Palladium-based nanocatalysts anchored on CNT with high activity and durability for ethanol electro-oxidation

Accepted Manuscript Palladium-based nanocatalysts anchored on CNT with high activity and durability for ethanol electro-oxidation Lina Ning, Xianhu Liu, Min Deng, Zhengzheng Huang, Aimei Zhu, Qiugen Zhang, Qinglin Liu PII:

S0013-4686(18)32672-0

DOI:

https://doi.org/10.1016/j.electacta.2018.11.188

Reference:

EA 33189

To appear in:

Electrochimica Acta

Received Date: 15 October 2018 Revised Date:

24 November 2018

Accepted Date: 26 November 2018

Please cite this article as: L. Ning, X. Liu, M. Deng, Z. Huang, A. Zhu, Q. Zhang, Q. Liu, Palladiumbased nanocatalysts anchored on CNT with high activity and durability for ethanol electro-oxidation, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.11.188. 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.

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Graphical Abstract:

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Lina Ning, Xianhu Liu, Min Deng, Zhengzheng Huang, Aimei Zhu*, Qiugen Zhang,

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Qinglin Liu

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Palladium-based nanocatalysts anchored on CNT with high activity and durability for ethanol electro-oxidation

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Department of Chemical & Biochemical Engineering, The College of Chemistry and

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Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China

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*Corresponding author

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Tel: +86-592-2188072, Fax: +86-592-2184822

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E-mail: [email protected] (Aimei Zhu), [email protected] (Lina Ning),

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[email protected] (Xianhu Liu), [email protected] (Min Deng), 416692905

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@qq.com

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[email protected] (Qinglin Liu)

Huang),

[email protected]

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(Zhengzheng

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(Qiugen

Zhang),

ACCEPTED MANUSCRIPT Abstract: The excellent catalyst need to meet low cost, long-time stability and high

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electrocatalytic activity in direct ethanol fuel cells (DEFC). Here, carbon

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nanotube-supported Pd-baesd (Pd/CNT, PdSn/CNT, PdNi/CNT and PdSnNi/CNT)

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catalysts were synthesized by microwave-assisted polyols and in-situ reduction.

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Compared with commercial Pd/C (JM), the as-prepared Pd-based catalysts have

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higher electrocatalytic activity and outstanding long-time durability toward ethanol

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electro-oxidation reaction in alkaline media. The enhanced electrocatalytic activity of

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Pd-based catalysts are owning to high electrochemically active surface area (ECSA)

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and modified electronic structure of Pd. After chronoamperometric test, the current

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densities of Pd-based catalysts can be reactivated by simple cycle potential scan in 1

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M NaOH, which indicated that all of them have excellent resistance to CO poisoning.

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Keywords:

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reactivation

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electro-oxidation

reaction;

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Ethanol

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Pd-based

catalyst;

Catalyst

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1. Introduction It is an urgent need that develops a green and pollution-free energy device due to

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resource shortage and environmental problems [1-2]. Direct alcohol fuel cell (DAFC)

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can efficiently transfer chemical energy to electrical energy without pollution, which

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has attracted an extensive attention because of environmental friendliness, easy

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storage and high power density [3]. Among of fuel cell devices, direct ethanol fuel

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cell (DEFC) is an idea candidate because of high energy density and non-toxic of

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ethanol [4-5]. However, the commercialization of fuel cell technology has been

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hindered by the low activity and durability and high cost of catalysts [4]. Pt, as a

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noble metal, has certain properties for the electrochemical oxidation of alcohol

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small-molecule [6-10]. Nonetheless, Pt is not only vulnerable to intermediate species,

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but also has poor performance in alkaline medium [11-12]. Compared with Pt-based

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catalysts, Pd has received widespread attention due to its facilitation in ethanol

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oxidation reaction (EOR) kinetics in alkaline environment [13-16]. In order to

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improve the electrocatalytic activity and durability and reduce the cost of catalysts,

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great efforts have been taken to meet the practical demand, such as introducing other

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metal or nonmetal, using support and so on [17-19].

