1

Coprecipitation:Anexcellenttoolforthesynthesisofsupported

metalcatalysts‐Fromtheunderstandingofthewellknown

recipestonewmaterials

M.Behrens1,2*

1Fritz‐Haber‐InstitutderMax‐Planck‐Gesellschaft.DepartmentofInorganicChemistry.

Faradayweg4‐6,14195Berlin,Germany

2UniversityofDuisburg‐Essen,FacultyofChemistryandCenterforNanointegration

Duisburg‐Essen(CENIDE),Universitätsstr.7,45141Essen,Germany

(*)malte.behrens@uni‐due.de

Keywords:Co‐precipitation,precursorchemistry,supportedmetalnanoparticles,Cu/ZnO

Abstract:

Constant‐pHco‐precipitationisastandardsynthesistechniqueforcatalystprecursors.

Thegeneralstepsofthissynthesisroutearedescribedinthisworkusingthe

successfullyappliedindustrialsynthesisoftheCu/ZnO/(Al2O3)catalystformethanol

synthesisasanexample.Therein,co‐precipitationleadstowell‐definedandcrystalline

precursorcompoundwithamixedcationiclatticethatcontainsallmetalspeciesofthe

finalcatalyst.Theanionsarethermallydecomposedtogivethemixedoxidesandthe

noblestcomponent,inthiscurrentcasecopper,finallysegregatesonanano‐metriclevel

toyieldsupportedanduniformmetalnanoparticles.Recentexamplesoftheapplication

ofthissynthesisconceptforsupportedcatalystsarereportedwithanemphasisonthe

layereddoublehydroxideprecursor(Cu,Zn,Al;Ni,Mg,Al;Pd,Mg,Al;Pd,Mg,Ga).This

precursormaterialisveryversatileandcanleadtohighlyloadedbasemetalaswellas

tomono‐andbi‐metallichighlydispersednoblemetalcatalysts.

Keywords:Catalystsynthesis,Co‐precipitation,Cu/ZnO,Layereddoublehydroxide

2

1.Introduction:

Constant‐pHco‐precipitationisastandardsynthesistechniqueforcatalystprecursors

[1]andreferredtoasthemethodoflowsupersaturation[2].Byproperadjustmentof

theprecipitationparametersthehomogeneousdistributionofdifferentmetalcationsin

amixedsolutioncanbecarriedovertoamultinarycatalystprecursorbyrapid

solidification.Contrarilytothealsocommonlyusedimpregnationmethodsforpre‐

formedsupports,thesematerialscontaintheprecursorspeciesforthesupportandfor

theactivecomponentsinthesamematerial[3].Highlydispersed,wellinter‐mixedand

uniformsupportedmetal/oxidecatalystscanbeobtainedfromsuchprecursorsby

decomposition(typicallycalcination)and/orreduction.However,thechemistrybehind

themanysynthesisstepsandtheirinterplayarecomplexandinvolved.Forapplied

systems,theempiricalsynthesisoptimizationisoftenfarmoreadvancedthantheexact

knowledgeoftheunder‐lyingchemistry.Thus,theevolutionofmanyappliedcatalyst

synthesesisusuallyacontinuouslong‐termprocessthattoalargeextentisbasedonthe

experienceofthemanufacturerandtheiraccumulatedknowledgeoftenleadsto

complexrecipes.Thiscomplexityissometimesgeneralizedasthe“blackmagic”of

catalystsynthesis.

Inthissituation,theretrospectiveinvestigationofaknownindustriallyapplied

synthesisprocedureisanidealstartingpointforreconstructingthechemistryof

catalystsynthesis.Onecanbesurethatthecriticaldetailsofallunitoperationshave

beentestedforrelevanceandaredirectlyimperativeforthebestfinalresult:ahighly

active,selectiveandstablematerialsuitableforindustrialapplication.Thegoalofsuch

studiesthusmustbetoupgradeempiricalsynthesisparameter‐functionrelationshipsto

synthesisparameter‐structure‐functionrelationshipsthatallowsforanunderstanding

andhopefullyamorerationalfurtheroptimizationofthecatalyst.

Aprominentexampleistheco‐precipitationofmixedCu,Zn,Alhydroxycarbonatesas

precursormaterialforCu/ZnO/Al2O3catalystswhichareemployedfortheindustrial

synthesisofmethanol[4‐6].Herein,therecentprogressthathasbeenmadeinanalyzing

andunderstandingthewell‐documentedindustrialsynthesisofthisimportantcatalyst

isbrieflyreviewed.Foramoredetailedtreatment,thereaderisreferredtorecent

reviews[7‐9].Thekeystepsofthesynthesisrecipeofthisparticularcatalystandthe

chemicalconceptbehindthe“blackmagic”areidentified,generalizedandtransferredto

othercatalystsystems.Herein,itisshownhowthelessonslearnedfromtheindustrial

3

recipecanbeappliedtosynthesizenewmaterialsthroughco‐precipitationofdifferent

precursorcompoundinvariouscatalystsystems.Thisworkreviewsourrecentresults

obtainedonCu/ZnO‐basedcatalysts[10‐13]aswellassupportedPd[14],intermetallic

Pd2Ga[15‐17]andNi[18]catalystswithafocusonco‐precipitatedlayereddouble

hydroxide(LDH)precursors.

