Direct propylene epoxidation is an important gas-phase reaction for the future industrial production of propylene oxide (PO). However, the mechanistic understanding of this reaction system is still elusive, including the role of side reactions. Herein, kinetic Monte Carlo (KMC) simulations are applied to explore the underlying mechanistic aspects of propylene epoxidation over Au/TiO2/SiO2, and consistent results are obtained with respect to experimental benchmarks. The present study systematically probes the reaction mechanism, which involves acrolein formation on Au nanoparticles as well as the formation of PO and several byproducts at the Au/Ti dual interface sites, and it highlights the synergistic effect of interface sites in heterogeneous catalysis. The origin of the negative effect of rising reaction temperature on PO selectivity has been clarified, as it is correlated with decreasing atomic oxygen coverage on the Au surface. The varying feed concentration of H-2/O-2/C3H6 leads to different coverages on the catalyst surface, emphasizing the importance of an optimum H-2/C3H6 inlet concentration ratio for enhancing PO selectivity and hydrogen utilization efficiency. Based on our model results, we propose that supports with acidic groups may promote hydroperoxy species transferring to the adjacent Ti sites, thereby decreasing its decomposition to atomic oxygen on the Au sites, which leads to byproduct formation. (C) 2018 Elsevier Ltd. All rights reserved.