The Design of MnOx Based Catalyst in Post-Plasma Catalysis Configuration for Toluene Abatement

32 Pages·2017·4.68 MB·English

catalysts
Review
The Design of MnO Based Catalyst in Post-Plasma
x
Catalysis Configuration for Toluene Abatement
ZhipingYe1,2,Jean-MarcGiraudon1,* ID,NathalieDeGeyter2,RinoMorent2and
Jean-FrançoisLamonier1 ID
1 Univ.Lille,CNRS,CentraleLille,ENSCL,Univ.Artois,UMR8181—UCCS—UnitédeCatalyseetChimie
duSolide,F-59000Lille,France;[email protected](Z.Y.);[email protected](J.-F.L.)
2 ResearchUnitPlasmaTechnology,DepartmentofAppliedPhysics,FacultyofEngineeringandArchitecture,
GhentUniversity,Sint-Pietersnieuwstraat41,9000Ghent,Belgium;[email protected](N.D.G.);
[email protected](R.M.)
* Correspondence:[email protected];Tel.:[phone]
Received:21December2017;Accepted:11February2018;Published:23February2018
Abstract: Thisreviewprovidesanoverviewofourpresentstateofknowledgeusingmanganese
oxide(MnO )-basedcatalystsfortolueneabatementinPPC(Postplasma-catalysis)configuration.
x
Thecontextofthisstudyisconciselysum-up. Afterbrieflyscreeningthemaindepollutionmethods,
theprinciplesofPPCareexposedbasedonthecouplingoftwomaturetechnologiessuchasNTP
(Nonthermalplasma)andcatalysis. Inthatrespect,thepresentationoftheabundantmanganese
oxides will be firstly given. Then in a second step the main features of MnO allowing better
x
performances in the reactions expected to occur in the abatement of toluene in PPC process are
reviewed including ozone decomposition, toluene ozonation, CO oxidation and toluene total
oxidation. Finally,inalastpartthecurrentstatusoftheapplicationsofPPCusingMnO ontoluene
x
abatementarediscussed. Inafirststep,theselectedvariablesofthehybridprocessrelatedtothe
experimentalconditionsoftolueneabatementinairareidentified. Theselectedvariablesarethose
expectedtoplayaroleintheperformancesofPPCsystemtowardstolueneabatement. Thenthe
descriptorslinkedtotheperformancesofthehybridprocessintermsofefficiencyaregivenand
theeffectsofthevariablesontheexperimentaloutcomes(descriptors)arediscussed. Thereview
wouldserveasareferenceguidefortheoptimizationofthePPCprocessusingMnO -basedoxides
x
fortolueneabatement.
Keywords: NTP (Non thermal plasma); principles of catalyst selection; MnO ; PPC (Post
x
plasma-catalysis);tolueneabatement;ozonedecompositionability;tolueneozonation;COoxidation;
toluenetotaloxidation
1. Introduction
Withtheincreasingconsumptionoftolueneincommercialandindustrialapplications[1],the
tolueneemissionproblembecomesofseriousconcernforcausingenvironmentalproblemaswell
as inducing negative health effects on humans [2,3]. So it is highly desired to find an economical
and green technique for toluene emission control. Comparing to traditional techniques such as
high-temperature-basedcombustionandcatalyticoxidation,non-thermalplasma(NTP)technology
is capable of operating at room temperature and ambient pressure. Meanwhile, easy operation is
anothermainadvantagesofNTPtechnique[4]. However,theenergyefficiencyandCO selectivity
2
still need to be improved for further industrial application. Hence, plasma-catalysis, including in
plasmacatalysis(IPC)andPPCconfigurations,canbeapromisingapproachfortolueneabatement
havingtheadvantagesofhighertolueneremovalefficiencyandlowerunwantedbyproductsatalow
operatingtemperature,atwhichitisnormallynotpossibletoactivatethecatalysts[5,6].
Catalysts 2018,8,91;doi:10.3390/catal8020091 www.mdpi.com/journal/catalysts
Catalysts 2018,8,91 2of32
Tobeginwith,thephysico-chemicalpropertiesoftoluenearepresentedanditsnegativeeffects
onenvironmentandhumanbeingaregiven. Then,afterbrieflyscreeningthemostencounteredVOCs
(VolatileOrganicCompounds)depollutionmethods,theprinciplesofPPCareexposedbasedonthe
couplingoftwomaturetechnologiessuchasNTPandcatalysis. Thedesignofmanganeseoxides
is discussed in a fourth part. The choice of MnO as potential catalysts in PPC configuration was
x
motivatedduetothefactthatexceptmetalsoftheplatinumgroup,characterizedbytheirhighprice,
themetaloxidecatalystscontainingmanganeseoxideexhibitthehighestactivityindecomposition
of gaseous ozone and also in catalytic oxidation of pollutants. In that respect, the presentation of
the abundant manganese is firstly given. Then, the important features of MnO allowing better
x
performancesinthemainreactionsexpectedtooccurintheabatementoftolueneinaPPCprocessare
reviewedincludingcatalyticoxidationoftoluene,ozonedecomposition,tolueneozonationandCO
oxidation. Finally,inalastpartthecurrentstatusoftheapplicationsofPPCusingMnO -basedtype
x
catalystsontolueneabatementisdiscussed. Theselectedvariablesofthehybridprocessrelatedto
theexperimentalconditionsoftolueneabatementinairarefirstidentified. Theselectedvariablesare
thoseexpectedtoplayaroleintheperformancesofthetwo-stageplasma-catalystsystemtowards
toluene abatement. The descriptors linked to the performances of the hybrid process in terms of
processefficiencyarealsopresented. Inasecondsteptheeffectsofthevariablesontheexperimental
outcomes(descriptors)arediscussed.
2. TheProperties,ApplicationsandHazardousEffectsofToluene
Toluene(C H CH )iswidelyinvestigatedasarepresentativeVOCsoftheBTX(Benzene,Toluene
6 5 3
and Xylene) family. Toluene is a colorless liquid with an aromatic odor. It is a mono-substituted
benzenederivative,consistingofaCH groupattachedtoaphenylgroup. Thebasicpropertiesof
3
toluenearepresentedinTable1.
Table1.Thephysicochemicalpropertiesoftoluene.
Property Value
Chemicalformula C H
7 8
No.CAS 108-88-3
Molarmass(g/mol) 92.14
Density(g/mL) 0.87(20◦C)
Meltingpoint(◦C) −95
Boilingpoint(◦C) 111
Solubilityinwater(g/L) 0.52(20◦C)
Vaporpressure(kPa) 2.8(20◦C)
Odorthreshold(ppmv,PartsPerMillionbyvolume) 0.17
Autoignitiontemperature(◦C) 480
Conversionfactor 1ppmv=3.76mg/m3(inair,25◦C)
Toluenecanreactasanaromatichydrocarboninelectrophilicaromaticsubstitution[7].Themethyl
grouphasagreaterelectron-releasingpropertythanthatofHatominthesameposition. Sulfation,
chlorinationandhalogenationhavebeenundergoneviathisproperty. Importantly,comparingtothe
phenylgroup,methylgroupismoresusceptibletooxidation[8].
