Surface Morphology of Cu Adsorption on Different Terminations of the Hagg 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 = 24, on the Fe5C2(010) surface for n = 2 and on the Fe5C2(001) surface for n = 24. 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.