CO adsorption, dissociation and coupling formation mechanisms on Fe2C(001) surface

Abstract

By means of density functional theory calculations and atomic thermodynamics, we systematically investigated the CO adsorption on the Fe2C(001) surface at different coverage. It has been found that CO prefers to adsorb on the surface iron atom at low coverage (1–8 CO); CO prefers to adsorb at the bridge site of Fe and C atoms at high coverage (9–12 CO). Eight CO molecules binding on the Fe2C(001) surface is favorable thermodynamically as indicated by the stepwise adsorption energy. The phase diagram shows that addition of more CO molecules up to a number of 8 is thermodynamically favorable, and that the incremental energy gained by adding one more CO molecule is almost constant up to 4 CO molecules, decreases up to 8 CO molecules, after which it becomes thermodynamically unfavorable to add more CO molecules. Probability distribution of different singe-CO adsorbed states on the Fe2C(001) surface as function of temperature shows that CO dissociation and coupling are least preferred, indicating that carbide mechanism is not dominant in the iron-based Fischer-Tropsch synthesis reaction. The projected density of states (PDOS) was used to analyze the CO adsorption mechanism.