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Ken-ichi Tanaka Dynamic Chemical Processes on Solid Surfaces

Chemical Reactions and Catalysis

DynamicChemicalProcessesonSolidSurfaces

DynamicChemicalProcesses onSolidSurfaces

ChemicalReactionsandCatalysis

Kashiwa,Chiba

Japan

ISBN978-981-10-2838-0ISBN978-981-10-2839-7(eBook) DOI10.1007/978-981-10-2839-7

LibraryofCongressControlNumber:2016958979

© SpringerNatureSingaporePteLtd.2017

Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart ofthematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations, recitation,broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped.

Theuseofgeneraldescriptivenames,registerednames,trademarks,servicemarks,etc.inthis publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse.

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Preface

Thedefi nitionofmaterialsisquitedifferentinchemistryandphysics.Thatis,a groupofmaterialsorcompoundshavingnovelfunctionalorchemicalpropertiesare considerednewmaterialsinchemistry.Fromthispointofview,thesurfaceofa solidisquiteinterestingasamaterial,becausechemicalpropertiesdependonthe crystalplanes;adifferentcrystalplanehasdifferentchemicalproperties.Forthis reason,thepropertyofsolidmaterialsknowntousisthatoftheirsurface,andno oneknowstherealchemicalpropertyofthesolidmaterialitself.

Ifweconsideratwo-dimensionalmaterialsuchasgraphene,graphite,and MoS2,theirsurfaceisaone-dimensionaledge.Thedifferentchemicalpropertyofa one-dimensionaledge,thesurface,isrevealedbybreakingitfromthatoftheparent material.GraphiteandMoS2 areverystablematerialschemicallyandthermally,but withtheirone-dimensionaledgeformedbybreaking,thesurfaceof two-dimensionalmaterialsprovidesactivenewmaterials.Infact,theedgeofMoS2 becomesanactivecatalystasshowninthisbook.

Ifthesurfaceformedbybreakingprovidesanewchemicalmaterial,thereaction ofsurfaceatomswouldprovideadditionalnewmaterials,althoughthesecouldnot beremovedfromthebulk.Accordingtothisidea,thenumerousphenomenataking placeonthesurfaceareresponsibleforthenewmaterialswhichcanexistonlyin thetwo-orone-dimensionalspace.Someofthesematerialsarepassive,butsome ofthemhaveuniquereactivityinchemicalreactionsandcatalysis,eventhough thesematerialscanexistonlyonaspeci ficsurface.Anadditionalimportantfeature ofthenewmaterialsisthatthechemicalreactiondynamicsarerestrictedintwo-or one-dimensionalspaceasdiscussedinthisbook.

About100yearsago,the firstscientifi cpageofcatalysisinmetalswasopened. Atthattime,ametalsurfacewasimaginedtobeanarrayofatomslikeachessboard.Inthelast50years,theremarkabledevelopmentofvacuumtechnologyand electronicshasenabledustodetectfeeblecurrentsandtoelucidatetheinherent propertiesofthesurfaceofsolidmaterials.However,mostworkhasfocusedonthe crystalstructureandtheelectronicstructureofthesurfaceinrelationtothoseof solidmaterials,whicharewellrationalizedbythebandstructureandthelattice energycalculationsinrelationtothoseofsolidmaterials.However,itcanbesaid

thatlittlestudyhasbeendonefromaviewpointthatthesurfacecomprisesnew chemicalmaterials,althoughmetalclustersarerecognizedasnewmaterial.

Thisbookthrowssomelightonthesurfaceasarealmofunexploredinteresting materialsexistingonthesolids.ThefollowingpassagefromRoaldHoffmannin SolidsandSurfaces AChemist’sViewofBondinginExtendedStructures is relevant.

Asurface beitofmetal,anionicorcovalentsolid,semiconductor isaformofmatter withitsownchemistry.Initsstructureandreactivity,itwillbearresemblancetoother formsofmatter:bulk,discretemoleculesinthegasphaseandvariousaggregatedstatesin solution.Anditwillhavedifferences.Justasitisimportantto findthesimilarities,itisalso importanttonotethedifferences.Thesimilaritiesconnectthechemistryofsurfacestothe restofchemistry,butthedifferencesmakelifeinteresting(andmakesurfaceseconomically useful).

Thepresentbookiscomposedoftwoparts: “DynamicChemicalProcesses onMetals” and “DynamicsofChemicalReactionsinCatalysis.” PartIillustrates variousunusualchemicalprocessesonmetalsurfacesregulatedbytwo-dimensional freedom,butthecharacteristicreactionsofsurfaceatomsarediffi culttoanticipate. WhenCu(100)andCu(110)surfacesareexposedtoO2 atroomtemperature,O2 moleculesdissociateintoO(a)atomsoneitheraCu(100)orCu(110)surface.TheO (a)atomsarestabilizedbyformingaminimumsizeofc(2 × 2)domains(madeby severalOatoms)onaCu(100)surface,whereasO(a)atomsonaCu(110)surface undergoreactionwithsurfaceCuatomsandone-dimensional(–Cu–O–)stringsare formedonaCu(110)surface,referredtoas “quasi-compounds” inthisbook.

Tounderstandrealcatalysis,wemustknowthedynamicchemicalprocesseson thesurface.AsdiscussedinPartII,ithasbeenacceptedthatthekineticequationof catalyticreactiondependsontheratedeterminingtheslowstepsofthereaction,but theintrinsicactivityofthesurfaceasacatalystdependsonthesteadyconcentration ofactivecompoundsformedonthesurface.

Theroleofpromotingmaterialsisquiteimportantinpracticalcatalysis,butit remainsanawkwardcommonproblemincatalysis.InPartII,Chap.10,Sects. 10.4 and 10.5,theroleofpromotingmaterialsinselectiveoxidationofCOinexcessH2 onmetalsiscarefullydiscussed.Itcouldbesaidthatthecooperationofdifferent functionalmaterialsoverthesamesurfaceisabigadvantageofheterogeneous catalysis.Ifefficientcooperationoftwoormorematerialsinaseriesofconnected reactionscouldbedesigned,thiswouldbethegoldenkeytoattaininga high-performancecatalyst,asdiscussedinSects. 10.4 and 10.5.Inthissense,this bookwillbeagoodguidenotonlyforthereaderinterestedinthebasicchemistry ofsolidsurfacesbutalsoforthereaderanticipatingthesurfacetobenewmaterials andhavinganinvaluableideafordesigninghigh-performancecatalysts.

Kashiwa,JapanKen-ichiTanaka September2016

Acknowledgements

Uponcompletingthewritingthisbook, firstofallImustexpressmygrateful acknowledgmenttoEmeritusProf.KenziTamaruofTheUniversityofTokyo,who firstenlightenedmeintheinterestingaspectsofthedynamicsofcatalysis.Iam indebtedtoEmeritusProf.YoshitadaMurataofTheUniversityofTokyo,who suggestedthatIbringtheprogressofsurfacesciencestocatalysis.Ialsoexpressmy sincerethankstoMr.MitsushiUmino,PresidentofAstecCo.,forhislong-time encouragementofmyresearchwork.Thegreatestappreciationistomywife, MasakoTanaka,forherlong-timesupportofmyresearchlife.

IdeeplyappreciateProf.FlemmingBesenbacheroftheUniversityofAarhusfor hisfruitfuldiscussionswithme.HisexperimentsusingtheSTMconvincedmeofa newconceptoflow-dimensionalquasi-compounds(one-orzero-dimensional compoundsonasurface)in1993.IalsoappreciateProf.BenE.Nieuwenhuys oftheUniversityofLeidenforhiscollaborationonaPt–Rh(100)singlecrystal, whichwasthestartofmyseriesworkonthePt–RhbimetalsandPt–Rhalloy catalystandtheideaoftheformationofa “newtwo-dimensionalPt–Rh-ordered alloy” onthesurface.IalsowanttoexpressmysincerethankstoProf.HongHe oftheResearchCenterforEco-EnvironmentalScience,ChineseAcademicof Sciences,andProf.Zhao-XiongXieandProf.YouzhuYuanofXiamenUniversity fortheircollaborationandmanyfruitfuldiscussionswithmeformyexperimental workattheSaitamaInstituteofTechnology.

Fortheexperimentsonwhichthisbookisbased,IamindebtedtomycollaboratorsandthegraduatestudentsattheResearchInstituteforCatalysis,theFaculty ofEngineeringofHokkaidoUniversity(1972–1984),theInstituteforSolidState PhysicsoftheUniversityofTokyo(1985–1998),andtheInstituteofAdvanced SciencesoftheSaitamaInstituteofTechnology(1999–2003).

MydeepappreciationgoestoMrs.QingcaiFengoftheResearchCenterfor Eco-EnvironmentalSciencesoftheChineseAcademyofSciencesforherhelpin completingthisbook,andtoDr.CatherineRiceforherverycarefulandsuitable improvementformydescriptions.IalsothankDr.ChristopherBorroni-Birdforhis help.

Finally,IwishtoexpressmygratitudetothelateProf.GeorgeBlyholderandthe lateProf.SamuelSiegeloftheUniversityofArkansasinFayetteville,Arkansas, USA,whohadagreatinfluenceonmeduringmystaythereasapostdoctoral researcher.

IalsoexpressmythankstoDr.ShinichiKoizumiofSpringerforhiswarm encouragementtopublishthisbook.

