Amorphous CuPt Alloy Nanotubes Induced by Na2S2O3 as Efficient

May 23, 2016 - ... anode electrocatalysts for direct methanol fuel cells. Lijun Zheng , Dachi Yang , Rong Chang , Chengwen Wang , Gaixia Zhang , Shuhu...
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Research Article pubs.acs.org/acscatalysis

Amorphous CuPt Alloy Nanotubes Induced by Na2S2O3 as Efficient Catalysts for the Methanol Oxidation Reaction Yige Zhao, Jingjun Liu,* Chenguang Liu, Feng Wang,* and Ye Song State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029 China S Supporting Information *

ABSTRACT: Here we propose amorphous CuPt alloy hollow nanotubes as efficient catalysts for the methanol oxidation reaction (MOR) prepared by Na2S2O3-assisted galvanic replacement reaction. The formation mechanism can be explained by the nanoscale Kirkendall-effect-induced hollowing process of the galvanic replacement reaction. The electrochemical tests suggest that the amorphous CuPt alloy exhibits better MOR activity and stability than the crystalline CuPt and commercial Pt/C catalysts, which can be ascribed to the enhanced CO tolerance ability of amorphous alloy. XPS measurements demonstrate that the enhanced anti-CO poison characteristic of amorphous CuPt alloy originates from the strong interaction between Pt and Cu atoms as a result of a unique crystallization state. This research not only provides a facile approach to synthesize amorphous alloy but also opens up an interesting way for amorphous Pt-based alloy to apply to the MOR. KEYWORDS: amorphous alloy, galvanic replacement reaction, methanol oxidation reaction, CO tolerance, bifunctional mechanism

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range, which allows the fine-tuning of electronic properties to meet the demands of highly catalytic performance.26 It has been demonstrated that electron transfer between Pt and another metal (M) for the Pt-based alloy is one of the major effects to modify the adsorption energy of CO adsorbed on the Pt surface during MOR.27 The amorphous alloys are likely to form a unique electron structure to promote the electron transfer between Pt and M atoms, leading to a lower electron density of Pt 5d orbital. The lowered electron density will lessen the electron backdonation from Pt 5d orbital to the CO 2π* orbital and thus destabilize the Pt-CO bond, resulting in the enhanced anti-CO poison capacity and improved MOR activity. Considering their special properties, amorphous alloys may open up an interesting way to gain efficient Pt-based catalysts for MOR. To our best knowledge, amorphous Pt-based alloys applied for MOR have rarely been researched. As for the synthetic routes of amorphous alloys, the two currently used methods are the rapid quenching and the chemical reduction methods.25,28 The rapid quenching method is technologically difficult and expensive for large-scale production of nanomaterials because it must guarantee a cooling rate of at least 105 to ∼106 K/s in vacuum or inert atmosphere. In terms of the chemical reduction method, the strong and exothermic reduction reaction probably results in serious aggregation, thus

INTRODUCTION Direct methanol fuel cells (DMFCs), as important alternative energy sources for transportation and portable electronics, have attracted extensive interest because of their low operation temperature, convenient storage of liquid fuel, and high energy density.1−6 Electrocatalysts are essential components in DMFCs because of the sluggish anode kinetics of the methanol oxidation reaction (MOR). Although platinum has been considered as the most common and efficient catalyst, the high cost and easy poisoning by intermediate CO during MOR confine its practical application.7−9 Up to now, Pt alloyed with a non-noble metal (M) is deemed to be the most effective way to protect Pt from CO poisoning,10−13 accompanied by reducing the costs. It is well-documented that catalytic performances of Pt−M alloy catalysts can be influenced by their morphology,14,15 chemical composition,16,17 and surface structure.18,19 These factors are strongly dependent on the atomic arrangement and coordination related to low-coordinated atoms, steps, kinks, and vacancies,20 which are easily able to interact with reactants and act as active sites to break chemical bonds, resulting in the excellent catalytic performances.21,22 Amorphous alloys, with long-range disordered but short-range ordered atomic arrangement, have more prominent catalytic properties relative to their crystalline counterparts.23 On the one hand, they possess abundantly unsaturated sites with high surface energy,24 which are crucial for the activation of reactants in catalysis to enhance the catalytic properties.25 On the other hand, the compositions of amorphous alloys can be regulated in a wide © XXXX American Chemical Society

