Theoretical Studies of the Stability and Oxidation of Pdn (n=1-7) Clusters on Rutile TiO2(110): Adsorption on the Stoichiometric Surface

First principles theoretical studies of the atomic and electronic structure of Pd-n (n = 1-7) clusters supported on a TiO2(110) surface, and O-2 activation by such clusters, have been carried out within a gradient corrected density functional approach. It is shown that the supported Pd-n cluster geometries are driven by competing effects including intracluster interactions favoring compact geometries and cluster-support interactions that favor geometries that flatten out in the TiO2(110) surface channel. When exposed to O-2, a single Pd atom only activates the O-O bond while all other clusters energetically favor a broken O-O bond. The differing behavior of Pd atom is proposed to originate from the minimal amount of charge transferred from Pd to O-2 and its spin excitation energy that also result in its observed lack of reactivity in experiments on CO oxidation. Our studies on PdnO clusters indicate an inverse correlation between the O binding strength to the cluster and the cluster's surface stability, which we define as the cluster binding energy to the surface minus the energetic cost of deformation when a Pd-n cluster conforms to the TiO2 surface channel. For PdnO2 (n = 2-7), it is shown that while the first O is adsorbed on the Pd-n cluster, the second O occupies a site above a lattice Ti site at the Pd-Ti interface and is indicative of spillover O atoms. The theoretical findings are compared with recent experiments on the structure and oxidation of CO by supported clusters in the presence of O-2.