Modelling of Pd and Pt supported on the {111} and {011} surfaces of cubic-ZrO2

The interaction of the noble metals Pd and Pt with the {111} and {011} surfaces of cubic-ZrO2 was examined by employing pseudo-potential plane-wave calculations within two Hamiltonians: the LDA and Perdew-Wang91 (GGA). The most favourable adsorption sites, adsorption energies, density of states and electron-charge density distributions are discussed, where the latter properties provide information on the bonding character between the ad-layer and the support, which is directly related to the catalytic activity. Owing to surface relaxation on the Pd/ZrO2{111} interface, we found a competition between the 1-fold and 3-fold oxygen sites; i.e. the differences in adsorption energies between the various adsorption sites are ca. 20 kJ mol(-1), which in addition suggests that the Pd-adlayer may have significant mobility. We anticipated the Pt-adlayer to be associated with lower surface mobility than the Pd-layer, due to the tendency to favour one particular adsorption site: DeltaE(ads) on top of O-S is similar to 100 kJ mol(-1), while on top of the 3-fold oxygen sites the energy is similar to 60 kJ mol(-1). Also, on the {011} surface, Pt is favoured by 1-fold oxygen coordination, while Pd exhibits higher oxygen coordination, and is adsorbed on the terraces of the {011} surface. Our calculations show the metal-support interactions to be mainly of an electrostatic nature, introduced by a polarisation of the metal ad-layers.