Thermodynamics Microcalorimetry and Thermal Desorption

Measurement of the enthalpies and entropies of adsorption and their variation as the pore space is filled is important to establish chemical models of the interactions and to enable chemical engineers to model and optimise catalytic reactors and large-scale separations. For physisorption of gases and vapours, the most widely available approach is to measure the equilibrium uptake as a function of adsorbate pressure and at a series of constant temperatures, giving a series of isotherms. This then permits the dependence of the equilibrium pressure with temperature to be derived for any value of the uptake, or fractional coverage, 0. The differential heat of adsorption (the heat at a particular coverage, 0 ) can then be calculated using the linearised form of the Clausius lapeyron equation  

For separation applications, it is important to measure adsorption in two- or multi-component mixtures. In these cases the compositions of both the adsorbed and the gas phase are needed to obtain a full description of the system. These can be obtained by measurement, or, if data on the single component adsorptions are available, the adsorption behaviour of the mixture can be simulated if the interaction parameters between the two adsorbates are known. 

For strongly chemisorbed species it can be difficult to obtain equilibrium uptakes below saturation at usual temperatures and measurable pressures (the adsorption isotherms rise very steeply with pressure to the monolayer coverage). Furthermore, chemisorption may be thermally activated, resulting in very long equilibration times. For these reasons other approaches are required to measure the thermodynamics of chemisorption. The two main ways are microcalorimetry and thermal desorption. 

Calorimetry is a highly accurate method to measure the heat of adsorption, be it physisorption or chemisorption. It is typically performed using microcalorimeters of the Tian-Calvet type, in which known volumes of the adsorbate are sequentially dosed onto the solid from the gas phase at the required temperature and the liberated heat is determined from the temperature rise. In this way very accurate plots of heats of adsorption against uptake can be obtained directly for both weakly and strongly bound sorbates. 

The chemisorption of basic molecules at acidic sites in microporous solids, for example, is also readily studied by thermal desorption to give details of acid site strength and type (Section 8.4.2.1). Not all chemisorption can be studied straightforwardly by TPD, however. For molecules such as alkenes, even moderate temperatures rapidly cause reaction of the adsorbed species, so that the experiment monitors reaction rather than desorption. The method beeomes temperature-programmed reaction and requires ehemical analysis of the produets. 

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