Abstract
The focus of this paper is centered on the thermal reduction of KMnO4 at controlled temperatures of 400 and 800 °C. The materials under study were characterized by atomic absorption spectroscopy, thermogravimetric analysis, average oxidation state of manganese, nitrogen adsorption–desorption, and impedance spectroscopy. The structural formulas, found as a result of these analyses, were and . The N2 adsorption–desorption isotherms show the microporous and mesoporous nature of the structure. Structural analysis showed that synthesis temperature affects the crystal size and symmetry, varying their electrical properties. Impedance spectroscopy (IS) was used to measure the electrical properties of these materials. The measurements attained, as a result of IS, show that these materials have both electronic and ionic conductivity. The conductivity values obtained at 10 Hz were 4.1250 × 10−6 and 1.6870 × 10−4 Ω−1cm−1 for Mn4 at 298 and 423 K respectively. For Mn8, the conductivity values at this frequency were 3.7074 × 10−7 (298) and 3.9866 × 10−5 Ω−1cm−1 (423 K). The electrical behavior was associated with electron hopping at high frequencies, and protonic conduction and ionic movement of the K+ species, in the interlayer region at low frequencies.
Highlights
From the 1960s, works of Wolkestein [1] state the importance of electron theory to elucidate the relation between the catalytic and electronic properties of a catalyst and the semiconductor nature of this type of material
In the study conducted by Herbstein et al [21] on the thermal decomposition of KMnO4 in a temperature range of 25 to 900 ◦C, in an atmosphere of air and nitrogen, it was found that the idealized equation for the decomposition of KMnO4 in air at 250 ◦C results in a soluble phase, the K2MnO4, and another phase that is insoluble, like this: 10 KMnO4 →2.65 K2MnO4 + [2.35K2O 7.75MnO2.05] + 6O2
The thermal decomposition of KMnO4 is a redox reaction in which the oxoanion oxygen is oxidized to molecular oxygen, while the oxidation number of manganese in the oxoanion is reduced from Mn (VII) to Mn (VI), from MnO−4 and MnO24−
Summary
From the 1960s, works of Wolkestein [1] state the importance of electron theory to elucidate the relation between the catalytic and electronic properties of a catalyst and the semiconductor nature of this type of material. Birnessite is a special structure of this manganese oxide family It is composed of Mn–O octahedra forming octahedral layers as clays, and it has monovalent or divalent ions surrounded by water molecules to compensate the electrical charge of its layers. Birnessite has been used in previous works as a catalyst in soot combustion processes and in methylene blue degradation, showing appreciable catalytic activity compared to traditional catalysts [9,11,19] For this reason, the present work focuses on the charge transport mechanism for two birnessite material types, Mn4 and Mn8, synthesized at 400 and 800 ◦C, respectively, with the intention for a deeper understanding of the nature of this type of material for advanced applications
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