Calvet calorimetry

The data may be obtained from calorimetric methods usually employed for the study of secondary reaction and thermal stability as DSC, Calvet calorimetry, and adiabatic calorimetry. 

The data required to answer this question may be obtained from reaction calorimetry for the accumulation in combination with DSC, Calvet calorimetry, or adiabatic calorimetry for the thermal stability. 

The thermochemical data may be determined by calorimetric methods. If no stirring is required for the reaction, DSC and Calvet Calorimetry may provide the required data (Figure 6.15). In the example given, a reactant is added in one shot... 

The volume of gas potentially released by a reaction (including secondary reactions in criticality classes 4 and 5) can be known from the chemistry or measured experimentally by appropriate calorimetric methods, as for example, Calvet calorimetry, mini-autoclave, Radex, or Reaction Calorimetry (as V at T k. and /Jmes). It must be corrected for the temperature to be considered, MTSR (class 2), MTT (class 3 or 4), or Tf (class 5). Where the gas stems from the main reaction, only the accumulated fraction (X) will be released ... 

With the approach using isothermal thermograms, the different thermograms must be checked for consistency. In certain cases when the peaks are well separated, as for consecutive reactions, they may be treated individually and the heat release rates can be extrapolated separately, and used for the TMRai calculation. The reaction that is active at lower temperature will raise the temperature to a certain level where the second becomes active, and so on. So under adiabatic conditions, one reaction triggers the next as in a chain reaction. In certain cases, in particular for the assessment of stability at storage, it is recommended to use a more sensitive calorimetric method as, for example, Calvet calorimetry or the Thermal Activity Monitor (see Section 4.3), to determine heat release rates at lower temperatures and thus to allow a reliable extrapolation over a large temperature range. Complex reactions can also easily be handled with the iso-conversional method, as mentioned below. 

Another powerful technique to provide thermal information on main and secondary reactions is to use Calvet calorimetry. The calorimeter C80, commercial-... 

The best accuracy achieved by calorimetric methods, in particular in Calvet calorimetry, is about 10 calthmol". That is about the same as... 

Figure 2. Comparison of heats of sorption for N2 and O2 on CaA and CO2 on NaX zeolites measured with SIM (open symbols) and Tian-Calvet calorimetry (full symbols).

Heat Calvet Calorimetry A Calvet calorimeter is a heat exchanging calorimeter with a cylindrical type detector working in isothermal and scanning modes. 

Unusual are set-ups combining Tian-Calvet calorimetry and high pressure volumetric lines. One of the few examples is reported in the work of Anikina and Verbetsky... 

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. 

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). 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

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... 

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. 

A. Rojas-Aguilar, A. Valdes-Ordonez. Micro-combustion Calorimetry Employing a Calvet Heat Flux Calorimeter. J. Chem. Thermodynamics 2004, 36, 619-626. 

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. 

A. Rojas-Aguilar, M. Martmez-Herrera. Enthalpies ofCombustion and Formation of Fullerenes by Micro-combustion Calorimetry in a Calvet Calorimeter. Thermochim. Acta 2005, 437, 126-133. 

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]). 

The above-mentioned method of deformation calorimetry has found a rather wide application. Modifications of the original design were constructed 72-75) and applied for investigating the thermomechanical behaviour of polymers and polymer composites. At the same time, the commercial Calvet-type calorimeters has been used in thermomechanical experiments on rubbers not only in the uniaxial mode 76-78 but also in torsion 79 80). Thus, deformation calorimetry has proved to be quite adequate in terms of sensitivity, specificity, rapidity and reliability and therefore seems to be the most promising experimental method of thermomechanical type. 

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). 

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). 

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. 

Figure 12.1 Closed-system setup for immersion calorimetry of powders in a Tian-Calvet heatflow microcalorimeter. (Adapted from [7].)...

The term differential scanning calorimetry has become a source of confusion in thermal analysis. This confusion is understandable because at the present time there are several entirely different types of instruments that use the same name. These instruments are based on different designs, which are illustrated schematically in Figure 5.36 (157). In DTA. the temperature difference between the sample and reference materials is detected, Ts — Tx [a, 6, and c). In power-compensated DSC (/), the sample and reference materials are maintained isothermally by use of individual heaters. The parameter recorded is the difference in power inputs to the heaters, d /SQ /dt or dH/dt. If the sample is surrounded by a thermopile such as in the Tian-Calvet calorimeter, heat flux can be measured directly (e). The thermopiles surrounding the sample and reference material are connected in opposition (Calvet calorimeter). A simpler system, also the heat-flux type, is to measure the heat flux between the sample and reference materials (d). Hence, dqjdi is measured by having all the hot junctions in contact with the sample and all the cold junctions in contact with the reference material. Thus, there are at least three possible DSC systems, (d), (c), and (/), and three derived from DTA (a), [b), and (c), the last one also being found in DSC. Mackenzie (157) has stated that the Boersma system of DTA (c) should perhaps also be called a DSC system. 

It should be noted that calorimetry is one of the oldest physicochemical experimental methods with a history of more than a century of scientific appUcation for which numerous experimental devices and techniques have been designed, tested, and applied. Since the first extensive review of the use of microcalorimetry in the fields of biochemistry, biotechnology, and biology by Calvet and Prat in 1956 [2], a wide variety of different microcalorimeters have been developed and employed in various branches of the life sciences. 

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. 

Tre] Calvet type calorimetry Room temperature / Cr2peS4... 

Kes] XRD, Calvet type calorimetry, chemical analysis Up to 700°C / Cr2Fe 4... 

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