CO and CO2 methanation over Ni/Al@Al2O3 core–shell catalyst

Article abstract of DOI:10.1016/j.cattod.2019.09.028

Core–shell Al@Al₂O₃, which was obtained by hydrothermal surface oxidation of Al metal particles, was used as the support in supported Ni catalysts for CO and CO₂ methanation. The core–shell micro-structured support (Al@Al₂O₃) helped develop a highly efficient Ni-based catalyst compared with conventional γ-Al₂O₃ for these reactions. Moreover, the deposition–precipitation method was shown to outperform the wet impregnation method in the preparation of the active supported Ni catalysts. The catalysts were characterized using various techniques, namely, N₂ physisorption, H₂ chemisorption, CO₂ chemisorption, temperature-programmed reduction with H₂, temperature-programmed desorption after CO₂ adsorption, X-ray diffraction, inductively coupled plasma-atomic emission spectroscopy, high-resolution transmission electron microscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy. Higher Ni dispersion when using Al@Al₂O₃ as the support and the deposition–precipitation method resulted in better catalytic performance for CO methanation. Furthermore, the higher density of medium basic sites and enhanced CO₂ adsorption capacity observed for Ni/Al@Al₂O₃ helped increase catalytic activity for CO₂ methanation.

Full text of DOI:10.1016/j.cattod.2019.09.028

CatalysisTodayxxx(xxxx)xxx–xxx
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Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
CO and CO2 methanation over Ni/Al@Al2O3 core–shell catalyst
Thien An Le, Jieun Kim, Jong Kyu Kang, Eun Duck Park⁎
Department of Chemical Engineering and Department of Energy Systems Research, Ajou University, 206, World cup-ro, Yeongtong-Gu, 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 catalyst
γ-Al2O3
Al@Al2O3
CO methanation
CO2 methanation
deposition–precipitation method
Core–shell Al@Al2O3, which was obtained by hydrothermal surface oxidation of Al metal particles, was used as
the support in supported Ni catalysts for CO and CO2 methanation. The core–shell micro-structured support
(Al@Al2O3) helped develop a highly efficient Ni-based catalyst compared with conventional γ-Al2O3 for these
reactions. Moreover, the deposition–precipitation method was shown to outperform the wet impregnation
method in the preparation of the active supported Ni catalysts. The catalysts were characterized using various
techniques, namely, N2 physisorption, H2 chemisorption, CO2 chemisorption, temperature-programmed reduc-
tion with H2, temperature-programmed desorption after CO2 adsorption, X-ray diffraction, inductively coupled
plasma-atomic emission spectroscopy, high-resolution transmission electron microscopy, and in situ diffuse
reflectance infrared Fourier transform spectroscopy. Higher Ni dispersion when using Al@Al2O3 as the support
and the deposition–precipitation method resulted in better catalytic performance for CO methanation.
Furthermore, the higher density of medium basic sites and enhanced CO2 adsorption capacity observed for Ni/
Al@Al2O3 helped increase catalytic activity for CO2 methanation.
1. Introduction
single-pass conversion. Moreover, temperature control while designing
the catalyst and reactor is essential to prevent the formation of hotspots.
CO2 conversion technologies are crucial for mankind, given their
ability to mitigate global warming [1]. Since CO2 is the most stable and
fully oxidized state of carbon, its chemical transformation requires
other highly reactive chemicals [2] or additional energy [3,4]. In order
to convert CO2 into energy or chemical feedstock without net produc-
tion of CO2 in the overall process, renewable energy should be used
directly [5] or indirectly [6]. The electrolysis of water using electricity
from wind or solar energy is an example of the indirect utilization of
renewable energy to produce H2. Power-to-gas (P2G) is the concept that
surplus electricity from a renewable energy source can be stored or
distributed in the form of synthetic natural gas which can be produced
from H2 and carbon oxides (CO and CO2) [7]. The carbon oxides can be
supplied by a carbon capture and storage facility or synthesized on site
by gasification of biomass or organic wastes [8].
From this viewpoint, the development of a very active catalyst at low
temperatures as well as the application of highly thermal conduction
material is extremely desirable.
Until now, Ni-based catalysts have been widely used in the in-
dustrial process because of their relative fair activity, low cost, and high
availability [9–17]. Various supports such as Al2O3 [10,11,18], CeO2
[10,19–21], ZrO2 [10,22], CeO2-ZrO2 [9,20], SiO2 [10,14,18,23,24],
Y2O3 [22,25], or zeolites [18,23,26] combined with novel preparation
methods [9,12,13,16,17,27] have been reported to fabricate active Ni-
based catalysts owning the high methanation catalytic activity at low
temperatures. Additionally, since the catalytic stability of Ni-based
catalyst has remained challenges due to a highly exothermic metha-
nation process, the development of core-shell structural catalysts to
prevent carbon deposition and Ni sintering has been reported [28–31].
