Application of Mossbauer Spectroscopy to the Study

PLEASESCROLLDOWNFORARTICLEThisarticlewasdownloadedby:On:24January2011Accessdetails:AccessDetails:FreeAccessPublisherTaylor&FrancisInformaLtdRegisteredinEnglandandWalesRegisteredNumber:1072954Registeredoffice:MortimerHouse,37-41MortimerStreet,LondonW1T3JH,UKCatalysisReviewsPublicationdetails,includinginstructionsforauthorsandsubscriptioninformation:~content=t713597232ApplicationofMössbauerSpectroscopytotheStudyofAdsorptionandCatalysisW.NicholasDelgassa;MichelBoudartaaStanfordUniversityStanford,CaliforniaTocitethisArticleDelgass,W.NicholasandBoudart,Michel(1969)'ApplicationofMössbauerSpectroscopytotheStudyofAdsorptionandCatalysis',CatalysisReviews,2:1,129—160TolinktothisArticle:DOI:10.1080/01614946908066542URL:::23:4624January2011130W.N.DELGASSANDM,BOUDARTI.INTRODUCTIONHeterogeneouscatalysisisinherentlyacomplexsubject,andprog-resstowardmakingitascienceratherthananarthasrequiredknowledgeandtechniquesinmanyfields.Inrecenttimesphysicaltoolssuchaselectronmicroscopyandopticalandmagneticreso-nancespectroscopyhaveseenincreasinguseandhaveyieldedmanyinterestingnewresults.ThispaperisadiscussionofMossbauerspectroscopy,atoolcapableofgivingnewinformationconcerningcatalysis.InthefirsthalfwereviewthephysicalparameterswhichplayaroleintheMossbauereffectanddiscussbrieflythetechniquesusedinitsmeasurement(thegeneralreferencesforthissectionare[l]and[2]).Inthelasthalfwediscussapplicationsoftheeffecttostudiesofadsorptionandcatalysiswithexamplesfromthelitera-tureandourownexploratorywork.11.OCCURRENCEOFTHEMOSSBAUEREFFECTTheMossbauereffectmaybedescribedastherecoil-freeemis-sionandresonantabsorptionofy-rays.Thespectroscopicconceptofresonanceisafamiliaroneasitisthebasisformuchatomicspectroscopy.Liketheatom,thenucleusisaquantizedsystem,andthenuclearenergylevelsarecharacterizedbytotalnuclearangularmomentum(spin)quantumnumbers.Thenuclearresonancefluores-cenceprocessisillustratedinFig.1.AsourceatomdecaysfromNUCLEAREXCITEDESTATE7GNUCLEARIGROUNDGSTATESOURCEABSORBERFIG.1.Resonancefluorescence.anexcitedenergyleveltoitsgrou2dstate,emittingay-rayofenergyequaltothetransitionenergyE,.Thaty-raymaythenbereso-nantlyabsorbedbyanidenticalabsorbernucleussincethetransitionen-ergyisthesame.Thissimpleanalogytoatomicspectroscopyis,how-ever,inadequateandwemustcompletethedescriptionoftheactualphysicalprocesstounderstandthedifficultiesinobservingthenu-clearresonanceeffect.Ifweconsiderthesourceatomtobefreeandatrest,thenconservationofmomentumrequiresthatarecoilmomentumbeimpartedtothenucleusduringtheemissionprocess.DownloadedAt:23:4624January2011MOSSBAUERSPECTROSCOPY131Theresultingrecoilvelocityvofthenucleusisgivenbyv=-Ey/Mc,whereMisthemassoftheparticle,cthespeedoflight,andEytheactualenergyoftheemittedy-ray.Conservationofenergythenre-quiresthatEybereducedfromthetransitionenergyE,byanamountE,xE2,/2Mc2becauseoftherecoilmotionofthesourceatom.Sim-ilarshiftsarefoundintheabsorptionprocess.Theimportanceoftheseshiftsisseenwhenoneconsidersthelinewidthofthenuclearemissionandabsorptionprocesses.TheselinewidthsaredeterminedbythelifetimeoftheexcitedstaterNthroughtheHeisenberguncer-taintyrelation:whereristhewidthoftheenergydistributionofthetransitionand2n