Rapidsynthesisofmesoporousceria–zirconiasolidsolutions

viaanovelsalt-assistedcombustionprocess

WeifanChena,*,FengshengLia,JiyiYua,

LeiliLiub,HailianGaoa

aNationalSpecialSuperfinePowderEngineeringResearchCenter,NanjingUniversityofScienceandTechnology,Xiaolingwei200,Nanjing,Jiangsu210094,PRChinabCollegeofChemicalEngineering,ShandongInstituteofLightIndustry,Jinan250100,PRChina

Received21January2006;receivedinrevisedform3April2006;accepted17April2006Availableonline5May2006

Abstract

Mesoporousceria–zirconiasolidsolutionswerepreparedbyanovelsalt-assistedcombustionprocessusingethyleneglycolasafuelandnitrateasanoxidant.Theeffectsofvariousoperatingconditionssuchasthefuel-to-oxidantratioandthenatureandamountofaddedsaltonthecharacteristicsoftheproductswereinvestigatedbyX-raydiffraction(XRD)andnitrogenadsorptionanalysis.Resultsfromtransmissionelectronmicroscopy(TEM)andhigh-resolutiontransmissionelectronmicroscopy(HRTEM)showedthattheintroductionofleachableinertinorganicsaltasahardagglomerationinhibitorintotheredoxmixtureprecursorledtothebreakupoffractalnanocrystalliteagglomeratesandthemesoporousstructureformedbythelooseagglomerationofmonodispersednanoparticles,whichwasalsoconfirmedbysmall-angleXRDandnitrogenadsorptionanalysis.Thepresenceofsaltwasfoundtoresultinamorethan10-foldincreaseinthespecificsurfaceareaoftheproductsfrom17.34to208.17m2/gatagivenmolarratioofethyleneglycol–nitrate.Amechanismschemewasproposedtoillustratethepossibleformationprocessesanddiscusstheroleofthesaltadditives.#2006ElsevierLtd.Allrightsreserved.

Keywords:A.Oxides;B.Chemicalsynthesis;D.Microstructure

1.Introduction

Ceriahasbeenwidelyusedasactivecomponentinso-calledthree-waycatalysts(TWCs)forautomotiveexhaust

treatment[1].OneofthemostimportantrolesofCeO2inthethree-waycatalystistoprovidesurfaceactivesitesandto

actasanoxygenstorage/transportmediumbyshiftingbetweenCe3+andCe4+underreductiveandoxidizing

conditions,respectively.Ithasbeenwelldocumentedthattheintroductionofzirconiaintocerianotonlyimprovesthe

oxygenstoragecapacity(OSC)andthermalresistance,butalsoenhancescatalyticactivityatlowertemperaturesby

theformationofceria–zirconiasolidsolutions[2].Itisimportantincatalysistofabricatehighsurfaceareaand

mesoporousmaterials,whichwouldofferalargenumberofactivesitesforcarryingoutcatalyticreactions[3].Hence,

muchefforthasbeenfocusedonthesynthesisofhighsurfaceareaandmesoporousceria–zirconiasolidsolutions,

includingmicroemulsionmethod[4],surfactant-assistedapproach[5],directsonochemicalroute[6],coprecipitation

www.elsevier.com/locate/matresbuMaterialsResearchBulletin41(2006)2318–2324

*Correspondingauthor.Tel.:+862584315529;fax:+862584315942.E-mailaddress:weifan-chen@163.com(W.Chen).

0025-5408/$–seefrontmatter#2006ElsevierLtd.Allrightsreserved.doi:10.1016/j.materresbull.2006.04.024followedbysupercriticaldrying[7],macromoleculesurfacemodifiedmethod[8],continuoushydrothermalsynthesis

inanear-criticalwaterflowreactor[9].

Amongtheavailablechemicalprocesses,self-sustainingsolutioncombustionsynthesisischaracterizedby

convenientprocessing,simpleexperimentalsetup,timeandenergysavingandhomogeneousproducts[10].Therefore,

therearemanyreportsaboutcombustionsynthesisofnanosizedceria–zirconiasolidsolutioninrecentyears.Aruna

andPatil[11]synthesizedceria–zirconiasolidsolutionwithspecificsurfaceareaintherangeof36–120m2/gviaa

solutioncombustionprocessusingoxalyldihydrazineandcarbohydrazineasthefuel,whichareexpensiveand

carcinogenic.FuandLin[12]preparedCexZr1ÀxO2powderswithspecificsurfaceareaintherangeof40–50m2/gbya

microwave-inducedcombustionprocessusingglycineasthefuel.Lascaleaetal.[13]increasedthespecificsurface

