Activation pathways taking place at molecular copper precatalysts for the oxygen evolution reaction

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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

Activation pathways taking place at molecular copper precatalysts for the oxygen evolution reaction

Cornelis J.M. van der Ham

, Furkan Is¸ ık

, Tiny W.G.M. Verhoeven

, J.W. (Hans) Niemantsverdriet

, Dennis G.H. Hetterscheid

aLeidenInstituteofChemistry,LeidenUniversity,2300RALeiden,TheNetherlands

bDepartmentofChemicalEngineeringandChemistry,EindhovenUniversityofTechnology,P.O.Box513,5600MBEindhoven,TheNetherlands

cSynCat@DIFFER,SyngaschemBV,P.O.Box6336,5600HHEindhoven,TheNetherlands

a r t i c l e i n f o

Articlehistory:

Received7September2016 Receivedinrevisedform 19December2016 Accepted21December2016 Availableonline7January2017

Keywords:

Wateroxidation Copperoxide Catalystactivation

a b s t r a c t

Theactivationprocessesof[Cu^II(bdmpza)2]inthewateroxidationreactionwereinvestigatedusing cyclicvoltammetryandchronoamperometry.TwodifferentpathswhereinCuOisformedweredistin- guished.[Cu^II(bdmpza)2]canbeoxidizedathighpotentialstoformCuO,whichwasobservedbyaslight increaseincatalyticcurrentovertime.When[Cu^II(bdmpza)2]isinitiallyreducedatlowpotentials,a moreactivewateroxidationcatalystisgenerated,yieldinghighcatalyticcurrentsfromthemomenta sufficientpotentialisapplied.Thisworkhighlightstheimportanceofcatalystpre-treatmentandthe choiceoftheexperimentalconditionsinwateroxidationcatalysisusingcoppercomplexes.

©2016PublishedbyElsevierB.V.

1. Introduction

The water oxidation reaction is important to ensure future energystorage andsustainability. Thereactionhasbeenexten- sivelystudiedinthepresenceof homogeneouswater oxidation catalyststhat predominantlyarebasedonnoblemetals suchas ruthenium[1–3]andiridium[4–9].Inparticularincaseofmolecu- larrutheniumsystemsbearingbipyridinetypeligands,mechanistic studieshaveprovidedthecommunitywithdetailedinsightshow wateroxidation catalysisoccurs[10–12].Incase ofotherwater oxidationcatalyststhetrueactivespeciesturnedouttobemetal oxidedepositsthatwereformedfromtheirorganometallicpre- cursorsundertheharshoxidativeconditionsapplied[13–16].In termsofatomabundanceandeconomicviability,complexesthat arebasedonfirstrowtransitionmetalsaremoreinterestingthan theirsecondandthirdrowcounterparts,albeitsuchsystemstypi- callydon’toperatewellunderacidicconditions.Duetosubstantial faster ligand dissociation kineticsat these first row transitions metals,control overthecatalyst structureis considerablymore cumbersome. Nevertheless,molecularcatalystsin case of man- ganese[17],iron[18–20],cobalt[21]andsinceveryrecentlycopper [22–30]havebeenreported.Especiallyincaseofthelatter,lig-

∗ Correspondingauthor.

E-mailaddress:d.g.h.hetterscheid@chem.leidenuniv.nl(D.G.H.Hetterscheid).

andexchangekineticsarefast,andconsequentlyseveralpapers haveappearedwhereincopperoxidesprovedtobethecompetent catalyticspeciesratherthantheirmolecularprecursors[31–35].A fruitfulstrategytopreventformationofcopperoxidesappearsto liewithmulti-denticity[29].Nevertheless,alsothecopperbipyri- dine complexes, first reportedby Mayer et al., appear to react exclusivelyviamolecularsites[22,23],suggestingthatdiscrimina- tionbetweenhomogeneousversusheterogeneouscatalysisismuch morecomplex.Fromearlycobaltpolyoxometallatewateroxidation chemistrythescientificcommunityhasalreadylearnedthatthe formationofwhichtypeofcatalyticspeciesisformedcanbelargely dependentontheexactreactionconditionsapplied,especiallyin caseofhighlydynamicsystems[36–39].

