State-Sensitive Monitoring of Active and Promoter Sites.
Applications to Au/Titania and Pt-Sn/Silica Catalysts by
XAFS Combined with X-Ray Fluorescence Spectrometry^
Yasuo Izumi^ Dilshad Masih,1^ Jean-Pierre Candy,* Hideaki Yoshitake,^1
Yasuko Terada,§ Hajime Tanida,§ and Tomoya Uruga§interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, * LCOMS-CPE, 43 bddu 11 Novembre 1918, 69616
Villeurbanne, France, f Graduate School of Engineering, Yokohama National University, Tokiwadai,
Hodogaya-ku, Yokohama 240-8501, Japan, and JASRI, Kohto 1-1-1, Sayo 679-5198, Japan
Abstract. State-sensitive XAFS was enabled combined with high-energy-resolution (AE = 0.3 eV@5.5 keV) X-ray
fluorescence spectrometry and applied to Au sites of Au/Ti02 and Sn promoter sites of Pt-Sn/Si02. Each state ofinterfacial Au sites located on Ti/O atoms and negatively/positively charged Aun clusters was discriminated.
Feasibility of more direct information of on-site catalysis via frontier orbital-sensitive XAFS was demonstrated.
Keywords: XAFS, fluorescence spectrometry, state sensitive, site selective, catalyst, gold, tin, frontier orbital
PACS: 61.10.Ht, 78.70.Dm, 78.70.En, 82.45Jn, 82.65.+r
catalyst (2.5 wt% Pt, Sn/Pt atomic ratio 1.0) was
prepared as in ref. 6.
X-ray measurements were performed at beamline
37XU of SPring-8. The storage ring energy was 8.0
GeV operated in top-up mode (ring current 99 - 97mA). A S i ( l l l ) monochromator and Rh-coatedmirror were used. The combination of X-ray beam
position monitoring [7] and monochromatorstabilization [8] minimized the deviation of beam
position at the sample. Sn and V data were measured
at beamline 10XU [6] and KEK-PF 9C, respectively.
Ge(555), (13,13,13), and (422) reflections wereused for Au Lai, Sn Kai, and V K$52 detection,respectively. The slit opening in front of / 0 ion

INTRODUCTION
X-ray absorption fine structure is able to pick up
active/promoter sites of catalysts. During catalytic
processes, dynamic state transformation of thesesites is taking place. Therefore, state-sensitive
monitoring of XAFS gives direct information of thecatalytic role. In this work, XAFS combined withhigh-energy-resolution
X-ray
fluorescence
spectrometry [1,2] was applied for state-sensitive
measurements to detect heterogeneity of Au clusters(average particle size 9.2±1.4 nm, Figure 1) on Ti0 2 .
Catalytic importance of perimeter sites, step edge
sites, and extra electrons on Ti0 2 support wassuggested [3]. To enable narrower size distribution
and higher loading of Au clusters, mesoporous Ti0 2(specific SA 1200 m2g_1) was used. In addition,better-resolved, steeper XANES spectra wereobtained to remove the core-hole lifetime width (I)of Au L3 and Sn K levels to identify the Sn
occupation site on Pt/silica.

chamber was 1.0 mm(V)x0.5 mm(H). The sample
was set in a nearly horizontal plane. Both sample
and SC were covered with lead box of ~ 3 mm
thickness except for in the beam paths.

METHODS
The Au/Ti0 2 catalyst was prepared by mixing
HAuCl4*4H20 (1.3 mM, 25.0 ml) [4] withmesoporous Ti0 2 (0.50 g) [5] at constant pH 8
followed by calcination at 523 K. The Pt-Sn/Si0 2#FIGURE 1. HR FE-TEM images for Au/mesoporousTi0 2 (JEM-2010F; CAMA, Tokyo Inst. Tech.).

This paper is part 19 in the series.

