Effect of Re addition on the activities of Co/CeO2 catalysts for water gas shift reaction

Article abstract of DOI:10.1016/j.molcata.2011.03.006

The catalytic activities of Re-Co/CeO₂ bimetallic catalysts for the water gas shift (WGS) reaction were investigated and compared with activities of Co/CeO₂. It was found that the rate of WGS reaction over Re-Co/CeO₂ bimetallic catalysts was higher than that of a single catalyst of Co on ceria. It seems that Re influences the catalysts and the catalyst performance in several ways. XRD and Raman spectroscopy results indicate that metal oxides were dispersed on ceria surface and H ₂-chemisorption indicated better dispersion of Co on the ceria surface upon addition of Re. X-ray absorption near edge structure (XANES) spectra of Re-Co/CeO₂ indicate that Re promotes the reduction of surface ceria to Ce₂O₃ and provides oxygen vacancies that facilitate the redox process at the surface. XANES also indicated that electron densities were withdrawn from Co d-state to Re, leading to weaker Co-adsorbate bonds. All of these effects contribute to an increase in the WGS reaction rate.

Full text of DOI:10.1016/j.molcata.2011.03.006

Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Effect of Re addition on the activities of Co/CeO2 catalysts for water gas shift
reaction
Kingkaew Chayakul, Tipaporn Srithanratana, Sunantha Hengrasmee∗
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic activities of Re–Co/CeO2 bimetallic catalysts for the water gas shift (WGS) reaction were
investigated and compared with activities of Co/CeO2. It was found that the rate of WGS reaction over
Re–Co/CeO2 bimetallic catalysts was higher than that of a single catalyst of Co on ceria. It seems that
Re influences the catalysts and the catalyst performance in several ways. XRD and Raman spectroscopy
results indicate that metal oxides were dispersed on ceria surface and H2-chemisorption indicated better
dispersion of Co on the ceria surface upon addition of Re. X-ray absorption near edge structure (XANES)
spectra of Re–Co/CeO2 indicate that Re promotes the reduction of surface ceria to Ce2O3 and provides
oxygen vacancies that facilitate the redox process at the surface. XANES also indicated that electron
densities were withdrawn from Co d-state to Re, leading to weaker Co-adsorbate bonds. All of these
effects contribute to an increase in the WGS reaction rate.
Received 11 October 2010
Received in revised form 9 February 2011
Accepted 9 March 2011
Available online 15 March 2011
Keywords:
Re–Co bimetallic catalyst
X-ray absorption near edge structure
Water gas shift reaction
Oxygen vacancies
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Au which are rare and expensive are the most promising promoters
because of their better catalytic activity at low temperature. Many
The water gas shift (WGS) reaction is an important step in reduc-
ing the amount of CO from H2 feed gas that will be used in fuel cells.
H2 feed gas is obtained as the major product of a reforming reaction
together with a small amount of by-products such as CH4, H2O, and
CO. Among these impurities, CO is poisonous to the Pt electrode in
a PEM fuel cell and decreases Pt activity. Removal of CO from the
feed gas is necessary, and the water gas shift reaction which can
reduce the amount of CO to less than 0.5–1% is a candidate for this
step.
research groups are trying to develop new types of catalysts from
metals that are less expensive and more abundant. Many studies
have shown that a combination of two metals can provide rather
good WGS activities. Many bimetallic systems have been studied
for many reactions, for example Pt–V/CeO2, Pt–Re/CeO2–ZrO2 and
Au–M on iron(III) oxide (M = Ag, Bi, Co, Cu, Mn, Ni, Pb, Ru, Sn and
Tl) have been found to increase the rates of WGS reactions [22–26],
while Pt–Sn/Nb2O5 is also active for selective CO-oxidation [27,28].
Among the second metals that have been used to form bimetal-
lic catalysts, Re is the most widely studied. Apart from increasing
the WGS rate, Pt–Re/carbon has also been reported to be active for
many other reactions such as hydrocarbon reforming [29] and con-
version of glycerol to synthesis gas [30]. Our initial investigation
found that Re also increased the rate of WGS reaction when it was
doped on Co/CeO2.
In the WGS reaction, carbon monoxide is converted to carbon
dioxide by water vapor. The WGS reaction is described by the fol-
lowing reaction,
CO + H2O ꢀ CO2 + H2
ꢀH = −41.2 kJ/mol
This reaction is exothermic and the rate equilibrium conver-
sion is lower with increasing temperature. In order to keep good
conversion rates at low temperature, catalysts must be employed.
The catalysts that are widely used are metal–ceria based cat-
alysts [1–6]. Ceria (CeO2) is a suitable support due to its high
oxygen storage capacity, high reducibility and high oxygen mobil-
ity [7–11]. Many precious metals such as Pt, Pd, Rh, Ru and Au
have been reported to show high WGS reaction rate when they
are doped on ceria supports [12–21]. Among these metals, Pt and
The manner in which Re improves the catalytic performance
varies. For the WGS reaction, Choung et al. [23] indicated that the
effect was partly due to better dispersion of Pt on the support, but
the actual role was more complex. Iida and Igarashi [25], in compar-
ing the rate of WGS reaction of Pt–Re/TiO2 (rutile) and Pt–Re/ZrO2,
indicated that the effect of Re on Pt dispersion and catalytic activ-
ity was largely affected by the state of the Re. In a recent paper on
conversion of glycerol by Pt–Re catalysts [30], Re was reported to
participate in weakening the binding energy of CO to the neighbor-
ing Pt sites. X-ray absorption spectroscopy was employed to study
the oxidation state of Re and Pt. Hilbrig et al. [31] demonstrated that
X-ray absorption near edge structure (XANES) can be used to deter-
mine the degree of reduction/interaction of bimetallic catalysts and
∗
Corresponding author. Tel.: +66 43202222x12243; fax: +66 43202373.
E-mail address: sunhen@kku.ac.th (S. Hengrasmee).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.03.006

40
K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
reported XANES-TPR studies of Pt–Re/Al2O3 and silica-supported
Pt/Ni catalysts [31,32]. Apart from identifying the binding energies
and oxidation state, XANES can also indicate changes in electron
density of the metal d-band. Study of Pd L3 near-edge absorption
in Ag-promoted Pd catalyst indicated increases in the Pd d-band
electron densities upon addition of Ag [33]. Pereira et al. [34] used
XANES to study bonding of CO on Pt-based bimetallic catalysts
and found increases of Pt 5d-band vacancies for Pt–M/C bimetal-
lic catalysts, which indicated a lowering of electron back-donation
to the CO molecules. In studying the activity of Re–Co/alumina in
the Fischer–Tropsch reaction, Re was reported to be a reduction
promoter that facilitated the rate of reduction of cobalt species
interacting with the support and generated more available active
cobalt metal sites to participate in the reaction [35].
solution was added dropwise to the prepared CeO2 support. The
impregnated supports were dried at 110 ◦C overnight and calcined
at 650 ◦C for 8 h.
