Structural Regulation And Hydrogen Storage Of Boron Clusters By Alkaline Earth Metals

Boron is a typical electron-deficient element in the periodic table and tends to form multi-center delocalization bonds to compensate for its electron-deficient nature.Its bonding characteristics are similar to those of carbon clusters but show more differences.And metal doping can effectively regulate the geometry and chemical bonding of boron clusters,resulting in a series of boron-based nanomaterials with special properties.In this work,the regulation of cluster B₁₂ by different numbers of alkaline earth metal Be atoms is investigated in terms of structural composition,chemical bonding and dynamics based on first-principles calculations and density functional theory,and then the hydrogen storage properties of alkaline earth metal-doped boron clusters are further explored to provide ideas for the study of hydrogen storage materials and also to expand the application fields of boron clusters.The results of this thesis show that:1.Different numbers of Be atoms doping into the cluster B₁₂ formed the stable quasi-planar structure C_s Be B₁₂ and the novel cage-like structures C_s Be₂B₁₂ and C_2v Be₃B₁₂,respectively,which have different structures and properties.Be atoms tends to be embedded in the B₇ or B₈unit ring on the surface of the cage-like structure of B₁₂,forming stable Be&B₇ and Be&B₈units through ionic and covalent interaction to stabilize the cage-like structure.Furthermore,at400K,C_s Be B₁₂ and C_s Be₂B₁₂ are stable,while C_2v Be₃B₁₂ shows structural fluxional phenomenon.The natural bond orbital（NBO）analysis shows that the three clusters have obvious electron transfer and the Be-B bonds is mainly ionic,covalent bonds exist at the same time.Bonding analysis shows that the quasi-planar structure C_s Be B₁₂ conforms to the（4n+2）（n=1）Hückel rule,and the cage-like structures C_s Be₂B₁₂ and C_2v Be₃B₁₂ follow the spherical aromaticity 2（n+1）²（n=1）electron counting rule,indicating that all three clusters are aromatic.2.Based on first-principles calculations,we perform detailed molecular dynamics simulations of the quasi-planar structure C_s Be B₁₂.It finds that when the temperature is increased to 800 K,the B₁₀ outer ring in Be B₁₂ rotates almost freely around the Be B₂ triangular core at a low energy barrier of 14.20 kcal mol^-1 with a rotation angle of 36°at each step.C_sBe B₁₂ is an isovalent electronic system to the experimentally observed C_2v B₁₃⁺(B₃@B₁₀⁺).Its global minimal structure C_s Be B₁₂（1,GM）and the transition state structure C_s Be B₁₂（2,TS）correspond to the transition state and the global minimal structure of B₁₃⁺,respectively.In the complete dynamic cycle,there are 10 equivalent GMs and 10 equivalent TSs alternating on its potential energy surface.Bonding analyses indicate that in addition to the ten localized periphery B-B bonds,both the C_s Be B₁₂（1）and C_s Be B₁₂（2）possess three delocalized bonding systems,satisfying the（4n+2）Hückel rule with n=0,2,and 1,respectively,making the neutral complex 2σ+πtriply aromatic in nature and highly stable in thermodynamics.This system is the smallest neutral quasi-planar alkaline earth metal-doped Wankel motor complex.3.Based on the density functional theory,we systematically investigate the hydrogen storage properties of the cage-like structures Be₂B₁₂ and Be₃B₁₂.The hydrogen storage calculations show that at the w B97XD level,Be₂B₁₂ and Be₃B₁₂ clusters can adsorb at least six H₂ molecules per Be site,corresponding to a weight density of 13.99 wt%and 18.69 wt%,respectively,which are higher than the hydrogen storage target proposed by the U.S.Department of Energy in 2017（5.5 wt%）.In addition,it shows that the adsorption and desorption dynamics of H₂ molecules on the clusters can also be regulated by the applied electric field.The adsorption mechanisms of Be₂B₁₂ and Be₃B₁₂ on H₂molecules are divided into two types,one for inter-orbit interaction and one for intermolecular forces.The molecular dynamics simulation results show that the reversible adsorption of H₂ molecules can be achieved by Be₂B₁₂ and Be₃B₁₂.It can be seen that Be₂B₁₂ and Be₃B₁₂ clusters are expected to be a promising new nano-hydrogen storage material.4.Based on the density functional theory,we investigate the electronic properties of the clusters M₂B₇（M=Be,Mg,Ca）and their hydrogen storage properties systematically.Extensive global search results show that the global minimal structures of all three systems（Be₂B₇,Mg₂B₇and Ca₂B₇）are heptagonal biconical structures,and the two alkaline earth metals M are located at the top of the biconical.Chemical bonding analyses show that the bonding nature of M₂B₇clusters is similar and haveσ+πdouble aromaticity.At the w B97XD level,the three systems have good hydrogen storage capabilities.The hydrogen storage densities are 23.03 wt%,13.87wt%,and 13.36 wt%,respectively,all of which far exceed the hydrogen storage target set by the U.S.Department of Energy in 2017.Their average adsorption energies of H₂ molecules all ranged from 0.1 e V/H₂ to 0.48 e V/H₂,which is fall in between physisorption and chemisorption.Molecular dynamics simulations show that the adsorbed H₂ molecules can be completely released from the three systems at a certain temperature.Therefore,M₂B₇ systems can achieve reversible adsorption of H₂ molecules,providing a good theoretical solution for new nano-hydrogen storage materials.