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HeterogeneousCatalysis
HeterogeneousCatalysis Fundamentals,Engineering,and Characterizations(withaccompanying presentationslidesandinstructor’smanual)
GiovanniPalmisano
DepartmentofChemicalEngineering,KhalifaUniversity,AbuDhabi, UnitedArabEmirates
SamarAlJitan
DepartmentofChemicalEngineering,KhalifaUniversity,AbuDhabi, UnitedArabEmirates
CorradoGarlisi
LuxembourgInstituteofScienceandTechnology,Luxembourg
1.2.1Fractionalcoverage
1.2.2Catalyticactivity
1.2.3Conversion,yield,andselectivity
1.3Importanceofheterogeneouscatalysisintoday’sindustry
1.3.1Ammoniasynthesis
2.3.3Potentialenergydiagrams
2.3.4TheElovichequationinchemisorptionkinetics
2.3.5Desorptionrate
3.1Adsorptionisothermsandtheirclassification .......................................................63
3.1.1Langmuirisotherm ....................................................................................68
3.1.2Henry’sisotherm.......................................................................................70
3.1.3Freundlichisotherm ..................................................................................71
3.1.4Temkinisotherm .......................................................................................72
3.1.5BETisotherm ............................................................................................72
3.1.6PotentialtheoryofPolanyi
3.1.7Recentapproachestomodeladsorptionisotherms
3.2Adsorptionisobarsandisosteres ..........................................................................79
3.3Modelsforsurfacereactions ................................................................................81
3.4Catalysts,cocatalysts,andsupports ......................................................................86
3.4.1Catalystsupports
3.4.2Cocatalysts
3.5Questionsandproblems
4.1Estimationofthesurfacearea ............................................................................101
4.1.1Gravimetricanddynamicmethods
4.1.2Volumetricmethods
4.2Estimationofporosityandporesize ..................................................................108
4.2.1Dubinin RadushkevichandDubinin Astakhovmethods ......................110
4.2.2Horvath Kawazoemethod .....................................................................110
4.2.3Saito Foleymethod
4.2.4Barrett Joyner Halendamethod
4.2.5Dollimore Healmethod
4.2.6Densityfunctionaltheory
4.2.7Mercuryporosimetry
4.3Hysteresisandcapillarycondensation ................................................................117
4.4Poremodels—morphology.................................................................................123
4.5Mechanismsofdiffusionwithincatalystpores ..................................................128
4.6Questionsandproblems .....................................................................................135
Chapter5:Catalyticreactionengineering
5.1Catalyticreactionsteps
5.1.1Externaldiffusion
5.1.3Adsorption
5.1.4Surfacereaction ......................................................................................147
5.1.5Desorption ..............................................................................................149
5.2Reactionmechanismandtherate-limitingstep
5.3Catalyticreactordesign
5.4Diffusionandreactioninheterogeneouscatalysis
5.4.1Masstransfer-limitedandreactionrate-limitedreactions
5.4.2Diffusionwithreactioninacatalystpellet
5.4.3Thielemodulus .......................................................................................168
5.4.4Internalandoveralleffectivenessfactors ................................................170
5.4.5Weisz PraterandtheMearscriteria
5.5Multiplesteadystatesandthermalhysteresis
5.6Catalystdeactivationandregeneration
5.7Questionsandproblems
6.1Conversionofbiomasstobiofuels
6.1.1Biomassfeedstock
6.1.2Traditionalthermochemicalprocessesforcatalyticconversionof
6.1.3Aqueous-phasereformingforhydrogenandalkanesproduction
6.2Electrocatalysis
6.2.1Fundamentalsofelectrocatalyticprocesses
6.2.2Waterelectrolysis
6.2.3ElectrochemicalCO2
6.3Photocatalysis
6.3.1Fundamentalsofphotocatalyticprocesses
6.3.2Waterandwastewaterpurification
6.3.3Organicsynthesis
7.1X-raydiffraction
7.5InfraredandRamanspectroscopy ....................................................................266
7.6Temperature-programedmethods .....................................................................272
7.7Electrochemicaltechniques ..............................................................................277
7.7.1Voltammetry .........................................................................................279
7.7.2Electrochemicalimpedancespectroscopy..............................................283
7.8UV visibleandphotoluminescencespectroscopy
7.8.1UV visiblespectroscopy
7.8.2Photoluminescencespectroscopy
7.9Solid-statenuclearmagneticresonanceandelectronparamagnetic resonancespectroscopies ..................................................................................293
7.9.1Nuclearmagneticresonance ..................................................................293
7.9.2Electronparamagneticresonance ..........................................................296
7.10Computationaltools:densityfunctionaltheoryandmolecular dynamicsimulations ........................................................................................301
7.11Questionsandproblems
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Preface
Heterogeneouscatalysisisamultidisciplinarysubjectofrelevancetoalargenumberof students,scholars,practitioners,andindustries,touchingindifferentwayschemical engineering,chemistry,andmaterialsscience.Itssignificanceisevidentlookingatthe currentapplicationsintheindustrialsynthesisofawiderangeofcommoditiesandfine chemicals,asalsoatthecontinuouseffortsinthescientificactivitiestargetingthe preparationofmoreefficientcatalystsfollowedbycharacterizationsaimedatthe understandingoftheirperformance,alongwiththeinvestigationsonnovelcatalytic processesandreactors.
Inthelastdecades,anumberoftextbooksandscientificbookshaveappeareddealingwith specificaspectsofcatalysisortryingtoachieveamoreholisticpresentationofthe fundamentalsandapplications.
Unlikeotherdisciplines,heterogeneouscatalysisdoesnotallowforastraightandrigorous theoreticaldevelopmentofitsprinciplestojustifyobservedphenomena,sinceanallembracingtheoryhasnotbeendevelopedtodate,duetothecomplexityofthemechanisms behindit.
