Ab initio electronic structure investigation of adsorption, coadsorption and reactions on selected transition metal surfaces
Ab initio electronic structure calculations based on density functional theory with the generalized gradient approximation and ultrasoft pseudopotentials have been applied to understand the effect of the local environment on the characteristics of chemisorption of selected atoms and molecules on low and high Miller index surfaces of transition metals. In some calculations the full-potential linearized augmented plane wave method was employed for a more accurate description of electronic struc¬ture. On a set of low and high Miller index surfaces of Cu with coordination ranging from 6 to 9 we find a decrease in CO adsorption energy with an increase in the local coordination. This general trend is found to be in qualitative agreement with experiment. The largest binding energy of CO is found on the kinked surface Cu(532). A similar analysis of CO adsorption on Au surfaces shows the strongest binding to be on Au(532). Analysis of vibrational dynamics of the systems reveals an increase in the frequency of the metal - C stretch mode with a decrease in the local coordination. Examination of the surface electronic structure shows the effect of CO adsorption on local densities of states of the substrate to be quite strong. Some energetics of surface diffusion of CO on (111) and (211) surfaces of Cu and Au are also provided. A detailed structural analysis of the kinked surface Cu(532) shows a dramatic oscillation in the relaxation pattern for the first ten layers with serious discrepancies in comparison to relaxations obtained with the embedded atom method. Full-potential band structure calculations reveal a dispersionless surface state for Cu(532). Pd being a catalyst in industrial processes provides a good model system for developing an understanding of the mechanisms by which adsorbates like C and S may cause a poisoning of the surface for chemical reactions. A hierarchy of adsorption sites are found for both C and S on the set of stepped Pd surfaces. A comparison of the results for Pd(211) and Pd(533) (vicinals of Pd(111)) shows an almost linear scaling of adsorption energies with effective coordination. The analysis of local densities of states indicates a strong hybridization between C p and Pd d states. A comparison of C and S adsorption on these surfaces shows a similar trend except for binding of C to the substrate atoms. The results of the energetics of adsorption on Pd(211) and Pd(533) (a surface with a wider terrace) do not show any significant effect of terrace width on the surface electronic structure. A comparison of the effect of coadsorbed C (a poison) and K (an experimentally well-known promoter) on the CO oxidation reaction on Pd(111) is also presented. Even at very low coverages (1/12 ML) of C and K the effect on the activation energy barrier was significant. The coadsorbed C was found to increase the barrier by about 6% whereas K shows exactly the opposite behavior. This effect was only there when coadsorbates were at a certain distance from the reaction path. The increase in coverage from 1/12 ML to 1/6 ML enhances the effect of coadsorbates. The detailed reaction pathways and energetics show the poison/promoter role of C and K on Pd surfaces which is in accordance with the general behavior seen experimentally.