J. Am. Chem. SOC.1993,115, 6462-6463

6462

CO-Breathing Ruthenium Carbido Clusters on MgO in
CO/Hz Reaction Conditions
Yasuo Izumi,fJ Teiji Chihara,t Hiroshi Yamazaki,t andYasuhiro Iwasawa*.t
Department of Chemistry, School of Science
The University of Tokyo, Hongo
Bunkyo- ku, Tokyo I 1 3, JapanThe Institute of Physical and Chemical Research
Wako-shi, Saitama 351 -01, Japan
Received January 14, 1993
Revised Manuscript Received March 30, 1993
Supported metal clusters are often transformed to ill-defined
microparticles or to degradated species under catalytic reaction
though limited examples for reversible decarbonylation of metal clusters have been reported.& The ruthenium
carbido cluster [Ru&(CO)~~(CHJ)]supported on Al203, Ti02,
MgO, and LazO3, keeping the hexamer framework [Ru~C],
catalyzed both ethene hydroformylation to form propanal andCO hydrogenation to form oxygenates with good selectivities, incontrast to the poor catalysis of the usual supported ruthenium
catalysts for these reaction^.^ In this study we have foundreversible structural change in the cluster framework [ R u ~ C ]with interstitial carbon by CO adsorption/desorptionunder CO/
Hz reaction conditions.
[N(CH~)~CH~P~]+[RU&(CO)I~(CH~)](I)*was reacted withMgO (200 mz g-l) as previously reported.7 The loading of Ru
was fixed at 1.8 wt %. The Ru K-edge EXAFS spectra (KEKproposal no. 9 1012) were analyzed by a curve-fitting method on
the basis of the formula of plane-wave single scattering theoryg
using empirical phase shift and amplitude functions extracted
from the cluster 1 for Ru-C, Ru-Ru (cluster), and Ru(-C-)Rubonds, [RuC12(C0)3]2 for the Ru(-C-)O bond, RuO2 powder
for the Ru-0 bond, and Ru powder for the Ru-Ru (metallic)bond.
Upon the reaction between the [Ru&(CO)~~(CH~)]cluster
and the MgO surface, the methyl ligand of the cluster reactedwith the surface hydroxyl group of MgO to form methane at 290
K, accompanied by the release of about two CO ligands from thecluster. The observation of Ru-Ru (nearest neighbor) (0.291nm, N = 4.1) and Ru(-C-)Ru (in diagonal line of octahedral[RU~C]framework) (0.410 nm, 1.0) by EXAFS (Figure l a andTable I) assures the retention of the [Ru&] structure.
The [ R u ~ Cframework]
was shrunk by evacuation at 523-623
K, as characterized by rRu-Ru = 0.263 nm (Table I, entry b).
Besides the Ru-Ru bonds, a shoulder peak at the lower distanceside of the Ru-Ru peak appearedin the Fourier transform (Figurelb) which was assigned to the Ru-O bond at 0.212 nm by theSchool of Science, The University of Tokyo.
8 The Institute of Physical and Chemical Research.
I Present address: Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama
227, Japan.
(1) Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dortrecht, The
Netherlands, 1986.
(2) Lamb, H.H.;Gates, B. C.; KnOzinger, H.Angew. Chem., Int. Ed.
Engl. 1988, 27, 1127.
(3) Metal Cluster in Catalysis; Gates, B. C.; Guczi, L.; KnOzinger, H.,
Eds.; Elsevier: Amsterdam, 1986.
(4) Asakura, K.; Bando, K.; Iwasawa, Y. J. Chem. Soc., Faraday Trans.

----.
--.
( 5 ) Sheu, L. L.; KnOzinger, H.;
Sachtler, W. H.M.J. Am. Chem. Soc.
1990. 86. 2645.
--1989, 1 1 1 , 8125.
(6) Lamb, H.H.;Gates, B. C. J . Phys. Chem. 1992, 96, 1099.
(7) Izumi, Y.; Chihara, T.; Yamazaki, H.;Iwasawa, Y. J. Chem. Soc.,
Chem. Commun. 1992, 1395.
(8) Chihara, T.; Aoki, K.; Yamazaki, H. J. Organomet. Chem. 1990,383,167.

(9) Teo, B. K. EXAFS Basic Principles and Data Analysis; Springer:
Berlin, 1986.

Distance r ( 10.' nm)

8 4.80xmY
L2 3.20LiL'
1.80

0104.00 I

234
Distance r ( 10" nm)5656A

3.20

5 2.40m
YL

1.60
ALiL'
0.80

010234
Distance r ( 10'' nm)

Figure 1. EXAFS Fourier transforms for [RU~C(CO),~(CHI)]-/M~O.
(a) Incipient supported species; (b) evacuated at 623 K and (c) treatedin CO (14.6 kPa) H2 (14.6 P a ) at 523 K for 1 h.

