examined. However, a systematic approach is still
required to find out the effect of each promoter on the catalytic activity
for CO and CO2 methanation.
gas composed of 1 mol% CO (or CO2), 50 mol% H2, and 49 mol% He.
The kinetic experiments were also performed separately under dif-
ferent reaction conditions as described in the supporting information.
The exit gas composition is analyzed using a gas chromatograph (YL
Instrument 6100GC) as described in the supporting information. CO
conversion, CO2 conversion, CO yield, and C1-C3 hydrocarbon yield are
calculated as described in the supporting information.
Recently, we reported that the core-shell Al@Al2O3 provided su-
perior heat conductivity and surface properties as a potential hetero-
geneous catalyst substrate for highly exothermic and endothermic re-
actions [40–43]. In this study, a series of promoters including Mn, Zr,
Ce, Mg, K, Zn and V were incorporated into Ni/Al@Al2O3 and Ni/γ-
Al2O3 catalysts. The promoter content was fixed to be 0.5 wt.% because
the addition of large amounts of the promoter was reported to result in
a decrease in catalytic activity because of the blockage of the active site
[25,26]. These catalysts were applied to CO and CO2 methanation to
find out the effect of each promoter on the catalytic activity. These
catalysts were also characterized to find out the relationship between
the catalytic activity and the physicochemical properties.
2.3. Characterization of catalysts
The prepared catalysts were characterized with various techniques
such as N2 physisorption, CO2 chemisorption, H2 chemisorption, tem-
perature-programmed reduction with H2 (H2-TPR), temperature-pro-
grammed desorption of CO2 (CO2-TPD), X-ray diffraction (XRD), high-
resolution transmission electron microscopy (HRTEM), X-ray photo-
electron spectroscopy (XPS), and in-situ diffuse reflectance infrared
Fourier transform spectroscopy (DRIFTS). The detailed procedure for
each technique is described in the supporting information.
2. Experimental
2.1. Catalyst preparation
3. Results and discussion
The Al@Al2O3 support was prepared as described in the supporting
information. For comparison, γ-Al2O3 (neutral, Alfa Aesar) was also
used as a support as received. Ni/Al@Al2O3 and Ni/γ-Al2O3 were pre-
pared by wet impregnation method as described previously [4]. Metal
(M)-promoted Ni catalysts were prepared by co-impregnation method
from an aqueous solution of Ni(NO3)2∙6H2O (Junsei Chemical Co., Ltd.)
and precursor of a promoter such as Mn(NO3)2∙4H2O (Aldrich), ZrO
(NO3)2·2H2O (Kanto Chemical Co., Ltd.), Ce(NO3)3·6H2O (Kanto Che-
mical Co., Ltd.), Mg(NO3)2·6H2O (Kanto Chemical Co., Ltd.), KNO3
(Daejung Chemicals & Metals Co., Ltd.), Zn(NO3)2·6H2O (Daejung
Chemicals & Metals Co., Ltd.), NH4VO3 (Aldrich), and Ni(NO3)2∙6H2O
(Junsei Chemical Co., Ltd.). After impregnation, all samples were col-
lected, dried in an oven at 110 ℃ overnight and then calcined in air at
500 ℃ for 3 h. The Ni and M content in supported Ni-M catalyst were
intended to be 10 wt.% and 0.5 wt.%, respectively.
3.1. Characterizations of the catalysts
The specific surface area (SBET), average pore volume, and average
pore diameter of supported Ni catalysts were determined by N2 physi-
sorption and are listed in Table 1. All Ni catalysts supported on
Al@Al2O3 show type IIb isotherms [44], as shown in Fig. S1. The ad-
dition of a promoter does not affect significantly the textural properties
of the supported Ni catalysts. The specific surface areas and pore vo-
lumes of the promoted Ni catalysts (except for Ni-K/Al@Al2O3 catalyst)
were found to be slightly lower than those of the corresponding Ni/
Al@Al2O3 catalyst. This is quite reasonable because the additional
promoter can be dispersed throughout the pores inside the catalyst.
However, the addition of K resulted in the similar SBET and pore volume
with Ni/Al@Al2O3 within the experimental error. The addition of metal
precursors can affect the further oxidation of the core Al metal in
Al@Al2O3 support resulting in the different fraction of Al core and
Al2O3 layer in the final catalyst as listed in Table S1.
