Uncovering the reduction mechanism of Np(vi) with N,N-diethyl hydroxylamine: a scalar-relativistic DFT investigation

Abstract

Salt-free reductants have been extensively studied in the plutonium uranium reduction extraction (PUREX) process for Np(VI) reduction. Hydroxylamine derivatives as a class of promising salt-free reductants can reduce Np(VI) to Np(V) in nitric acid solution. The reduction reaction and kinetic behavior of Np(VI) to Np(V) by diethylhydroxylamine (DEHA) were studied experimentally. Herein, we explored the reduction mechanism of Np(VI) by DEHA in aqueous solution using scalar-relativistic density functional theory. Four stages are included in the process of Np(VI) reduction to Np(V) by DEHA. The intermediate C₂H₅N(O)C₂H₄ is hydrolyzed to C₂H₅NHOH. The third and fourth stages of Np(VI) reduction are via two pathways due to different H atoms participating successively in the reduction process. The largest energy barrier for the four Np(VI) reduction processes is 12.3 kcal mol⁻¹ for the third stage of Pathway I, consistent with the experimental observations. The Np–O_yl bond distances and spin density on Np elucidate the reduction essence including outer-sphere electron transfer or hydrogen atom transfer. The analyses of bonding nature reveal the bonding evolution during the reduction process. This work elucidates the reduction mechanism of Np(VI) by DEHA and provides new insights into the reduction of Np(VI) by hydroxylamine derivatives.