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Physical and Electrochemical Characterizations of Microwave-Assisted Polyol Preparation of Carbon-Supported PtRu Nanoparticles Zhaolin Liu,*,† Jim Yang Lee,†,‡,§ Weixiang Chen,§ Ming Han,† and Leong Ming Gan† Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, and Singapore-MIT Alliance (SMA), National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Received July 3, 2003. In Final Form: October 19, 2003 PtRu nanoparticles supported on Vulcan XC-72 carbon and carbon nanotubes were prepared by a microwave-assisted polyol process. The catalysts were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The PtRu nanoparticles, which were uniformly dispersed on carbon, were 2-6 nm in diameter. All PtRu/C catalysts prepared as such displayed the characteristic diffraction peaks of a Pt face-centered cubic structure, excepting that the 2θ values were shifted to slightly higher values. XPS analysis revealed that the catalysts contained mostly Pt(0) and Ru(0), with traces of Pt(II), Pt(IV), and Ru(IV). The electro-oxidation of methanol was studied by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry. It was found that both PtRu/C catalysts had high and more durable electrocatalytic activities for methanol oxidation than a comparative Pt/C catalyst. Preliminary data from a direct methanol fuel cell single stack test cell using the Vulcan-carbonsupported PtRu alloy as the anode catalyst showed high power density.
Introduction PtRu alloys are currently the most active anode catalyst for the oxidation of methanol- or CO-contaminated H2 (e.g., H2 derived from reformed methanol) in low-temperature solid polymer electrolyte fuel cells such as direct methanol fuel cells (DMFCs)1-3 or indirect methanol fuel cells (IMFCs).4 In methanol reforming, methanol reacts with water to produce a reformate gas with a typical composition of 75% H2, 24% CO2, and about 1% CO. However, the performance of an IMFC is significantly affected by CO concentrations as low as a few parts per million.5 This is because of the strong adsorption of carbon monoxide on the Pt anode which inhibits the hydrogen oxidation reaction. PtRu alloys have similar H2 oxidation kinetics as compared to Pt electrocatalysts6 but a much higher CO tolerance.7 In the presence of Ru surface atoms, adsorbed CO is oxidized at potentials more negative than that on Pt. Thus, the Pt surface sites become more available for hydrogen adsorption and oxidation. * Corresponding author. E-mail:
[email protected]. Fax: +65-68720785. † Institute of Materials Research & Engineering. ‡ Department of Chemical and Environmental Engineering, National University of Singapore. § Singapore-MIT Alliance (SMA), National University of Singapore. (1) Cameron, C. S.; Hards, G. A.; Thompsett, D. In Direct MethanolAir Fuel Cells; Landgrebe, A. R., Sen, R. K., Wheeler, D. J., Eds.; The Electrochemical Society Proceedings Series; Electrochemical Society: Pennington, NJ, 1992; PV 92-14, p 10. (2) Ren, X.; Wilson, M. S.; Gottesfeld, S. J. Electrochem. Soc. 1996, 143, L12. (3) Wasmus, S.; Vielstich, W. J. Appl. Electrochem. 1993, 23, 120. (4) Oetjen, H. F.; Schmidt, V. M.; Stimming, U.; Trila, F. J. Electrochem. Soc. 1996, 143, 3838. (5) Lemons, R. A. J. Power Sources 1990, 29, 251. (6) Gasteiger, H. A.; Markovic, N.; Ross, P. N. J. Phys. Chem. 1995, 99, 8290. (7) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617.
