Environmental Accumulation,Chemical Preparation,and Performance Study Of Silver Single-Atom

Silver（Ag）nanomaterials are widely used in various fields owing to their low preparation cost and excellent antibacterial and catalytic performances.However,a great amount of Ag nanomaterials is released into the environment during the production and using process every year causing potential environmental risks.Due to the moderate redox potential of Ag,a cycle of Ag（0）and Ag（I）occurs under natural conditions,in which Ag ions are easily reduced to Ag（0）by reducing natural organic matters in the environment,and Ag（0）is transformed into Ag（I）through various complex reactions.However,the characterization and confirmation of real Ag species（such as Ag single-atom）and their formation mechanism in the cycling process remain obscure.This hinders the better understanding of the real environmental effect of Ag nanomaterials and raises the innovative difficulty of developing Ag-containing catalysts with outstanding performances.Single-atom catalysts（SACs）have the advantages of both heterogeneous and homogeneous catalysts,and have attracted numerous researches in various catalytic reactions.The 100%atom utilization efficiency due to atomic dispersion,uniform and stable active site and strong metal-support interaction make SACs exhibit excellent catalytic activity,stability,and selectivity.Meanwhile,multiple factors can affect the coordination environment of metal single-atom,resulting in different electronic structures and catalytic performance.Therefore,it is important to precisely control the coordination environment of active site at the atomic scale,to achieve the good structure-activity relationship between atomic structure and catalytic activity.However,due to the limitations of synthesis strategies and the complexity of atomic scale control,the preparation methods of SACs always exhibit unsatisfactory reproducibility.In addition,the diversity of coordination environments of active site is not conducive to the preparation of SACs with reasonable structure and performance relationships.This thesis studied the accumulation phenomenon of Ag single-atom under environmental conditions and explored its production mechanism.Furthermore,based on these findings,a series of Ag single-atom materials were prepared under mild conditions using diverse synthetic strategies.These Ag single-atom materials demonstrated excellent properties for various reduction reactions and antibacterial experiments,while the structure-activity relationship between coordination structure and activity was established through systematic studies.The thesis is divided into the following chapters:（1）The accumulation phenomenon of Ag single-atom under environmental conditions.The conversion process of Ag⁺to Ag（0）under light irradiation was studied in the presence of common natural organic matters（oxalic acid,humic acid,phenol）.The results show that in the humic acid and phenol system,not only zero-valent Ag nanoparticles（AgNP）are formed,but also many zero-valent Ag single-atom（AgSA）are accumulated,and the final accumulation amount of AgSA in humic acid system is higher than that in phenol system.But only AgNP formed in oxalic acid system.Through mechanism analysis,it is proposed that the phenolic hydroxyl structure in natural organic matters play an important role for the formation and stability of AgSA.Meanwhile,the accumulation phenomenon of AgSA has good universality,which indicates that AgSA may exist in the real environment.（2）Enhancing the accumulation of Ag single-atom by Ag nanoparticles under environmental conditions.Under irradiation,in the complex system where Ag⁺,AgNP and phenols coexisted,a large amount of zero-valent AgSA was generated,and AgNP can significantly promote the accumulation of AgSA.The mechanism shows that in phenol system,AgNP can photo-catalyze phenol to many intermediates including catechol,hydroquinone,and benzoquinone,which can promote the stabilization of AgSA more easily than phenol.And the content of AgNP is positively correlated with the generation rate of intermediates,thereby AgNP shows the promoting effect on the accumulation of AgSA.Meanwhile,this promotion phenomenon has good universality for a variety of phenolic substances and/or oxide supports system.（3）Benzoquinone-photo-assisted strategy for the synthesis of an Ag singleatom catalyst for efficient hydrogenation reactions.Ag single-atom catalyst（AgSA/Al₂O₃）with γ-Al₂O₃ as support was prepared by benzoquinone-photolysis method with poor solvents of strong ionic strength to limit Ag⁺ on the γ-Al₂O₃ surface.The characterization results indicate that the zero-valent AgSA is anchored by hydroxyl oxygen on the support surface to form an Ag-O bond.Compared with AgNP material,AgSA material exhibit better catalytic performance in the reduction of nitrobenzene,quinoline,and aldehydes by NaBH4.In the reduction reaction of nitrobenzene,AgNP is completely deactivated at pH≥ 10.4,while AgSA was not inactivated until pH≥13.4,so AgSA had better pH tolerance.When 10.4 ≤ pH<13.4,the utilization efficiency of AgSA material for NaBH4 can reach 100%.The mechanism shows that due to the high atomic dispersion efficiency and low oxidation state of AgSA,more stable Ag-H species will be generated during the catalytic process,which results in AgSA being able to transfer the H-atom to the substrate more quickly.The Ag-H species generated on the AgNP surface can move with smaller energy barriers to generate H2 thus exhibiting poor reduction reaction performance.（4）Pre-modified support strategy for the synthesis of Ag single-atom catalyst for efficient 4-nitrophenol reduction.The Ag single-atom catalyst（AgSA/TiO₂）with catechol-modified TiO₂ as support was prepared by impregnation-reduction method.And Ag nanoparticle catalyst（AgNP/TiO₂）were synthesized by the same procedure using unmodified TiO₂ as the support.The experimental results show that the phenolic hydroxyl group in catechol played an important role for the formation and stabilization of AgSA.AgSA exhibits a low valence state close to zero,and is simultaneously coordinated with the phenolic hydroxyl oxygen on the support surface and the hydroxyl oxygen on the TiO₂ surface to form an Ag-O bond.Like AgSA/Al₂O₃ in work（3）,AgSA/TiO₂ also showed better catalytic activity,pH tolerance and NaBH4 utilization efficiency than AgNP/TiO₂ in the reduction of 4-nitrophenol.Meanwhile,AgNP/TiO₂,exhibits a significant induction period under air atmosphere;But the AgSA/TiO₂ exhibits similar catalytic activity in air and nitrogen atmospheres without exhibiting an induction period.Therefore,AgSA/TiO₂ also has better oxygen tolerance.The mechanism study shows that AgSA can produce more lower oxidation state Ag-H species faster than AgNP,which results in that AgSA preferring catalytic reduction reaction and AgNP preferring catalytic hydrolysis reaction of NaBH4.（5）Preparation of Ag single-atom material by grinding method to achieve efficient antibacterial activity.An Ag single-atom material（AgSA/Al₂O₃）under environmental conditions was prepared by grinding a mixture of AgNP and hydroxylrich γ-Al₂O₃.The characterization results showed that during the grinding process,the Ag atoms on AgNP surface were trapped by the doubly bridging hydroxyl groups on the support surface and formed weak interaction Ag-O bond,which eventually led to the transformation of all AgNP into AgSA.And AgSA exhibits a low valence state close to zero,making it promising as high-performance heterogeneous antibacterial agent.The experimental results confirm AgSA material exhibit better performance in killing Escherichia coli（E.coli）compared to AgNP material,and the antibacterial activity is positively correlated with the content of AgSA.The mechanism indicates that AgSA can produce higher concentrations of Ag ions and superoxide radicals than AgNP,which can kill E.coli by destroying the cell wall.This work indicates that AgSA-based antibacterial agents have great potential as substitutes for AgNP-based antibacterial agents.