Reconstructions on Disordered Surface Alloys: Strain-Minimizing First Principles Treatments for Heterogeneous Catalysis

High-entropy alloys (HEAs) have attracted significant attention as candidates for improved heterogeneous catalysts, and their compositional flexibility allows an almost limitless number of catalyst structures to be explored. However, while the bulk structures of HEAs are generally acknowledged to consist of crystalline lattices with a random distribution of elements between lattice sites, considerably less understanding of the molecular structures of HEA surfaces, which control catalytic properties, exists. Herein, we analyze such structures using disordered CoMo bimetallic alloys, which are well-known catalysts for ammonia synthesis and decomposition, as a simple model for more complex HEAs. We present an approach which combines Density Functional Theory (DFT) calculations and a custom lattice matching algorithm to explore the stability of a large number of surface compositions and configurations, including both pseudomorphic and nonpseudomorphic (Moiré pattern) structures. The analysis reveals that thermodynamically stable surface compositions can differ significantly from corresponding bulk compositions. Furthermore, non-pseudomorphic structures, which reduce the magnitude of strain fields near the surface, are often found to be more stable than pseudomorphic patterns of the same composition, highlighting the relevance of Moiré-type surface reconstructions on disordered alloy surfaces. These fundamental insights into the interplay between surface composition, structure, and stability in disordered alloys lay the groundwork for understanding more complex multielement systems such as HEAs, in turn providing a pathway to an accurate and systematic evaluation of HEAs’ catalytic properties.