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Nanoparticles of Amorphous Ruthenium Sulfide Easily Obtainable froma TiO2-Supported Hexanuclear Cluster Complex [Ru6C(CO)16]2 :A Highly Active Catalyst for the Reduction of SO2 with H2Atsushi Ishiguro,[a] Takayuki Nakajima,[a] Tadahisa Iwata,[a] Masahiro Fujita,[a]Taketoshi Minato,[b] Fumitaka Kiyotaki,[b] Yasuo Izumi,*[b] Ken-ichi Aika,[b]Masaya Uchida,[c] Koji Kimoto,[c] Yoshio Matsui,[c] and Yasuo Wakatsuki*[a]Dedicated to Professor Horst Kisch on the occasion of his 60th birthday

Abstract: TiO2-supported rutheniummetal particles were derived from ananionic hexanuclear carbido carbonylcluster [Ru6C(CO)16]2 and comparedwith those prepared conventionally byimpregnation of TiO2 with a solution of
RuCl3 followed by reduction with H2 .
The average sizes of the metal particles
in both systems are similar, that is, 12 ä
for molecular cluster-derived particles
and 15 ä for those derived from the
RuCl3 precursor, although the size distribution is sharper in the former case.

These supported particles efficiently
promote the reduction of SO2 with H2
to give elemental sulfur. Their active
form is ruthenium sulfide as confirmed
by EXAFS and X-ray diffraction measurements. The nanoscale ruthenium sulfide particles, which originated from thecluster complex, have an amorphous
Keywords: amorphous materials ¥cluster compounds ¥ nanostructures¥ reduction ¥ ruthenium

Introduction
The structures and functions of very small metal particles,
which are synthesized with molecular cluster complexes asprecursors, have attracted considerable interest.[1] They often
show catalytic properties different from conventional sup-[a] Prof. Dr. Y. Wakatsuki, Dr. A. Ishiguro, Dr. T. Nakajima, Dr. T. Iwata,
Dr. M. Fujita
RIKEN (The Institute of Physical and Chemical Research)
Hirosawa 2-1, Wako-shi, Saitama 351-0198 (Japan)
Fax: ( 81) 48-462-4665
E-mail: waky@postman.riken.go.jp[b] Dr. Y. Izumi, T. Minato, F. Kiyotaki, Prof. Dr. K.-i. Aika
Interdisciplinary Graduate School of Science and Engineering
Tokyo Institute of Technology, 4259 Nagatsuta
Midori-ku, Yokohama 226-8502 (Japan)
Fax: ( 81) 45-924-5569
E-mail: yizumi@chemenv.titech.ac.jp[c] Dr. M. Uchida, Dr. K. Kimoto, Dr. Y. Matsui
Advanced Materials Laboratory
National Institute for Materials Science (AML-NIMS)
Tsukuba, Ibaraki, 304-0044 (Japan)
Fax: ( 81) 298-51-4976
E-mail: matsui.yoshio@nims.go.jp

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character and show activity even at low
temperature (463 K), whereas ruthenium sulfide formed from RuCl3-derived
metal dispersion is a pyrite-type RuS2
crystallite and needs a temperature
above 513 K to effect the same catalysis.
Amorphous ruthenium sulfide maintains its nano-sized scale ( 14 ä) regardless of the reaction temperature,
while RuS2 crystallite aggregates to form
larger nonuniform particles.

ported metals, and this has been attributed to, at least in part,the small size of the catalyst particles. However, most of thereactions examined with such nano-sized fine metal catalysts
involve the hydrogenation of olefins, hydroformylations,hydrogenolysis of alkanes, and in some limited cases thereformation of naphtha to produce aromatics. We wereoriginally interested if such a size effect could also be effectivein rather drastic reactions, such as SO2 reduction by H2 , since
the catalytic reduction of gaseous SO2 to elemental sulfur has
attracted much attention as a possible strategy to abate SO2from combustion processes.[2±5] In our studies along this line,
we previously reported that TiO2-supported ruthenium catalysts, which were prepared from two different precursors, thatis, the ruthenium carbido carbonyl cluster complex[(PPh3)2N]2[Ru6C(CO)16] (1) and the metal chloride RuCl3(2), showed the efficient reduction of SO2 with hydrogen to
give elemental sulfur at temperatures higher than 508 ±513 K.[6] One of the remarkable features of the Ru/TiO2
catalyst (Ru(A)), derived from the cluster complex 1, wasits quite high activity even at a lower temperature of 463 ±513 K, whereas TiO2-supported metal particles (Ru(B)),
made conventionally by the impregnation process from 2,
needed a reaction temperature higher than 513 K (Table 1).

