Preparation and Physical and Electrochemical Properties of Carbon

J. Phys. Chem. C 2008, 112, 1479-1492

1479

Preparation and Physical and Electrochemical Properties of Carbon-Supported Pt-Ru (Pt-Ru/C) Samples Using the Polygonal Barrel-Sputtering Method Mitsuhiro Inoue,† Hiroshi Shingen,‡ Tomohito Kitami,‡ Satoshi Akamaru,† Akira Taguchi,† Yasuhisa Kawamoto,‡ Akio Tada,‡ Kazuhiko Ohtawa,‡ Kanji Ohba,‡ Masao Matsuyama,† Kuniaki Watanabe,† Iwao Tsubone,‡ and Takayuki Abe*,† Hydrogen Isotope Research Center, UniVersity of Toyama, Gofuku 3190, Toyama 930-8555, Japan, and Nippon Pillar Packing Corporation, 541-1, Uchiba, Shimouchigami, Sanda, Hyogo 669-1333, Japan ReceiVed: July 11, 2007; In Final Form: September 26, 2007

Carbon-supported Pt-Ru alloy (Pt-Ru/C) catalysts were prepared using the “polygonal barrel-sputtering method”. From the preparation of a Pt-Ru alloy with Pt/Ru ) ca. 50:50 atom % on a glass plate as support, the optimum sputtering conditions were an Ar gas pressure of 0.9-0.7 Pa and room temperature. The amount of the sputtered Pt-Ru alloy was controlled by changing the ac power and the sputtering time. Subsequently, the Pt-Ru/C samples were prepared under the given optimum conditions. The Pt-Ru alloy was dispersed extensively in the form of nanoparticles on a carbon support. For the ac power levels of 130, 100, and 50 W, the size distributions were narrower when the ac power was lowered. The respective average particle sizes were 4.1 nm (130 W), 3.3 nm (100 W), and 2.2 nm (50 W). In the case of 30 W, however, the size distribution and the average particle size were almost identical to those for 50 W. In addition, when the Pt-Ru/C samples were prepared by changing the sputtering time, only the dispersion density of the alloy nanoparticles increased in the Pt and the Ru deposited without changing the particle size. The atomic ratios of Pt and Ru in individual Pt-Ru alloy nanoparticles for the prepared samples were similar to the sputtering ratio and homogeneous compared with those for the commercially available samples. With regard to the electrochemical properties for the prepared samples, the hydrodynamic voltammograms for H2 oxidation were identical to that of the commercially available sample. However, for CO oxidation, the peak shapes and the peak potentials for the prepared samples were sharper and ca. 20 mV lower than those for the commercially available samples, due to the uniform Pt and Ru atomic ratios of the individual alloy particles for the prepared samples. The coulomb charges of the CO oxidation reaction per amount of Pt and Ru for the prepared samples increased linearly in the reversed average particle sizes, while on the other hand, the charges for the commercially available samples were not proportional to the reversed sizes. This shows that the Pt-Ru alloy for the prepared samples was more efficiently utilized for electrochemical reactions rather than were the commercial ones. In addition, the cell performances for the alloy loading of 0.08 or 0.02 mg/cm2 using the prepared Pt-Ru/C samples were similar to those for 0.50 mg/cm2 using the commercially available sample.

Introduction It is expected that polymer electrolyte fuel cells (PEFCs) will be employed in vehicles or stationary applications due to their high-energy efficiency and low operating temperature. For the practical use of PEFCs, re-formed hydrogen gas will be supplied as fuel to an anode. However, when Pt is used as the anode electrocatalyst, carbon monoxide (CO), a constituent of the reformed hydrogen gas present at a few ppm, covers a considerably portion of the Pt surface. Thus, the hydrogen oxidation activity of Pt decreases dramatically.1-4 For this reason, the anode electrocatalyst needs to have better tolerance toward CO. Various Pt-based alloys with a high tolerance toward CO have been investigated, including Pt-Ru,5-14 Pt-Mo,15-17 PtSn,18-22 Pt-Fe,23-25 and others.14,26,27 In particular, Pt-Ru alloy has not only a high tolerance toward CO but also excellent chemical stability. Some researchers have reported that the PtRu alloy with the atomic ratio of Pt and Ru (hereafter, denoted * Corresponding author. E-mail: [email protected]. Tel: +8176-445-6933. Fax: +81-76-445-6931. † University of Toyama. ‡ Nippon Pillar Packing Corporation.

as Pt/Ru ratio) of 50:50 atom % had the highest CO oxidation activity.5-7 Hence, carbon-supported Pt-Ru alloy (Pt-Ru/C) samples with Pt/Ru ) 50:50 atom % are candidates for the anode electrocatalyst for practical PEFCs. However, Pt and Ru are precious metals and expensive, resulting in the high cost of PEFCs. Therefore, it is important to reduce the Pt and the Ru loadings in order to use PEFCs as economically viable power sources. To reduce the precious metal loading for the practical PEFCs, the following factors would be meaningful. The deposition of Pt-Ru alloy particles with a small and uniform size enables enlargement of their surface area, leading to high utilization of the precious metals for electrochemical reactions.28 In addition, since the CO oxidation activities of Pt-rich and Rurich Pt-Ru alloys are lower than that of the alloy with Pt/Ru ) 50:50 atom %,5-7 the Pt/Ru ratios of the individual Pt-Ru alloy particles deposited on the carbon support should be uniform at ca. 50:50 atom % to keep the high CO tolerance. Pt-Ru/C samples are frequently prepared using wet processes including the impregnation method,28-34 the Bo¨nnemann method,35-39 the alcohol reduction method,40,41 and others.42,43 In these processes, the Pt-Ru alloy is prepared through the

10.1021/jp075400o CCC: $40.75 © 2008 American Chemical Society Published on Web 01/12/2008

1480 J. Phys. Chem. C, Vol. 112, No. 5, 2008 adsorption of Pt and Ru precursors onto the support, followed by heat treatment at 200-350 °C to decompose the precursors. However, in the case of wet processes, the following disadvantages for the given Pt-Ru/C samples exist. (i) Wet processes involve multiple steps such as adsorption of precursors, filtration, washing, drying, and subsequent heating. Thus, wet processes are complicated. (ii) The size of individual Pt-Ru alloy particles deposited on a carbon support is not uniform since the heat treatment causes the growth of the alloy particles33,34 and particle aggregation. (iii) The Pt/Ru ratios of the individual Pt-Ru alloy nanoparticles are not homogeneous44 because the decomposition temperatures of Pt and Ru precursors are different.33,34,45 Further, dry processes such as the chemical vapor deposition (CVD) method and the sputtering method are also employed to prepare the Pt-Ru alloy catalysts for PEFCs.46-50 Sivakumar et al. have presented a novel CVD method for preparing PtRu/C samples.46,47 They have noted that the deposited Pt-Ru alloy nanoparticles have uniform sizes, though it seemed that the Pt/Ru ratios of the individual particles were varied. However, as seen in Figure 1 (e) in their report,46 it appears that the particle sizes were still not uniform. These results could be due to heating and using precursors with low vapor pressure. Thus, the CVD method cannot resolve problems ii and iii, as mentioned above. On the other hand, several researchers have prepared Pt-Ru alloys on an electrode surface using conventional sputtering equipment,48-50 which is not suitable to prepare Pt-Ru/C samples in powder form. Recently, we have developed a new surface-modifying process termed the “polygonal barrel-sputtering method”.51-56 This method can uniformly modify the surface of any powdered support with a thin film of foreign materials such as metals51-53,56 or metal oxides.54,55 When this method is employed for the preparation of a Pt-Ru/C sample, the following advantages are expected: (a) The method can prepare powdered Pt-Ru/C samples in a single process without the adsorption of precursors, filtration, washing, drying, and other steps. (b) The sputtering method makes it possible to deposit the materials as particles with a size range of several nanometers on the support.57,58 In addition, since the heat treatment for preparing the alloy is not always essential for sputtering,59,60 it is feasible to deposit the alloy particles with a uniform size. (c) The Pt/Ru ratio of the deposited Pt-Ru alloy can be made uniform because no precursors are needed and the atomic ratio of the sputtered Pt and Ru is constant. Due to these factors, it is most likely that disadvantages i-iii above can be overcome by employing the polygonal barrelsputtering method. In the present study, the optimum sputtering conditions for the Pt-Ru alloy with Pt/Ru ) ca. 50:50 atom % were studied, and subsequently, Pt-Ru/C samples were prepared under the obtained optimum conditions using the polygonal barrelsputtering method. The physical and electrochemical properties and the cell performance of the prepared Pt-Ru/C samples were also investigated. Experimental Section Sample Preparation. Figure 1A shows a schematic representation of the polygonal barrel-sputtering system used in the present study. This system is equipped with a target holder (length, 158 mm; width, 105 mm; and height, 89 mm), pumps (a rotary pump; pumping speed, 340 L/min; E2M18, Edwards,

Inoue et al.

