TiO2-Photocatalyzed Epoxidation of 1-Decene by H2O2 under Visible Light

Article abstract of DOI:10.1006/jcat.2001.3384

1-Decene was converted to 1,2-epoxydecane on UV-irradiated TiO2 powder using molecular oxygen as the oxygen source. Other main products were nonanal and 2-decanone. For anatase-form TiO2 powders, the reaction rate was hardly affected by addition of hydrogen peroxide to the solution. In contrast, for rutile-form TiO2 powders, the rate of epoxide generation was significantly increased by addition of hydrogen peroxide. In this case, the reaction occurred under visible light as well as UV light. The selectivity of the production of 1,2-epoxydecane was higher under visible light than under UV light. The conversion efficiency of an incident photon to 1,2-epoxydecane was about 2 percent when irradiated with visible light in the range 440-480 nm. UV-visible diffuse reflection spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy suggested the generation of a Ti-η²-peroxide on rutile TiO2 surface after treatment with hydrogen peroxide. The initial step of the reaction under visible light was attributed to a photochemical reaction of this peroxide with 1-decene.

Full text of DOI:10.1006/jcat.2001.3384

Journal of Catalysis 204, 163–168 (2001)
doi:10.1006/jcat.2001.3384, available online at http://www.idealibrary.com on
TiO -Photocatalyzed Epoxidation of 1-Decene by H O
2
2
2
under Visible Light
Teruhisa Ohno, Yuji Masaki, Seiko Hirayama, and Michio Matsumura1
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Received April 16, 2001; revised August 10, 2001; accepted August 10, 2001
We succeeded in increasingthe efficiencyfor the epoxida-
1
-Decene was converted to 1,2-epoxydecane on UV-irradiated tion of linear olefins by selecting appropriate TiO2 powders
TiO powder using molecular oxygen as the oxygen source. Other and optimizing experimental conditions (6, 7). However,
2
main products were nonanal and 2-decanone. For anatase-form
the efficiency is still not high enough for practical appli-
TiO powders, the reaction rate was hardly affected by addition
2
cations. In addition, the effective light for the reactions is
limited to the UV region as long as TiO2 is used as the pho-
tocatalyst. Further increase in the reaction efficiency and
expansion of the effective wavelength region are the keys
to the practical application of photocatalytic reactions.
In order to enhance the photocatalytic reactions for min-
eralization of organic compounds, it has been reported that
of hydrogen peroxide to the solution. In contrast, for rutile-form
TiO2 powders, the rate of epoxide generation was significantly
increased by addition of hydrogen peroxide. In this case, the re-
action occurred under visible light as well as UV light. The se-
lectivity of the production of 1,2-epoxydecane was higher under
visible light than under UV light. The conversion efficiency of an
incident photon to 1,2-epoxydecane was about 2% when irradi-
ated with visible light in the range 440–480 nm. UV–visible diffuse addition of hydrogen peroxide is effective (17, 18). In this
reflection spectroscopy, Fourier transform infrared spectroscopy, paper, we report that the epoxidation of 1-decene is also
and X-ray photoelectron spectroscopy suggested the generation of enhanced by addition of hydrogen peroxide. Furthermore,
2
a Ti–� -peroxide on rutile TiO surface after treatment with hy-
2
we found that the reaction occurs even under irradiation
with visible light, if rutile TiO2 particles are used as the
photocatalyst in the presence of hydrogen peroxide.
drogen peroxide. The initial step of the reaction under visible light
was attributed to a photochemical reaction of this peroxide with
-decene.
Key Words: titanium dioxide; epoxide; hydrogen peroxide; visible
light.
�
c 2001 Academic Press
1
EXPERIMENTAL
Materials
INTRODUCTION
Various kinds of titanium dioxide (TiO2) powders hav-
ing anatase and rutile crystal structures were obtained from
the Catalysis Society of Japan (JRC-TIO-3 and -5), Ishihara
Sangyo Co. (ST-01, PT-101), and Toho Titanium Co. (NS-
Semiconductor photocatalysts have attracted attention
because of their possible applications in treatment of wastes
and pollutants. As a result of these photocatalytic reactions,
organic compounds are usually mineralized (1–4). Under
some conditions, it is also possible to partially oxidize or-
ganic compounds using semiconductor photocatalysts, such
as TiO2 (5–11), ZnS (12), and CdS (13). The partial oxi-
dation is important from the viewpoint of organic synthe-
sis. We have reported that olefins, such as 1-decene and
5
1). The content of the rutile phase and the relative sur-
face area of these powders are as follows: JRC-TIO-3,
2
2
9
1
6
9.0% , 42.5 m /g; JRC-TIO-5, 90.7% , 2.2 m /g; ST-01, 0.1% ,
2
2
92.5 m /g; PT-101, 99.9% , 25.0 m /g; and NS-51, 98.6% ,
.5 m /g. 1-Decene, 2-decanone, nonanal, butyronitrile, and
2
acetonitrile were purchased from Wako Pure Chemical In-
dustries Co. 1,2-Epoxyhexadecane and titanium tetrachlo-
ride were purchased from Tokyo Kasei Co. They were all
guaranteed-grade reagents and used without further purifi-
cation.
Peroxotitanic acid was prepared according to the pro-
cedures described in the literature (19). Titanium perox-
ide was obtained as a fine yellow powder by completely
evaporating the peroxotitanic acid solution under reduced
pressure at 90˚C.
2
-hexene, are converted to the corresponding epoxides on
UV-irradiated TiO2 particles (6, 7). Kanno et al. (14) and
Fox and co-workers (15, 16) have also reported generation
of epoxides from olefins, especially from aromatic olefins
using TiO2 particles. However, in most of their results, the
main products were not epoxides, but carbonyl compounds
(
14–16).
1
To whom correspondence should be addressed. Fax: 81-6-6850-6699.
E-mail: matsu@chem.es.osaka-u.ac.jp.
163
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

