Comparative studies of adsorbed CO and methanol electrooxidation on carbon supported Pt and PtRu cat

RARE METALSVol. 25, No. 3, Jun 2006, p. 274Comparative studies of adsorbed CO and methanol electrooxidation oncarbon supported Pt and PtRu catalysts in acid solutionPENG Cheng), ZHANG Zhen'", CHENG Xuan?, and ZHANG Ying'1) Department of Applied Chemistry, South China University ofTechnology, Guangzhou 510641, China2)Department of Chemitry, State Key Laboratory for Plhysical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China3) Department of Materials Science and Engineering, Xiamen University, Xiamen 361005, China(Received 2005-06-01)Abstract: Physicochemical characterization of commercial Pt/C and PtRu/C catalysts was carried out by X-ray diffraction,X-ray photoelectron spectroscopy, and transmission electron microscopy. Comparative studies of methanol electrooxidationon carbon supported Pt and PtRu catalysts in acid solution were performed by means of an eletrochemical method. Theexperimental results showed that PtRu/C not only had a higher electrocatalytic activity for the electrocatalytic oxidation ofmethanol than PUC, but also for the electrocatalytic oxidation of adsorbed CO. Alloy formation of Ru with Pt modified notonly the characteristics of H2 adsorption on the surface of catalyst but also the characteristics of CO adsorption on the sur-face of catalyst. The interaction of CH2OH with Ru was a temperature-activated process requiring elevated temperature.Key words: electrochemistry; methanol oxidation; cyclic voltammetry; electrocatalyst[This work was financially supported by the National Natural Science Foundation of China No.20073036) and the Natu-ral Science Young Teacher Fund of South China University of Technology (No. BI5-E5050650.]1. Introductionhave to be heated to several hundred degrees centi-grade in a fuel processor. In addition, an emission ofDirect methanol fuel cells (DMFCs) have con-carbon monoxide as a by-product in the fuel proc-siderable advantages compared to gas feed (H/air)essor could be completely avoided in the DMFCs.polymer electrolyte membrane (PEM) fuel cellsOne of the main problems that hinder the com-[1-2]. The hydrogen PEM fuel cells use gaseous hy-mercialization of the DMFCs, however, is low activ-drogen as fuel. But there is no source and infra-ity and poisoning intolerance to methanol oxidationstructure established yet for hydrogen distributionintermediates on the anode catalyst side. For thisand storage, neither as a liquid nor as a gas. How-reason, a number of catalyst systems have been in-ever, the DMFC, similar to a proton exchangevestigated for their suitability as anode catalystsmembrane fuel cell (PEMFC), uses methanol fuel[3-4]. These studies have shown that PtRu alloys aredirectly for electric power generation and promisesthe best among the candidate catalysts for methanoltechnical advantages for a power train. A directelectrooxidation.methanol fuel-cell system offers higher system effi-In a fuel cell, the catalyst is usually deposited onciencies because it eliminates the fuel-processinghigh surface area conductive carbon. It is obvioussystem. Hence, a significantly smaller system size atthat the structure, surface, and morphology of thelower cost and comparable power densities may becatalyst中国煤化工olerance towardachievable. Furthermore, the DMFCs enable a quickadsorbe5ol electrooxida-startup procedure, because there are no units thattion.YHCNMHG。....rization.. of theCorresponding author: PENG ChengE-mail: chengpen@scut.edu.cnPeng C. et al, Comparative studies of adsorbed CO and methanol electrooxidation on...275catalyst is of fundamental importance for the inter-made by proportionally mixing the catalysts withpretation of electrochemical properties. In this article,isopropanol, 5% Nafion solutions, and DI water.we present a comparative study of adsorbed CO andCamel hairbrushes were used to carefully paint