Adsorption of cyclohexene on nAu/Pt(100) (n=0, 1, 2): A DFT study

Adsorption of cyclohexene on the nAu/Pt(100) (n = 0, clean Pt; n = 1, Pt covered with monolayer of gold (Au/Pt); n = 2, Pt covered with two layers of gold (2Au/Pt), respectively) has been investigated by using self-consistent (GGA-PW91) density functional theory combined with periodic slab model. It is found that on the clean platinum, there are two kinds of favorable adsorption sites, hollow sites and bridge sites, and the adsorption energy in the hollow site is higher than that in the bridge site. Two carbon atoms of the C=C moiety are reacted with the same platinum atom through 'pi' interaction, which forms the chemical bond including the back-donating bond. However, on the Au/Pt and 2Au/Pt surfaces, there are three kinds of adsorption sites, and the adsorption energies are alike in both of the bridge site and the top site. However, the most stable adsorption site is the hollowR45 site on the three surfaces, and the order of the adsorption energies is clean Pt (-1.35 eV) > Au/Pt (-0.63 eV) > 2Au/Pt (-0.59 eV) from the calculations. The configurations of cyclohexene molecule have been distorted a little during the geometry optimizations. The lengths of C-M (M = Pt or Au, in the top layer of the slab) bond are 2.08,2.77 and 3.07 angstrom, respectively, which can accord with the difference in adsorption energies. Furthermore, considering the conditions for the formation of the back-donating bond, the cl-band centers of clean Pt, Au/Pt and 2Au/Pt are calculated, they are -2.04, -2.73 and -3.04 eV, respectively, which are in line with the order of the adsorption energies. (C) 2008 Elsevier B.V. All rights reserved.