Chapter6.1

PlasmatechniqueforPollutionControlChemistryonPollutionControl等離子體等離子體是處于電離狀態的氣體，是美國科學家Langmuir于1927年在研究低氣壓下汞蒸汽中放電現象時命名的。等離子體由大量的電子、離子、中性原子、激發態原子、光子和自由基等組成，但整體表現出電中性。等離子體又稱物質第四態，它是包含足夠多的正負電荷數目近于相等的帶電粒子的非凝聚系統。

WhatisPLASMA?“FourthState”ofmatterIonizedgasathightemperaturecapableofconductingelectricalcurrentLightningisanexamplefromnature等離子體催化

等離子體催化凈化技術是利用高能電子射線激活、電離、裂解廢氣中各組分，從而使其發生一系列復雜的物理變化和化學反應，而隨后的催化劑將促進副產物的轉化，并降低反應的活化能，把有害物轉化為無害的或有用的物質加以回收。低溫等離子體催化技術解決了傳統的凈化方法所不能解決的問題。利用該技術來處理廢氣始于20世紀70年代，有的已進入應用階段。低溫等離子體催化可以處理廢氣，如VOCs，還原脫硫、聯合脫硝等。Whenanelectricfieldisappliedtoavolumeofgas,highlyenergeticelectronsaregenerated.Highlyenergeticelectronsrapidlycollidewithgasmoleculesinitiatingasuccessionofchemicalreactions.‘Active’plasmaspeciesisformed,includingfreeradicals,ions,excitedmolecules.Inthecaseofaplasmaformedinamixtureofmethaneandcarbondioxide,newgasmoleculessuchasH2

isformed.Dry:CH4+CO2=2CO+2H2Withwater:CH4+2H2O=CO2+4H2AppliedCatalysisB:Environmental94(2010)19–26Prepr.Pap.-Am.Chem.Soc.,Div.FuelChem.2004,49(1),180Processesofplasma

Non-thermalPlasmasElectricalProperties&ChemistryElectricalpropertiesDependingonreactortypePulsedcorona–largevolume/surfaceratio,largedischargegapDielectricbarrier–largesurface/volumeratio,smalldischargegap;barrierproperties;frequency(capacitivecoupling)Dielectricpackedbed–intermediatesurface/volumeratio&dischargegap;dielectricpackedbeproperties;frequency(localcapacitivecoupling)ChemistryFastradical&ionformation(E/n)inactivedischargeregionPulsedcorona–streamerheadDielectricbarrier–μ-discharges;ionizationwaves;fullydevelopedstreamers;AtmosphericPressureGlowDischargeSlowvolumechemistry(chainreactions)Plasma-CatalyticReactorConcepts1-stagereactor(plasmaincontacttocatalyticsurfaces)Example:DPBreactorDielectricpropertiesofcatalystneedtofittoapplicationDirectinteractionofplasma&catalystenabled2-stagereactor(nodirectplasma–catalystcontact)Example:DBD&catalystelectricalconductivityofthecatalystdoesn’tplayarolecatalyticreactionsinitiatede.g.bymetastableintermediateproductsPlasmaActivationofCatalyticReactionsPhysical/chemicaleffectsGasheatingCatalystheatingElectricfieldsVibrationalexcitationDissociationIonizationUVradiationIntermediateproductformationApplicationofplasmacatalysisCatalyticNOXremovalCatalyticmethaneconversionCatalyticVOCsdegradationCatalyticCO2reductionCatalystpreparation,regenerationApplicationPlasmaPlasma-CatalyticHybridProcessesforGasCleaningPlasmainduced/enhancedselectivecatalyticNOx-reductionforautomotiveexhaustgases(SAG,Ford,Chrysler,LLNL,PNNL,...)oxidationduetodielectricbarrierdischarge(DBD)pretreatmentnon-preciousmetalcatalyst&urea/fuelasareducingagentNOx-reductioninanoxidizingatmospherecatalyticactivityextendedtolowtemperaturessulfurtolerantPlasmaassistedcatalyticoxidationofVOCsforthedecontaminationofgroundwaterofindustrialsites(IUT)extractionofVOCsfromgroundwaterbyairstripping:largegasflows,lowgastemperatureefficientplasmapre-treatmentusingdielectricbarrierdischargescatalyticoxidationoftheVOCsatlowtemperature(<100°C)Sorption&plasmaassistedoxidationofVOCsusingoxygenplasma(AIST)SorptionofVOCfromoff-gasRegenerationofcatalyticsorbentbyoxygenplasmaIncreasedefficiencycomparedtodirectplasma-catalyticconversionCH4+CO2=2CO+2H2Fig.

