9022

J. Phys. Chem. B 2003, 107, 9022-9028

Nitric Oxide Reduction by Carbon Monoxide over Supported Hexaruthenium Cluster
Catalysts. 1. The Active Site Structure That Depends on Supporting Metal Oxide andCatalytic Reaction Conditions
Taketoshi Minato,† Yasuo Izumi,*,† Ken-ichi Aika,† Atsushi Ishiguro,‡
Takayuki Nakajima,‡ and Yasuo Wakatsuki*,‡Department of EnVironmental Chemistry and Engineering, Interdisciplinary Graduate School of Science andEngineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, and
RIKEN (The Institute of Physical and Chemical Research), 2-1, Hirosawa, Wako, Saitama 351-0198, Japan
ReceiVed: February 18, 2003; In Final Form: June 2, 2003

Ruthenium site structures supported on metal oxide surfaces were designed by reacting organometallic Rucluster [Ru6C(CO)16]2- or [Ru6(CO)18]2- with various metal oxides, TiO2, Al2O3, MgO, and SiO2. The surface
Ru site structure, formed under various catalyst preparation and reaction conditions, was investigated by the
Ru K-edge extended X-ray absorption fine structure (EXAFS). Samples of [Ru6C(CO)16]2-/TiO2(anatase)and [Ru6C(CO)16]2-/TiO2(rutile) were found to retain the original Ru6C framework when heated in the presenceof NO (2.0 kPa) or NO (2.0 kPa) + CO (2.0 kPa) at 423 K, i.e., catalytic reaction conditions for NO
decomposition. At 523 K, the Ru-Ru bonds of the Ru6C framework were cleaved by the attack of NO. In
contrast, the Ru site became spontaneously dispersed over TiO2 (anatase). When being supported over TiO2
(mesoporous), MgO, or Al2O3, the Ru6C framework split into fragments in gaseous NO or NO + CO evenat 423 K. The Ru6 framework of [Ru6(CO)18]2- was found to break easily into smaller ensembles in thepresence of NO and/or CO at 423 K on support. Taking into consideration the realistic environments inwhich these catalysts will be used, we also examined the effect of water and oxygen. When water was introduced
to the sample [Ru6C(CO)16]2-/TiO2(anatase) at 423 K, it did not have any effects on the stabilized Ru6C
framework structure. In the presence of oxygen gas, however, the Ru hexanuclear structure decomposed into
isolated Ru cations bound to surface oxygen atoms of TiO2 (anatase).

Introduction
A variety of heterogeneous catalysts has been developed tocontribute to solve environmental problems.1 These environmental catalysts often consist of complex chemical contents andin many cases it is not clear which site is responsible for thecatalytic performance. Supported cluster catalysts are attractive
for its unique and well-defined metal ensemble sites stabilized
on surface.2 The specific catalysis for the synthesis of chemicals
has been reported over supported clusters [Ru6C], [Ru6N], and[Rh10Se] prepared from the crystal [PPN]2[Ru6C(CO)16] (1, PPN) (PPh3)2N), [PPN][Ru6N(CO)16], and [PPN]2[Rh10Se(CO)22]
on MgO, La2O3, SiO2, Al2O3, TiO2, etc.3-6 The relationship of
the precious metal site structure and its specific catalytic
performance was clarified. For example, supported [Ru6C]/TiO2
catalysts exhibited selective oxygenate (formaldehyde, methanol,and dimethyl ether) synthesis starting from CO + H2. The
hexanuclear ruthenium unit was retained during the catalytic
reaction and reversible cluster framework expansion/contraction
was observed by the introduction/evacuation of CO + H2.3,4
The carbido carbon inside stabilized the Ru6 framework frominside and played a role of the center of “chemical bond spring”.4
The supported [Rh10Se]/TiO2 catalysts showed specific highreactivity for the catalytic ethanol synthesis starting from CO2+ H2. The stabilization effects of the Rh10 unit by the interstitial* Corresponding authors. E-mail: yizumi@chemenv.titech.ac.jp andwaky@postman.riken.go.jp.
† Tokyo Institute of Technology.
‡ RIKEN.

