Environmental science

Why Gold Becomes Active When in Form of Dispersed 3 nm Particles?
Reaction Intermediate Detected at SPring-8
Since ancient times, gold has been a symbol ofwealth and power because of its eternal brightness
due to its poor chemical reactivity. However, Au
becomes chemically active when it is in the form ofdispersed 3 nm particles, e.g., for the catalysis of lowtemperature CO oxidation [1]. The reason for thecontrasting reactivity of Au between the nanometerand centimeter scales is not yet known.
Various possibilities have been proposed as theactive site of Au nanoparticles: anionic Au clustersites, the interface sites between Au and the
supporting metal oxide, and lower-coordination stepedge sites [1]. It is difficult to determine the active site
because the site structure of Au nanoparticles cannot
be determined individually from the various chemical
states of Au.
X-ray diffraction (XRD) is insensitive to 3 nm
particles because there is insufficient repetition oflattices of Au particles. Also, XRD does notdiscriminate between different chemical states. In
contrast, X-ray photoelectron spectroscopy (XPS) canbe used to monitor chemical states and provides thepopulation of each state near the surface of thesample. However, XPS does not give structural
information. X-ray absorption fine structure (XAFS)
provides local structural information. However, theinformation is in the form of a statistical average if
more than two chemical states are present in theelement in the sample. A heterogeneous catalyst is atypical example of such a mixture of chemical states
for the active element, particularly during catalysis.
To determine each Au site structure in Au
nanoparticles on a porous, complex surface of TiO2,
we applied state-sensitive XAFS spectroscopy [2].
State-sensitive XAFS, which is an evolved version ofgeneral XAFS spectroscopy [3]. The statistical
disadvantage of XAFS that it gives average
information is not applicable to state-sensitive XAFS
because the chemical states of Au can be
distinguished by high-energy-resolution X-ray
fluorescence spectrometry. XAFS data for eachchemical state of Au was obtained by selecting X-ray
fluorescence signals originating from anionic, neutral,and cationic Au sites independently [2].
A homemade Rowland-type fluorescence
spectrometer [3] was set in the undulator beamline
BL37XU. With the excitation energy set to 11943 eV,
the Au Lα1 emission from the sample was analyzed

using a Johansson-type Ge(555) crystal. The
Rowland radius was 220 mm. The measured energy
resolution of this fluorescence spectrometer was 2.9eV at 9.7 keV including the contribution of thebeamline.
The fluorescence spectrometer was tuned to eachemission energy and the Au L3-edge XANES (X-ray
absorption near-edge structure) spectra were
measured. By tuning the energy to 9713.3 eV (peakof emission spectrum, Fig. 1, inset), Au Lα1-selecting
Au L3-edge XANES spectra were measured for Aumetal (Fig. 1, solid red line). The absorption edge at
11921.2 eV was steeper and the peaks at 11923.3,11935.3, 11941.3, and 11948 eV were sharper andmore resolved than those in the corresponding
spectrum measured in transmission mode (Fig. 1,dotted blue line). This sharpening effect is derived
from identical rationale used to discriminate between
chemical states in XANES combined with highenergy-resolution fluorescence spectrometry [4].
The particle size of Au nanoclusters on TiO2 wascontrolled between 2.9 and 8.7 nm by changing thedeposition-precipitation synthesis conditions of thecatalyst: the pH (NaOH or urea), the surface area ofTiO2 (60 - 1200 m2g–1), the reaction temperature, andthe duration of the deposition-precipitation reaction [2].
The Au particle size distribution was very narrow(standard deviation ≈26%).
The reason why there is critical difference in lowtemperature CO oxidation reactivity depending on the0.835.341.323.3

48

21.2
0.001

I f / I0

Normalized Intensity

1.20.4
09680

9700

9720

9740

Emission Energy (eV)011880Au metal
11920
11960
Energy (eV)12000

Fig. 1. Au Lα 1 -selecting Au L 3 -edge XANES
spectrum tuned to Au Lα1 peak (solid red line) and
XANES spectrum measured in transmission mode
(dotted blue line). (Inset) Au Lα1 emission spectrum.

Part 27 in the series.

