1

CoGa Catalysts for the Hydrogenation of CO2 to Methanol JosephA.Singh,1AngCao,2,3,JuliaSchumann,2,3TaoWang,2,3JensK.Nørskov,2,3FrankAbild-Pedersen,2,3

andStaceyF.Bent3

1. DepartmentofChemistry,StanfordUniversity,333CampusDrive,Stanford,California,94305,USA

2. SUNCATCenterforInterfaceScienceandCatalysis,SLACNationalAcceleratorLaboratory,2575Sand

HillRoad,MenloPark,CA94025,USA

3. SUNCATCenter for InterfaceScienceandCatalysis,DepartmentofChemicalEngineering,StanfordUniversity,443ViaOrtega,Stanford,California,94305,USA

Abstract Methanol is an important chemical compound which is used both as a fuel and as a platform

moleculeinchemicalproduction.Synthesizingmethanol,aswellasdimethylether,directlyfromcarbon

dioxideandhydrogenproducedusingrenewableelectricitywouldbeamajorstepforwardinenabling

an environmentally sustainable economy. We utilize density functional theory combined with

microkinetic modeling to understand the methanol synthesis reaction mechanism on a model CoGa

catalyst. A series of catalysts with varying Ga content are synthesized and experimentally tested for

catalytic performance. The performance of these catalysts is sensitive to the Co:Ga ratio, whereby

increasedGa content resulted in increasedmethanol anddimethyl ether selectivity and increasedCo

contentresultedinincreasedselectivitytowardsmethane.Wefindthatthemostactivecatalystshave

upto95%CO-freeselectivitytowardsmethanolanddimethyletherduringCO2hydrogenationandare

comparableinperformancetoacommercialCuZncatalyst.Usingin-situDRIFTSweexperimentallyverify

thepresenceofsurfaceformateduringCO2hydrogenationinsupportofourtheoreticalcalculations.

Introduction Fromcarbondioxideandhydrogen,methanolcanbesynthesizedandusedasaversatilemoleculeto

store energy and as a platformmolecule for chemical production.1 If thehydrogen is producedusing

renewableelectricity,thisroutetomethanolcouldrepresentanimportantstepinasustainablecarbon

cycle. Currently 70 million tonnes per year of methanol are produced, and methanol is already an

important component for polymer production and used as a fuel additive. Commercial methanol

productionusessyngasproducedfromfossilsourcesandcatalystsconsistingofCuandZnOsupported

onAl2O3.2ByutilizingCO2acarbonnegativeprocessbasedonCO2capturecanbedevised.

2

RecentworkhasdiscoveredaNi/Ga-based intermetallicmaterial as a promisingnew catalyst that

behavesdifferentlythantheconventionalCu-basedcatalystusedforCO2hydrogenationtomethanol.3,4

GahasalsoshownpromiseasapromoterforCu-basedmethanolsynthesiscatalysts.5,6 Inthepresent

work we show that the combination of Co and Ga is also a good catalyst for CO2 hydrogenation to

methanol. Recentwork has reported that CoGa catalysts have a significant selectivity towards higher

alcohols during CO hydrogenation.7,8 However, no prior reports have considered CoGa for the

hydrogenationofCO2tomethanol.

Inthiswork,weappliedadescriptor-basedapproachandmicrokineticmodelingondifferentmetal

surfaces to simulate the catalyticperformanceofmethanol synthesis fromCO2andH2. Basedon the

descriptors,aclassofCoGacatalystswereidentifiedasactiveandselectivetowardsmethanolformation.

Totestthedescriptor-basedresult,weexplicitlyperformedDensityFunctionalTheory(DFT)calculations

onthereactionintermediatesintheCO2hydrogenationprocessforCoGaandusedthisdatatoidentify

plausible reaction mechanisms and analyze activity/selectivity relationships. We performed

experimental synthesis and characterization of CoGa catalysts to explore the effects of catalyst

compositiononperformance.WediscoveredthatCoGacatalystsexhibitupto95%COfreeselectivityto

methanolanddimethylether.

Methods

Computational Methods AllcalculationswerecarriedoutusingtheQuantumEspressosoftware9 interfacedwiththeAtomic

SimulationEnvironment (ASE).10TheBEEF-vdW11exchangecorrelationfunctionalwasusedbecauseof

itsaccurateestimationofadsorptionenergies12anditsexplicit inclusionofvanderWaalsinteractions.

