催化剂表面碳物种及表征.docx
- 文档编号:8521369
- 上传时间:2023-01-31
- 格式:DOCX
- 页数:10
- 大小:371.38KB
催化剂表面碳物种及表征.docx
《催化剂表面碳物种及表征.docx》由会员分享,可在线阅读,更多相关《催化剂表面碳物种及表征.docx(10页珍藏版)》请在冰豆网上搜索。
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(K2O、MgO),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——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
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)afteritisseparatedfromoxygenandwaterinaGCcolumn;
∙detectionofCO,CO2andhydrocarbonaceouscompoundsaftermethanation;
∙quantificationofCO2withamassspectrometer;
∙monitoringtemperatureincrementaboveareferencesample,indifferentialthermalanalysis(DTA)equipment;
∙measurementofweightlossinthermalgravimetricanalysis(TGA)equipment.
∙detectionofCO2byFTIR
Temperature-ProgrammedHydrogenation(TPHy).
尤其适用于H2为反应物的催化反应——CharacterizationofcokebyTPHyisofspecialinterestwhenhydrogenisoneofthereactants,如:
∙reforming,
∙dehydrogenation
∙ methanolsynthesis
∙ Fischer-Tropschsynthesis
Temperature-ProgrammedGasification
与TPO相似,采用载气如He、Ar,也可以采用CO2代替O2
(2)ElectronMicroscopy
Thelocalization,natureandstructureofcokedepositshavebeenexaminedwithelectronmicroscopy。
Typically,theelectronmicroscopyalonedoesnotprovidemuchinformation,andisgenerallyusedincombinationwithrelatedspectroscopies
(3)ElectronEnergyLossSpectrocopy(EELS)
EELS能够提供的信息:
∙providesanalyticalandstructuralinformation,similartothatgivenbyX-rayabsorptionspectroscopy;
∙detectthelocationofthecoke(由于EELS具有高分辨率——1nm2);
∙providesqualitativeinformationregardingthetypeofcokepresentonthecatalyst(与参照样品对比).
局限性:
usedonlyinfewcasestocharacterizecokedepositsonheterogeneuscatalysts
(4)InfraredTechniques(FTIR,DRIFTS)
能够提供的信息:
∙obtainedwiththesetechniquesisthechemicalidentityofcompoundsthatformthecoke,suchasolefinic,saturatedoraromatic.
∙informationregardingthelocationofcokecanbeobtainedbyfollowingthesignalofcertaincatalystsurfacegroups,suchasBronstedOH.
∙observethedepositionofcarbonaceousmaterialsontheworkingcatalyst.
局限性:
limitedinformationonthenatureofcarbonaceousdepositsbecauseoftheircomplexityandofthedifficultytoassignunambiguouslyanIRbandtoparticularspecies。
波数归属情况
(5)LaserRamanSpectroscopy
ClassicalLaserRamanSpectroscopy(LRS)
提供信息:
provideinformationregardingcokestructure(pregraphiticorhighlyorganized)andontheaveragedimensionofthecrystallite,aslongasamonophasiccarbonisproduced
优点:
∙thehighsensitivitythatallowstheanalysisofcatalystswithlowcokecontent(0.3—0.5wt%)
∙thepossibilityoffollowingthegraphitizationofamorphouscarbon.
缺点:
∙结果解析困难;
∙surfacefluorescence干扰
UV-RamanSpectrometry(UV-RS).
解决了传统Raman的缺点
(6)DissolutionoftheSupportandSolventExtraction
原理:
afterthedissolutionofthesupportwithastrongacid,thecokeisextractedwithdifferentsolvents,然后采用GC、MS等进行分析洗脱物。
缺点:
thecokecouldbemodifiedduringthisprocedure
常采用的溶剂
(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.
∙1HNMR
alsousedtostudyHYzeolitescokedwithn-heptane
∙129XeNMR
Thistechniquehasbeenusedtostudycokelocationonzeolitecatalysts
∙29SiMASNMR.
