An In-Situ X-Ray Absorption Spectroelectrochemical Study of the Electroreduction of Uranium Ions in Hcl, HNO3, and Na2co3 Solutions

Radiochim. Acta 2016; 104(1): 1–9

Akihiro Uehara*, Toshiyuki Fujii, Hajimu Yamana, and Yoshihiro Okamoto An in-situ X-ray absorption spectroelectrochemical study of the electroreduction of uranium ions in HCl, HNO3, and Na2CO3 solutions

DOI 10.1515/ract-2015-2436 lution during the bulk electrolysis. The edge jump of the Received April 29, 2015; accepted August 24, 2015; published X-ray absorption near edge structure (XANES) spectrum online September 30, 2015 shifted from 17.164 to 17.163 keV, and the bond distances 2− Abstract: A spectroelectrochemical cell was fabricated for of U−Oax and U−O for CO3 increased from 1.78 to 1.88 Å in-situ X-ray absorption spectroscopy (in-situ XAS). The and from 2.42 to 2.53 Å, respectively, because of the reduction of the UO2+ to the UO+ carbonato complex. XAS spectra of the uranium 𝐿III edge were monitored 2 2 in electrolyte solutions during the electrochemical reduc- Keywords: Spectroelectrochemistry, redox potential, in- 4+ −3 tion. Tetravalent uranium, U ,in1moldm (M) hy- situ XAS, uranium, disproportionation, electrolysis. drochloric acid (HCl) was electrochemically prepared from 2+ hexavalent uranium, UO2 , by constant current electrolysis, and the extended X-ray absorption fine structure 2+ 1 Introduction (EXAFS) was analyzed. The concentration ratio of UO2 and U4+, which were formed via the disproportionation of + Uranium forms various oxidation states in aqueous solu- pentavalent uranium, UO2 , during the electrolysis, were calculated based on the intensity of the signal for the two tion (trivalent to hexavalent), and it is important to esti- 2+ mate the chemical state of uranium ions in the fields of axial oxygen atoms in the linear UO2 unit, the U−Oax, bond that had a radial structural function. The appar- nuclear fuel cycles and environmental science. These ox- 2+ 4+ idation states are prone to change via oxidation and dis- ent redox potential of the UO2 /U couple in 1M HCl was determined based on the Nernst equation using the proportionation reactions depending on the proton and 2+ 4+ oxygen concentration [1]. The ions in the aqueous solu- concentrations of UO2 and U . The electrode potential was shown to be close to the formal potential of the tion behave as strong Lewis acids, and the ions with oxi- 2+ + dation states that are higher than pentavalent form di- or UO2 /UO2 couple as reported previously. This result in- + tri-oxo ions. The speciation of the ions has been studied by dicates that the UO2 that was formed electrochemically at 2+ 4+ various spectroscopic methods. These include: electronic the electrode disproportionated to form UO2 and U in 2+ absorption spectroscopy, fluorescence spectroscopy, emis- the bulk solution. The in-situ XAS of UO2 in 0.1 M nitric acid was also performed. The U4+ that formed was par- sion lifetime analysis, and extended X-ray absorption 2+ − spectroscopy, better known as XAS analysis [2]. Detailed tially re-oxidized to UO2 by the NO3 present in the so- + information about the local structure around the atoms lution. The formation of the UO2 carbonato complex was observed by in-situ XAS in a 1M sodium carbonate so- of a specific element can be obtained from X-ray absorption fine structure (XAFS). Numerous XAFS studies of the coordination structure of uranium complexes have been conducted and reviewed [2–7]. The technique enables the *Corresponding author: Akihiro Uehara, Division of Nuclear Engineering Science, Research Reactor Institute, Kyoto identification of the valence states of uranium during the University, Asashironishi, Kumatori, Osaka, 590-0494, Japan, redox reactions from the X-ray absorption near-edge struc- e-mail: [email protected] ture (XANES). The bonds between uranium and inorganic Toshiyuki Fujii, Hajimu Yamana: Division of Nuclear Engineering − 2− 2− ligandssuchasU–O(forH2O, NO3 , CO3 , SO4 and Science, Research Reactor Institute, Kyoto University, OH−), and U–halide bonds may be characterized via the Asashironishi, Kumatori, Osaka, 590-0494, Japan Yoshihiro Okamoto: Quantum Beam Science Directorate, Japan analysis of the extended X-ray absorption fine-structure 2+ Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai, Naka, Ibaraki (EXAFS). EXAFS measurements for UO2 have been con- 319-1195, Japan ducted in various pH environments and electrolyte solu- 2 | A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions

tions, however, less is known about U4+ because it under- concentration that remained in the salt was less than goes oxidation under atmospheric conditions [8–13]. 1% by titration. After the uranium complexes were dis- To prepare the reduced products, electrochemical solved, the concentration was determined by absorption bulk electrolysis was combined with UV-Visible spec- spectroscopy [29]. A self-registering spectrophotometer V- + 4+ troscopy or XAFS measurements of UO2 [14–17]andU 350 (JASCO Co.) was used for the measurements at wave- in aqueous solutions [9–11, 18, 19]. Hennig and co-workers lengths between 350 and 800 nm. All other chemicals were studied the coordination sphere of U4+ ions in vari- used without further purification. ous electrolytes using an X-ray absorption spectroelectrochemical cell [9–11]. The U4+ coordination number report- edly decreased from 9 to 8 as the Cl− concentration was 2.2 In-situ X-ray spectroelectrochemical cell − increased from 0 (ClO4 )to9M[9]. In the presence of − 4+ NO3 ions, the water molecules of the U hydrate com- For the electrochemical measurements, a three-electrode plexes are successively replaced by planar bidentate co- system was used. A platinum wire (Nilaco Co.) or glassy ordinating nitrate ions upon increasing the HNO3 con- carbon fiber (Tokai Carbon Co.) was used as the working − centration, generating [U(H2O)x(NO3)5] as the major electrode to enhance the efficiency of the electrolysis, and product in 9.0 M HNO3 [11]. Ikeda and coworkers also asilver/silver chloride (Ag/AgCl) electrode was used as found that tetravalent UO2 (solid deposit) and pentava- the reference electrode. This reference electrode consists 5− lent [UO2(CO3)3] were formed by the electrochemical of a tube sealed at the end by a cellulose filter. The tube reduction in 0.1 M HClO4 and 0.8 M Na2CO3,respec- contained a silver wire (1mm diameter) coated in AgCl tively [11, 16]. that was immersed in an aqueous solution of LiCl (1M). In-situ X-ray spectroelectrochemistry [20–28]canbe The counter electrode was a silver wire, and a counter used to determine the formal potentials as well as the rel- phase containing 1MLiCl was separated from the objec- ative concentrations of the species at an applied potential, tive phase by a cellulose filter to avoid cyclic redox reac- even if sample solutions contain other redox-active species tions. Polyethylene terephthalate (PET) and polypropylene or there is an overlying optical transition that prohibits the (PP) were used as the cover materials for the reference direct determination of the concentrations. EXAFS spec- and counter electrodes, respectively. An electrochemical tra may undergo Fourier transform into frequency space, measurement system, IVIUM compactstat (IVIUM Co.) was providing a radial distribution function. The resultant “R- used for the bulk electrolysis. The concentration of U(VI) 2+ 4+ 3 space” clearly distinguishes UO2 from U via the pres- in the EXAFS samples was 0.05 M. Approximately 3cm of ence of a peak (U−Oax) that corresponds to the two axial each sample were sealed in the electrochemical cell that 2+ oxygen atoms of the linear UO2 complex. Therefore, in- was made of an acrylic plate and fabricated for the X-ray situ XAS measurements can provide the concentration ra- 2+ 4+ tio of UO2 and U , which is dependent on the equilib- rium potential in the solution. For safety and security, a closed system must be used to house a target sample that contains the aqueous ra- screwscrew capcap dioactive materials. In the present study, an in-situ XAS cell has been developed for the measurement of the ura- RE:RE: Ag|AgClAg|AgCl CECE RE WWEE CCE:E: AAgg nium valence states during the electrochemical reduction WWE:E: PtPt oror gglassylassy carboncarbon 2+ 2+ + 4+ acrylicacrylic boxbox of UO2 .TheredoxbehaviorofUO2 , UO2 ,andU were studied using XANES, EXAFS, and electrode potential in hydrochloric acid, nitric acid, and sodium carbonate solu- acrylicacrylic cellcell tions. KaptonKapton fil filmm solutionsolution

