Heat-flow microcalorimetry

R. L. Moss and L. Whalley Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis P. C. Gravelle... 

Heat-Flow Microcalorimetry and Its Application to Heterogeneous Catalysis... 

The adsorption of at least one reactant is the first step of the mechanism of any catalytic reaction. This step is followed by surface interactions between adsorbed species or between a gaseous reactant and adsorbed species. In many cases, these interactions may be detected by the successive adsorptions of the reactants in different sequences. Heat-flow microcalorimetry can be used with profit for such studies (19). 

Heat-flow microcalorimetry may be used, therefore, not only to detect, by means of adsorption sequences, the different surface interactions between reactants which constitute, in favorable cases, the steps of probable reaction mechanisms, but also to determine the rates of these surface processes. The comparison of the adsorption or interaction rates, deduced from the thermograms recorded during an adsorption sequence, is particularly reliable, because the arrangement of the calorimetric cells remains unchanged during all the steps of the sequence. Moreover, it should be remembered that the curves on Fig. 28 represent the adsorption or interaction rates on a very small fraction of the catalyst surface which is, very probably, active during the catalytic reaction (Table VI). It is for these... 

The calorimetric method which has been outlined in this section is not applicable to the study of surface interactions or of reaction mechanisms which occur between reversibly adsorbed species. But, even in these unfavorable cases, heat-flow microcalorimetry may still yield useful information concerning either the nature of the adsorbed species, the distribution of sites, or the irreversible modifications which occur frequently on the catalyst surface during the course of the reaction. 

P. C. Gravelle reviews Heat-Flow Microcalorimetry and shows its applications to the study of adsorption and heterogeneous catalysis. 

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]). 

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... 

More work on SMSI has to be done to elucidate the exact respective part - if any - of decoration and electron effects and a possible technique for that would be heat flow microcalorimetry. Acknowledgments The author thanks Dr. Pichat for his interest, and MM, J, Disdier and H, Courbon for technical assistance (photoconductance and CDIE analyses respectively),... 

Rossi, P.F., Adsorption heat-flow microcalorimetry applied to coal surlace properties. Adsorpt. Sci. Technol., 9(3), 148-189(1993). 

P. C. Gravelle, Heat-flow microcalorimetry and its application to heterogeneous catalysis, Advances in Catalysis, vol. 22, pp. 191-263, 1972. 

---