Supported metal catalysts characterization

Characterization of Metal Sites on Supported Metal Catalysts. Characterization of supported metals is usually more difficult. Considerable variation can frequently be found in the state of the reduced metal as a result of apparently minor differences in pretreatment, impurities in the support, or residual water or other contaminants. The problem is most severe with readily oxidizable metals. Ni (10), Mo (11), Re (12) and other metals can all show major variations depending on sample pretreatment and reduction procedures. Even in the case of platinum group metals many complications exist. The frequencies of bands observed when CO is adsorbed in a given manner (e.g. "linear" or "bridged") can shift by up to 100 cm 1 with coverage by CO or between different samples. 

The foregoing results characterizing structurally simple supported metal clusters can be generalized, at least qualitatively, to provide fundamental understanding that pertains to industrial supported metal catalysts, with their larger, nonuniform particles of metal. 

The use of EM (except in the special case of SEM) demands that the catalyst, whether mono-or multi-phasic, be thin enough to be electron transparent. But, as we show below, this seemingly severe condition by no means restricts its applicability to the study of metals, alloys, oxides, sulfides, halides, carbons, and a wide variety of other materials. Most catalyst powder preparations and supported metallic catalysts, provided that representative thin regions are selected for characterization, are found to be electron transparent and thus amenable to study by EM without the need for further sample preparation. 

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. 

In a few cases, adsorption on particulate nickel has been studied other than in the form of the conventional oxide-supported metal catalysts. Nash and De Sieno (73) exploded nickel wires in a rare-gas atmosphere to give Ni particles of ca. 20-nm diameter. Results were reported (but not illustrated) characterizing adsorption of ethyne they are similar to those found by Eischens and Pliskin. 

Zeolite-supported metal catalysts with atoms and clusters existing both inside and outside the zeolite channels are often examples of the situation depicted in Fig. Id. Again, XAFS has been an important tool for characterizing such catalysts (4-10). 

The performance of a catalyst is well known to be sensitive to its preparation procedure. For this reason, ideally an oxide-supported metal catalyst should be subjected to a number of characterization procedures. These may include measurements of the metal loading within the overall catalyst (usually expressed in wt%), the degree of metal dispersion (the proportion of metal atoms in the particle surfaces), the mean value and the distribution of metal particle diameters, and qualitative assessments of morphology including the particle shapes and evidence for crystallinity. These properties in turn can depend on experimental variables used in the preparation, such as the choice and amounts of originating metal salts, prereduction, calcination or oxygen treatments, and the temperature and duration of hydrogen reduction procedures. 

Characterization Data for Oxide-Supported Metal Catalysts Employed by Research Groups Studying the Adsorption of Hydrocarbons by Infrared (IR) or Raman (Ra) Spectroscopy... 

Concerning the first advantage, one can as easily study reactions on a single crystal metal surface as on a planar surface generated to duplicate the properties of a supported metal catalyst, as described above. Furthermore, with planar surfaces, LEED, Auger and photoelectron spectroscopies, along with many other analytical methods of surface science, can provide characterization of the surface composition and structure. 

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. 

A wealth of detailed evidence on the nature of supported metals can readily be obtained from infrared characterization studies, but correct interpretation of much of this evidence is still far from clear. The surface chemistry of supported metals is generally very complex, and assertions as to the origins of various band shifts and the exact nature of adsorption sites should be taken with some caution at present. Clearly, however, better understanding of the complex nature of supported metal catalysts should contribute greatly to the development of more efficient catalysts for many important industrial processes and to more efficient pretreatment and regeneration procedures. 

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