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international journal of hydrogen energy 34 (2009) 4312–4320 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Preparation and characterization of nano-sized Pt–Pd/C catalysts and comparison of their electro-activity toward methanol and ethanol oxidation F. Kadirgan*, S. Beyhan, T. Atilan Chemistry Department, Istanbul Technical University, Maslak-Istanbul, Turkey article info Article history: Received 25 December 2008 Received in revised form 11 March 2009 Acce
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  Preparation and characterization of nano-sized Pt–Pd/Ccatalysts and comparison of their electro-activity towardmethanol and ethanol oxidation F. Kadirgan*, S. Beyhan, T. Atilan Chemistry Department, Istanbul Technical University, Maslak-Istanbul, Turkey a r t i c l e i n f o Article history: Received 25 December 2008Received in revised form11 March 2009Accepted 13 March 2009Available online 10 April 2009 Keywords: Nano-sized Pt–Pd/CElectrocatalysisEthanolMethanolDirect alcohol PEM fuel cells a b s t r a c t In the present work nano-sized Pt–Pd alloys have been prepared by polyol process onVulcan XC72. The information on structural characteristics and surface chemistry of thenano-material was obtained using TEM, XRD and XPS.Their catalytic activities against methanol and ethanol oxidation were measured andcompared with that of commercial Pt/C (20wt% ETEK) by electrochemical methods. Themethanol and ethanol oxidation was studied in half cells at different temperatures. Theresults show that Pt–Pd/C catalysts were more active for oxidation of methanol thanethanol. The significant improvement in the catalytic activities for methanol and ethanoloxidation compared to that of the Pt/C is believed due to the higher electrochemical surfacearea and the synergistic effect between Pt and Pd. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rightsreserved. 1. Introduction Methanol and ethanol are the most studied alcohols for DirectAlcohol Fuel Cell (DAFC) application[1–7]. Use of ethanol asfuel in DAFCs provides many advantages over methanol dueto its renewability, low toxicity, safety, high energy density,and its easy production in great quantities from biomass[8–10].The main problems of DAFCs are poor performance of electrocatalysts, especially anode catalysts at lower temper-atures and the severe fuel crossover from anode to cathode,which leads to poisoning of cathode catalyst[11–13]. Theactivity improvement of anode catalysts is helpful to reducefuel permeation through electrolyte. Pt has been demon-strated as the only active and stable noble metal for alcoholoxidation, particularly in acid medium. However, it is wellknown that pure platinum is readily poisoned by CO-likeintermediates of methanol or ethanol electro-oxidation[14].On the other hand,thehigh cost ofthe platinumlimits its use.OneofthegrandchallengesinDMFCdevelopmentistoreducetheusageofPtpreciousmetal.Oneapproachtocostreductionis to use the Pt-based alloys to reduce the Pt loading. Anothereffective approach is to increase the utilization efficiency of Ptelectrocatalysts by exploring the high surface area supportssuch as high surface area carbon. Methanol or ethanoloxidation on Pt is only possible at potentials where adsorbedCOandotherpoisoningintermediatesareeffectivelyoxidized,leading to a significant overpotential and hence loss in effi-ciency. A higher efficiency at more negative potentials isobtained for PtRu[15–18]catalysts, which is generally attrib-utedtotheirsuperiorCOtoleranceduetoabifunctionaleffect,where CO is oxidized by OH species on Ru surface atoms. * Corresponding author . Tel.: þ 90 212 2853262; fax: þ 90 212 2856386.E-mail address:kadirgan@itu.edu.tr(F. Kadirgan). Available at www.sciencedirect.comjournal homepage:www.elsevier.com/locate/he 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2009.03.024 international journal of hydrogen energy 34 (2009) 4312–4320  However,theefficiencyoftheDMFCsoperatingonPtRuanodecatalysts is still insufficient for practical applications. Furtheroptimization of anode material for the DMFCs is thus impor-tant. Therefore a number of other catalyst systems have beeninvestigated for their suitability as methanol