催化剂表面碳物种及表征.docx
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催化剂表面碳物种及表征.docx
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催化剂表面碳物种及表征
何谓积碳
以含碳化合物为原料的催化反应的第一步通常是在催化剂表面上进行吸附形成含碳物种,如含碳物种经过分解、聚合等反应生成碳或焦炭沉积在催化剂表面,既所谓的积碳或结焦。
积碳会堵塞催化剂的活性位或催化剂的孔道,从而导
致催化剂的失活!
常发生积碳的催化反应
・催化裂化
・Pt重整
・加氢精制
*轻油制氢
积碳形态
积碳的形态一般分为无定形碳、层状石墨碳、管须状结晶碳以及粘稠状液态碳或焦油。
积碳原因
・热裂解
*催化裂解
*深度脱氢
・烯烃聚合
热裂解积碳是气相有机原料在高温下热裂解形成烟炱、焦油,它们在催化剂表
面上生成有序的或无序的碳;
催化积碳是在催化剂的作用下,烃类化合物发生催化积碳反应,它与催化剂的性质密切相关。
-氧化物、硫化物上主要是酸性积碳,积碳速率与催化剂的表面酸碱性有关;
*金属上的积碳是通过烃类深度脱氢和脱氢环化聚合产生积碳,金属颗粒大
小、分散度、合金化影响积碳;
*金属负载酸性载体催化剂的催化积碳则两者同时存在。
积碳防止
*合金化,如Pt重整催化剂中添加第二组分金属形成合金,可减缓催化剂的积碳;
*添加助剂,Pt重整催化剂采用少量Cl改性,可以减缓酸性积碳;
・利用载体活性组分的相互作用,降低积碳;
•对于酸性催化剂或载体,添加碱性助剂(K2OMgO,减弱酸性积碳
*通入水蒸气,消碳;
•添加其他元素如稀土,加速消碳,防止积碳。
积碳失活再生一一烧碳
催化剂因为积碳而失活,其催化剂再生通常采用烧碳的方法可以部分恢复催化剂的活性。
烧碳条件如气氛、温度、气体组分等都会影响到再生后催化剂的结构、物化性质以及催化性能,因此必须严加控制。
几条注意事项(三步骤):
1.除催化剂上的烃类和挥发物,必须在惰性气氛下进行;
2.除去硫和轻质碳,必须在低温250-350度下先使金属氧化除去硫,同时某些聚合物和轻质碳也随之除去;
3.最后除碳,根据碳的量以及类型在较高温度下进行。
积碳表征
催化剂积碳表征手段一览表
Technique
Application
RFTJR.DRIFTS
-Functionalgroupsofcoke(nature)Acidsitedeactivation
STEM-EELS
Cokelocadon
ElectronMicroscopy
Location,structure
TPOTPHyrJ*Hc
-morphology,kineticparameters,location
TGA
-cokereactivity
-volatilecompounds,oxygencontent
latticeoxygenreactivity
CakeExtraction
-CompositionofSolublefraction
-Coke{oration
LRSUV-RS
Structure(pregraphiticorhighlyorganized)Averagedimensionofcrystiillite(tnonophasiccarbon1
Oissoluiionof
Support
Cokecompositionandstructure
XRD
C〒、曽iMlitk?
strucliirt?
cokelocation
AES
Amountofcoke(approximately)卜fictionot<»morphous,graphiticVsctirbidic/nioleciiliir.
