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Unraveling the cooperative synergy of palladium/tin oxide/ aniline-functionalized carbon nanotubes enabled by layerby-layer synthetic strategy for ethanol electrooxidation Honglei Yang, Shuwen Li, Sihao Shen, Zeming Jin, Jun Jin, and Jiantai Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01197 • Publication Date (Web): 11 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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ACS Sustainable Chemistry & Engineering

Unraveling the cooperative synergy of palladium/tin oxide/aniline-functionalized carbon nanotubes enabled by layer-by-layer synthetic strategy for ethanol electrooxidation Honglei Yang, Shuwen Li*, Sihao Shen, Zeming Jin, Jun Jin and Jiantai Ma State Key Laboratory of Applied Organic Chemistry, Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China *

Corresponding authors

E-mail addresses: [email protected] (Shuwen Li) Address (all the authors): Tianshui South Road NO. 222, Lanzhou 730000, PR China.

1

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Abstract Aiming to unravel the cooperative synergy of palladium, tin oxide and aniline-functionalized carbon nanotubes for the ethanol oxidation reaction (EOR), a new

ternary

palladium/tin

oxide/aniline-functionalized

carbon

nanotubes

electrocatalyst (denoted as Pd/CNTs-NH2@SnO2) was constructed by layer-by-layer synthetic strategy. The aniline-groups were firstly grafted on the surface of CNTs by diazo-reaction under mild reaction conditions. Subsequently, the SnO2 and Pd nanoparticles were orderly anchored on the prepared-support by a facile and surfactant-free

approach.

Exhilaratingly,

the

Pd/CNTs-NH2@SnO2

presents

impressive electrochemical performance toward EOR in alkaline medium, including excellent electrocatalytic activity and high durability. The promoted intrinsic electrochemical performance of the Pd/CNTs-NH2@SnO2 is possible attributed to the efficient ternary synergism of the Pd, SnO2 and aniline-functionalized carbon nanotubes via the heterointerfaces. The tin oxide and aniline-groups can facilitate the well dispersion of small size nanoparticles and efficaciously prevent the detachment of Pd. Moreover, abundant OH species generated by neighboring SnO2 speeds up the oxidative removal of CO-like intermediates at Pd sites thereby enhancing the antipoisoning capability. Accordingly, the Pd/CNTs-NH2@SnO2 is an attractive alternative as electrocatalyst of EOR and the layer-by-layer synthetic strategy is a promising strategy to produce similar electrocatalysts with high-performance.

2


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Keywords: ethanol oxidation reaction, ternary synergy, palladium, tin oxide, aniline-functionalized carbon nanotubes

3



INTRODUCTION Direct fuel cells with high energy densities are able to convert chemical energy into electricity under low-to-zero pollutant emissions and play key roles to solving global energy and climate challenge as a promising future energy source.1 Among various possible fuels, ethanol as a carbon-neutral fuel source is extra attractive by right of its excessive theoretical energy density (comparable to that of gasoline), low toxicity and renewability.2-4 Unfortunately, the commercial applications of direct ethanol fuel cells (DEFCs) are still confronted to many obstacles, such as sluggish kinetics and inefficient of ethanol oxidation reaction (EOR) on electrocatalysts.5-7 Therefore, designing active and durable electrocatalysts toward EOR is urgently demand to practical application of DEFCs. So far, Pt and Pt-based electrocatalysts for EOR have been in limelight for several years.8-10 However, the expensive price and strongly prone to poisoning by CO-like intermediates of Pt and Pt-based electrocatalysts restrict their large-scale practical application of EOR11. Developing non-platinum electrocatalysts as well as exploring appropriate supports is an effective strategy to solve the above problems. Alternatively, Pd is a cost-effective choice because of its low price, high-efficiency and durability in the EOR.12-17 Despite Pd possesses prominent advantages over several Pt-based electrocatalysts for EOR, some issues are necessary to be addressed, such as enhancing oxidation kinetics and antipoisoning capability. It is reported that combining Pd with synergistic secondary metal oxides is an efficient approach to promote the electrocatalytic performance of EOR.18-20 Tin dioxide (SnO2) is a promising candidate owing to SnO2 can supply abundant OH species at interface sites to remove the surface CO-like intermediates.8,

