Selective methanol production from photocatalytic reduction of CO 2 on BiVO4 under visible light irradiation

Article abstract of DOI:10.1016/j.catcom.2012.08.008

Lamellar BiVO₄ was prepared through a surfactant-assisted hydrothermal process. The obtained lamellar BiVO₄ with monoclinic phase was used as photocatalyst for the reduction of CO₂, and exhibited a selective methanol production under visible-light irradiation. It is found that the methanol yield can be significantly enhanced by adding NaOH solution in the BiVO₄ suspension because the caustic solution can dissolve more CO₂ and the OH^- serves as a stronger hole-scavenger as compared to water. The possible mechanism for the selective methanol production from the present photocatalytic CO₂ reduction system was also discussed.

Full text of DOI:10.1016/j.catcom.2012.08.008

Catalysis Communications 28 (2012) 38–41
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective methanol production from photocatalytic reduction of CO on BiVO under
2
4
visible light irradiation
a
a,b,
a
a
a
Jin Mao , Tianyou Peng
⁎, Xiaohu Zhang , Kan Li , Ling Zan
a
College of Chemistry and Molecular Science, Wuhan University, Wuhan 430072, China
State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350108, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2012
Received in revised form 10 August 2012
Accepted 11 August 2012
Available online 18 August 2012
Lamellar BiVO4 was prepared through a surfactant-assisted hydrothermal process. The obtained lamellar BiVO4
with monoclinic phase was used as photocatalyst for the reduction of CO , and exhibited a selective methanol pro-
2
duction under visible-light irradiation. It is found that the methanol yield can be significantly enhanced by adding
−
NaOH solution in the BiVO
4
suspension because the caustic solution can dissolve more CO
2
and the OH serves as
a stronger hole-scavenger as compared to water. The possible mechanism for the selective methanol production
from the present photocatalytic CO
2
reduction system was also discussed.
©
Keywords:
CO reduction
2
2012 Elsevier B.V. All rights reserved.
Photocatalyst
Photocatalytic reduction
BiVO
4
1
. Introduction
2 4
CO . Recently, microspheric or lamellar BiVO was selectively prepared
through a hydrothermal process by using cetyltrimethylammonium
bromide (CTAB) as a template-directing reagent in our group [19]. The
Large-scale emission of CO
issue due to its greenhouse effect, and converting CO
useful fuels is known as a feasible way to solve the environmental
problem [1–3]. Since the photoelectrocatalytic reduction of CO to or-
ganic compounds in semiconductor suspension was first reported
in1979 [4], the focus has been on the use of TiO and water in order to
provide an approach to the photocatalytic reduction of CO to fuels
such as CH , CH OH and C OH [5–9]. However, the photocatalytic ef-
ficiency of CO reduction to fuels such as CH over TiO is very low
b0.1%) due to its bandgap wider than 3.0 eV [5–9]. Therefore, the ma-
jority of research is aimed at designing novel photocatalysts for the CO
2
has become a global environmental
2
efficiently to
microspheric BiVO
derived from a relatively low hydrothermal temperature (≤160 °C),
while the lamellar BiVO with a pure monoclinic phase was obtained at
200 °C, and showed a better visible-light-driven photoactivity for O
duction than the microspheric product [19].
During the O production over the monoclinic BiVO
4
with mixed tetragonal and monoclinic crystals was
2
4
2
pro-
2
2
2
4
, the byproducts
+
+
4
3
H
2 5
are photogenerated electrons and H ions because these H cannot
2
4
2
capture the electrons to produce H
between the conduction band of BiVO
water [18,19], whereas it is possible to convert CO
2
due to the unmatched energy levels
and the reduction potential of
to fuels by using
suspension [11,18]. Herein,
with a pure monoclinic phase was prepared and
used as a catalyst for photocatalytic CO reduction in a CO /NaOH suspen-
(
4
2
2
+
reduction in order to improve conversion efficiency, selectivity and/or
harvesting visible light [10–17]. For example, Jia and co-workers [10]
the above electrons and H in the BiVO
the lamellar BiVO
4
4
have found that C-doped LaCoO
under visible-light irradiation. Selective methanol production over
NiO/InTaO was also observed under visible light [11]. Moreover, the
improved efficiency was also demonstrated for some novel titania-
based nanomaterials [6,7], coupling TiO to dyes [8] or quantum dots
9]. Still, it remains as challenging tasks to find novel photocatalysts to
efficiently reduce CO under visible light [5,12–14].
Bismuth vanadate (BiVO ) has inspired a great deal of interest
3
is able to reduce CO
2
into HCOOH
2
2
sion, and the effects of the photoreaction conditions on the photoactivity
and stability were investigated. Moreover, the possible mechanism for
4
2
the selective methanol formation from the CO reduction system is also
2
discussed.
[
2
4
2. Experimental section
because of its low toxicity and excellent visible-light-responsive photo-
activity [18,19]. For example, Liu and co-workers [18] found that the
2.1. Material preparation
visible-light-driven photocatalytic CO
suspension can selectively produce C
2
reductions in monoclinic BiVO
OH under continuously bubbling
4
2
H
5
4
Monoclinic BiVO was prepared by a CTAB-assisted hydrothermal
process. More details on the material preparation and characterization
can be found in our previous report [19]. The obtained product hydro-
thermally treated at 200 °C shows well-defined lamellar structure with
⁎
Corresponding author. Tel./fax: +86 27 68752237.
E-mail address: typeng@whu.edu.cn (T. Peng).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.08.008

