Layered (nanosheet) oxides in the CaO - Mn2O3 - MnO2 system (CaMnO phases inspired by biological photosynthesis systems) are possible catalysts for water oxidation reactions and the reduction of CO for energy production. Two suites of CaMnO phases with varying surface areas and Mn3+/Mn4+ ratios were examined by calorimetric studies for enthalpies of formation, water adsorption and surface energies, and Mn3+ - Mn4+ oxidation enthalpies. Their improved catalytic activity is attributed to a combination of factors. Their low surface energy and less exothermic water adsorption energy both, thermodynamically and kinetically, facilitate transport between surface and aqueous solution. The constant internal oxidation energy of Mn3+ - Mn4+ is smaller in magnitude than that of oxidizing Mn2O3 to MnO2 and does not vary with Ca/Mn/H2O or the Mn3+/Mn4+ ratio in the structure composition, suggesting delocalization of electrons across the layered oxide surfaces. The nanosheet structure provides easy access to surfaces and lowers diffusion lengths. These factors combine to enhance catalytic activity by providing easy access for solutes and water and facile electron transfer between manganese in different oxidation states. Additionally, the thermodynamic drive to form and the internal policing of manganese oxidation states in the layer surfaces contributes to mineral phase stability - even while being consumed during catalysis they reform in response to the loss of structural integrity.

Mineral phase stability, constant internal oxidation enthalpy, and low surface energy contribute to catalytic ability of nanosheet CaMn-oxides

251st National Meeting (2016)

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