Materials Letters 61 (2007) 3833 – 3836
www.elsevier.com/locate/matlet

X-ray absorption fine structure combined with
X-ray fluorescence spectrometry
Part 18. Tin site structure of Pt–Sn catalyst
Yasuo Izumi a,⁎, Dilshad Masih a , Eric Roisin b , Jean-Pierre Candy b ,Hajime Tanida c , Tomoya Uruga caInterdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, Japanb
Laboratoire de Chimie Organometallique de Surface, UMR CNRS-CPE 43 bd du 11 Novembre 1918, F-69616 Villeurbanne, FrancecJapan Synchrotron Radiation Research Institute, Kohto 1-1-1, Sayo 679-5198, Japan
Received 13 May 2006; accepted 19 December 2006
Available online 8 January 2007

Abstract
Sn Kα1-detecting Sn K-edge XANES spectrum was successfully measured for Pt–Sn/SiO2 catalyst (2.5 wt.% Pt, Sn/Pt atomic ratio 1.0) in theenergy resolution 5.0 eV using Rowland-type fluorescence spectrometer equipped with Ge(13,13,13) bent crystal. The steeper, more clearly
resolved XANES spectrum thus obtained could be compared quantitatively to theoretical XANES spectra generated by ab initio calculations for
thirteen plausible Pt–Sn site models (alloy, adsorbed Sn, and mixture models). This spectral simulation suggested that fluorescence-detecting
XANES is a simple method to judge the plausible site structure without detailed spectral analyses. The comparison supported an adsorbed Sn sitemodel structure on highly dispersed Pt[–Sn] nanoparticles on SiO2.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Catalysts; Characterization methods; Nanocomposites; X-ray techniques; XANES

1. Introduction

2. Experimental

Pt–Sn binary catalysts have been widely investigated forselective hydrogenation of unsaturated aldehydes [1–3], dehydrogenation of alkanes [4–7], and reforming [5,8]. The presenceof tin favors the selective hydrogenation of carbonyl group, theformation of unsaturated olefins rather than smaller alkanes, andsuppresses coke formation. In this work, the feasibility of the tinsite structure study is demonstrated by Sn K-edge XANES (Xray absorption near-edge structure) combined with X-ray
fluorescence spectrometry for Pt–Sn binary catalyst. The corehole lifetime broadening is serious at higher-energy absorption
edge XANES [9]. Because obtained high energy-resolution SnKα1-detecting Sn K-edge XANES spectrum was sharper (moreresolved), it could be quantitatively compared to theoretical
XANES generated by ab initio calculations.

An aqueous slurry of PtII(NH3)4(OH)2 was mixed with silica(Degussa, 200 m2 g− 1) [4]. After filtration, obtained powder
was calcined and then in H2 at 673 K (Pt/SiO2). Sn(n-C4H9)4
was introduced to Pt/SiO2 in H2 (3.0 kPa) at 373 K (Pt–Sn/SiO2). Then, it was treated in H2 at 673 K and sealed in Pyrex
glass cell. The Pt loading was 2.5 wt.% and Sn/Pt atomic ratio
was 1.0. The amount of Sn supported was determined byelemental analysis and doubly checked by measuring unreactedSn(n-C4H9)4 by GC [4].
X-ray measurements were carried out at the undulator
beamline 10 XU and 37 XU in SPring-8. Storage ring energy
was 8.0 GeV and the current was 100–71 mA. Si(111) double
crystal monochromator was used. All the X-ray spectra weremeasured at 290 K. The Sn Kα1 fluorescence was analyzed by aRowland-type spectrometer [10] equipped with a Johanssontype Ge(13,13,13) crystal (Saint-Gobain, Rowland radius
220 mm) and NaI(Tl) scintillation counter. The Sn Kα1 emission⁎ Corresponding author. Tel./fax: +81 45 924 5569.
E-mail address: izumi.y.ac@m.titech.ac.jp (Y. Izumi).
0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2006.12.077

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Y. Izumi et al. / Materials Letters 61 (2007) 3833–3836

spectra were measured with the excitation energy at 29235.0 eV.
The spectrometer was tuned to the Sn Kα1 peak and Sn K-edge
XANES spectrum was measured. The undulator gap and theparallelism of the monochromator crystals were optimized tomaximize the X-ray beam flux at each data point.
Sn Kα1-selecting Sn K-edge XANES was compared to thattheoretically generated by FEFF8.2 in self-consistent field andfull multiple scattering calculation modes [11]. The Hedin–
Lundqvist self-energy was used for energy-dependent exchange-correlation potential. The calculated potential wascorrected by adding a constant shift to “pure imaginary opticalpotential”. Because the Sn Kα1-selecting XAFS was measured
under the condition that the energy resolution of fluorescence
spectrometer (5.0 eV) was smaller than the core-hole lifetime
width ΓK of Sn K level (8.49 eV [9]), the energy shift to “pureimaginary optical potential” was set to − 4.0 eV. The XANES
simulation with this method was already successful for tin metal[10]. The goodness of simulation was evaluated based on Rfactor (Rf = ∫|χobs(k) − χcalc(k)|2dk / ∫|χobs(k)|2dk). A code XDAP
2.1.2 was used for the EXAFS analyses [12]. The phase-shift
and amplitude parameters were extracted from EXAFS spectrum of SnO2 powder for Sn–O bond (2.053 Å) and theoretically generated using FEFF8.2 [11] for Sn–Pt bond.
3. Results and discussion
In the Sn Kα1-selecting Sn K-edge XANES spectrum (Fig. 1exp-f),
the first intense peak at 29206.3 eV (Table 1) became sharper andabsorption edge at 29200.3 eV became steeper compared tocorresponding XANES measured in transmission mode (exp-t).
When the high-energy resolution spectrum exp-f was convoluted

