ИСТИНА

Manganese oxides are known as promising catalytic materials due to their significant catalytic activity and chemical stability in alkaline media. It is not surprising that they are considered to be one of the main alternatives to the platinum in the oxygen reduction reaction in an alkaline medium. It is worth noting that the catalytic activity of materials based on manganese oxides significantly depends on their morphology and particles size, which are specified mainly by the synthesis method with which the material is obtained. Thus, a competent choice of the synthesis method of manganese oxides makes it possible to improve significantly the catalytic characteristics of the material. Usually, in methods for producing manganese oxides one uses an inorganic precursor, but an organic one can also be used to produce more dispersed and uniform materials. Usually to obtain disperse manganese oxides from organic precursor one can either dissolve precursor in liquid solvent with subsequent thermal treatment, or use chemical vapor deposition method. However, both approaches have certain shortcomings. The main disadvantage of the first method is the possibility of undesirable reactions between solvent and ligands. As for the chemical vapor deposition, it allows to obtain uniform thin films, but not disperse nanoparticles. One possible way to get rid of these drawbacks is to use an alternative solvent. In this work, we propose to use supercritical carbon dioxide (sc CO2) as such a solvent. Like a liquid, sc CO2 is highly dense and can be used as a solvent for various organometallic precursors of manganese. At the same time, the absence of surface tension-driven effects makes its application promising for obtaining particulate materials with small grain size and high specific area. The main aim of this work is to demonstrate a novel method of preparing manganese oxide nanoparticles from organic precursor using supercritical CO2. In our approach, we dissolve metalorganic precursor in supercritical CO2 and then thermally destroy organic phase in sc CO2 media, saturated by O2, to produce oxide nanoparticles. During basic synthesis procedure, a sample of a precursor was placed in a high-pressure vessel, then the vessel was closed and a pure oxygen pressure was injected therein. The vessel was then placed in a thermostat at a specified temperature and after reaching thermal equilibrium, a specified pressure was created in the vessel. After this, the temperature of the thermostat was raised to a certain exposure temperature and the exposure then was carried out for 24 hours, then the vessel was decompressed, cooled and opened. To study and characterize the obtained samples, methods of thermogravimetric and X-ray diffraction analysis, scanning and transmission electron microscopy were used. In addition, the cyclic voltammetry method and the rotating disk electrode method were applied to analyze the catalytic activity of the materials obtained. As a result of the conducted experiments, highly dispersed manganese oxides in the β-MnO2 phase with high specific surface and monodispersity were synthesized. In addition, the high electrocatalytic activity of the resulting material was found.