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Highly active carbon nanotube-supported Pd electrocatalystfor oxidation of formic acid prepared by etching coppertemplate method

Hong Zhao, T.S. Zhao*

Department of Mechanical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon,

Hong Kong SAR, China

a r t i c l e i n f o

Article history:

Received 10 August 2012

Received in revised form

1 November 2012

Accepted 3 November 2012

Available online 4 December 2012

Keywords:

Pd nanoparticles

Carbon nanotubes

Formic acid oxidation

a b s t r a c t

In this work, a carbon nanotube-supported Pd nano-catalyst (Pd/MCNTs) is prepared by the

etching copper template strategy. Cu nanoparticles (NPs) are formed onto MCNTs first as

the template and Pd NPs are then obtained through a galvanic displacement reaction

between Pd ions and Cu. TEM, XRD, and XPS characterizations show the crystalline of Pd

NPs with a typical diameter of 2e5 nm is homogeneously decorated onto MCNTs without

aggregation. Electrochemical characterizations reveal that the Pd/MCNTs materials exhibit

much higher catalytic activity for the formic acid oxidation than both conventional Pd/

MCNTs and commercial Pd/Vulcan catalysts do. The improved activity is mainly attributed

the fact that no surfactant is required in synthesis of the catalyst, eliminating the possible

passivation of catalytic sites associated with the use of surfactant in conventional

synthesis methods. In addition, the narrower distribution and better dispersion of catalyst

particles, as well as no defects of MCNTs are also beneficial for the improvement in the

catalytic activity. Another feature of the present synthesis method is the loading of Pd can

be adjusted by varying the amount of Cu ions.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

In the field of catalysis for fuel cells, palladium appears to be

particularly important among noblemetals. In order to further

maximize the activity of Pd and minimize the loading of the

precious metal, it is essential to obtain small nanostructures

with high activity on the surface of nanomaterials [1e3].

Excellent nanomaterial supports not only maximize the

availability of the nanosized electrocatalyst surface area for

electron transfer but also provide better transport of reactants

to the electrocatalyst [4e7]. This is due to their high surface

area, good electrical conductivity and low cost. As carbon

nanotubes (CNTs) have high surface areas, unique physical

properties and morphology, high electrical conductivity and

appropriate size and hollow geometry [8e12], they have been

widely investigated as a catalyst support [13e15], which

exhibit better performance in terms of catalytic activity and

durability than the conventional Vulcan XC-72 does. The large

majority of carbon-supported electrocatalysts are tradition-

ally prepared by chemical reduction in the presence of

a stabilizing agent. Historically, the most commonly used are

NaBH4, ethanol, ethylene glycol, hydrazine, tannic acid, for-

mic acid, hydrogen gas, supercritical CO2, and so on [16e20].

Typically, there exist two major disadvantages with

* Corresponding author. Tel.: þ86 852 2358 8647; fax: þ86 852 2358 1543.E-mail address: metzhao@ust.hk (T.S. Zhao).

Available online at www.sciencedirect.com

journal homepage: www.elsevier .com/locate/he

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conventional methods to synthesize metal/CNTs: One is to

use strong acid or alkali, which tends to destroy the electron

structure of CNTs and produces some defects sites onto the

surface of CNTs, reducing the electric conductivity. The other

disadvantage is associated with the use of stabilizers or

surfactants in traditional synthesis methods. As stabilizers or

surfactants are hard to be removed completely, the residuals

will reduce the electrochemical active sites, thereby

decreasing the electrochemical catalytic performance.

Therefore, it is essential to develop a simple and effective

synthetic route to fabricate uniformly sized and distributed

metal NPs onto the surface of pristine CNTs.

Here we extend our previous etching copper template

method [21e23] to preparation of Pd NPs on MCNTs. The as-

prepared Pd/MCNTs materials exhibit a much higher cata-

lytic activity for the formic acid oxidation than the commer-

cial Pd/Vulcan does. Moreover, the loading of Pd can be

adjusted by varying the amount of Cu ions.

2. Experimental section

2.1. Chemicals and reagents

Palladium chloride (PdCl2), copper sulfate (CuSO4�5H2O) and

dimethyl formamide (DMF) were purchased from Sigma-

eAldrich chemical reagent Co. Ltd, while MCNTs were

purchased from Shenzhen Nanotech Port Co. Ltd.

2.2. Preparation of Pd/MCNTs

100mg ofMCNTs are dispersed in 100mlmixture of water and

DMF (V:V ¼ 1:1). After being stirred and undergoing ultra-

sonication for 1 h, 80, 160 and 200 mg of CuSO4�5H2O are then

added into the dispersion, respectively, followed by ultra-

sonication for a further 1 h. The dispersion are then added

200 ml of NaBH4 solution (0.01% w/w), stirring for 4 h. The

resulting product are filtered and washed repeatedly with

deionized water and dried for 1 h at 60 �C in vacuum. The

obtained samples are designatedwith a, b and c, in a sequence

of increasing the content of Cu.

