Pulsed flow calorimetry

Finally, experimental procedures differing from that described in the preceding examples could also be employed for studying catalytic reactions by means of heat-flow calorimetry. In order to assess, at least qualitatively, but rapidly, the decay of the activity of a catalyst in the course of its action, the reaction mixture could be, for instance, either diluted in a carrier gas and fed continuously to the catalyst placed in the calorimeter, or injected as successive slugs in the stream of carrier gas. Calorimetric and kinetic data could therefore be recorded simultaneously, at least in favorable cases, by using flow or pulse reactors equipped with heat-flow calorimeters in place of the usual furnaces. 

A comparison between pulsed flow and conventional pulsed static calorimetry techniques for characterizing surface acidity using base probe molecule adsorption has been performed by Brown and coworkers [20, 21]. In a flow experiment, both reversible and irreversible probe adsorption occurring for each dose can be measured, and the composition of the gas flow gas can be easily modified. The AHads versus coverage profiles obtained from the two techniques were found to be comparable. The results were interpreted in terms of the extent to which NH3 adsorption on the catalyst surface is under thermodynamic control in the two methods. 

Table 1 Pulsed combustion flow calorimetry (PCFC) data for the polycarbonates Comparison of PCFC data for polymer 6 to that for BPA polycarbonate...

S.P. FeUx, C. SavUl-Jowitt, D.R. Brown, Base adsorption calorimetry for characterising surface aridity a comparison between pulse flow and conventional static techniques. Thermochim. Acta 433, 59-65 (2005)... 

The pumps required for flow calorimetry have to be carefully considered because they can be sources of error. Accentuated or irregular pulsing can cause severe baseline perturbation and the pump rollers can damage the cells. The choice of flow rate is crucial for several reasons. If it is too fast, heat will be carried away from the measuring vessel in the distal transmission tubing. If it is too slow, then there may well be problems of cell sedimentation and oxygen... 

Figure 11.5 Typical curve for a continuous titration calorimetry study of an exothermic reaction, using the calorimeter of Figure 11.1 in the heat flow isothermal mode of measurement./ is the frequency of the constant energy pulses supplied to the heater C in Figure 11.1 b. Adapted from [196,197],...

When the system is used in pulse mode, it allows the measurement of heats of adsorption of a gaseous reactant on a solid or interaction heats between a gaseous reactant and pre-adsorbed species. When used as a flow reactor, it allows the kinetic study of catalytic reactions as well as the study of the activation or the aging of the catalyst. This is also a suitable system to perform calorimetric temperature programmed reduction (TPR), temperature programmed oxidation (TPO) or temperature programmed desorption (TPD) experiments. In addition to calorimetry, temperature programmed desorption (TPD) of adsorbed probe molecules can in principle also be used to estimate heats of adsorption [19]. 

Liquid-flow microcalorimefry is a reliable method to measure simultaneously the enthalpy changes and amounts of adsorption under dynamic conditions. Calorimetry experiments may be carried out in two different ways by following a pulse or saturation operating mode [64, 78-83]. In the pulse mode, small aliquots of a stock solution at a known concentration are injected into the carrier liquid (pure solvent) flowing through the adsorbent bed placed inside the calorimetric cell. In this case, the calorimetric system contains an additional loop injection facility (a manual injection valve with appropriate injection loops). The interpretation of the enthalpy data obtained is straightforward only when the whole amount of the solute injected is irreversibly adsorbed on the solid surface. 

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