Calorimeter heat-flow Tian-Calvet

The heat-flow calorimeter of the Tian-Calvet type used for the determination of the adsorption heats of ammonia and the applied experimental technique were recently reported [9]. Ammonia adsorption was carried out at 80 C. All samples were pretreated under vacuum at 200 and 450 C, respectively, prior to any calorimetric measurement. 

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

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. 

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]). 

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],... 

Heat-flow adsorption microcalorimetry. The most important type of isothermal calorimeter in current use is that based on the principle of the heat flowmeter, which was first applied by Tian (1923) and improved by Calvet (Calvet and Prat, 1958,... 

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... 

On the other hand, for slow reactions, adiabatic and isothermal calorimeters are used and in the case of very small heat effects, heat-flow micro-calorimeters are suitable. Heat effects of thermodynamic processes lower than 1J are advantageously measured by the micro-calorimeter proposed by Tian (1923) or its modifications. For temperature measurement of the calorimetric vessel and the cover, thermoelectric batteries of thermocouples are used. At exothermic processes, the electromotive force of one battery is proportional to the heat flow between the vessel and the cover. The second battery enables us to compensate the heat evolved in the calorimetric vessel using the Peltier s effect. The endothermic heat effect is compensated using Joule heat. Calvet and Prat (1955, 1958) then improved the Tian s calorimeter, introducing the differential method of measurement using two calorimetric cells, which enabled direct determination of the reaction heat. 

The acid-base properties of the samples were investigated using adsorption of appropriate probe molecules, namely ammonia and sulfur dioxide, monitored by microcalorimetry. The microcalorimetric studies were performed at 353 K for sulfur dioxide adsorption and at 423 K for ammonia adsorption in a heat flow calorimeter of Tian-Calvet type (Setaram C80), linked to a conventional volumetric apparatus. Before each experiment the samples were outgassed overnight at 673 K. 

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). 

Busey et al. (1984) constructed a flow mixing device from a cylindrical coil of stainless steel tube which fitted into a high-temperature Tian-Calvet type, heat-flux calorimeter. Fluids were pumped at the same flow rate through both sample and reference cells. Platimun-rhodimn capillary... 

Heat capacities above 90K and heats of dissociation of several of the most Important gas hydrates have been measured with an automated Tian-Calvet heat flow calorimeter equipped for simultaneous measurement of the pressure. Hydrate compositions were determined by analysis of the decomposition products. The contribution of the water molecules to the heat capacity appears to be nearly the same as for the water molecules in ice. 

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... 

Instrument that is usually employed in this technique is differential heat flow calorimeter of Tian-Calvet type equipped by a stirring system. A programmable... 

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