Adsorption methods, characterization

Adsorption from solutions, research, 653-654 Adsorption isotherm, 253, 254f Adsorption methods, characterization of silica surface, 170, 175-177/... 

Tanahashi, I., Yoshida, A. and Nishino, A., Characterization of activated carbon fiber cloths for electric double layer capacitors by adsorption method. Carbon, 1991, 29(7), 1033 1037. 

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 structure of this review is as follows in Section II, the activation and templating methods for preparing the porous carbons are briefly summarized. Section III surveys the structural characteristics of the porous carbons by using gas adsorption method. In Section IV, the molecular probe method and the image analysis method for quantitative characterization of the pore surface irregularity and the size distribution irregularity based upon the fractal theory are discussed in detail. Section V is devoted to... 

Recently, Lee and Pyun have focused on the characterization of pore fractality of the microporous carbon powder specimens by using nitrogen gas adsorption method based upon the D-A adsorption theory in consideration of PSD with pore fractality. Figure 5 envisages the nitrogen gas adsorption isotherm obtained from the as-reactivated carbon powder specimen prepared by reactivation of the commercially as-activated carbon powder at 1000 °C in an atmosphere of C02/C0 gas mixture for 2 h. The solid... 

Subsequently, the characterization of the pore structures of the porous materials using gas adsorption method was discussed in detail. The types and characteristics of the adsorption isotherms and the hysteresis loops were introduced. In addition, the BET (Braunauer, Emmett, and Teller) theory92 for the determination of the surface area and various theoretical models for characterization of the pore structures according to the pore size range were summarized based upon the adsorption theory. 

All the aforementioned points about peculiarities of adsorption in micropores show that special attention is needed when microporous solids (i.e., activated carbons, ACFs, nanotubes, CMSs, charcoals, etc.) are characterized by the physical adsorption methods. 

As previously discussed in this chapter, gas adsorption techniques are the most common approaches to the characterization of the pore texture, nitrogen adsorption at 77 K being the most popular technique. However, as mentioned above, the SAXS technique represents an alternative to gas adsorption methods. 

It is the aim of this chapter to present a critical exposition of the use of adsorption methods for the characterization of heterogeneous catalysts. Such methods are applied to determine the texture of catalysts and therefore this concept has to be explained first. 

In this chapter, we introduce the currently most popular adsorption methods used for micropore size analysis our aim is to outline in general terms the relative merits and limitations of these procedures. Their application is discussed more fully in later chapters in relation to the characterization of particular adsorbents. 

The progress in the determination of porosity of various types of materials has arisen over the past ten years from advances in application of new spectroscopy techniques. In the present paper the application of small angle X-ray scattering (SAXS), positronium annihilation lifetime spectroscopy (PALS) and low temperature nitrogen adsorption methods to the characterization of mesoporosity is reviewed using different types of silica gels with chemically modified surface. The results from the three methods are compared and discussed. 

The problem of adsorption hysteresis remains enigmatic after more than fifty years of active use of adsorption method for pore size characterization in mesoporous solids [1-3]. Which branch of the hysteresis loop, adsorption or desorption, should be used for calculations This problem has two aspects. The first is practical pore size distributions calculated from the adsorption and desorption branches are substantially diflferent, and the users of adsorption instruments want to have clear instructions in which situations this or that branch of the isotherm must be employed. The second is fundamental as for now, no theory exists, which can provide a quantitatively accurate description of capillary condensation hysteresis in nanopores. A better understanding of this phenomenon would shed light on peculiarities of phase transitions in confined fluids. 

In the present work the meso- and macro-structural characteristics of the mesoporous adsorbent MCM-41 have been estimated with the help of various techniques. The structure is found to comprise four different length scales that of the mesopores, the crystaUites, the grains and of the particles. It was found that the surface area estimated by the use of small angle scattering techniques is higher, while that estimated by mercury porosimetry is much lower, than that obtained from gas adsorption methods. Based on the macropore characterization by mercury porosimetry, and the considerable macropore area determined, it is seen that the actual mesopore area of MCM-41 may be significantly lower than the BET area. TEM studies indicated that MCM-41 does not have an ideal mesopore structure however, it may still be treated as a model mesoporous material for gas adsorption studies because of the large radius of curvature of the channels. 

Two mesoporous silica molecular sieves synthesized by using n-octadecyl-ammonium bromide and n-dodecylammonium bromide as a templates were characterized for their pore size distribution by temperature programmed desorption method and low temperature nitrogen adsorption method. The pore size distributions and total pore volumes determined by the two methods agree quite well and are within experimental error. 

This chapter aims to give guidelines on how to use adsorption methods for the characterization of the surface area and pore size of heterogeneous catalysts. The information derived from these measurements can range from the total and available specific surface area to the pore sizes and the strength of sorption in micropores. Note that this spans information from a macroscopic description of the pore volume/specific surface area to a detailed microscopic assessment of the environment capable of sorbing molecules. In this chapter we will, however, be confined to the interaction between sorbed molecules and solid sorbents that are based on unspecific attractive and repulsive forces (van der Waals forces, London dispersion forces). 

The series of 10 chapters that constitute Part 3 of the book deals mainly with the use of adsorption as a means of characterizing carbons. Thus, the first three chapters in this section complement each other in the use of gas-solid or liquid-solid adsorption to characterize the porous texture and/or the surface chemistry of carbons. Porous texture characterization based on gas adsorption is addressed in Chapter 11 in a very comprehensive manner and includes a description of a number of classical and advanced tools (e.g., density functional theory and Monte Carlo simulations) for the characterization of porosity in carbons. Chapter 12 illustrates the use of adsorption at the liquid-solid interface as a means to characterize both pore texture and surface chemistry. The authon propose these methods (calorimetry, adsorption from solution) to characterize carbons for use in such processes as liquid purification or liquid-solid heterogeneous catalysis, for example. Next, the surface chemical characterization of carbons is comprehensively treated in Chapter 13, which discusses topics such as hydrophilicity and functional groups in carbon as well as the amphoteric characteristics and electrokinetic phenomena on carbon surfaces. 

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. 

Supports and catalysts were characterized (3,5) by different physicochemical techniques Specific surface area, (Sbet), was determined from N2 adsorption isotherms, X-ray diffractometry. Scanning Electron Microscopy (SEM) and IR spectroscopy. The pyridine adsorption method was used to identify by IR the nature and character of the surface acidic groups of supports, precursors and catalysts. 

Taking into account that formation of the propiophenone 2 is the most important side reaction, we were interested in determining the exact nature of the active sites responsible for the hydrolysis of the acetal moiety. Aimed at this purpose, a characterization of the Bronsted and Lewis acidity of the centers was accomplished for the HY, ZnHY and ZnNaY samples by means of the pyridine adsorption method. Pyridine when adsorbed on solid acids, shows in the IR spectra specific bands assignable to pyridinium Ion (1540 cm ) and Lewis adducts (1450 cm ), which intensities are directly related to the population of both types of centers... 

Pure silica Beta has been crystallized from alkali-free hydrogel containing tetraethyl-ammonium hydroxide and fumed silica at 413 K by the conventional hydrothermal synthesis method. Characterization has been done by XRD, IR, SEM, solid-state NMR, thermal analysis eind N2 adsorption. The results show that a highly crystalline pure silica Beta is formed. Si MAS NMR reveals that the pure silica Beta has a small number of sites originating from structural defects and almost half of sites are silanol groups. Thermal analysis shows that pure silica Beta possesses nonequivalent sites that are siloxy groups counterbalanced by TEA cations. 

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