Revealing the effect of water vapor pressure on the kinetics of thermal decomposition of magnesium hydroxide

Abstract

This study aims to establish an advanced kinetic theory for reactions in solid state and solid–gas systems, achieving a universal kinetic description over a range of temperature and partial pressure of reactant or product gases. The thermal decomposition of Mg(OH)₂ to MgO was selected as a model reaction system, and the effect of water vapor pressure p(H₂O) on the kinetics was investigated via humidity controlled thermogravimetry. The reaction rate of the thermal decomposition process at a constant temperature was systematically decreased by increasing the p(H₂O) value, accompanied by an increase in the sigmoidal feature of mass-loss curves. Under nonisothermal conditions at a given heating rate, mass-loss curves shifted systematically to higher temperatures depending on the p(H₂O) value. The kinetic behavior under different temperature and p(H₂O) conditions was universally analyzed by introducing an accommodation function (AF) of the form (P°/p(H₂O))^a[1 − (p(H₂O)/P_eq(T))^b], where P° and P_eq(T) are the standard and equilibrium pressures, respectively, into the fundamental kinetic equation. Two kinetic approaches were examined based on the isoconversional kinetic relationship and a physico-geometrical consecutive reaction model. In both the kinetic approaches, universal kinetic descriptions are achieved using the modified kinetic equation with the AF. The kinetic features of thermal decomposition are revealed by correlating the results from the two universal kinetic approaches. Furthermore, advanced features for the kinetic understanding of thermal decomposition of solids revealed by the universal kinetic descriptions are discussed by comparing the present kinetic results with those reported previously for the thermal decomposition of Ca(OH)₂ and Cu(OH)₂.