594

J. Phys. Chem. 1994, 98, 594-602

CO-Breathing Structure Change and Catalysis for Oxygenate Synthesis from CO/H2 on
Supported [RuaC] Clusters: Structural and Chemical Controls by Interstitial Carbido Carbon
Yasuo Izumi,tvl Teiji Chihara,: Hiroshi Yamazaki,* and Yasuhiro Iwasawa'qt
Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo- ku,
Tokyo 113, Japan, and The Institute of Physical and Chemical Research, Wako-shi, Saitama 351-01, Japan
Received: July 26, 1993; In Final Form: October 22, 1993'

Catalysis and structures of supported ruthenium carbido clusters [Ru&(CO) 16/(CH3)]-/0Xides were investigatedin comparison with those of supported non-carbido clusters prepared from [R h ( C 0 ) 1,312- and traditional ruthenium
catalysts prepared from Ru(NO)(NO&. Oxygenate synthesis (methanol, dimethyl ether, and formaldehyde)
in CO/H2 reaction was observed on the supported carbido clusters in contrast to the preferential formation of
methane and hydrocarbons on the conventional Ru catalysts and the supported non-carbido clusters. The active
structure for oxygenate synthesis crucially depended on the kind of oxides; on basic oxides (MgO and La203)
the cluster of framework was incorporated with surface oxygen atoms and expanded or shrunk under CO/H2
reaction conditions depending on the C O pressure. The reversible expansion-shrinking of the cluster framework
was correlated with the activated C O adsorption (CO breathing-induced structural change). On Ti02 the[RuaC] framework always held a shrunk structure as proved by EXAFS. The expanded clusters showed highselectivities in oxygenate synthesis. The shrunk [ R u ~ C/Ti02]
also showed much higher activity than the
non-carbido cluster or the traditional catalyst. IR and hydrogen isotope effects suggested the formation ofoxygenates through a p2-formyl intermediate. The switchover of reaction path from the formation of methaneand hydrocarbons to oxygenate synthesis is ascribed to the interstitial carbido carbon which has structural effectlike a central spring and electronic effect as a four-electron donor on the behavior of the cluster framework.

Introduction
The catalysis of metal clusters172 and supported metal cluster^^-^has extensively been studied thus far from chemical interests inthe unique metal ensembles as well as from industrial viewpointsof developing new catalytic multimetal systems. However,catalysis characteristic of metal cluster framework or that is notobserved on traditional metal catalysts is very rare. In addition,precursor clusters with originally well-defined structures are often
converted to ill-defined metal particles or degradated to heterogeneous metal sites by pretreatments or during catalytic
reactions. Octahedral carbido clusters of iron, ruthenium,
osmium, etc. are in general more stable than Ms(C0)12clusters(M = Fe, Ru, Os) and trigonal prismatic [M&] clusters (M =Co, Rh, etc).s7 We have reported that ruthenium carbidocluster[Ru6C(CO)16(CH3)]-supported
on SiO2catalyzesselectiveethenehydroformylation to form propanal, keeping the hexamer framework [RU&] under catalytic reaction condition^.^.^ Recently,
we also found that the ruthenium carbido clusters supported on
MgO, La2O3, TiOz, Zr02,and A1203catalyze CO hydrogenation
to form oxygenates with good selectivities in contrast to poor
catalysis of unusal supported ruthenium catalysts for oxygenate
synthesis fromCO/H2.10 Thecarbido-clusterframework on MgO
was also observed to expand or shrink under CO/H2 reaction
conditions;ll the structure change occurred reversibly between
the shrunk cluster (~R"-R" = 0.263 nm) and the expanded cluster(0.287 nm) at 523 K.
In this paper we report the catalysis of ruthenium carbido
clusters supported on inorganic oxides for oxygenate synthesis
from CO/H2 and the structural transformation of the supported
clusters. The present study is connected to chemical modificationof the cluster by the interstitial carbido carbon (promoter effectfrom inside) and to structural effect of carbido carbon like a
University of Tokyo.
*The Institute of Physical and Chemical Research.
1 Present address: Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama
227, Japan.
Abstract published in Advance ACS Absrrucrs, December 15, 1993.

0022-3654/94/2098-0594%04.50/0

central spring, by which the switchover of reaction pattern fromthe formation of methane and hydrocarbons (CO-dissociativehydrogenation) to oxygenate synthesis (CO-associative hydrogenation).
Experimental Section
The supported cluster catalysts were prepared in a similar way
to that described previously.8J0 The procedure is briefly summarized here. Ti02 (Degussa P25), ZrO2 (Soekawa, 99.9%),
A1203(Degussa alon C), and Si02 (Fuji-Davison, silica gel no.
952) were evacuated at 473 K before use as supports. ZnO(Kadox-15) was pretreated at 773 K. MgO and La203 wereprepared from Mg(OH)2 (Soekawa, 99.99%), and La(OH)3(Soekawa, 99.9%) was prepared by calcination at 773 K. Each
pretreated oxide was impregnated with a CH2C12 solution of thecluster [BTMA]+[Ru~C(CO)I~(CH~)]for 1 h at 290 Kin
argon (99.9999%), followed by removal of the solvent byevacuation. The catalyst derived from [Ru~C(CO)I~(CH~)]is
denoted as [Ru&]/oxide. The loadings of ruthenium were 3.0(SiOz), 2.0 (LazOs), 1.8 (MgO), 1.5 (TiO2, AlzO,), and 1.0 wt% Ru(Zn0, ZrO2). [BTMA]+2[Ry(C0)18]2-(2)13J4was also
supported in a similar manner to the case of 1 by using purified
acetone as a solvent (denoted as [Rub]/oxide). Conventional Ru
catalysts were prepared from aqueous solutionof Ru(NO)(NO3)3,
followed by calcination at 673 K and by reduction with H2 at 673
K (denoted as Ru/oxide). The Ru loadings in these systems
were adjusted to the loadings for the corresponding [RU&]
systems.
The CO/H2 reactions were carried out under CO (14.6 kPa)
and H2 (14.6 kPa) in the temperature range 423623 K in aclosed circulating system (dead volume 133mL) with a U-shaped
liquid nitrogen trap. The samples were evacuated at 623 K for
activation in situ before use as catalyst. The reaction products
C02, C2H6, C2H4, and C3H6were analyzed by a gas chromatograph equipped with a 2-m column of VZ-10. The oxygenatedcompounds like formaldehyde, dimethyl ether, acetaldehyde,
methanol, ethanol, and 2-propanol were analyzed with a 4-mcolumn of dioctyl sebacate, and CH4 was analyzed with a 2-m
0 1994 American Chemical Society

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 595

Structure and Catalysis of Supported Ru6C Clusters

TABLE 1: Turnover Frequencies (TOF) and Selectivities (S) for the Formation of Each Product of Oxygenates in CO (14.6
kPa) Hf(14.6 kpa) Reaction on the Ru Catalysts at 473 (A) and 523 K (B)

