Nanophase Manganese Oxides: Chemisorbed Water and Small Particle Size Promote Large Thermodynamically Driven Shifts in Oxidation-Reduction Equilibria

Manganese oxides are important in terrestrial and Martian settings, and changes in oxidation state (Mn 2+, 3+, 4+) produce different phases. This study focuses on changes in redox energetics at the nanoscale in the Mn-O system with water present. Nanophase hausmannite (Mn3O4), bixbyite (Mn2O3), and pyrolusite (MnO2) were synthesized using minor modifications of previously published methods, stored at room temperature, and then analyzed by powder-XRD, BET surface area measurement, and TGA for total water content. High-temperature oxide-melt drop solution calorimetry was performed on a series of characterized samples with known surface area and water content. The differential heat of water adsorption as a function of coverage was also measured. The surface enthalpies of manganese oxide phases, hausmannite (Mn3O4), bixbyite (Mn2O3), and pyrolusite (MnO2), were determined using the data from high-temperature oxide melt calorimetry and water adsorption calorimetry. Surface energy for the hydrous Mn3O4 tetragonal spinel phase is 0.96±0.08 J/m2, for Mn2O3 cubic phase is 1.29±0.10 J/m2, and for MnO2 cubic rutile phase is 1.64±0.10 J/m2. Surface energy for the anhydrous Mn3O4 is 1.31±0.08 J/m2, for Mn2O3 is 1.57±0.10 J/m2, and for MnO2 is 1.99±0.10 J/m2. Supporting preliminary findings, the spinel phase (hausmannite) has a lower surface energy than bixbyite, while the latter has a smaller surface energy than pyrolusite. We also observed phase changes, some of them rapidly reversible, associated with water adsorption/desorption for the nanophase manganese oxide assemblages. There are geochemical consequences. (1) At the nanoscale, both the pyrolusite/bixbyite and bixbyite/hausmannite equilibria are shifted to higher oxygen fugacity because the reduced phase has the lower surface energy. (2) The ready inter-conversion of phases with different oxidation states under aqueous conditions implies that, after a manganese oxide nanophase forms, it can easily transform to other phases with different oxidation states and water contents and perhaps record changes in environmental conditions after, as well as during, its initial formation.