ChineseJournalofCatalysis35(2014)1251–1259 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/chnjc
Review (Special Issue on Rare Earth Catalysis) TheuseofceriafortheselectivecatalyticreductionofNOxwithNH3
WenpoShana,b,FudongLiua,YunboYua,HongHea,*
ResearchCenterforEco‐EnvironmentalSciences,ChineseAcademyofSciences,Beijing100085,China
SchoolofEnvironmentalandBiologicalEngineering,NanjingUniversityofScienceandTechnology,Nanjing210094,Jiangsu,China a
b
A R T I C L E I N F O
A B S T R A C T
Articlehistory:
Received30March2014
Accepted8May2014
Published20August2014
Keywords:
Ceria
Dieselexhaust
Nitrogenoxidesabatement
Selectivecatalyticreduction
TheselectivecatalyticreductionofNOxwithNH3(NH3‐SCR)isoneofthewidelyusedNOxcontrol
strategiesforstationarysources(particularlyforpowerplants)andmobilesources(particularlyfor
diesel vehicles). The application is aimed at meeting the increasingly stringent standards for NOx
emissions. Recently, ceria has attracted much attention for its applications in NH3‐SCR catalysts
owingtoitsuniqueredox,oxygenstorage,andacid‐baseproperties.Inthisarticle,wecomprehen‐
sivelyreviewrecentstudiesonceriaforNH3‐SCRcatalystswhenusedassupport,promoter,orthe
mainactivecomponent.Inaddition,thegeneraldevelopmentofceriaforNH3‐SCRcatalystsisdis‐
cussed.
©2014,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.
PublishedbyElsevierB.V.Allrightsreserved.
1. Introduction
NOx, which mainly refers to NO and NO2, is considered a
major air pollutant owing to adverse effects on human health
andotherimpactsontheenvironment.Itcanleadtoacidrain
and photochemical smog and also contributes significantly to
theformationofhaze.Inhumans,itcancausedirectdamageto
the respiratory system. According to a recent estimate, NOx
emissions in China increased rapidly from 11.0 Mt in 1995 to
26.1 Mt in 2010. Power plants, industrial activities and trans‐
portationweremajorNOxsources.Basedoncurrentlegislation
and current implementation status, NOx emissions are esti‐
matedtoincreaseby36%by2030fromthe2010level.Failure
toimplementtheoperationoffluegasdenitrificationforpower
plants would be expected to increase NOx emissions dramati‐
cally in the next 5–10 years. Failure to control heavy diesel
vehicle emissions is expected to be associated with more ad‐
verseeffectsinthelongterm[1].
The reduction of NOx emissions has become one of the
greatestchallengesinenvironmentalprotection,especiallyfor
China. The selective catalytic reduction of NOx with NH3
(NH3‐SCR)isawidelyusedNOxcontrolstrategyforstationary
sources (particularly for power plants) and mobile sources
(particularlyfordieselvehicles).Ithasamajorroleinhelping
tomeettheincreasinglystringentstandardsforNOxemissions
[2]. There are some differences between the applications of
NH3‐SCRtostationarysourcesandtomobilesources.Forsafe‐
tyreasons,urea(inaqueoussolution)isapreferredreductant
rather than NH3 for mobile sources. In addition, the different
emission conditions of stationary sources and mobile sources
require the NH3‐SCR catalysts to work under different opera‐
tionalconditions,anddifferentspecificcatalyticpropertiesare
needed.For example,the catalyst for stationarysourcesis re‐
quiredtoresistsulfurpoisoningandminimizetheoxidationof
SO2 to SO3 owing to the relatively high SO2 concentrations in
fluegas.Thecatalystformobilesourcesneedstobeactiveina
*Correspondingauthor.Tel/Fax:+86‐10‐62849123;E‐mail:[email protected] ThisworkwassupportedbytheNationalBasicResearchProgramofChina(973Program,2010CB732304),theNationalHighTechnologyResearch
andDevelopmentProgramofChina(863Program,2013AA065301),theNationalNaturalScienceFoundationofChina(51308296,51108446),the
FundamentalResearchFundsfortheCentralUniversities(30920140111012),andtheQingLanProjectofJiangsuProvince.
