One-step synthesis of ordered mesoporous CoAl2O4 spinel-based metal oxides for CO2 reforming of CH4

Article abstract of DOI:10.1039/c5ra07249b

A series of ordered mesoporous CoAl₂O₄ spinel-based metal oxides with different Co content were facilely synthesized via a one-step evaporation-induced self-assembly method and utilized as catalysts for CO₂ reforming of CH₄. These mesoporous catalysts, which have unique textural properties and outstanding thermal stabilities, exhibited excellent catalytic performances under various reaction conditions. The ordered mesostructures in the catalysts could provide sufficiently accessible Co active sites for gaseous reactants and stabilize Co metallic nanoparticles via confinement effects within the mesoporous frameworks. These mesoporous catalysts exhibited better anti-coke abilities than non-mesoporous catalysts or traditional supported catalysts, thus acting as a promising series of potential catalyst candidates for CO₂ reforming of CH₄.

Full text of DOI:10.1039/c5ra07249b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
One-step synthesis of ordered mesoporous
CoAl2O4 spinel-based metal oxides for CO2
Cite this: RSC Adv., 2015, 5, 48256
reforming of CH4†
Leilei Xu,ab Jian Zhang,a Fagen Wang,ad Kaidi Yuan,c Liangjun Wang,c Kai Wu,be
abcd
Guoqin Xuabd and Wei Chen
*
A series of ordered mesoporous CoAl2O4 spinel-based metal oxides with different Co content were facilely
synthesized via a one-step evaporation-induced self-assembly method and utilized as catalysts for CO2
reforming of CH4. These mesoporous catalysts, which have unique textural properties and outstanding
thermal stabilities, exhibited excellent catalytic performances under various reaction conditions. The
ordered mesostructures in the catalysts could provide sufficiently accessible Co active sites for gaseous
reactants and stabilize Co metallic nanoparticles via confinement effects within the mesoporous
frameworks. These mesoporous catalysts exhibited better anti-coke abilities than non-mesoporous
catalysts or traditional supported catalysts, thus acting as a promising series of potential catalyst
candidates for CO2 reforming of CH4.
Received 21st April 2015
Accepted 13th May 2015
DOI: 10.1039/c5ra07249b
www.rsc.org/advances
gases (CO2 and CH4) can be simultaneously transformed into
the value-added synthesis gas resource (H2 + CO).7–14
1 Introduction
In comparison with other synthesis gas production
processes, such as steam reforming of methane (SRM, CH4 +
H2O ¼ CO + 3H2, DH ¼ +206 kJ molꢀ1) and partial oxidation of
methane (POM, 2CH4 + O2 ¼ 2CO + 4H2, DH ¼ ꢀ44 kJ molꢀ1),
the present CRM process can generate syngas with a low H2/CO
ratio, i.e., close to unity (1/1), which is highly desirable for
downstream chemical processes such as hydroformylation and
oxo synthesis.12,15–17 However, the highly endothermic nature of
this CRM process requires the reaction to be carried out at high
In the past decades, the excessive use of fossil fuels has caused
severe environmental problems such as increasing concentra-
tions of greenhouse gases (mainly CO2) in the atmosphere.1,2
Attempts to efficiently reduce CO2 levels have recently attracted
worldwide attention. Moreover, CO2 can be considered as a
valuable and sustainable carbon source for the synthesis of
value-added chemical fuels. Intensive research efforts have
been devoted to the development of efficient CO2 reduction and
conversion technologies.3,4 Among the strategies proposed for
converting CO2 into higher energy intermediates, heteroge-
neous catalysis-based processes are of great interest for their
ease of process scalability.5,6 Among them, the catalytic carbon
dioxide reforming of methane (CRM, CH4(g) + CO2(g) ¼ 2CO(g)
+ 2H2(g), DH298 ¼ +247 kJ molꢀ1) has been considered as one of
the most promising technologies for large scale CO2 reduction
and utilization. Moreover, under this process, two greenhouse
ꢁ
temperatures (>600 C) to achieve acceptable conversion rates.
Under such high temperatures severe thermal sintering of the
metallic active sites can result.11,18 The CRM process is oen
accompanied by undesirable side reactions such as methane
decomposition reaction (CH4(g) ¼ C(s) + 2H2(g), DH298
¼
+75 kJ molꢀ1) and Boudouard reaction (2CO(g) ¼ C(s) + CO2(g),
DH298 ¼ ꢀ172 kJ molꢀ1). Coke formed in these side reactions
can also passivate the active centers.9,12,17,19 Both factors can
cause the rapid deactivation of the catalyst, not allowing the
CRM process for large-scale industry applications. Therefore, it
is highly desirable to rationally design an efficient catalyst with
high resistance to thermal sintering and carbon deposition.
It has been widely reported that most of the group VIII
transition metals are active towards the CRM reaction.12,20–22
Among these metals, noble metals (such as Pt, Rh, Pd, and Ir)
usually have higher catalytic activity and anti-coke capacity than
non-precious metals (such as Ni and Co). However, it is more
practical to develop Ni- and Co-based catalysts for scalable
commercial applications due to their lower cost and inherent
availability.23–25 Supported Ni catalysts have been widely
aDepartment of Chemistry, National University of Singapore, 3 Science Drive 3,
117543, Singapore, Singapore. E-mail: phycw@nus.edu.sg; Fax: +65-6777-6126; Tel:
+65-6516-2921
bSingapore-Peking University Research Center for a Sustainable Low-Carbon Future, 1
CREATE Way, #15-01, CREATE Tower, Singapore 138602, Singapore
cDepartment of Physics, National University of Singapore, 2 Science Drive 3, 117551,
Singapore, Singapore
dNational University of Singapore (Suzhou) Research Institute, Suzhou, China
eCollege of Chemistry and Molecular Engineering, Peking University, Beijing 100871,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra07249b
48256 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
investigated because of their high initial activity that is
comparable with noble catalysts. However, Ni-based catalysts
are prone to carbon deposition over the catalyst surface due to
their strong C–H bond cleavage ability, leading to rapid cata-
lyst deactivation and blockage of the xed bed reactor.7,14,23 It
was found that the introduction of Co into a Ni-based catalyst
could signicantly suppress the carbon deposition during the
CRM reaction.7,19,26–28 Recently, it has been revealed that Co
showed interesting catalytic performance in the CRM
reaction.11,21,29–31
For Co-based catalysts, it is also very challenging to
prevent catalyst deactivation derived from thermal sintering
of the metallic active sites and carbon deposition.23 Based on
thermodynamic calculations, the methane decomposition
reaction can only be initiated at temperatures above 553 ꢁC
and the Boudouard reaction can take place at temperatures
below 700 ꢁC. Therefore, it is impossible to simultaneously
inhibit the occurrence of these two carbon deposition reac-
tions under typical CRM reaction conditions.12,32 Moreover,
the anti-coke performance of the catalyst is closely related to
the size of the Co metallic active sites. Large Co metal
ensembles easily stimulate carbon deposition, and therefore
the thermal sintering of the metallic active sites plays an
important role in carbon deposition during the CRM
process.33–36 However, under typical CRM reaction conditions,
the sintering of the Co particles can easily take place, making
it a big challenge to restrict Co metallic active centers within
nanometer size.
