Synergetic effect of Cu active sites and oxygen vacancies in Cu/CeO2-ZrO2for the water-gas shift reaction

Article abstract of DOI:10.1039/d0cy02462g

A series of Cu/CeO₂-ZrO₂catalysts with different Ce/(Ce + Zr) molar ratios and Cu loadings were prepared by a simple citrate sol-gel method, characterized by various techniques, and evaluated for water-gas shift (WGS) under H₂-rich and low H₂O/CO molar ratio conditions of the sorption enhanced WGS process. It was shown that the oxygen vacancy concentration and the Cu dispersion were important structural factors affecting the catalytic activity of Cu/CeO₂-ZrO₂, but neither of them alone was capable of correlating with the turnover frequency (TOF) for WGS based on kinetic analysis. By contrast, a positive linear correlation was established between the TOF and the ratio of oxygen vacancy concentration to Cu dispersion, demonstrating the synergetic effect of Cu active sites and oxygen vacancies for WGS. The stability of Cu/CeO₂-ZrO₂was found to be dependent on the crystal structure of CeO₂-ZrO₂solid solution, and the cubic to tetragonal phase transformation of CeO₂-ZrO₂led to a decreased stability. Among all Cu/CeO₂-ZrO₂investigated, the catalyst with a Ce/(Ce + Zr) molar ratio of 0.6 and a Cu loading of 50 wt% showed the highest activity and stability under 20% H₂, 40% CO and 40% steam in a gas hourly space velocity of 20?000 h^?1at 350 °C, with the CO conversion slightly varying from 77% to 75% over 60 h on stream and meanwhile 100% selectivities to H₂and CO₂

Full text of DOI:10.1039/d0cy02462g

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Synergetic effect of Cu active sites and oxygen
vacancies in Cu/CeO2–ZrO2 for the water–gas shift
reaction†
Cite this: Catal. Sci. Technol., 2021,
11, 2518
*
Yuanwu Hu, Na Wang and Zhiming Zhou
A series of Cu/CeO2–ZrO2 catalysts with different Ce/(Ce + Zr) molar ratios and Cu loadings were prepared
by a simple citrate sol–gel method, characterized by various techniques, and evaluated for water–gas shift
(WGS) under H2-rich and low H2O/CO molar ratio conditions of the sorption enhanced WGS process. It
was shown that the oxygen vacancy concentration and the Cu dispersion were important structural factors
affecting the catalytic activity of Cu/CeO2–ZrO2, but neither of them alone was capable of correlating with
the turnover frequency (TOF) for WGS based on kinetic analysis. By contrast, a positive linear correlation
was established between the TOF and the ratio of oxygen vacancy concentration to Cu dispersion,
demonstrating the synergetic effect of Cu active sites and oxygen vacancies for WGS. The stability of Cu/
CeO2–ZrO2 was found to be dependent on the crystal structure of CeO2–ZrO2 solid solution, and the cubic
to tetragonal phase transformation of CeO2–ZrO2 led to a decreased stability. Among all Cu/CeO2–ZrO2
investigated, the catalyst with a Ce/(Ce + Zr) molar ratio of 0.6 and a Cu loading of 50 wt% showed the
highest activity and stability under 20% H2, 40% CO and 40% steam in a gas hourly space velocity of
20000 h−1 at 350 °C, with the CO conversion slightly varying from 77% to 75% over 60 h on stream and
meanwhile 100% selectivities to H2 and CO2.
Received 24th December 2020,
Accepted 22nd January 2021
DOI: 10.1039/d0cy02462g
rsc.li/catalysis
temperatures (300–400 °C, the operating temperature window
1. Introduction
for MgO and hydrotalcites as CO2 sorbents) under H2-rich
As an important chemical and at the same time a clean
energy source, hydrogen is widely used in many processes.1,2
Nowadays, more than 80% of the world's hydrogen is
produced through steam methane reforming (SMR) and
water–gas shift (WGS).3 In particular, the WGS reaction plays
a significant role in an integrated gasification combined cycle
(IGCC) system that aims at converting CO and H2O into clean
hydrogen fuel for power generation. Recently, an
intensification scheme, namely sorption enhanced water–gas
shift (SEWGS), has been proposed by combining in situ CO2
removal with WGS.4,5 According to Le Chatelier's principle,
SEWGS offers advantages over conventional WGS (fulfilled in
two reactors followed by a CO2 capture unit), including
improved energy efficiency and reduced capital cost through
process simplification.6 Indeed, very recently, high-purity H2
(>99%) has been achieved using a single SEWGS reactor with
the aid of MgO-based sorbents.7,8
(10–20%) and low H2O/CO molar ratio (1–3) conditions.9–11
Currently, WGS catalysts are generally classified into high-
and low-temperature types, with Fe–Cr-based and Cu–Zn–Al-
based
mixed
metal
oxides
as
typical
catalysts,
respectively.12–18 However, there are some limitations to the
application of these catalysts in SEWGS. Fe–Cr-Based mixed
oxides are usually suitable for conditions of high H2O/CO
ratios (>4) and low R factors (R = (pCO + pH )/(pH O + pCO ) <
2
2
2
1.2), since otherwise the catalysts would deactivate due to
over-reduction of Fe3O4.19–21 For Cu–Zn–Al-based mixed
oxides, Cu nanoparticles are readily sintered at above 300
°C.22,23 Ni- and Co-based catalysts are also applied to WGS,
but they have high methanation activities and thus show
increased H2 consumption and decreased H2 yield.24,25
On the other hand, Cu–Ce-based catalysts are promising
candidates for WGS due to their oxygen storage capacity that
is derived from the Ce4+
↔
Ce3+ redox cycle.26,27
The success of the SEWGS process in IGCC depends on
the ability of WGS catalysts to operate at relatively high
Unfortunately, dopant-free Cu/CeO2 catalysts are normally
subject to severe Cu sintering at above 300 °C.28,29 After
doping with other metal oxides, the stability of catalysts can
be improved to some extent.30,31 However, when H2-
containing feedstocks are applied, a decay in activity usually
occurs, e.g., around 50% decrease in CO conversion on Cu/
Ce(La)Ox after 20 h of operation in 17% H2.30 In another
School of Chemical Engineering, East China University of Science and Technology,
Shanghai, 200237, China. E-mail: zmzhou@ecust.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy02462g
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study made by Jiang et al.,31 Cu/Ce0.59Zr0.41O2 showed good
stability in a simulated IGCC stream, but the selectivity to H2
was about 85% owing to coke formation. Therefore, current
Cu–Ce-based catalysts cannot meet the stability and
selectivity demands simultaneously when they are used for
SEWGS under H2-rich, low H2O/CO molar ratio and high
temperature conditions. Moreover, there is still a debate on
the major factors that influence the catalytic performance of
Cu–Ce-based catalysts for WGS. Wang et al. reported that the
catalytic activity was not directly correlated with the amount
of oxygen vacancies but depended largely on metallic Cu,32
while Jeong et al. claimed that oxygen vacancies were the
major factor affecting the activity.33 It would be desirable if
the major factors can be determined, which will undoubtedly
favor the research and development of high-performance Cu–
Ce-based catalysts for WGS.
