Selective photocatalytic oxidation of alcohols to aldehydes in water by TiO2 partially coated with WO3

Article abstract of DOI:10.1002/chem.201100166

Semiconductor TiO₂ particles loaded with WO₃ (WO ₃/TiO₂), synthesized by impregnation of tungstic acid followed by calcination, were used for photocatalytic oxidation of alcohols in water with molecular oxygen under irradiation at I>350 nm. The WO ₃/TiO₂ catalysts promote selective oxidation of alcohols to aldehydes and show higher catalytic activity than pure TiO₂. In particular, a catalyst loading 7.6 wt% WO₃ led to higher aldehyde selectivity than previously reported photocatalytic systems. The high aldehyde selectivity arises because subsequent photocatalytic decomposition of the formed aldehyde is suppressed on the catalyst. The TiO₂ surface of the catalyst, which is active for oxidation, is partially coated by the WO ₃ layer, which leads to a decrease in the amount of formed aldehyde adsorbed on the TiO₂ surface. This suppresses subsequent decomposition of the aldehyde on the TiO₂ surface and results in high aldehyde selectivity. The WO₃/TiO₂ catalyst can selectively oxidize various aromatic alcohols and is reusable without loss of catalytic activity or selectivity.

Full text of DOI:10.1002/chem.201100166

DOI: 10.1002/chem.201100166
Selective Photocatalytic Oxidation of Alcohols to Aldehydes in Water by
TiO2 Partially Coated with WO3
Daijiro Tsukamoto,[a] Makoto Ikeda,[a] Yasuhiro Shiraishi,*[a] Takayoshi Hara,[b]
Nobuyuki Ichikuni,[b] Shunsuke Tanaka,[c] and Takayuki Hirai[a]
Abstract: Semiconductor TiO2 particles
loaded with WO3 (WO3/TiO2), synthe-
sized by impregnation of tungstic acid
followed by calcination, were used for
photocatalytic oxidation of alcohols in
water with molecular oxygen under ir-
radiation at l>350 nm. The WO3/TiO2
catalysts promote selective oxidation of
alcohols to aldehydes and show higher
catalytic activity than pure TiO2. In
particular, a catalyst loading 7.6 wt%
WO3 led to higher aldehyde selectivity
than previously reported photocatalytic
systems. The high aldehyde selectivity
arises because subsequent photocata-
lytic decomposition of the formed alde-
hyde is suppressed on the catalyst. The
TiO2 surface of the catalyst, which is
active for oxidation, is partially coated
by the WO3 layer, which leads to a de-
crease in the amount of formed alde-
hyde adsorbed on the TiO2 surface.
This suppresses subsequent decomposi-
tion of the aldehyde on the TiO2 sur-
face and results in high aldehyde selec-
tivity. The WO3/TiO2 catalyst can selec-
tively oxidize various aromatic alcohols
and is reusable without loss of catalytic
activity or selectivity.
Keywords: alcohols · aldehydes ·
oxidation · photocatalysis · water
Introduction
tions in water, such as hydrocarbon oxidation,[6] aromatic
hydroxylation,[7] naphthalene oxygenation,[8] heterocycle
functionalization,[9] and cyclization of amino acids.[10] In
most cases, photocatalytic reactions on TiO2 in water pro-
ceed through the following steps:[11] 1) Generation of elec-
tron (eꢀ) and positive hole (h+) pairs by absorption of
supra-band gap photons; and 2) oxidation of substrates by
Since the discovery of redox properties of photoirradiated
titanium dioxide (TiO2),[1] heterogeneous photocatalysis
with TiO2 has mainly been employed for the decomposition
of harmful organic compounds in air and water, promoting
mineralization of these compounds into CO2 and H2O.[2] Ap-
plication of TiO2 photocatalysis for organic synthesis has
also attracted much attention. Several TiO2-based photoca-
talytic systems have been proposed so far;[3] however, many
of these systems need to be performed either in organic sol-
vents or in the gas phase.[4] Water is a desirable solvent for
chemical reactions, because of environmental concerns,
safety, and cost, and organic syntheses in water is currently
the focus of much attention.[5] There are, however, only a
few reports of selective photocatalytic organic transforma-
h+ or the hydroxyl radicals ( OH) formed by the reaction of
C
h+ with surface OH groups or adsorbed H2O molecules.
ꢀ
The oxidation step is, however, nonselective and usually
promotes further oxidation of the product (decomposition)
as well as the oxidation of substrate. Selective photocatalytic
transformation in water therefore requires selective promo-
tion of the substrate reaction while suppressing the product
reaction.[12]
Selective oxidation of alcohols to aldehydes is one of the
most important functional group transformations in organic
synthesis.[13] Photocatalytic reactions with TiO2-based cata-
lysts successfully promote selective alcohol oxidation when
using organic solvents.[14] Selective photocatalytic oxidation
in water is, however, difficult because the formed aldehyde
is decomposed by sequential photocatalytic reactions on the
TiO2 surface.[15] The highest aldehyde selectivity in water
has been achieved by the groups of Augugliaro and Palmisa-
no,[16] using low-crystalline TiO2 particles that are simply
prepared by hydrolysis of TiCl4 in water followed by aging
of the resulting gel. The aldehyde selectivity in the reaction
of benzyl alcohol, a typical aromatic alcohol, is only 42% at
50% alcohol conversion, although the reaction of 4-methox-
ybenzyl alcohol produces the corresponding aldehyde with
particularly high selectivity (72–74% at 50% alcohol con-
version). In addition, the catalytic activity of the catalysts is
much lower than that of crystalline TiO2.
