First-principles calculations of the adsorption, diffusion, and dissociation of a CO molecule on the Fe(100) surface

Abstract

First-principles pseudopotential plane wave calculations based on spin-polarized density functional theory (DFT) and the generalized gradient approximation (GGA) have been used to study the adsorption of CO molecules on the Fe(100) surface. Among several possible adsorption configurations considered here, the most stable corresponds to a fourfold state in which a CO molecule is tilted relative to the surface normal by 50\ifmmode^\circ\else\textdegree\fi{}. In this case, the CO bond is elongated to 1.32 \AA{} and has a low vibrational stretching frequency of 1246 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ to be compared with the experimental gas phase value of 2143 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The adsorption energy for this state is found to vary between 46.7 and 43.8 kcal/mol depending on the choice of exchange-correlation functional used in the DFT. A total of three adsorption sites have been located, and the relative adsorption energies are $E(\mathrm{fourfold})gE(\mathrm{twofold})\ensuremath{\approx}E(\mathrm{onefold})$ at lower surface coverage, and $E(\mathrm{fourfold})gE(\mathrm{onefold})gE(\mathrm{twofold})$ at higher coverage. A similar analysis performed for the C and O atoms indicates that the adsorption at the fourfold site is the most stable among various configurations, with adsorption energies of 186 and 145 kcal/mol, respectively. Additionally, we have demonstrated the possibility that a C atom embeds into the lattice in a twofold, bridgelike configuration with an adsorption energy of 154 kcal/mol. The minimum energy pathways for the surface diffusion of a CO molecule between selected pairs of local minima indicate that the barriers for these processes are generally quite small with values less than 2 kcal/mol. One exception to this is the diffusion out of the most stable fourfold site, where the barrier is predicted to be around 13 kcal/mol. Finally, the barriers for dissociation of CO bound in a fourfold site have been calculated to have values in the range of 24.5--28.2 kcal/mol, supporting the experimental observation that dissociation of CO bound to the surface seems to compete with CO desorption at 440 K.