- Email: [email protected]
C samples prepared on various carbon supports by using the barrel sputtering system
Vacuum 83 (2009) 658–663
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Physical and electrochemical properties of Pt–Ru/C samples prepared on various carbon supports by using the barrel sputtering system Mitsuhiro Inoue*, Satoshi Akamaru, Akira Taguchi, Takayuki Abe Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
a b s t r a c t Keywords: Barrel sputtering system Polymer electrolyte fuel cells (PEFCs) Anode electrocatalyst Physical and electrochemical properties Specific surface area of carbon support
The present study investigated physical and electrochemical properties of carbon-supported Pt–Ru alloy (Pt–Ru/C) samples prepared on various carbon supports by using a sputtering system with barrel-type powder sample holder (the barrel sputtering system). For this system, the deposited Pt–Ru nanoparticles had uniform sizes of less than 4 nm and homogeneous atomic ratios of Pt and Ru of ca. 50:50 at.% independent of a specific surface area of the carbon support. Further, the electrochemical properties of the prepared samples obtained from CO stripping voltammetry were almost identical. These were completely different from the result for wet process showing that the physical and electrochemical properties of samples prepared by an impregnation method changed with the specific surface area of the carbon support. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction A carbon-supported Pt–Ru alloy (Pt–Ru/C) electrocatalyst with an atomic ratio of the Pt and the Ru (denoted as Pt:Ru ratio) of 50:50 at.% is a candidate of an anode electrocatalyst for practical polymer electrolyte fuel cells (PEFCs) [1]. The electrochemical property of the Pt–Ru/C electrocatalyst is closely connected to its physical property, since CO oxidation activity of a Pt–Ru alloy depends on the Pt:Ru ratio [2,3] and the sizes of the alloy particles affect an electrochemical surface area and the utilization of the Pt and the Ru for electrochemical reactions [4–6]. Some researchers have investigated electrochemical properties of Pt–Ru/C samples prepared on carbon supports with various specific surface areas (hereafter referred to simply as ’surface area’) by wet process [7–9]. The samples used have been mainly prepared by heating the carbon supports adsorbing the Pt and the Ru precursors to decompose them into the Pt–Ru alloy. However, since the heating procedure causes particle growth [10] and the decomposition temperatures of the Pt and the Ru precursors are different [10–12], the sizes and the Pt:Ru ratios of the deposited alloy particles for wet process are not uniform. This leads to the difference of the physical properties of the prepared Pt–Ru/C samples, resulting in the dependence of their electrochemical properties on the
surface area of the carbon support. Thus, samples must be prepared with the same physical properties in order to investigate the true effects of the surface area of the carbon support toward the electrochemical properties of the Pt–Ru/C electrocatalyst. Recently, the authors have developed a sputtering system with barrel-type powder sample holder (the barrel sputtering system) for modifying surfaces of powdered particles [13]. For this system, the Pt–Ru/C sample is prepared by sputtering with oscillating the polygonal barrel to stir a powdered carbon support, leading to the high dispersion of the alloy particles on the support. Further, the sizes and the Pt:Ru ratios of the deposited alloy particles can be made uniform because no heating and no precursors are needed and the atomic ratio of the sputtered Pt and Ru is constant. These are completely different from the features of the preparation of the Pt–Ru/C sample for wet process as above-mentioned, suggesting that the barrel sputtering system could prepare the Pt–Ru/C samples with the same physical properties on various carbon supports. In the present study, the Pt–Ru/C samples were prepared with various carbon supports by using the barrel sputtering system and their physical and electrochemical properties were evaluated. 2. Experimental section 2.1. Preparation of Pt–Ru/C
* Corresponding author. Present address: Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka, Niigata 940-2188, Japan E-mail address: [email protected] (M. Inoue). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.042
In this study, KJB ECP600JD (surface area: 1270 m2/g; average size of primary particle: 34 nm, Ketjen), Vulcan XC72R (254 m2/g, 30 nm, Cabot), and MONARCH 280 (42 m2/g, 45 nm, Cabot) were used as carbon supports.
