Selective photocatalytic oxidation of benzyl alcohol and its derivatives into corresponding aldehydes by molecular oxygen on titanium dioxide under visible light irradiation

Article abstract of DOI:10.1016/j.jcat.2009.06.018

The photocatalytic oxidation of benzyl alcohol and its derivatives, such as 4-methoxybenzyl alcohol, 4-chlorobenzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alcohol, 4-(trifluoromethyl)benzyl alcohol, and 4-tertiary-butylbenzyl alcohol, into corresp

Full text of DOI:10.1016/j.jcat.2009.06.018

Journal of Catalysis 266 (2009) 279–285
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective photocatalytic oxidation of benzyl alcohol and its derivatives into
corresponding aldehydes by molecular oxygen on titanium dioxide under
visible light irradiation
a
a
b
a
Shinya Higashimoto a, , Naoya Kitao , Norio Yoshida , Teruki Sakura , Masashi Azuma ,
*
Hiroyoshi Ohue a, Yoshihisa Sakata b
a College of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan
b Graduate School of Science and Engineering, Yamaguchi University, 2-16-1 Tokiwadai, Ube 755-8611, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The photocatalytic oxidation of benzyl alcohol and its derivatives, such as 4-methoxybenzyl alcohol, 4-
chlorobenzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alcohol, 4-(trifluoromethyl)benzyl alcohol,
and 4-tertiary-butylbenzyl alcohol, into corresponding aldehydes proceeded at high conversion and
selectivity on a TiO2 photocatalyst under O2 atmosphere. The reaction was confirmed to proceed under
irradiation with both UV-light and visible light. In particular, the visible light response was observed
to be attributed to a characteristic surface complex formed by the adsorption of a benzyl alcoholic com-
pound on the TiO2 surface. Here, the properties of the surface complex as the active center for this selec-
tive photocatalytic oxidation and the mechanism behind the reaction have been investigated.
Ó 2009 Elsevier Inc. All rights reserved.
Received 2 May 2009
Revised 18 June 2009
Accepted 19 June 2009
Available online 19 July 2009
Keywords:
Selective photocatalytic oxidation
Titanium dioxide
Alcohol oxidation
Benzyl alcohol
Benzaldehyde
Visible light
Quantum yields
1. Introduction
visible light. They succeeded in the partial oxidation of various
kinds of alcohols into corresponding aldehydes on ruthenium
Aromatic aldehydes have attracted a great deal of attention as
important materials for the production of fine chemicals for such
applications as fragrances or flavorings and this work focuses on
an approach to their effective production. Two different processes
for the formation of aldehydes were investigated in order to devel-
op highly selective catalytic activity. One is the direct catalytic
hydrogenation process of carboxylic acids into corresponding
aldehydes [1–3] and the other is the direct selective oxidation of
alcohols into corresponding aldehydes [4,5].
Yamaguchi and Mizuno [4] have recently reported that a sup-
ported ruthenium catalyst, easily prepared by the treatment of
RuCl3 with c-Al2O3, is an efficient heterogeneous catalyst for the
oxidation of various kinds of alcohols in the presence of O2. In an-
other case, Tashiro et al. [5] have reported that various salicylalde-
hyde derivatives were produced from corresponding alcohols in
good to excellent yields by aerobic oxidation using a ruthenium
salen nitrosyl complex as a photocatalyst under irradiation with
catalysts.
Titanium dioxides (TiO2) are a promising nano-sized material in
photochemical applications [6–9] such as H2 production by water
splitting, the degradation of volatile organic compounds (VOCs),
organic synthesis, dye-sensitized solar cells, and super-hydrophilic
materials. Since the band gap of TiO2 with an anatase structure is
ca. 3.2 eV, it can only be activated by UV-light; however, only a
low percentage reaches the earth’s surface from the sun. In order
to utilize as much clean and safe solar energy as possible, the
development of visible light-sensitive nano-photocatalysts is
essential. In the case of organic synthesis, the selective photocata-
lytic oxidation of aromatic compounds in the presence of O2 under
only UV-light irradiation has previously been reported on TiO2 in
the gas phase [10], in acetonitrile [11,12], or in aqueous solution
[13–17].