J. Mao et al. / Catalysis Communications 28 (2012) 38–41
39
dimension in the range of 0.08–0.12 μm and pure monoclinic phase as
shown in Fig. S1 (see Supplementary data for details).
2
.2. Photocatalytic activity test
Photocatalytic CO
gas system. Supercritical fluid-grade CO
2
reduction was carried out in a homemade closed
was used as the reactant to
2
avoid any hydrocarbon contamination. Photoreaction was performed
in a Pyrex glass cell containing photocatalyst (0.2 g) and 1.0 M NaOH
solution (100 mL) with magnetic stirring. Prior to the light irradiation,
the above suspension was thoroughly degassed to remove air, and
then CO
2
gas was introduced continuously into the reactor for 30 min
solu-
to establish an adsorption–desorption balance and a saturated CO
2
tion. The reactor was tightly closed during the reaction with internal
pressure maintained at ~1.0 atm, and then was irradiated by 300 W
Xe-lamp (PLS-SXE300, Beijing Trusttech Co., Ltd.). If necessary, a cutoff
filter (Kenko L-42) was employed for the visible-light (λ≥420 nm)
irradiation. The photoreaction temperature was kept at 20 °C.
The products in the solution were qualitatively identified by gas
chromatography (SP6800A with GDX-502 columns) equipped with
a flame ionization detector (FID), and the outlet gases were sampled
through a sampling value and analyzed by gas chromatography
Fig. 1. Time course of the CH
system. Conditions: BiVO
diation of Xe-lamp.
3
OH and O
2
production from the photocatalytic CO
2
reduction
4
(0.2 g), 1.0 M NaOH solution (100 ml), and full spectrum irra-
H O þ 4hþ → O þ 4Hþ þ 4e
−
2
ð2Þ
ð3Þ
2
2
(
SP6800A with TDX-01 columns) equipped with thermal conductivi-
CO þ 6Hþ þ 6e → CH OH þ H O
−
ty detector (TCD). All peaks were checked with their corresponding
standards, and each experiment was repeated at least 5 times in parallel
to obtain an average value. After measuring the methanol production
amount under the same photoreaction condition, apparent quantum
efficiency (AQE) was calculated from the relationship 6×number of
evolved methanol molecules/number of incident photons [20–23]
2
3
2
To further confirm the above assumption, the effects of the photo-
reaction conditions on the CH OH production from the CO /BiVO
system containing 1.0 M NaOH solution were investigated. Fig. 1
shows the CH OH and O evolution amount as the functions of the ir-
radiation time. As can be seen, CH OH yield increases markedly with
prolonging the irradiation time to 6 h, and then a steady CH OH yield
remains at ~35 μmol upon further prolonging the irradiation time,
indicating the oxidation of the formed CH OH occurring on the cata-
lyst surface or the coverage of intermediates on the active site [21].
According to our previous report [19], the monoclinic BiVO is an ef-
3
2
4
3
2
(
see Supplementary data for details).
3
3
3
. Results and discussion
.1. Effect of photoreaction conditions on the photocatalytic CO
Control experiments showed no appreciable reduced C1 or C2
3
3
2
reduction
4
ficient photocatalyst for O production. As can be seen from Fig. 1, the
2
product in the absence of either photocatalyst or light irradiation,
illustrating that both photocatalyst and light irradiation are necessary
ratio of O /CH OH generation rate is approximately constant, ~1.22
2
3
rather than 1.5, which may be ascribed to the fact that only the O2
that existed in the outlet gas can be detected, and a part of the generated
O2 is dissolved in the reaction solution. Again, O2 yield also increases
slowly after 6 h light irradiation, implying that the re-oxidation of the
for the photocatalytic CO
nanoparticles (P25) were used as reference for the CO
no obvious reduced C1 or C2 product can be detected. Therefore, photo-
induced CO reduction in the BiVO suspension was firstly performed
under the full spectrum irradiation of Xe-lamp, it was found that
CH OH can be produced from the reaction system, and concomitant
traces of C OH are too small to be quantified. Moreover, no obvious
other compounds such as HCOOH are detectable, and the outlet gases
are determined to be O and CO but not CH or H . Furthermore,
there is also no obvious CH OH, C OH or other products detectable
from the BiVO suspension by introducing N instead of CO , demon-
strating the organic products are generated from the reduction of CO
Generally, the photocatalytic CO reduction efficiency and selec-
tivity mainly depend on the photocatalyst property and reaction con-
dition [5,6]. It has been reported that C OH can be selectively
produced from BiVO suspension with continuously bubbling CO at
sys-
2
reduction. In addition, commercial TiO
2
2
reduction, and
2
4
formed CH OH may occur [21].
3
Fig. 2 shows the effect of photocatalyst amount on CH OH production
3
3
in the CO reduction system. As can be seen, no appreciable CH OH can
2
3
2 5
H
2
2
4
2
3
2 5
H
4
2
2
2
.
2
2 5
H
4
−
2
1
the rate of 0.4 L min
[18]. In this continuously bubbling CO
2
tem, large amount of C1 intermediates can be produced due to the
sufficient carbon source, which can facilitate the dimerization of C1
2 5
intermediates to form C H OH [6,18]. Therefore, the selective forma-
tion of CH OH in the present closed gas system may be ascribed to the
3
limited carbon source. More importantly, the different catalyst struc-
tures may be another key reason for obtaining the different products
in the present system [7]. The corresponding overall reaction might
be described as follows.
Fig. 2. Effects of the amount of BiVO
4
on the CH
3
OH production from the photocatalytic
CO
2
reduction system. Conditions: 1.0 M NaOH solution (100 ml) and full spectrum
þ
−
BiVO þ hν → h þ e
ð1Þ
4
irradiation of Xe-lamp for 6 h.

