Engineering d-orbital of copper single-atom sites toward industrial-level electrocatalytic methanation

Domain: chemistry
Published: 2026-02-13
Editorial outcome: major_revision

Abstract
Copper(Cu)-basedsingle-atomcatalysts(SACs)enableelectrocatalyticCO2reductionintomethane(CH4)fuelforthermalpowerplantdecarbonization,yetconventionalCuSACsfaceindustrialdeploymentbarrierslikeinstabilityandsluggishkineticscausedbyd−porbitalcoupling.Herein,wedevelopaCu–TiO catalystwithlocalizedCusingle-atomsitesbyoxygenvacancy(Ov)-involvedorbitalengineering,achievingindustrial-levelCH4production.Theoreticalandin-situstudiesrevealtheintensificationofthed−dcouplingatCusitestriggeredby[Cu−Ov−Ti]motifs,whichenhancesd-π*polarinteractionsupon*CO2andacceleratesC−Obondcleavagein*OCH3intermediate.Asaresult,Cu–TiO achievesacompetitiveperformance,i.e.,thehighestFaradaicefficiencyof76%andapeakpartialcurrentdensityof670mAcm−2towardCH4(correspondingturnoverfrequency=24,930h−1),~3.5-foldpromotionoverconventionalCuSACs.Furthermore,itdemonstrateshighdurability(>1,230hours)atanindustrial-levelcurrentdensity,exceedingthelongevityofconventionalCuSACsbyover20times.Ourfindingshighlighttheprospectofd-orbitalengineeringinenablingindustrial-levelelectrocatalyticmethanation,offeringpromisingimplicationsfordecarbonizingtraditionalpowerplants.

Introduction
Carbonemissionsfromtraditionalthermalpowerplantssignificantlyhindercarbonneutralityefforts,e.g.,overseascoal-firedpowerplantscurrentlyemit0.53gigatonsofCO2peryear1.Toaddressthis,abundantdecarbonizationtechnologiesarebeingproposed,amongwhichtheelectrocatalyticCO2reductionreaction(CO2RR)emergesasacriticalpathwayforCO2valorizationandsustainablefuelproduction2,3.Specifically,electro-catalyticmethanation(i.e.,CO2-to-CH4conversion)usingfluegasprovidesapromisingdecarbonizationstrategybycouplingCO2fixationwithhigh-energyCH4(55.5MJkg−1)productionforthermalpowerplants4,5. Regarding catalyst candidates, copper (Cu)-based catalysts facilitate pathways for deeply reduced products, including CH4 (>2e⁻ transfer)6,7, but face challengesofsluggishkinetics(|Jtotal|<500mAcm−2)andpoorselectivity(<50%).

Fromamechanisticperspective8,9,spatiallycoupledCu–CusitespromoteC–CcouplingovertheCH4pathway(e.g., 0.53eV higher barrier for CH4 on bulk Cu10). Therefore, enhancing CH4 selectivity requires spatially isolated Cu single-atom catalysts (SACs). Current synthesis strategies for Cu SACs rely on embedding Cu into non-metallicmatriceslikecarbon,C3N4,andsoon(SupplementaryFig.1a)6. Forinstance,N-coordinatedCuSACshavebeenconfirmedtoowna favoredC1selectivity(e.g.,CO11,HCOOH12,andCH413,14).Nevertheless,themodernhard-softacid-base(HSAB)theory15,16illustratesthattheinherentlydelocalizedp-orbitals17maycausead-electronicdelocalizationviad–porbitalcoupling,thusweakeninghard-basicintermediateadsorptionlike*OCH3andfacilitatinghydrogenevolutionsidereaction18atCusites(DetailsinSupplementaryNote1)15.Moreover,Cu−X(X=C,N,O)coordinatesarethermodynamicallyunstable19,causingseverereconstructionsuchasagglomerationofCuatoms.ThismayfacilitateC−Ccoupling20andcauseperformancedegradationunderindustrial-levelcurrentdensity(>500mAcm−2)21.

Totheseissues,constructinglocalizedCusingle-atomsites(i.e.,harderacid15)byintroducingmatriceswithlocalizedd/fvalenceorbitalsmaybeessential22,andproposedsolutionsincludeconstructingCuMsingle-atomicalloys(M=f/d-blockelements)aswellasCu/MOxSACs(SupplementaryFig.1b,c).AlthoughCuMsystemsfeaturelocalizedd-orbitals,free-atom-likeCusites(i.e.,resemblingd−pcoupledmolecularmetalcomplexes)willcauseanunstablecondition23,24,thusfavoringatight*CObindingandpredominantCOselectivity25.ForCu/MOxSACs,theintrinsicchargepolarizationofM−Obonds26andspin-orbitinteractiontriggeredinsurmountableenergybarriersfordirectd−d/fcoupling27,28,finallycausingdominative[Cu−O−M]motifsthatfacilitateindirectd−p−d/fcouplingduringcatalystsynthesis29,30.Inthisindirectd−p−d/fcouplingstructure,thecontinuouselectron-polaronaccumulationatd-orbitalsduringCO2RRmaydestroytheCu−Ocoordinates31,significantlythreateningtherobustness(Fig.1a).Forinstance,Zhouetal.synthesizedaCu-Ce-Ox catalyst(i.e.,d−p−fcoupling)withisolatedCusites32,achievinganefficientCO2-to-CH4conversion(FEofCH4≈65%),butdurabilityremainspoor(<10h).

Unliketypicalelectron-polarontrappingatmetallicsites,electroninanatasetitaniumoxide(TiO2)prefersafree-carrierstate,andcanonlybetrappedatoxygenvacancy(Ov)31,33,providingavitalopportunityforO-inducedmetal-metalbondformation(strongd−dcoupling,Fig.1b),therebyconstructinglocalizedCusingle-atomsites(l-Cu sites) and stabilizing the active sites34,35. Inspired by these, employing O-rich TiO2 as matrices maybe a promising strategy torealize strong d−d coupling and facilitate selective and stable methanation.

Herein, we have developed an oxygen-vacancy-involved orbital engineering method to construct [Cu−Ov−Ti] motifs through Ov removalinCu-embeddedTiO2SAC(Fig.1c).As-synthesizedCu-basedSAC(markedasCu–TiO1.3)achievesanefficientCO2-to-CH4conversioninflowcells,includinganexcellentFECH4of76%at−0.88V(potentialsconvertingintothereversiblehydrogenelectrode(RHE),see“Methods”),andarecord-highturnoverfrequencyofCH4todate(24,390h−1at−1.0V).Furthermore,Cu–TiO1.3alsoexhibitsexceptionalstabilityof>1230hatanindustrial-levelcurrentdensity(600mAcm−2)inazero-gap electrolyzer,over12timeslonger thana conventionalCu/TiO2catalystwithmajor[Cu−O−Ti]motifs(markedasCu−O4(Ti)).Synergisticexperimental/theoreticalanalysesrevealthatOv-intensifiedd−dorbitalcouplingbetweenCuandTiatomsenhanceselectroniclocalizationofCuδ+sites(i.e.,harderaciditythanpristineCuδ+sitesinCu−O4(Ti)), strengthening d-π* polar interactions with adsorbed *CO (Fig. 1d) and stabilizing the Cu−O binding with hard-basic *OCH3. Simultaneously, in-situ time-field electrochemical electron magnetic resonance (E-EMR)revealsanewmechanisminvolvingpseudo-latticeoxygenformationthrough*OCH3insertionintoOv. This reversible pseudo-lattice oxygen formation route supplements a localized ultra-stable adsorption for *OCH3 surpassing the typical physico/chemical adsorption at conventional Cu single-atom sites (c-Cu sites) with majorly Cu−O coordination (i.e., bond dissociation enthalpy (BDE) of Cu−O=263.1 vs. 41.9kJmol−1). Therefore, an accelerated C−O bond cleavage (~16.8% lower BDE) and rapid methanation generation on Cu–TiO1.3 are achieved compared with Cu-O4(Ti) (Supplementary Fig. 2). Overall, this work establishes an orbital engineering method for constructing localized Cusingle-atomsitesandprovidesauniversalstrategytodesigndurableSACstowardindustrial-levelCO2-to-fuelconversion.

Results
Density functionaltheory(DFT) calculations werefirst employed to evaluatetheinfluenceofd−dorbitalcouplingatCusingle-atomsites.Thelocaldensityofstates(LDOS)forCuatomat[Cu−Ov−Ti]motifs exhibits intensified peaks relative to the conventional [Cu−O−Ti] structures,confirminglocalizedelectronicstatesofl-Cusites(Fig.2a,SupplementaryFig.3).Integratedprojecteddensityofstates(IPDOS)analysisidentifiesthedx2-y2orbitalasthefrontierorbitaloftheCusites(SupplementaryFig.4).Thehigherelectrondensitywithinthedx2-y2orbital(e.g., IPDOS=0.980 for up-spin states, Fig. 2b and Supplementary Table 1) of l-Cu site likely facilitates strong d-π* polar-interactions on adsorbed *CO2, corroborating accelerated CO2 activation36,37, and leading to an enhanced CO2-to-CH4 conversion trend as reported by Huang’s group37. Furthermore, crystal orbital Hamilton population (COHP) analyses reveal a significantly larger negative integrated COHP (−ICOHP) value for the Cu−Ti bond on l-Cu sites (2.422 a.u.) versus c-Cu sites (0.002 a.u., Fig. 2c), indicating robust d−d orbital coupling induced by oxygen vacancies. BDE assessments of *OCH3 confirm tighter Cu–O bonding post orbital engineering (BDE Cu–O =263.1kJmol−1onl-Cusites,Fig.2d)comparedwithunstableadsorptiononpristinec-Cusites(BDE Cu–O =41.9kJmol−1,SupplementaryTable2). Meanwhile, a 16.8% reduction in BDE C−O at l-Cu sites suggests potential acceleration of C−O bond cleavage during CO2 RR (inset slab model in Fig. 2d). Hence, d−d orbital coupling at l-Cu sites confers a kinetically enhanced methanation pathway.

Guided by computational insights, we designed Cu–TiO1.3 SAC withpartialCu–TibondingtowardCO2RR.Indetail,Cusingle-atomprecursorwasfirstfabricatedthroughahydrothermalmethod(Detailsin“Methods”),whereCusubstitutesTiin[TiO6]octahedraandformsCu–O–Timotifs38.Giventhatlatticeoxygenremovalfacilitatesadjacentmetallicbonding39,40,Cusingle-atomprecursorwasthencalcinedin10%H2/Arfor3h(surfaceCu:2.67wt%,SupplementaryTable3)tosynthesizeCu–TiO1.3.Asacontrolgroup,Cu−O4(Ti)catalystwaspreparedbyanair-calcinationofCusingle-atomprecursors.X-raydiffraction(XRD)patternsconfirmedthemajoranatasephase(PDF#71-1167,SupplementaryFig.5a)inCu–TiO1.3,whileCu-relatedpeakswereabsent,indicatingthelatticeincorporationofCuatoms.ComparedtopristineTiO2,the(101)diffractionpeakofCu–TiO1.3shiftedtoasmallerangle,whichmightresultfromthelatticeexpansion38. Meanwhile, Cu–TiO1.3 exhibited a higher 2θ angle for the (101) plane than Cu−O4(Ti),suggestingthelatticeshrinkagecausedbyOvaccumulation(SupplementaryFig.5b)41.

The morphologies and atomic structures of Cu–TiO1.3 and Cu−O4(Ti)werethenexaminedbytransmissionelectronmicroscopy(TEM)andaberration-correctedhigh-angleannulardark-fieldscanningtransmissionelectronmicroscopy(HAADF-STEM). The TEM images illustrated a retained TiO2 particle size of ~20 nm after Cu embedding (Supplementary Fig. 6). High-resolution transmission electron microscopy (HRTEM) of the Cu–TiO1.3 revealed an ~0.194nm interplanar spacingforthe(200)plane(d200,SupplementaryFig.6),indicatingalatticeexpansioncomparedwithTiO2(d200=0.189nm).Theenergy-dispersivespectroscopy(EDS)mappingconfirmedtheuniformdistributionofCu,O,andTiacrossThenanoparticles(SupplementaryFig.7).Inaddition,HAADF-STEMrevealedthedumbbell-shapedperiodicarraysconsistingofadjacentTiatompairsalongthe[100]directioninCu−O4(Ti)andCu–TiO1.3(Fig.3a,b).BrightspotsthatrefertoCuatomswereexclusivelylocatedattheoriginalTisites,indicatingtheon-sitesubstitutionandformationofCusingle-atomsitesinCu–TiO1.3andCu−O4(Ti).Moreover,inthe[100]-directedimages(Fig.3c),thedistancebetweenonebright(Cu)andonedarkspot(Ti)inCu–TiO1.3wasabout2.83Å,shorterthanthatinCu−O4(Ti)(2.97Å),suggestinganatomapproachingandapossibledirectCu−TibondformationtriggeredbyOv.

Interfacialelectronicstateswerethenassessed byX-rayphotoelectronspectroscopy(XPS). The high-resolution spectra of O 1s showedahigherOvpeakat531.2eV42forCu–TiO1.3thanCu−O4(Ti)(ca.18.5%vs.10.8%,SupplementaryFig.8).AsforCu2pspectra(SupplementaryFig.9a),Cu−O4(Ti)catalystshowedtwomaindeconvolutedpeaksataround951.7eVand932.0eV,respectively,attributedtoCu0/Cu+species15. These two peaks exhibited a ~0.40eV red-shift in Cu–TiO1.3,demonstratinganintensifiedelectrondensityatinterfacialCusites43.CuLMMAugerspectraunveiledthatbothcatalystsexhibitedtwofittedpeaksat917.7and916.7eV(SupplementaryFig.10),ascribedtotheCu2+andCu+species,respectively44.Indetail,Cu–TiO1.3possessedahigherratioofCu+with86.1%comparedwiththatofCu−O4(Ti)with67.9%,signifyingthelowvalencestateofsurfaceCu.Incontrast,Ti2p3/2and2p1/2peaksshowedaslightblue-shift(~0.13eV)postorbitalengineering,suggestinganelectronoutflowatinterfacialTisites(SupplementaryFig.9b)45.Takentogether,astrongd−dcouplingmayappearinCu–TiO1.3,whichmodulatesthelocalelectrondensitydistributionandresultsinl-Cusites46.Moreover,thestructuresofthese2catalystswerealsostudiedbyRamanspectroscopy(SupplementaryFig.11).Thebandsat637,517,395,200,and146cm−1wereobservedforallofCuSACsandTiO2,whichcouldbeascribedtothevibrationalmodeofEg(3),A1g&B1g(2),B1g(1),Eg(2),andEg(1),respectively45. The Eg(1) band in Cu–TiO1.3 showed the highestwavenumberamongallsamples,indicatingthehighestOvconcentration45.

To further investigate the electronic states and coordination environment at Cusites,X-ray absorption spectroscopy (XAS) was conductedattheCuK-edge.X-rayabsorptionnear-edgefinestructureexhibitedthattheadsorptionedgesandcorrespondingfirstderivativepeaksoftwoCuSACslaybetweenthoseofCu2OandCuOreferences(Fig.3dandSupplementaryFig.12),suggestingtheexistenceofmajorCuδ+speciesinCuSACs.ThepeakpositionofCu–TiO1.3catalyst(8982.3eV)waslowerthanCu−O4(Ti)catalyst(8982.7eV,SupplementaryFig.12),furtherindicatinglocalizedelectronicstatesofl-CusiteswhenOvaccumulation,consistentwiththeresultsofXPS.AsforextendedX-rayabsorptionfinestructure(EXAFS)spectra,theFourier-transformedEXAFSexhibitedanintensepeakatR≈1.92ÅforCu−O4(Ti)(Fig.3eandSupplementaryFig.13;SupplementaryTable4),whichmaybeascribedtoCu−Obonds15. Whilethewavelet-transformationanalysesrevealedtheabsenceofCu−CubondsinCu−O4(Ti),confirmingtheformationofCu−O−TimotifswhereanaverageCu−Tiinteratomicdistanceof2.97Åwasobserved(SupplementaryFig.14)16. While in Cu–TiO1.3, a peak at R=2.83Å appeared,andwavelet-transformedEXAFSrevealedahighenergyregionat~6.53Å−1(SupplementaryFig.14),whichtogetherconfirmstheformationofCu−Tibonds46.ThegenerationofCu−Tibonds,originatingfromOvaccumulation,impliesastrongd−dorbitalcouplinginCu–TiO1.3.

Electrochemicalmeasurements
ThecatalyticperformanceofCO2RRwasthentestedinaflowcellwith1MKOHelectrolyte(Detailsin“Methods”).ThelinearsweepvoltammetrycurveswiththeCO2flowrevealedahigherreactioncurrentforCu–TiO1.3thanCu−O4(Ti)(SupplementaryFig.15).In-situelectrochemicalimpedancespectroscopies(EIS)at–0.88Vwereexecuted,whichshowedthatCu–TiO1.3obtainedlowercharge-transferresistancecomparedwithCu−O4(Ti)(SupplementaryFig.16andTable5).Correspondingadvanceddistributedrelaxationtimesanalysesexhibitedtwocharacteristicpeaksat–0.88Vovertwocatalysts,i.e.,P2andP3,whichrefertothecharge-transferprocessofCO2RRandHER,respectively47.TheinterfacialchargetransferimpedancesofCO2RRonCu–TiO1.3werecalculatedas48.62Ω,lowerthanthatoverCu−O4(Ti)(123.7Ω),whilethetrendwasoppositetowardHER(SupplementaryTable6),indicatingamorefavorableCO2RRpathwayonCu–TiO1.3.

