Photocatalytic dehydrogenation of liquid alcohols by platinized anatase and other catalysts
The work described in this thesis was undertaken at the University of Nottingham between October 1981 and November 1984, under the supervision of Dr. R. Rudham. Except where indicated by reference, it is the original work of the author and has not been submitted for any other degree. The photocatalytic dehydrogenation of liquid alcohols (methanol, ethanol, propan-1-ol and propan-2-ol) by suspensions of platinum and other metals supported on anatase have been investigated by following carbonyl compound formation under a nitrogen atmosphere. Measurements were made over the temperature range 278-303 K using filtered 366 nm U.V. radiation. Reaction on photodeposited catalysts was consistently associated with an activation energy of 20 + 1 kJ mol-1 , although the activity fell in the sequence: PT/TiO2 > Pd/TiO2 > Rh/TiO2 > Au/TiO2 – O for catalysts with a metal content of 0.5 wt%. The activation energy is identical to that for photoreaction on the anatase support in the presence of oxygen and is believed to be associated with the transport of photoelectrons through the anatase to either metal particles or adsorbed oxygen. Activities and activation energies for carbonyl compound formation from the photocatalytic dehydrogenation of the individual alcohols were effectively identical on the same catalyst, indicating that the photocatalytic dehydrogenation reaction is not governed by the physical or chemical properties of the reactant alcohol. With platinized anatase prepared by hydrogen reduction there was an appreciable dark reaction, which was absent with catalysts prepared by photodeposition. It is believed that reduction in hydrogen at elevated temperature renders the anatase support non-stoichiometric, a process favoured by spillover of hydrogen atoms from the platinum particles. The low activation energy for photocatalysis on hydrogen reduced catalyst is considered to be associated with the non-stoichiometric nature of the anatase, which presumably provides an energetically favourable mechanism for photoelectron transport to the metal particles and negates the photoelectron traps responsible for an activation energy of 20 ± 1 kJ mol-1 on the non-reduced catalysts. A radical mechanism for photocatalytic dehydrogenation is proposed. This mechanism predicts a l1m1 ting quantum yield of one half, which is in good agreement with the value obtained from experiments at different light intensities.