The goal of this study is two-fold: (1) to identify the major
factors that affect the activity and stability of Cu/CeO2–ZrO2
catalysts for the WGS reaction; (2) to develop an efficient Cu/
CeO2–ZrO2 catalyst that can be used for SEWGS in an IGCC.
To this end, a series of Cu/CeO2–ZrO2 catalysts with different
Ce/Zr molar ratios and Cu loadings were prepared, and a
systematic exploration of the composition–structure–
performance relationship of the catalysts was performed. In
particular, a kinetic analysis was conducted over different
Cu/CeO2–ZrO2 catalysts to determine the dominant factor
affecting the specific activity for WGS.
Temperature programmed reduction (TPR) was carried out in
30% H2/N2 from room temperature to 400 °C at a heating
rate of 10 °C min−1 on a Micromeritics Autochem II 2920
apparatus. N2O chemisorption used to measure the Cu
dispersion of the catalyst was also performed on this
apparatus, with the catalyst reduced in 30% H2/N2 at 300 °C
for 2 h with a heating rate of 10 °C min−1. More details were
presented elsewhere.34 The element content of the catalyst
was acquired using inductively coupled plasma-optical
emission spectrometry (ICP-OES) on
a
Varian 710ES
spectrometer. The surface composition of the catalyst was
analyzed by X-ray photoelectron spectroscopy (XPS) on a
Thermo Scientific ESCALAB 250Xi electron spectrometer with
an Al Kα radiation source (hv = 1486.6 eV) and a pass energy
of 40 eV. The binding energy was referenced to the C 1s line
at 284.8 eV. The micromorphology of the catalyst was
analyzed by field emission scanning electron microscopy
(FESEM, Zeiss GeminiSEM 500) and high-resolution
transmission electron microscopy (HRTEM, JEOL JEM-2100)
equipped with an energy dispersive spectrometer (EDS, EDAX
Genesis XM2 system). Moreover, scanning transmission
electron microscopy (STEM) was performed on
a
ThermoFisher Talos F200X. High angle annular dark-field
(HAADF)-STEM images were recorded using a convergence
semi angle of 11 mrad, and inner and outer collection angles
of 59 and 200 mrad, respectively. EDS was carried out using 4
in-column Super-X detectors. The amount of carbon
deposition on spent catalysts was determined by
thermogravimetric analysis using a PerkinElmer TGA-4000
analyzer with heating to 900 °C at 10 °C min−1 in an air
stream. Raman analysis was made on a laser micro-Raman
spectrometer (Renishaw in Via, Renishaw, 532 nm excitation
wavelength), from which the concentration of oxygen
vacancies (N) was calculated using the spatial correlation
model from the relationship between correlation length (L)
and grain size (dg).35–38 The detailed equation is provided as
follows:
2. Experimental
2.1 Catalyst preparation
A series of Cu/CeO2–ZrO2 catalysts (labelled as xCuCeyZr1−y
)
with varying Cu loading (x, wt%) and Ce/(Ce + Zr) molar ratio
(y) were prepared by a citrate sol–gel method. Taking
20CuCe0.6Zr0.4 as an example, the preparation procedure was
as follows: first, 50 mL of citric acid and nitrate solution
(molar ratio of citric acid : Cu(NO3)2 : Ce(NH4)2(NO3)6 :
Zr(NO3)4
= 4.9 : 1.5 : 1.5 : 1) was heated to 80 °C and
51:8
ðΓ − 5Þ
maintained at this temperature for 8 h with vigorous stirring,
and then the formed sol was dried at 120 °C for 12 h; next,
the gel was ground into powder and calcined at 600 °C (with
a heating rate of 2 °C min−1) for 4 h in a muffle furnace. The
reason for 600 °C as the calcination temperature is that this
temperature can guarantee the complete removal of organic
species from the catalysts (evidenced by TG analysis and the
results are not shown here).
dgðnmÞ ¼
(1)
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢁ
2
h
i
À
Á
α
3
3
L ðnmÞ ¼
4dg2α þ dg − 2α
(2)
(3)
2dg
3
N ¼
4πL3
2.2 Catalyst characterization
where Γ is the half-width at half-maximum (HWHM) of the
Raman line (at around 460–480 cm−1) broadening and α is
the radius of CeO2 units (0.34 nm) determined from universal
constants.
The Brunauer–Emmett–Teller (BET) surface area, pore
volume and average pore diameter of the catalyst were
obtained by N2 adsorption–desorption at −196 °C on a
Micromeritics ASAP 2010 instrument. The crystalline
structure of the catalyst was acquired by X-ray diffraction
(XRD) on a Rigaku D/Max 2550 diffractometer with a Cu Kα
radiation source (λ = 0.15406 nm). The data were collected
with a 2θ range of 10–80° with a step size of 0.02°.
2.3 Catalyst test
The WGS activity and stability of the catalysts were evaluated
at 350 °C and 1 bar in a fixed-bed quartz reactor (inner
diameter of 8 mm) that was placed in a three-zone electric
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furnace. In each test, 0.2 g of catalyst with an average particle
size of 0.14 mm (100–120 mesh) was loaded into the center
of the reactor. Prior to the WGS reaction, the catalyst was
reduced at a heating rate of 10 °C min−1 from room
temperature to 300 °C in a flow of 30% H2/N2 (100 NmL
min−1), then held at 300 °C for 2 h. After reduction, the
reactor temperature was heated in N2 to 350 °C for the WGS
reaction. When the temperature was stabilized, the feedstock
composed of 20% H2, 40% CO, and 40% steam (obtained by
evaporation of H2O that was fed by a HPLC pump) was fed
and the total flow rate was fixed at 50 NmL min−1,
corresponding to a gas hourly space velocity (GHSV) of 20 000
h−1. A H2O/CO molar ratio of 1 followed the stoichiometry of
the WGS reaction, and a CO/H2 molar ratio of 2 simulated a
quenched syngas in an IGCC system.9 The dry product
leaving an ice trap (−10 °C) and a desiccant column was
analyzed on-line using a dual channel Micro-GC (INFICON
3000) with a known flow rate of N2 as an internal standard
gas. The conversion of CO (αCO) and selectivity of different
species (Si) are calculated by
Cu dispersion. The activation energy is obtained by the
Arrhenius equation
Ea
RT
lnðTOFÞ ¼ lnðAÞ −
(9)
where A and Ea are the pre-exponential factor and activation
energy, respectively. In order to avoid high conversion
conditions that may lead to mass and heat transfer
limitations, the experimental data used for calculating the
TOF values should be collected under differential reactor
conditions, i.e., at low CO conversions. To this end, the
kinetic experiments were conducted at 240–300 °C under a
higher GHSV of 80 000 h−1, which ensured that the CO
conversion data were less than 15% (Table S1†).
