SURFACE SCIENCE LETTERS LCAO-TYPE CALCU~TIONS OF THE EFFECT OF K AND Cl ATOMS ON THE DISSOCIATION OF CO AT THE Ni(lll) SURFACE

For important reactions such as oxidation and hydrogenation of carbon monoxide, transition metals are used as catalyst [l-3]. Both reactions seem to take place via a dissociation of CO prior to its reassociation with other adsorbed fragments. Depending on the reaction conditions, the dissociative adsorption of CO may become rate-determining [I] and is therefore of special interest. In a study performed on Ni surfaces it has been shown that adsorption of electro-positive elements such as K or Na strongly .enhances the dissociative adsorption of CO [4]. On the other hand, the presence of electronegative additives such as Cl or S inhibits the dissociation of CO IS]. Similar effects have been observed for these additives on a variety of transition metal surfaces [2]. The purpose of this letter is to obtain microscopic understanding of the way these atoms influence the dissociative adsorption of CO. Using an LCAO-type method, we calculate potential energy surfaces for the CO dissociation at the Ni( 111) surface without and with K and -Cl impieties. Calculations are performed for small clusters consisting of four Ni atoms with one adsorbed CO molecule. In the case of additives, one K or Cl atom is used as substitutional impurity. The geometry of the cluster is shown in the inset of fig. 1. For calculating the total energy, we use the Atom Superposition and Electron Delocalization (ASED) technique [6f. Here, the total energy consists of two parts. The binding part, due to electron delocalization, is given by the extended Hiickel one-electron energy. The atom superposition part is a sum of repulsive interatomic interactions. In previous calculations 171, this