A low energy ion scattering (LEIS) study of the influence of the vanadium concentration on the activity of vanadium-niobium oxide catalysts for the oxidative dehydrogenation of propane

A series of vanadium-niobium oxide catalysts in which the vanadia content varies between 0.3 and 18 mol% was prepared by coprecipitation. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and by catalytic testing in the oxidative dehydrogenation reaction of propane. The results of the surface analysis by XPS and LEIS are compared, It is concluded that the active site on the catalyst surface contains 2.0 ± 0.3 vanadium atoms on average. This can be understood by assuming the existence of two or three different sites: isolated vanadium atoms, pairs of vanadium atoms, or ensembles of three vanadium atoms. At higher vanadium concentration more vanadium clusters with a higher activity are at the surface. LEIS revealed that as the vanadium concentration in the catalyst increases, vanadium replaces niobium at the surface. At vanadium concentrations above 8 mol%, new phases such as ß-(Nb, V)2O5 which are less active because vanadium is present in isolated sites are formed, while the vanadium surface concentration shows a slight decrease.