Effect of Ionizing Radiation on the Redox Chemistry of Penta- and Hexavalent Americium

The recent development of facile methods to oxidize trivalent americium to its higher valence states holds promise for the discovery of new chemistries and critical insight into the behavior of the 5f electrons. However, progress in understanding high-valent americium chemistry has been hampered by americium’s inherent ionizing radiation field and its concomitant effects on americium redox chemistry. Any attempt to understand high-valent americium reduction and/or disproportionation must account for the effects of these radiolytic processes. Therefore, we present a complete, quantitative, mechanistic description of the radiation-induced redox chemistry of the americyl oxidation states in aerated, aqueous nitric acid, as a function of radiation quality (type and energy) and solution composition using multiscale modeling calculations supported by experiment. The reduction of Am­(VI) to Am­(V) was found to be most sensitive to the effects of ionizing radiation, undergoing rapid reductions with the steady-state products of aqueous HNO₃ radiolysis, i.e., HNO₂, H₂O₂, and HO₂^•, which dictated its practical lifetime under acidic conditions. In contrast, Am­(V) is only susceptible to radiolytic oxidation, mainly through its reactions with NO₃^•, and is notably radiation-resistant with respect to direct one-electron reduction to produce Am­(IV). Our multiscale modeling calculations predict that the lifetime of Am­(V) is dictated by its rate of disproportionation, 2AmO₂⁺ + 4H_aq⁺ → AmO₂²⁺ + Am⁴⁺ + 2H₂O, with a fourth-order dependence on [H_aq⁺] in agreement with previous experimental findings, giving an optimized rate coefficient of k = 2.27 × 10^–6 M^–5 s^–1. This disproportionation initially produces Am­(IV) and Am­(VI) species, but the lack of any spectroscopic evidence in our study for Am­(IV) suggests that solvent reduction of this cation occurs rapidly. The ultimate product of all the Am­(VI)/Am­(V) irradiations is Am­(III), which shows great stability in an irradiation field.