2.Results&Discussion:

2.1.Synthesisoftechnicalmethanolsynthesiscatalysts

Cu/ZnO‐basedcatalystsareindustriallyemployedinmethanolsynthesisfromsyngas.

TheroleofCudefectsanddisorder[19‐20]andofthe“synergetic”roleofZnO[21‐22],

whichexceedsthefunctionofamerephysicalstabilizerarevividlydebatedsincemany

years,butbeyondthescopeofthepresentpaper.Thesynthesisof(Al2O3‐promoted)

Cu/ZnOcatalystisanalyzedhereassumingthatthecoppernanoparticlesmustfulfill

threerequirementsforhighcatalyticperformance[23]:1.)exposealargecopper

surfacearea(SACu),2.)containsurfacedefectsand,3.)exhibitmanyreactive

(“synergetic”)interfacestoZnO.Theserequirementsareelegantlyrealizedbythenano‐

structuredandporousCu/ZnOarrangementshownintheTEMimageinFigure1that

resultsfromtheindustrialsynthesisrecipe.TheAl2O3promoterisusuallyaddedinlow

amountstoincreasethestabilityofthecatalyst,notasaclassicalaluminasupport.

Preparationofthisunique,butfragilemicrostructurerequiresahomogeneousand

maximizedintermixingoftheCuandZnspeciesinordertogenerateandstabilizethe

alternatingarrangementofsmallCuandZnOnanoparticles.Thus,themaingoalofthis

catalystsynthesisistocarryoverandmaintaintoamaximalextenttheperfectly

homogeneouscationdistributioninthestartingmixedsolutiontothefinalcatalyst.

4

Figure1:SchematicrepresentationofthemajorstepsoftheICIrecipeforCu/ZnO

catalystsynthesis.Favorableconditionsthatleadtohigh‐performancecatalystsare

indicatedintheFigure(takenfrom[7]).

Incaseofthemethanolsynthesiscatalyst,amultistepsynthesisroutetowardsCu/ZnO

catalystsintroducedbyICIinthe1960sachievesthisinaveryefficientmanner[4].Itis

schematicallyshowninFigure1anditcomprisesco‐precipitation[24]andageing[25‐

26]ofamixedCu,Zn,(Al)hydroxy‐carbonateprecursormaterial[27],thermal

decompositionyieldinganintimatemixtureoftheoxidesandfinallyactivationofthe

catalystbyreductionoftheCucomponent[28].Themajortrickofthissynthesisisthat

underbeneficialconditionstheco‐precipitatedprecursormaterialcanbesynthesizedas

asinglehomogeneousandwell‐definedprecursorphase,zincianmalachite

(Cu,Zn)2(OH)2CO3,withamixedcationicsub‐latticethatcontainsallcomponentsofthe

catalystandcanevenaccommodatesmallamountsoftheAl3+promoter[29]inasolid

solution.ThisperfectdistributionleadstoaneffectivedilutionoftheactiveCu

componentandisthebasisforasuccessfulnano‐structuringoftheprecursorupon

decompositionintoCuO/ZnOandforthehighCu/ZnO‐interdispersioninthefinal

reducedcatalyst.ThisconceptisschematicallyshowninFigure2.Consideringthe

synthesisofamaximallysubstitutedandsinglephasezincianmalachiteprecursor

materialastheprimarygoaloftheearlysynthesissteps,manydetailsofthesynthesis

recipeandparameterslikeCu:Znratio,pHandtemperatureduringprecipitationand

ageingcanindeedbeunderstood[7,24,26].

Inturn,theseconsiderationsprovideageneralguideforthesynthesisofcatalystsin

othersystemsorthroughotherprecursorphase:Toobtainuniformandhighly

5

interdispersedmaterialsoneshouldseeksynthesisparametersthatfavorthe

crystallizationofamixedcationiclatticeofallcomponentsofthecatalystwiththermo‐

labileanionslikehydroxide,carbonate,formate,oxalate,formate,acetate,etc.Thereby,

thesubstitutionshouldbesuchthatamaximalmixingofthecomponentscanbe

achievedwhilesegregationofotherphasesduringprecursorsynthesisisavoided.

Fig.2:Schematicrepresentationandelectronmicroscopyimagesofthemicrostructural

evolutionofanindustrialCu/ZnOcatalystsynthesizedbytherecipeshowninFigure1.

Theco‐precipitate(a)crystallizesyieldingzincianmalachiteneedles(b).During

calcinationtheindividualneedlesdecomposeintonanostructuredCuO/ZnO(c).Finally,

theCuOcomponentisreducedinhydrogenyieldingtheactivestateofthecatalystwith

auniquemicrostructureexhibitinghighporosityandhighCudispersion(takenfrom

[7])

2.2.AlternativeprecursorsfornewCu/ZnO‐basedcatalysts

Thisconceptthatthesolidstatechemistryoftheprecursordeterminesthesuccessof

thecatalystsynthesiscanbeusedtoexploreothermixedCu,Znprecursorcompounds

thanzincianmalachite.Forexample,theprinciplesshowninFigure2havebeen

transferredtoamixedCu,Znbasicformatesystem,(Cu1‐xZnx)2(OH)3HCO2[10].After

determiningtheproperco‐precipitationconditionsbymeansoftitrationexperiments,a

seriesofsolidsolutionswithvaryingxwaspreparedandstructurallycharacterized.The

systemshowedinterestingparallelswiththezincianmalachiteroute.Similartothe

6

malachitecase,ananisotropicchangeoftheunitcellparameterswasobservedasa

functionofZncontentthatallowedfindingacriticalcompositionatx=0.21beyond

whichsegregationofby‐phaseswasdetected.Needle‐likeparticleshavebeenobtained

asshowninFigure3a.Thesuccessfulnano‐structuringofthishomogeneousprecursor

compounduponthermaldecompositionandthegenerationofporescanbeseenin

Figure3bshowingtheCuO/ZnOpre‐catalystcalcinedinoxygenat200°C.Theabsolute

performanceofthisnoveltypeofCu/ZnOcatalystswascomparabletoabinaryzincian

malachitederivedbenchmarkofthesamecomposition,whilethepromotedindustrial

catalystsstillperformedsignificantlybetter[10].

