Surface Morphology of Cu Adsorption on Different Terminations of the Hägg Iron Carbide (χ-Fe<sub>5</sub>C<sub>2</sub>) Phase


Spin-polarized density functional theory computations have been carried out to investigate the surface morphology of Cu<sub><i>n</i></sub> adsorption on the Fe<sub>5</sub>C<sub>2</sub>(100), Fe<sub>5</sub>C<sub>2</sub>(111), Fe<sub>5</sub>C<sub>2</sub>(510), Fe<sub>5</sub>C<sub>2</sub>(001), and Fe<sub>5</sub>C<sub>2</sub>(010) surface terminations in different surface Fe and C ratios. On the Fe<sub>5</sub>C<sub>2</sub>(100), and Fe<sub>5</sub>C<sub>2</sub>(510) surfaces, aggregation is thermodynamically more favored than dispersion, while dispersion is more favored than aggregation on the Fe<sub>5</sub>C<sub>2</sub>(111) surface for <i>n</i> = 2–4, on the Fe<sub>5</sub>C<sub>2</sub>(010) surface for <i>n</i> = 2 and on the Fe<sub>5</sub>C<sub>2</sub>(001) surface for <i>n</i> = 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), Fe<sub>3</sub>C­(001), and Fe<sub>5</sub>C<sub>2</sub>(100) surfaces reveals that the Fe(110) surface has higher Cu affinity than the Fe<sub>3</sub>C­(001) and Fe<sub>5</sub>C<sub>2</sub>(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<sub><i>n</i></sub> 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 Fe<sub>3</sub>C­(010) surfaces, the adsorbed Cu atoms are negatively charged; while on the Fe<sub>3</sub>C­(100), Fe<sub>5</sub>C<sub>2</sub>(100), Fe<sub>5</sub>C<sub>2</sub>(111), Fe<sub>5</sub>C<sub>2</sub>(010), and Fe<sub>5</sub>C<sub>2</sub>(001) surfaces, the adsorbed Cu atoms are positively charged. On the Fe<sub>3</sub>C­(001) and Fe<sub>5</sub>C<sub>2</sub>(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

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oaioai:figshare.com:article/2181355Last time updated on 2/12/2018

This paper was published in FigShare.

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