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nanoparticles of 2.9 nm than for those with average
size of 8.7 nm. However, this Au δ– site was notaffected whether in air or in 5% CO.
Finally, the fluorescence spectrometer was tuned
to 9718 eV on the higher-energy side of Au L α 1emission. The Au L3-edge energy shift was +6.8 eVfor Au/TiO2(P25)-urea-16 of nanoparticles of 2.9 nm inair (Fig. 2(a-3)), greater than that in 5% CO (+3.9 eV,Fig. 2(b-3)) and that of nanoparticles of 8.7 nm(average) in air (+3.7 eV) [2]. Thus, it is suggested
that O2 molecules are activated on the cationic Auδ+site, probably near the interface with the TiO2 surfaceterminated with Ti atoms (the so called SMSI effect;Fig. 3(a)) by the further negative charge transfer fromAu δ+ to the adsorbed O 2 . The charge donation
observed in Fig. 2(a-3) is a snapshot of the reaction
intermediate of CO oxidation that proceeds on Au
nanoparticles of 2.9 nm, but does not occur on larger
Au nanoparticles [1].

0.8

(1)
9707.8eV-tune(a) Au / P25-urea-16
in air(3)9718.3eV-tune1.2

0.80.4(2)
9713.4eV-tune23.930.0

Normalized Intensity

0.4(2)
9713.1eV-tune

24.2
31.2

1.2

(1)
9708.1eV-tune(b) Au / P25-urea-16
in 5%CO
(3)
9718.6eV-tune01190011940

CO2OO CC(a) Ti-top
11980

12020

Energy (eV)Neutral Au0 site

Interface Auδ+ site
TiFig. 2. Au Lα1-selecting Au L3-edge XANES spectra
measured for Au/TiO2(P25) of 2.9 nm nanoclusters
synthesized at 353 K using urea in air (a) and in 5%
CO (balance gas: Ar) (b). 0.93 wt% Au. The energies
were tuned to 9707.8, 9713.1, and 9718.3 eV (a) and9708.1, 9713.4 and 9718.6 eV (b).

OC OCe– O2OO CCe–OC O (b) O-topCNeutral Au0 site
e– Interface AuO

δ–siteFig. 3. Au26 cluster models on anatasetype TiO2(001) surface terminated with
Ti atoms (a) and O atoms (b).

Au particle size [1] was clarified on the basis of the Au
Lα1-selecting Au L3-edge XANES spectra obtained bytuning the fluorescence spectrometer to 9708, 9713
and 9718 eV. The measured sample was a Au/TiO2catalyst of 2.9 nm nanoparticles synthesized using
urea at 353 K for 16 h on TiO2 (Degussa P25) in airand in CO(5%)/Ar.
The Au L 3-edge energy and spectral pattern ofXANES tuned to 9713 eV (Figs. 2(a-2) and 2(b-2))
resembled those for XANES measured for Au metal(Fig. 1, solid red line). Thus, the neutral Au 0 site
located on upper side of Au nanoparticles on TiO2 wasselected at 9713 eV (Fig. 3). When the energy wastuned to 9708 eV on the lower-energy side of Au Lα1emission, the Au L3-edge energy shifted by -4.6 to-4.7 eV toward the lower-energy side. The firstintense peak became a weak shoulder at 11922 eV(Figs. 2(a-1) and 2(b-1)). These trends indicatedgreater Au 5d occupation [5]. Thus, the formationof the anionic Auδ– site was suggested (Fig. 3(b)).
The population of Au δ– site was greater for Au

Yasuo Izumi
Graduate School of Science, Chiba University
E -mail: yizumi@faculty.chiba-u.jp

References[1] A. Cho: Science 299 (2003) 1684.
[2] Y. Izumi, D.M. Obaid, K. Konishi, D. Masih, M.
Takagaki, Y. Terada, H. Tanida and T. Uruga: Inorg.
Chim. Acta 361(4) (2008) 1149.
[3] Y. Izumi et al.: J. Electron Spectrosc. Relat. Phenom.
119 (2001) 193.
[4] Y. Izumi et al.: Anal. Chem. 77 (21) (2005) 6969.
[5] N. Weiher et al.: J. Am. Chem. Soc. 129 (8) (2007)
2240.

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