M. Inoue et al. / Vacuum 83 (2009) 658–663
The deposited amounts of Pt and Ru for the prepared sample were determined from the X-ray fluorescence analysis (XRF: PW2300, Philips) and the inductively coupled plasma-atomic emission spectroscopy (ICP-AES: Optima 3300XL, Perkin–Elmer). For the X-ray diffraction analysis (XRD: PW1825/00, Philips), the 2q value and the area of the diffraction peak corresponding to the Pt– Ru alloy were evaluated. The size of a Pt–Ru alloy particle was estimated by the transmission electron microscopy (TEM, JEM-2100, JEOL). The size distribution of the alloy particles was obtained by directly measuring the size of more than 100 randomly chosen particles in the magnified TEM images. The Pt:Ru ratio of the individual alloy particle was estimated by the energy dispersive X-ray analysis (EDX, JED-2300, JEOL). 2.3. Electrochemical measurement Electrochemical property of a prepared Pt–Ru/C sample was evaluated by CO stripping voltammetry at 40 C using a threecompartment electrochemical cell. A Pt wire (diameter 1 mmB 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 by fixing the sample of 40 mg on the mirror-finished glassy carbon electrode (diameter: 5 mm; geometric surface area: 0.196 cm2, Hokuto Denko) with 0.1% NafionÒ solution [15,16]. The working electrode was immersed in 1 N H2SO4 as a supporting electrolyte solution saturated with pure CO gas at 40 C for 30 min to adsorb CO completely on the surface of the alloy particles, 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. The CO stripping voltammogram was obtained with a sweep rate of 10 mV/s between 30 mV and 800 mV to prevent Ru dissolution [3,15].
3.1. Physical property of Pt–Ru/C Fig. 1 shows the XRD patterns of the prepared Pt–Ru/C samples. The Si(220) peak of 2q ¼ 47.30 was used as a standard position to decide exactly the 2q value. For W-KB (Fig. 1A), W-VX (Fig. 1B), and W-MN (Fig. 1C), the peak corresponding to the Pt–Ru alloy was observed at 2q ¼ 39.79 , 40.13 , and 40.28 , respectively. These 2q values were lower than 40.51 for the homemade bulk alloy with Pt:Ru ¼ 50:50 at.%, suggesting that the Pt:Ru ratios of the deposited alloy particles were not uniform. Further, each peak area drastically differed despite similar Pt and Ru deposited amounts, showing that the alloy particle sizes probably changed with the surface area of the carbon support. Whereas, for BS-KB (Fig. 1D), BS-VX (Fig. 1E), and BS-MN (Fig. 1F), no remarkable peak of the alloy was observed in the pattern. In a separate experiment, for the Pt–Ru/C samples with a similar XRD pattern prepared by our sputtering system, the deposited alloy particles had uniform sizes of less than 4 nm and homogeneous Pt:Ru ratios of ca. 50:50 at.%. Therefore, the sizes and the Pt:Ru ratios of the alloy particles for BS-KB, BS-VX, and BS-MN would be close to the above-mentioned results. Fig. 2 presents the typical TEM images and the size distributions of the alloy particles for the prepared samples. In all TEM images, the alloy nano-particles represented as black dots were observed on the carbon support as gray. The sizes of the alloy particles for WKB (Fig. 2A), W-VX (Fig. 2B), and W-MN (Fig. 2C) were distributed from 0.8 nm to 7.2 nm (particle count [n] ¼ 183), 0.8 nm to 12.8 nm (n ¼ 174), and 1.2 nm to 32.0 nm (n ¼ 126), respectively. These results indicate that the sizes of the alloy particles deposited by the impregnation method differed with the surface area of the carbon support. The Pt:Ru ratios of the individual alloy particles for W-KB, W-VX, and W-MN obtained by the EDX measurements widely deviated at 24.7:75.3–100.0:0.0 at.% (particle count [n] ¼ 26), 0.4:99.6–98.5:1.5 at.% (n ¼ 28), and 12.8:87.2–80.6:19.4 at.% (n ¼ 25). In addition, for W-KB, while the sizes of the alloy particles
Ru (100) 300
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intensity / cps
2.2. Physical characterization
3. Results and discussion
Si (220)
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The details of the barrel sputtering system have been reported [13]. The Pt–Ru/C samples with Pt:Ru ¼ ca. 50:50 at.% were prepared as follows. The metal plates of Pt and Ru (purity 99.99%, size W 25 mm L 100 mm) were used together as a target and their surface area ratio was adjusted at 6:10, resulting in an atomic ratio of the sputtered Pt and Ru of 52:48 at.%. Prior to sputtering, the air in a vacuum chamber that contained the hexagonal or octagonal barrel with a carbon support was evacuated at less than 8 104 Pa. The sputtering was performed under optimum conditions (Ar gas (purity 99.9999%) pressure: 0.8 Pa; temperature: room temperature; and AC power: 50 W) to deposit the alloy particles with the uniform sizes and the homogeneous Pt:Ru ratios. Sputtering time was controlled to maintain similar Pt and Ru deposited amounts. On sputtering, the barrel was oscillated (intervals and amplitude: 14 s and 75 (hexagonal) and 11 s and 60 (octagonal)) with a mechanical vibration. The Pt and the Ru deposited amounts of the prepared samples were obtained at 8.0 wt.%, 4.1 wt.% (KJB ECP600JD, denoted as BS-KB), 9.1 wt.%, 4.6 wt.% (Vulcan XC72R, BS-VX), and 8.2 wt.%, 4.3 wt.% (MONARCH 280, BS-MN), respectively. The Pt–Ru/C samples for wet process were also prepared by an impregnation method for comparison [7,10,14]. H2PtCl6$6H2O (Kanto Chemical) and RuCl3 (purity 95.1%, Kanto Chemical) were used as precursors. The sample was prepared by heating a carbon support adsorbing the precursors at 350 C for 3 h under flowing 10% H2 in N2 gas. The amounts of Pt and Ru deposited were 9.6 wt.%, 4.2 wt.% (KJB ECP600JD, denoted as W-KB), 8.9 wt.%, 4.9 wt.% (Vulcan XC72R, W-VX), and 8.0 wt.%, 4.5 wt.% (MONARCH 280, WMN), respectively.
