Mesoporous NiCu-CeO2 oxide catalysts for high-temperature water-gas shift reaction

Article abstract of DOI:10.1039/c4ra13142h

Mesoporous NiCu-CeO₂ oxide catalysts were synthesized by using the evaporation-induced self-assembly method applied to the high-temperature, water-gas shift reaction (HT-WGS) between 350 to 550 °C. Nickel and copper loadings on mesoporous ceria were tailored to achieve high activity and selectivity by suppressing methane formation in HT-WGS. Among the prepared catalysts, NiCu(1 : 4)-CeO₂ exhibited the highest selectivity to CO₂ and H₂ with 85% CO conversion at a very high GHSV of 83 665 h^-1. The higher activity of the catalysts was due to the mesoporous architecture, which provides more accessible active sites for the WGS reaction. Powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), N₂-adsorption/desorption isotherm, high-resolution transmission electron microscopy (HR-TEM), and H₂-temperature-programmed reduction (TPR) techniques were used to understand the role of mesoporosity and bimetallic composition of various NiCu-CeO₂ oxides in enhancing catalytic activity for HT-WGS. This journal is

Full text of DOI:10.1039/c4ra13142h

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PAPER
Mesoporous NiCu–CeO oxide catalysts for
2
high-temperature water–gas shift reaction†
Cite this: RSC Adv., 2015, 5, 1430
a
a
a
b
Ajay Jha, Dae-Woon Jeong, Won-Jun Jang, Chandrashekhar V. Rode
and Hyun-Seog Roh*a
Mesoporous NiCu–CeO
2
oxide catalysts were synthesized by using the evaporation-induced self-assembly
ꢀ
method applied to the high-temperature, water–gas shift reaction (HT-WGS) between 350 to 550 C.
Nickel and copper loadings on mesoporous ceria were tailored to achieve high activity and selectivity by
suppressing methane formation in HT-WGS. Among the prepared catalysts, NiCu(1 : 4)–CeO
the highest selectivity to CO and H with 85% CO conversion at a very high GHSV of 83 665 h . The
higher activity of the catalysts was due to the mesoporous architecture, which provides more accessible
2
exhibited
ꢁ
1
2
2
active sites for the WGS reaction. Powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), N
2
-
-
Received 25th October 2014
Accepted 20th November 2014
adsorption/desorption isotherm, high-resolution transmission electron microscopy (HR-TEM), and H
2
temperature-programmed reduction (TPR) techniques were used to understand the role of
DOI: 10.1039/c4ra13142h
2
mesoporosity and bimetallic composition of various NiCu–CeO oxides in enhancing catalytic activity for
www.rsc.org/advances
HT-WGS.
produced from steam reforming of hydrocarbons. In addition, the
presence of the CO and H in the waste-derived synthesis gas,
Introduction
2
2
Production of hydrogen has been a major focus of academic as which can drive the reverse WGS reaction as well as higher
well as industrial research because hydrogen is a promising concentration of CO, makes the system more severe. Therefore,
sustainable and clean energy carrier. Hydrogen is produced it is necessary to develop a novel HTS catalyst to force the reaction
from steam reforming of natural gas, partial oxidation of into a forward direction and withstand severe conditions. In our
1
petroleum oil and steam gasication of coal. However, the CO previous study, the CuFe
O
catalyst supported on mesoporous
2
4
level in the output gas of these processes is very high (>10%) alumina showed the stable CO conversion (80%) with high selec-
ꢀ
and H at 500 C at a GHSV of 41 821 h in a waste-
2
ꢁ1
and sufficient to poison the Pt electrode in the fuel cell. This tivity to CO
triggered immense interest in the water–gas shi reaction derived synthesis gas condition. In this work we observed that the
WGS; CO + H O / CO + H
). The WGS reaction is an indus- mesoporous nature of the catalyst played a major role in control-
2
8
(
2
2
2
trially important reaction to produce hydrogen for chemical ling catalytic activity via uniform dispersion of the metal oxide. The
processing and to remove CO contamination in feed streams for mesoporous structure of the catalyst facilitates the uninterrupted
2,3
fuel cell applications. The reaction involves CO oxidation and diffusion of molecules to and from active sites of the catalysts.