Fig.3:Scanningelectronmicrographsofthe(Cu1‐xZnx)2(OH)3HCO2needles(Cu:Zn=

78:22)before(a)andafter(b)calcinationinO2at200°C(takenfrom[7])

Anotherexample,well‐knownincatalystsynthesisscience,arelayereddouble

hydroxide(LDH)materials,MIIxMIII1‐x(OH)2(CO3)x/2∙mH2O.Thesematerialscrystallize

inalayeredstructurederivedfromthemineralbrucite,Mg(OH)2.Themetalcationsare

coordinatedbysixOHgroupsforminglayersofedge‐sharingoctahedra.IncaseofLDH,

sometrivalentcationsarealsopresentwithintheselayersandtheresultingpositive

excesschargeiscompensatedbyinter‐layeranions,typicallycarbonate(Fig.4a).LDHs

offerawidesubstitutionchemistryontheMIIandMIIIpositionsincludingCu,ZnandAl.

Allthreemetalspeciessharethesixfoldcoordinatedsitesinalayeredstructure,which

iscomposedofedge‐sharingoctahedra.Thus,theyareevenlydistributedonanatomic

levelwithinasinglephase.Hence,formationofcatalystsofahomogeneous

microstructurewithhighdispersionofthemetalspeciesandenhancedmetal‐oxide

interactioncanbeexpectedafterreduction.TheapplicationofLDHprecursorsfor

catalysishasbeencomprehensivelyreviewed[2]andmanyexamplescanbefoundin

theliterature.

7

SynthesisofCu,Zn,AlLDHinsteadofthezincianmalachiteprecursorrequires

adjustmentoftheco‐precipitationconditions[11]:ahigherAlcontentof30‐40%to

obtainphasepureLDHmaterialcomparedtotypicallylessthan15%,theuseofa

mixtureofNaOHandNa2CO3asprecipitatingagentinsteadofpureNa2CO3toavoid

formationofcarbonate‐richerphaseslikemalachite,anincreaseoftheprecipitationpH

fromca.7to8tofavorformationofthehydroxide‐richLDHphaseandalowerreaction

temperaturetoavoidoxolationofCuhydroxidespeciestoCuOatthishigherpH.

Cu,Zn,AlLDHwereobtainedwithaCucontentupto50mol%[11].Atthiscomposition

theZn:Alratiowascloseto1:2andtheoxidematrixtendedtoformZnAl2O4[13].The

resultingcatalystshowedadifferentmicrostructurethanthezincianmalachite‐derived

industrialCu/ZnO/(Al2O3)catalyst[11].DuetothelayeredstructureLDHprecursors

exhibitaplatelet‐likemorphology.ThelateralsizeoftheCu,Zn,AlLDHplateletsranged

frommorethan100downtoafewtensofnanometers,whiletheplateletthicknesswas

betweenapproximately20andlessthan5nm(Fig.4b).EDXmappingconfirmedthe

homogenouselementdistributionintheLDHmaterial.Theplatelet‐likemorphology

wasmaintainedinthefinalcatalystuponthermaltreatment.Comparedtoex‐zincian

malachitecatalysts,asmalleraverageCuparticlesizewasobservedintheseex‐LDH

platelets,whichisaresultofthelowertotalCucontent.However,theaccessibleCu

surfaceareawasconsiderablylower,aroundonly10m2g‐1comparedto20or30m2g‐1

determinedfortypicalindustrialCu/Zn/Al2O3catalystsusingtheN2Ochemisorption

method.Thisisaresultofthemuchstrongerembedmentofthesmallmetalparticlesin

theZnAl2O4matrix.Themajorchallengeinthepreparationofsuchex‐LDHCu/ZnAl2O4

catalystisthustooptimizethis“nuts‐in‐chocolate”‐likemorphologybyadjustingthe

LDHparticlesize,e.g.theprecursorplateletthickness,toaffectthedegreeof

embedmentandincreaseporosityinordertofindthebestcompromisebetweenCu

metal‐oxideinteractionsandexposedSACu.AstheLDH‐derivedcatalystsexhibithigher

thermalstability,increaseincalcinationtemperaturecanbeeffectiveinthisway[30].

Alsoamicroemulsionapproachfortheprecipitationoftheprecursorto“nano‐cast”the

plateletmorphologyoftheLDHphasewasshowntoincreaseSACuby75%comparedto

aconventionallyco‐precipitatedex‐LDHCu/ZnAl2O4catalyst(Cu:Zn:Al=50:17:33)[12]

(Tab.1).ItisinterestingtonotethatdespitethelowertotalSACu,theactivityperunit

areawasfoundtobehigherinthesesystemscomparedtomanyconventionalcatalysts.

