Enhanced performance in NOx reduction by NH3 over a mesoporous Ce-Ti-MoOx catalyst stabilized by a carbon template

Article abstract of DOI:10.1039/c4cy01371a

Mesoporous Ce-Ti-MoO_x catalysts have been prepared by stabilization with an in situ formed carbon template and used for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR). The Ce-Ti-MoO_x catalysts were characterized by N₂ adsorption-desorption, XRD, HR-TEM, H₂-TPR and XPS analysis. It was found that the stabilization with a carbon template obviously improved the NO_x conversion of the Ce-Ti-MoO_x catalyst and the pore distribution. The Ce-Ti-MoO_x catalyst stabilized with a carbon template at 850 °C in N₂ exhibited the best NH₃-SCR activity and showed a uniform mesopore distribution, and more than 90% NO_x conversions were obtained at 175-425°C with a GHSV of 60000 h^-1. The high stabilization temperature with a carbon template in N₂ not only led to an increase in specific surface area, pore volume and low-temperature SCR activity, but also contributed to the enhanced redox properties, highly dispersed titanium or molybdenum species and narrow mesopore distribution. The treatment of Ce-Ti-MoO_x with a high calcination temperature for removal of the carbon template led to a drastic decrease in surface area, an abrupt decrease in surface oxygen and cerium, production of rutile TiO₂, and a correspondingly low SCR activity. This journal is

Full text of DOI:10.1039/c4cy01371a

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Enhanced performance in NOx reduction by
NH3 over a mesoporous Ce–Ti–MoOx catalyst
Cite this: DOI: 10.1039/c4cy01371a
stabilized by a carbon template
*
Qiulin Zhang, Xin Liu, Ping Ning, Zhongxian Song, Hao Li and Junjie Gu
Mesoporous Ce–Ti–MoOx catalysts have been prepared by stabilization with an in situ formed carbon
template and used for selective catalytic reduction of NOx with NH3 (NH3-SCR). The Ce–Ti–MoOx catalysts
were characterized by N2 adsorption–desorption, XRD, HR-TEM, H2-TPR and XPS analysis. It was found
that the stabilization with a carbon template obviously improved the NOx conversion of the Ce–Ti–MoOx
catalyst and the pore distribution. The Ce–Ti–MoOx catalyst stabilized with a carbon template at 850 °C in
N2 exhibited the best NH3-SCR activity and showed a uniform mesopore distribution, and more than
90% NOx conversions were obtained at 175–425 °C with a GHSV of 60000 h−1. The high stabilization
temperature with a carbon template in N2 not only led to an increase in specific surface area, pore volume
and low-temperature SCR activity, but also contributed to the enhanced redox properties, highly dispersed
titanium or molybdenum species and narrow mesopore distribution. The treatment of Ce–Ti–MoOx with a
high calcination temperature for removal of the carbon template led to a drastic decrease in surface area,
an abrupt decrease in surface oxygen and cerium, production of rutile TiO2, and a correspondingly low
SCR activity.
Received 20th October 2014,
Accepted 15th January 2015
DOI: 10.1039/c4cy01371a
www.rsc.org/catalysis
ever-tightening emission regulations, great challenges such
as low-temperature activity and working windows must be
1. Introduction
Ammonia or urea selective catalytic reduction (SCR) is consid-
ered as the most effective technology for removal of NOx from
diesel exhaust gases. Vanadium-based catalysts IJV2O5/TiO2
promoted by WO3 or MoO3) as the most popular commercial
catalysts have achieved industrial success for heavy-duty diesel
exhausts for years.1 However, problems such as a narrow work-
ing window (300–400 °C),2 facile volatility, toxicology of vana-
dium species in the life cycle and difficulty of disposal of the
disused catalyst may be an obstacle for further application of
the vanadium-based catalysts.3 Cu or Fe exchanged zeolites
with uniform micro-mesopores and ultra-large surface area
have been regarded as the new generation SCR catalysts and
have attracted much attention for the abatement of NOx from
diesel engine exhaust. However, the expensive price, high
consumption of power in the production of zeolites and their
inferior SCR activity at low temperature are still the problems
for their wide application in practical processes.4 Since typical
working temperature of NH3(urea)-SCR for de-NOx process in
diesel exhausts ranging from 150 to 250 °C for light duty and
200 to 350 °C for heavy duty diesel engines or even lower
for advanced high-efficiency diesel engines.5,6 To meet the
overcome by de-NOx catalysts.
