A molecular water-reduction catalyst: Surface derivatization of TiO2 colloids and suspensions with a platinum complex

Abstract

Room-temperature irradiation of alkaline solutions (pH > 8) of [Pt(bpy)2]2+ (where bpy is 2,2′-bipyridine) in the presence of TiO2-SiO2 (or TiO2) colloids or TiO2 powder suspensions and an electron donor results in the loss of one bipyridyl ligand and the chemical fixation of Pt(bpy) to the TiO2 surface via the formation of Ti-O-Pt bonds. The chemical derivatization of TiO2 with Pt(bpy) is pH dependent and proceeds through a Ti-O-Pt(bpy)2 intermediary. X-ray photoelectron spectroscopy analysis indicates that the heterometallic surface complex Ti-O-Pt(bpy) exists in the Pt(0) and Pt(I) oxidation state. The surface complex exhibits a metal-to-ligand charge-transfer absorption band in the visible region of the spectrum. Direct excitation of either the semiconductor or the photosensitizer [Ru(bpy)3]2+, electrostatically adsorbed to colloidal SiO2-TiO2, initiates both the surface derivatization of TiO2 and the subsequent generation of H2. Large turnover numbers (H2 concentration/[Pt(bpy)2]2+ ≫ 1) are observed, indicating that the Pt(bpy) species on the TiO2 surface is catalytic and is therefore regenerated during the photoreduction of water. The rates of H2 evolution with Pt(bpy)-TiO2 particles are higher in acidic than basic media. The photocatalytic activity of these particles for H2 production compares favorably with that of other TiO2 systems charged with an equivalent Pt loading. © 1991 American Chemical Society.