Rh-and Ru-loaded Ti02-particles were produced from cluster precursors (Rh6 (CO)16, Ru3 (CO),,) and their activity in mediating H20-decomposition through band-gap excitation was investigated. Activity increases in the order Ru < RuO2 < Rh * Rh203. Bifunctional Rh/Ru02-loaded Ti02 exhibits optimal performance with overall light-to-chemical-energy conversion efficiency of 0.13%. Lack of Orappearance in the gas phase during photolysis observed with closed systems is due to photo-uptake of O2 by the Ti02-particles. In alkaline solution the capacity for 02-uptake is surprisingly high and the nature of the stored O2 is tentatively identified as a p-peroxo-bridged Ti-species.
Introduction. -Supported metal catalysts have previously been shown to be active in microheterogeneous systems that achieve H20-cleavage by visible and UV light [l-lo]. Noble metals deposited onto semiconductors such as Ti02 [l-31171, SrTi03 [4-61 and CdS [8] [9] appear to be particularly promising. To achieve optimum activity in H20-decomposition, high dispersion of the noble metal onto the support is generally required. This increases the H20-reduction and -oxidation rates per unit mass of catalyst employed. Moreover, there is recent evidence [ 111 that the recombination of H2 with O2 might be affected by the size of the noblemetal particle. Thus, it has been shown that the reduction of O2 on Pt-cathodes tested for fuel cell application is almost totally inhibited once the Pt-crystallite size decreases below 30 A. Reduction of O2 to H 2 0 is an undesirable back reaction in devices that afford simultaneous generation of H2 and O2 by light. In close relation to this effect stands the observation of Turkevich et al. [I21 that the activity of Pt-colloids in the catalytic decomposition of H202 drops from an extremely high value (close to that of catalase) to practically zero when the particle size was decreased below 30 A.
A particularly favorable situation for H20-decomposition would arise if these ultrafine Pt-particles, though inactive in 02-reduction, would still remain powerful catalysts for the generation of H2 from H20. That this concept is viable was shown by us in a recent investigation [13] where a systematic test of the effect of Pt-particle size on the rate of photochemical H2-generation in Pt/Ti02-dispersions was carried