+

____~

FAC MEd DMe other$

I:

Sb/%

0
0
0
0.17
0.10
0.052
0.060
0
0.006

0.10
0.50
0.052
0.18
0
0.14

0.055
0
0.017
0.43
1.4
0.19
0.53
0.1 1
0.37

48

0.12
0.68
0.066
0.24
0.11
0.22

0
0
0.009
0.04
0.12
0.019
0.058
0
0

24
92
53
35
53
34
34

0.18
0
0
1.1
5.1
0.80
1.2
2.7
0.27

0.24
0.12
0.039
0.39
2.9
0.86
1.1
0.98
0.32

0.25
0.094
0.051
0.46
2.4
0.72
0.98
0.090
0.15

0.036
0.023
0.006
0.11
0.61
0.14
0.092
0.073

0.70
0.23
0.10
2.1
11.0
2.5
3.4
3.8
0.74

49
10
10
49
44
28
39
27
6.2

0.032 0.023

000.004 0.004

0FAC

MEd DMe othersf

-

-

0

0.051 0.040 0.060

-

0

0

0

-

0

-

0.003

-

-

-

0.20

2.1

-

0.063

-

-

0.15

-

0.047 0.052 0.061

30

0
0
0

7.1

0

0

-

-

-

-

-

0.15
0.16
0.046 0.12

- - -

-

0

-

-

0.046
0.12

-

0.51

-

0.16

3.0

-

-

-

0.36
0.30

17
13

-

-

0.51

-

-

2.1

0
m i d . Total oxygenates mol 96. Formaldehyde. Methanol. Dimethyl ether. fOther oxygenates were mainly acetaldehyde, ethanol, andpropanal. 8 Cluster catalysts were reduced with H2 at 623 K.

column of SA molecular sieves. The column temperature wasare also listed in Table 1. The TOFs for [Ru&]/oxides werealways 353 K. The turnover frequency (TOF) for the supported
larger by 10 (Ti02) to 36 (A1203)times at 473 K and 5 (Ti02)
clusters was calculated assuming 100% dispersion of Ru atoms,
to 25 (A1203)times at 523 K than those for [Ru6]/oxides. The
and for conventionalRu catalysts it wascalculated from Hz uptake
traditional Ru/oxides (Ti02 and LazO3) showed no activity forat 290 K.
the oxygenate synthesis at 473 K and the activity at 523 K was
The Ru K-edge EXAFS (extended X-ray absorption finealso low. The most active [Ru&]/TiO2 produced formaldehyde(20%), methanol (12%), and dimethyl ether (9.6%) at 523 K,
structure) spectra were measured in a transmission mode by using
the EXAFS facilities installed at a beam line 10B of the Photoand the activity did not decrease over 15 h. The order of the
Factory in the National Laboratory for High Energy Physics
activity of [Ru&]/oxides for support at 473-523 K was Ti02>(proposal No. 91012). The EXAFS spectra were taken in an
AI203 2 ZrO2 2 La203 > Si02 H MgO > ZnO.
EXAFS cell without contacting air by using a Schlenk transfer
The supported ruthenium carbido clusters also exhibited higher
technique from a closed circulating system for sample preparation
selectivitiesto oxygenated compounds than [ h a ]/oxides or theto the cell. The EXAFS analysis was performed by a curveconventional ruthenium catalysts (Table 1). Particularly thefitting method on the basis of the formula of plane-wave single
traditional Ru/oxide catalysts made no sense in practice; thescattering theory,I5 using empirical phase shift and amplitudeselectivities at 523 K were as low as 17% (LazOs), 13% (TiOz),functions extracted from cluster 1 for Ru-C(carbony1) and Ru2.1% (Al204, and 3.0% (MgO) against 49, 44, 27, and 49%,(-C-)Ru bonds, [RuC12(CO)3]2 for Ru(-C-)O bond, RuOz
respectively, for the corresponding [ R u ~ C/oxide]
catalysts.
powder for Ru-0 bond, and Ru powder for Ru-Ru(metal1ic)
Among supported carbido clusters, [ R u ~ C/La203]
was most
bond. The phase shift and amplitude functions for Ru-Ruselective (92% at 473 K and 49% at 523 K). The order of the(cluster) bond were obtained by subtracting the peak of Ru(selectivity for support was La203 > MgO Ti02 2 ZrO2 >C-)O of [RuC12(CO)3]2 from the superimposingpeak of Ru-Ru
A1203> Si02 2 ZnO. The [RueC]/oxide catalysts preferentiallyplus Ru(-C-)O of 1, taking into account each coordination
produced the oxygenated compounds, while the conventional Ru
number. The Fourier transform of the k3-weighted EXAFS
catalysts produced mainly methane and hydrocarbons (ethene,
oscillation was carried out over the range 30-140 nm-l. In theethane, and propene). Even if various amounts of carbon were
curve-fitting analysis the Debye-Waller factors for each coordeposited on the metallic Ru particles of Ru/oxide by the
dination shell were fixed to be the same values as those obtained
Boudouard reaction (2CO C + C02), the oxygenate synthesisin the analyses for the model compounds [RU~C(CO)I~(CH~)]-, was not augmented. By carbon deposition the reaction rate
[RuCl2(C0)3]2, Ru02, and Ru powder.
monotonously decreased due to the partial cover of Ru surface.
FT-IR spectra of the supported clusters were recorded on an
The TOF for [Ru6]/Al203 without the interstitial carbon was
FTIR spectrometer (JASCO FTIR-7000) in an IR cell with two
low enough as compared with conventionalimpregnated catalysts,
NaCl windows (self-supporting disk) combined with a closed
probably due to the degradation of the [Rua] framework by thecirculating system. Support wafers were pretreated at a given
reaction with surface oxygen (see hereinafter).
temperature in the cell and impregnated by the drop of a CH2C12After the reduction of [Ru&]/Mgo and [RU&]/TiO2 insolution of the cluster.
hydrogen at 623 K the catalytic activities for oxygenate synthesis
decreased to 0 (473 K) to 32% (523 K) or 13 (473 K) to 22%
Results(523 K) of those before the treatment for [Ru&]/MgO or[Ru&] /TiOz, respectively (Table 1). The TOF and selectivity
OxygenateSynthesisonSupported[Ru&(CO)Is(CH&Clwter
for [Ru&]/TiO2 reduced at 623 K are nearly the same as those
Catalysts. Catalytic oxygenatesynthesisfrom CO/H2 was studied
for [RU6]/Ti02 derived from [Ru6(CO)ls12-.
under CO (14.6 kPa) and H2 (14.6 kPa) at 473-623 K. The
TOFs and the selectivities for the formation of oxygenated
The rates of oxygenate synthesis were also measured at low
compounds, methanol, formaldehyde, and dimethyl ether, at 473
CO pressure (0.10 kPa of CO and 29.3 kPa of H2) at 523 K on
and 523 K are shown in Table 1. For comparison, the results for[Ru&]/MgO and [Ru&]/La203. The TOFs were only 10the catalysts derived from [ R U ~ ( C O ) ~and~ ] ~Ru(NO)(NO&
17% of those for the reaction at the pressure of CO (14.6 kPa)

-

596

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

TABLE 2

Izumi et al.

