pubs.acs.org/Langmuir © 2010 American Chemical Society
Effect of Surface Segregation on the Methanol Oxidation Reaction in Carbon-Supported Pt-Ru Alloy Nanoparticles Tae-Yeol Jeon,† Kug-Seung Lee,‡ Sung Jong Yoo,‡ Yong-Hun Cho,§ Soon Hyung Kang,† and Yung-Eun Sung*,† † School of Chemical & Biological Engineering and Research Center for Energy Conversion & Storage, Seoul National University, Seoul 151-744, South Korea, ‡Fuel Cell Center, Korea Institute of Science and Technology, Seoul 136-791, South Korea, and §School of Advanced Materials Engineering, Kookmin University, 861-1 Jeongneung-dong, Seongbuk-gu, Seoul 136-702, South Korea
Received December 31, 2009. Revised Manuscript Received March 26, 2010 Ru and Pt-Ru (Pt:Ru = 1:1) nanoparticles supported on carbon black were prepared by the borohydride reduction method using oleylamine as a stabilizer in an anhydrous ethanol solvent. We investigated the effect of Pt segregation to the surface of alloy nanoparticles on the methanol oxidation reaction (MOR). As-prepared Pt1Ru1/C showed a narrow size distribution and a relatively uniform particle distribution on a carbon support. However, its electrocatalytic activity toward the MOR was poor due to the high surface concentration of Ru. As duration time of heat treatment at 200 °C was increased up to 2 h, the surface composition of Pt atoms was increased without significant particle growth due to thermally induced segregation of Pt atoms, which were revealed by TEM images, X-ray photoelectron spectroscopy (XPS) analysis, changes in the potentials of zero total charge (pztc), and increase in the oxidation charge of “reduced CO2”. In particular, from the combination of CO adlayer oxidation and “reduced CO2” oxidation charges, the increased surface concentration of Pt of alloy catalysts was relatively quantified when compared to its as-prepared state. Cyclic voltammograms in 0.1 M HClO4 solution with 0.5 M methanol showed that Pt1Ru1/C annealed for 2 h at 200 °C in a flow of mixture gas of Ar and H2 (5 vol %) had a less positive onset potential for the MOR. These results demonstrate a definitive contribution from segregation of Pt atoms to the MOR activity.
1. Introduction One metal component alloyed with another may enrich the surface region depending on the heat of segregation and the surface mixing energy.1 This segregation phenomenon is of interest as it may enhance or suppress electrocatalytic reactions.2,3 For catalysts in fuel cells, Pt has become the most widely used electrocatalyst and has been applied in the type of nanoalloys used for large electrochemical active surface area (EAS) and for maximum activity of an electrocatalytic reaction; therefore, surface segregation of Pt is of vital importance in electrocatalysis. In preparation of Pt-based alloy nanoparticles, when dissolved Pt ions are coreduced with precursors of other metals, such as Ni, Co, and Ru, by various reducing agents, one observes the low surface composition of Pt, even in the presence of stabilizers, mainly due to the higher redox potential of Pt relative to other transition metals, with the exception of Au.4,5 Discrepancy between surface and bulk compositions in as-prepared nanoalloys may be related to many factors, such as a redox priority of metal precursors,6 surface energy of each metal component,7,8 and *Corresponding author: Tel þ82-2-880-1889; Fax þ82-2-888-1604; e-mail
[email protected]. (1) Christensen, A.; Ruban, A. V.; Stoltze, P.; Jacobsen, K. W.; Skriver, H. L.; Norskov, J. K.; Besenbacher, F. Phys. Rev. B 1997, 56, 5822. (2) Ma, Y. G.; Balbuena, P. B. Surf. Sci. 2008, 602, 107. (3) Mayrhofer, K. J. J.; Juhart, V.; Hartl, K.; Hanzlik, M.; Arenz, M. Angew. Chem., Int. Ed. 2009, 48, 3529. (4) Hwang, B. J.; Kumar, S. M. S.; Chen, C. H.; Monalisa; Cheng, M. Y.; Liu, D. G.; Lee, J. F. J. Phys. Chem. C 2007, 111, 15267. (5) Park, S.; Wieckowski, A.; Weaver, M. J. J. Am. Chem. Soc. 2003, 125, 2282. (6) Cotton, F. A.; Daniels, L. M.; Liu, C. Y.; Murillo, C. A.; Schultz, A. J.; Wang, X. P. Inorg. Chem. 2002, 41, 4232. (7) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. J. Mol. Catal. A 1997, 115, 421. (8) Gautier, F.; Llois, A. M. Surf. Sci. 1991, 245, 191.
