Oxygen and Hydrogen Peroxide Reduction by 1,2-Diferrocenylethane

Oxygenandhydrogenperoxidereductionby1,2-diferrocenylethane atliquid/liquidinterface Haiqiang Dengaǡ Pekka Peljobǡ Fernando Cortés-Salazaraǡ Peiyu Geaǡ Kyösti KontturibǡHubertH.Giraulta,*

aLaboratoire d’Electrochimie Physique et Analytique, Ecole Polytechnique FédéraledeLausanne(EPFL),Station6,CH-1015Lausanne,Switzerland bDepartmentofChemistry,AaltoUniversity,P.O.Box16100,00076,Finland * CORRESPONDING AUTHOR FOOTNOTE

E-mail: [email protected]

Telephone number: +41-21-693 3145

Fax number: +41-21-693 3667

Acceptedmanuscript Journal of Electroanaytical Chemistry, 681 (2012) 16-23. http://www.sciencedirect.com/science/article/pii/S1572665712001907

1 Abstract: Molecular oxygen and hydrogen peroxide reduction by 1,2-diferrocenylethane (DFcE) was investigated at polarized water/1,2-dichloroethane (W/DCE) interface. The overall reaction points to proton-coupled electron transfer (PCET)mechanism,wherethefirststepconsistsoftheprotonationofDFcEto formtheDFcE-H+inDCEphase,eitherbyDFcEfacilitatedprotontransferacross the liquid-liquid interface or by the homogeneous protonation of DFcE in the presence of protons extracted in the oil phase by tetrakis(pentafluorophenyl)borate.TheformationofDFcE-H+isfollowedupby theO2reductiontohydrogenperoxideandfurtherreductiontowater.Thefinal productsofDFcEoxidation,namelyDFcE+or DFcE2+ǡwereinvestigatedbyion transfer voltammetry, ultramicroelectrode voltammetry and UV/visible spectroscopy.TheseresultsshowthatmostlyDFcE+isproduced,althoughDFcE+ can also reduce oxygen at longer time scales. Hydrogen peroxide reduction is actually faster than oxygen reduction, but both reactions are slow due to relativelylowthermodynamicdrivingforce. Keywords: ITIES, oxygen reduction, hydrogen peroxide reduction, 1,2- diferrocenylethane

2 1. Introduction The charge transfer processes across the interface between two immiscible electrolytesolutions(ITIES)areoffundamentalimportanceforvarietyof applications such as in storage and conversion of energy, solvent extraction, electroanalysis, and life sciences [1]. Within the context of green energy, vital processessuchasphotosynthesisandrespiration(i.e.oxygenreduction)taking placeatthelipidbilayersofbiomembranescanbestudiedattheITIES.Oxygen reduction reaction (ORR) at the water/1,2-dichloroethane interface (W/DCE) hasbeenstudiedformorethantenyears,sinceKiharaandco-workersshowed that tetrachlorohydroquinone in oil phase could reduce oxygen to water or hydrogenperoxide,dependingonthepotentialdifferenceappliedattheW/DCE [2].ThisworkwasoneoftheearlieststudiesofPCETreactionattheITIESas thereductionofoxygeninDCErequiressuitableelectrondonor(D)intheoil phaseandprotonsourceintheaqueousphase.Inrecentyears,wehavealso investigatedtheoxygenreductionattheITIESbydirectelectrondonorssuchas differentferrocenederivatives(forexampledecamethyl-(DMFc),1,1’-dimethyl- (DFc)andferrocene(Fc)[3,4])ortetrathiafulvalene(TTF)[5].Electrocatalysis ofoxygenreductionbydifferentporphyrins[6-11]anddodecylaniline[12]has also been studied, and Peljo et al. demonstrated novel fuel cell based on molecularcatalysisofoxygenreductionatliquid/liquidinterface[13].More recently,Olayaetal.observedthedirectfour-electronreductionofoxygenatthe ITIES catalyzed by self-assembled molecular rafts formed of two oppositely charged water-soluble porphyrins [14] and Peljo et alǤ investigated the mechanismofoxygenreductionbyso-calledcofacial“Pacman”typeporphyrins attheITIES[15].Thebiphasicsystemaforementionedappearssuperioroverthe extensively-investigatedhomogeneoussystemsinceontheonehand,theITIES provides physical separation of the reactants and products (water and hydrogen peroxide are transferred back to aqueous phase) resulting in easier productcollectionandhigheryieldsforsystemsatequilibrium(accordingtoLe Chateliere’sprinciple,theequilibriumcanbeshiftedonthefavourofproductsby extractionofproducts[16]);ontheotherhand,thereactionrateiscontrolledby the proton concentration in the oil phase, which is determined by the Galvani

3 potential difference across the interface that is conveniently controlled by modernelectrochemicaltechniques. ORRbymetallocenesatliquid/liquidinterfacehasbeenproposedtoproceed intwosteps:protontransferfromtheaqueoustotheoilphasefacilitatedbythe metallocenefollowedbyhomogenousoxygenreductionintheoilphase.Inthe caseofdecamethylferrocene(DMFc)formationofthehydrideDMFcH+withthe protonbindingtotheironisthefirststep[17].Densityfunctionaltheory(DFT) calculations suggest that instead of the coordination of triplet molecular oxygentotheironatom(spin-forbidden)[18]orinsertionintoFe–Hbond,the reaction with oxygen proceeds through delocalized triplet transition state, leadingtotheformationofDMFc+andhydrogenperoxylradical[17].Also, mechanism where molecular oxygen is coordinated between two protonated ferroceneshasbeenproposed[19].Thismechanismhassomesimilaritieswith theoxygenreductionbycofacialmetalpophyrins[20,21],mimickingtheoxygen reductionoccurringinthebimetalliciron/coppercenterofcytochromecoxidase [22].Becauseofthiswedecidedtostudyoxygenreductionby1,2- diferrocenylethane, multi-ferrocenyl compound, at the polarized water/DCE interface.Thiscompoundhasbeensuccessfullyusedasanelectrondonorfor electron transfer studies at the liquid/liquid interface [23, 24], and previous NMR results indicate that protonation of both ferrocenyl groups should take place in boron trifluoride monohydrate solution [25]. Thus, ORR to hydrogen peroxide could take place with molecular oxygen sandwiched between the protonated centers. The experimental results show in fact, that two DFcE moleculesareneededfortwo-electronoxygenreduction,andthuscastdoubton whether diprotonated DFcE is indeed formed in DCE. Hydrogen peroxide reductionislesswellunderstood,asthisreactionismentionedonlytoexplain observed four-electron oxygen reduction [18, 26]. mechanism suggested by

