Stanton Ching

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 2 S 2 O 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 0.26 Mg 0.13 MnO 2.04 (H 2 O) 1.26 , with the average Mn oxidation state being 3.55. Mg-todorokite has a composition of Mg 0.33 MnO 2.14 (H 2 O) 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 0.16 Mn)O 2.21 (H 2 O) 0.97 . If the Co percentages are applied to Mg-todorokite, a formula of Mg 0.19 (Mg 0.14 Mn)O 2.14 (H 2 O) 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 2 S 2 O 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.

Self-assembly of manganese oxide nanoparticles and hollow spheres. Catalytic activity in carbon monoxide oxidation

Reactions between MnSO(4) and KMnO(4) in the presence of carboxylic acids provide a facile, one-pot route to nanostructured manganese oxides with high surface areas. Acetic and propionic acid induce formation of hierarchical nanosphere morphologies whereas butyric acid promotes assembly of hollow spheres. The materials are active catalysts for CO oxidation.

Synthetic Routes to Microporous Manganese Oxides

Abstract Microporous manganese oxides with layer and tunnel structures have been synthesized by methods such as redox precipitation, thermal and hydrothermal alteration, sol-gel processing, and microwave heating. The open frameworks of these materials are composed of edge and corner shared MnO6 octahedra, with the internal pores being occupied by cations and water molecules. Birnessite-type (OL-1) manganese oxides have layered structures with interlayer spacings that depend on hydration. Hollandites (OMS-2) and todorokites (OMS-1) have tunnel structures that consist of respective 2 × 2 and 3 × 3 arrays of MnO6 octahedra. Microporous manganese oxides are of interest because of potential applications in heterogeneous catalysis, chemical absorption, and battery technology. This article describes some recent developments in the synthesis of microporous manganese oxides, with particular attention being given to birnessites. hollandites, and todorokites.

Synthesis of a new hollandite-type manganese oxide with framework and interstitial Cr(III)

Hollandite with Cr(III) in both tunnel and framework sites has been prepared hydrothermally from layered manganese oxide precursors.