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

Abstract

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 NH 4VO 3 and Sb 2O 3 in an aqueous slurry, with subsequent calcination, or via a solid-state reaction between Sb 2O 3 and V 2O 5. The characterisation techniques employed were X-ray diffraction (XRD), N 2 physisorption, electron microscopy, potentiometric titration, Mössbauer, EPR, X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), ultraviolet photoelectron spectroscopy (UPS), and IR 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) VSbO 4 for Sb/V=1, or a physical mixture of it with defective Sb 2O 4 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) VSbO 4, Sb 2O 4, and V 2O 5 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 V 2O 5 present, amorphous V oxide structures (highly dispersed species and aggregates) were supported on the Sb 2O 4 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 Sb 2O 4). No evidence is available for surface spreading of Sb species as discussed in the literature.