J. Phys. Chem. 1995,99, 10336-10345

10336

Preparation of [Ru&] Clusters on MgO, K+/MgO, Css/MgO, and A1203 and theReactivities with H2 and NZ
Yasuo Izumi* and Ken-ichi Aika*
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science andEngineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan
Received: January 26, 1995; In Final Form: April 3, 1995@

The supported ruthenium clusters [ R u a ] were prepared on MgO, K+/MgO, and Cs+/MgO from [Ru$\T(C0)161cluster as hydrogenation catalysts, stabilized, and chemically modified by nitrido nitrogen. The characterization
and reactivities with H2 and N2 were investigated by EXAFS (extended X-ray absorption fine structure) in
relation to their catalysis promoted by nitrido nitrogen. The [Ru~N]cluster unit was found to remain on
MgO after heating in vacuum at 813 K (decarbonylated) and in H2 at 588 K (reaction condition of N2hydrogenation), whereas the [RugN(C0)16]- cluster strongly interacted with the A1203 surface to degrade to[ R u ~ by] heating in vacuum at 813 K. The decarbonylated [ R u a ] framework also remained in H2 at 588 K
on K+/MgO and Cs+/MgO without aggregation or degradation. By changing the Ru loading from 0.48 to
3.9 wt % on MgO, the coordination number NR~-o,,(O,,, oxygen atom at surface) decreased from 1.2 to 0.3,
while the [ R u a ] cluster unit remained unchanged for samples with Ru loading up to -2.5 wt %. The preferable
reaction of [R@] clusters with MgO(001) flat surfaces was suggested for the sample at 2.5 wt % Ru, but
the cluster should have reacted mainly with lower coordination sites of MgO for the sample with lower Ru
loading (-0.5 wt %). The rRu-o,, was shorter for [RU6N] on K+/MgO and Cs+/MgO (2.00-2.03 A) than on
MgO (2.13 A), implying that the [Ru~N]was interacted with Os, atoms bonded to K+ or Cs+ ions to have a
direct support effect of basic oxide K+/MgO or Cs+/MgO on catalysis. H-induced structure changes wereobserved for [RUsN]/MgO and [Ru$\T]-Cs+/MgO as the reversible changes on bonding distance rRu-Ru of
0.03 and 0.08 A, respectively, by the adsorptioddesorption of Hz. The adsorption of NZwas simple adsorption
on [Ru~N]without structural change of [Ru~N]on MgO or Cs+/MgO.

+

Introduction
Metal clusters as homogeneous (in liquid) and heterogeneous(supported clusters) catalysts are expected to be effective
catalysts associated with their multiple metal sites. The specificcatalysis has been reported on supported carbido clusters [Ru~C]
and [Rh&] on MgO, La203, Sioz, A1203, TiO2, et^.'-^
Supported [Ru~C]clusters prepared from [Ru&(C0)16Me]produced oxygenated compounds (MeOH, H2C0, Me20) fromCO
H2 with high activities and sele~tivities.~Supported[Rh&] clusters prepared from [RhsC(C0)15]~-produced propanal from C2H4
CO
H2 in high selectivities.* Active
structures were indicated to be carbido metal hexamers [Ru~C]
and [Rh&] by means of EXAFS, and electron donation fromcarbido carbon to the cluster framework [M6] (M = Ru, Rh)
was suggested to have a chemical effect on catalysis. In relation
to the effects of structure on catalysis, reversible expansion/
contraction of the [RU&] framework was observed by theadsorptioddesorption of CO under in-situ conditions (in CO
H2 around 523 K).'32,4 These EXAFS results indicated that the
expansion of the [Ru~C]framework facilitated H2 molecules toaccess Ru cluster sites covered with CO(a) and to dissociateby elongating the Ru-Ru distance.',2 As a result of thishydrogen activation, the reaction path was switched over fromCO-dissociative (C& formation) to CO-associative type (oxygenate formation). Similarly, propanal was selectively formed
on CO-covered [RhfjC] clusters on Si02.2
These chemical (electron donation from C) and structural(cluster framework expansion) effects found for supported[RU6c] and [Rh&] clusters implied the possibility of investigat-

+

+

+

+

* To whom correspondence should be addressed. E-mail:yizumi@chemenv.titech.ac.jp.
Abstract published in Advance ACS Abstracts, June 1, 1995.
@

0022-3654/95/2099-10336$09.00/0

ing new catalyst systems of transition metal
main groupelement, such as ruthenium carbide or nitride and rhodium
carbide or nitride, although studies on these systems were veryrare because they cannot exist as bulk in usual thermodynamic
conditions. Recently, we reported the catalysis of nitrido-[Ru6]clusters [RU6N(C0)16]- on MgO, K+MgO, and Cs+/MgO,
which retained the hexamer [Ru~N]unit in the reaction
conditions. The interstitial nitrido nitrogen was suggested topromote catalysis through the comparison of activities with othersupported clusters and conventional Ru catalyst^.^ Similar to
the framework expansiodcontraction of [Ru~C]clusters by COadsorptioddesorption, expansiodcontraction of the [Ru~N]
framework was observed by the adsorptioddesorption ofhydrogen. These data suggested that the catalyst systems oftransition metal (Ru, Rh) carbon or nitrogen were impossible
as bulk, but possible as very small metal particles such assupported hexamer clusters carbon or nitrogen.
The information about the interactions of supported clusters
with H2 is important in relation to catalysis (hydrogenation),but research on the interactions has been difficult because ofthe "invisibleness" of hydrogen to most spectroscopies and theinstability of supported clusters in reaction conditions ofhydrogenation. The hydrogen effect on metal particle structure
was investigated for WAl2O3 after reduction (427 K) andsubsequent heating in vacuum (650 K) by means of EXAFS6
By heating at 650 K, the bonding distance qr-~r
and coordination
number N I ~ -decreased
I~from 2.71 to 2.64 8, (8, = lo-' nm)
and from 4.9 to 4.0, respectively. The interaction of Ir clusters
with H2 was not clear due to the change of cluster structure. It
was suggested that the cluster structure was transformed fromhemispherical to almost flat on the basis of the decrease of Nk-rr.
The conventional Pd/C (average Pd particle size 35 8,) and Pd/++

