Síntese de nanocompósitos a base de Fe2SiO4-Fe7Co3 dispersos na SBA-15: aplicação como adsorvente e fotocatalisador magneticamente separável

Abstract

The present study describes the synthesis of a new type of Fe2SiO4-Fe7Co3 based nanocomposite dispersed in the mesoporous material SBA-15 in order to apply in the dyes adsorption, in the photocatalytic degradation of methylene blue (MB) and consequent magnetic separation. The parameters studied were: the reduction temperature and amount of hydrogen used in the synthesis and how these affect the physicochemical properties of the oxide-alloy mixture with iron and cobalt, as well as the adsorption capacity and photodegradation of methylene blue. The prepared materials were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy (MS), X-ray photoelectron spectroscopy (XPS), vibratory sample magnetometry (VSM), temperature programmed reduction (H2-TPR), N2 or Ar adsorption and desorption isotherms , Scanning electron microscopy (SEM-FEG) and Transmission electron microscopy (TEM). The XRD results together with the TPR profiles confirmed the formation of Fe2SiO4-Fe7Co3-based nanocomposites at temperatures between 690 and 720 °C and amount of H2 between 0.5 and 2%. The increase of the reduction temperature and the increase of H2 content in the reduction mixture led to the formation of a higher amount of alloy compared to oxide according to the data obtained by the Rietveld refinement. Low-angle XRD, N2 isotherms, and TEM images show the formation of the SBA-15- based mesoporous support with a high specific area of 640 m/g². However, the surface area and mesoporous structure of the silica was slightly altered (to 420 m/g²) after Fe and Co insertion due to partial pore filling by the composite. For samples containing the metals dispersed on SBA-15, there may be an interconnection between mesopores according to scanning hysteresis loop from N2 and Ar isotherms accompanied by mathematical models. Adsorption tests confirmed that the material reduced at 700 °C using 2% of H2 had the highest adsorption capacity for methylene blue (MB) dye. The nanocomposites obtained have good adsorption capacity for MB dye and can be easily separated from the final dispersion by applying an external magnetic field after adsorption. The photocatalysis results confirmed that the silica support is essential to optimize the photocatalytic activity of the Fe and Co species in the dye degradation, favoring the accessibility of active sites. The attraction of the dye by the nanocomposite occurs mainly by π-π interactions and the mixing of Fe2SiO4 and Fe7Co3 phases leads to a synergistic effect which favor the methylene blue adsorption and the photocatalytic activity process compared to the same isolated phases. The degradation of the MB occur through a sequence of reactions both on the solid surface and on the solution.