Electrocatalytic oxidation of dimethyl ether on ruthenium modified platinum single crystal electrodes
Introduction
Dimethyl ether is regarded as a promising fuel for the fuel cell [1], [2], [3]. However, direct oxidation of DME is a sluggish reaction on noble metals therefore new electrocatalysts should be searched to improve the performance of direct DME fuel cells (DDFCs) [4], [5], [6]. Recently, we have shown that DME electrocatalytic oxidation on platinum electrodes is an extremely structure sensitive reaction [7], [8], [9] which takes place almost exclusively on surface sites with (1 0 0) symmetry. The poor kinetics of DME oxidation is mostly due to self-poisoning of the surface by reaction intermediates such as CO, which are formed during dehydrogenation of DME. Therefore, in order to improve the performance of DDFCs, anode electrocatalysts with an improved CO-tolerance are required.
The combination of Pt and Ru has been widely recognized as having a synergistic enhancement for the oxidation of CO and organic molecules [10], [11]. The mechanism of such an enhancement has been a subject of many studies. At present, there are two accepted explanations. One is the bifunctional mechanism [12], [13], it is assumed that Ru provides an oxygenated surface species by dissociating water at the Ru sites at lower potentials against pure Pt sites, leading to the accelerated CO2 formation and a decrease of the CO poisoning, thus improving the CO-tolerance of Pt catalysts. The ligand effect [14], [15], [16], on the other hand, known as the electronic effect, is based on the modification of Pt electronic structure by the presence of Ru which can reduce the Pt–CO bond strength and may facilitate CO oxidation.
Some groups have reported the enhancement of Ru to DDFCs by using bimetallic Pt/Ru electrodes [17]. However, no papers have been yet published in relation to the surface structure dependence of Ru modification for DME oxidation using the Ru-decorated Pt single crystal electrodes (Pt(h k l)/Ru). A. Wieckowski and co-workers reported for the first time that Ru could irreversibly adsorb onto Pt surfaces by spontaneous deposition and its coverage could be increased by repeating the procedure [18], [19]. The morphology and chemical state of Ru adlayer were investigated detailedly using scanning tunneling microscopy or X-ray photoelectron spectroscopy [20], [21]. It was proved that the catalytic activity of Pt(h k l) electrodes after Ru modification for methanol oxidation was enhanced to a great extent [22]. Although the surface structure may differ from that of Pt/Ru alloys, investigation of electrocatalytic activity on Pt(h k l)/Ru surfaces for the oxidation of small organic molecules is an essential step in obtaining a general understanding of fuel oxidation mechanisms on Pt/Ru surfaces and is of fundamental and practical significances.
In this paper, DME electrochemical behaviors are characterized by voltammetry and chronoamperometry on platinum single crystal electrodes modified by Ru through spontaneous deposition [19]. The structure dependence and coverage effects of Ru adlayer will be discussed.
Section snippets
Electrodes and instrumentation
The Pt(h k l) single crystal electrodes were prepared using Clavilier’s method. Details have been given elsewhere [23]. After annealing in H2 flame and cooling down in H2/Ar mixed atmosphere, the electrodes were transferred to an electrochemical cell with the protection of a droplet of water on its surface and then immersed into the electrolyte solution under potential control, typically at 0.05 V. A meniscus configuration was made between the electrode and the electrolyte solution (0.5 mol L−1 H2SO4
Results and discussion
Cyclic voltammograms (CVs) are employed to characterize the deposited surfaces in pure electrolyte. Typical CV profiles of the Pt(h k l) electrodes in 0.5 mol L−1 H2SO4 are shown as the solid lines in Fig. 1A (Pt(1 0 0)), B (Pt(1 1 0)) and C (Pt(1 1 1)). The curves obtained on the same surfaces deposited with ruthenium are shown as the dashed (one deposition), dotted (two depositions) and dash dotted (three depositions) lines. The current densities refer to the geometric surface area of the Pt(h k l)
Conclusions
Ru adlayer on Pt(h k l) electrode surfaces with different Ru coverage was obtained by repeated spontaneous deposition of ruthenium from a 1.0 × 10−3 mol L−1 RuCl3 + 0.1 mol L−1 HClO4 solution. Cyclic voltammograms and chronoamperometric curves are used to characterize the structure dependence of Ru adlayer on the electrocatalytic oxidation of DME. An enhancement of Ru modification has been found on Pt(1 0 0) surface both from CVs and chronoamperometric curves. The optimum Ru coverage for DME oxidation on
Acknowledgement
This work was supported financially by Natural Science Foundation of China (No. 20476020).
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