First-principles Study On Diffusion And Reactioins Of Small Molecules On Surfaces/Interfacess Of Graphene And C₃N₄

With the continuous improvement of computing methods and performance of computers,first-principles calculation has received more and more attention,and it has been applied to condensed matter physics,materials,and chemistry.With assist of first-principles calculation,we can not only explain the mechanisms in experiments,but also predict the properties of certain materials to provide guidance for experi-ments.On account of excellent optical,electrical and mechanical properties,graphene is considered to be a material with great potential for application in membrane separa-tion and catalysis.C3N4,which has a similar structure to graphene,has a strong nu-cleophilic ability due to its unique chemical composition and π-conjugated electronic structure.Hence,it can act as a Bronsted base and Lewis base in chemical reactions and can be used as a multifunctional catalyst in organic catalytic reactions.In this the-sis,the migration and reactivity of small molecules on graphene and C3N4 are studied by first-principle calculations.We first study the effect of wrinkles to the permeability of graphene.By con-structing ripples with different curvatures and planar graphene to compare the energy barriers for the migration of oxygen,the presence of ripples on graphene can improve the penetration performance of graphene,and the permeability increases with increase of curvature.What’s more,the concave surface is more favorable for the oxygen at-oms to pass through than the convex surface.By calculating the effect of copper sub-strate to the penetration performance of graphene,we come up with the conclusion that the copper substrate will lengthen the bond length of graphene,and the elongation of graphene bond length will greatly affect the penetration performance of the gra-phene.Secondly,we investigated the effect of graphene/copper interface on decomposi-tion of water molecules.The graphene/copper boundary that is originally closed can be easily decoupled through the reaction with water molecules.Then,other water molecules can migrate from the decoupling boundary into the graphene/copper inter-face and decompose to obtain hydrogenated graphene.The graphene/copper interface can decompose water molecules with a much lower energy barrier than the copper surface,indicating that the graphene/copper interface can promote the decomposition of water molecules.Finally,we studied the mechanism of selective hydrogenation of phenol on Pd@C3N4 surface.By calculating possible reaction pathways for hydrogenation of phenol,it indicates that when phenol is hydrogenated to 1-cyclohexen-l-ol,it is easy to isomerization to cyclohexanone.Further hydrogenation of cyclohexanone has an energy barrier much larger than desorption of cyclohexanone from substrate which results in cyclohexanone being more prone to get away from the substrate.Studies on the role of Pd atoms in the reaction have shown that Pd atoms can inhibit the further hydrogenation of cyclohexanone by promoting the regeneration of cyclohexanone when hydrogenation occurs.These factors lead to the selective hydrogenation of phenol on the surface of Pd@C3N4.In this thesis,the migration and reactivity of small molecules on graphene and C3N4 are studied,which provides a theoretical basis for the application of C3N4 and graphene in separation or catalysis.It has certain reference for the design of related materials and the investigations on material’s properties.