Theoretical study on the kinetic behavior of Np(VI) reduction with hydroxylamine and its derivatives: substituent effect

Abstract

Spent fuel reprocessing entails controlling the valence state of Np and its direction in the Plutonium-Uranium Reduction Extraction (PUREX) process. Hydroxylamine (HA) and its derivatives are effective salt-free reductants that can reduce Np(VI) to Np(V) without further reduction. Experimentally, hydroxylamine, N-methylhydroxylamine (MHA) and N, N-dimethylhydroxylamine (DMHA) reduce Np(VI) at different reaction rates. To investigate the impact of methyl substitution on the reduction mechanism, we have theoretically studied the Np(VI) reduction reaction by HA, MHA and DMHA. The reduction of Np(VI) involved hydrogen atom transfer from these reductants. The two Np(VI) reductions by HA are via hydrogen transfer. The Np(VI) reduction by MHA and DMHA both undergoes the hydrogen atom transfer at first and the outer-sphere electron transfer at second. The rate-determining step for MHA and DMHA is the first Np(VI) reduction, and the energy barrier for DMHA is lower than that for MHA, which are 6.2 and 7.7 kcal mol-1, respectively. It suggests the reaction rate of DMHA reducing Np(VI) is faster than that of MHA with the influence of methyl, which follows the experiments. Finally, we also analyzed the bonding evolution by the quantum theory of atoms in molecules (QTAIM), interaction region indicator (IRI), Mayer bond order (MBO), localized molecular orbitals (LMO) and spin density. This study gives us a kinetic insight into the effect of methyl substitution on the reduction of Np(VI) by hydroxylamine, which provides in-depth Np(VI) reduction by hydroxylamine derivatives in spent fuel reprocessing.