Fig. 1. Normalized Sn K-edge XANES spectra for Pt–Sn/SiO2 catalyst by theselective detection of Sn Kα1 (exp-f) or in transmission mode (exp-t). The data
exp-f was convoluted to be exp-f-c using a Lorentzian of 8.49 eV (width). Sn Kedge XANES spectra generated by FEFF for models A–I (spectra A–I,respectively). Dotted spectra D′ and H′ are for Sn site in the second layer inmodels D and H, respectively. Data J is the average theoretical spectrum for Snsite model inside the Pt bulk (33%) and of SnIIO (67%).

Table 1
Energy values of absorption edge and intense peak above the edge forexperimental XANES for Pt–Sn/SiO2 catalyst and theoretical XANES
generated for various models
Energy (eV)Absorption
edge

Intense peak(A) Experimental
Kα1-selecting
Transmission mode

29,200.3
29,201.1

29,206.3
29,209.8(B) Theoretical
Model A
Model B
Model C
Model D (surface)(second layer)
Model E
Model F
Model G
Model H (surface) a(second layer)
Model I a
Model J29,200.6
29,200.7
29,200.9
29,200.7
29,200.8
29,200.7
29,200.2
29,200.2
29,200.3
29,200.7
29,200.2
29,199.929,208.3
29,207.5
29,206.1
29,206.7
29,207.4
29,207.1
29,208.1 (29,203.5)
29,206.7 (29,203.3)
29,205.3
29,207.0
29,206.1
29,204.4a

Rf compared toexp-f spectrum (%)

2.71
2.78
2.63
2.44
2.60
2.33
2.20
1.86
1.80
2.91
1.27
1.47

Reasonable models for Sn site for Pt–Sn/SiO2 catalyst.

using a Lorentzian of ΓK for Sn, transformed data (Fig. 1exp-f-c)
resembled spectrum exp-t. The Sn Kα1-selecting XANES became
deconvoluted data of conventional XANES. The obtained Sn–O
(2.108 ± 0.007 Å) and Sn–Pt distances (2.713 ± 0.006 Å) for Sn K-edge
EXAFS were within the range reported: 2.053–2.224 Å [13] and 2.63–
2.80 Å [4,14–16], respectively. Following XANES simulation wasperformed within this range of bond distances.
Sn Kα1-selecting XANES was simulated based on models A–I(Fig. 2). Alloy Pt–Sn models A–E were based on face-centered cubicpacked Pt cluster within 7.0 Å from the central X-ray absorbing Sn
atom. The (111) face exposure was assumed. One third of surface Pt
atoms were replaced with Sn atoms in model B. The calculated Sn Kedge XANES spectra (Fig. 1A and B) resembled each other. The
substitution of neighboring Pt with Sn negligibly affected theXANES. In spectrum C for model C, XANES profile was similar
to those for spectra A and B, however, whole the pattern shifted
toward the lower energy side. The intense peak above the absorption
edge and next broad peak shifted by −1.4 to − 2.2 and − 5 to − 6 eV,respectively.
One third of Pt atoms were replaced with Sn atoms both at surface
and in bulk for model D. The spectrum D (solid line) calculated for thesurface Sn site was the intermediate on going from spectrum A/B to C
based on the energy values of absorption edge and post-edge peaks andtheir relative intensity. Spectrum D′ (dotted line) for the Sn site in thesecond layer resembled D (solid line) for surface Sn site. The post-edge
peak at 29231 eV either for surface or bulk Sn site in model D was notsimilar to exp-f.
Three-fold oxygen atoms (three atoms) were bonded to the central
Sn atom in model E (Fig. 2). Common broad peak for spectra A, B, C,D, D′, and E at 29,228–29,234 eV did not resemble that of exp-f(Rf = 2.33–2.78%, Table 1). Alloy models A–E were rejected as the Snsite model in Pt–Sn/SiO2 catalyst.
For isolated adsorbed Sn model F, the intense peak above theabsorption edge split into two and was broader (Fig. 1F) compared toexp-f. One third of surface was adsorbed with Sn atoms in adsorbed Snsite model G. Generated spectrum (Fig. 1G) was essentially identical

