Adiabatic adsorption calorimetry

In some respects, adiabatic calorimetry provides information which is complementary to that provided by heat-flow calorimetry. The latter allows a study to be made of the full composition range at constant temperature, whereas the adiabatic calorimetry study is carried out over the prescribed range of temperature with a constant amount of adsorptive in the adsorption cell (of course, this does not mean that a constant amount is adsorbed). Adiabatic calorimetry allows direct measurements of the heat capacities of adsorbed films, although they are difficult to make accurately 

This is now the most useful category for adsorption studies, especially for isothermal measurements. There are two main types. 

Phase-change adsorption calorimetry. This was the earliest type of diathermal-conduction calorimetry and was originally developed in the form of ice calorimetry by Lavoisier and Laplace (1783), who weighed the liquid water, and by Bunsen (1870), who measured the change of volume. Dewar (1904) devised an elegant adsorption calorimeter at liquid air temperature the heat was evaluated from the volume of air vaporized. Of course, the temperature of the calorimeter is imposed by the temperature of the phase change. Because these calorimeters lack adaptability and cannot be readily automated, they are mainly of historical interest. 

Heat-flow adsorption microcalorimetry. The most important type of isothermal calorimeter in current use is that based on the principle of the heat flowmeter, which was first applied by Tian (1923) and improved by Calvet (Calvet and Prat, 1958, 

This type of isothermal calorimeter is especially suitable for the study of open systems (i.e. with the introduction and withdrawal of gas) and is therefore highly recommended for the determination of energies of adsorption. 

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

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

Adiabatic calorimetry is particularly useful for the study of closed adsorption systems at low temperatures (where radiation losses are small) and for temperature scanning experiments. It is the preferred type of measurement for the determination of the heat capacity of adsorption systems, especially in the temperature range 4-300 K (Morrison et al., 1952 Dash, 1975). The temperature scan is obtained by means of the Joule effect applied to the sample container the sample heating coil shown in Figure 3.14 is used for this purpose. 

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