Se atom were reported on the basis of the Se and Rh K-edge
XAFS (X-ray absorption fine structure) analyses.6
In general, catalytic decomposition of nitric oxide (NO) is a
structure sensitive reaction.2 It has been recognized that thecatalytic performance (conversion and selectivity) is strongly
influenced by the morphology (size and shape) of supported
metal. The adsorption and catalytic decomposition of NO overmetal cluster catalysts have been reported.7-9 Palladium cluster
catalysts in H-ZSM-5 exhibited high NO conversion (85.0%)
to N2 in the NO + CH4 reaction. The correlation between its
catalytic performance and the morphology of Pd species wasreported by XAFS analyses. Pd2+ ions on acidic sites werereported to be catalytically active in the presence of O2.7
Supported [Pt12(CO)24]2-/NaY and [Pt9(CO)18]2-/NaY catalysts
showed activity 11-14 times higher compared to the case of
conventional Pt/Al2O3 catalysts for the NO + CO reaction.8
An intact Pt12 or Pt9 cluster framework inside the zeolite cage
was suggested to be the responsible site for the catalysis.9
Although ruthenium is one of the most reactive novel metals(Co, Ru, Rh, Pd, Pt) for the NO decomposition, only a little
has been reported about the effects of Ru morphology on the
catalytic performance of NO decomposition. At the atmosphericgas pressure conditions of NO (0.5%) + CO (2.0%) at 423523 K, the conversion to N2 was faster on Ru/Al2O3 than in thecases on Rh/Al2O3, Pd/Al2O3, or Pt/Al2O3.10 Recently, thereactions between Ru carbonyl clusters in solutions and polluting
gases (NO, SO2) have been investigated as models of adsorption
and catalytic removal of such gaseous molecules on metal

10.1021/jp034412g CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/16/2003

[Ru6C] Structure for Catalytic NO Reduction
ensemble site.11-13 Ishiguro et al. recently reported the application of cluster 1 to sulfur dioxide removal by supporting on
TiO2.14,15 Crystals of [PPN][Ru6C(CO)15(NO)] (2) were isolated
from a reaction mixture obtained by bubbling NO gas througha CH2Cl2 solution of complex 1 at room temperature. On further
bubbling of NO gas, [PPN][Ru6C(CO)12(NO)3] (3) and Ru5C(CO)14(NO)(NO2) (4) were formed. These model reactions showthat NO gas can easily substitute a carbonyl ligand of the original
hexanuclear Ru cluster even at room temperature, but further
addition of NO leads to stepwise cleavage of the metal-metalbond.11 Above 423 K in NO, decomposition of cluster 1 into
an ill-defined insoluble precipitate has been noted.
In this paper, the structure of supported [Ru6C(CO)16]2clusters is reported to establish the in situ structural benchmarkof the application to catalytic reduction of NO. Appropriate
inorganic oxide support material was investigated to finely
control the surface Ru site structure to be most active for thecatalytic removal of NO. To control the Ru site structure andaccordingly the catalytic reactivity, major control factors were
the basicity/acidity of the support, the extent of saturation of
the surface metal site, and the specific surface area of thecatalyst.
Experimental Section
Catalysts Preparation. The cluster 1 crystal was preparedby the method described in ref 16. MgO (specific surface area
200 m2 g-1) was prepared from Mg(OH)2 (Wako, 99.99%) byheating at 773 K for 2 h in a vacuum. Al2O3 (Aerosil C) (100m2 g-1), anatase (major phase, >83%) TiO2 (Aerosil P25) (50m2 g-1), rutile (the purity of TiO2 > 99.8%) TiO2 (JRC-TIO-3,
given by the Catalysis Society of Japan) (51 m2 g-1), mesoporous TiO2 (1 131 m2 g-1, given by H. Yoshitake at Yokohama
National University), and SiO2 (Fuji-Silysia C-9709008) (273
m2 g-1) were heated at 673 K for 2 h in a vacuum except forthe case of mesoporous TiO2 at 473 K to avoid sintering. Threekinds of TiO2 are denoted as A-TiO2, R-TiO2, and M-TiO2,
respectively. The crystal phase component ratio of A-TiO2 andR-TiO2 showed negligible change upon reaction with cluster 1
in a vacuum and in NO and/or CO gas at 423 K, on the basisof the X-ray diffraction (XRD) observations. In the case of
M-TiO2, the XRD data remained unchanged; i.e., it was always
in the amorphous state in these reaction conditions. Cluster 1
was reacted with MgO, Al2O3, A-TiO2, R-TiO2, M-TiO2, andSiO2 at 290 K for 2 h in purified tetrahydrofuran (Wako, Special
Grade, no stabilizer), and THF was subsequently removed in a
vacuum. The loading of Ru was 1.5 wt % for all the samples.
The supported carbido Ru clusters are denoted as [Ru6C]/
support.
Noncarbido [PPN]2[Ru6(CO)18] (5) cluster crystal was prepared by the method described in ref 17. Cluster 5 was supported
on A-TiO2 in THF solution in a manner similar to the case ofcluster 1. This supported Ru cluster is denoted as [Ru6]/A-TiO2.
The Ru loading was 1.5 wt %.
All the process of cluster catalysts preparation and transfer
to reactor or sample cell was carried out in an argon (99.99%)
or helium (99.99%) atmosphere. The incipient supported clusters
were heated in 2.0 kPa of NO for 30 min at 423-623 K.
Alternatively, the supported cluster was heated in NO (2.0 kPa)
+ CO (2.0 kPa) + O2 (0.56 kPa) or in NO + CO + H2O (3.1
kPa) for 3 h at 423 K.
Conventional Ru catalyst was prepared by the impregnationof A-TiO2 from an aqueous solution of Ru(NO)(NO3)3 (Ru 1.5
wt %). Obtained powder was calcined in air at 623 K for 2 h
and heated in 20 kPa of H2 at 623 K for 2 h (denoted as Ru/
A-TiO2).

