Available online at www.sciencedirect.com

Inorganica Chimica Acta 361 (2008) 1149–1156
www.elsevier.com/locate/ica

State-sensitive monitoring of gold nanoparticle sites on titania
and the interaction of the positive Au site with O2 by
Au La1-selecting X-ray absorption ﬁne structure q
Yasuo Izumi a,*, Diaa Mosbah Obaid b, Kazushi Konishi b, Dilshad Masih b,
Masafumi Takagaki c, Yasuko Terada c, Hajime Tanida c, Tomoya Uruga c,*baGraduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8502, Japanc
Japan Synchrotron Radiation Research Institute, Kohto 1-1-1, Sayo 679-5198, Japan

Received 17 April 2007; received in revised form 14 September 2007; accepted 16 September 2007
Available online 29 October 2007
Dedicated to Edward Solomon.

Abstract
The heterogeneity of gold sites in various Au/TiO2 catalysts was studied by means of state-sensitive Au L3-edge X-ray absorption ﬁne
structure (XAFS) combined with high energy-resolution X-ray ﬂuorescence spectrometry. A series of Au/TiO2 catalysts were preparedvia deposition-precipitation method on anatase-type or mesoporous (amorphous) TiO2 added with NaOH (lower Au loading) or urea
(higher Au loading). The mean Au particle size ranged between 29 and 87 Å based on high-resolution TEM (transmission electron microscope) measurements. The Au La1 emission peak energy for Au/mesoporous–TiO2 in air and Au/anatase–TiO2 in CO (5%) corresponded
to Au0 state. The emission peak energy for Au/anatase–TiO2 in air shifted toward that of AuI state. For relatively greater Au particles(average 87 Å) dispersed on mesporous TiO2, the major valence state discriminated by Au La1-selecting XANES (X-ray absorption
near-edge structure) spectrum tuned to Au La1 emission peak top was Au0, but the Aud state could be successfully monitored by
Au La1-selecting XANES tuned to the emission energy at 9707.6 eV, of which population was relatively small compared to the caseof smaller Au particles (average 29 Å) on anatase-type TiO2. On the other hand, negative charge transfer from Au 5d to support wasdemonstrated in Aud+-state sensitive XANES tuned to 9718.3–9718.7 eV. The Aud+-state sensitive XANES spectra resembled theoretically generated XANES for interface Aud+ sites model on TiO2 in contact with surface Ti sites. Further charge transfer was demonstrated from Au to adsorbed O2 for Au/anatase–TiO2 catalyst.
 2007 Elsevier B.V. All rights reserved.
Keywords: Gold; Titanium oxide; X-ray absorption ﬁne structure; Fluorescence spectrometry; Chemical state

1. Introduction
Gold is chemically inert, however, it was found to
become active when dispersed as nano-particles of size
30 Å or smaller in low temperature CO oxidation [1,2].
qPart 23 in the series.
Corresponding authors. Tel./fax: +81 43 290 3696.
E-mail addresses: yizumi@faculty.chiba-u.jp (Y. Izumi), urugat@
spring8.or.jp (T. Uruga).
*

0020-1693/$ - see front matter  2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.09.027

Various possibilities have been proposed as the active sites:
anionic Au cluster site, perimeter site, and lower-coordination step-edge site [1,2]. However, it is diﬃcult to observe
each site one by one among Au nano-particles dispersed
over porous, complex surface of TiO2.
In order to overcome the heterogeneity problem of Au
sites, we applied state-sensitive XAFS spectroscopy [3,4]to a series of Au/TiO2 samples. Due to the diﬀerences ofdeposition-precipitation (DP) conditions of Au compoundwith NaOH or urea, reaction time of DP, and crystalline

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Y. Izumi et al. / Inorganica Chimica Acta 361 (2008) 1149–1156

phase of TiO2 support (anatase/rutile or amorphous), particle size and loading amount of Au were systematicallycontrolled. Catalytically active Au nano-particle sites populated in narrow size distribution centered at 29 Å weremonitored state-sensitively in CO or O2 atmosphere to
extract activation sites for the low-temperature oxidation
reaction.
In this spectroscopic method, better-resolved, steeper
XANES spectra were obtained to remove the core-hole
lifetime width (C) of K levels for Cu [4,5] and Sn [4] andL3 levels for Dy [6], Pt [7], and Pb [4,8] by selecting theKa1 and La1 emission, respectively.