2.2. Catalytic activity test
H2 chemisorption was performed to determine the Ni dispersion and
the catalytically active surface area (CASA) of all catalysts. As listed in
Table 1, the noticeable improvement in Ni dispersion, as well as CASA,
is observed for Ni-Mn/Al@Al2O3 and Ni-Mn/γ-Al2O3 catalysts com-
pared to Ni/Al@Al2O3 and Ni/γ-Al2O3, respectively. Note that Ni/
Al@Al2O3 has much larger CASA than Ni/γ-Al2O3 and that a further
The CO and CO2 methanation were carried out in a fixed-bed quartz
reactor at atmospheric pressure in the reaction temperature range of
140–450 °C as described previously [4]. 0.10 g of the catalyst (45–80
mesh) was reduced at 500 ℃ for 1 h in a hydrogen stream with a flow
rate of 30 mL/min before being contacted with the 100 mL/min feed
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Fig. 1. CO2-TPD patterns of Ni-based catalysts (A) supported by Al@Al2O3, and (B) supported by γ-Al2O3.
increase in CASA is achieved by the introduction of Mn to Ni/Al@Al2O3
catalyst. There was no noticeable change in CASA and Ni dispersion for
Zr-promoted Ni/Al@Al2O3 catalysts, while V-, Mn-, Ce-, and Mg- pro-
moted increasing the CASA compared with those of Ni/Al@Al2O3.
Notably, among prepared Al@Al2O3-supported Ni catalysts, Ni-V/
Al@Al2O3 shows the largest CASA and Ni dispersion. Noticeable de-
crease in CASA and Ni dispersion was observed for Ni-Zn/Al@Al2O3
and Ni-K/Al@Al2O3. In the case of γ-Al2O3-supported Ni catalysts, the
CASA and Ni dispersion decreased in the following order: Ni-Mn/γ-
Al2O3 > Ni-Ce/γ-Al2O3 > > Ni-Zr/γ-Al2O3 ∼ Ni/γ-Al2O3. The strong
interaction between Ni and K or Zn was reported to cause the small
CASA by blocking the surface Ni site in the case of K- or Zn-promoted Ni
catalysts [26,27,35,45].
higher temperature than that of Ni/Al@Al2O3, which implies that the
additional moderate basic sites which are favorable for the activation of
CO2 are formed over them [6,7,10,17,46]. Fig. 1B reveals that similar
CO2-TPD patterns with those of Al@Al2O3-supported Ni catalysts were
obtained for all Ni catalysts supported on γ-Al2O3, wherein a TPD peak
for each catalyst appeared at ∼100 °C.
The XRD patterns of reduced samples are shown in Fig. 2 to confirm
any changes in the composition when the promoter was added to
supported Ni catalysts. Fig. 2A confirms that the core-shell structure of
Al@Al2O3 support remains after the co-impregnation of Ni and pro-
moter by the detection of Al (JCPDS 04-0787) and γ-Al2O3 (JCPDS 29-
0063). The crystallite size of the metallic Ni in the reduced catalyst
supported on Al@Al2O3 cannot be calculated because of the peak
overlap due to Ni and Al. In the case of Ni-based catalysts supported on
γ-Al2O3, the XRD peaks due to Ni (JCPDS 04-0850) are observed in
Fig. 2B indicating the transformation of NiO into Ni during the reduc-
tion process. Compared with those of Ni/γ-Al2O3 [8], rather broad XRD
peaks due to the metallic Ni were observed for Mn-, Ce-, and Zr-pro-
moted Ni/γ-Al2O3 catalysts indicating that these promoters increased
the Ni dispersion. The smallest crystallite size of Ni was obtained for Ni-
Mn/γ-Al2O3 catalyst among γ-Al2O3-supported Ni catalysts. Moreover,
the decrease in the Al core fraction was observed in the promoted Ni-
M/Al@Al2O3 (Table S1), implying that the core Al was further trans-
formed into Al2O3 during its preparation for this catalyst.