Traditionally, small particles can be obtained by the
impregnation of a porous support with large surface area in a
precursor-containing solution.37,38 However, it is difficult to
control metal dispersion because metallic particles have a
strong tendency to aggregate at high temperatures.18 Intensive
research efforts have been made to conne Co active sites in
precursors with dened structures such as spinel (CoAl2O4) and
solid solutions (Co–Mg–O).34,39,40 The formation of a strong
interaction between Co and the lattice structure can signi-
cantly improve the anti-sintering performance of the Co active
centers. However, these materials usually could not provide
enough accessible surface Co active sites because most of the
active metal species are homogeneously dispersed in the bulk.
To address these challenges, a series of ordered meso-
porous CoAl2O4 spinel-based metal oxides with different Co
content were designed and facilely synthesized via a one-step
2 Experimental
2.1 The synthesis of the ordered mesoporous CoAl2O4 spinel
based metal oxide
Ordered mesoporous CoAl2O4 spinel-based metal oxides with
different Co content were synthesized via a one-step evapora-
tion induced self-assemble (EISA) strategy.41,42 In a typical
synthesis procedure, approximately 1.0 g (EO)20(PO)70(EO)20
triblock copolymer (Pluronic P123, Mn ¼ 5800, Sigma-Aldrich)
was dissolved in 20.0 mL absolute ethanol with vigorous stir-
ring. 1.6 mL of 67.0 wt% concentrated nitric acid, 10 mmol
aluminum isopropoxide (>98%, Sigma-Aldrich), and quantita-
tive Co(NO3)2$6H2O (Sigma-Aldrich) were then sequentially
added. The pink transparent homogeneous sol was transferred
to a Petri dish aer stirring for at least 5 h at room temperature,
and then dish was placed into a temperature and humidity
controlled chamber at 60 ꢁC with 50% relative humidity to
undergo solvent evaporation for 48 h. A pink xerogel was
obtained aer EISA process. Calcination was carried out by
slowly increasing temperature from room temperature to 700 ꢁC
with 1 ꢁC minꢀ1 ramping rate and maintaining this nal
temperature for 5 h in air. The nal ordered mesoporous
materials were labeled as OMA-xCo, where x refers to the molar
percentage between Co and Al (x mol% ¼ nCo/nAl ꢂ 100%, x ¼ 3,
5, 7, 10, and 15, nAl ¼ 10 mmol).
To investigate the role of ordered mesostructures in
promoting catalytic properties, CoAl2O4 spinel-based metal
oxides without ordered mesostructures were also prepared. The
obtained non-mesoporous materials were denoted as NPA-xCo,
where the meaning of x was the same as that in OMA-xCo. The
method of preparing NPA-xCo was similar to that of OMA-xCo,
only without the addition of the Pluronic P123 template.
To compare the catalytic properties of the present OMA-xCo
catalysts with a traditional supported catalyst, a Co-based cata-
lyst supported on gamma-alumina (g-Al2O3) was prepared via an
incipient wetness impregnation method using Co(NO3)2$6H2O
(Sigma-Aldrich) as the precursor of cobalt. Aer impregnation,
ꢁ
the catalyst precursors were dried in an oven at 60 C for 24 h.
Finally, the catalyst precursors were calcined at 700 C for 5 h.
The obtained catalysts were denoted as x%Co/Al2O3, where x
refers to the molar percentage between Co and Al.
ꢁ
2.2 Catalyst characterizations
evaporation-induced self-assembly method. These catalysts X-ray diffraction (XRD) patterns were recorded on an X'pert Pro
possess outstanding structural properties and thermal multipurpose diffractometer (PANalytical, Inc., Netherlands)
stability. The Co species were homogeneously dispersed with Ni-ltered Cu Ka radiation (0.15046 nm). The N2 adsorp-
among the mesoporous matrix in the form of CoAl2O4 spinel, tion and desorption analyses were carried out using a NOVA
ꢁ
and thus the Co nanoparticles could be effectively stabilized 2200e (Quantachrome, US) at ꢀ196 C. Transmission electron
via the connement effect. This effectively suppressed the microscopy (TEM), selected area electron diffraction (SAED),
severe sintering of Co nanoparticles under CRM conditions. and energy-dispersed spectroscopy (EDS) measurements were
Furthermore, these mesoporous catalysts with large surface performed on a JEOL 2010F microscope (Japan) under a
areas, big pore volume, and uniform pore channels can ensure working voltage of 200 kV. X-ray photoelectron spectroscopy
and provide sufficient Co active sites for the gaseous feed- (XPS) analyses of the catalysts were performed on a VG ESCALAB
stock. This series of materials is promising as ideal catalysts 210 (Thermo Scientic, US) spectrometer. The XPS data were
towards CRM reaction and even other catalytic reactions such referenced to the carbon species with C 1s at the binding energy
as CO2 methanation.
of 284.8 eV. H2 temperature programmed reduction (H2-TPR)
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48256–48268 | 48257

View Article Online
RSC Advances
Paper
measurements for the as-prepared catalysts (100 mg) were increase of Co content from 3% to 15%. In contrast, the
performed on a ChemBET Pulsar TPR/TPD (Quantachrome, US) diffraction peaks for g-Al2O3 became weaker until they
utilizing 5 mol% H2/He steam (70 mL minꢀ1) as a reducing completely disappeared for OMA-15Co. This indicates that the
agent and the hydrogen consumption was recorded using a TCD incorporation of Co species into the framework is benecial for
detector. Temperature programmed hydrogenation (TPH) phase transformation toward CoAl2O4 by facilitating the
characterizations of the spent catalysts were carried out on the following reaction: CoO + Al2O3 ¼ CoAl2O4. However, it was very
same apparatus as described for H2-TPR. The long-term difficult to accurately calculate the crystallite size of CoAl2O4
endurance-tested cataꢀly1st (30 mg) was submitted to a heat spinel species by the Scherrer formula due to the broadening of
treatment (10 ꢁC min up to 900 ꢁC) in a gas ow (60 mL diffraction peak, which implied a high dispersion of Co species
minꢀ1) of the 5 mol% H2/He mixture. Thermogravimetric (TG) within the mesoporous skeleton.
analysis was performed using a TA instrument 2960 (Discovery,
Comparative studies for OMA-10Co, NPA-10Co, and 10%Co/
US) from room temperature to 900 ꢁC with a heating rate of Al2O3 catalysts prepared with different methods were also
10 C minꢀ1 under air stream.
carried out and their XRD patterns are displayed in Fig. 1(3).
