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C bimetallic nanoparticles synthesized by a radiolytic process
Applied Catalysis A: General 396 (2011) 68–75
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Methanol oxidation catalysis and substructure of PtRu/C bimetallic nanoparticles synthesized by a radiolytic process Takao A. Yamamoto a , Satoru Kageyama a,∗ , Satoshi Seino a , Hiroaki Nitani d , Takashi Nakagawa a , Ryo Horioka a , Yuji Honda a , Koji Ueno c , Hideo Daimon b a
Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka, 565-0871, Japan Hitach Maxell, Ltd., 1-1-88 Ushitora, Ibaraki, Osaka, 567-8567, Japan Japan Electron Beam Irradiation Service Ltd., Ozushima 5-3, Izumiohtsu, Osaka, 595-0074, Japan d Inst. Mater. Struct. Sci., KEK. Oho 1-1, Tsukuba, Ibaraki, 305-0801, Japan b c
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 23 January 2011 Accepted 28 January 2011 Available online 16 February 2011 Keywords: Methanol oxidation Bimetallic catalyst Radiolytic process Platinum Ruthenium
a b s t r a c t Nanoparticle catalysts of PtRu/C for the direct methanol fuel cell anode were synthesized by a radiolytic process. Bimetallic substructures were controlled by varying irradiation dose rate and by addition of NH4 OH or NaH2 PO2 . Material characterization was performed with the transmission electron microscopy, the X-ray diffraction, the X-ray fluorescence spectroscopy and the X-ray absorption fine structure techniques. Methanol oxidation activity was evaluated by the linear sweep voltammetry. We concluded that the structure of the radiolytically synthesized catalysts has a Pt-rich core/Ru-rich shell structure or incomplete alloy structure. A correlation between the substructures and catalytic activities was found by using a pairing factor defined from coordination numbers determined by the extend X-ray absorption fine structure analysis, which indicates the validity of the bifunctional mechanism in the PtRu nanoparticle system. This radiolytic process is promising for synthesizing advanced PtRu/C catalysts with well-mixed bimetallic substructures enhancing methanol oxidation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction PtRu bimetallic nanoparticles have been attracting much attention, because they can be applied to the anode catalysts for direct methanol fuel cells (DMFC) [1] and direct ethanol fuel cells (DEFC) [2], and to catalysts for CO preferential oxidation [3]. In particular, the application to the DMFC anode is the most efficient use of the PtRu nanoparticles owing to their high CO-tolerance. Although the PtRu nanoparticle catalysts have the high activity of methanol oxidation occurring at the DMFC anode, their high cost remains as the critical barrier to the commercial use. For the cost reduction, it is necessary to develop a new PtRu catalyst with a higher activity and an advanced synthesis process with mass-producibility [4]. However, there is no such process satisfying both these requirements because of difficulties in simul-
∗ Corresponding author. E-mail addresses: [email protected] (T.A. Yamamoto), [email protected] (S. Kageyama), [email protected] (S. Seino), [email protected] (H. Nitani), [email protected] (T. Nakagawa), [email protected] (R. Horioka), [email protected] (Y. Honda), kji [email protected] (K. Ueno), [email protected] (H. Daimon). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.01.037
taneous control of the bimetallic structure, the particle size and the morphology. Many researchers report that the catalytic properties of bimetallic catalysts are sensitive to their atomic substructure and its precise control leads to an advanced catalysis that we need [5,6]. In 1975, Watanabe and Motoo found that the PtRu bulk system exhibits an enhanced catalysis, which was explained by the bifunctional mechanism on the basis of Pt–Ru atomic pairs at the surface [7]. In the bifunctional mechanism, the high COtolerance of PtRu catalysts is explained by the strong adsorption of OH on Ru adjacent to Pt. This Ru–OH pair oxidizes the CO adsorbed on the Pt of catalytic active sites. Recent studies employed the first-principle calculation [8–10] and experiments with Rudecorated Pt thin film [11–14] to establish that the origin of the enhanced activity of bimetallic catalysis should be attributed to the modified electronic structures of Pt; such modifications are caused by the charge transfer between Pt and Ru, and by the strain of Pt lattice. In nanoparticle systems, it is no easy task to control and characterize the binary substructures correlating with the enhanced activity [15–21], although some authors have reported improved activities of bimetallic catalysts. Therefore, it is very important to obtain PtRu nanoparticle samples with wellcontrolled and well-characterized substructure. To control binary
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Fig. 1. TEM micrographs of the present samples: (a) sample ␥, (b) sample E and (c) TEC61E54.
substructure in nanoparticles, various studies were carried out, but they usually require a complicated process involving heat treatment, underpotential deposition, sol–gel process, and/or reflux processed [22–27]. Recently, a radiolytic process employing an electron beam (EB) has been applied to the synthesis of bimetallic PtCu nanoparticle catalysts with a high activity of CO oxidation [28,29]. This process is a simple one-pot process that is feasible for the mass production [30–32]. In this process, rapid reduction of ionic precursors occurs uniformly in the synthetic solution and then different species can be reduced simultaneously to form alloyed nanoparticles overcoming the restrictions imposed by their redox potentials. Thus, this radiolytic process has a high potential for synthesis of the PtRu bimetallic catalysts with a high activity originating from the random alloy structure. However, study on the radiolytic synthesis of PtRu is scarce, and researchers have reported only particle formation without any precise material characterization or catalysis evaluation [33–38]. In the present study, we synthesized the PtRu catalysts using this radiolytic process, and characterized the bimetallic structure with the X-ray absorption fine structure (XAFS) technique, and furthermore evaluated its catalytic activity. We found a good correlation between the activity and the mixing state of Pt–Ru after the procedure that Nitani et al. has used to confirm validity of the bifunctional mechanism even in the PtRu synthesized with a polyol process [39].
