Generation of organic radicals during photocatalytic reactions on TiO2

Using a variety of organic carbonyl molecules (R1C(O)R2) 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 η1 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.