First-principles Study Of The Adsorption Properties Of Atoms And Molecules On UN₂ Surface

UN₂ is one of the important nuclear fuels and has good thermal properties.At the same time,it is also one kind of accident-tolerant fuel.Compared to the traditional oxide nuclear fuel,uranium nitrides have higher thermal conductivity,melting point and density,which are very suitable for future high-temperature reactors.Therefore,it has been paid much attention in the energy field around the world.In the environment,UN₂ is easily contacted with O2,H2,H2O and other gases,and the redox reaction may be occurred,which can corrode the material.Therefore,it is particularly important for us to store the material effectively and block the reaction between the material and the corrosive gases in the atmosphere.In this paper,the first-principles method was used to simulate and calculate the possible reaction of UN₂ surface with O,H,O2,H2,OH and H2O.The adsorption characteristics of various adsorbates at different adsorption sites,the transferred charge and charge density difference were calculated.In this work,the first-principles calculation method based on density functional theory is used to study the interaction between UN₂ and a series of atoms and molecules systematically.The adsorption processes of the atoms and molecules in different adsorption sites and adsorption modes on the surface of the UN₂ were calculated.Besides,the most stable adsorption structures of these atoms and molecules on the surface and the strongest adsorption energy were obtained.The results show that the most stable adsorption site of O atom is bridge site,and the corresponding strong adsorption energy is-5.08 eV.The most stable adsorption site of the H atom is also the bridge site,corresponding to the strongest adsorption energy of-0.49 eV.Using the first-principles density functional theory approach and the Climbing Image Nudged Elastic Band?CI-NEB?method,we calculated the diffusion of O and H atoms on the surface.And the transferred charge of O and H atoms were calculated by Bader charge analysis method.For the adsorption of O2 molecules on the surface,when it is initially placed parallel to the surface at hollow site,it will dissociated completely,and corresponding to the strongest adsorption energy-10.21 eV.In the final state structure,two O atoms are adsorbed on the adjacent bridge sites,respectively.And when it is placed vertical to the surface at all of the adsorption sites,the O2 molecules are not dissociated.For the adsorption of H2 molecules on the surface,the interaction between H2 molecules and the surface is very weak,indicating it is just a physical adsorption,or even desorption.For the adsorption of OH on the surface,the adsorption structures of parallel placed ways in all adsorption sites will relax to the same stable structure:the OH is vertical to the surface with O atom interacting with the surface at the bridge site.This structure also corresponds to the strongest adsorption energy-5.73 eV.For the adsorption of H2O on the surface,we studied four kinds of placed approaches:H-parallel,H-up,H-down and H-up-down.The interaction between H2O molecules and the surface is relatively weak,there is no dissociated structure.Different adsorption sites and placed ways will lead to the change of adsorption energies,and there are three approaches which the final stable structures are the same corresponding to the strongest adsorption energy-0.69 eV.