oxidation cata-lysts including Pt alloy catalysts Pt–Sn[19–21], Pt–Pd[22, 26], Pt–Rh[23], Pt–Mo[24]other than PtRu. In order to improve the reaction kinetics it is necessary to develop new pluri-func-tional catalysts. Making alloys with a second or third metal isa convenient way to modify electrocatalytic properties of Pt inorder to overcome poisoning effects which results in lowersurface coverage by adsorbed CO[25,27].As known, the complete electro-oxidation of ethanolinvolves letting 12 electrons out per ethanol molecule. Moreactive electrocatalysts are needed to promote ethanol electro-oxidation than that of methanol at lower temperaturesbecause of additional activation barrier necessity related toboth of C–C bond cleavage and removal of more adsorbedintermediates and products formed during electro-oxidationof ethanol.The use of Pd is of interest as it is at least 50 times moreabundant on the earth than Pt. Palladium is a very goodelectrocatalyst for organic fuel electro-oxidation[28]. Whilevoltammetry has indicated stronger CO bonding on Pd 4 þ aspredicted[29], another study[30]showed that the release of  hydrogen occluded in palladium may provide a viable routefor lowering the surface concentration of adsorbed CO. ThePt–Pdbimetallic systemalso exhibitsa high resistanceagainstCO poisoning from the oxidation of formic acid[31]. Anenhancement of the activity for Pt in the presence of Pd hasbeen found by cyclic voltammetry and single direct ethanolfuel cell measurements[32]. However,C supported Pt–Pdalloyis also proposed as ethanol tolerant oxygen reduction catalystbecause in the presence of ethanol a larger increase in over-potential of the ORR on pure Pt than that on Pt–Pd was found,indicating a higher ethanol tolerance of the binary catalystwhen the carbon supported Pt–Pd catalyst with a Pt:Pd atomicratio 77:23 was prepared by reduction of metal precursorswithformicacid[33].Thispropertywasessentiallyascribedtoa reduced ethanol adsorption on Pt–Pd.In this paper, polyol process at 160  C, in the absence of surfactants was used to prepare nano-sized Pt–Pd/C elec-trodes. Their catalytic activities were evaluated toward elec-tro-oxidation of methanol and ethanol and compared to thatof commercial Pt/C ETEK. 2. Experimental 2.1. Catalyst preparation The Pt–Pd/C (with an atomic ratio of 1:1) electrocatalyst wasprepared using H 2 PtCl 6 $ 6H 2 O and PdCl 2 salts in ethylene glycolon carbon black (Vulcan XC 72) supports. All the samples con-tained30%metalinweightofcatalyst.Theamountofchemicalsin metal bases is calculated from the stoichiometric ratio of metals in precursors. For example 23.05 Â 10 À 3 gPdCl 2 and66.8 Â 10 À 3 gH 2 PtCl 6 $ 6H 2 O were used for Pt–Pd alloy prepara-tion. We prepared the samples containing 30% metal (total) inweight of catalyst (Pt–Pd). So, 130.6 Â 10 À 3 gC is mixed with39.18 Â 10 À 3 g Pt–Pd metal based salts. Catalyst containing inksolution was prepared by ultrasonic mixing of a solution con-taining 2 ml water, 5.4 ml Nafion 5% and 900 mg of catalysts.Thencalculatedquantityofink(dependingonthesurfaceused)was applied on a carbon paper electrode with an injection toobtain 0.4 mgcm À 2 total metal loading for Pt/C and Pt–Pd/C.First, palladium chloride was dissolved in ethylene glycoland the system was stirred overnight at room temperature.Then H 2 PtCl 6 $ 6H 2 O was added to this solution. Finally, NaOHsolution (in ethlylene glycol) was added to form a dark browncolloidalsolutionandthenheatedinanoilbathto160  Cundernitrogen purge for 3 h. Supporting particles (Vulcan XC 72)were added to the colloidal suspension after 3 h. The pH waslowered up to 1, to deposit unprotected catalyst metal nano-particlesontheVulcanXC72.Theresultantsamplewasfilteredand washed with de-ionized water and then dried at 110  Cunder vacuum overnight. The yield was 66.13%. 2.2. Characterization of catalysts 2.2.1. Spectroscopic methods The physical characteristics and surface chemical states of catalysts were studied using X-ray diffraction (XRD) analysisand X-ray photoelectron spectroscopy (XPS)[25]. X-raydiffraction pattern of Pt/C (ETEK) and Pt–Pd/C alloys wasobtained with commercial diffractometer using Cu K a radia-tion. The scan range was from 20 to 90  at a scanning rate of 0.5 degree/min. 2.2.2. Microscopic methods Pt–Pd/C catalyst samples were homogenously dispersed inmethanol and applied on a holey carbon grid to examine thedistribution and particle size by TEM (Philips CM200-FEG). Inaddition, the EDAX was also conducted for compositionalanalysis of the alloy catalyst samples. 