4–16 h in the absence of light [11]. Then, the ﬁltered powder was processed similar to the preparation using NaOH
and ﬁnally calcined at 573 K in air (Table 1d–g). Thus,
the obtained Au/TiO2 catalysts are denoted ‘‘Au/(type ofTiO2)-(alkaline used)-(DP time)-(calcined temperature)’’,e.g. Au/P25-urea-16-573.
The Au/TiO2 sample disks were sealed in on-reaction
cell for X-ray measurements purged with dry air or withthe mixture of CO (5%) and argon (95%). The window
was polyethylene naphthalate ﬁlm of 16 lm in thickness(Q51-16, Teijin).
2.2. Characterization

2. Experimental
2.1. Samples
K+[AuI(CN)2] (Wako) was thoroughly mixed withboron nitride (BN) to be 1.0 wt% Au and pressed into adisk of 20 mm in diameter. Au metal foil was controlled
to be the thickness of 6 lm and powders of AuICl[P(C6H5)3] (>99.9%, Aldrich), AuIII(OH)3 (Wako), andAuIII
2 O3 (Wako) were thoroughly mixed with BN andpressed into a disk of 20 mm in diameter, to make theAu L3 absorption edge jump to be 1.
The mesoporous TiO2 sample was synthesized in aqueous solution using dodecylamine [9] given by Professor
H. Yoshitake at Yokohama National University. The speciﬁc surface area (SA) was 1000 m2 g1 and wormhole-likepore size distributed in narrow range centered at 30 Å. The
Au/TiO2 catalyst was prepared by mixing 1 g of TiO2
(Degussa P25; anatase 70% and rutile 30%) or mesoporous
TiO2 with 50 ml of aqueous solution of HAuIIICl4 Æ 4H2O
(Wako, 1.3 mM) (Table 1a–c). The mixture was stirred at
328 K for 4.5–7 h. The pH of the solution was maintained
at 8 by adequately adding NaOH [10]. The powder was ﬁltered and washed several times with de-ionized, distilledwater. Dried powders were calcined at 523–673 K in air.
The Au/TiO2 catalyst was also prepared by mixing 1 g ofTiO2 (P25) with 100 ml of aqueous solution of HAuCl4 Æ 4H2O (4.2 or 0.51 mM) by adding 42 or 5.1 mM ofurea, respectively. The mixture was stirred at 353 K forTransmission electron microscope images were measured using ﬁeld-emission-type TEM (JEM-2010F, JEOL)
with an accelerating voltage of 200 kV at the Center ofAdvanced Materials Analysis, Tokyo Institute of Technology (Dr. A. Genseki). The powder was dispersed in carbon
tetrachloride using ultrasonic and mounted on carboncoated Cu grid.
The X-ray measurements were performed at 290 K at
the undulator beamline 37XU of SPring-8 (Sayo, Japan).
The storage ring energy was 8.0 GeV operated in top-up
mode at the ring current of 99–97 mA. A Si(1 1 1) monochromator and Rh-coated mirror were used. The undulator
gap value was optimized to maximize the X-ray beam ﬂux
at each data point. To stabilize the X-ray beam position on
the surface of the sample, a feedback mechanism using
MOSTAB (monochromator stabilization) [12] was applied.
No position change was detected within the range of 50 lm
throughout the beamtime.
A homemade Rowland-type ﬂuorescence spectrometer
was used to analyze Au La1 emission equipped with
Johansson-type Ge(5 5 5) crystal (d = 0.65321 Å, Saint
Gobain). The detail of the spectrometer was described inRefs. [4,13,14]. An ionization chamber was purged withthe mixture of nitrogen (30%) and helium (70%) in frontof the sample. The opening of slit 0 in front of the ionization chamber was 0.25–0.5 mm (horizontal) · 0.5–1 mm(vertical). The sample was placed in a plane near horizontal. The plane was tilted by 6 toward the incident X-ray

