EFFECT OF SURFACE ADSORPTIONS OF ALIPHATIC-ALCOHOLS AND SILVER ION ON THE PHOTOCATALYTIC ACTIVITY OF TIO2 SUSPENDED IN AQUEOUS-SOLUTIONS

Photoirradiation of the light of wavelength > 300 nm onto suspensions of titanium(IV) oxide (TiO2, anatase, TiO2(A), and rutile TiO2(R)) in aqueous solution of silver salts and aliphatic alcohols led to the evolution of molecular oxygen (O2), the deposition of silver metal (Ag) onto the surface of TiO2, and the oxidation of alcohols. The product distribution was studied by using some aliphatic alcohols (methanol, ethanol, 2-propanol, and tert-butyl alcohol) and inorganic and organic silver salts (sulfate, perchlorate, nitrate, fluoride, and acetate). Kinetic analyses were performed with an aqueous suspension containing 2-propanol and silver sulfate. Dependence of the initial formation rates of products (O2, acetone, and Ag) on the concentration of 2-propanol in suspension (C(p)) was analyzed on the assumption of a mechanism including reductive and oxidative trappings of pairs of photoexcited electron (e-)-positive hole (h+) by surface-adsorbed substrates. The mechanism implies facile recombination of the pair in the absence of the appropriate surface-adsorbed substrates, which react with the pair to produce free active species, e- and h+, with sufficient ability for subsequent redox reactions. It is presumed that coverage of surface sites with 2-propanol via Langmuirian adsorption determines the distribution of free h+ generated by the reductive trapping by silver ion (Ag+). The C(p) dependence of the rate in TiO2(A) suspension was consistent with the proposed mechanism and some kinetic and adsorption parameters were obtained. For the results in TiO2(R) suspension, it was necessary to suppose additional oxidation sites for not acetone but O2 production, suggesting the characteristics in the O2 evolution mechanism of TiO2(R). Dependence of the rate of acetone formation on the concentration of silver ion (C(a)) was also analyzed for both TiO2 suspensions. By using the parameters obtained from the C(p) dependence, the apparent C(a) dependence could be reasonably reproduced according to the proposed mechanism.