Received: February 22, 2016 Revised: April 24, 2016

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DOI: 10.1021/acscatal.6b00540 ACS Catal. 2016, 6, 4127−4134

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ACS Catalysis

Figure 1. (A) SEM image of amorphous CuPt nanotubes (Inset: TEM image of amorphous CuPt); (B) XRD patterns of amorphous CuPt, crystalline CuPt, and Cu nanowire. The green vertical lines represent the peak positions of pure Pt (PDF no. 04-0802). The inset shows the SAED image of amorphous CuPt; (C) STEM image of amorphous CuPt; (D−F) EDX mapping of Pt (D), Cu (E), and the composite Pt versus Cu (F). (G) Schematic illustration for the formation of amorphous CuPt alloy NTs.

Kirkendall-effect-induced hollowing process of the galvanic replacement reaction. The amorphous CuPt alloy exhibits better MOR activity and stability than the crystalline CuPt and commercial Pt/C catalysts, which can be ascribed to the enhanced CO tolerance ability of amorphous alloy. XPS measurements suggest that the enhanced anti-CO poison characteristic of amorphous CuPt alloy results from the strong interaction between Pt and Cu atoms.

decreasing the catalytic activity. In this regard, galvanic replacement reaction exhibits a fairly facile and mild route to the fabrication of bimetallic nanostructures.29,30 Over the past decades, the galvanic replacement reaction is usually applied to synthesize nanocrystalline materials.31−33 As demonstrated in our previous work,34 for the system involving Cu nanowires (NWs) and a H2PtCl6 precursor without any additive, the resulting product is crystalline CuPt alloy nanotubes (NTs). It seems difficult to generate amorphous CuPt NTs by employing the conventional galvanic replacement reaction. Herein, we report the Na2S2O3-induced formation of amorphous CuPt alloy NTs by galvanic replacement reaction between H2PtCl6 and Cu nanowires, where Na2S2O3 serves as the inhibitor of disproportionation reaction of CuCl. The formation mechanism can be explained by the nanoscale

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EXPERIMENTAL SECTION Preparation of CuPt Alloy Nanotubes. The amorphous CuPt nanotubes were fabricated by a Na2S2O3-aided galvanic replacement reaction in aqueous solution composed of H2PtCl6, copper nanowires, where Na2S2O3 served as the inhibitor of 4128

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of the Cu35Pt65 (atom %, determined by ICP−MS measurement) prepared with the addition of Na2S2O3. We can observe a hollow structure of the alloy with about 70−105 nm diameter, 15 nm wall thickness. Besides, their lengths reach to several micrometers (Figure S1), the value of which relies on the length of copper nanowire templates (Figure S2). As displayed in the XRD spectrum (Figure 1B), no obvious peaks can be seen for the sample fabricated in the presence of Na2S2O3, indicating that its structure is amorphous. Apart from that, the selected area electron diffraction (SAED) pattern of the amorphous specimen in Figure 1B inset exhibits a continuous hollow ring that is a characteristic diffraction pattern of an amorphous material, further confirming the amorphous structure. Moreover, a contrastive sample of crystalline CuPt alloy (Cu35Pt65, atom %) was also prepared without Na2S2O3 to study the effect of the additive on the crystallization state of Cu−Pt alloy. We can see that the appearance of the sample prepared without Na2S2O3 (Figures S3 and S4A) is similar to that of the amorphous sample. However, the XRD result of the contrastive sample (Figure 1B) shows that it presents a typically crystalline state with a facecentered-cubic structure, as proven by its SAED image (Figure S4B). In addition, according to the previous report,35 a high alloying degree contributes to the catalytic performance for the alloy catalysts. Therefore, we studied the alloying degree of the amorphous CuPt alloy by evaluating atomic-level scattering of Pt and Cu atoms in the NTs using energy-dispersive X-ray spectroscopy (EDX) in an aberration-corrected scanning transmission electron microscope (STEM), as shown in Figure 1(C−F). The Pt versus Cu image (Figure 1F) exhibits the uniform distribution of Pt and Cu atoms, suggesting that the amorphous alloy NTs have a high alloying degree. The possible formation mechanism of the amorphous CuPt alloy NTs can be explained by the nanoscale Kirkendall-effectinduced hollowing process of the galvanic replacement reaction between Cu NWs and PtCl62− ions with the help of Na2S2O3. The reactions are displayed in the following equations:36

