Electrochemical/computational analysis and the derivation of Pourbaix diagrams of redox-active structural and adsorbed species in uranyl peroxide clusters (U60)

Electrochemical analysis of U-60 nanoclusters (Li40K20[UO2(O-2)(OH)](60)(H2O)(214)) and their natural analog, the mineral studtite (UO2)O-2(H2O)(2)<middle dot>(H2O)(2), was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling to understand the mechanism, thermodynamics, and structural changes during redox transition (with a semi-quantitative kinetics evaluation). This research aims to better understand the redox behavior and transport of dissolved and structural (incorporated) uranyl units in the environment. Further analysis is performed to determine if redox switching of actinyl ions in clusters can be performed while leaving the cluster intact or at what redox transition it disintegrates. Quantum-mechanical calculations were applied to approximate electrochemical peak potential shifts and add them to standard reduction potentials to more accurately assign peaks to specific redox transitions between different species. These peak potentials are calculated from oxidation state-dependent binding energies between different uranyl species either within or adsorbed to U-60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. Voltammograms of U-60 in electrolyte solutions exhibit kinetically-inhibited coupled redox peaks assigned to the U(VI)/U(V) transition of cluster structural uranyl units at -0.34 V (vs. standard hydrogen electrode), indicative of U-60 reduction from UO22+ to UO2+. The U(V)/U(IV) transition at -0.71 V is assigned to the subsequent U-60 reduction of UO2+ to UO20. A method of observing these transitions in situ using synchrotron x-ray absorption spectroscopy was explored. Voltammetry, modeling, and peak analysis indicate a kinetically-inhibited two-step, one-e(-) (per step) reduction of U-60 from U(VI) to U(V) to U(IV). Voltammetry and x-ray absorption spectroscopy measurements performed before and after exhaustive cycling and the highly stable nature of U-60 provide evidence that clusters typically do not break apart upon reduction. Voltammograms of U-60 collected in varied uranyl solutions are indicative of UO22+ adsorption to the cluster surface and its subsequent reduction at +0.24 V (U(VI)/U(V)) and -0.05 V (U(V)/U(IV)). For uranyl solutions in electrolyte, the two-step, one-e(-) (per step) reduction of U-60 is superseded by uranyl adsorption from solution to the electrode surface with subsequent reduction via U(V) disproportionation. This study represents the sole electrochemical investigation of U-60 in the literature and only the second of any uranyl peroxide nanocluster. It is also one of the first to use quantum-mechanical approaches in combination with existing solution-based standard reduction potentials to calculate Pourbaix (E-H-pH) diagrams for adsorbed and structural species.