First-principles pseudopotential plane wave calculations, based upon spin-polarized density functional theory and the generalized gradient approximation, were used to study the adsorption of CO molecules on the Fe(100) surface. Among the several possible adsorption configurations which were considered, the most stable corresponded to a four-fold state in which a CO molecule was tilted, relative to the surface normal, by 50°. In this case, the CO bond was elongated to 1.32Å and had a low vibrational stretching

frequency of 1246/cm; as compared with the experimental gas-phase value of 2143/cm. The adsorption energy for this state was found to vary from 46.7 to 43.8kcal/mol; depending upon the choice of the exchange-correlation functional used in the density functional theory. Three adsorption sites were located, and the relative adsorption energies were E(4-fold) > E(3-fold) ≈ E(1-fold) at lower surface coverages, and E(4-fold) > E(1-fold) > E(2-fold) at higher coverages. A similar analysis, performed for C and O atoms, indicated that adsorption at the four-fold site was the most stable among the various configurations; with adsorption energies of 186 and 145kcal/mol, respectively. In addition, the possibility that a C atom was embedded into the lattice in a two-fold, bridge-like configuration with an adsorption energy of 154kcal/mol was demonstrated. The minimum-energy pathways for the surface diffusion of a CO molecule between selected pairs of local minima indicated that the barriers for these processes were generally quite small; with values of less than 2kcal/mol. One exception was that of diffusion out of the most stable 4-fold site, where the barrier was predicted to be about 13kcal/mol. The barriers to the dissociation of CO which was bound in a four-fold site were calculated to have values of 24.5 to 28.2kcal/mol; thus supporting the experimental observation that the dissociation of CO, bound to the surface, seemed to compete with CO desorption at 440K.

First-Principles Calculations of the Adsorption, Diffusion and Dissociation of a CO Molecule on the Fe(100) Surface. D.C.Sorescu, D.L.Thompson, M.M.Hurley, C.F.Chabalowski: Physical Review B, 2002, 66[3], 035416 (13pp)

Figure 2

Self-Diffusivity along Dislocations in α-Fe