Highly active carbon supported ternary PdSnPtx (x = 0.1–0.7) catalysts for ethanol electro-oxidation in alkaline and acid media

Accepted Manuscript Highly active carbon supported ternary PdSnPtx (x=0.1∼0.7) catalysts for ethanol electro-oxidation in alkaline and acid media Xiaoguang Wang, Fuchun Zhu, Yongwei He, Mei Wang, Zhonghua Zhang, Zizai Ma, Ruixue Li PII: DOI: Reference:

S0021-9797(16)30065-0 http://dx.doi.org/10.1016/j.jcis.2016.01.068 YJCIS 21051

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

9 August 2015 10 December 2015 27 January 2016

Please cite this article as: X. Wang, F. Zhu, Y. He, M. Wang, Z. Zhang, Z. Ma, R. Li, Highly active carbon supported ternary PdSnPtx (x=0.1∼0.7) catalysts for ethanol electro-oxidation in alkaline and acid media, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.01.068

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highly active carbon supported ternary PdSnPtx (x=0.1~0.7) catalysts for ethanol electro-oxidation in alkaline and acid media Xiaoguang Wang a,, Fuchun Zhu a,b, Yongwei He a, Mei Wang a, Zhonghua Zhang c, Zizai Ma a, Ruixue Li a

a Laboratory of Advanced Materials and Energy Electrochemistry, Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan, 030024, China. b State Key Lab of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China c School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan 250061, P.R. China

Abstract: A series of trimetallic PdSnPt x (x=0.1-0.7)/C catalysts with varied Pt content have been synthesized by co-reduction method using NaBH4 as a reducing agent. These catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and chronoamperometry (CA). The electrochemical results show that, after adding a minor amount of Pt dopant, the resultant PdSnPt x/C demonstrated more superior catalytic performance toward ethanol oxidation as compared with that of mono-/bi-metallic Pd/C or PdSn/C in alkaline solution and the PdSnPt0.2/C with optimal molar ratio reached the best. In acid solution, the PdSnPt0.2/C also depicted a superior catalytic activity relative to the commercial Pt/C catalyst. The possible enhanced synergistic effect between Pd, 

Corresponding authors. Tel./fax: +86 351 6010540 (X. Wang) Email addresses: [email protected];[email protected] (X. Wang) 1

Sn/Sn(O) and Pt in an alloyed state should be responsible for the as-revealed superior ethanol electro-oxidation performance based upon the beneficial electronic effect and bi-functional mechanism. It implies the trimetallic PdSnPt0.2/C with a low Pt content has a promising prospect as anodic electrocatalyst in fields of alkali- and acid-type direct ethanol fuel cells.

Keywords: Palladium-tin; Platinum; Electro-oxidation; Ethanol; Alkali and acid

1. Introduction Over the past few decades, a great deal of attention has been paid to low temperature fuel cells such as direct alcohol fuel cells (DAFCs) in convenient power sources for portable applications [1,2]. Among the alcohols which are suitable for DAFCs applications, ethanol is regarded as a green fuel owing to its facile production in large quantity from fermentation process, and it is less toxic and has higher energy density relative to methanol [3,4]. As one of the platinum group metals, Pd has attracted much attention in recent years due to many advantages such as, being more cost-effective than Pt and having remarkable electrocatalytic activity toward some small organic molecules in alkaline medium [5,6]. For further improving electrochemical activity of Pd, several strategies such as formation of Pd-M (M= Cu, Au, Pt, Ni, Ag, Sn, Ir, Co, etc) alloys with suitable atomic ratios or construction of heterogeneous clusters/composites (i.e., core@shell structure) have been intensively investigated [7-16]. It is generally considered that, the presence 2

of a second additive transition metal could lead to appropriate modification of electronic or/and structural properties of Pd surface, which not only has a significant impact on its activity and stability, but also can greatly reduce Pd loading so as to lower the usage of precious metals. Du et al. [17] investigated the ethanol oxidation reaction (EOR) on a series of alloyed Pd-Sn catalysts and found that an optimal Sn ratio of ca. 14 % can trigger a significant activity promoting effect in high ethanol concentration and/or high pH environment toward EOR, resulting from lower reaction energies for dehydrogenation of ethanol. In our former work, it was also found that combining Sn with Pd/CNTs can achieve more remarkable electrocatalytic activities for ethanol electro-oxidation in alkaline solution [18]. However, in comparison to Pt, the widely known intrinsic drawback of the Pd-based catalysts is the negligible electrocatalytic activity toward alcohol electro-oxidation in acid media. In previous studies, alloying of Pt with other additives such as Mo, Ru, Rh, W, Ni, Cr and Sn has been intensively investigated, and also found that Sn is an effective promoter for EOR in acid circumstance [19-26]. In view of the fact that Sn element is an intersection element for both active binary Pd- and Pt-based catalysts, it inspires us to merge them together to design a kind of trimetallic PdSnPtx catalyst with a low fraction of Pt content in order to widen its application in both alkaline and acidic solutions as well as reduce the cost as much as possible. To the best of our knowledge, no research to date has focused on the effect of low Pt content in PdSnPtx system on the electroactivity toward EOR in alkaline and acid solution. Herein, we successfully synthesized a series of carbon supported Pd, PdSn (molar ratio 1:0.2) and PdSnPt x

3

(x=0.1, 0.2, 0.3, 0.5 and 0.7) catalysts through borohydride reduction method. The influence of different amount of alloying element Pt on the electrochemical behavior as well as the electrocatalytic activity enhancement for EOR in both alkaline and acid media has been analyzed. Based upon the co-operative effect of Sn and the third additive Pt, these ternary catalysts reveal superior electrocatalytic activities toward EOR, and will find promising prospect in the applications of alkaline-/acid-type DEFCs.

