Regulation Of Single-atom Co-N_x Site Coordination Structure And Enhanced Persulfate Activation For Degradation Of Organic Pollutants In Wate

At present,the pollution situation of emerging contaminants in China is grim and have gradually become a new challenge in environmental treatment.Sulfate radical-based advanced oxidation processes（SR-AOPs）is considered to be an efficient and promising technology for the treatment of emerging contaminants.The development of catalysts with high activity and stability is essential for the practical application of SR-AOPs.Single-atom catalysts have unique structures,in which dispersed metal atoms interact strongly with the surrounding coordination atoms,which enables them to achieve maximum atomic utilization while maintaining high activity and stability,and even exhibit good catalytic selectivity.Therefore,single-atom catalysts have been increasingly applied to activate persulfates for the degradation of emerging contaminants with promising achievements.However,there are still great challenges in basic theory and engineering application.First,the reaction mechanism is unclear and there are many oxidation pathways,but its causes need to be further studied.Second,the coordination configuration of single-atom catalysts used activating persulfates is monotonic.Therefore,there is a lack of understanding between the structure and catalytic performance of catalysts,which is not conducive to the design of single-atom catalysts for practical environment.In this study,based on the excellent characteristics of carbon-based catalysts and the strong catalytic activity of cobalt（Co）to persulfate,we proposed to prepare carbon-based single-atom Co catalysts with different nitrogen coordination numbers based on nitrogen（N）-doped carbon-based materials.The details are as follows:（1）Two atomically dispersed cobalt catalysts with different nitrogen coordination numbers(denoted as Co _SA-N_x-C)were synthesized and firstly compared to activate peroxydisulfate（PDS）for bisphenol A（BPA）degradation.Theoretical calculations unveiled that lowering the Co-N coordination number from four to three can apparently increase the electron density of the single Co atom in Co _SA-N₃-C to enhance PDS conversion.The low-coordinated Co _SA-N₃-C with Co-N₃ coordination structure displays a high specific activity of 0.067 L min^–1 m^–2,which is 1.31 times greater than that of Co _SA-N₄-C with normal Co-N₄configuration(0.051 L min^–1m^–2)in PDS activation.Electron paramagnetic resonance（EPR）measurements and quenching tests confirmed the primary role of sulfate radical(SO₄^（?）–)in BPA oxidation over Co _SA-N₃-C with PDS.Moreover,Co _SA-N₃-C delivers favorable durability for PDS activation and potential practicability for realistic wastewater remediation.（2）A series of atomically dispersed cobalt catalysts with diverse coordination numbers（denoted as Co-N_x,x represents nitrogen coordination number）are synthesized and their peroxymonosulfate（PMS）conversion performance is explored.The catalytic specific activity of Co-N_x is found to be dependent on coordination number of single atomic Co sites,where the lowest-coordinated Co-N₂ catalyst exhibits the highest specific activity in PMS activation,followed by under-coordinated Co-N₃ and normal Co-N₄.Experimental and theoretical results reveal that reducing coordination number can increase the electron density of single Co atom in Co-N_x,which governs the Fenton-like performance of Co-N_x catalysts.Specifically,the entire Co-pyridinic N-C motif serves as active centers for PMS conversion,where the single Co atom,and pyridinic N-bonded C atoms along with nitrogen vacancy neighboring the unsaturated Co–pyridinic N₂ moiety account for PMS reduction and oxidation toward radical and singlet oxygen（¹O₂）generation,respectively.In practical applications,Co-N₂/PMS has shown efficient and stable performance in BPA degradation and actual food waste water treatment.These findings reveal the dependence between the structural configuration and catalytic performance of single-atom Co catalysts,providing a novel and useful avenue to coordination number modulation of SACs for wider environmental applications.