Solid-state characterization techniques heterogenization

Solid-state characterization techniques have been used to verify that the heterogenization procedure did not damage the catalyst. Fourier transform-infrared (FT-IR) spectra give useful indication about the structure of the catalyst heterogenized in the membrane. FT-IR spectra confirmed that the catalyst stmetore is preserved within the polymeric membrane (Fig. 27.1). The infrared spectrum of the catalytic membranes show the characteristic bands of Wio units (595, 803, 895, 958 cm ), as well as those typical of the employed alkylam-monium cation (2870 cm ). 

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. 

In this article the viability of solid-state NMR technique to characterize heterogeneous catalysts containing paramagnetic cations located on the surface is discussed. 

MRI to characterize hydrodynamics within reactors is already established. The extent to which the potential of MR to study both hydrodynamics and chemical conversion is fully realized will depend on our ability to integrate the well-established MR spectroscopy techniques in liquid- and solid-state NMR into imaging pulse sequences, and still provide quantitative data in the magnetically heterogeneous environments typical of catalysts and reactors. 

TG/DTA, TPR, and complementary techniques for characterizing catalysts in the working state (e.g., XRD Raman, IR, and UV-vis spectroscopies) can provide structural and metal valence information under reaction conditions. However, the capability of TR-XAFS spectroscopy to reveal quantitative phase composition and average metal valence together with the evolution of the local structure of a catalyst under varying (reaction) conditions, combined with a time resolution of 100 ms will continue to be a very powerful tool for kinetics investigations in solid-state chemistry and heterogeneous catalysis. 

Furthermore, with the advent of improved instriunentation and experimental techniques interesting in-situ investigations became possible which were related, for instance, to the synthesis of and heterogeneous catalysis on zeofites, catalyst deactivation, diffusion or solid-state ion exchange as well as other postsynthesis modifications. Combinations of IR spectroscopy with various characterization techniques such as, e.g., temperature-programmed desorption of probe molecules (TPD/IR, cf.[223,224]), electron spin resonance spectroscopy (ESR/IR, cf.[225,226]), UV-Vis spectroscopy [227,228], etc. were developed. 

This technique allows microscopic areas to be examined and, in the case of polymers, it has been used to study multicomponent systems, and in particular to characterize heterogeneities in solid-state polymer blends. The most popular types of analysis involve transmittance and reflectance techniques, whereby ETIR microscopes employ reflecting mirrors rather than the lenses which are used in microscopes operating with visible light (see Figure 2.27). 

Most of the adsorbents used in the adsorption process are also useful to catalysis, because they can act as solid catalysts or their supports. The basic function of catalyst supports, usually porous adsorbents, is to keep the catalytically active phase in a highly dispersed state. It is obvious that the methods of preparation and characterization of adsorbents and catalysts are very similar or identical. The physical structure of catalysts is investigated by means of both adsorption methods and various instrumental techniques derived for estimating their porosity and surface area. Factors such as surface area, distribution of pore volumes, pore sizes, stability, and mechanical properties of materials used are also very important in both processes—adsorption and catalysis. Activated carbons, silica, and alumina species as well as natural amorphous aluminosilicates and zeolites are widely used as either catalyst supports or heterogeneous catalysts. From the above, the following conclusions can be easily drawn (Dabrowski, 2001) ... 

The analysis of the data of PS I gave quite accurate information on the distance of the spin centres (25.4 0.3 A)301 that compared well with the crystal structure data.68 A problem is the extended it-spin density distribution in the donor and acceptor. For a solid comparison a centre of gravity for the spin must be calculated from experimental or theoretical spin density distributions of the two radicals. Similar data with almost unaltered distances were obtained for PS I with other quinones substituted into the Ai site.147-302This work has been extended to other electron acceptors,303 which show a larger heterogeneity in distances. It has been shown that the lifetime of the RP can also be measured and can even be controlled in the experiments by an additional mw pulse prior to the 2-pulse echo sequence.302 This pulse transfers population to triplet levels which cannot directly recombine to the singlet ground state. This has earlier been shown for the bRC.304,305 The OOP-ESEEM technique has also been applied to various mutants of PS I to characterize them by the measured distances between fixed donor and variable acceptors.254 256-263-264... 

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

The study of distribution function characterization of a solid surface is still in its infancy for lack of heterogeneous isotherms, but, in principle, theories and techniques are available to foster the development of this important bridge between the infinite and finite state in solid surface chemistry. 

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