Structural and electronic properties of cobalt carbide Co2C and its surface stability: Density functional theory study

Abstract

Density functional theory calculations have been performed to investigate the structural and electronic properties of bulk Co2C and the stability of low index Co2C surfaces. We found that the formation of Co2C is exothermic with the formation energy of −0.81eV/Co2C with respect to Co under the presence of syngas (mixture of CO and H2). While formed Co2C can be decomposed further to metal Co and graphite carbon with modest energy gain of 0.37eV/Co2C. This suggests that Co2C is only metastable in Fischer–Tropsch synthesis, which agrees well with experimental findings. The density of states (DOSs) reveals that the Co2C is paramagnetic and strong metallic-like. The difference of charge density analysis indicates that the bond of Co2C is of the mixtures of metallic, covalent, and ionic properties. A variety of low index Co2C surfaces with different terminations are studied. We find that the surface energy of low index stoichiometric Co2C highly relies on the surface area, the number of coordination of surface atoms and the surface dipole, with the decreased stability order of (101)>(011)>(010)>(110)>(100)>(001)=(111). Our results indicate that under Co-poor condition, the formation of non−stoichiometric surface (011) and (111) without terminated cobalt is energetically more favorable, while under Co-rich condition the formation of non−stoichiometric (111) surface with cobalt overlayer are preferential.