Figure 1. Schematic diagram of the polygonal barrel-sputtering system.

and a turbo molecular pump; pumping speed, 550 L/s; UTM500M, ULVAC), and a vacuum chamber (volume: 15 L) with a hexagonal barrel (diameter, 197 mm; and width, 100 mm). While performing sputtering, water was circulated in the target holder (flow rate: 0.65 L/min) to cool the target. ac power was supplied by a power generator (frequency, 13.56 MHz ( 5 kHz; RP-200C2, Pearl Kogyou). Metal plates of Pt and Ru (purity, 99.99%; length, 100 mm; and width, 25 mm) were simultaneously used, as shown in Figure 1B. In a separate experiment to determine the deposition rates of Pt and Ru, the atomic ratio of the sputtered Pt and Ru was estimated to be 62:38 atom %. On this basis, the surface area ratio of the Pt and the Ru targets was adjusted to 6:10 by using a stainless-steel target cover to obtain a sputtering ratio at ca. 50:50 atom % (see Figure 1B). When Pt and Ru were sputtered simultaneously on this modified target, the actual sputtering ratio was Pt/Ru ) 52:48 atom %. When a support was heated, a ceramic heater attached to the front of the target holder (Figure 1B) was used along with a heater equipped in the vacuum chamber (Figure 1A). The temperature was controlled with a thermocouple set on the support. The experimental procedure for preparing samples with the polygonal barrel-sputtering method can be described as follows: The hexagonal barrel containing a support was placed in the vacuum chamber, as shown in Figure 1A. The air in the chamber was carefully evacuated using the rotary and the turbomolecular pumps to a pressure of 8 × 10-4 Pa or less. Subsequently, Ar gas (purity: 99.9999%) was slowly introduced into the chamber to an appropriate pressure and then Pt and Ru were sputtered simultaneously. After sputtering, N2 gas (purity: 99.9998%) was gradually introduced to the atmospheric pressure wherein the prepared samples could be extracted. A SiO2 glass plate (22 mm × 22 mm, Matsunami Glass) was used as a support for studying the sputtering conditions such as Ar gas pressure (0.5-3 Pa), temperature (room temperature-350 °C), sputtering time (20-60 min), and ac power (30-130 W). In the case where the glass plate was used, the barrel was fixed

Pt-Ru/C Prepared by Barrel-Sputtering Method during sputtering. Hereafter, the sample prepared on the glass plate is referred to as the Pt-Ru/plate. Vulcan XC72R (specific surface area, 254 m2/g; and average size of the primary particle, 30 nm, Cabot) was used as a carbon support for preparing the Pt-Ru/C samples. Before using, as-received Vulcan XC72R was heated at 180 °C to prevent the aggregation of the carbon support by water. On sputtering, in order to fragment the secondary particles of Vulcan XC72R into primary carbon particles and stir them, the hexagonal barrel was oscillated (intervals of 14 s and amplitude of ( 75°) and mechanically vibrated, as shown in Figure 1A. To investigate the optimum preparation conditions for the Pt-Ru/C samples, glass-plate-sputtered Pt or Ru alone (denoted as Pt/plate and Ru/plate) and the bulk Pt-Ru alloy were used as reference samples. The Pt/plate and the Ru/plate samples were prepared in-house by sputtering. The bulk Pt-Ru alloy was prepared with the arc melting method12,61 using Pt (purity: 99.99%) and Ru (purity: 99.98%) metal powders purchased from Tanaka Kikinzoku Kogyo. On the other hand, physical properties such as the size and Pt/Ru ratio of the Pt-Ru alloy for the prepared Pt-Ru/C sample were also compared with commercially available Pt-Ru/C samples purchased from Tanaka Kikinzoku Kogyo (amount of precious metals: 49.4 wt % (Pt, 32.5 wt %; and Ru, 16.9 wt %), denoted as TK) and Johnson Matthey (43.9 wt % (Pt, 29.1 wt %; and Ru, 14.8 wt %), denoted as JM). Characterization of Samples. X-ray Diffraction Analysis (XRD: PW1825/00, Philips). For the XRD measurement, the 2θ value and the shape of the diffraction peak corresponding to the Pt-Ru alloy were evaluated. The XRD pattern was collected with a step size of 0.01° and a counting time of 0.5 s per step using Cu KR radiation operating at 30 mA and 40 kV. Si powder (purity: 99.999%, 200 mesh, Furuuchi Chemical) was used as an internal reference. The exact 2θ value of the alloy peak was determined by using the peak of Si (220) (2θ ) 47.30°) as a standard. It should be noted that several XRD measurements were performed using the prepared sample and the experimental errors in measuring the 2θ value and the full width at half-maximum (fwhm) value were estimated to be (0.05°. X-ray Fluorescence Analysis (XRF: PW2300, Philips). The deposited amounts of Pt and Ru for the prepared sample were determined from the XRF measurement using a Rh X-ray tube at 50 mA and 60 kV. A calibration curve was made by using the Pt/plate and the Ru/plate samples. Mixtures of TK and Vulcan XC72R with various weight ratios were also used as standard samples for calibration. The atomic ratio of the Pt and the Ru included in the prepared sample (denoted as overall Pt/ Ru) was estimated from the deposited amounts of the Pt and Ru measured by the XRF measurement. InductiVely Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES: Optima 3300XL, Perkin-Elmer). To estimate the Pt deposited amount of the prepared Pt-Ru/C sample, the sample (100 mg) was heated in aqua regia for 1 h, and the Pt content in the given solution diluted with distilled water was measured by the ICP-AES analysis. It should be noted that, since Ru did not dissolve in aqua regia during the above procedure, it was impossible to determine the Ru deposited amount by ICP. X-ray Photoelectron Spectroscopy (XPS). The chemical state of Pt and Ru for Pt-Ru/C was determined by XPS at room temperature. Non-monochromatized Mg KR radiation (1253.6 eV, PHI 04-151, Perkin-Elmer) was used for photoelectron excitation. The energy spectrum of photoelectrons was measured with a concentric hemispherical analyzer (PHOIBOS 100, SPECS).

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1481 Transmission Electron Microscopy (TEM: EM-002B, TOPCON) and Energy-DispersiVe X-ray Analysis (EDX: EDAX, TOPCON). The size of a Pt-Ru alloy particle for the Pt-Ru/C sample was estimated by TEM. The accelerating voltage and the magnification factor were 120 kV and 410 000, respectively. The Pt/Ru ratio of the individual Pt-Ru alloy particles on the carbon support was evaluated through EDX. Electrochemical Properties of the Pt-Ru/C Sample. The electrochemical properties associated with the H2 and CO oxidation of a Pt-Ru/C sample were evaluated by using a threecompartment electrochemical cell. A Pt wire (diameter 1 mm φ × length 20 cm) and a reversible hydrogen electrode (RHE) were used as a counter electrode and a reference electrode, respectively. The working electrode was prepared with the thinfilm electrode method.62,63 Specifically, a Pt-Ru/C sample was dispersed uniformly in a mixed solution of 2-propanol and Millipore water under ultrasonic agitation. A 20 µL aliquot of the dispersed solution of 0.2 mg/mL was loaded on a mirrorfinished glassy-carbon disk electrode (diameter, 5 mm; geometrical surface area, 0.196 cm2, HR2-D1-GC5, Hokuto Denko). After drying under flowing N2 gas, the deposited catalyst layer was covered with 0.1% Nafion solution diluted with ethanol. The supporting electrolyte was 1 N H2SO4, ultrapure concentrated H2SO4 (purity, 96%, Kanto Chemical) diluted with Millipore water. The potential of the working electrode was controlled with a potentiostat (HR101B, Hokuto Denko). Hydrodynamic voltammograms for H2 oxidation of the PtRu/C samples were recorded with a rotating disk electrode (RDE) method at room temperature in the electrolyte solution saturated with pure H2 gas (purity: 99.9999%). The potential of the working electrode was swept at 10 mV/s from 0 to 300 mV (vs RHE). The rotation rate of the electrode was modulated between 100 and 3600 rpm with a motor speed controller (HR202, Hokuto Denko). CO stripping voltammetry for the Pt-Ru/C samples was performed at 40 °C stagnant. For the measurements, the amount of sample loaded on the glassy-carbon disk electrode was 10fold larger than that used for recording the hydrodynamic voltammogram for H2 oxidation to obtain a high current without significant interference from traces of oxygen in the electrolyte.63 CO was adsorbed on the surface of the Pt-Ru alloy by immersing the working electrode for 30 min in the electrolyte solution saturated with pure CO gas (purity: 99.95%) at 40 °C, while the electrode potential was maintained at 70 mV (vs RHE). CO dissolved in the electrolyte solution was subsequently removed by bubbling N2 gas for 30 min. Following the sweep from 70 to 30 mV, the potential of the electrode was swept at 10 mV/s between 30 and 800 mV to prevent Ru dissolution.7,62 In the present study, a potential at which the CO oxidation current and the background current intersected was defined as an onset potential of the CO oxidation reaction.5 Cell performance of the Pt-Ru/C sample was evaluated by using a housing of the unit cell purchased from the Eiwa Corporation (Ex-1). The membrane electrode assembly (MEA, active area: 25 cm2) was prepared as follows.64-67 Nafion solution of 5% was added to the dispersed solution of the PtRu/C sample prepared through a method similar to the one mentioned earlier. A weight ratio of ionomer and carbon support was maintained at ca. 0.7:1 in the electrodes of all samples. The prepared solution was mechanically stirred for 30 min and then sprayed on one side of a Nafion 112 membrane (Dupont) used as a polymer electrolyte. The Pt/C purchased from Tanaka Kikinzoku Kogyo (amount of the Pt deposited: 46.6 wt %) as a cathode electrocatalyst was sprayed on the other side of the

