Unusual photocatalytic materials with UV-VIS-NIR spectral response: Deciphering the photothermocatalytic synergetic effect of Pt/LaVO4/TiO2

Article abstract of DOI:10.1039/c6ta06688g

In the present paper, we propose a new type of catalyst, Pt/LaVO₄/TiO₂ (PLVT), which combines the use of UV and visible light for photocatalysis on LaVO₄/TiO₂ (LVT) and the heating effect of infrared light for low-temperature thermocatalysis on Pt. Under simulated solar light, benzene as an objective pollutant can be 100% eliminated and converted into CO₂ and H₂O on PLVT at 70 °C with an obvious synergetic effect of photocatalysis and thermocatalysis. The photothermocatalytic activity at 70 °C is about three times the sum of the photocatalytic activity and thermocatalytic activity. The stable performance of PLVT in a 60 hour durability test also indicates its applicable potential. The roles of Pt and LVT in the synergetic effect are discussed by comparing PLVT with control samples regarding the interaction between Pt and LVT under photothermocatalytic conditions. On the basis of the above experimental results, a possible mechanism of the synergetic effect is proposed.

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Graphical Abstract
Unusual photocatalytic materials with UV-VIS-NIR spectral
response: deciphering the photothermocatalytic synergetic
effect of Pt/LaVO /TiO
4
2
Jialin Fang, Danzhen Li*, Yu Shao, Junhua Hu

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ARTICLE
Unusual photocatalytic materials with UV-VIS-NIR spectral
response: deciphering the photothermocatalytic synergetic effect
Received 00th January 20xx,
Accepted 00th January 20xx
of Pt/LaVO /TiO2
4
DOI: 10.1039/x0xx00000x
Jialin Fang, Danzhen Li*, Yu Shao, Junhua Hu
www.rsc.org/
In the present paper, we propose a new mode of catalyst, Pt/LaVO
photocatalysis on LaVO /TiO (LVT) and heating effect of infrared light for low-temperature thermocatalysis on Pt. Under
simulated solar light, benzene as the objective pollutant can be 100% eliminated and converted to CO and H O on PLVT at
0 °C with obvious synergetic effect of photocatalysis and thermocatalysis. The photothermocatalytic activity at 70 °C is
4 2
/TiO (PLVT), which combines UV and visible light for
4
2
2
2
7
about three times of the sum of photocatalytic activity and the thermocatalytic activity. The stable performance of PLVT in
sixty-hour durability test also indicates its applicable potential. The roles of Pt and LVT in the synergetic effect are
discussed by comparing PLVT with control samples about the interaction between Pt and LVT under photothermocatalytic
condition. On the basis of the above experimental results, the possible mechanism of synergetic effect is proposed.
1
4
and propane under simulated sunlight at about 160 °C . TiO
2 2
/CeO
1
. Introduction
and MnO /TiO were also used for degradation of benzene in a
x
2
stainless steel gas-phase reactor using the Xe lamp at about 200
As an attractive and sustainable strategy to solve the
1
5,16
°
C
.
These researches promote our notion about
environment and energy problems, photocatalysis has been broadly
1
photothermocatalysis, however, the high temperatures were
necessary to obtain high activity or extra apparatuses were needed
to get high temperatures for the degradation of refractory VOCs
such as benzene, which limits the direct use of solar light.
studied since 1972 . Persistent efforts have been made to enhance
2
2
-4
the utilization of solar light, such as the researches on TiO
,
5
,6,
7,8
9
ZnO CdS , N-doped TiO
the solar radiation spectrum can be approximately classified into
% of ultraviolet (UV), 43% of visible light (VIS) and 50% of infrared
IR), respectively. Accordingly, as most of photocatalysts reported
2
, etc. As we know, the proportions of
In this paper, we establish the composite materials of
7
Pt/LaVO
light
4
/TiO
to
2
(PLVT) to degrade benzene under simulated solar
implement our above prospection for
(
only exploit the UV and a small part of VIS, it is hard to generalize
the application of photocatalysis with its full superiority.
photothermocatalysis. Benzene is chosen as the objective pollutant
because of its widespread emission, persistence and carcinogenic
Herein, we propose a novel mode of catalysts which possess both
photocatalytic and low-temperature thermocatalytic activity.
Consequently, besides the traditional UV and VIS can be used for
photocatalysis, the heating effect of IR can also drive low-
temperature thermocatalysis. Several studies have been covered on
property. The reason why we choose LaVO
4 2
/TiO (LVT) as
photocatalyst is due to its outstanding activity in oxidation of
17
benzene under UV-Vis in our former study . The matched band
potential in this heterostructure makes it efficient for the
separation of photo-induced pairs of electrons and holes, which
contributes to the great photocatalytic activity. In addition, the
simple and low-cost synthesis and large output makes it more
promising to application. Platinum (Pt) has been broadly researched
the idea of combining photocatalysis and thermocatalysis. Pt/TiO
2
must be the first catalyst to be studied as photothermocatalyst
using in the degradation of ethanol, acetaldehyde and benzene.
