THEORETICAL STUDY OF CO ADSORPTION ON Ni(111), Pt(111) AND Pt/Ni(111) SURFACES

CO adsorption on a pseudomorphic Pt overlayer supported by Ni (111) has been studied with the use of extended Huckel calculations. Experimental information on the pure Pt (111) and Ni (111) single crystals was employed to select a consistent parameter set for our bimetallic system. This gives a good description of the chemisorption bond changes between the various systems considered in our study. The CO chemisorption energy on Pt/Ni (111) was found to be lowered in comparison with Pt (111) and Ni (111), in good agreement with experimental data on Pt-rich Pt–Ni surface alloys. This observation could be justified by the electronic changes of the Pt states (valence band broadening and decreasing density at the Fermi level). Indeed, they induce, in comparison with the pure substrates, a repulsion between Pt and CO although the 2π* population of the chemisorbed molecule increases. This points to the necessity of going beyond arguments based on an analysis of the 5σ donation and 2π* backdonation for a complete description of the chemisorption bond.

ADSORPTION SITE CHANGE FOR Cs, Rb OR K ADSORPTION ON Ag(111)

The adsorption geometries for the primitive (3×3), (2×2) and ([Formula: see text] structures of K, Rb and Cs on Ag(111) have been determined using low-energy electron diffraction. In the lower-coverage (3×3) and (2×2) structures, the adatoms occupy fcc hollow sites, while in the [Formula: see text]; structure they occupy the hcp hollow sites. The fcc hollow structures are accompanied by significant substrate rumpling. There is no significant coverage-dependent or site-dependent change in chemisorption bond length. However, there is a large coverage-dependent anisotropy of vibrational amplitude of the adatoms, with the parallel component as much as five times larger than the perpendicular component at low coverages.