Fomin indicates that the protonated ferrocene can react with H2O2 forming water,Fc+andOHʉradical,whichfurtherreactswithFcandprotontoproduce water [19]. From this point of view, DFcE seems ideal for hydrogen peroxide reduction,ashydrogenperoxidecanreactwithoneprotonatedferrocenylgroup andthenthegeneratedOHʉradicalcaneasilyoxidizetheotherferrocenylgroup. The experimental results obtained in this work show that hydrogen peroxide

4 reduction is faster than oxygen reduction, corroborating the proposed mechanism. 2. Experimental section 2.1 Chemicals All chemicals are analytical grade and used as received without further purification. 1,2-diferrocenylethane (DFcE) was purchased from Aldrich. Anhydrouslithiumchloride(LiCl),bis(triphenylphosphoranylidene)ammonium chloride (BACl), lithium sulfate (Li2SO4), 1,2-dichloroethane (DCE), sodium iodide(NaI),andtetramethylammoniumsulfate(TMA2SO4Ȍwereobtainedfrom Fluka. Lithium tetrakis(pentafluorophenyl)borate diethyl etherate (LiTB) was purchasedfromBoulderScientificandsulfuricacid(H2SO4ǡ 95-97%) was purchasedfromSigma-Aldrich.Potassiumbis(oxalato)-oxotitanate(IV)dihydrate was provided by Alfa Aesar. Bis(triphenylphosphoranylidene) ammonium tetrakis(pentafluorophenyl)borate (BATB) was prepared by metathesis of 1:1 mixturesofBAClandLiTB,inmethanol/water(v/vα2)mixture,followedby recrystallization in acetone. The aqueous solutions were prepared with ultrapurewater(18.2πcm)fromMillipore-Qsystem. 2.2 Two-phase reactions controlled by a common ion distribution (shake flask reactions) Two-phase shake flask reactions for oxygen reduction were performed in smallflaskunderstirring.Fortheseexperiments,equalvolumes(2mL)ofDCE and aqueous solutions containing the reactants (composition of both phases showninScheme1)weremixedtogetherandstirredvigorously.Afterreaction aqueousandorganicphaseswereseparatedandtheUV-VisspectrumoftheDCE phase was measured directly. The aqueous phase was treated with excess NaI

ԟ ԟ (equivalentto0.1M).HydrogenperoxidereactedwithI toproduceI3 ǡ which has an absorbance at 352 nm [4]. UV/visible (UV/Vis) spectra were obtained withanOceanOpticsCHEM2000spectrophotometerwithquartzcuvette(path length: 10 mm). To confirm the production of hydrogen peroxide, also the

5 titaniumoxalatemethodreportedbySellerswasused[27].Briefly,ͳmLsample of aqueous phase was acidified with sulfuric acid and mixed with potassium bis(oxalato)-oxotitanate(IV) solution to form yellow complex with hydrogen peroxide. For quantitative purposes, the absorption of the complex was

measuredat400nm[27]. TostudytheamountofDFcEconsumedinthereaction,theratioofdifferent DFcE species was determined by measuring cyclic voltammograms (CVs) at scanrateof20mV·s1 with Pt (25 P diameter), carbon fiber (10 P diameter) and glassy carbon (10 P diameter, Princeton Applied Research) ultramicroelectrodes (UMEs) with CHI900 electrochemical workstation (CH Instruments, Austin, USA). For comparison, CVs of freshly prepared DCE solutionofͷmMDFcEunderanaerobicconditionswerealsorecorded.For achievingtheanaerobicconditionsthesolutionwasdegassedbybubblingpure

N2 through it for30 min and then keeping N2 atmosphere over the solution duringthevoltammetricmeasurements.ForrecordingtheCVs,three-electrode system with Pt wire as the counter electrode and Ag/AgTB wire as the referenceelectrode(diameterα0.5mm,madebyelectrolysisofAgwirein10 mMLiTBsolution)wasemployed.Thepotentialscalewascalibratedwiththe additionofdecamethylferrocene(0.04vs.SHEinDCE[28])attheendofthe voltammetryexperiments. Fabrication of the Pt and carbon fiber UMEswas performed by sealing 25 Ɋm-diameterPtwireor10Ɋm-diametercarbonfiber(Goodfellow,Oxford,UK) atoneendofglasscapillary(i.e.innerdiameterͳmm,outerdiameter1.5mm, Bio-logic) by butane/propane/oxygen flame (C206 Super, CAMPINGAZ). Afterwards,theglasscapillarywithsealedPtwireorcarbonfiberissubjectto vacuumsystemforc.a.30min.Then,thecapillaryisslowlysealedontoPtwire orthecarbonfiberbyplacingitinsideresistorheatercoil(Model720,David KopfInstruments,USA).Specialattentionhastobepaidinordertoplacethe capillaryinthecenterofthecoiltoavoidcapillarydeformationsleadingtonon- straightUMEs.Aftercapillarysectionequaltoͳcm-lengthisproperlysealed, the electrical connection is made by melting given amount of tin powder betweenthePtwireorcarbonfiberandlargertin/copperleadwire.Lastly,the tin/copperleadwireisfixedtotheglasscapillarybytwo-componentepoxy

6 resin(Araldit,ReckittƬColmanAG),lettingfreetin/copperwiresection outsidetheglasscapillaryforelectricalconnectionpurposes[29].

