Adsorption of Gases on Solid Surfaces

The most important solid surface property is its interaction with gases or liquids. The adsorption of a gas on a solid surface has been known to be of very much important in various systems (especially in industry involved with catalysis, etc.) (Birdi, 2009 Bonzel, 2014 Do, 1998). The gas-solid surface phenomena can be analyzed as follows. 

The molecules in gas are moving very fast, but on adsorption (gas molecules are more or less fixed), there will be thus a large decrease in kinetic energy (thus decrease in entropy, AS). 

Movement of gas molecules In gas phase is much larger distances than when adsorbed on a solid surface. 

Adsorption takes place spontaneously, which means that is negative, which indicates that AHjj is negative (exothermic adsorption)  

The adsorption of gas can be of different types. The gas molecule may adsorb on the surface of a solid (as a kind of condensation process). It may under other circumstances react with the solid surface (chemical adsorption or chemisorption). In the case of chemoadsorption, one almost expects a chemical bond formation. On carbon while oxygen adsorbs (or chemisorb), one can desorb CO or CO2. The experimental data can provide information on the type of adsorption. On porous solid surfaces, the adsorption may give rise to capillary condensation. This indicates that porous solid surfaces will exhibit some specific properties. The most adsorption process in industry one finds in the case of catalytic reactions (e.g., formation of NH3 from N2 and H2). 

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The adsorption of gases on solid surfaces proceeds to such an extent that approximately 10 7 gr. is present per cm.2 in the equilibrium state. This is of the same order of magnitude as the strength of the limiting capillary layer of a liquid ( 184), hence it is not improbable, as suggested by Faraday (9) (1884), that the adsorbed gas is sometimes present in the liquid state. The adsorbed amount increases with the pressure and diminishes with rise of temperature. The first effect does not follow a law of simple proportionality, as in the case of the absorption of gases by liquids, rather the adsorbed amount does not increase so rapidly, and the equation ... 

In considering the adsorption of gases on solid surfaces we will suppose that m gr. adsorbent take up x c.c. gas, reduced to N.T.P. Then, since the solid adsorbent has a spongy structure,... 

Adsorption from solution is discussed by Adamson, but with emphasis on the equilibrium aspects, rather than the kinetics. The subject can conveniently be divided into adsorption of non-electrolytes and adsorption of electrolytes. The former can be treated for dilute solutions, in a similar manner to adsorption of gases on solid surfaces. Multilayer adsorption has been observed however, so that... 

The available literature on the entropy of adsorption of gases on solid surfaces is almost exclusively devoted to the experimental measurement of Aa< sS in real systems to obtain information about the adsorbed state and about the structure of the surface. Only scarce attempts have been made to calculate or simulate (by Monte Carlo techniques) the entropy change for more or less real situations, which differ so much from the two idealized models of adsorption described in Sect. 5.1.1.1. 

Adsorption of gases on solid surfaces whereby the bonding is by means of a weak intermolecular (van der Waals) attraction rather than by chemical bonding. 

The most important step in the study of adsorption came with a derivation by Brunauer, Emmett and Teller for the multilayer adsorption of gases on solid surfaces [22]. The multilayer adsorption theory, known generally as the BET equation, has occupied a central position in gas adsorption studies and surface area measurement ever since. 

The adsorption of gases on solids can be classified into physical and chemical adsorption. Physical adsorption is accompanied by a low enthalpy of adsorption, and the adsorption is reversible. The adsorption/desorption characteristics are in these cases often described by adsorption isotherms. On the other hand, chemical adsorption or segregation involves significantly larger enthalpies and is generally irreversible at low temperatures. It is also often accompanied by reconstruction of the surface due to the formation of strong ionic or covalent bonds. 

This isotherm finds use mainly in the study of the adsorption of gases on solids however, it can be useful in the study of adsorption of pollutants from aqueous systems, particularly onto solid phases. The heterogeneous nature of a solid surface (i. e., soils, sediments, suspended solids) would obviously invalidate the first assumption (i.e., a, above) used in developing the relationship. The third assumption (i. e., c, above) also would be invalid in a situation where one is dealing with multi-layer adsorption. 

The Langmuir equation (Eq. 5.1), derived originally to describe the adsorption of gases on solids, assumes that the adsorbed entity is attached to the surface at specific, homogeneous, localized sites, forming a monolayer. It is also assumed that the heat of adsorption is constant over the entire monolayer, that there is no lateral interaction between adsorbed species, that equilibrium is reached, and that the energy of adsorption is independent of temperature ... 

Physical adsorption of gases on solids is virtually always enthalpically driven (Att< 0). Entropically driven adsorption can exist but usually the entropy of molecules on a surface is much lower than in the gas phase. Vibrational, rotational, and also translational degrees of freedom are restricted on surfaces. 

The London forces are predominant in the adsorption of gases on solid substances such as carbon. In the adsorption on ionic lattices such as salt layers (CaF2 in electric lamps), silicic acid and aluminium oxide the adsorption of the first layer however depends mainly on polarization by electrostatic forces, in which isolated ions, corners and edges will give a larger heat of adsorption than a perfect crystal surface. In adsorption in multimolecular layers the Van der Waals-London energy is, however, predominant as in the cohesion energy. 

