Highly selective oxidation of sulfides on a CdS/C3N4 catalyst with dioxygen under visible-light irradiation

Article abstract of DOI:10.1039/c6cy01568a

A CdS/C₃N₄ composite photocatalyst was fabricated by a facile method, and its structure, composition, and morphology were characterized in detail. The catalyst exhibited high photocatalytic product selectivity towards the oxidation o

Full text of DOI:10.1039/c6cy01568a

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Highly selective oxidation of sulfides on a CdS/C3N4
catalyst with dioxygen under visible-light irradiation†
Cite this: DOI: 10.1039/x0xx00000x
Yong Xu,a ZiꢀCheng Fu,a Shuang Cao,a Yong Chena and WenꢀFu Fu*ab
A CdS/C3N4 composite photocatalyst was fabricated by a facile method, and its structure,
composition, and morphology were characterized in detail. The catalyst exhibited highly
photocatalytic products electivity towards the oxidation of sulfides into corresponding
sulfoxides even when the irradiation time was extended to 20 h. The synergistic effect between
CdS and C3N4 gave rise to efficient interfacial transfer of photogenerated electrons and holes
on both materials, as confirmed by transient photocurrent measurements. The best
photocatalytic activity for the catalyst prepared at 300 °C was achieved at a C3N4/CdS ratio of
0.3. Sulfides were efficiently oxidized to sulfoxides with dioxygen under visibleꢀlight
illumination in methanol at room temperature. The conversion efficiency of sulfides with
electronꢀwithdrawing groups was lower than that of those with a donating substituent, and the
conversion strongly depended on the steric hindrance effect of the substituent. A possible
photocatalytic mechanism was proposed based on electron spin resonance, trapping
experiments, and other experimental results.
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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catalyst, and demonstrated the occurrence of oxygen atom
transfer from water to the substrate through isotopic labeling
experiments.23 Sun and coꢀworkers developed a robust system
1. Introduction
Photocatalytic reactions have received growing interest owing
to the worsening energy crisis and urgent environmental
with an inorganic semiconductor material as photosensitizer
and a Ruꢀaqua complex as a catalyst to drive sulfide
oxidation.24 Additionally, an iron(IV)ꢀoxo species has been
used to oxidize the radical sulfide cations to the corresponding
−
problems.1 3 The copious amount of solar energy available as a
clean and economical energy source has promoted the
exploration of new methods to convert solar energy to chemical
−
5
sulfoxide
through
inꢀsituꢀgenerated
[RuIII(bpy)3]3+.25
energy.4
Sulfoxides have played an important role as
Nevertheless, these photocatalytic systems require a large
excess of sacrificial electron acceptors such as [Co(NH3)5Cl]Cl2
and Na2S2O8. From the viewpoint of green and sustainable
chemistry, molecular oxygen, an ideal oxidant, is omnipresent
and thus readily available for the oxidation of sulfides.26
However, the fact that the triplet ground state of oxygen is an
inactive species and singletꢀstate oxygen is highly reactive once
activated is a challenge. Chen and coꢀworkers employed TiO2
as a photocatalyst to achieve the aerobic oxidation of sulfide
and the aerobic oxidative formylation of amine with methanol
in the same system.27 Subsequently, they used triethylamine as
a redox mediator to realize the aerobic oxidation of sulfides into
sulfoxides under visibleꢀlight irradiation.28 A system including
mesoporous graphiteꢀlike carbon nitride (mpgꢀC3N4) and
isobutyraldehyde with highly visibleꢀlight catalytic activity
towards the selective oxidation of sulfide to sulfoxide was
reported by Zhang et al.29 This material has recently been used
intermediates in the synthesis of pharmaceuticals,
−
agrochemicals, and other fine chemicals.6 8 Generally, the most
common way to prepare sulfoxides is the oxidation of the
corresponding sulfides. Traditionally, oxidants with high
oxidative capacity have been used for this purpose, such as
trifluoroperacetic acid,8 Zn(MnO4)2,9 nitric acid,10 metaꢀ
−
14
chloroperoxybenzoic acid,11 and metallic oxides.12 However,
these oxidants have low product selectivity because of
uncontrollable overꢀoxidation to sulfones. Moreover, a great
excess of oxidant is used and accompanied by the generation of
toxic byproducts. In the last few decades, hydrogen peroxide
with catalytic amounts of catalysts such as metal
complexes,15,16 metalꢀsalts,17,18 inorganicꢀorganic hybrids,19 and
enzymes20 has also been studied as an oxidant. Nonetheless,
compared with photocatalysis, these thermal catalytic systems
are power wasting and environmentally hazardous, especially in
industrial application.
