Structural Regulation of PdCu2 Nanoparticles and Their

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Structural Regulation of PdCu Nanoparticles and Their Electrocatalytic Performance for Ethanol Oxidation Jing Xue, Guangting Han, Wanneng Ye, Yutao Sang, Hongliang Li, Peizhi Guo, and Xiusong Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13368 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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ACS Applied Materials & Interfaces

Structural Regulation of PdCu2 Nanoparticles and Their Electrocatalytic Performance for Ethanol Oxidation Jing Xue, Guangting Han, Wanneng Ye, Yutao Sang, Hongliang Li, Peizhi Guo* and X. S. Zhao

Institute of Energy and Environmental Materials, State Key Laboratory Breeding Base of New Fiber Materials and Modern Textile, School of Materials Science and Engineering, Qingdao University, Qingdao 266071 PR China

ABSTRACT: Two types of PdCu2 nanoparticles were prepared through one-pot synthesis and two-step reducing process, named as PdCu2-1 and PdCu2-2, respectively. The morphology and structure of as-prepared samples were investigated by transmission electron microscopy, high resolution

transmission

electron

microscopy,

X-ray diffraction,

X-ray

photoelectron

spectroscopy and inductively coupled plasma-optical emission spectrometry. Results showed that more Pd atoms were buried in the inside of PdCu2-1, whereas more available Pd sites were distributed on the surface of PdCu2-2. The electrochemical measurements indicated that both PdCu2-1 and PdCu2-2 nanoparticles showed a higher electrocatalytic activity than that for pure Pd nanoparticles. In particular, PdCu2-2 predictably exhibited a better stability and durability as well as a lower onset potential and a higher catalytic current density than that of PdCu2-1

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towards ethanol oxidation in alkaline media. On the basis of these studies, the formation mechanisms of both the PdCu2 catalysts and the relationship between their structure and properties were discussed in this paper.

KEYWORDS: PdCu2, Pd-based catalysts, structural regulation, solution-phase synthesis, electrocatalysis

1. INTRODUCTION

In response to the environment of energy shortages, many types of fuel cells have received extensive attentions as the potential candidates for energy conversion and power generation.1,2 Considering their high energy density and convert efficiency as well as zero/low emissions, direct alcohol fuel cells (DAFCs) are regarded as a kind of clean portable power source, becoming one of the hottest topics in the fuel cell community.3 Among them, direct ethanol fuel cells (DEFCs) gradually exhibit more favorable application prospect than conventional direct methanol fuel cells (DMFCs) due to a higher theoretical mass energy density for ethanol (8 kWh kg-1) compared with that for methanol (6.1 kWh kg-1).2 Furthermore, apart from its low toxicity and high fuel efficiency, ethanol also possesses advantages including low cost and desired sustainability for its obtaining way from renewable resources in large scale.4 To accelerate the wide application of DEFCs, designing highly efficient catalysts becomes the top priority to


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address the low reaction activity of ethanol electrooxidation. In addition to Pt-based materials, which are known as the most effective catalysts, some less expensive but relatively more abundant non-platinum catalysts, like Pd-based materials, are more likely to be put into practical use for DEFCs in alkaline media, owing to its good tolerance to the poisoning caused by intermediate species during ethanol oxidation process.5-11

To further lower cost and improve catalytic activity, besides the morphology control, synthesizing multiple-component Pd-based bimetallic or trimetallic catalysts with non-noble metals, such as Ru12, Sn13-15, Ag16-18, Au19,20, Co21, Ni22,23, Fe24-26, Cu27-29, has attracted increasing attentions. And these multi-metallic catalysts indeed show superior catalytic activity and stability than their corresponding monometallic catalysts. It is worth mentioning that Cu, with a full d band, is considered as a perfect compromise between lower cost and excellent properties and has been widely explored as an assistant component for bimetallic catalysts in various reactions.30 Considerable researches show that PdCu nanocomposites usually exhibit superior catalytic performance for ethanol oxidation.31,32 For instance, small-sized PdCu nanocapsules with thin wall thickness anchored on 3D graphene were successfully prepared and were proved to exhibit much enhanced electrocatalytic activity towards ethanol oxidation even after electrochemical activation.31 A novel Cu@PdCu/C catalyst with core-shell structure was also synthesized by galvanic replacement and showed improved catalytic activity, durability and anti-poisoning ability.32