HydrogenperoxidedecompositionwasinvestigatedbyhavingH2O2 solution incontactwithDFcEintheDCEphase(Scheme2),withoutpartitionof commonion.ThereactionwasmonitoredwithUV/VisspectroscopyandUME voltammetry. Hydrogenperoxidereductionwasstudiedingloveboxwithnitrogen atmosphere, using the cell described in Scheme 3. For this, equal volumes of deoxygenatedsolutionsweremixedtogetherandstirredvigorouslyfor30min, and the reaction products were analyzed as described above, with UV/Vis spectroscopyandUMEvoltammetrywithglassycarbonelectrode. 2.3 Electrochemical Measurements All the electrochemical measurements were performed at ambient temperature (20±2 °C) under aerobic conditions in Faraday cage. CVs at the W/DCE interface were obtained using an Autolab four-electrode potentiostat (PGSTAT30,Eco-chemie,theNetherlands).Twoglasscellsdesignedforliquid- liquidinterfaceexperimentswiththeinterfacialareaof0.159cm2(agenerous gift from prof. Zden¶ Samec, J. Heyrovský Institute of Physical Chemistry, Prague) or 1.53 cm2 were used in the experiments. Two reference electrodes

(Ag/Ag2SO4orAgquasi-referenceelectrode(AgQRE),andAg/AgCl)wereplaced inLuggincapillariestoreducetheiRdrop,andusedforcontrollingthepotential differenceacrosstheinterface,whiletungstenorplatinumcounterelectrodesin bothphasesprovidedthecurrent.Theorganicreferencephasehadcommon cation with the supporting electrolyte of the organic phase, as described in

w Scheme 4. The potential was converted to the Galvani potential scale ( 'o I ),

basedonCVmeasurementofthereversiblehalf-wavepotential 'wI 1/2 ofthe o TMA TMA+iontransfer(0.16inDCE[30]). 3. Results and Discussion

7 3.1 Redox properties of DFcE TheredoxpropertiesofDFcEinDCEwerestudiedbycyclicvoltammetryas showninFigure1.Fromthisfigure,itcanbeseenthatDFcEhastwooxidation waves corresponding to DFcE+/DFcE and DFcE2+/DFcE+inDCEwiththehalf- wavepotentialsE1/2at0.565and0.770vs.SHE(i.e.E1/2separation205mV), confirming that the oxidation of one ferrocenyl group increases the redox potential of the other group. This has been reported previously for DFcE in

Ϋ dichloromethane (E1/2 separation 180 mV), when TB was used as counter anion [31]. The solvent and supporting electrolyte affect the separation of the

Ϋ two oxidation waves: generally more strongly coordinating anions like ClO4 ǡ

Ϋ Ϋ BF4 and PF6 are able to stabilize the formed monocations by ion-pairing, decreasingthedifferencebetweentheobservedhalf-wavepotentialsofthetwo oxidation waves. Contrarily, the use of weakly coordinating anions like TBΫ increasestheseparationoftheobservedhalf-wavepotentials.Thesolventhas similar effect: highly polar solvents like dimethyl sulfoxide and solvents with significantdonorcharacter(forexampletetrahydrofuran,donornumberα20) arealsoabletostabilizetheformedmonocations,sothatsecondoxidationis easier [32]. For example, in polar solvent like dimethylformamide, DFcE has

Ϋ onlyoneoxidationwavewhenClO4 isusedascounteranion[33].

Voltammetry measurements show that the formal potential of DFcE species (0.565and0.770vs.SHE)isnotlowenoughforprotonreductiontooccurin theoilphase(standardredoxpotentialof0.55vs.SHEinDCE[34]),butoxygen reductioninthepresenceofprotons(standardredoxpotentialof1.17vs.SHE forhydrogenperoxideand1.75vs.SHEforwaterinDCE[6])byDFcEand DFcE+ is thermodynamically feasible. However, the reduction of oxygen to the superoxideishighlyunfavorable(standardredoxpotentialof–0.81vs.SHEin DCE [12]). This point will be detailed in section 3.2.1. The limiting currents measuredon10P carbon fiber UME were used to evaluate the diffusion coefficients of the neutral and DFcE+ species as 5.8 and 3.7×10Ϋ6 cmʹ sΫ1ǡ respectively. 3.2 Two-phase reactions controlled by a common ion distribution 3.2.1Oxygenreduction

8 Oxygen reduction by DFcE is described by equations ͳ or 2. Ferrocene derivatives also catalyze decomposition and also further reduce hydrogen peroxide,asdescribedbyequations͵and4. 2DFcE(o) O (o) 2H (w) o 2DFcE (o) H O (w) 2 2 2 (1) DFcE(o) O (o) 2H (w) o DFcE 2 (o) H O (w) 2 2 2 (2) 2H O (w) DFcEo O (w) 2H O(w) 2 2 2 2 (3) 2DFcE(o) H O (w) 2H (w) o 2DFcE (o) 2H O(w) 2 2 2 (4)

The reduction of O2 by DFcE was investigated by shake-flask experiments, where the Galvani potential difference across the interface is controlled by common ion distribution. When TB– was used as common ion, the Galvani potentialdifferenceisfixedatpotentialgreaterthan0.59V,sothatprotonsare extractedtooilphase.freshsolutionofDFcEinDCEhasbrowncolorand displays an absorption band in the UV/Vis spectrum at ɉmaxα436nm(dotted curveinFigure2).Afterthetwo-phaseshakeflaskreaction(i.e.reactiontimeα

10min),theDCEphaseturneddarkgreen,andbroadabsorptionbandatɉmaxα 619nm(dash-dottedcurveinFigure2)correspondingtoDFcE+wasobserved.

