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Role of Ru Oxidation Degree for Catalytic Activity in Bimetallic Pt/Ru Nanoparticles Huanhuan Wang,† Shuangming Chen,*,† Changda Wang,† Ke Zhang,† Daobin Liu,† Yasir A. Haleem,†

Xusheng Zheng,† Binghui Ge,‡ and Li Song*,†

†National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei 230029, China ‡Beijing National Laboratory for Condensed Mater Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

*S Supporting Information

ABSTRACT: Understanding the intrinsic relationship between the catalytic activity of bimetallic nanoparticles and their composition and structure is very critical to further modulate their properties and specific applications in catalysts, clean energy, and other related fields. Here we prepared new bimetallic Pt−Ru nanoparticles with different Pt/Ru molar ratios via a solvothermal method. In combination with X-ray diffraction (XRD), transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray absorption spectroscopy (XAS) techniques, we systematically investigated the dependence of the methanol electro-oxidation activity from the obtained Pt/Ru nanoparticles with different compositions under annealing treatment. Our observations revealed that the Pt−Ru bimetallic nanoparticles have a Pt-rich core and a Ru-rich shell structure. After annealment at 500 °C, the alloying extent of the Pt−Ru nanoparticles increased, and more Pt atoms appeared on the surface. Notably, subsequent evaluations of the catalytic activity for the methanol oxidation reaction proved that the electrocatalytic performance of Pt/Ru bimetals was increased with the oxidation degree of superficial Ru atoms.

■ INTRODUCTION Among various kinds of fuel cells, direct methanol fuel cells (DMFCs) have been considered to be promising power sources for future energy needs due to their high energy densities, low emissions, and facile fuel distribution and storage.1−3 Pt-based catalysts are the most efficient anode catalysts for the methanol oxidation reaction (MOR) in DMFCs.4 Nevertheless, challeng- ing issues of Pt-based catalysts such as the high cost, low abundance, and poison formation are the main obstacles to the commercialization of DMFCs.5 This has led to the develop- ment of Pt-based binary metallic systems, such as PtRu, PtMo, and PtSn, and ternary compounds, such as PtRuW, PtRuMo, and PtRuSn.6−8 PtRu alloy nanocrystals have been recognized as being greatly efficient electrocatalysts for methanol oxidation reaction.9 The effect of PtRu structural characteristics, such as composition, degree of alloying and Ru oxidation state, on the electrocatalytic activity for methanol oxidation has been reviewed.10 Guo et al. stated that the Pt−Ru (1:1) catalyst exhibited a highest methanol oxidation current and a lower poisoning rate.11 But Selda et al. found that a 0.25 Ru/Pt ratio is optimum at room temperature.12 An optimum ratio of 10− 30% Ru at room temperature for methanol oxidation has also been reported.13 There is also a debate on whether a PtRu bimetallic alloy or a Pt and Ru oxide mixture is the most effective methanol oxidation catalyst. Gasteiger et al. concluded that the high catalytic activity of Pt−Ru alloys for the electrooxidation of methanol is described very well by bifunctional action of the alloy surface.14 Huang et al. suggested

that the presence of crystalline RuO2 is essential to have a better methanol oxidation from Pt nanoparticles.15 On the other hand, Rolison et al. found that a commercial Pt−Ru catalyst composed of oxides of Pt and Ru could deliberately control the chemical state of Ru to form RuOxHy rather than Ru metal or particularly anhydrous RuO2 because of poor proton conduction.16 However, no unanimous conclusion has been reached until now. Therefore, understanding the intrinsic relationship between the catalytic activity of bimetallic nanoparticles and their composition and structure is very critical for further modulating their properties and specific applications in catalysts, clean energy, etc. The primary goal of the present work is to conclusively

establish the relative methanol oxidation activity of bimetallic Pt−Ru nanoparticles with different compositions and annealing treatments, using a consistent experimental approach. The catalyst samples were thoroughly characterized by physical and electrochemical technologies. Our detailed analysis of the bimetal’s catalytic activity for methanol oxidation reaction revealed that Pt/Ru nanoparticles with a Pt-rich core and Ru- rich shell structure promote increased electro-oxidation of methanol with the oxidation state of Ru atoms. This study provides useful insight for understanding the intrinsic relation- ship between catalytic property and structure/composition,

Received: December 15, 2015 Revised: February 26, 2016 Published: February 29, 2016

