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Application of Advanced Oxidation Processes (AOP) such as sono, photo and sonophoto catalysis in the purification of polluted water under ambient conditions involve the formation and participation of Reactive Oxygen Species (ROS) like ·OH, HO2·, O2(-), H2O2 etc. Among these, H2O2 is the most stable and is also a precursor for the reactive free radicals. Current investigations on the ZnO mediated sono, photo and sonophoto catalytic degradation of phenol pollutant in water reveal that H2O2 formed in situ cannot be quantitatively correlated with the degradation of the pollutant. The concentration of H2O2 formed does not increase corresponding to phenol degradation and reaches a plateau or varies in a wave-like fashion (oscillation) with well defined crests and troughs, indicating concurrent formation and decomposition. The concentration at which decomposition overtakes formation or formation overtakes decomposition is sensitive to the reaction conditions. Direct photolysis of H2O2 in the absence of catalyst or the presence of pre-equilibrated (with the adsorption of H2O2) catalyst in the absence of light does not lead to the oscillation. The phenomenon is more pronounced in sonocatalysis, the intensity of oscillation being in the order sonocatalysis>photocatalysis⩾sonophotocatalysis while the degradation of phenol follows the order sonophotocatalysis>photocatalysis>sonocatalysis>sonolysis>photolysis. In the case of sonocatalysis, the oscillation continues for some more time after discontinuing the US irradiation indicating that the reactive free radicals as well as the trapped electrons and holes which interact with H2O2 have longer life time (memory effect).

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SummaryRh-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