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

Abstract UV photolysis of aerated aqueous suspensions of TiO 2 (Degussa P25) containing CH 3 OH 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 [CuSO 4 ] while at higher concentrations it slows down. The enhancement of R HCHO at very low [CuSO 4 ] is attributed to catalytic dismutation of O 2 − forming H 2 O 2 and O 2 . At higher [CuSO 4 ], 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 [CuSO 4 ] 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 O 2 − radical ions, which are produced via O 2 reaction with the TiO 2 electrons, are important for the well known square root dependency. Unlike the mobile and trapped electrons, O 2 − 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 O 2 − 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 TiO 2 by added Cu(II) implies an apparent improvement of the TiO 2 quality as a photocatalyst.