Toluenehasnumerouscommercialandindustrialapplications: forexample,tolueneisusedasa
precursorinthesynthesisofchemicals,suchasxylene,benzene,toluenediisocyanate(TDI)forthe
manufactureofpolyurethanefoamandtrinitrotoluene(theexplosiveTNT).Tolueneisusedasan
octaneboosteringasolinefuels,whichareusedininternalcombustionengines. Inaddition,tolueneis
widelyusedasasolventbecauseofitsfavorableproperties,suchashighsolvency,lowflammability,
non-corrosivenessandhighstability.Inmanycommercialproducts,tolueneispresentinpaintthinners,
siliconesealants,nailpolishremover,glues,disinfectantsandcorrectionfluid. Theglobaldemandfor
toluenevaluedatoverUSD25.1billionin2015,isexpectedtoreachaboveUSD31.8billionin2021[9].
Catalysts 2018,8,91 3of32
Tolueneworldwideproductionisestimatedtobe19.6milliontonsin2020comparingto12.4millionin
2010[10].
However,itisnoteworthytomentionthat~86%ofthetolueneproducediseventuallyreleased
intothebiosphere(primarilyinthetroposphere)duetohighvolatilityandlowsolubilityinwater.
Thereareseveralman-madesourcesofemissionstotheatmosphere,includingthefollowing[11]:
• Inadvertentsources(65%),suchasemissionsfrommotorvehiclesandaircraftexhaust,losses
duringgasolinemarketingactivities,chemicalspills,cigarettesmoke,andhouseholdproducts.
• Industrialprocessesinwhichtolueneisused(33%).
• Tolueneproduction(2%).
Tolueneisbelievedtobeoneofthemostprevalenthydrocarbonsinthetroposphere.Itsdispersion
islargelydependentuponmeteorologicalconditionsanditsatmosphericreactivity. Thelifetimeof
toluenerangesfrom4days(athigh-altitudesduringthesummer)toseveralmonths(atlow-altitudes
duringthewinter). Theaveragehalf-lifeoftolueneresultingfromatmosphericoxidationisestimated
tobe12.8h[3]. Sothecontributionoftoluenetothephotochemicalsmogisconsiderable.
The presence of toluene in air has a negative impact on human health. Toluene is a main
contributor of indoor environmental pollution. Different sources such as paints, adhesives and
cigaretteareknowntocontributetotheincreaseoftolueneconcentrationinindoorair.
The report entitled “toxicological profile for toluene” published in 2000 conducted by U.S.
department of health and human services presents the health effects of toluene [2]. As shown in
Table2,therearethreemainexposureroutesforhuman: inhalation,oralanddermal. Amongthem,
a lot of data deal with toluene incorporation throughout inhalation. The high hazard profile of
toluene is highlighted as it covered all the possible health effects listed in Table 2. For example,
inVirginia,itwasreportedthat39deathswereattributedtoinhalantabusefrom1987to1996,and
Hobaraetal.[12]estimated1hofexposureto1800–2000ppmtoluenemaybefataltohumansvia
inhalation. Regardingoralroute,onlycasesofdeathandacuteeffecthavebeenreported. Concerning
thedermalroute,ithasbeenfoundthatexposureoftoluenecausedtheacuteeffectandgenotoxic
effect. Duetotheadverseeffectsonhumanhealth,WHO(WordHealthOrganization)haspublished
guidelineforthetolueneexposurelimit. EuropeanUnionhasfollowedtheinstructionofWHO[1]:
The lowest-observed-adverse-effect level on the central nervous system (CNS) from occupational
studies,isapproximately332mg/m3(88ppmv)andthelimitofoccupationalexposurefor8hofwork
is188mg/m3(46ppmv)inEuropeanUnion(EU).
Table2. Existinginformationonhealtheffectsoftoluene(blackdot: existingdata); adaptedfrom
reference[2].
HealthEffects Oral Dermal
Death
Acute
Intermediate
Chronic
Immunologic/Lyphoretic
Neurologic
Reproductive
Developmental
Genotoxic
Cancer
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3. TheTreatmentTechniquesfortheRemovalofToluene
3. The Treatment Techniques for the Removal of Toluene
The dangerousness of toluene involves researching an economic and green technique for its
The dangerousness of toluene involves researching an economic and green technique for its
abatement. There have been developing a few traditional types of VOCs abatement techniques,
abatement. There have been developing a few traditional types of VOCs abatement techniques,
mainlyfocusontwodirections: onedealswithnon-destructivemethods(adsorption,condensation,
mainly focus on two directions: one deals with non-destructive methods (adsorption, condensation,
membraneseparation),theotheroneconcernsdestructivemethods(biochemicalmethod,thermaland
membrane separation), the other one concerns destructive methods (biochemical method, thermal
catalyticoxidations). ThelimitsbasedonVOCsconcentrationandflowratefortheseconventional
and catalytic oxidations). The limits based on VOCs concentration and flow rate for these
methodshavebeendepictedinFigure1. Sinceeverytypeoftreatmenttechnologyhasadvantagesand
conventional methods have been depicted in Figure 1. Since every type of treatment technology has
disadavdavnatnatgagese,s nanodn ediissardevaalnlytasgaetsi,s nfaocnteo risy .really satisfactory.
Concentration
(g/m3)
100
CATALYTICAND
THERMALOXIDATION
CONDENSATION
10 ABSORPTION
MEMBRANES
ADSORPTION
1
TRAITEMENT
PHOTOCATALYBSIISOLOGIQUE PLASMA+ PLASMA-CATALYSIS
BIOFILTRATION
0.1
Flow rate
(m3/h)
10 100 1000 10000 100000

FigurFeig1u.rCe l1a.s Csilfiascsaiftiicoantioonf vofa vriaoruiosuas iarirp poollluluttiioonn ccoonnttrrooll tteecchhnnoololgoigeise bsabseadse odn oanir afliorwfl orawte raantde ainnitdiailn itial
VOCVcoOnCc econntrcaentitorant.ion.
Comparing to conventional methods, NTP technology has been claimed to be an alternative
Comparing to conventional methods, NTP technology has been claimed to be an alternative
method for reducing VOCs due to its wide applicable range of 10–50,000 Nm3·h−1 and (ppb level)-
meth1o0d,00f0o rprpemdvu c[i1n3g–1V6]O, Cesspedcuiaellyt oefiftesctwiveid efora pcponlicceanbtrleatiroanns gleowoefr 10th–a5n0 ,010000 pNpmm3v· h[−171].a nTdhe (ppb
level)a-p1p0l,i0c0a0tiopn pomf NvTP[1 o3n– 1t6o]lu,eensep aebcaitaelmlyenetf hfeacst iavtteracfoterd ccoonnsciednetrraabtlieo innsterleoswt beercatuhsaen of1 e0a0syp opnm/ovff [17].