2.2. Catalyst characterization
2.2.1. Standard characterization
XRD patterns of ceria based catalysts and all doped metal–ceria
based catalysts were obtained from Bruker XRD D8 Advance GX 280
˚
with Cu K radiation of wavelength 1.5406 A. The diffractograms
∝
were recorded in the range of 2ꢁ = 20–80◦ with a scan speed of 0.5 s
per step. The crystalline sizes of the samples, dhkl, were estimated
from Scherrer’s equation
In WGS reaction, the oxidation–reduction process is one of the
rate limiting steps that control the reaction rate. Many studies
reported that the reducible support (CeO2, ZrO2, TiO2, etc.) is usu-
ally used due to its participation in enhancing the WGS reaction.
CeO2 can store and release oxygen to undergo oxidation–reduction
cycles and promote catalytic activity for this reaction. Several stud-
ies have reported that metal-modified ceria has a higher oxygen
storage capacity and reducibility than pure ceria [36–38]. Sanchez
and Gazquez [39] proposed that oxygen vacancies in the fluo-
rite structure of the supported catalysts alter the morphology
and dispersion of the supported metal. In an attempt to dope
Pt/CeO2 catalysts with many metal cations, Panagiotopoulou and
co-worker [40] reported that the WGS activities of Pt/Ce–Me–O cat-
alysts (Me = metal cations) depended on the nature of the dopants
employed and the promotions affected the reducibility and oxy-
gen ion mobility of the CeO2 support. Creation of oxygen vacancies
lead to an improvement of oxygen mobility from the support to the
adsorbed species on the catalyst surface.
0.9ꢂ
FWHM cos ꢁ
dhkl
=
(1)
˚
where ꢂ is the X-ray wavelength of Cu K radiation (1.5406 A),
∝
FWHM is the broadening (in radians) at half-maximum of the
(1 1 1) crystallographic plane which is the most intense peak and
ꢁ is the diffraction angle corresponding to the (1 1 1) plane. Lat-
tice parameter (a) is calculated by Bragg’s equation with the same
crystallographic plane.
The specific surface areas of the catalysts were obtained from
the N2 adsorption isotherm at 77 K using the Brunauer Emmett
Teller (BET) method by Quantachrome Autosorb 1-C instrument.
The catalysts were degassed at 300 ◦C for 3 h prior to an adsorp-
tion experiment. The specific surface area was calculated by the
multipoint-BET method and the average pore volume and diameter
were determined at relative pressure P/P0 ∼ 0.99.
H2-chemisorption of all catalysts was studied by Quantachrome
Autosorb 1-C instrument. The catalysts were first pretreated by
Helium at 120 ◦C (rate 20 ◦C/min) for 30 min, then H2 (99.999%,
Thai Industrial Gas) was flowed over the sample while the tem-
perature was raised at the rate of 20 ◦C/min to reach 350 ◦C and
was held at this temperature for 120 min. After sample preparation,
the sample was cooled down under vacuum to room tempera-
ture (40 ◦C). The adsorbing gas (H2) was sequentially added to the
sample. An adsorbed volume (V) vs. Equilibrium pressure (P/P0)
isotherm was generated. The H2 chemisorbed and the percentage of
metal dispersion can be deduced from this chemisorbed isotherm.
The method used to obtain the volume of monolayer uptake (Vm) is
In this paper, we report the enhancement of WGS reaction rate
when Re is doped to Co/ceria catalysts. The catalysts were prepared
by the incipient-wetness impregnation method and character-
ized by X-ray diffraction (XRD), N2-adsorption, H2-chemisorption,
Raman spectroscopy and H2-temperature programmed reduction
(H2-TPR). XANES of the catalysts was employed to investigate the
movement of metal d-electrons in the catalysts.
2. Experimental
an extrapolation method. The monolayer of hydrogen uptake (Nm
is expressed in ␮mol of hydrogen per gram of sample, that is
)
2.1. Catalysts preparation
2.1.1. Preparation of support
Nm = 44.61Vm
(2)
CeO2 support was prepared by the urea gelation method which
was similar to the process described by Kundakovic and Flytzani-
Stephanopoulos [5] and Bickford et al. [41]. An appropriate amount
of cerium nitrate hexahydrate (Ce(NO3)3·6H2O, 99%, Aldrich) and
12 g of urea (H2NCONH2, 98%, Aldrich) were dissolved in 100 mL of
deionized water. The mixture was stirred and heated at 100–120 ◦C
until the salts were dissolved. While the mixture was heated, 2 mL
of ammonium hydroxide (NH4OH, Aldrich) was added dropwise to
obtain a yellow precipitate. The suspension was heated and stirred
for 4 h to remove excess ammonia and to age the support. After
aging, the precipitate was filtered and washed with boiling deion-
ized water. The filtered support was dried overnight at 110 ◦C in a
furnace and calcined at 450 ◦C for 4 h.
where Vm (cc/g) is determined by extrapolating the isotherm to
zero pressure. The percent metal dispersion (D) can be estimated
from the equation
N
mSM
D =
(3)
100L
where S is the adsorption stoichiometry of H2 which is equal to
2, M and L are the molecular weight of loading metal and percent
loading of the supported metal, respectively.
2.2.2. Raman spectroscopy
The Raman spectra of all catalysts were recorded by Jobin Yvon
T 64000 Raman spectrometer equipped with BX51 Olympus opti-
cal microscope. The Raman spectrometer was used to observe the
vibrational mode of prepared catalyst powder and the vibrational
signal was detected by Charge Coupled Device (CCD) detector. The
samples were irradiated by Ar ion laser with an output power
30 mW and wavelength of 514.532 nm. The Raman spectra were
2.1.2. Preparation of catalysts
The incipient wetness impregnation method with a minimum
amount of solvent was used to prepare the catalysts. The desired
amounts of ammonium perrhenate (NH4ReO4, 99%, Aldrich) and
cobalt(II)nitrate hexahydrate (Co(NO3)2·6H2O, 98%, Carlo Erba)
were dissolved in a minimal amount of deionized water. The
collected in the range of 200–2000 cm−1
.

K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
41
(331)
(400)
2.2.3. H2-temperature programmed reduction (H2-TPR)
(111)
(220)
(311)
(420)
(h) 1%Re-10%Co/CeO
(200)
H2-TPR was carried out using the Quantachrom Autosorb 1-C
instrument. The samples were pretreated by helium at 120 ◦C (rate
10 ◦C/min) for 30 min. After cooling down to room temperature, a
gaseous mixture of 5%H2 in 95%N2 was flowed over the pretreated
sample while the temperature was increased from 40 ◦C to 1000 ◦C
at a rate of 10 ◦C/min. The products were analyzed with TCD detec-
tor and H2-consumption was plotted as a function of temperature.