0 1995 American Chemical SocietySupported [ R u a ] Clusters and Reactivities
A1203 (34 8,) were treated in HZ at 373 K, and subsequently
the ambient gas was switched from HZ to Ar at the same
temperat~re.~During the desorption of H(a), the Qd-Pd was
decreased from 2.82 to 2.76 8, (PdK) and from 2.82 to 2.75 A(PdA1203) keeping the Npd-pd almost unchanged. The palladium hydride PdH, (x 0.44) formation/decomposition on
C or A1203 was suggested compared to a-phase PdH, (x <<0.6) and /3-phase PdH, (x 0.6) as bulk Pd hydride.
The spectroscopic detection of H(a) was reported by IR(infrared absorption) and NMR (nuclear magnetic resonance).
Different from the previous reports on supported metal catalysts,
multifold (bridging and 3-fold) H(a) were detected in addition
to the on-top H(a) on conventional R a g 0 and Ru-Cs+/MgO
at 183 K.8 Two peaks were observed for conventional Ru/SiOzaround -60 ppm in H2 (1.3-100 kPa) (a) and around -30
ppm (13-100 kPa) @) by solid 1H-NMR.9 The /3 peak was
implied to be weakly bound H species on Ru particles on thebasis of the smaller heat of adsorption (10 kJ mol-') and larger
H/Rusurfratio (3.3-5.6 at P H =~ 33-100 kPa).
In this paper, we report the preparation of [Ru~N]clusters
on MgO, K+/MgO, and Cs+/MgO compared to that on A1203
as the first subject. The stability of [Ru~C]clusters on MgO,
La2O3, TiOz, or A1203 was reported to be due to stabilizationof the [RU6] framework by carbido carbon,',l0 and similar
stabilization was expected for supported [Ru~N]. Detailed
characterization of supported [Ru~N]clusters was performed
to determine (1) the homogeneity of supported [Ru~N],(2) the
kind of MgO surface sites where [Ru~N]was attached, the (3)the nature of Ru-Os, bonds (Os,, oxygen atom at surface),through the EXAFS observations for samples at different Ru
wt % and the TPR (temperature-programmed reduction) measurements. Secondly, the reactivities of thus-characterized
supported nitrido clusters with HZ or N2 were investigated byin-situ EXAFS measurements. These studies on the reactivity
with H2 and Nz should be closely related with the catalysis(ammonia synthesis) on these nitrido clusters on a molecularlevel. The detailed discussions on the structures and theirchanges by the interaction with H2 or N2 are reported in thispaper. The extension to catalysis is described in the accompanying paper."-Experimental Section[PPN]+[Ru$J(co)l6]- (1) (PPN = N(PPh3)z)" ([PPNI'Cl-,
Aldrich Chem Co., 97%; NaN3, Wako Pure Chemical Ind., Ltd.,>90%; Ru~(CO)IZ,
Soekawa) was supported on MgO, K+/MgO,
Cs+/MgO, or A1203 by reaction at 290 K (1 h) in purified THF(tetrahydrofuran, Wako, Special Grade) and the subsequent
removal of THF in vacuum. The MgO (surface area 200 m2g-I) was prepared from Mg(0H)z (Wako, 99.99%) by heating
at 773 K (2 h) in vacuum. A water solution of CszC03 (Wako,'95%) or KzCO3 (Wako, >99.5%) was impregnated on MgO,
followed by treatments in 0 2 and then in HZ at 773 K. The
A120 (aerosil C) (100 mz g-l) was treated at 623 K (2 h) in
vacuum. The loadings of Ru were 2.5 wt % for MgO, K+/
MgO, and Cs+/MgO and 1.6 wt % for A1203 except for the
EXAFS experiments at different Ru wt % for MgO (0.48, 2.5,
and 3.9 wt a).The experiments at 0.48 and 3.9 wt % Ru were
specified in the text, and those without special notation for MgOsupported samples were at 2.5 wt % Ru. The molar ratios Cs+/
[Ru~N]and K+/[Ru&J] were fixed at 12 on the basis of thepreliminary activity tests for ammonia synthesis. The supported
nitrido-Ru clusters are denoted as [Rua]/oxide.
[PPN]+2[RU6(CO)1si2- (2) and [NMe3CHzPh]+[Ru6C(CO)I 6Me]- (3) ([NMe6CHzPh]+C1-, Wako; CH31, Wako, Special
Grade) were supported on MgO in THF solution in a similar

J. Phys. Chem., Vol. 99, No. 25, I995 10337
manner to the case of 1 or ref 1. The supported Ru clusters aredenoted as [Ru6]/MgO and [Ru6C]/MgO (2.5 wt % Ru). The
process of cluster catalyst preparation and transfer was carriedout in argon (99.99%) or helium (99.99%). As pretreatment,
incipient supported clusters were heated (elevating rate of
temperature 4 K min-I) in vacuum at 8 13 K except for [ R W ]Cs+/MgO and [Ru6N]-K+/MgO (heated at 673 K), followed
by treatment in HZ for 1 h (588-773 K).
The EXAFS spectra of the Ru K edge were measured forthese supported clusters at the beamline 10B and 6B (2.5 GeV,
current 360-260 A) of the Photon Factory in the National
Laboratory for High Energy Physics (Proposal No. 93GO10)
between June 1993 and November 1994. The X-ray was
monochronized through a double-crystal monochrometer of Si(311), and the spectra were obtained in the transmission modewith ionization chambers fitted with argon ( l o ) and krypton (I).
The energy resolution was -1 eV at 9 keV, and the photon
flux on the sample was >109/s. The sample was transferred toan EXAFS Pyrex cell (0.13 < A pt < 0.98 dependent on theRu wt %) with Kapton films on both sides by using the Schlenk
technique from a closed circulating system for sample preparation. The data collection was performed at 30-293 K utilizing
a closed cycle refrigerator (Cry0 System, LTS-21). The analysis
was performed by the program EXAFSH by Yokoyama et al.I3
The background subtraction was performed by calculating the
cubic spline with three blocks, and the obtained function put(k)- por(k) was normalized by using the Victoreen parameter po(A) = CA3 - DA4 to determine p&) and t. The Fouriertransform (FT) of the k3-weighted EXAFS oscillation was
carried out over the range of kmin = 3 and kmax = 13-16 8,-'according to the signahoise (S/N) of each spectrum. The
window function (Hanning function) was multiplied with a kwidth of (k,,, - km,,)/20 on both ends of the FT k range. The
inverse FT was performed in the r range of rmln= 1.3- 1.4 andr,,, = 2.9-3.0 8, multiplied by the window Hanning function
with the width 0.1 8, on both ends. The curve-fitting analysis
was performed by the method based on the formula of plane
wave single-scattering theory,I4using empirical phase shift andamplitude functions extracted from ruthenium powder for the
Ru-Ru bond (metallic), RuO2 for the Ru-0 bond, [RuClZ(CO)3]2 for the Ru(-C-)O bond, cluster 3 (Ru(-C-)Ru bond)for the Ru(-N-)Ru bond, and Ru3(C0)12 (Ru-C bond) for
the Ru-C and Ru-N bonds (in the case of N2 adsorbed on
supported [Ru~N]clusters). The parameters for the Ru-Ru
bond (cluster) were obtained by subtracting the oscillation of
the third shell of the complex [RuCl~(C0)3]2(Ru(-C-)0) fromsecond shell of cluster 1 (Ru-Ru
Ru(-C-)O), taking into
account each coordination number. The EXAFS spectra forthese reference compounds were taken at room temperature. The
residual factor (Rf)was calculated by the following equation:+in the k region of 4-13 8,-' (4-12 for [RusN]-Cs+/MgO in
H2 and 0.48 wt % [RusN]/MgO because of their worse S/N)
for the curve-fitting analysis. The HZ(99.99%) and NZ(99.99%)
gas were purchased from Toyo Sans0 Co., Ltd. The impurities(as molecular content) were less than the following values: H20< 10, NZ < 50,Oz -= 10, CO < 10, C02 < 10 ppm and totalhydrocabons < 10 ppm (as carbon content) in the H2 gas, andHz0 < 10, 0 2 < 2 ppm, and total hydrocarbons < 1 ppm (ascarbon content) in the NZgas. The HZor Nz was introduced to
the EXAFS sample cell after further purification by passage
through a liquid nitrogen trap.
FTIR spectra of supported nitrido clusters were recorded on
a FTIR spectrometer (JASCO FTIR-5000) in a quartz IR cell