Recently, many efforts are focused on the development of
cheap and environmentally-friendly catalysts with excellent
low-temperature SCR activity. Some transitional metal oxide
catalysts have been found to be cost-effective and active in
the NH3-SCR process.7–20 It is well known that the basic reac-
tion of NH3-SCR catalysts under oxygen-rich conditions is the
standard NH3-SCR of NO (4NH3 + 4NO + O2 → 4N2 + 6H2O)
since NO consists >90% of NOx in diesel exhausts,9 while the
excellent oxygen storage capability (OSC) of CeO2 could pro-
mote the standard NH3-SCR activity, due to the enhanced
activation of chemisorbed NH3 and the shift of NO to NO2.10
Anatase TiO2 with a high specific area and excellent sulfur
resistance is acclaimed as an ideal carrier for the NH3-SCR
process.3 WO3 and MoO3, as “structural” and “chemical” pro-
moters, can facilitate NH3 adsorption during the NH3-SCR
process.10,11 Compared with WO3 as a commercial SCR cata-
lyst, MoO3 exhibits the promising advantage of a low price.12
Ceria-based oxide catalysts with advantages of unique oxygen
storage capacity, excellent redox properties and good sulfur
resistance in the NH3-SCR process have been attracting more
attention.13–20 CeO2–TiO2 catalysts have been proven to be
active for the NH3-SCR process, even in the presence of H2O,
CO2 and C3H6.13,14 It has been reported that the introduction
of cerium oxides into titanium-based solid acid catalysts can
Faculty of Environmental Science and Engineering, Kunming University of Science
and Technology, Kunming, 650500, PR China. E-mail: ningping_58@126.com;
Tel: +86 871 65170905
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obviously enhance the SCR activity.15,16 In addition,
CeO2-containing solid acid catalysts also show remarkable
NOx conversion.17,18 Recently, CeO2–MoO3 and CeO2–WO3
catalysts have been proven to be promising SCR catalysts
for abating diesel exhaust.19–21 Liu et al. reported that the
CeO2/TiO2 catalyst doped with MoO3 exhibited excellent
NH3-SCR activity.22,23
concentrations of the simulated gases were as follows:
600 ppm NO, 600 ppm NH3, 5% O2, N2 balanced; the gas
hourly space velocity (GHSV) was 60 000 h−1. The concentra-
tions of NOx in the inlet and outlet gas were continually
analyzed by
a RBR ecom-J2KN flue gas analyzer. The
products were analyzed by using gas chromatography (Fuli,
9790) equipment with an electron capture detector for N2O.
The NOx conversion and the N2 selectivity were determined
by the following equations:
The non-silica materials with a uniform mesopore dis-
tribution have been regarded as potential candidates for the
NH3-SCR process.24 However, as far as we know, rare studies have
been reported for the preparation of tri-metal oxide NH3-SCR
catalysts with a uniform mesopore distribution and a large
surface area. Luo25 and Chen et al.26 reported that catalysts
with an ultra-small particle size, large surface area and
porous structures could be prepared by using a carbon tem-
plate, in which sintering of the catalysts could be inhibited.
Compared to various soft or hard template methods, the
in situ formation of a carbon template has advantages such
as cost-effective preparation and high yield of the catalyst.
Thus, the present work has attempted to prepare a meso-
porous Ce–Ti–MoOx catalyst stabilized with an in situ formed
carbon template by a modified sol–gel method. The influence
of the stabilization temperature in the presence of a carbon
template and the calcination temperature for removal of the
carbon template on the NH3-SCR properties has been studied.
XRD, XPS, HR-TEM, H2-TPR and N2 adsorption–desorption
techniques were implemented to characterize the structural and
chemical promotion properties of the Ce–Ti–MoOx catalysts.
NOx inlet  NO outlet


x 

NOx conversion 
100%
100%
(1)
(2)
NOx inlet


NOx inlet  NO outlet  2N O out
x  




2
SN

2
NOx inlet  NO outlet
x  


where NOx (inlet) represents the NOx concentration in the
feed stream, and N2O (out) and NOx (outlet) represent the
N2O and NOx concentrations in the effluent gases, respec-
tively. SN represents the N2 selectivity.