8

a)

b)

Figure4:IdealizedgeneralrepresentationoftheLDHcrystalstructure(a).Cross‐section

TEMofanaggregateofCu,Zn,AlLDHprecursorplatelets(b).Themicroscopysample

waspreparedbyembeddingthepowderinepoxyfollowedbymechanicalandAr‐ion

beamthinning.Thecross‐sectionofmanyplateletsappearsasneedles.TheEDX

Cu

Al

Zn

9

mappingshowstheuniformelementdistributionintheLDHprecursor(adoptedfrom

[11]).

2.3.SupportedNicatalysts

Thepeculiarmicrostructureofthereducedex‐LDHcatalystswithhighlyembedded

metalnanoparticlesisbeneficialnotonlyforenhancedmetal‐oxideinteraction,butalso

foranimprovedthermalstabilityofthecatalystagainstsintering.Thisfeaturewas

expoitedinthesynthesisofhighlystableNinanoparticlesfortheapplicationinthedry

reformingofmethaneathightemperatures.Forthisreaction,hightemperaturesare

favorable,becausethereactionishighlyendothermicandanincreaseintemperature

willincreasetheproductyield.Also,theoperationathightemperature

thermodynamicallysuppressestheexothermicBoudouardside‐reaction,whichistoa

largeextentresponsibleforundesiredcarbonfilamentgrowth[31‐32].Asimilar

syntheticapproachtostabilizeNinanoparticlesathightemperaturesagainstsintering

byincorporationintoastableoxidematrixwaspreviouslyalsoappliedforNi‐containing

perovskites[33]andspinels[34].AlsoLDHprecursorshavebeenalreadyappliedto

synthesizeNi‐basedcatalystsfordryreformingofmethane[35‐36].

Inourwork,LDHsofthenominalcompositionNixMg0.67‐xAl0.33(OH)2(CO3)0.17∙mH2O,

with0≤x≤0.5weresynthesizedbypH‐controlledco‐precipitationaccordingtothe

recipeintroducedabovefortheCu,Zn,Alsystem[18].ThehighestNicontentof50mol%

(metalbase)correspondstoa55wt.‐%Niloadingintheresultingcatalyst.Asdescribed

aboveforCu/ZnAl2O4,the1:2ratioofMgandAlisexpectedtoleadtospinelformation,

MgAl2O4‐asintering‐stableceramiccompound.Inaddition,beneficialeffectsonthe

cokingbehaviorofNicatalystshavebeenreportedonalumina,magnesiaandspinel

supports[37].

Thecharacterizationresultsshow,similartotheCu,Zn,Alcasedescribedabove,thata

phasepureLDHprecursorwiththetypicalplatelet‐likemorphologywasobtained.Upon

calcination,theLDHstructureiscompletelydecomposedat600°Cyieldingapoorly

crystallineoxidepre‐catalyst.Afterthisrelativelymildcalcination,noindicationfor

segregationofindividualspecieswasfoundandtheplateletshapeoftheparticleswas

maintained.Thus,thecalcinationproductisanamorphous,mixedNi,Mg,Aloxide,whose

10

homogenousdistributionofthemetalspecieshasbeenlargelyconservedduring

decompositionoftheLDHprecursor(Fig.5a).

Afterreductionofthecalcinedmaterialinhydrogenat800°C,electronmicroscopy

revealedthattheplatelet‐likemorphologyoftheLDHprecursorisstillpresent

indicatingastrongresistivityofthematerialagainstreconstructionsathigh

temperatures(Fig5b).Inaddition,brightdotscanbeobservedintheSEMmicrographs

homogeneouslydistributedovertheplateletsconfirmingthatananoscopicsegregation

ofthecomponentshastakenplace.

Figure5:SEMimagesoftheprecursormaterial(a)andthecatalystafterreductionat

800°C(b)andTEMmicrographsofthefreshNi/MgAl2O4catalystwith50mol.%Ni(x=

0.5;55wt.%)afterreductionat800(c)and1000°C(d,adoptedfrom[18]).

Indeed,XRDofthecatalystswithx=0.5clearlyconfirmedthepresenceofmetallicNi.

Theoxidiccomponentisstillonlypoorlycrystallineandnosharppeaksofspinelwere

detected.TEManalysisofindividualplateletsinthereducedmaterialrevealedan

averageparticlesizeofNiaround10nm,despitethehighreductiontemperatureand

thehighoverallloadingof55wt.%Ni(Fig.5c).TheNisurfaceareawasdeterminedby

hydrogenpulsechemisorptionandfoundtobe22m2gcat‐1ataBETsurfaceareaof226

m2g‐1afterreduction(Tab.1).Interestingly,atthesehighreductiontemperatures,there

wasnocleareffectoftheNiloadingontheNiparticlesizeandapproximately10nm

werealsoobservedforx=0.05.Evenanincreaseofthereductiontemperatureto900°C

didnotsignificantlyinfluencetheNiparticlesizeprovingthehighthermalstabilityof

a) b)

c) d)

11

thiscompositematerial.Onlyafterreductionat1000°C,asubstantialparticlegrowthto

19nmwasobserved(Fig.5d,Tab.1).Duetothishighlystablemicrostructure,whose

originistheprecursor‐inducedembedmentoftheNiparticlesintheceramicmatrix,the

catalystcanbeemployedatareactiontemperatureof900°Canditshoweda

remarkablestabilityover100hours[18].Itwasshownrecentlythatthecatalystloses

itsbeneficialmicrostructureinheritedfromtheLDHprecursoruponrepeatedTPR‐O

cyclesleadingtoanincreaseinNiparticlesizeto21nmafter21cycles[38].These

experimentsshowthegenerallimitoftheprecursorapproaches.Thecompositesystem

isonlykineticallytrappedinafavorable,butlabilemicrostructurewithfinitestability,

whichwasinheritedbythestructuralandmorphologicalpropertiesoftheprecursor.As

thesystemrelaxesathighertemperaturesoruponlongoperationtime,theseproperties

orthe“chemicalmemory”ofthecatalystmightalsoslowlyvanish.