OMA-10Co, synthesized with P123 template, presented weak
CoAl2O4 and g-Al2O3 diffraction peaks. As a comparison,
NPA-10Co without any template only displayed apparent
CoAl2O4 diffraction peaks that were considerably stronger in
intensity than those of OMA-10Co. This suggested that the
presence of P123 template was not favorable for phase trans-
formation toward CoAl2O4. The 10%Co/Al2O3 supported cata-
lyst displayed characteristic diffraction peaks for Co3O4 (JCPDS
Card no. 74-2120) species and g-Al2O3 (JCPDS Card no. 10-0425),
which could be attributed to the thermal decomposition of the
Co(NO3)2$6H2O precursor and g-Al2O3 support, respectively.
Moreover, the positions of the diffraction peaks of CoAl2O4 and
Co3O4 crystalline phases almost overlapped, which is consistent
with previous studies.43–46 CoAl2O4 and Co3O4 had the same
spinel cubic (Fd3m) crystallographic structure with only a slight
ꢁ
2.3 Catalytic properties measurement
The reaction was performed in a vertical xed bed quartz
reactor at atmospheric pressure. The temperature of the reac-
tion bed was monitored by a coaxial thermocouple, which was
protected by a small quartz thermocouple well. The ow rate of
reactant gas was precisely regulated by mass ow controllers
(Beijing Sevenstar Electronics Co., Ltd., China). For each test,
100 mg of catalyst was loaded on the quartz xed-bed reactor.
Prior to the reaction, the catalyst was pretreated in situ by
reducing in a mixed ow of H2 and N2 (10/20 mL minꢀ1) with a
heating rate of 1.5 ꢁC minꢀ1 rate to 800 ꢁC, and maintaining at
800 ꢁC for 120 min. Before introducing the reaction gases
(CH4/CO2 ¼ 1, without dilution), the catalyst bed was purged
with N2 for half an hour to remove adsorbed hydrogen. The
reaction products were analyzed online by a gas chromatograph
(Agilent 7890B) equipped with an automatic six-port valve and a
TCD detector. A TDX-01 packed column was used for the anal-
ysis of gaseous products. The conversions of CO2 and CH4 were
calculated based on the balance of carbon. Their formulas are
listed as follows:
˚
difference in the lattice constant (a ¼ 8.111 A for CoAl2O4 and
˚
a ¼ 8.0885 A for Co3O4). Therefore, they displayed similar XRD
peak patterns, and the positions of the peaks were nearly same.
However, we expected that these catalysts would exhibit
different Co 2p XPS and H2-TPR proles due to the difference in
their chemical coordination environments, which will be dis-
cussed later.
We also compared the XRD patterns of the as-reduced
OMA-10Co, NPA-10Co, and 10%Co/Al2O3 catalysts at 800 ꢁC
for 2 h, as exhibited in Fig. 1(4). It could be observed that the
intensities of metallic Co diffraction peaks (JCPDS Card no.
15-0806) for as-reduced OMA-10Co and NPA-10Co were weaker
than those of 10%Co/Al2O3. For as-reduced OMA-10Co, the
diffraction peaks of CoAl2O4 disappeared. As a comparison, the
intensities of g-Al2O3 peaks became stronger aer the reduction
F
ꢀ F
in;CH4
in;CH4
out;CH4
XCH
¼
ꢂ 100%
4
F
F
ꢀ F
out;CO2
in;CO2
in;CO2
XCO
¼
ꢂ 100%
2
F
process. The process of CoAl2O4 reduction (CoAl2O4 + H2
¼
g-Al2O3 + H2O) could account for this and the high dispersion of
metallic Ni nanoparticles within the mesoporous framework.
However, for the as-reduced NPA-10Co, its main crystalline
3 Results and discussion
3.1 Characterizations of the fresh catalysts
3.1.1 XRD analysis. Fig. 1(1) and (2) display the small-angle phase remained as CoAl2O4 spinel and its diffraction peaks were
(0.5–5.0ꢁ) and wide-angle (20–80ꢁ) XRD patterns of the as- considerable stronger than those of metallic Co. This suggests
synthesized OMA-xCo catalysts calcined at 700 ꢁC, respec- that the reduction of CoAl2O4 spinel species is more difficult.
tively. As shown in the gure, each sample presented a strong The difference in reducibility between OMA-10Co and
diffraction peak around 0.8ꢁ and another weak peak centered at NPA-10Co could be attributed to their different structural
around 1.4ꢁ, implying the presence of p6mm hexagonal properties. Compared with NPA-10Co, the larger surface area,
symmetry. Besides, most of the samples possessed diffraction bigger pore volume, and unblocked mesopore channels of
peaks for g-Al2O3 (JCPDS Card no. 10-0425) and CoAl2O4 spinel OMA-10Co made the reduction of CoAl2O4 spinel easier because
(JCPDS Card no. 82-2252). It is worth noting that the intensity of the H2 molecules has easier access to surface species than bulk
the CoAl2O4 spinel diffraction peaks increased with the gradual material. The pattern of the 10%Co/Al2O3 supported catalyst
48258 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 1 (1) Small-angle and (2) wide-angle XRD patterns of the as-synthesized OMA-xCo catalysts: (a) OMA-3Co, (b) OMA-5Co, (c) OMA-7Co, (d)
OMA-10Co, and (e) OMA-15Co; (3) XRD patterns of the as-synthesized and (4) as-reduced catalysts prepared by different methods: (a) OMA-
10Co, (b) NPA-10Co, and (c) 10%Co/Al2O3.
exhibited obvious metallic Co and g-Al2O3 diffraction peaks, Table 1. All the OMA-xCo samples calcined at 700 ꢁC exhibited
implying that most Co3O4 species had been reduced.
large BET surface areas up to 207.5 m2 gꢀ1, big pore volumes
3.1.2 Nitrogen adsorption–desorption analysis. The up to 0.31 cm3 gꢀ1, and average pore diameters in the range of
nitrogen adsorption–desorption isotherms and pore size 9.8–12.8 nm, which are considerably better than those of
distributions for the as-synthesized OMA-xCo with different Co NPA-10Co and 10%Co/Al2O3.
molar fractions are displayed in Fig. 2(1) and (2), respectively.