Here, we have investigated the selective photocatalytic oxida-
tion of benzyl alcohol and its derivatives on TiO2 in the presence
of O2 under irradiation with both UV-light and visible light. In par-
ticular, we have focused on the mechanisms behind the selective
photocatalytic oxidation of benzyl alcohol into benzaldehyde on
the TiO2 surface under visible light irradiation.
* Corresponding author. Fax: +81 6 69572135.
E-mail address: higashimoto@chem.oit.ac.jp (S. Higashimoto).
0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.06.018

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S. Higashimoto et al. / Journal of Catalysis 266 (2009) 279–285
100
50
0
2. Experimental
Photo-oxidations of benzyl alcohol and its derivatives were per-
formed on TiO2 photocatalysts at room temperature in the pres-
ence of O2. The photocatalysts (50 mg each) were dispersed in
1.5
1
(c)
acetonitrile solution (10 mL) involving the substrates (50 lmol)
in a test tube (volume: 20 mL) made of pyrex glass and the gas
phase was purged by O2 at 298 K. The following substrates were
used in this work: benzyl alcohol, 4-methoxybenzyl alcohol, 4-
chlorobenzyl alcohol, 4-nitrobenzyl alcohol, 4-methylbenzyl alco-
hol, 4-(trifluoromethyl) benzyl alcohol, 4-tertiary-butylbenzyl
0.5
(d)
(b)
alcohol, 4-hydroxybenzyl alcohol, and
investigation of the kinetic isotope effect.
a, a-d2 benzyl alcohol for
(a)
0
300
The photo-reaction cell was set at a photo-intensity of ca.
0.80 mW cmꢀ2 at 365 nm emitted from a black light measured
by a CSU-365 radiometer (Cosmo Bio Co., Ltd.) as well as at ca.
1.8 ꢁ 104 lux emitted from a blue LED lamp measured by an illumi-
nation meter, TMS 870 (TASCO Japan Co., Ltd.).
400
500
Wavelength / nm
600
700
Fig. 1. UV–Vis absorption spectra of (a) TiO2 by itself; (b) benzyl alcohol-adsorbed
TiO2; energy distribution emitted (c) from the black light; and (d) from the LED
lamp used in this experiment.
In order to estimate the quantum yields, the photo-oxidation of
benzyl alcohol into benzaldehyde was performed on TiO2 (10 mg)
in acetonitrile solution (2 mL) involving 10 lmol of benzyl alcohol
neither methanol nor benzene-adsorbed TiO2 exhibited absorption
in the visible region, suggesting that the absorption in the visible
region is unique on the benzyl alcohol-adsorbed TiO2. From these
results, absorption in the visible region of the surface complex is
expected to lead to photocatalytic reactions under visible light irra-
diation. Photocatalytic reactions were, therefore, carried out under
two different light sources: irradiation with UV-light emitted from
the black light having an energy distribution as shown in Fig. 1c;
and irradiation with visible light emitted from the LED lamp having
an energy distribution as shown in Fig. 1d.
in a quartz cell of 10 ꢁ 10 ꢁ 44 mm using a 500 W xenon lamp
through a monochromator (Koken, SG-100). The resolution of each
photo-irradiated wavelength was ca. 13.6 nm and the photo-en-
ergy was measured by a power-meter (Ophir, ORION/PD). After
the reaction, the catalysts were immediately separated from the
solution by filtration through a 0.20 lm membrane filter (Dis-
mic-25, Advantec). The solution was then analyzed by HPLC (Shi-
madzu LC10ATVP, UV–Vis detector, column: Chemcopak, mobile
phase: a mixture of acetonitrile and 1.0% formic acid aqueous solu-
tion) and the gas phase was analyzed by GC (Ohkura, Model-802,
column: porapak Q).
The infrared spectra of the adsorbed benzyl alcohol were exam-
ined by an IR transmission method. A self-supporting pellet
(50 mg) of the TiO2 photocatalyst was placed in an in situ infrared
cell connected to a closed gas circulation system which was
equipped with a vacuum line. The pellet was oxidized under O2
at 673 K for 6 h and then evacuated at 523 K for 30 min as a pre-
treatment in the cell. The benzyl alcohol used in the infrared exper-
iment was of analytical grade with a purity level of more than 99%.
