Efficacy And Mechanism Of Cobalt Single-Atom Catalysts For The Removal Of Benzene Ring-Containing Contaminants In Water

Catalysis accounts for around 40%of global industrial production value and plays an essential role in the fields of chemical engineering and environmental management.Single-atom catalysts have attracted great attention in environmental catalysis due to their high atomic utilization,adjustable coordination environment,and high selectivity.However,the high surface energy of single atoms makes them prone to aggregation and sintering during the synthesis process,which leads to a significant decrease in catalyst activity.Therefore,constructing highly efficient and stable catalysts has become a primary task in the field of catalysis.This study is conducted in the following three parts:research on the reduction performance and mechanism of nitrobenzene pollutants by cobalt single-atom/Co S catalyst;research on the efficiency and mechanism of cobalt single-atom catalyst activation of peroxymonosulfate（PMS）to remove sulfamethoxazole;and research on the application of cobalt single-atom catalyst in the treatment of actual antibiotic wastewater.Firstly,cobalt phthalocyanine（Co Pc）and trithiocyanuric acid were used as precursors to modify the electronic structure of the catalyst by sulfur-nitrogen co-doping.A high-graphitization,large-specific-area,and defect-rich single-atom catalyst（Co S@SNC）with Co S particles was obtained by high-temperature annealing.Based on this,the characteristics and mechanism of the reduction of 4-nitrophenol（4-NP）to 4-aminophenol（4-AP）by Co S@SNC were studied.The results showed that the conversion efficacy of 4-NP exceeded99%within 7 min.Co S particles in Co S@SNC gradually decomposed and decreased during catalytic reduction.Combined with characterization techniques such as XRD,XPS,TEM,AC-TEM,and synchrotron radiation,it was found that the morphology of Co S on the surface of Co S@SNC changed and gradually disappeared due to hydroxylation during the catalytic reduction process while a small amount of Co S inside Co S@SNC was stabilized by strong interaction with the carbon support.The synchrotron radiation analyses,Co K-edge fine structure fitting parameters,and the cyclic reduction of 4-NP experiments of Co S@SNC indicated that the typical active site Co-N-C of cobalt single-atom catalyst played a major role in the reduction of 4-NP to 4-AP.Furthermore,the characteristics and mechanisms of the activation of peroxymonosulfate（PMS）oxidation for the degradation of SMX using a carbon-supported single cobalt atom catalyst（Co@SNC）were investigated through comparative analysis.In terms of its activity,the removal efficacy of Co@SNC-activated PMS exceeded 92%within 15 min.In terms of mechanism,the similar systems Co（II）-PMS,Co Pc-PMS,and Co@SNC-PMS have different mechanisms for activating PMS to remove SMX.During the activation of PMS,there are differences in the main active species that activate SMX between divalent cobalt ions（Co（II））and Co Pc.As a molecular model of cobalt single atoms,Co Pc has a clear Co-N-C site.Its main active species in the process of activating PMS to degrade SMX is Co（IV）,while the contribution of hydroxyl radicals（·OH）and sulfate radical anions（SO₄·^-）were marginal.The mechanism of SMX degradation in the Co@SNC-PMS system is similar to that in the Co Pc-PMS system.The Co（II）-PMS,Co Pc-PMS and Co@SNC-PMS systems exhibited different mechanisms in the degradation of SMX.During PMS activation,the main active species involved in Co（II）-PMS and Co Pc-PMS were the divalent cobalt ion（Co（II））and the molecular model of phthalocyanine cobalt,respectively,whereas differences in the main active species existed in Co@SNC,which was cobalt single atoms.In the process of SMX degradation by Co Pc activated PMS with clear Co-N-C sites,the main active species was Co（IV）,and the contribution of hydroxyl radicals（·OH）and sulfate radicals（SO₄·^-）could be neglected.The mechanism of Co@SNC-PMS system for SMX degradation was similar to that of Co Pc-PMS.In the Co（II）-PMS system,the main active species were Co（IV）,·OH,and SO₄·^-,with SO₄·^-dominating the degradation of SMX.The Co@SNC in single atom catalysts did not simply enhance the activation of Co（II）in PMS but rather changed the activation pathway,transforming it from a radical-dominant degradation pathway to a non-radical dominant degradation pathway.Finally,the potential application of Co@SNC activated PMS for the selective degradation of SMX in pharmaceutical wastewater was evaluated.The results showed a selective removal efficacy of over 99%within 15 min.Thus,the Co@SNC-PMS system is suitable for the selective degradation of recalcitrant organic pollutants in wastewater or for advanced wastewater treatment.