Plasma

reactor.1–gasinlet2–highvoltage electrode3–

discharge

gap4–electricheater5–grounded electrode6–gasoutletAThereagentactivationbyelectronimpactorbyreactionswithatomicandmolecularactivespecies:CH4

+

e

=

CH3

+

H

+

eCH4

+

e

=

CH2

+

H2

+

eCH4

+

e

=

CH

+

H2

+

H

+

eCO2

+

e

=

CO

+

O

+

eBFasthomogeneousreactionsproceedinthegasphasebetweentheshortlivingactivespeciesCReactionsinducedbythesolidsurfacestakeplaceonelectrodesurfacesandinsidethe

discharge

volume.ThreestagesofPlasmacatalytic

reactionAppliedCatalysisB:Environmental94(2010)19–26ApplicationofplasmacatalysisApplicationofplasmacatalysisPlasmacatalytichybridprocessescansuccessfullybeappliedfortheabatementofodorous&noxiouscompoundsfromoff-gasesHybridprocessesweredemonstrated(PlasmaenhancedselectivecatalyticNOxreduction;PlasmaenhancedcatalyticoxidationofHCs&VOCs)Goodenergyefficiency,lowoperationcostscanbeexpected.Plasma-catalytichybridreactor&powersupplyconceptsareavailableSorptionpropertiesofcatalystareimportantCatalystneedstobetailoredforapplication(intermediateproductgeneratedbyplasma,plasmaregenerationofcatalyst,...)ApplicationofplasmacatalysisElectroncollisiondissociationofoxygenRadicalattackofHCsOxidationofNONO-Reductione+O2→e+O+OO+C2H4→HCO+CH3+HCH3+O2

→CH3O2H+O2

→HO2RO2+NO→NO2+ROO+NO+M→NO2+MO+NO2

→NO+O2NO+N→N2+OO2+N

→NO+OAdvantagesandshortcomingsofplasmacatalysis等離子體催化技術的優點用該項技術處理有機廢氣具有以下優點：能耗低，可在室溫下與催化劑反應，無需加熱,極大地節約了能源;使用便利，設計時可以根據風量變化以及現場條件進行調節；不產生副產物，催化劑可選擇性地降解等離子體反應中所產生的副產物；不產生放射物;尤其適于處理有氣味及低濃度大風量的氣體。CatalystCatalysisToday,2002,72:173–184Agoodcatalystmusthavecertaincharacteristics:provideasufficientlyhighreactionrateunderthespecificreactiveconditions;sustainitsactivityoveralongperiodoftime;exhibitlowsensitivitytopoisons,suchassulfurcompounds;havegoodmechanicalstrength;showselectivity,itshouldacceleratethedesiredreactiononly;bereducedbeforeusesincemostofcatalystsareproducedasoxides.

Thereductionperiodshouldbeasshortaspossibleinordertoavoidadecreaseintheproductionefficiency.等離子體催化中的催化劑等離子體催化凈化技術采用的催化劑包括幾類：鐵電體材料；分子篩；貴金屬催化劑；金屬氧化物及其金屬鹽催化劑；光催化劑；多性能顆粒／混合物等。CatalystpreparationCatalysisToday,2002,72:173–184Maintrends:plasmachemicalsynthesisofultrafineparticlecatalysts;plasmaassisteddepositionofcatalyticallyactivecompoundsonvariouscarriers,especiallyplasmasprayingforthepreparationofsupportedcatalysts;plasmaenhancedpreparationorplasmamodificationofcatalysts.

