Theoretical Study On Electrocatalytic Reduction Of CO₂ Over Two Dimensional Support-atomically Dispersed Active Site Catalysts

Carbon dioxide is the main greenhouse gas in the atmosphere,which is harmful to the environment and human health.The CO₂electrochemical reduction reaction（CO₂RR）is one of the most potential methods to eliminate and transform CO₂.Therefore,it is very important to design and develop CO₂RR catalysts with high activity,selectivity and stability.Through the design of two-dimensional support coordinated with atomically dispersed active sites catalysts,cheap and excellent nano catalytic materials were screened,and the reaction mechanism and catalytic nature of CO₂RR were deeply understood from the atomic scale,providing theoretical guidance and scientific basis for the study and development of novel and efficient CO₂RR catalysts.In this dissertation,several atomically dispersed active centers anchored on two-dimensional materials for electrocatalytic CO₂RR were studied by density functional theory（DFT）calculation.The main research contents are as follows:（1）The reaction mechanism of single-atom catalyst supported on Mo S₂in CO₂RR process was studied by theoretical calculation.The results show that monodisperse metal atoms can be stably supported on Mo S₂.Fe@Mo S₂,Co@Mo S₂,Ni@Mo S₂and Pt@Mo S₂show excellent electrocatalytic performance for CH₄production from CO₂RR,with limiting potential of-0.39V,-0.24V,-0.45V and-0.50V,respectively.The adsorption energy of the catalyst for the key intermediate HCOO can be used as an effective descriptor to screen suitable electrocatalysts.In addition,single-atom support on other two-dimensional chalcogenides（Mo Se₂,WS₂,WSe₂）also showed potential catalytic performance for CO₂RR.These results provide a theoretical basis for understanding the use of single-atom catalysts in CO₂RR.（2）The type of two-dimensional support has an important influence on the catalytic activity.From the results in（1）,it can be seen that the Co single-atom supported on Mo S₂has excellent catalytic performance for CO₂RR.We investigated the electrocatalytic performance of the single-atom Co anchored on a series of common two-dimensional support catalysts for CO₂RR to explore the influence rule of support on electroreduction reaction of CO₂.The calculation results show that different supports can not only adjust the geometric structure of the metal center,but also can control its electronic structure.By comparing the Gibbs free energy diagrams of CO₂RR on different catalysts,we found that different reaction products can be obtained by changing the support type,and the mechanism for generating the same product may be different on different catalysts.Among the studied catalysts,Co@Mo S₂,Co@Ti₂CS₂,Co@Ti₃C₂S₂and Co@GDY have excellent catalytic performance for CH₄production from CO₂RR,and the limiting potentials are-0.24V,-0.22V,-0.27V and-0.48V,respectively.Co@graphene-N₄is predicted to be promising CO₂RR catalyst for producing HCHO with the limiting potential of-0.65V.The product CH₃OH can be generated on Co@gra-N₃,and the limiting potential is-0.59V.These results indicate that changing the type of two-dimensional support can not only improve the activity of the catalyst,but also adjust the target product.It can provide theoretical guidance for revealing the interaction between the support and the active sites.（3）The two-atom and/or dimer catalyst is a supplement and improvement to the single-atom catalyst.We designed a series of Cu based dimer doped on nitrogen-containing graphene to study its electrocatalytic performance for CO₂RR.The calculation results show that the dimer doped on monolayer nitrogen-containing graphene has excellent stability.The most stable adsorption site of the intermediate in Cu-X-N₆catalyst was explored.The nitrogen atom between two atoms is the adsorption site of the reaction intermediate,while in other heteronuclear dimers,the active site is related to the specific metal species,which may be either nitrogen atom or metal atom.The main product on Cu-Mn-N₆and Cu-Fe-N₆is HCOOH,and the limiting potentials are-0.29V and-0.22V,respectively.Cu-Rh-N₆and Cu-Ir-N₆are excellent dimer catalysts for CH₃OH production,and the limiting potentials are-0.50V and-0.56V,respectively.These results provide theoretical guidance for designing heteronuclear dimer catalysts for CO₂RR.（4）It is more important to convert CO₂into higher value-added C₂hydrocarbons.Trimer catalysts with atomically dispersed active centers can provide suitable active sites to produce C₂products.Therefore,we constructed a series of transition metal trimers（Cr₃,Mn₃,Fe₃,Co₃,Ni₃,Cu₃and Ru₃）doped on the two-dimensional support g-C₃N₄,and calculated their CO₂RR reaction mechanism for generating C₁and C₂products.The theoretical calculation results show that the metal trimer can be stably anchored on the N-riched cavity stably.Cu₃@g-C₃N₄can generate C₁product CH₄with high activity,and its limiting potential is-0.42V.For the generation of C₂product,Fe₃@g-C₃N₄,Co₃@g-C₃N₄and Cr₃@g-C₃N₄are predicted to be promising CO₂RR catalysts for producing C₂H₄with the limiting potential of-0.45V,-0.64V and-0.64V,respectively.Its excellent performance makes the trimer catalyst become a potential catalyst for electroreduction of CO₂to C₂products.These results provide theoretical guidance for C₂H₄production through CO₂RR technology.（5）The bimetallic trimer catalysts have attracted much attention to researchers.DFT study of copper-based bimetallic trimer doped on C₂N carrier for CO₂RR was carried out by us.The results show that the binding ability of bimetallic trimer on C₂N is very strong,which makes the catalyst structure very stable.The overpotential of the catalysts for electrochemical reduction of CO₂to CH₄is in the range of 0.87-1.24V,indicating that it is difficult to produce C₁product.Cu₂Fe@C₂N is promising CO₂RR catalyst for producing C₂H₄with the limiting potential of-0.63V.The catalyst with better catalytic performance is selected based on Gibbs free energy of*COCO→*COCOH because the potential determining steps of all the studied catalysts to generate C₂H₄for CO₂RR are the process from*COCO→*COCOH.It is found that Fe₂Co@C₂N and Co₂Fe@C₂N also have excellent catalytic performance to produce C₂H₄,and the limiting potential are-0.58V and-0.66V,respectively.For the C₂H₄reaction from CO₂RR,the catalytic activity of bimetallic trimer catalyst is better than that of the corresponding single metal trimer catalyst.The above theoretical calculation results provide a new construction method and design idea for exploring efficient CO₂RR catalyst.