Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Abstract

Chemisorption of water onto anhydrous nanophase manganese oxide surfaces promotes rapidly reversible redox phase changes as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Surface reduction of bixbyite (Mn 2 O 3 ) to hausmannite (Mn 3 O 4 ) occurs in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. Additionally, transformation does not occur on nanosurfaces passivated by at least 2% coverage of what is likely an amorphous manganese oxide layer. The transformation is due to thermodynamic control arising from differences in surface energies of the two phases (Mn 2 O 3 and Mn 3 O 4 ) under wet and dry conditions. Such reversible and rapid transformation near room temperature may affect the behavior of manganese oxides in technological applications and in geologic and environmental settings.phase transformation | oxidation/reduction | water adsorption/desorption M anganese oxides are ubiquitous in the natural environment, often occurring as very fine grained (nanopahase) precipitates and coatings, and also find numerous technological applications, especially in catalysis. Easy variation of manganese oxidation state (Mn 2+ , Mn 3+ , Mn 4+ ) lends complexity to phase behavior and physical and chemical properties of manganese oxides. The thermodynamic stability of manganese oxide nanoparticles helps determine the robustness of such oxides and their behavior in soils and larger-scale geochemical cycles, as well as in applications in catalysis, renewable energy, and environmental remediation. The structure and energetics of nanoparticle surfaces are influenced by the phase present and its oxidation state and by the extent of surface hydration; thus dry surfaces are structurally and thermodynamically distinct from hydrated surfaces (1, 2). Using Mn 3 O 4 (hausmannite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite), previously, we showed that the position in temperature−oxygen fugacity space, of oxidation−reduction (redox) phase equilibria, is shifted at the nanoscale due to differences in nanophase surface energy as a function of particle size and surface hydration and that this thermodynamic shift is in favor of the lower surface energy phase (3). In the manganese oxides, the surface energy increases in the order hausmannite, bixbyite, pyrolusite, so small particle size favors the more reduced oxide phase.This paper summarizes observations in the nanoscale Mn 2 O 3 − Mn 3 O 4 system, which is found to undergo rapidly reversible redox phase transformations at room temperature induced by the adsorption/desorption of surface water. It is expected that the degree of reduction is a function of both average particle size and particle size distribution, but this work focuses on only one representative material to provide proof of concept. The rapid response of activated surfaces to changing hydration condition implies thermodynamic control and has implications for geochemistry, planetary scie...

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

“…2 A-D) than the binary Mn oxide phases previously studied (13), hausmannite, (Mn 3 O 4 , SE = 0.96 ± 0.08 J/m 2 ), bixbyite (Mn 2 O 3 ; SE = 1.29 ± 0.10 J/m2), and pyrolusite (MnO 2 ; SE = 1.64 ± 0.10 J/m2). The low SE values suggest that water is not strongly bound to the surfaces, interlayers, or tunnels, because water-adsorption enthalpy scales with the SE for many oxides (18)(19)(20)(21)(22)(23)(24)(25)28). There appears to be a small decrease in SE with decreasing Mn AOS (Fig.…”

Energetic Cost for Being “Redox-Site-Rich” in Pseudocapacitive Energy Storage with Nickel–Aluminum Layered Double Hydroxide Materials

“…Using in situ methodologies and from a thermodynamic perspective, our present study leads to the following general conclusions: first, as the Ni content increases, both as-made (hydrated) and dehydrated NiAl-LDHs tend to be energetically less stable, and all dehydrated NiAl-LDH samples are no longer stable, reflected by the positive formation enthalpies (ΔHf-dehy). Although decreased stability upon introduction of more redox defects is a common phenomenon seen for solid state oxide and hydroxide materials[37][38][39] , the endothermic enthalpies of formation for dehydrated NiAl-LDH are not typically seen, which indicates that upon removal of the intercalated water the NiAl-LDH is unstable and will readily transform to more energetically favorable phases. Hence, interlayer-confined species such as small molecules, ions, and/or materials are essential to minimize or neutralize the metastability generated by redox site enrichment.…”

Calorimetric study of the thermodynamic properties of Mn5O8

“…A previously reported synthesis method was followed with modifications in the way the precipitates were treated. About 70 g manganese nitrate hydrate (Mn(NO 3 ) 2 ·xH 2 O) was dissolved in 1.5 L Millipore water.…”