高指數(shù)晶面結(jié)構(gòu)PtPd納米催化劑的電化學(xué)制備與性能

附件論文中英文摘要作者姓名：田娜論文題目：高指數(shù)晶面結(jié)構(gòu)Pt、Pd納米催化劑的電化學(xué)制備與性能作者簡介：田娜，女，1976年02月出生，2002年09月師從于廈門大學(xué)孫世剛教授，于2007年12月獲博士學(xué)位。中文摘要鉑族金屬納米催化劑是燃料電池、石油催化重整、汽車尾氣凈化等能源和環(huán)境重大領(lǐng)域中不可替代、廣泛使用的催化劑。全世界每年用于催化劑的鉑族金屬中，僅鉑的用量就高達(dá)100噸，價(jià)值70多億美元。由于鉑族金屬的資源匱乏（在地殼中的豐度僅為0.003ppb）、價(jià)格昂貴，如何進(jìn)一步提高其催化活性、穩(wěn)定性和利用效率一直是相關(guān)領(lǐng)域的重大科學(xué)問題和關(guān)鍵工程技術(shù)問題。提高鉑族金屬納米催化劑的性能，一種方法是通過摻入其它組份形成合金，利用兩種或多種組份的協(xié)同效應(yīng)或電子結(jié)構(gòu)效應(yīng)；另一途徑則是通過調(diào)控納米粒子的形狀以改變表面原子排列結(jié)構(gòu)，利用催化反應(yīng)的表面結(jié)構(gòu)效應(yīng)。以金屬單晶面為模型催化劑的基礎(chǔ)研究表明：高指數(shù)晶面含有高密度的臺(tái)階原子和扭結(jié)原子，其配位數(shù)很低，非常容易與反應(yīng)分子發(fā)生相互作用，成為催化活性中心，其催化活性和穩(wěn)定性通常顯著優(yōu)于｛100｝、｛111｝等低指數(shù)晶面。但是本體金屬的高指數(shù)單晶面價(jià)格昂貴，比表面積很小，不可能作為實(shí)際催化劑。要充分利用催化反應(yīng)的表面結(jié)構(gòu)效應(yīng)，合成高指數(shù)晶面結(jié)構(gòu)的金屬納米粒子催化劑是具有重要基礎(chǔ)和應(yīng)用價(jià)值的課題，同時(shí)也極具挑戰(zhàn)性。因?yàn)楦咧笖?shù)晶面的表面能很高，根據(jù)晶體生長規(guī)律，晶體沿高指數(shù)晶面方向的生長速度遠(yuǎn)快于沿低指數(shù)晶面方向，致使高指數(shù)晶面趨于消失，最終僅得到由｛111｝、｛100｝等低指數(shù)晶面圍成的納米晶體。十余年來，雖然通過化學(xué)合成方法已經(jīng)制備了很多形狀（如四面體、八面體、截角八面體、立方體等）的鉑族金屬納米晶體，但均未能突破晶體生長規(guī)律的限制，其表面均為低指數(shù)晶面。高指數(shù)晶面可分為｛hk0｝、｛hkk｝、｛hhl｝及｛hkl｝（h>k>l>0）四種類型，相應(yīng)的晶體呈二十四面體、偏方三八面體、三八面體和六八面體。本論文中，我們發(fā)展了金屬納米晶體生長和表面結(jié)構(gòu)控制的電化學(xué)方波電位方法，成功制備出一系列由｛hk0｝、｛hkk｝和｛hkl｝型高指數(shù)晶面圍成的鉑、鈀納米單晶粒子及孿晶納米棒，揭示了這些晶體的生長規(guī)律，并研究它們對(duì)有機(jī)小分子電氧化的催化性能。論文的主要?jiǎng)?chuàng)新點(diǎn)和取得的主要研究結(jié)果如下：發(fā)展了金屬納米晶體表面結(jié)構(gòu)控制和生長的電化學(xué)方波方法。以玻碳為基底，利用方波電位處理多晶鉑納米球，首次成功制備出二十四面體鉑納米晶體催化劑，通過高分辨透射電鏡表征，確定其表面為｛730｝、｛520｝等高指數(shù)晶面。通過改變方波電位處理時(shí)間，可以控制納米晶體的粒徑，得到20?240nm的鉑二十四面體。研究證實(shí)鉑二十四面體不僅具有很高的電催化活性，對(duì)甲酸、乙醇氧化的電流密度為商業(yè)碳載鉑催化劑的2?4倍，同時(shí)還具有很高的化學(xué)和熱穩(wěn)定性，可耐高達(dá)800℃的加熱溫度。進(jìn)一步研究提出了納米晶體高指數(shù)晶面形成的相關(guān)機(jī)理：方波電位導(dǎo)致鉑表面發(fā)生周期性氧化/還原(即氧的反復(fù)吸脫附)，對(duì)于表面原子配位數(shù)較高的低指數(shù)晶面，氧傾向于侵入晶格，表面被擾亂；而對(duì)于表面原子配位數(shù)很低的｛hk0｝高指數(shù)晶面，氧傾向于在表面吸附，表面不會(huì)被擾亂，即在氧化條件下，｛hk0｝高指數(shù)晶面具有更高的穩(wěn)定性是鉑二十四面體形成的根本原因。該研究結(jié)果在Science(N.Tian,etal,Science,2007,316:732-735)上發(fā)表，同期的Science還配發(fā)了評(píng)論文章(Science,2007,316:699-700)，評(píng)價(jià)這一成果“是納米催化劑合成的重大突破”。該成果在國內(nèi)外學(xué)術(shù)界引起重大反響，被美國化學(xué)工程新聞周刊(C&EN)評(píng)為2007年度世界24項(xiàng)最重要研究成果之一(2007Chemistryhighlights),被英國皇家化學(xué)會(huì)的化學(xué)世界(ChemistryWorld)評(píng)為2007年度40項(xiàng)最重要進(jìn)展之一(Cuttingedgechemistryin2007),還被評(píng)為2007年度“中國高等學(xué)校十大科技進(jìn)展”，并入選“2007年度中國基礎(chǔ)研究十大新聞”。