Co3O4-ZrO2 and Co3O4-Nb2O5 crystalline-amorphous composites for H2O2 activation via Fenton-like and electroprotic processes – Proof of concept

Abstract

Generation of reactive oxygen species (ROS) via decomposition of hydrogen peroxide is associated with a cascade of reactions which involves either redox active centers of Fenton-like catalysts or acidic-base surface functional groups of non-redox materials. The latter mediate electroprotic reactions resulting in ROS formation. Such two functions can be utilized jointly by combining two oxide phases, a redox-active crystalline oxide and a non-redox amorphous oxide, into one composite material. As a proof of this concept, two amorphous-crystalline composites constituted by ZrO2 and Nb2O5 gels, and cobalt spinel Co3O4 nanocrystals were synthesized in this work. A thorough structural and chemical characterization was carried out using TEM/STEM combined with EDX spectroscopic mapping, XRD, and XRF techniques. Raman measurements were performed to inspect further the dual nature of the composite materials. The formation of surface-stabilized reactive species upon interaction with H2O2 was monitored with Raman and EPR spectroscopy. In parallel, the detection of hydroxyl radicals OH was performed using spin-trapping experiments with DMPO. The measurements proved the cooperative effect of the crystalline and amorphous phases in ROS generation. The Fenton-type mechanism of H2O2 decomposition was proposed, which involved formation of free hydroxyl radicals. It was augmented with the formation of superoxide radicals and stabilization on the amorphous component. The chemical reactivity of the investigated catalysts was tested in oxidation of OPD, a peroxidase substrate, and methylene blue discoloration exemplifying an advanced oxidation process (AOP).