In electrolysis experiments, Cu–Ti1O3 achieved a peak FECH4 of 76% at −0.88V, about 2.2-fold higher than Cu−O4(Ti) (Fig. 4a and Supplementary Fig. 17). The FE ratios of CH4 to 2e−-product (CO+HCOOH)werecalculatedas14.2onCu–TiO1.3,muchlargerthanthatofCu−O4(Ti)(Fig.4b and Supplementary Fig. 18), indicating a favorable multi-electron (>2e−) pathway on Cu–TiO1.3. Furthermore, partial current density of CH4 (JCH4)ontheCu–Ti1O3reachedamaximalvalueof670mAcm−2at−1.00V(i.e.,turnoverfrequencyofCH4(TOFCH4)=24,930h−1, Fig. 4c), surpassing Cu−O4(Ti) (JCH4=205mAcm−2, TOFCH4=7100h−1). The corresponding JCH4 normalizedbyelectrochemicallyactivesurfacearea(ECSA)wasalsocalculatedtoeliminatetheinfluenceofthereactionsurface,andCu–TiO1.3delivereda2.5timesgreatercurrentdensitythanCu−O4(Ti)(SupplementaryFig.19).Furthermore,theenergyefficiency(EE)ofCu–TiO1.3attainedapeakvalueof38.7%at−0.88V,exceedingthatonCu−O4(Ti)(19.2%,Fig.4d).BytuningtheCO2flowrateto50sccm,theCu–TiO1.3achievedthelargestFECH4of79%ataround−0.88V,whilethepeakFECH4oftheCu−O4(Ti)was38%(SupplementaryFig.20).ThepristineTiO2showedthatH2wasdominantproduct(FE>95%)withnodetectionofcarbonousproduct,confirmingthatCusitesonthesupportfacilitatetheCO2RR(SupplementaryFig.21).Formechanisticinsight,O-freeCu/MgO(Al)withoutotherd-blockelementswas then synthesized and tested (Details in Supplementary Note 2, XRD/XPS/TEM/EDSinSupplementaryFigs.22and23),exhibitingaFECH4oflessthan30%(SupplementaryFig.24),furthersuggestingtheessentialroleofOv-intensifiedd−dorbitalcoupling.Wethenstudytheinfluenceofimpuritiessuchassulfurdioxide(SO2)andoxygen(O2)ontheCO2RRperformanceofCu–TiO1.3at−0.88V.UnderpureCO2flow,theCu–Ti1O3realizedahighFECH4of~76%.However,uponintroducing1%SO2(~104ppm)att=0.5h,thetotalFE,alongwithFECH4,decreasedmarkedly(SupplementaryFig.25a),whichcanbeattributedtothepreferentialreductionofSO248. After 1h of electrolysis (including a feed of 1% SO2 for 30 min), the FECH4 recoveredto>66%,suggestingthattheinfluenceofSO2onthecatalyststructureislargelyreversible.A similar reversible trend was observed when feeding 10% O2 (~105ppm,SupplementaryFig.25b).

Bothtwocatalystswerefurtherintegratedintoa5-cm2zero-gapelectrolyzertoevaluatethepotentialofourstrategiesforpracticalapplicationstowardstheCH4formation(SupplementaryFig.26).TheCu–Ti1O3exhibitedahigherFECH4of75%thanthatofCu−O4(Ti)(37%)ataconstantcurrentdensityof–0.6Acm–2(SupplementaryFig.27).Catalyststabilitywasthenevaluatedat–0.6Acm−2. Cu–TiO1.3 maintainedcontinuousoperationfor>1230hatanindustrialcurrentdensityof–0.6Acm−2,exhibitingacellvoltageof−4.05±0.16VandanaverageFECH4of~71%(Fig.4e).Post-testXRD,HRTEM,aberration-correctedHAADF-STEM,andXPSconfirmedstructuralintegrityviapersistentd−dcoupling(SupplementaryFigs.28–32).Incontrast,Cu−O4(Ti)showedrapiddegradation:FECH4droppedto23%within64hunderidenticalconditions,withobservedCunanoparticleformationthatsuggestsCu−Obondcleavageandmetallicatomicaggregation.Inaddition,theCucontentofCu–TiO1.3afterreactionwasalmostunchanged,yieldinglessCudissolution(5ppbvs.65ppbforCu−O4(Ti),SupplementaryTables7and8),implyingscreenedCucorrosion.Fromtheperspectiveofreactionactivityandstability,Cu–TiO1.3surpassedmostoftheadvancedCuSACsinCH4productionmetrics(Fig.4f and Supplementary Table 9). Moreover, technoeconomic assessments based on a power plant model (Details in Supplementary Note 3 and Supplementary Fig. 33), utilizing established literature parameters21,49 and the durability data presented herein(SupplementaryTable10),demonstratethataprofitablepathwayisfeasibleforthisCO2-to-CH4conversionsystemunderagivencarbonprice(SupplementaryFig.34).

Mechanisminvestigations
Togaininsightintotheroleoforbitalengineeringinmodulatingthereactionmechanism,aseriesofinsituelectrochemicalspectroscopieswasemployed.In-situtime-fieldE-EMRtrackedOvdynamicsandintermediatesduringCO2RR.ForCu–TiO1.3,thetime-fieldscanssuggestedthatasignalrelatedtoOv(centeredatg-tensorof1.997)maintainedwellafter10-minCO2injection,excludingtheinfluenceofCO2adsorptionontheOvconcentrations(Fig.5aandSupplementaryFig.35).However,Ovgraduallyfadedoutunderappliedpotential,andrecoveredwhenthepotentialwasrevoked(Fig.5a).Onthecontrary,Cu−O4(Ti)catalystexhibitedamaintainedOvintensitythroughoutthecatalyticprocess(SupplementaryFig.36).Furtherquantitativespincounting(Fig.5b)showedaninitialOvconcentrationof1.08×1017spinsinCu–TiO1.3,approximately3timeshigherthanthatinCu−O4(Ti)(3.52×1016spins).Underappliedpotential,OvconcentrationofCu–TiO1.3droppedto3.78×1016spins,whilerecoveringto1.08×1017spinsatopen-circuitpotential(OCP),suggestinganintermediate-mediated(e.g.,*OCH3)reversibleOvoccupancyprocess.Wethenusedheavierdeuterium(D)fromD2OsolventasthesourceofHtoinvestigatetheOvconcentrationvariationduringCO2RR.Underappliedpotential,Ovconcentrationdecreasedto5.81×1016spins,higherthanthatusingH2Oassolventunderthesamecondition(SupplementaryFig.37).AshydrogenationactivityisweakenedinD2O50,lower*OCD3wasproducedduringCO2RR,hencecausingaloweroccupancyrateofOv. To reveal these short-lived intermediates, the spin traps were spiked under flow employing 5,5-dimethyl-1-pyrroline N-oxide (DMPO)51 (Supplementary Fig. 38). A typical six-finger peak (g=2.0055, AN=1.35mT, AHβ=0.74mT)appeared(Fig.5c),exhibitinganegativecorrelationwithOv,whichdemonstratedthereplacementofOvbyalkoxy(i.e.,*OCH3).Subsequently,thissignaldisappearedduringthepoweroutage,andsignalsassociatedwithOvgraduallyrevertedtotheirinitialstate.TheOvconcentrationofCu–TiO1.3wasalsoprobedduringwaterelectrolysis,whichretainedwell,indicatingthatH2Oand*OHadsorptionhaveonlyaminorinfluenceonOvstability(SupplementaryFig.39).SignalreversibilityduringpotentialinterruptionconfirmedOv’sdynamicparticipationinCO2RRonCu–TiO1.3,revealinga new*OCH3insertionmechanism(SupplementaryFig.40).Notably,asmethoxylradicals(*OCH3)aretypicallypoorlydetectableinbulkwatersolution,thisresultismoreindicativeofanassociationwiththeelectrochemicalinterface.Byregulatingthecalcinationcondition,theOvconcentrationcouldbecontrolledrationally(SupplementaryFig.41).EMRspectrashowedthattheOvconcentrationofcatalystincreasedtohighervalueswhen1.58×1017spinswhencalcinedin10%H2/Arfor4h,whichcouldfurtherimprovetheFECH4to78%.However,theTOFCH4perOvofCu–Ti1O3showedthemaximumvaluewith1.62×10−13h−1spin−1(SupplementaryFig.41e),indicatingthemostefficientCH4formationonl-Cusites.

Thequasi-operandoXPSalsorevealedthedynamicswitchofOvduringCO2RRfromasemi-quantitativeperspective.ForCu–TiO1.3,Ovcontentdecreasedto15.9%after10-min electrolysis (intermediate-retained state), while recovering upon degassing (Supplementary Fig. 42a). This reversible change indicates pseudo-lattice oxygen formation by adsorbed intermediates. Concurrent Ti2p binding energy upshift correlated with Ov depletion (Supplementary Fig. 42b), further evidencing intermediate-induced electronic perturbation. In stark contrast, Cu−O4(Ti)showednegligibleOv/Ti2pchanges(SupplementaryFig.42c,d),confirmingCu−O−Timotifs’minimalreactivityimpact.Complementaryin-situRaman(SupplementaryFig.43)alsorevealedareversibledownshiftoftheEg(1)band(147→143cm−1)inCu–TiO1.3duringreaction(Fig.5d),whileremaininginvariantinCu−O4(Ti)(Fig.5e),consistentwiththe*OCH3insertionmechanismmentionedabove.These results elucidate the unique mechanistic pathway governing CH4 formation on Cu–TiO1.3, mediated through d−d orbital coupling at l-Cusites.

CO2 temperature-programmed desorption (CO2-TPD) was then carried out to assess the CO2 adsorption capacity of the Cu SACs (Supplementary Fig. 44). Cu–TiO1.3 displayed a large chemisorption peakatahighertemperature(455°C),comparedwiththeweakchemisorptionpeakofCu−O4(Ti)at430°C,manifestingtheintensiveCO2adsorptionability52. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS, Supplementary Fig. 45) was then employed to probe intermediates. On Cu–TiO1.3, the 1355cm−1 peak (i.e., *COOH)12 at −0.1V disappeared distinctlyat−0.5V,indicatingrapidconversionto*CO(Fig.6a).Bothcatalystsexhibitedlinear-bonded*CO(*COL)at~2075cm−1(Fig.6a,b)53,54,withCu–TiO1.3showingstrongerintensityoverwiderpotentials,reflectinghigher*COcoverages.COLsignalonCu−O4(Ti)red-shiftedwithincreasingappliedpotential,ascribingtotheStarkeffect55. In contrast, COL peak on Cu–TiO1.3 intensified and blue-shifted with the potential increasing to –1.1V. This phenomenon is ascribed to dipole–dipole coupling resulting from the increased *CO coverage at localized Cuδ+ sites and adjacent Ti atom55,56. The in-situ ATR-SEIRAS spectra of pristine Au film, which served as the SEIRAS-active template, were also measured during CO2 RR (Supplementary Fig. 46). A peak observed at ~2100cm−1 wasassigned to adsorbed *CO57, confirming that the detected *COL peak on the Cu–TiO1.3 surface indeed stems from Cu–*CO interaction. The in-situ Raman spectra showedabandataround358cm−1(SupplementaryFig.47),whichwasattributedtothefrustratedrotationofadsorbed*COonCu58. Adistinct*COsignalatlowcathodicpotentialfurtherhighlightsthehigher*COcoverageonCu–TiO1.3surface.Moreover,C–H bending modes of *OCH3 appeared53, yet notably red-shifted on Cu–TiO1.3 (1380cm−1) versus Cu−O4(Ti)(1398cm−1).This18cm−1shiftsignifiesreducedC–Hvibrationalfrequency,consistentwithstabilized*OCH3insertionatOvmentionedabove35,59. Meanwhile, earlier emergence and intensified *OCH3 signal on Cu–TiO1.3 further evidence facilitated *CO hydrogenationtowardCH4.

DFT calculations were then carried out to assess the effect of orbital engineering oncatalytic performance (Model details in Supplementary Data 1). The CO2 adsorption features were first evaluated (Supplementary Table 11), and detailed charge density difference (CDD) analysesrevealedanenhancedelectrondonationfromCusites to*CO onCu–TiO1.3(Fig.6c).ThehighestCDDvalueonCu–TiO1.3wascalculatedas−1.8×10−3eBohr−3(63.6%largerthanthatonCu−O4(Ti)),showinganegativechargecloud (bluecolorin Fig.6c)surrounded*CO,indicatingastrongerd-π*polar-interaction.Moreover,thefreeenergydiagramsillustratehowtheCO2RRpathwaysproceedonCusingle-atomsites(Fig.6d).Theoverallfreeenergychange(ΔG)formethanation(i.e.,*CO→*+H2O+CH4(g))onbothCuSACsshowsenergydownhill,indicatingathermodynamicallyfavorablemethanation.WiththeBrønsted−Evans−Polanyi relation, we further assessed the reaction kinetics in CO2-to-CH4 conversion. For *CO→*COOHconversion,Cu–TiO1.3displayeda0.52eVsmallerenergyrise relative to Cu−O4(Ti), suggesting a promoted CO2 activation resulting from stronger d-π* polar-interactions36,37, which has been mentioned in Fig. 2b. For the vital *OCH3 intermediate, ΔG for *OCH3→*OCH2 step was only 0.01eV on Cu–TiO1.3, ~100% reduction from that on Cu−O4(Ti), demonstrating the accelerated kinetics inducedbythenew*OCH3insertionmechanismatl-Cusites.Meanwhile, ΔG for hydrogenation of *OCH3 was 1.57eV and 0.97eV on Cu−O4(Ti)andCu–TiO1.3,respectively(Fig.6d),revealingfaster*OCH3-to-*CH4convertingkineticsatl-Cusites.Takentogether,theDFTcalculationsindicateamorefavorableCO2-to-CH4conversiononCu–TiO1.3,whichisattributedtothelocalizationofCuelectronicstatesandsubsequentlyenhancedd-π*polar-interactionandCu–Obonding.

Methods
Chemicalsandmaterials
Potassium hydroxide (KOH, ≥85.0%), Sodium hydroxide (NaOH, ≥85.0%), acetone (99.7%), ethanol (99.7%), glycerol (99.7%), Copper chloride dihydrate (CuCl2·2H2O, ≥99.0%), Titanium oxysulfate dihydrate (TiOSO4·2H2O, ≥99.0%), Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, ≥99.0%) and Aluminum nitrate nonahydrate (Al(NO3)3·9H2O, ≥99.0%) were purchased from the Sinopharm ChemicalReagentCompany.NifoamwaspurchasedfromtheFangdouTechnologyCo.,LTD(99%metalbasis)with0.3mmthicknessand0.1mmporesize.TheNafionsolution(Dupont,D-520dispersion,5%w/winwaterand1-propanol)andtriethyleneglycolwerepurchasedfromAlfaAesar.Gasdiffusionlayer(SGLCarbon,28BC)waspurchasedfromFuelCellStore.Thehydroxideexchangemembrane(FumasepFAB-PK-130,3×2cm)fortheflowcellwaspurchasedfromFumatech,whichwasactivatedbysoakingin1MKOHfor24hforfurthertest.Thehydroxideexchangemembrane(Grade60,3×3cm)forthezero-gapelectrolyzerwaspurchasedfromDioxideMaterials,whichwasactivatedbysoakingin1MKOHfor24hbeforethetest.CO2gas(99.99%),Argas(99.99%),N2gas(99.99%),andallthestandardgaseswerepurchasedfromShanghaiTomoeGasCompany.

FabricationofCu–Ti1O3
In a typical procedure,2.2g TiOSO4·2H2O,0.15g CuCl2·2H2O,12mL deionized water,6mL glycerol, and 20mL ethanol were mixed and stirred for 1h. The mixed solution was then moved into a 100mL autoclaveandkeptat200°Cfor8h,followedbynaturalcoolingtoroom temperature. Finally, the precipitate was washed thoroughly with alcohol and dried at 60°C in a vacuum oven overnight. The samplewasthencalcinedat300°Cin10%H2/Arfor3htoobtaintheCu–Ti1O3catalyst.

FabricationofCu−O4(Ti)
Cu−O4(Ti)werepreparedinasimilarwayasCu–Ti1O3nanoparticles,exceptthatthefinalsamplewascalcinedat300°Cintheairfor3h.

Preparationofelectrodes
FortheCO2RRinaflowcell,awell-dispersedcatalystslurrycontaining18mg of catalysts, 3mL of acetone, and 120μL of Nafion (Dupont, D-520dispersion,5%w/winwaterand1-propanol)ionomersolutionwasslowlydroppedontoagasdiffusionlayer(SGLCarbon,28BC),attainingacatalystloadingof~0.3mgcm−2measuredby0.01mganalyticalbalance.

Materialscharacterizations
XRD was tested on a Bruker SMART APEX (II)-CCD. XPS data were characterizedbyaThermoScientificK-AlphaXPS.ScanningelectronmicroscopymeasurementswerecarriedoutbyaHitachiS4800SEM.HRTEMmeasurementswereperformedbyaJEM2100F(JEOL,Japan). 1H-nuclear magnetic resonance (1H-NMR) measurements were performed on a Bruker AVANCE AV III HD 500. Moreover,Athena and ArtemissoftwarewithastandardIFEFFITpackagewereusedtoprocessXASdata.

