Adsorption and activation of CO coadsorbed with K on Fe(100) surface: A plane-wave DFT study

Abstract

The adsorption and diffusion properties of K atoms together with the coadsorption effects induced upon CO activation on Fe(100) surface have been studied using spin-polarized plane-wave density functional theory (DFT) calculations and the generalized gradient approximation. Preferential adsorption of K atoms takes place at surface hollow sites and diffusion among these sites has a small activation energy of only 0.7 kcal/mol. Substitutional adsorption of K at a surface Fe site is also possible but only at high temperatures required to overcome a barrier of about 36.0 kcal/mol. A systematic analysis of the modifications of binding properties for molecular (CO) and atomic (C,O) species upon interaction with K has been performed both as function of the relative separations as well as coverage using a series of (4 × 4), (3 × 3) and (2 × 2) supercell models. The presence of K leads to stabilization of both C and O species but in the last case significant variations were observed only when O is bonded at a bridge site. For CO molecule a relatively large range of stabilization energies can be induced by coadsorbed K depending on the relative CO–K separation and K coverage. The largest stabilization effects are observed at small separations, when K is located at a hollow site adjacent to CO binding site. In such cases the increase in binding takes place with important red shifts of CO vibrational frequency and with relatively large bond elongations, independent of CO binding mode on the surface. Relative to the bare surface, the presence of K was also found to reduce CO activation energies by as much as 6.2–7.8 kcal/mol, i.e. 25–31%, function of the relative separation and molecular coverage. Such effects have been correlated with the charge transfer from K to Fe surface and to CO molecule leading to an increased stabilization primarily of O and somewhat less of C species in the transition state and to reduction of the bond competition between CO and the surface atoms.