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The pulse radiolysis technique has been used tostudy the reaction of OH* radicals with 13-nm Ti02 particles. Hydroxyl radicals react with these particles in a near-diffusion-controlled rate. The hydroxyl is trapped on the Ti02 and exhibits a broad absorption band centered at about 350 nm. Oxygen has no effect on this reaction and its product characteristics. The product decays via first-order kinetics which are assigned to the collapse of two species trapped on the same particle to yield peroxides. The oxidative abilities of this product were confirmed by oxidation of SCN" to give the anion radical (SCN) 2-, The identity of the product of the OH* reaction with the particles is discussed and is identified as a trapped hole at the particle surface. Mechanistic implications to photocatalysis are emphasized; it is argued that the trapped hole and a surface-bound OH* radical are indistinguishable species.