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A high-throughput method has been developed for screening and optimization of colloidal water oxidation catalysts. The catalysts are irradiated in parallel by visible light from an overhead projector in solutions containing tris(2,2'-bipyridyl)ruthenium(II) (Ru(bpy)(3)(2+)) and persulfate. The array of reaction solutions is held in a 96-well plate, and absorbance readings are taken intermittently using a bioassay plate reader. The absorbance at 430 nm is indicative of the amount of Ru(bpy)(3)(2+) remaining in solution. The best catalysts give the most persistent absorbance, because the oxygen evolution reaction is kinetically competitive with decomposition of Ru(bpy)(3)(3+). Reagent concentrations were varied using a factorial design-of-experiment approach in order to optimize reaction conditions for a IrO(2).xH(2)O colloidal catalyst. A higher colloid concentration, a lower Ru(bpy)(3)(2+) concentration, and a higher pH buffer doubled the number of turnovers relative to the original conditions. Metal oxide colloids consisting of IrO(2).xH(2)O doped with varying amounts of Pt, Ru, and Os were made using a parallel microwave synthesis technique and were tested both by the parallel screening method and by direct measurement of oxygen evolution. The correlation between the two methods was good, with Ir-Pt-Os oxide compositions showing the highest activity. The effect of adding small amounts of Pt and Os to IrO(2).xH(2)O appears to be predominantly to reduce the particle size of the colloids.
Morris et al. (Fri,) studied this question.