Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane

Abstract

Manganese oxides with layer and tunnel structures occur widely in nature and inspire technological applications. Having variable compositions, these structures often are found as small particles (nanophases). This study explores, using experimental thermochemistry, the role of composition, oxidation state, structure, and surface energy in the their thermodynamic stability. The measured surface energies of cryptomelane, sodium birnessite, potassium birnessite and calcium birnessite are all significantly lower than those of binary manganese oxides (Mn 3 O 4 , Mn 2 O 3 , and MnO 2 ), consistent with added stabilization of the layer and tunnel structures at the nanoscale. Surface energies generally decrease with decreasing average manganese oxidation state. A stabilizing enthalpy contribution arises from increasing counter-cation content. The formation of cryptomelane from birnessite in contact with aqueous solution is favored by the removal of ions from the layered phase. At large surface area, surface-energy differences make cryptomelane formation thermodynamically less favorable than birnessite formation. In contrast, at small to moderate surface areas, bulk thermodynamics and the energetics of the aqueous phase drive cryptomelane formation from birnessite, perhaps aided by oxidation-state differences. Transformation among birnessite phases of increasing surface area favors compositions with lower surface energy. These quantitative thermodynamic findings explain and support qualitative observations of phasetransformation patterns gathered from natural and synthetic manganese oxides.manganese oxides | birnessite | cryptomelane | calorimetry | thermodynamics C omplex manganese oxides, highly reactive fine-grained materials ubiquitous in nature, have served in a number of important capacities to benefit both the Earth and human society (1). These oxides have profoundly affected Earth's critical zone throughout geologic time, influencing the evolution of the atmosphere and the bioinorganic chemistry of organisms. They are effective in accumulation and recovery of economic ores and have recently inspired development of novel catalysts for green chemistry and energy sustainability technologies. Minerals based on MnO 2 include multiple classes, two of which are well known to geochemists: the 2 × 2 tunnel structure hollandite (the potassium -bearing variety is cryptomelane) and the layered structure phyllomanganates (e.g., birnessite) (2, 3). Layered Mn oxides, such as the mineral birnessite, derived from MnO 2 by inclusion of cations and water, with concomitant decrease in manganese average oxidation state (Mn AOS) from 4 to typical values between 3.5 and 3.9, with manganese in tetravalent, trivalent and sometimes divalent oxidation states, are among the most important Mn oxides in nature (4). These "nanosheet" Mn oxides readily transform among phases, influencing crucial environmental and technological processes, including biogeochemical cycles (1, 4, 5) water oxidation catalysis (6-8) and radionuclide confinement (9, ...

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