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interaction with various adsorbing species present during a preparation procedure.9-11 A surface with low Pt concentration results in low catalytic activity due to decreased EAS and the increased number of the second metal, which binds strongly with oxygen-containing species.12 When using proper stabilizers that induce stronger binding with Pt relative to the second metal, enrichment of the second metal in the surface can be suppressed. However, observance of the higher surface concentration of Pt relative to the nominal value is difficult. It is therefore required that the surface concentration of Pt be increased by use of several techniques, such as thermal heating in an appropriate atmosphere13-15 or chemical leaching of the non-noble metal16-19 in the surface layer. Heat treatment can be used not only for removing residual impurities and unwanted oxides but also for segregating Pt atoms at the surface. Pt segregation by heat treatment can be a novel method for improving undesirable surface composition. (9) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (10) Chakroune, N.; Viau, G.; Ammar, S.; Poul, L.; Veautier, D.; Chehimi, M. M.; Mangeney, C.; Villain, F.; Fievet, F. Langmuir 2005, 21, 6788. (11) Qiu, L. M.; Liu, F.; Zhao, L. Z.; Yang, W. S.; Yao, J. N. Langmuir 2006, 22, 4480. (12) Bligaard, T.; Norskov, J. K. Electrochim. Acta 2007, 52, 5512. (13) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (14) Christoffersen, E.; Stoltze, P.; Norskov, J. K. Surf. Sci. 2002, 505, 200. (15) Stamenkovic, V. R.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2006, 128, 8813. (16) Koh, S.; Leisch, J.; Toney, M. F.; Strasser, P. J. Phys. Chem. C 2007, 111, 3744. (17) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9. (18) Srivastava, R.; Mani, P.; Hahn, N.; Strasser, P. Angew. Chem., Int. Ed. 2007, 46, 8988. (19) Chen, S.; Ferreira, P. J.; Sheng, W. C.; Yabuuchi, N.; Allard, L. F.; Shao-Horn, Y. J. Am. Chem. Soc. 2008, 130, 13818.
Published on Web 04/08/2010
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Numerous theoretical studies have been carried out to elucidate the driving force and the tendency of Pt-based alloys toward surface segregation.1,7,14,20-29 First-principles approaches are usually implemented based on density functional theory (DFT).1,7,14,23-29 Using this method, trends in segregation energies in transition metal alloys have been reported.1,14,24,28,29 Christensen et al.1 constructed surface phase diagrams from surface energy as a function of the surface composition and calculated the heat of segregation and the surface mixing energy. In the same line with Christensen et al., Ruban et al.7,24 improved the accuracy of theoretical surface segregation energies and tabulated the qualitative picture of segregation energies containing various solvent and solute transition metals. According to their results, late-group transition metals as solute in the host of earlygroup transition metals usually show negative values. In addition, in the hosts of Fe, Ru, and Os, noble metal solutes with more than half-filled d-bands (e.g., Pd, Pt, Ag, and Au, etc.) have more negative segregation energies for lower surface energy, dependent mainly on the amount of surface core-level shifts (SCLSs). Recently, Greeley et al.30 showed that trends in the thermodynamics of surface alloy dissolution in acidic media generally follow trends in surface segregation energies and that the dissolution potential of Pt in “Pt skins” produced by surface segregation of Pt3M alloys (M = Fe, Co, or Ni) is higher than that of Pt3M alloy surfaces. The increased dissolution potential of “Pt skin” surface is consistent with the positive shift of OH adsorption to higher potential in the findings of Stamenkovic et al.31 for Pt3Ni(111). Therefore, surface segregation energy is one of the most important factors to be considered in the initial choice of alloy systems. Surface segregation of Pt can be of significance in electrocatalytic activities such as the methanol oxidation reaction (MOR)13 and oxygen reduction reaction (ORR)15,31 because surface Pt atoms are active sites for electrochemical reactions. Furthermore, modification of d-character of surface Pt atoms by segregation is normally desirable for stable electrode materials with higher dissolution potential in fuel cells30 because the surface energy of the alloy can be lowered by Pt segregation to the surface, as mentioned above. Ma et al.28 reported Pt surface segregation energies for 16 metals. In their results, the surface segregation energy for Pt3Ru(111) alloys is -0.83 eV. The value is more negative when compared to that of other transition metals, with the exception of Re and Mo. This means that Pt enrichment at the surface can readily occur in the Pt-Ru alloy system. Experimental results of Pt segregation in several alloy systems, including Pt-Ru, have already been reported.13,14 Despite this, electrochemical measurements of Pt segregation in carbon-supported Pt-Ru nanoparticles were scarce. CO tolerance and electrocatalytic activity of the MOR can be improved by alloying Pt with Ru. Ru is highly oxophilic and, (20) Chelikowsky, J. R. Surf. Sci. 1984, 139, L197. (21) Treglia, G.; Legrand, B. Phys. Rev. B 1987, 35, 4338. (22) Foiles, S. M.; Baskes, M. I.; Daw, M. S. Phys. Rev. B 1986, 33, 7983. (23) Schmid, M.; Hofer, W.; Varga, P.; Stoltze, P.; Jacobsen, K. W.; Norskov, J. K. Phys. Rev. B 1995, 51, 10937. (24) Ruban, A. V.; Skriver, H. L.; Norskov, J. K. Phys. Rev. B 1999, 59, 15990. (25) Lovvik, O. M. Surf. Sci. 2005, 583, 100. (26) Wang, G. F.; Van Hove, M. A.; Ross, P. N.; Baskes, M. I. J. Chem. Phys. 2004, 121, 5410. (27) Gonzalez, S.; Neyman, K. M.; Shaikhutdinov, S.; Freund, H. J.; Illas, F. J. Phys. Chem. C 2007, 111, 6852. (28) Ma, Y. G.; Balbuena, P. B. Surf. Sci. 2008, 602, 107. (29) Menning, C. A.; Chen, J. G. G. J. Chem. Phys. 2009, 130, 174709. (30) Greeley, J.; Norskov, J. K. Electrochim. Acta 2007, 52, 5829. (31) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (32) Falsig, H.; Hvolbaek, B.; Kristensen, I. S.; Jiang, T.; Bligaard, T.; Christensen, C. H.; Norskov, J. K. Angew. Chem., Int. Ed. 2008, 47, 4835.
