Differential gas adsorption manometry

Differential gas adsorption manometry. In the earliest form of this technique (Schlosser, 1959), represented in Figure 3.4, two carefully matched capillaries feed two bulbs (adsorption and reference) from a common reservoir of adsorptive. The pressure difference between the two sides provides a direct measurement of the amount adsorbed on the sample side, provided the gas flow rate through the two capillaries is the same this is no longer true when the difference between the two downstream pressures (on sample side and reference side) is too great For this reason, this technique is limited to downstream pressures lower than, say, 50 mbar. This allows one to determine surface areas by adsorbing Ar at 77 K, but not N2. 

A simplified form of differential gas adsorption manometry (Haul and Dxlmbgen, 1960, 1963) is represented in Figure 3.5. Nitrogen under atmospheric pressure is introduced into both sides at room temperature. The intermediate valve is closed and the two bulbs are immersed in liquid nitrogen. When equilibrium is reached, the pressure difference gives the amount adsorbed. No vacuum is needed (only flushing with... 

Figure 3.4. Differential gas adsorption manometry with capillaries (valves for initial evacuation of equipment or filling of reservoir not represented) (after Schlosser, 1959).

Figure 3.5. Differential gas adsorption manometry from atmospheric pressure (after Haul and Dflmbgen, 1960).

At the other extreme, a sophisticated form of differential gas adsorption manometry (Camp and Stanley, 1991 Webb, 1992) is shown in Figure 3.6. It can be considered as combining the best features of the equipment in Figures 3.3 and 3.5. The differential assembly allows one to eliminate the dead volume correction, provided that the dead volume of the reference side is properly adjusted by means of the glass beads. In addition, an integral measurement is possible by the use of the two reservoirs and an extra differential manometer. 

Figure 3.6. Differential gas adsorption manometry with double reservoir and triple pressure mea ment (after Camp and Stanley, 1991). 

In the case of differential or twin arrangements of adsorption manometry (cf. Figures 3.4-3.6), the dead volume determination is not required, but the volume equalization and the symmetry of the set-up are essential. The volume equalization is usually obtained with glass beads on the reference side and sometimes also with adjustable bellows or a piston. The check or adjustment is normally carried out at ambient temperature the introduction of an identical amount of gas on both sides must result in a zero pressure difference between them. 

Abstract This chapter is devoted to the study of coadsorption of gases in nanoporous solids by using the differential calorimetry. In the first part, the thermodynamic principles of adsorption of gases are recalled. Some of them have already presented in chapter one. However a special attention has been paid here to the determination of the adsorption enthalpies and entropies and we focused on the selective adsorption of binary mixtures. Then the specific experimental technique based on the combination of differential calorimetry with manometry and gas phase chromatography or mass spectrometry is shown in details. In the last part, the thermodynamic concepts on coadsorption are illustrated with experimental results taken from studies on gas separation by selective adsorption in mlcroporous solids. 

If the van t Hoff and isosteric methods are simple ways for estimating the adsorption enthalpy of single component from isothermal adsorption data, they have the disadvantage to not take into account the temperature dependence on the enthalpy and entropy and to be not enough accurate. Moreover they are not adapted to the adsorption of gas mixtures. The best mean to determine the adsorption and coadsorption enthalpy is to measure them by using a differential calorimetry technique coupled with others techniques allowing the measure of adsorbed amount and composition as for example the manometry and the chromatography. 

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