Distinguishing between homogeneous and heterogeneous hydrogen-evolution catalysis with molecular cobalt complexes

Article abstract of DOI:10.1039/c7cc02188g

Molecular proton-reduction catalysts can decompose under the acidic conditions required for hydrogen evolution, resulting in formation of secondary metal-containing materials. Using an electrochemical quartz crystal microbalance (EQCM), we report a new method for probing electrodeposition of catalytically active heterogeneous material from molecular precursors. The data collected suggest that EQCM can provide a direct measure of the homogeneity of molecular proton-reduction catalysts over short timescales.

Full text of DOI:10.1039/c7cc02188g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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DOI: 10.1039/C7CC02188G
JournalꢀNameꢀ
COMMUNICATIONꢀ
ꢀ
Distinguishingꢀbetweenꢀhomogeneousꢀandꢀheterogeneousꢀ
hydrogen-evolutionꢀcatalysisꢀwithꢀmolecularꢀcobaltꢀcomplexesꢀꢀ
Receivedꢀ00thꢀJanuaryꢀ20xx,ꢀ
Acceptedꢀ00thꢀJanuaryꢀ20xxꢀ
a
a,
DavidꢀJ.ꢀSconyers ꢀandꢀJamesꢀD.ꢀBlakemore *ꢀ
DOI:ꢀ10.1039/x0xx00000xꢀ
ww.rsc.orgꢀ
Molecularꢀ proton-reductionꢀ catalystsꢀ canꢀ decomposeꢀ underꢀ theꢀ
However,ꢀ severalꢀ groupsꢀ haveꢀ foundꢀ thatꢀ cobaloximesꢀ canꢀ
acidicꢀ conditionsꢀ requiredꢀ forꢀ hydrogenꢀ evolution,ꢀ resultingꢀ inꢀ beꢀ unstableꢀ underꢀ theꢀ electrochemicalꢀ conditionsꢀ usedꢀ toꢀ
formationꢀ ofꢀ secondaryꢀ metal-containingꢀ materials.ꢀ Usingꢀ anꢀ driveꢀcatalysis.ꢀForꢀexample,ꢀArteroꢀandꢀco-workersꢀdescribedꢀ
electrochemicalꢀquartzꢀcrystalꢀmicrobalanceꢀ(EQCM),ꢀweꢀreportꢀaꢀ cobaltꢀ metalꢀ formationꢀ fromꢀ 1ꢀ inꢀ aqueousꢀ solutionsꢀ uponꢀ
1
8
newꢀ methodꢀ forꢀ probingꢀ electrodepositionꢀ ofꢀ catalyticallyꢀ activeꢀ reduction. ꢀ Glyoximesꢀ canꢀ undergoꢀ hydrogenationꢀ andꢀ
heterogeneousꢀ materialꢀ fromꢀ molecularꢀ precursors.ꢀ Theꢀ dataꢀ decompositionꢀ followingꢀ reduction,ꢀ asꢀ shownꢀ byꢀ coulometricꢀ
1
9,20
collectedꢀsuggestꢀthatꢀEQCMꢀcanꢀprovideꢀaꢀdirectꢀmeasureꢀofꢀtheꢀ andꢀ spectroscopicꢀ methods.
homogeneityꢀ ofꢀ molecularꢀ proton-reductionꢀ catalystsꢀ overꢀ shortꢀ presenceꢀ ofꢀ strongꢀ acidꢀ (e.g.,ꢀ 4-cyanoanilinium,ꢀ pKaꢀ =ꢀ 7ꢀ ꢀ inꢀ
ꢀ Decompositionꢀ ofꢀ 1ꢀ inꢀ theꢀ
21
22
timescales.ꢀ
MeCN) ꢀhasꢀalsoꢀbeenꢀobservedꢀbyꢀUV-Visꢀspectroscopy. ꢀInꢀ
mostꢀ cases,ꢀ electrochemicalꢀ methodsꢀ and/orꢀ exꢀ situꢀ
spectroscopicꢀ investigationsꢀ wereꢀ usedꢀ toꢀ understandꢀ theꢀ
observedꢀreactivity.ꢀꢀ
Theꢀ developmentꢀ ofꢀ stableꢀ proton-reductionꢀ catalystsꢀ isꢀ keyꢀ
forꢀ theꢀ assemblyꢀ ofꢀ systemsꢀ capableꢀ ofꢀ storingꢀ reducingꢀ
1
,2
equivalentsꢀ asꢀ hydrogenꢀ fuel. ꢀ Molecularꢀ H2-evolvingꢀ
catalystsꢀhaveꢀattractedꢀsignificantꢀattentionꢀbecauseꢀtheyꢀareꢀ
readilyꢀ derivatizedꢀ andꢀ canꢀ beꢀ investigatedꢀ inꢀ solutionꢀ forꢀ
Weꢀ nowꢀ reportꢀ thatꢀ anꢀ electrochemicalꢀ quartzꢀ crystalꢀ
microbalanceꢀ (EQCM)ꢀ mayꢀ beꢀ usedꢀ toꢀ probeꢀ suchꢀ catalystꢀ
decompositionꢀinꢀsituꢀduringꢀelectrodeꢀpolarization.ꢀWithꢀthisꢀ
technique,ꢀaꢀmassꢀchangeꢀcanꢀbeꢀmeasuredꢀdueꢀtoꢀdepositionꢀ
ofꢀ materialꢀ onꢀ theꢀ electrodeꢀ surface;ꢀ inꢀ theꢀ caseꢀ ofꢀ aꢀ fullyꢀ
homogeneousꢀ catalyst,ꢀ noꢀ suchꢀ massꢀ changesꢀ wouldꢀ beꢀ
detected.ꢀ Additionally,ꢀ theꢀ EQCMꢀ canꢀ provideꢀ uniqueꢀ dataꢀ
regardingꢀformationꢀofꢀtransientꢀheterogeneousꢀmaterial—anꢀ
importantꢀconsideration,ꢀasꢀconditionsꢀforꢀH2ꢀevolutionꢀcanꢀbeꢀ
corrosiveꢀtoꢀmaterialsꢀdepositedꢀonꢀtheꢀelectrodeꢀsurface.ꢀꢀ
InꢀorderꢀtoꢀdevelopꢀthisꢀEQCMꢀtechnique,ꢀweꢀusedꢀtheꢀstrongꢀ
3
,4,5
mechanisticꢀinformation. ꢀHowever,ꢀmolecularꢀmechanismsꢀ
canꢀ beꢀ difficultꢀ toꢀ elucidate—eitherꢀ aꢀ molecularꢀ speciesꢀ orꢀ
secondaryꢀ heterogeneousꢀ materialꢀ producedꢀ inꢀ situꢀ couldꢀ
catalyzeꢀtheꢀreactionꢀofꢀinterest.ꢀAꢀnumberꢀofꢀmethodsꢀhaveꢀ
6,7,8
beenꢀ appliedꢀ toꢀ distinguishꢀ betweenꢀ theseꢀ cases.