3. Results and discussion
3.1 Effect of the Ce/(Ce + Zr) molar ratio
Six catalysts with constant Cu loading (20 wt%) but varying
Ce/(Ce + Zr) molar ratio (y = 0, 0.2, 0.4, 0.6, 0.8 and 1) are
prepared. The actual Cu loading and Ce/(Ce + Zr) ratio for
each catalyst measured by ICP-OES are close to the nominal
values (Table 1). Fig. 1 shows the N2 adsorption–desorption
isotherms and corresponding pore size distribution (PSD)
curves of the catalysts. All 20CuCeyZr1−y catalysts exhibit type
IV isotherms with H4-type hysteresis loops according to the
IUPAC classification,39 suggesting the co-existence of both
micropores (dP < 2 nm) and mesopores (dP = 2–50 nm), as
evidenced by the PSD curves. With increasing the Ce content
in 20CuCeyZr1−y, the BET surface area and pore volume first
increase to maximum values of 31.4 m2 g−1 and 0.085 cm3 g−1
at y = 0.6 and then decrease (Table 1, entries 1–6). From the
FESEM images (Fig. 2(a)), it can be seen that interconnected
porous networks are formed in all catalysts, which are mainly
formed by release of gases due to the decomposition of
organic species during calcination.40 However, both XRD
(Table 1) and HRTEM (Fig. 2(a)) analyses indicate that the
Cu/CeO2–ZrO2 catalysts have smaller average CuO particle
sizes than Cu/CeO2 and Cu/ZrO2. Likewise, the CeyZr1−yO2
FCO;in − FCO;out
αCO
¼
× 100%
× 100%
× 100%
× 100%
(4)
(5)
(6)
(7)
FCO;in
F
− F
H2;out
H2;in
SH
¼
2
FCO;in − FCO;out
F
CO2;out
SCO
¼
¼
2
FCO;in − FCO;out
F
CH4;out
SCH
4
FCO;in − FCO;out
where Fi,in and Fi,out denote the inlet and outlet molar flow
rates of species i. The turnover frequency (TOF) for the WGS
reaction is calculated based on the Cu dispersion as follows:
wCu
TOF ¼ FCO;inαCO
(8)
mcatxCuDCu
where wCu is the atomic weight of Cu, mcat is the mass of the
catalyst, xCu is the Cu loading of the catalyst, and DCu is the
Table 1 Physicochemical properties of xCuCeyZr1−y catalysts
Actual contente
a
a
b
c
c
d
d
(wt%)
SBET
Vp
dp
dCuO
dCuO
dCe Zr
SCu
DCu
(%)
Actual
y
Catalyst
(m2 g−1
)
(cm3 g−1
)
(nm)
(nm)
(nm)
(nm) 1−y
(m2 g−1
)
Cu
Ce
Zr
ye
20CuCe
17.3
18.0
31.4
24.0
16.9
14.1
28.5
23.0
8.4
0.053
0.055
0.085
0.066
0.039
0.031
0.079
0.053
0.030
0.006
16.1
14.1
11.7
16.7
15.1
10.9
11.0
9.9
11.1
7.2
5.2
6.2
8.3
14.3
5.9
7.2
10.7
—
17.2
9.3
5.4
12.2
7.2
5.6
6.0
7.3
14.0
5.3
5.0
4.6
—
8.1
6.0
21
22
20
19
21
22
43
53
63
—
59
53
36
28
15
0
29
22
14
—
0
1
20CuCe0.8Zr0.2
20CuCe0.6Zr0.4
20CuCe0.4Zr0.6
20CuCe0.2Zr0.8
20CuZr
40CuCe0.6Zr0.4
50CuCe0.6Zr0.4
60CuCe0.6Zr0.4
CuO
16.3
25.1
22.7
8.3
12.1
18.7
16.9
6.2
1.5
10.1
7.6
7.8
19
29
42
55
13
9.4
6.0
—
0.82
0.55
0.39
0.19
0
0.59
0.60
0.60
—
8.3
11.9
24.1
7.2
9.5
16.2
43.4
2.0
27.1
25.5
12.5
—
13.6
41.6
3.1
—
0.8
a
b
c
Calculated from the adsorption branch of the N2 isotherm by the BJH method. Determined by HRTEM analysis. Calculated by the
Scherrer equation from the XRD pattern. d Measured by N2O chemisorption analysis. e Determined by ICP-OES analysis.
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Fig.
1 (a) N2 adsorption–desorption isotherms and (b) pore size
distribution curves of various 20CuCeyZr1−y catalysts.
crystallite size is also smaller than those of CeO2 and ZrO2. It
appears that the higher the surface area of 20CuCeyZr1−y, the
smaller the average CuO particle size (Table 1). To further
distinguish CuO and the support, the HAADF-STEM-EDS
analysis is performed on 20CuCe0.6Zr0.4 (Fig. 2(b)). The bright
area corresponds to the CeO2–ZrO2 solid solution, while the
dark spots are Cu rich regions. The elemental mappings
demonstrate the uniform distribution of Cu species on CeO2–
ZrO2.
The active surface area (SCu) and dispersion (DCu) of Cu
determined by N2O chemisorption are summarized in
Table 1. Like the variation of BET surface areas with Ce
content, SCu increases from 2.0 to 8.3, 22.7 and 25.1 m2 g−1
with an increase in y from 0 to 0.2, 0.4 and 0.6, respectively,
and then decreases to 16.3 and 8.1 m2 g−1 with further
increasing y to 0.8 and 1.0, respectively. Accordingly, DCu first
increases from 1.5 at y = 0 to a maximum value of 18.7 at y =
0.6 and then decreases to 6.0 at y = 1. It means that SCu and
DCu can be tuned by adjusting the Ce/Zr ratio in the CeO2–
ZrO2 mixed oxide. Similar results are reported by Jiang
et al.31 and Jeong et al.,33 with a maximum DCu at y = 0.59
and 0.8, respectively. The above analysis reveals a generally
positive correlation between the surface area of 20CuCeyZr1−y
and the active surface area of Cu, showing the promoting
effect of the catalyst surface area on Cu dispersion.
Fig. 2 (a) FESEM (left panels) and HRTEM (right panels) images of
20CuCeyZr1−y catalysts (the insets in the right panels show the CuO
particle size distribution); (b) HAADF-STEM image and HAADF-STEM-
EDS mappings of 20CuCe0.6Zr0.4
.
and ZrO2 (2θ = 30.3, 35.3, 50.7 and 60.2°, JCPDS No. 50-1089)
are present in Cu/CeO2 and Cu/ZrO2, respectively. However,
neither CeO2 nor ZrO2 is detected in 20CuCeyZr1−y (y = 0.2,
0.4, 0.6 and 0.8); new diffraction peaks are formed instead,
which shift between the peaks of CeO2 and ZrO2. Because the
ion radius of Zr4+ (0.84 Å) is smaller than that of Ce4+ (0.97
Å), these shifts towards higher 2θ values with increasing y
indicate the substitution of Ce4+ with Zr4+ in the CeO2 lattice,
which consequently leads to the formation of CeyZr1−yO2
solid solution.41–43 The crystal structure and lattice parameter
Fig. 3 shows the XRD patterns of various 20CuCeyZr1−y. As
expected, besides the diffraction peaks assigned to CuO (2θ =
32.5, 35.5 and 38.7°, JCPDS No. 48-1548), the peaks belonging
to CeO2 (2θ = 28.5, 33.1, 47.5 and 56.3°, JCPDS No. 43-1002)
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Fig. 4 H2-TPR profiles of 20CuCeyZr1−y catalysts.
