Analysis of gas adsorption

The amount adsorbed M can be a function of pressure P at a fixed temperature T. This is called an adsorption isotherm, and is most often used for the qualitative analysis of gas adsorption. In contrast, the amount of adsorption as a function of... 

Pores can also be classified on the basis of their state, either open or closed. In order to identify the pores by gas adsorption (a method which has frequently been used for activated carbons), they must be exposed to the adsorbate gas. If some pores are too small to accept gas molecules they cannot be recognized as pores by the adsorbate gas molecules. These pores are called latent pores and include closed pores. Closed pores are not necessarily in small size. Pores in carbon materials have been identified by different techniques depending mostly on their sizes. Pores with nanometer sizes, that is, micropores and mesopores, are identified by the analysis of gas adsorption isotherms, mostly of nitrogen gas at 77 K [41-46]. 

Gas chromatographic analysis of the adsorption samples were made on a capillary gas chromatograph (Carlo Erba 2900) equipped with a modified injection system constructed at our department ( 3). ... 

The current study was aimed at generalization of the procedure for the quantitative composition determination for hexagonal/lamellar mixed phases using adsorption isotherms. Recent advances in adsorption on calcined MCM-41 silicas [29-31] made it possible to construct model adsorption isotherms for these materials with arbitrarily chosen pore sizes (or equivalently, capillary condensation pressures). Such model adsorption isotherms can be used instead of experimental adsorption isotherms for pure MCM-41 materials of the required pore size, significantly extending the range of HL materials, for which the phase composition analysis can conveniently be carried out on the basis of gas adsorption data. 

Experimentally, the investigation of adsorption from solution is much simpler than that of gas adsorption. A known mass of adsorbent solid is shaken with a known volume of solution at a given temperature until there is no further change in the concentration of the supernatant solution. This concentration can be determined by a variety of methods involving chemical or radiochemical analysis, colorimetry, refractive index, etc. The experimental data are usually expressed in terms of an apparent adsorption isotherm in which the amount of solute adsorbed at a given temperature per unit mass of adsorbent - as calculated from the decrease (or increase) of solution concentration - is plotted against the equilibrium concentration. 

Fozano-Castello D, Cazorla-Amords D, Linares-Solano A, Oshida K, Miyazaki T, Kim YJ, Hayashi T, and Endo M. Comparative characterization study of microporous carbons by HRTEM image analysis and gas adsorption. J. Phys. Chem. B, 2005 109 15032-15036. 

The main purpose of the present chapter is to introduce the underlying princ which will serve as a basis for the discussion of physisorption isotherms present later chapters. It is not our intention to give a general survey of the theorii physisorption at the gas-solid interface instead, our aim is to provide suffi information to enable the newcomer to surface science to appreciate the advan and limitations of the most widely used procedures for the analysis of experim adsorption isotherms. Our selection of theoretical material is necessarily some arbitrary in view of the vast literature on physisorption. The decision to include t ticular concept or equation is based on either its historical importance or its cu usage. Thus, a few equations were considered worthy of inclusion in this cht although for our purpose they do not merit further discussion or application in si quent chapters. 

J.A. Poulis, C.H. Massen, E. Robens Measurement of gas adsorption with Jantti s method using continuously increasing pressure. Journal of Thermal Analysis and Calorimetry 68 (2002) 2, 719-725. 

We now cite the types of experimental data in the literature, by which an analysis of surface adsorption effects is carried out. One common experiment involves measuring adsorption isotherms. By weighing or by volumetric techniques one determines as a function of equilibrium gas pressure the amount of gas held on a given surface at a specified temperature. Usually this quantity varies sigmoidally with rising pressure P, as sketched in Fig. 5.2.1 for a variety of temperatures 7). By standard methods that rely on the Brunauer, Emmett, Teller isotherm equa-tion one can determine the point on the isotherms at which monolayer coverage of the surface is complete it is usually is located fairly close to the knee of the isotherm. From the cross sectional area of the adsorbate molecules and from the amount needed for monolayer coverage one may then ascertain more or less quantitatively the surface area of the adsorbent. As-... 

The task of reviewing the thermodynamics of gas adsorption by carbons is complicated by several factors. One of them is the variabihty of carbon materials discussed in Chapter 2. Another is the large amount of published literature, produced over a long period and generated under very different environments and thus potentially very different conditions. This has prompted us to focus our analysis on recent work, assuming that older studies may be outdated by new ones thanks to advances in instrumentation and to the logical progress of science. 