[a] D. Tsukamoto, M. Ikeda, Dr. Y. Shiraishi, Prof. T. Hirai
Research Center for Solar Energy Chemistry
and Division of Chemical Engineering
Graduate School of Engineering Science
Osaka University
Toyonaka 560-8531 (Japan)
Fax : (+81)6-6850-6271
E-mail: shiraish@cheng.es.osaka-u.ac.jp
[b] Dr. T. Hara, Dr. N. Ichikuni
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering
Chiba University
Chiba 263-8522 (Japan)
[c] Dr. S. Tanaka
Department of Chemical, Energy
and Environmental Engineering
Kansai University
Suita 564-8680 (Japan)
9816
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FULL PAPER
The purpose of the present work was the development of
TiO2 photocatalytic systems that promote selective oxidation
of alcohols to aldehydes in water, while maintaining high
catalytic activity. It is known that the photocatalytic reactivi-
ty of organic compounds depends strongly on the extent of
adsorption onto the catalyst surface;[17] stronger adsorption
usually promotes reactivity enhancement. This implies that
one of the possible ways to enhance aldehyde selectivity is
the suppression of aldehyde adsorption onto the TiO2 sur-
face. The easiest way to achieve this is to coat the TiO2 sur-
face by metal oxide species; a different metal oxide layer, if
created on TiO2 particles, would lead to a decrease in the
amount of aldehyde adsorbed on the TiO2 surface and sup-
press sequential photocatalytic decomposition of aldehydes.
Several TiO2 particles coated
Results and Discussion
Catalyst preparation: Five kinds of WO3(x)/TiO2 catalysts
with
different
WO3
loadings
(x
(wt%)=
WO3/AHCTUNTGREG(NNUN WO3+TiO2)ꢁ100; x=1.4, 3.4, 7.6, 10.3, 18.5) were
synthesized by impregnation of tungstic acid (H2WO4) onto
the TiO2 particles followed by calcination according to a lit-
erature procedure[21] (see Experimental Section). Japan Ref-
erence Catalyst JRC-TiO-4 TiO2 particles (equivalent to De-
gussa P25; anatase/rutile=80:20) and the required amount
of H2WO4 were stirred in an ammonia solution at 353 K and
calcined at 673 K under O2, affording WO3(x)/TiO2 catalysts
as white powders. The properties of the catalysts are sum-
marized in Table 1. Figure 1 shows the diffuse reflectance
by metal oxide species such as
SiO2,[18] V2O5,[19] and MoO3,[20]
have been synthesized. In par-
ticular, the photocatalytic activ-
ity of TiO2 particles coated with
WO3 (WO3/TiO2) has been
studied extensively for their
ability to mediate decomposi-
tion of chloroaromatics in
water with O2.[21] The WO3/
TiO2 catalysts show higher ac-
tivity than pure TiO2 even
Table 1. Properties of catalysts and the results of photocatalytic oxidation of benzyl alcohol in water.[a]
Catalyst
SBET
[m2 gꢀ1
dp
Ebg
q
tirr
Benzaldehyde
select. [%][g]
CO2
ACHTUNGTRENNUNG[mmol]
[b]
G
]
[nm][c]
[eV][d]
[%][e]
[h][f]
1[h]
2
TiO2
54.0
56.5
58.2
53.1
49.7
44.4
23.7
23.8
24.0
24.1
25.0
27.2
3.13
3.06
3.06
3.02
3.02
3.00
0
15
34
58
76
89
9.0
5.5
3.3
5.0
5.7
13
28
47
56
53
55
28
61
55
54
121.5
60.2
12.3
6.7
9.1
7.7
63.7
4.8
6.2
WO
WO
WO
WO
WO
WO
3A
3A(3.4)/TiO2
3A(7.6)/TiO2
3A(10.3)/TiO2
3A(18.5)/TiO2
3A(7.6 wt%)+TiO2
3
G
4[i]
5
E
N
6
E
6.0
9.5
7[j]
8
N
WO3
3.0
127
2.80
24.0[k]
5.0
9[l]
10[m]
WO
3A
WO3A(7.6)/TiO2
C
5.0
7.4
though the area of the available [a] Reagents and conditions: benzyl alcohol (0.1 mmol), water (5 mL), catalyst (5 mg), O2 (1 atm), 298 K, l>
350 nm. [b] BET surface area. [c] Particle size of catalysts determined by dynamic light scattering analysis.
[d] Bandgap energies determined by a plot of the Kubelka–Munk function versus the energy of light absorbed
(Figure 1). [e] Surface coverage of TiO2 by WO3, determined by FTIR analysis (Figure 5). [f] The photoirradia-
tion time required for 50% alcohol conversion. [g] Calculated as [benzaldehyde formed]/[benzyl alcohol con-
TiO2 surface is reduced by the
WO3 coating. This is because
the charge separation between
eꢀ and h+ is facilitated by the
verted]ꢁ100. [h] (o-, m-, p-)Hydroxybenzyl alcohols (trace), (o-, m-, p-)hydroxybenzaldehydes (1.9 mmol), and
benzoic acid (20.1 mmol) were detected by GC analysis.[16a] The carbon balance {=100ꢁ[benzyl alcohol+ben-
zaldehyde+(o-, m-, p-)hydroxybenzyl alcohols+(o-, m-, p-)hydroxybenzaldehydes+benzoic acid+(CO2
formed)/7]/[initial amount of benzyl alcohol]} was determined to be 92%, in which nonvolatile or thermally-
degradable ring-opening products such as carboxylic acids are probably evolved as unidentified products.