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The presence of oxophilic metals such as Sn and Ni promotes the formation of

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OHads and drives the EOR without the generation of poisoning by-products, for

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instance, CO [19-20]. It is currently believed that the existence of the second additive

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transition metal could cause appropriate modification of electronic properties or/and

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atomic structure of Pd surface, which not only promotes its activity and stability, but 3

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also dramatically decreases Pd loading so much to lower the usage of noble metals

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[21-25]. In addition to elementary composition, the catalytic performance is related to the

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size, shape and surface morphology of the catalyst [26-29]. Microwave method is a

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simple and fast technique for the preparation of nanoparticles with narrow particle

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size. In general, nanoparticles are more likely to aggregate with the size reducing.

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Stabilizers, such as PVP, are used to protect nanoparticles from reuniting [30].

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Besides, the dispersity of nanoparticles is improved effectively by using appropriate

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support. Meanwhile, the performance of catalyst anchored on support is enhanced due

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to the electronic interactions between Pd and the support [31-32]. The support need to

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meet high surface area, well electrical and thermal conductivity, low cost and strong

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corrosion resistance [33]. Carbon materials, especially activated carbon, are the

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mostly widely used. However, it is susceptible to poisoning in the progress of

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catalytic reaction [34-37]. Carbon nanotube (CNT) has fascinating chemical stability

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and mechanical strength, in addition to the requirement of support, which make CNT

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a suitable candidate as the support for fuel cell applications [38].

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In this work, Pd-based catalysts (including Pd/CNT, PdSn/CNT, PdNi/CNT and

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PdSnNi/CNT) were successfully constructed by the combination of microwave

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assisted polyols and in-situ reduction. The as-prepared Pd-based catalysts showed

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excellent activity and superior stability toward EOR. Especially, after stability test, the

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as-prepared catalysts can be reactivated by a simple cycle potential scans in 1 M fresh

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NaOH electrolyte, which are attributed to the excellent resistance to poisoning. 4

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2. Experimental

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2.1 Materials Tin (II) chloride (SnCl2) was purchased from Aladdin. Palladium chloride (PdCl2)

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and NiCl2·6H2O were obtained from Shanghai Chemical Factory (Shanghai, China).

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Multi-walled carbon nanotube functioned with carboxylic acid and Nafion@117

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solution (0.5 wt. % solution in a mixture of ethanol and water) were obtained from

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Aldrich. All chemicals used in the work were of analytical grade. Ultrapure water (18

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MΩ cm) was provided by the Millipore system.

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2.2 Preparation of catalysts

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The synthesis process PdSn/CNT catalysts was shown in Fig. 1. Sn nanoparticles

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(Sn NPs) were synthesized firstly. 1.5 mL of SnCl2 (0.0294 M) solution was added to

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20 mL of ethylene glycol (EG), then adjusted the pH to 10 using 0.5 M NaOH/EG

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solution. After that, the mixture was kept in the microwave reactor at 130 ºC for 10

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min. The above Sn NPs sol was diluted with 20 mL of EG, followed by adding a

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certain amount of PdCl2 (0.09 M) solution. The mixed solution was heated to 130 ºC

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and kept for 3 h after adjusting the pH to 10. 10 mg of multi-walled carbon nanotube

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(MWCNT) was mixed with the PdSn composite nanoparticles sol by sonication. After

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2 h ultrasonic dispersion, the above solution was stirred at 60 ºC for 12 h. The product

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was collected by centrifugation and washed with Ultrapure water and ethanol several

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times. Afterwards, PdSn/CNT nanocomposites were dried in an oven at 60 ºC. The

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as-obtained catalysts prepared using 1.0, 1.8 and 2.0 mL PdCl2 were recorded as

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Pd2.0Sn/CNT, Pd3.3Sn/CNT and Pd5.0Sn/CNT, respectively. The preparation progress

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ACCEPTED MANUSCRIPT of Pd/CNT, PdNi/CNT and PdSnNi/CNT catalysts are similar to that of PdSn/CNT.

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Fig. 1. Schematic diagram for the preparation of Pd-based catalysts.