as
a
photocatalyst for the transformation of organic
Recently, a rutheniumꢀbased dyad system constituting a
photosensitizer and a catalyst, namely RuphotꢀRucatꢀOH2, has
been proved to selectively catalyze sulfide oxygenation.21,22
Our group previously reported the photocatalytic oxidation of
sulfide into the corresponding sulfoxide in neutral aqueous
solution using ruthenium(II) chromophoreꢀcatalyst dyad
−
33
substrates.30 Li and coꢀworkers also reported the successful
photocatalytic oxidation of sulfides on Pt/BiVO4 in water under
visibleꢀlight illumination; however, the conversion efficiency
was poor.34
CdS is one of the most attractive photocatalysts owing to its
highly efficient visibleꢀlight absorption.35 However, it also
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ARTICLE
DOI: 10.1039/C6CY01568A
suffers from several defects such as photocorrosion that impede 40, and 50 mg) were prepared. The samples were marked as
−
38
its wide application.36
Considering the relatively wellꢀ
cdcn(χ), where χ is the mass of C3N4. The catalysts used in the
matching band edges of CdS and C3N4, it is possible that present study were prepared at 300
C unless otherwise noted.
similar procedure was used to prepare CdS/C3N4
°
corrosive photogenerated holes from the valence band of CdS
can readily transfer to C3N4 and thereby facilitate electronꢀhole
pairs separation.39 Therefore, coupling CdS with C3N4 is
expected to be a simple method of enhancing the conversion
efficiency and product selectivity of the oxidation of sulfides to
corresponding sulfoxides.
Herein, we report a facile method of preparing an organicꢀ
inorganic heterogeneous CdS/C3N4 (cdcn) photocatalyst with
closely contacting interfaces. C3N4 can easily be exfoliated into
thin sheets by ultrasonication, thereby providing many binding
sites for anchoring CdS nanoparticles. This strategy reduces the
aggregation of CdS and improves the stability of the
photocatalyst. The resulting composite catalyst showed
significantly enhanced photocatalytic activity and product
selectivity for the oxidation of sulfides under visibleꢀlight
irradiation. Furthermore, the electronic and steric hindrance
effects of substituents on the sulfides were found to have an
intense effect on the photocatalytic sulfide to sulfoxide
conversion efficiency.
A
photocatalysts at 200, 250, 350, and 400 °C.
2.2 Characterization
The samples were characterized by powder Xꢀray diffraction
(XRD) on a Bruker AXS D8 Xꢀray diffractometer with Cu Kα
(λ =1.54056 Å) to identify their phases. Scanning electron
microscopy (SEM) and energy dispersive Xꢀray (EDX)
spectroscopy measurements were conducted on a Hitachi Sꢀ
4800 field emission scanning electron microscope. The particle
size and lattice fringes of the samples were analyzed on a
transmission electron microscope (TEM; JEM 2100F) with an
accelerating voltage of 200 kV. Fourier transform infrared
spectroscopy (FTIR) data was collected using the standard KBr
disk method at ambient temperature on a Bruker ALPHA FTIR
spectrometer from 4000 to 400 cm−1. Ultravioletꢀvisible diffuse
reflectance spectra (DRS) was recorded on a spectrophotometer
(Hitachi UVꢀ3010) using BaSO4 as
a
reference.
Photoluminescence spectra (PL) was measured at room
temperature under excitation at 325 nm (Hitachi Fꢀ4500). Xꢀray
photoelectron spectroscopy (XPS) was carried out on a PHI
5300 ESCA using an Al Kα Xꢀray source with a power of 250
W. The charge effect was calibrated using the binding energy
of C1s. Thermogravimetric analysis was performed on a TA
2. Experimental
2.1 Preparation
All reagents were purchased from commercial sources and used
as received without further purification. The C3N4 powder40
was obtained by directly heating urea (10 g) in a muffle furnace
Instrument Q600 SDT from room temperature to 800
°C.
Electron spin resonance (ESR) signals were recorded at room
temperature with a Bruker ESR E500 spectrometer.
at 550
furnace was cooled to room temperature, the resulting faint
yellow product was collected and milled into powder for further 2.3 Typical procedure for catalytic oxidation of methyl p-
°C for 4 h at a heating rate of 0.5 °
C min−1. After the
use.
methoxyphenyl sulfide and other sulfides
Typically, 3 mL of a solution containing 1 mmol substrate and
5 mg CdS/C3N4 photocatalyst was added to a 15ꢀmL cuvette
sealed with a rubber septum placed on top of a magnetic stirrer.
