Cooperative Catalysis of Nickel and Nickel Oxide for Efficient Reduction of CO2 to CH4

Article abstract of DOI:10.1002/cctc.201801896

The use of CO₂ to produce value-added energy chemicals is a promising means for renewable CO₂ transformation in low-carbon energy system, and CO₂ methanation has attracted ever-increasing interest. Herein, we report that t

Full text of DOI:10.1002/cctc.201801896

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Accepted Article
Title: Cooperative Catalysis of Nickel and Nickel Oxide for Efficient
Reduction of CO2 to CH4
Authors: Qingyuan Bi, Xieyi Huang, Guoheng Yin, Tianyuan Chen,
Xianlong Du, Jun Cai, Jing Xu, Zhi Liu, Yifan Han, and Fu-
Qiang Huang
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Cooperative Catalysis of Nickel and Nickel Oxide for Efficient
Reduction of CO2 to CH4
Qingyuan Bi,[a] Xieyi Huang,[a] Guoheng Yin,[a] Tianyuan Chen,[b] Xianlong Du,[c] Jun Cai,[d,e] Jing Xu,[b]
Zhi Liu,*[d,e] Yifan Han,*[b] and Fuqiang Huang*[a,e,f]
Abstract: The use of CO2 to produce value-added energy chemicals
is a promising means for renewable CO2 transformation in low-
carbon energy system, and CO2 methanation has attracted ever-
increasing interest. Herein, we report that the Ni/MCM-41 catalyst
with prominent cooperative effect of Ni and NiO is efficient for CH4
generation with CO2 conversion of 73.2% and CH4 selectivity of
91.6% at 400 oC and high gas hourly space velocity (GHSV) of
90000 mL gcat h1. Combined methodologies of in situ X-ray
diffraction, diffuse reflectance infrared Fourier transform spectrosc-
opy, and ambient-pressure X-ray photoelectron spectroscopy reflect
the structural evolvements of Ni/MCM-41 catalyst, the presence of
carbonyl intermediates, the co-existence of metallic and oxidized Ni
species with sufficient molar ratio of Ni0/Ni2+ under working
conditions. H2 and CO2 molecules are preferentially adsorbed and
chemically activated over Ni0 and Ni2+ species, respectively. The
possible four-step reaction mechanism involved carbonyl pathway
and the cleavage of C=O bond from CO2 as the rate-determining
step over the engineered Ni/MCM-41 catalyst was demonstrated.
transformation of CO2 is also an efficient strategy to alleviate the
serious problems of global greenhouse effect and ocean
acidification, and has attracted growing attention.[813] However,
the major challenge is the high thermodynamic stability of CO2
molecules and the activation energy barriers of the reaction
involved CO2 are always very high. Hydrogenation of CO2
toward methane (CO2 + 4H2 → CH4 + 2H2O), the well-known
Sabatier reaction or methanation reaction, has been extensively
studied using many kinds of active supported Ru, Pd, Fe, Co,
and Ni catalysts.[1418] Amongst them, Ni-based materials are the
most commonly investigated for industrial purposes because of
their high catalytic performance and low cost.[1720]
In spite of the great research efforts in the field of CO2
hydrogenation for methane formation,[1420] the understanding of
the active sites and surface chemical state of real catalyst on
practical reaction conditions as well as the potential key
intermediates for CO2 methanation is still limited. Many in situ
techniques can be employed to probe these significant and air-
sensitive species. In situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) is usually used to detect the
adsorption and activation of CO2 molecules and track the interm-
ediate species which exist in the steady-state or metastable
state. Based on the analysis of operando Raman and DRIFT
spectra, Duan and co-workers have demonstrated that Ru/CeO2
with more oxygen vacancy showed better performance than
Ru/α-Al2O3 for CO2 reduction.[21] The CO2 methanation process
underwent the formate intermediates and the post-pathway of
formate dissociation to methanol over the reducible CeO2
supported Ru nanoparticles, while the CO route was proved on
the surface of Ru nanoparticles anchoring on the inert α-Al2O3.[21]
Furthermore, the ambient-pressure X-ray photoelectron spectro-
scopy (AP−XPS) is recently to identify the surface chemical
state of solid catalyst and the nature of the adsorbed species in
CO2 conversion.[2224] With the AP−XPS methodology, Salmeron
and colleagues found that NiO was formed via CO2 dissociation
into CO and atomic oxygen in CO2 atmosphere over Ni(111)
surface, and the later addition of H2 gas can lead to the