[i] (o-, m-, p-)Hydroxybenzyl alcohols (2.7 mmol), (o-, m-, p-)hydroxybenzaldehydes (0.9 mmol), and benzoic
transfer of eꢀ photoformed on
the TiO2 particles to the surface
WO3 species.[22]
In the present work, the
WO3/TiO2 catalysts were em- acid (7.2 mmol) were detected by GC analysis. The carbon balance was 80%. [j] TiO2 (92.4 mg) and WO3
(7.6 mg) were mixed thoroughly and 5 mg of the mixture was used for reaction. [k] The benzyl alcohol conver-
sion was only 8% even after photoreaction for 24 h. [l] The 1st reuse of the catalyst (entry 4) after washing
with MeCN. [m] 2nd reuse.
ployed for photocatalytic oxida-
tion of alcohols in water. We
expected that the WO3 coating
would lead to a decrease in the
amount of aldehyde adsorbed
onto the TiO2 surface and suppress sequential decomposi-
tion, while retaining high catalytic activity. We clarified that
the WO3/TiO2 system successfully promotes the selective
production of aldehydes with higher catalytic activity than
pure TiO2. The aldehyde selectivity in benzyl alcohol oxida-
tion is 56% at 50% alcohol conversion, which is higher than
obtained in previously reported photocatalytic systems. Sur-
face analysis of the catalysts based on the Fourier-trans-
formed infrared (FTIR) spectroscopy, X-ray absorption
near-edge structure (XANES) spectroscopy, and X-ray pho-
toelectron spectroscopy (XPS) indicate that the high alde-
hyde selectivity is achieved by partial coverage of the TiO2
by WO3. The adsorption of photoformed aldehydes onto the
TiO2 surface is suppressed by the WO3 coating and the se-
quential reaction is suppressed significantly.
Figure 1. Diffuse reflectance UV/Vis spectra of respective catalysts:
a) TiO2, b) WO
N
3ACHTUNGTERNNUG(3.4)/TiO2, d) WO3ACHTUNGTNER(NUGN 7.6)/TiO2,
e) WO3A(10.3)/TiO2, f) WO CHTUGNTRENNUNG
C
3A(18.5)/TiO2, and g) WO3.
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Y. Shiraishi et al.
UV/Vis spectra of the catalysts. It can be seen that an in-
crease in WO3 loading leads to a red shift of the absorption-
edge of the catalysts, suggesting that WO3 is indeed loaded
on the TiO2 surface.[21]
diation time, along with an increase in the amount of CO2,
indicating that subsequent reaction of the aldehyde results
in decreased aldehyde selectivity. In contrast, with WO3-
AHCTUNGTERNGUN(N 7.6)/TiO2 (Figure 2b), the amount of CO2 scarcely increases
with time, and the aldehyde selectivity is almost unchanged
(ca. 55%).
Effect of WO3 loading on photocatalysis: The effect of WO3
loadings on selective alcohol oxidation in water was investi-
gated by studying the photoreaction of benzyl alcohol. The
reaction was performed by photoirradiation (l>350 nm) of
water (5 mL) containing catalyst (5 mg) and benzyl alcohol
(100 mmol) under an O2 atmosphere (1 atm) at 298 K.
Table 1 summarizes the photoirradiation time (tirr) required
for 50% benzyl alcohol conversion and the benzaldehyde
selectivity at this conversion. Pure TiO2 (Table 1, entry 1)
showed very low aldehyde selectivity (13%), whereas the
WO3/TiO2 catalysts (Table 1, entries 2–6) showed much
higher selectivity. The selectivity increased with WO3 load-
ing, and the catalysts with ꢁ7.6 wt% WO3 show >53% se-
lectivity, which is higher than the previously reported photo-
catalytic systems.[16]
During photocatalytic reaction with TiO2 in water, the al-
dehyde formed is sequentially decomposed to CO2 and
H2O.[15,16] As shown in Table 1 (entry 1), the amount of CO2
formed during the reaction with pure TiO2 was 122 mmol at
50% benzyl alcohol conversion (50 mmol benzyl alcohol re-
acted). In contrast, the WO3/TiO2 catalysts with ꢁ7.6 wt%
WO3 (entries 4–6) produced much less CO2 (less than
10 mmol), suggesting that sequential reaction of aldehyde is
significantly suppressed. Figure 2 shows the time-dependent
change in the amounts of substrate and product, and the
benzaldehyde selectivity during the reaction of benzyl alco-
To further clarify the reactivity of the aldehyde on TiO2
and WO3/TiO2 catalysts, benzaldehyde was used as the start-
ing material in the photocatalytic reactions. As shown in
Figure 3a, the aldehyde conversions on WO3/TiO2 are much
lower than that on TiO2, and catalysts with higher WO3
loadings suppressed the reaction more significantly. This
suggests that WO3 loading suppresses the subsequent de-
composition of the formed aldehydes and results in high al-
dehyde selectivity. As shown in Figure 3b, the amount of
CO2 formed during the reaction showed a similar trend.
This indicates that decomposition of the aldehyde initiates
degradation of the aromatic ring (complete decomposition
to CO2).