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2 / degree Fig. 1. XRD patterns of the Pt–Ru/C samples prepared by the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN) ((-$-$) Ru peak position, (/) Pt peak position, and (- - -) Si(220) peak (2q ¼ 47.30 )).
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particle size / nm Fig. 2. Typical TEM images and particle size distributions of the Pt–Ru/C samples prepared by the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN) (accelerating voltage: 200 kV; magnification factor: 400,000; and n: particle count).
with Pt rich ratios tended to be measured at more than 4 nm, the ones with Ru rich ratios were obtained at less than 4 nm, possibly implying the dependence of the Pt:Ru ratio on particle size. On the other hand, for the barrel sputtering system, the sizes of the alloy particles were obtained at less than 4 nm, as shown in Fig. 2(D)–(F). The Pt:Ru ratios of each alloy particle for BS-KB, BS-VX, and BS-MN were homogeneous at 50.4:49.6 (7.2) at.% (particle count [n] ¼ 32), 51.9:48.1 (6.3) at.% (n ¼ 27) and 52.6:47.4 (5.7) at.% (n ¼ 28), respectively. The above observations indicate that the sizes and the Pt:Ru ratios of the alloy nano-particles for the barrel sputtering system were almost identical independent of the surface area of the carbon support. It should be noted that the surface area of the carbon support changes with the amount of pores of less than 8 nm [17]. Thus, the deposition of the alloy particles in the
pores is possibly concerned with the dependence of physical properties of the Pt–Ru/C samples on the surface area of the carbon support. 3.2. Electrochemical property of Pt–Ru/C Fig. 3 exhibits the CO stripping voltammograms of the Pt–Ru/C samples for the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN). Inset is the coulomb charge of CO oxidation per amount of Pt and Ru (denoted as Q0 in mC/mgPtþRu) calculated from the area represented with a slash versus the surface area of the carbon support. For the impregnation method, a broad peak corresponding to CO oxidation was observed. The onset and the
M. Inoue et al. / Vacuum 83 (2009) 658–663
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peak potentials (vs. RHE) of CO oxidation were measured at 444 mV, 594 mV (W-KB), 444 mV, 509 mV (W-VX), and 389 mV, 474 mV (W-MN), indicating that CO oxidation activity of the samples prepared by the impregnation method changed with the surface area of the carbon support [2,3]. The respective Q0 values were estimated to be 0.62 mC/mgPtþRu, 0.46 mC/mgPtþRu, and 0.28 mC/mgPtþRu, decreasing in the surface area of the carbon support, as shown in the inset. This implies that the utilization of the Pt and the Ru for electrochemical reactions declined with the surface area of the carbon support since the Q0 represents the electrochemical surface area per amount of Pt and Ru [5,6]. The difference of the electrochemical properties of the prepared samples as above-mentioned was attributed to the sizes and the Pt:Ru ratios of the deposited alloy particles. In contrast, for the
barrel sputtering system, the peak shape of CO oxidation was sharp, as shown in Fig. 3(D)–(F). The onset and the peak potentials of CO oxidation were almost identical at 383 mV, 443 mV (BS-KB), 384 mV, 449 mV (BS-VX), and 389 mV, 449 mV (BS-MN), respectively. The Q0 values of each sample were also almost constant at 0.67 mC/mgPtþRu, 0.65 mC/mgPtþRu, and 0.65 mC/mgPtþRu independent of the surface area of the carbon support (inset). These indicate that for the Pt–Ru/C samples with the same physical properties, CO oxidation activity and the utilization of the Pt and the Ru were uninfluenced by the surface area of the carbon support. It should be noted that the peak potentials of CO oxidation and the Q0 values for the barrel sputtering system were lower and larger than those for the impregnation method, demonstrating that the electrochemical properties of the prepared
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Fig. 3. CO stripping voltammograms of the Pt–Ru/C samples prepared by the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN) (solid line: first scan; broken line: second scan; and inset: Q0 value versus surface area of carbon support) (measuring conditions: supporting electrolyte, 1 N H2SO4; temperature, 40 C; sweep rate, 10 mV/s; CO adsorption, 30 min; and N2 purge, 30 min (potential: 70 mV vs. RHE)).
