Heterogeneous photocatalysis. Oxidation of halide ions by oxygen in ultraviolet irradiated aqueous suspension of titanium dioxide

Abstract

In u.v. irradiated aqueous suspension of anatase, I^– ion are oxidised in the presence of O₂ at a conversion ≈ 80 times greater than that for Br^– ions, whereas Cl^– ions withstand oxidation. The quantum yields (≈ 2 % for I^–) have been determined and discussed. The nature of the final products depends on the pH, i.e., halogens or hypohalite ions in acidic or basic medium, respectively. However, the kinetics of the oxidation is not greatly influenced by the pH varying from 0 to 14. The reaction rate is proportional to the surface coverage in halide ions which is of the Langmuir type. We tentatively suggest that the reaction proceeds via the neutralization of adsorbed O^– ions by photogenerated holes producing activated oxygen species which oxidize adsorbed halide ions into hypohalite ions.