Flow Microcalorimeter

Figure Bl.27.8. Schematic view of Picker s flow microcalorimeter. A, reference liquid B, liquid under study P, constant flow circulating pump and 2, Zener diodes acting as heaters T and T2, thennistors acting as temperature sensing devices F, feedback control N, null detector R, recorder Q, themiostat. In the above A is the reference liquid and C2is the reference cell. When B circulates in cell C this cell is the working cell. (Reproduced by pemiission from Picker P, Leduc P-A, Philip P R and Desnoyers J E 1971 J. Chem. Thermo. B41.)...

III. Some Heat-Flow Microcalorimeters That Can Be Used in Heterogeneous Catalysis Research. 196... 

Calvet and Guillaud (S3) noted in 1965 that in order to increase the sensitivity of a heat-flow microcalorimeter, thermoelectric elements with a high factor of merit must be used. (The factor of merit / is defined by the relation / = e2/pc, where e is the thermoelectric power of the element, p its electrical resistivity, and c its thermal conductivity.) They remarked that the factor of merit of thermoelements constructed with semiconductors (doped bismuth tellurides usually) is approximately 19 times greater than the factor of merit of chromel-to-constantan thermocouples. They described a Calvet-type microcalorimeter in which 195 semiconducting thermoelements were used instead of the usual thermoelectric pile. 

In recent years, other heat-flow microcalorimeters equipped with commercially available semiconducting thermoelements have been described... 

Intrinsic Sensitivities of Some Heat-Flow Microcalorimeters... 

The value of the time constant depends upon the calorimeter itself p and upon the heat capacity of the calorimeter cell and of its contents p. Typical, but necessarily approximate, values of the time constant for some heat-flow microcalorimeters are given in Table II. 

It must be noted that the heat capacity of the calorimeter cell and of its contents p, which appears in the second term of Tian s equation [Eq. (12)], disappears from the final expression giving the total heat [Eq. (19)]. This simply means that all the heat produced in the calorimeter cell must eventually be evacuated to the heat sink, whatever the heat capacity of the inner cell may be. Changes of the heat capacity of the inner cell or of its contents influence the shape of the thermogram but not the area limited by the thermogram. It is for this reason that heat-flow microcalorimeters, with a high sensitivity, are particularly convenient for investigating adsorption processes at the surface of poor heat-conducting solids similar in this respect to most industrial catalysts. 

It is, of course, not necessary to use a heat-flow microcalorimeter in order to determine the heat released by rapid adsorption phenomena. Dell and Stone (74), for instance, using an isoperibol calorimeter of the Garner-Veal type, found an initial heat of 54 4 kcal mole-1 for the adsorption of oxygen on nickel oxide at 20°C. The agreement with the value (60 2 kcal mole-1) in Fig. 19 is remarkably good, particularly if it is considered that very different methods were used for the preparation of the nickel-oxide samples (19, 74)-... 

In the various sections of this article, it has been attempted to show that heat-flow calorimetry does not present some of the theoretical or practical limitations which restrain the use of other calorimetric techniques in adsorption or heterogeneous catalysis studies. Provided that some relatively simple calibration tests and preliminary experiments, which have been described, are carefully made, the heat evolved during fast or slow adsorptions or surface interactions may be measured with precision in heat-flow calorimeters which are, moreover, particularly suitable for investigating surface phenomena on solids with a poor heat conductivity, as most industrial catalysts indeed are. The excellent stability of the zero reading, the high sensitivity level, and the remarkable fidelity which characterize many heat-flow microcalorimeters, and especially the Calvet microcalorimeters, permit, in most cases, the correct determination of the Q-0 curve—the energy spectrum of the adsorbent surface with respect to... 

Heats of adsorption were determined with a Microscal Flow Microcalorimeter, using flow rates of one ml. per hour under a grav-itional head, and 70 to 80 mg of Sterling NS in the bed. The results of a typical run are shown in Figure 7, which illustrates the rapidity of adsorbing 2.25% OLOA 1200. The area under this peak corresponds to the generation of 6.8 millicalories of heat, a AH of -4.7 kcal/mole. The results of several such experiments are summarized in Table II. 

A liquid flow microcalorimeter, the thermal activity monitor (TAM), is commercially available from ThermoMetric (formerly LKB/Bofors). This instrument consists of two glass or steel ampules with a volume of 3 to 4 cm3 (25 cm3 ampule available with a single detector), placed in a heat sink block. Recently, an injection-titration sample vessel was developed which acts as a microreactor. This vessel is provided with flow-in, flow-out, and titration lines, with a stirring device. The isothermal temperature around the heat sink is maintained by a controlled water bath. Each vessel holder, containing an ampoule, is in direct contact with a thermopile array, and the two arrays are joined in series so that their output voltages subtract. The two pairs of thermopile arrays are oppositely connected to obtain a differential output,... 

Building a heat flow microcalorimeter is not trivial. Fortunately, a variety of modern commercial instruments are available. Some of these differ significantly from those just described, but the basic principles prevail. The main difference concerns the thermopiles, which are now semiconducting thermocouple plates instead of a series of wire thermocouples. This important modification was introduced by Wadso in 1968 [161], The thermocouple plates have a high thermal conductivity and a low electrical resistance and are sensitive to temperature differences of about 10-6 K. Their high thermal conductivity ensures that the heat transfer occurs fast enough to avoid the need for the Peltier or Joule effects. 