2.4.Supportednoblemetalcatalysts:Palladium

ThesamesynthesisapproachviaLDHprecursorscanalsobeusedfornoblemetalslike

Pd.ItisnotedthatPd2+isnoteasilyincorporatedintheLDHlayersduetoitslargerionic

sizecomparedtoMg2+,whichexceedstheempiricallimitofapproximately0.80Åforthe

incorporationintoLDH[39].Furthermore,Pd2+inaqueoussolutionsexhibitsatendency

towardsfour‐foldsquare‐planarcoordinationinsteadofanoctahedraloneasrequired

inLDH.ThereforeonlysmallamountsofPd2+canbeincorporatedintheLDHprecursor

andasecondbivalentcationasdiluent,likeMg2+,isneededtoachievecrystallizationof

allPdionsinasingleLDHphase.Astypicalloadingsofnoblemetalcatalystsareanyway

lowercomparedtobasemetalsforcostreasons,thelimitedsolubilityofPd2+inLDHis

usuallynotagreatproblem.

Pd,Mg,AlLDHprecursorsweresynthesizedunderslightlyadjustedconditionswitha

Pd:Mg:Alatomicratioofx:0.7‐x:0.3(0.001≤x≤0.025)[14].Theprecursorwasreduced

in5%H2inargonat500°Cwithoutpriorcalcination.Theresultingseriesof

Pd/MgO/MgAl2O4catalystshasbeencharacterizedandwasfoundtocontainPd

nanoparticles(Fig.6),whosesizecanbecontrolledtosomeextentbetween<1.9and

3.5nmbyadjustingthePdcontentduringsynthesis(Fig.7,Tab.1).Thesamplewiththe

lowestPdloadingof0.33wt‐%(0.1mol.%,x=0.001)showedamuchgreaterPd

dispersioncomparedtothosesampleswithPdcontentsbetween1.5and8wt.%(0.5–

12

2.5mol.%,0.005≤x≤0.025)makingthisseriesofcatalystsasuitablematerialsbasisfor

studyingparticlesizeeffectsinhydrocarbonactivation[40].

a) b)

Fig.6:SEMofthetypicalLDHmorphologyofthePd,Mg,Alprecursor(a)andTEMofthe

catalystobtainedafterreductionwithsmallPdnanoparticles(darkdots)formed

throughouttheoxideplatelets(adoptedfrom[14]).

Fig.7:DependencyofthePdnanoparticlesizedistributionontheloadingofPd2+inthe

LDHprecursorafterreductionat550°C(A:0.5%;B:1.0%;C:1.5%;D:2.5%,givenas

mol%ofmetalcationsubsititonintheLDH,takenfrom[14]).

TheseriesofLDH‐derivedPdcatalystswasstudiedinmethanechemisorption.The

adsorptioncapacityofthesecatalystswasgenerallyveryhighcomparedtoPdsamples

preparedbyothermethods[14].Inparticular,anextraordinaryhighcapacitywas

observedforthesamplewiththelowestloadingof0.33wt.%(0.1mol.%,x=0.001).Its

13

particlesdisplayedasignificantlyhigherintrinsicmethaneadsorptioncapacitythan

othermembersoftheseries,whoseintrinsiccapacitieswerecomparable(Fig.8).This

observationindicatestheexistenceofasizeeffectinPdnanoparticlesfortheadsorption

ofmethaneandconfirmsthestructure‐sensitivtyofthisreaction.Theex‐LDHsamples

haveshownthatacriticalPdclustersizebelow1.9nmisrequiredfortheenhanced

adsorptiontotakeplace.

Fig.8:Pdmass‐normalizedmethanechemisorptioncapacityat200and400°Coftheex‐

LDHPdcatalystsasafunctionofPdloading.Thefoursampleswithhighloading

correspondtothecatalystsshowninFigure7,thePdparticlesofthelowestloaded

catalystsweretoosmalltobecharacterizedbyTEM(takenfrom[14])

2.5.Supportedintermetalliccatalysts:Pd2Ga

Orderedintermetalliccompounds(IMCs)arediscussedasinterestingcatalyticmaterials

formanyreactions[41].Forexample,intheIMCPd2Gawasinvestigatedinselective

hydrogenationofacetylene[42].Forcatalyticapplications,suchIMCsshouldbepresent

informofnanoparticlessupportedonahighsurfaceareamaterial[43].