Article

pubs.acs.org/JPCC

© 2016 American Chemical Society 6569 DOI: 10.1021/acs.jpcc.5b12267 J. Phys. Chem. C 2016, 120, 6569−6576

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■ EXPERIMENTAL SECTION Sample Preparation. In a typical procedure for PtRu,

0.0889 g of poly(vinylpyrrolidone) (PVP), 400 μL of 0.2 M RuCl3(aq), and 800 μL of 0.1 M H2PtCl6(aq) were dissolved in 38.8 mL of ethylene glycol (EG) under constant magnetic stirring for 30 min. Then the mixed solution was transferred into a stainless autoclave having a 50 mL Teflon liner and heated in an oven at 200 °C for 12 h. After the autoclave was naturally cooled to room temperature, 23.68 mg of acetylene black was added to the resulting black solution and continuously stirred for 30 min. The final product was obtained by centrifugation, washed several times with deionized water and absolute ethanol, and dried in a vacuum oven at 60 °C for 12 h. The procedure for Pt2Ru and PtRu2 was the same as that for PtRu except that the molar ratio of RuCl3 and H2PtCl6 was changed to 1:2 and 2:1. To investigate the influence of annealing process, the resulting PtRu powder was calcined at 500 °C under 100 sccm H2/Ar flow for 4 h. Sample Characterization and XAFS Data Analysis. X-

ray diffraction was performed on a TTR-III high-power X-ray powder diffractometer employing a scanning rate of 0.02 s−1 in a 2θ range from 30° to 90° with Cu Kα radiation. The morphology of samples was characterized by transmission electron microscopy (TEM, JEM-2100F), equipped with energy-dispersive X-ray spectroscopy (EDX). The sample for TEM was prepared by placing a drop of ultrasonically dispersed ethanol solution onto a carbon-coated copper grid and allowing the solvent to be evaporated in air at room temperature. Metal concentrations were measured by inductively coupled plasma (ICP) atomic emission spectroscopy (AES) using an Atomscan Advantage Spectrometer. HAADF-STEM and EDX elemental mapping analysis were carried out in a JEOL ARM-200 microscope at 200 kV. X-ray photoelectron spectroscopy (XPS) experiments were performed at the Photoemission Endstation at the BL10B beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. This beamline is connected to a bending magnet and equipped with three gratings that cover photon energies from 100 to 1000 eV with a typical photon flux of 1 × 1010 photons/s and a resolution (E/ ΔE) better than 1000 at 244 eV. The Pt L3-edge and Ru K-edge XAFS measurements were made in transmission mode at the beamline 14W1 in Shanghai Synchrotron Radiation Facility (SSRF) and 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The X-ray was monochromatized by a double- crystal Si(311) monochromator, and the energy was calibrated using a platinum metal foil for the Pt L3-edge and a ruthenium metal foil for the Ru K-edge. The monochromator was detuned to reject higher harmonics. XAFS data were analyzed with WinXAS3.1 program.17 The energy thresholds were deter- mined as the maxima of the first derivative. Absorption curves were normalized to 1, and the EXAFS signals χ(k) were obtained after the removal of pre-edge and postedge back- ground. The Fourier transform (FT) spectra were obtained as k3χ(k) with a Bessel window in the range 3−12.5 Å−1 for the Pt L3-edge and 3.2−13.2 Å

−1 for the Ru K-edge. Theoretical amplitudes and phase-shift functions of Pt−Pt, Ru−Ru, Pt−O, and Ru−O were calculated with the FEFF8.2 code18 using the crystal structural parameters of the Pt foil, Ru foil, PtO2, and RuO2.

19−21 On the basis of a face-centered cubic (fcc) model,

the Pt−Ru bond was modeled. The S0 2 values were found to be

1.06 and 0.93 for Pt and Ru, respectively. Electrochemical Measurements. Electrochemical meas-

urements were taken using a conventional three-electrode system, with a Pt mesh electrode as counter electrode, a silver/ silver chloride electrode (Ag/AgCl) as the reference electrode, and a 3 mm diameter glassy carbon electrode as working electrode. The working electrode was prepared by coating a small amount of catalyst ink on glassy carbon electrode. Carbon-supported PtRu catalyst (2.0 mg) was dispersed into a solution containing 1 mL of ethanol and 10 μL of Nafion solution (5 wt %), followed by ultrasonic treatment for 30 min, and then the resultant suspension (ca. 10 μL) was pipetted onto glassy carbon electrode and dried at room temperature for 20 min. Prior to coating with catalyst ink, the glassy carbon electrode was polished with alumina paste and washed with deionized water. Cyclic voltammetry was carried out to study the methanol oxidation reaction (MOR) at room temperature in an electrolyte containing 1.0 M KOH and 1.0 M CH3OH between −0.8 and 0.3 V (vs Ag/AgCl) at a scan rate of 50 mV/ s. Prior to each cyclic voltammetry measurement, the electrolytic solution was purged with pure N2 for 30 min to remove dissolved oxygen.