1482 J. Phys. Chem. C, Vol. 112, No. 5, 2008

Inoue et al. TABLE 1: Summary of the XRD Measurement of the Pt-Ru/plate Samples under Ar Gas Pressure in the Range of 0.5-3 Paa for the Pt/plate, the Ru/plate, and the Bulk Pt-Ru Alloy (Pt/Ru ) 50:50 atom %) entry sample A B C D E F G H I J

P3 P2 P1 P0.9 P0.8 P0.7 P0.5 Pt/plate Ru/plate bulk Pt-Ru alloy (Pt/Ru ) 50:50 atom %)

Ar gas pressure/ Pa 3 2 1 0.9 0.8 0.7 0.5

2θ/deg 38.28 41.08 43.51 38.30 41.05 43.66 40.64 40.40 40.43 40.42 40.21 39.77 38.41 44.06 40.51

overall Pt/Ru ratio/atom % 36.7:63.3 ((0.1) 39.8:60.2 ((0.4) 45.4:54.6 ((2.4) 50.2:49.8 50.6:49.4 ((1.6) 50.1:49.9 ((0.8) 50.6:49.4

a The ac power, the sputtering time, and the temperature were fixed at 50 W, 20 min, and room temperature, respectively.

Figure 2. XRD patterns of the Pt-Ru/plate samples prepared at (A) 3 Pa (P3), (B) 2 Pa (P2), (C) 1 Pa (P1), (D) 0.9 Pa (P0.9), (E) 0.8 Pa (P0.8), (F) 0.7 Pa (P0.7), (G) 0.5 Pa (P0.5) (fixed sputtering conditions; ac power, 50 W; sputtering time, 20 min; temperature, room temperature), (H) the Pt/plate, (I) the Ru/plate, and (J) the Pt-Ru bulk alloy (Pt/Ru ) 50:50 atom %)) ((-‚-‚) Ru peak position, (‚‚‚‚‚) Pt peak position, (2θ ) 47.30°) Si (220) peak).

polymer electrolyte (Pt loading: 0.5 mg/cm2). The MEA with each catalyst sprayed on both sides was sandwiched between two Teflonized carbon papers (TGP-H-060; thickness, 190 µm; Toray) as a gas diffusion layer, and the sandwiched MEA was hot-pressed at 165 °C and 4.6 MPa for 10 min. H2 gas, including 100 ppm CO (denoted as CO/H2 gas), was supplied to the anode, while pure O2 gas was provided to the cathode. The gas utilization was set at 50%. Each gas was humidified by flowing through hot water at 80 °C. The cell polarization curve of the Pt-Ru/C sample was measured from 0 to 1000 mA/cm2 at 80 °C. The cell voltage as a function of the operation time was measured at a constant current mode of 400 mA/cm2. Data of the cell polarization curve and the cell potential against the operation time at the constant current mode was acquired with a fuel cell test system (890B, Toyo Corporation). Results and Discussion Evaluation of the Sputtering Conditions (Pt-Ru/plate Samples). Effect of Ar Gas Pressure. Figure 2 shows the XRD patterns of the Pt-Ru/plate samples prepared at (A) 3 Pa, (B) 2 Pa, (C) 1Pa, (D) 0.9 Pa, (E) 0.8 Pa, (F) 0.7 Pa, and (G) 0.5 Pa pressure of Ar gas, denoted as P3, P2, P1, P0.9, P0.8, P0.7, and P0.5, respectively. The ac power, the sputtering time, and the temperature were fixed at 50 W, 20 min, and room temperature, respectively. The XRD patterns of the Pt/plate, the Ru/plate, and the bulk Pt-Ru alloy (Pt/Ru ) 50:50 atom %) are also shown in Figure 2H-J. Table 1 summarizes the 2θ values of the obtained peak for each XRD pattern. The overall Pt/Ru ratios estimated by the XRF measurement are also listed in Table 1 (the values in parentheses represent the error of the

overall Pt/Ru ratios of the samples prepared at the same Ar gas pressure as that in the XRF measurements). The XRD patterns of P3 and P2 exhibited three broad peaks at 2θ ) 38.28°, 41.08°, and 43.51° and 38.30°, 41.05°, and 43.66°, respectively. The overall Pt/Ru ratios obtained for P3 and P2 were 36.7:63.3 ((0.1) and 39.8:60.2 ((0.4) atom %. It has been reported that for an overall Pt/Ru ratio of 20:80-40:60 atom %, the XRD pattern showed the mixed reflections of the Pt face-centered cubic (fcc) structure and the Ru hexagonal closed-packed (hcp) structure.61,68 The patterns for P3 and P2 presented similar reflections, as compared to the combined pattern between the Pt/plate (Figure 2H) and the Ru/plate (Figure 2I). Thus, the obtained peaks could be assigned to one peak of Pt(111) with some shift to a higher angle and the other peaks of the Ru(100) and the Ru(101) with some shift to a lower angle. On the other hand, the XRD patterns of P1, P0.9, P0.8, and P0.7 exhibited only one peak, as shown in Figure 2C-F. It is noteworthy that the peaks of Pt or Ru metal alone were not observed to the level of detection. For P1, the 2θ value of the obtained peak was 40.64°. In the case of P0.9, P0.8, and P0.7, the respective 2θ values were 40.40°, 40.43°, and 40.42°, which were lower than that of P1 and almost the same as that of the bulk Pt-Ru alloy observed at 40.51° (Figure 2J). Several researchers61,68-70 have described that, for the alloy with 40:60-100:0 atom %, the reflections characteristic of only the Pt fcc structure increased in the 2θ values of the peaks proportionally to the atomic ratio of Ru. Indeed, the overall Pt/Ru ratio of P1 was measured at 45.4:54.6 ((2.4) atom % and those of P0.9, P0.8, and P0.7 were obtained at 50.2:49.8, 50.6:49.4 ((1.6), and 50.1:49.9 ((0.8) atom %, respectively. In a separate experiment, it was noticed that at pressures exceeding 1 Pa, the deposition rate of the Pt reached a limit despite increasing the Ar gas pressure, but, on the other hand, the deposition rate of the Ru gradually increased with pressure. This is the reason why the overall Pt/Ru ratios for P3, P2, and P1 deviated from 50:50 atom %. In contrast, at pressures up to 0.9 Pa, the deposition rates of Pt and the Ru exhibited a linear relationship against the pressure, resulting in constant overall Pt/Ru ratios of ca. 50:50 atom % for P0.9, P0.8, and P0.7. As for P0.5, the overall Pt/Ru ratio was also estimated to be 50.6:49.4 atom % with the XRF measurement. As shown in Figure 2G, however, the XRD pattern of P0.5 displayed one peak at 40.21°, showing the formation of a Pt-rich alloy with an overall Pt/Ru ratio of 69.1:30.9 atom %. It is expected from this observation that a Ru-rich alloy was formed.71 For the pattern of P0.5, the tail of the peak was observed between 40.50° and 43.00°, probably showing the formation of the Ru-rich alloy.

Pt-Ru/C Prepared by Barrel-Sputtering Method

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1483

Figure 3. XRD patterns of the Pt-Ru/plate samples prepared at room temperature (DR), 100 °C (D100), 180 °C (D180), and 350 °C (D350) (fixed sputtering conditions; Ar gas pressure, 0.8 Pa; ac power, 50 W; sputtering time, 20 min).

