Structures and energies of Cu clusters on Fe and Fe3C surfaces from density functional theory computation

Abstract

Spin-polarized density functional theory computations have been carried out to study the stable adsorption configurations of Cu_n (n = 1–7, 13) on Fe and Fe₃C surfaces for understanding the initial stages of copper promotion in catalysis. At low coverage, two-dimensional aggregation is more preferred over dispersion and three-dimensional aggregation on the Fe(110) and Fe(100) surfaces as well as the metallic Fe₃C(010) surfaces, while dispersion is more favorable over aggregation on the Fe(111) surface. On the Fe₃C(001) and Fe₃C(100) surfaces with exposed iron and carbon atoms, the adsorbed Cu atoms prefer dispersion at low coverage, while aggregation along the iron regions at high coverage. On the iron surfaces, the adsorption energies of Cu_n (n = 2–7) are highest on Fe(111), medium on Fe(100) and lowest on Fe(110). On the Fe₃C surfaces, the adsorption energies of Cu_n (n = 1–3) are highest on Fe₃C(100), medium on Fe₃C(010) and lowest on Fe₃C(001), while, for n = 4–7 and 13, Fe₃C(010) has stronger adsorption than Fe₃C(100). On the basis of their differences in electronegativity, the adsorbed Cu atoms can oxidize the metallic Fe(110), Fe(100) and Fe₃C(010) surfaces and become negatively charged. On the Fe₃C(001) and Fe₃C(100) surfaces with exposed iron and carbon atoms, the adsorbed Cu atoms interacting with surface carbon atoms are oxidized and positively charged. Unlike the most stable Fe(110) and Fe₃C(001) surfaces, where the Fe(110) surface has stronger Cu affinity than the Fe₃C(001) surface, which is in agreement with the experimental finding, the less and least stable Fe₃C(010) and Fe₃C(100) surfaces have stronger Cu affinities than the Fe(110) and Fe(100) surfaces. Since less stable facets are not preferably formed thermodynamically, it is crucial to prepare such surfaces to explore Cu adsorption and promotion, and this provides challenges to surface sciences.