Rate and Stability of Photocatalytic Water Oxidation using [Ru(bpy)3]2+ as Photosensitizer

Abstract

The kinetics of homogeneous photocatalytic water oxidation is reported using [Ru(bpy)3]Cl2 as photosensitizer, Na2S2O8 as sacrificial electron acceptor, and three different water-oxidation catalysts: the ruthenium catalyst [Ru(bda)(isoq)2] ([1], H2bda = 2,2′-bipyridine-6,6′-dicarboxylic acid, isoq = isoquinoline), Co(NO3)2 ([2]), and [Ir(Cp*)(dmiz)(OH)2] ([3], Cp* = pentamethylcyclopentadienyl, dmiz =1,3-dimethylimidazol-2-ylidene). At pH 7.0, in a phosphate buffer and under blue light irradiation, the production of O2 at the catalyst is rate determining when [2] or [3] is used as a water-oxidation catalyst. However, when [1] is used as the catalyst under identical conditions the turnover at the water-oxidation catalyst is not the rate-limiting step of the photocatalysis. Instead, the step limiting dioxygen production is the transfer of electrons from the catalyst to the photooxidized photosensitizer [Ru(bpy)3]3+. Due to the instability of [Ru(bpy)3]3+ in neutral aqueous solutions, slow electron transfer ...