Fig. 3 XRD patterns of 20CuCeyZr1−y catalysts.
and weakly bound surface CuO; the medium-temperature
peak at 165–180 °C is assigned to the reduction of CuO
interacting with the support; and the high-temperature peak
at 185–210 °C is ascribed to the reduction of Cu2+
incorporated into the CeO2 lattice and bulk-like CuO.31,45–47
As for 20CuZr without CeO2, only the first two peaks are
present. ZrO2 cannot be reduced up to 800 °C, while CeO2 is
reduced at 600–800 °C (Fig. S1†), much higher than those of
CuO in 20CuCeyZr1−y. The effect of the Ce/Zr ratio on the
catalyst reducibility can be observed by plotting the
of CeyZr1−yO2 in 20CuCeyZr1−y are listed in Table 2. The
crystal structure of CeO2 in 20CuCe is cubic with a lattice
parameter of 5.4134 Å, while the crystal structure of ZrO2 in
20CuZr is tetragonal with lattice parameters of a = 3.6097 Å
and c = 5.1483 Å. As for the Cu/CeO2–ZrO2 mixed oxide
catalysts, the lattice parameters of CeyZr1−yO2 decrease with
increasing the Zr content indicative of a contraction of the
unit cell, which is consistent with the peak shifts in the XRD
patterns. In addition, there exists a phase transformation
from CeO2-rich cubic (20CuCeyZr1−y with y = 0.8 and 0.6) to
ZrO2-rich tetragonal (y = 0.4 and 0.2). The cubic to tetragonal
phase transformation in CeyZr1−yO2 solid solution has also
been reported by other researchers.33,44
maximum reduction temperature (Tred,max
) with y. As
presented in Fig. 5, Tred,max first decreases with increasing y
which in turn increases at y > 0.6. This pattern is just
opposite to that observed for the active surface area of Cu,
implying that 20CuCeyZr1−y with high reducibility has a high
Cu dispersion after reduction. Note that 20CuZr is an
exception because its SCu is as low as 2.0 m2 g−1, despite
having a low reduction temperature. The reason is probably
that most CuO in 20CuZr are readily sintered during
Fig. 4 shows the H2-TPR profiles of 20CuCeyZr1−y. For the
Ce-containing catalysts (y = 0.2–1), the H2-TPR profiles can be
generally fitted into three peaks: the low-temperature peak at
145–160 °C is attributed to the reduction of finely dispersed
Table 2 Lattice parameters and crystalline structure of CeyZr1−yO2 for
various xCuCeyZr1−y catalysts
CeyZr1−yO2 lattice
parameters
Catalyst
a (Å)
c (Å)
Phase
20CuCe
5.4134
5.3626
5.3011
3.7141
5.2525a
3.6327
5.1374a
3.6097
5.1050a
5.2805
5.2720
5.2610
—
—
—
5.2582
Cubic
Cubic
Cubic
Tetragonal
20CuCe0.8Zr0.2
20CuCe0.6Zr0.4
20CuCe0.4Zr0.6
20CuCe0.2Zr0.8
20CuZr
5.2119
5.1483
Tetragonal
Tetragonal
40CuCe0.6Zr0.4
50CuCe0.6Zr0.4
60CuCe0.6Zr0.4
—
—
—
Cubic
Cubic
Cubic
Fig.
5 Variation of Cu active surface area (SCu) and maximum
reduction peak temperature (Tred,max) with Ce/(Ce + Zr) molar ratio for
20CuCeyZr1−y catalysts (symbols: experimental data; lines: fitted data
by the Gaussian function).
a
Related to fluorite-type lattice.
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Table 3 XPS and Raman analysis results for 20CuCeyZr1−y catalysts
Surface compositiona
reduction due to the weak interaction with the support. With
this consideration, only the Ce-containing catalysts are taken
into account in Fig. 5. An interesting phenomenon is that
both SCu–y (0 < y ≤ 1) and Tred,max–y relationships can be
well fitted by Gaussian curves, with the highest SCu and
lowest Tred,max occurring at y close to 0.6.
Catalyst
Ce3+/Ce4+
Cur/Cu2+
Od/Ol
Nb (cm−3
)
20CuCe
0.22
0.35
0.48
0.38
0.32
—
0.21
0.62
1.21
0.90
0.46
0.05
0.99
1.13
1.22
1.16
0.61
0.36
2.6 × 1021
3.7 × 1021
5.2 × 1021
4.3 × 1021
1.8 × 1021
—
20CuCe0.8Zr0.2
20CuCe0.6Zr0.4
20CuCe0.4Zr0.6
20CuCe0.2Zr0.8
20CuZr
Fig. 6 shows the XPS spectra of Ce 3d, Zr 3d, Cu 2p and O
1s core levels for all 20CuCeyZr1−y. The total Ce 3d spectra (Ce
3d3/2 and Ce 3d5/2, Fig. 6(a)) can be deconvoluted into eight
peaks with binding energies (BEs) centered at around 882,
a
b
Surface composition is obtained by XPS analysis. Concentration
889 and 898 eV for Ce4+ 3d5/2, 901, 908 and 917 eV for Ce4+
of oxygen vacancies (N) is acquired by Raman analysis.
48,49
3d3/2, 885 eV for Ce3+ 3d5/2, and 903 eV for Ce3+ 3d3/2
.
The Ce3+/Ce4+ ratios calculated from the area of Ce3+ relative
to that of Ce4+ are summarized in Table 3. 20CuCe0.6Zr0.4 has
from which it is found that all Zr-containing catalysts except
20CuZr have larger Cur/Cu2+ ratio than Zr-free 20CuCe. This
is probably because the incorporation of Zr into the CeO2
matrix facilitates the reduction of Ce4+ to Ce3+ and
meanwhile improves the flexibility of Cu ions.54 In contrast,
20CuZr without Ce has a rather low Cur/Cu2+ ratio (0.05).
Among all 20CuCeyZr1−y, the catalyst with y = 0.6 has the
largest Cur/Cu2+ ratio (1.21).
the highest Ce3+/Ce4+ ratio (0.48) among all 20CuCeyZr1−y
,
showing a relatively higher surface concentration of Ce3+ as
compared to other catalysts. Considering that the reduction
of Ce4+ to Ce3+ is accompanied by oxygen vacancy
formation,50,51 it can be expected that 20CuCe0.6Zr0.4 would
have the largest number of oxygen defects.
For Zr-containing 20CuCeyZr1−y, two peaks of Zr 3d3/2 and
Zr 3d5/2 are centered at 184.4 and 182.0 eV, respectively
(Fig. 6(b)). The BEs are constant and the difference in BE
between the two peaks is 2.4 eV, which is in agreement with
the reported value,52 and implies a constant zirconium
oxidation state independent of composition.