Turnover Frequencies (10-3 m i d ) for CO/H2 and Related Reactions.
523

TIUC/K

523

473

523

523

PcolkPa
14.6
0.10
14.6
14.6
PH,/kPa
14.6
29.3
7.3
14.6
14.6
7.3
PD,/kPa
product
HC
ox
HC
ox
HD
HCOf
CH3OC
HCOzd
CH3Od
[R~~C(C~)I~(CH~)I-/A~Z~~
10
3.8
1.1
0.91
1.3
0
[ R U ~ ( C ~ ) I S ~ ~ - / A ~ Z ~2.0~
0.15
R~(NO)(NOsh/Alz03
24
0.51[RyC(CO)is(CH3)1-/MgO
0.70
0.70
0
0.12
7100
306
0
0.53
0
R4NO)(N03)3/MgO
5.1
0.16
1600
R ~ ~ C ( C O ) I ~ ( C H ~ ) ~ - / L ~2.1
ZO~
2.1
0.93
0.22
3 3b
0
1.2
0
Ru(NO)(NO3)3/LazO3
1.I
0.36~ R ~ ~ ~ ( ~ ~ ) I ~ ( C 14
H ~ ~ ~ 11
- / T ~ ~ Z
4100[RU~(CO)IS~~-/T~OZ
4.9
2.1
9900
Ru(NO)(N03)3/TiOz
2.0
0.30
630
HC, hydrocarbons; ox, oxygenates. At 473 K. The rate of the increase of species. The rate of the decrease of species.
TABLE 3: Hydro en Isotope Effects in CO/Hz Reaction on
the Supported ~U&(co)I6(cH3)~Clusters and a
Conventional Ruthenium Catalyst.
volvw

catalyst
CHI C2H4 C3H6 H2CO CH3OH (CHthO[Ru&]/TiOz
1.1
1.1
1.1
1.9
2.0
2.0
[RU&]/MgO 1.1
1.6
1.5
1.4
[RU6C]/Laz03 1.0 1.5
1.6
1.4
Ru/MgO
0.9
Reaction temperature = 523 K, CO, 14.6 kPa; HZ(or Dz), 14.6 kPa.
Not produced or negligible.
(Table 2). The catalytic activities in Table 1 were recoveredagain after the switch of the reaction gas from CO (0.10 kPa)
H2 (29.3 kPa) to CO (14.6 kPa) + H2 (14.6 kPa).
HrD2ExchangeinC0. H2-Dz (14.6kPa, 1:l) exchangerates
in CO ( 14.6 kPa) were observed on [Ru&] /oxides, [ h a ]/oxides,
and impregnated Ru catalysts. The rates at 473 K are listed inTable 2. Hz-Dz exchange rates were more than 3000 times fasterthan the rate of oxygenate synthesis on every catalyst.
Hydrogen Isotope Effect and Hydrogen Pressure Dependency
inCO/H2ReectiononSupported[RyC(CO)16(CH~)rCatalysts.
The hydrogen isotopeeffects for oxygenate synthesis wereobservedwith [Ru&]/Tioz, [Ru&]/La203, and [Ru&]/MgO. The
ratios of YD/YH for methanol, dimethyl ether, and formaldehyde
were larger than unity (Table 3) (1.4-2.0 (average -1,7)),
indicating an inverse isotope effect. The rates of hydrocarbon
formation had negligible hydrogen isotope effects.
The TOF for each product was measured as a function of PH*(4-28 kPa) at a fixed PCO(a14.6 kPa) at 523 K. The rate on[Ru&]/TiOz was given by a power law expression: r = UPH;(a, constant), where x ranged 0.76-0.99 for formaldehyde,
methanol, and dimethyl ether.
In more detail the reaction was expressed by the rate equation:r = kK~&,/(lK H ~ P H Jassuming,
the Langmuir-type
adsorption of hydrogen on the [RuaC] clusters on MgO. The
KH1was determined to be 0.012-0.045 kPa-I which is comparable
to 0.034 kPa-I for the quantitative acetaldehyde formation fromthe methyl ligand of [RU~C(CO)I~(CH~)]-/S~OZ
at 398 Kin CO+ H Z . The~ [RU&] clusters on La203 also exhibited a Langmuirtype dependency on P H (KH,
~ = 0.10-0.1 3 kPa-l).
In Situ EXAFS Studies of Supported [RII&(CO)I~(CH,)~
Catalysts in CO + Hz. Upon the reaction between[Ru6C(C0)16(CH3)]-clusterand each oxide surface, the methyl
ligand of the cluster reacted with the surface hydroxyl group of
MgO and La203 to form methane at 290 K, whereas for [ R u ~ C ( C ~ ) I ~ ( C H ~ ) ] - / S ~ ~ Zor, Ti02 systems no gas desorption
was observed at 290 K. The EXAFS spectra for the incipient
supported clusters on SiOz,
and TiOZ were almost thesame as that for 1, also indicating simple physisorption of 1 on
Si02, A1203, and Ti0z.8 The methyl ligand of the cluster reacted++with the surface OH groups of these supports at 373-473 K in
vacuum.
Figure la shows the EXAFS oscillation, its Fourier transform,
and the curve-fitting analysis for the incipient [Ru&] /MgO.
The best-fit results are listed in Table 4. The decrease in the
averaged coordination number (N) of Ru-C (2.82.1) andRu(-C-)O bonds (2.5 2.2) suggests the release of about two
CO ligands per one cluster upon supporting. These CO ligands
were trapped as surface formates on MgO (CO + OH(a)HCOO(a)). The amount of HCOO(a) corresponded to about
two CO ligands per one cluster (formate peaks: 2838 (m) and
2732 (w,sh) cm-l for YCH, 1660 (m), 1623 (s) and 1608 (w,sh)
cm-1 for vas(w0),and 1382 (m) and 1313 (m) cm-1 for vl(wo)).16
The observation of Ru-Ru (0.291 nm, N = 4.1) and Ru(-C-)Ru(0.410 nm, 1.0) assures the retention of the [RbC] structurecompared to the XRD crystallographic data of 1 (0.290 nm (N= 4.0) and 0.410 nm (l.O), respectively) (Table 4).
Next, the incipient [Ru6C]/MgO species were evacuated at523-623 K. The cluster framework was shrunk as characterized
by the decreaseof dRu-Ru from 0.29 1 to 0.263 nm (Table 4b). The[Ru&] cluster unit is suggested to be maintained because of thecoordinationnumber of about four (4.2) for Ru-Ru bond (Table
4b). The shrunk cluster did not show the Ru(-C-)Ru bond peak
as shown in Table 4. It is known that the intensity of the Ru( 4 - ) R u multiple scattering peak is greatly reduced when theeRu<-Ru angle decreases a little from ?r for the linear conformationin an octahedral symmetry." The shrunk clusters on MgO andLa203 are suggested to be distorted from an octahedralsymmetry.
In fact the interstitial carbon was detected as CD4, which exhibiteda peak around 573 K in temperature-programmedreaction in D2(Figure 2), while surfacecarbidesC(a) deposited on Ru particles/oxides showed a CD4peak around 473 K. The peaks higher than-610 K (peak maximum -673 K) may be derived from
[BTMA]+because thecarbidoand non-carbidoRy clwterswhich
both have thesamecountercation([BTMA]+)did show thesimilar
higher peak in Figure 2. The cluster frameworkon La203, A1203,
and Ti02 was also shrunk to the framework with ~R,,-R,, = 0.262,
0.263, and 0.261 nm,respectively.
Besides Ru-Ru bonds, in the cases of MgO and La203 ashoulder peak at the lower distance side of Ru-Ru peak appearedin the Fourier transform (Figure lb2), which was assigned to
Ru-0 bond at 0.212 nm (MgO) and 0.209 nm (LazO3) by the
curve-fitting analysis (Figure lb3 and Table 4).
Drastic change in EXAFS was observed with the MgO andLa203 systems in catalytic CO + H2 conditions at 523 K. For
the [Ru6C] cluster on MgO the Ru(-C-)Ru multiple scatteringpeak around 0.37 nm (phase-shift uncorrected) in the Fouriertransform appeared again under CO/H2 for 1 hat 523 K (Figurelc2). At first we performed a curve-fitting of the EXAFS
oscillation by assuming two waves of Ru-C(carbony1) and Ru(-Z-)O because carbonyl peaks were observed by FTIR. The