The adsorption of benzyl alcohol was carried out by introducing a
well-evacuated benzyl alcohol vapor (ca. 25 Pa) into the cell at
298 K and the gas phase was then evacuated after adsorption
was completed.
3.2. Photocatalytic oxidation of benzyl alcohol under UV-light
irradiation
The oxidation of benzyl alcohol on a TiO2 photocatalyst as a
function of time under the irradiation with UV-light emitted
from the black light is shown in Fig. 2. It was confirmed that
the oxidation reaction does not take place under photo-irradia-
tion without a TiO2 photocatalyst nor with a TiO2 photocatalyst
without irradiation, i.e., both TiO2 and irradiation are required
in combination for the oxidation reaction to occur. Before irradi-
ation, a decrease in the amount of benzyl alcohol originating
from its adsorption on TiO2 was observed. Photo-irradiation
The infrared spectra were recorded with a FT-IR (JASCO FT/IR
620) and measured with 100 scans at the resolution of 4 cmꢀ1
The spectrum of the adsorbed benzyl alcohol was obtained by
subtracting the spectrum of TiO2 from that of the benzyl alcohol-
adsorbed TiO2.
.
50
40
30
20
10
0
100
80
60
40
20
0
(e)
(b)
3. Results and discussion
3.1. Optical properties of benzyl alcohol-adsorbed TiO2
(a)
In order to understand the optical properties, the UV–Vis
absorption spectra of (a) TiO2 by itself and (b) the benzyl alco-
hol-adsorbed TiO2 were investigated and the results are shown in
Fig. 1. It was observed that TiO2 by itself exhibits absorption only
in the UV region with a band-gap transition at ca. 385 nm
(3.2 eV), as shown in Fig. 1a. In contrast, as can be seen in
Fig. 1b, it was confirmed that absorption in the visible region ap-
peared in the spectrum for benzyl alcohol-adsorbed TiO2. This
absorption is due to the surface complex between TiO2 and benzyl
alcohol, and is tentatively assigned to the charge transfer from
benzyl alcohol to TiO2 on the surface complex. On the other hand,
(c), (d)
0
10
20
Time / min
30
40
50
Fig. 2. Photocatalytic oxidation of benzyl alcohol on TiO2 (50 mg) under irradiation
with UV-light emitted from the black light. The initial amount of benzyl alcohol
added in the reaction cell was 50 lmol. Amounts of (a) benzyl alcohol; (b)
benzaldehyde; (c) benzoic acid; (d) CO2; and (e) percentage of total organic
compounds evolved in solution is plotted.

S. Higashimoto et al. / Journal of Catalysis 266 (2009) 279–285
281
was carried out after confirming the equilibrium of benzyl alco-
considered to be ca. 250
l
mol. When photo-oxidation was carried
hol adsorption. As shown in Fig. 2, upon UV-light irradiation, the
amount of benzyl alcohol decreased with time. In contrast, benz-
aldehyde is selectively produced while only negligible amounts
of CO2 or benzoic acid are formed. It was confirmed that the
reactions immediately cease when the light is turned off. Subse-
quently, after the light was turned on again, the reactions imme-
diately proceeded. The yield of benzaldehyde reached >95% and
the carbon balance in the liquid phase was >95% after photo-irra-
diation for 50 min. According to a previous report [11], the pho-
tocatalytic oxidation of benzyl alcohol led to the formation of
benzaldehyde and benzoic acid in addition to unidentified minor
compounds under full arc emitted from a 450-W medium pres-
sure Hg lamp. Therefore, the black light having an energy profile
as shown in Fig. 1 (c) seems to be suitable for the selective pho-
tocatalytic oxidation reaction. These results clearly demonstrate
that the photocatalytic oxidation of benzyl alcohol to benzalde-
hyde selectively proceeds in the presence of O2 on a TiO2 photo-
catalyst under UV-light irradiation.
out in acetonitrile solution (10 mL) involving 10 mmol of benzyl
alcohol on TiO2 under irradiation with visible light emitted from
the LED lamp for 36 h, the photo-formed benzaldehyde was ca.
