Characterization of Supported Metal Catalysts

5 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. -  

Traditionally, IR spectroscopy of adsorbed CO serves as a tool to gain knowledge about the electronic state and dispersion of supported metals. ° The spectra of adsorbed CO are known to be the result of the interplay of the 

There are indications in the literature suggesting the formation of electron deficient metal particles in e.g. AbOs-based and halogenated solid catalysts. However, the mechanism of this process and the nature of anchoring sites are not quite clear. Broensted acid sites, as well as strong Lewis acid sites may be considered as surface centers stabilizing small metal particles (Pt, Pd, Ir, Ni) and causing their positive charging.  

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Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are recognized as powerful and versatile tools for the characterization of supported metal catalysts, because real-space images of catalysts with spatial resolution down to 0.1 nm can be recorded and combined with high-spatially resolved spectroscopic information. However, TEM has been used mainly for ex situ characterization, for example, of catalysts after gas treatments. 

Characterization of Supported Metal Catalysts by X-ray Photoelectron Spectroscopy... 

In spite of this drawback, there is still much to be gained from XPS characterization of supported metal catalysts. Among these are the interconversion of metal salts into oxide and metal during catalyst pretreatment, the identification of poisons, and the distribution of metal within a zeolite or a porous pellet. 

Table 1 Characterization of supported metal catalysts Support properties...

Three aspects of the performance of supported catalysts are also discussed in this Chapter. With the development of techniques, as outlined above, for the characterization of supported metal catalysts, it seems timely to survey studies of crystallite size effect/structure sensitivity with special reference to the possible intrusion of adventitious factors (Section 5). Recently there has been considerable interest in the existence of (chemical) metal-support interactions and their significance for chemisorption and catalytic activity/ selectivity (Section 6). Finally, supported bimetallic catalysts are discussed for various reactions not involving hydrocarbons (hydrocarbon reactions over alloys and bimetallic catalysts have already been reviewed in this Series with respect to both basic research and technical applications ). References to earlier reviews (including some on techniques) that complement material in this Chapter are given in the appropriate sections. It might be useful, however, to note here some topics not discussed that also form part of the vast subject of supported metal and bimetallic catalysts and for which recent reviews are available, viz, spillover, catalyst deactivation, the growth and... 

The characterization of supported metal catalysts is a matter of some complexity and supported bimetallic catalysts even more so. Nevertheless the development and application of methods for determining catalyst structure is essential for an understanding of why the performance of a selected combination of metal(s) and support varies as a function of preparative variable, activation procedure, reaction conditions, or time. Although some aspects of catalyst structure can be routinely determined, the basic measurement of absolute metal dispersion by selective chemisorption/gas titration is still the subject of many publications and the necessity of cross-checking by instrumental methods is generally appreciated. The characterization of supported metal catalysts also involves some less accessible properties, e.g., the sites available on crystallites as a function of size, high-temperature... 

K.E. Birkeland, W.D. Harding, L. Owend, and H.H. Kung, in S.A. Bradley and J. Stencel (Editors) Chemistry and Characterization of Supported Metal Catalysts, Division of Petroleum Chemistry, Inc., American Chemical Society. 38, (1993), 880... 

IR bands of chemisorbed NO for the characterization of supported metal catalysts, e.g., M0/AI2O3, Pt-Re/Al203, cracking catalysts... 

It should now be evident that the modern AEM offers an incomparable level of versatility for the characterization of supported metal catalysts. The sheer variety of imaging and spectroscopic signals, all of which can be spatially resolved at, or near, the atomic-level, affords the catalyst chemist the opportunity to locally probe a host of catalytically relevant features within a catalyst sample. Recent... 

CHARACTERIZATION OF SUPPORTED METAL CATALYSTS BY SPECTROSCOPIC TECHNIQUES... 

Characterization of Supported, Metal Catalysts by Spectroscopic Techniques 143... 

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