Insights into the Reduction Mechanisms of Np(VI) to Np(V) by Carbohydrazide

An efficient approach to Np separation in the Plutonium Uranium Reduction EXtraction (PUREX) process is to adjust Np(VI) to Np(V) by free-salt reductants, such as hydrazine and its derivatives. Recently, carbohydrazide (CO(N₂H₃)₂), a derivative of hydrazine and urea, has received much attention, which can reduce Np(VI) to Np(V) in the extraction reprocessing of spent nuclear fuel. Herein, according to the experimental observations, we examine the reduction mechanism of four Np(VI) by one carbohydrazide molecule using multiple theoretical calculations. The fourth Np(VI) reduction with a 22.26 kcal mol^-1 energy barrier is the rate-determining step, which is in accordance with the experimental observations (20.54 ± 1.20 kcal mol^-1). The results of spin density reflect that the reduction of the first and third Np(VI) ion is an outer-sphere electron transfer, while that of the second and fourth Np(VI) ion is the hydrogen transfer. Localized molecular orbitals (LMOs) uncover that the breaking of the N-H bond and formation of the O_yl-H bond are accompanied by the reaction from initial complexes (ICs) to intermediates (INTs). This work offers basic perspectives for the reduction mechanism of Np(VI) to Np(V) by CO(N₂H₃)₂, which is also expected to design excellent free-salt Np(VI) reductants for the separation of Np in the advanced PUREX process.