ElectrocatalyticCO2reductionmeasurements
For CO2 RR in flow-cells, the electrochemical tests were conducted using a 1-cm2 cell with a Gamry (Model: Reference 3000) electrochemical workstation at room temperature (25±1°C). The flow-cell configurationconsistsofagasdiffusionlayerloadedwithcatalystsastheworkingelectrode,aNifoamastheanode,anAg/AgClelectrodeasthereference,andananionexchangemembranetoseparatethecathodeandanodechambers(SupplementaryFig.48).Thechambervolumeofthecathodeandanodeis0.5cm3. All applied potentials wereconvertedtotheRHEscaleaccordingtotheNernstequation:
ERHE =EAg/AgCl0 +0:197+0:059×pH (1)

The electrode potential was iR-corrected using the following equation:
Esample =Eapplied−0:9×Itotal×Rcell (2)

whereEsamplewasthecorrectedpotentialatthecathode,Eappliedwastheappliedpotential,andItotalwasthetotalcurrent.Rcellwasthecellresistanceof0.98±0.06Ω,whichwasmeasuredviapotentiostaticEIS. 1MKOHsolution(pH=14.00±0.02)wasutilizedastheelectrolyte thatwaspreparedbeforeelectrochemicalteststoensurefreshness. The reaction was performed with a steady stream of CO2 (30mLmin−1)measuredbyasoapbubbleflowmeter.Thegasproductwascollectedandanalyzedbyagaschromatograph(ShanghaiRamiin GC2060)whiletheliquidproductswerequantifiedby1H-NMRspectroscopy(BrukerAVANCEAVIIIHD500)usingamethodpreviouslyreported60.

ThegasproductFEwascalculatedbythefollowingequation:
FE(%)=N×F×Sx×C×v=I×100% (3)

whereNwasthenumberofchargetransfers(H2:2;CH4:6;C2H4:8);SxwasthepeakareaofthegasproductandSwasthepeakareaofcorrespondingstandardgas;Cwastheconcentrationofstandardgas;vwastheflowrateofCO2;Iwasthereal-timecurrentduringCORR;FwastheFaradayconstant(96485Cmol–1).

TheliquidproductFEwascalculatedbythefollowingequation:
FE(%)=N×V×C×F=Q×100% (4)

whereNwasthenumberofchargetransfers(Acetate:4;EtOH:8;n-PrOH:12); V was the electrolyte volume (20mL); C was the concentrationofproducts;Qwasthereactioncharge.

TheECSAwascalculatedbyassumingamonolayerofPbadatoms over a Cu surface and 2e− Pb oxidation with a conversion factor of 310μCcm−261.TheEISwasperformedatanopen-circuitpotentialstateinthefrequencyrangefrom100kHzto100MHz.ThePbunderpotentialdeposition was conducted in an Ar-saturated 0.1M HClO4 aqueoussolutionwith10mMPb(ClO4)2byusingcyclicvoltammetry.Thisisthedepositionofanadlayerofaforeignmetal(i.e.,Pb2+)onasubstrate(i.e.,surfaceCusite)atpotentialshigherthanthethermodynamicreductionpotentialoftheelectrodepositedmetal.TheforeignmetaltendstodepositontheCuratherthanTiO2duetohigherelectricalconductivity,leadingtoaccuratedepositionontheCusurface62.Beforetheexperiment,thepotentialwasfirstsetat−0.20Vvs.RHEfor150s,thensweptfrom−0.0840to0.161Vvs.RHEwithasweeprateof10mVs−1.

Stabilitytestsinazero-gapelectrolyzer
Thezero-gapelectrolyzer(ShanghaiKeqi,activearea5cm2,Supplementary Fig. 26) was utilized, which was established with a well-established membrane electrode assembly containing a cathode electrode,ananionmembrane(DioxideMaterials,Grade60),andananodeelectrode(Ti mesh loaded with 2mgcm−2 IrOx). 0.05M HCl-humidifiedCO2wassuppliedwithaflowrateof30sccmthroughtheflow channels inthe cathode flow field to reduce (bi)carbonate salt accumulationaccordingtopreviouswork63,and0.1MKHCO3(pH=6.83±0.03)wasfedintotheanodeflowchannelswithaflowrateof15sccm.Thestabilitytestswereperformedataconstantcurrentof−3A. During operation, the anolyte was replaced regularly, and the precipitated salt was further removed by separating the device to inhibitchannelblockandactivesiteshield.

CalculationforEE
Forflow-cellsystems,theEECH4wascalculatedbasedontheassumptionthattheoverpotentialofoxygenevolutioniszero.ThemethaneEEcanbecalculatedasfollows:
EECH4 = (1:23+(−ECH4))×FECH4 / (1:23+(−Eapplied)) (5)

whereEappliedisthepotentialappliedintheexperiment,FECH4isthemeasuredFEofCH4inpercentage,andECH4=0.17V vs. RHE for CO2 RR64.

Forthezero-gapelectrolyzer,EECH4iscalculatedasfollows:
EECH4 = (1:23+(−ECH4))×FECH4 / (−Efull-cellapplied) (6)

where Efull-cellapplied is the full-cell voltage applied in the zero-gap electrolyzer.

CalculationforTOF
TheTOFofCH4iscalculatedaccordingtothefollowingequation:
TOF(h−1)= (I×3600) / (nF×mcat×α) = MCu (7)

WherenisthenumberofelectronstransferredtoCH4with8,FistheFaradayconstantwith96485Cmol−1,mcatisthecatalystmassintheelectrode,αisthemassratioofsurfaceCuactivesitesinthecatalystmeasuredbyXPS,andMCuistheatomicmassofCu.

In-situelectrochemicalelectronmagneticresonance
Thein-situE-EMRwascarriedoutonaself-madein-situcellaccordingto our previous report with the employment of a magnetic resonance spectrometer(CIQTEK,EPR200,China)65.The0.17MofDMPO(DojindoLaboratories,Japan)wasselectedasthetrapagentwithoutadditionalpurification.IntheSchlenkline,thejust-preparedelectrodewascleanedin argon and then pumped with CO2. The electrochemical analyzer (CHI852D, CH Instrument, USA) applied multi-step potentials and recordedcurrentresponsewithsequential10minof0,−0.88,and0Vvs.RHE.Thecollaborationbetweenmagneticresonanceandelectro-analysis was ensured through a serial communication port. The obtained raw datawereprocessedandanalyzedbasedonEasyspin6.0.666,67.

In-situelectrochemicalinfraredspectroscopyexperiments
The in-situ attenuated total reflection surface enhanced infrared absorption(ATR-SEIRAS)measurementswereconductedonahomemadespectro-electrochemicalcell(SupplementaryFig.31).A~60-nm-thickSEIRA-activeAufilmwaspre-depositedonthereflectionplaneofasemicircularcylinderSiprism.Then,thecatalystwasdrop-castontheabovefilm.TheoperandoATR-SEIRASmeasurementswereperformedinanopticalsystemembeddedinthespectroscopicchamber.Toreachahighsignal/noiseratio,thespectrumwasobtainedbyaveraging256interferograms.TheincidentangleoftheIRbeamwassetto55°.ThespectrawereshownasabsorbancedefinedasA=‒logI/I0,whereIandI0representtheirradiationintensityofincidentandreflectivebeams,respectively.

For nano Au film preparation, a hemicylindrical Si prism was polishedandthencleanedwiththeRCAmethod:soakingitina1:1:5(v/v)solutionofNH3,H2O2,andH2O,andafterwardina1:1:5(v/v)solutionofHCl,H2O2,andH2Oatabout80°C,followedbyrinsingwithwater.Thentheretotalreflectingplane was immersed in 40% NH4F solutionfor90stoterminatetheSisurface.DepositionofAuwasperformedat60°Cbydroppingamixtureofaplatingsolutionand2%HF(2:1v/v)ontotheSisurface,andthedepositiontimewas60s. The composition of the plating solution was 0.015M NaAuCl4·2H2O+0.15M Na2SO3+0.05M Na2S2O3·5H2O+0.05M NH4Cl.Afterdeposition,theprismwasrinsedwithdeionizedwatertofinishthedeposition.

Forcatalystdeposition,awell-dispersedcatalystslurrycontaining18mg of catalysts, 3mL of acetone, and 120μL of Nafion ionomer solutionwasslowlydroppedontoAufilm,attainingacatalystloadingof~0.3mgcm−2thatcancovertheAufilm.

In-situRamanexperiments
ARamanspectrometer(HORIBAJobinYvonInc.,Model:HREvolution)coupledwithanopticalmicroscopewasusedforin-situRamanspectroscopy.Annear-infraredlaser(λ = 532nm)wasusedastheexcitationsource.Thetypesofobjectiveandgratingwere50XLand500nm,respectively.Ahomemadespectro-electrochemicalcellequippedwithaPtwirecounterelectrodeandanAg/AgClreferenceelectrodewasused,withtheworkingelectrodepreparedbycoatingthecatalystontoglassycarbon.Eachpotentialwasmaintainedforatleast10minbeforecollectingthespectra.

Quasi-in-situXPSmeasurement
All samples were prepared in the glovebox with an oxygen concentrationoflessthan4ppm.Thesampleswerepreparedthrough10-minCO2RRinAr-saturated1MKOHataround−0.88V,andwerethenpackagedwithliquidnitrogenprotectiontostabilizethesurfacereactionintermediateforstudyingitsinfluenceonOv. For the exploration of the Ov structure after reaction over catalysts, the samples were prepared in the same reaction conditions and were then driedundervacuumat120°Cfor24htoremovethesurfacespecies.

Theoreticalcomputational
DFTcalculationswereperformedwiththeViennaabinitiosimulationpackage(VASP),andthedetailedcalculationmethodsandparametersarepresentedinSupplementaryNote4andSupplementaryData1.Generally,allstructureswererelaxedbyaconjugategradientmethoduntiltheresidualforcecomponentwas<0.04eVÅ−1,andtheconvergencecriterionoftotalenergyintheself-consistentfieldmethodwassetto10−6eV.Theoriginalstructureofanatasewasemployedforcalculationswiththeparametersthata=b=3.7825Å,c=9.6150Å,andα=b=γ=90.0000°.Withtherelaxedcellstructure,the(100)plane wasutilizedforfurtherassessmentsaccordingtoXRDandTEMresults.DFT+UcalculationsonthedensityofstateswereperformedforbothCu–TiO1.3andCu−O4(Ti),andLobsterwasemployedforCOHPanalyses68.

Furthermore, (100) surface was also employed for CO2 RR evaluations,usingaslabmodelconsistingofthreeatomiclayersand2×1-unitcellsforthelateraldimensions,wherethethicknessofthevacuumwas set at 12Å to ensure that there was no superficial interaction betweendifferentlayers.Thetotalsizefortheestablishedslabmodelwas 7.6134×9.5168×21.3227 Å3, and α=β=γ=90.0000°. The Brillouin zoneofthesupercellwassampledbya3×3×1uniformk-pointmesh.Alltheenergycalculationswerebasedonatwo-stepprocessinvolvingthestructurerelaxationinvacuum(i.e.,electronenergy,EDFT)andasubsequentsingle-pointcalculationusinganimplicitsolvationmodel.

ThebonddissociationenthalpiesofCu–OandO–CinCu–*O–CH3werecalculatedbasedonasimilarmethodreportedpreviously15:
ΔHCu−O =H*+OCH3 −H*OCH3 (8)

ΔHO−C =H*O+CH3 −H*OCH3 (9)

whereH*+OCH3 andH*OCH3 aretheenthalpiesofthecatalystwithadsorptionofthecorrespondingmolecules,H*O+CH3 andH*O+CH3 aretheenthalpiesofthecorrespondingbond-breakingmodels.

TheGibbsfreeenergychangeofformation(atTK)foreachelementarystepwascalculatedas:
ΔG(T)=ΔEDFT+ΔEsol+ΔGcor(T) (10)

