Double-Aging Method for Preparation of Stabilized Na-Buserite and Transformations to Todorokites Incorporated with Various Metals.
A double-aging method has been developed to prepare and stabilize Na-buserite. In the first step, Na-buserite is synthesized by aging a MnO(x)() gel, which is produced from the oxidation of Mn(OH)(2) in NaOH solutions by KMnO(4) in the presence of Mg(2+). Stabilization of Na-buserite is done by further aging the as-synthesized buserite in distilled deionized water. Physical and chemical changes during the second aging (stabilization) have been investigated by using X-ray diffraction (XRD), infrared (IR), scanning electronic microscopy and energy-dispersive X-ray studies (SEM/EDX), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), cyclic voltammetry (CV), and temperature-programmed desorption combined with mass spectrometer (TPD-MS). The amount and type of metals incorporated into buserite and todorokite are greatly increased by the second aging treatment (including many lanthanides, whose incorporation has not been reported before). The metal species introduced in the layers considerably change the interlayer distances and, accordingly, the cell parameters. A criterion is obtained for the transformation of todorokite-type tunnel MnO(x)() materials from buserite-type layered MnO(x)() by hydrothermal treatment: only buserites which are stable at elevated temperatures in aqueous systems can convert to a todorokite structure; unstable buserites form a structure whose main d spacings are at 3.56 and 7.1 Å. Interconversions among several layered MnO(x)() are also discussed.