J. Phys. Chem. B, Vol. 107, No. 34, 2003 9023
TABLE 1: Catalytic N2 Formation Rate at 423 K from NO(2.0 kPa) and CO (2.0 kPa) on [Ru6C]/Oxides, [Ru6]/A-TiO2,
and Conventional Ru/A-TiO2 Catalyst

entry

catalystabcd[Ru6C]/A-TiO2

e
f
g
h
i
j
b

Tpreheata
(K)
423
523
623
423
423

[Ru6]/A-TiO2
conv Ru/A-TiO2[Ru6C]/Al2O3[Ru6C]/MgO[Ru6C]/SiO2

423
423
423
423
423

reaction
gasb
NO + CONO + CO + H2O(1.7 kPa)
NO + CO + O2
(0.56 kPa)
NO + CONO + CON2 formation rate
(10-7 mol ofN2 min-1
g of cat.-1)
5.1
2.9
2.5
10.9
2.6
3.3
0.41
1.6
1.5
0.83

a
Catalysts were heated in NO (2.0 kPa) before the catalytic test.
The pressure of NO (2.0 kPa) and CO (2.0 kPa) was fixed.

XAFS Measurements. The Ru K-edge XAFS spectra were
measured at 290 or 13 K using beamline 10B of the Photon
Factory in the National Laboratory for High Energy Physics.
The accumulation ring energy was 2.5 GeV, and the ring current
was 400-290 mA. The bending-magnet beamline 10B utilizeda channel-cut double crystal monochromator of Si(311). Samples
were placed vertically. The incident and transmitted X-rays weremonitored by ion chambers filled with Ar for I0 and Kr for I.
In situ prepared samples were transferred to a Pyrex glass cell,and transported to the beamline at Tsukuba.
EXAFS Analysis. The EXAFS spectra were analyzed by the
program EXAFSH.18 The postedge background subtraction was
performed by calculating the three-block cubic spline, followedby the normalization by using the Victoreen equation. Normalization was performed at k ) 4.0 Å-1 relative to the derivative
maximum of the absorption edge. The Fourier transform of k3weighted χ function was carried out over the range kmin ) 3.0
and kmax ) 13.0-14.0 Å-1 with the window function of 0.5
Å-1 on both sides of the region. The inverse Fourier transformwas performed in the filtering range r ) 1.0-4.5 Å.
The curve fitting analysis was performed in the range kmin )4 and kmax ) 12.0-13.0 Å-1 by the empirical phase shift andamplitude functions extracted from the EXAFS of modelcompounds: Ru powder for Ru-Ru bond (2.67 Å), RuO2
powder for Ru-O bond (1.98 Å), Ru3(CO)12 crystal for Ru-C
bond (1.93 Å), [RuCl2(CO)3]2 crystal for Ru(-C-)O bond (3.03
Å), and [PPN]2[Ru6C(CO)16] crystal for Ru(-C-)Ru bond (4.09
Å).5 The fit quality was evaluated by calculating the R factor
(Rf).

Rf )

∫|k3χobs(k) - k3χcalc(k)|2 dk/∫|k3χobs(k)|2 dk

Results
Effects of Support. The catalytic reactivity of [Ru6C]/ATiO2 for the NO reduction was preliminarily examined in theheating temperature range 423-623 K (Table 1a-c). Sample[Ru6C]/A-TiO2 previously heated in NO at 423 K showed
highest activity of N2 formation as high as 5.1 × 10-7 mol ofN2 min-1 gcat-1 in the NO + CO reaction. In the following
structural study, we focused on the TiO2 case because thecatalytic test results were best in the case rather than the cases
using other supports (Table 1).
The EXAFS spectrum for [Ru6C]/A-TiO2 heated in NO (2.0kPa) at 423 K is shown in Figure 1. The best-fit result of the9024 J. Phys. Chem. B, Vol. 107, No. 34, 2003

Minato et al.