2
2.3 Characterization of the catalysts
The X-ray diffraction (XRD) patterns were recorded on a
Bruker D8 ADVANCE diffractometer operating at 40 kV
and 40 mA with Cu Kα radiation (λ = 0.15406 nm) at a rate of
5° min−1. Specific surface areas (SSA) were determined by
nitrogen adsorption–desorption at 77 K using a TriStar II
3020 instrument. The pore volume and pore volume distribu-
tion were determined by the adsorption isotherms. The X-ray
photoelectron spectroscopy (XPS) studies were carried out on
ULVAC PHI 5000 Versa Probe-α equipment with Al Kα X-ray
radiation under UHV. All peaks were calibrated by using the
C1s peak (284.8 eV). High-resolution transmission electron
microscopy (HR-TEM) characterization was performed on a FEI
Tecnai G220 apparatus operating at 200 kV. H2-temperature
programmed reduction (H2-TPR) was carried out in a quartz
reactor, and 100 mg of sample was first pre-treated in N2
(30 mL min−1) at 400 °C and then cooled to room temperature.
After this, a flowing mixture of 5% H2/Ar (30 ml min−1)
was switched on with a linear heating rate of 8 °C min−1.
The H2 consumption signal was continuously monitored by a
thermal conductivity detector (TCD).
2. Experimental
2.1 Preparation of the Ce–Ti–MoOx catalysts
Ce–Ti–MoOx catalysts with a Ce/Ti molar ratio of 1 : 1 and a
nominal MoO3 content of 10% were prepared by a modified
sol–gel method. Cerium nitrate, ammonium molybdate and
citric acid were dissolved in a mixture of distilled water and
ethanol. Butyl titanate was dissolved in ethanol. The two
kinds of solutions were mixed with stirring and then evapo-
rated to form homogeneous gels. The gels were subsequently
calcined under an N2 atmosphere for 4 h at different temper-
atures for the in situ formation of a carbon template, and
then the resulting material was calcined in air for 3 h at
different temperatures for the removal of carbon species.
Pure MoO3 was prepared by calcining ammonium molybdate
at 600 °C for 3 h in air. Pure CeO2 and anatase TiO2 were
purchased from Aladdin Industrial Corporation. The
Ce–Ti–MoOx catalysts were denoted as NxAy; Nx and Ay repre-
sent the calcination temperature (x) under an N2 atmosphere
and the calcination temperature (y) in air, respectively.
3. Results and discussion
3.1 NH3-SCR catalytic activity
The effects of the stabilization temperature in the presence
of a carbon template and the calcination temperature for
removal of the carbon template on the NH3-SCR properties
have been studied for the Ce–Ti–MoOx catalyst. As shown in
Fig. 1(a), the Ce–Ti–MoOx catalyst with different stabilization
temperatures in the presence of a carbon template under an
N2 atmosphere shows a varied activity for the reduction of
NO by NH3. It can be seen that increasing the stabilization
temperature in the presence of a carbon template under an
2.2 NH3-SCR catalytic test
The NH3-SCR catalytic activity measurement was carried out
in a fixed-bed quartz flow reactor (i.d. = 10 mm). The reaction
temperature was monitored by a K-type thermocouple inserted
into the catalyst bed. Reactant gases were controlled by means
of mass-flow controllers before entering the reactor. The
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Fig. 1 NH3-SCR activities over Ce–Ti–MoOx catalysts. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5%, N2 balanced, GHSV = 60 000 h−1
.
N2 atmosphere increases the NOx conversion. The N850A400
catalyst shows over 90% NOx conversion at 170–430 °C. In terms
of cost and energy consumption, stabilization temperatures
above 850 °C were not recommended in the present study.
We further studied the effect of removing the carbon
template at different annealing temperatures in air on the
SCR activity of the Ce–Ti–MoOx catalyst. The results indicate
that the annealing temperature in air influences the SCR
activity significantly. From Fig. 1(b), the NOx conversions over
the Ce–Ti–MoOx catalysts decrease with the increase in
annealing temperature for removal of the carbon template in
air. The highest NOx conversion over the N550A850 catalyst is
only 50%, which indicates that a higher calcination tempera-
ture for removal of the carbon template can lead to an abrupt
deactivation of the Ce–Ti–MoOx catalysts.
The results of the N2 selectivity and the production of N2O
are shown in Fig. 2. It can be inferred from Fig. 2 that all of
the Ce–Ti–MoOx catalysts exhibit over 95% N2 selectivity and
no more than 25 ppm N2O concentration at 150–350 °C,
except for N550A850. The N850A400 catalyst even shows over
Fig. 2 N2 selectivities and N2O concentrations over the different catalysts. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5%, N2 balanced,
GHSV = 60 000 h−1
.
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Fig. 3 XRD patterns of (a) pure CeO2, TiO2 and MoO3; (b) and (c) Ce–Ti–MoOx catalysts under different treatment conditions.