Forthisreason,formerauthorshaveusuallycoveredonlyafewfundamentalaspectsof catalysis,movingforwardtodiscusstheimplicationsintermsofapplicationsandmaterials’ characterizations.
Thisbookpresentsanoverviewofthefundamentalsofheterogeneouscatalysisin Chapter1,Introduction,andChapter2,FundamentalsoftheAdsorptionProcess,covering thebasicconceptsandquantitiesforthelessexperiencedreaderwithfewexamplesof industrialcatalyticprocesses,followedbytheadsorptiontheorystartingwiththemolecular orbitalapproach.Thethermodynamicsandkineticsofadsorption/desorptionaredescribed withgraphicalexemplificationsforabetterunderstanding.Ahandfulofexamplestakenfor literaturearealsoprovidedtoguidethereaderthroughthecomplexinterpretationofthe catalysts’features.The Focus sectionsinthesetwochaptersreportcasestudiesonhowthe configurationandthesupportsofcatalystscanaffecttheirelectronicpropertiesandactivity, andpresentanumberofsolvednumericalexamples.
Chapter3,AdsorptionModels,SurfaceReaction,andCatalystArchitectures,describesthe traditionaladsorptionmodelswithanoverviewonrecentdevelopmentscoveredinthe Focus sections,whicharefoundthroughoutthebookandpointmostlytothecorrelation betweenmaterialspreparation/propertiesandcatalyticperformance,alongwithnumerical examples.Thechapterendswiththemodelsforsurfacereactionandtheroleofcocatalysts andsupports.
Chapter4,SurfaceAreaandPorosity,dealswiththeassessmentofthesurfaceareaand porosity,withinsightsontheoldandmorerecenthysteresisclassificationsandmodelsto estimateporosityandporesizedistribution.Thischapterpresentsasignificantnumberof examplestakenfromrecentliteraturethatareintendedtodevelopthereader’sskillsonthe interpretationofisothermswithhysteresisloopsandporesizedistributions. Focus sections describetheworkingprinciplesandtheequipmentusedfortheassessmentofthetextural propertiesofcatalysts,numericalexamplesontheBETequation,tortuosityanddiffusivity, alongwithacasestudyonthedesignofcatalystsfortheenhancementofdiffusion.
Overall,thefirstfourchaptersgiveauniqueoverviewofthefundamentalsofcatalysis, inclusiveofallthekineticandthermodynamicaspectsrelatedtoadsorption/desorption, followedbyadetailedandupdatedpresentationontheassessmentofsurfaceareaand porosity.
Chapter5,CatalyticReactionEngineering,coversthereactionengineeringtopicsof heterogeneouscatalysts,particularlyusefulforreaderswithoutabackgroundinchemical engineeringwhodidnotstudycatalyticreactionengineeringandreactormodeling.The Focus sectionsdescribedifferenttypesofcatalyticreactors,themodelingofdiffusionand transportinporouscatalysts,thedesignandpreparationofcatalystpelletsforfixedbed reactors,andtheyalsoincludesolvedreactionengineeringproblems.
Chapter6,GreenHeterogeneousCatalysis,isasurveyongreencatalysis,rangingfrom electrocatalysistophotocatalysisandthecatalyticconversionofbiomasstobiofuel.The chapterisfocusedontheapplicationsafteraquickintroductiontotheprinciplesofthese threegreenapproaches. Focus sectionscoverthepreparationaspectsofrecentlydeveloped catalystsandtheconnectionbetweencatalystpropertiesandactivity/selectivity.
Toconclude,themostpopularcharacterizationtechniquesusedforsolidcatalystsare discussedinChapter7,CharacterizationTechniques,wherethefundamentalsofall techniquesareexplainedfollowedbyexamplesoftheirapplications.Thischapteris approachablebybothanewcomerandamoreexpertreaderwhowouldliketobe introducedtoacharacterizationtechniquehe/sheisnotfamiliarwithandanalyzefewdata describingcatalystsreportedintheliterature.Theapplicationofin-situtechniques,the characterizationofcatalystsduringtheirsynthesis,andthedescriptionofasynchrotron facilityareamongthetopicscoveredinthe Focus sections.
Thebook,whichisreaderfriendlyandaccessibletoaheterogeneousreadershipranging fromseniorundergraduatestudentsuptoMaster’sandPhDlevelstudents,alongwith experiencedscholarsandpractitioners,issupplementedwith PresentationSlides foreach chapter,usefulforclasses,seminars,workshops.An Instructor’sManual isalsoavailable uponrequestwiththeanswerstotheproposedquestionsandproblems.Casestudies addressingmaterialspreparationamongothertopicsandnumericalexamplescanbefound inthe Focus sectionsofeachchapter.Endofchapter QuestionsandProblems areproposed forself-testsandasareferenceforassignments/examstobearrangedbyinstructors.
TheauthorsarethankfultoMr.MujeebOladipupoKareemforhiscontributiontothe Instructor’sManualandhisusefulsuggestions.
ToaccesstheInstructor’sManualincludinganswerstoproposedquestionsandproblems pleasegotothefollowingwebsiteandregistertouse: https://textbooks.elsevier.com/web/ Login.aspx?MREDIR=../web/Manuals.aspx?isbn=9780323898454
Introduction
Catalyticreactionsaresubdividedintotwodistinctcategoriesdependingonthephasesof boththecatalystandthereactingmixture.Ifthecatalystisinsolutionwithatleastoneof thereactants,thentheprocessisdefinedas homogenouscatalysis.Onthecontrary,ifthe processinvolvesmorethanonephase(e.g.,solidcatalystwithreactantsintheliquidorgas phase),thenitisdefinedas heterogeneouscatalysis.Ofthesetwotypes,heterogeneous catalysisismorecommon.Thisismainlyattributedtothesimpleandcompleteseparation ofthereactionproductsfromthesolidcatalyst.Catalystsarequitevaluableandtheirreuse isinhighdemand.Hence,thepossibilityofcatalystrecyclingmakesheterogeneous catalysiseconomicallymuchmoreattractive.