samples by our dry process were superior to those by the conventional wet process. 4. Conclusion Summarizing, the barrel sputtering system could prepare the Pt–Ru/C samples with same physical properties on various carbon supports. Further, these samples had almost identical electrochemical properties, which were superior to those for the impregnation method. These reveal that the electrochemical properties of the Pt–Ru/C samples with the same physical properties are independent of the surface area of the carbon support.
Acknowledgement This work was partially supported by a grant-in-aid for Science Research from the Ministry of Education, Science, Sports and Culture of Japan. References [1] Qi Z, Kaufman A. J Power Sources 2003;113:115–23. [2] Gasteiger HA, Markovic´ N, Ross Jr PN, Cairns EJ. J Phys Chem 1994;8:617–25. [3] Kabbabi A, Faure R, Durand R, Beden B, Hahn F, Leger J-M, et al. J Electroanal Chem 1998;444:41–53. [4] Radmilovic´ V, Gasteiger HA, Ross Jr PN. J Catal 1995;154:98–106.
M. Inoue et al. / Vacuum 83 (2009) 658–663 [5] Chen W, Sun G, Guo J, Zhao X, Yan S, Tian J, et al. Electrochim Acta 2006;51: 2391–9. [6] Wang Z-B, Yin G-P, Shi P-F. J Power Sources 2007;163:688–94. [7] Takasu Y, Kawaguchi T, Sugimoto W, Murakami Y. Electrochim Acta 2003;48: 3861–8. [8] Han K, Lee J, Kim H. Electrochim Acta 2006;52:1697–702. [9] Rao V, Simonov PA, Savinova ER, Plaksin GV, Cherepanova SV, Kryukova GN, et al. J Power Sources 2005;145:178–87. [10] Kawaguchi T, Sugimoto W, Murakami Y, Takasu Y. J Catal 2005;229:176–84. [11] Dickinson AJ, Carrette LPL, Collins JA, Fredrich KA, Stimming U. Electrochim Acta 2002;47:3733–9.
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[12] Guo J, Sun G, Wang Q, Wang G, Zhou Z, Tang S, et al. Carbon 2006;44: 152–7. [13] Abe T, Akamaru S, Watanabe K. J Alloys Compd 2004;377:194–201. [14] Rauhe Jr BR, McLarnon FR, Cairns EJ. J Electrochem Soc 1995;142: 1073–84. [15] Schmidt TJ, Noeske M, Gasteiger HA, Behm RJ, Britz P, Bo¨nnemann H. J Electrochem Soc 1998;145:925–31. [16] Schmidt TJ, Gasteiger HA, Sta¨b GD, Urban PM, Kolb DM, Behm RJ. J Electrochem Soc 1998;145:2354–8. [17] Uchida M, Fukuoka Y, Sugawara Y, Eda N, Ohta A. J Electrochem Soc 1996;143: 2245–52.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Physical and electrochemical properties of Pt–Ru/C samples prepared on various carbon supports by using the barrel sputtering system Mitsuhiro Inoue*, Satoshi Akamaru, Akira Taguchi, Takayuki Abe Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
a b s t r a c t Keywords: Barrel sputtering system Polymer electrolyte fuel cells (PEFCs) Anode electrocatalyst Physical and electrochemical properties Specific surface area of carbon support
The present study investigated physical and electrochemical properties of carbon-supported Pt–Ru alloy (Pt–Ru/C) samples prepared on various carbon supports by using a sputtering system with barrel-type powder sample holder (the barrel sputtering system). For this system, the deposited Pt–Ru nanoparticles had uniform sizes of less than 4 nm and homogeneous atomic ratios of Pt and Ru of ca. 50:50 at.% independent of a specific surface area of the carbon support. Further, the electrochemical properties of the prepared samples obtained from CO stripping voltammetry were almost identical. These were completely different from the result for wet process showing that the physical and electrochemical properties of samples prepared by an impregnation method changed with the specific surface area of the carbon support. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction A carbon-supported Pt–Ru alloy (Pt–Ru/C) electrocatalyst with an atomic ratio of the Pt and the Ru (denoted as Pt:Ru ratio) of 50:50 at.