Theoretical Exploration Of Novel Gas Separation Membranes And Catalytic Materials

With the continuous depletion of non-renewable resources such as oil and coal,energy shortage and environmental crisis have become more and more serious.Nowadays many traditional materials or methods have already not been able to satisfy the increasing demands of clear energy for human being.Thus the development of low-energy-consumption and environment-friendly functional materials has received more and more attention.Providentially,hydrogen,natural gas,fuel cells,solar cell,and other novel batteries,could meet people's requirements for environmentally clean energy.Benefiting from the advanced experimental methods and the development of computational materials science,researchers are able to synthesize a lot of new nanomaterials such as graphene,two-dimensional hexagonal boron nitride(h-BN),monolayer molybdenum disulfide(MoS2),nanotubes,single atom catalysts,etc.,and conduct in-depth studies on the properties and applications of these materials.For example,two-dimensional materials with suitable pores can be used for purifying hydrogen and enriching natural gas,and single atom catalysts or confinement catalysts are high efficient for fuel cells,water splitting,ammonia synthesis,carbon monoxide oxidation,etc.This dissertation,based on theoretical calculations,mainly studies the application of some novel nanomaterials in gas separation,oxygen reduction reaction(ORR),oxygen evolution reaction(OER),hydrogen evolution reaction(HER),ammonia synthesis and carbon monoxide(CO)oxidation.In chapter one,we first introduce the research background of novel nanomaterials with current status in energy,environment and materials science.Then we introduce several specific namomaterials,namely graphene,h-BN,MoS2,nanotubes and single atom catalysts,and discuss some important applications,namely gas separation,ORR,OER,HER,ammonia synthesis and CO oxidation.In chapter two,we briefly introduce fundamental theory and calculation methods used in the computational materials science.Here we mainly present two widely-used methods,namely first-principles density functional theory(DFT)calculation and the molecular dynamic(MD)simulation.The DFT method is used to study the ground state properties of materials based on quantum mechanics,and the MD method is employed to study the statistic physical properties based on classical mechanics.In chapter three,using DFT calculations and MD simulations,we explore the potential applications of h-BN for H2/CH4 separation and MoS2 monolayer for purifying hydrogen and enriching methane.We find that the h-BN membranes with appropriate pores possess excellent H2/CH4 selectivity(>105 at room temperature).Furthermore,the estimated permeability of H2 significantly exceeds the industrially accepted standard for gas separation over a broad temperature range.For laminar MoS2 material with triangular sulfur-edged nanopores,the permeance values for H2 and CO2 far exceed the industrial standard.Meanwhile,such a porous MoS2 membrane shows excellent selectivity in terms of H2/CO,H2/N2,H2/CH4,and CO2/CH4 separation at room temperature.In chapter four,based on DFT calculations,we propose high performance electrocatalysts,in which single 3d transition metal(Sc,Ti,V,Cr,Mn,Fe,Co,Ni,and Cu)atom is deposited on N-doped graphene,for ORR,OER,HER,and ammonia synthesis.These electrocatalysts combine the merits of both homogeneous and heterogeneous catalysts,which are uniform active sites,tunable coordination,and maximized atom utilization for homogeneous catalyst and durability,easy immobilization,and excellent recyclability for heterogeneous catalyst.Scaling relation is obtained between the calculated Gibbs free energy of OH*vs OOH*,which can help us to derive the minimum overpotential for ORR and OER.In addition,we find that there exists a volcano plot of the adsorption strength for ?GOH*and AGO*-?GOH*vs the overpotential of ORR and OER,respectively,which can be taken as a descriptor to evaluate catalytic performance.Our computations screen out a few quite promising single atom catalysts with high efficiency and good stability for ORR,OER,HER and N2 fixation.In chapter five,we theoretically study CO oxidation on a single Ni atom confined in a nitrogen vacancy on both the outside and inside surface of boron nitride nanotubes(BNNTs)as well as on the surface of h-BN.By exploring the Eley-Rideal mechanism,we find that Ni atom embedded on the interior surface of BNNTs exhibits a much higher catalytic activity for CO oxidation compared with Ni doping on the exterior surface.Moreover,the energy barriers of the rate-determining step for CO oxidation on Ni embedded on the inside wall of BNNT(5,5),BNNT(6,6)and BNNT(7,7)are 0.39 eV,0.29 eV and 0.33 eV,respectively.The calculated results illustrate the merit of confinement on CO oxidation.By analyzing the electronic structures of the initial state and transition state,we explain why the inside active site of nanotube has a higher catalytic activity.