The nudged elastic band (NEB)13 method was used to identify transition-state structures and their

energies.Allcalculationswereperformedwithaplane-wavecutoffof500eV,adensitycutoffof5000

eV,andaMonkhorst-Packtypek-pointsamplingof4×4×1.

Afourlayer(2×4)bcc(110)slabwasusedinthecalculationswiththetoptwolayersallowedtorelax

duringthegeometricoptimizationsuntiltheforceoneachatomwaslessthan0.03eV/Å.Theslabswere

separatedbyatleast15Åofvacuuminthe(110)direction.Dipolecorrectionwasincludedinallcasesto

decoupletheelectrostaticinteractionbetweenperiodicallyrepeatedslabs.

MicrokineticmodelingwascarriedoutusingtheCatMAPsoftwarepackage.14Ratesweredetermined

by numerically solving the coupled differential equations with the steady state approximation.

3

Selectivity was defined as the rate of formation of the product of interest divided by the rates of

formation formethane,methanol, and COwithout anyweighting for the number of carbons. In this

work,surfacesweremodeledusingtwosurfacesites:a“hydrogenreservoir”siteandasiteforallother

intermediatesasdoneinpreviouswork.15Furtherdetailsandallnumericalinputstothekineticmodel

canbefoundinpreviouswork.16,17

Forafastscreeningofpossiblecatalystcandidates,aone-descriptormodelwasusedbasedonwork

byStudtetal.4Electronicenergiesandzeropointenergycorrectionsforthe1Dvolcanoplotaswellas

theenthalpyandentropycorrectionsweretakenfromStudtetal.4

Catalyst Synthesis and Characterization Catalysts were synthesized via incipient wetness impregnation (IWI). Co(NO3)2*6H2O (Aldrich) and

Ga(NO3)3*xH2O(Aldrich)weredissolvedindeionizedwater.Themetalsaltmasswaschosentohave16

weightpercenttotalmetalcontentinthecatalyst.Thesolutionwasaddedtodegassedsilicagel(Davisil

643,Aldrich).Catalystsweredriedat80°Cfor15hours.CatalystsweresubsequentlyreducedinH2by

heatingto700°Cat5°C/minutethenholdingfor2hours.

CocatalystssupportedonsilicagelweresynthesizedlikewiseviaIWItoyieldcatalystswith10weight

%Cometalloading.TheCocatalystsweredriedatroomtemperatureandthencalcinedat350°Cinair.

ICP-OESwasperformedusingaThermoFisherICAP6300DuoViewICP-OESspectrometer.Samples

were first digested in hot nitric acid (TraceMetalGrade, Fischer). Elemental standardswere obtained

fromAldrichanddilutedwithdeionizedwater.

Nitrogen physisorption measurements were performed with a Micromeritics 3 Flex instrument.

Surface areas were calculated using the Brunauer-Emmett-Teller (BET) method. Pore volumes were

calculatedusingtheBarett-Joyner-Halenda(BJH)method.

PowderX-raydiffractionwasperformedusingaBrukerD8diffractometerwithMok-alphasource.

Samplesweremeasuredin1mmquartzcapillaries.

TEM was performed using a FEI Tecnai and FEI Titan microscopes operating in brightfield mode.

Catalysts were dropcast from ethanol onto TEM grids (TedPella). Scanning transmission electron

microscopy (STEM) and energy dispersive spectroscopy (EDS) were performed on the FEI Titan

microscope.STEM-EDSlinescansweresmoothedusing4pointmovingwindowaverage.

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In-situdiffuse reflectance infraredFourier transformspectroscopy (DRIFTS)wasperformedusinga

previously-described custom instrument.18 Catalysts were reduced in H2 at 450 °C for 1 hour. A

backgroundspectrumofacatalystwasacquiredafterreduction.Thecatalystwasthenpressurizedto9

barinflowing3:1H2:CO2at250°C.Spectrawereacquiredoncethepressurestabilizedina3:1H2:CO2

flow.

