Óxidos de ferro dopados para aplicação em processos oxidativos avançados: a mecanoquímica como conveniência de síntese verde

Abstract

Iron oxides, such as magnetite, have great potential to be applied as catalysts in advanced oxidative processes, including Fenton reactions. In these applications, the introduction of transition metals in the structure of these oxides can contribute to the enhancement of catalytic activity. Thus, aiming at the synthesis of iron oxides, some methodologies have been commonly used, among them coprecipitation. However, mechanochemistry presents itself as an alternative to conventional synthesis methods, with syntheses made from reagents in solid state by grinding processes, without the use of solvents, a simpler, faster and more suitable synthesis proposal principles of green chemistry. Therefore, mechanochemical syntheses of pure magnetites and modified with 20% mol of copper were carried out, an amount that was chosen to ensure the entry of copper into the structure of the oxide to be synthesized. For the syntheses, the grinding time was varied between 10, 20 and 30 minutes, obtaining three pure oxides and three modified with copper. These materials were compared with pure magnetite and a copper-modified one obtained from the coprecipitation methodology. As for the structural and morphological properties of these materials, it was found that iron oxides containing copper presented about 20% copper in relation to the number of iron moles, as was proposed in the syntheses, and presented irregular morphological characteristics, while the Pure oxides exhibited predominantly spherical clusters, compatible with what is proposed for magnetites. Regarding the materials obtained by mechanochemistry, the analysis of the surface area revealed that the oxides with copper had a significantly larger area than the pure ones. Through H2O2 decomposition tests, there was a greater activity of the doped materials, and those synthesized by mechanochemistry showed a higher rate of decomposition when compared to those obtained via coprecipitation. With the evaluation of the leachate from the tests, it is possible to verify the occurrence of heterogeneous catalysis, not being observed in the leaching of metal ions from the oxides into the solution. The catalytic and adsorptive tests using the black dye remazol also indicated greater activity of the oxides with copper when compared to the pure oxides, with no significant differences between the activities of the oxides obtained through the mechanochemical methodology with different grinding times. Thus, copper oxide synthesized by coprecipitation (MtCu-cop) and doped oxide synthesized via mechanochemistry with a shorter grinding time (MtCu-10) were selected and applied in cycles in order to assess their reuse capacity. Thus, it was found that, in consecutive degradation cycles, MtCu-10 showed activity over ten cycles, while MtCu-cop showed a marked reduction in catalytic activity after the fifth cycle. Thus, it is possible to verify the feasibility of employing mechanochemical methodologies for the synthesis of iron oxides that act as catalysts in the degradation of organic compounds. In addition, the potential that the incorporation of copper has to improve the catalytic activity of oxides of iron.