98% N2 selectivity and no more than 12 ppm N2O concentra-
tion at 150–350 °C. Although the Ce–Ti–MoOx catalysts pres-
ent inferior N2 selectivity above 375 °C, generally, the typical
operating temperatures of diesel engines are at 150–250 °C
for the light duty and 200–350 °C for the heavy duty diesel
engines.5,6 So the N2 selectivity of the Ce–Ti–MoOx catalysts
matches well with the operating temperature of light and
heavy duty diesel engines, indicating the application poten-
tial of the Ce–Ti–MoOx catalysts.
visible phase of Ce, Mo or Ti. But when the annealing tem-
perature in air is beyond 550 °C, the Ce–Ti–MoOx catalyst
exhibits an increasingly intense and sharp peak of cerianite
CeO2. The diffraction peaks of the monoclinic MoO2 phase
appear when the calcination temperature is at 550–650 °C.
However, when the calcination temperature rises up to
750 °C, the diffraction peaks of monoclinic MoO2 disappear.
When the Ce–Ti–MoOx catalyst is calcined at 750–850 °C in
air, the obvious phase of rutile TiO2 can be detected.
It is clear that the Ce–Ti–MoOx catalyst stabilization with a
carbon template can inhibit the generation of rutile TiO2 at
thermal treatment temperatures as high as 850 °C.
Otherwise, the Ce–Ti–MoOx catalyst stabilization with a
carbon template in N2 leads to the absence of molybdenum,
titanium and CeO2 crystallization phases, which indicates
that strong interactions exist among molybdenum, titanium
and cerium oxides. The Ce–Ti–MoOx catalyst treated at
increasing temperatures in air for removal of the carbon tem-
plate leads to an increase of crystallization of metal oxides.
As a result, the enhanced crystallization of the metal oxides
reduces the interactions among the metal oxides.
3.2 XRD analysis
Fig. 3(a) shows the XRD patterns of pure CeO2, MoO3 and
TiO2. The results indicate that all of the pure samples present
intense and sharp diffraction peaks. For pure CeO2, MoO3
and TiO2, crystalline phases of cerianite CeO2, orthorhombic
MoO3 and anatase TiO2 are observed, respectively. The XRD
patterns of the Ce–Ti–MoOx catalysts under different treat-
ment conditions are shown in Fig. 3(b) and (c). As shown in
Fig. 3(b), the Ce–Ti–MoOx catalyst with different stabilization
temperatures in the presence of a carbon template in an N2
atmosphere shows no obvious diffraction peaks of any
molybdenum and titanium oxides. Furthermore, no obvious
diffraction peaks of any cerium oxides are observed until the
stabilization temperature is 850 °C, but N850A400 only shows
a few diffraction peaks of cerianite CeO2.
The X-ray powder diffraction (XRD) patterns of the Ce–Ti–
MoOx catalysts treated at different temperatures in air for
removal of the carbon template are shown in Fig. 3(c). The
results indicate that the calcination temperature in air influ-
ences the crystalline phases significantly. The XRD pattern of
the Ce–Ti–MoOx catalyst treated at 450 °C in air shows no
3.3 N2 adsorption–desorption results
The specific surface area (SSA), pore volume and pore size of
the various catalysts are listed in Table 1. For the Ce–Ti–MoOx
catalyst, the increase in the stabilization temperature in the
presence of a carbon template under an N2 atmosphere leads
to an increasing trend of the surface areas and cumulative
pore volumes. But the Ce–Ti–MoOx catalyst treatment at
increasing temperatures in air for removal of the carbon
template leads to a sharp decrease in surface areas and
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Table 1 BET specific surface area and pore volume of the various
catalysts
indicating the presence of mesopores. The isotherms of
N450A450 and N550A550 show an uptake at a low relative pres-
sure IJP/P0 ≤ 0.03), indicating the presence of micropores,29
while the hysteresis loops of N550A550 and N550A650 approach
P/P0 = 1, indicating the presence of macropores (>50 nm).
Thus, it could be inferred that the pore size distributions of
N550A550 and N550A650 are wider than that of N450A450.
N550A750 and N550A850 exhibit a typical shape of type III
(IUPAC classification), indicating non-porous structures.
