Selective oxidation of sulfides on Pt/BiVO4 photocatalyst under visible light irradiation using water as the oxygen source and dioxygen as the electron acceptor

Article abstract of DOI:10.1016/j.jcat.2015.08.029

Photocatalytic selective oxidation represents an environment-friendly strategy for chemical transformation. Herein, we report the oxidation of sulfides into sulfoxides with high selectivity (up to 99%) on Pt/BiVO₄ photocatalyst in water under visible light illumination. The system exhibited excellent performance for the oxidation of sulfides in water compared with organic solvents. Isotopic oxygen (H₂¹⁸O) experiments clearly revealed that the oxygen atoms in the sulfoxide were mainly from water. Electron paramagnetic resonance demonstrated that the reactive oxygen species were the peroxy species, which were generated from water oxidation on BiVO₄ via the two-electron pathway. Dioxygen was reduced to water through proton-coupled electron transfer.

Full text of DOI:10.1016/j.jcat.2015.08.029

Journal of Catalysis 332 (2015) 95–100
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective oxidation of sulfides on Pt/BiVO photocatalyst under visible
4
light irradiation using water as the oxygen source and dioxygen as the
electron acceptor
Bao Zhang a,b,1, Jun Li a,1, Bingqing Zhang , Ruifeng Chong , Rengui Li , Bo Yuan , Sheng-Mei Lu , Can Li
a
a
a
a,b
a
a,
⇑
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, 457 Zhongshan Road,
Dalian 116023, China
b
University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic selective oxidation represents an environment-friendly strategy for chemical transforma-
tion. Herein, we report the oxidation of sulfides into sulfoxides with high selectivity (up to 99%) on
Received 16 July 2015
Revised 28 August 2015
Accepted 29 August 2015
Pt/BiVO
4
photocatalyst in water under visible light illumination. The system exhibited excellent perfor-
18
mance for the oxidation of sulfides in water compared with organic solvents. Isotopic oxygen (H
2
O)
experiments clearly revealed that the oxygen atoms in the sulfoxide were mainly from water. Electron
paramagnetic resonance demonstrated that the reactive oxygen species were the peroxy species, which
Keywords:
were generated from water oxidation on BiVO
4
via the two-electron pathway. Dioxygen was reduced to
Photocatalysis
Bismuth vanadate
Selective oxidation
Isotopic labeling
Peroxy species
Sulfide oxidation
water through proton-coupled electron transfer.
Ó 2015 Elsevier Inc. All rights reserved.
1
. Introduction
Recently, metal-complex catalysts using water as an oxygen
source have flourished via charge-transfer catalysis mediated by
Photocatalytic oxidation reactions have attracted significant
photosensitizers, such as organometallic complexes (Ru(bipy)2+
,
interest because of their potential ability to provide a sustainable
pathway for green synthesis [1–7]. Among the semiconductor-
based photocatalysts, TiO is frequently used as a model photocat-
2
alyst to explore various oxidation reactions and to understand the
fundamental processes of photocatalysis [8–10]. However, the
main obstacle lies in the unsatisfactory selectivity of the desired
etc.) [14–19] and semiconductors [20]. Several oxygenation reac-
tions of substrates containing sulfur [17–20] or alkene [16] have
shown substantial yields and excellent selectivity. The oxygenation
reactions have been carried on with water as the sole source of
oxygen atoms through the photogeneration of high-valence
metal–oxo species such as iron [19], ruthenium [14,15,17,18,20],
and manganese [16] molecular complexes. These highly oxidized
metal–oxo species derived from water oxidation pave a mild and
controllable pathway for photocatalytic oxidation reactions. How-
ever, such molecular systems are intrinsically limited because they
require a stoichiometric metal complex as a sacrificial acceptor of
electrons, which considerably reduces the sustainability of the pro-
cesses. Therefore, the development of systems that can directly use
dioxygen as an electron acceptor source is highly desirable. Unfor-
tunately, dioxygen is easily reduced to a variety of reactive oxyrad-
2
product for TiO photocatalytic oxidation reactions, which involve
many parallel oxidation pathways, such as direct oxidation by
Åꢀ
holes and indirect oxidation by various reactive oxyradicals (O
2
,
Å
Å
OOH, OH, etc.), derived from photogenerated electrons and holes,
respectively [8,10–12]. Even when the reactions are performed in
Å
inert organic solvent to prevent the generation of OH radicals
[
13], the selectivity is usually very low due to the nonselective
auto-oxidation of the photogenerated radicals [1]. Hence, the
rational control of reactive species derived from photogenerated
electrons and holes is an essential prerequisite for highly selective
oxidation reactions.