ThepresenceofH2O2intheaqueoussolutionaftertheshake-flaskreactionwas confirmed with the NaI and titanium oxalate (data not shown) methods, as

– representedbytheappearanceoftheI3 characteristicabsorptionbandatɉmaxα 352nm(solidcurveinFigure2).Onthecontrary,nosignalwasobservedinthe UV/Vis spectrum for the aqueous phase before the biphasic reaction (dashed curveinFigure2). Formation of the DFcE+ cation was also confirmed by the cyclic voltammogramsofcarbonfiber(diameterα10Ɋm)andPt(diameterα25 Ɋm)UMEsintheisolatedDCEphaseaftershake-flaskreaction,asillustratedin Figure3.After10minofshake-flaskreaction,threesteady-statecurrentwaves,

2+ 1 namely DFcEl DFcE ǡ DFcEl DFcE ǡ and HHo 2 2 were observed at the DMFc+/DMFc potential scale. The percentage of DFcE oxidized to DFcE+

9 couldbecalculatedfromthefirstoxidationwaveofDFcEtobeabout20%,socaǤ ͳmMDFcE+wasgeneratedduringthereaction.Thesumofthemagnitudesof cathodicandanodiccurrentforDFcE+l DFcE isclosetothatoffreshly preparedDFcEinDCE(datanotshown)showingalmostnoDFcE2+ was producedinthetimescaleofthisexperiment.Also,thehalf-wavepotentialE1/2 forH+reductionatcarbonUMEisabout577mVmorenegativethanthatatPt UME,asobservedinFigure3.ThismakescarbonUMEsmoresuitablefor studying the products of this reaction. The selectivity for the production of hydrogenperoxideoverwaterwaslessthan6%,calculatedbasedontheamount ofproducedhydrogenperoxideandtheamountofconsumedDFcE. shakeflaskexperimentwithLiTB:DFcE(5mMDFcE)molarratioofͶwas performedfor48hours,andtheconversionofDFcEwasmonitoredby voltammetrywithcarbonfiberUMEduringreaction.AbouthalfoftheDFcE had been oxidized to DFcE+afterͳhour,butfurtheroxidationtoDFcE2+ took muchlongertime.After18hoursabouthalfoftheDFcEwasconvertedto DFcE2+ǡandthereactionwasnotcompleteuntilafter26hoursofreaction(see Figure3).ResultsshowthatthekineticsofoxygenreductionbyDFcEisvery slow, and the produced DFcE+ is also able to reduce oxygen, although at even slowerrate,asexpectedfromthehigherredoxpotential.Ferrocenederivatives havebeenshowntocatalyzethedecompositionofhydrogenperoxide,andthey may also be able to further reduce hydrogenperoxide [26]. Thus, with such longtimescalethereactionswithhydrogenperoxidearenotnegligible,andthe exact efficiency of hydrogen peroxide production cannot be evaluated from shakeflaskexperiments.TheCVoftheorganicphaseafter26hoursofreaction showninFigure͵demonstratesthatthecompleteoxidationofDFcEtoDFcE2+is possible.ThediffusioncoefficientofDFcE2+calculatedfromthefirststeady-state cathodicprocessobservedwiththissolution(dottedlineinFigure3)isequalto 3.8×10Ϋ6cm2sΫ1ǡalmostthesameasthatofDFcE+Ǥ control shake-flask experiment without aqueous acid (Scheme 1, without aqueousH2SO4Ȍunderaerobicconditionswasalsoconducted(reactiontime10 min,datanotshown).ThecolorofDCEphaseturnedslightlygreenafterthetwo- phaseshakeflaskreaction,whilethecharacteristicabsorptionpeaksbothfor

+ ԟ DFcE inDCEphaseandthatforI3 inaqueousarenotevident,indicatingthat

10 the reaction is even slower in the absence of aqueous additional acid. The CV recordedwithglassycarbonUME(10Pdiameter)showedthatca.9.7%of

DFcEwasoxidized,whilethisvaluewasabout20%with50mMH2SO4 in the aqueousphase.ThepHinaqueousphaseincreasedfrominitialvalueof7.72(5 mMLiTB)to9.68duringtheexperiment.Thereactioncanbedescribedasthe oxygen reduction in alkaline conditions (equation 5). From the experimental pointofview,itisnoteasytodistinguishbetweenthereactiongoingthroughthe

Ϋ superoxideandthereactionproducingHO2 with direct reaction, but thermodynamically superoxide step is highly unfavourable (standard redox

Ϋ potentialof–0.81vs.SHEinDCE[12]),andthuslesslikelythantheHO2 path. TheprotonspresentintheaqueousphaseareextractedintotheoilphasebyTBΫ andmorewaterwilldissociatetoprotonsandOHΫtokeepthesystemin equilibrium. As the concnetration of protons at pH ε ͹ is very small, this extraction is slow. Protons react with oxygen and DFcE in the oil phase, and hencemoreprotonsareextractedtokeepthedistributionofionsbetweenboth phasesatequilibrium.TheconcentrationofOHΫincreasesintheaqueousphase, increasingthepH.

