Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

10B/2005G221

Tin K-edge XAFS study of supported Ir-Sn/SiO2 bimetallic catalysts for selective
propane dehydrogenation
Yasuo Izumi,1,* Dilshad Masih,1 Laura Sordelli,2 Matteo Guidotti,2 and Rinaldo Psaro2
1

Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, Japan
2
CNR-ISTM, Dipartimento CIMA, Via Venezian 21, 20133 Milan, Italy

Introduction
Multi-component catalysts have found widespread
usage because the combination of two or more metals
lead to increase activity, extend catalyst life, and improve
the selectivity. In this work, selective dehydrogenation of2–propane found for [Ir4(CO)10(SnCl3)2] (Scheme 1)
supported on amorphous SiO2 or MCM-41 wasinterpreted based on the Sn K-edge XANES incomparison with unselective impregnated Ir/SiO2 andinactive Ir+Sn/SiO2.
Experimental section+
CO[N(C2H5)4] 2[Ir4(CO)10(SnCl3OC2–CO)2 ]orIr+–[N(C2H5)4] [Ir6(CO)15(SnCl3)]
COOCCO(Scheme 2) crystal wasOCIrIr
impregnated on amorphousSnCl3SiO2 (specific surface area 290 Cl3SnIr
CO2 –12 –m g ) or MCM-41 (1024 m gCO1CO)([Ir4Sn2]/oxide,Scheme 1[Ir6Sn]/oxide). For comparison,
IrCl3 was impregnated on theOC OC COamorphous SiO2 followed byCO SnCl3Ir
the reaction with Sn(n-C4H9)4 OCCOCOIr
Ir(Ir+Sn/SiO2). The Ir content OCIr
IrOCCO
was fixed to 1 wt% in all theOCIr
COcatalysts and Ir/Sn ratio wasOCCOOCvaried between 1 and 6. TheseScheme 2
incipient samples were reducedin H2 at 773 K. Sn K-edge XAFS spectra were measured
at 290 K at beamline 10B in fluorescence detection mode
[1] using Lytle detector.
Results and discussion
The Sn K-edge XANES data for Ir+Sn/SiO2 aredepicted in Figure 1f – h. As the Sn amount in catalyst
decreased, the absorption edge progressively shifted
toward higher energy side. This trend implies that Sn was
effective electron donor to Ir and the donation extent per
a Sn atom was greater as Sn amount became relatively
smaller. When the Ir/Sn ratio was 6, the Sn state was 4+(Figure 1h, c).
The XANES spectra for [Ir4Sn2]/MCM-41 and[Ir6Sn]/MCM-41 were basically similar to those for
Ir+Sn/SiO2 (Ir/Sn = 2 and 6), respectively (Figure 1e, i, g,and h). The extent of edge energy positive shift wasslightly smaller (∆ = 0.2 – 0.3 eV) for supported

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Figure 1. Sn K-edge XANES spectra for Sn metal (a),SnIIO (b), SnIVO2 (c), [N(C2H5)4]+2[Ir4(CO)10(SnCl3)2]2–(d), [Ir4Sn2]/MCM-41 (e), Ir+Sn/SiO2 (f – h) (Ir/Sn = 1,2, and 6, respectively), [Ir6Sn]/MCM-41 (i), and[Ir4Sn2]/SiO2 (j).
bimetallic cluster on MCM-41 compared for Ir+Sn/SiO2.
This may be due to the difference of average metal
particle size: 15Å for supported bimetallic cluster samples
and 19Å for Ir+Sn/SiO2 samples (HR-TEM).
In the dehydrogenation reaction of propane for 18h in
101 kPa at 773 K on [Ir4Sn2]/MCM-41 and [Ir4Sn2]/SiO2
catalysts, the selectivity to propene reached as high as 88– 95% and the conversion decrease compared to Ir/SiO2
catalyst was acceptable (52 – 58%). The XANES
spectrum for [Ir4Sn2]/SiO2 (Figure 1j) even shifted more
toward higher energy (∆ = 0.25 eV) compared toIr+Sn/SiO2 (Ir/Sn = 6).
To obtain consistent trend of promoter Sn effect on theIr active site for selective dehydrogenation catalyst, theSn-Ir bond distance and the coordination number should
be analyzed in addition to Sn K-edge XANES above. Sn
K-edge EXAFS measurements are in progress at morebrilliant PF-AR beamline NW10A from 2006.
References[1] Y. Izumi, H. Nagamori, F. Kiyotaki, D. Masih, T. Minato, E.
Roisin, J.-P. Candy, H. Tanida, and T. Uruga, AnalyticalChemistry, 77(21), 6969 – 6975 (2005); ibid, 78(6), 2075 (2006).
*yizumi@chemenv.titech.ac.jp(contactpersoninJapan)