Adsorption of gases on solids

Adsorption isotherms, surface tension-concentration equations and two-dimensional equations of the surface pressure against area are all concepts that are unified or linked via the Gibbs adsorption equation. Equation 4.7. The interrelations are presented in Table 7.1 (fcg is the Boltzmann eonstant). 

The quality of the fit to the data is important but ean-not serve alone as a eriterion for selecting the best 

We start our discussion with the adsorption of gases on solids, which can be either of physical or chemical nature, and both are hereafter briefly discussed. Table 7.2 summarizes the differences between physical and chemical adsorption. The physical multilayer adsorption is often faster than the chemical adsorption but with a low heat of adsorption, 10 kcal mol ver- 

Surface tension-concentration Adsorption isotherm Two-dimensional equation 

and kc are rate constants for desorption (evaporation) and condensation respectively. The sites on die solid at which adsorption takes place were denoted as active sites by Langmuir. 

Because of the much shorter wavelength of elecuon beams, the Ewald sphere becomes practically planar in elecU on diffraction, and diffraction spots are expected in this case which would only appear in X-ray diffraction if the specimen were rotated. 

When a ledge is formed on an atomically smooth monolayer during tire formation of a thin film the intensity of the diffraction pattern is reduced due to the reduction in the beatrr intensity by inelastic scattering of electrons at the ledge-monolayer junction. The diffraction intensity catr thus be used during deposition of several monolayers to indicate the completion of a monolayer through the relative increase in intensity at tlris time. Observation of this effect of intensity oscillation is used in practice to count the number of monolayers which are laid down during a deposition process. 

B Surface vacant site C Single vacancy kink D Adatom E Kinked ledge F Terrace 

Adsorbed molecules are more strongly held at the sites where the weakest metal-metal bonding is to be found, and these conespond to the active sites of Langmuir. A demonstration of this effect was found in smdies of the adsorption of H2S from a H2S/H2 mixture on a single crystal of copper of which die separate crystal faces had been polished and exposed to die gas. The formation of copper sulphide first occuiTed on die [100] and [110] planes at a lower H2S partial pressure dran on die more densely packed [111] face. Thus die metal atoms which are less strongly bonded to odrer metal atoms can bond more strongly to die adsorbed species from die gas phase. 

A large number of reactions, many of them of great technological importance, involve the reaction of gases on soHd surfaces. The reactions 

In the interior of a sohd lattice, each unit (atom, molecule, or ion) is surrounded by others on all sides. On the surface, the units are not surrounded on one side and, therefore, they can form bonds to other species. While this process may take place by adsorption of molecules or ions from solutions, we are more concerned here with adsorption of gaseous molecules. It is also possible for gaseous reactants to penetrate below the surface of the solid in some cases. The sites on the sohd where the gases are adsorbed are called active sites. The solid material doing the adsorbing is caUed the adsorbent and the substance adsorbed is caUed the adsorbate. 

In general, it is believed that in cases of physical adsorption the bonding to the surface is so weak that the adsorbent molecules are changed only very shghtly by the adsorption process. Therefore, physical adsorption does not weaken the bonds in the adsorbate molecules significantly, and the adsorbent does not function as a catalyst. 

Surface atom of the catalyst Molecule of poison covering an active site 

Since the evaporation of a solid would occur at the kink sites because the bonding is weaker, atoms would diffuse also to these sites before evaporation. A demonstration of this is to be found on the morphologies of single crystals after a period of heating in vacuum to cause substantial evaporation. The resultant surface shows an increase in the number of ledges and kinks relative to the area of the terraces. It is also to be expected that dislocations emerging at the surface of catalysts, either as edge or screw dislocations, would play a 

Charcoal is used in gas masks to remove toxic gases. This is one example of a general phenomenon known as adsorption. 

In adsorption, a substance (the adsorbate) sticks to the surface of another material (the adsorbent). This is distinct from absorption, in which a substance penetrates another material. The difference between the two phenomena maybe remembered by thinking of a bath sponge. If dust sticks to the surface of the dry sponge then adsorption has occurred, but if water is taken into the sponge, absorption has occurred. 

It is often impossible to decide whether adsorption, absorption or a mixture of both are occurring. For this reason, the noncommittal term sorption is used for both processes. An example of this difficulty is found in chromatography, where a 

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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,... 

A.R. Miller, The Adsorption of Gases on Solids, Cambridge Univ. Press, 1949... 

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 ... 

An interesting phenomenon about adsorption of gases on solids and ion exchange of ions on resins is swelling. Some porous solids expand on exposure to the vapors of adsoiptives. 

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. 

Adsorption isotherms represent a relationship between the adsorbed amount at an interface and the equilibrium activity of an adsorbed particle (also the concentration of a dissolved substance or partial gas pressure) at a constant temperature. The analysis of adsorption isotherms can yield thermodynamic data for the given adsorption system. Theoretical adsorption isotherms derived from statistical and kinetic data, and using the described assumptions (see 3.1), are known only for the gas-solid interface or for dilute solutions of surfactants (Gibbs). Those for the system gas-solid are of a few basic types that can be thermodynamically predicted81. From temperature relations it is possible to calculate adsorption and activation energies or rate constants for individual isotherms. Since there are no theoretically founded equations of adsorption isotherms for dissolved surfactants on solids, the adsorption of gases on solides can be used as a starting point for an interpretation. 

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). 

Physical adsorption of gases on solids is virtually always enthalplcally driven < 0. Entropically driven adsorption can exist but it is very unlikely... 

H.C. van Ness, Adsorption of Gases on Solids. Review of the Role of Thermodynamics in Chemistry and Physics of Interfaces, Am. Chem. Soc. Publication (1971) p. 121. (Review, also covers mixtures of gases.)... 

W.A. Steele. The Physical Adsorption of Gases on Solids in Adv. Colloid Interface Set 1 (1967) 3. (Older but not outdated review many examples.)... 

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-. ... 

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 study of adsorption from solution is experimentally straightforward. A known mass of the adsorbent material is shaken with a solution of known concentration at a fixed temperature. The concentration of the supernatant solution is determined by either physical or chemical means and the experiment continued until no further change in the concentration of the supernatant is observed, that is, until equilibrium conditions have been established. Equations originally derived for the adsorption of gases on solids are generally used in the interpretation of the data, the Langmuir and Freundlich equations being the most commonly used. 

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. 

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... 

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