  The segregation behavior of the bimetallic alloys PtPd and CoCr in the case
of bare surfaces and in the presence of an oxygen ad-layer has been studied by
means of first-principles modeling based on density-functional theory (DFT).
For both systems, change of the d-band filling due to charge transfer between
the alloy components, resulting in a shift of the d-band center of surface
atoms compared to the pure components, drives the surface segregation and
governs the chemical reactivity of the bimetals. In contrast to previous
findings but consistent with analogous PtNi alloy systems, enrichment of Pt
atoms in the surface layer and of Pd atoms in the first subsurface layer has
been found in Pt-rich PtPd alloy, despite the lower surface energy of pure Pd
compared to pure Pt. Similarly, Co surface and Cr subsurface segregation occurs
in Co-rich CoCr alloys. However, in the presence of adsorbed oxygen, Pd and Cr
occupy preferentially surface sites due to their lower electronegativity and
thus stronger oxygen affinity compared to Pt and Co, respectively. In either
cases, the calculated oxygen adsorption energies on the alloy surfaces are
larger than on the pure components when the more noble components are present
in the subsurface layers.
