Incorporative Effect of Pt and Na₂CO₃ on TiO₂‑Photocatalyzed Degradation of Phenol in Water

Carbonate anions are often present in aqueous solution, but their effect on the semiconductor-photocatalyzed reaction has been rarely studied. In this work, we report a positive effect of Na₂CO₃ on the TiO₂-photocatalyzed degradation of phenol, 2,4-dichlorophenol, and H₂O₂ in an aerated aqueous suspension at initial pH 8.0. The rate of phenol degradation, upon the addition of 2.0 mM Na₂CO₃, 0.52 wt% Pt, and 2.0 mM Na₂CO₃ plus 0.52 wt % Pt, was increased by 1.78, 3.38, and 6.63 times, respectively. Such positive effect of carbonate was also observed from a TiO₂ and Pt/TiO₂ film electrode for the photoelectrochemical oxidation of phenol, but not water. However, the rates of phenol degradation over TiO₂ and Pt/TiO₂ became decreased as carbonate concentration exceeded 5.0 and 2.0 mM, respectively. It is proposed that CO₃^•– radicals are formed mainly from the hole oxidation of dicarbonate adsorbed on TiO₂, followed by phenol degradation. At a high concentration, the CO₃^•– radicals would recombine to a peroxocarbonate that easily decomposes into CO₂ and O₂. The carbonate-mediated hole transfer from TiO₂ to phenol would incorporate with the Pt-mediated electron transfer from TiO₂ to O₂, consequently resulting in a great improvement in the efficiency of charge separation for the reactions at interface.