CO adsorption on Pd: An XPS and density functional approach

XPS measurements and density-functional calculations are reported for the adsorption of carbon monoxide on the Pd (100) and (110) faces. In particular, the experimental C Is and O Is ionization potentials for CO adsorbed on the two faces of Pd single crystals were compared with the theoretical values. Although the experimental determination of the Ols binding energy was difficult because of its overlapping with the Pd 3p(3/2) ionization, a method was developed to extract the oxygen contribution based on the profile of the two spin-orbit components. For both Pd faces, two different sites, bridge and atop, have been investigated theoretically. The (100) and (110) surfaces have been modelled by clusters of appropriate shape and size, using up to 7 Pd atoms. While the C ls and O ls ionization potential values were found to oscillate depending on the cluster size, an almost independent quantity has been found, namely the Ols - Cls energy difference. This quantity was found to have values differing by 0.8-0.9 eV for the two adsorption sites. Our results for CO/Pd(100) and CO/Pd(110) are in agreement with previous experimental evidence indicating adsorption in the bridge site for both systems.