Photo–thermo Catalytic Oxidation over a TiO2-WO3-Supported Platinum Catalyst

Article abstract of DOI:10.1002/anie.202001701

Photo–thermo catalysis, which integrates photocatalysis on semiconductors with thermocatalysis on supported nonplasmonic metals, has emerged as an attractive approach to improve catalytic performance. However, an understanding of the mechanisms in operation is missing from both the thermo- and photocatalytic perspectives. Deep insights into photo–thermo catalysis are achieved via the catalytic oxidation of propane (C₃H₈) over a Pt/TiO₂-WO₃ catalyst that severely suffers from oxygen poisoning at high O₂/C₃H₈ ratios. After introducing UV/Vis light, the reaction temperature required to achieve 70 percent conversion of C₃H₈ lowers to a record-breaking 90 °C from 324 °C and the apparent activation energy drops from 130 kJ mol^?1 to 11 kJ mol^?1. Furthermore, the reaction order of O₂ is ?1.4 in dark but reverses to 0.1 under light, thereby suppressing oxygen poisoning of the Pt catalyst. An underlying mechanism is proposed based on direct evidence of the in-situ-captured reaction intermediates.

Full text of DOI:10.1002/anie.202001701

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: Photo-thermo catalytic oxidation over TiO2-WO3 supported
platinum catalyst
Authors: Leilei Kang, Xiao Yan Liu, Aiqin Wang, Lin Li, Yujing Ren,
Xiaoyu Li, Xiaoli Pan, Yuanyuan Li, Xu Zong, Hua Liu, Anatoly
I. Frenkel, and Tao Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001701
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Photo-thermo catalytic oxidation over TiO -WO supported
2
3
platinum catalyst
[
a]
[a]
[a]
[a]
[a],[b]
[a],[b]
[a]
Leilei Kang, Xiao Yan Liu,* Aiqin Wang, Lin Li, Yujing Ren,
Xiaoyu Li,
Xiaoli Pan,
[
c]
[d]
[a],[b]
[c],[e]
[b],[d]
Yuanyuan Li, Xu Zong, Hua Liu,
Anatoly I. Frenkel,
and Tao Zhang*
[
a]
Dr. L. Kang, Prof. Dr. X. Liu, Prof. Dr. A. Wang, Prof. Dr. L. Li, Dr. Y. Ren, X. Li, X. Pan, H. Liu
CAS Key Laboratory of Science and Technology on Applied Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023, China
E-mail: xyliu2003@dicp.ac.cn
[b]
[c]
[d]
[e]
Dr. Y. Ren, X. Li, H. Liu, Prof. Dr. T. Zhang
University of Chinese Academy of Sciences
Beijing 100049, China
E-mail: taozhang@dicp.ac.cn
Dr. Y. Li, Prof. Dr. A. Frenkel
Materials Science and Chemical Engineering Department
Stony Brook University
Stony Brook, New York 11794, United States
Prof. Dr. X. Zong, Prof. Dr. T. Zhang
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023, China
Prof. Dr. A. Frenkel
Chemistry Division
Brookhaven National Laboratory
Upton, New York 11973 United States
Supporting information for this article is given via a link at the end of the document.
Abstract: Photo-thermo catalysis that integrates photocatalysis on
semiconductors with thermocatalysis on supported nonplasmonic
metals has emerged as an attractive approach to improve the
catalytic performances. However, an understanding of the
mechanism from both of the thermo- and photo- catalytic
perspectives is missing. Here, the deep insights into the photo-
thermo catalysis are achieved via the catalytic oxidation of propane
Catalytic oxidation reaction using molecular oxygen as the
[
4]
oxidant is industrially important. Supported platinum is one of
the most active catalysts for this kind of reactions. Distinct from
[
5]
the CO poisoning in CO oxidation, the active Pt sites on the
surface of the catalysts can be easily poisoned by oxygen in
[
6]
most of the oxidation reactions. For instance, the activity of
supported Pt catalysts drops sharply with the augment of
[7]
(
C
3
H
8
) over the Pt/TiO
oxygen poisoning at high O
visible light, the reaction temperature for 70% conversion of C
2
-WO
3
catalyst that severely suffers from the
ratios. After introducing ultraviolet-
is
O
2
/propane (C
3 8
H ) ratios, which is a pervasive problem in
2
/C
3 8
H
supported platinum group metal (PGM) catalysts for
[
8]
H
3 8
hydrocarbon oxidation.
