A new synthetic route to todorokite-type manganese oxides

Microporous todorokite-type manganese oxides have been synthesized by a new route in which the key Na-birnessite precursor is prepared by oxidation of Mn(OH)(2) with K2S2O8 in aqueous NaOH. The reaction is promoted by foreign metal cations such as Mg2+, Co2+, Ni2+, and Cu2+, which are incorporated into the manganese oxide layer framework. These same divalent cations are used in a subsequent ion-exchange reaction that converts the Na-birnessite into a related layered material, buserite. Hydrothermal treatment of the buserite ultimately yields Mg-, Co-, Ni-, or Cu-todorokite. The todorokites have been characterized by powder X-ray diffraction, elemental analysis, Mn oxidation state determination, scanning electron microscopy, and cyclic voltammetry. The composition of Mg-doped Na-birnessite is Na0.26Mg0.13MnO2.04(H2O)(1.26), with the average Mn oxidation state being 3.55. Mg-todorokite has a composition of Mg0.33MnO2.14(H2O)(0.97), with a Mn oxidation state of 3.62. A mixed Co/Ni-todorokite has been synthesized to assess the distribution of foreign cation in framework and tunnel sites. For Co-todorokite. 42% of the Co is in the manganese oxide framework and 58% is in the interlayer galleries, which gives the formula Co-0.21(Co0.16Mn)O-2.21(H2O)(0.97). If the Co percentages are applied to Mg-todorokite, a formula of Mg-0.19(Mg0.14Mn)O-2.14(H2O)(0.97) is obtained. Thermal stability experiments reveal that Mg-todorokite is more robust compared to the other todorokites and remains intact up to 400 degrees C. The Co, Ni, and Cu-todorokites have similar thermal stabilities and their structures collapse at about 300 degrees C. Na-birnessite prepared by the Mn(OH)(2)/K2S2O8 route can further be used to generate other birnessite derivatives such as H-birnessite and alkylammonium-birnessites. These derivatives can be synthesized both with and without Mg2+, Co2+, Ni2+, and Cu2+ as isomorphous framework dopants. (C) 1999 Elsevier Science S.A. All rights reserved.