whereΔEsolwasthesolvationcorrection,ΔGcor(T)=ΔZPE−TΔS+ΔU(T)wasobtainedbyprocessingfrequencycalculationresults,andΔG was used to construct the free energy profiles. The computational hydrogen electrode model was also employed69, inwhichthechemicalpotentialof(H++e–)isequaltothatofhalfaH2moleculewithapHcorrectionofkBTln([H+]).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General Comment\nThis manuscript presents the rational design of copper single-atom catalysts (Cu SACs) supported on oxygen-vacancy-rich\nTiO2. The formation of [Cu–Ov–Ti] motifs enhances d–d orbital coupling, leading to highly efficient conversion of CO2 to\nCH4. The authors utilize DFT calculations along with a comprehensive array of in-situ and operando characterizations,\nincluding electrochemical EPR, Raman spectroscopy, XAS, ATR-SEIRAS. These analyses reveal that the oxygen vacancy-\ninduced orbital engineering improves CO adsorption, accelerates the C-O bond cleavage of *OCH3 intermediates, and\nstabilizes the active sites. Consequently, the Cu-Ti1O3 catalyst achieves an impressive CH4 Faradaic efficiency of up to\n76%, a record-high turnover frequency of approximately 24,000 h-1, and remarkable stability over 1,200 hours at industrially\nrelevant current densities.\nWhile the manuscript presents high-quality and impactful results, it requires further clarification and additional data support. I\nsuggest acceptance after major revision.\nSpecific Comments\n1. The procedure for in-situ FTIR Au layer deposition, along with the deposition amount and method of the catalyst ink, has\nnot been clearly described. To ensure experimental reproducibility, these aspects should be clarified in more detail.\n2. As is well known, Au itself is an active catalyst for CO2 reduction. If Cu single atoms are deposited on an Au layer, the\ninfluence of Au cannot be excluded. Therefore, in-situ FTIR data obtained from the Au layer alone (without the catalyst)\nwould be valuable as a reference.\n3. For Cu-Ti1O3, the COL peak appears to shift to higher wavenumber with increasing cathodic potential, while Cu-O4(Ti)\nexhibits a shift to lower frequencies. Because these trends are nearly opposite, a clear explanation for this contrasting\nbehavior is necessary.\n4. The authors have conducted in-situ Raman experiments; however, the data and the associated discussion presented are\nnot sufficiently detailed. It has been well established in previous studies that Raman measurements within the 200–600 cm-\n1 frequency range can effectively monitor Cu site-adsorbed intermediates under varying potentials. It is recommended that\nthe authors carry out these experiments and expand their discussion to strengthen this particular aspect of their research.\n5. The authors did not describe the experimental conditions for the in-situ Raman measurements. To ensure reproducibility,\ndetailed information such as the grating, laser source, objective, and other instrumental parameters must be included.\n6. Cu single-atom sites have been extensively reported in previous studies, especially those involving N-coordinated Cu\nsingle-atom catalysts. These catalysts often demonstrate a variety of product selectivities, such as CO, formate, or C2+\nproducts, which differ from the results presented in the current study. The authors should cite several of these previous works\nand provide additional discussion to better highlight their findings.\n7. The authors highlight industrial-level operation as a key strength of the developed catalyst. However, in real industrial flue\ngas, impurities such as SO2, NO, and O2 are typically present. To demonstrate the industrial relevance of this work, the\nauthors are encouraged to conduct additional experiments that examine how CH4 selectivity and overall catalyst\nperformance are affected when these impurities are co-fed with CO2.\n8. Although the authors have carried out various in-situ experiments, the manuscript does not provide morphological or\nstructural analyses of the catalyst after the long-term stability test. To support the durability claims, post-test characterizations\nsuch as SEM, XRD, or related analyses should be included.", "reviewer_2": "This manuscript presents oxygen vacancy (Ov)-involved orbital engineering for synthesizing a Cu–Ti1O3 catalyst with\nlocalized Cu single-atom sites toward CO2 electroreduction to methane. It was found that [Cu–Ov–Ti] motifs induced the\nenhancement of the d–d coupling at the Cu sites, leading to electronic localization. This construction boosted d-π* polar\ninteractions with adsorbed *CO2 and breakage of the C–O bond in *OCH3, which was proved by in-situ experiments and\ncalculations. In general, this work provides a new insight into regulating Cu single-atom sites for the generation of methane\nproduct from CO2 electroreduction. The work is novel and important to the field. I suggest the authors further improve this\nwork before it can be accepted. My detailed comments are listed below.\n1. It is worth investigating whether the CO2 flow rate can affect methane selectivity or activity. It is suggested to increase the\nCO2 flow rates to see whether it is possible to further boost methane production.\n2. Zero-gap electrolyzer was employed to evaluate the stability of catalysts in this work. In consideration of the distinct\ninterface microenvironment between zero-gap electrolyzer and flow cell, the influence of current density on the product\nselectivity for zero-gap electrolyzer should be further explored.\n3. The structure of Cu–Ti1O3 catalyst after reaction have been observed, which maintained well. In addition to the atomic\naggregation threatening the catalyst stability, the cathodic corrosion is also a big program (J. Am. Chem. Soc. 143, 3245–\n3255 (2021)). Is this Ov-involved construction can overcome this problem?\n4. To better understand the high selectivity towards methane on l-Cu sites of Cu–Ti1O3 catalyst, the authors will need to\nclearly identify the active site on the Cu–Ti1O3 catalyst rather than TiO2 substrate.\n5. The authors proposed that Cu–Ti1O3 could adsorb CO2 strongly due to stronger d-π* polar-interaction than Cu–O4(Ti),\nwhich was proved by the DFT calculations only. More experimental data is needed to support this conclusion.\n6. Some recently published related paper are suggested to be cited, for example, Nat Commun 16, 7943 (2025);\nElectrochem. Energy Rev. 2024, 7, 10; Nat Commun 14, 6849 (2023); Electrochem. Energy Rev. 7, 5 (2024); Nano Energy,\n2025, 133, 110457; Nano Energy, 2024, 128, Part A, 109873; Angewandte Chemie, 2025, 64 (15), e202424098; etc.\n7. In the Methods section, some experimental details were not given, such as the parameter of Ni form (i.e., thickness and\npore size), IrOx content, and the scan rate of LSV. The method of in-situ Raman should also be described.", "reviewer_3": "This manuscript presents a comprehensive study on Cu-based single-atom catalysts (SACs) supported on TiO2 with\noxygen-vacancy-induced orbital engineering for efficient CO2-to-CH4 electroreduction. The authors develop a Cu-Ti1O3\ncatalyst featuring [Cu-Oᵥ-Ti] motifs that enable strong d-d orbital coupling, leading to enhanced CH4 selectivity, high\nFaradaic efficiency (76%), and exceptional long-term stability at 600 mA/cm2. The study combines in-situ experiments and\nDFT calculations to elucidate the mechanistic origin of the improved methanation performance. Overall, this work presents a\npromising CO2 reduction electrocatalyst with impressive performance; however, further experimental validation is required\nto substantiate the proposed mechanism and confirm the robustness of the performance claims. After addressing the\nfollowing comments, the manuscript can be considered for publication in Nature Communications.\n1. While the manuscript highlights “industrial-level methanation”, the discussion of energy efficiency and techno-economic\nimplications is somewhat limited. The authors should provide a brief analysis comparing the energy efficiency (38.7%) and\noperational voltage (−4.05 ± 0.16 V) with existing industrial electrolysis benchmarks. A table summarizing key performance\nmetrics versus reported industrial-scale CO2RR systems would enhance practical relevance. Moreover, a supplementary\ntechno-economic analysis (TEA) should be incorporated to assess the feasibility and competitiveness of the proposed\nsystem under industrial operating conditions.\n2. The introduction of oxygen vacancy defects has been employed to construct d-d orbital interactions to optimize catalytic\nperformance in the authors’ work. The authors should clarify the structure-activity relationship between the oxygen vacancy\nconcentration and the CO2RR activity. Does there exist an optimal oxygen vacancy concentration that can maximize the\ncatalytic efficiency?\n3. The 1230-hour stability test is remarkable. However, since XRD and XPS have limited detection sensitivity for atomically\ndispersed Cu species, post-test EXAFS or HAADF-STEM analysis should be provided to confirm that Cu remains atomically\nisolated rather than aggregated into clusters.\n4. The manuscript presents DRT and impedance analyses but lacks the equivalent circuit model used for fitting. The\ninclusion of the circuit diagram and fitting parameters is essential to validate the extracted charge-transfer resistance values\nand ensure proper correlation between EIS and CO2RR kinetics.\n5. While the XPS results provide valuable information on the oxidation state of Cu, they alone cannot unambiguously\ndistinguish between Cu0 and Cu+ species due to the overlap of Cu 2p binding energies. It is therefore recommended to\ninclude Cu LMM Auger spectra to determine the Auger parameter, which offers a more reliable identification of the Cu\noxidation state. The combined XPS-Auger approach would help confirm whether the Cu sites in Cu–Ti1O3 are\npredominantly Cu+ or partially reduced Cu0 species, thereby strengthening the discussion of interfacial charge redistribution\nand d-d orbital coupling.\n6. The claim that reversible Ov occupancy occurs via *OCH3 intermediates is compelling but may benefit from additional\ncontrol experiments. It is suggested to perform isotope-labeling (e.g., 13CO2) or potential-step experiments to confirm that\nthe observed signal reversibility directly correlates with catalytic turnover rather than electrochemical reduction–oxidation of\nthe lattice.\n7. The reported TOFCH4 (24930 h-1) is impressive for single-atom catalysts and thus requires clear justification of the active\nsite quantification method (e.g., based on ICP-MS, XAS fitting, or XPS). Similarly, please elaborate on the ECSA\ndetermination method and its reliability, as the normalization directly affects the comparative analysis of catalytic activity.", "reviewer_4": "This manuscript reported an oxygen vacancy mediated d-orbital engineering of Cu-based single-atom catalysts (SACs) for\nelectrochemical CO2-to-CH4 conversion. The developed Cu–Ti1O3 catalyst could intensify d–d coupling at Cu sites and\nfurther facilitates the transformation of *OCH3 intermediate to CH4 by promoting C–O bond cleavage, enabling highly\nFaradaic efficiency of 76% under jCH4 of 670 mA cm–2 and an impressive durability over 1,230 h. Despite the mentioned\nremarkable achievement, several major experimental aspects are missing and the evidence provided seems not enough.\nTherefore, I would like to recommend its publication if the following issues are well solved:\n1. The authors stated that ‘electron enrichment at oxygen vacancy (Ov) occurs in titanium oxide (TiO2)’. Why are Ov\nparticularly highlighted in TiO2, given that Ov formation is also commonly observed and Ov engineering is widely applied in\nrare-earth oxides such as CeO2 (e.g., 10.1038/s41467-024-49981-9; 10.1038/s41467-024-46513-3).\n2. The authors emphasizes that this catalyst possesses outstanding performance at an industrial level. It is therefore\nrecommended to include an overall techno-economic analysis to substantiate this claim and provide a more convincing\nevaluation of its practical feasibility.\n3. Detailed atomic labels should be included in the diagrams (such as Supplementary Figure 1 and Figure 2) to enhance\nrigor and help readers more accurately interpret the structure information of the molecular models.\n4. This Cu–Ti1O3 catalyst exhibits exceptional stability for the electrochemical conversion of CO2 to CH4. However, as CH4\nis a deeply reduced products with 8-electron transfer process, the high local alkali concentrations and severe salt\nprecipitation usually hinder long-term stability test. Given this context, the main text should include more detailed\nexplanations and experimental descriptions on how on how these challenges were mitigated, to provide valuable technical\nguidance for researchers in the field.\n5. The authors should clarify the necessary of iR compensation in this work, since the reported reduction potential (e.g.,\nFECH4= 76% at −0.88 V in flow cells) appears significantly lower than values typically reported in the literature.\n6. Minor formatting adjustments are recommended. In particular, the orbital symbols in XPS results (such as 2p, 1s) should\nbe italicized for consistency with standard scientific conventions.\n7. The in-situ time-field E-EMR measurements indeed reveal intriguing phenomena regarding the Cu–Ti1O3 catalyst’s\nresponse to the applied potential. However, based on the current data, the observed decrease in Ov and the formation of\n*OCH3 species do not unambiguously demonstrate that *OCH occupies the vacancy sites. The authors are encouraged to\nfurther clarify how basis of this assignment and to rule out alternative possibilities such as the adsorption of H2O, CO2, or\n*OH species."}, "author_response": "Response:\nWe appreciate the reviewer’s very constructive comments for improving our manuscript.\nPlease see our point-by-point responses below and the corresponding revision in the\nmanuscript/supplementary information (SI).\n1) The procedure for in-situ FTIR Au layer deposition, along with the deposition amount and\nmethod of the catalyst ink, has not been clearly described. To ensure experimental\nreproducibility, these aspects should be clarified in more detail.\nResponse:\nWe thank for the reviewer’s suggestion, and have added the method of Au film deposition\nand information on the catalyst ink preparation in the revised SI to ensure experimental\nreproducibility.\n(Supplementary information, Page 11, Paragraph 2) “For nano Au film preparation,\nhemicylindrical Si prism was polished and then cleaned with the RCA method: i.e., soaking it in\na 1:1:5 (v/v) solution of NH , H O , and H O, and afterward in a 1:1:5 (v/v) solution of HCl,\n3 2 2 2\nH O , and H O at about 80 °C, followed by thorough rinsing with water. Then the total\n2 2 2\nreflecting plane was immersed in 40% NH F solution for 1.5 min to terminate the Si surface with\n4\nhydrogen. Deposition of Au was performed at 60 °C simply by dropping a mixture of a plating\nsolution and 2% HF (2:1 in volume) onto the hydrogen-terminated Si surface, and the deposition\ntime was 60 s. The composition of the plating solution was 0.015 M NaAuCl ·2H O + 0.15 M\n4 2\nNa SO + 0.05 M Na S O ·5H O + 0:05 M NH Cl. After deposition, the prism was rinsed with\n2 3 2 2 3 2 4\ndeionized water to finish the deposition.\nFor catalyst deposition, a well-dispersed catalyst slurry containing 18 mg of catalysts, 3 mL\nof acetone, and 120 μL of Nafion (Dupont, D-520 dispersion, 5% w/w in water and 1-propanol)\nionomer solution was slowly dropped onto nano Au film preparation, attaining a catalyst loading\nof ~ 0.3 mg cm−2 that could cover the Au film.”\n2) As is well known, Au itself is an active catalyst for CO reduction. If Cu single atoms are\n2\ndeposited on an Au layer, the influence of Au cannot be excluded. Therefore, in-situ FTIR data\nobtained from the Au layer alone (without the catalyst) would be valuable as a reference.\nResponse:\nWe appreciate the reviewer’s comments and have accordingly performed additional in-situ\nATR-SEIRAS measurements on the pristine Au layer. The spectra of the pristine Au film showed\na peak at 2100 cm−1, which could be assigned to *CO absorbed on the Au surface. In contrast,\nthe *CO adsorbed on Cu sites exhibited a peak at approximately 2075 cm−1 in in-situ\nATR-SEIRAS test. This further confirms that the *CO peak on the Cu–Ti O surface originates\nL 1 3\nfrom the Cu–*CO interaction instead of Au film.\nAction:\nWe have added Supplementary Figure 46 of the in-situ ATR-SEIRAS spectra on the\nAu film surface, following descriptions and corresponding reference (new Ref. 57) in the\nrevised manuscript.\n(Page 15, Paragraph 1) “The in-situ ATR-SEIRAS spectra of pristine Au film, which\nserved as the SEIRAS-active template, were also measured during CO RR (Supplementary\ndetected *CO peak on the Cu–Ti O surface indeed stems from Cu–*CO interaction.”\nL 1 3\nSupplementary Figure 46. In-situ ATR-SEIRAS spectra on pristine Au film in a potential\nwindow from OCP to −1.1 V.\n57. Guan, Y. et al. Exposure of Au Atom on Au(111) in Metal Nanoclusters for pH‐Universal\nElectrocatalysis. Angew. Chem. Int. Edit. 64, e202508459 (2025).\n3) For Cu-Ti O , the CO peak appears to shift to higher wavenumber with increasing cathodic\n1 3 L\npotential, while Cu-O (Ti) exhibits a shift to lower frequencies. Because these trends are nearly\n4\nopposite, a clear explanation for this contrasting behavior is necessary.