% is a candidate of an anode electrocatalyst for practical polymer electrolyte fuel cells (PEFCs) [1]. The electrochemical property of the Pt–Ru/C electrocatalyst is closely connected to its physical property, since CO oxidation activity of a Pt–Ru alloy depends on the Pt:Ru ratio [2,3] and the sizes of the alloy particles affect an electrochemical surface area and the utilization of the Pt and the Ru for electrochemical reactions [4–6]. Some researchers have investigated electrochemical properties of Pt–Ru/C samples prepared on carbon supports with various specific surface areas (hereafter referred to simply as ’surface area’) by wet process [7–9]. The samples used have been mainly prepared by heating the carbon supports adsorbing the Pt and the Ru precursors to decompose them into the Pt–Ru alloy. However, since the heating procedure causes particle growth [10] and the decomposition temperatures of the Pt and the Ru precursors are different [10–12], the sizes and the Pt:Ru ratios of the deposited alloy particles for wet process are not uniform. This leads to the difference of the physical properties of the prepared Pt–Ru/C samples, resulting in the dependence of their electrochemical properties on the
surface area of the carbon support. Thus, samples must be prepared with the same physical properties in order to investigate the true effects of the surface area of the carbon support toward the electrochemical properties of the Pt–Ru/C electrocatalyst. Recently, the authors have developed a sputtering system with barrel-type powder sample holder (the barrel sputtering system) for modifying surfaces of powdered particles [13]. For this system, the Pt–Ru/C sample is prepared by sputtering with oscillating the polygonal barrel to stir a powdered carbon support, leading to the high dispersion of the alloy particles on the support. Further, the sizes and the Pt:Ru ratios of the deposited alloy particles can be made uniform because no heating and no precursors are needed and the atomic ratio of the sputtered Pt and Ru is constant. These are completely different from the features of the preparation of the Pt–Ru/C sample for wet process as above-mentioned, suggesting that the barrel sputtering system could prepare the Pt–Ru/C samples with the same physical properties on various carbon supports. In the present study, the Pt–Ru/C samples were prepared with various carbon supports by using the barrel sputtering system and their physical and electrochemical properties were evaluated. 2. Experimental section 2.1. Preparation of Pt–Ru/C
* Corresponding author. Present address: Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, Kamitomioka 1603-1, Nagaoka, Niigata 940-2188, Japan E-mail address: [email protected] (M. Inoue). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.042
In this study, KJB ECP600JD (surface area: 1270 m2/g; average size of primary particle: 34 nm, Ketjen), Vulcan XC72R (254 m2/g, 30 nm, Cabot), and MONARCH 280 (42 m2/g, 45 nm, Cabot) were used as carbon supports.
M. Inoue et al. / Vacuum 83 (2009) 658–663
The deposited amounts of Pt and Ru for the prepared sample were determined from the X-ray fluorescence analysis (XRF: PW2300, Philips) and the inductively coupled plasma-atomic emission spectroscopy (ICP-AES: Optima 3300XL, Perkin–Elmer). For the X-ray diffraction analysis (XRD: PW1825/00, Philips), the 2q value and the area of the diffraction peak corresponding to the Pt– Ru alloy were evaluated. The size of a Pt–Ru alloy particle was estimated by the transmission electron microscopy (TEM, JEM-2100, JEOL). The size distribution of the alloy particles was obtained by directly measuring the size of more than 100 randomly chosen particles in the magnified TEM images. The Pt:Ru ratio of the individual alloy particle was estimated by the energy dispersive X-ray analysis (EDX, JED-2300, JEOL). 2.3. Electrochemical measurement Electrochemical property of a prepared Pt–Ru/C sample was evaluated by CO stripping voltammetry at 40 C using a threecompartment electrochemical cell. A Pt wire (diameter 1 mmB 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 by fixing the sample of 40 mg on the mirror-finished glassy carbon electrode (diameter: 5 mm; geometric surface area: 0.196 cm2, Hokuto Denko) with 0.1% NafionÒ solution [15,16]. The working electrode was immersed in 1 N H2SO4 as a supporting electrolyte solution saturated with pure CO gas at 40 C for 30 min to adsorb CO completely on the surface of the alloy particles, 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. The CO stripping voltammogram was obtained with a sweep rate of 10 mV/s between 30 mV and 800 mV to prevent Ru dissolution [3,15].
