THE ROLE OF REDUCING AGENT IN OXIDATION REACTIONS OF WATER ON ILLUMINATED TIO2 ELECTRODES

Oxidation of water on an illuminated TiO2 electrode was investigated u sing a rotating ring-disk electrode, focusing on the role of the reduc ing agent, SO32-, added in solution. The TiO2 disk was illuminated wit h a chopped-light source, and the corresponding ring response at the P t-ring, Delta I-R = I-R (light) - I-R (dark), was recorded. Although o xidation products were expected to be produced on the disk surface and carried to the ring electrode, Delta I-R was found to be negative in dilute Na2SO3 solution even at large negative potentials, e.g., -0.8 V vs. SCE. This phenomenon was observed in neutral and basic solutions. It is proposed that SO3- radical is formed at an illuminated TiO2-di sk and subsequently initiates a homogeneous free-radical chain oxidati on of sulfite ion. This chain reaction consumes oxygen to be supplied from the solution via the disk 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.