Tunneled manganese (Mn) oxides (TMOs), with both environmental and economic significance, form from topotactic transformation of layered manganese oxides (LMOs), but the transformation is deemed to be very slow at room temperature, and laboratory syntheses require elevated temperature. This study explores the possibility for LMOs to transform to TMOs at room temperature and enviornomenrally relevant pHs. Mn(II) pre-loaded δ-MnO2 (Mn(II)-MnO2) was incubated in 100 mM NaCl solution at 21 oC and pH 6 – 8 for 25 days. We found that the incubation resulted in rapid transformation of Mn(II)-MnO2 to a 4 × 4 TMO. The rapid transformation was triggered by the presence of a high amount of vacancy-adsorbed Mn(III) due to the comproportionation between the vacancy-adsorbed Mn(II) and Mn(IV) in δ-MnO2. The transformation is fastest at pH 7 but slower at pH 6 or 8 due to an insufficient produced Mn(III) (pH 6) or formation of a less viable precursor (triclinic birnessite) (pH 8). The presence of O2 favors the formation of triclinic birnessite at pH 8 and thus retards the transformation, whereas it enhances production of vacancy-adsorbed Mn(III) at pH 6 and 7 and promotes the transformation. Our work reveals information about the role of Mn(III) in the layered-to-tunneled structural transformation, and suggests that TMOs can form from LMOs in the low-temperature natural environment much faster than previously assumed. The study also provides an energy–efficient approach for synthesizing TMOs with large tunnels for technological applications.

Rapid transformation of layered to tunneled manganese oxide triggered by interlayer Mn(III)

255th National Meeting (2018)

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