Izumi and Aika

10338 .I.
Phys. Chem., Vol. 99, No. 25, 1995
10,

21.6

21.8

22

22.2

22.4

22.6

22.8

Photm Energy (keV1

23

23.2

23.4

23.6

-10

R (Ang8lfc.n)

5 ,

1

h

c
n

0

Y

U

4

2

4

6

8

10

12

14

16

18

k (1IAngdrom)

4

4

5

6

7

8
9
k (1IAngslmm)

10

11

1

2

1

3

Figure 1. Ru K edge EXAFS spectra observed at 293 K for [RudV(C0)16]-/MgO(2.5 wt % Ru). The sample was heated in vacuum (813 K) andin HZ (588 K): (a) raw spectrum, (b) k3-weighted EXAFS oscillation, (c) its associated Fourier transform, and (d) curve-fitting analysis, (-)
observed, (- - -) calculated.

with NaCl windows on both sides combined with a closed
circulating system. Support oxide disks were treated in the same
conditions as powders in the IR cell and impregnated by a dropof a THF solution of cluster 1. The temperature of the IR cell
can be maintained at 295-823 K by a coil would around thecell.
Results
Characterization of Supported [RwN] Clusters on MgO,
K+/MgO, Cs+/MgO, and A1203. 1 . Incipient Supported
Clusters. The E M S spectra were observed for incipient[Ru6N]/MgO, [ R u ~-Cs+/MgO,]
and [RU6N]/Al203. The
obtained coordination numbers N R ~ - cand N R ~ ( - c - ) owere 2.5,
2.1, and 2.4-2.5, respectively, by the curve-fitting analysis.
Corresponding numbers of carbonyl ligands per cluster would
be 15, 13, and 14-15, respectively, in accordance with the total
amount of desorbed CO
COz in TPD (temperatureprogrammed desorption) measurements up to 813 K separately
observed in vacuum (15, 12, and 16 on carbon basis). Although
the incipient [ R u a ] framework was not affected upon supporting on MgO and Cs+/MgO judging from the unchanged bonding
distances r and N for Ru-Ru (first coordination) andRu(-N-)Ru (diagonal line of octahedral (Oh)cluster [Ru~N])bondings, the Ru(-N-)Ru peak around 3.7 8, (phase shiftuncorrected) disappeared in the FT of k3x for incipient [ R u a ] /A1203 compared to the FT for unsupported cluster 1 (crystal).
This suggested the distortion of the [ R m ] framework (the angle
ORu-N-Ru was reduced from n)I5by strong interaction with theA1203 surface.

+

2. Heated and Reduced Clusters. The incipient [ R u a l N g O
and [Ru$rl]/AlzO3 were heated in vacuum at 813 K (up to thetemperature where CO ligands had totally desorbed) and thenin HZ at 588 K. The obtained EXAFS spectra were almost
identical after heating and after subsequent H2 treatment for[Rm]/MgO (Figure la,b). Also, the corresponding two spectra
were almost identical for [Ru$\T]/A1203. Therefore, common
ruthenium cluster species were suggested for samples with/without the H2 treatment at 588 K. The EXAFS spectrum for[Ru6N]/MgO after heating and in H2 is shown in Figure 1. A
strong peak ascribable to Ru-Ru bonding and a shoulder on
the shorter distance side of the Ru-Ru peak were observed(Figure IC). By the curve-fitting analysis with two waves (RuRu, Ru-0), the coordination number of Ru-Ru bonding( N R ~ - Rwas~ ) calculated to be 4.1 with
= 2.62 8, (Table
1 and Figure Id), similar to the value for cluster 1 (=4),
indicating that the [Ru~N]unit remained on MgO in H2 at 588
K. An alternative curve-fitting analysis was performed with
one wave (Ru-Ru) or three waves (Ru-Ru, two Ru-0), andthe Rfvalues were 6.8 and 2.8%, respectively. In the one-wave
fitting (Ru-Ru), the calculated curve did not trace the inversely
Fourier-transformed experimental curve in all wavenumber
regions. In the three-wave fitting (Ru-Ru, two Ru-0), thefitting was not improved by the addition of one more Ru-0
wave to two waves (Ru-Ru, Ru-0). Thus, the analysis with
two waves (Ru-Ru, Ru-0) was reasonable with smaller R$
2.5% (Table 1). The obtained N R ~ - o , ,(0.5) suggests that one
triangle Ru3 face of the @like [Ru$rl] framework was attachedon the MgO surface through three Ru-Os, bonds. If the o h like structure was destroyed to a raftlike structure by the

J. Phys. Chem., Vol. 99, No. 25, 1995 10339

Supported [ R u a ] Clusters and Reactivities

TABLE 1: Results for the Curve-Fitting Analysis of Ru K Edge EXAFS (Observed at 293 K) for Supported [R@(CO),J
Clusters on MgO, K+/MgO, Csf/MgO, and A1203 after Heating in Vacuum (819 or 673 K*) and in Hydrogen (588 K)eRu-RU
SUDDO~~ Ruwt%

N

P

Ru-0

AEo'

A(u')'

N

AEo

r

A(d)

Rr

~

MgO"
K+/h4gOb
CsC/MgOb

2.5
2.5
2.5
1.6

2.62
-7.4
4.0
0.5
2.13
12
-2.2
2.5
4.1
3.8
2.63
- 14
3.7
0.6
2.00
-13
0.87
5.2
4.0 (40.6) 2.63 (f0.016) -13 ( f 4 ) 3.6 ( f 1 . 2 ) 0.6 ( f 0 . 2 ) 2.03 (f0.027) -4.1 ( f 2 . 7 ) 0.94 ( f 0 . 6 7 ) 3.2 ( f 1 . 6 )
2.0
2.61
-8.9
0.49
1.9
2.16
1.7
1.9
0.6

a MgO and A1203catalysts heated at 813 K.
Kf/MgO and Csf/MgO catalysts heated at 673 K. K+ (or Cs+)/[RusN] = 12 in mol ratio. The
A2.
error (within the 50% increase of Rf)is shown in parentheses. In 8, (8, = lo-' nm). e In eV. f In