ꢀ
Electrode-drivenꢀ catalysisꢀ presentsꢀ special,ꢀ additionalꢀ
challengesꢀtoꢀdeterminingꢀmechanisms—theꢀcatalystꢀisꢀusuallyꢀ
inꢀ aꢀ diluteꢀ solution,ꢀ containingꢀ aꢀ highꢀ concentrationꢀ ofꢀ
electrolyte,ꢀandꢀonlyꢀaꢀsmallꢀfractionꢀofꢀmaterialꢀisꢀsufficientlyꢀ
+
–
organicꢀ acidꢀ protonatedꢀ dimethylformamideꢀ ([DMFH] [OTf] ,ꢀ
9
,10
23
nearꢀtoꢀtheꢀelectrodeꢀtoꢀbeꢀactive. ꢀꢀ
Cobaloximeꢀ complexesꢀ areꢀ amongꢀ theꢀ mostꢀ celebratedꢀ
pKaꢀ=ꢀ6.1ꢀinꢀMeCN) ꢀtoꢀunderstandꢀtheꢀchemistryꢀthatꢀcanꢀgiveꢀ
riseꢀtoꢀsecondaryꢀelectrodepositionꢀprecursorsꢀfromꢀ1ꢀandꢀ2.ꢀ
+
Monitoringꢀ ofꢀ treatmentꢀofꢀ1ꢀandꢀ2ꢀwithꢀ 2ꢀequiv.ꢀDMFH ꢀbyꢀ
11,12,13
molecularꢀcatalystsꢀforꢀprotonꢀreduction.
ꢀTheyꢀhaveꢀbeenꢀ
deployedꢀ inꢀ diverseꢀ electrochemical,ꢀ photochemical,ꢀ andꢀ
infraredꢀspectroscopyꢀrevealedꢀtheꢀappearanceꢀofꢀnewꢀsignalsꢀ
–1
atꢀ 614ꢀ cm ꢀ andꢀ 3300ꢀ cm ,ꢀ respectively.ꢀ Theꢀ signalꢀ atꢀ 3300ꢀ
1
4
–1
photoelectrochemicalꢀ systemsꢀ forꢀ hydrogenꢀ evolution. ꢀ Theꢀ
bis(difluoroboryl)-substitutedꢀ complexꢀ 1ꢀ (Schemeꢀ 1)ꢀ isꢀ
especiallyꢀ attractive,ꢀ asꢀ itꢀ canꢀ operateꢀ atꢀ modestꢀ
–1
cm ꢀ isꢀ consistentꢀ withꢀ theꢀ O-Hꢀ stretchingꢀ vibrationꢀ ofꢀ freeꢀ
dmgH2ꢀ(seeꢀESI).ꢀTheꢀabsenceꢀofꢀthisꢀsignalꢀinꢀtheꢀreactionꢀofꢀ1ꢀ
withꢀacidꢀsuggestsꢀthatꢀnoꢀfreeꢀdmgH2ꢀisꢀformedꢀinꢀthisꢀcase.ꢀꢀ
Toꢀ explainꢀ thisꢀ difference,ꢀ weꢀ turnedꢀ toꢀ nuclearꢀ magneticꢀ
1
5,16
overpotentials.
bridgesꢀ(2)ꢀhasꢀalsoꢀreceivedꢀattention. ꢀꢀ
ꢀ Theꢀ analogousꢀ complexꢀ withꢀ protonꢀ
17
11
resonanceꢀspectroscopyꢀ(NMR).ꢀTheꢀ BꢀNMRꢀspectrumꢀofꢀ1ꢀinꢀ
CD3CNꢀshowsꢀaꢀsingleꢀbroadꢀresonance,ꢀcenteredꢀatꢀ42ꢀppm,ꢀ
consistentꢀwithꢀitsꢀparamagneticꢀnatureꢀ(Sꢀ=ꢀ1/2).ꢀAdditionꢀofꢀ3ꢀ
+
equiv.ꢀofꢀDMFH ꢀresultsꢀinꢀattenuationꢀofꢀthisꢀbroadꢀsignalꢀandꢀ
formationꢀofꢀaꢀnew,ꢀsharpꢀsingletꢀatꢀ2ꢀppm.ꢀNoꢀotherꢀsignalsꢀ
wereꢀ present,ꢀ suggestingꢀ formationꢀ ofꢀ aꢀ singleꢀ newꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ1ꢀꢀ
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19
diamagneticꢀcompound.ꢀByꢀ FꢀNMR,ꢀ1ꢀisꢀNMRꢀsilent.ꢀAdditionꢀ undergoesꢀ aꢀ netꢀ two-electronꢀ reductionꢀ beyoVinewdꢀArt–ic1le.0OꢀnliVne,ꢀ
DOI: 10.1039/C7CC02188G
+
ofꢀ 3ꢀ equiv.ꢀ ofꢀ DMFH ꢀ resultsꢀ inꢀ theꢀ appearanceꢀ ofꢀ twoꢀ newꢀ resultingꢀinꢀformationꢀofꢀmetallicꢀcobaltꢀonꢀtheꢀelectrode.ꢀThisꢀ
signalsꢀ–ꢀaꢀsharpꢀsingletꢀatꢀ–152ꢀppmꢀandꢀaꢀbroaderꢀsignalꢀatꢀ– assignmentꢀ isꢀ confirmedꢀ byꢀ X-rayꢀ photoelectronꢀ (XP)ꢀ spectraꢀ
79ꢀppm.ꢀTheꢀsingletꢀatꢀ–79ꢀppmꢀisꢀconsistentꢀwithꢀtheꢀtriflateꢀ collectedꢀ onꢀ carbonꢀ electrodesꢀ polarizedꢀ atꢀ –1.5ꢀ Vꢀ forꢀ 5ꢀ
counterionꢀofꢀtheꢀaddedꢀacid.ꢀꢀ minutes,ꢀinꢀwhichꢀdistinctꢀcobaltꢀsignalsꢀareꢀobservedꢀ(seeꢀESI).ꢀ
