Pt−Ru Nanoparticles Supported on Carbon Nanotubes as Methanol

J. Phys. Chem. C 2007, 111, 2803-2808

2803

Pt-Ru Nanoparticles Supported on Carbon Nanotubes as Methanol Fuel Cell Catalysts Liang Li and Yangchuan Xing* Department of Chemical & Biological Engineering, UniVersity of MissourisRolla, Rolla, Missouri 65409 ReceiVed: August 27, 2006; In Final Form: December 11, 2006

Bimetallic Pt-Ru alloy catalysts have been demonstrated to be more active than pure Pt catalysts in the electrooxidation of methanol. We report here a study on Pt-Ru nanoparticle catalysts supported on sonochemically functionalized carbon nanotubes. The catalysts were prepared by directly reducing the corresponding salts, K2PtCl4 and K2RuCl5, in an ethylene glycol aqueous solution containing dispersed carbon nanotubes. Three catalysts of different Pt to Ru atomic ratios, namely, Pt53Ru47, Pt69Ru31, and Pt77Ru23, were prepared for investigation of the compositional effects. It was shown that highly dispersed bimetallic Pt-Ru alloy nanoparticles with no agglomeration can be synthesized on the carbon nanotubes with average particle sizes of less than 3.0 nm in diameter. The Pt-Ru nanoparticles are uniform and cover only the outside of the carbon nanotubes. It was found that the polyol process produced alloy compositions that are not consistent with the metal ratios in the precursors. It was also found that the lattice spacings of these catalysts are different due to the different compositions of the catalysts. Cyclic voltammetry showed that the catalysts were electrocatalytically active in the electrooxidation of methanol. Among the three catalysts, the Pt53Ru47 catalyst produced the best performance. This catalyst was found to be the most stable, while the other two deactivated faster in the oxidation of methanol. All three Pt-Ru catalysts have higher electrocatalytic activities than a commercial catalyst of Pt50Ru50 supported on carbon black. However, the Pt69Ru31 and Pt77Ru23 catalysts showed poorer stability that can be justified by the bifunctional mechanism of bimetallic Pt-Ru alloys.

Introduction Direct methanol fuel cells (DMFCs) are a promising power source for portable electronics due to their ease of handling, high-energy densities, and low operation temperatures.1 However, there are several problems in current DMFCs that need to be resolved to increase their efficiency and power density. One of the problems is the poisoning of the Pt electrocatalysts by carbon monoxide (CO).2 Pure Pt catalysts are prone to poisoning by CO since the CO can strongly adsorb onto the Pt surface and block the active sites. As a result, Pt catalysts rapidly deactivate due to the Pt-CO species formed in the electrooxidation of methanol.3 To alleviate this problem, bimetallic PtRu alloys have been studied as CO-resistant electrocatalysts.4 It has been shown that bimetallic Pt-Ru catalysts are effective in reducing CO poisoning. The adsorption of hydroxyls on the oxophilic Ru facilitates the removal of CO species adsorbed on the Pt and thus releases the occupied active sites.4b However, the effectiveness depends on the compositions of the Pt-Ru alloy. Current anode catalysts for DMFCs are Pt-Ru alloys supported on carbon black (PtRu/CB) in the form of highly dispersed nanoparticles with Pt to Ru atomic ratio of 1:1. Recent studies, however, have shown that Pt-Ru alloy nanoparticles supported on other forms of carbon, such as carbon nanofibers (CNFs)5 or carbon nanotubes (CNTs),6 displayed higher electrocatalytic activities. In the study by Steigerwalt et al.,6a for example, it was found that Pt-Ru nanoparticles supported on CNFs outperformed unsupported Pt-Ru black by 50% in the electrooxidation of methanol. Another study by Kim et al. found a 200% increase in the maximum power density in DMFCs with * Corresponding author. E-mail: [email protected]. Tel: 573-341-6772. Fax: 573-341-4377.