DOI:10.1016/S1872‐2067(14)60155‐8|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.35,No.8,August2014 WenpoShanetal./ChineseJournalofCatalysis35(2014)1251–1259
widetemperaturerangeunderveryhighspacevelocitiesowing
tothevariationofengineoperatingconditionsandthelimited
spaceonboardfortherequiredreactorsystem. Vanadium‐based NH3‐SCR catalysts, V2O5‐WO3(or MoO3)/
TiO2, were developed for NOx abatement from stationary
sourcesandalsofounduseinthedieselvehiclemarket,owing
to their effectiveness for NH3‐SCR reaction and resistance to
SO2 poisoning. However, the toxicity of active vanadium spe‐
cies,togetherwiththelowstabilityandlargeN2Oformationat
high temperatures, has limited their use as catalysts in diesel
vehicles. Although the use of vanadium‐based NH3‐SCR cata‐
lysts is still permitted in China and some other developing
countriesatpresent,thesecatalystswillberemovedfromthe
market for mobile applications in the next few years when
stricterenvironmentalprotectiondemandsareintroduced[3].
Thishasledtogreateffortsbeingmadetodevelopsubstitute,
environmentallybenignNH3‐SCRcatalysts.
Manytypesofcatalysts,includingoxidesandzeolites,based
ontransitionmetalsand/orrareearthmetalshavebeenstud‐
iedfortheNH3‐SCRreaction[4].Severaltransitionmetalssuch
asFe, Mn,and Cuhavebeen usedinNH3‐SCRcatalysts, while
the investigation of rare earth metals for NH3‐SCR has mainly
focusedonCe.Cehasbeenwidelyusedasacrucialcomponent
inthree‐waycatalysts(TWCs)for(gasoline)automotiveemis‐
sion control. Owing to its unique redox, oxygen storage, and
acid‐baseproperties,ceriahasattractedmuchattentionforits
applications in NH3‐SCR catalysts as support, promoter, or
main active component [4,5]. In this review, we will focus on
the recent studies of ceria for NH3‐SCR catalysts. In addition,
thefuturedevelopmentsinusingceriainNH3‐SCRapplications
willbediscussed.
2. Ceriaasacatalystsupport
Pureceriaisnotsuitableforuseasa support forNH3‐SCR
catalysts owing to its high reduction temperature and loss of
surface area by sintering. When zirconium oxide was added
intoceria,theoxygenstoragecapacityandthethermalstability
of the oxide were significantly increased [6]. This led to
CeO2‐ZrO2beinginvestigatedasanNH3‐SCRcatalystsupportin
somedetailbyseveralresearchers.
Sixtransitionmetaloxides(WO3,MoO3,Mn2O3,CrO3,Fe2O3,
and Co2O3) were deposited on CeO2‐ZrO2 to investigate their
catalyticactivitiesandthermalstabilities(Fig.1).Amongthese
catalysts, WO3/CeO2‐ZrO2 showed the highest NOx conversion
levels and exhibited good high temperature stability [6]. An‐
otherstudyonthesamecatalystsystemshowedthattheaddi‐
tionofWO3ledtoasignificantincreaseinNH3storagecapacity
(acidity)notinitiallypresentintheCe‐Zrmixedoxidesupport,
and this was reflected in a strong enhancement of catalytic
activityintheNH3‐SCRreaction[7].
Nickel and sulfate were impregnated on CeO2‐ZrO2 to en‐
hance the activity and N2 selectivity for the NH3‐SCR reaction
by Si et al. [8]. Ni addition improved the Lewis acidity of
CeO2‐ZrO2andtherebyenhancedthelow‐temperatureactivity.
Incontrast,Brönstedacidsites,introducedbysulfatemodifica‐
tion,werelessoxidativethantheLewisacidsites.Thesesites
100
GHSV = 90 000 h1
NO conversion (%)
1252
80
MoO3/CeO2-ZrO2
WO3/CeO2-ZrO2
60
Mn2O3/CeO2-ZrO2
40
CrO3/CeO2-ZrO2
Fe2O3/CeO2-ZrO2
20
Co2O3/CeO2-ZrO2
0
200
250
300
350
400
450
500
Temperature (C)
Fig. 1. NO conversion as a function of temperature over various
MOx/CeO2‐ZrO2 mixed oxide catalysts [6]. Reaction conditions: 1 mL
catalyst,totalflowrate=1500mL/min,550ppmNO,550ppmNH3,6
vol%O2,10vol%CO2,10vol%H2O,N2balanceandGHSV=90000h−1.
