H-2 DISSOCIATION ON A PT OVERLAYER ON NI(111)

The chemisorption of hydrogen atoms and the Hz molecule was studied by the use of extended Huckel calculations in a cluster model to simulat e a Pt(111) surface and a Pt overlayer in epitaxy on Ni(111). The resu lt of epitaxy on the chemisorptive properties of Pt is to disfavor the H atom adsorption on the higher coordination sites, and to increase t he activation energy of Hz dissociation. This is due to two factors. F irst, the Pt-Pt distance shortening increases the repulsion produced b y the Pt neighbors on the adatoms. Second, the study of the local elec tronic properties of the chemisorption site shows that the pure Pt sur face tends to chemisorb the H-2 molecule in the dissociated state wher eas the Pt/Ni system would favor the molecular adsorption only.