2. Experimental 2.1. Synthesis of PtRu/C H2 PtCl6 ·6H2 O (99.9%, Wako) and RuCl3 ·nH2 O (n = 2–3, 99.9%, Wako) were used as metal precursors. Non-metallic chemicals used were ultra pure water (18 M cm), carbon supports (Vulcan XC-72R, Cabot), 2-propanol (Wako), NaH2 PO2 ·H2 O (Wako), and ammonia water NH4 OH. Metal loading was adjusted to be approximately 10 wt.%. The concentrations of Pt, Ru precursors and phosphorus were 0.25 mM. 2-propanol was added to 1 vol.% as a reduction enhancer. Without addition of ammonia, pH = 3, after its addition pH = 4. A 100 ml glass vial containing water, these metal precursors and non-metallic chemicals was irradiated with 60 Co ␥-rays (2 kGy/h for 3 h, at the Institute of Scientific and Industrial Research, Osaka University) or an EB (3 kGy/s for 10 s, at EBIS) at room temperature under an ambient pressure. Radiation-induced radicals reduced the metal precursors, and the PtRu formed particles were stabilized on the carbon supports. The ␥-rays and the EB provide quite different irradiation conditions: the dose rate of the EB is 5400 times higher than that of the ␥-rays. After the irradiation, the powder was washed and dried to obtain catalyst samples. The purpose of the ammonia addition is to assist Ru reduction. In the radiolytic reduction, once reduced atoms or clusters, especially of base metals, are likely oxidized by protons, and this
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Fig. 2. Histogram of the present samples: (a) sample ␥, (b) sample E and (c) TEC61E54. The scales of the horizontal and vertical axes in the histogram of (c) are different from that in (a) and (b).
back oxidation may be suppressed by increasing the pH [30]. We expected the baser Ru would be reduced simultaneously with the nobler Pt to form bimetallic particles. The purpose of addition of NaH2 PO2 is size reduction of resultant metallic particles. Daimon and Kurobe demonstrated its effectiveness for the PtRu bimetallic catalyst synthesized with a polyol process [40]. As far as we know, its effectiveness for the radiolytic process was not examined yet. For comparison, a commercial PtRu catalyst TEC61E54 was purchased from TANAKA KIKINZOKU KOGYO K.K.
electrode-type beaker cell. A gold wire and a Ag/AgCl electrode (BAS, RE-1B) were used as counter and reference electrodes, respectively. A carbon paper with the catalyst powder was used as the working electrode. LSV measurements (0–0.8 V vs. NHE, 200 mV/s, 35 ◦ C) were performed in a 1.5 M H2 SO4 +20 vol.% CH3 OH solution under a nitrogen atmosphere. 3. Results and discussion 3.1. TEM observation, XRF, and XRD
2.2. Characterization Morphology of the samples was investigated using a transmission electron microscope (TEM; HITACHI, H-8100, 200 kV). Chemical composition was analyzed by an X-ray fluorescence spectrometer (XRF; RIGAKU ZSX100e). Crystallographic analysis was performed by an X-ray diffractometer (XRD; RIGAKU RINT2100Ultima with Cu K␣ radiation). XAFS measurements were performed at beamline BL01B1 of SPring-8 (Hyogo, Japan). Chemical states of metals were investigated by examining the XANES (X-ray absorption near edge structure) spectra around Pt-LIII edge and Ru-K edge. Chemical bonds were evaluated by the EXAFS (extend X-ray absorption fine structure) analyses at Pt-LIII edge and Ru-K edge. Experimental details of these XAFS analyses were described in our previous paper [41]. Methanol oxidation activity was evaluated by the linear sweep voltammetry (LSV) technique, using a three-
Fig. 1(a)–(c) show typical TEM micrographs of the samples obtained by the ␥-rays irradiation (sample ␥) and by the EB irradiation (sample E), and of the commercial catalyst (TEC61E54). Larger images with a weaker contrast stand for the carbon supports, and smaller images with a stronger contrast stand for metal particles. Note that the magnification of Fig. 1(c) is lower than that of the others. The histograms of particle size in Fig. 2 were obtained by measuring the sizes of more than 100 particle images excluding agglomerates. It is noticed that the metal particles obtained by the present radiolytic process are smaller and more narrowly distributed than in TEC61E54. Other samples by the radiolytic process exhibited similar morphologies. In Table 1, chemical composition, average sizes and its standard deviation are listed. The chemical compositions found in the samples were all around 6:4 in Pt:Ru ratio regardless of synthesis
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Table 1 Synthesis condition, composition and particle size of PtRu/C catalysts. Sample ID
Radiation
Additive
␥ ␥-NH3 ␥-P E E-NH3 E-P TEC61E54
␥-ray ␥-ray ␥-ray EB EB EB –
None NH4 OH NaH2 PO2 None NH4 OH NaH2 PO2 –
a
Composition (at.%) a
Size of PtRu particles (nm)
Pt
Ru
Average size
Standard deviation
59 59 63 61 58 56 40
41 41 37 40 42 45 60
2.2 1.6 1.3 1.7 1.5 1.4 3.7
0.4 0.4 0.4 0.3 0.3 0.3 1.6
Composition of TEC61E54 is nominal one.
conditions, though the initially loaded sources were 1:1. Average sizes of the synthesized samples were 1.3–2.2 nm, which are smaller than that of TEC61E54, 3.7 nm, and their size distributions, 0.3–0.4 nm in standard deviation, were narrower than that of TEC61E54, 1.6 nm. The particle sizes obtained with ammonia or phosphorus were smaller than those obtained without them, which shows the effectiveness of the addition of ammonia and phosphorus for the size reduction. Fig. 3 shows the XRD patterns of the samples, in which peaks corresponding to the fcc structure of Pt were observed but no peak corresponding to the hcp of Ru. Since Ru is indeed contained there as shown in Table 1, Ru would exist as a state which is not able to give significant diffraction. These peaks due to Pt (1 1 1) plane found in the samples obtained with ammonia or phosphorus are broader than those without the additives, indicating smaller Pt crystallite sizes. This is consistent with the average particle sizes determined from the TEM observation. The peaks due to Pt (1 1 1) and (2 2 0) of the sample E and TEC61E54 exhibited slight but significant shifts toward higher angles. These higher angle shifts indicate contractions of the Pt lattice caused by Ru taken into the lattice: therefore, by alloy formation. The high dose rate of the present EB has possibly assisted the alloy formation, as we have expected. Alloying in TEC61E54 could have been caused by an annealing treatment.
Fig. 3. XRD patterns of the present samples. The vertical lines denote positions of peaks in pure Pt metal.
