Why silver is the unique catalyst for ethylene epoxidation

The activities of Cu2O(0 0 1), Ag2O(0 0 1), and Au2O(0 0 1) surfaces for direct ethylene epoxidation and alternative paths for EO isomerization are studied. Among these three oxide surfaces, only Ag2O(0 0 1) surface enables direct path without a barrier. Au2O cannot regenerate surface oxygen, and overall reaction on Cu2O is endothermic. Furthermore, ring opening of ethylene oxide (EO) and subsequent acetaldehyde (AA) formation on Cu2O is more favorable than EO desorption. Ethylene adsorption on an oxygen vacancy results in the oxametallacycle (OMC) formation, which causes AA formation and reduces EO selectivity. Cl adsorption removes these surface vacant sites and hence prevents the formation of the OMC intermediate.