CHEMICAL EFFECTS AND SURFACE-PROPERTIES - THE NATURE OF AN ADSORBED COMPLEX

The general Born-Oppenheimer Hamiltonian of an adsorbate is expressed with reference to an AO basis by means of the second quantization tech nique, and the part representing the in situ adcomplex (admolecule pos sibly involving adsorption site atoms) is decomposed so as to show how contributions of different effects can be separated. A basic referenc e Hamiltonian of the adcomplex is defined, and the suggestion is made that it essentially represents a molecular system susceptible of treat ment in terms of ordinary localized bonds. This suggestion is shown to be valid for one thing in the highly unsaturated surface complex FeCO (compared with Fe5CO), provided maximum-localization hybrids are used as elements of the AO basis. The example treated has also been used t o show how two kinds of effects can be introduced and discussed: induc tive effects, which appear to result from hybridization and from feedb ack compensations to charge shifts, and charge transfer between the ad complex and the surface. The proposed local (''chemical'') description of an adcomplex appears to be an extremely useful tool for describing such complex adsorbates as those associated with Fischer-Tropsch cata lysts. (C) 1994 John Wiley and Sons, Inc.