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 informa tion 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 de scription of the chemisorption bond changes between the various systems con sidered in our study. The CO chemisorption energy on Pt/Ni(111) was found t o 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 broad ening and decreasing density at the Fermi level). Indeed, they induce, in c omparison with the pure substrates, a repulsion between Pt and CO although the 2 pi* population of the chemisorbed molecule increases. This points to the necessity of going beyond arguments based on an analysis of the 5 sigma donation and 2 pi* backdonation for a complete description of the chemisor ption bond.