\nResponse:\nWe thank for the reviewer’s suggestion. For Cu–O (Ti), the CO signal red-shifted with the\napplied potential increased, which was attributed to the Stark effect (Ref. 55: Sci. Adv. 6,\neabd2569 (2020)). Nonetheless, CO peak on Cu–Ti O showed a blue-shift when the potential\nL 1 3\nincreased to –1.1 V. This may be due to the adjacent Ti atoms, which could also display a weak\nadsorption of *CO (Ref. 56: Science 333, 736-739 (2011)). Under the higher cathodic potential,\nhigher *CO coverage may lead to a neighborly absorbed *CO on Ti sites near the localized Cuδ+\nsites, hence enhancing the dipole-coupling of *CO, finally resulting in the blue-shift\nphenomenon (Ref. 55: Sci. Adv. 6, eabd2569 (2020)).\nAction:\nWe have added the following sentences and corresponding references (new Ref. 56, 57)\nin the revised manuscript.\n(Page 15, Paragraph 1) “CO signal on Cu–O (Ti) red-shifted with increasing applied\nL 4\npotential, ascribing to the Stark effect55. In contrast, CO peak on Cu–Ti O intensified and\nL 1 3\nblue-shifted with the potential increasing to –1.1 V. This phenomenon is ascribed to dipole–\ndipole coupling resulting from the increased *CO coverage at localized Cuδ+ sites and adjacent\nTi atom55, 56.”\n55. Malkani, A. S., et al. Understanding the electric and nonelectric field components ofthe\ncation effect on the electrochemical CO reduction reaction. Sci. Adv. 6, eabd2569 (2020).\n56. Green, I. X., Tang, W., Neurock, M. & Yates Jr., J. T. Spectroscopic observation of dual\ncatalytic sites during oxidation of CO on a Au/TiO catalyst. Science 333, 736-739 (2011).\n2\n4) The authors have conducted in-situ Raman experiments; however, the data and the associated\ndiscussion presented are not sufficiently detailed. It has been well established in previous studies\nthat Raman measurements within the 200–600 cm-1 frequency range can effectively monitor Cu\nsite-adsorbed intermediates under varying potentials. It is recommended that the authors carry\nout these experiments and expand their discussion to strengthen this particular aspect of their\nresearch.\nResponse:\nWe really appreciate the reviewer’s valuable comments. Accordingly, we have further\nconducted the in-situ Raman measurements in a potential window from OCP to −1.1 V within\nthe required frequency range. The band at around 358 cm−1 was observed, which was ascribed to\nthe frustrated rotation of adsorbed *CO on Cu (Ref. 58: J. Am. Chem. Soc. 145, 21945–21954\n(2023)). Moreover, Cu–Ti O exhibited a stronger signal than that of Cu–O (Ti) at low cathodic\n1 3 4\npotential, further highlighting the higher *CO coverage.\nAction:\nWe have added Supplementary Figure 47 of the in-situ Raman spectra, following\ndescriptions and corresponding reference (new Ref. 58) in the revised manuscript.\n(Page 15, Paragraph 1) “The in-situ Raman spectra showed a band at around 358 cm−1\n(Supplementary Figure 47), which was attributed to the frustrated rotation of adsorbed *CO on\nCu58. A distinct *CO signal at low cathodic potential further highlights the higher *CO coverage\non Cu–Ti O surface.”\n1 3\nSupplementary Figure 47. In-situ Raman spectra of *CO on (a) Cu–Ti O and (b)\n1 3\nCu–O (Ti) catalysts in a potential window from OCP to −1.1 V.\n4\n58. Zhang, L., et al. Oxophilicity-controlled CO electroreduction to C alcohols over Lewis\n2 2+\nacid metal-doped Cuδ+ catalysts. J. Am. Chem. Soc. 145, 21945–21954 (2023).\n5) The authors did not describe the experimental conditions for the in-situ Raman measurements.\nTo ensure reproducibility, detailed information such as the grating, laser source, objective, and\nother instrumental parameters must be included.\nResponse:\nThanks for the reviewer’s comments. We have added the detailed experimental conditions of\nthe in-situ Raman measurements with a Raman spectrometer (HORIBA Jobin Yvon Inc., Model:\nHR Evolution), including the grating of 500 nm, objective of 50XL, and a near-infrared laser\n(λ = 532 nm).\nAction:\nWe have added the following sentences in the revised supplementary information.\n(Supplementary information, Page 11, Paragraph 4) “In-situ Raman experiments. A\nRaman spectrometer (HORIBA Jobin Yvon Inc., Model: HR Evolution) coupled with an optical\nmicroscope was used for in-situ Raman spectroscopy. A near-infrared laser (λ = 532 nm) was\nused as the excitation source. The types of objective and grating were 50XL and 500 nm,\nrespectively. A homemade spectro-electrochemical cell equipped with a Pt wire counter electrode\nand an Ag/AgCl reference electrode was used, with the working electrode prepared by coating\nthe catalyst onto glassy carbon. Each potential was maintained for at least 10 min before\ncollecting the spectra.”\n6) Cu single-atom sites have been extensively reported in previous studies, especially those\ninvolving N-coordinated Cu single-atom catalysts. These catalysts often demonstrate a variety of\nproduct selectivities, such as CO, formate, or C products, which differ from the results\n2+\npresented in the current study. The authors should cite several of these previous works and\nprovide additional discussion to better highlight their findings.\nResponse:\nWe thank for the reviewer’s suggestion. N-coordinated Cu SACs are extensively explored to\nenhance the performance of CO RR, among which C products like CO, formate, and CH are\n2 1 4\nthe predominant products for N-coordinated Cu SACs. For instance, the FE of CH over\nx\nsynthesized by Yang’s group (Ref. 14: Angew. Chem. Int. Edit. 64, e202424098 (2025)).\nMeanwhile, Pérez-Rodríguez et. al. proposed that copper-nitrogen doped carbon xerogel could\nproduce CO during CO RR (Ref. 11: J. Energy Chem. 88, 169-182 (2024)). In addition, Dong\n2\nand co-workers have reported that planar-symmetry-broken CuN SAC was favorable for\n3\nformate formation with a 94.3% selectivity (Ref. 12: Nat. Commun. 14, 6849 (2023)).\nMoreover, the dynamic restructuring of N-coordinated Cu single atoms—particularly their\nagglomeration induced by unstable Cu–N bonds—creates adjacent Cu sites that facilitate C–C\ncoupling, thereby leading to multi-carbon (C ) product formation. A recent report illustrates a\n2+\nhigh Faradaic efficiency of 35% toward C products (Ref. 20: J. Am. Chem. Soc. 147,\n2\n26425-26436 (2025)). All of the cases mentioned above have been added in the revised\nmanuscript.\nAction:\nWe have added the following sentences and corresponding references (new Ref. 11-14,\n20) in the revised manuscript.\n(Page 3, Paragraph 2) “For instance, N-coordinated Cu SACs have been confirmed to own\na favored C1 selectivity (e.g., CO11, HCOOH12, and CH 13, 14).”\n4\n(Page 3, Paragraph 2) “Moreover, Cu−X (X=C, N, O) coordinates are thermodynamically\nunstable19, causing severe reconstruction such as agglomeration of Cu atoms. This may facilitate\nC−C coupling20 and cause performance degradation under industrial-level current density (> 500\nmA cm−2)21.”\n11. Pérez-Rodríguez, S., et al. Single atom Cu-N-C catalysts for the electro-reduction of CO to\n12. Dong, J., et al. Continuous electroproduction of formate via CO reduction on local\n2\nsymmetry-broken single-atom catalysts. Nat. Commun. 14, 6849 (2023).\n13. Shen, W., et al. Self-healing Cu single-atom catalyst for high-performance electrocatalytic\nCO methanation. Nat. Commun. 16, 7943 (2025).\n2\n14. Cai, R., et al. The proximal protonation source in Cu-NH -C single atom catalysts\nx\nselectively boosts CO to methane electroreduction. Angew. Chem. Int. Edit. 64, e202424098\n2\n(2025).\n20. Lu, T., et al. Local Coordination Environment-Driven Structural Dynamics of Single-Atom\nCopper and the CO Electroreduction Pathway. J. Am. Chem. Soc. 147, 26425-26436 (2025).\n2\n7) The authors highlight industrial-level operation as a key strength of the developed catalyst.\nHowever, in real industrial flue gas, impurities such as SO , NO, and O are typically present.\n2 2\nTo demonstrate the industrial relevance of this work, the authors are encouraged to conduct\nadditional experiments that examine how CH selectivity and overall catalyst performance are\n4\naffected when these impurities are co-fed with CO .\nThanks for the reviewer’s suggestion, and we have conducted additional experiments\naccordingly. As is known, SO is a common industrial off-gas, and O exists in the air, both can\n2 2\nreadily contaminate the CO feed under realistic process conditions. To evaluate their impact, we\n2\nintroduced impurity streams containing 1% SO (~104 ppm), or 10% O (~105 ppm) during\n2 2\nCO RR tests. The FE of Cu–Ti O reached ~76% at around –0.88 V when pure CO was\ninitially supplied. Upon adding 1% SO at t = 0.5 h, both FE and FE decreased markedly,\n2 total CH4\nwhich was attributed to the lower energy barrier of SO reduction compared with CO RR (Ref.\n2 2\n50: J. Am Chem. Soc. 141, 9902-9909 (2019)). Importantly, the FE recovered to >65% after\nCH4\nstopping the SO flow, indicating that its effect on the catalyst structure is largely reversible. A\n2\nsimilar reversible influence was also observed upon introducing 10% O .\nWe have added a new figure for the catalytic performance with 1% SO and 10% O\n2 2\nfeed (Supplementary Figure 25) in the revised supplementary information. Meanwhile, the\nfollowing discussion and corresponding reference (new Ref. 48 and S34) in the revised\nmanuscript.\n(Page 11, Paragraph 1): “We then study the influence of impurities such as sulfur dioxide\n(SO ) and oxygen (O ) on the CO RR performance of Cu–Ti O at −0.88 V. Under pure CO flow,\n2 2 2 1 3 2\nthe Cu–Ti O realized a high FE of ~76%. However, upon introducing 1% SO (~104 ppm) at\n1 3 CH4 2\nt = 0.5 h, the total FE along with FE decreased markedly (Supplementary Figure 25a),\nCH4\nwhich can be attributed to the preferential reduction of SO 48. After 1 h of electrolysis (including\n2\na feed of 1% SO for 30 min), the FE recovered to > 66%, suggesting that the influence of\nSO on the catalyst structure is largely reversible. A similar reversible trend was observed when\n2\nfeeding 10% O (~105 ppm, Supplementary Figure 25b).”\n1 3 2\nfeed over the span of 3 h. (a) CO RR with 1% SO feed. The concentration of SO in flue gas is\n2 2 2\n~100 ppm (~0.01%), while SO concentration can be as high as ~10,000 ppm (1%) in\n2\nconcentrated CO streams that have not been desulfurizedS34. As a conservative limit, a stream of\n2\n1% SO was chosen in this study. (b) CO RR 10% O feed.\n2 2 2\n48. Luc, W., et al. SO -Induced Selectivity Change in CO Electroreduction. J. Am Chem. Soc.\n2 2\n141, 9902-9909 (2019).\nS34. Luc, W., et al. SO -Induced Selectivity Change in CO Electroreduction. J. Am. Chem. Soc.\n2 2\n141, 9902-9909 (2019).\n8) Although the authors have carried out various in-situ experiments, the manuscript does not\nprovide morphological or structural analyses of the catalyst after the long-term stability test. To\nsupport the durability claims, post-test characterizations such as SEM, XRD, or related analyses\nshould be included.\nResponse:\nWe thank for the reviewer’s comment, and have explored the structural durability of the\ncatalyst after the CO RR tests accordingly. Post-test SEM images exhibited that both catalysts\n2\ncould maintain ~20 nm particle size (Supplementary Figure 29). However, XRD patterns\nrevealed the Cu(111) diffraction peak of Cu–O (Ti), while Cu diffraction peak can hardly be\n4\nobserved for Cu–Ti O . These data suggest the breakage of Cu–O bonds in Cu–O (Ti), and a\n1 3 4\nrather stable Cu–Ti bonds in Cu–Ti O (Supplementary Figure 28). Moreover, post-test\n1 3\naberration-corrected HAADF-STEM images presented Cu nanoparticles on the Cu–O (Ti)\n4\nsurface, highlighting metallic atomic aggregation. For Cu–Ti O catalyst, the distance between\n1 3\nCu and Ti atoms was measured as 2.82 Å (Supplementary Figure 31), indicating the\npreservation of Oᵥ-intensified d−d orbital coupling.\nAction:\nWe have added figures of the post-test XRD, SEM, and aberration-corrected\nHAADF-STEM characterization (Supplementary Figure 28, 29, and 31) in the revised\nsupplementary information. And the following discussions have been updated in the\nrevised manuscript.\n(Page 11, Paragraph 2): “Post-test XRD, HRTEM, aberration-corrected HAADF-STEM\nand XPS confirmed structural integrity via persistent d−d coupling (Supplementary Figure\n28-32). In contrast, Cu–O (Ti) showed rapid degradation: FE dropped to 23% within 64 h\nunder identical conditions, with observed Cu nanoparticle formation that suggests Cu−O bond\ncleavage and metallic atomic aggregation.”\nSupplementary Figure 28. Post-test XRD patterns of Cu–O (Ti) and Cu–Ti O catalysts.\n4 1 3\nAppearance of Cu (111) peak in Cu–O (Ti) after CO RR demonstrates reconstruction of unstable\n4 2\nc-Cu sites, which is absent in the stable l-Cu sites of Cu–Ti O .\n1 3\nSupplementary Figure 29. Post-test SEM characterization. SEM images of (a) Cu–Ti O and\n1 3\n(b) Cu–O (Ti) catalyst after reaction.\nAberration-corrected HAADF-STEM images of (a) Cu–O (Ti) and (b) Cu–Ti O catalyst after\n4 1 3\nreaction. (c) Aberration-corrected HAADF-STEM images of Cu–Ti O after reaction along the\n1 3\n[100] direction after reaction. (d) The measured distance between the adjacent Cu and Ti atoms\non Cu–Ti O . Cu nanoparticle was observed in the Cu–O (Ti), while the distance of Cu and Ti\n1 3 4\natoms was measured as 2.82 Å, suggesting stable Cu–Ti bonds.\n========\nReviewer #2\nThis manuscript presents oxygen vacancy (O )-involved orbital engineering for synthesizing\nv\na Cu–Ti O catalyst with localized Cu single-atom sites toward CO electroreduction to methane.\n1 3 2\nIt was found that [Cu–O –Ti] motifs induced the enhancement of the d–d coupling at the Cu sites,\nv\nleading to electronic localization. This construction boosted d-π* polar interactions with\nadsorbed *CO and breakage of the C–O bond in *OCH , which was proved by in-situ\n2 3\nexperiments and calculations. In general, this work provides a new insight into regulating Cu\nsingle-atom sites for the generation of methane product from CO electroreduction. The work is\n2\nnovel and important to the field. I suggest the authors further improve this work before it can be\naccepted. My detailed comments are listed below.\nResponse:\nWe really appreciate the reviewer’s valuable comments for improving our work. We have\nmade careful revisions on the manuscript, with the point-by-point responses listed below.\n1) It is worth investigating whether the CO flow rate can affect methane selectivity or activity. It\n2\nis suggested to increase the CO flow rates to see whether it is possible to further boost methane\n2\nproduction.\nResponse:\nWe thank for the reviewer’s question. At 50 sccm CO , the Cu–Ti O catalyst could achieve\n2 1 3\nthe highest CH selectivity of 79% at around −0.88 V, compared to the Cu–O (Ti) at the same\n4 4\nCO flow rate. Nonetheless, as the CO flow rate below 20 sccm, the CH selectivity of both two\n2 2 4\ncatalysts decreased obviously.\nAction:\nAccordingly, we have added a new Supplementary Figure 20 in the revised\nsupplementary information, and the following descriptions in the revised manuscript.\n(Page 10, paragraph 2): “By tuning the CO flow rate to 50 sccm, the Cu–Ti O achieved\n2 1 3\nthe largest FE of 79% at around −0.88 V, while the peak FE of the Cu–O (Ti) was 38%\nCH4 CH4 4\n(Supplementary Figure 20).”\nSupplementary Figure 20. CO RR performance on (a) Cu–Ti O and (b) Cu–O (Ti) at\n2 1 3 4\naround −0.88 V with different CO flow rate.\n2\n2) Zero-gap electrolyzer was employed to evaluate the stability of catalysts in this work. In\nconsideration of the distinct interface microenvironment between zero-gap electrolyzer and flow\ncell, the influence of current density on the product selectivity for zero-gap electrolyzer should\nbe further explored.\nResponse:\nThanks for the reviewer’s suggestion. We have conducted additional experiments\naccordingly, to probe the effect of current density on the product selectivity. As a result,\nCu–Ti 1 O 3 showed a large FE CH4 of 75% at the constant current density of –0.6 A cm−2, while\nFE of Cu–O (Ti) was unsatisfied with 37% at the same condition.\nCH4 4\nAction:\nAccordingly, we have added a new Supplementary Figure 27 in the revised\nsupplementary information, and the following sentences in the revised manuscript.\n(Page 11, paragraph 2): “Both two catalysts were further integrated into a 5-cm2 zero-gap\nelectrolyzer to evaluate the potential of our strategies for practical applications towards the CH\n4\nformation (Supplementary Figure 26). The Cu–Ti 1 O 3 presented high FE CH4 of 75% at the\nconstant current density of –0.6 A cm–2, while the Cu–O 4 (Ti) displayed low value of FE CH4 (37%)\nat the same condition (Supplementary Figure 27).”\nSupplementary Figure 27. CO RR performance in a zero-gap electrolyzer. Tests with (a)\n1 3 4\nconstant-current-density measurement on Cu–Ti O and Cu–O (Ti) catalyst.\n1 3 4\n3) The structure of Cu–Ti O catalyst after reaction have been observed, which maintained well.\n1 3\nIn addition to the atomic aggregation threatening the catalyst stability, the cathodic corrosion is\nalso a big program (J. Am. Chem. Soc. 143, 3245–3255 (2021)). Is this O -involved construction\nv\ncan overcome this problem?\nResponse:\nWe thank for the reviewer’s comment, and have explored the structure of the catalyst and\ndissolved Cu after the CO RR tests accordingly. Cathodic corrosion can cause Cu dissolution\n2\nduring CO RR. We collected the Cu ions from the electrolyte and flushing water of membrane\n2\nand cathode chamber. Then we utilized XPS and ICP-OES to detect the content of Cu in the\ncatalyst after reaction, finding that the Cu content of Cu–Ti O after reaction was almost\n1 3\nunchanged. Moreover, the Cu ion concentration of Cu–Ti O was 5 ppb, much lower than that of\n1 3\nCu–O (Ti) (65 ppb), implying screened Cu corrosion. Hence, O -involved construction enables\n4 v\nto overcome cathode corrosion.