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therefore, dissociates adsorbed water into OHads and H at relatively low potential.32,33 The high activity of Pt-Ru alloy is attributed to bifunctional and electronic effects.33 For heterogeneous catalysis, bond formation between reactants (i.e., adsorbates and their intermediates) and electrodes is strongly dependent on the nature of the electrode materials, i.e., d-character of transition metals and their alloys.34 However, it has been proven that the bifunctional mechanism is dominant in the MOR, in which COads can be oxidized to CO2 only when adsorbed oxygencontaining species exist on the surface of catalysts; therefore, water activation by the second metal (i.e., Ru) is more important.35,36 It is worthy of note that methanol electrooxidation on Pt-Ru nanoparticles shows a volcano plot with increase of Ru content, as reported by Rigsby et al.36 According to their results, the maximum current density at a nominal composition of about 20-30% Ru cannot be explained by the electronic effect alone. Ru and Pt-Ru alloy nanoparticles are typically synthesized by a polyol reduction method,37-40 which utilizes hydrogenation of a polyol solvent, thereby reducing metal precursors.39 However, full reduction of metal precursors is difficult due to the weak reducing ability and bidentate chelating property41 of polyols. For this reason, sodium hydroxide (NaOH), which also acts as an enhancer of polyol’s dehydrogenation reaction and a charge stabilizer for nanoparticles of the desired size, is typically used. Unwanted oxidation of nanoparticles originated mainly from chemicals such as NaOH in the solution.42 It is well-known that RuO2 is a very good catalyst for oxygen evolution.43 This means that the free energy of the intermediates of the water-splitting reaction on RuO2 is lower than that of the metallic Ru surface. However, oxidized Ru atoms may be unfavorable to COads oxidation reaction in the bifunctional mechanism, and may cause slower MOR rates, because oxidation of Ru atoms can induce dealloying of Pt and Ru. In other words, since Ru can be easily oxidized by oxygen-containing species, it may lead to phase separation in Pt-Ru alloy and the Ru-rich surface.14,44 In this study, pure Ru and Pt1Ru1 (40 wt % of nanoparticles on Vulcan XC-72R carbon support) were synthesized by the NaBH4 reduction method using oleylamine as a stabilizer in an anhydrous ethanol solvent. Both Ru and Pt-Ru nanoparticles showed high dispersion and narrow size distribution. Our synthetic method is new and relatively simplified compared to other chemical routes for preparing Ru-based alloy nanoparticles.37-40 Synthesis of Ru nanoparticles on a carbon support was targeted for comparative studies with Pt-Ru alloy nanoparticles. In order to activate Pt segregation, as-prepared Ru/C and Pt-Ru/C were heated in a (33) Wang, J. G.; Hammer, B. J. Catal. 2006, 243, 192. (34) Bligaard, T.; Norskov, J. K. Electrochim. Acta 2007, 52, 5512. (35) Babu, P. K.; Kim, H. S.; Kuk, S. T.; Chung, J. H.; Oldfleld, E.; Wieckowski, A.; Smotkin, E. S. J. Phys. Chem. B 2005, 109, 17192. (36) Rigsby, M. A.; Zhou, W. P.; Lewera, A.; Duong, H. T.; Bagus, P. S.; Jaegermann, W.; Hunger, R.; Wieckowski, A. J. Phys. Chem. C 2008, 112, 15595. (37) Liu, Z. L.; Ling, X. Y.; Lee, J. Y.; Su, X. D.; Gan, L. M. J. Mater. Chem. 2003, 13, 3049. (38) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; Fievet-Vincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486. (39) Bock, C.; Paquet, C.; Couillard, M.; Botton, G. A.; MacDougall, B. R. J. Am. Chem. Soc. 2004, 126, 8028. (40) Liu, Z. F.; Ada, E. T.; Shamsuzzoha, M.; Thompson, G. B.; Nikles, D. E. Chem. Mater. 2006, 18, 4946. (41) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. Rev. 2004, 104, 3893. (42) Shengming, J.; Liangsheng, Y.; Ying, Z.; Guanzhou, Q.; Cuifeng, W. Mater. Res. Bull. 2006, 41, 2130. (43) Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Norskov, J. K. J. Electroanal. Chem. 2007, 607, 83. (44) Lewera, A.; Zhou, W. P.; Vericat, C.; Chung, J. H.; Haasch, R.; Wieckowski, A.; Bagus, P. S. Electrochim. Acta 2006, 51, 3950.