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2.3 Characterization

Transmission electron microscopy (TEM) was employed to analyze the

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morphology of the Pd-based catalysts. The element content and ratio were detected by

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an energy-dispersive X-ray spectroscope (EDX) analyzer attached to the scanning

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electron microscope (SEM). The chemical state of catalysts was analyzed by an X-ray

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photoelectron spectroscopy (XPS) analysis on a PHI QUANTUM 2000 XPS system

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with a monochromatic Al Kα source and a charge neutralizer. The X-ray diffraction

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(XRD) was used to analyze crystalline state of catalysts. It recorded on a Rigaku

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(miniflex) equipped with a Ni filter using Cu Kα radiation (λ = 1.54056 Å) at 35 kV

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and 15 mA.

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2.4 Electrochemical measurements

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All electrochemical tests were performed in a standard three electrode system with

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a CHI 660E electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, 6

ACCEPTED MANUSCRIPT China). A glass carbon (geometric area 0.07 cm2), an Ag/AgCl (3.0 M KCl, E0 = 0.22

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V vs. RHE) and a platinum wire served as the working electrode, the reference

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electrode and the counter electrode, respectively. The glassy carbon electrode (GCE)

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was polished with aluminium oxide power and ultrasonically washed for 15 min. 15

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µL catalyst ink was coated onto the GC surface in order to make a catalyst loading

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0.085 mg cm-2. Then, 10 µL nafion solution (0.5 wt. %) was carefully dropped on the

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catalyst layer dried at 25 ºC for electrochemical test. The cyclic voltammetry (CV)

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measurements were characterized in nitrogen-purged 1 M NaOH solution between

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-0.8 - 0.6 V at a scan rate of 50 mV s-1 to clean and activate the electrode surface. The

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activity and stability of the catalysts were characterized in nitrogen-purged 1 M

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NaOH + 1 M C2H5OH solution.

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3. Results and discussions

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Fig. 2 displays the typical TEM images of Pd-based catalysts. Fig. 2a-c shows that

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the nanoparticles were successfully decorated on the MWCNT without aggregation.

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Otherwise, a small part of nanoparticles gets inside the MWCNT due to its small size.

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In general, nanoparticles in cavity structure are prepared by impregnation reduction.

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The precursor solution can enter nanocage and be reduced in-site. In this work, the Sn

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NPs were prepared first in the EG solution. EG acts not only as a solvent, but also as a

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reductant. Nanoparticles can disperse well in EG without other protectants, due to EG

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is a viscous liquid. The nanoparticles are still isolated instead of aggregating in

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Pd5.0Sn/CNT catalyst, which is due to the microwave method provides a uniform and

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quick nucleation environment. Mapping images showed that both Pd and Sn are

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distributed homogeneously on the MWCNT, confirming the formation of

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Pd3.3Sn/CNT catalyst (Fig. S1). Pd nanoparticles anchored on MWCNT also has well

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distribution (Fig. 2d). Pd3.3Ni and PdSnNi composite nanoparticles with relatively small particle sizes

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successfully loaded on MWCNT (Fig. 2e-f). Though Pd3.3Ni and PdSnNi composites

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nanoparticles cannot expose many active sites like monodispersed nanoparticles, their

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unique network structure has its special advantages [30]. The metal loading on

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MWCNT was determined by EDX, as summarized in Table 1.

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Fig. 2. TEM images of (a) Pd2.0Sn/CNT, (b) Pd3.3Sn/CNT, (c) Pd5.0Sn/CNT, (d)

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Pd/CNT, (e) Pd3.3Ni/CNT and (f) PdSnNi/CNT catalysts.

