Reaction Intermediates Involved in the Epoxidation of Ethylene Over Silver Revealed by In Situ Photoelectron Spectroscopy

Ethylene oxide (EO) is a crucial building block in the chemical industry, and its production via ethylene epoxidation (EPO) is a pivotal process. Silver-based catalysts are known for their high selectivity and are currently largely used in the industrial process. Extensive research over the past 20 years has assumed the oxametallacycle (OMC) as a reaction intermediate, implying that ethylene reacts with adsorbed oxygen on the surface of silver. The OMC is suggested to be the common intermediate for both EO and acetaldehyde, with the latter rapidly converting into carbon dioxide. However, the detection of such intermediate is challenging. In this study, in situ X-ray photoelectron spectroscopy combined with density functional theory calculations is employed to investigate reaction intermediates formed during EPO on silver. The findings reveal that adsorbed EO is detected as a direct product of ethylene oxidation. Adsorbed ethylene is easily dehydrogenated to form C₂H_x (x = 1-3) species. C₂H_x, carbon monoxide, and carbonate are identified as precursors to carbon dioxide. A new methodical interpretation of complex spectral features is provided, which clarifies previous assignments. Notably, the OMC is detected neither under EPO nor under EO decomposition conditions, thus challenging the role of OMC in the reaction mechanism.