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flow of Ar þ H2 (5 vol %) mixed gas. These catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), including electrochemical characterization techniques, such as cyclic voltamograms in HClO4 solution with and without methanol. For detection of Pt segregation to the surface of alloy nanoparticles, “reduced CO2” oxidation and measurements of potentials of zero charge (pztc) were selected.
2. Experimental Section 2.1. Preparation and Characterization of Electrocatalysts. Carbon-supported Ru and Pt1Ru1 alloy nanoparticles were prepared via conventional borohydride reduction in anhydrous ethanol. All chemicals, with the exception of oleylamine (C18H37N, TCI), were obtained from Aldrich and were of analytical grade. Pt-Ru (40 wt %) alloy and pure Ru (40 wt %) nanoparticles were prepared through the same chemical route. Pt-Ru alloy nanoparticles were prepared as follows: 0.33 uL of oleylamine (1.03 mmol) was added to anhydrous ethanol (200 mL), and 0.15 g of carbon black (Vulcan XC-72R) was then dispersed in the solution. The mixed solvent was stirred for 30 min, followed by sonication for 30 min. 0.07 g (0.34 mmol) of PtCl4 and 0.11 g (0.34 mmol) of RuCl3 dissolved in anhydrous ethanol were added to the mixed solution, followed by constant mechanical stirring for 6 h. After additional sonication for 3 min, 0.13 g (3.4 mmol, 5 times the amount of total metals in a mole) of NaBH4 dissolved in 20 mL of anhydrous enthanol was quickly added with vigorous stirring. The mixture (300 mL) was stirred for 6 h in order to complete reduction of metal precursors and followed by filtration, washing with ethanol (purity 95%), and drying in a vacuum oven at 40 °C. Pure Ru nanoparticles supported on carbon black were prepared by the same method described above, except for the amount of oleylamine (0.08 mL, 0.25 mmol). In order to segregate Pt atoms to the surface of Pt-Ru alloy nanoparticles, heat treatment of as-prepared Pt-Ru alloy nanoparticles was performed at 200 °C in a flow of Ar þ 5 vol % H2 mixed gas. Prepared Pt-Ru electrocatalyst powder was examined by X-ray diffraction (XRD, Rigaku D/MAX 2500) with Cu KR radiation (40 kV, 200 mA). Size distribution, particle shape, and distribution were confirmed by high-resolution transmission electron microscopy (HR-TEM, JEOL 2010 at 200 kV). X-ray photoelectron spectra (XPS) were obtained from an Al KR source (Sigma probe, VG Scientifics). Binding energies were calibrated by referencing C 1s at 285 eV. Experimental data were curve-fitted using XPSPEAK4.1 software. Atomic ratios of different states were estimated from the area of the respective LorentzianGaussian peaks. 2.2. Electrochemical Measurements. The catalyst ink slurry was prepared by mixing carbon-supported nanoparticles with 200 μL of DI water, 572 μL of 5 wt % Nafion solution as a binding material, and 8 mL of 2-propanol per 0.1 g of electrocatalyst. Following mixing and ultrasonication, 7 μL of ink slurry was pipetted onto a glassy carbon substrate (geometric surface area, 0.283 cm2), leading to a metal loading of ca. 102 μgPtRu/cm2. The dried electrode was then transferred to the electrochemical cell, and cyclic voltammograms were recorded in argon-saturated 0.1 M HClO4. The pztc was calculated from the voltammetric contribution of specifically adsorbed anions that were displaced by CO as a neutral probe. The CO-displacement method described by Orts et al.45 was employed. Briefly, CO was introduced into the Ar-purged 0.1 M HClO4 electrolyte at a constant potential in which perchlorate ions were the dominant species adsorbed on the electrode surface. The resulting displacement current was (45) Orts, J. M.; Gomez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1994, 39, 1519.
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Article recorded versus time and then calculated into charge by integration. In order to calculate the pztc, the CO-displacement charge was subtracted from the so-called Hupd charge that was integrated from 0.15 V (CO dosing potential) to 0 V vs NHE. The pztc is a potential when an integrated charge of a cyclic voltammogram is equal to the CO-displacement charge. COads stripping voltammograms were followed by a similar procedure for CO-displacement, except for the potential for adsorption of CO. Potentials for displacement of adsorbed species with CO were 0.15 and 0.05 V in the pztc and the CO-stripping voltammograms, respectively. In order to detect Pt segregation to the surface, “reduced CO2” oxidation was chosen as an electrochemical method for distinguishing changes in the ratio of surface Pt and Ru atoms. As reported by Czerwi nski et al.,46 CO2 reacts with hydrogen adsorbed at a potential more positive than the reversible hydrogen potential and produces various adsorbates, such as CO, COH, CHO, and COOH radicals only on the surface of Pt and Rh among various transition metals. Using this selective adsorption of a reduced form of CO2, increase of the surface concentration of Pt in the surface of alloy nanoparticles was calculated. The measurement procedure for “reduced CO2” oxidation cyclic voltammogram was refererenced from results reported by Czerwi nski et al.47 In short, experiments were performed in a 0.1 M HClO4 solution at room temperature. The solution was deaerated with argon bubbling, followed by saturation with CO2 (purity 99.999%) gas for 20 min at a potential in the double-layer region, i.e. 0.35 V. After the electrolyte was saturated with CO2, the potential was switched to 0.01 V and maintained for 20 min for the reaction of adsorbed hydrogen and CO2. After that, saturated CO2 was removed from the electrolyte with an argon stream for 20 min, and then a cyclic voltammogram was measured. Electrochemical measurements were carried out using an Autolab potentiostat (PGSTAT128N) in a standard three-compartment electrochemical cell with a glassy carbon electrode, Pt wire, and saturated calomel electrode (SCE) as working, counter, and reference electrodes, respectively. All potentials are quoted with respect to normal hydrogen electrode (NHE), and measurements were conducted at room temperature.