High surface area alloy catalysts are generally prepared by coimpregnation,8 coprecipitation,9 absorbing alloy colloids,10-11 or surface organometallic chemistry techniques.12 For both alloy- and oxide-promoted catalytic systems, it is important that Pt and the second metal (or metal oxide) are in intimate contact.11 This close association of platinum and cocatalysts can be difficult to attain using conventional catalyst preparation techniques because the active components may be deposited at different sites on the support surface. The activity of these multicomponent catalysts has been shown to depend strongly on the preparation conditions.13 While activated carbon is still the most common support material for electrocatalysis, new forms of carbon such as fullerences and nanotubes, which have become more available recently, have also been investigated as catalyst supports. The deposition of Pt, Ru, and PtRu on carbon nanotubes (CNTs) has been reported,14-17 and the resulting supported catalysts have been superior to activated-carbon-supported catalysts.18-20 (8) Fendler, A. G.; Richard, D.; Gallezot, P. Faraday Discuss. 1991, 92, 69. (9) Watanabe, M.; Uchida, M.; Motoo, S. J. Electroanal. Chem. 1987, 229, 395. (10) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 925. (11) Bo¨nnemann, H.; Brinkmann, R.; Britz, P.; Endruschat, U.; Mo¨rtel, R.; Paulus, U. A.; Feldmeyer, G. J.; Schmidt, T. J.; Gasteiger, H. A.; Behm, R. J. J. New Mater. Electrochem. Syst. 2000, 3, 199. (12) Crabb, E. M.; Marshall, R.; Thompsett, D. J. Electrochem. Soc. 2000, 147, 4440. (13) Go¨tz, M.; Wendt, H. Electrochim. Acta 1998, 43, 3637. (14) Xue, B.; Chen, P.; Hong, Q.; Lin, J. Y.; Tan, K. L. J. Mater. Chem. 2001, 11, 2387. (15) Yu, R. Q.; Chen, L. W.; Liu, Q. P.; Lin, J. Y.; Tan, K. L.; Ng, S. C.; Chan, H. S. O.; Xu, G. Q.; Hor, T. S. A. Chem. Mater. 1998, 10, 718. (16) Rajesh, B.; Thampi, K. R.; Bonard, J. M.; Viswanathan, B. J. Mater. Chem. 2000, 10, 1575. (17) Lordi, V.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (18) Planeiz, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem. Soc. 1994, 116, 7395.
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In this paper, a simple microwave-assisted polyol procedure for preparing Pt-Ru nanoparticles supported on activated carbon or CNTs is reported. The polyol process, in which an ethylene glycol solution of the metal precursor salt is slowly heated to produce colloidal metal, has recently been extended to produce metal nanoparticles supported on carbon and Al2O3.21,22 In the process, the polyol solution containing the metal salt is refluxed at 393-443 K to decompose ethylene glycol to yield in situ generated reducing species for the reduction of the metal ions to their elemental states. The fine metal particles produced as such may additionally be captured by a support material suspended in the solution. Conductive heating is often used, but microwave dielectric loss heating may be a better synthesis option in view of its energy efficiency, speed, uniformity, and implementation simplicity.23 Experimental Section The PtRu/carbon black (20 wt % Pt and 10 wt % Ru on Cabot Vulcan XC-72) and PtRu/CNT (20 wt % Pt and 10 wt % Ru on carbon nanotubes synthesized by catalytic chemical vapor deposition) catalysts were prepared by microwave heating of ethylene glycol (EG) solutions of Pt and Ru salts. The atomic composition of the alloy was chosen to be close to Pt50-Ru50, the most active composition for the methanol electro-oxidation reaction.24 A typical preparation would consist of the following steps: In a 100 mL beaker, 1.0 mL of an aqueous solution of 0.05 M H2PtCl6‚6H2O (Aldrich, A.C.S. Reagent) and 0.05 M RuCl3 was mixed with 25 mL of ethylene glycol (Mallinckrodt, AR). KOH (0.4 M) was added dropwise up to a total volume of 0.75 (or 0.25 