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 sigma donation with little pi 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 sigma 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 from the subsequent increase of the repulsion with the occupied metal slates. This repulsion is responsible both for the larger top-bridge differential CO omega shift and for the different surface dipole which, in turn, determines the work function change. (C) 1997 Elsevier Science B.V.