BasedꢀonꢀhighꢀsymmetryꢀinꢀNMRꢀspectra,ꢀandꢀtheꢀobservedꢀ Additionꢀ ofꢀ acidꢀ toꢀ 5ꢀ resultsꢀ inꢀ aꢀ markedꢀ enhancementꢀ ofꢀ
uniqueꢀvibrationꢀinꢀtheꢀIRꢀspectrum,ꢀweꢀproposeꢀformationꢀofꢀ cathodicꢀcurrent;ꢀthisꢀcurrentꢀcanꢀdriveꢀbothꢀcobaltꢀdepositionꢀ
2
4
“
free-base”ꢀ3ꢀ(C2vꢀsymmetry)ꢀuponꢀstrongꢀacidꢀaddition. ꢀOut- andꢀ Coꢀ metal-catalyzedꢀ H2ꢀ generation.ꢀ Thisꢀ agreesꢀ wellꢀ withꢀ
of-planeꢀ N-Hꢀ vibrationsꢀ inꢀ free-baseꢀ porphyrinsꢀ haveꢀ beenꢀ theꢀ relativelyꢀ positiveꢀ thermodynamicꢀ potentialꢀ forꢀ H2ꢀ
–1
+/0 23
reportedꢀbetweenꢀ690-710ꢀcm ,ꢀsimilarꢀtoꢀtheꢀsignalꢀobservedꢀ evolutionꢀunderꢀtheseꢀconditionsꢀ(Eºꢀ≥ꢀ–389ꢀmVꢀvs.ꢀFc ). ꢀ
–1 25
forꢀtheꢀreactionꢀofꢀ1ꢀwithꢀDMFH ꢀ(614ꢀcm ). ꢀIntegrationꢀofꢀ
+
19
theꢀ FꢀNMRꢀsingletꢀatꢀ–152ꢀppmꢀversusꢀanꢀinternalꢀstandardꢀ
hexafluorobenzene)ꢀ indicatesꢀ thatꢀ ca.ꢀ 10%ꢀ ofꢀ theꢀ complexꢀ
decomposesꢀtoꢀformꢀthisꢀ“free-base”ꢀmacrocycleꢀ3.ꢀ
(
Protonationꢀ ofꢀ 1ꢀ orꢀ 2ꢀ inꢀ MeCNꢀ thusꢀ appearsꢀ toꢀ generateꢀ
dmgBF2)2H2ꢀ (3)ꢀ orꢀ dmgH2ꢀ (4),ꢀ respectively.ꢀ Massꢀ balanceꢀ
(
dictatesꢀco-generationꢀofꢀaꢀCo(II)ꢀspecies,ꢀpresumablyꢀbearingꢀ
MeCNꢀ ligands.ꢀ [Co(NCMe)6](BF4)2ꢀ (5)ꢀ canꢀ beꢀ preparedꢀ
2
6
chemicallyꢀandꢀisolated; ꢀtheꢀanalogousꢀ[Co(NCMe)6](OTf)2ꢀ(6)ꢀ
shouldꢀbeꢀgeneratedꢀinꢀsituꢀhere.ꢀTheꢀbroadenedꢀtriflateꢀsignalꢀ
19
+
inꢀ FꢀNMRꢀforꢀ1ꢀtreatedꢀwithꢀDMFH ꢀ(FWHMꢀ=ꢀ22ꢀvs.ꢀ5.5ꢀHzꢀ
withoutꢀ cobalt)ꢀ mayꢀ resultꢀ fromꢀ exchangeꢀ atꢀ paramagneticꢀ
Co(II).ꢀ
ꢀ
Figureꢀ1.ꢀCyclicꢀvoltammogramsꢀofꢀsolutionsꢀcontainingꢀcobaltꢀcomplexes.ꢀUpperꢀ
+
panel:ꢀ1ꢀ(grayꢀline)ꢀandꢀ1ꢀplusꢀ3ꢀequiv.ꢀofꢀDMFH ꢀ(blackꢀline).ꢀLowerꢀpanel:ꢀ5 (grayꢀ
ꢀ
+
2+
3
line)ꢀandꢀ5ꢀplusꢀ3ꢀequiv.ꢀofꢀDMFH ꢀ(blackꢀline).ꢀ[Co ]ꢀ=ꢀ2ꢀmM.ꢀ(CH CN,ꢀ0.1ꢀMꢀ
n
+
–
[
4 6
Bu N] [PF ] .ꢀWorkingꢀelectrode:ꢀhighlyꢀorientedꢀpyrolyticꢀgraphite).ꢀ
ꢀ
+
Schemeꢀ1.ꢀReactionsꢀofꢀcobaloximesꢀwithꢀstrongꢀacidꢀDMFH .ꢀꢀꢀ
Inꢀcyclicꢀvoltammetryꢀofꢀ1ꢀcarriedꢀoutꢀinꢀtheꢀpresenceꢀofꢀ3ꢀ
+
equiv.ꢀ ofꢀ DMFH ,ꢀ noꢀ anodicꢀ strippingꢀ currentsꢀ areꢀ observed,ꢀ
suggestingꢀveryꢀlittleꢀorꢀnoꢀcobaltꢀdepositionꢀonꢀtheꢀelectrodeꢀ
Havingꢀobservedꢀtheꢀreactivityꢀdescribedꢀabove,ꢀweꢀturnedꢀ
ourꢀattentionꢀtoꢀtheꢀelectrochemicalꢀbehaviorꢀofꢀtheꢀglyoximeꢀ
complexꢀ 1ꢀ andꢀ solventoꢀ speciesꢀ 5.ꢀ Cyclicꢀ voltammogramsꢀ
collectedꢀonꢀaꢀsolutionꢀofꢀ1ꢀwithꢀaꢀgraphiteꢀworkingꢀelectrodeꢀ
showꢀaꢀvirtuallyꢀreversibleꢀone-electronꢀreductionꢀcenteredꢀatꢀ
(
seeꢀESI).ꢀThisꢀisꢀsurprising,ꢀasꢀexperimentsꢀwithꢀsolutionsꢀofꢀ5ꢀ
ꢀ
always showꢀclearꢀformationꢀofꢀcobaltꢀmetal,ꢀregardlessꢀofꢀtheꢀ
presenceꢀofꢀacid.ꢀꢀ
Thus,ꢀ weꢀ turnedꢀ toꢀ anꢀ electrochemicalꢀ quartzꢀ crystalꢀ
microbalanceꢀ (EQCM)ꢀ toꢀ provideꢀ real-timeꢀ dataꢀ onꢀ cobaltꢀ