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Table 1 Relative contents of Pd, Sn and Ni in as-prepared Pd-based catalysts. Atomic % from EDX

Molar % from EDX

Sample Sn

Ni

Pd

Sn

Ni

Pd2.0Sn/CNT

0.96

0.48

–

7.23

4.03

–

Pd3.3Sn/CNT

1.43

0.43

–

10.42

3.50

–

Pd5.0Sn/CNT

2.00

0.41

–

14.03

3.21

–

Pd3.3Ni/CNT

1.55

–

0.47

PdSnNi/CNT

1.01

0.43

0.48

Pd/CNT

1.61

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–

1.91

7.45

3.54

1.95

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Pd

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11.96

Fig. 3 show the XRD patterns used to analyze the crystalline nature of the Pd/CNT,

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PdSn/CNT, Pd3.3Ni/CNT and PdSnNi/CNT catalysts. The broad peak at about 25.5º is

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evidently attributed to the (002) crystal plane of the MWCNTs [39]. The characteristic

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peaks concerning Pd of Pd-based catalysts included the peaks at around 39.8º, 46.2º,

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67.6º and 81.3º, which referred to (111), (200), (220) and (311) of Pd face centered

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cubic (fcc) crystal, respectively [40]. Compared with the standard peak position, the

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diffraction peaks of PdSn/CNT have a negative shift, which reflected the

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incorporation of Sn atom into Pd lattice. Because the atomic radius of Sn is larger

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than that of Pd, the doping of Sn atoms will increase the lattice parameters of Pd, then

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the diffraction peak will have a negative deviation [41]. There is a positive shift of Pd

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peak in Pd3.3Ni/CNT, indicating that the lattice parameter of Pd is shrunk because Pd

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ACCEPTED MANUSCRIPT atoms were doped with Ni that has smaller atomic radius. The diffraction peak of Pd

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in PdSnNi/CNT catalyst has no obvious deviation. Compared with PdSn/CNT catalyst,

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the doping of Ni in PdSnNi/CNT catalyst can cause lattice contraction, as the

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diffraction peak changes in Pd3.3Ni/CNT, thus reducing the overall change. However,

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the apparent inverse effect of Sn and Ni on Pd lattice constants makes it easy to adjust

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the geometric and electronic properties of Pd in the three-element catalyst.

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Fig. 3. XRD patterns of Pd/CNT, PdSn/CNT, Pd3.3Ni/CNT and PdSnNi/CNT

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catalysts.

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The XPS was carried out to investigate the chemical state of metallic nanoparticles

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and study the influence of Sn and Ni from the electronic effect. The spectrum of Pd 3d

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in Pd/CNT is deconvoluted into four peaks corresponding to Pd and Pd (II), as shown

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in Fig. 4a. The peaks at 340.8 and 335.4 eV are ascribed to Pd 3d3/2 and Pd 3d5/2 with

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the metallic Pd state, whereas the peaks at 342.6 and 337.1 eV correspond to Pd 3d3/2

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and Pd 3d5/2 with Pd (II), indicating that part of Pd exists as an oxidized state. The Pd

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peaks suggest that electron transfer from Pd to the CNT. Compared to pure Pd, the 3d

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peak of Pd have a positive shift in Pd3.3Sn/CNT catalyst, which means Pd is in 10

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ACCEPTED MANUSCRIPT different electronic environment due to the charge transfer between Pd and Sn (Fig.

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4b) [42-44]. That is to say, the core-level of Pd shifts down with respect to the Fermi

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level of Pd, corresponding to a downshift of the d-band center of Pd [19]. The

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d-banding center of the surface atoms can be used to describe the surface activity

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according to the reports [45-46]. A suitable downshift of d-band center can weaken

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the adsorption of both intermediates and reactants in a modest manner, favoring the

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EOR [47]. The binding energy of Pd 3d in Pd3.3Ni/CNT catalyst is approximate to that

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of Pd3.3Sn/CNT catalyst. The spectrum corresponding to Pd 3d5/2 and Pd 3d3/2 states in

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PdSnNi/CNT catalyst that appear around 335.8 and 337.3 eV are attributed to the

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species of Pd (0), while the two additional peaks at 341.1 and 342.6 eV were ascribed

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to Pd (II). Similarly, the Pd 3d peaks of Pd3.3Ni/CNT and PdSnNi/CNT catalysts are

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positively shifted, which indicated that there is empty valence electron charge [48].

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Fig. 4. Pd 3d spectra of (a) Pd/CNT, (b) Pd3.3Sn/CNT, (c) Pd3.3Ni/CNT and (d)

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PdSnNi/CNT catalysts.

