By means of first-principles density functional theory combined with atomistic thermodynamics, we have investigated the stability of several intermediates in the ethylene epoxidation reaction catalyzed by Ag-Cu alloys. We studied the surface phase diagrams of the low-index facets of Ag-Cu as a function of temperature and partial pressures of oxygen and ethylene, considering ethylene to be either physisorbed or chemisorbed in oxametallacycle and ethylenedioxy forms. We find that at high ethylene partial pressure or low temperature ethylene adsorbs as ethylenedioxy on a thin CuO layer formed on top of the silver particle. Subsurface oxygen can be present on the (111) facet at the interface between the CuO layer and silver. At temperatures and pressures relevant for industrial applications, though, the catalyst is not covered by ethylene and on all facets a thin CuO layers forms. The oxametallacycle intermediate is not predicted to be stable under any conditions. We have also investigated the shape of the catalyst particles as a function of the copper loading and temperature, showing that the dominant facet under conditions relevant for practical applications is the (100).
