A strategy for the fast generation of hydroxyl radicals (HO·) via photo-electro-reduction of oxygen by rerouting the electron transfer pathway was proposed. The rate-determining step of HO· production is the formation of H2O2 and the simultaneous reduction of H2O2. Engineering of F-TiO2 with single atom Pd bonded with four F and two O atoms favored the electrocatalytic 2-electron oxygen reduction to H2O2 with as high as 99% selectivity, while the additional channel bond HO-O···Pd-F-TiO2 facilitates the photogenerated electron transfer from the conduction band to single atom Pd to reduce Pd···O-OH to HO·. The optimized HO· production rate is 9.18 μ mol L-1 min-1, which is 2.6-52.5 times higher than that in traditional advanced oxidation processes. In the application of wastewater treatment, this proposed photoelectrocatalytic oxygen reduction method, respectively, shows fast kinetics of 0.324 and 0.175 min-1 for removing bisphenol A and acetaminophen. Around 93.2% total organic carbon and 99.3% acute toxicity removal were achieved. Additionally, the degradation efficiency was less affected by the water source and pH value because of the evitable usage of metallic active sites. This work represents a fundamental investigation on the generation rate of HO·, which would pave the way for the future development of photoelectrocatalytic technologies for water purification.
Keywords: electron transfer; hydroxyl radical; oxygen reduction reaction; photoelectrocatalytic process; single atom catalysis.