催化劑表面碳物種及表征

1、何謂積碳以含碳化合物為原料的催化反應(yīng)的第一步通常是在催化劑表面上進行吸附形成含碳物種，如含碳物種經(jīng)過分解、聚合等反應(yīng)生成碳或焦炭沉積在催化劑表面，既所謂的積碳或結(jié)焦。積碳會堵塞催化劑的活性位或催化劑的孔道，從而導(dǎo)致催化劑的失活！常發(fā)生積碳的催化反應(yīng)催化裂化Pt重整加氫精制輕油制氫積碳形態(tài)積碳的形態(tài)一般分為無定形碳、層狀石墨碳、管須狀結(jié)晶碳以及粘稠狀液態(tài)碳或焦油。積碳原因熱裂解催化裂解深度脫氫烯烴聚合熱裂解積碳是氣相有機原料在高溫下熱裂解形成煙炱、焦油，它們在催化劑表面上生成有序的或無序的碳；催化積碳是在催化劑的作用下，烴類化合物發(fā)生催化積碳反應(yīng)，它與催化劑的性質(zhì)密切相關(guān)。氧化物、硫化物上主要是

2、酸性積碳，積碳速率與催化劑的表面酸堿性有關(guān)；金屬上的積碳是通過烴類深度脫氫和脫氫環(huán)化聚合產(chǎn)生積碳，金屬顆粒大小、分散度、合金化影響積碳；金屬負載酸性載體催化劑的催化積碳則兩者同時存在。積碳防止合金化，如Pt重整催化劑中添加第二組分金屬形成合金，可減緩催化劑的積碳；添加助劑，Pt重整催化劑采用少量Cl改性，可以減緩酸性積碳；利用載體活性組分的相互作用，降低積碳；對于酸性催化劑或載體，添加堿性助劑（K20、MgO），減弱酸性積碳通入水蒸氣，消碳；添加其他元素如稀土，加速消碳，防止積碳。積碳失活再生燒碳催化劑因為積碳而失活，其催化劑再生通常采用燒碳的方法可以部分恢復(fù)催化劑的活性。燒碳條件如氣氛、溫度

3、、氣體組分等都會影響到再生后催化劑的結(jié)構(gòu)、物化性質(zhì)以及催化性能，因此必須嚴(yán)加控制。幾條注意事項（三步驟）：除催化劑上的烴類和揮發(fā)物，必須在惰性氣氛下進行；除去硫和輕質(zhì)碳，必須在低溫250-350度下先使金屬氧化除去硫，同時某些聚合物和輕質(zhì)碳也隨之除去；最后除碳，根據(jù)碳的量以及類型在較高溫度下進行。積碳表征催化劑積碳表征手段一覽表TeeimiqueAppircatimiIR,FT-IR.DRIFTS-Fundionulgroupsofcoke(ntdure)AcidsitedeactivationSThM-EELSCokelocationElectronMicroscopyLocation,st

4、ructureTPOTPHyTPHc-morphology,kineticparameterlocationTGA-cokereactivity-volatil亡compcnmd翼oxygencontent,latticeoxygenreactivityCokeExtriictioii-CompositionofSolublefraction-CoklocationLRSUV-RSStructuiepregrttphitieorhighlyorganized)AveragedimensionofcrystaBite(tnonophasiccarbonJDissolutioaofSupportC

5、okecompositionandstructureXRDCrysOLlirjestructurecokelocationAESAinonn：ofcoke(approximatety)FractionofamorphousgraphiticVscarbidic/moleculanSIMSHydrogen.contentofthecarbondeposits(qualitative)Sorptioncapacity-cokelocation(poremouthorsiteporevolumeaniysisblockingl(uncertaintyincokedensity)-microormes

6、oporeblockingrCCPMAS-NMR-typeofchemicalgroupsincoRe(nature)lHNMR-HCdistribution,ofhvdrogenintokcti9XeNMR沔iMASNMRXPSUV-VIS(aliphatic,aromatic,etc)-Cokedistribution-Cokedistributioncokelocationinzeolites!FunctionalgroupsofcokedeiectionofcarbciiiuiTiionsNeutronscatteringandAttenuationcokecontentreqnire

7、stlieCHbeknown)C/H(requiresthe%Cbeknown)Cokedistribution(requiresadditionalexjwrirneiiiiiliftforttiatiori)TEOM(microbalanceEPRweigtitgainCokeNature,radicalsduringcokelorniation表征手段具體介紹Temperature-ProgrammedTechniquesTemperature-ProgrammedOxidation(TPO)可獲取信息：providesdirectinformationregardingcokeoxid

8、ationrateobtainusefulinformationsuchas:location,composition(hydrogen/carbonratio)andmorphology(highlydispersedormultidimensionalparticles).具體如：DeterminationofH/CRatioDeterminationofCokeLocation.DeterminationofOxidationKineticsDeterminationofMorphologyDeterminationofCokeAmount方法：detectionofCO2byather