A stream of dioxygen was then passed into the reaction system
for 10 min and filled again every 2 h. White lightꢀemitting
diodes (LEDs) (30 × 3 W, λ ≥ 420 nm) were used as the
irradiation light source. The LEDs were positioned 3 cm away
from the sample, which was kept under continuous stirring at
room temperature. The mixture was stirred for 30 min before
irradiation. After the reaction, dimethyl pꢀbenzenedicarboxylate
was added to the mixture as an internal standard. The chemical
structure and the quantity of the products were determined by
1H nuclear magnetic resonance (NMR) spectroscopy
(BrukerAvance 400 spectrometer). The conversion of sulfide
and the selectivity for sulfoxide were calculated from the
equations:
2.1.1 Preparation of CdS nanoparticles
CdS nanoparticles were fabricated according to a modified literature
method.41 In a typical procedure, 400 mL (0.14 M) of aqueous Na2S
solution was added slowly to 500 mL (0.14 M) of Cd(OAc)2 solution
with vigorous stirring. The resulting yellow precipitate was stirred
for 24 h and then allowed to settle for 48 h. The yellow slurry was
filtered and washed with 200 mL of water, then transferred to a dried
Teflonꢀlined autoclave with a capacity of 100 mL. The autoclave
was sealed and heated at 200 °C for 72 h, after which the system
was allowed to cool to room temperature naturally. The
obtained yellow solid was filtered and then washed with 500
mL of water and 100 mL of ethanol consecutively. After drying
at 80 °C for 5 h in the oven, the obtained yellow solid was
ground into fine particles and used in the following step.
conversion (%) = nconsumed sulfide/ninitial sulfide× 100
selectivity (%) = nsulfoxide/nconsumed sulfide× 100
2.1.2 Preparation of CdS/C3N4 composite photocatalyst
The CdS/C3N4 hybrid photocatalyst was prepared as follows.
The asꢀprepared C3N4 was dispersed in 80 mL deionized water
and stirred for 30 min. The suspension was then transferred to
an ultrasonic environment for another 60 min to exfoliate it into
sheets. After that, 100 mg of the CdS powder was added to the
above solution and stirred for a further 30 min. The mixture
was then ultrasonicated for 2 h to completely disperse it.
Subsequently, the powder was collected by filtration, washed
Cycling tests: 3 mL of methanol containing 1 mmol methyl
pꢀmethoxyphenyl sulfide and 5 mg cdcn(30) was added into a
15ꢀmL cuvette sealed with a rubber septum. After the sample
filled with dioxygen was irradiated for 5 h, the obtained
mixture was centrifuged and washed 3 times with methanol.
The two parts of liquid was merged, the chemical structure and
the quantity of the products were determined by NMR analysis.
The remaining solid was dried in vacuum oven to use as the
with distilled water several times, and dried at 70
Finally, the sample was heated at 300
°C overnight.
°
C for 4 h in a muffle next time. Same process was conducted for next 3 times.
furnace. A series of CdS/C3N4 composites containing a fixed
amount of CdS (100 mg) and a varied mass of C3N4(10, 20, 30,
2 | J. Name., 2012, 00, 1-3
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2.4 Photoelectrochemical (PEC) measurements
Photocurrent measurements were conducted on a CHI 660C
electrochemical workstation (Chenhua Instrument, China) using
a
conventional threeꢀelectrode configuration. A catalyst
electrode served as the working electrode, with a platinum foil
as the counter electrode and an Ag/AgCl (saturated KCl)
electrode as the reference electrode. Then, 0.2 M of Na2SO4
aqueous solution (pH = 6.8) was used as the electrolyte,
through which nitrogen was bubbled for 20 min prior to
measurement. The white LED light source was used for
irradiation. The working electrodes were prepared as follows: 2
mg of the asꢀprepared catalyst sample was dispersed in 30ꢀL
N,Nꢀdimethyl formamide by sonication for 20 min. The slurry
was then evenly spread onto a 3.0 × 1.0 cm2 conducting indium
tin oxide glass substrate with an active area of about 1.0 cm2.
The film was dried in air and annealed at 150 °C for 1 h in
flowing Ar atmosphere. The photoresponses of the samples at
light on and off were measured at 0.0 V. Electrochemical
impedance spectroscopy (EIS) measurements were carried out
using an AC amplitude of 10 mV in the frequency range of 0.01
Hz–1000 kHz at 0.05 V.