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The electrochemical behavior of PdSn/CNT and Pd/C (JM) catalysts were shown in

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Fig. 5a-c. All catalysts were scanned 40 cycles during the testing process. Fig. 5a

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shows the CV curves of the catalysts performed in 1 M NaOH solution at the scan

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speed of 50 mV s-1. All of them display the typical response feature of Pd surface in

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alkaline solution. The peak in the range of -0.7 - 0.5 V is for the oxidation of the

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adsorbed and absorbed hydrogen: Pd–Habs/ads + OH− → Pd + H2O + e− [49-51]. The

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peak at the far negative potential, which partially overlaps the hydrogen desorption

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peak is attributed to the adsorption of OH−: Pd + OH− ↔ Pd + H2O + e− [51-53].

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When the chemisorbed OH− are transformed into higher valence oxides at higher 12

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ACCEPTED MANUSCRIPT potentials, the peak appearing above -0.26 V can be assigned to the formation of the

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PdO layer: Pd–OHads + OH− ↔ Pd–O + H2O + e−; Pd–OHads + Pd–OHads ↔ Pd-O +

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H2O [51-53]. The peak at around -0.47 V is attributed to the adsorption of OH−. It is

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less prominent of Pd/C (JM) in the CV, which can illustrate Pd in the as-prepared

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PdSn/CNT catalysts are more easy to absorb OH group compared with Pd/C (JM)

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[42]. At the cathodic scan, the reduction process of the PdO2 to PdO correspond to the

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peak between 0.14 - 0.29 V vs. Ag/AgCl. The peak at -0.40 V is assigned to the

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reduction of PdO to Pd [51-52, 54]. It is the basis for calculating the electrochemically

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active surface area (ECSA) of the catalyst, which is a vital parameter for the active

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site of the electro-catalyst. The ECSAs could be calculated by the charge required for

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oxygen desorption, which are the area of the reduction peak of PdO in the CVs of the

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PdSn/CNT catalysts in 1.0 M NaOH [55]. The ECSAs of catalysts are calculated by

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the equation ECSA = Q/(0.42 m), where Q is the coulombic charge according to

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integrating the peak area of the reduction of PdO (mC); 0.42 represents the charge

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required for the reduction of a PdO monolayer (mC cm-2Pd), and m is the total amount

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of Pd (mg) loaded on the catalyst [56]. The ECSAs of PdSn/CNT and Pd/C (JM)

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catalysts follow the order: Pd3.3Sn/CNT (188 m2 gPd-1) > Pd5.0Sn/CNT (125 m2 gPd-1) >

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Pd2.0Sn/CNT (70 m2 mgPd ) > Pd/C (JM) (50 m2 gPd-1), indicating that the exposed

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active sites of PdSn/CNT catalysts are more than that of Pd/C (JM), which will

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provide higher electro-catalytic activity.

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The electrocatalytic activity of the PdSn/CNT and Pd/C (JM) catalysts in the

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ethanol oxidation reaction was investigated in 1.0 M NaOH + 1.0 M C2H5OH solution 13

ACCEPTED MANUSCRIPT at a scan rate of 50 mV s-1. Fig. 5b shows the dominant characteristic peak of ethanol

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oxidation with PdSn/CNT and Pd/C (JM) catalysts. In the forward scan progress, the

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peak corresponds to the oxidation process of ethanol. In the early stage of the

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potential increase, toxic species are produced and the catalyst is poisoned, thus the

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current density goes up slowly. As the potential is further increased, the oxidation of

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ethanol occurs. The characteristic peak at around -0.11 V corresponded to the

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oxidation progress of chemisorbed species derived from ethanol. As the potential is

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further increased, the ethanol oxidation current drops rapidly owing to the formation

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of Pd oxide, which restraints the electrooxidation of ethanol molecule [57]. In the

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reverse scan progress, the characteristic peak on the left corresponds to the oxidation

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of residual carbon [58]. It is notable that the onset potential of PdSn/CNT catalysts is

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more negative than that of Pd/C (JM), indicating that the as-prepared catalysts are

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much more favorable for the electrooxidation of ethanol.