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It is well known that metal particles in nanoscale with high surface energy are easy to be chemically oxidized in air. Therefore, conducting the synthetic process in organic media is a practical alternative, in which metal nanoparticles (NPs) are stabilized by solvents or simple anions.33 Moreover, the chemical activity of the catalysts is closely dependent on not only the particle size or morphology34, but also the electrochemically available proportion of active Pd sites. In spite of the achieved remarkable progress in the cost control through alloying process, improving the effective Pd utilization still remains highly influential but is often considered to be posterior, which is a quite regrettable thought. Herein, with NaH2PO2 as moderate reducing agent, we successfully prepared two PdCu2 NPs in ethylene glycol to realize the structural regulation and optimization of Pd usage, following two routes: one-pot synthetic reaction and two-step reductive process, marked as PdCu2-1 and PdCu2-2, respectively. Both the bimetallic catalysts have improvement for catalytic oxidation of ethanol in some degree than pure Pd NPs synthesized by the same process as PdCu2-1 in the absence of Cu precursors, and PdCu2-2 with most of the Pd loaded on surface exhibits an optimized catalytic performance.

2. EXPERIMENTAL SECTION

2.1 Materials and regents

All the chemicals including palladium(II) chloride, copper(II) chloride dihydrate, sodium hypophosphite, ethanol, ethylene glycol (EG), polyvinyl pyrrolidone (PVP) (molar weight = 58 000) and acetone used in this article were of analytical grade and purchased from Sinopharm


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Chemical Reagent Company. Double distilled water was used in the experiments except for electrochemical measurements with ultrapure water (18.2 MΩ cm).

2.2 Synthetic procedures

Synthesis of PdCu2 NPs. In this paper, we synthesized PdCu2 NPs through two routes with NaH2PO2 as reductant and PVP as stabilizer in ethylene glycol. The whole experiment was conducted with continuous stirring under argon atmosphere.

One-pot synthetic reaction. A certain proportion of PdCl2 (0.0177 g) and CuCl2 2H2O (0.0184 g) was dissolved in 5 mL EG in advance. 0.0667 g PVP were added into 10 mL EG in a 50 mL three-necked round bottle flask. The mixture was heated to 60 °C with mechanical stirring for 30 minutes until mixing uniformly. Then the solution was transferred into the round bottle flask and further raised the temperature to 130 °C, at which point 1 mL freshly prepared NaH2PO2 (2 M) aqueous solution was injected into the above solution rapidly. During the heating process, the solution began to blacken gradually for the partial reduction of Pd2+ by ethylene glycol and then became completely black with the addition of NaH2PO2. The reaction proceeded for 3 hours at 130 °C to form the PdCu2-1 NPs.

Two-step reductive process. A certain amount of CuCl2 2H2O (0.0184 g) and PVP (0.0667 g) were added into 10 mL EG in a 50 mL three-necked round bottle flask. The mixture was heated to 60 °C with strong stirring until forming homogeneous solution and then further heated to 130


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°C. 1 mL freshly prepared NaH2PO2 (2 M) aqueous solution was injected into the above solution quickly. The color of the solution turned colorless or milky white immediately and then evolved to orange, brown and eventually aubergine after about 10 minutes, indicating the formation of Cu NPs. After stabilizing for a few minutes, Na2PdCl4/EG solution, prepared in advance by dissolving PdCl2 (0.0177 g) and NaCl (0.0977 g) in 5 mL EG with gentle heating, was added at a slow speed with the constant pressure funnel. The solution turned black slowly as the increasing concentration of palladium. The reaction proceeded for 3 hours at 130 °C after the Na2PdCl4 solution dripping off to form the PdCu2-2 NPs.

Synthesis of Pd NPs. PdCl2 (0.0177 g), NaCl (0.0977 g) and PVP (0.0667 g) were dissolved in 15 mL ethylene glycol in a 50 mL three-necked round bottle flask with the gentle heating and mechanical stirring until forming homogeneous solution. Then the solution was heated to 130 °C and 1 mL freshly prepared NaH2PO2 (2 M) solution was injected in quickly. The reaction proceeded for 3 hours to obtain Pd NPs.

