Chapter 5 - Kinetics and Mechanisms of Solid-Gas Reactions

Abstract

Abstract The kinetic modeling of chemical reactions in gas-solid systems necessitates a mathematical expression of the rate as a function of variables. For a long time the rate equations used for transformations of powders were different from those describing the oxidation of a metal or an alloy. Both approaches and a unified theory that intends to cover any type of reacting solids with the production/reaction of gases are discussed. Owing to kinetic assumptions of pseudo-steady-state and rate-determining step approximations, a general equation of the rate may be established. The rate d α dt is then a product of two functions, one depending on the thermodynamic variables and the other depending on the geometrical variables and time. According to the various possibilities of geometry, growth direction, rate-determining step localization, nucleation, and/or growth processes, about 40 kinetic models may be obtained for the variations of the kinetic rate with time. The mechanism decomposition in elementary steps is shown to get the calculation of the function, which governs the effect of the thermodynamic variables on the kinetic rate. A series of experimental tests (most of them based on the jump method) are shown to validate the kinetic assumptions and the geometrical modeling.