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Substantial differences in photocatalytic reactivity were observed on powdered TiO2 and ZnO, both Pt-loaded and unloaded, in the photocatalytic hydrogenation of CH3C≡CH and C3H6 in the presence of H2O. Similar reactivity was observed, however, in the photocatalytic oxidation of i-C4H8 and C3H6 by O2 on these same catalysts. Photoluminescence of TiO2 and ZnO in the presence and the absence of H2O and/or O2 indicate that H2O causes a downward band bending to increase the efficiency of recombination of photogenerated electrons and holes, with a greater effect being observed on ZnO than on TiO2. These results, together with the direct detection of reaction intermediates by ESR, are rationalized by differences in the yields of the photogenerated intermediates, i. e., H (formed from trapped electrons) and OH (formed from trapped holes). The steady state concentrations of these intermediates depend on the degree of the positively-charged adducts on the surfaces of these oxides. The addition of O2 causes an upward band bending of the oxide and suppresses recombination of photo-generated electrons and holes, by formation of negatively-charged surface adducts. With similar degrees of band bending, TiO2 and ZnO produce almost the same photocatalytic reactivities as are observed for these oxides as dry systems.

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