Preliminary water oxidation studies in our lab in the pres- enceof[Cu^II(bdmpza)₂](bdmpza⁻=bis(3,5-dimethyl-1H-pyrazol- 1-yl)acetate, Fig. 1), a structure similar to the aforementioned copperbipyridinesystem,revealedthattheobservedwateroxi- dation activity is strongly dependent on the electrochemical pretreatment of themolecularcatalyst, even thoughthe even- tualcatalyticexperimentswerecarriedoutundertheexactsame conditions.Inlightofthediscussionwhethercatalysisoccursat a homogeneousversusheterogeneous speciesand howonecan control theactivityof these catalytic species,the pretreatment dependencetriggeredustoinvestigatethecatalystactivationpath- waysof[Cu^II(bdmpza)₂]indetail.Inthiscontributionwediscuss twoindependentpathwaystotheformationofCuO,thetrueactive http://dx.doi.org/10.1016/j.cattod.2016.12.042

0920-5861/©2016PublishedbyElsevierB.V.

Fig.1. Pathsofactivationobservedfor[Cu^II(bdmpza)2].

species,whereintheobservedreactivityisgreatlyactivationpath dependent.

2. Experimental 2.1. Complexsynthesis

[Cu^II(bdmpza)₂] was obtained by dropwise addition of 0.33mmolbdmpzaNain25mlmethanoltoasolutionof0.33mmol Cu^II(OTf)₂in25mlmethanol.Afterstirringfor30min,partofthe methanol was evaporated and diethyl ether was added to the reactionmixturetoyieldablue-greenprecipitateovernight.The crystallinematerial wasdried in vacuoand recrystallized from methanolat−20^◦C,yielding[Cu^II(bdmpza)₂].Theinfraredspectra of[Cu^II(bdmpza)2]areingoodagreementwithpreviousreported data(seesupportinginfo)[40].ESIMSm/z(calc):558.2(558.2,[M]⁺ 580.2(580.2,[M+Na]⁺),612.2(612.2,[M+Na+MeOH]⁺).

2.2. Electrochemicalmethods

All experiments were performed on an Autolab PGSTAT 128N.Allelectrochemicalexperiments wereperformedin one- compartment25mlglasscellsinathree-electrodesetup,usinga goldworkingelectrode(WE).Inallcasesagoldwirewasusedasa counterelectrodeandallexperimentsweremeasuredagainstthe reversiblehydrogenelectrode.Theelectrochemicalcellwasboiled twiceinMilliporeMilliQwater(>18.2Mcmresistivity)priorto theexperiment.TheAuworkingelectrodeconstistedofadiscand wasusedinahangingmeniscusconfiguration.TheWEwascleaned byapplying10VbetweentheWEandagraphitecounterelectrode for30sin a 10%H₂SO₄ solution.Thiswasfollowed bydipping theWEina 6MHCl solutionfor 20s. Theelectrode wasflame annealed,followed byelectrochemicalpolishingin0.1MHClO₄, whilescanningbetween0and1.75VversusRHEfor200cycles at1Vs⁻¹.Eventually5␮lofa18mMsolutionof[Cu^II(bdmpza)2] inethanolwasdropcastedontotheworkingelectrodeanddried inair.TheelectrolytesolutionswerepreparedfromMilliQwater (>18.2Mcm resistivity) and ≥99.9995% NaOH obtained from Sigma-Aldrich.

Theelectrochemicalquartzcrystalmicrobalance(EQCM)exper- iments were performed in a 3ml Teflon cell purchased from Autolab.Asaworkingelectrode,anAutolabEQCMelectrodewas used,whereina200nmgoldlayer(1.5cm²)wasdepositedona quartzcrystal.SincethehydrogenbubblesoftheRHEreference electrodedisturbedthefrequencyduringtheEQCMmeasurements, aPd/H₂referenceelectrodewaspreparedbyapplyingapotential of−4.0VbetweenthePdwireandaplatinumcounterelectrode forapproximately10min.Priortotheexperiment thepotential ofthePd/H₂ electrode relative totheRHEwasdetermined.All EQCMdatawerecorrectedtotheRHEscale.Thesensitivitycoef- ficient of thequartz crystal (c_f)was determinedby deposition

therelationshipbetweenthefandtheamountofPbdeposited ontotheelectrode.Thesensitivitycoefficientwasdeterminedto be1.26×10⁻⁸gcm⁻²Hz⁻¹(Fig.S3).Forthecatalyticexperiments, 10␮lofan18mMsolutionof[Cu^II(bdmpza)₂]inethanolwasdrop- castedontotheEQCMelectrode,yetduetoitsgeometryit was impossibletoavoidthatsome[Cu^II(bdmpza)2]wasdepositedon thequartzpartoftheassembly. Theactualamountof catalytic materialthatisincontactwiththeworkingelectrodeistherefore overestimated.