1
64
OHNO ET AL.
Photocatalytic Reaction of 1-Decene on TiO2 Powder
ing energy due to surface charging was corrected using the
C 1s level at 285 eV as the internal standard. The XPS
peaks were assumed to have Gaussian line shapes and were
resolved into components by a nonlinear least-squares pro-
cedure after proper subtraction ofthe baseline. For the mea-
surements of XPS spectra of H O -treated TiO surfaces,
the single crystalline TiO2 samples were immersed in 30%
H2O2 for 24 h. The samples were introduced into the XPS
chamber after blow-drying.
Photocatalytic reactions were carried out in Pyrex glass
tubes (�, 10 mm), which contained TiO2 powder (200 mg),
water (0.2 g), acetonitrile (1.8 g), butyronitrile (1.5 g), and
1
-decene (0.5 g). In order to study the effect of hydro-
2
2
2
gen peroxide, 30% hydrogen peroxide (0.2 g) was added
to the glass tubes instead of water. The TiO2 powder was
suspended in the solution using a magnetic stirrer. The
suspension was photoirradiated using a 500 W Xe lamp
(
Wacom R&D, XDS-501I), which emits both UV and vis-
ible light over a wide wavelength. To limit the irradiation
wavelength, the light beam was passed through a UV-34, L-
RESULTS AND DISCUSSION
4
2, or Y-44 filter (Kenko Co.) to cut off wavelengths shorter Photocatalytic Epoxidation of 1-Decene on
than 340, 420, or 440 nm, respectively. Products were ana-
lyzed by a Shimadzu GC-8A gas–liquid chromatography.
The products were identified by coinjecting authentic sam-
ples. Details of the analysis have been described previously
Photoirradiated TiO2 Powder
1
,2-Epoxydecane, 2-decanone, and nonanal were the
main products obtained as a result of the photocatalytic
reaction of 1-decene on irradiated TiO2 particles in solu-
tion containing molecular oxygen as the electron accep-
tor. Figure 1a shows the products obtained after irradi-
ation for 4 h using four kinds of TiO2 powders, which
were suspended in solutions bubbled with pure oxygen.
In these reactions, light with wavelengths shorter than
(
6, 7). For determination of the conversion efficiency of an
incident photon to the reaction products, the suspensions
were irradiated using band pass filters (Kenko Co.) which
select the irradiation wavelength in a region from 440 to
4
80 nm. The power of incident light was determined using
a thermopile (Eppley Laboratory), and the number of inci-
dent photons was estimated by assuming that the average
wavelength of incident light was 460 nm.
3
40 nm was removed to prevent photochemical reactions
in solution. No reaction occurred in the dark or under
Deposition of TiO2 Films by RF Sputtering
TiO2 films were deposited on p-type Si(100) wafers in
a planar radio frequency (RF) sputtering apparatus using
a titanium disk (99.5% , 80 mm in diameter) as the target.
The target-substrate spacing was 25 mm, and the applied
RF power was 140–170 W. We used pure O2 or a mixture of
Ar and O2 as the sputtering gas. The thickness of the TiO2
film was about 550 nm. Pure rutile films were obtained by
heating the deposited TiO2 films at 800˚C for 2 h.
XRD, FT-IR, and XPS Measurements
The contents of anatase and rutile phases in the pow-
ders and films were determined by X-ray diffraction (XRD)
using a Philips X’Pert-MRD X-ray diffraction meter with
a CuK� source. The absorption and diffuse reflection
spectra were measured using a Shimadzu UV-2500PC
spectrophotometer. Fourier transform infrared (FT-IR)
spectra of TiO2 thin films deposited on Si wafers were ob-
tained using a Nicolet Nexus 370S FT-IR spectrometer with
a Harrick-type ATR unit. The FT-IR spectra were mea-
sured by internally multireflecting the IR beams in 20-mm-
long Si substrates. X-ray photoelectron spectra (XPS) of
single-crystalline TiO2(001) samples were measured using
a Shimadzu ESCA1000 photoelectron spectrometer with
an AlK� source (1486.6 eV). The TiO2(001) single crystals
FIG. 1. Photocatalytic activities of TiO2 powders for the photocat-
alytic reaction of 1-decene in solutions without (a) and with (b) H2O2. In
the reaction, the TiO2 particles suspended in the solution were irradiated
for 4 h with light at wavelengths longer than 340 nm using a Xe lamp.
The content of the rutile phase in the TiO2 powders is listed beneath the
were obtained from Earth Chemical Co. The shift of bind- figure.

