Natural photosynthesis, which achieves efficient solar energy conversion through the combined actions of many types of molecules ingeniously arranged in a nanospace, highlights the importance of a technique for site-selective coupling of different materials to realize artificial high-efficiency devices. In view of increasingly serious energy and environmental problems, semiconductor-based artificial photosynthetic systems consisting of isolated photochemical system 1 (PS1), PS2 and the electron-transfer system have recently been developed. However, the direct coupling of the components is crucial for retarding back reactions to increase the reaction efficiency. Here, we report a simple technique for forming an anisotropic CdS-Au-TiO2 nanojunction, in which PS1(CdS), PS2(TiO2) and the electron-transfer system (Au) are spatially fixed. This three-component system exhibits a high photocatalytic activity, far exceeding those of the single- and two-component systems, as a result of vectorial electron transfer driven by the two-step excitation of TiO2 and CdS.

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Nanometer-sized gold particles with varying mean size from 3.2 to 12.2 nm were loaded on the surfaces of TiO2 particles in a highly dispersed state with the loading amount maintained constant (0.46 +/- 0.02 mass %) using the deposition-precipitation method. Light irradiation (lambda(ex) > 300 nm) to a deaerated ethanol TiO2 particle suspension containing elemental sulfur (S8) led to the energetically uphill reduction of S8 to H2S. It has been found that this reaction is dramatically enhanced with such a low level of Au loading on TiO2 and that the zero-order rate constant of reaction increases with decreasing mean size of Au nanoparticles (d). The effects of reaction parameters (substrate concentration, light intensity, temperature) on the rate of reaction were studied to infer the essential reaction mechanism. Further, a kinetic analysis has led to a conclusion that the increase in the rate of reaction with decreasing d results from the improvement of the charge separation efficiency.

Catalytic combustion of ethyl acetate was investigated over various CeO 2 -supported precious metal catalysts prepared by impregnation method, and the effect of reduction treatment on the activity was examined. Among the catalysts tested, Ru/CeO 2 achieved the highest activity for ethyl acetate combustion, and the activity was almost unchanged by the heat treatment in a hydrogen atmosphere. In the cases of Pt/CeO 2 , Pd/CeO 2 , and Rh/ CeO 2 , the catalytic activity was enhanced by the reduction treatment at 400°C, though the activity of the reduced catalysts was still inferior to that of Ru/CeO 2 . It was confirmed by temperature-programmed reduction that the reduction of the ruthenium species was initiated at the lowest temperature among the CeO 2 -supported precious metals. The precious metal species reducible at lower temperatures should be responsible for the high activity in the complete oxidation of ethyl acetate.

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