Adsorption Enthalpies and Entropies

Klier et al. further scrutinized their model to determine whether the temperature dependences of the various adsorption equilibrium constants would yield reasonable values of adsorption enthalpies and entropies. These thermodynamic entities, calculated from data in Table IX, are summarized... 

The adsorption of the homologous alkane series on zeolites and other adsorbents has been extensively studied by many research groups. These studies demonstrate the existence of linear relationships between adsorption enthalpy and entropy, and the carbon number [1-4]. Contrarily, there are few studies dealing with the adsorption of alkanes in liquid phase. This can be explained by the lack of selectivity effects that occur in the adsorption of alkanes on adsorbents in liquid phase. Indeed, classical stationary phases for HPLC show no separation of alkane mixtures, as a result of the rather weak interactions between the molecules and the force field exerted by the surface of the amorphous material. 

The 10-membered ring zeolites (ZSM-22 and ZSM-23) were kindly provided by Prof. Martens (COK, KULeuven). Both of the zeolites have unidimensional pore structures without any intersection. The crystals are needle-like shaped for both materials. Zeolite ZSM-22 (belonging to the TON fhmily) has free pore dimensions of 0.44 X 0.55 nm and zeolite ZSM-23 (MTT fiimily) has free pore diameters of 0.45 X 0.42 nm. The framework structures are sketched in Figure 2. The low coverage adsorption properties were determined with the pulse chromatographic technique. The details of the experimental method are discussed elsewhere. The Henry constant was determined from the first moment of the response curve on the TCD detector alter injection of an alkane trace. Adsorption enthalpy and entropy were obtained by fitting the temperature dependence of the Henry constant to the van t Hoff equation. 

Unfortunately, in the continuous on-line experiments with transactinoid elements, the application of the above Second Law and related procedures has not been possible because of the very poor statistics of detected decays. Measurements of this type have been done only with long-lived lighter homologs of the transactinoids in batch tests. Packed columns and low flow rates of the carrier gas were employed to achieve narrow peaks. Major contribution came from Bachmann and co-workers. In Refs. [6,9-11] they measured the adsorption enthalpies and entropies on solid... 

Elements 108-116 are expected to be partially very noble metals. Thus, their electrochemical deposition on suitable electrode materials from aqueous solutions could be an attractive method for their isolation. It is known that the potential associated with the electrochemical deposition of radionuclides in metallic form from solutions of extremely small concentration is strongly influenced by the choice of the electrode material. This is reproduced by Eichler and Kratz (2000) in a model in which the interaction between the microcomponent A and the electrode material B is described by the partial molar adsorption enthalpy and entropy. By combination with the thermodynamic description of the electrode process, a potential is calculated that characterizes the process at 50% deposition ... 

The adsorption of dihydrogen onto the zeohtes Na-ZSM-5 and K-ZSM-5 rendered the fundamental H-H stretching mode as IR active [05A1]. The corresponding IR adsorption bands were fonnd at 4101 and 4112 cm for H2/Na-ZSM-5 and H2/K-ZSM-5, respectively. By means of variable-temperature IR spectroscopy, the standard adsorption enthalpy and entropy were determined. 

Testing the fit of the rate equation to the experimental data and calculating the confidence interval of the parameters should be part of any kinetic modeling study but it is not sufficient yet. With the mechanistic insight incorporated to a maximum extent into the models, the parameters of the latter should satisfy well-established physicochemical laws. As mentioned already, the rate coefficients have to obey the Arrhenius temperature dependence. Boudart et al. [1967] also derived constraints on the adsorption enthalpies and entropies which are too often overlooked. 

Interpretations can be offered on two levels, phenomenological or in terms of a model. The former usually rests on thermodynamics and generally helps to establish generally valid conclusions. Moreover, by establishing the signs of the adsorption enthalpy and entropy A S, basic information on the driving forces may be extracted. Having established these. 

Rudolph, J., Bachmann, K. Determination of adsorption enthalpies and entropies by on-line-gas chromatography with short-lived nuclides. Radiochim. Acta 27, 105-108 (1980)... 

Tsalas, S., Bachmann, K., Heinlein, G. The application of non-analytical radio gas-chromatography for the determination of adsorption enthalpies and entropies of inorganic bromides. Radiochim. Acta 29, 217—221 (1981)... 

Abstract This chapter is devoted to the study of coadsorption of gases in nanoporous solids by using the differential calorimetry. In the first part, the thermodynamic principles of adsorption of gases are recalled. Some of them have already presented in chapter one. However a special attention has been paid here to the determination of the adsorption enthalpies and entropies and we focused on the selective adsorption of binary mixtures. Then the specific experimental technique based on the combination of differential calorimetry with manometry and gas phase chromatography or mass spectrometry is shown in details. In the last part, the thermodynamic concepts on coadsorption are illustrated with experimental results taken from studies on gas separation by selective adsorption in mlcroporous solids. 

Calculation of Adsorption Enthalpy and Entropy from Single Adsorption Isotherms... 

The adsorption enthalpie and entropie of single component can be extracted from adsorption data obtained under isothermal conditions by using two different methods. The first one is based on the van t Hoff equation and the second one on the Clausius-Clapeyron equation. These methods only need a set of adsorption isotherms measured at various temperatures. 

Thus, by plotting the curve LnA (r) = f(l/T) it is easy to determine the adsorption enthalpy and entropy from the slope and the intercept of this straight-line (Fig. 7.1 la). In this method we have to keep in mind that we consider that the adsorption enthalpy and entropy are constant i.e. independent on the temperature and the adsorbed... 

Fig. 7.11 Grapliic determination of adsorption enthalpy and entropy from isothermal adsorption data measured at various temperatures, a Van t Hoff method applied to the adsorption equilibrium constant deduced from the parameter of a thermodynamic model like Henry or Langmuir fitting well the adsorption isotherms, h Clausius-Clapeyron or isosteric method the isosters he below a limit Une, which corresponds to the liquefaction equihbrium of the adsorptive...

Thus the adsorption enthalpy and entropy can be determined by plotting Ln ( ) =... 

In the case of multi-components adsorption, the partial molar differential adsorption enthalpies and entropies of each component i present in the gas mixture cannot be directly measured by experiments. However, it is possible to estimate them by mean of the tangent method based on the well-known Gibbs-Duhem relation. 

---