disproportionated reaction of CuCl generated by the replace reaction between the Cu and H2PtCl6. First, copper nanowires with about 100 nm in diameter and several micrometers in length were prepared through a simple chemical reduction as follows: 20 mL of copper nitrate (0.1 M) was dissolved into NaOH (10 M) aqueous solution in a round-bottom flask with stirring, and then ethylenediamine (4 mL) and 1 mL of hydrazine (85%) were added. When all the reactants mixed uniformly, the flask was put in a water bath at 80 °C for 1 h. Subsequently, the sedimentary products were filtered and washed using deionized water and ethanol to gain the copper nanowires. Second, the copper nanowires (64 mg) and Na2S2O3 (79 mg) were dissolved into 100 mL of N2-saturated deionized water in a three-necked flask and dispersed by ultrasonic agitation. Afterward, the H2PtCl6 solution was added dropwise to the flask to promote the galvanic reaction, which lasted for half an hour, producing the amorphous CuPt nanotubes. Finally, the as-prepared CuPt nanotubes were filtered and washed with deionized water and ethanol. For comparison, the crystalline CuPt nanotube was also prepared by the same method, except for no addition of Na2S2O3 in the galvanic replacement reaction. Physical Characterizations. The morphologies of the CuPt alloy nanotubes were observed by scanning electron microscopy (SEM, JEOL 6701F) and transmission electron microscopy (TEM). The TEM and selected area electron diffraction (SAED) characterization were performed with a JEOL TEM 2010 microscope. The structure analysis was carried out using X-ray diffraction (XRD) with Cu Kα radiation at λ = 1.5406 Å. The atom dispersion was explored by an aberration-corrected scanning transmission electron microscope (STEM, JEOL ARM 200F) and energy dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectra (XPS, ESCALAB 250) were obtained from a monochromator (Al KR source) calibrated compared to the C (1s) peak at 284.6 eV. The alloy compositions were researched by inductively coupled plasma mass spectrometry (ICP−MS) measurements. Electrochemical Measurements. The MOR activities and stabilities of the CuPt nanotubes were evaluated through cyclic voltammetry (CV) measurements in N2-saturated 0.1 M HClO4 and 0.5 M CH3OH solution in a conventional three-electrode system by Autolab PGSTAT30. For the electrochemical cell, a glassy carbon (GC) electrode was used as the working electrode. The Pt foil and saturated calomel electrode (SCE) with a salt bridge acted as the counter electrode and reference electrode, respectively. All potentials are calibrated to RHE. To prepare the working electrode, 10 mg of catalysts were dispersed ultrasonically in alcohol (2 mL) and Nafion (100 μL). Afterward, 10 μL of this above catalyst ink was pipetted onto a GC substrate (0.071 cm2). For the CO stripping tests, CO was preadsorbed on the GC electrode coated with electro-catalyst by bubbling in the electrolytes for 30 min while keeping the electrode potential at −0.15 V (vs SCE) in 0.1 M HClO4. After ventilation with N2 for another 30 min to remove any dissolved CO in the electrolyte, the adsorbed CO was oxidized by CV at a sweep rate of 50 mV/s, obtaining the CO stripping curve. Two consecutive CV curves were also recorded to confirm the complete oxidation of the adsorbed CO gas. All electrochemical measurements were performed at room temperature and at ambient pressure.

Cathode: PtCl 6 2 − + 2e− → PtCl4 2 − + 2Cl− (0.68 V vs SHE) PtCl4 2 − + 2e− → Pt + 4Cl−

(0.73 V vs SHE)

(1)

Anode: Cu + Cl− → CuCl + e−

(0.137 V vs SHE)

CuCl + xS2 O32 − → [CuCl(S2 O3)x ]2x − (2)

Overall: 4Cu + 4xS2 O32 − + PtCl 6 2 − → Pt + 4[CuCl(S2 O3)x ]2x − + 2Cl−

(3)

It is well-known that the disproportionated reaction of CuCl (2CuCl → Cu + Cu2+ + 2Cl−) easily happens at room temperature. However, Na2S2O3, as a complexing agent for CuCl, can restrain its disproportionation, making the anode reaction (eq 2) occur. Since the standard reduction potential of PtCl62−/Pt pairs (0.68 V) is significantly more positive than that of Cu/CuCl (0.137 V), the overall reaction (eq 3) will take place spontaneously. On the basis of the stoichiometric relationship in eq 3, the deposition of one Pt atom will consume four Cu atoms, which leads to redundant lattice vacancies on the surface of Cu