Herein, the photo-thermo catalysis of semiconductor-
supported Pt catalyst (Pt/TiO -WO ) is found to be capable of
dramatically enhancing the catalytic oxidation of C at low
temperatures and high O /C ratio (volume ratio: 20). With the
irradiation of ultraviolet-visible light, the reaction temperature for
o
o
lowered to a record-breaking 90 C from 324 C and the apparent
activation energy drops from 130 kJ/mol to 11 kJ/mol. Furthermore,
2
3
the reaction order of O
2
is -1.4 in dark but reverses to 0.1 under light,
3 8
H
which, therefore, suppresses the oxygen poisoning of the Pt catalyst.
The underlying mechanism is proposed based on the direct
evidences of the in situ captured reaction intermediates.
2
3 8
H
o
o
70 % C
correspondingly, the apparent activation energy (E
more than ten times. The reaction order (n) of the reactants
changes sharply, especially for the O increased from -1.4 to 0.1,
thus, the oxygen poisoning of the Pt catalyst can be overcome.
The peroxycarbonate (-OCO ) is found as the intermediate for
3
H
8
conversion (T70) decreases from 324 C to 90 C,
a
) reduces
Introduction
2
Photocatalysis and thermocatalysis have been developed
separately with their own principles. While, integrating photo-
generated charge carriers with thermocatalysis, namely photo-
3
this reaction for the first time by in situ diffuse reflectance
infrared Fourier-transform spectroscopy (DRIFTS) and the
[
1]
thermo catalysis, has been developing as a burgeoning field. It
is distinguished from the photo-thermal reaction that merely
-
superoxide anion (O
2
) is determined as the active oxygen
species by in situ electron paramagnetic resonance (EPR) under
the photo-thermo reaction process. The origin of the synergistic
effect between the photo- and thermo- catalysis is proposed.
The photo-induced charge carriers promoted by rising
temperature are determined as the most important factors that
facilitate the activation and desorption of the adsorbed oxygen
on the Pt surface.
[
2]
emphasizes the photo-induced heating effect on the reactions.
The remarkable advantages that couple the photonic and
thermal stimuli over the plasmonic metals (Au, Ag, Cu) have
[
3]
been intensively studied in many typical reactions. By contrast,
an in-depth understanding of the photo-thermo catalysis on
semiconductor supported nonplasmonic metals (Pt, Pd, Rh, etc.)
from both of the thermo- and photo- catalytic point of view is still
lacking.
1
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RESEARCH ARTICLE
a 2 3 3 8
Figure 1. (a) Catalytic activity, (b) Arrhenius plots as well as (c) T70 and E of the Pt/TiO -WO catalyst under light with different power density. (d) C H
2 3
conversion on the Pt/TiO -WO catalyst under continuous illumination without heating and at 200 °C with and without light irradiation (power density: 400
2
mW/cm ).
−
1
−1
mass space velocity (S.V.) was 30000 mL gcat h . The wide
passband infrared filter was used in the Xenon lamp to avoid the
localized thermal effect of infrared light on the surface of the
catalyst. Moreover, the temperature of the catalyst bed was
measured by a thermocouple of the furnace so that the light-
induced heating can be balanced. Prior to reaction, the catalyst
Results and Discussion
2 3
The Pt/TiO -WO catalyst was synthesized by an impregnation
method. By balancing both of the light absorption (the results of
ultraviolet-visible spectroscopy (UV-vis) in Figure S1) and the
was reduced in flowing hydrogen (10% H
2
/He) to improve the
activity (Figure S6), since metallic Pt is the most active site for
specific surface area (the results of N
isotherms in Figure S2) of the different supports, TiO
molar ratio of 1:1 was selected as the support of the catalyst.
