DFT Calculation Analysis of the Infrared Spectra of Ethylene Adsorbed on Cu(110), Pd(110), and Ag(110)

A density functional theory (DFT) calculation was performed on the cluster models of ethylene on Cu(110), Ag(110), and Pd(110) to clarify the correlation between the IR spectra of the adsorbate and the modes of ethylene−surface interaction. The metal surfaces were modeled by two- or three-layered clusters consisting of 13−34 metal atoms. Four kinds of adsorption sites were considered:  atop bonding sites with the CC bond parallel and perpendicular to the 〈1&onemacr;0〉 direction (ST and LT sites), a short bridge site with the CC bond parallel to the 〈1&onemacr;0〉 direction (SB site), and a long bridge site with the CC bond perpendicular to the 〈1&onemacr;0〉 direction (LB site). The results of calculations for three-layered models consisting of more than 20 metals could be compared reasonably with the experimental data. The comparison indicated that (i) upon increasing surface coverage, ethylene on Cu(110) converts its adsorption site from an SB to an ST site, (ii) ethylene adsorbs at an LT site of Ag(110), and (iii) ethylene on Pd(110) takes on an ST site. These conclusions are consistent with those derived from STM and other spectroscopic measurements including UPS and NEXAFS, indicating that the DFT calculation on the cluster models is efficient for the analysis of the IR spectra of ethylene adsorbed on metal surfaces, which delineates the adsorption modes. The contribution of donation and back-donation of electrons to the ethylene−metal bonding was estimated by calculating the projections to the π-bonding and π*-antibonding orbitals of the isolated ethylene in the adsorbed geometries. The results proved that both the π donation and π* back-donation make appreciable contributions to the ethylene−surface interaction on Cu(110), whereas the π* back-donation is negligible in the ethylene−Ag(110) interaction. It was suggested that the frequency increase of the CH<INF>2</INF> out-of-plane wagging vibration from that of the free ethylene observed for ethylene on Ag(110) is a measure of the contribution of the π donation to the ethylene−surface interaction.