depositionꢀ duringꢀ electrodeꢀ polarization.ꢀ EQCMꢀ reliesꢀ onꢀ theꢀ
converseꢀpiezoelectricꢀeffectꢀtoꢀmonitorꢀmassꢀchangesꢀatꢀtheꢀ
working-electrodeꢀ surfaceꢀ duringꢀ anꢀ electrochemicalꢀ
+/0
–
withꢀ theꢀ squareꢀ rootꢀ ofꢀ theꢀ scanꢀ rate,ꢀ asꢀ expectedꢀ forꢀ aꢀ
0.89ꢀVꢀvs.ꢀFc ꢀ(Figureꢀ1).ꢀTheꢀpeakꢀcurrentsꢀincreaseꢀlinearlyꢀ
+
diffusionalꢀ species.ꢀ Uponꢀ additionꢀ ofꢀ 3ꢀ equiv.ꢀ ofꢀ DMFH ,ꢀ theꢀ
reductionꢀeventꢀbecomesꢀfullyꢀirreversibleꢀandꢀtheꢀcurrentꢀisꢀ
enhanced,ꢀconsistentꢀwithꢀH2ꢀevolutionꢀcatalysis.ꢀNoticeably,ꢀinꢀ
theseꢀ scans,ꢀ weꢀ consistentlyꢀ observeꢀ anꢀ additionalꢀ minorꢀ
reductiveꢀfeatureꢀnearꢀ–1.3ꢀV,ꢀsuggestingꢀformationꢀofꢀaꢀnewꢀ
redox-activeꢀspeciesꢀuponꢀacidꢀaddition.ꢀꢀ
2
9
experiment. ꢀ Briefly,ꢀ ifꢀ aꢀ changeꢀ inꢀ massꢀ orꢀ inꢀ viscoelasticꢀ
pressureꢀ occursꢀ atꢀ theꢀ workingꢀ electrode,ꢀ theꢀ resonantꢀ
vibrationꢀ ofꢀ theꢀ electrodeꢀ shiftsꢀ inꢀ magnitudeꢀ andꢀ sign.ꢀ Theꢀ
Sauerbreyꢀ equationꢀ relatesꢀ theꢀ changeꢀ inꢀ frequencyꢀ toꢀ aꢀ
changeꢀinꢀmass,ꢀassumingꢀnoꢀcomplicatingꢀviscoelasticꢀeffects.ꢀ
Theꢀ EQCMꢀ isꢀ exceptionallyꢀ sensitive,ꢀ and,ꢀ underꢀ theꢀ presentꢀ
Forꢀ aꢀ solutionꢀ ofꢀ 5,ꢀ weꢀ observeꢀ aꢀ single,ꢀ irreversibleꢀ
27
reductionꢀwithꢀonsetꢀnearꢀ–1.1ꢀVꢀthatꢀpeaksꢀatꢀ–1.3ꢀV. ꢀThisꢀ
processꢀisꢀdiffusionalꢀ(seeꢀESI)ꢀandꢀvirtuallyꢀirreversible.ꢀAfterꢀ
excursionꢀtoꢀnegativeꢀpotential,ꢀanꢀanodicꢀwaveꢀappearsꢀnearꢀ
experimentalꢀconditions,ꢀcanꢀmonitorꢀmassꢀchangesꢀasꢀsmallꢀasꢀ
±
2
5ꢀ ngꢀ (frequencyꢀ units:ꢀ ±5ꢀ Hz;ꢀ electrodeꢀ area:ꢀ 0.205ꢀ cm ).ꢀ
EQCMꢀ techniquesꢀ haveꢀ beenꢀ usedꢀ toꢀ measureꢀ aqueousꢀ
depositionꢀandꢀstrippingꢀofꢀthinꢀlayersꢀofꢀcobaltꢀmetalꢀinꢀpriorꢀ
2+ work,ꢀencouragingꢀitsꢀapplicationꢀhere. ꢀꢀ
+
similarꢀ toꢀ thoseꢀ measuredꢀ forꢀ anodicꢀ strippingꢀ ofꢀ cobaltꢀ inꢀ
0.3ꢀ Vꢀ (seeꢀ ESI).ꢀ Theꢀ positionꢀ andꢀ shapeꢀ ofꢀ thisꢀ waveꢀ areꢀ
30
28
aqueousꢀ solutions. ꢀ Weꢀ thusꢀ concludeꢀ thatꢀ [Co(NCMe)6] ꢀ
2
ꢀ|ꢀJ.ꢀName.,ꢀ2012,ꢀ00,ꢀ1-3ꢀ
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2+
EQCMꢀstudiesꢀonꢀsolutionsꢀcontainingꢀ[Co(NCMe)6](BF4)2ꢀ(5)ꢀ ofꢀ[Co(NCMe)6] —thisꢀisꢀsupportedꢀbyꢀbothꢀspectroscopyꢀandꢀ
View Article Online
DOI: 10.1039/C7CC02188G
confirmꢀ cathodicꢀ electrodepositionꢀ (Figureꢀ 2(a)).ꢀ Onꢀ theꢀ firstꢀ electrochemicalꢀ findings.ꢀ Theꢀ EQCMꢀ confirmsꢀ thatꢀ cobaltꢀ isꢀ
2+
cathodicꢀscanꢀofꢀaꢀsolutionꢀcontainingꢀ2ꢀmMꢀ5ꢀandꢀ3ꢀequiv.ꢀofꢀ depositedꢀ uponꢀ reductionꢀ ofꢀ [Co(NCMe)6] ,ꢀ andꢀ thatꢀ
+
DMFH ,ꢀanꢀirreversibleꢀcurrentꢀisꢀmeasuredꢀwithꢀaꢀpeakꢀnearꢀ– depositionꢀdoesꢀnotꢀrequireꢀpresenceꢀofꢀacid.ꢀInꢀlineꢀwithꢀtheseꢀ
1.25ꢀ V.ꢀ Duringꢀ theꢀ returnꢀ scanꢀ toꢀ positiveꢀ potentials,ꢀ anꢀ observations,ꢀXPꢀspectraꢀcollectedꢀonꢀelectrodesꢀpolarizedꢀtoꢀ–