The loading of Pt in the Pt/TiO -WO catalyst was determined to
2
adsorption-desorption
[
10]
C
3
H
8
oxidation.
2 3
As a contrast, the activity of the TiO -WO
2
-WO with
3
support is sluggish under the same reaction conditions (Figure
S7). Figure 1a shows the light-off curves under light with
2
3
2 3
temperature. It is obvious that the activity of the Pt/TiO -WO
be 0.48% by inductively coupled plasma spectrometer (ICP-
AES). Figure S3a shows scanning transmission electron
enhances with the increase of the light intensity. The best
performance is achieved when the light intensity is 1000
microscopy (STEM) image of the reduced Pt/TiO
The size of Pt clusters dispersed on the TiO -WO
0.8 nm. In the energy dispersive spectroscopy (EDS) mappings
2
3
-WO catalyst.
2
mW/cm . In this case, the reaction temperature for 70%
2
3
support is
o
o
conversion of C
3 8
H (T70) is as low as 90 C, which is 234 C
~
lower than that in dark. In addition, no obvious hysteresis can be
of Pt, W and Ti elements (Figure S3b), one can see that the
distribution of Pt is in accordance with that of W instead of Ti,
portending that Pt would preferentially deposit onto the surface
found in the light-off curves of the fresh and used Pt/TiO
catalysts, demonstrating the excellent stability (Figure S8).
2 3
-WO
To get deep insight into the origin of the enhancement, the
Arrhenius plot is given in Figure 1b. The E of our catalyst is 130
kJ/mol in dark, while it decreases to 11 kJ/mol under strong light.
Such small value is beyond the contribution of
thermodynamics. For clarity, these results are summarized in
3
of WO because of the relatively better affinity between Pt and
[9]
WO
composed of anatase and rutile TiO
obvious diffraction peak of Pt crystal is found, indicating no large
Pt nanoparticle in the Pt/TiO -WO catalyst (Figure S4).
3
.
Moreover, the XRD patterns of the TiO
2 3
-WO
support are
a
2
, and monoclinic WO
3
. No
a
E
a
2
3
Figure 1c. It is interesting to find that the E
proportional to the power density of the light (I):
=65.3-0.054*I
According to this equation, the E
kJ/mol when the light intensity is infinitely close to zero.
a
value is inversely
The catalytic testing was carried out at atmospheric pressure
in a self-designed continuous flow fixed-bed quartz reactor,
which allows all of the feed gas to go through the catalyst bed.
The cross-section of the reactor is profiled in Figure S5. The
E
a
(1)
a
can be extrapolated to be 65.3
2
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RESEARCH ARTICLE
2
Figure 2. Dependence of reaction rate on partial pressure of (a) O
2
and (b) C
3
H
8
at 200 °C with and without light irradiation (power density: 500 mW/cm ) as well
2 3 8
as (c) O and (d) C H at 400 °C in dark.
Considering that the E
a
is as high as 130 kJ/mol in dark (dashed
independent of O
2
concentration under this condition. The n of
arrow in Figure 1c), we speculate that the mechanism of the
photo-thermo catalysis is different from that of the thermal
catalysis. To differentiate the contributions of light and heat, we
compared the activities of photocatalysis, thermocatalysis and
photo-thermo catalysis, respectively (Figure 1d). Under
C H is 0.9 in dark (Figure 2b), which is consistent with the result
3 8
[12]
of the previous literature. It has been reported that activating
C-H bond of hydrocarbon molecule is the kinetically relevant
steps (KRS) in catalytic oxidation reaction, which occurs on the
[
13]
metallic Pt surface.
the competitive absorption between O
contrary, the n of C declines to 0.3 upon light illumination,
suggesting that more C molecules can reach the surface of
This high n value mainly originates from
[
14]
continuous illumination without heating, the conversion of C
3
H
8
2
and C
3
H
8
.
