Chemistry Letters 2000
909
further enhancement by the addition of H O to the solution.
2
2
Contrarily, in the case of rutile TiO , the rate can be enhanced
2
by the addition of H O , because the electron transfer to oxygen
2
2
is an uphill process in energetics.
As regards the oxidation power of holes generated in pho-
toirradiated TiO particles, we have evidence indicating that the
2
holes in rutile particles are more reactive than those in anatase
particles as follows. On rutile particles, water can be efficiently
oxidized to oxygen in the presence of suitable electron accep-
4
,5
tors, such as Fe(III) and Ag(I) ions. In contrast, the oxidation
of water hardly proceeds on anatase particles even when these
electron acceptors were added to the solution. On anatase parti-
cles as well as on rutile particles, alcohols, which are chemical-
ly reactive, are easily photo-oxidized. These results suggest
that the hole transfer from the valence band of anatase TiO to a
2
species in solution is not an easy process, if it is hard to oxidize.
Since naphthalene is rather difficult to oxidize, the hole transfer
from the valence band of anatase particles is probably a slow
process, which controls the reaction rate. Hence, in this case no
enhancement is expected by the addition of H O .
2
2
In conclusion, we found that dihydroxylation of naphtha-
lene on photoirradiated TiO powder is enhanced by the addi-
no enhancement was observed for anatase TiO powders. In
2
2
tion of H O when rutile TiO is used as the photocatalysts.
the case of P25, which has 30% rutile phase and 70% anatase
phase, the reaction was enhanced by 2.6 times by the addition
of H O .
2
2
2
This effect is considered to be applicable to other organic syn-
thetic reactions on TiO photocatalysts, especially to hydroxyla-
2
2
2
tion reactions.
The influence of H O concentration on the photocatalytic
2
2
reaction is shown in Figure 2, where TIO-5 (rutile) was used as
the photocatalyst. The reaction rate was drastically enhanced
by the addition of H O at concentrations higher than 0.02 M.
The quantum efficiency of the production of dihydroxylated
compounds (1,8- and 1,3-dihydroxynaphthalenes) reached as
Reference and Notes
1
M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahnemann,
Chem. Rev., 95, 69 (1995).
P. Sawunyama, L. Jiang, A. Fujishima, and K. Hashimoto, J. Phys.
Chem. B, 101, 11000 (1997).
2
2
2
3
4
J. Stark and J. Rabani, J. Phys. Chem. B, 103, 8524(1999).
T. Ohno, D. Haga, K. Fujihara, K. Kaizaki, and M. Matsumura, J.
Phys. Chem. B, 101, 6415 (1997); errata, 101, 10605 (1997).
K. Fujihara, T. Ohno, and M. Matsumura, J. Chem. Soc., Faraday
Trans., 94, 3705 (1998).
K. Sayama and H. Arakawa, J. Chem. Soc., Faraday Trans., 93,
1647 (1997).
high as 76% at an H O concentration of 0.058 M. The effi-
2
2
ciency was determined on the assumption that 4 holes are nec-
essary to form one dihydroxynaphthalene molecule. When
5
6
7
8
9
H O was added to the solution, this high reaction efficiency
2
2
was obtained without bubbling oxygen. From the analysis of
the reaction products, it was found that 55 and 35 µmol of 1,8-
and 1,3-dihydroxynaphthalenes were generated after photoirra-
diation for 1 h, during which 127 µmol of naphthalene (17% of
the initial amount) was consumed.
L. Cermenati, C. Richter, and A. Albini, Chem. Commun., 1998,
8
05.
M. Dusi, C. A. Muller, T. Mallat, and A. Baiker, Chem. Commun.,
999, 197.
T. Ohno, T. Kigoshi, K. Nakabeya, and M. Matsumura, Chem. Lett.,
998, 877.
1
1
Enhancement of photocatalytic reactions by the addition of
H O has generally been attributed to the formation of ·OH rad-
1
1
0
1
J. Jia, T. Ohno, Y. Masaki, and M. Matsumura, Chem. Lett., 1999, 963.
T. Ohno, K. Nakabeya, and M. Matsumura, J. Catal., 176, 76
2
2
icals as the result of the reaction with electrons in the conduc-
tion band of TiO2.18 However, in the present case, these ·OH
radicals are not considered to be the main oxidant, judging from
the absence of 1- and 2-naphthols in the products. The
enhancement may be attributable to the strong electron accept-
ing ability of H O . This leads to efficient separation of elec-
(1998).
12 E. Baciocchi, T. D. Giacco, M. I. Ferrero, C. Rol, and G. V.
Sebastiani, J. Org. Chem., 62, 4015 (1997).
B. Ohtani, J. I. Kawaguchi, M. Kozawa, S. I. Nishimoto, T. Inui, and
13
K. Izawa, J. Chem. Soc., Faraday Trans., 91, 1103 (1995).
14 “Semiconductor Nanoclusters–Physical, Chemical, and Catalytic
Aspects”, ed. by P. V. Kamat and D. Meisel, Elsevier, Amsterdam
2
2
(
1996), Vol. 103, p. 417.
E. Pelizzetti, V. Carlin, C. Minero, and M. Grätzel, New J. Chem.,
5, 351 (1991).
trons and holes, and to the improved oxidation of the reactants.
However, this model is not sufficient to explain why the reac-
tion is enhanced only in the case of rutile particles. To discuss
the effect of H O , the energy levels and oxidation power of
15
1
16 I. Poulios, M. Kositzi, and A. Kouras, J. Photochem. Photobiol., A:
Chem., 115, 175 (1998).
2
2
1
7
K. Tanaka, T. Hisanaga, and K. Harada, New J. Chem., 13, 5 (1989).
rutile and anatase TiO powders are the important criteria.
2
18 R. Doong and W. Chang, J. Photochem. Photobiol., A: Chem., 107,
The conduction band edge of anatase TiO is reported to be
239 (1997).
2
at ca. –0.7 V vs NHE at pH 7, and that of rutile TiO at ca. –0.5
19 K. E. O’Shea, I. Garcia, and M. Aguilar, Res. Chem. Intermed., 23,
2
V vs NHE.21 On the other hand, the reduction potential of
325 (1997).
2
2
0
1
R. W. Matthews, J. Chem. Soc., Faraday Trans. 1, 80, 457 (1984).
M. V. Rao, K. Rajeshwar, V. R. Pai Verneker, and J. DuBow, J.
Phys. Chem., 84, 1987 (1980).
“Encyclopedia of Electrochemistry of the Elements”, ed. by A. J.
Bard, Marcel Dekker Press (1974), Vol. II, p.193.
2
2
molecular oxygen at this pH is at –0.563 V vs NHE. Because
of the higher (more negative) energy level of the conduction
2
2
band of anatase TiO , the electron transfer to oxygen is consid-
2
ered to be an efficient process, and there may be no room for