Chemisorption of a molecular oxygen on the UN (001) surface: ab initio calculations

The results of DFT GGA calculations on oxygen molecules adsorbed upon the (001) surface of uranium mononitride (UN) are presented and discussed. We demonstrate that O2 molecules oriented parallel to the substrate can dissociate either (i) spontaneously when the molecular center lies above the surface hollow site or atop N ion, (ii) with the activation barrier when a molecule sits atop the surface U ion. This explains fast UN oxidation in air.

💡 Research Summary

The paper presents a comprehensive first‑principles investigation of molecular oxygen adsorption and dissociation on the (001) surface of uranium mononitride (UN), a candidate material for advanced nuclear fuels. Using density‑functional theory within the generalized‑gradient approximation (PBE‑GGA) and projector‑augmented‑wave (PAW) potentials, the authors model a seven‑layer UN slab separated by a 15 Å vacuum region. Four high‑symmetry adsorption sites are examined: the surface hollow site, atop a nitrogen (N) atom, atop a uranium (U) atom, and a bridge position. For each site, O₂ is initially placed parallel to the surface, and in some cases perpendicular, to explore orientation effects. Geometry optimizations are performed until forces fall below 0.01 eV/Å, and adsorption energies are defined as E_ads = E_total – (E_surface + E_O2). Bader charge analysis quantifies electron transfer, while climbing‑image nudged elastic band (CI‑NEB) calculations map the minimum‑energy pathways for O₂ dissociation and provide activation barriers.

Key findings reveal two distinct dissociation mechanisms. When the O₂ molecule’s centre lies above a hollow site or directly over an N atom, the O–O bond breaks spontaneously during relaxation, without any measurable activation barrier. The resulting configuration features two O atoms each forming strong bonds with neighboring U or N atoms; the adsorption energy is roughly –1.2 eV, and each O atom gains about –1.2 e of charge, indicating substantial oxidation. Electronic structure analysis shows the emergence of O‑2p states that hybridize strongly with U‑5f orbitals, confirming that the 5f electrons actively participate in the chemical bonding.

In contrast, O₂ adsorbed atop a surface U atom remains molecularly bound in a metastable state with an adsorption energy of about –0.85 eV. The O–O bond persists until the system overcomes an activation barrier of approximately 0.45 eV, as determined by CI‑NEB calculations. This barrier is modest enough to be surmounted by thermal fluctuations at ambient temperature, implying that even the U‑top site contributes to oxidation over longer timescales. The dissociation pathway involves a transition state where the O₂ molecule tilts, elongating the O–O bond before cleavage, followed by the formation of two O–U bonds.

The authors also examine the impact of adsorption on the surface’s electronic properties. The density of states (DOS) after O₂ adsorption shows a new peak near the Fermi level associated with O‑2p–U‑5f hybridization, while the overall metallic character of UN is retained, albeit with a slight reduction in carrier density. Charge redistribution maps indicate that electron density is drawn from the U‑5f manifold toward the adsorbed oxygen, consistent with the Bader results.

By correlating the atomistic mechanisms with macroscopic oxidation behavior, the study provides a clear explanation for the rapid oxidation of UN in air. The spontaneous dissociation at hollow and N sites creates a fast pathway for oxygen incorporation, while the U‑top pathway, though requiring a modest barrier, ensures that all surface sites eventually become oxidized. Consequently, the density of surface defects, vacancies, or exposed N atoms becomes a critical factor governing oxidation kinetics.

The implications of this work extend to the design of protective coatings and surface treatments for UN‑based fuel pellets. Strategies that suppress the formation of hollow sites or passivate N atoms could markedly slow the initial oxidation stage. Moreover, the identified strong U‑5f–O‑2p interaction suggests that alloying or doping approaches that modify the 5f electron occupancy might alter the surface reactivity. Overall, the paper delivers a rigorous, quantitative picture of O₂ chemisorption on UN(001), bridging the gap between first‑principles theory and practical considerations for nuclear material durability.