+curve-fitting analysis (Table I). The Ru-O bonds are similar to
those for other supported clusters recently reported.4JO-12 Theloss of one CH3 and two CO ligands in the incipient supported
cluster mentioned above implies the advantageous interaction
betweenaclusterfacecomposedofthreeRuatomsandthesupport
surface. In this geometricarrangement of the supported cluster,
three of the six Ru atoms are able to attach to the MgO surface.
The averaged coordination number of the Ru-O bond wasobserved to be nearly one (1.1; Table I). Thus we propose the[Ru&(p-oMg)~]cluster structure with three bridging oxygen(10) Asakura, K.;Iwasawa, Y. J. Chem. Soc., Faraday Trans. 1990,86,2657.
(1 1) Kirlin, P. S.;van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C.
J. Phys. Chem. 1990,94, 8439.
(12) Chang, J. R.; Gron, L. U.; Honji, A.; Sanchez, K. M.;Gates, B. C.
J . Phys. Chem. 1991, 95, 9944.

0002-7863/93/1515-6462$04.00/00 1993 American Chemical Society

Communications to the Editor

J. Am. Chem. SOC.,Vol. 115, No. 14, 1993 6463

Table I. Results of the Curve-Fitting Analysis of Ru K-Edge EXAFS for [RUgC(C0)16(CH3)]-on MgO under Various Conditions4

Ru-C
entryb N rlnm MIeV N

Ru-RU
Ru(-C-)O
Ru(-C-)RU’rlnm AEIeV N rlnm AEIeV N rlnm U l e V Na0.291
0.263
0.287
0.269

2.1 0.190

-4.5

4.1
4.2
3.9
4.2bCd

1.6 0.188
1.0 0.187

-5.8
-7.8

-0.3
-3.4
-3.6
-5.5

2.2 0.301
1.7 0.299
1.2 0.300

-1.9
-4.3
-6.0

1.0 0.410

0.8 0.410

Ru-O
Ru-RU
rlnm AEleV N rlnm AEIeV Rfd/%

0.8

0.8

1.1 0.212
0.8 0.204
0.7 0.211

3.1
4.5
4.7

1.0 0.381

-6.0

1.7
3.2
2.6
1.7

XRD 2.8 0.190
4.0 0.290
2.5 0.302
1.0 0.410
0 Each Debye-Waller factor was taken to be equal to that obtained in the analyses for the model compounds. a, incipient supported species; b,a was evacuated at 523-623 K, c, b was treated in CO (14.6 kPa) + H2 (14.6 H a ) at 523 K, d, b was reduced in H2 (14.6 kPa) at 623 K and treatedin CO (14.6 kPa) + Hz (14.6 kPa) at 523 K. A multiple-scattering shell correspondingto a diagonal line of octahedral [RbC] framework. Residuale XRD analysis for [N(CH~)~CHZP~]+[R~&(CO)~~(CH~)]-.
factor = Jlk3xh(k) - k3x~(k)lUk/Slk3x0”(k)l2dk).

Scheme I. Structure Transformation of the CO-Breathing
Ruthenium Carbido Clusters on MgO[R%C(CO)I~(CH~)~( r(Ru-Ru)= 0.290nm )

290K

IMgO

[R&C(CO)la,l/MgO (X ii 2)[R~g(cO)6(~1OMg)sl/MgO( r(Ru-Ru)= 0.269nm )( r(Ru-Ru)= 0.291 nm )(a)

[RuaC(P-OMg)3l/MgO( r(Ru-Ru)= 0.263 nm )5 3K

CO+Hcvac.

(shrunk framework)(b)

1(shrunk framework)[Rt&(CO)l ~ ( -pOMg)3]/MgO( r(Ru-Ru)= 0.287 nm )(expanded framework)(e)atoms incorporated from the MgO surface (Scheme Ib). The
shrunk [Ru6C(p-OMg)3] cluster on MgO is suggested to be
distorted from an octahedral symmetry because of no observationof the multiple-scattering Ru(-C-)Ru peak by EXAFS.I3(1 3) Binsted, N.; Cook,S.L.; Evans, J.; Greaves, G. N.; Price, R.J. Am.
Chem. Soc. 1987,109,3669.

TheRu(-C-)Rupeakaround0.37 nm (phaseshift uncorrected)
in the Fourier transform was observed again under CO/H2 for
1 h a t 523 K (Figure IC). The Ru(-C-)Ru distance (0.410nm)
and its coordination number (0.8) in Table I are almost the sameas those for the incipient species. The distance of the nearestneighbor Ru-Ru bond also increased from 0.263 to 0.287 nm,
similar to 0.290 nm for the original cluster (XRD). The
coordinationnumber of 1.6-1.7for Ru-CO implies 1 1 carbonyl
ligands per [RusC]. The CO uptake value was consistent withthose determined by adsorption measurement and IR peakintensity. The amount of CO adsorption on the [ R y C ] clusters
on MgO was much larger at 523 K (10.7)than at 290 K (2.9),
indicating that the expansion of the cluster framework is anactivated process induced by CO uptake. The expanded cluster
was shrunk again by CO release by evacuationor at lower pressureof CO at 523 K. The expansion-shrink cycle of the [RU~C]
framework reversibly takes place in conjunction with CObreathing.
The octahedral [ R u ~ ]structure without interstitial carbon
characterized by the next-nearest Ru-Ru bond at 0.381 nm byEXAFS revealed no significantexpansion of the [ R u ~framework](Table I and Scheme Id). These results demonstrate that theinterstitial carbon has a key role, like a central spring to makethe cluster structure flexible. Only the expanded [ R y C ] clusters
on MgO selectively produced oxygenated compounds from CO/-H2.