Duringtheonlineelectrochemicalmassspectrometry(OLEMS) measurementsthegaseousproductsformedattheworkingelec- trodewerecollectedviaahydrophobictip(KEL-Fwithaporous Teflonplug)incloseproximitytothesurfaceoftheworkingelec- trodeandanalyzedinaPfeifferQMS200massspectrometer.An Ivium A06075 potentiostat was used in combination with the OLEMSexperiments.AdetaileddescriptionoftheOLEMSsetupis availableelsewhere[40].

2.3. XPS

XPS measurements were carried out with a Thermo Scien- tificK-Alpha,equippedwithamonochromatic small-spotX-ray sourceanda180^◦doublefocusinghemisphericalanalyzerwitha 128-channeldetector.Spectrawereobtainedusinganaluminium anode (AlK␣=1486.6eV) operating at 72W and a spot size of 400␮m.Surveyscansweremeasuredataconstantpassenergy of 200eV and region scans at 50eV. The background pressure was2×10⁻⁸mbarandduringmeasurement4×10⁻⁷mbarArgon becauseofchargecompensation.

Samples for XPS were prepared by chrono amperometry in 0.1MNaOH at pH13, using 0.8cm² pyroliticgraphite discs as workingelectrodes.Priortouse,theelectrodesweresandedwith waterproof 2500 grit sandpaper. A total amount of 180nmol [Cu^II(bdmpza)2]wasdropcastedontotheelectrodesandthediscs wereusedinahangingmeniscusconfiguration.

3. Results

Thebdmpza⁻ ligands of[Cu^II(bdmpza)₂] are centrosymmet- ricallyarrangedaroundthecopperion,forminga trans-CuN₄O₂ complex,whereinthecoppersiteiscoordinativelysaturated[41].

However,itisnotunprecedentedthatoneoftheligandarmsof bdmpza⁻dissociatesinfavorofcoordinationofwater[42],pro- vidinganentryintocatalysisata molecularspecies.Theredox chemistry of [Cu^II(bdmpza)₂] was explored by dropcastingthe complex ontoa gold workingelectrode (WE). Fig.2 showsthe cyclicvoltammogramof90nmol[Cu^II(bdmpza)2]dropcastedonto a0.050cm²(geometricsurfacearea)goldelectrodeina0.1Maque- ousNaOHsolutionatpH13.Theexperimentwasstartedat1.2V versusRHEandscannedtowardspositivepotentialsinitially.Inthe firstscanrelativelylittlecatalyticcurrentisobserved,whichcon- traststhesecondandthirdscans.Scanningthepotentialupto2.0V versusRHEresultedinasmallpeak(designated1inFig.2)inthe cyclicvoltammogram,whichdoesnotexceedthecurrentdisplay- ingthatofablankgoldelectrodeunderthesameconditions(seeFig.

S4).Whilestartingabove1.2VversusRHEandscanningintoapos- itivedirectionfirstorscanninginnegativedirectionimmediately doesnotresultinchangesinthereductionchemistry.Below1.2V aseriesofsharpreductionpeaks(2–5inFig.2)canbeobserved that lieon topof a broad negative baselinecurrent that starts roughlyat0.8VversusRHE.Thenegativebaselinecurrentcontin- uesuponscanningintothepositivedirectionuntil0.8Vafterwhich avery sharpoxidativepeak(7) isobservedat0.9V versusRHE.

Fig.2.Thefirstthreescansofacyclicvoltammetryexperimentof[Cu^II(bdmpza)2]in0.1MNaOHata100mVs⁻¹scanrate(left).Thefirstscanofthecyclicvoltammetry (red)isdepictedinmagnifiedviewseparately(right)fromthe2nd(blue)and3rdscan(green).(Forinterpretationofthereferencestocolourinthisfigurelegend,thereader isreferredtothewebversionofthisarticle.)

Withanonsetofroughly1.6VversusRHEasubstantialcatalytic waveisobservedinthecyclicvoltammetrythatgreatlyexceeds theoxidative currentthatwasobservedinthefirstscan. When aninitialstartingpotentialwasselectedbelow0.8VversusRHE, suchacatalyticcurrentcanalreadybeobservedattheveryfirst oxidativescan,suggestingitistriggeredbyaninitialreductionof [Cu^II(bdmpza)₂].Inthesecondreductivescanabroadfeatureat 0.5VversusRHEisobservedfollowedbyacatalyticreductivecur- rentwithanonsetat0.2VversusRHE.Thislattercatalyticfeatureis mostlikelyduetoreductionofdioxygenthatisformedabove1.7V versusRHEinthe2ndscan.Fromhereontheredoxfeaturesinthe cyclicvoltammogramdonotfurtherchangeuponpotentialcycling.

