Influence of the metal center of metalloprotoporphyrins on the electrocatalytic CO2 reduction to formic acid

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ContentslistsavailableatScienceDirect

Catalysis

Today

jo u rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d

Influence

of

the

metal

center

of

metalloprotoporphyrins

on

the

electrocatalytic

CO

2

reduction

to

formic

acid

Yuvraj

Y.

Birdja

a

,

Jing

Shen

a,b

,

Marc

T.M.

Koper

a,∗ aLeidenInstituteofChemistry,LeidenUniversity,POBox9502,2300RALeiden,TheNetherlands bChemistryandChemicalEngineeringDepartment,HunanInstituteofEngineering,Xiangtan,China

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22August2016

Receivedinrevisedform23January2017 Accepted27February2017

Availableonline23March2017

Keywords:

Carbondioxidereduction Hydrogenevolution Formicacidformation Immobilizedmolecularcatalysts Metalloporphyrins

a

b

s

t

r

a

c

t

Electrocatalyticconversionofcarbondioxidehasgainedmuchinterestforthesynthesisofvalue-added chemicalsandsolarfuels.Importantissuessuchashighoverpotentialsandcompetitionofhydrogen evolutionstillneedtobeovercomefordeeperinsightintothereactionmechanisminordertosteerthe selectivitytowardsspecificproducts.Hereinwereportonseveralmetalloprotoporphyrinsimmobilized onapyrolyticgraphiteelectrodefortheselectivereductionofcarbondioxidetoformicacid.Noformic acidisdetectedonCr-,Mn-,Co-andFe-protoporphyrinsinperchloricacidofpH3,whileNi-,Pd-, Cu-andGa-protoporphyrinsshowonlyalittleformicacid.Rh,InandSnmetalcentersproducesignificant amountsofformicacid.However,thefaradaicefficiencyvariesfrom1%to70%dependingonthemetal center,thepHoftheelectrolyteandtheappliedpotential.Thedifferentiationofthefaradaicefficiency forformicacidonthesemetalloprotoporphyrinsisstronglyrelatedtotheactivityoftheporphyrinfor thehydrogenevolutionreaction.CO2reductiononRh-protoporphyrinisshowntobecoupledstronglyto

thehydrogenevolutionreaction,whilstonSn-andIn-protoporphyrinsuchstrongcouplingbetweenthe tworeactionsisabsent.TheactivityforthehydrogenevolutionincreasesintheorderIn<Sn<Rhmetal centers,leadingtofaradaicefficiencyforformicacidincreasingintheorderRh<Sn<Inmetalcenters. In-protoporphyrinisthemoststableandshowsahighfaradaicefficiencyofca.70%,atapHof9.6and apotentialof−1.9VvsRHE.Experimentsinbicarbonateelectrolytewereperformedinanattemptto qualitativelystudytheroleofbicarbonateinformicacidformation.

©2017TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

Inthepastfewdecadestheconsequencesofanthropogenic car-bondioxideaccumulationintheatmospherehavebeenaddressed repeatedly.Itisnowadaysgenerallyacceptedthattheincreasing carbondioxidelevels intheatmosphere poseseriousproblems ifnoactionistaken[1].TheemissionofCO2hasincreasedsince

theindustrialrevolution,leadingtoanincreasedamountofCO2in

theatmosphere[2].AccumulationofatmosphericCO2leadstothe

greenhouseeffectwhichcontributestoglobalwarmingandclimate change.Anotherimportantsustainabilityissueisthedepletionof fossilfuelsourceswhichiscausedbyanincreasingworld popu-lationandachanginglifestyle,resultinginanincreasingenergy demand.Mankindisstillstronglydependentonfossilfuelsforits energyconsumption.Adrawbackoftheuseoffossilfuelsisthe

∗Correspondingauthor.

E-mailaddress:m.koper@chem.leidenuniv.nl(M.T.M.Koper).

productionofCO2aftercombustionwhichisoftensimplyreleased

in the atmosphere. In the past couple of years much research hasbeendonetomitigateCO2accumulation[3–6]andsearchfor

renewableenergysources[7–9].ApromisingwaytoutilizeCO2

isbyelectrochemicalCO2 conversion.Comparedtoother

meth-odstheadvantageofelectrochemicalconversion,whenoperational onanindustrialscale,istwo-fold:itreducestheCO2emissionsin

theatmosphereononehand,anditproducesrenewablefuelsand commoditychemicalsontheother.Moreover,additional advan-tagesarethefactthattheprocesscanbecarriedoutatambient conditions,watercanbeusedashydrogensourceand,ifthe elec-tricityusedisproducedfromrenewablesources,onecancontribute toacompletelysustainablecarboncycle.However,therearestill significanthurdleswhich shouldbeovercomeorcircumvented, suchasthecompetitionofthehydrogenevolutionreaction,high overpotentialsforCO2reduction,poorsolubilityofCO2inaqueous

mediaandthepoorselectivityforspecificfuels.For implementa-tioninthecurrentinfrastructure,liquidfuelsaremoreconvenient thangaseousfuels.Thereforemuchworkhasbeenfocusedonthe

http://dx.doi.org/10.1016/j.cattod.2017.02.046

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selectiveproductionofmethanol,ethanolandformicacid.These fuelscanbeemployeddirectlyinfuelcellstoproduceelectricity (e.g.DirectMethanolFuelCellandDirectFormicAcidFuelCell). ElectroreductionofCO2toformicacidislesscomplexand

there-foreeasiertooptimizecomparedtoalcohols,asonlytwoelectrons needtobetransferred.