Åꢀ
Å
icals (O , OOH, etc.), which are difficult to control and lead to poor
2
selectivity of the desired products [8,10–12].
4
Here, we present the oxidation of sulfides using a Pt/BiVO pho-
⇑
Corresponding author. Fax: +86 411 84694447.
E-mail address: canli@dicp.ac.cn (C. Li).
URL: http://www.canli.dicp.ac.cn (C. Li).
tocatalyst under visible light irradiation in aqueous solution.
Water, as the major oxygen source, is activated to generate reactive
peroxy species via a two-electron pathway, while dioxygen, as the
1
These authors are equally contributed to this work.
http://dx.doi.org/10.1016/j.jcat.2015.08.029
021-9517/Ó 2015 Elsevier Inc. All rights reserved.
0

9
6
B. Zhang et al. / Journal of Catalysis 332 (2015) 95–100
sacrificial electron acceptor, is thoroughly reduced to water via a
four-electron pathway (see Scheme 1). The system does not
involve oxyradicals derived from photogenerated holes and elec-
trons, effectively avoiding unselective radical oxidation pathways.
Meanwhile, the experimental evidence strongly suggests that the
for 5 min to remove the catalyst particles. The remaining solution
was analyzed with an Agilent Gas Chromatograph (GC6890)
equipped with a flame ionization detector and an Agilent Technol-
ogy HP-INNOWAX 19091N-213 capillary column using diphenyl
ether as the internal standard. The chemical structures of products
1
active oxygen species derived from water oxidation on BiVO
responsible for the selective oxidation of organic substrates.
4
are
were confirmed by H NMR. Conversion of sulfide and selectivity
for sulfoxide were defined as follows:
Conversion ð%Þ ¼ ½ðC
0
ꢀ CsulfideÞ=C ꢂ ꢃ 100;
0
2
. Experimental section
Selectivity ð%Þ ¼ ½Csulfoxide=ðC
where C
0
ꢀ CsulfideÞꢂ ꢃ 100;
2
.1. General information
0
was the initial concentration of sulfide and Csulfide and
Csulfoxide were the concentrations of the substrate sulfide and the
Chemicals without special descriptions were purchased from
commercial sources and were used without further purification.
Bi(NO O, NH VO (NH Mo 24ꢁ4H O, CH CN, and
ꢁ5H
NaClO O were supplied by Tianjin Kermel Chemical Reagent
ꢁH
Co., Ltd. Na
Sinopharm Chemical Reagent Co., Ltd.
corresponding sulfoxide, respectively.
)
3 3
2
4
3
,
4
)
6
7
O
2
3
4
2
2.2.3. Electron paramagnetic resonance measurements
2
WO
4
ꢁ2H
2
O, C
2
H
5
OH, and AgNO
3
were obtained from
The active oxygen species generated in the photocatalytic pro-
cess were probed using electron paramagnetic resonance (EPR)
spectra recorded on a Bruker EPR A 200 spectrometer by trapping
with DMPO. Samples containing BiVO or 0.5 wt.% Pt/BiVO (1 g/L),
18
H
2
O,
C
6
H
5
CF3, and
5
,5-dimethyl-1-pyrroline N-oxide (DMPO) were purchased from
1
8
J&K Scientific. The isotopic enrichment of H
2
O was 98%. Various
4
4
sulfides were obtained from Aladdin Industrial Corporation.
PtCl O was purchased from Reagent No. 1 Factory of
DMPO (0.045 M), AgNO3 aqueous solution (0.1 M), or thioanisole
(12 L) were vacuumized and purged with argon. Then, the mix-
H
2
6
ꢁ6H
2
l
Shanghai Chemical Reagent Co., Ltd. Sulfoxides were synthesized
tures were oscillated to make the catalyst blend evenly, added into
an EPR quartz tube, and irradiated with a 300 W Xe lamp (CERA-
MAX LX-300) with a 420 nm cutoff filter. The settings for the ESR
spectrometer were as follows: center field = 3310 G, microwave
frequency = 9.30 GHz, sweep width = 140 G, modulation fre-
quency = 100 kHz, and power = 6.36 mW.
1
as reported [21]. H NMR spectra were recorded on a Bruker 400
spectrometer. Chemical shifts were reported in parts per million
1
(
d) relative to TMS (0.0 ppm) for H NMR data. Mass spectra were
measured on an Agilent GC–MS 6890N/5973 mass spectrometer.
The BET specific surface area was measured using a NOVA 4200e
instrument at 77 K.
Note: Twice deionized water must be carefully redistilled in the
ꢀ
5
presence of 4 ꢃ 10 M KMnO
4
and NaOH at pH > 12 for 36 h, and
then stored in conventional plastic bottles.