The aoppperliactiaotnio anndo finNexTpPenosnivteonleusse n[1e8a,1b9a].t eGmeneenrtalhlya, sNaTtPtr acoctnesdistcso onfs eidneerrgaebtilce eilnetcetrroensts, bieocnas,u ascetiovef easy
on/orfafdoipcaelrsa, ntieountraaln pdarinticelxeps eanndsi UveVn pehssot[o1n8s, 1a9s] s.hGowenne irna Flliyg,uNreT 2P. Tchoenses issptsecoiefse anree rpgreotdicuceelde cwtrhoenns a, ions,
sufficient strong electric field was applied allowing gas discharge. The electron energy is usually
activeradicals,neutralparticlesandUVphotonsasshowninFigure2. Thesespeciesareproduced
considered to range from 1 to 20 eV while the temperature of the carrier gas keeps close to room
when a sufficient strong electric field was applied allowing gas discharge. The electron energy is
temperature. These high energetic electrons can promote toluene abatement via a primary process
usuallyconsideredtorangefrom1to20eVwhilethetemperatureofthecarriergaskeepsclosetoroom
such as charge transfer, ionization, excitation, dissociation reactions [20]. The secondary process is
temperature. Thesehighenergeticelectronscanpromotetolueneabatementviaaprimaryprocess
characterized by the subsequent chemical reactions involving the products of primary process
such as charge transfer, ionization, excitation, dissociation reactions [20]. The secondary process
(electrons, radicals, ions, and excited molecules). However, the overall process is not selective into
is chathrae cdteesriirzeedd tabrygetth wehsiuchb sise qCuOe2n. tToc hoevmericcoamler elaacckti oonf sCOin2v soellevcitnivgittyh, erepseraordchuecrtss hoafvpe rfiomcuasreyd porno cess
(electermonesr,grinagd itceaclhsn,iiqouness, arnesdueltxincgit efdrommo tlheec ucloeusp).liHngo wofe vtwero, tmheatouvree rValOlCprso acbeastseimsennott tseeclhencotilvoegiienst. othe
desireAdmtoanrgge tthwoshei,c phlaissmCaO-ca.tTaolyosivse irsc oonmee olfa tchkeo mfCosOt prsoemleicstiinvgit. yA,rttersaecatirvcehneersss hoaf vtehifso hcuybseridd opnroecmesesr ging
2 2
technsitqeumess frreosmu ltthine gpforsosmibiltihtye tcoo utapklein fgulol fatdwvaonmtagaetus roef VthOe Ctwsoa btaetcehmnoelnogtiteesc: hhnigohlo sgeileesc.tiAvimty ofnrogmt hose,
catalysis and the easy operation from NTP. Thus, previous works have emphasized the beneficial
plasma-catalysisisoneofthemostpromising. Attractivenessofthishybridprocessstemsfromthe
role of plasma-catalysis in VOCs removal allowing to decrease the energy cost as well as minimizing
possibilitytotakefulladvantagesofthetwotechnologies: highselectivityfromcatalysisandtheeasy
the production of hazardous by-products [17,21,22].
operationfromNTP.Thus,previousworkshaveemphasizedthebeneficialroleofplasma-catalysisin
According to the location of catalyst, the plasma catalysis system can be divided into two
VOCsremovalallowingtodecreasetheenergycostaswellasminimizingtheproductionofhazardous
categories, namely, IPC and PPC configurations. In this study, we will focus only on PPC applied to
by-products[17,21,22].
toluene removal.
According to the location of catalyst, the plasma catalysis system can be divided into two
categories,namely,IPCandPPCconfigurations. Inthisstudy,wewillfocusonlyonPPCappliedto
tolueneremoval.
Catalysts 2018,8,91 5of32
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 34

Figure 2. Time-scale of elementary processes in a non thermal process [20].
Figure2.Time-scaleofelementaryprocessesinanonthermalprocess[20].
4. Design of Catalysts for Toluene Abatement in PPC Process
4. DesignofCatalystsforTolueneAbatementinPPCProcess
4.1. Introduction
4.1. Introduction
In PPC configuration the catalyst is located downstream from the non-thermal plasma reactor.
IInn PthPaCt wcoayn fihgomuroagteionneotuhse ccoamtapllyexs treisacltoiocantse odccduorw bnetswtreeeanm plfarsomma tahned nthoen -ctahrreiremr gaalsp mlaoslmecaulreesa ctor.
Inthasutcwh aays Oho2,m No2,g peonsesiobules Hco2Om apnldex three aVcOtiCo nwshoocsceu nrubmebtewr eaennd pexlatesnmt acaann bde ttuhneecda brryi ecrhagnagsinmg othleec ules
sucheanseOrg2y, Npu2t, ipno tshsei bplleasHm2aO. Dauned tot htheeV loOwC swelehcotisveitny uomf tbhee rNaTnPd, aelxotnegn tthcea VnObCe touxnideadtibony icnhtoa nCgOin2,g the
energiyts ptruantsifnortmheatpiolans ims aalw. Dayuse actocotmhepalnoiweds beyle tchtei vfoitrymoatfiothne ofN uTnPw,aanlotendg htahzearVdoOuCs boyxpirdoadtuioctns (ien.tgo., CO ,
2
itstrapnlasfsomram gaetnioernatiesda VlwOaCyss (arecaccotmionp ainntieerdmbedyiathtees)f,o CrOm, aOti3o, nanodf NuOnwx).a Tnhtiesd phroabzlaermd ocauns bbey rpersoodlvuecdt sa (e.g.,
priori through the high selectivity of the catalyst. For instance, the proper catalyst should be able to
plasmageneratedVOCs(reactionintermediates),CO,O ,andNO ). Thisproblemcanberesolved
3 x
oxidize the plasma generated organic by-products and the plasma non processed initial VOC as well
apriorithroughthehighselectivityofthecatalyst. Forinstance,thepropercatalystshouldbeable
as CO to CO2 and decompose O3 and NOx generally in moist air at a temperature close to room
tooxidizetheplasmageneratedorganicby-productsandtheplasmanonprocessedinitialVOCas
temperature or moderate (≤ 150 °C) at the same time. The basic idea is to take advantage of the O3
well as CO to CO and decompose O and NO generally in moist air at a temperature close to
decomposition 2efficiency of the catal3yst to suppxly active species enabling the total oxidation of
roomptleamsmpae nraotnu preroocersmsedo dhearzaatredo(≤us1 s5p0ec◦iCes). aWththene rseagmareditnimg teh.eT nhaetubrea soifc tihdee caatiaslytostst,a nkoebaled mvaentatlasg eof
theOandde ctroamnspitoiosnit iomneteaflfi coixeindceys ohfatvhee bceaetanl yfsotutnods utop pbley athceti vmeosspte caicetsiveen asubblisntagntchese tfootra loozxoindea tion
3
of pladsemcoamnpoosnitiponro. cHesosweedvehra azmarodnogu tshes pdeecvieelso.peWd hceantalryesgtsa, rMdinnOgx tahree ncoantusirdeeroefd thtoe bcea ttahley smtso,snt oble
metalesffiacniedntt raannds ictoiostn-emffeecttaivleo fxoird eoszohnaev edebcoemenpofsoiutinond. tAodbdeititohneallmy,o tshtea cptoisvseibsiluitbys toafn icnecsrefaosrinog zone
moderately the temperature of the catalyst allows to activate some oxygen of the catalyst. Based on
decomposition. Howeveramongthedevelopedcatalysts,MnO areconsideredtobethemostefficient
x
that observation, efficient catalysts for total oxidation of VOC cannot be discarded. Again, the same
andcost-effectiveforozonedecomposition. Additionally,thepossibilityofincreasingmoderatelythe
families of catalysts should be taken into account and again MnOx can be good candidates for total
temperatureofthecatalystallowstoactivatesomeoxygenofthecatalyst. Basedonthatobservation,
oxidation of toluene. Lastly MnOx are also recognized as efficient catalysts in CO oxidation which is
efficientcatalystsfortotaloxidationofVOCcannotbediscarded. Again,thesamefamiliesofcatalysts
also an important reaction to consider.
shouldbeTthaiks erenviinewto aattcecmouptnst toa nsudmamgaairnizMe onuOr xprceasnenbt estgaoteo odf ckannodwildedagtees ufsoinrgt oMtanlOoxx-bidasaetdio cnatoaflytsotslu ene.