2 Experimental standstand 2.1 Chemicals Acrylic box (included oxygen absorber ) Uranyl chloride (UO2Cl2) hydrate, was prepared by evap- + orating aqueous solutions of UO2Cl2 and HCl. The H3O Fig. 1: The in-situ X-ray spectroelectrochemical cell. A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions | 3

2 measurement (Figure 1), and the cell contained an acrylic duction factor, 𝑆0, was defined as 0.9 in the FEFF calcula- window (3×3cmand 1mmthick). The sample solutions tion and was held at that value during the EXAFS fittings. were purged with nitrogen for 30 min and the cells were The 𝛥𝐸0, the fitted energy shift parameter, was linked for prepared in a glove box under a nitrogen atmosphere to all shells. A Bessel function was used as a window func- avoid the oxidation of the uranium ions. The three elec- tion for the Fourier transforms (FT). trodes were sealed into the cell by epoxy resin to avoid oxygen from the atmospheric air and to prevent leaking of the sample solution. We confirmed that the cell made 3 Results and discussion from acrylic, PET, and PP provided satisfactory chemical and X-ray resistance for the duration of this study. The 3.1 In-situ electrochemical XAFS cell was placed in the first inner acrylic container and was 2+ measurement of UO2 in hydrochloric partially covered with Kapton film for the X-ray transmis- acid media sion. This container was installed for in-situ XAFS measurements and remained unopened at the beam line. An The EXAFS measurements of 0.05 M UO2+ in 1M HCl oxygen absorber (AGELESS, Mitsubishi Gas Chemical Co.) 2 were performed before the electroreduction; 5 scans were was placed in the inner and outer containers. The pre- collected and averaged, as shown in curve 1 of Figure 2. pared uranium solutions were perfectly sealed in the elec- The 𝑘3 weighted EXAFS spectra for the uranyl complexes trochemical cell at the Hot Laboratory at the Kyoto Univer- were obtained before and after electrolysis, and their cor- sity Research Reactor Institute and were transported to the responding FT are shown in (a) and (b) of Figure 3.Curve photon factory in the High Energy Accelerator Research 1inFigure3b shows the typical peaks corresponding to Organization, KEK. a single scattering path from the axial oxygen atoms of the 2+ linear UO2 unit (U−Oax) and oxygen atoms of the hydrated water in the equatorial plane perpendicular to the 2.3 XAFS measurement and data analysis 2+ UO2 axis (U−Oeq)at𝑅 + 𝛥 = 1.25 and 1.92 Å, respectively. The XAFS measurements were performed at the BL27B To monitor the electrode potential during the electrol- beam line [30] at the Photon Factory in KEK, Japan. Hard ysis, constant current electrolysis was employed. A fixed X-rays ranging from 5–20 keV are available at this beam current of −1 mA was applied for approximately 32 000 s, line with a Si(111) double-crystal monochromator. XAFS during which time the in-situ XAS measurements were measurements based on the U𝐿 -absorption edge (𝐸 = III 0 repeated. Here, a platinum wire was used as the work- 17.153 keV) were performed. All the solutions were mea- ing electrode. Five scans were averaged. XANES, the 𝑘3- sured in fluorescence mode using a Ge solid state detector. weighted EXAFS spectra, and their corresponding FTs are Asinglescanfrom16.865to18.265 keV took 564 seconds. shown in curves 2–9 of Figures 2, 3aandb,respectively. The X-ray was focused on an area closest to the working The potential change during the electrolysis was moni- electrode. The beam size was approximately 3×2mm.The electrodes were connected to the measurement system by five meters of cable enabling control over the electrolysis from outside the beam line hatch. An XAFS scan was measured before the current was applied to obtain an initial spectrum and was then repeated during the electrolysis. S

The XAFS spectra were analyzed using WinXAS ver. E Fig. 2: The 𝐿 III-edge XANES N

