Natural manganese oxides are generally formed in surficial environments that are near ambient temperature and water-rich, and may be exposed to wet-dry cycles and a variety of adsorbate species that influence dramatically their level of hydration. Manganese oxide minerals are often poorly crystalline, nanophase, and hydrous. In the near-surface environment they are involved in processes that are important to life, such as water column oxygen cycling, biomineralization, and transport of minerals/nutrients through soils and water. These processes, often involving transformations among manganese oxide polymorphs, are governed by a complex interplay between thermodynamics and kinetics. Manganese oxides are also used in technology as catalysts, and for other applications.
The major goal of this dissertation is to examine the energetics of bulk and nanophase manganese oxide phases as a function of particle size, composition, and surface hydration. Careful synthesis and characterization of manganese oxide phases with different surface areas provided samples for the study of enthalpies of formation by high temperature oxide melt solution calorimetry and of the energetics of water adsorption on their surfaces. These data provide a quantitative picture of phase stability and how it changes at the nanoscale.
The surface energy of the hydrous surface of Mn3O4 is 0.96 ± 0.08 J/m2, of Mn2O3 is 1.29 ± 0.10 J/m2, and of MnO2 is 1.64 ± 0.10 J/m2. The surface energy of the anhydrous surface of Mn3O4 is 1.62 ± 0.08 J/m 2, of Mn2O3 is 1.77 ± 0.10 J/m 2, and of MnO2 is 2.05 ± 0.10 J/m2. Supporting preliminary findings (Navrotsky et al., 2010), the spinel phase (Mn3O4) has a lower surface energy (more stabilizing) than bixbyite, while the latter has a smaller surface energy than pyrolusite. These differences significantly change the positions in oxygen fugacity—temperature space of the redox couples Mn3O4-Mn2O 3 and Mn2O3-MnO2 favoring the lower surface enthalpy phase (the spinel Mn3O4) for smaller particle size and in the presence of surface hydration.
Chemisorption of water onto anhydrous nanophase Mn2O 3 surfaces promotes rapidly reversible redox phase changes at room temperature as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Water adsorption microcalorimetry (in situ) at room temperature measured the strongly exothermic integral enthalpy of water adsorption (-103.5 kJ/mol) and monitored the energetics of the redox phase transformation. Hydration-driven redox transformation of anhydrous nanophase Mn(III) 2O3, (high surface enthalpy of anhydrous surfaces 1.77 ± 0.10 J/m2) to Mn(II,III)3O4 (lower surface enthalpy 0.96 ± 0.08 J/m2) occurred during the first few doses of water vapor. Surface reduction of nanoparticle bixbyite (Mn 2O3) to hausmannite (Mn3O4) occurs under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials.
Layered structure manganese oxides contain alkali or alkaline earth cations and water, are generally fine-grained, and have considerable thermodynamic stability. The surface enthalpies (SE) of layered and tunnel structure complex manganese oxides are significantly lower than those of the binary manganese oxide phases. The SE for hydrous surfaces and overall manganese average oxidation state (AOS) (value in parentheses) are: cryptomelane 0.77 ± 0.10 J/m 2 (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 are: δCa 0.39MnO2.3nH2O 0.75 ± 0.10 J/m2 (3.89) and δCa0.43MnO2.3nH2O 0.57 ± 0.12 J/m2 (3.68). The surface enthalpy of the complex manganese oxides appears to decrease with decreasing manganese average oxidation state, that is, with greater mixed valence manganese (Mn 3+/4+). Low surface energy suggests loose binding of H2O on the internal and external surfaces and may be critical to catalysis in both natural and technological settings.
|Commitee:||Casey, William, Kauzlarich, Susan|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 76/10(E), Dissertation Abstracts International|
|Keywords:||Calorimetry, Catalysts, Layered- and tunnel- structures, Manganese oxides, Nanoparticles, Thermochemistry|
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