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RESULTS AND DISCUSSION Characterizations of Amorphous CuPt NTs. Figure 1A and its inset show the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, respectively, 4129

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Figure 2. (A) CV curves in N2-saturated 0.1 M HClO4 and 0.5 M CH3OH solution at a scan rate of 10 mV/s at room temperature; (B) onset potentials; (C−E) long-term stability measurements: CV curves of (C) amorphous CuPt, (D) crystalline CuPt, and (E) commercial Pt/C before and after 1000 cycles; (F) The loss values of Jf after 1000 cycles.

vacancies to balance the diffusivity difference.38 As the reaction time increases, the vacancies inside the NWs will reach saturation and condense to form Kirkendall voids.39 Subsequently, these voids tend to grow and collapse in the center, until the interior material disappears, eventually leaving amorphous Pt−Cu alloy NTs behind (Figure 1G). Electrocatalytic Measurements. The catalytic activity of amorphous CuPt alloy (Cu35Pt65, at%) toward MOR was studied by a three-electrode system in a N2-purged 0.1 M HClO4 and 0.5 M CH3OH solution at a scan rate of 10 mV/s, and the samples of crystalline CuPt alloy (Cu35Pt65, at%) and a commercial Pt/C (ETEK, 40 wt %) were also tested for comparison. The current densities are normalized by the geometric area of the electrode (0.071 cm2). From the resultant cyclic voltammetry (CV) curves shown in Figure 2A, it can be seen that the onset potential decreased in the sequence: amorphous CuPt > crystalline CuPt > Pt/C. The amorphous CuPt exhibits a marked negative shift of 21 mV and 25 mV with respect to the crystalline CuPt and commercial Pt/C (Figure 2B), respectively. Moreover, the

NWs template because less Pt atoms cannot compensate the vacancies left by Cu atoms. However, for the system without Na2S2O3, the overall reaction is shown below34. 2Cu + PtCl 6 2 − → Pt + 2Cu 2 + + 6Cl−

(4)

We can see that the generation of one Pt atom only requires two Cu atoms. Therefore, for the reaction system with Na2S2O3, there will be more lattice vacancies, resulting in higher surface free energy. In this case, the atoms are inclined to rearrange randomly because the energy of amorphous structure is lower than that of crystalline structure with lots of defeats.37 In other words, the amorphous structure is more favorable than the crystalline structure in this case. As the reaction proceeds continuously, vast Cu atoms are consumed and the high surface energy will trigger the accelerated outward diffusion of Cu atoms from the interior to the surface. Meanwhile, Pt atoms will keep on depositing on the surface and diffuse inward to form the Pt−Cu alloy. As a consequence, a total outward Cu atoms flux from the interior to the surface occurs, accompanying the inward flux of 4130

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Figure 3. (A−C) CO stripping voltammograms; (D) onset potentials for CO oxidation; (E) schematic illustration of the CO tolerance abilities for the amorphous CuPt, crystalline CuPt, and commercial Pt/C catalysts.

at a scan rate of 200 mV/s to accelerate the reaction. The obtained CV curves before and after 1000 cycles are shown in Figure 2(C−E). It can be observed that the amorphous CuPt shows a least loss of Jf (9.6%) and exhibits the highest stability among these catalysts (Figure 2F). To compare our catalysts with the commercial Pt−Ru catalyst, we further performed the MOR measurement for the commercial Pt−Ru/C catalyst (Johnson Matthey), as shown in Figure S7. We can observe that our amorphous CuPt catalyst has relatively poor MOR activity compared with the Pt−Ru alloy. The reason why the amorphous catalyst does not exhibit more excellent activity than the commercial Pt−Ru alloy may be that the present alloy composition (Cu35Pt65) is not optimal. It will be our further

amorphous CuPt has the maximum forward peak current density (Jf) values (7.8 mA.cm−2, Figure S5), higher than the corresponding values of the crystalline CuPt (7.1 mA.cm−2) and Pt/C (6.7 mA·cm−2). If only considering the mass of Pt, the mass activities (Jf values normalized to Pt loading amount, Figure S6) of the amorphous alloy (373.7 mA.mg−1Pt) is still higher than that of the crystalline CuPt (340.1 mA·mg−1Pt) and Pt/C (281.8 mA·mg−1Pt). These outcomes suggest that the amorphous CuPt catalyst exhibits remarkably enhanced activity toward MOR, compared with the crystalline CuPt and Pt/C catalysts. Besides, the stabilities of the above catalysts were estimated by potential cycling at a scan rate of 10 mV/s before and after 1000 cycles, while the intermediate process (1−1000 cycles) was performed 4131