20-21

Moreover, it is conducive to

4


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uniformly dispersion of nanoparticles (NPs) and modification electronic state of metal through the strong metal-metal oxide interaction.21 Another feasible strategy to facilitate the EOR performances of Pd-based catalyst is exploring a robust support.20-22 Carbon nanotubes (CNTs) with unique tubular geometry have been used as an electrocatalyst backbone, due to their chemically inert, mechanically stable and high conductivity.23-24 Nevertheless, the “pristine” CNTs lack the binding sites to decorate the precursor metal ions or NPs, leading to sluggish dispersion and agglomeration of NPs. Hence, the applicable functionalization of CNTs is condignly adopted. It is needed that the binding sites introduced by functionalization of CNTs not only have small impact on the conductivity but also is firmly grafted on the surface of CNTs. In the present study, we have fabricated a new ternary palladium/tin oxide/aniline-functionalized

carbon

nanotubes

electrocatalyst

(denoted

as

Pd/CNTs-NH2@SnO2) through layer-by-layer synthetic strategy to unravel the cooperative synergy of palladium, tin oxide and aniline-functionalized carbon nanotubes for EOR (Fig. 1). The aniline-groups were firstly grafted on the surface of CNTs by diazo-reaction under mild reaction conditions to introduce binding sites. Subsequently, the SnO2 and the Pd NPs were orderly anchored on the prepared-support

by

a

facile

and

surfactant-free

approach.

Finally,

the

obtained-electrocatalysts was tested as catalysts of EOR through the different electrochemical techniques under half-cell conditions. EXPERIMENTAL SECTION Materials The chemicals and materials used in this part of work are summarized in Supporting Information. 5



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Preparation of the catalysts The Pd/CNTs-NH2@SnO2 was synthesized by layer-by-layer synthetic strategy as presented in Fig. 1 and the particular processes are as follows. The CNTs-NH2 was synthesized by diazo-reaction under mild reaction conditions. First, CNTs (60 mg) was dispersed in acetonitrile (50 mL) and sonicated for 0.5 h. P-phenylenediamine (12 mg) were subsequently put into above suspension. After stirring for 0.5 h, isoamylnitrite (1.2 mL) was quickly injected into the suspension and the mixture suspension was stirred continuously for 12 h at 60 °C under an inert atmosphere. Then, the suspended solid was filtered, rinsed with deionized water and ethanol absolute for several times respectively. Finally, the CNTs-NH2 was got after drying overnight at 60 °C in vacuum oven. The SnO2 NPs was covered on the surface of CNTs-NH2 (or CNTs) based on the literatures.21, 25 30 mg of CNTs-NH2 (or CNTs) was dispersed in 40 mL of water after ultrasonic treatment for 0.5 h, followed by the addition of 45 mg of SnCl2·H2O and 50 μL of concentrated HCl (38%). After ultrasonic treatment for 0.5 h, 30 mg of urea was put into and the mixture suspension was stirred continuously at 90 °C for 8 h. Then, the sediment was filtered and washed completely by water. Finally, the CNTs-NH2@SnO2 (or CNTs@SnO2) can be acquired after drying overnight at 60 °C in vacuum oven. The

Pd/CNTs-NH2@SnO2,

as

well

as

Pd/CNTs,

Pd/CNTs-NH2

and

Pd/CNTs@SnO2, was prepared by a similar facile and surfactant-free approach. 30 mg of CNTs-NH2@SnO2 (or CNTs@SnO2, or CNTs-NH2, or CNTs) was ultrasonicated in 30 mL water for 0.5 h. Subsequently, 4.5 mL of K2PdCl4 (11.3 mmol mL-1) solution was dropped into the mixture under drastic stirring at ambient temperature and stirred for 10 min. Then, 10 mL of L-ascorbic acid (11.3 mmol mL-1) 6


