Thermogravimetry Kinetic modelling and mechanisms of solid-gas reactions

Although widespread in various sectors of industrial chemistry and metallurgy, and studied for a very long time, chemical transformations of divided or massive solids, such as thermal decompositions or reactions between a solid and one or more gases, are difficult for the neophyte engineer or researcher to understand and master. On the one hand, the production of oxides with specific usage properties (CaO, UO ₂ , ZrO ₂ , CeO ₂ ...), gas purification (H ₂ S, SO ₂ ...), chemical energy storage, etc. are all chemical processes that involve one or more reactions taking place in a heterogeneous reactor, the modeling of which always poses the same problem: which reaction rate equation should be taken? On the other hand, the development of materials that are increasingly resistant to aggressive high-temperature gaseous environments (superalloys, ceramics, cermets, etc.) requires the addition of elements or the search for new microstructures, and the answer to the question: what are the mechanisms involved in material degradation? Heterogeneous kinetics is a powerful method for providing answers to these questions, since it enables us to establish a rate equation for a given chemical reaction and determine its mechanism.

While it's easy to obtain kinetic curves from commercial thermobalances, the search for and validation of realistic models is less straightforward than it might seem, given the operating software supplied by manufacturers. You need to understand the basics of kinetic model construction to avoid the pitfalls of automated searches for parameters to fit mathematical models. The aim of this article is to pass on the knowledge and know-how essential for those who want the most realistic possible description of the course of the reaction at the level of the solid phases and the elementary mechanisms of the processes involved: the germination of the new phase and the growth of these germs.

In the vast majority of cases, the simplifying assumptions used in heterogeneous kinetics (pseudo-stationarity, determining step, simple geometric form of the solid) allow us to interpret the kinetic curves obtained by thermogravimetry under conditions of constant temperature and partial pressure over time. The general equation used to express the reaction rate, based on the separation of thermodynamic and morphological variables, takes the form of a product of the two functions φ (T, P _i ...) and S _m (time, geometry, γ) where φ and γ are characteristic quantities...