The core–shell metal@metal oxide particle can be considered as a high
thermal-conducting support material given the high thermal con-
ductivity of the metal and good textural properties of the metal oxide
layer (e.g., high surface area and enhanced stability against thermal and
chemical attack) [32]. Among various core–shell composite candidates,
Al@Al2O3 can be prepared by the hydrothermal surface oxidation
(HTSO) of Al metal particles in an aqueous solution at elevated tem-
peratures (120–200 ℃) [33]. It provides superior heat conductivity and
The Sabatier reaction involves the synthesis of CH4 from H2 and
carbon oxides (CO and CO2) as follows.
CO (g) + 3H2 (g) ↔ CH4 (g) + H2O (g) ΔH2o98 K = −206 kJ/mol
(1)
CO2 (g) + 4H2 (g) ↔ CH4 (g) + 2H2O (g) ΔH2o98 K = −165 kJ/mol (2)
Since these reactions are thermodynamically limited and highly
exothermic, the low-temperature operation is preferred to achieve high
⁎ Corresponding author.
E-mail address: edpark@ajou.ac.kr (E.D. Park).
https://doi.org/10.1016/j.cattod.2019.09.028
Received 19 April 2019; Received in revised form 20 August 2019; Accepted 18 September 2019
0920-5861/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:ThienAnLe,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2019.09.028

T.A. Le, et al.
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surface properties and thus can be regarded as a potential hetero-
geneous catalyst substrate for highly exothermic and endothermic re-
actions [33–35].
Pulsed CO2 chemisorption and temperature-programmed desorption
(TPD) of CO2 (CO2-TPD) were carried out on a Micrometrics Autochem
2910 instrument to analyze the basicity of the catalyst surface. Pulsed
CO2 chemisorption was conducted at room temperature by injection of
0.50 mL of 15 mol% CO2 balanced with He in a He stream at a flow rate
of 30 mL/min. CO2-TPD was conducted in the He stream at a flow rate
of 30 mL/min for the temperature range of 40–900 °C at a heating rate
of 10 °C/min. The ion signals recorded at m/e = 44 were utilized for
monitoring the desorbed CO2.
In-situ diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) experiments were carried out on a NICOLET 6700 (Thermo
Scientific) spectrometer equipped with a ZnSe window with a resolu-
tion of 3.857 cm–1. Before CO2 adsorption, the sample was reduced in-
situ in the DRIFTS cell at 500 °C for 1 h using H2 at a flow rate of 30 mL/
min, and cooled under He flow to 40 ℃. The cell was purged with He
before the introduction of CO2 at a flow rate of 50 mL/min. A back-
ground spectrum was recorded under He flow. CO2 adsorption was
performed for 20 min at 40 ℃, and the spectra were recorded while
purging with He at a flow rate of 100 mL/min at different temperatures.
The spectra were also recorded during CO and CO2 methanation under
the same reaction conditions as described for the catalytic activity test.
At each reaction temperature, the signal was recorded after 20 min of
reaction time.
In this work, the core–shell microstructural Al@Al2O3 support was
applied as the support for supported Ni catalysts in CO and CO2 me-
thanation. Two different preparation methods, namely wet impregna-
tion (WI) and deposition–precipitation (DP), were compared for the
supported Ni catalysts. Improved catalytic activity was observed over
the Ni/Al@Al2O3 catalyst prepared by the DP method. The catalytic
activity was closely related to Ni dispersion and preferential uptake of
the reactant, and these findings were supported by various character-
ization techniques.
2. Experimental
2.1. Catalyst preparation
The core–shell Al@Al2O3 support was prepared by the HTSO
method [33]. The detailed procedure is described in the supporting
information. The Al@Al2O3-supported Ni catalyst prepared from the
aqueous solution of Ni(NO3)2∙6H2O (Junsei Chemical Co., Ltd.) and
home-made Al@Al2O3 by the conventional WI method is denoted as Ni/
Al@Al2O3 (WI). For comparison, the Al2O3-supported Ni catalyst was
also prepared with γ-Al2O3 (neutral, Alfa Aesar) by the WI method and
is designated as Ni/γ-Al2O3 (WI). Both Ni/γ-Al2O3 (WI) and Ni/
Al@Al2O3 (WI) were prepared after calcination in air at 500 ℃ and
subsequent reduction in the H2 stream at 500 ℃. Moreover, the
Al@Al2O3-supported Ni catalyst was prepared using the DP method.