mL). Vulcan XC-72 carbon (0.040 g), with a specific Brunauer-Emmett-Teller (BET) surface area of 250 m2/g and an average particle size of 40 nm, or CNTs prepared by chemical catalytic vapor deposition were added to the mixture and sonicated. The beaker and its contents were heated in a household microwave oven (National NN-S327WF, 2450 MHz, 700 W) for 50 s. The resulting suspension was filtered, and the residue was washed with acetone and dried at 373 K overnight in a vacuum oven. The catalysts were examined by transmission electron microscopy (TEM) on a JEOL JEM 2010. A JEOL JSM-5600LV was used to determine the metal contents in the samples by energy-dispersive X-ray analysis (EDX). For microscopic examinations, the samples were first ultrasonicated in acetone for 1 h and then deposited on 3 mm Cu grids covered with a continuous carbon film. The samples were also analyzed by X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB MKII spectrometer. Narrow scan photoelectron spectra were recorded for C 1s, O 1s, Ru 3p, and Pt 4f, and a vendor-supplied curve-fitting program (VGX900) was used for spectral deconvolution. A Philips X’Pert diffractometer using Cu KR radiation and a graphite monochromator was used to obtain the powder X-ray diffraction patterns. An EG&G model 273 potentiostat/galvanostat and a conventional three-electrode test cell were used for electrochemical measurements. The working electrode was a thin layer of Nafionimpregnated catalyst cast on a vitreous carbon disk held in a Teflon cylinder. The catalyst layer was obtained in the following way: (i) a slurry was first prepared by sonicating for 1 h a mixture of 0.5 mL of deionized water, 60 mg of Pt/C or PtRu/C catalyst, and 0.5 mL of Nafion solution (Aldrich, 5 w/o Nafion); (ii) 8 µL of the slurry was pipetted and spread on the carbon disk; (iii) the (19) Liu, Z. L.; Lin, X.; Lee, J. Y.; Zhang, W.; Hang, M.; Gan, L. M. Langmuir 2002, 18, 4054. (20) Li, W. Z.; Liang, C. H.; Qiu, J. S.; Zhou, W. J.; Han, H. M.; Wei, Z. B.; Sun, G. Q.; Xin, Q. Carbon, 2002, 40, 791. (21) Chen, W. X.; Lee, J. Y.; Liu, Z. L. Chem. Commun. 2002, 25882589. (22) Miyazaki, A.; Balint, I.; Aika, K. I.; Nakano, Y. J. Catal. 2001, 203, 364. (23) Galema, S. A. Chem. Soc. Rev. 1997, 26, 233. (24) Takasu, Y.; Fujiwara, T.; Murakami, Y.; Sasaki, K.; Oguri, M.; Asaki, T.; Sugimoto, W. J. Electrochem. Soc. 2000, 147, 4421.
Liu et al. electrode was then dried at 90 °C for 1 h and mounted on a stainless steel support. The surface area of the vitreous carbon disk was 0.25 cm2, and the catalyst loading was therefore 1.92 mg/cm2 based on this geometric area. Pt gauze and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. All potentials in this report are quoted against SCE. All electrolyte solutions were deaerated by highpurity argon for 2 h prior to any measurement. For the measurement of hydrogen electrosorption curves, the potential was cycled between +0.25 and -0.25 V at 10 mV/s to obtain the voltammograms of hydrogen adsorption in Ar-purged electrolytes. For cyclic voltammetry of methanol oxidation, the electrolyte solution was 2 M CH3OH in 1 M H2SO4, which was prepared from high-purity sulfuric acid, high-purity grade methanol, and distilled water. The MEA (membrane electrode assembly) for the DMFC test cell was made by hot-pressing pretreated Nafion 117 together with an anode sheet and a cathode sheet. The anode sheet was a carbon paper (SGL, Germany) with a carbon-supported PtRu catalyst layer. The cathode sheet was a carbon paper with a carbon-supported 40 wt % Pt catalyst layer supplied by E-TEK. The catalyst loadings at the anode and cathode were 4 and 3 mg/cm2, respectively, and the effective electrode area was 6 cm2. The fuel was 2 M CH3OH delivered at 2 mL/min by a micropump, and oxygen flow was regulated by a flowmeter at 500 cm3/min.
