Michel Soustelle

Experimental methods useful in the kinetic modelling of heterogeneous reactions

Abstract A theorem on the separation of models is reported which shows how it may be possible to describe the kinetics of a reaction involving a solid: a physico-chemical model in which ‘reactivity’ has been defined, and a geometrical model to which a ‘space function’ corresponds. Then a series of rigorous experimental methods based on this theorem are presented. Their use improves and validates the kinetic modelling of a heterogeneous reaction.

Reduction of uranium oxide U3O8 to UO2 by hydrogen

Abstract The reduction of U3O8 by hydrogen at 510 °C has been studied by thermogravimetry. After a first reduction to UO2, the sample is oxidised; then the analysis of the shape of the kinetic curves of a second reduction leads to the determination of the specific growth rate, according to a contracting-sphere model provided that the rate-limiting step occurs on the external surface of the particles. A mechanism of reduction is then proposed which accounts for the observed variations of the rate with the partial pressure in hydrogen.

Characterisation of the chemical reactivity of a CaCO3 powder for its decomposition

Abstract The decomposition of calcium carbonate has been studied by isothermal and isobaric thermogravimetry at 700 °C, under various pressures of carbon dioxide. The kinetic curves, which have a sigmoidal shape, have been interpreted with a model involving nucleation and growth processes based on the Mampels model. It was supposed that nucleation occurs at the surface of all the particles and that growth proceeds isotropically inward in each of them. This model allowed to characterise the chemical reactivity of several CaCO3 powders by means of the determination of both their specific nucleation frequency and their specific growth rate.