For the DP method, 2.91 g of Ni(NO3)2∙6H2O (Junsei Chemical Co.,
Ltd.) was dissolved in 50 mL of deionized (DI) water. This solution was
contacted with 5.3 g of Al@Al2O3, and 1.0 M aqueous NH4OH solution
(Samchun Pure Chemical Co., Ltd.) was added to this slurry drop by
drop until a final pH of 9 was reached under stirring for 12 h at room
temperature. The slurry was filtered and washed several times with DI
water. The recovered powder was dried in an oven at 110 °C for 12 h.
This dried sample was further reduced in the H2 stream at 500 ℃ for
1 h. The resulting catalyst is denoted as Ni/Al@Al2O3 (DP110). In order
to assess the effect of the calcination temperature, the dried sample was
calcined in air at 500 ℃ for 3 h and subsequently reduced in the H2
stream at 500 ℃ for 1 h to obtain Ni/Al@Al2O3 (DP500). The Ni content
for all supported Ni catalysts was intended to be 10 wt.% and confirmed
using inductively coupled plasma-atomic emission spectroscopy (ICP-
AES).
Thermogravimetric analysis (TGA) and differential thermal analysis
(DTA) were performed on a thermogravimetric analyzer (NETZSCH
STA 409 PC/PG) in air with a flow rate of 50 mL/min from room
temperature to 1000 ℃ at a heating rate of 10 ℃/min.
Temperature-programmed oxidation (TPO) was conducted over
0.10 g of each sample in a 2% O2/He stream by heating the sample in
the temperature range of 30–900 °C at a heating rate of 10 ℃/min while
monitoring the thermal conductivity detector (TCD) signals (Autochem
2910, Micromeritics) and online mass spectrometer signals corre-
sponding to CO2 (m/z = 44) (Cirrus 2 Quadrupole Mass Spectrometer)
after the sample was purged with He at room temperature for 1 h.
High-resolution transmission electron microscopy (HRTEM) images
were obtained using Tecnai G2 TEM (FEI) operating at 200 kV with an
energy dispersive (EDS) detector.
Inductively coupled plasma-atomic emission spectroscopy (ICP-
AES) was carried out on a Thermo Scientific iCAP 6500 instrument to
determine the Ni content for each catalyst.
2.3. Catalytic activity test
CO and CO2 methanation were carried out in a fixed-bed quartz
reactor, as described previously [10]. Briefly, 0.10 g of the catalyst with
particulate sizes of 45–80 mesh was reduced at 500 ℃ for 1 h in a
30 mL/min H2 stream and then contacted with the feed gas composed
of 1 mol% CO (or CO2), 50 mol% H2, and 49 mol% He at a flow rate of
100 mL/min. The reaction was conducted at atmospheric pressure in
the reaction temperature range of 140–450 ℃. The kinetic experiments
were performed separately at low reaction temperatures under different
reaction conditions, wherein 0.10 g of the catalyst was diluted with
0.20 g of α-alumina and then contacted with the feed gas. The CO and
CO2 conversions were controlled to be less than 15%. The activation
energy (Ea) over each catalyst was calculated based on the Arrhenius
equation.
2.2. Catalyst characterization
N2 physisorption was analyzed using a Micromeritics ASAP 2020
instrument in which the supports and catalysts were degassed under
vacuum for 6 h at 200 ℃ before the analysis. The specific surface area
(SBET
)
of the sample was determined according to the
Brunauer–Emmett–Teller method. The pore size distribution for each
catalyst was obtained using the Barrett–Joyner–Halenda desorption
method.
X-ray diffraction (XRD) patterns were detected by a Rigaku D/Max
instrument with a Cu Kα source to assess the bulk crystalline structure
of the samples.
The temperature-programmed reduction with H2 (H2-TPR) was
performed with a Micromeritics 2910 Autochem instrument to check
the reducibility of the nickel oxide species in the sample. All samples
except for Ni/Al@Al2O3 (DP110), which was used only in the dried
form, were calcined in air at 500 ℃. Then, 0.20 g of the sample was
contacted with 10 mol% H2/Ar at a flow rate of 30 mL/min in the
temperature range of 40–900 ℃.
