Mn(II) oxidation catalyzed by nanohematite surfaces and manganite/hausmannite core-shell nanowire formation by self-catalytic reaction

Journal Article · Thu Aug 01 00:00:00 EDT 2019 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2019.05.011· OSTI ID:1567184

Inoue, Sayako ^[1]; Yasuhara, Akira ^[2]; Ai, Haruka ^[3]; Hochella, Michael F. ^[4]; Murayama, Mitsuhiro ^[1]

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1. Virginia Tech
2. JEOL Ltd, Tokyo Japan
3. JEOL Ltd, Tokyo, Japan
4. BATTELLE (PACIFIC NW LAB)

The present study investigated the solid products of heterogeneous catalytic oxidation of aqueous Mn(II) by O2 in the presence of hematite nanoparticles (NPs) using transmission electron microscopy (TEM), scanning TEM (STEM), and electron energy-loss spectroscopy (EELS). The oxidation experiments were conducted at room temperature (22±2?) in solutions containing 10-3 M Mn(II) at pH 7.5 in the dark. During 48 hours of reaction, a single-phase manganite (a Mn oxyhydroxide) nanowire was formed by way of metastable groutite and feitknechtite nanowires, both of which are polymorphs of manganite. Between 48 and 168 hours, the manganite further altered to core-shell structured nanowires with hausmannite (a mixed valent Mn oxide) forming a very thin (2 nm) outer-shell on the manganite wire core. The formation of Mn-oxyhydroxide nanowires was catalyzed by the hematite NPs surface through electrochemical pathways. This is described via the electron transfer across a redox couple from adsorbed Mn(II) to another site with adsorbed O2 via the band structure of the semiconducting hematite. On the other hand, the Ostwald step rule was operative in the sequential crystallization from metastable groutite and feitknechtite to stable manganite, which could have resulted from the differences in the surface free energies, sizes, and morphologies of product polymorphs at the nanoscale. The final product (at least in the time range of our experiments) exhibited manganite/hausmannite core-shell structures. This product is most likely to be formed by the reactions catalyzed by the manganite nanowires themselves. Microscopic investigation of interfacial changes in structure and chemistry at the nanoscale is key, and it can be essential to understand the kinetic and thermodynamic phenomena related to the redox chemistry and phase stability of nanoparticles in complex, heterogeneous systems such as natural environments.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1567184
• Report Number(s): PNNL-SA-143235
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 258
• Country of Publication: United States
• Language: English