Synergetic effect between photocatalysis on TiO2 and thermocatalysis on CeO2 for gas-phase oxidation of benzene on TiO2/CeO2 nanocomposites

Article abstract of DOI:10.1021/acscatal.5b00292

TiO₂/CeO₂ nanocomposites of anatase TiO₂ nanoparticles supported on microsized mesoporous CeO₂ were prepared and characterized by SEM, TEM, BET, XRD, Raman, XPS, and diffuse reflectance UV-vis absorption. The formation of the TiO₂/CeO₂ nanocomposites considerably enhances their catalytic activity for the gas-phase oxidation of benzene, one of the hazardous volatile organic compounds (VOCs), under the irradiation of a Xe lamp compared to pure CeO₂ and TiO₂. A solar-light-driven thermocatalysis on CeO₂ is found for the TiO₂/CeO₂ nanocomposites. There is a synergetic effect between the photocatalysis on TiO₂ and the thermocatalysis on CeO₂ for the TiO₂/CeO₂ nanocomposites, which significantly increases their catalytic activity. The CO₂ formation rate (r_CO2) of the TiO₂/CeO₂ nanocomposite with the Ti/Ce molar ratio of 0.108 under the synergetic photothermocatalytic condition is 36.4 times higher than its r_CO2 under the conventional photocatalytic condition at near room temperature. CO temperature-programmed reduction (CO-TPR) with the irradiation of the Xe lamp and in the dark reveals that the synergetic effect, which occurs at the interface of the TiO₂CeO₂ nanocomposite, is due to the considerable promotion of the CeO₂ reduction by the photocatalysis on TiO₂.

Full text of DOI:10.1021/acscatal.5b00292

Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Article
Synergetic Effect between Photocatalysis on TiO2 and Thermocatalysis on
CeO2 for Gas-Phase Oxidation of Benzene on TiO2/CeO2 Nanocomposites
Min Zeng, Yuanzhi Li, Mingyang Mao, Jilin Bai, Lu Ren, and Xiujian Zhao
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00292 • Publication Date (Web): 20 Apr 2015
Downloaded from http://pubs.acs.org on April 21, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
Synergetic Effect between Photocatalysis on TiO and Thermocatalysis on CeO for Gas-Phase
2
2
Oxidation of Benzene on TiO
2
/CeO
2
Nanocomposites
Min Zeng, Yuanzhi Li*, Mingyang Mao, Jilin Bai, Lu Ren, Xiujian Zhao
State Key Laboratory of Silicate Materials for Architectures (Wuhan University of Technology), 122
Luoshi Road, Wuhan 430070, P. R. China. Email: liyuanzhi66@hotmail.com
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Abstract. TiO
mesoporous CeO were prepared and characterized by SEM, TEM, BET, XRD, Raman, XPS, and
diffuse reflectance UVꢀVis absorption. The formation of the TiO /CeO nanocomposites considerably
enhances their catalytic activity for the gasꢀphase oxidation of benzene as one of hazardous volatile
organic compounds (VOCs) under the irradiation of a Xe lamp compared to pure CeO and TiO . A
solar light driven thermocatalysis on CeO is found for the TiO /CeO nanocomposites. There is a
synergetic effect between the photocatalysis on TiO and the thermocatalysis on CeO for the
TiO /CeO nanocomposites, which significantly increases their catalytic activity. The CO formation
rate (rCO2) of the TiO /CeO nanocomposite with the Ti/Ce molar ratio of 0.108 under the synergetic
2 2 2
/CeO nanocomposites of anatase TiO nanoparticles supported on microsized
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
photothermocatalytic condition is 36.4 times higher than its rCO2 under the conventional photocatalytic
condition at near room temperature. CO temperatureꢀprogrammed reduction (COꢀTPR) with the
irradiation of the Xe lamp and in dark reveals that the synergetic effect, which occurs at the interface of
the TiO
2 2 2
/CeO nanocomposite, is due to the considerable promotion of the CeO reduction by the
photocatalysis on TiO
2
.
Keywords: TiO
2
, TiO
2
/CeO
2
nanocomposite, photocatalysis, thermocatalysis, photothermocatalysis,
synergetic effect, VOCs
1
ACS Paragon Plus Environment

ACS Catalysis
Page 2 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1. Introduction
Volatile organic compounds (VOCs) as major air pollutants in indoor air, polluted urban
atmospheres, factories of petrochemical, fine chemical, paint, and so on are harmful to human health as
well as the environment. It is highly desirable to develop efficient technology for the abatement of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1,2
VOCs. Heterogeneous photocatalysis by nano semiconductors is a promising technology for the
abatement of VOCs as it is energyꢀsaving, and operates under mild conditions, such as ambient
temperature, atmospheric pressure, and using oxygen (air) as the oxidizing agent. Among the various
photocatalysts, nanostructured TiO
2
is one of the most prominent photocatalysts due to its excellent
3
catalytic performance, chemical stability, low cost, and nontoxicity. However, there are three
bottlenecks which greatly retard its extensive application for the removal of VOCs. One is its low
4,5
quantum efficiency due to fast recombination of photogenerated electrons and holes. The other is that
it is only photoactivated by UV, which accounts for ~ 5% of the sunlight, due to its wide band gap (3.2
6
ꢀ8
eV for anatase, and 3.0 eV for rutile). The two disadvantages make the photocatalysis only efficient
for the removal of low concentration VOCs. The third is that it is prone to deactivation mainly due to
9
ꢀ11
the deposition of less reactive byproducts on the TiO
2
surface. Another efficient technology for the
removal of VOCs is heterogeneous thermocatalysis that has been widely applied in industry. Expensive
1
2
noble metals are conventionally used as the most efficient thermocatalysts. The main advantages of
thermocatalysis are that it is efficient for the removal of high concentration VOCs, and has good
thermocatalytic durability. However, compared to the photocatalysis, thermocatalysis is
energyꢀconsuming, and must operate at higher temperature by using additional heater. Therefore,
finding a novel strategy for the efficient abatement of VOCs with the advantages of both the
photocatalysis and thermocatalysis, which could utilize renewable solar energy and avoid their
disadvantages, would be scientifically and technologically significant.
Nanostructured CeO
2
is one of the efficient thermocatalysts besides the expensive noble metals
4+
3+
for the removal of air pollutants including VOCs owing to its remarkable Ce /Ce redox
1
3ꢀ20
properties.
{100}, {110}),
nanostructured CeO
activity under UV and/or visible irradiation,
Its thermocatalytic activity can be improved by controlling the size, exposed facets (e.g.
1
4ꢀ16
17ꢀ20
and surface structure, such as oxygen vacancies,
etc. On the other hand,
has photocatalytic
and its photocatalytic activity can be improved by
2
as nꢀtype semiconductor with band gap narrower than TiO
2
2
1ꢀ24
2
5,26
controlling its morphology, doping metal ions,
2
and forming nanocomposite with TiO to increase
2
ACS Paragon Plus Environment

