Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

7C/2005G032

EXAFS study on PtRu nanoparticle catalyst for DMFC
Hiroaki NITANI*1, Takashi NAKAGAWA1, Takahiro ONO1, Yusuke HONDA1, Akiko KOIZUMI1,Hideo DAIMON2, Yukiko KUROBE2, Takao A. YAMAMOTO1
1
Graduate School of Engineering, Osaka Univ., 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
2
Development & Technology Division, Hitachi Maxell Ltd., 6-20-1 Kinunodai, Yawara-mura,
Tsukuba-gun, Ibaraki, 300-2496 Japan

28

Users' Report

the “Artemis” and “FEFF7” code assuming the single
scattering path of Pt-Pt and Pt-Ru bonds. Optimized
values of structural parameters are shown in Table I.
Coordination numbers of Pt-Ru bonding in PtRu
nanoparticle synthesized at pH 3.0 and at pH 5.5 are 1.4
and 0.6, respectively. This result indicates that Pt and Ru
atoms of PtRu nanoparticle synthesized at pH 3 exist
more randomly compared with PtRu catalyst synthesized
at pH 5.5. Based on bifunctional mechanism theory, whenPt and Ru atoms exist contiguously each other, Ru
becomes an associate catalyst for oxidation of CO whichchemisorbs on active site of Pt [2]. LSV measurement andEXAFS analysis revealed that PtRu nanoparticle in which
Pt and Ru atoms stand close shows higher catalytic
activity for methanol oxidation, which strongly supports
bifunctional mechanism theory.

40pH 3.0
pH 5.530
201000

EXAFS oscillation χ(k)k350-2

Current density (mA / cm )Introduction
Direct methanol fuel cell (DMFC) is expectedcandidate for the next generation's mobile power source.
PtRu has been studying as an anode catalyst of DMFC,
because it shows excellent CO tolerance during oxidation
reaction of methanol. However, effective cell voltage ismuch lower compared with thermodynamic one due tovery high overpotential at anode. Therefore, it isimportant to improve catalytic activity of PtRu anodecatalyst for realization of DMFC. In this work, wesynthesized PtRu nanoparticle catalyst and investigated a
relationship between catalytic activity and atomic
structure of it.
Experimental
PtRu nanoparticles supported on carbon particle
catalysts were synthesized by polyol reduction method inwhich ethylene glycol was used as solvent and reducer [1].
Appropriate amount of Vulcan XC72R carbon support,Pt(acac)2 and Ru(acac)3 were added into ethylene glycol
so as to keep an atomic ratio of Pt and Ru ions 1:1. In
addition, pH of synthetic solution was adjusted to 3.0 and
5.5 by adding sulfuric acid. Then the solution wasrefluxed at 473 K for 4 hours with mechanical stirring
under nitrogen atmosphere. Catalytic activity formethanol oxidation of synthesized PtRu nanoparticles
were estimated by linear sweep voltammetry (LSV).
XAFS spectrum of PtRu nanoparticles were obtained at
Pt-LIII edge. All the measurements were performed at theBL-7C beam line of KEK-PF in air at room temperaturein the transmission mode. Synchrotron radiation wasmonochromated by using an Si(111) crystal. Unwanted
high harmonics were eliminated with detuning. Incident
and transmitted intensities of X-rays were measured withionization chambers filled with N2(85 %)+Ar(15 %)mixed gas and Ar(100 %) gas, respectively. The energy of
X-ray was calibrated by setting the first inflection pointon the LIII edge jumps of Au foil to 11913 eV.
Results and Discussions
The result of LSV measurements is shown in Fig. 1. It
indicated that the PtRu nanoparticle synthesized at pH 3.0
has higher catalytic activity for methanol oxidationcompared with one synthesized at pH 5.5. However, XRD
and TEM analyses did not find out a significant difference
between these two samples.
EXAFS oscillation of PtRu nanoparticle synthesized at
pH 3.0 and theoretically calculated fitting curve are
depicted in Fig. 2. The theoretical fit was performed withExperimental
Calculated

40 60 80 100 120
0.1
0.2
0.30.4
Photoelectron wave vector k (nm-1)
Potential (V vs. NHE)

Fig. 1. Catalytic activity for Fig. 2. Experimentally andtheoretically obtained
methanol oxidation reaction
EXAFS oscillation of PtRu
of synthesized PtRu.
synthesized at pH 3.0.
Table 1: Optimized structural parameters evaluated from
Pt-LIII edge EXAFS analysis
22Sample BondN
R [nm] σ [nm ] ∆E [eV]-5
Pt-Pt
5.9 0.272
7.1x105.1
pH 3.0-5
Pt-Ru 1.4 0.270
5.0x10
3.4-5
Pt-Pt
6.2 0.273
6.5x105.1pH 5.5-5
Pt-Ru 0.6 0.271
3.3x10
3.4
References[1] H. Daimon et al., Catal. Today 111, 182 (2006).
[2] M. Watanabe et al., J. electroanal. chem. interfacial
electrochem. 60, 267 (1975).
* h-nitani@mit.eng.osaka-u.ac.jp