Surface electrochemistry of UO2 in dilute alkaline hydrogen peroxide solutions: Part II. Effects of carbonate ions

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<ul><li><p>Electrochimica Acta 51 (2006) 32783286</p><p>Surface electrochemistry of UO2hydrogen peroxide sol</p><p>onShoichmoer 20</p><p>r 2005</p><p>Abstract</p><p>The elect um dinfluenced b pecieallowing H2 -freecarbonate io . Ovecatalyzed by e H2solutions tha the diOH. 2005 Elsevier Ltd. All rights reserved.</p><p>Keywords: Uranium dioxide; Hydrogen peroxide reduction; Mechanism; Corrosion; Nuclear waste disposal</p><p>1. Introdu</p><p>The devcorrosion rlong-termof containewith grounredox condthat failureand gammatime, onlyof the -raconcern. Aexerts the g</p><p>Our preistry of UOwas found</p><p> CorresponE-mail ad</p><p>dwshoesm@u1 ISE mem</p><p>0013-4686/$doi:10.1016/jction</p><p>elopment of models to predict nuclear fuel (UO2)ates is a primary requirement in the assessment ofnuclear waste disposal scenarios [1]. In the eventr failure, the fuel surface could come into contact</p><p>dwater, and the corrosion rate would be controlled byitions in the disposal vault. It is reasonable to assumeshould not occur during the time period when betaradiation fields are high (</p></li><li><p>J.S. Goldik et al. / Electrochimica Acta 51 (2006) 32783286 3279</p><p>More recently, de Pablo et al. [10], have shown that, incarbonate-free solution at pH 6, the H2O2 consumption ratewas greateto the form[(UVIO2)8Ooxides coumay also acconsumptiothat the surtion, consis[7,8]. Ekerthe H2O2 cto solution.diate, was</p><p>UO2 + H2Ofollowed b</p><p>2UO2(surf)+</p><p>or</p><p>UO2(surf)+</p><p>All of thesanodic disscircuit corUO2 dissotic interprehave studiecal means.develop arated into afuel dissolu</p><p>2. Experim</p><p>A standain all experas the referThe counte5 cm2) sporeference afrom the mLuggin capin the elect</p><p>A Solariments. Th(supplied bspectroscoppled withlyzer. A 10data accum102 Hz. Zand to perfat steady sdata pointsin a groun</p><p>current interrupt method was used to compensate for the ohmicpotential drop, which occurs mainly in the (SIMFUEL) working</p><p>de, dity hFU</p><p>c Enmpoed istudhich</p><p>des whe Ssed wsionde fus exes abas</p><p>Thea2COutionarbo</p><p>frousinpH/aterOC gel din ofy Fiseachinedains</p><p>ults</p><p>IMF</p><p>. 1 coEL.1 mhe v</p><p>are a</p><p>(UOEL</p><p>of therven ren; thnglyxidation pcathn thets (Ulutioed cr than the UO2 dissolution rate. This was attributedation of surface oxides on UO2 (possibly schoepite,</p><p>2(OH)12]12H2O). The authors suggested that theseld catalyze the H2O2 decomposition process, whichcount for some of the difference in the rates of H2O2n and UO2 dissolution. The same authors concludedface oxide phase was not formed in carbonate solu-tent with the XPS evidence of Shoesmith and Sunderoth and Jonsson [11] found that HCO3 increasedonsumption rate as well as the UO22+ release rateA Fenton-like mechanism, involving a UV interme-</p><p>proposed for the reaction between H2O2 and UO2:</p><p>2 UO2(surf)+ + OH + OH</p><p>y either</p><p> UO22+ + UO2</p><p>+ OH UO22+ + OH</p><p>e previous studies dealt with either the rate of theolution reaction, or with the kinetics of the open-</p><p>rosion process, and while these revealed how thelution process depends on H2O2, a full mechanis-tation has not yet been attempted. In this work, wed the cathodic reduction of H2O2 by electrochemi-A primary rationale for studying this reaction is to</p><p>currentpotential relationship that may be incorpo-mixed potential model to predict the rate of nucleartion under disposal conditions [12].