Heterogeneous catalysis 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... 

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

J.A. (1992) Applications of adsorption microcalorimetry to the study of heterogeneous catalysis. Adv. Catal.,... 

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

Microcalorimetry has gained importance as one of the most reliable method for the study of gas-solid interactions due to the development of commercial instrumentation able to measure small heat quantities and also the adsorbed amounts. There are basically three types of calorimeters sensitive enough (i.e., microcalorimeters) to measure differential heats of adsorption of simple gas molecules on powdered solids isoperibol calorimeters [131,132], constant temperature calorimeters [133], and heat-flow calorimeters [134,135]. During the early days of adsorption calorimetry, the most widely used calorimeters were of the isoperibol type [136-138] and their use in heterogeneous catalysis has been discussed in [134]. Many of these calorimeters consist of an inner vessel that is imperfectly insulated from its surroundings, the latter usually maintained at a constant temperature. These calorimeters usually do not have high resolution or accuracy. 

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]). 

On solids, the amount and strength of acid or basic sites are quite independent parameters, so both of them must be analyzed independently for a complete characterization. Additionally, several different families of acid sites may occur in the same solid surface, so their distribution must be characterized. The key to the effective utilization of microcalorimetry in heterogeneous catalysis is the judicious choice of gas-phase molecules for study. 

The measurement of heats of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis in the past few decades to gain more insight into the nature of gas-surface interactions and the catalytic properties of solid surfaces. Specific attention will be focused on group IIIA containing samples in this section. 

The main objective of this article is to present a survey of theoretical and applied aspects of microcalorimetry to heterogeneous catalysis with particular emphasis on the determination of acid-base properties of metal oxides and mixed metal oxides. This review is not meant to be comprehensive but to provide an overview of recent work done in the area. Additional applications can be found in recent reviews 1-4). 

Importantly, the combination of such measurements allows correlations to be sought between surface thermodynamic and kinetic properties. We anticipate that this approach will be a growing application of microcalorimetry in heterogeneous catalysis. 

For most effective utilization in heterogeneous catalysis research, adsorption microcalorimetry must be used in combination with other techniques which probe the nature of the surface-adsorbed species. In the case of acidity studies, for example, IR spectroscopy is needed to identify which regions of the acid strength distribution correspond to Lewis verus Brpnsted acid sites. As the application of adsorption microcalorimetry in heterogeneous catalysis evolves from studies involving primarily probe molecules to studies involving more reactive molecules, it will become even more important to combine these calorimetric studies with surface spectroscopic investigations. 

Adsorption microcalorimetry is the measure of the heat of adsorption evolved when dosing measured small amounts of a vapor probe on a surface. Cardona-Martinez and Dumesic [36] summarized the results obtained for oxides, zeolite, and metal catalysts before 1992. Summaries of the application of these techniques to gas-solid interactions and heterogeneous catalysis have been published recently [37-39]. As done by Auroux and Gervasini [40] for a number of binary metal oxides, calorimetric studies of the acidity and basicity are mostly performed using ammonia as an acidity probe and carbon dioxide as a basicity probe [41]. 

The key for effective utilisation of microcalorimetry in heterogeneous catalysis is the judicious choice of gas-phase molecules for study. Although total number of surface active sites and potentially active centres can be estimated by this technique, the obtained values are strongly dependent on the nature and size of the probe molecule. As a general rule, no matter which surface property is examined (acidic, basic or redox) probe molecule should be stable with temperature and with time. Furthermore, as discussed before, the adsorbed probe should be sufficiently mobile to equilibrate on active sites at temperature of the experiment. 

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

This series provides systematic and detailed reviews of topics of interest to scientists and engineers in the catalysis field. The coverage includes all major areas of heterogeneous and homogeneous catalysis, and also specific applications of catalysis such as NO control, kinetics and experimental techniques such as microcalorimetry. Each chapter is compiled by recognised experts within their specialist fields, and provides a summary of the current literature. 

---