September2016Ken-ichiTanaka

Chapter1 Introduction

Abstract Variousuniquereactionsanddynamicmotionsofatomsandmolecules areknowntooccuronsolidsurfaces.Inthischapter,theauthorthrowslighton theseunusualnewmaterialsandtheirdynamicprocessesisonthesurface. Removingunnecessarydynamicmotionofadsorbedspeciesonthesurfaceisan importantstrategytosolvethereactionmechanismsandtherolesofpromoting materialsincatalysis.

Keywords Intermediates Dynamics Pt–Rhalloy Quasi-compounds Adsorption STM(scanningtunnelingmicroscopy) LEED(low-energyelectron diffraction) Activationofsurface

Solidmaterialsarenecessarilysurroundedbytwo-dimensional(2-D)materials formedbybreakingasshowninFig. 1.1a,andthepropertyofsurfaceisrecognized asthepropertyofsolidmaterials.Ifweconsiderthetwo-dimensionalmaterials suchasgraphiteandMoS2,theedgemadebybreaking,Fig. 1.1b,givesentirely differentpropertyfromthatofthematerials.Similarly,one-dimensionalmolecule serveszero-dimensionalradicalorionattheterminalendasshowninFig. 1.1c. Thearrayofatomsonthesurfaceiswellexplainedbythelatticeenergycalculations,andtheelectronicstructureofthesurfaceisalsowelldescribedbytheband structureatthesurface,butitisquitediffi culttoassertthechemicalpropertiesofthe surface.Ifthesurfaceisconsideredasone-dimensionallylowerednewmaterials, thenthechemicalreactionstakingplaceonthesurfacearethedynamicsofatoms andmoleculesrestrictedinone-dimensionallyloweredspace.Asdescribedinthis book,thedynamicsofatomsormoleculesonthesurfacearequitedifferentfrom ourknownreactionsin3-Dspace,becauseasurfaceprocesscannotbefreefromits mothermaterial.

IfweconsiderthelamellarcompoundMoS2,thesurfacemadebybreaking (definitionofthesurface)istheedgeofMoS2,whichisdifferentfromthe two-dimensionalbasalplaneofMoS2 (Sulfursheet);thatis,onedegree-of-freedom loweredmaterialsareformedbybreaking.Iftheatomsonthesurfacereactwith additionalatomsormolecules,thenadditionalnewcompoundsareformedalong

© SpringerNatureSingaporePteLtd.2017

K.Tanaka, DynamicChemicalProcessesonSolidSurfaces, DOI10.1007/978-981-10-2839-7_1

Fig.1.1 One-dimensionallylowerednewmaterialsareformedbybreaking: a Surfaceis two-dimensionalmaterialsformedbybreakingthree-dimensionalsolids. b Edgeis one-dimensionalmaterialsmadebybreakingtwo-dimensionalmaterials. c Zero-dimensional radialorionisformedbybreakingoflongmolecules

theedge.Thesenewlyformedcompoundsareevidentlydifferentfromadsorbed atomsormoleculesonthesurface,becauseadsorbedmoleculescanbetakenout fromthesurface,butthesenewlyformedcompoundsonthesurfacecannotbetaken outfromthesurfaceandcanexistonlyonthemothermaterial.

Fromthisviewpoint,aSTMexperimentonaNi(110)surfaceexposedtoO2 and H2 reportedbyBesenbacherandhiscoworkers[1]in1990wasagroundbreaking result,whichproveddefinitivelythedifferenceofareactionwithsurfaceatoms fromtheadsorptionofatomsonthesurface.Withthisexperiment,werealizedthe formationofunknowncompoundsbythereactionofadsorbedO(a)andH(a)with surfaceNiatoms.Thesecompoundshavetheirownstoichiometryandcharacteristicoflow-dimensionalmolecule-likestructureonspeci ficmetalsurfaces,buttheir featuresareentirelydifferentfromthoseofourknownmoleculesorcompounds. Thesenewmaterialsarediffi culttotakeoutthemfromthesurface;thatis,these materialscanexistonlyonthesurface.Takingthesefactsintoaccount,Tanaka[2] namedthesecompounds “pseudo-molecules” or “quasi-compounds” asis explainedindetailinChap. 4 in PartI

Formationofquasi-compoundsisevidentlydifferentfromtheadsorptionof atomsand/ormolecules,whichisclearlyshownontheCu(100)andCu(110) surfaceexposedtoO2 asshowninFig. 4.4 of PartI.Thatis,theadsorptionofO(a) takesplaceontheCu(100)surface,whereastheformationofthequasi-compound (–CuO–)proceedsontheCu(110)surface.Furthermore,onecanprepareadditional quasi-compound(–CuO–)onaAg(110)surfacebyusingthereactionof(–AgO–) onAg(110)withCuatoms.Newlyformedquasi-compoundof(–CuO–)ontheAg (110)undergoesreversiblereaction,(Cu2)3 +O2 ⇌ (–CuO–),whichprovides anotherquasi-compound(Cu2)3,ontheAg(110)surface.Furthermore,thereaction ofCuatomswiththequasi-compounds(–AgO –)andAg(CO3)preparedonAg (110)cancontroltheselectivitybytheelectrostaticpotentialbyusingaCu-coated STMW-tip,asshowninFigs. 5.1 and 5.2 in PartI.

Fromthispointofview,thereactionofheteroatomicmoleculessuchasCO,NO, andNH3 withsurfacemetalatomsformedisinterestingbecausetwoormore quasi-compoundsaresimultaneouslyformedonthesurface.ThereactionofNO moleculesonaCu(110)surfaceisaprominentexample.TheadsorbedNO

moleculesontheCu(110)surfaceprovide(–CuO–)stringsandCu3Ndots accordingtotheequationsofEq.(1.1).IfthissurfaceisfollowedbyexposingH2 at roomtemperature,then(–CuO–)stringsreactwithH2,butCu3Ndotscannotreact withH2.Accordingly,itisevidentthattheCu(110)surfaceisinactiveforthe catalyticreductionofNOwithH2.

Takingtheseresultsintoaccount,onecanexpectmoreinterestingchemical processesonbimetalsurfaces.Infact,averystrikingphenomenonwasobserved notonlyonthePt–Rh(100)alloysurface,butalsoonthePt/Rh(100)orRh/Pt(100) bimetalsurfaceswhenthesesurfaceswereexposedtoO2 orNOat400K.Pt(100) andRh(100)arenotactiveforthecatalyticreductionofNOwithH2,andaPt–Rh (100)alloyandtheRh/Pt(100)orPt/Rh(100)bimetalsurfacesarealsonotactivefor thecatalyticreductionofNOwithH2.However,thealloyandbimetallicsurfaces changetohighlyactivecatalystsat400KinO2 orNO,andthecatalyticreduction ofNOwithH2 wellproceedsattemperatureshigherthan400K.Thatis,ahighly activesurfaceisformedbyachemicalreconstructionofthesurfaceswithO2 orNO at400K.ItwasfoundthattheactivesurfaceforthereactionofNOwithH2 is composedofanorderedarrayofPtandRh,whichisanactivesurfaceresponsible forhighlyactivePt–Rhthree-waycatalyst[3, 4],buttheroleofeachPtandRh atominthecatalysisisstillunclear.

Toappreciatethepresentstatusofresearchoncatalystandcatalysis,abrief historyofcatalysisandthedevelopmentofsurfacesciencemaybehelpful.The fundamentalconceptofcatalysiswasestablishedbykineticstudiesonmetalsby IrvingLangmuir(NobelPrize1932)[5],buttherealroleofthesurfaceincatalysis wasanenigmaforalongtime,becausethesurfaceitselfhasbeenanimpenetrable blackbox,aswasjokinglydescribed, “thebulkwasmadebyGodbutthesurface wasmadebytheDevil,” byWolfgangPauli(NobelPrizein1945)in1930.Infact, knowledgeofthesurfacewasspeculativeuntilitsorderedstructurewasdeducedby thetoolofelectrondiffraction.Despitethis,thepotentialofcatalysiswashighlightedintheearlypartofthetwentiethcentury,whichmaderemarkabledevelopmentchemicalindustrybyusingtheso-calledheterogeneouscatalysts.The hydrogenationofunsaturatedhydrocarbonsusingaNicatalystbyPaulSabatier (NobelPrizein1912)andthesynthesisofammoniabythehydrogenationofN2 usingFeandOsmetalsbyFritzHaber(NobelPrizein1918)areseminalworks.

Asiswellknown,adiffractionphenomenonsuggestingthewavenatureof electrons,predictedbyDeBroglie(NobelPrize1929),wasaccidentallyfoundona contaminatedNisurfacebyC.J.Davison(NobelPrize1937)andL.H.Germer, whichwasthemostimportantgroundbreakingeventsinremovingthemysterious

veilofthesurface.Thestudyofsolidsurfaceswasrapidlyadvancedafterthe SecondWorldWar(1945)bythelow-energyelectrondiffraction(LEED)optics improvedbyL.H.Germer,whomadeavitalcontributiontothedevelopmentof surfacestudies.In1964,hewroteinPhysicsToday: “Themostsigni ficantresult thathasbeenobtaineduptothistimefromLEEDstudiesistheobservationof Reconstruction” [6].

Itisnownotdiffi culttoevaluatetheshiftingofatomsinthe firstseveralsurface layersbyintensityanalysisofLEEDpatterns,butwearestillsurprisedbyvarious unexpectedpropertiesofthesurfaceasmaterialsinvariouschemicalprocesses. Scanningtunnelingmicroscopy(STM),inventedbyBinnigandRohrer[7, 8]in 1983 (NobelPrizeinPhysics, 1987),causedarevolutioninthestudyofconductive materialsurfaces,anditisnowpossibletodirectlytouchthesurfaceofvarious materialsanddetecttheatomsandmoleculesadsorbedonasurfaceorthecompoundsformedbythereactionwithsurfaceatomsinatomicresolution.Adsorption ofatoms,andmolecules,andtheformationofintermediateswillcreatealocal distortionofthesurfaceatoms,butthedynamicchangesofthelocaldistortionwith adsorption,reaction,anddesorptionarediffi culttocaptureatthepresenttime.

