Characterization catalysts

Characterization is an important field in catalysis. Spectroscopy, microscopy, diffraction and methods based on adsorption and desorption or bulk reactions (reduction, oxidation) all offer tools to investigate the nature of an active catalyst. With such knowledge we hope to understand catalysts better, so that we can improve them or even design new catalysts. 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

Therefore, in many fundamentally oriented studies the complex catalyst is replaced by a simplified model, which is better defined. Such models range from supported particles from which all promoters have been removed, via well-defined particles deposited on planar substrates, to single crystals (Fig. 4.1). With the latter we are in the domain of surface science, where a wealth of informative techniques is available that do not work on technical catalysts. 

In industry, the emphasis is mainly on developing an active, selective, stable and mechanically robust catalyst. To accomplish this, tools are needed which identify those structural properties that discriminate efficient from less efficient catalysts. All information that helps to achieve this is welcome. Empirical relationships between those factors that govern catalyst composition (e.g. particle size and shape, and pore dimensions) and those that determine catalytic performance are extremely useful in catalyst development, although they do not always give fundamental insights into how the catalyst operates on the molecular level. 

Concepts of Modem Catalysis and Kinetics. I. ChorkendorfF, J. W. Niemantsverdriet Copyright 2003 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30574-2 

Characterization of a catalyst requires a detailed knowledge of the structure and composition of the various components present, and this in turn, requires the use of a combination of techniques. The examples in this section have been chosen to illustrate the ways in which radiotracers can be used to provide information which would be difficult to obtain by other experimental techniques. Applications are restricted to the characterization of working catalysts. Examples of the use of radioactivity in other aspects of surface science can be found in an earlier review.  

Theoretical models based on first principles, such as Langmuir s adsorption model, help us understand what is happening at the catalyst surface. However, there is (still) no substitute for empirical evidence, and most of the papers published on heterogeneous catalysis include a characterization of surfaces and surface-bound species. Chemists are faced with a plethora of characterization methods, from micrometer-scale particle size measurement, all the way to angstrom-scale atomic force microscopy [77]. Some methods require UHV conditions and room temperature, while others work at 200 bar and 750 °C. Some methods use real industrial catalysts, while others require very clean single-crystal model catalysts. In this book, I will focus on four main areas classic surface characterization methods, temperature-programmed techniques, spectroscopy and microscopy, and analysis of macroscopic properties. For more details on the specific methods see the references in each section, as well as the books by Niemantsverdriet [78] and Thomas [79]. 

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Deviney M L and Gland J L (eds) 1985 Catalyst Characterization Science Surface and Solid State Chemistry (Washington, DC American Chemical Society)... 

R. J. Farrauto and M. C. Hodson, "Catalyst Characterization," Enyclopedia of Physical Science andTechnology, Vol. 2, Academic Press, Inc., New York, 1987. 

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

Rc-Pt [Re2Pl(CO)i2] 197 K-AI2O3 Catalyst characterization (IR. XPS. TPR, chemisorption) Catalyst characterization (EXAFS. chemisorption) and methylcyclohe.xane dehydrogenation 203 204... 

Figure 4.2. Catalyst characterization techniques The circle represents the sample under study, the inward arrows denote excitation processes, and the outward arrows indicate how the information should be extracted.

X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline phases inside catalysts by means of lattice structural parameters, and to obtain an indication of particle size. 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 4.5 gives an example where XRD readily reveals the Pd reflections in two Pd/Si02 catalysts. 

The strength of XRD for catalyst characterization is that it gives dear and unequivocal structure information on particles that are sufficiently large, along with an estimate of their size, and it can reveal this information under reaction conditions. The limitation of XRD is that it can not detect partides that are either too small or amor-... 

In this chapter we have limited ourselves to the most common techniques in catalyst characterization. Of course, there are several other methods available, such as nuclear magnetic resonance (NMR), which is very useful in the study of zeolites, electron spin resonance (ESR) and Raman spectroscopy, which may be of interest for certain oxide catalysts. Also, all of the more generic tools from analytical chemistry, such as elemental analysis, UV-vis spectroscopy, atomic absorption, calorimetry, thermogravimetry, etc. are often used on a routine basis. 

We have already mentioned that fundamental studies in catalysis often require the use of single crystals or other model systems. As catalyst characterization in academic research aims to determine the surface composition on the molecular level under the conditions where the catalyst does its work, one can in principle adopt two approaches. The first is to model the catalytic surface, for example with that of a single crystal. By using the appropriate combination of surface science tools, the desired characterization on the atomic scale is certainly possible in favorable cases. However, although one may be able to study the catalytic properties of such samples under realistic conditions (pressures of 1 atm or higher), most of the characterization is necessarily carried out in ultrahigh vacuum, and not under reaction conditions. 

What are the aims of catalyst characterization in the context of (a) industrial catalysis and (b) fundamental research ... 

Caris-Veyrat, C. et al., Cleavage products of lycopene produced by in vitro oxidations characterization and mechanisms of formation, J. Agric. Food Chem., 51, 7318, 2003. Caris-Veyrat, C. et al., Mild oxidative cleavage of beta, beta-carotene by dioxygen induced by a ruthenium porphyrin catalyst characterization of products and of some possible intermediates, New J. Chem., 25, 203, 2001. 

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