O reduction to give CO and H
. As the WGS reaction is Moreover, because of the ascendant textural properties of meso-
H
2
2
2
limited by equilibrium, at an industrial scale the reaction is porous materials, well-dispersed nanoparticles over mesoporous
implemented in two stages: the rst, at high temperatures support can provide more accessible active sites. In the continua-
(
HT-WGS) using Fe and Cr based catalysts; and the second, at tion of this work reported here, mesoporous NiCu–CeO
2
oxide
4,5
low temperatures (LT-WGS), employing Cu–Zn–Al catalysts.
catalysts for waste-derived synthesis gas were employed for the
Recently, waste to energy has received much attention due to its HT-WGS reaction.
6,7
potential to become a major hydrogen source. In general, gasi-
cation can convert waste into valuable synthesis gas (H + CO) many areas of chemistry, materials science, and physics.
2
Ceria (CeO ) has attracted much interest in recent decades in
9–12

2
including CO
waste-derived synthesis gas is higher than the typical synthesis gas outstanding oxygen storage capacity, that is, its ability to
13,14
2 4 2
, CH and N
, etc. However, CO concentration in the Widespread applications mainly originate from the its
4
+
3+
repeatedly and rapidly pass through redox (Ce /Ce ) cycles.
a
This is usually related to the ease of formation of labile oxygen
vacancies, and particularly, those that promote relatively high
Chemical Engineering & Process Development Division, CSIR-National Chemical mobility of bulk oxygen species at the surface of solid ceria.
Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil, Wonju,
Gangwon 220-710, South Korea. E-mail: hsroh@yonsei.ac.kr; Fax: +82-33-760-2571
b
15–17
Laboratory, Pune-411008, India
Electronic supplementary information (ESI) available: H
composition, and WGS reaction results. See DOI: 10.1039/c4ra13142h
Relative to other oxide supports, ceria also enhances the
performance of transition metal catalysts in a variety of
†
2
-TPR patterns, surface
1430 | RSC Adv., 2015, 5, 1430–1437
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reactions, including water–gas shi, steam reforming of
oxygenates, and preferential oxidation of CO, all of which hold
18,19
promise for enabling a hydrogen economy.
In particular, the
CuO/CeO catalyst has attracted great attention to the WGS due
2
to its ability to rmly anchor Cu, reducing its tendency to sinter.
Although the Cu/CeO catalyst performed well in the LT-WGS
2
reaction, it is not suitable for the HT-WGS reaction and
suffers from rapid deactivation at high temperatures
ꢀ
20–22
(
>300 C).
On the other hand, nickel-based catalysts are
currently providing greater scope in the WGS reaction than the
23
copper catalyst due to its high CO adsorption efficiency.
However, hydrogenation of CO and CO is an undesired side
2
reaction (in methane formation), reducing the efficiency of
24
nickel-based catalysts for the WGS reaction.
To overcome the drawbacks of lower thermal stability of Cu
and side reactions of Ni-based catalysts in the WGS reaction,
bimetallic Cu–Ni catalysts were developed for HT-WGS reac-
tions. Lin et al. suggested that the presence of copper in the
bimetallic Cu–Ni catalysts suppresses undesirable side effects,
25
whereas the nickel component enhanced WGS activity. Saw
et al. observed the formation of Ni–Cu alloy in the bimetallic
NiCu–CeO
2
catalyst that enhanced the CO adsorption at high
23
reaction temperatures and increased its thermal stability.
Doping of nickel to the copper oxide led to signicant changes
in the electronic properties of the catalyst via formation of active
surface structures, new chemical bonding and surface coordi-
nation environment, which subsequently considerably inu-
26,27
enced catalytic performance.