reduction of NiO species.[25] The initial generation of carbonate
(CO32) can be destroyed by H2, and the evolved intermediate
CO via the reverse water-gas shift (RWGS, CO2 + H2 → CO +
H2O) can be hydrogenated to the final product of CH4.[25]
1

Introduction
Utilization of CO2 as an abundant, non-toxic, renewable, and
environmentally friendly C1 building block for synthesizing value-
added chemicals or low-carbon fuels is promising as a comple-
mentary approach to fossil-derived resources.[17] The chemical
[a]
Dr. Q. Bi, X. Huang, Dr. G. Yin, Prof. Dr. F. Huang
State Key Laboratory of High Performance Ceramics and Superfine
Microstructures, Shanghai Institute of Ceramics, Chinese Academy
of Sciences, Shanghai 200050, China
E-mail: huangfq@mail.sic.ac.cn
[b]
[c]
[d]
T. Chen, Prof. Dr. J. Xu, Prof. Dr. Y. Han
State Key Laboratory of Chemical Engineering, East China
University of Science and Technology, Shanghai 200237, China
E-mail: yifanhan@ecust.edu.cn
Dr. X. Du
Key Laboratory of Interfacial Physics and Technology, Shanghai
Institute of Applied Physics, Chinese Academy of Sciences,
Shanghai 201800, China
J. Cai, Prof. Dr. Z. Liu
State Key Laboratory of Functional Materials for Informatics,
Shanghai Institute of Microsystem and Information Technology,
Chinese Academy of Sciences, Shanghai 200050, China
E-mail: zliu2@mail.sim.ac.cn
Because of the air-sensitivity, Ni nanoparticles with small size
are difficult to exist in oxygen environment. The general data
from conventional characterizations are hard to accurately
reflect the structural evolvements of Ni-based catalysts under
practical reaction conditions and the lack of in situ results may
give us some controversial understandings for the structure-
activity/selectivity relationships. In situ X-ray diffraction (XRD)
investigation can collect the real information on the structure
change and phase transformation.[26] Therefore, the in situ tech-
[e]
[f]
J. Cai, Prof. Dr. Z. Liu, Prof. Dr. F. Huang
School of Physical Science and Technology, ShanghaiTech
University, Shanghai 201203, China
Prof. Dr. F. Huang
State Key Laboratory of Rare Earth Materials Chemistry and
Applications, College of Chemistry and Molecular Engineering,
Peking University, Beijing 100871, China
Supporting information for this article is given via a link at the end of
the document.
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niques, including DRIFT, AP−XPS, XRD, and the combination of
them, are very appropriate means to study the active sites,
surface chemical states, and structure evolvements of Ni-based
catalysts under indicated reaction conditions. These are very
important to obtain an in-depth insight into the CO2 methanation
reaction mechanism and to rationally design the synthesis of
large-scale catalyst for industrial applications.
(a)
(c)
(b)
(d)
Herein, we present Ni/MCM-41 catalysts with different
precipitation agents during preparation process as the target and
control samples for CO2 methanation and try to give a clear
understanding on cooperative catalysis of Ni and NiO. MCM-41
zeolite is a mesoporous crystalline material with unique pore
structure nature and high specific surface area (~800 m2 g1)
showing advantages of the dispersion and anchoring of active
sites (e.g., Ni-based species) as well as the convenient transpor-
tation and diffusion of small molecules. Also, MCM-41 owns
great application potential in industry because of its affordable
price. Combined methodologies of in situ XRD, DRIFTS, and
AP−XPS were used to study the synergistic effect of NiNiO
components, active sites, surface chemical states, and potential
key intermediates of Ni/MCM-41 catalyst under practical reaction
conditions. In situ XRD analysis shows the structural evolvem-
ents of Ni-based catalysts in the mixed CO2/H2 environment on
the elevated temperatures. AP−XPS and in situ DRIFTS data
reveals the co-existence of Ni0 and Ni2+ species under working
conditions and the presence of carbonyl (C=O) intermediates in
CO2 reduction. The cooperative catalysis of Ni and NiO can
greatly enhance the efficiency of CO2 transformation. Furtherm-
ore, the mechanism of CO2 methanation and the thermal stability
of the engineered Ni/MCM-41 sample were demonstrated. With
the rapid advance of in situ techniques, the observation of
intrinsic active sites and corresponding reaction mechanism can
be achieved and thus provide an effective avenue to design
high-performance catalysts.