On the
merely maintains at ~5%. We further tracked the steady-state
3 8
H
o
reaction at 200 C with and without light irradiation. The
3 8
H
3
3
conversion of C
dark (off). Also, the light enhancement process is reproducible.
Therefore, the superior activity of the Pt/TiO -WO catalyst for
catalytic oxidation of C can be attributed to the synergic
3
H
8
under light (on) surges to ~9 times of that in
the metallic Pt and be activated. To examine the effect of the
possible localized heat on the catalyst surface under light, the
2
3
dependence of reaction rates on the partial pressure of O
were further measured at 400 °C in dark. As shown in
Figure 2c, the n of O is increased to -0.6 in this case, indicating
that heating is beneficial for relieving the oxygen poisoning to
some extent. Nevertheless, it is still difficult for C molecules
to approach to the Pt surface, since the n of C still keeps at
2
and
3
H
8
3 8
C H
effect of light and heat. Furthermore, the specific reaction rates
based on the Pt content under different conditions are calculated
2
and compared (Table S1). For the Pt/TiO
2
-WO
can be achieved at
20 C in photo-thermo catalysis, which is almost 5 times of the
3
catalyst, the
3 8
H
-1 -1
specific reaction rate of 457.3 μmolgPt
2
s
3 8
H
o
0.9 (Figure 2d). The results unambiguously verify that the photo-
thermo catalysis is capable of accelerating the KRS of the
oxidation reaction by suppressing the poisoning of the supported
-
best results reported for supported Pt catalysts (96.44 μmolgPt
1
-1
[11]
s ) at the same reaction temperature,
even though the light
2
intensity is only 250 mW/cm .
Pt catalyst by oxygen and improving the accessibility of C
molecules.
3 8
H
To investigate how light and heat influence the reaction, the
relationship between the reaction rates and the partial pressure
To monitor the valence state of the Pt in photo-thermo
of O
with respect to O
excessive O in the reaction system would speed up the
coverage and oxidation of Pt, leading to the degradation of the
catalyst. That is, the oxygen can poison the surface of the
supported Pt catalyst. However, the n changes to 0.1 under light
illumination, which means that the reaction rate is almost
2
and C
3
H
8
were measured at 200 °C. It is found that the n
reaction, in situ X-ray absorption spectroscopy (XAS)
2
is -1.4 in the absence of light (Figure 2a). The
experiment was conducted on the Pt/TiO
2 3
-WO catalyst, in which
2
the content of Pt was increased to 1 wt% and that of W to 4 wt%
[
15]
for better signal noise ratio. As shown in the normalized X-ray
absorption near edge structure (XANES) spectra (Figure 3a), the
white line intensities of the activated and reacted catalysts are
comparable, and slightly higher than that of the Pt foil but lower
3
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RESEARCH ARTICLE
than those of the PtO
2
and Pt(NH
3
)
4
(NO
3
)
2
and calcined catalyst,
carbonaceous species of C
3
H
8
oxidation in dark. These bands
suggesting that the Pt was mainly at metallic state during the
continuous photo-thermo reaction (Figure S9). The metallic
nature of the activated and reacted catalysts can also be verified
by the dominant metallic Pt-Pt coordination in the extended X-
ray absorption fine structure (EXAFS) spectra (Figure S10) and
the corresponding fitting results (Table S2). In comparison with
XAS, in situ DRIFTS of CO absorption can afford the first hand
remain unchanged with time. In contrast, a remarkable new
band appears by introducing light illumination, which is
highlighted by a red rectangle in Figure S12b. To identify the
origin of the band, C
into the sample cell under light. As shown in Figure 4a, the
DRIFT spectrum of the C collected over the catalyst under
light is identical to that of the C
that the interaction between C
3 8 2
H and O were successively introduced
3
H
8
[
19]
3
H
8
gas phase. This indicates
[16]
information on the charges of Pt on the surface of the catalyst.
