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

Silica-, alumina-, and zirconia-supported tantalum oxide and supported vana dia-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 sup port, at low coverages tetrahedral TaO4 species exist, with a Ta=O bond and three bridging Ta-O-support bonds, At higher coverages, octahedral TaO6 sp ecies occur. On Al2O3 and ZrO2 both surface species exist already at consid erably low Ta loading. The acidic properties of the catalysts were investig ated by an FTIR study of adsorbed pyridine. Supported tantala is clearly mo re Lewis acidic in comparison to vanadia. The oxidation of methanol was use d to probe the catalytic performance of the supported oxide species and fro m the activity and product distribution it is concluded that supported tant ala predominantly shows acidic over redox behavior.