Promoting Effect of SiOx Monolayer Coverage of TiO2 on the Photoinduced Oxidation of Cationic Surfactants

The effect of SiO x monolayer coverage on the rate of the TiO 2 photoinduced oxidation of cetylpyridinium bromide (CPB) in aqueous solutions has been studied. The rate of the reaction increased ca. 3.4-fold with the surface modification in the 5−6 pH range. The promoting effect was emphasized as the initial concentration of CPB decreased. Adsorption experiments showed that the adsorption strength for CPB (β/M -1 ) increases from 3.7 × 10 3 (TiO 2 ) to 2.25 × 10 4 (SiO x /TiO 2 ), while the saturated amount of adsorption (Γ s /mol m - 2 ) is not so varied (Γ s ∼ 3 × 10 - 6 ). A drastic change in the point of zero charge from 7.5 (TiO 2 ) to 3.2 (SiO x /TiO 2 ), which is near to the value of SiO 2, was observed in titration experiments. A modified Langmuir−Hinshelwood model, where the rate of the surface reaction is assumed to be proportional to the coverage of CPB at the photostationary state that is determined by the balance between the rate of adsorption and the rate of decomposition, was proposed for dilute reaction systems. As the result of the kinetic analyses, the enhancement of the reaction with the SiO x monolayer coverage of TiO 2 could be mainly attributed to the increase in the electrostatic attraction of cetylpyridinium ion (increase in β).