Mixed-valent Fe-MOF accelerated Fe(III)/Fe(II) cycle for highly efficient photo-Fenton-like catalytic degradation of organic pollutants: Boosting mechanism and degradation pathways

Fe-based metal-organic frameworks (Fe-MOFs) show great potential as effective Fenton catalysts in practical pollutants degradation. However, the sluggish cycling rate of the Fe(III)/Fe(II) pair severely impedes their catalytic performance, and thus hinders the framework's effectiveness. In this work, by employing a mixed-valent Fe-MOF, we proposed an innovative strategy to address the limitation of Fe(III)/Fe(II) cycle of Fe-MOF in photo-Fenton-like reaction. We pioneered a direct Fe(II) proportional substitution method to synthesize a variety of mixed-valent Fe-MOFs by adjusting the FeIII/FeII ratio in the precursor, utilizing MIL-53(Fe)-NH2 as the template material. Notably, with superior accelerated Fe(III)/Fe(II) cycle, the constructed Fe-MOF-2 with a distinctive FeIII/FeII ratio of 3:1 displayed exceptional oxytetracycline (OTC) catalytic degradation capabilities, with an impressive degradation rate of 92.6 % within 30 min. The FeIII/FeII mixed-valent structure was proven to expedite the rate-determining step of PMS activation, which involved the reduction process of the Fe(III)/Fe(II) cycle. The production of more iron-unsaturated sites effectively promoted the production of SO4•− and •OH, which contributed significantly to the OTC degradation. Additionally, the possible removal intermediates, mechanism and pathways of OTC degradation in the Fe-MOF-2-based system were elucidated. This study provides a pioneering perspective, and demonstrates the potential application of mixed-valent Fe-MOF as a photo-Fenton-like catalyst.