第5章-气固界面-1吸附等温式.ppt

第五章 气固界面 李硕琦山东科技大学材料学院2015 4 8Thegas solidinterfaces adsorption catalysis 8 1Introduction引言 Asolidsurfaceincontactwithgasusuallyattractsanadsorbedlayerofgasmolecules Whenadsorptionoccurstheprocessisspontaneous whichmeansthefreeenergychange G isnegative Inmostcasesthereisalsoadecreaseinentropy 熵 S becausethegaslosesdegreesoffreedom Hence toensuethat Gisnegative theenthalpy 焓 H negativeandlargeenoughtooutweightheentropyterm 固体表面和气体接触时会吸附一层气体分子 如果该过程是自发的 即意味着 G是负的 而此时 S下降 所以 H一定为负 而且超过 S项 G H T S 8 1Introduction引言 Theadsorptionofgasesplaysanimportantroleinmanyprocesser inparticularheterogeneouscatalysis 异相催化 气体的吸附在很多进程中其重要作用 尤其是异相催化 8 1Introduction引言 Itistheweakeningorbreakingofchemicalbondsthatoccursuponadsorptionthatmakessomesolidsurfaceseffectivecatalysts 催化剂 forgasphasereaction 吸附的同时发生化学键的减弱或者加强 导致某些固体成为气相反应的催化剂 8 2Physicalandchemicaladsorption Theadsorptionofagas oftencalledtheadsorbate吸附质 onasolidsurface oftencalledtheadsorbent吸附剂 mayinvolveonlyphysicalinteractionsduetovanderWaalsforce ortheremaybeachemicalinteractionbetweenthesolidandthegas 吸附质和吸附剂之间可能存在范德华力的物理相互作用 或者化学相互作用 Activatedcarbonisusedasanadsorbent 8 2Physicalandchemicaladsorption Thesetwoprocessesarereferredtorespectivelyasphysicaladsorptionandchemicaladsorption Thedistinctionbetweenthemisveryimportantastheadsorptionprocessesandthebehavioroftheadsorbedlayersarequitedifferent 物理吸附化学吸附 Activatedcarbonisusedasanadsorbent 8 2Physicalandchemicaladsorption Physicaladsorption becauseinvolvesonlytheforcesofmolecularinteraction isverysimilartothecondensationofavaportoformaliquid 物理吸附只涉及分子间力 所以和气体的凝聚类似 8 2Physicalandchemicaladsorption Theenthalpy 焓变 ofthephysicaladsorptionisroughly 20kJ mol thesameastheenthalpyofcondensationandinmanywaystheadsorbedmaterials especiallywhenmanylayershavebeenadsorbed behaveslikeatwodimensionalliquid 物理吸附的焓变大约是20kJ mol 和凝聚的焓变相同 8 2Physicalandchemicaladsorption Inchemisorption ontheotherhand achemicalbondisformed Consequentlytheenthalpiesoftheadsorptionaremuchgreater 80kJ mol thanforphysicaladsorption Thereareoccasionalexceptions e g forthechemisorptionofhydrogenonglasstheenthalpyisonlyabout12kJ mol 化学吸附存在化学键的形成 其焓变大得多 80kJ mol 不过也有例外 8 2Physicalandchemicaladsorption Furthermore theadsorbedatomsarelocalizedatparticularsitesonthesolidsurfaceandonlyonelayerofadsorbatemaybechemisorbed 化学吸附的位点是特定的 吸附的层数只能为一 Fig 3OptimizedstructureofPt4 Al2O3system a sideview b topview RyotaIshimoto ChanghoJung HideyukiTsuboi MichihisaKoyama AkiraEndou MomojiKubo CarlosA DelCa Periodicdensityfunctionalandtight bindingquantumchemicalmoleculardynamicsstudyofcatalyticpropertieson Al2O3supportedPtcatalysts AppliedCatalysisA General Volume305 Issue1 2006 64 69 http dx doi org 10 1016 j apcata 2006 02 049 氧化铝表面的P4催化剂 Fig 4CalculationmodelsofNOadsorptiononPt4 Al2O3 a on topsite b bridgesite and c three foldsite RyotaIshimoto ChanghoJung HideyukiTsuboi MichihisaKoyama AkiraEndou MomojiKubo CarlosA DelCa Periodicdensityfunctionalandtight bindingquantumchemicalmoleculardynamicsstudyofcatalyticpropertieson Al2O3supportedPtcatalysts AppliedCatalysisA General Volume305 Issue1 2006 64 69 http dx doi org 10 1016 j apcata 2006 02 049 NO在P4上的三种化学吸附的形式 