2.2.3. X-ray photoelectron spectroscopy A Specs spectrometer was used for X-ray photoelectronspectroscopy (XPS) measurements using K a lines of Mg (1253.6 eV, 10 mA) as an X-ray source. All lines were refer-encedtotheC1slineat284.6 eV.Peakfittingsweredoneusing a Gaussian function. 2.2.4. Electrochemical methods Electrochemical experiments were performed using a Parstat2273 potentiostat. Suspended catalyst containing ink solutionwas prepared by ultrasonic mixing of a solution containing 2 ml water, 5.4 ml Nafion 5% and 900 mg of catalysts for15 min. Then calculated quantity of ink was applied ona carbon paper electrode with an injection to obtain0.4 mgcm À 2 totalmetalloadingforPt/CandPt–Pd/C.Electrodewas dried in air at room temperature for 15 min.Electrolyte solution was prepared from 0.1 M HClO 4 (Merck,suprapur) and 1 M methanol or 1 M ethanol. Alcohols were of analytical grade and used as received. All experiments werecarried out under pure nitrogen gas atmosphere. A saturatedcalomelelectrode(SCE)andaPtcagewereusedasthereferenceand counter electrode, respectively. All the electrochemicalmeasurements were carried out at 25 mVs À 1 sweep rate. international journal of hydrogen energy 34 (2009) 4312–4320 4313  3. Results and discussion 3.1. Physicochemical characterization of Pt–Pd catalysts Pt and Pd metal colloids are obtained mainly by reduction of halogenated precursors (H 2 PtCl 6 and PdCl 2 ) by means of ethylene glycol (EG). Apart from monometallic colloids,bimetallic colloids are also obtainable by means of reductivemethods, but the metals used should have a similar redoxpotential because otherwise bimetallic colloids are notformed. When this reaction is performed in an alcoholmedium, an oxidation of ethylene glycol does occur at thesame time. Although the chain products for a moderatedoxidation of EG have been well known, the nature of theproducts formed in the course of such metal precursorreduction has been the subject of an investigation[34]. Nogeneral scheme of EG oxidation can be drawn. Through thisstudy, we assumed that the reduction of the precursor intometal comes with a total oxidation of EG into CO 2 and H 2 O,this situation corresponding to the maximum reducing powerof the alcohol.CH 2 OH–CH 2 OH þ 3 PdCl 2 / 3Pd þ 2CO 2 þ 6HClSince the catalytic activity of the catalyst is stronglydependent on the particle size, to able to compare thecatalytic activities of synthesized catalyst with commercialPt–ETEK, the particle sizes of both catalysts have beenchecked.Fig. 1a–c shows the low magnification and high resolutiontransmission electron microscope (TEM) images withelementalanalysisresultsbyEnergyDispersiveX-rayanalysisfor the Pt–Pd/C electrocatalysts, respectively. Particle sizedistribution of Pt–Pd alloy is given inFig. 2. TEM images of Pt–ETEK catalyst were investigated and particle size distributionwas calculated (Fig. 3). It is observed from these figures thatboth Pt–ETEK and Pt alloy nano-particles are uniformlydistributed with particle size range approximately 3 nmwithout any agglomeration, on the exterior of the carbonparticles. There was no other impurity.Fig. 4shows the XRD diffractograms for Pt/C, Pt–Pd/VulcanXC-72R.TypicalofPt–PdalloyformationswereconfirmedfromtheXRDpattern.CorrelationofXRDandTEMresultsallowsusto define theoretical calculated particle size of catalyst.From the shift of the XRD peaks it is possible to calculate thenew cell parameter a Pt–Pd from the following equation:sin q ¼  l  h 2 þ k 2 þ l 2  1 = 2  = 2 a ð for a cubic structure Þ The lattice parameters of Pt–Pd catalysts reflect the forma-tion of an alloy calculated from Vegard’s law for a pure Pt–Pd(58:42) alloy involving the incorporation of Pt and atoms intothe fcc structure (Pt lattice parameter: 3.9161 nm; Pd latticeparameter: 3.8823 nm; Pt–Pd lattice parameter: 3.896 nm). Theaverage particle size of platinum–palladium catalysts wasfound to be about 3 nm.Fig. 5shows X-ray photoelectron spectra