Clusters, chemisorption, and catalysis

The electronic structures of small clusters of atoms at solid interfaces and of polyatomic aggregates in supporting materials are of central importance to chemisorption and catalysis. Self-consistent field (SCF) molecular orbital theory provides a conceptually simple approach to cluster bonding, but it is of quantitative significance to chemisorption and catalysis only if accurate numerical applications to real systems are practical. A critique of the traditional Hartree-Fock molecular orbital approach to chemisorption is presented, along with a discussion of recent semiempirical “LCAO” calculations for chemisorption on graphite. The recently developed SCF-Xa “scattered-wave” method allows one to calculate highly accurate ground and excited electronic states of polyatomic clusters (as functions of cluster size, composition, geometry, and supporting environment), yet requires only moderate amounts of computer time. Current applications of this technique to chemisorption at transition-metal interfaces and to catalytically active metal aggregates are described.