CNT supported Pt-Ru catalysts (PtRu/CNT), when compared to the PtRu/CB catalyst.6d CNTs have been under intensive investigation in recent years for their use as support for a variety of metallic catalysts.7 Almost all of the reported studies have found an enhanced catalytic activity of CNT supported catalysts. Although the underlying mechanisms for the activity enhancement are still not well understood, it was suspected that the surface structures and electronic properties of CNTs may be well responsible.8 It is well-known that metal-support interactions affect the activity of supported metal catalysts and the surface structures of carbon support can play an important role.9 A recent study on carbon nanocoil supported Pt-Ru nanoparticles concluded that high crystallinity of the nanocoil surface contributed to the observed enhanced catalytic activity.10 Since different carbons (e.g., CB, CNFs, CNTs, and fullerenes) have different surface structures, they therefore would affect the activity of supported metal nanoparticles. In this paper we report a study on CNT-supported Pt-Ru bimetallic nanoparticle catalysts. We have used a previously developed sonochemical technique for the functionalization of multiwalled CNTs. It was found that the sonochemically functionalized CNTs can result in high quality Pt-Ru catalysts of uniformly dispersed Pt-Ru nanoparticles with sizes of less than 3.0 nm in diameter, ideal for use as DMFC catalysts. The study has focused on the preparation and characterization of the PtRu/CNT catalysts and their activity in the electrooxidation of methanol. Three catalysts with different Pt to Ru atomic ratios were prepared. It was found that they exhibit different activities with the catalyst of close to 1:1 atomic ratio showing the best performance.

10.1021/jp0655470 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007

2804 J. Phys. Chem. C, Vol. 111, No. 6, 2007 Experimental Methods Catalyst Preparation. Multiwalled CNTs with ∼95% in purity, 20-50 nm in diameter and 5-20 µm in length, were purchased from NanoLab, Inc. (Newton, MA). The as-purchased CNTs were functionalized using a sonochemical oxidation technique before depositing Pt-Ru bimetallic nanoparticles on them. The sonochemical process has been reported elsewhere.8a,11 Briefly, the CNTs were first dispersed in a concentrated H2SO4-HNO3 mixture (8.0 M for each acid) and then placed in an ultrasonic bath (Fisher Scientific, 130 W and 40 kHz) for treatment at a bath temperature of 60 °C with a duration of 2 h. This treatment created functional oxygenated groups on the CNTs, which act as anchor sites for the bimetallic Pt-Ru nanoparticles. The synthesis of Pt-Ru bimetallic nanoparticles on the functionalized CNTs was carried out by reducing Pt and Ru salt precursors, namely, K2PtCl4 and K2RuCl5 (reagent grade from Alfa Aesar), in an ethylene glycol-water solution.6c,12 In a typical experiment, the surface-functionalized CNTs were placed in a 50 mL flask, to which 15.0 mL of ethylene glycolwater solution (2:1 v/v ratio) was added. An appropriate amount of stock solutions (0.01 M) of the precursors was added, depending on the expected weight loadings and compositional ratios of each metal. The reduction reactions were performed under reflux conditions for 2 h with continuous magnetic stirring. The catalysts were then separated from the solution in a centrifuge (CL2, ThermoIEC) and thoroughly washed with deionized water. The catalysts were dried in a vacuum furnace at ∼70 °C for 24 h. The separated supernatant solution was used for trace metal elemental analysis. Since the supernatant solutions contain a high concentration of ethylene glycol, they were first digested to eliminate ethylene glycol. For each sample, 2 mL of supernatant was added into 15 mL of HNO3, digested at 65 °C for ∼12 h, and then diluted to 50 mL. The samples were analyzed with induction-coupled plasma mass spectroscopy (ICP-MS) (ELan DRC-e, Perkin-Elmer). The actual metal loading and composition were calculated from the trace analysis. Catalyst Characterization. Transmission electron microscopy (TEM) analysis was carried out on a Philips EM430 microscope at an operating voltage of 300 kV. The TEM samples were prepared by ultrasonic dispersion of a PtRu/CNT catalyst in ethanol for about 10 min, followed by dropping a small amount of the dispersion on a 400-mesh carbon-coated copper grid (CF400-Cu, Electron Microcopy Sciences) and letting it dry in air. The X-ray diffraction (XRD) analysis was performed on a Scintag 2000 diffractometer with a scan rate of 0.01 deg/min to ensure obtaining fine crystalline structures of the Pt-Ru nanoparticles. Electrochemical Measurements. Voltammetric techniques were used to study the Pt-Ru catalysts. Cyclic voltammetry (CV) was conducted in 1.0 M H2SO4 to evaluate the electrochemical activity in the hydrogen adsorption/desorption regions and was used to investigate methanol electrooxidation in 1.0 M H2SO4 solution containing 2.0 M methanol. The experiments were performed at room temperature using an Electrochemical Workstation (BAS 100, Bioanalytical Sciences). A thin film electrode technique was used to prepare the working electrodes. The technique was previously developed to characterize high surface area electrocatalysts.13 Briefly, a glassy carbon disk (3 mm in diameter, or 0.071 cm2 in area) was polished to a mirror finish with 0.05 µm alumina pastes before each measurement and served as the substrate for the PtRu/CNT catalysts. Aqueous suspensions of the catalysts with a concentration of 1.0 mg/mL were made by ultrasonically dispersing 5 mg of the PtRu/CNT

