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 sur faces are very similar to their heats of adsorption on closely packed planes, even though the coordination numbers of the atoms in the stepp ed surfaces can be quite different from those in the flat surface. At present, these experimental observations are largely unexplained. In t his 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 step ped surface changes due to surface relaxations. We find that surface r elaxation reduces the coordinative unsaturation. We define an electron ic coordination number for the surface atoms and show that the average electronic coordination number of the atoms in a relaxed stepped surf ace is always about nine: i.e. almost the same as Pt(lll). This simila rity of the electronic coordination numbers among surfaces after relax ation provides a possible explanation of why heats of adsorption of ga ses on platinum vary so little with surface structure. (C) 1998 Elsevi er Science B.V. All rights reserved.