Thedynamicsofadsorbedatoms,molecules,andintermediatespeciesonthe surfacearestilladifficultsubject,becauseitisessentiallydifferentfromthe dynamicsofmoleculesinthegasphaseorintheliquidphase.Agoodexampleis thesimultaneousdesorptionofN2 andNOfromaPd(110)surfaceat490K.When aPd(110)surfacewithcoadsorbedNO(a)andN(a)washeatedinvacuum,NOand N2 moleculesaresimultaneouslydesorbedat490K.DesorbedNOmoleculestake onaperpendiculardistributiontothesurface,butthesimultaneouslydesorbedN2 moleculesexhibittwospatialdistributions,oneofwhichisnormaltothesurface, buttheothertakesca.38°off-normalfromtheperpendiculartothesurfacealong the[001]directionasshowninFig. 8.2 in PartII.However,theN2 molecules formedbythecatalyticreactionofNOwithH2 onPd(110)surfaceshowedacos θ distribution.Catalysisisaseriesofconnectedchemicalreactionsoverthesurface, whichisfarmorecomplexcomparedtothoseinthegasphaseorintheliquid phase,andinterestingdynamicprocessestakingplaceonthesurfacesarediscussed indetailinChap. 8 of PartII.

Itshouldberememberedthatafreshlypreparedsurfaceisactiveforthe adsorptionandreactionofmolecules,butitisnotnecessarilyanactivesurfaceasa catalyst.Activefunctionalsurfaceormaterialsareformedduringcatalysisas schematicallydescribedinFig. 1.2 [3],andeachactivationprocesswillbediscussedforpracticalcatalyticreactionsin PartII

Itshouldbepointedoutthattherealmechanismofcatalysiscannotbeclari fied bythecharacterizationofcatalystorbythe firstprinciplecalculationbyusing statisticdataattainedbythecharacterization;thatis,experimentalevidencesupportingthedynamicsofaseries-connectedreactionisessential.Weshouldbe remindedthatacatalyticreactionisaccomplishedbyaseries-connectedchemical processesonthesurface,butthecontributionofsomeextraprocessesmakes confusetherealmechanismofcatalysis.OnegoodexampleistheHoriuti–Polanyi mechanism,inwhichthetwoindependentreactionscannotbedistinguishedby

(i) Catalysis by the original surface.

(Oxidation of CO with O2 on Pt)

(ii) Catalysis involving surface atoms as a reactant.

(Methanation of CO with H2 on Ni)

(iii) Catalysis by forming a new functional surface.

(NO reduction with H2 on Pt-Rh)

(iv) Catalysis by forming specific functional sites.

(Isomerization, Hydrogenation, Metathesis of olefins)

(v) Catalysis by multi-functional materials.

(PROX reaction of CO in H2 enhanced by H2O.)

Fig.1.2 Activecatalystisformedbyvariousactivationprocessesofthesurface.Practical examplesareshowninparentheses[3]

rapidsurfacescramblingofadsorbedhydrogenatoms.Onestrategytoclarifythe realmechanismincatalysisistoremoveunnecessarymigrationofintermediates duringcatalysis.Basedonthisidea,theisomerizationandthehydrogenationof olefinswereperformedonMoS2,wherethereactionshouldtakeplaceonthesites existingintheone-degreeloweredspaceoftheedgeofMoS2.Asaresult,twoalkyl intermediates,oneofwhichisactivefortheisomerizationandtheotherforthe hydrogenationofolefi ns,areclearlydistinguishedontheMoS2 catalyst[9, 10].

Theimportanceofreactiondynamicsincatalysisisdiscussedpreciselyin PartII. Asthereactionratedependsontherate-determiningslowstep,theimportanceof rapidprocessesincatalysisisveryoftenignored.Infact,ifarapidprocessisdisturbed,thecatalyticreactionstops,whichisaparadoxtoconsiderinthereaction kineticsofcatalysis.Ontheotherhand,itshouldberememberedthatthedynamic cooperationoftwoormorefunctionalsitesormaterialsonthesurfaceisan advantageousfunctionofheterogeneouscatalysis.Ifacatalyticreactionaccomplishedbythecooperationoftwodifferentfunctionalsitesisconsidered,catalysis willbedisturbedbystoppingthetransportofanintermediatebetweenthetwosites. However,itisdifficulttoconfirmwhethertheactivityisdisturbedbystoppingthe transportofintermediatesorthefunctionaldistortionofactivesites.Anillustrative exampleisthecatalyticoxidationofCOwithO2 enhancedbyH2 (actuallyH2O)on thePt/CNT(carbonnanotubes)catalystinferredfrom PartII, Sects. 10.4 and 10.5. InthecaseofPt/CNT,theCNThaveNi–MgOattheterminalend.IftheNi–MgOis removed(CNT–p),thePt/CNT–phasnocatalyticactivityfortheoxidationofCO enhancedbyH2 atlowtemperature[11, 12].Inthiscase,Ptparticlesonthetrunkof CNTarecompletelyseparatedfromtheNi–MgOlocalizedattheterminal

endoftheCNT,butthesetwoareindispensablefortheoxidationofCOenhancedby H2 (actuallyH2O).Takingthesefactsintoaccount,anewconceptisproposedforthe selectivity,whichdependsonthedynamics.AsitispreciselymentionedinChap. 10, Sect.10.4,theroleofaH2Omoleculeisexpressedbytheequation CO+ ½ O2 +nH2O → CO2 +nH2O,whereH2OisadynamicworkingmolecularcatalystoverthePt/CNTcatalyst[11–14].

Inthiscase,itisevidentthattheroleofNi–MgOisneitherthe “activationofPt metal” northe “formationofanactiveformofPt,” butprovidesanewoxidation reactionofCOtakingplacebyrapidtransportofOH– ionfromNi–MgOtothePt particlesasdiscussedindetailinSect. 10.5 of PartII.Thedynamicmechanismof theoxidationofCOenhancedbyH2 (H2O)wasclari fiedbyinsituDRIFTspectroscopy,andanewconceptfortheselectivitywasproposed,whichisgivenby repeatingdynamiccontributionofoneH2Omolecule.Anotherinterestingphenomenonincatalysisistheselectivecatalysisregulatedbytherestricteddynamic motionofintermediates;thatis,theselectivityisgivenbyselectiveinternalrotationalmotionofintermediatesasshowninFig. 10.5 in PartII.Inthiscase,the internalrotationofalargegroupoftheintermediatewasprevented,sothatthe correspondingcatalyticreactionwasselectivelyrestricted.Suchanapparentparadoxincatalysiscanobscuretheessentialrolecatalysis.Infact,wecanobservesuch aparadoxicalphenomenoninthehydrogenexchangereactionbetweenpropene[2H]0 andpropene-[2H]6 onMoS2 asshowninFig. 10.6 in PartII

Takingtheseresultsintoaccount,anewconceptof “dynamicworkingofH2O moleculeasamolecularcatalyst” isproposedinSects. 10.4 and 10.5 inthisbook. Wecanrationalizevariousunsolvedphenomenainthehighlyselectivepreferential oxidation(PROX)reactionofCOinexcessH2 bythisnewconcept.Forexample,a highlyselectivePROXreactionofCOwasattainedbyPtnanorodsheldinsideSiO2 nanotubes,whilenosuchPROXreactionwascatalyzedonPtrodssupportedonthe outersurfaceofSiO2 nanotubes,asshowninFig. 10.25 in PartII. Morestrange phenomenawereobservedinthePROXreactionofCOinH2 onaAu/CeO2 catalyst,onwhichselectiveoxidationofCOinH2 hadaremarkedeffectofthe crystalshapeofCeO2 althoughtheoverallconsumptionofO2 wasnotaffectedby thecrystalshapesofCeO2,asshowninFig. 10.31.Thesephenomenaarewell rationalizedbytheconceptof “dynamicworkingofH2Omoleculeasacatalyst”; thatis,theefficiencydependsontheresidencetimeofH2Oonthecatalystsurface. Infact,theselectivityforthePROXreactionofCOinH2 ontheAu/CeO2 catalyst dependsontheratio(Au+ +Au3+)/(Au0 +Au+ +Au3+),whichhasaninfluenceon theresidencetimeofH2O.However,thesitesresponsiblefortheoxidationreaction ofCOcatalyzedbyH2Oarestillnotclearlyidentifi ed.Theeffectofalkalimetal ions(Li+,Na+,andK+)ontheselectiveoxidationofCOinH2 isalsoaninteresting subject.Tillnow,theroleofalkalimetalionshasbeenexplainedbytheformation ofactivesites,suchasPtO(OH)x alkalimetalionsandAu–O(OH)x–(NaorK),but theoxidationreactionhasbeenpremisedinalmostallcasesasthesamereaction mechanismthathasbeenconfirmedtooccuronthePtsurface.Theoxidation reactionofformaldehyde(HCHO+O2 → H2O+CO2)onPt/TiO2 [15]andthe watergasshiftreaction(CO+H2O → H2 +CO2)onAuparticles[16]arealso

improvedbyalkalimetals.Suchacommoneffectofalkalimetalshasbeenmiscast asacommoneffectcausedbytheformationofanactiveformofthemetals,ora commonsynergisticeffectonthemetals.However,therealroleofpromoting materialsisfarmorecomplex,aswillbediscussedinmoredetailinSect. 10.5 of

PartII.

