Adsorbent activity parameter

Hb describes the molecular cross section occupied on the adsorbent surface by the solvent molecule b in units of 0.085 nm2 (which is 1/6 of the area of an adsorbed benzene molecule, corresponding to the effective area of an aromatic carbon atom on the adsorbent surface), while the parameter a describes the activity parameter of the adsorbent surface. The adsorbent activity parameter is a measure of the adsorbent s ability to interact with adjacent molecules of solute or solvent and is constant for a given adsorbent. For silica gel a value of 0.57 is given by Poole (2003). The molar solvent ratio is calculated by Eq. 4.6 with the molar volumes of the molecules (Eq. 4.7). 

Equations (12-3) and (6-6) are seen to be of the same form, if we define a temperature-dependent adsorbent activity parameter a. ... 

Variation of Adsorbent Activity Parameter ay with Separation Temperature... 

Eq. (12-3) (with 297/a = 0.13) or Eq. (12-3c) will be generally reliable, since the variability of a normally small quantity 291 ja) should itself be small. As in the case of Eq. (6-6), the reliability of Eqs. (12-3) and (12-3c) will be greatest for weak or moderately strong solvent systems. Several other studies [e.g.. Refs. (4-/0)] have confirmed the fact that sample adsorption (i.e., the value of A ) generally decreases with increasing T, as required by Eqs. (12-3) and (12-3c). A quantitative comparison of these latter relationships with literature data is not possible in the absence of data on the adsorbents used (values of V and a). Table 12-2 shows how the adsorbent activity parameter ay varies with temperature. 

In eadi specific case the choice of an adsorbent, electrophysical parameters and the method of registration of its change as well as the choice of various pre-adsorption treatment techniques of the surface of adsorbent is dictated by the type and nature of analytical problem to be solved. For instance, if particles active from the standpoint of the change in electrophysical parameters of semiconductor adsorbent occur on the surface of the latter due to development of a chemical reaction involving active particles, it is natural to use either semiconductor material catalyzing the reaction in question or if this is not possible specific surface dopes accelerating the reaction. Above substances are used as operational element of the sensor. If such particles occur as a result of adsorption from adjacent volume, one can use semiconductor materials with maximum adsorption sensitivity to the chosen electrophysical parameter with respect to a specific gas as operational element. 

Two different adsorbents, activated carbon Norit R 0.8 Extra (Norit N.V., The Netherlands) and molecular sieve (type 4A, Merck), were used to study tert-butylbenzene, cyclohexane, and water vapour breakthrough dynamics. Structural parameters of the carbon adsorbent were calculated from benzene vapour adsorption-desorption isotherms measured gravimetrically at 293 K using a McBain-Bakr quartz microbalance, and nitrogen adsorption-desorption isotherms recorded at 77.4 K using a Micromeritics ASAP 2405N analyzer described in detail elsewhere.22,24 Activated carbon Norit has a cylindrical... 

The measurement of ket for single electron-transfer reactions is of particular fundamental interest since it provides direct information on the energetics of the elementary electron-transfer step (Sect. 3.1). As for solution reactants, standard rate constants, k t, can be defined as those measured at the standard potential, E, for the adsorbed redox couple. The free energy of activation, AG, at E°a is equal to the intrinsic barrier, AG t, since no correction for work terms is required [contrast eqn. (7) for solution reactants] [3]. Similarly, activation parameters for surface-attached reactants are related directly to the enthalpic and entropic barriers for the elementary electron-transfer step [3],... 

Adsorbent activity function, proportional to adsorbent surface energy a = I for standard" absorbent Parameters defined by Hammers et al. (S5, 36) which measure importance of delocalization due to intramolecular effects (/3) or to site competion (y)... 

To use the isothenns, constants are empirically determined by running an experiment. This is done by adding increasing amonnts of the adsorbent to a sample of adsorbate solntion in a container. For each amonnt of adsorbent added, the equilibrium concentration [CJ is determined. The pairs of experiment trial values can then be used to obtain the desired parameter valnes from which the constants are determined. Once the constants are determined, the resnlting model is used to determine M, the amonnt of adsorbent (activated carbon) that is needed. From the derivation, the adsorption capacity of activated carbon is a = (XJM) i,. From this ratio, the absorption capacity of activated carbon is shown as the maximnm value of the XIM ratios. This ratio corresponds to a concentration equal to the maximum possible solute equilibrium concentration. 

Fig. 8.9 Plot of the fraction FA/rAm against the logarithm of adsorbate activity aa according to the Frumkin isotherm (equation (8.6.20)) with the interaction parameter ad/FAm equal to 0, 2, and 5.