在成功制備｛hk0｝晶面結(jié)構(gòu)鉑二十四面體的基礎(chǔ)上，通過改變添加劑、鍍液等條件，進(jìn)一步拓展方波電位方法，制備出其它類型高指數(shù)晶面圍成的納米晶體，如出口｝晶面結(jié)構(gòu)的鉑凹四十八面體、｛hk0｝晶面結(jié)構(gòu)的鈀二十四面體、｛hkk｝晶面結(jié)構(gòu)的鈀偏方三八面體和｛hkl｝晶面結(jié)構(gòu)的鈀凹四十八面體等，這些結(jié)果表明所發(fā)展的方波電位方法是制備高指數(shù)晶面結(jié)構(gòu)鉑、鈀等金屬納米晶體的通用方法。除了單晶納米粒子，利用方波電位方法，還制備了由｛hk0｝高指數(shù)晶面圍成的五重孿晶結(jié)構(gòu)鉑納米棒(Z.Y.Zhou,N.Tian,etal,FaradayDiscussions,2008,140:81-92)。與典型五重孿晶納米棒不同，鉑納米棒兩端呈十棱錐形狀，側(cè)面由一系列上下起伏的小晶面圍成。觀察到鉑納米棒各部位由于生長速度存在差異，導(dǎo)致晶面指數(shù)呈有規(guī)律變化：頂端較尖銳，生長速度最快，表面為臺(tái)階原子密度較低的｛410｝晶面；底端較鈍，生長速度最慢，表面為臺(tái)階原子密度很高｛320｝、｛210｝、｛730｝等晶面；側(cè)面主要為臺(tái)階原子密度中等的｛520｝晶面。研究結(jié)果指出，除了方波電位，通過控制晶體的生長速度還可以更加精細(xì)地調(diào)控高指數(shù)晶面的指數(shù)。這種不對(duì)稱的鉑納米棒有利于局域修飾和定向組裝等，有望用于納米器件。此外還首次觀察到鉑五重孿晶納米棒的亞晶之間在生長過程中存在分裂現(xiàn)象。進(jìn)一步通過改變方波電位處理時(shí)間，研究分裂過程，豐富了對(duì)五重孿晶結(jié)構(gòu)納米棒生長過程的認(rèn)識(shí)。進(jìn)一步發(fā)展方波電位方法，將其應(yīng)用于直接電沉積制備高指數(shù)晶面結(jié)構(gòu)的五重孿晶鈀納米棒(N.Tian,etal,Chem.Commun.,2009,1502-1504,2008年12月15日被接收)。通過改變方波的上、下限電位，實(shí)現(xiàn)調(diào)控鈀納米棒的長徑比和高指數(shù)晶面的類型。在較低的方波上限電位得到由｛hkk｝高指數(shù)晶面圍成鈀納米長棒，其截面為五邊形；而在較高上限電位則得到由｛hk0｝高指數(shù)晶面圍成的十棱雙錐狀的鈀納米短棒。實(shí)驗(yàn)證實(shí)鈀納米棒對(duì)乙醇在堿性介質(zhì)中的電氧化有較高的催化活性，氧化峰的電流密度比商業(yè)鈀黑催化劑提高了一倍多。利用方波電位電沉積法制備了刺球狀鉑納米粒子。鉑納米刺非常尖銳，呈側(cè)面內(nèi)凹的四棱錐形狀，為單晶結(jié)構(gòu)。刺球狀鉑納米粒子給出顯著的表面增強(qiáng)拉曼效應(yīng)，測(cè)得吸附態(tài)吡啶分子的環(huán)呼吸振動(dòng)峰強(qiáng)度達(dá)140cps，拉曼增強(qiáng)因子約為2000，比通常電化學(xué)粗糙法得到的表面提高了一個(gè)數(shù)量級(jí)，這歸結(jié)于鉑納米刺尖端的曲率半徑非常小(?1nm)，產(chǎn)生很強(qiáng)的電場(chǎng)，即避雷針效應(yīng)(N.Tian,etal,Chem.Commun.,2006,4090-4092)；刺球狀鉑納米粒子還表現(xiàn)出較強(qiáng)的增強(qiáng)紅外效應(yīng)，對(duì)于吸附態(tài)一氧化碳，紅外增強(qiáng)因子約為17。簡單的制備方法及較高的拉曼、紅外增強(qiáng)因子，表明刺球狀鉑納米粒子電極適合用作SERS和紅外基底，以提高鉑表面吸附物種的檢測(cè)靈敏度。本文發(fā)展的金屬納米催化劑表面結(jié)構(gòu)控制和生長的電化學(xué)方波電位方法，制備出由不同類型高指數(shù)晶面圍成的一系列形狀的鉑、鈀納米晶體，豐富了納米晶體表面結(jié)構(gòu)控制生長的內(nèi)涵，深化對(duì)金屬晶體生長規(guī)律的認(rèn)識(shí)。上述成果還被J.Phys.Chem.邀請(qǐng)撰寫FeatureArticle(N.Tian,etal,J.Phys.Chem.C,2008,112:19801-19817)。本文所研制的高指數(shù)晶面結(jié)構(gòu)鉑族金屬納米催化劑不僅活性高而且穩(wěn)定性好，開辟了一條通過控制納米粒子表面原子排列結(jié)構(gòu)提高催化劑性能的嶄新途徑，也是將模型電催化劑的基礎(chǔ)研究推進(jìn)到實(shí)際催化劑設(shè)計(jì)和研制過程中的一個(gè)重大進(jìn)展。關(guān)鍵詞：鉑族金屬，納米晶體，高指數(shù)晶面，電催化ElctrochemicalPreparationandHighPerformanceofPlatinumand