AfeasiblesynthesisroutetosupportedPd2Gananoparticlesistheabove‐described

approachthroughaco‐precipitatedPd,Mg,GaLDHprecursortoevenlydistributethe

constituentelementsoftheintermetalliccompoundaswellasofthesupportinasingle

precursorphase[15].SimilartothemonometallicPdcatalystswithAlinsteadofGa

14

introducedintheprevioussection,Pdnanoparticlesareformedintheinitialreduction

stepfromsuchaprecursorunderreducingconditions.Uponfurtherincreaseofthe

reductiontemperature,partialreductionofthegalliumspeciesbyspilloverofatomic

hydrogenfromthemetallicPdsurface[44]setsinleadingtotheformationoftheIMC

[45],whileunreducedcomponentsoftheprecursorconstitutetheoxidesupport.Like

thissupportedanddispersedIMCsaregeneratedbyreactionbetweennoblemetaland

thereducibleoxidesupportinafeasibleandreproduciblemanner.Thehomogeneous

distributionandthelowamountofPdintheLDHprecursorhelptheformationofvery

smallandreactivePd0particles[14].ThesinglephaseLDHprecursorisexpectedto

favortheformationofuniformandnano‐sizedPd2GaIMCbecauseitprovidesa

homogenousmicrostructureconcerningPdparticlesizeandPdmetal‐oxide

interactions.

APd,Mg,GaLDHprecursorwithx=0.025(molarratio2.5:67.5:30)wassynthesizedby

controlledco‐precipitationatpH=8.5and55°Cbyco‐feedingappropriateamountsof

mixedaqueousmetalnitrateandsodiumcarbonatesolutionasprecipitatingagent.The

precursorwasreducedin5%H2inargonat550°CtoobtainthesupportedPd2Ga

intermetalliccompound.ThereductionprocessoftheGa‐specieswasmonitoredbyin‐

situPdK‐edgeXANES(Fig.9)showingtheformationofmono‐metallicPdatlowand

IMCformationathigherreductiontemperature.HighresolutionTEMofthereduced

catalystsalsoshowsthatthePd2Gaphasewassuccessfullyformedwithintheoxidic

plateletsandthattheaverageparticlesizewasbelow5nm(Fig.10,Tab.1).

Fig.9:IMCformationfromPd,Mg,GaLDHprecursorsuponreductionmonitoredbyin‐

siteuXANES(takenfrom[17])

15

Fig.10:TEMandHRTEMimagesofthereducedPd,Mg,GaLDHprecursorshowingthe

plateletmorphology(a),thepresenceofmetalnanoparticles(b)andevidenceforthe

formationofthePd2Gaphase(c,d,takenfrom[17])

3.Conclusion

RecentexamplesofsynthesisofsupportedcatalystsderivedfromLDHprecursorsand

theirapplicationshavebeendescribed.Anoverviewandselectedpropertiesare

summarizedinTable1.Thisprecursormaterialisveryversatileandcanleadtohighly

loadedbasemetalaswellastomono‐andbi‐metallichighlydispersednoblemetal

catalystswithanincreasedthermalstability.Thegeneralstepsofthissynthesisroute

areanalogoustothosesuccessfullyappliedintheindustrialsynthesisofmethanol

synthesiscatalysts.Co‐precipitationleadstowell‐definedandcrystallineprecursor

compoundswithamixedcationiclatticethatcontainsallcomponentsofthefinal

catalyst.Theanionsarethermallydecomposedtogivethemixedoxidesandthenoblest

componentsfinallysegregateonanano‐metricleveltoyielduniform

metal/intermetallicnanoparticles.

16

Tab.1:OverviewontheLDH‐derivedcatalystsreviewedinthisarticlewithselected

propertiesandthereactionsystemsapplied.

Cations Composition

/mol.%

TReduction

/°C

SAMetal/

m2gcat‐1,or

dispersiona

Metalparticle

sizeb/nm

Reactionc,

remarks

reference

Cu,Zn,Al 50/17/33 300 8.3 7.7 MSR[12],MS

[46]

13.8 7.8 MSR,micro‐

emulsion

[12]

Ni,Mg,Al 50/17/33 800 22 10.4 DRM

[18,47]900 19 8.9

1000 6 19.4

5/62/33 1000 3 9.3

Pd,Mg,Al 0.1/69.9/30 550 67% n.d. CH4chem.

[14]0.5/69.5/30 17% 1.9

1.0/69.0/30 25% 2.2

1.5/68.5/30 14% 3.5

2.5/67.5/30 16% 3.3

Pd,Mg,Ga 2.5/67.5/30 n.d. 4.8(Pd2Ga) Sel.Hydr.

[15,17]adeterminedusingN2O(Cu),H2(Ni)orCO(Pd)asprobemolecules.

bdeterminedbyTEM.

cMSR=methanolsteamreforming,MS=methanolsynthesis,DRM=dryreformingofmethanol,CH4chem.=

methanechemisorption,Sel.Hydr.=selectivehydrogenationofacetylene.

Acknowledgements

ThethoroughexperimentalworkofmygroupmembersattheFritz‐Haber‐Instituteis

greatlyacknowledged.IalsothankRobertSchlöglforhiscontinuoussupportandfor

fruitfuldiscussionsduringmytimeinBerlin.

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References

1. Schüth,F.,M.Hesse,andK.K.Unger,PrecipitationandCoprecipitation,inHandbookofHeterogeneousCatalysis.2008,Wiley‐VCHVerlagGmbH&Co.KGaA.

2. Cavani,F.,F.Trifirò,andA.Vaccari,Hydrotalcite‐typeanionicclays:Preparation,propertiesandapplications.CatalysisToday,1991.11(2):p.173‐301.