oxidativeꢀstrippingꢀprocessꢀoccursꢀatꢀ–0.45ꢀV,ꢀconsistentꢀwithꢀ 1.5ꢀ Vꢀ forꢀ 5ꢀ min.ꢀ inꢀ solutionsꢀ containingꢀ [Co(NCMe)6] either
2+ꢀ
ꢀ
0
30
Co ꢀoxidationꢀonꢀAu. ꢀMassꢀdepositsꢀcathodicallyꢀbeyondꢀ–0.8ꢀ aloneꢀorꢀwithꢀacidꢀshowꢀsignalsꢀforꢀcobaltꢀ(seeꢀESI).ꢀꢀ
V,ꢀandꢀmovingꢀpositiveꢀofꢀ–0.8ꢀVꢀonꢀtheꢀanodicꢀsweepꢀresultsꢀinꢀ
aꢀ rapidꢀ dropꢀ inꢀ mass,ꢀ consistentꢀ withꢀ theꢀ observedꢀ strippingꢀ
current.ꢀ Subsequentꢀ cyclesꢀ areꢀ comparable;ꢀ weꢀ measureꢀ ca.ꢀ
730ꢀ ngꢀ ofꢀ cobaltꢀ metalꢀ reversiblyꢀ depositedꢀ forꢀ eachꢀ cycle.ꢀ
Similarꢀmassꢀdepositsꢀinꢀtheꢀabsenceꢀofꢀacid.ꢀ(SeeꢀESI.)ꢀ
Basedꢀ onꢀ chargeꢀ passedꢀ inꢀ theꢀ voltammetryꢀ withoutꢀ acidꢀ
present,ꢀ theꢀ calculatedꢀ massꢀ depositedꢀ (353ꢀ ng)ꢀ agreesꢀ wellꢀ
withꢀtheꢀpredictedꢀmassꢀstrippedꢀ(356ꢀng).ꢀHoweverꢀtheꢀEQCMꢀ
measuresꢀaꢀlargerꢀactualꢀmassꢀchangeꢀ(795ꢀng,ꢀseeꢀESI),ꢀevenꢀinꢀ
theꢀ absenceꢀ ofꢀ acid.ꢀ Itꢀ isꢀ possibleꢀ thatꢀ tightlyꢀ boundꢀ orꢀ
encapsulatedꢀ solventꢀ mayꢀ increaseꢀ theꢀ apparentꢀ electrodeꢀ
massꢀ(ca.ꢀ1.75ꢀMeCNꢀperꢀCo).ꢀThisꢀisꢀconsistentꢀwithꢀformationꢀ
ofꢀ smallꢀ cobaltꢀ metalꢀ “islands”ꢀ drivenꢀ byꢀ rapidꢀ reductionꢀ ofꢀ
solubleꢀcobalt.ꢀUponꢀadditionꢀofꢀacid,ꢀtheꢀmeasuredꢀamountꢀofꢀ
cobaltꢀ depositedꢀ isꢀ similarꢀ (730ꢀ ng).ꢀ Thus,ꢀ reductiveꢀ currentꢀ
drivesꢀbothꢀCoꢀdepositionꢀandꢀCo-catalyzedꢀH2ꢀgeneration.ꢀ
EQCMꢀexperimentsꢀcarriedꢀoutꢀonꢀaꢀsolutionꢀofꢀcatalystꢀ1ꢀinꢀ
theꢀabsenceꢀofꢀacidꢀareꢀshownꢀinꢀFigureꢀ2(b).ꢀAsꢀexpectedꢀforꢀ
theꢀ pureꢀ compound,ꢀ weꢀ observeꢀ aꢀ singleꢀ virtuallyꢀ reversibleꢀ
Co(II/I)ꢀ couple.ꢀ Noꢀ significantꢀ massꢀ changeꢀ atꢀ theꢀ electrodeꢀ
surfaceꢀisꢀobservedꢀduringꢀvoltammetry.ꢀThisꢀisꢀconsistentꢀwithꢀ
priorꢀchemicalꢀwork—bothꢀtheꢀCo(II)ꢀandꢀCo(I)ꢀformsꢀofꢀ1ꢀareꢀ
essentiallyꢀsolubleꢀinꢀMeCN.ꢀꢀꢀ
Uponꢀ additionꢀ ofꢀ acidꢀ toꢀ theꢀ solutionꢀ containingꢀ 1ꢀ (Figureꢀ
(c)),ꢀ theꢀ usualꢀ currentꢀ responseꢀ isꢀ encountered:ꢀ lossꢀ ofꢀ
2
reversibilityꢀforꢀtheꢀCo(II/I)ꢀcoupleꢀandꢀanꢀincreaseꢀinꢀcathodicꢀ
currentꢀsignifyingꢀmolecularꢀcatalysis.ꢀTheꢀmassꢀdataꢀfromꢀtheꢀ
EQCM,ꢀ however,ꢀ revealꢀ contributionꢀ fromꢀ theꢀ behaviourꢀ ofꢀ
2
+
[
Co(NCMe)6] ꢀ(6)ꢀgeneratedꢀuponꢀadditionꢀofꢀacid.ꢀSpecifically,ꢀ
nearꢀtheꢀonsetꢀofꢀtheꢀpseudo-catalyticꢀcurrentꢀatꢀ–0.75ꢀV,ꢀweꢀ
observeꢀ aꢀ significantꢀ massꢀ increaseꢀ atꢀ theꢀ electrodeꢀ surfaceꢀ
thatꢀ continuesꢀ throughoutꢀ theꢀ cathodicꢀ sweepꢀ (ca.ꢀ 50ꢀ ng).ꢀ
Immediatelyꢀprecedingꢀthisꢀmassꢀincrease,ꢀaꢀshallowꢀdecreaseꢀ
ꢀ
Figureꢀ2.ꢀPotential-dependentꢀcurrentꢀ(blackꢀlines)ꢀandꢀmassꢀ(blueꢀlines)ꢀdataꢀforꢀ
+
inꢀ massꢀ isꢀ measured,ꢀ consistentꢀ withꢀ formationꢀ ofꢀ hydrogenꢀ solutionsꢀcontainingꢀ(a)ꢀ5ꢀandꢀ3ꢀequiv.ꢀofꢀDMFH ,ꢀ(b)ꢀ1,ꢀandꢀ(c)ꢀ1ꢀandꢀ3ꢀequiv.ꢀofꢀ
