Iridium oxide hydrosols as catalysts for the decay of zinc porphyrin radical cations in water

Abstract

Iridium oxide hydrosols, containing both Ir^III and Ir^IV sites within a complex oxyhydroxide structure, have been prepared and their interaction with water-soluble zinc porphyrin radical cations has been studied. The colloids possess a negative surface charge so that the positively charged zinc porphyrins are bound to the colloid by electrostatic attraction whilst negatively charged porphyrins are repelled from the surface. Oxidation of Ir^III sites can be accomplished with fairly mild oxidants and, at most pH values, the zinc porphyrin radical cations are capable of driving this process. The resultant Ir^IV sites can undergo further redox reactions, the outlet for any stored oxidizing equivalents being dependent upon the type of porphyrin used. With positively charged porphyrins, discharge of the colloid involves oxidation of surface-bound porphyrin. Using negatively charged porphyrins, the colloid catalyses the oxidation of water to O₂ with remarkably high efficiency. Evolution of O₂ is restricted, however, to pH > 7 owing to thermodynamic factors associated with the porphyrin. The photo-oxidation of water to O₂, under sacrificial conditions, is found to proceed with an initial quantum efficiency of 72% for an optimized system.