Today,heterogeneouscatalysisplaysakeyroleinmanyimportantindustrialprocesses, sinceitinvolvesc.80%oftheindustrialchemicalconversions.Assuch,itisappliedin (1)theproductionoforganicandinorganicchemicals,(2)crudeoilrefiningand petrochemistry,(3)environmentalprotection,and(4)energy-conversionprocesses. Heterogeneouscatalysistakesplacethroughsurfacereactions,withadsorptionofatleast oneofthereactantsonthecatalystsurfaceastheinitialstep.
A catalyst isdefinedasasubstancethatincreasestherateofachemicalreactionwithout itselfundergoingachemicalchange.Agoodcatalystmustpossesshighactivity,thedesired selectivity,andlong-termstability.Theactivityofacatalystmaybeassessedviadifferent parameters,includingits turnoverfrequency (TOF), conversion, yield,and,most importantly, selectivity.Thesefundamentalcatalysistermswillbethoroughlydefinedand explainedwithinthischapterafterahistoricalaccountofheterogeneouscatalysis.
1.1Historicalbackground
TheSwedishchemistJo ¨ nsJacobBerzeliushasbeencreditedwithcoiningtheterm catalysis (fromtheGreek kata-,“down,”and lyein,“loosen”)in1835.Inanattempttoconnecta seriesofobservationsmadebyotherscientistsinthelate18thandearly19thcenturies, Berzeliusconcludedthatinadditiontoaffinity,therewasafurtherforce,namely,the catalyticforce,abletotriggerdecompositioninbodiesandformnewcompounds.Someof themostsignificantstudiesthatservedasabackgroundforBerzeliustoformulatethe
HeterogeneousCatalysis. DOI: https://doi.org/10.1016/B978-0-323-89845-4.00001-1 © 2022ElsevierInc.Allrightsreserved.
conceptofcatalysiswerethetransformationofstarchtosugarbyacidsobservedbyGustav Kirchhoff,theenhancedcombustionofavarietyofgasesinthepresenceofplatinum observedbyHumphryDavy,andtheoxidationofalcoholtoaceticacidinthepresenceof fineplatinumpowder [1].
In1834MichaelFaradayobservedthecatalyticactionofplatinumthatcatalyzedthe spontaneouscombustionofhydrogenandoxygeninwaterelectrolysis.Hewasthefirstto introducetheconceptofwhatweknowasadsorptionandhypothesizethatcatalytic reactionoccursonthecatalystsurfacefollowingthesimultaneousadsorptionofthe reactantsinvolvedintheprocess [2].Presently,waterelectrocatalysisisbeingwidely appliedintheindustrialproductionofhigh-purityhydrogen [3].
AnimportantcontributiontocatalysisasweknowitnowwasgivenbyWilhelmOstwald in1897,whodefinedacatalystasamaterialthat modifiesthereactionratewithout appearinginthefinalproducts,therebyrecognizingcatalysisasakineticphenomenonfor thefirsttime [4].Inthesameyears,PaulSabatierinvestigatedthehydrogenationreaction catalyzedbymetals,suchasnickel,otherthanplatinum.Heproposedtheformationand decompositionofintermediatecomplexeswiththesurfacecatalysts,resultinginalowering oftheGibbsenergyofthesystem [5].
Afundamentalstepincatalysisoccurredin1909,whenthechemistFritzHaberdiscovered thatasignificantamountof ammonia couldbegeneratedfromhydrogenandnitrogeninthe presenceofosmiumanduraniumcatalysts.Thepotentialforindustrialapplicationofthese processescouldbeincreasedbyworkingathighpressure,andtothatend,theHaber’s laboratoryapparatuswasconvertedtoahigh-pressurelarge-scaleindustrialapparatusby CarlBosh.Afterward,BadischeAnilinundSodaFabrik(BASF)startedalargeprogramto developacheaperandeffectivecatalyst.AlvinMittasch,togetherwithhiscolleagues,Wolf andStern,identifiedmagnetite(Fe3O4)asthebestcandidateforammoniaproduction [6]. Inthewakeoftheseresults,in1923,BASFalsodevelopedthefirstplantforthesynthesis ofmethanolfromsyngasproducedfromcoalinthepresenceofZnO/Cr2O3 catalysts operatingatc.400 Cand200bar [7].
In1938FischerandTropschstudiedthe conversionofsyngas tohydrocarbonsandalcohols catalyzedbymetalssuchascobaltandiron(i.e.,Fischer Tropschsynthesis) [8].
Intheearly20thcentury,agreaterunderstandingofthemechanismsbehindthecatalytic processeswasgainedthankstothestudiesconductedbyIrvingLangmuiron adsorption andHughStottTayloronsurface activesites.Thelatterespeciallyfocusedonthe heterogeneityofthecatalystsurface,whichusuallypresentsvacancies,kinks,terraces, ledges,etc.,beinghostingsiteswheremetalatomshavedifferentcoordinationnumbersand thuscatalyticactivities [2].Meanwhile,StephenBrunauer,PaulHughEmmett,andEdward Tellerexplainedthephysicaladsorptionofgasmoleculesonasolidsurface;EricRideal,
MikhailTemkin,MichelBoudart,andmanyothersprovidedavaluablecontributiontoa greaterunderstandingofthekineticsofcatalyticreactions [9].
