A First-Principles Study on the Adsorption and Desorption of Nitrogen on UN and U(N,C) Surfaces

This work employs first-principles calculations to investigate the adsorption of nitrogen atoms and nitrogen molecules on the uranium mononitride (UN) surface and the desorption behavior of nitrogen atoms, and to construct a surface nitrogen vacancy stability boundary diagram. The UN(001) surface is identified as the thermodynamically most stable low-index surface. Nitrogen atoms preferentially adsorb at hollow sites, while the most stable N2 adsorption configuration has one N atom at a hollow site and the other bonded to a neighboring U atom. Thermodynamic analysis shows that under high temperature and low nitrogen partial pressure, nitrogen atoms tend to desorb from the UN surface. The effect of carbon doping on the stability of the UN surface was further investigated. For the configurations considered, as the carbon content increases, the desorption energy of N atoms generally increases, and the surface nitrogen vacancy stability region on UN(001) is significantly enlarged, thereby effectively suppressing the desorption of N atoms.