A review of chromium(VI) use in chlorate electrolysis: Functions, challenges and suggested alternatives
Balázs Endr Å di
a,b, Nina Simic
c, Mats Wildlock
c, Ann Cornell
a,*
aAppliedElectrochemistry,SchoolofChemicalScienceandEngineering,KTHRoyalInstituteofTechnology,SE100-44Stockholm,Sweden
bDepartmentofPhysicalChemistryandMaterialsScience,UniversityofSzeged,H-6720Szeged,Hungary
cAkzoNobelPulpandPerformanceChemicals,SE445-80Bohus,Sweden
ARTICLE INFO
Articlehistory:
Received16December2016
Receivedinrevisedform24February2017 Accepted26February2017
Availableonline28February2017
Keywords:
chlorateelectrolysis cathodeselectivity hydrogenevolutionreaction chromate
oxygentolerantcathode
ABSTRACT
Sodiumchlorateisindustriallyproducedbyelectrolysisofanaqueoussaltsolution,inwhichchromium (VI) constitutesanimportantexcipient component. Itisaddedtoaconcentrationof afewgrams Na2Cr2O7/litertotheelectrolyteandhasseveralfunctionsintheprocess,themostimportantbeingto increasetheFaradaicefficiencyforhydrogenevolutionintheundividedelectrochemicalcells.Athinfilm ofCr(OH)3nH2Oformedbyreductivedepositiononthecathodesdecreasestherateofunwantedside reactions, while still enabling hydrogen evolution to occur. In addition chromium(VI) buffers the electrolyteattheoptimumpHforoperationandpromotesthedesiredhomogeneousreactionsinthe electrolytebulk.Chromium species also affecttherates of hydrogenand oxygenevolutionat the electrodesandaresaidtoprotectthesteelcathodesfromcorrosion.
Althoughchromium(VI)staysinaclosedloopduringchlorateproduction,chromateisahighlytoxic compoundandnewREACHlegislationthereforeintendstophaseoutitsuseinEuropefrom2017.A productionwithoutchromium(VI),withnootherprocessmodificationsisnotpossible,andtodaythere arenocommerciallyavailablealternativestoitsaddition.Thus,thereisanurgentneedforEuropean chlorateproducerstofindsolutionstothisproblem.Itisexpectedthatchromium-freeproductionwillbe arequirementalsoinotherpartsoftheworld,followingtheEuropeanexample.
Asthechromium(VI)additionaffectsthechlorateprocessinmanywaysitsreplacementmightrequire acombinationofsolutionstargetingeachfunctionseparately.Theaimofthispaperistoexplaintherole andimportanceofchromium(VI)inthechloratemanufacturingprocess.Previousachievementsinits replacementaresummarizedandcriticallyevaluatedtoexposethecurrentstateofthefield,andto highlightthemostpromisingavenuestobefollowed.Anattemptisalsomadetorevealconnectionswith otherresearchfields(e.g.photochemicalwatersplitting,corrosionscience)facingsimilarproblems.
Alliedeffortofthesedifferentcommunitiesisexpectedtoopenupresearchavenuestothemutual benefitofthesefields.
©2017ElsevierLtd.Allrightsreserved.
1.Introduction
Sodiumchlorateisabulkchemicalproducedbyelectrolysisof sodiumchloridesolutions[1,2].Itsindustrialmanufacturedates backtothe19thcenturyandhasovertheyearsevolvedtobeoneof the major electrochemical industries behind chlor-alkali and aluminaproduction[3].Thetotalannualproductionofmerchant chloratewas3.6milliontonsin2015,whereof3.2wasdirectedfor productionofchlorinedioxideusedin environmentallyfriendly Elemental Chlorine Free (ECF) bleaching of pulp [4]. The ECF
bleachingiscurrentlyregardedastheBestAvailableTechnique[5]
andisthereforethepreferredchoicefornewbleachedchemical pulpproducingplants.Thisdirectlyinfluencestheneedforsodium chlorateonthemarket.Theworldwidesodiumchloratedemand hasincreasedwithover30%overthelast10–15years(Fig.1)asthe consumption of bleached paper and board has increased. The growthisexpectedtocontinueastheuseoftissue(kitchenrolls, bathroomtissueetc.)andpackagingboardincreases.
Intheearlydayslargeamountsofcalcium(II)saltswereadded totheelectrolytetoachieveacceptablecurrentefficiency[6,7].In theend of the 19thand beginningof the 20thcenturyseveral researchgroupsworkedwiththechlorateprocessandatthattime itwasdiscoveredthattheadditionofmultivalentmetalionscould enhance the current efficiency [8]. Especially chromium was
*Correspondingauthor.
E-mailaddress:amco@kth.se(A.Cornell).
http://dx.doi.org/10.1016/j.electacta.2017.02.150 0013-4686/©2017ElsevierLtd.Allrightsreserved.
ContentslistsavailableatScienceDirect
Electrochimica Acta
j o u r n a lh o m e p ag e : w w w . e l s e vi e r . c o m / l o c a t e / e l e c t a c t a
mentionedand theuseof chromatewas patentedin 1897byJ.
Landin[9].AlmostatthesametimeImhoffwasgrantedasimilar patentinGermany,1898,andin theUnitedStates,1899[10].It shouldalsobenotedthatBischoffandFoersterproposedatheory forthefunctionofcalciumsaltsin1898describingtheformationof adiaphragm,whichalsobecamethemainexplanationfortherole ofchromate[11].Theinventionwasdescribedin1899byE.Müller as“theelectrolysiswithchromatepresentrepresentsoneofthe mostidealdiaphragmprocessesonecanimagine”[12].Compared tothepreviouscalciumsaltbasedelectrolytecurrentefficiencies wereimprovedfrom40%toalmost90%[7].
Inchloratemanufacturethechromium(VI)substanceismost often referred toas sodium dichromate (Na2Cr2O7)but in the electrolytethetypeofchromiumsubstancevaryduetochemical andelectrochemicalreactionsandsolutionpHandcanbefoundas forexampleH2CrO4,HCrO4,Cr2O72,CrO42andCrO3Cl.Forthe convenienceofreadingallchromium(VI)compoundswillinthis paper be referred to as chromium(VI) except when citing references.
Theuseofchromium(VI) insodiumchlorateproduction has been accepted as Best Available Technique by the European Commission[1].Besidesallthetechnicalbenefits;chromium(VI) substanceshaveforalongtimebeenknowntopossesshazardous properties and are classified as toxic, corrosive, oxidizing and dangeroustotheenvironment[13].Inadditiontothisthereisalso aconcernforlong-termeffectsonhumanhealthbyexposureof chromium(VI)duetoitscarcinogenic,mutagenicandreprotoxic (CMR) classifications. Therefore chromium(VI) substances have beenidentified as“substances of veryhighconcern”[14,15].In 2013sodiumdichromatewas included inAnnex XIV of REACH meaningthatanauthorizationmustbegrantedbytheEuropean commissionforcontinuedindustrialuseafterthesunsetdate21/
09/2017[16].AsoneofmanyindustriescoveredbyAnnexXIV(e.g.
metalfinishing),chlorateproductionwasidentifiedandtherefore thereisanurgentneedtofindasolutiontocircumventtheuseof chromium(VI)intheprocess[13].However,arecentreportofan independent third-party organization issued by the chlorate industrystatesthattheuniquepropertiesofchromium(VI)serve thechlorateprocessinseveralimportantways,whichmakesits replacementverydifficult[16].Thesocio-economicimpactofa completestopofusingsodiumdichromatein theproductionof sodium chlorate and the consequences on the European pulp industryaredescribedingeneraltermsinthepublicversionsof Socio-Economic Analysis provided to the European Chemicals Agency(ECHA)bytheproducersofsodiumchlorate[17–20].Asall these documents conclude there is no currently available technologywhich couldreplace theuse of chromium(VI). It is
therefore predictedin thesestudies thattheEuropeanchlorate plantsofthesecompaniesmustbecloseddownifanauthorization isnotgrantedforthecontinueduseofchromium(VI).