Figure 1. Ru K-edge EXAFS for [Ru6C]/A-TiO2 in NO (2.0 kPa) at 423 K: (a) raw spectrum; (b) the k3-weighted EXAFS oscillation; (c) its
associated Fourier transform; (d) the curve fit analysis. Key: (-) observed; (‚‚‚) calculated.

TABLE 2: Structural Parameters Based on the Curve Fit Analysis for the Ru K-Edge EXAFS for Supported Ru Cluster
Catalysts and Reference Metric Data of Cluster Crystals

entry
a
b
c

catalysts[Ru6C]/A-TiO2

TPreheatb
(K)
423

d
e
f
g
h
i
j
k
l
m
n
o
p
q

[Ru6C]/R-TiO2[Ru6C]/M-TiO2[Ru6C]/SiO2

523
623
423

[Ru6C]/Al2O3[Ru6C]/MgO[Ru6]/A-TiO2
Ru/A-TiO2[Ru6C(CO)16]2- d[Ru6C(CO)12(NO)3]- d-reaction
gasc

Ru-C or
Ru-N

Ru-O

N r (Å) ∆(σ2)a N r (Å) ∆(σ2)a

3.0
NO-CO
1.5
NO-CO-H2O
2.0(3.1 kPa)
NO-CO-O2 1.7 2.05
5.9 0.6(0.56 kPa)1.9 2.00 -2.6
1.7 2.08 -0.3 0.8
3.0
1.8 2.10
2.9 1.0
2.5
NO-CO
1.3 2.06 -1.7 0.9
1.3 2.03 -3.1 1.0
2.2 2.04
0.1 1.0
1.9 2.05
1.6 0.8NO-CO
1.9 2.05
5.7 0.6

1.91
9.0
2.11 -0.2
2.12
2.8

N

Ru-Ru

Ru(-C-)O or
Ru(-N-)Or (Å) ∆(σ2)aN

3.5 2.87
3.5 2.87
3.5 2.87

2.1
4.2
5.6

1.83 -2.9
2.02 -1.7
1.91
7.4
1.97
5.5
1.87
1.3
2.01 -1.7
1.97 -0.9
1.97
1.1
1.76
8.7
1.83
3.3

3.3 1.95
3
1.93

2.4
3.6
1.4
4.1
4.0
0.8
2.7

2.66
2.86
2.65
2.82
2.65
2.63
2.65

4.9
4.9
9.9
2.7
8.7
4.6
9.1

11.1 2.68
4.0 2.89
4
2.89

1.2

Ru(-C-)Ru
Rfr (Å) ∆(σ2)a N r (Å) ∆(σ2)a (%)

3.0 3.05
2.1 3.08
2.3 3.08

1.4
3.2
7.3

0.6 2.94

0.11

4.0

2.87
2.4
3.04
1.5
2.95
5.0
2.98
0.5
2.89 -0.7
2.88
2.5
2.87
2.6
2.90
2.0
2.93
1.5

2.0
0.0
3.8
0.2
3.5
0.9
3.4
2.3
1.5
3.2
0.8

1.0
3.0
1.4
2.5
0.9
1.2
1.0
0.8
0.6

3.3 3.03
3
3.03

0.7 3.99
2.3
0.6 4.05 -0.2
0.6 4.07
1.4

0.7 4.10

0.6

0.8 4.12
0.0
0.3 4.05 -1.7

1.7
2.6
3.8

1.0 4.09
1
4.09

a
The unit of ∆(σ2) is 10-3 Å2. b In NO (2.0 kPa) for 30 min. c NO (2.0 kPa) and CO (2.0 kPa). Temperature ) 423 K. d Determined by X-ray
crystallographic analysis. The contribution of NO ligand was considered for cluster 3.

curve-fitting analysis for the spectrum is listed in Table 2a. In
its associated Fourier transform (Figure 1c), the peaks at 1.32.0, 2.3, and 3.5 Å (phase shift uncorrected) were ascribed to
Ru-C, Ru-Ru + Ru(-C-)O, and Ru(-C-)Ru bonds, respectively. The first and second peaks also include the contribution of Ru-N and Ru(-N-)O, respectively, because theaverage bond length of Ru-N (1.76 Å) is similar to that of
Ru-C (1.97 Å) and the average bond length of Ru(-N-)O(3.00 Å) is similar to that of Ru(-C-)O (3.03 Å) for cluster
3.19 The obtained bond lengths (r) and coordination numbers(N) of Ru-C (1.91 Å with N ) 3.0), Ru-Ru (2.87 Å with N) 3.5), Ru(-C-)O (3.05 Å with N ) 3.0), and Ru(-C-)Ru