3. Results and Discussion 3.1. Preparation and Characterization of Carbon-Supported Pt-Ru Alloy Nanoparticles. The conventional NaBH4 method using oleylamine as a stabilizer in anhydrous ethanol was used for preparation of 40 wt % Ru/C. X-ray powder diffraction patterns of as-prepared and heat-treated Ru nanoparticles are shown in Figure 1. The curve denoted by a dotted line represents as-prepared Ru/C, which has an amorphous structure. Figure 1S of the Supporting Information shows electron diffraction (ED) images of (a) as-prepared and (b) heat-treated Ru/C. While the asprepared Ru/C demonstrates characteristics of an amorphous structure, the heat-treated sample shows clear rings resulting from a polycrystalline structure. In order to change the poor crystallinity to the ordered structure of metallic Ru, heat treatment in a flow of Ar þ H2 (5 vol %) mixed gas was introduced. As can be seen in Figure 1, temperature of 200 °C is sufficient for our purpose, i.e., Pt segregation to the surface of nanoparticles while minimizing sintering. Heat treatment of 300 °C resulted in increased intensity and decreased full width at half-maximum (fwhm). This indicates that the temperature of 300 °C is too high for as-prepared Ru nanoparticles to become ordered with a minimization of sintering. Using a reference to the XRD results of pure Ru/C, we prepared 40 wt % Pt1Ru1/C catalysts by the same method as (46) Sobkowski, J.; Czerwinski, A. J. Phys. Chem. 1985, 89, 365. (47) Grden, M.; Paruszewska, A.; Czerwinski, A. J. Electroanal. Chem. 2001, 502, 91.
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Figure 1. Powder XRD patterns of as-prepared and heat-treated 40 wt % Ru/C at 200 °C (2 h) and 300 °C (1 h).
Figure 2. XRD profiles of 40 wt % Pt1Ru1 alloy nanoparticles supported on a carbon black before and after heat treatment at 200 °C for 1 and 2 h. The bottom figure with black and gray lines represents diffraction patterns of the fcc Pt phase and hcp Ru phase, respectively.
that used in synthesis of pure Ru/C. Figure 2 shows XRD powder scans of as-prepared and heat-treated Pt-Ru electrocatalysts with different durations of heat treatment. The first peaks of Pt-Ru alloys were shown at 40.1° in all XRD patterns, regardless of heat treatment. That is, peaks of Pt-Ru alloy nanoparticles shifted to higher angles when compared to those of pure Pt (peak positions of pure Pt and Ru can be seen in the below indication of Figure 2), thereby indicating formation of alloy with facecentered cubic (fcc) structure and contraction of the lattice upon substitution of Pt with Ru. It should be noted that Ru(101) and Ru(102) were detected, and their intensities increased as duration of heating increased. This means that as heating time increases, phase separation between Pt and Ru becomes larger. Pure Ru/C and Pt1Ru1/C were characterized by HR-TEM, as shown in Figure 3. Pure Ru nanoparticles are highly dispersed on the surface of the carbon support, as shown in Figure 3a. The average diameter of Ru particles was 1.9 nm. To the best of our knowledge, the uniform interparticle distance and narrow size distribution of Ru nanoparticles on a carbon support have never been achieved by use of a chemical reduction method. Moreover, experimental results on preparation of Ru/C by NaBH4 reduction 9126 DOI: 10.1021/la9049154
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were difficult to find. Figures 3b-e show as-prepared Pt1Ru1/C and samples heated for 1, 2, and 5 h at 200 °C, respectively; the isolated Pt-Ru nanoparticles have an average diameters of ca. 2.7 nm (see Figure S2 in the Supporting Information). Compared to as-prepared Pt-Ru, however, heat-treated samples commonly show agglomerated particles. The fact that average diameter of the isolated alloy nanoparticles is nearly constant supports that the agglomeration by heating arises from contact between particles. From these TEM images, particle growth by sintering seems to be completed within the first 1 h of heating. Determination of Pt segregation from structural and morphological analysis, such as XRD and TEM, is difficult. Therefore, we performed electrochemical analysis such as calculations of pztc and Pt surface composition by measuring the oxidation charges of the adsorbed CO and CO2 as well as the methanol oxidation cyclic voltammograms, as discussed in the next section. 