-

-

-

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 597

Structure and Catalysis of Supported Ru& Clusters

1s

s

3

sr3

K

"r

z

iP n

in

I'

4's

5.4

3

4

5 6 7 8 0 1 0 1 1 I21314
Wave Number k (10 n d )3

5 6 7 8 9 1 0 1 1 121314
WN. Number k (10 nm.')

4

.LIo

3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4Wava Number k (10 nm-')

a.71

z
& 2.0
a

-

U
a
. 1.M
e.4

0

0

1

D i n a m r ( l o 1 nm)

4

5

6

7

8 9 1 0 1 1
Wave Number k (1Onm")

1

2

4

5

2
3
4
Distanw r ( 10.1 nm)

1

6

0

12

4

1

"

6

7
8
9 1 0 1 1
WawNumber k (10nm")

5

2
3
4
Dirtam r ( IO.( nm)

v.,

,
" '
6
7
8
9
10
Wave Numkr k (10 nml)
'

5

,

6

, . J
11

12

Figure 1. EXAFS spectra for [ R u & ( C O ) I ~ ( C H ~ ) ] - / M ~ (a)O ; incipient supported species, (b) after evacuation at 623 K in vacuum, and (c) after
CO (14.6 kPa)/H2 (14.6 kPa) reaction at 523 K for 1 h; (1) k3-weighted EXAFS oscillation, (2) its associated Fourier transform, and (3) curve-fitting
analysis, (-) observed, (- -) calculated.

-

TABLE 4(B).

Results of the Curve-Fitting Analysis of Ru K-Edge EXAFS for Supported [Ru&(CO)ls(CHs)r (A) and [Rur(CO)&Ru-C

N
Si02 (c)A1203 0 )A1203 (c)Ti02 (b)TiOl(c)MgO(a)
MgO (b)MgO(c)

Ru-RU

dlnm AEIeV N dlnm EIeV

0.8

0.186

-11

0.6
2.1

0.184
0.190

-7.7
-4.5

(1.4
(1.5
(1.5
1.6
1.0

0.185
0.186
0.184
0.188
0.187

-19
-10
-14
-5.8
-7.8

0.188
0.192

-5.8
-0.1

MgO(d)
L a 2 0 3 (b)LazO>(c) 0.9
All03 (b) 0.9
Ti02 (b)
XRD*c
2.8

0.190

6.6
4.4
3.8
3.2
3.5
4.1
4.2

Ru(-C-)O

Ru(-C-) Rud

Ru-O

Ru-RU

N dlnm AEleV N dlnm AEIeV N dlnm AEleV N dlnm AEIeV Rt/%

0.265 -8.4
0.263 -3.7
0.267 -5.5 0.8 0.300
0.261 -10
0.263 -11
0.6 0.299
0.291 -0.3 2.2 0.301
0.263 -3.4
1.3 0.306
2.3 0.284 -11
1.3 0.305
2.2 0.280 -18
1.3 0.306
3.9 0.287 -3.6 1.7 0.299
4.2 0.269 -5.5 1.2 0.300
3.3 0.262 -4.8
3.1 0.277
0.3 0.9 0.301
1.9 0.261 -3.7 0.7 0.299
3.8 0.264 -6.4
4.0 0.290
2.5 0.302

-1.9
-10
-1.9

1.0 0.410

0.3-1.0
0.1 0.9 0.409-4.3 0.8 0.410
-6.0
-1.0
-2.7

0.8

-0.4
0.8

1.1 0.212

3.1

0.8 0.204
0.7 0.211
0.7 0.209
0.5 0.198
1.8 0.206

4.5
4.7
4.5
0.2
1.4

1.0 0.381

-6.0

3.8
5.8
3.8
5.3
5.4
1.7
3.2
12)
6.2)
5.2)
2.6
1.7
3.7
2.2
4.5
5.5

1.0 0.410

a Debye-Waller factor was taken to be equal to that obtained in the analysis for the model compounds. (a) Incipient supported species; (b), (a)was evacuated at 523-623 K (c) (b) was treated in CO(14.6 H a ) Hz(14.6 kPa) at 523 K, and (d) (b) was reduced in Hz(14.6 Wa) at 623 K andtreated in CO(14.6 kPa) + Hz(14.6 P a ) at 523 K. *Residual factor (= Jlk3xh(k) - k3xdC(k)J2 dk/Jlk3xob(k)12 dk). CXRD analysis for[N(CH~)~CH~P~]+[RU&(CO)~~(CH~)]-.
A multiple scattering shell corresponding to a diagonal line of the octahedral [ R u C ] framework.

+

two-wave fitting in Figure 3a did not fit the experimental dataat all (Rr1295 in Table 4). Next the three-wave fitting of RuC(carbonyl), Ru-Ru, and Ru(-C-)O was conducted in Figure
3bbecause the shrunkcluster holds the nearest-neighbor Ru-Ru
bond. The fitting was improved, but the oscillation in the higher
wavenumber region was not reproduced (Rf 6.2% in Table 3).