Ea
RT
⎛
⎞
⎠
k = Aexp −
(3)
⎝
where k denotes the reaction rate constant, A is the frequency factor, Ea
is the activation energy, R is the gas constant, and T is the absolute
temperature. The exit gas composition was analyzed using a gas chro-
matograph (YL Instrument 6100GC), equipped with a packed column
filled with Carbosphere® for TCD and a capillary Poralot Q column for
The catalytically active surface area (CASA) and Ni dispersion for
each catalyst were determined by H2 chemisorption using
Micromeritics ASAP 2020 instrument, as described previously [11].
a
the flame ionization detector (FID). CO conversion (
X
CO), CO2 con-
version (XCO2), CO yield ( CO), and C1-C3 hydrocarbon yield (YC
Y
)
H
x
y
2

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Fig. 1. N2 adsorption (filled points) and desorption (unfilled points) isotherms (A) and pore size distribution (B) of Ni/Al@Al2O3 (WI) (a), Ni/Al@Al2O3 (DP110) (b),
and Ni/Al@Al2O3 (DP500) (c).
were calculated using the following equations.
prepared by the WI method (NiO/Al@Al2O3 (WI)). On the other hand,
very weak XRD peaks owing to NiO were observed over NiO/Al@Al2O3
(DP), indicating the high dispersion of NiO. α-Al2(OOH)2 and γ-Al2O3
were detected for the unreduced NiO/Al@Al2O3 (DP110) and NiO/
Al@Al2O3 (DP500), respectively, as the bulk crystalline phase of alu-
minum oxides. However, γ-Al2O3 was the only phase as aluminum
oxide, and all the XRD peaks due to NiO disappeared for all supported
Ni catalysts after reduction. The crystallite size of the metallic Ni in the
reduced catalyst could not be calculated due to the overlap of XRD
peaks corresponding to Ni and Al.
H2 chemisorption was performed to determine the Ni dispersion and
the CASA of all the catalysts. As shown in Table 1, Ni/Al@Al2O3 (WI)
shows higher Ni dispersion and larger CASA than Ni/γ-Al2O3 (WI).
Compared with the WI method, a further increase in Ni dispersion and
CASA was achieved by adopting the DP method for the Al@Al2O3-
supported Ni catalyst. In the case of the Ni/Al@Al2O3 (DP) catalyst,
merely drying and reducing Ni/Al@Al2O3 (DP110) facilitates higher Ni
dispersion and larger CASA than calcining and reducing Ni/Al@Al2O3
(DP500) at 500 ℃.
[CO]in − [CO]out
XCO (%) =
× 100
[CO]in
(4)
(5)
(6)
[CO2]in − [CO2]out
XCO (%) =
× 100
2
[CO2]in
x [Cx Hy ]out
YC
(%) =
× 100
H
y
x
[CO]in + [CO2]in
where [CO]in, [CO2]in, [CO]out, [CO2]out, and [Cx Hy ]out are the CO con-
centration in the feed stream, CO2 concentration in the feed stream, CO
concentration in the exit stream, CO2 concentration in the exit stream,
and CxHy concentration in the exit stream, respectively.
3. Results and discussion
3.1. Characterization of the catalysts
Fig. 1A shows the N2 adsorption and desorption diagram of each
catalyst. All catalysts have the type-IIb isotherms [36]. A rather narrow
pore diameter distribution was obtained for each Ni/Al@Al2O3 catalyst
(Fig. 1B). The textural properties of the prepared catalysts probed by N2
physisorption are summarized in Table 1. Compared with Al@Al2O3,
whose surface area, pore volume, and average pore diameter were
determined to be 142 m2/g, 0.19 cm3/g, and 5.4 nm, respectively, Ni/
Al@Al2O3 (WI) had a smaller surface area, pore volume, and average
pore diameter, indicating that the Ni particles were well-dispersed over
the support. However, small increases in the specific surface area, pore
volume, and average pore diameter were noted for the Ni/Al@Al2O3
(DP) catalysts compared with those for the support, indicating that the
textural properties of the Al@Al2O3 were further modified during the
DP process.