2
2
.2. Experimental procedures
.2.1. Preparation of photocatalysts
2.2.4. Electrochemical measurements
BiVO
method [22]. The metal-loaded samples were prepared as follows:
BiVO powder was suspended in methanol aqueous solution
50 vol%) containing chloroplatinic acid in a photoreactor under
vacuum conditions. The photoreactor was irradiated by a 300 W
Xe lamp (PLS-SXE 300, Beijing Perfectlight Co., Ltd.) for 3 h, which
4
samples were synthesized using a previously reported
Electrochemical measurements were made using a computer-
controlled electrochemical station (CHI Model 730D) with a con-
ventional three-electrode cell at room temperature. A saturated
calomel electrode (SCE) was used as the reference electrode, while
a platinum foil was used as the counter electrode. The working
4
(
electrode was prepared as follows: BiVO
alyst (5 mg) and Nafion (5 wt.%, Du Pont Corp.) (50
persed in absolute ethanol (1 mL) using sonication. The slurry
(20 L) was spread on the glassy carbon disk, which was dried in
air at room temperature. Linear sweep voltammetry (LSV) was car-
ried out in Ar- or O -saturated NaClO (0.1 M) solution and the
4
or 0.5 wt.% Pt/BiVO
4
cat-
formed Pt-loaded BiVO
using the same method. Bi
were prepared using the published method. TiO
tase, 20% rutile) purchased from Degussa was sintered at 300 °C for
4
. Other metals were deposited on BiVO
MoO [23] and Bi WO [24] samples
(P25, ca. 80% ana-
4
lL) were dis-
2
6
2
6
l
2
4
h.
2
4
electrode potential was scanned between ꢀ1.2 and 0.2 V vs. SCE
with a scan rate of 0.1 V/s.
2.2.2. Evaluation of photocatalytic activity
Photocatalytic oxidation of sulfides was carried out in mixed
CH
loaded BiVO
quantity of 0.1 wt.% metal-loaded BiVO
suspended in 1.5 mL mixed CH CN and H
2:1 v/v) including thioanisole (50 mol). The system was con-
3
CN and water solvents (2:1 v/v) in suspensions of metal-
under irradiation by visible light (k > 420 nm). A
powder (50 mg) was
3. Results and discussion
4
4
3.1. Photocatalytic conversion of thioanisole on BiVO -based
photocatalysts under various conditions
4
16
18
3
2
O or H
2
O solvents
(
l
nected to a balloon filled with pure molecular oxygen. The mixture
was stirred for 30 min to blend evenly in the solution before visible
light irradiation. The reaction temperature was controlled at 20 °C
by a water-cooling system. The suspensions were irradiated by a
BiVO4 photocatalysts possess a suitable valence band (VB)
(+2.53 V vs. standard hydrogen electrode (SHE)) [25] for photocat-
alytic water oxidation [22] and a conduction band (CB) (+0.13 V vs.
SHE) [25], and their optical absorption edge is 530 nm. The oxida-
tion of thioanisole is chosen as a model reaction because of the fol-
lowing merits: (1) thioanisole is a substrate that does not contain
any oxygen atoms and both the sulfoxide and sulfone products
are oxygenated; and (2) the functional group that contains oxygen
atoms in its products does not exchange with water or dioxygen
3
00 W Xe lamp equipped with a 420 nm cut filter with continuous
stirring. After reaction, the mixture was centrifuged at 10,000 rpm
(Fig. S1, ESI), so it is possible to trace whether the oxygen atoms
in the functional group originate from water or dioxygen. The reac-
tions were investigated under continuous illumination with visible
light from a Xe lamp with power 300 W equipped with a 420 nm
Scheme 1.

B. Zhang et al. / Journal of Catalysis 332 (2015) 95–100
97
cutoff filter. Metal nanoparticles as reduction cocatalysts were
loaded onto BiVO by the photodeposition method.
The reaction conditions are screened and the results are sum-
marized in Table 1. The preliminary results show that no reaction
took place in the absence of a photocatalyst, light, or dioxygen. In
dry CH CN, BiVO gave very low conversion, ca. 3%, in an aerobic
3 4
atmosphere under visible light illumination. No reaction took place
in dry benzotrifluoride, which had high solubility for dioxygen
0.1
.0
-0.1
0.2
-0.3
4
0
-
0
.271
-0.4
-0.5
-0.6
-0.7
[
26]. Nor did a reaction take place in the protic solvent ethanol,
BiVO /Ar
4
-
0.079
although this solvent was favorable in some cases where dyes were
used as photosensitizers [27,28]. To improve the photogenerated
charge separation, BiVO
CH CN. However, the activity was still very poor, ca. 4% conversion.
