Catalysts supported metal nanoclusters

Atomic-Scale Imaging of Supported Metal Nanocluster Catalysts in the Working State... 

II] ATOMIC-SCALE IMAGING OF SUPPORTED METAL NANOCLUSTER CATALYSTS 79... 

High-resolution transmission electron microscopy (HRTEM) has matured markedly in the preceding decade and has emerged as a powerful technique for investigation of nanostructured metal catalysts at the atomic level, even under working conditions. The ability to image the dynamic structure and morphology of supported metal nanocluster catalysts in such detail makes HRTEM an essential complement to the arsenal of spectroscopic techniques used for characterization of... 

This section of this chapter includes a brief review of methods of preparation and properties of supported metal nanoclusters only catalysts that have been relatively well characterized and found to be nearly uniform are considered. The nanoclusters described here lack the structural definition... 

Hansen PL, Helveg S, Datye A. Atomic-scale imaging of supported metal nanocluster catalysts in the working state. J Catal. 2006 50 77. 

XAS can be used in several different ways to determine local structural information about catalysts in reactive atmospheres. This structural information may be static or dynamic it may be geometric or electronic. The depth of information that can be ascertained is often dependent upon the type of catalyst, for example, supported metal nanoclusters versus bulk or surface oxides. It may also be controlled by some property of the catalyst, for example, the concentration of the element in the catalyst that is being investigated. In this section a few examples are provided to highlight the importance and relevance of XAFS in catalyst characterization. The examples are focused on (1) structural information characterizing samples in reactive atmospheres, (2) transformation of one species to another, (3) oxidation state determination, (4) determination of supported metal cluster size and shape, and (5) electronic structure. These examples illustrate the type of information that can be learned about the catalyst from XAFS spectroscopy. 

In the preparation and activation of a catalyst, it is often the case that the chemical form of the active element used in the synthesis differs from the final active form. For example, in the preparation of supported metal nanoclusters, a solution of a metal salt is often used to impregnate the oxide support. The catalyst is then typically dried, calcined, and finally reduced in H2 to generate the active phase highly dispersed metal clusters on the oxide support. If the catalyst contains two or more metals, then bimetallic clusters may form. The activity of the catalyst may depend on the metal loading, the calcination temperature, and the reduction temperature, among others. 

Besides the obvious computational challenge, we would like to point out two aspects of transition metal particles of nanoscale size. First, such nanoclusters offer an excellent opportunity for mimicking so-called model supported metal catalysts which are characterized by a notably reduced complexity compared to the corresponding real metal catalysts on oxides [2,206]. Thus, such supported metal nanoclusters represent the limiting case of supported metal species opposite to metal atoms and dimers considered in Section 5. At variance with these smallest metal species on oxides, the electronic properties of metal nanoclusters are supposed to be essentially independent of the support in most of... 

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