In order to study the bulk crystalline structure of the supported Ni
catalysts before and after reduction, XRD patterns of all Ni/Al@Al2O3
samples were obtained. As shown in Fig. 2, the presence of Al metal was
confirmed for all Ni catalysts supported on Al@Al2O3 before and after
reduction. Note that the XRD peaks due to Al (JCPDS 04-0787) become
weaker for Ni/Al@Al2O3 prepared by the DP method. This aspect is
closely related to the changes in the textural properties of Ni/Al@Al2O3
(DP) compared with Ni/Al@Al2O3 (WI). Using NH4OH solution in the
DP process caused the partial transformation of the aluminum in the
core into α-Al2(OOH)2 (Fig. 2B) resulting in the decrease in the in-
tensity of the XRD peak due to Al metal. Therefore, the fractions of core
Al metal in the Ni/Al@Al2O3 (DP) were smaller than that in the Ni/
Al@Al2O3 (WI), as listed in Table S1. The strong XRD peaks due to NiO
(JCPDS 47–1049) were observed for unreduced supported Ni catalysts
H2-TPR was carried out to probe the reducibility of NiO in each
sample before reduction. Fig. 3 shows that the sample prepared using
the WI method show TPR peaks at higher temperatures than the sam-
ples prepared using the DP method. It is generally accepted that the
TPR peaks at low and high temperatures are due to NiO interacting
weakly and strongly with the support, respectively [17,18]. Compared
with the TPR profile for Ni/γ-Al2O3 (WI), in which only broad TPR peak
centered at 680 ℃ was observed [17], the TPR peak shifted to the lower
temperature for Ni/Al@Al2O3 (WI). This indicates that the Ni/
Al@Al2O3 (WI) has weaker interactions between NiO and the support
than the Ni/γ-Al2O3 (WI). Ni/Al@Al2O3 (DP110) was reduced at much
lower temperatures than Ni/Al@Al2O3 (DP500), which implies that the
interaction between NiO and the support was strengthened in the cal-
cination step at 500 ℃. The reduction degree of NiO for each supported
Ni catalyst was estimated and reported in Table 1. It increased from
45% for Ni/Al@Al2O3 (DP500) to 81% for Ni/Al@Al2O3 (DP110),
confirming the higher reducibility of the Ni/Al@Al2O3 (DP110) cata-
lyst. As a result, a larger amount of NiO must be reduced for Ni/
Al@Al2O3 (DP110) than Ni/Al@Al2O3 (DP500) when both samples
were reduced at 500 ℃. In the case of the samples supported on
Al@Al2O3, a small negative TPR peak is observed at approximately
600 °C, which can be attributed to the physical melting of the Al core.
Based on the H2-TPR data, Al@Al2O3 appears to interact more weakly
with NiO than γ-Al2O3 and the DP method outperforms the WI method
with regard to increasing the reduction degree and fraction of metallic
Ni at the same reduction temperature.
In order to assess the surface property of the prepared catalyst, CO2
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chemisorption and CO2-TPD were carried out to measure the number of
basic surface sites and the strengths of the basic sites, respectively. As
listed in Table 1, the amounts of chemisorbed CO2 for Ni/γ-Al2O3 (WI),
Ni/Al@Al2O3 (WI), Ni/Al@Al2O3 (DP110), and Ni/Al@Al2O3 (DP500)
are 28, 31, 42, and 39 μmol/g, respectively. This result implies that the
DP method provides a larger number of basic sites than the WI method.
Fig. 4 reveals that similar CO2-TPD patterns were obtained for Ni/γ-
Al2O3 (WI) and Ni/Al@Al2O3 (WI), wherein a desorption peak at ap-
proximately 100 °C, assigned to CO2 desorption from weak basic sites,
was observed [11,15]. The only difference between these patterns is the
latter shows a TPD peak maximum at a higher temperature than the
former, indicating that the basic sites of Ni/Al@Al2O3 (WI) can interact
with CO2 more strongly than Ni/γ-Al2O3 (WI). Interestingly, the Ni/
Al@Al2O3 (DP) samples show additional CO2-TPD peaks in the tem-
perature range of 200–350 ℃ compared with the samples prepared by
the WI method. This result implies that the DP method provides the
additional moderate basic sites, which cannot be formed by the WI
method. These moderate basic sites have been reported to be favorable
for the activation of CO2 [12,13,25,37]. The comparison between Ni/
Al@Al2O3 (DP110) and Ni/Al@Al2O3 (DP500) reveals that there is the
slight shift of moderate basic sites to a higher temperature for the
former catalyst, demonstrating the stronger binding of CO2 on the
former catalyst compared with the latter one. Additionally, very weak
CO2-TPD peaks were also observed in the temperature range of 550 to
750 ℃. However, these strong basic sites might have a limited effect on
the catalytic activity for CO2 methanation because of their weak peak
intensity. Each band corresponding to a different basic site over each
catalyst was deconvoluted and quantified in Table S2. The fraction of
strong basic site (peak δ) for all samples was below 0.05. The total area
for each catalyst from the CO2-TPD profile in Table S2 is consistent with
the CO2 uptake measured at room temperature in Table 1.
Fig. 5 illustrates the TEM images of the Al@Al2O3-supported Ni
catalysts. As revealed in the particle size distribution of Ni metal (Fig.
S1), a rather uniform particle size distribution of Ni metal can be found
for Ni/Al@Al2O3 (DP) catalysts compared with Ni/Al@Al2O3 (WI). The
average particle sizes of Ni metal for Ni/Al@Al2O3 (DP110) and Ni/
Al@Al2O3 (DP500) are determined to be 2.8 and 3.7 nm, respectively.
Conversely, the slight agglomeration of Ni particles is observed for Ni/
Al@Al2O3 (WI) catalyst, with the mean particle size of Ni being ap-
proximately 5.0 nm. The measurements are consistent with the H2
chemisorption data, which shows that Ni dispersion decreases with
increasing average Ni particle size. The average particle size of Ni in-
creases in the following order: Ni/Al@Al2O3 (DP110) < Ni/Al@Al2O3
(DP500) < Ni/Al@Al2O3 (WI). Moreover, the typical STEM dark field
image and corresponding elemental maps confirm that the Ni, Si, and O
elements are well distributed in each sample (Fig. 5).