Molecularcatalystshave been gaining more attentionlately as they are relatively inexpensive and more abundantly avail-ablecomparedto(noble)metalcatalysts.Theyusuallyshowhigh activitiesandgood selectivitiesforvarious reactionswhich can betunedbymodifyingthecatalystwithadditionalligands,using electron-donating or electron-withdrawing groups [10–13]. In homogeneouselectrocatalysis,thesecatalystsareoftendissolved innon-aqueous solvents,astheyare poorlysoluble inaqueous electrolytes.Furthermore,largeamountsofcatalystareneededto dissolveintheelectrolyte.CO2canbindtoametalcenterwhich

resultsinitsactivation[14]bya weakenedC–Ointeractiondue totransferofelectrondensity.Aftercoordinationtoametal cen-ter,reactionscantakeplacewhichwereinitiallynotpossiblefor freeCO2.Manydifferentmolecularcatalystshavebeenreported

forCO2 reductionsuchasporphyrins,phthalocyanines,cyclams,

phosphinesandpolypyridines[15,16].

Immobilizationofmolecularcatalystsshouldinprinciple com-binethebestofbothworlds:molecularcatalysisonheterogeneous surfaces[17,18].Asystemwithhighefficiencyandselectivitycan be createdwhereby the catalyst structure can be modified by additionofspecificligandse.g.tochangethecatalystelectronic propertieswhichisveryusefulformechanisticstudiesor control-lingtheselectivity.Furthermore,onlyasmallamountofthecatalyst isrequiredandevencatalystsinsolubleincertainsolventscanstill beusedwhenimmobilized onasurface.Deactivation processes oftenencounteredinhomogeneoussystems,suchasdimerization andaggregationofthecatalyst,canbecircumvented.Inthe lit-eraturemanydifferent systemsbased onimmobilizedcatalysts havebeendescribed:covalentattachmentofthecatalystbyusing e.g.aryldiazoniumsalts,4-aminopyridine[19–21],non-covalent attachmentofthecatalyst(drop-casting)[22,23]anddispersionof thecatalystinpolymerfilms[24–27].

In this work we will focus on one type of molecular cata-lysts,namelymetalloprotoporphyrins(MPPs)whichareasubgroup of the more general metalloporphyrins. Metalloporphyrins are complexesthatconsistofametalcenterwithinaheterocyclic mac-rocyclecomposedoffourpyrrolegroupsattachedtoeachother withmethinebridges.Protoporphyrinshavetwovinyl,two pro-pionicacidandfourmethylgroupsattachedtotheporphyrinring asshowninFig.1.Metalloprotoporphyrinsareanimportant pre-cursorforessentialmoleculesinbiologysuchastheheme-group inourredbloodcellsandchlorophyllsinplants(protoporphyrins withrespectivelyanFe2+andMg2+metalcenters).Furthermore

ithasbeenshownthat (immobilized)metallo(proto)porphyrins aregoodcatalystsfore.g.theoxygenreductionreaction,hydrogen evolutionreaction,carbondioxidereductionandnitratereduction [28,10,16,29].SavéantandRobertandcoworkershaveconducted extensiveresearchonmolecularcatalystsforCO2 reductionand

howtoinfluencetheselectivity[30,31].Theyrecentlyshowedthat COorHCOOHisproducedbychangingthemetalcenterofthe com-plex[32].Theselectivityissueisimportantfroma fundamental pointofviewandthereforealsothesubjectoftheoreticalstudies [33,34],inwhichithasbeenshownthatadifferentbindingmode ofCO2tothemetalcenterleadstoeitherCOorHCOOH.

Inthispaper,theselectivitytowardsformicacidisinvestigated experimentallywheretheroleofthemetalcenterisscrutinized bystudyingthepH effect,theconcomitanthydrogenevolution and thenature of the electroactivespecies (i.e., CO2 or HCO−3)

on different metalloprotoporphyrins immobilized on pyrolytic graphite.

Fig.1. Chemicalstructuremetalloprotoporphyrins.

