Heteropolytungstates as catalysts for the photochemical reduction of oxygen and water

Abstract

A series of polytungstate anions [XW12O40]n–(X = P, Si, Fe, Co, or H2; n= 3,4,5,6, and 6 respectively), spanning a range of reduction potentials, have been studied as sensitizers for the photoreduction of water and O2. [SiW12O40]4– was the most efficient sensitizer for H2 evolution in the presence of colloidal platinum. Saturation kinetics were found with respect to the concentrations of Pt,[SiW12O40]4–, and CH3OH as predicted by a simple kinetic scheme. The maximum rate depended on competition between the natural decay of the excited polyanion and quenching by alcohol. Electron transfer from photoreduced polyanions to O2 was also investigated by flash photolysis. Rate constants depended on the reduction potential of the polyanion and increased by a factor of 3 700 on going from [PW12O40]3– to [FeW12O40]5–, in line with the Marcus equation for adiabatic electron-transfer reactions.