Tian-Calvet

Figure Bl.27.11. Schematic diagram of a Tian-Calvet heat-flux or heat-conduction calorimeter.

Fig. 2. Vertical section of a Tian-Calvet microcalorimetric element (16) inner vessel (A) and hollow truncated cone (B) wedged in the heat sink (C). Reprinted from Calvet and Prat (23) with permission of Dunod.

Calvet and Persoz (29) have discussed at length the question of the sensitivity of the Calvet calorimeter in terms of the number of thermocouples used, the cross section and the length of the wires, and the thermoelectric power of the couples. On the basis of this analysis, the micro-calorimetric elements are designed to operate near maximum sensitivity. The present-day version of a Tian-Calvet microcalorimetric element, which has been presented in Fig. 2, contains approximately 500 chromel-to-constantan thermocouples. The microcalorimeter, now commercially available, in which two of these elements are placed (Fig. 3) may be used from room temperature up to 200°C. 

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. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

A heat-flow calorimeter of Tian-Calvet type from Setaram maintained at a desired temperature, from room temperature up to 400°C, was used in connection with a volumetric apparatus equipped with a Me Leod gauge. Sample weights were typically 100 mg and ammonia doses 0.1 cm NTP. 

M. Laffitte. Trends in Combustion Calorimetry. The Use of the Tian-Calvet Microcalorimeter for Combustion Measurements. In Experimental Chemical Thermodynamics, vol. 1 S. Sunner, M. Mansson, Eds. IUPAC-Pergamon Press Oxford, 1979 chapter 17 3. 

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 adsorption experiment is conducted until a relatively high pressure is reached without significant evolution of heat and the adsorbed amount becomes negligible. Owing to the high sensitivity of the method, only small quantities of sample are required. The error may be around 1%, as for the Tian-Calvet calorimeter [129]. 

Measurements of the change of temperature of a solid on being stretched or compressed were first made by Joule in the 19th century and extensive studies on rubbers by Muller about 20 years ago. For plastics little work has been done until the recent studies of Haward and his associates40 and the work, using the Tian-Calvet microcalorimeter, of Godovskii et al.41 ... 

FIGURE 6.5 Schematic of Tian-Calvet calorimeter at the Canadian National Research Council. (Reproduced from Handa, Y.P., Calorimetric Studies of Laboratory Synthesized and Naturally Occurring Gas Hydrates, paper presented at AIChE 1986 Annual Meeting Miami Beach, November 2-7, 28 (1986b). With permission.)... 

As was explained in the previous section, when an adsorbate contacts an adsorbent, heat is released. The thermal effect produced can be measured with the help of a thermocouple placed inside the adsorbent and referred at the room temperature (see Figure 6.3) [3,31,34,49], This is a version of the Tian-Calvet heat-flow calorimeter [50], This calorimetric technique is distinguished by the fact that the temperature difference between the tested adsorbent and a thermostat is measured. Consequently, in the Tian-Calvet heat-flow calorimeter, the thermal energy released in the adsorption cell is allowed to flow without restraint to the thermostat [3,31,34,49],... 

The simultaneous measurement of the heat of adsorption and the adsorbed amount of H20 was performed by means of a Tian-Calvet microcalorimeter, operating at 303 K, connected to a volumetric apparatus. The samples were pretreated in vacuo at the chosen temperature and subsequently transferred into the calorimeter without further exposure to air. Small doses of water were subsequently admitted onto the sample, the pressure being continuously monitored by a transducer gauge (Baratron MKS, 0-100 Torr). 

When several thermocouples are connected to each other in series, then a multijunction thermocouple, or thermopile (e.g., Tian -Calvet thermopile), is obtained, with a much higher potential difference about 500 thermocouples may be combined to form a single thermopile. 

In contrast, DSC, designed in 1960 by Watson184 and O Neill,185 is a newer, more quantitative technique that does measure Ts and TR, but also measures very precisely the electrical energy used by separate heaters under either pan to make Ts = TR (this is power-compensated DSC, useable below 650° C). The power input into S minus the power input into R is plotted against Tr. High-temperature DSC (useful for TR > 1000°C) measures the heat fluxes by Tian-Calvet thermopiles rather than the electrical power, as a function of Tr. In a heat-flux DSC, both pans sit on a small slab of material with a calibrated heat resistance. The temperature of the calorimeter is raised linearly with time. A schematic DSC curve is shown in Fig. 11.80. 

We shall examine here the two major procedures for gas adsorption calorimetry (cf. Section 3.3.3). Both procedures make use of a diathermal, heat-flowmeter, Tian-Calvet microcalorimeter (cf. Section 3.2.2). 

Figure 3.15. Gas adsorption cell in a Tian-Calvet thermopile.

Gravimetry can be associated with adsorption calorimetry, either by using two samples (one in the microcalorimeter, the other in the microbalance) in contact with the same atmosphere of adsorptive (Gravelle, 1972) or using a single sample, located in the cylindrical pan of a microbalance and surrounded by a Tian-Calvet thermopile (LeParlouer, 1985). 

Two operational arrangements fulfilling the above requirements are represented in Figures 5.16b and 5.16c. For convenience, both are incorporated in a Tian-Calvet microcalorimeter with large cells (i.e. c. 100 cm3). The first device uses a disc stirrer (up and down movement) and cancels any temperature difference between the added solution and the adsorbent by placing both the adsorbent and the solution reservoir in the top part of the microcalorimetric cell (Rouquerol and Partyka, 1981). The second device uses a propeller which is given very fast half-turns (c. 10 per minute) by means of a hindered magnetic transmission which serves to damp the vibrations from the motor. 

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. 

Immersion calorimetry can be used to study either the surface chemistry or the texture of active carbons. A sensitive Tian-Calvet microcalorimeter is adaptable for either purpose, the main difference being in the choice of wetting liquids. 

The effect of surface dehydroxylation of a mesoporous silica on the Ar and N2 energetics of adsorption is illustrated in Figure 10.12. In the work of Rouquerol et al. (1979b) Tian-Calvet microcalorimetry was used to determine the variation of the differential enthalpy of adsorption as a function of surface coverage. Although strong... 

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. 

This work is a continuation of our earlier study [1] of the hydrogen interaction with intermetallic compound (IMC) AB2-type Tio.9Zro.1Mn . 3V0.5. The measurements were carried out in twin-cell differential heat-conducting Tian-Calvet type calorimeter connected with the apparatus for gas dose feeding, that permitted us to measure the dependencies of differential molar enthalpy of desorption (AHdes.) and equilibrium hydrogen pressure (P) on hydrogen concentration x (x=[H]/[AB2]) at different temperatures simultaneously. The measurements were carried out at 150°C, 170°C and 190°C and hydrogen pressure up to 60 atm. 

Calorimetric measuring techniques give additional information on thermodynamic data. The measuring technique is sophisticated and specialised instruments are not yet on the market. In general Tian-Calvet calorimeters are used by which the heat of adsorption is measured (Fig.l). From this the adsorption enthalpy and the adsorbed mass in principle can be calculated. 

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... 

In a joint work with A. A. Isirikyan with the participation of G. U. Rakhmat-Kariyev, we carried out direct measurements of differential heats of adsorption of water vapors on crystalline and molded zeolite NaA at 22 °C using a Tian-Calvet-type calorimeter. The calorimetric installation enabled us to measure thermal effects for each point of the adsorption isotherm for a period of 300 hours and more (Figure 1). The squares and circles in the upper part of the graph denote our data for... 

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