LastlyforM tnoOluxenaer eabaalsteomreencto ginn iPzPedC acsoneffifigcuireantitonc.a tIanl ytshtast inresCpOecto xthide aptrioesnenwtahtiiocnh iosf atlhseo aabnunimdapnot rtant
reactimonantgoacnoenses iodxeidr.es will be firstly given. Then in a second step the main features of MnOx allowing
Tbhetitserr epveierfworamttaenmceps tsint otosluumenme atoritzael ooxuirdpatrieosne,n otzsotantee doefckonmopwolseitdiogne, utsoilnugenMe noOzon-abtaiosend acnadt aClyOs tsfor
x
toluenoxeidaabtaiotenm weinllt bine PsuPcCcecsosinvfielgyu brraiteifolyn .rIenvitehwaetdr.e sFpineacltlyt,h ienp trhees elanstta ptiaornt othfet hceurarbenutn sdtaatnutsm ofa nthgea nese
oxideasppwliiclaltiboensfi orfs PtlPyCg uivsienng. MTnOhex noni ntolauesneec aobnadtemsteenpt wthilel bme adiinscufessaetdu.r es of MnO allowing better
x

performancesintoluenetotaloxidation,ozonedecomposition,tolueneozonationandCOoxidation
willbesuccessivelybrieflyreviewed. Finally,inthelastpartthecurrentstatusoftheapplicationsof

PPCusingMnO ontolueneabatementwillbediscussed.
x
Catalysts 2018,8,91 6of32
4.2. PresentationoftheMainManganeseOxideStructures
Catalysts 2018, 8, x FOR PEER REVIEW 6 of 34
Manganese(Mn)isamultivalentelement. Eachvalencestatehasacharacteristicoxideoroxides.
MnO occurnaturallyasmineralsinatleast30differentcrystalstructuresinarangeofmanganese
x 4.2. Presentation of the Main Manganese Oxide Structures
oxidationstates. Manganeseoxidesaretechnologicallyimportantmaterialsandhavewidelybeen
Manganese (Mn) is a multivalent element. Each valence state has a characteristic oxide or oxides.
usedaseffectiveandinexpensivecatalystsandaselectrodematerialsforbatteries. Interestingly,some
MnOx occur naturally as minerals in at least 30 different crystal structures in a range of manganese
MnO crystalstructuresexhibittunneland/orlayeredstructureswithvariedproportionsofMnin
x oxidation states. Manganese oxides are technologically important materials and have widely been
different oxidation states (+2, +3, and +4). The known binary oxides of manganese include MnO,
used as effective and inexpensive catalysts and as electrode materials for batteries. Interestingly,
Mn O ,Mn O andMnO . Likemanymonoxides,MnOadoptstherocksaltstructure,wherecations
2 3some3 Mn4Ox crystal st2ructures exhibit tunnel and/or layered structures with varied proportions of Mn
andaniionn dsifafreerebnot tohxoidcattaihone dsrtaatlelys (c+o2o, r+d3i, naantde d+4.)T. hTeheα k-Mnonw2nO b3inpahrays oexiisdnesa mof emdabnigxabnyeistee .inTchluedMe Mn3n+Oio, ns
areoctaMhne2dOr3a, lMlync3Ooo4 radnidn MatneOd,2.w Lihkiel emtahneyO mioonnosxihdaesv,e M4nMO nadnoepitgs hthbeo rros.ckT shaelt bsitxrubcytuitree,s wtrhuecrteu craeticoanns be
viewedaansda acnlioosnes- apraec bkoetdh loactttaicheedorfaMllyn cowoirtdhinOatieodn. sTfihell iαn-gMtnh2rOe3e pqhuaaser tiesr nsaomfetdh ebitxebtryaithee. dTrhael Minnte3+r isotnitsi als
inapatatreer nocwtahitehdrIaal3lys ycomormdientartye.dH, wahuislme tahne nOit eionMs nhaOve 4h Masna nsepigihnbeolrsst.r Tuhcetu briexbhyaitvei nstgruactcuorme cpaons bitei on
3 4
formulavieMwne2d+ a(Ms an c3lo+s)eO-pa,ckwedh elarteticMe nof2 +Mino wnsitho Occ iuopnys ftilhliengt ethtrraehe eqduraartlesrsit oefs thaen tdetrMahne3d+rathl ientoecrsttaihtiaelds ral
2 4
sites[23in]. a pattern with Ia3 symmetry. Hausmannite Mn3O4 has a spinel structure having a composition
formula Mn2+(Mn3+)2O4, where Mn2+ ions occupy the tetrahedral sites and Mn3+ the octahedral sites
Along these oxides is the metastable layered Mn O having a compositional formula
[23]. 5 8
((Mn2+) (Mn4+) O ). Manganese dioxide, MnO , for its own part, shows polymorphism. MnO
2 Alon3g 8these oxides is the metastable 2layered Mn5O8 having a compositional formula 2
hasseveralcrystallinestructures,includingα-,β-,γ-,δ-,andλ-MnO . Thedifferenttunnelstructures
((Mn2+)2(Mn4+)3O8). Manganese dioxide, MnO2, for its own part, sh2ows polymorphism. MnO2 has
canbedseevscerriabl ecdrybstyaltlhinee ssitzreucotfurthese,i irntculundnienlgs αd-e, tβe-r,m γ-i,n δe-d, abnyd tλh-MennuOm2. bTehre odfifofecrteanhte tdurnanlesl ustbruunctiutsre(sn c×an m),
asdepicbtee ddeisncrFiibgeudr bey3 .thαe- sMizne Oof t(hheoirl ltaunndnietles) dceotnesrmistinseodf dbyo uthbel encuhmabiners ooff oecdtaghee-sdhraalr isnugbuMnintsO (no ×c mta)h, easd ra,
2 6
whichadreepliicntekde dina Ftigcourren e3r. sα-tMofnoOr2m (h1oDlla(n2d×ite)2 )coannsdist(s1 o×f d1o)utbulen nchealsinisn otfh eedgteet-rsahgaorinnagl MunnOit6c oecltla.hTehderas,i ze
of the (w2h×ich2 )arteu lninnkeeldi sat0 c.o4r6nenrms t,ow fohrmich 1Dis (2a ×l a2r)g aendtu (n1 n×e 1l) ftournninelsse irnt itohne /teetxratrgaocntaiol nunoitf caelllk. aTlhiec asitzieo ns.
