The Effect of Coadsorbed Oxygen on the Adsorption and Diffusion of Potassium on Rh(110): A First-Principles Study

Abstract

The adsorption and diffusion of potassium and oxygen on Rh(110), as well as the co-adsorption of K and O and its effect on K diffusion, has been studied using periodic density functional theory (DFT) calculations (PW91-GGA). On both the non-reconstructed (1x1) and the missing-row (MR) reconstructed surfaces, O prefers the short bridge site at low coverage, with a binding energy of ca. -5.2 eV at 1/4 ML. At theta_O>1/2 ML O atoms occupy alternating threefold sites along the ridge and forms a zigzag pattern. Interaction with the ridge sites is enhanced by the MR reconstruction. K prefers to be in the trough, with a binding energy of -2.3 eV on the (1x1) surface and -2.9 eV on the MR surface at 1/8 ML. Thus the adsorption of both O and K at low to medium coverage promotes the MR reconstruction. The co-adsorption of K and O enhances the binding energy of K to a maximum of -3.6 eV at the highest oxygen coverage studied, 1-3/8 ML. Oxygen adsorption is also stabilized by K, though to a smaller extent on a per-O atom basis. On both surfaces, K prefers to diffuse in the [110] direction with a barrier of ca. 0.05more » eV. O diffusion also prefers the [110] direction on the MR surface but is not clearly anisotropic on the (1x1) surface. The barrier to O diffusion ranges from 0.6~0.8 eV depending on the coverage and reconstruction. In the presence of co-adsorbed O, the diffusion barrier of K tops out at ca. 0.12 eV, much lower than earlier estimates based on mean-field models. Possible reasons for this apparent contradiction are discussed.« less