Using the technique of in situ photoprecipitation, a comparative study of the structurally homologous ionic species hexachloroplatinate ([PtC&12-), hexachloroosmiate ( [OsC&12-), and hexachloroiridates was conducted for photoconversion to metallocatalysts for photosynthetic hydrogen evolution. As with earlier [PtC1612-studies, [osc16l2-can be photoconverted to a metallocatalyst at photosystem I (PSI), although at a rate about 50% slower than that of [PtC&]2-. However, once photoprecipitated, its catalytic action for H2 production was 3 times as high as that of metallic platinum. Simultaneous photoevolution of 02 and H2 was observed in [osc16l2--photoprecipitated thylakoids. Maximum hydrogen evolution rate was 113 nmol-h-'.mg chl-'. Surprisingly, neither [IrC&12--nor [IrC&13-treated thylakoids were able to produce H2. Analysis indicated that [IrC1612-was able to accept only one electron by transformation to [IrC&]3-which was completely unable to acquire subsequent electrons from PSI. The inability of [IrC&l" to be reduced to metallic iridium is presumably due to a high energy level barrier of [IrC&]3-reduction.
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