Lastoskie, C.M., Quirke, N., and Gubbins, K.E. (1997). Structure of porous adsorbents analysis using density functional theory and molecular simulation. In Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces, Studies in Surface Science and Catalysis, Elsevier, Vol. 104 (W. RudziAski, W.A. Steele and G. Zgrablich, eds). pp. 745—75. 

The bulk and framework compositions of the H-GaAlMFI and Ga/H-ZSM-5 zeolites are given in Table 1. The zeolites were prepared by the procedures described elsewhere (6,9). The MFI structure of the zeolites was confirmed by XRD. The incorporation of Ga in the framework of GaAlMFI zeolite was confirmed by the Ga MAS NMR peak at +156 ppm. The framework (FW) Si/Ga and Si/Al ratios of the zeolites were obtained from their 29gi mAS NMR peaks, Si(0Ga or OAl) at about -112 ppm and Si(lGa or lAl) at about -104 ppm by the method described earlier (10). The crystal size and morphology of the zeolites was studied by SEM. The zeolite bulk chemical composition was determined by the chemical analysis of Ga, Al, Si and Na. Strong acid sites on the zeolites were measured in terms of the pyridine chemisorbed at 400 C, using the GC adsorption/desorption method (11). 

Frequency response methods have been found useful in both theoretical and experimental analysis of gas mixing in fluidized beds. Experiments in a fluidized-bed reactor related to mixing theory were made by Bamstone and Harriott 24). Testin and Stuart have measured diffusion coefficients in gas-solid adsorption studies 25). 

Gas adsorption is the preferred method of surface-area determination. An isotherm is generated of the amount of gas adsorbed against gas pressure, and the amount of gas required to form a monolayer is determined. The surface area can tTien be calculated using the cross-sectional area of the gas molecule. Outgassing of the powder before analysis should be conducted very carefully to ensure reproducibility. Commonly, nitrogen at liquid nitrogen vapor pressure is used but, for low surface-area powders, the adsorbed amounts of krypton or xenon are more accurately found. Many theories of gas adsorption have been advanced, but measurements are usually interpreted by using the BET theory [Brunauer, Emmett, and Teller, J. Am. Chem. Soc., 60,309 (1938)]. 

Comparison of STM image analysis with gas adsorption. Rejainted with permission Horn Ref. [13]. Copyright 1999 Materials Research Society. 

Gas adsorption is an important method for characterization of nanoporous carbons because it allows for evaluation of the specific surface area, pore volume, pore size, pore size distribution and surface properties of these materials [1, 10-12]. Although various techniques for measurement of gas adsorption data and methods of their analysis pear to be well established, an accurate and reliable evaluation of adsorption properties is still a difficult task. This can be attributed to the inherent features of many porous carbonaceous materials, namely, to their strong surface and structural heterogeneity. The effects of structural and surface heterogeneity in adsorption on nanoporous carbons are often difficult to separate. 

Since there are three equilibrium constants (i.e., Ka, Kb, and / eq.Rxi) and one kinetic rate constant that one seeks to determine via linear least-squares analysis, it is necessary to have an independent measurement of either Ka or via adsorption studies on the same high-surface-area catalyst in a pulverized state. For example, if this catalyst is exposed to pure gas A and Ka is determined via linear least-squares analysis of its adsorption isotherm, then some of the other temperature-dependent parameters in the Hougen-Watson model are calcnlated as follows from ao, ai and a2. 

Recently, the influence of gas adsorption on physical properties of carbon nanotubes has attracted a considerable interest. It is caused by a possibility to create gas nanosensors [1,2]. The analysis of calorimetric data has shown that the gas adsorption in space between nanotubes gives the main effect on physical properties [3]. The measurement of the conductivity is simple and convenient method to detect a response of multiwalled nanotubes with respect to an external influence. This response can be used to construct a resistive sensor. 

That equation and its integral form soon gained enormous popularity among scientists investigating the kinetics of gas adsorption on solid surfaces. Correlations of experimental data usually started with the application of that equation, and the frequently noted deviations from "Elovich behavior" were the form of analysis in dozens of published papers. Even reviews followed that general trend, discussing the Elovich equations and "deviations" from it. 

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