[Ru6C] Structure for Catalytic NO Reduction

J. Phys. Chem. B, Vol. 107, No. 34, 2003 9025

Figure 2. Ru K-edge EXAFS for [Ru6C]/M-TiO2 in NO (2.0 kPa) at 423 K: (a) raw spectrum; (b) the k3-weighted EXAFS oscillation; (c) its
associated Fourier transform; (d) the curve fit analysis. Key: (-) observed; (‚‚‚) calculated.

(3.99 Å with N ) 0.7) (Table 2a) were essentially identical tothe corresponding values based on XRD crystallographic dataof cluster 3 (Table 2q),11-13 taking the maximum error of
coordination number by EXAFS (≈30%) into account. It is
known that the intensity of the Ru(-C-)Ru multiple scatteringpeak is greatly reduced when the Ru6C framework distorts fromoctahedral to lower symmetry. According to the symmetry
change, the θRu-C-Ru angle decreases from π.20 This may be
the reason that the N for the Ru-C-Ru bond (0.7) was smaller
by 30% than the case of cluster 3 crystal.
The EXAFS analysis results of [Ru6C]/R-TiO2 heated in NO(2.0 kPa) at 423 K were essentially identical to the above caseof [Ru6C]/A-TiO2 (Table 2a,g). The numbers of CO and NO
ligands that remained in the EXAFS sample of [Ru6C]/A-TiO2
were evaluated by the gas chromatograph (GC). They were 12.1
and 1.9, respectively, per one Ru6C cluster unit. The numberof NO is 3 in the crystal of cluster 3. The GC data suggest thatcluster 1 was transformed to cluster 3 by the reaction of NO at
423 K, supported on the TiO2 surface. The Ru site toaccommodate one more NO molecule may be used instead toform bond(s) with surface oxygen atom(s) of TiO2.
The EXAFS spectrum for [Ru6C]/M-TiO2 heated in NO (2.0kPa) at 423 K is shown in Figure 2. The peak at 3.5 Å in the
Fourier transform (c) was not fit well with the empiricalparameters of the Ru(-C-)Ru bond (Table 2h). The Ru-Ru
bond distance decreased from 2.89 for cluster 1 crystal to 2.65
Å. This value is a typical bond distance for the hcp metallic Ru
crystal. On the basis of the bond distance of Ru-Ru firstcoordination, the peak at 3.5 Å may be due to the next-nearest
Ru site.21 The obtained N value for Ru-Ru first coordination

was 1.4. The Ru hexanuclear structure framework should havesplit into smaller fragments in this case.
In the cases of [Ru6C]/Al2O3 and [Ru6C]/MgO, no peak
appeared in the region of Ru(-C-)Ru bond in the k3-weighted
Fourier transform of EXAFS χ function (not shown). Best fit
values are listed in Table 2k,l. On the basis of the rRu-Ru values(2.63-2.65 Å) and the N values (0.8-2.7), the smaller Rucluster split from the six-atom ensemble and/or atomicallydispersed Ru cations should be distributed on the Al2O3 andMgO surface at 423 K in NO.
In the case of [Ru6C]/SiO2 (Table 2i), best-fit result was
similar to the case of [Ru6C]/A-TiO2 and [Ru6C]/R-TiO2 andaccordingly to that of the cluster 3 crystal. The GC evaluationof the CO ligand number for the EXAFS sample of [Ru6C]/
SiO2 was 13.9 per a Ru6C cluster unit. No NO gas derived fromthe sample was detected (<6 × 10-9 mol). The [Ru6C] cluster
supported on SiO2 retained the Ru6 framework in NO at 423
K, probably due to the lower reactivity between the cluster andthe SiO2 surface. Essentially intact cluster 1 may remain on the
SiO2 surface. (Structurally) unperturbed cluster 1 is less reactivewith NO gas rather than partially decarbonylated cluster.3-6
In summary, the transformation of cluster 1 to 3 wassuggested on A-TiO2 and R-TiO2 in NO at 423 K. The cleavageof Ru-Ru bond in the Ru6C cluster was typically observed for[Ru6C]/M-TiO2, [Ru6C]/Al2O3, and [Ru6C]/MgO. The retained
Ru6C framework in the case of [Ru6C]/SiO2 was suggested to
be nonreactive with the NO gas.
Effects of Heating Temperature in NO. The Fouriertransforms of EXAFS spectra for [Ru6C]/A-TiO2 in NO (2.0kPa) at 423-623 K are illustrated in Figure 3. The results of