Page 3 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
2
7ꢀ39
eꢀh separation efficiency.
Recently, a synergetic effect of photo/thermoꢀcatalysis was reported by
several groups. With the synergetic effect, significant enhancement in the catalytic activity was
40
41, 42
achieved on mesoporous CeO
2
and Mn, Bi doped CeO
2
for the abatement of VOCs such as
43
benzene, formaldehyde, and on Y doped CeO
2
for the photodegradation of dye.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Recently, we developed a facile templateꢀfree approach to synthesize microsized mesoporous
CeO
thermocatalytic activity as compared to CeO
Herein, we load TiO
2
and we reported that the microsized mesoporous CeO
2
exhibited a significant enhancement of
2
0
2
nanoparticles and nanocubes without mesopores.
2
2 2
nanoparticles on the microsized mesoporous CeO to form TiO /CeO
2
nanocomposites. We achieve a perfect combination of photocatalysis and thermocatalysis on the
TiO /CeO nanocomposites. They exhibit efficient catalytic activity for the gasꢀphase benzene
oxidation under the full solar spectrum irradiation and visibleꢀinfrared irradiation. A solar light driven
thermocatalysis on CeO is found for the TiO /CeO nanocomposites. We find a novel synergetic effect
between the light driven thermocatalysis on CeO and the photocatalysis on TiO for the TiO /CeO
nanocomposites, which considerably increases their catalytic activity. We reveal that the synergetic
effect, occurring at the interface of the TiO /CeO nanocomposites, is due to the considerable
promotion of the CeO reduction by the photocatalysis on TiO
2
2
2
2
2
2
2
2
2
2
2
2
2
.
2
. Experimental
Preparation. Microsized mesoporous CeO
2
was prepared according to the procedure reported in
2
0
our previous work: 60.000 g of Ce(NO ) ·6H O and 24.896 g of urea were added into 160 ml of
3 3 2
distilled water under magnetic stirring until they were dissolved. Then, the solution was added to a 200
mL Teflon bottle, which was sealed tightly in a stainlessꢀsteel autoclave. The autoclave was put in an
electric oven, heated to 180 °C and kept at 180 °C for 16 h. After the autoclave cooled to ambient
temperature, the precipitate was thoroughly washed with distilled water and dried at 90 °C for 12 h.
The power obtained was the uncalcined ceria. Finally, the powder was calcined at 400 °C for 4 h in a
Muffle furnace.
2 2
TiO /CeO samples with different Ti/Ce molar ratio were prepared according to the following
procedure: A known amount of titanium butoxide (e.g. 0, 0.341, 0.852, 1.705 g) was dissolved to 70
mL of ethanol in a beaker. 4.742 g of the uncalcined ceria was added to the titanium butoxide solution.
o
The beaker was placed into a water bath at 70 C. The mixture was magnetically stirred until the
3
ACS Paragon Plus Environment

ACS Catalysis
Page 4 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ethanol was evaporated. The product was thoroughly washed with distilled water, dried at 90 °C for 12
h in an electric oven, and finally calcined at 400 °C for 4 h in a Muffle furnace. The obtained
TiO
2
/CeO
/CeO
Pure nano TiO
addition of the uncalcined ceria.
2
samples with Ti/Ce molar ratio of 0.043, 0.108, 0.216 in reactants are denoted as
ꢀA, TiO /CeO ꢀB, TiO /CeO ꢀC, respectively.
2 2
was prepared by the same procedure as the TiO /CeO samples except for no
TiO
2
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
Characterization. SEM images and energy dispersive Xꢀray spectroscopy (EDX) were obtained
with a ULTRA PLUSꢀ43ꢀ13 scanning electron microscope. Transmission electron microscopy (TEM)
images were obtained on a JEMꢀ100CX electron microscope. The measurement of surface area was
2
conducted by using N adsorption at ꢀ196 °C on ASAP2020. Xꢀray diffraction (XRD) was taken on a
Rigaku Dmax Xꢀray diffractometer with Cu Ka radiation. Raman spectra were recorded on a Renishaw
inVia Raman microscope with the excitation of 514.5 nm laser light. Xꢀray photoelectron spectroscopy
(XPS) analysis was taken on a VG Multilab 2000 Xꢀray photoelectron spectrometer with Mg Kα
radiation. Diffuse reflectance UVꢀVis absorption was measured on a UVꢀ3600 spectrophotometer.
CO temperatureꢀprogrammed reduction (COꢀTPR) was performed on a TPꢀ5080 multifunctional
adsorption apparatus equipped with a TCD detector. The sample was put in a tubular quartz reactor. A
quartz window was connected to one of the end of the quartz reactor. A Xe lamp (CHFꢀXM500) was
2
put in the front of the quartz window. The samples were preꢀtreated in 5 vol% O /He at 200 °C for 1 h.
COꢀTPR was conducted by heating the preꢀtreated samples in the flow of 5 vol% CO/He with the
irradiation of the Xe lamp or in dark.
Photothermocatalytic activity. The photothermocatalytic activity of the samples for benzene
oxidation was measured on a closed cylindrical stainless steel gasꢀphase reactor with a quartz window
under the irradiation of a Xe lamp (CHFꢀXM500) as schematically illustrated in Scheme 1. To reduce
energy loss during the irradiation, 0.1000 g of the sample was coated on a thermal insulation slice. To
measure the photothermocatalytic activity under the visibleꢀinfrared irradiation from the Xe lamp, a
cutoff filter that can filter out the irradiation with wavelength less than 420 or 480 nm was placed
between the Xe lamp and the quartz window. The products and reactants were analyzed by gas
chromatograph. The experimental procedure and the light intensity from the Xe lamp were detailed in
4
4
our recently published work.
4
ACS Paragon Plus Environment

