A density functional theory study of CO and atomic oxygen chemisorption on Pt(111)

Abstract

Ab initio total energy calculations within a density functional theory framework have been performed for CO and atomic oxygen chemisorbed on the Pt(111) surface. Optimised geometries and chemisorption energies for CO and O on four high-symmetry sites, namely the top, bridge, fcc hollow and hcp hollow sites, are presented, the coverage in all cases being 0.25 ML. The differences in CO adsorption energies between these sites are found to be small, suggesting that the potential energy surface for CO diffusion across Pt(111) is relatively flat. The 5σ and 2π molecular orbitals of CO are found to contribute to bonding with the metal. Some mixing of the 4σ and 1π molecular orbitals with metal states is also observed. For atomic oxygen, the most stable adsorption site is found to be the fcc hollow site, followed in decreasing order of stability by the hcp hollow and bridge sites, with the top site being the least stable. The differences in chemisorption energies between sites for oxygen are larger than in the case of CO, suggesting a higher barrier to diffusion for atomic oxygen. The co-adsorption of CO and O has also been investigated. Calculated chemisorption energies for CO on an O/fcc-precovered surface show that of the available chemisorption sites, the top site at the oxygen atom's next-nearest neighbour surface metal atom is the most stable, with the other four sites calculated being at least 0.29 eV less stable. The trend of CO site stability in the coadsorption system is explained in terms of a ‘bonding competition’ model.