The WO3/TiO2 catalysts show higher catalytic activity for
alcohol oxidation than pure TiO2. As shown in Table 1, in-
creased WO3 loadings shorten the photoirradiation time re-
quired for 50% benzyl alcohol conversion. Among the cata-
lysts used, WO3ACHTNUTRGENNGU(3.4)/TiO2 and WO3ACHUTNGTREN(NNGU 7.6)/TiO2 showed better
activity; these catalysts attained 50% alcohol conversion
within 3.3 and 5.0 h photoirradiation, respectively, while
pure TiO2 required 9.0 h irradiation. Further WO3 loadings
(ꢁ10.3 wt%), however, decreased the catalytic activity. The
aldehyde selectivity and the catalytic activity data suggest
that WO3ACHTNUTRGENNG(U 7.6)/TiO2 shows the best catalytic performance.
As shown in Table 1 (entry 7), the physical mixture of TiO2
and 7.6 wt% WO3 shows much lower aldehyde selectivity
hol with TiO2 or WO3ACHTNURGTNEG(UN 7.6)/TiO2. In the case of TiO2 (Fig-
ure 2a), the aldehyde selectivity decreased with photoirra-
and catalytic activity than WO3ACTHUNGTRENNU(G 7.6)/TiO2 (entry 4). This sug-
gests that the WO3 loading on
the TiO2 surface is essential for
both high aldehyde selectivity
and catalytic activity.
Surface structure of the cata-
lysts: The TiO2 surface of WO3-
AHCTUNGRTEN(GNUN 7.6)/TiO2 is partially coated by
a WO3 layer. The impregnation
of H2WO4 on the TiO2 surface
followed by calcination leads to
the formation of a WO3 layer
consisting of branched chains of
pentahedral WO5 units and ter-
minal tetrahedral WO4 units,[23a]
as schematically shown in
Scheme 1. Figure 4 shows the
W L1- and L3-edge XANES
spectra of WO3/TiO2 catalysts;
the spectra for Na2WO4 and
WO3 powders are also shown as
reference spectra for the WO4
and WO5 units, respectively. In
the W L1-edge spectra (Fig-
Figure 2. Time-dependent change in (top) the benzaldehyde selectivity and (bottom) the concentrations of
(open circle) benzyl alcohol and (closed circle) benzaldehyde and (triangle) the amount of CO2 formed,
during photoreaction of benzyl alcohol in water with a) TiO2 and b) WO
tions are identical to those in Table 1.
3ACHTUNGTNER(NUNG 7.6)/TiO2 catalysts. Reaction condi-
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Photocatalytic Oxidation of Alcohols
FULL PAPER
Figure 4. Normalized A) W L1-edge and B) W L3-edge XANES spectra
of a) Na2WO4, b) WO
and e) WO3.
3ACHTUNGTRNENU(NG 3.4)/TiO2, c) WO3AHCTNURGTEG(NNUN 7.6)/TiO2, d) WO3ACHTNUGTRENUN(GN 18.5)/TiO2,
Figure 3. Time-dependent changes in a) the benzaldehyde conversion and
b) the amount of CO2 formed during the reaction of benzaldehyde in
water with TiO2 or WO3(x)/TiO2. Reagents and conditions: water (5 mL),
benzaldehyde (0.5 mmol), catalyst (5 mg), O2 (1 atm), 298 K, l>350 nm.
(Figure 4B). Na2WO4 (line a) shows a sharp absorption at
approximately 10210 eV, which can be assigned to tetrahe-
dral WO4 units.[23] In contrast, WO3 particles (line e) show a
broad absorption due to the small number of WO4 units.
The absorption of WO3/TiO2 catalysts (lines b–d) becomes
broader with increasing WO3 loading. This again suggests
that the catalysts with higher WO3 loadings contain larger
numbers of WO5 units than WO4 units. These data indicate
that, as shown in Scheme 1, the catalysts with lower WO3
loading contain narrow WO3 islands on the TiO2 surface,
and increases in the WO3 loading leads to a coalescence of
these islands (producing larger WO3 islands). These data
suggest that the TiO2 surface of the catalyst is covered by a
WO3 layer and that the coverage increases with WO3 load-
ing.
The percent coverage of the TiO2 surface by WO3, given
as q, can roughly be determined by diffuse reflectance FTIR
analysis. As shown in Figure 5, the strong absorption at
ꢀ1
ꢀ
3691 cm , assigned to the Ti OH species on the TiO2 sur-
Scheme 1. Proposed surface structure of WO3(x)/TiO2 catalysts.
face,[24] decreases with increasing WO3 loading due to the
adhesion of WO3. As shown in the inset of Figure 5, the q
value determined from the absorption decrease increases
with increasing WO3 loading; WO CHUTNGTREN(NUNG 7.6)/TiO2 has 58% cov-
erage and WO
(89%).
The catalyst surface was further characterized by TEM
and XPS analysis. As shown in Figure 6a and Figure 6b, cat-
alysts with ꢂ7.6 wt% WO3 have a transparent surface, indi-
cating that monolayer WO3 forms on the TiO2 surface.[25] In
ure 4A), Na2WO4 (line a) shows an intense pre-edge peak at
approximately 12102.5 eV, which was assigned to the tetra-
hedral WO4 units.[23] In contrast, WO3 particles (line e) show
a broad pre-edge peak that can be assigned to the pentahe-
dral WO5 units.[23a] As shown by the lines b–d, the peak in-
tensity of WO3/TiO2 becomes broader with increasing WO3
loadings, indicating that the catalysts with higher WO3 load-
ings contain larger numbers of WO5 units than WO4 units.
This result was also confirmed by the W L3-edge spectra
3A
3ACHTUNGTNER(NUNG 18.5)/TiO2 shows much higher coverage
contrast, WO3ACHTNUTRGNEUNG(18.5)/TiO2 shows dark flecks (Figure 6c), sug-
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9819

Y. Shiraishi et al.
Figure 7. Relationship between the intensity ratio of W 4d and Ti 2p sig-
nals (IW4d/ITi2p) and the WO3 loading, x, for WO3(x)/TiO2 catalysts, ob-
tained by XPS measurements.