-035

,

I

I

0

2

1

3

5

4

6

R (Angstrom)
5

4

4

3

3
2
2
-

5

1

s
X
a

Y

o

1

C o
8

-1

-1

2

2

3
3

4

-5

4

4

6

8
10
k (l/AnQatrom)

12

14

16

4

5

5

7

8

9

1
0
k (1IAngrtrom)

1

1

1

2

1

3

Figure 2. Ru K edge EXAFS spectra observed at 293 K for [ R U ~ V ( C O ) I J / A ~ ~(1.6O ~wt % Ru). The sample was heated in vacuum (813 K) andin HZ(588 K). The captions for (a)-(d) are the same as in Figure 1 .
interaction with the MgO surface, NR,,-R~
should be smaller thanthe observed 4.1. Hence, we suppose the formulation [Ru&f&-OSu)3l(NR,,-o,, = 2.13 8,). The cluster 1 was suggested tointeract with the A1203 surface and be distorted to lose oh
symmetry of the [ R u a ] framework, as indicated by the disappearance of Ru(-N-)Ru bonding for the incipient cluster (inthe previous section). The EXAFS spectra for [RusN]/A1203
after heating and in H2 are shown in Figure 2, and curve-fitting
result is listed in Table 1. The NR,,-R,, decreased to 2.0, andNR,,-o$"was 1.9. The formation of the trimer [Ru3f&2-Osu)3]
cluster was suggested after the fragmentation from hexamer
nitrido cluster 1 by strong interaction with the A1203 surface.
The coordination of Os,, should be p2 (coordinate to two Ru
atoms) in order to be consistent with NR"-o,, (1.9) taking into
account the [ R u ~cluster]
size and the population of 0," at theA1203 surface (see the Discussion section).
Also, [Ru~N]-K+/MgO and [Ru6N]-Cs+/MgO heated in
vacuum at 673 K showed very little change of EXAFS spectra
by subsequent H2 treatment at 588 K. The EXAFS spectra areshown in Figure 3 for [Rua]-Cs+/MgO heated in vacuum at
673 K and in H2 at 588 K. Corresponding curve-fitting results
are listed in Table 1. The result was similar to that for [Rua]/
MgO, and NR,,-R,, of 3.8 on K+/MgO and 4.0 on Cs+/MgO
suggest retention of the [ R u a ] unit on K+/MgO and Cs+/MgO.
However, the P-R,,-o,,values were 2.00-2.03 8, for [Ru6N]K+/MgO and [Rua]-Cs+/MgO, which were shorter by 0.100.13 8, than that for [RusN]/MgO (2.13 A) (Table 1).
3. The Change of [ R u a ] Cluster Structure and MgO Sites
at Dzrerent Ru wt %. The EXAFS spectra were also measured
for [Rua]/MgO at 0.48 and 3.9 wt % Ru. The EXAFS
spectrum of a sample at 0.48 wt % Ru is shown in Figure 4.
Compared to the Fourier transform for the 2.5 wt % Ru sample(Figure IC), the peak around 1.7 8, (Ru-0) (phase shiftuncorrected) became stronger, while the peak around 2.3 8, did
not show significant change (Figure 4c). The best fit result islisted in Table 2 and shown in Figure 4d. The NR,,-R,,(4.0)
suggested that the [Ru~N]cluster unit was retained also at 0.48
wt % Ru on the MgO surface.

Izumi and Aika

10340 J. Phys. Chem., Vol. 99, No. 25, 1995
4 1 .

.0.2

.0.5

-0.55

t (a> k
~

0

2

1

3
R (Anpalrom)

5

4

6

I

4 ,

2

4

8

8

10

k (1IA~sIrwn)

12

11

16

18

4

5

6

7

8

9

k (1IAngrlrom)

1

0

1

1

1

2

1

3

Figure 3. Ru K edge EXAFS spectra observed at 293 K for [Ru~(CO),~]--Cs+lMgO(2.5 wt % Ru, Csf/[Ru6N] = 12 in mol ratio). The sample
was heated in vacuum (673 K) and in Hz (588 K). The captions for (a)-(d) are the same as in Figure 1.

The NR,,-R,, for [RugN]/MgO at 3.9 wt % Ru was 5.0 withthe same mu+,,(2.62 A) as for the sample at 2.5 wt % Ru
(Table 2). The [ R u a ] clusters should have aggregated to larger
Ru particles similar to [RugN]/MgO (2.5 wt % Ru) in H2 at
653-773 K (NR,,-R,,= 4.9-6.6, EXAFS). The surface cluster
species [RugN] were stable in H2 or in reaction conditions of
N2 4-H2, but aggregated to larger metal particles in more severe
conditions or with larger Ru loadings (Ru
4 wt %). The
NR,,-R,,of [RUs]/MgO (without interstitial atom) prepared from
non-nitrido cluster 2 was 5.9 in vacuum at 813 K and 6.2 in H2
at 588 K by EXAFS, indicating the aggregation of the [ R u ~ ]cluster to larger particles in vacuum at 813 K andor in H2 at
588 K in the case of cluster 2 (without nitrido N).
4 . TPR Measurements for [Rudv]iMgO, [Rua]-Cs+/MgO,
and [Ru&MgO. We studied TPR in H2 by mass spectroscopy
for incipient [Rua]/MgO, [Ru&"-Cs+/MgO, and [Rus]/MgO(Figure 5a). These spectra had common peaks of NH3 around510 K, but higher temperature peaks of NH3 were observed
around 660 K only for [RusN]/MgO and [Ru6N]-Cs+/MgO.
As the former desorption temperature (around 510 K) coincidedwith the disappearing temperature of the IR peak of [PPN]+(cation of clusters 1 and 2) (1 117 cm-I) (Figure Sb), we believethat the peaks around 660 K originated from nitrido N of 1.
This TPR result supported the structure [Rua@-0,,)3] on MgO,
K+/MgO, and Cs+/MgO.
The Reactivity of Supported [ R W ] Clusters on MgO andCs+/MgO with H D 2 . 1. With H2. The FT of the EXAFS at
193 K for [Ru$rT@-O,,)3] on MgO in H2 (76 kPa) slightly shiftedto the longer distance side compared to those in vacuum (Figurel), and the obtained curve-fitting data indicated the change ofbond distance of Q,,-R,, from 2.62 to 2.65 8, with the change of-m,,-o,, from 2.13 to 2.17 8, by the adsorption of H2 (Table 3).
The sample after evacuation of H2 at 193 K was very similar
to the EXAFS of the initial supported cluster before H2
introduction (Figure l), suggesting that this structure changeby the hydrogen adsorption/desorption was reversible. The
control EXAFS experiment was carried out for [Ru,#]/MgO
in vacuum at 30-193 K. The obtained Q,,-R,, (2.62 A) at 30
K) in Table 3 was the same as that for [RusN]/MgO in vacuumat 293 K (Table l), excluding the possibility of the expansion/
contraction of the [Ru$J] cluster framework by coolingheating.
The reactivity of [Ru&j-Cs+/MgO with H2 (76 kPa) was
also examined by EXAFS. Compared to k3x observed in
vacuum (Figure 3b), we observed a drastic change of EXAFS
in HZ(Figure 6b). In its associated FT (Figure 6c), the bonding
distance of the main peak (Ru-Ru) became larger (phase shiftuncorrected) than those in vacuum (Figure 3). The TR,,-R,,
obtained by curve fitting (Figure 6d) was 2.71 A for the sample
in H:! (Table 3), longer by 0.08 A than for the sample in vacuum(2.63 A, Table 1). Additional analyses were performed bychanging the FT range, CF range, N , or r, independently, andalso for another EXAFS spectrum of the same sample in thesame conditions in order to estimate the errors. The errors form,,-~,,were f0.016 and f0.015 A for spectra corresponding
to Figures 3 and 6 (Table 1 and 3). Therefore, it is valid tothink that the Q,,-R,, change (f0.08 A) was induced by hydrogen
expansion/contraction.
As a reference, incipient [Ru&]/MgO was heated in vacuum
and in H2 at 623 K. The non-nitrido Ru hexamer cluster [Rug@-O&] was suggested to be formed from the equimolar
desorption data for C h ' and the EXAFS data mu+,,(2.63 A)
and NR,,-R,, (4.2) (Table 4). The number x should be 3-4