+
2+
n
+
–
DMFH .ꢀ[Co ]ꢀ=ꢀ2ꢀmM.ꢀ(CH
coatedꢀquartzꢀdisk).ꢀ
3 4 6
CN,ꢀ0.1ꢀMꢀ[ Bu N] [PF ] .ꢀWorkingꢀelectrode:ꢀgold-
bubblesꢀ atꢀ theꢀ electrodeꢀ surfaceꢀ dueꢀ toꢀ catalysis—bubbleꢀ
formationꢀdecreasesꢀtheꢀviscoelasticꢀpressureꢀofꢀtheꢀcontactingꢀ
electrolyteꢀ mediumꢀ andꢀ registersꢀ asꢀ anꢀ apparentꢀ massꢀ
However,ꢀweꢀwereꢀsurprisedꢀtoꢀfindꢀthatꢀXPꢀspectraꢀcollectedꢀ
29
decrease. ꢀ Onꢀ theꢀ returnꢀ anodicꢀ scan,ꢀ theꢀ 50ꢀ ngꢀ ofꢀ onꢀelectrodesꢀpolarizedꢀtoꢀ–1.3ꢀVꢀforꢀ5ꢀmin.ꢀinꢀtheꢀpresenceꢀofꢀ
electrodepositedꢀ materialꢀ isꢀ rapidlyꢀ lostꢀ fromꢀ theꢀ surfaceꢀ ofꢀ 1ꢀandꢀacidꢀshowꢀnoꢀcobaltꢀonꢀtheꢀsurface.ꢀPolarizationꢀatꢀ–0.34ꢀ
theꢀ electrode,ꢀ returningꢀ theꢀ recordedꢀ massꢀ toꢀ nearꢀ itsꢀ initialꢀ Vꢀforꢀ5ꢀmin.ꢀalsoꢀgivesꢀspectraꢀfreeꢀofꢀcobaltꢀsignalsꢀ(seeꢀESI).ꢀ
valueꢀ(i.e.,ꢀΔmꢀ≈ꢀ0ꢀng).ꢀꢀꢀ
Toꢀinvestigateꢀthis,ꢀweꢀcarriedꢀoutꢀlinearꢀsweepꢀvoltammetryꢀ
Notably,ꢀ useꢀ ofꢀ aniliniumꢀ triflateꢀ asꢀ aꢀ protonꢀ sourceꢀ (pkaꢀ =ꢀ usingꢀtheꢀEQCMꢀonꢀsolutionsꢀcontainingꢀ1ꢀandꢀacidꢀ(seeꢀESI).ꢀ
2
1
1
0.6ꢀinꢀMeCN) ꢀdoesꢀnotꢀresultꢀinꢀdetectableꢀdecompositionꢀ Theꢀ linearꢀ cathodicꢀ sweepꢀ appearsꢀ asꢀ itꢀ doesꢀ inꢀ cyclicꢀ
ofꢀ1ꢀasꢀjudgedꢀbyꢀNMRꢀorꢀIRꢀspectroscopy.ꢀEQCMꢀstudiesꢀofꢀ1ꢀ voltammetry,ꢀbutꢀallowedꢀusꢀtoꢀobserveꢀanyꢀmassꢀchangesꢀthatꢀ
withꢀ aniliniumꢀ triflateꢀ doꢀ notꢀ showꢀ massꢀ depositionꢀ duringꢀ immediatelyꢀfollowꢀtheꢀpolarizationꢀthatꢀdepositsꢀcobaltꢀmetal.ꢀ
electrodeꢀ polarizationꢀ (seeꢀ ESI).ꢀ Theseꢀ resultsꢀ agreeꢀ withꢀ Aꢀ voltageꢀ scanꢀ fromꢀ 0ꢀ toꢀ –1.35ꢀ Vꢀ wasꢀ carriedꢀ out,ꢀ andꢀ theꢀ
recentꢀ detailedꢀ studiesꢀ ofꢀ molecularꢀ catalysisꢀ withꢀ 1ꢀ andꢀ aꢀ instrumentꢀ wasꢀ thenꢀ immediatelyꢀ returnedꢀ toꢀ openꢀ circuit,ꢀ
2
2
seriesꢀofꢀsubstitutedꢀaniliniumꢀacids. ꢀꢀ
endingꢀ theꢀ polarization.ꢀ Theꢀ massꢀ profileꢀ showsꢀ aꢀ steadyꢀ
Theꢀavailableꢀdataꢀthereforeꢀsuggestꢀthatꢀtheꢀdecompositionꢀ readoutꢀ (Δmꢀ ≈ꢀ 0)ꢀ priorꢀ toꢀ polarization,ꢀ followedꢀ byꢀ massꢀ
+
pathwayꢀofꢀ1ꢀuponꢀtreatmentꢀwithꢀDMFH ꢀinvolvesꢀformationꢀ depositionꢀuponꢀexcursionꢀbeyondꢀ–0.94ꢀV,ꢀasꢀexpectedꢀbasedꢀ
Thisꢀjournalꢀisꢀ©ꢀTheꢀRoyalꢀSocietyꢀofꢀChemistryꢀ20xxꢀ
J.ꢀName.,ꢀ2013,ꢀ00,ꢀ1-3ꢀ|ꢀ3ꢀ
Pleaseꢀdoꢀnotꢀadjustꢀmarginsꢀ

Pleaseꢀd Co hꢀ ne omt ꢀ Ca do j mu smt ꢀmarginsꢀ
Page 4 of 4
COMMUNICATIONꢀ
JournalꢀNameꢀ
onꢀtheꢀresultsꢀdescribedꢀabove.ꢀHowever,ꢀfollowingꢀtheꢀreturnꢀ ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ
View Article Online
5
DOI: 10.1039/C7CC02188G
toꢀopenꢀcircuit,ꢀtheꢀmassꢀgainedꢀviaꢀcobaltꢀdepositionꢀ(ca.ꢀ30ꢀ ꢀJ.ꢀT.ꢀMuckermanꢀandꢀE.ꢀFujita,ꢀChem.ꢀCommun.,ꢀ2011,ꢀ47,ꢀ12456-
ng)ꢀisꢀlostꢀwithinꢀ10ꢀsecondsꢀ(seeꢀESI).ꢀThisꢀisꢀconsistentꢀwithꢀ
