Basic adsorption parameters

A first stage in characterization of adsorption properties of active carbons is usually determination of their surface area and pore volume. The surface area is normally determined flxim equilibrium adsorption isotherm of a gas or vapor measured in a range of relative pressures fium 0.01 to 03. Currently, there are two major methods used to evaluate specific surface area from gas adsorption data the Brunaucr-Emmett-Teller (BET) method [1, 12, 13, 64] and the comparative plot analysis [12,13]. 

The evaluation of the specific area by the BET method is based on the evaluation of the monolayer capacity (i.e., the number of adsorbed molecules in the monolayer on the surface of a material) by fitting experimental gas adsorption data to the BET equation  

(5) is used to determine the monolayer capacity, 0, which is necessary for the evaluation of the surface area, Sset- If the cross-sectional area a for a single molecule in the monolayer formed on a given surface is known, the surface area Sbet can be evaluated by using the following formulae  

Despite all these problems and limitations, the BET method is currently a standard way for evaluation of the specific surfece area of solids. For several reasons, nitrogen (at 77 K) is generally atnsidered to be the most suitable adsorbate for surface area determination [12, 13, 

65] and it is usually assumed that the nitrogen monolayer is close-packed so that 0) 2 = 0.162 nm. The specific surface area for the active carbons under study (WV-A900, BAX 1500 and NP5) was assessed using the standard BET method (see Fig. 5). In the case of the WV-A900 and BAX 1500 active carbons the adsorption data measured at relative pressures from 0.05 to 0.3 were used, whereas for the NP5 active carbon a more narrow range (0.01 - 0.2) was employed. The monolayer capacity, a , constant C and the BET specific surface area, Sbet, for these active carbons are listed in Table 1. 

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Basically, three types of experiments are carried out for measurement of the adsorption parameters of a given rock sample. 

Basic adsorption isotherms have been described in this chapter. For micro-porous membranes, the use of the DR equation to describe micropore filling has been shown to be quite adequate. Techniques for the determination of surface area and pore size distribution have ben presented. The use of potential functions for the determination of pore size distribution in microporous materials has been described. Although the potential function techniques give consistent and satisfactory results, caution must be exerted in using these techniques for the calculation of the pore size distribution, due to the uncertainty involved in the values of the parameters used in the calculation and the simplifying assumptions employed in the derivation of the model equations. 

The extensive study of adsorption of various gases (argon, krypton, xenon and methane) on lamellar dihalides [127] have provided a rather unique opportunity to find correlations between the properties of the adsorbed films and the basic geometrical parameters characterizing the surface lattice. 

Basic information of diffusion and adsorption have been discussed in some details in the last five chapters (7 to 11). In this chapter and the subsequent chapters we will present various methods devised to determine the diffusion coefficient. We start with a method of time lag, which was introduced in 1920 by Daynes. This method can be exploited for the determination of diffusion and adsorption parameters. The concept of this method is simple. Basically a porous medium is mounted between two reservoirs (Figure 12.1-1). 

In the subsequent BOC-MPAJBI-QEP relationships, the basic energetic parameter is the Morse constant Qo in equation 6.31, which corresponds to the maximum two-center M-A bond energy, Qoa, for atom A adsorbed on an on-top site. This value is not directly obtainable, but it can be readily determined from the experimental heat of adsorption value, Qa (the atomic binding energy), associated with the M -A bond energy, Q , namely... 

The model is intrinsically irreversible. It is assumed that both dissociation of the dimer and reaction between a pair of adjacent species of different type are instantaneous. The ZGB model basically retains the adsorption-desorption selectivity rules of the Langmuir-Hinshelwood mechanism, it has no energy parameters, and the only independent parameter is Fa. Obviously, these crude assumptions imply that, for example, diffusion of adsorbed species is neglected, desorption of the reactants is not considered, lateral interactions are ignored, adsorbate-induced reconstructions of the surface are not considered, etc. Efforts to overcome these shortcomings will be briefly discussed below. 