Thermogravimetric (TGA) experiments have long established that some sites can be more easily reduced than others. The choice of support and activation parameters was found to influence the temperature of reduction with H2 [44,46,47]. CO can also be used to reduce the catalyst, and it can even be used to discriminate between sites [377], CO reduction yields highly coordinatively unsaturated Cr(II) [52,215,217-219,250-252,322-325,339,347]. If the catalyst is cooled in CO, Cr(II) strongly adsorbs CO, which irreversibly poisons the catalyst. In contrast, Cr(VI) does not adsorb CO. Thus by partial reduction of a catalyst in CO, followed by cooling it in CO, some Cr(II) sites are formed, which chemisorb CO and are subsequently inactive for polymerization. Other sites, more resistant to reduction, remain Cr(VI) and are therefore unaffected by the CO, becoming active for polymerization upon later exposure to ethylene. Thus, one can selectively poison only those sites that are most easily reduced as indicated in Scheme 15. 

Where a single activity scale can be set up for a class of adsorbents of given type, a value for a standard sample-solvent combination fully defines adsorbent activity. That is, V and a can be tabulated as functions of AT values for a standard sample and solvent. This concept is implicit in most previous standardization tests. Either a value is measured in a standard equilibrium or chromatographic system using the adsorbent to be tested, or (which is equivalent) the amount of sample taken up by the adsorbent from a standard solution is determined. Table 6-1 tabulates Af values (/ ,) for standard sample-solvent combinations for alumina and silica, so that the activity of these adsorbent types can be measured easily (assuming that a single-parameter adsorbent activity scale applies). A previous study (13) has shown that the concept of a single-parameter activity scale applies approximately to a number of aluminas described in the literature. 

Changes in K" with Changes in Adsorbent Activity or Water Content The basic equation for these calculations is Eq. (8-12) with Ag s = 0 [i.e., Eq. (6-6)]. Given that log R for the elution of 6-chloroquinoline from 3.7% H. O-AlaO by benzene is equal to 0.45, what is log for elution of this same compound by benzene from 1.7% H2O-AI2O3 From Table 6-1 we can interpolate the following adsorbent parameters ... 

C" values, 264 surface activity, 161-162 surface structure, 156-160 Va and a values, 136 Va and a values in TLC, 145 water deactivation of, 159, 162 Silica gel, see Silica Silicic acid, see Silica Silver ion, as adsorbent additive, 173 Single-parameter adsorbent activity scale, 149-152... 

C. Single-Parameter Adsorbent Activity Scales Symbols Used in Chapter 6. ... 

An earlier derivation of Eq. (9-2a) (8), where the term (—aja) was defined as e , assumes that the quantity is proportional to adsorbent surface activity a. This is not generally true, however. As we have seen in Chapters 6 and 7, a tends to change markedly with changes in adsorbent activation temperature or the addition of water to the adsorbent, corresponding to the appearance or disappearance of strong adsorbent sites. Dispersion interactions and are much less sensitive to changes in adsorbent surface structure than are selective interactions and a. The parameter exhibits much less variation with differences in adsorbent treatment than does a. This is illustrated in Fig. 9-2, where values of... 

The influence of air moisture is due to competition between water molecules and adsorbate. Activated carbon is generally useful at room temperature until a relative humidity of about 70 - 80 % for pollutant concentrations ranging from 1 to 1,000 mg m . However, the most important parameter is the ratio of molecule and water concentration. That is to say, a high humidity will inhibit the removal of solvent traces. In contrast, a high humidity will have no effect on high solvent concentration removal. 

In order to prevent auto-ignition of activated carbon adsorbers activated carbons are tested in standardized procedures for their tendency to self-heating. These tests deliver parameters, which include a number of properties of the tested material, but important properties like the permeability for gasflow, etc. are neglected. The field of velocities in the adsorber plays an important role too. The calculation results discussed show that it is necessary to take not only the parameters of the material but of the whole system into account. On the other hand the calculaton results show that REBOS is a efficient tool... 

The regression constants, R are in domain of 1 < R< 0.997. The next step in calculating the function cp(x) was the fitting of the Toth equation (387) to isotherms of ethane, ethylene, carbon dioxide, propane, and propylene measured on microporous adsorbents (activated carbons) at different temperatures below the critical one. These measured data are collected in Valenzuela and Myers handbook [19]. The adsorbents are activated carbons BPL, Nuxit, Columbia L, BPL-P, and Fiber Carbon KF-1500. AH isotherms are Type I without a multilayer plateau. The temperature domains are those shown in Fig. 44. The specific surface areas determined by the BET method, a (N2, 77 K) and the saturation pressures Pq ai collected in Ref 19. The parameters Kj-, and t are also known from the fitting procedure of the Toth equation (387). 

Another limitation of tire Langmuir model is that it does not account for multilayer adsorption. The Braunauer, Ennnett and Teller (BET) model is a refinement of Langmuir adsorption in which multiple layers of adsorbates are allowed [29, 31]. In the BET model, the particles in each layer act as the adsorption sites for the subsequent layers. There are many refinements to this approach, in which parameters such as sticking coefficient, activation energy, etc, are considered to be different for each layer. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

Active solids are widely used as adsorbents of gases and vapours, and the specific surface is the most important parameter for characterizing their... 

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