On the stability and oxidation of Pdn (n=1-7) clusters on rutile TiO2(110)

First principles theoretical studies of the atomic and electronic structure of Pd$_n$ (n=1-7) clusters supported on a TiO$_2$(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 intra-cluster interactions favoring compact geometries and cluster support interactions that favor geometries that flatten out in the TiO$_2$(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 the Pd atom is proposed to originate from the minimal amount of charge transferred from Pd to O$_2$ and its spin excitation energy. For Pd$_n$O$_2$ (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 spill over O atoms. The theoretical finding are compared with recent experiments on the structure and oxidation of CO by supported clusters in the presence of O$_2$.