The Cu 2p3/2 spectra ranging from 938–928 eV (Fig. 6(c))
are deconvoluted into two peaks centered at 934–935 eV and
932–933 eV, which correspond to CuO (Cu2+) and reduced Cu
species (Cur), respectively.53 In addition, the satellite peaks
located at 938–948 eV are assigned to CuO. The presence of
both CuO and reduced Cu may result from the oxygen
vacancies that can stabilize Cu+ species.45 The Cur/Cu2+ ratios
determined from XPS measurements are listed in Table 3,
The O 1s spectra (Fig. 6(d)) can be deconvoluted into three
peaks with BEs centered at 531.6–532.2 eV, 530–531 eV and
529.5–530 eV. The first two peaks are assigned to defect
oxygen (Od), and the third one is ascribed to lattice oxygen
(Ol).55,56 The defect oxygen refers to mobile oxygen ions such
as O2− and O− as well as adsorbed oxygen species from water
and hydroxyl species on the surface. It is observed that the Ol
peak gradually shifts to higher BEs with increasing the
amount of Zr, indicating the interaction of Zr with Ce owing
to the higher BE of Ol in ZrO2 than in CeO2.33 The Od/Ol ratio
that reflects the surface concentration of oxygen defects is
calculated and outlined in Table 3. 20CuCe0.6Zr0.4 once again
shows the largest Od/Ol ratio (1.22), indicating the largest
amount of oxygen defects. This result is in line with the
analysis of the Ce3+/Ce4+ ratio. The dependence of Od/Ol, Cur/
Cu2+ and Ce3+/Ce4+ ratios on the Ce/(Ce + Zr) ratio is shown
in Fig. S2.† All dependence can be well depicted by the
Gaussian function just like SCu–y and Tred,max–y relationships
(Fig. 5); moreover, the Od/Ol, Cur/Cu2+ and Ce3+/Ce4+ ratios
follow the same dependence on y, i.e., first increases with y
and then passes through a maximum at around y = 0.6.
In order to acquire the exact concentration of oxygen
vacancies of 20CuCeyZr1−y, Raman analysis is carried out. The
Raman spectra (Fig. S3†) confirm the cubic structure of CeyZr1−y
O2 for 20CuCe, 20CuCe0.8Zr0.2 and 20CuCe0.6Zr0.4, and the
tetragonal structure of CeyZr1−yO2 for 20CuCe0.4Zr0.6, 20CuCe0.2
-
-
Zr0.8 and 20CuZr. In addition, the band of the F2g vibration mode
of CeO2 (460–480 cm−1) that is connected with oxygen vacancies
is present on the Raman spectra.36–38 Note that with decreasing
the Ce/Zr molar ratio, the intensity of this peak becomes weaker
as a result of reduced CeO2 content. The calculated vacancy
concentrations (N) for various 20CuCeyZr1−y catalysts are given in
Table 3, which are in the order 20CuCe0.6Zr0.4 > 20CuCe0.4Zr0.6
>
20CuCe0.8Zr0.2 > 20CuCe > 20CuCe0.2Zr0.8, in agreement with
Fig. 6 XPS spectra of (a) Ce 3d, (b) Zr 3d, (c) Cu 2p and (d) O 1s of
20CuCeyZr1−y catalysts.
the order of the Od/Ol ratio derived from XPS analysis.
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Fig. 7 presents the CO conversion curves of 20CuCeyZr1−y
Catalysis Science & Technology
catalysts over 30 h time on stream at 350 °C. 20CuZr exhibits
the lowest apparent activity among all catalysts, which is
mainly attributed to the low amount of oxygen defects. In
addition, the very small Cu active surface area (2.0 m2 g−1) or
dispersion (1.5%) contributes to the low WGS activity. In
contrast, the Ce-containing catalysts possess
a certain
amount of oxygen defects and relatively high active Cu
surface area, which result in greatly improved initial activities
when compared to 20CuZr. The initial activities increase in
the order 20CuZr < 20CuCe0.2Zr0.8 < 20CuCe < 20CuCe0.8
-
Zr0.2 < 20CuCe0.4Zr0.6 < 20CuCe0.6Zr0.4, which completely
coincides with the order of N (or Od/Ol ratio) and almost
follows the order of DCu (or SCu) except for 20CuCe0.2Zr0.8 and
20CuCe. After 30 h of operation, these catalysts are tested
over a temperature range of 250 to 450 °C. The results
indicate that the apparent activities at different temperatures
Fig. 8 Arrhenius plots of turnover frequency (TOF) of CO conversion
obtained over 20CuCeyZr1−y catalysts (reaction conditions: H2 : CO :
H2O = 1 : 2 : 2 (molar ratio), P = 0.1 MPa, GHSV = 80 000 h−1).
are generally ranked as 20CuCe0.6Zr0.4 > 20CuCe0.4Zr0.6
>
20CuCe0.8Zr0.2 > 20CuCe > 20CuCe0.2Zr0.8 > 20CuZr (Fig.
S4†). The oxygen vacancy concentration of spent 20CuCey-
Zr1−y (Table S2†) changes slightly as compared to that of its
fresh counterpart, but the sequence of oxygen vacancy
concentrations remains unchanged.
By using the conversion data listed in Table S1,† the TOFs
of 20CuCeyZr1−y can be calculated according to eqn (8), which
are also presented in Table S1.† Next, by plotting ln(TOF) as a
function of temperature (Fig. 8), apparent activation energies
of 53.5, 49.9, 46.6, 48.4, 56.3 and 66.5 kJ mol−1 are determined
structural factors affecting the activity of 20CuCeyZr1−y.
Indeed, for CeO2 supported Cu-based catalysts, it is known
that active Cu sites adsorb and activate CO, while oxygen
vacancies facilitate H2O dissociation.57–59 Next, the TOFs for
WGS are correlated with N and DCu of 20CuCeyZr1−y. As
shown in Fig. 9, neither N nor DCu can yield a satisfactory
linear correlation with TOF at any temperature from 240 to
300 °C, implying that these two factors individually cannot
account for the variation in TOF. However, when TOF is
correlated with the ratio of N to DCu (N/DCu), a positive linear
relationship is found. This indicates that N/DCu is superior to
either N or DCu in determining the specific activity of
for 20CuCe, 20CuCe0.8Zr0.2, 20CuCe0.6Zr0.4, 20CuCe0.4Zr0.6
,
20CuCe0.2Zr0.8 and 20CuZr, respectively. With an increase in
Zr content, the activation energy first decreases and later
increases, with 20CuCe0.6Zr0.4 having the lowest activation
energy. It is interesting that the order of activation energy is
opposite to the order of apparent activity in WGS.
20CuCeyZr1−y
, reflecting the synergistic effect of oxygen
vacancies and active Cu sites. To the best of our knowledge,
it is the first time that N/DCu is regarded as the dominant
factor affecting the specific activity of Cu/CeO2–ZrO2. In a
sense, N/DCu represents the amount of oxygen vacancies
The above analyses have demonstrated that oxygen
vacancy concentration and Cu dispersion are two important
Fig.
9 Correlation between TOF and (a1–d1) Cu dispersion, (a2–d2)
oxygen vacancy concentration, and (a3–d3) ratio of oxygen vacancy
concentration to Cu dispersion at different temperatures ((a1–a3) 240 °C,
(b1–b3) 260 °C, (c1–c3) 280 °C, (d1–d3) 300 °C) (symbols: (square)
20CuCe, (circle) 20CuCe0.8Zr0.2, (up triangle) 20CuCe0.6Zr0.4, (down
triangle) 20CuCe0.4Zr0.6, (diamond) 20CuCe0.2Zr0.8; dashed lines: linear fit).
Fig. 7 CO conversion curves of 20CuCeyZr1−y catalysts during 30 h
time on stream (the dashed line represents the equilibrium conversion;
reaction conditions: H2 : CO : H2O = 1 : 2 : 2 (molar ratio), T = 350 °C, P
= 0.1 MPa, GHSV = 20 000 h−1).