Figure 1. HRTEM images of (a) Ni/MCM-41-NaOH, (b) Ni/MCM-41-(NH4)2CO3,
(c) Ni/MCM-41-NH3·H 2O, and (d) NaOH-treated Ni/MCM-41-NH3·H 2O.
ned after nickel loading.
High-resolution transmission electron microscopy (HRTEM)
images depicted in Figure 1 show that the typical morphology
and mesoporous structure are observed and the NiO nanopartic-
les are uniformly dispersed on the MCM-41 support with
comparable particle size of ca. 6 nm. These indicate the regular
mesopores are still present even at high Ni loadings, which are
well in line with the N2 adsorption-desorption data (Figure S2
and Table S1). XPS results demonstrate that there is no Cl resi-
dual (Figures S3 and S4) and only Ni2+ species on the catalyst
surface (Figure S5), which are consistent with the XRD patterns.
For the as-prepared samples, several Fourier transform infrared
(FTIR) bands are observed between 400 and 4000 cm1, as
Results and Discussion
Ni/MCM-41 samples were prepared via deposition-precipitation
(DP) route with different alkaline precipitation agents, such as
NaOH, (NH4)2CO3, and NH3·H 2O. For comparison, the sample
made by NH3·H 2O-DP was further treated with NaOH solution.
The powder XRD patterns (Figure S1) clearly show the well-
defined crystal of dominant NiO phase but without any metallic
Ni species supporting on the aluminum silicate substrate. There
is no visible structural difference among these four samples. The
characteristic type-IV isotherms with a clear hysteresis loop for
the as-prepared catalysts are observed (Figure S2 and Table
S1).[27] The Ni loading is ca. 19 ± 0.5 wt% according to the
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) analysis. Note that this relatively high content of Ni is
proved to be an optimal value for CO2 methanation[17,18] and can
enhance the actual signal during in situ processes. The specific
surface area is ranging from 528 m2 g1 for Ni/MCM-41-NaOH to
742 m2 g1 for Ni/MCM-41-(NH4)2CO3 (Table S1), which are in
good agreement with the previously reported results.[28] The data
of pore diameter and volume suggest that the typical mesopor-
ous structure feature of intrinsic MCM-41 support is still maintai-
Ni/MCM-41-NaOH
Ni/MCM-41-(NH4)2CO3
Ni/MCM-41-NH3H2O
NaOH-treated Ni/MCM-41-NH3H2O
4000 3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
500
Figure 2. FTIR spectra of Ni/MCM-41 catalysts.
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indicative of the better reducible feasibility of Ni/MCM-41-
NH3·H 2O with finely dispersed nickel oxide species. Control
experiment of NaOH-treated Ni/MCM-41-NH3·H 2O with 2.5 wt%
NaOH solution (Figure 3a) further confirms the negative effect of
Na on reducibility of NiO at relatively low temperature (< 400 oC).
Moreover, the results of temperature-programmed desorption of
CO2 (CO2TPD) in Figure 3b show the different behaviors of
CO2 adsorption and chemical activation over these Ni-based
samples. The total amount of adsorbed CO2 on Ni/MCM-41-
(NH4)2CO3 and Ni/MCM-41-NH3·H 2O is superior to that of
Ni/MCM-41-NaOH and NaOH-treated Ni/MCM-41-NH3·H 2O
(Table S2). The precipitation agent with relatively weak basicity
used in the preparation process is favorable for the dispersion of
nickel oxide species and thus the CO2 adsorption. The
properties of CO2 adsorption and chemical activation are crucial
for CO2 transformation.
