The catalytic effects of copper ions on photo-oxidation in TiO₂ suspensions: The role of superoxide radicals

UV photolysis of aerated aqueous suspensions of TiO₂ (Degussa P25) containing CH₃OH at pH 8.5 produces HCHO. Addition of CuSO ₄ affects the rate of HCHO formation (R_HCHO) in two opposite ways. Below 2 μM, R_HCHO increases with increasing [CuSO₄] while at higher concentrations it slows down. The enhancement of R_HCHO at very low [CuSO₄] is attributed to catalytic dismutation of O₂^- forming H₂O₂ and O₂. At higher [CuSO₄], R_HCHO decrease is attributed to removal of mobile holes by copper. Square root dependency of HCHO yield on the absorbed light density is observed in the entire [CuSO₄] range used. The square root dependency is generally attributed to competition between second-order electron-hole recombination and first-order trapping, although the lifetime of both electrons and holes is too short to enable their accumulation in the same nano-volume. It is shown here that the electron-hole recombination path involves reaction of the mobile holes with adsorbed O ₂^-. The experimental results provide indirect evidence that adsorbed O₂^- radical ions, which are produced via O₂ reaction with the TiO₂ electrons, are important for the well known square root dependency. Unlike the mobile and trapped electrons, O₂^- has a relatively long lifetime and its accumulation in the nano-volume upon successive absorptions of photons is feasible. It is concluded that the reaction between the accumulated O₂^- and mobile holes is responsible for the square root law. The decrease of K _d, which has been suggested as a standard parameter for the nature of TiO₂ by added Cu(II) implies an apparent improvement of the TiO ₂ quality as a photocatalyst.