Titanium-Silicon Binary Oxide Catalysts
J. Phys. Chem. B, Vol. 102, No. 30, 1998 5875
coordinatively saturated Ti3+ species having H2O or OH- as
nescence spectrum attributed to the radiative decay from the
charge-transfer excited state of these species. The excited state
of these tetrahedrally coordinated titanium oxide species plays
a significant role in the liquid-phase photocatalytic oxidation
of 1-octanol to 1-octanal as an active site, their roles being
similar to the photocatalysis in the gas phase.
-
ligands. These H2O or OH species are formed by the processes
of the photoreduction of Ti4 ions by H2 at 77 K. On the other
hand, the Ti/Si binary oxide having a large Ti content (TS-80)
exhibits an intense and sharp ESR signal with a value of g⊥ )
+
1
.990. This characteristic ESR signal indicates the presence
3
2
of the aggregated octahedral titanium oxide species.
Acknowledgment. The present work has been supported in
part by the Grant-in-Aid on Priority-Area-Research on “Catalytic
Chemistry of Unique Reaction Fields” (07242264), “Electro-
chemistry of Ordered Interfaces” (09237258), “Carbon Alloys”
UV irradiation of the Ti/Si binary oxides in the 1-octanol-
acetonitrile solution in the presence of O2 led to the photocata-
lytic oxidation of 1-octanol to 1-octanal as the major reaction.
The selectivity for the formation of 1-octanal at the initial stage
of the reaction was >95% on the binary oxides having Ti
content lower than 20 wt % TiO2. No products could be
detected in the dark under these same reaction conditions.
Figure 8 shows the specific photocatalytic reactivities of the
catalysts per unit weight of TiO2. A remarkable increase in
the specific photocatalytic reactivity of the catalysts can be seen
for the catalysts having a low Ti content while a slight increase
can be seen for the catalysts having a high Ti content. The
specific photocatalytic reactivity of these binary oxides having
Ti content of 0.4-5 wt % TiO2 was found to be much higher
than that of the “standard TiO2 catalyst”, anatase TiO2 (Degussa
P-25). The quantum yields determined with the binary oxide
at 1 wt % TiO2 under the UV irradiation (λ ) 300 nm) of light
(09243232), and International Joint Project Research (07044162)
of the Ministry of Education, Science, Sports, and Culture of
Japan. We thank the Japan/U.S. Cooperative Program in
Photoconversion and Photosynthesis Research for a fellowship
to H.Y. that made his visit to Austin possible. This work at
the University of Texas was supported by the National Science
Foundation. Useful XPS measurements by Dr. M. F. Arendt
and Prof. J. M. White are gratefully acknowledged.
References and Notes
(
1) Photocatalytic Purification and Treatment; Ollis, D. F., Al-Ekabi,
H., Eds.; Elsevier: Amsterdam, 1993.
2) Anpo, M. Res. Chem. Intermed. 1989, 9, 67. Anpo, M. Catal. SurV.
Jpn. 1997, 1, 169.
(
(3) Anpo, M.; Yamashita, H. In Surface Photochemistry; Anpo, M.,
17
3
flux of 1 × 10 /min cm was 22%, while it was 12% for TiO2
Ed.; Wiley: West Sussex, 1996; p 117.
(4) Photocatalysis: Fundamentals and Applications; Serpone, N.,
Pelizzetti, E., Eds.; John Wiley & Sons: New York, 1989.
(P-25) and 4% for TiO2 powder prepared by the same sol-gel
method. These findings indicate that Ti/Si binary oxides,
especially the oxides having a low Ti content prepared by the
sol-gel method, are useful and promising photocatalysts to be
used in liquid-phase reactions.
(
5) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341.
(6) Kamat, P. V. Chem. ReV. 1993, 93, 267.
(7) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Chem. ReV. 1995, 95, 69.
(
(
8) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 95, 735.
9) Zamaraev, K. I.; Khramov, M. I.; Parmon, V. N. Catal. ReV.sSci.
As described above, in situ photoluminescence, UV-vis
reflectance, FT-IR, ESR, XAFS, XRD, and XPS spectroscopic
investigations of these Ti/Si binary oxides indicated that the
titanium oxide species are highly dispersed in the SiO2 matrixes
and exist in a tetrahedral coordination. Such titanium oxide
species exhibited the distinct and characteristic photolumines-
cence, and its yield increased when the Ti content in the Ti/Si
binary oxides was decreased. As shown in Figure 8, a parallel
relationship between the specific photocatalytic reactivities of
the titanium oxide species and the yields of the photolumines-
cence of the Ti/Si binary oxide catalysts can clearly be seen. It
is clear that the high photocatalytic reactivity of the Ti/Si binary
oxides is closely associated with the formation of the charge-
transfer excited complexes of the highly dispersed tetrahedral
titanium oxide species and their high reactivities.