3. Results and discussion
Fig. 1 (a) Powder XRD pattern and (b), (c) SEM images of cdcn(30), (d)
HRTEM image of cdcn(30), (e) TEM image of cdcn(30), and
corresponding EDX mappings (f, CꢀK; g, NꢀK; h, SꢀK; i, CdꢀL).
3.1 Characterization of photocatalysts
The XRD patterns of the asꢀprepared pure C3N4, pure CdS, and
CdS/C3N4 composite synthesized with 30 mg C3N4 (mass ratio
of C3N4/CdS, 30/100; marked as cdcn(30)) are shown in Fig. 1a.
spectrum (Fig. S3) of cdcn(30) showed that C, N, S, Cd, and a
small amount of O elements existed in the sample. Fig. 2a–d
show the highꢀresolution XPS spectra of C 1s, N 1s, S 2p, and
Cd 3d. The C 1s spectrum could be separated into three peaks,
at 284.4, 285.6, and 288.1 eV. The former peak was ascribed to
graphitic sp2 C˗C bonds, while the last was identified as sp2
hybridized C atoms on the Nꢀcontaining aromatic rings
(N˗C=N), which represent the major carbon species in C3N4.
The weak middle peak (285.6 eV) was assigned to sp3˗bonded
carbon from defects on the C3N4 surface.43,44 Peaks at 398.2,
399.7, and 401 eV were identified in the N 1s spectrum (Fig.
2b). The peak located at 398.2 eV was assigned to the
sp2˗bonded N in the triazine units (C˗N=C)4, while the weak
peaks at 399.7 and 401 eV originated from central N in the
triazine aromatic cycles and amino groups with a hydrogen
atom (C˗N˗H), respectively.39,45 Comparison of the XPS
spectrum of C3N4 with that of CdS/C3N4 revealed that the C 1s
peak (285.6 eV) was shifted to higher binding energy, while the
peaks at 398.2 eV and 401 eV for N 1s were shifted to lower
binding energy. These shifts indicated electronic interaction
between C3N4 and CdS, which is consistent with the FTIR (Fig.
S4) results. Additionally, the XPS spectrum of S 2p (Fig. 2c)
showed two peaks, at 161.3 eV (S 2p3/2) and 162.4 eV (S 2p1/2),
which were ascribed to S2− in CdS. The Cd 3d peaks (Fig. 2d)
were observed at 404.9 eV and 411.7 eV, attributed to Cd 3d5/2
and Cd 3d3/2 for Cd2+ in CdS.46 Moreover, the binding energies
of S 2P (161.2 eV) and Cd 3d (411.6 eV) in cdcn(30) were
slightly lower than those of the pure materials, which also
suggests an interaction between CdS and C3N4.
Two distinct diffraction peaks were found at 13.1° and 27.4°
for the pure C3N4 sample (JCPDS Card No. 87˗1526),
corresponding to the interplanar separation and interlayer
stacking of the conjugated aromatic segments, respectively, and
indexed as the (100) and (002) planes.42 After ultrasonic
exfoliation, the peak at 27.4° was much weaker (Fig. S1); this
decreased intensity indicated that the interlayer structure was
destroyed. The pure CdS sample was identified as hexagonal
wurtzite CdS (JCPDS Card No. 65˗3414). The two peaks of
C3N4 nearly disappeared from the pattern of the cdcn(30)
sample, suggesting that CdS nanoparticles without the
surfactant asꢀprepared, with size ranging from 30 nm to 50 nm,
either loaded on the surface of C3N4 sheets or embedded into
the layers of C3N4. The SEM (Fig. 1b, c) and TEM (Fig. 1d, e)
images intuitively showed that exfoliated C3N4 were decorated
with CdS nanoparticles. Additionally, oxygenꢀcontaining
defects and amino groups on the surface of the C3N4 likely
served as anchoring sites to immobilize the CdS nanoparticles,
which reduced their aggregation. The lattice spacing of 0.337
nm was ascribed to the (002) plane of CdS, the peak of which
(26.5°) was very close and overlapped with that of C3N4 (27.4°).
EDX analysis (Fig. S2) carried out on randomly selected areas
revealed an approximate S:Cd ratio of 1:1 based on the
calculated peak areas, confirming the successful formation of
the desired phase. The peak of Si was attributed to the silicon
pellet. EDX mapping further revealed that all elements were
uniformly distributed in the cdcn (Fig. 1f–i).