These studies were mainly proceeded under high temperatures
18,19
1
0-12
with its outstanding thermocatalytic performance
. We expect to
(
about 200 °C) and UV light due to the light absorption of TiO
Other catalyst such as Ce1−xBi 2−δ was reported to oxidize
formaldehyde (200 ppm) to 100 ppm CO in three hours under
2
.
combine photocatalysis on LVT and low-temperature
thermocatalysis on Pt and these two parts have synergetic effect
without mutual inhibition. In this way, the general defects of
photocatalysis like unable to accomplish total degradation of
benzene and the main weakness of thermocatalysis such as high
temperature needed to complete elimination of benzene would be
overcome. Since benzene could be degraded over PLVT under
simulated solar light without other facilities at low temperature,
photothermocatalysis should be a promising way to truly make the
most of solar energy for the elimination of benzene or other VOCs.
x
O
2
13
photothermocatalytic condition at 80 °C . Co O nanosheets were
3
4
electrodeposited on stainless steel mesh to eliminate propylene
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2
. Experimental Section
(30 °C) and the photothermocatalytic activity under simulated
solar light with different temperatures was controlled by
cooling fan. The simulated solar light was realized by Xe-arc
lamp without any cut-off filter (500 W, Beijing changtuo. Co.),
while the visible light was obtained via a 420<λ<800 nm filter.
The irradiation spectra of these two light sources are shown in
Figure S1 and S2.
2.1
Preparation of samples
The preparation of PLVT was divided into two parts, the synthesis
of LVT and the loading of Pt. Firstly, the LVT was prepared using a
simple sol-gel method reported previously by our group, and the
1
7
ratio of LaVO
LaVO powders synthesized by hydrothermal method were added
into TiO sol. After ultrasonic dispersed for 0.5 h and stirred for 24
4 2
to TiO is fixed at 1wt% . Briefly speaking, 1.0 wt%
4
In the experiments of gas-phase degradation, the
conversion of benzene (abbreviated as COB in main text) has
2
h, these mixed colloids were dried in a microwave oven to remove
solvents. The resultant xerogel was calcined at 500 °C for 6 h to
obtain LVT samples and sifted to 50-70 mesh. Secondly, the LVT was
impregnated by an aqueous solution of chlorine platinum acid
0 0 0
been defined as (C – C)/C . C is the initial concentration of
pollutant when adsorption-desorption equilibrium is achieved
in the degradation system and C is the residual concentration
after degradation. Mineralization was defined as the ratio of
-2
-1
(
H
2
PtCl
Then the catalyst was reduced by an excess solution of sodium
borohydride (NaBH , 0.1 M) in sodium hydroxide (NaOH, 0.1 M).
6 2
·6H O, 2.57×10 mol·L ) and then dried at 80 °C for 12 h.
2
the amount of practically produced CO and the theoretical
one which was calculated by the complete mineralization of
pollutants.
4
Prior to being characterized and tested, the samples were calcined
at 400 °C for 4 h. Different loading amount of Pt by 0.5 wt%, 1.0
wt%, 2.0 wt% and 5.0 wt% over LVT were prepared.
COB = (C ×100%
0
– C) / C
0
Mineralization = [CO ]produced / [6(C – C)]×100%
2 0
The synthesis of control samples are as follows. The synthesis of 2.3
Conditions of characterization
The X-ray diffraction (XRD) patterns were recorded on a
PLVT-A was the same as the process of PLVT, except the Pt was
4
unreduced by NaBH /NaOH solution. The unactivated PLVT-B Bruker D8 Advance X-ray diffractometer using Cu Kα1
sample was synthesized by the same method above except that the irradiation (λ =1.5406 Å) to identify the phase constitutions in
samples were not calcined at 400 °C for 4 h after reduction. The samples. The morphology and microstructure of the composite
synthesis of PLVT-C was divided into two parts. Firstly, 67.5 mg PVP were further investigated by transmission electron microscopy
2 6 2
and 49.5 mg H PtCl ·6H O were dissolved in 6 ml ethylene glycol (TEM) and high-resolution TEM (HRTEM) using a JEOL JEM
(EG)，respectively, then they were respectively dropped into 28 ml 2010F microscope at an accelerating voltage of 200 kV. The
EG. After stirring for 0.5 h, the mixture was transferred into 100 ml measurements of specific surface area were performed by
Teflon-line stainless steel autoclaves by hydrothermal treatment at Brunauers-Emmetts-Teller (BET) analyzer with a Coulter
110 °C for 3 h. The product was washed by acetone for 6 times and Ominisor 100 CX automatic physisorption analyzer through
then dispersed in ethanol as stocking solvent. Secondly, LVT and isotherms. X-ray photoelectron spectroscopy (XPS) analysis
PVP stabilized Pt stocking solvent were mixed together at the was conducted on a ESCALAB 250 photoelectron spectroscope
-10
loading ratio of 1.0 wt% Pt. After ultrasonic treatment for 10 min (Thermo Fisher Scientific) at 3.0×10
mbar with
and stirring for 5 h, the products were purified by centrifugation monochromatic Al Kα radiation (E =1486.2 eV). The UV-vis
with the help of acetone. Finally the sample was dried at 60 °C diffuse reflectance spectra (DRS) were obtained by a Varian
overnight and then sifted to 50-70 mesh for the further use. The Cary 500 UV-VIS-NIR spectrophotometer.