Inthe1930scatalyticcrackingwasintroducedinthepetroleumrefiningindustry.This processallowedtobreaktheC Cbondsinlargepetroleummolecules,convertingthem intohigh-octanefuelsusinganaluminumchloridecatalyst(i.e.,Friedel Craftscatalysts). Yet,thefoundationofmodernpetroleumrefiningwasestablishedin1942withthe commercializationof fluidcatalyticcracking (FCC).Theseprocessesenabledtomaintain thecatalystparticlesinsuspensionbyastreamofvaporizedhydrocarbonsblownthrough thereactorandsuccessivelypassedthroughtheregenerator [7].
Animportantlandmarkwastheadventofsynthetic zeolites,aluminosilicatecatalysts,inthe 1960s,whenamorphoussilica aluminacatalystsusedinFCCweregraduallyreplacedby syntheticfaujasites(zeolitesXandY)(Fig.1.1).Thezeolitecatalystswereordersof magnitudemoreactivethantheconventionalonesandensuredamuchhigheryieldof gasoline,thekeyproductinFCCplants.Thistypeofcatalyst,characterizedbyabroad varietyofstructures,rapidlyfoundapplicationsinfurtherprocessesinpetroleumrefining andbasicpetrochemistry. Fig.1.1 showssomeofthetypicalstructures,alongwiththe distinctivemicroporesystemsanddimensionsofzeolites.Amongthese,thesynthetic zeoliteZSM-5isoneofthemostusedindustrially,beingtheprototypeofshape-selective catalystsandfindingapplicationinthesynthesisofethylbenzene,conversionofmethanol togasoline,andisomerizationofxylenes [10].
Figure1.1 Fourexamplesofzeolitestructureswiththeirmicroporesystemanddimensions. Source:Reproduced from [10]
Fromthe1960s,theevolutionofthetransportationindustry,theincreaseinindustrial activity,andthesevereriseinair-bornpollutantshaveledtotheestablishmentofnewlaws thatregulatevehicleandstationaryemissions [11].Drivenbytheseregulations,global attentionandwidespreadinterestinenvironmentalcatalysisgrewsubstantially.
Environmentalcatalysis addressesproblemsrelatingdirectlytoemissioncontrol,NOx removal,volatileorganiccompoundconversion,andwastetreatment.Presently,the researchfocusofenvironmentalcatalysishasexpandedfrompollutantabatementtoinclude pollutantprevention.Accordingly,newcatalyticrouteshavebeenexploredforthesynthesis ofvalue-addedproductswithouttheformationofundesirablepollutantsandforthe productionofnewcleanfuelswithlowsulfurcontent [12].
Theengineeringofsemiconductors,especiallyoftitaniumoxide,hasledtogrowing attentiontowardheterogeneousphotocatalysisduringthe20thcentury,whichnowadaysisa hottopicinappliedcatalysisforthepossibilitytoefficientlydegradeanyorganicpollutants andproducealternativecleanfuelsbytheirradiationofacatalyst.Theearliestreferenceon photocatalysisdatesbackto1911,whenAlexanderEibnerstudiedtheeffectofthe illuminationofZnOonthebleachingofPrussianblue.However,intheabsenceofpractical applications,theseprocessesremainedacuriosityformuchofthe20thcentury.Inthe 1970sAkiraFujishimaandKenichiHonda [13] observedthe electrochemicalphotolysisof water inacellconsistingofanionicallyconductingseparatorandarutileTiO2 electrode irradiatedbynear-UVlightandconnectedtoaplatinumcounterelectrodeviaanelectrical load(Fig.1.2).TheHonda Fujishimaeffectattractedtheattentionofthewholescientific
Figure1.2
SchematicoftheelectrochemicalcellemployedbyFujishimaandHondaforthephotolysisof water:(1)rutileelectrode,(2)platinumcounterelectrode,(3)ionicallyconductingseparator, (4)gasburette,(5)loadresistance,and(6)voltmeter. Source:Reproducedfrom [14]
communitybecauseofitspotentialtoovercomethedependenceonpetroleumbyproducing cleanhydrogenfromabundantandinexpensivewaterandsunlight.
Energy-efficientcatalyticprocessessuchasthosethatutilizerenewableenergysources havealsoseenaveryfastdevelopment [12].Oneexampleistheindustrialproductionof hydrogenthroughrenewableenergy-poweredwaterelectrocatalysisinaprocessknownasa power-to-gas(PtG). Fig.1.3 showsthemaincomponentsofaPtGsysteminwhich electricityisconvertedintohydrogenbywaterelectrocatalysis.Theproducedhydrogenis storedtobelaterreconvertedbackintoelectricityeitherthroughfuelcellsorhydrogen combustionengines.Otherapplicationsofhydrogenarepresentedin Fig.1.3.Solarand windenergyarethemostcommonlyappliedrenewableenergysourcesinPtGsystems.Due tostrongfluctuationsinsupply,however,renewablepowerisfrequentlycoupledwith publicelectricityfromthegrid.Asof2013,12pilot-scalePtGsystemshaveutilizedsolar energyasasourceofrenewablepowerforwaterelectrocatalysis,whilewindenergyhas beenappliedinnineotherpilot-scaleplantsofthesame [15].