XPS was employed to confirm the formation of the CdS/C3N4
heterogeneous catalyst and study its surface chemical state. The XPS
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Fig. 3 (a) Ultravioletꢀvisible DRS of C3N4, CdS, and cdcn, (b)
corresponding plots of (αhν)2 versus energy (hν) for the band gap
energy, (c) transient photocurrent response under visible light, and (d)
EIS Nyquist plots of CdS and cdcn.
Fig. 2 Highꢀresolution XPS spectra of (a) C 1s, (b) N 1s, (c) S 2p, and
(d) Cd 3d in the samples.
Figure 3a depicts the typical ultravioletꢀvisible DRS of C3N4,
CdS, and cdcn. A clear redshift occurred from C3N4 to cdcn(30),
which is favorable for the use of visible light. The band gaps of
the samples were determined using the Tauc/DavidꢀMott model
from 200 °C to 400 °C in 50 °C steps. To determine the optimal
catalyst, the catalytic oxidation of methyl pꢀmethoxyphenyl
sulfide was performed using the asꢀprepared samples. Fig. 4a
shows that the highest catalytic activity was obtained when the
mass ratio of C3N4 to CdS was 0.3 and the preparation
described by the equation:47 (αhν)1/n = A(h
ν − Eg). The value of
the exponent n denotes the nature of the sample transition and
is defined as 0.5 for a directly allowed transition. The fitting
results showed in Fig. 3b indicate that the band gap of pure
C3N4, CdS, and cdcn(30) was approximately 2.87 eV, 2.301 eV,
and 2.327 eV, respectively. Because of the relatively high
optical absorption coefficient of CdS in the visible region, the
absorbance of cdcn(30) was slightly enhanced compared with
that of C3N4. Fig. 3c shows the transient photocurrent response
of CdS and cdcn(30) during several onꢀoff cycles. It was clear
that the photocurrent response of cdcn(30) was significantly
higher than that of CdS under the same visibleꢀlight
illumination. The generated photocurrent was mainly the result
temperature was 300 °C. With increasing C3N4 content, the
CdS nanoparticles would have been better dispersed and a
larger number of active catalytic sites provided, but when the
limiting value was reached, the adsorption of substrate
molecules and light would have been restrained. The catalytic
activity of the cdcn samples also exhibited a dependence on the
catalyst preparation temperature. After thermal treatment, the
interaction between CdS and C3N4 becomes stronger, which
was proved by comparison of cdcn(30) and mechanical mixing
of CdS and C3N4. However, a significant decline in activity was
observed when the preparation temperature was increased to
of photoinduced electrons diffusing to the indium tin oxide.48 350
°
C owing to the partial decomposition of CdS (inset in Fig.
Therefore, the enhanced photocurrent implied that a more
effective charge transfer was achieved after the formation of the
heterojunction. The strength of the photocurrent was in line
with the order of photocatalytic activity. To further verify the
above results, electrochemical impedance spectroscopy, a
powerful tool used to analyze charge transfer properties and the
efficiency of photogenerated charge carrier separation, was also
performed.49 As presented in Fig. 3d, the impedance radius of
cdcn(30) was smaller than that of CdS and other cdcn samples.
Generally, a smaller arc radius on EIS (Nyquist plots)
represents a faster interfacial charge transfer and higher
separation efficiency of photogenerated charge carriers. The
results therefore demonstrated that cdcn(30) more favorably
transferred and separated photogenerated charge carriers than
4b). Thus, cdcn(30) prepared at 300 C was selected as the
°
photocatalyst for the following sulfide oxidation experiments.
The surface areas of CdS and cdcn(30) are 32.039 m2/g and
47.356 m2/g respectively, which was estimated from the
amount of N2 adsorbed by using the BET equilibrium equation.
the
CdS
particles.
C3N4
displayed
an
intense
photoluminescence, but the emissive intensity of cdcn(30) was
remarkably weakened, indicating efficient transfer of
photogenerated electrons and holes between CdS and C3N4 (Fig.
S5).
Fig. 4 (a) Photocatalytic activity of various cdcn samples prepared at
different temperatures and different mass ratios of C3N4 to CdS. Methyl
pꢀmethoxyphenyl sulfide was used as the substrate, 3 mL CH3OH was
used as the solvent. (b) Thermogravimetric analysis results for CdS,
C3N4, and cdcn(30).