After cooling the solutions to the room temperature, about 50 mL acetone was added separately, followed by centrifuging and washing with the mixture of EG/acetone for three times and acetone once finally, then dried the samples in an oven at 60 °C for 6 h for later use.


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2.3 Characterization

Transmission electron microscopy (TEM) was carried out with a JEM-2000EX at an accelerating voltage of 120 kV. High resolution TEM (HRTEM) images and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy dispersive Xray (EDX) elemental mappings were measured with a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV, respectively. The X-ray diffraction (XRD) patterns were conducted on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 0.15418 nm) from 10 to 80 degrees (2θ) using a solid detector. X-ray photoemission spectroscopy (XPS) was recorded with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was conducted using an Optima8000 instrument.

2.4 Electrochemical measurements

All the electrochemical measurements were performed on a CHI760D electrochemical workstation with a typical three-electrode cell at room temperature. A platinum foil was used as counter electrode and a saturated calomel electrode (SCE) in acidic solutions, or an Hg/HgO electrode in alkaline media as the reference electrode. The working electrode was prepared using a modified glassy carbon electrode (GCE, 3 mm in diameter) with 10 µL of the catalyst ink dropcoated on the surface of GCE and drying under the air to obtain a thin film. The ink containing


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1.0 mg of electrocatalyst with per 1 mL of ultrapure water was obtained by ultrasonic for 1 ~ 2 h to be uniformly dispersed. Cyclic voltammetry (CV) of ethanol electrooxidation was carried out in the 1 M KOH electrolyte containing 1 M C2H5OH at a sweep rate of 50 mV s-1. Voltammetric stripping curves of underpotential deposited copper (UPD-Cu) was performed in Ar-saturated 0.5 M H2SO4 + 2 mM CuSO4 and the base CVs recorded in 0.5 M H2SO4 were given for reference at at a scan rate of 20 mV s-1. The UPD-Cu was performed in 0.5 M H2SO4 + 2 mM CuSO4 at 50 mV vs. SCE for 180 s.

3. RESULTS AND DISCUSSION

XRD patterns of the as-prepared samples are depicted in Figure S1 to illustrate the successful formation of bimetallic NPs and their crystalline structures. Three diffraction peaks at 40.1º, 46.7º and 68.1º are respectively indexed to (111), (200) and (220) planes of a face-centered cubic (fcc) crystalline Pd (JCPDS PDF No. 46-1043) as well as 43.3º, 50.4º and 74.1º for the (111), (200) and (220) planes of fcc-Cu (JCPDS PDF No. 04-0836). The XRD pattern of Pd NPs exhibits the completely consistent diffraction peaks with pure Pd, whereas for PdCu2-1 and PdCu2-2 catalysts, these three characteristic peaks are located between those specialized ones of pure fcc-Pd and Cu, with a slight positive shift of about 0.3º ~ 0.6º for PdCu2-1 and 0.4º ~ 0.9º for PdCu2-2. And no obvious diffraction peaks related to pure Pd, Cu, or their oxides are observed. All these results indicate the formation of single-phase fcc-PdCu alloys.


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The structural features of PdCu2-1 and PdCu2-2 NPs were investigated by TEM and HRTEM. As shown in Figure 1, the TEM images at low and high magnification (Figure 1A, B) of PdCu2-1 show that dispersed NPs are obtained, whereas the size distribution is not uniform with an average diameter of ca. 7±3 nm. Note that PdCu2-2 NPs have a relatively homogeneous particle size distribution with the average size of ca. 4±1 nm, as shown in Figure 1D, E. The inset d in D image is the TEM image of Cu NPs obtained as the precursor for the synthesis of PdCu2-2. The particle size is smaller than PdCu2-2, indicating the growth and evolution from Cu NPs to PdCu22 NPs. The TEM images of Pd NPs with an average diameter of about 6±3 nm are demonstrated in Figure S2, but the dispersibility was not so good as the PdCu2 alloy NPs. The three corresponding particle size distribution patterns are shown in Figure S3. According to the corresponding selected area electron diffraction (SAED) patterns, both PdCu2 NPs display a well crystalline nature.