2. Experimental 2.1 Preparation of catalysts The activation of carbon black support (Cabot Vulcan XC-72R) was performed in a concentrated HNO3 (20 %) at 60 °C for 1 h and then washed repeatedly with ultra-purified water and evaporated to dryness. Each Pd/C, PdSn (with an atomic ratio of 1:0.2)/C and PdSnPtx (x=0.1,0.2,0.3,0.5,0.7)/C were synthesized using PdCl2, SnCl2·2H2O and K2PtCl6 as metal sources, NaBH4 as reducing agent and active carbon black as support. All the chemicals were analytical grade and used without further purification. The required stoichiometric amount of metal source salts were dissolved in ethylene glycol, and then an appropriate amount of as-treated active carbon black (ca. 20 wt. % metal / 80 wt. % carbon) was added and ultrasonically stirred for 4 h. After that, a mixing solution of NaBH4 and NaOH with a pH value of ca. 12 was added, drop by drop, to the well-dispersed mixtures under vigorous stirring to facilitate the decoration of metals on the carbon surface. Finally, the as-obtained

4

suspensions were washed several times to remove residual salts, centrifuged and dried. 2.2 Characterization of catalysts X-ray diffraction (XRD) profiles were collected using a Bruker-D8-Advance X-ray diffractometer with Cu Kα radiation (λ=0.15406 nm) generated at 40 kV and 40 mA. The microstructure and chemical composition were analyzed using a high resolution transmission electron microscopy (HRTEM, JEM-2100, JEOL) and an energy dispersive X-ray analyzer (EDX, Bruker-QX-200). X-ray photoelectron spectroscopy (XPS) spectra were recorded using a K-Alpha (Thermo Scientific ESCALAB 250) spectrometer equipped with monochromatic Al Kα radiation operating at 15 kV and 10 mA, and the binding energies (BE) were corrected by referencing the C1s peak to 284.8 eV. 2.3 Electrochemical measurements All electrochemical measurements were conducted in a standard three-electrode cell using a CS-350 Potentiostat. The working electrode was prepared by thin-film electrode method as follows: The catalyst of 5 mg was firstly ultrasonically mixed with 400 μL isopropanol and 100 μL Nafion solution (0.5 wt. %) to form a well-dispersed catalyst ink. Then, 2 μL of the ink obtained was pipetted and spread on a polished glassy carbon electrode (GCE, Ø4 mm) as the working electrode [27]. The counter electrode was a bright Pt plate, and a saturated calomel electrode (SCE) or an Hg/HgO (1.0 M KOH) electrode (MMO) was used as the reference electrode, depending on the experimental requirements. The voltammetric behavior was 5

characterized in a 0.5 M H2SO4 solution and corresponding electrocatalytic activities towards ethanol oxidation were measured in the solutions of 1.0 M KOH + 1.0 M EtOH and 0.5 M H2SO4 + 1.0 M EtOH, respectively. Before tests, the electrolytes were de-oxygenated by bubbling N2 for 30 min. In order to compare the inherent activity of different catalysts, the current obtained was normalized by electrochemical active surface area (SEASA). The CO stripping experiments were performed by firstly maintaining the working electrode in the 0.5 M H2SO4 solution saturated with high purity CO gas at the potential of -0.09 V (vs. SCE) for the period of 600 s, and then transferring the electrode into a clean 0.5 M H2SO4 solution followed by CV scan at a scan rate of 50 mV s-1. Moreover, we also benchmarked the electrochemical catalytic activities of the as-synthesized catalysts against the JM Pt/C (40%, Johnson Matthey) commercial catalyst under the identical experimental conditions. If no specific emphasis, the electrochemical experiments were performed at ambient temperature (~ 25 ºC).

3. Results and discussion 3.1 Physicochemical characterization of electrocatalysts EDX results for all the prepared catalysts are presented in Table 1, which indicates that each metal is present in a composition close to the nominal values. XRD patterns of Pd/C, PdSn/C, PdSnPt0.1/C, PdSnPt0.2/C, PdSnPt0.3/C, PdSnPt0.5/C and PdSnPt0.7/C are shown in Fig. 1. Typical peaks of face-centered cubic (fcc) structure including (111), (200), (220) and (311) planes are marked on all the patterns at ca. 40°, 6

47°, 68° and 82° , respectively (Fig. 1a). For all the samples, the broad peak at about 25° is associated with the (002) crystalline plane of graphite with a hexagonal structure for Vulcan XC-72R carbon [28]. No diffraction peaks, which indicate the presence of either pure crystalline Sn or Sn oxides, appear in the XRD patterns, suggesting that Sn form an alloy with Pd/Pt or exist as oxides in amorphous phase. In addition, the PdSn/C and PdSnPt x/C showed a gradual peak shift to lower angles relative to Pd structure. The peak shift can be clearly noticed in the expanded view of the (111) reflections (Fig. 1b). This phenomenon should be ascribed to the reason that the incorporation of Sn and Pt with larger atom radii into Pd environment induces the lattice expansion of fcc Pd crystal. Meanwhile, the peaks of PdSnPt x/C become slightly broader than the ones of the Pd/C and PdSn/C. The d spacings of (111) peaks can be calculated via the Bragg law: d=λ/2sinθ, where λ is the X-ray wavelength (λ=1.5406 Å) and θ is the angle of (111) peaks. The lattice parameter (a) and average crystalline size (L) can be estimated according to the equation: dhkl= a/(h2+k2+l2)1/2 and L=0.9λ/B2θcosθ [29,30], where L is the mean size of particles (nm), λ is the wavelength of X-ray (λ=1.5406 Å), θ is the angle of (111) peak, and B2θ is the width (in radians) of (111) peak at half height. The average crystallite sizes and the lattice parameters calculated from the XRD result are illustrated in Table 1. A decrease of the average crystallite sizes was observed with the incorporation of Sn and Pt into Pd. Previous studies revealed that Pd tends to form twinned or multiple twinned seeds while Pt prefers to form single crystal seeds [31,32], which may be responsible for the smaller average crystallite and less agglomeration of Pt-containing PdSnPt x/C