Ni/MCM-41-NaOH
(a)
608
Ni/MCM-41-(NH4)2CO3
Ni/MCM-41-NH3H2O
NaOH-treated
660
Ni/MCM-41-NH3H2O
387
100 200 300 400 500 600 700 800
Temperature (oC)
Upon establishing that the Ni/MCM-41 catalysts gave conside-
rable surface properties and CO2 adsorption, we sought to
explore the potential applications for CO2 transformation. The
catalytic activities for CO2 methanation were conducted in a
homemade fixed-bed reactor at 200400 oC and at a gas hourly
Ni/MCM-41-NaOH
(b)
Ni/MCM-41-(NH4)2CO3
Ni/MCM-41-NH3H2O
1
space velocity (GHSV) of 90000 mL gcat h1 with the initial
722
NaOH-treated
558
Ni/MCM-41-NH3H2O
H2/CO2 molar ratio of 4/1. As depicted in Figures 4 and S7,
elevated temperature can improve the catalytic activity and
Ni/MCM-41-NH3·H 2O shows the best performance for CO2
methanation with the conversion of 73.2% and the CH4
131
315
o
selectivity of 91.6% at 400 C. Ni/MCM-41-(NH4)2CO3 displays a
698
moderate activity. By contrast, Ni/MCM-41-NaOH exhibits the
lowest activity with CO2 conversion of only 46.5% (CH4
selectivity of 64.8%) probably due to the absence of sufficient
metallic Ni species and the lack of prominent synergy between
Ni and NiO on the catalyst surface. Note that the Ni/MCM-41-
100 200 300 400 500 600 700 800
o
Temperature (oC)
NH3·H 2O catalyst pretreated by H2 at 650 C displays very low
activity (less than 5% of CO2 conversion), indicative of the indis-
pensable role of NiO in CO2 methanation. NiO, as an alkaline
oxide can enhance the adsorption and chemical activation of the
acidic CO2 molecules during the target reaction. Meanwhile, the
Figure 3. (a) H2TPR and (b) CO2TPD profiles of Ni/MCM-41 catalysts.
shown in Figure 2. These are the O–Si–O bending mode at 468
cm1, the symmetric υSi–O band at 798 cm1 and the asymmetric
one at 1086 cm1.[29] The δOH vibration of MCM-41 supported
nickel oxide would be situated at 677 cm1.[30] The broad band
centered at 3449 cm1 can be assigned to water molecules
adsorption on hydrogen-bonded SiOH groups.[28] Adsorbed
water is responsible for the characteristic bending vibration at
1635 cm1. A weak absorption at 3624 cm1 shows an additional
indication for a silicate with a two tetrahedral SiO4 sheets.[30]
Surface physicochemical properties of Ni/MCM-41 catalysts
were further investigated. H2 temperature-programmed reduction
(H2TPR) profiles depicted in Figure 3a show that there are two
reduction peaks at low and high temperatures ascribed to the
weak and strong interaction of NiO and MCM-41, respectively.[28]
However, Ni/MCM-41-NaOH gives only a high-temperature (608
oC) reduction peak probably due to the strong interaction of NiO
and aluminum silicate support. The presence of Na element as
oxygen-fixing media in Ni/MCM-41-NaOH can weaken the
reducibility of NiO (Figure S6).[31,32] Notably, there is an obvious
reduction peak at 387 oC for Ni/MCM-41-(NH4)2CO3 and Ni/MCM
-41-NH3·H 2O catalysts, and the latter shows stronger intensity,
100
80
73.2%
69.6%
60
48.1%
46.5%
40
20
0
Ni/MCM-41-NaOH
Ni/MCM-41-NH3H2O
NaOH-treated
Ni/MCM-41-NH3H2O
Ni/MCM-41-(NH4)2CO3
Figure 4. Catalytic hydrogenation of CO2 over Ni/MCM-41 catalysts. Reaction
conditions: 50 mg catalyst, GHSV = 90000 mL gcat h1, 400 oC for 8 h.
Selectivity: CH4, CO; Conversion: CO2. The dotted line shows
the equilibrium conversion of CO2 on the indicated reaction conditions.
1

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