To illustrate the detailed features, HRTEM measurement was carried out. The lattice spacings across the ligament are calculated to be ~ 2.23 Å and 2.14 Å for PdCu2-1 (Figure 1C) as well as ~ 2.21 Å and 2.17 Å for PdCu2-2 (Figure 1D), in accordance with their own interplanar distance of (111) and (200) lattice planes. Moreover, both angles between the crystalline planes are measured as about 53º, which agree well with the theoretical one.35 The reduced lattice spacing of (111) plane compared with that of Pd, with the value of 2.36 Å according to its HRTEM image (Figure S2 C), is the result of the alloying action with Cu, whose value is about 2.08 Å as


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reported previously.36 Figure S4 shows the HAADF-STEM images of a single large nanoparticle and their corresponding elemental mappings for PdCu2-1 (Figure S4 A-D) and PdCu2-2 (Figure S4 E-F). The overall distribution of Cu and the relatively large density derive from the copper net and thus the signal of Pd seems to be weaker. Although the drawbacks exist, it clearly reveals the coexistence of Pd and Cu elements for both the PdCu2 NPs and the content of Pd is indeed smaller than that of Cu. In addition, Pd and Cu elements are evenly distributed among the nanoparticle for PdCu2-1. While for PdCu2-2, more Cu is coated in the core and more Pd locates on the surface, which is believed to be in favor of the superior electrochemical performance.

Figure 1. TEM (A, B, D, E) and HRTEM (C, F) images of PdCu2-1 (A, B, C) and PdCu2-2 (D, E, F). The insets at top right corner in A and D are the relevant SAED patterns, while the inset d in D is the TEM image of Cu nanoparticles for the two-step reductive synthesis process.


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The XPS survey spectra were carried out to further analyze their alloy composition. According to the XPS data, the Pd/Cu atomic ratio is 1/2.29 for PdCu2-1 and 1/2.12 for PdCu2-2, which is consistent with the results from other technology containing ICP-OES analysis as 1/2.11 and 1/2.05 respectively and from the EDX testing shown in Figure S5. In fact, PVP as the surfactant is very difficult to be removed completely, so the test error may be caused by the coverage of Pd active sites on the surface by residual PVP37, which influences the analytic results to some extent. To further verify the surface structure, the high-resolution XPS spectra of Pd 3d and Cu 2p regions were recorded and shown in Figure 2. Here we analyze the spectra with the relative intensity. As revealed in Figure 2A, two pairs of doublets of Pd 3d peak assigned to their own Pd0 and the corresponding Pd2+ can be observed and the Pd0 is predominant in both catalysts. The peaks located at ca. 335.5 eV and 340.8 eV are attributed to Pd 3d5/2 and Pd 3d3/2 banding energies, respectively.

Figure 2. High-resolution XPS spectra of Pd 3d (A) and Cu 2p (B) regions of PdCu2 samples.


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The emergence of slight Pd2+ is because of the formation of Pd-Oads and surface PdO, just as the common results of noble metals.30,38-41 In comparison, the proportion of Pd2+ markedly increases for pure Pd NPs as shown in Figure S6 A, so the alloying with Cu hinders this process and thus improves the availability of Pd sites. In the Cu 2p region (Figure 2B), the deconvoluted peaks of PdCu2-1 are at about 931.49 eV and 951.43 eV for Cu0, 933.79 eV and 953.64 eV for Cu2+ and the satellite peak of CuO at ~ 942.0 eV32. But for PdCu2-2, there are only one pair of broad peaks at 932.60 eV and 952.75 eV for Cu0 and the corresponding satellite signal of CuO. The peak-fit of Cu 2p reveals the coexistence of Cu0 and Cu2+ on the surface of catalysts with the relative area ratio of 49.5%/50.5% in PdCu2-1 and 84.4%/15.6% in PdCu2-2. The oxidation of Cu is due to the long time exposure in the air before the XPS detection. The more content of Cu distributes on the surface, the easier it is oxidized.28 Accordingly, most of the Pd locates on the surface of PdCu2-2 while relatively more Cu is exposed for PdCu2-1, as derived from the results of HAADF-STEM measurement. Besides, combined with the HR-XPS spectra of pure Pd and Cu shown in Figure S6, an approximately 0.5 eV positive shift of the banding energy for Pd 3d can be observed in both bimetallic catalysts. While for Cu 2p region, there is an about 1.1 eV negative shift for PdCu2-1 whereas negligible change for PdCu2-2.