Therefore, the Ru(-C-)Ru bond which should be observed in theoctahedralstructureof [ h a c ] wasaddedin
in Figure 3c, exhibiting a good fit in the higher wavenumber
region (Rf5.2% in Table 3). However, in the curve-fitting of
Figure 3c the lower wavenumber oscillation was not reproduced.
Since the shrunk [RuaC] species were linked to MgO surface

Izumi et al.

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

598

m 1.o

I

I

1

al

t

/\

3.

Figure 2. Temperature-programmed reduction spectra in Dz for (1)[R&C(CO)16(CH3)1-/Ti02,(2) lR&C(CO)ls(CH3)1-/MgO,,(3)[RW(CO)1s]~/Ti02,(4) C(a)/conventional Ru/MgO. The heating rate 4
k min-I.

clusters and the conventional catalysts are shown in Table 6. The
main peaks for the [Ru&]/oxides were observed at lower
frequencies by 97, 58, and 21 cm-l for A1203,TiO2, and MgO,
respectively, than those for the corresponding conventional Ru
catalysts.
In situ IR spectra for supported [RuaC] catalysts in CO/H2
reaction showed formate peaks at 1606 (u,(wo)), 1388 (u,(wo)),and 1343cm-l (us(oco))for [R&C]/Mg0,16at 158Ocm-i ( u M ~ ~ ) )for [Ru&]/LazO3, and at 1592 (u,(oco)), 1392 ( ~ c H )and
, 1380cm-1 ( ~ ~ ( 0 ~for0 )[ )R U & ] / A ~ ~ O On
~ . ~the~ ,A1203~ ~ system,
methoxy species at 1460 cm-1 ( ~ c Hwas
) also observed.22 The
formation and decomposition rates of formate and methoxy species
are shown in Table 2. For the AI203 system, both rates of theformation and decomposition of formate and methoxy species
were smaller than the TOF for oxygenates. In the cases of Ti02
and La203 the increase of formates was much faster than theoxygenate synthesis, but the decrease of formates was slower
than the oxygenate synthesis (Table 2). These results suggestthat the two types of adsorbates have no relation to the catalytic
process and are only situated probably on the supports.
The IR peaks in the region 1700-1200-~m-~in CO/H~reaction
on [Ru&]/TiO2 weremuch weaker than thoseon [Ru&]/MgO,[Ru&]/LazO3, or [Ru&]/A1203 (Figure 4). This made theobservation of reaction intermediate like formyl on Ru possible.
We observed a weak peak at 1576 cm-i in CO/H2 reaction. It
shifted to 155 1 cm-i immediately after the switch of ambient gas
from CO H2 to CO + D2 at 523 K. The peak disappearedvery
rapidly by the evacuation at 523 K, so that the species at 1576
cm-i is observable only under the reaction conditions. Two peaks
at 1619 and 1374 cm-i still remained after the evacuation for 10
min at 523 K which are assigned to formates on Ti02 surface.
The vco of formyl and formaldehyde together with the uwo offormates on organometalliccomplexes and catalyst surfaces arelisted in Table 7. The 1576-cm-i peak may be assigned to formyl
species rather than formates. The similar peaks at 1587 cm-1 on
Rh/Si02z7 and 1584 cm-l on [RuCo~]/Si02~8
have been assigned
to formyl species (Table 7), which also exhibited H/D-isotope
shift as in our case. The peaks at 3020 (2260) and 1306 cm-l
are due to methane in gas phase (in parentheses; in CO D2,
also in the next sentence).29 The peaks at 2965 (2221), 2877
(2186), 1441 (1427), and 1346 (1332) cm-I can be assigned tomethyl, and the peaks at 2929 (2203) and 2856 (2137) cm-l are
the VCH of methylene species.
We observed a weak absorption peak around 1954 cm-i forterminal hydride in H2 atmosphere (without CO) at 290 K forthe [Ru&]/Ti02 preheated at 623 K. The peak was replacedby 1430 cm-1 in D2. In the CO/H2 reaction conditions at 523
K the terminal hydride peak was not observed. Any r2-bridging
hydride peak was not observed either unlike the observation of
p2-bridging hydrides on ruthenium clusters.3w

through the surface oxygen atoms, we finally performed the fivewave fitting by Ru-C(carbonyl), Ru-Ru, and Ru(-C-)0, Ru(42-)Ru, and Ru-O,. Figure 3d shows the best fit with Rr =2.6%. The Ru(-C-)Ru distance and its coordination number
were determined to be 0.410 nm and 0.8, respectively, in Table
3, which are almost the same as those for the original cluster andthe incipient [Ru&]/MgO. Thedistanceof the nearest-neighbor
Ru-Ru bond increased from 0.263 to 0.287 nm which is similar
to 0.290 nm for the original cluster (XRD).
This expansion of the cluster framework is suggested to becaused by CO adsorption on the [RU&] framework. Table 4showsthe coordination number of 1.6-1.7 for Ru-CO bonds (~R,,-c= 0.188 nm, d ~ ~ ( 4=-0.299
p nm). The value implies 1 1 carbonyl
ligands per [Ru~C]. The number of CO ligands per cluster
determined by CO adsorption and IR intensity for carbonyl peaks
isshown inTable 5. Thesevaluescoincide with thevalueestimatedby EXAFS. The amount of CO adsorptionon the shrunk [RuaC]
clusters on MgO and La203 was much larger at 523 K than at
290 K, which indicates that the expansionof the cluster frameworkobserved by EXAFS is an activated process induced by CO uptake.
For the expanded cluster the bridging CO was observed at 1795
cm-i besides the linear CO peaks at 2045 (w,sh), 1989 (s), and1933 (w,sh) cm-i for the sample on MgO. The expanded cluster
was shrunkagain by CO release by evacuation or at a low pressureof CO at 523 K. This expansion-shrink cycle of the [RU&]
framework reversiblytookplacein conjunctionwith CO breathing.
In the case of the shrunk [ R U ~ Ccluster](~R,,-R,, = 0.262 nm)
on La2O3, the Ru-Ru distance was elongated to 0.277 nm under
CO/H2 reaction conditions. The distance is shorter than 0.287
nm for the caseof MgO. Itscoordination number was 3.1 which
is similar to 3.3 for the shrunk species. The coordinationnumberof Ru-CO bond was 0.9 which is much smaller than 1.6-1.7 forthe [Ru&]/MgO in Table 4. The incomplete expansion isconsistent with the less coordination of CO ligand to ruthenium
atom.
In order to examine the behavior of the non-carbido cluster the
Discussion
carbon was removed as CH,(g) by the treatment with H2 at 623
It was found that the supported ruthenium carbido clusters
K (Figure 2), and then the sample was exposed to CO/H2 at 523
preferentially produced methanol, dimethyl ether, and formalK. The EXAFS analysis of the obtained sample revealed no
dehyde in CO hydrogenation, which is contrastedto the formation
significant expansion of the [ R b ] framework, where the distancesof methane and hydrocarbons on conventional Ru catalysts orand coordinationnumbers (N) of Ru-Ru and Ru-0 bonds were
[R~6(C0)18]~-derived
catalysts (Table 1). Thus the associativeobserved at 0.269 nm with N = 4.2 and 0.21 1 nm with N = 0.7,
type hydrogenation of CO without CO dissociation is favorable
respectively (Table 4). Besides the nearest-neighbor Ru-Ru aton the supported carbido clusters, whereas the dissociative-type
0.269 nm the next-nearest Ru-Ru bond was observed at 0.381
hydrogenation of CO through CO bond-breaking mainly proceeds
nm with N = 1.0 (Table 4).
on the traditional and non-carbido-clustercatalysts. The catalytic
The result for [ R U ~ ( C O ) ~ ~ ] ~ after- / A ~evacuation~O~
at 623
performance of the [Ru~C]clusters on inorganic oxide supports
K indicated that the Ru6 cluster degradated to small clusters likeis connected to the active structure, structural change, andRu3 species (&R,,= 0.261 nm, NR,,-R,,= 1.9) (Table 4) inelectronic state of the clusters in CO/H2 reaction conditions
contrast to the retention of the framework for [Ru&]/MgO,
hereinafter.
La203, Ti02, and A1203.
In Situ Infrared Spectra for Supported [ R ~ C ( C O ) I ~ C H J ) ~ The active structures of the [RyC] clusters on MgO, La2O3,
TiO2, Al2O3. and Si02 under the CO/H2 reaction conditions
Clusters in CO/H2 Reaction Conditions. The stretching frewere examined by EXAFS. In the case of MgO- and LazO3quencies of carbonyl peaks (adsorbed CO) for the supported++