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Fig. 5. CV of Pd-based catalysts measured in (a, d) 1 M NaOH and (b, e) 1 M NaOH

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+ 1 M C2H5OH at a scan rate of 50 mV s-1; (c, f) CA of Pd-based catalysts measured

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in 1 M NaOH + 1 M C2H5OH at -0.2 V vs. Ag/AgCl.

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The activity of PdSn/CNT catalysts is significantly higher than that of Pd/C (JM)

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due to the large electrocatalytic active area, synergistic and electronic effects (Fig. 5b).

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The current densities on Pd2.0Sn/CNT, Pd3.3Sn/CNT, Pd5.0Sn/CNT and Pd/C (JM)

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catalysts are 3055, 3156, 3434 and 627 A g-1, respectively. Though Sn cannot directly 15

ACCEPTED MANUSCRIPT adsorb or oxidize organic molecules, it is expected to adsorb −OH more easily than Pd.

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What is more, the addition of Sn modified the electronic structure of Pd atom, which

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can improve the electrocatalytic activity of PdSn/CNT catalysts. The peak current

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density ratio of If/Ib can be used to reflect the resistance of a catalyst to accumulation

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of carbonaceous intermediates, where If and Ib represent the peak current density in

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the forward and reverse scan, respectively [59]. The If/Ib of as-prepared Pd2.0Sn/CNT,

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Pd3.3Sn/CNT and Pd5.0Sn/CNT catalysts are 1.6, 1.4 and 1.7, respectively.

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To further evaluate the stability, chronoamperometry curves of PdSn/CNT and Pd/C

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(JM) catalysts were performed in 1 M NaOH + 1 M C2H5OH at -0.2 V during the

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4000 s test. It is obvious that the original current densities of as-prepared PdSn/CNT

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catalysts are much higher than that of Pd/C (JM) (Fig. 5c), indicating a large number

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of active sites exposed in PdSn/CNT catalysts, which could be attributed to the well

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dispersion of nanoparticles and the electronic effect of Pd in PdSn/CNT catalysts. The

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enhanced current density values of PdSn/CNT catalysts demonstrate that the

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PdSn/CNT catalysts are much more favorable for ethanol oxidation. After 500 s, the

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current densities of Pd2.0Sn/CNT and Pd3.3Sn/CNT catalysts remained almost constant,

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indicating an excellent tolerance to the carbonaceous species generated in the process

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of ethanol oxidation. What is more, the residual current densities of PdSn/CNT

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catalysts can also keep outstanding level after 4000 s I-t test. The residual current

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densities of Pd2.0Sn/CNT (1153 A g-1), Pd3.3Sn/CNT (1253 A g-1), Pd5.0Sn/CNT (1064

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A g-1) catalysts are much excellent than that of Pd/C (JM) (116 A g-1) catalyst. The

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current density of Pd3.3Sn/CNT can keep at a stable and high level. So, there is an

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ACCEPTED MANUSCRIPT optimum ratio of Pd and Sn. At the beginning of test, almost all the active sites are

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exposed which can provide high oxidation current. Then, a part of active sites is

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poisoned by intermediate species produced in the EOR, resulting in a significant

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decrease in current density. Afterwards, all of PdSn/CNT and Pd/C (JM) catalysts

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trend to be stable. The promotion of the activity and stability of PdSn/CNT catalysts is

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because of the incorporation of Sn. The Sn in PdSn/CNT catalysts can enhance the

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tolerance to CO like intermediate species and facilitate the kinetics of ethanol

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oxidation conforming to electronic effect, which favors outstanding activity and

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stability toward ethanol electrooxidation [60]. The excellent performance of

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PdSn/CNT catalysts is ascribed to the use of CNT and the properly facilitate the

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remove of the intermediate species from the Pd sites.

