Preparation and thermal stability of manganese-containing hydrotalcite, [Mg0.75MnII0.04MnIII0.21(OH)2](CO3)0.11·nH2O

Abstract

A magnesium–manganese hydroxycarbonate with the hydrotalcite structure has been synthesized by coprecipitation from aqueous solutions of Mg²⁺ and Mn²⁺. According to temperature-programmed reduction data, 84% of the manganese is oxidized to Mn³⁺ during synthesis. Fourier-transform infrared spectroscopy, powder X-ray diffraction and transmission electron microscopy data support the hydrotalcite structure. Thermal decomposition in air at 390 °C leads to an amorphous material with a simultaneous increase of the specific surface area. Further calcination at higher temperatures (670 and 1000 °C) leads to crystallization of MgO and Mg₂MnO₄ spinel and to sintering. Reduction of the spinel takes place in two separate steps, probably through formation of intermediate Mn₃O₄ and finally MnO.