Theapplicationofautomotivecatalystsasprincipalemission-controltoolsinthe detoxificationofexhaustgaseshasseenunqualifiedsuccess.Here,hydrocarbonsand carbonmonoxideareoxidizedintoCO2 andwater,whileNOx isreducedtoN2 andO2. ShortlyfollowingtheirpositiveimplementationinUSvehiclesofthemodelyear1975, automotivecatalystswerealsolateradoptedinJapanandEuropesometimeinthe1980s [16].Initially,PtandPd,indifferentproportions,werechosenascatalystsfortheoxidation reaction.However,afterexhibitingasignificantlyenhancedactivityinthecatalytic reductionofnitrogenoxides,Rhwasalsolaterintroducedintothe automotivecatalytic system [17].Currently,Pd,Pd/Rh,Pt/Rh,andPt/Pd/Rhcatalystsareallincommercialuse.
Figure1.3
Themaincomponentsofapower-to-gassystemandthevarioustypesofapplicationsforit.
Source:Reproducedfrom [15]
Inthe21stcentury, catalyticasymmetricsynthesis methods,developedbyKnowles,Noyori, andSharpless,haveseengreatsignificanceintheindustrialproductionofpharmaceuticals andagro-chemicals [18].Catalyticasymmetricsynthesisreferstocatalyticmethodsin whichoneoftheenantiomersisformedinpreferencetotheother.Here,achiralcatalystis usedtoproducelargequantitiesofanopticallyactivecompoundfromaprecursorthatmay bechiralorachiral.Onthisbasis,manyorganicchemistshaverecentlydeveloped numerousmethodsthatconvertprochiralsubstratesintochiralproductswithhigh enantioselectivity.
Pd-catalyzedcarbon carbonbond-formingreactions,developedbyHeck,Negishi,and Suzuki,havealsohadahugeinfluenceonsyntheticorganicchemistryduringthe21st century [19].Alargenumberofnaturalproductsandbiologicallyactivecompoundsinthe finechemicalandpharmaceuticalindustriesarepresentlybeingsynthesizedbythesePdcatalyzed cross-couplingreactions.Thisismainlyattributedtothemildreactionconditions andthetoleranceofawiderangeoffunctionalgroups.InPd-catalyzedcross-coupling reactions,twomoleculesassembleonPdviatheformationofmetal carbonbonds.The carbonatomsboundtoPdare,therefore,inverycloseproximitytooneanother. Accordingly,thetwocarbonatomscoupletogetherandanewcarbon carbonsinglebond isformed.
Focus1.1:EffectofZn:CrratioonthestructureandreactivityofZnO/Cr2O3 catalystsformethanolsynthesis
AdvancesontheZnO/Cr 2 O 3 catalystdevelopedbyBASFin1923forthesynthesisof methanolfromsyngashavebeenrecentlyachievedbySongetal. [20] whostudiedthe effectofdifferentZn:Crratios—from100:0to0:100—onthecatalyticperformance.The catalystswithratiosfrom65:35to55:45consistedofZnCr 2 O 4 spinelwithalowdegreeof crystallinityandhighsurfacearea.Inparticular,thesamplewith65:35exhibitedthe highestreactivityduetotheoptimumZn :Crratiocorrespondingtothatofthe hydrotalcite-likeprecursorZn 4 Cr 2 (OH) 12 CO 3 .Thisprecursor,formedduringthesynthesis ofthecatalystsbycoprecipitation,decomposedduringcalcinationtothe nonstoichiometricZn Crspinel,whichistheactivephaseofZnO/Cr 2 O 3 inmethanol synthesis.Ontheotherhand,theloweractivityofCr-richcatalystswasmainlyascribedto theformationofchromatesduringthecalc ination,havingadetrimentalimpacton methanolsynthesis.
Densityfunctionaltheorycalculationsshowedthat(100)surfaceofthespinelhasfavorable oxygenvacancyformationenergies. Fig.1.4 displaysthecalculatedlocalstructureforthe spinelwithexcess60%Zncontent(26.66%doping).Thenonstoichiometricsurfaceis terminatedbyanamorphous-likethinlayerofZnO,whichistheactivephaseformethanol production,sinceitpossessesalowoxygenvacancyformationenergy,aswellasnottoo strongadsorptionofhydrogenandcarbonmonoxide.
(Continued)
Focus1.1:EffectofZn:CrratioonthestructureandreactivityofZnO/Cr2O3 catalystsformethanolsynthesis(Continued)
Figure1.4
Localstructureforthespinelstructurewith26.66%Zndoping. Source:Reproducedfrom [20].
1.2Fundamentalconceptsandquantitiesincatalysis
1.2.1Fractionalcoverage
Theextentofsurface fractionalcoverage θ indicatestheportionofcatalyticsitesoccupied byanadsorbedspecies(adsorbate)anditcanbedefinedinthefollowingway:
5
Numberofadsorptionsitesoccupied Totalnumberofadsorptionsites (1.1)
Thisquantityisusuallycalculatedfromthevolumeofadsorbateadsorbed(V)by θ 5 V/Vm, where Vm isthevolumeofadsorbatecorrespondingtoacomplete monolayer (i.e.,asingle layerofatomsormoleculesadsorbedonthesurfaceofacatalyst).Thevolumes V and Vm areofthefreegasmeasuredinthesameconditionsoftemperatureandpressure [21].When θ reachesthevalueof1,thesurfacecatalyticsitesarecompletelyoccupied,andadsorption offurtherlayersispossibleonlyontopofthefirstone(multilayeradsorption),asitwillbe widelydiscussedinthefollowingchapters.