However, the reason behind the formation of the Ru-rich alloy for P0.5 is presently unknown. The above results show that, when the Ar gas pressure is between 0.9 and 0.7 Pa, the Pt-Ru alloy with Pt/Ru ) ca. 50: 50 atom % can be prepared. Hereafter, the Ar gas pressure was fixed at 0.8 Pa during sputtering. Effect of Temperature. Figure 3 shows the XRD patterns of the Pt-Ru/plate samples prepared at room temperature (denoted as DR), 100 °C (denoted as D100), 180 °C (D180), and 350 °C (D350). The ac power and the sputtering time were fixed at 50 W and 20 min, respectively. Although the XRD measurements for each sample were conducted from 2θ ) 35° to 50°, Figure 3 shows the pattern obtained between 36° and 45°. As for DR, the obtained pattern was the same as that for P0.8 shown in Figure 2E. For D100, one peak was observed at 2θ ) 40.43°, which was identical to that of DR, indicating that the Pt-Ru alloy with Pt/Ru ) ca. 50:50 atom % was prepared up to 100 °C. On the other hand, in the case of D180, the peak was observed at 40.11°, which shifted at a lower angle than those of DR and D100. The tail of the peak was also observed from 40.45° to 42.45° in the pattern. The pattern of D350 indicated that new broad peaks appeared at 37.92° and 43.14° with the main peak at 40.22°. These results suggest that the Pt/Ru ratio in a part of the prepared alloy deviated from 50:50 atom %. Gasteiger et al. have reported that when the Pt-Ru alloy was heated at 800 °C after the preparation, Pt segregation was observed.61 Therefore, the deviation of the Pt/Ru ratios for D180 and D350 would also be due to Pt segregation. It should be noted that since heating was performed during sputtering in the present study, Pt segregation probably occurred even at 180 °C. These results indicate that the polygonal barrel-sputtering method can fabricate Pt-Ru alloy with ca. 50:50 atom % even at room temperature. In the following experiments, all samples were prepared at room temperature. Effect of Sputtering Time. Figure 4,I presents the XRD patterns of the Pt-Ru/plate samples at the sputtering time of 60 min (denoted as M60), 40 min (M40), and 20 min (M20). The ac power was fixed at 50 W. Each XRD pattern displayed one peak at 2θ ) 40.40° (M60), 40.44° (M40), and 40.43° (M20) without a noticeable tail. These results indicate that the Pt/Ru ratio of the prepared alloys was ca. 50:50 atom %. In addition, as shown in the inset of Figure 4,I, the peak intensities of Pt LR and Ru KR obtained from the XRF measurement were

Figure 4. (I) XRD patterns of the Pt-Ru/plate samples prepared at 60 min (M60), 40 min (M40), and 20 min (M20) (inset; the intensities of the Pt LR and the Ru KR peaks for the prepared Pt-Ru/plate samples obtained from the XRF measurements as a function of the sputtering time) and (II) the peak areas of the Pt-Ru alloy obtained from the XRD measurements for the prepared Pt-Ru/plate samples as a function of the sputtering time (fixed sputtering conditions; Ar gas pressure, 0.8 Pa; ac power, 50 W; temperature, room temperature).

proportional to the sputtering time, and the Pt/Ru ratio estimated from each slope was also ca. 50:50 atom %. Figure 4,II shows the relationship between the sputtering time and the integrated areas of the XRD peak. The peak area increased linearly with the sputtering time, implying that the amount of the sputtered alloy was controlled by the sputtering time. Effect of ac Power. The effect of the ac power was evaluated from the XRD measurements of the Pt-Ru/plate samples prepared at 130 W (denoted as W130), 100 W (W100), 50 W (W50), and 30 W (W30). The sputtering time was fixed at 20 min. As shown in Figure 5, one symmetrical peak appeared and the respective 2θ values were 40.45° (W130), 40.40° (W100), 40.43° (W50), and 40.45° (W30). The peak areas obtained for each sample were 2.95 × 105 counts (W130), 2.05 × 105 counts (W100), 1.14 × 105 counts (W50), and 6.44 × 104 counts (W30), increasing linearly with the ac power. These results show that the ac power also related to the amount of the alloy sputtered. On the basis of the study of the sputtering conditions, PtRu alloy with a Pt/Ru ratio of ca. 50:50 atom % can be prepared under an Ar gas pressure of 0.9-0.7 Pa at room temperature. The amount of the alloy sputtered on the support is controlled by changing the sputtering time and the ac power. In the case of preparing the Pt-Ru/C samples, the Ar gas pressure and the temperature were fixed at 0.8 Pa and room temperature. The Pt


Inoue et al. TABLE 2: Summary of the Preparing Conditions (ac Power and Sputtering Time), Yield, and Amounts of Pt and Ru Deposited for the Prepared Pt-Ru/C Samplesa deposited amounts/wt % sample SW130 SW100 SW50h8 SW30 SW50h4 SW50h2

ac sputtering power/W time/h yield/% 130 100 50 30 50 50

3.5 4 8 13.5 4 2

97 98 96 94 92 95

Pt

Ru

(total)

9.6 10.0 9.4 8.0 4.6 2.5

4.7 5.1 4.6 4.1 2.4 1.2

(14.3) (15.1) (14.0) (12.1) (7.0) (3.7)

a The Ar gas pressure and the temperature were fixed at 0.8 Pa and room temperature, respectively.

Figure 5. XRD patterns of the Pt-Ru/plate samples prepared at 130 W (W130), 100 W (W100), 50 W (W50), and 30 W (W30) (fixed sputtering conditions; Ar gas pressure, 0.8 Pa; sputtering time, 20 min; temperature, room temperature).

Figure 6. (I) Photograph of the prepared Pt-Ru/C sample immediately after the chamber was opened and (II) XRD patterns of the prepared Pt-Ru/C sample (A, after burning (Pt, 18 wt %; Ru, 9 wt %); B, nonburning (Pt, 14.7 wt %; Ru, 8.0 wt %); inset, magnified pattern at 2θ ) 33-38°) ((-‚-‚) Ru peak position, (‚‚‚‚‚) Pt peak position, (---) RuO2 peak position).

and the Ru deposited amounts were adjusted by the ac power and the sputtering time. Preparation of the Pt-Ru/C Sample. On the commercially available samples, the amounts of Pt and Ru deposited were ca. 30 and 15 wt %. Therefore, the preparation of the Pt-Ru/C sample with Pt and Ru deposited amounts of ca. 30 and 15 wt % was attempted using the polygonal barrel-sputtering method. However, when such a sample was prepared at 100 W and 12 h, fire and smoke were observed for 3-5 min as soon as the chamber was opened after the preparation, as shown in Figure 6,I. The same phenomenon was also observed in preparing the sample with Pt and Ru deposited amounts of 18

and 9 wt %. On the other hand, in preparing the sample with 14.7 and 8.0 wt % (ac power, 100 W; sputtering time, 6 h), no burning of the sample occurred. Figure 6,II shows the XRD patterns of (A) the burned sample (Pt and Ru deposited amounts: 18 and 9 wt %) and (B) the unburned sample (14.7 and 8.0 wt %). For the burned sample, two dominant peaks were observed at 2θ ) 35.13° and 39.90°; for the unburned sample, one broad peak was observed at 40.45°. For these patterns, the peaks of 39.90° and 40.45° correspond to the PtRu alloy. The 2θ value of the former was lower than that of the bulk Pt-Ru alloy (40.51°) in Figure 2J, showing the formation of the Pt-rich alloy. In contrast, the 2θ value of the latter was almost the same as that of the bulk alloy, indicating that the Pt-Ru alloy with Pt/Ru ) ca. 50:50 atom % was prepared. It should be emphasized that the peak broadness for the unburned sample was not due to the deviation of the Pt/Ru ratio, as described later. On the other hand, the peak of 35.13° was assigned to RuO2 with a tetragonal structure. For the burned sample, the XPS measurement exhibited a peak of RuO2 at 463.1 eV.37 It can be explained from these results that a part of Ru in the alloy was oxidized, resulting in the formation of RuO2 and Pt-rich alloy in the burned sample. In the case of the unburned sample, however, the peak of RuO2 was not detected in the XRD (inset) and XPS measurements. The Pt-Ru/C samples with various deposited amounts were also prepared at the constant ac power of 130 or 50 W by changing the sputtering time. For 130 W, burning was observed for the sample with Pt and Ru deposited amounts of 20 and 10 wt % and not for the sample with 16.0 and 8.5 wt %. At 50 W, the sample with 13 and 6.5 wt % burned and the sample with 9.4 and 4.6 wt % did not burn. The detailed mechanism of the burning of the prepared samples will be discussed later. From the above results, it appears that the physical properties of the prepared Pt-Ru/C samples are influenced by the ac power and the sputtering time. Therefore, the Pt-Ru/C samples were prepared at various ac power levels and sputtering times and the given Pt-Ru/C samples were characterized by comparison with the physical properties of the commercially available samples. The preparation conditions such as the ac power and the sputtering time, yields, and the exact amounts of Pt and Ru deposited for the prepared Pt-Ru/C samples are summarized in Table 2. As shown in Table 2, the yields for each prepared sample were more than 90%, independent of the difference in the preparation conditions. Characterization of the Pt-Ru/C Sample. XRD Measurement. Figure 7 shows the XRD patterns of the commercially available samples (A, TK; B, JM) and the prepared samples by changing the ac power (C, SW130; D, SW100; E, SW50h8; F, SW30) and the sputtering time (G, SW50h4; H, SW50h2). In the patterns of TK and JM, single broad peaks appeared at 2θ


