Study of the photocatalytic reforming and oxidation of Glycerol over Ag-Pd/TiO2

The photocatalytic reaction of glycerol was conducted over 0.1 at. % Ag -0.3 at.% Pd/TiO2 catalyst in order to monitor the switch over between the photo-reforming to hydrogen and CO2 and photo-oxidation to H2O and CO2. This alloy of Ag-Pd has shown to be more active than Pd for hydrogen ion reduction previously. Up to a partial pressure of about 0.2 atm. of molecular oxygen the rates of hydrogen production were mostly unaffected. There seems to be a threshold for the effect of O2 on the inhibition of hydrogen production. This is found when the ratio of dissolved molecular oxygen to surface Ti cations is about 2. Also, the order of reaction with respect to PO2 was found to be-2. Hydrogen production was found to be largely insensitive to glycerol concentrations (above 0.05 vol %) as expected from a zero order reaction kinetics in the liquid phase, where the rate is determined by surface coverage. The considerable amounts of dissolved CO2 found indicate that monitoring gas phase CO2 is not a true measurement of the photo-reaction rates. The dissolved CO2 concentration was 15-20% of the gas phase concentration of CO2 at any reaction time investigated. The fast decrease of the pH of the aqueous solution within the first 30 min points to the build up of oxidation products of glycerol. Liquid phase analysis showed the presence of formaldehyde, hydroxy acetone and ethylene glycol as the main reaction products. At high glycerol concentration the main product in the liquid phase was hydroxy acetone while at low concentrations it was formaldehyde. At low glycerol concentrations, the rise of formaldehyde concertation in the liquid phase with time was concomitant with hydrogen production, its decay with time, however, was not accompanied by further in-crease in hydrogen production. This is most likely because of the competition between decomposition in the presence of water (to H2 and CO2) and oxidation (to formates).(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.