Theolefi nmetathesisreactionontheMoO3-x catalyst,discussedinSect. 10.3 of PartII,isatypicalexchangereactionofalkylidenegroups(=CHR)asdescribedby 2R1CH=CHR 2 → R1CH=CHR1 +R2CH=CHR 2.Inthemetathesisreaction ofpropene,ethyleneandbut-2-enearerecognizedastheproductsofareaction givenby2CH2 =CH–CH3 → CH2 =CH2 +CH3–CH=CH–CH3.Asdescribed indetailinSect. 10.3,wherethereactionispromotedbythealternatereactionof propenewithMo=CH2 andMo=CRCH3 sites,buttheequalnumberoftwosites doesnotmeanequalcontributionofthetwositesincatalysis.Thatis,anundetectablehiddenmetathesisreaction,namedasthedegeneratemetathesisreaction,is repeatedonthesamesites.Ifisotope-labeledmoleculesareused,onecandetermine thefrequencyofthehiddenreactionof(CH2 =CH–CH3 +*CH2 =*CH–*CH3) → (*CH2 =CH–CH3 +CH2 =*CH–*CH3).Surprisingly,theturnover frequencyofthishiddenmetathesisreactionofpropeneontheMo=CH–CH3 site isabout900timesfasterthanthatontheMo=CH2 siteonMoO3-x,asshownin Sect. 10.3 [17].Asdescribedintheoutlineofthisbook,anunderstandingofthe realcatalysisbasedonthedynamicprocessestakingplaceonthesurfacecanbe deducedonlybywell-plannedexperiments.Inthisrespect,weshouldbedeeply skepticalofexperimentallyunsupporteddynamicmechanismssuchastheone constructedtoexplainthecatalyticoxidationofethanolbyMo2(η–C3H5)4 supportedonamorphousSiO2,wherethecatalyticoxidationofethanolwasconjecturedtoinvolvedynamicconversionbetween(O=Mo6+ =O)2 ⇌ (O=Mo5+–O–Mo5+ =O)withoutanyexperimentalevidencesupportingthisdynamicprocess, onlybyanaverageMo–ModistanceonasurfacederivedbyusingEXAFS[18, 19].Itwillbeclearthatsuchalong-distancereversibleconformationalconversion linkedwiththefrequencyofreactionontheamorphousSiO2 surfaceisimplausible ifyouunderstandtherealdynamicstakingplaceonthesurface,whichisdescribed inthisbook.

References

1.F.Jensen,F.Besenbacher,E.Laegsgaard,I.Stensgaard,Phys.Rev.B.,42 (1990)9206.

2.K-I.Tanaka,Jpn.J.Appl.Phys.,32 (1993) 1389.

3.K-I.TanakaandHongHe,Catal.Today,201 (2013) 2.

4.K-I.Tanaka,ACSSymposiumSeries656Solid-LiquidElectrochemicalInterfaces,Chapter 18 (1997) 245–273.Edt.ByG.Jerkiewicz,M.P.Soliaga,K.Uosaki,andA.Wieckowski.

5.I.Langmuir,Trans.FaradaySoc.,17 (1921) 607.

6.L.H.Germer,Phys.,Today,17,No.7 (1964) 19.

7.G.Binnig,H.Rohrer,Ch.GerberandE.Weibel,Phys.Rev.Lett.,50 (1983) 120.

8.G.BinnigandH.Rohrer,Surf.Sci.,126 (1983) 236.

9.K-I.Tanaka,AdvancesinCatalysis,Vol.33,99–158.Acad.Press (1985).

10.W.M.H.Sachtler,FaradayDiscuss.Chem.Soc.,72 (1981) 7.

11.K-I.Tanaka,M.Shou,andY.Yuan,J.Phys.Chem.,C114 (2010) 16917.

12.K-I.Tanaka,HongHe,andY.Yuan,RSC,Advances,5 (2015) 949.

13.K-T.Tanaka,HongHe,M.Shou,X.Shi,Catal.Today,175 (2011) 467.

14.K-I.Tanaka,M.Shou,HongHe,andX.Shi,Catal.Letters,110 (2006) 185.

15.C.Zhang,F.Liu,Y.Zhai,H.Ariga,N.Yi,Y.Liu,K.Asakura,M.Flytzani-Stephanopoulos andHongHe,Angew.Chem.Int.,31 (2012) 9628.

16.Y.Zhai,D.Pierre,R.Si,W.Deng,P.Ferrin,A.UNilekar,G.Peng,J.A.Herron,D.C.Bell, H.Saltsburg,N.MavrikakisandM.F.Stephanopoulos,Sci.,329 (2010) 1633.

17.K-I.Tanaka,Appl.Catalysis.A,General,188 (1999)37.

18.Y.Iwasawa,K.Asakura,H.Ishii,andH.Kuroda,Z.Phys.Chem.,144 (1985) 105.

19.Y.Iwasawa,Adv.inCatal.35 (1985) 187.

Chapter2

SurfaceStructuresandtheCrystalHabit ofGrowingParticles

Abstract Atruncatedsingle-crystalsurfacevariestoamorestable two-dimensionalarrayofatomsbyraisingthetemperature.Ontheotherhand, metalparticlesmaygrowviaatwo-dimensionalprecursorarrayofatoms.The (1 × 1)Au(111),Au(110),andAu(100)surfacesundergoreconstructioninelectrolytesolutionsatacertainnegativeelectrodepotential(vs.SCE).Theicosahedral ordecahedralAuparticlesarepreferentiallygrownonanelectrodeheldatthesame negativeelectrodepotentialmakingreconstructionofAucrystalsurfaces.The resultsuggeststhatmetalparticlesmaygrowviaprecursorstatesofmetalarray whicharesimilartotwo-dimensionalarrayofthereconstructedsurfaces. Multi-twinPtparticlesarealsoformedathighlynegativeelectrodepotential.

Keywords Reconstructionofmetalsurfaces Pt(100) Au(111) Au(100) Au (110) Si(111) Electrodepotential Multi-twinmetalparticles Potential-induced surfacereconstruction Quasi-compounds UPD(under-potentialdeposition) Cu–AualloyparticlesformedbyUPD

A(1 × 1)-Pt(100)surfaceisstabilizedbyformingahexagonal(5 × 20)-Pt(100) surface,anda(1 × 1)-Si(111)surfaceisreconstructedtoa(7 × 7)-Si(111)surface. Thereconstructedsurfaceisdirectlyobservedbythescanningtunnelingmicroscopy(STM)asshowninFig. 2.1aandb[1].Inthecaseofatruncated(1 × 1)-Si (111)surface,everySi-atomhasoneunsaturatedcovalentbondcalleda “dangling bond,” thesurfaceisstabilizedbyloweringthenumberofSiatomswithadangling bondinvacuumatca.1200K,andacharacteristic(7 × 7)-Si(111)surfaceis formed.

Ifatomsormoleculesareadsorbedonastabilizedsurface,thesurfacemay undergoadditionalreconstruction,theso-calledadsorption-inducedreconstruction, wherebythetotalenergyincludingtheadsorptionenergyisloweredbythe reconstruction.Thereversereconstructionfromastabilized(5 × 20)-Pt(100)clean surfacetoametastable(1 × 1)-Pt(100)surfaceisattainablebyusingadsorptionof COandO2 [1].Thatis,theheatofadsorptionofO2 onthe(1 × 1)-Pt(100)surface ishigherthanthatonthehex-(5 × 20)-Pt(100)surface,whichisthedrivingforce

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K.Tanaka, DynamicChemicalProcessesonSolidSurfaces, DOI10.1007/978-981-10-2839-7_2

(22x 3)-Au(111); Herring-bone Structure

Fig.2.1 Typicalexamplesofthesurfacetakinga2-Dstabilizedconformation: a hex-(5 × 20)Pt (100)surface[1], b (7 × 7)-Si(111)surface,and c (22 × √3)-Au(111)surface,namedherringbone structure,formedbyalternatedstackingofhcpandfccregions,withbrightridgelinesappearing betweenthem,asshownbyamodelof(22 × √3)-Au(111)[3]

forthereversereconstructionfrom(5 × 20)-Pt(100)toametastable(1 × 1)-Pt (100)surface.Bythisprocess,ametastable(1 × 1)-Pt(100)surfaceisattainableby removingtheadsorbedoxygenbyreactingwithH2 atlowtemperature. ReconstructionoftheAu(111)surfaceisanotherinterestingexample,induced byanisotropicshorteningofthelatticedistanceinthetopmostlayer.Inthiscase,an anisotropicshorteningof * 4.2%oftheAu–Aulatticedistance,from0.50to 0.48nm,occursinthetopmostlayeralongthe 〈211〉 directiononthe(1 × 1)-Au (111)plane.Asaresult,theAuatomsinthetopmostlayermakeanarrayof ABC/ABC(fccstructure)areasandAB/AB(hcpstructure)stackingareasseparated byridgesofatoms.Figure 2.1cshowsanSTMimageofareconstructedAu(111) surface,theso-calledherringbone(22 × √3)-Au(111)surface,wherethewiderdark stripecorrespondstoafccstackingarea,thenarrowdarkstripecorrespondstoa hcpstackingarea,andthebrightlinebetweenthemisaridgeofAuatoms(the (1 × 1)-Au(111)surfacehasthreeequivalentaxes)[3].TheAu–Audistanceofthe reconstructed(22 × √3)-Au(111)structureisreducedfromthebulkdistanceof0.50 to0.48nmalongthe 〈1 10〉 axis,whichisaca.4.2%reductionintheAu–Au distance.Asaresult,a(22 × √3)-Au(111)surfacewiththeslightlydistorted chevronor “herringbone” patternshowninFig. 2.1cisformed.The(22 × √3)-Au (111)surfaceisenergeticallyinhomogeneousduetolatticestrain,whichinfluences

thepreferentialgrowthofnanoparticlesofAu[2],Co[3],Fe[4],Ni[5],andAg[6] attheelbowsitesonthe(22 × √3)-Au(111)surface.

