Colloidal platinum catalysts for the reduction of water to hydrogen, photosensitised by reductive quenching of zinc porphyrins

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Abstract
EDTA transfers an electron to the triplet excited state of ZnTMPyP4+ in aqueous solution and the one-electron reduction product of the zinc porphyrin reduces water to H2. A Pt catalyst is required to promote this latter reaction and it was found that the nature of the support used to stabilise the colloidal Pt particle against flocculation had a great effect upon the efficiency of the Pt catalyst. The most effective support was Carbowax 20M and irradiation of ZnTMPyP4+ in the presence of EDTA (0.1 mol dm–3) and Pt–Carbo (10–4 mol dm–3) resulted in formation of H2 with an optimum quantum yield (ϕ½ H2 ) of 0.07. The average lifetime of the reduced zinc porphyrin was increased by binding the porphyrin to a polymer and by using a surfactant derivative of the porphyrin. In the latter case, the surfactant porphyrin was used as a support for colloidal Pt particles and upon irradiation in the presence of EDTA H2 was produced with ϕ½ H2 of ca. 0.004; this was increased significantly when methyl viologen was present in the aqueous solution.