Pore structures

A catalyst is porous and contains pores. The pores are of course of a very irregular shape. 

When the pore volume is measured, one usually find pores of two distinct sizes  

In the study of reaction mechanisms, industrial catalysts are complicated to work with. For this reason, mechanistic studies are often performed on model systems, such as single crystals. For this purpose, single crystals are available of most metals in sizes of e g 10 by 10 mm 

Experience has shown that it is often much easier to study a number of single crystal surfaces, deduce the reaction mechanism and then verify this for the real catalyst, than to attempt to deduce the reaction mechanism directly from studies of the catalyst. 

Attaining an economic space-time yield (Equation 2.1-3) requires a sufficient number of active centers.This is achieved by having a large internal surface area in the form of pores. Table 2.1-6 lists the standard classification of pores according to their diameter. Methods for determining the pore size distribution and the mean pore diameter are mercury porosimetry and the BET method (see Section 2.1.3.3) [Kast 1988, Wijngaar-den 1998]. 

The thermal decomposition of coal (Chapter 13) emphasizes the need for access of reagents such as hydrogen to regions of the coal undergoing conversion. Thus, a key consideration in coal conversion is the mass transfer of reagents and products which occurs by means of coal s extensive pore network. 

A wide variety of pore types exist in coals of different ranks—ranging from large macropores (2300 A) to micropores (12 A) (Gan et al., 1972). The total volume of such pores, the pore size distribution, and the effective surface area of the organic material are needed to anticipate coal s potential reactivity. 

Understanding the pore chemistry and physics of complicated materials such as coal can make a significant long-term contribution to advanced coal conversion technology. 

In conclusions, there is much to be gained by the analysis of coal and understanding the influence of coal composition on use (Chapters 8 and 9) but, as noted elsewise (Chapter 3), giving consideration to (1) the different chanical precursors of coal, (2) the varied geological environment, (3) the maturation conditions, and (4) the variation of coal analysis within a seam results in the rationale for an average structure of coal being of little value. 

Analytical Methods for Coal and Coal Products, C. Karr, Jr. (Ed.). Academic Press, Inc., New York, Vol. m. Chapter 56. 

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Instead of considering the pore structure in detail, it would alternatively be possible to characterize the medium by a single "effective... 

The adsorption of a gas by a solid can, in principle, be made to yield valuable information as to the surface area and pore structure of the solid. In practice the range of suitable adsorptives is quite narrow, by far the most commonly used one being nitrogen at its boiling point, 77 K. 

These models, though necessarily idealized, are sufficiently close to the actual systems found in practice to enable useful conclusions to be drawn from a given Type IV isotherm as to the pore structure of a solid adsorbent. To facilitate the discussion, it is convenient to simplify the Kelvin equation by putting yVJRT = K, and on occasion to use the exponential form ... 

Everett concludes that in systems where pore blocking can occur, pore size distribution curves derived from the desorption branch of the isotherm are likely to give a misleading picture of the pore structure in particular the size distribution will appear to be much narrower than it actually is. Thus the adsorption branch is to be preferred unless network effects are known to be absent. 

The pressures involved in porosimetry are so high (e.g. 1000 atm = 6-6 ton in" ) that the question as to whether the pore structure is damaged by mercury intrusion naturally arises. This possibility was recognized by Drake, but as a result of several intrusion-extrusion runs at pressures up to 4000 atm on a number of porous catalysts Drake concluded that any deformation caused by compression was elastic and therefore not permanent. 

Similarly, Johula and Wiig in three successive experiments on the same sample of charcoal which was soft and susceptible to crushing found that the three penetration curves agreed closely, indicating that the pore structure had suffered no permanent damage. 

Hysteresis, which is invariably present, adds to the complications its interpretation is, if anything more complex than with capillary condensation, inasmuch as it can depend not only on the pore structure of the solid but also on the magnitude of the applied pressure. 

Despite these various limitations, mercury pwrosimetry constitutes an indispensable tool for the quantitative study of pore structure, but it needs to be supplemented by other techniques, if a reliable picture of the pore system is to be built up. 

K.. Unger and H. Fischer, in Proceedings of RILEM/IUPAC International Symposium on Pore Structure and Properties of Materiails (eds. S. Modry and... 

