Highly efficient visible-light-induced photocatalytic hydrogenation of nitrobenzene to aniline in water

Article abstract of DOI:10.1039/c3ra40935j

Highly efficient visible-light-induced photocatalytic hydrogenation of nitrobenzene to aniline in water was observed over a Bi₂MoO ₆ photocatalyst under N₂ atmosphere in the presence of (NH₄)₂C₂O₄ as a hole scavenger. 100% of the nitrobenzene was converted and the selectivity of aniline was ～97% after 20 min of visible light irradiation (λ ≥ 400 nm). The Bi ₂MoO₆ photocatalyst showed good catalytic stability for the hydrogenation of nitrobenzene. Further experimental results revealed that both (NH₄)₂C₂O₄ and the N ₂ atmosphere were indispensable for the photocatalytic hydrogenation of nitrobenzene to aniline over the Bi₂MoO₆ photocatalyst. On the basis of the electron spin resonance analysis results, photoinduced electrons of the Bi₂MoO₆ photocatalyst were found to be the main active species for the hydrogenation of nitrobenzene. Moreover, a mechanism was proposed to explain the visible-light-induced photocatalytic hydrogenation of nitrobenzene to aniline over the Bi₂MoO₆ photocatalyst. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra40935j

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Highly efficient visible-light-induced photocatalytic
hydrogenation of nitrobenzene to aniline in water3
Cite this: RSC Advances, 2013, 3,
10894
Weiming Wu, Rui Lin, Lijuan Shen, Ruowen Liang, Rusheng Yuan and Ling Wu*
Highly efficient visible-light-induced photocatalytic hydrogenation of nitrobenzene to aniline in water was
observed over a Bi2MoO6 photocatalyst under N2 atmosphere in the presence of (NH4)2C2O4 as a hole
scavenger. 100% of the nitrobenzene was converted and the selectivity of aniline was y97% after 20 min
of visible light irradiation (l ¢ 400 nm). The Bi2MoO6 photocatalyst showed good catalytic stability for the
hydrogenation of nitrobenzene. Further experimental results revealed that both (NH4)2C2O4 and the N2
atmosphere were indispensable for the photocatalytic hydrogenation of nitrobenzene to aniline over the
Bi2MoO6 photocatalyst. On the basis of the electron spin resonance analysis results, photoinduced
electrons of the Bi2MoO6 photocatalyst were found to be the main active species for the hydrogenation of
nitrobenzene. Moreover, a mechanism was proposed to explain the visible-light-induced photocatalytic
hydrogenation of nitrobenzene to aniline over the Bi2MoO6 photocatalyst.
Received 22nd February 2013,
Accepted 19th April 2013
DOI: 10.1039/c3ra40935j
www.rsc.org/advances
methods (such as photocatalysis8,9 and electrolytic reduc-
tion10,11). Among these methods, photocatalysis is a promising
1. Introduction
Nitrobenzene is used extensively in chemical manufacturing,
especially in the synthesis of other industrial chemicals and
intermediates (such as aniline, benzidine and azobenzene).1
However, nitrobenzene is very toxic (threshold limit value, 5
mg m23) and can cause acute poisoning via absorption
through the skin or respiratory tract.2 It has been classified
by the international agency for research on cancer (IARC) as a
Group 2B carcinogen which is possibly carcinogenic to
humans.3 Therefore, it is necessary to eliminate this com-
pound when it occurs as an organic pollutant in water.
Until now, various advanced oxidation processes such as
ozonation and O3–UV processes,4 photocatalysis,5 photo-
assisted Fenton oxidation6 and supercritical oxidation7 have
been reported to degrade nitrobenzene into CO2 and H2O in
water. While oxidative methods try to destroy the conjugated
structure of nitrobenzene, reductive methods pay increasing
attention to the nitro group of nitrobenzene. Hydrogenation of
nitrobenzene to aniline has been achieved by several reductive
technique for the hydrogenation of nitrobenzene. It is
desirable to eliminate nitrobenzene and to allow reuse of the
end product by utilizing sunlight. In previous work, TiO2 has
been reported to be an active catalyst for the photocatalytic
hydrogenation of nitrobenzene under UV light irradiation.8,9
In order to efficiently utilize the sunlight, the development of
highly efficient visible-light-induced photocatalysts for the
hydrogenation of nitrobenzene is required. We have found
that PbBi2Nb2O9 and CdS show photocatalytic activities for the
hydrogenation of 4-nitroaniline.12,13 They may be highly
efficient photocatalysts for the hydrogenation of nitrobenzene
under visible light irradiation. But these catalysts contain toxic
metal ions (Pb2+ and Cd2+), which may limit their industrial
applications in the photocatalytic hydrogenation of nitroben-
zene to aniline.
