Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

10B/2003G250

Pd K-edge XAFS study on palladium-polymer micelle catalyst
2
3Tomoko YOSHIDA1, Hisao YOSHIDA, Ryo AKIYAMA3 and_ , Kuniaki OKAMOTO
3Shu KOBAYASHI
1Division of Quantum Science and Energy Engineering, Department of Materials, Physics andEnergy Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
2
Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Furo-cho,Chikusa-ku, Nagoya 464-8603, Japan
3
Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo
113-0033, Japan

15126

Users' Report

30
15Pd-Pd

10Pd-05-10

Pd-01050FTFT-5-5-10-5-10a)a)c)-150123R/Å

4560-20-10a)
b)

-15

Pd-Pd
Pd-Pd

2010Pd-Pd

Pd-0
Pd-0(C)

50-30
-15

0

123R/Å

4

560

123R/Å

456Fig.1 FT of k3-weighted Pd K-edge EXAFS of a) PdO, b) Pd catalyst and c)
Pd foil.
101.59871.0

6540.532Coordination number (CN)Diameter of the cluster / nmResults and Discussion
Fig. 1 shows the radial structure function (RSF) ofPdO, Pd catalyst and Pd metal, which were obtained by
3
Fourier transforming of k -weighted Pd K-edge EXAFS
spectra of these samples. A peak observed at 2-4 Åshows the presence of the second-neighboring Pd atoms.
It is noteworthy that the peaks due to second neighboring
Pd atoms for Pd catalyst (Fig. 1b) and reference Pd metal(Fig. 1c) appeared at the same position. This confirmsthat the Pd was in metallic environment. However, thepeak magnitude for Pd catalyst was much lower than thatfor the reference Pd metal. This result demonstrates thedecrease in the number of second-neighboring Pd atoms,
supporting the formation of small Pd metal particles in Pd
catalyst. On the other hand, the RSF peaks appearing
around 1.6 Å are due to the backscattering from the15

Pd-Pd

10

FT

Experimental Section
The cross-linking of palladium-polymer micelles wasprepared as described elsewhere.[1]X-ray absorption
experiments of the palladium-polymer micelles catalysts
were carried out on the beam line 10B at Photon Factoryin High Energy Accelerator Research Organization
Institute of Materials Structure Science (KEK-PF),
Tsukuba Japan, with a ring energy of 2.5GeV and stored
current of 450-380 mA. X-ray absorption spectra wererecorded in a transmission mode at room temperature witha Si(311) channel cut monochromator. The intensities ofthe incident and transmitted X-ray were measured with
17-cm ion chamber with a Ar gas flow and a 62-cm ion
chamber with a Ar gas flow, respectively. Energy
calibration was carried out using Pd K-edge of Pd foil.

adjacent carbon or oxygen atoms. In the RSF of PdO,
the peak around 1.6 Å is assignable to Pd-O shell. In theRSF of Pd catalyst, very small peak can be noticed in thesimilar position, which is probably due to carbon atomsof polymer coordinated with Pd atoms (Pd-C shell) oroxygen atoms on the surface of Pd metal particles (Pd-Oshell), in our opinion. In any case, this would be minor
fraction.
We also performed nonlinear curve-fitting analyses of
the Fourier-filtered EXAFS of the first and the second
shells, corresponding Pd-C(or Pd-O) and Pd-Pd shells, bya least-squares. From this analysis for Pd catalyst thedistance of the Pd-Pd shell was estimated 2.76 Å, which
was almost consist with that of the reference Pd metal.
The coordination number (CN) was calculated to be 4.4,
which was much lower than that in reference Pd metal,suggesting that the size of the palladium metal in Pd
catalyst was extremely small like small clusters.
The sphere-like particle of fcc structure wasconsidered as a stable model cluster to estimate thecluster size of Pd metal from the coordination number asplotted in Fig. 2. The cluster whose coordination
number was 4.4 would be larger than 4-atom tetrahedra
and less than 13-atom cubotetrahedron; the estimatednumber of Pd was around 9 and diameter was around 0.8
nm, indicating the formation of nano-size palladium
metal cluster in Pd catalyst.

FT
FT

Introduction
Colloidal metal particles and metal clusters have attracted
much interest because of their unique properties andpotential applicability to microelectronics, chemicalsensing, data storage, catalysts, etc.
In this paper, we describe the formation of stable,
subnanometer Pd clusters contained within micelles
produced from random copolymers using a novel method.
To the best of our knowledge, the Pd clusters formed inthis work are the smallest reported to date (average cluster
diameter ~ 0.7 nm). The clusters were stabilized inseveral micelle morphologies, and exhibited efficient
catalytic activity in hydrogenation and Heck reactions.

Fig. 2
Geometricalcalculation of diameter ofsmall sphere Pd cluster andthe average coordination
number in the cluster.

10001020304050

Number of Pd atoms in cluster model

References[1] K. Okamoto, R. Akiyama, H. Yoshida, T. Yoshida andS. Kobayashi, J. Am. Chem. Soc., 127, 2125 (2005)