(222)
2
(g) 1%Re-7.5%Co/CeO
2
(f) 1%Re-5%Co/CeO
2
2.3. Water gas shift catalytic activity test
(e) 1%Re-2.5%Co/CeO
2
WGS catalytic activities of all samples were tested with a flow
reactor in the temperature range of 150–600 ◦C. 20 mg of catalysts
were packed and sandwiched between two layers of quartz wool in
the quartz tube reactor with 0.6 cm ID, 0.8 cm OD × 30 cm length.
The quartz tube was placed inside the temperature controllable
tube furnace. The temperature was monitored by K-type thermo-
couple that was placed at the top of the catalyst bed. The flow of
feed gases was controlled by mass flow controllers (Dywer). Both
temperature and flow rate were monitored by Data Logger Wisco
DL2100B. The initial feed gas, consisting of CO and He at a flow rate
of 5 and 85 mL/min respectively, was flowed into a water-saturator
in which the temperature was kept at 47 ◦C to pick up water vapor.
The mixed feed gas containing 5%CO, 10%H2O and 85%He, was
allowed to pass through the catalyst bed of the reactor. The reac-
tants and products were analyzed by on-line gas chromatography
(Agilent 6890N Series, Agilent Technology) equipped with HEYSEP
D Packed Column and TCD detector. The percentage conversion of
CO was plotted as a function of temperature.
(d) 1%Re-1%Co/CeO
2
(c) 5%Co/CeO
(b) 1%Re/CeO
2
2
(a) CeO
2
20
30
40
50
2theta
60
70
80
Fig. 1. XRD patterns of (a) CeO2, (b) 1%Re/CeO2, (c) 5%Co/CeO2 and (d)–(h)
1%Re–X%Co/CeO2 (X = 1.0, 2.5, 5.0, 7.5 and 10.0) catalysts.
reduction. Data reduction was carried out by pre-edge background
removal and normalization by division of the height of the adsorp-
tion edge.
The rates of reaction were obtained from separate experiments
under differential conditions, where the conversions of reactants
were lower than 10% and with this condition heat and mass transfer
effects were negligible. The rates were calculated by the following
expression
3. Results and discussion
3.1. Standard characterization
FCO × XCO
XRD patterns of CeO2, Co/CeO2 monometallic and Re–Co/CeO2
bimetallic catalysts are shown in Fig. 1. All of these samples exhibit
diffraction peaks which correspond to the fluorite type structure of
CeO2. Neither Re nor Co oxide phases were observed in all spectra.
This suggests that the metal oxide species were highly dispersed
on the ceria surface [43,44].
The lattice parameters of ceria and metal–ceria based catalysts
were calculated by (1 1 1) crystallographic planes which are based
on Bragg’s equation. For pure CeO2 support, the lattice parameter
is 0.541 nm which is in agreement with other works [40,45]. The
lattice parameter of other catalysts (Co/CeO2, and Re–Co/CeO2) also
displayed similar values varying from 0.544 to 0.547 nm. It can be
concluded that addition of Co or Re did not lead to modification of
crystal structures.
rCO
=
(4)
Wcat
where rco is the conversion rate of CO (mol g−1 s−1), Fco is the molar
flow rate of CO (mol s−1), Wcat is the mass of the catalysts (g) and
Xco is the conversion of CO which is defined as
in
CO
out
CO
C
− C
XCO
=
(5)
in
C
CO
The rco reported in this manuscript is an average of three repro-
ducible experiments.
2.4. X-ray adsorption near edge structure
X-ray absorption spectroscopy (XAS) measurements were stud-
ied in the XANES region on Beamline 8 [42] at the Synchrotron
Light Research Institute (SLRI), Nakhon Ratchasima, Thailand. The
measurements were carried out in fluorescence mode for Re M5
and Co K absorption edges and transmission modes for Ce L3
absorption edge. Synchrotron radiation is tunable by a fixed-exit
double crystal monochromator (DCM) equipped with InSb(1 1 1)
and Ge(2 2 0) crystals for low and high energy ranges, respectively.
The monochromatic flux at the sample was 108–1010 phs/s. The
samples were deposited over the Kapton window which was placed
on the sample frame. Ion chambers were filled with a mixture of
nitrogen and helium for Re M5 absorption measurement and with
argon and helium for Ce L3 and Co K absorption edge measure-
ments. These chambers were installed in front of and behind the
sample which continuously detected the incident (Io) and transmit-
ted (I1) X-ray beams. A Lytle detector was used to detect sample
fluorescence. The software Athena was used to process the data
The results from particle analysis together with the XRD results
are summarized in Table 1. Increases in crystalline sizes were
observed for all samples of doped ceria. This result is suggested
to be due to agglomeration and growth of metal–ceria crystallites
after calcination at high temperature [46]. The BET surface area
of CeO2 is 68.3 m2/g which is in agreement with others [5,36,47].
Introduction of metals onto the ceria surface by the impregnation
method led to particle pore blocking and gave rise to lowering of
surface area and pore volume and consequently the pore diameter
was enlarged.
The results from H2-chemisorption and % active metal disper-
sion are also displayed in Table 1. H2 was not chemisorbed on Re
and the data on H2-chemisorption and % active metal dispersion
are referred to Co metal. The% metal dispersion data indicate that
Re helped to disperse Co metal on ceria support. However, the
dispersion decreased with an increase of Co. This result, as also
indicated by XRD, was due to agglomeration and the growth of

42
K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
Table 1
Specific surface area, crystalline size, lattice constant, H2 chemisorbed and % dispersion of various catalysts.
Sample
SBET
(m2/g)
Crystalline
sizea (nm)
Lattice
H2 chemisorbed
(␮mol/g)
% dispersion
constanta (nm)
CeO2
1%Re/CeO2
5%Co/CeO2
1%Re–1%Co/CeO2
1%Re–2.5%Co/CeO2
1%Re–5%Co/CeO2
1%Re–7.5%Co/CeO2
1%Re–10%Co/CeO2
68.3
26.8
28.0
22.9
26.9
22.9
25.3
22.9
14.2
22.6
21.6
23.5
19.5
21.5
26.0
19.0
0.541
0.545
0.544
0.547
0.544
0.545
0.545
0.544
–
–
–
–
39.1
45.2
46.0
45.3
45.5
46.6
9.2
53
22
11
7.2
5.5
a
Calculated from the (1 1 1) crystallographic plane.
In our Raman spectra, the 464 cm−1 band was assigned to Ce–O
vibration and other peaks were assigned to the existence of small
crystallites of Co3O4 which are present at the surface of CeO2. The
occurrence of the CoO phase may also be possible since the 484
and 691 cm−1 CoO bands coincide with the Co3O4 band [52]. The
Re–O vibration band was not observed due to very low Re con-
tent. Upon investigating the 464 cm−1 band of CeO2, it is clearly
seen that the addition of Re or Co did not lead to additional peak
of other metal oxide species and this result indicates the preserva-
tion of the bulk structure. However, the intensities of the 464 cm−1
band were gradually lowered and a slight red shift of 2–3 cm−1 was
observed with the increase of the added metals. This result may due
to the presence of Ce3+at the catalyst surface which is produced
upon reduction of surface Ce4+ by Co and by Re.
metal–ceria crystallites. As the result of better metal dispersion,
the H2-chemisorption increased.