Li and Xing TABLE 1: Atomic Compositions of Pt-Ru Catalysts samples

precursor Pt:Ru

actual Pt:Ru

composn (%)

metal loading (wt %)

S S1 S2 S3

1:1 1:1.2 2:1 3:1

1.27:1 1.13:1 2.23:1 3.35:1

Pt56Ru44 Pt53Ru47 Pt69Ru31 Pt77Ru23

18.62 19.28 19.41 19.39

catalyst in 5 mL of deionized water. A 30 µL aliquot of the suspension was pipetted onto the glassy carbon substrate. After evaporation of the water, 10 µL of a diluted Nafion solution (5 wt %, Alfa Aesar) was put on top of the catalyst, which acted as both a binder and the electrolyte. The thin film electrode was then put in the electrochemical cell in a Faraday cage (C3 Cell, Bioanalytical Sciences) for measurement. Potentials of the working electrode were measured against a Ag/AgCl reference electrode, and a platinum wire coil was used as the counter electrode. During the experiments, ultrahigh-purity N2 was introduced into the cell above the electrolyte solution as the protection atmosphere. All potentials used in this paper were converted to potentials relative to the reversible hydrogen electrode (RHE) potential, unless otherwise noted. Results and Discussion Preparation and Characterization. To prepare Pt-Ru catalysts with desired compositions for study, we tried to obtain Pt to Ru atomic ratios of 1:1, 2:1, and 3:1 by using a predetermined amount of salt precursors that would give these ratios. However, it was found that the actual Pt to Ru ratios are higher than expected. This was evidenced by the ICP-MS analysis that showed a non-negligible amount of Ru left in the supernatant solutions. Table 1 shows the initial and actual atomic ratios that we have obtained. It can be seen that, for all three samples that were to have 1:1, 2:1, and 3:1 ratios, the actual catalysts had less Ru than expected, indicative of that the Ru precursors were not completely reduced to metallic Ru. The Pt, however, was consistent with the precursor used, evidenced by the fact that there was negligible amount of Pt left in the supernatant. To prepare a catalyst that is close to having Pt:Ru ratio of 1:1, we tried to increase the Ru precursor in the preparation process. It was found that although there was an increase in the Ru content, the more Ru precursor used, the more Ru was lost in the solution. We eventually settled with the sample that would give a ratio of 1 to 1.2 but with an actual ratio of 1.13 to 1. This sample was designated as S1 (Table 1). No experimental effort in this work was made to match the 2:1 or 3:1 ratio. The catalysts for the expected 2:1 and 3:1 ratios were as synthesized and are designated S2 and S3, respectively. All samples were expected to have the same metal loading of 20 wt % although the actual metal loading is lower (Table 1) due to the loss of Ru. The loss of Ru was believed to be due to the hydrolysis of the salt precursor K2RuCl5.14 In a study by Liu et al.,6c incomplete reduction of Ru salts was also observed, although RuCl3 was used in that study. Figure 1 shows typical TEM images of the CNT supported Pt-Ru catalysts of S1-S3. It can be seen that for all samples the Pt-Ru nanoparticles were formed uniformly on the external walls of the CNTs. Well-dispersed Pt-Ru nanoparticles can be found in any microregions in the sample view on the TEM grid. The nanoparticles have spherical morphologies. Their sizes were obtained by measuring the nanoparticles on TEM images using imaging analysis software, Scion (NIST). More than 400 nanoparticles were measured to ensure statistically significant representation of the nanoparticle sizes. The mean nanoparticle

