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 (Mn3O4), bixbyite (Mn2O3), and pyrolusite (MnO2), were determined using high-temperature oxide melt solution calorimetry in conjunction with water adsorption calorimetry. The energy for the hydrous surface of Mn3O4 is 0.96 +/- 0.08 Jim(2), of Mn2O3 is 1.29 +/- 0.10 J/m(2), and of MnO2 is 1.64 +/- 0.10 J/m(2). The energy for the anhydrous surface of Mn3O4 is 1.62 +/- 0.08 Jim', of Mn2O3 is 1.77 +/- 0.10 J/m(2), and of MnO2 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-Mn3O4 relative to Mn2O3 and Mn2O3 relative to MnO2. We also report rapidly reversible structural and phase changes associated with water adsorption/desorption for the nanophase manganese oxide assemblages.