Calvet microcalorimeter

Since heat exchange between the calorimeter vessel and the heat sink is not hindered in a heat-flow calorimeter, the temperature changes produced by the thermal phenomenon under investigation are usually very small (less than 10 4 degree in a Calvet microcalorimeter, for instance) (23). For most practical purposes, measurements in a heat-flow calorimeter may be considered as performed under isothermal conditions. 

The Calvet microcalorimeter (16) is an improved version of the first heat-flow calorimeter described by Tian in 1924 (25). In this micro-... 

Fig. 3. Vertical section of the Calvet microcalorimeter (16) microcalorimetric element (A) the metal block (B) metallic cones (C and C ) thick metal cylinder (D) thermostat consisting of several metal canisters (E) electrical heater (F) switch (G) thermal insulation (I) and thermal lenses (J and J ). Reprinted from Calvet and Prat (S3) with permission of Dunod.

The basic principle of heat-flow calorimetry is certainly to be found in the linear equations of Onsager which relate the temperature or potential gradients across the thermoelements to the resulting flux of heat or electricity (16). Experimental verifications have been made (89-41) and they have shown that the Calvet microcalorimeter, for instance, behaves, within 0.2%, as a linear system at 25°C (41)-A. heat-flow calorimeter may be therefore considered as a transducer which produces the linear transformation of any function of time f(t), the input, i.e., the thermal phenomenon under investigation]] into another function of time ig(t), the response, i.e., the thermogram]. The problem is evidently to define the corresponding linear operator. 

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-...

Fig. 15. Volumetric line used in connection with a Calvet microcalorimeter (67).

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). 

Calvet microcalorimeters are particularly convenient for such studies. Figure 19 show s, for instance, the evolution of the differential heat of adsorption of oxygen, measured at 30°C with a Calvet calorimeter, as a function of the total amount of oxygen adsorbed on the surface of a sample (100 mg) of nickel oxide, NiO(200) (19, 73). The volume of the first... 

One of the conclusions deduced from the thermochemical cycle 2 in Table V, for instance, is that in the course of the catalytic combustion of carbon monoxide at 30°C, the most reactive surface sites of gallium-doped nickel oxide are inhibited by the reaction product, carbon dioxide. This conclusion ought to be verified directly by the calorimetric study of the reaction. Small doses of the stoichiometric reaction mixture (CO + IO2) were therefore introduced successively in the calorimetric cell of a Calvet microcalorimeter containing a freshly prepared sample of gallium-doped... 

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... 

Other instruments include the Calvet microcalorimeters [113], some of which can also run in the scanning mode as a DSC. These are available commercially from SETARAM. The calorimeters exist in several configurations. Each consists of sample and reference vessels placed in an isothermally controlled and insulated block. The side walls are in intimate contact with heat-flow sensors. Typical volumes of sample/reference vessels are 0.1 to 100 cm3, The instruments can be operated from below ambient temperatures up to 300°C (some high temperature instruments can operate up to 1000°C). The sensitivity of these instruments is better than 1 pW, which translates to a detection limit of 1 x 10-3 W/kg with a sample mass of 1 g. 

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. 

S. Murata, M. Sakiyama, S. Seki. Construction and Testing of a Sublimation Calorimetric System Using a Calvet Microcalorimeter. J. Chem. Thermodynamics 1982, 14, 707-721. 

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. 

Calorimeters, see Microcalorimeters Calorimetry, surface acidity, 27 121 Calvet microcalorimeter, 22 197-201, 38 172-... 

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]). 

Methods. The differential heats of adsorption of reagents and the differential heat of their interaction on the nickel oxide surface were measured in a Calvet microcalorimeter with a precision of 2 kcal. per mole. The apparatus has been described (18). For each adsorption of a single gas, small doses of gas are allowed to interact with a fresh nickel oxide sample (100 to 200 mg.) placed in the calorimeter cell maintained at 30°C. At the end of the adsorption of the last dose, the equilibrium pressure is, in all cases, 2 torr. Duplication of any adsorption experiment on a new sample gives the same results within 2 kcal. per mole of heat evolved and 0.02 cc. of gas adsorbed per gram. Electrical conductivities of the nickel oxide sample are measured in an electrical conductivity cell with platinum electrodes (1) by a d.c. bridge. 

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 ... 

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). 

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). 

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. 

Heat-flow microcalorimetry was developed originally by Calvet (39). He modified a calorimeter previously conceived by Tian (40). In the Calvet microcalorimeter, heat flow is measured between the system and the heat block... 

Differential heats of adsorption of oxygen on NiO(200°) were measured at 30° with a Calvet microcalorimeter (44). The surface coverage corresponding to the break in the adsorption isotherm (Fig. 3) is indicated in Fig. 4 by an arrow. The irreversible adsorption, which is fast, yields high heats (initial value, 60 kcal/mole) (Fig. 4), whereas the heats produced during the partially reversible adsorption are smaller and are evolved slowly. Heats of desorption and readsorption of oxygen have also been measured at 30° (2-4 kcal/mole). These low heats are explained by a molecular adsorption (25). 

Heats of oxygen adsorption and amounts of a weakly bound oxygen were determined using a high temperature Calvet microcalorimeter, TPD and electrochemical method [13]. 

Heats of adsorption were measured at 303 K with a Tian-Calvet microcalorimeter (Setaram), connected to a vacuum/gas volumetric apparatus, that enabled the simultaneous determination of adsorbed amounts by the stepwise contact with subsequent doses of the adsorptive [5]. 

Heats of adsorption of water vapour on J7-A1203 have been determined by means of a Tian-Calvet microcalorimeter at temperatures between 25 and 740°C. 8... 

The aim of this classification is mainly historical. It introduces the reader to the way of thinking of the thermochemists prior to 1912 and does not pretend to be comprehensive and modem. Quite logically, it does not include, for instance the Tian-Calvet microcalorimeter. 

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