Generation of Reactive Oxygen Species via Electroprotic Interaction of H2O2 with ZrO2 Gel: Ionic Sponge Effect and pH-Switchable Peroxidase- and Catalase-Like Activity

Formation of reactive oxygen species (ROS) is of vital importance in catalytic oxidation chemistry. In this paper we have shown that a nonredox system such as amorphous zirconium dioxide (a-ZrO2) is highly active in ROS formation via H202 decomposition. Interaction between a-ZrO2 and H2O2 in aqueous solution was investigated by means of EPR, HYSCORE, Raman, and UV-vis, along with auxiliary FTIR, TG-MS, and XPS techniques, in a broad range of pH values and H2O2 concentrations. Various reaction intermediates such as superoxide (O-2(center dot-)) and hydroxyl ((OH)-O-center dot) radicals as well as peroxide (O-2(2-)) species were identified. At pH <5.3 the superoxide and hydroxyl radicals were generated simultaneously in large amounts with the peak concentration being reached around the isoelectric point of the gel catalyst. In this pH region, the ZrO2 gel exhibited peroxidase-type activity, quantified by an o-phenylenediamine assay. At pH >5.3 formation of O-2(2-) is accompanied by a substantial release of O-2 due to the pronounced catalase-like activity of a-ZrO2. The role of electroprotic processes (an interfacial proton transfer coupled with an intermolecular electron transfer) in H2O2 decomposition and ROS formation was elucidated, and a plausible mechanism of this reaction, HO2(surf)- + H2O2(aq) + -> center dot OH(aq) + Zr+-O-2(center dot-)(surf) + H2O, was proposed. The surface of a-ZrO2 covered with hydroxyl groups plays a role of an ionic sponge, which controls the electroprotic equilibrium by capturing the charged reaction intermediates. Unlike the amorphous gel, crystalline zirconia exhibits only weak activity in the production of the O-2(center dot-) and (OH)-O-center dot radicals, and a different mechanism is involved. It is worth mentioning that the activity of the zirconia gel catalyst in ROS generation, as gauged by the Michaelis Menten constant, is comparable (ca. 40%) to that of the Fenton -type oxides (Fe3O4, Co3O4).