Theaimofthispaperistoexplainthemultifunctionalrolethat chromium(VI)hasinthechloratemanufacturingprocessandto showthecomplexityin findingits replacement,topresent and discussalternatives tochromium(VI)suggestedin theliterature and topoint atdirections forfuture researchanddevelopment towardsachromium(VI)-freechlorateprocess.
2.Thechlorateprocess
Electrochemical production of sodium chlorate is an energy intensiveprocess,as5–6000kWhenergyisrequiredtoproducea tonofNaClO3[21].Thecoreoftheprocessistheelectrolysiswhich takesplaceinundividedcells.Theelectrolytecompositionandthe processconditionsaresummarizedinTable1.
On the dimensionally stable anodes(DSA)chloride ions are oxidizedtochlorine,whichthendissolvesintheelectrolyteand forms chloratethroughacomplexseriesofreactions(Reactions (1)–(4)).Themostimportantintermediatesarethehypochlorous acid and hypochlorite ion, the sumof which will hereafterbe referredtoashypochlorite.
2Cl!Cl2þ2e ð1Þ
pKh=2.98(T=RT)[22] Cl2+H2OÐHOCl+HCl (2)
pKa=7.0(T=70C)[23] HOClÐClO+H+ (3)
2HOClþClO!ClO3þ2Clþ2Hþ ð4Þ Onthecathode,mostusuallyofmildsteel,waterisreducedto hydrogen gas. Someunwantedreactions, most importantlythe reduction of hypochlorite and chlorate ions can also proceed, lowering the energy efficiency of the process. Their negative impactcanbeminimizedtoanalmostinsignificantlevelbyadding chromium(VI) tothe solution, which is undoubtedly the most beneficialroleofthisadditiveintheprocess.Thechromium(VI)is notconsumedintheprocess.
Asitwillbediscussedthroughoutthemanuscript,theoverall efficiencyofthechloratecellisinfluencedbyanodicandcathodic lossesduring theelectrolysis,as wellas byunwantedhomoge- neousreactions.Reliablecalculationandreportingofthecurrent (and/orenergy)efficiencythereforenecessitateseitherthedirect measurement of the actual chlorate production rate, or the measurementofthecelloff-gases,andthesubsequentcalculation of the chlorate formation rate according to the mass-balance equations[24].
Beyond the electrolysis cell, there are several other unit operations involved in the manufacture of sodium chlorate Fig.1.Theannualglobalsodiumchloratedemandinthe21stcentury.
Datasource:IHSChemicalEconomicsHandbook,2015[4].
Table1
Theappliedconditionsandthesolutioncompositionintheelectrolyticchlorate productionprocess[21].
Cellvoltage(V) 2.9–3.7
Currentdensity(kAm2) 1.5–4
Temperature(C) 65–90
NaClconcentration(gdm3) 70–150
NaClO3concentration(gdm3) 450–650
NaOClconcentration(gdm3) 1–5
Na2Cr2O7concentration(gdm3) 1–6
ElectrolytepH 5.5–7.0
Electrodedistance(mm) 3
whereofthemajorparts aresalt purification andcrystallization (Fig. 2).Environmental, health and economic reasonsrequire a closedloopoperationofthechlorateplants,whichinfluencesall partsof theprocess. Thissecuresa highdegreeof recyclingof chromium(VI)whichminimizes itsoutput.Different techniques for recyclingthe chromiumare described in literature (see for exampletheCyclochrome processinRef.[25]).Theclosedloop operationhoweverresultsin anaccumulationoftheimpurities thatentertheprocessviavariousrawmaterials.Hencethebrine purification is an important and significant part of a chlorate productionplant[26].
3.Thecomplexandmanifoldroleofthechromium(VI)additive inthechlorateprocess
Theadditionofchromium(VI)influencestheprocessinmany differentways,almostexclusivelytoitsbenefit.Inthissectionthe reportedeffectsonthereactionsatthecathodes,intheelectrolyte bulkandat theanodesaredescribedtoprovidea basisforthe searchforalternativesubstances.
3.1.Filmformationanditseffectontheselectivityofthecathode process
Themostimportanteffectofthechromium(VI)additiveisthat itincreasesthecathodiccurrentefficiencyofthechlorateprocess byhinderingthereductionofoxy-chlorinespeciesashypochlorite andchlorateions.Duringelectrolysischromium(VI)isreducedto Cr(III)onthecathodeandformsafilmofchromium(III)hydroxide accordingtoEq.(5).
CrO24 þð4þnÞH2Oþ3e!CrðOHÞ3nH2Oþ5OH ð5Þ The film has poor electrical conductivity, contains high amountsof waterand its compositionhasbeen determinedas Cr(OH)3nH2O [27]. It hinders different cathodic reactions, amongthesethereductionofCr(VI)andtherebyitsowngrowth, so after a period of electrolysis in a chromium(VI) containing electrolyteitreachesafinalthickness[28,29].Thefilmisthin,less than10nmwhenformedin1MNaOH,anditsthicknessdepends on the substrate material, the electrolyte composition, the chromium(VI)concentrationandthedegreeofcathodicpolariza- tion [29]. Experiments where a chromium hydroxide film was formedex-situandtheelectrodethentransferredtoachromium (VI)freesolutionshowsimilarresultsasfor thein-situformed films.Thusitisthefilmitselfandnotthedissolvedchromium(VI) thataccountsforthehinderingeffects[30–33].
Twodifferentmechanismshavebeenproposedtoexplainthis effect.Accordingtooneofthehypotheseschromium(VI)additions result in a porous diaphragm on the cathode surface, which negatively charged ions cannot penetrate due to the adverse potential gradient [34,35]. The other hypothesis explained the hinderingeffectbytheformationofathinfilmhinderingcertain reactions irrespective of the charge of the involved species [32,36,37].Laterresultsfromfurthertheoreticalandexperimental studies [28,38] supported this second hypothesis and also convincingly showed that the theory of the adverse potential gradient cannot be correct. The detailed mechanism through whichthefilmhinderscertaincathodicreactionsisstillnotknown but may be found in the properties of selective transport of differentspeciesthroughthefilm.
OClþH2Oþ2e!Clþ2OH ð6Þ
ClO3þ3H2Oþ6e!Clþ6OH ð7Þ
Intheabsenceofchromium(VI)inthesolutionthetwomajor cathodic loss reactions are the reduction of hypochlorite and chlorate ions, Reactions (6) and (7). While Reaction (6) is controlled by mass transport [39], Reaction (7) is kinetically controlledanditsratedependsonthecatalyticpropertiesofthe cathodematerial[40].Chloratereductionhasbeenshowntobe veryfastoncertainmaterialssuchassomeoxides/hydroxidesof iron[40]andruthenium[41],whereasitisslowoncobalt,nickel, molybdenum,titanium,mercuryandcarbon[40].Asteelcathode coveredbyathick,brownlayerofironoxidesproducednovisible hydrogen gas evolution when electrolyzing at an industrially relevantcurrentdensityinchlorateelectrolytewithnochromium (VI)addition[35].Similarresultswereobtainedforaruthenium dioxidecathode[41]. Hydrogenevolvedin bothcasesat ahigh currentefficiencyafteradditionofchromium(VI)totheelectrolyte.
Ithasbeenshownthatverylowconcentrationsofchromium (VI), inthe micromolarrange,is sufficient toforma protective cathode film that hinders Reaction (6) on smooth steel and titanium [42]. Commercial sodium chlorate crystals typically contain up to8ppm Cr,a concentrationthat will lead toover 90
m
Mchromium(VI)intheelectrolyteifpreparinga600gdm3 NaClO3-solution.Whenstudying alternativesolutions tohinder Reactions (6) and (7) it is therefore important to analyze the chromiumcontentofthechloratesaltand,ifnecessary,purifythe crystals by e.g. recrystallization. On the other hand, on rough, catalyticsurfacesmuchhigherconcentrationsareneededtoforma protectivefilm.Corrodedsteelcathodesrequirehigherchromium Fig.2. Schematicmodelofthemainstepsofthechlorateprocessincludingthemostimportantinputsandoutputsaswellastheinternalloops.(VI)concentrations compared tosmooth,non-corroded steelto obtainahighcurrentefficiencyinchlorateelectrolyte[43].
