Energy partitioning studies of adsorbate repulsion: Chemisorption and coadsorption of CO and NO on the Pd(1 1 1) surface

Abstract

Hamilton population (HP) analysis—a bond-by-bond partitioning of the electronic energy—is used to elucidate the physical mechanisms by which direct (through-space) and surface-mediated interactions between adsorbates are propagated in ordered CO, NO and mixed CO/NO phases on the Pd(1 1 1) surface. Analysis of ( 3 × 3 ) R 30 ° -CO and NO phases at one-third coverage, c(4 × 2)-2CO, 2NO, and mixed CO/NO phases at half-coverage, and p(2 × 2)-3CO and 3NO phases at high (0.75 ML) coverage demonstrates that, in each case, the interaction between neighboring adsorbates is principally surface-mediated. Within the HP description, direct interactions between neighboring adsorbates are found to be of little consequence, even at high (0.75 ML) adsorbate coverages. At each coverage, the surface-mediated interaction between adsorbates primarily results from a reduction in surface–adsorbate bonding that, in turn, results from the competition between neighboring adsorbates for bonding to individual surface atoms. The magnitude of the surface-mediated interaction is shown to be strongly influenced by the relative orientation of the adsorbates about intervening surface atoms. As such, the surface-mediated interaction between adsorbates is not found to be a simple function of adsorbate separation. The reduction in surface–adsorbate bonding about shared surface atoms is shown to be influenced by the extent to which neighboring adsorbates compete for σ and π bonding with the intervening surface atoms. The extent to which the interaction between neighboring CO and/or NO molecules is mediated by Pd–Pd bonds within the surface is also shown to be a function of the respective ability of CO and NO to act as σ-donors and π-acceptors.