Catalyst performance was evaluated using an Altamira Benchcat 4000 HP reactor. In each

measurement, a catalyst was packed into a bed between plugs of quartz wool in a 0.25 inch outer

diameterSilcolloy(SilcoTek)coatedstainlesssteeltube.Thecatalystwasreducedat450°Cfor1hourin

H2.Catalystswereheatedtothedesiredreactiontemperatureandpressurizedin3:1H2:CO2.Products

were quantified using an online Agilent 7890B gas chromatograph-mass spectrometer (GCMS).

Hydrocarbonproductswerequantifiedusingaflameionizationdetectorandcarbonmonoxideusinga

thermalconductivitydetector.Testingwasperformedunderdifferentialconditions.

A commercial CuZn methanol synthesis catalyst was used for comparison from Alfa Aesar. The

catalyst’s primary components are CuO, ZnO, Al2O3, andMgO as reported by themanufacturer. The

catalystpelletsweregroundusingamortarandpestleandtestedasdescribedabove.

Results and Discussion

CoGa Catalyst Discovery and Mechanism

Figure1.Calculatedturnoverfrequencies(TOF)relativetocopper(TOFCu)asafunctionofoxygen(ΔEO)binding energies. Calculations were performed for 250 °C with 6.67 bar carbon dioxide, 23.33 barhydrogen,0.01barwater,and0barmethane,methanol,andcarbonmonoxide.

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In our analysis, we combined DFT calculations and microkinetic modelling to study CO2

hydrogenation on metal surfaces. We have used energy scaling relations to describe reaction and

transition-stateenergiesfortheCO2hydrogenationreactionnetworkintermsofcarbonmonoxideand

oxygenbindingenergiesonlyinaccordancewithpriorwork.4,19Weusedavolcanogeneratedfromthis

approachbyStudtetal.toperformaninitialscreening.4Thevolcanoallowsforestimatingtheturnover

frequency(TOF)ofmethanolsynthesisasafunctionofObindingenergyandtheresultsofperforming

thisanalysisarepresentedinFigure1.

AscanbeseeninFigure1,thecalculatedvalueofObindingenergyonCoGaislocatedintheregion

ofhighmethanolactivity.CoGaisexpectedtohavereasonableactivitytowardsmethanol,comparable

tomonometallicCu.ThissuggeststhatCoGacouldbeagoodcatalystcandidateformethanolsynthesis.

Figure 2. Free energy diagrams of methane (black), methanol (red), and carbon monoxide (blue)formationat250°ContheCo1Ga1(110)surface.Freeenergiesofreactants(CO2,H2)arecalculatedfor30bar3:1H2:CO2.Allotherfreeenergiesarecomputedatthestandardstate.

Inthefollowing,weusedDFTtoinvestigatetheCoGacatalystinmoredetail.Weperformedexplicit

calculationsofreactionandactivationenergiesforthepathwayofCO2hydrogenationtomethanolon

theCo1Ga1(110)surface.Co1Ga1hasaBCCcrystalstructureandwechosethestableclose-packed(110)

surfaceasourmodel structure.Thecalculated reactionenergiesandbarriersofelementary reactions

involved intheformationofmethane,methanol,andcarbonmonoxidearepresented inTableS1and

fromthisweidentifiedthemostfavorablepathwaysformethane,methanol,andCOformation.Similar

toNiGa4andCuZnO20,wefoundthatmethanolisformedviaaformatepathway.ComparedwithNiGa,

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the hydrogenation of formate on CoGa proceeds via a modified pathway where the C-OH bond of

HCOOHwassplitratherthanH2COOHasinthecaseofNiGa.4

To formhydrocarbons, suchasmethane,we found that themost energetically favorablepathway

required splitting of the C-O bond in the CH3O intermediate. The CH3O intermediate splitting had a

barrierof1.40eVwhereasthebarriertohydrogenateCH3OtoCH3OHwasonly0.75eV.Thisindicated

thatCoGawouldformmethanolratherthanmethaneoranyotherhydrocarbonsinwhichtheC-Obond

splitinCH3Oisrequired.

ThemostfavorablewaytoformCOwasviathedirectreversewatergasshiftpathway,notinvolving

aformateintermediate(Figure2).InthisprocessCO2(g)directlysplitstoCO*andO*withtheCO*being

desorbedasCO(g)andtheO*reactingwithH*formingH2O(g).

