Influence of Zn(II) adsorption on the photocatalytic activity and the production of H2O2 over irradiated TiO2

The photocatalytic activity of semiconductor oxides, in particular TiO2 powders or colloids, is a complex function of bulk (light absorption and scattering, charge carrier mobility and recombination rate) and surface (structure, defects and reconstruction, charge, presence of adsorbate, surface recombination centers) properties. Among surface modifications, the inner sphere surface complexation of metal cations can change the surface charge of the metal oxide, thus changing the surface activity coefficient of ionic substrates, the band edge positions, as well as the mechanism and kinetic of interfacial electron transfer by blocking surface trapping sites for photogenerated carriers ( Ti-OH). In this work we show that in anatase/water systems under band-gap irradiation, both the organic substrate (formate) oxidation initiated by photogenerated valence band holes and the formation of hydrogen peroxide from O-2 reduction (by conduction band electrons) is strongly influenced by the presence of Zn2+ cations. Depending on the pH, the formate oxidation rate can be enhanced or nearly completely inhibited. The observed result can be rationalized by considering the fraction of Ti-OH surface sites blocked by inner sphere complexation of Zn2+ as a function of pH. When this fraction is low, the more positive surface charge favors formate oxidation, whereas when the fraction is high the almost complete blockage of Ti-OH surface sites by Zn2+ stops almost entirely formate oxidation. Interestingly, the surface complexation of Zn2+ is accompanied by an increasing production of H2O2 during formate degradation in the presence of O-2. Zn(II) cations are not complexed by peroxide/superoxide species derived from O-2 reduction. When Ti-OH sites are blocked by Zn2+, the complexation on the TiO2 surface of peroxide/superoxide species is inhibited, hindering their further transformation. The results presented demonstrate that the combined effect of pH and surface complexation of redox inert cations greatly influences both the oxidative and reductive processes during the photocatalytic process over TiO2.