The objective of the present work is to study effects of the
catalyst's mesoporous architecture catalyst on performance of
the NiCu–CeO
tion of methane formation. The mesoporous bimetallic
NiCu–CeO catalysts were prepared by the evaporation-induced
2
oxide for the HT-WGS reaction with minimiza-
2
self-assembly (EISA) method. A series of NiCu–CeO2 oxide
catalysts were prepared by varying the weight percentage ratio of
Ni/Cu from (1 : 1) to (1 : 4). The amount of CeO was xed to
2
7
0 wt% in all cases investigated in this study. Performances of
the mesoporous NiCu–CeO oxide were compared to CuO–CeO
and NiO–CeO oxide catalysts in HT-WGS with the aim of
2
2
2
understanding the effect of Ni doping with respect to catalytic
activity and selectivity.
Results and discussion
The catalysts were studied by XRD to analyze their composition Fig. 1 XRD patterns of (A) CuO–CeO , NiO–CeO , and NiCu(1 : 4)–
2
2
CeO
NiCu(1 : 4)–CeO
NiCu(1 : 4)–CeO
2
oxides, (B) NiCu(1 : 1)–CeO
2
,
NiCu(1 : 2)–CeO
2
,
and
, and
and phase purity. Fig. 1A shows the XRD patterns of calcined
2
oxides, and (C) reduced Cu–CeO
2
, Ni–CeO
2
samples of CuO–CeO , NiO–CeO and NiCu(1 : 4)–CeO . The
2
2
2
2
catalysts.

uorite-type oxide diffraction pattern of CeO
2
was present in all
catalysts. X-ray powder diffraction patterns of CuO–CeO
2
ꢀ
ꢀ
exhibited peaks at 2q ¼ 35.4 and 38.5 , which correspond to
CuO (PDF00-001-1117), while NiO–CeO exhibited peaks at
q ¼ 37.2 , 43.1 , and 62.6 , which correspond to NiO
Fig. 1B exhibits that intensity of the diffraction peaks
increased with higher Cu loading, indicating greater crystalline
nature of the catalysts. The Scherrer equation was applied to
2
ꢀ
ꢀ
ꢀ
2
(
25
PDF00-004-0835). The XRD pattern of the NiCu(1 : 4)–CeO
2
estimate the crystallite size of the catalysts by using the peak at
catalyst also exhibited the corresponding peaks of both CuO
and NiO at the same 2q values.
Fig. 1B shows the XRD patterns of NiCu–CeO series catalysts
2
prepared by varying the weight percentage ratio of Ni/Cu.
ꢀ
2
q ¼ 28.4 (111) corresponding to the cubic phase of CeO , and
2
the crystallite size values are given in Table 1. From this table it
is evident that the crystallite size of the NiCu–CeO catalysts
2
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increased along with increased copper loadings. This may be
due to agglomeration of excess CuO on the surface of ceria.
Fig. 1C represents XRD patterns of the reduced catalysts.
Metallic phases of Cu and Ni were observed in the XRD patterns
of Cu–CeO
2
and Ni–CeO
2
, respectively, aer being tested in the
ꢀ
WGS reaction (up to 550 C). These results indicate that oxide
phases of the Cu and Ni were reduced to their corresponding
metallic phases under WGS conditions. Crystallite sizes of all
catalysts increased aer the water–gas shi reaction, which
suggests the agglomeration of catalysts that led to a decrease in
surface area (Table 1). In the case of NiCu(1 : 4)–CeO oxide
2
aer the WGS reaction, three new reections appeared that
ꢀ
belong to the (111), (200), and (220) crystal planes, at 2q ¼ 43.3 ,
ꢀ ꢀ
5
0.45 , and 74.1 , respectively, which are related to the metallic
27
phase of copper; we did not observe the metallic phase of
nickel in the reduced NiCu(1 : 4)–CeO catalyst. This may be
because of segregation of copper on the surface layer during the
2
Fig. 2
2
H -TPR patterns of the catalysts.
11
WGS reaction, and thus lower surface energy.