ReproducedwithpermissionfromtheRSC.
facilitatedNH3adsorptioninsteadofNH3oxidationandthereby
enhancedhigh‐temperatureactivityandselectivity.Phosphates
were also impregnated on CeO2‐ZrO2 to improve its NH3‐SCR
catalyticperformance[9].Inaddition,CeO2‐ZrO2wasusedasa
support for Mn‐based catalysts for the low temperature
NH3‐SCR reaction and contributed significantly to catalytic
performance[10,11].
3. CeriaasanNH3‐SCRcatalystpromoter
Ceriumhasbeenwidelyusedasanadditivetoenhancethe
catalytic performance of various catalysts. For NH3‐SCR cata‐
lysts, Ce has also been shown to be an effective catalyst pro‐
moter.AdditionofCecouldexertapromotionaleffectontradi‐
tionalV‐basedcatalysts.Chenetal.[12]foundthatCeaddition
to V2O5‐WO3/TiO2 could enhance the adsorption and then ac‐
celerate the SCR reaction owing to a synergistic interaction
betweenCe,V, and W species (Fig. 2A). The added Ce species
existedmainlyintheformofCe3+oxideinthecatalyst,which
wasbeneficialfortheoxidationofNOtoNO2.Moreover,theCe
additive on V2O5‐WO3/TiO2 could provide stronger and more
active Brönsted acid sites, which were beneficial for the SCR
reaction.Ceria‐modifiedV2O5‐ZrO2/WO3‐TiO2catalystwasalso
evaluatedfortheNH3‐SCRofNOxindieselengines[13].Com‐
pared with the V2O5/WO3‐TiO2 catalysts having only Zr addi‐
tion,the co‐additionof Ce greatlyenhancedthelow‐ tempera‐
ture activity of the catalyst, but the material obviously deac‐
tivated with age. Characterization measurements suggest that
enrichmentofCe3+andenhancedredoxpropertiestakeplace.
In addition, the more active adsorbed nitrates on CeO2‐ modi‐
fiedcatalystsaidedtheNH3‐SCRreaction.Catalystdeactivation
was mainly owed to sintering and segregation of CeO2 on the
catalyst’ssurface,consistentwithapoorhydrothermalstability
of the Ce component. However, the additional NO2 will com‐
pensate forthe activitylossowingtohydrothermal agingand
significantly improve the low temperature SCR activity. This
suggests a high sensitivity of the Ce component towards NO2
WenpoShanetal./ChineseJournalofCatalysis35(2014)1251–1259
20
20
1
GHSV = 28 000 h
0
200
300
400
0
GHSV=332 000 h1
200
300
400
500
60
40
MnTi catalyst
40
80
Ce0.07MnTi catalyst
60
NO conversion (%)
40
V0.1W6Ti catalyst
60
1253
(C) 100
Fe(1.9%)-ZSM-5
80
NO conversion (%)
80
V0.1W6Ce10Ti catalyst
(B) 100
NOx conversion (%)
(A)100
Fe(1.6%)-Ce(2.5%)-ZSM-5
20
GHSV = 40 000 h1
0
80
120
160
Temperature (C)
Temperature (C)
Fig.2.EnhancementsofcatalyticperformanceowingtoadditionofCeto(A)V2O5‐WO3/TiO2[12],(B)Fe‐ZSM‐5[17],and(C)Mn/TiO2[19]catalysts.
Reactionconditions:(A)500mgsample,500ppmNO,500ppmNH3,3%O2,N2asbalancegas,GHSV=28000h−1;(B)20mgsample(0.025mL),2000
ppmNO,2000ppmNH3,3%O2,balanceHe,totalflowrate138.3mL/min,GHSV=332000h−1;(C)2mLsample,1000ppmNO,1000ppmNH3,3%
O2,3%water,balanceN2,GHSV=40000h−1.ReproducedwithpermissionfromtheACS,theRSC,andElsevier,respectively.
Temperature (C)
[13].Inanotherstudy,itwasfoundthattheadditionofceriato
an Sb‐V2O5/TiO2 catalyst could enhance the total acidity and
redoxpropertiesofthecatalyst,leadingtohigherNOxconver‐
sionsinawidetemperaturewindow[14].
Fe‐exchangedZSM‐5hasreceivedmuchattentionforappli‐
cations on diesel vehicles as an NH3‐SCR catalyst [15]. During
the development of Fe‐ZSM‐5 for NH3‐SCR by Long and Yang
[16],Cewasfoundtobeaneffectivepromoterforthecatalyst.
