Adsorption classification

Figure Bl.26.3. The lUPAC classification of adsorption isothemis for gas-solid equilibria (Sing K S W, Everett D H, Haul RAW, Mosoul L, Pierotti R A, Rouguerol J and Siemieiiiewska T 1985 Pure. Appl. Chem. 57 603-19).

Calorimetry is the basic experimental method employed in thennochemistry and thennal physics which enables the measurement of the difference in the energy U or enthalpy //of a system as a result of some process being done on the system. The instrument that is used to measure this energy or enthalpy difference (At/ or AH) is called a calorimeter. In the first section the relationships between the thennodynamic fiinctions and calorunetry are established. The second section gives a general classification of calorimeters in tenns of the principle of operation. The third section describes selected calorimeters used to measure thennodynamic properties such as heat capacity, enthalpies of phase change, reaction, solution and adsorption. 

Fig. I.l The five types of adsorption isotherm, I to V, in the classification of Brunauer, Deming, Deming and Teller (BDDT), together with Type VI, the stepped isotherm.

The basis of the classification is that each of the size ranges corresponds to characteristic adsorption effects as manifested in the isotherm. In micropores, the interaction potential is significantly higher than in wider pores owing to the proximity of the walls, and the amount adsorbed (at a given relative pressure) is correspondingly enhanced. In mesopores, capillary condensation, with its characteristic hysteresis loop, takes place. In the macropore range the pores are so wide that it is virtually impossible to map out the isotherm in detail because the relative pressures are so close to unity. 

Isotherms of the type now characterized as Type IV have played an essential part in the development of adsorption theory and practice, as being the first kind of isotherm to be studied in detail. Already in 1888, half a century before the BDDT classification had appeared, van Bemmelen had... 

A new classification of hysteresis loops, as recommended in the lUPAC manual, consists of the four types shown in the Figure below. To avoid confusion with the original de Boer classification (p. 117), the characteristic types are now designated HI, H2, H3 and H4 but it is evident that the first three types correspond to types A, E and B, respectively, in the original classification. It will be noted that HI and H4 represent extreme types in the former the adsorption and desorption branches are almost vertical and nearly parallel over an appreciable range of gas uptake, whereas in the latter they are nearly horizontal and parallel over a wide range of relative pressure. Types H2 and H3 may be regarded as intermediate between the two extremes. 

In a typical amorphous adsorbent the distribution of pore size may be very wide, spanning the range from a few nanometers to perhaps one micrometer. Siace different phenomena dominate the adsorptive behavior ia different pore size ranges, lUPAC has suggested the foUowiag classification ... 

Pore size is also related to surface area and thus to adsorbent capacity, particularly for gas-phase adsorption. Because the total surface area of a given mass of adsorbent increases with decreasing pore size, only materials containing micropores and small mesopores (nanometer diameters) have sufficient capacity to be usehil as practical adsorbents for gas-phase appHcations. Micropore diameters are less than 2 nm mesopore diameters are between 2 and 50 nm and macropores diameters are greater than 50 nm, by lUPAC classification (40). 

Fig. 12. Classification of adsorptive separations where NG = natural gas and S = sulfur.

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Processes and/or unit operations that fall under this classification include adsorption, ion exchange, stripping, chemical oxidation, and membrane separations. All of these are more expensive than biological treatment but are used for removal of pollutants that are not easily removed by biomass. Often these are utilized in series with biologic treatment but sometimes they are used as stand-alone processes. 

In order to elucidate the pore structure of Csx, the adsorption-desorption isotherm of N2 was first measured. Tsrical results are given in Figure 4. H3PW12O40 exhibited a Type II isotherm (according to the lUPAC classification... 

Distinguishing between physical and chemical adsorption using the value of adsorption heat cannot lead to unambiguous results, too. An arbitrary classification of physical adsorption as having small heats Q - 0.01 + 0.2 eV typical for and attributing - 1 eV to diemisorption is often violated. A typical example can be provided by a dissipative chemisorption diaracterized by small total heat effect. 

Thus, as of today, there is no reliable classification of various types of adsorption. Presumably, it would be most correct to consider various types of adsorption interactions consistent with classification of chemical bonds [19]. 

It has been proven by experiment that there are donor acceptor atoms and molecules of absorbate and their classification as belonging to one or another type is controlled not only by their chemical nature but by the nature of adsorbent as well (see, for instance [18, 21, 203-205]). From the standpoint of the electron theory of chemisorption it became possible to explain the effect of electron adsorption [206] as well as phenomenon of luminescence of radical recombination during chemisorption [207]. The experimental proof was given to the capability of changing of one form of chemisorption into another during change in the value of the Fermi level in adsorbent [208]. 

C. H. Giles, T. H. MacEwan, S. N. Natchwa and D. Smith, Studies in Adsorption, Part XI A System of Classification of Solution Adsorption Isotherms and its Use in Diagnosis of Adsorption Mechanism and its Measurement of Specific Surface Area of Solids, J. Chem. Soc., p. 3973,1960. 

Taking into account the electron density relocation (Table 2.4) two routes of NO adsorption can be distinguished. Thus, the nitric oxide coordinates to the monovalent Cr, Ni, and Cu ions in an oxidative way (A<2M > 0), whereas for the rest of the TMIs in a reductive way (AgM < 0). Although this classification is based on the rather simplified criteria, it is well substantiated by experimental observations. Examples of reductive adsorption are provided by interaction of NO with NinSi02 and NinZSM-5, leading at T > 200 K to a Ni -NOs+ adduct identified by the characteristic EPR signal [71]. At elevated temperatures, similar reduction takes place for ConZSM-5 [63], whereas in the case of Cu ZSM-5 part of the monovalent copper is oxidized by NO to Cu2+, as it can readily be inferred from IR and EPR spectra [72,73], This point is discussed in more detail elsewhere [4,57],... 

In practice one can usually distinguish between chemisorption and physical adsorption on the basis of experimental evidence. However, it is frequently necessary to consider several criteria in making the classification. 

The adsorption capacities of the adsorbents are usually determined from modeling of the adsorption isotherms according to the Giles s classification [36] (Figure 15.1). 

Table 16-4 shows the IUPAC classification of pores by size. Micropores are small enough that a molecule is attracted to both of the opposing walls forming the pore. The potential energy functions for these walls superimpose to create a deep well, and strong adsorption results. Hysteresis is generally not observed. (However, water vapor adsorbed in the micropores of activated carbon shows a large hysteresis loop, and the desorption branch is sometimes used with the Kelvin equation to determine the pore size distribution.) Capillary condensation occurs in mesopores and a hysteresis loop is typically found. Macropores form important paths for molecules to diffuse into a par-... 

The pore structure of a solid can contribute to the disintegration, dissolution, adsorption, and diffusion of a drug material [26,27]. Because of this, porosity and pore size distribution measurements have been used extensively to study tablets [28-30], granules [31,32], and excipients [33]. The following classification system of pore sizes has been developed based on the average pore radii [6] ... 

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