DFT + U Study of the adsorption and dissociation of water on clean, defective, and oxygen-Covered U₃Si₂{001}, {110}, and {111} Surfaces

The interfacial interaction of U₃Si₂ with water leads to corrosion of nuclear fuels, which affects various processes in the nuclear fuel cycle. However, the mechanism and molecular-level insights into the early oxidation process of U₃Si₂ surfaces in the presence of water and oxygen are not fully understood. In this work, we present Hubbard-corrected density functional theory (DFT + U) calculations of the adsorption behavior of water on the low Miller indices of the pristine and defective surfaces as well as water dissociation and accompanied H₂ formation mechanisms. The adsorption strength decreases in the order U₃Si₂{001} > U₃Si₂{110} > U₃Si₂{111} for both molecular and dissociative H₂O adsorption. Consistent with the superior reactivity, dissociative water adsorption is most stable. We also explored the adsorption of H₂O on the oxygen-covered U₃Si₂ surface and showed that the preadsorbed oxygen could activate the OH bond and speed up the dissociation of H₂O. Generally, we found that during adsorption on the oxygen-covered, defective surface, multiple water molecules are thermodynamically more stable on the surface than the water monomer on the pristine surface. Mixed molecular and dissociative water adsorption modes are also noted to be stable on the {111} surface, whereas fully dissociative water adsorption is most stable on the {110} and {001} surfaces.