The addition of a small amount of Ce to Fe‐ZSM‐5 could not
onlyincreasetheactivitybutalsoplayastabilizationrole,en‐
hancing the catalyst’s SO2/H2O resistance and hydrothermal
stability.Thepoorlow‐temperatureactivityisamajorproblem
for Fe‐ZSM‐5 catalysts. Carja et al. [17] significantly improved
the low‐temperature catalytic performance of the Fe‐ZSM‐5
catalystbytheadditionofCe.Further,theydemonstratedthat
thejointactionofCeandFewithinthezeoliteframeworkgave
risetoahigh activitycatalyst (Fig.2B).In addition,theincor‐
poration of CeO2 into an Fe3+‐exchanged TiO2‐pillared clay
(Fe‐TiO2‐PILC) was also found to lead to an improvement in
catalytic activity.This was attributed to an increase intheac‐
tivity of NO oxidation to NO2 by O2 (NO2 being an important
intermediatefortheSCRreactiononthiscatalyst[18]).
Manganese oxides are the most active components for
NH3‐SCR at low temperatures. Therefore, Mn‐based oxide cat‐
alysts have been studied extensively for NOx abatement for
bothstationaryandmobilesources.However,thelowN2selec‐
tivity and pronounced SO2/H2O negative impact on perfor‐
mancearebigchallengesfortheapplicationofMn‐basedcata‐
lysts[20].Cehasbeen provedtobeaneffectivepromoter for
Mn oxide to improve its catalytic performance [21−24]. A
Mn‐Ce mixed oxide catalyst developed by Qi and Yang [21]
showed excellent low‐temperature NH3‐SCR activity together
withhighN2selectivityandgoodSO2/H2Oresistance.Thecat‐
alyticperformanceofMnOx‐CeO2couldbefurtherimprovedby
the addition of other metal elements such as Fe, Pr, and Nb
[22,23]. In addition, it was found that Ce addition could im‐
provethecatalyticactivityofMn/TiO2owingtotheincreasein
chemisorbedoxygen,improvedacidity,andanenhancementof
redoxproperties(Fig.2C)[19].Moreover,theresistanceofthe
Mn/TiO2catalysttoSO2couldbegreatlyenhancedbyCeaddi‐
tion. The improved behavior was associated with the preven‐
tionofformationofmetalsulfatesandtheinhibitingeffectsof
(NH4)2SO4andNH4HSO4deposition[25].
4. CeriaasthemainactivecomponentforNH3‐SCR catalysts
Labileoxygenvacanciesandbulkoxygenspecieswithrela‐
tively high mobility are easily formed on cerium oxide during
theredoxshiftbetweenCe3+andCe4+underoxidizingandre‐
ducing conditions, respectively. Therefore, Ce‐based oxide
could be used effectively as a main active component for
NH3‐SCRcatalysts.
Pure CeO2 oxides usually possess poor NH3‐SCR activity
(Fig.3A)[26,28].However,CeO2‐zeolites,obtainedbythecom‐
bination of CeO2 with zeolites (BEA, ZSM‐5, MOR, and FER)
usingsimplephysicalmixingmethods,couldachieveexcellent
NOx conversions at very high space velocities, owing to the
synergeticeffectbetweentheacidicsitesofzeoliteandtheox‐
idation component present (Fig. 3B) [27]. The catalytic per‐
formance of CeO2 could also be greatly improved by surface
sulfation. This process could result in an enrichment of Ce3+
(leadingtoanincreaseinactiveoxygencontent)andcouldalso
leadtostrongacidsites(favoringNH3chemisorptionandacti‐
vation)ontheCeO2surface(Fig.3A)[25].Yangetal.[29]pro‐
posedanoveleffectofsulfationontheSCRreactionoverCeO2.
In this system the adsorption of NH3 on CeO2 was promoted,
enhancing the Eley‐Rideal mechanism. The sites for –NH2 ad‐
sorptionandtheoxidizingagentsfor–NH2oxidizationonCeO2
wereseparatedafterthesulfation,resultinginaninhibitionof
thecatalyticoxidationof–NH2toNO.Asaresult,theSCRactiv‐
ityofCeO2obviouslyincreasedaftersulfation.
