Synergism between rutile and anatase TiO2 particles in photocatalytic oxidation of naphthalene

Photocatalytic oxidation of naphthalene was investigated in a mixed solution of acetonitrile and water using various kinds of titanium dioxide (TiO2) powders as the photocatalysts and molecular oxygen as the electron acceptor. The main product from naphthalene is 2-formylcinnamaldehyde. For this reaction, anatase small TiO2 particles, which are commonly used as photocatalyst, are inactive, probably because band bending is necessary for the oxidation of naphthalene. If the particles are not extremely small, pure rutile and pure anatase powders show fairly high activity, and those containing both anatase and rutile phases show the highest activity. When a pure anatase powder is partly (about 90%) converted to the rutile form by heat treatment, the activity is largely enhanced. The activity of pure rutile particles is also enhanced by physically mixing them with a small amount of small-sized anatase particles, which are inactive for this reaction. These results can be explained by the synergism between rutile and anatase particles. We consider that electrons are transferred from rutile particles to anatase particles, i.e. naphthalene is mainly oxidized on rutile particles and oxygen is mainly reduced on anatase particles. This electron transfer process is supported by electrochemical properties of TiO2 electrodes for reduction of oxygen. (C) 2002 Elsevier Science B.V. All rights reserved.