2. Experimental

The electrochemical experiments were performedin a one-compartment three-electrode cell at room temperature and ambientpressure.Allglasswarewasfirstcleanedbyboilingina 1:1mixtureofconcentratedsulfuricandnitricacid.Beforeeach experimenttheglasswarewasboiledthreetimesinultrapurewater (MilliporeMilliQgradientA10system,18.2Mcm).Thelong-term electrolysisexperimentswerecarriedoutinatwo-compartment three-electrode cell(H-type cell), where theworking electrode (WE)andcounterelectrode(CE)compartmentswereseparatedby anafionmembrane(Nafion115).

TheWEwasapyrolyticgraphite(PG)discwithadiameterof 5mmor 10mm usedina hanging meniscusconfiguration.The largePGelectrodes wereusedfor HPLC measurementsto gen-erate largeramounts ofproducts.The reportedcurrent density isnormalized bythegeometricsurface areaoftheWE.A plat-inumgauzeandareversiblehydrogenelectrode(RHE)inthesame electrolytewereusedasCEandreferenceelectrode(RE), respec-tively.Unlessmentionedotherwise, thepotentials inthispaper arereferredtothisreferenceelectrode.Priortoeachexperiment, theWEwaspolishedwithsandpaper(firstP600andthenP1000) andultrasonicatedforapproximately2–3mininwater.Blankcyclic voltammogramswererecordedatascanrateof500mVs−1until

astablevoltammogramwasobtained(typicallyaround50cycles) toensureacleanPGsurface.Afterimmobilizationofporphyrinsa blankcyclicvoltammogramwasrecordedagaininorderto qualita-tivelyverifytheimmobilizationoftheporphyrin(asshowninFig. S.1intheSI).Metalloprotoporphyrins(MPPs)wereimmobilizedon PGbydropcastingfroma0.01MboratesolutionofpH10inwhich theporphyrinwasdissolvedtoaconcentrationof0.5mM[35].

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Fig.2. BulkpHoftheusedelectrolytesbeforeandafterCO2saturation.

purgingthecellwithCO2(Linde,Carbondioxide4.5)foratleast

20min.Hydrogen6.0Scientific(Linde)wasusedfortheRHE ref-erence electrode. A -AutolabType III(MetrohmAutolab B.V.) wasusedfor voltammetricexperiments withsamplecollection andanIviumStatorCompactStat(IviumTechnologies)wasused toperformchronoamperometryandelectrochemicalimpedance spectroscopy.Experimentswerecarriedoutinunbuffered perchlo-ricacidofpH1and3andinavarietyofdifferentphosphatebuffer solutionsinthepHrangeof3–12(seeTableS.1inthe Suppor-tingInformation).AlthoughtheactualpHwillbelowerwhenthe electrolyteissaturatedwithCO2,thedifferentelectrolyteswillbe

referredtobytheirpHasmeasuredpriortoCO2saturation.Fig.2

showstherelationbetweenthepHbeforeandafterCO2saturation

indicatingthatthepHafterCO2 saturationisapproximatelythe

same(6.6)foralmostallneutralandalkalinephosphatebuffers. Aswewillseelater,therearedifferencesbetweentheseinitially neutralandalkalineelectrolytesintermsoffaradaicefficiencies and product distribution. Therefore for a better discrimination betweenthedifferentelectrolyteswerefer tothem bythe ini-tialbulkpH.ForcorrectmeasurementsversustheRHEscale,the LuggincapillaryandtheRHEcompartmentwerealsofilledwith CO2 saturated electrolyte. Furthermore,under reduction

condi-tions,especially whenH2 isevolved,thepHneartheelectrode

(“localpH”)maydifferfromthepHinthebulk.Thismayaffectthe onsetpotentialforproductformationaswellasproduct selectiv-ity.Asquitehighcurrentswereobtained,especiallyattheø10mm PGelectrode,ohmicdropcouldnotbeneglected.Hence,the solu-tionresistancewasdeterminedbypotentiostaticelectrochemical impedancespectroscopy.Theobtainedvaluewasusedtocorrect thevoltammogramsaftertheelectrochemicaldatawascollected. Forchronoamperometry(electrolysisexperiments)the potentio-stat’sIRcompensationfunctionwasusedtocompensateforohmic dropduringmeasurement.DetailscanbefoundintheSupporting InformationSection2.

OnLineElectrochemicalMassSpectrometry(OLEMS)was uti-lizedforthedetectionofvolatilereactionproducts.Atip,which is placedclose (≈10␮m)totheelectrode surface, continuously collects volatilereaction productsfrom theelectrode interface. Thetiphasadiameterof0.5mmandconsistsofaporousteflon cylinder(averageporesize10–14␮m)inaKel-Fholderandis con-nectedtoanEvoLutionmassspectrometer(EuropeanSpectrometry SystemsLtd.)byaPEEKcapillary[36].AQuadrupoleMass Spec-trometerPrismaQMS200(Pfeiffer)isbroughttovacuumwitha TMH-071Pturbomolecularpump(60ls−1,Pfeiffer)andaDuo2.5

rotaryvanepump(2.5m3h−1,Pfeiffer).Priortoexperiments,the

tipwascleanedin0.2MK2Cr2O7 in2MH2SO4 andrinsedwith

ultrapurewater.ASEMvoltagebetween1200Vand2400Vwas usedforthedifferentmassfragments.Allmassfragmentsshoweda decayduringmeasurementwhichistheresultofslowequilibration ofthepressureinthesystem.Thiswascorrectedforbysubtracting adoubleexponentialfittodatapointswherenochangeinactivity isobservedfromthewholedataset.Themassfragmentsshownin thispaperareallbackgroundcorrectedinthismanner.