In addition to Reactions (6) and (7) the chromium oxide/
hydroxidefilmhinders alsothecathodicreductionof dissolved molecular oxygen [28,44], nitrate and nitrite ions [45] and hexacyanoferrate(III)[28]. Lindbergh et al. concluded that the suppressionofthecathodicreactionwasmuchmoreefficientfor reactions involvingadsorbed intermediates,asthereduction of hypochloriteandoxygen,thanforsimpleouter-spherereactions, suchasthereductionofhexacyanoferrate(III)[28].Kolthoffetal.
studiedseveraldifferentcathodicaswellasanodicreactionsona pre-filmed electrode [32]. The reduction of Fe(III) and the oxidation of Fe(II)wereclearly hindered,whereas reduction of Ag(I)andofTl(I)couldeasilyproceedinthepresenceofthefilm.
Yoshidaetal.concludedthatpre-filmedplatinumelectrodeswere activeforthehydrogenoxidationreaction(HOR)[46].
Thechromiumhydroxidefilmhasrecentlyfoundanapplication inphotocatalyticwatersplitting,whereH2andO2areformedon catalyticnanoparticles[47].Bycoatingthenoblemetalco-catalyst with a Cr(III) oxide/hydroxide layer (Fig. 3), the O2 reduction reactionishindered.Thissignificantlyincreasestheefficiencyof thewatersplittingprocessasthebackwardreactionofH2andO2
formingH2Oisinhibited[46–48].Thechromiumcontaininglayer wasproducedbyphotodepositioninaK2CrO4solutionandwas foundtobeafewnminthickness[47].Themajorityofthepapers mentionedabovesuggestthatselectivepermeabilityofdifferent speciesthroughtheCr(III)oxide/hydroxidefilmisthereasonfor theincreasedcathodiccurrentefficiencyinitspresence.
3.2.Theeffectoftheformedfilmonthekineticsofthehydrogen evolutionreaction
Asopposedtothecathodicreductionofoxygen,hypochlorite species etc., hydrogen evolution can proceed on an electrode surface covered by the chromium oxide/hydroxide film. The reactionseemstotakeplaceattheinterfacebetweenthefilmand thesubstrate[46,49].Hydrogenionsand/orhydroxideionscan thuspenetratethefilm,whichalsocontainslargeamountsofthe reactant water. The product leaving the active sites is likely
molecular dihydrogen (H2), which diffuses through thefilm to reachthefreeelectrolyte[46].
The effectof thefilm onthekineticsof theelectrochemical hydrogenevolutionreaction(HER)forasteelcathodeinchlorate electrolyteisdifficulttointerpret,asintheabsenceofthefilmthe cathodereactionisamixofhydrogenevolutionandreductionof oxy-chlorinespecies(chlorateandhypochloriteions).Anincrease in electrode potential of about 350mV at a constant current densityobservedforasteelcathodeafteradditionofchromium(VI) toachlorateelectrolytedoesthereforenotrepresentchangesin the HER kinetics [49]. Polarization curves recorded in sodium hydroxideelectrolytesolutionwithandwithouttheadditionof chromium(VI) have shown that thepresence of the chromium hydroxidefilmincreasestherateofhydrogenevolutionforpoor electrocatalystsasAu[49]and Pb[50],whereasitdecreaseson electrocatalyststhataremorefavorableforhydrogenevolution,as rhodium[46],iron[49]andrutheniumdioxide[41].Asseen in Fig.4.,theTafelslopesfortheHERonachromiumhydroxidefilm covered RuO2 or Fe electrode are similar tothose onthe bare cathodes.
TheTafelslopescharacteristicforthetwomaterialsareabout 40mV decade1 of current for RuO2 and 160mV decade1 of currentforFe,irrespectiveofthepresenceofchromium(VI)inthe solution. Thisindicatesthat hydrogen evolution takes placeon active sites of theRuO2 orFe substrate. A parallel shiftof the polarization curves in the presence of chromium(VI) to more negativepotentialsindicatesthattheinsituformedfilmblocksa partoftheactivesites.Resultsfromcyclicvoltammetry[28]and in-situIRAS[46] onPtcoveredwithchromiumhydroxidefilms indicatethat75–80%and80–90%,respectively,ofthePtsitesare stillactiveforprotonreduction.
For Auanactivationof 100–200mVhasbeenobservedafter additionofchromium(VI)toa1MNaOHelectrolyteandtheeffect wasobtainedalsoonpre-filmedelectrodes[49].OnPb,whichisa verypoorHERcatalyst,anoverpotentialdecreaseofover200mV was found[50].Theactivationobserved indicatesthat thefilm must in this casebe activein thehydrogenevolution reaction.
Likelythereductiontakesplaceattheinnermostlayerofthefilm, mostpossiblyonchromiumspeciesasactivesites.
3.3.Theinfluenceofchromium(VI)onthechemicalreactions Beyondtheabovementionedveryimportantroleinincreasing thecathodeselectivity,chromium(VI)alsoplaysasignificantand complex role in the bulk chemistry. To fully understand this
Fig.3. SchematicmodeloftheH2evolutionreactiononcore/shellnoble-metal/
Cr2O3particulatesystemasacocatalystforphotocatalyticoverallwatersplitting [46].
Fig.4. PolarizationcurvesofFeandRuO2electrodesin1MNaOHwithandwithout 4gdm3Na2Cr2O7inthesolution.Rotatingdiscelectrodes,at3000rpmrotation rate.
complexity it is important to elaborate on the actual chlorate formationreactionsandthepropertiesoftheelectrolytesystem.
3.3.1.Thehomogeneousreactionsofhypochlorite
Inthechlorateprocess(asdetailedabove),chlorineisformed fromtheoxidationofchlorideontheanode.Chlorineisrapidly hydrolyzedforminghypochlorousacidandhypochlorite(andHCl, accordingtoEq.(2)).Inasubsequentdisproportionationreaction thehypochloriteandhypochlorousacidformchlorate,asshownby Eq. (4). The chlorate formation is a slow reaction at room temperature but proceeds at a considerable rate at elevated temperatureslike80C.Inweaklyalkalineandacidsolutionsthe reactionorderforthechlorateformationapproaches3[51,52].The reaction rate is pH dependent and its maximum occurs when [HOCl]2[OCl](Fig.5),whichisatpH=pKa0.3,whereKaisthe aciddissociationconstantofhypochlorousacid.ThevalueofpKais alsodependentonthetemperatureandinadilutesolution,at25C itisreportedtobe7.24,whileitequalsto6.98at70C[23].
Thisishowevernottheonlyreactionpathwaythathypochlorite can follow. An alternative reaction is the decomposition of hypochloritewhich leadstounwantedoxygen asbyproduct,as shownbythefollowingreactions:
Inthesolution 2HOCl!O2þ2Clþ2Hþ ð8Þ
Inthesolution 2OCl!O2þ2Cl ð9Þ The decomposition of hypochloriteformingoxygenis a loss reactioncausingbothcurrentefficiencylossesaswellasincreased oxygenconcentrationinthehydrogencontainingcellgaswhichis asafetyissue.Fortunatelythisdecompositionisslowerthanthe
concurrentchlorateformationreactionintheabsenceofcatalysts.
However,itisstronglycatalyzedbyimpuritiesintheelectrolyte suchasCoII>NiII>CuII>>FeIII>MnII,giveninorderof catalyzing effect [53]. It is therefore of outmostimportance tokeep such catalystsoutoftheprocess.Therateofthedecompositionreaction formingoxygenisalsodependentonthepH,anditsmaximumrate occursinthesamepHrangeinwhichchlorateformationhasits optimum. Therefore it was suggested that the same reaction intermediate is involved in theformation of both chlorateand oxygen[23].
3.3.2.Bufferingeffectofchromium(VI)
TheHCrO4/CrO42systemhasitsmaximalbuffercapacityin thepHregion6–6.5,whichcoincidesverywellwiththeoptimalpH of the chlorate formation reaction, see Fig. 5. The acid base equilibriumisdescribedinReaction(10)anditsaciddissociation constant(pKa,Cr)isreportedtobeintherangeof5.8–6.6[54–57].