2.2 mmol, i.e., the TON was estimated to be ca. 9, clearly indicating
that this reaction proceeds photocatalytically. From the results
shown in Figs. 1 and 3, selective photocatalytic oxidation under
visible light irradiation was observed to proceed on an active sur-
face complex initiated by the adsorption of benzyl alcohol on the
TiO2 surface involving OH groups.
In order to understand the states of benzyl alcohol-adsorbed
TiO2 at 298 K, the infrared spectra were measured and the results
are shown in Fig. 4. The infrared spectrum of the benzyl alcohol
molecules can be clearly confirmed from Fig. 4a [20]. When the
spectrum of the benzyl alcohol adsorbed on TiO2, as shown in
Fig. 4b, is compared with that of the benzyl alcohol by itself in
Fig. 4a, the characteristic IR bands for benzyl alcohol adsorbed on
TiO2 are shown to be substantially the same as those for the benzyl
alcohol molecules. Moreover, the following unique features caused
by the interaction between the benzyl alcohol molecules and TiO2
surface are observed in Fig. 4a: (i) a downward negative band at
3.3. Photocatalytic oxidation of benzyl alcohol under visible light
irradiation
3715 cmꢀ1 assigned to
served; (ii) instead of a negative band at 3715 cmꢀ1, a very broad
band attributed to
(OH) appears between 3500 and 2500 cmꢀ1
m(OH) of the surface hydroxyl group is ob-
Fig. 3 shows the oxidation of benzyl alcohol on the TiO2 photo-
catalyst as a function of time under irradiation with visible light
emitted from the LED lamp. The photocatalytic oxidation of benzyl
alcohol took place on TiO2 under visible light and this reaction was
completed within 240 min, as shown in Fig. 3. Moreover, the yield
of benzaldehyde reached ca. 95% and the carbon balance in the li-
quid phase was ca. 95% after photo-irradiation for 240 min. In or-
der to clarify the contribution of the surface states of TiO2 on the
photocatalytic oxidation, the reactions were carried out on fluori-
nated TiO2 (F-TiO2) on which the surface OH was exchanged to
fluoride by diluted HF aqueous solution [18]. Although benzyl alco-
hol-adsorbed F-TiO2 exhibits absorption in the visible region, the
photocatalytic activity drastically decreased under visible light
irradiation. This result suggests that the surface OH groups play a
significant role in the photocatalytic oxidation reaction.
m
;
and (iii) a band intensity at 1615 cmꢀ1 attributed to the phenyl
ring vibration increases markedly. These characteristics indicate
that the benzyl alcohol molecules interact with the surface hydro-
xyl group by the adsorption on the TiO2 surface. Taking the previ-
ous results in the interaction of benzene with surface OH group of
TiO2 into consideration [21], the CH2OH group and/or phenyl ring
of the benzyl alcohol in the form of molecules interacts with the
surface OH group of TiO2.
3.4. Selective photocatalytic oxidation of benzyl alcohol and its
derivatives
Photocatalytic reactions of benzyl alcohol and its derivatives
into corresponding aldehydes were carried out on TiO2 under
irradiation with both visible light and UV-light. As shown in Table
1, benzyl alcohol and its derivatives substituted by –OCH3, –Cl, –
NO2, –CH3, –CF3, and –C(CH3)3 groups were converted to corre-
sponding aldehydes with a high conversion (>99%) and high selec-
tivity (>99%) on TiO2, while no other products were observed. Upon
UV-light irradiation, benzyl alcohol and its derivatives as men-
tioned above were also converted to corresponding aldehydes with
a high conversion (>99%) and high selectivity (>99%) on TiO2, while
Here, the turnover numbers (TONs) were evaluated as the num-
ber of benzaldehyde molecules produced per one surface OH group
involved on TiO2. Since the amount of OH groups on the TiO2 sur-
face is generally estimated to be ca. 10 nmꢀ2 [19], the amount of
OH groups involved in 50 mg of TiO2 (BET surface: 300 m2 gꢀ1) is
100
80
60
40
20
0
50
40
30
20
10
0
(e)
(b)
(a)
0.02
(a)
1615
3715
(b)
(c), (d)
0
60
120
Time / min.
