Graphene-supported CoS2 particles: An efficient photocatalyst for selective hydrogenation of nitroaromatics in visible light

Article abstract of DOI:10.1039/c7cy00356k

CoS₂/graphene composites fabricated by a facile hydrothermal method exhibit excellent photocatalytic performance for selective hydrogenation of nitroaromatics to the corresponding aniline employing molecular hydrogen as a reducing agent under visible light irradiation (400-800 nm). The rate constant of the composite catalyst for nitrobenzene hydrogenation can achieve as high as 35.50 × 10^-3 min^-1 with a selectivity of 100% toward the target product under mild conditions (30°C and 0.25 MPa pressure of H₂). The catalyst also shows high recyclability, and there is no decrease in the catalytic activity after five successive cycles. There exists a synergistic effect between the graphene support and the CoS₂ particles: conductive graphene as the support can rapidly extract the photoexcited electrons and effectively suppress the recombination of photogenerated charges in CoS₂ particles, and then improve the photocatalytic performance. The photocatalytic reduction of nitrobenzene over the CoS₂/graphene catalyst to aniline occurs through the direct pathway in the presence of H₂.

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Graphene-supported CoS2 Particles: an Efficient Photocatalyst for
Selective Hydrogenation of Nitroaromatics in Visible Light
Ben Ma,a,b Yingyong Wang,*a Xili Tong,a Xiaoning Guo,a Zhanfeng Zheng a and Xiangyun Guo*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
CoS2/graphene composite fabricated by a facile hydrothermal method exhibits excellent photocatalytic performance for
selective hydrogenation of nitroaromatics to corresponding aniline employing molecule hydrogen as reducing agent under
visible light irradiation (400-800 nm). The rate constant of the composite catalyst for nitrobenzene hydrogenation can
achieve as high as 35.50×10-3 min-1 with a selectivity of 100% toward the target product under mild conditions (30 oC and
0.25 MPa pressure of H2). The catalyst also shows high recyclability, and there is no decrease in the catalytic activity after
five successive cycles. There exists a synergistic effect between the graphene support and the CoS2 particles: conductive
graphene as the support can rapidly extract the photoexcited electrons and effectively suppress the recombination of
photogenerated charges in CoS2 particles, and then improve the photocatalytic performance. The photocatalytic reduction
of nitrobenzene over the CoS2/graphene catalyst to aniline occurs through the direct pathways in the presence of H2.
www.rsc.org/
potential safety problems.14 From the viewpoint of sustainable
development and green chemistry, it is highly essential to
1. Introduction
Aniline and its derivatives are important chemicals and
intermediates for the synthesis of pharmaceuticals, dyes,
pigments, polyurethanes, and agricultural products.1,2
However, selective reduction of nitro group in the presence of
other reducible groups is a challenge in organic synthesis, and
undesirable by-products are often produced in the
processes.3,4 The conventional processes for amine production
include catalytic hydrogenation of nitroaromatics, reduction of
nitroaromatics with iron and iron salts, and amination of
develop green catalytic routes for selective reaction of
nitroaromatics with H2 to the corresponding amines.
Photocatalytic reduction of nitroaromatics is such a novel and
promising process. However, the proposed photocatalysts are
mainly noble-metal-based catalysts, such as Au,15,16 Ag17,18 and
Pt,18-20 which are always limited by their scarce resources and
high cost.
