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Abstract:

Birnessites containing Na, K or Li in the interlayer have been prepared by oxidation of Mn(II) cations with H2O2 in a basic medium with different alkaline cation/Mn molar ratios. The solids prepared have been characterised by elemental chemical analysis, powder X-ray diffraction, thermal analyses (differential thermal analysis and thermogravimetric analysis), FT-IR spectroscopy and surface texture assessment by adsorption of N2 at −196◦C. Crystalline birnessites are obtained for A/Mn ratios (A = K, Li) larger than 3.4, but MnO(OH) has been also identificed when such a ratio is smaller than 3.4. Ion exchange is topotactic, but is not complete for exchanging Na, K, or Mg for pre-existing Li. The solids are stable up to 400◦ C, and formation of spinels and solids with tunnel structures is observed at this temperature. Li-containing birnessites are transformed to LiMn2O4 spinel at 400◦C, and co-crystallization of bixbyte (Mn2O3) is observed at higher temperatures. Bixbyte and cryptomelane are formed at 500◦C for the K-containing birnessites.

Abstract:

Manganese oxide catalysts were synthesized by direct reaction between manganese acetate and permanganate ions, under acidic and reflux conditions. Parameters such as pH (2.0–4.5) and template cation (Na?,K? and Cs?) were studied. A pure cryptomelane-type manganese oxide was synthesized under specific conditions, and it was found that the template cation plays an important role on the formation of this kind of structure. Cryptomelane was found to be a very active oxidation catalyst, converting ethyl acetate into CO2 at low temperatures (220 C). This catalyst is very stable at least during 90 h of reaction and its performance is not significantly affected by the presence of water vapour or CO2 in the feed stream. The catalyst performance can be improved by the presence of small amounts of Mn3O4.

Low Temperature Synthesis of Nano-Sized Lithium Manganese Oxide Powder by the Sol-Gel Process Using PVA

Lithium manganese oxide (LiMn2O4) powder with spinel structure has been synthesized by a sol-gel method using an aqueous solution of metal nitrates containing polyvinyl alcohol (PVA). The role of PVA and the calcination conditions for the formation of LiMn2O4 have been studied. Homogeneity and reactivity of the precursor powder are enhanced with an increase in the amount of PVA in the starting solution. When the amount of PVA is low, an impurity phase-Mn2O3 is formed at low temperature. On the other hand, when the vinyl alcohol monomer unit of PVA to metal ion ratio is 2 : 1 in the starting solution, only spinel phase is formed at 180± C and organic-free LiMn2O4 powder is obtained at as low as 400± C. Nanosized LiMn2O4 particles with a narrow size distribution have been successfully prepared by this technique. This method with proper amount of PVA results in much lower calcination temperature and shorter calcination time for producing the single spinel phase in comparison with the conventional solid state reaction and other solution techniques.