Table 1
Preparation conditions via deposition-precipitation (DP) method and the property of various Au/TiO2 samplesa
Entry

Alkaline

TiO2

pH

DP reaction
temperature (K)DP reaction time(h)Calcined
temperature (K)Au loading(wt%)b

Average Au particle
size (Å)

a
b
c
d
e
f
g

NaOH
NaOH
NaOH
urea
urea
urea
urea

mesoporous
mesoporous
P25
P25
P25
P25
P25

8
8
8
3!8
3!8
4!8
4!8

328
328
328
353
353
353
353

7
4.5
7
4
16
4
16

523
673
673
573
573
573
573

0.21
0.21
0.09
7.1
6.8
0.97
0.93

87 ± 2378 ± 3032 ± 2137 ± 1035 ± 1929 ± 1029 ± 18abGold precursor was HAuCl4 Æ 4H2O.
Evaluated based on the Au La1 emission and Au L3 absorption edge intensity.

Y. Izumi et al. / Inorganica Chimica Acta 361 (2008) 1149–1156

1151

beam and by 7 toward the Ge crystal [4,13,14]. A receiving
slit 1 was placed at 100 mm from the sample. The opening
was 8 mm(h) · (0.5–4) mm(v). The sample, Ge(5 5 5) crystal, and slit 2 in front of the NaI(Tl) scintillation counter(SC) were controlled on Rowland circle (the radius
220 mm). The opening of slit 2 was 8 mm(h) · 2 mm(v).
The sample and SC were covered with a lead plate housingof 1–3 mm thickness except for the incident X-ray beam
entrance and the exit of ﬂuorescence X-rays toward the
direction of Ge crystal.
With the excitation energy set to 11943.1 eV, Au La1emission (M5 ! L3; Bragg angle hB = 77.700) spectra
were measured. The scan step was 0.9 eV and the accumulation time was 60 s for a data point. The ﬂuorescence
spectrometer was tuned to each emission energy and theAu L3-edge XANES spectra were measured. The scan steps
were 7.5, 0.5, and 2.4 eV in the pre-edge, edge, andpost-edge energy regions, respectively. The accumulation
time was 40–100 s for a data point. The Au La1 ﬂuorescence and Au L3-edge energy values were calibrated to
9713.3 and 11921.2 eV, respectively, for the spectra of Au
metal [15]. The energy positions of the monochromatorand the ﬂuorescence spectrometer were reproduced within±0.1 and ±0.2 eV, respectively. Some of the XANES spectra for standard Au samples were also measured in transmission mode at beamline NW10A of Photon Factory
Advanced Ring at KEK (Tsukuba, Japan). The obtained
data resolution was essentially the same as that measured
at beamline 37XU at SPring-8 in the transmission mode.

3. Results

2.3. XAFS data analysis

3.2. X-ray spectroscopy for standard Au compounds

The XAFS data analysis was performed using XDAP
version 2.2.7 (XAFS Services International) based on
works of M. Vaarkamp, H. Linders, and D. Koningsberger. The pre-edge background was approximated by themodiﬁed Victoreen function C2/E2 + C1/E + C0. The background of post-edge oscillations was approximated by asmoothing spline function calculated by the following
equation for the number of data points N.

