Direct gas-phase epoxidation of propylene to propylene oxide (PO) by molecular oxygen was studied over the MoO3-modified supported-silver catalyst. The effect of promoter and support on the performance of the Ag-MoO3 catalyst and their role were investigated by XPS, XRD, SEM, BET surface area, NH3-TPD, CO2-TPD techniques and so on. As the promoter, MoO3 plays a great role of electron- and structure-type bi-functional promoter. Of all the supports studied, ZrO2 is the most suitable one for the Ag-MoO3 catalyst. The presence of support can regulate the size of the Ag-MoO3 particles and the pore radii of catalyst, and decrease the loading of MoO3 from similar to50 wt.% in the unsupported Ag-MoO3 catalyst to similar to4 wt.% in the Ag-MoO3/ZrO2 catalyst. The properly weak Lewis acidic sites, the larger size of the Ag-MoO3 particles and pore channel on the Ag-MoO3/ZrO2 catalyst are beneficial to improve epoxidation performance of the catalyst. The effect of the reaction temperature and space velocity on the catalytic epoxidation of propylene was also investigated. The low temperature or high space velocity can decrease the deep oxidation of propylene oxide to improve the selectivity to PO. Over the 20%Ag-4%MoO3/ZrO2 catalyst at 400degreesC, 0.1 MPa and space velocity of 7500 h(-1), the selectivity to PO of 60.3% was achieved with the O-2 conversion of 4.8%; under the space velocity of 12 000 h-1, the selectivity to PO was 71.5% with 2.5% O-2 conversion. (C) 2004 Elsevier B.V. All rights reserved.