Chem Soc Rev

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Catalysis of the electrochemical reduction of carbon
dioxide†
Citethis:Chem.Soc.Rev.,2013,
42,2423
Cyrille Costentin, Marc Robert and Jean-Michel Save´ant*
A
0
6
3
5
S3 The direct and catalyzed electrochemistry of CO2 partake in the contemporary attempts to reduce this
C
2 inert molecule to fuels by means of solar energy, either directly, after conversion of light to electricity,
C
9/ or indirectly in that all elements of comprehension derived from electrochemical experiments can be
33
0110 used in the design and interpretation of photochemical experiments. Following reviews of the activity
y 210. inthefielduntil2007–2008,thepresentreviewreportsmorerecentfindingseveniftheirinterpretation
versité Paris 7 on 25 Februar2 on http://pubs.rsc.org | doi: RDweOwcIe:wi1v.e0rsd.1c.03o30r9tgh//ccA2sucrsg3u5s3t6200a12 rttfteahhhmveaeotaeprtialenerbracsflntoeursronpmrdocireaeleenrmtcpaoearifnsete.saoercIfitniadetallxlyiasnisodathdinndeidtgetieovcaernaorleotgiapnellysinkhseuetosolsymeuwfstouhogcleaennbtnoeatelthyoitosehuinsesn.seoctcahbetajsaestlcaytatrilycolodfwsayatscaatteniavmaerlesyfzaauinvntagudirleaaobnnwldeot.chrAokemmecpormuanpcrgiihnatalghsceimshgeoisemrnepiecurariatglloortornroleeunthsdoleysf
Uni201
CD ber 1 Introduction negative as (cid:3)1.97 V vs. SHE in an aprotic solvent like N,N0-
d by SDecem Among the contemporary energy objectives, the reduction of dstirmateetghiyelsfohramvaemthiduesb(DeeMnFd),e6vealonpded(cid:3)t1o.9a0voiindtwhaeteinr.t7erCmaetadliyatcicy
wnloaded on 11 cCaOrb2oinntodifouxeidlsebiysmameaanjsoroifsssoulea.rIdeneaelrlgyy,.oOnneedwreaaymissttoodcoirnevcetlryt omfatyheexCpOec2tafnroiomntrhadeieclaelcftororcohbetmainicianlgrethdeucvtairoinouosfpCrOod2uatctlsesosneer
Dohe use light to achieve this conversion. The other involves the energeticcosts.8Mostoftheseproductsappearinthediagram
s
bli preliminary conversion of light into electricity and then the represented in Fig. 1. Not represented on the diagram are
u
P
electrochemical reduction of CO2. The first of these approaches multi-carbon products starting with oxalate and other even
has been the object of recent reviews1–3 with particularly note- more important condensation products. Fig. 1 is actually a
worthyexamples.4,5Wewillthereforeconcentrateontheelectro- setofPourbaixdiagramsrelatingthestandardpotentialofeach
chemicalreductionofCO2,havinghoweverinmindthatthedata, CO2 conversion reaction to the pH in water as the solvent,
modelsandallelementsofmechanisticcomprehensionderived pointing to the fact that in most cases not only electrons but
fromelectrochemicalexperimentscanbeusedinthedesignand also protons are involved. This observation should be borne
interpretationofphotochemicalexperiments. in mind when discussing these energetically more favorable
Beforeaddressingthestrategiesdevelopedforcatalyzingthe strategies, in water as well as in non-aqueous solvents. These
electrochemical reduction of CO2, it is useful to remind what strategies essentially consist of catalyzing the target reaction
products are obtained by direct reduction on an electrode by means of a homogeneous or a heterogeneous catalyst.
materialthatdoesnotinterfereinthereactionandthefactors Homogeneous catalysis has essentially, but not exclusively,
thatgovernthecompetitionbetweentheirformations.Thiswill involvedreducedstatesoftransitionmetalcomplexes.Following
betheobjectofthesecondsectionofthisreview. the pioneering work on Ni and Co macrocyclic complexes as
Directreductiononsuchan‘‘outersphereelectrode’’entails potential catalysts for electrochemical CO reduction,10 many
2
theinitialformationofthehighlyenergeticCO2anionradical. metal complexes have been used for this purpose. Recent
The standard potential of the CO2=CO(cid:2)2(cid:3) couple is indeed as reviews11,12 have listed them, sometimes in great detail, until
2007–2008, with however rather cursory descriptions of their
performances. In particular, comparison between the various
Universit´eParisDiderot,SorbonneParisCit´e,Laboratoired’Electrochimie catalystscouldnotbeperformedonsoundbases.Thesearethe
Mol´eculaire,Unit´eMixtedeRechercheUniversit´e–CNRSNo7591,
reasons that the present review pursues the following goals: (i)
BaˆtimentLavoisier,15rueJeandeBa¨ıf,75205ParisCedex13,France.
reportmorerecentfindingseveniftheirinterpretationremains
E-mail:[email protected]
†Partofthesolarfuelsthemedissue. uncertain on the one hand and to analyze and (ii) compare
Thisjournalis c TheRoyalSocietyofChemistry2013 Chem.Soc.Rev.,2013, 42,2423--2436 2423
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ReviewArticle ChemSocRev
A Scheme1ReductionofCO2ataninertelectrode.
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s 7 on 25 Fpubs.rsc.or (aofrfuelClR2liOTnl4neH)1,(cid:3)0H/(CFROTwl2niHt1h,0Ht/h2CeFO)e.2x(cid:3)c(edpatsiohnedoflinthee),hCo2Oriz4oHn2t,aCl2lOin4eHs(cid:3)a,nCd2Oo4f2t(cid:3)h.e9Tfohremsalotipoens
versité Pari2 on http:// mneocreessrairgyodraotuasalyretahveaiplaebrlfeo.rTmhaenlcaettseroofbtjhecetivceatraelqyustisreswrheecnalltinhge
Uni201 or describing of some notions that are deemed useful to this
CD ber purpose.ThisistheobjectofSection3inwhichemphasiswill
Sm
d by Dece boveeprpuotteonntitahleforerlaptiroenpsahraiptisvethsactallienkextpuerrnimoveenrtfsr.eqItuewnicllyaalnsdo
de1
a1 describe the way in which cyclic voltammetry can provide an
Downloshed on acacctaelsyssttsocrteheensiengr.eSleactitoionnsh4ipresviienwsththeefhraommeowgeonrkeouofscaatqaluyisctks dFiegn.s2ityOoxafl1a.t6emyieAldcmin(cid:3)t2haetp0re1pCaarastaivefuenlcetciotrnoloysfisCOof2CcOon2ciennDtrMatFioant.1a6current
bli that convert CO essentially to CO (which can be further
u 2
P
converted to fuels, like, e.g., methanol or hydrocarbons).