2DFcE + O222o H O HO OH 2DFcE (5) It should be noted that under aqueous acidified conditions, reaction in equationͷwillbethesameasdescribedinequation1. 3.2.2Hydrogenperoxidedecompositionandreduction Theeffectofhydrogenperoxidedecompositionontheobservedselectivityfor two-electron oxygen reduction was studied by mixing equal volumes of DCE solution containing DFcE and aqueous hydrogen peroxide solution in vial (Scheme 2). The amount of hydrogen peroxide determined with TiOx method decreasedby5%after10minutesand76%after60minutes(inseparate experiments).Theaqueousphaseturnedslightlybluishafter60minexperiment, indicating that DFcE partitions into the aqueous phase and reduces hydrogen peroxidetoformDFcE+Ǥ NooxidationofDFcEintheDCEphasewasobserved, buttheamountofDFcEinDCEdecreasedsignificantly,4%after10minutesand 30%after60minutes,confirmingthatDFcEpartitionsintotheaqueousphase where it reduces hydrogen peroxide. Partition coefficient (Log PȌofDFcE

11 betweenDCEandwaterwasdeterminedtobe3.7-3.9(seesection3.3),sothe equilibriumconcentrationofDFcEinwaterisδ0.4PM.Thus,30%ofDFcElost fromtheDCEphase(0.6mM)after60minofreactionisoxidizedtoDFcE+by stoichiometricamountof0.3mMofhydrogenperoxide.Thisaccountsfor40%of thelossofH2O2ǡandtherestislostbydecomposition.Hydrogenperoxideisalso slightlypartitioningintotheDCEphase,butfurtherreductionisnowimpossible asnoprotonsareavailableintheDCEphase. Hydrogenperoxidereductionwasinvestigatedintheabsenceofoxygen,in glove box. ʹ mL of DCE solution was mixed with ʹ mL of hydrogen peroxide solutionandstirredvigorouslyfor30minutes(Scheme3).Theoilphasewas analyzedwithUMEvoltammetryandUV/Visspectroscopybeforeandafterthe reaction,andthehydrogenperoxidewasanalyzedwithTiOxandNaI–methods. Now45%ofDFcE(1.2mM)wasoxidizedtoDFcE+ǡandDFcEΪpeakwasclearly observedinUV/VisspectraoftheDCEphase(FiguresͶand5).96%ofhydrogen peroxide was consumed (93% when analyzed with TiOx method) during the reaction.Asthestoichiometricamountofreducedhydrogenperoxidewouldbe 0.6mMor60%(eq.4),partofthehydrogenperoxidewasdecomposedduring thereaction(eq.3).Nowthisreactioncantakeplaceinbothphases,asprotons and DFcE (< 0.4 P in aqueous phase) are both available everywhere in the system. However, the reaction is thermodynamically more favorable in the oil phase(standardredoxpotentialof2.31vs.SHEinDCEand1.76vs.SHEin water [12]), and proceeds much faster than observed in the absence of LiTB

(60%ofH2O2reducedafter30minutesvsǤ30%ofH2O2reducedafter60minutes intheabsenceofLiTB).Incomparison,60minaerobicshakeflaskreactionwith ʹ mM DFcE Ϊ ͷ mM BATB and ͷ mM LiTB Ϊ 10 mM HCl had only 33% conversion of DFcE to DFcE+ǡ indicating that hydrogen peroxide reduction by DFcEisfasterthanoxygenreduction.Thisexplainswhysosmallamountsof hydrogen peroxide are detected in the shake flask experiments, as oxygen is firstlyreducedtohydrogenperoxide,followedbyfasterhydrogenperoxide reductionstep. Ferroceneanditsderivativeshavebeenusedaselectrondonorsforoxygen reductionintheabsence[26]orpresenceofcatalysts[35-37]ǡbuttheseresults show that certain care has to be taken when determining the selectivity of

12 oxygen reduction to water, as the electron donor itself can reduce hydrogen peroxide, thus increasing the observed apparent selectivity of the catalyst towards four-electron reduction of molecular oxygen. This ability to reduce hydrogenperoxideisexpectedtobeevenstrongerinthecaseofstronger reductantslikedecamethylferrocene,socontrolexperimentsbothforhydrogen peroxidereductionanddecompositionshouldalwaysbeperformed. 3.3 Four electrode cell measurements The CVs obtained with the four-electrode electrochemical cell described in SchemeͶareshowninFigure6.

WhenH2SO4/Li2SO4andBATBwereusedasthehydrophilicandhydrophobic electrolytes in aqueous and DCE phases respectively, polarized potential window (PPW) from about –0.30 to 0.40 in the Galvani potential scale at aqueouspHαͳcanbeobtained,asdisplayedbythecyclicvoltammogramshown inFigure͸(dottedline).ThepotentialwindowislimitedbythetransferofH+

2– and SO4 fromwatertoDCEatpositiveandnegativepotentials(i.e.watervs. DCE),respectively.Frommechanisticpointofview,ithasbeenproposedthat the first step in the oxygen reduction at the ITIES is the protonation of the electrondonor.Thisisobservedasfacilitatedprotontransferfromaqueousto oilphasebytheelectrondonoractingasligand(L).TheNernstequationfor thisfacilitatedprotontransferprocesscanbedescribedasshownineq.͸[38]: §· w 1/2 w 0'RTDL 2.303 RT 2.303 RT w 'II 'ln¨¸ pK pH (6) ooLHH2FDFF¨¸ a ©¹LH+ where 'wI 1/2 and 'wI 0' are the formal transfer potentials of LH+andH+ o LH o H (0.55 at the W/DCE interface), respectively. LH+ stands for the protonated complex,Drepresentsthediffusioncoefficientofspeciesintheoilphase,Kais theequilibriumconstantoftheprotonationreactionofandpHwistheaqueous pH.HencetheITIESessentiallyfunctionsasprotonpump.Thenextstepisthe reduction of oxygen by the formed protonated complex homogeneously to producehydrogenperoxideorwater.