Supported [ R u a ] Clusters and Reactivities
0.35

J. Phys. Chem., Vol. 99, No. 25, 1995 10341

,

I

8,

1

h

5

P

Y

C
m

'

I

405
216

218

222
224
226
Phdm Enemy (keV)

22

228

23

I

232

1

5

5

4

4

3

3

2

c

1
h

3
(Agltrom)

6

5

4

2

1

h

J

2

o

H

;

*)

A

O

L

-1

u

2

2

3

3

d

-4

5

6

4

8
k (IIAnpstrom)

5

12

10

14

4

5

6

7

8

9

10

11

I2

k Il//\nprlroml

Figure 4. Ru K edge EXAFS spectra observed at 293 K for [Ru6N(C0)16]-/MgO (0.48 wt % Ru). The sample was heated In vacuum (813 K) andin HZ(588 K) The captions for (a)-(d) are the same as in Figure 1.
TABLE 2: Results for the Curve-Fitting Analysis of Ru K
Edge EXAFS (Observed at 293 K) for [Rufl(CO)&/MgO
at Different Ru wt % (0.48, 2.5, and 3.9 wt %p
Ruwt %
048
2.5
3.9

A

Ru-RU
Ru-0
AEo' A(o*)~ Nr
AEo A(u2) Rf
4.0 2.65 -0.4
4.6
1.2 2.09 4.1 -3 0 4.8
4.1 2.62 -1.4
40
0 5 2.13 12-2 2 2.5NP

5 0 262-94

3.5

0 3 2.20

13

-5.0

4.7

The samples were treated in the same conditions as in table 1. IneV d ~ 10-3
nA2

judging from NR"-o,, (0.6). In H2, the cluster showed nostructural change of cluster framework (Table 4).
2 . With N2. The EXAFS spectrum was observed for [ R u a @-Os,)3] on MgO in NZ(25 kPa) at 193 K. In its FT,a shoulder
peak on the lower distance side than Ru-Ru bonding became
stronger than in Figure IC. The data were best fitted with threewaves, Ru-Ru, Ru-Os,, and Ru-N, with the distances 2.62,
2.13, and 1.95 8, (Rf 1.7%, in Table 3) rather than two waves(Ru-Ru, Ru-Os,) (Rf 4.0%). The N R ~ - (0.4)
N by three-wave
fitting suggested two or three on-top N2(a) or one or two
bridging N2(a) on the Ru cluster because molecular N2 should
not be dissociated at 193 K. The N and r for Ru-Ru and RuOs, bondings did not change by the adsorption of N2 (NR,-R,= 4.1 f 0.1, m,-~,,= 2.62 A, NR,-o,, = 0.5, m,-o,, = 2.13 8,(Tables 1 and 3).
The interaction with N2 (25 kPa) was also examined for[R@]-Cs+/MgO.
The curve-fitting analysis with three waves,
Ru-Ru, Ru-Os,, and Ru-N, improved the fitting comparedto two-wave (Ru-Ru, Ru-Os,) fitting, especially in the lower
k region. The obtamed r for Ru-Ru and Ru-Os, bondings by

three-wave fitting (Table 3) showed slight changes ( A ~ , - R=,
0.02 A, ArRu-o,, = 0.04 8)compared to the sample in vacuum(Table 1). The
was 2.02 A, larger by 0.07 A than forN2(a) on [RuaIMgO (Table 3).
Discussion-

Cluster Structures. The [RUa(co)16]- cluster 1 interactedwith MgO to form [Ru&(C0)16-~]~-(xl), releasing one
CO at room temperature, and transformed to [Ru6N@-Osu)3](Figure 7) by the decarbonylation in vacuum at 813 K (Figure
1 and Table 1). The attached cluster framework structure did
not change by subsequent treatment in H2 at 588 K or thetreatment in N2 and H2 at 588 K (m,-~,,= 2.62 A, r~,-o,, =2.13 A). On Cs+-doped MgO and K+-doped MgO, the cluster
1 interacted with Cs+/MgO or K+/MgO to form [Ru&(C0)16-xlm- (x 3-4), releasing three to four CO ligands fromthe cluster, and transformed to [Ru6N@-Os,)3] (Figure 7) by
the evacuation at 673 K and hydrogen treatment at 588 K (mu-,.,,= 2.63 8,mU-o,,= 2.03 A on Cs'/MgO and m , , - ~=, 2.63 8,,~R,,-o,,= 2.00 8,on K+/MgO, in Table 1). In contrast, cluster
1 transformed to [Ru3@-2-Osu)3]on A1203by the evacuation at
813 K (Figure 7), and the species were very stable in H2 at
588-773 K (Table 1). This bangle cluster (Q,-R,, = 2.61 A,
mU-o,, = 2.16 A) should be similar to [Ru3(p2-OSu)3]on A1203
prepared from cluster 2 (~R,,-oJ = 2.06
Considering the
crystal structures of A1203 (corundum for a and spinal for y )and MgO (NaC1-t pe), the smallest interatormc distances of twoO;, atoms at 2.8 for A1203 and 2.98 A for MgO. Comparedto the ~-R,-R,, of attached Ru clusters (2.61 8, for A1203 and2.62 8,for MgO, Table l), the obtained NR,-o,, (1.9) for [ R u ~ ]species on A1203 can be interpreted only as the p2 coordination-

8:

