Modeling coverage dependent adsorption energies
Many adsorbates have coverage dependent adsorption energies on late transition metals due to interactions between the adsorbates and the surface. For example, here we examine the coverage dependence of oxygen adsorption on late transition metal surfaces.
The coverage dependence is caused by modifications of the surface electronic structure, particularly the d-band. Here it is apparent that the d-band gets broader as the coverage increases, although the d-band filling is hardly changed. This leads to a reduction in energy of the d-band and a weaker interaction with subsequent adsorbates.
The common electronic structure modification mechanism leads to correlations in the adsorption properties of these adsorbates, provided the geometry of the configurations are similar:
We can show more quantitatively that the quality of the correlation is related to the geometric dissimilarity. Below the size of the circles is proportional to how dissimilar a configuration is to the same configuration on Pd(111). The more dissimilar a configuration is, the lower the quality of the correlation.
These results are far from complete; many configurations have not been considered. Unfortunately, DFT is too slow to even try computing every possible configuration. We use the cluster expansion methodology to expand the number of configurations. The cluster expansion works basically like this:
The red dots represent basis functions which we use to expand the DFT results. By solving for the J's we can compute the energies of arbitrary configurations.
We gratefully acknowledge funding from DOE-BES Grant No. DE-FG02-07ER15919