Electrodeposition of Cu-Pd alloys onto electrophoretic deposited carbon nanotubes for nitrate electroreduction

doi:10.1016/j.apsusc.2014.04.119

Applied Surface Science

Volume 308, 30 July 2014, Pages 113-120

https://doi.org/10.1016/j.apsusc.2014.04.119 Get rights and content

Highlights

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CNTs were uniformly and stably deposited on Ti plate via EPD.
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Cu-Pd alloy was formed by potentiostatic coelectrodeposition on Ti/CNTs.
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Ti/CNTs/Cu-Pd electrodes were applied for nitrate reduction.
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Ti/CNTs/Cu5-Pd5 possessed the best electrocatalytic performance.
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CNTs improved the electrocatalytic activity and stability of the alloy.

Abstract

Copper-palladium (Cu-Pd) alloys have been electrodeposited onto carbon nanotubes, which were uniformly and stably deposited on Ti plates via electrophoretic deposition. Electrodes with a wide range of Cu/Pd atomic ratios were fabricated by potentiostatic coelectrodeposition of Cu and Pd onto Ti/CNTs. They were characterized by energy-dispersive X-ray analyzer, X-ray diffraction and tested for nitrate electroreduction. The electrode deposited in bath with 5 mM Cu²⁺ and 5 mM Pd²⁺ (Ti/CNTs/Cu5-Pd5) possessed outstanding stability as well as the highest electrocatalytic activity with the best nitrate conversion yield and proper N₂ selectivity, indicating a synergistic effect of Cu and Pd. X-ray photoelectron spectroscopy and scanning electron microscopy analysis of Ti/CNTs/Cu5-Pd5 and Ti/Cu5-Pd5 revealed that CNTs played a remarkable role in the homogeneous formation of the bimetal, significantly improving the alloy's electrocatalytic activity and stability. The fabricated Ti/CNTs/Cu5-Pd5 was proved to be a promising electrode for nitrate electroreduction.

Graphical abstract

Introduction

The reduction of nitrate in water has gained renewed attention since it causes serious problems both on aquatic ecosystem (e.g. eutrophication, aquatic hypoxia) and human health (e.g. methemoglobinemia, gastric cancer) [1]. Accordingly, the World Health Organization has classified nitrate as a Group 2A substance or “possibly human” carcinogen and recommends a maximum limit of 45 mg L⁻¹ of nitrate in drinking water [2]. The usual techniques (biofiltration, ion-exchange or membrane separation) have major disadvantages (e.g. continuous monitoring, slow kinetics, generation of byproduct, etc.) [3], [4], [5], [6]. Electrochemical reduction is highly effective for nitrate removal based on its ability to treat highly concentrated nitrate, absence of sludge production, high construability, ease of implementation and relatively low capital cost [7], [8], [9]. All these merits make electrochemical method promising for practical and engineering applications in the field of nitrate elimination.

So far, nitrate electroreduction has been investigated on wide variety of metal electrodes, including Cu, Fe, Ni, Sn, Bi, Pt, Pd, Rh, and Ir [7], [8], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], as well as diamond electrodes [23], [24]. Studies suggest that the reduction products show remarkable dependence on the nature of electrode material. Up to now, Cu-Pd alloys appear as the one of the most promising materials with superior selectivity for nitrogen [5], [12], [14].

Carbon nanotubes (CNTs) are demonstrated to be excellent supports for catalysts [25], [26], [27] for their ultra-high mechanical strength, excellent electrical properties, large speciﬁc area, as well as high aspect ratios [28], [29]. CNTs supported Cu-Pd (CNTs/Cu-Pd) catalysts, prepared by a successive impregnation-drying-reduction process, have been proved to be very active and selective catalysts in the catalytic reduction of nitrate with H₂ by Pereira's group [30], [31], [32], [33], [34]. A study on electrochemical reduction of nitrate by CNTs/Cu-Pd is interesting, and to the best of our knowledge, has never been reported, possibly because of the difficulty to uniformly and stably deposit CNTs onto the substrate. Recently, electrophoretic deposition (EPD) is considered to be a simple and efficient method for producing CNTs films with high homogeneity, large area and good stability [15], [35], [36].

In the present study, we report on the fabrication of Ti/carbon nanotubes supported Cu-Pd alloys (Ti/CNTs/Cu-Pd), their characterization by scanning electron microscopy (SEM), energy dispersive X-ray analyzer (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. The main goal is to evaluate their electrocatalytic activity for nitrate reduction and clarify the role of CNTs in changing characteristics of Cu-Pd alloys. Moreover, as it is known that the film composition is the key factor influencing the electrocatalytic activity and nitrate reduction performance, Cu/Pd atomic ratio was investigated to obtain the optimal electrocatalytic activity, stability and N₂ selectivity.

Section snippets

Electrophoretic deposition of CNTs on Ti plate

Ti plates (99.6%, 10 mm × 10 mm) were pretreated as reported in our previous studies [37] and used as the substrates for CNTs deposition. Pt plate with the same area was used as counter electrode. The deposition experiment was carried out at room temperature (25 °C) under quiescent conditions.

Multi-walled carbon nanotubes (30–50 nm in diameter and 10–20 μm in length, Nanotech Port Co., Shenzhen, China) were pretreated by chemical oxidation in a 3:1 (v/v) mixture of concentrated H₂SO₄/HNO₃ solution to

EPD of CNTs

Since the presence of carboxylic groups on the surface of acid oxidized CNTs provides a negative surface charge [41], [42], the deposition of CNTs took place on the anode during EPD. Besides, the acidic treatment has the secondary effect of making the CNTs shorter and less entangled, leading to suspensions of well-dispersed individual, charged nanotube segment.

The inset of Fig. 1 shows the SEM image of Ti/CNTs. Electrophoretically deposited CNTs randomly dispersed on Ti plate and formed a

Conclusions

In this paper, Ti/CNTs/Cu-Pd electrodes were successfully fabricated, for the first time, via a simple electrochemical method. Mixed Cu-Pd film could be obtained by potentiostatic coelectrodeposition on Ti/CNTs over the entire composition range simply by changing the composition of the deposition bath. XRD measurements revealed that all the bimetallic films were made of mono-phased Cu-Pd alloy. From the electrochemical analysis, Ti/CNTs/Cu5-Pd5, of which electrocatalytic activity and stability

Acknowledgements

This work was financially supported by the Foundation of State Key Laboratory of Pollution Control and Resource Reuse of China, the Natural Science Foundation of China (No. 51008154), the Fundamental Research Funds for the Central University (No. 1112021101), and the Scientific Research Foundation of Graduate School of Nanjing University (No. 2012CL19).

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Electrodeposition of Cu-Pd alloys onto electrophoretic deposited carbon nanotubes for nitrate electroreduction

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Electrophoretic deposition of CNTs on Ti plate

EPD of CNTs

Conclusions

Acknowledgements

Chem. Eng. J.

Appl. Catal. B-Environ.

Chem. Eng. J.

J. Hazard. Mater.

Electrochim. Acta

Chem. Eng. J.

Electrochim. Acta

J. Mol. Catal. A: Chem.

J. Electroanal. Chem.

J. Mol. Catal. A: Chem.

Water Res.

Electrochim. Acta

Electrochim. Acta

Electrochem. Commun.

J. Electroanal. Chem.

Sep. Purif. Technol.

Chem. Eng. J.

Mater. Chem. Phys.

Appl. Catal. B-Environ.

Carbon

Chem. Eng. J.

Desalination

Carbon

Chem. Eng. J.

J. Electroanal. Chem.

J. Catal.