Synthesis of manganese oxides with defects for MO degradation

Abstract

A low-temperature (30 °C) procedure for the synthesis of mixed-valent nanoparticles of manganese oxides is developed through the MnO₄⁻ reduction by ascorbic acid (AA) and potassium iodide (KI) with 3 molar ratios (AA: KI = 1:3, 1:1, and 3:1). The samples were dried at 90 °C and calcined at 600 °C. The dried products exhibited low crystallinity with ratios 1:3 and 1:1 and a more developed one with the ratio 3:1. A new tetragonal MnO₂-type was formed with AA: KI = 1:1 at 90 °C while the calcined samples showed the formation of crystalline manganese oxides with defects. The application of the calcined products in the oxidation of Methyl orange (MO) showed that Mn_0.98O₂ formed by the ratio AA: KI = 1:3 at 600 °C was more active, registering 92.4 % of degradation after 1 h at 40 °C. The activities of AK11 and AK31 formed by α-MnO₂, α-Mn₂O_3, and Mn_0.965O followed the content of α-MnO₂ and registered 89.37 and 85.86 %, respectively, under the same conditions. The synthesized samples exhibited antiferromagnetic behavior with coercivity values ranging from 31 to 48.6 Oe and exchange bias values from 67 to 132 Oe for the dried samples at 90 °C and 9 to 66.5 Oe for the calcined samples at 600 °C. The three products formed by nanoparticles reacted through direct oxidation processes via complex redox mechanisms that depended on the nature of manganese oxide, the presence of metal defects and potassium content.