Calvet-type microcalorimeter

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. 

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]). 

In the work of Schirmer et al. (1980), a Tian-Calvet type microcalorimeter was used to determine the energetics of adsorption for n-hexane, cylohexane and benzene on NaY zeolite. The differentia] adsorption energies for n-hexane and benzene are plotted in Figure 11.17 as a function of the amounts adsorbed. 

Microqalorimetry. Following a pretreatment at 623 K under vacuum (<10 torr) overnight, the differential heat of adsorption of ammonia was measured at 423 K using a SETARAM - Tian-Calvet type microcalorimeter associated with a volumetric equipment allowing the simultaneous determination of the adsorption isotherm. 

A new experimental procedure based on the isothermal desorption of vapour is proposed to extend the domain of characterisation of porous solids and powders by capillary condensation until the macropore range. The set-up is based on the use of a Tian-Calvet type microcalorimeter that insures a lull control of temperature gradients around the sample and allows the desorption isotherm to be determined very close to the saturation pressure. The principle of the experiment is described and the first results obtained for water desorption are compared to measurements based on gravimetry as well as to pore size distributions obtained by mercury porosimetry. 

In a gas-solid open system, as the one described above (i.e., a Tian-Calvet type microcalorimeter connected to a high-vacuum gas-volumetric apparams) the system will exchange with the environment not only heat but also work and matter. Work is due to the reversible, isothermal transfer of matter to both gas (V dp) and adsorbed (RT dn ) phase. So, the isothermal heat measured in the calorimeter at constant T is defined by the Eq. 1.17 ... 

A Calvet-type microcalorimeter (see Figure 4.4) enables us to measure the heat flux that is released or absorbed by the sample that is in the cell. It can be easily proven that the reaction speed is related to the heat flux using the following equation ... 

The acidic and adsorptive properties of the samples in gas phase were evaluated in a microcalorimeter of Tian-Calvet type (C80, Setaram) linked to a volumetric line. For the estimation of the acidic properties, NH3 (pKa = 9.24, proton affinity in gas phase = 857.7 kJ.mol-1, kinetic diameter = 0.375 nm) and pyridine (pKa = 5.19, proton affinity in gas phase = 922.2 kJ.mol-1, kinetic diameter = 0.533 nm) were chosen as basic probe molecules. Different VOC s such as propionaldehyde, 2-butanone and acetonitrile were used in gas phase in order to check the adsorption capacities of the samples. 

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 adsorption calorimetric measurements were carried out at 423 K on a SETARAM microcalorimeter of calvet-type connected with a standard volumetric adsorption apparatus. The pressure measurements were made using a MKS Baratron membrane manometer. Prior to the ammonia adsorption, the samples (900 mg) were carefully calcined in high vacuum at 673 K for 15 h. 

Microcalorimetric studies of the adsorption of ammonia (Critical diameter 0.3 nm) have been carried out using a Tian-Calvet type heat-flow microcalorimetric (C-80 model, Setaram, France). The microcalorimeter has been connected to a volumetric vacuum adsorption unit for catalyst treatment and probe molecule delivery. A validyne low pressure transducer (USA) attached with vacuum system has been used for precision pressure mecisurement. 

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. 

Various commercial calorimeters are now available for routine heat of immersion measurements. For research it is preferable to use a calorimetric technique which is consistent with thermodynamic requirements. We recommend the employment of a Tian-Calvet type of microcalorimeter, which by means of two thermopiles composed of a large number of thermocouple junctions allows the heat flux to be measured accurately at practically constant temperature (AT < 10 K). Whichever technique is used, the experiments must be devised in a manner which will allow the evaluation of a number of corrective terms due to partial evaporation of the liquid, bulb breaking, stirring and effect of atmospheric pressure. In practice, this does not present difficulties because the detailed procedures and calculations are described in the literature. ... 

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. 

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). 

A batch microcalorimetric experiment, very similar to the one just described, is possible with a diathermal heat flowmeter type of microcalorimeter, which is less versatile than the Tian-Calvet microcalorimeter (especially in its temperature range and ultimate sensitivity), but of a simpler design. In the Montcal microcalorimeter (Partyka et al., 1989), the thermopile with up to 1000 thermocouples is replaced by a few thermistors. 

The SETARAM C 80 calorimeter is particularly well adapted for the investigation of thermal storage reactions. It is a modern type of Calvet microcalorimeters with a large sample volume (Fig. 1) (2). 

Apparatus, Calvet microcalorimeter, low-temperature type heating rate, 0.5 °C h" . 

The best-known calorimeter of this type was developed by Tian and Calvet (Fig. 17). Here the defined heat-conduction path to the thermostated surroundings (a large aluminum block) consists of a large number of differential thermocouples coupled in series (thermopile). This arrangement permits optimum determination of the heat flow rate to the surroundings, and such an instrument can be very sensitive (microcalorimeter). 

A Tian-Calvet microcalorimeter (model BT 2.15, Setaram, France) was used to measure the enthalpies of adsorption of propane and propylene at room temperature. The samples (0.1 g) were treated under different conditions (i) vacuum at 523 K, (ii) vacuum at 773 K, (iii) He at 1073 K and (iv) H2 at 1073 K, all for 4h. Then, thqr were s ed into a Pyrex RMN tube in pure He and placed into the microcalorimetric celL A conventional manometric system coupled to the microcalorimet was used to m isare the amount adsorbed employing a (type 660) manometer witii a pr xsion of 0.001 Torr. The maximum apparent leak rate of the manometric system (including tire calorimetric cells) was 10 Torrmiri in a volume of about 60 cm. ... 

An adsorption micro calorimeter for the simultaneous determination of the differential heat of adsorption and the adsorption isotherm for gas-solid systems are designed, built, and tested. For this purpose, a Calvet heat-conducting microcalorimeter is developed and is connected to a gas volumetric unit built in stainless steel to record adsorption isotherms. The micro calorimeter is electrically calibrated to establish its sensitivity and reproducibility, obtaining K=154.34 0.23 WV h The adsorption microcalorimeter is used to obtain adsorption isotherms and the corresponding differential heats for the adsorption ofCO on a reference solid, such as a NaZSM-5 type zeolite. Results for the behavior of this system are compared with those obtained with commercial equipment and with other studies in the literature. 

However, the same type of sensor can also be used in terrestric investigations, yet with a strongly reduced sensitivity. A Beckman-Monitor-System (type 123301 O2/T) was adapted to 100-mL vessels of a Calvet microcalorimeter. The electrode was placed high enough above the heat flux meter that no interference between the two signals could be observed. This system was - e.g. - applied during investigations of crabs [64] or snakes and lizards [38,65],... 

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