Photocatalytic oxidation of iodide ions by titanium dioxide

Abstract

The photocatalytic oxidation of aqueous I^–, usually as neutral 0.1 mol kg^–1 KI, has been investigated using suspensions of pure anatase and rutile, five doped rutiles and two commercial anatase pigments. Reaction only proceeded in the presence of both O₂ and u.v. radiation to yield I₂, which reacted with excess I^– to form I₃^–; there was no evidence for IO^– or IO₃^– formation. The dependence of initial reaction rate on the square root of the intensity of the incident 366 nm radiation, together with low quantum yields, reflect the recombinative loss of photoelectrons and photoholes greatly exceeding their consumption in productive reaction steps. No activation energy for I₂ formation was observed with any of the catalysts, and in this respect the photo-oxidation of I^– differs from that of liquid alcohols on the identical catalysts. A mechanism is proposed in which I₂ and OH^– result from electron transfer between charged and neutral species adsorbed on the TiO₂ surface, leading to an overall reaction: 4I^–+ O₂+ 2H₂O +hvâ†� 2I₂+ 4OH^–.