synthetic process of Pt/TiO
2
and Pt/LaVO
or TiO
4
were basically the same
was added.
The photoelectrochemical experiment was performed using
a CHI-660D electrochemical work station (Chenhua
as that of PLVT except that no LaVO
4
2
Instruments, Co., Shanghai) in a conventional three-electrode
electrochemical cell, filled with 0.1 M of Na SO electrolyte (30
2
.2
Measurement of catalytic activity
3
2
4
Thermocatalytic activity was tested in
a
20×15×1.0 mm
mL). A Pt plate was used as the counter electrode, and an
Ag/AgCl/sat. KCl electrode was used as the reference
electrode. The samples were deposited as a film form on a 5
mm × 5 mm ITO conductive glass that served as the working
electrode.
rectangle fixed-bed quartz reactor by feeding a benzene cylinder
gas (DL Gas, China), which contained 281 ppm benzene with mixing
−1
synthetic air (80% N
2 2
: 20% O ), at a flow rate of 20 mL·min over
0.27 g of catalyst (50-70 mesh). Thermocouple was inserted to
contact with catalyst and monitor the temperature. Different
temperatures were obtained through heating plates. The gas hourly
The ESR measurement was performed by a Bruker model
-
A300 spectrometer (Bruker Instruments, Inc.). ·OH and O ·
2
−1
space velocity (GHSV) was 6000 h . The flow was measured online
by Agilent 6890 GC. The concentrations of benzene and carbon
dioxide were determined by using the flame ionization detector
radicals were trapped by 5,5-dimethyl-l-pyrroline N-oxide
DMPO) in ultrapure water and methanol, respectively. For the
(
test of photocatalysis, in order to exclude IR heating effect of
the 500 W Xe-arc lamp, a 300<λ<800 nm filter was used to
obtain UV-VIS light, and the spectra of irradiation under this
condition is shown in Figure S3. In order to ensure the test of
photothermocatalysis only increases the IR part compared
with test of photocatalysis, a filter with λ>300 nm was used to
obtain simulated solar light, and its irradiation is shown in
Figure S4. Similarly, the thermocatalysis was measured under
(
FID) and thermal conductivity detector (TCD), respectively. The
adsorption/desorption equilibrium of benzene gas on the catalysts
was obtained after 8 h in the dark before the activity measurement.
And the activity test at each temperature was proceeded for 10 h
for the data collecting.
Photocatalytic and photothermocatalytic activity were
measured in the same reactor as the thermocatalytic one. The
photocatalytic activity was obtained at normal temperature
2
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the irradiation of the Xe-arc lamp with a λ>800 nm filter S6. The BET surface area of PLVT is 76 m g , which is close to
-1
2 -1 17
that of LVT (79.94 m g ) .
(Figure S5).
O
2
-TPD tests were performed on Autochem 2920
instrument (Micrometric Co.). Typically, 80 mg samples were
placed in a quartz glass tube and pretreated at a flow of He gas
at 500 °C for 1 h. The adsorption of oxygen was performed in a
2
flow of 3 vol % O /Ar for 1 h under the following three
conditions, namely at 70 °C without irradiation, at ambient
temperature with irradiation (spectra of irradiation as Figure
S3) and at 70 °C with irradiation (spectra of irradiation as
Figure S4). Afterward, the samples were heated to 600 °C for
desorption using TCD as a signal recorder. All the gas flow
−1
rates were set as 25 mL·min .
3
Results and discussion
3
.1
Characterization of PLVT
Figure 2 (a) TEM image of PLVT. (b) HRTEM image of PLVT. (c) The
FFT of the boxed section in (b). (d) HRTEM image of Pt in PLVT. (e)
The FFT of (d)
4
Figure 1 XRD patterns of the LaVO and PLVT.
Figure 1 shows the XRD patterns of the LaVO
4
and PLVT. As
for the LaVO , all the peaks are in line with the standard
4
spectra (JCPDS No.50-0367, space group: P21/n). The XRD
patterns of PLVT show its mixed crystal phase mainly
composed of anatase as well as rutile and a little brookite. No
4
diffraction peaks of Pt and LaVO are observed due to low
loading amount, good dispersion and small particle size.
