  Recent experiments on model catalysts have shown that Ag-Cu alloys have
improved selectivity with respect to pure silver for ethylene epoxidation. In
this paper we review our first-principles investigations on the (111) surface
of this alloy and present new findings on other low index surfaces. We find
that, for every surface orientation, the presence of oxygen leads to copper
segregation to the surface. Considering the alloy to be in equilibrium with an
oxygen atmosphere and accounting for the effect of temperature and pressure, we
compute the surface free energy and study the stability of several surface
structures. Investigating the dependence of the surface free energy on the
surface composition, we construct the phase diagram of the alloy for every
surface orientation. Around the temperature, pressure and composition of
interest for practical applications, we find that a limited number of
structures can be present, including a thin layer of copper oxide on top of the
silver surface and copper-free structures. Different surface orientations show
a very similar behavior and in particular a single layer with CuO
stoichiometry, significantly distorted when compared to a layer of bulk CuO,
has a wide range of stability for all orientations. Our results are consistent
with, and help explain, recent experimental measurements.