Samples
SSA (m2 g−1
)
Pore volume (cm3 g−1
)
N450A400
N550A400
N650A400
N750A400
N850A400
N450A450
N550A550
N550A650
N550A750
N550A850
42
60
62
86
91
55
19
6
0.060
0.066
0.074
0.074
0.106
0.084
0.077
0.032
0.009
0.003
The pore size distributions calculated from the N2 adsorp-
tion isotherms of various Ce–Ti–MoOx catalysts are shown in
Fig. 5. It can be seen from Fig. 5 that the Ce–Ti–MoOx cata-
lysts treated at different stabilization temperatures in the
presence of a carbon template under an N2 atmosphere
exhibit narrow pore size distributions in the main range of
2–22 nm. The full width at half maximum (fwhm) of below
4 Å measured for the pore size distribution in the main range
of 2–10 nm for N750A400 and N850A400 indicates the
uniform mesopore dimensions. N850A400, which shows
the largest pore volume and SBET, exhibits a higher NH3-SCR
activity than the others. N650A400 and N550A400, which
exhibit wider pore size distributions at 2–10 nm and no more
than 4 nm of fwhm, also shows uniform mesopore distri-
butions. N450A450 reflects a bimodal pore size distribu-
tion in the range of 2–12 nm. In comparison, it can be seen
from Fig. 5 that N550A550 and N550A650 exhibit a broad
diameter range of pore size distribution. Above all, the treat-
ment of the Ce–Ti–MoOx catalysts with increasing stabiliza-
tion temperatures in the presence of a carbon template under
an N2 atmosphere may contribute to the decreasing diameter
range of pore size distributions. In contrast, the ranges of
pore size distributions can be extended by increasing the
treatment temperatures at 450–650 °C in air for removal of
the carbon template. Ce–Ti–MoOx catalyst calcination above
650 °C in air for removal of the carbon template may lead to
the collapse of the pore structure. To our great interest, it
can be inferred that the narrow diameter range of the pore
3
1
cumulative pore volumes. It could be seen from Table 1 that
N850A400 exhibits the largest specific surface area (91 m2 g−1
)
and cumulative pore volume (0.106 cm3 g−1), while N550A850
shows the smallest specific surface area (1 m2 g−1) and cumu-
lative pore volume (0.003 cm3 g−1). Therefore, it could be con-
cluded that the stabilization treatment in the presence of a
carbon template can prevent the Ce–Ti–MoOx catalysts from
being sintered at thermal treatment temperatures as high as
850 °C.
The nitrogen sorption isotherms of the Ce–Ti–MoOx cata-
lysts with different stabilization temperatures in the presence
of a carbon template under an N2 atmosphere are listed in
Fig. 4(a). It can be inferred from Fig. 4(a) that the isotherms
of the Ce–Ti–MoOx catalysts exhibit a typical shape of type IV
(IUPAC classification), indicating the presence of mesopores
(2–50 nm).27 The narrow hysteresis loops of the catalysts listed
in Fig. 4(a) exhibited a typical type H2 (IUPAC classification),
indicating a narrow distribution of pore diameters.28 The
nitrogen sorption isotherms of the Ce–Ti–MoOx catalysts with
different annealing temperatures in air for removal of the
carbon template are listed in Fig. 4(b). The nitrogen sorption
isotherms of N450A450, N550A550 and N550A650 can also be
assigned to the typical shape of type IV (IUPAC classification),
Fig. 4 N2 adsorption–desorption isotherms of the various catalysts at
77 K.
Fig. 5 Pore size distributions of the different Ce–Ti–MoOx catalysts.
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size distributions improves the low-temperature NO conver-
sion over the Ce–Ti–MoOx catalysts as described in Fig. 1.
(110),33,34 respectively. It is noteworthy that rutile TiO2 and
cerianite CeO2 crystals of N550A850 have a crystallite size of
more than 20 nm (Fig. 6(c)), indicating the high crystallinity
of cerium and titanium oxides, while the lattice fringes of
anatase TiO2 and MoO3 could not be observed. Thus, it can
be inferred that a majority of molybdenum oxide exists in the
form of surface amorphous species, while a majority of
surface titanium and cerium oxides is transformed into
bulk TiO2 and CeO2 as the calcination of Ce–Ti–MoOx at
850 °C in air for removal of the carbon template occurs. To
our best knowledge, the decrease in surface amorphous
titanium and cerium oxides may lead to a relative increase in
surface molybdenum species and a decrease in the interactions
among amorphous surface molybdenum, titanium and cerium
oxides. Hence, further studies on temperature programmed
reduction and surface analysis are expected to be explored as
well as interactions among the tri-metal oxides.
3.4 HR-TEM analysis
High-resolution transmission electron microscopy (HR-TEM)
analysis was used to further explore the phase composition,
microstructure and particle size distribution of the catalysts.
