School of Chemical Sciences and the F. Seitz Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801, United States.
J Am Chem Soc. 2011 Mar 16;133(10):3582-91. doi: 10.1021/ja110033g. Epub 2011 Feb 22.
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 γ-Al(2)O(3). Deposition of the molecular bimetallic cluster [Ir(3)Pt(3)(μ-CO)(3)(CO)(3)(η-C(5)Me(5))(3)] on γ-Al(2)O(3), and its subsequent reduction with hydrogen, provides highly dispersed supported bimetallic Ir-Pt nanoparticles. Using spherical aberration-corrected scanning transmission electron microscopy (C(s)-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(2)O(3) support. Combined nanobeam diffraction (NBD) and high-resolution transmission electron microscopy (HRTEM) data suggest the polycrystalline γ-Al(2)O(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.
尺寸在 1-2nm 范围内的组成明确的 Ir-Pt 纳米簇，负载在 γ-Al(2)O(3)上。将分子双金属簇[Ir(3)Pt(3)(μ-CO)(3)(CO)(3)(η-C(5)Me(5))(3)]沉积在 γ-Al(2)O(3)上，并随后用氢气还原，得到高度分散的负载型双金属 Ir-Pt 纳米颗粒。使用球差校正扫描透射电子显微镜（C(s)-STEM）和基于同步加速器的 X 射线吸收光谱（XAS）测量的理论建模，我们的研究为这个模型催化体系提供了明确的结构分配。原子分辨率的 C(s)-STEM 图像显示，在簇中存在强烈而特定的晶格定向应变，这些应变遵循 γ-Al(2)O(3)载体的局部键合构型。结合纳米束衍射（NBD）和高分辨率透射电子显微镜（HRTEM）数据表明，多晶γ-Al(2)O(3)载体材料主要暴露（001）和（011）表面平面（与双金属簇经常表现出的晶带轴取向一致）。数据表明，负载的双金属簇表现出复杂的结构动力学模式，这些模式证明了核心为 Ir 富集的扁/半球形立方八面体核心几何形状的扰动（这是基于表面能的模型的结果，该模型有利于 Pt 作为环境簇的终止），这是由于 M-M 键对吸附物和金属-载体相互作用的具体情况的动态响应。总的来说，这些数据表明，发生了强烈的温度依赖的电荷转移效应，这些效应可能由簇-载体、簇-吸附物和金属间键合相互作用以不同的方式介导。