The morphology of PLVT was further observed using TEM
and HRTEM. In the TEM image of PLVT (Figure 2a), the dark
spots circled denote Pt nanoparticles due to their higher
electron density. These uniformly sized Pt nanoparticles are 3-
Figure 3 XPS spectra for (a) Pt 4f of PLVT, (b)Ti 2p, (c) O 1s of LVT
5
nm and well-dispersed on the surface of LVT. Several
different lattice fringes can be obtained from the HRTEM
image of PLVT in Figure 2b. The lattice fringes of = 0.243 nm
match the (12 2) crystallographic planes of LaVO
nanoparticles and the lattice fringes of = 0.351 nm match the
101) crystallographic planes of anatase phase of TiO , which
and PLVT, (d) La 3d and V 2p of LaVO4, LVT and PLVT.
d
The composition of the catalysts was further probed by XPS

4
(
Figure 3). The Pt 4f XPS spectra exhibit peak 4f7/2 and peak
d
4
f5/2 at 70.3 and 73.6 eV, respectively, which are
2
0
(
2
corresponding to zero-valence Pt and verify the totally
reduction of impregnated H PtCl on LVT. The detailed spectra
can find their corresponding diffraction spots in the fast-
Fourier transform (FFT) patterns (Figure 2c). The lattice fringes
of the dark dot in HRTEM in Figure 2d match the (111)
crystallographic of Pt, which can also be identified by its
characteristic diffraction spots in Figure 2e. The nitrogen
adsorption-desorption isotherm and pore size distribution plot
2
6
of Ti 2p3/2 and 2p1/2 show peaks at 458.5 and 464.2 eV,
respectively, for both LVT and PLVT. In spectra of O 1s, the
peaks at 529.8 and 529.9 eV of LVT and PLVT are related to
lattice oxygen while the peaks at 531.9 and 531.6 eV of LVT
and PLVT denote the surface oxygen species. The surface
oxygen species account for 30 and 40 percent of all the oxygen
(inset) of the as-prepared PLVT sample is presented in Figure
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species for LVT and PLVT, respectively. This might be due to 100 °C. As shown in Figure 5a, the temperatures of TCB are
the reduction of Pt in PLVT is done under alkaline condition, 70 °C and 90 °C under simulated solar light and VIS,
which can lead to more hydroxyl groups chemisorbed on the respectively. As for thermocatalytic degradation, the
2
1,22
surface of PLVT
. As the surface oxygen species have higher temperature of TCB is 100 °C. In order to confirm the
mobility than lattice oxygen, it is more helpful to the catalytic photothermocatalytic synergetic effect, the photocatalytic
activity. Comparing the spectra of La 3d and V 2p of LVT and COB (at 30 °C) under simulated solar light and VIS, respectively,
PLVT with that of LaVO
basically the same, which further confirms the existence of shown in Sum S and V in Figure 5a. For example, the
LaVO in LVT and PLVT. Besides, in order to verify no remaining thermocatalytic COB at 70 °C is 20% while the photocatalytic
4
(Figure 3d), the binding energy is are added to the thermocatalytic COB at each temperature, as
4
Cl, Na and B during the synthesis process, the XPS spectra of COB (at 30 °C) is 12%, so the addition value is 32%, which is far
Cl, Na and B for PLVT are shown in Figure S7. The results show lower than the photothermocatalytic COB (100%) under
that no Cl, Na or B remain in the obtained PLVT.
simulated solar light at 70 °C. It can be clearly figured out that
The UV-vis diffuse reflection spectra (DRS) of PLVT, LVT and P25 the photothermocatalytic activity is not merely the sum of
(
TiO , as a standard sample) can be obtained from Figure 4. photocatalytic and themocatalytic activity but some synergetic
2
Compared with LVT, PLVT shows an increase in absorbance due to effect exists. In
a
word, PLVT shows notable
Pt.
photothermocatalytic synergetic effect and the temperature of
TCB is 70 °C under simulated solar light, which shows
promising prospect in industrial application.
Figure 4 UV-Vis DRS of P25,LVT and PLVT
3
.2
Comparison of thermocatalytic activity
4 2
As the optimum ratio of LaVO to TiO and calcination
1
7
temperature had been studied in our previous work of LVT .
Herein, different loading amount of Pt with 0.5 wt%, 1.0 wt%,
2.0 wt% and 5.0 wt% over LVT had been synthesized. As shown
in Figure S8, the thermocatalytic activity of PLVT is enhanced
obviously with the increase in loading amount of Pt. Without
loading of Pt, LVT presents the lowest catalytic activity, and
the COB is lower than 6% at 110 °C. When the amount of Pt is
0.5 wt%, the temperature of total conversion of benzene
(when COB=100%, total conversion of benzene is abbreviated
as TCB) is 110 °C. When the loading amount of Pt is further
increased to 1.0 wt%, the temperature of TCB is 100 °C.