themethanol electrooxidation on commercial catalystsinks onto the polished mirror-imaging surface of thePt/C and PtRu/C in acid solution, and at the sameGC electrode. After painting two to three layers, thetime, the physicochemical characterizations of theelectrode was dried (120°C, 1 h) and weighed againtwo catalysts are to be carried out by means of X-rayto obtain the catalyst loading.diffraction, X -ray photoelectron spectroscopy, andElectrochemical measurements were performedtransmission electron microscopy.using an Autolab general purpose electrochemicalsystem (Ecochemie, Netherlands). The data were2. Experimentalacquired with a personal computer. The solutionswere continuously purged by nitrogen gas for at2.1. Materialsleast 30 min before each measurement. The nitrogenGlassy carbon (GC) rods with a diameter of 5atmosphere was also maintained during the electro-mm and a length of 10 cm were purchased from Alfachemical measurement. Unless stated otherwise, allAesar. Commercial catalysts, 30% PtRu (1:1 inpotentials in this article are referred to the SCE.atomic ratio) and 40% Pt catalysts, both of whichwere supported on a high surface area conductive2.3. Characterization of catalystscarbon black (Vulcan XC-72)(E-TEK)， and 5%X-ray diffraction (XRD) analysis was carried outNafion solutions were purchased from Du Pont. Thewith a Rigaku D/max-rC diffractometer usinga Cugases used in this work including nitrogen and car-Ka source. X-ray photoelectron spectroscopy (XPS)bon monoxide were of high purity (> 99.99%) andanalysis was performed using an Escalab MkII pho-the chemicals were of analytical grade. The deion-toelectron spectrometer. The X-ray source was Mgized water of 18 M9 was used to prepare all aque-Ka operating at a power of 22 W. XPS spectra wereous solutions.obtained with a pass energy of 50 eV for wide scansand 15 eV per individual elements.2.2. Electrochemical measurementsTransmission electron microscopy (TEM) analy-A conventional 3-electrode cell was used in elec-sis was performed with a JEOL JEM-100CX IItrochemical measurements. The working electrodeselectron microscope, working at 100 kV acceleratingwere GC, Pt/C on GC, and PtRu/C on GC electrodes,voltage. Specimens were prepared by grinding therespectively. The counter electrode was platinizedraw catalysts and ultrasonically suspending the par-platinum and the reference electrode was a standardticles in alcohol. The suspensions were deposited oncalomel electrode (SCE), with an elecrolyte bridgea standard Cu grid covered with carbon film, and al-to avoid chloride contamination in the cell.lowed to dry before the microscopic analysis.The GC working electrode was prepared by in-serting a piece of 2 cm-thickness-5 mm-diamenter3. Results and discussionGC rod into a PTFE cap. The electric contact wasmade by connecting the PTFE cap protected GC3.1. Characterization of commercial catalystswith another piece of copper rod protected by aTypical X-ray diffaction patterns measured onPTFE tubing. A platinum wire as a lead was used toboth commercial catalysts, Pt/C and PtRu/C, arecontact the copper rod.compared in Fig. 1. In Fig.l, both samples exhibitThe GC electrode was polished with diamondonly the characteristic diffraction peaks of the Pt fccpaste (0.25 um, Beulhur) on a polishing cloth,structu中国煤化Iificant shift to .cleaned ultrasonically in DI water, dried in an ovenhigherthe diffractionf片CNMHG(120°C, 1 h), and then weighed. Catalyst inks werepeaks in ue I uxwC vaialysu wiul cspect to the Pt/C276RARE METALS, Vol. 25, No. 3, Jun 2006catalyst. The lattice parameters, Cic, were 0.3906 nmmeasurement. The dispersion of metal particles inin Pt/C and 0.3892 nm in PtRu/C catalysts, respec-the PtRu/C sample appeared slightly less homoge-tively, as determined by the peak profile fitting ofneous than that in the Pt/C catalyst (Fig. 3(b)). Smallthe (220) reflection ilustrated in Fig. 2. The ob-agglomerates of metal particles