Thermodynamics of manganese oxides: Effects of particle size and hydration on oxidation-reduction equilibria among hausmannite, bixbyite, and pyrolusite

The surface enthalpies of manganese oxide phases, hausmannite (Mn ₃O ₄), bixbyite (Mn ₂O ₃), and pyrolusite (MnO ₂), were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The energy for the hydrous surface of Mn ₃O ₄ is 0.96 ± 0.08 J/m ², of Mn ₂O ₃ is 1.29 ± 0.10 J/m ², and of MnO ₂ is 1.64 ± 0.10 J/m ². The energy for the anhydrous surface of Mn ₃O ₄ is 1.62 ± 0.08 J/m ², of Mn ₂O ₃ is 1.77 ± 0.10 J/m ², and of MnO ₂ is 2.05 ± 0.10 J/m ². Supporting preliminary findings (Navrotsky et al. 2010), the spinel phase (hausmannite) has a lower surface energy than bixbyite, whereas the latter has a smaller surface energy than pyrolusite. Oxidation-reduction phase equilibria at the nanoscale are shifted to favor the phases of lower surface energy-Mn ₃O ₄ relative to Mn ₂O ₃ and Mn ₂O ₃ relative to MnO2. We also report rapidly reversible structural and phase changes associated with water adsorption/desorption for the nanophase manganese oxide assemblages.