Hence its presence in the chlorate electrolyte promotes the chlorateformation.
pKa;Cr¼5:97ðT¼75C; I¼5MÞ HCrO4 ÐCrO24þHþ ð10Þ
lgKd¼1:46ðT¼75C; I¼5MÞ 2HCrO4 ÐCr2O27 þH2O ð11Þ Inadditionhydrogenchromateisinequilibriumwithitsdimer dichromateaccordingtothedimerizationReaction(11)ofwhich equilibriumconstantisintherangeoflgKd=1.46–2.2[56–59].In thedistributiondiagramspresentedinFig.6.itcanbeseenthatthe dominant chromium(VI) species in basic solution is chromate, CrO42, while in the pH=3–6 range hydrogen chromate ion, HCrO4andthedimer,dichromateCr2O72dominate.Thehigher thechromium(VI)concentrationthemorerelativeamountofthe dimerisformed.
WithinthecellthepHisaffectedbytheelectrodereactions resultinginlocalpHgradients,asshowninFig.7A.Thebuffering effectofchromium(VI)(Fig.7B)alsoinfluencesthesegradients.We note, that atvery low pH(typicallybelow 1)there is a riskof chlorinedioxideformation fromthehomogeneousreductionof chloratebychloride[60–62].Undernormaloperationconditions no chlorine dioxide is formed in the chlorate cell. However, withoutabufferlikechromium(VI),theelectrolyteinthevicinity oftheanodemayreachaciditieswherechlorinedioxideformation can occur. In chlor-alkalimembrane and diaphragm processes, whichoperate undermuchmoreacidicconditions,thechlorine dioxideformationfromthebyproductchlorateisaknownproblem [63].
At the cathode hydrogen evolution can proceed via the reductionofprotonsorwatermolecules,asdepictedinReactions Fig.5. Initial rate of hypochlorite decomposition, with [HOCl]0=0.0233M at
T=50C,determinedwithUV-visspectroscopy[52].
Fig.6.Chromium(VI)speciationdiagraminfunctionofpHforthetotalconcentrationof(A)4mMand(B)40mM.Theequilibriumconstantsforthedeprotonationof hydrogen-chromateandthedimerizationprocessweretakentopKa=5.97andKd=1.46,asreportedin[57]forthecaseofI=5.00MNaCl,T=75C.
(12) and (13).At intermediatepH, likein thechlorateprocess, Reaction(12)dominatesatlowcurrentdensities.Asthecurrent densityisincreased,andsothepHclosetothecathode,atransition favoringthewaterreduction(Reaction(13))takesplace.Thiscan beobserved asa limiting current densityfor protonreduction, whichdependsonfactorsasmasstransportandbuffercapacityof theelectrolyte.Experimentalandsimulatedpolarizationcurvesfor an iron RDE in chlorate electrolyte at varying dichromate concentrationareshown in Fig.8,and it is clearlyseen that a higher concentration (and hence buffer capacity) results in a higherlimitingcurrentdensityforprotonreduction[64].
2Hþþ2e!H2 ð12Þ
2H2Oþ2e!H2þ2OH ð13Þ
Asa consequence of aboveit is obvious that thepH of the electrolytehasadecisiveroleonboththeelectrochemicalandthe homogeneouschemical reactions. Toachieve maximumcurrent efficiencyandplantsafetythepHoftheelectrolyteiscontinuously monitored and adjusted to a narrow pH range during the operation.Avariationineitherdirectionwillnotonlyinfluence therateofthechlorateformationreaction,itwillalsoaffectother factorssuchasdesorptionofchlorineandhypochlorousacidtothe cellgasandtherateofoxygenevolution[62].
3.3.3.Catalyticeffectofchromium(VI)
Aselucidatedbefore,itiswellknownthatchromium(VI)inthe chlorateprocesspromotesthehomogeneous chlorateformation reaction by buffering the electrolyte in the optimal pH range
[12,35,65–67].However,isthepromotingeffectofchromium(VI)a purebufferingeffectoristhereanadditionalcatalyticeffect?The catalyticeffectofchromium(VI)onthechlorateformationreaction hasbeensuggested[65,68].Ontheotherhandnocatalyticeffect was foundin twootherstudies[23,30].Howevernoneof them wereconductedunderconditionssimilartotheconditionsinthe chlorateprocess.Mostrecentlytheinfluenceofchromium(VI)on thechlorateformationratewasinvestigatedinchlorateelectrolyte and a concentration dependent rate enhancing effect, likely catalytic,wasdemonstrated[68].
Wenoteherethatadecreasedhypochloriteconcentrationin the chlorateelectrolyte would lead toa decreasein the losses related to the unwanted side reactions of hypochlorite. It is thereforeexpected, thatanadditivewhich increasestherateof chlorate formation (without increasing the rate of oxygen formation) could increase the energy efficiencyof the process andhencecouldhaveasignificantcontributiontothereplacement ofchromium(VI).
3.4.Effectofchromium(VI)ontheselectivityoftheanodetowards chlorineformation
Oxygenistodaythemostimportantunwantedby-productin the chlorate process. It is formed in several reactions in the electrolytebulkaswellasontheanode(fordescriptionsofthe reactionsseethereviewbyKarlssonetal.inref[21]).Attheanode oxygen has been suggested to be formed mainly from anodic reactionsinvolvinghypochlorite[69–71],orfromtheoxidationof water[40].Fig.9.showstheanodiccurrentefficiencyforactive chlorineevolution,calculatedbasedonanalysesofoxygeninthe cellgas,asafunctionofhypochloriteconcentration.Itisclearly seenthatthecurrentefficiencydecreaseslinearlywithhypochlo- riteconcentrationandasimilardependencyhasbeenreportedin severalotherstudies[39,69].
The effectof chromate additionon oxygenformation is the subjectofsomecontroversyintheliteratureasanincrease[66],a decrease[62,68]aswellasnoeffect[69]havebeenreported.The deviationmaybeexplainedbythewaythemeasurementswere performed, asthesystem is complex and parametersas anode material,electrolytecompositionandmasstransport conditions caninfluencetheresults.InthestudybyHardeeetal.[69]itwas emphasizedthatanincreaseinoxygenconcentrationinthecellgas in the absence of chromium(VI) can be due to a low cathodic currentefficiencyforhydrogenevolution.Eventhoughtheoxygen levelincreasedinthecellgasintheabsenceofchromium(VI),the volumetricproductionofoxygenremainedthesamevalue.
Threewaysinwhich chromium(VI)caninfluencetheoxygen formationarethroughadsorptiontoactivesitesontheanode,by bufferingthepHandbyloweringtheconcentrationofhypochlorite Fig.7. (A)SchematicpH-profileinthechloratecellduringtheelectrolysis(B)pHtitrationcurvesinsolutionscontaining450gdm3NaClO3and150gdm3NaCl(T=70C, V=100cm3)[35].
Fig.8.Experimental(lineswithsymbols)andsimulated(solidlines)polarization curvesofanironrotatingdiscelectrode(at3000rpmrotationrate),inchlorate electrolytewith3or9gdm3Na2Cr2O7inthesolution,atpH=6.5andT=70C[64].
intheelectrolyte.Anincreasedanodepotentialhasbeenobserved for DSA-type anodes [72–74] and this has been explained by adsorptionof chromium(VI)ionstoactivesites.Experiments at pH=2,where buffereffects arenegligible,resultedin aparallel shiftofthepolarizationcurvethatindicatedadecreaseinactive anode surface area [74]. The adsorption may promote oxygen formingreactions[66]andleadtoashorterlifetimeoftheanodes [72,73].
The pH in the vicinity of the anode surface is more acidic comparedtothatintheelectrolytebulk(Fig.7A).ThispHgradient isinfluencedbythebuffercapacityoftheelectrolyteandthusby the chromium(VI) concentration. Reactions where oxygen is formedfromhypochloriteions,dependonpHwhichdetermines theratioofthereactantsHOCl/ClO[75].
