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 fo r 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 an d the character of their exposed surfaces. The V-Sb oxides were prepared vi a a redox reaction between NH4VO3 and Sb2O3 in an aqueous slurry, with subs equent calcination, or via a solid-state reaction between Sb2O3 and V2O5. T he characterisation techniques employed were X-ray diffraction (XRD), N-2 p hysisorption, 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 homogene ous 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 defect ive Sb2O4 for Sb/V = 2. Their bulk Sb/V ratio was found also for the surfac e region, the outmost part of which was enriched in vanadium to a small ext ent. Materials prepared via the slurry route consisted of (non-stoichiometr ic) 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 specie s and aggregates) were supported on the Sb2O4 surface, which caused a signi ficant vanadium excess in the overall surface composition in samples of Sb/ V > 1. The average oxidation degree of the surface V species was higher tha n 4+. Use of these catalysts in the propane ammoxidation reaction caused th e surface V oxidation degree to approach 4+ and diminished the degree of su rface enrichment in vanadium. This was due both to a disappearance of the d ispersed V entities and a decreased detectability of V oxide aggregates (ag gregate growth or solid-state reaction with supporting Sb2O4). No evidence is available for surface spreading of Sb species as discussed in the litera ture. (C) 2000 Published by Elsevier Science B.V.