As an important kind of the transition metal dichalcogenides,
cobalt disulphide (CoS2) has excellent electronic conductivity
and thermal stability. Due to good photoinduction effect, CoS2
phenol.5 The use of stoichiometric reducing agent, such as
9,10
oxalic acid,6 NaSH,7 NaBH4,8 BH3NH3
and hydrazine
particles
also
show
commendable
photocatalytic
monohydrate,11 usually results in large amounts of toxic
wastes. The molecule hydrogen is the cleanest and most
environmentally friendly reducing agent and has been used as
the hydrogen source in many reduction processes of
nitrobenzene. For instance, Keane et al. found that Mo2C-
supported Au-Pd could efficiently catalyze reduction of
nitrobenzene, p-chloronitrobenzene and p-nitrobenzonitrile to
the target amine.12 Serp et al. showed that Ru@C60 catalyst
exhibited high catalytic activity for the chemoselective
hydrogenation of nitrobenzene with H2.13 Nevertheless, using
H2 as the hydrogen source, the reactions are usually conducted
under elevated temperature or high H2 pressure, which not
only increases the energy consumption, but also causes
performances in degradation of organics. For example, Oh’s
group reported that CoS2 particles and their composites, such
as CoS2/graphene, CoS2-graphene/TiO2, CoS2-C60/TiO2, CoS2-
CNTs/TiO2 etc., could catalyze the degradation of methylene
blue and Texbrite BA-L under visible light irradiation.21-24 Peng
et al. found that CoS2 and nitrogen-doped-carbon coated CoS2
showed superior performance for photocatalytic degradation
of methylene bule.25 Although CoS2 particles have been
investigated in various fields, their photocatalytic selective
hydrogenation performance has not been reported as far as
we know.
Graphene, a 2D material of sp2 bonded carbon atoms,
presents high conductivity, superior electron mobility and high
surface area, which can not only improve the photocatalytic
performance of semiconductor, but also protect the supported
particles from oxidation.26-28 Herein we report that graphene-
supported CoS2 particles, as
a novel photocatalyst, can
catalyse selectively hydrogenation of nitroaromatics to
corresponding aromatic amines under visible light irradiation
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o
and mild conditions ( 30 C and 0.25 MPa pressure of H2). It is
the first report for selective photocatalytic hydrogenation of
nitroaromatics with H2 as hydrogen sources under visible light.
The composite catalyst shows excellent photocatalytic activity,
selectivity and stability.
changed. At last, the adsorption capacities of the catalyst
samples were calculated by the spectra of H2.
2.3 Photocatalytic hydrogenation of nitroaromatics
The photocatalytic selective hydrogenation of nitroaromatics
reactions were conducted in a 50 mL sealed stainless autoclave
with a quartz window for light irradiation. A typical reaction
process is described as follows: 1.0 mmol of nitroaromatics
and 40 mg of CoS2/graphene catalyst were dispersed in 10 ml
of neat ethanol, and the suspension was then sealed in the
autoclave under 0.25 MPa of H2 with magnetically stirring. A
300 W Xe lamp was employed as the light source (light
wavelength from 400 to 800 nm), and the light intensity was
0.45 W/cm2. The reaction temperature was carefully
controlled at 30 oC by the circulating cooling water for 1.5
hours unless otherwise specified. The dependence of the
catalytic activity on the irradiation intensity (0.2-0.45 W/cm2)
was investigated by adjusting the electric current of light
source. A series of optical pass filters were used to adjust light
source to obtain a specified wavelength range of irradiation
while the same irradiation intensity was kept at 0.45 W/cm2.
For example, a 700 nm optical filter allows the light with a
wavelength range from 700 to 800 nm to pass. After reaction,
the products were collected and analysed by Gas
chromatography mass spectroscopy (GC-MS, Bruker SCION SQ
456 GC-MS system, Karlsruhe, Germany).
2. Experimental section
2.1 Catalyst preparation
The synthesis of CoS2/graphene composite was carried out in a
Teflon-lined stainless steel autoclave. Typically, 5 mmol of
cobalt nitrate (Co(NO3)2•6H2O) and 50 mg of graphene (~500
m2/g) were dispersed into 40 mL of distilled water, and the
suspension was magnetically stirred to form a homogeneous
mixture of cobalt nitrate and graphene. After stirring for 2 h,
another 20 mL of aqueous solution containing 5 mmol of
sodium hyposulfite (Na2S2O3•5H2O) and 500 mg of
polyvinylpyrrolidone (PVP, K30) were added dropwise into the
mixture while keeping stirring for another 0.5 h. In the
subsequent step, the suspension was transferred into a 100
mL stainless steel autoclave, and maintained at 160 oC for 24 h.