This result indicates that the reaction is not driven distinctly
Pt/BiVO /Ar
4
BiVO /O
2
4
loaded with 0.1 wt.% Pt was tested in
4
0
.164
3
Pt/BiVO /O
4
2
-
0.208
-0.6 -0.4 -0.2
1
through the active singlet oxygen ( O
2
) pathway, which in certain
0.0
0.2
0.4
0.6
0.8
1.0
cases is dominant when dyes are used as photosensitizers [27,28].
The activity in organic solvents, although very poor, may be attri-
butable to this pathway, in which the photogenerated holes react
with the substrate directly.
E(V / vs. SHE)
Fig. 1. Dark current (J)—potential (E/V vs. SHE) curves for the BiVO
Pt/BiVO electrodes in NaClO (0.1 M) solutions under Ar and O
4
and 0.5 wt.%
4
4
2
.
When H
what higher conversion, ꢄ7%. Unexpectedly, Pt/BiVO
0% conversion in the CH CN/H
fold higher than BiVO . Virtually no overoxidation compounds
were produced. The result strongly suggests the important role of
water in the reaction.
2
O was added to the reaction system, BiVO
4
gave some-
gave up to
1 2
the corresponding cathodic peaks E and E shifted to 0.164 and
4
7
3
2
O (2:1 v/v) mixture, about 10-
ꢀ0.208 V, respectively. Moreover, the cathodic current density
increased significantly and the onset shifted toward a more posi-
tive potential. For instance, the cathodic current increased by about
2.5 times at ꢀ0.20 V, and the onset shifted positively to about
100 mV, indicating that the addition of Pt nanoparticles promoted
the dioxygen reduction reaction. This dioxygen reduction property
4
To differentiate the roles of photogenerated holes and electrons
in the reaction, the dioxygen reduction was further optimized
before the effect of H
2
O was studied. Various metal nanoparticles,
of Pt/BiVO
[29], in which surface modification with Cu(acac)
with the ability of multielectron dioxygen reduction. The photoex-
cited electrons in the conduction band of BiVO are theoretically
incapable of the one-electron process of dioxygen reduction
4
is similar to the result reported by Kobayashi and Tada
including Au, Ag, and Pd, were investigated as cocatalysts. The
nanoparticles Ag and Au as cocatalysts exhibited slightly promot-
ing effects, and Pd gave a moderate promoting effect (Table S1,
2
endowed BiVO
4
4
ESI). BiVO
4
with 0.5 wt.% Pt exhibited the highest activity, at least
h
Åꢀ
2
3
0-fold enhanced compared with that of BiVO (Fig. S2, ESI).
4
(E (O
2
/O
) = ꢀ0.284 V vs. SHE) [29], but could enable four-
h
electron and two-electron pathways (E (O
2
/H
2 2
O ) = +0.695 V and
h
3.2. The dioxygen reduction species in the reaction
E (O
peaks E
2 2
/H O) = +1.229 V vs. SHE) [30,31]. Therefore, the cathodic
h
Åꢀ
2
1
2 2
and E , more positive than E (O /O ), can be ascribed,
A linear sweep voltammetry test was performed to investigate
the dioxygen reduction properties of BiVO , as shown in Fig. 1. In
the Ar-saturated solution, only a very weak current was obtained
for both BiVO and Pt/BiVO . In the O -saturated solution, the lin-
ear sweep voltammetry (LSV) scan showed two peaks centered at
= 0.271 V and E
= ꢀ0.079 V vs. SHE on the cathodic sweep of
BiVO , indicating that dioxygen reduction occurred through at
respectively, to the four-electron and two-electron dioxygen
reduction pathways.
4
We further detected H
by iodometric titration. At low (5%) or moderate (35%) conversion,
the concentration of H was below the detection limit of the cur-
2 2
O as the product of dioxygen reduction
4
4
2
2 2
O
E
1
2
rent method (Fig. S3, ESI) [32]. These results indicate the four-
electron dioxygen reduction pathway. Hence, the Pt nanoparticles
promote not only the separation of the photogenerated charge, but
4
4
least two electrochemical pathways. When Pt/BiVO was used,
Table 1
The results of photocatalytic oxidation of thioanisole under various conditions:
Solvent
Photocatalyst
Conv. (%)a
Sel. (% sulfoxide:sulfone)a
CH
CH
3
CN
CH
BiVO
BiVO
BiVO
Pt/BiVO
BiVO
4
4
4
3
>99:1
–
–
>99:1
99:1
–
3
2
OH
<1
<1
4
7
<1
70
C
6
H
5
CF
CN
3
b
CH
3
CH
3
CH
3
CH
3
4
CN/H
CN/H
CN/H
2
2
2
Oc
O
O
4
c
d
b
Pt/BiVO
Pt/BiVO
4
4
c
98:2
Notes: Reaction conditions: catalyst 50 mg, thioanisole 50
lmol, total volume 1.5 mL, O
2
0.1 MPa, reaction temperature 20 °C, k > 420 nm, reaction time 5 h.
a
Based on GC.
b
0
.1 wt.% Pt loading.