In order to assess the surface basicity of the prepared catalysts, CO2
chemisorption and CO2-TPD were carried out to measure the number of
basic surface sites and the strengths of the basic sites, respectively. As
presented in Table 1, the amounts of chemisorbed CO2 were dependent
on the promoter. Irrespective of support, Mg-promoted Ni catalysts
show the largest CO2 uptake at room temperature. The CO2 uptake for
Al@Al2O3-supported Ni catalysts decreased in the following order: Ni-
Mg/Al@Al2O3 > Ni-Mn/Al@Al2O3
Al@Al2O3 > Ni-Zr/Al@Al2O3
∼
Ni-Ce/Al@Al2O3
∼
Ni-K/
∼
Ni/Al@Al2O3 > > Ni-Zn/
Al@Al2O3 > Ni-V/Al@Al2O3. Note that the addition of Zn or V de-
creased the CO2 uptake significantly. Fig. 1A reveals that most CO2
molecules adsorbed onto the catalyst are desorbed at ∼100 °C only
except for Ni-Ce/Al@Al2O3, which indicates that most CO2 molecules
are adsorbed on weak basic sites [5,9]. The CO2 desorption peaks of Ni-
Ce/Al@Al2O3, Ni-Mn/Al@Al2O3, and Ni-Mg/Al@Al2O3 shift to the
Fig. 3 shows the H2-TPR profiles of the Ni-based catalysts calcined at
500 ℃. The H2 uptake of each catalyst calculated based on H2-TPR
profiles in the temperature range of 35–500 ℃ is presented in Table 1.
In Fig. 3A, the low-temperature TPR peak is observed for all samples
Fig. 2. XRD patterns of reduced Ni catalysts (A) supported by Al@Al2O3, and (B) supported by γ-Al2O3. (•) Al JCPDS 04-0787, (▼) Ni JCPDS 04-0850, and (▪) γ-
Al2O3 JCPDS 29-0063.
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Fig. 3. H2-TPR profiles of Ni-based catalysts calcined at 500 ℃; (A) supported by Al@Al2O3, and (B) supported by γ-Al2O3.
only except for Ni-K/Al@Al2O3 in the temperature range of 260–310 ℃,
which implies the presence of NiO interacting weakly with the support.
In the case of Ni-V/Al@Al2O3, this low-temperature TPR peak can also
be assigned to the reduction of some V-containing phase [47]. The TPR
peaks at high temperatures (400–500 ℃) indicate the reduction of NiO
species interacting strongly with the support. A small negative TPR
peak at ∼ 600 ℃ is attributed to the physical melting of Al core. The H2
uptake of Ni-Mn/Al@Al2O3 is larger than the others only except for Ni-
V/Al@Al2O3. Similarly, the low-temperature TPR peaks are observed
for Mn-, Zr-, and Ce-promoted Ni/γ-Al2O3 samples, as shown in Fig. 3B.
The highest H2 consumption (0.19 mmol/gcat.) was obtained for Ni-Mn/
γ-Al2O3. Ni/Al@Al2O3 [41] and Ni/γ-Al2O3 [8] show the only high-
temperature TPR peak with a peak maximum at above 500 °C indicating
the low reduction degree of nickel oxides when the catalyst was re-
duced at 500 ℃. Consequently, the addition of promoters such as Mn,
Zr, Ce, Mg, and V can improve the reducibility of NiO resulting in
higher reduction degree of NiO and possibly higher catalytic activity
than the unpromoted supported Ni catalysts. On the other hand, K- or
Zn-promoted Ni/Al@Al2O3 samples have lower H2 uptakes than the Ni/
Al@Al2O3 sample, which implies that the former catalysts would have
lower reduction degree of NiO than the latter catalyst when they are
reduced at 500 ℃. This is responsible for their low Ni dispersions,
CASAs, and catalytic activity for CO and CO2 methanation.
Each fraction was also quantified and listed in Table S2. Similar dis-
tribution of Mn(II), Mn(III), and Mn(IV) species are confirmed for both
Ni-Mn/γ-Al2O3 and Ni-Mn/Al@Al2O3.
3.2. CO methanation
The catalytic activity for CO methanation over Al@Al2O3-supported
Ni catalysts was obtained and displayed in Fig. 6A. The addition of V,
Mn, Ce, Mg, and Zr into Ni/Al@Al2O3 improved the catalytic perfor-
mance for CO methanation. Note that these catalysts have higher H2
uptakes determined by H2-TPR and higher Ni dispersion as well as
CASA calculated based on H2 chemisorption. On the other hand, the
negative effect on CO methanation was observed over Zn-, and K-doped
Ni/Al@Al2O3 catalysts which can be due to the significant decrease of
CASA (Table 1). In terms of product yield, methane is a predominant
product at all reaction temperatures, while ethane and propane are also
detected as byproducts (Fig. S3).
