Density Functional Theory Study of Oxygen Adsorption and Dissociation on Lower Miller Index Surfaces of ThN

Abstract

Thorium nitride (ThN), a promising metallic nuclear fuel with higher actinide density and higher thermal conductivity, is investigated as a candidate nuclear fuel for the next generation of reactors. In this article, a systematic investigation of the adsorption mechanism of molecular (O2) and dissociated oxygen (O–O) for several configurations, on clean ThN (100), (110), and (111) surfaces, was performed using the density functional theory calculations. To elucidate the interfacial interaction of ThN with O2, an extensive examination of the relaxed adsorption structures, adsorption energies, electronic structure, changes in bond lengths, and the Bader charge transfer were conducted. The energetics and the most stable adsorption configurations of O2 at higher coverages are also reported. The results obtained indicate that the (100) surface is the most stable with a surface energy of 0.98 J/m2, followed by (111) and (110) with an energy of 1.68 J/m2 and 1.77 J/m2 respectively. The relaxed adsorption structures revealed that O2 has a stronger affinity for the thorium (Th) atom than the nitrogen (N) atom. The calculated interatomic bond distances of the Th–O bond was found to be consistent with the measured bond length of thorium oxide (ThO2), indicating the formation of the thorium oxide layer on the surface of ThN. Among all the different configurations considered on the (100) surface, only the O2 molecule placed at the Th–N bridge site has undergone spontaneous dissociation, whereas, in the case of (111) surface, the O2 molecule placed at the bridge site and the side-on Th site has undergone spontaneous dissociation. However, the O2 molecule adsorbed on the (110) surfaces of ThN requires activation energy for dissociation to occur. These observations in ThN surfaces are in stark contrast with the previously reported cases of uranium nitride (UN) and α-U (001), where all the configurations have undergone a spontaneous dissociation. Oxidation behavior of ThN is a critical issue that needs to be understood before researchers can push for its use in commercial reactors. In line with this thought, a molecular-level understanding of the surface chemistry of ThN in an oxygen environment is expected to provide information on the oxidation mechanisms.