EXAFS Study on Nanosized PtRu Catalyst for Direct Methanol Fuel Cell
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EXAFS Study on Nanosized PtRu Catalyst for Direct Methanol Fuel Cell Hiroaki Nitani1, Takahiro Ono1, Yusuke Honda1, Akiko Koizumi1, Takashi Nakagawa1, Takao A. Yamamoto1, Hideo Daimon2 and Yukiko Kurobe2 1 Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, 565-0871 Osaka, Japan. 2 Development & Technology Division, Hitachi Maxell Ltd., 6-20-1 Kinunodai, Yawara-mura, Tsukuba-gun, 300-2496 Ibaraki, Japan. ABSTRACT Nano-sized PtRu catalysts supported on carbon nanoparticles were synthesized by a polyol process. The PtRu catalyst prepared at pH=3 indicated higher catalysis for methanol oxidation than one prepared at pH=5.5. The samples were analyzed by techniques of the extended X-ray absorption fine structure (EXAFS), transmission electron microscope (TEM), X-ray diffraction (XRD) and X-ray fluorescence (XRF). Their results showed that the pH of the precursor solution during the polyol process affected the substructure of the PtRu nanoparticles. The correlation of the substructure with the catalytic activity was studied. INTRODUCTION Fuel cell has been attracting much attention of scientists and engineers since it is regarded as a next generation’s power source in various fields [1,2]. One of the most promising fuel cell is the direct methanol fuel cell (DMFC), which employs fuel of methanol not as other cell types using hydrogen gas. However, effective cell voltage of DMFC is lower compared with the thermodynamic one because of significant overpotential caused by methanol oxidation occurring at anode surface. To reduce the overpotential, the DMFC usually uses Pt catalysts for oxidizing the methanol on its anode, but Pt is poisoned easily by CO. To resolve this problem, many scientists have been studying new Pt based alloys with a high tolerance for CO poisoning [3,4]. Of many alloys so far reported, PtRu alloy has the highest CO tolerance and its high tolerance was reported as described by “bi-functional mechanism” by Watanabe et al. in 1975 [5]. In this work, we synthesized PtRu alloy nanoparticles loaded on porous carbon support catalyst and studied correlation between its structure and catalytic activity. We successfully made PtRu particles in a polyol process, and found that their catalytic performance measured by the linear sweep voltammetry (LSV) changes with pH of the precursor solution. The reason was attributed to the atomic arrangement of Pt and Ru revealed by combining results of the extended X-ray absorption fine structure (EXAFS) measurement and the “bi-functional mechanism”.
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EXPERIMENTAL Sample preparation The present PtRu catalysts were synthesized by a polyol process employing ethylene glycol as a reductant and a solvent [6-8]. The ethylene glycol solution containing 1.69 mmol of Pt(acac)2 and 1.69 mmol of Ru(acac)3 was prepared. Then a commercial carbon nanoparticle powder (Valcan XC-72R, Cabot Corp.) was added into the mix solution as a support material. The amount of carbon was 0.5 g, twice of the noble metals element in weight. To adjust the pH
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