Photon Factory Activity Report 2014 #32 (2015) B

AR-NW10A/ 2012G644, 2014G558

EXAFS Studies of Pd Nanoparticles-Abnormal elongation of bond distanceTadashi Ohba， Hiromitsu Uehara, Satoru Takakusagi, and Kiyotaka Asakura
1Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
Pd nanoparticles have been reported to exhibit abnormal behavior where the Pd lattice expands as the size of the Pd
nanoparticles decreases. We measured X-ray absorption
finestructure (XAFS) of Pd nanoparticles andconfirmed that the Pd-Pd bond distance in small Pd nanoparticles with a diameter of 3 nm was elongated by 2%
compared with that in Pd foil. The origin for the unusual bond elongation was discussed.

1 Introduction
Metal Nanoparticles (MNPs) have drawn muchattention because of its fundamental and practicalinterests. In many MNPs show the contraction of metalmetal bonds. Yamauchi and Teranishi independently
found that the Pd MNPs showed the lattice expansion.1
We have invested this unique phenomenum using XAFS
and confirmed the elongations of Pd-Pd distance.22 Experiment
MNP samples were prepared by a solution reduction
method under inert atmosphere to avoid oxidation of thePd PVP poly(N-vinyl-2-pyrrolidone)-protected MNPs
reported elsewhere. We prepared three samples withdifferent particle size.
TEM images of the Pd PVP
MNPs showed that the Pd MNPs had uniform sturcturewith an average diameter of 2.7 ± 0.5 nm, 4.5 ± 1.1 nm,
and 7.6 ± 2.3 nm for the samples hereafter called Pd(3),Pd(5), and Pd(8), respectively. XAFS measurements
were carried out in NW10A in PF-AR using atransmission mode.
3 Results and DiscussionFigure 1 showed the Fourier transforms of Pd samples
with different particles sizes. The main peak could be
assigned to Pd-Pd distance. The peak was shifted to thelonger side in Pd(3) while Pd(8) had the peak at the same
position as that of Pd foil, indicating that the Pd(3) had
the expanded lattice.
Table 1 summarized the curve
fitting results . The Pd-Pd bond distances increased withthe order of 2.75 Å for Pd(8) < 2.78Å for Pd(5) < 2.81 Åfor Pd(3). We confirmed the bond elongation with thePd(3). Pd(5) had the intermediate distance between thePd(3) and Pd(8). Looking at longer distances we could
find the 2nd to 4th shells well corresponding to those of
the fcc Pd lattice indicating the crystal structure wasmaintained in Pd(3). The origin of the bond elongation
was not clear yet. Pd easily creates the Pd hydride when
Pd is exposed to hydrogen where Pd-Pd bond distance islonger than the pure Pd foil keeping the fcc structure. .
However , the formation of Pd hydride can be rejected
because of the following reasons.
First, smaller
nanoparticles can dissolve fewer hydrogen. In addition,
Yamauchi found that the elongated Pd PVP MNPs could
absorb hydrogen. Hydrogen-absorbed Pd PVP MNPs
show less expansion with decreasing particle size thanthat found for the larger ones. Finally, the absorbedhydrogen can be easily removed under a hydrogen-freeatmosphere. We measured MNP’s XAFS under the N2
atmosphere so that hydrogen should not be absorbed.
Therefore hydride formation could not explain the bondelongation. We are still investigating the origin for theelongation of Pd-Pd distance

Figure 1. Fourier transformations of EXAFS spectra of a)Pd(3), b) Pd(5), and c) Pd(8). Gray dashed lines indicate thepositions of the 1st to 4th peaks for Pd(8).
1M. Yamauchi, H. Kitagawa, Synth. Met. 2005, 153, 353.;T.
Teranishi, M. Miyake, Chem.Mat. 1998, 10, 594.
2 Ohba et al. Chem Lett. in press
askr@cat.hokudai.ac.jp

Table 1. XRD and XAFS analyses of Pd PVP MNPs
EXAFS

NrPd-Pd / Å2 /10-3·Å 2

E /eV

C3 / 10-4·Å 3Pd(8)9.2±1.0

2.75 ±0.02

6.6 ±0.50±3

0.1±1.0

0.3

Pd(5)
Pd(3)

8.5±1.0
8.3±1.0

2.78 ±0.02
2.81 ±0.02

7.4 ±0.6
7.9 ±0.7

0±30±31.2±1.0
0.8 ±1.0

0.3
0.5

kas

R factor /%