3.2. CO methanation
The catalytic activity for CO methanation over the supported Ni
catalysts was evaluated, and the CO conversions as a function of the
reaction temperature are displayed in Fig. S2A. The catalytic activity
increased in the following order: Ni/γ-Al2O3 (WI) < Ni/Al@Al2O3
(WI) < Ni/Al@Al2O3 (DP500) < Ni/Al@Al2O3 (DP110). Note that
Al@Al2O3 outperformed γ-Al2O3 and that the DP method is better than
the WI method for achieving high catalytic activity for CO methanation.
This catalytic activity is directly related to Ni dispersion and CASA. In
terms of product selectivity, CH4 is a predominant product at all reac-
tion temperatures, while C2H6 and C3H8 are also detected as byproducts
(Fig. S3). The Arrhenius plots for the CH4 formation rate under CO
methanation over all the catalysts were determined and are presented
in Fig. 6A. The Ni/Al@Al2O3 (DP110) catalyst shows the highest CH4
formation rate and has the lowest apparent activation energy (95 kJ/
mol) among the tested catalysts. The activation energies were de-
termined to be 110, 103, and 96 kJ/mol for Ni/γ-Al2O3 (WI), Ni/
Al@Al2O3 (WI), and Ni/Al@Al2O3 (DP500), respectively. The activity
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Fig. 2. X-ray diffraction patterns of Ni/Al@Al2O3 (WI) (A), Ni/Al@Al2O3 (DP110) (B), and Ni/Al@Al2O3 (DP500) (C) before (a) and after reduction at 500 ℃ (b). (●)
Al (JCPDS 04-0787), (○) γ–Al2O3 (JCPDS 29-0063), (◼) NiO (JCPDS 47–1049), (▼) Ni (JCPDS 04-0850), and (□) α-Al2(OOH)2 (JCPDS 05-0190).
3.3. CO2 methanation
The catalytic activity for CO2 methanation was measured over the
supported Ni catalysts, and the CO2 conversions at different reaction
temperatures are presented in Fig. S2B. Similar to the results for CO
methanation, Ni/Al@Al2O3 (DP110) shows the highest catalytic ac-
tivity for CO2 methanation among the tested catalysts. This catalytic
activity is related to the CASA, CO2 uptake capacity, and binding
strength of CO2 onto the catalyst. The CASA is critical to supply a high
concentration of surface H for the hydrogenation of intermediate spe-
cies in the rate-determining step [16,17]. The high CO2 uptake capacity
and strong interactions between CO2 and the catalyst surface are also
essential for the high catalytic activity needed for CO2 methanation
[9,12,15,37]. The Arrhenius plots for the CH4 formation rate under CO2
methanation over the supported Ni catalysts are displayed in Fig. 6B.
Ni/Al@Al2O3 (DP110) has the highest CH4 formation rate and the
lowest apparent activation energy (74 kJ/mol) among the tested cata-
Fig. 3. H2-TPR profiles of Ni/Al@Al2O3 (WI) (a), Ni/Al@Al2O3 (DP110) (b),
lysts. The apparent activation energies of Ni/γ-Al2O3 (WI), Ni/
and Ni/Al@Al2O3 (DP500) (c). The deconvoluted peaks are plotted in a dashed
Al@Al2O3 (WI), and Ni/Al@Al2O3 (DP500) were determined to be 99,
line.
79, and 77 kJ/mol, respectively. The results are comparable to previous
reports (Table S4) in which the activation energies for CO2 methanation
were found to be 80 kJ/mol and 89 kJ/mol over 10 wt.% Ni/Al2O3 [38]
and 10 wt.% Ni/SiO2 [18], respectively. During CO2 methanation, only
CH4 was formed over all the catalysts (Fig. S4). The activity comparison
for CO2 methanation between the Ni/Al@Al2O3 catalysts and other
supported Ni catalysts reported previously reveals that the Ni/
Al@Al2O3 catalysts are superior to their counterparts in terms of the
CH4 formation rate under similar reaction conditions (Table S4).
