Molecular orbital investigation of chemisorption. I. Hydrogen on tungsten (100) surface

Abstract

The relative bonding energies of hydrogen chemisorbed at three symmetric sites on a W(100) surface were obtained by means of the extended Hückel molecular orbital theory (EHMO). The preferred site for hydrogen chemisorption was found to be the single coordination number (1 CN) site or the site above a surface tungsten atom. The W(100) surface was represented by finite arrays of tungsten atoms which were shown to be adequate for obtaining semiquantitative results. The basis set for the calculations contained the valence orbitals of tungsten and, initially, the 5p orbitals which were nonbonding but provided the necessary repulsion at small internuclear separation. The repulsive energy provided by these orbitals was replaced by an analytical exponential repulsive energy term. This allowed the 5p orbitals to be omitted from the basis set to simplify computation. Functionally, the energy change for the reaction Wn + H → Wn H was calculated for various assumed configurations of the Wn H ``molecule.'' The bonding between tungsten atoms was found to be changed as a result of Wn H formation, and the change varied with hydrogen position. Energy barriers to surface diffusion were also calculated and found to agree reasonably with experimental values.