Based on the energetics shown in the free energy diagram in Figure 2, there is a competition

betweendissociationofCO2*toCO,and thehydrogenationstep to formHCOO*.These twostepsare

critical in determining the overall product selectivity. As CO2 hydrogenation is a complex multistep

process,weperformedmicrokineticmodelling tomakequantitativepredictionsof theoverall activity

andselectivity.

Figure3.Calculatedturnoverfrequencies(TOF)asafunctionoftemperatureandpressureformethanol(a), carbon monoxide (b), and methane (c), and corresponding selectivities (d-f) on Co1Ga1 (110).Modelling was performed under the conditions of 3:1 H2:CO2, 0.01 bar water, and 0 bar methane,methanol,andcarbonmonoxide.

Theactivityandselectivityformethanol,carbonmonoxide,andmethaneformationcalculatedasa

functionof temperature andpressure are presented in Figure 3. The rate ofmethane formationwas

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muchlowerthanthatoftheothertwoproductsduetothehigherbarrierforsplittingtheC-Obondin

CH3O*.Therefore,themethaneselectivitywouldbeverylowontheCo1Ga1(110)surface

Atlowtemperatures,ahigheractivityandselectivityformethanolovercarbonmonoxideformation

was observed.With increasing temperature, the rate of carbonmonoxide formation increased faster

thantherateofmethanolformation.Thus,theselectivityformethanoldecreased,whiletheselectivity

forCOincreased.Methanol,therefore,wouldbetheprimaryproductonCo1Ga1(110)surfaceatlower

temperatures,andCOwouldbemorefavorableathighertemperatures.

CoGa Catalyst Characterization and Evaluation To test the computational predictions, CoGa catalysts were synthesized and experimentally

evaluated.Theelementalcompositionofthecatalyst,surfacearea,andporosityarepresentedinTable

1. A variety of compositions were considered as our computational predictions refer to a surface

compositionandinpracticethesurfacestructureandcompositionofacatalystcandeviatedfromthe

bulk composition and structure. The elemental composition of each catalyst closely matched the

expected ratio based on the mass of Co and Ga precursors used in each catalyst’s synthesis. The

prepared catalysts had similar surface areas and porosities, with the Co5Ga1 catalyst having slightly

lowersurfaceareaandporosity.

Table1.Co/Gamolarratios,BETsurfaceareas,andBJHporevolumesofCoGaCatalysts

Co/Gamolarratioexpected

Co/Gamolarratioexperimental*

BETSurfaceArea(m2/g)

BJHPoreVolume(cm3/g)

Co5Ga1 5.00 5.07 240 0.84

Co5Ga3 1.67 1.73 290 1.0

Co1Ga1 1.00 1.07 300 1.0

*CalculatedusingvaluesfromICP-OES

X-raydiffraction(XRD)wasusedtodeterminethestructureofthecatalysts.Basedontheseresults,

we can compare the structure of the synthesized catalysts with the structures used in our DFT

calculations.XRDpatternsofCo5Ga1,Co5Ga3,andCo1Ga1catalystsarepresentedinFigure4.TheCo5Ga1

catalystwasinaFCCCophase(PDFCard00-015-0806).ThepeakswereshiftedrelativetothoseofCo

towardssmaller2θangles.GahasalargeratomicradiusthanCoanditsincorporationintotheFCCCo

latticewouldresultinlatticeexpansionandthusashifttosmaller2θangles.

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Figure4.MeasuredXRDpatternsofCo5Ga1,Co5Ga3,andCo1Ga1catalysts (a). In (b), theFCCCo (220)peaksfrom(a)areshownwiththepositionoftheFCCCo(220)reflectionmarkedwithacircle.

TheCo5Ga3catalysthadpeaksduetobothCo1Ga1(PDF04-001-3339)andFCCCophases.TheFCCCo

peakswerefurthershiftedtowardssmaller2θanglesthan intheCo5Ga1catalyst.Theemergenceofa

Co1Ga1phaseandfurthershiftingoftheFCCCopeaksagreewiththegreaterGacontentofthiscatalyst.

The Co1Ga1 catalyst had no detectable FCC Co peaks and only contained peaks corresponding to a

Co1Ga1alloyphase.The solepresenceofaCo1Ga1alloyphaseagreeswith thecatalyst stoichiometry.