The promoting effect of chromium(VI) on the chlorate formation (Reaction (4)) will lead to a lower hypochlorite concentration in the electrolyte at steady state operating conditions.Astheanodicoxygenformationisdirectlyproportional totheconcentrationofhypochlorite,seeFig.9,anindirecteffectof chromateadditionisthatitlowerstheoxygenproductionthrough thismechanism[68].
Morestudiesareneededtobetterunderstandtheimportance of the different effects of chromium(VI) on the O2 byproduct formation.Inparticularitwouldbevaluabletoanalyzetheoxygen production rate (rather than %O2 in the cell gas) at varying chromium(VI)concentrationshavingaconstantlevelofhypochlo- riteintheelectrolyte.
3.5.Effectofchromium(VI)additiononthecorrosionofthecathode The most frequently used cathode material in the chlorate processis low-carbonsteel. Althoughduring theoperationthe electrode is cathodically protected, its corrosion cannot be overlookedduringoperationalstops(e.g.servicehours,mainte- nance).The chlorate electrolyte is highly corrosive and conse- quentlythe dissolution rate of the steel electrode can befast, leadingtofrequentneed ofelectrode replacement.Further,the corrosionproductscontaminatethechlorateproductandcanlead toshortcircuitsinthenarrowcellgaps.Therateofsteelcorrosion in a chlorate cell is determined by the limiting current for hypochlorite reduction, the cathode reaction in the corrosion process[76].
In studiesregarding thecorrosionofcopper[44],steel [77], aluminum[78]ornickel[79],chromium(VI)iswell-knownofits inhibition properties, which is caused by the formation of a passivation chromium(III) oxide/hydroxide layer on the metal
surface.Similarly,inthecaseofthesteelcathodeinthechlorate process,thefilmprovidingcorrosioninhibitionisaCr(III)oxide/
hydroxidefilm.Aslongasthefilmstaysontheelectrodesurfaceit protectsthesteelfromcorrosionbybeingabarrierforoxidantslike oxygen and hypochlorite.However, duringshut down onopen circuit,thisfilmisoxidizedbyhypochloriteinthesolution[80]
leaving the steel to corrode. Measurements of open circuit potentialsonsteelshowedsimilarvaluesirrespectiveofwhether the electrolyte contained chromium(VI) [67], indicating that chromium(VI)doesnot inhibitcorrosiontoany largedegree.If cathodicprotection is usedduringshut down[62],thecurrent neededtobeapplieddependsontheconcentrationofchromium (VI),whichreducestherateofhypochloritereduction[76].Thusa lower current is sufficient in the presence of chromium(VI) comparedtothechromium-freecase.
In bipolarcells chromium(VI) canreduce thecorrosion rate duringproductionstopsevenatopencircuitconditions.During shutdown,thesteelelectrodestartstocorrode,whileoxychloric species arereduced ontheDSAelectrode. Iftheelectrodes are connected(simulatingthecaseofachloratecellsetupwithbipolar electrodes), the rate of these processes can be monitored by recordingtheappearing corrosioncurrent,which isofreversed polarity as compared to that of the electrolysis process [81].
Additionofchromium(VI)totheelectrolytereducesthese“reverse currents”,thushinderingthecorrosionprocess[43].
Although the corrosion of steel cathodes is an important problemin thechlorate process,the inhibitingproperty of the chromium(VI)additiveisnotofmajorconcernwhenconsidering thechromium-freeprocess.
4.Suggestedalternativestochromium(VI)inthechlorate process
Summarizingalltheabovedetailedeffectsofthechromium(VI) additiveinthechlorateprocess,onemustadmirehowcomplex roleitplays,andhowperfectlyitschemicalpropertiesfitinthe process.Itisthereforenotsurprising,thatnosinglecandidatehas beenfoundsofartoreplaceitinallitsfunctions.Despiteofthis, manypromising stepshavebeentakenin this direction.In the followingsections,wesummarizetheseearlierfindings,directly relatedtothechlorateprocess.Moreover,relevantachievementsof other research fields (photocatalytic water splitting, corrosion science)willbealsobrieflylookedover.Note,thatdespiteofthe significantlydifferentconditions,thesefieldsshareanimportant needforprotected,selectivecatalyticsurfaces.Inphotocatalytic watersplittinghydrogenevolutionshouldtakeplaceatahighrate fromreductionofwaterwhileoxygenreductionshouldbeavoided toachieveahighefficiencyforhydrogenproduction.Suppression oftheoxygenreductionreactionisalsoveryimportantincorrosion science.Alliedeffortofthesedifferentcommunitiesmaytherefore openupresearchavenuestothemutualbenefitofthesefields.
Withrespecttothehighinvestmentcostandlonglifetimeofa chlorate plant a direct replacement of chromium(VI) to the electrolyteisseenasthepreferredsolutionbutalsonewelectrode materialsandcompletelynewcelldesignsareconsidered.
4.1.Alternativewaystomaintainselectivitytowardshydrogen evolutionatthecathode
4.1.1.Additionofsolutionspecies
4.1.1.1.Alkalineearthandrareearthmetalsalts. Amongtheearliest studies aiming for the energy efficiency improvement of the process, in situ formed diaphragms of alkaline earth metal hydroxides/carbonateswerefoundtohavebeneficialeffects[7].
This attempt was however shortly replaced by the more Fig.9.Anodiccurrentefficiencyforactivechlorineevolutionasafunctionofthe
hypochloriteconcentrationinthesolution(DSAanode,300gdm3NaCl,2gdm3 Na2Cr2O7,T=80C)[71].
advantageous usage of dichromate additive, and therefore the scientificandindustrialinterestdedicatedtoadditivesofCa(II)and Mg(II)saltsdiminished.Thisattemptwasrevisitedandstudiedin more detailsalmost a century afterthese first reports [82]. As found in this study, thick hydroxide films are formed on the cathodebecauseoftheincreasedpHlevelinthevicinityof the electrode (see Fig. 7A). Although the MgCl2 additive has insignificanteffectonthecurrentefficiency,theformationof a Ca(OH)2 layer leads to the suppression of the electrochemical reductionofhypochlorite.Thethicknessoftheformeddepositsis limitedonlybythemechanicalerosionand bythepHgradient.
Consequentlythesedepositscanbeintherangeofcouplehundred micrometers,orevenmillimetersthickandcanthereforeleadto blockageinthenarrowcellgapsofabout3mm.
Very similarly, in situ formed cathode films of yttrium-, samarium-andlanthanumhydroxidecanalsosuppresshypochlo- ritereductiononironandgoldinelectrolytesofhighionicstrength andelevatedtemperature.Additionofthesesaltswereshownto activatethehydrogenevolutionreaction.Howeverahighchloride concentration(5MNaCl)wasdetrimentaltotheactivatingeffect, inparticularatelevatedtemperature.Thelowsolubilityoftherare earthmetalionsinthechlorateelectrolytemakesthemunsuitable forindustrialapplications,andtheiruseisthereforenotarealistic alternative[83].
Whatiscommoninthesetwogroups(alkalineearthandrare earthmetals)isthattheprotectivelayeronthecathodeisformed as a consequence of the increasedlocal pH. Its thickness, and morphologyisthereforenotcontrolled,itgrowsrandomlyduring theelectrolysis.Weemphasizehere,thatoneofthekeysuccess factorofthechromium(VI)additiveisthattheprotectivelayeris formed in a Faradaic reaction, leading to a well-defined, thin compactcoverage.
4.1.1.2. Chromium compounds with a valence state lower than +6. Chromium(III)chlorideandsimilarcompounds(e.g.Cr2O3), inwhichthevalencestateofchromiumislowerthan+6havebeen suggestedasanalternativebyHedenstedtetal.inarecentpatent [84].Whenaddingsuchcompoundstothechlorateelectrolyte,its initiallyappearingblueishcoloralmostinstantaneouslyturnsto darkorange,indicatingtheformationofCr(VI)inthesolutiondue to the oxidation of Cr(III) by hypochlorite. The addition of chromium(III)compoundscircumventstherisksassociatedwith the handling of sodium dichromate before addition to the electrolyte but chromium(VI) will be still present in the electrolyte and to some extent in the final product. This approachthereforecannotbeconsideredasalong-termsolution.