3.4. CO and CO2 methanation mechanism
In order to probe the surface species during CO methanation over
the best Ni/Al@Al2O3 (DP110), in-situ DRIFTS spectra were obtained
by increasing temperature from 100 to 300 ℃ as shown in Fig. 7. The
recorded spectra at 2180 cm−1 is ascribed to Al3+−CO originated from
CO adsorption on Al2O3 at low temperatures [39]. Its intensity de-
creased with increasing temperature above 200 °C. Besides, the band
which appears at 2120 cm−1 suggests the linear C]O vibration of
Fig. 4. CO2-TPD profiles of Ni/γ-Al2O3 (WI) (a), Ni/Al@Al2O3 (WI) (b), Ni/
Ni−CO species, which rapidly vanished to form the subcarbonyl Ni
species at 2060 cm−1 [40] while increasing the reaction temperature.
These subcarbonyl Ni species are present alongside the linear carbonyls
(2023–2043 cm−1) [41], which were consumed to form CH4 at 1304
and 3016 cm−1 as the reaction temperature increased. The linear CO
Al@Al2O3 (DP110) (c), and Ni/Al@Al2O3 (DP500) (d). The deconvoluted peaks
are plotted in a dashed line.
comparison for CO methanation between the Ni/Al@Al2O3 catalysts
and other supported Ni catalysts reported previously reveals that the
Ni/Al@Al2O3 catalysts are superior to their counterparts in terms of the
CH4 formation rate under similar reaction conditions (Table S3). All the
catalysts are reported to have activation energies ranging from 69 to
132 kJ/mol (Table S3).
species are more active than the bridged CO species (1845 - 1930 cm−1
)
toward CO hydrogenation. The CeH bending mode at 1392 cm−1 was
detected from 150 °C, whereas the CeH stretching modes are somewhat
covered by the rotational fine structure of CH4 [40]. Moreover, the
adsorbed species at 1590 and 2904 cm−1 matched formate species,
which were formed due to H-assisted CeO bond breaking and the
partial dehydroxylation on Al2O3 surface at low temperatures [40]. The
5

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Fig. 5. HRTEM images, and elemental mapping of (A) Ni/Al@Al2O3 (WI), (B) Ni/Al@Al2O3 (DP110), and (C) Ni/Al@Al2O3 (DP500).
formate species were then further hydrogenated to CH4.
basic sites to weak basic sites, which is consistent with the CO2-TPD
profiles described in Fig. 4 and percentage of peak-fitting data observed
in Table S2. The Ni catalysts supported on Al@Al2O3 are confirmed to
possess higher densities of medium and strong basic sites than Ni/γ-
Al2O3 (WI), which is beneficial for the high catalytic activity required
for CO2 methanation [9,12,15,25,37].
Fig. 9 shows the various surface intermediates formed during CO2
methanation in the temperature range of 150–400 ℃. New bands ap-
pear at 3016, 2904, 1595, 1376, and 1304 cm−1 while the bands cor-
responding to carbonates at 1650, 1530, 1440, and 1230 cm−1 are at-
tenuated at low reaction temperatures. The peaks at 3016 and
1304 cm−1 are assigned to CH4 [9,25,46,47]. The peaks at 2904, 1595,
and 1376 cm−1 correspond to bidentate formate species and the band
at 1340 cm−1 is assigned to monodentate formate species
[9,12,13,18,23,25,46]. With increasing reaction temperature, the bands
of formate species reached a maximum at 300 °C with the continuous
consumption of carbonates, suggesting the transformation of
In-situ DRIFTS experiments were performed to study the evolution
of surface species during CO2 methanation over the Ni/γ-Al2O3 (WI),
Ni/Al@Al2O3 (WI), Ni/Al@Al2O3 (DP500), and Ni/Al@Al2O3 (DP110)
catalysts. Fig. 8 shows the infrared spectra recorded after the adsorption
of CO2 for 20 min at 40 ℃ and 300 ℃ to differentiate basic surface sites.
Three different basic sites on the catalyst can be distinguished: bi-
carbonate (at ν = 1650, 1450, and 1230 cm−1), monodentate carbo-
nate (ν = 1390 and 1530 cm−1), and bidentate carbonate (ν = 1590
and 1320 cm−1) [12,37,42–45]. Each band corresponding to a different
basic site was deconvoluted (Fig. S5) and quantified as seen in Table 2.
The fraction of moderate basic sites forming bidentate carbonate and
strong basic sites responsible for monodentate carbonate [12,22,37,42]
seems to be slightly higher for the Ni/Al@Al2O3 (DP) catalysts. On the
other hand, the fraction of weak basic sites forming bicarbonate seems
to be high for Ni/γ-Al2O3 (WI) and Ni/Al@Al2O3 (WI). Ni/Al@Al2O3
(DP110) appears to have the highest ratio of total medium and strong
Fig. 6. CH4 formation rate of Ni/γ-Al2O3 (WI) (a), Ni/Al@Al2O3 (WI) (b), Ni/Al@Al2O3 (DP110) (c), and Ni/Al@Al2O3 (DP500) (d) on: (A) CO methanation and (B)
CO2 methanation. All the catalysts were reduced in H2 at 500 ℃. Reaction conditions: 1 mol% CO/CO2, 50 mol% H2, 49 mol% He, F/W =1000 mL/min/gcat.