areaofthecombustion-derivedCe–Zrmixedoxideto93m2/gbyapH-controllednitrate–glycinegel-combustion

process.Potdaretal.[14]preparednanosizedCe0.75Zr0.25O2porouspowderswithaverywideporesizedistributionin

therangeof2–250nmandsurfaceareaofabout40.0m2/gviaaglycine-nitrateautocombustionprocess.The

synthesizedpowdershavehighersurfaceareasthanthepowderspreparedbyconventionalsolid-statemethods[10],

however,thesemethodsarenottoosatisfyingwithrespecttohighsurfaceareaandfeasibilityofcommercial

production.Hence,combustionsynthesisofhighsurfaceareaceria–zirconiasolidsolutionsemployingsafetyand

cheapfuelhasbeenstillachallengingissue.

Here,wereport,forthefirsttime,thesynthesisofhighsurfaceareamesoporousceria–zirconiasolidsolutionbythe

simpleintroductionofsaltinthecombustionreactionusingethyleneglycolasanovelfuelandnitrateasanoxidant.The

resultsrevealthattheintroductionofsaltintotheredoxmixturesolutionresultsinadrasticincreaseinsurfaceareaand

providesanewmeanstocontrolthepropertiesoftheproductsfortheconventionalcombustionsynthesis(CCS).

2.Experimental

Allchemicalsandreagentsusedwerehigh-puritycommerciallyavailableCe(NO3)3Á6H2O(99.5%),

Zr(NO3)4Á5H2O(99.5%),ethyleneglycol(99%),NaCl(99.9%),KCl(99.9%),andLiCl(99.9%),andwereused

asreceived.AccordingtothestoichiometricratiosofCe0.75Zr0.25O2,therequiredethyleneglycol–nitratemolarratios

andthedesiredamountofsaltaddictives,theproperamountofceriumnitrate,zirconiumnitrate,ethyleneglycoland

saltaddictivesweredissolvedinaminimumvolumeofdeionizedwaterinaquartzbeakertoobtaintransparent

solution,theresultingsolutionwasevaporatedonahotplateadjustedat1108C.Atthisstage,theviscousliquids

swelled,followedbytheevolutionofbrowngases,andself-propagatingsolutioncombustionslowlyoccurred,

yieldinglooselightyellowpowder.Toremovesalt,theas-burnedpowderwasboiledindeionizedwater,filteredand

washedwithdeionizedwaterandethanol.Finally,theproductwasdriedinanovenat808Cfor2h.

Wide-angleandsmall-angleX-raypowderdiffraction(XRD)patternswereobtainedusingaBrukerAdvanceX-ray

D8diffractometer,withCuKaradiation.Thecrystallitesizeswerecalculatedfromlinebroadeningofthe(111)XRD

peakbySherrer’sformula:D=0.89l/bcosu(scanningatarateof0.58/min).Transmissionelectronmicrographs

(TEMs)andhigh-resolutiontransmissionelectronmicrographs(HRTEMs)were,respectively,takenwithaJEOL

TEM-200CXmicroscopeandaJEOLTEM-2010.Thespecificsurfaceareaandporesizedistributionofthepowders

weremeasuredwithaCOULTERSA3100analyzerusingthemultipointBrunauer,Emmett,andTeller(BET)

adsorptiontechnique.Theparticlesizeswereestimatedfromtheformula:DBET=6/rSBET.

3.Resultsanddiscussion

Ithasbeenwelldocumentedthattheconventionalsolutioncombustionsynthesismethodoftenproduces

nanocrystalliteagglomerateswhicharenanometerincrystallitesizecalculatedbytheScherrer’sformula[10–15].As

thenanocrystallitesareagglomerated/sinteredtogetherandvirtuallyinseparable,theobtainedproductsdonotshow

highspecificsurfaceareascorrespondingtotheirsmallcrystallitesizes.Recently,wefoundthattheintroductionofa

solublesaltintoredoxmixturesolutionbreaksupfractalnanocrystalliteagglomeratesintonanoparticlesandprevent

nanoparticlesfromforminghardagglomerationduringthecombustionprocess.Wedenotethemethodassalt-assisted

combustionsynthesis(SCS)incomparisontotheconventionalcombustionsynthesis,i.e.salt-freecombustion

synthesis.

Accordingtotheprincipleofpropellantchemistry[16],forstoichiometricredoxreactionbetweenafuelandan

oxidizer,theratioofthenetoxidizingvalencyofthemetalnitratetothenetreducingvalencyofthefuelshouldbeW.Chenetal./MaterialsResearchBulletin41(2006)2318–23242319