Finally, the autoclave was cooled to room temperature
naturally. The precipitate was filtered off, washed with neat
ethanol and distilled water respectively, and dried in a vacuum
environment at 40 oC for 12 h.
For comparison, 10-CoS2/G, 20-CoS2/G, 30-CoS2/G and 40-
CoS2/G were prepared by the similar procedure in the present
of 10 mg, 20 mg, 30 mg and 40 mg graphene, respectively.
Pure CoS2 particles were prepared using the same method in
the absence of graphene.
As the reaction process was the pseudo-first-order kinetics
with respect to the concentration of nitrobenzene, the rate
constant (κ) could be calculated following the equation 1:
κ = ꢀꢁ ꢂꢃ ꢄꢄ
ꢅ
(1)
ꢆ
where Ct and C0 were the concentrations of nitrobenzene at
the reaction time t and 0, respectively.
2.2 Catalyst characterization
The crystalline phases of composites were measured by X-ray
diffractometer (XRD, Rigaku D-Max/RB). The transmission
electron microscopy (HRTEM, JEM-2100F) at accelerating
voltage of 200 kV was used to investigate microstructures of
the catalysts. Scanning electron microscopy (SEM, JSM-5600,
Jeol, Tokyo, Japan) was used to observe the morphology and
structure. X-ray photoelectron spectroscopy (XPS) was
measured by using Al Ka (hk=1486.6 eV) X-ray line on a Kratos
XSAM800 spectrometer. UV-vis diffuse reflectance spectra
were recorded on a UV-visible absorption spectra (UV-3600,
Shimadzu) using Al2O3 as a reference at room temperature.
The fluorescence spectrophotometer (F-700 FL 220-240 eV,
Tokyo, Japan) was used to measure the photoluminescence (PL)
spectra of samples, and the excitation wavelength was 280 nm.
The hydrogen temperature-programmed desorption (H2-
The contribution of different wavelengths was calculated as
the following: contribution of light induced by different
wavelengths (%) =[conversion of nitrobenzene at a specific
wavelength range (%)
×
selectivity (%)]/[conversion of
selectivity (%)
selectivity (%)].
nitrobenzene (%) without any filter
×
–
conversion of nitrobenzene in dark (%)
3. Results and Discussion
3.1 Catalyst characterization
×
From the field-emission scanning electron microscopy (FESEM)
image shown in Fig. 1A, CoS2 particles show a subsphaeroidal
morphology with a mean diameter of about 300 nm. The
interplanar crystal spacing of the particles is 0.32 nm (inset of
Fig. 1A), which matches well with the (111) plane of CoS2.29 Fig.
1B shows a FESEM image of the CoS2/graphene composite. The
subsphaeroidal CoS2 particles with a mean size of about 300
nm are uniformly dispersed on the graphene sheets. Similarly,
the lattice spacing of CoS2 particles in the composite also
corresponds to the CoS2 (111) crystal face (inset of Fig. 1B),
indicating that graphene does not change the crystal lattice of
CoS2 in the composite. The X- ray diffraction (XRD) patterns of
CoS2 particles and the CoS2/graphene composite are shown in
Fig. 1C. The strong diffraction peaks are assigned to a cubic
TPD) experiments were conducted by
a
commercial
chemisorption analyser (TP-5080, Xianquan, China). In the H2-
TPD experiments, 100 mg of catalyst sample was pre-treated
in N2 flow (25 mL/min) at 573.15 K (5 K/min) for 2 h. After the
pre-treatment, the temperature was naturally decreased to
303.15 K in N2 flow (25 mL/min). Then 0.081 mL H2/N2 (~10%
of H2 in volume) was inducted to expose the catalyst sample
for 1 min and the spectra of H2 were recorded. The adsorption
was repeated 10 times until the spectra of hydrogen were not
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Table
1 Experimental results of photocatalytic hydrogenation of
nitrobenzene under given reaction conditions.