TiO2-PHOTOCATALYZED EPOXIDATION OF 1-DECENE
165
irradiation with visible light for any kind of TiO2 pow-
der. When the reaction was carried out under UV irradi-
ation in neat 1-decene, the reaction rate was significantly
increased, and the anatase powder (ST-01) showed the
highest activity (7). In this study, we did the measure-
ments in mixed solutions containing TiO2 powder (200 mg),
water (0.2 g), acetonitrile (1.8 g), butyronitrile (1.5 g), and
1
-decene (0.5 g). In this solution, the difference in the activ-
ity among the TiO2 powders is smaller than that observed in
neat 1-decene. An exception is the rutile powder (TIO-3),
which exhibited a very low activity, as shown in Fig. 1a.
When H2O2 is added to the solution, the activity in-
creases significantly for rutile TiO2 powders, as shown in
Fig. 1b. No reaction occurred in the dark. The rutile TIO-3
powder, which exhibited very poor photocatalytic activity,
FIG. 3. Photocatalytic activities of four kinds of rutile TiO2 powders
shows drastic increase in its activity by addition of H2O2. for the reaction of 1-decene under visible light (�ꢀ 440 nm) in the solution
containing H2O2. A Xe lamp was used as the light source, and irradiation
wavelength was controlled using a cutoff glass filter (Y-44). The suspension
was photoirradiated for 4 h. The surface area of the TiO2 powders is listed
beneath the figure.
The other rutile powders also show an increase in their
activity by addition of H2O2. In contrast, for the anatase
ST-01 powder, practically no enhancement of the activity is
observed. Other anatase TiO2 powders did not show an in-
crease in the activity by addition of H2O2 either (not shown
in Fig. 1). The enhancement of the activity of rutile TiO2
as a photocatalyst. The photocatalytic reaction proceeds
powders by addition of H2O2 has been observed for pho-
even by photoirradiation at wavelengths longer than
tocatalytic oxidation of naphthalene (9). In this reaction,
4
40 nm. The content of 1,2-epoxydecane in the products
the enhancement is attributed to the increase in the rate
of electron transfer from rutile TiO2 to the electron accep-
tor (H2O2) in solution because H2O2 is a stronger electron
acceptor than is molecular oxygen.
It is interesting to note that we found that the reac-
tion of 1-decene on rutile TiO2 particles occurs under visi-
ble light irradiation when H2O2 is added to the solution.
The wavelength dependence of the photocatalytic reac-
tion was measured using different cutoff glass filters and
a xenon lamp. Typical results are shown in Fig. 2, for TIO-3
increases when the UV component is removed from irradi-
ation. More precisely, the yield of 1,2-epoxydecane (based
on consumed 1-decene) reaches 52% for irradiation longer
than 440 nm, while it is only 42% for irradiation lower
than 340 nm. In contrast, production of nonanal is lower
when UV light is removed from the light. The main path
for the production of nonanal is probably oxidation of the
olefin by photogenerated holes, as reported by Fox and
co-workers for the reaction of aromatic olefins (15, 16).
Under visible light, the production of nonanal is lower be-
cause visible light does not generate electrons and holes in
TiO2.
Other rutile TiO2 powders also show photocatalytic ac-
tivity under visible light in the solution containing H2O2,
as depicted in Fig. 3. Contrarily, practically no reaction oc-
curs under visible light when anatase particles are used as
the photocatalysts. Among rutile TiO2 powders, the TIO-3
shows the highest activity under visible light irradiation,
although this powder shows very low activity under UV ir-
radiation, as shown in Fig. 1. This powder has the largest
surface area among the rutile powders we used. Hence, the
reaction under visible light isconsidered to be due to species
generated on the surface of rutile particles.
Using the TIO-3 powder, the conversion efficiency of
an incident photon to 1,2-epoxydecane was estimated to
be about 2% by irradiation in a wavelength region from
FIG. 2. Effect of irradiated wavelengths on the TiO2-photocatalyzed
reaction of1-decene in the solution containingH2O2. A rutile TiO2 powder
TIO-3) was used as the photocatalyst, and the suspension was irradiated
for 4 h. A Xe lamp was used as the light source, and irradiation wavelength
was controlled using cutoff glass filters.
4
40 to 480 nm. The real quantum efficiency is higher
than this value because in this estimation we assumed
that all the incident photons were absorbed by the sus-
pension.
(

