Plane-Wave Density Functional Theory Investigations of the Adsorption and Activation of CO on Fe₅C₂ Surfaces

A systematic analysis of the adsorption properties of CO on a set of seven low Miller index surfaces ((010) 0.25, (111̅) 0.00, (110) 0.00, (111) 0.00, (111̅) 0.50, (110) 0.50, and (100) 0.00)) of the Hägg iron carbide (Fe₅C₂) phase has been performed. Calculations were based on spin-polarized plane-wave density functional theory (DFT) within the generalized gradient approximation (GGA). Three general groups of adsorption configurations have been identified corresponding to CO binding exclusively to surface Fe atoms (Fe-only states), to mixed Fe and C(s) atoms (mF−C states), and exclusively to surface C(s) atoms (0F−C states), respectively. Among these, the most stable adsorption configurations correspond to adsorption at Fe-only sites with maximum binding energies ranging from 44.4 to 48.5 kcal/mol, depending on the crystallographic orientation. A diverse bonding scheme for CO was found to exist with formation of one up to six different bonds to the Fe atoms. In the case of CO adsorption at mixed mF−C states or exclusively on top of C(s) atoms, lower adsorption energies are observed ranging from 18.5 to 35 kcal/mol. Despite the lower binding energies, adsorption at mF−C states is shown to lead to significant weakening of the CO bonds, as reflected by large bond elongations and red shifts of the vibrational frequencies. The analysis of the dissociation properties of CO indicates that the most stable adsorption configurations at Fe-only sites have also large activation energies for dissociation, in excess of 40 kcal/mol. Decrease of the activation energy of dissociation was found to take place only for a limited number of cases in which the molecule adsorb in a lying down configuration, where both the C and O ends are bonded to the surface by a total of at least five bonds. Molecular dissociation from mixed mF−C states requires significantly lower activation energies, consistent to the weakening of the CO bonds observed in adsorption studies. In such instances activation energies as low as 15.6 kcal/mol have been determined. Formation of small carbon chains is preferential upon molecular dissociation from such states.