To further verify the thermal stability of the ordered mes-
All the isotherms can be attributed to the type IV with ostructures of the OMA-xCo catalysts, the nitrogen adsorp-
H1-shaped hysteresis loops, which are the typical characteris- tion–desorption analyses of the as-reduced OMA-xCo catalysts
tics for the presence of mesopores with cylindrical shape. For were carefully carried out and their results are presented in
the adsorption isotherms with steep condensation steps, the Fig. S1(1) and (2) (ESI†). Identical to their corresponding
corresponding pore size curves displayed a very narrow steep precursors, the as-reduced samples also fully displayed type IV
condensation step, and the corresponding pore size distribu- isotherms with H1-shaped hysteresis loops and narrow pore
tion curves possessed a narrow distribution around 10.0 nm.
size distributions around 10.0 nm, demonstrating excellent
To investigate the role of the P123 template in the thermal stabilities. In addition, the isotherms and pore size
construction of ordered mesoporous channels, comparative distribution curves of the as-reduced NPA-10Co and 10%Co/
analyses between OMA-10Co and NPA-10Co were carried out Al2O3 are shown in Fig. S1(3) and (4) (ESI†), respectively.
and the results are displayed in Fig. 2(3) and (4). Unlike Similar to the as-synthesized samples, they also possessed type
OMA-10Co, which has a type IV H1-shaped hysteresis loop IV H3-shaped hysteresis loops and wide pore size distribu-
and a narrow pore size distribution, NPA-10Co exhibited a tions. These structural properties of the as-reduced catalysts
type IV H3-shaped hysteresis loop and a wide pore size are also summarized in Table 1. Compared with their corre-
distribution, suggesting the absence of uniform mesopores. sponding precursors, the specic surface areas and pore
This demonstrated that the P123 template plays a crucial role volumes suffered some decline aer H2 reduction treatment,
in constructing ordered mesopores. In addition, the N2 which might be caused by the shrinkage of the skeleton and
adsorption–desorption analysis result of 10%Co/Al2O3 sup- phase transformations at high temperature.
ported catalysts are also depicted in Fig. 2(3) and (4), showing
3.1.3 TEM analysis. To conrm the presence of the ordered
a type IV H3-shaped hysteresis loop and a wide pore size mesoporous channels, TEM characterization of OMA-xCo was
distribution around 6.0 nm. It is proposed that the mesopores carried out, as displayed in Fig. 3. For all the samples, the
in 10%Co/Al2O3 might be blocked due to the disordered alignment of the cylindrical shaped pores along the [1 1 0]
accumulation of g-Al2O3 support particles. The values of direction was observed. The SAED patterns of the ordered
specic surface areas, pore volumes, and average pore mesostructure domains are also provided as insets in Fig. 3.
diameters of the as-synthesized catalysts are summarized in Their diffraction rings are very fuzzy, indicating the low
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48256–48268 | 48259

View Article Online
RSC Advances
Paper
Fig. 2 Nitrogen adsorption–desorption isotherms ((1), (3)) and pore size distribution curves ((2), (4)) of the as-synthesized catalysts.
crystallinity of the mesoporous walls. This was consistent with
The TEM characterization of the as-reduced OMA-xCo cata-
previous XRD patterns that only exhibited weak diffraction lysts was also performed. The as-reduced OMA-3Co, OMA-5Co,
peaks. An EDS measurement of the ordered mesoporous region and OMA-10Co catalysts were chosen as representatives and
for OMA-10Co was conducted and its prole is shown in their TEM images are displayed in Fig. S2 (ESI†). The alignment
Fig. 3(g). The characteristic peaks for Co, Al, and O elements of cylindrical pores along the [1 1 0] direction for the as-reduced
could be clearly observed. This suggested that the Co element catalysts was successfully preserved aer the reduction process
ꢁ
was successfully introduced into ordered mesoporous alumina under severe conditions (800 C for 2 h), demonstrating excel-
matrix via the present one-step synthesis method.
lent thermal stability. Furthermore, there was no evident
agglomeration of metallic Co within the ordered mesoporous
channels, indicating the high dispersion of metallic Co species
within the whole mesoporous framework.
Table 1 Structural properties of the as-synthesized, as-reduced, and
60 h endurance-tested catalysts
3.1.4 XPS analysis. XPS measurements were used to deter-
mine the chemical oxidation state of the surface Co species in
as-synthesized OMA-xCo, NPA-10Co, and 10%Co/Al2O3. Their
Co 2p XPS proles are shown in Fig. 4. It is worth noting that in
Fig. 4(1) all the as-prepared OMA-xCo samples presented
similar peak shape proles. In particular, all the samples dis-
played an obvious Co 2p3/2 peak around 781.83 eV, a weak
shake-up satellite peak around 786.86 eV, and a Co 2p1/2 peak
around 798.40 eV. Compared with the Co 2p3/2 characteristic
binding energy values reported in the literature for cobalt
compounds, such as Co(OH)2 (780.9 eV), Co3O4 (780.0 eV), and
CoAl2O4 (781.0 eV),45,47–49 the oxidation state of Co species in the
present OMA-xCo was ascribed to Co2+ in the form of CoAl2O4
spinel, which is in good agreement with the XRD result in
Fig. 1(2).
Specic
surface area Pore volume Average pore
Isotherm
diameter (nm) type
Samples
(m2 gꢀ1
)
(cm3 gꢀ1
)
OMA-3Co
OMA-5Co
OMA-7Co
OMA-10Co
10Co/Al2O3
NPA-10Co
OMA-15Co
148.4
196.5
152.5
207.6
99.3
0.28
0.27
0.24
0.31
0.17
0.08
0.25
0.27
0.28
0.19
0.25
0.18
0.05
0.19
0.18
0.15
0.16
0.09
9.9
9.8
9.8
9.8
6.0
—
12.8
9.8
9.8
9.9
9.9
6.7
—
IV H1
IV H1
IV H1
IV H1
IV H3
IV H3
IV H1
IV H1
IV H1
IV H1
IV H1
IV H3
IV H3
IV H1
IV H1
IV H1
IV H3
IV H3
85.6
150.4
ARa-OMA-3Co 107.7
AR-OMA-5Co
AR-OMA-7Co
124.1
87.2
AR-OMA-10Co 133.8
AR-10Co/Al2O3 96.9
AR-NPA-10Co
AR-OMA-15Co
SP-OMA-7Co
77.5
89.2
Fig. 4(2) presents the comparative analyses of Co 2p XPS
proles for OMA-10Co, NPA-10Co, and 10%Co/Al2O3 catalysts
with the same Co loading. As for NPA-10Co, it exhibited similar
patterns to OMA-10Co in both peak position and shape. This
suggested that its surface Co species was also present in the
form of CoAl2O4 spinel, as veried by XRD analysis in Fig. 1. We
12.6
8.1
8.0
4.3
—
125.6
SPb-OMA-10Co 119.8
SP-10Co/Al2O3
SP-NPA-10Co
89.2
61.4
AR stands for “as-reduced”. b SP stands for “60 h spent”.
a
48260 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 3 TEM, SAED, and EDS measurements of the as-synthesized OMA-xCo catalysts: (a) OMA-3Co, (b) and (c) OMA-5Co, (d) OMA-7Co, (e) and
(g) OMA-10Co, and (f) OMA-15Co.
