Energetics of adsorption

The data showed that, as the partial pressure of n-butane increased, the 3600 cm  

For propane, n-pentane and n-hexane the differential heats of adsorption over FER dropped more rapidly, right after 1 molecule was adsorbed per Bronsted acid site. Similar results were obtained with TON. In contrast, with MOR and FAU the drop in the differential heats of adsorption for n-alkanes occurred at lower coverages, indicating that only a certain fraction of the Bronsted acid sites were accessible to the adsorbing alkane probe molecules. With MFI the drop did not occur until 2 molecules of n-alkane were adsorbed per Bronsted acid site, suggesting perhaps a higher stoichiometry of about two n-alkanes per Bronsted acid site. In the cases of i-butane and i-pentane the drop occurred around one alkane per Bronsted acid site. Finally, n-butane adsorption isotherms measured over TON framework type catalysts having three different A1 contents (Si/Al2 = 90, 104, 128) showed Henry coefficients to increase with increase in the A1 content [5], Based 

As we have seen, an adsorption isotherm is one way of describing the thermodynamics of gas adsorption. However, it is by no means the only way. Calorimetric measurements can be made for the process of adsorption, and thermodynamic parameters may be evaluated from the results. To discuss all of these in detail would require another chapter. Rather than develop all the theoretical and experimental aspects of this subject, therefore, it seems preferable to continue focusing on adsorption isotherms, extracting as much thermodynamic insight from this topic as possible. Within this context, results from adsorption calorimetry may be cited for comparison without a full development of this latter topic. 

The approach we follow is essentially that used to derive the Clapeyron equation (Atkins 1994). Suppose we consider an infinitesimal temperature change for a system in which adsorbed gas and unadsorbed gas are in equilibrium. The criterion for equilibrium is that the free energy of both the adsorbed (subscript s) and unadsorbed (subscript g) gas change in the same way  

The following equations may be written for these two quantities if the temperature change is assumed to cause no change in the amount of adsorbed material  

Substituting Equations (85) and (86) into Equation (84) and rearranging gives 

In writing this last result, it has been explicitly noted that the number of moles of adsorbed gas ns is constant. If the process under consideration is carried out reversibly, Sg — Ss may be replaced by qsl/T, where qst is known as the isosteric (the same coverage) heat of adsorption  

The measurement and interpretation of the thermodynamic function of the adsorbed phase, such as the free energy of adsorption, the enthalpy of adsorption, and the entropy of adsorption have been the subject matter of large number of investigations. These functions have been evaluated using adsorption isotherms and are compared with those obtained from theoretical considerations. In all 

The thermodynamic functions of the adsorbed phase are a function of pressure, temperature, and concentration of the adsorbed molecules on the surface of the solid so that it is essential to distinguish between the molar and partial molar properties. As the derivation of the thermodynamic functions is beyond the scope of this chapter, we shall only define the functions and discuss only the calculations of the enthalpy of adsorption or the heat of adsorption. 

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This chapter introduces the topic of adsorption, giving examples of both physical adsorption and chemical adsorption, and discusses the similarities and differences between the two. The standard nomenclature of surface science is given from within this context. The energetics of adsorption are explained in terms of the enthalpies of bond formation A/Tadsr Next, isotherms are discussed. 

In addition, (-potential measurements on PCP showed no significant effect of ionic strength on adsorption characteristics [123]. This finding suggests that no ion pair is formed, but also indicates that screening of the membrane surface by ions has little effect on the energetics of adsorption, i.e. PCP is buried at some depth below the membrane surface. 

Fubini, B., Della Gata, G. and Venturello, G., 1978. Energetics of adsorption in alumina-water systems. Microcalorimetric study on the influence of adsorption temperature on surface processes. 3. of Colloid and Interface Sci., 64 470. 

The majority of adsorbents applied in industry has porous sizes in the nanometer region. In this pore-size territory, adsorption is an important method for the characterization of porous materials. To be precise, gas adsorption provides information concerning the microporous volume, the mes-opore area, the volume and size of the pores, and the energetics of adsorption. Also, gas adsorption is an important unitary operation for the industrial and sustainable energy and pollution abatement applications of nanoporous materials. 

