Synthesis and Characterization of Metallic and Bimetallic ... - MatDL

Synthesis and Characterization of Metallic and Bimetallic Nanoparticlesfor Fuel Cell ApplicationsMariaJulieth BallesterosDept. of Chemistry, The City College of New York, New York, NYFrank DiSalvo and Chandrani Roy ChowdhuryDept. of Chemistry and Chemical Biology, Cornell University, Ithaca, NYFuel cells are an alternative source of energy.Even though this technology has been around since1839, the materials that are used in fuel cells areold and need improvements. A fuel cell in generalconsists of an anode, a cathode and a semipermeable membrane that not only is the supportfor the electrodes but also allows the transfer ofions and electrons. Pure platinum (Pt) and its alloysare very common used as anodic and cathodicmaterials; however Pt poisoning by CO during thereduction-oxidation process lowers the efficiencyand the lifetime of the cell. Pt alloys are lesssusceptible to CO poisoning than pure platinum butare not stable during long term operatingconditions.In methanol fuel cells the use of pureplatinum as a material for the anode and cathodereduces the efficiency of the fuel cell because of itsfast CO poisoning. So for all proton exchangemembrane (PEM) fuel cells, there is a need fornovel materials which would be cheap, tolerant toCO and sulphur poisoning, and durable.In the Cornell Fuel Cell Institute,intermetallic compounds are being considered as anew class of electrocatalysts. These have orderedcrystal structures as opposed to alloys which havedisordered structures. The focus of this project is toprepare Pt containing intermetallics in nanoparticleform, so as to have higher surface area, and in turn,enhanced electrocatalytic activity.During this summer one of the goals was toreproducibly synthesize nanoparticles of twointermetallic phases, namely BiPt and PbPt.Different precursors, such as disodiumhexachloroplatinate (Na 2 PtCl 6 ) and platinichexachloro acid(H 2 PtCl 6 ) for platinum, lead 2 ethyl hexanoateand lead MOEEAA for lead, bismuth 2 ethylhexanoate and bismuth MOEEAA for bismuth,were used for the chemical reduction. Anothergoal was to study the solubility and reductionbehavior of different metal precursors inmethanol. This solubility and reduction test gaveus information on ease of reduction, particle size(from Scherrer Equation and SEM) and stabilityof the reduced compound. This informationwould be used in future for the synthesis of other,novel intermetallic phases.Results and discussion Synthesis ofdisodium hexachloro platinate (Na 2 PtCl 6 ) fromplatinic hexachloro acid (H 2 PtCl 6 ) was anawkward process even though it was a reportedprocedure (1).H 2 PtCl 6 (aq) + NaCl (excess) Na 2 PtCl 6 (1)The chief problem with the reaction wasthat the sodium hexachloroplatinate did notprecipitate out of the solution, probably due tounsaturation of the mother liquor. Extra steps likeconcentrating the solution, dissolving the solid inhot ethanol, and keeping the solution at therefrigerator for weeks instead of ice bath for 20minutes were done in order to get the sodium saltto precipitate; with these steps, we were able toget a considerable amount of disodiumhexachloroplatinate (Na 2 PtCl 6 ) but with someimpurities. An interesting observation during thewhole process was that, when the solution waskept cold for a long time, the pXRD of theproduct had some peaks corresponding toNa 8 PtO 6 .The potassium salt, K 2 PtCl 6 was notconsidered a good precursor because of its lowsolubility in methanol, which was the solventused for the synthesis.For the synthesis of the intermetallics BiPtand PbPt reactions were carried out using sodiumborohydride (NaBH 4 ) as a reducing agent (table1). All reactions were done under argon.

