Abstract
Recent studies on the detection and behavior of the active species which play important roles in TiO₂ photocatalytic reactions are briefly reviewed. This article follows the previous review article published in this journal five year ago and includes following topics. Sterically hindered cyclic amines are oxidized by TiO2 photocatalysts and form the corresponding nitroxide radicals. Though these amines are used for detecting singlet oxygen (¹O₂), the method cannot be applicable to photocatalysis. The formation of 1O2 in photocatalysis was detected by monitoring phosphorescence at 1270 nm band. Although the lifetime of ¹O₂ is as short as 2 s because of a strong interaction with TiO₂ surface, the reaction with pyrrole and folic acid occurs within about 1 s. Hydroxyl radicals (●OH) was detected for anatase photocatalysts but not for rutile photocatalysts in aqueous suspension. On the other hand, ●OH was detected for both crystalline phases in gaseous reaction systems, where LIF method has been used. The reactivity and the state of surface OH groups were changed by heat treatment of photocatalysts as evidenced by the decomposition rates of amino acids and dipeptides.

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