Theoretical insights into the reduction mechanism of neptunyl nitrate by hydrazine derivatives

In the advanced spent fuel cycle, the control and adjustment of neptunium valence state is greatly important for the highly efficient separation of neptunium. Hydrazine and its derivatives as salt-free reagents can selectively reduce Np(VI) to Np(V), but their reduction mechanisms are still unclear. We explored the reduction of [Np${}^{VI}$O${}_2$(H${}_2$O)${}_2$(NO${}_3$)${}_2$] by N${}_2$H${}_4$ and its two derivatives HOC${}_2$H${}_4$N${}_2$H${}_3$ and CHON${}_2$H${}_3$ using scalar relativistic density functional theory. The thermodynamic energy of the reactions [Np${}^{VI}$O${}_2$(H${}_2$O)${}_2$(NO${}_3$)${}_2$] with three reductants are sensitive to the substitution group, HOC${}_2$H${}_4$N${}_2$H${}_3$ enhances thermodynamic ability of the reaction and CHON${}_2$H${}_3$ shows contrary result. Both HOC${}_2$H${}_4$N${}_2$H${}_3$ and CHON${}_2$H${}_3$ have lower energy barrier compared to N${}_2$H${}_4$ based on the potential energy profiles (PEPs), which probably attributes to the intramolecular hydrogen bond of hydrazine derivatives. The nature of these redox reactions is that the hydrogen atom of reductants is gradually transferred to the axis oxygen atom of neptunyl, which accompanies the N-H bond dissociation and O${}_{ax}$-H bond formation. The reduction of Np(VI) with HOC${}_2$H${}_4$N${}_2$H${}_3$ is the most favorable reaction based on the thermodynamic and kinetic results. This work provides theoretical perspective into the reduction of Np(VI) to Np(V), which is beneficial to the development of more effective free-salt reductants for the separation of neptunium from uranium and plutonium in spent fuel reprocessing.