Catalyst characterization tools

Catalysis is still very much a black box discipline, and catalyst characterization tools help us look inside this box. Characterization is done on several levels On the first, the... 

Development of fundamental kinetics for improved understanding of complex reaction systems is another frontier. More advanced catalyst characterization tools, including on-line and in-line measurements, need to be developed to provide better understanding of critical catalyst parameters. This should involve application of predictive chemistry capability to design better catalysts which carry out desired conversions in complex reaction systems. 

Several spectroscopic, microscopic and diffraction techniques are used to investigate catalysts. As Fig. 4.2 illustrates, such techniques are based on some type of excitation (in-going arrows in Fig. 4.2) to which the catalyst responds (symbolized by the outgoing arrows). For example, irradiating a catalyst with X-ray photons generates photoelectrons, which are employed in X-ray photoelectron spectroscopy (XPS) -one of the most useful characterization tools. One can also heat a spent catalyst and look at what temperatures reaction intermediates and products desorb from the surface (temperature-programmed desorption, TPD). 

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

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. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The industrial view on catalyst characterization is different. Here the emphasis is mainly on developing an active, selective, stable and mechanically robust catalyst. In order to accomplish this, tools are needed which identify those structural properties that discriminate between efficient and less efficient catalysts. All spectroscopic information that helps to achieve this is welcome. Establishing empirical relationships between the factors that govern catalyst composition, particle size and shape and pore dimensions on the one hand and catalytic performance on the other are extremely useful in the process of catalyst development, although such relationships may not give much fundamental insight into how the catalyst operates in molecular detail. 

Catalyst characterization is a lively and highly relevant discipline in catalysis. A literature survey identified over 4000 scientific publications on catalyst characterization in a period of two years [14]. The desire to work with defined materials is undoubtedly present. No less than 78% of the 143 papers presented orally at the 1 llh International Congress on Catalysis [15] contained at least some results on the catalyst(s) obtained by characterization techniques, whereas about 20% of the papers dealt with catalytic reactions over uncharacterized catalysts. Another remarkable fact from these statistics is that about 10% of the papers contained results of theoretical calculations. The trend is clearly to approach catalysis from many different viewpoints with a combination of sophisticated experimental and theoretical tools. 

One of the most studied aspects of catalysis science is the relationship between structure and function. Some general themes are weU estabUshed by now, but specific connections between catalyst characteristics and performance attributes remain elusive in most cases. The crystalline geometry of zeolites makes them relatively more amenable to study by a variety of powerful modern characterization tools, but there remain many key unanswered questions in the catalytic application... 

The input parameters can be taken from measurements on model systems. If the structure of the catalyst is known and one has a suspicion which is the active crystal surface and do experiments on this model with all the chemical phase, then one can isolate this phase, usually in the form of a single and structural characterization tools available in surface science. 

The characterization tools to investigate cobalt-based Fischer-Tropsch catalysts are mostly used to study the catalyst materials under conditions far from industrially relevant reaction conditions i.e., in the presence of CO and H2, as well as of the reaction products, including H2O at reaction temperatures and at high pressures. Since catalytic solids are dynamic materials undergoing major changes under reaction conditions it can be anticipated that the currently obtained information on the active site is at least incomplete. This holds also for the active state and location of the promoter element under reaction conditions. For example, an electronic elfect on the cobalt active phase induced by a promoter element can maybe exist only at high pressures and will remain -due to the lack of the appropriate instrumentation - unnoticed to the catalyst... 

Summarizing, there are still many scientific challenges and major opportunities for the catalysis community in the field of cobalt-based Fischer-Tropsch synthesis to design improved or totally new catalyst systems. However, such improvements require a profound knowledge of the promoted catalyst material. In this respect, detailed physicochemical insights in the cobalt-support, cobalt-promoter and support-support interfacial chemistry are of paramount importance. Advanced synthesis methods and characterization tools giving structural and electronic information of both the cobalt and the support element under reaction conditions should be developed to achieve this goal. 

HT technology for catalysts-automated synthesis and testing appears to be reasonably adapted to date, but further improvements are expected for HT catalysts characterization, which is still restricted to costly and in general ex-situ spectroscopic techniques. These tools would provide the new catalyst descriptors needed to improve the ability to predict catalytic performances without testing. 

The objective of the work presented here is to combine activity studies using a model reaction with STM and AFM studies on model catalysts and to determine structural correlations between catalytic activity and morphology. Other characterization tools are also used to determine compositional effects induced by pretreatment or the reaction. The model reaction used is the hydrogenation of 1,3 butadiene hydrogenation due to its high reactivity on low-surface area Pd model catalysts and its well-studied mechanism [17-19]. X ray photoelectron spectroscopy (XPS) was used to determine surface composition. 

A Data Procurement for Knowledge-based Systems Progress in analytical characterization of catalysts plays an important role in their further development and improvement. Synergistic effects of complimentary characterization tools by which different properties of the catalytic materials arc determined are claimed to be beneficial in catalyst design. If this is so, then an expert system for assisting in catalyst selection should be designed in such a way that it accounts for different chemical and physico-chemical properties and their relation to catalytic performance of solid materials. 

Catalyst characterization, if properly utilized, can be both the cornerstone of the science of catalysis and the "catalyst in its own right for industrial progress". As a long term research endeavor, its goal should be an understanding of the catalytic act on a molecular level, a clear elucidation of the nature of the individual catalyst sites and of their interdependence and interaction with each other as well as with the reactants. It must also provide a stimulus for the development of instrumental capabilities and instrumental and chemical techniques which are the tools for this research effort. A basic knowledge of catalysis and the ability to conceptualize a working hypothesis is a most desirable prerequisite for such studies. 

All of the above comments have a commonality - a high appreciation for the new techniques but, at the same time, they emphasize that adsorption - desorption techniques remain indespensable tools for catalyst characterization. 

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