The favorable role of acid addition is emphasized and the
section closes with an attempt to benchmark the various in Scheme 1.13–16 They are the results of the homogeneous
catalysts described in the literature. Section 5 is devoted to chemistry of the CO2 anion radical. Dimerization, leading to
thehomogeneouscatalyststhatgiverisetoproductsotherthan oxalate, is a manifestation of the radical character of CO(cid:2)2(cid:3)
CO, i.e., formate and oxalate. Attempts to produce CO + H entailingaone-electronstoichiometry.
2
mixtures in prescribed proportions are discussed. The last CO and carbonate, on the one hand, and formate, on the
sectionisdedicatedtoheterogeneouscatalystsputtingempha- other, result from CO(cid:2)2(cid:3) being a Lewis base and a Brønsted
sis, thanks to a few examples, on the role of the metal of the base,respectively,leadingtoatwo-electronstoichiometry.The
electrode. dimerizationoftheanionradicalisfastinDMF6andinwater
(from pulse radiolysis experiments17), close to the diffusion
limit.Thismechanismwassubstantiatedbyacarefulanalysis
2 Direct electrochemical reduction at of the product ratio oxalate/CO as a function of the concen-
tration of CO . The oxalate/CO product ratio (in the absence
inert electrodes 2
of purposely added water), is an increasing function of the
Mercuryandleadmaybeconsideredasthebestapproximation following parameter: prr¼ krr I (I is the cur-
ofelectrodematerialsthatdonotinterfereinthechemistryof rs ðkrsÞ3=2½CO2(cid:4)3=2D1=2F
CO reduction. Three products are formed upon electrolysis rent density, D the CO diffusion coefficient. k and k are
2 2 rr rs
in DMF at the level of the very negative cyclic voltammetric defined in Scheme 1). The good agreement between the theo-
CO reduction wave,6 namely carbon monoxide, oxalate reticalpredictionsandexperimentsprovides(Fig.2)arigorous
2
and formate, with a quantitative global faradaic yield. The proof of the mechanism, a rare event in direct and catalyzed
reactionpathwaysthatleadtothesethreeproductsareshown electrochemicalreductionofCO .
2
2424 Chem.Soc.Rev.,2013, 42,2423--2436 Thisjournalis c TheRoyalSocietyofChemistry2013
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3 Useful notions
Werecallinthissectionnotionsconcerningcatalysisingeneral
that may be useful in the mechanism analysis and practical
designofcatalyticsystemsfortheelectrochemicalreductionof
CO . The distinction between redox catalysis and chemical
2
catalysis is not a recently uncovered notion but seems worth
recalling each time a booming, and necessarily somewhat
anarchic,attentionispaidtoanewcatalysisfieldasispresently
the reduction of CO . This is the object of the following first
2
subsection.Re-examiningthenotionofturnovernumber,and
turnover frequency, and their possible dependence on over-
A potential is also a timely task for the same reasons. Clarifica-
0
36 tionofthesequestionsinpreparative-scaleconditionsforboth
5
S3 heterogeneousandhomogeneous catalysisistheobjectofthe
C Fig.3Redoxandchemicalcatalysisofelectrochemicalreactions.1:redox
2 following second subsection. Fast screening of the catalytic
C catalysis,2:one-stepchemicalcatalysis,3:two-stepchemicalcatalysis.
9/ properties of molecules by means of a non-destructive technique
33
10
01 such as cyclic voltammetry, serving as guidelines for preparative-
ebruary 2g | doi:10. smcaaldeeesliegcntrifoilcyasnets,pirsogarlessoseas.tTimhieslyisqtuheestoiobnje,cwthoficfhollhoawsinrgectehnitrldy esters and nitriles, which give rise to a quasi-redox catalysis
on 25 Fs.rsc.or psurobvsiedcetiothne. Tbhaseesnootfiomnesadneinveglfouplecdominpathriessoentwbeotwlaesetnsuthbesevcatriioonuss, leadingexclusivelytooxalate.21
versité Paris 7 2 on http://pub cr3ea.1dtaulcyRttiiocendmoooxfleaCcnOudl2ecsahsthedamitsihccuaasvlsecebadetaeinnlytsphirseopfoolsleodwifnorgtsheecteiloenctsr.ochemical 3aTwn.hi2tdehhrOteehavteecertrciopoogonnetdensnieettoiqiouaunlseankcnac0edtcaldtyueskrpi,nsir2coe2tvepedrrefsirneenqStuscehanencmyaeilnm2,hoscotommgoepgnleeenrmaeoleuntwsteod-
Uni201 Inredoxcatalysis,18thecatalystsimplyshuttleselectronsbetween electronstoichiometryreactionscheme.23Thecatalyticcurrent
D er theelectrodeandthesubstrate,actingasanoutersphereelectron in preparative-scale electrolysis then results from the combi-
Cb
d by SDecem tmraonresfeprhryesaicgaelntth.aTnhechveemryicraeal:sothnethelaetctcraotnaslystoisbiseotrbatnaisnfeerdreids nstaetaiodny-sotfateelelcintreoadredeilffecutsriovne ttrraannssfpeorrst, cshkeetmchiceadl rienacFtiigo.n4s,afnodr
ade11 are dispersed in a three-dimensional space instead of being both heterogeneous catalysis (where a film of catalytic mole-
wnlod on confined within a two-dimensional space. Electrochemists are culeshasbeendepositedontheelectrode surface)andhomo-
Dohe keen designing porous electrodes, so porous that they are geneous catalysis (where the catalytic molecules are dispersed
s
bli sometimes named ‘‘volumic electrodes’’. Redox catalysis is inthewholecellcompartment).24,25Itfollowsthat:
u
P
the equivalent of such an electrode where ‘‘volumization’’ dCb SD (cid:2)dC (cid:3) IS
reachesthemolecularscale. C¼(cid:3) A C ¼
dt V dx 2FV
Incontrastwithredoxcatalysis,wherethecatalystactsasan x¼d
outersphereelectrontransferagent,chemicalcatalysisinvolves
more intimate interactions between the active form of the In practically interesting systems, the catalytic reaction is
catalystandthesubstrate.Twosituationsmayariseaccording I
fast,inwhichcase: ¼kC0G intheheterogeneouscaseand
to the strength of these interactions. In the first of these, 2F A Q
bonded interactions in the transition state confer to electron I ¼qffi2ffiffikffiffiCffiffiffiffi0ffiffiffiDffiffiffiffiffiffiðC Þ inthehomogeneouscase.
