Molecular-Orbital Studies of Transition-and Noble-Metal Clusters by the SCF-X(alpha) Scattered-Wave Method.

摘要

The electronic structures of small copper, nickel, palladium, and platinum clusters having simple-cubic and cubo-octahedral geometries have been calculated, using the self-consistent-field X-alpha scattered-wave approach to molecular-orbital (MO) theory. As the cluster size and coordination number are increased, the MO results show increasing similarity to the electronic structures of the corresponding crystalline metals. The calculated ionization potentials decrease gradually with increasing cluster size and for the 13-atom cubo-octahedral clusters are approximately 2 eV greater than the corresponding average bulk work functions. Localized d-electron states are also observed for the cubo-octahedral transition-metal clusters. They appear to be cluster analogs of the 'surface states' postulated for crystalline transition metals and probably play an important role in the chemisorption and catalytic activities of small metal aggregates of the type utilized as the active centers of heterogeneous catalysts. The SCF-Xalpha-SW MO results for transition-metal clusters are critically compared with those obtained for similar clusters by the extended-Huckel (EH) and complete-neglect-of-differential-overlap (CNDO) methods. The implications of these results in surface-cluster studies of chemisorption on transition metals are discussed.