The Au La1 emission spectra were measured for Au
metal and K+[AuI(CN)2]. The C values for Au L3 andM5 levels are 5.41 [17] and 2.18 eV [19], respectively.
Based on the FWHM (full width at the half maximum)
value of the Au La1 peak for Au metal (6.5 eV), theenergy resolution of ﬂuorescence spectrometer was estimated to 2.9 eV including the contribution of beamline
at 9.7 keV. The chemical shift of Au La1 emission peaks
was 0.2 eV for K+[AuI(CN)2] compared to that for
Au metal (Table 2). Because the shift value was comparable to the reproducibility of energy position of the ﬂuorescence spectrometer, the chemical shift was experimentally
checked three times and the shift was reproduced andsigniﬁcant.
By tuning the ﬂuorescence spectrometer to each emission peak top, Au La1-selecting Au L3-edge XANES spectra were measured for Au metal and K+[AuI(CN)2]
(Fig. 2). Satisfying the condition that the energy resolution
to monitor the X-ray ﬂuorescence was smaller than that C
for Au L3 level [4,18], absorption edge became steeper andthe peaks at 11923.3, 11935.3, 11941.3, and 11 948 eV
depicted in Fig. 2a and those at 11926.8 and 11 948 eV
depicted in Fig. 2b were sharper and more resolved compared to the corresponding spectra measured in transmission mode (dotted line).

N
Xðlxi  BGi Þ2
6 smoothing factor
2i¼1 expð0:075k i Þ

For Au cluster site models (Ti-top and O-top) supported
on anatase-type TiO2(0 0 1) surface, Au L3-edge XANES
spectra were theoretically generated using FEFF 8.2 [16]in self-consistent ﬁeld and full multiple scattering calculation modes. The Hedin–Lundqvist self-energy was used
for energy-dependent exchange-correlation potential. The
calculated potential was corrected by adding a constantshift to ‘‘pure imaginary optical potential’’. Because the
Au La1-selecting XANES was measured under the condition that the energy resolution to detect the X-ray ﬂuorescence (2.9 eV) was smaller than the C value of Au L3 level(5.41 eV) [17], the constant energy shift value was set tonegative (1.0 eV) [4,18].

3.1. TEM observation of Au/TiO2 catalysts
High-resolution TEM images were observed for a seriesof Au/TiO2 catalysts (Fig. 1). The average Au particle size
and the standard deviation are listed in Table 1. The average Au particle size was in the range 29–37 Å supported onanatase-type TiO2(c–g) versus relatively greater particles
size in the range 78–87 Å supported on mesoporous
TiO2(a,b). The Au particle size distribution was especiallynarrow (standard deviation 10–19 Å) in the cases of d–g
prepared using urea. Observed Au particle sizes (Table 1)
were basically consistent with values reported in Refs.
[10,11].
The degree of Au loading was reﬂected as the population of spherical darker dots (Au) dispersed on amorphous TiO2 (a,b) or anatase-type TiO2 crystalline (c–g)(Fig. 1). The population of Au dots was smaller when
NaOH was used during the DP reaction (a–c). The population of Au dots was greater when urea was used (d–g)
and the population seems proportional to the initial concentration of HAuCl4 Æ 4H2O:4.2 mM for Fig. 1d and e
and 0.51 mM for Fig. 1f and g. The Au loading amountevaluated based on the Au La1 emission and Au L3 edge
jump intensity (Table 1) was in accord with the trend of
Au nano-particles population detectable in the TEM
images.

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Y. Izumi et al. / Inorganica Chimica Acta 361 (2008) 1149–1156

Fig. 1. High-resolution TEM images for Au/TiO2 catalysts. The images (a–g) correspond to catalyst samples listed in Table 1, entries a–g, respectively.
Table 2The Au La1 emission peak and Au L3 absorption edge positions for Au standard compounds and Au/TiO2 catalysts in air or 5% CO by tuning thesecondary ﬂuorescence spectrometer to each energy
SampleAu metal
K+[Au(CN)2]
AuCl[P(C6H5)3]
Au/mesoporous–TiO2–NaOH-7-523

Au/mesoporous–TiO2–NaOH-4.5-673
Au/P25–NaOH-7-673
Au/P25–urea-16-573 (0.93 wt% Au)aPurged gas

Au La1 peak energy (eV)Tune energy (eV)Au L3 edge energy (eV)9713.3
9713.1

9713.3
9713.1air

9713.3

air
air
air

9713.4
9713.09713.1

CO (5%) + Ar (95%)

9713.49707.6
9713.09718.7
9713.29713.09707.89713.1
9718.39708.1
9713.49718.611921.2
11923.5
11923.0a
11917.1
11920.8
11924.5
11921.1
11921.1
11916.5
11921.1
11927.911916.4
11921.1
11925.0

Measured in transmission mode.