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3260 ± 3268
Table 1. Reduction of SO2 with H2 to elemental sulfur over TiO2supported ruthenium catalysts carried out at various temperatures.[a]

443

463

Conversion [%]
T [K]
473
493
503

2.8
1.7

81.9
1.9

84.3
3.2

Precursor
1
2

85.1
5.1

86.1
9.8

513

523

90.1
87.6

87.1
86.8

[a] See ref. [6] for the reaction conditions.

The metal dispersion on TiO2 derived from the zero-valentmononuclear complex, that is, [Ru(cyclooctadiene)(cyclooctatriene)], as the precursor exhibited exactly the same
temperature dependency as in the case of Ru(B), andtherefore even if a trace amount of Cl ion remains on themetal surface from RuCl3 , it should have no effect in thepresent SO2 reducing reaction.[6] In this work, the TiO2supported particles derived from 1 and 2, before and after
catalysis at various temperatures, were characterized in detailto elucidate the origin of the difference in their activities.
Characterization based on transmission electron microscopy(TEM), Ru K-edge X-ray absorption fine structure (EXAFS)
spectroscopy, and X-ray diffraction (XRD) measurement
indeed showed that Ru(A) and Ru(B), as well as their sulfide
derivative, have significantly different characteristics in thepresent reaction involving sulfur. However, the main reason
for this difference was concluded to be not the size of thecatalyst particles, as initially expected, but rather theirmorphology. The present results suggest the existence of
unprecedented amorphous ruthenium sulfide as a nanoparticle dispersion and that it plays a significant role in catalytic
activity.

Results
The TiO2-supported cluster complex was prepared by impregnation of TiO2 with a solution of 1 in THF and transformed to metal dispersion Ru(A) by heating the incipient
sample at 573 K for 1.5 h under vacuum and then heating at
the same temperature for three hours under a stream of H2 .
By using the same procedure, precursor 2 was converted to
TiO2-supported metal dispersion Ru(B). The catalytic reduction of SO2 at T Kelvin was interrupted after two hours, andthe gray powders of [RuS(A)]T and [RuS(B)]T, which wereformed from Ru(A) and Ru(B), respectively, were subjected
to EXAFS and XRD spectra measurements, as well as theobservation of particle size by TEM.
Transmission electron microscopy: The TEM image of Ru(A)(Figure 1a) shows an exceptionally narrow size distribution of
metal particles dispersed on TiO2 , with an average size of
12  1.7 ä. The image of Ru(B) (Figure 1b) indicates that the
average particle size is 15  2.6 ä, and the particles are not as
uniform as in the case of Ru(A). Energy-dispersive X-ray(EDX) analysis was used to confirm that the dark spots
observed were actually images of Ru metal.
When Ru(A) was exposed to a gaseous mixture of SO2/H2(1/2 v/v) at 443 K for two hours, only a slight conversion(2.8 %) of SO2 to elemental sulfur was observed (Table 1).
However, the particle-size changed during that process: theresulting particles, denoted [RuS(A)]443 , became smaller(average size 8  2.6 ä), but still maintained a sharp size
distribution (Figure 2a). In contrast, upon treating Ru(B)under the same conditions, the resulting particle [RuS(B)]443
tended to be larger with an even wider distribution compared

Figure 1. TEM images and histograms of the TiO2-supported Ru particles. a) Ru(A); b) Ru(B).
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Figure 2. TEM images and histograms of the TiO2-supported particles obtained from the reaction with SO2/H2 at 443 K. a) [RuS(A)]443 ; b) [RuS(B)]443 .

to that of the parent Ru(B) (Figure 2b). This property of[RuS(B)] for aggregation is more pronounced at higher
temperatures, as shown in Figure 3b: [RuS(B)]473 paricles
have average sizes of 30  7.8 ä. Likewise, the avarage size for[RuS(B)]523 particles was observed to be 30  6.3 ä, which act
as a catalyst at 523 K (Table 1). The changes in particle sizeand distribution with the reaction temperature are much lessin the system with Ru(A); [RuS(A)]473 still has a narrow
distribution, with a particle size of 14  1.8 ä (Figure 3a),while [RuS(A)]523 has a mean particle size of 14  3.5 ä, assummarized in Table 2.
Ruthenium EXAFS spectra: Figure 4 displays the originalraw EXAFS data. Figures 5 and 6 show the normalized

Figure 3. TEM images and histograms of the TiO2-supported particles obtained from the reaction with SO2/H2 at 473 K. a) [RuS(A)]473 ; b) [RuS(B)]473 .