propose that during the one-step synthesis process, the presented similar hydrogen consumption proles in terms of
precursors of Co and Al species could mix thoroughly and shape regardless of Co content, displaying one pronounced H2
sufficiently react with each other. As a result, Co species could reduction peak in the region above 1080 ꢁC and one broad
enter into the bulk alumina matrix during the process of shoulder peak in the region of 746–937 ꢁC. There was no
ꢁ
calcination, promoting the formation of CoAl2O4 spinel via the reduction peak observed in the range of 300–450 C for all the
following process: CoO + Al2O3 ¼ CoAl2O4. However, for the mesoporous catalysts. This implied the absence of free CoO
10%Co/Al2O3 catalyst, its Co 2p spectrum was considerably and/or Co3O4 species that did not interact with the mesoporous
different from OMA-10Co in both peak position and shape. In framework.43,48,50 The XRD and XPS measurements discussed
particular, the Co 2p3/2 (780.33 eV) binding energy of 10%Co/ above conrmed the presence of CoAl2O4 spinel in the OMA-xCo
Al2O3 was considerably lower than that of OMA-10Co framework. CoAl2O4 spinel can only be reduced at high
(781.83 eV), which was attributable to a Co3O4 species.45,47,49 In temperatures due to the strong interaction between Co and the
addition, there was no shake-up satellite peak accompanying mesoporous framework.48,51,52 Therefore, these two reduction
the Co 2p3/2 peak for 10%Co/Al2O3, which is a typical feature for peaks in the different temperature regions should be ascribed to
the CoAl2O4 spinel species.48
the reduction of Co2+ from +2 to 0 valence state, which are in
3.1.5 H2-TPR analysis. To investigate the interaction different coordination environments. As for the reduction peaks
between Co species and mesoporous frameworks, the H2-TPR in the region of 746–937 ꢁC, they might be assigned to the
measurements of the as-synthesized OMA-xCo catalysts were reduction of surface Co species. On the one hand, the unsatu-
carried out, as shown in Fig. 5(1). The intensities of the H2 rated coordination of surface Co2+ made them easier to reduce;
reduction peaks gradually increased with the increase of Co on the other hand, the surface Co2+ was more easily accessible to
content for most of the samples, indicating the increase in H2 the H2 stream owing to their large surface areas and uniform
consumption. It is also noteworthy that most of the samples pore channels. As a result, they could be reduced in the relatively
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48256–48268 | 48261

View Article Online
RSC Advances
Paper
Fig. 5 (1) H2-TPR profiles of the OMA-xCo catalysts: (a) OMA-3Co, (b)
OMA-5Co, (c) OMA-7Co, (d) OMA-10Co, and (e) OMA-15Co; (2)
H2-TPR profiles of the catalysts prepared by different methods: (a)
OMA-10Co, (b) NPA-10Co, and (c) 10%Co/Al2O3.
Fig. 4 (1) XPS profiles of Co 2p for the as-synthesized OMA-xCo
catalysts: (a) OMA-3Co, (b) OMA-5Co, (c) OMA-7Co, (d) OMA-10Co,
and (e) OMA-15Co; (2) XPS profiles of Co 2p for the as-synthesized
catalysts prepared by different methods: (a) OMA-10Co, (b)
NPA-10Co, and (c) 10%Co/Al2O3.
(CoO or/and Co3O4). In the case of the 10%Co/Al2O3 catalyst, its
lower temperature region. The reduction peaks in the higher
temperature region above 1080 ꢁC could be ascribed to the
reduction of the saturated coordinated Co2+, which has strong
metal-support interactions with the mesoporous framework.
Furthermore, the comparative analysis of H2-TPR proles for
OMA-10Co, NPA-10Co, and 10%Co/Al2O3 catalysts are summa-
rized in Fig. 5(2). For the NPA-10Co catalyst, it presented
obvious reduction peaks in a high temperature region similar to
OMA-10Co. It displayed high reduction peaks in the high
temperature regions, which were also attributed to the reduc-
tion of CoAl2O4 spinel species in different coordination envi-
ronments. In addition, the small uptake peaks in the low
temperature region around 586 ꢁC could be attributed to the
reduction of Co oxides, which had weak interactions with the
bulk catalyst. This might be caused by the absence of a self-
assembly step around the P123 template during its synthesis.
As a result, the Co species could not be completely transformed
into CoAl2O4 and parts of Co species existed as cobalt oxides
prole displayed three obvious reduction peaks centered
around 607, 786, and 989 C, which could be attributed to the
ꢁ
rst step reduction of Co3O4 and subsequent reduction of CoO
species. The XRD and XPS analyses had conrmed the presence
of Co3O4 crystalline phase over 10%Co/Al2O3 supported cata-
lyst. The reduction peak at 607 ꢁC could be attributed to the rst
step reduction of Co3O4 (I) (Co3O4 (I) + H2 ¼ 3CoO (I) + H2O),
which had weak interactions with the Al2O3 support. The peak
ꢁ
centered at 786 C, however, might stem from both the subse-
quent reduction of CoO (I) (CoO (I) + H2 ¼ Co + H2O) and the
rst step reduction of Co3O4 (II) that was strongly interacted
with the support. The last peak around 989 ꢁC was attributed to
the further reduction of CoO (II) derived from Co3O4 (II), which
also had strong interactions with the Al2O3 support.
3.2 Catalytic performances
3.2.1 Effect of reaction temperature. To investigate the
inuence of reaction temperature, the catalytic performance of
48262 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
CRM reaction over OMA-xCo catalysts with different Co content the decrease in the residence time over the catalyst surface and
at different temperatures under particular conditions (CH4/CO2 the limited number of active sites with respect to the increasing
¼ 1, GHSV ¼ 15 000 mL (gꢀ1 hꢀ1), 1 atm) were investigated and amount of reactants were the main reasons. With the increase
the results are summarized in Fig. 6(1) and (2). The conversions of GHSV, the velocity for the gaseous reactants through the
of CH4 and CO2 were greatly promoted with an increase in the catalyst bed became faster and residence time over the catalyst
reaction temperature. Typically, they displayed the highest bed became shorter. However, as GHSV increased, the number
activity at 800 ꢁC, regardless of Co content due to the intensively of reactants moving through the catalyst bed largely increased,
endothermic nature of the CRM reaction.12 The relationship making the active sites insufficient for redundant reactants. The
between catalytic activity and Co content for OMA-xCo catalysts effect of Co content on the catalytic activity for OMA-xCo cata-
was also studied. With an increase in Co content, the conver- lysts under different GHSV could also be investigated. Among
sions of CH4 and CO2 rapidly increased in the initial stage and the OMA-xCo catalysts under given GHSV, OMA-10Co still
reached a maximum value at 10%Co content for all the possessed the highest CH4 and CO2 conversion among all the
temperature stages. This could be attributed to the increasing catalysts studied. It was of great interest to observe that the
number of Co active sites, providing the gaseous feedstock with OMA-10Co always displayed the best catalytic activity among the
more accessible metallic active sites under similar reduction OMA-xCo catalysts under different GHSVs and at different
conditions. However, the higher content of Co up to 15% caused temperatures.
a sharp drop in CH4 and CO2 conversion, which most likely
appeared due to the thermal agglomeration of the Co active role of the ordered mesoporous framework in promoting CRM
centers and hence the decrease of active sites. catalytic activity, the comparative analyses of OMA-10Co,
3.2.3 Effect of ordered mesostructure. To investigate the
3.2.2 Effect of gas hourly space velocity (GHSV). The NPA-10Co, and 10%Co/Al2O3 catalysts with identical Co
inuence of gas hourly space velocity (GHSV) on the conver- content were systematically carried out under different reaction
sions of CH4 and CO2 was investigated under the given condi- conditions. As shown in Fig. 7, CH4 and CO2 conversions for the
tions (CH4/CH4 ¼ 1, 750 ꢁC, 1 atm) over OMA-xCo catalysts. As OMA-10Co mesoporous catalyst were considerably higher than
shown in Fig. 6(3) and (4), with an increase of feed ow from those for NPA-10Co and 10%Co/Al2O3 catalysts under different
7200 to 45 000 mL (gꢀ1 hꢀ1), the conversions of CH4 and CO2 reaction conditions. This demonstrated that the ordered
rapidly decreased (e.g. decreased from 88.9% to 60.5% for the mesostructure could signicantly promote catalytic activity.