Ce‐basedcompositeoxidecatalystsaremoreattractivethan
single oxide catalysts because other metal elements can pro‐
motethecatalyticpropertiesofCeO2.Inourpreviousstudy,Xu
etal.[30]developedapromisingCeO2/TiO2catalyst,prepared
1254
WenpoShanetal./ChineseJournalofCatalysis35(2014)1251–1259
() 100
( )
100
NOx conversion (%)
NO conversion (%)
Sulfated CeO2
80
GHSV = 60 000 h1
60
40
20
0
Fresh CeO2
200
250
300
350
400
450
60
GHSV = 500 000 h1
40
75% CeO2-H-BEA
75% CeO2-H-ZSM-5
75% CeO2-H-MOR
75% CeO2-H-FER
20
0
500
Temperature (C)
80
300
400
500
Temperature (C)
600
Fig.3.ImprovedcatalyticperformanceofCeO2bysurfacesulfation(A)[26]andcombinationwithzeolites(B)[27].Reactionconditions:(A)[NH3]=
[NO]=1000ppm,[O2]=3vol%,N2balance,GHSV=60000h−1;(B)1000ppmNO,1000ppmNH3,10vol%O2,9vol%H2O,balanceN2,GHSV=
500000h−1.ReproducedwithpermissionfromElsevierandtheRSC,respectively.
byanimpregnationmethod,andthisshowedhighSCRactivity
and N2selectivityat275−400C. Acomparativestudyinvolv‐
ing three preparation methods for CeO2/TiO2 catalysts was
reported by Gao et al. [31], and the results indicated that the
catalystpreparedbyasinglestepsol‐gelmethodhadthehigh‐
estSCRactivityandSO2resistance.Highsurfaceareaandgood
redoxpropertiesareimportantforcatalyticactivity,whilethe
stronginteractionbetweenCeandTiandahighconcentration
ofamorphous,orhighlydispersednanocrystalline,ceriacould
explaintheexcellentperformanceofthecatalyst.Usingvarious
methods, Li et al. [32] confirmed that the active site of a
CeO2/TiO2 catalyst was the Ce‐O‐Ti short‐range ordered spe‐
cieswiththeinteractionbetweenCeandTibeingattheatomic
level. To improve the resistance to alkali metal poisoning, a
titanate nanotube in which CeO2 was confined was designed
and synthesized by Chen et al [33]. To enhance the catalytic
activity and the SO2 resistance of CeO2/TiO2, Liu et al. [34]
supported CeO2 on TiO2‐SiO2, while Chen et al. [35,36] co‐im‐
pregnated CeO2 and WO3 onto TiO2. Furthermore, Peng et al.
[37]improvedthelow‐temperatureactivityofCeO2‐WO3/TiO2
by SiO2 addition. Other transition metals such as Mo [38], Fe
[39],Zr[40],andNb[41]werealsoinvestigatedasmodifying
agents.
Recently,manystudieshavefocusedontheCe‐basedmixed
oxide catalysts for the NH3‐SCR reaction. Chen et al. [42] pre‐
paredaCeO2‐WO3catalystusingacoprecipitationmethod.The
catalyst exhibited high activity, high N2 selectivity, and good
SO2durabilityinabroadtemperaturerangeof175–500Cata
spacevelocityof47000h−1.GeandMnwereusedbyChanget
al.[43]forfurtherimprovingtheCeO2‐WO3catalyst.Liuetal.
[44]comparedCeO2‐WO3catalystspreparedbyvariousmeth‐
odsandconcludedthatthehighNH3‐SCRactivitycouldbeat‐
tributedtolargesurfacearea,highsurfaceconcentrationsofCe
andCe3+,enhancedNOoxidizationability,andhighconcentra‐
tion of surface acid sites. Based on an in situ IR and Raman
spectroscopic study, Peng et al. [45] suggested an NH3‐SCR
reactionmechanismof CeO2‐WO3consistingoftwoindepend‐
entcycles.Thesewere denotedas aredoxcycle,owing to the
excellent oxygen storage capability and reducibility of cubic
fluorite‐structured CeO2 (for NH3 activation), and an acid site
cycle. The latter resulted from Brönsted acid sites formed on
theW‐O‐WspeciesofCe2(WO4)3(forNH3adsorption).
In addition, Liu et al. [46] developed a superior Cu‐Ce‐Ti
oxidecatalystwithdualredoxcycles(Cu2++Ce3+Cu++Ce4+,
Cu2++Ti3+Cu++Ti4+)anddemonstratedthatthedualredox
cycles playkey rolesinthe catalyticbehavior. Pengetal.[47]
preparedaMoO3‐CeO2catalystandextensivelyinvestigatedits
structure‐activityrelationshipfortheNH3‐SCRreaction.Caiet
al. [48] synthesized three‐dimensional ordered macroporous
(3DOM)Ce0.75Zr0.2M0.05O2‐δ(M=Fe,Cu,Mn,Co)usingacolloi‐
dal crystal template method for NH3‐SCR. A novel nio‐
bia‐ceria‐based catalyst was reported by Casapu et al. [49] to
beusefulforNOxabatementaswellasthecatalyticregenera‐
tion of diesel particulate filters (DPF) in diesel engines. The
catalyst is successful because of its multi‐functionality in
NH3‐SCR, the hydrolysis of urea to NH3, and the oxidation of
soot. The relationship between structure and performance of
theniobia‐ceriacatalystforNH3‐SCRwasexaminedbyQuetal.