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Model I can be acceptable when surface density of adsorbed Sn was
greater and core Pt particle size was small enough to enable
experimental atomic ratio Sn/Pt 1.0, e.g. adsorbed Sn model on thePt13 core (Fig. 2, bottom). The structure I was consistent with EXAFS
curve fit results (N(Sn–O) of 3.0 ± 0.5 and N(Sn–Pt) of 4.0 ± 1.0).
Because theoretical Sn K-edge XANES profile was insensitive to thepresence of oxygen atoms adsorbed (Fig. 1B versus E; F versus I), thepresence of adsorbed oxygen(s) could not be judged in model H. Similaradsorbed Sn model was reported on small metal cluster inside zeolite
cage [6]. For Pt–Sn/SiO2 (1.4 wt.% Pt, Sn/Pt atomic ratio 1.0), average
metal particle size was 19 Å and 44% of surface Sn species and 56% ofalloy PtSnx species were reported [5]. The surface Sn species may
correspond to adsorbed Sn sites in model H or I.
Finally, mixed multiple phase model was considered. A central Sn
atom site inside Pt particle bulk [N(Sn–Pt) = 12] and Sn site in SnIIO[N(Sn–O) = 4] were mixed with the ratio 1:2 [resultant N(Sn–Pt) = 4and N(Sn–O) = 2.67]. In the obtained spectrum (Fig. 1J), the energy
values of absorption edge, the first strong peak, and a post-edge
peak (29,232 eV) were inconsistent with exp-f (exp-f-c) (Table 1)(Rf = 1.47%). Thus, mixed phase model of alloy and Sn oxide sites wasinappropriate.
The whiteline peak energy in spectrum exp-f corresponded to that for
SnIIO. The absorption edge energy was relatively constant for models A–I(Table 1) comparable to 29,200.3 eV for Pt–Sn/SiO2 (Fig. 1exp-f). Thus,
partial electron transfer was suggested from Sn to Pt. The extent ofelectron transfer was greater when the Sn site was incorporated in alloy,
e.g. models A–E and bulk section of model H (29,200.6–29,200.9 eV)compared to those for adsorbed Sn sites in models F, G, H, and I(29,200.2–29,200.3 eV) [7].
Fig. 2. Sn local site models on/in Pt[–Sn] nanoparticles. Gray: Pt, green: Sn, andred: O atoms. Sn–Pt bond distance was assumed to be equal to Pt–Pt bonds(2.775 Å) in models A–E and for bulk section in model H. The Sn–Pt distance(surface) in models F–I was varied within 2.63–2.80 Å and typically set to2.713 Å. The Sn–O and Pt–O distance in models E and I was varied within
2.053–2.224 Å and typically set to 2.108 Å. In model I′, the Sn–Pt distance waselongated to 2.800 Å.

with F. The addition of adsorbed Sn on neighboring Pt negligiblyaffected the XANES pattern.
Further, one third of the bulk section in model G was substituted withSn sites (Fig. 2H). For the surface Sn site (Fig. 1H, solid line), theintense peak at 29,205.3 eV became stronger compared to that inspectrum F or G. The post-edge pattern beyond 29,220 eV wasrelatively flat. A weak peak appeared at 29,258 eV similar to the broad
peak for exp-f (29,055 eV). Theoretical spectrum for the Sn site in thesecond layer in model H (Fig. 1H′, dotted line) was more similar tospectrum A, B, D, D′, or E. Because experimental XANES spectrum isthe statistical average for all the Sn sites in sample, model H wasacceptable as the Sn catalyst site model if the contribution of adsorbed
Sn sites was predominant, i.e. ultimately high metal dispersion of Pt–Sn
nanoparticles, e.g. adsorbed Sn model on the Pt13 core (Fig. 2, bottom).
Three-fold oxygen atoms (three atoms) were added to model F(model I). Note that to simplify, adsorbed Sn site was only one inmodel I because the contribution of adsorbed Sn atoms on neighboring
Pt sites was negligible to XANES (Fig. 1F and G). Theoretical XANES
for the model I resembled most exp-f (exp-f-c) (Rf = 1.27%) based on
the intensity and broadness of whiteline peak and post-edge peak
(Table 1). When the Sn–Pt bond distance was elongated to 2.80Å
(model I′) from 2.713Å (model I), the calculated spectrum changed
negligibly (Fig. 1I′ and I).

4. Conclusions
Sn Kα1-selecting Sn K XANES spectrum was successfully
obtained at highly brilliant undulator beamline in SPring8 combined with fluorescence spectrometry, with the energy
resolution of 5.0 eV for Pt–Sn/SiO2 catalyst. The Sn content
was as small as 1.5 wt.%. The selective XANES was comparedto theoretical XANES for various Pt–Sn alloys, adsorbed Snsites, and mixture models. Adsorbed Sn site models on
ultimately dispersed Pt[–Sn] core particles were demonstrated
to be appropriate for the Pt–Sn/SiO2 catalyst.
Acknowledgements
X-ray experiments were performed under the approval ofSPring-8 Program Review Committee (2004A0122-NX-np,
2003B0386-NXa-np). This work was supported from theGrant-in-Aid for Scientific Research (17550073) and thePriority Area “Molecular Nano Dynamics” (432-17034013)
from the Ministry of Education, Culture, Sports, Science, andTechnology.
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