For the physical adsorption of gases on solids the attraction between the molecules and the surface is almost the exclusive driving force. Thermodynamically this means that such gas adsorption is exothermic. Usually the enthalpy of adsorption per molecule depends on 0 because of heterogeneity (upon filling an adsorbent with adsorbate the "highest energetic" parts are covered first) and because, with increasing 0, lateral interaction also increases (this contribution may be attractive or repulsive). 

The determination of the adsorption of gases on solids can be a time consuming matter. Already in 1969 Jantti suggested to measure three points of the initial course of the kinetic adsorption curve and to extrapolate the equilibrium value (3PM). When the specific molecular model of the adsorption of a gas on a solid surface is known and when it can be expected that only one kind of adsorption is at stake, this method delivers good results and allows a very fast stepwise measurement of adsorption isotherms-. ... 

The extent of adsorption of gases onto solid surfaces can be determined experimentally using a wide variety of apparatus and techniques, and the literature on this subject is extensive. In general, measurements fall into one of two categories either the volume of the gas adsorbed is determined manometrically, or gravimetric methods are used, where the mass adsorbed on the solid is determined directly. 

The total surface area is calculated from the amount of physical adsorption of nitrogen at 77 K. During the thirties Brunauer, Emmett, and Teller [1,2] presented a theory dealing with the multilayer adsorption of gases on solids. They assumed that the first layer of gas molecules is adsorbed more strongly than subsequent layers, and that the heat of adsorption of subsequent layers is constant. They also assumed the absence of lateral interaction between adsorbed molecules. On the basis of these much criticized assumptions they derived an adsorption isotherm, which describes the experimentally determined adsorption isotherms excellently. From the adsorption isotherm a value corresponding to the volume of the adsorbed monolayer is calculated. With physical adsorption the amount of gas adsorbed is usually plotted as a function of the relative pressure, that is the pressure... 

The mathematical models that have been applied to the physical adsorption from liquid solutions are generally extensions of the theories that have been developed to describe the sorption of gases on solid surfaces with modifications to account for the competition between the solute and solvent for the adsorption sites. Two of these models have been applied to the adsorption isotherms of nonelectrolytes from solution they are the Langmuir model and the Brunauer, Emmett, and Teller (BET) model in addition the Freundlich empirical equation has also been used. In the Langmuir model it is assumed that the adsorbed species forms a monolayer on the surface of the adsorbent, that the adsorbed molecules... 

Both the BET and the Duhinin models are widely thought to adequately describe the physical adsorption of gases on solid carbons. BET surface areas from many microporous carbons range from 500 to 1500 m g . However, values of up to 4000 m g" are found for some super-activated carbons and these are unrealistically high. 

The Langmuir equation was originally developed to describe adsorption of gases on the surfaces of solids. It has been frequently used to describe phosphate adsorption onto solid surfaces. This equation was derived with the following assumptions ... 

It is also possible to perform kinetic analysis of reactions involving adsorption of gases on solids by representing the fraction of the surface-covered means of the Fremdlich isotherm,... 

Prior to 1910 many different theories of adsorption had been proposed, but none of them had been very successful. In most of these theories the increased concentration of the adsorbed substance near the surface was thought to be analogous to the retention of the earth s atmosphere by the gravitational attraction of the earth. An adsorbed gas was thus regarded as a kind of miniature atmosphere extending out a short distance from a solid substance. In general such theories were called upon to account only for qualitative aspects of the adsorption of gases on solids [10]. 

The only gas chromatographic method used for the measurement of diffusion coefficients of gases on solid surfaces is the RF-GC technique validating a recent mathematical analysis, also permitting the estimation of adsorption and desorption rate constants, local adsorbed concentrations, local isotherms, local monolayer capacities, and energy distribution functions." The RF-GC technique has been successfully applied for the time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species of O2, CO, and CO2 on heterogeneous surfaces of Pt/Rh catalysts supported on Si02." All calculations for the... 

Fan, L.T., et al The mtister equation for linear adsorption and desorption of gases on solid surfaces, Chem. Eng. Commun., 108. 127-146(1991). 

In the following, we consider the equilibrium and kinetics of adsorption of surfactants at the air-water interface on the basis of Langmuir s theory of adsorption of gases on solids. According to Langmuir s theory, it is assumed that the adsorption surface consists of sites, which can be occupied by adsorbed molecules. These sites correspond to the minimum of surface free energy. At achieving the balance between the adsorbed molecules and molecules of gas, only some parts of the potentially available adsorption sites are occupied by gas molecules. This part is equal to 0, and the total number of molecules Na adsorbed on the surface obeys the ratio... 

Adsorptions not described by Langmuir s theory especially the physical multilayer adsorption of gases on solids can be best described by the BET (Bmnauer-Emmett-Teller) theory discussed next. BET can also be written in a linear form, for reduced pressures (P/Po) up to about 0.4, and yield values of the monolayer coverage and of the specific solid surface area. The BET theory is a very useful tool in describing physical adsorption, although it does... 

Adsorption from solution on solids is, in some respects, more complex than adsorption of gases. There are several complications related to the adsorption of solutes on solid surfaces. The most important one is the role of the solvent. For example, the adsorption of stearic acid on carbon black reaches different limiting values in different solvents ranging from 10 mmol kg when benzene is the solvent up to more than 40 mmol kg when cyclohexane is the solvent. The stearic acid is probably adsorbed with the acyl chain parallel to the surface and the measured adsorption values are consistent with that arrangement. The role of the solvent is indeed very important. 

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