Figure 7. XRD patterns of the commercially available samples (A, TK; B, JM), the prepared Pt-Ru/C samples (C, SW130; D, SW100; E, SW50h8; F, SW30; G, SW50h4; H, SW50h2), (I) the heated SW50h8 sample (300 °C for 2 h under an Ar gas flow), and (J) the physically mixed TK sample with Vulcan XC72R (weight ratio 1:2 (Pt, 10.8 wt %; Ru, 5.6 wt %)) ((-‚-‚) Ru peak position, (‚‚‚‚‚) Pt peak position).

) 40.36° and 40.54°, respectively. While the 2θ value for JM was similar to that of the bulk Pt-Ru alloy, the one for TK was different from that of the bulk alloy, suggesting that the Pt/Ru ratio of the alloy for TK was different from 50:50 atom %. The XRD patterns of SW130 (Figure 7C) and SW100 (Figure 7D) exhibited single broad peaks at 40.46° and 40.45°, respectively. These 2θ values were almost identical to that of the bulk alloy, implying that the Pt/Ru ratio of the deposited alloys was ca. 50:50 atom %. The broad peak areas were obtained at 1.32 × 104 and 1.13 × 104 counts, making them about 3 times smaller than those of TK (4.27 × 104 counts) and JM (4.29 × 104 counts). To clarify the difference in the areas, TK was physically mixed with Vulcan XC72R in the weight ratio of 1:2 and the XRD pattern of the mixed sample was measured (Figure 7J). The peak area was estimated to be 1.58 × 104 counts, which was almost the same as those of SW130 and SW100, revealing that the difference of the area as mentioned above was attributed only to the amount of deposited alloy. In contrast, in the case of SW50h8 (Figure 7E) and SW30 (Figure 7F), the peak of the alloy was not remarkably observed in the patterns, though the amounts of the Pt and the Ru deposited for SW50h8 and SW30 were almost the same as those for SW130 and SW100. The XRD patterns of SW50h4 and SW50h2 also showed no remarkable peak of the alloy, as shown in parts G and H of Figure 7. When SW50h8 was heated at 300 °C for 2 h under an Ar gas flow, however, the XRD pattern indicated one broad peak at 40.44° (Figure 7I). The peak area (2.14 × 104 counts) was similar to those of SW130 and SW100. Thus, it is expected that no observation of the alloy peak shown

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1485 in Figure 7E-H related to the microscopic properties of the prepared alloy. Subsequently, the TEM measurement of each sample was performed. TEM Measurement. Parts A and B of Figure 8 show the typical TEM images for TK and JM. In the images, the black dots indicate the Pt-Ru alloy and the gray portions represent the carbon support. It is evident that the nanosized alloy particles were deposited on the carbon support. However, aggregations of the alloy nanoparticles were frequently observed. The size distributions of the alloy particles are also shown in Figure 8A,B. For TK, the alloy nanoparticles with sizes between 1.6 and 13.6 nm (number of particle counts n ) 157) were deposited on the carbon support. As for JM, the deposited alloy particles had sizes between 0.8 and 12.8 nm (n ) 170). The average sizes of the nanoparticles for TK and JM were estimated to be 4.5 and 4.0 nm, respectively, as listed in Table 3. On the other hand, the TEM images of SW130 and SW100 shown in parts C and D of Figure 8 demonstrate that the alloy nanoparticles are highly dispersed on the carbon support and the aggregation of the nanoparticles is not seen at all. The size of the alloy particles for SW130 ranged from 1.6 to 7.2 nm (n ) 144), and the average size of the alloy particles was 4.1 nm. In comparison with the observations of TK and JM, the size distribution of the alloy particles for SW130 narrowed slightly, though the average particle size was similar value. In the case of SW100, the particle sizes ranged from 1.6 to 6.4 nm (n ) 146) and the obtained size distribution was narrower than that of SW130. The average size of SW100 was 3.3 nm, which was 20% smaller than that of SW130. For the TEM images of SW50h8 and SW30 (parts E and F of Figure 8), it appears that smaller-sized nanoparticles were highly dispersed on the carbon support. The particle sizes on SW50h8 and SW30 ranged from 0.8 to 3.6 nm (n ) 176) and 0.8 to 4.0 nm (n ) 153); evidently, the distributions were drastically narrower than that of SW100. Moreover, the average sizes of SW50h8 and SW30 were 2.2 nm, which was ca. 30% smaller than that of SW100. It can be stated from the TEM observations for SW130, SW100, and SW50h8 that the size distribution and the average size of the alloy particles depend on the ac power. However, at 50 and 30 W, no dependence of the particle size on the ac power was observed. Parts G and H of Figure 8 give the typical TEM images for SW50h4 and SW50h2. In these images, the alloy nanoparticles were also highly dispersed on the carbon support. The alloy particle sizes ranged from 0.8 to 3.6 nm (average size of 2.2 nm, n ) 162) and 0.8 to 4.0 nm (2.1 nm, n ) 195), ranges which almost corresponded to that for SW50h8. These imply that the sizes of the alloy particles are independent of the sputtering time. However, in the magnified TEM images (30 nm × 30 nm) for SW50h8, SW50h4, and SW50h2, the number of alloy nanoparticles per unit area was estimated to be 0.10, 0.05, and 0.03 particles/nm2, revealing that only the dispersion density of nanoparticles of the same size is affected by the sputtering time. It should be noted that in the case of the samples prepared at 50 and 30 W, the alloy nanoparticles with a size greater than 4 nm were completely absent. Zhang et al. have claimed that for alloy particles with a size of less than 4 nm, the peak was not detected in the XRD pattern because of detection limitations.31 Thus, the reason for the lack of a significant alloy peak was the absence of nanoparticles with sizes greater than 4 nm. For the heated SW50h8 sample, as shown in Figure 8I, the aggregations of the alloy particles were observed. The particle sizes were dispersed between 1.6 and 8.8 nm (average size of 4.3 nm, n ) 122), and the distribution was wider than that of SW50h8 and similar to those of TK and


Inoue et al.

Figure 8. Typical TEM images and size distributions of the Pt-Ru alloy nanoparticles of the commercially available samples (A, TK; B, JM), the prepared Pt-Ru/C samples (C, SW130; D, SW100; E, SW50h8; F, SW30; G, SW50h4; H, SW50h2), (I) the heat-treated SW50h8 sample (300 °C for 2 h under an Ar gas flow), and (J) the burned sample (Pt, 18 wt %; Ru, 9 wt %) (accelerating voltage, 120 kV; magnification factor, 410 000; n, number of particle counts).

Pt-Ru/C Prepared by Barrel-Sputtering Method TABLE 3: Summary of the Average Particle Sizes and Pt/Ru Ratios of Individual Pt-Ru Alloy Particles from the TEM and the EDX Measurements sample

average particle size (range)a/nm

atomic Pt/Ru ratioa/atom %

TK JM

commercially available samples 4.5 (1.6-13.6, n ) 157) 51.0:49.0-89.4:10.6 (n ) 22) 4.0 (0.8-12.8, n ) 170) 46.6:53.4-100:0 (n ) 25)

SW130 SW100 SW50h8 SW30 SW50h4 SW50h2

prepared Pt-Ru/C samples 4.1 (1.6-7.2, n ) 144) 54.3:45.7 ((5.6) (n ) 29) 3.3 (1.6-6.4, n ) 146) 53.9:46.1 ((6.1) (n ) 33) 2.2 (0.8-3.6, n ) 176) 52.9:47.1 ((5.3) (n ) 128) 2.2 (0.8-4.0, n ) 153) 50.5:49.5 ((7.0) (n ) 32) 2.2 (0.8-3.6, n ) 162) 52.8:47.2 ((6.1) (n ) 18) 2.1 (0.8-4.0, n ) 195) 54.7:45.3 ((7.2) (n ) 24)

a

n ) number of particle counts.