3.1. Physical property of Pt–Ru/C Fig. 1 shows the XRD patterns of the prepared Pt–Ru/C samples. The Si(220) peak of 2q ¼ 47.30 was used as a standard position to decide exactly the 2q value. For W-KB (Fig. 1A), W-VX (Fig. 1B), and W-MN (Fig. 1C), the peak corresponding to the Pt–Ru alloy was observed at 2q ¼ 39.79 , 40.13 , and 40.28 , respectively. These 2q values were lower than 40.51 for the homemade bulk alloy with Pt:Ru ¼ 50:50 at.%, suggesting that the Pt:Ru ratios of the deposited alloy particles were not uniform. Further, each peak area drastically differed despite similar Pt and Ru deposited amounts, showing that the alloy particle sizes probably changed with the surface area of the carbon support. Whereas, for BS-KB (Fig. 1D), BS-VX (Fig. 1E), and BS-MN (Fig. 1F), no remarkable peak of the alloy was observed in the pattern. In a separate experiment, for the Pt–Ru/C samples with a similar XRD pattern prepared by our sputtering system, the deposited alloy particles had uniform sizes of less than 4 nm and homogeneous Pt:Ru ratios of ca. 50:50 at.%. Therefore, the sizes and the Pt:Ru ratios of the alloy particles for BS-KB, BS-VX, and BS-MN would be close to the above-mentioned results. Fig. 2 presents the typical TEM images and the size distributions of the alloy particles for the prepared samples. In all TEM images, the alloy nano-particles represented as black dots were observed on the carbon support as gray. The sizes of the alloy particles for WKB (Fig. 2A), W-VX (Fig. 2B), and W-MN (Fig. 2C) were distributed from 0.8 nm to 7.2 nm (particle count [n] ¼ 183), 0.8 nm to 12.8 nm (n ¼ 174), and 1.2 nm to 32.0 nm (n ¼ 126), respectively. These results indicate that the sizes of the alloy particles deposited by the impregnation method differed with the surface area of the carbon support. The Pt:Ru ratios of the individual alloy particles for W-KB, W-VX, and W-MN obtained by the EDX measurements widely deviated at 24.7:75.3–100.0:0.0 at.% (particle count [n] ¼ 26), 0.4:99.6–98.5:1.5 at.% (n ¼ 28), and 12.8:87.2–80.6:19.4 at.% (n ¼ 25). In addition, for W-KB, while the sizes of the alloy particles
Ru (100) 300
Ru (002)
Pt (111)
Pt (200) Ru (101)
intensity / cps
2.2. Physical characterization
3. Results and discussion
Si (220)
A B
C
300
intensity / cps
The details of the barrel sputtering system have been reported [13]. The Pt–Ru/C samples with Pt:Ru ¼ ca. 50:50 at.% were prepared as follows. The metal plates of Pt and Ru (purity 99.99%, size W 25 mm L 100 mm) were used together as a target and their surface area ratio was adjusted at 6:10, resulting in an atomic ratio of the sputtered Pt and Ru of 52:48 at.%. Prior to sputtering, the air in a vacuum chamber that contained the hexagonal or octagonal barrel with a carbon support was evacuated at less than 8 104 Pa. The sputtering was performed under optimum conditions (Ar gas (purity 99.9999%) pressure: 0.8 Pa; temperature: room temperature; and AC power: 50 W) to deposit the alloy particles with the uniform sizes and the homogeneous Pt:Ru ratios. Sputtering time was controlled to maintain similar Pt and Ru deposited amounts. On sputtering, the barrel was oscillated (intervals and amplitude: 14 s and 75 (hexagonal) and 11 s and 60 (octagonal)) with a mechanical vibration. The Pt and the Ru deposited amounts of the prepared samples were obtained at 8.0 wt.%, 4.1 wt.% (KJB ECP600JD, denoted as BS-KB), 9.1 wt.%, 4.6 wt.% (Vulcan XC72R, BS-VX), and 8.2 wt.%, 4.3 wt.% (MONARCH 280, BS-MN), respectively. The Pt–Ru/C samples for wet process were also prepared by an impregnation method for comparison [7,10,14]. H2PtCl6$6H2O (Kanto Chemical) and RuCl3 (purity 95.1%, Kanto Chemical) were used as precursors. The sample was prepared by heating a carbon support adsorbing the precursors at 350 C for 3 h under flowing 10% H2 in N2 gas. The amounts of Pt and Ru deposited were 9.6 wt.%, 4.2 wt.% (KJB ECP600JD, denoted as W-KB), 8.9 wt.%, 4.9 wt.% (Vulcan XC72R, W-VX), and 8.0 wt.%, 4.5 wt.% (MONARCH 280, WMN), respectively.