13 Under aerobic conditions, the current increased remarkably on the positive limitofthepotentialwindowuponadditionofͷmMDFcE,andnoclearreturn peak was observed for the transferred protons (solidline in Fig.6), indicating thattheywereconsumedinhomogeneousreactionaccordingtoeqs.ͳor2. ThetransferofDFcE+generatedintheoxygenreductionisobservedintherange of–0.2to–0.05withthehalf-wavepotentialat–0.12V,andtheonsetpotential of the proton transfer took place at lower potentials, indicating the assisted protontransferbyDFcE,asdescribedby equation5.smallcurrentresulting from proton transfer from the oil phase after reversal of the scan direction at around0.4showsthatoxygenreductionisnotfastenoughtoconsumeallthe transferred protons in the time scale of the experiment. Similar results have beenreportedpreviouslybySuetal.whenDMFcwasusedaselectrondonorfor theoxygenreductionattheITIES[4].TheforwardcurrentenhancementbyDFcE is less than that by DMFc, indicating most likely slower kinetics due to the differenceinelectrochemicaldrivingforce(0.04forDMFcvs.0.57vs.SHEfor DFcE).ThesetwosystemsareclearexamplesofPCETreactionsoccurringatthe ITIES.Ineffect,DFcEactsaslipophilicbasetocomplexH+duringtheforward potentialscan,whilethepolarizedITIESactsaspumptodriveH+transferfrom aqueoustoorganicphase. Figure͹illustratesthepHdependenceofthepresentsystem,inwhichthe onset potential for assisted H+ transfer shifts positively with the increasing aqueouspH.Itcanbenoticedthatthepotentialwindowdecreasesasthe aqueouspHdecreases,therebyconfirmingthatthepotentialwindowislimited

+ 2– bythetransferofH andSO4 Ǥ Thescanratedependenceoftheelectrochemicalsignalobserved byCVwas alsoinvestigated(seeFigure8),showingthatthecurrentmagnitudeforboth DFcE+transferandassistedH+transferincreaseswiththeincreaseinthescan rate.Specifically,itisproportionaltothesquarerootofthescanrate(seeinset inFigure8).Thisscanratedependenceindicatestheoverallprocessislimitedby thelineardiffusionofDFcE+(iontransfer)orDFcE(assistedprotontransfer)to theinterface.Besides,theCVsinFigureͺshowso-called“Osakaimechanism” [39], represented by positive current offset between 0.1 and 0.3 that increases with increasing scan rate. The latter indicates that DFcE partitions

14 betweenthetwophasesandreactswithaqueousprotonsandoxygen,forming DFcEΪthatistransferredbacktooilphaseatpotentialshigherthantheobserved half-wave potential for DFcE+ transfer and thus producing positive current offsetintheCVs.similarbehaviorhasbeenobservedforDMFcsystem[12]. Moreover,theincreasingcurrentsresultingfromthetransferofprotonsfromoil toaqueousphasewiththescanrateduringthereversescanat0.3-0.4 indicatesthatthekineticsofPCETisnotfast. TheuncompensatedresistancebroadenstheobservedpeakofDFcE+-transfer, but if the peak potentials are plotted as function of the scan rate and extrapolatedtozeroscanrate,thepeakseparationapproaches68mV(datanot shown), confirming that the observed species is indeed DFcE+Ǥ Potential was scannedtoevenmorepositivepotentialstoseeifanyadditionalpeaksfromthe formationofDFcE2+orDFcE–HΪcouldbeobserved,buttheonlyeffectswerethe increase of the proton back transfer peak and the peak of DFcE+-transfer, confirming that formation of DFcE2+isslow.Thisisprobablyduetothelow protonaffinityoftheDFcE–H+ǣsecondprotonationofthemoleculeisnotlikely totakeplaceduetotherepulsionofthepositivechargeofDFcE–H+ǡbutinstead theprotonwillfavorassociationwithfreeDFcEorTB–Ǥ The0.1mMDFcE2+ DCE solution prepared by 26 biphasic reaction was studiedinfourelectrodecell.ThecomparisonofCVsofDFcEandDFcE2+ solutionsareshowinFigure9. FigureͻshowsthepeaksforreversibletransferofDFcE+(formedafterPCET, wheretheinitialspeciesinDCEisDFcE)andDFcE2+acrosstheW/DCEinterface atΫ0.16and0.00Vǡrespectively.DFcE+wasformedaccordingtoequationsͳ or2,asdescribedearlier.ThepeakseparationofthetransferpeakofDFcE2+was about 30 mV, confirming that the transferred species had charge equal to 2. The peak current of DFcE2+transfer depended on the square root of the scan rate.TheGibbsfreeenergyoftransferforbothspecieswascalculatedwitheq.7.

wow0wo 'Gtroo zFii'|'II zF ' (7) Thetransferenergyoftheneutralspeciescanbeestimatedbysubtractingthe chargedependentpartofthetransferenergy(eq.8)[40].Thiscanbecalculated

15 forexamplewiththetheoreticalmodelofBorn(eq.9)orAbraham-Liszi,orwith semiempiricalmodelbyOsakaietalǤ[40].