OnlineHighPerformanceLiquidChromatography(onlineHPLC) isthetechniqueemployedtoanalyzenon-volatilereaction prod-ucts.AsimilartiptotheoneforOLEMS,however,withoutaporous tefloncylinder,isplacedneartheelectrodesurface[37].Samples witha volumeof60␮lwerecollectedwitha fractioncollector (FRC-10A,Shimadzu)atarateof60␮lmin−1(LC-20ATpump,

Shi-madzu).Sincethescanrateofthepotentialsweepduringsample collectionwas1mVs−1,eachsampleheldtheaveraged

concentra-tionofa60mVpotentialdifference.Thesampleswereanalyzed aftervoltammetrywithHPLC(ProminenceHPLC,Shimadzu).The sampleswereplacedinanauto-sampler(SIL-20A)whichinjects 20␮lofthesampleintothecolumn.AnAminexHPX87-H (Bio-Rad)columnwithaMicro-GuardCationHCartridge(Bio-Rad)in frontwereused.Theeluentwas5mMH2SO4andtheeluentflow

rate0.6mlmin−1.Thecolumnand therefractiveindex detector

(RID-10A)weremaintainedatatemperatureof35◦C.

The reported results were all reproduced at least twice andallelectrochemicalmeasurements(CV, OLEMS,onlineHPLC andChronoamperometry)showedqualitativelythesameresults (HCOOH trend,current densities,onset potentials, etc.).Froma quantitativepointofview,theresultssometimescouldbeslightly different,which is ascribedtoa differentPGsurface each time afterpolishingthePGelectrodeascanbeobservedfromtheblank voltammogramsrecordedpriortotheexperiments.Thisisalsothe reasonthatforthestandarderroranalysisintheHPLCresults,only theconcentrationanalysisofliquidphase istakenintoaccount (concentrationsareoftenlowandthebaselinenoisy,whichleads todifferentpeakareasfordifferentexperiments).

3. Resultsanddiscussion

3.1. ActivityofmetalloprotoporphyrinsinperchloricacidpH3

Foraproperattributionoftheeffectofthemetalcenteronthe activityorselectivityfor theelectrocatalyticCO2 reduction, the

influenceofthebaresubstrate(PG)andtheporphyrinmacrocycle shouldfirstbeknown.Therefore,CO2reductiononpristinePGand

onmetal-lessorfreebasePPimmobilizedonPG(denotedas PP-PG)wasinvestigated.WithonlineHPLCduringvoltammetryitwas confirmedthatnoformicacidorotherliquidproductsareproduced atanypotentialonpristinePGnoronPP-PG.On-Line Electrochem-icalMassSpectrometryshowedthattheonlyproductfromCO2

reductiononPGandPP-PGisH2 (seeFig.S.2intheSupporting

Informationandalsoreference[38]).

Someoftheinvestigatedprotoporphyrinswithcertainmetal centersproducenoformicacidoramountslowerthanthe detec-tionlimit.TheseprotoporphyrinshaveaCr,Mn,CoorFecenter. WithOLEMS,nogaseousproductsotherthanH2areobservedon

MnPP,InPP,CrPP,SnPPandGaPP(Fig.S.4).OnFePP,RhPPandCuPP smallamountsofCH4aredetectedandonNiPPsmallamountsof

COandCH4aredetectedasshowninFig.S.5.CoPPwasrecently

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Fig.3. LinearsweepvoltammetryinCO2saturated0.001MHClO4+0.099MNaClO4. Scanrate:1mVs−1.

evolutionreaction(HER)orCO2reduction,whilePPpresumably

blocksactivesitesforHERandCO2reductiononPG.Thisconfirms

thatthemetalcenterisofimportanceforcatalysis.Thefluctuations intheCVatverynegativepotentialsaretheresultofH2 bubble

formationandtheirdetachmentfromtheelectrodesurface.The currentspikesappearskewedinsomeofthelatervoltammograms becauseofthepost-measurementOhmicdropcorrection.

InadditiontotheMPPswhich arenotactiveforformicacid productionfromCO2,thereisasetofMPPswhichproducetrace

amountsofformicacid,asshowninFig.4a.ThesearetheMPPswith Ni,Ga,PdorCumetalcenters.Theresultshavebeenreproducedand thestandarderrorsintheconcentrationareshowninthegraphsas well.OLEMSresultsagaindonotshowsignificantamountsofother CO2reductionproductsbesidesH2.

