THEORETICAL STUDY OF CO ADSORPTION ONNi(111),Pt(111)ANDPt/Ni(111)SURFACES

CO adsorption on a pseudomorphic Pt overlayer supported byNi(111)has been studied with the use of extended Huckel calculations. Experimental information on the purePt(111)andNi(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 onPt/Ni(111)was found to be lowered in comparison withPt(111)andNi(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 the2π*population of the chemisorbed molecule increases. This points to the necessity of going beyond arguments based on an analysis of the5σdonation and2π*backdonation for a complete description of the chemisorption bond.