9026 J. Phys. Chem. B, Vol. 107, No. 34, 2003

Figure 3. Fourier transforms of k3-weighted Ru K-edge EXAFS χ
function for [Ru6C]/A-TiO2 in NO (2.0 kPa) at 423 (a), 523 (b), and
623 K (c).

the curve-fitting analysis are listed in Table 2a,e,f. The resultat 423 K was already described in the previous section. The
peak originating from Ru(-C-)Ru and Ru-Ru bonds disappeared at 523 K (Figure 3b). All the Ru-Ru bonds in the Ru6C
framework were cleaved and hence the Ru cation was atomicallydispersed, stabilized by the bonding with approximately two(in average) surface oxygen atoms at the distance of 2.00 Å.
With heating in NO at 623 K, a peak due to Ru-Ru bonds
appeared again at 2.4 Å in Figure 3c (phase shift uncorrected).
The Ru-Ru first coordination average distance was determined
to be 2.66 Å. The diagonal Ru(-C-)Ru coordination peak was

Minato et al.
not observed. As the NRu-Ru value was as small as 2.4, lessthan six Ru atoms reconstituted one Ru cluster (unit) in average.
The carbido carbon should not remain inside the Ru cluster,and accordingly, the Ru microparticle smaller than 1.0 nm had
the metallic nature rather than ruthenium metal carbide.
In summary, retained Ru6C framework at 423 K entirely split
into single Ru cations at 523 K. At 623 K, the metallic Ru cluster
was reorganized and the particle size should be very small(smaller than 1.0 nm) because the Ru-Ru coordination number
was only 2.4.
Effects of Carbido Carbon. The catalytic reactivity of NO
decomposition for noncarbido [Ru6]/A-TiO2 was compared tothat of carbido [Ru6C]/A-TiO2 both heated in the presence ofNO (2.0 kPa) at 423 K. The catalytic N2 formation rate startingfrom NO + CO gas at 423 K on [Ru6]/A-TiO2 was found to be
only 65% of that on [Ru6C]/A-TiO2 (Table 1a,f). The EXAFS
data for [Ru6]/A-TiO2 treated in NO (2.0 kPa) at 423 K are
depicted in Figure 4. In our control EXAFS measurement for
unsupported cluster 5 crystal, the peak attributed to second
nearest Ru-Ru bond (diagonal line of octahedral, 3.96-4.09Å) was not observed in the Fourier transform of Ru K-edge
EXAFS spectrum at 290 K. Due to this reason, only the fit valueof the first coordination Ru-Ru peak was used to judge theretention of the Ru6 framework in the case of supported
noncarbido cluster. The best fit consisted of Ru-O, Ru-C (orRu-N), and Ru(-C-)O (or Ru(-N-)O) (Table 2m). The fit
including the Ru-Ru bond contribution was rather worse. A
plausible Ru site structure was spontaneously atomicallydispersed Ru cations and/or a very small Ru cluster (less thansix Ru atoms ensemble) on the TiO2 surface, in contrast with

Figure 4. Ru K-edge EXAFS for [Ru6]/A-TiO2 in NO (2.0 kPa) at 423 K: (a) raw spectrum; (b) the k3-weighted EXAFS oscillation; (c) its
associated Fourier transform; (d) the curve fit analysis. Key: (-) observed (‚‚‚) calculated.

[Ru6C] Structure for Catalytic NO Reduction

J. Phys. Chem. B, Vol. 107, No. 34, 2003 9027
SCHEME 1: Structure Transformation of the Ru
Cluster Active Site Based on EXAFSa

Figure 5. Fourier transforms of k3-weighted Ru K-edge EXAFS χ
function for [Ru6C]/A-TiO2 in NO (2.0 kPa) + CO (2.0 kPa) (a), NO+ CO + H2O (3.1 kPa) (b), and NO + CO + O2 (0.56 kPa) (c), all at
423 K.