The Arrhenius plots for CH4 formation rate for CO methanation over
Al@Al2O3-supported Ni catalysts (Fig. 6B) provide the activation en-
ergies (Ea) in the range of 93–120 kJ/mol (Table 2). The most active
catalyst (Ni-V/Al@Al2O3) shows the highest CH4 formation rate of
0.242 mol·h−1·g−1 at 200 ℃ with the low apparent Ea of 96 kJ/mol
among tested catalysts.
Fig. 4A illustrates the HRTEM image of the Ni-Mn/Al@Al2O3 cata-
lyst. There is a rather uniform particle size distribution of Ni metal. The
average particle size of Ni metal is determined to be around 5 nm
(Fig. 4B). The typical STEM dark field image and corresponding ele-
mental maps (Fig. 4C) confirm that Mn, Ni, Al, and O elements are well
distributed in the Ni-Mn/Al@Al2O3 catalyst.
The improvement of the CO methanation activity was also observed
over Mn-, Ce-, and Zr-promoted Ni/γ-Al2O3 catalysts (Fig. S4), which is
consistent with the previous reports [6,12,15,31,32,34,36,37,
39,52,53]. The CO methanation activity is dependent on the surface
area of the metallic Ni, which affects the CO activation and dissociation
and the production of highly active surface H species. The enhanced
catalytic activity of promoted Ni catalysts was reported to be related to
the improvement of Ni dispersion [5]. The Ce-doped Ni/γ-Al2O3 cata-
lyst was reported to promote CO methanation due to high Ni dispersion
[37,38]. Hu et al. [26] demonstrated that the addition of Zr, Co, Ce, Zn,
and La possessed high catalytic activity for CH4 formation and good
catalytic stability, which were attributed to the formation of smaller Ni
nanoparticles with high metal dispersion. Zr promoter was also re-
ported to be able to produce oxygen vacancies, thereby increasing the
ability of CO to adsorb and dissociate [36]. Zr promoter was also
claimed as the best promoter among Zr, Mg, Ba, and Ca to enhance
coking resistance by reducing the rate of polymeric carbon formation
through CO methanation [28]. 2–4 wt% of MgO adding to Ni/Al2O3
was reported to enhance the CO methanation activity via the small NiO
particles formation and improve the catalytic stability by reducing the
carbon deposition [29]. MnOx promoter was reported to restrain the
sintering and aggregation of Ni due to the formation of relatively small
Ni nanoparticles and to provide more oxygen vacancies, which is con-
ducive to the removal of carbonaceous species for anti-coking [12].
X-ray photoelectron spectroscopy (XPS) studies were carried out on
Ni-Mn/Al@Al2O3 and Ni-Mn/γ-Al2O3 samples with to determine the
oxidation state of the metals and the surface composition in the reduced
state. The bands observed at 851–852 and 854–856 eV (Fig. 5B) can be
assigned as Ni 2p3/2 XPS spectra corresponding to the metallic Ni [48]
and nickel oxide/hydroxide [49,50], respectively. The above results
indicated that Ni species existed as Nio and NiO. Note that the fraction
of the metallic Ni in Ni-Mn/Al@Al2O3 was higher than those in Ni-Mn/
γ-Al2O3 and Ni/Al@Al2O3 (Table S2). This is consistent with the fact
that Ni-Mn/Al@Al2O3 has a higher H2 uptake than Ni-Mn/γ-Al2O3 and
Ni/Al@Al2O3 during H2-TPR (Table 1). The Ni 2p3/2 XPS spectra ob-
tained for Ni/Al@Al2O3, Ni-K/Al@Al2O3, and Ni-Zn/Al@Al2O3 confirm
that the fraction of the metallic Ni was decreased when K or Zn was
added as a promoter (Fig. S2). This is also consistent with the result that
Ni-K/Al@Al2O3 and Ni-Zn/Al@Al2O3 have lower H2 uptake during H2-
TPR than Ni/Al@Al2O3 (Table 1). From the spectra of Fig. 5C, Mn(II),
Mn(III), and Mn(IV) species are confirmed to coexist because the cor-
responding XPS spectra were found in the range of 630–650 eV [51].
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Fig. 4. (A) HRTEM image, (B) size distribution, and (C) elemental mapping of Ni-Mn/Al@Al2O3 catalyst.
