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.