3 8
H and the catalyst is weak,
Figure 3b depicts the typical DRIFT spectra of CO chemisorbed
on the Pt catalyst with and without light at 20 C. When the light
excluding the possibility of the direct activation of
molecules and confirming the participation of O to form the new
species. Concurrent with the reduction of IR features of C
exhilaratingly, an increasing characteristic band at 1676 cm
presents in the time-resolved DRIFT spectra in the presence of
(Figure 4b). This newly generated principal band can be
ascribed to the stretching vibration of peroxycarbonate in
3 8
C H
o
2
off, besides the absorption bands of CO gas phase, there is a
3 8
H ,
-1
-1
vibrational stretch at 2083 cm , which can be assigned to the
oscillation of CO molecules linearly adsorbed on low-
[17]
coordinated metallic Pt. Upon light illumination, a new band at
O
2
-
1
2
2
093 cm appears accompanied by the decrease of the band at
-
1
-1
083 cm , suggesting that the surface of Pt is slightly electron-
comparison with the -OCO
(Ph P) PtOCO .
3 2 3
3
band at 1678 cm
When rising the temperature, the
decomposition of the peroxycarbonate can be accelerated,
evidenced by the increase of the bands of gaseous CO at 2331
for
[
5]
[20]
deficient. The photo-induced thermal effect can be excluded
since heating would make the absorption band shift towards low
wavenumbers (Figure S11). Therefore, this result demonstrates
that the photoexcited electrons can be pumped from the 5d
orbital of Pt to the vacant 2π* orbital of CO by the π electron
back donation under light, which is responsible for weakening
2
-1
-1
cm and 2364 cm (Figure 4c). This result definitely confirms
that the peroxycarbonate should be the intermediate of the
3 8
photo-thermo catalytic oxidation of C H . Serving as the reaction
[
18]
the Pt-CO bond. Besides, the phenomena are reversible with
the light on and off, which indicates the photo-induced electron
could be produced reversibly. This is in good agreement with the
catalytic result in Figure 1d.
intermediate, peroxycarbonate has been verified by density
functional theory (DFT) in CO oxidation on supported Pt
[
21]
cluster.
Figure 3. (a) Normalized XANES spectra at the Pt
L
II-edge of PtO
2
,
Pt(NH (NO calcined Pt/TiO -WO activated Pt/TiO
3
)
4
3
)
2
,
2
3
,
2
-WO reacted
3
,
Pt/TiO
Pt/TiO
2
-WO
3
and Pt foil. (b) In situ DRIFTS of CO adsorbed on activated
o
2
-WO
3
at 20 C with and without light irradiation.
To capture the reaction intermediate, in situ DRIFTS of the
catalyst in the reaction gas was performed. Figure S12a exhibits
the evolution of the DRIFT spectra of the oxygenated
Figure 4. In situ DRIFT spectra of the Pt/TiO
contacted with (a) C H , (b) O
3 8 2
2
-WO
at 70 C under light illumination and then (c)
heated to different temperatures.
3
catalyst successively
o
4
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RESEARCH ARTICLE
[
26a]
3+
-
-
The great change of the n of O
2
under light suggests that the
Degussa P25 lattice.
Pt/TiO -WO should be the evidence of the electron transfer
from TiO to WO due to the proper band alignment. To validate
3
The absence of Ti , O and O over the
photo-thermo reaction follows a very distinct pathway, in which
light would induce the evolution of oxygen species to drive the
reaction. Therefore, identifying active oxygen species is crucial
for understanding the underlying mechanism. To this end, EPR
was employed, which is a robust technique for detecting
2
3
2
3
this assumption, the photoluminescence (PL) spectra were
collected for comparing the ability of charge separation (Figure
S15). We can clearly see that the addition of WO
substantially reduce the PL intensity of TiO , indicating the
suppressed charge recombination. Besides, the participation of
WO endows the support with increased utilization of light
3
can
[
22]
unpaired electron.
Figure 5a shows the in situ EPR spectra
-WO with and
2
obtained at 100 K in air on the reduced Pt/TiO
2
3
without light illuminations. In dark, the symmetric signal at
g=2.003 is due to unpaired electron trapped at the oxygen
3
irradiation (Figure S1).
