Site preference of CO chemisorbed on Pt(1 1 1) from density functional calculations

Chemisorption of carbon monoxide on monocoordinated and tricoordinated sites of Pt(1 1 1) is studied using various computational methods based on density functional theory and a series of cluster and periodic models. Calculated results for geometries and binding energies are provided. We demonstrate that both types of models, irrespective of the density functional approximation used, always favour CO adsorption at the threefold coordinated hollow site instead of on-top, monocoordinated CO, as already suggested in the paper of Feibelman et al. [J. Phys. Chem. B 105 (2001) 4018]. This is at variance with experimental evidence and indicates a possible limitation of common approximate density functional theory methods. It is shown that small clusters, that do not correctly describe the substrate environment of the active site, are not adequate models to obtain adsorption energies or adsorption energy differences. However, with increasing cluster size, cluster results are very close to results of periodic calculations. The new insight is that hybrid functionals including a part of the exact exchange decrease the energy difference between the two positions, suggesting a stabilization of the on top site relative to the threefold hollow site in the limit of extended models. Arguments are presented that the energetic preference of the threefold hollow site is due to an inadequate description of the HOMO-LUMO gap.