20mg of above obtained Cu/MCNTswith different contents

of Cu (a, b and c) are dissolved with water, followed by the

addition of 1L PdCl2 (10�4 M), respectively. The mixtures are

stirred for 8 h at room temperature until the CuNPs are oxided

completely, with ultrasonication. The solution is then centri-

fuged andwashed several timeswith deionized water. Finally,

the Pd/MCNTs composite sampleswith different content of Pd

are obtained and also named sample a, b and c, corresponding

to the different content of Pd, respectively. In the following

discussion, the as-prepared Pd/MCNTs are denoted as AP-

MCNTs. For comparison, conventional Pd/MCNTs were

prepared by using NaBH4 as reduction regent after treating

MCNTs with HNO3.

2.3. Characterization

Transmission electron microscopy (TEM) images are obtained

by using a high-resolution JEOL 2010F TEM system operating

with a LaB6 filament at 200 kV. Carbon-coated nick grids are

used as sample holders for TEM analysis. The valence state of

the prepared samples is carried out by the X-ray photoelec-

tron spectroscopy (XPS) technique, which is equipped with

a Physical Electronics PHI 5600 multi-technique system using

Al monochromatic X-ray at a power of 350 W. The laser beam

is focused onto the sample with a 50 � objective. The X-ray

diffraction (XRD) patterns are obtained with a Philips powder

diffraction system (model PW 1830) using a Cu Ka source

operating at 40 keV at a scan rate of 0.025 S�1.

Electrochemical measurements were performed using

a three-electrode test cell at room temperature. A Pt gauze

was used as the counter electrode, and a saturated calomel

electrode (SCE) served as the reference electrode. The glassy

carbon (GC) electrode (4 mm in diameter) coated with the

catalyst is used as the work electrode. The preparation of the

work electrode is as follows, 4 mg of the catalyst is dispersed

into 2 ml ethanol by sonication. 8.9 mL of the catalyst

suspension is dropped onto the surface of the GC electrode

and dried with an infrared lamp. 4.5 mL of nafion solution

(0.5%) is placed on the surface of the GC electrode modified

with the above material and dried before electrochemical

experiments.

3. Results and discussion

3.1. The structure characterization of Pd/MCNTscomposite

The whole preparation strategy for constructing the AP-Pd/

MCNTs is shown in Scheme 1. Cu NPs are formed onto

MCNTs through the reduction reaction of Cu2þ with NaBH4

first. The AP-Pd/MCNTs are then obtained by reduction reac-

tion of Pd2þ ions with the resulting Cu NPs. Because the

standard electrode potential of Pd2þ (0.799 V) is higher than

the one of Cu2þ (0.340 V), the Cu can be replaced thoroughly

with the sufficient amount of Pd2þ ions [21]. It should also be

mentioned that because theMCNTs can shuttle electrons back

and forth between these two half reactions Cu¼ Cu2þþ 2e and

Pd2þ þ2e ¼ Pd, the two half reactions may occur at spatially

separated locations [24e26]. In other words, the galvanic

displacement process not only occurred between Pd2þ and Cu

contacted each other as shown in Fig. 1-1, but also between

Pd2þ and Cu uncontacted as shown in Figs. 1 and 2. For the

above reason, highly dispersed Pd NPs can be homogeneously

decorated onto MCNTs without aggregation. Also, since the

growth of the as obtained Pd NPs is restricted by the strong

van der Waals forces between the MCNTs and the Pd NPs, the

size of the obtained Pd NPs is usually small.

Furthermore, the morphological structure, the particle

size, and the dispersion of AP-Pd/MCNTs are examined by

transmission electron microscopy (TEM). The different

loading of samples a, b and c are shown in Fig. 2a, b and c,

respectively. It is confirmed that Pd NPs are decorated onto

MCNTs with uniform size and good dispersity. With an

increase in the amount of Cu, the resulting distribution of Pd

NPs becomes more and more crowded. However, the change

in the size of Pd NPs is little. Thus, the electrocatalytic activity

of AP-Pd/MCNTs can be tuned by changing the amount of Cu

to result in different loading of Pd NPs. Fig. 3 display the

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further TEM images of Sample bwith differentmagnifications.

From this figure, we can see that in addition to the uniform

size (around 2e5 nm) and good dispersity of Pd NPs, their

single-crystalline structure with highly ordered and contin-

uous fringe patterns are also seen from the HRTEM image of

AP-Pd/MCNTs. The measured inter planar spacing for the

lattice fringes is 0.227 nm, which corresponds to the (111)

lattice plane of face-centered cubic (fcc) Pd. The crystal

structure of the hybrid is further characterized by X-ray

diffraction (XRD) and is shown in Fig. 4. It is found that MCNTs

are crystalline and the XRD pattern has peaks at 2q ¼ 26.2�,which correspond to (0 0 2) crystal planes of graphite. And

these diffraction peaks, which locate at 2q ¼ 39.6�, 46.9�and68.24�, can be indexed to the (1 1 1), (2 0 0), and (2 2 0) planes of

the face centered cubic structure of Pd, respectively. This

result is in accordance with the previous the HRTEM results.

