Super-ambient temperature gas phase photocatalysis

Abstract

This thesis is concerned with the use of a continuous flow gas phase photocatalytic system to produce hydrogen as a cleaner fuel source. M/TiO2 photocatalysts (where M is Pt, Pd, Au and Ag) was used in a continuous flow gas phase photocatalytic system. Two main reactions were examined in detail, the reaction of water and methanol to form carbon dioxide and hydrogen, and the water gas shift reaction, i.e. the reaction of water and carbon monoxide to form carbon dioxide and hydrogen. The catalysts were made via a wet impregnation method, typically between 0.01 to 10 wt.%.

It was observed for both these reactions that as the metal loading increased so did the rate of reaction for hydrogen production. This usually occurred until the metal loading reached a loading of around 0.2-0.5 wt.%, where after this loading, say > 0.5 wt.% until around 1 or even 10 wt.%, the rate of reaction greatly plummets, until very little or no rate of hydrogen production is observed. A theoretical rationale was applied and the metal support interface (MSI) model was replaced with a new expanding photocatalytic area overlap model (EPAO). This model gives a good fit to the data reported in this thesis but also to the data reported in the literature. A visual representation of this model is also provided and explored as a function of metal and particle or ‘dot’ size.

Further to this, selectivity in the methanol steam reforming reaction was explored for various M/TiO2 photocatalysts showing differing selectivity, either towards methanol oxidation to CO2 or methanol degradation to CO. The water gas reaction was probed further as a function of temperature, where Arrhenius kinetics are applied, and as a function of reactant concentration, where Langmuir-Hinshelwood kinetics are observed.