Growth of graphene-supported hollow cobalt sulfide nanocrystals via MOF-templated ligand exchange as surface-bound radical sinks for highly efficient bisphenol A degradation

Graphene nanosheet-supported hollow cobalt sulfide nanocrystals (Co3S4@GN, CoS@GN) were fabricated via a facile ligand exchange route using metal-organic frameworks (MOFs) as self -templates. Subsequent thermal annealing induced the phase transformation of Co3S4 to CoS. This synthesis strategy drove the cobalt ions inside zeolitic imidazolate frameworks (ZIF-67) to migrate outwards, forming a highly reactive shell composed of abundant exposed active sites to activate peroxymonosulfate (PMS) in the sulfate radical (SO4 center dot-)-based advanced oxidation process (SR-AOP). The graphene support exhibited excellent efficiencies in the enrichment of targeted pollutant as well as the charge transfer between absorbed molecules and radicals. The nanocatalysts were fully characterized and applied to the catalytic degradation of bisphenol A (BPA). Benefitting from the unique structure characteristic, the as-synthesized nanocomposites showed superior catalytic activities over a broad pH range. The degradation efficiency of BPA reached similar to 100% within 8 min by using CoS@GN, and the kinetic constant (0.62 min(-1)) was higher than those of most reported heterogeneous catalysts by 1-2 orders of magnitude. Furthermore, the critical roles of graphene support in regulating the variety and action site of radicals were addressed for the first time. The adsorptive and conductive graphene made the SO4 center dot- once produced was consumed immediately, which limited the diffusion of SO4 center dot- out of catalyst surface and the generation of center dot OH. The catalyst served as a surface-bound SO4 center dot- sink for the in-situ degradation of adsorbed BPA. Catalyst characterizations and the Density-Functional-Theory (DFT) calculation confirmed the excellent activity of CoS@GN in yielding SO4 center dot- with Co(II) as the active center. A CoS@GN-coated membrane reactor was constructed to avoid catalyst loss and worked well in consecutive 3 cycles, suggesting the satisfactory catalyst reusability and system robustness. Overall, this work paved a new way for MOFs in the environmental application and provided a novel family of Co -based nanocatalysts to produce surface-bound radicals for recalcitrant contaminant degradation by SR-AOP.