Entry
Catalyst
Conv.(%)
Select.(%)
κ
(10-3 min-1)
1
/
0
0
0
2
Graphene
CoS2
0
0
0
3
4a
72
12
99
23
65
100
100
100
100
100
100
13.75
1.45
35.50
3.30
11.50
CoS2
5
CoS2/graphene
CoS2/graphene
CoS2/graphene
CoS2/graphene
6a
7b
8c
2
~0
a
b
c
Without light irradiation. Water as solvent. Under an argon
atmosphere.
×
10-3 min-1 (entry 3, Table 1). However, the
Fig. 1 FESEM images of CoS2 (A) and CoS2/graphene (B) catalysts, XRD
patterns (C) and UV-vis diffuse reflection spectra (D) of CoS2 and
CoS2/graphene catalysts; the inset pictures in A and B are the size
distributions of CoS2 particles and HRTEM images.
13.75
CoS2/graphene composite exhibits more excellent catalytic
activity under the same conditions. The conversion of
nitrobenzene exceeds 99%, and the rate constant as high as
35.50×
10-3 min-1 (entry 5, Table 1). Evidently, the catalytic
phase of CoS2 with a space group of Pa-3 (JCPDS Card No. 89-
activity of the composite catalyst is higher than that of lone
CoS2 particles. According to the thermodynamic parameters
for the hydrogenation of nitrobenzene in the temperature
1492). No characteristic diffraction peaks for graphene are
observed in the XRD patterns of the CoS2/graphene composite.
This could be due to the low amount (~7.5 wt. %) and the
relatively low diffraction intensity of graphene.30 The
ultraviolet-visible (UV-vis) diffuse reflection spectra of CoS2
particles and the CoS2/graphene composite are shown in Fig.
1D. CoS2 particles and the composite have strong absorption at
about 300 nm, and in the range of 450-800 nm, indicating that
both CoS2 particles and CoS2/graphene composite have good
ability for UV and visible light absorption. According to
literature, we speculate that the strong absorption around 300
nm may be associated with the charge-transfer from the
valence band to the conduction band within CoS2 samples.31
The absorption band in the range of 450-800 nm could be
o
range between 30 and 60 C under irradiation,33 the activation
energies of photocatalytic hydrogenation of nitrobenzene over
CoS2 and CoS2/graphene catalysts are calculated to be 74.4
and 48.6 kJ/mol respectively, based on Arrhenius equation
(Figure S2).34 The evident difference in the activation energy
indicates that the photocatalytic reaction undergoes different
potential energy surfaces.35 Compared to some reports of
nitrobenzene hydrogenation reactions which were conducted
in the presence of hydrogen, it can be observed that
CoS2/graphene catalyst can efficiently drive the reaction to
obtain a good aniline yield under milder conditions (Table S1).
Employing water as the solvent, the activity of the composite
catalyst decreases to 11.50
×
10-3 min-1 (entry 7, Table 1). This
ascribed to the d-d transitions of Co(
Ⅱ) ions in CoS2 catalysts,
in which Co is in octahedral coordination.32 Comparing the two
absorption spectra, it can be seen that the composite exhibits
stronger UV and visible light absorption than the CoS2 particles
alone, suggesting that the light energy can be better exploited
by the composite.
decrease is possibly due to the low solubility of nitrobenzene
in water or H2O molecules occupying the catalytic active sites
of CoS2/graphene catalyst. When the reaction is conducted
under an argon atmosphere, almost no nitrobenzene is
converted (entry 8, Table 1), indicating that H2 is the sole
hydrogen source of the reaction. For comparison, the
performance of CoS2 and CoS2/graphene catalysts in dark were
also investigated. Under the same conditions, the activity are
3.2 Photocatalytic performance
The catalytic performances of graphene, CoS2 and
CoS2/graphene for the hydrogenation of nitrobenzene were
investigated, and the results are summarized in Table 1. The
rate constants are calculated following equation 1 (Figure S1).
From the table, it is clear that the hydrogenation reaction
cannot occur under the given conditions without catalyst or
using only graphene as the catalyst (entries 1 and 2, Table 1).
