Why do heats of adsorption of simple gases on platinum surfaces vary so little with surface structure?

Heats of adsorption of atoms on metal surfaces have been measured for many years. One key unexplained observation is that experimental heats of adsorption vary only modestly with changing surface structure. The heats of adsorption of most simple molecules on stepped or kinked surfaces are very similar to their heats of adsorption on closely packed planes, even though the coordination numbers of the atoms in the stepped surfaces can be quite different from those in the flat surface. At present, these experimental observations are largely unexplained. In this paper, we discuss one possible reason why heats of adsorption show so little variation with surface structure. The embedded atom method was used to calculate how much the coordinative unsaturation of a stepped surface changes due to surface relaxations. We find that surface relaxation reduces the coordinative unsaturation. We define an electronic coordination number for the surface atoms and show that the average electronic coordination number of the atoms in a relaxed stepped surface is always about nine: i.e. almost the same as Pt(111). This similarity of the electronic coordination numbers among surfaces after relaxation provides a possible explanation of why heats of adsorption of gases on platinum vary so little with surface structure.