Title: 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 paper 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 (Mn₃O₄, Mn₂O₃, and MnO₂), 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. Finally, these quantitative thermodynamic findings explain and support qualitative observations of phase-transformation patterns gathered frommore » natural and synthetic manganese oxides.« less

Authors:

Birkner, Nancy ^[1]; Navrotsky, Alexandra ^[2]

---

+ Show Author Affiliations

1. Peter A. Rock Thermochemistry Laboratory, University of California, Davis, CA 95616,, Nanomaterials in the Environment, Agriculture, and Technology, University of California, Davis, CA 95616,, Pratt School of Engineering, Duke University, Durham NC 27708
2. Peter A. Rock Thermochemistry Laboratory, University of California, Davis, CA 95616,, Nanomaterials in the Environment, Agriculture, and Technology, University of California, Davis, CA 95616,

Publication Date:

2017-01-27

Research Org.:

Univ. of California, Davis, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1341471

Alternate Identifier(s):

OSTI ID: 1465689

Grant/Contract Number:  

FG02-97ER14749

Resource Type:

Published Article

Journal Name:

Proceedings of the National Academy of Sciences of the United States of America

Additional Journal Information:

Journal Name: Proceedings of the National Academy of Sciences of the United States of America Journal Volume: 114 Journal Issue: 7; Journal ID: ISSN 0027-8424

Publisher:

Proceedings of the National Academy of Sciences

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; manganese oxides; birnessite; cryptomelane; calorimetry; thermodynamics