Photocatalytic Water Oxidation in a Buffered Tris(2,2‘-bipyridyl)ruthenium Complex-Colloidal IrO2 System

Abstract

A buffered tris(2,2‘-bipyridyl)ruthenium complex-colloidal IrO₂ system was studied as a photocatalyst for the production of O₂ from water. Phosphate buffer, which has historically been used to control the pH in this system, accelerates the decomposition of the photosensitizer and inhibits O₂ evolution, whereas sodium hexafluorosilicate (Na₂SiF₆)−base solutions are ideal buffers for the reaction. Na₂SiF₆-containing buffers poise the solution under visible light irradiation at ca. pH 5, preventing the pH drop that accompanies oxidation of water in unbuffered solutions. Decomposition of the photosensitizer is not kinetically competitive with oxygen evolution in these buffers. In particular, the Na₂SiF₆−NaHCO₃ buffer greatly improves the turnover number of the photosensitizer, relative to previously used phosphate buffers, without any decrease in activity. Photocatalytic reactions studied under various conditions suggest that adsorbed carbonate or bicarbonate on the surface of the colloidal IrO₂ particles contributes to the increased turnover number of the photosensitizer.