Study On The Catalytic Mechanism Of SACs-based Fenton-Like Systems Based On The Redox Characteristics Of Pollutants

With the rapid development of industrialization and the extensive use of various chemicals,the hazards of emerging contaminants to the ecological environment and human health have gradually appeared.Although some progress has been achieved in the treatment,detection and evaluation of emerging contaminants,many problems still exist.Advanced oxidation technologies based on sulfate radicals(SR-AOPs)could effectively degrade and mineralize multiple pollutants by activating peroxymonosulfate(PMS)to generate diverse radicals.Singleatom catalysts(SACs)with unique electronic and coordination structures exhibit excellent catalytic performance in SR-AOPs and have attracted domestic and international attention.SACs/PMS systems are highly selective to the type of organic pollutants while the mechanisms remain ambiguous.In this work,we carried out experimental and theoretical investigations to reveal the origins of selectivity of radical and nonradical pathways in a designated SACs/PMS system.The main research contents and conclusions are as follows:(1)Preparation and characterization of cobalt single-atom catalysts and nitrogen-carbon catalysts.In this study,cobalt single-atom catalysts(Co-N4-C)and nitrogen-carbon catalysts(N-C)were firstly prepared by one-step calcination pyrolysis using lignin and dicyandiamide as precursors,and characterized by various instrumental methods.The results show that the Co atoms and N atoms in Co-N4-C are uniformly dispersed on the carbon-based substrate,where the Co atoms are coordinated with N atoms exclusively,and the coordination structure is CoN4.The doping of metal atoms results in a larger graphite N content,defect degree and specific surface area of Co-N4-C,which is more favorable for electron transfer and provide more active sites for PMS activation and contaminant degradation.(2)Investigating the structure-activity relationships between pollutant structure and degradation performance and mechanism in catalytic systems.Organic pollutants with different structural characteristics were selected as target pollutants,examining the relationship between the discrepancy of degradation performance and the structural characteristics of pollutants in the SR-AOPs.It was found that pollutants with electron-giving groups such as hydroxyl groups were easily degraded,while pollutants with electron-absorbing groups such as carboxyl groups were almost non-degradable.Two typical pollutants[bisphenol A(BPA)and metronidazole(MNZ)]with different molecular structures were employed for further analysis of the limiting factors and mechanisms of degradation.The results showed that radical and nonradical pathway co-existed in the Co-N4-C/PMS and N-C/PMS systems.MNZ was degraded primarily by radicals(HO·,SO4·-,while the oxidization of BPA was dominated by electron transfer pathway(ETP).Besides,the contribution of different oxidation pathways for MNZ and BPA degradation was related to PMS concentration.The sulfate radical contributed more than 80%for MNZ degradation at low PMS dosage,and then it was gradually decreased with the increasing PMS dosage.The relative contribution of ETP to BPA degradation was approximately 24 times greater than that of radicals,and progressively offset the effect of radicals with increasing doses of PMS.In addition,further theoretical calculations revealed that the discrepancy in the degradation mechanism was significantly associated with the energy gap difference between the highest occupied molecular orbitals(HOMO)of pollutants and the lowest unoccupied molecular orbital(LUMO)of Co-N4-C-PMS*complexes.Benefiting from the narrow energy gap of BPA and Co-N4-C-PMS*complexes,PMS could readily extract electrons from the(BPA),while the energy gap between HOMO of MNZ and LUMO of Co-N4-C-PMS*complexes was much broader,MNZ could not be oxidized via ETP.(3)Clarify the origin of the superior catalytic performance of SACs and their active sites.The open-circuit potential,electron paramagnetic resonance and ion chromatography revealed that Co-N4-C showed a prominent advantage in the activation of PMS,and the Co-N4 site could significantly promote the adsorption and activation of PMS with subsequent enhancement of electron transfer and radical oxidation processes.(4)Analysis the relationship between degradation mechanisms and the redox characteristics&adsorption of pollutants.In order to clarify the generality of the selective degradation mechanism of Co-N4-C/PMS system,the relationship between the degradation performance and mechanism with the redox potential and adsorption of pollutants was investigated.The results showed that the redox potential of the refractory pollutants was higher and hardly adsorbed by Co-N4-C,which could not be oxidized by ETP,and the increasing radical concentration at high PMS concentration could promote the effective oxidation of the refractory pollutants.The redox potential of the readily degradable pollutants was lower,and ETP occurred easily in the catalyst surface region due to the strong adsorption of Co-N4-C,and the contribution of ETP gradually offset the effect of radicals as PMS concentration increased.