PHYSICAL ADSORPTION AND THE CHARACTERIZATION OF POROUS ADSORBENTS

In discussing the fundamentals of adsorption it is useful to distinguish between physical adsorption, involving only relatively weak intermolecular forces, and chemisorption which involves, essentially, the formation of a chemical bond between the sorbate molecule and the surface of the adsorbent. Although this distinction is conceptually useful there are many intermediate cases and it is not always possible to categorize a particular system unequivocally. The general features which distinguish physical adsorption chemisorption are as follows  

Low heal of adsorption ( 2 Of 3 times latent heal of evaporation.) 

No electron transfer although polarization of sorbate may occur. 

May involve dissociation. Possible over a wide range of temperature. 

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Ruthven, D.M. 1984. Physical adsorption and the characterization of porous adsorbents. In Principles of Adsorption and Adsorption Processes, pp. 29-61. John Wiley and Sons, New York. 

Because of its small size (collision diameter 0.20 nm), helium would appear to be a useful probe molecule for the study of uitramicroporous carbons. The experimental difficulty of working at liquid helium temperature (4.2 K) is the main reason why helium has not been widely used for the characterization of porous adsorbents. In addition, since helium has some unusual physical properties, it is to be expected that its adsorptive behaviour will be abnormal and dependent on quantum effects. 

Abstract Unsteady liquid flow and chemical reaction characterize hydrodynamic dispersion in soils and other porous materials and flow equations are complicated by the need to account for advection of the solute with the water, and competitive adsorption of solute components. Advection of the water and adsorbed species with the solid phase in swelling systems is an additional complication. Computers facilitate solution of these equations but it is often physically more revealing when we discriminate between flow of the solute with and relative to, the water and the flow of solution with and relative to, the solid phase. Spacelike coordinates that satisfy material balance of the water, or of the solid, achieve this separation. Advection terms are implicit in the space-like coordinate and the flow equations are focused on solute movement relative to the water and water relative to soil solid. This paper illustrates some of these issues. 

The next two chapters deal mainly with the use of adsorption to characterize porous solids. In the case of activated carbon fibers (Chapter 17), methods to characterize microporosity, and particularly ultramicroporosity, by physical adsorption are of particular relevance for understanding the behavior of these adsorbents and extending the range of their applications. Moreover, in Chapter 18 the pore structure of ordered mesoporous carbons is shown to differ greatly from that of conventional activated carbons for which most of the available data treatment methods have been developed. Therefore, suitable procedures for correctiy analyzing the pore structure of these novel carbons are proposed in this chapter. 

Gas adsorption is one of the most widely used techniques for characterization of porous materials. In this chapter wc arc focused on physical adsorption (physisorption), which in contrast to chemisorption, occurs due to the Van der Walls forces. The amount adsorbed, a, expressed per unit mass of solid (adsorbent) is dependent on the gas (adsorbate) pressure, p, temperature, T, properties of the adsorbent and the nature of the gas-solid interactions [55]. Thus, for a given adsorbate adsorbed on a particular adsorbent one can write ... 

FIGURE 11.12 Physical characterization and diagram of mesoporous CDC and its impact on cytokine removal, (a) Scanning electron micrograph of the synthesized mesoporous CDC. (b) Superior performanee of mesoporous CDC (800°C) as compared to other materials with respect to the ability to remove cytokines from human Wood plasma concentration of IL-6 in the plasma solution initially and after 5, 30, and 60 min of adsorption. (c,d) Schematics of protein adsorption by porous carbons (c) surface adsorption in microporous carbon. Small pores do not allow proteins to be adsorbed in the bulk of carbon particles (d) adsorption in the bulk of mesoporous CDC. Large mesopores are capable to accommodate a larger fraction of the proteins. (Adapted from Yushin, G. et al. Biomaterials 27, 5755-5762, 2006.)... 

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) ... 

Physical adsorption of gases and vapors is a powerful tool for characterizing the porosity of carbon materials. Each system (adsorbate-adsorbent temperature) gives one unique isotherm, which reflects the porous texture of the adsorbent. Many different theories have been developed for obtaining information about the solid under study (pore volume, surface area, adsorbent-adsorbate interaction energy, PSD, etc.) from the adsorption isotherms. When these theories and methods are applied, it is necessary to know their fundamentals, assumptions, and applicability range in order to obtain the correct information. For example, the BET method was developed for type II isotherms therefore, if the BET equation is applied to other types of isotherms, it will not report the surface area but the apparent surface area. 

The author of this book has been permanently active during his career in the held of materials science, studying diffusion, adsorption, ion exchange, cationic conduction, catalysis and permeation in metals, zeolites, silica, and perovskites. From his experience, the author considers that during the last years, a new held in materials science, that he calls the physical chemistry of materials, which emphasizes the study of materials for chemical, sustainable energy, and pollution abatement applications, has been developed. With regard to this development, the aim of this book is to teach the methods of syntheses and characterization of adsorbents, ion exchangers, cationic conductors, catalysts, and permeable porous and dense materials and their properties and applications. 

Pores, and especially mesopores (with sizes between 2 and 50 nm) and micropores (with sizes less than 2 nm), play an essential role in physical and chemical properties of industrially important materials like adsorbents, membranes, catalysts etc. In addition to pore structural characterization described above, the description of transport phenomena in porous materials has received attention due to its importance in many applications such as drying, moisture transport in building materials, filtration etc. Although widely different, these applications present many similarities since they all depend on the same type of transport phenomena occurring in a porous media environment. In particular, transport in mesoporous media and the associated phenomena of multilayer adsorption and capillary condensation have been investigated as a separation mechanism for gas mixtures [29]. 

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