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CATALYSIS1.IntroductionHomogeneouscatalysis,324Heterogeneouscatalysis,340Catalysis,isthekeytoefficientchemicalprocessing.Mostindustrialreactionsandalmostallbiologi-calreactionsarecatalytic.ThevalueoftheproductsmadeintheUnitedStatesinprocessesthatatsomestageinvolvecatalysisisapproachingseveraltrilliondollarsannually,whichismorethanthegrossnationalproductsofallbutafewnationsoftheworld.Productsmadewithcatalysisincludefood,clothing,drugs,plastics,detergents,andfuels.Catalysisiscentraltotechnologiesforenviron-mentalprotectionbyconversionofemissions.Thisarticleisanintroductionandsurveythatstatesthefundamentalprin-ciplesanddefinitionsofcatalysis,demonstratestheunityofthesubject,andplacesitinanappliedperspective.Theselectionofindustrialcatalyticprocessesdiscussedhasbeenmadeforthesakeofillustratingprinciplesandrepresenta-tivecharacteristicsofcatalysisandcatalyticprocesses.Acatalystisasubstancethatincreasestherateofapproachtoequilibriumofachemicalreactionwithoutbeingsubstantiallyconsumeditself.Acatalystchangestheratebutnottheequilibriumofthereaction.ThisdefinitionisalmostthesameasthatgivenbyOstwaldin1895.Thetermcatalysiswascoinedin1835byBerzelius,whorecognizedthatmanyseeminglydisparatephenomenacouldbedescribedbyasingleconcept.Forexample,fermentsaddedinsmallamountswereknowntomakepossibletheconversionofplantmaterialsintoalcohol;andtherewerenumerousexamplesofbothdecompositionandsynthesisreactionsthatwereapparentlycausedbyadditionofvariousliquidsorbycontactwithvarioussolids.Berzeliusattributedcatalyticactiontoill-definedforces,andthevalueofOstwald’smorelastingdefinitionisthatitidentifiedcatalysisasaphenomenonthatwasconsistentwiththeemergingprinciplesofphysicalchemistry.Nowitiswellrecognizedthatcatalystsfunctionbyformingchemicalbondswithoneormorereactants,therebyopeninguppathwaystotheirconversionintoproductswithregenerationofthecatalyst.Catalysisisthuscyclic;reactantsbondtooneformofthecatalyst,productsaredecoupledfromanotherform,andtheinitialformisregenerated.Thesimplestimaginablecatalyticcycleisthereforedepictedasfollows:CRCRPwhereRisthereactant,Ptheproduct,Cthecatalyst,andRCanintermediatecomplex.Theintermediatecomplexesincatalysisareoftenhighlyreactiveandnotobservable.Kirk-OthmerEncyclopediaofChemicalTechnology.CopyrightJohnWiley&Sons,Inc.Allrightsreserved.10.1002/0471238961.0301200107012005.a01.pub2Vol.1CATALYSIS499Ideally,thecatalystwouldcycleforeverbetweenCandRCwithoutbeingconsumed.Butinrealitytherearecompetingreactions,andcatalystsareconvertedintospeciesthatarenolongercatalysts.Inpractice,catalystsmustberegeneratedandreplaced.Catalystmanufactureisalargeindustry;catalystsworthsomeseveralbilliondollarsaresoldannuallyintheUnitedStates.Catalystsmaybegases,liquids,orsolids.Mostcatalystsusedintechno-logyareeitherliquidsorsurfacesofsolids.Catalysisoccurringinasinglegasorliquidphaseisreferredtoashomogeneouscatalysis(ormolecularcatalysis)becauseoftheuniformityofthephaseinwhichitoccurs.Catalysisoccurringinamultiphasemixturesuchasagas–solidmixtureisreferredtoashetero-geneouscatalysis;usuallythisissurfacecatalysis.Biologicalcatalystsareproteins,ie,poly(aminoacids),calledenzymes,andmetalloenzymes,whichareproteinsincorporatinginorganiccomponents,eg,ironsulfideclusters.Someenzymesarepresentincytoplasmicsolutionincellsandothersareanchoredincellmembranesoronsurfaces.Traditionally,homogeneouscataly-sis,heterogeneouscatalysis,andenzymiccatalysishavebeenregardedasalmostseparatedisciplines,andthelanguageandliteraturehavedevelopedwithoutmuchcoherenceandoverlap.Theperformanceofacatalystismeasuredlargelybycriteriaofchemicalkinetics,asacatalystinfluencestherateandnottheequilibriumofareaction.Thecatalyticactivityisapropertyofacatalystthatmeasureshowfastacata-lyticreactiontakesplaceandmaybedefinedastherateofthecatalyticreaction,arateconstant,oraconversion(ortemperaturerequiredforaparticularconver-sion)underspecifiedconditions.Theselectivityisameasureofthepropertyofacatalysttodirectareactiontoparticularproducts.Thereisnosingledefinitionofselectivity,butitissometimesdefinedasaratioofactivities,suchastheratiooftherateofadesiredreactiontothesumoftheratesofallthereactionsthatdepletethereactants.Selectivitymayalsoberepresentedsimplyasaproductdistribution.Becausecatalyststypicallyloseactivityand/orselectivityduringoperation,theyarealsoevaluatedintermsofstabilityandlifetime.Thestabilityofacatalystisameasureoftherateoflossofactivityorselectivity.Inpracticaltermsthestabilitymightbemeasuredasarateofdeactivation,suchastherateofchangeoftherateofthedesiredcatalyticreactionorastherateatwhichthetemperatureofthecatalystwouldhavetoberaisedtocompensatefortheactivityloss.Catalyststhathavelostactivityareoftentreatedtobringbacktheactivity,ie,reactivated;theregenerabilityisameasure,oftennotpreciselydefined,ofhowwelltheactivitycanbebroughtback.Technologicalcatalystsarealsoeval-uatedintermsofcost;inatypicalprocess,thecostofthecatalystisasmallfrac-tionofthetotalprocessingcost,oftenonlyafewcentsorlessperkilogramofproduct.Liquid-phasecatalystsareusedinbatchandflowreactors.Batchreact