of the catalysts,both the Pd3d and Pt4f regions, and our deconvoluted 2 peakscorresponding to Pd (0) and Pd (x þ ). For the deconvolutionprocess the XPSPEAKFIT program available freely from theweb is used. For the Pd3d we have taken the spin-orbit-split-ting as 5.326 eV. Since the signal is very weak, obviously thereis some ambiguity in terms of the deconvolution of the peak abc 020004000600080001000012000100200300400    C  o  u  n   t  s Energy (eV) Pt-Pd/CarbonEDAX dataTEM Philips CM200CarbonPt L 1 β Pt M 1 β Pt L 1 β Pd L 1 β Cu K 1 β Pt L 2 β Pd L 1 α Pt M 1 α Cu K 1 α Fig. 1 – (a) High resolution TEM image of Pt–Pd/Vulcan XC-72R (scale bar [ 20 nm). (b) Low magnification of TEMimage of Pt–Pd/Vulcan XC-72R (scale bar [ 5 nm). (c) EDAXdata of Pt–Pd/Vulcan XC-72R. international journal of hydrogen energy 34 (2009) 4312–4320 4314  intensities and/or areas. But, it is obvious from the figure thatthere is a significant contribution ( > 50%) of the ionic Pd(x þ )moieties.The XPSPEAKFIT program fitted the Pt4f region to 2 peakswith the spin-orbit-splitting of 3.33 eV. The program correctlytakesintoaccountoftheintensityratioofthe4fdoublets(4:3),but does not show them separately. The inset is expandedcorresponding to the Pd3d and Pt4f regions for better visualperception the Pd3d and Pt4f7/2 and 4f5/2 peaks indicate twodifferent types of platinum and palladium (Fig. 5). These havebeen identified as Pt (0), Pt (x þ ), Pd (0) and Pd (x þ ) which couldbe platinum or palladium oxide or hydroxide. Surface atomicratio of Pt:Pd was also found 1:1 from XPS data. The peakscorresponding to F and S come from the nafion solution usedto prepare the ink. 3.2. Electrochemical measurements The molecular structure of the electroactive species hasa great influence on its electro-activity. The nature andstructure of the electrode material also play a key role inthe adsorption and electro-oxidation of organic fuels,particularly aliphatic alcohols. The enhancement of activitytoward methanol oxidation is generally related to theirsuperior CO tolerance as explained by the bifunctionalmechanism according to which adsorbed CO species areoxidized by OH species. CO stripping of the Pt–Pd catalystswas investigated and compared with that of Pt/C (E-TEK)under the same conditions[25]. An improvement of COtolerance for the Pt–Pd/C electrocatalysts was observed. Butit is previously stated that[26]bulk Pd and Pt have verysimilar properties, the coverage by OH species occurs in thesame potential range, although Pd is slightly more easilyoxidizable than Pt. Additionally Pd was completely inactivein acid medium toward oxidation of methanol[26]. Thesrcin of this synergistic effect may explain with enhance-ment of the overall reaction rate by decreasing the elec-trode poisoning.Now, if we focus first on comparison of electro-activity of Pt/C ETEK catalyst Pt–Pd/C synthesized electrode for meth-anol oxidation, considering the 1 M concentration of meth-anol, which is close to the working conditions in fuel cells, at25  C, for potentials higher than 0.4 V vs. RHE, the platinumsupported catalyst becomes the less active one (Fig. 6). Thiscan be explained by the fact that from this potential, plat-inum becomes less active for methanol oxidation and ispoisoned by adsorbed CO species. For DMFC application, theinteresting working potentials are located between 0.3 and0.5 V vs. RHE to obtain a cell voltage from 0.6 to 0.4 Vassuming a potential of about 0.9–1 V vs. RHE for thecathode, with the highest current density. Oxidation of methanol begins at 0.45 V for both of Pt/C and Pt–Pd/Celectrodes. Current density observed becomes two timesmore at 0.5 V on bimetallic alloy electrode. This activityincrease becomes more important at potential valuesbetween 0.5 and 1 V vs. RHE. The differences in activity as Fig. 2 – Particle size distribution of Pt–Pd/C catalyst. 123456789100,00,10,20,30,4    F  r  e  q  u  e  n  c  e Particle size (nm) Pt-ETEK (40 wt%) d NA = 3.0nm, D VA = 3.7nmd V = 4.0nmStd.Dev.= 0.9nm Fig. 3 – TEM image and particle size distribution of Pt–ETEK (20 wt%) catalyst (based on the observation of 974 particles). international journal of hydrogen energy 34 (2009) 4312–4320 4315
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