7

electrocatalysts. In general, the calculated mean diameter values of the catalysts based upon the XRD patterns are almost comparable to those obtained from the TEM observation. The typical microstructure and composition analysis of the PdSnPt0.2/C catalyst with the best performance are shown in Fig. 2. From Fig. 2a and b, it is clear that the as-synthesized metallic nanoparticles were well dispersed on carbon black with a narrow size distribution centered at ca. 5 nm, slightly smaller than the value based upon the XRD evaluation. From the high magnification shown in Fig. 2c, the d-spacing of 0.226 nm was attributed to the expanded (111) plane of as-alloyed fcc Pd crystals, due to the introduction of Sn and Pt element (Fig. 2c) [33]. To further examine the elemental distribution, high-angle annular dark-field scanning TEM (HAADF-STEM) was performed. Fig. 2d shows a typical HAADF-STEM image and the corresponding elemental maps of Pd, Sn, and Pt, which confirm that the main elements are uniformly distributed across a nano-particulate range. It indicates the three main elements mixed well with each other to form an alloy state and no segregation was discernible. A typical EDX spectrum of the PdSnPt0.2/C catalyst further reveals the co-existence of Pd, Sn and Pt elements, and the determined atomic composition ratio is very close to the nominal value (Fig. 2e and Table 1). XPS was also employed to investigate the nature of surface species for the PdSn/C and PdSnPt0.2/C and the corresponding spectra are shown in Fig. 3. It is clear that all the binding energies for Pd 3d5/2 and Sn 3d5/2 are located at 335.9 eV and 487.4 eV, respectively, indicating that the Pd and Sn elements exist in the same

8

chemical state within the superficial layer of PdSn/C and PdSnPt0.2/C catalysts. The comparison with the data from literature suggests that the main part of Pd element is in metallic state [34]. As reported, the binding energy (Sn 3d5/2) of the elemental Sn (0) and Sn (+4) is located at 484.8 eV and 487.2 eV, respectively [35]. Hence, it is reasonable to assume that the Sn element on the topmost layer of PdSn/C and PdSnPt0.2/C catalysts mainly exists in the form of SnO2 oxide due to the oxophilic character of Sn atom, which is similar to the previous reports [36,37]. In the entire Pt spectra of PdSnPt0.2/C, the 4f7/2 signal locates at around 71.8 eV, which exhibits a slightly positive shift relative to the reported 4f7/2 value of either 70.5 eV from bulk Pt-metal [38] or 71.3 eV from Pt/C catalyst [39]. On one hand, it is reasonable to consider that although bulk noble metals such as Pt are resistant to being oxidized, nano-particles are prone to suffering oxidation at their surface in air at ambient temperature [40]. The formation of partial O-metal binds (Pt-Ox) on superficial surface may induce an overlapping with zero-valent Pt and Pt-Ox, leading to a positive shift of binding energy value. On the other hand, Wang et al. [41] reported that the Pt 4f7/2 binding energy of core@shell-type Pt/PdSn-SnO2 also centered at ca. 72 eV and ascribed the positive binding energy shift to the widespread strain state on the outmost Pt shell [42]. Based upon the XPS spectra, the topmost layer contains ca. 38.7 at.% Pd, 42.1 at.% Sn and 19.2 at.% Pt for the PdSnPt0.2/C. In the previous studies, Baranova et al. [43] and Caballero et al. [44] revealed that PdxPt1-x(x=0~1) clusters are prone to formation of Pt enriched surface for high Pd concentration. Herein, it can be found that both Pt and Sn content on the surface are also a bit larger than the bulk. The

9

enrichment of Pt and Sn dopant at the surface region may form a superficial Pt-Sn(O)-Pd shell with a comparable atomic proportion of elements. Moreover, the positive binding energy shift of Pt 4f may also be derived from a metal/support (core/shell) interaction or small cluster-size effects [45]. In addition, Villullas et al. [46] found that the neighboring Ru oxide sites can give rise to the positive binding energy shift of the Pt 4f. It was also reported that the interaction between noble metal (Pt and Pd) and oxides can lead to the positive shift of binding energies [47,48]. Therefore, it could be presumably concluded that there may exists an interaction between the Pt, Pd and Sn(O) in the present case, which will generate both electronic and geometric modification toward the Pt atoms, being beneficial to the catalytic oxidation reaction towards small organic molecules. 3.2 Cyclic voltammetry and CO stripping analysis Before the catalysis evaluation, base CV curves of the catalysts were characterized in the 0.5 M H2SO4 solution and the results are present in Fig. 4. It is clear that the base CVs of Pd/C, PdSn/C and PdSnPt x (x=0.1-0.7) catalysts exhibit three characteristic regions including typical underpotentially-deposited hydrogen (Hupd) adsorption/desorption peaks, oxide formation/reduction peaks and a charging/discharging double layer region in between. Here, Pd/C resembles the similar characteristic features to those of nano-scaled Pd electrode (Fig. 4a) [49]. Relative to the Pd/C, the hydrogen adsorption/desorption peaks of the PdSn/C sample appeared to be depressed, possibly originating from the blockage of Sn-containing species

on

Pd

sites

(Fig.

4b).

