Direct observation of the one-electron reduction of methyl viologen mediated by the CO2 radical anion during TiO2 photocatalytic reactions.

The one-electron reduction of methyl viologen (MV(2+)) mediated by the carbon dioxide radical anion (CO(2)(*-)) during photocatalytic reactions in a colloidal TiO(2) aqueous solution (pH 2) has been investigated by time-resolved absorption spectroscopy. The formation of MV(*+) generated from the one-electron reduction reaction with CO(2)(*-), which is generated from the one-electron oxidation reactions with the photogenerated holes (h(+)), was directly observed. The spectral features of the photogenerated charge carriers and the kinetic analysis of the formation process of MV(*+) revealed that the CO(2)(*-), desorbed from the surface, reacts with MV(2+) via a homogeneous electron-transfer process in the bulk solution.