Pt-Ru Nanoparticles Supported on Carbon Nanotubes

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2805

Figure 1. TEM images of Pt-Ru nanoparticles deposited on the sonochemically functionalized CNTs of S1-S3, respectively, from left to right, and their nanoparticle size distributions. The scale bar is applicable to all images.

TABLE 2: Properties of the Pt-Ru Catalysts samples

nanoparticle size (nm)

active surf area (cm2/mg)

S1 S2 S3 E-TEK

2.28 ( 1.1 2.49 ( 1.3 2.65 ( 1.5

893.0 852.2 719.9 795.9

size (diameter) for each catalyst was calculated and is listed in Table 2. All three samples have a mean size of less than 3 nm. The size distributions of these Pt-Ru nanoparticles (Figure 1) are fairly consistent with lognormal distributions, typical for particles formed in colloids. The fits of the size distributions give geometric means of 2.28, 2.49, and 2.65 nm for the samples S1-S3, respectively, showing that the nanoparticles in the 3:1 catalyst have the largest particle size and broadest size distribution. Figure 2 shows typical XRD patterns of the three catalysts, from which the crystalline lattice fringes of the bimetallic alloy nanoparticles were confirmed. It has been shown in previous studies that Pt-Ru alloys would take the face-centered cubic (fcc) structure of Pt, if Ru content is below 60 at. %.15 The XRD patterns displayed the (111), (200), (220), and (311/222) reflections, confirming that the nanoparticles are consistent with fcc structures. The first peak near 2θ ∼ 26° in Figure 2 originates from the graphitic carbon of CNTs. Lattice parameter values, i.e., d-spacings, for Pt(111) were obtained by fitting the XRD spectra and are listed in Table 3. For each catalyst, three samples were prepared. When one obtains the d-spacings, a standard peak of the multiwalled CNTs16 at 2θ ) 25.94° was used for calibration. The data in Table 3 are the averages. The d-spacing of a commercial E-TEK catalyst of carbon blacksupported Pt-Ru with a 1:1 Pt to Ru ratio (Pt50Ru50) was also obtained and listed in Table 3. The shifts of peak positions relative to those of pure Pt are indicative of structure changes in the Pt-Ru catalysts. However, all d-spacings of the CNT supported catalysts are larger than that of the commercial catalyst, even for sample S1 which has the closest compositions to the commercial catalyst. This leads us to speculate that not all of the Ru formed alloy with Pt. Previous studies17 have shown the existence of unalloyed Ru, which can form hexagonal

Figure 2. Typical X-ray diffraction patterns of the PtRu/CNT catalysts prepared with different Pt to Ru atomic ratios.