However, when the content of Pt increases to more than 2.0
wt%, the temperature of TCB has no significant decrease or
even a miner decrease in activity at low temperature, which
might be due to the decrease in dispersion of Pt at higher
Figure 5 (a) Conversion of benzene versus temperature over PLVT
under simulated solar light, VIS and without irradiation for the
thermocatalysis. Sum S and V refer to the sum of thermocatalytic
and photocatalytic activity (conversion at 30 °C) under simulated
solar light and VIS, respectively. (b) Mineralization versus
23
loading amount . As a result, to achieve efficient use of the
precious metal resources as much as possible, the loading
amount of Pt was fixed at 1.0 wt% in the following sections.
temperature over PLVT under simulated solar light, VIS and without
3
.3
Photothermocatalytic activity of PLVT
irradiation for the thermocatalysis, inserting the amount of CO2
The activity of benzene degradation on PLVT was tested
under these conditions.
under simulated solar light and VIS irradiation for the
photocatalysis at 30 °C, for photothermocatalysis within 50-
90 °C and for thermocatalysis without irradiation within 60-
4
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The results of concentration of CO produced and the thermocatalytic activity within 30-100 °C and the COB of LVT
mineralization are shown in Figure 5b. Obviously, the trends of does not change with temperature irradiated by simulated
the amount of CO versus temperature are basically the same solar light. As a result, it can be inferred that the generation of
as that of COB versus temperature. But the photothermocatalytic synergy of PLVT does not come from
photothermocatalytic mineralization under simulated solar Pt/TiO , Pt/LaVO and LVT itself.
light and VIS are different with that of the thermocatalytic one. Furthermore, it is interesting that no synergy is observed on
As for the thermocatalytic one, once the thermocatalytic Pt/TiO and Pt/LaVO while PLVT shows intense synergy. The
reactions are initiated, the mineralization sharply increases to difference among Pt/TiO Pt/LaVO and PLVT is the
almost 100%. Nevertheless, the photocatalytic mineralization component, namely TiO , LaVO and LVT, which mainly work
under simulated solar light and VIS ascend with temperature. as photocatalysts. Our previous study about LVT has declared
For temperature below 60 °C, the mineralization of these two that, compared with TiO and LaVO , the heterostructure of
2
2
4
2
4
2
,
4
2
4
2
4
irradiation conditions have the same trends but the simulated LVT could help to improve the separation efficiency of photo-
solar light one has higher value. When temperature reaches induced pairs of electrons and holes and greatly increase the
1
0 °C, the mineralization of these two are almost alike. It can photocatalytic activity . Herein, the photocurrent of TiO ,
7
6
2
be figured out that photocatalysis plays a dominated role Pt/TiO , LaVO , Pt/LaVO , LVT and PLVT is measured as shown
2
4
4
below 60 °C while thermocatalysis mainly influences the in Figure S15. Compared with TiO and LaVO , LVT has the
4
2
mineralization above 60 °C.
biggest photocurrent. Furthermore, loading of Pt can help
From the test of durable conversion of benzene over PLVT at increase the photocurrent. It can be inferred that the excellent
the irradiation of simulated solar light in Figure S9, it is known separation efficiency of photo-induced pairs of electrons and
that PLVT has great stability in activity. The XPS spectra of PLVT holes of LVT probably is one of the reasons for the
before and after reactions in Figure S10 show no obvious photothermocatalytic synergy of PLVT.
difference, which indicates the commendable structural
3
.5
From the above discussion, the control samples of Pt/TiO2,
The exclusion of the photothermocatalytic synergy of Pt/LaVO4 and LVT have been excluded from the origin of
The role of Pt for the photothermocatalytic synergy
stability of PLVT.
3
.4
Pt/TiO , Pt/LaVO and LVT
photothermocatalytic synergy of PLVT. Meanwhile, the
important impact of the separation efficiency of photo-
induced pairs of electrons and holes in LVT on
photothermocatalytic synergy is also revealed. Considering the
important impact of Pt may have on the synergy, control
samples about Pt have been synthesized according to the main
steps in loading of Pt, in which the impregnation of Pt is then
2
4
reduced by NaBH /NaOH and following the activation at 400
4
°
C. As a comparison, the unreduced PLVT-A sample,
unactivated PLVT-B sample and PLVT-C sample of PVP
stabilized Pt nanoparticles mixing with LVT were prepared as
shown in Figure 7a. The DRS of these three samples are shown
in Figure S16. Their photocatalytic activity at room
temperature, thermocatalytic and photothermocatalytic
activity at 70 °C are also studied as shown in Figure 7b.
As for the unreduced sample, PLVT-A, the XPS spectra are
Figure 6 Conversion of benzene versus temperature over LVT under shown in Figure S17. Without reduction, Pt in PLVT-A is mainly
2
+
4+
2+
simulated solar light, without irradiation for the thermocatalysis
and the sum of thermocatalytic and photocatalytic activity
Pt and Pt . The generation of Pt may be due to the
calcination after impregnation in H PtCl . In the activity test of
2
6
(
conversion at 30 °C) under simulated solar light.