O (0.5 mL).
Under Ar.
c
H
2
d

9
8
B. Zhang et al. / Journal of Catalysis 332 (2015) 95–100
also the four-electron dioxygen reduction process. However, this
does not fully exclude the possibility that two-electron dioxygen
from 16O
lel pathways. The percentage of H
the product using BiVO was higher than that of the product using
Pt/BiVO at a similar conversion (see Table 2). The percentage of
O in the product decreased as the conversion increased, and
2
-derived oxygen atoms, indicating the presence of paral-
1
8
2
O-derived oxygen atoms in
reduction formed H
.3. The effect of water
Next, we further investigated the effect of water. To promote
2
O
2
and led to further rapid decomposition.
4
4
1
8
3
1
8
16
the final atomic ratio of O/ O was ca. 3.4, possibly due to the
1
6
incorporation of the H
2
O generated as a product of the sacrificial
. The results unambiguously suggest that both H
participate in the reaction, but the majority of the oxygen
in the product is from H O.
1
6
the mass transfer of the substrate, the conversion of thioanisole
was controlled to below 30%. Through addition of a trace amount
acceptor
and O
O
2
2
O
2
of water (1.3% v/v) to CH
observed compared with the conversion for dry CH
3
CN, a sevenfold enhancement was
CN (Fig. S4,
2
3
ESI). As the water concentration increased, the increase in the
amount of product was almost proportional to the water concen-
tration. The optimal percentage of water concentration was about
3
.4. Proposed mechanism
In view of the proportion of 18O originating from H
18O and 16
2
O
3
3% (v/v) under the experimental conditions. The reaction was evi-
16
originating from
2
O in the product, there are at least two path-
dently suppressed by excess water because of incomplete dissolu-
tion of the substrate, even though the reaction was much faster in
ways involved the reaction (Fig. 3). Due to the inability to reduce
dioxygen to form radical species on BiVO
atoms are incorporated into the product directly through the pho-
togenerated hole-dominated oxidation pathway. The adsorbed
4
thioanisole on the surface of BiVO can be oxidized by photogener-
16
4
, the
2
O -derived oxygen
3
water than in CH CN.
To gain further insight into the effect of water in the reaction,
1
8
18
2
O-labeled H O, with an isotope abundance of 98%, was used
to trace the origin of the oxygen atom of sulfoxide using 0.5 wt.%
ated holes to produce sulfide radical cations, which are
1
8
Pt/BiVO
conversion of ca. 9%, the proportion of H
atoms incorporated into the product was 83%, of which 17% was
4 3 2
in CH CN/H O (2:1 v/v). As shown in Fig. 2, at a
18
2
O-derived oxygen
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Conv.
Sel.
1
8
Content of O-sulfoxide
0
1
2
3
4
5
6
Time (h)
Fig. 2. Photocatalytic oxidation of thioanisole in the presence of H 1
Pt/BiVO . Reaction conditions: catalyst 20 mg, thioanisole 100 mol, total volume
.5 mL, CH 0.1 MPa, reaction temperature 20 °C, k > 420 nm.
8O with 0.5 wt.
2
Fig. 3. Two possible reaction pathways of photocatalytic oxidation of thioanisole on
Pt/BiVO4.
%
1
4
l
18
3 2 2
CN/H O 2:1 v/v, O
Table 2
Photocatalytic oxidation of thioanisole with catalysts in the presence of H
2
18O:
2
Conv. (%)a
18O–atom from water (%)
Photocatalyst
Surface area (m /g)
The activity of thioanisole
Product
ꢀ
2
ꢀ1
(lmol m
h )
18O
16O
TiO b
67
38
17
2
6
6
4
3
7
9
26
10
35
89
81
74
90
65
11
19
26
10
36
91
83
2
Bi
Bi
2
WO
MoO
6
0.2
0.4
7
2
6
BiVO
Pt/BiVO
4
c
4
6
150
3 2 2
Notes: Reaction conditions: catalyst 50 mg, thioanisole 50 lmol, total volume 1.5 mL, CH 0.1 MPa, reaction temperature 20 °C, k > 420 nm, reaction time
CN/H 18O 2:1 v/v, O
5
h.
a
Based on GC.