659
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2 / degree Fig. 1. XRD patterns of the Pt–Ru/C samples prepared by the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN) ((-$-$) Ru peak position, (/) Pt peak position, and (- - -) Si(220) peak (2q ¼ 47.30 )).
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M. Inoue et al. / Vacuum 83 (2009) 658–663
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4
8
12
16
20
24
28
32
28
32
particle size / nm
B
distribution / %
15
10 n = 174 5
20 nm 0
4
8
12
16
20
24
particle size / nm
C distribution / %
10
20 nm
n = 126
5
0
4
8
12
16
20
24
28
32
particle size / nm Fig. 2. Typical TEM images and particle size distributions of the Pt–Ru/C samples prepared by the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN) (accelerating voltage: 200 kV; magnification factor: 400,000; and n: particle count).
with Pt rich ratios tended to be measured at more than 4 nm, the ones with Ru rich ratios were obtained at less than 4 nm, possibly implying the dependence of the Pt:Ru ratio on particle size. On the other hand, for the barrel sputtering system, the sizes of the alloy particles were obtained at less than 4 nm, as shown in Fig. 2(D)–(F). The Pt:Ru ratios of each alloy particle for BS-KB, BS-VX, and BS-MN were homogeneous at 50.4:49.6 (7.2) at.% (particle count [n] ¼ 32), 51.9:48.1 (6.3) at.% (n ¼ 27) and 52.6:47.4 (5.7) at.% (n ¼ 28), respectively. The above observations indicate that the sizes and the Pt:Ru ratios of the alloy nano-particles for the barrel sputtering system were almost identical independent of the surface area of the carbon support. It should be noted that the surface area of the carbon support changes with the amount of pores of less than 8 nm [17]. Thus, the deposition of the alloy particles in the
pores is possibly concerned with the dependence of physical properties of the Pt–Ru/C samples on the surface area of the carbon support. 3.2. Electrochemical property of Pt–Ru/C Fig. 3 exhibits the CO stripping voltammograms of the Pt–Ru/C samples for the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN). Inset is the coulomb charge of CO oxidation per amount of Pt and Ru (denoted as Q0 in mC/mgPtþRu) calculated from the area represented with a slash versus the surface area of the carbon support. For the impregnation method, a broad peak corresponding to CO oxidation was observed. The onset and the
M. Inoue et al. / Vacuum 83 (2009) 658–663
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30
distribution / %
D
20
n = 121 10
20 nm 0
4
8
12
16
20
24
28
32
28
32
28
32
particle size / nm
E distribution / %
30
20 n = 128 10
20 nm 0
4
8
12
16
20
24
particle size / nm
F distribution / %
30
20 n = 119 10
20 nm 0
4
8
12
16
20
24
particle size / nm Fig. 2. (Continued)
peak potentials (vs. RHE) of CO oxidation were measured at 444 mV, 594 mV (W-KB), 444 mV, 509 mV (W-VX), and 389 mV, 474 mV (W-MN), indicating that CO oxidation activity of the samples prepared by the impregnation method changed with the surface area of the carbon support [2,3]. The respective Q0 values were estimated to be 0.62 mC/mgPtþRu, 0.46 mC/mgPtþRu, and 0.28 mC/mgPtþRu, decreasing in the surface area of the carbon support, as shown in the inset. This implies that the utilization of the Pt and the Ru for electrochemical reactions declined with the surface area of the carbon support since the Q0 represents the electrochemical surface area per amount of Pt and Ru [5,6]. The difference of the electrochemical properties of the prepared samples as above-mentioned was attributed to the sizes and the Pt:Ru ratios of the deposited alloy particles. In contrast, for the
