A new synthetic route to todorokite-type manganese oxides

Abstract

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 K 2S 2O 8 in aqueous NaOH. The reaction is promoted by foreign metal cations such as Mg 2+, Co 2+, Ni 2+, and Cu 2+, 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 Na 0.26Mg 0.13MnO 2.04(H 2O) 1.26, with the average Mn oxidation state being 3.55. Mg-todorokite has a composition of Mg 0.33MnO 2.14(H 2O) 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(Co 0.16Mn)O 2.21(H 2O) 0.97. If the Co percentages are applied to Mg-todorokite, a formula of Mg 0.19(Mg 0.14Mn)O 2.14(H 2O) 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°C. The Co, Ni, and Cu-todorokites have similar thermal stabilities and their structures collapse at about 300°C. Na-birnessite prepared by the Mn(OH) 2/K 2S 2O 8 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 Mg 2+, Co 2+, Ni 2+, and Cu 2+ as isomorphous framework dopants.