The role of reducing agent in oxidation reactions of water on illuminated TiO2 electrodes

Oxidation of water on an illuminated TiO2 electrode was investigated using a rotating ring-disc electrode, focusing on the role of the sulphur trioxide ion reducing agent, added in solution. Although oxidation products were expected to be produced on the disc surface and carried to the ring electrode, the difference between ring currents in light and dark was found to be negative in dilute Na2SO3 solution even at large negative potentials. This phenomenon was observed in neutral and basic solutions. It is proposed that an excited sulphur trioxide radical forms at an illuminated TiO2-disc and subsequently initiates a homogeneous free-radical chain oxidation of sulphite ion. This chain reaction consumes oxygen to be supplied from the solution via the disc to the ring, reducing the ring current associated with the reduction of oxygen. As a result, the ring current is lower under illumination than in the dark. 12 refs.