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Table 2. Particle size [ä] of Ru-nanoparticles measured from TEM
images.
Before reaction
Ru(A)
Ru(B)
[RuS(A)]T[RuS(B)]T

443

Reaction temperature [K]473523

12 1.7
15 2.6
8 2.616 4.6

14 1.830 7.8

14 2.930 6.5

oscillation c(k) and the associated k3-weighted Fourier transforms, respectively, for the supported Ru catalysts before (aand e) and after (b ± d and f, g) the catalytic run at eachreaction temperature. Figure 7 shows the results of respective
curve-fitting analysis.
The Fourier transform for the initial metal dispersion ofRu(A) exhibits a single strong peak (Figure 6a). By comparison with the EXAFS oscillation and its associated Fouriertransform spectra of authentic Ru metal powder, this peak
was confirmed to represent the nearest neighboring Ru Ru
bond. TiO2-supported Ru metal has been reported to lack the
Ru O bond.[7, 8] Furthermore, in the present case, the oneshell Ru Ru model gave a satisfactory fit, as shown in
Figure 7a, while two-shell (Ru O  Ru Ru) fitting did notgive reasonable parameters. The corresponding Fourier transform for Ru(B) (Figure 6e) again has a strong single peak due

Figure 4. Original Ru K-edge EXAFS data. a) Ru(A), b) [RuS(A)]443 , c) [RuS(A)]473 , d) [RuS(A)]523 , e) Ru(B), f) [RuS(B)]473 , and g) [RuS(B)]523 .

Figure 5. Ru K-edge EXAFS oscillations c(k) of a) Ru(A), b) [RuS(A)]443 , c) [RuS(A)]473 , d) [RuS(A)]523 , e) Ru(B), f) [RuS(B)]473 , and g) [RuS(B)]523 .
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Figure 6. Fourier transforms of k3-weighted EXAFS data of a) Ru(A), b) [RuS(A)]443 , c) [RuS(A)]473 , d) [RuS(A)]523 , e) Ru(B), f) [RuS(B)]473 , andg) [RuS(B)]523 .

Figure 7. Inverse Fourier transforms (lines) and calculated curve-fitting (dots) of a) Ru(A), b) [RuS(A)]443 , c) [RuS(A)]473 , d) [RuS(A)]523 , e) Ru(B),f) [RuS(B)]473 , and g) [RuS(B)]523 .

to the Ru Ru bond and agrees well with the EXAFS
spectrum reported in the literature for Ru metal supported
on TiO2 .[8] Structural parameters are summarized in Table 3.
Table 3. Structural parameters of Ru(A) and Ru(B) determined fromEXAFS data.

Ru(A)
Ru(B)

N

d [ä][a]Ru Ru
DE0 [eV]Ds [ä]

Rf [%]

7.0
9.2

2.67
2.66

4.9
9.8

0.023
0.037

1.7
1.8

[a] The Ru Ru distance determined by X-ray analysis of the metal is2.65 ä.

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The coordination number of the Ru Ru bond was 7.0 for
Ru(A) and 9.2 for Ru(B), which is consistent with the smaller
size of Ru(A) observed by TEM (Table 2).
The Fourier transform of EXAFS data for [RuS(A)]443 ,which is the species formed when Ru(A) is treated with a SO2/
H2 gas mixture at 443 K (Table 1), is shown in Figure 6b. The
spectrum is completely different from that of the parent
Ru(A). The direct Ru Ru bond is now very weak, while onestrong peak is seen for the Ru S bond, as assigned based on
the spectrum of authentic RuSx (x  2). The curve-fitting
analysis based on two shells (Ru S  Ru Ru) gave thereasonable parameters listed in Table 4, while the fitted curve¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002

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Table 4. Structural parameters of catalysts [RuS(A)] and [RuS(B)] determined from EXAFS data.
Catalyst[RuS(A)]443[RuS(A)]473[RuS(A)]523[RuS(B)]473[RuS(B)]523

Nd [ä][a]Ru S
DE0 [eV]Ds [ä]Nd [ä][a]Ru Ru
DE0 [eV]Ds [ä]