H -TPR patterns of NiO–CeO , CuO–CeO and bimetallic
2
2
2
indicates the easy reducibility of the catalyst. Several authors
attribute the strong interaction between highly dispersed metal
NiCu–CeO oxide catalysts presented in Fig. 2 show very inter-
2
esting patterns. In the case of CuO–CeO
appears at 125 C, corresponding to the reduction of CuO
2
, the shoulder peak
oxides (CuO and NiO) and CeO
2
for shiing reduction peaks
ꢀ
32,33
toward lower temperatures.
Doping of Ni- and Cu- together
nanoparticles highly dispersed on CeO
2
. The major peak at
in the ratio of 1 : 4 into CeO produced a complex TPR prole
2
ꢀ
28
204 C, could be ascribed to the reduction of bulk CuO. In
ꢀ
(
Fig. 2), giving four maxima at 82, 107, 168 and 259 C. The rst
contrast, the TPR of NiO–CeO
2
exhibited a distinct rst peak at
ꢀ
two reduction peaks at 82 and 107 C were due to the reduction
of interfacial and highly dispersed CuO, respectively. The two
remnant peaks at 168 and 259 C were possibly due to reduction
ꢀ
the higher temperature of 208 C followed by another major
34
ꢀ
ꢀ
peak at 300 C and a small hump at 369 C. The rst peak at
ꢀ
ꢀ
2
08 C corresponded to the reduction of NiO nanoparticles
2
of bulk CuO and NiO on the reducible mesoporous CeO .
A typical ordered mesoporous material is characterized by a
SAXS (Fig. 3), indicating the existence of a meso-structure and a
ꢀ
highly dispersed on the CeO
2
, while the second peak at 300 C
29
could be ascribed to the reduction of bulk NiO. The third
ꢀ
shoulder peak at 369 C could be due to the partial reduction of
type-IV adsorption isotherm obtained through N adsorption/
2
30
2
ceria. In the case of NiCu–CeO oxide catalysts, reduction of
desorption measurements, indicating the textural property of
CuO and NiO shied towards lower temperature as compared to
CuO–CeO and NiO–CeO catalysts. This indicates that Ni and
catalysts. SAXS of CuO–CeO
CeO showed a sharp peak and shied to the higher 2q value.
Interestingly, the peak of NiCu(1 : 4)–CeO oxide was similar to
that of NiO–CeO in spite of higher copper loading. This
observation suggested that the incorporation of Cu- and Ni-
2
exhibited a broad peak while NiO–
2
2
2
Cu together on mesoporous CeO have a combined inuence on
2
2
the reducibility of NiCu–CeO oxide. This is clearly evident from
2
2
the fact that the reduction temperature of highly dispersed CuO
in the NiCu–CeO catalysts decreased signicantly with the
2
increase in copper loadings (Fig. 2). Interestingly, the reduction
peak of bulk CuO and NiO remained at the same position in the
31
bimetallic series. The lowest reduction temperature in the
ꢀ
range of 82–168 C observed for the NiCu(1 : 4)–CeO2 oxide
Table 1 Textural property of bimetallic NiCu–CeO
2
catalysts
Crystallite size
CeO (nm)
2
2
ꢁ1
a
S
BET (m g
)
Catalyst
Fresh
Used
Fresh
Used
CeO
CuO–CeO
NiO–CeO
NiCu(1 : 1)–CeO
NiCu(1 : 2)–CeO
NiCu(1 : 4)–CeO
2
81
98
78
86
98
—
8
22
31
39
37
10
12
6
5
6
—
15
11
10
9
2
2
2
2
2
102
8
10
2 2
Fig. 3 SAXS patterns of CuO–CeO , NiO–CeO , and NiCu(1 : 4)–
a
Calculated from XRD measurement.
CeO catalysts.
2
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together into the CeO
2
did not inuence the mesoporous
framework of the catalyst.
Fig. 4 displays the N -adsorption/desorption isotherm plots
2
of the CuO–CeO , NiO–CeO and NiCu(1 : 4)–CeO oxide cata-
2
2
2
lysts. Isotherms of CuO–CeO
2
reveal that with increasing relative
o 2
pressure (p/p ), an uptake of N -adsorption volume increased
slowly, which is the characteristic feature of type-I isotherm,
35
typical of microporous materials. CuO–CeO
2
sample displayed
H4-shaped hysteresis loop, implying the existence of the
36
micropores in the sample. However, N adsorption–desorption
2
Fig. 5 (A) SEM and (B) HR-TEM image of fresh NiCu(1 : 4)–CeO
catalyst.