■ RESULTS AND DISCUSSION XRD and TEM Characterization. Figure 1 shows the

comparison of XRD patterns for different samples. The

characteristic peaks for a face-centered cubic phase (fcc) were clearly observed in all samples. No additional peaks, such as those attributed to Pt or Ru oxides, can be detected. Interestingly, the characteristic peaks shifted to a higher angle with increasing Ru percentage, indicating the contraction of the lattice parameter due to formation of the Pt−Ru alloy. In addition, the diffraction peaks shifted to higher angles and became slightly sharper after annealing. This suggests that the annealing process can reduce the lattice parameter and slightly increase the grain size and the alloying extent of the Pt/Ru nanocrystals. The particle size and corresponding histograms of size

distribution of different samples are shown in Figure 2. Most particles of PtRu, Pt2Ru, and PtRu2 are monodisperse with an average size about 3−4 nm. After annealing, the particles became slightly larger in size (Figure 2d). The compositions of the catalyst were measured by ICP-AES and EDX and are

Figure 1. XRD patterns of PtRu, PtRu-annealed, Pt2Ru, and PtRu2.

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shown in Figure S1 and Table S1 (Supporting Information), in which the overall chemical compositions for PtRu, Pt2Ru, and PtRu2 alloy nanoparticle electrocatalysts are well confirmed with 1:1, 2:1, and 1:2 Pt:Ru atomic ratios. The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image and the corresponding EDX elemental mapping image of PtRu are shown in Figure 2e and Figure 2f. These observations reveal that the prepared PtRu particles are formed by Ru and Pt elements. The EDX elemental mapping image indicates that Ru atoms have a degree of dispersion higher than that of Pt atoms. XANES and XPS Analysis. To identify the microstructure

of Pt/Ru bimetals, we performed synchrotron-based X-ray absorption spectroscopy (XAS) of the samples. The X-ray absorption near-edge structure (XANES) spectra of the Pt L3- edge and Ru K-edge are shown in Figure 3. In the Pt L3-edge of Figure 3a, all samples exhibited more intense white line peaks than that of Pt foil. It is known that the Pt L3-edge white line corresponds to the excitation of 2p3/2 electrons to empty 5d orbitals,22 which means more unoccupied 5d states of Pt atoms in these Pt/Ru alloy nanoparticles in contrast to Pt foil. In general, this explanation can be ascribed to three effects: size effect, alloying effect, and surface oxidation effect. However, as the Pt atoms in pure Pt nanoparticles have more d electrons than that in bulk,23 the influence of the size effect can be eliminated. To clarify the alloying effect, we investigated the Pt L3-edge

XANES spectrum of Pt−Ru alloy and compared it with the

spectrum of pure Pt. In the calculations, we modeled the Pt L3- edge XANES spectra of Pt−Ru alloy by replacing some of the 12 nearest-neighbored Pt atoms around the central Pt atom with Ru atoms. As shown in Figure 3b, Pt/Ru alloy has a slightly weaker white line peak compared to pure Pt. That means the alloying effect cannot cause the increase in white line peak intensity. Finally, we suggest that the increase can be attributed to a surface oxidation effect. More precisely, it originates from the oxidation of some surface Pt atoms. Besides, it is worth noting that the white line intensity for PtRu, Pt2Ru, and PtRu2 was almost constant while PtRu-annealed exhibited a distinct increase, which can be explained by the increased oxidized Pt atoms after annealing. However, strong oxidation of Pt in these Pt−Ru alloy nanoparticles should be ruled out based on the direct comparison with bulk Pt and PtO2. For the Ru K- edge XANES spectra in Figure 3c, the Ru atoms in sample PtRu, Pt2Ru, and PtRu2 were partially oxidized where the order of oxidation degree is Pt2Ru > PtRu > PtRu2. Similarly, strong oxidation of Ru should also be eliminated. Notably, there is almost no oxidation of Ru in PtRu after annealing. This means that oxidized Ru atoms in PtRu were reduced by the annealing process. To further investigate the electronic structure of these Pt−Ru

nanoparticles, XPS spectra for the Pt 4f and Ru 3d core level region for all samples were measured as shown in Figure 4. As shown in Figure 4a, the binding energies (BE) of Pt 4f7/2 for all PtRu, Pt2Ru, and PtRu2 are almost the same while a slight right shift to higher BE can be observed for PtPu-annealed,