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was slowly injected into the mixture suspension. After stirring for 2 h at ambient temperature, the resultant product was filtered and repeatedly washed with water and ethanol absolute. Finally, the Pd/CNTs-NH2@SnO2 (or Pd/CNTs, or Pd/CNTs-NH2, or Pd/CNTs@SnO2) was obtained after drying overnight at 60 °C in vacuum oven. Characterization The morphologies, sizes, dispersion and chemical composition of the obtained catalysts were characterized by systematic characterizations and the related instrumentation was listed in Supporting Information. Electrochemical measurements In present work, all electrochemical tests were conducted at ambient temperature under half-cell conditions. The relevant experimental descriptions have been summarized in Supporting Information.

Fig. 1 Schematic illustration of the synthesis of Pd/CNTs-NH2@SnO2.

7



RESULTS AND DISCUSSION

Fig. 2 TEM images of (A) CNTs@SnO2, (B) Pd/CNTs, (C) Pd/CNTs@SnO2, (D) CNTs-NH2@SnO2, (E) Pd/CNTs-NH2 and (F) Pd/CNTs-NH2@SnO2. Insets are the HRTEM images of NPs.

Fig. 3 (A) HAADF-STEM image of Pd/CNTs-NH2@SnO2. (B-F) HAADF-STEM-EDS elemental mapping images of C, N, O, Sn and Pd for Pd/CNTs-NH2@SnO 2.

Fig. 2 shows the typical transmission electron microscopy (TEM) images of the 8


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obtained-catalysts to reveal the characterization of the morphology and size distribution for the catalysts. As shown in Fig. 2A and 2C, ultrafine SnO2 NPs are covered on the surface of CNTs and CNTs-NH2. Different from the uniform distribution of SnO2 NPs on the surface of CNTs-NH2, the partial of SnO2 NPs agglomerate on the CNTs, which indicates the aniline-groups are beneficial to the uniform distribution of NPs. It is further confirmed by the TEM images of Pd/CNTs and Pd/CNTs-NH2 (Fig. 2B and 2E). The Pd NPs with wide size distribution range are adhered on the CNTs and some NPs agglomerate, which are not observed in the bulk space of the CNTs. And relatively uniform Pd NPs with the average sizes of 30 nm are immobilized on the CNTs-NH2. Notably, the ultra-small Pd NPs overlap with the neighboring SnO2 on the CNTs@SnO2 and CNTs-NH2@SnO2, manifesting that coating the SnO2 can generate small Pd NPs and can block the aggregation of Pd NPs to some extent. Similarly, the Pd NPs are anchored on the CNTs-NH2@SnO2 more evenly in comparison with Pd/CNTs@SnO2. Moreover, the HRTEM images of NPs show lattice fringes of ~0.23 nm and ~0.34 nm, which are corresponding to the (111) plane of Pd crystals and the (110) plane of SnO2 crystals, respectively.19 The spatial distribution of Pd, Sn and N in the Pd/CNTs-NH2@SnO2 was verified by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and the HAADF-STEM image and elemental mapping are displayed in Fig. 3. Obviously, small NPs are uniformly decorated on the CNTs-NH2 as illustrated in Fig. 3A. The elemental mapping images accurately identify the successful introduction of aniline-groups, SnO2 and Pd on the CNTs. Moreover, it is distinctly observed that the C,

N,

O,

Sn

and

Pd

species

are

homogeneously

distributed

in

the

Pd/CNTs-NH2@SnO2. Consequently, introducing the aniline-groups and coating the SnO2 are the effective approaches to improve the distribution of NPs and limit their 9



growth, thereby further enhancing the electrocatalytic activity, durability and utilization efficiency of noble metal. Table S1 summarizes the weight percentages of N, Sn and Pd in the obtained-catalysts based on the conventional combustion method and inductive coupled plasma atomic emission spectrometer (ICP-OES) analysis.

