Adsorption Enthalpy on Homogeneous Surface

Adsorption (desorption) energies or enthalpies of molecules and atoms on various surfaces are of primary and major interest in the experimental gas-phase radiochemical studies of the heaviest elements. In practice, pertinent data can be obtained almost exclusively in the experiments based on chromatographic principles. In the pioneering works [1-3] the required values were derived using the simplest description of the processes in columns in terms of molecular kinetics (see Sect. 4.2). Later [4] the task of finding the adsorption enthalpies was examined using a thermodynamic approach. It revealed that the molecular-kinetic treatment 

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For a better understanding of catalysis we shall now derive the Langmuir equation on the basis of chemisorption on the active centers of the catalyst. Langmuir assumed the simple case of an energetically homogeneous catalyst surface, so that the adsorption enthalpy is independent of the degree of coverage of the surface 6a-... 

Many reactions take place on solid surfaces, and solid materials catalyse such reactions. Surface catalysis is very important in industry, and makes important contributions to some atmospheric reactions. Molecules will be adsorbed on solid surfaces through physicosorption in which the attractive force between the reactant molecules and the surface is of the van der Waals type. Enthalpy of adsorption, AHgj, of physicosorption is relatively small, about -40 kj moH. With chemical sorption the value for the enthalpy of adsorption is large because a strong bond is formed between the chemical molecule and the surface. Catalytic reactions can be classified as homogeneous catalysis or heterogeneous catalysis. 

These theoretical considerations of the thermochromatographic process presume that the adsorption entropy and enthalpy do not depend on the temperature. It was also postulated that the adsorbent was homogeneous, its surface was not saturated with the adsorbate (monolayer or less), and the carrier (reagent) gas was unsorbable. Diffusion in the solid phase (adsorbent) and surface diffusion were ignored. Furthermore, in the theoretical considerations the effect of the carrier (reagent) gas pressure on the substance transport was not taken into account, which, however, should be considered in the case of TC at reduced reactant gas pressures and vacuum TC or with densely filled columns. 

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