Metal oxide crystals were determined by the interplanar distance
measurement. It can be obviously seen from Fig. 6(b) and (d)
that the N850A400 catalyst has a mean particle diameter of
~22 nm (range from 9 to 27 nm), while the N550A850 catalyst
has a mean particle diameter of ~282 nm (range from 165
to 356 nm). Therefore, it is evident that the stabilization
treatment in the presence of a carbon template under an
N2 atmosphere is rather critical to stop the Ce–Ti–MoOx
catalysts from being sintered. The lattice fringes of N850A400
(Fig. 6(a)) with an interplanar distance of 0.271 nm reveal the
(200) lattice plane of the cubic cerianite cell,30 indicating the
existence of a cerianite CeO2 structure. In addition, it can be
seen from Fig. 6(a) that N850A400 shows no more than
10 nm cerianite crystallite size. Over half of the areas adja-
cent to the cerianite CeO2 (Fig. 6(a)) could be attributed to
the semi-crystalline or amorphous zone of non-crystalline
molybdenum, titanium and cerium oxides. The lattice fringes
of anatase, rutile as well as MoO3 could not be observed from
the image of N850A400, which is consistent with our XRD
results, indicating high dispersions of amorphous metal
oxides and strong interactions among tri-metal oxides. In
comparison, it can be seen from the HR-TEM images of
N850A400 in Fig. 6(c) that the lattice fringes with interplanar
distances of 0.314 nm and 0.324 nm correspond to the crys-
tallite orientations of cerianite CeO2 (111)30–32 and rutile TiO2
3.5 H2-TPR and XPS results
3.5.1 H2-TPR results. The redox properties of cerium-based
catalysts are rather critical for the oxygen storage capacity
(OSC) in the standard NH3-SCR process (4NH3 + 4NO + O2 →
4N2 + 4H2O) in the presence of excess oxygen since NO
consists >90% of NOx in diesel exhausts.9 Thus, H2-TPR is a
useful tool in evaluating the redox properties of SCR cata-
lysts. Fig. 7 presents the H2-TPR profiles of the various cata-
lysts. A gradual consumption of H2 at 260 to 532 °C could be
assigned to the removal of surface oxygen-containing func-
tional groups,35 partial reduction of surface Ce4+ → Ce3+,36–38
and well-dispersed high-valent MoIJVI) species.39,40 It is well
known that the reduction of titanium oxides is harder than
that of cerium and molybdenum oxides below 700 °C;36,41
Fig. 6 High-resolution transmission electron microscopy (HR-TEM) images of IJa)–IJb) N850A400, and IJc)–IJd) N550A850 catalysts.
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concentrations of Ce, Ti, Mo and O were determined, while
the peak areas and the full width at half maximum (fwhm)
were determined by using the XPS PeakFit (Version 4.0, AISN
Software Inc.) program. In our present study, the surface
atomic ratios of Ce3+/Ce4+ were calculated according to the
following equation:
S2  S6
Ce3 % 


(3)
S1  S2  S3  S4  S5  S6  S7  S8
where S1, S2, S3, S4, S5, S6, S7 and S8 represent the peak areas
of the corresponding u, u′, u″, u‴, v, v′, v″ and v‴ lines,
respectively.
The surface atomic ratios of Osurface/Ototal were determined
according to the following equation:
Ssurface
Osurface / Ototal % 
100%


(4)
Fig. 7 H2-TPR profiles of the various catalysts.
Ssurface  Slattice
where Ssurface and Slattice are the peak areas corresponding to
O2− and OH−/CO3 respectively.
2−
therefore, the peaks of the various catalysts listed below
700 °C could be assigned to the reduction of surface CeO2
and reduction of MoIJVI) to MoIJIV). It is well known that the
relative amount of reducible components can be assessed by
H2 consumption in H2-TPR. The total H2 consumption over
different catalysts at 400–800 °C is calculated and listed in
Table 2. As shown in Table 2, the H2 consumptions of the
various catalysts are in the order: N850A400 > N750A400 >
N550A750 > N550A850. It can also be seen from Fig. 7 that
the reduction peak of N850A400 exhibits a higher intensity of
H2 consumption and a lower reduction temperature than the
others listed, indicating that N850A400 has the highest
oxygen storage capacity. In terms of N850A400 and
N750A400, it could be inferred that the Ce–Ti–MoOx with a
higher stabilization temperature in the presence of a carbon
template under an N2 atmosphere enhance the redox capacity.