Results and Discussion Physicochemical Characterization of PtRu/Vulcan Carbon and PtRu/CNT Nanocomposites. In our approach, bimetallic nanoparticles are prepared and directly deposited on the carbon surface by microwave heating of EG solutions of Pt and Ru salts. The supported bimetallic catalysts prepared as such are expected to maintain good electrocatalytic activity and CO tolerance in the direct methanol oxidation reaction at room temperature. To the best of our knowledge, we are not aware of any other report on the rapid and direct synthesis of electrochemically active PtRu alloy nanoparticles supported on carbon using microwave irradiation. The size and composition of the alloy particles were analyzed by TEM and point-resolved EDX measurements. Figure 1a,b shows typical TEM images of Vulcan-carbonsupported and CNT-supported catalysts, showing a remarkably uniform and high dispersion of metal particles on the carbon surface. The particle size distributions of the metal in the supported catalysts were obtained by directly measuring the size of 150 randomly chosen particles in the magnified TEM images (e.g., Figure 1c for the PtRu-Vulcan carbon sample). The average diameters of 3.9 ( 0.3 nm for PtRu/Vulcan carbon and 3.5 ( 0.3 nm for PtRu/CNTs were accompanied by relatively narrow particle size distributions (2-6 nm). This is an improvement over our previous effort to prepare Vulcan-carbonsupported PtRu particles by microemulsion-based techniques which resulted in an average diameter of 4.3 nm and a broad particle size distribution ranging from 1 to 8 nm.25 The microwave-assisted heating of H2PtCl6/RuCl3/ KOH/H2O in ethylene glycol had evidently facilitated the formation of smaller and more uniform PtRu particles and their dispersion on either the Vulcan carbon or CNT support. It is generally agreed that the size of metal nanoparticles is determined by the rate of reduction of the metal precursor. The dielectric constant (41.4 at 298 K) and the dielectric loss of ethylene glycol are high, and hence rapid heating occurs easily under microwave irradiation. In ethylene glycol mediated reactions (the “polyol” process), ethylene glycol also acts as a reducing agent to reduce the metal ion to metal powders. The fast (25) Liu, Z. L.; Lee, J. Y.; Han, M.; Chen, W. X.; Gan, L. M. J. Mater. Chem. 2002, 12, 2453.
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Figure 1. TEM images of microwave-synthesized PtRu nanoparticles supported on different carbon samples: (a) Vulcan XC-72 carbon; (b) carbon nanotubes (nominal Pt loading, 20 wt %; Ru loading, 10 wt %). (c) Particle size distribution for the PtRu/Vulcan carbon. (d) EDX spectra of Vulcan-carbon-supported PtRu alloy nanoparticles.
heating by microwave accelerates the reduction of the metal precursor and the nucleation of the metal clusters. The easing of the nucleation-limited process greatly assists in small particle formation. Additionally, the homogeneous microwave heating of liquid samples reduces the temperature and concentration gradients in the reaction medium, thus providing a more uniform environment for the nucleation and growth of metal particles. The carbon surface may contain sites suitable for heterogeneous nucleation, and the presence of a carbon surface interrupts particle growth. The smaller and nearly single dispersed Pt nanoparticles on carbon XC-72 prepared by microwave irradiation can be rationalized in terms of these general principles. EDX measurements (Figure 1d) showed PtRu contents of 29.4 wt % for PtRu/Vulcan carbon and 30.3 wt % for PtRu/CNT. The Pt/Ru atomic ratios were around 1.0:1.01.1, which agree well with the stoichiometric ratio of 1:1 used for the preparation. The powder X-ray diffraction (XRD) patterns for PtRu/ Vulcan carbon and PtRu/CNT are shown in Figure 2 alongside the diffraction patterns of a Vulcan-carbonsupported Pt catalyst used as a comparison. All PtRu electrocatalysts displayed the characteristic patterns of Pt face-centered cubic (fcc) diffraction, except that the 2θ values were all shifted to slightly higher values. For example, the 2θ values of the (111) peak for PtRu/Vulcan carbon and PtRu/CNT were 40.20° and 40.40°, respectively, whereas the value was 39.85° for pure Pt. The same trend was replicated for the Pt(220) diffraction, which was 68.10° for Pt/Vulcan carbon, 68.05° for Pt/CNT, and
Figure 2. XRD patterns of microwave-synthesized PtRu/C catalysts.
67.65° for Pt/Vulcan carbon. These observations are consistent with the results of Chu and Gilman26 who found that Pt-Ru alloys containing up to 52 at. % of Ru would show only Pt reflections with some shifts in the position of each diffraction peak. This is the indication of a solid solution formed at the atomic level with a basically (26) Chu, D.; Gilman, S. J. Electrochem. Soc. 1996, 143, 1685.
