Oxidation of phenol by radiolytically generated ^•OH and chemically generated SO₄^•-. A distinction between ^•OH transfer and hole oxidation in the photolysis of TiO₂ colloid solution

Yields and distribution of the hydroxylation products of phenol in aqueous solutions were measured under the following conditions: (i) γ-radiolysis, (ii) reaction with SO₄^•-, and (iii) photolysis in the presence of TiO₂ colloid particles. Cupric ions as oxidants of OH adducts were present in all three systems. At relatively low phenol concentrations, the photochemical and the γ-radiolysis systems show similar behavior, with the exception that the dependence of the product yield on pH is shifted toward lower pH's in the photochemical system as compared to the radiolysis system. This is expected if the photolytic hydroxylation reaction takes place at the TiO₂ surface where the positive charge of the TiO₂ colloid particles induces a higher local pH compared with that in the bulk of the solution. At relatively high phenol concentrations, the photochemical system resembles the SO₄^•- system. This concentration effect results from the oxidation of adsorbed phenol by mobile TiO₂ holes, which are the precursors of the localized holes. The mobile holes oxidize adsorbed phenol by electron transfer, while the latter react with phenol via an ^•OH radical transfer. Our results indicate that localized TiO₂ holes are relatively long-lived and are capable of reacting with nonadsorbed molecules.