3.2. XAFS analysis 3.2.1. Xanes In Fig. 4, normalized Pt-LIII edge XANES spectra of the samples are shown together with those of reference materials, Pt metal and PtO2 . Since the XANES region at the Pt-LIII edge reflects the 2p–5d transition state, the Pt–O bonds sharpen the peak shape near the absorption edge. Therefore, the integral intensity of the white line is a good indicator of oxidation. The spectrum of PtO2 has a sharper white line at 11,554 eV reflecting the electron transition to a narrow energy band of localized orbitals, while that of Pt metal has a broadened line at 11,552 eV reflecting the transition to a broad conduction band. The difference in these peak positions originates from a difference in the lowest levels of 5d-band in metal and oxide. The spectra of the samples resemble that of the metal in peak shape, and the peaks slightly shifted toward the direction corresponding to PtO2 . These features may indicate that a part of the Pt atoms in the samples are oxidized. These spectra were found to be successfully described by the linear combinations analyses of the reference spectra of the Pt metal and the PtO2 in the region of −20 to 50 eV from the absorption edge. We used Athena code; associated R-factors were all less than 0.0004. The calculation resulted in Pt metal:PtO2 ratios of 67:33 for TEC61E54, 85:15 for ␥, 74:26 for ␥-NH3 , 41:59 for ␥-P, 94:6 for E, 88:12 for E-NH3 and 56:44 for E-P. It is noted that the smaller particles by the TEM observation, ␥-P and E-P, showed higher PtO2 fractions, indicating the materials well oxidized. In Fig. 5, normalized Ru-K edge XANES spectra of the samples are shown along with those of reference materials of Ru metal and RuO2 . The XANES region at the Ru-K edge reflects the 1s–5p transition state and the Ru–O bonds influence the peak shape near the absorption edge. The spectrum of the reference Ru metal has two
Fig. 4. Normalized XANES spectra of the present samples at the Pt-LIII edge. The two vertical lines denote positions of peaks in reference material Pt metal and PtO2 .
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Fig. 5. Normalized XANES spectra of the present samples at the Ru-K edge. Measurement of the sample ␥ was not performed. The two vertical lines denote positions of two peaks in reference material Ru metal.
Fig. 7. Radial structure function and Fourier back-transformed EXAFS oscillation in the inset of the sample E-P at the Ru-K edge.
peaks, as indicated by the two vertical lines (22,085 and 22,110 eV). The absorption edge of the reference RuO2 arises at higher energy than that of the Ru metal. In the spectrum of the radiation synthesized samples, the absorption edges arise at higher energy, resembling RuO2 in shape. In contrast, metallic features are easily seen in the spectra of the sample E and TEC61E54. The sample E keeps a trace of Ru metal at 22,110 eV, which corresponds to the second peak of the reference Ru metal. In TEC61E54, the two peaks as the metallic features are seen clearly. The linear combination analyses of the spectra were performed with the reference spectra of the Ru metal and the RuO2 in the area of −20 to 50 eV from the absorption edge. The results showed Ru metal:RuO2 ratios of 78:22 for TEC61E54, 31:69 for ␥-NH3 , 11:89 for ␥-P, 60:40 for E, 43:57 for E-NH3 and 28:72 for E-P. All the R-factors were less than 0.0002. The sample TEC61E54 and E showed lower RuO2 fractions. In addition, metallic features are easily seen in the spectra of the sample E and TEC61E54. These features are consistent with the promotion of PtRu alloying confirmed by XRD.
obtained at Pt-LIII and Ru-K edges, respectively. Broken lines stand for the Hanning window functions used in the analyses. Structural parameters were optimized by curve fits in q-space assuming single scattering paths. The curve fits were performed with the Artemis code [42,43], and backscattering amplitudes, phase shifts, and mean free paths were calculated by using the FEFF7 code [44]. Dots denote experimental values of back-transformed oscillations, and solid lines are drawn based on the optimized parameters. Note that the calculated values well reproduce the experimental plots. The optimized parameters in the present Pt-LIII edge EXAFS analyses assuming Pt–O, Pt–Ru and Pt–Pt bonds, are listed in Table 2, and those in the Ru-K edge assuming Ru–O, Ru–Ru and Ru–Pt bonds, are listed in Table 3. In these tables, interatomic distance and coordination number from A to B are denoted as RA–B and NA–B , respectively. The reliability of the present EXAFS analyses was checked by referring to the R-factors [43] listed in the last columns. They were all smaller than 0.044, which indicates enough reliability to allow us to discuss the bimetallic substructures. Judging from values of NPt–Pt , Pt–Pt bonds occur often in the radiolytic samples, while there are certain Pt–Ru and Pt–O bonds, though less than the Pt–Pt. In contrast, NRu–Ru values are far smaller than the NPt–Pt . These findings indicate that Pt atoms form relatively larger metallic particles, but Ru does not. This is consistent with the metallic features found in the Pt-LIII XANES and the XRD peaks from Pt (1 1 1) plane. NRu–O values in radiolytic samples are large, as compared to other bonds from Ru. Combining with the present data sets of the XANES and XRD, one may conclude that a major part of Ru atoms are considerably oxidized with too low crystallinity to give discernible diffraction. The number of Pt–Ru bonds should increase as both atoms are mixed well, and would be reflected on coordination numbers. In the present EXAFS analyses, NPt–Ru and NRu–Pt values neither diverged nor converged to physically absurd values. Therefore, certain occurrence of Pt–Ru would have been reflected on such determined values, although these NPt–Ru and NRu–Pt values did not agree. Even with such uncertainty, we may point out that relatively large coordination numbers are indeed seen for the samples synthesized with the EB, in other words, Pt–Ru mixing has been enhanced. This conjecture is better supported when the data sets of TEC61E54 are also taken into account. This sample is no doubt alloyed, judging from its XRD pattern and smaller RPt–Pt than those of pure Pt, and the samples ␥, ␥-NH3 , ␥-P and E-NH3 are presumably less mixed. Regarding the existence state of P, we tried EXAFS analyses assuming Pt–P and Ru–P bonds, but our parameters did not converge to physically plausible values. Therefore, we
3.2.2. Exafs Figs. 6 and 7 are typical radial structure functions (RSF) and Fourier back-transformed EXAFS oscillations of the sample E–P
Fig. 6. Radial structure function and Fourier back-transformed EXAFS oscillation in the inset of the sample E-P at the Pt-LIII edge.
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Table 2 Structural parameters obtained from the analysis of EXAFS at the Pt-L III edge. Pt coordination a
Ru coordination
O coordination
Sample ID
RPt–Pt (nm)
NPt–Pt
RPt–Ru (nm)
NPt–Ru
RPt–O (nm)
NPt–O
R-factor
␥b ␥-NH3 ␥-Pc E E-NH3 E-P TEC61E54
0.276 0.276 – 0.275 0.276 0.271 0.273
9.3 5.8 – 8.2 8.1 4.8 3.0
– 0.272 – 0.272 0.271 0.270 0.271
– 0.2 – 1.6 0.4 1.6 3.1
– 0.200 – 0.198 0.199 0.200 0.198
– 2.2 – 0.4 1.0 2.8 1.8
0.003 0.006 – 0.004 0.001 0.044 0.034
a b c
RPt–Pt = 0.277 nm in Pt metal. Assuming Pt–Pt bonds only. No physically reasonable parameter was obtained.
