Electro-oxidation of ethanol on ternary Pt–Sn–Ce/C catalysts
Graphical abstract
Introduction
In view of a possible use as anode materials in direct ethanol fuel cells (DEFCs), the electro-catalytic activity for the ethanol oxidation reaction (EOR) in acid medium of Pt-based catalysts has been widely investigated [1], [2], [3], [4], [5], [6], [7], [8], [9]. Among various Pt-based binary catalysts, Pt–Sn has been reported as the most effective for the electro-oxidation of ethanol [5], [6]. The addition of tin to platinum not only increases the activity of the catalyst towards the oxidation of ethanol and, as a consequence, the DEFC performance, but also changes the product distribution, improving the ethanol oxidation to acetaldehyde and acetic acid [10]. In both alloyed and non-alloyed catalysts Sn provides OH species to oxidize adsorbed residues, and enhances the formation of acetaldehyde and acetic acid. To further increase its EOR activity and stability and to promote CC bond cleavage the addition of a third metal, in particular Ru [11], Ni [12], [13], [14] and Rh [12], [13], [15], [16], [17], [18], to Pt–Sn has been investigated. Recently, the addition of rare earth metals to platinum-based catalysts for low-temperature fuel cells has been reported [19]. Among rare earths, the use of cerium in low-temperature fuel cells was mostly investigated. Cerium oxide, CeO2, is one of the most widely used rare earth metal oxide as a promoter for metallic catalysts such as Pt. CeO2 is a fluorite-structured oxide with high oxygen storage capacity associated with its rich oxygen vacancies and low redox potential between Ce3+ and Ce4+. CeO2 is regarded as a kind of oxygen tank to adjust oxygen concentration at the catalyst surface under reaction condition and it has the ability to act as an oxygen buffer [20]. Many studies have been addressed to the effect of the addition of CeO2 to Pt-based binary and ternary catalysts on the electrochemical oxidation of low molecular weight alcohols [19]. More recently, a sphere-like monodispersed Ni-doped CeO2 (Ni-CeO2) nanoparticles prepared by a facile thermal decomposition method was used to promote the catalytic properties of the Pt/C catalyst in DEFCs [21]. The effect of ceria on the electrocatalytic activity of an alloyed Pt–Sn/C (3:1) catalyst for the EOR was investigated, and the results indicated that the addition of Ce to Pt–Sn/C increases the EOR activity [22]. The dependence of the EOR activity of alloyed Pt–Sn–Ce/C catalysts on Ce content went through a maximum. In this work the effect of Ce addition on the EOR activity and stability of Pt–Sn–Ce/C catalysts was evaluated. Pt–Sn–Ce/C and, for comparison, Pt–Sn/C and Pt–Ce/C electrocatalysts were prepared by a modified formic acid method [23]. The as-prepared materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) and were tested for carbon monoxide and ethanol oxidation in acid medium by CO stripping, linear sweep voltammetry (LSV) and chronoamperometry (CA) measurements.
Section snippets
Catalysts preparation
Pt–Sn/C, Pt–Ce/C and Pt–Sn–Ce/C catalysts were obtained by a modified formic acid procedure. The methodology was adding carbon Vulcan® in 0.5 mol L−1 of formic acid (Sigma Aldrich; 98.0%) on pH adjusted to 12.5 and heat the mixture at 80 °C in a reducing in carbon monoxide (CO) atmosphere. A solution of H2PtCl6·6H2O (Alfa Aesar, 99.9%) and CeCl3 (Sigma Aldrich, 56.81 wt% Ce) and SnCl2 (Sigma Aldrich, 99.0%). The precursor solution was added in an interval of 15 min. After the addition, the time of
Structural characterization of the Pt–Sn–Ce/C catalysts
The EDX compositions of the carbon supported Pt–Sn–Ce, Pt–Sn/C and Pt–Ce electro-catalysts are reported in Table 1: the average compositions were in agreement with the nominal compositions. The XRD patterns of the Pt–Sn–Ce/C and Pt–Sn/C electrocatalysts and of the commercial Pt/C electrocatalyst by E-TEK are shown in Fig. 1. The first broad peak located at the 2θ value of about 25° is attributed to the (0 0 2) phase of the hexagonal structure of carbon support. The other five diffraction peaks
Conclusions
Carbon supported Pt–Sn–Ce catalysts were prepared by a modified formic acid method, and their electrocatalytic activity for ethanol oxidation and short-term stability was compared with that of Pt–Sn/C, Pt–Ce/C and Pt/C catalysts. The EOR activity of ternary catalysts was considerably higher than that of Pt–Ce/C and Pt/C, mainly due to Sn presence in the catalysts. The onset potential for ethanol oxidation decreased with increasing Sn content on the catalyst surface. The effect of Ce presence in
Acknowledgements
The authors thank the Grants 2011/50727-9 and 2012/12189-8, São Paulo Research Foundation (FAPESP) and thank CNPq (Grant 307623/2012-2). The authors also wish to thank National Synchrotron Light Laboratory, Brazil (LNLS) for assisting with the XPS measurements (Project SXS – 15166).
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