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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