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

Authors:
Gebarski, Benjamin
Becker, Udo

Method:
Data set was generated using a potentiostat/galvanostat (EG&G Instruments Princeton Applied Research Model 263A) with computer output using the PowerSuite software package was used for voltammetry as described in the associated journal article.

Description:
Files are uploaded in text-based format for representing cyclic voltammetry.
These files contain the CVs for each figure 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: 7 files in text format each corresponding to the CV figures in the journal article.

XAS_Fig_S3a.txt
-Contains the raw data for the XAS spectra in Figure S.3 (t = 0 hrs)

XAS_Fig_S3b.txt
-Contains the raw data for the XAS spectra in Figure S.3 (t = 9 hrs)

XAS_Fig_S4a.txt
-Contains the raw data for the XAS Fourier transform spectra in Figure S.4 (t = 0 hrs)

XAS_Fig_S4b.txt
-Contains the raw data for the XAS Fourier transform spectra in Figure S.4 (t = 9 hrs)

XAS_Fig_S5.txt
-Contains the raw data for the XAS spectra in Figure S.5 (studtite)

XAS_Fig_S6a.txt
-Contains the raw data for the XAS spectra in Figure S.6 (studtite)

XAS_Fig_S6b.txt
-Contains the raw data for the XAS spectra in Figure S.6 (U60)



Definition of Terms and Variables:
No glossary is necessary for the uploaded data points.

Use and Access:
The file uploaded (.txt) are useable by any plotting software. Any version of OriginPro 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), and the derivation of Pourbaix diagrams for structural and adsorbed species [Dataset] (2023) Geochimica et Cosmochimica Acta. (2022) Geochimica et Cosmochimica Acta.