Effects of concentration of chlorohydrocarbon and reaction time on the catalytic performance of 20% Ag-0.1% Y2O3-0.1% K2O/alpha-Al2O3 catalyst for the epoxidation of propylene by molecular oxygen were investigated, in which the role of chlorohydrocarbon in increasing selectivity of propylene oxide (PO) was characterized by XRD, SEM-EDS and XPS. With an increase in the concentration of chlorohydrocarbon in the feed gas, PO selectivity increased significantly and propylene conversion decreased remarkably and then the catalytic performance remained nearly constant when the concentration of chlorohydrocarbon was more than 125 ppm. PO selectivity increased from 46.8% to 75.6% and propylene conversion declined from 4.0% to 0.77% after 10 h induction period, and then the catalytic performance remained nearly constant for 140 h, under the reaction conditions of 245 degrees C, 0.1 MPa, GHSV of 2000 h(-1), and 125 ppm 1,1,1-trichloroethane (TCE). A small amount of TCE could effectively control the surface morphology of the catalyst and restrain the agglomeration of Ag crystallites, in which TCE was dissociated on the surface of Ag crystallites to form AgCl and the coexistence of Ag and AgCl was favorable to increase PO selectivity. The presence of Cl could withdraw electrons from nearby Ag and thus make Ag electropositive, which was beneficial to produce more active sites where electrophilic oxygen species could be absorbed to increase PO selectivity. Y2O3 played a role of electron-type promoter that could strongly polarize electron cloud of nearby Ag, which made the absorbed oxygen species hold proper electrophilic character and then attack C=C bond of propylene to produce PO. (C) 2011 Elsevier B.V. All rights reserved.