Catalysis isothermal experiments

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

One could think about measuring b directly in a separate adsorption experiment. However, it has been found that the adsorption thus measured has no quantitative relation to the adsorption leading to a catalytic reaction. Not only the amounts adsorbed, but even the shape of the isotherm and the sensitivity to poisoning or surface alterations are totally different. The reason is that the adsorption experiment measures the total surface under favorable conditions (B.E.T. method), whereas the catalysis takes place on a quantitatively entirely different active ... 

This LANGMUIR isotherm is still very popular in the interpretation of adsorption and catalysis experiments, although it is obvious that the underlying hypotheses are rather restrictive ... 

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