Figure 2. TEM images and histograms of particle-size distributions of (a) PtRu, (b) Pt2Ru, (c) PtRu2, and (d) PtRu after annealing treatment. (e) HAADF-STEM image. (f) The corresponding EDX elemental mapping image of PtRu.

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suggesting an increase in the d-vacancy of the Pt atoms.24 The Ru 3d core level region was deconvoluted as shown in Figure 4b−e, as described by Roblison et al.25 The corresponding deconvoluted results are summarized in Table 2. The XPS data suggest that there are three Ru species (Ru metal, RuO2, and RuO2·xH2O) present on the surface of the Pt−Ru catalyst. The percentages of Ru−OH species (RuO2·xH2O) and Ru-oxide increase in the following trend: Pt2Ru > PtRu > PtRu2 > PtRu- annealed, consistent with the XANES analysis. EXAFS Analysis. To further study the structure, the

corresponding extended X-ray absorption fine structure (EXAFS) of the samples was analyzed. The k3-weighted EXAFS signals of the Pt L3-edge and Ru K-edge are shown in Figure S2 (Supporting Information). It has been noted that EXAFS oscillations of all samples were lower in amplitude compared to that of bulk Pt and bulk Ru in both the Pt L3-edge and Ru K-edge, which can be attributed to the size effect of the nanoparticles. In contrast to amplitude, the phase of EXAFS oscillations of PtRu, Pt2Ru, and PtRu2 were similar to that of bulk sample, which indicates that these nanoparticles are more likely to be a core−shell structure rather than an alloying structure. For the control sample, the EXAFS oscillations of PtRu-annealed were slightly phase-shifted at each edge, indicating the increased alloying extent after the annealing process. Particularly, the comparison with the EXAFS signals of standard Pt and Ru oxides further confirmed that strong oxidation of Pt and Ru could be eliminated in our samples. Figure 5a and 5b shows the corresponding Fourier-

transformed EXAFS spectra of the Pt L3-edge and Ru K- edge. It is observed that the Pt L3-edge for PtRu, Pt2Ru, and PtRu2 exhibit similar local structure around Pt. However, there is a significant change in local structure around Pt in PtRu after

annealing. On the basis of the Ru K-edge, we can conclude that Ru atoms in Pt2Ru have the highest oxidation degree. EXAFS data analysis was carried out by simultaneously fitting both the Pt L3-edge and the Ru K-edge. The comparisons of experimental and fitting data for the Pt L3-edge and Ru K- edge are shown in Figures S3 and S4 (Supporting Information), and corresponding fitting parameters are summarized in Table S2 (Supporting Information). According to previously reported literature,26 we can

determine atomic distribution and alloying extent in bimetallic nanoparticles based on four parameters: Pobserved(NPt−Ru/NPt‑i), Robserved(NRu−Pt/NRu‑i), Prandom, and Rrandom. For PtRu and PtRu- annealed samples, Prandom and Rrandom can be taken as 0.5, as the atomic ratio of Pt and Ru is 1:1. For the Pt2Ru sample, Prandom and Rrandom can be taken as 0.33 and 0.67, respectively, as the atomic ratio of Pt and Ru is 2:1. Conversely, Prandom and Rrandom can be taken as 0.67 and 0.33 for PtRu2. Then alloying extents of Pt (JPt) and Ru (JRu) can be calculated using the following equations:

= ×J P P( / ) 100%Pt observed random (1)

= ×J P P( / ) 100%Ru observed random (2)

All the calculated results based on this method are summarized in Table 1. The observed parameter relationships ∑NPt−M > ∑NRu−M and JRu, JPt < 100% indicate that all of the as-synthesized Pt−Ru nanoparticles adopt a Pt-rich core and Ru-rich shell structure. The larger JPt and JRu values in PtRu- annealed indicate the increased extent of atomic dispersion and alloying extent after the annealing process, which is consistent with the above analysis. The higher values of Robserved and JRu suggest a higher alloying extent of Ru atoms compared with Pt.

