Impact of Evolved Gas on the Reaction Kinetics in Inorganic Solid-Gas Systems

摘要

The effect of the partial pressure of evolved gas (p(gas)) on the kinetics of reversible reactions in solid-gas systems has long been studied for gaining more rigorous kinetic descriptions of many important reactions [1]. Three different phenomena have been observed as being caused by the influence of evolved gas. Generally, the effect of p(gas) on the reversible thermal decomposition of inorganic solids is the normal effect with respect to the chemical equilibrium, but the inverse effect is also observed in some selected reaction systems. In the thermal decomposition of some basic carbonates [2,3] and hydrogen carbonates [4,5], the normal and inverse effects are observed with respect to evolved CO2 and water vapor, respectively. The evolved gas also influences on the kinetics of the subsequent reaction in a scheme of consecutive reactions regulated by the physico-geometrical constraints. Typical examples are the thermally induced carbonation of alkaline and alkaline earth hydroxides in a CO2 atmosphere [6], in which the water vapor produced by the primary reaction catalyses the subsequent carbonation reaction. In this presentation, the mechanistic interpretation and possible kinetic description of these three different types of the p(gas) effects are discussed by reviewing our recent results. Concerning the normal effect of p(gas) on the reversible thermal decomposition of solids, the theoretical background of the possible accommodation function of p(gas) to be introduced into the fundamental kinetic equation is discussed on the basis of classical nucleation and interface reaction theories. Using an accommodation function a(p(gas), Peq(T)), an universal kinetic approach to the thermal decomposition of solids under different p(gas) is demonstrated. The inverse effect of p(gas) is described by focusing on the catalytic effect of water vapor on the crystal growth of the product solids in the surface product layer and as exemplified by the formation process of nano-sized metal and metal oxides via the thermal decomposition of precursors [7]. For the catalytic effect of the evolved gas on the subsequent chemical reaction, the kinetic feature is revealed through the kinetic analysis of the consecutive reactions in solids. The catalytic action is discussed based on the physico-geometrical reaction mechanism, providing a suggestion for the effective CO2 capture by the alkaline and alkaline earth oxides.