Moreover, 10

it

represents

much

bigger

charging/discharging double layer region than Pd/C does, which is similar to the characteristic feature of Ru-containing catalysts [50]. It is reasonable to assume that there may exist an activation of H2O molecule on Sn sites same as the role of Ru sites from PtRu counterpart. With increasing of the Pt content, it is clear that the CV profiles of PdSnPtx/C gradually evolve to the curves resemble to that of Pt-based electrode (Fig. 4c-g). From the zoomed profiles of hydrogen desorption and oxides reduction regions, it further confirms that, when introducing a small fraction of Pt content, the hydrogen oxidative desorption peaks exhibit a negative shift as compared with the Pd/C and PdSn/C (Fig. 4h). For example, the hydrogen desorption peak potential of the Pd/C locates at ca. -0.174 V while the PdSnPt0.2/C shifts to ca. -0.233 V. It indicates that the exposure of Pt atoms on the surface may limit the strong bond of H with the surrounding Pd atoms. Relative to the Pd/C, the PdSn/C exhibits a more negative oxide reduction potential value in the reverse scan direction, being consistent well with the fact that the Sn belongs to a well-known oxophilic element. To the contrary, the cathode peak potential tends to shift positively for reducing oxides on the Pt-containing PdSnPtx/C catalysts (Fig. 4i), in good agreement with those of the previously-reported results relevant to the Pd/Pt co-existing electrocatalysts [33,51]. To reduce strongly adsorbed CO species formed on the surface or to favor CO oxidation at low potential plays an important role in improving the performance of electrocatalysts [52,53]. CO stripping can provide valuable information on the CO-tolerant nature of electrocatalysts. Fig. 5 shows the CO stripping curves of the Pd/C, PdSn/C and PdSnPt x (x=0.1-0.7) catalysts. It is clear that pre-adsorbed CO has

11

been oxidized completely in the first scan (solid line), generating a broad anodic CO stripping peak at the high potential range. During the second potential scan (dashed line), no CO oxidation reaction is monitored and the voltammograms go back to the original CV curves, indicating the recovery of active sites on catalyst surface. The CO-stripping parameter of the as-synthesized catalysts was analyzed and summarized in Table 2. It is obvious that, the onset potential for the oxidation of adsorbed CO on PdSn/C is lower than that on Pd/C (with a negative potential shift of ca. 200 mV). Furthermore, the addition of a minor amount of Pt makes the onset potential for CO stripping much lower than the PdSn/C catalyst, suggesting the PdSnPtx/C with low Pt content can efficiently reduce the overpotential of CO oxidation reaction on the surface. Among them, the PdSnPt0.2/C reveals the lowest CO oxidation onset potential (ca. 144 mV vs. SCE) and peak potential (ca. 382 mV vs. SCE), indicating the lowest CO adsorption strength on the PdSnPt0.2/C surface. In other words, the PdSnPt0.2/C

has

superior

tolerance

to

CO

intermediate

during

alcohol

electro-oxidation process among all the catalysts. The CO oxidation current density is reflective of the ability of electrocatalyst for the oxidation of CO from oxyhydroxide species via a bi-functional mechanism. If the transfer ratio of OHads is accelerated, i.e., there are more available OHads sites, the oxidation rate for CO-like intermediates at the surface can be promoted, which increases the activity for the oxidation of ethanol as well as the tolerance of the electrocatalyst for CO poisoning. The electrochemical active surface area (SEASA) of the catalysts can also be estimated from the CO stripping peak by assuming a charge of 420 μC cm-2 for electro-oxidizing the CO monolayer on

12

Pt and Pd surface [26,41,52-53]. It is noticed that, there exist a large SEASA value when the Pt content is low (i.e., around ca 0.1 and 0.2), then it decreases with the continuous augment of Pt element (Table 2). As mentioned above, the doping of Pt with low content can effectively decrease the size of nanoparticles, thus enlarging the number of exposed active sites for electrochemical reactions. However, excessive existence of Pt content on the nanoparticle surface facilitates the strong adsorption of COad species. The formation of strong Pt-COad bonding interaction on continuous Pt surface further impairs the enlargement of SEASA.

3.3 Electrocatalytic activities for ethanol electro-oxidation in alkaline and acidic media The activities toward ethanol electro-oxidation were evaluated in both alkaline and acidic solution. In the alkaline solution, the CVs of the catalysts were performed in solutions of 1.0 M KOH + 1.0 M EtOH. It is clear that the EOR on the as-synthesized PdSnPtx/C (x=0.1-0.7) and PdSn/C catalysts represent almost the same profiles with two typical well-defined current peaks on the forward (peak I) and reverse scan (peak II), as illustrated in Fig. 6a. It is generally recognized that, in the forward scan, the oxidation peak (I) is correlated with the oxidation of freshly chemisorbed species which come from EtOH adsorption, and is normally used to evaluate the catalytic activity of electrocatalysts [54,55]. The reverse scan peak (II) is primarily associated with the removal of carbonaceous species which are not completely oxidized in the forward scan [56,57]. Their onset oxidation potential, oxidation peak potential and specific activity for the forward scan were analyzed and 13

their values are shown in Tables 3 corresponding to the EOR in alkaline solution. The CV curve of the as-synthesized mono-metallic Pd/C was also characterized for ethanol electro-oxidation and illustrated in Fig. 6b. As compared with the as-synthesized mono-metallic Pd/C, the Sn and Pt doped PdSn/C and PdSnPt x/C catalysts exhibit a negative shift of onset potential for ethanol oxidation (Table 3). Although the oxidation peak potentials slightly shifted to positive values with the addition of Pt to PdSn/C catalyst, a significant activity enhancement is observed for the PdSnPtx catalyst compared with Pd/C and PdSn/C. Moreover, with increasing of the Pt content in PdSnPt x/C, it depicts a rough Pt-doping dependent “volcano-like” activity enhancement trend and the PdSnPt0.2/C shows the highest specific activity of ca. 7.53 mA cm-2, that is ca. 2.18 times that of Pd/C (ca. 3.46 mA cm-2) and ca. 2 times that of PdSn/C (ca. 3.71 mA cm-2). Moreover, the as-synthesized catalysts were also compared with the commercial Pt/C for ethanol oxidation in alkaline solution as shown in Fig. 6b. Apart from the mono-metallic Pd/C, the PdSnPt0.2/C also exhibits a more superior catalytic activity relative to the Pt/C. According to the result, the peak current density of Pt/C attains to ca. 3.05 mA cm-2, which is even slightly lower than that of Pd/C. In the previous research, Xu et al. [58] also reported that Pd is a good electrocatalyst for ethanol oxidation in alkaline media than Pt. Further evaluation for catalytic durability of the electrocatalysts was conducted by chronoamperometry (CA), as shown in Fig. 6c. It can be clearly seen that the currents for ethanol oxidation at -100 mV (vs. MMO) dropped rapidly at first and then became relatively stable. The initial decline of the current is possible due to the charging current or the catalyst