(9)AugerElectronSpectroscopy(AES)
Augerelectronspectroscopy(AES)wasusedincombinationwithsecondaryionmassspectrometry(SIMS)todistinguishbetweenfourtypesofcarbonaceousdeposits,onmetalfoils(rhodium,iridiumandplatinum).Thefoilswerecokedbyexposingtoethyleneatlowpressure.Augerspectroscopycandistinguishbetweenmolecularorcarbidicontheonehand,andgraphiticoramorphouscarbonontheother.
(10)X-Raydiffraction(XRD)
CokestructurecanbecharacterizedbyX-Raydiffractionanalysis.Thistechniquemakesitpossibletodetermineifthereiscokewithcrystallinestructureonthecatalyst.
缺点:
thesensitivityofthistypeofdeterminationisratherlow,beingitdifficulttodeterminethefractionand/oramountofcokeinthecrystallineform.
(11)SecondaryIonMassSpectrometry(SIMS)
SIMSisamongthefewsurfacesensitivetechniqueswhicharecapableofdetectinghydrogencontentofthedeposits.IthasbeenusedincombinationwithAEStoanalysecokeonmetalfoils
缺点:
However,itwasnotpossibletoobtaintherealH/Cratioofthecarbondeposit
(12)SorptionCapacity:
SurfaceAreaandPoreVolume
原理:
Adsorptionmeasurementsallowthedeterminationofcokelocation.Whenthevolumeoccupiedbycokeismuchsmallerthanthevolumeinaccessibletoadsorbates,itmeansthatthereisaporeblockage.
缺点:
However,inmanycasestheadsorptionstudyiscarriedoutatadifferenttemperaturethanthereaction,andthereforediffusivitycouldbequitedifferent.Anotheraspectthatshouldbetakenintoaccountisthatifthepretreatmentforadsorptionmeasurementrequirestemperatureshigherthanthereactiontemperature,animportantfractionofcarbonaceousdepositscouldbestrippedoffthecatalystand,therefore,theporevolumemeasuredinthiswaywillbehigherthantheactualvolumeunderreactionconditions.
(13)X-RayPhoto-electronSpectroscopy(XPS)
(14)UltraViolet-VisibleSpectroscopy(UV-VIS)
TheUV-VISspectroscopycanbeusedtodeterminethechemicalidentityofthecokecomponentes.UV-VIS,unlikeNMRandIR,caneasilydetectalkylandalkenylcarbeniumions,essentiallyduetoitsmuchhighersensitivity55.Thisisusuallycarriedoutundervaccum,andtherefore,themorevolatilecompoundscouldbelostundertheseconditions.
(15)ElectronParamagneticResonance(EPR)
TheEPR(orESR)techniqueallowsthestudyoftheradicalsthataccompanythecokeformation,andthusestimateroughlytheamountofcokeandobtaininformationregardingitsnature.
Oneoftheadvantagesofthistechnique,isthatitcanbeusedbothunderstaticorunderon-streamconditions.
(16)CokeFormationRate
Theamountofcokethatisbeingdepositedonacatalysthasbeentraditionallyfollowedwithconventionalmicrobalances.However,duetotheinherentlimitationsofthisequipment,inwhichitisalmostimpossibletoavoidfeedby-passeffectanddiffusionaleffects,thistechniquehasnotbeenveryusefultodeterminecokingkineticsasafunctionoffeedcomposition.Arecycleelectrobalancereactorhasbeendesignedtoavoidthisundesirableeffect。
(17)其他
特殊表征手段,只针对某些特殊催化剂、反应有效
Someofthetechniqueslimitedtosinglecrystalsorpolycrystallinefoils,suchaslowenergyelectrondiffraction(LEED),Hescattering,coreelectronenergylossspectroscopy(CEELS),andmetastabledeactivationspectroscopy(MDS)
而通常情况下,催化剂若积炭后在烧炭过程中都会伴随重量的变化,鉴于热分析特别是热重法可以原位定量检测,所以采用热分析技术来研究催化剂的积炭行为不仅可行而且也十分方便。
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 催化剂 表面 物种 表征