transfer an innersphere character, which results in a lower F A P Q x¼0
Usingtheclassical definition of the turnovernumber, TON, as
activation energy than for an outersphere electron transfer of
the ratio between the number of molecules transformed by the
same driving force (Fig. 3). Stronger interactions may involve
catalyticreactionandthenumberofcatalystmoleculesemployedto
theformationofanadduct(Fig.3),whichshouldnothowever
achievethistransformation,oneobtainsintheheterogeneouscase:
betoostableforanefficientcatalysistooperate.
Amongothers,thereductiveconversionofvicinaldihalides molC Cb (cid:5)V G
TON¼ ¼ C ¼kC0 Q(cid:5)t ð1Þ
intothecorrespondingolefinshasprovidedaparticularlyclear molðPþQÞ G0 (cid:5)S AG0
illustration of redox and chemical catalysis.19,20 In the case of P P
CO reduction,unambiguousexamplesofredoxcatalysiscould andthus,theturnoverfrequency:
2
notbeestablishedbecauseofdeactivationofthecatalystinthe G
courseofthecatalyticprocess.Indeed,aromaticmolecules,the TOF¼kCA0GQ0 ð2Þ
P
anionradicalsofwhichusuallyserveasredoxcatalysts,undergo
carboxylation, preventing them to act as electron mediators. Inthehomogeneouscase,theaboveexpressionofthecurrent
A noteworthy exception to this rule is provided by aromatic densityisvalidforfastcatalyticprocesses,suchastheonesof
Thisjournalis c TheRoyalSocietyofChemistry2013 Chem.Soc.Rev.,2013, 42,2423--2436 2425
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ReviewArticle ChemSocRev
A
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y 210.
on 25 Februars.rsc.org | doi: Scheme2Generaltwo-electronstoichiometrycatalyticreactionscheme(P/Q:catalystcouple,Asubstrate,Cproduct,B:intermediate).
s 7 pub interestinthepresentdiscussion.Then‘‘purekineticconditions’’ Counting only the catalyst molecules present in the reaction
versité Pari2 on http:// aecolreemctaprcoehdnieseavsetuidrofnaccooerf,roecsfaptaaoslnytdetiaicndgrye-sattocattietoh,newhaenisctdhabcrleaisstauhllmytssetfnrdot,imffcultoshsieoenmt:outtuhael ltTahhyeiesrpaeisrpfpothrromeabacnhasciiesssoaoftfavhaefratieairrnocagenednweimothuesacnuainrnrdgefnhutlopmcroaomcgtepicnaeer,ioswuoshniccbahetatwtlayesketesns.
SCD Unimber 201 DPdd2xC2Q(cid:3)2kCA0CQ¼0; itnhsetedaedtetrhmeincaattaiolynstopfrtehseenTtOinNtahnedwThOolFe(ssoeleutSieocntiionnto4a).cTcohuisntniont
ownloaded by ed on 11 Dece Within this reawctiitohnð–CdQiffÞxu¼s1ion¼l0a;yer,(cid:2)tdhdCexQQ(cid:3)-xp¼r1of¼ile0, obtained by ogtinhentenleyercofuaeuntrasedl,cyuasmltyt.aadlTkyishsiniaesdg,vvboatuhnlutetmaalcgseoeo-smtodhp-osoaeumrsrisfnoaogocnetenrbreeaefolttueiwocsetceotanhfteatthlahyecsetiuscveatallolrpwiowrauoorpsduelrhdreteipetthoesrreootnsf-
DPublish cspoancdeitiinotnegsriast:ionoftheabovedifferentialequationandboundary mouetasnidiengtlheses.reWaectinoontelathyeart tmheaycastearlvyestsaspraeseunsetfiunl tshtoecskolwuthioenn
deactivationofthecatalystinthecourseofelectrolysisoccurs,a
0 sffiffiffiffiffiffiffiffiffiffiffiffi 1
2kC0 possibilitythatisnotavailableinheterogeneouscatalysis.
CQðxÞ¼ðCQÞx¼0exp@(cid:3) D AxA Inanumberofcases,electrontransferbetweentheelectrode
P
andthecatalystissofastthattheNernstlawisobeyed.Then,in
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi boththeheterogeneousandhomogeneouscases:
corresponding to an approximate thickness m¼ D =2kC0.
P A
kC0
Keeping the same definition of TON as in the case of hetero- TOF¼ A ð3Þ
1þexp½fðE(cid:3)E0 Þ(cid:4)
geneouscatalysis,weconsiderthetotalamountofthecatalyst, cat
including both forms, per unit surface area in the reaction wheref=F/RTandE0 isthecatalyststandardpotential,i.e.the
cat
layer:
standardpotentialoftheP/Qcouple.
