Layered (birnessite) and tunneled (cryptomelane) manganese oxides contain Ca, Na, and/or K, in addition to water. They are typically nanophase, and charge balance is achieved by interlayer or tunnel cations and by vacancies. Earlier studies determined the energetics of formation these phases (Fritsch et al. 1997, 1998) but did not consider the effects of particle size and surface energy. Thus, we returned to these materials with the hypothesis that they, like the nanosheet calcium manganese oxide catalysts, have low surface energies which help stabilize them as fine-grained materials. By controlling the ionic strength of a background electrolyte during synthesis, it was possible to synthesize several series of these materials with virtually identical cation contents and average manganese oxidation states (AOS). Different particle sizes were achieved by variation of aging time. Enthalpies of formation and surface energies were measured previously (Birkner and Navrotsky 2017). For the birnessites, cryptomelane, and calcium manganese oxide (CaMnO) nanosheets an increase in the counter cation content has a thermodynamically stabilizing influence on the enthalpy of formation, in accord with our earlier work. The surface energies for hydrous surfaces and overall manganese average oxidation state (AOS) (value in parentheses) for complex manganese oxides are: cryptomelane 0.77 ± 0.10 J/m2 (3.78), sodium birnessite 0.69 ± 0.13 J/m2 (3.56), potassium birnessite 0.55 ± 0.11 J/m2 (3.52), and calcium birnessite 0.41 ± 0.11 J/m2 (3.50). Surface enthalpies of hydrous surfaces of the calcium manganese oxide nanosheets as Ca0.39MnO2.3nH2O 0.75 ± 0.10 J/m2 (3.89) and Ca0.43MnO2.3nH2O 0.57 ± 0.12 J/m2 (3.68). Surface energy of the complex manganese oxides appears to decrease with decreasing manganese average oxidation state, that is, with greater mixed manganese valence. The surface energies of the layer and tunnel structured complex manganese oxides are confirmed to be significantly lower than those of binary manganese oxide phases, which were previously measured as: hausmannite 0.96 ± 0.08 J/m2 (Mn3O4), bixbyite 1.29 ± 0.10 J/m2 (Mn2O3), and pyrolusite 1.64 ± 0.10 J/m2 (MnO2). The current work adds to these energetics measurements with a comparison of conductivity for this systematic set of nanomaterials.

Energetics of nanophase layer and tunnel structure manganese oxides

2019 Spring National Meeting

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