Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

12C/2002G287

Local Structures of Au(III) species Adsorbed on Cerium Oxide Using XAFS
Shuji MATSUO*1, Shinya SHUFUMOTO2, Hironori OHASHI2, Yoshihiro OKAUE2,Takushi YOKOYAMA2, Hisao YAMASHIGE3, Hisanobu WAKITA1,3
1Adv. Mater. Inst., Fukuoka Univ., Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan
2
Dept. of Chem., Faculty of Sci., Kyushu Univ., Ropponmatsu, Chuo-ku, Fukuoka 810-8560, Japan
3
Dept. of Chem., Faculty of Sci., Fukuoka Univ., Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan

Experimental
The adsorption experiments were carried out at3ambient condition. A 0.5 dm of Au(III) sample solutions-3(5 mg dm for Au) with a constant ionic strength (0.12 MNaCl) were prepared by diluting an H[AuCl4] standard
solution with deionized water. 50 mg of cerium oxide
2-1
powder (SBET = 166 m g ) was added to the sample
solution, and the suspended solution was then adjusted topH 6, 8, or 10 with a NaOH aqueous solution. After
stirring for 24 and 48 h, each suspension was filtered with
a 0.45 µm membrane filter and dried under the vacuumfor 24 h. The adsorption rate of gold was ca. 99 %. The
obtained sample powders were hardened on a piece ofacryl board using a press machine to make a flat surface.
XAFS data for Au L3-edge were collected at ambientcondition by a fluorescence mode. The fluorescence Xrays were detected by 19-elements Ge-SSD. Data
accumulation time was 5 s / step. Data analysis wasperformed using the WinXAS ver. 2.1 package andFEFF6 code for the theoretical backscattering amplitudes
and phase shifts. The curve fittings were carried out for
the first-shell filtered data obtained by an inverse Fouriertransform range of 0.9 – 2.2 Å.

Results and Discussion
Figure 1 shows the Au L3-edge XANES spectra for thepowder samples of Au(III) complex ion adsorbed on thecerium oxide together with those for H[AuCl4] and goldfoil. The spectra indicate that most of gold adsorbed on
cerium oxide at pH 6, 8, or 10 for 48 h exist as Au(III)species.
Table 1 shows the result of the curve fitting of anEXAFS spectrum for the powder samples of Au(III)complex ion adsorbed on cerium oxide at pH 6 for 24 h.
This result suggests that there are two different Au(III)species as the adsorption structure on cerium oxide; one iscoordinated by –OH and –Cl, and another is coordinatedonly by –OH. Existence of the two different Au(III)197
species have been also observed by Au Mössbauer
spectroscopy.

Relative Intensites / a.u.

Introduction
We have found in the previous studies that Au(III)complex ions adsorbed on the surface of the powders foraluminum, titanium, and manganese oxides were
spontaneously reduced to atomic gold without a specificreducing agent or heat treatment, and consequently
formed gold particles on each oxide[1]. However, thereduction of Au(III) complex ions did not occur on the197
surface of the powder of cerium oxide as studied by Au
Mössbauer measurements, although it has the adsorption
capacity as well as the powder of manganese oxide. Thisphenomenon for cerium oxide is useful in studying on thelocal structures of Au(III) complex ions on a metal oxidein the first step of the adsorption. In the case of a metaloxide on which the reduction occurred like manganeseoxide, on the other hand, it is difficult to study on it
because the adsorption behavior is not distinguishable
from the reduction behavior. In the present study, XAFS
measurements were carried out for Au(III) complex ionsadsorbed on the powder of cerium oxide to examine theirlocal structures.

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pH 10pH 8
pH 6K[AuCl4]
Gold foil

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Fig. 1. Au L3-edge XANES spectra for the powder
samples of Au(III) ion adsorbed on the cerium oxide at
pH 6, 8, or 10 for 48 h, H[AuCl4], and gold foil.
Table 1: Result of curve fitting of EXAFS spectrum using
two atomic pairs: sample prepared at pH 6 for 24 h22E0 / eV
CNr/Å
σ /Å
4.59
Au-O
3.3
1.96
0.0058
15.73
Au-Cl
0.7
2.30
0.0075
Reference[1] S. Matsuo et.al., X-ray Spectrom. 32, 158 (2003).
* mashu23@hotmail.com

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Users' Report

11.96

Energy / keV