No abstract

X-ray diffraction (XRD), infrared (IR), and scanning electron microscopy (SEM) techniques have been used to monitor the crystallization of birnessite (Na4Mn14O27·9H2O, JCPDS card 32-1128) by aging MnO x , which was produced from the oxidation of Mn(OH)2 by KMnO4 in an NaOH solution in the presence of Mg2+. The crystallization process of birnessite can be divided into three stages:  an induction period, a fast crystallization period, and a steady-state period. The crystallization of birnessite was accompanied by the crystallization and phase transformation of feitknechtite (β-MnOOH, JCPDS card 18-0804). Increasing the temperature will reduce the induction period and accelerate the crystallization and phase transformation. Increasing the basicity has almost the same effect as increasing the aging temperature. The crystallization process is also influenced by varying the molar ratio of MnO4 -/Mn2+. A diagram of produced phases at room temperature at different basicities and ratios of MnO4 -/Mn2+ is obtained. The interconversion between birnessite and buserite (Na4Mn14O27·21H2O, JCPDS card 23-1046) is also discussed.

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.

No abstract

No abstract