JM. This result implies that the alloy nanoparticles were aggregated and increased in size by heating. It is noteworthy that the formation of the particles of more than 4 nm size by heating induced the appearance of the XRD peak. Thus, the growth and aggregation of the alloy nanoparticles occur during heating in order to decompose the precursors for the wet process. On the basis of the TEM observations, the mechanism of the burning of the Pt-Ru/C samples, as mentioned before, is discussed. For the prepared Pt-Ru/C samples, the alloy nanoparticles were smaller and were more highly dispersed on the carbon support than in the case of the commercially available samples. It has been reported that several kinds of metal nanoparticles undergo rapid oxidation upon exposure to air.72-74 Since RuO2 was observed in the XRD and XPS measurements for the burned sample (Pt, 18 wt %; Ru, 9 wt %), the burning of the prepared sample is probably initiated by the Ru oxidation reaction. Ru atoms located near the surface of the alloy particles were probably oxidized, based on the observation of the PtRu alloy peak in the XRD pattern. However, if the burning was caused only by the Ru oxidation reaction (an exothermic reaction), it is difficult to explain the observation of fire and smoke over a span of 3-5 min. Thus, it is most likely that the burning is induced by the combustion of the carbon support subsequent to the Ru oxidation reaction. Assuming a particle size of 3.3 nm and a number of alloy nanoparticles per unit area of 0.10 particles/nm2, the amount of heat released by the Ru oxidation reaction was calculated to be 1.80 × 10-21 kJ/ nm2. This value is 10 times larger than the heat amount needed for the spontaneous combustion of carbon calculated at 2.84 × 10-22 kJ/nm2 and sufficient to burn the carbon support. A large intensity and a small fwhm value, as shown in Figure 6,II (A), are probably attributed to the growth of the alloy nanoparticles caused by the burning of the sample. Indeed, the alloy particles with a large size of ca. 20-50 nm were observed in the TEM image of the burned sample (Figure 8J). On the other hand, for the unburned sample (Pt, 14.7 wt %; Ru, 8.0 wt %), the Ru oxidation reaction should occur upon exposing the samples to air. However, no burning of the sample was observed, implying that the total heat amount generated by the Ru oxidation reaction was insufficient to burn the carbon support. Further, the peak of RuO2 was not detected in the XRD and XPS measurements for the unburned samples, suggesting that the oxidized Ru was reduced again. The reduction of the oxidized Ru is possibly related to the carbon support, since carbon is a reducing agent.75 In addition, no burning was observed for the sample with 14.7 and 8.0 wt % prepared at 100 W, while burning appeared for the sample with 13 and 6.5 wt % at 50 W. It has been described that the sizes of the alloy nanoparticles decrease with lowering ac power. Reducing the particle size increases the total surface

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1487 area of the alloy, leading to an increase in the total heat amount released by the Ru oxidation reaction. Therefore, it is expected that the combustion of the carbon support easily occurred for the sample prepared at lower ac power. In the case of the commercially available samples, since the deposited alloy particles had a larger size, the burning of the sample was not observed at all. That is, the alloy particles that could induce the burning of the sample would disappear from the commercially available samples through the particle growth by heating to decompose the Pt and Ru precursors. EDX Measurement. The EDX measurement was conducted to obtain the Pt/Ru ratios of the individual alloy nanoparticles in the samples. The obtained Pt/Ru ratios are listed in Table 3. The Pt/Ru ratios for TK and JM were observed from 51.0:49.0 to 89.4:10.6 atom % (n ) 22) and 46.6:53.4 to 100:0 atom % (n ) 25), ratios which widely deviated. Pozio et al. have reported that the Pt/Ru ratios for the commercially available sample were different from the molar ratio of the Pt and Ru precursors containing 50:50 atom %.76 Rauhe et al.30 and Kawaguchi et al.33 have also reported that the Pt metal alone or the Pt-rich alloy existed in the Pt-Ru/C samples prepared with the impregnation method. Moreover, Dickinson et al. have reported that the Pt/Ru ratios were varied owing to the different decomposition temperatures of the Pt and Ru precursors.45 On the other hand, it can be seen that the obtained Pt/Ru ratios for the prepared samples were approximated to be the atomic ratio of the sputtered Pt and Ru (Pt/Ru ) 52:48 atom %), as shown in Table 3. In addition, the deviation of the Pt/Ru ratios was much smaller than those for the commercially available samples. These observations were due to not using the precursors. On the basis of the TEM and EDX measurements, the reason for the broadness of the Pt-Ru alloy peak for the unburned sample shown in Figure 6, II (B) has been discussed. The fwhm value of the peak was obtained at 2.54°, resulting in an average alloy particle size calculated at 3.3 nm from the Scherrer equation. Whereas on the TEM measurement of the unburned sample, the average size of the alloy particles was estimated to be 3.5 nm (size distribution, 1.2-6.4 nm; n ) 166), which was almost the same as the calculated size. This result indicates that the broadness of the peak for the unburned sample can only be attributed to the particle size of the alloy. A similar result was also obtained for SW130 (calculated, 4.1 nm; observed, 4.1 nm) and SW100 (3.5 and 3.3 nm). On the other hand, the average sizes of the alloy particles for TK and JM were calculated at 2.9 nm (fwhm: 2.94°) and 2.5 nm (3.36°) with the Scherrer equation, ca. 1.6 times smaller than the observed sizes of 4.5 and 4.0 nm. It has already been explained that the 2θ value of the alloy peak depends on the Pt/Ru ratio. This indicates that the shape of the alloy peak is broader through not only the decrease in the particle sizes but also the deviation of the Pt/ Ru ratios. Since the Pt/Ru ratios for TK and JM deviated widely, the calculated particle sizes were underestimated against the observed sizes. That is, the Scherrer equation can be applied for evaluating the particle size only in the case of alloys with uniform Pt/Ru ratios. Electrochemical Properties of Pt-Ru/C Sample. As described above, the Pt-Ru/C samples with various particle sizes and amounts of Pt and Ru deposited were prepared by changing the ac power and the sputtering time. Thereafter, the electrochemical properties of the prepared samples were evaluated to compare with those of the commercially available samples. H2 Oxidation. Figure 9 shows the hydrodynamic voltammograms for H2 oxidation of (A) SW50h2 and (B) TK loaded on the glassy-carbon disk electrode (geometric surface area: 0.196


Figure 9. Hydrodynamic voltammograms for the H2 oxidation of (A) SW50h2 and (B) TK by a RDE method (inset: Levich-Koutecky plot obtained at 200 mV vs RHE) (measuring conditions: electrolyte, 1 N H2SO4 saturated by pure H2 gas; temperature, room temperature; sweep rate, 10 mV/s; disk rotation rate (ω), 100-3600 rpm).

cm2). In both voltammograms, an anodic current corresponding to H2 oxidation increased rapidly from 0 mV (vs RHE) and subsequently a diffusion-limited current was observed. In addition, the diffusion-limited current densities obtained at each rotation rate of the working electrode were identical for both samples. It should be noted that similar hydrodynamic voltammograms were also obtained for SW130, SW100, SW50h8, SW30, and SW50h4. The inset in Figure 9 presents the LevichKoutecky plot for the H2 oxidation reaction obtained from the current density (i) and the rotation rate of the working electrode (ω) at 200 mV. The reversed values of the slopes for SW50h2 and TK were estimated to be 6.71 × 10-2 and 6.76 × 10-2 mA/cm2‚rpm-1/2, almost identical and in good agreement with the calculated value (6.54 × 10-2 mA/cm2‚rpm-1/2).11,63,77 The values obtained for other prepared samples were almost equal to those for TK and SW50h2, e.g., the value for SW50h8 was 6.76 × 10-2 mA/cm2‚rpm-1/2. CO Oxidation. Figure 10 shows the CO stripping voltammograms of the commercially available samples (A, TK; B, JM) and the prepared Pt-Ru/C samples (C, SW130; D, SW100; E, SW50h8; F, SW30; G, SW50h4; H, SW50h2). During the first scan (solid line), no current corresponding to H desorption between ca. 30 and 200 mV (vs RHE) was observed in all voltammograms, showing that the surfaces of the Pt-Ru alloy nanoparticles were fully covered by CO. CO oxidation current was subsequently observed. During the second scan (broken line), H desorption current was measured and no peak of CO oxidation appeared, indicating that CO adsorbed on the surface of the alloy nanoparticles was completely oxidized. As for the prepared samples, the onset of CO oxidation was observed at

