Neptunium redox speciation

Insights about the redox speciation of neptunium in an aqueous mineral acid electrolyte were obtained through a combination of in situ EXAFS (extended X-ray absorption fine structure) spectroelectrochemistry, density functional theory (DFT), and simple geometric modeling. A single solution of neptunium in 1 M perchloric acid was used to extract metrical information about the Np coordination environment, in terms of hydration numbers (n) and Np-O interatomic distances. Four aquo ions - Np³⁺·nH₂O, Np⁴⁺ · n´H₂O, [Np⁵⁺O₂]⁺ · n´´H₂O, and [Np⁶⁺O₂]²⁺ · n´´´H₂O - were electrolytically prepared and precisely maintained by use of constant potential bulk electrolysis (with coulometry) throughout the simultaneous EXAFS data acquisition. For the Np(III) and Np(IV) aquo ions, the experiments revealed a contraction of the average Np-O bond lengths from 2.48(2) to 2.37(2) Å, respectively. The data analyses suggest that there are 9 water molecules in the first or inner hydration sphere about Np³⁺ in [Np(OH₂)₉]³⁺ and Np⁴⁺ in [Np(OH₂)₉]⁴⁺. The DFT calculations reveal 8-9 water molecules coordinated to Np(III), supporting the EXAFS results. Simple geometric modeling supports a coordination number of 8 for both trivalent and tetravalent Np. For the Np(V) and Np(VI) aquo ions, the EXAFS revealed bond length contractions. The average interatomic distances for the trans-dioxygen atoms in [NpO₂]⁺ and [NpO₂]²⁺ decreased from 1.80(2) Å for Np(V) to 1.73(2) Å for Np(VI). The average interatomic distances to the oxygen atoms of the coordinated H₂O molecules decreased from 2.44(3) Å to 2.36(3) Å, respectively. The oxygen coordination numbers were identical, suggesting that 5 water molecules are bound to Np⁵⁺ in [NpO₂(OH₂)₅]⁺ and to Np⁶⁺ in [NpO₂(OH₂)₅]²⁺.