Gas adsorption calorimetry

We shall examine here the two major procedures for gas adsorption calorimetry (cf. Section 3.3.3). Both procedures make use of a diathermal, heat-flowmeter, Tian-Calvet microcalorimeter (cf. Section 3.2.2). 

In some respects the technique is less demanding than gas adsorption calorimetry. For example, the calorimeter and pre-adsorption rig are separate and therefore easy to handle. 

The first experiments of gas adsorption calorimetry by Favre (1854) were made with an isoperibol calorimeter. More recently, refinements were introduced by Beebe and his co-workers (1936) and by Kington and Smith (1964). Because of the uncontrolled difference between the temperature of the sample and that of the surroundings, Newton s law of cooling must be applied to correct the observed temperature rise of the sample. In consequence, any slow release of heat (over more than, say, 30 minutes), which would produce a large uncertainty in the corrective term, cannot be registered. For this reason, isoperibol calorimetry cannot be used to follow slow adsorption equilibria. However, its main drawback is that the experiment is never isothermal during each adsorption step, a temperature rise of a few kelvins is common. The corresponding desorption (or lack of adsorption) must then be taken into account and, after each step, the sample must be thermally earthed so as to start each step at the same temperature. In view of these drawbacks,... 

Finally, when integral molar energies of adsorption are directly measured by gas adsorption calorimetry, it is possible to obtain the corresponding integral molar entropies of adsorption from Equations (2.65) and (2.66). 

Since the data provided by the above type of immersion calorimetry experiment can be directly related to the results obtained by gas adsorption calorimetry the question arises which type of experiment is to be preferred In fact, immersion calorimetry, although time-consuming, has certain advantages because of the difficulty in handling easily condensable vapours in adsorption manometry. 

Gas adsorption calorimetry 62 Adiabatic adsorption calorimetry 63 Diathermal-conduction adsorption calorimetry 64 Diathermal-compensation adsorption calorimetry 66 Isoperibol adsorption calorimetry 66... 

When coupled to gas adsorption data, calorimetric data can be very useful for the textural characterization of carbons. The use of chemical probes with different molecular sizes allow determining the pore size distribution [288-295]. On the other hand, relevant information concerning chemical properties of the carbon surfaces and their influence on the sorption properties of carbons can be obtained when using the appropriate calorimetric technique. Immersion, flow adsorption and gas-adsorption calorimetry have been employed for the study of surface chemistry of carbons. For instance, immersion calorimetry provides a direct measurement of the energy involved in the interaction of vapor molecules of the immersion liquid with the surface of the solid. This energy depends on the chemical nature of the solid surfajoe and the probe molecules, i.e. the specific interaction between the solid and the liquid. Comparison between enthalpies of immersion into liquids with different polarities provides a picture of the surface chemistry of the solid. Although calorimetric techniques are not able to completely characterize the complex surface chemistry of carbons, they represent a valuable complement to other techniques. 

An excellent example of work of this type is given by the investigations of Benson and co-workers [127, 128]. They found, for example, a value of = 276 ergs/cm for sodium chloride. Accurate calorimetry is required since there is only a few calories per mole difference between the heats of solution of coarse and finely divided material. The surface area of the latter may be determined by means of the BET gas adsorption method (see Section XVII-5). 

Microcalorimetry has gained importance as one of the most reliable method for the study of gas-solid interactions due to the development of commercial instrumentation able to measure small heat quantities and also the adsorbed amounts. There are basically three types of calorimeters sensitive enough (i.e., microcalorimeters) to measure differential heats of adsorption of simple gas molecules on powdered solids isoperibol calorimeters [131,132], constant temperature calorimeters [133], and heat-flow calorimeters [134,135]. During the early days of adsorption calorimetry, the most widely used calorimeters were of the isoperibol type [136-138] and their use in heterogeneous catalysis has been discussed in [134]. Many of these calorimeters consist of an inner vessel that is imperfectly insulated from its surroundings, the latter usually maintained at a constant temperature. These calorimeters usually do not have high resolution or accuracy. 

As we have seen, an adsorption isotherm is one way of describing the thermodynamics of gas adsorption. However, it is by no means the only way. Calorimetric measurements can be made for the process of adsorption, and thermodynamic parameters may be evaluated from the results. To discuss all of these in detail would require another chapter. Rather than develop all the theoretical and experimental aspects of this subject, therefore, it seems preferable to continue focusing on adsorption isotherms, extracting as much thermodynamic insight from this topic as possible. Within this context, results from adsorption calorimetry may be cited for comparison without a full development of this latter topic. 

It is with this type of equation that, for instance, Micale el al. (1976) was able to check the consistency of the isosteric approach (from gas adsorption isotherms) with immersion calorimetry, for the water-microciystalline Ni(OH)2 system. 

It should be kept in mind that any change in surface area, surface chemistry, or microporosity will result in a change in the energy of immersion. Because immersion calorimetry is quantitative and sensitive, and because the technique is not too difficult to apply in its simplest form, it can be used for quality testing. The preliminary outgassing requires the same care as for a BET measurement, but, from an operational standpoint, energy of immersion measurements are probably less demanding than gas adsorption measurements. 

This is an indirect way of assessing the energetics of gas adsorption in micropores. The pre-adsorbed vapour can be that of the immersion liquid or it can be another adsorptive for instance, to study the water filling mechanism in microporous carbons, Stoeckli and Huguenin (1992) devised an experiment with water pre-adsorption prior to immersion calorimetry (in water or in benzene). 

Adsorption calorimetry consists of the coupling of a heat flow calorimeter with a system able to monitor the adsorption of a probe molecule by determining the amount of probe gas that has reacted with the solid under study. It is probably the most direct method for describing in detail both the quantitative and energetic features of surface sites. The adsorption of a probe molecule is an exothermic phenomenon (AHj s < 0), while desorption processes are associated with endothermic peaks (AHdes > 0). Heats of reduction are generally associated with an... 

A system that links microcalorimetry to the volumetric measurement of quantities of adsorbed reactants makes it possible to study gas-solid interactions and catalyhc reactions. This system works under stahc vacuum. The admission of gases into the calorimeter can be performed either in a discontinuous way (by successive doses) by means of a valve, or in a continuous manner by means of a capillary. The classical technique of adsorption calorimetry by doses is the most appropriate way to measure the energy of interaction between the adsorbed species and the catalyst. If the surface can be a priori considered as heterogeneous, the heat of adsorption, the amount adsorbed and the kinetics of adsorption must be measured for very small successive doses of the adsorbate so as to obtain accu-... 

The above information from electronic, atomic, and morphological levels must be integrated together with gas adsorption data to understand structures and functions of nanoporous systems. We must stress the importance of calorimetry and molecular... 

Table 2 Results obtained with nitrogen (gas or liquid) at 77 K immersion calorimetry and gas adsorption...

A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [11,15] and neutron [14] scattering, inverse gas chromatography [12,13], differential thermal analysis [12], Fourier transform infrared [12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect... 

E. Robens, J.A. Poulis, C.H. Massen Fast gas adsorption measurements for complicated adsorption mechanisms. In V.A. Tertykh, V. Pokrovskiy (eds.) Proceedings of the 28 International Conference on Vacuum Microbalance Techniques, 1999, Kyiv. Journal of Thermal Analysis and Calorimetry 62 (2000) 429-433. 

Let us now consider the most common situation in adsorption calorimetry, a gas-solid open system in which adsorption is brought about by continous or stepwise admission of the adsorptive at constant temperature but not necessarily at the same temperature as the calorimeter. Initially the system contains... 

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