Table 3 Structural parameters obtained from the analysis of EXAFS at the Ru-K edge. Sample ID
␥b ␥-NH3 ␥-P E E-NH3 E-P TEC61E54 a b
Pt coordination
Ru coordination
O coordination
R-factor
RRu–Pt (nm)
NRu–Pt
RRu–Ru (nm)a
NRu–Ru
RRu–O (nm)
NRu–O
– 0.272 0.272 0.272 0.271 0.270 0.271
– 1.6 0.6 2.3 1.5 2.3 5.8
– 0.270 0.257 0.268 0.269 0.266 0.268
– 2.4 0.2 4.0 3.5 1.3 1.9
– 0.201 0.206 0.199 0.200 0.200 0.198
– 4.4 4.2 2.4 3.2 5.3 2.1
– 0.036 0.041 0.005 0.009 0.029 0.003
RRu–Ru = 0.266 nm in Ru metal. Not measured.
may conclude that phosphorus does not exist in the particles. We did not estimate the particle sizes of the samples with the coordination numbers determined by the EXAFS analysis, because of the difference of the particle shape or disorder of crystal structure [45–48] among the samples, as shown in the XRD patterns and TEM micrographs. 3.2.3. Methanol oxidation activity Fig. 8 shows the methanol oxidation current vs. potential curves obtained with the present samples, in which the oxidation current was normalized to the PtRu weight contained in each sample. It should be pointed out that the radiolytically synthesized material works as an electrode catalyst for the DMFC even without any post-treatment with heat and chemicals. They were obtained just by mixing the source materials, irradiation, washing, and drying.
Fig. 8. Oxidation current vs. potential curves of the present samples.
The oxidation currents exhibited by the samples ␥-P and E-P were comparable to that of the commercial catalyst TEC61E54, when evaluated at the potential of 0.45 vs. NHE which is supposed to be the effective potential during operation of the DMFC [49]. Thus evaluated catalytic activities of the present samples accord to the following order: ␥ < ␥-NH3 < ␥-P, E < E-NH3 < E-P. These relationships imply that the additives have been effective for size reduction and suppression of Ru back oxidation, whichever radiation was employed. 3.2.4. Correlation between PtRu structure and catalytic activity The present characterization results with XRF, XRD, XANES and EXAFS are now discussed in combination with the redox potentials of Pt and Ru. In general, the substructure of bimetallic nanoparticles formed by reducing aqueous ions should be influenced by the redox potentials of the constituent metals. The species with a higher redox potential is first reduced and precipitates and the other one follows it, likely resulting in a core/shell substructure [50–52]. The standard redox potentials of Pt2+ /Pt and Ru3+ /Ru are 1.18 and 0.69 V vs. NHE, respectively, so that a core/shell structure of Pt/Ru or individual particle formation is anticipated rather than alloy particles. The present characterization results indicated that not fully but partially randomized alloy phase is formed in the particles when the EB is used. The most reliable clue of alloying must be the XRD data, and clear peak shifts were in fact observed with the E in Fig. 3, but these shifts are not enough to account for the full randomness. The alloying possibly proceeded only halfway even with the EB and the additive for enhancing reduction. The bimetallic substructure occurring in the present samples would be of Pt-rich core and Ru-rich shell, still reflecting the restriction enforced by the redox potentials. The Ru-rich shell would be considerably oxidized so as not to give discernible diffraction, while the core would be virtually metallic in which possibly some alloying proceeds incompletely and gradually along the radial direction. This conjecture is supported by the XRD peak shift, the XANES features indicating Pt’s slight oxidation and Ru’s considerable oxidation, and Pt–Ru coordination significant but midway. The present data would in total
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alysts exhibited the enhanced activity, in spite of the incomplete alloying. Acknowledgments The ␥-ray irradiation was performed at the Institute of Scientific and Industrial Research, Osaka University. The authors thank the staff of the Institute of Scientific and Industrial Research for their assistance with the irradiation experiments. This research was partially supported by a Grant-in-Aid for Challenging Exploratory Research, (no. 19651042). References
Fig. 9. Correlation between methanol oxidation current and pairing factor PRu calculated from the results of the Ru-K edge analyses.
indicate that the degree of the alloying depends on the synthesis conditions. These proofs for the substructure are similar to those of PtRu/C that was synthesized with a polyol process. Its substructure was investigated by the XAFS correlating with its catalyst activity by Nitani et al. as follows. They defined the pairing factor PRu , which corresponds to the frequency of Pt–Ru pair in the bimetallic sample.
