How Do Metal Oxides Mislead Spin-Trapping Electron Paramagnetic Resonance Analysis?

Limited by the detection techniques, the reactive species involved in catalytic water purification processes are difficult to be clarified. Spin-trapping electron paramagnetic resonance (EPR) analysis is recognized as a reliable tool for radical identification, in which 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) is usually used as the radical trapper. However, it is questioned that the detection of adducts of DMPO definitively indicates the generation of radicals. In this work, the DMPO transformation caused by transition metal oxides is monitored by EPR, and abundant spin signals are observed. MnO₂, Mn₂O₃, and NiO could oxidize DMPO into DMPOX through direct oxygen transfer. Besides, the dissolved transition metal ions could transform DMPO into misleading DMPO–OH and DMPO–R. The findings in this work, e.g., the interactions between DMPO and different metal oxides and the quenching behavior of the different pathways, would help with reliable identifications of reactive species in both engineered systems and natural environments.