Porous characterization

Tamon H, Ishizaka H (1998) Porous characterization of carbon aerogels. Caibtm 36 1397-1399. 

Table 1.13 compiles the benzene and toluene adsorption capacities for pristine AC obtained by numerical integration of breakthrough curves similar to those of Figure 1.36. It includes the porous characterization of commercial samples developed for removal of hydrocarbons as well as of ACs prepared by different activation methods and activating agents. 

Liu, P.K.T., Surface and porous characterization of two commercial activated carbons with polymer and petroleum coke as precursors, Ind. Eng. Chem. Res., 31(9), 2216-2222 (1992). 

The polymer formed in this way is soluble in water. When the crosslinking process is carried out at higher temperature, or at a longer time, the polymers of higher molecular weight are obtained. The VOCs can be readily entrapped in p CD cavities due to good size fitting and hydrophobicity of the CD cavity. The BET porous characterization of the prepared polymer was made. 

As a general rule, adsorbates above their critical temperatures do not give multilayer type isotherms. In such a situation, a porous absorbent behaves like any other, unless the pores are of molecular size, and at this point the distinction between adsorption and absorption dims. Below the critical temperature, multilayer formation is possible and capillary condensation can occur. These two aspects of the behavior of porous solids are discussed briefly in this section. Some lUPAC (International Union of Pure and Applied Chemistry) recommendations for the characterization of porous solids are given in Ref. 178. 

Recommendations for the Characterization of Porous Solids, Pure Appl. Chem., 66, 1739 (1994). 

Increasing the surface-to-bulk ratio of the sample to be studied. This is easily done in the case of highly porous materials, and has been exploited for the characterization of supported catalysts, zeolites, sol-gels and porous silicon, to mention a few. 

The second edition, like the first, is addressed to those workers in academic laboratories or industrial laboratories who are not necessarily specialists in the field of gas adsorption, but whose work is concerned either directly or indirectly with the characterization of finely divided or porous solids. 

Solid Density. SoHds can be characterized by three densities bulk, skeletal, and particle. Bulk density is a measure of the weight of an assemblage of particles divided by the volume the particles occupy. This measurement includes the voids between the particles and the voids within porous particles. The skeletal, or tme soHd density, is the density of the soHd material if it had zero porosity. Fluid-bed calculations generally use the particle... 

Fig. 3. Microporous membranes are characterized by tortuosity, T, porosity, S, and their average pore diameter, d. (a) Cross-sections of porous membranes containing cylindrical pores, (b) Surface views of porous membranes of equal S, but differing pore size.

The Michaehs-Menten equation and other similar nonhnear expressions characterize immobihzed enzyme kinetics. Therefore, for a spherical porous carrier particle with enzyme molecules immobilized on its external as well as internal surfaces, material balance of the substrate will result in the following ... 

Volume 39 Characterization of Porous Solids. Proceedings of the lUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Krai Volume 40 Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11,1987 edited by J. Koukal... 

Volume 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the lUPAC Symposium (COPS II), Alicante, May 6-9,1990... 

Volume 87 Characterization of Porous Solids III. Proceedings of the lUPAC Symposium (COPS III), Marseille, France, May 9-12,1993... 

Adsorbents, and activated carbon in particular, are typically characterized by a highly porous structure. Adsorbents with the highest adsorption capacity for gasoline or fuel vapors have a large pore volume associated with pore diameters on the order of 50 Angstroms or less. When adsorption occurs in these pores, the process is comparable to condensation in which the pores become filled with hquid adsorbate. Fig. 5 depicts the adsorption process, including transfer of adsorbate molecules through the bulk gas phase to the surface of the solid, and diffusion onto internal surfaces of the adsorbent and into the pores. 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

We may begin by describing any porous medium as a solid matter containing many holes or pores, which collectively constitute an array of tortuous passages. Refer to Figure 1 for an example. The number of holes or pores is sufficiently great that a volume average is needed to estimate pertinent properties. Pores that occupy a definite fraction of the bulk volume constitute a complex network of voids. The maimer in which holes or pores are embedded, the extent of their interconnection, and their location, size and shape characterize the porous medium. 

A microscopic description characterizes the structure of the pores. The objective of a pore-structure analysis is to provide a description that relates to the macroscopic or bulk flow properties. The major bulk properties that need to be correlated with pore description or characterization are the four basic parameters porosity, permeability, tortuosity and connectivity. In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous. Due to this complexity, pore-structure description is most often a statistical distribution of apparent pore sizes. This distribution is apparent because to convert measurements to pore sizes one must resort to models that provide average or model pore sizes. A common approach to defining a characteristic pore size distribution is to model the porous medium as a bundle of straight cylindrical or rectangular capillaries (refer to Figure 2). The diameters of the model capillaries are defined on the basis of a convenient distribution function. 

Porous media is typically characterized as an ensemble of channels of var ious cross sections of the same length. The Navier-Stokes equations for all chaimels passing a cross section normal to the flow can be solved to give ... 

When a dilute solution of a polymer (c << c ) is equilibrated with a porous medium, some polymer chains are partitioned to the pore channels. The partition coefficient K, defined as the ratio of the polymer concentration in the pore to the one in the exterior solution, decreases with increasing MW of the polymer (7). This size exclusion principle has been used successfully in SEC to characterize the MW distribution of polymer samples (8). 

A research group in Lehigh University has extensively studied the synthesis and characterization of uniform macroporous styrene-divinylbenzene copolymer particles [125,126]. In their studies, uniform porous polymer particles were prepared via seeded emulsion polymerization in which linear polymer (polystyrene seed) or a mixture of linear polymer and solvent were used as inert diluents [125]. The average pore diameter was on the order of 1000 A with pore volumes up to... 

Thermal insulation is available over a wide range of temperatures, from near absolute zero (-273 C) ( 59.4°F) to perhaps 3,(1()0°C (5,432°F). Applications include residential and commercial buildings, high- or low-temperature industrial processes, ground and air vehicles, and shipping containers. The materials and systems in use can be broadly characterized as air-filled fibrous or porous, cellular solids, closed-cell polymer foams containing a gas other than air, evacuated powder-filled panels, or reflective foil systems. 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

Since electrochemical promotion (NEMCA) studies involve the use of porous metal films which act simultaneously both as a normal catalyst and as a working electrode, it is important to characterize these catalyst-electrodes both from a catalytic and from an electrocatalytic viewpoint. In the former case one would like to know the gas-exposed catalyst surface area A0 (in m2 or in metal mols, for which we use the symbol NG throughout this book) and the value, r0, of the catalytic rate, r, under open-circuit conditions. 

Catalyst films for electrochemical promotion studies should be thin and porous enough so that the catalytic reaction under study is not subject to internal mass-transfer limitations within the desired operating temperature. Thickness below 10 pm and porosity larger than 30% are usually sufficient to ensure the absence of internal mass-transfer limitations. Several SEM images of such catalyst films have been presented in this book. SEM characterization is very important in assessing the morphological suitability of catalyst films for electrochemical promotion studies and in optimizing the calcination procedure. 

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