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Paper
located near each active Cu site. A higher N/DCu means more
oxygen vacancies available for H2O dissociation and
subsequent CO oxidation once CO is activated on Cu sites,
which gives rise to a higher TOF value. Note that although
20CuCe has the highest TOF in WGS among all 20CuCeyZr1−y
,
its apparent activity is relatively low due to a small amount of
active Cu sites. In contrast, 20CuCe0.6Zr0.4 shows the highest
apparent activity, but its TOF is not the best owing to a large
number of active Cu sites.
When it comes to stability, the conversion decay rates for
20CuCe, 20CuCe0.8Zr0.2, 20CuCe0.6Zr0.4, 20CuCe0.4Zr0.6 and
20CuCe0.2Zr0.8 are 0.40, 0.33, 0.30, 0.57, and 1.07% per hour,
respectively. 20CuCe0.4Zr0.6 and 20CuCe0.2Zr0.8 show lower
stability compared to the other three catalysts. It is worth
noting that cubic crystal structures are found for CeyZr1−yO2
in 20CuCe, 20CuCe0.8Zr0.2 and 20CuCe0.6Zr0.4
,
while
tetragonal structures in 20CuCe0.4Zr0.6 and 20CuCe0.2Zr0.8
(Table 2). It seems that the cubic to tetragonal phase
transformation in CeyZr1−yO2 deteriorates the catalyst
stability, which is probably due to the fact that the tetragonal
phase hinders the oxygen mobility during the repetitive redox
process.60,61 Among the three cubic 20CuCeyZr1−y, 20CuCe0.6
-
Zr0.4 with the highest oxygen vacancy concentration
undoubtedly possesses the highest oxygen storage capacity,
which helps to retard the textural modification derived from
the expansion/contraction of the lattice parameter during the
redox process.61 Therefore, 20CuCe0.6Zr0.4 exhibits the best
stability among all 20CuCeyZr1−y. In view of the highest
apparent activity and stability of 20CuCe0.6Zr0.4 in WGS, in
the next section, the Ce/(Ce + Zr) molar ratio is fixed at 0.6 to
prepare xCuCe0.6Zr0.4 catalysts with different Cu loadings.
Fig. 10 FESEM (left panels) and HRTEM (right panels) images of
xCuCe0.6Zr0.4 catalysts (the insets in the right panels show the CuO
particle size distribution; the insets in the bottom panel show the fast
Fourier transformation patterns of 50CuCe0.6Zr0.4).
homogeneously distributed even in the case of a high Cu
loading. This result further demonstrates that the sol–gel
method can ensure the homogeneity of the elemental
distribution in the catalyst.
3.2 Effect of Cu loading
Besides 20CuCe0.6Zr0.4
,
three other xCuCe0.6Zr0.4 with
different Cu loadings of 40, 50 and 60 wt% are prepared. In
addition, pure CuO is synthesized using the same technique.
The BET surface area and pore volume of xCuCe0.6Zr0.4
decrease with an increase in Cu loading (Table 1, entries 3
The XRD patterns of xCuCe0.6Zr0.4 are shown in Fig. S6.†
Only CuO and Ce0.6Zr0.4O2 are detected, which are also
identified by the lattice fringes of 50CuCe0.6Zr0.4 shown in
Fig. 10 (the lattice spacings determined by fast Fourier
transformation analysis of selected areas are 0.271 and 0.309
nm, which correspond to the (110) plane of CuO and the
(111) plane of Ce0.6Zr0.4O2, respectively). In addition, the
diffraction peaks of CuO become stronger with increasing x,
suggesting larger CuO particles; no peak shift is observed for
Ce0.6Zr0.4O2, indicating that the crystalline phase remains
unchanged and independent of x. Le Bail refinement of the
and 7–9), implying that more CuO enters the pores of Ce0.6
-
Zr0.4O2 leading to the blockage of some pores. As an extreme
sample, pure CuO has almost no pores (Table 1, entry 10).
The variation of textural data can be reflected from the
surface morphology. As illustrated in Fig. 10, with increasing
x, the macroporous networks gradually disappear on the one
hand, and the grains become larger on the other hand.
Furthermore, the agglomeration and growth of CuO particles
become more apparent, with the average particle size
increasing from 5.2 to 10.7 nm when x varies from 20 to 60.
Accordingly, the Cu dispersion decreases with x (Table 1).
The active surface area of Cu varies slightly in the range of x
= 20–50 (SCu = 25.1–27.1 m2 g−1), but decreases dramatically
to 12.5 m2 g−1 at x = 60, which is a trade-off between Cu
loading and dispersion.
XRD data confirms the stable cubic crystalline phase of Ce0.6
-
Zr0.4O2 (Table 2). However, the lattice parameter decreases
with increasing x, which is related to the amount of oxygen
vacancies and Cu atoms in the matrix, as the substitution of
Cu2+ with Ce4+ will generate oxygen vacancies and cause a
contraction of the lattice cell.32,62 This is in accordance with
the TPR results that the amount of H2 consumption of the
high-temperature TPR peak increases with x (Fig. S7†). As for
the concentration of oxygen defects, the Od/Ol ratio increases
Fig. S5† gives the HRTEM-EDS mappings of 50CuCe0.6
-
Zr0.4 which clearly show that Cu, Ce and Zr are
,
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Catalysis Science & Technology
from 1.22 to 1.40 when x from 20 to 50 (Fig. S8†), implying
more oxygen defects; but it decreases sharply to 0.8 at x = 60,
suggesting less oxygen defects. It is known that the
substitution of Ce4+ by Cu2+ favors the generation of oxygen
vacancies, but a higher Cu loading in xCuCe0.6Zr0.4 also
brings about a lower Ce content, and hence a compromise is
made.
Fig. 11(a) compares the apparent activity and long-term
stability of different xCuCe0.6Zr0.4 catalysts. Compared to
Ce0.6Zr0.4O2 supported catalysts, pure CuO exhibits much
lower activity and stability, with the CO conversion dropping
from around 43% to near zero after 15 h time on stream.
This is understandable because Cu particles are readily
sintered at 350 °C in the case without a support. As for
xCuCe0.6Zr0.4, the initial activity increases in the order
the highest concentration of oxygen defects and therefore, it
shows the highest apparent activity.
Note that there exists a self-activation phenomenon on
60CuCe0.6Zr0.4, i.e., the catalyst activity increases with the
reaction time up to 30 h. This phenomenon is probably
associated with the reduction of some unreduced CuO
particles (because of too much CuO in 60CuCe0.6Zr0.4) during
the WGS reaction. However, 60CuCe0.6Zr0.4 is unstable and
its activity gradually decreases after 30 h. Among all xCuCe0.6
-
Zr0.4, 50CuCe0.6Zr0.4 exhibits the best stability, with the CO
conversion slightly varying from around 77% to 75% after 60
h time on stream. In addition, the selectivities to H2 and CO2
are 100% during the whole period, while the selectivity to
CH4 is zero (Fig. 11(b)). The long-term stability of 50CuCe0.6
-
Zr0.4 is mainly due to the cubic structure and the high
concentration of oxygen vacancies, as analyzed in the above
section. The morphology of the spent 50CuCe0.6Zr0.4 is well
preserved as compared to the fresh one and the Cu metal
particle size is only slightly increased (8.9 nm vs. 7.2 nm)
(Fig. S9†). Furthermore, no coke is formed on the spent
50CuCe0.6Zr0.4 as evidenced by no weight loss at
temperatures above 400 °C in the TGA curve (inset in
Fig. 11(b)). The weight gain at 200–400 °C is attributed to the
oxidation of Cu in air. Therefore, 50CuCe0.6Zr0.4 is a potential
candidate for the SEWGS process of the IGCC system.