CH4 conversion over OMA-10Co) for all the catalysts investi- Compared with NPA-10Co, which does not have any obvious
gated. It has been proposed that the reduction in the residence mesoporous structure, the mesoporous catalyst possessed
time of the reactants on the catalyst surface and the drop in larger surface area, bigger pore volume, and more barrier-free
catalyst bed temperature could result in these phenomena.53 pore channels. In contrast, most of the Co species were
However, for the present OMA-xCo catalysts, we propose that located in the bulk of the material for NPA-10Co. Therefore, the
Fig. 6 The curves of (1) CH4 conversion and (2) CO2 conversion versus reaction temperature for OMA-xCo catalysts with different Co content;
reaction conditions: CH4/CO2 ¼ 1, GHSV ¼ 15 000 mL (gꢀ1
h
ꢀ1), 1 atm. The curves of (3) CH4 conversion and (4) CO2 conversion versus gas
hourly space velocity (GHSV) for OMA-xCo catalysts with different Co content; reaction conditions: CH4/CO2 ¼ 1, 750 ꢁC, 1 atm.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48256–48268 | 48263

View Article Online
RSC Advances
Paper
mesoporous catalysts could provide much more exposed Co Previous studies have revealed that metallic active centers with
active centers for the gaseous reactants, which accounted for smaller sizes possess strengthened capacity of coke suppres-
the higher catalytic activity of OMA-10Co over that of NPA-10Co. sion, also known as the size effect.33,34 For the OMA-10Co and
The 10%Co/Al2O3 supported catalyst was prone to severe OMA-7Co catalysts, the growth of the Co particles was effec-
thermal agglomeration of the metallic Co active sites during the tively suppressed by the connement effect of the mesoporous
processes of catalyst reduction and CRM reaction due to the framework during the processes of catalyst reduction and
weak interactions between metal and support. At this point, the CRM reaction, thus leading to good catalytic stability. For
OMA-10Co catalyst presented better performance than 10%Co/ NPA-10Co, the absence of ordered mesostructure might cause
Al2O3. The Co species could be effectively conned by the thermal sintering of the metallic active centers. At the same
mesoporous framework due to the advantage of the one-step time, due to the weak interaction between metal and support,
synthesis strategy, which effectively stabilized the Co metallic the thermal agglomeration of the Co species could take place
active sites against severe reduction and CRM reaction condi- on the 10%Co/Al2O3 supported catalyst, thus leading to a
tions. Therefore, the decrease in the number of Co active gradual continuous deactivation. The large metallic clusters
centers caused by thermal sintering could be effectively avoided facilitated methane decomposition and carbon deposition,
to some degree, which accounted for the higher catalytic activity hence resulting in the rapid deactivation of the catalyst and
compared to 10%Co/Al2O3.
large amount of surface coke.33,34 All the H2/CO ratios over
3.2.4 Long-term stability tests of the catalysts. Laboratory- these catalysts were less than 1, as shown in Fig. 8(3). This was
scale 60 h long-term stability tests over OMA-10Co, OMA-7Co, derived from reverse water gas shi side reaction (RWGS,
NPA-10Co, and 10%Co/Al2O3 catalysts were performed under CO2(g) + H2(g) ¼ H2O(g) + CO(g), DH298 ¼ +41 kJ molꢀ1), which
specic reaction conditions: 750 ꢁC, GHSV ¼ 15 000 mL consumes H2 and generates CO.12 The H2/CO ratios over
(gꢀ1 hꢀ1), CH4/CO2 ¼ 1, 1 atm. As shown in Fig. 8, OMA-10Co OMA-10Co and OMA-7Co mesoporous catalysts were constant
and OMA-7Co catalysts, which have ordered mesostructures, around 0.87 and 0.83, respectively, indicating that equilibrium
exhibited both high catalytic activities and excellent catalytic between the coke deposition reaction and elimination reaction
stabilities during the whole 60 h test. On the other hand, had been achieved.53,54 As a comparison, the H2/CO ratio for
NPA-10Co and 10%Co/Al2O3 samples suffered a gradual the NPA-10Co sample decreased continuously from 0.99 to
decline in CH4 and CO2 conversions (e.g. from 79.9% to 50.6% 0.76, implying that a balance between carbon deposition and
for CH4 conversion over NPA-10Co). It is well known that coke elimination could not be achieved. The rate of carbon
developing a catalyst with enhanced stability is signicant for deposition was considerably faster than that of the coke
the future large-scale industrialization of CRM process. elimination reaction. Therefore, coke deposition over the
Fig. 7 The curves of (1) CH4 conversion and (2) CO2 conversion versus reaction temperature for catalysts prepared with different methods;
reaction conditions: CH4/CO2 ¼ 1, GHSV ¼ 15 000 mL (gꢀ1
h
ꢀ1), 1 atm. The curves of (3) CH4 conversion and (4) CO2 conversion versus gas
hourly space velocity (GHSV) for catalysts prepared with different methods; reaction conditions: CH4/CO2 ¼ 1, 750 ꢁC, 1 atm.
48264 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig.
9
TG curves of the 60
h
endurance-tested catalysts: (a)
OMA-7Co, (b) OMA-10Co, (c) NPA-10Co, and (d) 10%Co/Al2O3.
deposition over OMA-7Co, OMA-10Co, NPA-10Co, and 10%Co/
Al2O3 catalysts aer 60 h long-term stability tests were examined
by TG analysis. As shown in Fig. 9. The weight losses over
OMA-7Co, OMA-10Co, 10%Co/Al2O3, and NPA-10Co catalysts
were 6.7%, 10.0%, 16.0%, and 43.5%, respectively. In contrast to
10%Co/Al2O3 and NPA-10Co catalysts, OMA-7Co and OMA-10Co
mesoporous catalysts possessed considerably less carbon depo-
sition and hence better anti-coke performance. The mesoporous
framework could stabilize the nano-sized Co particles during the
processes of reduction and CRM reaction via its connement
effect, thus signicantly suppressing the coke formation owing
to the size effect of the metallic active sites.