Page 5 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. The schematic diagram of the photothermocatalytic setꢀup.
Photocatalytic activity. The photocatalytic activity of the samples for the oxidation of benzene
under the Xe lamp irradiation at near ambient temperature was measured according to the procedure
4
4
described in our recent publications.
Thermocatalytic activity. The thermocatalytic activity of the samples for the oxidation of
benzene was tested in a continuous flow fixedꢀbed quartz tubular reactor at different temperature. The
2
experimental procedure was detailed in our previous publication.
3
3
. Results and Discussion
.1. Characterization. The TiO
2
/CeO
nanoparticles on the microsized mesoporous CeO
SEM images with the distribution of elements and energy dispersive Xꢀray spectroscopy of the
TiO /CeO samples. It can be seen from Figure 1, titanium is well distributed on the microsized CeO
for all the TiO /CeO samples (Figure 1A, 1B, and 1C). This is confirmed by the SEM images with the
separate distribution of Ti, Ce, and O for the TiO /CeO samples (Figure S1, Supporting information).
Ce, O, Ti, and adventitious carbon are detected by EDX (Figure 1D, IE, and 1F). The Ti/Ce molar ratio
of TiO /CeO ꢀA, TiO /CeO ꢀB, and TiO /CeO ꢀC is 0.021, 0.068, 0.174, respectively (Table 1), which
is slightly lower than the corresponding data in reactants (0.043, 0.108, 0.216). This is due to the
diffusion of titanium butoxide into the pores of the microsized mesoporous CeO during the preparation
see Experimental). Figure 2 shows TEM images of the TiO /CeO samples. TiO nanoparticles are
observed on microsized mesoporous CeO . With increasing the Ti/Ce molar ratio from 0.043 to 0.108,
2
nanocomposites with different Ti/Ce molar ratio were prepared
by loading different amount of TiO
2
2
. Figure 1 shows
2
2
2
2
2
2
2
2
2
2
2
2
2
2
(
2
2
2
2
0
.216, more TiO
2
nanoparticles are observed. When the Ti/Ce molar ratio increases to 0.216
5
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(TiO
CeO
this case, mesopores cannot be observed on microsized CeO
HRTEM shows that TiO nanoparticles with {101} planes of anatase for all the TiO
closely contacted to CeO
nanocomposites of anatase TiO
Table 1. Ti/Ce molar ratio, BET surface area, pore volume, dominant pore size, and Ce fraction
of the samples.
2
/CeO
2
ꢀC, Figure 2C), there is observed a thin layer of TiO
2
nanoparticles formed on microsized
2
2 2 2
. The size of TiO nanoparticles in the TiO /CeO samples is estimated by TEM to be 2~17 nm. In
2
as they are blocked by TiO
2
nanoparticles.
samples are
/CeO
2
2
/CeO
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
with {111} planes. This result indicates the formation of TiO
nanoparticles supported on microsized mesoporous CeO
2
2
2
2
.
3+
6
ACS Paragon Plus Environment

Page 7 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2 2
Figure 1. SEM images with the distribution of elements and EDX of TiO /CeO –A (A, D),
TiO
2
/CeO
2
–B (B, E), and TiO
2
/CeO
2
–C (C, F): Ti (red), Ce (cyan), and O (green).
7
ACS Paragon Plus Environment

ACS Catalysis
Page 8 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 2. TEM images of TiO
2
/CeO
2
2 2 2 2
–A (A, D), TiO /CeO –B (B, E), and TiO /CeO –C (C, F).
8
ACS Paragon Plus Environment

Page 9 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
The N
2
2 2
adsorption–desorption isotherm indicates that the TiO /CeO samples have a desorption
hysteresis due to the capillary condensation of N
TiO /CeO ꢀA, TiO /CeO ꢀB, TiO /CeO ꢀC is 3.8, 1.5, 1.5, 1.3 nm, respectively (Figure S2, Table 1).
Their corresponding total pore volume is 0.0545, 0.0589, 0.0575, 0.0612 cm g , respectively. The
2 2
in pores (Figure S2). The dominant pore size of CeO ,
2
2
2
2
2
2
3
ꢀ1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
ꢀ1
BET surface area of CeO
respectively (Table 1).
2 2 2 2 2 2 2
, TiO /CeO ꢀA, TiO /CeO ꢀB, TiO /CeO ꢀC is 84.0, 71.6, 70.3, 73.1 m g ,
The XRD analysis reveals that CeO
JCPDS 89ꢀ8436) as shown in Figure 3A. There is a weak peak observed at 25.3° for TiO
with higher Ti/Ce molar ratio of 0.216 (Table 1), which belongs to {101} planes of anatase (JCPDS
9ꢀ4921). This is in agreement to the observation by HRTEM. But no XRD peaks of anatase are
observed for TiO /CeO ꢀA and TiO /CeO ꢀB with lower Ti/Ce molar ratio (Table 1). This is attributed
to the lower fraction of anatase in the TiO /CeO nanocomposites than the detection limit by XRD.
Figure 3B shows the Raman spectra of the samples. For TiO
2
in all the TiO
2
/CeO
2
samples has a cubic fluorite structure
(
2
/CeO ꢀC
2
8
2
2
2
2
2
2
2
/CeO
2
ꢀA, there is a weak Raman peak
ꢀ
1
11, 45
observed around ~143 cm , which is attributed to Eg mode of anatase.
Increasing the Ti/Ce molar
ratio from 0.043 to 0.108 and 0.216 leads to a gradual enhancement of the intensity for the Eg mode of
anatase.
9
ACS Paragon Plus Environment

ACS Catalysis
Page 10 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. XRD pattern (A) and Raman spectra (B) of CeO
TiO /CeO ꢀC (d).
Figure 4A shows diffuse reflectance UVꢀVis spectra of the samples. The microsized mesoporous
and TiO /CeO nanocomposites have absorption up to ~500 nm. The visible absorption is
attributed to the presence of Ce in the microsized mesoporous CeO
the band gap of CeO , which is due to the indirect O2p—Ce4f transition along the L highꢀsymmetry
2 2 2 2 2
(a), TiO /CeO ꢀA (b), TiO /CeO ꢀB (c), and
2
2
CeO
2
2
2
3
+
40, 46
2
and TiO
2
/CeO
2
samples
as
2
3+
46
3+
lines of the Brillouin zone, is correlated with the Ce concentration. The existence of Ce in the
samples is confirmed by analyzing their Ce3d XPS spectra (Figure 4B). Six peaks labeled as v, v´´, v´´´
4
+
(3d5/2), u, u´´, u´´´ (3d3/2) referring to three pairs of spinꢀorbit doublets are characteristic of Ce 3d.
3
+
27,47
Four peaks labeled as v
0
, v´(3d5/2), u
0
, u´ (3d3/2) correspond to Ce 3d.
The molar ratio of
10
ACS Paragon Plus Environment

Page 11 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
3
+
3+
4+
2 2 2 2 2 2 2
Ce /(Ce +Ce ) in CeO , TiO /CeO ꢀA, TiO /CeO ꢀB, and TiO /CeO ꢀC is estimated by the
deconvolution of their Ce3d XPS spectra to be 0.30, 0.31, 0.33, 0.35 (Table 1), respectively.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
Figure 4. Diffuse reflectance UVꢀVis absorption (A) and Ce3d XPS spectra (B) of CeO (a),
2 2 2 2 2 2
TiO /CeO ꢀA (b), TiO /CeO ꢀB (c), and TiO /CeO ꢀC (d).
3
.2. Photothermocatalytic activity. The photocatalytic activity of the samples was measured by
evaluating the rate of CO formation from the gasꢀphase oxidation of benzene under the irradiation of a
Xe lamp. The catalytic oxidation of benzene is chosen as benzene is carcinogenic and recalcitrant, and
one of main VOC pollutants. As shown in Figure 5A, the microsized mesoporous CeO exhibits
efficient catalytic activity for benzene oxidation. After the irradiation for 60 min, the concentration of
2
2
ꢀ
3
CO
2
produced is 3587.4 mg m . Loading a small amount of TiO
2
nanoparticles on the microsized
mesoporous CeO
2
(TiO /CeO ꢀA) leads to a significant enhancement of its photocatalytic activity.
2
2
11
ACS Paragon Plus Environment

