PNNL

Abstract

It is generally held that radicals form and participate in heterogeneous photocatalytic processes on oxide surfaces, although understanding the mechanistic origins and fates of such species is difficult. In this study, photodesorption and thermal desorption techniques show that acetone is converted into acetate on the surface of TiO2(110) in a two step process that involves, first, a thermal reaction between acetone and coadsorbed oxygen to make a surface acetone-oxygen complex, followed second by a photochemical reaction that ejects a methyl radical from the surface and converts the acetone-oxygen complex into acetate. Designation of the photodesorption species to methyl radicals was confirmed using isotopically labeled acetone. The yield of photodesorbed methyl radicals correlates well with the amount depleted of acetone and with the yield of acetate left on the surface, both gauged using post-irradiation temperature programmed desorption (TPD). The thermal reaction between adsorbed acetone and oxygen to form the acetone-oxygen complex exhibits an approximate activation barrier of about 10 kJ/mol. A prerequisite to this reaction is the presence of surface Ti³? sites that enable O2 adsorption. Creation of these sites by vacuum reduction of the surface prior to acetone and oxygen co-adsorption results in an initial spike in the photodecomposition rate, but replenishment of these sites by photolytic means (i.e., by trapping excited electrons at the surface) appears to be a slow step a sustained reaction. Evidence in this study for the ejection of organic radicals from the surface during photo-oxidation catalysis on TiO2 provides support for mechanistic pathways that involve both adsorbed and non-adsorbed species.