TheXRDresultssupportasuccessfulsynthesisofalloyedCoGacatalysts.

TEM characterization was used to investigate the structure of the catalysts at the nanoscale, in

particulartodeterminewhetherthesurface isanalloystructureorthere issegregationofCoandGa.

WeinvestigatedtheCo5Ga3catalystdueto itscomparativelyhighactivityandselectivityformethanol

whichwill bediscussed in the subsequent sections. TEMcharacterizationof theCo5Ga3 catalyst after

reduction is presented in Figure 5. The catalyst primarily consisted of CoGa domains with the

observanceof latticefringescorrespondingtoaCo1Ga1crystalstructure(Figure5a) inagreementwith

theXRDresults.Basedontheimaging,thecatalysthadaparticlesizedistributionof17±4nm(Figure

S1). Elemental analysis of an individual catalyst nanoparticle using STEM-EDS (Figure 5b) indicated a

uniform distribution of Co and Ga without evidence for segregation of either species. Elemental

mappingofthecatalystovertheregionshowninFigure5cispresentedinFigure5d.Themajorityofthe

CoandGasignalswerecolocalizedinnanoparticles.SomeCoandGasignalswereobservedinpartsof

themappingwithoutaclearnanoparticlepresent.Weattributedthesesignalstonoiseassimilarsignals

arepresentinregionsofthemappingwherenosamplewaspresentwhencorrelatingwiththedarkfield

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STEMimage(Figure5c).TheTEMresultsindicatedthecatalysthasnosignificantsegregationofCoand

Ga.ThecombinedXRDandTEMresultsdemonstratedtheformationofalloyedCoGacatalysts.Sincewe

successfully formed CoGa catalysts, we can use these catalysts to evaluate our computational

predictionsforCoGacatalysts.

Figure5.MicrographsandmicroanalysisofCo5Ga3catalysts.(a)brightfieldTEMmicrograph,(b)STEM-EDSlinescan,and(c)STEMdarkfieldmicrographand(d)correspondingSTEM-EDSelementalmappingwithCoinredandGaingreen.

Catalytic Performance Basedonthecharacterizationresults,itisevidentthatwesuccessfullysynthesizedCoGacatalystsfor

comparisonwithourcalculatedpredictions.CoGacatalystswiththecompositionscharacterizedabove

wereevaluatedforCO2hydrogenationperformanceandtheresultsarepresentedinTable2.Methanol

wasthemajorproductforallcatalystcompositionsfollowedbycarbonmonoxide,whilehydrocarbons

weretraceproductswithnohydrocarbonsheavierthanpropanedetected.TheCo5Ga3catalysthadthe

highest selectivity for methanol. The decrease inmethanol selectivity from Co5Ga3 to Co1Ga1 closely

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correlatedwith an increased selectivity for dimethyl ether between these two catalysts. As shown in

Table 2, the increase in dimethyl ether (DME) formation tracked the ratio of Co:Ga, with increasing

amountsofGacorrelatedwith increasedDMEformation.Dimethyletherformation isknowntooccur

bythebimoleculardehydrationofmethanol.1Thereaction iscatalyzedbysolidacidssuchasγ-Al2O3.1

PriorworkusingpyridinechemisorptionfoundthattheadditionofGa2O3increasestheacidityofSiO2.21

Hence,we speculate thatwith increasing amounts of Ga, gallium oxide sitesmay form that catalyze

DME formation due to their acidic nature. Because DME formation most likely occurs on sites not

related to the CoGa surfaces and is formed through a secondary reaction from methanol product,

dimethyletherformationwasnotconsideredinourtheoreticalcalculations.Further,whencomparing

ourcalculationswithexperimentalresults,DMEwillbetreatedasmethanolproducedbyCoGa.

