Isothermal Flow-Microcalorimetry

Isothermal Flow-Microcalorimetry Principles and Applications for Industry... 

Isothermal flow microcalorimetry is now an indispensable tool for the study and characterization of a variety of reaction systems within the chemical industry. It permits the study of these systems as a function of time in a non-destructive, non-inva-sive manner and hence allows accurate, quantitative information for both the... 

Adsorption measurements should preferably be supplemented by microcalorimetry, such as immersion and flow-microcalorimetry (see Figure 4.8. The lUPAC classification of adsorption isotherm shapes see also Figure 8.4). These techniques give additional information about the nature of surfaces of the adsorbent and the mode or mechanism of adsorption. 

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. 

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,... 

In a standard set-up for heat-flow gas adsorption microcalorimetry, the adsorbent bulb, (which is located in the thermopile (Figure 3.15)), is connected to a device to allow the simultaneous determination of the adsorption isotherm by one of the techniques listed in the previous sections. The simultaneous determination of the amount adsorbed and corresponding heat evolution allows one to assess the energy of adsorption (for the exact procedure, see Section 3.3.3) provided the following points are... 

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... 

An adsorption isotherm is a necessary but not sufficient way of describing the thermodynamics of ionic surfactant adsorption because a full description of the phenomenon requires knowledge of mutual interactions between all the components of the system. Such opportunity is offered by flow and batch liquid adsorption microcalorimetry [25-30]. 

Wadso (11) states that one of the main applications for isothermal microcalorimetry is the investigation of noncovalent binding processes by means of titration techniques. This use of the instrument employs a cell with a liquid (pure liquid, solution, or suspension), which is stirred. It is then possible to inject small quantities of another liquid and to measure the heat flow from the events that take place. One event will be the dilution of the injected material into the fluid in the cell thus, it is necessary to correct for these dilution effects. Other responses will be due to any interaction between the injected material and the solute or suspended matter in the cell. If it is assumed that the titrated material all interacts with the solute and suspended material, then there will be almost zero free concentration of the titrated sample. This will allow calculations of the enthalpy for the binding process and the stoichiometry. 

The adsorption of three argon/nitrogen binary mixtures at 310 K and up to 0.6 bar are presented. A continuous, quasi-equilibrium flow technique of adsorptive introduction was used to allow high-resolution isotherms to be obtained. These are compared to differential enthalpies of adsorption determined using adsorption microcalorimetry. 

Chapter 9, by Kiraly (Hungary), attempts to clarify the adsorption of surfactants at solid/solution interfaces by calorimetric methods. The author addresses questions related to the composition and structure of the adsorption layer, the mechanism of the adsorption, the kinetics, the thermodynamics driving forces, the nature of the solid surface and of the surfactant (ionic, nonionic, HLB, CMC), experimental conditions, etc. He describes the calorimetric methods used, to elucidate the description of thermodynamic properties of surfactants at the boundary of solid-liquid interfaces. Isotherm power-compensation calorimetry is an essential method for such measurements. Isoperibolic heat-flux calorimetry is described for the evaluation of adsorption kinetics, DSC is used for the evaluation of enthalpy measurements, and immersion microcalorimetry is recommended for the detection of enthalpic interaction between a bare surface and a solution. Batch sorption, titration sorption, and flow sorption microcalorimetry are also discussed. 

Adsorption microcalorimetry of N2 and Ar at 77K was carried out with an equipment described by Rouquerol (ref. 8) and which associates quasi equilibrium adsorption volumetry with isothermal low temperature microcalorimetry (using Tian Calvet heat flow-meters) so that two curves are continuously recorded (heat flow and quasi equilibrium pressure) as a function of the amount of gas introduced into the systems. Continuous plots of the adsorption isotherm and of the derivative enthalpy of adsorption h vs surface coverage may easily be doived (refs. 4,7). 

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