Parameter k of Equation (4.10) is an expression of the breadth of the Gaussian distribution of the cumulative micropore volume IF over the normalized work of adsorption sfifi, and is therefore determined by the pore structure. Thus B also (cf. Equation (4.13)) is characteristic of the pore structure of the adsorbent, and has accordingly been termed the structural constant of the adsorbent. ... 

A discussion of the adsorption of water on oxides would be incomplete without some reference to the irreversible effects which are often encountered when samples of oxide, hydroxide or oxide-hydroxide are exposed to the vapour. These effects ( low-temperature ageing ), which manifest themselves in changes in surface area, in pore structure and sometimes in the lattice structure itself, are complex and difficult to reproduce exactly. ... 

As pointed out earlier (Section 3.5), certain shapes of hysteresis loops are associated with specific pore structures. Thus, type HI loops are often obtained with agglomerates or compacts of spheroidal particles of fairly uniform size and array. Some corpuscular systems (e.g. certain silica gels) tend to give H2 loops, but in these cases the distribution of pore size and shape is not well defined. Types H3 and H4 have been obtained with adsorbents having slit-shaped pores or plate-like particles (in the case of H3). The Type I isotherm character associated with H4 is, of course, indicative of microporosity. 

The book is addressed to those workers whether in academic institutions or in industrial laboratories, whose work is concerned, either directly or indirectly, with the surface area or the pore structure of finely divided or porous solids. 

S. Modry and M. Svata, eds.. Pore Structures, Properties, and Materials, Proceedings of the International Symposium, Scademia, Prague, Czechoslovakia, 1974. 

The gas rate at which coalescence begins to reduce the effectiveness of dispersion appears to depend not only on the pore size and pore structure of tlie di.spersiiig medium but also on the li( iiid properties, li( iiid depth, agitation, and other features of the pin giiig environment coalescence is strongly dependent on the concentration of... 

For a porous adsorbent, the amount adsorbed in the pore structure per unit mass of adsorbent, based on surface excess, is obtained by the difference... 

In a particle having a bidispersed pore structure comprising spherical adsorptive subparticles of radius forming a macroporous aggregate, separate flux equations can be written for the macroporous network in terms of Eq. (16-64) and for the subparticles themselves in terms of Eq. (16-70) if solid diffusion occurs. 

For particles with a bidispersed pore structure, the mass-transfer parameter in the LDF approximation (column 2 in Table 16-12) can be approximated by the series-combination of resistances... 

Leaching is the removal of a soluble fraction, in the form of a solution, from an insoluble, permeable sohd phase with which it is associated. The separation usually involves selective dissolution, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid or liquid and it may be incorporated within, chemically combined with, adsorbed upon, or held mechanically in the pore structure of the insoluble material. The insoluble sohd may be massive and porous more often it is particulate, and the particles may be openly porous, cellular with selectively permeable cell walls, or surface-activated. 

The rate of destruction of active sites and pore structure can be expressed as a mass-transfer relation for instance, as a second-order reaction... 

Hea vy particulate loadings can damage pore structure of filter bed... 

Preconditioning for Particulates Heavy particulate loading of the inlet gas with dust, grease, oils, or other aerosols can be very dam-aging to the pore structure of the filter bed, resulting in an eventual pressure-drop increase. Oils and heavy metals that are deposited on the filter bed can be poisonous to the microorganisms that live within the biofilm. Particulate APC equipment such as fabric filters and venturi scrubbers are generally adequate for this level of particulate removal. 

Endo, M., Takeuchi, K., Sasuda, Y., Matsubayashi, K., Oshida, K. and Drcsselhaus, M. S., Fractal analysis on pore structure for activated carbon fibers. Electron. Commun. Jpn., Part II Electron., 1994, 77(6), 98 107. 

Economy, J., Daley, M., Hippo, E. J. and Tandon, D., Elucidating the pore structure of activated carbon fibers through direct imaging using scanning tunneling microscopy (STM), Carbon, 1995, 33(3), 344 345... 

Alcaniz-Monge, J., Cazorla-Amoros, D., Linares-Solano, A., Yoshida, S. and Oya, A., Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibers. Carbon, 1994, 32(7), 1277 1283. 

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