Novel Raney-like nanoporous Pd catalyst with superior electrocatalytic activity towards ethanol electro-oxidation

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Abstract

A novel Raney-like nanoporous Pd catalyst has been fabricated through the combination of ball-milling with alkali-dealloying strategy. The microstructure of this catalyst has been characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results show that the as-fabricated Raney Pd powders are several microns in size, and each particle exhibits an open, bicontinuous interpenetrating ligament-channel structure with a length scale of 3–7 nm. Electrochemical measurements demonstrate that this Raney Pd catalyst has a high electrochemical active surface area and shows remarkable electrocatalytic activity and stability towards ethanol oxidation. Due to the advantages of simple preparation and superior performance, this Raney Pd catalyst can find promising application as a candidate for the anode catalyst of direct ethanol fuel cells.

Highlights

► Ball-milling and alkali-dealloying of Al-Pd can be used to produce Raney Pd catalyst. ► Powder-shaped configuration provides a high electrochemical active surface area. ► Raney Pd catalyst shows superior activity towards ethanol electro-oxidation.

Introduction

It is well known that Raney catalysts, especially Raney nickel, are widely used in a large number of industrial processes and in organic synthesis reactions because of their stability and high catalytic activity at room temperature [1], [2]. The Raney catalysts, which are also called skeletal catalysts, can be synthesized through selective leaching partial Al element from corresponding Al–M (M = Ni, Cu, Fe) alloys in concentrated sodium hydroxide solutions (this process is also called dealloying). Generally, the Raney catalysts are usually limited to skeletal Raney nickel, copper, iron/iron oxide powders, especially in the industrial application field. However, several precious Raney metals, such as Raney Pt or Pd, are rarely reported till now. As is known to all, the noble metals, such as Au, Ag and some platinum group metals (Pt, Pd), reveal more positive electrochemical potential and larger electrode potential gap compared to Al (i.e. −1.706 V (vs. SHE) for Al/Al3+ and 0.83 V (vs. SHE) for Pd/Pd2+). In our previous work, a series of ribbon or rod-like nanoporous metals have been successfully fabricated by dealloying corresponding Al-based precursor alloys [3], [4], [5]. These nanoporous metals with well-controlled feature sizes and unique properties have promising applications in fields of catalysis, sensors, actuators, micro-flow control, etc [6], [7], [8]. In some fields (i.e. sensors or actuators), apart from high surface areas, other requirements are also of significant importance, such as well-shaped continuity and superior mechanical property. In other fields, however, such as catalysis, more attention should be concerned about whether a high active surface area can be acquired. In this case, there is no special requirement on the overall continuity of the sample. So catalysts are normally produced in powders in order to greatly enhance the specific surface area. In general, most of the previously reported nanoporous metals mainly involved with a bulk configuration (ribbons or films) inherited from the initial shape of precursor alloys. It is obvious that these nanoporous metallic ribbons in our previous work can be used for sensors and actuators, but not suitable for catalytic applications.

Nowadays, fuel cells have aroused tremendous interest from both energy and environmental considerations [9], [10]. Among them, direct ethanol fuel cells (DEFCs) have attracted more and more attention since ethanol is non-toxic and can be easily produced in great quantities by fermentation of sugar-containing raw materials [11], [12]. Due to high cost and limited resource of Pt, Pd has been intensively studied as an alternative electrocatalyst by virtue of its superior electrocatalytic activity towards ethanol oxidation in alkaline circumstance as well as its abundance on the earth [13], [14]. Till now, a series of Pd catalysts with various morphologies such as nanoparticles, nanowire arrays and nanoflowers, are generally synthesized through chemical reduction of corresponding metal salts, with assistance of surfactants and reducing agents, to form low dimensional species in order to acquire large surface areas and high utilization of precious metals [15], [16], [17]. However, the massive use of surfactants and reducing agents (generally organic agents) may pose additional economic issues and environmental problems, thus hindering their industrial applications. Contrarily, through the combination of ball-milling of precursor alloys and subsequent dealloying treatment, Raney-like nanoporous metallic powders will be directly synthesized. During the synthesis process, bulk- or ribbon-like alloys will be firstly ground into fine particles with sizes of microns or submicrons. And then, nanoporous powders can be obtained through a simple dealloying reaction. It is clear that the as-dealloyed nanoporous powders can provide a large active surface area compared to those nanoporous ribbon-like samples or other non-porous powders. Moreover, the nanoporous structure can enable easy access of reactant to the electrode/electrolyte interface, which is beneficial to liquid catalytic reactions, such as ethanol electro-oxidation reaction. In addition, with the help of ball-milling, the dealloying reaction time will be greatly shortened, and the catalyst can be produced in mass to meet the requirement of industrialization.

In the present paper, the Raney-like nanoporous Pd will be synthesized through the combination of ball-milling and a simple alkali leaching treatment. Moreover, we particularly focus on its electrochemical behavior and electrocatalytic property toward ethanol oxidation with an eye to the potential application as high efficiency catalyst in DEFCs.

Section snippets

Preparation

Pure elemental Al (99.95 wt.% purity) and Pd (99.9 wt.% purity) were melted with a nominal atomic ratio of 77:23 in a quartz crucible using a high-frequency induction furnace. Using a single roller melt spinning apparatus, the pre-alloyed ingots were remelted by high-frequency induction heating in a quartz tube and then melt-spun onto a copper roller with a diameter of 0.35 m at a rotation speed of 1000 revolutions per minute in a controlled argon atmosphere. The as-obtained Al77Pd23 alloy ribbons

Microstructural characterization of nanoporous Raney Pd catalyst

Fig. 1a shows the morphology of the melt-spun Al77Pd23 alloy ribbons, generally with the size of 2–4 mm in width and several centimeters in length. Through the milling treatment, the Al–Pd alloy ribbons have been transmitted into powder-like samples with the same nominal composition. It is clear that the starting alloy ribbons are bright silvery white, however, both the as-milled alloy powders and as-dealloyed samples look black (Fig. 1b and c). During the dealloying treatment, the Al element in

Conclusions

In summary, a novel Raney-like nanoporous Pd catalyst can be fabricated through the combination of ball-milling and alkali-dealloying strategy. This Raney Pd catalyst consists of Pd particles with sizes of several microns and submicrons, in which there exists a typical 3-dimensional bicontinuous ligament-channel structure with the length scale of 3–7 nm. This Raney Pd catalyst has an enlarged active surface area of ∼37 m2 g−1. Moreover, compared with those traditional Pd catalysts (i.e. Pd

Acknowledgement

The authors gratefully acknowledge financial support by the National Natural Science Foundation of China under grant (50971079), and Independent Innovation Foundation of Shandong University (2010JQ015). Z.H. Zhang acknowledges the support from the Alexander von Humboldt Foundation (Germany). X. G. Wang acknowledges the support from Taiyuan University of Technology Talents Fund.

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