9、malconductivitydetector(TCD)afteritisseparatedfromoxygenandwaterinaGCcolumn;detectionofCO,CO2andhydrocarbonaceouscompoundsaftermethanation;quantificationofCO2withamassspectrometer;monitoringtemperatureincrementaboveareferencesample,indifferentialthermalanalysis(DTA)equipment;measurementofweightlossi

10、nthermalgravimetricanalysis(TGA)equipment.detectionofCO2byFTIRTemperature-ProgrammedHydrogenation(TPHy).尤其適用于H2為反應(yīng)物的催化反應(yīng)一-CharacterizationofcokebyTPHyisofspecialinterestwhenhydrogenisoneofthereactants,如：reforming,dehydrogenationmethanolsynthesisFischer-TropschsynthesisTemperature-ProgrammedGasificat

11、ion與TPO相似，采用載氣如He、Ar,也可以采用CO2代替O2ElectronMicroscopyThelocalization,natureandstructureofcokedepositshavebeenexaminedwithelectronmicroscopy。Typically,theelectronmicroscopyalonedoesnotprovidemuchinformation,andisgenerallyusedincombinationwithrelatedspectroscopiesElectronEnergyLossSpectrocopy(EELS)EELS能

12、夠提供的信息：providesanalyticalandstructuralinformation,similartothatgivenbyX-rayabsorptionspectroscopy;detectthelocationofthecoke(由于EELS具有高分辨率1nm2);providesqualitativeinformationregardingthetypeofcokepresentonthecatalyst(與參照樣品對比).局限性：usedonlyinfewcasestocharacterizecokedepositsonheterogeneuscatalystsInfr

13、aredTechniques(FTIR,DRIFTS)能夠提供的信息：obtainedwiththesetechniquesisthechemicalidentityofcompoundsthatformthecoke,suchasolefinic,rmationregardingthelocationofcokecanbeobtainedbyfollowingthesignalofcertaincatalystsurfacegroups,suchasBronstedOH.observethedepositionofcarbonaceousmat

14、erialsontheworkingcatalyst.局限性：limitedinformationonthenatureofcarbonaceousdepositsbecauseoftheircomplexityandofthedifficultytoassignunambiguouslyanIRbandtoparticularspecies。波數(shù)歸屬情況氏沁衣、用1)伽IgnttK笊740-760Mono-siibstiti!tcdarcmatics790-840CHdeformationinalkenes10361045vCCCH?inclirbutenesiCbondnIMOlinear

15、butenes120WiggingCH2longchuiipnratlins1336waggingCHiC4=dimeric1370-1380bumchedchainoligonrtrs1385aromatics-卩isCH3139()olefinicspecies1410linearbutenes1420.1430linearbutenes/ft=CHvinyl1437CH?asymmetricdeforinaiion(attachedtoaromatics1450/1463aromaticskeletalringbreathing5CH51471CHjdeformation(attache

16、dtouromatics)15131524yC=Caromatics1583.159YC=Caroniatics；polyaromatics1589niycrocristiillinepolycycliciiromatics1615.1617；C=Caroniatios；polyaroniatici1622C1=Cstretchniginbutenes1630.1625Cbondtt/oieiinicspecies2865saturatedCHStretching2925saturatedCHstretching2960saturatedCHpolyoleins十a(chǎn)fbraatic2970/2

17、990saturatedCH5tretchingCHolcii11itspecies3045/305SvCHaromatic5)LaserRamanSpectroscopyClassicalLaserRamanSpectroscopy(LRS)提供信息：provideinformationregardingcokestructure(pregraphiticorhighlyorganized)andontheaveragedimensionofthecrystallite,aslongasamonophasiccarbonisproduced優(yōu)點：thehighsensitivitythata

18、llowstheanalysisofcatalystswithlowcokecontent(0.30.5wt%)thepossibilityoffollowingthegraphitizationofamorphouscarbon.缺點：結(jié)果解析困難；surfacefluorescence干擾UV-RamanSpectrometry(UV-RS).解決了傳統(tǒng)Raman的缺點Table2Ranntabuihlretaiucdliy(lrairbonsRanidnshift(cmRtiinaubandasigtrnieiilx1605-1615!36O-14iO1200-12101545-1550

19、1483RingrichesofpolypiroinulkspecieRingstreTchesofpolyarnmalicspeciesC-CstretchesofpolyaromalicspeciesCCstretchesOfconjugatedolefinsIn-phaseC=CstreichofcyclopcinadicnlspeciesDissolutionoftheSupportandSolventExtraction原理：afterthedissolutionofthesupportwithastrongacid,thecokeisextractedwithdifferentso

