Catalyst materials, characterization

Table I lists the catalyst materials characterized and available for use as reference materials. For each property measured, the number of the ASTM Standard Test Method used in the determination is identified the material is specified the consensus mean value determined is listed the interlaboratory reproducibility and the intralaboratory repeatability from round robin tests are presented and the number of the ASTM research report describing the round robin data is listed. These round robins were conducted in accordance with ASTM E-691 -- Standard Practice for Conducting an Interlaboratorv Study to Determine the Precision of a Test Method.

In present study, thermo-chemical-induced crosslinking of PTFE has been demonstrated by using fluorinated-compound without any other catalyst materials. Characterization of crosslinked PTFE, which was obtained by radiation and chemical crosslinking, has been studied by various measurement techniques, such as thermal analysis (DSC, TGA), mechanical test, solid-state 19F-NMR spectroscopy, and so on. 

T. H. Tsai, J. W. Lane, and C. S. Lin Temperature-Programmed Reduction for Solid Materials Characterization, Alan Jones and Brian McNichol Catalytic Cracking Catalysts, Chemistry, and Kinetics,... 

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. 

The electron crystallography method (21) has been used to characterize three-dimensional structures of siliceous mesoporous catalyst materials, and the three-dimensional structural solutions of MCM-48 (mentioned above) and of SBA-1, -6, and -16. The method gives a unique structural solution through the Fourier sum of the three-dimensional structure factors, both amplitude and phases, obtained from Fourier analysis of a set of HRTEM images. The topological nature of the siliceous walls that define the pore structure of MCM-48 is shown in Fig. 28. 

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. 

In the application of XAS to the study of fuel cell catalysts, the limitations of the technique must also be acknowledged the greatest of which is that XAS provides a bulk average characterization of the sample, on a per-atom basis, and catalyst materials used in low temperature fuel cells are intrinsically nonuniform in nature, characterized by a distribution of particle sizes, compositions, and morphologies. In addition, the electrochemical reactions of interest in fuel cells take place at the surface of catalyst par-... 

The importance of materials characterization in fuel cell modeling cannot be overemphasized, as model predictions can be only as accurate as their material property input. In general, the material and transport properties for a fuel cell model can be organized in five groups (1) transport properties of electrolytes, (2) electrokinetic data for catalyst layers or electrodes, (3) properties of diffusion layers or substrates, (4) properties of bipolar plates, and (5) thermodynamic and transport properties of chemical reactants and products. 

The University of Patras works on design and synthesis of materials, characterization of materials, catalyst development and evaluation, advanced electrochemical reactors, SOFCs, electrodes, and the reforming of fuels. 

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. 

Thorsteinson et al.21 found a material consisting of mixed oxides of composition Mo0.6i V0.3iNb0.o8 to have optimum properties for the oxy-dehydrogenation of ethane. The better catalysts are characterized by a broad X-ray diffraction band near 4.0 A which may point to the presence of the compound VMo3Oii+x described by Andrushkevich.132 The presence of Nb stabilizes the catalyst against oxidation and reduction. 

X-ray powder diffraction has been the primary tool used in zeolite structure research. With new high-flux sources, the size requirement of useful single-crystals for structure determination studies has decreased significantly. In addition, refinements of atomic coordinates of known structures using Rietveld powder techniques have become common (24). The solution of a dozen or more new zeolite structure types within the last several years has added to our knowledge base for looking at unknowns (for examples, see references 25-31), and has made us better able to characterize catalyst materials and to correlate synthesis, sorptive, catalytic, and process parameters to their structures (32,33). 

The identification and characterization of new catalyst materials are important and often very complex tasks. When the catalysts are crystalline solids, diffraction techniques have clear advantages over most other characterization tools. Since, as discussed, every atom in a crystal contributes to every observed diffraction peak, XRD powder patterns are truly representative of the material being studied. The arrangement of atoms dictates the "d-spacings" and intensities observed in the XRD powder pattern. As that arrangement of atoms is characteristic of the material, then too is the XRD powder pattern. 

It has been shown how various factors can affect the appearance of XRD patterns and how the subtle differences in those patterns can be used to gain valuable structural and characterization information. It is clear that in order to understand a material and define it properly, all of these factors must be examined carefully. Techniques in addition to XRD, particularly electron microscopy, but also sorption and spectroscopy, should be utilized when attempting to understand the nature of a new catalyst material. Finally, it must be recognized that published XRD powder data for a given material can tell much about its structural nature but, to interpret the XRD data properly, it is also necessary to be aware of sample history, data collection parameters, morphology, etc. In short, one must know as much as possible about the various factors that affect the x-ray diffraction characteristics of catalyst materials. 

Through a series of round robin tests conducted by participating laboratories, ASTM Committee D-32 on Catalysts has characterized a variety of catalyst materials using standard test methods. Materials include fluid cracking catalysts, zeolites, silicas, aluminas, supported metals, and a gas oil feedstock. Properties characterized include surface area, crush strength, catalytic microactivity, particle size, unit cell dimensions and metal content. These materials are available from the National Institute of Standards and Technology as reference materials. 

Although several standard test methods have been developed for the chemical analysis of catalysts only small samples of supported platinum and palladium reference materials are available. Zeolites have been characterized for zeolite area, unit cell dimensions, and relative x-ray diffraction intensity. The crush strength of alumina pellets has also been determined. As the needs of catalyst users and producers change so will the materials characterized. To the extent that adequate amounts of material can be donated, standard test methods developed, and round robin tests performed Committee D-32 on catalysts will continue to make them available through NIST as reference materials. 

Many minerals or their synthetic equivalents are of industrial importance, and the possibilities of greater understanding of their behavior and properties through application of the methods described in this book is a major stimulus to research. Two examples are chosen for discussion here The first is the zeolites, a group of framework alumino-silicates (with interstitial Na+, Ca +, and HjO), characterized by very open frameworks with large interconnecting spaces or channels the second is the transition-metal sulfide catalysts, materials already generally discussed in Chapter 6, but considered here specifically in relation to their catalytic properties. 

Characterization of Catalytic Materials, edited by l.E. Wachs in the Materials Characterization Series (Buttcrworth-Hcincmann, Stoneham, MA 02180 1992) and Characterization of Solid Catalysts, volume 2, chapter 3 of the Handlnmlc of Heterogeneous Catalysis, edited by G. Ertl, H. Knozinger and J. Weitkamp (Wiley-VCH, Weinheim 1997). 

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