The vapor-phase selective oxidation of propylene (H2C=CHCH3) to acrolein (H2C=CHCHO) was investigated over supported V2O5/Nb2O5 catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 degrees C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VOx species on the Nb2O5 support in the sub-monolayer region (< 8.4 V/nm(2)). Surface allyl species (H2C=CHCH2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VOx coverage. Two surface VOx sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir-Hinshelwood mechanism with first-order in propylene and half-order in O-2 partial pressures. C3H6-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O-2 was critical for maintaining the surface VOx species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging V-O-Nb support bond. (c) 2006 Elsevier B.V. All rights reserved.