Bismuth molybdate (Bi2MoO6, band gap y2.7 eV) is an
attractive material as a candidate for non-toxic and highly
efficient visible-light-induced photocatalysts for the oxidative
degradation of organic pollutants in the field of contaminated
environment remediation.14,15 However, as far as we know,
State Key Laboratory Breeding Base of Photocatalysis, Research Institute of
Photocatalysis, Fuzhou University, Fuzhou 350002, P. R. China.
there is
a lack of systematic studies on the catalytic
E-mail: wuling@fzu.edu.cn; Fax: 86 591 83779105; Tel: 86 591 83779362
hydrogenation reactions over the Bi2MoO6 photocatalyst.
Herein, the photocatalytic hydrogenation of nitrobenzene to
aniline over the Bi2MoO6 photocatalyst in water was investi-
gated in detail under visible light irradiation (l ¢ 400 nm),
including the activity and stability of the catalyst, the effects of
the addition of hole scavengers and the reaction atmosphere,
and the active species. Our results may allow us to provide a
highly efficient approach to the photocatalytic hydrogenation
of nitrobenzene in water utilizing sunlight.
Electronic supplementary information (ESI) available: The experimental details
3
of the GC-MS analysis; TEM and high-resolution TEM images for the Bi2MoO6
sample; Liner relationship between the concentration (C) and the absorbance (A)
at 229 nm for aniline; GC-MS spectra of the nitrobenzene aqueous solution
before and after the reaction; Photocatalytic hydrogenation of nitrobenzene over
the TiO2 photocatalyst under UV light irradiation (l ¢ 300 nm) in the presence
of HCO2NH4 or (NH4)2C2O4; XRD patterns of the Bi2MoO6 photocatalyst before
and after the reaction; Photocatalytic hydrogenation of nitrobenzene over the
Bi2MoO6 photocatalyst in the presence of (NH4)2C2O4 as the hole scavenger
without N2 atmosphere. See DOI: 10.1039/c3ra40935j
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Fig. 2 UV-vis spectral changes of the nitrobenzene aqueous solution over the
Bi2MoO6 photocatalyst as a function of irradiation time in the presence of
(NH4)2C2O4 under visible light irradiation (l ¢ 400 nm).
Fig. 1 XRD pattern for the as-prepared sample. The inset is the UV-vis DRS
spectrum of the sample.
ity of aniline is y97%, since there is a liner relationship (C =
11.618A + 0.0675, R2 = 0.9999) between the concentration (C)
2. Results and discussion
and the absorbance (A) at 229 nm for aniline (see Fig. S2, ESI ).
The GC-MS result further indicates that nitrobenzene is
completely reduced to aniline, and no other products are
An X-ray diffraction (XRD) pattern of the as-prepared sample is
shown in Fig. 1. All diffraction peaks of the pattern for the
sample can be indexed to a pure orthorhombic phase of
Bi2MoO6 (JCPDS card no. 071-2086). The UV-vis diffuse
reflectance spectroscopy (UV-vis DRS) spectrum (see the inset
in Fig. 1) indicates that the absorption edge of the as-prepared
sample is located at y460 nm, corresponding to a band gap of
ca. 2.69 eV. This reveals that Bi2MoO6 can be a highly efficient
photocatalyst under visible light irradiation. Furthermore, the
transmission electron microscopy (TEM) image of the sample
shows that nanoplate-like Bi2MoO6 with a width of 50 nm and
thickness of 10 nm has been obtained by the solvothermal
3
detected (see Fig. S3 and S4 in ESI ).
