Applications to Catalysts

There is a growing interest in modeling transition metals because of its applicability to catalysts, bioinorganics, materials science, and traditional inorganic chemistry. Unfortunately, transition metals tend to be extremely difficult to model. This is so because of a number of effects that are important to correctly describing these compounds. The problem is compounded by the fact that the majority of computational methods have been created, tested, and optimized for organic molecules. Some of the techniques that work well for organics perform poorly for more technically difficult transition metal systems. 

Atomic force microscopy (AFM) or, as it is also called, scanning force microscopy (SFM) is the most generally applicable member of the scanning probe family. It is based on the minute but detectable forces - order of magnitude nano-Newtons -between a sharp tip and atoms in the surface [39]. The tip is mounted on a flexible arm called a cantilever, and is positioned at a subnanometer distance from the surface. If the sample is scanned under the tip in the x-y plane, it feels the attractive or repulsive force from the surface atoms and hence is deflected in the z direction. Various methods exist to measure the deflection, as described by Sarid [40]. Before we describe equipment and applications to catalysts, we will briefly look at the theory behind AFM. 

In this chapter, we introduce some of the most common spectroscopies and methods available for the characterization of heterogeneous catalysts [3-13], These techniques can be broadly grouped according to the nature of the probes employed for excitation, including photons, electrons, ions, and neutrons, or, alternatively, according to the type of information they provide. Here we have chosen to group the main catalyst characterization techniques by using a combination of both criteria into structural, thermal, optical, and surface-sensitive techniques. We also focus on the characterization of real catalysts, and toward the end make brief reference to studies with model systems. Only the basics of each technique and a few examples of applications to catalyst characterization are provided, but more specialized references are included for those interested in a more in-depth discussion. 

Toshima, N., Takahashi, T., and Hirai, H., Polymerized micelle-protected platinum clusters-preparation and application to catalyst for visible light-induced hydrogen generation, J. Macromol. Sci. -Chem., A25, 669,1988. 

Holena, M., Baerns, M., Feedforward neural networks in catalysis, a tool for the approximation of the dependency of yield on catalyst composition, and for knowledge extraction, Catal. Today 2003, 81, 485-494. Serra, J. M., Corma, A., Argente, E., Valero, S., Botti, V., Neuronal networks for modeling of kinetic reaction data applicable to catalyst scale up and process control and optimization in the frame of combinatorial catalysis, Appl. Catal. A 2003, 254, 133-145. 

In this method metal chlorides or oxychlorides are made to react with gaseous hydrocarbons in the vicinity of a localized heat source (1400-2100 K). Clearly, the reaction is thermodynamically favorable (Tables 3 and 4). The method was first used by Van Arkel in 1923 with an incandescent tungsten filament to make carbides of tantalum and zirconium [40]. Although the reaction variables have been studied extensively, problems remain with control of the process and with low productivity. Application to catalyst synthesis has been moderate [41],... 

More severe tests are applicable to catalysts used in moving bed reactors. Some testsuse a high velocity stream of gas to cause attrition of the catalyst. In some tests, a small-scale plant which simulates the commercial plant is used. The amount of attrition measured in all these tests is very dependent on the exact procedure used. Reproducibility among laboratories is only likely to be possible if precise details of the method are given. 

Mechanism of Rapid Zeolite Crystallizations and Its Applications to Catalyst Synthesis... 

This review does not deal with the general characterization of catalysts by Raman spectroscopy but is focused instead on the application to catalysts in reactive environments. Such experiments are often described as in situ," a term that is minimally used in this volume. The term "in situ, Latin for "on site," implies that the sample is analyzed at the location where it has been treated or is being treated. Several levels of such experiments are described here ... 

The same slurry-coating method is applicable to catalyst particles, and the same reasoning holds for an optimal particle size. Different is the impact of fhe use of binder material. 

The theoretical basis of Mossbauer spectroscopy as well as its applications to catalyst characterization was reviewed in Advances in Catalysis in 1989 (2). This thorough article summarizes the physical basis of the technique and significant contributions to the characterization of solid catalysts. Since 1989, Mossbauer spectroscopy has not underwent major developments, and its applications to catalysis have been largely limited to catalysts that were not in reactive atmospheres, notwithstanding the impressive advances that have been made with other techniques in characterizing catalysts under working conditions. 

J. Fraissard, T. Ito and L.C. De Menorval, Nuclear Magnetic Resonance study of Xenon adsorbed on zeolites and metal-zeolites. Application to catalyst research, in Proceedings 8th International Conference on Catalysis, Berlin, 1984, pp. 25-35. 

In its application to catalysts, the preceding paragraph suggests that pseudo-acids or bases will be ineffective catalysts compared with other acids or bases of the same strength i.e., they should exhibit negative deviations from the Bronsted relation. There are a number of isolated observations which confirm this idea. Thus Bronsted and Pedersen (15) found that the nitrourethane ion had an unexpectedly small catalytic... 

In view of the general importance that seems likely to be attached to this new tool for measuring catalyst surface areas, it seems worthwhile to restate occasionally its details and to discuss critically new developments in its theory and application in catalytic work. Accordingly, in the present chapter there is first presented a review of the gas adsorption method as originally developed and published, and then a critical discussion of recent papers dealing with its new modifications, derivations, and applications to catalysts and catalytic materials. 

As stated in the beginning of this section, there have been quite a number of versions of the gas-solid reaction analysis. Further detail has been provided by Bischoff, Luss, and Yoshida, et al. [K.B. Bischoff, Chem. Eng. Sci., 46, 7111 (1963) D. Luss, Can. J. Chem. Eng., 46, 154 (1968) R. Yoshida, D. Kunii and F.J. Shimizu, Chem. Eng. Japan, 8, 417 (1976)]. A more detailed model, considering the granular nature of most porous solids, has been given by Szekely and coworkers [H.Y Sohn and J. Szekely, Chem. Eng. Sci., 25, 1091 (1970) J. Szekely and J.W. Evans, Chem. Eng. Sci., 27, 763 (1972) 29, 630 (1974) J. Szekely, J.W. Evans, and H.Y. Sohn, Gas-Solid Reactions, Academic Press, New York, NY, (1976)]. Some more studies of a similar nature, but with application to catalyst regeneration, will be given in the next section. 

New catalysts using precious metals have been developed by altering the ligand, but that approach is less applicable to catalysts based on non-traditional metals. 

Applications to catalyst deactivation and activity are given by Rudd [131], and use of these concepts for reactor-regenerator systems (e.g., in catalytic cracking) is analyzed by Petersen [132], Gwyn and Colthart [133], and Jacob [134] an outline of the developments is given in Chapter 13. 

Percolation theory represents the most advanced and most widely used statistical framework to describe structural correlations and effective transport properties of random heterogeneous media (Sahimi, 2003 Torquato, 2002). Here, briefly described are the basic concepts of this theory (Sahimi, 2003 Stauffer and Aharony, 1994) and its application to catalyst layers in PEFCs. 

Another promising spectroscopic method for the infrared analysis of samples which do not lend themselves readily to normal preparation techniques is that of photoacoustic detection 4,12,17,18) Examples of the application to catalyst, poiymer, chromatographic sections or spots, and coals, tar sands and shale oils, have been discussed (F5-7,9,10,13,16) most novel applications of infrared photo-... 

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