ThemostinterestingMPPsforthisstudyareSnPP,InPPandRhPP astheseareabletoproducesignificantamountsofformicacidfrom CO2asseeninFig.4b.AswiththeotherMPPsthereareno

signif-icantamountsofreactionproductsotherthanH2 observedwith

OLEMS.Alloftheseprotoporphyrinsshowthesametrendinformic acidconcentrationduringLinearSweepVoltammetry,withSnPP andRhPPhavingaslightlylessnegativeonsetpotentialforHCOOH formationthanInPP.InandSnmetalcenterswerealsoidentifiedto beactiveforCO2reductiontoformicacidonphtalocyanineswhich

aresimilarmolecularcatalyststoporphyrins[39].More interest-ingisthefactthatRhPPproducessignificantamountsofformic

Fig.5. CO2reductiononPGinRhPPcontainingelectrolytesandimmobilizedRhPP onPG.Electrolytesolution0.001MHClO4+0.099MNaClO4.Scanrate:1mVs−1.

acidfromCO2.IthasbeenreportedthatInandSnmetalelectrodes

mainlyproduceHCOOH,whileRhmetalmostlyformsH2[40].Rh

metalonlyshowsactivityforCO2reductionatelevatedpressures

[41].Rhcomplexeshavebeenshowntobeactivefor hydrogena-tionofCO2 toformicacid,albeitthattheyoftendonotoperate

inaqueousmediawithoutspecificligands[42–44].Muchresearch hasbeendoneonCO2hydrogenationtoformicacid,but

electrocat-alyticreductionofCO2toformicacidonRhporphyrinsorsimilarRh

molecularcatalystshasnotbeeninvestigatedindepth.Olderwork existsonRhcomplexesdissolvedinnon-aqueousmedia[45,46]for whichformicacidproductionwasalsoobserved.

InFig.5,acomparisonismadebetweenimmobilizedRhPPon PGanddifferentamountsofRhPPintheelectrolytewithapristine PGelectrode.ThecurrentdensitiesandthetrendsinHCOOH con-centrationaresimilar.However,atacertainpotentialtheformic acidconcentrationreachesamaximumandstartstodecreasefor RhPPinsolution,whiletheRhPP-PGelectrodecontinuesto pro-duceformicacid.ThemaximumforRhPP-PGishigherandatmore negativepotentialscomparedtoRhPPinsolution.Another obser-vationisthatalargerconcentrationofRhPPinsolutionleadsto aloweramountofformic acidproduced. Thiscanbeexplained byinhibition of activesiteswithmore RhPPmolecules present in solution. These results show thesuperiority of immobilized

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Fig.6.CO2reductiononformicacidproducingMPPsin(a)0.1MHClO4and(b)phosphatebufferofpH6.8.Scanrate:1mVs−1.

metalloprotoporphyrinswithrespecttometalloprotoporphyrins dissolvedintheaqueouselectrolyte.

3.2. Activityofmetalloprotoporphyrinsinotherelectrolytes

TheformicacidproducingMPPsfromthepreviousparagraph werestudiedfortheirpHdependenceisinvestigatedinorderto obtainmoremechanisticinsight.Theaimistodeducetheoriginof theactivityofRhPPforCO2reductiontoformicacidandidentify

possibledifferencesbetweentheMPPsaswellastoidentifythe optimalconditionsforformicacidformation.

Inthe0.1MHClO4electrolyte,alloftheseMPPsproduce

signif-icantlylessformicacidthaninpH=3HClO4electrolyteasshown

inFig.6a.Interestingly,adifferencebetweenthethreeformicacid producingMPPsisobserved.InpH1InPPandRhPPstillproduce someHCOOHwhileSnPPonlyshowsnegligibleamountsofHCOOH. ThecurrentgeneratedbytheInPPismuchsmallercomparedto thatoftheotherporphyrins.Thisisascribedtoa loweractivity forhydrogenevolution,whichalsoinfluencesthefaradaic efficien-ciesaswillbediscussedinafollowingsection.Moreover,theonset potentialsofthecurrentprofilesoftheMPPsarequitedifferent, inthatRhPPhastheleastnegativeonsetpotentialandInPPthe mostnegativeonsetpotential.Thisisprobablyassociatedwitha differenceinactivityfortheHERontheseMPPs.InpH6.8(0.1M

phosphatebuffer)comparableamountsofformicacidareformed asinpH3,asillustratedinFig.6b.