40
J. Mao et al. / Catalysis Communications 28 (2012) 38–41
Fig. 4. Repeated experiments of the photocatalytic CO
tion from BiVO (0.2 g) suspension containing 1.0 M NaOH solution (100 ml) under
full spectrum irradiation.
2 3
reduction for CH OH produc-
Fig. 3. Effects of NaOH concentration on the CH
CO reduction system. Conditions: BiVO (0.2 g), NaOH solution (100 ml), and full
spectrum irradiation of Xe-lamp for 6 h.
3
OH production from the photocatalytic
4
2
4
be detected in the absence of photocatalyst, illustrating BiVO
sary for the photocatalytic CO reduction. Usually, a higher photocatalyst
amount is expected to correspond to greater light absorption, and thus a
higher CH OH yield. However, CH OH yield obviously increases with en-
hancing the photocatalyst dosage from 0.1 to 0.2 g, and then decreases
when the addition amount exceeds 0.2 g (Fig. 2). It can be concluded
that the light penetration is blocked by the aggregation of nanoparticles
owing to the large quantity of photocatalyst in the suspension [21].
4
is neces-
CH
3
OH detected under the visible-light irradiation. The maximum
OH production achieved 0.24% and 0.22% for
2
AQE values for the CH
3
the full spectrum and visible-light irradiation, respectively. The less
satisfactory visible-light-responsive efficiency can be ascribed to the
narrow visible-light absorption band with an absorption edge at
~550 nm (Fig. S2a) [19]. Moreover, the limited increase in the photo-
catalytic efficiency under the full spectrum can be attributed to the
small spectral distribution of UV-light with wavelength b420 nm
(Fig. S2b).
3
3
Fig. 3 shows the effect of NaOH concentration on CH
tion in the CO reduction system. As can be seen, traces of CH
can be detected even without NaOH, which is ascribable to the limited
CO dissolved in the suspension. Whereas CH OH yield substantially
3
OH produc-
2
3
OH
Fig. 4 shows the photostability of the present CO
2 4
/BiVO system
for CH OH production under 6 h light irradiation of full spectrum
for each run. The photocatalyst was recovered every 6 h by centrifu-
gation, washing and drying at 80 °C. As can be seen, the photoinduced
3
2
3
increases with enhancing the NaOH concentration, indicating NaOH
solution is crucial in the present system. This phenomenon may be
due to the following reasons: Firstly, NaOH solution can dissolve more
2
CO reduction reaction shows comparatively good reproducibility
and consistency in three consecutive repeated runs of accumulatively
18 h. To further check the stability of the photoreaction, the photo-
catalyst recovered by centrifugation but without washing and drying
processes was immediately reused to the repeated experiments. It
CO
2
than water. The pH value decreased from 14 (1.0 M NaOH) to 7.5
−
after bubbling CO
with a small amount of CO
ic CO
2
, indicating the solution mainly contains HCO
3
or
2
−
3
, which might accelerate the photocatalyt-
−
2
reduction [6,11]. In addition, OH can act as hole-scavenger
can be found that the CO
tency in the repeated runs (Fig. S3). Moreover, XRD pattern (Fig. S1)
and XPS spectra (Fig. S4) of the recovered BiVO after the repeated
experiment for 18 h are almost identical to that of the as-prepared