To better understand the photothermocatalytic synergy of
PLVT, investigating the synergy of Pt/TiO , Pt/LaVO , and LVT is
necessary. Firstly, the catalytic activity over Pt/TiO and
Pt/LaVO were studied as a comparison. As shown in Figure
S11 and 12, H PtCl has been well reduced to zero-valent Pt in
Pt/TiO and Pt/LaVO as Pt in PLVT. However, the
photothermocatalytic activity is closed to thermocatalytic
activity for Pt/TiO in Figure S13 while almost no activity is
observed on Pt/LaVO in Figure S14. No photothermocatalytic
synergy is shown on Pt/TiO and Pt/LaVO . Secondly, the
PLVT-A, no thermocatalytic activity and almost no
photothermocatalytic synergetic effect are observed at 70 °C.
From this control experiment of PLVT-A, it can be inferred that
the metallic Pt is important to the thermocatalytic activity,
which is the basis of photothermocatalytic synergy.
2
4
2
4
As for the unactivated sample, the HRTEM image shows that
the Pt has almost the same size as that of PLVT. Besides, PLVT-
B has compatible photocatalytic and thermocatalytic activity
as PLVT, but really weak photothermocatalytic synergy is
observed compared with PLVT. In this case, we infer that the
activation of Pt which is related to interaction between Pt and
2
6
2
4
2
4
2
4
2
4
LVT is important to the synergy. Li et al. found that after the
thermocatalytic and photothermocatalytic activity of LVT are
also investigated. As can be seen in Figure 6, LVT shows no
totally reduced Pt/TiO
2
being calcinated at 400 °C, the Pt
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atoms thermally diffused from Pt particles into the lattice of observed, which is more confirmed that the strong interaction
2
+
4+
TiO
2
and substituted Pt for Ti by means of XPS in situ between Pt and support is really indispensable for synergy.
+
measurement of Ar sputtering experiment. So, they proposed
Moreover, the photocurrent of these samples in Figure S19
that as a result of calcinations after reduction, the contact indicates that the interaction truly influence the transference
resistance in the interface was decreased, which could help of electrons. From the above discussion about the role of Pt, it
the transference of photogenerated electrons. Since the can be inferred that the benign and direct contact between Pt
+
spectra of XPS without measurement of Ar sputtering only and LVT should be necessary to the photothermocatalytic
detected Pt on the surface, no obvious difference of XPS could synergetic effect.
be found between PLVT-B (Figure S18) and PLVT here. It can be
3
.6
Discussions about the origin of photothermocatalytic
synergy
inferred that the PLVT has better contact in interface of Pt and
LVT than PLVT-B does due to activation and may cause
decrease in Schottky barrier, which means better electrons 3.6.1
and mass transfer and finally influence the
photothermocatalytic synergy.
The mechanism of thermocatalysis
To investigate the origin of photothermocatalytic synergetic
effect, the thermocatalytic mechanism is discussed firstly. In
the
former
discussions
of
mechanism
about
photothermocatalytic synergetic effect, the thermocatalysis
was widely considered to follow the Mars-van Krevelen
1
2,13,16
mechanism on metal oxides
. However, from the above
discussion, the thermocatalytic activity here is mainly related
to Pt, on which the oxidation of benzene is always attributed
2
5
to the model of L-H mechanism . First step of this mechanism
is the adsorption of molecular oxygen and its dissociation to
form atomic oxygen, as depicted in equation (1). In the next
step, the adsorbed benzene is oxidized by atomic oxygen.
ꢃꢄ
ꢀ
ꢁ → ꢅꢁ ꢆ → ꢅꢁꢆꢇꢈꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ(1)
ꢂ
ꢂ ꢇꢈ
3
.6.2
The
probable
active
species
related
to
photothermocatalytic synergetic effect
Combining the above discussions about the interaction
between Pt and LVT, the photothermocatalytic synergy is
related to the excellent efficiency of separation of photo-
induced electrons and holes and the intimate contact between
Pt and LVT. Considering the mechanism of thermocatalysis and
photocatalysis, we think the factors which bridge the gap
between them should be focus on the interaction of the active
species of both thermocatalysis and photocatalysis.
For photocatalysis, the mechanism is related to the species
-
like photoinduced electrons (e ) and holes (h ) and the
+
-
2
following generated active oxygen species (e.g. ·OH and O · )
and subsequent degradation of benzene. Under
photothermocatalytic condition, the oxygen absorbed on Pt
-
-
2
will capture the photoinduced e from LVT and make O ·
Figure 7 (a) The schematic diagram of preparation of PLVT, PLVT-
A,B and C, inserting the HRTEM images with the scale bar of 2 nm
and the photos of these samples. (b) Conversion of benzene under
conditions of photocatalysis, thermocatalysis at 70 °C, the sum of
these two and photothermocatalysis at 70 °C for the sample PLVT-A,
B, C and PLVT, respectively.
