Effects of nano-Ag particles loading on TiO 2 photocatalytic reduction of selenate ions

Abstract

The photocatalytic reduction of selenate Se(VI) ions was studied using unmodified TiO 2 and Ag-loaded TiO 2 (Ag-TiO 2) photocatalysts. In the presence of formic acid, both the TiO 2 and Ag-TiO 2 photocatalysts were effective in reducing Se(VI). The reaction proceeded through the reduction of Se(VI) ions to elemental selenium Se and then to hydrogen selenide gas (H 2Se). When unmodified TiO 2 photocatalyst was used, the Se formed from the reduction of Se(VI) was further reduced to Se 2− in the form of H 2Se upon the exhaustion of Se(VI) in solution. In the presence of the Ag-TiO 2 photocatalysts, hydrogen selenide gas was generated simultaneously with the reduction of Se(VI). It was found that the maximum Se(VI) reduction rate occurred at pH 3.5 and at a 0.5 at.% Ag loading while the maximum hydrogen selenide gas generation occurred at pH 3.5 and at 2.0 at.% Ag loading. The simultaneous reduction of Se(VI) to hydrogen selenide gas can be attributed to efficient charge separation due to the mediation of photogenerated electrons by the Ag particles. A mechanism is proposed in terms of the TiO 2-Ag-Se electronic interaction during UV irradiation.