4.1.1.3. Molybdate. Molybdate has emerged as a promising alternativeto chromium(VI) in the chlorateprocess [42,85,86].
Duringelectrolysis,aMo/MoOxfilmisdepositedontheelectrode surface. In thecase of a Ticathode, in situ additionof Mo(VI)
activatedtheHERinbothalkalineandpHneutralelectrolyte[86].
Moreimportantly,thepolarizationcurvesfortheHERweresimilar for titanium and molybdenum cathodes when molybdate was added to the solution. It was concluded that the formed film determinestheelectrocatalyticpropertiesofthecathode.Thefilm was significantly thickerthan in thecase of thechromium(VI), which indicatesthat thelayer growthis notself-limitedinthis case.
Similarlytochromium(VI),molybdatealsoactsasabuffering agentduring thechlorateprocess,accordingtotheequilibrium shown in Eq. (14) [85]. It is worth mentioning however, that molybdatewas foundtobuffereffectivelyina lowerand more narrowpHrangethanchromium(VI).
pKa;Mo56 HMoO4 ÐMoO24 þHþ ð14Þ Toachievecurrentefficienciesmatchingthepresentindustrial requirements,highconcentrationofmolybdateneedstobeadded tothesolution[42].Thisleadstoanincreasedoverpotentialand increasedoxygenformationontheanode(Fig.10).Thisisnotonly unfavorablefor theoverallenergyefficiencyof theprocess,but leadstoseveresafetyconcernsasexplosivegasmixturesofH2and O2areformed.
Attempts were made to decrease the concentration of the molybdate additive and to increase the current efficiency by addingsmallamountsofchromium(VI)tothesolution[42].When adding both molybdate and chromium(VI) to the solution, an increaseinthecurrentefficiencywaswitnessedascomparedto thecaseswhenonlyoneoftheadditiveswasused.Hencethesame current efficiency could be achieved at a significantly lower chromium(VI)concentration.
4.1.2.Newelectrodematerialsandelectrodecoatings
4.1.2.1. Chromium containing cathode materials. The unwanted electrochemicalreduction reactionsonastainlesssteelcathode canbeeffectivelysuppressedbyplatingathinchromiummetal film onit [87]. Althoughit has notbeen studiedin detail, the reportedsignificantweightlossoftheelectrodeduringelectrolysis suggests,thattheincreasedselectivityiscausedbythetwo-step formationofachromium(III)oxide/hydroxidefilm.Asthefirststep the chromium metal is oxidized (and dissolved) by the hypochloriteformedin thesolution.Subsequently,a chromium hydroxide film is deposited on the cathodethat increases the currentefficiencyoftheprocess.
InarecentstudyofHedenstedtetal.,Cr2O3andCr(OH)3films wereelectrodepositedex-situ,characterizedandstudiedelectro- chemicallyinthepresenceofhypochlorite[33].Itwasfoundthat bothlayershinderhypochloritereduction,andtheCr2O3ismore active towards HER. It was proposed that semiconducting properties of the chromium film could be a reason for the selectivityofthesefilms.
Fig. 10.(A)CurrentefficiencyinfunctionofthesolutionpH,measuredbycollectingcelloff-gas.(B)Off-gasO2levelintheproducedgasduringtheelectrolysis.Bothmeasured ina300gdm3NaCl,8gdm3Na2MoO4solutionatT=80C,usingamildsteelcathodeandaDSAanode[85].
Nanocrystallinecoatingscontainingmixedoxideswithdiffer- ent Ti/Ru/Fe atomic ratios were prepared by high-energy ball milling[88–90].Thisapproachleadstocatalystswithhighactivity towardsHER.Incorporationofchromiummoieties–eitherinthe formofCr(0)orCr2O3–inthesecatalystresultedina significant increaseintheHERcurrentefficiencywhenappliedascathodesin achlorateelectrolyte(ascomparedtothemeasurementwiththe chromiumfreeelectrodes,intheabsenceofchromium(VI))[91].
TheelectrocatalyticactivityofthecatalystsfortheHERwasnot influenced by the presence of the chromium species in the electrode. The analysis of the surface of this catalyst after electrolysisrevealedtheformationofachromium-hydroxidefilm – similar in its chemical nature to the deposit formed when dichromateisaddedtothesolution.
Althoughcompleteremovalofchromiumfromtheprocessis notachievableintheseways,andthelong-termsuppressionofthe side reactions have not been proven, these approaches are promising tosignificantlyreduce the chromiumcontentof the electrolyteandthesodiumchloratefinalproduct,andhencethe handlingrisks.
4.1.2.2. Zirconium oxide. A detailed study has confirmed that zirconiumoxidelowersthereductionrateofhypochloriteinNaOH solution[92].Zirconiumplateswereoxidizedin differentways, andthe electrochemicalactivity and long-termbehavior ofthe formedelectrodeswerecompared.Althoughthethermallyformed oxideshoweda fairlyhighinitialselectivity fortheHER,it was shownthatitceasescontinuouslyduringtheelectrolysis,which was attributed to the reduction of the layer. It was therefore concluded, that zirconium dioxide cannot be considered as an alternativeelectrodematerialinthechlorateproductionprocess [92].
4.1.2.3. Molybdenum containing electrodes and coatings. Fe-Mo alloys and MoO2 (with MoO3 traces on the catalyst surface) containing chromium (Cr-MoO2) electrodes were tested as cathodes for the electrochemical reduction of hypochlorite in neutralandslightlyalkalinesolutions,respectively[93,94].Inboth cases,anincreasedactivitytowardstheHERwasfoundwiththe electrodesofoptimized composition ascompared tomildsteel cathodes. In achieving high current efficiency however, the presenceof a Cr(OH)3/Cr2O3 film onthe electrode surface was crucialinbothcases.Thechromium(III)filmformedduringthe electrolysisinthecaseofCr-MoO2electrodes(throughasimilar mechanism as we discussed above for the chromium-plated electrodes),whileadditionofsmallamount(0.1gdm3)ofsodium dichromatetotheusedphosphatebufferwasnecessaryinthecase of the Fe-Mo alloy. Despite of this, the amount of the used chromium compound can be significantly reduced by these approaches.
4.1.2.4. Polymer coated chlorate cathodes. Polymer coatings, directlycastedonthe cathode, wereconsidered tobepossible alternativesin apatent byTilakandco-workers [95]. Cathodes covered by perhalogenated polymeric materials (the chemical compositionisnotspecifiedinthepatent)showedsimilarcurrent efficiencies to those with a thin Cr2O3 layer, close to 100% in chloratecontainingelectrolytes,atelevatedtemperature.Despite ofthesepromisingresults,tothebestofourknowledgenofurther patentsorscientificarticleswerepublishedonthistopic,except forsomeverylimiteddata[35].
4.1.3.SelectiveHERcatalystsinphotocatalyticwatersplitting A selective catalyst for HER is desirable also in other applicationssuchasphoto(electro)chemicalwatersplitting.The resultsandresearchstrategiesusedmightbethereforerelevant
alsoforsolvingthecathodicselectivityinthechlorateprocess.The reductionofatmosphericoxygenisanunwantedprocessduring photo(electro)chemicalwatersplittingasitleadstoaloweroverall efficiency. In addition, theproducts formedin theuncomplete reduction of O2 (e.g. radicals, hydrogen peroxide) cause the deactivation and leaching of the catalysts and degrade the supporting materials and the ionomers (e.g. the membrane in the water electrolyzers) as well, hence leading to shorter operationallifetime. To avoid this, several strategieshave been proposedtosuppresstheoxygenreductionreaction,includingthe usageofdifferentenzymesandmolecularcomplexes[96].While theseavenuesareunlikelytobesuitableforthechlorateprocess, anotherbranchofthesestudies,namelytheapplicationofoxygen tolerantcatalyticsurfacesmayreadilyfitintheprocess.