3
For comparison, the photocatalytic hydrogenation of
nitrobenzene over a commercial TiO2 photocatalyst (P25,
Degussa Co.) under UV light irradiation (l ¢ 300 nm) has
been carried out in this work. As shown in Fig. S5 (ESI ), the
3
TiO2 photocatalyst shows catalytic activity for the photocata-
lytic hydrogenation of nitrobenzene, giving a nitrobenzene
conversion of 100% and aniline selectivity of 98% after 30 min
of UV light irradiation. However, as mentioned above, 100% of
nitrobenzene is converted and the selectivity of aniline is
y97% over the Bi2MoO6 photocatalyst after 20 min of visible
light irradiation (l ¢ 400 nm). The results indicate that
Bi2MoO6 is a highly efficient visible-light-induced photocata-
lyst for the photocatalytic hydrogenation of nitrobenzene to
aniline.
method (see Fig. S1a, ESI ). Clear lattice fringes are also
3
observed in the high-resolution TEM image (see Fig. S1b, ESI ).
3
The lattice spacing of d = 0.275 nm matches that of the (002)
crystallographic plane of Bi2MoO6, and the fringes of d = 0.270
nm correspond to the interplanar spacing of the (210) plane
for Bi2MoO6.
Generally, the stability of a catalyst is a very important
factor for its practical applications. As shown in Fig. 3, the
photocatalytic activity of the Bi2MoO6 photocatalyst does not
obviously decrease in the recycling experiment. Its catalytic
activity can remain at y100% in the 3th cycle of testing. XRD
Fig. 2 shows the UV-vis spectral changes of the nitroben-
zene aqueous solution over the Bi2MoO6 photocatalyst as a
function of irradiation time in the presence of (NH4)2C2O4
under visible light irradiation (l ¢ 400 nm), in which
(NH4)2C2O4 aqueous solution is used as a reference solution.
There is a rapid decrease in the absorption of nitrobenzene at
268 nm along with a simultaneous appearance of two peaks at
229 and 278 nm. The peaks observed at 229 and 278 nm are
the characteristic peaks of aniline. This reveals that the
photocatalytic hydrogenation of nitrobenzene to aniline can be
achieved over the Bi2MoO6 photocatalyst under visible light
irradiation (l ¢ 400 nm). It is noted that the absorbance of
aniline at 229 nm in the UV-vis spectrum is 1.24 after visible
light irradiation for 20 min. Meanwhile, the full amount of
nitrobenzene is completely converted. Therefore, the selectiv-
patterns (see Fig. S6, ESI ) indicate that the crystal structure of
3
the Bi2MoO6 photocatalyst is intact before and after the
reaction. The XRD patterns of the catalyst can be well indexed
to orthorhombic Bi2MoO6 (JCPDS card no. 071-2086). The
results confirm that the Bi2MoO6 photocatalyst has good
stability for the photocatalytic hydrogenation of nitrobenzene
under visible light irradiation (l ¢ 400 nm) in the presence of
(NH4)2C2O4 as the hole scavenger upon purging with N2.
As a part of this work, the photocatalytic hydrogenation of
nitrobenzene to aniline has been carried out under different
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be attributed to the photoreduction of oxygen by photoin-
duced electrons of the Bi2MoO6 photocatalyst (O2 + e2 A O2
,
2
?
E = 20.28 V vs. SHE at pH 7;16
E
= 20.32 V vs.
conduction band
SHE at pH 717). Therefore, fewer photoinduced electrons
participate in the hydrogenation of nitrobenzene. In previous
work, HCO2NH4 has also been found to be an efficient hole
scavenger for photocatalytic reduction reactions.13,18 However,
the Bi2MoO6 photocatalyst shows negligible catalytic activity
for the hydrogenation of nitrobenzene in the presence of
HCO2NH4 as the hole scavenger (see Fig. 4d). This reveals that
(NH4)2C2O4 is a more efficient hole scavenger for the Bi2MoO6
photocatalyst compared to HCO2NH4. Therefore, on the basis
of the above experimental results, both (NH4)2C2O4 and the N2
atmosphere are indispensable for the photocatalytic hydro-
genation of nitrobenzene to aniline over the Bi2MoO6
photocatalyst.
A 5-dimethyl-1-pyrroline N-oxide (DMPO) spin-trapping
electron spin resonance (ESR) technique has been introduced
to investigate the photocatalytic hydrogenation of nitroben-
zene over the Bi2MoO6 photocatalyst under visible light
irradiation (l ¢ 400 nm). For comparison, similar work has
also been carried out over the commercial TiO2 photocatalyst
Fig. 3 Reusability of the Bi2MoO6 photocatalyst for the photocatalytic hydro-
genation of nitrobenzene.
conditions (see Fig. 4). The control experiment (nitrobenzene
and (NH4)2C2O4 without the Bi2MoO6 photocatalyst) exhibits
negligible photocatalytic hydrogenation of nitrobenzene
(Fig. 4a), which indicates the photolysis of nitrobenzene can
be ignored under visible light irradiation (l ¢ 400 nm). Fig. 4b
shows that no significant hydrogenation of nitrobenzene is
observed after 20 min of visible light irradiation (l ¢ 400 nm)
in the absence of (NH4)2C2O4 as the hole scavenger.