Inoue et al. ca. 380 mV and one sharp peak appeared at 440-449 mV, as shown in parts C-H of Figure 10 and Table 4. It should be noted that the peak currents of CO oxidation increased with Pt and Ru deposited amounts. By comparison, the onset potential of CO oxidation for TK and JM was almost the same as those for the prepared samples. However, the broad peaks were observed at 464 mV (TK) and 462 mV (JM), ca. 20 mV higher than those for the prepared samples (parts A and B of Figure 10). Gasteiger et al. and Kabbabi et al. have reported that for the 50:50 atom % bulk alloy, a sharp CO oxidation peak was observed at the lower potential.5,7,11 In addition, they have stated that the CO oxidation peak on a Pt-Ru alloy shifts to a higher potential as the Pt/Ru ratio of the alloy deviates from 50:50 atom %.5,7 As shown in Table 3, the Pt/Ru ratios of the individual alloy nanoparticles for the prepared samples were ca. 50:50 atom % and homogeneous compared with those for the commercially available samples. Thus, the sharper peak and the lower peak potential for the prepared samples were due to CO oxidation on the alloy particles with ca. 50:50 atom %. That is, the broader peak and the higher peak potentials for the commercially available samples were attributed to an overlap of CO oxidation peaks on the alloy particles with various Pt/ Ru ratios. A superposition of the peaks for TK and SW50h8, of which the peak currents were normalized, is presented in the figure enclosed by the solid line in Figure 10, and the meshed area is in accord with the amount of CO adsorbed on the alloys with a lower CO oxidation activity, namely, of the Pt-rich alloy or the Ru-rich alloy. The meshed area for TK was estimated to be ca. 42% toward a total area of CO oxidation (48% for JM). Afterward, the charge associated with CO oxidation (denoted as Q in mC) for the Pt-Ru/C samples was estimated from the shaded area in each CO stripping voltammogram shown in Figure 10. The obtained Q values are listed in Table 4. Figure 11,I presents the Q value against the total amount of the Pt and Ru deposited. In the case of SW50h8 (E), SW30 (F), SW50h4 (G), and SW50h2 (H), the Q value was proportional to the total amount of deposited Pt and Ru, showing a linear relationship between the electrochemical surface area of the alloy and the total amount of deposited Pt and Ru.78 This is coincident with the fact that the dispersion density of the nanoparticles of less than 4 nm increased in the deposited amount. As for SW130 (C) and SW100 (D), however, the Q values deviated slightly from the solid line in Figure 11,I. In addition, large deviations were observed for TK (A) and JM (B). Furthermore, the Q per unit amount of Pt and Ru (denoted as Q0 in mC/µgPt+Ru) was calculated from the obtained Q value. It should be noted that the Q0 value expresses the utilization of the deposited Pt-Ru alloy for electrochemical reactions. Each Q0 value is also listed in Table 4 and displayed as a bar graph in Figure 11,II. In the case of SW50h8 (E), SW30 (F), SW50h4 (G), and SW50h2 (H), the Q0 values were almost constant at ca. 0.7 mC/µgPt+Ru. For SW130 (C) and SW100 (D), the Q0 values were 0.44 mC/µgPt+Ru and 0.50 mC/µgPt+Ru, obviously lower than those for 50 and 30 W. On the other hand, the Q0 values for TK (A) and JM (B) were 0.28 mC/µgPt+Ru and 0.34 mC/µgPt+Ru, ca. 2-fold as small as those for 50 and 30 W. The relation between the Q0 value and the size of the nanoparticles has already been reported as follows:78

Q0 ) (60q(1/F))(1/d) where q is the coulomb charge of the CO oxidation reaction per unit surface area of Pt-Ru alloy (in mC/cm2), F is the density of the Pt-Ru alloy with Pt/Ru ) 50:50 atom % (in g/cm3), and d is the diameter of the Pt-Ru alloy particles (in


J. Phys. Chem. C, Vol. 112, No. 5, 2008 1489

Figure 10. CO stripping voltammograms of the commercially available samples (A, TK; B, JM) and the prepared Pt-Ru/C samples (C, SW130; D, SW100; E, SW50h8; F, SW30; G, SW50h4; H, SW50h2) (figure enclosed by the solid line; superposition of the normalized CO oxidation peak for TK and SW50h8) (solid line, first scan; broken line, second scan. Measuring conditions: electrolyte, 1 N H2SO4; temperature, 40 °C; sweep rate, 10 mV/s. CO adsorption conditions: immersing time, 30 min (N2 purge 30 min); potential, 70 mV vs RHE; temperature, 40 °C).

TABLE 4: Summary of CO Stripping Voltammetry for the Pt-Ru/C Samples potential/mV vs RHE sample

onset

TK JM

375 384

SW130 SW100 SW50h8 SW30 SW50h4 SW50h2

384 388 384 384 379 379

peak

coulomb charge of CO oxidation Q/mC

commercially available samples 464 5.50 462 5.93 prepared Pt-Ru/C samples 449 2.53 449 2.99 440 3.88 448 3.27 442 1.96 447 1.05

Q0/mC/µgPt+Ru 0.28 0.34 0.44 0.50 0.69 0.68 0.70 0.71

nanometers). Figure 12 shows Q0 as a function of 1/d when the average size of alloy particles obtained from TEM observation was used as d. It can be seen that the Q0 values for all prepared samples were proportional to 1/d. The slope of the straight line obtained in the figure was 1.20, which was close to the calculated value of 1.37 from the reported q (0.42 mC/cm2)79,80 and F (18.37 g/cm3) values.78,81 This result indicates that the deviation for SW130 and SW100 shown in Figure 11,I was

attributed to the size of the alloy particles deposited. Although the average size for the commercially available samples was similar to those for SW130 and SW100, on the other hand, the Q0 values for TK (A) and JM (B) were evidently plotted below ca. 32% and ca. 23% from the line. This result implies a lower utilization of the deposited alloy for the commercially available samples. The likely causes of the lower utilization for TK and JM are considered as follows. If the size distribution of the alloy particles is symmetrical in shape, the average size is the typical size of whole particles. It can be seen that a symmetrical size distribution was observed for the prepared samples and not for the commercially available samples. Thus, the larger-sized particles outside the symmetrical size distribution caused the lower utilization of the alloy. In the case of TK, when a symmetrical size distribution centered at 3.6 nm was assumed, alloy particles of ca. 20% were not included in the assumed distribution. In the case of JM, under the same assumption, the value was ca. 15%. These values were about 1.5 times smaller than the values for TK (32%) and JM (23%) estimated in Figure 12, showing that the lower utilization for the commercially available samples was not only due to the existence of the larger-


Inoue et al.

Figure 13. Polarization curves of TK (Pt-Ru alloy loading: A, 0.50 mg/cm2; B, 0.15 mg/cm2), (C) SW50h8 (0.08 mg/cm2), and (D) SW50h2 (0.02 mg/cm2) (MEA; active area, 25 cm2; cathode electrocatalyst, Pt/C (Pt loading: 0.5 mg/cm2), cell temperature, 80 °C; reactant gas (gas utilization), anode, 100 ppm CO/H2 gas (50%); cathode, pure O2 gas (50%)).

Figure 11. (I) Q value as a function of total amount of Pt and Ru deposited and (II) bar graph of the Q0 value (A, TK; B, JM; C, SW130; D, SW100; E, SW50h8; F, SW30; G, SW50h4; H, SW50h2).