ACS Catalysis
Page 12 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
After the irradiation for 60 min, the concentration of CO
2
produced increases from 3587.4 to 5303.8
ꢀ
3
mg m . Increasing the Ti/Ce molar ratio from 0.043 to 0.108 results in a further improvement in the
2
photocatalytic activity. After the irradiation for 60 min, the concentration of CO produced increases to
ꢀ3
5
954.8 mg m (TiO
2
/CeO
2
ꢀB). However, further enhancing the Ti/Ce molar ratio to 0.216 does not
/CeO ꢀC has the almost same photocatalytic activity as
ꢀB. This means that the optimum Ti/Ce molar ratio is 0.108 for the TiO /CeO
nanocomposites. It is well known that TiO (P25), a mixture of anatase (80%) with band gap of 3.2 eV
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
increase the photocatalytic activity. TiO
TiO /CeO
2
2
2
2
2
2
2
3
and rutile (20%) with band gap of 3.0 eV, exhibits very good photocatalytic activity under UV
irradiation, thus is widely used as benchmark photocatalyst. For comparison, we measured the
photocatalytic activity of TiO
2
(P25) for benzene oxidation under the irradiation of the Xe lamp. After
ꢀ3
the irradiation for 60 min, the concentration of CO
activity is much lower than the microsized mesoporous CeO
also measured the photocatalytic activity of pure nano TiO for benzene oxidation under the irradiation
of the Xe lamp. The pure nano TiO has anatase crystalline structure with band gap of 3.2 eV and its
BET surface area is 46.9 m g . After the irradiation for 60 min, the concentration of CO produced is
nanocomposites. Figure
2
produced is 2706.4 mg m . Its photocatalytic
2
2 2
and the TiO /CeO nanocomposites. We
2
2
2
ꢀ1
2
ꢀ
3
3
362.9 mg m . Its photocatalytic activity is much lower than the TiO
2
/CeO
formation rate per unit mass of catalyst (rCO2). rCO2 of TiO
, TiO (P25), nano TiO , respectively. As the catalyst has
formation rate per unit surface area of
ꢀB (0.14 µmol m min ) is 2.0, 1.8 times higher than CeO and
(P25), respectively. This result reveals that the formation of the TiO /CeO nanocomposites
considerably enhances its photocatalytic activity. The photocatalytic durability of TiO /CeO ꢀB for
2
ꢀ
1
5
B shows the CO
2
2 2
/CeO ꢀB (10.1 µmol g
ꢀ1
min ) is 1.7, 2.2, 1.8 times higher than CeO
2
2
2
different surface area (Table 1), we calculate the rate of CO
2
ꢀ
2
ꢀ1
catalyst (rSCO2). rSCO2 of TiO
2
/CeO
2
2
TiO
2
2
2
2
2
benzene oxidation under the irradiation of the Xe lamp was tested. As shown in Figure 5C, its
photocatalytic activity keeps unchanged when the catalyst was recycled for 30 times, suggesting that
TiO
2
/CeO
As the TiO
whether they exhibit visible photocatalytic activity, we measured the photocatalytic activity of
TiO /CeO ꢀB for benzene oxidation under the visibleꢀinfrared irradiation above 420 nm or 480 nm
from the Xe lamp. As shown in Figure 5D, TiO /CeO ꢀB exhibits photocatalytic activity under the
visibleꢀinfrared irradiation above 420 nm or 480 nm. After the visibleꢀinfrared irradiation above 420
2
ꢀB exhibits a good photocatalytic durability.
2
/CeO nanocomposites have absorption up to ~500 nm (Figure 4), in order to confirm
2
2
2
2
2
12
ACS Paragon Plus Environment

Page 13 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
ꢀ
3
nm or 480 nm for 60 min, the concentration of CO
The rCO2 of TiO /CeO ꢀB under the visibleꢀinfrared irradiation above 420 nm or 480 nm is 2.37, 0.68
µmol g min , respectively. The considerable decrease of the photocatalytic activity is attributed to the
decreased absorption above 420 nm or 480 nm for TiO /CeO ꢀB (Figure 4A).
2
produced is 1398.3, 401.1 mg m , respectively.
2
2
ꢀ
1
ꢀ1
2
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Time course of CO
2
produced from benzene oxidation (A), rCO2 for benzene oxidation on the
catalysts (B), the durability of TiO /CeO ꢀB for the benzene oxidation (the reaction time of every cycle,
2
2
6
0 min) under the Xe lamp irradiation (C), and time course of CO
2
produced from benzene oxidation
on TiO /CeO ꢀB under the irradiation above 420 nm or 480 nm (D): CeO (a), TiO /CeO ꢀA
2
2
2
2
2
2 2 2 2 2 2
(b),TiO /CeO ꢀB (c), TiO /CeO ꢀC (d), TiO (P25) (e), and TiO (f).
3
3
.3. Mechanism. Nano TiO
2
has been regarded as one of the most effective UV photocatalysts. Some
2
1ꢀ24
researchers reported that nano CeO shows photocatalytic activity under UV or visible irradiation.
2
The photodegradation of organic pollutants on TiO
2
follows the wellꢀknown photocatalytic
3
mechanism. To confirm whether the TiO /CeO nanocomposites follow the conventional
2
2
2 2 2
photocatalytic mechanism, the photocatalytic activity of CeO and TiO /CeO ꢀB for benzene oxidation
13
ACS Paragon Plus Environment

ACS Catalysis
Page 14 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
under the Xe lamp irradiation at near ambient temperature was measured. As can be seen from Figure
6
, TiO
2
/CeO
2
2
ꢀB shows photocatalytic activity while CeO has almost no photocatalytic activity for
o
benzene oxidation at ~40 C. But, rCO2 of TiO
2
/CeO
2
ꢀB under photothermocatalytic condition is 36.4
o
times higher than that of TiO
2
/CeO
2
ꢀB under the photocatalytic condition at ~40 C (Figure 6). This
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
result clearly indicates the presence of a solar light driven thermocatalysis (schematically illustrated in
Scheme 2) except for the photocatalysis for benzene oxidation on TiO /CeO ꢀB under the irradiation of
the Xe lamp: The irradiation of the Xe lamp on the TiO /CeO nanocomposites results in a considerable
increase of their temperature due to the photothermal conversion and the infrared heating effect (See
.3.1. Photothermal conversion). When the temperature increases to the thermocatalytic lightꢀoff
temperature (Tlightꢀoff) of the TiO /CeO nanocomposites, the thermocatalytic oxidation of benzene starts
See 3.3.2.Thermocatalysis).
2
2
2
2
3
2
2
(
Scheme 2. Schematic illustration of solar light driven thermocatalysis and the synergetic effect
between the photocatalysis on TiO and thermocatalysis on CeO for the TiO /CeO nanocomposites.
2
2
2
2
14
ACS Paragon Plus Environment