Table2.CatalyticperformanceofpreparedCoGacatalyststestedat250°Cusing30bar3:1H2:CO2

Selectivity (C%)

MeOH CH4 HC* CO DME**

MeOH + DME CO Free Selectivity

Activity***

Co5Ga1 62% 9% 1% 27% 1% 86% 0.19 Co5Ga3 63% 4% 0.1% 26% 7% 95% 0.12 Co1Ga1 50% 1% 1% 27% 22% 97% 0.06

*ethaneandpropane**dimethylether***μmolC/gcatalyst/s

In addition to differences in DME selectivity, the catalysts showed different selectivities toward

methane.TherelativelyhighermethaneselectivityontheCo5Ga1catalystmaybetheresultofisolated

CositesonthisCo-richcatalystsinceCohasahighmethanationactivity.22Theoverallhighselectivity

towardsmethanoland its secondaryproductDMEagreeswithourprediction thatCoGacatalystswill

exhibitgoodselectivitytowardmethanolduringCO2hydrogenation.

Table2showsthatwithincreasingCocontent,theactivityofthepreparedcatalystsincreased.The

exactoriginofthisbehavior isunclear.Onepossibleexplanation is thatCo-sitesaretherelevantones

forCO2hydrogenation.IncatalystswithhighGacontenttherewouldbelessexposedCositesduetoa

reductioninCocontentandexcessGacoveringsurfaceCosites.WithincreasingCocontenttherewould

bemoreactivesitesandthereforehigheractivity.

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Figure 6. Selectivity of Co5Ga3 catalyst (squares) and a commercial CuZnmethanol synthesis catalyst(circles)duringCO2hydrogenationasafunctionoftemperature.ThecalculatedselectivitiesfromFigure3areshownasdashedlines.

Selectivities of Co5Ga3 and a commercial CuZn catalyst are compared in Figure 6. As temperature

increased,theCo5Ga3catalystshowedanoveralltrendofdecreasingselectivitytowardsmethanoland

dimethyl ether and increasing selectivity towardsCO in agreementwithour calculations.Although at

highertemperatures,thecommercialCuZncatalysthadhigherselectivityforthedesiredmethanoland

dimethyl ether products than did our Co5Ga3 catalyst, at 200 °C the selectivity of the Co5Ga3 catalyst

closelymatchedthatofthecommercialCuZnmethanolsynthesiscatalyst.Notingthatthecommercial

CuZn catalyst was highly optimized, we conclude that Co5Ga3 is highly promising as laboratory-

synthesizedcatalystsinceevenunoptimized,itcanreachsimilarperformancelevelstothecommercial

catalysts. Moreover, the predicted methanol and CO selectivity closely matched that of the Co5Ga3

catalyst. The calculationswereperformedon aCo1Ga1 (110) surfacewhile a real catalystwill contain

othersurfacestructures.Thediscrepancybetweenthecalculatedandexperimentalvaluesmost likely

resultedfromthecalculationsonlyconsideringtheCo1Ga1(110)surface.

Toinvestigatethesurfacechemistryunderconditionssimilartotheoperatingconditionsandverify

thatformateispresentaswepredictedtheoretically, in-situDRIFTSwasperformedat9barinflowing

3:1H2:CO2forCoandCo5Ga3catalysts.TheresultsarepresentedinFigure7.GasphaseCO2,CO,H2O,

andCH4specieswereobserved.MuchhigherCO,H2O,andCH4signalswereobserved forCothan for

Co5Ga3 (Figure 7a), in agreementwith the lowmethane and CO selectivity of Co5Ga3. In the formate

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stretchingregionofCo5Ga3apeakwasobservedat1613cm-1.PriorworkstudyingCO2hydrogenation

on Co/Al2O3 catalysts assigned a peak at 1597 cm-1 to formate on Co during CO2 hydrogenation.23 A

formatepeak at 1595 cm-1wasobservedonCu/SiO2 catalystsduringCO2hydrogenation.24A formate

peakat1583cm-1wasobservedonNi/SiO2catalystsduringformicaciddecomposition.24Theobserved

peakon theCo5Ga3catalystwasshifted towardshigher frequencies thanreported for formateonCo,

Cu, and Ni. Ga has a lower electronegativity than Co, Ni, and Cu and therefore a higher formate

stretching frequency would be expected for a CoGa alloy than for the puremetals discussed above.

Hence,theDRIFTSspectrasupportthepresenceofformatespeciesontheCo5Ga3catalyst.Thesecond

expectedformatepeak23near1380cm-1wasnotobserved;however,thispeakcannotberuledoutdue

to the lowsignal tonoiseat this frequency. For theCocatalyst, formate featureswerenotdetected;

however, the presence of H2O (due to the hydrogenation of CO2 to CH4)may have obscured aweak

formatefeature.