Figure 3. XANES spectra at the (a) Pt L3-edge and (c) Ru K-edge for Pt foil, Ru foil, PtO2, RuO2, and all samples. (b) The comparison of the calculated Pt-L3 edge XANES spectra of pure Pt and Pt−Ru alloy with some Pt atoms substituted by Ru atoms in the first shell.

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This means that most of the Ru atoms were reduced and involved in alloying after the annealing process. Meanwhile, some Pt atoms migrated to the surface and were then oxidized by air, according to the XANES and XPS analysis. Here we can summarize that as-grown Pt/Ru nanoparticles have a Pt-rich core and Ru-rich shell structure. After the annealing process, the alloying extent of Pt/Ru nanoparticles had been increased, and more Pt atoms appeared on the surface. The structures of Pt/Ru nanoparticles are schematically shown in Figure 5c. Catalytic Performance in the Methanol Electro-

oxidation. Cyclic voltammetry experiments were performed in N2-saturated freshly prepared 1 M KOH solution by sweeping the electrode potential from −0.8 to 0.3 V vs Ag/ AgCl at a scan rate of 50 mV/s, to measure the electrochemical active surface area (ECSA) of the catalysts, as shown in Figure S5 (Supporting Information). The integrated charge in the hydrogen adsorption/desorption peak area in the CV curves represents the total charge concerning H+ adsorption, QH, and has been used to determine ECSA by employing the following equation:27

μ μ

= ×

Q

ECSA [m /g of Pt]

charge [ , C/cm ]

210 [ C/cm ] electrode loading [g of Pt/m ]

2

H 2

2 2

The trend in ECSA values varied in the following order: Pt2Ru (80.71 m2/g) > PtRu (64.01 m2/g) > PtRu2 (52.08 m

2/g) > PtRu-annealed (27.63 m2/g). Among these electrocatalysts, Pt2Ru was ascertained to possess the greatest electrochemical activity. Accordingly, it is rational to assume that the higher ECSA value may signify the better electrocatalyst that has more catalyst sites available for electrochemical reaction. To investigate the effect of Pt/Ru bimetal structure on the

catalytic property, a methanol electro-oxidation experiment was carried out. Figure 6a displays cyclic voltammograms (CVs) of methanol oxidation on Pt2Ru, PtRu, PtRu2, and PtRu-annealed in 1.0 M KOH containing 1.0 M CH3OH solution. Two well- defined oxidation peaks can be clearly observed: one in the forward scan is produced because of oxidation of freshly chemisorbed species coming from methanol adsorption, and the other in the reverse scan is primarily ascribed to removal of incompletely oxidized carbonaceous species formed during the forward scan. As known, the oxidation peak during the forward

Figure 4. XPS spectra of (a) Pt 4f and C 1s + Ru 3d for (b) PtRu, (c) PtRu-annealed, (d) Pt2Ru, and (e) PtRu2. The entire Ru 3d + C 1s envelope was deconvolved for all spectra, but for clarity, only the fits for Ru 3d5/2 lines are shown. The envelopes are fitted with three Ru 3d5/2 peaks.

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scan can be used to evaluate the catalytic activity of the catalyst. It is estimated that the values of current density increase in the following trend: Pt2Ru > PtRu > PtRu2. This phenomenon is attributed to two probable reasons: one is increasing oxidation degree of surface Ru atoms in these samples (Pt2Ru > PtRu > PtRu2), which is consistent with the order of catalytic activity of the catalysts, while the other is increasing Pt concentration in

these Pt/Ru catalysts. However, with the same composition, the PtRu-annealed sample with the lowest oxidation degree of Ru atoms and more Pt atoms on the surface exhibits the worst catalytic activity. Thus, we can suggest that the higher methanol oxidation catalytic activity originates from the increasing oxidation degree of surface Ru atoms in Pt/Ru bimetals. This is probable due to the content of Ru−OH increasing with the oxidation degree of surface Ru atoms, as Ru−OH is a critical component of the MOR of the Pt−Ru catalyst which determines the electrocatalytic activity of Pt−Ru.25 Further- more, the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used as an important index to evaluate the catalyst tolerance to CO accumulation.28,29 Our calculation indicates that Pt2Ru, PtRu, and PtRu2 have almost the same If/Ib value, while the If/ Ib value of PtRu-annealed is obviously larger. This may be attributed to the increasing alloying extent after the annealing process, as it has been proved that the tolerance to CO accumulation by the Pt−Ru alloying catalyst will increase with the alloying degree.30 Thus, the best catalyst for oxidation of accumulated CO is not necessarily the best one for methanol oxidation.10