Page 15 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Time course of CO
2
produced from benzene oxidation (A) and rCO2 for benzene oxidation on
the catalysts (B) under the different cases: TiO
photothermocatalytic, a), TiO /CeO ꢀB (b) and CeO (c) under the Xe lamp irradiation at near room
temperature (photocatalytic), TiO /CeO ꢀB under the irradiation above 420 nm with higher light
and CeO with the same Ti/Ce molar ratio as TiO /CeO ꢀB under the
2 2
/CeO ꢀB under the Xe lamp irradiation
(
2
2
2
2
2
intensity (d), a mixture of TiO
Xe lamp irradiation (e).
2
2
2
2
3
.3.1. Photothermal conversion. To prove the solar light driven thermocatalysis, first, we measured
the temperature evolution of the samples with the irradiation of the Xe lamp. Under the Xe lamp
irradiation, the temperature of all the samples quickly increases to a plateau (Figure 7A). The
temperature increase is attributed to the photothermal conversion due to the absorption of the
UVꢀvisible irradiation by the samples (Figure 4A) as well as the heating effect of the infrared
irradiation from the Xe lamp. When the photothermal conversion and the heating effect establish
equilibrium with the energy dissipation from the sample to the surroundings, a plateau temperature is
15
ACS Paragon Plus Environment

ACS Catalysis
Page 16 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2 2 2 2 2 2 2
observed. The plateau temperature of CeO , TiO /CeO ꢀA, TiO /CeO ꢀB, TiO /CeO ꢀC is 212, 213,
o
2
05, 193 C, respectively (Figure 7B). This result indicates a slight decrease of the plateau temperature
with increasing the Ti/Ce molar ratio, which is due to the reflectance of the infrared light by TiO
2
as
white pigment. The temperature evolution of TiO /CeO ꢀB under the irradiation above 420 and 480 nm
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
was also measured. In this case, the plateau temperature under the irradiation above 420 and 480 nm is
o
1
88, 174 C, respectively.
Figure 7. Temporal evolution of the temperature on TiO
the samples (B) under the Xe lamp irradiation: CeO (a), TiO
TiO /CeO ꢀC (d).
2
/CeO
2
ꢀB (A) and the plateau temperature of
/CeO ꢀA (b), TiO /CeO ꢀB (c), and
2
2
2
2
2
2
2
3.3.2. Thermocatalysis. To confirm whether the plateau temperature of the catalysts can reach to
the lightꢀoff temperature (Tlightꢀoff) for benzene oxidation, we studied the effect of reaction temperature
on the thermocatalytic activity of the catalysts for the oxidation of benzene in a flow fixedꢀbed reactor.
16
ACS Paragon Plus Environment

Page 17 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
2
As shown in Figure 8, compared to the thermocatalytic activity of the microsized mesoporous CeO ,
2 2 2 2
loading a small amount of TiO nanoparticles on the microsized mesoporous CeO (TiO /CeO ꢀA,
TiO
to 0.216 (TiO
lower thermocatalytic activity of the nano TiO
2
/CeO
2
ꢀB) does not obviously alter its thermocatalytic activity. Further increasing the Ti/Ce molar
o
2
/CeO
2
ꢀC) leads to a decrease of thermocatalytic activity above ~200 C due to the much
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
(Figure 8, curve e) than CeO
nanocomposites, benzene starts to be oxidized around ~ 160
and the TiO /CeO
2
2
. For the microsized
2 2 2
mesoporous CeO and all the TiO /CeO
o
C. All the plateau temperatures of the microsized mesoporous CeO
2
2
nanocomposites under the Xe lamp irradiation (Figure 7B) are higher than their corresponding Tlightꢀoff
o
(
~160 C). Thus, the solar light driven thermocatalytic oxidation of benzene can take place. The plateau
2 2
temperature of TiO /CeO ꢀB under the irradiation above 420 nm or 480 nm is higher than its
2 2
corresponding Tlightꢀoff. This is why TiO /CeO ꢀB exhibits visibleꢀinfrared light driven thermocatalytic
activity for benzene oxidation.
Figure 8. Benzene conversion versus reaction temperature over the catalysts for benzene oxidation
−
3
−1
−1
under the condition of benzene concentration = 2.0 g m , space velocity (SV) = 48000 mL g catalyst h ,
ambient pressure: CeO (a), TiO /CeO ꢀA (b), TiO /CeO ꢀB (c), TiO /CeO ꢀC (d), and nano TiO (e).
2
2
2
2
2
2
2
2
3.4. Synergetic effect. We tested the photocatalytic activity of TiO
2
/CeO
2
ꢀB under the irradiation
above 420 nm with higher irradiation intensity by reducing the distance between the Xe lamp and the
reactor (Figure 6). By increasing the visibleꢀinfrared irradiation intensity, its plateau temperature is
o
equal to that under the full solar spectrum irradiation of the Xe lamp (205 C, Figure 7). In this case,
only visibleꢀinfrared light driven thermocatalysis on CeO
2 2
occurs as anatase TiO can not be
photoactivated by the visible irradiation above 420 nm because of its large band gap (3.2 eV, 386 nm).
17
ACS Paragon Plus Environment