In this study, the CASA was not increased noticeably when 0.5 wt%
in which V-promoted Ni/Al2O3 improved CO methanation due to its
large H2 uptake and high Ni dispersion [36,55–57]. They stated that the
CO dissociation was improved by electron transferring from V species to
Nio. In this study, we also observed the high H2 consumption during
TPR experiment.
Zr was added. Therefore, it can be said that the nature of support, such
as ZrOx-Al2O3, might affect the CO dissociation rate, which is the in-
termediate step in CO methanation [34,52,54]. Moreover, it was re-
ported that Zr was effective to reduce carbon formation [28,52]. Among
tested catalysts, the V-promoted Ni catalyst is the best one because it
has the largest CASA. The promotional effect of V in supported Ni-V
catalyst on CO methanation is still controversial. There are some reports
Zn was reported to make strong interactions with Ni, cover Ni
particles, and decrease the chemisorption amount of CO, which is not
favorable for the CO dissociation on Ni-Zn alloy [33]. The specific
Fig. 5. XPS spectra of reduced Ni-Mn/Al@Al2O3 and Ni-Mn/γ-Al2O3 catalysts; (A) full scan, (B) Ni 2p, and (C) Mn 2p.
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Fig. 6. Catalytic performance of Ni-based catalysts supported by Al@Al2O3 for CO methanation; (A) CO conversion, and (B) CH4 formation rate. All the catalysts were
calcined in air at 500 ℃ and reduced by H2 at 500 ℃. Reaction conditions: 1 mol% CO, 50 mol% H2, 49 mol% He, F/W =1000 mL/min/gcat. The data for Ni/
Al@Al2O3 [41] are included for easy comparison.
3.3. CO2 methanation
Table 2
Activity comparison for CO and CO2 methanation among Ni-based catalysts.
The catalytic activity for CO2 methanation was measured over the
Al@Al2O3-supported Ni catalysts. As shown in Fig. 7A. Ni-Mn/
Al@Al2O3 shows the highest catalytic activity for CO2 methanation
among the tested catalysts. This result is related to the combination of
the high CASA, high CO2 uptake capacity, and moderate binding
strength of CO2 by the introduction of Mn promoter. High CASA sup-
plies high surface H concentration for the hydrogenation of inter-
mediates species is the rate-determining step [5,8,20], and the high CO2
uptake capacity coupled with strong interactions with catalyst surface
promotes the high catalytic activity for CO2 methanation [6,9,20,58].
Interestingly, Ni-V/Al@Al2O3 with a higher Ni dispersion but smaller
CO2 uptake shows the higher catalytic activity for CO2 methanation
than Ni/Al@Al2O3, while Ni-K/Al@Al2O3 with a lower Ni dispersion
but higher CO2 uptake obtains the lower activity. This result reveals the
synergic effect between Ni dispersion and CO2 uptake in CO2 metha-
nation. Methane was a major product during CO2 methanation (Fig.
S5). The formation of C2H6 was observed over Ni-Ce/Al@Al2O3 and Ni/
Al@Al2O3 catalysts. CO formation via reverse water gas shift (RWGS)
reaction was confirmed over Ni(-K, -Mg, -Zn, -V)/Al@Al2O3 catalyst.
These promoters could promote the RWGS reaction. The beneficial ef-
fect of Mn, Ce, and Zr in promoted Ni catalysts supported on γ-Al2O3 on
CO2 methanation was also observed (Fig. S6). The catalytic activity for
CO2 methanation decreased in the following order: Ni-Mn/γ-
Al2O3 > Ni-Ce/γ-Al2O3 > Ni-Zr/γ-Al2O3 > Ni/γ-Al2O3.
Catalyst
CO Methanation
CO2 Methanation
Ea
Specific reaction
Ea
Specific reaction
(kJ/mol) rate at 200 °C
(mol
(kJ/mol) rate at 220 °C
(mol
CH4·h−1·g−1
)
CH4·h−1·g−1
)
Ni/Al@Al2O3 [41] 115
0.099
0.239
0.225
0.170
0.198
0.073
0.058
0.242
83
82
76
68
82
80
83
74
0.219
0.462
0.332
0.263
0.320
0.162
0.185
0.280
Ni-Mn/Al@Al2O3
Ni-Ce/Al@Al2O3
Ni-Zr/Al@Al2O3
Ni-Mg/Al@Al2O3
Ni-Zn/Al@Al2O3
Ni-K/Al@Al2O3
Ni-V/Al@Al2O3
106
93
106
120
103
114
96
amount of K was reported to inhibit methane formation [26]. ZnO is
more reactive towards CO than CeO2 [33], and the strong interaction
between Ni and ZnO led to the CH4 suppression in CO methanation
[35]. A small amount of alkali metal such as Na and K was reported to
improve the CO methanation performance by increasing the metal
dispersion [25,26]. Conversely, promotional effects were not always
positive; for instance, using K can inhibit the CH4 formation in Ni-K/
Al2O3 [26,27], and increase the selectivity of undesired higher hydro-
carbons in products as a consequence of direct K-Ni interactions [27].
