Why silver is the unique catalyst for ethylene epoxidation

The activities of Cu(2)O(001), Ag(2)O(001), and Au(2)O(001) surfaces for direct ethylene epoxidation and alternative paths for EO isomerization are studied. Among these three oxide surfaces, only Ag(2)O(001) surface enables direct path without a barrier. Au(2)O cannot regenerate surface oxygen, and overall reaction on Cu(2)O is endothermic. Furthermore, ring opening of ethylene oxide (EO) and subsequent acetaldehyde (AA) formation on Cu(2)O 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. (C) 2011 Elsevier Inc. All rights reserved.