Recrystallization effect during the dehydration of magnesium hydroxide

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

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)/Peq(T))b], where P° and Peq(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.

Doping magnesium hydroxide with sodium nitrate: a new approach to tune the dehydration reactivity of heat-storage materials

Thermochemical energy storage (TES) provides a challenging approach for improving the efficiency of various energy systems. Magnesium hydroxide, Mg(OH)2, is known as a suitable material for TES at temperature T>300 °C. In this work, the thermal decomposition of Mg(OH)2 in the absence and presence of sodium nitrate (NaNO3) is investigated to adapt this material for TES at T<300 °C. The most notable observations described for the doped Mg(OH)2 are (1) a significant reduction of the decomposition temperature Td that allows tuning the dehydration reactivity by varying the NaNO3 content. The Td decrease by 25 °C is revealed at a salt content Y≤2.0 wt %. The maximum Td depression of some 50 °C is observed at Y=15-20 wt %; (2) the NaNO3-doped Mg(OH)2 decomposes considerably faster under conditions typical for closed TES cycles (at T>300 °C in vapor atmosphere) than a pure Mg(OH)2; (3) the morphology of the dehydration product (MgO) dramatically changes. Differential scanning calorimetry, high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and vibrational spectroscopy (IR and Raman) are used to study the observed effects and to elucidate possible ways the NaNO3 influences the Mg(OH)2 dehydration and morphology of the dehydration product. The mechanism involving a chemical interaction between the salt and the hydroxide accompanied by nitrate embedding into brucite layers is discussed.