Abstract
The energetics of 14 synthetic manganese dioxides, including framework phases with todorokite structure and layer phases with birnessite and buserite structures, were studied using high-temperature solution calorimetry in molten 3Na2O4·MoO3 at 977 K. The composition of these phases with a general formula of NaxMgyMnO2·nH2O was systematically varied. Using the measured enthalpies of drop solution, the enthalpies of formation from oxides based on 1 mol of NaxMgyMnO2·nH2O at 298 K, ΔHfo(oxides), were obtained. The ΔHfo(oxides) data show no obvious correlation with either content of charge-balancing cations or content of water for the layer and framework phases. Using a new parameter, the hydration number of the charge-balancing cations (N(hyd) = the number of water molecules per cation), all of the ΔHfo(oxides) data can be linearly correlated. The linear correlations are ΔHfo(oxides) = 16.5N(hyd) − 73.3 for the layer phases, and ΔHfo(oxides) = 11.7N(hyd) − 64.4 for the framework phases. The average N(hyd) values for the birnessite, buserite, and todorokite phases are 2, 4, and 2, respectively.
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