Crystal field surface orbital — Bond-energy bond- order (CFSO-BEBO) model for chemisorption: Application to hydrogen adsorption on a platinum (111) surface

Abstract

A model describing adsorption which combines features of the bond-energy bond-order relationships of gas phase kinetics and spectroscopy, and the concept of crystal field orbitals has been formulated. This model is thus termed the crystal field surface orbital — bond energy bond order (CFSO-BEBO) model of chemisorption. The calculation is empirical in nature since single bond energies for metal-metal and metal-gas bonds are deduced from available data on bulk compounds or transition metal complexes. Alternatively, the single order bond energies may be estimated from suitable mixing rules or deduced from sublimation energies and spectroscopic gas phase data. Spectroscopic data are also used for the bond energies of the stable gas phase species and for the correlation between bond energy and bond order. The bond-energy bond-order correlation is assumed to be linear for metal-metal and metal-gas bonds.This model is used to describe the interaction of hydrogen with platinum (111) in a detailed way, i.e., the potential energy of interaction along the reaction (or adsorption) coordinate. The trapping probability of hydrogen on platinum is consistent with this CFSO-BEBO model, and the energies of the two predicted chemical adsorption states correspond to the observed experimental heats of adsorption, 17 and 31 kcal/mole, with the low energy state being molecular. LEED and flash desorption experiments also give strong support for the calculated potential energy along the reaction coordinate.