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 gamma-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 V(5+), which may be reduced, while V(4+) 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 degrees 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. (C) 2011 Elsevier B.V. All rights reserved.