SIMS
Hydrogencontentofthecarbondeposits(qualitative)
Sorptioncapacity--cokelocation(poremouthorsitePorevolumeanalysisblocking)
(uncertaintyincokedensity)-microormesoporeblocicing
pCCPMAS-NMR
-typeofchemicalgroupsincokematurel
lHNMR
-HCdistributionofhydrogenincoke(aliphatic,aromatic,cte)
NMR
-Cokedistribution
^SiMASNMR
-Cokedistribution
XPS
cokelocation(inzeolites)
UV^VIS
Functionalgroupsof匚okedelectionofcarbeniuniions
Neutronscattcrinc
cokecontent andAttenuation C/H(requireslhe%Cbeknown) Cokedistribution(requiresadditionalexperimentalinformation) TEOM (microbalance) weight^ EPR CokeNature,radicalsduringcokefonnation 表征手段具体介绍 (1)Temperature-ProgrammedTechniques Temperature-ProgrammedOxidation(TPO)可获取信息: *providesdirectinformationregardingcokeoxidationrate *obtainusefulinformationsuchas: location,composition(hydrogen/carbonratio) andmorphology(highlydispersedormultidimensionalparticles). 具体如: *DeterminationofH/CRatio *DeterminationofCokeLocation. *DeterminationofOxidationKinetics *DeterminationofMorphology *DeterminationofCokeAmount 方法: «detectionofCO2byathermalconductivitydetector(TCD)afteritisseparated fromoxygenandwaterinaGCcolumn; «detectionofCO,CO2andhydrocarbonaceouscompoundsaftermethanation; «quantificationofCO2withamassspectrometer; «monitoringtemperatureincrementabovearefereneesample,in differentialthermalanalysis(DTA)equipment; «measurementofweightlossinthermalgravimetricanalysis(TGA)equipment. «detectionofCO2byFTIR Temperature-ProgrammedHydrogenation(TPHy). 尤其适用于H2为反应物的催化反应CharacterizationofcokebyTPHyisof specialinterestwhenhydrogenisoneofthereactants,如: «reforming, «dehydrogenation *methanolsynthesis *Fischer-Tropschsynthesis Temperature-ProgrammedGasification 与TPO相似,采用载气如HeAr,也可以采用CO2代替02 (2)ElectronMicroscopy Thelocalization,natureandstructureofcokedepositshavebeenexaminedwithelectronmicroscopy。 Typically,theelectronmicroscopyalonedoesnotprovidemuchinformation,andisgenerallyusedincombinationwithrelatedspectroscopies (3)ElectronEnergyLossSpectrocopy(EELS) EELS能够提供的信息: *providesanalyticalandstructuralinformation,similartothatgivenbyX-ray absorptionspectroscopy; *detectthelocationofthecoke(由于EELS具有高分辨率1nm2); *providesqualitativeinformationregardingthetypeofcokepresentonthe catalyst(与参照样品对比) 局限性: usedonlyinfewcasestocharacterizecokedepositsonheterogeneuscatalysts (4)InfraredTechniques(FTIR,DRIFTS) 能够提供的信息: «obtainedwiththesetechniquesisthechemicalidentityofcompoundsthatform thecoke,suchasolefinic,saturatedoraromatic. «informationregardingthelocationofcokecanbeobtainedbyfollowingthesignal ofcertaincatalystsurfacegroups,suchasBronstedOH. «observethedepositionofcarbonaceousmaterialsontheworkingcatalyst. 局限性: limitedinformationonthenatureofcarbonaceousdepositsbecauseoftheir complexityandofthedifficultytoassignunambiguouslyanIRbandtoparticularspecies 波数归属情况 BanJ(ctnr; (5)LaserRamanSpectroscopy ClassicalLaserRamanSpectroscopy(LRS) 提供信息: provideinformationregardingcokestructure(pregraphiticorhighlyorganized)andon theaveragedimensionofthecrystallite,aslongasamonophasiccarbonisproduced 优点: «thehighsensitivitythatallowstheanalysisofcatalystswithlowcokecontent (0.3—0.5wt%) «thepossibilityoffollowingthegraphitizationofamorphouscarbon. 缺点: •结果解析困难; *surfacefluorescenee干扰 UV-RamanSpectrometry(UV-RS). 解决了传统Rama的缺点 I4hie2JitJi/rttHband“卜心屮hfmh小时retdinedh\ifrtH\trbon^ Riimttnshift(cm'丿bandassiynmems 1605-3615 1360-1410 121XK121O 1545-3550 1483 Ringstretch^ofpolyamrnaticspevies Ringstrcichcsofpolyarcmuliespecies C<\irekhesorpolyaroni;uicsecies (6)DissolutionoftheSupportandSolventExtraction 原理: afterthedissolutionofthesupportwithastrongacid,thecokeisextractedwithdifferentsolvents,然后采用GCMS等进行分析洗脱物。 