When only CoS2 particles are used as the catalyst, the
conversion of nitrobenzene is 72% and the rate constant is
only 1.45
× ×
10-3 min-1 (entry 4, Table 1) and 3.30 10-3 min-1
(entry 6, Table 1), respectively, which is far lower than the
reaction rate driven by light. Therefore, the irradiation of
visible light plays the dominative role in the hydrogenation of
nitrobenzene.
3.3 Effect of light intensity and wavelength
To further confirm the contribution of light irradiation to the
catalyst activity, the reactions were conducted under different
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Table 2. The photocatalytic hydrogenation of various nitroaromatics
over CoS2/graphene catalyst.
a The reaction time is 3 h.
both industry and organic synthesis.37 Hence the high
chemoselectivity of catalyst for the reduction of the nitro
group is highly desirable. A series of nitroaromatics were
employed to investigate the chemoselectivity of
CoS2/graphene catalyst in this category of hydrogenation
reactions. From the results shown in Table 2, the yield of all
target products is 99% except p-dinitrobenzene and o-nitro
anisole. The reaction of p-dinitrobenzene takes longer reaction
time than others because two nitro groups need to be
hydrogenated (entry 1, Table 2). The activity of o-nitro anisole
is slightly lower than others, possibly due to the strong
sterically hindering effect between adjoining nitro and
methoxy group (entry 8, Table 2). However, no matter
electron-withdrawing (entries 1-5, Table 2) or electron-
donating substitutes (entries 6-9, Table 2) in nitrobenzene,
only nitro group could be reduced to the amino group under
the giving condition. It can be explained that the
chemoselectively of photocatalytic nitroaromatics reduction
depends on the ability of nitro group and other functional
group to accept photogenerated electrons.14 Obviously the
nitro group is easier to be activated by photogenerated
electrons than other reducible groups in this work.
Fig. 2 Dependence of the catalytic performance of CoS2/graphene for
nitrobenzene hydrogenation on the irradiation intensity (A) and
irradiation wavelength (B).
light intensities. The reaction results are shown in Fig. 2A. It
can be seen that the conversion of nitrobenzene grows linearly
with the irradiation intensity. This indicates that the
photocatalytic process is first order in photon and the reaction
is dominated by a single photon adsorption.36 Fig. 2B shows
the light wavelength dependence of the photocatalytic activity.
Without any filter, the light wavelength ranges from 400 to
800 nm, the conversion of nitrobenzene is 99%. When the
irradiation is limited in a wavelength range of 450-800, 500-
800, 550-800, 600-800, 650-800 and 700-800 nm, the
conversion of nitrobenzene is 93%, 84%, 72.5%, 58.1%, 41.6%
and 31.1%, respectively. By eliminating the contribution of
heating, the contribution induced by 400-450 nm light is 7.9%
((99-93) / (99-23) × 100%). Similarly, the contribution of 450-
500, 500-550, 550-600, 600-650, 650-700 and 700-800 nm
light accounts for 11.8%, 15.1%, 18.9%, 21.7%, 13.8% and
10.7%, respectively. It indicates that the light in the 500-700
nm makes
a major contribution to the photocatalytic
hydrogenation, which is well in accord with the UV-vis diffuse
reflection spectrum.
3.5 Proposed mechanism
Under the irradiation of visible light, the electrons of CoS2 are
excited from its valence band (VB) to its conduction band (CB),
and thus photogenerated holes are left at VB. When CoS2
particles are supported on graphene, the conductive graphene
3.4 Selective hydrogenation of nitroaromatics
Aniline derivatives, such as p-chloroaniline, are important for
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Fig. 3 Photoluminescence spectra of CoS2 (a) and CoS2/graphene (b) Fig.
4
The conversion of nitrobenzene in the dark reaction and
(excitation wavelength, 280 nm).
photocatalytic reaction with 0.6 mL of TEA.
Table 3. The adsorption capacities of hydrogen and photocatalytic graphene can rapidly transfer the photoexcited electrons and
performance of different catalysts for hydrogenation of nitrobenzene.
effectively suppress the charge recombination of CoS2 particles.
Secondly, as a high surface area support, graphene can offer
more adsorption and catalytic reaction sites.
