Photoinduced oxygen formation and silver-metal deposition in aqueous solutions of various silver salts by suspended titanium dioxide powder

Abstract

The photochemical reaction of Ar-purged aqueous solutions containing various silver salts and TiO₂ powder in suspension has been studied at room temperature. Photoirradiation (λ_ex < 300 nm) resulted in O₂ formation and deposition of Ag metal on the TiO₂ particles. The apparent reaction rate depended on the anions of the silver salt and increased in the order ClO^–₄ < NO^–₃≈ SO^2–₄ < F^–(≈ PO^3–₄). The sensitizing activity of the rutile [TiO₂(R)] was comparable to that of the anatase [TiO₂(A)]. The molar ratio of deposited Ag metal to liberated O₂, which was independent of the reaction rate, was equal to ca. 5 except for the case of TiO₂(A)/AgClO₄. The pH of the reaction mixture decreased with irradiation time, resulting in deactivation of the TiO₂. Although O₂ formation and Ag-metal deposition did not occur at pH < 2, the photosensitizing activity of the TiO₂ powder was recovered by the addition of NaOH. The addition of propan-2-ol to the deactivated acidic suspension of TiO₂ was also effective for Ag-metal deposition but not for O₂ formation.