Supported tantalum oxide and supported vanadia-tantala mixed oxides:: Structural characterization and surface properties

Silica-, alumina-, and zirconia-supported tantalum oxide and supported vanadia-tantala mixed oxide catalysts were investigated by XRD, UV-vis-DR, FTIR, and FT-Raman spectroscopy to determine the nature of the surface tantala species. Two types of surface TaOx species were identified. On the SiO2 support, at low coverages tetrahedral TaO4 species exist, with a Ta=O bond and three bridging Ta-O-support bonds, At higher coverages, octahedral TaO6 species occur. On Al2O3 and ZrO2 both surface species exist already at considerably low Ta loading. The acidic properties of the catalysts were investigated by an FTIR study of adsorbed pyridine. Supported tantala is clearly more Lewis acidic in comparison to vanadia. The oxidation of methanol was used to probe the catalytic performance of the supported oxide species and from the activity and product distribution it is concluded that supported tantala predominantly shows acidic over redox behavior.