3.2 Photocatalytic oxidation of sulfides
To optimize the reaction conditions for sulfide oxygenation,
Photocatalysts (cdcn) with C3N4 to CdS mass ratios of 0.1–0.5 different solvents (Table 1, entry 1˗6) were chosen with methyl
were also prepared, and the reaction temperature was varied
pꢀmethoxyphenyl sulfide as a model substrate. Methanol was
found to be the preferable medium for this oxidation reaction
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Table 1 Photocatalytic oxidation of methyl pꢀmethoxyphenyl sulfide
conversion efficiencies obtained for substrate molecules with
different groups clearly demonstrated the more important role
of the electronic donor or acceptor group in the photocatalytic
oxidation reaction (Table 2). Based on the above analysis and
previous reports,27,28 it can be concluded that the presence of
electronꢀdonating groups accelerated the conversion of sulfide
into sulfoxide, whereas a large steric hindrance effect was
unfavorable to the photocatalytic oxidation.
under various conditions
Sel. (%)
Entry
Catalyst
Solvent
Conv. (%)
1a
1b
Table 2 Photocatalytic oxidation of sulfides with different substitutes.
1
2
3
4
5
6
7
8
9a
10b
11c
12d
13e
14f
15g
16h
cdcn(30)
cdcn(30)
cdcn(30)
cdcn(30)
cdcn(30)
cdcn(30)
CdS
CH3OH
CH3CN
CH2Cl2
acetone
H2O
88.1
17.0
2.1
1.6
7.1
6.0
61.0
3.1
0
66.9
0
0
44.6
100
52.6
11.5
97
100
100
100
100
100
97
100
—
97
—
—
97
97
100
99
3
0
0
0
0
0
3
0
—
3
—
—
3
3
0
Entry
Substrate
Product
Time (h) Conv. (%)
Sel. (%)
99
1
2
3
4
5
6
6
6
6
6
85.8
73.6
61.8
6.5
THF
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
99
C3N4
cdcn(30)
CdSꢀC3N4
—
cdcn(30)
cdcn(30)
cdcn(30)
cdcn(30)
cdcn(30)
99
100
100
5.6
1
Reaction conditions: substrate 0.3 mmol, cdcn(30) 5 mg, methanol 3
mL, O2 1 atm, at room temperature.
Reaction conditions: methyl pꢀmethoxyphenyl sulfide 1 mmol, catalyst
5 mg, solvent 3 mL, reaction filled with oxygen (1 atm) every 2 h, at
1H NMR spectra of the substrate and product in the
photocatalytic oxidation reaction measured at different reaction
time are shown in Fig. 5a, in which changes in the peaks were
observed with increasing reaction time to 5 h. However, no
difference in selectivity was observed even when the
photocatalytic system was irradiated for 20 h, implying a good
room temperature, white LEDs (30
× 3 W, λ ≥ 420 nm), 4 h irradiation.
aIn the dark. bMechanical mixing of CdS and C3N4 (mass ratio, 100:30),
5 mg. cWithout cdcn. dFilled with N2. eIn air. f5 h irradiation. gMethyl oꢀ
h
methoxyphenyl sulfide (1 mmol) as substrate, 5 h irradiation. Methyl
mꢀmethoxyphenyl sulfide (1 mmol) as substrate, 5 h irradiation.
Conversion and selectivity was determined by 1H NMR.