TABLE 3: Lattice Parameter Values for Pt(111) sample

d-spacing (Å)

S1 S2 S3 E-TEK

2.2749 ( 0.0195 2.2865 ( 0.0301 2.2862 ( 0.0220 2.2435 ( 0.0186

closed packed (hcp) structure without alloying with Pt. Considering the fact that Ru salt cannot be completely reduced in the polyol process, it is quite possible that the reaction rates of the two salt precursors are different, which may cause elemental segregation in the nanoparticles.18 Although more quantitative studies of the Pt-Ru nanoparticles are needed to clarify their detailed crystalline structures, the d-spacings may qualitatively indicate that sample S2 with the largest d-spacing have the most unalloyed Ru. This sample will be shown later to have the poorest performance in methanol oxidation, corroborating with this conclusion. Electrochemical Performance. Cyclic voltammograms of the catalysts obtained in 1.0 M H2SO4 without methanol are shown in Figure 3. The CV scan rate was 20 mV/s, and the potential ranges were from 0 to 1.2 V. The CV curves look similar to those of pure Pt. However, the hydrogen adsorption/ desorption region is not well defined for bimetallic Pt-Ru catalysts. The hydrogen adsorption feature at ∼0.12 V, which

2806 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Li and Xing

Figure 3. Cyclic voltammograms of the PtRu/CNT catalysts in 1.0 M H2SO4 solution saturated by N2 with scan rate of 20 mV/s. Also shown is the CV for the commercial PtRu catalyst supported on carbon black obtained under the same experimental conditions.

is correlated with Pt(110) sites,19 can be still recognized, but the more positive hydrogen adsorption peak at ∼0.25 V correlated with Pt(100) sites is indistinguishable. As the Ru content was increased, the double layer charging regions became larger due to that Ru is more hydrophilic than Pt. It has been shown that at certain potentials (∼0.2 V) water discharging occurs on Ru sites to form Ru-OH groups on the catalyst surface.20 From Figure 3 it also can be seen that the potentials of oxide reduction shifts negatively with the increase of Ru content, and that was attributed to the slow kinetics of oxide reduction. These observations in the characteristic CV curves are consistent with those observed for the Pt-Ru alloy electrodes in previous studies.10,21 The electrochemically active surface areas of the Pt-Ru nanoparticles was obtained from the CV curves by integrating the areas under the curves in the hydrogen adsorption region (∼0.05-0.3 V). To obtain the integrated areas for the hydrogen adsorption peaks (i.e., the hatched area in Figure 3 for the S3 catalyst), a dashed vertical line was drawn to separate the molecular hydrogen region and a dashed horizontal line was drawn to correct the double-layer charging.18 Active surface areas for the catalysts obtained this way are given in Table 2. Although the trend of the active surface areas is consistent with the trend calculated from the nanoparticles sizes, i.e., smaller particles have larger specific surface areas, the nanoparticle sizes cannot be fully applicable for the Pt-Ru alloys since Ru is not supposed to participate in the hydrogen adsorption/desorption. However, the catalyst, Pt53Ru47, was observed to have the highest active surface area, despite that the catalyst has the least percentage of Pt. Reducing particle size to the nanoscale leads to the increase of catalytic sites/unit mass; however, it may be coupled with a structural change in the increase of the turnover number for certain structure-sensitive reactions.22 It was shown that adsorbed hydrogen exhibits strong dependence on factors such as particle size and surface crystallographic orientation.23 The underpotential deposition of atomic hydrogen, Hupd, is a structuresensitive catalytic process, characteristic of Pt for the Pt(100), Pt(110), and Pt(111) single-crystal planes.24 It was shown that much of the H responsible (peak at ∼0.25 V vs RHE) is related to H adsorption at defect and edge sites.25 It is possible that the different atomic ratios of Pt and Ru can result in different crystal structures seen from the lattice spacing changes in Table 3. The structures may play a more important role in the hydrogen adsorption and desorption than the compositions of the Pt-Ru nanoparticle catalysts.

Figure 4. Methanol oxidation plots of the PtRu/CNT catalysts and the PtRu/CB catalyst performed in 1.0 M H2SO4 solution containing 2.0 M methanol saturated by N2 at room temperature with a sweep rate of 20 mV/s: (a) current densities based on catalyst mass; (b) current densities based on active catalyst surface areas.

