Adsorption isotherms, distinguishing

In order to accurately determine the chemisorbed amount from the overall adsorption isotherm, the sample can be further outgassed at the same temperature to remove the physically adsorbed amount, after which a new adsorption procedure is carried out to obtain isotherm II. The difference between the first and second isotherm gives the extent of irreversible adsorption ( ) at a given temperature (Figure 13.5b), and can be considered as a measurement of the amount of strong sites in the catalyst. However, in the first approximation, the magnitude of the heat of adsorption can be considered as a simple criterion to distinguish between physical and chemical adsorption. 

Adsorption isotherms are used to quantitatively describe adsorption at the solid/ liquid interface (Hinz, 2001). They represent the distribution of the solute species between the liquid solvent phase and solid sorbent phase at a constant temperature under equilibrium conditions. While adsorbed amounts as a function of equilibrium solute concentration quantify the process, the shape of the isotherm can provide qualitative information on the nature of solute-surface interactions. Giles et al. (1974) distinguished four types of isotherms high affinity (H), Langmuir (L), constant partition (C), and sigmoidal-shaped (S) they are represented schematically in Figure 3.3. 

Figure 1.21. Distinguishing between localized (drawn lines) and mobile (dashed curves) adsorbates. Figure (a), adsorption isotherms. Figure (b), two-dimensional equations of state. In figure (a) K[ =Kv=100 m N figure (b) is independent of K. The lateral interaction parameter (in units of kT] is indicated.

Three regions can be distinguished, indicated in the schematic adsorption Isotherms in fig. 5.8. 

The distinguishing feature of dehydrated zeolites as microporous aluminosilicate adsorbents lies in the presence in their voids—i.e., micropores—of cations. These cations compensate excess negative charges of their aluminosilicate skeletons. The cations form, in the zeolite micropores, centers for the adsorption of molecules with a nonuniform distribution of the electron density (dipole, quadrupole, or multiple-bond molecules) or of polarizable molecules. These interactions, which will be called, somewhat conventionally, electrostatic interactions, combine with dispersion interactions and cause a considerable increase in the adsorption energy. As a result, the adsorption isotherms of vapors on zeolites, as a rule, become much steeper in the initial regions of equilibrium pressures as compared with isotherms for active carbons. 

In interfacial science the term monolayer is used in a number of different ways. Although this rarely leads to confusion one needs to be aware of them. In the chapters on adsorption on solids (chapters II. 1 and 2) the notion of monolayer was sometimes used to distinguish it from bilayer or multilayer, and implying that all adsorbed molecules are in contact with the adsorbent. Whether or not this layer is completely filled does not matter in that case. However, the packing did matter in other instances where the amount adsorbed in a completely filled layer was at issue, as for example in the plateau of the Langmuir adsorption isotherm (r(< )), or in the volume in the BET theory, corresponding to the volume of gas that would... 

Figure 2 presents the CO2 adsorption isotherms obtained at 273 K for samples of series CS (a) and CW (b). The amount of carbon dioxide adsorbed increases, for both series, with bum-off. Isotherms are rather similar for samples with low burn-off (CS-2 to CS-8 on one hand, CW-1 and CW-2 on the other) what makes it difficult to distinguish them only with these measurements. When plotted in Dubinin-Radushkevich (D-R) coordinates, these isotherms become straight lines as corresponds to samples with a narrow and uniform homogeneous microporosity. The micropore volumes of the different samples, obtained by application of the D-R equation to the CO2 adsorption data are reported in Table 1. Micropore volumes increase from 0.22-0.23 cm g- in the less activated CMS to 0.32 cm g- in samples with the highest burn-off in each series although, due to the different reactivity of the starting materials, they are obtained after very different activation times (16 h for CW series and 70 h for CS series). 

The difference between initial and final pH is often substantial and these two quantities have to be clearly distinguished. It is certainly easier to carry out experiments at constant initial pH than at constant final pH, but significance of adsorption isotherms at constant initial pH is questionable. In some publications initial and final pH values are reported, but only the final value is given in Tables 4.1, 6. In a few publications only the initial pH is reported. 

From the point of view of gas adsorption, which is the main objective of this chapter, there are no differences in the results obtained between the ACFs and the granular and powder ACs, except for the kinetics of gas adsorption due to the special pore structure of the ACF [2, 40, 41] and for the higher packing density that can be obtained with them due to their fiber shape [3]. In this way, although there are important differences in the pore structure and distribution of porosity among the ACFs and the conventional ACs (this aspect will be described in a next section), the adsorption isotherms are not sensitive to them and do not allow to distinguish the shape of the porous carbons (i.e., fiber, granular, powder, monohth). 

