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.