14

poisoning during ethanol oxidation. However, it is noticed that, the PdSnPt0.2/C still demonstrates the highest current densities in the targeted test period, indicating its superior catalytic durability for ethanol oxidation in alkaline media. Moreover, it can be found that the nano-particulate configuration and uniform distribution of main elements (i.e., Pd, Sn and Pt) are both well preserved, as shown in Fig. 6d. No obvious either etching and loss of carbon support or aggregation of nanoparticles occurs even after suffering through a long-term electrochemical polarization, further indicative of its good microstructure and electrochemical durability under such a simulated operating condition. The activities of the as-synthesized PdSnPtx/C (x=0-0.7) catalysts for EOR in acid media are characterized by CVs in the solution of 0.5 M H2SO4 + 1.0 M EtOH, as shown in Fig. 7a. As expected, the Pt-free PdSn/C reveals the identical voltammogram in ethanol-containing H2SO4 solution to that in blank H2SO4 solution, suggesting it exhibits no electrocatalytic activity for ethanol oxidation in acid media. However, the introduction of a minor amount of Pt content can greatly promote the electrocatalytic activity of PdSn-related catalysts in acid solution, especially when the Pt content exceeds 0.1. With increasing from Pt0.2 to Pt0.7, two oxidation current peaks, I and II, appear upon the positive potential scan while a single anodic peak, III, is evolved in the reverse potential scan at lower potential. Normally, the oxidation peak I is related to the dehydrogenation of ethanol, which can be used to evaluate the electrocatalytic activity of the catalyst. The oxidation peak II at higher potential very likely refers to the oxidation process involving with surface-adsorbed oxygen-like

15

species [33]. The oxidation peak III at ca. 0.4 V (vs. SCE) in the reverse scan is generally attributed to the removal or oxidation of surface adsorbed CO ads. The onset oxidation potential, oxidation peak (I) potential and specific activity for the forward scan on the as-synthesized catalysts were also analyzed and their values are shown in Table 4 corresponding to the EOR in acid solution. Among the PdSnPt x/C catalysts, PdSnPt0.2/C shows the lowest onset potential (ca. 130 mV) and more superior specific activity (ca. 0.912 mA cm-2). Fig. 7b reveals a further activity comparison between the as-synthesized PdSnPt0.2/C, Pd/C and commercial Pt/C. Similar to the PdSn/C, mono-metallic Pd/C also depicts no activity at all in acid medium. The EOR on the Pt/C gave rise to a peak (I) current density of ca. 0.4 mA cm-2 at ca. 0.64 V (vs. SCE), similar to that of the previous report in acidic situation [19]. It is obvious that the as-synthesized PdSnPt0.2/C exhibits a more superior catalytic performance. The CA results for EOR on the PdSnPtx/C (x=0.1-0.7) and Pt/C at a fixed potential of 0.6 V (vs. SCE) are shown in Fig. 7c. The decay rate of current density with time was slow, implying that the addition of Sn and Pt into Pd induced both high activity and good anti-poisoning ability. Among them, the PdSnPt0.2/C demonstrates the highest current densities in the targeted test period. And the stability sequence of the catalysts for ethanol oxidation in acid media is in good agreement with the CV results. It is noticeable that the CV result gives much higher activity than the CA tests at the same potential. This is because CV is a dynamic test and the COads poisons can be oxidized at a higher applied potential whereas the COads poisons will gradually accumulate on the catalyst surface and deteriorate the catalyst activity in the CA test. As shown in

16

Fig. 7d, the uniform nanoparticle dispersion as well as elemental distribution across the whole nanoparticle can also be observed after the long-term CA test in acid solution, demonstrating an excellent anti-aggregating feature same as the situation in alkaline circumstance above mentioned. Previous studies revealed that both Pd and Pt exhibit electro-activities toward EOR in alkaline media, however, there exist different ethanol decomposition progress on them. Ma et al. [59] compared the EOR on Pt/C and Pd/C catalysts in alkaline media and found that, the Pt/C is more active for C-C bond cleavage. The EOR on Pt/C consists of C2 and C1 pathway. In the C2 pathway, the C-C bonds do not break down, and the final product exists in the form of acetate in alkaline solution. In the C1 pathway, the C-C bonds break down and generate C1 species, converting eventually to carbonate. In comparison, the C2 pathway is the main reaction pathway on Pd/C for EOR. Yang et al. [60] also investigated the binary Pd-Pt catalyst for EOR in alkaline medium. It is revealed that one of the elements may alter the electronic properties of the other (e.g., increase of Pt d-electron vacancy) to yield a more active catalytic surface, which will be more effective at breaking the C-C bond of ethanol and enhance the ratio of C1 pathway. Ren et al. [7] found that the binary Pd-Pt catalyst with a high Pd/Pt ratio (3:1) represents more superior catalytic activity toward EOR in alkaline solution and ascribed the causation to the change in the electronic structure of Pt and Pd. The d-band center of Pd will be shifted upward when it is combined with Pt because the lattice constant of Pt (3.92 Ǻ) is larger than that of Pd (3.89 Ǻ) [61]. Thus, the adsorption of OH- can be promoted, facilitating the oxidation of CH3COads.