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Figure 4. (A) XPS spectra showing Pt 4f peaks; (B) fitted Pt(0) 4f7/2 core-level binding energies; (C) schematic illustration of the mechanism of enhanced anti-CO poison property.

catalysts (Figure 3E), leading to the highest MOR activity and stability. To further investigate the CO tolerance ability of the amorphous CuPt alloy, we also performed the CO stripping cyclic voltammetry (CV) for the commercial Pt−Ru/C catalyst (Johnson Matthey) to make a comparison with the amorphous alloy, as shown in Figure S9. We can see that the CO anodic oxidation on commercial Pt−Ru/C catalyst starts earlier (0.50 V) than that on the amorphous CuPt alloy (0.72 V), indicating that the commercial Pt−Ru/C catalyst has a stronger CO oxidation and better CO tolerance abilities than our amorphous CuPt alloy. As shown in Figure S7, the Pt−Ru catalyst displays better MOR activity compared to the amorphous CuPt catalyst. It reveals that the avoidance for CO poisoning is mainly responsible for the MOR activity for the Pt-based catalysts. This conclusion has been verified by previous published papers.4,7,8,41 As well-known, the activity of alloy catalyst strongly depends on their compositions. So, if the CO tolerance ability of the alloy is further improved by adjusting the atomic ratio of Pt and Cu atoms, the MOR activity of the amorphous would be further improved. If so, the amorphous CuPt alloy will have promising commercial value. Origin of Enhanced CO Tolerance. To gain further insight into the mechanism of enhanced anti-CO ability for the amorphous CuPt alloy, X-ray photoelectron spectra (XPS) experiments were carried out to give detailed analysis of the

investigation work to further improve the MOR activity of the amorphous CuPt catalyst by adjusting the atomic ratio of Pt and Cu atoms. To eliminate the carbon effect, we also performed the MOR measurement for the Pt black catalyst, as shown in Figure S8. We can observe that our amorphous CuPt catalyst has higher forward peak current than the Pt black catalyst, indicating that our amorphous CuPt catalyst still has better MOR activity than the Pt black catalyst. Moreover, we performed the CO stripping cyclic voltammetry (CV) for the three catalysts in 0.1 M HClO4 solution with a sweep rate of 50 mV/s, as shown in Figure 3A−C). For the three catalysts, the hydrogen desorption peaks were completely suppressed in the first positive scan, which was due to the saturated coverage of COads on the active surface sites. A broad peak corresponding to CO anodic oxidation appeared in the potential range from 0.7 to 1.1 V. In the reverse sweep, the hydrogen adsorption peak arose since the active sites were freed after the oxidation removal of CO. Along with the oxidation removal of adsorbed CO, the hydrogen adsorption/desorption peaks reappeared in the second scan.40 Significantly, we can see that the CO anodic oxidation on the amorphous CuPt alloy (0.72 V) starts much earlier than that on the crystalline CuPt (0.76 V) and Pt/C catalysts (0.79 V), as shown in Figure 3D. This result indicates that the amorphous CuPt catalyst has the weakest CO adsorption and best CO tolerance ability among the three 4132

