First-Principles Study On Single-Atom Catalytic Reactions Inside Confined Nanotubes

With the continuous consumption of non-renewable resources such as petroleum and coal,environmental and energy problems have become more and more severe.There is an urgent need to research and develop new catalysts with high efficiency,low cost,high selectivity and high stability to solve these difficult problems.The efficiency of heterogeneous catalysis depends on the ratio and degree of dispersion of surface active atoms.Single-atom catalysts can achieve single-atom dispersion,which greatly improves the utilization of metals,especially precious metals.The confinement effect in nanotubes can improve catalytic activity and selectivity.In this thesis,we use first-principles-based density functional theory and molecular dynamics methods to simulate and design a series of confined carbon nanotubes,boron nitride nanotubes(BNNT)and graphite-like carbon nitride(g-C₄N₃)single-atom catalysts on the interior surface of nanotubes,and studied their activity and catalytic mechanism for the catalytic reactions of hydrogen evolution reaction(HER),CO oxidation and nitrogen(N₂)electrochemical reduction.The first chapter introduces the basic concepts and research progress of single-atom catalysis and confined catalysis,and briefly reviews the nanotube materials,hydrogen evolution reaction,CO oxidation reaction and N₂ electrochemical reduction reaction involved in this article.The second chapter describes the theoretical basis and related software involved in this thesis,including density functional theory,first-principles-based molecular dynamics methods,and models describing free energy changes in electrocatalytic reactions.Chapter 3 designs a single-atom catalyst in nitrogen-doped carbon nanotubes with high conductivity and high catalytic activity.By calculating the change of the free energy of adsorption of hydrogen atoms,group VIII transition metal single-atom catalyst with good catalytic activity for HER in nanotubes was screened out.Compared with the outer surface of nitrogen-doped carbon nanotubes,the internal environment of the tube can promote the activation of hydrogen by single metal atoms,thereby improving the reaction efficiency.The size of the tube is also an important factor in regulating the catalytic activity in the nanotube.Chapter 4 briefly introduces the catalytic mechanism of CO oxidation reaction by single-atom catalysts confined on the inner and outer surfaces of BNNT.Through theoretical screening of 3d non-noble transition metals,the most suitable single-atom catalyst for adsorption at the N vacancy in the BNNT was found.By calculating the Eley-Rideal(ER)reaction mechanism,it is found that the Cu single-atom catalyst confined in the BNNT(6,6)tube exhibits higher catalysis for CO oxidation than BNNT(5,5)and BNNT(7,7)The energy barrier for the decisive step is only 0.24eV.Therefore,the diameter of the nanotube can be used to adjust the energy barrier for CO oxidation.On this basis,a comprehensive study of platinum group noble metal(PGM)single-atom catalysts confined to different active sites on the inner and outer surfaces of BNNT(6,6)nanotubes,and their use in ER and Langmuir–Hinshelwood(LH)The catalytic mechanism in the CO oxidation reaction path.The results show that for PGM single-atom catalysts,N vacancies on the inner surface of nanotubes are more suitable confined catalytic centers than B vacancies.According to the LH reaction mechanism,the energy barrier of CO oxidation reaction under the action of Pt single-atom catalyst is only 0.28eV.The interior surface of BNNT promotes charge transfer and increases the activation of reactants O₂ and CO.At the same time,the spatial confinement of BNNT on reactive molecules(O₂ and CO)makes it distribute along the axis of the nanotube,which promotes the CO reaction.Chapter 5 designs a single-atom catalyst based on graphitic carbon nitride(g-C₄N₃)nanotubes for electrochemical reduction of nitrogen.Through the calculation of metal binding energy,highly stable transition metal single-atom catalysts are screened out;through the calculation of N₂ adsorption performance,single-atom catalysts that can chemisorb and fully activate N₂ are screened out;finally,the electrochemical reduction of N₂ to form NH₃ products is studied The optimal reaction path and free energy change of the Mn single-atom catalyst is found to be only 0.32V,which is expected to become an excellent catalyst for the nitrogen fixation reaction.