3.1 code developed by Ressler [31]. Multiple XAFS scans A 1 spectra of 0.05 M uranium X ion in 1MHCl during the d 2

were collected from for each sample at ambient temper- e

z constant current electro-

i 3 l ature (298 K),andtheresultswereaveraged.TheEXAFS a 4 lysis applied at −1 mA.Five m data were fitted using a theoretical phase and ampli- r 5 scans were averaged. o 6 tudes calculation from the program, FEFF 8, by Rehr et N A platinum wire was used 7 2+ as the working electrode. al. [32]. For UO2 , the single scattering path was calcu- 8 9 Curve 1: before the lated based on the structures of UO2Cl2 ⋅H2O [33]and 4− 10 electrolysis, Curves 2 to 9: [UO2(CO3)3] [34]. Due to the increasing noise levels at during the electrolysis, 17.13 17.18 17.23 the higher 𝑘-range, the data analysis for curve fitting was Curve 10 (dotted line): U(IV) −1 E (keV) restricted to a 𝑘-range of 2.5–12.5 Å .Theamplitudere- in 1.5 M HClO4 [13]. 4 | A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions

at the end of the electrolysis, the potential was − 0.319 V at 32 000 s.

The H2O to uranium coordination number (𝑁H2 O)

andbonddistanceoftheU−OH2 (𝑅U−OH2 ), including the

m phase correction, were calculated during the EXAFS shell r o f fitting. The 𝑁 of curve 1 that was measured before the

n H2O a r electrolysis was 4.4 at 𝑅 = 2.42 ± 0.02 Å, which was T U−OH2

r e

3 similar to the reported in [9, 35, 36]. i 𝑁H2O 1M HClO4 r k u

o A significant U-Cl bond was not observed in the curve fit-

F 2+ ting because 56% of the total concentration of UO2 forms + + UO2Cl based on the stability constant of UO2Cl [37]. At

the end of the electrolysis, the 𝑁H2 O of curve 9 was 9.5 at 4+ 𝑅U−OH2 = 2.40 ± 0.02 Å, which agrees with 𝑁H2O of U 2.5 5 7.5 10 0 1 2 3 4 in 1.5 M HClO4 (curve 10 in Figure 3b) [13]. These results R + ( ) k ( -1) 2+ 4+ indicate that UO2 was reduced to U as shown in the following reaction (Eq. 1): 3 Fig. 3: Uranium 𝐿 -edge 𝑘 -weighted EXAFS data (a) and III −1 corresponding Fourier transforms (b) taken over 𝑘=2.5–10.5 Å in 2+ + − 4+ UO2 +4H +2e 󴀕󴀬 U +2H2O (1) 1MHCl during the electrolysis. Five scans were averaged. The dotted lines in Figure 3a are curves fitted using the initial spectrum The electrode potential in Figure 4 was between −0.14V before the electrolysis (curve 1) and final spectrum for U(IV) (curve and −0.18V, which is not near the redox potential of the 9). Curve 10: U(IV) in 1.5 M HClO4 [13]. 2+ 4+ 2+ + UO2 /U couple but is similar to the UO2 /UO2 couple [38–41]. In other words, the following reaction, shown as Eq. (2), proceeds at the electrode:

) 2+ − + E UO +e 󴀕󴀬 UO (2) S 2 2 S

s + v The pentavalent UO is very unstable in acidic solu-

V 2 (

2+ 4+ l tion [39, 40] and disproportionates to form and

a UO U

i 2 t

n immediately [42–44], as shown as Eq. (3). e t o P + + 2+ 4+ 2UO2 +4H 󴀕󴀬2 UO +U +2H2O (3)

As a result, UO2+ was reduced to U4+ by a two electron re- Time (s) 2 action. Curves 9 in Figures 2, 3aand3b were very close 4+ Fig. 4: Potential change under the constant current electrolysis to curve 10 of U in 1.5 M HClO4, which indicates that 2+ 4+ applied at −1 mA. The closed circle (∙) shows the average time point UO2 was quantitatively reduced to form U by the elec- from the 5 spectra measured. Numbers in the figure correspond to trolysis and disproportionation. The electrode potential the numbers shown in Figures 2 and 3. shifted to a more negative gradient over 25 000 s (plots 8 and 9 in Figure 4), which suggests that the main species tored (Figure 4). The time points plotted in Figure 4 cor- was U4+. 2+ 4+ respond to the average time point from the 5 scans. Dur- The relative concentrations of UO2 and U were de- ing the continued electrolysis, the edge jump for U𝐿III termined using the FT peak corresponding to the U−Oax shifted to a slightly lower energy, as shown in curves 3 bond in which curves 1 and 9 in Figure 3b are assumed 2+ 4+ and 4 of Figure 2. The peak for the U−Oax bond at 𝑅+ to be the spectrum of 100% UO2 and U , respectively. 2+ 𝛥 = 1.25 Å decreased, while the peak at 𝑅 + 𝛥 = 1.92 Å The result of this (Figure 5a) shows that the UO2 con- which corresponds to oxygen atoms of the hydrated wa- centration (closed circle) decreased with an increasing 4+ ter, U−OH2, increased in Figure 3b. This result indicates U concentration (closed triangle) throughout the elec- 2+ 2+ 4+ that the U−Oax bond of UO2 was replaced by water trolysis. For comparison, the UO2 (open circle) and U molecules during the electrolysis. The electrode potential (open triangle) concentration ratio was also calculated shifted slightly from −0.139 to − 0.196 V for 16 300 s,and using the 𝑘3-weighted EXAFS spectra in which curves 1 and 9 in Figure 3a are again assumed to be the spectra A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions | 5