rapidꢀ corrosionꢀ ofꢀ nascentꢀ cobaltꢀ materialꢀ onꢀ theꢀ surfaceꢀ byꢀ
12458.ꢀ
6
7
ꢀ
ꢀ
R.ꢀH.ꢀCrabtree,ꢀChem.ꢀRev.,ꢀ2015,ꢀ115,ꢀ127-150.ꢀ
G.ꢀ M.ꢀ Whitesides,ꢀ M.ꢀ Hackett,ꢀ R.ꢀ L.ꢀ Brainard,ꢀ J.ꢀ Lavalleye,ꢀ A.ꢀ F.ꢀ
Sowinski,ꢀ A.ꢀ N.ꢀ Izumi,ꢀ S.ꢀ S.ꢀ Moore,ꢀ D.ꢀ W.ꢀ Brownꢀ andꢀ E.ꢀ M.ꢀ
Staudt,ꢀOrganometallics,ꢀ1985,ꢀ4,ꢀ1819-1830.ꢀ
Y.ꢀLinꢀandꢀR.ꢀG.ꢀFinke,ꢀInorg.ꢀChem.,ꢀ1994,ꢀ33,ꢀ4891-4910.ꢀ
J.ꢀJ.ꢀStrackeꢀandꢀR.ꢀG.ꢀFinke,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2011,ꢀ133,ꢀ14872-
+
DMFH ꢀ inꢀ solution.ꢀ Thisꢀ observationꢀ agreesꢀ withꢀ theꢀ CVꢀ
experiment—thereꢀisꢀnoꢀanodicꢀstrippingꢀwaveꢀyetꢀmassꢀlossꢀ
fromꢀtheꢀsurfaceꢀisꢀdetected.ꢀ
Thisꢀ process,ꢀ analogousꢀ toꢀ corrosionꢀ ofꢀ cobaltꢀ metalꢀ inꢀ
8
9
ꢀ
ꢀ
3
1
aqueousꢀsolution, ꢀshouldꢀresultꢀinꢀhydrogenꢀevolution:ꢀꢀ
+ 2+
Coꢀ(s)ꢀ+ꢀ2DMFH ꢀ→ꢀCo ꢀ+ꢀH2ꢀ(g)ꢀ+ꢀ2DMFꢀ
1
4875.ꢀ
10
ꢀ
N.ꢀD.ꢀSchley,ꢀJ.ꢀD.ꢀBlakemore,ꢀN.ꢀK.ꢀSubbaiyan,ꢀC.ꢀD.ꢀIncarvito,ꢀF.ꢀ
D'Souza,ꢀR.ꢀH.ꢀCrabtreeꢀandꢀG.ꢀW.ꢀBrudvig,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ
2011,ꢀ133,ꢀ10473-10481.ꢀ
ꢀ W.ꢀ T.ꢀ Eckenhoff,ꢀ W.ꢀ R.ꢀ McNamara,ꢀ P.ꢀ Duꢀ andꢀ R.ꢀ Eisenberg,ꢀ
Biochim.ꢀBiophys.ꢀActa,ꢀBioenerg.,ꢀ2013,ꢀ1827,ꢀ958-973.ꢀ
ToꢀconfirmꢀtheꢀabilityꢀofꢀelectrodepositedꢀcobaltꢀtoꢀevolveꢀH2ꢀ
+
inꢀ theꢀ presenceꢀ ofꢀ DMFH ,ꢀ weꢀ preparedꢀ anꢀ electrodeꢀ withꢀ aꢀ
1
1
smallꢀamountꢀofꢀCoꢀonꢀtheꢀsurfaceꢀbyꢀreductionꢀofꢀ5,ꢀpassingꢀ
2
43ꢀmCꢀduringꢀanꢀelectrodeposition.ꢀExposureꢀofꢀthisꢀelectrodeꢀ
+
1
1
2
3
ꢀ
ꢀ
G.ꢀN.ꢀSchrauzer,ꢀAcc.ꢀChem.ꢀRes.,ꢀ1968,ꢀ1,ꢀ97-103.ꢀ
P.ꢀ Connollyꢀ andꢀ J.ꢀ H.ꢀ Espenson,ꢀ Inorg.ꢀ Chem.,ꢀ 1986,ꢀ 25,ꢀ 2684-
toꢀaꢀsolutionꢀcontainingꢀ6ꢀmMꢀDMFH ꢀresultsꢀinꢀgenerationꢀofꢀ
0ꢀµLꢀofꢀH2,ꢀconfirmingꢀcorrosionꢀofꢀcobaltꢀmetalꢀunderꢀtheseꢀ
conditionsꢀ(seeꢀESI).ꢀꢀ
2
2
688.ꢀ
1
4
ꢀ
ꢀ
ꢀ
L.ꢀM.ꢀUtschig,ꢀS.ꢀC.ꢀSilver,ꢀK.ꢀL.ꢀMulfortꢀandꢀD.ꢀM.ꢀTiede,ꢀJ.ꢀAm.ꢀ
Chem.ꢀSoc.,ꢀ2011,ꢀ133,ꢀ16334-16337.ꢀ
M.ꢀRazavet,ꢀV.ꢀArteroꢀandꢀM.ꢀFontecave,ꢀInorg.ꢀChem.,ꢀ2005,ꢀ44,ꢀ
Theꢀ presentꢀ studyꢀ showcasesꢀ theꢀ potentialꢀ ofꢀ theꢀ EQCMꢀ
techniqueꢀ asꢀ aꢀ usefulꢀ methodꢀ inꢀ futureꢀ studiesꢀ ofꢀ proton- 15
reductionꢀ catalysis.ꢀ Formationꢀ ofꢀ heterogeneousꢀ materialꢀ isꢀ
4
786-4795.ꢀ
X.ꢀ L.ꢀ Hu,ꢀ B.ꢀ S.ꢀ Brunschwigꢀ andꢀ J.ꢀ C.ꢀ Peters,ꢀ J.ꢀ Am.ꢀ Chem.ꢀ Soc.,ꢀ
007,ꢀ129,ꢀ8988-8998.ꢀ
1
1
1
6
7
8
challengingꢀ toꢀ detectꢀ inꢀ electrochemicalꢀ cells,ꢀ andꢀ ourꢀ
experimentsꢀshowꢀthatꢀsuchꢀmaterialꢀcanꢀevenꢀbeꢀtransient—
onlyꢀ presentꢀ duringꢀ reductiveꢀ polarizationꢀ thatꢀ cathodicallyꢀ
2
ꢀJ.ꢀHawecker,ꢀJ.ꢀM.ꢀLehnꢀandꢀR.ꢀZiessel,ꢀNouv.ꢀJ.ꢀChim.,ꢀ1983,ꢀ7,ꢀ
271-277.ꢀ
ꢀ S.ꢀ Cobo,ꢀ J.ꢀ Heidkamp,ꢀ P.-A.ꢀ Jacques,ꢀ J.ꢀ Fize,ꢀ V.ꢀ Fourmond,ꢀ L.ꢀ