sffiffiffiffiffiffiffiffiffiffiffiffi In a more general case the kinetics of electron transfer to
D
molðPþQÞm¼S 2kCP0CP0 the catalyst is not so fast and may therefore interfere. Then,
A
assuming the validity ofthe Butler–Volmer rate lawwitha 0.5
leadingtoexactlytheexpressionsofTONandTOFsimilarbut transfercoefficient:27
notidenticaltotheirheterogeneouscounterparts(eqn(1)and F 1
(2),respectively): I ¼ 2kC0G0exp½(cid:3)fðE(cid:3)E0 Þ(cid:4)
A P cat
molC ðC Þ
TON¼ ¼kC0 Q x¼0(cid:5)t ð10Þ 1 1
molðPþQÞ A C0 þ þ
m P khet;catG0exp½(cid:3)fðE(cid:3)E0 Þ=2(cid:4) 2kC0G0
S P cat A P
TOF¼kC0ðCQÞx¼0 ð20Þ in the heterogeneous case, where khSet,cat is the heterogeneous
A C0
P electron transfer standard rate constant (corresponding to a
2426 Chem.Soc.Rev.,2013, 42,2423--2436 Thisjournalis c TheRoyalSocietyofChemistry2013
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ChemSocRev ReviewArticle
intheheterogeneouscase:
qffiffiffiffiffiffiffiffiffi (cid:5) (cid:6)
1 expð(cid:3)fZÞ 2 kCA0 exp (cid:3)f2Z
¼ þ p
TOF TOF0 khSet;cat ffiTffiffiffiOffiffiffiFffiffiffiffi0ffiffi ð4Þ
exp½(cid:3)fðE0 (cid:3)E0 Þ(cid:4)
þ AC cat
TOF
0
andinthehomogeneouscase:
1 ¼expð(cid:3)fZÞþpffi2ffiffiDffiffiffiffiPffiffiepxp(cid:5)(cid:3)f2Z(cid:6)
TOF TOF0 kcSat ffiTffiffiffiOffiffiffiFffiffiffiffi0ffiffi
exp½(cid:3)fðE0 (cid:3)E0 Þ(cid:4)
A þ AC cat ð5Þ
60 TOF0
3
5
3 with:
S
C
C2 TOF ¼kC0 exp½(cid:3)fðE0 (cid:3)E0 Þ(cid:4)
9/ 0 A AC cat
33
0110 inbothcases.
s 7 on 25 February 2pubs.rsc.org | doi:10. tTbcuaehrttianIwstloeytvsreheetnurcwescfnraaaetstpqalylpucyheeseanatmsrrcsayeicnrtatghencearodditnzotethcvrdoeaernsrbptecyoewitpstiewttnahotaditalihelnlofedfiwnopesriprteeeeavanircodephuleranpsettacinoirspotneitacsirouhacnlimoapmrtehbctpaeaeattrtwrasei,lseayoeicsthnnts.
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Uni201 Fig.4Catalysisofelectrochemicalreactions.Electrodeelectrontransfer, Fig. 5, where three linear variation zones can be delineated:
D er chemicalreactionsandsteadystatelineardiffusivetransportinheterogeneous independencefromZatlargevaluesof Z,linearvariationwith
Cb
Sm andhomogeneousmolecularcatalysisofanelectrochemicalreactionaccording an F/RTln10 slope at small values of Z, and, in between, an
ded by 1 Dece tCo0A,Sicshmemaient2ai(nwedithcokn0sctankt),dinurtinhgecthaseewwhhoelerectohuersseubofsteraletcetrboulylksisc.o26ncxe:ndtirsatatinocne, Fco/2nRtrTilbnu1t0ionsloofpeelelcintreoanrtrvaanrisafetirotnotchoerrceastpaolynsdt.inTghetloatttehresekgimneetnict
wnload on 1 tdo:dthiffeuesiloenctrlaoydeerstuhrifcakcnee,sms.:Dfilsmubs(ctrioppt:)doiffruresiaocntioconelffiayceiern(btso.tGtosumbs)crtihpti:cksunrefsasc,e decreases in size and eventually vanishes as electron transfer
Doshe concentrations.Csubscript:volumeconcentrations.G0P,C0P:totalsurfaceand becomesfasterandfaster.
bli volumeconcentrationsofthecatalyst.S:electrodesurfacearea,V:cell Itthusappearsthatoneofthebestwaystocharacterizethe
u
P compartmentvolume.I:currentdensity,k:catalyticsecondorderrate intrinsic catalytic properties of a molecule is the value of its
constant(seeScheme2). TOF ,theturnoverfrequencyatoverpotentialzero.
0
Thelowoverpotentiallinearvariationzoneineqn(4)maylook
atfirstsightasaformofaTafelplot.A‘‘Tafelplot’’isaclassical
reactantattachedtotheelectrodesurface,dimensionoftime(cid:3)1),
and:
F 1
¼
I qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2kC0D C0exp½(cid:3)fðE(cid:3)E0 Þ(cid:4)
A P P cat
1 1
þ þ
kcSatCP0exp½(cid:3)fðE(cid:3)Ec0atÞ=2(cid:4) qffi2ffiffikffiffiCffiffiffiffiA0ffiffiffiDffiffiffiPffiffiffiCP0
in the homogeneous case, where kcat is the electron transfer
S
standardrateconstantcorrespondingtoareactantinsolution
(dimensionofspace(cid:5)time(cid:3)1)
Iftheoverpotential,Z,definedasZ=E0 (cid:3)E(whereEisthe
AC
electrode potential at which electrolysis is run and E0
AC
thestandardpotentialcorrespondingtothetransformationof
the substrate A into the product C), is introduced in the Fig.5Anexampleofturnoverfrequencyvs.overpotentialrelationship.
precedingequations: EA0C(cid:3)Ec0at=0.64V,logTOF0ðs(cid:3)1Þ=(cid:3)4.6,pffi2ffiffiDffiffiffiffiPffiffi=kS¼2qffikffiffiCffiffiffiffiA0ffiffi=khSet;cat¼0:03s1=2.
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linear approximation of the relationship between the current, tobeusedtoestimatetheoverpotentialandthecatalyticefficiency,29
orcurrentdensity,andtheoverpotentialofanelectrochemical successfulstrategies,asdetailedbelow,forderivationoftheTOF–Z
reaction, kinetically controlled by electron transfer. It is char- relationshipfromcyclicvoltammetricdataarerecent.22
acterized by a transfer coefficient, a, giving rise to the slope In the homogeneous case, Fig. 6 shows a zone diagram
aF/RTln10 and an exchange current density I defined as depicting the various diffusion/reaction regimes that can be
0
thecurrentdensityatzerooverpotential.Inthepresenthetero- observedbymeansofcyclicvoltammetryaccordingtothevarious
geneouscase: interveningparameters(thesehavebeendefinedinthecaptionof
Fig.4,nisthescanrate).30,31Thecatalyticsystemsofinterestare
a¼1 and I =2F ¼kC0G0exp½(cid:3)fðE0 (cid:3)E0 Þ(cid:4)
0 A P AC cat those in which k is large enough for the representative point of
¼TOF (cid:5)G0 the system to stand in one of the three upper zones in which
0 P
‘‘pure kinetic conditions’’ are achieved. Pure kinetic conditions
are exactly the same as already introduced in preparative-scale
0A It is seen that, unlike TOF0, I0 is not a molecular intrinsic conditions, corresponding to the same steady-state situation.
