TIO2 based photocatalytic gas purification: the effects of co-catalysts and process conditions

Photocatalysis is highly promising as a technology to mitigate environmental pollution. In this thesis the focus will be on air purification by photocatalytic oxidation of volatile organic compounds (VOC’s). A major challenge in photocatalysis is the low photonic efficiency, due to high electron hole recombination rates. One of the solutions proposed for this issue, is the use of (metal) nanoparticles as co-catalyst. However, currently knowledge on how nanoparticles affect the photocatalytic performance of TiO2 is limited, especially because of the often complex relation between chosen reaction conditions (light intensity, oxygen and pollutant concentration) and rates obtained for TiO2 (and co-catalysts). The aim of this thesis is to improve the understanding of the effects of Pt nanoparticles on the rates obtained in photocatalytic oxidation of propane and ethanol over TiO2. Control of the properties of TiO2 and co-catalysts is essential for such fundamental study. Therefore, methods allowing the synthesis of well-defined TiO2 morphology, and co-catalysts have been explored and used, respectively supersonic cluster beam deposition and a spark-generator system. First, the effects of crystallinity, surface area and quantity of OH-groups of Hombikat (TiO2) on photocatalytic activity, with focus on the applied reaction conditions are tested for propane oxidation. The activity of both TiO2 and Pt-TiO2 was tested in the oxidation of ethanol and propane at variable light intensity and oxygen concentrations. The functionalization of TiO2 with Pt resulted in a significant increase in activity compared to TiO2 in ethanol oxidation, and in a minor decrease in activity for propane. It is proposed that the also observed change in selectivity by the addition of Pt is a result of changes in rate limiting steps in the reaction sequence of conversion of the reactants to CO2. The effect of the size of the Pt nanoparticles on the photocatalytic activity is described for both propane and ethanol oxidation. It is proposed that the negative effect of the addition of Pt, is a result of back-donation of electrons from Pt into TiO2 trap states, and that this back donation is more pronounced for larger particles. The practical implications of the results are also discussed.