Photocatalytic oxidation of sulfur dioxide in aqueous suspensions of α-Fe_2O_3)

The kinetics and mechanism of the photoassisted autoxidation of S(IV) in aqueous colloidal suspensions of α-Fe_2O_3 have been studied over the pH range of 2-10.5. Similar kinetic behavior toward S(IV) was observed for colloidal suspensions of TiO_2. Quantum yields, Ф, 4, ranged from 0.08 to 0.3 with a maximum yield found at pH 5.7. Upon band-gap illumination conduction-band electrons and valence-band holes are separated; the trapped electrons are transferred either to surface bound dioxygen or to Fe(III) sites on or near the surface while the trapped holes accept electrons from adsorbed S(IV) to produce S(V). The formation of S(V) radicals indicates that the reaction proceeds via successive one-electron transfers. The relatively high quantum yields are attributed in part to the desorption of SO_3^- from the α-Fe_2O_3 surface and subsequent initiation of a homogeneous free radical chain autoxidation of S(IV) to S(VI). Kinetic and thermodynamic models for the photoassisted surface chemistry of S(IV) on α-Fe_2O_3 are presented