6
53 expression of the catalytic capabilities of the catalyst since it ThemostclassicalcyclicvoltammetricresponseistheS-shaped
3
S dependsontheamountofcatalystonehasbeenabletodeposit waveintheupperright-handzoneofFig.6,ofequation:32,33
C
C2 ontheelectrodesurface.Moreover,Tafelplotswithunitytransfer qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
y 201310.1039/ ccoalelffiyccoienntrtosllaerdebqyueilteectrraornetfroarnsefleerc.tTrohcihsermeciaclalsltroeaucstitohnastekqinne(t5i)- FiS¼1þexp2½k(cid:3)CfA0ðDEP(cid:3)CEP0c0atÞ(cid:4) ð6Þ
ebruarg | doi: iosvenroptoatennaticatlupalloTtacfhealrpalcotte.rIitziinsgraatdheeproasittuerdnmovoelrefcruelqaurecnactaylyvsst. (i:current,S:electrodesurfacearea).Itisindependentofscanrate
on 25 Fs.rsc.or intLhiekelwowiseovinerpthoetehnotimalorgeegnieoonu.scasewhere: arensdpothneserecvoerrrseesptroancdesistoidaefnatsictaclattoaltyhtiecfroerawctairodntwraicthe.nTehviesrtthypeleesosf
Université Paris 7 2012 on http://pub eTqanfel(p5)lotasn.d thIe0=pFlo¼tsqinffi2ffiffiDffiffiffiFffiPffiiffi=ffigffikffi.ffiffiCffiffi5ffiA0ffiffi,ffiC8P0(cid:5)anTdOF100; are not actual nlehoalfeeeyglectplhrtire,geodiadbrnbeleed.yveAcdaornisnvafiiasbldyuslsitemnisegPploe/ttQfichoteenrwxopconaaefvtrettairhmlatyenhetissnacuftteabcrilusstbrrreroeaesbttunweslteetisnerbvnybtehydtthehmeierneepcaateanchattsaiekoloynacfsu–btedrsqareiffnennnudc(ts6e,it)ohioi0pnesf,
CD ber 3.3 Analysisofcatalyticresponsesincyclicvoltammetry catalysis(e.g.intheabsenceofsubstrate):34,35
Sm
ded by 1 Dece Ccaatrarlyyisntg,doeutetrmaisnyesttehmeabteicstsceorniedsitoiofnesleocftirtosluyssees,atvooiedvianlugastiedea- Fip0S¼0:446(cid:5)CP0pffiDffiffiffiPffiffipffifffivffiffi ð7Þ
a1
nloon phenomenasuchasohmiclosses,self-inhibition,deactivation
Dowhed etc...isanexhaustingtask.Thisisthereasonthatnon-destructive Thenormalizationprocedurethatthusconsistsofreplacing
blis techniques,suchascyclicvoltammetry,28arecurrentlyusedto eqn(6)byeqn(8):36
Pu rapidly gauge what can be the catalytic significance of a
rffiffiffiffiffiffiffiffiffi
moleculeandestablishthebestconditionsforpreparativescale 2kC0
2:24 A
electrolyses. In spite of previous attempts to define the i ¼ fv ð8Þ
potentialofthecyclicorrotating-diskelectrodevoltammogram ip0 1þexp½fðE(cid:3)Ec0atÞ(cid:4)
is a straightforward manner of avoiding the determination of
thecatalystdiffusioncoefficientandtheelectrodesurfacearea.
Arepresentationofeqn(8)isgiveninFig.7a.
Eqn (6) has been used extensively to measure the global
catalytic rate constant and thereof to analyze mechanism
through determination of reaction orders in catalyst, CO
2
or molecules favoring catalysis such as protons donors (see
Section4.1).37–42Cautionhassometimesbeentakentousethis
equation when it is indeed applicable, i.e., when the cyclic
voltammetric response is (or is close to) an S-shaped curve
independentfromthedirectionofthepotentialscanandfrom
the scan rate.41 In a large number of cases it has nonetheless
beenundulyappliedwhenthecatalyticresponseispeak-shaped
and–comingtogether–scan-ratedependent.Theoddsituation
ofascan-ratedependentcatalyticrate‘‘constant’’thusarises.40
One may consequently wonder what the reliability of reactions
ordersderivedintheseconditionsisandthereofofthemecha-
Fig.6Diffusion/reactionregimesandshapeofthecyclicvoltammetricresponses
incatalyticreactionsofthetyperepresentedinScheme2. nistic conclusions drawn.37–40,43–46 Various side-phenomena
2428 Chem.Soc.Rev.,2013, 42,2423--2436 Thisjournalis c TheRoyalSocietyofChemistry2013
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ChemSocRev ReviewArticle
A
0
6
3
5
3
S
C
2
C
9/
33
10
01
y 210.
ebruarg | doi:
on 25 Fs.rsc.or
s 7 pub
Université Pari2012 on http:// caF(roi,iggna.hs0:7ut)CmCf0Aoapr=ttaiao1lynttyM.ipcF,icrc2oyakcmllsi=cybsv1ltuoe0elm0tat,smo(cid:3)v1my=,ee0nllto.o1rwysVur:ebsCs(cid:3)s0Apt1or,(aMnDtseP)e==sco(11lne0,sf(cid:3)0ut5).m1ac,pmn0td2.i0ofs1no(cid:3),.o10b,t-C.,o00Pbf0-=05t:h;1seu2m-bkwsM=atrv,a1eTt0ea=0n2sa(cid:3)9ly18s.eKs.
