Theoretical Study Of Two-Dimensional Materials For Separation And Electrochemical Reduction Of CO₂

The massive emission of carbon dioxide(CO2)has brought many negative effects on the global climate.In recent years,how to solve the CO2-caused problems and achieve carbon neutrality have received considerable attention.Typically,industrial emissions are gas mixtures containing CO2,and the separation and capture of CO2 is particularly important.Among the emerging materials for gas separation,twodimensional membrane materials are considered to have good application prospect due to their small thickness,large specific surface area and high gas separation efficiency.Early studies mostly focused on the sieving effect of pore size for molecule gases.Herein,we systematically investigated the influence of charge distribution on gas diffusion through the BN-doped graphdiyne.In addition,CO2 is an important source of carbon,and it is highly required to catalyze it into high value-added chemical compounds and fuels.However,how to activate CO2 still remains challenging due to its high thermal stability.In this thesis,we designed various two-dimensional materials containing different SiNx moieties and found that CO2 can be easily activated on these 2D monolayers by forming Si-CO2 bonds.Furthermore,we explore CO2 electroreduction on these nanosheets.The main research contents and conclusions of this thesis are as follows:(1)Two BN-doped graphdiyne membranes(GDY-BN and GDY-fBN)were designed through boron nitride(BN)doping,and their performances for H2,N2,CO,CO2 and CH4 separation were investigated.The first-principles calculations show that BN-doped graphdiynes obtain better permeability for these gases than the GDY membrane.The presence of an external electric field can reduce the diffusion barrier of CO2 and CO through these graphdiyne membranes,while H2,N2 and CH4 are almost not influenced.The present results indicate that doping and an external electric field may change the charge distribution of gas/membrane,and further regulate the diffusion behavior of gas molecules.Molecular dynamics simulations reveal that the efficiency of gas passing through the membrane is affected by its translation and rotation motion.Increasing the temperature is conducive for gas translation through the membrane,but it will also accelerate the rotation motion of gas molecules and reduce the possibility of gas passing through the membrane in the optimal axial direction.(2)Based on the molecular configuration of planar tetracoordinate silicon(ptSi),a novel SiN4C4 nanosheet with dispersed SiN4 sites was designed and its interaction with CO2 was further investigated.Theoretical calculations reveal that the SiN4C4 sheet obtains high thermodynamic and dynamic stability.CO2 can be easily chemically adsorbed at the Si site,and charge transfer plays an important role in CO2 activation.The external electric field can enhance the interaction between CO2 and SiN4C4 nanosheets,but has little effect on CH4,N2 and H2.Such unexpected effect may be applied for CO2 separation from mixed gases.Furthermore,SiN4C4 nanosheets show excellent activity for CO2 electroreduction(CO2ER)and can reduce CO2 to HCOOH,CH3OH and CH4 at a very low limit potential of-0.46 V.Therefore,the novel SiN4C4 nanosheet has a good application prospect in CO2 capture and electrochemical reduction.(3)CO2 can only be physically adsorbed on the surface of h-BN sheet,while after replacing one B atom with the Si atom,CO2 can be easily captured.DFT calculations show that the chemisorption of CO2 on Si-doped h-BN sheet is exothermic with almost barrier free,and the Si-pz orbital plays an important role in the bonding between Si and CO2.Interestingly,under high doping concentration,the Si-doped h-BN sheet can still capture CO2 efficiently.(4)Based on two-dimensional graphene sheet,two types of single-atom Si embedded N-doped graphene nanosheets were designed for CO2 activation and electroreduction.In general,the more N atoms coordinate with Si in both SiNxC3-x and SiNxC4-x,the more remarkable the CO2 activation is.For SNxC3-x,the activation of CO2 at the Si site is dominated by both the electron population on Si atom and the Sipz band center,while for SiNxC4-x,deeper Si-pz band center is beneficial to the activation of CO2.The reason for such discrepancy is that the Si atom acts as an electron donor in CO2 activation on SiNxC3-x,while it acts as an electron shuttle for SiNxC4-x.Furthermore,the SiN3C0 sheet is predicted to be quite a promising electrode material for CO2 electrochemical reduction to HCOOH,CH3OH,and CH4 with quite low limiting potentials.