3.2. Raman spectroscopy
Raman spectroscopy is a powerful technique sensitive to
observing M–O bond arrangement and lattice defects [48]. Ceria
with fluorite structure (space group Fm3m) has a very simple vibra-
tion structure with only one Raman active mode. The Raman
spectra of pure CeO2 displays only one prominent peak at 464 cm−1
due to F2g Raman active mode of the fluorite structure [49] as shown
in Fig. 2. This vibration mode is a symmetric breathing mode of 8
oxygen atoms around each Ce cation. Incorporation of some ions of
similar size but different charges into the CeO2 lattice would lead to
shifting and lowering of the intensity of the 464 cm−1 peak, along
with the appearance of a small additional peak due to vibration of
oxygen around the added atom, and these phenomena indicate the
existence of oxygen vacancies [50].
In studying the Raman spectra of cobalt oxide on ␥-alumina,
Zacharaki et al. [51] reported 5 characteristic bands due to the crys-
talline Co3O4 spinel with Co2+ and Co3+ located at tetrahedral and
octahedral sites, respectively. These bands were identified as A1g
(694 cm−1), F2g (622 cm−1), F2g (527 cm−1), Eg (487 cm−1) and F2g
(198 cm−1).
3.3. H2-temperature programmed reduction
H2-TPR profiles of ceria, monometallic (Re/CeO2 and Co/CeO2)
and bimetallic catalysts (Re–Co/CeO2) are presented in Fig. 3. For
CeO2, there are two main reduction peaks at 520 and 880 ◦C which
are attributed to reduction of the uppermost layer of Ce4+ in CeO2
and the main peak at high temperature is assigned to the reduction
of bulk ceria [53]. Addition of Re or Co onto CeO2 brought up addi-
tional reduction peaks around 300–380 ◦C. In the case of Re/CeO2,
two reduction peaks were observed. The first one is located at
300 ◦C and the other at 530 ◦C. The lower temperature reduction
peak is attributed to reduction of rhenium oxide species [54], and
689
487
523
464
o
(h) 1%Re-10%Co/CeO2
380
C
o
o
880
880
C
(g) 1%Re-7.5%Co/CeO2
(f) 1%Re-5%Co/CeO2
(f) 1%Re-10%Co/CeO2
o
410
C
C
(e) 1%Re-2.5%Co/CeO2
C
(e) 1%Re-5%Co/CeO2
(d) 1%Re-1%Co/CeO2
o
395
880 oC
(d) 1%Re-1%Co/CeO2
(c) 5%Co/CeO2
o
380
C
o
o
o
300
C
880
C
520
C
(c) 5%Co/CeO2
(b) 1%Re-CeO2
(a) CeO2
o
300
C
o
880
880
C
(b) 1%Re/CeO2
(a) CeO2
o
530
520
C
C
o
C
o
200
400
600
800
1000
0
200
400
600
800
1000
wavenumber (cm-1)
Temperature (ºC)
Fig. 2. Raman spectra of (a) CeO2, (b) 1%Re/CeO2, (c) 5%Co/CeO2 and (d)–(h)
1%Re–X%Co/CeO2 (X = 1.0, 2.5, 5.0, 7.5 and 10.0) catalysts.
Fig. 3. H2-TPR profiles of (a) CeO2, (b) 1%Re/CeO2, (c) 5%Co/CeO2 and (d)–(f)
1%Re–X%Co/CeO2 (X = 1.0, 5.0 and 10.0) catalysts.

K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
43
the peak at 530 ◦C is due to CeO2. It is not clear what the final
100
(a)
state of Re is after the reduction, since it seems to depend on
the state of the supports. For examples, Johnson and LeRoy [55]
reported that rhenium on alumina is reduced exclusively by hydro-
gen to the Re4+ state. In contrast, Webb [56] reported that alumina
supported rhenium is completely reduced at 400 ◦C. Jacobs et al.
[35], upon studying Re–Co/Al2O3, reported that Re promoted the
reduction of CoO to Co0, and Re remains on the surface as iso-
lated Re atoms in intimate contact with Co metal clusters. Extensive
study is required in order to determine the final state of Re/CeO2
upon hydrogen reduction. For Co/CeO2, two additional peaks at 300
and 380 ◦C point to a two-step reduction process of cobalt oxide
(Co3+ → Co2+ → Co0). Luo et al. [57] reported the reduction of Co3+
at the interface between Co3O4 and CeO2 to Co2+ in the temperature
range of 240–320 ◦C and the reduction of Co3O4 that weakly inter-
acts with CeO2 to Co in the range of 320–480 ◦C. Variations of TPR
spectra of Co/CeO2 have been reported [58–60] and the differences
of the spectra have been discussed to be due to various factors such
as precursor salts and treatment conditions. Our results seem to
agree with the results of Luo et al. [57]. The existence of reduction
peaks of metal oxides that are separated from CeO2 reduction peaks
indicates that these oxide phases are separate and probably reside
on the surface of ceria, leading to a weaker surface ceria signal.
The H2-TPR of the bimetallic catalysts Re–Co/CeO2 are different
from those of monometallic catalysts. In this case the surface reduc-
tion temperature of ceria at 520 ◦C disappears and a concurrent
reduction of metal oxide species appears as a broad peak ranging
from 300 to 480 ◦C with peak center situated around 380–410 ◦C.
Electron density transfer between Re, Co, and Ce may occur. As the
result of electron density transfer, reduction of the catalyst surface
is easier.
(g)
(f)
(e)
80
60
40
20
0
(d)
(c)
(c)-(g)
(b)
(a)
(b)
(a)
(a)
equilibrium
100
200
300
400
500
600
Temperature (ºC)
(b)
1%Re-10%Co/CeO
1%Re-7.5%Co/CeO
1%Re-5%Co/CeO
2
2
1E-5
1E-6
1E-7
1E-8
2
1%Re-2.5%Co/CeO
2
1%Re-1%Co/CeO
2
5%Co/CeO
1%Re/CeO
2
2
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
-1
1000/T (K )
3.4. Water gas shift catalytic activity
Fig. 4. (a) WGS catalytic activities of (a) 1%Re/CeO2, (b) 5%Co/CeO2, and (c)–(g)
1%Re–X%Co/CeO2 (X = 1.0, 2.5, 5.0, 7.5 and 10.0) catalysts. (b) Arrhenius plot of reac-
tion rates over 1%Re/CeO2, 5%Co/CeO2 and 1%Re–X%Co/CeO2 (X = 1.0, 2.5, 5.0, 7.5 and
10.0) catalysts.
