Metal–Organic Framework MIL-101(Fe) Nanoparticles Decorated with Ag Nanoparticles for Regulating the Photocatalytic Phenol Oxidation Pathway for Cr(VI) Reduction

Photocatalysis is a promising technology to treat dilute phenol–Cr(VI) mixture, where photoinduced electrons are commonly thought of as the main active species for Cr(VI) reduction. However, it generally depends on the surface adsorption to achieve efficient electron transfer. Meanwhile, the possible contribution of reductive quinone derivatives oxidized from phenol to Cr(VI) reduction has been rarely explored. The key is to explicitly understand the relation between the phenol oxidation pathway and Cr(VI) reduction. Herein, Ag/AgCl/MIL-101(Fe) prepared by directly loading Ag nanoparticles on MIL-101(Fe) was applied to the joint treatment of phenol and Cr(VI). The role of quinone derivatives in reducing Cr(VI) was revealed by excluding the surface adsorption. During the phenol oxidation, •OH radicals were more consumed in the initial stage for the ring opening of phenol. Meanwhile, 1O2 evolved from •O2– gradually caused the accumulation of reductive quinone intermediates, which dramatically accelerated the Cr(VI) reduction afterward. This study demonstrates the significance of controlling the evolution process of active oxygen species for the joint photocatalytic treatment of phenol–Cr(VI) mixture.