Degradação oxidativa de compostos orgânicos em meio aquoso por via catalítica heterogênia com magnetita e goethita dopadas com nióbio

Abstract

This work was divided into two main parts: (i) Synthesis, characterization and study of catalytic properties of niobium-doped goethites and (ii) Transformation of the goethites described in part (i) via chemical reduction with H2 to obtain magnetite. The catalysts weretested in the oxidation reaction of organic compounds in aqueous medium, employing the organic dyes methylene blue and rhodamine B as model organic molecules to simulate the pollutant substrate. The magnetic iron oxides, isostructurals to the spinel, have someinteresting properties, such as the presence of Fe2+ and Fe3+ simultaneously in the structure to be applied as catalysts in this redox processes. The magnetites were also characterized andtested in the same reactions as the goethite precursors.The physicochemical characterizations and textural properties of the materials allowed the identification of the iron compounds formed, as well as their key characteristics that reveal iron oxides with promising catalytic properties. For the oxidation reaction tested usinghydrogen peroxide and organic dyes in aqueous medium, the goethites containing niobium were the most active comparatively to magnetites, with approximately 90% of solution discoloration. It was carried out an identification study of the intermediates formed during themethylene blue dye oxidation, using the mass spectrometry technique with electrospray injection, allowing it to elucidate, at least partially the catalytic mechanism involved. The experimental search for elucidation of chemical mechanisms, based on the identification of the intermediates nature formed during the oxidation of organic dye, by massspectrometry with electrospray injection, led to important evidences about the steps involved in the catalytic oxidation process. The presence of Nb in the structure increases the catalytic efficiency of goethite in the oxidation of organic compounds by a mechanism involving theformation of peroxo species with niobium, with strong oxidizing character, on the surface of the grains. These oxidizing species generated in situ can also be regenerated by the action of hydrogen peroxide, added to the reactionary medium.