Solid-Vapor Adsorption Isotherms

It is well known in the science and art of heterogeneous catalysis that the presence of small amounts of certain materials can greatly improve or disastrously ruin a catalytic reaction. Where improvement is found the additive is referred to as a promoter (a catalyst catalyst ). When disaster results, the additive is termed a poison.  

The exact role of promoters is not very well understood in many cases, but it is now generally accepted that it is related to the formation of specific electronic surface states necessary for the given catalytic reaction. It apparently does not matter how that electronic state is produced that is, whether it is formed in the preparation of the native catalyst surface or by the presence of some other component which induces the necessary state. As an example, the presence of small amounts of aluminum and potassium oxides on iron-iron oxide catalyst in the Haber ammonia synthesis greatly improves its activity. Either promoter alone has no significant effect on the process. Why Such questions remain as fodder for further industrial or graduate research. 

Catalyst poisons are materials that significantly alter, reduce, or completely destroy the activity of a given catalyst. Such materials generally function by binding strongly and (effectively) irreversibly to the specific surface sites necessary for the functioning of the desired process. Particularly troublesome materials in that sense are sulfur-containing compounds, especially thiols and thioethers. For example, the catalytic converters used to oxidize hydrocarbon residues in automobile exhausts will rapidly lose their effectiveness if exposed to such materials. 

On the other hand, poisoned catalysts can have their uses. In some hydrogenations of organic molecules, for example, it may be desirable to produce a reaction between hydrogen and one functional group in the molecule, while leaving untouched another functionality that would normally react as well. By selectively poisoning the catalyst, surface states necessary for the desired reaction may be left untouched while those for the unwanted reaction are blocked. 

Clearly, chemisorption and related catalytic processes are quite complex and remain a relatively poorly understood area of surface science. Modern surface analytical techniques have added much to our understanding of the molecular processes involved, but much remains in the realm of art (or perhaps black magic). 

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Characterization. When siHca gel is used as an adsorbent, the pore stmcture determines the gel adsorption capacity. Pores are characterized by specific surface area, specific pore volume (total volume of pores per gram of solid), average pore diameter, pore size distribution, and the degree to which entrance to larger pores is restricted by smaller pores. These parameters are derived from measuring vapor adsorption isotherms, mercury intmsion, low angle x-ray scattering, electron microscopy, gas permeabiHty, ion or molecule exclusion, or the volume of imbibed Hquid (1). 

Coal is microporous, with certain partial molecular sieve properties. (A microporous solid herein refers to that which contains pores with diameters of a few tens of A. or less.) Micropores can be considered as entities capable of sorbing foreign molecules, and it is known that additivity of their sorption potential fields enhances the sorption owing to dispersion interactions. As the pores become progressively narrower, the vapor adsorption isotherm (Figure 1) in the initial region up to point B becomes progressively steeper (toward the... 

Satisfactory integration cannot be performed unless adsorption data are obtained at very low pressures (II). The lowest pressure that could be measured reliably in this series of experiments was 8.5 microns of Hg. An empirical equation fitted to the experimental points at low pressure was used to extrapolate the isotherm to zero pressures. calculated from each of the isotherms is plotted in Figure 4. Values of the decrease in surface energy, 7r, range upward to 200 ergs cm.-2 at room temperature, a value about the same as is found in other solid-vapor adsorption systems and less than ten times as high as is obtained by the spreading of insoluble monolayers on water (7). 

The calculation of the specific surface area should be taken in three steps (1) determination of the gas (vapor) adsorption isotherm on the solid materials to be investigated, (2) calculation of the function v /(p) from the measured isotherm, and (3) based on the function y(p) selection of the appropriate isotherm equation and the calculation of the specific surface area from this equation which has to include the value of the total monolayer capacity ( ). 

Still another manifestation of mixed-film formation is the absorption of organic vapors by films. Stearic acid monolayers strongly absorb hexane up to a limiting ratio of 1 1 [272], and data reminiscent of adsorption isotherms for gases on solids are obtained, with the surface density of the monolayer constituting an added variable. 

The present discussion is restricted to an introductory demonstration of how, in principle, adsorption data may be employed to determine changes in the solid-gas interfacial free energy. A typical adsorption isotherm (of the physical adsorption type) is shown in Fig. X-1. In this figure, the amount adsorbed per gram of powdered quartz is plotted against P/F, where P is the pressure of the adsorbate vapor and P is the vapor pressure of the pure liquid adsorbate. 

A rather different approach is to investigate possible adsorption isotherm forms for use with Eq. X-43. As is discussed more fully in Section XVII-7, in about 1914 Polanyi proposed that adsorption be treated as a compression of a vapor in the potential held U x) of the solid with sufficient compression, condensation to liquid adsorbate would occur. If Uq(x) denotes the held necessary for this, then... 

The adsorption isotherm corresponding to Eq. X-51 is of the shape shown in Fig. X-1, that is, it cannot explain contact angle phenomena. The ability of a liquid him to coexist with bulk liquid in a contact angle situation suggests that the him structure has been modihed by the solid and is different from that of the liquid, and in an enmirical way, this modihed structure corresponds to an effective vapor pressure F , F representing the vapor pressure that bulk liquid would have were its structure that of the... 