3. SynthesisofSolidCatalysts,ed.K.P.d.Jong.2009:Wiley‐VCHVerlagGmbH&Co.KGaA.I‐XX.

4. Waller,D.,etal.,Copper‐zincoxidecatalysts.Activityinrelationtoprecursorstructureandmorphology.FaradayDiscussionsoftheChemicalSociety,1989.87:p.107‐120.

5. Baltes,C.,S.Vukojevic,andF.Schuth,Correlationsbetweensynthesis,precursor,andcatalyststructureandactivityofalargesetofCuO/ZnO/Al2O3catalystsformethanolsynthesis.JournalofCatalysis,2008.258(2):p.334‐344.

6. Li,J.L.andT.Inui,Characterizationofprecursorsofmethanolsynthesiscatalysts,copperzincaluminumoxides,precipitatedatdifferentpHsandtemperatures.AppliedCatalysisa‐General,1996.137(1):p.105‐117.

7. Behrens,M.andR.Schlögl,HowtoPrepareaGoodCu/ZnOCatalystortheRoleofSolidStateChemistryfortheSynthesisofNanostructuredCatalysts.ZeitschriftfüranorganischeundallgemeineChemie,2013.639(15):p.2683‐2695.

8. Behrens,M.,MethanolSteamReforming,inCatalysisforalternativeenergygeneration(eds:L.Guczi,A.Erdohelyi),M.Armbruster,Editor.2012,Springer.

9. Behrens,M.andE.Kunkes,MethanolChemistry,inChemicalEnergyStorage,R.Schlögl,Editor.2012,WalterdeGruyter:Berlin.p.413‐442.

10. Behrens,M.,etal.,Knowledge‐baseddevelopmentofanitrate‐freesynthesisrouteforCu/ZnOmethanolsynthesiscatalystsviaformateprecursors.ChemicalCommunications,2011.47(6):p.1701‐1703.

11. Behrens,M.,etal.,Phase‐PureCu,Zn,AlHydrotalcite‐likeMaterialsasPrecursorsforCopperrichCu/ZnO/Al2O3Catalysts.ChemistryofMaterials,2009.22(2):p.386‐397.

12. Kuhl,S.,etal.,Cu,Zn,Allayereddoublehydroxidesasprecursorsforcoppercatalystsinmethanolsteamreforming‐pH‐controlledsynthesisbymicroemulsiontechnique.JournalofMaterialsChemistry,2012.22(19):p.9632‐9638.

13. Kühl,S.,etal.,Cu‐BasedCatalystResultingfromaCu,Zn,AlHydrotalcite‐LikeCompound:AMicrostructural,Thermoanalytical,andIn SituXASStudy.Chemistry–AEuropeanJournal,2014.20(13):p.3782‐3792.

14. Ota,A.,etal.,Particlesizeeffectinmethaneactivationoversupportedpalladiumnanoparticles.AppliedCatalysisA:General,2013.452(0):p.203‐213.

15. Ota,A.,etal.,IntermetallicCompoundPd2GaasaSelectiveCatalystfortheSemi‐HydrogenationofAcetylene:FromModeltoHighPerformanceSystems†.TheJournalofPhysicalChemistryC,2010.115(4):p.1368‐1374.

16. Ota,A.,etal.,Comparativestudyofhydrotalcite‐derivedsupportedPd2GaandPdZnintermetallicnanoparticlesasmethanolsynthesisandmethanolsteamreformingcatalysts.JournalofCatalysis,2012.293(0):p.27‐38.

17. Ota,A.,etal.,DynamicSurfaceProcessesofNanostructuredPd2GaCatalystsDerivedfromHydrotalcite‐LikePrecursors.ACSCatalysis,2014:p.2048‐2059.

18. Mette,K.,etal.,StablePerformanceofNiCatalystsintheDryReformingofMethaneatHighTemperaturesfortheEfficientConversionofCO2intoSyngas.ChemCatChem,2014.6(1):p.100‐104.

18

19. Behrens,M.,etal.,TheActiveSiteofMethanolSynthesisoverCu/ZnO/Al2O3IndustrialCatalysts.Science,2012.336(6083):p.893‐897.

20. Gunter,M.M.,etal.,ImplicationofthemicrostructureofbinaryCu/ZnOcatalystsfortheircatalyticactivityinmethanolsynthesis.CatalysisLetters,2001.71(1‐2):p.37‐44.

21. Fujitani,T.,etal.,ThekineticsandmechanismofmethanolsynthesisbyhydrogenationofCO2overaZn‐depositedCu(111)surface.SurfaceScience,1997.383(2–3):p.285‐298.

22. Nakamura,J.,Y.Choi,andT.Fujitani,OntheissueoftheactivesiteandtheroleofZnOinCu/ZnOmethanolsynthesiscatalysts.TopicsinCatalysis,2003.22(3‐4):p.277‐285.

23. Zander,S.,etal.,TheRoleoftheOxideComponentintheDevelopmentofCopperCompositeCatalystsforMethanolSynthesis.AngewandteChemieInternationalEdition,2013.52(25):p.6536‐6540.

24. Behrens,M.,etal.,Understandingthecomplexityofacatalystsynthesis:Co‐precipitationofmixedCu,Zn,AlhydroxycarbonateprecursorsforCu/ZnO/Al(2)O(3)catalystsinvestigatedbytitrationexperiments.AppliedCatalysisa‐General,2011.392(1‐2):p.93‐102.

