Manganese oxides (Mn-oxides) are ubiquitous in the near surface environment and they play important roles in processes critical to geochemistry and life on Earth, including water column oxygen cycling, nutrient transport, surface adsorption, and sequestration of heavy metals and radionuclides.Previously we found that nanoparticle surface energy and hydration energy differences promote significant Gibbs free energy shifts in redox phase equilibria at the nanoscale. (Navrotsky et al. 2010). The spinel phase Mn3O4 (hausmannite) has a lower surface enthalpy than Mn2O3 (bixbyite), while the latter has a smaller surface enthalpy than MnO2 (pyrolusite). (Birkner and Navrotsky 2012) Thus the stability field of Mn3O4is greatly extended at the nanoscale.We examined the role of water chemisorption on dry nanoparticle Mn2O3 and found thermodynamically controlled rapidly reversible mineral phase transformation. Surface reduction of Mn2O3 to Mn3O4occurred in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. The occurrence and the reversibility of the transformation process are due to thermodynamic control arising from differences in surface energies of the two phases under wet and dry conditions. (Birkner and Navrotsky, in progress)We focused on the energetics of nanosheets of calcium manganese oxide (CaMnO). We found that these layered phases are highly thermodynamically stable and have significantly lower surface enthalpy than the binary manganese oxides Mn3O4, Mn2O3, and MnO2. (Birkner et al. 2013) The enthalpy of oxidation of CaMnO materials was found to be independent of the average oxidation state and significantly less exothermic that the oxidation of Mn2O3 to MnO2. We suggest that the above factors are important in enhancing the catalytic activity of the layered CaMnO materials by facilitating adsorption/desorption and surface transport of water, ions, and electrons.

Energetics of manganese oxides: Implications for phase stability and catalysis in the near surface environment

247th National Meeting (2014)

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