17

Herein, it is reasonable to assume that the presence of an enhanced synergistic effect between Pd, Sn/SnO2 and Pt in the ternary PdSnPt0.2/C catalyst may trigger a hybrid C1 and C2 pathway with an optimal ratio, leading to the superior catalytic performance for the EOR in alkaline media. For the EOR on PdSnPt x/C in acid media, the synergy of Pt and Sn/SnO2 should play the main function for generating oxidation current because the Pd is proven to be inert for acid-type alcohol electro-oxidation. In acid media, adsorption and decomposition of ethanol and its intermediate occurs on active Pt sites. The neighboring Sn expanded the Pt lattice parameter, which influences the adsorption of these organic molecules and the following oxidation process [21]. In addition, there is an electronic effect that decreases the d band center of Pt, weakening adsorption of the intermediates such as CO and releasing the active electrocatalytic sites. Moreover, partial of Sn oxides on the surface of electrocatalyst may contribute to the oxidative removal of CO and formation of CO 2 via the bi-functional mechanism [62]. It can thus be reasonable to conclude that, the as-revealed superior activity of the PdSnPt0.2/C should be attributed to the Pd/Sn induced beneficial electronic modification of Pt and to the presence of Sn/Sn(O) species resulting in a combination of electronic effect and bi-functional mechanism in acid media [63]. Considering the fact that the trimetallic catalyst system is much more complicated than the binary ones, the in-depth mechanism should be further explored in detail relying on more sophisticated instruments in the future such as the two-dimensional (2D) electron energy loss spectroscopy (EELS), three-dimensional (3D) tomographic reconstruction, time-resolved surface-enhanced infrared absorption

18

(SEIRA) spectroscopy, and so forth.

4. Conclusions In summary, a series of trimetallic PdSnPtx (x=0.1-0.7)/C catalysts with varied Pt content have been synthesized by co-reduction method using NaBH4 as a reducing agent. Although the monometallic Pd and bimetallic PdSn have been proven to obtain good activity toward EOR in alkaline medium, the results herein showed that after adding a minor amount of Pt dopant, the resultant PdSnPt x/C demonstrated more superior catalytic performance and the PdSnPt0.2/C with optimal molar ratio reached the best. With the assistance of Pt dopant, the PdSnPt0.2/C also depicted a superior EOR catalytic activity even in acid circumstance. Moreover, the PdSnPt0.2/C acquired better catalytic activities than the Pt/C in both ethanol-containing alkaline and acid solutions. The possible enhanced synergistic effect between Pd, Sn/Sn(O) and Pt in an alloyed state may contribute to the as-revealed superior EOR catalytic performance owing to the beneficial electronic effect and bi-functional mechanism. These findings make the PdSnPt0.2/C as a promising Janus catalyst within a wide pH range, being beneficial to the development of advanced catalysts with excellent performance in DEFCs.

19

Acknowledgments The authors acknowledge financial support by National Natural Science Foundation of China (51201113), Natural Science Youth Foundation of Shanxi Province (2013021011-4), the 51st China Postdoctoral Science Foundation (2012M510781), Program for Distinguished Young Talents Cultivation from Taiyuan University of Technology (tyut-2013Y010) and Talents Introduction Fund from Taiyuan University of Technology (tyut-rc201115a).

20

References [1] L. Carrette, K. A. Friedrich, U. Stimming, Fuel Cells 1 (2001) 5. [2] E. Antolini, J. Power Sources 170 (2007) 1. [3] C. W. Xu, H. Wang, P. K. Shen, S. P. Jiang, Adv. Mater. 19 (2007) 4256. [4] H. S. Kheshgi, R. C. Prince, G. Marland, Annu. Rev. Energy Environ. 25 (2000) 199. [5] E. Antolini, Enery Environ. Sci. 2 (2009) 915. [6] C. Bianchini, P. K. Shen, Chem. Rev. 109 (2009) 4183. [7] F. F. Ren, H. W. Wang, C. Y. Zhai, M. S. Zhu, R. R. Yue, Y. K. Du, P. Yang, J. K. Xu, W. S. Lu, ACS Appl. Mater. Interfaces 6 (2014) 3607. [8] Z. H. Zhang, C. Zhang, J. Z. Sun, T. Y. Kou, C. C. Zhao, RSC Adv. 2 (2012) 11820. [9] Z. Qi, H. R. Geng, X. G. Wang, C. C. Zhao, H. Ji, C. Zhang, J. L. Xu, Z. H. Zhang, J. Power Sources 196 (2011) 5823. [10] S. T. Nguyen, H. M. Law, H. T. Nguyen, N. Kristian, S. Y. Wang, S. H. Chan, X. Wang, Appl. Catal. B 91 (2009) 507. [11] Q. G. He, W. Chen, S. Mukerjee, S. W. Chen, F. Laufek, J. Power Sources 187 (2009) 298. [12] F. C. Zhu, M. Wang, Y. W. He, G. S. Ma, Z. H. Zhang, X. G. Wang, Electrochim. Acta 148 (2014) 291. [13] S. Y. Shen, T. S. Zhao, J. B. Xu, Electrochim. Acta 55 (2010) 9179. [14] C. W. Xu, Z. Q. Tian, P. K. Shen, S. P. Jiang, Electrochim. Acta 53 (2008) 2610. 21