CP882, X-ray Absorption Fine Structure—XAFS13
edited by B. Hedman and P. Pianetta© 2007 American Institute of Physics 978-0-7354-0384-0/07/$23.00
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X-ray spectra were measured at 290 K. Au Lai,
Sn Ka l 9 and V Kp 5 2 emission spectra weremeasured. Then, the fluorescence spectrometer wastuned and state-sensitive XAFS spectra weremeasured.
The undulator gap (SPring-8) wasoptimized to maximize the beam flux at each data
point.

0.00200.00006
0.0015_o 0.00004

0.0010 £0.000020.0005RESULTS
Au State-Sensitive Measurements for
Au/Mesoporous-Ti02 Catalysts
Au La! emission spectra were measured for
Au/mesporous-Ti0 2 (Figure 2a). When theexcitation energy (Eexc) w a s between 11922.0 and11948.6 eV, the peak energy and intensity changednegligibly (9713.0 - 9713.3 eV, (3) - (1)). When theEexc value decreased to 11919.1 eV, the peak shifted
downward to 9711.0 eV and the intensity decreased
to 66% (4).
Corresponding to the emission peaks in (1) - (3),state-sensitive Au L3-edge absorption spectra weremeasured (Figure 2b). When tuned to 9713.0 eV(emission peak top), the obtained spectrum (3, dots)
was similar to the one measured for Au metal (3,line). The Au La r detecting XANES was sharper
because the energy resolution (3.3 eV) was smaller
than the l v a l u e for Au L3 (5.41 eV) [6,9].
When the spectrometer was tuned to 9707.6 eV(A = -5.4 eV), the obtained spectrum 1 shifted by 3.5 eV from spectrum 3. This negative shift of
XANES corresponded to the emission peak shift to9711.0 eV (Figure 2a-4). When the spectrometer
was tuned to 9719.7 eV (A = +6.7 eV), the obtained
spectrum 4 shifted by +4.4 eV from spectrum 3.
Alternatively, spectrum 4 was similar to that forAutKPPhg) (Figure 3b).
The peak intensity just above the absorption edge
varied depending on the fluorescence tune energy.
The peak at 11924.1 eV in Figure 2b-3 was mostintense among the four absorption spectra. Thisfeature corresponded to the theoretical XANES
spectrum generated by FEFF8.1 [10] for interfacial
Au atom sites located on Ti atoms (Figures 3c and4a). Charge may transfer from Au 5d to form Ti3+
states at the interface [11]. The peak was not
resolved from the absorption edge for interfacial Au
atom sites located on O atom (Figures 3d and 4b)[12-14], similar to Figure 2b-1 or 2b-2.
In summary, interfacial Au sites located on Ti(Figure 2b-3) and on O (2b-2) and Au~1+„ [13] andAux"w clusters [3] on amorphous Ti0 2 (Figures 2b-4
and 2b-1, respectively) were extracted using statesensitive Au L3-edge XAFS.

COEo11900

12000
Energy (eV)FIGURE 2.
(a) Au Lo^ emission spectra for
Au/mesoporous-Ti02 with the excitation energy set to11948.6 (1), 11934.1 (2), 11922.0 (3), and 11919.1 eV (4).
The vertical opening of the receiving slit for spectrum 1
was 16 times greater than in other cases, (b) Au L3-edge
XANES (dots) measured tuning the fluorescence
spectrometer to 9707.6 (1), 9710.4 (2), 9713.0 (3), and9719.7 eV (4). Reference spectrum for Au metal was alsoplotted as line (3).

