First-principles Study Of Nitrogen Adsorption And Dissociation On Uranium Surface

Uranium (U) is one of the heaviest elements in nature, and it can start nuclear fission reaction spontaneously, which has been used widely in military and energy. Due to its strong chemical reactivity, uranium metal will be easily eroded by the oxidation reactions in atmosphere. Nitridation can develop good anticorrosion properties of U by producing thin nitride surface layers on the metal surface. U shows large potentials in catalytic chemistry. As early as 1909, it was found that uranium is one of the most efficient catalysts used for the synthesis of ammonia during the Haber-Bosch process. In recent decades, numerous reactions were shown to proceed efficiently with participation of uranium-containing catalysts. It can be seen that, whether in the nitriding process of U surface or during the U-containing catalytic reactions, the interaction between U and nitrogen are both involved. Thus, the study of the interaction between U and nitrogen is of great significance.In the present paper the adsorption and dissociation behaviors of U and nitrogen are investigated systematically with first-principles density functional theory (DFT) calculations. The adsorption and dissociation of nitrogen molecular on U surface have been studied, and the calculated results indicate that the nitrogen molecules adsorbed horizontally on the long-bridge site will dissociate completely, and the corresponding adsorption energies are about-4 eV. It is found that the adsorbed nitrogen atoms only seize electrons from the top-most uranium layer, which indicates that nitrogen molecular only interact with the top-most uranium atoms during the dissociated process. The adsorption properties of different numbers of nitrogen atoms on fixed size of U surfaces, and the most stable configurations of various nitrogen coverages are calculated. We analyzed how the adsorption energies, structural parameters and Bader charges vary with the increase of nitrogen coverage. Based on ab initio atomistic thermodynamics, the surface phase diagram for nitrogen adsorption on U surface is obtained. We investigate how the thermodynamically stable phase changes with increasing the pressure of nitrogen under different temperatures. With first-principles DFT approach and the Climbing Image Nudged Elastic Band method, we have investigated the nitrogen adsorption, surface diffusion, subsurface absorption and surface penetration on three different sizes of U surfaces. Our calculated results indicate that, regardless of the nitrogen coverage, a nitrogen atom prefers to locate the hollow1 site and the Oct site on and below the surface, respectively. The energy barriers for on-surface and penetration diffusion are the lowest at the coverage of 1/2 ML. The site preferences, interstitial diffusion and vacancy trapping of nitrogen atom in bulk U and been studied, and the calculated results indicate that nitrogen atom prefers to occupy the Oct site in bulk uranium and high energy barriers are observed during the diffusions between the neighboring Oct sites. There exists significant attractive nitrogen-vacancy interaction at the trapping center site, and the interstitial nitrogen atom can be captured by its nearby vacancy with a low energy barrier of 0.3 eV.