The influence of alkali-ion-doping on supported vanadium oxide was investigated for selective photocatalytic oxidation of propylene with molecular oxygen. V2O5/Al2O3 shows higher selectivity to propenal formation than V2O5/SiO2. Alkali-ion-doping resulted in a significant enhancement of the photocatalytic capability for the propenal formation. Among alkali ions of Li, Na, K, and Rb, rubidium ion was found to be the most effective modifier. The Rb-ion-modified V2O5/Al2O3 was characterized by means of XAFS, Fr-IR, Raman, and UV-vis diffuse reflectance spectroscopies. The terminal V=O bond of monomeric VO4 tetrahedron was elongated by an interaction with a Rb ion. The photocatalytic active site was found to be the Rb-ion-modified VO4 monomer bound to alumina surface through three V-O bonds. In contrast, metavanadate chains were not effective for the photocatalytic reaction. (c) 2006 Elsevier B.V. All rights reserved.