Interactions among adsorbates on metal surfaces

The low coverage adsorption modes of CN on Pt(1 1 1) have been studied by means of density functional theory and both cluster and slab models; geometry, adsorption energy and vibrational frequencies have been computed. The most favourable adsorption mode is C-down on top, with the CN axis perpendicular to the surface. Hollow sites are also stable minima, but the bridge site is a second order saddle point on the potential energy surface. Energetic differences between different sites are very small, suggesting a high mobility of the CN species on the surface. N-down structures are always less stable than C-down, being the difference of ∼1 eV for each site. The metal CN bond is described as a mixing of covalent and ionic contributions with a charge on chemisorbed CN of ∼−0.5e. The analysis of the bond permits to understand the changes in both adsorption energies and vibrational frequencies.