Thecalculationsforthesupportedsystemclarifythatthe Al2O3supportincreasesNOactivationforthree foldsites whileitdecreasesitforon topandbridgesites 8 2PhysicalandchemicaladsorptionComparingphysicalandchemicaladsorption Incomparingthecharacteristicofphysicalandchemicaladsorptionitisusefultousetheanalogieswithcondensationandchemicalreaction Itwillbeseenthatnosinglecharacteristiccanbeusedtodistinguishthesetwotypesofadsorptionasthereareoftenexceptions soseveralcharacteristicsshouldbeexamined 8 2PhysicalandchemicaladsorptionComparingphysicalandchemicaladsorption Enthalpyofadsorption吸附焓变Theenthalpyofphysicaladsorptionisusuallynogreaterthan20kJmol 1 whereastheenthalpyofchemisorptionisusuallymuchgreaterthanandisrarelylessthan80kJmol 1 物理吸附的焓变不超过20kJmol 1 但是化学吸附的焓变很少低于80kJmol 1 8 2PhysicalandchemicaladsorptionComparingphysicalandchemicaladsorption Specificity特异性Aschemicaladsorptioninvolvesachemicalbetweengasmoleculesandgroupsonthesurfaceofthesoliditishighlyspecific Physicaladsorption ontheotherhand andoccurswithanygas solidcombinationundertheappropriateconditionsoftemperatureandpressure 化学吸附因为存在化学反应而有特异性 但物理吸附则只要有合适的温度和压强即可发生 8 2PhysicalandchemicaladsorptionComparingphysicalandchemicaladsorption Reversibility可逆性Physicaladsorptioncanbereversedbysimplereducingthegaspressureandusuallywithoutraisingofthetemperature 物理吸附在降压甚至不用升温的条件下即可逆转 8 2PhysicalandchemicaladsorptionComparingphysicalandchemicaladsorption Thicknessoftheadsorbedlayer吸附层的厚度Chemicaladsorptionislimitedtooneadsorbedlayer butinphysicaladsorptionthereareusuallyseverallayersadsorbed 化学吸附只有单层分子 但是物理吸附可以是多层吸附 Physicaladsorptionontopofachemisorbedlayerispossibleiftheconditionsareappropriate 化学吸附层上的物理吸附是可以的 8 2PhysicalandchemicaladsorptionComparingphysicalandchemicaladsorption Conditionsofadsorption吸附的条件Theexperimentalconditionsfavoringphysicaladsorptionandchemicaladsorptionaresimilartothosefavouringcondensationandchemicalreaction respectively forphysicaladsorption lowtemperature andpressuresclosetothoseforliquefaction forchemisorption hightemperaturesandawiderangeofpressures 8 3Measurementofgasadsorption气体吸附量的测量 Gasadsorptionisusuallymeasuredbyoneoftwobroadmethods observingthedecreaseintheamountofgaseousadsorbate 吸附质 presentafterexposureofthegastothesolid ortheuseofasensitivemicrobalancetomeasuretheincreaseinweightofthesolidafterexposuretothegas 两种测量吸附量的方法 测定吸附前后吸附质量的减少 或用微天平测定吸附后吸附剂质量的增加 8 3Measurementofgasadsorption气体吸附量的测量 Ineachcase theamountofgasadsorbedisplottedasafunctionofequilibriumgaspressureas 吸附等温线 将气体吸附量和气压做曲线 得到的就是吸附等温线 8 3 Measurementofgasadsorption Routine automatedmeasurementofgasadsorptionisusuallyperformedwiththehelpofaflowsystem wheretheadsorbategasismixedwithadiluentandchangesinconcentrationofthegasmixtureoccurasthegasisadsorbedonthesurface Anexamplecommonlyusedisamixtureofnitrogeninhelium 测量氮气浓度的变化 直读动态流动法比表面积分析仪基于直读动态流动法比表面积分析仪是美国康塔仪器公司 Quantachrome 为快速比表面测定设计的直读动态比表面积分析仪 可在6分钟内完成比表面测试 8 3 Measurementofgasadsorption 以He作为载气 N2为被吸附气体 当外界温度降低到氮气沸点温度 195 8 即氮气的相变温度时 氮分子能量降低 在范德华力作用下被固体表面吸附 达到动态平衡 形成近似于单分子层的状态 当混气中氮气的分压在B E T公式要求的0 05 0 35范围内时 固体对氮分子的吸附量与其总比表面积成线性关系 即可以用被吸附氮气量来定量表征固体的总表面积 由于氮分子直径相对于固体的各种物理表面结构及物理空隙形态都足够小 其能充分的布满及进入固体的各种物理结构形态中 所以能准确而全面的反映固体表面积大小 