2
6
-
radicals on Pt . The O
that of absorbed oxygen [O
while the latter is 118 kcal/mol) , which means that it is easier
2
· radicals have lower bond energy than
2
]
ad (the former is 94.5 kcal/mol,
2
4
-
-
for O
on Pt. At the same time, O would further turn into ·OH with
the H O from the degradation process (equation (3)), which is
2
· to dissociate into oxygen atoms and O (equation (2))
-
2
To further verify the important influence of contact helpful for photocatalysis. Finally, the generated species as
between Pt and LVT to photothermocatalytic synergy, the mentioned would oxidize benzene efficiently under
sample of PVP stabilized Pt particles mixing with LVT namely photothermocatalytic condition, as seen in equation (4).
ꢊ
ꢃꢄ
ꢉ
PLVT-C was synthesized. With Pt nanoparticles being covered
by PVP, the contact of Pt and LVT would be further weakened
or even cut off. Similarly, there is no big difference of the
morphology of Pt in PLVT-C from PLVT as shown in Figure 7a.
And in the activity test, no photothermocatalytic synergy is
ꢀꢀꢀꢁ → ꢅꢁ ꢆ ꢋꢌ ꢁꢂ → ꢅꢁꢆꢇꢈ ꢎ ꢅꢁꢍꢆꢇꢈꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢏ2ꢐ
∙ꢍ
ꢀ
ꢂ ꢂ ꢇꢈ
ꢀꢁꢍ ꢎ ꢑ ꢁ →ꢒ ꢁꢑ ꢎ ꢁꢑ ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢏꢓꢐ
ꢍ
ꢂ
ꢅꢁꢆꢇꢈ, ꢁ∙
ꢍ
,ꢒ ꢁꢑ, ꢔ ꢎ ꢖꢉꢗꢘꢉꢗꢉ → ⋯ → ꢀꢙꢁ ꢎ ꢀꢑ ꢁꢀꢀꢀꢀꢏꢚꢐ
ꢕ
ꢂ
ꢂ ꢂ
6
| J. Name., 2012, 00, 1-3
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3
.6.2.1
The measurement of ESR
consumption of electrons greatly accelerates the separation
In order to discuss the process in equation (2) and (3), efficiency of photo-induced pairs of electrons and holes in
-
2
ideally the signals of O · and ·OH radicals can be in-situ PLVT. As a result, the O2·- radicals increase compared with
detected. However, under the restriction of testing means in photocatalytic condition. Meanwhile, due to the equation (3),
-
gas phase, DMPO trapping O · and ·OH radicals in methanol the ·OH radicals also increase.
2
and aqueous solution detected by ESR techniques is always
used for discussions of radicals in gas-phase photocatalytic
2
7-30
systems
. Herein
· and ·OH radicals on LVT, PLVT and PLVT-A, B, C. The UV-vis
light (Figure S3), the simulated solar light (Figure S4) and the
，ESR was used as a method to investigate
-
O
2
-
2
near infrared (NIR) (Figure S5) are used to study the O ·
and ·OH radicals for photocatalysis, photothermocatalysis and
thermocatalysis, respectively.
As can be seen in Figure 8, PLVT shows the strongest signals
-
of DMPO-·OH and O · spin adduct among these samples under
-
Figure 9 ESR signals of (a) DMPO-·OH and (b) O
VIS light (denoted as PLVT-photo) and simulated solar light
denoted as PLVT-photothermo)
· of PLVT under UV-
2
2
UV-VIS light for photocatalysis. Compared with signals from
(
LVT, the introduction of Pt in PLVT increases the amount
-
of ·OH and O · radicals under UV-VIS light while other samples
-
Particularly, during the test of O · under simulated solar
2
-
light, the signals of DMPO-O · spin adduct of PLVT increase
2
2
have no obvious enhancement. Since photoinduced electrons
of LVT could transfer into Pt, the separation efficiency of
firstly then decrease, while under UV-Vis light the signals still
increase within the same time as well as the signals over LVT
under both conditions (Figure S22). This is due to the
competitive reaction between the production and
photo-induced pairs of electrons and holes in PLVT was
-
enhanced and thus more ·OH and O · radicals were produced.
2
This result can also verify the former deduction about efficient
contact between Pt and LVT in PLVT compares with other
samples.
-
consumption of O2· radicals according to equation (2).
Especially we need to point out, the solvent in this test,
+
methanol, was oxidized by holes (h ) to ·CH OH over PLVT
2
under simulated solar light, while no similar phenomena were
found over PLVT under UV-VIS light or LVT under both these
two light sources. This might be due to that the processes of
equation (2) and (3) greatly enhance the separation efficiency
of photoinduced electron-hole pairs of PLVT under simulated
+
solar light and a large amount of h are produced over PLVT
3
then oxidize methanol to ·CH OH , which further illustrates
1
2
the appearance of ·CH OH signal in the ESR test should be
2
related to the photothermocatalytic synergetic effect.