Since the photogenerated electrons in CoS2 have been
extracted to the graphene sheets, especially on the interface
between CoS2 and graphene, abundant holes are left in the
CoS2 particles. When 0.6 mL of triethanolamine (TEA), a hole
scavenger, 41 was employed to trap the photoexcited holes of
CoS2, the conversion of nitrobenzene decreased sharply from
100% to 27% at a H2 pressure of 0.25MPa. The value is almost
same as that in the dark reaction. With increasing the H2
pressure, the conversion of nitrobenzene shows a similar
increase curve with that in the dark (Fig. 4). Similarly, the
addition of 5,5-Dimethyl-1-pyrroline N-oxide (DMPO), an
Entry
Catalyst
CoS2
H2/μmol•g-1
71.10
Conv.(%)
72
Select.(%)
100
1
2
3
4
5
6
10-CoS2/G
20-CoS2/G
30-CoS2/G
40-CoS2/G
CoS2/graphene
93.03
77
100
124.52
163.85
204.39
241.69
81
100
88
100
93
100
99
100
sheets can rapidly extract photogenerated electrons from CoS2
particles. As a result, the recombination of photogenerated
electrons and holes is effectively suppressed, which can be
proved by the obvious decrease of the photoluminescence
intensity of the CoS2/graphene composite compared with that
of CoS2 particles (Fig. 3).38,39 Besides, as a high surface area
support, graphene offers more adsorption and catalytic
reaction sites.28,30 For comparison, 10-CoS2/G, 20-CoS2/G, 30-
CoS2/G and 40-CoS2/G were prepared by the similar procedure.
The adsorption capacities of hydrogen on the catalysts were
measured in a flow of H2/N2 (10% in volume) by H2-TPD
experiments and the catalytic activities of catalysts with
different content of graphene were tested under giving
condition. As shown in Table 3, the H2 adsorption capacities of
the catalysts increase with the content of graphene in catalyst,
which leads to the increase of the photocatalytic activity of
hydrogenation.28,40 Besides, within a certain range, the higher
content of graphene in catalyst is more effectively suppressed
recombination of photogenerated electrons and holes, which
can be certified by the photoluminescence (PL) spectra shown
in Figure S3. In summary, graphene enhances the
photocatalytic performance of the catalyst in two ways. Firstly,
electron-trapping agent,42 also resulted in
reactivity of photocatalytic hydrogenation. And the
a decline of
conversion of nitrobenzene decrease to 28% when the volume
of DMPO increases to 0.9 mL (Figure S4). The above
phenomena indicate that the light induced hydrogenation
reaction is almost quenched in the absence of photoexcited
electrons or holes. Based on the above results, it is proposed
that the photoexcited holes on the surface of CoS2 can activate
H2 molecules and produce active hydrogen species.
Simultaneously, the N-O bonds in the nitrobenzene which
adsorbed on the surface of catalyst are activated by the
photogenerated electrons. Then the active hydrogen species
migrate to activated nitrobenzene molecules and couple
together to form the aniline and water.
Despite many reports on the reduction of nitrobenzene to
aniline, the reaction mechanism has not been clearly
understood. As early as 1898, Haber have proposed a
mechanism for the reduction of nitrobenzene, including direct
and indirect reaction pathway (Scheme S1), which is generally
accepted so far.43 According to the mechanism,
nitrosobenzene and hydroxylamine are intermediates for the
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Table 4. Experimental results of photocatalytic hydrogenation of
various intermediates under the given reaction conditions. a
Fig. 6 Photocatalytic stability of CoS2 and CoS2/graphene catalysts in
five cycles.
azobenzene gave a lower yield of aniline than nitrobenzene
(entries 5 and 6, Table 4). Therefore, it is suggested that aniline
is produced in direct pathway in our work. On the basis of the
results described above, a proposed photocatalytic mechanism
of the hydrogenation of nitrobenzene on the CoS2/graphene
catalyst is sketched in Fig. 5.
a
Reaction conditions: 40 mg of catalyst, 1.0 mmol reactant, 10 mL of
o
ethanol at 30 C, 0.25 MPa of H2 with light intensity of 0.45 W/cm2 for
1 h. b 0.5 mmol nitrosobenzene and 0.5 mmol hydroxylamine.
