Complete debromination of 2,2′,4,4′-tetrabromodiphenyl ether by visible-light photocatalysis on g-C3N4 supported Pd

Deep debromination of polybrominated diphenyl ethers (PBDEs) by visible light photocatalysis is still a challenge in the field of halogenated pollutants control. Here, an efficient photocatalyst (Pd/g-C3N4) was prepared by a simple impregnation-chemical reduction method with bulk g-C3N4 and PdCl2 as precursors. Under preferred reaction conditions (visible light irradiation for 120 min), the use of the 0.5% Pd/g-C3N4 composites (0.4 g L-1) yielded an almost complete debromination of BDE47 (10 mu mol L-1) in methanol-water mixture, while the debromination of BDE47 did not occur on pure g-C3N4. A Pd-promoted active H atom species attack and C-Br bond cleavage route was proposed according to the identified degradation intermediates. A catalytic mechanism was further clarified: Pd nanoparticles exerted affinity interaction with bromine atoms, and the storing of electrons on Pd would increase the binding interaction. In the debromination process, methanol acts as both a hole scavenger and a hydrogen donor to provide active H atom species through the reaction of H+ and photogenerated electrons on Pd nanoparticles. The initiation of BDE47 debromination on Pd nanoparticles required an induction period to enrich a critical amount of electrons for stretching the C-Br bond and its subsequent being attacked by active H atoms. The generated 2,2'-dibromodiphenyl ether (BDE4) would be further reduced to bromine-free products by hydro-debromination process or coupling reaction.