| The adsorption, photo- and thermal-chemistry of adsorbates, and the photoactivation of TiO2(110) single crystalline surfaces are the central topics investigated and presented in this thesis.; The hole-induced photodesorption of chemisorbed O2 from a defective TiO2(110) surface was studied to monitor the kinetics of electron-hole pair formation, recombination and hole-trapping. Two distinct O2 desorption processes are found which are characteristic of low and high photon fluxes. At a critical photon flux, the slow O2 desorption process converts to a fast process as a result of the saturation of hole trapping sites in the TiO2 crystal. Both the slow and fast O2 desorption processes are governed by the flux of UV radiation, Fhv1/2, indicating that the steady state concentration of photogenerated holes can be described by second-order electron-hole pair recombination kinetics. A hole scavenger is used to probe the role of added hole trap centers on the photodesorption rate.; The photodesorption for O2 from TiO2(110) was found to exhibit fractal kinetic behavior where the photodesorption process is described by a rate coefficient that varies throughout the measurement. A model is proposed in which the electrons associated with O-vacancy defects on the surface percolate from vacancy site to vacancy site via the filled orbitals at these sites to neutralize photo-produced holes. This electron percolation, causing e-h recombination, reduces the efficiency of charge transfer between a photo-produced hole and an O2-(a) species localized at a vacancy defect site, causing the rate of O2 photodesorption to follow a fractal rate law. We postulate that the fractal electron conduction path across the surface is one-dimensional.; The adsorption and thermal desorption of CO2 on TiO2 (110) was investigated as a probe of the concentration of oxygen vacancy defects. For the stoichiometric and fully oxidized surface, a single thermal desorption feature (Ed = 48.5 kJ/mol) is measured and is attributed to CO2 bound to regular 5-fold coordinated Ti4+ atoms. For the partially reduced TiO2(110) surface, CO2 binds not only to regular sites, but also to oxygen vacancy sites (E d = 54.0 kJ/mol), created by thermal annealing. The variation in the characteristic CO2 desorption kinetics was measured as a function of the surface reduction temperature and the systematic production of increasing levels of surface defects is observed in the temperature range of 600 K-1100K.; Finally, the thermal- and photo-chemistry of the mustard gas simulant molecule, 2-chloroethyl ethyl sulfide (2-CEES), was investigated on both TiO 2(110) single crystals and TiO2 powdered materials. 2-CEES decomposition occurs through the incorporation of lattice oxygen from the TiO2 material into the oxidized products. (Abstract shortened by UMI.)... |