[15] X. G. Wang, G. S. Ma, F. C. Zhu, N. M. Lin, B. Tang, Z. H. Zhang, Electrochim. Acta 114 (2013) 500. [16] Z. H. Zhang, C. Zhang, J. Z. Sun, T. Y. Kou, Q. G. Bai, Y. Wang, Y. Ding, J. Mater. Chem. A 1 (2013) 3620. [17] W. X. Du, K. E. Mackenzie, D. F. Milano, N. A. Deskins, D. Su, X. W. Teng, ACS Catal. 2 (2012) 287. [18] F. C. Zhu, G. S. Ma, Z. C. Bai, R. Q. Hang, B. Tang, Z. H. Zhang, X. G. Wang, J. Power Sources 242 (2013) 610. [19] H. D. Herrera-Méndez, P. Roquero, M. A. Smit, L. C. Ordóñez, Int. J. Electrochem. Sci. 6 (2011) 4454. [20] F. H. B. Lima, E. R. Gonzalez, Electrochim. Acta 53 (2008) 2963. [21] P. E. Tsiakaras, J. Power Sources 171 (2007) 107. [22] R. Crisafulli, R. M. Antoniassi, A. O. Neto, E. V. Spinacé, Int. J. Hydrogen Energy 39 (2014) 5671. [23] A. Kowal, M. Li, M. Shao, K. Sasaki, M. B. Vukmirovic, J. Zhang, N. S. Marinkovic, P. Liu, A. I. Frenkel, R. R. Adzic, Nat. Mater. 8 (2009) 325. [24] S. Beyhan, J. M. Léger, F. Kadirgan, Appl. Catal. B 103-131 (2013) 305. [25] Y. Wang, S. Q. Song, G. Andreadis, H. Liu, P. Tsiakaras, J. Power Sources 196 (2011) 4980. [26] Y. J. Ma, H. Wang, S. Ji, V. Linkov, R. F. Wang, J. Power Sources 272 (2014) 142. [27] S. H. Yan, S. C. Zhang, Int. J. Hydrogen Energy 36 (2011) 13392.

22

[28] G. L. Li, L. H. Jiang, Q. Jiang, Electrochim. Acta 56 (2011) 7703. [29] B. Xue, P. Chen, Q. Hong, J. Y. Lin, K. L. Tan, J. Mater. Chem. 11 (2011) 2378. [30] M. Wang, Y. W. He, R. X. Li, Z. Z. Ma, Z. H. Zhang, X. G. Wang, Electrochim. Acta 178 (2015) 259. [31] Y. Xiong, Y. Xia, Adv. Mater. 19 (2007) 3358. [32] O. Winjobi, Z. Y. Zhang, C. H. Liang, W. Z. Li, Electrochim. Acta 55 (2010) 4217. [33] R. F. Wang, Z. Y. Liu, Y. J. Ma, H. Wang, V. Linkov, S. Ji, Int. J. Hydrogen Energy 38 (2013) 13604. [34] M. Hakamada, K. Tajima, K. Yoshimura, Y. Chino, M. Mabuchi, J. Alloys Compd. 494 (2010) 309. [35] T. Matsui, K. Fujiwara, T. Okanishi, R. Kikuchi, T. Takeguchi, K. Eguchi, J. Power Sources 155 (2006) 152. [36] S. Zignani, V. Baglio, J. J. Linares, G. Monforte, E. R. Gonzalez, A. S. Arico, Electrchim. Acta 70 (2012) 255. [37] J. Thepkaew, S. Therdthianwong, A. Therdthianwong, A. Kucernak, N. Wongyao, Int. J. Hydrogen Energy 38 (2013) 9454. [38] E. Lee, A. Murthy, A. Manthiram, Electrochim. Acta 56 (2011) 1611. [39] X. Zhang, H. Wang, J. Key, V. Linkov, S. Ji, X. Wang, Z. Lei, R. Wang, J. Electrochem. Soc. 159 (2012) B270. [40] L. K. Ono, J. R. Croy, H. Heinrich, B. R. Cuenya, J. Phys. Chem. C 115 (2011) 6856.

23

[41] H. Wang, Z. Y. Liu, S. Ji, K. L. Wang, T. B. Zhou, R. F. Wang, Electrochim. Acta 108 (2013) 833. [42] G. Chen, D. Xia, Z. Nie, Z. Wang, L. Wang, L. Zhang, J. Zhang, Chem. Mater. 19 (2007) 1840. [43] E. A. Baranova, N. Miles, P. H. J. Mercier, Y. L. Page, B. Patarachao, Electrochim. Acta 55 (2010) 8182. [44] G. E. R. Caballero, P. B. Balbuena, Mol. Simul. 32 (2006) 297. [45] W. J. Zhou, S. Q. Song, W. Z. Li, Z. H. Zhou, G. Q. Sun, Q. Xin, S. Douvartzides, P. Tsiakaras, J. Power Sources 140 (2005) 50. [46] H. M. Villullas, F. I. Mattos-Costa, L. O. S. Bulhoes, J. Phys. Chem. B 108 (2004) 12898. [47] W. L. Qu, Z. B. Wang, Z. Z. Jiang, D. M. Gu, G. P. Yin, RSC Adv. 2 (2012) 344. [48] R. Awasthi, R. N. Singh, Int. J. Hydrogen Energy 37 (2012) 2103. [49] X. G. Wang, W. M. Wang, Z. Qi, C. C. Zhao, H. Ji, Z. H. Zhang, Electrochem. Commun. 11 (2009) 1896. [50] E. M. Cunha, J. Ribeiro, K. B. Kokoh, A. R. Andrade, Int. J. Hydrogen Energy 36 (2011) 11034. [51] H. X. Zhang, C. Wang, J. Y. Wang, J. J. Zhai, W. B. Cai, J. Phys. Chem. C 114 (2010) 6446. [52] E. Lee, I. S. Park, A. Manthiram, J. Phys. Chem. C 114 (2010) 10634. [53] X. G. Wang, W. M. Wang, Z. Qi, C. C. Zhao, H. Ji, Z. H. Zhang, Int. J. Hydrogen Energy 37 (2012) 2579.

24

[54] Z. P. Sun, X. G. Zhang, R. L. Liu, Y. Y. Liang, H. L. Li, J. Power Sources 185 (2008) 801. [55] X. G. Wang, W. M. Wang, Z. Qi, C. C. Zhao, H. Ji, Z. H. Zhang, J. Alloys Compd. 508 (2010) 463. [56] C. W. Xu, Y. L. Liu, D. S. Yuan, Int. J. Electrochem. Sci. 2 (2007) 674. [57] Y. W. Lee, S. B. Han, K. W. Park, Electrochem. Commun. 11 (2009) 1968. [58] C. W. Xu, L. Q. Cheng, P. K. Shen, Y. L. Liu, Electrochem. Commun. 9 (2007) 997. [59] L. Ma, D. Chu, R. R. Chen, Int. J. Hydrogen Energy 37 (2012) 11185. [60] X. Yang, Q. D. Yang, J. Xu, C. S. Lee, J. Mater. Chem. 22 (2012) 8057. [61] B. Hammer, J. K. Nørskov, Surf. Sci. 343 (1995) 211. [62] L. S. Parreira, J. C. M. Silva, M. D’Villa-Silva, F. C. Simões, S. Garcia, I. Gaubeur, M. A. L. Cordeiro, E. R. Leite, M. C. Santo, Electrochim. Acta 96 (2013) 243. [63] E. V. Spinacé, M. Linardi, A. O. Neto, Electrochem. Commun. 7 (2005) 365.