The a scale of solvent acidity (hydrogen-bond donor) and the (3 scale of solvent basicity (hydrogen-bond acceptor) are parameters derived from solvatochromic mea-siuements used in adsorption chromatography [51,54,55]. 

The correlation of Snyder s solvent strength e° with molecular dipolarity and polarizability (7t ) and the hydrogen-bond acidity (a) and the hydrogen-bond basicity ((3) solvatochromic parameters for adsorption chromatography can be achieved, although most papers on solvatochromic parameters deal with reversed-phase systems [18]. 

Equation (89) shows that the allowance for the variation of the charge of the adsorbed atom in the activation-deactivation process in the Anderson model leads to the appearance of a new parameter 2EJ U in the theory. If U — 2Er, the dependence of amn on AFnm becomes very weak as compared to that for the basic model [see Eq. (79)]. In the first papers on chemisorption theory, a U value of 13eV was usually accepted for the process of hydrogen adsorption on tungsten. However, a more refined theory gave values of 6 eV.57 For the adsorption of hydrogen from solution we may expect even smaller values for this quantity due to screening by the dielectric medium. 

As we have already noted, the parameters i °, rr, and n+ are of special significance in the electronic theory. They enter into all the basic formulas of the theory. For one thing, they are the quantities on which the adsorption capacity and the catalytic activity of a surface depend. 

Nitrogen adsorption/desorption isotherms of all the activated carbons are of Type I, i.e. characteristic of basically microporous solids. There is a lack of adsorption/desorption hysteresis. More careful analysis permits to notice significant differences in the porous texture parameters depending on precursor origin. 

Table 4.2. The basic parameters of the models under discussion for some adsorption systems. The dephasing (2f 4)) and the relaxation (2T 3)) contributions to the full spectral linewidth for local vibrations as well as the...

A similar system, (CH3)2C=CH X, was studied by Endrysova and Kraus (55) in the gas phase in order to eliminate the possible leveling influence of a solvent. The rate data were separated in the contribution of the rate constant and of the adsorption coefficient, but both parameters showed no influence of the X substituents (series 61). A definitive answer to the problem has been published by Kieboom and van Bekum (59), who measured the hydrogenation rate of substituted 2-phenyl-3-methyl-2-butenes and substituted 3,4-dihydro-1,2-dimethylnaphtalenes on palladium in basic, neutral, and acidic media (series 62 and 63). These compounds enabled them to correlate the rate data by means of the Hammett equation and thus eliminate the troublesome steric effects. Using a series of substituents with large differences in polarity, they found relatively small electronic effects on both the rate constant and adsorption coefficient. 

One of the parameters in the broad class of liquid adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. The acidity of zeolitic adsorbent is normally affected by the zeolite Si02/Al203 molar ratio, the ionic radii and the valence of the cations exchanged into the zeolite. In this contribution, Sanderson s model of intermediate electronegativity of zeolitic adsorbent acidity (SjJ can be calculated as a representation of the strength of the adsorbent acidity based on the following equation ... 

The foundation of equilibrium-selective adsorption is based on differences in the equilibrium selectivity of the various adsorbates with the adsorbent While all the adsorbates have access to the adsorbent sites, the specific adsorbate is selectively adsorbed based on differences in the adsorbate-adsorbent interaction. This in turn results in higher adsorbent selectivity for one component than the others. One important parameter that affects the equilibrium-selective adsorption mechanism is the interaction between the acidic sites of the zeolite and basic sites of the adsorbate. Specific physical properties of zeolites, such as framework structure, choice of exchanged metal cations, Si02/Al203 ratio and water content can be... 

One of the parameters in the broad class of equilibrium-selective adsorption mechanisms is the interaction between the acidic and basic sites of the adsorbent and the adsorbate. ZeoUtes can be ion-exchanged with a variety of metal cations... 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

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