1.2.2Catalyticactivity
Thecatalyticactivityisrelatedtotheabilityofacatalysttochangethereactionrate,which,in turn,canbedefinedinmanywaysdepending onthereactionsystem.Ingeneral,the reactionrate (R)isafunctionoftemperatureandpartialpressureorconcentrationofthereactants.Itcanbe
expressedastheproductofanapparentcoefficient(k)andafunction( f )ofthepartialpressure (gas solidsystem)orconcentration(liquid solidsystem)ofthereactant i:
where pi and ci arethepartialpressureandconcentrationofthereactant i,respectively.The reactionrateexpressesachangeinthenumberofmoles(ofareactantorproduct)perunit timeperunitmass(orsurface)ofacatalyst.The rateconstant canbeexpressedthroughthe Arrheniusequation:
where A isthepreexponentialfactorno tchangingwithtemperature, R istheuniversal gasconstant, T istheabsolutetemperature,and E istheapparent activationenergy . E isusuallydifferentfromthetrueactiva tionenergysincetheconcentrationofthe reactantatthecatalystsurfacemaybetemperaturedependent,althoughthecatalyst structuremaynotbeaffectedbytemperaturechanges.Toavoidresortingtothe activationenergy,mostofthetimecatal yticactivityisexpressedintermsof TOF , whichisdefinedasthenumberofmolecularreactionsorcatalyticcycles( n )peractive siteofthecatalystperunittime.Beingareactionrate, TOF dependsonthereaction conditionsincludingtemperatureandcompos itionofthereactingsystem.Ingeneral, TOF canbeexpressedas:
where t standsforreactiontimeand S indicatesthenumberofactivesites.Forthemost relevantheterogeneouscatalyticreactionsinindustry,TOFisintherangeof 10 2 102 s 1.ThemaindifficultyindeterminingTOFliesincountingactivesites. Moreover,sitesmaynotbeallidentical.Insuchsituations, S isusuallyreplacedbythe totalexposedarea,mass,orvolumeofthecatalyst [7].
Fig.1.5 presentstheresultsofastudyconductedonthecatalyticgas-phasehydrogenation of p -nitrobenzonitrile(p-NBN)to p-aminobenzonitrile(p -ABN)usingaseriesofoxides assupportforthecatalystconsistingo fgold(Au)particleswithameansize3 8nm. DifferentsupportsresultedindifferentAunanoparticlesizes,impactingthehydrogen chemisorption(Fig.1.5A )and TOF ( Fig.1.5B ).Thevariationofthe TOF withthemean AusizecoincidedwiththatobservedforH 2 uptake,indicatingastrongcorrelation betweenthehydrogenationrateandH 2 dissociationcapacity.Thereduced TOF atsmall Ausize(,3nm)isduetothesemiconductingpropertiesofthecatalyststothedetriment ofthemetallicones [22].
Figure1.5
Variationof(A)H2 uptakeand(B)TOFofp-NBNwithmetalparticlesizeforthehydrogenation ofp-nitrobenzonitrileoverAucatalystssupportedonvariousoxides. Source:Reproducedfrom [22].
Focus1.2:Single-atomcatalysisforhydrogenevolutionreaction
Single-atomcatalysishasbeenahottopicinresearchoverthelastfewyears.Giventhehighcostof manycatalyticmaterials,especiallynoblemetals,reducingthenanostructurestoatomically distributedmetalcenterssupportedonasuitable catalystcanconsiderablydecreasemetalusage andmaximizeatomefficiency.Arecentstudyinthisareareportstheenhancedcatalyticactivityofa single-atomPtsupportedonnanoporous(np)nonstoichiometriccobaltselenide(denotedasPt/npCo0.85Se)catalyst [23].Theatomicengineeringofnp-Co0.85SebysinglePtatomsdopingisan effectivestrategytoproducearobustelectrocatalystforhydrogenevolutionreaction(HER),ensuring aTOFmuchhigherthanthoseachievedbynp-Co0.85Sealoneandmanyreportedcatalysts.
Fig.1.6 depictstheschematicoftheHERmechanismoverPt/np-Co0.85Seinneutralmedia. ThewatermoleculesareselectivelyadsorbedontheCosites.Lateron,H2Oisdissociatedinto intermediateHads andOHads byCosites.TheformedHads canbeadsorbedonanearby emptyCositeorPtsiteandbefinallyconvertedintoH2.
Figure1.6
SchematicillustrationoftheHERmechanismovernp-Co0.85Secatalyst. HER,Hydrogenevolution reaction. Source:Readaptedfrom [23]
Thestabilityofthereactionintermediatesformedatthesurfaceofthecatalystplaysa significantroleindeterminingtherateofacatalyticreaction.Accordingto Sabatier’sidea, theintermediateshouldbestableenoughtobeformedbutnottoostable,asitmust decomposeandevolveintotheproducts.Thisprinciplecanberepresentedgraphicallyby volcano-shapedplots thatwerefirstintroducedbyBaladin.Inthesediagrams,ameasureof thecatalyticactivitysuchasthereactionrate, TOF,temperatureatafixedconversion/ reactionrate,etc.isplottedagainstameasureofthe stabilityofthereactionintermediate suchasitsenthalpyofformation.
Fig.1.7A showsthetypicalvolcanoplotforthemetal-catalyzeddecompositionofformic acid,wherethetemperatureforaspecificreactionrateisplottedagainsttheheatof formation(ΔHf)ofthereactionintermediate,namely,metalformate,atafixedconversion (Section1.2.3) [24].Atlow ΔHf, therateofadsorptionislowandrate-limitingduetothe littlepredispositionofthemetalformatetoform.Ontheotherhand,athigh ΔHf,thelow rateofdesorptionbecomesrate-limiting.Thepeakofthevolcano,namely,thehighest catalyticactivity,isobservedforplatinumgroupmetals,whicharecharacterizedby intermediatevaluesof ΔHf,forwhichthetwoconflictingeffectsarebalancedandthe resultingreactionrateisacombinationoftherateofadsorptionandrateofdesorption.