Furthermore, as shown in Fig. 4c, the photocatalytic hydro-
genation of nitrobenzene can be achieved over the Bi2MoO6
photocatalyst in the presence of (NH4)2C2O4 without N2
atmosphere. But the Bi2MoO6 photocatalyst requires a longer
under UV light irradiation (l ¢ 300 nm). Sextet characteristic
2
?
?
peaks of the DMPO– OH and DMPO– O2 adducts can be
observed in the TiO2 water and methanol suspensions under
UV light irradiation (see Fig. 5a and b), respectively. These
confirm that photoinduced holes (h+) and electrons (e2) of the
illuminated TiO2 photocatalyst can react H2O and O2
2
?
?
molecules to produce OH and O2 radicals in water (H2O +
h A H + OH, E = 2.28 V vs. SHE at pH 7;16 O2 + e2 A O22, E
+
+
?
?
= 20.28 V vs. SHE at pH 716), respectively. It is also found that
2
?
?
the OH and O2 radicals can be detected in the Bi2MoO6
photocatalyst system (E
= 20.32 V and E
conduction band
= 2.37 V vs. SHE at pH 717). However, as shown in
time (30 min, see Fig. S7, ESI ) to convert nitrobenzene into
3
valence band
aniline under visible light irradiation (l ¢ 400 nm). This may
Fig. 5a, the intensity of the characteristic peaks of the DMPO–
?
OH adduct is very weak in the Bi2MoO6 water suspension. The
?
negligible signal of the DMPO– OH adduct in the ESR
spectrum may be attributed to the low redox potential of the
valence band for the Bi2MoO6 photocatalyst. Therefore, the
TiO2 photocatalyst can produce photoinduced electrons and
?
OH radicals to participate in the hydrogenation of nitroben-
zene under N2 atmosphere, when the main active species in
the Bi2MoO6 system are photoinduced electrons and holes.
?
Generally, OH radicals and photoinduced holes are the
active species for photocatalytic oxidation reactions.
Therefore, the oxidation of nitrobenzene rather than the
hydrogenation of nitrobenzene may be observed in the
4–7
?
presence of the OH radicals or/and photoinduced holes.
This further confirms that the addition of a hole scavenger is
indispensable for the photocatalytic hydrogenation of nitro-
benzene to aniline under N2 atmosphere. As a part of this
work, the DMPO spin-trapping ESR spectra of different
samples water suspensions in the presence of (NH4)2C2O4 (or
HCO2NH4) as the hole scavenger have been obtained (see
Fig. 6). The results reveal that sextet characteristic peaks of the
Fig. 4 Photocatalytic hydrogenation of nitrobenzene under different conditions:
(a) without the Bi2MoO6 photocatalyst, (b) without (NH4)2C2O4, (c) without the
N2 atmosphere and (d) with HCO2NH4 as the hole scavenger.
2
?
DMPO– CO2 adduct have been clearly observed in the
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Fig. 6 DMPO spin-trapping ESR spectra of different samples water suspensions
in the presence of (NH4)2C2O4 or HCO2NH4 as the hole scavenger.
photocatalyst. As a result, the photoinduced electrons of the
Bi2MoO6 photocatalyst are the main active species for the
photocatalytic hydrogenation of nitrobenzene in the present
system.