β-MnOof (tphye r(o2l u× s2i)t etu)nisneclo ims 0p.4o6s endmo, fwshiincgh lies sat rlaarngde stuonfneedl gfoers ihnsaerritniognM/exntrOactioocnt aohf eadlkraalit ocaftioornms. aβ-1D
2 6
(1×1)MtunnOn2e (lp.yBreocluasuistee) oisf ctohmepnoasrerdo wof (s1in×gle1 )sttruanndnse olfs iezdege(0s .h1a8r9inngm M),nβO-6M ocntaOhedcraan tnoo ftoarmcc oa m1Dm (o1d ×a te
2
cations.1T) thuennsterlu. Bcteucareusoef oγf -tMhen nOarrioswr a(1n d× o1m) tuinnnteerl gsrizoew (0th.1s89o fnrma)m, βs-dMenllOite2 c(a1n×no2t )acacnodmpmyordoalutes citaeti(o1ns×. 1)
The structure of γ-MnO2 is r2andom intergrowths of ramsdellite (1 × 2) and pyrolusite (1 × 1) domains.
domains. δ-MnO isa2Dlayeredstructurewithaninterlayerseparationof0.7nm. Ithasasignificant
δ-MnO2 is a2 2D layered structure with an interlayer separation of 0.7 nm. It has a significant amount
amountofo wfwateart earnda nstdabsitlaizbiinlgiz ciantgiocnast sioucnhs assu Ncha+a osr NK+a +beotwreKe+n btheet wsheeeents tohfe MsnhOee6 toscotafhMednraO. 6λ-oMctnaOh2e ids ra.
λ-MnO2a 3isDa s3pDineslp sitnruecltsutrreu. cture.

(a) (b)
Figure F3i.guIlrleu s3t. raIltluiosntraotifonth oef ctrhyes tcarlyssttarlu scttruurcetusroesf osef lseeclteecdtedc ocmommmoonn mmaannggaanneessee ooxxiiddees:s :(a()a )MMnOn2O
2
includiningc:luαd-iMngn: IVαO-Mn(IhVOol2l a(nhodliltaen)d;irtue)t;i lreuβti-leM βn-IMVOnIVO(p2 y(proylruolsuitseit)e;)R; -RM-MnnIVIVOO2 ((rraammssddeellliltiete);) ;γγ-M-MnnIVIOV2O
(intergrowth); 2δ-MnIVO2 (birnessite); λ-MnIVO2 (s2pinel); Here, The purple an2d yellow atoms represent 2
(intergrowth);δ-MnIVO (birnessite);λ-MnIVO (spinel);Here,Thepurpleandyellowatomsrepresent
spin-up and spin-2down Mn, respectively, w2hile the red atoms represent O [24]; (b) Crystal structures
spin-upoaf nMdnsOp,i nα-d-Mown2On3M ann,dr eMspne3Oct4i. vTehlye, wgrheeilne, tbhleuere, danadto rmeds raetpomress ernetpOres[e2n4t] ;M(bn)IIIC, rMysntIIa, lasntrdu cOt2u−,r es
of MnOr,esαp-eMctinv2eOly3 [2a5n].d Mn3O4. The green, blue, and red atoms represent MnIII, MnII, and O2−,
respectively[25].
The richness in MnOx structures thus provides a rigorous test system to explore the significance
of atomic positions on the catalytic efficiency towards toluene oxidation.
TherichnessinMnO structuresthusprovidesarigoroustestsystemtoexplorethesignificance
x
ofatomicpositionsonthecatalyticefficiencytowardstolueneoxidation.
Catalysts 2018,8,91 7of32
4.3. MnO inCatalyticOxidationofToluene
x
ThehighreactivityofMnO towardsVOCsabatementisgenerallyascribedtoeasyvariationof
x
manganeseoxidationstate(Mn2+,Mn3+,Mn4+),oxygenstoragecapacityinthecrystallinelatticeand
highmobilityoflatticeoxygen[26]. KimandShim[27]investigatedtheeffectofO/Mnstoichiometry
using Mn O , Mn O and MnO . It was found that the catalytic activity decreased in the order
3 4 2 3 2
of Mn O > Mn O > MnO in line with the decrease of the specific surface areas and oxygen
3 4 2 3 2
mobilityoftheMnO . Mn O [28],Mn O [29,30]andMnO [28]werealsopreviouslyinvestigated
x 3 4 2 3 2
separatelybydifferentresearchteamsfortolueneoxidation. F.Wangetal.[31]reportedontheexternal
morphologyeffectofMnO (rod-, wire-, tube-, andflower-likemorphologies)fortheoxidationof
x
toluene. Itwasfoundthattheoxygenad-speciesconcentration(relatedtotheMn3+ concentration)
and low-temperature reducibility decreased in the order of rod-like α-MnO > tube-like α-MnO
2 2
> flower-like Mn O > wire-like α-MnO , in good agreement with the sequence of the catalytic
2 3 2
performanceofthesesamples.Thebest-performingrod-likeα-MnO catalystcouldeffectivelycatalyze
2
the total oxidation of toluene at low temperatures (T = 210 ◦C and T = 225 ◦C at a GHSV of
50 90
20,000mL·g−1·h−1).Itwasconcludedthattheexcellentcatalyticperformanceofα-MnO nanorods
2
mightbeassociatedwiththehighoxygenad-speciesconcentrationandlow-temperaturereducibility.
Verykiosetal.[32]showedthatγ-MnO whichwaslessaffectedbythepresenceofwaterthanother
2
MnO polymorphscouldbesuitablefortolueneremoval. Thiscatalystwasclaimedtobemoreactive
2
thannoblemetalcatalysts.
Inasameway,variousstructuresofMnO i.e.,α-,β-,δ-MnO werepreparedandtheircatalytic
2 2
performancesintolueneoxidationwereassessedtothoseofaγ-likeMnO obtainedbytheselective
2
removalofLa3+fromanacid-treatedthree-dimensionalmacroporousandmesoporousLaMnO [33].
3
Thebestperformanceofthisγ-likeMnO overtheotherMnO polymorphswasascribedbothtothe
2 2
openthree-dimensionalmacroporousandmesoporousmorphologyleadingtohigherBETsurface
areasthanconventionalsyntheses,enhancedsurfaceadsorbedoxygen(O )tolatticeoxygen(O )
ads latt
molar ratio (O /O ) and easier reducibility. Similarly Liao et al. [34] investigated the external
ads latt
morphologyeffectoverMn O havinghollowandsolidspheremorphologiesfortolueneabatement.
3 4
ThebettercatalyticactivitywhenusinghollowspheresofMn O wasclaimedtoresultfrombeneficial
3 4
combinedeffectssuchasthecavitymorphologyleadingtohighsurfacearea,higherO /O molar
ads latt
ratio, believed to be proportional to active oxygen, and higher Mn average oxidation state (AOS).
MnO supported catalysts were affected by the same factors too. To conclude catalytic activity is
x
generallyassociatedwithseveralfactors,suchastheBETsurfacearea,presenceofoxygenvacancies,
reducibility,andexternalmorphologyofthecatalyst.
