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

Anatase TiO 2 is the most studied environmental photocatalyst, but its efficiency is still not high enough to enable application. Herein we report a positive effect of Ni 2+ ions on the photocatalytic reaction in neutral aqueous solution. On addition of 1 mM Ni(ClO 4 ) 2 , the rates of phenol oxidation on TiO 2 and 0.5 wt.% Pt/TiO 2 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, Ni 2+ concentration did not change with irradiation time. Moreover, a maximum rate of phenol degradation was observed with Ni 2+ ions, occupying half the surface of TiO 2 or Pt/TiO 2 , where Ni(OH) + was more active than Ni(OH) 2 . X-ray photoelectron spectroscopy identified a Ni 3+ species produced in absence of phenol. A (photo)electrochemical study revealed that Ni 2+ oxidation was favored over H 2 O oxidation, whereas Ni 2+ and Pt inhibited and facilitated O 2 reduction, respectively. It is proposed that the adsorbed Ni(OH) + on TiO 2 is oxidized to Ni 3+ by the photogenerated holes of TiO 2 , followed by regeneration through phenol oxidation. The Ni 2+ -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 Ni 2+ ions as the hole mediator of organic oxidation over a semiconductor.