The WGS catalytic activities of 5%Co/CeO2 monometallic and
1%Re–X%Co/CeO2 bimetallic catalysts are presented in Fig. 4(a),
where the conversion of CO is plotted as a function of tempera-
ture. It was found that doping 1% of Re onto Co/CeO2 with various
loading of Co to form bimetallic catalysts result in shifts of the
conversion curves toward a lower reaction temperature (lines b–f)
compared with that of Co on ceria (line a). The Re–Co/CeO2 bimetal-
lic catalysts exhibit higher CO conversion rate compared to the
monometallic one, which become active at temperatures higher
than 220 ◦C while Xco increases with increasing temperature and
reaches equilibrium conversion around 390 ◦C. The effect of Co
loading on catalytic activities is also illustrated in Fig. 4(a). It is
found that further increases of Co content do not shift the conver-
sion curve toward lower temperature (lines b–f). 1%Re–5%Co/CeO2
is the most active catalyst at this condition and gives measur-
able conversion at temperatures lower than 250 ◦C. The WGS
activity of 1%Re/CeO2 was plotted for comparison (line a) and
it is clearly seen that 1%Re/CeO2 is not very active for the WGS
reaction.
The Arrhenius plots of monometallic and bimetallic catalysts
are illustrated in Fig. 4(b). These plots also display that the rate
of CO conversion of bimetallic catalyst is higher than the rate of
monometallic catalysts. Table 2 summarizes the rate of WGS reac-
tion and apparent activation energies which are obtained from the
slope of the Arrhenius plot. The rates (per gram of catalyst) of
Re–Co/CeO2 bimetallic catalysts are very much higher than those
of Co/CeO2. For example, the WGS rate upon 1%Re–5%Co/CeO2 is
33.9 ␮mol/g s while the rate on 5%Co/CeO2 is only 3.83 ␮mol/g s.
Further addition of Re does not further raise the rate of the reac-
tion and the appropriate amount of Re that is enough to maximize
the WGS rate is 1 weight percent. When attention is drawn to the
variation of Co content, it appears that the bimetallic catalyst with
1%Re and 5%Co gives the highest WGS rate under this working con-
dition. Lowering of the apparent activation energy is also observed.
For reaction on 5%Co/CeO2 the activation energy is 84 kJ/mol while
the activation energies of bimetallic catalysts are between 76 and
80 kJ/mol. These values do not vary with variation of Co or Re
contents which could indicate that under these experimental con-
ditions, the rate limiting step is not affected by variation of Co or Re
contents. Other parameters that did not change with the increase
of metal contents are surface area and H2-chemisorption (Table 1).
This result indicates that the WGS reaction activity depends on the
physical state of the catalyst and not on the metal contents. The
calculated values of Ea are in agreement with those reported for
various metals on ceria supports [40,61,62].
Table 2
The rate of reaction at 290 ◦C and apparent activation energy (Ea) for all doped
catalysts.
Sample
Rate at 290 ◦
C
Ea
(␮mol/g s)
(kJ/mol)
5%Co/CeO2
1%Re/CeO2
3.83
2.80
33.9
30.8
32.4
25.3
23.9
33.9
21.3
23.0
84
90
76
77
79
77
77
76
80
77
1%Re–5%Co/CeO2
2%Re–5%Co/CeO2
5%Re–5%Co/CeO2
1%Re–1%Co/CeO2
1%Re–2.5%Co/CeO2
1%Re–5%Co/CeO2
1%Re–7.5%Co/CeO2
1%Re–10%Co/CeO2

44
K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
allowed transition of Ce 2p to Ce 4f 15d final state [64]. For Ce in
the tetravalent state (CeO2) a double white line is usually observed.
These two peaks were assigned as transitions of Ce 2p to a mixed
valance state of cerium, one with ground state electronic configura-
tion 4f0 and another with 4f1 [65]. Apart from 2 major peaks A and
B, a small low-energy shoulder (c) and a pledge peak (D) are usu-
ally observed for CeO2. The lower energy shoulder was due to the
crystal field splitting of the Ce 5d state for bulk CeO2 or is assigned
to a Ce3+ impurity [64]. The feature D was assigned to the dipole
forbidden 2p3/2 → 4f transition or a transition to the unoccupied Ce
d state at the bottom of the CeO2 conduction band [66]. Our XANES
spectra of 1%Re/CeO2, 5%Co/CeO2 display features that are similar
to those of CeO2.
The jump in energy in XANES spectra indicates energy that
is required to raise electrons up from core level to unoccupied
valence band and it is called the edge energy. The edge energy
can be accurately derived by differentiating the XANES spectra and
assigning the turning point of the derivative spectra as the edge
energy. Various investigators have found a relationship between
metal oxidation states and the energy shifts of the absorption edge
[31,67]. The Ce L3 edge energy shifts of 1%Re/CeO2, 5%Co/CeO2,
1%Re–5%Co/CeO2, Ce(NO3)3·6H2O and CeO2 were plotted relative
to the edge energy of Ce0 in Fig. 5(b). A linear fit was constructed
through L3 energy shifts of Ce0, Ce3+ and Ce4+. The edge energy
shifts of 5%Co/CeO2, 1%Re/CeO2, and 1%Re–5%Co/CeO2 relative to
Ce0 was placed on this straight line at appropriate ꢀE and the
apparent oxidation states of Ce of these compounds lay between
+3 and +4. The result indicates the coexistence of Ce3+ and Ce4+
in which Co reduced surface Ce4+ in CeO2 by transferring electron
density into d-orbitals of Ce4+, leading to a small amount of Ce3+
.
Evidence that electrons were transferred from Co2+ will be shown
by increases of Co oxidation states in the next section. Among
these three catalysts, the oxidation state of Ce in 1%Re–5%Co/CeO2
was the lowest. This result seems to indicate that Re assists Co in
reducing Ce4+ giving rise to more Ce3+at the surface of the ceria
support.
Fig. 5. (a) XANES spectra of the Ce L3 absorption edge for Ce(NO3)3·6H2O (Ce3+),
CeO2 (Ce4+), 1%Re/CeO2, 5%Co/CeO2 and 1%Re–5%Co/CeO2. (b) Relationship between
edge energy shift of Ce compounds and the oxidation states. Filled squares are Ce0,
Ce(NO3)3·6H2O (Ce3+) and CeO2 (Ce4+) standards, open triangles are 1%Re/CeO2,
5%Co/CeO2 and 1%Re–5%Co/CeO2.