Moreover, as mentioned above, the photoinduced holes are
the main active species in the photocatalytic reactions for the
2
Bi2MoO6 photocatalyst. In this work, compared to HCO2
22
ions, C2O4 ions will react relatively easily with the photo-
induced holes (h+) due to their stronger electrostatic attrac-
tion. Therefore, more photoinduced electrons can escape from
the pair recombination, and participate in the hydrogenation
of nitrobenzene. This can explain why (NH4)2C2O4 rather than
HCO2NH4 is a more efficient hole scavenger for the photo-
catalytic hydrogenation of nitrobenzene over the Bi2MoO6
photocatalyst. However, in contrast to the Bi2MoO6 photo-
catalyst, the photoinduced holes of the TiO2 photocatalyst can
Fig. 5 DMPO spin-trapping ESR spectra of (a) the samples water suspensions
and (b) the samples methanol suspensions.
presence of the TiO2 photocatalyst and (NH4)2C2O4 (or
22
2
2
HCO2NH4) under UV light irradiation (l ¢ 300 nm). The
?
react with the C2O4 and HCO2 ions to produce the CO2
radicals. But they can also react with H2O molecules to give the
2
?
formation of the CO2 radical can be attributed to the
reaction between the photoinduced holes of the TiO2 photo-
?
?
OH radicals. The OH radicals may be trapped relatively easily
22
2
by the HCO22 ions as compared to the C2O422 ions because of
their weaker electrostatic repulsion. The analysis results of the
DMPO spin-trapping with ESR indicate that only the ESR
catalyst and the C2O4 within (NH4)2C2O4 (or HCO2 within
HCO2NH4). Furthermore, the CO22 radical has been proved to
?
be an efficient species for the photoreduction of benzenes due
to its strong reductive ability (E ( CO22/CO2) = 22.21 V vs. SHE
?
2
?
signal of the DMPO– CO2 adduct is detected in the TiO2
2
at pH 7).12,13,19 Therefore, the CO2 radicals and photo-
?
water suspension under UV light irradiation in the presence of
induced electrons (E conduction band = 20.36 V vs. SHE at pH 720
)
HCO2NH4 as the hole scavenger, whereas the ESR signal of the
2
can reduce nitrobenzene to aniline (E (nitrobenzene/aniline) =
?
?
DMPO– CO2 and DMPO– OH adducts is observed in the TiO2
system in the presence of (NH4)2C2O4 as the hole scavenger
0.15 V vs. SHE at pH 78) in the TiO2 system. However, as shown
in Fig. 6, there are no obvious signals of DMPO– CO22 adduct
?
(see Fig. 6). Therefore, as shown in Fig. S5 (ESI ) the TiO
3
2
over the Bi2MoO6 photocatalyst in the presence of the hole
scavenger ((NH4)2C2O4 or HCO2NH4). This can be attributed to
the fact that (NH4)2C2O4 or HCO2NH4 is directly oxidized into
CO2. The total organic carbon (TOC) values of the nitroben-
zene solution before and after the reaction are 45 and 37 mg
photocatalyst shows high catalytic activity for the hydrogena-
tion of nitrobenzene under UV light irradiation (l ¢ 300 nm)
in the presence of HCO2NH4.
On the basis of the above experimental results, the
mechanism for the visible light-induced photocatalytic hydro-
genation of nitrobenzene to aniline over the Bi2MoO6
photocatalyst in the presence of hole scavengers under N2
L
21, respectively, which confirms the oxidation of the hole
scavengers by the photoinduced holes of the Bi2MoO6
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atmosphere is proposed as follows: When the Bi2MoO6
photocatalyst is illuminated under visible light (l ¢ 400
nm), electron-hole pairs are created. Then, the photoinduced
charge carriers migrate to the surface of the photocatalyst and
participate in the redox reaction. The photoinduced holes (h+)
XRD patterns were collected on a Bruker D8 Advance X-ray
diffractometer with Cu-Ka radiation. The data were recorded
in a 2h range of 15–85u at a scan rate of 1.2u min21. UV-vis DRS
spectra were measured on a Cary 500 UV-vis-NIR spectro-
photometer (Varian Co.). BaSO4 was used as a reflectance
standard. TEM images were obtained using a FEI Tencai 20
transmission electron microscope at an accelerating voltage of
200 kV. Samples for TEM imaging were prepared by placing a
drop of the sample ethanol suspension on a copper grid
coated with carbon film and dried in the atmosphere. DMPO
spin-trapping ESR spectra were recorded with a Bruker A300
spectrometer at room temperature. General instrument
settings are as follows: microwave power, 6.35 mW; modula-
tion amplitude, 3 G; receiver gain, 1 6 103; time constant,
10.24 ms; sweep time, 42 s; center field, 3507 G; sweep width,
80 G.
22
will react easily with C2O4 ions to produce CO2 due to their
strong electrostatic attraction, whilst the photoexcited elec-
trons (e2) will directly participate in the redox reaction in
aqueous solution under N2 atmosphere because of the
suppression of the photoreduction of oxygen (O2 + e2
A
O22). The photoinduced electrons have strong reductive
?
ability (E
= 20.32 V vs. SHE at pH 7 17), and
conduction band
therefore can participate in the hydrogenation of nitrobenzene
8
to aniline (E (nitrobenzene/aniline) = 0.15 V vs. SHE at pH 7 ).