woo woo woo 'Gtr, neutral 'Gtr, ion 'Gtr (charge dependent) (8)

where NA is Avogadro’s constant, e is the elementary charge, H0 is the permittivityofvacuum,andHandHoaretherelativepermittivityofaqueousand oilphase,respectivelytakenas78.54[40]and10.42[41].Theradiusof1,2- diferrocenylethanewasestimatedtobecloseto10%(Crystallographicradiusof ferroceneisreportedas3.65%[42]andthelengthoftheethanechainisthree times C-C bond length of 1.54 % [43]). Eq. ͺ was also used to calculate the transferenergyoftheneutralspecies,andthepartitioncoefficientofDFcE(log PȌbetweenDCEandwaterwasalsocalculatedwitheq.10[44]. 'G wo0 log P tr, neutral 2.3RT (10) ThedifferenceofthetransferenergiesofDFcE+andDFcE2+wascalculatedto be17.3kJ/mol,whilethemeasuredvaluewas16.7kJ/mol.LogPforDFcEwas determinedas3.89or3.75,whenthetransferenergiesmeasuredforDFcE2+and DFcE+ were used, respectively. The values are in reasonable agreement with eachother,butithastobetakenintoaccountthateq.ͻisverysensitivetothe molecularradius,andtheaccurateestimationofthatisuncertain. Themeasuredtransferpotentials,redoxpotentialsanddiffusioncoefficients ofallthespeciesaretabulatedinTable1. Table1:Diffusioncoefficients(D),formalpotentialofoxidationvs.SHEinDCE(E’),theformal w ՜o transferpotentials( 'o I ’)andGibbsfreeenergiesoftransferfromwatertoDCE('Gtr Ȍforall observedspeciesofDFcE. 6 2 Ϋͳ w ՜o Ϋ1 D×10 Ȁcm s E’/vs.SHE 'o I ’ 'Gtr ȀkJmol DFcE 5.8 0.565 Ϋ Ϋ

16 DFcE+ 3.7 0.77 -0.162 -15.6 DFcE2+ 3.8 Ϋ 0.005 0.96 3.4 Mechanism ThemeasurementsindicatethatoxygenreductionbyDFcEdoesnotproceed throughoxygenmoleculesandwichedinsidethetwoprotonatedironcentersof DFcE,ashypothesizedbyFomin[19],asnoDFcE2+isobservedinfour-electrode cellmeasurements.Thusitismorelikelythatthereactionproceedsasproposed byGiraultetal.fordecamethylferrocenethroughdelocalizedtriplettransition state, leading to the formation of DFcE+ and hydrogen peroxyl radical [17].

+ HydrogenperoxylradicalwillthenreactfastwithDFcEandH toformH2O2Ǥ Alternatively,thereactioncouldstillproceedthroughmolecularoxygenlocated betweentwoDFcE–H+molecules,butthiswouldrequiretrimolecularreaction deemingitmoreunlikely.Anotherinterestingquestionis,whethertheelectron is donated by the protonated iron or the non-protonated one. In this case, probablytheprotonationwilltakeplaceoutsideofthemoleculeduetothesteric hindranceoftheotherferrocenylgroup,andthusthenon-protonatedironwould betoofartohaveaneffectonthereaction. Hydrogen peroxide reduction by ferrocene derivatives has been briefly mentioned previously in the literature to explain observed four-electron reductionofmolecularoxygenbutithasbeenlesswellstudied[18,26]. mechanism for ferrocene oxidation by hydrogen peroxide suggested by Fomin

+ indicatesthattheprotonatedferrocenecanreactwithH2O2toformwater,Fc andOHʉradical,whichfurtherreactswithFcandprotontoproducewater[19]. Fromthispointofview,DFcEseemsidealforhydrogenperoxidereduction,as hydrogen peroxide can react with the protonated ferrocenyl group and the generated OHʉ radical can then easily oxidize the other ferrocenyl group. The experimental results show that hydrogen peroxide reduction is faster than oxygenreduction,corroboratingtheproposedmechanism.

Taking into account all the previous results, it can be proposed that O2 reductionbyDFcEcantakeplaceattheW/DCEinterfaceorinthebulkDCE phase(seeScheme5B).ThefirststepconsistsoftheprotonationofDFcEtoform

17 the DFcE-H+ in DCE phase, as observed with other metallocenes. Then, the

+ formedDFcE-H isattackedbydissolvedO2inDCEsideattheW/DCEinterface

+ orintheDCEbulktoproduceH2O2orwaterandDFcE Ǥ Hydrogen peroxide reductionisassumedtoproceedinsimilarmanner,butthereactionisassumed totakeplaceclosetotheinterfaceduetothesmallsolubilityofH2O2inDCE. DFcEcanalsopartitionintoaqueousphaseandreactwithaqueousoxygenor hydrogenperoxide(seeScheme5A),asshownbyH2O2 decomposition experiments.InthatcasenoprotonswerepresentinDCE,sotheobservedDFcE+ inaqueousphasewasthereactionproductofH2O2 reduction by partitioned DFcE.Inbiphasicshake-flaskexperiments,theprotonsareextractedtotheoil phasebyTBΫǤ This fast extraction is followed by slower oxygen reduction initially by DFcE and later by DFcE+inthebulkDCEphase(seeScheme5C). Fromvoltammetryandshake-flaskresults,itcanbeconcludedthatthereaction is controlled by the Galvani potential difference applied at the ITIES, which mainlyfunctionsasthedrivingforcefortheprotonpumpatthesoftmolecular interface. 4. Conclusions

Insummary,wehaveshownthatO2andH2OʹreductionbyDFcEoccursat polarizedwater/DCEinterface.ThefirststepconsistsoftheprotonationofDFcE toformtheDFcE-H+ in DCE phase, either by DFcE facilitated proton transfer observed in the four-electrode cell experiments or by the homogeneous protonationofDFcEinthepresenceofprotonsextractedintheoilphasebyTBΫǤ