PRu =
NRu–Pt NRu–Pt + NRu–Ru
Since Ru exists preferentially in a near surface layer of the bimetallic particle, PRu calculated only from coordination numbers determined by Ru-K edge measurement may be regarded as an indicator for Pt–Ru pair occurrence on the surface. We calculated PRu values for the present samples and plotted them with the methanol oxidation currents in Fig. 9, where the oxidation current at 0.45 V vs. NHE was adopted. A clear correlation between PRu and current was found, except for the plot for TEC61E54. This correlation is similar to that reported by Nitani et al. though their conditions were different from ours with respect to temperature, solution concentration, and catalyst loading. Remembering that the bifunctional mechanism attributes the high CO tolerance of PtRu bimetallic catalysts to Pt–Ru atomic pairs, we may conclude this mechanism is valid in the PtRu nanoparticle system. The deviation of the plot of TEC61E54 from the others could be caused by its larger size and more enhanced alloying which could be given by a post annealing. 4. Conclusion We have successfully synthesized PtRu/C catalysts by the radiolytic process employing the ␥-rays and the EB. The PtRu nanoparticles were found to be smaller and have a more narrow size distribution using TEM observation. From the results of XRD and XAFS analyses, we concluded that the structure of PtRu is a Ptrich core/Ru-rich shell, which can be attributed to the difference of the redox potentials. The mixed state of PtRu at their surface was controlled by using high dose rates and the addition of ammonia and phosphorus. A correlation between the mixed state and activity of methanol oxidation was investigated using EXAFS analysis and employing the pairing factor. This investigation confirmed the validity of the bifunctional mechanism in the PtRu nanoparticle system synthesized by the radiolytic process. Our radiolytic cat-
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Contents lists available at ScienceDirect
Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Methanol oxidation catalysis and substructure of PtRu/C bimetallic nanoparticles synthesized by a radiolytic process Takao A. Yamamoto a , Satoru Kageyama a,∗ , Satoshi Seino a , Hiroaki Nitani d , Takashi Nakagawa a , Ryo Horioka a , Yuji Honda a , Koji Ueno c , Hideo Daimon b a
Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka, 565-0871, Japan Hitach Maxell, Ltd., 1-1-88 Ushitora, Ibaraki, Osaka, 567-8567, Japan Japan Electron Beam Irradiation Service Ltd., Ozushima 5-3, Izumiohtsu, Osaka, 595-0074, Japan d Inst. Mater. Struct. Sci., KEK. Oho 1-1, Tsukuba, Ibaraki, 305-0801, Japan b c
a r t i c l e
i n f o
Article history: Received 8 September 2010 Received in revised form 23 January 2011 Accepted 28 January 2011 Available online 16 February 2011 Keywords: Methanol oxidation Bimetallic catalyst Radiolytic process Platinum Ruthenium
a b s t r a c t Nanoparticle catalysts of PtRu/C for the direct methanol fuel cell anode were synthesized by a radiolytic process. Bimetallic substructures were controlled by varying irradiation dose rate and by addition of NH4 OH or NaH2 PO2 . Material characterization was performed with the transmission electron microscopy, the X-ray diffraction, the X-ray fluorescence spectroscopy and the X-ray absorption fine structure techniques. Methanol oxidation activity was evaluated by the linear sweep voltammetry. We concluded that the structure of the radiolytically synthesized catalysts has a Pt-rich core/Ru-rich shell structure or incomplete alloy structure. A correlation between the substructures and catalytic activities was found by using a pairing factor defined from coordination numbers determined by the extend X-ray absorption fine structure analysis, which indicates the validity of the bifunctional mechanism in the PtRu nanoparticle system. This radiolytic process is promising for synthesizing advanced PtRu/C catalysts with well-mixed bimetallic substructures enhancing methanol oxidation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction PtRu bimetallic nanoparticles have been attracting much attention, because they can be applied to the anode catalysts for direct methanol fuel cells (DMFC) [1] and direct ethanol fuel cells (DEFC) [2], and to catalysts for CO preferential oxidation [3]. In particular, the application to the DMFC anode is the most efficient use of the PtRu nanoparticles owing to their high CO-tolerance. Although the PtRu nanoparticle catalysts have the high activity of methanol oxidation occurring at the DMFC anode, their high cost remains as the critical barrier to the commercial use. For the cost reduction, it is necessary to develop a new PtRu catalyst with a higher activity and an advanced synthesis process with mass-producibility [4]. However, there is no such process satisfying both these requirements because of difficulties in simul-
∗ Corresponding author. E-mail addresses: [email protected] (T.A. Yamamoto), [email protected] (S. Kageyama), [email protected] (S. Seino), [email protected] (H. Nitani), [email protected] (T. Nakagawa), [email protected] (R. Horioka), [email protected] (Y. Honda), kji [email protected] (K. Ueno), [email protected] (H. Daimon). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2011.01.037
taneous control of the bimetallic structure, the particle size and the morphology. Many researchers report that the catalytic properties of bimetallic catalysts are sensitive to their atomic substructure and its precise control leads to an advanced catalysis that we need [5,6]. In 1975, Watanabe and Motoo found that the PtRu bulk system exhibits an enhanced catalysis, which was explained by the bifunctional mechanism on the basis of Pt–Ru atomic pairs at the surface [7]. In the bifunctional mechanism, the high COtolerance of PtRu catalysts is explained by the strong adsorption of OH on Ru adjacent to Pt. This Ru–OH pair oxidizes the CO adsorbed on the Pt of catalytic active sites. Recent studies employed the first-principle calculation [8–10] and experiments with Rudecorated Pt thin film [11–14] to establish that the origin of the enhanced activity of bimetallic catalysis should be attributed to the modified electronic structures of Pt; such modifications are caused by the charge transfer between Pt and Ru, and by the strain of Pt lattice. In nanoparticle systems, it is no easy task to control and characterize the binary substructures correlating with the enhanced activity [15–21], although some authors have reported improved activities of bimetallic catalysts. Therefore, it is very important to obtain PtRu nanoparticle samples with wellcontrolled and well-characterized substructure. To control binary
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Fig. 1. TEM micrographs of the present samples: (a) sample ␥, (b) sample E and (c) TEC61E54.
substructure in nanoparticles, various studies were carried out, but they usually require a complicated process involving heat treatment, underpotential deposition, sol–gel process, and/or reflux processed [22–27]. Recently, a radiolytic process employing an electron beam (EB) has been applied to the synthesis of bimetallic PtCu nanoparticle catalysts with a high activity of CO oxidation [28,29]. This process is a simple one-pot process that is feasible for the mass production [30–32]. In this process, rapid reduction of ionic precursors occurs uniformly in the synthetic solution and then different species can be reduced simultaneously to form alloyed nanoparticles overcoming the restrictions imposed by their redox potentials. Thus, this radiolytic process has a high potential for synthesis of the PtRu bimetallic catalysts with a high activity originating from the random alloy structure. However, study on the radiolytic synthesis of PtRu is scarce, and researchers have reported only particle formation without any precise material characterization or catalysis evaluation [33–38]. In the present study, we synthesized the PtRu catalysts using this radiolytic process, and characterized the bimetallic structure with the X-ray absorption fine structure (XAFS) technique, and furthermore evaluated its catalytic activity. We found a good correlation between the activity and the mixing state of Pt–Ru after the procedure that Nitani et al. has used to confirm validity of the bifunctional mechanism even in the PtRu synthesized with a polyol process [39].
