Characterization of the Solid Structure

The reader wishing to know more about these and other detectors for binary gas mixtures should consult texts on gas chromatography [13]. Obviously, a gas chromatograph can be used to analyze samples withdrawn from the downstream gas. Unfortunately, however, the time required to complete an analysis is of the order of one minute. For some fast reactions this may require numerous gas samples to be taken and stored for later analysis. 

We saw in the earlier chapters that the rate of reaction between a solid and a gas may depend quite strongly on the size of the solid particles (grains) that make up the solid agglomerate. It is therefore appropriate to make a brief mention of the various techniques for particle size measurement. Further information may be found in the monographs by Irani and Callis [14] and by Allen [59] or in a recent review article by Davies [15]. 

Probably the simplest technique for particle size measurement is sieving. Conventional woven-wire screens can be used to analyze powders, most of whose particles fall into the range of 50 jum to 1 cm. The range can be extended downward to 10 fim by using electroformed screens, which are more accurate than woven wire screens up to about 200 m. It is advisable to 

This last point is important in all particle size measuring techniques and it may be necessary to use a liquid sieving technique, together with dispersing agents, in difficult cases. 

Many automatic and semiautomatic devices are available for measuring particle size distributions at the microscope or from photomicrographs. Although these make the measurements much more rapid, their cost is not usually justified merely for investigations connected with gas-solid reactions. 

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A proper description of heterogeneously catalyzed oxidation reactions must treat several difficult problems simultaneously. First is the characterization of the solid surface in its reactive state. What oxygen species exist on this surface and what reactions does each species undergo What other sites for adsorption are present Second is the problem of reaction path. What steps are involved in the reaction What are the structures and relative energy contents of the intermediates Third is the problem of reaction velocity, a general and difficult problem in all chemistry. What are transition states, activation energies, and reaction probabilities for the various steps ... 

Now let us overview the theoretical adsorption models for characterization of the pore structures according to the pore size range. For physical adsorption of the gas molecules on such microporous solids as activated carbons and zeolites, Dubinin and Radushkevich95 developed an empirical equation, which describes the volume filling process in the micropores. Their theory incorporates earlier work by Polanyi96 in regard to the adsorption potential Ad defined as... 

French AD, Roughhead WA, Miller DP (1986) The structures of cellulose characterization of the solid state. Alalia RH (ed) ACS Symposium Series, No 340, pp 15-50... 

In this chapter, we consider basic concepts of crystallographic symmetry, which are essential to the understanding of how atoms and molecules are arranged in space and how they form crystalline solids. Furthermore, the detailed knowledge of crystallographic symmetry is important to appreciate both the capabilities and limitations of powder diffraction techniques when they are applied to the characterization of the crystal structure of solids. We begin with the well-established notions of the three-dimensional periodicity... 

Structural characterization of the solids by X-ray diffraction showed that the materials ZSM-5 and USY zeolites studied had a high crystallinity, while a mordenite structure was observed for the natural zeolite traces of clinoptilolite were observed in this sample. 

The final mixture was loaded into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 40 h without agitation. The solid product was filtered off, washed with distilled water, and dried at 105 °C. The product was uniform small crystals with a size of 6 pm. Powder XRD pattern characterization of the solid product showed a fully crystalline MFI structure. Elemental analysis gave an Si/Al ratio of 12-13.5, which is tunable by adjusting the ratio of source materials in the precursor gel. The absence of A1 species in the present synthetic system leads to the formation of the high-quality all-silica ZSM-5-type molecular sieve termed silicalite-I. Moreover, high-quality [B]-ZSM-5, [Fe]-ZSM-5, or [Ti]-ZSM-5-type molecular sieves could be obtained by the addition of B203, Fe2(S04)3, or Ti(OC2H5)4 to the synthetic system of silicalite-I and crystallization at 200, 180, or 175 °C for several days, respectively. 

Characterization of the pore structure of amorphous adsorbents and disordered porous catalysts remains an important chemical engineering research problem. Pore structure characterization requires both an effective experimental probe of the porous solid and an appropriate theoretical or numerical model to interpret the experimental measurement. Gas adsorption porosimetry [1] is the principal experimental technique used to probe the structure of the porous material, although various experimental alternatives have been proposed including immersion calorimetry [2-4], positron... 

Characterization of the chemical structure of highly cross-linked polymers, and of the chemical changes that accompany degradation processes, relies on spectroscopic methods. Solid-state nuclear magnetic resonance techniques have the potential to allow a more detailed characterization than before possible of the chemical environment and structure of chemical crosslinks in elastomers and thermoset epoxies. Degradation processes in cross-linked systems have been studied by using infrared spectroscopy, solid-state NMR, and electron spin resonance. 

D. L. A. VanderHart andR. H. Atalla, Further C NMR evidence for the co-existence of two crystallines forms in native celluloses, The Structures of Cellulose, Characterization of the Solid States, ACS Symposium Ser., 1987, pp. 88-118. 

In this chapter a survey is presented on standardised methods for the characterization of the surface structure of dispersed or porous solids and to determine its sorption properties. The survey includes also reference materials which can be used as a comparative sample and to adjust and control the measuring equipment. 

Standards for characterization of the surface structure of dispersed or porous solids... 

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