The photo-oxidation of water by sodium persulfate, and other electron acceptors, sensitised by TiO₂

A number of different electron acceptors are tested for efficacy in the oxidation of water to oxygen, photocatalysed by titanium dioxide. The highly UV-absorbing metal ion electron acceptors, Ce⁴⁺ and Fe³⁺, appear ineffective at high concentration (10^-2 M), due to UV-screening, but more effective at lower concentrations (10^-3 M). The metal-depositing electron acceptor, Ag⁺, is initially effective, but loses activity upon prolonged irradiation due to metal deposition which promotes electron-hole recombination as well as UV-screening the titania particles. Most striking of the electron acceptors tested is persulfate, particularly in alkaline solution (0.1 M NaOH). The kinetics of the photo-oxidation of water by persulfate, photocatalysed by titania are studied as a function of pH, [S₂O₈ ^2-] and incident light intensity (I). The initial rate of water oxidation increases with pH, is directly proportional to the concentration of persulfate present and depends upon I^0.6. The TiO₂/alkaline persulfate photosystem is robust and shows very little evidence of photochemical wear upon repeated irradiation. The results of this work are discussed with regard to previous work in this area and current mechanistic thinking. The formal quantum efficiency of the TiO₂/alkaline persulfate photosystem was estimated as ca. 2%.