D er
Cb
Sm
ded by 1 Dece hreasvpeobneseenwehveonkeadntSo-sehxapplaedinrtehspeoanpspeeaisraenxcpeecotfeda.4p0e,4a7k,4-8shaped
a1
nloon The most obvious of these side-phenomena is substrate
owed consumption. Passing indeed from the upper-right zone of
Dh
Publis Fcoign.su6mptotiotnhoeftthweosulebfstt-hraatned,thzeondeiffsuseinotnaiolsfwahnichinbcerecaosminegs Feilegc.t8roCgaetnaelyrsaitseodfFteh(e0)eTlPePct.rCoycchliecmvioclatlarmemduecttriyonofoFfeCTOPP2t(1omCOMb)yinDMF+0.1M
thesolerate-determiningstepin‘‘totalcatalysis’’conditions.In n-Bu4NBF4,inthepresenceof0.23MCO2and0.1(a,a0),3(b,b0)MPhOHatascan
the intermediatezone, theinterference ofsubstrate consump- rateof0.1Vs(cid:3)1.a,b:cyclicvoltammetricresponses.Solidline:experiments;
dashedline:simulationofthecorrespondinghypotheticalunperturbedcatalytic
tion decreases when one approaches the foot of the catalytic
responses.a0,b0:foot-of-the-waveanalyses.c:Resultingturnoverfrequencyvs.
wave.Thisobservationsuggestsastrategyconsistingofplotting
overpotentialrelationships(thicklines)derivedfromthefoot-of-the-wave
i=i0againstf1þexp½fðE(cid:3)E0 Þ(cid:4)g(cid:3)1.Inthecasewherethecatalytic analysesat0.1,1,10,50Vs(cid:3)1(green,red,magentaandbluerespectively).The
p cat
reactionisnotperturbedbysidephenomena,astraightlineisthus threeyellowdotsrepresenttheresultsobtainedbymeansofpreparativescale
electrolysisinthesameconditionsasina.
obtained over the whole range of f1þexp½fðE(cid:3)E0 Þ(cid:4)g(cid:3)1, as
cat
pictured in Fig. 7a, a0. This is no longer the case if substrate
consumption comes into play (Fig. 7b, b0), but remains true chargeisincreased,e.g.inhibitionbyproducts,48ordeactivation
at the foot of the wave within an interval that decreases when ofthecatalyst.22The‘‘foot-of-the-wave’’strategyappliestothese
thisside-phenomenoninterferesincreasingly.Theslopeofthis cases too and to any other side-phenomena that interfere
linearportionmaythenbeusedfordeterminingofkandfinally increasinglyuponincreasingthefaradaicchargepassed.
of the TOF–Z relationship for any catalyst without taking sub- An illustrative example is given in Fig. 8 where the catalyst
strateconsumptionintoaccount.Thisisduetothefactthatat forCO -to-COreductioniselectrogeneratediron(0)tetraphenyl-
2
the foot of the wave, the faradaic charge passed is small and porphyrin(Fe(0)TPP)inthepresenceofphenol22(thefavorable
therefore the amount of substrate transformed is also small. roleofaddingacidwillbefurtherdiscussedinSection4.1).Itis
Raising the scan rate also results in a decrease of the faradaic worth noting that the results of preparative-scale electrolysis
charge passed in a given potential interval, thus extending the are consistent with the predictions made on the basis of the
linearportionoftheplot.22Thereareotherphenomena,which foot-of-the-waveanalysis,thusvalidatingthisstrategy.
may be considered as side-phenomena, vis a` vis the catalytic Extension to the case where electron transfer between the
reaction,whichinterfereinanincreasingmannerasthefaradaic catalystandtheelectrodeisnotunconditionallyfastispossible.
Thisjournalis c TheRoyalSocietyofChemistry2013 Chem.Soc.Rev.,2013, 42,2423--2436 2429
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ReviewArticle ChemSocRev
Thismaybecomenecessaryevenwithfastelectronexchanging of CO to CO.49 However, the catalyst deactivates after a few
2
catalystswhencatalysisissofastthatelectrontransferwiththe turnovers.AdditionofLewis50,51andBrønstedacids47,52consider-
electrode starts to interfere in the overall kinetics. Then, the ably boosts catalysis. In the latter case, weak acids such as
functionthatshouldbeplottedagainstf1þexp½fðE(cid:3)E0 Þ(cid:4)g(cid:3)1is alcohols,particularlytrifluoroethanol,wereusedtoavoidcatalysis
cat
nolongeri=i0 but,rather: ofprotonreductionasobservedwiththesameFe(0)TPP,whenan
p acid as strong as Et NH+ is used.53 It has been shown more
3
i
FITðE(cid:3)Ec0atÞ ¼ 1(cid:3)0:446ipffiDffiffiPffiffipip0ffifffivffiffiexphfðE(cid:3)Ec0atÞi raelscoentblyoothsatstacdadtiatiloysnisofwmhoidleerastteilllysytireolndginagcidess,sseuncthiaallsyphCeOnolass,
ip0 kS 2 product.22Amechanism,basedonreactionordersderivedfrom
rffiffiffiffiffiffiffiffiffi cyclicvoltammetricresponsesclosetothecanonicalS-shape,has
2kC0
2:24 A been proposed (Scheme 3 in ref. 47). More reliable and more
fv
¼ 1þexp½fðE(cid:3)E0 Þ(cid:4) detailed mechanistic pictures are expected from the systematic
A cat
0 applicationofthefoot-of-the-waveanalysisofcyclicvoltammetric
6
53 ksisthestandardrateconstantofelectrontransferbetweenthe responses as illustrated in Section 3.3 (Fig. 8) with, precisely,
3
S catalyst and the electrode, as derived from the cyclic voltam- theexampleofphenoladdition.54
C
C2 metryofthecatalystcoupleintheabsenceofcatalysis.kisthen Similar studies of the effect of weak Brønsted acids on the
13039/ obtainedfromtheslopeoftheinitiallinearportion,oftheFIT catalysis of the conversion of CO2 to CO by means of electro-
y 2010.1 plot, leading finally to the TOF–Z relationship. An example of reduced[Re(bpy)(CO)3(py)]2(cid:3)havebeenperformed,55inaceto-
ebruarg | doi: tshtriastemgyorisegsiovepnhiisnticSaetcetdionap4p.2li.cation of the foot-of-the-wave- ncaittraillyes,t.f5o6lMlowecihnagntihsteicpcioonnceleursiinognsstwuedrieessimofilathretoRteh(obspeyd)(rCaOw)n3
on 25 Fs.rsc.or tionApopflitchaetiognlobofalthciastasltyrtaictegraytehacsontwstoanotustcaosmaesf:u(ni)ctdioenteromfitnhae- irneptehaetecdasereocfenFetl(y05)7puosripnhgyrtihnes.sTahmeesawmeaekexBpreørnimsteendtsachiadvse,bweiethn
s 7 pub concentrationofthevariousreactants,andhencethemechanism determination of reaction orders and of H/D kinetic isotope
versité Pari2 on http:// moesftocasabttlaeislffiyhsmcisieebnnyttcmoofentadhniestiTooOnfsFth–oeZfrpreeraleacpttiiaoornnastoihvriedp-esarcssaltaehgueulseidcdteertoetolrymssieisnleaecdnt;dt(hitioe) ecthyffceelycictds.vooAnlptoapmtlicpmalatoitogernaauom.fsIenqanrfae(c8st),cwathnaes-ractotwenosiincdhdeaerpreadecntadesreisnsattfiecesbveecncaantunhsooetutghbhee
Uni201 comparetheintrinsicpropertiesofthecatalystsproposed. separated,buttheresultsaremostprobablyreliablebecausethe
D er peaksareveryshallow,closetoaplateaubehavior.