被吸附氮气量的确定由热导池检测器及数据采集处理系统来完成 热导池检测器可以通过检测混合气体热导率的变化而确定混气中气体比例的变化 8 3 Measurementofgasadsorption 被吸附氮气量的确定由热导池检测器及数据采集处理系统来完成 热导池检测器可以通过检测混合气体热导率的变化而确定混气中气体比例的变化 8 3Measurementofgasadsorption 3H 2000 型全自动氮吸附比表面积测试仪 8 3Measurementofgasadsorption 仪器型号 ASAP2020型购置日期 2009 8 3国别厂家 美国Micromeritics公司金额 403710元 8 3Measurementofgasadsorption AUTOSORB 1C化学物理吸附分析仪 8 3Measurementofgasadsorption Amicrobalanceisjustaverysensitiveweighingdevice usuallycomprisingamodernformoftheclassicbeambalance 杠杆式天平 Microbalancecanbesensitivetoweightdifferenceofnanograms Withsuchextremesensitivityitispossibletomeasuretheweightchangecausedbytheadsorptionofasinglemonolayeronasolidifthesurfaceareaislarge IGA智能重量分析仪IntelligentGravimetricAnalysersHiden公司生产的IGA系列全自动重量分析系统代表着目前世界重量法吸附仪的最高水准 Hiden率先将电子微量天平引入全自动的吸附科学研究系统 在设计上不采用任何折中办法就能适应较宽范围材料的表征 BEAConfidential 32 8 4Adsorptionisotherms 吸附等温线 博鲁瑙尔匈牙利犹太人1921年赴美读书二战时候中校军衔Uponhisdeath theAmericanChemicalSocietycalledhim undoubtedlyoneofthemosthighlyrespectedsurfacescientistsofourtime 横坐标 压强 纵坐标 吸附量 8 4Adsorptionisotherms Oftenthepressureisreferenced 参考 tothesaturationpressure p0 attheselectedtemperature Thesaturationpressureisthegaspressureatwhichcondensationwouldcommence inotherwords theequilibriumvapourpressureoftheliquefied 液化 attheselectedtemperature 在选定温度下 一般压强会和饱和压强做比 饱和压强是冷凝开始的压强 或者可以说 是选定温度下蒸气开始液化的压强 8 4Adsorptionisotherms Itiscommontousetherelativepressure whichistheratiooftheactualgaspressuretothesaturationpressure p p0 astheindependentvariableinadsorptionisothersms 通常使用的就是相对压强 Fig 1Arepresentativenitrogenisothermforthecharreddolomitesorbentsinvestigated 1hat800 C A Duffy G M Walker S J Allen Investigationsontheadsorptionofacidicgasesusingactivateddolomite ChemicalEngineeringJournal Volume117 Issue3 2006 239 244 http dx doi org 10 1016 j cej 2005 11 016 氮气在白云石吸附剂上的吸附等温线 8 4Adsorptionisotherms Understandingofadsorptionatthegas solidinterfacewasgreatlyenhancedwhenBrunauer 1940 realizedthatthenumerousisothermsthathadbeenreportedtothattimecouldbeclassifiedbytheirshapesintofivegroups TheseshapesaretermedTypeI TypeII TypeV 博鲁瑙尔发现所有吸附等温线一共分为五类 8 4Adsorptionisotherms 吸附等温线 Fig 1TheIUPACclassificationforadsorptionisotherms TypesI VI withthex axisrepresentingtherelativepressureofthefluidandthey axisdenotingamountofgasadsorbed OnumaCarmody RayFrost YunfeiXi SergeKokot Surfacecharacterisationofselectedsorbentmaterialsforcommonhydrocarbonfuels SurfaceScience Volume601 Issue9 2007 2066 2076 http dx doi org 10 1016 j susc 2007 03 004 8 4Adsorptionisotherms ThetypeIisothermsareformedwhenthegasischemisorbedandlimitedtoonelayerofcoverage I型的吸附等温线对应的化学吸附的单层情况 Fig 8AdsorptionisothermsofCO2onH STIatvarioustemperatures JunLi JinQiu YaojunSun YingcaiLong龙英才 StudiesonnaturalSTIzeolite modification structure adsorptionandcatalysis MicroporousandMesoporousMaterials Volume37 Issue3 2000 365 378 http dx doi org 10 1016 S1387 1811 99 00282 6 CO2在H HSI沸石上的吸附等温线 8 4Adsorptionisotherms Theotherfourtypesareconsideredtoderivefromphysicaladsorption 其他4种类型包含物理吸附 8 4Adsorptionisotherms TypeIIandIVshowsteeprisesinadsorptionatlowgaspressuressuggestingahighaffinitybetweengasandsolid whereasTyepIIIandVexhibitlowadsorptionatlowgaspressuresindicatingrelativelyweakaffinitybetweengasandsolid