Theoretical Simulation And Study Of Small Molecules For Photoelectrocatalytic Energy Of Low Dimensional Materials

As fossil energy materials has been consumed gravely and human's demand for energy grows rapidly,it is imminent for us to develop environmentally friendly,sustainable and renewable energy.Hydrogen energy,due to its high energy density and environmental friendliness,is thought to be the most potential sustainable green energy.The use of solar energy for photocatalytic hydrogen production is a candidate scheme for producing hydrogen energy and has attracted widespread attention.At the same time,industrial synthetic ammonia,the cornerstone of modern agriculture,has also become a major issue that needs to be resolved urgently for sustainable development due to the high energy consumption of the Haber-Bosch process.Inspired by the natural nitrogen-fixing enzymes to achieve nitrogen fixation under mild conditions,people are committed to the use of electrocatalytic reduction technology to produce ammonia to replace the century-old Haber-Bosch process.This is also a novel way to store renewable energy(including electricity and hydrogen)in the form of chemical bonds.Since 2015,nitrogen reduction reaction(NRR)to produce ammonia has made remarkable progress.The first chapter of this paper is divided into two parts.The first part introduces different water splitting hydrogen production technologies,especially photocatalytic technology.Among them,the properties and composition of the photocatalyst determine the performance of the photocatalyst,and the performance of the photocatalyst determines the efficiency of water splitting and hydrogen production.Therefore,this chapter also discusses different photocatalytic hydrogen production materials,such as metal oxides,metal sulfides and nanocomposites.In addition,the advantages and-disadvantages of the materials used are discussed in order to select the best raw materials to split water to produce hydrogen.The second part introduces the theoretical study of the electrocatalytic nitrogen fixation reaction,focusing on the mechanism of nitrogen reduction to ammonia.The theoretical and experimental research progress of transition metal surfaces,metal single atoms,nitrides,carbides and oxides as catalysts for nitrogen reduction reactions are reviewed.Finally,it presents its views on current challenges and possible opportunities in the field.In Chapter Two,density functional theory(DFT)based on first principles was introduced.The Schrodinger equation can be simplified by various approximations(valence electron approximation,adiabatic approximation,Hartree-Fock approximation).Then,the Kohn-Sham theorem was used to transform the multi-particle system into a single-particle one without interaction,and the exchange correlation functional was introduced to solve the ground state characteristics of the system.Meanwhile,it is necessary for different material systems to select the most proper functional in terms of time-consuming and accuracy.Finally,different software was developed to simulate and study the system.Based on the first principles,two schemes of photocatalytic hydrolysis for hydrogen production were designed(Chapter 3 and Chapter 4),and one scheme of monoatomic electrocatalytic nitrogen fixation was designed(Chapter 5).In Chapter Third,in order to solve the problem that the limitation of light excitation leads to the close of photocatalytic reduction and oxidation sites,designed was a system combined by carbon nanotubes(CNT)and carbon nitrogen nanotubes(CNNT).In this system,carbon nitrogen nanotubes are wrapped by carbon nanotubes.The rigid structure of nanotubes keeps the oxidation and reduction sites close to each other in the range of interlayer spacing,which ensures effective charge separation.And yet the selectivity of the six membered carbon ring which only allows proton penetration isolates the oxidation and reduction sites from the reactants,thus making them separate.First principles calculations show that the system is excited by light to get electron-hole pairs,and the electrons and holes are respectively transferred to the reduction site of CNNT and the oxidation site of CNT.The water molecules adsorbed on the nanotube will split with the help of holes,and generate protons.The generated protons will penetrate the nanotube surface and enter the reduction site of CNNT to generate hydrogen.As hydrogen molecular is not able to penetrate the nanotubes,it will be isolated within the system,which inhibits the occurrence of reverse reaction,to realize the safe preparation and transportation of hydrogen.In Chapter Four,Inspired by the CNT/CNNT hybrid structure,we developed a new hybrid structure design scheme.Using first-principles simulations,we propose a hybrid structure design where metal clusters of TM4(TM=Au/Pt)are encapsulated in a boron carbon nitride nanotube(BCNNT)decorated with CuN3 group.It can readily absorb ultraviolet-visible solar light.The photo-generated electrons and holes would be separately delivered to the reduction site of TM4 and oxidation site of BCNNT layer,respectively.Then protons generated by water dissociation at the BCNNT layer will penetrate through BCNNT,and consequently meet electron charges at the TM4 site for reducing to H2.As a selective sieve,BCNNT prevents oxygen species from going inside and H2 from going out,so that H2 can be completely isolated.Further,the enough space of tubular cavity endows the transport of produced H2 along the nanotube for collection and utilization.The proposed design combines photocatalytic hydrogen production and safe delivery,which may help to develop practical solution for photo-driven hydrogen production.In Chapter Five,through first principles calculation,a kind of single atom catalyst(SAC)was designed based on the system combined by graphene and hexagonal boron nitride(h-BN)to achieve stable and efficient electrocatalytic nitrogen fixation.In this system,by changing the ratio of h-BN to graphene,the problems of excessive energy band and poor conductivity of h-BN is solved,and the energy band regulation of the hybrid system is realized.The calculation results show that the Mo@BCN system designed by us is highly stable,and its unique metal properties can facilitate charge transfer to the active sites.to drive the electrocatalytic nitrogen reaction.At the same time,in this system,hydrogen evolution as a competitive reaction is effectively inhibited,which improves the selectivity of nitrogen fixation reduction.Based on this system design,an efficient and stable electrocatalytic nitrogen fixation is realized based on h-BN,providing a new idea and method for nitrogen fixation with monoatomic catalyst.