The modification of copper by vanadium significantly enhanced its catalytic performance of propylene epoxidation by oxygen Synergistic effects existed between copper and vanadium and the catalysts with V/Cu atomic ratios of 0 11-0 20 exhibited better propylene oxide (PO) formation activity The dispersion of copper was enhanced by vanadium and this might contribute to the Increase in catalytic activity The pre-reduction of catalyst resulted in better performances than the oxidative pretreatment and an induction period was observed for PO formation over the reduced catalyst In situ XRD measurements revealed that Cu-0 in the reduced catalyst was partially transformed into Cu2O in propylene oxidation and the presence of VOx promoted this transformation Structure-performance correlations demonstrate that Cu-1 accounts for propylene epoxidation Vanadium species at lower valence states (V-III and V-IV) may participate in the activation of oxygen The presence of VOx also suppressed the reactivity of lattice oxygen in the working catalyst (C) 2010 Elsevier Inc All rights reserved ; National Natural Science Foundation of China [20773099, 20625310, 20923004]; National Basic Research Program of China [2010CB732303]; Research Fund for the Doctoral Program of High Education [20090121110007]; Key Scientific Project of Fujian Province [200