Nonstoichiometric Uranium Dinitride as a Potential Catalyst for Ammonia Synthesis: A Theoretical Study of Surface Science

Uranium nitrides UN _{2− x} are excellent candidates as catalyst for ammonia synthesis, as demonstrated by their historical precedent. However, despite their potential, uranium nitrides have not been extensively investigated. In this study, we theoretically investigated the bulk and surface properties of pure and defective UN ₂ (UN _1.75 ) implementing DFT + U approach. Uranium nitride UN ₂ is not stable, and vacancies are spontaneously formed leading to N/U ratio between 1.7 and 1.8 depending on the synthesis conditions. The vacancy formation energy of UN ₂ was calculated as −0.70 eV. To identify the most stable surface, surface energy was calculated for both UN ₂ and UN _1.75 catalyst built in four Miller indices (100), (110), (111), and (121). It was found that (111) surface is the most stable one. Moreover, considering differences in distribution of vacancies in UN _1.75 , a random vacancy distribution for (111) surface shows the lower surface energy of 0.51 J.m ⁻² . The surface can easily be covered by layer of oxygen at very low oxygen pressure even at high temperature of 1500 K. However, the regeneration of the initial deoxygenated surface is possible under reaction conditions, (N ₂ /H ₂ ; 1:3) at 1000 K temperature. Regeneration is further facilitated in the presence of ammonia in the gas mixture.