AnothernotablefeatureoftheAu(111)surfaceisareversiblereconstructionin aqueoussolutionthatcanbeinducedbyanelectrochemicalpotential.The potential-inducedreconstructionoftheAusurfacewas fi rstsuggestedbyHamelin [7]toexplainthehysteresisoftheelectro-capacitancecurveofanAu(111)surface, andthereconstructionwasconfi rmedbyKolbetal.byusingLEED[8].A(1 × 1)Au(111)surfacechangestoa(22 × √3)structurewhenitisbiasedatacertain negativeelectrochemicalpotentialrelativetothesaturatedcalomelelectrode (SCE)inanelectrolytesolution,whichissimilartothereconstructiontakingplace invacuumonraisingtemperature.ItisnotablethatAu(111),Au(100),andAu(110) surfacesallundergoreconstructionatanequallynegativepotential,buttheelectrochemicalpotentialforthereversereconstructiontothe(1 × 1)surfacedepends onthecrystalplane,Au(110)<Au(100)<Au(111),dependingonthedensityof Auatoms,asshowninFig. 2.2a[9, 10].Thisresultsuggeststhatthearrayofthe topmostAuatomsdependsontheelectrochemicalpotentialinaqueoussolution.If thestructureofthetwo-dimensionalarrayofAuatomsdependsonthe

Fig.2.2a StablesurfacestructuresofAu(111),Au(110),andAu(100)dependingonthe electrochemicalpotential(E)withrespecttoSCEin0.1MHClO4 solution. b Auparticlesgrown onaC-coated-Au-meshelectrodein50mMHClO4 +1mMHAuCl4 solution.Crystalhabitof Auparticlesdependsontheelectrochemicalpotentials[11]. c Relativepotentiallevelsofan electrode(0ϕM)inanelectrolytesolutionwithrespecttotheSCEandtheFermilevelinthe vacuum

Fcc Octahedral Particles
Icosahedral & Decahedral Particles (b)

electrochemicalpotential,thegrowthofAuparticlesfromAu3+ ionsonanelectrodemaybeinfluencedbytheelectrodepotential,whichwouldbethearrayofAu atomsinaprecursorstateofthegrowthofacrystal.Ifthisisthecase,theelectrode potentialwillgivesomeeffectonthegrowingformofcrystals.Aspredicted, multi-twinAuparticlesgrowonacarbon-coatedAugauzeheldattheelectrochemicalpotentialinducingthereconstructionofAu(111),Au(100),andAu(110) surfacesasshowninFig. 2.2b[11, 12].Amoreinterestingfactisthatasimilar potentialeffectisobservednotonlyonAuparticlesbutalsoonmostfccmetal particles,althoughpotential-inducedsurfacereconstructionshavebeenreported onlyonAusurfaces.

Ifweconsidertheelectrodepotential(0ϕM)fromthevacuumlevel,itisgivenby theequation: 0ϕ

+EasillustratedinFig. 2.2c,wherethe electrodepotentialE(V)isreferredtoSCE(thestandardsaturatedcalomelelectrode).IftheFermileveloftheelectrolytesolution(0ϕS)canbepresumedtobein therangeof0.1 * 0.2V[13],onecanestimatetheelectrodelevel(0ϕM)tothe potentialleveloftheSCE(0ϕR)fromthevacuum.Bytheshiftofthevibrational frequencyofadsorbedCOonaPt(111)electrodeasafunctionoftheelectrostatic potential,onecanevaluatetheFermilevelofthemetalinsolutioncomparedtothat invacuum.Whentheelectrodepotentialisheldatthezero-charge(Ezc)potential, thestretchingvibrationalfrequencyofadsorbedCOonthePtelectrodebecomes equaltothatofadsorbedCOonPt(111)invacuum[14].Takingtheseresultsinto account,theFermilevelofanelectrode(0ϕM)fromthevacuumlevelrelativeto SCE(E)isschematicallyshowninFig. 2.2c.AsshowninFig. 2.2b,whenan electrodeisheldatanegativepotentialwithrespecttoSCE,thefractionsof decahedralandicosahedralAuparticlesincreasewithdecreasing(morenegative) potential.

Theleft-hand-sideTEMimageshowninFig. 2.3ashowsanordinaryoctahedral Aucrystalparticleformedat+0.33Vanditselectrondiffractionpatternalong the 〈100〉 direction,andtheothertwoTEMimagesofAucrystalparticlesare formedat 0.30and 0.55V,respectively[11, 12].ThefractionsofAucrystal particlesformedonanAu-meshelectrodeinaHAuCl4 (1 × 10 3 M)+HClO4 (0.5M)solutionatpotentialsrangingfrom0.35to 0.55V(SCE)areshownin Fig. 2.3b.ItisknownthattheshapeofAucrystalparticlesisclearlyaffectedbythe electrochemicalpotentialasshownbytheTEMimage,andthedecahedralAu particlesformedat 0.30VandicosahedralAuparticlesat 0.55Vareshownin Fig. 2.3a.Theratioofmulti-twinparticlestothetotalparticlespermm2 (B/A) linearlyincreaseswithrespecttothenegativeelectrodepotentialasshownin Fig. 2.3b,andmorethan60%ofAuparticleswereeithericosahedralordecahedral,withthedecahedralAuparticlesbeingpreferred.

Potential-inducedsurfacereconstructionoftheAu(111)surfaceiscausedbyan anisotropicshorteningoftheAu–Audistancein4–5%,andradiallyshaped nanometer-sizedAuparticlesresultedfromthegrowthofmultiplytwinnedparticles [15].Formationofmulti-twinAuparticles,aswellasdecahedralandicosahedral Auparticles,was fi rstobservedbyInoandOgawa[16]onaheatedNaClcrystal planeinvacuumandalsoobservedonaheatedgraphitesurfacebyWaymanand

Fig.2.3a TEMimagesofoctahedralAuparticlesgrownat+0.33V,andofdecahedraland icosahedralAuparticlesattainedat 0.30,and 0.55V[11].Electrondiffractionpatternalong the 〈100〉 directionofthefccoctahedralAuparticlesgrownat0.35V. b TotalnumberofAu particles(A)permm2 formedin60s,andthenumberofdecahedralandicosahedral multi-twinparticles(B) dependingonthepotential(E).TheB/Aratioshowspreferential growthofmulti-twinparticles(B)atnegativeelectrochemicalpotentials[12]

Darby[17].ThedecahedralandicosahedralAuparticleswereexplainedas resultingfromtheaggregationof fi veandtwentytwinparticles.Takingtheseresults intoaccount,thetotalsurfaceenergyofparticlesisloweredbyformingtwin boundaries,whichisthedrivingforceforthegrowthoficosahedralordecahedral multi-twinAuparticles.

ThereconstructedstructureisreleasedbykeepingtheAusurfaceatapositive potential(SCE)insolution,andthe(1 × 1)surfaceisrecovered.However,the icosahedralanddecahedralAuparticlescannotbechangedtooctahedralfccparticlesbyholdingthematthecorrespondingpositivepotentialsinsolution.Inother words,thetwo-dimensionalarrayofAuatomsisreversibledependingonthe electrochemicalpotential,butonceformed,bulkstructureofAuparticlescannotbe reformedbythechangeinsurfacestructure.Itshouldbepointedoutthat potential-inducedsurfacereconstructionhasnotbeenobservedonotherthanAu surfaces,butthegrowthofdecahedralandicosahedralparticleswasobservedfor Cu,Ni,Pd,andAgparticlesatnegativeelectrodepotential(SCE)[18].Ptisa highlystablemetalgivingnobulkphasereconstruction,butdecahedraland/or icosahedralPtparticleswereformedat 0.9V(SCE)inasolutionof0.05M CsClO4 +1mMPtCl4 at 0.5V,whereneitherCl norClO4 ionswereadsorbed onthePtsurface[19].Theseresultsmightreflectthearrayofmetalatomsinthe precursorstateofnano-sizedmetalparticlesgrowinginsolutionundertheelectrostatic field.

Asisknown,theelectrochemicalself-depositionofmetalionsoccursatthe Nernstequilibriumpotential(Er),butnoelectrochemicaldepositionproceedsat potentialsmorepositivethanthereversibleNernstequilibriumpotentialEr However,thedepositionofforeignmetalatomsoccursatpotentialsmorepositive thantheNernstself-depositionpotential,whichisknownastheunder-potential deposition(UPD)offoreignmetalatoms.Figure 2.4 showstheunder-potential deposition(UPD)ofCu2+ ionsonaAu(111)surfaceinaH2SO4 solutiongivenby Kolbetal.[20].A(√3x √3)R30structureisadoptedontheAu(111)surfacewith UPD-depositedCu+ ions,anditchangestoa(1 × 1)structureondepositionofCu0 asillustratedinFig. 2.4.Inessence,UPDofCu2+ ionscreatesa2/3monolayerof Cu+ ionsontheAu(111)surface,andonemonolayerdepositionofCu0 isformedat thelowerelectrodepotential[21, 22].Manneetal.[23]studiedastripped-down electrodepositedCulayeronaAu(111)surfaceatanUPDpotentialindifferent electrolytesbyatomicforcemicroscopy(AFM),showedthattheUPDmonolayer formeddifferentstructuresindifferentelectrolytes,andshowedthatthedeposited CuatomsonAu(111)surfaceadoptaclose-packedarraywithaspacingof0.29nm inaperchloricacidelectrolytesolution,buttakeonamoreopenlatticespacingof 0.49nminasulfateelectrolyte.

