Sol−Gel Route to the Tunneled Manganese Oxide Cryptomelane

Abstract

The sol-gel reaction between KMnO 4 and fumaric acid in a 3:1 mole ratio generates a flocculant gel that serves as a precursor to the tunneled manganese oxide, cryptomelane. The elemental composition of sol-gel cryptomelane has been determined to be K 0.12 MnO 2.0 -(H 2 O) 0.09 . Further characterization has been performed using powder X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy. The sol-gel process is heavily dependent on reactant concentration. Solutions that are too concentrated produce the layered manganese oxide birnessite, whereas overly dilute reactions yield mixtures of cryptomelane and Mn 2 O 3 . The preference for cryptomelane over birnessite correlates with low potassium content in the gel. The sol-gel procedure for synthesizing cryptomelane is not easily transferred to the preparation of analogous manganese oxides with different tunnel cations. Reactions that employ permanganates other than KMnO 4 generally yield Mn 2 O 3 , with cryptomelane being a minor product at best. Thermal analyses of cryptomelane gels indicate that calcination proceeds through a series of stages that involve loss of water, loss of residual organics, conversion to cryptomelane, and finally degradation to Mn 3 O 4 . The extraction of potassium ions from sol-gel cryptomelane by various foreign cations is minimal, with the loss of K + being on the order of 10%.

Sonochemical synthesis and chracterization of Mn3O4 nanoparticles

“…The oxygen octahedral is tetragonally distorted due to Jahn-Teller effect on Mn 3+ ions [6]. Various methods including high temperature sol-gel [7], sonochemical [8], co-precipitation [9], solvothermal [10], hydrothermal [11], chemical bath deposition [12], oxidation [13,14], microwave irradiation [15] and surfactant-assisted methods [16] have been reported for its synthesis.…”

Manganese oxide thin films prepared by nonaqueous sol–gel processing: preferential formation of birnessite

“…In particular, Li-birnessite has shown a propensity for the spinel form at temperatures ranging from 110 o C to 600 o C [5,31,32]. K-birnessite is known to transform into cryptomelane at temperatures in the range of 500-600 o C [12], although this conversion was not observed in nonaqueous bulk sol-gel syntheses [13].…”

“…This work was built on much earlier observations of "manganese dioxide jellies" reported by Witzemann in 1915 [10]. With potassiumcontaining gels, there was an additional discovery that partial extraction of K + from the gel matrix led to formation of cryptomelane rather than birnessite [9,11,12]. More recently, Ching and Suib reported nonaqueous sol-gel routes to microporous manganese oxides using reactions of alkylammonium permanganates with methanol in the presence of alkali metal cations as dopants [13].…”

Cr-hollandite: Breaking tradition with todorokite-type manganese oxides

“…It can be prepared by a variety of methods, including reflux heating, hydrothermal treatment, and sol-gel processing. [3][4][5] It is also generated in syntheses of hierarchical urchin-like nanostructured manganese oxides. [6,7] The 3x3 todorokite structure is typically synthesized from layered buserite precursors under hydrothermal conditions.…”