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 599

Structure and Catalysis of Supported Ru& Clusters

2.10
1.26
0.42
-0.42
n
Y
v

-1.26

x

% -2.1011

'

'

5

4

0

'

'

'

6

'

'

' .",

8

7

'

'

'

'

'

9 1 0 1 1

'

12

-2.10

Q)

0.46 .

ll v , I I

UP I

I

I I

.

-2.30 4 l

5

a
6

7 38

9 *1 0 1 ;1

12U

Wave Number k (1 0 nm" )Figure 3, Curve-fitting analyses for inversely Fourier-transformed EXAFS oscillation in Figure IC; (a) two-wave (Ru-C, Ru(-C-)0) fitting; (b)three-wave (Ru-C, Ru-Ru, Ru(-C-)0) fitting; (c) -four-wave (Ru-C, Ru-Ru, Ru(-C-)0, Ru(-C-)Ru) fitting; and (d) five-wave (Ru-C, Ru-Ru,
Ru(-C-)0, Ru(-C-)Ru, Ru-O ,) fitting; (-) observed; (- -) calculated.

-

TABLE 5 Amounts of CO Uptake on the Shrunk [ R G ]in
CO(14.6 kPa) + Hz(14.6 kPa)
no. of C O per R u C

COuptaktO
IRintensityb

2.9
3.2

10.7
10.6

4.0
3.6

7.9
8.1

4.0
5.8

6.1
7.6

a Determined taking into account the amount of formates on oxides.

6 Calculated from the vco peak area by using the incipient ruthenium

carbido cluster as reference.

TABLE 6 IR Absorption Peaks of CO Adsorbed on
Supported [R&(CO)ldCH3)r and [RudCO)d- Clusters
and Conventional Ruthenium Catalysts
vlcm-1
Ah03

2040 (m,sh)
1976 (s)1752 (w,br)
2072 (m,sh)2002 (s)1975 (w,sh)

Ti022143 (w)2054 (m,sh)1978 (s)2139 (m)2074 (s)2042 (s,sh)2006 (w,sh)

2045 (w,sh)1989 (s)1933 (w,sh)1795 (w,br)

MgO+

2137 (w)'"2073 (s)2050 (m,sh)2140 (m)'"2085 (w,sh)
2060 (s)2010 (s,br)bJ+

a In COH2 at 523 K. In CO H2 at 548 K. In C O at room
= 1.4 nm).l* e From Ru(NO3)3 (daYc
temperature. d From RuCl3= 3.0 nm).lgfFrom RuCl,.=

supported system the incipient [RU&] cluster was incorporated
with surface oxygen atoms by evacuation at 523-623 K (Table
4b, and Figure lb). The bond distance of Ru-O, were similar
to others recently r e ~ r t e d . ~ > 3 9The surface oxygen(0,)incorporated structures [Ru3(p-OI)3] or [Ru3(C0)6(r-O1)p]have
been proposed for [ R u ~ ( C O ) IMg0.363~]
Upon supporting, one
CH3 and two CO ligands per cluster were lost, which implies theadvantageous interaction between a cluster face composed ofthree ruthenium atoms and support surface. In an octahedral

I

I

I

I

s8 88 8

.

.

.

.

Wavenumber I cm'

I

.

.

s

0

Figure 4. In situ FT-IR spectrum of [ R U ~ C ( C O ) I ~ ( C H I ) ] - /inT CO~~~(14.6 kPa) and H2 (14.6 Wa) at 523 K (after 40 min).

structure of the cluster the opposite face to the face attached tothe surface cannot interact with the surface. The coordination
number of Ru-0, bond was determined to be about unity (1.1
(MgO) and 0.7 (La203); Table 4b), suggesting that eachrutheniumatom of the bottom face directly attached to the surface
is bonded to two oxygen atoms because the EXAFS coordination
number is of averaged one for total Ru atoms of a cluster. Fromthese results we propose the [Ru&(p-o,)~] cluster structure withthree bridging oxygen atoms incorporated from the surface asshown in Figure 5b.
In the catalytic CO/H2 conditions the surfaceattached [RyC( P - O ~ )species~]
showed activated CO adsorption at 523 K (- 1 1
and -8 CO/ [Ru6C]on MgO and La2O3, respectively, comparedto 3-4 CO/[Ru6C] at 290 K, as shown in Table 5 . This COadsorption caused the expansion of the [Ru&] framework as
proved by the observation of the multiple scattering Ru(-C-)Ru
peak (0.410 nm, N = 0.8;Table 4c and Figure IC) and theelongation of the nearest Ru-Ru bond (0.287nm, N = 3.9;Table4c) on MgO. We conducted the systematiccurve-fitting analyses
with two, three, four, and five waves for the expanded species(Table4; MgO-c) as described above. Only the five-wave analysis
reproduced the observed EXAFS data with a convincingreliability
over a whole wavenumber region (Figure 3). The EXAFS analysis

Izumi et al.

