Introduction microcalorimeter

Figure 15 gives a diagrammatic representation of a volumetric line which is used in connection with a high-temperature Calvet microcalorimeter 67). Other volumetric lines which have been described present the same general features (15, 68). In the case of corrosive gases or vapors, metallic systems may be used 69). In all cases, a sampling system (A in Fig. 15) permits the introduction of a small quantity of gas (or vapor) in a calibrated part of the volumetric line (between stopcocks Ri and Ro in Fig. 15) where its pressure Pi is measured (by means of the McLeod gage B in Fig. 15). The gas is then allowed to contact the adsorbent placed in the calorimeter cell C (by opening stopcock Ro in Fig. 15). The heat evolution is recorded and when it has come to completion, the final equi-...

The enthalpies of phase transition, such as fusion (Aa,s/f), vaporization (AvapH), sublimation (Asut,//), and solution (As n//), are usually regarded as thermophysical properties, because they referto processes where no intramolecular bonds are cleaved or formed. As such, a detailed discussion of the experimental methods (or the estimation procedures) to determine them is outside the scope of the present book. Nevertheless, some of the techniques addressed in part II can be used for that purpose. For instance, differential scanning calorimetry is often applied to measure A us// and, less frequently, AmpH and AsubH. Many of the reported Asu, // data have been determined with Calvet microcalorimeters (see chapter 9) and from vapor pressure against temperature data obtained with Knudsen cells [35-38]. Reaction-solution calorimetry is the main source of AsinH values. All these auxiliary values are very important because they are frequently required to calculate gas-phase reaction enthalpies and to derive information on the strengths of chemical bonds (see chapter 5)—one of the main goals of molecular energetics. It is thus appropriate to make a brief review of the subject in this introduction. 

One sees that the use of Equation (2.79) requires a knowledge of the following experimental quantities dQm (heat measured by the calorimeter), dna (amount adsorbed), dp (increase in equilibrium pressure) and Vc (dead volume of the part of the cell immersed in the heat-flowmeter of the microcalorimeter cf. Figure 3.15.). If the conditions of small and reversible introduction of adsorptive are not fulfilled, the quantity assessed by Equation (2.79) can be described as a pseudo-differential enthalpy of adsorption (see Figure 3.16a). 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

The heats of adsorption of the probe molecules were measured in a heat-flow microcalorimeter of the Tian-Calvet type from Setaram, linked to a glass volumetric line to permit the introduction of successive small doses of gases [6]. The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics). Successive doses were sent onto the sample until a final equilibrium pressure of 133 Pa was obtained. The adsorption temperature was maintained at 353 K in order to limit physisorption interactions between the probe molecules and the zeolites. All the samples were pretreated at 773 K under vacuum overnight prior to any calorimetric measurement. 

The introduction of a capillary-contained sample of CB melted at 80°C into the microcalorimeter cell precooled at -10°C resulted in its rapid quenching at a rate in... 

It is worth recalling that the entropy of adsorption may be obtained from calorimetric experiments only if the heat exchange is reversible. A formula for evaluating the standard adsorption entropy Aa5 °from a reversible adsorption volumetric-calorimetric data was proposed by Garrone et al. [91] and applied to a selection of quasi-ideal systems, [97] consisting of CO adsorbed on non d/d metal oxides, at the surface of which cus cations acting as Lewis acidic sites were exposed. An isothermal microcalorimeter with a discontinuous (stepwise) introduction of the adsorptive, as the one described here, was fruitfully employed. 

In the reported examples, an isothermal microcalorimeter with a discontinuous (stepwise) introduction of the adsorptive was fmitfully employed. An alternative, suitable way to collect the experimental data required for determining the enthalpy and entropy changes accompanying the adsorption process is to follow the procedure implying the slow-and-constant adsorptive introduction which was extensively described in Ref. [99]. 

Most commonly used are heat-flow microcalorimeters of the Han-Calvet type [5, 8]. The detailed theory and operation of this calorimeter can be found elsewhere [11]. The apparatus is composed of an experimental vessel, where the studied system is located, which is placed into a calorimetric block (Fig. 3.1). The temperature of the block, which functions as heat sink, is controlled very precisely. The heat generated in the system flows to the heat sink and is accurately measured by means of detector. This is made of a large numbers of identical thermocouples (a thermopile) that surrounds the vessel and connected to the block (Fig. 3.2) in such a way that vessel and block temperature are always close to each other. A signal is generated by the detector that is proportional to the heat transfer per unit time. Undesired signals due to the external temperature fluctuations in the calorimetric block are minimized by connecting in opposition two heat flow detectors from two identical vessels, one of which is used to perform the experiment, the other being used as a reference. Heat related to the introduction of the probe and other parasitic phenomena are thus compensated. 

The results of CO adsorption microcalorimetry described in this overview were collected with a differential and isothermal microcalorimeter (Tian-Calvet Microcalorimeter) linked to a static volumetric system. The equipment permits the introduction of successive small doses of CO onto the catalyst. Both the calorimetric and the volumetric data were stored and analyzed by microcomputer processing. The obtained data are presented as differential heats versus the amount of CO adsorbed... 

Isothermal titration calorimetry (ITC) has long been recognized as a useful tool for the evaluation of binding constants [51]. The field of calorimetry changed markedly during the early 1990s with the introduction of commercial titration microcalorimeters [52]. These devices, available from several suppliers, operate with volumes near one to two milliliters. Virtually all the data reported to date on the thermody-... 

---