Rf [%]

5.3
3.8
6.5
6.0
6.2

2.31
2.33
2.33
2.31
2.31

0.1
8.1
4.6
1.6
0.3

0.017
0.054
0.014
0.020
0.015

0.4
1.6
±
±
±

2.72
2.77
±
±
±

-6.8
6.1
±
±
±

0.020
0.199
±
±
±

0.7
2.0
1.8
1.3
3.8

[a] The Ru S distance in pyrite RuS2 crystals is 2.351 ä.

is shown in Figure 7b. The NRu Ru value was only 0.4; the NRu S
was 5.3. The Ru S bond length of 2.31 ä is comparable to thecorresponding value of 2.351 ä found in the pyrite-type single
crystals of RuSx. At 473 K and 523 K, the cluster-derived
particles showed good catalytic activity (Table 1) and thecorresponding EXAFS data (Figures 5c, 6c; 5d, 6d) of the
supported particles [RuS(A)]473 and [RuS(A)]523 were essentially the same as those for [RuS(A)]443 . For the curve-fitting
shown in Figure 7d ([RuS(A)]523), a one-shell approximation
based on Ru S was good enough (Table 4).
The EXAFS data for [RuS(B)]473 and [RuS(B)]523 , obtained
from noncatalytic and catalytic runs, respectively (Table 1),
were similar to those of the corresponding [RuS(A)] analogues, as shown in Figures 5f ± 7f and 5g ± 7g. The Fouriertransformed data in the range of 1.5 ± 2.5 ä show a single
peak. These were fit (Figure 7f and g) with curves generatedusing one-shell (Ru S) parameters (Table 4).
All of these EXAFS data prove that the initially formed
TiO2-supported metal nanoparticles, Ru(A) and Ru(B), areconverted to metal sulfide species upon reacting with a SO2/
H2 gas mixture above 443 K, regardless of whether thetemperature is high enough to achieve active catalysis.
X-ray diffraction (XRD) analysis: Since the metal loading of
1.5 wt % used for TEM and EXAFS studies was too low for
XRD measurements, TiO2 was loaded with the metal in 15
wt %. Figure 8a shows the XRD of blank TiO2 . As expectedfrom the EXAFS data, both [RuS(B)]473 and [RuS(B)]523(Figure 8b) show two peaks at 2q  32 and 468, which areassignable to the [200] and [220] faces, respectively, of the
RuS2 pyrite phase. The RuS2 [311] face at 2q  548 isoverlapped by the strong peaks of TiO2 .
In contrast, both [RuS(A)]503 and [RuS(A)]523 showed thesame XRD pattern (Figure 8e), which had a very broad peakin the range 2q  30 ± 558, as is evident by comparison with the
XRD pattern of pure TiO2 . Annealing of the [RuS(A)]523
sample under Ar at 723 K for two hours (Figure 8d) andfurther at 773 K for two hours (Figure 8c) yielded a sample
with an XRD pattern that was essentially the diffraction of
pyrite-type RuS2 crystallite. To confirm further that the broad
XRD peak observed in Figure 8e is due to ruthenium sulfide
and not to impurities, a typical sulfidation method withconditions for metals was applied for Ru(A), that is, thereaction was performed at 573 K with H2S/H2 (1/9 v/v) inplace of SO2/H2 . The XRD pattern of the resulting ruthenium
sulfide had exactly the same broad peak as that in Figure 8e,
and annealing of this sample at 773 K gave the expected [200]and [220] peaks of the crystalline RuS2 . Similar sulfidation ofChem. Eur. J. 2002, 8, No. 14

Figure 8. XRD patterns of a) blank TiO2 sample, b) [RuS(B)]473 and[RuS(B)]523 , c) [RuS(A)]523 sample after annealed at 773 K, d) [RuS(A)]523
sample after annealed at 723 K, and e) [RuS(A)]503 and [RuS(A)]523 .

Ru(B) with H2S/H2 proceeded only above 673 K, and its
XRD pattern was very close to that shown in Figure 8b.
Based on these observations, the broad peak present in
Figure 8e is assigned to the amorphous phase of ruthenium
sulfide.
XRD peaks of the parent metal dispersions Ru(A) andRu(B) could be observed only in low 2q region due to low
intensities of the diffraction. XRD of Ru(B) showed, albeit
unresolved, overlapped peaks assignable to [002] and [101]faces of the hcp phase of the metal crystallite, whereas thecorresponding diffraction for Ru(A) is broader and the peak
top position is apparently different from those of theauthentic Ru metal (Figure 9).