2
isotherms of NiO–CeO2 and NiCu(1 : 4)–CeO2 oxide demon-
strated typical type IV isotherms and an uptake of N -adsorbed
2
volume reected in the adsorption branches at the relative
pressure range p/p
nitrogen into mesopores, which is a characteristic feature of
ordered mesoporous material. This result is consistent with the
SAXS shown in Fig. 3. NiO–CeO and NiCu(1 : 4)–CeO samples
displayed an H3-shaped hysteresis loop, implying the presence
o
¼ 0.5–1.0, due to capillary condensation of
represents the hexagonal lattice fringe pattern with a d-spacing
value of 0.29 nm corresponding to the ceria oxide phase.
37
2
2
Catalytic performance for WGS reaction
36
of sheet-like aggregates of metal oxides. Interestingly, a
signicant change in the surface area and textural properties of
The catalytic activities of mesoporous NiCu–CeO oxide cata-
2
lysts were compared with the monometallic catalysts for WGS
the CeO
Cu- into CeO
Ni- led to a decrease in surface area. In contrast, NiCu–CeO
oxides exhibited higher surface area than pure Ni–CeO , and the
2
was observed upon doping with Ni- or Cu-. Doping of
ꢀ
reaction in the temperature range of 350–550 C, the results of
2
increased the surface area; however, doping with
which are shown in Fig. 6A. Nickel-based catalysts were found to
2
2
surface area was found to further increase with greater Cu-
loadings (Table 1). Enhanced surface areas could be correlated
to formation of ordered mesoporous structure of the sample
with increased Cu- loadings in the case of bimetallic catalysts.
Fig. 5 shows SEM and HR-TEM images of the fresh meso-
2
porous NiCu(1 : 4)–CeO oxide. The SEM image displayed
agglomerated particles having uniform spherical morphology
with the diameter in the range of 8–12 nm, which is well
matched with the XRD result. Fig. 5B shows the lattice fringe
patterns of the fresh catalyst, which exhibits the interplaner
spacing of 0.28 nm and 0.31 nm corresponding to the (1 0 0) and
37
(
1 1 1) planes of the ceria cubic phase, while 0.19 nm corre-
sponds to the monoclinic phase of the CuO. The inset in Fig. 5B
Fig. 6 (A) CO conversion and (B) selectivity to CH
4
as a function
Fig. 4
NiO–CeO
N
2
-adsorption/desorption isotherms of (a) CuO–CeO
, and (c) NiCu(1 : 4)–CeO catalysts.
2
, (b) of reaction temperature (H O/(CH + CO + CO
2
4
2
) ¼ 2.0; GHSV ¼
ꢁ
1
2
2
83 665 h ).
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2
be more highly active than Cu–CeO . The higher activity of the
catalysts was related to higher CO adsorption efficiency of Ni-
23
ꢀ
than the Cu-. The catalytic activity at 350 C occurs in the
following order: CuO–CeO < NiCu(1 : 4)–CeO < NiCu(1 : 2)–
2
2
CeO
2
< NiCu(1 : 1)–CeO
2
< NiO–CeO
2
. This observation indi-
cates that CO conversion was directly proportional to the nickel
loadings. In general, Ni supported on metal oxide is a well-
38
known catalyst for methane formation. Since formation of
CH consumes H , the higher the CH yield, the lower the H
4
2
4
2
yield in the WGS reaction. Fig. 6B shows methane selectivity
during WGS reaction for all prepared catalysts as a function of
reaction temperature. Fig. 6A and B display that Ni–CeO cata-
2
lyst was more active but gave the highest selectivity to
ꢀ
CH
4
(28%) at 350 C. In contrast, selectivity to CH
4
was negli-
catalysts at all
reaction temperatures employed. For NiCu–CeO oxide cata-
lysts, selectivity to CH was found to progressively decrease with
2 2
gible for the Cu–CeO and NiCu(1 : 4)–CeO
2
4
increases in Cu- loadings (Fig. 6B). Among the NiCu–CeO oxide
2
catalysts, NiCu(1 : 4)–CeO was found to be a better catalyst in
2
the WGS reaction, where we observed that the addition of only
6
wt% of Ni– to Cu–CeO
to 65%, with complete selectivity to CO
catalysts showed reduction in CH formation compared to
Ni–CeO that may be related to formation of a copper-rich
2
increased the CO conversion from 30%
2
and H . NiCu–CeO
2
2
4
2
surface on the top of the catalyst aer activation that sup-
pressed the methanation reaction. This fact is also supported by
XPS analysis of the fresh NiCu(1 : 4)–CeO2 catalyst (Fig. 7).
