| Apparent catalyst deactivation is a frequently observed phenomenon during the continuous gas-phase photocatalytic oxidation of aromatic contaminants on titanium dioxide. Even low aromatic concentrations (∼10 ppm), contaminant removal rates may decline substantially over time. This apparent deactivation has been attributed to the accumulation of recalcitrant, strongly bound, partially oxidized reaction intermediates on the catalyst surface.; This hypothesis is examined in this thesis, which develops several transient kinetic models for the photocatalytic oxidation of benzene, toluene, and xylene. One proposed model, which relied on two distinct types of catalyst sites, was most capable of reproducing experimental data collected during the photocatalytic oxidation of aromatic contaminants. Further development of this Two-Site kinetic model also allowed for the simulation of catalyst regeneration via photooxidation of the accumulated reaction intermediates in the presence of uncontaminated, humidified air and UV illumination.; Previous studies in our lab have suggested that the use of an HCl pretreated titania photocatalyst can delay the onset of apparent deactivation in the photocatalytic oxidation of branched aromatic contaminants, but not of benzene, through the generation; of reactive chlorine radicals. In the present study, a broad range of pretreatments was examined experimentally. An acidic pretreatment solution (HCl) was found to be necessary to produce enhanced activity from a chloride-based pretreatment. The presence of a proton (H+) source appeared to be necessary in order to facilitate the photocatalytic generation of chlorine radicals. However, no other halide acids (HF, HBr, or HI) were found to produce enhanced photocatalytic activity.; A thermodynamic analysis indicates that bromine and iodine radicals, if produced, are not sufficiently reactive to participate in the oxidation of aromatic contaminants. Fluorine radicals could produce enhanced photocatalytic activity, but the energy required to generate these radicals is not available in the titania photocatalytic system examined. Chlorine radicals were calculated branched aromatic contaminants, but not from the aromatic ring. This explanation is consistent with our current and previous experimental findings, which demonstrated enhanced activity towards branched aromatics, but not towards benzene. |