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The electrochemical properties of Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT

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catalysts were also studied. The distinct peaks between -0.6 to -0.36 V are attributed

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to the absorption of OH− in the alkaline solution (Fig. 5d), which illustrated OH− can

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be readily absorbed on the surface of Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT

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catalysts. Moreover, the oxidation and reduction peaks of Ni species appears. The

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sharp peaks at about -0.45 V in the forward sweep correspond to the reduction peak of

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palladium oxide. The peak potential of Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT

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catalysts are -0.47 and -0.45 V, respectively. The reduction potential of Pd/CNT,

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Pd3.3Ni/CNT and PdSnNi/CNT catalysts have a certain negative shifts compared with

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Pd/C (JM) catalyst. The ECSAs of Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT are 134,

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250 and 267 m2 gPd-1, respectively. The enhancement of active sites of Pd/CNT

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ACCEPTED MANUSCRIPT catalyst is ascribed to the good dispersion of the Pd nanoparticles. Additionally, the

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CNT support provides an accessible conductive path for the charged ion transmission.

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The incorporation of Ni and the use of CNTs as support leads to the enhanced ECSAs

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of Pd3.3Ni/CNT and PdSnNi/CNT catalysts, thus improving the electrocatalytic

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activity of Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT catalysts. The characteristic peaks

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of ethanol electrooxidation on Pd3.3Ni/CNT and PdSnNi/CNT catalysts were obvious

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(Fig. 5e). The onset potential of ethanol oxidation on Pd/CNT, Pd3.3Ni/CNT and

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PdSnNi/CNT catalysts are similar to that of the PdSn/CNT catalysts and have a

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negative shift, indicating Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT catalysts have

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preferable intrinsic electrocatalytic capability for EOR. Although the reaction

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mechanism of ethanol oxidation progress is indistinct, it has been accepted that the

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ethanol electrooxidation on Pd surface in alkaline environment is initialized through a

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dehydration process, and then the adsorbed hydroxyl groups (OHads) play a significant

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role in the oxidation process, as described by [42, 61-62]

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CH3CH2OH + 3OH− → CH3COads + 3H2O + 3e−

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CH3COads + OHads → CH3COOH (rate limiting step)

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CH3COOH + OH− → CH3COO− + H2O Thus, the ethanol electrooxidation activity is related to the OHads on the surface of

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Pd [63]. The oxophilic metals (Ni and Sn) in PdNi/CNT and PdSnNi/CNT catalysts

20

can facilitate the formation of OH radicals, and that combine with CH3CO radicals on

21

the adjacent Pd active sites to generate acetate ions (Fig. S1). That is to say, the

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resulting oxygen-containing species can contribute to the oxidation of carbonaceous 18

ACCEPTED MANUSCRIPT intermediates and release the Pd active sites, thus drives the ethanol electrooxidation

2

reaction. What is more, Ni has catalytic properties for EOR in nature. The oxidation

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current density of ethanol in PdSnNi/CNT catalyst is much higher than that of Pd/C

4

(JM) catalyst. Compared with PdSn/CNT catalysts, the current density value of

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PdSnNi/CNT catalyst is also more outstanding while the peak potential is more

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negative. The If/Ib of Pd3.3Ni/CNT and PdSnNi/CNT catalysts are 2.6 and 2.1,

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respectively, which were higher than that of PdSn/CNT catalysts. It means that

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Pd3.3Ni/CNT and PdSnNi/CNT catalysts have excellent tolerance to carbonaceous

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intermediates.

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The stability of Pd/CNT, Pd3.3Ni/CNT and PdSnNi/CNT catalysts were also

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evaluated (Fig. 5f). At the initial stage, the current densities of Pd3.3Ni/CNT and

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PdSnNi/CNT catalysts occurred a certain drop, which is attributed to the formation of

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CO poisoning species during the ethanol electrooxidation process in alkaline

14

environment. Afterwards, the current densities of all as-prepared catalysts tend to be

15

stable and keep at a high level. The above results demonstrated that PdNi/CNT and

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PdSnNi/CNT catalysts possess commendable electrochemical stability towards EOR

17

(Fig. S1). Among all the catalysts, PdSnNi/CNT catalyst have the highest initial

18

current density, the slowest rate of descent and the most stable residual current density,

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indicating the best catalytic stability and resistance to intermediate species poisoning.