Eqn (1) shows a probable stoichiometry of the photocatalytic
hydrogenation of nitrobenzene to aniline in the presence of
(NH4)2C2O4.21
For the photocatalytic hydrogenation of nitrobenzene, an
ozone-free 300 W Xe lamp (PLS-SXE300C, Trusttech Co. Ltd,
Beijing, intensity: y1.20 W cm22 at l = 400 nm) with an
infrared filter and a cutoff filter of 400 nm was used as the
light source (l ¢ 400 nm). Prior to the catalytic test, 80 mg of
Bi2MoO6 powders was suspended in 80 mL of nitrobenzene
solution (20 mg L21, A.R., Sinopharm Chemical Reagent Co.)
in a Pyrex reactor (100 mL). After adding 20 mg of a hole
scavenger (ammonium oxalate, (NH4)2C2O4, A.R., Sinopharm
Chemical Reagent Co.), the suspension was stirred in the dark
for 30 min to ensure the elimination of oxygen from the
system by purging with nitrogen. As the reaction proceeded, 4
mL of the suspension was taken at a certain time interval and
was filtrated. The nitrobenzene and aniline concentrations
during the reaction were analyzed by measuring the absor-
bance at 268 and 229 nm with a Cary 50 UV-vis spectro-
photometer (Varian Co.), respectively. The whole
photocatalytic process was carried out under N2 bubbling
with a flow rate of 60 mL min21. Moreover, the TOC values of
the nitrobenzene aqueous solution before and after the
reaction were measured using a TOC analyzer (TOC-V CPH,
Shimdzu Co.), and the presence of nitrobenzene and aniline
was detected using GC-MS (for a more detailed experimental
22
Ph–NO2 + 3C2O4 + 4H2O A Ph–NH2 + 6CO2 + 6OH2
(1)
3. Conclusions
The Bi2MoO6 photocatalyst showed highly efficient catalytic
activity and good stability for the photocatalytic hydrogenation
of nitrobenzene to aniline under visible light irradiation (l ¢
400 nm). Further experimental results indicated that both hole
scavengers and N2 atmosphere were indispensable for the
hydrogenation of nitrobenzene over the Bi2MoO6 photocata-
lyst. (NH4)2C2O4 was found to be an efficient hole scavenger
for the Bi2MoO6 photocatalyst due to the strong electrostatic
attraction between the photoinduced holes of the Bi2MoO6
photocatalyst and the C2O4
22
ions within (NH4)2C2O4.
Moreover, the analysis results of the DMPO spin-trapping
ESR revealed that the photoinduced electrons of the Bi2MoO6
photocatalyst were identified as the main active species for the
visible-light-induced photocatalytic hydrogenation of nitro-
benzene in the present system.
section, see the ESI ).
3
4. Experimental section
Acknowledgements
The Bi2MoO6 photocatalyst was prepared using a solvothermal
method.14 Typically, 7.0 mmol Bi(NO3)3?5H2O (A.R.,
Sinopharm Chemical Reagent Co.) and 0.5 mmol
(NH4)6Mo7O24?4H2O (A.R., Sinopharm Chemical Reagent Co.)
were firstly dissolved in 60 mL of ethylene glycol (A.R.,
Sinopharm Chemical Reagent Co.) in a 100 mL Teflon liner.
Then, the pH value of the solution was adjusted to y9 by
dripping in 5 wt% aqueous ammonia (A.R., Sinopharm
Chemical Reagent Co.). After being stirred for 30 min (1000 r
min21), the Teflon liner was sealed in a stainless steel
autoclave and maintained at a temperature of 160 uC for 6 h.
Yellow Bi2MoO6 powders were collected, were washed with
deionized water several times, and were dried in air at 60 uC.
This work was supported by National Natural Science
Foundation of China (21177024 and 21273036), Natural
Science Foundation of Fujian Province, China (2011J01041)
and National Key Basic Research Program of China (973
Program: 2011CB612314).
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