Whenacarbon-coatedAu-meshelectrode(C-coatedAu)isheldattheNernst potentialofAu3+ ioninasolutionof(Au3+ +Cu2+)ions,Auparticlesareformed, butnogrowthofindividualCu0 particlesoccurs.However,theUPDofCu2+ ions createsaCu+ orCu0 layerontheAuparticlesformedontheC-coatedelectrode; thatis,Au3+ andCu2+ ionsarealternatelydepositedoneaftertheotherattheUPD potentialoftheCu2+ ion.ThecompositionofCu/AuUPDparticlesformedonan Au-meshelectrodeinasolutionof1mMHAuCl4 +0.1MHClO4 + ξ MCu (ClO4)2 (ξ =0.10M * 0.001M)variesdependingontheconcentrationofCu2+ ionsandelectrodepotentialasshowninFig. 2.5b,andtheCu/Auratioofalloy particlesextrapolatedto(E Er ≃ 0)inFig. 2.5bsuggeststheformationof50% Cuand50%Aualloyparticles[12, 24–27.Theresultsareingoodagreementwith

Au(111)

Random Au(111)

(√3 x √3)R30 Cu+ Au(111)

(1 x 1) Cu0

Au(111)

Deposition of bulk-Cu

Fig.2.4 Under-potentialdeposition(UPD)ofCu2+ iononaAu(111)surface. a Cycliccurrent–potentialcurve(1mV/sec)showingunder-potentialdeposition(UPD)ofCu2+ ionsonanAu(111) surfacein0.05MH2SO4 +1mMCuSO4 b ThestructureofAu(111)surfacechangedbythe depositionofCu+ andCu0,wherethecoveragewasderivedfromthechargeofUPDCuions[20].

c IllustrationofthedepositedofCu+ andCu0 ontheAu(111)surface

theUPDmechanismfortheformationofCu–Aualloyparticlesasonemonolayer Cu0 depositsontheAu(111)surface,asshowninFig. 2.4

WhentheelectrodepotentialisheldathigherthantheUPDpotentialEr (E Er >0),thedepositionrateofAu3+ ionstoAu0 isincreasedwithincreasing potential,whereastheUPDrateofCu2+ ionsmayremainataconstantrate,sothat thefractionofAuintheCu–Auparticlesincreaseswithelectrodepotentialina 1mMAu3+ ionsolution.ItisanoteworthyresultthattheCu/Auratiomaintainsa constantvalueoverawideconcentrationrangeofCu2+ ions(0.1–0.001M)at potentialsmorepositivethantheNernstpotentialofAu3+ ion(E Er ≃ 0).The layer-by-layerdepositionofAu3+ andCu2+ ionsformingCu–AualloywasconfirmedbytheDebye–ScherrerringsasshowninFig. 2.5a,wherethelatticeofthe Cu–Aualloy(Cu30%)particleswasshortenedcomparedtothatofAu.Ifthe experimentisperformedatlowerthantheNernstequilibriumpotentialofCu2+ ion (E<Er),thedepositionrateofCudependsontheconcentrationofCu2+ ion,sothat

Fig.2.5 FormationofCu–AualloyparticlesbysimultaneouselectrochemicaldepositionofCu2+ andAu3+ ionsintheUPDregionofCu2+ depositiononAuparticles. a Debye–ScherrerringofAu particles(left)preparedin1mMHAuCl4 +0.1MHClO4 at 0.5V(SCE)andofCu–Aualloy particles(Cu30%)formedin1mMHAuCl4 +0.1MHClO4 +50mMCu(ClO4)2 at0.1V (SCE). b Au(%)inCu–Aualloyparticlesdependingontheelectrodepotentialinasolution1mM HAuCl4 +0.1MHClO4 + ξ MCu(ClO4)2,wheretheconcentrationofCu2+ ion(ξ)waschanged from0.1Mto0.001M.Er isthereversibleNernstpotentialofCu2+ ion.Arrowsindicatethe potentialforsimultaneousformationofCu2O. c AmodelofUPDofCu+ onanOPDAulayer, whichchangestoaCu–Aualloyparticle[24]

thecompositionoftheCu–AualloyparticlesdependsontheconcentrationofCu2+ ioninsolution,asshownwiththearrowsinFig. 2.5b.ItisnotablethatCu–Au alloyparticlesalsoformmulti-twinparticles,althoughtheUPDofCu2+ iononAu particlesoccursatapotentialmorepositivethanthatrequiredforthe potential-inducedreconstructionofAusurfaces,andalsomorepositivethanthe formationpotentialofthedecahedralandicosahedralmulti-twinAuparticles showninFig. 2.2aandb[18, 24, 28, 29].Thisresultmayindicatethatthe shorteningofthelatticedistancebyformingaCu–Ausurfacealloyisresponsible fortheformationofmulti-twinCu–Aualloyparticles.

References

1.A.Borg,A.-M.Hilmen,andE.Bergene,Surf.Sci.,306 (1994) 10. 2.C.A.Lang,M.M.Dovek,J.Nogami,andC.F.Quate,Surf.Sci.,224 (1989) L947.

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8.M.D.Kolb,G.Lehmpfuhl,andM.S.Zei,J.Electroanal.Chem.,179 (1984) 289.

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Chapter3

Self-AssembledArrayofAtoms andMoleculesonMetals

Abstract Moleculesadsorbedonametalsurfacearestabilizedonthesurfaceby adsorptionorbyreactingwithsurfaceatoms.Byeitheradsorptionorbyreacting withsurfaceatoms,theyprovidenewchemicalmaterials.Atypicalexampleisa two-dimensionalcrystallayerofn–C17H36 andn–C36H74 moleculesformedona (22 × √3)-Au(111)surface.Theymakeatwo-dimensionaleutecticcrystallayeron a(22 × √3)-Au(111)surface,whichisaninterestingtwo-dimensionalnewmaterial. AsdiscussedinChap. 9,thephenomenonissimilartotheformationofanordered newalloylayeronaPt–Rh(100)surfaceat490KinO2,althoughPt–Rhisa randommixingalloy.Thereactionofadsorbedatomormoleculewithsurface atomsprovidesadditionalnewtwo-dimensionalmaterialsnamedas “quasi-compound” inChap. 4.

Keywords SurfacereconstructionofAu(111) Self-assembledarrayofalkane molecules Two-dimensionalcrystallayerofn–C25H52 andn–C28H58 on (22 × √3)-Au(111)surfaces Two-dimensionaleutecticcrystallayer Structureof domainboundary Orderingofalkanemoleculeswithgraphitesurface

WhenametalsurfaceisexposedtosimplemoleculessuchasH2,O2,orN2, adsorbedmoleculesreactoftenwithmetalatomsonthesurfacetoformnew compounds.WhenheteroatomicmoleculessuchasNOandCOareadsorbedona metalsurface,thereactionwithmetalatomsprovidestwodifferentkindsofcompoundsonthesurface.Astheincreaseofcompounds,theymakecharacteristictwo differentarraysoverthesurfacedependingonthecrystalplane,whichissimilarto thearrayoftwosortsofmoleculesoverthesurface.Whenweconsiderthereaction ofadsorptionmoleculeswithsurfaceatoms,wehavetoconsideragainthefeatures ofsurfacedefinedinFig. 1.1.Considerthecoadsorptionoftwon-alkanemolecules withonetwicethelengthoftheotherona(22 × √3)-Au(111)surfaceandon graphite.(22 × √3)-Au(111)isareconstructedAu(111)crystalplane,butthebasal

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Title: Happy ending

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Language: English

Original publication: United States: Standard Magazines, Inc, 1948

Credits: Greg Weeks, Mary Meehan, Alex White & the online Distributed Proofreaders Canada team at https://www.pgdpcanada.net. *** START OF THE PROJECT GUTENBERG EBOOK HAPPY ENDING ***

HAPPY ENDING

Out of the Future emerge the Robot and Tharn—while James Kelvin fights them blindly, knowing not friend from foe!

[Transcriber's Note: This etext was produced from Thrilling Wonder Stories, August 1948. Extensive research did not uncover any evidence that the U.S. copyright on this publication was renewed.]

The android uttered a protesting cry as Kelvin sent a wave of mental energy at him

This is the way the story ended:

James Kelvin concentrated very hard on the thought of the chemist with the red mustache who had promised him a million dollars. It was

simply a matter of tuning in on the man’s brain, establishing a rapport. He had done it before. Now it was more important than ever that he do it this one last time. He pressed the button on the gadget the robot had given him, and thought hard.

Far off, across limitless distances, he found the rapport. He clamped on the mental tight beam.

He rode it....

The red-mustached man looked up, gaped, and grinned delightedly.

“So there you are!” he said. “I didn’t hear you come in. Good grief, I’ve been trying to find you for two weeks.”

“Tell me one thing quick,” Kelvin said. “What’s your name?”

“George Bailey. Incidentally, what’s yours?”

