Properties of O2•- and OH• formed in TiO2 aqueous suspensions by photocatalytic reaction and the influence of H2O2 and some ions

We have investigated the effect of H2O2 on the behavior Of O-2(.-) and OH. produced in photocatalysis of aqueous TiO2 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 H2O2, since the consumption of photoinduced valence band holes (h(+)) in the oxidation of H2O2 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 TiO2 particle. The decay process might be caused by trapped h(+), which cannot react with water and then remains on the TiO2 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 H2O2 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 H2O2, indicating the direct reduction of adsorbed H2O2 on the TiO2 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 H2O2.