Nanostructured Al-doped maghemite: a low-cost and eco-friendly material tested for methylene blue removal from water and as an accelerator in ammonium nitrate decomposition

Abstract

Maghemite´s properties for roles as removable and reusable catalysts or photocatalysts in reactions that promote green chemistry can be enhanced by doping with other metals. We investigated nanostructured aluminum-doped magnetic iron(III) oxides (AlxFe2-xO3, x = 0.33, 0.67, and 1) synthesized via the autocombustion method and characterized using various techniques. These materials were tested as photocatalysts for methylene blue dye degradation without peroxide and as catalysts for the thermal decomposition of ammonium nitrate. The results revealed that the cubic spinel structure predominates in all compositions. Aluminum doping led to decreased cell parameters and grain size and loss of microgranular structure. Saturation magnetization decreased with aluminum doping, reaching a low value (∼2 emu/g) for x = 1. The optical band gaps were red-shifted with respect to pure maghemite, with values near 2 eV. Among compositions, x = 0.33 showed the best photocatalytic efficiency, closely matching incident light energy (1.95 eV). The highest degradation efficiency was around 37 % after 80 min. The catalytic effect on ammonium nitrate decomposition was strongly correlated with aluminum concentration, turning the reaction exothermic with energy release of 200 and 400 J/g for x = 0.33 and 0.67, respectively.