Properties of O2•- and OH• Formed in TiO2 Aqueous Suspensions by Photocatalytic Reaction and the Influence of H2O2 and Some Ions

Abstract

We have investigated the effect of H 2 O 2 on the behavior of O 2 • - and OH • produced in photocatalysis of aqueous TiO 2 suspensions by means of luminol chemiluminescence probing and terephthalic acid fluorescence probing, respectively. The reduction of O 2 by photoinduced conduction band electrons (e - ) was increased by the addition of H 2 O 2, since the consumption of photoinduced valence band holes (h + ) in the oxidation of H 2 O 2 caused the repression of e - −h + recombination. After the end of the light irradiation, the amount of O 2 • - decreased based on the fractal-like kinetics at the heterogeneous surface of the TiO 2 particle. The decay process might be caused by trapped h +, which cannot react with water and then remains on the TiO 2 particle after the irradiation. The energy level of the trapped h + was estimated to be above the redox potential of SCN -, since it could react with the adsorbed H 2 O 2 and I - ions but not with other ions such as SCN -, Br -, and Cl - . The formation rate of OH • was increased by the addition of H 2 O 2, indicating the direct reduction of adsorbed H 2 O 2 on the TiO 2 surface. The quantum efficiencies of the formation of O 2 • - and OH • were increased by 3.0 and 3.6 times by the addition of 0.2 mM H 2 O 2 .