Stability of Pt Monolayers on Ir-Co Cores with and without a Pd Interlayer

Density functional theory calculations are used to examine surface segregation and stability of adsorbed oxygenated structures on core shell structure models. The structures consist of a surface Pt-skin monolayer over an IrCo or Ir3Co core, with or without a Pd interlayer between the Pt surface and the Ir-Co core. The study is aimed to further elucidate the role of the Pd interlayer in the core-shell structures that have shown higher activity for the reduction of oxygen relative to Pt(111) surfaces. It is found that, without the Pd interlayer, Ir and Co may segregate from the core to the surface, thus considerably weakening the stability of the structure, whereas the Pd interlayer inhibits such segregation of the core elements. At pH = 0, water oxidation on the Pt-skin surfaces takes place at a higher potential (0.9-1.0 V), compared to 0.7 V for Pt(111). Adsorbed phases of relatively low coverage: 0.25 ML O and 0.125 ML OH are stable up to 1.2 V, while on Pt(111), O and OH coverage increase rapidly with the increase of potential and reach 0.33 ML O at 1.2 V. Thus, the alloy surfaces possess more available sites that lead to higher oxygen reduction reaction activity than in pure Pt(111) surfaces. The calculations show that core elements segregation is the key challenge on these Pt-skin structures.