3.3.2 TPH analysis. TPH technique could be used to study
the types of coke deposited on the catalysts. The different types
of coke possessed different hydrogenation performances for the
generation of CH4. Generally, coke with high activity usually
presented CH4 signal peak in the low temperature region, while
the species with low activity exhibited CH4 signal peak in the
Fig. 8 60 h long-term stability tests over different catalysts: (1) CH4
conversion, (2) CO2 conversion, (3) H2/CO ratio; reaction conditions:
CH4/CO2 ¼ 1, GHSV ¼ 15 000 mL (gꢀ1 hꢀ1), 750 ꢁC, 1 atm.
catalyst surface was very serious and the catalyst subsequently
suffered quick deactivation.
3.3 Characterization of the spent catalysts
3.3.1 TG analysis. In view of the carbon balance, most
carbon atoms of the feed gases (CH4 and CO2) were converted
into the nal product in the form of CO. However, a portion of
the carbon atoms was deposited on the surface of the catalyst in
the form of the coke due to the presence of side reactions under
CRM reaction condition such as methane decomposition and
Fig. 10 TPH profiles of the 60 h endurance-tested catalysts: (a)
Boudouard reaction. Therefore, the properties of carbon OMA-7Co, (b) OMA-10Co, (c) NPA-10Co, and (d) 10%Co/Al2O3.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48256–48268 | 48265

View Article Online
RSC Advances
Paper
high temperature region.55–57 TPH characterization was carried morphology of carbon deposition over the catalyst surface, as
out over 60 endurance-tested OMA-7Co, OMA-10Co, shown in Fig. 11. The OMA-7Co and OMA-10Co ordered mes-
h
NPA-10Co, and 10%Co/Al2O3 catalysts to study the characteris- oporous catalysts retained uniform cylindrical mesopores
tics of the surface coke species and their proles are shown in along the [1 1 0] direction and hexagonal honeycomb-like
Fig. 10. Most of the samples displayed CH4 signal peaks with mesopores along the [0 0 1] direction even aer 60 h CRM
similar shapes, with one weak peak around 400 ꢁC and one reactions, which is in good agreement with their type IV H1
strong peak around 640 ꢁC (or even higher temperatures around hysteresis loops and narrow pore size distributions in Fig. S3
ꢁ
750 C), which could be assigned to two types of carbon depo- (ESI†). This suggested that these ordered mesoporous catalysts
sition with different reactivities toward hydrogenation. The possessed good thermal stability. It is worth noting that there
weak shoulder peaks around 400 ꢁC were proposed to arise from was no obvious large and aggregated Co metallic cluster
the hydrogenation of amorphous carbon deposition,55–58 which observed in these mesoporous catalysts. This further
was responsible for CO formation during the reaction.10,55,56,59,60 conrmed that the connement effect of the mesoporous
The main peaks around 640 ꢁC could be attributed to the framework played an important role in stabilizing the Co
hydrogenation peak of carbon nanotube species, which had a metallic nanoparticles during the long-term stability test. In
considerably lower reactivity toward H2 than amorphous carbon addition, carbon nanotubes were the main coke residue for all
and nally caused the deactivation of the catalyst by covering the catalysts studied, as predicted by TPH characterization,
metallic active sites and blocking pore channels.56,58,61
that caused the rapid deactivation of the catalysts.61,62 In
3.3.3 Morphology analysis of the long-term endurance- contrast, there was no apparent amorphous carbon species
tested catalysts. The TEM analysis of the 60 h endurance- found among the catalysts. We propose that amorphous
tested OMA-10Co, OMA-7Co, NPA-10Co, and 10%Co/Al2O3 carbon might uniformly distribute among the mesoporous
catalysts was performed to further conrm the thermal channels and catalyst surface, which could be further elimi-
stability of the ordered mesostructure and investigate the nated in the subsequent process of CO production.
Fig. 11 TEM and SAED (inset) images of the 60 h endurance-tested catalysts: (a), (b), and (c) OMA-7Co; (d), (e), and (f) OMA-10Co; (g) NPA-10Co;
(h) and (i) 10%Co/Al2O3.
48266 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
16 J. R. H. Ross, A. N. J. vanKeulen, M. E. S. Hegarty and
K. Seshan, Catal. Today, 1996, 30, 193–199.
17 Y. H. Hu and E. Ruckenstein, Catal. Rev.: Sci. Eng., 2002, 44,
423–453.
18 N. Sun, X. Wen, F. Wang, W. Wei and Y. Sun, Energy Environ.
Sci., 2010, 3, 366–369.
19 K. Takanabe, K. Nagaoka, K. Nariai and K. Aika, J. Catal.,
2005, 232, 268–275.
20 S. Liu, L. Guan, J. Li, N. Zhao, W. Wei and Y. Sun, Fuel, 2008,
87, 2477–2481.
21 P. Ferreira-Aparicio, A. Guerrero-Ruiz and I. Rodriguez-
Ramos, Appl. Catal., A, 1998, 170, 177–187.
22 J. H. Edwards and A. M. Maitra, Fuel Process. Technol., 1995,
42, 269–289.
23 A. W. Budiman, S. H. Song, T. S. Chang, C. H. Shin and
M. J. Choi, Catal. Surv. Asia, 2012, 16, 183–197.
24 C. Shi and P. Zhang, Appl. Catal., B, 2012, 115, 190–200.
25 X. Y. Quek, D. Liu, W. N. E. Cheo, H. Wang, Y. Chen and
Y. Yang, Appl. Catal., B, 2010, 95, 374–382.
26 M. S. Fan, A. Z. Abdullah and S. Bhatia, Appl. Catal., B, 2010,
100, 365–377.
4 Conclusion
In conclusion, a series of ordered mesoporous CoAl2O4 spinel-
based metal oxides with different Co content were designed
and facilely prepared by a one-step evaporation induced self-
assemble method for the rst time. The obtained materials
exhibited excellent structural properties and good thermal
stability, and were utilized as catalysts for the CRM reaction.
Compared with non-mesoporous and traditional supported
catalysts, these mesoporous catalysts exhibited higher catalytic
activities and better stabilities. The ordered mesoporous
structure could signicantly improve catalytic activity and
stability by providing sufficiently accessible Co metallic active
sites for gaseous feed stocks and stabilizing the Co nano-
particles via the connement effect of the mesoporous frame-
work during the CRM reaction, simultaneously. In addition, the
OMA-xCo catalysts had a strong capacity for coke resistance due
to the size effect of the Co nanoparticles. Our results demon-
strated that these OMA-xCo materials could be considered as a
series of promising catalysts for the CRM reaction.
27 J. Zhang, H. Wang and A. K. Dalai, J. Catal., 2007, 249, 300–
310.
28 P. Djinovic, I. G. O. Crnivec, B. Erjavec and A. Pintar, Appl.
Catal., B, 2012, 125, 259–270.
29 E. Ruckenstein and H. Y. Wang, Appl. Catal., A, 2000, 204,
257–263.
30 E. Ruckenstein and H. Y. Wang, J. Catal., 2002, 205, 289–293.
31 N. Wang, W. Chu, L. Huang and T. Zhang, J. Nat. Gas Chem.,
2010, 19, 117–122.