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unaltered fcc structure. The broader diffraction peaks for the two catalysts also led to smaller average alloy particle size as calculated by the Scherrer equation:27
L ) 0.9λKR1/(B2θ cos θB) where L is the average particle size, λKR1 is the X-ray wavelength (1.54056 Å for Cu KR1 radiation), B2θ is the peak broadening, and θB is the angle corresponding to the peak maximum. The calculation results, which estimated the average size of 3.9 ( 0.3 nm for PtRu/Vulcan carbon and 3.5 ( 0.3 nm for PtRu/CNTs, are in good agreement with the TEM measurements. Besides a small shift to a slightly higher value, X-ray scattering from the CNT support was similar to that from Vulcan carbon and was detectable around 2θ ) 24°-26°. XPS was used to determine the surface oxidation states of the metals. As most of the atoms in small particle clusters are surface atoms, the oxidation state measured as such would also reflect well the bulk oxidation state. As the binding energy (BE) for the Ru3d line of zerovalent ruthenium at 284.3 eV28 is very close to the C1s line resulting from adventitious carbonaceous species, the Ru3p spectrum was used instead for the analysis of Ru oxidation state. Figure 3 shows the C1s, Pt4f, and Ru3p regions of the XPS spectrum of the PtRu/Vulcan carbon catalyst. The Pt4f signal consisted of three pairs of doublets. The most intense doublet (71.07 and 74.4 eV) was due to metallic Pt. The second set of doublets (72.4 and 75.7 eV), which was observed at a BE 1.4 eV higher than that of Pt(0), could be assigned to the Pt(II) chemical state as in PtO and Pt(OH)2.29 The third doublet of Pt was the weakest in intensity and occurred at even higher BEs (74.2 and 77.7 eV). These are the indications that these peaks were most likely caused by a small amount of Pt(IV) species on the surface. The slight shift in the Pt(0) peak to higher binding energies is a known effect for small particles, as has been reported by Roth et al.30 The C1s spectrum appears to be composed of graphitic carbon (284.6 eV) and -CdO-like species (285.83 eV).29 A small amount of surface functional groups with high oxygen contents was also noted in the spectrum (BE 286.8 eV). The overlap of the large C1s signal from the carbon support at about 284.6 eV with the Ru3d3/2 peak (284 eV) prevented the accurate determination of the Ru oxidation state at this BE value. Nevertheless, the small peak at 280.5 eV in the tail end of the C1s region appeared to arise from zerovalent Ru.31 The Ru3p3/2 signal could be deconvoluted into two distinguishable pairs of peaks of different intensities located at BE ) 461.1 and 462.7 eV which corresponded well with Ru(0) and RuO2,32 respectively. Electrochemical Performances. The real surface area of platinum for the Pt/C and PtRu/C catalysts could be estimated from the integrated charge in the hydrogen absorption region of the cyclic voltammogram (hatched area in Figure 4). The areas in m2/g were calculated from the following formula assuming a correspondence value (27) Warren, B. E. In X-ray Diffraction; Addison-Wesley: Reading, MA, 1996. (28) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992. (29) Woods, R. J. Electroanal. Chem. 1976, 9, 1. (30) Shukla, A. K.; Ravikumar, M. K.; Roy, A.; Barman, S. R.; Sarma, D. D.; Arico`, A. S.; Antonucci, V.; Pino, I.; Giordano, N. J. Electrochem. Soc. 1994, 141, 1517. (31) Roth, C.; Goetz, M.; Fuess, H. J. Appl. Electrochem. 2001, 31, 793. (32) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. In Handbook of X-ray Photoelectron Spectroscopy; Mullemberg, G. E., Ed.; Perkin-Elmer: Eden Prairie, MN, 1978.
Figure 3. X-ray photoelectron spectra of the microwavesynthesized PtRu/Vulcan carbon catalyst.
of 0.21 mC/cm2 (calculated from the surface density of 1.3 × 1015 atom per cm2, a value generally admitted for polycrystalline Pt electrodes29) and the Pt loading.
