Plane-Wave DFT Investigations of the Adsorption, Diffusion, and Activation of CO on Kinked Fe(710) and Fe(310) Surfaces

The adsorption, diffusion, and dissociation properties of CO on kinked Fe(710) and Fe(310) surfaces have been analyzed using spin-polarized plane-wave density functional theory (DFT) calculations within the generalized gradient approximation (GGA). Several one-, two-, three- and 4-fold binding configurations have been identified among which the preferential adsorption takes place at a 4-fold hollow site near the top of the step while the one with the smallest activation energy for dissociation is located at a 4-fold site near the bottom of the step. In the case of the individual atomic species, the adsorption takes place preferentially at the hollow site for the C atom and at the pseudo 3-fold site on the step for the O atom. By the increase in coverage, there is an overall decrease of the adsorption energies for either molecular (CO) or atomic (C,O) species. The diffusion barriers among different local minima at the steps or on the terraces of both surfaces have been determined, and their values were found to be smaller than the barriers for CO dissociation. The most activated configuration at the bottom of the step can be populated by direct diffusion over the step, from the upper terrace, or by reorientation of the CO molecule within the same hollow site. This last process requires an activation energy of 6.5 kcal/mol on Fe(710) and 9.3 kcal/mol on the Fe(310) surface. The barrier heights for dissociation of CO molecules on Fe(710) (Fe(310)) were found to vary between 15.4 and 20.5 (16.7 and 20.9) kcal/mol depending on the specific location of the hollow site relative to the step edge and to the particular molecular orientation within a given hollow site. For the set of crystallographic Fe surfaces (110), (100), (211), (710), (310), and (111), we found that dissociation on Fe(710) and Fe(310) requires the smallest activation energies in the regime of low coverages. The analysis of the activation properties of CO on this six-member set of flat, stepped, and kinked surfaces indicates the existence of a direct correlation between the apparent activation energy and the rebonding energy of the noninteracting products of the reaction.