6 research outputs found
Orbital interactions and chemical reactivity of metal particles and metal surfaces
A review is presented with 101 refs. on chem. bonding to metal surfaces and small metal particles demonstrating the power of symmetry concepts to predict changes in chem. bonding. Ab-initio calcns. of chemisorption to small particles, as well as semiempirical extended Hueckel calcns. applied to the study of the reactivity of metal slabs are reviewed. On small metal particles, classical notions of electron promotion and hybridization are found to apply. The surroundings of a metal atom (ligands in complexes, other metal atoms at surfaces), affect bonding and reactivity through the prehybridization they induce. A factor specific for large particles and surfaces is the required localization of electrons on the atoms involved in the metal surface bond. At the surface, the bond energy is found to relate to the grou8p orbital local d. of states at the Fermi level. The use of this concept is extensively discussed and illustrated for chemisorption of CO and dissocn. of NO on metal surfaces. A discussion is given of the current decompn. schemes of bond energies and related concepts (exchange (Pauli)-repulsion, polarization, charge transfer). The role of non-orthogonality of fragment orbitals and of kinetic and potential energy for Pauli repulsion and (orbital) polarization is analyzed. Numerous examples are discussed to demonstrate the impact of those concepts on chem. bonding theor
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 sit