Photon-driven reduction of Zn2+ to Zn metal.

Easily oxidized metals are of interest as a means of storing solar energy in the form of fuels. While their efficient metal/air batteries make them attractive solar fuel candidates, the photoreduction of the corresponding metal ions remains difficult. Accordingly, this work describes the photon driven reduction of Zn(2+) by an iridium(III) photosensitizer (PS) and catalyst. Ir(ppy)2(dtbbpy) (ppy = 2-phenylpyridine, dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) was found to be the most robust photocatalyst, and the use of ZnCl2 as the Zn(2+) starting material and acetonitrile as the solvent afforded the highest yield of Zn metal product. Under these conditions, a maximum of 430 catalyst turnover numbers were achieved. Cyclic voltammetry of ZnCl2 in different solvents and of different zinc salts in acetonitrile (MeCN) demonstrated the roles of MeCN and Cl(-) in the photoreduction mechanism. Kinetics measurements revealed a first order dependence of the initial rate on both Ir(ppy)2(dtbbpy) and ZnCl2. A first order decay of the reaction rate was also observed.