Eng. 1994, 36, 617.
(10) Heller, A. Acc. Chem. Res. 1995, 28, 503.
(
11) Anpo, M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Louis, C.;
Giamello, E. J. Phys. Chem. 1985, 89, 5017, 5689.
12) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207.
(
(13) Yamashita, H.; Ichihashi, Y.; Harada, M.; Stewart, G.; Fox, M.
A.; Anpo, M. J. Catal. 1996, 158, 97.
(14) Yamashita, H.; Ichihashi, Y.; Anpo M.; Hashimoto, M.; Louis, C.;
Che, M. J. Phys. Chem. 1996, 100, 16041.
(15) Anpo, M.; Yamashita, H.; Ichihashi, Y.; Fujii Y.; Honda, M. J.
Phys. Chem. B 1997, 101, 2632.
(
16) Yamashita, H.; Ichihashi, Y.; Zhang, S. G.; Matsumura, Y.; Souma,
Y.; Tatsumi, T.; Anpo, M. Appl. Surf. Sci. 1997, 121&122, 305.
17) Avnir, D. Acc. Chem. Res. 1995, 28, 328.
(
(18) Negishi, N.; Matsuoka, M.; Yamashita, H.; Anpo, M. J. Phys. Chem.
993, 97, 5211.
1
(19) Moon, S. C.; Fujino, M.; Yamashita, H.; Anpo, M. J. Phys. Chem.
B 1997, 101, 369.
On the other hand, such photocatalytic reactivity could not
be observed for Ti/Si binary oxides in the middle region of Ti
content of about 50 wt % TiO2. XPS and XRD investigations
of the catalysts suggested that the segregation of SiO2 in the
surface regions of the binary oxides is remarkable for the catalyst
having a Ti content of about 50 wt % TiO2. The covering of
the titanium oxide species with such SiO2 moieties suppresses
and disturbs the accessibility of the reactant molecules with the
active site of the titanium oxide species, resulting in a dramatic
decrease in the photocatalytic reactivity of the catalysts.
(20) Anpo, M.; Nakaya, H.; Kodama. S.; Kubokawa, Y.; Domen, K.;
Onishi, T. J. Phys. Chem. 1985, 90, 1633.
(
21) Imamura, S.; Nakai, T.; Kanai, H.; Ito, T. J. Chem. Soc., Faraday
Trans. 1995, 91, 1261.
22) Liu, Z.; Davis, R. J. J. Phys. Chem. 1994, 98, 1253.
(23) Davis, R. J.; Liu, Z. Chem. Mater. 1997, 9, 2311.
24) Yamashita, H.; Matsuoka, M.; Tsuji, K.; Shioya, Y.; Anpo M.; Che,
M. J. Phys. Chem. 1996, 100, 397.
25) Camblor, M. A.; Corma, A.; P e´ rez-Pariente, J. J. Chem. Soc., Chem.
(
(
(
Commun. 1993, 557.
(26) Stakheev, A. Y.; Shpiro, E. S.; Apijok, J. J. Phys. Chem. 1993, 97,
668.
5
(27) Anpo, M.; Kawamura, T.; Kodama, S. J. Phys. Chem. 1988, 92,
4
38.
Conclusions
(28) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina,
Ti/Si binary oxides having different Ti content were prepared
using the sol-gel method and used as photocatalysts. When
the Ti content is decreased, the crystalline structure of the
titanium oxides changes from aggregates in an anatase phase
to ultrafine species in an amorphous state. In Ti/Si binary oxides
having lower Ti content the isolated titanium oxide species in
tetrahedral coordination were present separately from each other
in the SiO2 matrixes and exhibited a characteristic photolumi-
A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic,
G. J. Phys. Chem. 1994, 98, 1253.
(
29) Bonneviot, L.; On, D. T.; Lopez, A. J. Chem. Soc., Chem. Commun.
1
993, 685.
(30) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995,
378, 159.
(31) Yoshida, S.; Takenaka, T.; Tanaka, T.; Hirano, H.; Hayashi, H.
Stud. Surf. Sci. Catal. 1996, 101, 871.
(32) Anpo, M.; Shima, T.; Fujii, T.; Suzuki, S.; Che. M. Chem. Lett.
1987, 1997.