Adsorption behavior of O₂ on U-Nb surface: a first-principles study

Oxygen is a chemically active gas. Metal uranium will be rapidly oxidized when exposed to oxygen directly, forming a complex and uncompacted oxide layer on the surface. The U-O₂ reaction system is a very complex metal oxidation system. The surface oxidation reaction of uranium-niobium alloy is more complex, and it is also the main corrosion form of uranium-niobium alloy. Exploring the microscopic mechanism of surface oxidation corrosion of uranium-niobium alloy is of great significance for understanding the surface corrosion phenomenon in practical applications. The adsorption of O₂ on U-Nb (5at.%) surface was investigated by first-principles calculation using a periodic slab model within the density functional theory (DFT). The effect of different levels of Nb doping on O adsorption was investigated, and the stability of O-U and O-Nb adsorption was studied in conjunction with the Bader charge distribution, differential charge density map, electron density of states, and surface work function. The results showed that physical adsorption happens when O₂ is vertically adsorbed at the top site of Nb atom, whereas dissociative adsorption happened in all other configurations considered here. The interaction between adsorbed O₂ and U-Nb surface was essentially the hybridization between the O/2p orbital electrons and the U/6d, U/5f, Nb/4p, Nb/5s, and Nb/4d orbital electrons to form a relatively stable ionic bond. When Nb was doped into the second layer of the surface, the most stable O₂ adsorption configuration was top-site horizontal adsorption. The adsorption energy was - 22.38 eV, which was more negative than the adsorption energy of - 21.38 eV for the first doped layer horizontal adsorption at hollow-site. The interaction was essentially the hybridization between O/2p orbital electrons and U/6d and U/5f electrons and between O/2s orbital electrons and U/6p orbital electrons to form a relatively stable ionic bond.

In this paper, the adsorption and dissociation of oxygen molecules on the surface of U-Nb system were systematically studied by first-principles calculations, in order to explore the surface oxidation corrosion mechanism of uranium-niobium alloy from the micro level. The VASP program uses PAW pseudopotential, which belongs to the density functional method, and uses DSP.