Reaction Mechanism and Role of Chlorine in Ethylene Epoxidation Revealed by in situ XPS

Abstract Ethylene epoxidation is a cornerstone process in the chemical industry and the primary route for ethylene oxide (EO) production. Despite its significance, the reaction mechanism and the structure of the active surface remain under debate. Here, we employ in situ photoelectron spectroscopy supported by DFT to probe surface intermediates on Ag/Al2O3 under ethylene epoxidation conditions, with and without vinyl chloride promotion. Under these conditions, combustion is triggered by ethylene dehydrogenation at surface defects, assisted by lattice oxygen and yielding CO2. Vinyl chloride suppresses this pathway by reducing defect density, hindering lattice oxygen formation, and inhibiting ethylene adsorption. Chlorine binds preferentially at defect sites, passivating undercoordinated silver atoms to disfavor C–H activation. Ethylene reacts with one of the oxygen atoms of adsorbed dioxygen on reconstructed silver surfaces through a direct oxidation pathway to produce EO. These findings provide molecular-level insights and offer a rational basis for optimizing ethylene epoxidation catalysts.