The occurrence of these shifts is ascribed to the strong electron interaction between Pd and Cu elements, indicating the formation of alloy.32,42 The layout of Cu relative to the Pd species will strongly influence the catalytic activity because of the related atomic ensemble effect.6 Moreover,


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alloying with Cu is in favor of the tolerance to catalyst poisoning due to the increased distance of Pd-Pd band and thus hindering the binding of carbonaceous species in the bridge sites.14 Actually, the surface structure and composition of nano-sized electrocatalysts, in which only the atoms located on surface and sub-surface are the available reactive sites, are also the significant decisive factors for their catalytic properties.16 Concerning the PdCu catalyst with rich Pd sites in the surface, there is a certain lattice strain effect and the adsorption strength of OHads species is greatly associated with the strain magnitude.6 Namely, with respect to the Feimi level of Pd, the core-level as well as the corresponding d-band center of Pd shifts down instead due to the alloying with Cu, which weakens the adsorption of the MeOH oxidation intermediates.43 All these synergistic effects contribute together to the superior catalytic activity.43

Figure 3 shows the TEM images of intermediary products collected at different intervals during the sample preparation to investigate the formation process. Combined with the analysis results all above, we can infer the different formation mechanisms of PdCu2-1 and PdCu2-2 (Scheme 1). For PdCu2-1 synthesis system, Pd2+ was partly reduced by ethylene glycol during the heating process.44,45 Then the three-dimensional branched PdCu nanostructure was obtained due to the rapid reduction of Cu and residual Pd precursors and then self-assembly as the addition of the reductant NaH2PO2 in the presence of some Pd nanocrystals serving as the seeds. The width and length of the branches are in the range of 5-8 nm and 20-25 nm, respectively (Figure 3a). As the reaction time prolongs, the branches gradually fractured into short rods of


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varying length and then nanoparticles with an average diameter of ca. 3-8 nm and eventually uniformly dispersed after about 3 h (Figure 3b-d). In fact, the irregularly embedded Pd in the branches can be considered as a structural defect, coupled with the continuous increase of internal stress at high temperatures to provide enough energy and thus accelerate the fracture and finally form the relatively stable sphere-shaped nanoparticles. However, one cannot neglect the fact that PdCu2-1 prepared in this reduction process make quite a part of Pd atoms buried inside so that the catalytic activity will be affected.

Figure 3. TEM images of intermediary products obtained at different intervals of 0.5 h, 1 h, 2 h and 3 h during the formation process of PdCu2-1 after the addition of NaH2PO2 (a-d) and PdCu22 after the addition of Na2PdCl4 (e-h), respectively.

Unlike PdCu2-1, well-dispersed small-sized Cu NPs of ca. 2 ~ 3 nm (the inset d in Figure 1D) were obtained firstly as the seeds by the reduction of relatively strong reducing agent NaH2PO2


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in the PdCu2-2 system. After the dropwise addition of [PdCl4]2-, Pd precursors were reduced slowly and most of the Pd atoms attached on the surface of Cu NPs. There was also a small portion infiltrating into the relatively inside positions because of the galvanic replacement or the reassembly between the adjacent atoms. After 3 hours, the system became stable and the uniformly dispersed PdCu2-2 NPs were obtained ultimately and grew larger than Cu NPs (Figure 3e-h). The exposure of more Pd sites on the surface can better fit the catalytic reaction, as schematically illustrated in Scheme 1.46 In addition, from the XRD patterns of the intermediary products at different intervals (Figure S7), although the macromorphology changes and evolves, the crystalline structures of the both bimetallic catalysts have reached stability in the initial phase.

Scheme 1. The formation mechanisms of PdCu2-1 and PdCu2-2.