ones. The above results indicated the BiVO is stable in the present
CO reduction system.
Based on the above results and discussion, the possible photocata-
lytic mechanism is proposed and illustrated in Fig. 5. Accordingly, lots
of photogenerated carriers can transfer to the BiVO surfaces under
visible-light irradiation. The absence of H in the outlet gases suggests
that the proton failed to capture the photogenerated electrons, which
confirmed the previous report that BiVO cannot produce H due to
2
reduction reaction still shows good consis-
to promote the carrier separation, and resulting in an improved CO
reduction efficiency. Moreover, CH OH yield increases slowly once the
2
3
4
NaOH concentration is higher than 1.0 M, which can be ascribed to
−
the formation of the saturated HCO
3
solution after bubbling CO
2
.
4
Since the alkaline substance with excessive concentration would dam-
age the chromatographic column, 1.0 M NaOH solution was chosen
for the following experiments.
2
4
3
.2. Photocatalytic CO
2
reduction efficiency and stability
2
Table 1 summarizes the CH
quantum efficiency (AQE) over the monoclinic BiVO
trum or visible-light (λ≥420 nm) irradiation of Xe-lamp for 6 h. As
3
OH production rate and apparent
4
2
4
under full spec-
−
1
can be seen, the CH
3
OH production rate is 3.76 μmol h
under
visible-light irradiation, with ~31% reduction as compared to that
under full spectrum, indicating UV-light also contributes to the se-
lective methanol production. Moreover, there is still no other than
Table 1
Photocatalytic CH
3
OH production rates and AQE values from BiVO
4
suspension.a
Visible light
Light source
Full spectrum
CH
AQE (%)
3
OH yield rate (μmol h−1)
5.52
0.24
3.76
0.22
a
Condition: BiVO
4
(0.2 g), 1.0 M NaOH solution (100 ml), and full spectrum or
Fig. 5. Proposed mechanism of CH
containing BiVO
3 2
OH production from the CO reduction system
visible-light (λ≥420 nm) irradiation of Xe-lamp for 6 h.
4
.

J. Mao et al. / Catalysis Communications 28 (2012) 38–41
41
its energy band structures are unmatched with the water reduced
potential [18,19]. Considering that the potential can be lowered by
Appendix A. Supplementary data
2
−
−
nearly 0.7 V when CO
2
was changed to CO
3
or HCO
3
[11], it is reason-
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.08.008.
−
2−
3+
able to think that those HCO
3
or CO
3
can anchor to the surface Bi
sites through weak Bi…O bonds so that they can efficiently receive
−
the electrons from the V3d-block bands of BiVO
anions, and give rise to methoxyl (•OCH
after protonation [18,24], while O is produced from H
scavenging holes on the BiVO surfaces [4,19,21,24]. These procedures
can reduce the carrier recombination. Obviously, the photoreduction
of CO and the decomposition of H O proceed competitively on the
BiVO surfaces, and CH OH is preferentially produced since the protons
4
to form CO
) radicals and then to CH
2
•
radical
3
3
OH
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2
2
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This work was supported by the Natural Science Foundation of
China (20973128, 20871096, 20573078) and the National High-Tech
Research and Development Program (2006AA03Z344).