Electron Spin Resonance Evidence for Electro-generated Hydroxyl Radicals

Electro-generated hydroxyl radicals (center dot OH) are of fundamental importance to the electrochemical advanced oxidation process (EAOP). Radical-specific electron spin resonance (ESR) evidence is still lacking in association with the direct electron transfer (DET) reaction of spin trap (e.g., S,S-dimethyl-1-pyrroline-N-oxide; DMPO) and side reactions of the DMPO-OH adduct in the strongly oxidative environment offered by anodic polarization. Herein, we showed ESR identification of electrogenerated center dot OH in EAOP based on the principle of kinetic selection. Excessive addition of a DMPO agent and fast spin trapping allowed suitable kinetic conditions to be set for effective spin trapping of electro-generated center dot OH and subsequent ESR identification. Otherwise, interferential triplet signals would emerge due to formation of paramagnetic dimer via dehydrogenation, DET oxidation, and dimerization reactions of the DMPO OH adduct. The results demonstrate that center dot OH formation during spin-trapping on the titanium suboxide (TiSO) anode could be quantified as 47.84 +/- 0.44 mu M at current density of 10 mA cm(-2). This value revealed a positive dependence on electrolysis time, current density, and anode potential. The effectiveness of ESR measurements was verified by the results obtained with the terephthalic acid probe. The ESR identification not only provides direct evidence for electro-generated center dot OH from a fundamental point of view, but also suggests a strategy to screen effective anode materials.