Formation of small particles of gold on alumina support films and their behaviour in oxygen and hydrogen atmospheres

Attempt has been successfully made to disperse ultrafine gold particles on alumina support films by keeping the substrate at elevated temperatures during metal deposition under vacuum of ≈ 10^-5 Torr. It is found that reasonable dispersion of particles occurs for a mean thickness of 0.5 nm at a substrate temperature of 125 °C. The resulting samples turn out to be quite suitable for model studies of the behaviour of gold/alumina catalyst. Electron microscope observations coupled with diffraction have provided evidence for emergence of gold particles without any compound formation with the residual gases present during deposition at 10^-5 Torr. The nature of dispersion and average particle size are shown to depend on substrate temperature and the amount of metal evaporated. Heat treatments in hydrogen and oxygen at 200–500 °C for various lengths of time led to an increase in particle size with a simultaneous decrease in number density (i.e. coarsening of particles) following, in all probability, the Ostwald ripening mechanism. The oxidizing atmosphere has been shown to be more favourable to coarsening or deactivation of the catalyst. Finally, it is indicated that gold particles prefer to facet giving well-defined shapes (e.g. hexagonal, pentagonal or rhombohedral) with faces corresponding to planes of relatively smaller interfacial energies.