[
23]
vacancies in TiO
2
,
and the prominent signal at g=1.979 is
3+
[24]
attributed to Ti in the rutile lattice of Degussa P25.
continuous illumination for min, set of paramagnetic
(gxx=2.002, gyy=2.010, gzz=2.026) signals
Upon
5
a
-
superoxide anion O
were observed on the reduced Pt/TiO
2
[
25]
2
-WO
3
catalyst.
-
Meanwhile, the EPR signals of O
under light once C is introduced, signifying O
even at 100 K (Figure 5b). This process is reversible with
and without light illumination, further confirming the importance
2
are weakened obviously
-
3
H
8
2
can react with
3 8
C H
-
2
of photo-induced O in this reaction.
Figure 6. (a) Catalytic activity of the Pt/TiO
2
-WO
3
catalyst mixed with SiO
2
,
(NH
4 2
) C
O
2 4
and K
Cr O under light illumination. (b) O -TPD profiles of the
2 2 7 2
Pt/TiO
2
-WO catalyst under different conditions.
3
The origination of the photogenerated electrons was studied
by comparing the catalytic activities of the Pt/Al with and
without light irradiation (Figure S16). Compared with the Pt/TiO2-
WO catalyst (Figure 1a), the catalytic performance of the
Pt/Al catalyst under light is slightly better than that in dark, for
which the direct photo-excitation of hybridized Pt-O states may
Besides, the light absorption of the Pt/TiO
is similar to that of the TiO -WO (Figure S17). The above
2
O
3
2 3
Figure 5. In situ EPR spectra of the reduced Pt/TiO -WO catalyst in (a) air
and (b) reaction gas with and without light irradiation.
3
2
O
3
The role of WO
measurements. Distinct from the reduced Pt/TiO
signals from the reduced Pt/TiO at g=2.026, 2.017 and 2.014
were seen (Figure S13), which can be assigned to O
3
in our catalyst can also be clarified by EPR
2
[
27]
2
-WO , the EPR
3
be responsible.
WO
2
-
2
3
2
3
-
-
2
, O
, the exclusive
) produced on the Pt/TiO -WO catalyst
should be responsible for its high activity (Figure S14). Besides,
3
and
results indicate there is no plasmonic effect over the Pt
-
[26]
[5]
O , respectively.
Compared with the Pt/TiO
2
nanoparticles. Therefore, the electronic effect should be from
-
oxygen intermediate (O
2
2
3
2 3
the TiO -WO support. To further verify the contributions of the
charge carriers, the potassium dichromate and the ammonium
3
+
a sharp signal at g=1.990 responsible for Ti in the anatase
oxalate are used as electron and hole scavengers,
5
5
[28]
lattice appears over the reduced Pt/TiO
2
under light, in parallel
respectively.
3 8
The conversion of C H is ~77% when the
3+
with the increase of the signal intensity of Ti in the rutile lattice.
In this regard, it can be interpreted that the excess
photogenerated electrons induce the reduction of Ti
catalyst was grinded with the inert quartz sand. The trappings of
electrons and holes result in the decrease of the equilibrium
conversion of C H (Figure 6a), respectively. These results
4+
in
3
8
5
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RESEARCH ARTICLE
Figure 7. Proposed reaction mechanisms of (a) thermal and (b) photo-thermo oxidation of C
3
H
8
over the Pt/TiO
2
-WO
3
catalyst at low temperatures.
manifest that the trapping of holes or electrons would inhibit the
reaction. In order to understand the activation and desorption of
oxygen on the catalyst, the temperature-programmed desorption
We can infer that the elevated temperature may be conductive
to transfer more energetic charge carriers to the active sites of
the catalyst. From the thermal catalytic point of view, heating
would also profit the desorption and/or transform of the
intermediates on the catalyst. Thus, the synergy between
photocatalysis and thermocatalysis could be reflected.