Advantages:ahighlydistributedactivespecies;reducedenergyrequirements;enhancedcatalystactivation,selectivity,andlifetime;shortenedpreparationtime.Catalystpreparation,regenerationPlasma-chemicalsynthesisandregenerationofcatalystsCatalysisToday,2002,72:213–2211:electric-arcdcplasmatron;1a:thoriatedtungstencathode;1b:copperwater-cooledanode;1c:plasticadjustingring;2:CWPCR;3:quenchingdevice;4:copperwater-cooledsectionsforthequenchingdevice;5:powder-trappingchamber;6:filter;7:vibrationpowder-feeding;8:currentrectifier;9:flow-meters;10:bottleswithplasma-forming,powdercarryingandquenchinggases;T1:temperatureofinletwater;T2:temperatureofoutletwater.Schematicdrawingoftheplasma-chemicalinstallationforsynthesisandregenerationofcatalysts.Catalystpreparation,regenerationPlasma-chemicalsynthesisandregenerationofcatalystsCatalysisToday,2002,72:213–221Electronmicroscopephotographofsamplesphericalinshapeequivalentdiametersfellwithintherangeof10–30nmPlasma-chemicalsynthesisiseffectiveforregeneratingspentindustrialcatalysts.CatalysisToday,2002,72:173–184(a)glowdischarge;(b)microwaveplasma;(c)plasmaspraying.SchematicallyrepresentativesofelectrodeconfigurationsofdischargephenomenaappliedforcatalystpreparationDevicesforcatalystpreparationCommercialPlasmaTorchPlasmatorchinoperationPlasmaassisteddepositionofcatalyticallyactivecompoundsonvarioussupportsCatalysisToday,2002,72:173–184CatalystPlasmaMetalandmetaloxidesofFe,Ni,Cr,Cu,Au:supportedonasupportsPlasmaplatingFe–CrorFe–Cr–Ni/supportPlasmasprayedalloyCoandCuoxidesonAl2O3fibersheetPlasmavapordepositionPtsupportedonporousNi–CralloyThermalplasmasprayingDual-layerceramiccoatedsurfacePlasmadepositionHoneyPtsupportplasma,coatedwithPdPlasmadepositionCatalystforcombustion(Pt,Au)Plasmasprayingonametalsupportα-Al2O3andγ-Al2O3supportedonNi,Ti,Co–LaElectric-arcplasmatorchNicatalystforhydro-treatingHighfrequencyplasmaCo–FePlasmasprayingTable.IllustrativecatalystspreparedusingplasmasprayingCatalysisToday,2002,72:173–184ThermalplasmaColdplasmaPressureAtmosphericorhigherLow(<0.1bar)formostcasesAppearanceFilamentaryinhomogeneousHomogeneousTemperaturesHighelectrontemperatureHighelectrontemperatureHighgas(bulk)temperatureLowgastemperatureDe-excitationrateingasphaseHighLowTypesPlasmajet;dccoronatorch;arcGlow;radiofrequency;microwaveUsesUltra-fineparticlesspraying;sputteringModificationortreatmentofcatalystsurfaceTable.CharacteristicsofthermalandcoldplasmasusedforcatalystpreparationCatalystregenerationSurfaceofCatalysttreatedbydischargeplasmaCatalysisToday,2006,115:205–210Theschematicrepresentativeofsetupforglowdischargecatalysttreatment.CatalystregenerationSurfaceofCatalysttreatedbydischargeplasmaCatalysisToday,2006,115:205–210Thecatalystpreparedbyplasmatreatmentshowsanimprovedlow-temperatureactivityandanenhancedstability.Comparedtothereportedwork,methaneconversionovertheplasmatreatedcatalysthasaca.20%increaseatthesamereactiontemperature.Theplasmatreatedcatalystpossessesabetteranti-carbondepositperformanceforcarbondioxidereformingofmethane,comparedtothereportedwork.Theplasmatreatmentfollowedbycalcinationthermallyinducesagenerationofspecificnickelparticlesonthesupport.Highlydispersedmetalspecieswouldinducebetterlow-temper