1
66
OHNO ET AL.
Diffuse Reflectance Spectra of TiO2 Powders
To identify the surface species, which are effective for
the reaction under visible light, we measured diffuse re-
flectance spectra of TiO2 powders before and after treat-
ment with 30% H2O2 for 8 h. As shown in Fig. 4, all TiO2
powders after the H2O2 treatment show absorption up to
about 550 nm. The absorption becomes stronger with the
increase in surface area of TiO2 powders. This result sug-
gests that peroxides are generated on the surface of TiO2
particles by the treatment with H2O2 (20).
The photocatalytic activity of the H2O2-treated TiO2
powders under visible light must be related to this absorp-
tion in the visible region. However, we have to recall that
the anatase powder (ST-01) shows very poor photocatalytic
activity under visible light, although its absorption in the
FIG. 5. X-ray photoelectron spectra of the O1s band of a TiO2(001)
single crystal before (a) and after (b) the treatment with H2O2, and that
of a Ti–peroxide powder (c).
visible region after the treatment with H2O2 is very strong.
Hence, further information about the species generated on
the H2O2-treated TiO2 surface is necessary.
XPS Analysis
The O 1s XPS spectrum of a rutile TiO2(001) single crys-
tal before the treatment with H2O2 is shown in Fig. 5a. The
4+
band at 529.8 eV is assigned to the lattice oxygen (Ti –O)
3
+
3+
21� 23
(
22). The one at 531.8 eV is attributed to Ti (Ti –O)
4+
and Ti –OH (24). The spectrum did not changed by im-
mersing the sample in pure water for 24 h. On the other
hand, after the H2O2 treatment, a new band appears at
5
33 eV, as shown in Fig. 5b. This band is considered to
be due to the peroxide generated on the surface. This as-
sumption is confirmed by comparing the peak to that of a
Ti–peroxide powder, which shows a strong peak at 532.8 eV,
as shown in Fig. 5c. The synthesis of the Ti–peroxide powder
FIG. 4. Diffuse reflection spectra of TiO2 powders before and after
the treatment with H2O2 : ST-01 (a), TIO-3 (b), and TIO-5 (c). The content
of the rutile phase and the surface area of the TiO2 powders are shown in
the figure.