2. Experimental 2.1. Synthesis of PtRu/C H2 PtCl6 ·6H2 O (99.9%, Wako) and RuCl3 ·nH2 O (n = 2–3, 99.9%, Wako) were used as metal precursors. Non-metallic chemicals used were ultra pure water (18 M cm), carbon supports (Vulcan XC-72R, Cabot), 2-propanol (Wako), NaH2 PO2 ·H2 O (Wako), and ammonia water NH4 OH. Metal loading was adjusted to be approximately 10 wt.%. The concentrations of Pt, Ru precursors and phosphorus were 0.25 mM. 2-propanol was added to 1 vol.% as a reduction enhancer. Without addition of ammonia, pH = 3, after its addition pH = 4. A 100 ml glass vial containing water, these metal precursors and non-metallic chemicals was irradiated with 60 Co ␥-rays (2 kGy/h for 3 h, at the Institute of Scientific and Industrial Research, Osaka University) or an EB (3 kGy/s for 10 s, at EBIS) at room temperature under an ambient pressure. Radiation-induced radicals reduced the metal precursors, and the PtRu formed particles were stabilized on the carbon supports. The ␥-rays and the EB provide quite different irradiation conditions: the dose rate of the EB is 5400 times higher than that of the ␥-rays. After the irradiation, the powder was washed and dried to obtain catalyst samples. The purpose of the ammonia addition is to assist Ru reduction. In the radiolytic reduction, once reduced atoms or clusters, especially of base metals, are likely oxidized by protons, and this
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Fig. 2. Histogram of the present samples: (a) sample ␥, (b) sample E and (c) TEC61E54. The scales of the horizontal and vertical axes in the histogram of (c) are different from that in (a) and (b).
back oxidation may be suppressed by increasing the pH [30]. We expected the baser Ru would be reduced simultaneously with the nobler Pt to form bimetallic particles. The purpose of addition of NaH2 PO2 is size reduction of resultant metallic particles. Daimon and Kurobe demonstrated its effectiveness for the PtRu bimetallic catalyst synthesized with a polyol process [40]. As far as we know, its effectiveness for the radiolytic process was not examined yet. For comparison, a commercial PtRu catalyst TEC61E54 was purchased from TANAKA KIKINZOKU KOGYO K.K.
electrode-type beaker cell. A gold wire and a Ag/AgCl electrode (BAS, RE-1B) were used as counter and reference electrodes, respectively. A carbon paper with the catalyst powder was used as the working electrode. LSV measurements (0–0.8 V vs. NHE, 200 mV/s, 35 ◦ C) were performed in a 1.5 M H2 SO4 +20 vol.% CH3 OH solution under a nitrogen atmosphere. 3. Results and discussion 3.1. TEM observation, XRF, and XRD
2.2. Characterization Morphology of the samples was investigated using a transmission electron microscope (TEM; HITACHI, H-8100, 200 kV). Chemical composition was analyzed by an X-ray fluorescence spectrometer (XRF; RIGAKU ZSX100e). Crystallographic analysis was performed by an X-ray diffractometer (XRD; RIGAKU RINT2100Ultima with Cu K␣ radiation). XAFS measurements were performed at beamline BL01B1 of SPring-8 (Hyogo, Japan). Chemical states of metals were investigated by examining the XANES (X-ray absorption near edge structure) spectra around Pt-LIII edge and Ru-K edge. Chemical bonds were evaluated by the EXAFS (extend X-ray absorption fine structure) analyses at Pt-LIII edge and Ru-K edge. Experimental details of these XAFS analyses were described in our previous paper [41]. Methanol oxidation activity was evaluated by the linear sweep voltammetry (LSV) technique, using a three-
Fig. 1(a)–(c) show typical TEM micrographs of the samples obtained by the ␥-rays irradiation (sample ␥) and by the EB irradiation (sample E), and of the commercial catalyst (TEC61E54). Larger images with a weaker contrast stand for the carbon supports, and smaller images with a stronger contrast stand for metal particles. Note that the magnification of Fig. 1(c) is lower than that of the others. The histograms of particle size in Fig. 2 were obtained by measuring the sizes of more than 100 particle images excluding agglomerates. It is noticed that the metal particles obtained by the present radiolytic process are smaller and more narrowly distributed than in TEC61E54. Other samples by the radiolytic process exhibited similar morphologies. In Table 1, chemical composition, average sizes and its standard deviation are listed. The chemical compositions found in the samples were all around 6:4 in Pt:Ru ratio regardless of synthesis
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Table 1 Synthesis condition, composition and particle size of PtRu/C catalysts. Sample ID
Radiation
Additive
␥ ␥-NH3 ␥-P E E-NH3 E-P TEC61E54
␥-ray ␥-ray ␥-ray EB EB EB –
None NH4 OH NaH2 PO2 None NH4 OH NaH2 PO2 –
a
Composition (at.%) a
Size of PtRu particles (nm)
Pt
Ru
Average size
Standard deviation
59 59 63 61 58 56 40
41 41 37 40 42 45 60
2.2 1.6 1.3 1.7 1.5 1.4 3.7
0.4 0.4 0.4 0.3 0.3 0.3 1.6
Composition of TEC61E54 is nominal one.
conditions, though the initially loaded sources were 1:1. Average sizes of the synthesized samples were 1.3–2.2 nm, which are smaller than that of TEC61E54, 3.7 nm, and their size distributions, 0.3–0.4 nm in standard deviation, were narrower than that of TEC61E54, 1.6 nm. The particle sizes obtained with ammonia or phosphorus were smaller than those obtained without them, which shows the effectiveness of the addition of ammonia and phosphorus for the size reduction. Fig. 3 shows the XRD patterns of the samples, in which peaks corresponding to the fcc structure of Pt were observed but no peak corresponding to the hcp of Ru. Since Ru is indeed contained there as shown in Table 1, Ru would exist as a state which is not able to give significant diffraction. These peaks due to Pt (1 1 1) plane found in the samples obtained with ammonia or phosphorus are broader than those without the additives, indicating smaller Pt crystallite sizes. This is consistent with the average particle sizes determined from the TEM observation. The peaks due to Pt (1 1 1) and (2 2 0) of the sample E and TEC61E54 exhibited slight but significant shifts toward higher angles. These higher angle shifts indicate contractions of the Pt lattice caused by Ru taken into the lattice: therefore, by alloy formation. The high dose rate of the present EB has possibly assisted the alloy formation, as we have expected. Alloying in TEC61E54 could have been caused by an annealing treatment.
Fig. 3. XRD patterns of the present samples. The vertical lines denote positions of peaks in pure Pt metal.