25

Figure captions: Fig. 1 (a) XRD patterns of the as-synthesized catalysts and (b) the zoomed view of the (111) reflections. Fig. 2 (a) TEM and (b,c) HRTEM images of the PdSnPt0.2/C catalysts. (d) HAADF-STEM image together with elemental maps for Pd, Sn and Pt. (e) the typical EDX spectrum. The inset in (b) is the corresponding particle size distribution histogram. Fig. 3 X-ray photoelectron spectra of (a) Pd 3d in PdSn/C, (b) Sn 3d in PdSn/C, (c) Pd 3d in PdSnPt0.2/C, (d) Sn 3d in PdSnPt0.2/C and (e) Pt 4f in PdSnPt0.2/C. Fig. 4 CV curves of the as-synthesized (a) Pd/C, (b) PdSn/C, (c) PdSnPt0.1/C, (d) PdSnPt0.2/C, (e) PdSnPt0.3/C, (f) PdSnPt0.5/C and (g) PdSnPt0.7/C catalysts in the 0.5 M H2SO4 solution. Scan rate: 50 mV s-1. (h) and (i) correspond to the zoomed Hupd desorption and oxide reduction curves. Fig. 5 CO stripping curves of the as-synthesized catalysts in the 0.5 M H2SO4 solution (solid line: the 1st scan; dashed line: the 2nd scan). Scan rate: 50 mV s-1. Fig. 6 (a) CV curves of the as-synthesized PdSnPt x/C (0≤x≤0.7) and (b) activity comparison of as-synthesized PdSnPt0.2/C, Pd/C and commercial Pt/C in the solution of 1.0 M KOH + 1.0 M EtOH. Scan rate: 50 mV s -1. (c) CA curves of the catalysts in the same solution at a fixed potential of -100 mV (vs. MMO). (d) Morphology and elemental mapping of the PdSnPt0.2/C after the CA test in (c). Fig. 7 (a) CV curves of the as-synthesized PdSnPt x/C (0≤x≤0.7) and (b) activity comparison of as-synthesized PdSnPt0.2/C, Pd/C and commercial Pt/C in the 26

solution of 0.5 M H2SO4 + 1.0 M EtOH. Scan rate: 50 mV s -1. (c) CA curves of the catalysts in the same solution at a fixed potential of 600 mV (vs. SCE). (d) Morphology and elemental mapping of the PdSnPt0.2/C after the CA test in (c).

27

Tables: Table 1 Atomic compositional ratio, average crystallite size and lattice parameters of the as-synthesized catalysts Catalyst

Pd/C PdSn/C PdSnPt0.1/C PdSnPt0.2/C PdSnPt0.3/C PdSnPt0.5/C PdSnPt0.7/C

Nominal

Actual

Lattice parameter (Ǻ)

Crystallite sizes (nm)

compositional ratio

compositional ratio

---

---

3.895

10.179

1：0.2

1：0.25

3.901

9.163

1：0.2：0.1

1：0.15：0.06

3.902

8.380

1：0.2：0.2

1：0.2：0.16

3.908

7.379

1：0.2：0.3

1：0.15：0.28

3.914

6.995

1：0.2：0.5

1：0.24：0.49

3.925

5.640

1：0.2：0.7

1：0.29：0.72

3.931

5.142

Table 2 CO-stripping parameter of the as-synthesized catalysts Catalyst Pd/C PdSn/C PdSnPt0.1/C PdSnPt0.2/C PdSnPt0.3/C PdSnPt0.5/C PdSnPt0.7/C

Onset potential /(mV vs SCE)

Peak potential /(mV vs SCE)

SEASA /(cm mg-1 metal)

598 406 248 144 204 272 302

735 726 571 382 595 581 561

67.87 80.27 292.10 152.60 41.66 30.97 53.62

2

Table 3 Cyclic voltammogram parameters for ethanol oxidation on as-synthesized catalysts in the electrolyte of 1.0 M KOH + 1.0 M EtOH. Catalyst

Onset potential /(mV vs MMO)

Peak potential /(mV vs MMO)

Specific activity /( mA cm-2)

Pd/C PdSn/C PdSnPt0.1/C PdSnPt0.2/C PdSnPt0.3/C PdSnPt0.5/C PdSnPt0.7/C

-544 -580 -548 -551 -548 -571 -545

-151 -171 -138 -152 -154 -109 -41

3.46 3.71 4.84 7.53 5.57 4.52 5.35

28

Table 4 Cyclic voltammogram parameters for ethanol oxidation on as-synthesized catalysts in the electrolyte of 0.5 M H2SO4 + 1.0 M EtOH. Catalyst

Onset potential /(mV vs SCE)

Peak potential /(mV vs SCE)

Specific activity /( mA cm-2)

Pd/C

-

-

-

PdSn/C PdSnPt0.1/C PdSnPt0.2/C PdSnPt0.3/C PdSnPt0.5/C

251 130 188 156

684 687 685 682

0.397 0.912 0.759 0.791

PdSnPt0.7/C

132

687

0.866

29

Figures:

Fig. 1

30

Fig. 2

31

Fig. 3

32

Fig. 4

33

Fig. 5

34

Fig. 6

35

Fig. 7

36

Graphical Abstract