There is a number of very pleasing and instructive relationships between adsorption from a binary solution at the solid-solution interface and that at the solution-vapor and the solid-vapor interfaces. The subject is sufficiently specialized, however, that the reader is referred to the general references and, in particular, to Ref. 153. Finally, some studies on the effect of high pressure (up to several thousand atmospheres) on binary adsorption isotherms have been reported [154]. Quite appreciable effects were found, indicating that significant partial molal volume changes may occur on adsorption. 

The nitrogen adsorption isotherm is determined for a finely divided, nonporous solid. It is found that at = 0.5, P/P is 0.05 at 77 K, gnd P/F is 0.2 at 90 K. Calculate the isosteric heat of adsorption, and AS and AC for adsorption at 77 K. Write the statement of the process to which your calculated quantities correspond. Explain whether the state of the adsorbed N2 appears to be more nearly gaslike or liquidlike. The normal boiling point of N2 is 77 K, and its heat of vaporization is 1.35 kcal/mol. 

In the first step, in which the molecules of the fluid come in contact with the adsorbent, an equihbrium is established between the adsorbed fluid and the fluid remaining in the fluid phase. Figures 25-7 through 25-9 show several experimental equihbrium adsorption isotherms for a number of components adsorbed on various adsorbents. Consider Fig. 25-7, in which the concentration of adsorbed gas on the solid is plotted against the equilibrium partial pressure p of the vapor or gas at constant temperature. At 40° C, for example, pure propane vapor at a pressure of 550 mm Hg is in equilibrium with an adsorbate concentration at point P of 0.04 lb adsorbed propane per pound of silica gel. Increasing the pressure of the propane will cause... 

Equation 10.27 is generally known as Freundlich equation. Equation 10.27 with concentration replaced by pressure was also used to describe the adsorption isotherms of gases on solids, suggesting the incorrect idea that adsorption from solution by a solid could be paralleled with gas or vapor adsorption on the same adsorbents. Whereas in some cases the restriction to dilute solutions was imposed by the solubility of solids (e.g., benzoic acid in water or stearic acid in benzene) it was not imposed on the investigation of mixtures of completely miscible liquids, e.g., acetic acid in water. 

The data which are plotted as isotherms in the case of adsorption from liquid solutions on solid adsorbents are different in nature from those of gas (or vapor) adsorption on the same adsorbents. In fact, while the isotherm for adsorption of a single gas by a solid represents directly the quantity (weight or volume under standard conditions) of gas adsorbed per unit weight of the solid, the experimental measurement in adsorption from solution is the change in concentration of the solution which results from adsorption. The fact that a change in concentration is measured emphasizes that there are at least two components in the solution [13]. 

A plot of W versus P, at constant T, is referred to as the adsorption isotherm of a particular vapor-solid interface. Were it not for the fact that E, the interaction potential, varies with the properties of the vapor and the solid and also changes with the extent of adsorption, all adsorption isotherms would be identical. 

As noted above, the range of pressures over which gas adsorption studies are conducted extends from zero to the normal vapor pressure of the adsorbed species p0. An adsorbed layer on a small particle may readily be seen as a potential nucleation center for phase separation at p0. Thus at the upper limit of the pressure range, adsorption and liquefaction appear to converge. At very low pressures it is plausible to restrict the adsorbed molecules to a mono-layer. At the upper limit, however, the imminence of liquefaction suggests that the adsorbed molecules may be more than one layer thick. There is a good deal of evidence supporting the idea that multilayer adsorption is a very common form of physical adsorption on nonporous solids. In this section we are primarily concerned with an adsorption isotherm derived by Brunauer, Emmett, and Teller in 1938 the theory and final equation are invariably known by the initials of the authors BET. 

Vapor Pressures and Adsorption Isotherms. The key variables affecting the rate of destruction of solid wastes are temperature, time, and gas—solid contacting. The effect of temperature on hydrocarbon vaporization rates is readily understood in terms of its effect on liquid and adsorbed hydrocarbon vapor pressures. For liquids, the Clausius-Qapeyron equation yields... 

An adsorption isotherm is a graph of the amount adsorbed versus the pressure of the vapor phase (or concentration in the case of adsorption from solution). The amounts adsorbed can be described by different variables. The first one is the surface excess I in mol/m2. We use the Gibbs convention (interfacial excess volume Va = 0). For a solid surface the Gibbs dividing plane is localized directly at the solid surface. Then we can convert the number of moles adsorbed Na to the surface excess by... 

Physical adsorption of gases and vapors is a powerful tool for characterizing the porosity of carbon materials. Each system (adsorbate-adsorbent temperature) gives one unique isotherm, which reflects the porous texture of the adsorbent. Many different theories have been developed for obtaining information about the solid under study (pore volume, surface area, adsorbent-adsorbate interaction energy, PSD, etc.) from the adsorption isotherms. When these theories and methods are applied, it is necessary to know their fundamentals, assumptions, and applicability range in order to obtain the correct information. For example, the BET method was developed for type II isotherms therefore, if the BET equation is applied to other types of isotherms, it will not report the surface area but the apparent surface area. 

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