Cb
Sm
aded by 11 Dece 4ofHCoOm2otogeCnOeous catalysis of the reduction 4to.2a. veArtytaecffihicniegntthceaatacliydsigsroupstothecatalystmoleculeleads
nloon 4.1 Favorableroleofacids
wd Basedontheaboveobservationsofthefavorableroleofacids,
oe
Dsh In the series of palladium–phosphine catalysts, addition of an iron(0) tetraphenylporphyrin with OH substituents in ortho,
ubli acidhasbeenshowntobebeneficialsincethebeginning.37–42Itis ortho0positionsofthephenylgroups(Fig.9,thephenylgroups
P
remarkablethatinspiteofthechoiceofastrongacid,HBF ,to areinfactperpendiculartotheporphyrinring)provedtobea
4
accelerateCO conversion,COremainsthemainproductwithno particularly efficient catalyst of the CO to CO electroreduc-
2 2
or little hydrogen production. In spite of the above-mentioned tion.58 Fig. 9a shows the very large catalytic current obtained
uncertaintiesonreactionorders,themechanismislikelytobe withthismolecule.Asinmanyaforementionedcases,itdoesnot
ofthetypeshowninScheme3ofref.42. plateauoff.Thisisthereforeatypicalcasewherethefoot-of-the-
Electrogeneratediron(0)porphyrins,essentiallytetraphenyl- wave analysis shows its usefulness. The catalysis rate constant
porphyrin(TPP)hasbeenshowntobeacatalystfortheconversion canbederivedfromthelinearportionoftheplot(Fig.9b)aswell
Fig.9Aniron(0)porphyrincatalystwiththeacidgroupsattachedtothemolecule.a:cyclicvoltammetryofFeTDHPP(1mM)inDMF+0.1Mn-Bu4NBF4,intheabsence
(black)andpresence(red)ofCO2(0.23M)atascanrateof0.1Vs(cid:3)1.b:Foot-of-the-waveanalysis(seetext).
2430 Chem.Soc.Rev.,2013, 42,2423--2436 Thisjournalis c TheRoyalSocietyofChemistry2013
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5 Homogeneous catalysis of the reduction
of CO to other products
2
5.1 Formate(vs.CO,vs.H )
2
Formateisalsoaninterestingproductsinceformicacidmaybe
usedforhydrogenstorageorasafuel.63Whenthereductionof
CO takestheanionradicalpathway,formateresultsfromthe
2
reaction with water and is indeed the only product of electro-
lysisinwateronaninertelectrode.Inhomogeneouscatalysis,
formateappearsasaside-productinmanycasesbutthereare
catalyststhatgiveamorespecificaccesstoitsproduction.For
Fig.10Correlationbetweenturnoverfrequencyandoverpotentialfortheseries example, substantial amounts of formate were obtained with
0A ofCO2-to-COelectroreductioncatalystslistedinTable1.Thickredandblue Rh(diphos)2 (diphos = 1,2-bis(diphenylphosphino)ethane) as
6
3 segments:TOFvaluesderivedfrom‘‘foot-of-the-wave-analysis’’ofthecyclic catalyst at a potential where the intermediacy of the rhodium
5
S3 voltammetriccatalyticresponsesofFeI/0TDHPPandFeI/0TDMPP,respectively, hydride appears likely,64 based on previous investigations of
9/C2C iFneIt/h0TeDpMrePsPen(bcoetotofm2)M.ThHe2Ost.aDrainshdeicdatleinseas:pprleoptasrfaotrivFee-0sTcaDlHePePxp(teorpim)eanntdwith the electrochemistry of this complex.65 Substantial amounts
1303 Fe0TDHPPascatalyst. of formate were obtained together with CO and H2 with
ebruary 20g | doi:10.1 astheTOF–Zrelationship.ThisisshowninFig.10whereitis Rypirueel(sdbepniync)e2(fCoorfOma)2aptaresotioncanctraedlaoyssnetosri.nw66itaIhcteittshoeninitptreKirlaeesotoifnrtghmteoetphnraoontteoonlthidantontthhoeer
on 25 Fs.rsc.or (csoemepSaercetdiotno4o.t3h)e.rBceactaaulyssetsooffththeehCigOh2tcoatCaOlyteiclecratrtoe,reedlueccttrioonn pobretaseinnetd. wSiitmhirlaerdluyceidmcpiso-r[tRahn(tbpyyi)e2lXd2s]+(oXf=fCorl(cid:3)moartethewterriefluaolrsoo-
s 7 pub transferkineticsbetweentheelectrodeandthecatalysthadto methane-sulfonate anion)67 or with another rhodium complex
versité Pari2 on http:// bI(snetattrahkiinesnFciiangs.teo10tao)cocf,oaultlnshteininrelitsnhueeltwanoitafhlyptsrhiesepapasrraedtdeiivtcaetiiloescdnailonefStehelecetciftoornoolty3-so.3ifs-. ca[(alZlto5aw-lMyesdet5Casnh5)oRewhvne(bnpinyb)eCtSltc]e+hr6e8smpieenci3faiacc,eittyion,nwitarictiehleto.sntRiiltelrcialeennt+onu5-sn%eegolwifgaittbehlree,
Uni201 the-wave analysis. That the remarkable catalytic properties of productionofhydrogen(15%).69
D er this molecule are related to the presence of the phenolic OH Amuchbetterselectivityisobtainedwhenusingatungsten-
Cb
d by SDecem gcoronufiprsmiendtbhyecmomolpecaurilseocnlowsiethtoththeepiorropnh(y0r)incamtaolylteiccucleenwtehreries ceolencttaroindiengsufrofarmceaitne wdaetheyrdwroitghenaahseighentzuyrmnoeveardsfroerqbueednctoy atnhde
ade11 alltheOHgroupshavebeenreplacedbymethoxysubstituents averylowoverpotential.70
wnlod on (FeTDMPP). Rather high TOF values are also obtained but Several indications from the papers cited above point to a
Doshe at the price of a much larger overpotential, resulting in a firstcompetitionbetweentheadditionofCO2andofH+onthe
bli TOF value one billion times smaller than with FeTDHPP. activereducedformofthecatalyst.TheformerleadstoCOand
u 0
P
