Journal of Electroanalytical Chemistry 681 (2012) 16-23. © 2012 by authors and © 2012 Elsevier. Preprinted by permission of Elsevier. 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