3.2. XAFS analysis 3.2.1. Xanes In Fig. 4, normalized Pt-LIII edge XANES spectra of the samples are shown together with those of reference materials, Pt metal and PtO2 . Since the XANES region at the Pt-LIII edge reflects the 2p–5d transition state, the Pt–O bonds sharpen the peak shape near the absorption edge. Therefore, the integral intensity of the white line is a good indicator of oxidation. The spectrum of PtO2 has a sharper white line at 11,554 eV reflecting the electron transition to a narrow energy band of localized orbitals, while that of Pt metal has a broadened line at 11,552 eV reflecting the transition to a broad conduction band. The difference in these peak positions originates from a difference in the lowest levels of 5d-band in metal and oxide. The spectra of the samples resemble that of the metal in peak shape, and the peaks slightly shifted toward the direction corresponding to PtO2 . These features may indicate that a part of the Pt atoms in the samples are oxidized. These spectra were found to be successfully described by the linear combinations analyses of the reference spectra of the Pt metal and the PtO2 in the region of −20 to 50 eV from the absorption edge. We used Athena code; associated R-factors were all less than 0.0004. The calculation resulted in Pt metal:PtO2 ratios of 67:33 for TEC61E54, 85:15 for ␥, 74:26 for ␥-NH3 , 41:59 for ␥-P, 94:6 for E, 88:12 for E-NH3 and 56:44 for E-P. It is noted that the smaller particles by the TEM observation, ␥-P and E-P, showed higher PtO2 fractions, indicating the materials well oxidized. In Fig. 5, normalized Ru-K edge XANES spectra of the samples are shown along with those of reference materials of Ru metal and RuO2 . The XANES region at the Ru-K edge reflects the 1s–5p transition state and the Ru–O bonds influence the peak shape near the absorption edge. The spectrum of the reference Ru metal has two
Fig. 4. Normalized XANES spectra of the present samples at the Pt-LIII edge. The two vertical lines denote positions of peaks in reference material Pt metal and PtO2 .
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Fig. 5. Normalized XANES spectra of the present samples at the Ru-K edge. Measurement of the sample ␥ was not performed. The two vertical lines denote positions of two peaks in reference material Ru metal.
Fig. 7. Radial structure function and Fourier back-transformed EXAFS oscillation in the inset of the sample E-P at the Ru-K edge.
peaks, as indicated by the two vertical lines (22,085 and 22,110 eV). The absorption edge of the reference RuO2 arises at higher energy than that of the Ru metal. In the spectrum of the radiation synthesized samples, the absorption edges arise at higher energy, resembling RuO2 in shape. In contrast, metallic features are easily seen in the spectra of the sample E and TEC61E54. The sample E keeps a trace of Ru metal at 22,110 eV, which corresponds to the second peak of the reference Ru metal. In TEC61E54, the two peaks as the metallic features are seen clearly. The linear combination analyses of the spectra were performed with the reference spectra of the Ru metal and the RuO2 in the area of −20 to 50 eV from the absorption edge. The results showed Ru metal:RuO2 ratios of 78:22 for TEC61E54, 31:69 for ␥-NH3 , 11:89 for ␥-P, 60:40 for E, 43:57 for E-NH3 and 28:72 for E-P. All the R-factors were less than 0.0002. The sample TEC61E54 and E showed lower RuO2 fractions. In addition, metallic features are easily seen in the spectra of the sample E and TEC61E54. These features are consistent with the promotion of PtRu alloying confirmed by XRD.
obtained at Pt-LIII and Ru-K edges, respectively. Broken lines stand for the Hanning window functions used in the analyses. Structural parameters were optimized by curve fits in q-space assuming single scattering paths. The curve fits were performed with the Artemis code [42,43], and backscattering amplitudes, phase shifts, and mean free paths were calculated by using the FEFF7 code [44]. Dots denote experimental values of back-transformed oscillations, and solid lines are drawn based on the optimized parameters. Note that the calculated values well reproduce the experimental plots. The optimized parameters in the present Pt-LIII edge EXAFS analyses assuming Pt–O, Pt–Ru and Pt–Pt bonds, are listed in Table 2, and those in the Ru-K edge assuming Ru–O, Ru–Ru and Ru–Pt bonds, are listed in Table 3. In these tables, interatomic distance and coordination number from A to B are denoted as RA–B and NA–B , respectively. The reliability of the present EXAFS analyses was checked by referring to the R-factors [43] listed in the last columns. They were all smaller than 0.044, which indicates enough reliability to allow us to discuss the bimetallic substructures. Judging from values of NPt–Pt , Pt–Pt bonds occur often in the radiolytic samples, while there are certain Pt–Ru and Pt–O bonds, though less than the Pt–Pt. In contrast, NRu–Ru values are far smaller than the NPt–Pt . These findings indicate that Pt atoms form relatively larger metallic particles, but Ru does not. This is consistent with the metallic features found in the Pt-LIII XANES and the XRD peaks from Pt (1 1 1) plane. NRu–O values in radiolytic samples are large, as compared to other bonds from Ru. Combining with the present data sets of the XANES and XRD, one may conclude that a major part of Ru atoms are considerably oxidized with too low crystallinity to give discernible diffraction. The number of Pt–Ru bonds should increase as both atoms are mixed well, and would be reflected on coordination numbers. In the present EXAFS analyses, NPt–Ru and NRu–Pt values neither diverged nor converged to physically absurd values. Therefore, certain occurrence of Pt–Ru would have been reflected on such determined values, although these NPt–Ru and NRu–Pt values did not agree. Even with such uncertainty, we may point out that relatively large coordination numbers are indeed seen for the samples synthesized with the EB, in other words, Pt–Ru mixing has been enhanced. This conjecture is better supported when the data sets of TEC61E54 are also taken into account. This sample is no doubt alloyed, judging from its XRD pattern and smaller RPt–Pt than those of pure Pt, and the samples ␥, ␥-NH3 , ␥-P and E-NH3 are presumably less mixed. Regarding the existence state of P, we tried EXAFS analyses assuming Pt–P and Ru–P bonds, but our parameters did not converge to physically plausible values. Therefore, we
3.2.2. Exafs Figs. 6 and 7 are typical radial structure functions (RSF) and Fourier back-transformed EXAFS oscillations of the sample E–P
Fig. 6. Radial structure function and Fourier back-transformed EXAFS oscillation in the inset of the sample E-P at the Pt-LIII edge.
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Table 2 Structural parameters obtained from the analysis of EXAFS at the Pt-L III edge. Pt coordination a
Ru coordination
O coordination
Sample ID
RPt–Pt (nm)
NPt–Pt
RPt–Ru (nm)
NPt–Ru
RPt–O (nm)
NPt–O
R-factor
␥b ␥-NH3 ␥-Pc E E-NH3 E-P TEC61E54
0.276 0.276 – 0.275 0.276 0.271 0.273
9.3 5.8 – 8.2 8.1 4.8 3.0
– 0.272 – 0.272 0.271 0.270 0.271
– 0.2 – 1.6 0.4 1.6 3.1
– 0.200 – 0.198 0.199 0.200 0.198
– 2.2 – 0.4 1.0 2.8 1.8
0.003 0.006 – 0.004 0.001 0.044 0.034
a b c
RPt–Pt = 0.277 nm in Pt metal. Assuming Pt–Pt bonds only. No physically reasonable parameter was obtained.