because it is the strongest organic protic solvent and is redox product selectivity. Additionally, cdcn(30) exhibited high
active.27 When CH3OH was replaced with inert organic photocatalytic activity for sulfide oxygenation compared with
solvents, only slight conversion of sulfide was obtained. It has those of previously reported photocatalysts (Table S1) in terms
been reported that Pt/BiVO4 systems exhibit excellent of per unit mass. The reaction kinetics (Fig. 5b) for the
performance for the oxidation of sulfides in water compared
photocatalytic oxidation of methyl pꢀmethoxyphenyl sulfide
with in organic solvents.34 However, in the present catalytic under visibleꢀlight irradiation was also investigated in air while
system, low conversion was observed when water was used as attempting to maintain the concentration of dioxygen. Methyl
the solvent, which might be because of the poor solubility of pꢀmethoxyphenyl sulfide was almost linearly oxidized,
sulfides in water (the amount of sulfides we used is large). The
maintaining a high selectivity towards sulfoxide. Thus, the
experimental results of control experiments (Table 1, entries 7– conversion of sulfide almost followed zeroꢀorder reaction
12), showed that no reaction took place in the absence of kinetics. As a heterogeneous catalyst, cdcn could be separated
photocatalyst, light, or dioxygen. The catalytic activity of from the reaction solution easily by simple filtration or
mechanically mixed CdS and C3N4 was higher than that of bare
centrifugation. Therefore, the photostability of the hybrid
CdS or C3N4, but incomparable with that of cdcn(30). This catalyst, which is an important factor for practical application,
reaction conducted in air resulted in a relatively low conversion could be evaluated using recycling tests (Fig. 5c). After four
efficiency compared with that when the system was filled with successive cycles, the catalyst retained its high photocatalytic
dioxygen (1 atm; Table 1, entry 13). When methyl pꢀ
activity. The XRD pattern (Fig. S6) of the used catalyst showed
methoxyphenyl sulfide was replaced with methyl oꢀ no significant differences with that of the asꢀprepared catalyst,
methoxyphenyl sulfide, the conversion suddenly dropped to suggesting that the crystal structures of the composite were not
52.6 %. Moreover, a conversion of only 11.5 % was achieved altered during the photocatalytic reaction. In addition, we
measured the concentration of Cd2+ by method of linear sweep
when methyl mꢀmethoxyphenyl sulfide was used as the
substrate. These results strongly suggest that electronic and voltammetry with a bismuth film electrode (Fig. S7). The
steric hindrance effects of the substituents in the aryl sulfides obtained results showed that the concentration of cadmium ion
played an important role in the catalytic reaction using cdcn. As changed from1.8 mg/L to 38.3 mg/L after 5 h irradiation (Fig.
known, electronꢀdonating groups (ED) are easier to donate S8), implying the composited catalyst is stable.
electrons than electronꢀwithdrawing groups (EW). This may be
the reason for the low conversion of mꢀmethoxyphenyl sulfide.
Additionally, the steric hindrance of methyl oꢀmethoxyphenyl
sulfide led to a decrease in conversion.34 Furthermore, the
3.3 Proposed mechanism
To acquire more insight into the reaction mechanism, an ESR
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ARTICLE
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Fig. 6 Changes in ESR spectra for a system containing DMPO, cdcn(30)
(2 mg) in methanol under visibleꢀlight irradiation in air. (a) Without
substrate, light irradiation for 2 min, (b) for 4 min, (c) with methyl pꢀ
methoxyphenyl sulfide for 2 min, and (d) with methyl pꢀmethoxyphenyl
sulfide for 4 min.
Table 3 Results obtained after adding different quenchers to the
photocatalyzed sulfide oxidation system.
1
Fig. 5 (a) H NMR (DMSOꢀd6) spectra of methyl pꢀmethoxyphenyl
sulfide after different irradiation time. (b) Reaction kinetics plot for
photocatalytic oxidation of methyl pꢀmethoxyphenyl sulfide under
visibleꢀlight irradiation in air. (c) Results of cycling test of cdcn(30).
Reaction conditions: methyl pꢀmethoxyphenyl sulfide 1 mmol, cdcn(30)
5 mg, CH3OH 3 mL, 1 atm O2, at room temperature.
Sel. (%)
Entry
Quencher
Quenching group
free radicals
Conv. (%)
1a
1b
1
2
hydroquinone
pꢀbenzoquinone
0
3.7
—
100
—
0
─
⋅O2
3
4
5
HCOONH4
tertiary butanol
CCl4
hole
.OH
e─
21.5
54.5
88.7
97
98
98
3
2
2
spinꢀtrapping technique with 5,5’ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide
(DMPO) was employed to probe the reactive oxygen species
generated during visibleꢀlight irradiation. The four
─▪
characteristic peaks of the DMPO˗O2 adduct (Fig. 6a, b) were
observed. Furthermore, this species was thermodynamically
Reaction conditions: methyl pꢀmethoxyphenyl sulfide 1 mmol, cdcn(30)
5 mg, methanol 3 mL, O2 1 atm, visibleꢀlight irradiation for 4 h, at
stable for the transfer of photogenerated electrons to the room temperature.