Electrochemical measurement was carried out to report the catalytic performance of asprepared catalysts. Figure 4 shows the typical voltammetric stripping curves of UPD-Cu on


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PdCu2-1, PdCu2-2 and Pd recorded in 0.5 M H2SO4 + 2 mM CuSO4 at a scan rate of 20 mV s-1, as the dotted line displayed. Several copper UPD stripping peaks with different adsorption energies can be observed in the potential range from 0.1 to 0.5 V (vs. SCE). While the corresponding base CVs performed in 0.5 M H2SO4 exhibit strong peaks associated with hydrogen adsorption/desorption in the range of -0.2 ~ 0.1 V (vs. SCE) and Pd oxidization/reduction above 0.4 V (vs. SCE). The additional anodic peak observed at around 0.32 V (vs. SCE) for PdCu2-1 and PdCu2-2 compared with pure Pd is ascribed to the oxidation of exposed Cu.47

Figure 4. Voltammetric stripping curves (dotted line) of UPD-Cu on PdCu2-1, PdCu2-2 and Pd NPs in Ar-saturated 0.5 M H2SO4 containing with 2 mM CuSO4 at at a scan rate of 20 mV s-1 and their respective base CVs (solid line) in 0.5 M H2SO4 without CuSO4 at the same scan rate. The underpotential deposited Cu was performed in 0.5 M H2SO4 + 2 mM CuSO4 at 50 mV vs. SCE for 180 s.


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The electrochemical active surface area (ECSA) of an electrode is an important parameter for evaluating the catalytic activity and various methods containing the desorption of hydrogen48-50, stripping of UPD-Cu monolayer on the Pd surface51,52 and the reduction region of palladium oxide13 expounded as below in Figure S8 are usually used to estimate the ECSA. According to the CV data, the corresponding ECSAs of PdCu2-1, PdCu2-2 and Pd are calculated according to the above methods and listed in Table S1. Among them, the ECSAs obtained from Cuupd are 22.98, 50.67 and 24.83 m2 g-1 for PdCu2-1, PdCu2-2 and Pd, respectively, matching well with that from Hdes although with a slight decrease. As such, more Pd atoms located in the core of PdCu2-1 NPs cause a smaller ECSA than that of PdCu2-2 or pure Pd NPs in accord with aforementioned results of XPS and HAADF-STEM measurements. PdCu2-2 NPs exhibits the largest surface area and the most catalytic active sites as expected, and this is favorable to the electrocatalytic oxidation towards ethanol.

The corresponding CVs in 1 M KOH with a scan rate of 20 mV s-1 are shown in Figure S8. The peaks at around -0.24 V vs. SCE for PdCu2-1 and PdCu2-2, with a slightly positive shift than that of Pd (-0.27 V vs. SCE), are considered to be the reduction of oxygenated Pd species (PdOHads or PdOx). The shift suggests that alloying Pd with Cu will make it easier for the reduction of PdOx due to the ability to transfer the -OHads between Pd and Cu sites and thus retain the activated -OHads species or remove the unwanted ones.4 From this point, both the bimetallic catalysts exhibit a higher ECSA and the inconformity for PdCu2-1 compared to the two other


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results is acted out, with the ECSA values of 34.52, 59.27 and 27.78 m2 g-1 for PdCu2-1, PdCu2-2 and Pd, respectively. Conceivably, it also plays an important role in the catalytic property for ethanol oxidation in alkaline medium explicated as below.

The electrooxidation reaction of ethanol was investigated in alkaline medium, and the mass activity was used to evaluate the catalytic performance of the electrodes. Two well-separated anodic peaks can be observed during the forward and reverse scans of ethanol oxidation, and the amount of oxidized ethanol at the catalyst electrodes is directly proportional to the current density magnitude of the anodic peak in the forward scan29,53, as depicted in Figure 5A. Among the three kinds of catalysts, PdCu2-2 delivers the best catalytic performance with the highest mass current density and lowest onset potential. In detail, the current density values for the reactions on PdCu2-1, PdCu2-2 and pure Pd electrodes are 1290 mA mg-1, 1630 mA mg-1 and 1120 mA mg-1, respectively. In addition, the onset potential for ethanol oxidation on PdCu2-2 catalyst is ca. -0.6 V (vs. Hg/HgO), with more negative about 100 mV than the others at around 0.5 V (vs. Hg/HgO), just as the magnification for the onset potential of the inset curves in Figure 5A. Not much difference of the curves is observed between PdCu2-1 and Pd until after the onset potential. The information about onset anodic potential suggests the significant enhancement of PdCu2-2 in the kinetics of the ethanol oxidation reaction.29


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Figure 5. CVs of PdCu2-1, PdCu2-2 and Pd NPs in 1 M KOH/1 M C2H5OH at 50 mV s-1 normalized by mass (A) and surface area (B); the inset curves in A are the magnification of the onset potentials for ethanol oxidation in A.

