Improvement of methanol oxidation catalytic activities of radiochemically synthesized PtRu/C nanoparticles by post annea
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Improvement of methanol oxidation catalytic activities of radiochemically synthesized PtRu/C nanoparticles by post annealing process Satoshi Seino, Masato Morisue, Yuji Ohkubo, Junichiro Kugai, Takashi Nakagawa and Takao A. Yamamoto Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka, 565-0871, Japan ABSTRACT Electrode catalysts composed of carbon supported PtRu nanoparticles (PtRu/C) synthesized by radiochemical process were annealed to control the PtRu substructure to enhance catalytic activity. The substructure of the PtRu nanoparticles synthesized by using high-energy electron beam under acidic condition was Pt-rich core/Ru-rich shell type, reflecting the redox potentials of each precursor ions. The material characterization techniques revealed that the reductive annealing led to the mixing of PtRu both in the core and on the surface. The sample with annealing temperature of 300oC for 5 hour showed the highest methanol oxidation current, 2.3 times higher than that obtained with before annealing. INTRODUCTION Much attention has been paid to direct methanol fuel cells (DMFCs) as power sources for mobile devices in near future. However, the high cost of Pt in the catalysts in DMFCs is a critical barrier to their commercial application. Bimetallic nanoparticle catalysts of PtRu immobilized on carbon support (PtRu/C) have been reported to show high activity for methanol oxidation reaction. It has been reported that the control of PtRu structure to a well-mixed state is one of the key factors for improved catalytic activity [1-4]. To keep the activity over a long period of time, the Ru should be mixed with the Pt not only at the surface but also at the inside of the particle, because the surface layer would be dissolved out in operating condition [5]. The synthesis of PtRu/C catalysts in an aqueous solution system is preferable from the viewpoint of mass production and cost reduction. However, it is not an easy task to obtain wellmixed PtRu nanoparticles in solution systems. In general, the bimetallic substructures in nanoparticles formed by reducing aqueous ions are influenced by the redox potentials of the constituent metals. In the PtRu system, Pt ions are first reduced and Ru ions follow it because of the difference in redox potentials, and usually result in Pt–core/Ru–shell or Pt–core/PtRu–shell structure [6]. We have reported successful preparation technique of PtRu/C nanoparticles by using radiochemical process [7-9]. In our previous study, we have succeeded in synthesizing PtRu grains with controlled size of ca. 2 nm which are well dispersed on the surface of carbon support by controlling the solution pH = 3 [9]. However, material characterization indicated that the substructure was Pt-rich core/Ru-rich shell. To obtain higher catalytic activity, the substructure of the PtRu needs to be improved to a more mixed state. The target of this study is to control the substructure of the radiochemically synthesized PtRu nanoparticles by a simple post annealing process. As annealing condition
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