(461a) DFT Study of the Effect Surface Carbonates on Ethylene Epoxidation

Ethylene epoxidation is a process to produce ethylene oxide, an important precursor for the synthesis of antifreeze, detergents, and plastics. It is extensively produced using silver-based catalysts, with an annual global capacity exceeding 34.5 million tons. However, epoxidation also leads to the undesired byproducts CO₂ and H₂O, which necessitates efforts to increase the selectivity towards ethylene oxide. Previous experimental studies have indicated that carbonate can be present on the Ag surface under reaction conditions and may affect selectivity; however, the effects of carbonate on the surface structure and reactivity have not been computationally studied and are still unclear.

In this work, we studied the effect of carbonate on the state of the surface and on the predicted selectivity of ethylene epoxidation. In addition to studying the stability of carbonate on various known surface structures—such as the p-(4x4) reconstruction—we created phase diagrams that include the effect of both oxygen and ethylene. These phase diagrams predict that the most stable state for Ag(111) under reaction conditions is a metallic surface with adsorbed carbonates, consistent with multiple previous experimental studies. To address the influence on selectivity, we calculated reaction barriers for forming ethylene oxide, as well as various non-selective pathways, with and without coadsorbed carbonate. These calculations indicate that carbonate can improve selectivity. Therefore, surface carbonate may be an important factor in determining the selectivity of ethylene epoxidation.