Theoretical Study on the Reduction Mechanism of Np(VI) by Hydrazine Derivatives

Abstract

The key to effective separation of neptunium from the spent fuel reprocessing process is to adjust and control its valence state. Hydrazine and its derivatives have been experimentally confirmed to be effective salt-free reductants for reducing Np(VI) to Np(V). We theoretically studied the reduction reactions of Np(VI) with three hydrazine derivatives (2hydroxyethyl hydrazine (HOC 2 H 4 N 2 H 3 ), methyl hydrazine (CH 3 N 2 H 3 ), and formyl hydrazide (CHON 2 H 3 )) and obtained the free radical ion mechanism and the free radical mechanism. Their potential energy profiles (PEPs) suggest that the free radical mechanism is the most probable reaction. Based on the energy barrier of the free radical ion mechanism, the trend of the reduction ability of the three hydrazine derivatives is HOC 2 H 4 N 2 H 3 > CH 3 N 2 H 3 > CHON 2 H 3 , which is in excellent agreement with the experimental results. Lastly, the analyses of natural bond orbitals (NBOs), quantum theory of atoms-in-molecules (QTAIM), and electron localization function (ELF) have been carried out to explore the bonding evolution of the structures along the reaction pathways. This work provides an insight into the reduction mechanism of Np(VI) with hydrazine derivatives from the theoretical perspective and helps to design more effective reductants for the separation of U/Np and Np/Pu in spent fuel reprocessing.

Theoretical Insights into the Reduction Mechanism of Np(VI) with Phenylhydrazine

“…In addition, for the two free radical ion mechanisms, the energy barrier for the first stage (12.89 kcal/mol) is much higher than those for the second stage (2.79 and 3.07 kcal/mol), which indicates that the first stage is the rate-determining step. The rate-determining step is different from those of hydrazine, 2-hydroxyethyl hydrazine, methyl hydrazine, and formyl hydrazine reported in previous works, probably due to different hydrogen bonds in these neptunyl species with hydrazine derivatives. Additionally, the order of energy barriers of the rate-determining step for these hydrazine derivatives is phenylhydrazine (12.89 kcal/mol) < 2-hydroxyethyl hydrazine (15.15 kcal/mol) < methyl hydrazine (16.57 kcal/mol) < formyl hydrazine (22.81 kcal/mol) .…”

“…The rate-determining step is different from those of hydrazine, 2-hydroxyethyl hydrazine, methyl hydrazine, and formyl hydrazine reported in previous works, probably due to different hydrogen bonds in these neptunyl species with hydrazine derivatives. Additionally, the order of energy barriers of the rate-determining step for these hydrazine derivatives is phenylhydrazine (12.89 kcal/mol) < 2-hydroxyethyl hydrazine (15.15 kcal/mol) < methyl hydrazine (16.57 kcal/mol) < formyl hydrazine (22.81 kcal/mol) . Therefore, based on the calculated results, the order of their reduction ability is phenylhydrazine > 2-hydroxyethyl hydrazine > methyl hydrazine > formyl hydrazine, which is in excellent agreement with the experimental result. , These results indicate that the different substituents of hydrazine can not only affect the energy barrier but also change the rate-determining step for their redox reaction.…”

“…Additionally, the order of energy barriers of the rate-determining step for these hydrazine derivatives is phenylhydrazine (12.89 kcal/mol) < 2-hydrox- yethyl hydrazine (15.15 kcal/mol) < methyl hydrazine (16.57 kcal/mol) < formyl hydrazine (22.81 kcal/mol). 32 Therefore, based on the calculated results, the order of their reduction ability is phenylhydrazine > 2-hydroxyethyl hydrazine > methyl hydrazine > formyl hydrazine, which is in excellent agreement with the experimental result. S1) and LMOs (Figure S2).…”

“…21 The reduction reactions of Np(VI) with hydrazine and its hydrazine derivatives have been investigated, such as conditions, mechanism, and kinetics, 14,22,29,30 which demonstrate that hydrazine and its derivatives merely reduce Np(VI) to Np(V) without reducing U(VI) and Pu(IV) under certain conditions. We have theoretically investigated the reduction reaction of Np(VI) with hydrazine 31 hydroxyethyl hydrazine, methyl hydrazine, and formyl hydrazine), 32 and the calculated trend of their reduction ability is in good agreement with experimental observation. In addition, previous works showed that phenylhydrazine exhibits better reduction ability for Np(VI) compared to other hydrazine derivatives.…”

Reduction of Np(vi) with hydrazinopropionitrileviawater-mediated proton transfer

“…We previously theoretically investigated the reduction mechanism of Np( vi ) with hydrazine, 41 2-hydroxyethyl-, methyl-, formylhydrazine, 42 and phenylhydrazine. 43 Here we report results for reduction of Np( vi ) by HzPn via a water-mediated proton transfer.…”