First-Principles Descriptors of CO Chemisorption on Ni and Cu Surfaces
A comprehensive analysis of low coverage CO adsorption on Ni and Cu low-index miller surfaces – (100), (110), and (111) - over all the possible adsorption sites is presented. Systems are theoretically studied within an accurate adsorption model using RPBE density functional calculations to obtain electronic and geometrical structure predictions along with their corresponding adsorption energy computations. Firstly, we developed a consistent correlation of factors that determine the final C-O structure upon adsorption. The resulting C-O bond length is depicted to be directly dependent on structural parameters, such as depth of adsorption and metal-adsorbate bonding distances, and the quantity of charge being transferred from the surface to the CO molecule. Those factors are put into an equation that defines the final C-O bond: the “C-O equation”, and for each adsorption site, the final C-O bond length is calculated numerically using the equation and compared with the DFT predictions. The comparison displayed a consistent match over all sites, with marginal deviations at sites where the action of long-range dispersive force is more pronounced. Deeper analysis of the adsorbed C-O molecule is also presented from a molecular orbital level. Density of states (DOS) charts were exploited to investigate the perturbations to the 3σ and 1π orbitals that hold the internal C-O bond. From this analysis, a consistent link between the degree destabilization of the orbitals and the final C-O bond length is found for both Cu and Ni surfaces, offering a deeper understanding of the final adsorbate structure. Energetically, adsorption thermodynamics on Cu and Ni surfaces are compared within the Blyholder-Nilsson and Petersson (B-NP) model. The frontier (5σ and 2π*) orbital energies relative to the d-band center of the metal surfaces are displayed, which implicitly defines the thermodynamics of adsorption. Finally, the controversial repulsive nature of the σ-interaction proposed in NP model has been tested by tracking the charge redistribution within the metallic states, including the broad sp-states. Based on charge transfer results, the attractive nature of π-interaction is confirmed, however, for the σ-interaction, a dual nature is proposed with the existence of both partially attractive and partially repulsive natures simultaneously.