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

Abstract

First principles theoretical studies of the atomic and electronic structure of Pdn (n = 1–7) clusters supported on a TiO2(110) surface, and O2 activation by such clusters, have been carried out within a gradient corrected density functional approach. It is shown that the supported Pdn 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 O2, 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 O2 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 Pdn cluster conforms to the TiO2 surface channel. For PdnO2 (n = 2–7), it is shown that while the first O is adsorbed on the Pdn 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 O2.