The Atomic Structural Dynamics of γ-AI_2O_3 Supported Ir-Pt Nanocluster Catalysts Prepared from a Bimetallic Molecular Precursor:A Study Using Aberration-Corrected Electron Microscopy and X-ray Absorption Spectroscopy

摘要

This study describes a prototypical, bimetallic heterogeneous catalyst: compositionally well-defined Ir-Pt nanoclusters with sizes in the range of 1-2 nm supported on y-Al2O3. Deposition of the molecular bimetallic cluster [Ir_3Pt_3(μ-CO)_3(CO)_3(η-C_5Me_5)_3] on γ-A1_2O_3, and its subsequent reduction with hydrogen, provides highly dispersed supported bimetallic Ir-Pt nanoparticles. Using spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) and theoretical modeling of synchrotron-based X-ray absorption spectroscopy (XAS) measurements, our studies provide unambiguous structural assignments for this model catalytic system. The atomic resolution C_S-STEM images reveal strong and specific lattice-directed strains in the clusters that follow local bonding configurations of the γ-Al_2O_3 support. Combined nanobeam diffraction (NBD) and high-resolution transmission electron microscopy (HRTEM) data suggest the polycrystalline γ-Al_2O_3 support material predominantly exposes (001) and (011) surface planes (ones commensurate with the zone axis orientations frequently exhibited by the bimetallic clusters). The data reveal that the supported bimetallic clusters exhibit complex patterns of structural dynamics, ones evidencing perturbations of an underlying oblate/hemispherical cuboctahedral cluster-core geometry with cores that are enriched in Ir (a result consistent with models based on surface energetics, which favor an ambient cluster termination by Pt) due to the dynamical responses of the M-M bonding to the specifics of the adsorbate and metal-support interactions. Taken together, the data demonstrate that strong temperature-dependent charge-transfer effects occur that are likely mediated variably by the cluster-support, cluster-adsorbate, and intermetallic bonding interactions.