Theoretical Studies On The Two-dimensional Single/double-atom Catalysts Based On Phosphorene Or Hexagonal Boron Nitride For CO₂/CO Electrocatalytic Reduction

With the advancement of industrialization,humans consume a large amount of fossil fuel,which puts great pressure on the natural environment.One of the consequences is that the emission of CO₂ increases year by year.Because CO₂ is an important component of greenhouse gases,a large number of climatological studies have found that the increasing CO₂ content in the air will lead to serious global warming.In recent years,the frequent occurrence of various extreme weather phenomena is likely induced by global warming.Therefore,research and development of new methods to capture and transform CO₂ have received a lot of attention.Many approaches have been explored,with electrochemical reduction of CO₂ to fuels and industrial feedstocks such as methane(CH₄),methanol(CH₃OH),and ethylene(C₂H₄)being the most anticipated.However,there are still many problems to be solved for the efficient electrochemical reduction of CO₂,among which the most urgent one is to find catalysts that can efficiently catalyze CO₂ into single-carbon or multi-carbon products.With the development of first-principles calculation and electrochemical surface reaction theory,it has become a frontier topic in the field of electrocatalysis to analyze the catalytic performance of catalysts and screen new electrocatalysts through theoretical calculation.It is very necessary to carry out theoretical research on novel and efficient catalysts for CO₂ reduction reaction(CRR)by first-principles methods to promote China's carbon neutrality development strategy and accelerate the recycling of carbon resources in space.Meanwhile,as CO appears as the input reactant of the second step in the two-step scheme of CO₂ electrocatalytic reduction,it is also very important to explore novel efficient catalysts for CO reduction reaction(CORR).A large number of theoretical and experimental studies have shown that co-doping of transition metal and nitrogen atom can obtain high catalytic activity centers in three-dimensional and two-dimensional materials.Two-dimensional transition metal single-atom catalysts have attracted extensive attention because of their high metal atomic utilization efficiency,large specific surface area,strong modifiability,and excellent catalytic activity.Therefore,it is necessary to theoretically explore the catalytic activity of novel two-dimensional catalysts with transition metal atom-nitrogen coordination structures for electrocatalytic CRR/CORR,and reveal the role of these structures in the electrocatalytic reaction.In this paper,we have constructed three kinds of two-dimensional single/double-atom catalysts with transition metal atom-nitrogen coordination structures based on two typical two-dimensional materials(phosphorene and two-dimensional hexagonal boron nitride).The catalytic activity for electrocatalytic CRR/CORR was studied by first-principles calculations theory,and a variety of candidate catalysts with high catalytic activity were screened out,some of which have reached the frontier level in this field.It was found that these two-dimensional catalysts share common sources of high activity.The transition metal atom-nitrogen coordination structures play a key role.The research content of this paper is as follows:1.Phosphorene is a unique two-dimensional anisotropic material,which is a suitable substrate for novel two-dimensional catalysts.In chapter 3,we investigated the catalytic activity of a series of two dimensional single-atomic catalysts based on black phosphorene monolayer,denoted as MN₃@P(M=Mo,Mn,Fe,Co,Cr,Ru,Rh,Pt,Pd,V,W atom),for CORR by first-principles calculations.Two excellent catalysts,Mn N₃@P and Mo N₃@P,were found to be capable of electrocatalytic reduction of CO to CH₄ with a limit potential(U_L)of-0.59 and-0.31 V(vs.RHE),respectively.The high catalytic activity of Mo N₃@P for CORR is attributed to the special bonding mode between Mo atoms and C atoms in the adsorption molecule CO caused by the transition metal atom-nitrogen coordination structure.High migration barriers(>7.2e V)of metal atoms ensure that Mo N₃@P and Mn N₃@P can avoid inactivation due to the accumulation of anchored metal atoms.It is found that Mn N₃@P has a zero band gap and Mo N₃@P has a 0.5 e V band gap.If the double-layer phosphorene is used as the substrate,the bandgap of Mo N₃@P can be reduced to 0.22 e V.Because CORR is the key to the two-step solution of CRR,this study has provided two efficient candidate catalysts for the conversion of CO₂/CO into high-value-added fuel and chemical feedstock by electrochemical CORR and CRR.2.The catalytic activity of single-atom catalysts can be significantly improved by changing the coordination environment of transition metal atoms in the center of single-atom catalysts through coordination engineering,which is a hot topic in the research of single-atom catalysts.Considering that hexagonal boron nitride monolayers have been extensively synthesized in experiments,in chapter 4,we use light atoms(C,O,and B atoms)to carry out coordination engineering on two-dimensional single-atom catalysts of boron nitride monolayer loaded with transition metals.It is found that only C and O atoms can effectively stabilize the anchored metal atoms,and two kinds of catalysts,M@BN?C and M@BN?O,are obtained.Pt@BN?C,Co@BN?C,Co@BN?O and Au@BN?O were identified as potential high-efficiency catalysts for CRR.Pt@BN?C has the most outstanding catalytic performance and can catalyze the reduction of CO₂to methane with an ultra-low U_L of-0.18 V(vs.RHE).The positive vacancy formation energy(2.1 e V)of transition metal atoms ensures the long-term use of Pt@BN?C.Based on the analysis of valence charge transfer between the various parts of Pt@BN?C and the electronic structure of key intermediates in Pt@BN?C in the electrocatalytic CRR process,we believe that the ultrahigh activity of Pt@BN?C comes from the C atom's adjustment of valence charge transfer and the special bonding between the adsorbed molecule and Pt atoms in some intermediates due to the transition metal atom-nitrogen coordination structure with adjustment of C atom.We believe this work will provide guidance for the coordination engineering of two-dimensional single-atom catalysts based on boron nitride monolayers.3.Compared with single-atom catalysts,double-atom catalysts contain synergies between metal atoms and have one more degree of freedom in the regulation of active sites,which has gradually attracted extensive attention from researchers.In chapter 5,a series of novel double-atom catalysts with transition metal atom-nitrogen coordination structures were constructed by loading same kind of double transition metal atoms onto large vacancies in two-dimensional boron nitride.By analyzing the reaction free energy of the CRR,we identified two potentially efficient catalysts with low U_L:Cr₂@BN and Mn₂@BN.Mn₂@BN can reduce CO₂ to CH₃OH with a U_L of-0.66 V(vs.RHE),while Cr₂@BN can reduce CO₂ to CH₄ with a U_L of-0.35 V(vs.RHE).By calculating the total state density,it is found that Mn₂@BN and Cr₂@BN have small band gaps,about 0.55 and 0.13 e V,respectively,indicating that they have good conductivity.Careful analysis of the electronic structure shows that the high activity of Cr₂@BN is due to the special bonding between Cr atom and the C atom in adsorbed molecules COOH,which is caused by the transition metal atom-nitrogen coordination structure.This study can provide useful guidance for the study of double atom catalysts based on two-dimensional hexagonal boron nitride.