Characterization of catalysts

The above-mentioned variables that characterize the performance of a catalyst depend not only on the external process parameters (T, p, Ci, etc.) but also in a complex manner on a series of other quantities  

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W-Rh IWCRht/oCOMCOljD/VCsHO ,) 193, Si02 Hydrogenation of CO and characterization of catalyst (FTIR. CO chemisorption. TPR. EPR) E)9... 

Volume 149 Fluid Catalytic Cracking VI Preparation and Characterization of Catalysts... 

Transmission electron microscopy is one of the techniques most often used for the characterization of catalysts. In general, detection of supported particles is possible, provided that there is sufficient contrast between particles and support - a limitation that may impede applications of TEM on well-dispersed supported oxides. The determination of particle sizes or of distributions therein is now a routine matter, although it rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. 

It is beyond the scope of this book to go though all the specifics of catalyst testing and to discuss all pitfalls that may arise. Instead we list the Ten Commandments for the Testing of Catalysts. This is a set of guidelines that have been provided by experts of a company called Catalytica [F.M. Dautzenberg in Characterization of Catalyst Development An Iterative Approach (Eds. S.A. Bradley, M.J. Gattuso, R.J. Ber-tolacini), ACS Symposium Series, Vol. 411 (1989)]. 

Christenn C, Steinhilber G, Schulze M, Friedrich KA (2007) Physical and electrochemical characterization of catalysts for oxygen reduction in fuel cells. J Appl Electrochem 37 1463-1474... 

STM has particularly great potential for in situ chemical studies. While our present knowledge of the atomic structure of catalyst surfaces is largely limited to those structures which are stable in ultra-high vacuum before and after reaction, STM may provide an insight into both adsorbate and catalyst surface structure in situ during the reaction. The following issues to be characterized by STM may be most relevant to characterization of catalysts and catalysis ... 

Perl, J. B. Characterization of catalyst surfaces by computerized infrared spectroscopy. Prepr., Div. Pet. Chem.,... 

In addition to the information enumerated above that is important in the characterization of catalysts, we also require as much knowledge as possible... 

Catalysis is a dynamic process, and deeper insights into its phenomenology are extractable from in situ measurements than from characterizations of catalysts before and after catalysis. A number of notable in situ experiments have relied on modifications of standard TEM operations under vacuum. The main functions of the EM depend on a high-vacuum environment, and the pressure in a TEM is usually of the order of 10-7-10-6 mbar. Because the influence of the reaction environment on the structure and activity of a catalyst is critical (3), the high-vacuum environment of a conventional EM is inappropriate for investigating a catalytic reaction, as are characterizations of catalysts in post-reaction environments (e.g., when the catalyst has been taken out of the reaction environment and cooled to room temperature). 

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

The fact that LEIS provides quantitative information on the outer layer composition of multi-component materials makes this technique an extremely powerful tool for the characterization of catalysts. Figure 4.19 shows the LEIS spectrum of an alumina-supported copper catalyst, taken with an incident beam of 3 keV 4He+ ions. Peaks due to Cu, A1 and O and a fluorine impurity are readily recognized. The high intensity between about 40 and 250 eV is due to secondary (sputtered) ions. The fact that this peak starts at about 40 eV indicates that the sample has charged positively. Of course, the energy scale needs to be corrected for this charge shift before kinematic factors Ef/E-, are determined. 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

Vibrational spectroscopy techniques are quite suitable for in situ characterization of catalysts. Especially infrared spectroscopy has been used extensively for characterization of the electrode/solution interphases, adsorbed species and their dependence on the electrode potential.33,34 Raman spectroscopy has been used to a lesser extent in characterizing non-precious metal ORR catalysts, most of the studies being related to characterization of the carbon structures.35 A review of the challenges and applications associated with in situ Raman Spectroscopy at metal electrodes has been provided by Pettinger.36... 

The characterization OF catalyst structures has undergone revolutionary developments in recent years. Powerful novel techniques and instrumentation are now used to analyze catalyst structure before, during, and after use. Many of these advances are responsible for placing the field of catalysis on an improved scientific basis. These developments have resulted in a better understanding of catalytic phenomena, and therefore improvements in commercial catalysts and the discovery of new systems. The application of advanced electronics and computer analysis has optimized many of these analytical tools. These developments are especially evident in spectroscopy, zeolite structure elucidation, and microscopy several other techniques have also been developed. Thus, the difficult goal of unraveling the relationships between the structure and reactivity of catalytic materials is finally within reach. 

The origins of analytical electron microscopy go back only about 15 years when the first x-ray spectra were obtained from submicron diameter areas of thin specimens in an electron microscope [1]. Characterization of catalyst materials using AEM is even more recent[2,3] but is currently a very active research area in several industrial and academic laboratories. The primary advantage of this technique for catalyst research is that it is the only technique that can yield chemical and structural information from individual submicron catalyst particles. 

Raman spectroscopy offers an alternative for the vibrational characterization of catalysts, and has been used for the study of the structure of many solids, in particular of oxides such as Mo03, V205,... 

The analytical methods for the characterization of catalysts are described extensively in other chapters of this book. Here, only a brief overview on methods of predominant importance for the investigation of micro- and mesoporous materials will be given... 

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