Surface Stability of Alloys and Intermetallics during Electrochemical CO Reduction

Alloy catalysts for electrochemical CO2 and CO reduction are often screened on idealized, bulk-truncated surfaces, yet adsorbates and the applied potential can change which surface is actually stable under reaction conditions. Here, we combine density functional theory with surface thermodynamics to test whether active sites identified from *CO and *H adsorption energies persist in atomically dispersed single-, dual-, and tri-atom alloys, substitutional alloys, and intermetallic compounds. Two-dimensional surface phase diagrams, spanning the electrode potential and the CO chemical potential, show that adsorbates can reshape both surface composition and termination. *CO keeps Pd at the surface of the Pd1Au single-atom alloy, while the stable termination of intermetallic compounds such as SnPd and GaNi depends strongly on the elemental chemical-potential reference. Depending on this reference, the *CO and *H adsorption energies shift by up to ∼1 eV, so a site that appears active on an idealized surface may not exist under reaction conditions. Adsorption-energy descriptors should therefore be evaluated on thermodynamically accessible surfaces, and predictions for intermetallic candidates are most robust when their predicted active surface is independent of the chemical-potential reference.