As we already mentioned, the unwanted back-reaction in photochemicalwatersplittingofH2andO2toH2Oonthecatalytic surfacescanbeeffectivelysuppressedbycoatingtheactivesites with a Cr(III) oxide/hydroxide layer [47]. Similar effect can be achieved by coating different photocatalysts with NiO [97], amorphous TiO2 [98], different lanthanide oxides [99], mixed amorphous Ti-Sn oxohydrates[100] and SiO2 [101]. Recently, ammoniumtetrathiomolybdatewas showntoincrease theHER selectivityofgoldsurfacesatneutralpH[102].Inthis case,the molybdate forms a couple nm thick, highly hydrated self- assembled layer, which prevents O2 to reach the surface (Fig.11).ThislayerisalsobeneficialfortheHERkineticsonthe goldsubstrate.
Thecommonin allthesecases,isthattheformedcoverages preclude theO2 toreachtheactivecatalytic sites,where H2 is formed.Atthesame time,thewater splittingreactioncanstill proceed–eitherthroughthedirectreductionofthewatercontent ofthehighlyhydratedfilms,orthroughanindirectmechanism,e.g.
aligandbasedprotoncoupledelectrontransferprocess[102].We highlight, that except for the chromium films, none of these compoundshavetothebestofourknowledgebeentestedinthe chlorateprocess.
4.1.4.Corrosionprotectionlayerswithoxygenreductionhindering properties
Thecorrosionrateofmetalscanbemoderatedbysuppressing the rate of the metal oxidation and/or the coupled reduction reaction, which is most typically the reduction of atmospheric oxygen. Ahigh scientific interestis therefore dedicated tofind
Fig.11.Schematicsoftheverticalcrosssectionofaprobablethree-dimensional ammoniumtetrathiomolybdateassemblyonanAuelectrode[102].
solutionstothesuppressionofthislatterprocess.Onesolutionfor this is to cast protective layers on the otherwise corroding substrates, preventing the oxygen to reach the metal surface.
Similar chromium compounds which ensure the high current efficiency of the chlorate process are very frequently used as corrosionprotectionlayers[44,77–79].
Thedesiretoprotectmetalsurfacesfromcorrosiondatesback asearlyasMankindstartedusingdifferentmetals,andthereforea very significant knowledge has been treasured in this field [103,104]. Suggested alternatives to chromium for effective corrosionprotectionareforexamplerare-earthmetalcompound coatings[105],sol-gelformedzirconatesandtitanates[106],Ce- basedcoatings[107]orevenlayereddoublehydroxides[108].The useoforganiccoatings(e.g.polyestersorpolyurethanes)represent anothersignificantclassofthesestudies[109].Inmostcasesthese layersserveasaphysicalbarrierbetweenthemetalsurfaceand oxygen.Thispropertymakesthesematerialsattractivecandidates for chlorate cathodes, but we emphasize, that several other requirements,mostimportantlyhighHERactivity,mustbealso met.
4.1.5.Applicationofnewcelldesignsinchlorateproduction Physicalseparationoftheoxychlorideproducts,formedonthe anodeandinthesubsequent chemicalreactionsinthesolution, may be the ultimate solution to circumvent their unwanted reductiononthecathode.Importanttorememberthatinallsuch applicationstherewillbeaneedtocontrolpHandwaterbalance, whichisalreadyapartofanoptimizedundividedcell.Alsonote, thatchangingfromundividedtoseparatedelectrochemicalcells wouldrequireamajorinvestmentfromtheindustrialparticipants andalongoptimizationprocess.Thereforethisrouteisconsidered asalong-termalternativetothecurrenttechnology.
4.1.5.1.Ion-exchangemembraneseparatedchloratecell. Including anion-exchangemembranebetweentheanodeandthecathode chambers leads to the complete inhibition of the unwanted reductionofchlorateandhypochlorite.Itcanbeenvisioned,that suchaprocesscanbeoperatedwithaninherentlyhighcurrent efficiency without the addition of chromium(VI). Further,
remarkable development of membranes in the sense of conductivity, physicaland chemical resistivityand temperature tolerancemaymakeitpossibletoseparatetheelectrodeswithout causingastrikingdecreaseintheenergyefficiencyoftheprocess [110,111].
A divided-cell setup with an anion-exchange membrane separatorwastestedintheelectrochemicalhypochloriteproduc- tion [112]. Solutions of different chloride and hypochlorite concentrations wereelectrolyzed ata constant current density, and the current efficiency was monitored by both volumetric analysisofthegaseousproductsandpotentiometrictitrationofthe formedchlorinederivatives.Themostimportantfindingsofthis studywasthattheoverallcurrentefficiencyofthecellisdictated onlybytheanodiclosses(O2formationfromH2Oandhypochlorite oxidation),theparasiticcathodereactionsarecompletelycircum- vented by the application of the membrane. Using an anion- exchangemembraneseparatedcellisinprincipleanoptimalidea, realizingtheneedediontransport,but–accordingtothebestof our knowledge – until now no anion-exchange membrane is available with sufficient stability againstalkaline and oxidizing conditions.
Cation-exchange membrane separated cells are extensively applied in the chlor-alkali process (schematics of the cell is depictedinFig.12.).Insuchcellstheusedmembranesshowgood selectivityandlong-termstability.Althoughtheconditions(pH,T, electrolyte concentration) of the chlorate process are different from that of the chlor-alkali production, the solution species present,andtheconsequentoxidativenatureoftheelectrolyteis very similar in the two cases. Therefore, the stability of the membranesusedinthechlor-alkaliproductionispromising for theirusealsoatchlorateprocessconditions.
4.1.5.2.Membraneseparatedchloratecellswithoxygendepolarized cathodes. Anotherpossibilitytoincreasetheenergyefficiencyof the chlorate process is to change the cathode reaction from hydrogen evolution to a thermodynamically and/or kinetically morefavoredreactiononthecathodeandhencedecreasethecell voltage.
Fig.12.Comparisonofchlor-alkalielectrolysisinamembraneseparatedelectrolysiscellwith(A)hydrogenevolvingand(B)oxygendepolarizedcathodes[113].
Table 2
Effect of formerly investigated catalysts on the decomposition rate of hypochlorite (N.A.: not analyzed, RT: room temperature).
Solution composition [OCl]0/M T/C pH Catalyst Catalyst amount Effect on the rate of Ref.
HOCl decomp. ClO3formation O2evolution
0.32 M NaOH + NaCl 0.8–1.4 30–60 Alkaline Mn, Fe oxide 1.5–5 mg dm3 No/negligible effect [53]
Co, Ni, Cu oxide + No effect +
2 M NaCl + NaOH 0.8–1.6 30–60 Alkaline Ionic strength + N.A. N.A [121–123]
2 M NaCl + NaOH/Buffers 0.136 20–40 7–11 Na2IrCl6 1–7 mg dm3 + + (neutral) (+) (alkaline) [124,125]
1 M NaCl 1 25 Alkaline Ni, Co peroxide 0.02–2 g dm3 + N.A. + [126]
Cu(OH)2 (+) N.A. (+)
Al, Fe, Mn, Hg oxide Negligible effect
1.14 M NaCl + 0.28 M NaOH, 1.14 25–50 Alkaline Co peroxide mM range + N.A. + [127]
2 M NaCl 2 M 40–70 Alkaline Cu(II) 1–2 ppm + N.A. + [128]
0.05–0.1 M NaOH 0.4–0.8 30–45 Alkaline Cu, Fe oxide 50–250 mg dm3 + N.A. + [129,130]
Cu oxide + N.A. +
RT MgO promoted Cu oxide 20–400 mg dm3 + N.A. + [131]
0.3–0.7 M NaCl 0.3–0.7 25 7–11 Co-oxide 2 g dm3 + (+) neutral + [132]
Dilute aqueous solution 310 ppm RT No inf. Fe oxide-hydroxide 1.67 g dm3 No effect [133]
Cu oxide-hydroxide (+) N.A. N.A.
Ni, Co oxide-hydroxide + N.A. N.A.
0–4 M NaOH 0.2 25 Alkaline Cu(OH)2 10–100mM + N.A. + [134]
Dilute aqueous solution min. 5% 75 Alkaline MnO2 1 w/v % No/negligible effect [135]
CuO 1 w/v % + N. A. N. A.