3.3. X-ray spectroscopy for Au/TiO2 catalysts
The Au La1 emission spectra for Au/TiO2 catalysts are
depicted in Fig. 3. The peak top energy (9713.3–
9713.4 eV) for Au/mesoporous–TiO2–NaOH-7-523 andAu/mesoporous–TiO2–NaOH-4.5-673 in air and Au/P25–
urea-16-573 (0.93 wt%) in 5% CO was essentially the same

as for Au metal (Table 2). The peak shifted to 9713.0–
9713.1 eV for Au/P25–NaOH-7-673 and Au/P25–urea-16573 both in air. The energy corresponds to the value for
K+[AuI(CN)2] (Table 2).
Next, the Au La1 emission spectra were measured for
Au/mesoporous–TiO2–NaOH-7-523 in air by progressively
changing the excitation energy (Eexc) on going from

Y. Izumi et al. / Inorganica Chimica Acta 361 (2008) 1149–1156

1153

Fig. 2. Au L3-edge XANES spectra measured by tuning the ﬂuorescence
spectrometer to Au La1 peak top (solid line, 1) and in transmission mode
(dotted line, 2) for Au metal (a) and K+[AuI(CN)2] (b).

11943.1 eV to 11919.1 eV [3]. In the Eexc range between
11943.1 and 11922.0 eV, the emission peak energy andintensity negligibly changed. When the Eexc value decreased
to 11919.1 eV, the Au La1 emission peak shifted to9711.0 eV and the intensity decreased to 66% [3].
The ﬂuorescence spectrometer was tuned to each peaktop (9713.0–9713.4 eV, the arrows in Fig. 3) and Au La1selecting Au L3-edge XANES spectra were measured for
Au/TiO2 catalysts listed in Table 1a–c and g. The absorption edge was at 11920.8–11921.1 eV (Table 2) similar tothe value for Au metal (11921.2 eV). The sharp, resolved
XANES pattern for these Au/TiO2 catalysts (Fig. 4a-(2),b-(2), and c-(2)) was similar to Au La1-selecting XANES
for Au metal (Fig. 2a-(1)).
Then, tune energy was set lower than Au La1 peak top,
corresponding to peak intensity of 1/3 (9707.6–9708.1 eV,
the arrows in Fig. 3) for Au/mesoporous–TiO2–NaOH-7523 in air and Au/P25-urea-16-573 (0.93 wt% Au) in air
and in 5% CO. The Au L3 edge shift compared to the data
tuned to Au La1 peak top was 3.7 eV for Au/mesoporous–TiO2NaOH-7-523, smaller than shifts for Au/P25–
urea-16-573 in air and in 5% CO (4.6 to 4.7 eV) (Table2). The ﬁrst intense peak at 11923.9–11924.2 eV above
absorption edge observed in Fig. 4a-(2), b-(2), and c-(2)became a weak shoulder feature at 11 922 eV (Fig. 4a-(1),Fig. 3. Au La1 emission spectra measured for Au/mesoporous–TiO2–
NaOH-7-523 in air (a) and Au/P25–urea-16-573 (0.93 wt% Au) in air (b)and in 5% CO (c). The arrows are tune energy in the Au L3-edge XAFS
measurements in Fig. 4.

b-(1), and c-(1)). The post-edge pattern looked to shift by3.7 to 4.7 eV compared to the spectra tuned to Au
La1 peak top.
Subsequently, the ﬂuorescence spectrometer was tuned
to the energy corresponding to peak intensity of 1/3, on
higher-energy side (9718.3–9718.7 eV, the arrows in
Fig. 3). The Au L3 edge shift compared to the data tuned
to Au La1 peak top was +6.8 eV for Au/P25–urea-16573 in air, greater than shifts for Au/mesoporous–TiO2–
NaOH-7-523 in air and Au/P25–urea-16-573 in 5% CO(+3.7 to +3.9 eV) (Table 2). The intensity of ﬁrst intense
peak at 11930.0–11931.2 eV above absorption edge wasin the order for spectra of Au/P25–urea-16-573 in air

