Behavior of superoxide radicals formed on TiO2 powder photocatalysts studied by a chemiluminescent probe method

Abstract

A luminol chemiluminescence (CL) probe method was successfully applied to the investigation of the superoxide radical (O₂⁻˙) formed on photoirradiated TiO₂ powders. For several kinds of commercially available TiO₂ photocatalysts and their calcined samples, the amount of O₂⁻˙ produced at the steady state was measured and found to increase with the secondary particle size or the degree of aggregation. The decay of O₂⁻˙ for the non-calcined samples was as long as several hundred seconds and obeyed second order kinetics, indicating that disproportionation is the main deactivation pathway. For some TiO₂ photocatalysts, oxidative species such as OH˙ radicals are suggested to exist for 1 s after irradiation. On the other hand, reductive photoinduced electrons may remain for several seconds after irradiation since the decay of O₂⁻˙ starts after a delay of more than 1 s. On calcination at temperatures up to 1173 K, O₂⁻˙ decays by a mechanism other than disproportionation, suggesting the prolonged lifetime of an oxidative species that can react with O₂⁻˙.