Theoretical study for the Pt2Au- and PtAu2-ethylene interaction

A theoretical study was done for the on-top coordination mode of the ethyle ne molecule chemisorbed over the Pt(100) surface, which was modeled by the Pt2Au and PtAu2 systems. In this on-top reaction mode, the C-C axis is para llel to the Pt-Pt, Pt-Au, or Au-Au axis, respectively. Calculations were do ne with the Turbomole program, which is a density functional theory based m ethod. Relativistic energy corrections were included and electron-core pote ntials were used. Orbital basis sets of DZP quality were employed for all t he atomic species. The gradient-dependent BLYP functional was used for the description of the exchange-correlation energy. It was found that the ethyl ene moiety is severely perturbed by this metallic surfaces. We have found t hat in Pt-3-C2H4, which contains two Pt surface atoms and one Pt bulk atom, the moiety is chemisorbed with a binding energy (BE) of 50.3 kcal/mol. Sub stitution of the bulk Pt atom by an Au atom reduces the BE to 37.6 kcal/mol , 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 a ble to chemisorb the ethylene moiety with a BE of 13.31 kcal/mol. Similarly , replacement of one surface Pt atom and one bulk Pt atom by Au atoms, resp ectively, gives a Pt-Au, system where the BE is 14.2 kcal/mol. Finally, rep lacement of the two surface Pt atoms by Au atoms produces a Pt-Au-2 system which is unable to chemisorb the C2H4 molecule, since here there is an ener gy barrier of 12.72 kcal/mol. Thus, the substitution of the Pt bulk (surfac e) atoms by Au particles enhances (diminishes) the chemisorption of C2H4. ( C) 1999 John Wiley & Sons, Inc.