Preparation,characterization And Electro-catalytic Properties Of Single-atom Catalysts

Metal supported catalysts are the most widely studied heterogeneous catalysts in the field of electrocatalysis.The size of the metal nanostructure is an important parameter that affects the activity of the catalysts.Since the active center of electrocatalysis is the metal atoms with unsaturated coordination,reducing the particle size of the catalysts can greatly increase the specific activity of each metal atom.The single atom catalysts(SACs)with a single metal atom anchored on the support has attracted much attention in energy conversion and storage because of its maximum atom utilization,high selectivity and efficiency.However,the surface free energy of metal atoms increases and tends to aggregate into nanoparticles or nanoclusters with the particle size decreasing.In order to prevent the arbitrary movement and aggregation of atoms,it is necessary to develop a suitable support to form a strong interaction with metal atoms and to obtain stable and uniformly dispersed active sites.In addition,modifying the support material by introducing of defects,vacancies and heteroatom doping will increase the metal loading with the interaction between a single atom and support optimizing.Target at the problems above,this paper focuses on the preparation and application of SACs.Based on the modifying of support and through the use of template and heteroatom doping,the relationship between the morphology and electronic structure of catalysts and performance is studied to optimize the reaction process.The main research content and results are as follows:(1)Aiming at the problems of carbon stacking and metal aggregation during hightemperature synthesis of carbon-based single atom catalysts,a synthesis strategy using montmorillonite as a template was developed to prepare single atom catalysts with transition metal atom supported on graphene nanosheets.By inserting the complex of phenanthroline and metal into the montmorillonite layers,the carbon precursor with twodimensional growth under the action of the interlayer confinement,and inhibits the metal overgrowth and aggregation during the high-temperature pyrolysis process.After the etching of template,a three-dimensional carbon material with single metal atom uniformly dispersed is obtained.Compared with the bulk carbon material obtained without the template,the layered structure catalysts using template exhibits an ultra-high specific surface area.Among them,Ni-SAC has excellent CO2 RR activity,and the electronic structure analysis of Ni-SAC obtains the valence state of Ni is between 0valence and +2 valence.The coordination environment of the central metal atom is NiN4.The experimental results show the Ni-SAC catalyst with this special structure makes excellent electrocatalytic activity in CO2 RR.In the large potential range of-0.6 to-1.1 V,the CO Faraday efficiency of Ni-SAC is greater than 90%,and it can remain stable in a continuous reaction at-0.9 V vs.RHEt for 30 hours with the CO FE greater than 95%.(2)By using montmorillonite as template and phenanthroline as carbon precursor,a series of Fe single-atom catalysts with different calcination temperature were prepared.With higher calcination temperature,the Fe loading was slightly reduced,but the content of pyridine nitrogen increased.The pyridine nitrogen is identified as the nitrogen species that coordinates with metal.This change of carbon surppot results in different coordination environment for Fe,which affects the activity of the catalyst in ORR.Among them,the Fe-SAC-900 which calcined at 900 oC shows specific ORR activity consistent with commercial Pt/C both in acid and alkaline electrolytes,and its stability exceeds that of commercial Pt/C in long-term potentiostatic stability test and methanol oxidation resistance test.(3)The aniline was coated on the surface of halloysite nanotubes which played a role of template.The template is removed by acid etching after high-temperature pyrolysis to obtain Fe single atom supported on a three-dimensional carbon surpport with orderly arrangement of hollow nanotubes.The catalysts have specific surface areas above1000 m2·g-1.In addition,the introduction of phytic acid in the aniline coating process can modify the catalysts by phosphorus doping.Phosphorus doping can regulate the coordination environment of catalytic center formed by metal sites and support,and then affects the catalyst performance on electrochemical application.Experimental results show that the nitrogen-phosphorus co-doped structure formed after the introduction of phosphorus has better ORR activity than which with pure nitrogen doping,and the ORR reaction process is a four-electron process.(4)The single-atom Sn doped Cu O is used to control the selectivity of CO2 RR,and by changing the doping amounts of Sn,over 90% Faraday efficiency of CO can be obtained.The addition of single-atom Sn not only changes the morphology of Cu O to nanowires,but also regulates the electronic valence state of Cu and changes the electronic structure of the catalyst.In-situ attenuated Surface Enhanced Infrared Spectroscopy confirms that the mechanism of Sn doping is to change the linear adsorption of intermediate CO on the electrode to bridge adsorption,thereby inhibiting the further reduction of reactants and making the product singularized into CO.