Enhanced photocatalytic degradation of phenol over anatase TiO2 in neutral water on addition of iron(iii) substituted polyoxotungstate

Abstract

PW₁₂O₄₀-type polyoxometalates have been widely used as electron mediators of TiO₂ photocatalysis for organic degradation in water, but they are stable only at pH 1–2, which greatly limits their application for water treatment. Herein we report an iron(III)-substituted PW₁₁O₃₉ (PW₁₁Fe) capable of mediating the photocatalytic degradation of phenol and 2,4-dichlorophenol in an aerated aqueous suspension of anatase TiO₂ at pH 2.0–7.2. As the initial concentration of PW₁₁Fe or the initial pH of the suspension increased, the rate of phenol degradation increased, and then decreased. A maximum reaction rate was achieved at 2.0 mM PW₁₁Fe and pH 5.5, which was 3.9 times higher than that measured without PW₁₁Fe. In all cases, the rates of phenol degradation were of the first order in phenol, implying the recycling behavior of PW₁₁Fe. Through electrochemical measurement, a possible mechanism is proposed, which involves the interfacial electron transfer from the irradiated TiO₂ to PW₁₁Fe, and the reduction of O₂ by the reduced PW₁₁Fe. This would improve the efficiency of the charge separation of TiO₂, and consequently increase the rate of phenol degradation at interfaces.