3.5.2. Oxidation states of Co and Re
The plot of XANES spectra and the edge energy shift of Co2+
and Co3+ (K absorption edge) relative to the edge energy of Co0
are displayed in Fig. 6. A linear fit was constructed through K edge
energy shift of Co0, Co2+ and Co3+ in Fig. 6(b). The edge energy shifts
of 10%Co/CeO2, 1%Re–10%Co/CeO2 and 10%Re–10%Co/CeO2 when
placed on this straight line indicate that the oxidation state of Co in
these compounds lies above +2. For 10%Co/CeO2, the oxidation state
of Co appears to be ∼+2.1. Together with the result from Fig. 5(b) it
can be concluded that electron density was transferred from Co2+
to Ce4+ leading to increase in oxidation state of Co2+ and lower-
ing of oxidation state of Ce4+. For 1%Re–10%Co/CeO2, the oxidation
state of Co2+ is found to further increase to ∼+2.7. This result indi-
cates that Co2+ also donates some electrons to Re leading to further
increase in its oxidation state. With higher content of Re (10%Re)
further increase in Co oxidation state was observed. From these
results we can conclude that Co2+ donates its electron to both Ce4+
3.5. X-ray absorption spectroscopy
It is interesting to try to understand the role of Re in enhancing
the WGS activity of Co on ceria. The results from the H2-TPR study
indicate that the catalyst surfaces were easier to reduce at tem-
peratures lower than the surface reduction temperature of ceria
when the catalysts were doped with 1%Re. Since the reduction
process involves electron transfer among the involved species, it
is interesting to study the electron movement within the catalyst
surfaces and XAS was employed to study the electronic nature of
our catalysts. In the XAS technique, an intense X-ray beam is used
to excite core electrons of atoms at the surface to the unoccupied
valence shell. The absorption spectra contain two main features:
X-ray absorption near edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS). Information that can be deduced
from XANES includes oxidation states and valence shell electron
population [63]. The XANES technique was practiced in our study
to determine the oxidation states and d-electron density of metals
in the catalysts.
and Re7+
.
Fig. 7 shows XANES spectra and a plot of edge energy shift of
Re M5 absorption edges for 1%Re–10%Co/CeO2, 1%Re–20%Co/CeO2
and 1%Re/CeO2 as compared to Re0 powder and Re7+ in NH4ReO4. A
linear fit was constructed through M5 energy shift of Re0 and Re7+
.
Although only two standards (Re0 and Re7+) were used in this study,
we are confident in drawing a straight line for edge energy shift
of these two standards since many papers have reported a linear
correlation between Re valence and its chemical shift with respect
to metallic Re [30,31]. The edge energy shift of 1%Re–10%Co/CeO2,
and 1%Re–20%Co/CeO2, when placed on this straight line, indicates
the oxidation state of Re to be a little above +6 while the oxidation
3.5.1. Oxidation states of Ce
Fig. 5(a) shows XANES spectra recorded at the Ce L3 edge
for standards Ce(NO3)3·6H2O, CeO2, 1%Re/CeO2 and 5%Co/CeO2.
It is clearly seen that XANES spectra of standard Ce(NO3)3·6H2O
exhibits a single white line at 5725.5 eV. This peak characterizes
the Ce in the trivalent state and the transition was labeled as dipole

K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
45
Fig. 7. (a) XANES spectra of the Re M5 absorption edge for Re metal (Re0),
NH4ReO4 (Re7+), 1%Re/CeO2, 1%Re–10%Co/CeO2 and 1%Re–20%Co/CeO2. (b) Rela-
tionship between edge energy shift of Re compounds and the oxidation states.
Filled squares are Re powder (Re0) and NH4ReO4 (Re7+) standard, open triangles
are 1%Re/CeO2, 1%Re–10%Co/CeO2 and 1%Re–20%Co/CeO2.
Fig. 6. (a) XANES spectra of the Co
K
absorption edge for Co foil (Co0),
Co(NO3)2·6H2O (Co2+), Na3Co(NO2)6 (Co3+), 10%Co/CeO2, 1%Re–10%Co/CeO2 and
10%Re–10%Co/CeO2. (b) Relationship between edge energy shift of Co com-
pounds and the oxidation states. Filled squares are Co0, Co(NO3)3·6H2O (Co2+) and
Na3Co(NO2)6 (Co3+) standards, open circles are 10%Co/CeO2, 1%Re–10%Co/CeO2 and
10%Re–10%Co/CeO2.
absorption for 10%Co in the vicinity of 1%Re and 10%Re. The higher
peak belongs to 10%Re, which also indicates that Co with 10%Re has
less d-electrons than Co with 1%Re. This result indicates that higher
content of Re withdraws more electrons from Co. A catalyst with
higher content of Re withdraws more electrons from Co than a cat-
alyst with less content of Re. Therefore, electron movement from
Co to Re is confirmed. The XANES spectra of Re in Fig. 8(a) con-
tains rather high background due to limitations of the instrument.
The source energy was low and Re absorption was taken at the M5
absorption edge. However, six spectra were averaged and care was
taken in placing the pre-edge and post-edge of the spectra at the
same position. Although the differences of the white line intensi-
ties were low, the direction of electron density transfer indicated
by the white line intensities corresponds well with the information
obtained from Figs. 6 and 7.
The results from oxidation states and white line intensity com-
parison for both Re and Co in the catalyst tend to indicate that
electron densities are transferred from Co to Re d-orbital, giving
rise to lowering of Re oxidation state and increasing of electron
density in Re d-states. The role of Re in enhancing WGS activity of
Re–Co/CeO2 can be explained by the electron withdrawing effect
of Re. As a result of the electron withdrawing effect of Re, the d-
electrons density of Co are partially transferred to the d-orbital
of Re, and so the d-electrons density of Co that are available for
back donation to the ␲* orbital of adsorbed CO are less, the Co-
adsorbate bonds are weaker and the CO2 product can easily leave
the catalyst surface. Apart from an electron withdrawing effect of
state of Re in 1%Re/CeO2 was +7. Lowering of Re oxidation state
in bimetallic Re–Co/CeO2 indicates movement of electron density
into the d-state of Re.
Study of oxidation states of all metals in the catalysts seems to
indicate that Co2+ donates electron density to both CeO2 and Re
in the catalysts, leading to a higher oxidation state of cobalt and
lowering of rhenium and cerium oxidation states.
3.5.3. The role of Re in enhancing WGS activity
To further confirm the direction of electron movement among
the metals in the catalysts, the white line intensities of Re and Co
in the catalysts were compared. The intensity of the white line is
related to occupancy of the d-state of the element. A highly occu-
pied d-state means that fewer core electrons can be excited into
its d-state, therefore the intensity of the absorption peak is low.