-
Figure 8 ESR signals of (a) DMPO-·OH adduct and (b) DMPO-O2·
adduct of LVT, PLVT, PLVT-A,B and C under UV-VIS light for
photocatalytic condition (see the irradiation graph in Figure S3)
3.6.2.2
The measurement of O -TPD
2
As a main method to investigate the oxygen species on catalyst at
different catalytic conditions, the tests of O -TPD were performed
-
· spin
2
The comparisons of the signals of DMPO-·OH and O
2
with the processes of oxygen adsorption at 70 °C without
irradiation, under irradiation at ambient temperature simulating
photocatalytic condition and under irradiation at 70 °C simulating
photothermocatalytic condition, respectively (as depicted in A, B
and C in Figure 10, respectively).
adduct for PLVT, LVT, PLVT-A, B, and C under simulated solar
light (photothermocatalysis) and UV-VIS light (photocatalysis)
are shown in Figure 9 and Figure S20. For PLVT, both the
-
· spin adduct under simulated
signals of DMPO-·OH and O
2
solar light are stronger than those under UV-VIS light.
However, for other samples, the signals are almost the same
under these two light sources. Particularly, no signals are
detected under NIR for thermocatalytic test of PLVT (Figure
Generally, the surface adsorbed oxygen species proceed as the
-
following transformation steps with electron gain: O
2
(ad) → O
2
(ad)
-
2-
→
2
O (ad) → O (lattice) . O (ad) as the physically adsorbed oxygen is
basically removed by purging argon before desorption analysis. The
S21) or other samples (not shown). The results indicate that
-
-
-
surface active-oxygen species O
2
and O are always observed to
more ·OH and O
2
·
radicals are produced by PLVT in
32
desorb below 400 °C successively . Herein, the peak at around
photothermocatalysis than photocatalysis. As other samples
have almost the same intensity of radical signals in these two
-
2
2
40 °C corresponds to O species while the peak at 325 °C
-
corresponds to O species. As seen in Figure 10, the area of peak B is
catalytic conditions, we can deduce that the increase in the
-
· radicals for PLVT are accelerated by
smaller than that of peak A, which might be due to photodesorption
amount of ·OH and O
2
33,34
under irradiation
. It can be also inferred that photodesorption
photothermocatalytic conditions. According to equation (2),
-
· radicals will
-
2
mainly influence the adsorption of O species because the decrease
the electrons are captured by O
2
on Pt then O
2
-
-
of peak situating below 300 °C while the peak at 325 °C for O
further resolve into oxygen atoms and
O
under
species keeps basically unchanged. However, compared peak C with
photothermocatalytic condition. The increase in the
-
peak B, there is still diminishment of O
2
species. In other words,
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-
2
under photothermocatalytic condition, O species further decrease energy. The discussions about the roles of Pt and LVT indicated the
-
besides the influence of photodesorption for
O
2
species. mutual interaction between these two parts and the
-
Meanwhile, the peak of O species in C increases slightly compares interrelationship of photocatalysis and thermocatalysis, which at
with those in A and B. In this case, the process in equation (2) in last lead to the synergetic effect for the efficient activity. It is a
-
-
-
which the O
2
· species (equals to O
2
species here) dissociate into O promising way to combine photocatalysis and thermocatalysis for
-
can lead to the O
2
-TPD results of decrease in O
2
species while truly use of solar energy in environmental purification or other
-
increase in O species under photothermocatalytic condition here. fields.
However, the O -TPD profiles of Pt/TiO with the process of oxygen
2
2
adsorption under UV-Vis irradiation at ambient temperature and
simulated solar irradiation at 70 °C in Figure S23 are almost the
same, which indicates that the process in equation (2) does not
2
happen on Pt/TiO .
Scheme 1 Possible mechanism of photothermocatalytic oxidation of
-
benzene over PLVT: the photoinduced e from LVT to Pt can be
-
captured by the [O
2
]ad on Pt and making O
2
· , which is easier to
2
dissociate into oxygen atoms on Pt than [O ]ad. Such process
happens photothermocatalytically. Those species circled
synergeticly photothermocatalytic oxidize benzene.
Figure 10 O -TPD profile of PLVT with the process of oxygen
2
adsorption under (a) 70 °C, (b) UV-vis irradiation (irradiation spectra
in Figure S3) at ambient temperature, (c) simulated solar irradiation
Acknowledgements
(irradiation spectra in Figure S4) at 70 °C and (d) the merge image of
This work was financially supported by the NNSF of China (2117304
(
a), (b) and (c).
7
and 21373049).
-
Further, it should be noted that the decrease in O species here
2
-
is collating with the variation tendency of O · species in ESR test for
2
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