Among alkali metal ions, Cs+ was found to be the most efficient promoter of the CuOx/SiO2 catalyst for the epoxidation of propylene to propylene oxide (PO) by O-2. Stronger interactions between Cs+ and CuOx nanoparticles were proposed to favor the selective formation of PO. Kinetic studies indicated that PO was formed as a primary product in parallel with acrolein over the CuOx/SiO2 catalyst, but PO readily underwent isomerization to ally] alcohol, followed by oxidation to acrolein, without Cs+ modification. The modification by Cs+ inhibited the isomerization of PO because of the weakened acidity, contributing to the increase in PO selectivity. The inhibition of the reactivity of the lattice oxygen in the presence of Cs+ also promoted PO selectivity by suppressing the allylic oxidation route. In situ X-ray diffraction, CO-adsorbed Fourier-transform infrared, and pulse-reaction studies suggest that Cu-I sites generated during the reaction account for the epoxidation of propylene by O-2. (c) 2012 Elsevier Inc. All rights reserved.