Differentphosphatebuffersolutionswereusedforthestudyin electrolyteswithapHrangefrom3till12.Eventhoughthebulk pHwillbelowerwhentheelectrolyteissaturatedwithCO2,the

differentelectrolyteswillbereferredtobytheirpHasmeasured priortoCO2saturation.Theformicacidproductionduringthe

neg-ativepotentialsweeponthethreeHCOOH-producingMPPsinthe differentelectrolytesisshowninFigs.7a–9a.Thecurrentprofilesof RhPPshowaplateaubetween−0.9Vand−1.5V(seeinset)while InPPandSnPPshownosuchplateaucurrent(thisisalsovisible inFigs.4b and6b).OnRhPPtheformic acidconcentration pro-fileshowsamaximumwhichappearstobeatpotentialswithin thisplateauregion.Moreover,theHCOOHconcentrationprofiles forRhPPareslightlyshiftedtowardspositivepotentialsforhigher pHwhereasthoseforInPPandSnPPdonotshowsuchapotential shift.Thisisbettervisiblewhentheconcentrationprofilesare nor-malizedbytheirmaximumconcentration(asshowninFig.S.6in theSI).InthesameFigs.7b–9bthehydrogenevolutioncurrenton thethreeMPPsisshowninthesameelectrolytes.Inthese voltam-mogramsnocurrentplateauregionisobservedasforCO2reduction

onRhPP,implyingthatthisplateaufeatureisspecificallyrelatedto CO2reduction.Moreover,thereisacleardistinctionbetweenthe

currentprofilesinthedifferentphosphatebufferelectrolytesforall

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Fig.8. CO2reduction(a)andhydrogenevolution(b)onInPPinelectrolyteswithdifferentpHs.

Fig.9. CO2reduction(a)andhydrogenevolution(b)onSnPPinelectrolyteswithdifferentpHs.

MPPs.Theonsetofhydrogenevolutionseemsstronglyrelatedto thepHofthephosphatebuffers.AthighpH(11.6)thereisless neg-ativeonsetofH2 evolutionandveryhighcurrents,whilelowpH

(4and5.8)showsamorenegativeonsetpotentialforHERandthe currentsarelow.Ingeneral,thecurrentsaremuchhigherforRhPP comparedtoInPPandSnPP,indicatingthebetteractivityofRhPPfor theHER.ComparingthecurrentprofilesforCO2reductionandHER

onallthreeMPPs,theHERissomewhatinhibitedbyCO2reduction

asthereductiononsetisdelayedtomorenegativepotentialand thecurrentsaremuchlower.ForRhPPtheonsetoftheHERisclose to/withintheplateauregion.ThemaximumHCOOHproduction alsolieswithinthispotentialrangeandshiftstopositive poten-tialswithhigherpH.AsthemaximumofHCOOHproductioncan beinterpretedtobetheresultofthecompetitionbetween hydro-genevolutionandCO2 reduction,itseemsplausibletoassociate

thecurrentplateauwiththiscompetition.Thefactthatthecurrent plateauisnotobservedforInPPandSnPPcouldbeduetothe decou-plingorweakcouplingbetweenCO2reductionandHERonthese

materials.TheonsetpotentialoftheHERismorenegativeforSnPP andInPPcomparedtoRhPP.ThecompetitionofCO2reductionand

HERisimportantforhighfaradaicefficienciestowardsHCOOHas willbediscussedinalatersection.

To investigate thepH effect more quantitatively, we define the onset potential for HCOOH and H2. The onset potential is

determinedbased ontheHCOOHconcentrationprofileandthe currentprofiles.Theonsetpotentialbasedontheconcentration isdefinedastheaveragepotentialofthepotentialswherethe con-centrationofHCOOHreaches0.01mM,0.03mMand0.05mM.The potentialscorrespondingtothesedifferentconcentrationsleadto asimilartrend.ThisisshowninFig.S.7intheSupporting Infor-mation.Similarly,theonsetpotentialbasedonthecurrentdensity isdefinedastheaverageofthepotentialscorrespondingto dif-ferentcurrentdensitieswithintherangeof0.25–1mAcm−2.The

onsetpotentialsareconverted totheNHEscale,and thetrends forthe3MPPsareshowninFig.10.Thedatawithintheshaded areacorrespondtothephosphatebufferelectrolytes.Notethatthe analysisoftheonsetpotentialsisonlyperformedforabetter com-parisonbetweenthedifferentMPPsandnottoderiveunderlying mechanisticinformation.ThedifferenceinpHbeforeandafterCO2

saturationasmentionedintheexperimentalsection,willnotaffect theseresults,sincealltheMPPsaremeasuredinthesame elec-trolyte.Thebuffercapacityoftheelectrolytemayplayanimportant roleontheselectivityforCO2reduction[47,48].AsshowninFig.

