Correlations in coverage-dependent atomic adsorption energies on Pd(111)

The adsorption energy of an adsorbate can depend strongly on its coverage. We present density-functional calculations of the adsorption energy of C, N, and O on the fcc sites of Pd(111) at coverages from 0.2 to 1 ML. The adsorption energy generally increases (gets weaker) with coverage in a near linear fashion for the most stable configurations. We observed a practically constant $d$-band filling at all coverages and interpret the coverage dependence in terms of a simple $d$-band model where the $d$ band is broadened and lowered in energy by the interaction with the adsorbates. We find the Pd(111) $d$-band center is approximately linearly correlated with the coverage of each adsorbate. A consequence of this electronic structure-coverage correlation for each adsorbate is that the adsorption energies of each adsorbate configuration are strongly correlated with each other. All of the exceptions to these correlations can be traced to surface reconstruction or adsorbate configurations that are not geometrically similar to adsorption in the fcc hollow sites. Finally, we show that all the coverage-dependent adsorption energies can be collapsed into a single configurational correlation by scaling of the adsorption energies.