180
240
0.025
0
4000
3000
Wavenumber/ cm -1
2000
1000
Fig. 3. Photocatalytic oxidation of benzyl alcohol on TiO2 (50 mg) under irradiation
with visible light emitted from the LED lamp. The initial amount of benzyl alcohol
added to the reaction cell was 50 lmol. Amounts of (a) benzyl alcohol; (b)
benzaldehyde; (c) benzoic acid; (d) CO2; and (e) percentage of total organic
Fig. 4. FT-IR spectra of (a) benzyl alcohol by itself; and (b) benzyl alcohol adsorbed
compounds evolved in solution is plotted.
on TiO2.

282
S. Higashimoto et al. / Journal of Catalysis 266 (2009) 279–285
Table 1
Selective photocatalytic oxidation of benzyl alcohol and its derivatives into corresponding aldehydes in the presence of O2 under irradiation with visible light emitted from the
LED lamp for 4 h.
Reactants
Products
Conversion (%)
>99
Selectivity (%)
>99
CHO
CH2OH
>99
>99
>99
>99
>99
>99
>99
>99
CHO
CH2OH
H3CO
H3CO
CH2OH
CH2OH
CH2OH
CHO
CHO
CHO
Cl
Cl
O2N
O2N
H3C
F3C
H3C
>99
>99
>85
>99
>99
23
CH2OH
CH2OH
CH2OH
CHO
CHO
CHO
F3C
t-H9C4
t-H9C4
HO
HO
no other products were observed. Thus, it was confirmed that
various kinds of benzyl alcoholic compounds mentioned above
were effectively converted to corresponding aldehydes under irra-
diation with both visible light and UV-light. A kinetic study of the
selective photocatalytic oxidation will be discussed in a future
report.
On the other hand, the benzyl alcoholic compound substituted
by the –OH group such as 4-hydroxybenzyl alcohol was unselec-
tively photo-oxidized into 4-hydroxybenzaldehyde as well as some
unidentified products, as shown in Table 1. From now on, we inves-
tigate the reasons for the high selectivity on the oxidation of benzyl
alcohol, compared with that of 4-hydroxybenzyl alcohol.
sorbed TiO2 are shown in Fig. 5. Compared with the spectrum of
the benzyl alcohol-adsorbed TiO2, the 4-hydroxybenzyl alcohol-
adsorbed TiO2 exhibits intensive absorption in the visible region,
as shown in Fig. 5c. It is known that the surface OH groups of
TiO2 react with phenolic compounds (Ph–OH) to form a linkage
of „Ti–O–Ph, which exhibits strong absorption in the visible re-
gion due to the ligand-to-metal charge transfer (LMCT) [22]. The
4-hydroxybenzyl alcohol is, therefore, strongly adsorbed on TiO2
to form the intensive linkage of „Ti–O–Ph–CH2OH. On the other
hand, the UV–Vis spectra of the TiO2 adsorbed benzyl alcoholic
derivatives listed in Table 1, with the exception of 4-hydroxyben-
zyl alcohol, showed absorption in the visible light region in a sim-
ilar manner as that of benzyl alcohol.
3.5. Interaction of 4-hydroxybenzyl alcohol with the TiO2 surface
The uptakes in a mixture of benzyl alcohol and benzaldehyde as
well as those of 4-hydroxybenzyl alcohol and 4-hydroxybenzalde-
The UV–Vis absorption spectra of (a) the TiO2 by itself, (b) ben-
zyl alcohol-adsorbed TiO2, and (c) 4-hydroxybenzyl alcohol-ad-
hyde (50
lmol for each) on TiO2 as a function of time under dark
conditions are shown in Fig. 6. The uptake of every molecule on

S. Higashimoto et al. / Journal of Catalysis 266 (2009) 279–285
283
0.6
0.4
0.2
0
60
1.5
1
40
20
(c)
(b)
0.5
(a)
0
300
400
500
600
700
Wavelength / nm
0
350
400
450
500
550
600
650
700
Fig. 5. UV–Vis absorption spectra of (a) TiO2 by itself; (b) benzyl alcohol-adsorbed
TiO2; and (c) 4-hydroxybenzyl alcohol-adsorbed TiO2.
Wavelength / nm
Fig. 7. Action spectrum for the photocatalytic oxidation of benzyl alcohol into
benzaldehyde on TiO2; and the absorption spectrum of benzyl alcohol-adsorbed
TiO2.