Methanol Oxidation. The activities of the PtRu/CNT catalysts in the electrooxidation of methanol were investigated in a half-cell reaction. For comparison, an E-TEK catalyst of carbon black-supported Pt-Ru with a 1:1 Pt to Ru ratio (Pt50Ru50) was also studied under the same experimental conditions. Typical CV curves of the catalysts are plotted in Figure 4. The maximum potential scanned was 0.759 V vs RHE to avoid Ru dissolution.26 It was found that the methanol oxidation currents on the three PtRu/CNT catalysts were much higher than that obtained on the PtRu/CB catalyst, with the catalyst of Pt to Ru ratio closest to 1:1, i.e., Pt53Ru47/CNT, having the best performance. However, S2 (Pt69Ru31/CNT) was outperformed by S3 (Pt77Ru23/CNT). This may be explained by that the amount of Ru alloying with Pt in the Pt-Ru nanoparticles of S2 was less than that in S3. According to the bifunctional theory,4b increasing the number of Pt-Ru pair sites leads to improvement of the catalytic activity. Therefore, 1:1 atomic ratio of Pt and Ru would produce the best results in the electrooxidation of methanol. It has been demonstrated in several prior studies that Ru produces the most pronounced effect on the methanol oxidation rate when it is alloyed with Pt in a 1:1 ratio.27 The Pt-Ru catalysts on CNTs in this study do not have exact 1:1 ratios, but rather the catalysts have a higher Pt content. However, all catalysts demonstrated better performance than the 1:1 commercial catalyst of Pt-Ru on carbon black. If we use the electrochemically active surface areas listed in Table 2 as the base, a new plot can be obtained as shown in Figure 4b. We can see that the PtRu/CNT catalysts still have better methanol oxidation performance, even though the active surface area of the commercial Pt50Ru50/CB catalyst is larger than that of Pt77Ru23/CNT catalyst. We believe that the CNTs support must have enhanced the catalytic activity of the Pt-Ru catalysts.

Pt-Ru Nanoparticles Supported on Carbon Nanotubes

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2807

Electrooxidation of methanol to form CO2 can be via dualpath mechanisms consisting of non-CO and adsorbed CO reactive intermediates:28

Pt(CH3OH)ads + H2O f CO2 + 6H+ + 6e Pt(CH3OH)ads f Pt(CO)ads + 4H+ + 4e The non-CO reaction pathway is preferred for methanol oxidation for which it does not involve CO, a poison for Pt. The adsorbed CO reaction pathway often presents, however, in which the intermediates via (CO)ads are mostly in the form of linearly bonded CO, i.e., PtdCdO.29 Accumulation of this complex on the catalyst surface leads to CO poisoning. When Ru presents in the bimetallic catalyst, it assists the oxidation of CO through chemisorbed -OH on the Ru sites:20

Ru + H2O f Ru-OH + H+ + e Ru-OH + Pt(CO)ads f Ru + Pt + CO2 + H+ + e In this way the poisoned Pt is regenerated and can again participate in the oxidation of methanol. Due to the monospecies of CO and OH on Pt and Ru, respectively, the best results can be obtained when the Pt to Ru atomic ratio is 1:1. For the CNTsupported Pt-Ru catalysts with higher atomic ratios, i.e., Pt69Ru31 and Pt77Ru23, the amount of Pt content is more than that of the Pt53Ru47 catalyst. The excessive amounts of platinum do not form the Pt-Ru atom-pair sites and can be poisoned by the intermediates (i.e., CO or other carbonaceous species) produced during the methanol oxidation. The Pt53Ru47 catalyst has the closest atomic ratio to the 1:1 and therefore showed the best catalytic activity. However, since all three catalysts have shown better performance than the commercial Pt50Ru50/CB catalyst despite their imbalanced Pt and Ru compositions, there must be other factors affecting the catalyst activity. It has been known that metalsupport interactions can significantly affect the activity of supported catalysts.30 CNTs have more organized graphitic structures than carbon black. They therefore would result in different interactions with the supported metal, which happen to be positive to the catalyst activity in the electrooxidation of methanol. Hills et al.8b reported a study on the carbon support effects on bimetallic Pt-Ru nanoparticles. They found that carbon black resulted in structural disorders in the alloy nanoparticles in which the nearest coordination numbers were smaller. In contrast, the nanoparticles deposited on fullerene soot have a more ordered crystalline structure. This structural difference would be responsible for the observed activity enhancement, since methanol electrooxidation has been known to be structure-sensitive. In a previous study by Chrzanowski and Wieckowski,31 it was shown that for Pt-Ru the catalytic activity is maximized with the presence of (111) crystallographic planes. From the XRD patterns shown in Figure 3, the Pt53Ru47/CNT catalyst (S1) shows the sharpest and most intense diffraction peak for (111) plane among the three CNT-supported Pt-Ru catalysts. This may be indicative of that the highest degree of crystallization of (111) has improved the Pt-Ru catalyst activity. The data reported in Liu et al. study6c allowed us to compare our results with theirs. This was done for the Pt53Ru47/CNT catalyst and their carbon-black-supported Pt52Ru48 catalyst. The two catalysts have almost the same compositions. To correct the scan rate effect, we have assumed that the oxidation current

