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

[Display omitted] •Simple addition of Ni2+ greatly accelerated phenol degradation and mineralization.•Photoactivity increase of Pt/TiO2 by Ni2+ was much larger than that of TiO2.•Ni2+ concentration did not change with time, either in absence or presence of phenol.•NiOOH is formed, reactive to phenol and its reaction intermediates, but not to H2O.•The adsorbed NiOH+ on the oxide was more active than the deposited Ni(OH)2. 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 O2 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 O2 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.