Fig. 4 (A) Raman spectra of CNTs and CNTs-NH2, (B) XRD patterns of CNTs, CNTs-NH2, CNTs@SnO2 and CNTs-NH2@SnO2, (C) XRD patterns and XPS spectra of (D) N 1s, (E) Sn 3d and (F) Pd 3d for (a) Pd/CNTs, (b) Pd/CNTs-NH2, (c) Pd/CNTs@SnO2 and (d) Pd/CNTs@SnO 2.

Raman spectroscopy was used to assess the chemical functionalization of CNTs and the related spectra were presented in Fig. 4A. Three pronounced peaks are observed 10


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for CNTs and CNTs-NH2. The D band at 1330 cm-1 is originated from the disordered structural disorder or defects of graphitic lattice, and the G band at 1590 cm-1 is related to sp2 electronic configuration of graphitic carbon.26-27 Accordingly, the intensity ratio between D and G band (ID/IG) can offer the information for structural changes due to the functionalization of CNTs.24 Fortunately, the ID/IG value of CNTs-NH2 is 1.16 and slightly higher than that of CNTs (1.09), which confirms the success of surface aniline-functionalization and tiny damage towards the conjugated structure of CNTs.28 X-ray diffraction (XRD) analysis was performed to confirm the structural features of the obtained-samples. As shown in Fig. 4B, the CNTs-NH2 as well as CNTs displays the sharp diffraction peaks of the characteristic CNTs, which further confirms that the conjugated structure of CNTs is slightly destroyed by the aniline-functionalization.29

Meanwhile,

the

presence

of

SnO2

crystals

for

CNTs@SnO2, CNTs-NH2@SnO2, Pd/CNTs@SnO2 and Pd/CNTs-NH2@SnO2 results in the diffraction peaks at 26.3o, 33.8o, 37.8o, 51.4o, 61.2o and 65.5o as displayed in Fig. 4B and 4C, assigning to the (110), (101), (200), (211), (310) and (301) crystalline planes of rutile SnO2 (JCPDS NO. 41-1445).8, 21, 30 Moreover, the diffraction peaks observed at 40.0o, 46.5o, 68.0o, 82.0o and 86.5o correspond to the (111), (200), (220), (311) and (222) crystalline planes of the face-centered cubic (fcc) structure for all the obtained-catalysts (Fig. 4C), consistent with the standard metallic Pd diffraction pattern (JCPDS NO. 46-1043).22 Notably, the diffraction peaks of fcc-Pd for Pd/CNTs@SnO2 and Pd/CNTs-NH2@SnO2 are distinct broader than that of Pd/CNTs and Pd/CNTs-NH2, implying the small Pd NPs immobilized on the CNTs@SnO2 and CNTs-NH2@SnO2. The surface chemical composition and electronic structure of the prepared-catalysts was examined by X-ray photoelectron spectroscopy (XPS). Fig. 4D shows the N 1s 11



regions of the catalysts. An evident peak is detected at binding energy (EB) of approximately 400 eV for the Pd/CNTs-NH2 and Pd/CNTs-NH2@SnO2, further indicative of the success of surface aniline-functionalization. The deconvolution peaks at EB of 487.2 eV and 495.6 eV in the Sn 3d regions of Pd/CNTs@SnO2 and Pd/CNTs-NH2@SnO2 is ascribed to Sn 3d5/2 and Sn 3d3/2 of Sn(Ⅳ), which can be inferred the presence of stoichiometric SnO2 in the Pd/CNTs@SnO2 and Pd/CNTs-NH2@SnO2 (Fig. 4E).8,

20

Fig. 4F presents the high-resolution XPS

spectrum of Pd 3d regions of the prepared-catalysts. And two spin-orbit splitting peaks of 3d5/2 and 3d3/2 are observed. All the Pd 3d spectra can be resolved into two pairs of peaks corresponding to the metallic Pd(0) and Pd(Ⅱ) species of PdO or Pd(OH)2 and the detailed data is summarized in Table S2.9, 15 In particular, the EB values of Pd(0) 3d5/2 peaks in Pd/CNTs@SnO2 and Pd/CNTs-NH2@SnO2 positively shift as compared with that in Pd/CNTs and Pd/CNTs-NH2, which is mainly from the contribution of the strong interaction between Pd and SnO2.31-32 Furthermore, the aniline-functionalization and SnO2-covered can adjust the Pd(Ⅱ) species contents of the catalysts. A suitable amount of Pd(Ⅱ) species in the Pd-based catalysts favors the electrochemical catalytic oxidation. 15, 33