adsorbed O2 to produce superoxide radicals ( O2─). After adding
•
─
▪
sulfide to the system, the signals of the DMPO˗O2 adduct
were remarkably weakened (Fig. 6c, d), which implies that the
superoxide radical played an important role in oxygenation of
sulfide in this catalytic system. Notably, hydroxyl radicals were
not detected in the ESR analysis. Methanol solvent can serve as
a quencher of hydroxyl radicals. A series of active species
trapping experiments (Table 3) were further performed to
investigate the photocatalytic oxidation mechanism. It is very
interesting that the reaction did not proceed when hydroquinone,
a free radical scavenger, was added to the sulfide oxygenation
system, which indicated that the reaction involved a free radical
chain pathway. When pꢀbenzoquinone was added to quench the
superoxide radical, the conversion surprisingly dropped to
3.7 %, revealing that the generated superoxide radicals were the
major oxidative species involved in the selective oxidation of
sulfide to sulfoxide. Besides the superoxide radicals, valence
band holes also had an effect on sulfide oxygenation to some
Scheme 1. Under visibleꢀlight irradiation, electrons are excited
from the valence band of CdS and C3N4 to the conduction band,
leaving holes in the valence band. The electrons on the
conduction band would be trapped by electrophilic O2 to form
superoxide radicals, which is the major active species that
induces sulfide oxygenation. Sulfides with electronꢀdonating
groups easily deliver electrons to the holes of the photocatalyst
compared with those containing electronꢀwithdrawing groups.
Furthermore, the stability of the persulfoxide can be
dramatically influenced by nearby groups of the sulfide, which
with electronꢀdonating groups can contribute electron density to
the persulfoxide sulfur. It has been reported that collinear
donorꢀpersulfoxide sulfurꢀoxygen is beneficial for sulfide
oxygenation.50 Conversion is therefore the highest when
electronꢀdonating groups are located at the paraꢀposition. The
superoxide radical can attack the sulfur atom, forming
persulfoxide, which is recognized to be the key active
intermediate in the whole oxidation process and is confirmed
extent.
While
the
quenching
of
electrons
with
−
53
by computational and experimental evidence.51
The
tetrachloromethane had almost no influence on the conversion.
These active species trapping results demonstrated that
molecular oxygen trapped photogenerated electrons to produce
superoxide radicals, preventing the recombination of the
photogenerated charge carriers.
persulfoxide can in fact undergo either an unproductive
intersystem crossing (path a) or product formation (path b).
Additionally, the process of product formation in protic
On the basis of the above results, a possible mechanism for
the photocatalytic oxidation of sulfide on cdcn is illustrated in
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C6CYA01R5T68ICALE
new insights for the design of highꢀperformance catalysts for
the oxidation of sulfides.
Acknowledgements
This work was financially supported by the National Key Basic
Research Program of China (973 Program 2013CB834804), the
Ministry of Science and Technology (2012DFH40090). We
thank the National Natural Science Foundation of China
(21273257, 21267025, 21471155 and U1137606) for financial
support.
Notes and references
Scheme 1 Possible reaction mechanism for photocatalytic sulfide
aKey Laboratory of Photochemical Conversion and Optoelectronic
Materials and HKUꢁCAS Joint Laboratory on New Materials,
Technical Institute of Physics and Chemistry, University of Chinese
Academy of Sciences, Chinese Academy of Sciences, Beijing 100190,
P.R. China
oxygenation.
solvents involves an interaction between persulfoxide and
solvent molecules, resulting in a second intermediate (path c).51
In addition, we conducted a set of experiments by adding acetic
acid into acetonitrile, and the conversion changed from 17.0 %
to 36.4 % (Table S2), supporting that protonation of weakly
basic persulfoxide is the intermediate. In general, the
conversion of substrate molecules with electronꢀwithdrawing
substituents is lower owing to the decreased electron density
around the sulfur atom. However, path c is more efficient than
path b. This is attributed to hydrogenꢀbonded persulfoxide,
which is a stronger electrophile and thus facilitates oxygen
transfer to a second sulfide molecule. The presence of
substituents in the orthoꢀposition in the chain makes the
reaction less effective because the steric hindrance encountered
by the approaching sulfides, which makes oxygen transfer less
competitive leading to the decay of the persulfoxide. In this
case, some effect is also exerted when the substituent is in the
metaꢀposition. Additionally, the corresponding sulfone may be
generated via hydroxyl group migration to the sulfur atom.
However, this migration is relatively difficult owing to the
steric hindrance and electronic effects.
Eꢁmail: fuwf@mail.ipc.ac.cn
bCollege of Chemistry and Chemical Engineering, Yunnan Normal
University, Kunming 650092, P.R. China
†Electronic Supplementary Information (ESI) available. See
DOI:10.1039/b000000x/
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Graphical abstract:
CdS/C3N4 visibleꢀlight catalyst exhibits high product
selectivity towards photocatalytic oxidation of sulfides
into corresponding sulfoxides with dioxygen in
methanol.
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