Figure 5. Chronoamperometry collected for 1 h at 0.7 V vs RHE for the PtRu/CNT catalysts and the PtRu/CB catalyst in 1.0 M H2SO4 solution containing 2.0 M methanol saturated with N2 at room temperature.

is linearly proportional to the scan rate.32 The peak methanol oxidation current at 0.759 V (the maximum scanned potential in our work) for our catalyst was found to be 31% higher than that of theirs. This may further demonstrate the positive effect of the CNTs as a Pt-Ru catalyst support. Chronoamperometry data for the three PtRu/CNT samples and the commercial PtRu/CB sample were compared in Figure 5. Before the chronoamperometry was performed each time, the electrolyte (1 M H2SO4 + 2 M CH3OH) was deaerated with N2 for 30 min. The PtRu/CNT catalyst and PtRu/CB catalyst were biased at 0.7 V vs RHE, and the changes in the oxidation current density with time were recorded. As shown in Figure 5, all catalysts had a decay current density and reached a steady state within about 1000 s. However, the CNT-supported Pt53Ru47 catalyst shows the highest current density and therefore the best catalyst stability. Although Pt69Ru31/CNT and Pt77Ru23/ CNT showed much higher current than that of the commercial catalyst at the beginning, the stable currents of these two catalysts were found to be lower. This was probably due to the composition effects of unpaired Pt and Ru atoms. Liu and coworkers6c have also observed similar results in their studies. They compared the stability of a Pt52Ru48 catalyst on carbon black to other catalysts with higher Pt contents and found that the Pt52Ru48 catalyst was the most stable in the methanol oxidation reaction. Conclusions In this paper we have reported the preparation of Pt-Ru nanoparticles on carbon nanotubes. By reducing the Pt and Ru salt precursors in an ethylene glycol-water solution dispersed with sonochemically functionalized CNTs, we were able to prepare highly dispersed bimetallic Pt-Ru alloy nanoparticles at various compositions. Under our experimental conditions, the K2PtCl4 precursor was completely reduced, but K2RuCl5 was not. Consequently, there was a loss of Ru content in the synthesized bimetallic nanoparticles. To compensate for this loss, extra Ru precursors were used and a Pt53Ru47/CNT catalyst was obtained, having the closet ratio to 1:1. With this technique, highly dispersed Pt-Ru catalysts with no particle agglomeration can be prepared on CNTs with mean particle sizes of less than 3 nm and a narrow size distribution. Cyclic voltammetry for hydrogen adsorption-desorption showed that Pt53Ru47/CNT catalyst has the largest active surface. Study on the electrooxidation of methanol on the catalysts showed that they all have a much higher specific activity than that of the commercial PtRu catalyst supported on carbon black, even though the Pt and Ru atoms are paired in the commercial catalyst. The activity

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