ACS Catalysis
Page 18 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Its thermocatalytic activity should be the same as under the full solar spectrum irradiation of the Xe
lamp. Interestingly, rCO2 of TiO
2 2
/CeO ꢀB under the full solar spectrum irradiation is 1.9 times higher
than the sum of its rCO2 under the irradiation above 420 nm with higher irradiation intensity (5.15 µmol
ꢀ1
ꢀ1
g min ) and its rCO2 under the full solar spectrum irradiation at near ambient temperature. This result
suggests the presence of a synergetic effect between the light driven thermocatalysis on CeO and the
photocatalysis on TiO for TiO /CeO ꢀB under the full solar spectrum irradiation
In order to further prove the synergetic effect, we tested the photocatalytic activity of a mixture of
the nano TiO and the microsized mesoporous CeO with the same Ti/Ce molar ratio as TiO /CeO ꢀB
under the Xe lamp irradiation. In this case, rCO2 of the mixture of the nano TiO and the microsized
ꢀB (Figure 6). These
results undoubtedly indicate that there is a synergetic effect between the light driven thermocatalysis on
CeO and the photocatalysis on TiO for TiO /CeO ꢀB, and the synergetic effect takes place at the
interface of the TiO /CeO nanocomposites as schematically illustrated in Scheme 2:
Upon UV excitation, electrons on the valence band of TiO are excited to the conduction band,
leaving hole on the valence band. The photogenerated holes and electrons move to the surface of TiO
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
2
2
2
2
2
2
ꢀ
1
ꢀ1
2 2 2
mesoporous CeO (4.95 µmol g min ) is 2.0 times lower than that of TiO /CeO
2
2
2
2
2
2
2
2
.
2 2
The electrons reduce the electron acceptor (e.g. O ) adsorbed on the surface of TiO to form active
ꢀ
oxygen (e.g. O
2
) due to the lower oxidation potential of e (ꢀ0.18 V vs NHE at pH = 1) than that of O
2
ꢀ
48, 49, 50
(
e.g. O
TiO
organic molecules (e.g. benzene )
those of hydroxyl groups (e.g. OH·/OH , 1.89 V vs NHE) and organic molecules (e.g. benzene, 2.995
2
/O
2
, ꢀ0.16 V vs NHE).
2
The holes oxidize donors: adsorbed H O or hydroxyl group on
2
to produce hydroxyl radical (•OH), and adsorbed organic molecule (e.g. benzene) to form active
+
3, 48ꢀ51
due to the higher oxidation potential of h (3.02 V vs NHE) than
ꢀ
50
5
2
V vs NHE). Thus, the photocatalytic oxidation of organic molecule (e.g. benzene) proceeds.
Meanwhile, the light driven thermocatalysis proceeds on the microsized mesoporous CeO
2
. The
is as
widely accepted Mars–van Krevelen mechanism for the thermocatalytic oxidation on CeO
follows: organic molecule adsorbed on the surface of CeO
2
2
2
is oxidized by the lattice oxygen of CeO ,
1
4, 17, 18, 20
and the reduced ceria is subsequently reꢀoxidized by gas phase oxygen.
+
The active benzene (e.g. benzene ) produced by the photocatalysis on TiO
2
is more active than
benzene according to molecular orbital theory as the electron number in the bonding molecular orbital
+
53
of benzene is less than that of benzene. Thus the reduction of CeO
2
by the active benzene is
through the interface
thermodynamically favorable. The active benzene undoubtedly migrates to CeO
2
18
ACS Paragon Plus Environment

Page 19 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
of the TiO
2
/CeO
2
nanocomposite, promoting the reduction of CeO
2
(Scheme 2) because the active
, does not leads
(clumn a in Figure 5B and clumn e in Figure 6B).
are
. Thus the
2 2 2
benzene, also existed on TiO in the mixture of the nano TiO and the microsized CeO
to an enhancement in the catalytic activity of CeO
2
ꢀ
The active oxygen (e.g. O
2
) and hydroxyl radical (•OH) produced by the photocatalysis on TiO
2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
more active than gas phase oxygen (O
2
) in the conventioanl thermocatalysis on CeO
2
ꢀ
oxidation of the reduced ceria by the active oxygen (e.g. O
2
) and hydroxyl radical (•OH) is
migrate to
nanocomposite, accelerating the oxidation of the reduced ceria
formed by thermocatalyis (Scheme 2). Therefore, the synergetic effect between the photocatalysis on
thermodynamically favorable. The active species formed by the photocatalysis on TiO
CeO via the interface of the TiO /CeO
2
2
2
2
TiO
2
2
and the light driven thermocatalysis on CeO
/CeO nanocomposites under the Xe lamp irradiation.
.5. Origin of synergetic effect. It is widely accepted that the reducibility of CeO
2
considerably increases the catalytic activity of the
TiO
2
3
2
plays a decisive
2
is much slower than the reꢀoxidation
role in its thermocatalytic activity, because the reduction of CeO
14, 17, 18, 20
of reduced ceria.
In order to put insight in the origin of the synergetic effect, the effect of the
is investigated by CO temperatureꢀprogrammed reduction under
irradiation on the reduction of CeO
2
the irradiation of the Xe lamp and in dark (Experimental). Under the dark condition without the
o
irradiation of the Xe lamp, no peak is observed for pure TiO
2
. A negative peak around 130 C is
(Figure 9A). There are observed two
observed for TiO
2
/CeO
2
ꢀB due to the desorption of CO on CeO
2
o
broad TPR peaks around ~276 , ~394 C, which are attributed to the oxidation of CO by two types of
1
4, 17
surface lattice oxygen on CeO
2
.
Interestingly, under the irradiation of the Xe lamp, the two broad
o
TPR peaks shift to lower temperature of ~259, ~353 C, respectively. This observation indicates that
the irradiation of the Xe lamp reduces the oxidation temperature of CO by the lattice oxygen on CeO
Moreover, the intensity of the two TPR peaks is considerably enhanced. We quantitatively measure the
amounts of CO consumed for TiO /CeO ꢀB by calculating the area of their TPR profiles calibrated by
the reduction of a known amount of CuO by CO. The total amounts of CO consumed for TiO /CeO ꢀB
in dark is 580.1 µmol g . Under the irradiation of the Xe lamp, the total amounts of CO consumed
2
.
2
2
2
2
−
1
−
1
increases to 1033.1 µmol g . In striking contrast, for a mixture of the nano TiO
mesoporous CeO with the same Ti/Ce molar ratio as TiO /CeO ꢀB, the irradiation of the Xe lamp does
not lead to an obvious evolution of its COꢀTPR profile as compared to that in dark (Figure 9B). This
result reveals that the irradiation of the Xe lamp significantly accelerates the reduction of CeO by CO
2
and the microsized
2
2
2
2
19
ACS Paragon Plus Environment

