Origin of Oxidation and Support-Induced Structural Changes in Pd4 Clusters Supported on TiO2

摘要

Theoretical first-principles studies of the effect of oxidation on the atomic and electronic structure of a Pd4 cluster and the nature of cluster support interactions have been carried out by investigating a Pd4 cluster deposited on a rutile TiO2(110) surface. Our studies based on a gradient-corrected density functional approach indicate that the deposited Pd4 has a compact ground state with an energetically close pseudoplanar structure that is expected to coexist with the ground state. The compact ground state undergoes a transition to a planar structure upon the absorption of a single O atom. The addition of a second O, however, generates two different structures: (1) a pseudoplanar structure where a Pd site mediates the interaction between the two O atoms, and (2) an energetically more stable species with a planar Pd4 structure and O in a "spillover" mode. Experimental evidence supporting this spillover O is discussed. Detailed analysis of the electronic states and charge densities reveals that the interaction between the Pd and the lattice O sites is mainly responsible for the variations in structure of the deposited species as well as the relaxation of the underlying lattice. Specifically, variations in the charge state of the lattice Ti and O sites are shown to lead to large displacements of Ti ions providing a microscopic mechanism of observed strong metal support interactions.