12


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Table 1 Electrochemical parameters of the different catalysts obtained from CV curves of EOR. Catalysts

Eonset

ja

ja

jb

(V

(mA cm-2)

(mA

(mA

mgPd-1)

mgPd-1)

vs.Hg/HgO)

ja/jb

Pd/CNTs

-0.481

2.75

313.8

677.2

0.46

Pd/CNTs-NH2

-0.512

4.76

628.4

1452.8

0.43

Pd/CNTs@SnO2

-0.573

5.49

856.0

1656.8

0.52

Pd/CNTs-NH2@SnO2 -0.601

8.15

1451.1

1888.5

0.77

Fig. 5 (A) CVs of the different catalysts in nitrogen-saturated 1 M KOH solution at a scan rate of 50 mV s-1. (B) CO stripping curves in 1 M KOH solution at a scan rate of 50 mV s -1. (C) CVs of the different catalysts in nitrogen-saturated 1 M KOH and 1 M C2H5OH solution at a scan rate of 50 mV s-1. (D) The mass activities and specific activities of the different catalysts. Catalysts: (a) Pd/CNTs, (b) Pd/CNTs-NH2, (c) Pd/CNTs@SnO2 and (d) Pd/CNTs-NH2@SnO2.

Cyclic voltammetry was initially carried out in 1 mol L-1 KOH solution to assess the electrochemically active surface area (ECSA) and the cyclic voltammograms 13



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(CVs) of the obtained-catalysts was shown in Fig. 5A. The distinctive peaks observed around -0.3 V are attributed to the reduction peaks of palladium oxide for all the obtained catalysts.27 The ECSA of the obtained catalysts (listed in Table S3) can be acquired from the Coulombic charge of the reduction of palladium oxide on the grounds of the equation S1 (see the Supporting Information). The calculated ECSA values of the catalysts are 11.4 (Pd/CNTs), 13.2 (Pd/CNTs-NH2), 15.6 (Pd/CNTs@SnO2) and 17.8 (Pd/CNTs-NH2@SnO2) m2 gPd-1, respectively. Based on the full utilization efficiencies of Pd (448 m2 gPd-1), the Pd utilization efficiencies of the catalysts are 2.5% (Pd/CNTs), 2.9% (Pd/CNTs-NH2), 3.5% (Pd/CNTs@SnO2) and 4.0% (Pd/CNTs-NH2@SnO2), respectively.7 Therefore, it is declared that the aniline-functionalization and SnO2-covered can enlarge the ECSA and improve the Pd utilization efficiency. As known, the chemisorbed CO-like intermediate species on the surface of active sites have been verified as a major poisoning species for EOR. Accordingly, the CO stripping study is frequently used to examine the antipoisoning capability of the obtained catalysts (Fig. 5B). All the CO stripping curves present a characteristic peak of CO oxidation with small difference in the peak potential for CO oxidation (Ecooxidation, listed in Table 1). It is clear that the Ecooxidation of Pd/CNTs-NH2@SnO2 (-0.101 V) is more negative compared to Pd/CNTs (-0.079 V), Pd/CNTs-NH2

(-0.083

V),

aniline-functionalization

and

Pd/CNTs@SnO2 SnO2-covered

are

(-0.094

V),

conducive

to

indicating

the

enhance

the

anti-poisoning ability of the catalysts.32 Electrocatalytic performance of the prepared-catalysts for EOR was evaluated in 1 mol L-1 KOH solution containing 1 mol L-1 ethanol at 50 mV s-1. Fig. 5C displays the representative CVs of EOR for the prepared-catalysts and two well-defined oxidation peaks are observed. The related data of electrocatalytic performance from CVs are 14