Based on the area calculation we observed that the surface of
the NiCu(1 : 4)–CeO
Cu, which is almost equal to the 1 : 4 ratio of Ni/Cu into the Fig. 7 XPS spectrum of fresh (A) Cu 2p3/2 and (B) Ni 2p3/2 of
NiCu(1 : 4)–CeO catalyst.
2
oxide was composed of 21% Ni and 79%
catalyst (Table S1†). The HR-TEM result of the NiCu(1 : 4)–CeO
2
2
catalyst also showed the Cu-rich surface with the (2 0 2) plane of
the CuO.
The importance of the catalyst's mesoporous architecture catalyst, resulting in enhanced activity of the WGS at high
was proven by comparing reaction results with the same cata- temperatures. In the case of the NiCu-oxide without CeO
,
2
lytic composition, that is, Ni/Cu(1 : 4), but prepared through the conversion of CO increased with rising temperature up to
ꢀ
impregnation method (I–NiCu/CeO
NiCu oxide). Fig. 8 depicts the CO conversion prole as a
function of reaction temperature over mesoporous NiCu(1 : 4)–
CeO , I–NiCu/CeO and NiCu-oxide catalysts. It was found that
2
) and also without ceria 400 C; aer that the CO conversion began to decrease notably,
(
2
2
the CO conversion increased with increasing temperature, but
the shape of conversion versus temperature curve was affected
by the catalyst nature reecting the inuence of catalyst
composition. The CO conversion over mesoporous NiCu(1 : 4)–
ꢀ
CeO
2
catalyst was approximately four times higher at 350 C as
compared to the catalyst prepared by the impregnation method
2
ꢁ1
(
I–NiCu(1 : 4)/CeO
2
, surface area ¼ 14 m g ). The higher WGS
activity of the NiCu(1 : 4)–CeO
2
catalyst could be explained by
the presence of high surface area and mesoporous nature of the
catalysts, which can allow uniform distribution of the active
sites on the catalyst surface. TPR results showed that the
NiCu(1 : 4)–CeO could be reduced at lower temperature than
2
2
the I–NiCu(1 : 4)/CeO catalyst (Fig. S1†). This indicates that the
mesoporous architecture of the catalysts helps to enhance the
4
+
3+
redox capability of the Ce /Ce via formation of nanocrystallite
CeO , facilitating oxygen mobility. As a consequence, the EISA
method helps to increase reducibility of NiCu–CeO
oxide + CO
Fig. 8 CO conversion as a function of reaction temperature over
NiCu(1 : 4), I–NiCu–CeO , and NiCu–CeO catalysts (H O/(CH + CO
) ¼ 2.0; GHSV ¼ 83 665 h ).
2
2
2
2
4
ꢁ1
2
2
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2 2 2
CeO and more selective than Ni–CeO , NiCu(1 : 1)–CeO and
NiCu(1 : 2)–CeO catalysts towards the WGS reaction. The higher
2
selectivity of the NiCu(1 : 4)–CeO2 catalyst was due to the
formation of Cu- rich surface that suppressed the methanation,
which was also supported by the XPS and HR-TEM results.