>7.6 wt% WO3. This indicates that monolayer WO3 islands
form at ꢂ7.6 wt% WO3 (Scheme 1b and c), and that multi-
layer WO3 islands form at higher WO3 loadings
Figure 5. Diffuse reflectance FTIR spectra of TiO2 and WO3(x)/TiO2.
(Inset) Percent coverage (q) of TiO2 surface by WO3.
(Scheme 1d).[26] These data suggest that WO
3ACHTNUGTRNEUNG(7.6)/TiO2 cata-
lysts possessing high catalytic activity and aldehyde selectivi-
ty have surface structures consisting of an exposed TiO2 sur-
face (42%) and a monolayer WO3 surface (58%).
Mechanism for high catalytic activity: Photoirradiation of
WO3/TiO2 catalysts promotes excitation of both TiO2 and
WO3. The WO3 surface is, however, inactive for oxidation,
and the exposed TiO2 surface behaves as the oxidation site.
The inactivity of the WO3 layer for oxidation can be ex-
plained by the valence band potentials of TiO2 and WO3. As
shown in Scheme 2, the valence band potential of WO3
(+3.20 V vs. NHE) is more positive than that of TiO2
(+2.95 V vs. NHE),[27] indicating that, in the WO3/TiO2
system, h+, even if formed on the photoexcited WO3, is
transferred exothermically to TiO2.[22a] The valence band po-
tentials of WO3 and TiO2 suggest that the WO3 layer on the
WO3/TiO2 catalyst is not an oxidation site, leaving the ex-
posed TiO2 surface as the functioning oxidation site.
As shown in Table 1, the WO3/TiO2 catalysts show much
higher activity for alcohol oxidation than pure TiO2. This is
because the photogenerated eꢀ on TiO2 is transferred to the
WO3 layer, which promotes efficient charge separation be-
tween the eꢀ and h+ pairs.[22] As shown in Scheme 2, the
Figure 6. TEM images of catalysts: a) WO
and c) WO3A(18.5)/TiO2.
3ACHTUNGTERNNUG(3.4)/TiO2, b) WO3ACHTUNGTNER(NUGN 7.6)/TiO2,
CHTUNGTRENNUNG
gesting that the adhesion of larger amounts of WO3 produ-
ces multilayered WO3. XPS analysis also confirms this. As
shown in Figure 7, the intensity ratio of the W 4d and Ti 2p
signals (IW4d/ITi2p) increases linearly with increasing WO3
loading at ꢂ7.6 wt% WO3, but becomes gentler at
Scheme 2. Valence and conduction band potentials for TiO2 and WO3.
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Photocatalytic Oxidation of Alcohols
FULL PAPER
conduction band potential of TiO2 (ꢀ0.18 V vs. NHE) is
more negative than that of WO3 (+0.40 V vs. NHE),[27] sug-
gesting that the eꢀ formed on TiO2 is transferred exothermi-
cally to the WO3 layer. The eꢀ on the conduction band of
the WO3 layer is not consumed by one-electron reduction of
ꢀ
Cꢀ
O2 because the reduction potential (O2 +e !O2 , ꢀ0.13 V
vs. NHE)[28] is more negative than the conduction band po-
tential of WO3 (Scheme 2). The eꢀ is consumed by multi-
electron reduction of O2, which occurs exothermically be-
cause the reduction potentials (O2 +2H+ +2eꢀ!H2O2,
+0.68 V vs. NHE; O2 +4H+ +4eꢀ!2H2O, +1.23 V vs.
NHE)[29] are more positive than the conduction band poten-
tial of WO3 (Scheme 2). This mechanism is supported by the
Figure 9. The amount of H2O2 formed during photocatalytic oxidation of
benzyl alcohol in water with respective catalysts. H2O2 concentration was
determined by titration with KMnO4. Reagents and conditions: water
(5 mL), benzyl alcohol (0.1 mmol), catalyst (5 mg), O2 (1 atm), 298 K, l>
350 nm, irradiation time: 3 h.
Cꢀ
analysis of O2 and H2O2 formed during reaction of benzyl
alcohol. Figure 8a shows the electron spin resonance (ESR)
spectra obtained upon photoirradiation of a benzyl alcohol
solution with respective catalysts in the presence of 5,5-di-
methyl-1-pyrroline N-oxide (DMPO), a spin-trapping re-
agent. All of the systems show distinctive signals that can be
Table 1, the surface area of TiO2 is 54.0 m2 gꢀ1, whereas that
of commercial WO3 is only 3.0 m2 gꢀ1, suggesting that the
surface area of the WO3 layer on TiO2 is much larger. Sever-
al literature reports have shown that WO3 materials with
large surface areas show higher catalytic activity than com-
mercial WO3 particles.[31] The larger surface area of the
WO3 layer on TiO2 probably promotes efficient multi-elec-
tron reduction of O2. As shown in Table 1 (entries 5 and 6),
catalysts with higher WO3 loadings (ꢁ10.3 wt%) show de-
creased catalytic activity, which is consistent with the
amount of H2O2 formed (Figure 9). Coverage of the TiO2
surface by the WO3 layer increases with increasing WO3
loading, and catalysts with ꢁ10.3 wt% WO3 have very large
coverage (ꢁ76%). This leads to a decrease in the amount
of exposed TiO2 surface (decrease in the oxidation site) and,
hence, results in decreased catalytic activity. Choosing a cat-
alyst with an appropriate WO3 coverage is therefore impor-
tant for high catalytic activity.
