Faujasite-supported Ir-4 clusters: A density functional model study of metal-zeolite interactions

The interaction of a metal cluster, Ir-4, and a zeolite support was investi gated computationally with the aid of a density functional method and a clu ster 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 zeolit e 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 A ngstrom for various transition and noble metal clusters on zeolite (and met al oxide) supports, including Ir-4 in the supercages of zeolite NaY. The ca lculations indicate an Ir-lr distance of about 2.5 Angstrom, only a few hun dredths of an Angstrom ngstrom more than the value calculated for the free Ir-4 cluster, but about 0.2 Angstrom less than the values observed repeated ly by EXAFS spectroscopy for zeolite-supported clusters approximated as Ir- 4. The experimental distances characterizing the zeolite-supported clusters an in close agreement with the crystallographic and calculated value repor ted for the coordinatively saturated cluster Ir-4(CO)(12) and favor the sug gestion that the supported clusters investigated with EXAFS spectroscopy we re not entirely ligand free i.e., that their formation by decarbonylation o f the parent Ir-4(CO)12 did not proceed by simple, complete removal of CO l igands. Consequently, calculations were performed for unsupported model clu sters Ir-4 With single H or C atoms as ligands; the results match the EXAFS data characterizing the Lr-lr distance and favor the suggestion of carbon on the zeolite-supported clusters. The bonding of a single CO molecule to t he supported Ir-4 at the on-top site was also modeled to probe changes in t he electronic structure of the metal cluster in comparison with an unsuppor ted metal cluster. The results show that the interaction of the metal clust er with the zeolite fragment induces a notable electron donation from the s upport to the metal cluster; it also causes a moderate charge rearrangement in the bonding region between the Lr and O centers, accompanied by a consi derable polarization of the electron density toward the apex of the cluster not interacting directly with the zeolite. Generalizing this result, we su ggest that small noble metal dusters interacting mainly with basic oxygen a toms of zeolite and metal oxide supports are nearly zerovalent or slightly negatively charged and that some effects of supports in catalysis may be ex plained by this charge transfer and by polarization.