Heat-flow microcalorimeters

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... 

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. 

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]). 

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. 

A Tian—Calvet heat-flow microcalorimeter which, due to the 480 thermocouples of its thermopile, ensures all at once a good isothermicity of the experiment and a high sensitivity allowing to use a small sample (for an activated carbon, typically 50-100 mg) relatively easy to wet. 

X-ray fluorescence spectrometer (XRF) is used to estimate the elements silicon and aluminum present in the ZSM-5 samples. These values are further utilized to estimate bulk Si02/Al203 ratios of the samples. The number and strength of acid sites in the ZSM-5 samples were determined by measuring the heat of adsorption of ammonia in a high vacuum system coupled to Setaram C-80 heat flow microcalorimeter (France). 

The measurement of the heat of adsorption by a suitable calorimeter is the most reliable method for evaluating the strength of adsorption (either physical or chemical). Tian-Calvet heat-flow microcalorimeters are an example of high sensitivity apparatus which are suitably adapted to the study of gas-solid interactions when connected to sensitive volumetric systems [10-14, 50-55]. Volumetric-calorimetric data reported in the following were measured by means of either a C-80 or MS standard heat-flow microcalorimeter (both by Setaram, F), connected to ahigh vacuum (residual pressure... 

From Eq. 1.23 it turns out that the experimental heat measured in a gas-solid open system, operating in a differential assembly of calorimetric cells, represents the enthalpy change associated to the adsorption. This result applies to adsorption processes performed in a gas-solid open system through the admission of the adsorptive on the solid material kept isothermally within a heat-flow microcalorimeter consisting of two cells in opposition. 

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. 

In the case of water pollution, the estimation of adsorption affinity of potential solid adsorbent toward the specific pollutant can be done using the so-called liquid microcalorimetry. The instruments used for this purpose are differential heat flow microcalorimeters modified to allow continuous stirring of liquid samples. The adsorbate is added to both sample and reference cells simultaneously using a programmable twin syringe pump, linked to the calorimeter. The heat evolved as a result of adsorption can be obtained by integration of the area under the calorimeter signal, for each particular injection (dose). The output of typical microcalorimetric experiment of this type is shown in Fig. 10.9. 

The volumetric-calorimetric data obtained by a Tian-Calvet heat-flow microcalorimeter connected to a gas-volumetric apparatus (as the one described in Chap. 1) being intrinsically molar quantities, their molecular interpretation often requires a multi-techniques approach. 

---