were sporadicallyserved smaller lattice parameter for the PtRu/C sam-observed. The particle size ranged between 2.0 andple accounts for the presence of solid solution of Pt4.0 nm with an average dimension of 3.0 nm.and Ru. According to Vegard's law, the crystal latticeof a material contracts if it forms a solid solutionwith the atoms having smaller radii (rp:= 0.138 nm;rRu= 0.1322 nm) [5]. Average particle sizes for themetal crystallites of 3.2 and 2.6 nm in the PUC andPtRu/C catalysts, respectively, were also determined目Pt/Cfrom the broadening of the (220) diffraction peaksEtshown in Fig. 2 using the Debye-Sherrer equation.The dispersion of metal particles on carbon blackPIRuCwas also investigated by TEM for both Pt/C and30405(50708(90PtRu/C catalysts. The Pt/C catalyst showed homo-201()geneous dispersion of Pt particles on the carbonFig. 1. XRD patterns of the commercial Pt/C andsupport with an average particle size of 3.5 nm (Fig.PtRu/C.3(a)), which is in good agreement with the XRD180[12(b)160100g 80-120; 10060 F80|40-600一626466687072?7462 64666870 72 7420/()Fig. 2. Peak profile ftting of the (220) reflection in the commercial Pt/C (a) and PtRu/C (b) catalysts.a)(b)_中国煤化工50mm, PFHCN M H G50nmFig. 3. TEM micrographs of the commercial Pt/C (a) and PtRu/C (b) nanoparticle catalysts.Peng C. et al, Comparative studies of adsorbed CO and methanol electrooxidation on...277The commercial catalyst powders were also ana-present. Meanwhile, another possible reason for thelyzed by XPS to study the electronic structure andlarger positive shift of the bimetallie sample is thatchemical environment. The photoemission spectrahe Pt electronic structure is modified by the pres-of Pt (4f) doublets are reported in Fig. 4. The peakence of Ru. This feature of the resulting shift wouldmaximum for Pt (4f72) shifted to higher values bybe in agreement with the work of McBreen and0.4 eV for the PVC catalyst with respect to the stan-Mukerjee [8]. By XAS measurements, they founddard Pt sample (70.8 eV). This may be ascribed tothat in the PtRu alloy, Ru increases vacancies in thethe presence of a platinum-support electronic effectPt valence band, leading to more tightly bound elec-[6]. The specific metal-support interaction is throughtrons in core levels. In conclusion, the Pt (4f) peakelectron transfer from platinum clusters to the car-shift in the spectra toward a higher binding energy ofbon d-band. The metal-support interaction will bePtRu/C is on account of stronger platinum-carbonhelpful to the improvement of catalytic propertiesinteractions and metal-metal interaction.and to the increase of the stability in the electrocata-lyst. A close inspection of the plot also reveals that3.2. Electrochemical studiesthere is a further shift to higher values in Pt (4fr2) byFig.5 ilustrates representative cyclic voltammo-about 0.4 eV in the PtRu/C sample. This further shiftgrams of two catalysts obtained in the half-cellin PtRu/C with respect to Pt/C is probably related (1)without methanol, recorded at 10 mV.s' in 0.5to different oxidation states of platinum, or (2) tomol:L-' H2SO4. The shape of the voltammetric curvemetal-metal interaction, or (3) to metal-support in-of the Pt/C catalyst is very similar to that of the un-teraction. We have found that the Pt (4f) spectra ofsupported Pt electrode. From Fig. 5(a), we can seethe two catalysts can be deconvoluted into threethat the potential region of the hydrogen adsorption/doublets of the same binding energy (i.e, the samedesorption is separatedfrom the reversi-components) and intensity (i.e., the same amounts),ble/irreversible oxide formation by the double layerindicating the absence of different Pt oxidation states.potential region. Comparing the voltammetry ofThus this effect could be very lttle. The presence ofPt/C with that of PtRu/C, it can be seen that alloyRu precursors and their decomposed species can af-formation of Ru with Pt has changed