CO adsorption on Ni(1 1 1)––a density functional theory study

Density functional theory calculations have been used to investigate the adsorption of carbon monoxide on the (1 1 1) surface of nickel. For various surface coverages between Θ=1/12 and 1 ML with CO occupying different adsorption sites geometric structure, adsorption energy, CO stretch frequency (and intensities) and O1s and C1s binding energies have been calculated. By comparison to experimental data from the literature, the site occupation with coverage could be traced: at low coverage (Θ<0.5 ML) CO occupies mainly bridge sites, accompanied by small patches of hollow (for Θ<0.2 ML) and on-top (for Θ>0.2 ML) adsorbed species. The relatively high-dynamic dipole moment of the linear bond species (especially at low coverage) leads to a distinct signal even for very small amounts. At Θ=0.5 the well established c(2×4) structure with a mixed occupation of fcc and hcp adsorbed molecules is formed. In this structure strong dipole interaction between the adsorbates shift the CO stretch frequency to a value that is traditionally assigned to bridging species. For even higher coverage more complicated structures within 7×7 (Θ=4/7 ML) and c(23×4)rect (Θ=5/8 ML) cells are discussed. A further fundamental observation of this study is a very pronounced coverage dependence of the dynamic dipole moment for the CO stretch frequency and hence of the corresponding peak intensities in vibrational spectroscopy.