+ TheformedDFcE-H isthenattackedbydissolvedO2inDCEsideattheW/DCE

+ interfaceorintheDCEbulktoproduceH2O2andDFcE Ǥ Hydrogenperoxideis reducedbytheprotonatedDFcEinsimilarmanner.Theproducts,H2OʹorH2O, generatedinDCEphaseareextractedintotheaqueousphase.Thisseparationof productsfromreactantsisoneadvantageofthebiphasicsystemusedherein. The reaction can be turned on by driving protons to the oil phase with the Galvani potential difference across the interface, which can be easily tuned by the chemical way (common ion distribution) or by the external potential

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23 Scheme1.Schematicrepresentationoftheinitialcompositionsoftheaqueous phase and the organic phase for studying oxygen reduction in biphasic reaction. Scheme 2. Schematic representation of the initial composition of the aqueous phaseandtheorganicphaseforstudyinghydrogenperoxidedecompositionin biphasicreaction. Scheme 3. Schematic representation of the initial composition of the aqueous phase and the organic phase for studying hydrogen peroxide reduction in biphasicreaction. Scheme 4. Schematicdepiction of the electrochemical cell composition used in thefour-electrodeconfiguration. Scheme5.ProposedmechanismofO2reductionbyDFcEattheW/Ointerface. αaqueousphase,αDCEphase.Hydrogenperoxidecanreplaceoxygeninthe scheme,producingwater.(A):DFcEpartitionstoaqueousphaseandreactswith protons and oxygen or hydrogen peroxide to produce H2O2orH2 (“Osakai mechanism”); (B): Protons facilitated transferred into DCE phase by DFcE followedbyoxygenorhydrogenperoxidereductiontoH2O2orH2partitioning back to aqueous phase; (C): Biphasic shake-flask reaction where TBΫ extracts protonstoDCEphase,followedbyoxygenandhydrogenperoxidereductionin thebulkofDCEphase.

Figure 1. Cyclic voltammogram (20 mV·s–1Ȍ of freshly prepared ͷ mM DFcE solutionunderN2atmosphereoncarbonfiberUME(diameterα10Ɋm).The potentialscalewasreferredtotheDMFc+/DMFccouple. Figure2.UV/Visspectraoftheaqueousphasebefore(control,dashedline)and after (target, solid line) 10 min of aerobic two-phase reaction under stirring conditions:bothoftheaqueoussolutions(controlandtarget)weretreatedwith 0.1 NaI prior to UV/Vis measurements. The dotted and dash-dotted traces correspondtotheUV/VisspectraofDFcEsolutionsinDCEbeforeandafter10 min of aerobic two-phase reaction under stirring conditions (diluted by half), respectively. For the two-phase reaction: theaqueous phase contained 50 mM H2SO4ΪͷmMLiTB(2mL);theDCEphasecontainedͷmMDFcEΪͷmMBATB (2mL). Figure3.CyclicvoltammogramsatPt(diameterα25Ɋm,solidline)and carbonfiberUME(diameterα10Ɋm)locatedinDCEsolutioncontainingͷmM DFcEandͷmMBATBafter10min(dashedline)ofthetwo-phaseshakeflask reactionunderaerobicconditions,inwhichthepotentialscalewasreferredto theDMFc+/DMFccouple.Forcomparison,cyclicvoltammogramofDFcE2+/DFcE+ solution obtained by 26 shake flask experiment (dotted line) under aerobic

24 conditionsoncarbonfiberUME(diameterα10Ɋm)isincluded.Scanrateis20 mV·s–1ǤThehorizontallinedepictsthepositionofzerocurrent.

Figure4.Cyclicvoltammograms(with10PdiameterglassycarbonUME)of2.6 mMDFcEΪͷmMBATBinDCEbefore(solidline)andafter(dashedline)30min biphasichydrogenperoxidereductioninsideglovebox.Thepotentialscalewas calibratedwithrespecttotheDFcE+/DFcEcouple.

Figure5.UV/Visspectraof2.6mMDFcEΪͷmMBATBinDCEbefore(solidline) andafter(dash-dottedline)30minbiphasichydrogenperoxidereductioninside glovebox. Figure6.Cyclicvoltammogramsobtainedwiththeelectrochemicalcellshownin schemeͶintheabsenceofDFcE(dottedline,aerobic,xα0,yα50,zα10)andin thepresenceofDFcE(solidline,aerobic,xα5,yα50,zα10)intheDCEphase; scanrate:50mV·s–1Ǥ Figure7.CyclicvoltammogramshowingthepHdependenceofassistedproton transfer by DFcE under aerobic conditions. The electrochemical cell employed wasshowninschemeͶ(x α 5, yα50,5,0.5correspondingtopH1,2,͵of aqueousphase,respectively,zα0).Scanrateα50mV·s–1Ǥ Figure8.Cyclicvoltammogramsobtainedwiththeelectrochemicalcellshownin schemeͶinthepresenceofDFcE(aerobic,xα5,yα50,zα10)atvariousscan rates: 10, 20, 50, 80, 100 mV·s–1frominnertoouter.Theinsetshowsthe dependence of the cathodic peak current (square) and the assisted proton transfercurrentat0.4(triangle)onthesquarerootofthescanrate. Figure9.ComparisionofiRcompensatedvoltammogramsofͷmMDFcE(dotted line)and0.1mMDFcE2+ (solid line) solutions in the electrochemical cell described in Scheme Ͷ (aerobic, yα50,zα0).Forcomparison,blankCV recordedwithiRcompensationintheelectrochemicalcelldescribedinSchemeͶ (dashedline,aerobic,xα0,yα50,zα0)isalsoincluded.Scanrateα50mV·s–1Ǥ