11900

12000
Energy (eV)FIGURE 3. Au L3-edge XANES spectra for Au metal (a),AuICl(PPh3) (b), and interfacial Au sites located on Ti andO atoms (c and d, respectively), a, b: experimental, intransmission mode, c, d: generated for model sites inFigure 4a and b using FEFF [10].
Au26(a)(b)

FIGURE 4. Au cluster models on anatase TiO2(001)surface terminated with Ti atoms (a) and O atoms (b).
589

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Sn Site Identification for Pt-Sn/Si0 2Catalyst
With the excitation energy set at 29235.0 eV, theSn Koci emission peak for Pt-Sn/Si0 2 appeared at25270.1 eV, shifted by -1.2 eV from that for Snmetal. The Sn Ka r detecting XANES spectrum wasobtained (Figure 5a) by tuning the fluorescence
spectrometer to 25270.1 eV.
An intense peak above the absorption edge
appeared at 29206.3 eV, between 29204.9 eV for
Sn n O and 29209.3 eV for Sn IV 0 2 .
Thecorresponding peak for Sn metal (29201.7 eV)shifted to 29208.3 eV in theoretical XANES
generated using FEFF8.1 [9] for a Sn atom model
buried in Pt particles, suggesting electron transferfrom Sn to neighboring Pt atom(s). Among several
Pt-Sn site uniform/mixture models [15], a Pt-Sn
alloy model illustrated in Figure 5b and a mixturesite model of Sn metal and Sn0 2 (c) were found tobe acceptable. Sn K-edge EXAFS analysis indicatedthe presence of Sn-metal bonds at 2.71 A [6]significantly shorter than a typical Sn-Sn bondlength (2.81 A). Thus, the dispersed alloy
nanoparticle model was concluded to be mostplausible, rather than a metallic/oxidic Sn site for thePt-Sn/Si0 2 catalyst.
2r-f 29206.3 eV(a)1.5

absorption edge structure resembled conventional
spectrum for VIV. In this way, V 3d frontier orbitalsensitive insights of V+Ti0 2 catalysts will be given
under on-site conditions of catalytic ethanol
dehydration under visible light [16].
i VIV site, Kfe-tuned>.
+->-w
cV H+*c 1•-o<D.N/lllr\lInly HIHIUIJUMn J/ T V IHJ'^U ^ffltitiUilil 1 llwii•fj-oiEoi™ up i|pwri' -//0•f l i J transmission data
\ j j r \ f forVlvO(S04)vTH20I•w

5460

5480

5500

5520

Energy (eV)FIGURE 6. V K-edge XANES tuned to 5459.0 eV for aphysical mixture of VIV0(S04)*wH20 and Na3Vv04 (1:1)(Red/Gray) and for VIV0(S04)*wH20 measured intransmission mode (Blue/Black).

ACKNOWLEDGMENTS
This work was supported from the Grant-in-Aidfor Scientific Research (C17550073) and the Priority
Area "Molecular Nano Dynamics" (432-17034013)
from MEXT. X-ray experiments were performed
under the approval of the SPring-8 Program Review
Committee(2006A1156,2004A0122, and2003B0386) and that of PF Proposal Review
Committee (2005P014).

REFERENCES"° 1"TOo 0.5(c)/029150(b)

Pt-Sn alloymodel structure

29200
29250
Energy (eV)29300

FIGURE 5. State-sensitive Sn K-edge XANES for PtSn/Si02 catalyst (a). Spectrum generated using FEFF for
the Pt-Sn alloy model (inset) (b). Average spectrum for Sn
metal and Sn02 (ratio 1:1) (c). Spectrum (c) was broader
because the spectrum for Sn02 was measured intransmission mode.

FUTURE PROSPECT
A more direct approach to study catalytic sites
than state-sensitive studies is frontier orbitalsensitive XAFS by the discrimination of fluorescencelines due to the electron transition from a frontier
orbital to a core hole.
The feasibility wasdemonstrated for the physical mixture of VIV and V v
compounds (Figure 6). Tuned to the energy of K(35for the VIV site (5459.0 eV), the VIV state wasextracted and the ls-> 3d pre-edge peak at 5468.2 eV
was enhanced due to resonance excitation effects.
The density of states near the Fermi level seems toconsist significantly of V 3d because the V K

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