The photoelectrochemical properties of Ti02 single-crystal electrodes on which a small amount of RuQ2 is deposited were investigated. From the increase of cathodic current around the flat band potential and the negative potential shift of Ru02 on Ti02 under illumination, Ru02 on n-type semiconductors such as Ti02 and CdS is concluded to play a role as a reduction catalyst for the water-splitting reaction and other hydrogen-evolution reactions. A remarkable catalytic effect of Ru02 on powdered Ti02 for hydrogen evolution and the isotope effect on hydrogen evolution from an ethanol-water mixture support this idea. The dependence of oxygen-evolution rate on the molar ratio of Ru02 to Ti02 behaves quite differently depending on the kind of electron acceptor. In the presence of Ag+ as an electron acceptor, RuOz on powdered TiÓ2 suppresses photocatalytic oxygen evolution. A negative shift of the potential of RuQ2 on a Ti02 single crystal under irradiation indicates that RuOa on Ti02 functions as a reduction site for photocatalytic oxygen evolution in the presence of a strong electron acceptor such as Ag+ or Fe3+.

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