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Y. Izumi et al. / Inorganica Chimica Acta 361 (2008) 1149–1156(Fig. 4b-(3)) > Au/P25–urea-16-573 in 5% CO (Fig. 4c-(3))>Au/mesoporous–TiO2–NaOH-7-523 in air (Fig. 4a-(3)).
This resonance peak was reported to be more intense as
the Au 5d states were less occupied [20]. The post-edge pattern looked to shift by +3.7 to +6.8 eV compared to thespectra tuned to Au La1 peak top.
Au La1-detecting EXAFS (extended X-ray absorption
ﬁne structure) spectra for Au/mesoporous–TiO2–NaOH7-523 (0.93 wt%) tuned to 9707.6 and 9718.7 eV (Fig. 3a,arrows) were very similar to each other in the wave-number
region 2–9 Å1 in contrast to the typical diﬀerence of statesensitive spectra in the XANES region (Fig. 4a). A peak
due to Au–Au bonding appeared at 2.3 Å (phase shiftuncorrected) in the Fourier transform of EXAFS for Au/
mesoporous–TiO2–NaOH-7-523 tuned to 9707.6 and9718.7 eV and Au/P25–urea-16-573 (0.93 wt% Au) in air
tuned to 9713.1 eV (Fig. 3b, arrow), however, a peak due
to Au–O bonding [21,22] was unclear/unresolved in the
Fourier transform for all the EXAFS spectra.
4. Discussion
State-sensitive Au L3-edge XANES spectra tuned to Au
La1 peak (9713.0–9713.4 eV) for Au/TiO2 catalysts(Fig. 4a-(2), b-(2), and c-(2)) resembled Au La1-selecting
XANES for Au metal (Fig. 2a-(1)). The Au0 state among
Au/TiO2 catalysts was discriminated with this range oftune energy.
When the ﬂuorescence spectrometer was tuned to9707.6–9708.1 eV, the ﬁrst intense peak above the edge
became a weak shoulder at 11 922 eV (Fig. 4a-(1), b-(1),and c-(1)) suggesting greater Au 5d occupation [20,21].
Two types of Au cluster models were considered locatedeither on Ti or on O atom of anatase-type TiO2(0 0 1) surface (Fig. 5a and b, respectively). The spectral pattern,
especially in the absorption edge region, was similar to
the theoretical XANES generated by FEFF for O-top
interfacial Au sites (Figs. 4d-(6) and 5b). The Au L3-edge
XANES spectra tuned to 9710.4–9710.6 eV for Au/meso-

Fig. 4. Au La1-selecting XANES spectra measured for Au/mesoporous–
TiO2–NaOH-7-523 in air (a) and Au/P25–urea-16-573 (0.93 wt% Au) inair (b) and in 5% CO (c). Tune energy was 9707.6, 9713.0, and 9718.7 eV(a), 9707.8, 9713.1, and 9718.3 eV (b), and 9708.1, 9713.4, and 9718.6 eV(c). (d) Au L3-edge XANES spectra for gold metal (1), AuICl[P(C6H5)3](2), AuIII(OH)3 (3), AuIII
2 O3 (4), and interfacial Au sites located on Ti andO atoms (5 and 6, respectively). 1–4: Experimental, measured intransmission mode. 5, 6: generated for model sites in Fig. 5a (Ti-top)and b (O-top), respectively, using FEFF [16].

Fig. 5. Au cluster models on anatase-type TiO2(0 0 1) surface terminatedwith Ti atoms (Ti-top; a) and O atoms (O-top; b).