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Fig.10.Onsetpotentialsbasedon(a)thecurrentforCO2-saturatedsolution,(b)theHCOOHconcentrationprofile,(c)themaximumconcentrationofHCOOHand(d)the currentoftheHER.

isdominanthere.AscanbeseeninFig.10a,theonsetpotential showsalineardependenceonpHforallMPPs.ThepHdependence (slope)isnotthesameforthethreeMPPs.Theonsetpotentialof SnPPshiftswith53±2mVpH−1whileforInPPandRhPPtheyshift

with36±5 and35±4mVpH−1.Aslopeof 59mVpH−1 onthe

NHEpotentialscalewouldindicatea concertedproton–electron transfermechanism[49].ItisdifficulttosayifSnPPfollowsa con-certedproton–electron transfermechanism solelybased onthe analysisoftheonsetpotentials.However,theresultsindicatethat thereisprobablyadifferenceinmechanismbetweenSnPPonone handand RhPPand InPPontheotherhand.Anotherdistinction betweenSnPPandRhPPorInPPisthefactthatatpH11.6only smallamountsofHCOOHareformedonSnPP,whilereasonable amountsofHCOOHareproducedonInPPandRhPP.ForRhPP sim-ilarpHdependencesareobservedfortheonsetofHCOOHandfor H2 formation(Fig.10aandd),indicatingthattheCO2 reduction

toHCOOHonRhPPiscoupledtotheconcomitantHER,asalready suggestedbefore.InPPandSnPPdonotexhibitaclearlinearpH dependenceforHER(seeFig.10d),whichsuggeststhatthetrend observedinFig.10aisrelatedtotheCO2reductionratherthanto

theHER.OnInPPandSnPP,CO2reductionandHERarenotstrongly

coupled,asconcludedbefore.OnRhPPthepotentialsofmaximum HCOOHproduction(Fig.10c)showasimilarslopetothatofthe onsetpotentialsofHCOOH(Fig.10b)confirmingtheearlier obser-vationofapotentialshiftoftheconcentrationprofileswithpH,in contrasttoInPPandSnPPwhereasimilarslopeisonlyobserved

fortheHCOOHmaximaandonsetpotentialofthecurrent(Figs.10a andc).Furthermore,theonsetpotentialsofRhPParealwaysatmore positivepotentialscomparedtothoseofInPPandSnPP.This differ-enceinonsetpotentialsconfirmsthatRhPPismoreactiveforthe HER.

3.3. Faradaicefficiencies

ForafurthercomparisonoftheactivitybetweentheMPPs,the faradaicefficienciesweredeterminedduring2helectrolysisina two-compartmentcell.Thefaradaicefficienciesforformicacidin HClO4,pH3forallthethreeMPPsatdifferentpotentialsareshown

asfunctionoftimeinFig.11.ForRhPPandSnPPthefaradaic effi-cienciesalwaysdecaywithtimetovaluesof≈1–2%.Interestingly InPPseemstoreachasteadystatevalueof≈10–15%.Thisindicates thattheimmobilizedporphyrinsonPGarenotverystableor deac-tivateratherquickly,especiallyRhPP-PGandSnPP-PG.Improving theperformanceoftheimmobilizedporphyrinsisbeyondthescope ofthispaper.However,webelievethatthedeactivationisrelated tothecatalystinsteadofdepositionofpoisoningspeciesonthe surfaceorblockageoftheactivesitesbyintermediates.Asshown inFig.S.9theblankvoltammogramoftheimmobilizedporphyrin onPGiscomparedwiththeblankvoltammogramafterCO2

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Fig.11.Faradaicefficienciesin0.001MHClO4+0.099MNaClO4ondifferentMPPs asfunctionoftime.

deactivationoftheporphyrinstructureordetachmentofthe por-phyrinfromPG.Itis assumedthat thevery negativepotentials ultimatelyaredestructivefortheimmobilizedporphyrins.RhPP showslowerfaradaicefficienciescomparedtoSnPPwhichinturnis lowercomparedtoInPP.Thefaradaicefficienciesdeterminedafter 10mininHClO4pH=3areplottedagainsttheappliedpotentialsin

Fig.13a.Thepotentialwherethehighestfaradaicefficienciesare obtainedinpH3isaroundE=−1.3VforRhPP,aroundE=−1.9V forInPP,andapproximatelyE=−1.5VforSnPP.Thistrenddepends

ontheelectrolyte,ascanbeseeninFig.12b,wherethesameis depictedforaphosphatebufferofpH5.8.Whenonlyexperiments atanappliedpotentialofE=−1.5Vareconsidered,theinfluence ofthepHcanberevealedasseen inFig.S.8intheSI. The ini-tialfaradaicefficiencyofthedifferentMPPsasa functionofpH isshowninFig.13.ThefaradaicefficiencyofInPPatpH9.6seems tobeoptimalandreachesavaluecloseto70%.

3.4. Electroactivespecies

ThefactthateveninquitealkalineelectrolytesHCOOHis pro-duced,couldindicatethatnot(only)CO2istheelectroactivespecies

butforinstancebicarbonateisinvolvedintheformationofformate. IntheliteratureonelectrocatalyticCO2reductiontherehasbeen

controversyabouttherealelectroactivespeciesduringCO2

reduc-tionatdifferentpHandatdifferentcatalysts.OftendissolvedCO2

hasbeenidentifiedastheelectroactivespecies[50,51],butthere arealsosystemswherebicarbonatehasbeensuggestedtobethe keyreactantspecificallyfortheformationofformicacid/formate [52–56].