the Ru6C framework being retained in the case of [Ru6C]/ATiO2 in NO (2.0 kPa) at 423 K.
Ru Site Structure during Catalysis. The Ru cluster site
structure of [Ru6C]/A-TiO2 was investigated during catalysis
in the presence of NO (2.0 kPa) + CO (2.0 kPa) at 423 K. The
reaction condition corresponds to the best removal rate of NO
in the series of model catalysts (Table 2b). The catalysts under
the in situ catalytic reaction condition were quenched and the
XAFS spectrum was measured at 290 K. The NRu-Ru andNRu(-C-)Ru values were 3.5 and 0.6, being effectively equivalent
to the case of Ru cluster site in only NO at 423 K (3.5 and 0.7,
respectively, Table 2a). The addition of gas-phase carbon
monoxide did not have significant effects on the Ru cluster
structure supported over A-TiO2.
In contract, the NRu(-C-)Ru value in the case of [Ru6C]/SiO2
in NO (2.0 kPa) + CO (2.0 kPa) at 423 K decreased from 0.8
to 0.3 (Table 2j), suggesting the deformation of Ru octahedral
framework. The Ru6 unit should have contracted to microparticles of hcp metallic nature on the basis of the obtained
Ru-Ru distance of 2.65 Å. The reason of the difference of theRu site structure is not clear whether in the presence of NO orin the presence of NO + CO (Table 2i,j).
Effects of O2 and Water. The N2 formation rate at 423 K
starting from NO (2.0 kPa) + CO (2.0 kPa) on [Ru6C]/A-TiO2
increased by 112% when 1.7 kPa of water was mixed with thereaction gas (Table 1d). When 0.56 kPa of O2 was mixed withthe reaction gas, the N2 formation rate decreased by 49% (Table
1e).
Figure 5 compares the Fourier transforms of k3-weighted
EXAFS χ function in NO + CO (a), in NO + CO + H2O (b),and in NO + CO + O2 (c). The results of curve fitting analysis
are listed in Table 2a,c,d. Added water did not have a significant
effect on the FT data (Figure 5a,b). Upon addition of O2 to thereaction mixture, the peak around 3.5 Å disappeared and thepeak at 2.4 Å became very weak (Figure 5c). The transformed
Ru site structure may be similar to that in NO at 523 K (Figure
3b, Table 2e) except that one CO or NO ligand remained to
one/two Ru atom(s) when the O2 gas was introduced to [Ru6C]/
A-TiO2 at 423 K (Table 2d).
Discussion
The transformation of the Ru active site is summarized in
Scheme 1. The carbido Ru hexanuclear framework was retained
at 290 K when cluster 1 was supported on weakly acidic metalaThe ligand is not drawn for clarity.

oxides such as A-TiO2, R-TiO2, and SiO2. On other supports(M-TiO2, Al2O3, and MgO), the Ru6C framework appeared to
split into smaller fragments on surface. Upon supporting
carbonyl clusters on metal oxide, a general reaction we need toassume is the following water gas-shift type.22