Table 3 Structural parameters obtained from the analysis of EXAFS at the Ru-K edge. Sample ID
␥b ␥-NH3 ␥-P E E-NH3 E-P TEC61E54 a b
Pt coordination
Ru coordination
O coordination
R-factor
RRu–Pt (nm)
NRu–Pt
RRu–Ru (nm)a
NRu–Ru
RRu–O (nm)
NRu–O
– 0.272 0.272 0.272 0.271 0.270 0.271
– 1.6 0.6 2.3 1.5 2.3 5.8
– 0.270 0.257 0.268 0.269 0.266 0.268
– 2.4 0.2 4.0 3.5 1.3 1.9
– 0.201 0.206 0.199 0.200 0.200 0.198
– 4.4 4.2 2.4 3.2 5.3 2.1
– 0.036 0.041 0.005 0.009 0.029 0.003
RRu–Ru = 0.266 nm in Ru metal. Not measured.
may conclude that phosphorus does not exist in the particles. We did not estimate the particle sizes of the samples with the coordination numbers determined by the EXAFS analysis, because of the difference of the particle shape or disorder of crystal structure [45–48] among the samples, as shown in the XRD patterns and TEM micrographs. 3.2.3. Methanol oxidation activity Fig. 8 shows the methanol oxidation current vs. potential curves obtained with the present samples, in which the oxidation current was normalized to the PtRu weight contained in each sample. It should be pointed out that the radiolytically synthesized material works as an electrode catalyst for the DMFC even without any post-treatment with heat and chemicals. They were obtained just by mixing the source materials, irradiation, washing, and drying.
Fig. 8. Oxidation current vs. potential curves of the present samples.
The oxidation currents exhibited by the samples ␥-P and E-P were comparable to that of the commercial catalyst TEC61E54, when evaluated at the potential of 0.45 vs. NHE which is supposed to be the effective potential during operation of the DMFC [49]. Thus evaluated catalytic activities of the present samples accord to the following order: ␥ < ␥-NH3 < ␥-P, E < E-NH3 < E-P. These relationships imply that the additives have been effective for size reduction and suppression of Ru back oxidation, whichever radiation was employed. 3.2.4. Correlation between PtRu structure and catalytic activity The present characterization results with XRF, XRD, XANES and EXAFS are now discussed in combination with the redox potentials of Pt and Ru. In general, the substructure of bimetallic nanoparticles formed by reducing aqueous ions should be influenced by the redox potentials of the constituent metals. The species with a higher redox potential is first reduced and precipitates and the other one follows it, likely resulting in a core/shell substructure [50–52]. The standard redox potentials of Pt2+ /Pt and Ru3+ /Ru are 1.18 and 0.69 V vs. NHE, respectively, so that a core/shell structure of Pt/Ru or individual particle formation is anticipated rather than alloy particles. The present characterization results indicated that not fully but partially randomized alloy phase is formed in the particles when the EB is used. The most reliable clue of alloying must be the XRD data, and clear peak shifts were in fact observed with the E in Fig. 3, but these shifts are not enough to account for the full randomness. The alloying possibly proceeded only halfway even with the EB and the additive for enhancing reduction. The bimetallic substructure occurring in the present samples would be of Pt-rich core and Ru-rich shell, still reflecting the restriction enforced by the redox potentials. The Ru-rich shell would be considerably oxidized so as not to give discernible diffraction, while the core would be virtually metallic in which possibly some alloying proceeds incompletely and gradually along the radial direction. This conjecture is supported by the XRD peak shift, the XANES features indicating Pt’s slight oxidation and Ru’s considerable oxidation, and Pt–Ru coordination significant but midway. The present data would in total
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alysts exhibited the enhanced activity, in spite of the incomplete alloying. Acknowledgments The ␥-ray irradiation was performed at the Institute of Scientific and Industrial Research, Osaka University. The authors thank the staff of the Institute of Scientific and Industrial Research for their assistance with the irradiation experiments. This research was partially supported by a Grant-in-Aid for Challenging Exploratory Research, (no. 19651042). References
Fig. 9. Correlation between methanol oxidation current and pairing factor PRu calculated from the results of the Ru-K edge analyses.
indicate that the degree of the alloying depends on the synthesis conditions. These proofs for the substructure are similar to those of PtRu/C that was synthesized with a polyol process. Its substructure was investigated by the XAFS correlating with its catalyst activity by Nitani et al. as follows. They defined the pairing factor PRu , which corresponds to the frequency of Pt–Ru pair in the bimetallic sample.
PRu =
NRu–Pt NRu–Pt + NRu–Ru
Since Ru exists preferentially in a near surface layer of the bimetallic particle, PRu calculated only from coordination numbers determined by Ru-K edge measurement may be regarded as an indicator for Pt–Ru pair occurrence on the surface. We calculated PRu values for the present samples and plotted them with the methanol oxidation currents in Fig. 9, where the oxidation current at 0.45 V vs. NHE was adopted. A clear correlation between PRu and current was found, except for the plot for TEC61E54. This correlation is similar to that reported by Nitani et al. though their conditions were different from ours with respect to temperature, solution concentration, and catalyst loading. Remembering that the bifunctional mechanism attributes the high CO tolerance of PtRu bimetallic catalysts to Pt–Ru atomic pairs, we may conclude this mechanism is valid in the PtRu nanoparticle system. The deviation of the plot of TEC61E54 from the others could be caused by its larger size and more enhanced alloying which could be given by a post annealing. 4. Conclusion We have successfully synthesized PtRu/C catalysts by the radiolytic process employing the ␥-rays and the EB. The PtRu nanoparticles were found to be smaller and have a more narrow size distribution using TEM observation. From the results of XRD and XAFS analyses, we concluded that the structure of PtRu is a Ptrich core/Ru-rich shell, which can be attributed to the difference of the redox potentials. The mixed state of PtRu at their surface was controlled by using high dose rates and the addition of ammonia and phosphorus. A correlation between the mixed state and activity of methanol oxidation was investigated using EXAFS analysis and employing the pairing factor. This investigation confirmed the validity of the bifunctional mechanism in the PtRu nanoparticle system synthesized by the radiolytic process. Our radiolytic cat-
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