Moreover, chronoamperometry (CA) was also performed to investigate the long-term stability of those catalysts under the same conditions. Figure 6b shows CA curves performed in 1.0 M KOH + 1.0 M CH3OH at −0.2 V (vs Ag/AgCl) for 2500 s. After a sharp drop in the initial period of around 300 s, the currents decay at a much slower speed and then approach a flat line. During the whole time, it was clear that current density produced on the Pt2Ru catalyst was higher than the current density produced on the PtRu, PtRu2, and PtRu-annealed catalysts. These results are in agreement with those of the cyclic voltammetry measurements, indicating that Pt/Ru bimetals

Figure 5. Fourier-transformed EXAFS spectra of the (a) Pt L3-edge and (b) Ru K-edge for Pt foil, Ru foil, PtO2, RuO2, and all samples. (c) Schematic representation of the structure of the Pt−Ru nanoparticles having different molar ratios synthesized by EG reduction and after annealing.

Table 1. Alloying Extent Values of All Samples

sample ∑NPt‑M ∑NRu‑M Pobserved Robserved JPt(%) JRu(%)

PtRu 10.2 7.3 0.09 0.26 0.18 0.52 PtRu-annealed 10.1 6 0.19 0.53 0.38 1.06 PtRu2 10.8 7.5 0.1 0.2 0.15 0.61 Pt2Ru 10.1 7.5 0.07 0.12 0.21 0.18

Table 2. Binding Energies of Ru Species Obtained from Curve-Fitted Ru 3d5/2 XPS Spectra for PtRu Catalysts

catalysts binding energy/

eV assignment relative

concentration/%

PtRu 280.0 Ru metal 58.25 280.9 RuO2 19.42 282.2 RuO2·xH2O 22.33

PtRu-annealed 279.8 Ru metal 62.16 280.8 RuO2 21.62 282.2 RuO2·xH2O 16.22

Pt2Ru 280 Ru metal 45.46 280.9 RuO2 27.27 282.3 RuO2·xH2O 27.27

PtRu2 280 Ru metal 61.54 280.9 RuO2 19.23 282.2 RuO2·xH2O 19.23

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with higher Ru oxidation degree can pose better methanol oxidation catalytic activity.

■ CONCLUSIONS Bimetallic Pt−Ru nanoparticles with different Pt/Ru molar ratios were synthesized by a solvothermal method and characterized by various methods. Our observations revealed that these Pt−Ru nanoparticles have a Pt-rich core and a Ru- rich shell structure. After annealing at 500 °C, the alloying extent of Pt/Ru nanoparticles increased, a portion of Pt atoms migrated to surface, and most of the surficial oxidized Ru atoms were reduced and involved in alloying. The evaluations of methanol electro-oxidation activity elucidated that electro- catalytic performance improved with the increasing oxidation degree of superficial Ru atoms. This study provides useful information and deep insight for understanding the relationship of electrocatalytic performance of bimetallic nanoparticles with their structure, which may help us to further tune the bimetal structure, composition, and catalytic activity for specific applications.

■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12267.

EDX analyses of Pt2Ru, PtRu, and PtRu2. Comparison of compositions determined from EDX and ICP. Compar- ison of k3-weighted EXAFS signals, experimental data, and the fitting curves for Pt L3-edge and Ru K-edge. Cyclic voltammograms (CVs) of all samples in 1 M KOH. Best fit parameters of the Pt L3-edge and Ru K- edge EXAFS spectra (PDF)

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS Financial support comes from 973 program (2014CB848900), NSF (U1232131, U1532112, 11375198, 11574280), the Postdoctoral Science Foundation of China (2015M581990), the Fundamental Research Funds for the Central Universities (WK2310000053), and User with Potential from CAS Hefei Science Center (CX2310000080). We also thank the SSRF (BL14W1), BSRF (1W1B), MCD, and Photoemission Endstations in NSRL for help with synchrotron-based measurements and the USTC Center for Micro and Nanoscale Research and Fabrication.

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Figure 6. (a) Cyclic voltammograms and (b) chronoamperometric curves at −0.2 V for 2500 s toward methanol electro-oxidation of Pt−Ru nanoparticles. Electrolyte solution was 1.0 M KOH + 1.0 M CH3OH (scan rate: 50 mV/s).

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