Degussa P25 15 mg, Xe lamp 0.5 h.
Catalyst 20 mg, thioanisole 100 lmol, 0.5 wt.% Pt loading, reaction time 0.5 h.
b
c

B. Zhang et al. / Journal of Catalysis 332 (2015) 95–100
99
subsequently recombined by interacting with dioxygen. Due to the
hydrophilic surface of BiVO and the weak adsorption of thioani-
sole on the surface of BiVO , the hole-dominated oxidation
pathway is generally suppressed. The pathway by which
Table 4
4
Photocatalytic oxidation of various sulfides with 0.5 wt.% Pt/BiVO .
4
Conv. (%)a
96
Sulfoxide sel. (%)b
98
4
Substrate
Time (h)
.5
5
1
8
2
H O-derived oxygen atoms are incorporated into the product is
an indirect one through a water-oxidized intermediate.
To confirm whether such a mechanism is common to other
12
8
96
99
semiconductor-based photocatalysts, TiO
2
(P25), Bi
2 6
WO , and
18
Bi
Table 2). The percentages of H
6%, 10%, and 36% for TiO , Bi
and these percentages were much lower than for BiVO
Pt/BiVO . It is noteworthy that BiVO under visible-light irradiation
delivered somewhat higher activity than TiO after normalization
by surface area, and Pt/BiVO exhibited the highest activity. The
result further indicates that the excellent performance of BiVO
2 6 2
MoO were also tested using H O-labeled isotope experiments
18
(
2
2
O-derived oxygen atoms were
WO , and Bi MoO , respectively,
and
99
95
98
97
2
2
6
2
6
4
4
4
17.5
2
4
4
1
1
0
1
97
97
98
98
is due to its unique ability to follow both the water oxidation
and four-electron dioxygen reduction pathways simultaneously.
3.5. The reactive oxygen species derived from water
A control experiment was conducted to reveal the relationship
Notes: Reaction conditions: catalyst 20 mg, thioanisole 100
l
mol, O
2
0.1 MPa, total
volume 1.5 mL, CH
3
CN/H
2
O 2:1 v/v, reaction temperature 20 °C, k > 420 nm.
between photocatalytic water oxidation and photocatalytic oxida-
a
+
Based on GC.
tion of thioanisole (Table 3). Using Ag as a sacrificial electron
b
Based on 1H NMR.
4 2
acceptor, BiVO produced about 197 lmol O under irradiation.
In contrast, when thioanisole was added to the same reaction solu-
tion, oxygen production was completely suppressed, resulting in
about 162 lmol sulfoxide. The result clearly shows that water oxi-
dation and oxidation of thioanisole are closely correlated. The
active oxygen species that are available for thioanisole oxidation
are mainly derived from the oxidation of water by photogenerated
ficial reagent. As shown in Fig. 4, an EPR signal change was
observed in the presence of BiVO or Pt/BiVO photocatalysts when
the reaction mixture was under visible light irradiation. A seven-
line paramagnetic signal was obtained for BiVO and Pt/BiVO . This
signal is a typical feature of DMPO-X [33–36] and is generated
from the oxidation of DMPO by peroxide [37,38]. This indicates
that the peroxy species are the most likely reactive oxygen species
4
4
4
4
4
holes on BiVO .
To identify the active oxygen species generated in the photocat-
alytic process, the EPR-spin-trap technique (with DMPO as a trap-
ping reagent) was used to detect the intermediate species in the
derived from water on BiVO
OH was not observed, implying that the OH radical may not be
4
. The typical four-line peak of DMPO-
Å
3
anaerobic system using AgNO aqueous solution as electron sacri-
involved in the photocatalytic oxidation of thioanisole on BiVO
39,40]. When thioanisole was added to the BiVO /H O/AgNO sys-
4
[
4
2
3
tem, the seven-line EPR signal originating from the peroxy species
disappeared, further confirming that these peroxy intermediate
species involved the selective oxidation of thioanisole.
Table 3
4
Photocatalytic water oxidation in the absence or presence of thioanisole on BiVO .
Substrate
O
2
(
lmol)
Sulfoxide (lmol)
ꢀ
197
1
–
162
3.6. Photocatalytic oxidation of various sulfides on Pt/BiVO
4
+
Notes: Reaction conditions: catalyst 150 mg, thioanisole 0.05 M, AgNO
CH CN/H O 2:1 v/v (100 mL), reaction temperature 20 °C, k > 420 nm, reaction time
.5 h.