3.6 Stability
The stability and recyclability is one of the most important
properties of heterogeneous catalysts. However, the main
issue of non-noble-metal-based materials such as FeS2 is
instability in the successive reactions.46 To investigate the
recyclability of lone CoS2 and CoS2/graphene catalysts, the
catalysts were tested for five rounds at the same reaction
conditions. The experiment results in Fig. 6 show that there is
no significant decrease in the catalytic activity of
CoS2/graphene after five successive reactions, suggesting that
the CoS2/graphene catalyst has excellent stability. However, a
slight decrease in the activity of CoS2 is observed. From the X-
ray photoelectron spectroscopy (XPS) profiles of used CoS2 and
CoS2/graphene, it can be seen that the intensity of the peak of
2-
SO4 in CoS2 catalyst significantly increases after five cycles
2-
(Figure S5A), indicating a part of S2 have been oxidized to
Fig.
5 The proposed nitrobenzene hydrogenation photocatalytic
mechanism of CoS2/graphene composite.
2-
SO42-, whereas no SO4 was produced in CoS2/graphene
(Figure S5B).47 The oxidation of CoS2 leads to the decrease in
the catalyst activity. Comparing the stability of CoS2 and
CoS2/graphene catalysts, it is further proved that the graphene
sheets can protect CoS2 particles from oxidization. The FESEM
analysis confirms that the morphology of CoS2/graphene and
CoS2 particles in the used catalysts has no obvious change
(Figure S6).
hydrogenation of nitrobenzene to aniline in the direct pathway,
whereas nitrobenzene was firstly reduced to azoxybenzene
and azobenzene in the indirect pathway.44 In order to clarify
the photocatalytic reaction path of nitrobenzene in the
presence of H2, nitrosobenzene, hydroxylamine, azoxybenzene
and azobenzene are used as reactants in the giving reaction
conditions.45 When using nitrosobenzene or hydroxylamine as
reactants, the conversions are higher than the conversion of
nitrobenzene (79%) (entries 1-3, Table 4), and the selectively
of aniline are 100%. However, when the mixture of
nitrosobenzene and hydroxylamine are used as reactants, the
conversion of reactant is 92%, and only a small quantity of
azoxybenzene is detected (Selectively of 3%, entry 4, Table 4),
indicating that it is hard to produce azoxybenzene in our
catalytic system. Most importantly, azoxybenzene and
4. Conclusions
In summary, the present work shows that CoS2/graphene
composite can efficiently photocatalyze the chemoselective
hydrogenation of nitroaromatics into corresponding amines
under mild conditions (30 oC and 0.25 MPa pressure of H2)
with excellent activity, high selectivity and good stability. Using
H2 as hydrogen source, the photocatalytic reduction of
6 | J. Name., 2012, 00, 1-3
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21 Z. D. Meng and W. C. OH, Chinese. J. Catal. 2012, 33, 1495-
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performance of CoS2/graphene catalyst. The catalytic system
not only provides
a
low energy-consumption and
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amines, but also enriches the strategies for the design of
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Graphene-supported CoS2 Particles: an Efficient Photocatalyst for Selective
Hydrogenation of Nitroaromatics in Visible Light
Ben Ma,a,b Yingyong Wang,*a Xili Tong,a Xiaoning Guo,a Zhanfeng Zheng a and Xiangyun Guo*a
CoS2/graphene composites fabricated by a facile hydrothermal method show excellent photocatalytic performance for selective
hydrogenation of nitroaromatics to corresponding aniline employing molecule hydrogen as reducing agent under visible light
irradiation (400-800 nm). There exists a synergistic effect between the graphene support and the CoS2 particles: conductive
graphene as the support can rapidly extract the photoexcited electrons and effectively suppress the recombination of
photogenerated charges in CoS2 particles, and then improve the photocatalytic performance.
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