Ab initio study of element segregation and oxygen adsorption on PtPd and CoCr binary alloy surfaces

TL;DR

This study uses first-principles DFT calculations to analyze element segregation and oxygen adsorption on PtPd and CoCr alloy surfaces, revealing how charge transfer and oxygen presence influence surface composition and reactivity.

Contribution

It provides new insights into segregation behavior and oxygen affinity in PtPd and CoCr alloys, highlighting the role of d-band shifts and subsurface composition effects.

Findings

Pt enriches the surface in PtPd alloys despite lower surface energy of Pd

Oxygen prefers Cr and Pd surface sites due to electronegativity and affinity

Oxygen adsorption energies increase with noble component presence in subsurface layers

Abstract

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.