Acetylene on Cu and Pd(111) surfaces: A comparative theoretical study of bonding mechanism, adsorption sites, and vibrational spectra

We have used ab initio cluster model wavefunctions to study the adsorption of acetylene on Cu(111) and Pd(lll) surfaces. Clusters of different sizes have been employed to model the adsorption modes of acetylene on bridge (B) and three-hollow (H) sites. The interactions on Cu and Pd are different because of the occupation of the sp conduction band, which is larger on Cu than on Pd. One manifestation of the role of the 4sp electrons in Cu is a larger Pauli repulsion with the adsorbed molecule. This often causes a crossing of electronic states to reduce the repulsion as the molecule approaches the cluster. A direct measure of the extent of the Pauli repulsion on Cu and Pd has been obtained by decomposing the interaction energy into the sum of Various contributions, including intra-unit polarization and charge transfer. The sp electrons substantially contribute to the back bonding from the surface to the adsorbate hence to the distortion of the acetylene molecule which is slightly more activated on Cu than on Pd despite the longer distance from the surface and the weaker bonding. The B and Il sites have similar stabilities, but the H site is slightly preferred. The geometrical parameters and the vibrational spectra of C2H2 on Pd(lll) are in good agreement with the experiment (both theory and experiment indicate a C-C distance very close to 1.3 Angstrom); for the Cu case we reproduce the experimental vibrational spectra but not the C-C distance which, in recent experiments, was found to be Very elongated (1.48 +/- 0.10) Angstrom, while the computed value is around (1.37 +/- 0.02) Angstrom. Finally, our data show that the vibrational spectra are characteristic of the metal (Cu is different from Pd) but not of the adsorption site, Very similar spectra are computed for the B and H sites of the same metal surface.