Cꢀ
assigned to the DMPO–O2 spin adduct (aN =12.9 G; abH =
10.4 G; agH =1.4 G; g=2.0059);[30] however, the signal inten-
sity decreases with increasing WO3 loading (Figure 8b). This
indicates that, on WO3/TiO2, the eꢀ on the conduction band
of the WO3 layer is not typically consumed by one-electron
reduction of O2. Figure 9 shows the amount of H2O2 formed
during the reaction. The amount of H2O2 increases with in-
creasing WO3 loading, and catalysts with 3.4–7.6 wt% WO3
produce larger amounts of H2O2. The obtained profile of
H2O2 generation is consistent with the oxidation activity of
benzyl alcohol (Table 1). These data indicate that the eꢀ on
the conduction band of WO3 is consumed by multi-electron
reduction of O2.
It is known that commercially-available WO3 particles do
not promote multi-electron reduction of O2 well,[29] as also
shown in Figure 9; however, the present WO3/TiO2 catalysts
do promote such a reduction process. The promotion of
multi-electron reduction on WO3/TiO2 is probably due to
the large surface area of the WO3 layer. As shown in
Mechanism for high aldehyde selectivity: The high aldehyde
selectivity of WO3/TiO2 catalysts arises because the WO3
loading leads to a decrease in the area of exposed TiO2 sur-
face. This leads to a decrease in the amount of aldehyde ad-
sorbed onto the TiO2 surface and suppresses sequential de-
composition, as schematically shown in Scheme 3. Adsorp-
tion experiments were carried out to clarify the adsorption
properties of aldehyde onto the catalyst surface. The respec-
tive catalyst (5 mg) was added to water (5 mL) containing
different amounts of benzaldehyde (0.01–1.0 mmolLꢀ1) and
stirred at 298 K for 3 h. Figure 10A shows the adsorption
isotherm, in which Ce is the equilibrium concentration of al-
dehyde in solution and Q is the amount of aldehyde ad-
sorbed on the catalysts, respectively. All of the curves show
Langmuir-type profiles, and the Q values for WO3/TiO2 (b–
f) are higher than that for pure TiO2 (Figure 10Aa). Fig-
ure 10Bshows the first-order linear transform of the iso-
therm obtained by applying Equation (1),[32] in which Qmax
and K are the total number of adsorption sites on the cata-
lysts and the adsorption equilibrium constant, respectively.
Figure 8. a) ESR spectra of DMPO-O2·ꢀ spin adduct signals obtained by
photoirradiation of a benzyl alcohol solution containing DMPO with
TiO2 (x=0) and WO3(x)/TiO2 (see Experimental section). b) The relative
intensity obtained by double integration of the spin adduct signals be-
tween 3405–3455 G, for which the intensity obtained with TiO2 (x=0)
was set as 1.
Chem. Eur. J. 2011, 17, 9816 – 9824
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9821

Y. Shiraishi et al.
larger than that for pure TiO2 and become larger with in-
creasing WO3 loading. This suggests that the WO3 layer has
higher adsorption affinity with benzaldehyde than the TiO2
surface.
To clarify the adsorption profile of aldehydes on the ex-
posed TiO2 surface of WO3/TiO2, the number of adsorption
sites on the exposed TiO2 surface (QTiO ) and the WO3 sur-
2
face (QWO ) were determined. The QTiO values were ob-
3
2
tained from the Qmax value for pure TiO2 (4.33ꢁ
10ꢀ2 mmolgꢀ1), using the WO3 loading (x) and the surface
coverage (q) in Equation (2) and the QWO values for WO3/
3
TiO2 can, therefore, be expressed by Equation (3)
QTiO ¼ 4:33 ꢃ 10ꢀ2 ꢃ ð1ꢀx=100Þ ꢃ ð1ꢀq=100Þ
ð2Þ
ð3Þ
2
Q
WO3
¼ QmaxꢀQ
TiO2
As summarized in Table 2, the QTiO values decrease with
Scheme 3. Schematic representation of surface reactions on a) TiO2 and
b) WO3/TiO2 catalysts.
2
increasing WO3 loading, along with an increase in QWO . The
3
percentage of aldehyde adsorbed on the TiO2 surface in the
total amount of the adsorbed aldehyde, defined as QTiO
/
2
(QTiO +QWO ), decreases with increasing WO3 loading. This
2
3
suggests that the WO3 loading leads to a decrease in the al-
dehyde adsorption on the exposed TiO2 surface, at the same
time as increasing the adsorption onto the WO3 surface,
which is inactive for aldehyde decomposition. This suppress-
es sequential decomposition of the aldehyde on the TiO2
surface. As shown in Figure 3, photodecomposition of ben-
zaldehyde is suppressed with increasing WO3 loading, which
is consistent with the adsorption results. These data indicate
that the increased aldehyde selectivity during photocatalytic
alcohol oxidation (Table 1) is due to decreased aldehyde ad-
sorption on the TiO2 surface.
Selective oxidation of other alcohols: The present photoca-
talytic system with WO3ACHTNUGTRNEUNG(7.6)/TiO2 is capable of selective ox-
idation of other benzylic alcohols (Table 3). As shown in
entry 1, the aldehyde selectivity in the reaction of 4-methox-
ybenzyl alcohol with pure TiO2 is only 23% at 50% alcohol
conversion. In contrast, WO
hyde with >54% selectivity (Table 3, entries 2 and 3). In ad-
dition, WO3A(7.6)/TiO2 successfully oxidizes a range of substi-
3ACHTUNGRTEN(NUNG 7.6)/TiO2 produces the alde-
CTHUNGTRENNUNG
tuted benzylic alcohols to the corresponding aldehydes with
higher yields than achieved with pure TiO2.
