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 3 O 4 ), bixbyite (Mn 2 O 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 3 O 4 is 0.96 ± 0.08 J/m 2 , of Mn 2 O 3 is 1.29 ± 0.10 J/m2, and of MnO 2 is 1.64 ± 0.10 J/m 2 . The energy for the anhydrous surface of Mn 3 O 4 is 1.62 ± 0.08 J/m 2 , of Mn 2 O 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 3 O 4 relative to Mn 2 O 3 and Mn 2 O 3 relative to MnO 2 . We also report rapidly reversible structural and phase changes associated with water adsorption/desorption for the nanophase manganese oxide assemblages.