3.2. Electrochemical Characterization. Mass-specific surface area (SM)48 was calculated by Hupd and COads oxidation charges, as was listed and plotted in Table 1 and Figure S3 (see the Supporting Information). With the exception of the case of the asprepared state with low CO2 charge, SM calculated from the Hupd and COads oxidation charges showed a steady increase. For monometallic nanoparticles, it is expected that a small increase in particle size results in decreased SM. The Pt-Ru alloy, however, showed that SM was increased with duration time of heat treatment. The low active area of the as-prepared one might have originated from the strong interaction of the Ru-rich surface with oxygen or water and the resulting low COads coverage. Because theoretical binding energy of H or CO to Ru is higher than that of Pt,32 metallic Ru atoms in the surface layer strongly bind with H and CO. Therefore, the increase in the surface composition of Pt cannot significantly contribute to increased hydrogen coverage. However, it may result in increased CO coverage induced by the increased number of free sites for adsorption of CO because oxygen-contaning species show strong chemisorptions on Ru, and CO must compete with anions and oxygen-containing species for occupying free sites,49 whereas Pt is relatively free from surface oxidation due to its intrinsic properties, such as the highest work function among transition metals, with the exception of Au. Figure 4 shows cyclic voltammograms for methanol oxidation on Pt1Ru1/C with three different states: as-prepared and heattreated samples for 1 and 2 h. All currents were divided by the total mass of Pt and Ru, the so-called mass-specific current density. MOR activities of the three Pt1Ru1/C catalysts with different post-treatments showed obvious differences. The asprepared sample was nearly inactive in the mixed solution of 0.1 M HClO4 with 0.5 M CH3OH. Samples heated for 1 h showed greatly enhanced current densities relative to the as-prepared one. Further heating from 1 to 2 h induced a negative shift in onset potentials of the MOR and the highest current density. Consequently, heat treatment of Pt-Ru alloy nanoparticles resulted in a thermodynamically favorable electrocatalyst with MOR kinetics faster than that of its as-prepared state. This enhancement may originate from Pt segregation to the surface and/or removal of oxygen bound with Pt or Ru by heat treatment under Ar and H2. The latter was checked by XPS. In short, as discussed with XPS data in the next section, the metallic Ru was decreased by a small fraction after heat treatment. Since no evidence regarding the increase of Ru(0) was obtained from XPS data, it should be (48) Park, I. S.; Lee, K. S.; Choi, J. H.; Park, H. Y.; Sung, Y. E. J. Phys. Chem. C 2007, 111, 19126. (49) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 16757.
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Figure 3. TEM images of carbon-supported Ru and Pt-Ru nanoparticles: (a) as-prepared Ru, (b) as-prepared Pt1Ru1, and Pt1Ru1 alloys heated for (c) 1 h, (d) 2 h, and (e) 5 h. Table 1. Experimentally Determined Active Surface Area and Charges from Hupd and Oxidation of COads and Adsorbed CO2
samples
QHupd (C/g)
metal-specific active area from QHupd (SM/m2 g-1)
as-prepared heat treatment, 1 h heat treatment, 2 h heat treatment, 5 h
30.8 33.9 38.3 60.8
14.7 16.2 18.2 28.9
Figure 4. Cyclic voltammograms for methanol oxidation on as-prepared and heat-treated Pt1Ru1/C catalysts at 200 °C for 1 and 2 h. All data were measured for 0.5 M methanol in 0.1 M perchloric acid.
revealed that Pt atoms segregated to the alloy surface by measuring the surface properties, such as the pztc and the adsorption of “reduced CO2”. Figure 5a shows the calculation and shift of the pztc of Pt1Ru1/ C with different heating periods from 1 to 5 h at 200 °C. The pztc and the potential of zero free charge (pzfc) are known as the potential of zero charge (Epzc). The pztc can be equal to the pzfc only when no interfacial charge transfer occurs.50 This means that the pzfc is directly related to the work function of the electrode surface. In an electrolyte solution, however, specific adsorption leads to deviation of the pztc from the pzfc. Therefore, the pzfc is difficult to measure with typical electrochemical techniques. It is worthy of note that the pztc is also dependent mainly on the work function of the electrode surface. It is expected, therefore, that (50) Frumkin, A. N.; Petrii, O. A. Electrochim. Acta 1975, 20, 347.