Oppositely, if the d-state is only slightly occupied, more core elec-
trons can be excited and move into it, and so the absorption peak is
strong. In comparison, higher white line intensity indicates lower
electron density in d-states. Fig. 8(a) compares white line intensity
of Re M5 absorption for 1%Re in the vicinity of 10% and 20%Co. In
this figure, the white line intensity of 1%Re with 10%Co is a little
higher than that of 20%Co which indicates that Re in the vicinity
of 10%Co has less d-electrons than Re with 20%Co. The result indi-
cates that 10%Co donates less d-electrons to Re than 20%Co. On
the contrary, Fig. 8(b) illustrates the white line intensity of Co K

46
K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
The XANES spectra of Ce L3 absorption edge indicates the presence
of Ce2O3 which produces oxygen vacancies. The XANES spectra of
Re M5 and Co K absorption edges and white line comparison indi-
cate that Re withdraws electron density from Co and lowers the
back-donation of d-electron from cobalt to ␲* orbital of adsorbed
CO molecules. In conclusion, it seems that Re influences the cata-
lysts and the catalyst performance in several ways. Re increases the
reducibility of the surface ceria, and facilitates the redox process at
the surface. Re also increases the metal active sites and weakens
the active metal-adsorbate bonds. All of these effects contribute to
an increase in the WGS reaction rates.
Acknowledgements
The authors would like to thank the Center of Excellence for
Innovation in Chemistry (PERCH-CIC), Commission on Higher Edu-
cation, Ministry of Education and the National Nanotechnology
Center, National Science and Technology Development Agency for
financial support. Generous beam time from the Synchrotron Light
Research Institute (SLRI) is gratefully acknowledged.
References
[1] R.J. Gorte, S. Zhao, Catal. Today 104 (2005) 18–24.
[2] S. Hilaire, X. Wang, T. Luo, R.J. Gorte, J. Wagner, Appl. Catal. A: Gen. 258 (2004)
271–276.
[3] X. Qi, M. Flytzani-Stephanopoulos, Ind. Eng. Chem. Res. 43 (2004) 3055–3062.
[4] M. Asadullah, K. Fujimoto, K. Tomishige, Ind. Eng. Chem. Res. 40 (2001)
5894–5900.
[5] L. Kundakovic, M. Flytzani-Stephanopoulos, J. Catal. 179 (1998) 203–221.
[6] G. Jacobs, E. Chenu, P.M. Patterson, L. Williams, D. Sparks, G. Thomas, B.H. Davis,
Appl. Catal. A: Gen. 258 (2004) 203–214.
[7] E. Mamontov, T. Egami, R. Brezny, M. Koranne, S. Tyagi, J. Phys. Chem. B 104
(2000) 11110–11116.
[8] H. Cordatos, T. Bunluesin, J. Stubenrauch, J.M. Vohs, R.J. Gorte, J. Phys. Chem.
100 (1996) 785–789.
[9] A.D. Mayernick, M.J. Janik, J. Phys. Chem. C 112 (2008) 14955–14964.
[10] A. Martnez-Arias, M. Fernndez-Garca, L.N. Salamanca, R.X. Valenzuela, J.C.
Conesa, J. Soria, J. Phys. Chem. B 104 (2000) 4038–4046.
[11] T. Kim, J.M. Vohs, R.J. Gorte, Ind. Eng. Chem. Res. 45 (2006) 5561–5565.
[12] A. Martnez-Arias, J.M. Coronado, R. Catalua, J.C. Conesa, J. Soria, J. Phys. Chem.
B 102 (1998) 4357–4365.
Fig. 8. (a) Comparison of white line intensity of Re M5 absorption edge
for1%Re–10%Co/CeO2 and 1%Re–20%Co/CeO2. (b) Comparison of white line intensity
of Co absorption edge for 1%Re–10%Co/CeO2 and 10%Re–10%Co/CeO2.
Re, H2-chemisorption study also displays that Re increases the Co
dispersion on the ceria support and the active sites available for
WGS reaction increase.
[13] P. Bera, A. Gayen, M.S. Hegde, N.P. Lalla, L. Spadaro, F. Frusteri, F. Arena, J. Phys.
Chem. B 107 (2003) 6122–6130.
[14] X. Wang, R.J. Gorte, Appl. Catal. A: Gen. 247 (2003) 157–162.
[15] S.H. Oh, G.B. Hoflund, J. Phys. Chem. A 110 (2006) 7609–7613.
[16] J. Zhou, D.R. Mullins, J. Phys. Chem. B 110 (2006) 15994–16002.
[17] A. Gayen, K.R. Priolkar, P.R. Sarode, V. Jayaram, M.S. Hegde, G.N. Subbanna, S.
Emura, Chem. Mater. 16 (2004) 2317–2328.
Another factor that may be of more importance is the effect that
Re contributes to reduction of Ce4+ to Ce3+. The presence of Ce2O3
at the ceria surface gives rise to oxygen vacancies which facilitate
the electron movement at the surface leading to ease of surface
reduction. This result agrees with the TPR results that display sur-
face reduction peaks at a temperature lower than the ceria surface
reduction temperature of 520 ◦C. Many papers have reported that
the WGS mechanism involves the redox mechanism on the ceria
surface [1,6,68]. The major role of Re in enhancing the WGS activity
of Co/CeO2 may be the ability to promote the reduction of the ceria
surface leading to lowering of reduction temperature, increased
oxygen vacancies, and increased surface redox property.
[18] T. Utaka, T. Okanishi, T. Takeguchi, R. Kikuchi, K. Eguchi, Appl. Catal. A: Gen.
245 (2003) 343–351.
[19] A. Basin´ ska, K. Pawełczyk, React. Kinet. Catal. Lett. 89 (2006) 325–331.
[20] V. Idakiev, T. Tabakova, A. Naydenov, Z.Y. Yuan, B.L. Su, Appl. Catal. B: Environ.
63 (2006) 178–186.
[21] W. Deng, A.I. Frenkel, R. Si, M. Flytzani-Stephanopoulos, J. Phys. Chem. C 112
(2008) 12834–12840.
[22] A.M. Duarte de Farias, P. Bargiela, M.G.C. Rocha, M.A. Fraga, J. Catal. 260 (2008)
93–102.
[23] S.Y. Choung, M. Ferrandon, T. Krause, Catal. Today 99 (2005) 257–262.
[24] R. Radhakrishnan, R.R. Willigan, Z. Dardas, T.H. Vanderspurt, Appl. Catal. B:
Environ. 66 (2006) 23–28.
[25] H. Iida, A. Igarashi, Appl. Catal. A: Gen. 303 (2006) 192–198.
[26] A. Venugopal, J. Aluha, M.S. Scurrell, Catal. Lett. 90 (2003) 1–6.
[27] P. Marques, N.F.P. Ribeiro, M. Schmal, D.A.G. Aranda, M.M.V.M. Souza, J. Power
Sources 158 (2006) 504–508.
4. Conclusion
[28] A. Katayama, J. Phys. Chem. 84 (1980) 376–381.
The catalytic activities of Re–Co/CeO2 bimetallic catalysts for
WGS reaction were investigated and the results show that the WGS
reaction rate provided by Re–Co/CeO2 was much higher than the
rate provided by Co/CeO2. XRD and Raman spectroscopy results
indicate that metal oxides were dispersed on ceria surface and H2-
chemisorption indicated better dispersion of Co on the ceria surface
upon addition of Re. H2-TPR results reveal that doping of Re onto
Co/CeO2 to form Re–Co/CeO2 bimetallic catalysts leads to lowering
of surface reduction temperature. The effect of Re on enhancing the
catalytic activity for the WGS reaction was described by XANES.