Thecatalyticcurrentdoesincreasesomewhatfromscan2–3,indi- catingthatstillmoreactivewateroxidationsitesareformedonthe WE.

The displayed catalytic activity in the presence of [Cu^II(bdmpza)₂] was further evaluated by chrono amperome- tryexperimentsusing both gold(Fig.3)and pyrolytic graphite electrodes. In this series of experiments 1.2V versus RHE was selected as a standby potential as no oxidative or reductive currentswereobservedatthis potentialin thefirstscanofthe voltammogramof [Cu^II(bdmpza)₂]. Ongoldaninitialcurrentof 7.2␮Acm⁻² wasobserved after 120s of amperometryat 2.0V thatsteadilyincreadesto12␮Acm⁻² after 30min(Fig.3).This suggeststhatsomeactivationof[Cu^II(bdmpza)₂]toa(more)active catalytic species takes place under these oxidative conditions.

Inlinewiththeoxidativecurrentobservedintheamperometry, onlineelectrochemicalmassspectrometry(OLEMS)datadoshow formationofsomedioxygenoverthecourseoftime(Fig.3,top panel).

X-rayphotoelectronspectroscopyof[Cu^II(bdmpza)2]thatwas kept at 2.0V versus RHE for 10min dropcasted on a pyrolytic graphiteelectrodeshowstwoindependentsignalsinthebinding energyregionofthe2pelectronsofcopperat931.1and933.3eV.

Thelow bindingenergypeak(931.3eV)canbeduetoeitherCu metaloraCu(I)species,suchasCu₂OTheCuLMMAugerpeaks (notshown),fallinginthebindingenergyrange,excludethepres- enceofmetallicCu(whichhasadistinctpeakat565,5eV),and, interestingly,alsothatofCu(OH)₂.Thesignalat933.3eVissimi- lartothatofCuOforwhichwefindabindingenergyof933.4eV (slightlyhighervalueshavebeenreportedinliterature:i.e.933.9eV [34],933.7eV[43],933.8eV[44]).Theshake-upstructureismostly characteristicofCuO,exceptforthesmallbutvisiblefeatureonthe highbindingenergyside(about942eV),whichisalsopresentinthe

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2

j / mA cm-2

1500 1000

500 0

t / s

Oxygen production / a.u.

Fig. 3.Chrono amperometry of 360nmol [Cu^II(bdmpza)2]dropcasted onto a 0.72cm²goldelectrode(geometricalsurfacearea)in0.1MNaOH.Thepotential wassetat2.0VversusRHEfor600s,thensetto0.0Vforanother600sandthen returnedto2.0Vforthelast600s(bottompanel).Theevolutionofdioxygenwas followedsimultaneouslyusingOLEMS(toppanel).

spectrumofCu2O.HenceweinterprettheexsituXPSmeasurement of[Cu^II(bdmpza)2]keptat2.0VversusRHEfor10mintoamixture ofCu(I)andCu(II)oxides,whilethespectrashownoevidencefor CumetalnorCu(OH)_2.

Apparently [Cu^II(bdmpza)2] slowly converts to CuO at 2.0V versusRHE.SinceCuOisaknownwateroxidationcatalyst[45–47]

andthecatalyticactivityincreasesuponprolongedelectrolysisit seemslikelythatCuOisresponsibleformost–ifnotall–catalytic current. Infact atthis pointwe have noreason tobelievethat anyoftheobservedcatalyticactivityshouldbedescribedtothe [Cu^II(bdmpza)2]speciesitself.