Inaddition to formatespecies, chemisorbedCOspecieswereobserved.Basedonourcalculations,

methanolformationoccursviaformate,andweexpectthechemisorbedCOspeciestoactasspectators

in thecaseofCo5Ga3with theCO formedthrough reversewatergas shift.ForbothCoandCo5Ga3,a

peakwasobservedat2009cm-1.ThispeakagreeswellwiththeexpectedlocationforCOonaCosite.25

TheCo catalyst’s peakwasmuchbroader than that of the Co5Ga3 catalyst. TheCo catalyst peakwas

asymmetricandskewedtowardslowerfrequencies.TheselowerfrequenciesmaycorrespondtoCOon

bridgingsites.Anadditionalpeakat1877cm-1wasobservedontheCo5Ga3catalystthatcanbeassigned

toCOadsorbedatthreefoldsites.26

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Figure 7. DRIFTS of Co and Co5Ga3 catalysts at 250 °C in 9 bar 3:1 CO2:H2. (b) is the surface CO andformateregionof(a).

ThroughDFTcalculationsandmicrokineticmodelling,wepredictedanewCoGamethanolsynthesis

catalyst.Ourcalculations indicatedthatthehydrogenationofCO2tomethanolproceedsviaaformate

intermediate. Our experimental results proved that CoGa is indeed a highly selective catalyst for

methanolduringCO2hydrogenationinagreementwithourpredictions.Usingin-situDRIFTSweverified

thatsurfaceformatespecieswerepresent,inlinewithourproposedmechanism.

Conclusions Althoughnotfullyoptimized,CoGacatalystsshowedpromisingselectivity forthehydrogenationof

CO2 to methanol. DFT was used to investigate the pathway for CO2 hydrogenation and explain the

selectivityofCoGacatalystsformethanol.TheperformanceofthecatalystswassensitivetotheCoGa

ratio with increasing Ga content resulting in increased dimethyl ether selectivity and increasing Co

contentincreasingselectivitytowardsmethane.Theproductselectivityofthepreparedcatalystsclosely

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matchedourDFTpredictions.Thepreparedcatalysts,althoughlowerinperformancethanacommercial

CuZn methanol synthesis catalyst, are comparable in selectivity. With additional optimization, the

performance of CoGa methanol syntheses catalysts may exceed that of current state-of-the-art

catalysts.

Acknowledgments We acknowledge financial support from the U.S. Department of Energy, Office of Basic Energy

SciencestotheSUNCATCenterforInterfaceScienceandCatalysis.Theauthorsgratefullyacknowledge

theuseoftheStanfordNanoSharedFacilities(SNSF)ofStanfordUniversityforsamplecharacterization.

The authors would like to thank Andrew Riscoe for performing the nitrogen physisorption

measurements. AngCaogratefullyacknowledgesfinancialsupportfromChinaScholarshipCouncil.

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Supporting information TableS1.Allpossibleelementary reactions involved in the formationofCO,CH3OHandCO fromCO2hydrogenation together with the reaction energies (ΔE) and activation energies (Ea) on Co1Ga1 (110)surface.

Elementaryreaction ΔE(eV) Ea(eV)

CO2+H→HCOO R1-1 -0.51 0.56

CO2+H→COOH R1-2 0.12 1.72

CO2→CO+O R1-3 -0.09 1.02

COOH+H→CO+OH R2-1 -0.53 0.25

HCOO+H→HCO+OH R3-1 0.45 0.74

HCOO+H→HCOOH R3-2 0.51 1.07

HCOOH+H→H2COOH R4-1 -0.02 1.55

H2COOH→H2CO+OH R5-1 0.39 0.42

HCO+H→CHOH R6-1 0.11 0.79

HCO+H→CH2O R6-2 0.02 0.66

CH2O+H→CH2OH R7-1 -0.13 1.20

CH2O+H→CH3O R7-2 -0.63 0.47

CH3O+H→CH3+OH R8-1 -0.08 1.40

CH3O+H→CH3OH R8-2 -0.33 0.75

CH3+H→CH4 R9-1 -0.99 0.50

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FigureS1.ParticlesizedistributionofCo5Ga3catalystmeasuredusingTEM.