20、lvents,然后采用GC、MS等進行分析洗脫物。缺點：thecokecouldbemodifiedduringthisprocedure常采用的溶劑Tabk3Cokejiinctiomilitymuls&hcutaffinityrrrjfjj.xfwrf(ftfrtpfnnids1Hesatie1Henaiie-benzene(E5i15)ChlorGrormChloroform-dietliylether(90:10)Dteihyletier-eth；H)(il73i7Chiorofami-ethanolFyridine-ethatliplS：nttrtdhvdracarbonArotii

21、aLicJiydrocsrbonsPohiTiironiaik.norr-biisich-ctcrocyctcMonophenolsllasicnitrogpnatedheterocyclesHigldyfundioiialnioi-cculesPolyphenolsMoleculeswith址highcontentofNandONeutronScatteringandAttenuation可提供信息：measurecokecontentandC/Hratio優(yōu)點：Thistechniquehastheadvantagethatthecokecontentandthereforethecoke

22、profilealongthecatalystbed,canbemeasuredin-situ.NuclearMagneticResonance(NMR)提供信息：TheNMRtechniqueisapowerfultechniquetoinvestigatethenatureofcarbonaceousdepositsdetectbindingsbetweenaromaticrings,alkylfragments,andeventertiarycarbeniumion-likespecies.缺點：TheinformationprovidedbyNMRisnotquantitative.種

23、類：13CCP/MAS-NMR.1920Talk4t3CCPMASNMRsignalussiLfnHrentsSitfuaifppNiAssi(fninenrmethylgroupconnectedwithsecondarycarbon,aliphaticmethylgroupboundtop-aralliniccarbonHHthylgrcupin3dto;iroirbrt!iesecondarycarbon,aliphaticmethylgroupbmmdtoMcfiniGc鳳bonCHZgrouplinkedtomethylgroupsotherparaHiiiicCH；groupsme

24、thylgrvupalquaternarycarbonsecondaryandtertiaryparaffinicCcirbonaromaticC-Hgroupsaromiiii匚C-Hgroupssp2bendedcarbonEtlkylinedaroniLitics,carbonbridgesbiwccnaiv)maticEingsxubstiimcdamtnaticcarbons,bridgedciirbonsiticondensedaromLitics.olcfiincCHgroupspoly毀11;、ibOinnt沁CObondaroniLiric30and323030-50125L

25、28130B513841140-501601HNMRalsousedtostudyHYzeolitescokedwithn-heptane129XeNMRThistechniquehasbeenusedtostudycokelocationonzeolitecatalysts29SiMASNMR.AugerElectronSpectroscopy(AES)Augerelectronspectroscopy(AES)wasusedincombinationwithsecondaryionmassspectrometry(SIMS)todistinguishbetweenfourtypesofca

26、rbonaceousdeposits,onmetalfoils(rhodium,iridiumandplatinum).Thefoilswerecokedbyexposingtoethyleneatlowpressure.Augerspectroscopycandistinguishbetweenmolecularorcarbidicontheonehand,andgraphiticoramorphouscarbonontheother.X-Raydiffraction(XRD)CokestructurecanbecharacterizedbyX-Raydiffractionanalysis.

27、Thistechniquemakesitpossibletodetermineifthereiscokewithcrystallinestructureonthecatalyst.缺點：thesensitivityofthistypeofdeterminationisratherlow，beingitdifficulttodeterminethefractionand/oramountofcokeinthecrystallineform.SecondaryIonMassSpectrometry(SIMS)SIMSisamongthefewsurfacesensitivetechniqueswh

28、icharecapableofdetectinghydrogencontentofthedeposits.IthasbeenusedincombinationwithAEStoanalysecokeonmetalfoils缺點：However,itwasnotpossibletoobtaintherealH/CratioofthecarbondepositSorptionCapacity:SurfaceAreaandPoreVolume原理：Adsorptionmeasurementsallowthedeterminationofcokelocation.Whenthevolumeoccupi

29、edbycokeismuchsmallerthanthevolumeinaccessibletoadsorbates,itmeansthatthereisaporeblockage.缺點：However,inmanycasestheadsorptionstudyiscarriedoutatadifferenttemperaturethanthereaction,andthereforediffusivitycouldbequitedifferent.Anotheraspectthatshouldbetakenintoaccountisthatifthepretreatmentforadsorp

30、tionmeasurementrequirestemperatureshigherthanthereactiontemperature,animportantfractionofcarbonaceousdepositscouldbestrippedoffthecatalystand,therefore,theporevolumemeasuredinthiswaywillbehigherthantheactualvolumeunderreactionconditions.X-RayPhoto-electronSpectroscopy(XPS)UltraViolet-VisibleSpectroscopy(UV-VIS)TheUV-VISspectroscopycanbeusedtodeterminethechemicalidentityofthecokecomponentes.UV-VIS,unlikeNMRandIR,caneasilydetectalkylandalkenylcarbeniumions,essentiallyduetoitsmuchhighersensitivity55.Thisisusuallycarriedoutundervaccum,andtherefore,themorevolatilecompoundscould