Figure 12. Q0 value as a function of 1/d (d; average particle size obtained from the TEM measurement) (A, TK; B, JM; C, SW130; D, SW100; E, SW50h8; F, SW30; G, SW50h4; H, SW50h2; I, heated SW50h8 sample).

sized alloy particles outside the symmetrical size distribution. It has already been mentioned that the aggregation of the alloy nanoparticles was frequently observed for TK and JM, as shown in parts A and B of Figure 8. The aggregation may contribute to the lower utilization, since it is expected that CO hardly covers the surface of particles at the adjacent portions and within the aggregation. It should be emphasized that the Q0 value for the heated sample was 0.39 mC/µgPt+Ru, which deviated slightly from the straight line (Figure 12I), though the size distribution shown in Figure 8I was symmetrical in shape. This result suggests the likelihood that the lower utilization of the alloy was caused by the existence of the aggregation of the nanoparticles. Whereas Uchida et al. and Han et al. have stated that the nanoparticles were probably deposited in the pores as well as on the outside surface of the carbon support by wet process,82,83 Marie et al. have claimed that the alloy particles deposited in the pores of the carbon support were only partially covered by CO.84 Thus, the existence of the alloy particles deposited in the pores supposedly led to the lower utilization

for TK and JM, though the alloy particles in the pore could not be clearly distinguished in the TEM images in the present study. Cell Performance. It can be seen from the CO stripping voltammetry that the prepared Pt-Ru/C sample has the higher utilization of Pt and Ru for electrochemical reactions compared with the commercially available samples. It is expected from this result that the amount of Pt-Ru alloy loaded on the cell will be reduced by using the prepared samples as an anode electrocatalyst. Thus, the cell performances for SW50h8 and SW50h2 were investigated by comparison with those for TK. Figure 13 (A) shows the polarization curve for TK used as an anode electrocatalyst. The Pt-Ru alloy loading on the anode side of MEA was adjusted at 0.50 mg/cm2. When 100 ppm CO/ H2 gas was supplied to the anode, the cell voltage decreased from an open circuit voltage of ca. 1.0 V down to ca. 0.6 V with increasing current density. A similar result for the commercially available sample has been already reported with the 100 ppm CO/H2 gas used as the fuel.85 However, for the alloy loading of 0.15 mg/cm2 (Figure 13 (B)), the cell voltage decayed steeply from the current density of 400 mA/cm2. Gasteiger et al. have stated that the minimum alloy loading with the commercially available sample was 0.2 mg/cm2, using 100 ppm CO/H2 gas,65 indicating that the results of Figure 13 (A and B) are not strange. Figure 13 (C and D) presents the polarization curves for SW50h8 and SW50h2. When the volume of the used samples was adjusted to that for TK with 0.50 mg/ cm2, the respective alloy loadings were 0.08 and 0.02 mg/cm2, ca. 1/3 and 1/10 times the reported minimum loading (0.2 mg/ cm2). Surprisingly, the obtained polarization curves almost corresponded to that for 0.50 mg/cm2 using TK and no serious decline of the cell voltage was observed. As mentioned before, the prepared samples did not have the Pt-rich alloy, the Rurich alloy, or alloy particles with a larger size. From the comparison of the utilization of the alloy between the prepared sample and the commercially available sample, the minimum loading for the prepared sample was estimated to be ca. 0.06 mg/cm2. However, the alloy loading for SW50h2 was ca. 3 times smaller than the estimated minimum loading. Han et al. have mentioned that the alloy nanoparticles, which were selectively deposited on the outside surface area of the carbon support, could easily come in contact with the polymer electrolyte, leading to the high utilization of the alloy.83 This indicates that the polygonal barrel-sputtering method accomplished the selective deposition of the Pt-Ru alloy nanoparticles on the outside surface of the carbon support, resulting in the linear relationship for the prepared samples in Figure 12. Figure 14 shows the cell voltages at 400 mA/cm2 of (A) TK (0.50 mg/cm2), (B) SW50h8 (0.08 mg/cm2), and (C) SW50h2


Figure 14. Cell voltage at 400 mA/cm2 as a function of operating time of (A) TK (Pt-Ru alloy loading: 0.50 mg/cm2), (B) SW50h8 (0.08 mg/cm2), and (C) SW50h2 (0.02 mg/cm2) (MEA; active area, 25 cm2; cathode electrocatalyst, Pt/C (Pt loading: 0.5 mg/cm2), cell temperature, 80 °C; reactant gas (gas utilization), anode, 100 ppm CO/H2 gas (50%); cathode, pure O2 gas (50%)).

Figure 15. Typical TEM image and size distribution of the Pt-Ru alloy nanoparticles of SW50h2 after the operation of 60 h at constant current mode of 400 mA/cm2 and cell temperature of 80 °C (accelerating voltage, 120 kV; magnification factor, 410 000; n, number of particle counts).

(0.02 mg/cm2) against the operation time. Gas of concentration 100 ppm CO/H2 was still used as the fuel. The cell voltage for TK was constant at ca. 0.7 V for 60 h. In the case of SW50h8 (Figure 14 (B)) and SW50h2 (C), similar results were observed, despite the lower alloy loading. In addition, for SW50h8, the decline of the cell voltage was not observed for even 120 h. Figure 15 presents the typical TEM image of SW50h2 used in the cell test. It should be noted that the contrast of the image was slightly low, since the Pt-Ru/C sample was covered by the thin film of Nafion. This image was almost the same as that of as-prepared SW50h2 shown in Figure 8H. It can be seen that the alloy nanoparticles were still highly dispersed on the carbon support after the cell test, implying that the growth and the aggregation of the alloy particles did not occur during the cell operation. This result shows the physical stability of the prepared Pt-Ru/C sample. Conclusions In the present study, the Pt-Ru/C samples were prepared by using the polygonal barrel-sputtering method and their physical and electrochemical properties were evaluated. The obtained results are described as follows.

J. Phys. Chem. C, Vol. 112, No. 5, 2008 1491 (1) From the study of the preparation conditions, Pt-Ru alloy with a Pt/Ru ratio of ca. 50:50 atom % could be prepared under an Ar gas pressure of 0.9-0.7 Pa at room temperature. No heating procedure was required to prepare the sample. The amounts of Pt and Ru sputtered were controlled by changing the sputtering time and the ac power. (2) The Pt-Ru/C samples were prepared in a single process under the given optimum conditions. For 50 and 30 W, no remarkable peak of the Pt-Ru alloy was observed in the XRD patterns, resulting in the absence of particles with a size greater than 4 nm. (3) The sizes of the alloy nanoparticles for the prepared samples depended on the ac power when more than 50 W was applied. However, between 50 and 30 W, no dependence of the particle size on the ac power was observed. The Q0 values of the samples prepared at 50 and 30 W were obtained ca. 0.7 mC/µgPt+Ru, which was ca. twice the value of those for the commercially available samples. This implies that the alloys deposited in the prepared samples were efficiently utilized for the H2 and CO oxidation reaction. (4) The Pt/Ru ratios of the individual alloy particles in the prepared samples were ca. 50:50 atom % and extremely uniform compared with those in the commercially available samples. The sharper peak and the lower peak potential of CO oxidation in the voltammograms were attributed to the uniformity of the Pt/Ru ratios. (5) The polarization curve and the cell voltage at the constant current of 400 mA/cm2 against the operation time for the prepared sample with the alloy loading of 0.08 and 0.02 mg/ cm2 were similar to those for the commercially available sample with 0.50 mg/cm2. These results indicate that when the prepared sample is used as the anode electrocatalyst, the precious metal loading can be reduced by 1/10 times versus the present minimum loading. Acknowledgment. This work was partially supported by a Grant-in-aid for Science Research from Ministry of Education, Science, Sports and Culture in Japan. References and Notes (1) Lemons, R. A. J. Power Sources 1990, 29, 251. (2) Igarashi, H.; Fujino, T.; Watanabe, M. J. Electroanal. Chem. 1995, 391, 119. (3) Costamagna, P.; Srinivasan, S. J. Power Sources 2001, 102, 242. (4) Wee, J.-H.; Lee, K.-Y. J. Power Sources 2006, 157, 128. (5) Gasteiger, H. A.; Markovic´, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (6) Ianniello, R.; Schmidt, V. M.; Stimming, U.; Stumper, J.; Wallau, A. Electrochim. Acta 1994, 39, 1863. (7) Kabbabi, A.; Faure, R.; Durand, R.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 41. (8) Gasteiger, H. A.; Markovic´, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (9) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N., Jr.; Jiang, X.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91. (10) Gasteiger, H. A.; Markovic´, N.; Ross, P. N., Jr.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795. (11) Gasteiger, H. A.; Markovic´, N. M.; Ross, P. N., Jr. J. Phys. Chem. 1995, 99, 8290. (12) Gurau, B.; Viswanathan, R.; Liu, R.; Lafrenz, T. J.; Ley, K. L.; Smotkin, E. S.; Reddington, E.; Sapienza, A.; Chan, B. C.; Mallouk, T. E.; Sarangapani, S. J. Phys. Chem. B 1998, 102, 9997. (13) Koper, M. T. M.; Lukkien, J. J.; Jansen, A. P. J.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 5522. (14) Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E. S.; Sung, Y.-E.; Kim, H.; Thomas, S.; Wieckowski, A. J. Phys. Chem. B 2000, 104, 3518. (15) Grgur, B. N.; Zhuang, G.; Markovic, N. M.; Ross, P. N., Jr. J. Phys. Chem. B 1997, 101, 3910. (16) Massong, H.; Wang, H.; Samjeske´, G.; Baltruschat, H. Electrochim. Acta 2000, 46, 701.

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Preparation and Physical and Electrochemical Properties of Carbon

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