\nAction:\nAccordingly, we have added a new Supplementary Table 7 and Supplementary Table 8\nin the revised supplementary information, and the following sentences in the revised\nmanuscript.\n(Page 11, paragraph 2): “In addition, the Cu content of Cu–Ti O after reaction was almost\n1 3\nunchanged, yielding less Cu dissolution (5 ppb vs. 65 ppb for Cu–O (Ti), Supplementary Table\n4\n7 and Table 8), implying screened Cu corrosion.”\nSupplementary Table 7. Cu atomic ratio of Cu–Ti O and Cu–O (Ti) catalyst after stability\n1 3 4\ntest measured by ICP-OES and XPS.\nICP-OES XPS\nSample\natomic ratio mass ratio atomic ratio mass ratio\nCu–Ti O 0.50% 1.19% 1.09% 2.58%\n1 3\nCu–O (Ti) 0.30% 0.72% 0.58% 1.38%\nflushing water of membrane and cathode chamber before and after stability test over\nCu–Ti O and Cu–O (Ti) catalyst measured by ICP-OES.\n1 3 4\nSample Before (ppb) After (ppb)\nCu–Ti O Not detected 5\n1 3\nCu–O (Ti) Not detected 65\n4\n4) To better understand the high selectivity towards methane on l-Cu sites of Cu–Ti O catalyst,\n1 3\nthe authors will need to clearly identify the active site on the Cu–Ti O catalyst rather than TiO\n1 3 2\nsubstrate.\nResponse:\nWe thank for the reviewer’s comment. The CO RR performance of TiO was then measured.\n2 2\nIt showed that H was the predominant product of TiO with >95% FE, and carbonous products\n2 2\ncould hardly be detected. This confirmed that Cu sites on the support were the key active sites\nfor CO RR.\nAccordingly, we have added a new Supplementary Figure 21 in the revised\nsupplementary information, and the following sentences in the revised manuscript.\n(Page 10, paragraph 2): “The pristine TiO showed that H was dominant product (FE >\n2 2\n95%) with no detection of carbonous product, confirming that Cu sites on the support facilitate\nthe CO RR (Supplementary Figure 21).”\n2 2\nflow cell.\n5) The authors proposed that Cu–Ti O could adsorb CO strongly due to stronger d-π*\n1 3 2\npolar-interaction than Cu–O (Ti), which was proved by the DFT calculations only. More\n4\nexperimental data is needed to support this conclusion.\nResponse:\nWe appreciate the reviewer’s suggestion. CO -TPD analysis was then performed to evaluate\n2 1 3 4\ndesorption peak at around 147 °C, which could be ascribed to the physisorption of CO (Ref. 53:\nchemisorption peak of Cu–O (Ti) at 430 °C, Cu–Ti O showed a stronger chemisorption peak at\n4 1 3\na higher temperature (455 °C), indicating the enhanced CO adsorption capacity (Ref. 53: Angew.\nAction:\nAccordingly, we have added a new Supplementary Figure 44 of CO -TPD in the\n2\nrevised supplementary information. The following sentences and corresponding reference\n(new Ref. 52) have also been updated in the revised manuscript.\n(Page 14, paragraph 2): “CO temperature-programmed desorption (CO -TPD) was then\n2 2\ncarried out to assess the CO adsorption capacity of the Cu SACs (Supplementary Figure 42).\n1 3\nthe weak chemisorption peak of Cu–O (Ti) at 430 °C, manifesting the intensive CO adsorption\n4 2\nability52”\nCO -TPD Cu–Ti O\n2 1 3\nCu–O (Ti)\n200 400 600\n).u.a(\nytisnetnI Physisorption\nTemperature (°C)\nSupplementary Figure 44. CO -temperature programmed desorption (TPD) of Cu–Ti O\n2 1 3\nand Cu–O (Ti) catalysts.\n4\n52. Yao, F., et al. Ordered copper triangular atomic sites for industrial-grade electromethanation\nof CO via self-regulated adsorption of reactants. Angew. Chem. Int. Edit. 64, e202511459\n2\n(2025).\n6) Some recently published related paper are suggested to be cited, for example, Nat Commun\n16, 7943 (2025); Electrochem. Energy Rev. 2024, 7, 10; Nat Commun 14, 6849 (2023);\nElectrochem. Energy Rev. 7, 5 (2024); Nano Energy, 2025, 133, 110457; Nano Energy, 2024,\n128, Part A, 109873; Angewandte Chemie, 2025, 64 (15), e202424098; etc.\nResponse:\nWe appreciate the reviewer’s advise, and have cited these papers in this work to strengthen\nthe viewpoint.\nAction:\nAccordingly, we have added the following references in the revised manuscript and\nsupplementary information.\n(Page 3, paragraph 1): “To address this, abundant decarbonization technologies are being\nproposed, among which the electrocatalytic CO reduction reaction (CO RR) emerges as a\n2 2\ncritical pathway for CO valorization and sustainable fuel production2, 3.”\n2\n3. Rakhsha, A., et al. Tandem gold/copper catalysis and morphological tuning via wrinkling to\nboost CO electroreduction into C products. Nano Energy 133, 110457 (2025).\n2 2+\n(Page 3, paragraph 2): “From a mechanistic perspective8, 9, spatially coupled Cu–Cu sites\npromote C–C coupling over the CH pathway (e.g., 0.53 eV higher barrier for CH on bulk\n4 4\nCu10).”\n9. Wu, M., et al. Emerging atomically precise metal nanoclusters and ultrasmall nanoparticles for\nefficient electrochemical energy catalysis: synthesis strategies and surface/interface engineering.\nElectrochem. Energy Rev. 7, 10 (2024).\n(Page 3, paragraph 2): “For instance, N-coordinated Cu SACs have been confirmed to own\na favored C selectivity (e.g., CO11, HCOOH12, and CH 13, 14).”\n1 4\n12. Dong, J., et al. Continuous electroproduction of formate via CO reduction on local\n2\nsymmetry-broken single-atom catalysts. Nat. Commun. 14, 6849 (2023).\n13. Shen, W., et al. Self-healing Cu single-atom catalyst for high-performance electrocatalytic\nCO methanation. Nat. Commun. 16, 7943 (2025).\n2\n14. Cai, R., et al. The proximal protonation source in Cu-NH -C single atom catalysts selectively\nx\nboosts CO to methane electroreduction. Angew. Chem. Int. Edit. 64, e202424098 (2025).\n2\n(Supplementary information, page 4, paragraph 2): “Moreover, most of reported\nsingle-atom sites were stabilized through interacting with NS7, S8.”\nS7. Hossain, M. N., Zhang, L., Neagu, R. & Rassachack, E. Free-standing single-atom\ncatalyst-based electrodes for CO reduction. Electrochem. Energy Rev. 7, 5 (2024).\n2\nelectroreduction. Nano Energy 128, 109873 (2024).\n7) In the Methods section, some experimental details were not given, such as the parameter of Ni\nfoam (i.e., thickness and pore size), IrO content, and the scan rate of LSV. The method of in-situ\nx\nRaman should also be described.\nResponse:\nThanks for the reviewer’s comment. We have supplemented the information of Ni foam with\n0.3 mm thickness and 0.1 mm pore size, IrO loading with 2 mg cm–2 and LSV scan rate of 50\nx\nmV s–1. Moreover, the detailed information on in-situ Raman was also supplemented in the\nsupplementary information\nAction:\nAccordingly, we have added the following sentences in the revised supplementary\ninformation.\n(Page 7, paragraph 1): “Ni foam was purchased from the Fangdou Technology Co., LTD\n(99% metal basis) with 0.3 mm thickness and 0.1 mm pore size.”\n(Page 9, paragraph 2): “The zero-gap electrolyzer (Shanghai Keqi, active area 5 cm2,\nSupplementary Figure 26) was utilized, which was established with a well-established\nmembrane electrode assembly (MEA) containing a cathode electrode, an anion membrane\n(DioxideMaterials, Grade 60), and an anode electrode (Ti mesh loaded with 2 mg cm–2 IrO ).”\nx\n(Page 11, paragraph 4): “In-situ Raman experiments. A Raman spectrometer (HORIBA\nJobin Yvon Inc., Model: HR Evolution) coupled with an optical microscope was used for in-situ\nRaman spectroscopy. A near-infrared laser (λ = 532 nm) was used as the excitation source. The\ntypes of objective and grating were 50XL and 500 nm, respectively. A homemade\nspectro-electrochemical cell equipped with a Pt wire counter electrode and an Ag/AgCl reference\nelectrode was used, with the working electrode prepared by coating the catalyst onto glassy\ncarbon. Each potential was maintained for at least 10 min before collecting the spectra.”\nSupplementary Figure 15. Linear sweep voltammetry (LSV) curves of TiO , Cu–O (Ti),\n2 4\nand Cu–Ti O catalyst at a scan rate of 50 mV s–1.\n1 3\n========\nReviewer #3\nThis manuscript presents a comprehensive study on Cu-based single-atom catalysts (SACs)\nsupported on TiO with oxygen-vacancy-induced orbital engineering for efficient CO -to-CH\n2 2 4\nelectroreduction. The authors develop a Cu-Ti O catalyst featuring [Cu-Oᵥ-Ti] motifs that\n1 3\nenable strong d-d orbital coupling, leading to enhanced CH selectivity, high Faradaic efficiency\n4\n(76%), and exceptional long-term stability at 600 mA/cm2. The study combines in-situ\nexperiments and DFT calculations to elucidate the mechanistic origin of the improved\nmethanation performance. Overall, this work presents a promising CO reduction electrocatalyst\n2\nwith impressive performance; however, further experimental validation is required to\nsubstantiate the proposed mechanism and confirm the robustness of the performance claims.\nAfter addressing the following comments, the manuscript can be considered for publication in\nNature Communications.\nResponse:\nWe appreciate the reviewer’s very constructive comments. Our point-by-point responses\nwere presented in the following.\n1) While the manuscript highlights “industrial-level methanation”, the discussion of energy\nefficiency and techno-economic implications is somewhat limited. The authors should provide a\nbrief analysis comparing the energy efficiency (38.7%) and operational voltage (−4.05 ± 0.16 V)\nwith existing industrial electrolysis benchmarks. A table summarizing key performance metrics\nversus reported industrial-scale CO RR systems would enhance practical relevance. Moreover, a\n2\nsupplementary techno-economic analysis (TEA) should be incorporated to assess the feasibility\nand competitiveness of the proposed system under industrial operating conditions.\nResponse:\nThanks for the reviewer’s suggestion. First of all, we have updated the EE and operation\nvoltage of this work and previously reported work in Supplementary Table 9. Moreover,\nCO -to-CH conversion may not be suitable for manufacturing bulk commodities (i.e., CH ) for\n2 4 4\nselling due to the low price of methane (<$200 t−1) in the market, which has been presented by\nprevious reports (Ref. S10: Chem. Eng. J. 523, 167510 (2025)). Herein, we employ a power\nplant model (Supplementary Figure 33) for assessments, which base on key parameters including\nelectrolyzer cost (550 $ kW−1), operating time (350 days year−1) and MEA replacement cost\n(1180 $ m−2), as well as carbon-trading price predicted as $250.0~$350.0 (Ref. S14: Nat. Energy\n6, 1133-1143 (2021)), $409.8 (Ref. S15: Applied Energy 293, 116914 (2021)) or $585.4 (Ref.\nS16: Carbon Price Projections, https://www.enerdata.net/research/carbon-price-projections.html)\nfor 1 ton of CO by 2050. The calculation results revealed that the gross profit may be positive\n2\nunder a specific carbon price (Supplementary Figure 34).\nAction:\nAccordingly, we updated the TEA-related information in the revised manuscript.\nMeanwhile, we have also updated Supplementary Note 2, Supplementary Figure 33, Figure\n34, Supplementary Table 9, and 10 in the revised supplementary information.\n(Page 12, Paragraph 1): “Moreover, techno-economic assessments based on a power plant\nmodel (Supplementary Figure 33), utilizing established literature parameters21, 49 and the\ndurability data presented herein (Supplementary Table 10), demonstrate that a profitable\npathway is feasible for this CO -to-CH conversion system under a given carbon price\n2 4\n(Supplementary Figure 34).”\n21. Lv, X., Liu, Z., Yang, C., Ji, Y. & Zheng, G. Tuning structures and microenvironments of\nCu-based catalysts for sustainable CO and CO electroreduction. Acc. Mater. Res. 4, 264-274\n2\n(2023).\n49. Zhou, B., et al. Electrosynthesis of CO from an electrically pH-shifted DAC post-capture\nliquid using a catalyst:support amide linkage. Joule 9, 101883 (2025).\n(Supplementary Information, Page 4, Paragraph 3) “Supplementary Note 2. Energy\nassessments and techno-economic assessments (TEA) based on a power plant model.\nDifferent from general power-to-X plants in previous CO RR evaluations, CO -to-CH\n2 2 4\nconversion is not suitable for manufacturing bulk commodities (i.e., CH ) for selling due to the\n4\nlow price of methane (<$200 t −1)S10 in the market. In contrast, it may be a promising\nCH4\nemission-reducing approach with the excess electricity output (Supplementary Figure 33). As\nsuch, the energy balance and carbon prices may be the key factors accounting for its economic\nvalue (as mentioned in the Introduction).\n1. Upstream feedstock preparation (purified CO production from the flue gas)\n2\npost-combustion CO capture technology. The overall energy consumption was estimated to be\n2\n4.1 GJ t −1, and the evaluated economic cost is around $46 t −1 according to previous\nCO2 CO2\nreportsS11.\n2. Electrocatalytic conversion process (CO RR)\n2\nassembly for conversion to CH . Based on the test parameters from this study and literature\n4\nvalues (Supplementary Table 10), the overall electricity demand is estimated at ~17.8 GJ t −1,\nCO2\nwhile the combined capital cost and operational costs amount to $247.9 t −1 according to Jiao’s\nCO2\nmodelS12.\n3. Downstream utilization (CH -based electricity production and carbon emission trading)\nCH4, and results in 27.1 GJ of electricity output in a natural gas combined cycle (NGCC) power\nplantS13. Using the conversion parameters in the above electrocatalysis process, the resulting\nelectricity output is equal to 3.1 GJ t −1 (i.e., ~ $25.8 t −1). Moreover, a power plant can\nCO2 CO2\nfurther conduct quota trading on carbon emission trading platforms (e.g., Shanghai Environment\nand Energy Exchange), the carbon-trading price is predicted to be $250.0~$350.0S14, $409.8S15,\nor $585.4S16 for 1 ton of CO by 2050 based on different models.\n2\n4. Overall assessments\nBased on the above evaluations, the potential economic value of CO -to-CH conversion\n2 4\ntechnology was calculated to be −$18.1~$81.9, $144.2, or $319.8 per ton of CO under different\n2\npredicted carbon prices (Supplementary Figure 34).”\nS10. Garcia-Vallejo, M. C., et al. A new approach to improve the economic and environmental\nattractiveness of biomethane from biomass gasification with redox-activated CO\n2\nsorbents. Chem. Eng. J. 523, 167510 (2025).\nS11. Raganati, F. & Ammendola, P. CO post-combustion capture: A critical review of current\n2\ntechnologies and future directions. Energy & Fuels 38, 13858-13905 (2024).\nS12. Shin, H., Hansen, K. U. & Jiao, F. Techno-economic assessment of low-temperature\ncarbon dioxide electrolysis. Nat. Sustain. 4, 911-919 (2021).\nS13. Kuehn, N. J., Mukherjee, K., Phiambolis, P., Pinkerton, L. L., Varghese, E. & Woods, M.\nC. Current and future technologies for natural gas combined cycle (NGCC) power plants.\nUnited States; 2013 10.\nS14. Mercure, J. F., et al. Reframing incentives for climate policy action. Nat. Energy 6,\n1133-1143 (2021).\nS15. Pietzcker, R. C., Osorio, S. & Rodrigues, R. Tightening EU ETS targets in line with the\nEuropean Green Deal: Impacts on the decarbonization of the EU power sector. Applied\nEnergy 293, 116914 (2021).\nS16. Enerdata. Carbon Price Projections. 2025 [cited 2025 December 12th] Available from:\nhttps://www.enerdata.net/research/carbon-price-projections.html\nSupplementary Figure 33. A simplified power-plant model for techno-economic\nassessments.\nSupplementary Figure 34. Gross profit for processing 1 ton of CO in power plants under\n2\ndifferent situations. Details in Supplementary Note 2.\nSupplementary Table 10. Parameters used for TEA.\nParameter Value\nProduction rate (t day−1) 10,000\nCH4\nElectricity price ($ kWh−1) $0.03\nElectrolyzer cost ($ kW−1) 550\nSystem lifetime (year) 20\nOperating time (days year−1) 350\nZero-gap electrolyzer replacement Cost ($ m−2) 1180\nPressure swing adsorption operating cost (kWh m−3) 0.25\nCell voltage (V) 4.05\nCurrent density (A cm−2) 0.60\n2) The introduction of oxygen vacancy defects has been employed to construct d-d orbital\ninteractions to optimize catalytic performance in the authors’ work. The authors should clarify\nthe structure-activity relationship between the oxygen vacancy concentration and the CO RR\n2\nactivity. Does there exist an optimal oxygen vacancy concentration that can maximize the\ncatalytic efficiency?\nResponse:\nThanks for the reviewer’s valuable comments. We have modulated the O concentration by\nv\nregulating the calcination condition (including calcination in 10% H /Ar for 2, 3 and 4 h, as well\n2\nas in air for 3h). EMR spectra found that O concentration was positively correlated with the\nv\ncalcination time in 10% H /Ar. When the calcination time increased to 4 h, the O concentration\n2 v\nincreased to higher values with 1.58×1017. This construction exhibited the highest FE of 78%.\nCH4\nNonetheless, the TOF per O of Cu–Ti O exhibited highest value of 1.62×10−13 h−1 spin−1,\nCH4 v 1 3\nindicating most efficient CH formation on l-Cu sites. This further suggested that more O is not\n4 v\nfavorable for increasing catalytic efficiency of active site.\nAction:\nAccordingly, we have added a new Supplementary Figure 41 in the revised\nsupplementary information, and the following sentences in the revised manuscript.\n(Page 13, paragraph 1): “By regulating the calcination condition, the O concentration\nv\ncould be controlled rationally (Supplementary Figure 41). EMR spectra showed that the O\nv\nconcentration of catalyst increased to higher values with 1.58×1017 spins when calcined in 10%\nH /Ar for 4 h, which could further improve the FE to 78%. However, the TOF per O of\nCu–Ti O showed the maximum value with 1.62×10−13 h−1 spin−1 (Supplementary Figure 41e),\n1 3\nindicating most efficient CH formation on l-Cu sites.”\nv 2\ndoped TiO catalyst calcined in 10% H /Ar for 2, 3 and 4 h, as well as in air for 3 h. (a) XRD\n2 2\npatterns, (a) EMR spectra, and (b) corresponding quantitative spin counts of signals related to O .