600 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

TABLE 7: IR Absorption Peaks of Formyl and Formaldehyde Ligands in OrganometallicCompounds and Formyl Adsorbed on
Catalyst Surface
wavenumber/cm-l
species
wn
wo
vcop
vco:
ref
Os(CHO)H(CO)z(PPh3)2
2760 w, 2680 w, 2540 m
1601
23
24
2620 w, 2540 w
1610 sOS(CHO)Cl(CO)2(PPh3)2
2690,2540
Nat [Fe(CHO)(CO)d]25
1577[EtdNI [Fe(CHO)(CO)s(P(OPh)3)12515 w
26
1584 mOS(~~~-CH~O)(CO)Z(PP~)~
2910 w, 2820 m
1017 m23
27
1587
CHO(a)/Rh/SiOl
CHO(a)/[RuCo3]/SiOz
CHO(a)/ [RUsC]/Ti02(1 545)
1584(1575)
1576(1551)

28

this work

The values in parentheses are for deuterium-labeled species.
[ O ~ I O C ( C ~ ) Z ~[P~Is(CO)XI~ ~ - / M ~ ~ , ~ Or [RhdCO)161/
A 1 ~ 0 3the, ~ contribution of oxide surfaces was unclear.
The interstitial carbido carbon has structural and electronic
effectson thecluster frameworkin thespecificoxygenatesynthesis.
The [Ru&]/Al203 and /Ti02 hold the [Ru&] structure in CO/
Hz conditions (type 11), whereas the [Ru6]/A1203 derived from[Ru~(CO),~]“without carbido carbondegradated to [Ru,(C0)3d(RU-Ru)(ec.-0)3]36J7as suggested by EXAFS (Table 4). The reversible
structure change of [RU&] on MgO and La203 depending on the
CO pressure (type I) indicates thermodynamical stability of the
carbido cluster because other non-carbido Ru clusters nevershowedsuch phenomenon. The [Ru6]/MgOobtained by removalof the interstitial carbon atom with H2 reduction showed only alittle change of Ru-Ru bond (0.263 0.269 nm) compared withI
523 K/
thelarge expansion from 0.263 0.287 nm for the [RU&]/
523 - 623 K evac.
/co + H~
MgO. These results demonstrate that the interstitial carbon hasd(RU-RU)
623%= 0.287 nm
a key role like a central spring to make the cluster structure
flexible.
Generally the structures of type I (La203 and MgO) wereselective for oxygenate synthesis, while the structures of type I1(Ti02, ZrO2, and A1203) were less selective but more active
compared with the type I. A major difference between the[Ru&] /MgO, [Ru&] /La203 selective for oxygenate synthesis
and the traditional Ru catalysts selective for methane andhydrocarbon formation may be ascribed to the ability of CO
Figure 5. Structure transformation of [ ~ u ~ ~ ( ~ ~ ) ~ ~ dissociation.
( ~ ~ ~ ) The] -amounts/ ~ g of~CO adsorbed on the [RU&] /MgO
including reversible expansion-shrink of the cluster framework.
and [Ru&]/LazO3 at 523 K were as large as eleven or eight
CO/cluster, respectively, as shown in Table 3. Particularly on
indicates the recovery of a nearly ideal octahedral metal
the three Ru atoms of the upper triangle face in the supported
framework for [RU~C(CO)II(~~.-O~)~]/M~O
by co uptakeat 523
cluster in Figure 5, three CO per Ru are likely adsorbed. This
K (Figure 4c) and the shrinking of the cluster structure by COsituation prevents CO dissociating on the cluster. On the otherrelease. The expansion and shrinking of the structure washand, the structures coordinated with the large amount of CO
reversible on MgO (~R,,-R,, = 0.263-0.287 nm). Similar but
are disadvantageous for H2 adsorption. This may be a reason
incomplete expansion-shrink cycle of [RU&] was observed onthat [Ru&]/MgO and [Ru&]/La203 are less active than
La203 (~R,,-R,, = 0.262 s 0.277 nm; Table 4; LazO3). The[Ru6C]/TiO2. However, fortunately in view of catalysis, thesupported clusters like [Ru&]/MgO and [RU&]/La203 which[ R u ~ C cluster]
framework is expanded remarkably under theexhibit a reversible expansion-shrink cycle are classified as “typeCO/H2 reaction conditions, which provides the coordination spaceI”.
available for the dissociative adsorption of H2.42 In fact theOn Ti02 and AlzO3 the EXAFS spectra after evacuation atreaction order with respect to H2 pressure for [Ru&]/MgO and523-623 K were simply fitted with one wave of the nearest Ru[ R u ~ C/La203]
showed a Langmuir-type dependency, while it
Ru bond (Table 4;TiO2-b, A1203-b). There is no evidence of
was about unity (straight h e ) for [Ru&]/TiO2, suggesting a
Ru-O, bond on Ti02 and A1203. These [Ru6C] species on Ti02
more facile H2 adsorption on the former samples at 523 K. Theseand A1203 hardly expand in the CO/Hz conditions (Table 4) andadvantageous expansion of the cluster structure made the carbidoare classified as “type 11”. The six-Ru unit aggregated to be thecluster catalysts much more active for the oxygenate synthesis
species which shows the NR,,-R,,of 6.6 (classified as “type 111”).
compared with the corresponding traditional Ru catalysts in TableThe expansion-shrink of cluster framework induced by CO
1, Two competitive factors, the number of CO adsorption per
breathing was observed with the supported clusters which havecluster and the coordination space for H2 dissociative adsorption,
the interstitial carbido carbon and are incorporated by surface
should make type I selective and type I1 less selective. These
oxygen atoms of MgO or La203 (basic oxides). The surface
factors are partly affected by the support composition throughoxygen incorporated to the cluster seems important in thethe cluster structures. It was found that the expanded cluster inreversible transformation of shrunk [Ru6C(ec.-O,]31 F? expanded
Pco = 14.6 kPa was much more active and selective for the[ R U ~ C ( C O ) I I ( C ( - ~on~ )MgO.
~]
In the previous reports foroxygenate synthesis than the shrunk cluster in PCO= 0.10 kPa