As an example of the above statement. Fig. 17.3 contains the Nj adsorption isotherms for powder AC vidth different adsorption capacities [3]. These isotherms, compared with those in Figs. 17.1 and 17.2, clearly demonstrate that the adsorption isotherms do not permit neither to distinguish the ACF from the AC nor to deduce differences in the pore size distribution. However, the unique fiber shape and porous structure of the ACF are advantages that permit to deepen into the fundamentals of adsorption in microporous solids [31]. ACFs are essentially microporous materials [13, 31], with sht-shaped pores and a quite uniform pore size distribution [42, 43]. Thus, they have simpler structures than ordinary granulated ACs [31] and can be considered as model microporous carbon materials. For this reason, important contributions to the understanding of adsorption in microporous solids for the assessment of pore size distribution have been made using ACF [31, 33, 34, 39, 42-46], which merit to be reviewed. 

The approach to proton adsorption from aqueous solution must be different from the approach to adsorption of other solutes, because water molecules can provide or absorb a practically unlimited number of protons (higher by several orders of magnitude than the concentration of any other species in solution and the concentration of surface sites) to balance the changes induced by adsorption. Thus, adsorption isotherms based on the concept of a distribution of a limited amount of adsorbate molecules between solution and surface are not applicable. Most authors accept this obvious fact, but a few others have used the same formalism for proton adsorption as is used for other solutes. For example, in [205], the surface charging of alumina is discussed in terms of adsorption isotherms (amount adsorbed vs. equilibrium concentration). Positive adsorption of protons is equivalent to negative adsorption of OH , and vice versa. In adsorption experiments, uptake of protons and release of OH cannot be distinguished. Only the net result of uptake/releasc of H and OH can be obtained, and independent curves of 11 and OH adsorption reported in the literature [206,207] must be based on measurements of other quantities. 

A plot of F versus C2 gives the adsorption isotherm. Two types of isotherms can be distinguished a Langmuir type for reversible adsorption of surfactants (Figure 18.17) and a high-affinity isotherm (Figure 18.18) for the irreversible... 

Physical chemists distinguish between adsorption and absorption. Adsorption is a surface phenomenon. Consider a solid or liquid phase (the adsorbent), in contact with another, fluid, phase. Molecules present in the fluid phase may now adsorb onto the interface between the phases, i.e., form a (usually monomolecular) layer of adsorbate. This is discussed in more detail in Section 10.2. The amount adsorbed is governed by the activity of the adsorbate. For any combination of adsorbate, adsorbent, and temperature, an adsorption isotherm can be determined, i.e., a curve that gives the equilibrium relation between the amount adsorbed per unit surface area, and the activity of the adsorbate. Powdered solid materials in contact... 

Structural perturbations are to be distinguished from what are essentially phase changes. The perturbations represent relatively minor deviations from bulk liquid, and adsorption isotherms are usually nearly invariant if x is plotted against P/P°. That is, the heat of adsorption from the vapor phase is very close to that of condensation. By contrast, and as an example, in the case of water on PE, while this near invariance in isotherm shape holds around 20°C, the isotherms at -9°C and -24°C are very flat, with an Xq of only about l.sX. It would appear that the adsorbed film... 

Brunauer, Deming, Deming, and Teller [3] later distinguished five different physical adsorption isotherms. The Type I adsorption isotherm is characteristic of chemisorption, for which the first layer is adsorbed much more strongly than subsequent layers. The type II isotherm is characteristic of the multilayer adsorption exhibited with physical adsorption near the boiling point of the adsorbate. Type III isotherms are obtained for multilayer physical adsorption with condensation of the adsorbate in narrow pores whereas Type IV isotherms are obtained when the first layer is adsorbed with a lower heat than the heat of condensation of the adsorbate. Finally, Type V isotherms are characteristic of adsorption according to Type IV on an adsorbent with narrow pores. 

Brunauer-Emmett-Teller Isotherm (BET Isotherm) An adsorption isotherm equation that accounts for the possibility of multilayer adsorption and different enthalpy of adsorption between the first and subsequent layers. Five types of adsorption isotherm are usually distinguished. These are denoted by roman numerals and refer to different characteristic shapes. See Adsorption Isotherm. 

The described results demonstrate very well how the adsorption kinetics allows to distinguish between the contributions of different compounds of a mixture to the measured surface tension change. Using the correct adsorption model (adsorption isotherm and equation of state) a quantitative description of the adsorption kinetics of mixtures would be possible, however, no programmes are available at present and analytical solutions do not exist. 

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