Mono- and Bimetallic Supported Catalysts

3 Mono- and Bimetallic Supported Catalysts. - The key factor in designing supported metal catalysts is the knowledge about the reaction mechanisms and information about the role of different types of active sites in a given step of the catalytic reaction. The performance of supported mono-functional monometallic catalysts is governed by the metal particle size, metal dispersion, overall morphology of the metal nanocluster, the character of metal-support interaction, and the electronic properties of the metal. In bifunctional supported metal catalysts in addition to the above listed factors the metal/acid balance, and the type and strength of the acid function play a key role in the overall performance. 

In case of bimetallic catalysts, other properties, such as surface composition and the potential stabilization of one of the metal components in ionic form, are the most crucial determining the performance of the catalyst. It is noteworthy that combination of modern methods enables the chemist to characterize both active sites of supported metals and the reaction intermediates formed. Additionally, quantum chemical calculations become more and more powerful tools in understanding chemical interaction controlling and governing both the catalyst structure and the catalytic performance.  

In the last decade much attention has been paid to metal nano-clusters including supported nanoparticles as one of the promising advanced nanoscopic materials. Elements easily forming supported metal nanoclusters are Group VIII and IB transition metals as follows Pt, Ir, Pd, Rh, Ru, Ni, Co, and Au, Ag, Cu. It is interesting to note that the heat of formation of the oxides of these metals is low (usually below -AHf = 40 kcal/mol at 25 °C referred to one oxygen atom ). 

Based on a careful literature survey and recent results published by the authors of this review, it can be assumed that in a number of heterogeneous catalytic reactions metal ion - metal nanocluster ensemble sites are operative. In these catalysts the metal ions have to be located in atomic closeness to the metal nanocluster. 

As far as the action of supported bimetallic catalysts is concerned, the main theories suggest either geometric and/or electronic effects to account for the improved catalytic properties. For instance, in platinum based naphtha reforming catalysts, the electronic modification of platinum particles may be induced by an interaction with an oxide layer of the promoter or by alloy formation. The electronic modification results in a change in the Pt-C bond strength of adsorp- 

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For the sake of clarity the most important consequences of spillover are summarized and discussed in the following four subsections. First, the enhanced adsorption by mono- and bimetallic supported catalysts is described. This deals mainly with abnormal hydrogen adsorption quantities... 

Characterization of Supported Metal Catalysts. - Chemisorption of different probe molecules and Temperature Programmed Reduction (TPR) studies are frequently used to study the metal dispersion, surface composition and oxidation state of metals in mono- and bimetallic supported catalysts. Combined use of CO, hydrogen and oxygen chemisorption as well as oxygen-hydrogen titration can provide information about the dispersion and surface composition of metal nanoclusters. TPR studies of bimetallic catalysts can give information about the type, the reducibility, and the oxidation state of metal components. In addition, the position of TPR peaks can be used to characterize the type of interactions of the metal species in the catalysts. - ... 

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