NaCl + NaOH 1 RT Alkaline Cr, Fe, Mn, U, Bi, Pd, Os, Ta, V oxide 0.1 g dm3 No/negligible effect [136]
Co, Ni, Ir oxides 0.1 g dm3 + N. A. N. A.
Dilute aqueous solution 80 mM 80 6.5 FeCl3, Fe3O4, CeCl3, Na2Cr2O7, Na2MoO4, RuCl3, RuO2 10mM No/negligible effect [23]
AgCl 100 ppm No/negligible effect
Al2O3 9 ppm No/negligible effect
CoCl2 10mM + No effect +
IrCl3 10mM + + +
8B.EndrÅdietal./ElectrochimicaActa234(2017)108–122
Following this thread, a specific variant of the membrane separatedcellcanbeenvisionedbyutilizingaporouselectrodeas thecathode,whichiscontinuouslyfedwithpressurized,oxygen containinggas(Fig.12B).Applicationofthisoxygendepolarized cathodeconceptleadstoalargereductioninthechlor-alkalicell voltage[113].Wenotethatthisisnotonlysupportedbyacademic studies,butplant(s)workingonthisprinciplehavealreadybeen builtand are in operation[114].Although several studieshave addressed the applicability of such devices also for chlorate production,[115,116]theirindustrialapplicationhasstillnotbeen realized.Themostimportantreasonsforthisaretherequirement ofcontinuous,pressurizedoxygenfeeding(thepurity isalsoan important factor), and that the H2 produced in the “original” chlorateprocessisinmanycasesavaluableproductwhichisnot produced here.Further, in the incompletereduction of oxygen H2O2maybeformed,whichmaydegradeboththecatalystandthe appliedporoussubstrate,andcouldleadtotheintenseformation ofoxygeninthecell.Also,precipitationofsodiumperoxidemay occurinthecell,leadingtotheblockageofboththeactivesurface areaofthecatalyst,andtheporesofthegasdiffusionelectrode [113,117].
Asanotherpromisingexample,averysimilarsetupcanbeused forthereductionofCO2duringchlorineproduction[118].Byfine- tuningthereactionparameters(e.g.catalyst,flow-rate,tempera- ture etc.), a simultaneous production of synthesis gas (CO-H2 mixture)onthecathode,andchlorineformationontheanodecan be achieved. As important raw chemicals are formed on both electrodes,suchcellscanbeimportantunitsinawell-integrated commoditychemicalsynthesisplant.Asafurtheradded-value,a harmfulgreenhouse-gas,CO2isconsumedintheprocess.
4.1.5.3. Chlorate formation from chlorine gas and caustic soda produced in a chlor-alkali plant. State-of-the-art membrane separated chlor-alkali plants produce chlorine at a power consumption of 2100–2600kWht1 Cl2 [119]. Using this chlorine for thechemical production of NaClO3 in an external reactor,thespecificpowerconsumptioncanbecalculatedtothis latter(consideringthestoichiometryofthechlorinedisproportion reactionandthechemicalchlorateformationreaction,asdepicted inEqs.(2)and(4))tobe5000–5500kWht1NaClO3.Thisenergy consumptionisfairlycomparabletotheenergyneedofchlorate formationinthecurrentlyappliedundividedelectrochemicalcells.
In this case, however,no chromium(VI) needstobe added for currentefficiencyreasons,leadingtothecompletediminishment ofthesafetyhazardsrelatedtoitshandling.Further,theproduced chlorateisfreeofanychromium(VI)contamination.Alsonote,that inthecaseofthechlor-alkalicellverypureH2(>99.9%),withlow oxygenandchlorinecontentisproducedonthecathode.Thisisof highervaluethanthegasmixtureformedinthecaseoftheactual chlorateprocess,contributingtotheeconomicfeasibilityofthis concept[120].
3Cl2þ6NaOH!5NaClþNaClO3þ3H2O ð15Þ Reaction(15) illustratestheoverallchemical reactiontaking placeoutsideoftheelectrochemicalreactor.Asseen,fivechloride ionsareformedforeachchlorateion.Theformedchlorideneedsto berecycledtothecells,whereaschlorateshouldbeseparatedas the product of the process. It is of high importance that the recycledchloridestreamdoesnotcontainanychloratethatcould formchlorinedioxideintheacidicanolyte.Anotherconcernabout thisconcept,isthatfortheavailablemembranestheconcentration ofthecausticsodais limitedto32wt%.Using thisfor chlorate production (according to Eq. (15)) the achievable chlorate concentrationisnothighenoughforcrystallization.Thesolution mustthereforebeconcentratede.g.byevaporatinga significant
portionof thesolvent,while watermustbefed tothecathode compartment,leadingtoconsiderableadditionalcosts.
4.2.Catalysisofthehomogeneouschlorateformationreaction Theconcentrationofhypochloritehasasignificantimpacton theenergyefficiencyofthechlorateprocessasitinfluencesboth theanodeandthecathodereactionsasexplainedintheprevious sections.Findingacatalystwhichselectivelyincreasestherateof chlorateformationishighlydesired.Itwouldleadtolowersteady- stateconcentrationsofhypochloriteinthechloratesolution,thus less oxygen produced, and possibly to more efficient process designsassmallerreactorvolume(seeFig.2)wouldbeneededfor thechlorateformingreaction.Severalstudieswereconductedto identifytheeffectofdifferentcompoundsonthedecomposition rate of hypochlorite and on the oxygen formation rate, as summarizedinTable2.
Several transition metal derivatives, e.g. CuO, Co-oxide, Ni- oxidewereshowntocatalyzethedecompositionofhypochloriteto oxygen,withouthavingsignificanteffectonthechlorateforma- tion.Othercompounds,e.g.MnO2,Fe-oxideswereshowntohave no effect on the decomposition rate of hypochlorite. More importantly, only iridium-salts were found to catalyze the formation of chlorate [23,124,125]. The presence of chromium (VI)wasshowntohavenoeffectonneitherthechloratenorthe oxygen formation rate in dilute, neutral/alkaline hypochlorite solutions[23,30,136].
It is important to highlight, however, that none of these measurementswereperformedunderthecircumstances(solution composition, temperature, pH) of the actual chlorate process.
Therefore theseresultsmust behandled withprecautions, and shouldberepeatedatindustriallymorerelevantconditions.
4.3.Theuseofalternativebuffersinthechlorateprocess
Thebufferingeffectofthechromium(VI)additiveiscrucialfor thechlorateprocess.ThesolutionpHhasasignificantimpacton thehomogeneouschlorateformationrate,andhenceonseveral differentaspectoftheprocess,e.g.ontheoxygenproductionrate, asdescribed intheprevious sections[52]. Insolutionswithno chromium(VI) addition, the decreased buffer capacity may necessitatetheadditionofsomebufferingagent.
The applicability of phosphate buffer has been studied [42,66,94].Thesestudiesconcluded,thatinamembraneseparated electrochemical cell the cathode current efficiency is slightly increased.Inparallelwiththis,anincreasedO2productionratewas witnessedontheanode,possiblycausedbythespecificadsorption of thephosphateions.Thephosphatewas alsoidentifiedasan anodepoisonbyanodeproducers[137].Anotherspecificproblem in the existing chlorate process is the formation of stable precipitatesbetweenphosphateanddi-ortrivalentcationssuch asCa2+andFe3+[138].IncontradictiontothisSpasojevicandhis co-workersfound,thatpartialreplacementofchromium(VI)with thephosphatebufferisbeneficialfortheanodecurrentefficiency forcertainanodecoatings[65,139].
5.Conclusionsandfutureoutlooks
Chromium(VI)servesseveralrolesinthechlorateproduction andhasaneffectonalmosteverysub-stepoftheprocess,ranging fromtheelectrodereactionstothebufferingofthesolution.Most importantoftheseistheincreasedcurrentefficiency,ensuredby thesuppressionoftheunwantedelectrochemicalprocessesonthe cathode.Thisoriginatesfromtheformationofacoherent,compact, thinfilmofCr(III)hydroxidewithpoorconductivity.Thislayeris permeableforthereactant(H2O/H+)andthereductionproducts