the H2 adsorp-fect the acid-base properties of the carbon supporttion/desorption characteristics of Pt/C, leading to the[7]. This effect can produce a strong metal-supportabsence of distinct hydrogen adsorption/desorptioninteraction, which will lead to the change of thepeaks on the PtRuC electrode (Fig. 5()). We canelectronic configuration of carbon where platinum isalso find that in the double layer region of the curvefor the PtRw/C catalyst, the charging current is larger,probably on account of the presence of the large(1amounts of oxygenated species [9]. In addition, thePt4fmetal oxide reduction peaks on the cathodic sweepof PtRu/C is shifted cathodically about 35 mV withrespect to Pt/C.Fig. 6 shows cyclic voltammograms of Pt/C (a)Mand PtRu/C (b) in 0.5 mol:L-l H2SO4 and 0.5mol:L CH3OH solutions at different temperatures.A close inspection of the plot reveals that as thetemperature is raised from 22 to 60°C, the increase66870727476788in current density at the positive potential limit isBinding energy / eVmuchI中国煤化工han on the Pt/CFig. 4. X-ray photoelectron spectra of Pt/C (1) andelectrodIYCNMHurent density atPtRu/C (2) catalysts.the postuve porcttual 1 ID luch more on the278RARE METALS, Vol. 25, No. 3, Jun 2006PtRu/C than on the Pt/C electrode when the tem-acter toward CH;OH is activated. At the same time,perature is raised from 60 to 80°C. This may beit can be seen that the onset of methanol oxidation isprobably related to the interaction of CH3OH withshifted negatively with the increase in temperature atRu, which is a strongly activated process requiringboth electrodes. This negative potential shift is be-elevated temperature [10]. The CHzOH cannot because of the restructuring of the oxygen-containingadsorbed on Ru sites at room temperature. But whenspecies on Pt when the temperature is raised [10]the temperature is raised to above 60°, its inert char-a)3(61⑤0个0自-15-2-3---0.2 0.00.0.4 0.6 0.8-4-0.20.0 0.20.40.60.8E/V (vs. SCE)E1 V (vs. SCE)Fig. 5. Cyelic voltammograms of Pt/C (a) and PtRu/C (b) in 0.5 mol:L-1 H2SO4 at 22°C with a scan rate of 10mVs5'.b)80C60; 80'C4(40,60C,60"Cj22C2022C0个-20七-20 t-0.2 0.0.40.-0.20.00.2.4E/ V (vs. SCE)E1V (vs. SCE)Fig. 6. Cyclic voltammetry of Pt/C (a) and PtRu/C (b) in 0.5 molL-' H2SO4 and 0.5 mol:L-' CH3OH at differenttemperatures with a scan rate of 20 mV/s.The above potentiodynamic analysis providesties are compared at 0.5 V vs. RHE (reversible hy-important transient information of the electrochemi-drogen electrode). The corresponding mass activitiescal kinetics on the PUC and PtRu/C electrodes. Theare calculated and summarized in Table 1. The gen-potentiostatic study provides a steady state per-eral trends in potentiodynamic measurements in Figs.formance of the catalysts. Fig. 7 shows the potentio-5 and 6 are also correspondent to the steady-statestatic oxidation curves of Pt/C (a) and PtRu/C (b) atmeasurements: (1) kinetics on the PtRu/C electrodetwo different temperatures. The initial high currentsis much faster than on the Pt/C electrode; (2) thecorrespond largely to double-layer charging. Thecatalyst中国煤化工an increase incurrents decay with time in a hyperbolic trend andtemper:reach an apparent steady state within 500 s. ForTheMYHc N M H Grbon monoxide,steady-state analysis the mass specific current densi-an intermediate product of anodic methanol oxida-Peng C. et al, Comparative studies of adsorbed CO and methanol electrooxidation on...27920a)|presence of adsorbed CO, indicating nearly full COcoverage. The Pt/C electrode showed a major CO”15-oxidation peak at about 0.54 V. This value was morepositive than that on the bulk polycrystalline plati-num surfaces (about 0.48 V), but similar to that兵5|found with Pt (111) single crystal surfaces [10]. In_60C|addition to the main peak, a lower potential shoulder0was observed at ca. 0.46 V, which indicated thepresence of two different CO binding features on the4008001200Pt particles. The PtRu electrode, however, onlyTime/sshowed a single CO oxidation peak at ca. 0.24 V,60which shifted to negative approximately 300 mV in命50comparison with that on the Pt electrode. On thesecond scan, the voltammograms showed that CO香40was fully oxidized and the hydrogen adsorp-30tion/desorption peaks reappeared as shown in Fig. 5.