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Table 2
Basic site distribution of each Ni-based catalyst determined by deconvolution of
DRIFTS spectra after CO2 adsorption at 300 ℃.
Catalyst
Percent of each basic site in wavenumber range
1130–1910 cm−1 (%)
Weak basic
sites
Moderate basic sites
Strong basic
sites
Ni/γ-Al2O3 (WI)
55.5
46.3
27.4
42.2
11.6
11.8
36.6
12.0
32.9
41.9
36.0
45.8
Ni/Al@Al2O3 (WI)
Ni/Al@Al2O3 (DP110)
Ni/Al@Al2O3 (DP500)
first 10 h of operation but showed steady-state values for the remaining
time. The change in the CO or CO2 conversions during the stability test
was less than 3%. The results of the XRD analysis confirm that the bulk
crystalline structure is maintained (Fig. S6). No XRD peak is detected
for crystalline carbon (graphite or whisker carbon) and NiO. There is no
noticeable change in the relative peak intensity attributable to Al and γ-
Al2O3 for the catalyst after the stability test, indicating that the core–-
shell structure is maintained. No apparent morphological change is
observed via TEM analysis after the stability test (Fig. S7). However, the
particle size of Ni appears to have increased slightly from 2.8 to 3.8 nm
after the stability test.
TGA and DTA analyses of the fresh and spent catalysts were also
performed to monitor the weight loss in the sample as a function of
temperature. The TGA curves show a weight-loss up to 8% from room
temperature to 600 ℃ (Fig. S8). The small difference (of up to 1%) in
the TGA profiles between the fresh and spent samples, and the over-
lapping of the DTG curves indicate negligible coke deposition.
Moreover, TPO profiles confirm slight coke deposition in the spent
catalysts (Fig. S9). The amount of CO2 produced during the TPO ex-
periments appears to be larger after CO methanation than that after CO2
methanation. In conclusion, the Ni/Al@Al2O3 (DP) catalyst calcined at
low temperatures and subsequently reduced, exhibiting high Ni dis-
persion, large CASA, and stable catalytic activity.
Fig. 7. In situ DRIFTS during CO methanation over Ni/Al@Al2O3 (DP110) from
100 to 300 ℃. The catalyst was contacted with the feed gas composed of 1 mol
% CO, 50 mol% H2, and 49 mol% He with a total flow of 50 mL/min.
carbonates into formate species in the presence of H2. The monodentate
formate species are reported to react more quickly with H2 than the
bidentate formate species [13,18]. The peaks of the formate species
decrease with the continuous formation of surface CH4 species.
Therefore, it can be said that the possible complete reaction route of the
CO2 methanation over the Ni/Al@Al2O3 (DP110) catalyst is as follows.
CO2 is first chemisorbed onto the catalyst surface to form carbonates.
Then, it is hydrogenated into formate, and finally, it transforms into
CH4. This is consistent with the findings of past studies [9,13,18,37].
There are two main different opinions on the CH4 formation process
and the nature of the intermediate involved in CO2 methanation. The
first is that the process might involve the conversion of CO2 to CO
before being hydrogenated to CH4 [47–49]. The second posits the direct
hydrogenation of CO2 to CH4 without forming a CO intermediate
[9,18,37,46,49]. In this study, no CO was observed under the given
reaction conditions, which suggests that the catalytic mechanism might
not involve the CO intermediate.
4. Conclusions
3.5. Stability evaluation
The outstanding catalytic performance of the Ni catalysts supported
on core–shell Al@Al2O3 with regard to both CO and CO2 methanation
owing to the high dispersion of Ni and high reduction degree of NiO
nanoparticles offers insights into a new catalyst design strategy.
Deposition–precipitation with mild thermal pretreatment is confirmed
The most active catalyst, Ni/Al@Al2O3 (DP110), was selected in
order to evaluate the catalyst stability during CO and CO2 methanation.
As shown in Fig. 10, CO and CO2 conversions decreased slowly in the
Fig. 8. In situ DRIFTS after CO2 adsorption at 40 ℃ (A) and 300 ℃ (B) on (a) Ni/γ-Al2O3 (WI), (b) Ni/Al@Al2O3 (WI), (c) Ni/Al@Al2O3 (DP110), and (d) Ni/
Al@Al2O3 (DP500).
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Fig. 9. In situ DRIFTS during CO2 methanation over Ni/Al@Al2O3 (DP110) from 150 to 400 ℃. 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.
online version, at doi:https://doi.org/10.1016/j.cattod.2019.09.028.
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