I--

Structure and Catalysis of Supported RU& Clusters

The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 601

asshown in Table 2. These resultsdemonstrate the advantageous
catalysis of the expanded [Ru&]/oxide for the oxygenate
synthesis from CO/H2.
The reaction mechanisms via formyl, hydroxycarbene, andformate have been proposed for methanol synthesis.43 The rateof formate reduction with H2 was slower than the: rate of thesteady-state oxygenate synthesis as proved by FT-IR (Table 2),
indicating that oxygenate synthesisoccurs mainly on RU& entity
under the present conditions. Both formyl and hydroxycarbene
are known to be thermodynamically unstable.4M-48The in situ IR
observation revealed the presence of a peak at 1576 cm-' (Figure4) (15 5 1 cm-1 in CO/D2 as listed in Table 7) and this species wasvery reactive at 523 K. The peakof 1576cm-' cannot be assigned
to hydroxycarbene with a single bonding of carbon and oxygen,but is assignable to the formyl whose end oxygen has interaction
Figure 6. Proposed reaction intermediate for oxygenate synthesis on thewith the adjacent Ru atom, The frequency of the formyl peakTiO2-supported ruthenium carbido cluster. Carbonyls are not shown tois compared to the reported values for organometallic formyl
avoid complication of the figure.
complexes in Table 7.
Kellner and Bell reported49inverse isotope effects with respect(Table 6), which are larger than general vcoshifts by the change
to hydrogen for methane formation in CO/H2 reaction at 453of Ru particle size in R ~ / A 1 2 0 ~ . The
1 ~ larger amount of CO
543Kwith 1.oX 1O20r 1.ox 103kPa~fCO/H2(1:3);r~~4/T~~4
adsorbed on Ru would cause a blue shift by a dipole coupling,= 1.4-1.6 on Ru/A1203. Minor formations of acetaldehyde on
but this is reverse to the present observation. These YCO peaks
Ru/SiOz and methanol on Ru/A1203 also showed inverse isotope
for the [Ru&]/oxides were observed after the activation at 623
effects: rCD3CDO/rCH3CH0 2.0 and rcD30D/rcH,oH= 1.4-1 .6.50
K, but at this temperature the counter cation [BTMA]+ had
The oxygenate synthesis on [Ru&]/TiO2, [Ru&]/MgO, andalready been decomposed, excluding the effect of the cation on[Ru&]/La2Os showed inverseisotopeeffectsof 1.4-2.0asshown
the peak shift. Thus, the red shift for the supported carbido
in Table 6. The isotope effect in the equilibrium constant for theclusters compared to those for the conventionalRu catalysts may
step, CO(a) H(a) s HCO(a), can be calculated on the basis
be due to electron donation from the carbido carbon to theof the vibrational partition functions neglecting the secondary
ruthenium f r a m e w ~ r k . ~ ~ ~ ~ ~ ~ ~isotope effects.

-+

KD/KH = exp[-(CDDCo - CHHCo)- 2(CDDC0C H ~ ~ O )(sD-sH) (vasD- vasH) (sD-sH)]h/(2k~]

+

+

+

Conclusions
1. Supported ruthenium carbido clusters prepared from

[R~~C(CO)~~(CH3)]-exhibited
selectiveoxygenatesynthesisfrom

Adsorbed hydrogenon [RhC] /oxides was consideredas pz-bridge
CO/H2 in contrast to the preferential formation of methane andtype with the analogy of a preferable formation of p2-bridging
hydrocarbonson supported clusters prepared from [ R U ~ ( C O ) ~ ~ ] ~ hydride in metal-hydride clusters determined by XRD.3@34The
without interstitial carbido-carbon or on conventionalRu catalysts.
inverseisotopeeffect was calculated to be 1.9by using the reported
2. Oxygenate-selective [Ru&]/La203 and [ R u ~ C/MgO]
values for the vibrational modes23,32.49,51
and the present formyl
showed reversibleexpansion-shrink transformation of the [RbC]
peak; ~M(RUHRU)(RU~H~(CO)IZ)
1585, VS(RUHRU)(RU~H~(CO)I~)framework. The [RuaC]structurewasexpanded (~R,,-R,, = 0.277= 1290, VM(RUDRU)(RU~D~(CO)I~))= 1153, ~ s ( R u D R u ) ( ~ ~ 4 ~ 4 ( ~ ~ ) 10.287
2 ) nm) under CO/H2 reaction conditions and shrunk (~R,,-R,,= 909, ~ M =H 716-810, v~H(O~(CHO)H(CO)~(PP~~)~)
2540,= 0.263 nm) at low CO pressures or in vacuum at 523 K.
and bcH(a1dehyde) = 1390 cm-I. The calculated value of 1.9
3. The expansion of the cluster structure was induced by CO
coincides with the observed value of 1.7 (*0,3). This indicatesadsorption which is an activated process. The large CO uptakethat the transition state [CHO]* may be similar to CHO(a) and(8-1 1 CO per cluster) on [Ru6C]/MgOand /La203 may preventthe step CO(a) H(a)CHO(a) may be rate-determining.
CO dissociating to increase the selectivity of oxygenate synthesis.
The rate of H2-D2 exchange in the presence of CO was 3000
4. The expanded [ R u ~ Cstructures]
on MgO and La203 weretimes larger than the steady-state rate of oxygenate synthesis
more active than the shrunk ones probably due to the larger(Table 2). Therefore, the dissociative adsorption of hydrogen on
accessibilityof H2 todissociatively adsorbon the expandedcluster.
thecarbido cluster should be in equilibriumunder CO/H2 reaction
5 . [Ru&]/TiO2 was most active among the catalysts with theconditions. Accordingly, a catalytic reaction cycle for the
shrunk structure under the reaction conditions.
oxygenate synthesis on the supported carbido clusters may be
6. The H2-D2 exchange reaction on [Ru&]/oxides in CO
shown in Figure 6, where the formyl intermediate is interacted
was far faster (>3000 times) than the steady-state oxygenate
with the neighboring ruthenium atom because pl-formyl is
synthesis. The hydrogen exchange rate on the carbido clusters
unstable thermodynamically27+43and also because of the observed
was larger than that on the traditional Ru catalysts.
red shift of vco peak (1576 cm-1). Three oxygenated compounds,
7. The inverse isotope effects ( V D / V H ) were observed for themethanol, dimethyl ether, and formaldehyde, may be produced
oxygenate synthesis on the supported carbido clusters, which can
via the same intermediate because the three products showed
be explained by the zero-point energy in the key step CO(a) +similar hydrogen isotope effects and reaction orders. DimethylH(a)HCO(a).
ether is believed to be formed by the dehydration of methanol on
8. Theobservedpeakat 1576cm-l(1551 cm-')maybeassigned
oxide supports.43
to formyl intermediate.
The formation of hydrocarbons which is more favorable on
9. Interstitial carbido carbon is regarded as a central spring
contracted [RU&]/TiOz and /A1203 may be initiated by the CO
for the reversible structure transformation and also as an electron
dissociation on the clusters. Negligible isotope effects (YD/YH donor to the cluster framework.
1.0, Table 3) imply that this CO dissociation process would
be rate-determining.
Referen'@ and Notes
The difference of vco(main peak) for carbonyls on [Ru,&]/oxides and the corresponding conventional Ru catalysts were as(1) The Chemistry of Metal Cluster Complexes; Shriver, D. F., Knew.
H.D., A d a m , R. D., Us.;
VCH Publishers: New York, 1990.
large as 21 Cm-' (MgO), 58 Cm-' (TiOz), and 97 Cm-' (A1203)

-

+

-

-

-

602 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994

Izumi et al.

(2) Muettertes, E. L.; Krause, M. J. Angew. Chem., Int. Ed. Engl. 1983,
22, 135.
(3) Metal Cluster in Catalysis; Gates, B. C., Guczi, L.; Knbinger, H.,Us.;
Elscvier: Amsterdam, 1986.
(4) Lamb, H. H.; Gates, B. C.; Kn6zinger. H. Angew. Chem., Inr. Ed.
Engl. 1988, 27, 1127.
( 5 ) Vargas, M. D.; Nicholls, J. N. Adv. Inorg. Chem. Radiochem. 1986,
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