\nv\n(d) CO RR performance at around –0.88 V. (e) corresponding TOF per O .\n3) The 1230-hour stability test is remarkable. However, since XRD and XPS have limited\ndetection sensitivity for atomically dispersed Cu species, post-test EXAFS or HAADF-STEM\nanalysis should be provided to confirm that Cu remains atomically isolated rather than\naggregated into clusters.\nResponse:\nThanks for the reviewer’s suggestion, we have added the post-test aberration-corrected\nHAADF-STEM analysis of catalysts after reaction accordingly (Supplementary Figure 31). For\nCu–O (Ti), Cu nanoparticle was observed, indicating the breakage of Cu–O bonds and\n4\naggregation of Cu atoms. However, Cu nanoparticle is absent for Cu–Ti O , and the distance of\n1 3\nCu and Ti atoms was measured as 2.82 Å, suggesting stable Cu–Ti bond.\nAction:\nAccordingly, we have added Supplementary Figure 31 of post-test aberration-corrected\nHAADF-STEM images, and the following sentences in the revised manuscript.\n(Page 11, paragraph 2): “Post-test XRD, HRTEM, aberration-corrected HAADF-STEM,\nand XPS confirmed structural integrity via persistent d−d coupling (Supplementary Figure\n28-32). In contrast, Cu–O (Ti) showed rapid degradation: FE dropped to 23% within 64 h\nunder identical conditions, with observed Cu nanoparticle formation that suggests Cu−O bond\ncleavage and metallic atomic aggregation.”\nSupplementary Figure 31. Post-test aberration-corrected HAADF-STEM characterization.\nAberration-corrected HAADF-STEM images of (a) Cu–O (Ti) and (b) Cu–Ti O catalyst after\n4 1 3\nreaction. (c) Aberration-corrected HAADF-STEM images of Cu–Ti O after reaction along the\n1 3\n[100] direction after reaction. (d) The measured distance between the adjacent Cu and Ti atoms\non Cu–Ti O . Cu nanoparticle was observed in the Cu–O (Ti), while the distance of Cu and Ti\n1 3 4\natoms was measured as 2.82 Å, suggesting stable Cu–Ti bond.\n4) The manuscript presents DRT and impedance analyses but lacks the equivalent circuit model\nused for fitting. The inclusion of the circuit diagram and fitting parameters is essential to\nvalidate the extracted charge-transfer resistance values and ensure proper correlation between\nEIS and CO RR kinetics.\nThanks for the reviewer’s advice. We have fitted the in-situ EIS and DRT spectra according\nto the equivalent circuit model (Ref. S32: J. Electrochem. Sci. Technol. 11, 1-13 (2020)). The\nfirst semicircle of the EIS spectrum originated from the impedance of the layer that forms on the\ninterface between the electrode and electrolyte, which was called as the solid electrolyte\ninterphase (SEI). During CO RR, carbonate formed due to the alkaline electrolyte, hence\n2\ninducing interface impedance. Cu–O (Ti) exhibited higher R with 40.8 Ω than that of\nCu–Ti O (4.17 Ω), suggesting more CO consumed by OH–. Moreover, the results of in-situ EIS\n1 3 2\nshowed that Cu–Ti O has lower charge-transfer resistance (R ) compared with Cu–O (Ti).\n1 3 ct 4\nFor DRT fitting results, the peaks of P1, P2, P3, and P4 could be ascribed to the mass\ntransfer of CO , charge-transfer process of CO RR and HER, and ionic transfer of K+. Especially,\n2 2\nCu–Ti O possessed lower interfacial charge transfer impedances of CO RR with 48.62 Ω than\n1 3 2\nthat of Cu–O (Ti) (123.7 Ω), whereas trend of HER was opposite. This suggested a more\n4\nfavorable CO RR pathway on Cu–Ti O .\n2 1 3\nAction:\nAccordingly, we have updated Supplementary Figure 16, Supplementary Table 5 and\nTable 6 in the revised supplementary information, and the following sentences in the\nrevised manuscript.\n(Page 9, paragraph 2): “In-situ electrochemical impedance spectra at –0.88 V were\nexecuted, which showed that Cu–Ti O obtained lower charge-transfer resistance compared with\n1 3\nCu–O (Ti) (Supplementary Figure 16 and Table 5). Corresponding advanced distributed\n4\nrelaxation times analyses exhibited two characteristic peaks at –0.88 V over two catalysts, i.e.,\nP2 and P3, which refer to the charge-transfer process of CO RR and HER, respectively47. The\n2\ninterfacial charge transfer impedances of CO RR on Cu–Ti O were calculated as 48.62 Ω, lower\n2 1 3\nthan that over Cu–O (Ti) (123.7 Ω), while the trend was opposite toward HER (Supplementary\n2 1 3\nSupplementary Figure 16. In-situ electrochemical impedance spectroscopy. (a) Nyquist plots\nand (b) DRT plots of Cu–Ti O and Cu–O (Ti)catalysts in the frequency range from 105 to 10−3\n1 3 4\nHz at –0.88 V. The curves were fitted according to the equivalent circuit modelS32. The first\nsemicircle of the EIS spectrum originated from the impedance of the layer that forms on the\ninterface between the electrode and electrolyte, which is called as the solid electrolyte interphase\n(SEI). This was generated by the carbonate formation on the catalyst surface during CO RR due\n2\nto alkaline electrolyteS33. Cu–O (Ti) catalyst exhibited higher R with 40.8 Ω than that of\nCu–Ti O (4.17 Ω), indicating more CO consumption by OH–.\n1 3 2\nSupplementary Table 5. The fitting results of EIS data for Cu–Ti O and Cu–O (Ti)\n1 3 4\ncatalysts at –0.88 V.\nCPE - CPE -\n1 2\nR R CPE - R CPE -\ns SEI 1 ct 2\nSample T T\n(Ω) (Ω) P (Ω) P\n(10-4) (10-4)\n0.98\nCu–Ti O 4.17 2.87 0.675 77.1 5.86 0.856\n1 3\n1\n0.98\nCu–O (Ti) 40.8 0.829 0.731 89.2 7.90 0.843\n4\nrelaxation time plots. The peaks of P1, P2, P3 and P4 could be ascribed to the mass transfer of\nCO , charge-transfer process of CO RR and HER, and ionic transfer of K+.\n2 2\nSample P1 (Ω) P2 (Ω) P3 (Ω) P4 (Ω)\nCu–Ti O 23.06 48.62 5674 5135\n1 3\nCu–O (Ti) 34.43 123.7 226.1 4939\nof Electrochemical Impedance Spectroscopy (EIS) for Lithium-ion Batteries. J. Electrochem. Sci.\nTechnol. 11, 1-13 (2020).\nS33. Feng, J., et al. CO electrolysis to multi-carbon products in strong acid at ampere-current\n2\nlevels on La-Cu spheres with channels. Nat. Commun. 15, 4821 (2024).\n5) While the XPS results provide valuable information on the oxidation state of Cu, they alone\ncannot unambiguously distinguish between Cu0 and Cu+ species due to the overlap of Cu 2p\nbinding energies. It is therefore recommended to include Cu LMM Auger spectra to determine\nthe Auger parameter, which offers a more reliable identification of the Cu oxidation state. The\ncombined XPS-Auger approach would help confirm whether the Cu sites in Cu–Ti O are\n1 3\npredominantly Cu+ or partially reduced Cu0 species, thereby strengthening the discussion of\ninterfacial charge redistribution and d-d orbital coupling.\nResponse:\nThanks for the reviewer’s advice. We have added the results of Cu LMM auger spectra of\ntwo catalysts, which displayed two peaks at 917.7 and 916.7 eV that could be attributed to the\nCu2+ and Cu+ species, respectively (Ref. 44: Surf. Sci. 630, 294-299 (2014)). Moreover,\nCu–Ti O displayed a higher ratio of Cu+ (86.1%) than Cu–O (Ti) with 67.9%, highlighting the\n1 3 4\nlow valence state of surface Cu, which may result from the d-d overlapping between Cu and Ti\natoms.\nAction:\nAccordingly, we have added Supplementary Figure 10 of Cu LMM Auger spectra in the\nrevised supplementary information, and the following sentences in the revised manuscript.\n(Page 8, paragraph 2): “Cu LMM Auger spectra unveiled that both catalysts exhibited two\nfitted peaks at 917.7 and 916.7 eV (Supplementary Figure 10), ascribed to the Cu2+ and Cu+\nspecies, respectively44. In detail, Cu–Ti O possessed higher ratio of Cu+ with 86.1% compared\n1 3\nwith that of Cu–O (Ti) with 67.9%, signifying low valence state of surface Cu.”\n1 3 4\ncatalyst.\n44. Pauly, N., Tougaard, S. & Yubero, F. LMM Auger primary excitation spectra of copper. Surf.\nSci. 630, 294-299 (2014).\n6) The claim that reversible O occupancy occurs via *OCH intermediates is compelling but\nv 3\nmay benefit from additional control experiments. It is suggested to perform isotope-labeling (e.g.,\n13CO ) or potential-step experiments to confirm that the observed signal reversibility directly\n2\ncorrelates with catalytic turnover rather than electrochemical reduction–oxidation of the lattice.\nResponse:\nThanks for the reviewer’s suggestion. Given that the hydrogenation rate would be decreased\ndue to the kinetic isotope effect (Ref. 51: Angew. Chem. Int. Edit. 64, e202417008 (2025)), using\nD O as solvent during CO RR would restrict the formation of *OCH . As a result, we carried out\n2 2 3\nthe control experiment in D O under applied potential. It is noted that O concentration of\n2 v\nCu-Ti O decreased to 5.81×1016 spins in D O solvent, significantly higher than that in H O\n1 3 2 2\nsolvent, further confirming the O occupancy by the intermediate.\nv\nMoreover, O concentration of Cu–Ti O catalyst maintained well during CO injection and\nv 1 3 2\nwater electrolysis, excluding the influence of CO , H O, and *OH adsorption on the O\n2 2 v\nconcentration. Combined the results of in-situ time-field E-EMR spectra with spin-trap agent, we\ncan propose that the decrease of O concentration was induced by the *OCH insertion.\nv 3\nAction:\nAccordingly, we have added a new Supplementary Figure 35, 37, and 39 in the revised\nsupplementary information, and the following sentences in the revised manuscript.\n(Page 12, paragraph 2): “In-situ time-field E-EMR tracked O dynamics and intermediates\nv\nduring CO RR. For Cu–Ti O , the time-field scans suggested that a signal related to O (centered\n2 1 3 v\nat g-tensor of 1.997) maintained well after 10-min CO injection, excluding the influence of CO\n2 2\nadsorption on the O concentrations (Figure 5a and Supplementary Figure 35).”\nv\nSupplementary Figure 35. In-situ E-EMR spectra of Cu–Ti O . Signals were recorded at the\n1 3\nbeginning and after 10-min CO injection.\n2\n(Page 12, paragraph 2): “We then used heavier deuterium (D) from D O solvent as the\n2\nsource of H to investigate the O concentration variation during CO RR. Under applied potential,\nv 2\nO concentration decreased to 5.81×1016 spins, higher than that using H O as solvent under the\nv 2\nsame condition (Supplementary Figure 37). As hydrogenation activity is weakened in D O50,\n2\nlower *OCH was produced during CO RR, hence causing a lower occupancy rate of O .”\n3 2 v\nSupplementary Figure 37. In-situ time-field E-EMR spectra of Cu–Ti O . (a) Signals were\n1 3\nrecorded before and after 10-min CO RR using D O as solvent at −0.88 V. (b) Quantitative spin\n2 2\ncounts of signals related to O versus time.\nv\n50. Li J, et al. Copper-catalysed electrochemical CO methanation via the alloying of single\n2\ncobalt atoms. Angew. Chem. Int. Edit. 64, e202417008 (2025).\n(Page 13, paragraph 1): “The O concentration of Cu–Ti O was also probed during water\nv 1 3\nelectrolysis, which retained well, indicating that H O and *OH adsorption have only a minor\n2\ninfluence on O stability (Supplementary Figure 39).”\nv\nSupplementary Figure 39. In-situ time-field E-EMR spectra of Cu–Ti O during water\n1 3\nelectrolysis. Signals were recorded before and after 10-min CO RR at −0.88 V. There is almost\n2\nno change during the water electrolysis.\n7) The reported TOF (24930 h-1) is impressive for single-atom catalysts and thus requires\nCH4\nclear justification of the active site quantification method (e.g., based on ICP-MS, XAS fitting, or\nXPS). Similarly, please elaborate on the ECSA determination method and its reliability, as the\nnormalization directly affects the comparative analysis of catalytic activity.\nResponse:\nThanks for the reviewer’s comments. The mass ratio of surface Cu utilized in TOF was\nCH4\ncalculated by XPS. UPD is the deposition of an adlayer of a foreign metal (i.e. Pb2+) on a\nsubstrate (i.e. surface Cu site) at potentials higher than the thermodynamic reduction potential of\nthe electrodeposited metal. The foreign metal tends to deposit on the Cu rather than TiO due to\n2\nhigher electrical conductivity, hence leading to precise deposition on the Cu surface (Ref. S26: J.\nElectroanal. Chem. 896, 115446 (2021)).\nFurthermore, the ECSA was measured by assuming a monolayer of Pb adatoms over a Cu\nsurface and 2e− Pb oxidation with a conversion factor of 310 μC cm−2 according to previous\nwork (Ref. S27: Nat. Commun. 13, 5898 (2022)).\nAction:\nAccordingly, we have added the following sentences in the revised supplementary\ninformation.\n(Page 8, paragraph 4): “The Pb underpotential deposition (UPD) was conducted in an\nAr-saturated 0.1 M HClO aqueous solution with 10 mM Pb(ClO ) by using cyclic voltammetry.\n4 4 2\nThis is the deposition of an adlayer of a foreign metal (i.e., Pb2+) on a substrate (i.e., surface Cu\nsite) at potentials higher than the thermodynamic reduction potential of the electrodeposited\nmetal. The foreign metal tends to deposit on the Cu rather than TiO due to higher electrical\n2\nconductivity, leading to accurate deposition on the Cu surfaceS26. Before the experiment, the\npotential was first set at −0.20 V vs. RHE for 150 s, then swept from −0.084 to 0.161 V vs. RHE\nwith a sweep rate of 10 mV s−1. The ECSA was calculated by assuming a monolayer of Pb\nadatoms over a Cu surface and 2e− Pb oxidation with a conversion factor of 310 μC cm−2\naccording to previous workS27.”\nS26. Sebastián-Pascual, P. & Escudero-Escribano, M. Surface characterization of copper\nelectrocatalysts by lead underpotential deposition. J. Electroanal. Chem. 896, 115446 (2021).\nS27. Gao, J., et al. Solar reduction of carbon dioxide on copper-tin electrocatalysts with energy\nconversion efficiency near 20%. Nat. Commun. 13, 5898 (2022).\n========\nReviewer #4\nThis manuscript reported an oxygen vacancy mediated d-orbital engineering of Cu-based\nsingle-atom catalysts (SACs) for electrochemical CO -to-CH conversion. The developed\n2 4\nCu–Ti O catalyst could intensify d–d coupling at Cu sites and further facilitates the\n1 3\ntransformation of *OCH intermediate to CH by promoting C–O bond cleavage, enabling\n3 4\nhighly Faradaic efficiency of 76% under jCH of 670 mA cm–2 and an impressive durability over\n4\n1,230 h. Despite the mentioned remarkable achievement, several major experimental aspects are\nmissing and the evidence provided seems not enough. Therefore, I would like to recommend its\npublication if the following issues are well solved:\nResponse:\nWe really appreciate the reviewer’s valuable comments. We have made a careful revision of\nthe manuscript, with the point-by-point responses listed below.\n1) The authors stated that ‘electron enrichment at oxygen vacancy (O ) occurs in titanium oxide\nv\n(TiO )’. Why are O particularly highlighted in TiO , given that O formation is also commonly\n2 v 2 v\nobserved and O engineering is widely applied in rare-earth oxides such as CeO (e.g.,\nv 2\n10.1038/s41467-024-49981-9; 10.1038/s41467-024-46513-3).\nResponse:\nThanks for the reviewer’s comments. CeO has been explored as a support extensively due\n2\nto its facile formation of O . However, the strong spin-orbit interaction of f-orbitals caused a high\nv\nenergy barrier for direct d−f coupling (Ref. 26: Science 374, 459-464 (2021); Ref. 27: Adv. Mater.\n35, 2302462 (2023); Ref. 28: Angew. Chem. Int. Edit. 64, e202414721 (2025)), triggering\ndominant [Cu−O−Ce] motifs with d−p−f coupling instead. In contrast, for anatase TiO , the\n2\nelectron prefers a free-carrier state, and can only be trapped at around O (Ref. 33: Phys. Rev.\nv\nLett. 113, 086402 (2014)), which is favorable for filling the d orbital of Cu and Ti and bridging\ntheir distance for boosting the formation of [Cu−Ti] motifs with d−d coupling. This enables the\nconstruction of localized Cu single-atom sites and increases structural stability.\nAction:\nAccordingly, we have added the following sentences in the revised supplementary\ninformation.\n(Page 4, paragraph 2): “For Cu /MO SACs, the intrinsic charge polarization of M−O\n1 x\nbonds26 and spin-orbit interaction triggered insurmountable energy barriers for direct d−d/f\ncoupling27, 28, finally causing dominative [Cu−O−M] motifs that facilitate indirect d−p−d/f\ncoupling during catalyst synthesis29, 30.”\n(Page 4, paragraph 2): “Unlike typical electron-polaron trapping at metallic sites, electron\nin anatase titanium oxide (TiO ) prefers a free-carrier state, and can only be trapped at oxygen\n2\nvacancy (O )31, 33, providing a vital opportunity for O -induced metal-metal bond formation\nv v\n(strong d−d coupling, Figure 1b), thereby constructing localized Cu single-atom sites (l-Cu sites)\nand stabilizing the active sites34, 35.”\n2) The authors emphasizes that this catalyst possesses outstanding performance at an industrial\nlevel. It is therefore recommended to include an overall techno-economic analysis to substantiate\nthis claim and provide a more convincing evaluation of its practical feasibility.\nResponse:\nThanks for the reviewer’s questions. In a general power-to-X plant (i.e., the common model\nused by previous TEA), CO -to-CH conversion is not suitable for manufacturing bulk\n2 4\ncommodities (i.e., CH ) for selling due to the low price of methane (<$200 t−1) in the market.\n(2025)). Herein, we employ a power plant model (Supplementary Figure 33) for assessments,\nand calculation results revealed that the gross profit may be positive under a specific carbon\nprice (Supplementary Figure 34).\nAction:\nAccordingly, we updated the TEA-related information in the revised manuscript.\nMeanwhile, we have also updated Supplementary Note 2, Supplementary Figure 33, Figure\n34, and Supplementary Table 10 in the revised supplementary information.\n(Page 12, Paragraph 1): “Moreover, techno-economic assessments based on a power plant\nmodel (Supplementary Figure 33), utilizing established literature parameters21, 49 and the\ndurability data presented herein (Supplementary Table 10), demonstrate that a profitable\npathway is feasible for this CO -to-CH conversion system under a given carbon price\n2 4\n(Supplementary Figure 34).”