Correlation between cation distribution and catalytic production of free radicals in mixed ferrite nanoparticles

Abstract

The role of divalent 3d Fe, Zn, and Ni metallic ions in heterogeneous Fenton reactions promoted by magnetic nanoparticles with ferrite crystalline structures was studied. The coordination and oxidation state of each cation in the ferrite nanoparticles were determined by analyzing the results of X-ray absorption spectroscopy experiments. The morphology and final composition of each system were determined using transmission electron microscopy (TEM) and particle-induced X-ray emission (PIXE). By correlating these data with the catalytic activity of the nanoparticulated systems to decompose H₂O₂ in reactive oxygen species (ROS) under acid conditions measured by electron paramagnetic resonance (EPR), it was possible to establish the role of the oxidation state and coordination of the surface metal ions in the ROS species produced, as well as their concentration. The Fe²⁺ ion in the octahedral coordination plays a major role in the catalytic activity, resulting in an increase in the amount of the OH˙ free radical. At the same time, the presence of Ni²⁺ and Zn²⁺ in the octahedral and tetrahedral sites, respectively, apparently has no catalytic effect on the formation of oxygen-based free radicals due to the decomposition of H₂O₂ detected by EPR experiments.