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QCO (C/g)
metal-specific active area from QCO (SM/m2 g-1)
QCO2 (C/g)
96.3 217.3 227.3 243.9
22.9 51.7 54.1 58.1
2.8 57.7 94.4 73.2
change in the surface composition of alloy nanoparticles induces shift of the pztc. The procedure for the CO-displacement experiment45,51,52 has already been described in our previous work on Pt-Ni alloy catalysts for oxygen reduction reaction (ORR).53 If Pt atoms segregate to the surface of Pt-Ru alloy nanoparticles, the surface Pt concentration increases, and therefore, the number of neighboring Pt atoms surrounding a Pt atom also increases. As surface Pt concentration increases, the pztc generally shifts toward a higher potential due to increased surface coverage by Pt, with a higher work function relative to Ru. Of course, the shift of the pztc is dependent on many factors, such as defect density,54 nanoparticle size,56 clustering of Pt in the surface layer,53 and an electronic modification by alloying. As shown in Figure 5a, the pztc was increased from 42 to 88 mV as heating duration increased up to 5 h. Upon increasing the heating time from 2 to 5 h, the increase in the pztc was only 3 mV. It might be originated from completion of Pt segregation and further phase separation in whole nanoparticles. Figure 5b shows that reduction peaks of the reactive oxygen species57 from cathodic (backward) scans in cyclic voltammograms of samples with various heating durations. The reduction peak was observed after heat treatment for 1 h, and then the peak area increased. This also supports that the nanoparticle surface becomes less oxophilic54 due to increased Pt concentration in the surface layer. The positive shift of the pztc was not sufficient to confirm Pt segregation by thermal activation because, as mentioned above, it (51) Climent, V.; Gomez, R.; Orts, J. M.; Rodes, A.; Aldaz, A.; Feliu, J. M. Interfacial Electrochemistry; Marcel Dekker: New York, 1999; p 463. (52) Clavilier, J.; Albalat, R.; Gomez, R.; Orts, J. M.; Feliu, J. M. J. Electroanal. Chem. 1993, 360, 325. (53) Jeon, T. Y.; Yoo, S. J.; Cho, Y. H.; Lee, K. S.; Kang, S. H.; Sung, Y. E. J. Phys. Chem. C 2009, 113, 19732. (54) Domke, K.; Herrero, E.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 2003, 552, 115. (55) Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc. 2005, 127, 6819. (56) Mayrhofer, K. J. J.; Blizanac, B. B.; Arenz, M.; Stamenkovic, V. R.; Ross, P. N.; Markovic, N. M. J. Phys. Chem. B 2005, 109, 14433. (57) Giner, J. Electrochim. Acta 1963, 8, 857.
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Figure 5. Cyclic voltammetric profiles at (a) anodic and (b) cathodic sweep of as-prepared and heat-treated Pt1Ru1/C catalysts in 0.1 M HClO4. For the determination of the pztc in (a), CO gas was introduced to displace the adsorbed species on electrodes at the potential of 0.15 V. Arrows point to positions of the potentials of zero total charge.
is related to many factors that should be considered in determination of the pztc of Pt-based alloy nanoparticles using the COdisplacement technique. Therefore, “reduced CO2” oxidation cyclic voltammograms were employed to obtain direct evidence regarding increased surface Pt concentration, that is, Pt segregation. Here, “reduced CO2”, which was first observed by Giner,57 is an indicatation of the reaction products of dissolved CO2 and Hads. Dissolved CO2 can react with hydrogen adsorbed on Pt and Rh electrodes. Czerwinski et al.,46,58,59 by focusing on the oxidation mechanism of adsorbed CO2, first reported surface coverage of Pt in Pt-Pd alloy. In their studies, the real surface area of the electrode was calculated from the reduction charge of surface oxides in the alloy surface layer, according to a standard procedure established by Woods et al.60 However, Pt-Ru alloy nanoparticles did not show the clear reduction peak of the alloy surface oxide. Therefore, application of the standard procedure for calculations of the real surface area of Pt-Ru alloy nanoparticles was difficult. In this study, the change in surface concentration of Pt was calculated by measuring “reduced CO2” oxidation voltammograms, as listed in Table 1. The CO adlayer oxidation voltammograms were also obtained. Although heat treatment can activate the sintering between adjacent particles, QCO increases as heating duration increases. Furthermore, QHupd and mass-specific active area from QHupd showed the same trend as QCO. SM was calculated by converting QCO into the active area using the conversion factor of 420 μC/cm2 for Pt surface and dividing weight of metal loaded on a working electrode. The increase of all these values support that the surface concentration of Pt increases through the surface segregation during heat treatment. QCO2 originates from hydrogen-covered Pt atoms in the Pt-Ru electrode because CO2, in the reduced form, does not adsorb on surface Ru atoms. As heating duration increased, the resulting QCO2 was also increased. This means that heat treatment induced the increase in the surface coverage of “reduced CO2”. (58) Grden, M.; Paruszewska, A.; Czerwinski, A. J. Electroanal. Chem. 2001, 502, 91. (59) Siwek, H.; Lukaszewski, M.; Czerwinski, A. Phys. Chem. Chem. Phys. 2008, 10, 3752. (60) Woods, R. In Bard, A. J., Ed.; Chemisorption at Electrodes in Electroanalytical Chemistry; Marcel Dekker: New York, 1976; Vol. 9.
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Figure 6. Cyclic voltammograms for oxidation of adsorbed CO2 and CO. Commercial Pt/C (40 wt %, Johnson-Matthey) are denoted by a gray line.
