Cs-promoted Active Species for Ag-catalyzed Ethylene Epoxidation

Ethylene epoxidation on Ag catalysts is a fundamental reaction in chemical industry. Although electrophilic oxygen species are previously attributed as potential active species for this reaction, their precise structure and formation mechanisms have remained unsolved. In this study we have revealed that Cs promotes the formation of an unreported zigzag [1 ^- 10] Ag-O chain on Ag(110) surface. This chain characteristic of electrophilic oxygen and embedded Ag dimers, in sharp contrast with the [001] Ag-O chains which comprise nucleophilic oxygen and form in the absence of Cs. Further experiments have shown that the electrophilic oxygen with neighboring Ag dimers together serves as the active species for completely selective production of ethylene oxide. However, the [001] Ag-O chains predominantly react with ethylene to form water. Theoretical calculations further corroborate that the oxametallacycle intermediate is stabilized due to the matching of the frontier orbitals between ethylene and the [1 ^- 10] Ag-O chain and responsible for the distinct selectivity observed in these Ag-O chains. This atomic-scale understanding of the active species in ethylene epoxidation necessitates reassessing the role of alkali metals in catalysis and developing strategies to enhance the selectivity of the ethylene epoxidation process.