</p><p>ental</p><p>rd three-electrode, three-compartment cell was usediments. A saturated calomel electrode (sce) was usedence, and all potentials are reported against this scale.r electrode was a platinum sheet (approximately</p><p>t-welded to a 15 cm platinum wire (Alfa Aesar). Thend counter electrode compartments were separatedain compartment of the cell by sintered glass frits. Aillary was used to minimize the ohmic potential droprolyte solution.tron model 1287 potentiostat was used in all exper-e data were analyzed using CorrwareTM softwarey Scribner Associates). Electrochemical impedancey (eis) was performed using the above setup cou-</p><p>a Solatron model 1255B frequency response ana-mV sinusoidal potential waveform was applied, andulated as a function of frequency from 100 kHz toplotTM software was used to collect the eis resultsorm equivalent circuit fitting. That the system wastate was checked by recording a small number ofon a reverse frequency scan. The cell was housed</p><p>ded Faraday cage to minimize external noise. The</p><p>electroresistiv</p><p>SIMAtomiThe codescribin thisvalue welectroeter. Tand rinimmerelectroPrevioproduc</p><p>Theicals).with Ning soltotal crangedfirmedTriodepore wgas (Bto expduratioplied bbeforedetermtion ag</p><p>3. Res</p><p>3.1. S</p><p>FigSIMFU(b) a 0tion. T0 mVoxideSIMFUlutionare obssolutiosolutioto stroface oreducton theished ideposifree sodissolvue to its small resistivity. Previously, the electrodeas been measured to be 60 cm [3].</p><p>EL electrodes were cut from pellets fabricated byergy of Canada Limited (Chalk River, Ont., Canada).sition and microstructure of this material has been</p><p>n previous publications [13,14]. The SIMFUEL usedy replicates UO2 fuel taken to 1.5 at.% burn-up, awould be a little low for real used CANDU fuel. Theere approximately 3 mm thick and 1.18 cm in diam-</p><p>IMFUEL was polished on wet 1200 grit SiC paperith methanol and deionized water prior to use. Upon</p><p>in the cell, a potential of 1.6 V was applied to theor 10 min in order to remove any air-formed oxides.periments and XPS evidence confirm that this stepclean surface [15].</p><p>e electrolyte used was 0.1 mol L1 NaCl (ACP chem-carbonate/bicarbonate concentrations were adjusted</p><p>3 and NaHCO3 (Merck). The pH values of the result-s were all 9.7 0.2, unless stated otherwise, and the</p><p>nate concentration ([CO3]tot = [CO32] + [HCO3])m 103 to 0.2 mol L1. The solution pH was con-g an Orion 250A+ pH meter with an Orion 91-07</p><p>ATC probe. The solutions were prepared from Milli-( = 18.2 M cm), and were purged with UHP argonases) for at least 20 min prior to experiments in orderssolved O2. The Ar purging was continued for theall experiments. Hydrogen peroxide (3% w/v, sup-her Scientific) was introduced to the cell immediatelyexperiment. The H2O2 concentration in the cell wasupon completion of each experiment by redox titra-</p><p>t standardized KMnO4 (Aldrich).</p><p>UEL electrochemistry</p><p>mpares cyclic voltammograms obtained on a 1.5 at.%electrode in (a) a 0.1 mol L1 NaCl solution, andol L1 NaCl + 0.1 mol L1 Na2CO3/NaHCO3 solu-ery small anodic currents for potentials 200 tottributed to surface oxidation to a mixed UIV/UV</p><p>2+x), consistent with our previous observations on[3]. Large anodic currents for the oxidative disso-is UO2+x layer to soluble uranyl (UO22+) speciesd at higher potentials (&gt;300 mV). This oxidative dis-action is clearly accelerated in carbonate containingis is indicative of the ability of HCO3/CO32 ionscomplex UO22+. In the absence of carbonate, sur-ion to UO2+x on the anodic scan produces a smalleak (A) in the potential range 600 to 900 mVodic scan. This reduction peak is noticeably dimin-carbonate solution. The formation of hydrated UVI</p><p>O3yH2O), which has been confirmed in carbonate-ns [3], is inhibited when the electrolyte containsarbonate. The large increase in cathodic current for</p></li><li><p>3280 J.S. Goldik et al. / Electrochimica Acta 51 (2006) 32783286</p><p>Fig. 1. Cyclic voltammograms of 1.5 at.% SIMFUEL in 0.1 mol L1 NaCl(dashed line), and 0.1 mol L1 NaCl + 0.1 mol L1 Na2CO3/NaHCO3 (solidline), both solutions at pH 9.7. = 16.7 Hz; = 10 mV s1.</p><p>potentials negative to 1.0 V is due to H2O reduction, a processbelieved to be catalyzed by the metallic -phase in the SIMFUEL[14].