Whenthepotentialaftertheinitialamperometryexperimentat 2.0Vissetat0.0Vfor10minandthenplacedbackat2.0Vversus RHE,aconsiderablyhighercatalyticcurrentisobserved.Thisisin linewithOLEMSdataatthis stage,whichshowsthat aconsid- erableamountofdioxygenis produced(Fig.3,toppanel).After 60sacurrentof56␮Acm⁻²wasrecordedthatslowlydecreased

Fig. 4. Electrochemistry of [Cu^II(bdmpza)2] combined with a quartz crystal microbalanceshowingthelossofmassoftheelectrode(toppanel)duringcyclic voltammetry(bottompanel)ina0.1MaqueousNaOHsolutionofpH13.Estart=1.3V, scanrate=1mVs⁻¹.

to28␮Acm⁻².Evenhighercatalyticcurrentsareobtainedwhen [Cu^II(bdmpza)₂]isimmediatelyreducedat0Vandthenbroughtto 2.0V.X-rayphotoelectronspectroscopynowonlyshowsasingle peakat933.3eVsuggestingthatfullconversiontoCuOhastaken place.Thedecreaseofthecatalyticcurrentuponprolongedelectrol- ysisismostlikelyduetodepletionofCu²⁺fromtheelectrodeunder theseconditions.Itislikelythatuponreduction[Cu^II(bdmpza)2] convertstometalliccopper(0)attheelectrode interfacethat in turnisoxidizedtoCuO.Forseveralrelatedsystemsconversionof molecularspeciestocopper(0)hasbeenobservedinrelationtothe catalytichydrogenevolutionreaction[48–50].

FromthecyclicvoltammetryinFig.2itwasobservedthatthe reductionof[Cu^II(bdmpza)2]ontheelectrodehasagreatinfluence onthecurrentobservedintheoxidativeregimeand isbelieved toproceedviainitialformationofmetalliccopper.Electrochemi- calquartzcrystalmicrobalance(EQCM)incombinationwithcyclic voltammetryisapowerfultooltogaininsightintotheprocesses takingplaceontheelectrode.Inthesestudiesroughly180nmol [Cu^II(bdmpza)2]inEtOHwasdropcastedontotheEQCMelectrode.

Fig.4showstheEQCMof[Cu^II(bdmpza)2]between1.3and0.2V versusRHE.Thebottompanelshowsthepotential–currentrelation- shipfromtheCV,whereasthetoppanelshowsthecorresponding masschangeofthequartzcrystal,determinedfromthechangein oscillationfrequencyofthequartzcrystalsimultaneouslywiththe cyclicvoltammetryexperiment.Atascanrateof1mVs⁻¹abroad andclearlyvisiblereductionpeakisobservedwithanonsetof0.7 versusRHE,duetoreductionof[Cu^II(bdmpza)₂]tocopper(0).The featuresobservedinthecyclicvoltammetryuponreductionlead- ingtoformationofcopperarestronglyscanratedependent.Similar tothecyclicvoltammetrydepictedinFig.2,anegativebaselineis observedinbothnegativeandpositivescan.Inadditiontoreduc- tionof[Cu^II(bdmpza)2]thisinpartmaybeduetoreductionofsome dioxygenthatleaksintotheTeflonEQCMcell.Thetoppanelof Fig.4showsthattheinitialmassoftheelectrodedoesnotchange uponscanningthepotentialfrom1.3to0.7VversusRHE.Beyond

Fig.5.CyclicvoltammogramofapolycrystallineCuelectrodeat100mVs⁻¹in0.1M aqueousNaOHsolution(pH13).Theexperimentisstartedat0.5VversusRHEand scannedtowardspositivepotentialsfirst.

theonsetofthereductionwaveat0.7VversusRHEinthecyclic voltammogram,themassofthecrystalstartstodecrease,indicat- ingthatdesorptionofmaterialfromtheelectrodetakesplace.This massdecreasecontinuesinthepositivegoingscanandreachesa plateauat0.4VversusRHE.

Thesharpoxidationpeakobservedat0.8VversusRHEisconsid- erablylessdependentonthescanrate.Thisismostlikelyastripping peak[51],illustratedbyasmallandabruptdecreaseinmass.The secondandthirdscanshowconsiderablylesscurrentandsmaller masschangesthanthefirstscanandisinagreementwithahigh conversionof[Cu^II(bdmpza)₂]tometalliccopperatthisstage.In Fig.4,atotalchargeof8.5mChaspassedthroughtheWEtoreduce [Cu^II(bdmpza)₂],whichequalsatotalof88nmolelectrons.Asome- whatlargeramountof180nmol[Cu^II(bdmpza)₂]wasdropcasted ontothe goldelectrode, butit proved difficulttoexcludelarge amountsofmaterialfromendinguponthequartzratherthanon thegoldsurfacewhichiselectrochemicallyactive.Moreoversome ofthechargeflowmaybeduetoreductionofdioxygenasitproved tobedifficulttoexcludeairleakingintotheTeflonEQCMcell.Nev- erthelessthesenumbersareinlinewithaconsiderablepartofthe dropcastedmaterialtobereducedtocopperintheEQCMexper- imentandagreewithfullconversionof[Cu^II(bdmpza)2]totake placeduringprolongedamperometryat0.0VversusRHE.

