钠基纳米流体中钠原子吸附行为特性模拟计算

Theoretical Study on Adsorption of Sodium Atoms in Nanofluid

钠基纳米流体利用过渡金属纳米颗粒吸附钠原子的性质,有效降低了钠原子的反应性,进而有效抑制钠火及钠水事故严重性,但目前相关吸附行为及规律尚不明确。研究基于密度泛函理论和电子结构分析,计算分析了钛、铁和铜纳米团簇(TM_(n),TM=Ti、Fe、Cu,n=2~13)及其与钠原子间形成复合物(Na-TM_(n))的结构和性质,分析了TM_(n)的稳定性以及其与Na原子间相互作用。结果表明,Ti_(n)具有最高的稳定性,但其吸附钠原子的能力低于Fe_(n)和Cu_(n)。钠原子主要通过范德华作用吸附于TM_(n)表面,且两者间的电荷转移行为使得TM_(n)带负电荷。

Sodium-cooled fast reactor(SFR)is one of the major the GenerationⅣnuclear reactors,which utilizes the liquid sodium as the coolant.Despite of the excellent heat-transfer characteristic and low neutron absorption cross-section,the liquid sodium suffers the serious accident risks due to the high chemical reactivity of sodium atoms,especially when liquid sodium contacts with water or steam during the break accident.Recently,the liquid sodium nanofluid,obtained by dispersing the transition metal nanoparticles in liquid sodium(Nano-LS),have been gained wide consideration due to the enhancement to the coolant performance,especially the suppression of the reactivity of sodium atoms.However,the reported experimental or theoretical works mainly focused on the Nano-LS doped with titanium nanoparticles.The influence of transition metal type and the suppression mechanism is still ambiguous from the atomic scale.Theoretical chemistry is an effective tool to reveal the special natures of the Nano-LS.Hence,detailed theoretical computations based on the density functional theory and electronic structure analysis were performed to reveal the stability of three representative 3d transition metal clusters,TM_(n)(namely,Ti,Fe and Cu,with atom number n from 2 to 13),and clarify the interaction characteristic between TM_(n)and sodium atoms.The TM_(n)representative structures were accessed according to the artificial bee colony algorithm for cluster global optimization using ABCluster code.Structures and energies of TM_(n)and Na-TM_(n)with different multiplicities were further calculated to obtain the most energy stable isomers based on TPSS functionals with the double zeta basis sets def2-SVP and triple zeta basis sets def2-TZVPP,respectively.The D3 Becke-Johnson damping correction was used to consider the dispersion interaction.The structure,electronic energy,electron affinity,ionization potential,and electronegativity of TM_(n)were calculated to compare the cluster stability.And the adsorption sites on the TM_(n)for sodium atom were predicted by the electrostatic potential mapped molecular van der Waals surface.Furthermore,the chemical stability,intra-molecular interaction(namely,independent gradient model based on Hirshfeld partition analysis),and charge transfer analysis were combined to illustrate the strength and nature of the interaction between sodium atom and TM_(n).All the DFT calculations were performed using the ORCA 5.0.4 program,and the wavefunction analysis were realized utilizing the Multiwfn 3.8 code.The results indicate that with the increase of atom numbers n,the average binding energies EA(n)for Ti_(n),Fe_(n)and Cu_(n)increase.Meanwhile,the vertical electron affinity and vertical ionization potential tend to increase and decrease when the size of TM_(n)becomes larger.The values of Mulliken electronegativity of different TM_(n)shows size-independent.The electrostatic potential mapped molecular surface show that the Electrostatic potential minimum point always tends to located over the center of three transition metal atoms,which is considered as the interaction site to adsorb Na atoms.Overall,the Ti_(n) is more stable compared with Fe_(n)and Cu_(n),while are less attractive for sodium atom.Mulliken electronegativity and electrostatic potential analysis are effective tools to predict the interaction strength and sites for Na-TM_(n)complexes.The sodium atom is absorbed on the TM_(n)mainly through van der Waals interaction,and has no significant effects on the structure of TM_(n)clusters.The obtained atomic dipole moment corrected Hirshfeld population charges for Na-TM_(n)indicate that the electron transfers from sodium atom to TM_(n).Hence,TM_(n)are always carry the negative charge,which contributes to the suspension stability of TM_(n)in liquid sodium.