Note that the CVs for ethanol oxidation in 1 M KOH/1 M C2H5OH were obtained after scanning two cycles in 0.5 M H2SO4 to remove impurities and activate the catalysts. The greater electrocatalytic activity of PdCu2-2 results from the more Pd active sites on the surface as well as the synergistic effect generated by the intimate contact between Pd and Cu.32 However, the comparison of catalytic activities normalized by specific surface area from ECSA values exhibits the opposite performance, as shown in Figure 5 B. Actually, there is a barrier to separate the charge contributions of hydrogen absorption or adsorption in the Hdes region and metal dissolution or multi-layer oxide formation in the oxide reduction region, so both the methods have their own limitations.52 Thus here we adopt the ECSAs obtained by the stripping of UPDCu. The catalytic current densities can reach 5.60 mA cm-2, 3.22 mA cm-2 and 4.51 mA cm-2 for PdCu2-1, PdCu2-2 and pure Pd electrodes, respectively. In fact, the catalytic activity is closely related to the adsorption strength of reactive intermediates, which is dependent on the surface


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structure and chemical bonds. As a whole, PdCu2-2 can be regarded as the Pd clusters on the Cu nanoparticles and thus results in the specific electronic effect. Along with the change in Pd atomic distance, the adsorption of intermediates could be hindered and thereby the electrocatalytic performance can be enhanced.54-56 But when considering the Van der Waals forces or interactions of chemical bonds between Pd atoms in PdCu2-2 for its specific structure, when splitting to per unit Pd area, undesired effect comes to light.50 Thus although possessing the superior mass activity, PdCu2-2 shows inferior availability for per unit Pd area. On the contrary, even if analyzing per equivalent Pd in PdCu2-1, the incorporation of surrounding Cu can weaken the adsorption strength of oxygenated intermediates on the Pd surface.57 In this light, PdCu2-1 exhibits a higher activity per real surface area and more effective utilization of per active Pd site. However, the PdCu2-1 catalyst as a whole displays a poor stability because of the unreasonable distribution of Cu, as illustrated below.

Herein, the electrochemical performance of our PdCu2 NPs and those from literatures for Pdbased bimetallic catalysts are collected in Table S2. Note the inconformity of the units and the different contents of active Pd. Here the activities of our PdCu2 NPs were normalized to the total mass of the catalysts. From the Table S2, in terms of the values, it can be seen that the PdCu2-2 modified electrode exhibits a relatively high mass activity for ethanol oxidation, but lower than that reported in Ref. S2 due to the well dispersibility supported with carbon black. As for ECSA, the value of PdCu2-2 is lower than the activated one reported in Ref. 31 and that in Ref. 5 but


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higher than others. Considering the specific structure and composition, efficient Pd usage and high activity, it can be found that PdCu2-2 indeed possesses an excellent property for the electrooxidation of ethanol in alkaline media.

In order to further confirm the relationship of catalytic activity and structure for the bimetallic catalysts, CVs for ethanol oxidation of PdCu2-1 (Figure 6A) and PdCu2-2 (Figure 6B) after scanning different cycles in 0.5 M H2SO4 were carried out. For PdCu2-1, the anodic peaks display a negative shift of about 0.06 V compared to that without CV pretreatment, but the catalytic activity shows no measurable increase at the beginning and then even declines significantly with the extended cycle numbers instead. As for PdCu2-2, the peak current density displays an obvious enhancement of about 320 mA mg-1 after two cycles and 170 mA mg-1 after five cycles than that without treatment in 0.5 M H2SO4 before the electrocatalysis of ethanol. However, the catalytic activity begins to decrease and the position of anodic peak is slightly negative after 10 scan cycles in 0.5 M H2SO4. Based on these performances, it can be deduced that the catalysts are activated with scanning two cycles in H2SO4, most likely due to the remove of impurities and the dissolution of surface Cu so as to increase the available active sites of Pd species. However, the continuous CV treatment leads to the structural damage and then the activity decline. The undesirable electrocatalytic performance on the PdCu2-1 modified electrode is mainly ascribed to the unstable structure and relative high content of Cu atoms on the surface of PdCu2-1, which is consistent with the above results.