Izumi and Aika

10342 J. Phys. Chem., Vol. 99, No. 25, 1995
I

400

500

600

700

surface OH groups were estimated to be 2.3 x 1O2I and 1 x
lo2' per gram of MgO, respectively. On MgO, OH groups wereobserved as a sharp strong peak at 3757 cm-I in addition to a
weaker broad peak around 3614 cm-I after pretreatment at 773
K, and the peak intensity of these peaks did not change by theinteraction with cluster 1 in THF and/or heating in vacuum at
813 K. The number of attached [Ru6N] clusters was 4.8 x
lo'*, 2.4 x lOI9, and 4.0 x 1019 per gram of MgO in the caseof 0.48, 2.5, and 3.9 wt % Ru samples, respectively. It wasdifficult to monitor the MgO surface sites which were reactedwith [Ru&] clusters because the reaction of incipient [Ru6N(C0)16-xln- (partially decarbonylated) with the MgO surface(to form Ru-Os, bondings) occurred during heating in vacuum(8 13 K), accompanying the total decarbonylation. However,the number of surface OH groups was larger by about 30 times
than that of [ R u a ] for the 3.9 wt % Ru sample, and the NR,-o,,changed very critically with different Ru loadings (Table 2). In
this context, we believe the reaction of the cluster is with Os,,not with OH. As the (001) face is believed to be predominantly
exposed for MgO even as a powder,I7 two models of supported[Ru$J@-O,,),] were proposed on major sites of MgO(001)(Figure 8a) and on fewer sites on MgO(001) with steps (Figure8b). We think that the [Ru6N] with low Ru loading (Ru 0.5
wt %) mainly interacted with the lower coordination MgO
surface to form [ R u ~ @ - O , , ) ~(x] = 5-7), such as in Figure
8b (NR,-o,, = 0.83 as a model), on the basis of the obtained
NR,-o,, by EXAFS (1.2, Table 2). The [Ru6N] at moderate Ru
loading (Ru 2.5 wt %) mainly interacted with relatively flat
MgO planes to form [Ru6N@-Osu)3], similar to Figure 8a(NR,-o,, = 0.5 as a model), on the basis of the obtained NR,-o,,by EXAFS (0.5, Table 2). Heterolytic fragmentations of [ R u a ]by heating in vacuum at 813 K, such as from [Ru~N]to [Ru~N]
Ru, seemed improbable on the basis of wavenumbers andthe peak pattem of adsorbed CO (in IR) on [Rua]/MgO (heatedat 813 K and in H2 at 588 K) at 0.06 wt % Ru (1978w(sh),1965s) similar to that at 2.5 wt % (2059w(sh), 2045w(sh),1992s)," without specific twin or triple peaks ascribable tosurface Ru(CO), species.'8-20
The nuclearity of [ R u a ] on MgO, Cs+/MgO, or K+/MgO
did not change by heating in vacuum or in H2 probably due tothe strong binding of [ R u ~by] nitrido nitrogen on the basis of
the experimental facts of the retention of nitrido N below -600
K in H2 in TPR (Figure 5a) and the NR,-R, (about 4) by EXAFS(Tables 1-3). The 100%-raft [ R u ~on] flat MgO(001) cannot
explain the observed NR,-o,, = 1.2 (r~,-o,,= 2.09 A) with
NR,-R, = 4.0 (mu+, = 2.65 A) at 0.48 wt % Ru (Table 2)
because the NR,-R, should be around 3 for raft [RU6]. We
believe the selective reaction of O ~ - [ R U ~ Nwith] a lower
coordination MgO surface at lower Ru loadings was quite
probable due to higher reactivity of lower coordination surface
sites. The rmetal-0,,distances were 2.09-2.17 A for [Ru6N]/
MgO (Tables 2 and 3), 2.00 8, for [Ru6N]-K+/MgO (Tablel), and 2.03-2.07 8, for [Ru6N]-Cs+/MgO (Tables 1 and 3).
The ~ , , - o , ,distances were shorter on K+- or Cs+-doped MgO(2.00-2.03 A) than on alkali-undoped MgO (2.13 A) after theincipient clusters were heated at 813 or 673 K (Table 1). The
distances ~R,-o,, for [Ru6N]/MgO were always longer (2.092.20 A) than [Ru~N]-A+/M~O(A = K, CS) (2.00-2.07 A) at
any Ru wt % and in H2 or N2 (Tables 1-3), implying the [Ru$rT@-Os,)3] clusters were coordinated to Os, bonded to K+ or Cs+
ions on K+/MgO or Cs+/MgO (Figure 7).
Similar to the supported nitrido [Ru6N] clusters, the carbido
clusters 3 transferred to [ R ~ C ~ - O s , ) (m,-~,31= 2.63 A, ~R,-o,,= 2.12 A) on MgO by heating in vacuum at 623 K.',' Byfurther treatment in H2 at 623 K, it lost carbido carbon and was

-

400

~

500

600

760

Temperature / K
Figure 5. Temperature-programmed reduction (TPR) in H2 for(A),andincipient [Ru6N(c0)16]-/MgO (e),[Ru$~T(CO)~~]--CS+/M~O[Ru6(C0)18]*-/Mg0 (m): (a) NH3 desorption monitored by mass; (b)the decrease rate of the IR peak at 1117 cm-I. The heating rate is 4
K min-I.

of three Os, atoms. The NR,-o,, (0.5) for [Ru6N] species on
MgO can be understood as three Os, atoms coordinated to one
triangle face [ R u ~ of] the Oh-like [Ru~N]species as the p1
coordination. This coordination style of [ R m ] species on MgO
was ensured by the Ru wt % dependence of NR,-o,, (see below).
The other examples of metal-Os, bond distances (2.00-2.83
A) were summarized in ref 2 for the supported Ru, Rh, Pd, Re,
Os, Ir, or Pt clusters, and the distances were classified as either
in the range 2.00-2.24 8, or in the range 2.63-2.83 A.
Observed ~R,-o, values for [Ru6N@-Osu)3]on MgO (2.13 A),
K+/MgO (2.00
Cs+/MgO (2.03 A), or A1203 (2.16 A) were
within the range 2.00-2.24 A, similar to other supported Ru,
Rh, Re, or Os clusters. In the curve-fitting analyses in Tables
1-3 for Nz-free samples, all the Fourier-filtered k3x were wellfitted with two waves (Ru-Ru, Ru-Os,). The addition of a
Ru-N,,, (Ni,,, interstitial nitrogen) wave (2.06 A) was needed
to fit the EXAFS of cluster 1 (powder) as the bonding between
Ru and NInt. The reason that the additional wave was not needed
to fit the experimental data in Tables 1-3 may be electron
donation from N,,, to [RUG],as suggested by IR and the distorted
framework structure of [Ru6N] from its oh symmetry when
~nsupported.'~Similarly, Ru K edge EXAFS for supported[RU&] was well fitted without a Ru-CInt wave probably due
to the chemical state of C,,, (with less electrons) as a backscattering atom in EXAFS and the distortion of the [RU&]A),

