Square-pyramidal subsurface oxygen [Ag4OAg] drives selective ethene epoxidation on silver

Ag-catalysed ethene epoxidation is the only viable route for making ethene oxide (EO) in industry, but the active site remains elusive due to the lack of tools to probe this reaction under high temperature and high-pressure conditions. Here, aided by advanced machine-learning grand canonical global structure exploration and in situ experiments, we identify a unique surface oxide phase, namely O5 phase, grown on Ag(100) under industrial catalytic conditions. This phase features square-pyramidal subsurface O and strongly adsorbed ethene, which can selectively convert ethene to EO. The other Ag surface facets, although also reconstructing to surface oxide phases, only contain surface O and produce CO2. The complex in situ surface phases with distinct selectivity contribute to an overall medium (50%) selectivity of Ag catalyst to EO. Our further catalysis experiments with in situ infra-red spectroscopy confirm the theory-predicted infra-red-active C=C vibration of adsorbed ethene on O5 phase and the microkinetics simulation results. Ethylene oxide is a key platform chemical that is produced industrially from the epoxidation of ethylene on silver catalysts, but the precise mechanism remains elusive. Now, in a joint computational–experimental effort, a phase of the silver catalyst grown on (100) facets that contains square-pyramidal subsurface oxygens and is stabilized by strongly adsorbed ethylene is identified as the active phase, and the mechanism is revealed.