First Principles Study Of Single Atom Catalysis In Two-dimensional Hexagonal Membrane

Methane（CH₄）,as one of the main components of natural gas,widely exists in nature and has potential application prospects in many fields.Like methanol and aromatic hydrocarbons,methane（CH₄）plays a significant role in many chemical applications.However,methane is a highly volatile greenhouse gas,posing a challenge to transportation and storage.Therefore,in order to facilitate transportation and reduce methane emissions,making it into liquid fuel is a reasonable and important method.This thesis mainly studies the catalytic oxidation of methane（CH₄）to methanol（CH₃OH）by nitrogen-modified graphene doped with a single metal using the first-principles.Cobalt（Co）atoms can be stably isolated in nitrogen-modified graphene（Co/N₄-Gr）,and Co/N₄-Gr is further oxidized to Co O/N₄-Gr by oxidants,thus playing an effective role in catalytic oxidation of single atom catalysts（SACs）.The Co O/N₄-Gr provides excellent catalytic activity for the oxidation of CH₄ to CH₃OH.Methane The C-H bond activation of methane only overcomes the low barrier of 0.56 e V.The dissociation of physically adsorbed CH₃OH on Co/N₄-Gr requires overcoming a reaction barrier of 0.46 e V.The low barrier is conducive to the oxidation of methane to methanol by single Co atoms in Co O/N₄-Gr under the action of electron buffering.At the same time,all reactants and products are not adsorbed on the catalyst surface after one reaction cycle,making the surface restore to its original state,which is conducive to the preparation of the catalyst for the next catalytic cycle.Therefore,the Co O/N₄-Gr is a promising catalyst for the oxidation of CH₄ to CH₃OH.Firstly,the VASP software was used to optimize the nitrogen-modified double-vacancy graphene to form a stable substrate,and the doping of cobalt atom was further relaxed to form a stable substrate（Co/N₄-Gr）.In the Co/N₄-Gr,the adsorption energy of Co atom was-8.45 e V,and the difference between adsorption energy and binding energy was-8.36 e V.The negative values indicate that the Co atom can exist stably on N₄-Gr as an isolated single atom,instead of forming Co atomic clusters.Therefore,Co atom doped nitrogen-modified graphene is very stable.Second,further realization of methane oxidation to methanol requires the provision of an O atom.After investigation and calculation,it is found that O₃ can be used as an oxygen supply substance to provide an O atom for the substrate（Co/N₄-Gr）,thus forming Co O/N₄-Gr and breaking down the clean gas O₂.It is known that the desorption energy of O₃ to O₂ in natural environment is 2.11 e V.After calculation,it is found that on Co/N₄-Gr substrate,the energy barrier for the conversion of O₃ to O₂ only needs 1.42 e V,so the oxidation of catalyst Co/N₄-Gr（Co O/N₄-Gr）can be realized.Finally,Finally,CH₄ was relaxed and combined with Co O/N₄-Gr to form CH₄-Co O/N₄-Gr.CH₄ reacted with the substrate to break C-H bond in methane at the optimal catalytic site,requiring a potential barrier of0.56 e V.Furthermore,the CH₃OH was further promoted on the substrate（CH₃OH-Co O/N₄-Gr）,the separation barrier of CH₃OH physically adsorbed on the substrate was only 0.46 e V,which could realize the cyclic application of the substrate.In conclusion,Co atom doped nitrogen-modified graphene can catalyze the oxidation of methane to methanol.The main content of this paper is divided into the following steps:Chapter 1describes the research background and status quo of methane conversion,the application status quo of graphene materials,and the design and research process of single metal doped nitrogen-modified graphene.In chapter 2,the density functional theory and first principles calculation used in this study are introduced in detail.In chapter 3,the whole reaction process of methane oxidation to methanol is discussed in detail,and the feasibility analysis of methane conversion to methanol is given by using structural parameters,Bader charge,CI-NEB and DOS diagram.In chapter 4,this study is summarized and a conclusion is drawn that the oxidation of methane to methanol can be achieved by monatomic catalysis with low barrier energy.