Gas adsorption isotherms

Jaroniec, M., Gilpin, R. K., Kaneko, K. and Choma, J., Evaluation of energetic heterogeneity and microporosity of activated carbon fibers on the basis of gas adsorption isotherms, Langmuir, 1991, 7(1 1), 2719 2722. 

During ball milling the mean particle diameter, d, gas adsorption isotherm specific surface area (BET) and contamination (wt% contaminant) conform to the general law ... 

Figure 10.3 An isotherm of amount of gaseous cyclohexane adsorbed (as y ) against pressure p (as x ), depicted as a function of temperature. The substrate is a catalyst comprising a mixture of metal oxides, called Stirling-FTG. (Figure reproduced by permission of Pergamon from the paper Towards a general gas adsorption isotherm , G. M. Martinez and D. Basmadjian, Chem. Eng. Sci.,...

On the basis of the theory of Pfeifer et al.,108-112 the surface fractal dimension can be determined from the gas adsorption isotherms according to the following equation ... 

For single-component gas permeation through a microporous membrane, the flux (J) can be described by Eq. (10.1), where p is the density of the membrane, ris the thermodynamic correction factor which describes the equilibrium relationship between the concentration in the membrane and partial pressure of the permeating gas (adsorption isotherm), q is the concentration of the permeating species in zeolite and x is the position in the permeating direction in the membrane. Dc is the diffusivity corrected for the interaction between the transporting species and the membrane and is described by Eq. (10.2), where Ed is the diffusion activation energy, R is the ideal gas constant and T is the absolute temperature. 

Experimental gas adsorption isotherms are traditionally classified into one of the five types shown in Figure 9.4. 

FIG. 9.4 Qualitative shapes of the five general types of gas adsorption isotherms. (See text for a discussion of their physical significance.)... 

Prepare a plot of the FHH isotherm using n = 3 and K = 0.1 and comment on the resemblance of this isotherm to actual gas adsorption isotherms as shown in Chapter 9. 

Nitrogen adsorption measurements were done using a Micromeritics model ASAP 2010 adsorption analyzer (Norcross, GA). Adsorption isotherms were measured at -196°C over the interval of relative pressures from 10 6 to 0.995 using nitrogen of 99.998% purity. Before each analysis the sample was degassed for 2 hours at 150°C under vacuum of about 10 Torr in the degas port of the adsorption apparatus. 

A new model for determining the pore size distribution of micro and mesoporous materials from gas adsorption isotherm has been successfully proposed and tested. The present model was found to be successful in predicting the pore size distribution of pure as well as binary physical mixtures of MCM-41. 

Measurements of xenon gas adsorption isotherms were performed at temperatures between 273 and 299.5 K (this latter temperature being that of the 129Xe NMR probe). 

Figure 1. Plots of differential pore volume against pore diameter calculated from the N2 gas adsorption isotherms obtained from meso/macroporous carbon specimens I (-0-), II (- -), and III (-A-) using Barrett-Joyner-Halenda (BJH) method. Reprinted with permission from G. -J. Lee and S. -I. Pyun, Carbon, 43 (2005) 1804. Copyright 2005, with permission from Elsevier.

Table 1. As the relative pressure increases, the thickness of adsorbed molecule layers on the pore surfaces increases as well, and then the pore filling process caused by capillary condensation occurs first in the small pores simultaneously with the multilayer adsorption on the larger pores. For specimen III, the value of 4d,max is significantly larger than those values for the other specimens, which is ascribed to the fact that as a result of the pore filling process in the larger macropores the adsorbed volume starts to increase abruptly only near the saturation vapor pressure in the gas adsorption isotherm. From the above results, they suggested that 4d,max / 4d,mm is closely related to rmax, that is, the larger rmax, the wider ranges the length-scale of the fractal regime in value.

On the other hand, it is impossible to apply the SP method to the correct description of gas adsorption in the micropores, since the adsorption in the micropores does not occur by multilayer adsorption but by micropore volume filling process. In this case, the pore fractal dimension gives a physical importance for the description of structural heterogeneity of the microporous solids. Terzyk et al.143"149 have intensively investigated the pore fractal characteristics of the microporous materials using gas adsorption isotherms theoretically simulated. 

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... 

It is with this type of equation that, for instance, Micale el al. (1976) was able to check the consistency of the isosteric approach (from gas adsorption isotherms) with immersion calorimetry, for the water-microciystalline Ni(OH)2 system. 

Assessment of wettability from the gas adsorption isotherm. If the solid is covered with a liquid film in equilibrium with saturated vapour p°, the spreading pressure of the film can be derived from Equation (2.22). Thus ... 

J.U. Keller, W. Zimmermann, A. Schein, R. Staudt Determination of absolute gas adsorption isotherms by eombined calorimetric and dielectric measurements. Adsorption (2002) in print. 

A known mass of degassed (free of water and volatile compounds) powder with a total surface of more than 2 m is used for analysis. By adding defined doses of the analysis gas and measuring the equilibrium pressure, gas adsorption isotherms can be obtained (Equation 10.21). From this measurement the... 

The key impurities present in a typical ROG for the recovery of H2 by a PSA process are bulk Ci and C2 and dilute C3 and C4 hydrocarbons. Figures 10.11 and 10.12 describe the pure gas adsorption isotherms of the components of ROG at 30 °C on the BPL activated carbon and a silica gel sample (Sorbead H produced by Engelhard Corp.), respectively.31 These data were also measured in Air Products and Chemicals, Inc. laboratories. It may be seen that the carbon adsorbs C3+ hydrocarbons very strongly. Consequently, desorption of these hydrocarbons from the carbon by H2 purge becomes rather impractical requiring a large volume of purge gas. 

Of the established static techniques, which we have considered here, that involving gas adsorption isotherm measurements remains one of the most powerful and widely applicable. It is indeed very accessible with the availability of automated commercial equipment and the variety of data treatment facilities available. Nevertheless, it is still circumscribed by the assumptions implicit in the choice of a pore shape model in the case of mesoporous materials. Its application to microporous structures has recently advanced considerably, although there are here certain reservations which still exist concerning the general application of theories to describe adsorption in such small pores in ill defined structures. 

Adsorption equilibria determine the thermodynamic limits of the specific amounts of adsorption (mol/g) of a pure gas or the components of a fluid mixture (gas or liquid) under a given set of conditions [pressure (P), temperature (T), and mole function (y or Xi) of component /] of the bulk fluid phase. The simplest way to describe adsorption equilibria of pure gas i is in the form of adsorption isotherms where the amount adsorbed (n ) is plotted as a function of gas pressure (P) at a constant temperature (P). The pure gas adsorption isotherms can have various shapes (Types I-V) by Brunauer classification depending on the porosity of the adsorbent (microporous, mesoporous, or nonpo-rous) and the system temperature (below or above the critical temperature of the adsorbate). However, the most common isotherm shape is Type I, which is depicted by most microporous adsorbents of practical use. These isotherms exhibit a linear section in the very low-pressure region (Henry s law region) where the amount adsorbed is proportional to the gas pressure [ n ) = KiP]. The proportionality constant is called... 

Fig. 3 Pure gas adsorption isotherms for (A) nitrogen and (B) oxygen on various zeolites.

Fig. 4 Binary gas adsorption isotherms for nitrogen (1) and oxygen (2) mixtures on sodium mordenite.

In the following section, we shall describe a different approach for the description of the surface properties of divided solids the determination of the surface energetic heterogeneity starting with the gas adsorption isotherms. 

---