[50]. Because there have been many reports on Ce‐based ox‐
idesforbothNH3‐SCRandsootoxidation,thefurtherdevelop‐
mentofmulti‐functionalCe‐basedoxidecatalystsmeritsmore
attention.
5. High‐efficiencyCe‐basedNH3‐SCRcatalysts
There are limitations to the catalyst volume that can be
placedondieselvehicles,whichrequiresthatthecatalystpos‐
sessessuperiorNH3‐SCRperformanceunderhighspaceveloc‐
ity conditions. However, the above mentioned Ce‐based oxide
catalystshavebeentestedatrelativelylowGHSVs(<150000
h−1).BecausethereductionofNH3‐SCRcatalystvolumeisone
ofthemainchallengesfordieselvehicleapplications,itisvery
importanttodevelophighlyefficientNH3‐SCRcatalystscapable
ofoperatingsuccessfullyathighspacevelocities[2].
WenpoShanetal./ChineseJournalofCatalysis35(2014)1251–1259
In our previous study, a Ce‐Ti oxide catalyst was prepared
byafacilehomogeneousprecipitationmethod[51].Compared
withtheCe‐Tioxidecatalystspreviouslyreported,thiscatalyst
showeda remarkablyimprovedlow‐temperature SCR activity
and, in turn, a significantly wider reaction temperature win‐
dow.Furtheroptimizationofthepreparationmethodresulted
in significantly enhanced high‐temperature activity and an
evenfurtherbroadenedtemperaturewindow[52].Inaddition,
the SCR activity at high space velocity conditions was also
clearlyimproved.
A superior Ce‐W‐Ti oxide catalyst was prepared by doping
WintotheCe‐Tioxidecatalyst[53].TheCe‐W‐Tioxidecatalyst
showed both enhanced low‐temperature activity and high‐
temperatureactivitysimultaneously,combinedwithenhanced
N2selectivity,comparedwiththeundopedCe‐Tioxidecatalyst.
The effects of W species in the Ce‐W‐Ti oxide catalyst were
investigated,andtheresultsshowedthattheintroductionofW
species increased the concentration of surface oxygen vacan‐
cies and enhanced the redox properties of the catalyst. The
latter attribute can benefit the low‐temperature activity by
facilitatingthe“fastSCR”reaction.TheintroductionofWspe‐
cies could also simultaneously increase the amount of surface
Brönsted and Lewis acid sites, which, in turn, enhances both
thehigh‐temperatureactivityandtheN2selectivitybyinhibit‐
ingtheunselectiveoxidationofNH3athightemperatures.
Investigations on the Ce‐W‐Ti oxide catalyst showed that
the roleofTispecies,suchas acidity promotion,could beful‐
filledbyWspecies.Therefore,anovelCe‐Woxidecatalystwith
aCe/Wmolarratioof1:1wasdevelopedfortheNH3‐SCRreac‐
tion [54]. The Ce‐W oxide catalyst showed much higher SCR
activity than the previous Ce‐Ti and Ce‐W‐Ti oxide catalysts
(Fig. 4). Further, the catalyst exhibited a near 100% NOx con‐
version over a wide temperature range from 250 to 425 C
underanextremelyhighGHSVof500000h−1.TheCe‐Woxide
catalyst also exhibited excellent N2 selectivity, good stability,
and high resistance to poisoning. Under the same test condi‐
tions, the Ce‐W oxide catalyst showed much better SCR per‐
formance than V2O5‐WO3/TiO2 and Fe‐ZSM‐5 catalysts, which
1255
have been industrially and commercially used for NOx abate‐
mentfromdieselengineexhausts. 6. Perspectives
Cerium is relatively cheap and accounts for a large part of
therareearthelementmarket.Withtheincreaseintheindus‐
trialapplicationofheavyrareearthelements,coproducedlight
rare earthelements, such as Ce,appearsto be surplusto cur‐
rent demands, especially in China [55]. Therefore, the devel‐
opment of new applications for Ce is urgently needed. The
pursuit of NH3‐SCR applications of Ce, especially in the devel‐
opment of Ce‐based NH3‐SCR catalysts, is a very promising
undertaking. Despite much progress, there remain some problems and
challenges for the use of Ce‐based NH3‐SCR catalysts. For sta‐
tionary applications, Ce has been shown to be an effective
promoter forV2O5‐WO3/TiO2catalysts[12].However,the cat‐
alysts with Ce as a main active component are inferior to
V‐based catalysts regarding SO2 resistance [56]. For mobile
applications,CeisagoodpromoterforFe‐ZSM‐5andenhances
its catalytic activity, hydrothermal stability, and SO2/H2O re‐
sistance[16,17].Incontrast,thethermalstabilitiesoftheoxide
catalystswithCeO2‐ZrO2asthesupportorCeasthemainactive
componentaregenerallylowerthanthoseofzeolitecatalysts.