\n21. Lv, X., Liu, Z., Yang, C., Ji, Y. & Zheng, G. Tuning structures and microenvironments of\nCu-based catalysts for sustainable CO and CO electroreduction. Acc. Mater. Res. 4, 264-274\n2\n(2023).\n49. Zhou, B., et al. Electrosynthesis of CO from an electrically pH-shifted DAC post-capture\nliquid using a catalyst:support amide linkage. Joule 9, 101883 (2025).\n(Supplementary Information, Page 4, Paragraph 3) “Supplementary Note 2. Energy\nassessments and techno-economic assessments (TEA) based on a power plant model.\nDifferent from general power-to-X plants in previous CO RR evaluations, CO -to-CH\n2 2 4\nconversion is not suitable for manufacturing bulk commodities (i.e., CH ) for selling due to the\n4\nlow price of methane (<$200 t −1)S10 in the market. In contrast, it may be a promising\nCH4\nemission-reducing approach with the excess electricity output (Supplementary Figure 33). As\nsuch, the energy balance and carbon prices may be the key factors accounting for its economic\nvalue (as mentioned in the Introduction).\n1. Upstream feedstock preparation (purified CO production from the flue gas)\n2\npost-combustion CO capture technology. The overall energy consumption was estimated to be\n2\n4.1 GJ t −1, and the evaluated economic cost is around $46 t −1 according to previous\nCO2 CO2\nreportsS11.\n2. Electrocatalytic conversion process (CO RR)\n2\nassembly for conversion to CH . Based on the test parameters from this study and literature\n4\nvalues (Supplementary Table 10), the overall electricity demand is estimated at ~17.8 GJ t −1,\nCO2\nwhile the combined capital cost and operational costs amount to $247.9 t −1 according to Jiao’s\nCO2\nmodelS12.\n3. Downstream utilization (CH -based electricity production and carbon emission trading)\nCH4, and results in 27.1 GJ of electricity output in a natural gas combined cycle (NGCC) power\nplantS13. Using the conversion parameters in the above electrocatalysis process, the resulting\nelectricity output is equal to 3.1 GJ t −1 (i.e., ~ $25.8 t −1). Moreover, a power plant can\nCO2 CO2\nfurther conduct quota trading on carbon emission trading platforms (e.g., Shanghai Environment\nand Energy Exchange), the carbon-trading price is predicted to be $250.0~$350.0S14, $409.8S15,\nor $585.4S16 for 1 ton of CO by 2050 based on different models.\n2\n4. Overall assessments\nBased on the above evaluations, the potential economic value of CO -to-CH conversion\n2 4\ntechnology was calculated to be −$18.1~$81.9, $144.2, or $319.8 per ton of CO under different\n2\npredicted carbon prices (Supplementary Figure 34).”\nS10. Garcia-Vallejo, M. C., et al. A new approach to improve the economic and environmental\nattractiveness of biomethane from biomass gasification with redox-activated CO\n2\nsorbents. Chem. Eng. J. 523, 167510 (2025).\nS11. Raganati, F. & Ammendola, P. CO Post-combustion Capture: A Critical Review of\nS12. Shin, H., Hansen, K. U. & Jiao, F. Techno-economic assessment of low-temperature\ncarbon dioxide electrolysis. Nat. Sustain. 4, 911-919 (2021).\nS13. Kuehn, N. J., Mukherjee, K., Phiambolis, P., Pinkerton, L. L., Varghese, E. & Woods, M.\nC. Current and Future Technologies for Natural Gas Combined Cycle (NGCC) Power\nPlants. United States; 2013 10.\nS14. Mercure, J. F., et al. Reframing incentives for climate policy action. Nat. Energy 6,\n1133-1143 (2021).\nS15. Pietzcker, R. C., Osorio, S. & Rodrigues, R. Tightening EU ETS targets in line with the\nEuropean Green Deal: Impacts on the decarbonization of the EU power sector. Applied\nEnergy 293, 116914 (2021).\nS16. Enerdata. Carbon Price Projections. 2025 [cited 2025 December 12th] Available from:\nhttps://www.enerdata.net/research/carbon-price-projections.html\nSupplementary Figure 33. A simplified power-plant model for techno-economic\nassessments.\nSupplementary Figure 34. Gross profit for processing 1 ton of CO in power plants under\n2\ndifferent situations. Details in Supplementary Note 2.\nSupplementary Table 10. Parameters used for TEA.\nParameter Value\nProduction rate (t day−1) 10,000\nCH4\nElectricity price ($ kWh−1) $0.03\nElectrolyzer cost ($ kW−1) 550\nSystem lifetime (year) 20\nOperating time (days year−1) 350\nMEA replacement Cost ($ m−2) 1180\nPressure swing adsorption operating cost (kWh m−3) 0.25\nCell voltage (V) 4.05\nCurrent density (A cm−2) 0.60\n3) Detailed atomic labels should be included in the diagrams (such as Supplementary Figure 1\nand Figure 2) to enhance rigor and help readers more accurately interpret the structure\ninformation of the molecular models.\nResponse:\nThanks for the review’s valuable comment. We have added the detailed atomic labels in\nFigure 2, as well as Supplementary Figure 1, Figure 2, and Figure 28 to help readers understand\nthe configuration of the catalyst and reaction intermediates.\nAction:\nAccordingly, we updated the information of Figure 2 in the revised manuscript, as well\nas Supplementary Figure 1, Figure 2, and Figure 40 in the revised supplementary\ninformation.\nFigure 2. Assessments on the electronic states of two distinct Cu single-atom site\nconfigurations. (a) LDOS of Cu atom at l-Cu and c-Cu sites. (b) IPDOS of Cu−O−Ti and l-Cu\nsites around the Fermi level. (c) COHP analysis for d−d orbital coupling at c-Cu and l-Cu sites.\n(d) BDE evaluations illustrate that strong d−d coupling promotes C−O bond cleavage of *OCH\n3\non l-Cu sites compared with c-Cu sites.\nSupplementary Figure 1. Scheme illustration of three types of conventional Cu single-atom\ncatalysts (SACs). (a) Conventional Cu SACs (e.g., Cu-N-C catalyst) display a weak d−p orbital\ncoupling due to the intrinsically delocalized p-orbitals. (b) Free-atom-like Cu sites introduced by\nthe single-atom alloy strategy result in CO preference on Cu M. (c) Indirect d−p−d/f coupling\n1\ninvolving lattice oxygen causes a weak metallic atom interaction.\nSupplementary Figure 2. Scheme illustration of sluggish kinetics on Cu–O (Ti)surface. On\n4\na Cu–O (Ti)surface, the orbital coupling of Cu and Ti is weak due to indirect d−p−d coupling,\n4\ncausing an unstable Cu–O coordination and weak *OCH adsorption at the c-Cu single-atom site.\n4\nthe tendency on the l-Cu single-atom site of Cu−Ti O .\n1 3\nSupplementary Figure 40. Scheme illustration of *OCH insertion mechanism on Cu–Ti O\n3 1 3\ncatalyst. *OCH intermediates play a key role in the CO -to-CH conversion as their reversible\n3 2 4\ninsertion promotes the strong Cu−O bonding and facilitates the C−O cleavage.\n4) This Cu–Ti O catalyst exhibits exceptional stability for the electrochemical conversion of\n1 3\nCO to CH . However, as CH is a deeply reduced products with 8-electron transfer process, the\n2 4 4\nhigh local alkali concentrations and severe salt precipitation usually hinder long-term stability\ntest. Given this context, the main text should include more detailed explanations and\nexperimental descriptions on how these challenges were mitigated, to provide valuable technical\nguidance for researchers in the field.\nResponse:\nThanks for the reviewer’s comments. (Bi)carbonate salt formation has been widely\nrecognized as a primary factor in the poor operational stability of the CO RR. We utilized 0.05\n2\nsalt accumulation according to previous work (Ref. S28: Science 388, eadr3834 (2025).).\nMoreover, we also refresh the anolyte and disassemble the electrolyzer after 380-h reaction and\n800-h to clear the deposited salt in the chamber and on the back of the gas diffusion electrode.\nThese methods together could help to reduce the negative influence of salt precipitation on the\nstability test.\nAction:\nAccordingly, we have added the following sentences in the revised supplementary\ninformation.\n(Supplementary Information, Page 9, Paragraph 2) “Stability tests in a zero-gap\nelectrolyzer. The zero-gap electrolyzer (Shanghai Keqi, active area 5 cm2, Supplementary\nFigure 26) was utilized, which was established with a well-established membrane electrode\nassembly (MEA) containing a cathode electrode, an anion membrane (DioxideMaterials, Grade\n60), and an anode electrode (Ti mesh loaded with 2 mg cm-2 IrO ). 0.05 M HCl-humidified CO\nx 2\nwas supplied with a flow rate of 30 mL min−1 through the flow channels in the cathode flow field\nto reduce (Bi)carbonate salt accumulation according to previous workS28, and 0.1 M KHCO was\n3\nfed into the anode flow channels with a flow rate of 15 mL min−1. The stability tests were\nperformed at a constant current of −3 A. During operation, the anolyte was replaced regularly,\nand the precipitated salt was further removed by separating the device to inhibit channel block\nand active site shield.”\nS28. Hao, S., et al. Acid-humidified CO gas input for stable electrochemical CO reduction\n2 2\nreaction. Science 388, eadr3834 (2025).\n5) The authors should clarify the necessary of iR compensation in this work, since the reported\nreduction potential (e.g., FE = 76% at −0.88 V in flow cells) appears significantly lower than\nCH4\nvalues typically reported in the literature.\nResponse:\nThanks for the reviewer’s suggestion. Due to the creation of localized electronic states of\nl-Cu site, the Cu–Ti O catalyst exhibited high activity of CO RR (i.e., large current density).\n1 3 2\nTherefore, the voltage decrease caused by the internal resistance of devise increased obviously,\nwhich would influence the evolution of the intrinsic reaction potential. Hence, it is necessary to\nuse iR-correction to assess the intrinsic reaction performance. To evaluate the practical\nperformance of Cu–Ti O , we also calculated and listed the applied potential without\n1 3\niR-correction, as well as the corresponding EE . In consideration of the internal resistance, the\nCH4\nEE of Cu–Ti O could still reach > 30% at –0.88 V.\nCH4 1 3\nAction:\nAccordingly, we have updated the following Supplementary Table 9 in the revised\nsupplementary information.\nSupplementary Table 9. Electrocatalytic performances for methanation over reported\nCu-based catalysts. Cu–Ti O displays an outstanding performance toward methanation,\n1 3\nachieving the best durability and the highest TOF among the reported Cu-based catalysts.\nCH4\nPotential FE CH4\nCH 4 |J CH4 |\nEE CH4 TOF CH4\nStabil\nCatalyst selectivity (mA ity Ref.\n(V ) (%) (%) (h-1)\nRHE (%) cm-2) (h)\n–0.88 39\n76 94 476 17,711 /\n(–1.42#) (31#)\nThis\n–1.00 32\nCu–Ti O\n1 3 68 90 670 24,930 /\n(–1.86#) (23#) work\n–4.05* 71 91 426 20 15,851 1235\n–1.08 19\n34 56 147 5,073 /\n(–1.46#) (13#) This\nCu–O (Ti)\n4\nwork\n–4.39* 31 52 186 9 6,418 64\nCuBIM –1.08 52 69 672 24 558 6 S27\nCu-SAs/HG\n–1.10 72 90 230 33 2,756 12 S28\nDY\nBNC-Cu –1.46 73 92 462 29 5,108 8 S29\nLa Cu –3.40* 45 91 182 15 61 6 S30\n5 95\nCa CuO –0.30 52 63 517 35 445 6 S31\n2 3\nH-CuO @C –1.60 73 73 434 23 36 8 S32\nx\nIr −Cu N/Cu\n1 3\n–1.30 75 86 240 31 356 0.5 S33\nO\n–1.20 81 90 200 35 409 11 S34\nSAs/GDY\nCu-DBC –0.90 80 93 162 39 241 2.5 S35\nPul(3)-Cu –1.87 57 76 234 19 23 15 S36\nCuPEDOT –1.58 62 76 354 23 146 15 S37\nCu-N/IPCF –1.21 74 85 223 32 605 3 S38\nCu /PCN –1.50 71 86 427 28 741 10 S39\n1\n*Note: Full-cell voltages in these tests performed in zero-gap electrolyzers.\n#Note: Potential without iR-correction and corresponding EE .\nCH4\n6) Minor formatting adjustments are recommended. In particular, the orbital symbols in XPS\nresults (such as 2p, 1s) should be italicized for consistency with standard scientific conventions.\nResponse:\nThanks for the reviewer’s question. We have italicized the orbital symbols in XPS as well as\nthe orbital symbols in other parts of this paper to meet the standard of scientific writing.\nAction:\nAccordingly, we have updated the following Figure and following sentences in the\nrevised manuscript and revised supplementary information.\n(Page 4, paragraph 2): “To these issues, constructing localized Cu single-atom sites (i.e.,\nharder acid15) by introducing matrices with localized d/f valence orbitals may be essential22, and\nproposed solutions include constructing Cu M single-atomic alloys (M = f/d-block elements) as\n1\nwell as Cu /MO SACs (Supplementary Figure 1b, c). Although Cu M systems feature\n1 x 1\nlocalized d-orbitals, free-atom-like Cu sites (i.e., resembling d−p coupled molecular metal\ncomplexes) will cause an unstable condition23, 24, thus favoring a tight *CO binding and\npredominant CO selectivity25. For Cu /MO SACs, the intrinsic charge polarization of M−O\n1 x\nbonds26 and spin-orbit interaction triggered insurmountable energy barriers for direct d−d/f\ncoupling27, 28, finally causing dominative [Cu−O−M] motifs that facilitate indirect d−p−d/f\ncoupling during catalyst synthesis29, 30.”\n(Page 5, paragraph 1): “Synergistic experimental/theoretical analyses reveal that\nOᵥ-intensified d−d orbital coupling between Cu and Ti atoms enhances electronic localization of\nCuδ+ sites (i.e., harder acidity than pristine Cuδ+ sites in Cu-O (Ti)), strengthening d-π*\n4\npolar-interactions with adsorbed *CO (Figure 1d) and stabilizing the Cu−O binding with\n2\nhard-basic *OCH .”\ncatalysts (SACs). (a) Conventional Cu SACs (e.g., Cu-N-C catalyst) display a weak d−p orbital\ncoupling due to the intrinsically delocalized p-orbitals. (b) Free-atom-like Cu sites introduced by\nthe single-atom alloy strategy result in CO preference on Cu M. (c) Indirect d−p−d/f coupling\n1\ninvolving lattice oxygen causes a weak metallic atom interaction.\nSupplementary Figure 2. Scheme illustration of sluggish kinetics on Cu–O (Ti)surface. On\n4\na Cu–O (Ti)surface, the orbital coupling of Cu and Ti is weak due to indirect d−p−d coupling,\n4\ncausing an unstable Cu–O coordination and weak *OCH adsorption at the c-Cu single-atom site.\n4\nthe tendency on the l-Cu single-atom site of Cu−Ti O .\n1 3\nSupplementary Figure 4. Integrated PDOS (IPDOS) analysis. IPDOS of Cu 3d orbitals at (a)\nc-Cu and (b) l-Cu single-atom sites. Calculations performed using corresponding (100) plane\nslabs. Plots were collected from static calculations and post-processing by the VASPKIT\npackageS22.\nSupplementary Figure 8. Interfacial O vacancy assessment. O 1s XPS spectra of Cu–Ti O ,\n1 3\nCu–O (Ti),and TiO reveal a semiquantitative analysis of interfacial oxygen vacancies in these\n4 2\nsamples, indicating a significant accumulation of O .\nv\nSupplementary Figure 9. Evaluation of the interfacial electronic states at metallic sites by\nXPS. (a) Cu 2p XPS spectra of Cu–O (Ti) and Cu–Ti O catalyst. (b) Ti 2p XPS spectra of TiO ,\n4 1 3 2\nCu–O (Ti), and Cu–Ti O catalyst. Despite small amounts of Cu2+, Cu0/Cu+ species dominate the\n4 1 3\ninterfacial distributions, while a d−d orbital coupling leads to a significant red-shift of the Cu\npeak in Cu–Ti O .\n1 3\nSupplementary Figure 22. Structural analysis of Cu /MgO(Al). (a) XRD pattern, (b) Cu 2p\n1\nand (c) O 1s XPS spectra of Cu /MgO(Al) sample.\n1 3\ntest. Interfacial O concentrations display a reversible characteristic in Cu–Ti O .\nv 1 3\nSupplementary Figure 42. Quasi-in-situ XPS tests. XPS spectra for (a) O 1s and (b) Ti 2p\npeak over Cu–Ti O during 10-min CO RR at −0.88 V. Quasi-in-situ XPS spectra for (c) O 1s\n1 3 2\nand (d) Ti 2p peak over Cu–O (Ti) during 10-min CO RR at −0.88 V.\n4 2\n7) The in-situ time-field E-EMR measurements indeed reveal intriguing phenomena regarding\nthe Cu–Ti O catalyst’s response to the applied potential. However, based on the current data,\n1 3\nthe observed decrease in O and the formation of *OCH species do not unambiguously\nv 3\ndemonstrate that *OCH occupies the vacancy sites. The authors are encouraged to further\n3\nclarify how basis of this assignment and to rule out alternative possibilities such as the\nadsorption of H O, CO , or *OH species.\n2 2\nResponse:\nThanks for the reviewer’s question. For the influence of CO adsorption, in-situ\nCO on the Cu surface through Cu–C interaction. And the O concentration of Cu–Ti O catalyst\n2 v 1 3\nmaintained well after CO injection, highlighting the weak influence of CO on the O\n2 2 v\nconcentration. For the influence of H O and *OH adsorption over Cu–Ti O , in-situ time-field\n2 1 3\nE-EMR spectra at around −0.88 V during water electrolysis without CO injection displayed that\n2\nthe O concentration was almost unchanged. This further demonstrated the faint affect of H O\nv 2\nand *OH adsorption on the O concentration.\nv\nAction:\nAccordingly, we have added a new Supplementary Figure 35 and 39 in the revised\nsupplementary information, and the following sentences in the revised manuscript.\n(Page 12, paragraph 2): “In-situ time-field E-EMR tracked O dynamics and intermediates\nv\nduring CO RR. For Cu–Ti O , the time-field scans suggested that a signal related to O (centered\n2 1 3 v\nat g-tensor of 1.997) maintained well after 10-min CO injection, excluding the influence of CO\n2 2\nadsorption on the O concentrations (Figure 5a and Supplementary Figure 35).”\nv\nSupplementary Figure 35. In-situ E-EMR spectra of Cu–Ti O . Signals were recorded at the\n1 3\nbeginning and after 10-min CO injection.\n2\n(Page 13, paragraph 1): “The O concentration of Cu–Ti O was also probed during water\nv 1 3\nelectrolysis, which retained well, indicating that H O and *OH adsorption have only a minor\n2\ninfluence on O stability (Supplementary Figure 39).”\nv\nSupplementary Figure 39. In-situ time-field E-EMR spectra of Cu–Ti O during water\n1 3\nelectrolysis. Signals were recorded before and after 10-min CO RR at −0.88 V. There is almost\n2\nno change during the water electrolysis."}]

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