But Kelvin didn’t answer He had suddenly remembered the other thing the robot had told him about that gadget which established rapport when he pressed the button. He pressed it now—and nothing happened. The gadget had gone dead. Its task was finished, which obviously meant he had at last achieved health, fame and fortune. The robot had warned him, of course. The thing was set to do one specialized job. Once he got what he wanted, it would work no more.

So Kelvin got the million dollars. And he lived happily ever after

This is the middle of the story:

As he pushed aside the canvas curtain something—a carelessly hung rope—swung down at his face, knocking the horn-rimmed glasses askew. Simultaneously a vivid bluish light blazed into his unprotected eyes. He felt a curious, sharp sense of disorientation, a shifting motion that was almost instantly gone.

Things steadied before him. He let the curtain fall back into place, making legible again the painted inscription: horoscopes—learn your future—and he stood staring at the remarkable horomancer.

It was a—oh, impossible!

The robot said in a flat, precise voice, “You are James Kelvin. You are a reporter. You are thirty years old, unmarried, and you came to Los Angeles from Chicago today on the advice of your physician. Is that correct?”

In his astonishment Kelvin called on the Deity. Then he settled his glasses more firmly and tried to remember an exposé of charlatans he had once written. There was some obvious way they worked things like this, miraculous as it sounded.

The robot looked at him impassively out of its faceted eye.

“On reading your mind,” it continued in the pedantic voice, “I find this is the year Nineteen Forty-nine. My plans will have to be revised. I had meant to arrive in the year Nineteen Seventy. I will ask you to assist me.”

Kelvin put his hands in his pockets and grinned.

“With money, naturally,” he said. “You had me going for a minute. How do you do it, anyhow? Mirrors? Or like Maelzel’s chess player?”

“I am not a machine operated by a dwarf, nor am I an optical illusion,” the robot assured him. “I am an artificially created living organism, originating at a period far in your future.”

“And I’m not the sucker you take me for,” Kelvin remarked pleasantly. “I came in here to—”

“You lost your baggage checks,” the robot said. “While wondering what to do about it, you had a few drinks and took the Wilshire bus at exactly—exactly eight-thirty-five post meridian.”

“Lay off the mind-reading,” Kelvin said. “And don’t tell me you’ve been running this joint very long with a line like that. The cops would be after you. If you’re a real robot, ha, ha.”

“I have been running this joint,” the robot said, “for approximately five minutes. My predecessor is unconscious behind that chest in the corner. Your arrival here was sheer coincidence.” It paused very briefly, and Kelvin had the curious impression that it was watching to see if the story so far had gone over well.

The impression was curious because Kelvin had no feeling at all that there was a man in the large, jointed figure before him. If such a thing as a robot were possible, he would have believed implicitly that he confronted a genuine specimen. Such things being impossible, he waited to see what the gimmick would be.

“My arrival here was also accidental,” the robot informed him. “This being the case, my equipment will have to be altered slightly. I will require certain substitute mechanisms. For that, I gather as I read your mind, I will have to engage in your peculiar barter system of economics. In a word, coinage or gold or silver certificates will be necessary. Thus I am—temporarily—a horomancer.”

“Sure, sure,” Kelvin said. “Why not a simple mugging? If you’re a robot, you could do a super-mugging job with a quick twist of the gears.”

“It would attract attention. Above all, I require secrecy. As a matter of fact, I am—” The robot paused, searched Kelvin’s brain for the right phrase, and said, “—on the lam. In my era, time-traveling is strictly forbidden, even by accident, unless government-sponsored.”

There was a fallacy there somewhere, Kelvin thought, but he couldn’t quite spot it. He blinked at the robot intently. It looked pretty unconvincing.

“What proof do you need?” the creature asked. “I read your brain the minute you came in, didn’t I? You must have felt the temporary amnesia as I drew out the knowledge and then replaced it.”

“So that’s what happened,” Kelvin said. He took a cautious step backward. “Well, I think I’ll be getting along.”

“Wait,” the robot commanded. “I see you have begun to distrust me. Apparently you now regret having suggested a mugging job. You fear I may act on the suggestion. Allow me to reassure you. It is true that I could take your money and assure secrecy by killing you, but I am not permitted to kill humans. The alternative is to engage in the barter system. I can offer you something valuable in return for a small amount of gold. Let me see.” The faceted gaze swept around the tent, dwelt piercingly for a moment on Kelvin. “A horoscope,” the robot said. “It is supposed to help you achieve health, fame and fortune. Astrology, however, is out of my line. I can merely offer a logical scientific method of attaining the same results.”

“Uh-huh,” Kelvin said skeptically. “How much? And why haven’t you used that method?”

“I have other ambitions,” the robot said in a cryptic manner. “Take this.” There was a brief clicking. A panel opened in the metallic chest. The robot extracted a small, flat case and handed it to Kelvin, who automatically closed his fingers on the cold metal.

“Be careful. Don’t push that button until—”

But Kelvin had pushed it....

He was driving a figurative car that had got out of control. There was somebody else inside his head. There was a schizophrenic, doubletracked locomotive that was running wild and his hand on the throttle couldn’t slow it down an instant. His mental steering-wheel had snapped.

Somebody else was thinking for him!

Not quite a human being. Not quite sane, probably, from Kelvin’s standards. But awfully sane from his own. Sane enough to have mastered the most intricate principles of non-Euclidean geometry in the nursery.

The senses get synthesized in the brain into a sort of common language, a master-tongue. Part of it was auditory, part pictorial, and there were smells and tastes and tactile sensations that were sometimes familiar and sometimes spiced with the absolutely alien. And it was chaotic.

Something like this, perhaps....

“—Big Lizards getting too numerous this season—tame threvvars have the same eyes not on Callisto though—vacation soon— preferably galactic—solar system claustrophobic—byanding tomorrow if square rootola and upsliding three—”

But that was merely the word-symbolism. Subjectively, it was far more detailed and very frightening. Luckily, reflex had lifted Kelvin’s finger from the button almost instantly, and he stood there motionless, shivering slightly.

He was afraid now.

The robot said, “You should not have begun the rapport until I instructed you. Now there will be danger. Wait.” His eye changed color. “Yes ... there is ... Tharn, yes. Beware of Tharn.”

“I don’t want any part of it,” Kelvin said quickly. “Here, take this thing back.”

“Then you will be unprotected against Tharn. Keep the device. It will, as I promised, ensure your health, fame and fortune, far more effectively than a—a horoscope.”

“No, thanks. I don’t know how you managed that trick—sub-sonics, maybe, but I don’t—”

“Wait,” the robot said. “When you pressed that button, you were in the mind of someone who exists very far in the future. It created a temporal rapport. You can bring about that rapport any time you press the button.”

“Heaven forfend,” Kelvin said, still sweating a little.

“Consider the opportunities. Suppose a troglodyte of the far past had access to your brain? He could achieve anything he wanted.”

It had become important, somehow, to find a logical rebuttal to the robot’s arguments. “Like St. Anthony—or was it Luther?—arguing with the devil?” Kelvin thought dizzily. His headache was worse, and he suspected he had drunk more than was good for him. But he merely said:

“How could a troglodyte understand what’s in my brain? He couldn’t apply the knowledge without the same conditioning I’ve had.”

“Have you ever had sudden and apparently illogical ideas? Compulsions? So that you seem forced to think of certain things, count up to certain numbers, work out particular problems? Well, the man in the future on whom my device is focused doesn’t know he’s en rapport with you, Kelvin. But he’s vulnerable to compulsions. All you have to do is concentrate on a problem and then press the button. Your rapport will be compelled—illogically, from his viewpoint —to solve that problem. And you’ll be reading his brain. You’ll find out how it works. There are limitations, you’ll learn those too. And the device will ensure health, wealth and fame for you.”

“It would ensure anything, if it really worked that way. I could do anything. That’s why I’m not buying!”

“I said there were limitations. As soon as you’ve successfully achieved health, fame, and fortune, the device will become useless. I’ve taken care of that. But meanwhile you can use it to solve all your problems by tapping the brain of the more intelligent specimen in the future. The important point is to concentrate on your problems before you press the button. Otherwise you may get more than Tharn on your track.”

“Tharn? What—”

“I think an—an android,” the robot said, looking at nothing. “An artificial human ... However, let us consider my own problem. I need a small amount of gold.”

“So that’s the kicker,” Kelvin said, feeling oddly relieved. He said, “I haven’t got any.”

“Your watch.”

Kelvin jerked his arm so that his wrist-watch showed. “Oh, no. That watch cost plenty.”

“All I need is the gold-plating,” the robot said, shooting out a reddish ray from its eye. “Thank you.” The watch was now dull gray metal. “Hey!” Kelvin cried.

“If you use the rapport device, your health, fame and fortune will be assured,” the robot said rapidly. “You will be as happy as any man of this era can be. It will solve all your problems—including Tharn. Wait a minute.” The creature took a backward step and disappeared behind a hanging Oriental rug that had never been east of Peoria.

There was silence.

Kelvin looked from his altered watch to the flat, enigmatic object in his palm. It was about two inches by two inches, and no thicker than a woman’s vanity-case, and there was a sunken push-button on its side.

He dropped it into his pocket and took a few steps forward. He looked behind the pseudo-Oriental rug, to find nothing except emptiness and a flapping slit cut in the canvas wall of the booth. The robot, it seemed, had taken a powder. Kelvin peered out through the slit. There was the light and sound of Ocean Park amusement pier, that was all. And the silvered, moving blackness of the Pacific Ocean, stretching to where small lights showed Malibu far up the invisible curve of the coastal cliffs.