Platinum Bismuth orprecursor Lead precursorProductH 2 PtCl 6 Bi-MOEEAA BiPtH 2 PtCl 6Bi 2-ethylhexanoateBiPt, PtNa 2 PtCl 6Bi-2-ethyl Bi, Pt, Na,hexanoate Bismuth OxidesH 2 PtCl 6 Pb-MOEEAA PbPtNa 2 PtCl 6Pb-2-ethylhexanoatePbPt, PbTable 1 List of the attempts to prepare the intermetallicsBiPt and PbPt and its product. MOEEAA {2,[2,(2methoxy), ethoxy]ethoxy}Acetic AcidThe highlighted reactions are the ones thatproduced ordered intermetallic phase products;the other reactions showed some Bi and Pb oxideimpurities and elemental platinum. Themorphology of the pure intermetallic phasesobtained were studied by scanning electronmicroscopy (SEM) to get accurate information onparticle size and agglomeration effects. The SEMshowed agglomerates of nanoparticles asexpected due to the absence of the surfactants,which can separate nanoparticle clusters intosingle or smaller nanoparticles clusters.between the MOEEAA and ethyl hexanoateprecursors, a chloride test was done. LithiumChloride (LiCl) was added to the Bi and Pbprecursors and we looked for the formation of aprecipitate. This test gave us some pXRD database useful for understanding the possibleformation of PbCl 2 and BiCl 3 during thepreparation of the intermetallics. The chloridetest was also tried for sodium hexachloropalladate.Precursor Precipitate XRDBi MOEEAA yes BiOCl, BiOPbMOEEAAno noBi 2-ethylhexanoateyes BiO Cl, BiOPb 2-ethylNo (stickyyeshexanoateprecipitate)*Sodiumhexachloro no nopalladateTable 2 List of the trials for chloride precipitation testusing lithium chloride (in bold primary product).*XRD for Pb 2-ethyl hexanoate was not taken because welost a lot of the sample that got stuck to the mortar.And I tried just for fun, the following reactionNa 2 PdCl 6 4H 2 O + H 2 PtCl 6 + NaBH 4 PtPdThe pXRD of the reduction of platinum (Pt) andpalladium (Pd) shows single peak for both metalsbecause they form an alloy with identical pXRD.100nm scale(a)200nm scaleFigure 2 SEM picture of the alloy PdPt. The spikes shapevisi ble in the picture shows the typical behavior of an alloyho wever surprisingly we found some areas with roundshapes.200 nm scale (b)Figure 1(a) (b) SEM pictures of the intermetallics clusters(a) BiPt nanoparticles cluster (b) PbPt nanoparticles clustershowing some plate like areas.In order to understand the possible effect ofhalogen in the synthesis and the differencesAs mentioned before some precursors were testedfor solubility and reduction using sodiumBorohydrate (NaBH 4 ). This procedure wascarried out under argon and the solvent( methanol) was degassed for 2 hours. In order toachieve the solubility of the salt sonication, and

heating were done (if necessary). The followingtable shows the list of salts used for solubilitytest.PrecursorSodiumhexachloropalladateAmmonium-ptungstateCobalt( II)oxalatedehydrateGallium(III)Nitrate hydrateSilver acetateanhydrousSol.MeOHRed.Part. domainsize (nm)yes yes 10.21no no nono no noyesnotred.nono no noNickel sulfate* yes yes noNickelnotpartiallycarbonatered.noSilvercarbonateno no noSilver cyanate sl ight no noIndiumphosphatesl ight no noGermaniummonosulphateslight no noThalliumsulfidesl ight no noIndiumsulphate**yes yes noT able 3 List of salts used for solu bility test and particledomain size* Nickel particle domain size was not calculated becausethe sample was exposed to air making some of the nickel tooxidize back.**The solubility in methanol of indium sulfate requiredheating then at the moment we added the reduction agentthe sample was hot, and it seem to be an incompletereductionConclusionsWe were able to obtain the sodiumhexachloroplatinate precursor by modifying theprocedure from literature; in addition, we noticedthat the precipitate starts forming from themother liquid after 1 or 2 weeks in therefrigerator at about 4 o C. The next step for thisreaction is to try to get the precipitate using thestandard procedure, the same NaCl salt but with asaturated solution of platinic hexachloro acid(H 2 PtCl 6 ).We were able to reproduce thenanoparticle synthesis of two intermetallicphases, namely BiPt and PbPt by the chemicalreduction of different precursors. However forsome reactions the impurities or the unreactedmetals need further study. From solubility andreduction test we got relevant information forpalladium, once it was the only compound thatreally showed solubility and stability after beingreduced. Further reactions may be consideredusing this compound as a precursor for newintermetallics.The SEM pictures for the twointermetallics compounds showed very differentmorphology of the sample due to a differentagglomeration pattern of the nanoparticles. PbPtnanoparticles clusters showed two types ofagglomerates one like a flat shape and the othercloud-like shape; meanwhile for BiPt we did notobserve any major change in the nanoparticlesclusters. PdPt alloy show an interesting roundshape among the spikes shape ones typical of analloy.AcknowledgementsThanks to…. Frank DiSalvo, mentor, for lettingme be part of his group during this summer.To Chandrani Roy Chowdhury for teaching meall she could related with the fuel cells projectand for giving me the chance to try new reactionsand work with fancy equipment. I hope you gotsomething relevant or at least the not to do list!Also thanks to DiSalvo’s group for being helpfulwhen I needed.Cornell Center for Materials Research, Thanks alot. I had learn a lot and I had so much fun atCornell.References1. Chandrani Roy Choudhury and Francis JDiSalvo Chem mater.2005, XX,XXXX2. Raimondi, F.; G.G.Scherer; R. Kotz, and A.Wokaum Angew. Chem. int. Ed 2005, 44, 2190-22093. Casado-Rivera, E.; D. J. Volpe; L. Alden; C.Lind; C. Downie; T. Vazquez-Alvarez; A.C.D.Angelo; F. Di Salvo and H. Abruna J. Am.Chem.Soc. 2004, 126, 4043-40494. CornellFuel Cell Institute web page