The dependence of cluster structure on Ru wt % was
examined for [RusN]/MgO at 0.48, 2.5, and 3.9 wt % Ru. Oh
the basis of the surface area of MgO (200 m2 g-l), mg-0(2.106
A), and the number of surface OH groups of MgO heated at
773 K (4nm-*),I6 the numbers of surface 0 atom (Os,)and-+J. Phys. Chem., Vol. 99, No. 25, 1995 10343

Supported [Ru~N]Clusters and Reactivities

TABLE 3: Results for the Curve-Fitting Analysis of Ru K Edge EXAFS (Observed at 193: loop and 30 Kc) for
IRuaN(CO)161-Mg0and [RuaN(CO)161--Cs+/Mg0in HZor Nzd
Ru-RU
support

gas

N

Ru-N

Ru-0

AEd

P

N

A(u2)g

r

AEo

A($)

N

Rf

AEo A ( d )rMgOvacuumC 3.9
2.62
-11
0.36
0.7
2.15
7.0
-0.05
4.9
Hza
4.2
2.65
-7.9
3.6
0.5
2.17
5.2
-1.5
2.3
N2a
4.2
2.62
-9.9
6.8
-3.9
0.4 1.95 -13
10 1.73.0
0.5
2.13
4.0 ( f 0 . 5 ) 2.71 (f0.015) -14 (h5)3.5 (f0.6) 0.6 (f0.2) 2.06 (fO.O1O) 1.2 ( f l . l ) -2.1 (f2.2)
W h i g 0 H2'
3.8 (f1.9)
2.65
Nf
3.7
-6.9
5.8
0.7
2.07
6.4
-3.0
0.7 2.02
8.9 4.3 1.9

a MgO catalyst in H2 and N2 and Cs+/MgO catalyst in N2 observed at 193 K.
Cs+/MgO catalyst in H2 observed at 100 K. MgO catalyst in
vacuum observed at 30 K. The samples were treated in the same conditions as Table 1. The error (within the 50% increase of Rf) is shown inA*.
parentheses. e In A. f h eV. 8 In

0

1

2

3
R (Anprlrwn)

5

4

6

5
4

3

2 -

2
h

5

'

mN

0

Y

1

2

4 -

3
4

2

3

4

5

6

7
8
k IlIAnprIrom)

S

1

0

1

1

1

2

1

5

3

r

8

9

10

I1

12

k (l/Anprlrom)

Figure 6. Ru K edge EXAFS spectra observed at 100 K for [Ru&(CO)l&-Cs+/MgO(2.5 wt % Ru, Cs+/[RusN] = 12) in H2 (76 Wa). The
sample was heated in vacuum (673 K) and in H2 (588 K). The captions for (a)-(d) are the same as in Figure 1

TABLE 4: Results for the Curve-Fitting Analysis of Ru K
Edge EXAFS (Observed at 293" and 100 Kb) for[Ru&Os.)3-4] on MgO Prepared from [Ru&(CO)&le]- in
Vacuum and in HZat 623 Kc
Ru-RU

P
AEo'
vacuum" 4.2 2.63 -10
Hzb
3.7 2.63 -7.9
gas

N

Ru-0

A(u2)f
2.2
1.5

Nr
AEo A(02) Rf
0.6 2.10 0.7 -0.3 4.2
0.7 2.11 2.3 -0.5 3.3

In vacuum at 293 K. In H2 at 100 K. The samples were treatedin the same conditions as in Table 1. In A. e In eV. f In
A2.

transformed to [RU6@-Osu)3-4](Q,"-R,,= 2.63 A, mu-oSy= 2.10
A) on MgO (Table 4). Corresponding TPR in H2 for [Ru&]/
MgO had a peak of C h around 560 K (carbido C desorbed asC&),I similar to the peak of NH3 around 660 K for [Ru6N]/
MgO and [Rua]-Cs+/MgO (Figure Sa). The cluster 2 without
an interstitial atom readily aggregated to larger Ru particles on

MgO during the evacuation at 813 K and/or subsequent H2
treatment at 588 K (NR~-R,,= 5.9-6.2). Thus, nitrido N (orcarbido C) was suggested to maintain the [RU6] cluster
framework by binding from the inside even at high temperatures
or in H2.
Cluster Structure Changes in H2 or N2. In the EXAFS
measurements in H2 or N2, a drastic change of k3x was observedfor [RusN]-Cs+/MgO in H2 (Figure 6 and Table 3) compared
to no change in vacuum at 30-293 K (Tables 1 and 3). Smallerchanges of k3x (and the fitted values) were observed for [Rua]/
MgO by the introduction of H2 (Table 3). It should be notedthat initial EXAFS spectra in vacuum were reproduced by theevacuation for these two cases in H2, indicating these EXAFS
changes by the adsorptioddesorption of H2 were reversible.
The causes of the spectral changes in H2 or N2 were classified
as (1) simple adsorption of gas and (2) adsorption-induced
structure changes of cluster framework (AQ,,-R,,) and cluster/Izumi and Aika

10344 J. Phys. Chem., Vol. 99, No. 25, 1995

/MgO

\g+go

7pYY?ona72,:zz
yz;F&// / / /

Figure 7. Structures of supported clusters on MgO, K+/MgO, Cs+/
MgO, and A1203 prepared from [Ru6N(CO)&. The samples were
heated in vacuum (8 13 or 673 K) and in Hz (588 K).

structure (a)structure (b)Figure 8. Structure models for [Ru~N]clusters on MgO: (a) on flat
MgO face; (b) on stepped MgO face. Average NR”-o,, are 0.5 and0.83 in model structures (a) and (b), respectively. The ~ , , - o , ,are basedon EXAFS data in Table 2 (2.13 for (a) and 2.09 A for (b)). Only Ru3
triangle faces of [ R u ~ N are] drawn for clarity.

