Titanium dioxide photocatalysis: studies of the degradation of organic molecules and characterization of photocatalysts using mechanistic organic chemistry

This dissertation explores the use of physical organic chemistry in order to define the specific chemical mechanisms that lead to the degradation of organic molecules in the presence of titanium dioxide photocatalysts and light. The first study focuses effects of photocatalyst particle size on the early chemical steps of degradation of two probe molecules, 4-methoxyresorcinol (MR) and 1-para-anisyl-1-neopentanol (AN) when the Millennium PC series anatase titania catalysts are used. These catalysts differ in particle size based on the amount of thermal annealing. Product formation ratios and kinetic rates of loss show that changes in primary particle size between 5 and 100 nm do not affect the mechanisms by which MR and AN degrade, and only moderate degradation rate increase as particle size increased was noted. Therefore, primary particle size is in the range studied had an effect on the same order as varying other experimental parameters like pH. Next, multiple tungsten-doped titania catalysts were studied in order to explore their reactivity in terms of improving visible light absorption and improve degradation efficiency. Using quinoline (Q) and AN as molecular probes, kinetic and product studies were performed. Results showed that doping W exclusively on the surface as WOx showed a marked increase in degradation efficiency compared to pristine TiO2, and that bulk doping had a minor positive effect. Visible light activity was studied by using 420 nm bulbs. Regardless of the type of doping, visible light activity was minimal at best, and attributable to red-edge absorption of titanium dioxide, not a W-based species. In conclusion, surface doping of tungsten helps improve the efficiency of photocatalytic degradation, but only at UV wavelengths. Finally, a set of biphenylcarboxylic acid-based molecular probes have been proposed for use in characterizing new titanium dioxide catalysts for activity. Using a standard titanium dioxide catalyst, the kinetics and product formations of these biphenyl probes were evaluated. For each probe, the major hydroxylation products were identified and quantified. The downstream single electron transfer (SET) products were also identified. Control reactions were also employed in order to determine the product mixtures when only hydroxyl radicals were available as reactive species. These studies showed that variation of the electronic demand on the carboxyl-bearing ring played a major role in the reactivity of the probe, including degradation rate and product ratios. A simple ab initio computational method was also developed and employed in order to shed light on the experimental trends found in product formation for photocatalytic degradations. First, Q was used and experimental trends for hydroxylation and SET-based reactions were reproduced faithfully. Second, the same analysis was performed on the biphenyl probes, and the results were again faithful to experiments, though the details were less clear-cut. With more use and refinement, this method could lead to a simple predictive tool for identifying possible and likely products of photocatalysts for probe molecules