The Structure of Heterogeneous Catalysts

HgDre 9-1 The pores in a catalyst The lAuuM area is solid matmal The unshaded area is the pores, 4iidi contain a gas or liquid. The solid, faladc areas represent islands or clusters of an arrive oonqxxient attadied to die walls of die pores. 

In some cases, the ceramic or metallic material that comprises most of the catalyst actnally catalyzes the leactioiL For example, Y-alumina (Y-AI2O3) catalyzes the fommiiai of dimefliyl her fiom methanol. 

The reaction takes place on acidic sites on the walls of the pores in the alumina. The exact nature of these sites is relatively unimportant at this stage of discussion. However, the sites generally will be distributed more or less evenly along the pore walls. 

In other cases, the material that makes up most of the catalyst does not catalyze the reaction. For example, the hydrogenation of benzene to cyclohexane 

In this example, the metal (e.g., Pt or Ni) is referred to as the active component. The alumina is referred to as a support Its function is to provide the surface to which the active component is anchored, as well as the mechanical strength that is required to use the catalyst in a reactor. Desirably, the metal clusters should be very small in order to make effective use of an expensive metal such as Pt. It is common to find metal nanoparticles with dimensions as small as a few nanometers in heterogeneous catalysts. 

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The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

Electron probe and X-ray fluorescence methods of analysis are used for rather different but complementary purposes. The ability to provide an elemental spot analysis is the important characteristic of electron probe methods, which thus find use in analytical problems where the composition of the specimen changes over short distances. The examination of the distribution of heavy metals within the cellular structure of biological specimens, the distribution of metal crystallites on the surface of heterogeneous catalysts, or the differences in composition in the region of surface irregularities and faults in alloys, are all important examples of this application. Figure 8.45 illustrates the analysis of parts of a biological cell just 1 pm apart. Combination of electron probe analysis with electron microscopy enables visual examination to be used to identify the areas of interest prior to the analytical measurement. 

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. 

There is a continuing debate about the usefulness of XRD as a characterization method in catalysis, because heterogeneous catalysis is clearly related to the structures of solid catalyst surfaces. XRD provides surface structural information only for atomically flat surfaces at grazing incidence of radiation from high-brilliance sources. 

In this chapter we first discuss the salient features of polyethylene and polypropylene manufacture by heterogeneous catalytic processes. We also discuss the structural features of metallocene complexes that are used as homogeneous catalysts and the relationship between the structure of these catalysts and the structures of the resultant polymers. 

The results described in a previous paper [7] and this one indicate that the use of chelated metal precursors for the preparation of heterogeneous catalysts can suitably be extended to mesoporous support materials. The mechanisms underlying the fundamental processes occurring during catalyst preparation appear to be the same for both types of support materials. Therefore no limitations appear to exist to apply a wide variety of other elements into the pores of several types of mesoporous supports, with retention of the unique textural and structural properties of the support materials. Catalysts thus prepared will feature very high dispersions of active phase as well as very small particles with sizes even smaller tlmn on conventional support materials (due to the limiting size of the pores of mesoporous support materials). 

This chapter does not cover the most common aspects of the solid-state NMR techniques employed in the study of heterogeneous catalysts such techniques are described in Chapter 4. Since this chapter emphasizes the surface characterization of silica and alumina systems and silica aluminas by NMR methods, only those technical aspects highly relevant to surface characterization and not otherwise emphasized in this volume are explicitly discussed here. NMR studies of zeolites and clays are treated in separate chapters, and the bulk structures of silica and alumina systems are covered by Eckert. Unavoidably this chapter is also concerned with dynamics at the surface, although the amount of detailed work on that subject to date is limited. With the increasing availability of variable-temperature solid-state NMR equipment, however, one can expect that attention devoted to dynamics at surfaces will increase markedly during the next few years. 

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