A quantum-chemical study of Pd atoms and dimers supported on TiO2(110) and their interaction with CO

The interaction of Pd atoms and dimers with the (110) surface of TiO2 has been studied by means of gradient corrected density functional theory (DFT) calculations. (TiO2)(n) (n=3-15) clusters have been used to simulate the surface of rutile. The role of embedding in point charges and total ion model potentials has been analyzed in detail. Adsorption sites along the protruding oxygen and the basal five-fold coordinated Ti rows have been compared. A considerable stability of the results versus cluster size has been achieved. At low coverage we found a preference for Pd adsorption on the two-fold coordinated protruding oxygens where the bond strength is about I eV. Considerable adhesion is found also along the Ti rows, with interaction energies of 0.5-0.6 eV. A coverage of 0.25 monolayers (ML) of Pd has been simulated by performing periodic supercell DFT calculations. In this case there is an indication for a similar stability of the two sites, with the Ti rows slightly preferred. Adsorbate-induced reconstructions are not included in these models. Pd dimers adsorbed on the TiO2 surface lose most of the Pd-Pd interaction due to the relatively strong bond with the substrate. Adsorption of a CO molecule on the supported Pd dimers shows that the major effect of the bond at the interface is not a modification of the ability of the metal to donate electrons to CO but that of preventing a polarization of the metal electrons away from the CO molecule, thus increasing the Pauli repulsion contribution to the CO-Pd bonding. (C) 1999 Elsevier Science B.V. All rights reserved.