The H-induced structure change of the Ru(0001) surface wasmonitored by VLEED (very low-energy LEED (low-energy
electron diffra~tion)).~~
It was found that the first layer Ru array
expanded by 1.7%in the [liOO] direction and by 0.9% in the[OOOl] direction by H adsorption (Tad < 72 K). The otherexamples of H-induced structure change were reported for Co,
Ni, Pd, Mo, and W single-crystal surfaces.25 The expansionsof O d - p d on conventional Pd/c and Pd/A1203 by hydride or
carbide formation were reported by means of EXAFS.’ The
O d - p d increased by 2.0 and 2.4% with hydride formation andby 1.2 and 0.3%with carbide formation for Pd/C and Pd/A1203,
respectively. The expansion of O d - p d of Pd/ZrO2 by carbide
formation was mesured during the CO disproportionation by
XRD (X-ray diffraction).26 The carbon was incorporated up to
15 atom % in Pd, and the O d - p d was linearly increased by up
to 2.6%. The change of(A = 0.08 A, 3.0%) for our[Ru6N]-Cs+/MgO (Table 3) was larger than the above Hinduced reconstruction of Ru(0001) surface (0.9- 1.7%) or“hydrogen-absorptive” or “carbon-absorptive” supported Pd
catalysts (0.3-2.6%). This large expansion of the [Ru~N]
framework may be due to the fact that all the Ru atoms of[RugN@-Osu)3]were exposed and able to interact with H(a).
The adsorbed amount of hydrogen atoms were three to four on[RusN]/MgO at 185 K and five to six on [Ru6N]-Cs+/MgO at
77 K.”
The catalytic effect of [RU6C] expansions of [Ru6C]/A1203,/La2O3, or /MgO induced by CO was discussed and summarized
to afford enough space on [Ru~C]framework to dissociate H2
molecules, and CO-associative oxygenate compounds wereselectively produced on the expanded [Ru~C].’,~
The catalytic
effects of [Ru~N]expansiodcontraction of [Ru6N/MgO and[Ru6N]-Cs+/MgO induced by hydrogen adsorptioddesorption
are described in the accomanying paper.
Conclusions

1. Supported [Ru$l@-0,,,)3] clusters were prepared on MgO,
support interface (Ar~,,-o,,). The adsorption of N2 should be
K+/MgO, or cs+/Mgo from [RusN(C0)16]- clusters (2.5 wt
simple nondissociative adsorption. Examples are reported for% Ru). The cluster [Ru,jN(C0)16]- was cleaved to the trimer
the distance rmeta1-~
for adsorbed N2 on metal catalysts or N2[Ru3@2-Osu)3]on A1203 by heating at 813 K. These clusters
coordinated organometallic complexes. The mi-N were reported
were stable in H2 at 588 K.
to be 1.91-1.94 A for n-coordinated N2 in [(PhLi)&N22. The m,,-o,,,were in the range 2.09-2.17 A for [Ru6N]/(Et20)2]2.21 The W - N were reported to be 1.832-1.833 A for
MgO, and they were shorter (2.00-2.07 A) for [Ru6N]-K+/
nearly straight V-N-N-V
bonding in @~-N~)[{(o-M~~NCH~)-MgO and [RusN]-Cs+/MgO. The [Ru~N]was implied to beC6H4)}2V(Py)]2(THF)2.22 The mi-N were reported to be 1.857reacted with three Os,,atoms bonded to K+ or Cs+ ions on K+/
2.181 A for ,u3-N2 in @3-N2)[(r5:rl5-C~oHs)(~-C5H5)2Ti2I*[(rl1:
MgO and Cs+/MgO.
r15-C5H4)(rl-C5H5)3Ti21’[(~-c5H5)I403)TiIc6H
2(C6H
I 403.23 The
3. The [ R u a ] was suggested to inteact with lower coordinamu+ were 1.95-2.02 A for N2(a) on [R@]/MgO andtion sites of the MgO surface, such as step sites, on the basis of[RusN]-Cs+/MgO (Table 3). The adsorptions of molecular N2
the increase of NR,,-Q,, when the Ru loadings were changed fromwere not accompanied by the expansiodcontraction of cluster
2.5 to 0.5 wt %.
framework [Ru~N],keeping the Q,,-R~ (almost) unchanged for
4. The expansion and contraction of the [Ru~N]framework[RusN]/MgO (2.62 A) and for [RusN]-Cs+/MgO (2.64 f 0.01
were observed by the hydrogen adsorption and desorption forA) (Table 3).
[Ru$l]/MgO (Am,,+,, = 0.03 A) and [Ru6N]-Cs+/MgO
The adsorption of H2 must be accompanied by adsorption(Amu+,, = 0.08 A). This expansiodcontractionof the [Ru~N]
induced structure changes of cluster framework [Ru~N]. The
framework was found to be reversible.
R,,-R,,changes from 2.62 to 2.65 A for [Ru6N]/MgO and from
5. N2 was adsorbed on [Ru@I]/MgO and [Ru6N]-Cs+/MgO
2.63 to 2.71 8, for [RusN]-Cs+/MgO (Table 3) by theat 193 K at m,,-~= 1.95-2.02 A observed by EXAFS without
adsorption of hydrogen. These H-induced structure changes also
structural modifications of the [Ru~N]framework.
induced smaller changes of cluster/support interface; the Q,,-o,,References and Noteschanged from 2.13 to 2.17 A for [RusN]/MgO and from 2.03
to 2.06 A for [RusN]-Cs+/MgO (Table 3). The changes of(1) Izumi, Y.; Chihara, T.; Yamazaki, H.; Iwasawa, Y. J . Phys. Chem.
mu+,,(1.1 % for [Ru$l]/MgO and 3.0%for [Ru6N]-Cs+/MgO)
1994, 98, 594.
( 2 ) Izumi, Y.; Iwasawa, Y. Chemrech 1994, July, 20.
are smaller than the [Ru~C]framework expansiodcontraction(3) Izumi. Y.; Chihara, T.; Yamazaki, H.; Iwaswa, Y. J . Chem. Soc.,
induced by CO (9.1%) for [Ru&]/MgO and [Ru6C]/La203
Chem. Commun. 1992, 1395.
(5.7%)and comparable to that induced by CO for [Ru&]/A1203(4) Izumi, Y.; Chihara. T.; Yamazaki. H.; Iwasawa, Y. J . Am. Chem.
(1 .5%).’s2
Soc. 1993, 115, 6462.

Supported [Ru~N]Clusters and Reactivities(5) Izumi, Y.; Aika, K. Chem. Lett. 1995, 137.
(6) Kamper, F. W. H.; Koningsberger, D. C. Faraday Discuss. Chem.
SOC. 1990, 89, 137.
( 7 ) McCaulley, J. A. J . Phys. Chem. 1993, 97, 10372.
(8) Izumi, Y.; Hoshikawa, M.; Aika, K. Bull. Chem. SOC.Jpn. 1994,
67, 3191.
(9) Bhatia, S.; Engelke, F.; Pruski, M.; Gerstein, B. C.; King, T. S. J.
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(10) Izumi, Y.; Liu, T.; Asakura, K.; Chihara, T.; Yamazaki, H.;
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(11) Izumi, Y.; Aika, K. J . Phys. Chem. 1995, 99.
(12) Blohm, M. L.; Gladfelter, W. L. Organometallics 1985, 4, 45.
(13) Yokoyama, T.; Hamamatsu, H.; Ohta, T. Analysis Program
EXAFSH2; The University of Tokyo, 1994.
(14) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer:
Berlin, 1986.
(15) Binsted, N.; Cook, S. L.; Evans, J.; Greaves, G. N.; Price, R. J .
Am. Chem. SOC. 1987, 109, 3669.

J. Phys. Chem., Vol. 99, No. 25, 1995 10345(16) Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dordrecht,
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