Óxidos de ferro nanoestruturados parcialmente substituídos: efeito do íon isomórfico na degradação de compostos orgânicos

Abstract

The different applications of materials based on iron oxides are directly related to the dimensions and morphologies of their structures. The presence of a second metal in the structure of these oxides can promote interesting modifications in the crystalline arrangement, capable of increasing its catalytic activity in chemical oxidation processes. In addition, greater stability and dispersion of these oxides can be achieved by combining with polymeric materials. In this work, the synthesis of iron oxides stabilized by chitosan (QSFe) and isomorphically replaced by Cu 2+ ions (QSFeCu5, QSFeCu10, QSFeCu15) was performed. The composites were synthesized in spherical format, which facilitated the reuse process. The beads were subjected to heat treatment under N2 atmosphere to eliminate the organic matrix and formation of active phase. The chemical, morphological and structural characterization of the materials were carried out using different analyzes. The isomorphic substitution of Fe by Cu in situ was verified mainly by means of XRD and Mössbauer Spectroscopy. The catalytic potential of the materials was evaluated from O2 evolution tests and methylene blue (AM) dye degradation. The substituted materials showed very different morphological and crystalline characteristics of the QSFe composite. This was attributed to the identification of different phases of iron oxides present in the spheres, in which the QSFe material presented goethite (α-FeOOH) as the major phase. With the increase in the isomorphic ion content, the predominant phase formed was magnetite (Fe3O4). The decomposition profile of hydrogen peroxide (H2O2) suggests a combined mechanism of hydrogen peroxide activation, with a predominance of vacant sites. The catalytic activity of the substituted materials (QSFeCux) in the AM degradation was higher than that of the non-substituted material (QSFe), corroborating the H2O2 decomposition data and evidencing the importance of the Cu as substituent. Material QSFeCu10, with 10% Fe 3+ replaced by Cu 2+ (mol/mol ratio), was able to degrade the model dye by more than 92% after optimization of the experimental variables. The isomorphically substituted catalysts showed to be more stable as the release of the active phases of Fe and Cu, with low levels of leaching, inferring that the catalytic process occurs exclusively in heterogeneous phase. No significant adsorption was observed, the removal being essentially by degradation of the molecule. Studies on the life of the composite indicate the possibility of use in different consecutive cycles, which shows the absence of catalytic poisoning and significant loss of activity. Thus, the incorporation of Cu in the QSFe spheres presents a promising material for the oxidation of organic contaminants, which associates robustness and catalytic activity.