Spectroscopic studies of molybdate species deposited on a Nb2O5 support

Abstract

Three series of molybdate–Nb₂O₅ catalyst samples were prepared by impregnating Nb₂O₅ either at pH 0.5 (series A) or at pH 5.5 (series B) and at pH of equilibrium adsorption (0.5)(series C). X.r.d., Raman, F.t.i.r., diffuse reflectance, ESCA and FABMS-SIMS techniques have been used to characterize structural properties of the samples. The nature and structure of surface molybdate species were observed to depend on Mo loading and to a lesser extent on the method of preparation involving in particular the pH value of the impregnation solution. It is shown that polymeric and not monomeric molybdate species are formed first up to ca. 5 wt % MoO₃. At higher Mo loadings MoO₃ crystallites are also formed. SIMS data showed that for low Mo loading the top surface layer is much richer in oxygen ions than for MoO₃ crystallites. This is presumably related to its poor catalytic partial oxidation properties and to its acidic features. This behaviour is very different from that observed for molybdate species deposited on an SiO₂ support.