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Figure 9. XRD peaks of the TiO2-supported Ru metal-particles: a) Ru(A),b) Ru(B). The marked peaks are those due to TiO2 back-ground.

Discussion
Molecular metal cluster complexes have been used as some of
the most effective sources for supported ultimately fine metal
particles.[1] With regard to hexaruthenium carbonyl clusters
with an interstitial carbide, a methyl derivative[Ru6C(CO)16(CH3)] has been used as a precursor to bringabout strong interaction with the supporting material (MgO,
Al2O3 , TiO2) and to maintain the [Ru6C]/support structure
framework after the organic ligands are removed undervacuum at 523 ± 623 K.[9±11] When the interaction between themetal and the support is not strong enough, aggregation of theinitially formed metal cluster particle may take place to someextent. In a series of studies based on the [PtRu5C(CO)16]
complex supported on carbon black, it has been shown that
reduction with H2 at 673 K yields bimetallic nanoparticles(average diameter of 16 ä) with a narrow size distribution
packed in an fcc structure rather than a conventional hcp
structure.[12±14] In the case of the present dispersed metal
Ru(A) derived from impregnated 1 in 1.5 wt %, it is evident
that some aggregation also occurred during treatment for
decarbonylation at 573 K. The TEM study indicates that the
average size of Ru(A) is 12 ä with a significantly narrow size
distribution, which roughly corresponds to the condensationof five [Ru6] metal units. By similar treatment of the metal
chloride precursor 2, dispersed metal Ru(B), with an average
diameter of 15 ä, is obtained (Figure 1). Although the size
distribution of Ru(A) is markedly sharper than that of Ru(B),it is difficult to explain the difference in the catalytic reactivity
for SO2 reduction (Table 1) by this difference in the size ofthese metal particles.
The Ru K-edge EXAFS spectra suggest that when a gas
mixture of SO2/H2 (1/2 v/v) is passed over these supported
ruthenium-metal particles at temperatures higher than 443 K,
the Ru Ru bond is almost completely lost, while ruthenium
sulfide becomes the predominant chemical component. Thus,
the active catalyst species in the reduction of SO2 with H2 isconfirmed to be ruthenium sulfide. Therefore, the ruthenium
sulfide [RuS(A)], which originates from cluster complex 1
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and is formed in the SO2 reducing reaction of metal Ru(A), ishighly active as a catalyst at a low temperature of 463 ± 503 K.
In contrast, the other ruthenium sulfide [RuS(B)], formed
from Ru(B), is inactive in the same temperature range(Table 1) and works only at higher temperatures.
XRD can provide information on structural differences
between the two ruthenium sulfides. Sulfide [RuS(B)] isevidently an ubiquitous pyrite-type crystallite of RuS2 supported on TiO2 , while [RuS(A)] is amorphous (Figure 8).
Successful curve-fitting analysis of the EXAFS data for[RuS(A)], with calculations based on crystalline RuS2 (Figure 7), suggests that the relative orientation of the Ru andnearest S atoms in the amorphous phase is similar to that in
the more common pyrite-type form (Table 4). Although there
have been a number of reports concerning Ru sulfide, only
two have reported the preparation of an amorphousform.[15, 16] The reported XRD of amorphous Ru sulfide
consists of two broad overlapping humps with 2q ranging fromapproximately 20 ± 558, although the peak-head positions areevidently different in the two reports. To the best of ourknowledge, [RuS(A)] obtained in the present study is the firstexample of a supported amorphous species of a ruthenium
sulfide dispersion and also the first case in which reactivity ofamorphous ruthenium sulfide was examined.
Comparison of the ruthenium sulfides prepared here by
TEM indicates that the sulfidation of Ru(A) during thereaction with SO ?/H2 causes partial fragmentation of themetal particles into smaller metal sulfide particles at low
temperature, that is, [RuS(A)]443 has an average size of 8 ä(Figure 2a). Although some re-aggregation takes place at
higher temperatures, the particle size remains rather small, forexample, [RuS(A)]473 and [RuS(A)]523 each measured  14 ä
(Table 2). Thus, it is very likely that amorphous ruthenium
sulfide [RuS(A)] has weak interaction between the RuS2
units, which does not allow greater aggregation. In the caseof Ru(B), the behavior toward sulfidation through thereaction with SO2/H2 is very different from that with Ru(A);at 443 K, the reaction induced partial aggregation rather thanfragmentation and resulted in a broad distribution of theparticle size of 20 ± 25 ä (Figure 2b). The tendency of
crystalline RuS2 for rather random aggregation is even more
pronounced at higher temperature, at which it gives ruthenium sulfide [RuS(B)] with the particle size scattered over awide range of 18 ± 47 ä (473 K) and 17 ± 33 ä (523 K), as
shown in Figure 3b.
The striking difference between the two ruthenium sulfide
catalysts, one originating from the [Ru6] cluster complex 1 andthe other from ruthenium salt 2, may be traced back to thedifferent morphologies of the initially formed metal particles
Ru(A) and Ru(B), which are intermediate precursors for thesulfide species. Aside from sulfidation by a gas mixture of
SO2/H2 (1/2 v/v) as discussed above, the more common
sulfidation process with a gas mixture of H2S/H2 (1/9 v/v)
also gave amorphous ruthenium sulfide from Ru(A) andpyrite-type crystallite from Ru(B), as observed by XRD
analysis (Figure 8). Furthermore, the fact that a higher
temperature is needed for the sulfidation of Ru(B) (673 K)
by H2S/H2 relative to the reaction with Ru(A) (573 K) is
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XRD observations displayed in Figure 9 suggest that TiO2supported Ru(A) and Ru(B) vary with regard to structure:
Ru(A) is much more reactive probably because it has
significant structural defects or more amorphous-like, compared to crystalline metal Ru(B), and furnishes nanoparticles
of amorphous metal sulfide on sulfidation.