Fig.1.7B displaysthevolcanoplotfortheelectrocatalyticHERonvariouscatalysts [25]. Thecurrentdensity J0 (i.e.,rateofoxidationandreductionatanequilibriumelectrode)is plottedversustheGibbsfreeenergyforhydrogenadsorption ΔGH ,whichisagood descriptoroftheintrinsicactivityofametalcatalystforthisreaction.Thecatalyststothe leftofPtbindhydrogenatomstoostrongly,blockingtheactivesitesandfailingtogiverise
Figure1.7
Volcanoplotfor(A)decompositionofformicacid onvarioustransitionmetalsand(B)hydrogen evolutionreactionondifferentcatalysts. Source:(A)Reproducedfrom [24].(B)Reproducedfrom [25]
tothedevelopmentofhydrogen.Conversely,thecatalyststotherightofPtbindhydrogen tooweakly,thusnotstabilizingtheintermediatestateandprecludingthereactiontooccur.
Cocatalysts canbeusedtoboostcatalyticactivitybyactingasfurtherreactionsites, improvingthestabilityofthecatalystorpromotingthechargeseparationandtransport,the latterbeingthecaseofphotocatalyticprocesses.Here,theloadingofthelight-harvesting photocatalystwithametalcocatalystsuchasplatinum,palladium,andcopperisthemost popularwaytopromotetheactivity,forexample,inthewaterphotosplitting.The cocatalystcanindeedcapturethephotogeneratedelectronsandsuppresshole electron recombination,lowertheactivationenergyforH2 generation,aswellasactasareaction siteforthereductionofprotons [26].Cocatalystscanalsoserveasinitiatorsof polymerizationreactions.Forinstance,cocatalystssuchasmethylaluminoxane(MAO)act asactivatingspeciesforthecatalystinmetal-catalyzedpolymerizationprocesses [27].
Catalystsupport alsoplaysanintegralpartinthecatalyticperformance,primarilybecauseit bringsoutthecapabilityofthesupportedcatalysttoactasanactivecenterforthetarget reaction.Thesupportmaybeinertorparticipateinthecatalyticprocess.Typicalmaterialsfor catalystsupportsincludealumina,titania,graphite,activatedcarbon,andsilica,whichexhibit ahighsurfacearea,chemicalandmechanicalstability,andabilityfordispersingcatalyst particlesoverthesurface [28].Thereaderisreferredto Section3.4 formoredetailson cocatalystsandsupports.
1.2.3Conversion,yield,andselectivity
Theprogressofaheterogeneousreactioncanbecharacterizedbythreemainquantities: conversion,yield,andselectivity.
The conversion designatestheconsumptionofthereactant(s).Itcanbedefinedastheratio ofthenumberofmolesofthereactantAconsumedtothenumberofmolesofAfed.Ata constantfluiddensity,theconversionofAatthetime t (XA(t))canbecalculatedbythe followingequation:
where C 0 A istheinitialconcentrationand CA ðt Þ istheconcentrationatthetime t.Ifdensity changes,concentrationsmustbereplacedbymolarquantitiesinbatchreactorsorbymolar flowsinflowreactors.Inthepresenceofmultiplereactantschargedinnonstoichiometric amounts,theconversioniscalculatedwithrespecttothelimitingreactant.
The yield referstotheproductformations.TheyieldofaproductBatatime t(YB(t)) canbe expressedastheratioofthenumberofthelimitingreactantAconvertedtoBtotheinitialnumber
ofmolesofthelimitingreactantA.Ataconstantdensity,forthereaction ν A A-ν B B,where ν A and ν B aretherelevantstoichiometriccoefficients,theyieldofBcanbecalculatedasfollows:
The selectivity ofacatalystisindicativeofitsabilitytodirectthereactiontoyielda particularproduct,whileinhibitingtheformationofothers.Acatalystaffectstheselectivity ofareaction,resultinginreactionpathways,andthusproducts,whicharenotpossiblein thesamenoncatalyticsystem.Atconstantfluiddensity,theselectivityoftheproductBata time t(SB(t))canbecalculatedas:
Theselectivitycanalsobedefinedinrelationtothereactionrates.Inthecaseofparallel reactions,thereactantAcanreacttogivetwoproducts,BandC,atrates R1 and R2, respectively,overthesamecatalyst(Eqs.1.2 1.3).Duringthereaction,Bcanalsoreactto giveCatarate R3.Thesituationisdepictedin Scheme1.1.
Theselectivity S toproductBcanbeexpressedas:
IfBisnotconvertingtoC(R3 5 0),theselectivitytoBbecomes:
Theformationofthedesiredproductcanbethusacceleratedbyusinganappropriate catalystabletomodifythereactionkineticsaccordingly,althoughtheformationof undesirableproductsmaybethermodynamicallyfavored.Acatalysttherebychangesthe ratiooftheratesbyselectivelyincreasingtherateatwhichthetargetedreactionproceeds withrespecttotheothercompetingreactions.
Scheme1.1
Schematicofparallelreactionsinwhichoneofthetwoproductscanevolvetotheother.
Itisclearthat acatalyticreactioncanproceeddifferentlydependingonthecatalyst employed.Oneimportantexampleisthereactionofcarbonmonoxideandhydrogenwhich canleadtotheformationofdifferentproductsusingdifferentcatalyticsystems.Anickel catalystcanselectivelyfavortheformationofmethane [29].Ontheotherhand,when copper-basedand/oramixtureofzincoxideandchromiumoxide(ZnO Cr2O3)isusedas acatalyst,methanolisthemainproduct [30].Still,formaldehydeistheselectiveproductin theaqueousphaseinthepresenceofacatalystconsistingofamixtureofrutheniumand nickel alumina(Ru Ni/Al2O3) [31].
