Carbon-supported bimetallic PdIr catalysts for ethanol oxidation in alkaline media
Introduction
Recently, anion-exchange membrane direct alcohol fuel cell (AEM DAFC) has been gaining more and more attention as one of the most promising power sources for mobile and portable electronic devices due to its many important advantages [1], [2], [3]. More importantly, the kinetics of both alcohol anodic oxidation and oxygen cathodic reduction in alkaline media become more facile than in acidic media, making it possible to use non-noble and low-cost metal catalysts. Among the various types of AEM DAFCs, anion-exchange membrane direct ethanol fuel cell (AEM DEFC) is more attractive because ethanol has higher energy density than methanol (8.0 kWh kg−1 vs. 6.1 kWh kg−1), is less toxic, and can be produced in large quantities from agricultural products or biomass [4], [5], [6]. Palladium is recognized as the most efficient material for the ethanol oxidation reaction (EOR) in alkaline media [7], [8]; moreover, Pd is more abundant in nature than Pt, and thus the catalyst cost can be substantially reduced. Xu et al. [9] proved that Pd has both a higher catalytic activity and better steady-state behavior than Pt for the EOR in alkaline media. Nevertheless, both the catalytic activity and stability of Pd for the EOR in alkaline media need to be further enhanced. In this regard, various Pd and Pd-based catalysts are explored and investigated.
Since the catalytic activity depends on both the shape and size of catalysts, some efforts have been focused on studying the activities of different Pd nanostructures for the EOR in alkaline media, including Pd spherical nanoparticles [10], multitwinned particles [10], spherical spongelike particles (SSPs) [10], nanowires [11], [12], nanoballs [13], nanoporous structures [14], and so on. It is well known that bimetallic catalysts often exhibit enhanced catalytic performances for alcohol oxidation, in terms of activity, selectivity, and stability, compared to the separate component. The combination of Pd with another metal M (M = Au, Sn, Ru, Ag, Ni, Pb and Cu) [15], [16], [17], [18], [19], [20], [21], [22], [23], [24] can enhance both the catalytic activity and stability of Pd for the EOR in alkaline media. As far as we know, no bimetallic PdIr catalysts for the EOR in alkaline media have been reported in the literature. Although iridium itself shows much less overall activity for the EOR, it shows a more negative onset potential and higher current density at much lower potentials than other noble metals [25], [26]. Previous studies demonstrated that the incorporation of Ir to PtRu or to PtSn improved both the catalytic activity and stability of these catalysts for the hydrogen, carbon monoxide, methanol and ethanol oxidation in acidic media [27], [28], [29], [30], [31]. The improved catalytic performance can be due to the fact that the hydroxyl groups are more easily adsorbed on both metallic Ir and iridium oxide (IrO2) at lower potentials, assisting in the oxidation of COads or other adsorbed intermediates. Driven by the curiosity of whether alloying Ir with Pd can improve the catalytic performance of the single Pd catalyst for the EOR in alkaline media, in this work, for the first time carbon-supported bimetallic PdIr catalysts were synthesized by the simultaneous reduction method reported in our previous work [19] and by using citrate as complexing agent and stabilizer [32], [33]. The obtained PdIr/C catalysts were characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalytic activity and stability of the PdIr/C catalysts with different Pd/Ir atomic ratios were examined and compared with that of the Pd/C catalyst by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronopotentiometry (CP) methods through the EOR in alkaline media.
Section snippets
Catalyst synthesis
All chemicals used were of analytical grade, and all solutions were prepared in deionized (DI) water. Palladium chloride (PdCl2) (Aldrich), hydrogen hexachloroiridate hydrate (H2IrCl6) (Aldrich), potassium citrate (K3C6H5O7), sodium borohydride (NaBH4), potassium hydroxide (KOH), hydrochloric acid (HCl), and ethanol (CH3CH2OH) (all from Merk KGaA) were used as received. Vulcan XC-72 carbon (particle size 20–40 nm) was purchased from E-TEK, while 5 wt% polytetrafluoroethylene (PTFE) emulsion was
XRD, TEM and XPS characterizations
To obtain the structural information, including metal particle size, alloy formation and morphology, the obtained samples were first characterized by XRD and TEM. Fig. 1a shows the XRD diffraction patterns of the Pd/C, ci-Pd/C and PdIr/C catalysts with different Pd/Ir atomic ratios. The first broad peak, locating at the 2θ value of about 25°, refers to the graphite (0 0 2) facet of the carbon powder support, and the other four peaks are characteristics of the face-centered cubic (fcc) Pd
Conclusions
In this work, for the first time carbon-supported bimetallic PdIr catalysts were successfully synthesized by the simultaneous reduction method using NaBH4 as reductant and citrate as complexing agent and stabilizer. An important advantage of the Pd7Ir/C catalyst is that its onset potential is much more negative than that of the Pd/C catalyst. The CP results indicated that the Pd7Ir/C catalyst was more resistant to the poisoning caused by adsorbed ethoxi intermediates than the Pd/C catalyst, as
Acknowledgement
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. 623709).
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