Theoretical Exploration And Mechanistic Study On Two-Dimensional Porous Materials And Their Application Toward Photo-/Electro-catalytic CO₂ Reduction And N₂ Reduction

With the rapid development of the global economy and excessive consumption of fossil fuels,the energy crisis and global warming become increasingly severe.It is urgent to explore novel,clean,low-carbon,and sustainable energy conversion systems.At present,photochemical and electrochemical energy conversions,including photo-/electro-catalytic CO₂ reduction,N₂ reduction,etc.,have become the latest research hotspot due to its green and environmental advantages.Hence,it is of great significance to exploit novel,stable,low cost,highly active and selective photocatalysts and electrocatalysts.Numerous researches have shown that two-dimensional(2D)porous materials{including 2D metal-organic frameworks(MOFs),2D covalent-organic frameworks(COFs),2D carbon-based porous materials}possess noticeable superiorities among many reported catalysts,such as larger specific surface area,abundant and easily accessible active sites,easily tuned structures and functions,etc.,and have been widely applied in the fields of photo-/electro-catalysis.Therefore,by means of density functional theory(DFT)computations,this thesis have designed and screened out several promising catalysts with prominent advantages for photo-/electro-catalytic CO₂ reduction and N₂reduction reactions among 2D porous materials.The dominant contributions are as follows:1.The catalytic potential of the 2D cobaltporphyrin-based organic frameworks linked with phenyl,experimentally reported Co O-cluster,Zn O-cluster,and Zr O-cluster as CO₂ reduction photocatalysts(abbreviated as Co P,Co-PMOF,Zn-PMOF,and Zr-PMOF,respectively)was contrastively investigated by using DFT computations.The DFT computational results demonstrate that the optimal reduction pathway from CO₂ reduction to CH₄ catalyzed by the Co P,Co-PMOF,Zn-PMOF,and Zr-PMOF monolayers is the same:CO₂?*COOH?*CO?*CHO?*CH₂O?*CH₂OH?CH₃OH?*CH₃?CH₄.Especially,the 2D Co-PMOF exhibits superior catalytic activity than other MOFs in CO₂ reduction and rate-determining?G is only0.39 e V.The superior photocatalytic performance should be ascribed to highly accessible active sites on the double sides and the collaborative contribution from Co O-cluster and cobaltporphyrin during CO₂ reduction.Therefore,this research provides a key basis,reference mechanism,and important data for the potential applications of the experimentally synthesized 2D porphyrin-based MOFs in the field of photocatalytic CO₂ reduction.2.The potential of 2D reductive TM-PMOFs(where TM=Fe,Co,Ni,Cu,Zn,Ru,Rh,Pd,Os,Ir,and Pt)as CO₂ reduction electrocatalysts was fully investigated by using DFT computations and combining the experimental facts,which was constructed through the Mo?N triple bond between arylamides and four equatorial molybdenums from 4-connected tetra-(4-aminophenyl)metalloporphyrin(TM-TAPP)and reductive Lindqvist-type clusters[Mo₆]^2e/2H as building structs.Through the complete computational screening,the 2D Co-PMOF is found to be the most competitive because of its optimal catalytic activity for the reduction from CO₂ to CH₄ with the lowest limiting potential(-0.41 V).This work not only narrows the scope of exploration,but also provides reliable theoretical guidance for experimental studies in this field.3.The potential of single TM atoms supported on g-C(CN)₃ monolayer{TM-C(CN)₃,TM=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,respectively}as CO₂reduction electrocatalysts was systematically evaluated.The computational results display that these TM atoms can be firmly supported on the porous g-C(CN)₃substrate,revealing their high stability of TM-C(CN)₃ monolayers.Meanwhile,these single-atom catalysts show high CO₂ reduction activity and selectivity.Particularly,the Sc-,Co-,and Ni-C(CN)₃ are identified as practical CO₂ reduction electrocatalysts due to their low limiting potentials(-0.28,-0.42,and-0.46 V,respectively).This work provides important theoretical guidance for the development of single TM atoms supported on g-C(CN)₃ monolayer as single-atom catalysts toward CO₂electroreduction.4.The catalytic performance and mechanism of Cu dimer anchored in g-CN(Cu₂@CN)monolayer as electrocatalyst for CO₂ reduction was systematically evaluated by means of DFT calculations and combining the advantages of Cu in catalyzing CO₂ reduction.The computational results indicate that the Cu₂@CN monolayer can electrochemically convert CO₂ to HCOOH,in which the hydrogenation of*OCHO to form*HCOOH is identified as the potential-determining step with a low limiting potential(-0.16 V).Especially,CO₂ can be electrochemically converted to C₂H₄ on the Cu₂@CN monolayer via direct*CO dimerization,where CO₂?*COOH?*CO?*COCO?*COCOH?*C₂O?CHCO?*CH₂CO?*CH₂CHO?*CH₂CHOH?*C₂H₃?C₂H₄ is the most favorable pathway with a low limiting potential(-0.52 V).This work provides a new research idea for the design of novel and effective double-atomic catalysts for obtaining valuable C2products.5.The catalytic performances of a series of stable and conductive 2D TM-based covalent organic frameworks(TM-COFs,TM=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Nb,Mo,Ru,Rh,Pd,Ag,W,Ir,Pt,and Au,respectively)toward the N₂ reduction reaction were explored by means of DFT calculations,which were constructed by the robust linkage between 2,3,9,10,16,17,23,24-octaamino-metallophthalocyanine and pyrene-4,5,9,10-tetraone.Combining the screening of key steps and detailed theoretical simulations,the results show that the 2D conductive Mo-COF exhibits the highest electrocatalytic performance for N₂ fixation with a very low overpotential of0.16 V among the 20 potential candidates,and can effectively inhibit the competitive hydrogen evolution reaction.This work will promote follow-up experimental research of 2D COFs in N₂ fixation and the discovery of more highly active Mo-based COF catalysts.