Dual Optimization Strategies For Photocatalytic Degradation Of Tetracycline By Single Atom Mo Dispersed Graphitic Carbon Nitride: Morphology Regulation And Defect Construction

Tetracycline（TC）antibiotics have huge production and consumption in the world every year.The environmental pollution caused by the production and use of TC antibiotics poses a great threat to the ecosystem.As a highly efficient advanced oxidation technology,photocatalysis has many advantages in the degradation of antibiotics,such as low energy consumption,rapid reaction,mild conditions,and strong mineralization ability.Single atom catalyst is a research hotspot in the field of catalytic materials.The unsaturated coordination metal atoms exposed on the surface of the carrier do not contact and interact with each other.The atom utilization rate is very high,and the significant catalytic activity is guaranteed while consuming a small amount of metal materials.However,the agglomeration and low load density of metal atoms are urgent problems to be solved,and the selection and optimization of support materials are effective measures to solve these problems.Graphitic carbon nitride（g-C₃N₄）is not only an excellent semiconductor photocatalyst,but also a suitable metal single atom catalyst support.On the one hand,it is easy to modify,and on the other hand,hepazine ring structure unit provides many anchoring sites for metal atoms.Therefore,g-C₃N₄ can be used as a link between photocatalysis and single atom catalysis,thus showing excellent performance in the field of TC degradation.This study is divided into the following two parts:In part 1（chapter 3）,tubular porous g-C₃N₄ with N-vacancy（Nv-TCN）was successfully synthesized by pretreatment of the precursor and secondary calcination in N₂ atmosphere,and then single atom Mo was loaded on the surface of Nv-TCN（Mo/Nv-TCN）through a solvothermal process.The tubular morphology provides more anchor sites for Mo atoms.N vacancies induce the formation of stable and special coordination structures between Mo atoms and g-C₃N₄.Mo atoms and Nv-TCN carriers are connected by Mo-2C/2N bonds,and Mo mainly exists in the form of Mo⁶⁺.In the process of photocatalytic reaction,photogenerated electrons tend to transfer to Mo atoms locally.Mo-2C/2N bond acts as a bridge of charge transfer,and Mo atoms become the active site of the reaction.The single atom modification improves the absorption of visible light,promotes the separation and transfer of photogenerated electrons and holes（h⁺）,increases the charge density,and optimizes the band structure of the carrier.In part 2（chapter 4）,the degradation performance of TC in visible light was compared between modified composite and bulk g-C₃N₄（CN）.The performance improvement mainly comes from single atom Mo loading.The optimal 10-Mo/Nv-TCN sample has a TC removal rate of 94.45%within 1 h,and the apparent rate constant is 0.049 min^-1,which is 4.46 times that of bulk CN.In addition,the sample has good performance and structural stability.Analysis results of reactive species showed that the reactive species in the degradation process of TC included h⁺,superoxide radical（·O₂^-）,singlet oxygen（¹O₂）,and hydroxyl radical（·OH）,among which·O₂^-played the most important role,with a contribution rate of 92%.In this study,by modifying the morphology and defects of the support,the single atom can be stably and high-density loaded,which provides a reference for the construction of the single atom material system,and proves that the support selection and modification is an important prerequisite for the successful construction of the single atom catalyst.In addition,this study also fully demonstrates the research space and prospect of single atom design concept in the field of photocatalysis,proves the potential of single atom photocatalysis system,and provides practical reference and theoretical basis for the removal of antibiotics in environmental water.