Surface Morphology of Cu Adsorption on Different Terminations of the Hägg Iron Carbide (χ-Fe₅C₂) Phase

Spin-polarized density functional theory computations have been carried out to investigate the surface morphology of Cu_n adsorption on the Fe₅C₂(100), Fe₅C₂(111), Fe₅C₂(510), Fe₅C₂(001), and Fe₅C₂(010) surface terminations in different surface Fe and C ratios. On the Fe₅C₂(100), and Fe₅C₂(510) surfaces, aggregation is thermodynamically more favored than dispersion, while dispersion is more favored than aggregation on the Fe₅C₂(111) surface for n = 2–4, on the Fe₅C₂(010) surface for n = 2 and on the Fe₅C₂(001) surface for n = 2–4. The difference in structures and stability at low coverage depends on the stronger Cu–Fe interaction over the Cu–Cu interaction as well as the location of the adsorption sites. The adsorption energies do not correlate with the surface Fe and C ratios. Comparison among the most stable Fe(110), Fe₃C­(001), and Fe₅C₂(100) surfaces reveals that the Fe(110) surface has higher Cu affinity than the Fe₃C­(001) and Fe₅C₂(100) surfaces; and the carbide surfaces have close Cu affinities; in agreement with the experimental observations. On all these iron and carbide surfaces, two-dimensional monolayer surface adsorption configurations are energetically more favored than the adsorption of three-dimensional Cu_n clusters, and it can be expected that the adsorbed Cu atoms should grow epitaxially as a layer-by-layer mode at the initial stage. On the metallic Fe(110), Fe(100), Fe(111), and Fe₃C­(010) surfaces, the adsorbed Cu atoms are negatively charged; while on the Fe₃C­(100), Fe₅C₂(100), Fe₅C₂(111), Fe₅C₂(010), and Fe₅C₂(001) surfaces, the adsorbed Cu atoms are positively charged. On the Fe₃C­(001) and Fe₅C₂(510) surfaces, the adsorbed Cu atoms mainly interacting with surface Fe atoms are very slightly negatively charged. This trend is in line with their difference in electronegativity. Our results build the foundation for further study of the Cu-promotion effect in Fe-based FTS in particular and for metal-doped heterogeneous catalysis in general.