TiO2-PHOTOCATALYZED EPOXIDATION OF 1-DECENE
167
FIG. 6. FT-IR spectra of TiO2 thin films with the anatase form (80% ) (a) and with the rutile form (100% ) (b). The spectra were measured just
after H2O2 treatment, and 20 min after the treatment. The TiO2 films were about 550 nm thick, and the light beam was internally reflected 20 times.
was described in the experimental section, and the sample 940–820 cm 1 and 800–740 cm , seen in Fig. 6, are attri-
�
� 1
contained both Ti–peroxide and TiO2; the corresponding buted to the stretching vibrations of the –O–O– bonds of
2
O 1s peaks are seen in Fig. 5c.
Ti–� -peroxide and Ti–�-peroxide, respectively.
We also measured the XPS spectra of TiO2 powders be-
fore and after the H2O2 treatment. For these powder sam- Ti–� -peroxide in the rutile TiO2 film is much higher than
From the spectra of Figs. 6a and 6b, the content of
2
ples, however, we were unable to isolate the band at 533 eV that in the anatase (80% ) TiO2 film. The small absorption
�
1
because the band at 531 eV was too intense. Although we in the region 940–820 cm for the anatase (80% ) film may
did not observe the O 1s band due to the Ti–peroxide on be due to the rutile component (20% ) included in the film.
�
1
the surface of TiO2 powders, it is reasonable to assume the On the other hand, the absorption band (800–740 cm
)
generation of the Ti–peroxide on the surface, which was due to Ti–�-peroxide is larger for the anatase film than for
confirmed on the surface of TiO2(001) single crystals.
the rutile film. These results strongly suggest that the active
species formed on rutile TiO2 powders for the epoxidation
of 1-decene under visible light is Ti–� -peroxide.
2
IR Spectra of TiO2 Thin Films
Proposed Mechanism of the Epoxidation under
Visible Light
In order to obtain information about the Ti–peroxide
generated on the surface of TiO2, we measured the atten-
uated total reflection (ATR) FT-IR spectra of TiO2 films
which were deposited on the Si(100) substrates. The spec-
tra of the TiO2 films were measured just after the treat-
ment with 30% H2O2, and 20 min after the treatment.
Figures 6a and 6b display the difference spectra of the
H2O2-treated samples and water-treated samples (used as
backgrounds) for anatase (80% ) and rutile (100% ) films,
On the basis of the results, we propose a mechanism,
shown in Scheme 1, for the production of 1,2-epoxydecane
on the surface of TiO2 under visible light. The Ti–� -
peroxide generated on the surface of rutile TiO2 is consi-
dered to be the active species. Under visible light, one of the
2
�
1
respectively. Bands in the range 950–700 cm are ascribed
to different Ti–peroxo species (25–28). These absorption
bands disappear 20 min after the H2O2 treatment, suggest-
2
ing that they are very reactive. Ti–� -peroxide is reported to
�
1
have absorption bands in the range 932–800 cm (25, 28).
On the other hand, Ti–�-peroxide is reported to have ab-
�
1
sorption bands in the range 770–700 cm ,which are often
observed for solid-state peroxides (27, 28). On the basis
of these assignments, the absorption bands in the ranges
SCHEME 1

1
68
OHNO ET AL.
oxygen atoms in the peroxo group is assumed to be trans-
ferred to the olefin via a transition state, shown in Scheme 1.
A similar transition state was proposed for the catalytic
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2
(
29, 30). After the detachment of the epoxide, the Ti–� -
(
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6. Ohno, T., Kigoshi, T., Nakabeta, K., and Matsumura, M., Chem. Lett.
77 (1998).
peroxide complex is regenerated by H2O2.
8
The generation of the epoxide on rutile TiO2 was also en-
hanced under UV light by H2O2. This result suggests that
the intermediate is efficiently generated by UV light. Under
UV irradiation, the production of nonanal and 2-decanone
was also enhanced by the addition of H2O2. These com-
7
8
. Ohno, T., Nakabeya, K., and Matsumura, M., J. Catal. 176, 76 (1998).
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(
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1
1
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H2O2 helps the separation of electrons and holes in TiO2,
leading to improved reaction efficiency.
1
The important point of the reactions under visible light
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1
1
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1
2
2
2
2
2
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(
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CONCLUSION
We found that epoxidation of 1-decene on photoirradi-
ated TiO2 powder is enhanced by addition of H2O2, when
rutile TiO2 is used as the photocatalyst. Furthermore, rutile
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(
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2
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