On the composition of bimetallic near-surface alloys in the presence of oxygen and carbon monoxide

• We studied surface segregation in bimetallic Pt alloys in the presence of oxygen or CO.
• Phase diagrams of PtRu alloys as a function of (O2, CO) pressure and temperature
• In an oxygen atmosphere, surface becomes rich in more oxophilic metal.
• CO provides a weaker driving force than oxygen for adsorbate-induced segregation.

Periodic, self-consistent density functional theory calculations (GGA-PW91) are used to examine surface segregation in close-packed bimetallic Pt-overlayer alloy surfaces (Pt*/M, M = Au, Ag, Cu, Pd, Ir, Rh, Os, Ru, and Re) in different environments. In particular, we find that the thermodynamically stable surface termination in these Pt*/M alloys can be inverted from Pt-terminated in vacuum to M-terminated under exposure to oxygen (for an M that is more oxophilic than Pt). Interestingly, in many of these alloys, Pt is not driven into the bulk; rather it remains in the first subsurface layer where it enhances oxygen binding through a ligand interaction with the surface metal atoms. On the other hand, exposure to CO provides a much milder driving force for the surface composition inversion. To quantify segregation under catalytically relevant conditions, we constructed approximate phase diagrams for the PtRu system as a function of O2 and CO chemical potential (temperature, pressure). The results show that the surface termination inverts with many orders of magnitude higher CO pressure than with O2.

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