Conclusion
By comparing the TiO2-supported metal particles Ru(A) andRu(B) and their sulfide derivatives [RuS(A)] and [RuS(B)],several features became clear.
1) Though the size distribution is sharper in Ru(A), theaverage particle size of the two forms is rather similar(Ru(A) 12 ä, Ru(B) 15 ä). However, they differ instructures; Ru(A) probably has significant structuraldefects, while Ru(B) is a conventional hcp crystallite.
2) Ru(A) readily undergoes sulfidation with a H2S/H2 gas
mixture at 573 K to give amorphous ruthenium sulfide,
while Ru(B) requires a temperature of 673 K and gives
ruthenium sulfide in the form of pyrite-phase crystallite.
3) Upon reaction with a gas mixture of SO2/H2 , both metal
species are converted to Ru sulfide. Ru sulfide derived
from Ru(A) consists of amorphous RuS2 , while Ru(B)gives the conventional RuS2 pyrate phase. The amorphousruthenium sulfide formed in situ from Ru(A) catalyzes the
reduction of SO2 to elemental sulfur by H2 at a much lower
temperature (463 K) than does the crystalline ruthenium
sulfide derived from Ru(B) (513 K).
4) Ruthenium sulfide crystallite [RuS(B)] aggregates to alarger and nonuniform dispersion at higher temperature.
In contrast, amorphous ruthenium sulfide [RuS(A)] maintains a small particle size under various temperatures forthe catalytic reaction. This probably is due to the much
weaker interunit interaction among RuS2 units in theamorphous phase.
5) Ru EXAFS analysis suggests that the relative orientationof Ru and S atoms in the RuS2 species of the amorphous
phase is similar in average to that in the crystalline pyrite
phase.
Ruthenium cluster complexes are thus good precursors foramorphous metal sulfide supported on TiO2 , which is
expected to have high activity under rather mild conditions.
Further studies on its reactivity in other catalytic reactions are
now in progress.