60CuCe0.6Zr0.4
<
20CuCe0.6Zr0.4
≈
40CuCe0.6Zr0.4
<
50CuCe0.6Zr0.4 (see the inset). 60CuCe0.6Zr0.4 has the lowest
concentration of oxygen defects and the smallest active Cu
surface area and thus shows the lowest initial activity.
20CuCe0.6Zr0.4
,
40CuCe0.6Zr0.4 and 50CuCe0.6Zr0.4 have
similar active Cu surface areas, but 50CuCe0.6Zr0.4 possesses
4. Conclusions
A series of Cu/CeO2–ZrO2 catalysts with varying Ce/(Ce + Zr)
molar ratio (y = 0–1) and Cu loading (20–60 wt%) were
prepared for WGS under 20% H2, 40% CO and 40% steam at
350
°C.
The
composition–structure–performance
relationships of Cu/CeO2–ZrO2 were deeply explored. It was
found that the dependences of SCu (active Cu surface area),
Od (defect oxygen)/Ol (lattice oxygen), Cur (reduced Cu)/Cu2+
and Ce3+/Ce4+ ratios of the catalysts on y were well described
by the Gaussian function, with maximum values at
approximately y = 0.6. The WGS activity of the catalysts was
closely associated with oxygen vacancies and active Cu sites.
In general, for Cu/CeO2–ZrO2 with the same Cu loading (20
wt%) but different Ce/Zr ratios, the higher the concentration
of oxygen vacancies and the Cu dispersion, the higher the
apparent activity. However, when it came to the specific
activity (or TOF), neither oxygen vacancy concentration nor
Cu dispersion was able to individually account for the
variation in TOF. The ratio of oxygen vacancy concentration
to Cu dispersion was found to be the dominant factor
influencing the specific activity. Kinetic investigation showed
that the lower the activation energy of the WGS reaction, the
higher the apparent activity of Cu/CeO2–ZrO2. As for the
catalyst stability, it turned out that the tetragonal-to-cubic
phase transformation in CeyZr1−yO2 increased the stability of
Cu/CeO2–ZrO2. Among all the catalysts studied, a sol–gel-
derived Cu/CeO2–ZrO2 catalyst with a Ce/(Ce + Zr) ratio of 0.6
and a Cu loading of 50 wt% exhibited the best WGS activity
and stability: the CO conversion was only slightly decreased
Fig. 11 (a) CO conversion curves of xCuCe0.6Zr0.4 catalysts and (b)
selectivity of various species over 50CuCe0.6Zr0.4 during long-term
WGS tests (the dashed line in panel (a) represents the equilibrium
conversion; the inset in panel (b) shows the TGA curve of spent
50CuCe0.6Zr0.4; reaction conditions: H2 : CO : H2O = 1 : 2 : 2 (molar
ratio), T = 350 °C, P = 0.1 MPa, GHSV = 20 000 h−1).
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from 77% to 75% during 60 h time on stream while the
selectivities to H2 and CO2 were maintained at 100%; in
addition, the catalyst morphology was well preserved after 60
h and no coke formation occurred.
17 D. Devaiah, T. Boningari and P. G. Smirniotis, Mol. Catal.,
2018, 451, 20–32.
18 D. Devaiah, D. Jampaiah, A. Welton, P. Boolchand, A.
Arvanitis, J. Dong and P. G. Smirniotis, Catal. Today,
2020, 355, 921–931.
19 C. Rhodes, G. J. Hutchings and A. M. Ward, Catal. Today,
1995, 23, 43–58.
20 C. Rhodes, B. P. Williams, F. King and G. J. Hutchings,
Catal. Commun., 2002, 3, 381–384.
21 J. Y. Lee, D. W. Lee, K. Y. Lee and Y. Wang, Catal. Today,
2009, 146, 260–264.
Author contributions
Yuanwu Hu: conceptualization, methodology, investigation,
data curation, writing
–
original draft. Na Wang:
methodology, investigation. Zhiming Zhou: supervision,
writing – review & editing, funding acquisition.
22 P. Kowalik, W. Próchniak and T. Borowiecki, Catal. Today,
2011, 176, 144–148.
Conflicts of interest
23 P. Kowalik, M. Konkol, K. Antoniak, W. Próchniak and P.
Wiercioch, J. Mol. Catal. A: Chem., 2014, 392, 127–133.
24 S. D. Senanayake, J. Evans, S. Agnoli, L. Barrio, T. L. Chen, J.
Hrbek and J. A. Rodriguez, Top. Catal., 2011, 54, 34–41.
25 Y. L. Lee, A. Jha, W. J. Jang, J. O. Shim, K. W. Jeon, H. S. Na,
H. S. Kim, D. W. Lee, S. Y. Yoo, B. H. Jeon, J. W. Bae and
H. S. Roh, Top. Catal., 2017, 60, 721–726.
26 P. Djinović, J. Batista, J. Levec and A. Pintar, Appl. Catal., A,
2009, 364, 156–165.
27 D. Devaiah, L. H. Reddy, S. E. Park and B. M. Reddy, Catal.
Rev.: Sci. Eng., 2018, 60, 177–277.
There are no conflicts to declare.
Acknowledgements
Financial supports from the National Natural Science
Foundation of China (21776088) and the Fundamental
Research Funds for the Central Universities (222201718003)
are acknowledged. The authors thank the Research Center of
Analysis and Test of East China University of Science and
Technology for the help on the characterization.
28 S. S. Maluf, P. A. P. Nascente and E. M. Assaf, Fuel Process.
Technol., 2010, 91, 1438–1445.
Notes and references
1 D. B. Bukur, B. Todic and N. Elbashir, Catal. Today,
2016, 275, 66–75.
2 G. C. Laredo, P. M. Vega Merino and P. S. Hernández, Ind.
Eng. Chem. Res., 2018, 57, 7315–7321.
29 S. Y. Ahn, H. S. Na, K. W. Jeon, Y. L. Lee, K. J. Kim, J. O.
Shim and H. S. Roh, Catal. Today, 2020, 352, 166–174.
30 X. Qi and M. Flytzani-Stephanopoulos, Ind. Eng. Chem. Res.,
2004, 43, 3055–3062.
3 M. Zhu and I. E. Wachs, ACS Catal., 2015, 6, 722–732.
4 J. Boon, V. Spallina, Y. van Delft and M. Van Sint Annaland,
Int. J. Greenhouse Gas Control, 2016, 50, 121–134.
5 J. Boon, K. Coenen, E. van Dijk, P. Cobden, F. Gallucci and
M. van Sint Annaland, Chapter One – Sorption-Enhanced
Water–Gas Shift, in Advances in Chemical Engineering, ed. A.
A. Lemonidou, Academic Press, 2017, pp. 1–96.
6 M. Gazzani, E. Macchi and G. Manzolini, Fuel, 2013, 105,
206–219.