20
(c)
dehyde on TiO2. The quantum yield (/) is defined in the following
Eq. (1):
15
ðamount of product formedÞ
/ ¼
ꢁ 100
ð1Þ
(d)
ðamount of photons absorbedÞ
10
It was observed that the photo-response is extended toward ca.
700 nm and the quantum yields are estimated to be ca. 4.3% under
photo-irradiation at 500 6.8 nm, ca. 5.5% at 460 6.8 nm, and ca.
39% at 420 6.8 nm, as shown in Fig. 7. An action spectrum for the
photocatalytic activity shows a red-shift from the UV–Vis spec-
trum, as can be seen in Fig. 7. This result suggests that the surface
complex formed by adsorption of benzyl alcohol on the TiO2 is
highly sensitive to the visible light.
(a)
5
(b)
0
0
2
4
6
8
10
Time / min
Fig. 6. Uptakes of (a) benzyl alcohol; (b) benzaldehyde; (c) 4-hydroxybenzyl
alcohol; and (d) 4-hydroxybenzaldehyde on TiO2 in the dark. A mixture of benzyl
alcohol and benzaldehyde or of 4-hydroxybenzyl alcohol and 4-hydroxybenzalde-
hyde (50 lmol each), respectively, was added to an acetonitrile suspension
involving TiO2 (50 mg).
3.7. Kinetic isotope effect on the photocatalytic oxidation of
benzyl alcohol
a, a-d2
The photocatalytic oxidation of (a) normal benzyl alcohol and
-d2 benzyl alcohol into corresponding benzaldehyde is
(b)
a, a
shown in Fig. 8[I]. The reaction rate constants of kH and kD for (a)
and (b) shown in Fig. 8[I] have been, respectively, evaluated from
the first-order rate law in Eqs. (2) and (3):
TiO2 is saturated within 1 min. It was noted that the amount of
benzaldehyde adsorbed on TiO2 was negligible compared to that
of benzyl alcohol, as is also shown in Fig. 6. These results indicate
that the interaction between benzaldehyde and TiO2 is fairly weak.
Therefore, once benzaldehyde is produced by the oxidation of ben-
zyl alcohol, benzaldehyde is immediately released into the bulk
solution and may be one of the reasons benzaldehyde is selectively
formed.
dx
¼ kða ꢀ xÞ
ð2Þ
ð3Þ
dt
ꢀ
ꢁ
a
kt ¼ ln
ða ꢀ xÞ
Eq. (3) shows that if ln [a/(a ꢀ x)] is plotted against t, a first-or-
der reaction gives a straight line for slope k. Here, the amount of
added benzyl alcohol and that of the photo-formed benzaldehyde
as a function of time (t) are defined as a and x, respectively.
Although the initial reactions do not completely fit with a first-or-
der reaction due to the induction period, the reaction rate con-
On the other hand, the amount of 4-hydoxybenzaldehyde ad-
sorbed on TiO2 is around the same as that of 4-hydoxybenzyl alco-
hol. This suggests that the adsorbed 4-hydoxybenzaldehyde on
TiO2 produces a stable species due to the formation of strong link-
ages, as mentioned above. From these results, the reasons for the
low activity for the oxidation of 4-hydoxybenzyl alcohol compared
with the other benzyl alcoholic derivatives may be considered as
follows: 4-hydroxybenzyl alcohol is so strongly adsorbed to form
a „Ti–O–Ph–CH2OH linkage that it leads to further oxidation un-
der photo-irradiation; and/or the –CH2OH group exists far from
the TiO2 surface.
stants for normal benzyl alcohol and a, a-d2 benzyl alcohol were
roughly estimated to be 2.06 ꢁ 10ꢀ4 sꢀ1 and 5.18 ꢁ 10ꢀ5 sꢀ1 for
(c) and (d), respectively (cf. Fig. 8[II]). The ratio of the reaction rates
(kH/kD) was, therefore, estimated to be ca. 3.9. The first-order iso-
tope effect shows that the C–D bond is cleaved more slowly than
the C–H bond. It should be noted that the value of kC-H/kC-D is the-
oretically ꢂ8 when the C–H bond is completely cleaved at the tran-
sitional state. Here, from the value of kC-H/kC-D at ꢂ3.9, the kinetic
3.6. Action spectrum for the photocatalytic oxidation of benzyl alcohol
on TiO2
isotope effect shows that an abstraction of the a-hydrogen atom
from the –CH2OH group is a rate-determining step while the C–H
bond is partially left in the transitional state.