缺点: thecokecouldbemodifiedduringthisprocedure 常采用的溶剂 1Hc\ime 2HevmebenzeneIKS: I5i 3Chlorotbrm 4cI\lon)fbrm-dietIbIcther 5Diethylether-elhanol197: ^1 ■ 6MellianaE 7ChlorotornidhuiHi-lI^";3> 8Pyridinv-elhjnol SttiieniortnixtureElutetliwtipdtmds S: uunitedhydrocurhon^ XrtMiiLilii: hvdrocarbois ■r Pohraromnti-cs,non-Ki^ichdcrccAcks ■Monophench BisicnitroueiuuedhcLerocvcIe^; V■ Highly(unctionilmolecules JAMjphenols Molecule*;^iihahighcontenlo『兀;indO (7)NeutronScatteringandAttenuation 可提供信息: measurecokecontentandC/Hratio 优点: Thistechniquehastheadvantagethatthecokecontentandthereforethecokeprofilealongthecatalystbed,canbemeasuredin-situ. (8)NuclearMagneticResonance(NMR) 提供信息: *TheNMRtechniqueisapowerfultechniquetoinvestigatethenatureofcarbonaceousdeposits *detectbindingsbetweenaromaticrings,alkylfragments,andeventertiarycarbeniumion-likespecies. 缺点: TheinformationprovidedbyNMRisnotquantitative. 种类: *13CCP/MAS—NMR. Table4JJCCPM4SNMRsiipuilassHpiments SiiirtalIppm}Assitfitinenr 13 methylgroupconnectedwithsecondarycarbon,aliplialic 15 methylgroupboundtoparaffiniccarbon 19 methyltirouplinkedtanromatic 20 sccondar)curboiLalipluiic 21 methylgroupboundtoofciiniccurbon 23 CH2冲uptinkedtonieihylgroups SOand32 otherpnraNiT: icCH? groups 30 methylgrcupat ;ut>on 30-50 seconda[yandtertiaryparafliniccarbon 125 aroin(nicC-Hgroups L28 aronmiicC-Hgroups 130 sp2bondedcarbon 135 Eilkylaicdiiromiitics.carbonbridgC'ibetweenEiremalicring© 138-141 subshtuted^romiticcarbons,bridgedcarbonsincondensedurontiiiics,olefinicCHgroups 140-150 pi^kalkylat^djrOdiLLtic 160 C^Obonds,aromatic ・1HNMR alsousedtostudyHYzeolitescokedwithn-heptane *129XeNMR Thistechniquehasbeenusedtostudycokelocationonzeolitecatalysts ・29SiMASNMR. (9)AugerElectronSpectroscopy(AES) Augerelectronspectroscopy(AES)wasusedincombinationwithsecondaryionmass spectrometry(SIMS)todistinguishbetweenfourtypesofcarbonaceousdeposits,onmetal foils(rhodium,iridiumandplatinum).Thefoilswerecokedbyexposingtoethyleneatlowpressure.Augerspectroscopycandistinguishbetweenmolecularorcarbidiconthe onehand,andgraphiticoramorphouscarbonontheother. (10)X-Raydiffraction(XRD) CokestructurecanbecharacterizedbyX-Raydiffractionanalysis.Thistechniquemakes itpossibletodetermineifthereiscokewithcrystallinestructureonthecatalyst. 缺点: beingitdifficultto thesensitivityofthistypeofdeterminationisratherlowdeterminethefractionand/oramountofcokeinthecrystallineform. (11)SecondaryIonMassSpectrometry(SIMS) SIMSisamongthefewsurfacesensitivetechniqueswhicharecapableofdetectinghydrogencontentofthedeposits.IthasbeenusedincombinationwithAEStoanalysecokeonmetalfoils 缺点: However,itwasnotpossibletoobtaintherealH/Cratioofthecarbondeposit (12)SorptionCapacity: SurfaceAreaandPoreVolume 原理: Adsorptionmeasurementsallowthedeterminationofcokelocation.Whenthevolume occupiedbycokeismuchsmallerthanthevolumeinaccessibletoadsorbates,itmeansthatthereisaporeblockage. 缺点: However,inmanycasestheadsorptionstudyiscarriedoutatadifferenttemperaturethanthereaction,andthereforediffusivitycouldbequitedifferent.Anotheraspectthatshouldbetakenintoaccountisthatifthepretreatmentforadsorptionmeasurementrequirestemperatureshigherthanthereactiontemperature,animportantfractionofcarbonaceousdepositscouldbestrippedoffthecatalystand,therefore,theporevolumemeasuredinthiswaywillbehigherthantheactualvolumeunderreactionconditions. (13
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- 关 键 词:
- 催化剂 表面 物种 表征