The common oxidation catalytic reactions generally involved three types of mechanism [35]:
Langmuir-Hinshelwood,Eley-RidealandMars-VanKrevelen(MkV)mechanism[33]. Itiswellknown
thatMnO exhibitstrongoxygenstorage/releaseabilityduetothefactthattheyeasilyundergoa
x
rapidreduction–oxidationcyclethroughtheinteractionwithreducingoroxidantagentsaccompany
bytheformationofmanganeseionsinvariousoxidationstates. Theinvolvementoflatticeoxygen
ofcatalystsintotaloxidationoftolueneandthestrongcorrelationbetweenlabilelatticeoxygenand
catalyticactivitysuggestthatthereactioncouldproceedviatheMvKmodel[8,36]. InthecaseofVOCs
catalyticoxidation,thefirststepcorrespondingtothereductionofthecatalystsurfaceisgenerallythe
ratedeterminingstep[37].
4.4. MnO inOzoneDecomposition
x
Decomposition of ozone in the gas phase was carried out on various metal oxides by
S.Imamuraetal.[38]. Itwasshownthattheactivityofthemetaloxidecatalystsincreasedroughly
in the order of the increase in their surface area and in the amount of surface oxygen on them.
Theseobservationsareveryinteresting,asthesecharacteristicsresembletothoseoftotaloxidation
catalysts because low surface area and a small amount of surface oxygen are generally targeted
forpartialoxidationcatalysts. Interestingly,metaloxideconductanceincreasedthroughoutozone
Catalysts 2018,8,91 8of32
introductionforthemostreactiveoxidessuchasAg O≈NiO,CuO,Co O whicharep-typeoxides
2 3 4
suggestingthatnegativelychargedoxygenspecieswereformedontheirsurface. Ag Owasclaimed
2
tobethemosteffectivecatalystindecomposingozone,increasingitsactivityasthereactionproceeds
whilesometransition-metaloxidesaredeactivatedduringthereactionduetoaccumulationofoxygen
speciesthatcovertheiractivesites. ThehighreactivityoftheoxygenspeciesonAgwasinvokedtobe
duetotheespeciallyweakbondbetweenoxygenandsilvercomparedwiththosebetweenoxygen
andothermetals. Additionally,thehighreactivityofsuchcatalystforCOoxidationwasbelievedto
occurthankstotheproductionofveryactiveoxygensuchasoxygenionO−withouttotallydiscarding
oxygen(O)andozonideion(O −)resultingfromthedecompositionofO −.
3 2
Catalysts 2018, 8, x FOR PEER REVIEW 8 of 34
Inthatway,acomprehensivesurveyofdifferentoxides(MnO ,Co O ,NiO,Ag O,Fe O ,Cr O ,
2 3 4 2 2 3 2 3
CeO2,MgO,Veff2eOcti5v,eC cautaOlysat nind dMecoomOpo3s)insgu opzponoer,t iendcreoansinγg -iAts la2cOtiv3itfyo ars othzeo rneaectdioenc poromcepedoss iwtihoilne ,sobmuet thistimein
thepresenceotrfanwsiatitoenr-m(RetaHl o(xRideelsa atriev edehacutimvaiteddi tdyu)r:in4g0 t%he) ,rewacatisonp deurfeo trom acecudmbuylatBio.nD ohf oaxnydgeanp sapneicieest al.[39]. Itis
that cover their active sites. The high reactivity of the oxygen species on Ag was invoked to be due to
foundagainthatp-typeoxidesemiconductorsarethemostactivesubstancesforozonedecomposition.
the especially weak bond between oxygen and silver compared with those between oxygen and other
Among the inmvetealsst. iAgdadteitdiontarlalyn, sthieti ohinghm reeacttaivlitoyx oifd seuschM cantaOly2st sfohro CwOe odxidthateionh iwgahs ebsetlieavcetdi vtoi toycceuxr plaining its
prevalence inthapnakts etno tthde ipsrcoldouscutiorne so.f vTerhy eactoivzeo onxeygdene csuocmh apso osxiytgioenn iopn aOth− wwitahyouot tvoetarllyM dniscOardisnug pported on
x
γ-Al O wasoinxyvgeesnt (iOg)a atnedd obzoyniTd.e Oioyn a(Om3−a) reestualtli.ngT fhroemr ethaec dtieoconmpproosictieoen dofs Oth2−.r oughtwoirreversiblesteps,the
2 3 In that way, a comprehensive survey of different oxides (MnO2, Co3O4, NiO, Ag2O, Fe2O3, Cr2O3,
adsorptionofCoeOzo2, nMegOo,n Vt2Oh5e, CcuaOta alnyds tMsouOr3f)a scueppaonrtdedt ohne γd-Aels2Oo3r pfotri ooznonoef dmecoomlepcousiltaiorn,o bxuyt gtheisn ti[m4e0 ,in4 1]. Ozoneis
firstdecompothsee pdr/esreendceu ocfe wdatbery (RMHn (Rne+lactiavtei ohunmiindtitoy)O: 420−%),a wnads pOerf.orTmheedn byp Br.o Ddhuancdtiaopnanio eft aalc. t[i3v9]e. IOt 2− andO
2 2 2
is found again that p-type oxide semiconductors are the most active substances for ozone
wasproposedviareactionofO2−withanotherO moleculeinthepresenceofMn(n+2)+. Next,unstable
decomposition. Among the investigated transi3tion metal oxides MnO2 showed the highest activity
O22− reducedexMplanin(nin+g2 )i+tsb parcevkalienntcoe Min npnat+enat nddiscdloessuorersp. tTihoen oozofnaen doecxoymgpeonsitmiono lpeacthuwleayo ocvceur rMednO[4x 2,43]:
supported on γ-Al2O3 was investigated by T. Oyama et al. The reaction proceeds through two
irreversible steps, the adsoOrpti+onM ofn onz+on→e onO th2−e c+ataMlysnt (snu+r2fa)+ce +anOd the desorption of molecular (1)
oxygen [40,41]. Ozone is first3 decomposed/reduced by Mnn+ cation into 2O2− and O2. Then production
oMf na(cn+t2i)v+.e N Oe2x2−t ,a unnds tOaO2b l3we a+Os 2Op2−r 2roe−pdous+ceedMd v Mina n(rn(en++a22c)+)t +iboan→c ko fi OnOto22− 2 Mw−int+nh+ aaMnnodnt hd(eners+ oO2r)3p+ mti+oonlOe ocfu2 laen i no xtyhgee pnr emseonlecceu olef (2)
occured [42,43]:
O 2− +Mn(n+2)+ →Mnn++O (3)
2 O3 + Mnn+ → O2− + Mn(n+2)+ + O2 2 (1)
Oyamaetal. foundthatthedifOfe3 r+e On2c− e+ Minn(nt+h2)+e →s tOr2u2−c +t Murne(n+2o)+f +t Oh2e manganeseactive(c2e) nterwasthe
origin of support effect on the activity. At low Mn loadings (<6 wt %) the active phase was to be
welldispersedonAl O butnotwellunOd2e2−r +s Mtono(nd+2).+ →Si Mgnnni+fi +c Oa2n tprogresseswereachie(v3e) din2001by
2 3
R.RadhakrishnnOaynameta aetl .alb. yfouunsdi nthgate txhet ednifdfeeredncXe -inr athye astbruscoturpret ioof nthefi mnaensgtarnuescet aucrtieve( EceXntAerF wSa)s ttohee valuatethe
origin of support effect on the activity. At low Mn loadings (<6 wt %) the active phase was to be well
localbondingenvironmentintheMnO /γ-Al O catalyst[43]. Thecatalyst(3wt%)wasprepared
dispersed on Al2O3 but not well unxderstood2. Si3gnificant progresses were achieved in 2001 by R.