Activity comparison results between the NiCu–CeO
CeO and NiCu-oxide catalysts exhibited that the mesoporous
NiCu–CeO was highly active compared to I–NiCu–CeO and
NiCu-oxide catalysts. The higher activity of NiCu–CeO was due to
2
, I–NiCu–
2
2
2
2
the lowest reduction temperature and mesoporous architecture
of the catalyst. The catalyst without ceria (NiCu-oxide) was least
active and showed deactivation at higher temperature. N2-
adsorption/desorption and SAXS measurements showed that
CuO–CeO
the surface area as well as mesoporosity of the samples. H
of the NiCu(1 : 4)–CeO exhibited the lowest reduction tempera-
2
has a microporous nature, and doping of Ni increased
2
-TPR
2
2
Fig. 9 CO conversion with time on stream over NiCu(1 : 4)–CeO ,
ture peak for the highly dispersed CuO compared to pure CuO–
I–NiCu/CeO
T ¼ 450 C; GHSV ¼ 83 665 h ).
2
and Cu–CeO
2
catalysts (H
2
O/(CH
4
2
+ CO + CO ) ¼ 2.0;
ꢀ
ꢁ1
CeO catalysts, which could be the result of strong interaction
2
between the metal oxide and CeO . Furthermore, the NiCu(1 : 4)–
2
CeO catalyst exhibited stable activity with 80% CO conversion
reaching 0% at 550 C (Fig. 8). The decrease in activity at higher aer 45 h on stream at a high GHSV of 83 665 h . The
2
ꢀ
ꢁ1
2
temperatures was due to sintering of the catalyst, which may lead NiCu(1 : 4)–CeO catalyst should be considered a promising
2
ꢁ1
to substantial reduction in surface area (from 11 to 0.6 m g ). catalyst for the high-temperature WGS reaction.
The poor performance of the bare NiCu-oxide in the WGS reaction
could be due to its lower capability for water dissociation, which
Experimental section
is one of the important steps in the WGS reaction assisted by
39
ceria. Supporting the NiCu oxide on CeO
2
facilitates the water Catalyst preparation
dissociation, and easily continues the catalytic cycle in the
production of hydrogen as we observed in the case of
2
The mesoporous NiCu–CeO oxide catalysts were prepared by
the evaporation-induced self-assembly method using triblock
NiCu(1 : 4)–CeO catalyst. Furthermore, ceria possesses oxygen
2
copolymers (EO) (PO) (EO) , P123. A series of NiCu–CeO
2
2
0
70
20
vacancies, which stabilize the transition metal nanoparticles
supported on oxide surfaces against sintering. Our results
clearly demonstrate that presence of ceria in the catalyst
formulation is absolutely necessary for activity and stability in
the WGS reaction.
oxide catalysts were prepared by varying the weight percentage
ratio of Ni/Cu from (1 : 1) to (1 : 4). The amount of CeO was
2

xed to 70 wt% in all cases. In a typical synthesis, solution “A”
was prepared by adding 1.0 g of P123 in 10 ml of iso-butanol.
Similarly, solution “B” was made with the combination of 10 ml
of iso-butanol, 1.0 g P123 and corresponding nitrates precursors
of copper and nickel. Both solutions A and B were allowed to stir
separately for 1 h. Meanwhile, 4.32 g of Ce(NO ) $6H O was
For possible industrial applications, both activity and
stability of the catalyst are necessary. To compare stability of
NiCu(1 : 4)–CeO , I–NiCu(1 : 4)/CeO and Cu–CeO catalysts in
2
2
2
3
3
2
ꢀ
WGS, CO conversion data were collected at 450 C and at a very
3
dissolved into 10 ml of iso-butanol and 1.6 ml of 67 wt% HNO .