Y. Izumi et al. / Inorganica Chimica Acta 361 (2008) 1149–1156

porous–TiO2–NaOH-7-523 in air [3] and Au/P25–urea-16573 (0.93 wt% Au) in 5% CO (not shown) resembled thetheoretical data of Fig. 4d-(6) even more. Thus, negative
charge transfer from interfacial O atoms to Au is suggested[23–25] at the interface of Au/TiO2 catalysts, successfully
discriminated by tuning the spectrometer to 9707.6–
9710.6 eV.
Among the Au La1-selecting XANES spectra tuned to9707.6–9708.1 eV, the Au L3 absorption edge shift compared to XANES spectra tuned to Au La1 peak top was
smaller for Au/mesoporous–TiO2–NaOH-7-523 in air(3.7 eV) versus 4.6 to 4.7 eV for Au/P25-urea-16-573
in air and in 5% CO (Table 2). This diﬀerence can be
explained due to the diﬀerence of average Au nano-particle
size: 87 and 29 Å, respectively. As the Au nano-particle size
increases, the eﬀect of charge transfer at the particle-support interface becomes relatively small per Au particle
and thus the population of Au0 state was greater. Instead,this diﬀerence may be explained based on the report that
smaller Au particles were relatively more negatively
charged [21].
First intense peak above the edge was observed at
11923.9–11924.2 eV by tuning the ﬂuorescence spectrometer to peak top and at 11930.0–11931.2 eV by tuning it to9718.3–9718.7 eV (Fig. 4). An intense peak appeared at
11925.7 eV for theoretically generated XANES for Ti-top
interfacial Au sites (Figs. 4d-(5) and 5a) [24]. Thus, negative charge transfer was suggested from Au 5d to support[22,26] for Au states discriminated by tuning to 9718.3–
9718.7 eV. The charge transferred from Au 5d to reduce
to Ti3+ states at the interface (Fig. 5a).
Among the Au La1-selecting XANES spectra tuned to9718.3–9718.7 eV, the Au L3 absorption edge shift compared to XANES spectra tuned to Au La1 peak top was
greater for Au/P25–urea-16-573 in air (+6.8 eV) versus+3.7 to +3.9 eV for the same catalyst in 5% CO and Au/
mesoporous–TiO2–NaOH-7-523 in air. This diﬀerence
demonstrates that the negative charge transfer from Au
to O2 is only detected for smaller Au particles (mean size
29 Å, Fig. 5a), not for Au/mesoporous–TiO2–NaOH-7523 (mean size 87 Å). The AuI species were detected byMössbauer and X-ray photoelectron spectroscopy [26]and suggested to be active for CO oxidation. Negative
charge transfer from Au to O2 was supported for Au nanoparticle model on rutile-type TiO2(1 1 0) by ultraviolet photoelectron spectroscopy [27] and density functional theory(DFT) calculations [28]. The eﬀect of O2 was suggested
based on the diﬀerence XANES spectrum measured intransmission mode before/after the introduction of O2 to
Au/TiO2 [20]. The eﬀect of O2 was directly demonstrated
by state-sensitive XANES in this work (Fig. 4b-(3)).
The Aud+ site to interact with O2 may be at the interface
based on the similarity of spectral pattern between Fig. 4b(3) (state-sensitive XANES) and D-5 (theoretical XANES
generated for interface Aud+ site model, Fig. 5a), as also
proposed based on on-reaction EXAFS measurements[29] and DFT calculations [30].

1155

The pattern of positively-shifted XANES spectra(Fig. 4b-(3)) due to the eﬀects of negative charge transferfrom Au to support and/or O2 was similar to the spectrum
for Au metal (Fig. 2A1) shifted by +6.7 eV rather thanones for K+[AuI(CN)2]AuI (Fig. 2b-(1)), AuICl[P(C6H5)3], AuIII (OH)3, or AuIII
2 O3 (Fig. 4d-(2)–(4)). Thus,
the Au states discriminated by tuning the spectrometer to9718.3 eV for Au/P25–urea-16-573 in air were for electron-deﬁcient Au nano-particle sites.
Acknowledgements
This work was supported from the Grant-in-Aid for Scientiﬁc Research (Y.I., C17550073) and the Research Foundation for Opto-Science and Technology (Y.I. 2005–2006).
The X-ray measurements were performed under theapprovals of the SPring-8 Program Review Committee(2006A1156 and 2006B1486) and Photon Factory Proposal
Review Committee (2005G220).
References[1] A. Cho, Science 299 (2003) 1684.
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