XPS and Mossbauer characterization of Au/TiO2 propene epoxidation catalysts

Gold catalysts supported on TiO2 and TiO2/SiO2 were used in gas-phase propene epoxidation with a hydrogen-oxygen mixture. The catalysts were characterized by Au-197 Mossbauer absorption spectroscopy (MAS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Gold particle sizes of 1 wt % Au catalysts calcined at 673 K ranged from 3 to 6 nm. Two Au contributions were found in Mossbauer spectra and assigned to bulk metallic Au atoms in the core of a gold particle and metallic gold on the outer surface of this particle. By MAS, no evidence for charge transfer from support to Au particle could be found. The Auger parameters confirmed that the surface layer of 3-5 nm gold particles is metallic. Deactivation is not due to a change in the active gold species but is related to the TiO2 support. In the preparation via deposition-precipitation, Au(OH)(3) species are converted to metallic gold during calcination. Gold particles do not gradually grow during calcination, possibly due to the simultaneous conversion of Au(OH)(3) Moieties with dehydroxylation of the TiO2 support. Epoxidation activity increases with the amount of surface metallic gold. No evidence for oxidized gold under reaction conditions was found.