Screening of polyoxometalates, semiconductor metal oxides and zeolites for photocatalytic activity and selectivity

The present study reports the treatment performance and selectivity of two photochemically active materials, namely TiO₂ (1 g/L, pH = 2.8) and the heteropolyacid H₄SiW₁₂O₄₀ (1.44 g/L or 0.5 mM in aqueous solution, pH = 2.8) for the oxidation of phenol, isopropyl alcohol and formic acid selected as the recalcitrant index pollutants, using UV-C (20 W) and UV-A (125 W) light sources in two different photoreactors. In separate experiments, the effect of the zeolites Y (1 g/L; Si/A1 = 5.2/1 and 80/1) and Beta (1 g/L; Si/A1 = 75/1) as TiO₂ and H₄SiW ₁₂O₄₀ catalyst supports has also been studied under the same reaction conditions. It could be demonstrated that H₄SiW ₁₂O₄₀ is more selective than TiO₂, especially for the charge transfer type isopropanol oxidation. It was also found that the zeolitic supports improved the photocatalytic activity of polyoxometalates (POM) with a pronounced effect for UV-C (short - UV) irradiation. A significantly reduced photoactivity (12.5 % for UV-A and 9.5 % for UV-C) in comparison with TiO₂-mediated photocatalysis was observed for the combination POM + TiO₂, speculatively due to the conduction band electron "short-circuit" effect of POM, i.e. ecb^- + POM → POM_red.