Surface segregation in the AgAuCuPdPt high entropy alloy: insights from molecular simulations†

Abstract

High entropy alloys (HEAs) are emerging as a novel class of superior catalysts for diverse chemical conversions. The activity of a catalyst is intimately related to the composition and atomic structure at its surface. In this work, we have used embedded atom (EAM) potential based Monte Carlo–Molecular Dynamics simulations to study surface segregation in the equimolar AgAuCuPdPt HEA that was recently shown to be an efficient catalyst for CO₂ electrochemical reduction. Firstly, EAM potentials were extensively validated against experimental segregation data for several different binary and ternary compositions. Subsequently, simulations on the HEA were carried out for four different surface orientations and spherical and cubic nanoparticles to obtain detailed structural and concentration profiles normal to the surface. In all cases, Ag atoms were found to preferentially segregate to the surface while the subsurface layer mainly consisted of Au atoms. No Pt atoms were found on the surface layer for all systems. A detailed analysis of the neighborhood of each surface site revealed that the atoms formed a finite number of chemically unique clusters. The percentage of chemically unique sites was larger for elements with lower concentration at the surface. Together with the physical diversity surrounding each site, the enrichment of one or more element(s) at the surface also increased the number of unique catalytically active sites. The results from our work suggest that HEAs are prone to surface segregation and that such effects must be taken into consideration while modeling the surface chemistry of these materials.