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ACS Catalysis surface electronic state. The obtained Pt 4f core-level binding energy is shown in Figure 4A. It can be seen that Pt has diverse valence states such as Pt0 (metallic Pt) and Pt2 (PtO and Pt(OH)2) in these catalysts. By comparing their relative intensities, we conclude that Pt0 dominates among the components because it can offer more active sites for MOR than Pt.3,42 Thus, the Pt0 binding energies represented by Pt0 4f7/2 are compared and depicted in Figure 4B. We can observe that the Pt0 4f7/2 peak of the amorphous CuPt shifts to a higher binding energy (71.6 eV) in contrast to that of crystalline CuPt (71.3 eV) and Pt/C (70.9 eV) catalysts. Owing to the higher electronegativity of Pt (2.28) than that of Cu (1.90), Pt atoms will withdraw the electrons from Cu in their alloy.43 In general, the increased electron density around Pt would cause downshift of Pt 4f binding energy in its alloys.8 However, in our case, the observed Pt 4f binding energy of the amorphous alloy is upshifted as compared to that of the other Pt-based catalysts. The upshift of Pt 4f binding energy in the amorphous CuPt alloy can be explained by the difference in work function of Pt and CuPt alloy and the rehybridization of both d-band and sp-band in the CuPt alloy, according to the previous report.44 Because of Pt alloying with Cu, the change of its work function leads to the upshift of the reference level, that is, the Fermi energy (EF), in photoelectron measurements.45,46 This will result in the upshift of Pt 4f binding energy in the amorphous alloy. In addition, Mukerjee et al. revealed that the total electron number per Pt atom increased, but the Pt 5d electron decreased, when Pt was alloyed with a second metal like Cu.47 Therefore, in our amorphous alloy, although Pt withdraws electron from Cu, Pt 5d electron still decreased. That is, Pt atom in the amorphous alloy still has more 5d vacancies. According to the reported literature,48 the core-levels and valence bands of the metal are more sensitive to the variations of the d electron occupancy than to those of the s, p orbitals. Therefore, the loss of Pt 5d electrons in amorphous alloy may also lead to the positive shifts of Pt 4f binding energies. Like an Au−Al alloy system with different electronegativity,48 the loss of 5d electrons at the Pt site is perhaps due to the valence electron redistribution caused by intra-[Pt(5d) → Pt(6s, 6p)] orbital hybridization, as Pt is alloyed with Cu. As reported, the formation of the Pt−CO bond depends on the electron donation from the 5σ orbital of CO to the empty Pt d orbital and the back-donation from the filled Pt d orbital to the 2π* orbital of CO.49 In our work, although the amorphous CuPt has more 5d vacancies, the Pt atoms in the amorphous Pt− Cu alloy have withdrawn some electrons from Cu, making the total electrons increase, which possibly makes the adsorption of CO as electron-donating species more difficult. Moreover, it has been proposed that the back-donation will strengthen the Pt-CO bond by removing the Pt excess negative charge.50 As for the amorphous CuPt, it has more 5d orbital vacancies and a less filled 5d orbital, which reduces the back-donation and weakens the stability of Pt-CO bond (Figure 4C). More importantly, according to the previous report,49 the Pt atom with less d electron density may be highly active for the oxidation of the adsorbed water described by eq 5. Considering the so-called “bifunctional mechanism” (eq 6) proposed by Watanabe and Motoo,51 the formative Pt−OH can contribute to the removal of the CO intermediate (Figure 4C). On the basis of the above reasons, we can conclude that the unique interaction between Pt and Cu atoms in the amorphous CuPt alloy contributes to the enhanced ability of CO removal, improving the MOR activity and stability.

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Pt − OH 2 → Pt − OH + H+ + e−

(5)

Pt − CO + Pt − OH → 2Pt + CO2 + H+ + e−

(6)

CONCLUSION In summary, we have fabricated amorphous CuPt alloy NTs by Na2S2O3-assisted galvanic replacement reaction between PtCl62− and Cu NWs. The possible formation mechanism of the amorphous CuPt alloy can be explained by the nanoscale Kirkendall-effect-induced hollowing process of the galvanic replacement reaction. As a result, the amorphous CuPt alloy catalyst exhibits better MOR activity and stability than the crystalline CuPt and commercial Pt/C. The outstanding performance of amorphous CuPt catalyst is due to its enhanced anti-CO poison characteristic, which can be ascribed to the strong interaction between Pt and Cu atoms as a result of special electronic state. This work not only provides a facile approach to fabricate amorphous CuPt alloy with improved electrochemical performance but also opens a new direction for amorphous Ptbased alloy to apply to MOR.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00540. SEM, TEM, SAED images; CV curves for MOR; CO stripping test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-10-64411301. *E-mail: [email protected]. Tel: +86-10-64451996. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Funds of China (Grant No. 50972003, 51125007). REFERENCES

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DOI: 10.1021/acscatal.6b00540 ACS Catal. 2016, 6, 4127−4134

Research Article

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DOI: 10.1021/acscatal.6b00540 ACS Catal. 2016, 6, 4127−4134