AEL (m2/(g catalyst)) ) QH/(0.21 × 10-3 C × (g catalyst)) From the calculation results in Table 1, the Pt surface area was expectedly lower in the two supported PtRu catalysts. All measurements are made with nearly the same catalyst loading at the 0.25 cm2 disk electrode as shown in Table 1. If it is assumed that each noble metal retains its bulk properties, Ru would not chemisorb hydrogen above 0.2 V (normal hydrogen electrode (NHE))
Preparation of Carbon-Supported PtRu Nanoparticles
Figure 4. Hydrogen electrosorption voltammetric profiles for the microwave-synthesized PtRu/C catalysts in 1 M H2SO4 with a scan rate of 50 mV/s at room temperature. The hatched area represents the amount of charge of the electrosorption of hydrogen on Pt. Table 1. Real (Active) Surface Areas of Pt and PtRu Catalysts as Determined by Hydrogen Electroadsorption origin of PtRu/C catalyst
onset catalyst AELc potential QHa SELb loading (mg/ (m2/(g 2 2 (V) (mC) (cm ) 0.25 cm ) catalyst))
PtRu/Vulcan carbon 8.2 PtRu/CNTs 8.5 Pt/Vulcan carbon 10.8 Pt/CNTs 10.4
39.1 40.5 51.4 49.5
0.42 0.45 0.42 0.43
9.3 9.0 12.2 11.5
0.19 0.20 0.27 0.28
a Q : charges exchanged during the electroadsorption of hyH drogen on Pt or PtRu. b SEL: real surface area obtained electrochemically. c AEL: real surface area obtained electrochemically per gram of catalyst.
while Pt would adsorb hydrogen strongly.26 The addition of Ru therefore appears to have caused the platinum surface sites to be partially covered. Therefore, the PtRu/C has lower real surface area than Pt/C. The use of Vulcan carbon or CNT did not result in significant differences in the Pt surface area. This is not surprising in view of the similarly small metal particle size in these supported bimetallic systems. The Pt and PtRu alloy nanoparticles were characterized by cyclic voltammetry (CV) in electrolytes of 1 M H2SO4 and 1 M H2SO4/2 M CH3OH. For cyclic voltammetry in 1 M H2SO4 at room temperature, the potential was scanned between -0.25 and 1.20 V (vs SCE) at the rate of 50 mV/s. In this article, all voltammograms refer to the features in the third cycle, where steady-state response was obtained. The voltammogram of the Pt/Vulcan carbon electrode in Figure 5 indicates the presence of polycrystalline Pt. The peaks for hydrogen and oxygen adsorption and desorption on the Pt surface in H2SO4 are clearly shown, and they concur well with the literature.33-34 On the other hand, the PtRu/Vulcan carbon electrode shows a very thick double layer suggesting the presence of RuOH and higher oxides in the surface layer. The alloying effect due to ruthenium (in PtRu/Vulcan carbon and PtRu/CNT) is shown in the voltammogram of methanol oxidation as a lower onset potential for oxidation. The comparison of onset potentials for methanol oxidation (Table 1) between the two bimetallic catalysts shows that the onset potential for PtRu/CNT (0.19 V) is a little lower than that for PtRu/ (33) Frelink, T.; Visscher, W.; Cox, A. P.; Veen, J. A. R. Electrochim. Acta 1995, 40, 1537. (34) Park, K. W.; Choi, J. H.; Kwon, B. K.; Lee, S. A.; Sung, Y. E.; Ha, H. Y.; Hong, S. A.; Kim, H.; Wieckowski, A. J. Phys. Chem. B 2002, 106, 1869.
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Figure 5. Voltammograms of the microwave-synthesized Pt/C and PtRu/C in 1 M H2SO4 and 2 M CH3OH/1 M H2SO4 with a scan rate of 50 mV/s at room temperature.
Vulcan carbon (0.20 V). However, the polarization curves for PtRu/Vulcan carbon and PtRu/CNT intercept at about 0.4 V. The origin of the crossing of polarization curves in Figure 5 for PtRu/CNT and PtRu/Vulcan carbon will be further investigated. During the methanol oxidation experiments, current density above the background level was detected as early as 0.1 V and began to escalate rapidly at 0.2 V (Figure 5). This may be interpreted in terms of a water discharge reaction producing OH species that were chemisorbed on the Ru sites. The anodic overpotential for the water discharge reaction on Ru sites and the formation of surface Ru-OH groups are kinetically more facile than that on the Pt sites. It is general believed that the Pt sites in Pt-Ru alloy are involved in the methanol dehydrogenation step and the strong chemisorption of the methanolic residues. At 0.2 V, the water discharge reaction occurs mostly on the Ru sites of the catalyst surface. The final step is the reaction between Ru-OH groups and neighboring methanolic residues on Pt to yield carbon dioxide. The effect of methanol concentration on the oxidation reaction for Pt/Vulcan carbon and PtRu/Vulcan carbon electrodes in 1 M H2SO4 at 25 °C is shown in Figure 6. For the Pt/Vulcan electrode in the low potential (