This is especially true of the recently developed Cu‐based
small‐pore zeolite catalysts [13,48]. In addition, although the
combinationofCeO2andWO3hasbeenshowntobeveryeffec‐
tive for the NH3‐SCR reaction and can form the basis for high
efficiencycatalysts,thehighcostofWO3impliesthatthereisa
needforreducingoreliminatingWO3insuchsystems.
References
[1] ZhaoB,WangSX,LiuH,XuJY,FuK,KlimontZ,HaoJM,HeKB,
CofalaJ,AmannM.AtmosChemPhys,2013,13:9869
[2] GrangerP,ParvulescuVI.ChemRev,2011,111:3155
[3] LiuFD,ShanWP,PanDW,LiTY,HeH.ChinJCatal,2014,doi:
10.1016/S1872‐ 2067(14)60048‐6
[4] LiuFD,ShanWP,ShiXY,ZhangCB,HeH.ChinJCatal,2011,32:
NOx conversion (%)
100
1113
[5] HeH,LiuFD,YuYB,ShanWP.SciSinChim,2012,42:446
[6] LiY,ChengH,LiDY,QinYS,XieYM,WangSD.ChemCommun,
80
GHSV = 500 000 h1
2008:1470
[7] CanF,BerlandS,RoyerS,CourtoisX,DuprezD.ACSCatal,2013,3:
60
1120
40
[8] SiZC,WengD,WuXD,YangJ,WangB.CatalCommun,2010,11:
Ce-Ti oxide catalyst
Ce-W-Ti oxide catalyst
Ce-W oxide catalyst
20
1045
[9] SiZC,WengD,WuXD,RanR,MaZR.CatalCommun,2012,17:
146
[10] KoJH,ParkSH,JeonJK,KimSS,KimSC,KimJM,ChangD,ParkY
0
K.CatalToday,2012,185:290
200
250
300
350
400
450
Temperature (C)
Fig.4.NOxconversionasafunctionoftemperatureoverCe‐Ti,Ce‐W‐Ti,
and Ce‐W oxide catalysts. Reaction conditions: 0.06 mL catalyst, total
flow rate = 500 mL/min, 500 ppm NO, 500 ppm NH3, 5 vol% O2, N2
balance,GHSV=500000h−1.
[11] ShenBX,WangYY,WangFM,LiuT.ChemEngJ,2014,236:171
[12] ChenL,LiJH,GeMF.JPhysChemC,2009,113:21177
[13] WangXQ,ShiAJ,DuanYF,WangJ,ShenMQ.CatalSciTechnol,
2012,2:1386
[14] LeeKJ,KumarPA,MaqboolMS,RaoKN,SongKH,HaHP.Appl
CatalB,2013,142‐143:705
1256
WenpoShanetal./ChineseJournalofCatalysis35(2014)1251–1259
GraphicalAbstract
Chin.J.Catal.,2014,35:1251–1259 doi:10.1016/S1872‐2067(14)60155‐8
TheuseofceriafortheselectivecatalyticreductionofNOxwithNH3
N
ResearchCenterforEco‐EnvironmentalSciences,ChineseAcademyof Sciences;NanjingUniversityofScienceandTechnology [16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
N
H
H
N
H
H
O
O
Ce
Thisreviewpresentstheprogressinresearchontheuseofceriafortheselective
catalyticreductionofNOxwithNH3.
