First-principles study of the mechanism of ethylene epoxidation over Ag-Cu particles

Silver-copper alloys have been proposed as catalysts for ethylene epoxidation due to their superior selectivity compared to pure silver, the predominant catalyst for this reaction. Under reaction conditions it has been previously shown that, rather than a two-dimensional (2D) Ag-Cu alloy, a thin copper oxide-like layer forms on top of silver, and several possible surface structures were identified (Phys. Rev. Lett., 2010, 104, 035503). By means of density-functional theory calculations, we study the mechanism of ethylene epoxidation catalyzed by the thin oxide-like surface structures. We identify different reaction pathways that will compete and/or synergetically interplay in the catalysis. In general, the reaction mechanism is structure-dependent and the reaction does not always proceed through the formation of (meta)stable intermediates, in contrast to clean Ag and the 2D alloy. Analyzing the competing reactions, we discuss how the addition of Cu improves the selectivity and stress the overall importance of accounting for the effect of ambient conditions