[29] G.M. Bickle, J.N. Beltramini, D.D. Do, Ind. Eng. Chem. Res. 29 (1990) 1801–1807.
[30] E.L. Kunkes, D.A. Simonetti, J.A. Dumesic, W.D. Pyrz, L.E. Murillo, J.G. Chen, D.J.
Buttrey, J. Catal. 260 (2008) 164–177.
[31] F. Hilbrig, C. Michel, G.L. Haller, J. Phys. Chem. 96 (1992) 9893–9899.
[32] A. Jentys, B.J. McHugh, G.L. Haller, J.A. Lercher, J. Phys. Chem. 96 (1992)
1324–1328.
[33] D.C. Huang, K.H. Chang, W.F. Pong, P.K. Tseng, K.J. Hung, W.F. Huang, Catal. Lett.
53 (1998) 155–159.
[34] L.G.S. Pereira, V.A. Paganin, E.A. Ticianelli, Electrochim. Acta 54 (2009)
1992–1998.
[35] G. Jacobs, J.A. Chaney, P.M. Patterson, T.K. Das, B.H. Davis, Appl. Catal. A: Gen.
264 (2004) 203–212.

K. Chayakul et al. / Journal of Molecular Catalysis A: Chemical 340 (2011) 39–47
47
[36] Y. Li, Q. Fu, M. Flytzani-Stephanopoulos, Appl. Catal. B: Environ. 27 (2000)
179–191.
[37] X. Wang, J.A. Rodriguez, J.C. Hanson, D. Gamarra, A. Martıˇınez-Arias, M.
Fernandez-Garcıia, J. Phys. Chem. B 110 (2006) 428–434.
[38] T. Tabakova, V. Idakiev, J. Papavasiliou, G. Avgouropoulos, T. Ioannides, Catal.
Commun. 8 (2007) 101–106.
[53] G.L. Markaryan, L.N. Ikryannikova, G.P. Muravieva, A.O. Turakulova, B.G.
Kostyuk, E.V. Lunina, V.V. Lunin, E. Zhilinskaya, A. Aboukais, Colloids Surf. A:
Physicochem. Eng. Aspects 151 (1999) 435–447.
[54] Y. Yuan, Y. Iwasawa, J. Phys. Chem. B 106 (2002) 4441–4449.
[55] M.F.L. Johnson, V.M. LeRoy, J. Catal. 35 (1974) 434–440.
[56] A.N. Webb, J. Catal. 39 (1975) 485–486.
[39] M.G. Sanchez, J.L. Gazquez, J. Catal. 104 (1987) 120–135.
[40] P. Panagiotopoulou, J. Papavasiliou, G. Avgouropoulos, T. Ioannides, D.I. Kon-
darides, Chem. Eng. J. 134 (2007) 16–22.
[41] E.S. Bickford, S. Velu, C. Song, Catal. Today 99 (2005) 347–357.
[42] W. Klysubun, P. Sombunchoo, N. Wongprachanukul, P. Tarawarakarn, S.
Klinkhieo, J. Chaiprapa, P. Songsiriritthigul, Nucl. Instrum. Meth. Phys. Res. A
582 (2007) 87–89.
[57] J.Y. Luo, M. Meng, X. Li, X.G. Li, Y.Q. Zha, T.D. Hu, Y.N. Xie, J. Zhang, J. Catal. 254
(2008) 310–324.
[58] T.K. Das, G. Jacobs, P.M. Patterson, W.A. Conner, J. Li, B.H. Davis, Fuel 82 (2003)
805–815.
[59] C.L. Pieck, M.B. González, J.M. Parera, Appl. Catal. A: Gen. 205 (2001) 305–312.
[60] L.F. Liotta, G. Di Carlo, G. Pantaleo, A.M. Venezia, G. Deganello, Appl. Catal. B:
Environ. 66 (2006) 217–227.
[43] Y. Sato, Y. Soma, T. Miyao, S. Naito, Appl. Catal. A: Gen. 304 (2006) 78–85.
[44] W. Cai, F. Wang, E. Zhan, A.C.V. Veen, C. Mirodatos, W. Shen, J. Catal. 257 (2008)
96–107.
[45] Q. Fu, W. Deng, H. Saltsburg, M. Flytzani-Stephanopoulos, Appl. Catal. B: Envi-
ron. 56 (2005) 57–68.
[61] O. Thinon, F. Diehl, P. Avenier, Y. Schuurman, Catal. Today 137 (2008) 29–35.
[62] A.A. Phatak, N. Koryabkina, S. Rai, J.L. Ratta, W. Ruettinger, R.J. Farrauto, G.E.
Blau, W.N. Delgass, F.H. Ribeiro, Catal. Today 123 (2007) 224–234.
[63] Y. Iwasawa (Ed.), X-ray Absorption Fine Structure for Catalysts and Surfaces,
World Scientific Publishing, Singapore, 1995, p. 60.
[46] X. Wu, J. Fan, R. Ran, D. Weng, Chem. Eng. J. 109 (2005) 133–139.
[47] Q. Fu, A. Weber, M. Flytzani-Stephanopoulos, Catal. Lett. 77 (2001) 87–95.
[48] R.D. Monte, J. Kaspar, J. Mater. Chem. 15 (2005) 633–648.
[49] V.G. Keramidas, W.B. white, J. Chem. Phys. 59 (1973) 1561–1562.
[50] J.R. McBride, K.C. Hass, B.D. Poindexter, W.H. Weber, J. Appl. Phys. 76 (1994)
2435–2441.
[64] J. Zhang, X. Ju, Z.Y. Wu, T. Liu, T.D. Hu, Y.N. Xie, Chem. Mater. 13 (2001)
4192–4197.
[65] A.M. Shahin, F. Grandjean, G.J. Long, T.P. Schuman, Chem. Mater. 17 (2005)
315–321.
[66] P. Nachimuthu, W.C. Shih, R.S. Liu, L.Y. Jang, J.M. Chen, J. Solid State Chem. 149
(2000) 408–413.
[51] I. Zacharaki, C.G. Kontoyannis, S. Boghosian, A. Lycourghiotis, Ch. Kordulis,
Catal. Today 143 (2009) 38–44.
[52] D. Gallant, M. Pzolet, S. Simard, J. Phys. Chem. B 110 (2006) 6871–6880.
[67] T. Ressler, J. Wienold, R.E. Jentoft, T. Neisius, J. Catal. 210 (2002) 67–83.
[68] S. Hilaire, X. Wang, T. Luo, R.J. Gorte, J. Wagner, Appl. Catal. A: Gen. 215 (2001)
271–278.