Photooxidation processes on single crystal titanium dioxide(110) Surfaces

There is considerable interest in light induced chemistry on TiO 2 surfaces due to its numerous applications, one of the most common being the destruction of hazardous bacterial matter and organic pollutants in wastewaters or air. In some cases, the complete oxidation of organic compounds to CO2 and H2O is observed. The work presented here focuses on thermally induced or photoinduced processes occurring on TiO2 surfaces with the goal of better understanding the decomposition mechanism of such pollutants and identifying the intermediates involved. Final state velocity distributions of molecules ejected from the TiO2 surface upon photoexcitation were determined under ultrahigh vacuum (UHV) conditions to gain insight into the energy transfer processes that occur. The velocity distributions were measured by using a pump-probe, time-of-flight (TOF) method along with one photon, vacuum ultraviolet (VUV) ionization of gas phase products for product detection. Temperature program desorption (TPD) studies were employed to identify surface bound products.;Adsorbed molecular oxygen has been shown to be essential in the removal of organic compounds on TiO2, hence initial investigations concentrated on the interaction of O2 with single crystal rutile TiO2(110) surfaces. Velocity distributions of molecular oxygen desorbed from the surface via a photo-induced process were determined and are shown to be consistent with a hole capture, substrate mediated excitation mechanism. In addition, the distributions indicate that different initial states of adsorbed oxygen, perhaps different O2 species (i.e. O2- or O22-), binding sites, or geometries exist on the surface of TiO2(110).;An investigation of the photodecomposition mechanism of organic molecules was performed by exploring the dynamics of desorbates produced from photoreactions of model carbonyl compounds (acetone and 2-butanone) on TiO2(110) surfaces. Consistent with previous studies, it was found that in order for the system to be active for photodecomposition adsorbed molecular oxygen is required. Evidence that an active oxygen species involved is an O atom is presented. The primary photooxidation pathway yields two products, a gas phase alkyl radical and surface bound carboxylate which are generated by photo-induced fragmentation of the ketones. Velocity distributions of the alkyl radicals were determined to be bimodal indicating two channels for photodecomposition operate under the conditions explored. In the case of 2-butanone the two channels may be described by the slow channel involving excess energy partitioning into excitation of the pyramidal bending mode of the methylene group of the ejected ethyl radical. Additionally, no evidence for secondary chemistry involving alkyl radicals was observed under the UHV conditions employed.