A LOW-ENERGY ION-SCATTERING (LEIS) STUDY OF THE INFLUENCE OF THE VANADIUM CONCENTRATION ON THE ACTIVITY OF VANADIUM-NIOBIUM OXIDE CATALYSTSFOR THE OXIDATIVE DEHYDROGENATION OF PROPANE

A series of vanadium-niobium oxide catalysts in which the vanadia cont ent varies between 0.3 and 18 mol% was prepared by coprecipitation, Th ese catalysts were characterized by X-ray diffraction (XRD), X-ray pho toelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and b y catalytic testing in the oxidative dehydrogenation reaction of propa ne. 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 assum ing the existence of two or three different sites: isolated vanadium a toms, pairs of vanadium atoms, or ensembles of three vanadium atoms, A t higher vanadium concentration more vanadium clusters with a higher a ctivity are at the surface, LEIS revealed that as the vanadium concent ration in the catalyst increases, vanadium replaces niobium at the sur face, At vanadium concentrations above 8 mol%, new phases such as beta -(Nb, V)(2)O-5 which are less active because vanadium is present in is olated sites are formed, while the vanadium surface concentration show s a slight decrease. (C) 1995 Academic Press, Inc.