of O
moieties can be detected when the O
conducted under light, indicating that light illumination would
restrain the O adsorption onto the catalyst. Two prominent
peaks appear at around 188 C and 294 C in dark, which
correspond to different kinds of the chemically adsorbed O
2
(O
2
-TPD) was conducted. As shown in Figure 6b, no O
2
2
adsorption process was
2
o
o
2
*
Conclusion
[
29]
species. In sharp contrast, the O
2
* species almost begins to
o
desorb at room temperature and attain the summit at 105
C
In summary, photo-thermo catalysis has been demonstrated to
be an effective strategy to promote the catalytic oxidation
reactivity over the semiconductor supported nonplasmonic metal
Pt, especially at low reaction temperatures. The inversely linear
dependence of the apparent activation energy on the intensity of
light and the great change of the reaction orders unravel the
essence of the advantages for the photo-thermo catalytic
oxidation reaction, in which overcoming the oxygen poisoning of
the supported Pt catalyst is the key factor to accelerate the
activation of C-H on Pt surface. The roles of the nonplasmonic
metal Pt and the semiconductor support in the catalytic photo-
thermal oxidation process are clarified: the decreased electron
density of the Pt could enhance the activation of oxygen; while
under light. The baseline drifts downward upon illumination at
the beginning, which may stem from the consumption of
physically adsorbed O . These results clearly suggest that light
2
illumination is helpful for the activation and desorption of oxygen
species.
Based on our results, the corresponding mechanisms are
2
proposed. For thermocatalysis, O would preferentially adsorb
onto the surface of the active Pt cluster due to the higher affinity,
resulting in the oxygen poisoning (Figure 7a). Higher
temperature enables the adsorbed oxygen species with high
activity, thereby driving the catalytic oxidation reaction. Yet, the
excess O
2
would not only suffer competitive adsorption with
hydrocarbons but also oxidize the metallic Pt and lead to the low
2 3
the TiO -WO support could not only promote the utilization of
[
8a]
performance of the catalyst. As illustrated in Figure 7b, photo-
thermo catalysis over the Pt/TiO -WO catalyst indeed brings
about a great change. The O adsorbed onto the Pt surface can
the light but could suppress the charge recombination.
Combined with the carbonaceous intermediate and active
oxygen species detected by the in situ characterizations, a new
reaction route of the photo-thermo catalysis at low temperatures
is proposed, which provides an in-depth insight into the photo-
thermo catalysis of the semiconductor supported nonplasmonic
metal catalysts.
2
3
2
δ-
be activated to generate anionic superoxide (Pt-O-O ) by
photogenerated electrons from the semiconductors (in situ EPR
-
results in Figure 5a). The active O
desorb (O
adjacently activated C
results in Figure 5b), followed by the formation of
2
species would readily
2
-TPD results in Figure 6b) and react with the
* to produce carbonate (in situ EPR
3
H
8
[
30]
peroxycarbonate with the mediation of holes
(in situ DRIFT
Acknowledgements
results in Figure 4 a, b). Subsequently, the decomposition of the
peroxycarbonate can be accelerated by heating to produce
We are grateful for the support from the National Key R&D
Program of China (2016YFA0202801), the Strategic Priority
Research Program of the Chinese Academy of Sciences
gaseous CO
chemisorbed O
2
(in situ DRIFT results in Figure 4 c) and
*. The E can be reduced obviously due to the
2
a
above steps. For improving the reaction rate, suitable heating is
of the essence in photo-thermo catalysis. The conductivity of
(
XDB17020400), the National Natural Science Foundation of
China (21776271, CSC 201804910082), CAS Interdisciplinary
[
31]
semiconductors increases exponentially with the temperature.
6
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RESEARCH ARTICLE
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RESEARCH ARTICLE
Entry for the Table of Contents
Compared with thermocatalysis, photo-thermo catalytic oxidation of propane (C
3
H
8
) on Pt/TiO
2
-WO
3
catalyst exhibits an excellent
2 3 8
activity at high O /C H ratios and low temperatures. By investigating the apparent activation energy and reaction orders, the enhanced
performance can be attributed to breaking through the oxygen poisoning of supported Pt catalyst.
8
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