AN ABINITIO MOLECULAR-ORBITAL STUDY ON ADSORPTION AT THE MGO SURFACE .1. H-2 CHEMISORPTION ON THE (MGO)4 CLUSTER

The hydrogen molecule chemisorption on the low coordination or corner site of the MgO surface has been studied with the ab initio method, adopting the (MgO)4 cluster as the surface site model. The effect of basis functions and correlation have been examined. Two physisorbed complexes have been found; one is an end-on complex on the oxygen site and the other a side-on complex on the Mg site. These complexes are led to a common transition state (TS) and then to a dissociative chemisorption product. A substantial polarization of charges in the hydrogen molecule takes place at the TS, from where the dissociative chemisorption proceeds ionically. The best estimates of energies of the physisorbed complexes, the TS and the product, relative to the isolated reactants are -2, +2, and -21 kcal/mol, respectively, for this cubic cluster model. When the effect of the Madelung potential is taken into account, the chemisorbed product is further stabilized by 4 kcal/mol.