reaction, elemental sulfur remaining on the catalyst surface was flushedwith argon for 2 h at the reaction temperature. The supported catalyst
sample was then transferred under Ar to a glass cell with Kapton films on
both sides and subjected to EXAFS measurements. Sulfidation of the
supported metals Ru(A) and Ru(B) by H2S was carried out under amixture of H2S/H2 (1/9 v/v), with a flow rate of 44 cm3 min 1 at atmosphericpressure. In this manner, Ru(A) and Ru(B) underwent sulfidation at 573 K
and 673 K, respectively. After 3 h, the sample was cooled to room
temperature and flushed with nitrogen for 1 h.
EXAFS measurements and analysis: Ru K-edge EXAFS measurements
were performed at room temperature at beamline 10B of the Photon
Factory at the High Energy Accelerator Research Organization (KEK-PF)
in Tsukuba. The accumulation ring energy was 2.5 GeV and the ring current
was 450 ± 300 mA. The beamline 10B makes use of the channel-cut double
crystal monochrometer of Si(311). The incident and transmitted X-rays
were monitored by ion chambers filled with Ar for I0 and Kr for I. The
photon energy was calibrated by equalizing the derivative maximum of theabsorption edge of Ru powder at 22119.3 eV. The background was
subtracted by fitting the polynomial to the pre-edge region and extrapolating it to the postedge region. The edge jump of the subtracted spectrum was
normalized at 22131.8 eV.
The EXAFS data were analyzed by the EXAFSH program.[18] Backgroundsubtraction was performed by calculating the three-block cubic spline,
followed by normalization using the Victoreen parameter. Fourier transformation of the k3-weighted c function was carried out over the range
kmin  3 and kmax  17.0 ± 19.0 ä 1 according to the signal/noise (S/N) of each
spectrum. The window function (Hanning function) was multiplied by the kwidth of (kmax kmin)/20 on both ends of the FT k range. Inverse FT was
performed in the r range of rmin  1.3 ± 1.4 and rmax  3.9 ± 4.1 ä multipliedby the window Hanning function with width 0.1 ä on both ends. Curvefitting analysis was performed by using a method based on the formula ofthe plane wave single-scattering theory, with empirical phase shift andamplitude functions extracted from ruthenium metal powder for theRu Ru bond (metallic), from RuO2 for the Ru O bond, and from RuSx
(x  2) for the Ru S bond. The coordination number (N), bond length (d)and Debye ± Waller factor (s) were determined in the fitting procedure. In
Tables 3 and 4, Debye-Waller factors thus obtained are listed in the form of
Ds in ä, that is, with reference to the corresponding values for the standard
compounds (Ru metal, RuO2 , and RuS2).
Transmission electron microscopy: Microscopy studies were performed byusing a field emission Vacuum Generator JEM-2000EXII operated at
200 kV. The spatial resolution of the microscope was estimated to be  3 ä.
Specimens were prepared by dipping a copper-mesh-supported holey
carbon grid into an ethanol suspension of the sample.
X-ray diffraction measurement: The amount of Ru metal loaded on TiO2
was 15 wt % in XRD studies. XRD analysis of the catalyst samples([RuS(A)] and [RuS(B)]) was performed on a MAC Science M18XHF
X-ray diffractometer with rotating anode generators and a monochromatic
detector. CuKa radiation (l  1.542 ä) was used at 45 kV and 400 mA. For
crystal phase identification, the typical operation parameters were
divergence slit of 18, scattering slit 18, receiving slit 0.3 mm, and a scanning
rate of 28 min 1 with a 0.028 data interval. XRD patterns were collected in
the 2q domain ranging from 10 to 1008. XRD measurement of intermediate
metal particles (Ru(A) and Ru(B)) was carried out on a Rigaku RINT/
RAPID diffractometer with CuKa (l  1.542 ä) radiation at 40 kV/36 mA
with radiation time of 15 min.

Experimental Section
Sample Preparation: The anionic carbido carbonyl cluster complex,[(PPh3)2N]2[Ru6C(CO)16] (1), was prepared by the method described in
the literature.[17] Cluster 1 was supported on TiO2 (Japan Aerosil P25, mean
particle size 21 nm) by impregnation at 298 K for 1 h with dry THF solution
under an Ar atmosphere, and subsequent removal of THF in vacuo. The
amount of Ru metal loaded was fixed at 1.5 wt % unless stated otherwise.
Dried incipient catalysts were heated in vacuum for 1.5 h at 573 K and thenreduced for 3 h under a stream of hydrogen at 573 K to give Ru(A). In thesame manner, RuCl3 ¥ 3 H2O (2) on TiO2 was reduced to give Ru(B). The
catalytic reduction of SO2 was carried out in a gas flow system (flow rate
60 cm3 min 1) as described in a previous report.[6] After the catalytic
Chem. Eur. J. 2002, 8, No. 14

Acknowledgement
Finantial support of this work by Eco-molecular Science Program at
RIKEN is gartefully acknowledged. The authors appreciate the help of Dr.
M. Saigo for the XRD analysis on the dispersed metal particles.

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¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002

Received: December 27, 2001 [F 3764]

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Chem. Eur. J. 2002, 8, No. 14