Fig. 7 shows the action spectrum for the formation of benzalde-
hyde in the photocatalytic oxidation of benzyl alcohol into benzal-

284
S. Higashimoto et al. / Journal of Catalysis 266 (2009) 279–285
50
40
30
20
10
0
and H2O is shown in Fig. 9. The surface complexes, which are
[I]
formed by the interaction of the –CH2OH group or possibly the
phenyl ring of benzyl alcohol with the surface OH group, induce
absorption in the visible region. Upon visible light irradiation,
when the surface complex is photo-excited to form holes (h+)
and electrons (eꢀ), the holes abstract hydrogen atoms from the
CH2OH group of benzyl alcohol. Subsequently, the photo-induced
benzyl alcoholic radicals may automatically release another
electron to form benzaldehyde due to the current-doubling
effect.
On the other hand, the photo-induced electrons are scavenged
by O2 as shown in Fig. 9. The effect of the Ag+ ions as electron
acceptors on the photo-oxidation of benzyl alcohol was also inves-
tigated in a suspension of benzyl alcohol with TiO2 in the absence
of O2 under visible light irradiation. It was observed that only neg-
ligible amounts of benzaldehyde were formed by the photo-oxida-
tion of benzyl alcohol under visible light irradiation, while the Ag+
ions were reduced to Ag particles by the photo-induced electrons.
These results indicate that the photo-excited surface complex does
not produce benzaldehyde simply by the photo-induced holes, but
also in combination with the assistance of O2 which plays a signif-
icant role as electron acceptors; and possibly leads to the re-gener-
ation of surface hydroxyl groups on TiO2, as shown in Fig. 9.
Selective photocatalytic oxidation with a high efficiency was, thus,
realized on a TiO2 surface under visible light and a plausible reac-
tion mechanism is proposed.
(a)
(b)
0
10000
20000
30000
40000
50000
Time / sec
4
3
2
1
0
[II]
(c)
(d)
4. Conclusions
0
10000
20000
30000
40000
50000
Time / sec
The selective photocatalytic oxidation of benzyl alcohol and its
derivatives into corresponding aldehydes proceeded at a high con-
version rate of >99% and high selectivity of >99% on TiO2 in the
presence of O2. To the best of our knowledge, this is the first report
of such reactions taking place under visible light irradiation, i.e., a
surface complex was formed by the adsorption of benzyl alcohol
on TiO2 involving surface OH groups, leading to the absorption of
visible light and the selective photocatalytic oxidation reaction.
Moreover, the reaction mechanism behind the photocatalytic oxi-
dation of benzyl alcohol under visible light irradiation was also
demonstrated.
Fig. 8. Photocatalytic oxidation of (a) normal benzyl alcohol and (b) a, a-d2 benzyl
alcohol on TiO2 (50 mg) under irradiation with visible light emitted from the LED
lamp [I], and the plots for ln [a/(a ꢀ x)] against the time [II] for (c) normal benzyl
alcohol and (d) a, a-d2 benzyl alcohol.
3.8. Reaction mechanism for selective photocatalytic oxidation under
visible light irradiation
The tentative reaction mechanism for the oxidation of benzyl
alcohol on TiO2 under visible light irradiation into benzaldehyde
hv (visible light)
HOH2C
CH2OH
HOH2C
CH2OH
h+
h+
OH
OH
OH
OH
Ti
e-
-
O2 e
OH
OH
OH
O
OH
hv (visible light)
Ti
O
Ti
O
Ti
O
O
O2
O
O
O
O
O
+ 2 H2O
2
OHC
CH2OH
2
OH
OH
Ti
OH
OH
Ti
O
O
O
O
O
Fig. 9. Tentative reaction mechanism for the selective photocatalytic oxidation of benzyl alcohol in the presence of O2 on TiO2 under visible light irradiation.

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