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The CO poisoning effects of the Pd-based catalysts were also investigated by

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obtaining CO-stripping voltammograms in 1 M NaOH solution (Fig. 6). The onset

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potentials of Pd-based catalysts were about -0.18 V. The potential value on 19

ACCEPTED MANUSCRIPT PdSnNi/CNT is more negative than that on PdSn/CNT, PdNi/CNT and Pd/CNT,

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which indicated PdSnNi/CNT possesses better COads intermediate tolerance capability

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during the ethanol oxidation reaction.

Fig. 6. CVs of CO stripping on Pd-based catalysts in 0.1 M NaOH solution

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Fig. 7. 5 consecutive sweeps of CAs on the (a-c) PdSn/CNT, (d) Pd/CNT, (e)

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Pd3.3Ni/CNT and (f) PdSnNi/CNT catalysts obtained in 1 M NaOH + 1 M C2H5OH at

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-0.2 V vs. Ag/AgCl.

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Interestingly, after the 4000 s chronoamperometry stability test, the partially

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deactivated Pd/CNT, PdSn/CNT, Pd3.3Ni/CNT and PdSnNi/CNT catalysts could be

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easily reactivated with several CV cycles in 1.0 M fresh NaOH (Fig. 7). The simple

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cycle potential scanning in an alkaline solution is sufficient to remove the 21

ACCEPTED MANUSCRIPT carbonaceous species absorbed on Pd surface and recover the electrocatalytic activity

2

of the as-prepared Pd-based catalysts. Consecutive reactivated cycles of Pd-based

3

catalysts present similar electrocatalytic activity. The above results demonstrate that

4

the as-prepared Pd-based catalysts have good ability to strip CO adsorbed on Pd

5

surface, which can improve the electrocatalytic activity and stability. There is almost

6

no loss of current density of as-prepared catalysts after the 5 prolonged cycles of CV

7

reactivation (Fig. 7 and Fig. S2). In summary, Pd/CNT, PdSn/CNT, PdNi/CNT and

8

PdSnNi/CNT catalysts show prominent electrochemical performance toward EOR

9

with respect to the more negative onset potential, the higher catalytic activity and

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poisoning tolerance ability, which are attributed to the optimized electronic structure

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of Pd and the effect of CNT.

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Conclusion

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In summary, a simple route was developed for the fabrication of Pd-based catalysts

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with excellent electrocatalytic activity and prominent durability toward EOR in

16

alkaline media. Pd and Pd-based composite nanoparticles were well loaded on carbon

17

nanotubes without aggregation, which may provide high ECSAs of Pd-based catalysts.

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XRD and XPS spectra demonstrate that the electronic structure of Pd atom was

19

modified by Ni and Sn. Pd/CNT catalyst exhibits good catalytic activity and superior

20

durability compared with Pd/C (JM) catalyst, which are ascribed to the well-dispersed

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Pd nanoparticles as well as the introduction of one-dimensional CNT. PdSn/CNT,

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PdNi/CNT and PdSnNi/CNT catalysts exhibited enhanced catalytic activity and

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ACCEPTED MANUSCRIPT superior durability, which are ascribed to the improved electronic structure of Pd by

2

the addition of oxophilic metals (Sn and Ni). Moreover, after stability test, all the

3

as-prepared catalysts can possess as high as the origin or even higher current densities

4

by transferring them to the fresh alkaline electrolyte with CV cycles. These results

5

indicated that as-prepared Pd-based catalysts can efficiently remove CO from the

6

surface of Pd.

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Acknowledgements

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The authors thank the project was supported by the Natural Science Foundation of

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Fujian Province of China (No. 2017J01022) and the National Natural Science

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Foundation of China (No. 21576226).

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Highlights Pd-based catalysts were synthesized by simple and efficient method.




The use of CNT improved the performance of the catalyst.




The modified electronic structure of Pd enhanced the performance of the catalyst.




All catalysts showed excellent catalytic activity and stability for EOR




All catalysts can be reactivated by a simple cycle potential scan in 1 M NaOH.

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