25. Kniep,B.L.,etal.,Rationaldesignofnanostructuredcopper‐zincoxidecatalystsforthesteamreformingofmethanol.AngewandteChemie‐InternationalEdition,2004.43(1):p.112‐115.

26. Zander,S.,B.Seidlhofer,andM.Behrens,InsituEDXRDstudyofthechemistryofagingofco‐precipitatedmixedCu,Znhydroxycarbonates‐consequencesforthepreparationofCu/ZnOcatalysts.DaltonTransactions,2012.41(43):p.13413‐13422.

27. Behrens,M.andF.Girgsdies,StructuralEffectsofCu/ZnSubstitutionintheMalachite–RosasiteSystemZeitschriftfüranorganischeundallgemeineChemie,2010.636(6):p.919‐927.

28. Behrens,M.,Meso‐andnano‐structuringofindustrialCu/ZnO/(Al(2)O(3))catalysts.JournalofCatalysis,2009.267(1):p.24‐29.

29. Behrens,M.,etal.,PerformanceImprovementofNanocatalystsbyPromoter‐InducedDefectsintheSupportMaterial:MethanolSynthesisoverCu/ZnO:Al.JournaloftheAmericanChemicalSociety,2013.135(16):p.6061‐6068.

30. Tang,Y.,etal.,High‐performanceHTLcs‐derivedCuZnAlcatalystsforhydrogenproductionviamethanolsteamreforming.AIChEJournal,2009.55(5):p.1217‐1228.

31. Swaan,H.M.,etal.,Deactivationofsupportednickelcatalystsduringthereformingofmethanebycarbondioxide.CatalysisToday,1994.21(2–3):p.571‐578.

32. Tsipouriari,V.A.,etal.,ReformingofmethanewithcarbondioxidetosynthesisgasoversupportedRhcatalysts.CatalysisToday,1994.21(2–3):p.579‐587.

33. Choudhary,V.R.,B.S.Uphade,andA.A.Belhekar,OxidativeConversionofMethanetoSyngasoverLaNiO3PerovskitewithorwithoutSimultaneousSteamandCO2ReformingReactions:InfluenceofPartialSubstitutionofLaandNi.JournalofCatalysis,1996.163(2):p.312‐318.

34. Guo,J.,etal.,Dryreformingofmethaneovernickelcatalystssupportedonmagnesiumaluminatespinels.AppliedCatalysisA:General,2004.273(1–2):p.75‐82.

35. Tsyganok,A.I.,etal.,Dryreformingofmethaneovercatalystsderivedfromnickel‐containingMg–Allayereddoublehydroxides.JournalofCatalysis,2003.213(2):p.191‐203.

19

36. Perez‐Lopez,O.W.,etal.,EffectofcompositionandthermalpretreatmentonpropertiesofNi–Mg–AlcatalystsforCO2reformingofmethane.AppliedCatalysisA:General,2006.303(2):p.234‐244.

37. Chen,Y.‐g.andJ.Ren,Conversionofmethaneandcarbondioxideintosynthesisgasoveralumina‐supportednickelcatalysts.EffectofNi‐Al2O3interactions.CatalysisLetters,1994.29(1‐2):p.39‐48.

38. Mette,K.,etal.,RedoxDynamicsofNiCatalystsinCO2ReformingofMethane.CatalysisToday,2014.submitted.

39. DeRoy,A.,LamellarDoubleHydroxides.MolecularCrystalsandLiquidCrystalsScienceandTechnology.SectionA.MolecularCrystalsandLiquidCrystals,1998.311:p.173‐193.

40. Klier,K.,J.S.Hess,andR.G.Herman,Structuresensitivityofmethanedissociationonpalladiumsinglecrystalsurfaces.TheJournalofChemicalPhysics,1997.107(10):p.4033‐4043.

41. Studt,F.,etal.,DiscoveryofaNi‐Gacatalystforcarbondioxidereductiontomethanol.NatChem,2014.6(4):p.320‐324.

42. Armbrüster,M.,etal.,Pd−GaIntermetallicCompoundsasHighlySelectiveSemihydrogenationCatalysts.JournaloftheAmericanChemicalSociety,2010.132(42):p.14745‐14747.

43. He,Y.,etal.,PartialhydrogenationofacetyleneusinghighlystabledispersedbimetallicPd–Ga/MgO–Al2O3catalyst.JournalofCatalysis,2014.309(0):p.166‐173.

44. Collins,S.E.,etal.,Gallium‐HydrogenBondFormationonGalliumandGallium‐PalladiumSilica‐SupportedCatalysts.JournalofCatalysis,2002.211(1):p.252‐264.

45. Penner,S.,etal.,Pd/Ga2O3methanolsteamreformingcatalysts:PartI.Morphology,compositionandstructuralaspects.AppliedCatalysisA:General,2009.358(2):p.193‐202.

46. Kühl,S.,etal.,TernaryandquaternaryCrorGa‐containgex‐LDHcatalysts‐influenceoftheadditionaloxidesontothemicrostructureandactivityofCu/ZnAl2O4catalysts.CatalysisToday,2014.submitted.

47. Düdder,H.,etal.,RoleofcarbonaceousdepositsintheactivityandstabilityofNi‐basedcatalystsappliedinthedryreformingofmethane.Catal.Sci.Technol.,2014.accepted.