ꢁ
1
high GHSV of 83 665 h for 45 h; corresponding results are
Once dissolved it was added to solution A, and then solution B
was also mixed with solution A. To get a homogeneous solution,
the entire mixture solution was kept 5 h for stirring. Aer that
displayed in Fig. 9. Under these conditions, the NiCu(1 : 4)–
CeO
2
catalyst had higher stability than I–NiCu(1 : 4)/CeO
2
and
Cu–CeO
2
catalysts, showing only a slight decrease in the CO
ꢀ
the gel formed was kept at 120 C for 48 h in an oven for solvent
conversion (85–80% for CO conversion aer 45 h). This result
proved the potential application of the catalyst's mesoporous
architecture, which makes it highly active, and the doping of Ni
evaporation. The nal catalyst NiCu–CeO
2
was obtained aer
and NiO–CeO were
ꢀ
calcination at 500 C for 5 h. The CuO–CeO
2
2
also prepared by the same method. The NiCu-oxide (without
ceria) was prepared by using only solution “B,” keeping
remaining conditions the same. NiCu(1 : 4)-oxide supported on
2
stabilizes the NiCu(1 : 4)–CeO oxide during the HT-WGS
reaction.
ceria (I–NiCu/CeO ) was also prepared by the impregnation
2
Conclusions
method. CeO used for this purpose was prepared by the
2
2
ꢁ1
4
precipitation method having a surface area of 117 m g
.
A series of mesoporous NiCu–CeO
NiO–CeO catalysts were prepared by the evaporation-induced
self-assembly method using a non-ionic P123 template to
2 2
oxides and CuO–CeO and
2
Catalyst characterization
understand the effect of mesoporosity on catalytic activity and X-ray diffractograms of the catalysts were recorded in the 2q
ꢀ
selectivity towards the HT-WGS reaction. Among these catalysts, range of 20–80 using a Rigaku D/MAX-IIIC diffractometer (Ni-
2
NiCu(1 : 4)–CeO was found to be more highly active than Cu– ltered Cu-Ka radiation, 40 kV, 100 mA). The crystallite size
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4
0,41
was estimated using the Debye–Scherrer equation.
Small
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angle X-ray scattering (SAXS) was collected on the same
instrument over a 2q range of 0.3–5 . The BET surface area and
the type of isotherm were determined by the N adsorption/
ꢀ
2
desorption method at 77 K using an ASAP 2010 Micromeritics.
Hydrogen-temperature programmed reduction (H -TPR) exper-
2
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H
2
-TPR was performed using 10% H
2
in Ar with a heating rate of
ꢀ
ꢁ1
ꢀ
10 C min , from room temperature to 800 C. The sensitivity
of the detector was calibrated by reducing a known weight of
42,43
NiO.
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reactor with an inner diameter of 4 mm. The catalyst charge was
.035 g. A T-union was employed at the exit of the quartz reactor
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N. L ´o pez and D. Teschner, ACS Catal., 2013, 3, 2256.
0
to install a thermocouple. A thermocouple was inserted into the 14 N. Pal, E.-B. Cho and D. Kim, RSC Adv., 2014, 4, 9213.
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2
/N
2
ꢁ1
16 M. A. Henderson, C. L. Perkins, M. H. Engelhard,
ꢀ
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gas consisted of 17.02 vol.% CO, 9.55 vol.% CO
2
, 1.03 vol.%
752.
CH , 13.14 vol.% H , 55.20 vol.% H O, and 4.06 vol.% N , which 18 Q. Fu, H. Saltsburg and M. Flytzani-Stephanopoulo, Science,
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2
2
2
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44,45
enter the WGS reactor in a waste gasication system.
feed H O/(CH + CO + CO
avoid coke formation. The GHSV of 83 665 h was used to 20 D.-W. Jeong, W.-J. Jang, J.-O. Shim, W.-B. Han, H.-S. Roh,
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4
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) ratio was intentionally xed at 2.0 to
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46
chromatograph.
62, 201.
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2
2
2
2
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Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Science, ICT and Future Planning
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(2013R1A1A1A05007370). This work is nancially supported by
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Korea Ministry of Environment (MOE) as “Knowledge-based
environmental service (waste to energy recycling) human
resource development project”.
7 Q. Wu, L. D. L. Duchstein, G. L. Chiarello, J. M. Christensen,
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