</p><p>3.2. H2O2</p><p>Fig. 2 sh8 103 mand (b) 0.1the supportined the cyalkaline Naon the cathon the anodon the forwUO22+ from</p><p>Fig. 2. CyclicH2O2 solutio0.1 mol L1 Nat pH 9.7. Thelated from Eq</p><p>Fig. 3. Cycliclimits in a 0.1with [H2O2] = = 16.7 Hz; </p><p>the currentrevived uto 900 mV</p><p>influn th0 m</p><p>tentiticalress</p><p>nFc</p><p>n iss (nH2O</p><p>.71 ution</p><p>10de (ans expression [19]:reduction on SIMFUEL</p><p>ows voltammograms on a SIMFUEL electrode in anol L1 H2O2 solution, with (a) 0.1 mol L1 NaCl,mol L1 NaCl + 0.1 mol L1 Na2CO3/NaHCO3 as</p><p>ing electrolyte. In our previous study [3], we exam-clic voltammetric behaviour of SIMFUEL in slightlyCl solutions containing H2O2. The currents observedodic scan are suppressed relative to those recordedic scan, Fig. 2. This was attributed to the formationard scan of a UO3yH2O layer by precipitation of</p><p>solution. This layer has insulating properties and</p><p>clearlyrents oto 20for potheorethe exp</p><p>jL = whereprocestion of(D = 1the solof 1.0 electroNewmvoltammograms of 1.5 at.% SIMFUEL in an 8 103 mol L1n. The electrolytes were 0.1 mol L1 NaCl (dashed line) andaCl + 0.1 mol L1 Na2CO3/NaHCO3 (solid line), both solutionsdotted line shows the theoretical diffusion limited current calcu-</p><p>. (1). = 16.7 Hz; = 10 mV s1.</p><p> =1 + 0.</p><p>where Sc isreduction csolution, ththe current</p><p>In the pin Fig. 2) tcathodic scthe carbonaanodic limi</p><p>1 The equilbe significantcoefficient iscarbonate solvoltammograms of 1.5 at.% SIMFUEL taken to different anodicmol L1 NaCl + 0.1 mol L1 Na2CO3/NaHCO3 solution (pH 9.7)8 103 mol L1. The curves are offset by 2 mA cm2 for clarity.= 10 mV s1. The markers (+) indicate the zero line for each plot.</p><p>for electron transfer to H2O2 is blocked, and is notntil this layer is reduced in the potential range 600</p><p>(region A in Fig. 1). The presence of carbonate ionsences the shape of the currentpotential profile. Cur-</p><p>e anodic scan are suppressed for potentials cathodicV, while those on the cathodic scan are suppressedals negative to 700 mV. The dotted line shows thediffusion-limited current value (jL), calculated usingion [16]:</p><p>bD2/3v1/61/2 (1)</p><p>the number of electrons transferred in the reduction= 2), F the Faraday constant, cb the bulk concentra-</p><p>2 in mol cm3, D the diffusion coefficient of H2O2105 cm2 s1 [17]1), v the kinematic viscosity of(assumed to be the same for both solutions; a value</p><p>2 cm2 s1 was used), the rotation frequency of the= 16.7 Hz) and is a numerical coefficient given by1.55532980(Sc)1/3 + 0.14514(Sc)2/3 (2)</p><p>the Schmidt number (Sc = v/D). While the peroxideurrent approaches the diffusion-limited value in NaCle presence of carbonate ions causes a suppression ofbelow the theoretical diffusion-limited value.otential region 200 to +100 mV (labeled region Bhere is a modest enhancement of the current on thean in the carbonate-containing solution compared tote-free solution. Fig. 3 shows the effect of varying thet (Ean) on the voltammetric scan. The catalytic effect</p><p>ibrium H2O2 + HCO3 = HCO4 + H2O has been determined toat 25 C [18]. How this affects the value of the effective diffusion</p><p>not currently known. Furthermore, the higher ionic strength of theution should also lower the diffusion coefficient of H2O2.</p></li><li><p>J.S. Goldik et al. / Electrochimica Acta 51 (2006) 32783286 3281</p><p>Fig. 4. Leviccyclic voltamreverse scan;[H2O2] = 2 </p><p>in region Brecorded indent on botFig. 4 showat 800 anand reversecurrents artransport cregion. Thesuggests thsignificantl</p><p>In ordercatalytic efwas subtraenables usthe cathodiboth the ancurrent diff(mV s1). Bsubtractionsus 1/2 is lBoth thesetrochemicaB, obtainedwhich jr ison the bulktotal carbonconfirms thest carbonaproportionaate concentelectrode (icontain thein region BUO2 latticelished resu</p><p>Plots of the residual current for H2O2 reduction (jr jf) in region Bhe applied potential on a 1.5 at.