The Arrhenius plots for the CH4 formation rate during CO2 metha-
nation displayed in Fig. 7B confirm the trend observed in Fig. 7A. The
CH4 formation rate decreased in the following order: Ni-Mn/
Fig. 7. Catalytic performance of Ni-based catalysts supported by Al@Al2O3 for CO2 methanation; (A) CO2 conversion, and (B) CH4 formation rate. All the catalysts
were calcined in air at 500 ℃ and reduced by H2 at 500 ℃. Reaction conditions: 1 mol% CO2, 50 mol% H2, 49 mol% He, F/W =1000 mL/min/gcat. The data for Ni/
Al@Al2O3 [41] are included for easy comparison.
6

T.A. Le, et al.
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Al@Al2O3 > Ni-Ce/Al@Al2O3 > Ni-Mg/Al@Al2O3 > Ni-V/Al@Al2O3
Immediate appearance of new bands at 3016, 2904, 1595, 1376, and
1304 cm−1 was observed with accompanying apparent attenuation of
the carbonates bands at 1650 and 1530 cm−1 at low reaction tem-
peratures. The disappearance of peaks at 1590, 1440, 1390, 1320 and
1230 cm−1 can be explained by the transformation of carbonates spe-
cies into intermediates during CO2 hydrogenation. The new peaks can
be classified into two groups: (i) 2904, 1595 and 1376 cm−1, and (ii)
3016 and 1304 cm−1. The former and the latter group can be assigned
to bidentate formate species and methane, respectively
[6,7,16,17,58,64,65]. With increasing reaction temperature, the frac-
tion ratio of formate species to carbonates species reached a maximum
at 300 ℃ with the continuous consumption of carbonates, suggesting a
transformation of carbonates into formate species with the steady
supply of H2 [7]. The peak intensity of formate species decreased along
with the continuous formation of surface methane species. According to
the IR data, we can conclude that CO2 methanation over Ni-Mn/
Al@Al2O3 proceeds with the initial formation of carbonates due to CO2
adsorption, their hydrogenation into formate species, and finally sub-
sequent transformation into methane. There are two main different
mechanisms on CO2 methanation. One is that the process might involve
the conversion of CO2 to CO before being hydrogenated to methane
[64,66,67]. The other involves the direct hydrogenation of CO2 to
methane without forming a CO intermediate [11,46,58,59,65,67].
Based on the IR study, it can be said that direct hydrogenation of CO2
proceeds over Ni-Mn/Al@Al2O3 catalysts.
∼
Ni-Zr/Al@Al2O3 > Ni/Al@Al2O3 > Ni-K/Al@Al2O3 > Ni-Zn/
Al@Al2O3. The apparent Ea of all catalysts in the kinetic experiment for
CO2 methanation were found to be in the range of 68–83 kJ/mol as
listed in Table 2.
Ni/γ-Al2O3 catalysts doped with Ce was reported to enhance the
CO2 methanation performance thanks to the possible activation of CO2
[14,39]. Hu et al. [26] demonstrated that the addition of Zr, Co, Ce, Zn,
and La possessed high catalytic activity for CH4 formation and good
catalytic stability, which were attributed to the formation of smaller Ni
nanoparticles with high metal dispersion. Mg was reported to be a
potential promoter for CO2 methanation by improving the Ni disper-
sion, as well as the active CO2 amount over Ni-Mg-USY zeolite catalysts
[30]. La, K, and Na were proposed to increase CO2 methanation activity
in the coexistence of CO and CO2 condition [24]. The active sites that
are responsible for CO2 methanation reaction were proposed as the
basic metallic surface centers and oxygen vacancy sites [59].