CO (ligand) + OH (surface) f
CO2 + H (ligand) + O-Ru (bond)In the cases that this reaction proceeds dominantly, loss ofcarbonyl ligands should lead to destabilization of the cluster
framework. It has been reported that the reactivity of surfacehydroxyl group of MgO and Al2O3 toward 1 as well as
analogous anionic Ru carbonyl clusters is higher than those of
TiO2 and SiO2 surfaces.3,5At 423 K in NO, the Ru hexanuclear framework of [Ru6C]/
A-TiO2 was retained, whereas at 523 K in NO, the cleavage of
Ru-Ru bonds occurred and resulted in decomposition of the
Ru hexanuclear structure. The effect of carbido carbon instabilizing the Ru hexanuclear structure of [Ru6C(CO)16(CH3)]-/
A-TiO2 has already been noted either in CO + H2 or in a
vacuum at 523 K.3 In a homogeneous system with saturated
NO gas at room temperature, stepwise (partial) cleavage of RuRu bonds in the Ru cluster begins to take place when morethan three NO molecules coordinate to the cluster, transforming
cluster 1 to 2, and then to 3.11 At higher temperatures of 423523 K, nearly all the Ru-Ru bonds of cluster 1 were cleavedby the effects of NO and precipitated in solution. In contrast,on the A-TiO2 surface, a split single (or Ru ensemble consistedof less than six atoms) Ru atom was stabilized by thecoordination of surface oxygen atoms (Scheme 1). When heatedto 623 K, the Ru-Ru first coordination peak reappeared but
the bond distance was shortened by 0.21 Å (Figure 3c, Table
2f).
The overall trend was that the Ru hexanuclear framework
became Ru ensemble units of fewer atom numbers (even single
Ru atoms) and then the Ru ensemble was reorganized, consistingof fewer than approximately six Ru atoms when the temperature
in NO was raised from 423 to 623 K. This trend is understandable to consider the combination of two opposing factors: (1)9028 J. Phys. Chem. B, Vol. 107, No. 34, 2003
the cleavage of Ru-Ru bonds by NO and (2) the higher surface
diffusion rate of metal atom (ensemble) at higher temperature.
As the temperature was raised, the former factor reduced the
Ru ensemble size whereas the latter factor inversely grew theensemble size. Thus, the Ru ensemble size should reach theminimum at 523 K.
The third and fourth factors are (3) the kind of support and(4) the presence/absence of carbido carbon inside the starting
Ru cluster crystal framework. Relatively acidic support and thepresence of carbido carbon were found to stabilize the Ru6cluster framework. The effects of factors (1), (3) (basic support),and (4) (noncarbido cluster) were dominant in the case on[Ru6C]/M-TiO2, Al2O3, MgO, and [Ru6]/A-TiO2 at 423 K.
Mononuclear surface Ru ion species was suggested as in
Scheme 1 in the cases of [Ru6C]/A-TiO2 in NO + CO + O2 at
423 K (Table 2d), [Ru6C]/A-TiO2 in NO at 523 K (e), andnoncarbido [Ru6]/A-TiO2 in NO + CO at 423 K (n). In thepresence of CO gas, 0.6 of Ru-C (or Ru-N) and R(-C-)O{or Ru(-N-)O} coordination was observed (Table 2d,n), but
only the Ru-O single peak was observed for [Ru6C]/A-TiO2
in NO at 523 K (e). Therefore, one CO molecule coordinatedto one/two mononuclear surface Ru ion(s). Mononuclear
ruthenium carbonyl species, Ru(CO)2 and Ru(CO)3 werereported in the case of [Ru(NH3)6]/NaY when heated at 393
K.23 The Ru3(CO)12 cluster supported in hydroxylated γ-Al2O3
was reported to transform into mononuclear [Ru(CO)2]2+
species.24
The Ru metal surface of conventional Ru/TiO2 catalysts isreported to be predominantly occupied by NO under NO + CO
gas.25 In the case of [Ru6C]/A-TiO2 (this study), the number of
CO and NO ligands was estimated to be 12.1 and 1.9 per one
Ru6C cluster unit on the basis of the EXAFS and GC analysis.
The difference of adsorbent (ligand) ratio may be the reason
that [Ru6]/A-TiO2 exhibited a remarkably higher catalytic NO
reduction rate at 423 K than the case of conventional Ru/TiO2
catalyst (Table 1). The preferable CO adsorption was also
reported in the case of [Pt12(CO)24]2-/NaY during the NO +CO reaction.8,9Another plausible explanation of the higher NO reduction
rate on [Ru6C]/A-TiO2 is the effect of bridging CO (µ2-CO) on
the Ru cluster site. On conventional Ru catalysts, only terminal(on-top) CO adsorption was observed by FT-IR (Fouriertransformed infrared absorption spectroscopy).26 On the contrary,
three bridging CO exist in the cluster 3, sharing a common Ru
atom with a terminal NO ligand. A similar cluster structure wasdemonstrated in the case of [Ru6C]/A-TiO2 in NO at 423 K(Figure 1 and Table 2a).
In a practical point of view, O2 and/or H2O effects wereconsidered. Structural transformation of the ruthenium cluster
was observed in the presence of O2 (Scheme 1), resulting in
the partial deactivation of catalytic N2 formation (Table 1).
Water addition exhibited positive effects on this catalysis,
however, the reason was not clarified by EXAFS study in thispaper.
Conclusions(1) The Ru hexanuclear framework was retained for [Ru6C]/
A-TiO2, [Ru6C]/R-TiO2, and [Ru6C]/SiO2 heated in NO at 423
K. Stabilized Ru hexanuclear structure showed high NO
conversion and high N2 selectivity in the NO + CO reaction at
423 K. The retained Ru6C core was found to be the best,
designed surface site for the environmental NO removal.
(2) The Ru6 framework stabilized by the effects of carbido
carbon and TiO2 surface split into smaller ensemble and finally

Minato et al.
single Ru atoms dispersed on TiO2 surface until 523 K. This
structural transformation was in the negative direction for thecatalytic NO decomposition. The dispersed Ru atoms reorganized Ru ensemble consisted of less than six Ru atoms until
623 K.
(3) NO addition and basic support tend to cleave the RuRu bonds. In contrast, carbido carbon stabilized the Ru6framework from inside. Elevation of reaction temperature led
to the aggregation of Ru atoms.
(4) The addition of O2 to the reaction mixture NO + CO
broke the Ru6C framework into atomically dispersed Ru atoms.
The catalytic reactivity of NO decomposition over the Ru
species was lower. No evident structural change was observedby the addition of H2O.
Acknowledgment. The experiments were performed under
the approval of the KEK-PF Program Review Committee(2001G092).
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