% SIMFUEL electrode in a 0.1 mol L1.1 mol L1 Na2CO3/NaHCO3 solution. The potential scan rates () are:V s1; () 25 mV s1; () 20 mV s1; () 15 mV s1; (+) 10 mV s1;</p><p>V s1. [H2O2] = 8 103 mol L1 and = 16.7 Hz.h-type plots for H2O2 reduction currents recorded during ametric experiment: () 800 mV, forward scan; () 800 mV,</p><p>() 200 mV, forward scan; () 200 mV, reverse scan.103 mol L1.</p><p>is only observed when Ean is &gt;100 mV. The currentsregion B on the reverse scan are only weakly depen-</p><p>h potential and the rotation frequency of the electrode.s a Levich plot (j versus 1/2) for currents recordedd 200 mV (within region B) on both the forwardpotential scans. For the more negative potential, the</p><p>e clearly rotation rate dependent, consistent with aontribution to the kinetics of H2O2 reduction in this</p><p>very weak current dependence on at 200 mVat the reduction process occurring in region B is noty influenced by gain a better understanding of the nature of the</p><p>fect in region B, the current on the anodic scan (jf)cted from the current on the cathodic scan (jr). Thisto separate the catalytic process, observed only onc scan, from the non-catalyzed process, observed onodic and cathodic scans. Fig. 5 shows plots of this</p><p>Fig. 5.against tNaCl + 0() 30 m() 5 merence (jr jf) for a series of potential scan rates, oth the peak potential (EP) and the peak current after(jP) depend on . Fig. 6 shows that the plot of jP ver-inear, while EP is linear when plotted against log().dependencies are diagnostic for an irreversible elec-l process [16]. The integrated charge (Q) in region</p><p>by integrating (jr jf) over the potential range forgreater than (as an absolute value) jf, is dependentH2O2 concentration. This is shown in Fig. 7 for</p><p>ate concentrations of 0.1, 0.13 and 0.2 mol L1, andat the peak involves H2O2 reduction. For the low-te concentration the total reduction charge is directlyl to [H2O2]. Evidently, Q is smaller at higher carbon-rations. Experiments performed on a split SIMFUELdentical to 1.5 at.% SIMFUEL except that it does notnoble metal -phase) also showed the catalytic effect, confirming that this catalytic effect is related to the</p><p>and is not associated with the -particles [unpub-lts]. A more extensive study of the H2O2 reduction</p><p>Fig. 6. Plots of the peak current (jP) and peak potential (EP) from Fig. 5 asfunctions of the potential scan rate ().</p></li><li><p>3282 J.S. Goldik et al. / Electrochimica Acta 51 (2006) 32783286</p><p>Fig. 7. Plots of the integrated charge (Q) in region B against the hydrogen perox-ide concentration. Values of [CO3]tot are: () 0.1 mol L1; () 0.13 mol L1;() 0.2 mol L1.</p><p>reaction on SIMFUELs with different dopant concentrations isunderway.</p><p>Fig. 8 shows a cyclic voltammogram for the reduction ofH2O2 in which the initial cathodic scan (immediately follow-ing the anodic scan to +300 mV) is reversed in direction at300 mV (region B).does not focally formereduction ithan 300B). As show1200 mVsecond sca</p><p>Fig. 8. Cyc2 103 molNaCl + 0.1 moinset. The dot(to 300 mV300 mV).</p><p>Fig. 9. Tafel-solutions of diThe values o() 2 1022 101 mol</p><p>Fig. 9 sha functiondeterminedto Fig. 2) b</p><p>Kout</p><p>1jkjL i</p><p>anne</p><p>mitet ats eximmove</p><p>ur prials 0.3 V thenhanced by carbonate, consistent with the catalytic</p><p>ribed above.</p><p>s of pH</p><p>ograms for the reduction of H2O2 on SIMFUEL inolutions of pH 8.6 and 13.0 are compared in Fig. 10.reduction wave is shifted in the cathodic directionrease in pH. The magnitude of this shift is approxi-V/pH for j = 0 on the forward scan (points C and D</p><p>Fig. 11 shows a series of Tafel plots for H2O2 reduc-erent pH values. The kinetic currents were obtainedmanner as in Fig. 9. Between pH 8.6 and 10.6, thereincrease in the cathodic current with increasing pH.</p><p>10.6 and 11.4, the H2O2 reduction rates show verypendence. At pH 13.0, there is a significant inhibition2 reduction currents.</p></li><li><p>J.S. Goldik et al. / Electrochimica Acta 5...</p></li></ul>


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