A theoretical study was done for the on-top coordination mode of the Ž . ethylene molecule chemisorbed over the Pt 100 surface, which was modeled by the Pt Au and PtAu systems. In this on-top reaction mode, the C-C axis is parallel to the 2 2
Pt-Pt, Pt-Au, or Au-Au axis, respectively. Calculations were done with the Turbomole program, which is a density functional theory based method. Relativistic energy corrections were included and electron-core potentials were used. Orbital basis sets of DZP quality were employed for all the atomic species. The gradient-dependent BLYP functional was used for the description of the exchange᎐correlation energy. It was found that the ethylene moiety is severely perturbed by this metallic surfaces. We have found that in Pt -C H , which contains two Pt surface atoms and one Pt bulk atom, the 3 2 4
Ž . moiety is chemisorbed with a binding energy BE of 50.3 kcalrmol. Substitution of the bulk Pt atom by an Au atom reduces the BE to 37.6 kcalrmol, but it also produces strong structural changes on ethylene. Substitution of one surface Pt atom by one Au atom produces a Pt-PtAu system, which is able to chemisorb the ethylene moiety with a BE of 13.31 kcalrmol. Similarly, replacement of one surface Pt atom and one bulk Pt atom by Au atoms, respectively, gives a Pt᎐Au system where the BE is 14.2 kcalrmol. Finally, 2 replacement of the two surface Pt atoms by Au atoms produces a Pt-Au system which 2 is unable to chemisorb the C H molecule, since here there is an energy barrier of 2