Comparison with the kinetics of catalysis by Fe(0)TPP in the the latter to a metal hydride. The hydride may then react
presence of phenol (Fig. 8c) shows that the eight phenolic competitively with CO and H+, yielding formate and H ,
2 2
groupsinFe(0)TDHPPareequivalenttoa150Mconcentration respectively. One would thus expect that increasing the
ofphenolinsolution. medium acidity would make the product distribution pass
from CO to formate and finally to H . This is indeed what
2
seems to happen with the catalyst shown in Scheme 3b.71
4.3 Catalystsbenchmarking
Formate is the ‘‘major product’’ (no figures provided) in the
The intrinsic catalytic properties of most of the proposed presenceofbenzoicacidwhereashydrogenistheonlyproduct
homogeneous catalysts are compared in Fig. 10, based on the in the presence of the stronger toluenesulfonic acid. In this
value of the turnover frequency and overpotential as derived case,thethreecorneredcompetitionbetweenCO,formateand
from preparative-scale experiments described in the literature H formation under the effect of added acids of various
2
(the list is limited to catalysts with a reasonable stability, strengths clearly requires further investigation. This is also
at least several hours and to electrode materials that do not thecasewithiron(0)porphyrin,whichappearsasanexcellent
interfereinthecatalyticprocess).Theintrinsiccatalyticproper- catalyst for H evolution in the presence of a strong acid,
2
ties of the various molecules may be compared according to protonated triethylamine,53 whereas addition of weak (e.g.
the location of the representative point in the log TOF vs. trifluoroethanol), and moderately weak (e.g. phenol) acids
Z plane. The characteristics of each catalyst are summarized acceleratestheformationofCO.Inotherwords,theconditions
inTable1,inwhichcomparisoncanalsobemadethroughthe for producing syngaz mixtures with adjustable proportions of
TOF values. It is seen that the two most efficient catalysts COandH usingasinglecatalystarenotyetathand.Another
0 2
areFeTDHPPandRe(bpy)(CO) ,withaslightadvantageforthe approachtothe samegoalconsistsofthecombination oftwo
3
former, which also involves a mustmore common metal than catalyst systems, as in ref. 72 (Re(bpy-tBu)(CO) Cl and p-Si
3
thesecond. underillumination).
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ReviewArticle ChemSocRev
Table1CatalysisofCO2reductionintoCO.Correlationbetweenturnoverfrequencyandoverpotentialfortheseriesofcatalystslisted
Solvent Catalyst
E0 (Vvs.SHE) E0 (Vvs.SHE) Z(V) logTOF(s(cid:3)1) logTOF (s(cid:3)1) Ref.
CO2=CO cat 0
DMF+2MH O Fe0TDHPP 0.41–0.56 2.0–3.9 (cid:3)4.9 58
2
(cid:3)0.690 (cid:3)1.333
Fe0TDMPP 0.89–0.99 1.0–2.2 (cid:3)14.2
(cid:3)1.69
Re(bpy)(CO) 0.57 3.0 (cid:3)6.1 56
3
(cid:3)1.25
DMF+HBF {m-(triphos) [Pd(CH CN)] } 0.80 0.37 (cid:3)7.8 59
4 2 3 2
(cid:3)0.260a (cid:3)0.76
CH CN+5%H O (cid:3)1.16 0.51 (cid:3)0.35 (cid:3)8.7 60
A 3 2
0 (cid:3)0.650
6
3
5
3
S
C
2
C
9/
33
10
01
ebruary 2g | doi:10. C(cid:3)H0.36C5N0 (cid:3)1.30 0.87 1.2 (cid:3)9.8 61
on 25 Fs.rsc.or
s 7 pub
versité Pari2 on http:// (cid:3)1.25 0.81 1.2 (cid:3)9.1
Uni201
D er
Cb
Sm
ded by 1 Dece 1(cid:3):04.6H502OCH3CN (cid:3)1.30 0.55 1.9 (cid:3)7.4 62
a1
nloon
wd
oe
Dh
s
bli aThelargechangeinE0 isduetothepresenceofastrongacid,HBF ,muchstrongerthan(CO +H O).
Pu CO2=CO 4 2 2
quantitativeinacetonitrileonaglassycarbonelectrodeatavery
low overpotential, prior to the reduction of dioxygen, allowing
the capture and reduction of CO from air selectively to a C2
2
product, oxalate. It should however be noted that no simple
routetoreductivelytransformoxalateoroxalicacidtovaluable
carbonderivativesisavailablesofar.
Scheme3CatalystsforthereductionofCO2toformate. 6 Heterogeneous catalysis of the reduction
of CO
2
5.2 Oxalate 6.1 ReductiontoCObymeansoftransitionmetalcomplexes
atmercuryelectrodes
OxalateisformedinhighyieldstogetherwithCOinthedirect
reductionofCO oninertelectrodesinaproticmedia,bymeans In spite of the fact that reduced nickel cyclams have been
2
ofdimerizationoftheinitialCO anionradical(Section3).Itis shown to catalyze the reduction of CO to CO on an inert
2 2
formedexclusivelyinthesamemediumwithanionradicalsof electrodesuchasglassycarbon,62,74thesamecomplexesgivea
aromatic esters and nitriles as catalysts (Section 3.1). In both muchbettercatalysisonmercurymakingthesesystemsonesof
cases, a rather large overpotential is required to make the the most selective and efficient catalysts for the CO to CO
2
reaction go. A remarkable catalytic system in this connection conversion.75–78Carefulstudieshaveshownthatthemostlikely
isshowninScheme4.73Theformationofoxalateispractically catalystisthenickel(I)complexchemisorbedonmercury.74,79,80
2432 Chem.Soc.Rev.,2013, 42,2423--2436 Thisjournalis c TheRoyalSocietyofChemistry2013