Role of Ni2+ ions in TiO2 and Pt/TiO2 photocatalysis for phenol degradation in aqueous suspensions

Anatase TiO2 is the most studied environmental photocatalyst, but its efficiency is still not high enough to enable application. Herein we report a positive effect of Ni2+ ions on the photocatalytic reaction in neutral aqueous solution. On addition of 1 mM Ni(ClO4)(2), the rates of phenol oxidation on TiO2 and 0.5 wt.% Pt/TiO2 increased by factors of 2.1 and 8.0, respectively. Meanwhile, the formation of organic intermediate was reduced, and the removal of total organic content was enhanced. Interestingly, Ni2+ concentration did not change with irradiation time. Moreover, a maximum rate of phenol degradation was observed with Ni2+ ions, occupying half the surface of TiO2 or Pt/TiO2, where Ni(OH)(+) was more active than Ni(OH)(2). X-ray photoelectron spectroscopy identified a Ni3+ species produced in absence of phenol. A (photo)electrochemical study revealed that Ni2+ oxidation was favored over H2O oxidation, whereas Ni2+ and Pt inhibited and facilitated O-2 reduction, respectively. It is proposed that the adsorbed Ni(OH)(+) on TiO2 is oxidized to Ni3+ by the photogenerated holes of TiO2, followed by regeneration through phenol oxidation. The Ni2+-catalyzed organic oxidation then promotes the Pt-catalyzed O-2 reduction, and vice visa, resulting into the greatly improved efficiency of charge separation. This work highlights the possibility of Ni2+ ions as the hole mediator of organic oxidation over a semiconductor.