The magnitude of the CO2 oxidation charge was dramatically increased during the first 1 h and then showed a nearly constant rate of increase. The sudden increase during the first 1 h may originate from the sintering and/or the surface segregation of Pt. It should be noted that the further increase from 57.7 to 94.4 C/g (from 1 to 2 h) was observed. This increase in QCO2 is higher than that of QCO, suggesting that Pt segregation to the nanoparticle surface occurred. Figure 6 shows the oxidation currents of the adsorbed CO2 together with the CO adlayer. The COads oxidation charge of as-prepared Pt1Ru1/C was relatively low (96.3 C/g); in the same line, QCO2 also showed a very small value of 2.8 C/g. As heating duration increased from 1 to 2 h, the negative shift of peak potentials in the CO-stripping voltammograms was observed and was similar to the cathodic shifts of the onset potential in the MOR and the CO2 oxidation peaks of Figures 4 and 6, respectively, suggesting that Pt segregation occurred continuously during heat treatment, and therefore, the surface concentration of Pt reached the maximum with heat treatment of 2 h. Although Pt segregation was apparent from various electrochemical measurements, all of these results could have been Langmuir 2010, 26(11), 9123–9129
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resulted in the large increase in the “reduced CO2” oxidation charge after heat treatment. Therefore, chemical analysis by XPS led us to conclude that the increased activity of the MOR did not originate from reduction of the oxygenated metals by H2 during heat treatment.
Figure 7. Area ratios of various states fitted with XPS spectra of Pt 4f of Pt1Ru1/C (left) and Ru 3p of Ru/C (right).
influenced by reduction of Ru oxide upon heating under argon and hydrogen. As described in the next section, XPS was performed in order to reveal that removal of oxygen present in as-prepared nanoparticles contributed to increased MOR activity and CO2 oxidation charge. 3.3. XPS Characterization. X-ray photoelectron spectroscopy (XPS) was performed with as-prepared and heat-treated Pt1Ru1/C samples. Figure 7 shows quantitative area ratios of Pt and Ru. Spectra of Pt and Ru and their fitted curves are presented in Figures S4 and S5 (see the Supporting Information), respectively. It was noticed that Pt exists in various states, such as 71.73 eV of Pt(0), 72.52 eV of Pt(II), and 74.14 eV of Pt(IV), from curvefitting of Pt 4f and that the relative amount of Pt(0) was increased from 46.1% to 54.4% after heat treatment of 2 h, as shown in the left part of Figure 7. Ru states were comprised of metallic Ru (462 eV of Ru(0)) and Ru oxide (484 eV of Ru(IV)).61 The deconvolution of the Ru 3p spectra suggested that the relative amount of Ru(0) was decreased from 62.9% to 56.8% by heat treatment, which was calculated with the fitted curves from Ru 3p spectra in Figure S5 (see the Supporting Information), as compared with the right part of Figure 7. The increase of the metallic Pt may originate from surface segregation of Pt atoms and/or reduction of Pt oxides. It should be noted that the Ru(IV) increased from 37.1% to 43.2%. This increase can be ignored when considering the fact that the increase of metallic Ru results in the enhanced MOR activity.36 In addition, it is not acceptable that the small increase of the Ru oxide (61) Bock, C.; MacDougall, B.; LePage, Y. J. Electrochem. Soc. 2004, 151, A1269.
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4. Conclusions Pt segregation of carbon-supported Pt-Ru nanoparticles has been studied. We have succeeded in preparing pure Ru and Pt-Ru alloy nanoparticles highly dispersed on a carbon support by the NaBH4 reduction method using oleylamine as a stabilizer in an anhydrous ethanol solvent. Cyclic voltammograms in acidic media with methanol, however, showed that as-prepared Pt1Ru1/ C had low MOR activity due to the low surface concentration of Pt. In order to modify the surface concentration of Pt, heat treatment at 200 °C was chosen, because Ru as a host strongly segregates Pt as a solute. Pt1Ru1/C heated for 2 h showed the lowest onset potential and the highest current density. The pztc and “reduced CO2” oxidation cyclic voltammograms were measured to reveal the increased surface concentration of Pt after heat treatment. The pztc shifted toward higher potential as heating duration increased. This upshift may originate from Pt segregation, reduction of metal oxides generated during the preparation procedure, and aggregation of nanoparticles. Therefore, oxidation charges of CO2 with the selective adsorption behavior in reduced forms were measured and compared along with the heat treatment. Consequently, Pt-Ru heated for 2 h have 34 times “reduced CO2” oxidation charge representing the surface Pt concentration of Pt-Ru, as compared to its as-prepared state. Since the increase in metallic Ru was not observed after heat treatment from XPS, it was finally confirmed that the enhanced activity of the Pt-Ru alloy nanoparticles heated for 2 h resulted mainly from Pt segregation. Acknowledgment. This work was supported by the Research Center for Energy Conversion & Storage and the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science & Technology (R31-10013). The work at Kookmin University was supported by Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2009-0093814). Supporting Information Available: Electron diffraction (ED) images of Ru/C; particle size distribution from TEM images; plots of the mass-specific active surface areas calculated from COads stripping peaks and Hads/des currents; XPS spectra of Pt 4f of Pt1Ru1/C and Ru 3p of 40 wt % Ru/C. This material is available free of charge via the Internet at http://pubs.acs.org.
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