3.4. In-situ DRIFTS analysis
In-situ DRIFTS experiments were performed to study the evolution
of surface species during CO2 methanation over the most active Ni-Mn/
Al@Al2O3 catalyst. Fig. S7 shows the infrared spectra of the surface
species formed on this catalyst after the adsorption of CO2 at different
temperatures for 20 min in order to probe the surface basicity of these
catalysts. Three kinds of basic sites distinguished on the catalyst can be
classified as bicarbonate (at ν = 1650 cm−1
,
ν = 1440 cm−1 and
(ν=1390 cm−1and
3.5. Stability
ν = 1230 cm−1),
monodentate
carbonate
ν=1530 cm−1), and bidentate carbonate (ν = 1590 cm−1and
ν = 1320 cm−1) [6,46,60–63]. The band intensity at 1650, 1440, and
1230 cm−1 corresponding to the weak bicarbonate decreased sig-
nificantly with increasing temperature. On the other hand, the bands
due to monodentate carbonate and bidentate carbonate were weakened
a little after increasing temperature up to 300 ℃. Each band corre-
sponding to a different basic site was deconvoluted and quantified in
Table S3. Compared with Ni/γ-Al2O3 and Ni/Al@Al2O3 catalysts [41],
the fraction of moderate basic sites forming bidentate carbonate species
is predominantly found over Ni-Mn/Al@Al2O3. On the other hand, the
fraction of weak basic sites forming bicarbonate species decreased in
In order to evaluate the stability of the most active catalysts (Ni-Mn/
Al@Al2O3 and Ni-Mn/γ-Al2O3) during CO and CO2 methanation, the
long-term stability test was carried out. As shown in Fig. 9, CO and CO2
conversion were maintained over these two catalysts for 50 h. The bulk
crystalline structure is confirmed not to be changed by XRD analysis for
the catalyst before and after the stability test (Fig. S8). No detectable
signal was found due to crystalline carbon (graphite and whisker
carbon) or NiO. There is no change in the intensity ratio of peaks due to
Al and Al2O3 for the Ni-Mn/Al@Al2O3 after the stability test, which
indicates that the core-shell structure of Al@Al2O3 support is main-
tained.
the
following
order:
Ni/γ-Al2O3
(55.5%) > Ni/Al@Al2O3
(46.3%) > Ni-Mn/Al@Al2O3 (28.7%). Considering the largest CO2 up-
take capacity of Ni-Mn/Al@Al2O3 (Table 1), we can say that this cat-
alyst is confirmed to possess the higher density of medium and strong
basic sites than Ni/Al@Al2O3 and Ni/γ-Al2O3. This is beneficial for
higher catalytic activity for CO2 methanation.
4. Conclusion
The catalytic activity for CO and CO2 methanation was strongly
dependent on the kind of promoter (M = Mn, Ce, Zr, Mg, K, Zn, or V) in
Ni-M/Al@Al2O3 catalysts. Mn, Ce, Mg, V, and Zr have a positive effect
on both reactions. On the other hand, K and Zn have a negative effect
on both reactions. The Ni dispersion and CASA are critical factors to
Fig. 8 shows the formation of various surface intermediates during
CO2 hydrogenation at different temperatures from 150 °C to 300 ℃.
Fig. 8. In situ DRIFTS spectra over Ni-Mn/Al@Al2O3 observed during CO2 methanation from 150 to 300 ℃. The catalyst was contacted with the feed gas composed of
1 mol% CO2, 50 mol% H2, and 49 mol% He with a total flow of 50 mL/min.
7

T.A. Le, et al.
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Fig. 9. Stability test over Ni-Mn/γ-Al2O3 and Ni-Mn/Al@Al2O3 for (A) CO methanation, and (B) CO2 methanation during 50 h. Reaction conditions for CO and CO2
methanation were the same as those in Figs. 6 and 7, respectively.
affect the catalytic activity for CO methanation. Besides them, CO2
uptake and moderate binding strength of chemisorbed CO2 are further
required to guarantee the high catalytic activity for CO2 methanation.
The most positive effect of Mn promoter for both CO and CO2 metha-
nation was also confirmed in Ni-Mn/Al@Al2O3 catalysts. The direct
hydrogenation of surface carbonate species seems to be the main
pathway for CO2 methanation over Ni-Mn/Al@Al2O3 catalyst. The
catalytic stability for methanation process is guaranteed over Ni-Mn/
Al@Al2O3 and Ni-Mn/γ-Al2O3 for 50 h.
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