喜208)营10_60C60F22c4800 1200Fig. 7. Potentiostatic oxidation of 0.5 mol:L 1 CH;OHS 0一in N-saturated 0.5 mol:L' H2SO4 on Pt/C (2) and2tPtRu/C (b) catalysts at different temperatures. Poten-tial-step from 0.075 to 0.5 V vs. RHE.-0.4-0.2 0.0 0.20.40.6 0.E1V (vs. SCE)Table 1. Mass- specific current densities at 0.5 V vs.0 ()I/ (mA-mg )25FE/V (vs. RHE) Pt/C (40 wt%) PtRu/C (30 wt%)20 F22°C 60°C22°C60°C5-0.51.2.522.927.28tion, on the Pt surface, poisons the catalyst, and thusfurther hinders the oxidation of methanol. For acomparison to the methanol oxidation, the electro-chemical oxidation of carbon monoxide has also- 10been investigated in this article. The CO was-0.4-0.20.00.20.40.6 0.8pre-adsorbed on the electrodes at -0.15 V vs. SCE inE/V(vs. SCE)0.5 molL-1 H2SO4 at 22°C. The cyclic voltammo-Fig. 8. ptrirnin . of Pt/C (a) andgrams were obtained and shown in Fig. 8. For eachPtRu/C中国煤化工so, at 22C withelectrode, during the first forward scan, the hydro-ascanMHC N M H Ges represent thegen peaks observed in Fig.5 were suppressed by thefirst cycle and the dashed lines the second.280RARE METALS, Vol. 25, No. 3, Jun 20064. Conclusions4] Shukla A.K., Arico A.S, and EIkhatib K.M, X-rayphotoelectron spectroscopic study on the effect of Ru(1) There are metal-metal interactions andand Sn additions to platinized carbons, Appl. Surf.stronger metal-carbon interactions on the PtRu/CSci, 1999, 137 (1-4): 20.catalyst.5] Wilson M.S., Garzon FH, Sickafus K.E, and Got-(2) The alloy formation of Ru with Pt changes thetesfeld S., Surface area loss of supported platinum incharacteristics of H2 adsorption/desorption on thepolymer electrolyte fuel cells, J. Electrochem. Soc.,surface of the Pt/C catalyst and also changes the1993, 140 (10): 2872.characteristics of CO adsorption on the surface of6] Shukla A.K, Ravikunar M.K, Roy A., Barman S.R.,Sarma D.D, Arico A.S, Antonucci V, Pino L, andthe Pt/C catalyst.Giordano NJ, Electro-oxidation of methanol in sul-(3) The PtRu/C catalyst shows a better catalyticfuric acid electrolyte on platinized- carbon electrodesactivity to methanol oxidation and CO oxidationwith several functional-group characteristics, J. Elec-than Pt/C.trochem. Soc, 1994, 141 (16): 1517.(4) The oxidation of CH;OH on Ru is a tempera-7] Antolini E., Formnation of carbon-supported PtM al-ture-activated process.loys for low temperature fuel cells: a review, Mater:Chem. Phys., 2003, 78 (1): 563.References8] McBreen J. and Mukerjee S., In situ X-ray absorptionstudies of a PtRu electrocatalyst, J. Electrochem. Soc,[1] Ren X., Piotr Z, Sharon T, John D., and Shimshon1995, 142 (10): 3399.G, Recent advances in direct methanol fuel cells at9] Ticianelli EA, Beery J.G, Paffett M.T, and Gottes-Los Alamos National Laboratory, J. Power Sources,feld S., An electrochemical, llipsormetric and surface2000, 86 (1-2):111.science investigation of the PtRu bulk alloy surface,J.[2] Sharon T, Ren x, Shimshon G, and Piotr Z, DirectElectroanal. Chem, 1989, 258: 61.methanol fuel cells: progress in cell performance and[10] Gateiger HA. and Markovic N, Ross P.N, andcathode research, Electrochim. Acta, 2002， 47Cairms EJ, Temperature-dependent methanol elec-(22-23): 3741.tro-oxidation on well-characterized PtRu alloys, J.[3] Iwasita T, Hoster H, and John-annacker A., Metha-Electrochem. Soc, 1994, 141 (7): 1795.nol oxidation on PtRu electrodes: Influence of sur-[11] Gasteiger H.A, Markovic N, Ross P.N. and Caimsface structure and PtRu atom distribution, Langmuir,E.J, CO electrooxidation on well-characterized PtRu2000, 16 (2): 522.alloys,.J. Phys. Chem, 1994, 98 (2): 617.中国煤化工MHCNMHG