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given in Table 1. The order of onset potential for EOR (Eonset) in the forward scan is observed to be Pd/CNTs-NH2@SnO2 (-0.601 V) < Pd/CNTs@SnO2 (-0.573 V) < Pd/CNTs-NH2 (-0.512 V) < Pd/CNTs (-0.481 V). As well known, the more negative Eonset signifies faster kinetics of EOR.34 Consequently, the aniline-functionalization and SnO2-covered have definitely accelerated the kinetics process of EOR. In addition, the order of peak potential for EOR during positive scan is in accord with the order of Eonset, suggesting the Pd/CNTs-NH2@SnO2 possesses lower activation energy for EOR. Similarly, the peak current densities in the forward scan (ja), which has been normalized to the mass of Pd loading (mass activity) and the ECSA (specific activity), is found to follow the order of Pd/CNTs-NH2@SnO2 > Pd/CNTs@SnO2 > Pd/CNTs-NH2 > Pd/CNTs as shown in Fig. 5D and Table 1. More importantly, the ja of Pd/CNTs-NH2@SnO2 is greater than those of many similar state-of-the-art electrocatalysts in the literatures as summarized in Table S4 (Supporting Information). The results confirm that the aniline-functionalization and SnO2-covered have certainly promoted the electrocatalytic EOR activity of the catalyst. As reported, the forward scanning peaks are derived from the oxidation of freshly chemisorbed ethanol, and the peaks during the reverse scan are attributed to the removal of the intermediate carbonaceous species formed in the forward scan.27, 35-37 And the ja to the peak current densities in the reverse sweep (jb) ratio (ja/jb) can get insight of the reaction pathway. 38 The order of ja/jb is similar as well as ja (Table 1), suggesting the aniline-functionalization and SnO2-covered can improve the efficient EOR process.7

15



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Fig. 6 (A) ja on the different catalysts versus the cycle number. (B) The retention ratio of EOR for the different catalysts after 500 cycles. (C) CA curves at -0.20 V in nitrogen-saturated 1 M KOH and 1 M C2H5OH solution. (D) js based on the CA tested after 7200 s. Catalysts: (a) Pd/CNTs, (b) Pd/CNTs-NH2, (c) Pd/CNTs@SnO2 and (d) Pd/CNTs-NH2@SnO2.

Table 2 Electrochemical parameters of the different catalysts obtained from ATD and CA. Catalysts

Retention ratio (after 500 cycles, %)

js (mA mgPd-1)

Pd/CNTs

14.7

14.9

Pd/CNTs-NH2

35.3

21.4

Pd/CNTs@SnO2

46.8

24.6

Pd/CNTs-NH2@SnO2

60.9

30.1

The long-term stability and antipoisoning capability of the obtained-catalysts for EOR

was

investigated

by

accelerated

degradation

tests

(ATD)

and

chronoamperometry (CA). Fig. 6A plots the ja of the obtained-catalysts as a function of the cycle number. The ja increases during the initial cycles owing to efficient elimination of the contaminants from the active sites and gradually decreases after 16


