TY - JOUR
T1 - Mn(II) oxidation catalyzed by nanohematite surfaces and manganite/hausmannite core-shell nanowire formation by self-catalytic reaction
AU - Inoué, Sayako
AU - Yasuhara, Akira
AU - Ai, Haruka
AU - Hochella, Michael F.
AU - Murayama, Mitsuhiro
N1 - Funding Information:
This material is based upon work supported by the National Science Foundation (NSF) and the Environmental Protection Agency (EPA) under NSF Cooperative Agreement EF-0830093 and DBI-1266252, Center for the Environmental Implications of NanoTechnology (CEINT). Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not been subjected to EPA review and no official endorsement should be inferred. Funding for this project was partly provided by DOE BES Geosciences (DE-FG02-06ER15786) and facilities were made available through Virginia Tech's Institute for Critical Technology and Applied Science Nanoscale Characterization and Fabrication Laboratory (ICTAS-NCFL).The authors also used shared facilities at the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure (NanoEarth), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), supported by NSF (ECCS 1542100). The authors are grateful to Dr. F. Marc Michel for supporting XRD analyses. We also thank Dr. Deborah Aruguete at PennState Behrend and Dr. Harish Veeramani at Carleton University for assistance with the initial experiment setup.
Funding Information:
This material is based upon work supported by the National Science Foundation ( NSF ) and the Environmental Protection Agency ( EPA ) under NSF Cooperative Agreement EF-0830093 and DBI-1266252, Center for the Environmental Implications of NanoTechnology ( CEINT ). Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the NSF or the EPA. This work has not been subjected to EPA review and no official endorsement should be inferred. Funding for this project was partly provided by DOE BES Geosciences ( DE-FG02-06ER15786 ) and facilities were made available through Virginia Tech’s Institute for Critical Technology and Applied Science Nanoscale Characterization and Fabrication Laboratory ( ICTAS-NCFL ).The authors also used shared facilities at the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure (NanoEarth), a member of the National Nanotechnology Coordinated Infrastructure ( NNCI ), supported by NSF ( ECCS 1542100 ). The authors are grateful to Dr. F. Marc Michel for supporting XRD analyses. We also thank Dr. Deborah Aruguete at PennState Behrend and Dr. Harish Veeramani at Carleton University for assistance with the initial experiment setup.
Publisher Copyright:
© 2019 Elsevier Ltd
PY - 2019/8/1
Y1 - 2019/8/1
N2 - 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 °C) in solutions containing 10−3 M Mn(II) at pH 7.5 in the dark. During 48 h 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 h, 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.
AB - 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 °C) in solutions containing 10−3 M Mn(II) at pH 7.5 in the dark. During 48 h 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 h, 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.
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U2 - 10.1016/j.gca.2019.05.011
DO - 10.1016/j.gca.2019.05.011
M3 - Article
AN - SCOPUS:85066458436
SN - 0016-7037
VL - 258
SP - 79
EP - 96
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
ER -