Furthermore, the XPS patterns of the resulting AP-Pd/MCNTs

(shown in Fig. 5) indicate significant individual peaks at

337.2 and 342.7 eV corresponding to Pd 3d5/2 and Pd 3d3/2

binding energies, respectively. The Pd mass loading of AP-Pd/

MCNTs (sample b) is around 30% as shown in Fig. S1, which

has been evaluated by thermogravimetry analysis.

3.2. Electrochemical catalyst activity of AP-Pd/MCNTscomposite for formic acid oxidation

Fig. 6 shows the cyclic voltammograms (CV) of electrodes

coated with the AP-Pd/MCNTs, conventional Pd/MCNTs and

commercial Pd/Vulcan (Pd mass loading: 30%) catalysts in

0.5 M H2SO4, the potential is scanned from �0.2e1.0 V vs. SCE

at a scan rate of 50 mV s�1. And the CV curves of each elec-

trode were obtained from the stabilized curve after scanning

20 cycles. The electrochemically active surface area (ECSA) of

AP-Pd/MCNTs, Pd/MCNTs and the Pd/Vulcan catalysts are

calculated based on: ECSA ¼ QH/0.21*[Pd] [27e29], where QH is

the charge due to the hydrogen adsorption/desorption in the

hydrogen region of the CVs, 0.21 mC cm�2 is the electrical

charge constant associated with monolayer adsorption of

Fig. 1 e Scheme of the fabrication of AP-Pd/MCNTs.

Fig. 2 e TEM images of AP-Pd/MCNTs with different Pd loading.

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hydrogen on Pd particles [30], and [Pd] is the loading of Pd on

the working electrode. The results show that the resultant

electrode coated with AP-Pd/MCNTs exhibits the large elec-

trochemical surface area of 68.25 m2/g, and is much higher

than those on electrodes coated with conventional Pd/MCNTs

(52.20 m2/g) and Pd/Vulcan (31.11 m2/g). Fig. 7 displays the

mass activities of the electrodes coated with Pd/MCNTs and

Pd/Vulcan for the catalysts in 0.5 M HCOOHþ 0.5 M H2SO4; the

potential is scanned from �0.2e1.0 V vs. SCE at a scan rate of

50 mV s�1, respectively. As can be seen, the electrode coated

with AP-Pd/MCNTs exhibits a much higher current density of

402.1 mA mg�1 Pd than those of the electrodes coated with

conventional Pd/MCNTs (278.7 mA mg�1 Pd) and Pd/Vulcan

(196.7 mA mg�1 Pd). For the precious metal Pd, it is very

significant that the AP-Pd/MCNTs catalyst have high mass

activity. Fig. 8 demonstrates the chronoamperometric (CA)

results for the electrodes coated with AP-Pd/MCNTs,

Fig. 3 e TEM images of AP-Pd/MCNTs with different magnification.

Fig. 4 e XRD curve of the AP-Pd/MCNTs. Fig. 5 e XPS curve of the AP-Pd/MCNTs.

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conventional Pd/MCNTs and Pd/Vulcan at a constant potential

of 0.15 V, respectively. The current density at 1200 s on the

electrode coated with AP-Pd/MCNTs is 35.31 Amg�1, much

higher than those of the electrodes coated with conventional

Pd/MCNTs (16.76 mA mg�1) and Pd/Vulcan (4.33 mA mg�1 Pd).

This indicates that AP-Pd/MCNTs catalyst has the higher

catalytic stability toward formic acid oxidation than conven-

tional Pd/MCNTs and Pd/Vulcan catalysts do.

The excellent electrochemical catalytic performance of the

obtained Pd/MCNTs catalyst can be attributed to the three

major reasons [21]: i) as no surfactants in the process of Pd/

MCNTs synthesis are needed, the catalyst activity sites can be

ensured; ii) due to the reduced size and better improved

dispersion of Pd NPs, AP-Pd/MCNTs enable a higher mass

electro-activity [31]; iii) as no strong acid and alkali is are used

in the synthesis process, the surface of MCNTs will not be

affected, maintaining a good electric conductivity.

4. Conclusion

In this work, small-sized and highly dispersed Pd NPs are

loaded onto pristine CNTs via the etching copper template.

The electrochemical results demonstrate the as-prepared Pd/

MCNTs are a promising catalyst for formic acid oxidation. This

synthesis strategy represents a new route as complementary

to conventional method. Moreover, one of themost important

features of this method is that the loading of Pd can be

adjusted by varying the amount of Cu ions.

Acknowledgments

The work described in this paper was fully supported by

a grant from the Research Grants Council of the Hong Kong

Special Administrative Region, China (project no. 623010).

Appendix. Supplementary data

Supplementary data related to this article can be found online

at http://dx.doi.org/10.1016/j.ijhydene.2012.11.009.

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