Adsorption isobar

Alternatively, data may be plotted as n versus T at constant pressure or as P versus T at constant n. One thus has adsorption isobars and isosteres (note Problem XVII-2). 

The adsorption of a component j in a given system depends on temperature T and on the component s concentration, Cyj, in the bulk phase. The overall adsorption equation can be written as Aj =f(T, Cyj). The relation between adsorption and the adsorbate s bulk concentration (or pressure, in the case of gases) at constant temperature is called the adsorption isotherm the relation between adsorption and temperature at constant concentration is called the adsorption isobar. From the shape of the adsorption isotherms, the adsorption behavior can be interpreted. In the case of monolayer adsorption, the isotherms are usually written in the form 9 =f(Cyj). (The subscript j is dropped in what follows.)... 

The capacity of an adsorbent to adsorb an adsorbate can be represented by adsorption isotherms, as shown in Figure 10.7a, or adsorption isobars, as shown in Figure 10.7b. The general trend can be seen that adsorption increases with decreasing temperature and increases with increasing pressure. 

Also considered are adsorption isobars, which are the dependences of q(T) at p — const., and adsorption isosters, that is, dependence ofp T) at q = const. [53]. 

Figure 17.4. Equilibrium data for the adsorption of ammonia on charcoal1 10) (a) Adsorption isotherm (b) Adsorption isobar (c) Adsorption isostere...

An additional difficulty in studies of slow chemisorption is evidenced by the Taylor-Liang technique of measuring adsorption isobars at successively higher temperatures without pumping between runs, a technique that has been reviewed in an earlier volume of this series (8). The ad-sorption-desorption phenomena observed between room temperature and about 150°C. clearly show in this temperature range the existence on the surface of zinc oxide of two types of hydrogen chemisorption (9). 

As a matter of fact there is a striking parallelism between both systems, H2-ZnO and O2-NiO. Thus the adsorption isobar (60 mm. Hg) corresponding to the amount of oxygen taken up after 100 min. by NiO between room temperature and 700°C. has the characteristic shape of hydrogen adsorption lobars a curve with a flat minimum between 200° and 300°C. (37). 

Fig. 9. Adsorption isobar of hydrogen on zinc oxide (after Taylor and Strother).

We show in Fig. 9 the adsorption isobar of CO on our ZnO in the lower temperature range chemisorption rapidly decreases, the minimum being found at about 150° above this temperature the quantity adsorbed increases somewhat. The second type of chemisorption is associated with hysteresis and occurs over the temperature range in which oxygen exchange is readily measurable. 

Figure 5.3 Schematic adsorption isobar showing the transition between physical adsorption and chemisorption...

Taylor (37) describes the shape of adsorption isobars for hydrogen on an iron catalyst and curves showing desorption, readsorption when the temperature is increased stepwise. These observations are best explained by the existence of an a priori heterogeneity. This heterogeneity may well be connected with the existence of abnormal lattice distances on the surface. 

Fig. 4. Abnormal shapes of the adsorption isobars of H2 on zinc oxide, P = 760 mm. Hg (Taylor and Strother) promoted iron, P = 760 mm. Hg (Emmett and Harkness) reduced nickel, P = 0.01 mm. Hg (Eucken) and evaporated nickel film presintered at 400°C., P = 0.1 mm. Hg. (The ordinates are arbitrarily), (Beeck).

Finally we pay attention to the ideal frontal TC (cf. Fig. 4.1). The high temperature front of the zone profile is obviously proportional to the adsorption isobar and so, at least for the localized adsorption model, to the adsorption constant. As such, it would obey Eq. 5.14. It holds for the activities which do not appreciably decay in the course of run. As for the shorter-lived nuclides, both the elution and the formally frontal TC result in non-ideal frontal chromatograms. Their shapes are close to what would arise from ideal processing during t . but they are smeared due to the random lifetimes of nuclei. Still the initial part of the thermochromatogram might be useful for evaluation of the required quantity, provided that the statistics of detected decay events is good. 

Bachmann and co-workers interpreted their data for molecules halides in columns loaded with alkaline halides as evidence for the formation of complexes between the adsorbate and the crystalline halide. Such complexes were considered for application in radiochemical separations earlier by Zvara and co-workers in [3,13,14], who estimated the adsorption enthalpies from the rough adsorption isobar obtained in thermochromatographic experiments using Eq. 5.14. The adsorption enthalpy for NbCls on KC1 [14] is intermediate between the data of Table 5.1 for the adsorbents NaCl and CsCl, so that the results of the two groups are consistent. 

In the ideal frontal thermochromatography, the high temperature branch of the TC peak must obviously approach the adsorption isobar... 

Figure 22.11 Adsorption of CH4 on PX 21 activated carbon for storage (a) adsorption isotherms at various temperatures, (b) adsorption isobars and operating lines for CH4 fill and discharge.

The Chreedimensional diagram obtained could serve both theoretical and technological computations concerning oxygen enrichment of air on K-clinoptilolite. It is a basis for determining of adsorption isobars, fictitious and real isotherms, as well as thermodynamic data for adsorption from binary C /Nj mixtures. Freundlich and Henry constants (for the real adsorption isotherms) correlated with the equilibrium concentration are useful for computation of the amounts adsorbed at arbitrary partial pressures and equilibrium concentrations. 

Fig. 4 Ratio of peak areas of TPO profiles Fig.5 H2 adsorption isobar...

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