ACS Catalysis
Page 20 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
for the TiO
2
/CeO
2
2
nanocomposite, and the promotion of CeO reduction by CO with the irradiation
2 2
occurs at the interface of the TiO /CeO nanocomposite. MunozꢀBatista repoted that the reaction rate
behavior in the TiO
2
/CeO
2
composite system was dominated by the availability of holes at the surface
27
of the material, and the photodegradation was a holeꢀtriggered reaction. For COꢀTPR in dark, there is
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
only the reduction of CeO
2
by CO. For COꢀTPR under the irradiation of the Xe lamp, the
+
photogenerated hole (h) on TiO
2
reacts with CO to form active CO (e.g. CO ) due to the higher
5
4
+
oxidation potential of h than that of CO (0.64 V vs RHE on Pt electrode). The active CO (e.g. CO ) is
more active than CO according to molecular orbital theory as the electron number in the bonding
+
53
molecular orbital of CO is less than that of CO. Thus the reduction of CeO
2
by the active CO (e.g.
CO ) is thermodynamically favorable. The active CO (e.g. CO ) produced by photocatalysis on TiO
undoubtedly migrates to CeO through the interface of the TiO /CeO nanocomposites, promoting the
reduction of CeO (Figure 9A) because the active CO, also existed on TiO in the mixture of the nano
TiO and the microsized CeO , does not leads to the promotion of CeO reduction (Figure 9B). This
accounts for the shift of the COꢀTPR peaks to lower temperature as well as the enhancement of CO
consumption with the irradiation of the Xe lamp for the TiO /CeO nanocomposite (Figure 9A). It
should be noted that in principle, photogenerated electrons and holes could be produced on the
+
+
2
2
2
2
2
2
2
2
2
2
2
2
7, 31
conduction band and valence band of CeO
the photogenerated holes on CeO does not promote the reduction of CeO
considerable promotion of CeO reduction by the photocatalysis on TiO
photocatalysis on CeO improves the solar light driven thermocatalytic activity of CeO
TiO /CeO nanocomposite as illustrated in Scheme 2.
2
upon UV or visible irradiation, respectively.
(Figure 9B). The
rather than by the
for the
However,
2
2
2
2
2
2
2
2
20
ACS Paragon Plus Environment

Page 21 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 9. COꢀTPR profile of TiO
2
/CeO
2 2 2
ꢀB (A), and a mixture of TiO and CeO with the same
Ti/Ce molar ratio as TiO
2
/CeO
2
ꢀB (B) in dark and with the irradiation of the Xe lamp.
4
. Conclusion
In summary, TiO
2
/CeO
2
nanocomposites exhibit enhanced catalytic activity for benzene oxidation
and TiO . A solar light driven
nanocomposites. There is a synergetic effect
and the thermocatalysis on CeO for the TiO /CeO
nanocomposites, which significantly increases their catalytic activity. The synergetic effect, which
occurs at the interface of the TiO /CeO nanocomposites, is due to the considerable promotion of the
CeO reduction by the photocatalysis on TiO . The novel strategy using the synergetic effect between
the photocatalysis on TiO and the solar light driven thermocatalysis on CeO is applicable for
under the irradiation of the Xe lamp compared to pure CeO
thermocatalysis on CeO is found for the TiO /CeO
between the photocatalysis on TiO
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
21
ACS Paragon Plus Environment

ACS Catalysis
Page 22 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
designing other nanocomposite catalysts for the environmental purification using renewable solar
energy.
Acknowledgement
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
This work was supported by National Natural Science Foundation of China (21273169,
2
1473127), and Research and Development Project of Hubei Province (2013BAA045).
Supporting Information
SEM and N adsorptionꢀdesorption of the catalysts. This material is available free of charge via the
2
Internet at http://pubs.acs.org.
Reference.
(1) Huang, Q. W.; Tian, S. Q.; Zeng, D. W.; Wang, X. X.; Song, W. L.; Li, Y. Y.; Xiao, W.; Xie, C.
S. ACS Catal. 2013, 3, 1477ꢀ1485.
(
(
(
2) Hou, J. T.; Li, Y. Z.; Liu, L. L.; Ren, L.; Zhao, X. J. J. Mater. Chem. A 2013, 1, 6736ꢀ6741.
3) Chen, X. B.; Mao, S. S. Chem. Rev. 2007, 107, 2891ꢀ2959.
4) Kong, M.; Li, Y. Z.; Chen, X.; Tian, T. T.; Fang, P. F.; Zheng, F.; Zhao, X. J. J. Am. Chem. Soc.
2
011, 133, 16414ꢀ1617.
5) Tang, J. W.; Durrant, J. R.; Klug, D. R. J. Am. Chem. Soc. 2008, 130, 13885ꢀ13891.
(
(6) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269ꢀ271.
(7) Shi, N.; Li, X. H.; Fan, T. X.; Zhou, H.; Ding, J. A.; Zhang, D.; Zhu, H. X. Energy Environ.
Sci. 2011, 4, 172ꢀ180.
(
(
(
(
8) Li, Y. F.; Xu, D. H.; Oh, J. I.; Shen, W. Z.; Li, X.; Yu, Y. ACS Catal. 2012, 2, 391ꢀ398.
9) MendezꢀRoman, R.; CardonaꢀMartinez, N. Catal. Today 1998, 40, 353ꢀ365.
10) Wu, W.; Liao, L.; Lien, C.; Lin, J. Phys. Chem. Chem. Phys. 2001, 3, 4456ꢀ4461.
11) Li, Y. Z.; Huang, J. C.; Peng, T.; Xu, J.; Zhao, X. J. ChemCatChem 2010, 2, 1082ꢀ1087.
(12) Liotta, L. F. Appl. Catal. B 2010, 100, 403ꢀ412.
(13) Zhang, D. S.; Du, X. J.; Shi, L. Y.; Gao, R. H. Dalton Trans. 2012, 41, 14455ꢀ14475.
(14) Wu, Z. L.; Li, M. J.; Overbury, S. H. J. Catal. 2012, 285, 61ꢀ73.
22
ACS Paragon Plus Environment