2+ + to the formal potential of the UO2 /UO2 couple that has been previously reported [45]; that report also showed that + the UO2 concentration was negligible due to the fast dis- + k proportionation reaction of UO2 in 1MHCl. k 2+ 4+ R The concentrations of UO2 and U were also cal- R culated using the FT peak of the U−OH2 bond (𝑅+𝛥= 1.92 Å) in which curves 1 and 9 in Figure 3b are assumed 2+ 4+ to be the spectra of 100% UO2 and U , respectively. The 2+ 4+ concentration ratio of UO2 and U that was calculated based on the FT peak at the U−OH2 bond agreed with the calculation based on the FT peak at the U−Oax bond. Thus, the uranium coordination sphere consists of U−Oaq and U−OH2 bonds in 1MHCl during the redox reaction 2+ 4+ of the UO2 /U couple.

2+ 3.2 Electrochemical reduction of UO2 in nitric acid

The in-situ XAFS measurements were performed in 0.1 M HNO instead of HCl with a lower concentration of H+ 2+ 4+ 3 Fig. 5: (a) Concentration ratio of UO2 remaining and U formed by during the constant current electrolysis. Three scans were the electrolysis and plotted as a function of the potential measured 2+ 4+ averaged and plotted in Figures 6, 7aand7bshowingthe in Figure 4. The concentration ratio of UO2 (∘)andU were 3 calculated by fitting the spectra before (curve 1) and after (curve 9) XANES, 𝑘 -weighted EXAFS and their corresponding FT 2+ 2+ the electrolysis in Figure 3a. the concentration ratio of UO2 (∙)and spectra, respectively. The XAFS spectrum of UO2 before 4+ U (󳵳) were calculated by U−Oax peak intensity in Figure 3b. (b) the electrolysis, shown in curve 1 of Figure 6, agreed with 2+ 4+ The relationship between log[UO2 ]/[U ] calculated by using the that in HCl as shown in curve 1 of Figure 3. The electrolysis U−Oax peak intensity in Figure 3b and the electrode potential. was then conducted by applying a fixed current of −1 mA, and the XAFS measurements were repeated during the 2+ 4+ of 100% UO2 and U , respectively. The calculated lines process. The variation in potential by electrolysis was are shown as dotted lines in Figure 3a. The concentra- monitored and is shown in Figure 8. The time points that tion ratios plotted in Figure 5a (open circle and triangle) are plotted in Figure 8 indicate the average time point for 3 show similar behavior with those obtained using FT peak scans, and the numbers correspond to those in Figures 6 (closed circle and triangle). The apparent redox potential and 7. The electrode potential attained −0.34Vwithin 2+ 4+ ∘󸀠 of the UO2 /U couple, 𝐸U(VI)/U(IV), is expressed based 7000 s possibly because the surface area of the glassy car- on the Nernst equation as Eq. (4):

∘󸀠 2+ 4+ 𝐸=𝐸U(VI)/U(IV) + (2.30𝑅𝑇/2𝐹) log([UO2 ]/[U ]) (4) 2+ 4+ Here, 𝐸, 𝐹, 𝑅, 𝑇,[UO2 ], and [U ] are the electrode potential, the Faraday constant, the gas constant, the ab- 2+ 4+ solute temperature, and the UO2 and U concentrations, respectively. The [ 2+]and[ 4+] that were based S UO2 U E

