Cobalt(II) tetrasulfophthalocyanine on titanium dioxide. 2. Kinetics and mechanisms of the photocatalytic oxidation of aqueous sulfur dioxide

Abstract

Cobalt(I1) 4,4'4″4‴-tetrasulfophthalocyanine, Co^(II)TSP, covalently linked to the surface of titanium dioxide particles, TiO_2-CoTSP, is shown to be an effective photocatalyst for the oxidation of sulfur(IV) to sulfur(VI) in aqueous suspensions. Upon band-gap illumination of the semiconductor, TiO_2, conduction-band electrons and valence-band holes are separated; the electrons are channeled to the bound Co^(II)TSP complex resulting in the reduction of dioxygen while the holes react with adsorbed S(IV) to produce S(VI) in the form of sulfate. The formation of S(V) radicals indicates that the reaction proceeds via successive one-electron transfers. Quantum yields in excess of unity were observed and attributed to desorption of SO_3^- from the TiO_2 surface and subsequent initiation of a homogeneous free radical chain reaction. Observed quantum yields between 0.5 and 300 depend on the concentration and nature of free radical inhibitors present in the suspension. A kinetic model that integrates the photon absorption properties of the solid, the heterogeneous redox reactions on the catalyst surface, and the homogeneous reactions of S(IV) is presented.