Faujasite-supported Ir₄ clusters: A density functional model study of metal-zeolite interactions

The interaction of a metal cluster, Ir₄, and a zeolite support was investigated computationally with the aid of a density functional method and a cluster model of a zeolite, i.e., a six-ring consisting of six O atoms and six T (Si or Al) atoms facing a supercage of a faujasite framework. Structural parameters are reported for an Ir₄ tetrahedron interacting with the zeolite six-ring. The calculations indicate two Ir-O distances, which match those reported on the basis of EXAFS spectroscopy at about 2.1-2.2 and 2.5-2.7 Å for various transition and noble metal clusters on zeolite (and metal oxide) supports, including Ir₄ in the supercages of zeolite NaY. The calculations indicate an Ir-Ir distance of about 2.5 Å, only a few hundredths of an Angstrom more than the value calculated for the free Ir₄ cluster, but about 0.2 Å less than the values observed repeatedly by EXAFS spectroscopy for zeolite-supported clusters approximated as Ir₄. The experimental distances characterizing the zeolite-supported clusters are in close agreement with the crystallographic and calculated value reported for the coordinatively saturated cluster Ir₄(CO)₁₂ and favor the suggestion that the supported clusters investigated with EXAFS spectroscopy were not entirely ligand free (i.e., that their formation by decarbonylation of the parent Ir₄(CO)₁₂ did not proceed by simple, complete removal of CO ligands). Consequently, calculations were performed for unsupported model clusters Ir₄ with single H or C atoms as ligands; the results match the EXAFS data characterizing the Ir-Ir distance and favor the suggestion of carbon on the zeolite-supported clusters. The bonding of a single CO molecule to the supported Ir₄ at the on-top site was also modeled to probe changes in the electronic structure of the metal cluster in comparison with an unsupported metal cluster. The results show that the interaction of the metal cluster with the zeolite fragment induces a notable electron donation from the support to the metal cluster; it also causes a moderate charge rearrangement in the bonding region between the Ir and O centers, accompanied by a considerable polarization of the electron density toward the apex of the cluster not interacting directly with the zeolite. Generalizing this result, we suggest that small noble metal clusters interacting mainly with basic oxygen atoms of zeolite and metal oxide supports are nearly zerovalent or slightly negatively charged and that some effects of supports in catalysis may be explained by this charge transfer and by polarization.