The EQCM data in Fig. 4 shows that relatively little ligand (∼20nmolbdmpzaequaling5%ofdropcastedbdmpza)islostfrom theelectrodeinterfaceduringthereductionof[Cu^II(bdmpza)₂]to copper(0).Alsotheamountofcopperlosttothesolutionatthe copperstrippingpeakat0.8VversusRHEislimited(after three scans∼18nmolCu,10%ofalldropcastedcopper).

In the cyclic voltammetry of polycrystalline copper a small oxidation waveis observedat 0.6V versusRHEthat isascribed tooxidation ofcopper(0)toCu2O(Fig.5,signal1)[52]. Avery similaroxidationwaveis observedat0.6Vincase ofdeposited [Cu^II(bdmpza)₂](Figs.2and4).FurtheroxidationtoCuO,however, isconsiderablymorefacilein caseofdeposited[Cu^II(bdmpza)2] comparedtopolycrystallinecopper,whichshowsbroadfeatures (signals 2 and 3 in Fig. 5)as the result of oxidation of differ- entcrystaldomains [51–53].Thecopperdeposit obtainedfrom [Cu^II(bdmpza)₂]onlyshowsasmallandverysharpoxidationwave (Fig.2,signal7).Wehavebeenunabletoreproducetheseelectro- chemicalfeaturesofusingothersourcesofcopper,includingcopper oxidenanoparticles,Cu(OTf)₂ and apolycrystallinecopperelec- trode(seeFig.5andthesupportinginfo).Inallthesecasescopperis considerablyeasierremovedfromtheelectrodesurfacecompared

tosamplesof[Cu^II(bdmpza)2].Inlinewiththeremarkablestability ofthecoppercatalystobtainedfrom[Cu^II(bdmpza)₂]comparedto othersourcesofcopper,alsotheobservedcatalyticcurrentismore persistentandsignificantlyhigher.Itappearsthatthebdmpzalig- andhascleareffectonthestabilityofthecopperparticlesthatare formed,andseemstopreventsolvationofCu²⁺uponreoxidation ofcoppertothe+IIoxidationstate.Inlinewithsuchahypothesis thepresenceofconcentratedsolutionsofcarbonate[54]andespe- ciallyborate[34,55]haveadramaticinfluenceonthestabilityand thereforeactivityofcopperdepositsunderoxidativeconditions.It seemsthatsimilartocoordinatinganions,thebdmpzaligandhas astabilizingeffectoncopperoxideversussolvation,therebypos- inganinterestingapplicationoftheuseoforganicligandsand/or additivesinheterogeneouswateroxidationchemistry.

4. Conclusions

Twopathshavenbeenidentified wherein[Cu^II(bdmpza)₂] is convertedtoCuO,whichisthetrueactivespeciesinthewateroxi- dationreaction.Underoxidativeconditions[Cu^II(bdmpza)2]slowly convertstoCuOandleadstomoderateactivityonly.Initialreduc- tion of [Cu^II(bdmpza)₂] below 0.8V versus RHE leads to initial formationofcopper(0),whichultimatelyconvertstoCuOunder catalyticconditions.Thispathleadstoasubstantialhighercatalytic activityundertheconditionsexplored.Onethereforehastobevery carefulwhichstandbyand/orstartpotentialisselectedpriortocat- alyticwateroxidationmediatedbymolecularcoppercomplexes, asthesesettingsmayhaveadramaticeffectonthedisplayedcat- alyticactivityasillustratedabove.Alsothebdmpza⁻ligandplays animportantrolein theobservedcatalyticactivity,sincedrop- castingvariousothercoppersourcesresultsinmediocrestability andactivity.Clearlyonecannotusesuchalternativecoppersources convincinglyasacontrolfortheformationofactiveCuOnanopar- ticles.The precisemechanismwhereinbdmpza⁻ influencesthe catalyticactivityisnotwellunderstoodatpresent.

Acknowledgements

TheauthorsgratefullyacknowledgeDr.TomvanDijkmanand Prof.Dr.ElisabethBouwmanforsupplyingthebdmpzaNaligands usedinthiswork.ThisworkwasfinanciallysupportedbytheERC (StG637556).

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod.2016.12.

042.

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