Guetaz,ꢀ B.ꢀ Jousselme,ꢀ V.ꢀ Ivanova,ꢀ H.ꢀ Dau,ꢀ S.ꢀ Palacin,ꢀ M.ꢀ
FontecaveꢀandꢀV.ꢀArtero,ꢀNat.ꢀMater.,ꢀ2012,ꢀ11,ꢀ802-807.ꢀ
3
2
protectsꢀ againstꢀ corrosion. ꢀ Thisꢀ isꢀ especiallyꢀ problematicꢀ asꢀ
reductiveꢀ polarizationꢀ isꢀ requiredꢀ toꢀ driveꢀ electrocatalysis.ꢀ
However,ꢀweꢀnoteꢀthatꢀlackꢀofꢀaꢀmassꢀchangeꢀdoesꢀnotꢀexcludeꢀ
formationꢀofꢀsolubleꢀsecondaryꢀmaterialsꢀbyꢀactionꢀofꢀacidꢀonꢀ
catalystꢀprecursors.ꢀNonetheless,ꢀconsideringꢀtheꢀchallengesꢀinꢀ
distinguishingꢀ betweenꢀ homo-ꢀ andꢀ heterogeneousꢀ catalysis,ꢀ
1
9
ꢀ
E.ꢀAnxolabehere-Mallart,ꢀC.ꢀCostentin,ꢀM.ꢀFournier,ꢀS.ꢀNowak,ꢀM.ꢀ
RobertꢀandꢀJ.-M.ꢀSaveant,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2012,ꢀ134,ꢀ6104-
6
107.ꢀ
weꢀ proposeꢀ thatꢀ theꢀ EQCMꢀ canꢀ provideꢀ usefulꢀ real-timeꢀ 20
informationꢀonꢀcatalystꢀstability.ꢀSuchꢀinformationꢀcouldꢀaidꢀinꢀ
ꢀ
ꢀ
ꢀ
E.ꢀ Anxolabehere-Mallart,ꢀ C.ꢀ Costentin,ꢀ M.ꢀ Fournierꢀ andꢀ M.ꢀ
Robert,ꢀJ.ꢀPhys.ꢀChem.ꢀC,ꢀ2014,ꢀ118,ꢀ13377-13381.ꢀ
J.ꢀ T.ꢀ Muckerman,ꢀ J.ꢀ H.ꢀ Skone,ꢀ M.ꢀ Ningꢀ andꢀ Y.ꢀ Wasada-
Tsutsui,ꢀBiochim.ꢀBiophys.ꢀActa,ꢀBioenerg.,ꢀ2013,ꢀ1827,ꢀ882-891.ꢀ
E.ꢀS.ꢀRountree,ꢀD.ꢀJ.ꢀMartin,ꢀB.ꢀD.ꢀMcCarthyꢀandꢀJ.ꢀL.ꢀDempsey,ꢀ
ACSꢀCatal.,ꢀ2016,ꢀ6,ꢀ3326-3335.ꢀ
ꢀA.ꢀM.ꢀAppelꢀandꢀM.ꢀL.ꢀHelm,ꢀACSꢀCatalysis,ꢀ2013,ꢀ4,ꢀ630-633.ꢀ
Burdinski,ꢀF.ꢀBirkelbach,ꢀM.ꢀGerdan,ꢀA.ꢀX.ꢀTrautwein,ꢀK.ꢀWieghardtꢀ
andꢀP.ꢀChaudhuri,ꢀJ.ꢀChem.ꢀSoc.,ꢀChem.ꢀCommun.,ꢀ1995,ꢀ963-
2
1
theꢀ discoveryꢀ andꢀ developmentꢀ ofꢀ highlyꢀ activeꢀ systemsꢀ forꢀ
storingꢀcleanꢀenergyꢀinꢀchemicalꢀfuels.ꢀꢀ
2
2
2
2
3
4
Acknowledgementsꢀ
ꢀ
TheꢀauthorsꢀthankꢀDr.ꢀJustinꢀDouglasꢀandꢀSarahꢀNeuenswanderꢀ
forꢀassistanceꢀwithꢀNMRꢀspectroscopy,ꢀProf.ꢀMishaꢀBarybinꢀforꢀ
useꢀofꢀanꢀIRꢀspectrometer,ꢀandꢀDr.ꢀPremꢀThapaꢀforꢀassistanceꢀ 25
withꢀ X-rayꢀ photoelectronꢀ spectroscopy.ꢀ Thisꢀ workꢀ wasꢀ
9
64.ꢀ
ꢀ
ꢀ
J.ꢀO.ꢀAlben,ꢀS.ꢀS.ꢀChoi,ꢀA.ꢀD.ꢀAdlerꢀandꢀW.ꢀS.ꢀCaughey,ꢀAnn.ꢀN.Y.ꢀ
Acad.ꢀSci.,ꢀ1973,ꢀ206,ꢀ278-295.ꢀ
R.ꢀ A.ꢀ Heintz,ꢀ J.ꢀ A.ꢀ Smith,ꢀ P.ꢀ S.ꢀ Szalay,ꢀ A.ꢀ Weisgerberꢀ andꢀ K.ꢀ R.ꢀ
Dunbar,ꢀInorg.ꢀSynth.,ꢀ2002,ꢀ33,ꢀ75-83.ꢀ
ꢀJ.ꢀKe,ꢀW.ꢀSu,ꢀS.ꢀM.ꢀHowdle,ꢀM.ꢀW.ꢀGeorge,ꢀD.ꢀCook,ꢀM.ꢀPerdjon-
Abel,ꢀP.ꢀN.ꢀBartlett,ꢀW.ꢀZhang,ꢀF.ꢀCheng,ꢀW.ꢀLevason,ꢀG.ꢀReid,ꢀJ.ꢀ
Hyde,ꢀJ.ꢀWilson,ꢀD.ꢀC.ꢀSmith,ꢀK.ꢀMallikꢀandꢀP.ꢀSazio,ꢀProc.ꢀNatl.ꢀ
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2
2
6
supportedꢀ byꢀ theꢀ Universityꢀ ofꢀ Kansas.ꢀ Theꢀ authorsꢀ alsoꢀ
acknowledgeꢀ theꢀ Nationalꢀ Scienceꢀ Foundationꢀ andꢀ Nationalꢀ
InstitutesꢀofꢀHealthꢀforꢀsupportꢀofꢀNMRꢀinstrumentationꢀ(CHE-
7
0
320648,ꢀS10OD016360,ꢀandꢀS10RR024664)ꢀusedꢀinꢀthisꢀstudy.ꢀ
2
8
ꢀ
K.ꢀMech,ꢀP.ꢀŻabińskiꢀandꢀR.ꢀKowalik,ꢀJ.ꢀElectrochem.ꢀSoc.,ꢀ2013,ꢀ
60,ꢀD246-D250.ꢀ
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