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Figure 6. CVs of PdCu2-1 (A) and PdCu2-2 (B) in 1 M KOH/1 M C2H5OH after scanning different cycles in 0.5 M H2SO4.

The durability of the three catalysts towards ethanol electrooxidation was monitored by chronoamperometry technique at 0.3 V and the cyclic stability measurement, as illustrated in Figure 7. All the polarization currents (Figure 7A) show a rapid decay in the initial period for the three catalysts, probably due to the formation of poisonous intermediate species during the ethanol electrooxidation process in alkaline media at the beginning, followed by a gradual decrease of electrocatalytic activity under the subsequent measurements.29 Nevertheless, the current density platforms for PdCu2-1, PdCu2-2 and Pd change from 516, 611 and 447 mA mg-1 after a few seconds to 177, 275 and 217 mA mg-1 at the end of 1000 s, respectively. It can be observed that the current density on the PdCu2-1 electrode becomes lower than that on the Pd electrode after 800 s (Figure 7A), which is consistent with their corresponding cyclic stability curves after around 226 cycles where the current of PdCu2-1 begins to decrease sharply (Figure 7B). It can also be observed from Figure 7B that both the PdCu2 catalysts have a more drastic increase in the initial cycles, especially for PdCu2-2 reaching the maximum value only at about


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15 cycles. At the end of 300 cycles, the current densities of PdCu2-2 and Pd still remain about 71.5% and 64.8% of their highest values while only 15.8% for PdCu2-1. Its disappointing longterm performance and poor tolerance to poisoning probably attribute to the low structural stability for more Cu distributing on the surface of PdCu2-1. All these results suggest that PdCu22 possesses the highest catalytic activity and favorable stability and durability towards ethanol electrooxidation for its desired electronic effect and structure-induced strain (geometry) effect58 as supported by aforementioned characterizations, which also obviously reduce the usage of noble metal Pd and the cost of the electrocatalyst.

Figure 7. (A) Chronoamperometric curves of as-prepared electrodes at 0.3 V; (B) The variation of current density along with the cycle number for ethanol oxidation in 1M KOH/1M C2H5OH.


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4. CONCLUSION

In brief, well-dispersed PdCu2-1 and PdCu2-2 NPs catalysts were prepared successfully by onepot synthesis and two-step reductive process respectively to realize the structural regulation and optimization of Pd usage. Both the bimetallic catalysts exhibit a higher catalytic activity to ethanol electrooxidation in alkaline media compared with pure Pd NPs, although the long-term performance of PdCu2-1 is poor instead due to the unexpected distribution with more Cu on surface. In comparison, PdCu2-2 NPs exhibit an optimized electrocatalytic performance, with a lower onset potential, higher mass activity and an excellent catalytic stability and durability ascribed to the smaller size, more Pd active site on surface and the good synergistic effects between Pd and Cu atoms. Our results would be helpful to the synthesis of high-performance electrocatalysts with novel nanostructures.

ASSOCIATED CONTENT

Supporting Information.

Additional supporting figures (Figure S1-S8) and tables (Table S1, S2) for the morphology of pure Pd NPs, HAADF-STEM measurements of PdCu2 NPs, size distribution and EDX spectra of the PdCu2 catalysts, XPS spectra of pure Pd and Cu NPs, XRD patterns of the intermediate


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products as well as the electrochemical information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; Tel: +86 532 85953982

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. U1232104 and 11674186), National Key Project on Basic Research (No. 2012CB722705) and the Taishan Scholars Advantageous and Distinctive Discipline Program for supporting the research team of energy storage materials of Shandong Province and the Foundation of Taishan Scholar program of Shandong Province, China.


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Table of Contents


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Structural Regulation of PdCu2 Nanoparticles and Their

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