Interactions Of Hydrogen Molecules With Transition Metal Modified Defective Graphene

Transition metal modification is an effective way to improve the hydrogen storage performance of carbon materials at room temperature.However,due to the high polymerization energy of transition metals,the metals are easy to agglomerate together,which greatly affects the hydrogen storage performance.Therefore,the study of metal-modified carbon materials as hydrogen storage substrates can be divided into two categories:one is to transfer the object of study to the metal complex with smaller structure,which is used as the basic unit of hydrogen storage for research,and on this basis to construct two-dimensional and three-dimensional porous materials.The other is to modify the substrate of carbon materials to increase the binding between the metal and the substrate to make it become a stable hydrogen storage material.The third is the hydrogen storage performance of metal clusters on the surface of carbon materials.For single metal-modified carbon materials,metal is usually considered as the adsorption site of hydrogen molecules,and the adsorption mechanism is electrostatic interaction and Kubas interaction.However,many current studies have found that dissociation of the first and even the second hydrogen molecule occurs at the 3d metal adsorption site.The dissociation of hydrogen molecules provides the possibility of further hydrogen migration,which will change the understanding of the mechanism of hydrogen adsorption.Therefore,the study of H₂ dissociation and hydrogen migration on a single 3d transition metal modified carbon material is of great importance to better understand the mechanism of hydrogen storage.Theoretical and experimental results show that the introduction of defects can greatly improve the stability of metal-modified carbon materials by regulating the electron distribution.However,will hydrogen molecules disintegrate at this time?The study on hydrogen storage in carbon materials modified by metal clusters shows that the mechanism of adsorption of hydrogen molecules by metal clusters is not purely Kubas,and hydrogen spillover may exist.At present,the most studied hydrogen spillover is on carbon materials modified by noble metal clusters such as Pt,Pd and Ru.It is not clear whether hydrogen spillover can also occur on metal clusters such as 3d transition metals Ni and Ti.Therefore,the research contents and main conclusions of this paper are as follows:（1）The interaction between hydrogen molecule and 3d transition metal complex TMC₆H₆The six-membered carbon ring is an important connecting molecule of some metal-organic frameworks and covalent organic frameworks,and is also the basic unit of C₆₀,graphene and carbon nanotubes.Therefore,the 3d transition metal complex TMC₆H₆ was used as the research object to clarify the bonding morphology in the hydrogen adsorption process,to find the lowest energy path of physical adsorption and chemical adsorption,and to clarify the hydrogen storage mechanism of the 3d transition metal complex.It was found that all hydrogen molecules would dissociate,and the dissociation energy barriers ranged from 1.50 to 17.58 e V,and the migration energy barriers ranged from 11.82 to 28.96 e V.Therefore,the process of hydrogen molecular dissociation and migration on 3d transition metal complexes is very common.（2）Hydrogen storage of graphene modified by a single metal（TM=Sc,Ti,V）The adsorption energies of Sc,Ti and V atoms on the defective graphene substrates are-6.93,-8.82 and-9.30 e V,respectively.It is 4-5 times that of complete graphene,effectively increasing the stability of the adsorbed substrate.Although the dissociation form energy of the first H₂ is low on the Sc/PG system,there is a dissociation energy barrier（0.12 e V）.The Sc,Ti and V atoms in the complete graphene system will eventually adsorb five,four and four hydrogen molecules by physical adsorption,and the adsorption energy of hydrogen molecules is-0.49～-0.74e V.After the introduction of defects,the number of hydrogen molecules adsorbed was seven,three and five,and the adsorption energies of hydrogen molecules were-0.13,-0.19 and-0.18 e V/H₂,respectively.（3）Hydrogen storage properties of graphene modified by transition metal cluster Ti_n（n=1-7）.Among them,the Ti₃ and Ti₅ clusters modified graphene were more stable,and the average metal binding energies are-5.15 e V and-5.62 e V respectively.The most stable adsorption site of hydrogen molecules in Ti_n/graphene system is the bridging bond position of metal.Ti_n/graphene system can saturate and adsorb 4,5,6,8,9,10,10 H₂ molecules respectively.