31 L. Jiang, H. Zhu, R. Razzaq, M. Zhu, C. Li and Z. Li, Int. J.
Hydrogen Energy, 2012, 37, 15914–15924.
32 X. Wang, J. A. Rodriguez, J. C. Hanson, D. Gamarra, A.
Martínez-Arias and M. Fernández-García, J. Phys. Chem. B,
2006, 110, 428–434.
33 D. W. Jeong, H. S. Na, J. O. Shim, W. J. Jang and H. S. Roh,
Catal. Sci. Technol., 2015, 5, 3706–3713.
34 Y. Wan, Z. Zhou and Z. Cheng, Chin. J. Chem. Eng., 2016, 24,
1186–1194.
7 C. H. Lee and K. B. Lee, Appl. Energy, 2017, 205, 316–322.
8 Y. Hu, H. Cui, Z. Cheng and Z. Zhou, Chem. Eng. J.,
2019, 377, 119823.
9 C. C. Cormos, Int. J. Hydrogen Energy, 2009, 34, 6065–6077.
10 C. C. Cormos, Energy, 2012, 42, 434–445.
11 A. M. Cormos, C. Dinca and C. C. Cormos, Appl. Therm. Eng.,
2015, 74, 20–27.
35 I. Kosacki, T. Suzuki, H. U. Anderson and P. Colomban, Solid
State Ionics, 2002, 149, 99–105.
36 P. Trogadas, J. Parrondo and V. Ramani, ACS Appl. Mater.
Interfaces, 2012, 4, 5098–5102.
37 J. O. Shim, H. S. Na, A. Jha, W. J. Jang, D. W. Jeong, I. W.
Nah, B. H. Jeon and H. S. Roh, Chem. Eng. J., 2016, 306,
908–915.
12 W. Fu, Z. Bao, W. Ding, K. Chou and Q. Li, Catal. Commun.,
2011, 12, 505–509.
13 C. Wang, C. Liu, W. Fu, Z. Bao, J. Zhang, W. Ding, K. Chou
and Q. Li, Catal. Today, 2016, 263, 46–51.
14 D. W. Jeong, A. Jha, W. J. Jang, W. B. Han and H. S. Roh,
Chem. Eng. J., 2015, 265, 100–109.
38 J. O. Shim, W. J. Jang, K. W. Jeon, D. W. Lee, H. S. Na, H. M.
Kim, Y. L. Lee, S. Y. Yoo, B. H. Jeon, H. S. Roh and C. H. Ko,
Appl. Catal., A, 2018, 563, 163–169.
39 M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F.
Rodriguez-Reinoso, J. Rouquerol and K. S. W. Sing, Pure
Appl. Chem., 2015, 87, 1051–1069.
15 D. Devaiah and P. G. Smirniotis, Ind. Eng. Chem. Res.,
2017, 56, 1772–1781.
40 R. O. Fuentes and R. T. Baker, J. Phys. Chem. C, 2009, 113,
914–924.
16 D. Devaiah and P. G. Smirniotis, Curr. Opin. Chem. Eng.,
2018, 21, 103–110.
41 Z. Zhang, Y. Zhang, Z. Mu, P. Yu, X. Ni, S. Wang and L.
Zheng, Appl. Catal., B, 2007, 76, 335–347.
This journal is © The Royal Society of Chemistry 2021
Catal. Sci. Technol., 2021, 11, 2518–2528 | 2527

View Article Online
Paper
Catalysis Science & Technology
42 R. Zhang, W. Y. Teoh, R. Amal, B. Chen and S. Kaliaguine,
J. Catal., 2010, 272, 210–219.
54 Y. Z. Chen, B. J. Liaw and H. C. Chen, Int. J. Hydrogen
Energy, 2006, 31, 427–435.
43 Y. Cui, L. Xu, M. Chen, X. Lian, C. E. Wu, B. Yang, Z. Miao,
F. Wang and X. Hu, Catal. Sci. Technol., 2019, 9, 5605–5625.
44 M. Yashima, H. Arashi, M. Kakihana and M. Yoshimura,
J. Am. Ceram. Soc., 1994, 77, 1067–1071.
55 D. W. Jeong, W. J. Jang, H. S. Na, J. O. Shim, A. Jha and H. S.
Roh, J. Ind. Eng. Chem., 2015, 27, 35–39.
56 D. Devaiah, T. Tsuzuki, T. Boningari, P. G. Smirniotis and
B. M. Reddy, RSC Adv., 2015, 5, 30275–30285.
45 L. Kundakovic and M. Flytzani-Stephanopoulos, Appl. Catal.,
A, 1998, 171, 13–29.
46 L. Kundakovic and M. Flytzani-Stephanopoulos, J. Catal.,
1998, 179, 203–221.
47 H. Zou, X. Dong and W. Lin, Appl. Surf. Sci., 2006, 253,
2893–2898.
57 J. A. Rodriguez, P. Liu, X. Wang, W. Wen, J. Hanson, J.
Hrbek, M. Pérez and J. Evans, Catal. Today, 2009, 143, 45–50.
58 J. A. Rodriguez, J. Graciani, J. Evans, J. B. Park, F. Yang, D.
Stacchiola, S. D. Senanayake, S. Ma, M. Pérez, P. Liu, J. F.
Sanz and J. Hrbek, Angew. Chem., Int. Ed., 2009, 48,
8047–8050.
48 M. Alifanti, B. Baps, N. Blangenois, J. Naud, P. Grange and
B. Delmon, Chem. Mater., 2003, 15, 395–403.
49 D. G. Cheng, M. Chong, F. Chen and X. Zhan, Catal. Lett.,
2008, 120, 82–85.
50 M. Nolan, J. E. Fearon and G. W. Watson, Solid State Ionics,
2006, 177, 3069–3074.
51 A. Migani, G. N. Vayssilov, S. T. Bromley, F. Illas and K. M.
Neyman, Chem. Commun., 2010, 46, 5936–5938.
52 A. E. Nelson and K. H. Schulz, Appl. Surf. Sci., 2003, 210,
206–221.
53 Q. F. Deng, T. Z. Ren, B. Agula, Y. Liu and Z. Y. Yuan, J. Ind.
Eng. Chem., 2014, 20, 3303–3312.
59 K. Mudiyanselage, S. D. Senanayake, L. Feria, S. Kundu, A. E.
Baber, J. Graciani, A. B. Vidal, S. Agnoli, J. Evans, R. Chang,
S. Axnanda, Z. Liu, J. F. Sanz, P. Liu, J. A. Rodriguez and D. J.
Stacchiola, Angew. Chem., Int. Ed., 2013, 52, 5101–5105.
60 P. Fornasiero, R. Dimonte, G. R. Rao, J. Kaspar, S. Meriani,
A. Trovarelli and M. Graziani, J. Catal., 1995, 151, 168–177.
61 P. Fornasiero, G. Balducci, R. Di Monte, J. Kašpar, V. Sergo,
G. Gubitosa, A. Ferrero and M. Graziani, J. Catal., 1996, 164,
173–183.
62 X. Wang, J. A. Rodriguez, J. C. Hanson, D. Gamarra, A.
Martínez-Arias and M. Fernández-García, J. Phys. Chem. B,
2005, 109, 19595–19603.
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This journal is © The Royal Society of Chemistry 2021