Direct Visualization of Oxametallacycle and Ethylenedioxy Intermediates on Ag(111)

The selective epoxidation of ethylene on silver remains one of the most industrially important yet mechanistically debated catalytic processes. Central to this debate is the identity of the intermediate that leads to selective epoxidation. However, unambiguous detection and characterization of chemical intermediates in situ is very challenging. Here, we take a well-defined single-crystal approach that enables direct atomic-scale imaging with scanning tunneling microscopy (STM) at cryogenic temperatures interpreted with density functional theory (DFT) and report how ethylene interacts with two different oxygen phases on Ag(111). At lower deposition temperatures, NO2 produces triangular six-oxygen-ring assemblies encircling Ag vacancies, whereas higher temperature exposures yield the well-known oxygen p(4 × 4) reconstruction, which is often attributed to nucleophilic oxygen. STM imaging reveals the formation of oxametallacycle (OMC)-like intermediates when ethylene interacts strongly with the oxygen-ring assemblies, supported by DFT calculations and simulated images. The strong interaction of ethylene in the OMC complexes leads to restructuring and the formation of chains of oxygen rings linked by ethylene. This contrasts with the weaker and less site-specific binding of ethylene to the p(4 × 4) surface. Together these model studies provide direct evidence for OMC-like intermediates often invoked in mechanistic studies of epoxidation and show they are more complex than the current one-oxygen, one-ethylene model.