Title:
Electrochemical and computational analysis of the thermodynamics of oxidation/reduction reactions of uranyl peroxide clusters (U60), including the derivation of Pourbaix diagrams for adsorbed species [Dataset]

Authors:
Gebarski, Benjamin
Becker, Udo

Method:
Data set was generated using Biovia Materials Studio 2019 as described in the associated journal article.

Description:
Files are uploaded as crystallographic information files (.cif), the standard text file format for representing crystallographic information.
These files contain the optimized molecular models for uranium-60 nanoclusters (U60) reduction as described in the associated journal article.

This research was conducted by Benjamin Gebarski and Udo Becker at the University of Michigan-Ann Arbor and was submitted for publication on 9/9/2022.

This research was supported by U.S. Department of Energy: Heavy Element Chemistry and Geosciences grant.

Research Overview:
Electrochemical analysis of U60 nanoclusters (Li40K20[UO2(O2)(OH)]60(H2O)214) and their natural analog, the mineral studtite (UO2)O2(H2O)2·(H2O)2, was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling. The purpose of this research is to study the redox behavior, thermodynamics, and kinetics of uranyl peroxide materials in order to understand their behavior and transport 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 the cluster 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 U60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. 
Voltammograms of U60 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 a reduction from UO22+ to UO2+. The U(V)/U(IV) transition at -0.71 V is assigned to the subsequent reduction of UO2+ to UO2. 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 U60 from U(VI) to U(V) to U(IV). Exhaustive cycling and the stabilization of U(V) via peroxide provide evidence that the majority of clusters do not break apart upon reduction. 
Voltammograms collected in varied uranyl solutions are indicative of UO2 adsorption to the U60 cluster surface and its subsequent reduction at +0.24 V (U(VI)/U(V)) and -0.05 V (U(V)/U(IV)). In uranyl solutions in electrolyte, the two-step, one-e- (per step) reduction of U60 is superseded by uranyl adsorption from solution to the electrode surface, and subsequent reduction via U(V) disproportionation.
This study represents the sole electrochemical investigation of U60 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 (Eh-pH) diagrams for adsorbed species.

File inventory: 12 Molecular models in .cif format
All models are enclosed in a P1 Triclinic crystal lattice in order to preserve orientation in .cif form. To replicate exact results from the associated article you must first remove the lattice.

U60_6.cif
-U60 nanocluster in the hexavalent state
U60_5.cif
-U60 nanocluster in the pentavalent state
U60_4.cif
-U60 nanocluster in the tetravalent state

U60_6_SO4.cif
-U60 nanocluster in the hexavalent state with sulfate
U60_5_SO4.cif
-U60 nanocluster in the pentavalent state with sulfate
U60_4_SO4.cif
-U60 nanocluster in the tetravalent state with sulfate

U60_6_UO2_6.cif
-U60 nanocluster in the hexavalent state with adsorbed uranyl in the hexavalent state
U60_6_UO2_5.cif
-U60 nanocluster in the hexavalent state with adsorbed uranyl in the pentavalent state
U60_6_UO2_4.cif
-U60 nanocluster in the hexavalent state with adsorbed uranyl in the tetravalent state

U60_6_UO2_6_SO4.cif
-U60 nanocluster in the hexavalent state with adsorbed uranyl sulfate in the hexavalent state
U60_6_UO2_5_SO4.cif
-U60 nanocluster in the hexavalent state with adsorbed uranyl sulfate in the pentavalent state
U60_6_UO2_4_SO4.cif
-U60 nanocluster in the hexavalent state with adsorbed uranyl sulfate in the tetravalent state

Definition of Terms and Variables:
No glossary is necessary for the uploaded molecular models.

Use and Access:
The file uploaded (.cif) are useable by any molecular modelling software. Any version of Materials Studio is recommended.

Suggest a Citation for the Dataset:
Gebarski, B. B. and Becker, U. Electrochemical and computational analysis of the thermodynamics of oxidation/reduction reactions of uranyl peroxide clusters (U60), including the derivation of Pourbaix diagrams for adsorbed species. (2022) Geochimica et Cosmochimica Acta.