PalladiumNanocatalystswithHigh-IndexFacetsTianNaABSTRACTPlatinum-groupmetalnanomaterialsarewidelyusedascatalystsappliedinfuelcells,petroleumcatalyticreform,automotivecatalyticconvertersandotherimportantfields.Amongtheplatinum-groupmetalsusedascatalystsintheworld,theplatinumisconsumedashighas100tonsperyear,whichvaluesmorethan7billionUSdollars.Thepriceofplatinum-groupmetalisextremelyhighduetotheirrarereserveontheearth.So,thekeyscientificandtechnologicalissueofplatinumcatalystsistofurtherimprovetheiractivity,stabilityandutilizationefficiency.Generally,catalyticpropertiesofnanocrystalscanbefinelytunedeitherbytheircomposition,whichmediateselectronicstructure,orbytheirshape,whichdeterminestheirsurfaceatomicarrangementandcoordination.Fundamentalstudiesofmodelcatalystsusingsingle-crystalplaneshavedemonstratedthathigh-indexplanesofplatinum-groupmetalsexhibitgenerallymuchhighercatalyticactivityandstabilitythanthoseofthelow-indexplanes,suchas{111},{100},andeven{110},becausethehigh-indexplaneshaveahighdensityofatomicsteps,ledges,andkinks,whichrepresentactivesitesforbreakingchemicalbonds.However,high-indexplanesofmetalsinglecrystalcannotbeusedasrealcatalystsbecauseoftheirlowspecificsurfaceareaandhighcost.Theexcitingwayistosynthesizeplatinum-groupmetalnanocrystalsboundedbyhigh-indexfacetsforhighercatalyticactivityandstability.Itis,however,ratherchallengingtosynthesizenanocrystalsthatareenclosedbyhigh-indexfacetsbecauseoftheirhighsurfaceenergy.Fromthecrystalgrowthlaw,therateofcrystalgrowthinthedirectionperpendiculartoahigh-indexfacetwithhighsurfaceenergyismuchfasterthanthatalongthenormaldirectionofalow-indexfacet,whichresultsinarapiddisappearanceofhigh-indexfacetsduringnanoparticleformation.Asaconsequence,onlylow-indexfacets,suchas{111}and{100},boundednanocrystalsofplatinum-groupmetalshavebeenobtainedbyconventionalchemicalsynthesismethods.Inthepastdecade,avarietyofplatinum-groupmetalnanocrystalswithwell-definedshapeshavebeensynthesized,suchastetrahedron,octahedron,cuboctahedron,cubeandrhombicdodecahedron,butnearlyallofthemareboundedbythelow-indexplanes.High-indexfacetscanbeclassifiedas{hk0},{hkk},{hhl}and{hkl}(h>k>l>0),thecorrespondingshapesofnanocrystalsaretetrahexahedron,trapezohedron,trisoctahedronandhexoctahedron