Anotherexampleconcernstheselectiveoxidationofhydrocarbons.Thenoncatalytic processyieldsthetwomostthermodynamicallyfavoredproducts,namely,CO2 andH2O. Nevertheless,selectivecatalyticoxidationcangiverisetothecorrespondingaldehydesand/ orketones,byselectiveinsertionofoxygen,oralkenes,byselectiveremovalofhydrogen [32].Finally,intheautomotivethree-wayconverters,NOreactswithCOandisreducedto thedesiredproduct,namely,N2,overrhodiumcatalyst,whileitcanbeconvertedtoa pollutant,thatis,NH3,ifthereactioniscatalyzedbyPtcatalyst [11]
Itisimportanttonotethattheproduct selectivitycanbecontrolledandadjusted inmany wayssuchasthestructural,chemical,electronic,compositional,andmorphological alterationofthecatalyst.Yet,certaincatalystsrelyonthemolecularsieveeffect,showing selectivitytowardareactantorproductdependingonitsshapeorsizeofmolecules.Two catalystsidenticalfromthechemicalstandpoint,butwithporesofdifferentsizes,mayshow differentselectivities.Forinstance,zeolitesaremicroporouscatalystswithcagesof moleculardimensionthatselectivelyacceptorrejectcertainreactantsorproducts dependingontheirporesize.Inadditiontoreactantandproductselectivity,theymayalso presentrestrictedtransition-stateselectivemechanisms,preventingcertainreactionsdueto therestrictedspaceavailableforthecorrespondingtransitionstates [33]
Focus1.3:Enzymesasasourceofinspirationforthedevelopmentoftailor-made catalysts
Thewaythecatalystinteractswiththereactingmolecules,andultimatelyitsactivityand selectivity,dependsonanumberoffactorsthatincludeexperimentalsetup,operating conditions(e.g.,temperatureandpressure)andphysicochemicalpropertiesofthecatalyst.
(Continued)
Focus1.3:Enzymesasasourceofinspirationforthedevelopmentoftailor-made catalysts(Continued)
Intherationaldesignoftailor-madecatalysts,enzymes(naturallyoccurringbiocatalysts)haveacted asasourceofinspirationforscientiststotunethephysicochemicalpropertiesofcatalysts [34].Many enzymesrelyonawell-confinedspace,thatis,thecage,aroundtheactivecenterstoensurethe proximityofthesetothesubstratemolecules.Moreover,theycanpreorganizethesubstrateina favorableenergyconformationinthecage,which canalsodestabilizeintermediatesandreducethe transitionstatebarrierleadingtoanincreasedreactivity [35,36].Theseconceptshavebeenextremely usefulforthedevelopmentofcatalystssuchas zeolites,metal-organicframeworks(MOFs),covalent-organic frameworks(COFs),whichrelyontheso-calledconfinementeffecttogoverntheactivityandselectivity.
MOFsarearelativelynovelclassofcatalystsconsistingoftwomaincomponents,thatis,inorganic vertices(metalionsorclusters)andorganiclinkersarrangedinanordered,highlyporous heterogeneousnetwork(Fig.1.8).Themoleculardiversityandthetunablecharacterofthesebuilding blocksenableanalmostinfinitevarietyofMOFs.Themainadvantageofthistypeofcatalystisindeed theeaseofadjustingtheirphysicochemicalpropertiesthroughmodifyingtheirorganicligands,metal centers,activefunctionalgroups,oryetbytuningtheirmorphology [37].Moreover,theconfined spaceofMOFsinvolvesahighsurfaceareawithenhancedandmonodisperseloadingofcatalytically activespecies,whichcanleadtobetterperformancecomparedtotraditionalcatalysts.
Figure1.8
SchematicstructureoftheMOF. MOF,Metal-organicframework. Source:Reproducedfrom [38]
Focus1.4:Numericalexamplesoffundamentalquantitiesincatalysis
● Surfacecoverage
TheadsorptionofO2 onmesoporousAl2O3 leadstotheformationofamonolayerwith volume Vm 5 60cm3 g 1.Calculatethevolume V ofadsorbedO2 when θ 5 0.25.
(Continued)
Focus1.4:Numericalexamplesoffundamentalquantitiesincatalysis(Continued)
Solution
Byrearranging Eq.(1.1), V canbecalculatedas:
1
● ReactionrateandTOF
Considerthefollowingsecond-ordercatalyticliquid-phasereactionrunatatemperatureof500K:
2A-B
Amixtureofzincoxideandironoxide(ZnO/Fe3O4)isusedasacatalystforthereaction.The initialconcentrationofA(CA0)is0.12molL 1.ThepreexponentialfactorAintheArrhenius equationis1200L2 mol 1 g 1 s 1 andtheactivationenergy Ea is3.0kJmol 1.Knowingthatthe numberofadsorptionsitesinthecatalystis0.63molg 1,calculatetheinitialTOF.
Solution
Thesecond-orderrateconstantcanbecalculatedbytheArrheniusequation:
Theinitialreactionrate R0 canbeobtainedasfollows:
TheinitialTOFpergramofcatalystcanbeobtainedasfollows:
● Conversion,selectivity,yield
Aphotocatalyticbatchreactorisfedwithanaqueoussolutionoftoluene(A)havinganinitial concentrationof0.75M.Tolueneisselectivelyoxidizedtobenzaldehyde(B)usingaTiO2 catalystunderUVirradiation.Duringthephotocatalyticprocess,asmallfractionofthe formedbenzaldehydeisconvertedtobenzoicacid(C).After30minofreaction,theproduct containsthethreecompoundswiththefollowingconcentrations: CA 5 0.21M, CB 5 0.45M, CC 5 0.05M.Calculate XA, SB, SC, YB,and YC
Solution