The reduction peaks of N550A750 and N550A850 shift to
673 °C, which is obviously higher than that of the others,
indicating a decrease in the interaction among metal oxide
species and/or a drastic decrease in well-dispersed surface
metal oxides during the calcination process in air for removal
of the carbon template, which is consistent with our XRD
(Fig. 3(c)) and HR-TEM (Fig. 6(c) and (d)) results. It is note-
worthy that N550A850 exhibits a lower intensity of H2 con-
sumption than N550A750 at 650 to 775 °C, indicating a sharp
decrease in amorphous surface molybdenum and/or cerium
oxides. However, in order to determine this decrease in the
reduced surface species, further studies on the surface composi-
tion and chemical states of the metal species are still expected.
3.5.2 XPS analysis results. X-ray photoelectron spectroscopy
(XPS) analysis was used to clarify the surface composition
and chemical states of the catalysts. The surface atomic
Table 3 shows the binding energies and full width at half
maximum (fwhm) of surface titanium species of N850A400
and N550A850 determined from XPS analysis. The two peaks
of the Ti 2p orbital located at around 458.5 and 464.3 eV
indicate that titanium species of N850A400 and N550A850
exist in the form of titaniumIJIV) species. N550A850 shows a
smaller full width at half maximum (fwhm) than N850A400
which indicates that N550A850 has a higher degree of
order,42,43 while a high degree of order may further indicate
the high crystallinity of surface titanium species,44 which is
Table 3 Fwhm of surface titanium species determined from XPS analysis
Positions (eV)
2p 3/2
Fwhm (eV)
2p 3/2
Samples
2p 1/2
2p 1/2
N850A400
N550A850
458.6
458.7
464.4
464.5
1.43
1.13
2.13
1.92
Table 2 H2 consumption of the various catalysts
Samples
N850A400
8296
N750A400
7256
N550A750
3747
N550A850
3172
Areas (a.u.)
Fig. 8 XPS patterns of the O 1s spectra over Ce–Ti–MoOx catalysts.
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different calcination temperatures for removal of the carbon
template have little effect on the valence states of titaniumIJIV)
species.44,45 It could be seen from Fig. 8(a) and (b) that the
obvious peaks and shoulders located at 529.2–530.3 and 530.4–
531.2 could be attributed to lattice oxygen (O2−), hydroxyl and/
or carbonate species and adsorbed oxygen, respectively.46,47 No
obvious signals of surface adsorbed molecular water could be
detected.
The Ce 3d XPS spectra are shown in Fig. 9(a) and (b). The
peaks located at 883.2 0.2 eV, 889.3 0.4 eV, and 899.0
0.3 eV (denoted with u, u″ and u‴, respectively) represent the
spin–orbital doublets of Ce 3d5/2 and the peaks located at
901.8 0.3 eV, 908.3 0.3 eV and 917.3 0.3 eV (denoted
with v, v″ and v‴, respectively) represent the Ce 3d3/2, indicat-
ing the presence of Ce4+. The shoulders located at 886.0
0.3 eV and 904.1 0.4 eV (denoted with u′ and v′, respectively)
could be assigned to Ce3+.31,48 The peaks of u, u″ and u‴
representing Ce 3d5/2 could be attributed to the multielectron
configuration of the 3d94f2 (O 2p4), 3d94f1 (O 2p5) Ce4+ and
3d94f0 (O 2p6) Ce4+ final states, while the shoulder u′ located
at 886.0 0.3 eV could be attributed to the multielectron con-
figuration of the 3d94f1 (O 2p6) Ce3+ final state.49–52 Thus, it
can be concluded that a majority of the surface cerium spe-
cies is Ce4+, while the Ce3+ oxidation state is indistinctly iden-
tified through the Ce 3d spectra. The XPS spectra of Mo 3d for
various catalysts are listed in Fig. 10(a) and (b). All of the spec-
tra contained duplet signals corresponding to MoIJVI) oxide
with a binding energy of ~232.5 eV, which corresponds to the
binding energy of Mo 3d3/2, indicating the presence of Mo6+
surface species.53 Combining with our HR-TEM and XRD
analysis results, it could be concluded that Mo exists in the
form of amorphous Mo6+ and Mon+ (0 < n < 6) species.53
To the best of our knowledge, the presence of surface OH
groups may facilitate NO adsorption in the presence of oxy-
gen,54 and surface adsorbed H2O and OH groups may play an
important role in the adsorption activation of NH3 in the
NH3-SCR process.55 In addition, a suitable surface molar
ratio of Ce3+/Ce4+ plays an important role in the Ce3+ ⇌ Ce4+
redox cycle for the NH3-SCR process in the presence of oxy-
gen.10 It can be seen from Table 4 that the surface cerium,
titanium, molybdenum, oxygen atomic concentrations,
Fig. 9 XPS patterns of the Ce 3d spectra: (1) N450A400, (2) N550A400,
(3) N650A400, (4) N750A400, (5) N850A400, (6) N450A450, (7) N550A550,
(8) N550A650, (9) N550A750 and (10) N550A850.