Modern heat flow microcalorimeters employ a diversity of heat sinks and cells, depending on the applications for which they were designed. The heat sinks can be water baths, kept at a constant temperature ( 5 x 10-4 K) and typically operating in the range of 20-80 °C, or metal blocks, allowing much wider temperature ranges (e.g., -196°C to 200°C, 20°C to 1000°C). In some cases it is possible to scan the temperature at a predetermined rate (see chapter 12). 

The number of chemical reactions that have been examined with the heat flow microcalorimeter of figure 10.2 is still fairly small. We have selected reaction 10.17, the photochemical isomerization of trans- to cz s-azobenzene, to illustrate the method. 

It is pertinent to ask why Dias et al. decided to use one unit instead of two (we add that their microcalorimeter has not two but four of those units ). The cost was obviously not an issue in their case. However, by testing this new approach they have shown that it is possible to use other types of heat flow microcalorimeters—containing only two cells (or one unit)—in photocalorimetric studies. 

As is often the case, we have become involved in microemulsions somwehat by accident. In the last five years or so we have been making systematic studies of the thermodynamic properties of aqueous organic mixtures and of electrolytes in these mixed solvents. Of particular interest were our heat capacity measurements. With a differential flow microcalorimeter it is possible to... 

The apparent molal heat capacity of DEC was measured in BE-H2O mixtures with a flow microcalorimeter following the usual... 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

The densities and volumetric specific heats of some alkali halides and tetraalkylammonium bromides were undertaken in mixed aqueous solutions at 25°C using a flow digital densimeter and a flow microcalorimeter. The organic cosolvents used were urea, p-dioxane, piperadine, morpholine, acetone, dime thy Isulf oxide, tert-butanol, and to a lesser extent acetamide, tetrahydropyran, and piperazine. The electrolyte concentration was kept at 0.1 m in all cases, while the cosolvent concentration was varied when possible up to 40 wt %. From the corresponding data in pure water, the volumes and heat capacities of transfer of the electrolytes from water to the mixed solvents were determined. The converse transfer functions of the nonelectrolyte (cosolvent) at 0.4m from water to the aqueous NaCl solutions were also determined. These transfer functions can be interpreted in terms of pair and higher order interactions between the electrolytes and the cosolvent. 

Standard heat capacities of transfer can be derived from the temperature dependence of standard enthalpies of solution (8). While this technique can give general trends in the transfer functions from water to mixed solvents (9), it is not always sufficiently precise to detect the differences between similar cosolvents, and the technique is rather laborious. Direct measurements of the difference between heat capacities per unit volume of a solution and of the solvent a — gq can be obtained with a flow microcalorimeter (10) to 7 X 10 5 JK 1 cm-3 on samples of the order of 10 cm3. A commercial version of this instrument (Picker dynamic flow calorimeter, Techneurop Inc.) has a sensitivity improved by a factor oi about two. 

There are shortcomings in this work, however, and we expect to solve these soon. Adsorption is a slower process than most of us realize (25), and at 25°C the adsorption of pyridine onto iron oxide takes about three days to reach equilibrium. The results of Figure 7 with pyridine and those with triethylamine were obtained in about one hour. However Fm was the same for the two temperatures, for the slopes are exactly equal for the two lines. We are now using a flow microcalorimeter to measure the evolution of heat upon adsorption and we are adding a UV sensor to detect concentration changes this combination should give accurate heats of adsorption and desorption. We will then be able to compare these direct measurements of heats of adsorption with those obtained from the temperature coefficients of adsorption isotherms. 

FIGURE 7.7 Thermostat of the heat-flow microcalorimeter showing the sample cell with the solution and the ion exchanger separated by a membrane to prevent ion exchange and to guarantee the initial state of the experiment. 

Oxidation of ethene on silver catalysts to yield ethene oxide is a good example of an industrial catalytic process with a high selectivity. In order to confirm a possible correlation between the catalysts affinity towards oxygen and their activity in ethene epoxidation, a heat-flow microcalorimeter equipped with a pulse flow reactor has been used to study the reaction of oxygen at 473 K with a series of silica-supported silver catalysts [71]. At 473 K, adsorption of oxygen at the surface of silver is a fast process incorporation of oxygen into deeper metal layers, though present, is a slow process. 

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. 

There are two possible modes of operation for flow microcalorimeters used for solution-phase studies. 

It is apparent from these studies that all flow microcalorimeters should be validated in this way, if quantitative information is sought. To date, similar studies have not been conducted for validation of gas-phase flow calorimeters. It is likely, however, that the problems associated with the removal of heat by the flowing medium will not be as significant in such systems since the associated heat capacity is much lower for gaseous/vapour systems and hence the amount of heat lost in this way is significantly reduced. 

The first investigation of the influence of particle mass transfer on the reaction kinetics in a flow microcalorimeter, dealing with properties of urease immobilized on controlled pore glass, was published in 1985 [25]. More recently, the evaluation of microcalorimetric data in the case of particle-diffusion limitation was improved and simplified by introducing the principle of the differential bed [28,29]. 

Fig. 9. Kinetic measurement of the calcium pectate gel immobilized penicillin acylase in the flow microcalorimeter. Cell loading concentrations (mg of dry weight per mL of gelling suspension) ( ) 11.9 ( ) 23.7 ( ) 29.6 [29]...

Fig. 13. Operational stability of D-amino acid oxidase fixed in cells of Trigonopsis variabilis CCY 15-1-3 entrapped in standard (A) and hardened (a) calcium pectate gel and standard (O) and hardened calcium alginate gel ( ). The relative activity was monitored by continuous processing, with the substrate (cephalosporin C) solution in the flow microcalorimeter [39]...

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