Bulk and surface structure and composition of V-Sb mixed-oxide catalysts for the ammoxidation of propane

Vanadium-antimony mixed oxides, which are active and selective catalysts for the ammoxidation of propane to acrylonitrile, were obtained via different preparation routes and studied with a number of bulk and surface-sensitive techniques to elucidate the bulk composition of these complex materials and the character of their exposed surfaces. The V-Sb oxides were prepared via a redox reaction between NH4VO3 and Sb2O3 in an aqueous slurry, with subsequent calcination, or via a solid-state reaction between Sb2O3 and V2O5. The characterisation techniques employed were X-ray diffraction (XRD), N-2 physisorption, electron microscopy, potentiometric titration, Mossbauer, EPR, X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), ultraviolet photoelectron spectroscopy (UPS), and LR spectroscopy. It was found that samples obtained by the solid-state reaction were more homogeneous than those prepared via the slurry route. The former consisted of (non-stoichiometric) VSbO4 for Sb/V = 1, or a physical mixture of it with defective Sb2O4 for Sb/V = 2. Their bulk Sb/V ratio was found also for the surface region, the outmost part of which was enriched in vanadium to a small extent. Materials prepared via the slurry route consisted of (non-stoichiometric) VSbO4, Sb2O4, and V2O3 if Sb/V = 1; the latter was missing for Sb/V = 2 and Sb/V = 5. While the character of the mixed surfaces was not determined by the V2O5 present, amorphous V oxide structures (highly dispersed species and aggregates) were supported on the Sb2O4 surface, which caused a significant vanadium excess in the overall surface composition in samples of Sb/V > 1. The average oxidation degree of the surface V species was higher than 4+. Use of these catalysts in the propane ammoxidation reaction caused the surface V oxidation degree to approach 4+ and diminished the degree of surface enrichment in vanadium. This was due both to a disappearance of the dispersed V entities and a decreased detectability of V oxide aggregates (aggregate growth or solid-state reaction with supporting Sb2O4). No evidence is available for surface spreading of Sb species as discussed in the literature. (C) 2000 Published by Elsevier Science B.V.