Fig. 10 XPS patterns of the Mo 3d spectra over Ce–Ti–MoOx catalysts.
consistent with our XRD (Fig. 3(b) and (c)) and HR-TEM
(Fig. 6(a) and (c)) analysis results. Therefore, it can be con-
cluded that the different stabilization temperatures in the pres-
ence of a carbon template under an N2 atmosphere and the
O
surface/Ototal and Ce3+/Ce4+ of all the Ce–Ti–MoOx catalysts
Table 4 Surface atomic concentrations of the Ce–Ti–MoOx catalysts determined by the XPS analysis
Surface atomic concentration (%)
Samples
Ce
Ti
Mo
O
Osurface/Ototal
Ce3+/Ce4+
Ce : Ti : Mo
N450A400
N550A400
N650A400
N750A400
N850A400
N450A450
N550A550
N550A650
N550A750
N550A850
37.03
35.32
37.65
36.57
36.43
36.27
35.76
37.32
39.20
24.02
9.45
3.93
5.40
4.30
5.67
4.10
4.17
4.90
3.96
3.84
6.65
49.58
50.01
48.81
49.15
49.55
50.85
49.68
49.08
48.2
0.314
0.281
0.317
0.266
0.321
0.294
0.317
0.302
0.346
0.231
0.148
0.157
0.164
0.158
0.143
0.161
0.191
0.164
0.191
0.186
1 : 0.25 : 0.11
1 : 0.26 : 0.15
1 : 0.25 : 0.11
1 : 0.24 : 0.16
1 : 0.27 : 0.11
1 : 0.24 : 0.11
1 : 0.27 : 0.14
1 : 0.26 : 0.11
1 : 0.22 : 0.10
1 : 0.58 : 0.28
9.27
9.24
8.61
9.92
8.71
9.66
9.64
8.75
14.06
55.27
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listed show no obvious differences within systematic errors
except for N550A850, which shows a drastic decrease in
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concentrations. Thus, it can be inferred that a shift in the
calcination temperature for removal of the carbon template
in air from 750 to 850 °C leads to an abrupt increase in
surface titanium, molybdenum and oxygen atomic concen-
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is consistent with our H2-TPR analysis. It is interesting that
N550A750 and N550A850 show no significant difference
in SSA, pore size properties and phase compositions, while
N550A850 shows an abrupt decrease in NOx conversion
as described in Fig. 1(b). Therefore, it could be inferred
that different surface atomic ratios are one of the major
factors that contribute to the significant difference in
NH3-SCR activity.
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4. Conclusion
The present work has successfully synthesized a mesoporous
Ce–Ti–MoOx catalyst stabilized by an in situ formed carbon
template. The higher stabilization temperature in the pres-
ence of a carbon template under an N2 atmosphere and the
lower thermal treatment temperature for removal of the
carbon template have promoted and influenced the NOx
reduction activity. The Ce–Ti–MoOx catalyst stabilized with a
carbon template at 850 °C under an N2 atmosphere and
calcined at 400 °C in air for removal of the carbon template
exhibits a nanoparticle size of 22 nm, the largest surface area
of 91 m2 g−1 and the highest SCR activity at 150–450 °C. The
higher the stabilization temperature in the presence of a
carbon template under an N2 atmosphere, the higher the
surface area and low-temperature NOx conversion obtained.
The stabilization treatment with a carbon template under
an N2 atmosphere can stop the crystallization of molyb-
denum, titanium and cerium oxides. The higher the thermal
treatment temperature for removal of the carbon template,
the lower the surface areas and NOx conversions gained.
Otherwise, the increasing calcination temperature for
removal of the carbon template contributes to the increased
crystallinity of titanium and increased collapse of the pore
structures.
Acknowledgements
This work is supported by the National Natural Science
Foundation of China (no. U1137603, no. 21307047) and the
Opening Project of the Key Laboratory of Green Catalysis of
the Sichuan Institutes of Higher Education (LYJ1309).
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