Conversion of surface carbidic to subsurface carbon on cobalt (0001) - a theoretical-study

Preliminary HFS-LCAO calculations of a nine cobalt cluster reveal a minimal energy difference between surface carbidic and subsurface carbon configurations. The electron withdrawing power, and therefore the poisoning effect on potential CO adsorption, is maximal for subsurface C, but localized to immediately neighboring metal atoms. If the metal lattice is kept fixed, the barrier for moving the carbon atom between the two sites is high (4.0 eV) because of steric repulsion. If the three-fold hollow of the cobalt cluster is stretched slightly by only 1%, the barrier is reduced by nearly 50%. By analogy to effective medium calculations, we may expect thermally active phonon modes to allow as much as a 10% lattice relaxation, which can reduce the surface to subsurface carbon barrier by 90% (0.49 eV). Coadsorption with oxygen favours the subsurface carbon site