So he came back inside the booth and looked around. A fat man in a swami’s costume was unconscious behind the carved chest the robot had indicated. His breath, plus a process of deduction, told Kelvin that the man had been drinking.

Not knowing what else to do, Kelvin called on the Deity again. He found suddenly that he was thinking about someone or something called Tharn, who was an android.

Horomancy time rapport no! Protective disbelief slid like plate armor around his mind. A practical robot couldn’t be made. He knew that. He’d have heard—he was a reporter, wasn’t he?

Sure he was.

Desiring noise and company, he went along to the shooting gallery and knocked down a few ducks. The flat case burned in his pocket. The dully burnished metal of his wrist-watch burned in his memory. The remembrance of that drainage from his brain, and the immediate replacement burned in his mind. Presently bar whiskey burned in his stomach.

He’d left Chicago because of sinusitis, recurrent and annoying. Ordinary sinusitis. Not schizophrenia or hallucinations or accusing voices coming from the walls. Not because he had been seeing bats or robots. That thing hadn’t really been a robot. It all had a perfectly natural explanation. Oh, sure.

Health, fame and fortune. And if— THARN!

The thought crashed with thunderbolt impact into his head.

And then another thought: I am going nuts!

A silent voice began to mutter insistently, over and over. “Tharn— Tharn—Tharn—Tharn—”

And another voice, the voice of sanity and safety, answered it and drowned it out. Half aloud, Kelvin muttered:

“I’m James Noel Kelvin. I’m a reporter—special features, leg work, rewrite. I’m thirty years old, unmarried, and I came to Los Angeles today and lost my baggage checks and—and I’m going to have another drink and find a hotel. Anyhow, the climate seems to be curing my sinusitis.”

Tharn, the muffled drum-beat said almost below the threshold of realization. Tharn, Tharn. Tharn.

He ordered another drink and reached in his pocket for a coin. His hand touched the metal case. And simultaneously he felt a light pressure on his shoulder.

Instinctively he glanced around. It was a seven-fingered, spidery hand tightening—hairless, without nails—and white as smooth ivory.

The one, overwhelming necessity that sprang into Kelvin’s mind was a simple longing to place as much space as possible between himself and the owner of that disgusting hand. It was a vital requirement, but one difficult of fulfilment, a problem that excluded everything else from Kelvin’s thoughts. He knew, vaguely, that he was gripping the flat case in his pocket as though that could save him, but all he was thinking was:

I’ve got to get away from here.

The monstrous, alien thoughts of someone in the future spun him insanely along their current. It could not have taken a moment while that skilled, competent, trained mind, wise in the lore of an unthinkable future, solved the random problem that had come so suddenly, with such curious compulsion.

Three methods of transportation were simultaneously clear to Kelvin. Two he discarded; motorplats were obviously inventions yet to come, and quirling—involving, as it did, a sensory coil-helmet—was beyond him. But the third method—

Already the memory was fading. And that hand was still tightening on his shoulder. He clutched at the vanishing ideas and desperately made his brain and his muscles move along the unlikely direction the future-man had visualized.

And he was out in the open, a cold night wind blowing on him, still in a sitting position, but with nothing but empty air between his spine and the sidewalk.

He sat down suddenly.

Passersby on the corner of Hollywood Boulevard and Cahuenga were not much surprised at the sight of a dark, lanky man sitting by the curb. Only one woman had noticed Kelvin’s actual arrival, and she knew when she was well off. She went right on home.

Kelvin got up laughing with soft hysteria. “Teleportation,” he said. “How did I work it? It’s gone ... Hard to remember afterward, eh? I’ll have to start carrying a notebook again.”

And then—“But what about Tharn?”

He looked around, frightened. Reassurance came only after half an hour had passed without additional miracles. Kelvin walked along the Boulevard, keeping a sharp lookout. No Tharn, though.

Occasionally he slid a hand into his pocket and touched the cold metal of the case. Health, wealth and fortune. Why, he could— But he did not press the button. Too vivid was the memory of that shocking, alien disorientation he had felt. The mind, the experiences, the habit-patterns of the far future were uncomfortably strong.

He would use the little case again—oh, yes. But there was no hurry. First, he’d have to work out a few angles.

His disbelief was completely gone....

Tharn showed up the next night and scared the daylights out of Kelvin again. Prior to that, the reporter had failed to find his baggage tickets, and was only consoled by the two hundred bucks in his wallet. He took a room—paying in advance—at a medium-good hotel, and began wondering how he might apply his pipe-line to the future. Very sensibly, he decided to continue a normal life until something developed. At any rate, he’d have to make a few connections. He tried the Times, the Examiner, the News, and some others. But these things develop slowly, except in the movies. That night Kelvin was in his hotel room when his unwelcome guest appeared.

It was, of course, Tharn.

He wore a very large white turban, approximately twice the size of his head. He had a dapper black mustache, waxed downward at the

tips like the mustache of a mandarin, or a catfish. He stared urgently at Kelvin out of the bathroom mirror.

Kelvin had been wondering whether or not he needed a shave before going out to dinner. He was rubbing his chin thoughtfully at the moment Tharn put in an appearance, and there was a perceptible mental lag between occurrence and perception, so that to Kelvin it seemed that he himself had mysteriously sprouted a long moustache. He reached for his upper lip. It was smooth. But in the glass the black waxed hairs quivered as Tharn pushed his face up against the surface of the mirror.

It was so shockingly disorienting, somehow, that Kelvin was quite unable to think at all. He took a quick step backward. The edge of the bathtub caught him behind the knees and distracted him momentarily, fortunately for his sanity. When he looked again there was only his own appalled face reflected above the wash-bowl. But after a second or two the face seemed to develop a cloud of white turban, and mandarin-like whiskers began to form sketchily.

Kelvin clapped a hand to his eyes and spun away. In about fifteen seconds he spread his fingers enough to peep through them at the glass. He kept his palm pressed desperately to his upper lip, in some wild hope of inhibiting the sudden sprouting of a moustache. What peeped back at him from the mirror looked like himself. At least, it had no turban, and it did not wear horn-rimmed glasses. He risked snatching his hand away for a quick look, and clapped it in place again just in time to prevent Tharn from taking shape in the glass.

Still shielding his face, he went unsteadily into the bedroom and took the flat case out of his coat pocket. But he didn’t press the button that would close a mental synapse between two incongruous eras. He didn’t want to do that again, he realized. More horrible, somehow, than what was happening now was the thought of reentering that alien brain.

He was standing before the bureau, and in the mirror one eye looked out at him between reflected fingers. It was a wild eye behind the gleaming spectacle-lens, but it seemed to be his own. Tentatively he took his hand away....

This mirror showed more of Tharn. Kelvin wished it hadn’t. Tharn was wearing white knee-boots of some glittering plastic. Between them and the turban he wore nothing whatever except a minimum of loin-cloth, also glittering plastic. Tharn was very thin, but he looked active. He looked quite active enough to spring right into the hotel room. His skin was whiter than his turban, and his hands had seven fingers each, all right.

Kelvin abruptly turned away, but Tharn was resourceful. The dark window made enough of a reflecting surface to show a lean, loinclothed figure. The feet showed bare, and they were less normal than Tharn’s hands. And the polished brass of a lamp-base gave back the picture of a small, distorted face not Kelvin’s own.

Kelvin found a corner without reflecting surfaces and pushed into it, his hands shielding his face. He was still holding the flat case.

Oh, fine, he thought bitterly. Everything’s got a string on it. What good will this rapport gadget do me if Tharn’s going to show up every day? Maybe I’m only crazy. I hope so.

Something would have to be done unless Kelvin was prepared to go through life with his face buried in his hands. The worst of it was that Tharn had a haunting look of familiarity. Kelvin discarded a dozen possibilities, from reincarnation to the déjà vu phenomenon, but—

He peeped through his hands, in time to see Tharn raising a cylindrical gadget of some sort and leveling it like a gun. That gesture formed Kelvin’s decision. He’d have to do something, and fast. So, concentrating on the problem—I want out!—he pressed the button in the surface of the flat case.

And instantly the teleportation method he had forgotten was perfectly clear to him. Other matters, however, were obscure. The smells— someone was thinking—were adding up to a—there was no word for that, only a shocking visio-auditory ideation that was simply dizzying.

Someone named Three Million and Ninety Pink had written a new flatch. And there was the physical sensation of licking a twenty-fourdollar stamp and sticking it on a postcard.

But, most important, the man in the future had had—or would have— a compulsion to think about the teleportation method, and as Kelvin snapped back into his own mind and time, he instantly used that method....

He was falling.

Icy water smacked him hard. Miraculously he kept his grip on the flat case. He had a whirling vision of stars in a night sky, and the phosphorescent sheen of silvery light on a dark sea. Then brine stung his nostrils.

Kelvin had never learned how to swim.

As he went down for the last time, bubbling a scream, he literally clutched at the proverbial straw he was holding. His finger pushed the button down again. There was no need to concentrate on the problem; he couldn’t think of anything else.

Mental chaos, fantastic images—and the answer

It took concentration, and there wasn’t much time left. Bubbles streamed up past his face. He felt them, but he couldn’t see them. All around, pressing in avidly, was the horrible coldness of the salt water....

But he did know the method now, and he knew how it worked. He thought along the lines the future mind had indicated. Something happened. Radiation—that was the nearest familiar term—poured out of his brain and did peculiar things to his lung-tissue. His blood cells adapted themselves....

He was breathing water, and it was no longer strangling him.

But Kelvin had also learned that this emergency adaptation could not be maintained for very long. Teleportation was the answer to that. And surely he could remember the method now He had actually used it to escape from Tharn only a few minutes ago.

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