3
0.1 M,
Pt/BiVO was tested for the photocatalytic oxidation of a series
4
3
2
of sulfides under the previously optimized conditions, and the
results are summarized in Table 4. A number of methyl phenyl
sulfides bearing electron-withdrawing or electron-donating
substituents on the phenyl ring were efficiently oxidized, with both
complete conversion and excellent selectivity for the corresponding
sulfoxides. In addition, the location of substituents on the phenyl
ring significantly affected the reaction activity. For instance,
o-chlorophenyl methyl sulfide showed much lower oxidation
activity to the corresponding sulfoxide than the m- and p-isomers
because of steric hindrance effects. Thioanisole reacted faster than
the methyl phenyl sulfides with substituents, possibly implying that
it is difficult for hydrophobic substrates to access the surface of
0
Light-10
Light-5
Dark
Without catalyst
BiVO4
Pt/BiVO4
BiVO /PhSCH3 Pt/BiVO /PhSCH3
4
4
4
BiVO .
4
. Conclusion
3300
3350 3300
3350
3300
3350 3300
3350
Fig. 4. EPR spectra of DMPO-spin-trapped radical species upon photocatalytic
oxidation of H O and thioanisole on BiVO and Pt/BiVO under argon under visible
light illumination. Reaction conditions: BiVO or 0.5 wt.% Pt/BiVO 1 g/L, DMPO
.045 M, AgNO aqueous solution 0.1 M, or thioanisole 12 L. The signals obtained
before irradiation are denoted as Dark, after irradiation for 5 min as Light-5, and
after irradiation for 10 min as Light-10.
4
In summary, Pt/BiVO can act as an efficient photocatalyst for
2
4
4
the oxidation of sulfides to sulfoxides, with excellent selectivity
in aqueous solution under visible light irradiation. The oxygenation
of sulfides is conducted mainly with the peroxy species generated
through the two-electron photocatalytic water oxidation pathway.
4
4
0
3
l

1
00
B. Zhang et al. / Journal of Catalysis 332 (2015) 95–100
[11] M.A. Lazar, W.A. Daoud, RSC Adv. 3 (2013) 4130–4140.
Dioxygen as an electron acceptor is reduced to water through the
four-electron pathway, avoiding the use of noble and rare com-
plexes as sacrificial reagents. The study provides a fundamental
understanding on the incorporation pathway of oxygen atoms in
[
12] H. Kisch, Angew. Chem., Int. Ed. 52 (2014) 812–847.
[
13] X.J. Lang, H.W. Ji, C.C. Chen, W.H. Ma, J.C. Zhao, Angew. Chem., Int. Ed. 50
(
2011) 3934–3937.
[14] W. Chen, F.N. Rein, R.C. Rocha, Angew. Chem., Int. Ed. 48 (2009) 9672–9675.
[
15] S. Ohzu, T. Ishizuka, Y. Hirai, S. Fukuzumi, T. Kojima, Chem. Eur. J. 19 (2013)
563–1567.
16] S. Fukuzumi, T. Mizuno, T. Ojiri, Chem. Eur. J. 18 (2012) 15794–15804.
4
the oxidation of compounds on BiVO photocatalysts. Importantly,
1
the study provides a promising strategy for selective photocatalytic
oxidation reactions, as the method does not involve oxyradical-
induced processes. Semiconductors endowed with multielectron
dioxygen reduction can significantly reduce the limitations of the
thermodynamic potential for electron transfer and consumption.
Consequently, a number of visible-light-respondent photocatalysts
with the inability to implement single-electron dioxygen reduction
could be extended to selective oxidation in organic synthesis.
[
[17] P. Guillo, O. Hamelin, P. Batat, G. Jonusauskas, N.D. McClenaghan, S. Menage,
Inorg. Chem. 51 (2012) 2222–2230.
[
18] F. Li, M. Yu, Y. Jiang, F. Huang, Y.Q. Li, B.B. Zhang, L.C. Sun, Chem. Commun. 47
2011) 8949–8951.
(
[19] A. Company, G. Sabenya, M. González-Béjar, L. Gómez, M. Clémancey, G.
Blondin, A.J. Jasniewski, M. Puri, W.R. Browne, J.M. Latour, L. Que, M. Costas, J.
Pérez-Prieto, J. Lloret-Fillol, J. Am. Chem. Soc. 136 (2014) 4624–4633.
[
20] X. Zhou, F. Li, X. Li, H. Li, Y. Wang, L.C. Sun, Dalton Trans. 44 (2015) 475–479.
[21] F. Izquierdo, A. Chartoire, S.P. Nolan, ACS Catal. 3 (2013) 2190–2193.
[
22] R.G. Li, F.X. Zhang, D.E. Wang, J.X. Yang, M.R. Li, J. Zhu, X. Zhou, H.X. Han, C. Li,
Nat. Commun. 4 (2013) 1432–1438.