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initial cycles on account of the catalyst surface poisoning.39 Apparently, the retention ratio of ja after 500 cycles to the maximum-ja for the Pd/CNTs-NH2 @SnO2 is 60.9%, which is greater in comparison with that of Pd/CNTs@SnO2 (46.8%), Pd/CNTs-NH2 (35.3%) and Pd/CNTs (14.7%) as shown in Fig. 6B and table 2. In addition, the morphology of Pd/CNTs-NH2@SnO2 after the 500 cycles was no obvious changes as shown in Fig.S1. Fig. 6C depicts the typical CA curves of the obtained-catalysts, which were performed in 1 mol L-1 KOH solution containing 1 mol L-1 ethanol with the fixed potential at -0.20 V for 7200 s. As time go by, the current intensities of the obtained-catalysts in the primary stage rapidly attenuate due to the catalyst surface poisoning by CO-like intermediates, and follows by a torpid damping until a pseudo-steady state.40 Obviously, the CA profile of the Pd/CNTs-NH2@SnO2 presents lower current decay and higher current intensity than that of other tested-catalysts during polarization time. Furthermore, the residual current density (js) after testing 7200 s obeys the order of Pd/CNTs-NH2@SnO2 (30.1 mA mgPd-1) > Pd/CNTs@SnO2 (24.6 mA mgPd-1) > Pd/CNTs-NH2 (21.4 mA mgPd-1) > Pd/CNTs (14.9 mA mgPd-1), which is consistent well with the ATD results (Fig. 6D and Table 2). The results of ATD and CA testing demonstrate the aniline-functionalization and SnO2-covered are favorable for a good catalytic activity, antipoisoning property and long durability towards EOR. The superior electrochemical performance of Pd/CNTs-NH2@SnO2, including enlarged ECSA, negatively shifted Eonset, enhanced electrocatalytic activity and promoted durability toward EOR, is believed to derive from the efficient ternary synergism of CNTs-NH2, SnO2 and Pd NPs via the heterointerfaces. Firstly, the SnO2 NPs (or Pd NPs) can be firmly modified on the surface of CNTs-NH2 by the interaction between aniline-groups and NPs.41-42 Moreover, the small Pd NPs can be 17



observed to be evenly distributed on the surface of CNTs-NH2@SnO2 through the strong metal (Pd)-metal oxide (SnO2) interaction.19 Thus, the aggregation of NPs as well as the detachment from the CNTs-NH2 is efficaciously prevented, thereby enhancing the catalytic activity and long-term stability for EOR. As a platform for facile OH adsorption (OHads), on one hand, SnO2 NPs affords abundant OHads species. Hence, the removal of CO-like intermediates at Pd sites can be facilitated by the OHads species generated at neighboring SnO2 sites due to the versatile interface formed between the Pd NPs and SnO2 NPs thereby accelerating the regeneration of Pd active sites.8, 19, 43 On the other hand, the electronic state of Pd is altered by SnO2, which can boost the EOR electrocatalytic activity.18 Furthermore, the electrochemical corrosion of CNTs-NH2 can be mitigated by the SnO2-covering, which can further strengthen the long-term stability of catalyst.21 Therefore, the intimate interaction via the heterointerfaces of Pd, SnO2 and CNTs-NH2 through layer-by-layer synthetic strategy is the key factor for the promoted intrinsic EOR performance of Pd/CNTs-NH2@SnO2. CONCLUSION The Pd/CNTs-NH2@SnO2 was synthesized by layer-by-layer synthetic strategy to accomplish the cooperative synergy of Pd, SnO2 and CNTs-NH2. As an electrocatalyst for EOR, the Pd/CNTs-NH2@SnO2 achieves promoted electrocatalytic activity and remarkable durability. Based on the systematic electrochemical tests and the in-depth characterization, the efficient ternary synergism of CNTs-NH2, SnO2 and Pd NPs via the heterointerfaces is a key factor to the superior electrochemical performance of the Pd/CNTs-NH2@SnO2. Therefore, the Pd/CNTs-NH2@SnO2 will be used as an attractive alternative electrocatalyst for the future commercialization of DEFCs. Furthermore, this research demonstrates the layer-by-layer synthetic strategy is a 18


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promising strategy to fabricate similar electrocatalysts with high-performance. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Detailed

information

of

Sample

characterization

and

Electrochemical

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potentiodynamics of the CO2 generation during the electro-oxidation of ethanol. J. Catal. 2017, 353, 335-348, DOI: 10.1016/j.jcat.2017.07.033.

23



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Graphical Abstract

Highlights Aiming to unravel the cooperative synergy of palladium, tin oxide and aniline-functionalized carbon nanotubes for the ethanol oxidation reaction (EOR), a new

ternary

palladium/tin

oxide/aniline-functionalized

carbon

nanotubes

electrocatalyst (denoted as Pd/CNTs-NH2@SnO2) was constructed by layer-by-layer synthetic strategy.

24


Aniline

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