throughoutinRacdiphaikernisthnwnaent nete sals. bimy upsirnegg enxatetnidoend uX-sriany gabasomrpationng fainnee sstreucatcueret a(EteXApFrSe) ctou resvaolru.atAe tfhtee rheatingat
170◦Cfor6hlo,ctahl beosnodilnigd ewnvairsoncmaelcnitn ine dthea Mt4n0O0x/γ◦-CAl.2OB3a csaetadlysat l[s4o3].o Tnhei ncastailtyust R(3a wmt a%n) wspase pcrterpoasrecdo pyanalysis
throughout incipient wetness impregnation using a manganese acetate precursor. After heating at 170
performedbeforeandduringozonationreactionandabInitiocalculationsascomplementarytoolsa
°C for 6 h, the solid was calcined at 400 °C. Based also on in situ Raman spectroscopy analysis
fourmononupcelrefoarrmsepd ebecfioerse awnda dsupriongs touzolantaetidon aresactthioen manad nabg Ianniteios cealacucltaitvioensc aesn ctoemrpwlemitehntaaryM tono–lsO a distanceof
210pmasshofowurn minonFoniguculeraer 4sp.eDcieusr winags ptohsetuolazteodn aes dtheec moamngpaonessieti aocntivree caecnttieor nw,itahn a aMdns–oOr bdiesdtanpcee roof xidespecies
wasobserved21o0n pmth aes schaotwanly ins tFisguurref a4.c Deu(rRinag mthea onzolnine edeacotm8p8o0sictimon− re1a)c.tion, an adsorbed peroxide species
was observed on the catalyst surface (Raman line at 880 cm−1).

Figure4.GeoFmigeutrer y4.- oGpeotmimetriyz-eodptifmoiuzerdo fxouyrg oexnygceno ocorodrdininaattee MMnn strsutcrtuucret uusriengu tshien HgFt/6h-3e1H1GF m/e6th-3od1 1[4G3].m ethod[43].
Only a few works have investigated the activity of manganese dioxides with specific crystal
Only a fpehwasew [4o4,r4k5]s. Mhaa evt eal.i n[4v6]e psrtoipgoasteedd a stchheemaec otfi voziotyne odfecmomapnogsitaionne osveerd triaonxsiitdioens mwetaitl hdosppede cific crystal
phase[44,45].crMypatometelaanl.e–[4ty6p]e pmraonpgaonseesed oaxidsec hcaetmalyestso (fαo-MzonOn2e) idmepclyoimng pthoes Mitino3+n/4+ omvixeerd tvraalennsciet iino nthem etaldoped
decomposition/reduction of O3 into O2 and O−. Consequently, the main differences here stem in the
cryptomelane–typemanganeseoxidecatalysts(α-MnO )implyingtheMn3+/4+mixedvalenceinthe
2
Catalysts 2018,8,91 9of32
decomCaptaloysstist 2i0o1n8,/ 8r, ex dFOuRc tPiEoEnR oRfEVOIE3Win toO2 andO−. Consequently,themaindifferencesherest9e omf 34i nthe
natureoftheimpliedredoxcoupleofmanganeseandinthenatureofthereactiveoxygenspecies: O−
insteandatuorfeO of 2t−he. implied redox couple of manganese and in the nature of the reactive oxygen species: O−
2
instead of O22−. O +(Mn3+)→O +O− (Mn4+) (4)
3 2 ads
O +OO3 +− (Mn(M3+) n→4+ O)2→ + O2O−ads(M+n(M4+)n 3+) (4) (5)
3 ads 2
This scheme highlights the impOo3 r+t Oan−acdes(Mofn4t+h) →e m 2Oix2 e+d (Mvanl3+e)n ce (Mn4+/3+) in α-MnO c(a5)t alysts.
2
BasedonTthhiiss smcheecmhaen hiisgmhlitghhetsh tihgeh iemrpcoornttaenncte ooff Mthen m3+ix(seud rvfaalceencoex (yMgenn4+/3v+)a icna nαc-Miesn)Oc2a ctaiotanlyssptsr.e Bsaesnetda tthe
surfaocne othfitsh meeccahtaalnyissmti stheex pheigchteedr tcoonpteronmt ooft eMonz3o+ n(seudrfeaccoe moxpyogseinti ovna.cancies) cations present at the
Isnutrefarcees toifn tghley ,ctahtaelyesftf eisc texopfeMctendO to sptrroumctoutree oozonnoez doenceomdpecoosimtiopno.s itionwasrecentlyinvestigatedon
2
α-,β-andInγte-rMesntiOng.lyT, hthee aecffteivcti toyf oMfntOhe2 ssetruthctruereeM onn oOzonpeo dlyemcoomrppohssitfioonr owzaos nreecdenetcloym inpvoesstiitgioatnedfo olnlo wed
2 2
theorαd-,e βr-o afnαd- γ>-Mγ-n>Oβ2. -TMhen Oacti.vTithye osfu thpeesrei otrhrpeeer MfonrmO2a pnocleysmoofrαp-hMs fnoOr ozwoneree deexcpomlapinoesditiboyn tfholelocwomedb ined
2 2
the order of α- > γ- > β-MnO2. The superior performances of α-MnO2 were explained by the combined
largest specific surface area and the lowest Mn AOS. Additionally, it was found that the catalytic
largest specific surface area and the lowest Mn AOS. Additionally, it was found that the catalytic
activity of MnO strongly depended on the density of oxygen vacancies in line with the presence
activity of M2nO2 strongly depended on the density of oxygen vacancies in line with the presence of
ofMnM3n+3.+.T Thhee oozzoonnee ddeeccoommppoossitiitoionn mmecehcahnainsmis mbabseads eodn otnhet hinevionlvvoemlveenmt eanndt arnedcyrcelicnygc loifn goxoyfgoenx ygen
vacanvcayca(nVcyO )(VwO)a swpasr opproopseodsedb ebleolwowa nanddt hthee ppeerrttiinneenntt ppooinintst swwereer egrgarsapsepd eidn iFnigFuirgeu 5r.e O5n.ceO Mncne3+M n3+
appeaaprspeinartsh ien mthea nmgaanngeasneesdei odxioidxied,eo, xoxyyggeennv vaaccaanncciieess wweerere ggeneenreartaedte tdo tmoaminatainint aeilnecetrloescttartoics tbaatliacnbcae lance
accoradcicnogrdtiongth toe tfhoell foowlloinwginpgr opcroecsess(sE (qEuquaatitoionn( (66))))::
4Mn4++4OM2n−4+ →+ O42−M →n 44+M+n42+ e+− 2/e−V/VO ++ 11//22OO2 →→ 2M2nM4+n +4 +2M+n23+M +n V3O+ ++ 1V/2O2+ 1/2O (6) (6)
O 2 O 2
O3 + VO → O2 + O2− (7)
O +V →O +O2− (7)
3 O 2
O O+3 O+ 2O−2− →→ OO2 ++ OO22− 2− (8) (8)
3 2 2
OO2−22− →→ OO2 ++ VVO (9) (9)
2 2 O

Figure5.Ozonedecompositionmechanismbasedontheinvolvementandrecyclingofoxygenvacancy
Figure 5. Ozone decomposition mechanism based on the involvement and recycling of oxygen
(VO)v[4a4ca].ncy (VO) [44].
