Is CO chemisorbed on Pt anomalous compared with Ni and Pd? An example of surface chemistry dominated by relativistic effects

Relativistic gradient-corrected density functional calculations have been performed for Co adsorbed at on-top and bridge sites of Ni, Pd, and Pt cluster models of the corresponding (100) surfaces. The calculations reproduce correctly important features of the bonding of CO on these metals: the larger differential CO vibrational frequency shift between top and bridge adsorption sites and a much smaller change in the substrate work function for Pt(100)/CO, In the past, this experimental observation has been rationalized by a different kind of bonding for CO on Pt compared with Ni and Pd. In particular, it has been suggested that the bonding on Pt is dominated by σ donation with little π back donation. We show that the extent of the back donation in Pt is similar, if not larger, than on the other metals of the group and that the σ bonding is not the reason for the observed anomalies of the Pt-CO system. Rather, these arise from the large relativistic contraction of the Pt-CO distance for on-top adsorption and form the subsequent increase of the repulsion with the occupied metal states. This repulsion is responsible both for the larger top-bridge differential CO ω shift and for the different surface dipole which, in turn, determines the work function change,