X-ray Absorption Spectroscopy Studies on the Phase Transition
of PtRu Bimetallic Nanoparticles
Project No.: 93-I0037-J4
April 1, 2005 to Jan 15, 2006
Tsang-Lang Lin, T.-Y. Chen, B.-Y. Lao, J.-C. Wu, and J.-M. Lin
Department of Engineering and System Science, National Tsing-Hua University
Abstract
A 10 wt.% Pt50Ru50/C sample was prepared
atomic structure of Pt-Ru NPs [1-5]. Both Pt
by the incipient wet impregnation method. The
were analyzed to reveal the detail structure of
freshly prepared PtRu alloy particles have sizes in
Pt-Ru bimetallic NPs. The Pt-Ru nanoparticles
the range of 1.5 to 2.5 nm as found by TEM.
studied in this research were synthesized by Prof.
Effects of oxidation treatments in temperature
Yeh’s group. These naoparticles were synthesized
ranged from 300 to 570 K on the structure of
using incipient wet impregnation method. The Pt
dispersed alloy nanoparticles were characterized
to Ru atomic ratio was 1 to 1. Different oxidation
by X-ray absorption spectra (XAS). In the freshly
conditions were processed from room temperature
prepared PtRu nanoparticle, the atomic structure
to 570 K for an hour to elucidate the phase
was found to be Ru in the core and Pt at the shell.
transition behavior between metallic Pt domain
Upon mild oxidation, a concomitant coalesce of
and oxide RuO2/Pt domain. The atomic structure
dispersed
of
particles
and
segregation
of
the
LIII-edge and Ru K-edge X-ray absorption spectra
PtRu/C
NPs
upon
different
oxidation
components were noticed. Upon severe oxidation
temperature conditions were investigated in this
at 573 K, a phase separation of RuXPt1-xO2 domain
study.
from Pt-rich domains was found by EXAFS
analysis.
2. Experimental
XAS measurements were carried out at the
1. Introduction
BL-17C1 (for Pt LIII-edge) and BL-01C1 (for Ru
For the development of direct methanol fuel
cell, it is essential to have high efficiency catalysts.
K-edge) of National Synchrotron Radiation
Research Center (NSRRC), Hsinchu, Taiwan.
Pt-Ru bimetallic nanoparticles (NPs) are found to
be very effective as the catalysts for such
3. Results and Discussions
applications. The performance of the Pt-Ru
catalysts depends greatly on the atomic structure
In this research we employed XAS analysis
of the Pt-Ru nanoparticles. We have developed
in order to investigate the atomic structure of PtRu
the
Pt-Ru
NPs on carbon supports. Different oxidation
nanoparticles as well as the synchrotron X-ray
treatments were processed to re-modify the atomic
characterization
X-ray
structure arrangements of supported PtRu NPs.
Absorption Fine Structure (EXAFS) and X-ray
Figure 1 shows the Radial Structure Function
diffraction were employed to determine the
(RSF) of EXAFS data and the fitting results at Pt
methods
of
synthesizing
methods.
the
Extended
1
LIII-edge of Pt-Ru nanocatalysts for the hydrogen
reduced
samples
at
different
temperatures.
4. Summary
According to EXAFS fitting analysis it was
determined to have Pt-O peak with R = 1.98 Å
In this study, we showed that synchrotron
and coordination number CN = 3.8; Pt-O peak
X-ray EXAFS measurements can be very useful in
with R = 2.01 Å, and CN = 4.2; Pt-Pt peak with R
determining the atomic structure of Pt-Ru
= 3.1 Å and CN = 1.22; and Pt-Ru peak with R =
nanocatalysts. The Pt-Ru nanocatalysts after H2
3.08 Å and CN = 0.76 at 300 K, where R is the
reduction at 623 K were found to have a Pt shell
atomic distance and CN is the coordination
and Ru core structure from Pt LIII and Ru K edge
number.
XANES spectra.
Fig. 2 shows the RSF profiles of PtRu/C
samples at Ru K-edge (Pt：Ru = 1：1) upon
570 K for 1 hour, the structure probably turned
With further O2 treatment at
into a PtRu and Ru1-XPtXO2 segregated domains.
different oxidation conditions from 300 to 570 K
for 1 hour.
Phase transition of Pt was observed
by radial structure function of PtRu/C samples
Acknowledgment
We would like to thank the National
between 550 and 570 K.
The phase transition
Synchrotron Radiation Research Center (NSRRC)
was driven by the recrystallization of shell PtOx
for the help and allocation of beam time for the
and the sintering between individual particles
EXAFS measurements.
during the decomposition of carbon support at
high temperature. The XAS spectra indicated that
Reference
both Pt and Ru were oxide phase from 300 to 550
1.
Shibata, T.; Bunker, B. A.; Zhang, Z. ; Meisel, D. ;
K. The increase of oxidation temperature induced
Vardeman II, C. F. and Gezelter, J. D. J. AM. CHEM.
sintering. The XRD analysis showed that Pt and
SOC. 2002, 124, 11989-11996.
Ru atoms were well deposited nano-clusters about
2.
Besenbacher, F.; Chorkendorff, I.; Clausen, B. S.;
1-2 nm in diameter at temperature 300 to 520 K.
Hammer, B.; Molenbroek, A. M.; Nørskov, J. K.;
An distinct diffraction peak of RuO2 appeared at
Stensgaard, I. Science, 1998, 297, 1913-1915.
520 K. The Pt fcc peak can only be observed at
570 K.
3.
According to TEM observation the
particle size of PtRu NPs on carbon supports
Liu, D.-G.; Lee, J.-F.; and Tang, M.-T. Journal of
Molecular Catalysis A: Chemical 2005, 240, 197–206.
4.
Nashner, M. S.; Frenkel, A. I.; Somerville, D.; Hills, C.
remains constant at 550 K which is important in
W.; Shapley, J. R. and Nuzzo, R. G. J. Am. Chem. Soc.
discussing the phase transition behaviors by
1998, 120, 8093-8101.
EXAFS.
EXAFS analysis was employed to
5.
Nashner, M. S.; Frenkel, A. I.;
Adler, D. L.; Shapley,
investigate the atomic distribution of Pt and Ru
J. R. and Nuzzo, R. G. J. Am. Chem. Soc. 1997, 119,
atoms in alloyed NPs.
7760-7771.
There was segregation of
PtO and RuO2 domains driven by heterogeneous
nucleation between oxides domains. Also, there
was
the
dissolution
of
Pt
atoms
upon
decomposition of PtO2 and the sintering between
RuO2 crystals at high temperatures (the M.T. of
PtO2 at 450 K).
2
Figure 2. RSF profiles of PtRu/C samples at Ru K-edge
(Pt：Ru = 1：1) upon different oxidation conditions from 300
to 570 K for 1 hour. The number mark (1) represents the
first Ru-O scattering peak; (2) Ru-Ru scattering peak; (3)
the second Ru-O peak and (4) anomalous scattering peak.
Figure 1. RSF profiles of PtRu/C samples at Pt LIII-edge
(Pt：Ru = 1：1) upon different oxidation conditions from 300
to 570 K for 1 hour.
3