Faujasite-Supported Ir₄Clusters:  A Density Functional Model Study of Metal−Zeolite Interactions

Abstract

The interaction of a metal cluster, Ir 4 , 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 Ir 4 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 Ir 4 in the supercages of zeolite NaY. The calculations indicate an Ir-Ir distance of about 2.5 Å, only a few hundredths of an Ångstrom more than the value calculated for the free Ir 4 cluster, but about 0.2 Å less than the values observed repeatedly by EXAFS spectroscopy for zeolite-supported clusters approximated as Ir 4 . The experimental distances characterizing the zeolite-supported clusters are in close agreement with the crystallographic and calculated value reported for the coordinatively saturated cluster Ir 4 (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 Ir 4 (CO) 12 did not proceed by simple, complete removal of CO ligands). Consequently, calculations were performed for unsupported model clusters Ir 4 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 Ir 4 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.

Supported Iridium Cluster Catalysts for Propene Hydrogenation: Identification by X-Ray Absorption Spectra Measured During Catalysis

“…However, theoretical calculations by density functional methods have shown that Ir 4 clusters supported on zeolite NaY have essentially this same distance when only a single carbon or hydrogen ligand is present in addition to the support. [18] Thus, the Ir À Ir distances observed in the present work are in accord with related reports indicating the presence of ligands on the clusters, but they are not sufficient to determine how many ligands were present or whether the supported clusters were coordinatively saturated. (The EXAFS parameters characterizing the Ir-low-Z contributions do not shed additional light on the ligands because of the relatively large uncertainties in the metal-low-Z coordination numbers and the inability of the EXAFS spectroscopy to distinguish between low-Z scatterers such as oxygen and carbon.…”

“…[16] Thus, the data confirm the implicit assumption of earlier work [15] in which conclusions were drawn about the effects of cluster size [16] and support [17] on the catalytic activity for toluene hydrogenation. Suggestions have been made Tables 2 and 3. elsewhere [15,18] about the possible causes of the cluster size effect on the activity, and the issues are unresolved.…”

“…[18] This result, combined with the theoretical result that the effect of the zeolite support, modeled as a sixring, is to increase the IrÀIr distance by only a few hundredths of an ngstrom unit, leads to a question about the Ir À Ir distances of the supported clusters under vacuum (Table 3); these are 2.67 and 2.68 and are much closer to the values characterizing the ligated clusters than to the theoretical value characterizing the clusters without ligands other than the zeolite support. We suggest that a possible explanation for the apparent discrepancy is that the clusters formed by decarbonylation from the precursor metal carbonyls were not entirely free of ligands besides the support; for example, carbon might have remained on the metal, as has been suggested before.…”

The Decomposition of Hydrazine in the Gas Phase and over an Iridium Catalyst

“…Petrova, Rösch, and coworkers have used twelve-membered rings with three-fold symmetry: balanced by three Na + ions [95] 3 , which is neutralized by O protonation [96][97][98][99], as models for zeolite supports of Ir 4 . The lower energy structure of the former model [95] binds three Ir atoms to three oxygens. The hydroxyl groups in the latter model [96][97][98][99] surround but do not bond to the three attached Ir atoms.…”

Density functional studies of the adsorption for hydrated Zn(II) ion on anatase TiO2 (101) surface

“…Density functional theory (DFT) arises as one of the most useful theoretical tools in this field [1][2][3], mainly because of its ability of accounting for the correlation energy (important in the treatment of metal systems) in a very efficient way [4], which is usually applied to describe the adsorption of molecules on a surface as a localized interaction. It has been used to give an insight into adsorption processes at the atomic level and to contribute to the interpretation of experimental results from SPM, AFM, and EX-AFS [5].…”