Figure 10. A) Adsorption isotherms and B) the linear-transformed Lang-
muir plots of benzaldehyde obtained by adsorption experiments in water
at 298 K for 3 h with various catalysts: a) TiO2, b) WO3ACHTUNTRGNE(UNG 1.4)/TiO2,
It must be noted that the WO
ble for further reactions. The WO
for photocatalytic oxidation of benzyl alcohol was recovered
(7.6)/TiO2 catalyst is reusa-
CTHUNGTRENNUNG
c) WO
3A
f) WO3ACHTUNGTRENNUNG(18.5)/TiO2.
Table 2. Adsorption parameters for benzaldehyde on various catalysts.[a]
Catalyst
Qmax
[mmolgꢀ1
K
A
QTiO
A
QWO
G
QTiO /(QTiO +QWO
ꢁ100 [%]
)
2
3
2
2
3
G
]
[Lmmolꢀ1
]
[mmolgꢀ1
]
[mmolgꢀ1
]
Ce=Q ¼ 1=KQmax þ Ce=Qmax ð1Þ
TiO2
4.33ꢁ10ꢀ2
4.87ꢁ10ꢀ2
5.19ꢁ10ꢀ2
5.59ꢁ10ꢀ2
6.04ꢁ10ꢀ2
6.73ꢁ10ꢀ2
4.67
5.12
6.97
7.57
7.58
8.62
4.33ꢁ10ꢀ2
3.63ꢁ10ꢀ2
2.76ꢁ10ꢀ2
1.68ꢁ10ꢀ2
0.93ꢁ10ꢀ2
0.39ꢁ10ꢀ2
WO
WO
WO
WO
WO
3A
3A(3.4)/TiO2
3A(7.6)/TiO2
3A(10.3)/TiO2
3A(18.5)/TiO2
1.24ꢁ10ꢀ2
2.43ꢁ10ꢀ2
3.91ꢁ10ꢀ2
5.11ꢁ10ꢀ2
6.34ꢁ10ꢀ2
75
53
30
15
6
Table 2 summarizes these
values, which were determined
from the slope and intercept of
the line in Figure 10B. The Qmax
CTHUNGTRENNUNG
CTHUNGTRENNUNG
CTHUNGTRENNUNG
and K values for WO3/TiO2 are [a] Reagents and conditions: water (5 mL), benzaldehyde (0.05–5.0 mmol), catalyst (5 mg), 298 K, 3 h.
9822
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9816 – 9824

Photocatalytic Oxidation of Alcohols
FULL PAPER
Table 3. Photocatalytic oxidation of various alcohols on TiO2 and WO3-
the semiconducting surface, may contribute to the develop-
ment of photocatalytic systems promoting selective organic
transformations.
ACHTUNGTRENNUNG
(7.6)/TiO2.[a]
R
Catalyst
TiO2
tirr
Aldehyde
CO2
ACHTUNGTRENNUNG
Experimental Section
[h][b]
select [%][c]
Materials: All reagents were purchased from Wako, Tokyo Kasei, or
Sigma–Aldrich and used without further purification. Water was purified
by the Milli-Q system. Japan Reference Catalyst JRC-TIO-4 TiO2 was
kindly supplied by the Catalysis Society of Japan. The WO3(x)/TiO2 cata-
lysts with different WO3 loadings [x (wt%)=WO /
x=1.4, 3.4, 7.6, 10.3, 18.5] were synthesized by an impregnation method
as follows;[21] TiO2 (1 g) was stirred in an ammonia solution (1.0 molLꢀ1
50 mL) containing an appropriate amount of H2WO4. The obtained solu-
tion was dried at 353 K under vigorous stirring. The powders formed
were calcined at 673 K for 2 h under O2 flow (0.5 Lminꢀ1), affording
white powders of catalysts. WO3 was purchased from Kojundo Chemical
Laboratory Co. and used as a reference.
1
2
p-methoxy
8.0
6.0
18.5
5.5
4.0
5.0
3.5
6.0
2.5
9.5
4.8
6.0
1.5
23
54
62
10
42
19
44
32
60
23
50
23
41
108
8.4
13.0
55.2
4.3
34.3
1.6
52.1
3.9
71.6
5.5
33.3
1.8
WO
WO
3A
3[d]
4
3A(7.6)/TiO2
p-methyl
m-methyl
p-chloro
m-chloro
p-bromo
TiO2
WO3ACTHNGUTER(NNUG 7.6)/TiO2
TiO2
WO3ACTHNUTRGNE(NUG 7.6)/TiO2
TiO2
WO3ACTHNGUTER(NNUG 7.6)/TiO2
TiO2
WO3ACTHNGUTER(NNUG 7.6)/TiO2
3 ACHTUNGTREN(UNNG TiO2+WO3)ꢁ100;
5
6[e]
7[e]
8
,
9
10
11
12[e]
13[e]
TiO2
WO3ACTHNUTRGNE(NUG 7.6)/TiO2
Photoreaction: Each catalyst (5 mg) was suspended in water (5 mL) con-
taining the substrate within a Pyrex glass tube (f 10 mm; capacity,
20 mL). The tube was sealed with a rubber septum cap. The catalyst was
dispersed by ultrasonication for 5 min and O2 was bubbled through the
solution for 5 min. The solution was photoirradiated with magnetic stir-
ring by a 450 W high-pressure mercury lamp (USHIO Inc.), filtered
through a glass filter to give light wavelength of l>350 nm. The light in-
tensity at 350–400 nm was 2.81 mWcmꢀ2. The temperature of the solution
was kept at 298 K in a water bath during photoirradiation. After photoir-
radiation, the gas-phase product was analyzed by GC-TCD (Shimadzu;
GC-14B). The catalyst was recovered by centrifugation and washed with
MeCN (5 mL). The combined solution was analyzed by GC-FID (Shi-
madzu; GC-1700); the substrate and product concentrations were deter-
mined with authentic samples. Identification of the products was per-
formed by GC-MS (Shimadzu; GC–MS-QP5050 A) analysis.
