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The structural properties and oxidation state of vanadium in alumina-supported vanadium-phosphorous oxides at nearly monolayer coverage influence its catalytic performances. The population distribution VVO and VIVO species in the amorphous VPO phase strongly depends on the sample history. By using alumina-supported VPO instead of conventional bulk catalysts, we facilitate enhanced discrimination between active phase and bulk phase signals in the vanadium-phosphorous oxygen system. This brings light on the speciation of VPO catalysts in general. Our observations were achieved by using mainly in situ EPR operando Raman–GC spectroscopic techniques at relevant temperatures and gas conditions, showing that the (VivO)2P2O7 phase is formed during propane ammoxidation at 753K. Such transformation is less intense in inert atmosphere at 753K. Ammonia adsorption on the VPO catalyst surface leads to the formation of VO2+ sites, probably bearing Brønsted and/or Lewis acidity. Propane ammoxidation appears to promote the formation of mixed valence compounds. [ABSTRACT FROM AUTHOR]

Abstract: Much attention has recently been devoted to the surface of bulk catalysts. It has been proved that the most critical phenomena occur at the surface, which differs significantly from the bulk. To evaluate the role of the bulk in VPO catalyst, vanadium and phosphorus were supported on γ-alumina by incipient wetness impregnation at a total V+P of two monolayers so that nanoscaled VPO phases (n-VPO) form on alumina, such size maximizes the surface-to-bulk ratio. Alumina support stabilizes V5+, which may be reduced, while V4+ predominates in bulk vanadyl pyrophosphate phase. In addition, the support modifies acid–base and textural properties. The presence of VPO nanocrystals supported on alumina (n-VPO) facilitates investigating the interaction between alumina-dispersed vanadia and VPO-lattice vanadium ions. Both, bulk and supported phases are active for propane ammoxidation affording acrylonitrile yields of 41% and 48%, respectively, at 500°C. The yield to acrylonitrile per mass unit of VPO increases by 150% at 500°C in the alumina-stabilized n-VPO system when compared to the bulk VPO. [Copyright &y& Elsevier]