II型IV型在低压时上升很快 说明气体和固体之间吸附较强 III型V型在低压时吸附量低 说明气体和固体之间吸附较小 Fig 1Idealizedadsorptionisotherms Thesefiveisothermtypesarerepresentativeofthoseexperimentallyobtainedforvariousgasesandadsorbentsandpublishedintheliterature Vadsistheamountofgasadsorbedatagiven GordonLJendrasiak RalphLSmith Theeffectofthecholineheadgrouponphospholipidhydration ChemistryandPhysicsofLipids Volume113 Issues1 2 2001 55 66 http dx doi org 10 1016 S0009 3084 01 00174 8 吸附强 吸附弱 8 4Adsorptionisotherms Atverylowgaspressuresalloftheisothermstypesapproximatetoastraightline Asthisbehaviorisreminiscent remi nisnt 怀旧的 ofHenry slaw solubilityofagasisproportionaltoitspressure thissectionoftheisothermsisoftencalledtheHenry slawregion 亨利定律区域 Thisfigureshowsaseriesofisothermsforvariousgasesat25C ExceptforCO2 allthegasesremainintheHenry sLawregimeupto13bar 8 4Adsorptionisotherms 对吸附等温线的研究的落脚点 是对催化剂 一般是多孔材料 对气体吸附性能和比表面积性质的研究 最终找到合适的 有效的催化剂 Fig 2SurfaceSEMimagesofPVDF PEO x ZSM 5compositemicroporousmembranes a x 5 b x 10 c x 20 d x 30 SolidStateIonics Volume177 Issues7 8 2006 709 713 Fig 1TEMimagesof a calcinedSBA 15recordedalongthe 001 zoneaxisand b calcined 3 Fe SBA 15recordedalongthe 110 zoneaxis MicroporousandMesoporousMaterials Volume86 Issues1 3 2005 335 340 8 5Adsorptionequations吸附等温式 Therehavebeenanumberofattemptstoderiveanequationthatwillfittheexperimentaladsorptionisothermsandseveralofthesehavebeenreasonablysuccessful However theabilityofanequationtofittheshapeofanexperimentalisothermdoesnotprovideasensitivetestforthemodelonwhichtheequationwasbased 下一步的努力就是用数学公式为吸附等温线建模 Amorestringent 严格的 testistocomparetheenthalpyofadsorption 吸附焓变 deducedfromtheisothermsusingtheequationwiththevaluemeasuredcalorimetrically 如果能用公式计算处吸附焓变 则更为严格 8 5Adsorptionequations Singlelayeradsorption Langmuir 1918 Multilayeradsorption Brunauer EmmettandTeller 1938 从右到左 8 5AdsorptionequationsTheLangmuirequation朗格缪尔吸附等温式 TheLangmuirequationisbasedonachemisorptionmodelinthattheadsorptionislimitedtoasingleadsorbedmonolayeronthesolidsurface 单层化学吸附层 Theequationis Wheren istheamountadsorbed n mistheamountrequiredforcompletemonolayercoverageofthesurface themonolayercapacity and isdefinedby E1enthalpyofdesorption解吸附焓变 8 5Adsorptionequations Theequationcanberearrangedintoalinearform Thus byplottingexperimentalvaluesofp n againstpastraightlineshouldbeobtainedfromwhichthevaluesof andn m 斜率 themonolayercapacity canbecalculated 通过p n 和p可做直线 从而可以得到of 和n m 最终可以得到单分子层的容量 8 5Adsorptionequations ApplicationoftheLangmuirequationPlotsoftheLangmuirequationareshowninFigure8 5ItisclearthattheequationgeneratescurvesthatfittheTypeIshapeandthattheadsorptionreachesalimitwhenonecompletemonolayerhasbeenadsorbed 8 5Adsorptionequations Increasinginthevalueof andhenceinthevalueofE1 showincreasingsteepnessintheisothermsatlowpressuresreflectingincreasingaffinitybetweenadsorbateandadsorbent 8 5Adsorptionequations Fordatathatfitthelinearformoftheequation 8 3 valuesofthemonolayercapacitycanreadilybeobtained Ifthesizeoftheadsorbatemoleculesisknownthevalueofn myieldsanestimateofthesurfaceareaofthesolid 利用单分子层容量和吸附质分子的体积计算固体的表面积 BEAConfidential 54 BEAConfidential 55 BEAConfidential 56 