[a] Reagents and conditions: water (5 mL), alcohol (0.1 mmol), catalyst
(5 mg), O2 (1 atm), 298 K, l>350 nm. [b] The photoirradiation time re-
quired for 50% alcohol conversion. [c] Calculated as [aldehyde formed]/
[alcohol converted]ꢁ100. [d] 0.5 mmol of alcohol was used. [e] 0.05 mmol
of alcohol was used due to low solubility of the alcohol in water.
by simple centrifugation. As shown in Table 1 (entries 9 and
10), the catalyst, when reused for further reaction after
washing with MeCN, shows almost the same aldehyde selec-
tivity and catalytic activity as the virgin catalyst (Table 1,
entry 4). This indicates that the catalyst can be reused for se-
lective oxidation of alcohols at least twice without loss of se-
lectivity and activity.
Analysis: The total amounts of W and Ti in the catalysts were deter-
mined with an X-ray fluorescence spectrometer (Seiko Instruments, Inc.;
SEA2110). Diffuse reflectance UV/Vis spectra were measured with a
UV/Vis spectrophotometer (Jasco Corp.; V-550 with Integrated Sphere
Apparatus ISV-469) with BaSO4 as a reference. FTIR spectra were mea-
sured with an infrared spectrophotometer (Jasco Corp.; FTIR-610) using
CaF2 as a reference. Particle size distribution was determined with a
Horiba LB-500 dynamic light-scattering particle size analyzer. BET sur-
face area was measured at 77 K using an AUTOSORB-1-C/TCD ana-
lyzer (Yuasa Ionics Co., Ltd.). The W L-edge XANES spectra were mea-
sured with the apparatus at the NW10A, Photon Factory (PF) at the
High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
(KEK-PF, proposal No. 2009G069). Synchrotron radiation emitted from
a 6.5 GeV storage ring with a Si (311) double crystal monochromator.
TEM images were recorded with a JEOL JEM-2010 microscope at an ac-
celeration voltage of 200 kV. XPS measurement was performed with a
JEOL JPS-9000MX spectrometer using MgKa radiation as the energy
source.
Conclusion
The WO3/TiO2 catalysts promote selective oxidation of alco-
hols to aldehydes in water with O2 under photoirradiation at
l>350 nm. Catalysts containing approximately 8 wt% WO3
show high catalytic activity and possess aldehyde selectivity
much higher than the previously reported photocatalytic sys-
tems. The high activity is due to the transfer of eꢀ from the
conduction band of TiO2 to the surface WO3. This leads to a
charge separation between eꢀ and h+ and promotes efficient
alcohol oxidation on the TiO2 surface. The high aldehyde se-
lectivity is due to a decrease in the available TiO2 surface
due to the WO3 coating. This suppresses further reaction of
the formed aldehydes on the TiO2 surface and results in
high aldehyde selectivity. It is known that several catalytic
(nonphotocatalytic) systems achieve selective alcohol oxida-
tion in water with very high yields.[33] The conversion and al-
dehyde selectivity of the present photocatalytic system is
much lower than those of the nonphotocatalytic systems;
therefore, further improvement of catalytic performance is
necessary for practical application. Nevertheless, the basic
concept presented here, which is based on the creation of
metal oxide surface behaving as an electron acceptor site on
ESR measurement: ESR spectra were recorded at the X-band with a
Bruker EMX-10/12 spectrometer with a 100 kHz magnetic field modula-
tion at a microwave power level of 10.5 mW; microwave power saturation
of the signals did not occur.[12] The magnetic field was calibrated using
1,1’-diphenyl-2-picrylhydrazyl (DPPH) as standard. The measurement
was carried out as follows:[30] each catalyst (0.5 gLꢀ1) was suspended in
water containing benzyl alcohol (10 mmolLꢀ1
(100 mmolLꢀ1
and dispersed well by ultrasonication. An aliquot
)
and DMPO
)
(100 mL) of the suspension and DMSO (900 mL) were introduced into a
flat ESR cell [10ꢁ20ꢁ0.3 mm (path length)], and O2 was bubbled
through the solution for 1 min. The cell was placed in the ESR sample
cavity and photoirradiated using a 500 W Xe lamp through a glass filter
to give light wavelengths of l>350 nm at RT. After photoirradiation for
1 min, the irradiation was turned off and the measurement was started
immediately.
Chem. Eur. J. 2011, 17, 9816 – 9824
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www.chemeurj.org
9823

Y. Shiraishi et al.
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Acknowledgements
We thank the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan (MEXT) for a Grant-in-Aid for Scientific Research (Nos.
20360359 and 23360349). D.T. thanks the Japan Society for Promotion of
Science (JSPS) for Young Scientist and the Global COE Program
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Received: January 17, 2011
Revised: April 19, 2011
Published online: July 6, 2011
9824
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 9816 – 9824