Acknowledgements
The authors acknowledge the nancial support from Singapore
MOE grant R143-000-542-112, Singapore National Research
Foundation CREATE-SPURc program R-143-001-205-592, and
Jiangsu Province Government Research Platform Grant and
NUSRI Seed Fund.
32 S. B. Wang, G. Q. M. Lu and G. J. Millar, Energy Fuels, 1996,
10, 896–904.
References
1 A. Baiker, Appl. Organomet. Chem., 2000, 14, 751–762.
2 I. Omae, Catal. Today, 2006, 115, 33–52.
33 J. H. Kim, D. J. Suh, T. J. Park and K. L. Kim, Appl. Catal., A,
2000, 197, 191–200.
3 K. Mette, S. Kuehl, H. Duedder, K. Kaehler, A. Tarasov, 34 H. Y. Wang and E. Ruckenstein, Appl. Catal., A, 2001, 209,
M. Muhler and M. Behrens, ChemCatChem, 2014, 6, 100–104.
4 C. S. Song, Catal. Today, 2006, 115, 2–32.
5 W. Wang, S. Wang, X. Ma and J. Gong, Chem. Soc. Rev., 2011,
40, 3703–3727.
6 R. Schloegl, ChemSusChem, 2010, 3, 209–222.
7 A. T. Ashcro, A. K. Cheetham, M. L. H. Green and
P. D. F. Vernon, Nature, 1991, 352, 225–226.
8 W. Chen, G. Zhao, Q. Xue, L. Chen and Y. Lu, Appl. Catal., B,
2013, 136, 260–268.
9 Z. Liu, J. Zhou, K. Cao, W. Yang, H. Gao, Y. Wang and H. Li,
Appl. Catal., B, 2012, 125, 324–330.
10 M. Yu, K. Zhu, Z. Liu, H. Xiao, W. Deng and X. Zhou, Appl.
Catal., B, 2014, 148, 177–190.
207–215.
35 W. Zhu, A. Borjesson and K. Bolton, Carbon, 2010, 48, 470–
478.
36 Y. H. Hu, Catal. Today, 2009, 148, 206–211.
37 K. Bourikas, C. Kordulis and A. Lycourghiotis, Catal. Rev.:
Sci. Eng., 2006, 48, 363–444.
38 S. Damyanova, B. Pawelec, K. Arishtirova, J. L. G. Fierro,
C. Sener and T. Dogu, Appl. Catal., B, 2009, 92, 250–261.
39 L. Y. Mo, J. H. Fei, C. J. Huang and X. M. Zheng, J. Mol. Catal.
A: Chem., 2003, 193, 177–184.
40 L. Y. Mo, X. M. Zheng, C. J. Huang and J. H. Fei, Catal. Lett.,
2002, 80, 165–169.
41 Q. Yuan, A. X. Yin, C. Luo, L. D. Sun, Y. W. Zhang,
W. T. Duan, H. C. Liu and C. H. Yan, J. Am. Chem. Soc.,
2008, 130, 3465–3472.
11 S. Zeng, X. Zhang, X. Fu, L. Zhang, H. Su and H. Pan, Appl.
Catal., B, 2013, 136, 308–316.
12 M. C. J. Bradford and M. A. Vannice, Catal. Rev.: Sci. Eng., 42 S. M. Morris, P. F. Fulvio and M. Jaroniec, J. Am. Chem. Soc.,
1999, 41, 1–42.
2008, 130, 15210–15216.
13 M. S. Fan, A. Z. Abdullah and S. Bhatia, ChemCatChem, 2009, 43 H. Y. Wang and E. Ruckenstein, Catal. Lett., 2001, 75, 13–18.
1, 192–208.
44 R. Liu, M. Yang, C. Huang, W. Weng and H. Wan, Chin. J.
Catal., 2013, 34, 146–151.
14 C. J. Liu, J. Ye, J. Jiang and Y. Pan, ChemCatChem, 2011, 3,
529–541.
15 B. Q. Xu and W. M. H. Sachtler, J. Catal., 1998, 180, 194–206.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 48256–48268 | 48267

View Article Online
RSC Advances
Paper
45 N. Srisawad, W. Chaitree, O. Mekasuwandumrong, 54 L. Xu, H. Song and L. Chou, Catal. Sci. Technol., 2011, 1,
P. Praserthdam and J. Panpranot, J. Nanomater., 2012,
1032–1042.
2012, 95.
55 L. Xu, H. Song and L. Chou, Appl. Catal., B, 2011, 108, 177–
190.
46 L. Ji, J. Lin and H. C. Zeng, J. Phys. Chem. B, 2000, 104, 1783–
1790.
56 M. Rezaei, S. M. Alavi, S. Sahebdelfar, P. Bai, X. Liu and
Z.-F. Yan, Appl. Catal., B, 2008, 77, 346–354.
57 M. Rezaei, S. M. Alavi, S. Sahebdelfar and Z.-F. Yan, J. Nat.
Gas Chem., 2008, 17, 278–282.
47 R. L. Chin and D. M. Hercules, J. Phys. Chem., 1982, 86, 360–
367.
48 H. Xiong, Y. Zhang, K. Liew and J. Li, J. Mol. Catal. A: Chem.,
2005, 231, 145–151.
58 J. Guo, H. Lou and X. Zheng, Carbon, 2007, 45, 1314–1321.
´
´
´
´
49 Z. Ferencz, K. Baan, A. Oszko, Z. Konya, T. Kecskes and 59 H. M. Swaan, V. C. H. Kroll, G. A. Martin and C. Mirodatos,
}
A. Erdohelyi, Catal. Today, 2014, 228, 123–130.
Catal. Today, 1994, 21, 571–578.
50 H. F. Xiong, Y. H. Zhang, J. L. Li and Y. Y. Gu, J. Cent. South 60 Y. G. Chen and J. Ren, Catal. Lett., 1994, 29, 39–48.
Univ. Technol., 2004, 11, 414–418.
51 L. Ji, S. Tang, H. C. Zeng, J. Lin and K. L. Tan, Appl. Catal., A,
2001, 207, 247–255.
61 C. Wang, N. Sun, N. Zhao, W. Wei, J. Zhang, T. Zhao, Y. Sun,
C. Sun, H. Liu and C. E. Snape, ChemCatChem, 2014, 6, 640–
648.
52 J. L. Li, X. D. Zhan, Y. Q. Zhang, G. Jacobs, T. Das and 62 H. Liu, C. Guan, X. Li, L. Cheng, J. Zhao, N. Xue and W. Ding,
B. H. Davis, Appl. Catal., A, 2002, 228, 203–212.
53 J. M. Wei, B. Q. Xu, J. L. Li, Z. X. Cheng and Q. M. Zhu, Appl.
Catal., A, 2000, 196, L167–L172.
ChemCatChem, 2013, 5, 3904–3909.
48268 | RSC Adv., 2015, 5, 48256–48268
This journal is © The Royal Society of Chemistry 2015