SEM图片 BEAConfidential 57 BEAConfidential 58 BEAConfidential 59 BEAConfidential 60 BEAConfidential 61 BEAConfidential 62 BEAConfidential 63 吸附等温线 BEAConfidential 64 直线形式 BEAConfidential 65 8 5AdsorptionequationsTheBETequation TheLangmuirmodelhasbeenextendedbyBrunauer EmmettandTeller 1938 toincludemultilayeradsorption givingtheequationnowknownastheBETequation TheBETequationis WhereZisdefinedby E1为第一吸附层的解吸附焓变 Ev为物理吸附层的解吸附焓变 8 5AdsorptionequationsTheBETequation ApplicationoftheBETequationTheBETequationcanberearrangedforlinearplotting 直线形式 Thus iftheexpressionontheleftisplottedagainstrelativepressure p p0 astraightlineshouldresultifthedatafollowtheBETisotherm Fromsuchastraightlinethevaluesofn mandZcanbedetermined 等式左侧对p p0可做直线 从而求出n m和Z 8 5AdsorptionequationsTheBETequation Thevalueofn menablesthesurfacearea 表面积 ofthesolidtobecalculatediftheareaoccupiedbyanadsorbatemoleculeisknown Usuallysuchmeasurementsaremadewithnitrogengasat77Kandtheareaoccupiedbyeachmoleculeisthen0 162nm2 通常使用的氮气分子面积为0 162nm2 8 5AdsorptionequationsTheBETequation Z和曲线形状的关系 LowvalueofZgiveTypeIIIisothermsandcorrespondtoweakinteractionbetweenadsorbateandadsorbent andconsequentlowadsorptionvaluesatlowrelativepressures 8 5AdsorptionequationsTheBETequation Z和曲线形状的关系 WhenZislargethereisstronginteractionbetweenadsorbentandadsorbateandtheBETequationgivesisothermsofTypeII 8 5AdsorptionequationsTheBETequation ItisworthnotingthattheadsorptionwheretheisothermsforhighZstarttostraightenout 直线 isclosetothemonolayercapacity 8 5AdsorptionequationsTheBETequation PointBmethod surfaceareadetermication 8 5AdsorptionequationsTheBETequation 适用范围 TestsoftypeIIexperimentaldataagainstthelinearformoftheBETequationnearlyalwaysgiveagoodlinearplotintheintermediaterangeofrelativepressures 0 05 p p0 0 3 实验结果表明 多数催化剂的吸附实验数据按BET作图时的直线范围一般是在p p00 05 0 35之间 BET曲线p p00 05 0 35呈直线 8 6Capillarycondensation 毛细管凝聚 However adsorptionisnottheonlymechanismbywhichgasmoleculescaninteractwithasolidsurface thereisalsothepossibilityofcondensationwithinthecavities k v ti 孔穴 ofaporous p r s 多孔的 solid 8 6Capillarycondensation Condensationmayoccurwhentheactualpressureexceedstheequilibriumvapourpressure 当实际的压强超过平衡蒸汽压时 就会发生凝聚 8 6Capillarycondensation Iftheliquidsurfaceiscurved theKelvinequationshowsthatthislatterpressuremaybesignificantlylowerthatp0 Thuscondensationmayoccuratp p0 1 aphenomenonknownascapillarycondensation 但是当液面弯曲时 开尔文公式显示 蒸汽压将会比p0低 所以凝聚会在p p0 1时发生 是为毛细管凝聚 8 6Capillarycondensation 升压时 曲率半径R 0 蒸汽压变大 凝聚变得更难 8 6Capillarycondensation 凝聚变得更难 更大的压强才能凝聚 8 6Capillarycondensation 减压时 曲率半径R 0 蒸汽压变小 蒸发变得更难 8 6Capillarycondensation Capillaryemptyingwilloccuratalowerrelativepressurethatcapillaryfillingandtheisothermwillshowadsorption desorptionhysteresis 滞后现象 用来判定多孔结构的存在 蒸发变得更难 更小的压强才能解吸附 Fig 8 a Capillarycondensationhysteresiscurves and b poresizedistributionforthemediafromFig 7 Franti ek t p nek Miroslav o PavolRajniak Characterisationofporousmediabythevirtualcapillarycondensationmethod ColloidsandSurfacesA PhysicochemicalandEngineeringAspects Volume300 Issues1 2 2007 11 20 http dx doi org 10 1016 j colsurfa 2006 10 018 Fig 2Nitrogenadsorptionisothermat77 4KforporoussiliconsamplesA symbol andB symbol Ininsett