The study of zeolites as adsorbent materials began in 1938 when Professor Barrer published a series of papers on the adsorptive properties of zeolites [28], In the last 50 years, zeolites, natural and synthetic, have turned out to be one of the most significant materials in modem technology [27-37], Zeolites have been shown to be good adsorbents for H20, NH3, H2S, NO, N02, S02, C02, linear and branched hydrocarbons, aromatic hydrocarbons, alcohols, ketones, and other molecules [2,31,34], Adsorption is not only an industrial application of zeolites but also a powerful means of characterizing these materials [1-11], since the adsorption of a specific molecule gives information about the microporous volume, the mesoporous area and volume, the size of the pores, the energetics of adsorption, and molecular transport. 

In any investigation of the energetics of adsorption, a choice has to be made of whether to determine the differential or the corresponding integral molar quantities of adsorption. The decision will affect all aspects of the work including the experimental procedure and the processing and interpretation of the data. 

As was noted earlier, the level of surface OH concentration has very little effect on either the isotherms or the energetics of adsorption of non-polar molecules such as argon or the alkanes. Specific interactions become significant when the adsorptive... 

The effect of surface dehydroxylation of a mesoporous silica on the Ar and N2 energetics of adsorption is illustrated in Figure 10.12. In the work of Rouquerol et al. (1979b) Tian-Calvet microcalorimetry was used to determine the variation of the differential enthalpy of adsorption as a function of surface coverage. Although strong... 

Inspection of Table 11.3 reveals that there are relatively small differences between the corresponding values of H and E0 for NaY and HY. This is to be expected since the adsorbent-adsorbate interactions are essentially non-specific (see Chapter 1). Decationization of zeolite Y thus has a minimal effect on the energetics of adsorption of the paraffins. The molecular shape of the adsorptive is also unimportant. In accordance with the results in Figures 1.S and 1.6, the molar mass (number of carbon atoms) is much more important than the molecular shape. As before, there is a linear relation between E0 and Nc. An exponential increase of kH with Nc is of course consistent with the form of Equation (4.3). 

In the work of Schirmer et al. (1980), a Tian-Calvet type microcalorimeter was used to determine the energetics of adsorption for n-hexane, cylohexane and benzene on NaY zeolite. The differentia] adsorption energies for n-hexane and benzene are plotted in Figure 11.17 as a function of the amounts adsorbed. 

There are a number of different factors which may affect the level of uptake and the energetics of adsorption from solution the chemistry and electrical properties of the solid surface and the molecular/micellar/polymeric structure of the solution must all be taken into account. Whenever possible, a study of both adsorption isotherms and enthalpies of displacement is worthwhile, but it is often necessary to complement these measurements with others including electrophoretic mobilities, FI7R spectra-and various types of microscopy. 

Terzyk A.P, Rychlicki G., Empirical relationship describing energetics of adsorption at low coverages of macroporous carbons, J. Thermal Anal., 55 (1999) pp 1011-1020. 

The spectrum of adsorption mechanisms is wide and depends on the specific properties of a given adsorption system. It comprises induced dipolar or polarization effects, hydrogen bond formation, acid-base affinity, and other interactions lying somewhere between the strictly nonpolar dispersion and ion-ion coulombic forces. They can alter the extent and mode of the process. For example, if the adsorbate contains an electron-rich group and the solid support has strongly polarizing sites, attraction between them markedly enhances the energetics of adsorption [77]. 

There are several physical factors affecting the energetics of adsorption of ionic surfactants onto adsorbents with oppositely charged sites as well as its effectiveness, i.e., the amount adsorbed at surface saturation. The role of several important parameters, like the overall charge density, pH and temperature of the aqueous phase and the spatial distribution of surface charge (i.e., topography), has been discussed in the previous paragraphs. 

The theoretical treatments of other electrocatalytic reactions are very limited. Even semiquantitative treatments are important since they provide insight as to the role of adsorption sites and surface interactions involving reactants, intermediates, and/or products. Of special interest are theoretical treatments of the energetics of adsorption on various sites using molecular orbital and X- scattered wave calculations in combination with experimentally evaluated adsorption isotherms and in situ spectroscopic measurements on single-crystal electrode surfaces. 

The differential enthalpy of adsorption, also called isosteric heat of adsorption, is often used to characterize the energetics of adsorption phenomena. It depends upon the properties of the adsorbent and the adsorbate and reflects the magnitude of gas-solid interactions. It can be measured by calorimetry or calculated from adsorption isotherms at constant coverage using the expression ... 

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