barrel sputtering system, the peak shape of CO oxidation was sharp, as shown in Fig. 3(D)–(F). The onset and the peak potentials of CO oxidation were almost identical at 383 mV, 443 mV (BS-KB), 384 mV, 449 mV (BS-VX), and 389 mV, 449 mV (BS-MN), respectively. The Q0 values of each sample were also almost constant at 0.67 mC/mgPtþRu, 0.65 mC/mgPtþRu, and 0.65 mC/mgPtþRu independent of the surface area of the carbon support (inset). These indicate that for the Pt–Ru/C samples with the same physical properties, CO oxidation activity and the utilization of the Pt and the Ru were uninfluenced by the surface area of the carbon support. It should be noted that the peak potentials of CO oxidation and the Q0 values for the barrel sputtering system were lower and larger than those for the impregnation method, demonstrating that the electrochemical properties of the prepared
0.8
Q0 value / mC/ gPt+Ru
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A B C
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F
E
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0.0 10
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D
125
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10000
1000
surface area of the carbon support / m2/g
A
125
0.8
150
current / A
current / A
0
B
150
E
0
0
120
120
F
C
0
0
0
200
400
600
800
potential / mV vs. RHE
0
200
400
600
800
potential / mV vs. RHE
Fig. 3. CO stripping voltammograms of the Pt–Ru/C samples prepared by the impregnation method (A: W-KB; B: W-VX; and C: W-MN) and the barrel sputtering system (D: BS-KB; E: BS-VX; and F: BS-MN) (solid line: first scan; broken line: second scan; and inset: Q0 value versus surface area of carbon support) (measuring conditions: supporting electrolyte, 1 N H2SO4; temperature, 40 C; sweep rate, 10 mV/s; CO adsorption, 30 min; and N2 purge, 30 min (potential: 70 mV vs. RHE)).
samples by our dry process were superior to those by the conventional wet process. 4. Conclusion Summarizing, the barrel sputtering system could prepare the Pt–Ru/C samples with same physical properties on various carbon supports. Further, these samples had almost identical electrochemical properties, which were superior to those for the impregnation method. These reveal that the electrochemical properties of the Pt–Ru/C samples with the same physical properties are independent of the surface area of the carbon support.
Acknowledgement This work was partially supported by a grant-in-aid for Science Research from the Ministry of Education, Science, Sports and Culture of Japan. References [1] Qi Z, Kaufman A. J Power Sources 2003;113:115–23. [2] Gasteiger HA, Markovic´ N, Ross Jr PN, Cairns EJ. J Phys Chem 1994;8:617–25. [3] Kabbabi A, Faure R, Durand R, Beden B, Hahn F, Leger J-M, et al. J Electroanal Chem 1998;444:41–53. [4] Radmilovic´ V, Gasteiger HA, Ross Jr PN. J Catal 1995;154:98–106.
M. Inoue et al. / Vacuum 83 (2009) 658–663 [5] Chen W, Sun G, Guo J, Zhao X, Yan S, Tian J, et al. Electrochim Acta 2006;51: 2391–9. [6] Wang Z-B, Yin G-P, Shi P-F. J Power Sources 2007;163:688–94. [7] Takasu Y, Kawaguchi T, Sugimoto W, Murakami Y. Electrochim Acta 2003;48: 3861–8. [8] Han K, Lee J, Kim H. Electrochim Acta 2006;52:1697–702. [9] Rao V, Simonov PA, Savinova ER, Plaksin GV, Cherepanova SV, Kryukova GN, et al. J Power Sources 2005;145:178–87. [10] Kawaguchi T, Sugimoto W, Murakami Y, Takasu Y. J Catal 2005;229:176–84. [11] Dickinson AJ, Carrette LPL, Collins JA, Fredrich KA, Stimming U. Electrochim Acta 2002;47:3733–9.
663
[12] Guo J, Sun G, Wang Q, Wang G, Zhou Z, Tang S, et al. Carbon 2006;44: 152–7. [13] Abe T, Akamaru S, Watanabe K. J Alloys Compd 2004;377:194–201. [14] Rauhe Jr BR, McLarnon FR, Cairns EJ. J Electrochem Soc 1995;142: 1073–84. [15] Schmidt TJ, Noeske M, Gasteiger HA, Behm RJ, Britz P, Bo¨nnemann H. J Electrochem Soc 1998;145:925–31. [16] Schmidt TJ, Gasteiger HA, Sta¨b GD, Urban PM, Kolb DM, Behm RJ. J Electrochem Soc 1998;145:2354–8. [17] Uchida M, Fukuoka Y, Sugawara Y, Eda N, Ohta A. J Electrochem Soc 1996;143: 2245–52.