Acknowledgments
[
[
23] J.H. Bi, J.G. Che, L. Wu, M.H. Liu, Mater. Res. Bull. 48 (2013) 2071–2075.
24] L.S. Zhang, W.Z. Wang, Z.G. Chen, L. Zhou, H.L. Xu, W. Zhu, J. Mater. Chem. 17
(2007) 2526–2532.
This work was financially supported by the National Natural
Science Foundation of China (NSFC 21473190) and the DICP Funda-
mental Research Program for Clean Energy (DICP M201302).
[
[
25] R. Negishi, S. Naya, H. Tada, J. Phys. Chem. C 119 (2015) 11771–11776.
26] Q. Wang, M. Zhang, C.C. Chen, W.H. Ma, J.C. Zhao, Angew. Chem., Int. Ed. 49
(
2010) 7976–7979.
27] L. Quan, W.H. Lin, T.T. Sun, Z.G. Xie, Y.B. Huang, X.B. Jing, Catal. Lett. 144 (2013)
08–313.
28] J. Dad’ová, E. Svobodová, M. Sikorski, B. König, R. Cibulka, ChemCatChem 4
2012) 620–623.
29] S. Naya, T. Niwa, R. Negishi, H. Kobayashi, H. Tada, Angew. Chem., Int. Ed. 53
2014) 13894–13897.
30] S. Kitano, N. Murakami, T. Ohno, Y. Mitani, Y. Nosaka, H. Asakura, K. Teramura,
T. Tanaka, H. Tada, K. Hashimoto, H. Kominami, J. Phys. Chem. C 117 (2013)
11008–11016.
[
[
[
[
3
Appendix A. Supplementary material
(
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2015.08.029.
(
References
[
31] T. Arai, M. Horiguchi, M. Yanagida, T. Gunji, H. Sugihara, K. Sayama, J. Phys.
Chem. C 113 (2009) 6602–6609.
[
[
[
[
[
1] X.J. Lang, X.D. Chen, J.C. Zhao, Chem. Soc. Rev. 43 (2014) 473–486.
2] S. Protti, M. Fagnoni, Photochem. Photobiol. Sci. 8 (2009) 1499–1516.
3] S. Fukuzumi, K. Ohkubo, Chem. Sci. 4 (2013) 561–574.
4] M.A. Fox, Acc. Chem. Res. 16 (1983) 314–321.
5] G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem. Commun. (2007)
ꢀ
5
[32] Note: The detection limit of the iodometric titration is below 6 ꢃ 10 M.
[33] S.D. Stan, M.A. Daeschel, J. Agric. Food Chem. 53 (2005) 4906–4910.
[34] C. Bernofsky, B.M.B. Bandara, O. Hinojosa, Free Radical Biol. Med. 8 (1990)
2
31–239.
35] S.V. Verstraeten, S. Lucangioli, M. Galleano, Inorg. Chim. Acta 362 (2009)
305–2310.
[
3
425–3437.
2
[
[
6] D. Ravelli, M. Fagnoni, A. Albini, Chem. Soc. Rev. 42 (2013) 97–113.
7] D. Ravelli, D. Dondi, M. Fagnoni, A. Albini, Chem. Soc. Rev. 38 (2009) 1999–
[36] S.D. Stan, J.S. Woods, M.A. Daeschel, J. Agric. Food Chem. 53 (2005) 4901–4905.
[37] R.F. Chong, J. Li, Y. Ma, B. Zhang, H.X. Han, C. Li, J. Catal. 314 (2014) 101–108.
[38] R.F. Chong, J. Li, X. Zhou, Y. Ma, J.X. Yang, L. Huang, H.X. Han, F.X. Zhang, C. Li,
Chem. Commun. 50 (2014) 165–167.
2
011.
8] Y. Shiraishi, T. Hirai, J. Photochem. Photobiol., C: Photochem. Rev. 9 (2008)
57–170.
9] X.J. Lang, W.H. Ma, C.C. Chen, H.W. Ji, J.C. Zhao, Acc. Chem. Res. 47 (2014) 355–
63.
[
[
1
[
[
39] J. Zhang, Y. Nosaka, J. Photochem. Photobiol., A: Chem. 303–304 (2015) 53–58.
40] Z.H. Wang, W.H. Ma, C.C. Chen, H.W. Ji, J.C. Zhao, Chem. Eng. J. 170 (2011) 353–
3
362.
[
10] V. Augugliaro, M. Bellardita, V. Loddo, G. Palmisano, L. Palmisano, S. Yurdakal,
J. Photochem. Photobiol., C: Photochem. Rev. 13 (2012) 224–245.