The EXAFS Analysis of the Atomic Structure of Pt-Ru Nanocatlyst
Project No.: 93-I0037-J4
April 1, 2005 to August 31, 2005
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
method. The Pt to Ru atomic ratio was 1 to 1.
One sample was treated by H2 reduction at 623 K,
then left in air for some weeks. The other sample
was also treated by H2 reduction at 623 K, then by
In this study, we showed that synchrotron
X-ray EXAFS measurements can be very useful in
determining the atomic structure of Pt-Ru
nanocatalysts. Both Pt and Ru were all oxidized.
The Pt-Ru nanocatalysts after H2 reduction at 623
K were found to have a Pt core and Ru shell
structure. With further O2 treatment at 573 K for
1 hour, the structure probably turned into a Pt and
Ru segregated structure, with oxidized Ru and
non-oxidized Pt.
O2 treatment at 573 K for 1 hour.
2. Experimental
EXAFS measurements were carried out at
the BL-17C1 (for Pt LIII-edge) and BL-01C1 (for
Ru K-edge) of National Synchrotron Radiation
Research Center (NSRRC), Shinshuu, Taiwan.
1. Introduction
3. Results and Discussions
For the development of direct methanol fuel
cell, it is essential to have high efficiency catalysts.
Pt-Ru bimetallic nanoparticles (NPs) are found to
be very effective as the catalysts for such
applications. The performance of the Pt-Ru
catalysts depends greatly on the atomic structure
of the Pt-Ru nanoparticles. We have developed
the methods of synthesizing the Pt-Ru
nanoparticles as well as the synchrotron X-ray
characterization methods. Extended X-ray
Figure 1 shows the EXAFS data and the
fitting results at Pt LIII-edge of Pt-Ru
nanocatalysts for the sample treated by H2
reduction at 623 K. It as determined to have
Pt-O peak with R = 1.98Å and CN = 3.8; Pt-O
peak with R = 2.01Å, and CN = 4.2; Pt-Pt peak
with R = 3.1Å and CN = 1.22; and Pt-Ru peak
with R = 3.08Å and CN = 0.76, where R is the
atomic distance and CN represents the
Absorption Fine Structure (EXAFS) and X-ray
diffraction were employed to determine the atomic
structure of Pt-Ru NPs. Both Pt LIII-edge and Ru
K-edge X-ray absorption spectra were analyzed to
reveal the detail structure of Pt-Ru bimetallic NPs.
The Pt-Ru nanoparticles studied in this research
were synthesized by Prof. Yeh’s group. These
naoparticles were synthesized using impregnation
coordination number. The high Pt-O CN number
indicates high contents of Oxygen atoms in the
nanoparticle. Also, the CN of Pt-Pt is higher
than the CN of Pt-Ru, which indicates the Pt
atoms could form clusters and the Ru could be
dispersed around the Pt clusters. If a single
cluster of Pt is assumed, it would means the Pt
form the center core with Ru surrounding at the
1
surface. Figure 2 shows such a model.
For the second sample, treated with O2
treatment at 573 K for 1 hour, the EXAFS data
and the fitting results are shown in Fig. 3. The
major difference of the second sample from the
first sample is that the Pt-O peak is no longer
visible. This means the Pt is not oxidized now.
The analysis results are: Pt-Pt peak with R =
2.77Å and CN = 7.0; Pt-Ru peak with R = 2.79Å
and CN = 1.45. The much higher CN of Pt-Pt and
the low CN of Pt-Ru indicates the phase
segregation of Pt with Ru. Further studies of
measuring the Ru K-edge, as shown in Fig. 4, it
7
H623
Fitting
Pt LIII-edge
6
Pt-O = 1.98-2.01Å
4
3
FT k (R)
5
3
Pt-O
Pt-Pt
Pt-Ru
2
1
0
0
1
2
3
4
5
6
R (Å)
was found there were high Ru-O peaks. This
means Ru is higher oxidized. The corresponding
atomic structural model is shown in Fig. 5, which
shows the phase segregation of Pt and Ru, and the
Figure 1 EXAFS data and the fitting results
at Pt LIII-edge of Pt-Ru nanocatalysts. The
Pt to Ru atomic ratio was 1 to 1. The
sample was treated by H2 reduction at 623
K, then left in air for some weeks.
Ru is highly oxidized while Pt is not.
4. Summary
In this study, we showed that synchrotron
X-ray EXAFS measurements can be very useful in
determining the atomic structure of Pt-Ru
nanocatalysts. Both Pt and Ru were all oxidized.
The Pt-Ru nanocatalysts after H2 reduction at 623
K were found to have a Pt core and Ru shell
structure. With further O2 treatment at 573 K for
1 hour, the structure probably turned into a Pt and
Ru segregated structure, with oxidized Ru and
non-oxidized Pt.
Acknowledgment
We would like to thank the
National
Synchrotron Radiation Research Center (NSRRC)
for the help and allocation of beam time for the
EXAFS measurements.
Figure 2 Atomic structural model based on
the EXAFS analysis results of Fig. 1. The
Pt to Ru atomic ratio was 1 to 1. The
sample was treated by H2 reduction at 623
K, then left in air for some weeks.
2
20
Pt : Ru = 1 : 1, H623K + O573K 1h
Fitting
Pt LIII-edge
Pt-Pt = 2.7721Å, CN = 7.0
Pt-Ru = 2.7900Å, CN = 1.4
FT k (R)
15
3
10
5
0
0
1
2
3
4
5
6
R (Å)
Figure 3 EXAFS data and the fitting
results at Pt LIII-edge of Pt-Ru
nanocatalysts. The Pt to Ru atomic ratio
was 1 to 1. The sample was treated by
H2 reduction at 623 K, then by O2
treatment at 573 K for 1 hour.
18
Figure 5 Atomic structural model based on
the EXAFS analysis results of Fig. 3. The
Pt to Ru atomic ratio was 1 to 1. The
sample was treated by H2 reduction at 623
K, then by O2 treatment at 573 K for 1
hour.
PtRu 1:1 H2 reduction + O2 oxidation 573K
Ru K-edge
Ru-O
Ru-Pt
3
FT k (R)
Ru-O
12
6
0
0
1
2
3
4
5
6
R(Å)
Figure 4 EXAFS data and the fitting results
at Ru K-edge of Pt-Ru nanocatalysts. The
Pt to Ru atomic ratio was 1 to 1. The
sample was treated by H2 reduction at 623
K, then by O2 treatment at 573 K for 1
hour.
3