2008,50:492
LongRQ,YangRT.JAmChemSoc,1999,121:5595
CarjaG,DelahayG,SignorileC,CoqB.ChemCommun,2004:1404
LongRQ,YangRT.ApplCatalB,2000,27:87
WuZB,JinRB,LiuY,WangHQ.CatalCommun,2008,9:2217
LiJH,ChangHZ,MaL,HaoJM,YangRT.CatalToday,2011,175:
147
QiGS,YangRT.ChemCommun,2003:848
QiGS,YangRT,ChangR.ApplCatalB,2004,51:93
CasapuM,KröcherO,ElsenerM.ApplCatalB,2009,88:413
LiL,WangLS,PanSW,WeiZL,HuangBC.ChinJCatal,2013,34:
1087
WuZB,JinRB,WangHQ,LiuY.CatalCommun,2009,10:935
GuTT,LiuY,WengXL,WangHQ,WuZB.CatalCommun,2010,
12:310
Krishna K, Seijger G B F, van den Bleek C M, Calis H P A. Chem
Commun,2002:2030
ZhangL,PierceJ,LeungVL,WangD,EplingWS.JPhysChemC,
2013,117:8282
YangSJ,GuoYF,ChangHZ,MaL,PengY,QuZ,YanNQ,WangCZ,
LiJH.ApplCatalB,2013,136‐137:19
XuWQ,YuYB,ZhangCB,HeH.CatalCommun,2008,9:1453
GaoX,JiangY,FuYC,ZhongY,LuoZY,CenKF.CatalCommun,
2010,11:465
LiP,XinY,LiQ,WangZP,ZhangZL,ZhengLR.EnvironSciTech‐
nol,2012,46:9600
ChenXB,WangHQ,WuZB,LiuY,WengXL.JPhysChemC,2011,
115:17479
LiuCX,ChenL,LiJH,MaL,ArandiyanH,DuY,XuJY,HaoJM.En‐
vironSciTechnol,2012,46:6182
ChenL,LiJH,GeMF,ZhuRH.CatalToday,2010,153:77
ChenL,LiJH,GeMF.EnvironSciTechnol,2010,44:9590
PengY, LiuCX, Zhang XY,LiJH. ApplCatalB,2013,140‐141:
276
N
O
[15] BrandenbergerS,KröcherO,TisslerA,AlthoffR.CatalRev‐SciEng,
H
O
WenpoShan,FudongLiu,YunboYu,HongHe*
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
Ce
Ce
Ce
Ce
Ce
LiuZM,ZhangSX,LiJH,MaLL.ApplCatalB,2014,144:90
ShuY,SunH,QuanX,ChenS.JPhysChemC,2012,116:25319
ShenYS,MaYF,ZhuSM.CatalSciTechnol,2012,2:589
MaZ,WengD,WuXD,SiZC,WangB.CatalCommun,2012,27:97
ChenL,LiJH,AblikimW,WangJ,ChangHZ,MaL,XuJY,GeMF,
ArandiyanH.CatalLett,2011,141:1859
ChangHZ,LiJH,YuanJ,ChenL,DaiY,ArandiyanH,XuJH,HaoJ
M.CatalToday,2013,201:139
LiuCX,ChenL,ChangHZ,MaL,PengY,ArandiyanH,LiJH.Catal
Commun,2013,40:145
PengY,LiKZ,LiJH.ApplCatalB,2013,140‐141:483
LiuZM,YiY,LiJH,WooSI,WangBY,CaoXZ,LiZX.ChemCom‐
mun,2013,49:7726
PengY,QuRY,ZhangXY,LiJH.ChemCommun,2013,49:6215
CaiSX,ZhangDS,ZhangL,HuangL,LiHR,GaoRH,ShiLY,Zhang
JP.CatalSciTechnol,2014,4:93
CasapuM,BernhardA,PeitzD,MehringM,ElsenerM,KröcherO.
ApplCatalB,2011,103:79
QuRY,GaoX,CenKF,LiJH.ApplCatalB,2013,142‐143:290
ShanWP,LiuFD,HeH,ShiXY,ZhangCB.ChemCatChem,2011,3:
1286
ShanWP,LiuFD,HeH,ShiXY,ZhangCB.CatalToday,2012,184:
160
Shan W P, Liu F D, He H, Shi X Y, Zhang C B. Appl Catal B, 2012,
115‐116:100
ShanWP,LiuFD,HeH,ShiXY,ZhangCB.ChemCommun,2011,
47:8046
ZhanWC,GuoY,GongXQ,GuoYL,WangYQ,LuGZ.SciSinChim,
2012,42:433
XuWQ,HeH,YuYB.JPhysChemC,2009,113:4426
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