Kinetics and mechanism of glycerol photo-oxidation and photo-reforming reactions in aqueous TiO₂ and Pt/TiO₂ suspensions

The kinetics and mechanism of glycerol photo-oxidation and photo-reforming reactions have been investigated over irradiated TiO₂ and Pt/TiO ₂ suspensions. It has been found that, in the presence of O ₂, oxidation of glycerol to CO₂ takes place readily over TiO₂ photocatalyst and that the reaction rate is enhanced substantially in the presence of dispersed Pt crystallites. The rate of the photo-reforming reaction is very slow over bare TiO₂ and increases significantly over the platinized sample. Analysis of reaction intermediates and final products in the gas and liquid phases indicates that both oxidation and reforming reactions proceed via the same general reaction pathways. The initial transformation steps involve hydrogenolysis of glycerol to propylene glycol and dehydration of glycerol to glyceraldehyde. Subsequent dehydration, dehydrogenation and decarbonylation reactions lead to the formation of various reaction intermediates, including glycoaldehyde, 2-oxopropanol, acetaldehyde, acetone, ethanol and methanol, which are eventually transformed to gas-phase CO₂ (and H₂ under photo-reforming reaction conditions). It is concluded that oxidation of glycerol to CO₂ over irradiated TiO₂ photocatalysts may take place by using either O₂ or H₂O as the oxidizing species. The overall reaction rate depends on the nature of the oxidant and is enhanced in the presence of Pt co-catalyst.