Page 23 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
(15) Mai, H. X.; Sun, L. D.; Zhang, Y. W.; Si, R.; Feng, W.; Zhang, H. P.; Liu, H. C.; Yan, C. H.
J. Phys. Chem. B 2005, 109, 24380ꢀ24385.
(
16) Zhang, J.; Kumagai, H.; Yamamura, K.; Ohara, S.; Takami, S.; Morikawa, A.; Shinjoh, H.;
Kaneko, K.; Adschiri, T.; Suda, A. Nano Lett. 2011, 11, 361ꢀ364.
17) Liu, X. W.; Zhou, K. B.; Wang, L.; Wang, B. Y.; Li, Y. D. J. Am. Chem. Soc. 2009, 131,
140ꢀ3141.
(18) Lawrence, N. J.; Brewer, J. R.; Wang, L.; Wu, T. S.; WellsꢀKingsbury, J.; Ihrig, M. M.;
Wang, G. H.; Soo, Y. L.; Mei, W. N.; Cheung, C. L. Nano Lett. 2011, 11, 2666ꢀ2671.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
3
(
19) Hua, G. M.; Zhang, L. D.; Fei, G. T.; Fang, M. J. Mater. Chem. 2012, 22, 6851ꢀ6855.
20) Shi, W. Q.; Li, Y. Z.; Hou, J. T.; Lv, H. Q.; Zhao, X. J.; Fang, P. F.; Zheng, F.; Wang, S. J.; J.
(
Mater. Chem. A 2013, 1, 728ꢀ734.
21) Ji, P. F.; Zhang, J. L.; Chen, F.; Anpo, M. J. Phys. Chem. C 2008, 112, 17809ꢀ17813.
(
(22) Valente, J. S.; Tzompantzi, F.; Prince, J. Appl. Catal. B 2011, 102, 276ꢀ285.
(23) Chaudhary,Y. S.; Panigrahi, S.; Nayak, S.; Satpati,B.; Bhattacharjee, S.; Kulkarni, N. J.
Mater. Chem. 2010, 20, 2381ꢀ2385.
(24) HernandezꢀAlonso, M. D.; Hungrıa, A. B.; MartınezꢀArias, A.; FernandezꢀGarcıa, M.;
Coronado1, J. M.; Conesa, J. C.; Soria, J. Appl. Catal. B 2004, 50, 167ꢀ175.
(25) Yue, L.; Zhang, X.M. J. Alloys Compd. 2009, 475, 702ꢀ705.
(26) Channei, D.; Inceesungvorn, B.; Wetchakun, N.; Ukritnukun, S.; Nattestad, A.; Chen, J.;
Phanichphant, S. Sci. Rep. 2014, 4, 5757.
(27) MunnozꢀBatista, M. J.; GomezꢀCerezo, M. N.; Kubacka, A.; Tudela, D.; FernandezꢀGarcia,
M. ACS Catal. 2014, 4, 63ꢀ72.
(
28) Parasupree, S.; Suzuki, Y.; PrisaꢀArt, S.; Yoshikawa, S. J. Solid State Chem. 2005, 178,
28ꢀ134.
29) Ghasemi, S.; Setayesh, S. R.; HabibiꢀYangjeh, A.; HormoziꢀNezhad, M. R.; Gholami, M. R.
1
(
J. Hazard. Mater. 2012, 199- 200, 170ꢀ178.
(30) Li, G.; Zhang, D.; Yu, Y. C. Phys. Chem. Chem. Phys. 2009, 11, 3775ꢀ3782.
(31) Tian, J.; Sang, Y. H.; Zhao, Z. H.; Zhou, W. J.; Wang, D. Z.; Kang, X. L.; Liu, H.; Wang , J.
Y.; Chen, S. W.; Cai, H. Q.; Huang, H. Small 2013, 9, 3864ꢀ3872.
23
ACS Paragon Plus Environment

ACS Catalysis
Page 24 of 25
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(32) Cao, T. P.; Li, Y. J.; Wang, C. H.; Wei, L. M.; Shao, C. L.; Liu, Y. C. Mater. Res.
Bullet.2010, 45, 1406ꢀ1412.
(33) Wang, Y. G.; Li, B.; Zhang, C. L.; Cui, L. F.; Kang, S. F.; Li, X.; Zhou, L. H. Appl. Catal. B
2
013, 130- 131, 277ꢀ284.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(34) Yang, H. M.; Zhang, K.; Shi, R. R. J. Am. Ceram. Soc. 2007, 90, 1370ꢀ1374.
(35) Karunakaran, C.; Gomathisankar, P. ACS Sustainable Chem. Eng. 2013, 1, 1555ꢀ1563.
(36) Alessandri, I.; Zucca, M.; Ferroni, M.; Bontempi, E.; Depero, L. E. Small 2009, 5, 336ꢀ340.
37) Lin, H.; Wang, M.; Wang, Y.; Liang, Y.; Cao, W.; Su, Y. J. Photochem. Photobiol. A 2011,
23, 157ꢀ162.
(
2
(38) Wang, Y.; Li, B.; Zhang, C.; Cui, L.; Kang, S.; Li, X.; Zhou, L. Appl. Catal, B 2013, 130-131,
2
77ꢀ284.
(39) MunnozꢀBatista, M. J.; Kubacka, A.; GomezꢀCerezo, M. N.; Tudela, D.; FernaandezꢀGarcia,
M. Appl. Catal. B 2013, 140-141, 625ꢀ635.
(40) Li, Y. Z.; Sun, Q.; Kong, M.; Shi, W. Q.; Huang, J. C.; Tang, J. W.; Zhao, X. J. J. Phys.
Chem. C 2011, 115, 14050ꢀ14057.
(41) Jiang, D.; Wang, W. Z.; Gao, E.; Zhang, L.; Sun, S. J. Phys. Chem. C 2013, 117,
2
4242ꢀ24249.
(42) Jiang, D.; Wang, W. Z.; Zhang, L.; Qiu, R. H.; Sun, S. M.; Zheng, Y. L. Appl. Catal. B 2015,
1
65, 399ꢀ407.
(43) Liyanage, A. D.; Perera, S. D.; Tan, K.; Chabal, Y.; Balkus, Jr. K. J. ACS Catal. 2014, 4,
577ꢀ584.
(44) (a) Hou, J. T.; Li, Y. Z.; Mao, M. Y.; Yue, Y. Z.; Greaves, G. N.; Zhao, X. J. Nanoscale 2015,
7, 2633ꢀ2640. (b) Ma, Y.; Li, Y. Z.; Mao, M. Y.; Hou, J. T.; Zeng, M.; Zhao, X. J. J. Mater. Chem. A,
2015, 3, 5509–5516. (c) Mao, M. Y.; Li, Y. Z.; Hou, J. T.; Zeng, M.; Zhao, X. J. Appl. Catal. B 2015,
174, 496–503.
(45) Zhang, Y. H.; Ebbinghaus, S. G.; Weidenkaff, A.; Kurz, T.; von Nidda, H. A. K.; Klar, P. J.
Gungerich, M.; Reller, A. Chem. Mater. 2003, 15, 4028ꢀ4033.
(46) Patsalas, P.; Logothetidis, S.; Sygellou, L.; Kennou, S. Phys. Rev. B 2003, 68, 035104.
(
47) Salvi, A. M.; Decker, F.; Varsano, F.; Speranza, G. Surf. Interface Anal. 2001, 31, 255ꢀ264.
48) Linsebigler, A. L.; Lu, G. Q.; Yates, Jr. J. T. Chem. Rev. 1995, 95, 735ꢀ758.
(
24
ACS Paragon Plus Environment

Page 25 of 25
ACS Catalysis
1
2
3
4
5
6
7
8
9
(49) D. T. Sawyer, J. S. Valentine, Acc. Chem. Res. 1981,14, 393ꢀ400.
(50) Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2007, 111, 5259ꢀ5275.
(51) d'Hennezel, O.; Pichat, P.; Ollis, D. F. J. Photochem. Photobio. A 1998, 118, 197ꢀ204.
(52) Luo, L.; Li, Y. Z.; Hou, J. T.; Yang, Y. Appl. Surf. Sci. 2014, 319, 332ꢀ338.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
53) Zhou, G. D.; Duan, Y. L. The Basic of Structural Chemistry, Peking University Press, 2008.
ꢀ363.
(54) Ianniello, R.; Schmidt, V. M.; Stimming, U.; Super, J.; Wallau, A. Electrochimica Acta 1994,
1
3
9, 1663ꢀ1869.
TOC
25
ACS Paragon Plus Environment