Abstract: VPO ultrafine particles were first successfully prepared by the supercritical fluid drying(SCFD). BET, XRD, TEM and XPS techniques were used to investigate the specific surface area, the phase composition, the grain size and the oxidation state of various samples. The VPO ultrafine particles after SCFD were activated in the reactant mixture or nitrogen at 673K for 6h. The results indicated that the obtained VPO ultrafine particles have a larger specific surface area, much smaller particle size and an unidentified crystalline phase in some cases after SCFD and activation. The kind and amount of promoter used and the activation atmosphere affect the physical properties of the VPO ultrafine particles. An ultrafine particle with Zr⁴⁺and Mo⁶⁺promoters, a surface area of 175.6 m²/g and grain size of smaller 50 nm, can be obtained by activating in the nitrogen atmosphere at 673 Kfor 6 h. The oxidation state of surface vanadium atoms of ultrafine particles was around +4, similar to that of vanadium in (VO)₂P₂O_7.The surface P/V molar ratio is in the range of 1.04-1.46, and is a little higher among the samples activated in N₂atmosphere. Primary property evaluating results indicated that the small VPOparticles showed very high selectivity but relatively low conversion.

Key words: VPO oxide, Ultrafine particle, Supercritical fluid drying.