WheWnhoezno noezognaes gflaosw flsowthsr othurgohugMh nMOnO,2t,h teheo zooznoneem mooleleccuullee bbiinnddss ttoo MMnnOO2 ssuurfrafcaec ebyb yinisnesretirntgin gan
2 2
an O atom into an oxygen vacancy site. The oxygen vacancy is 2-electron donor and transfers 2
Oatomintoanoxygenvacancysite. Theoxygenvacancyis2-electrondonorandtransfers2electrons
electrons to an O atom of ozone [47,48], thus forming an oxygen species (O2−) in the oxygen vacancy
toanOatomofozone[47,48],thusforminganoxygenspecies(O2−)intheoxygenvacancysiteandan
site and an oxygen molecule, which desorbs into the air (see Equation (7)). Then another ozone
oxygenmolecule,whichdesorbsintotheair(seeEquation(7)). Thenanotherozonemoleculereacts
molecule reacts with O2− to produce a gas-phase oxygen molecule and a bridging O2 dimer (peroxide,
withOO222−−), twohpicrho dwuacse oabsgearsv-epdh bayse ino xsyitgue Rnammaonle scpuelcetraonsdcoapyb r(isdeeg iEnqguOat2iodni m(8)e)r. F(pinearlolyx,i dthee, Ope2r2o−x)id,we hich
wasospbesceirevs e(dO2b2−y) idnecsoitmupRosaems aton rsepleeacster oasnc ooxpyyg(esne emEolqeucualteio (nse(e8 E))q.uFaitnioanll y(9,)t)h. eThpee rdoexciodmepsopseitcioiens o(fO 22−)
decompeprooxsiedset sopreeclieesa sweaasn beolxieyvgeedn tom boel ethceu rleat(es-eliemEitqinuga tsitoepn. ( 9)). Thedecompositionofperoxidespecies
wasbelievThedis tsotubdey tbhreinrgast eim-lipmorittainngt gsuteidpe.lines for the design of future MnO2-based catalyst for ozone
Trehmisovsatlu. dy brings important guidelines for the design of future MnO -based catalyst for
2

ozoneremoval.
Catalysts 2018,8,91 10of32
4.5. MnO inOzonationReactions
x
Catalyticozonationisaninnovativemethodforoxidationofpollutantsinliquidorgasphase.
OzonationreactionsmainlyreferredtocatalyticVOCsoxidationbyozone. Asthisreactionconsists
ofozonedecompositioninthepresenceofanorganicsubstrateitisnotsurprisingthatMnO ,which
x
areamongthemostactivetransitionmetaloxidesforlowtemperaturedecompositionofozoneto
generate active oxygen species, are required for oxidation of VOCs by ozone. Application of this
methodingasphasehasopenedanewcatalyticapproachforlowtemperatureoxidationofVOCs
attraceconcentrationlevels. Resultsoftheliteraturereviewshowthatapparentactivationenergy
ofVOCsoxidationcanbegreatlyreducedbyuseofozonecomparedtothatofoxygen,makingtotal
oxidationofVOCspossibleattemperaturesaslowas100◦Corevenlower.
EffectofmanganeseloadingontotaloxidationoftoluenebyozoneusingMnO /γ-Al O was
x 2 3
investigatedbyE.Rezaietal.[49]. ActivitiesoffourloadingsofMn(1%, 5%, 10%and20%)were
studiedinthetemperaturerangeof22–100◦C.AdirectrelationshipbetweenMnloadingandaverage
oxidationstateofmanganesewasfound.Catalystsatlowerloadings,upto10%,weremostlycomposed
ofMn O whileamixtureofMnO andMn O waspresentincatalystswithloadingshigherthan10%.
2 3 2 2 3
Allcatalystsbecamedeactivatedatroomtemperatureandtheiractivitieswereimprovedbyanincrease
oftemperature. ItwasobservedthatlowerMnloadingshavehigheractivityinoxidationoftoluene.
Itwasproposedthatloweroxidationstatesofmanganesearemorefavorableindecompositionof
ozoneresultinginhigherrateoftolueneoxidation. ALangmuir–Hinshelwoodmechanismbasedon
activationoftoluenemoleculeviaabstractionofhydrogenfromthemethylgroupwaspostulatedto
occuroverMnOx/γ-Al O (10wt%)catalyst[50]explainingtheeffectoftolueneandozonepartial
2 3
pressuresontolueneoxidationrate. HencetheC–Hbondcleavagewasassumedtobetherelevant
kineticstepintheoxidationoftoluenebyozone.
The mechanism of oxidation of VOCs by ozone is based on ozone decomposition due to
Equations (10)–(14) generating atomic oxygen to react with VOCs in the oxidation reaction
(Equation(14))assumingthatVOCsareadsorbedonMnsites(Equation(13))andthereactionproceeds
basedonLangmuir-Hinshelwoodmechanismbetweenadsorbedspecies(Equation(14))[49].
O +Mn→O +O-Mn (10)
3 2
O +O-Mn→O +O -Mn (11)
3 2 2
O -Mn→O +Mn (12)
2 2
VOC+Mn→VOC-Mn (13)
VOC-Mn+O-Mn→Products (14)
ItisbelievedthathigherdispersionofMnatomsincatalystswithlowerloadinglevelsdecreases
oxidationstateofMn.
XRDstudiesshowedthepresenceofβ-MnO andMn O (bixbyite)intheMnloadingof10%.
2 2 3
ThepercentagesofMn O andMnO inthecatalystestimatedfromXANESspectrawere91.0%and
2 3 2
9.0%,respectively. Theactivationenergyofthereactionwasdeterminedtobe31kJ/molobtainedbya
powerlawmodel. Thepowerlawmodelalsodeterminedthereactionorderswithrespecttotoluene
andozoneas−1and2,respectively[50].
OzonationoftoluenecatalyzedbyMnO supportedonsiliceousMCM-41wasalsoinvestigated
x
bythesameteam[51]. TheperformanceswerefoundtobesimilartothoseofMnO /γ-Al O atroom
2 2 3
temperature,theactivitydecreasingwithreactiontimeasthesupportedaluminacatalyst.
MnO /graphenesamplesweredevelopedforcatalyticozonationoftoluenebyM.Huetal.[52].
2
Thehydrothermalmethodwasusedasaneffectivewaytopreparetightlyanchoredbirnessite-type
MnO ongraphene. Thehighesttoluenedegradationrateof7.89×10−6mol·min−1·g−1obtainedover
2
the64.6wt%MnO /graphenesamplewasattributedtoasynergeticeffectofgrapheneandMnO .
2 2