ToprobetheroleofHCO−3,westudieddifferentconcentrations ofKHCO3aselectrolytewithandwithoutCO2saturation.InFig.14a

itisshownthatahigherconcentrationofKHCO3leadstoalarger

amountofHCOOHformedonRhPPwhennoCO2isspargedthrough

thesolution.IftheelectrolyteissaturatedwithCO2anevenlarger

amountofHCOOHisproduced.ThisisobservedforInPPandSnPP aswell,asshowninFig.14b.TheHCOOHconcentrationonInPPand SnPPisincreasedbyafactorofmorethan10whenCO2ispurged

throughthesolution.TheseresultsgiveafirstimpressionthatCO2

shouldbethedominantelectroactivespecies,however,itisstill notconclusiveenoughtoruleoutHCO−3asreactivespecies.OLEMS experimentsinKHCO3 withandwithoutCO2bubblingshownin

Fig.15,indicatethatCO2doeshaveaninfluenceastheH2evolution

andCO2consumptionaredelayed.EvenwithoutCO2 saturation,

theCO2masssignaldecreasesduringthenegativepotentialsweep

implyingthattheoriginoftheformedHCOOHistheKHCO3

elec-trolyteitself.However,duringthenegativepotentialsweepthepH inthevicinityoftheworkingelectrodeincreasesasaresultofthe reactionsshowninEqs.(1)and(2).ThishigherlocalpHcouldlead toalocalconversionofCO2toHCO−3 (Eq.(3)),whichmayinfluence

thedirectbicarbonatereductionmechanismtoformate.In previ-ousstudies,directbicarbonatereductionhasbeensuggestedon palladium,leadandcopperelectrodes[53–56].Theexperiments performedinthisstudyonlyprovidequalitativeinformationabout theinfluenceofCO2 andHCO−3.Thesimultaneousmeasurement

ofthelocalconcentrationsofformate,CO2andbicarbonate

dur-ing voltammetryis crucial in order to shedmore light on this debate.

2H++2e−→H2 (1)

2H2O+2e−→H2+2OH− (2)

CO2+OH−→HCO−3 (3)

4. Conclusions

Inthisstudyweinvestigatedtheinfluenceofthemetal cen-terofmetalloprotoporphyrinsfortheelectrocatalyticreductionof CO2towardsformicacid.WefoundthatSn,InandRhmetal

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Fig.12. Faradaicefficienciesdeterminedatt=10min,asfunctionofpotentialin(a)0.001MHClO4+0.099MNaClO4and(b)phosphatebufferofpH5.8.

Fig.13.Faradaicefficienciesdeterminedatt=10min,atE=−1.5VasfunctionofpH.

Thehydrogenevolutionreactionplaysanimportantroleinthe differenceinactivitytowardsHCOOHformationonRhPP,InPPand SnPP.RhPPisthemostactiveforHERandInPPtheleastactive. Consequently,InPPshowshighfaradaicefficiencytowardsHCOOH

formationfromCO2andRhPPlowfaradaicefficiency.SnPPliesin

betweenwithmoderateactivityforHERand faradaicefficiency towardsHCOOH.Allcatalystsdeactivatewithtime,however,the deactivation of InPP seems to be less compared to RhPP and SnPP.

AllthreeHCOOH-producingMPPsshowapHdependencefor CO2reductionaswellasforHER.AtverylowpH,theproton

reduc-tionisdominantwhichresultsinlittleornoHCOOHformed.Atvery alkalinepHs,theHERalsoseemstobedominant,leadingtopoor selectivitytowardsHCOOH.TheoptimalpHisaround9.6forInPP andbetween7and10forSnPP.Theidealsituation(highestfaradaic efficiencies)isdifferentforthethreeMPPsintermsofelectrolyte solutionandappliedpotential.Thebestperformanceobservedin thisstudyisforInPPinpH9.6electrolytewherefaradaicefficiencies of≈70%wereobtained.

This workhighlights some important properties of metallo-protoporphyrinsforelectrocatalyticCO2reductiontoformicacid.

However,thereisnoconsensusyetabouttheelectroactivespecies for the formation of formic acid. Quantitative measurement of HCO−3,CO2andHCOOHconcentrationsduringvoltammetrywould

benecessary inthis respect.For a detailed investigationof the mechanism,thesupportoftheoreticalcalculationssuchasinRef. [34]isalsodesirable.

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Fig.15. H2andCO2signalsduringCO2/HCO3reductionin0.5MKHCO3.Scanrate:1mVs−1.

Acknowledgement

J.S.acknowledgestheawardofagrantfromtheChinese Schol-arshipCouncil.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod.2017.02. 046.

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