Generation of organic radicals during photocatalytic reactions on TiO ₂

Using a variety of organic carbonyl molecules (R₁C(O)R ₂) and the rutile TiO₂(1 1 0) surface as a model photocatalyst, we demonstrate both experimentally and theoretically that ejection of organic radicals from TiO₂ surfaces is likely a prevalent reaction process occurring during heterogeneous photooxidation of organic molecules. Organic carbonyls react with coadsorbed oxygen species to form organic diolates which are more strongly bound to TiO₂ than are the parent carbonyls. The parent carbonyls, when bound to TiO₂(1 1 0) in an ν¹ configuration, are photo-inactive toward valence band holes. However, the diolates are shown to photodecompose by ejection of one of the two R substituents from the surface into the gas phase, leaving behind the carboxylate of the other R group. Theoretical calculations using DFT show that in most cases the choice of which R group is ejected can be predicted based on the C-R bond energies and, to a lesser extent, the stability of the ejected R group.