N Fig. 6: 𝐿 III-edge XANES on the FT peak of the U−Oax bond were plotted against A X spectra of 0.05 M uranium

the electrode potential in Figure 5b. The slope of the plots d

e ions in 0.1 M HNO3 during z i was 34 mV per order of magnitude, which is similar to l the constant current a m the value for a reversible two electron reaction (30 mV). r electrolysis applied at −1 mA. + o N Three scans were averaged. In other words, the UO2 concentration was negligible because of the rate of disproportionation that was en- A glassy carbon fiber was + − used as the working hanced by both the H and Cl concentrations [42–44]. electrode. Curve 1: before the 𝐸 was calculated to be − 0.175 V from the inter- 17.13 17.18 17.23 U(VI)/U(IV) E (keV) electrolysis, Curves 2 to 7: section point of the line and x-axis. This potential is close during the electrolysis. 6 | A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions ) E S S

. s v

m V r (

o l f a i n t a n r 3 e T t

k r o e i P r u o F

Time (s)

Fig. 8: Potential change under the constant current electrolysis 2.5 5 7.5 10 0 1 2 3 4 applied at −1 mA. The closed circle (∙) shows the average time point -1 from the 3 scans measured. The numbered positions in this figure k ( ) R + ( ) correspond to those shown in Figures 6 and 7.

3 Fig. 7: U𝐿 -edge 𝑘 -weighted EXAFS data (a) and the corresponding III −1 Fourier transforms (b) taken over 𝑘=2.5–10.5 Å in 0.1 M HNO3 during the electrolysis. Total concentration of uranium was 0.05 M. Curve 1: before the electrolysis, Curves 2 to 9: during the electrolysis.

bon fiber electrode that was used as the working electrode was over four times larger than the platinum wire electrode that we used previously. During the electrolysis process,

the U𝐿III edge jump shifted to lower energy because of the 2+ reduction from UO2 to U(IV), as shown in curve 3 and 4 of Figure 6.TheU−Oax peak in the FT decreased as the 2+ + bond increased (Figure 7b). The bond was Fig. 9: The concentration ratio of uranyl (UO2 and UO2 )anduranus U−OH2 U−Oax 4+ still observed in curve 5 in Figure 7b in which the electrode (U ) formed by the electrolysis plotted as a function of the potential measured in Figure 8. The concentration ratio of uranyl (∙) potential is , although the electrode potential − 0.296 V and uranus ( ) were calculated using the peak intensity in ∘󸀠 󳵳 U−Oax was approximately 0.15 V negative of the 𝐸U(VI)/U(V).This Figure 7b. 2+ + result suggests that both UO2 and UO2 are present when + the disproportionation of UO2 exhibits a lower rate con- + stant, as shown in Eq. (3), at a lower H concentration be- The U−Oax bond peak increased and the U−OH2 cause the rate is controlled by the concentration of H+ [42– bond peak remained as shown in curve 7 in Figure 7b 3+ 44]. Figure 9 shows the U−Oax bond peak corresponding at −0.56V.TheU(OH) that formed may have been 2+ + 2+ − to UO2 and UO2 , which was converted into the concen- chemically re-oxidized to form UO2 by NO3 in the 0.1 M tration ratio and plotted as a function of the electrode po- HNO3 [46], as shown in Eq. (5); tential. The concentration ratio of U−Oax free complexes, 4+ 3+ − + 2+ such as U , can be calculated from the ratio of the uranyl U(OH) +2NO3 +H 󴀕󴀬2 UO +2NO2 +H2O (5) complexes. Approximately 30% of the uranyl complexes remained at − 0.296 V. However, only 10% of the uranyl Although UO2 was formed in the 0.1 M HClO4 [11], complexes should be present at − 0.384 V, indicating that a black precipitate was not observed during the measure- 90% U4+ was formed, as shown on the Plot 6 in Figure 8. ment.

An increase in the U-OH2 bond peak due to the formation of U4+ complexes was not observed, as shown in curve 6 2+ in Figure 7b. This outcome occured because the formation 3.3 Electrochemical reduction of UO2 in of U4+ hydrolysis species, i.e., U(OH)3+,aresignificant, carbonate media and poly-nuclear hydroxide is not negligible in 0.1 M H+ based on the stability constants [37]. To prepare U(V), in-situ XAFS was performed in 1M

Na2CO3 solution by applying a constant current at A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions | 7

Table 1: EXAFS structural parameters for the uranyl tricarbonato complexes.

