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/TiO2 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 H-2 under photo-reforming reaction conditions). It is concluded that oxidation of glycerol to CO2 over irradiated TiO2 photocatalysts may take place by using either O-2 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. (C) 2012 Elsevier B.V. All rights reserved.