,respectively.Inthisthesis,wehavedevelopedanelectrochemicalmethodofsquare-wavepotentialtocontrolthesurfacestructureandthegrowthofmetalnanocrystals,andpreparedsuccessfullynanocatalystsofplatinum,palladiumboundedbyhigh-indexfacetsof{hk0},{hkk}and{hkl}.Thegrowthmechanismandthecatalyticpropertiesofthesynthesizednanocrystalstowardstheoxidationofsmallorganicmoleculeswereinvestigated.Thestudyhasillustratedthatthesynthesizednanocatalystsexhibithighactivityandstability.Themainresultsareasfollowing:Tetrahexahedral(THH)Ptnanocrystalshavebeenpreparedforthefirsttimeusingthedevelopedelectrochemicallyshape-controlledsynthesismethod.ThesurfacestructureoftheTHHPtnanocrystalsisidentifiedas{730}and{520}high-indexfacetsthroughHRTEM.Byvaryingthegrowthtime,THHPtnanocrystalsofdifferentuniformsizevaryingfrom20to240nmwereobtained.IthasdemonstratedthattheTHHPtnanocrystalsexhibit2~4timeshighercatalyticactivitythanthatofcommercialPt/Ccatalyststowardselectrocatalyticoxidationofformicacidandethanolintermsofcurrentdensity.TheTHHPtnanocrystalsshowalsohighchemicalandthermalstability,andcanmaintaintheirshapeandfacetsupto800℃.IthasconfirmedthatthekeyreasonforthegrowthofTHHPtnanocrystalsconsistsinthatthehigh-indexfacetsofPtpossessahigherstabilitythanlow-indexfacetsunderperiodicoxidation-reductionconditionsgeneratedbysquare-wavepotential.TheseaboveresultswerepublishedinScience(N.Tian,etal,Science,2007,316:732-735),andwereevaluatedas“abreakthroughinthesynthesisofnanoscalecatalysts”byacommentarticlepublishedinthesamevolume(Science,2007,316:699-700).Theresultshavereceivedsignificantimpacts:theywereevaluatedas“2007Chemistryhighlights”byC&EN,as“Cuttingedgechemistryin2007”byChemistryWorld,andevaluatedasoneof“TenmostsignificantprogressesinscienceandtechnologymadeinChineseUniversitiesfortheyear2007”andasoneof“Thetop10newsof2007inbasicresearchinChina”.BasedonthesuccessofsynthesizingTHHPtNCs,wehavealsopreparedsuccessfullytetrahexahedral,trapezohedralandconcavedhexaoctahedralPdnanocrystalsandconcavedhexaoctahedralPtnanocrystalsbyusingthedevelopedelectrochemicallyshape-controlledsynthesismethod,anddemonstratedthatthesquare-wavepotentialmethodisversatileforthepreparationofmetalhigh-indexfacetednanocrystalssuchasPtandPd.Besidessinglecrystallinenanocrystals,wehavepreparedfive-foldtwinnedPtnanorodsboundedbyhigh-indexfacetsof{hk0}throughthedevelopedelectrochemicallyshape-controlledsynthesismethod(Z.Y.Zhou,N.Tian,etal,FaradayDiscussions,2008,140:81-92).Unliketheshapeofcommonfive-foldtwinnednanorod,thetipsofthePtnanorodsareofdecagon-basedpyramidalshape,andthesidesurfacescontainaseriesofconcavo-convexsubfacets.Wehaveobservedthatthefacetsvariedwiththegrowthrate:theuppertip,withthehighestgrowthrate,hasfacetswiththelowestdensityofsteppedatoms;thelowertip,withthelowestgrowthrate,hasfacetswiththehighestdensityofsteppedatoms,whichindicatethatthefacetscanbefinelytunedbyvaryingthegrowthrateofnanocrystals.Wehavealsoobservedforthefirsttimethephenomenonofsplitoffive-foldtwinnednanorodsatthetwinedboundaries,andinvestigatedthesplittingprocessbyvaryingthegrowthtime,whichdeepentheunderstandingofthegrowingprocessofthefive-foldtwinnednanocrystals.Wehavefurtherdevelopedtheelectrochemicalmethod,andapplieditdirectlytopreparefive-foldtwinnedPdnanorodsbyelectrodeposition(N.Tian,etal,Chem.Commun.,2009,1502-1504).IthasbeendemonstratedthattheaspectratioandsurfacestructureofPdnanorodscanbetunedbyvaryingthelower(El)andupper(Eu)limitofthesquare-wavepotential.IthasrevealedthatPdnanorodsgrownatrelativelylowEushowtwoendsofpentagonalpyramidshapeboundedbyhigh-indexfacetsof{hkk};whilenanorodsgrownathighEugiverisetotwoendsofdecagon-basedpyramidalshapeboundedbyhigh-indexfacetsof{hk0}.IthasillustratedalsothatthePdnanorodsexhibithighercatalyticactivitythancommercialPdblackcatalyststowardstheelectro-oxidationofethanolinalkalinesolution.Ptnanothornswerepreparedalsobythesquare-wavepotentialelectrodeposition.ThetipofPtnanothornisverysharp,showsatetragonalpyramid-likeshapewithslightlycurvedside.ThePtnanothornshaveaveryhighsurfaceenhancedRamanscattering(SERS)activity.Anenhancementfactorof2000foradsorbedpyridinewasmeasured,whichisaboutoneorderofmagnitudebiggerthanthatmeasuredonaPtelectroderoughenedbyconventionalelectrochemicallyoxidation/reductionmethod(N.Tian,etal,Chem.Commun.,2006,4090-4092).ThePtnanothornsalsoshowahighinfraredenhancement:IRbandintensity(peakheight)ofCOadsorbedonthePtnanothornsisashighas3%,yieldinganenhancementfactorof17.TheseresultsimplythatthePtnanothornelectrodeisagoodcandidateforSERSandIRstudyintermstoincreasesignificantlythedeterminingsensitivityofsurfaceadsor