U(VI) U(V) Ref S 2 2 2 2 E 𝑁𝑅(Å) 𝜎 (Å ) 𝑁𝑅(Å) 𝜎 (Å ) N A X

U−Oax 2 1.78 0.0019 2 1.88 0.0023 d

e -edge XANES 1.80 0.005 1.90 0.007 [15] z Fig. 10: 𝐿 III i l

a spectra of uranium ions in 1M 1.81 0.0015 1.91 0.0023 [16] m r Na2CO3 before (solid line) U−O 6 2.42 0.0070 6 2.53 0.0103 o

N and after (dotted line) the 2.43 0.014 2.50 0.017 [15] electrolysis. Five scans were 2.44 0.0059 2.50 0.0070 [16] averaged. A platinum wire U−C 3 2.90 0.0017 3 2.97 0.0031 was used as the working 2.89 0.007 2.94 0.011 [15] 17.13 17.18 17.23 electrode. The total concen- 2.92 0.0030 2.93 0.0030 [16] E (keV) tration of uranium was 0.05 M. −1 Errors in distances 𝑅 are ±0.02 Å. A 𝑘 range of 2.5–12.5 Å was used.

−0.1mA for 19 000 s. For this experiment, a platinum + wire electrode was used as the working electrode. Ac- is different from that of UO2 shown in curve 2. It has 2+ + cording to the chronopotentiogram by constant current been reported that UO2 and UO2 carbonato complexes electrolysis, the electrode potential was −0.96Vvs. SSE form bidentate-coordinated tricarbonato complexes i.e., 4− 5− after 4100 s. This rate is more than the 120 mV nega- [UO2(CO3)3] and [UO2(CO3)3] , respectively. The 2+ + 4− tive of the standard potential of the UO2 /UO2 cou- known crystal structure of [UO2(CO3)3] [34]wasem- ple in the carbonate solution (−0.492 V vs. NHE [19]), ployed for the curve fitting analysis. The coordination which means that over 99% of the total concentration numbers were assumed as U−Oax 𝑁(U−Oax ) =2, U−O for + 2− 2− of uranium was reduced to form UO2 based on the CO3 𝑁(U−O) =6,andU−C for CO3 𝑁(U−C) =3. The bond Nernst equation. Figure 10 shows the XANES spectra be- distances of , for 2− ,and U−Oax 𝑅(U−Oax) U−O CO3 𝑅(U−O) 2− 2+ fore and after electrolysis. The edge jump slightly shifted U−C for CO3 𝑅(U−C) for UO2 complexes could then from 17.164 to 17.163 keV (Figure 10), and the spectra be obtained as 1.78, 2.42,and2.90 Å, respectively. Those were similar to those reported previously [15, 16]. The so- value were significantly shorter than the values for the lution was light yellow before the electrolysis and col- + complex ( , and UO2 𝑅U−Oax =1.88 𝑅U−O =2.53 𝑅U−C = orless and transparent afterwards, which is typical of 2.97.SeeTable1)[15, 16, 47]. The fits are shown as dashed + 3 UO2 .The𝑘 weighted EXAFS spectra for the uranyl lines in Figure 11a and b, and these results agree with carbonato complexes that were obtained before and af- the previous reports [15, 16]. The electrode potential of + ter electrolysis and their corresponding FT are shown −0.95VwashighenoughtoformUO2 complexes (see in(a)and(b)ofFigure11. The EXAFS oscillation pat- Figures S1 and S2) because the FT peaks corresponding to 2+ tern of UO2 is shown in curve 1 of Figure 11a, and it U−Oax and U−O were almost constant. The FT curves of 3 m k r o f n a r T

r e i

2 4 6 8 10 12 r 3 u Fig. 11: U𝐿III-edge 𝑘 -weighted EXAFS -1 o k ( ) F data (a) and the corresponding Fourier transforms (b) taken over −1 𝑘=2.5–12.5 Å in 1MNa2CO3 before (curve 1) and after (curve 2) electrolysis. 012345 Solid line: experimental data. Dashed R ) line: theoretical fit. 8 | A. Uehara et al., An in-situ XAS study of the electroreduction of uranium ions

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