In this work a series of supported Ag catalysts was studied for propylene epoxidation with molecular oxygen as the oxidant. It was found that alpha-Al2O3 and CaCO3 were suitable supports for propylene epoxidation and that on the latter Ag particles between 400 and 700 nm gave the highest selectivity to propylene oxide (PO). Ball-milling treatment of the CaCO3 catalyst and promotion with NaCl resulted in improved catalytic performance. The highest PO selectivity (45%) was obtained on a ball-milled catalyst with a silver loading of 56 wt.% supported on CaCO3 and promoted with 1 wt.% NaCl (Ag(56)-NaCl(l)/CaCO3). The catalysts were tested with reactant flow rates of C3H6:O-2:He = 5:10:15 cm(3) min(-1), a gas hourly space velocity (GHSV) of 1800 h(-1), a reaction pressure of 0.3 Wa, and a reaction temperature of 533 K (260 degrees C). Addition of 500 ppm of ethyl chloride (EtCl) to the reactant gases enhanced the stability of the Ag(56)-NaCl(l)/CaCO3 catalyst. X-ray diffraction (XRD) of the Ag(56)NaCl(l)/CaCO3 catalyst detected the existence of AgCl in the catalyst and ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy confirmed the presence of surface Ag+ species. Scanning electron microscopy showed a roughening of the Ag particles by the ball-milling treatment as well as by the addition of NaCl. The effect of NaCl on the enhancement of the catalytic performance was probably due to both physical and electronic changes in the properties of the catalyst. The NaCl not only helped increase the dispersion of the silver on the CaCO3 Support, but also probably increased the quantity of electrophilic oxygen species favorable for epoxidation. In situ UV-vis spectra suggested that the rapid reduction of Ag+ species on the surface of the Ag(56)-NaCl(l)/CaCO3 catalyst could be the cause of a decline in PO selectivity observed during reaction. (c) 2006 Elsevier B.V. All rights reserved.