J. Phys. Chem. 1995,99, 10346-10353

10346

Promoted Catalysis by Supported [Ru~N]Clusters in N2 and/or H2: Structural andChemical Controls
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@

Catalysis (ammonia synthesis) on supported nitrido clusters [Ru~N]was investigated as a potential catalysis
system of transition metal main group element in relation to the framework structure (change) of [ R u d ]clusters in HZor NZas found in the accompanying paper. The [RU6N@-O)su)3](Os,, oxygen atom at surface)clusters were prepared from the [RugN(C0)16]- cluster on MgO, K+-doped MgO, and Cs+-doped MgO, andthe stability in reaction conditions of ammonia synthesis was probed by EXAFS (extended X-ray absorption
fine structure). The reaction rates on these nitrido clusters were found to be faster than non-nitrido [ R u ~ ]clusters prepared from [ R U ~ ( C O ) ~ ~degraded]~-,[Ru3@2-Osu)3]clusters or aggregated Ru clusters (NR"-R,,=
6.2-6.6) prepared from [RugN(C0)16]-, or conventional Ru catalysts. Also, the H2-D2 exchange reactions(in the presence/absence of N2) proceeded faster on supported [Ru~N]clusters than the other catalysts. The
Ru wt % dependence of ammonia synthesis activities on [RusN]/MgO suggested the importance of the Ruhexamer ensemble and cluster/support interface for the catalysis. Related to the coordination structures of H
or NZ and structure changes of the [ R u d ] framework in H2 or N2 in the accompanying paper, the promoted
reaction mechanism of ammonia synthesis on supported [ R u a ] clusters is discussed in terms of (1) nuclearityof Ru, (2) clustedsupport interface, (3) structural effect through expansionkontraction of the [ R M ] framework,and (4) electron donation by nitrido nitrogen, based on in-situ EXAFS, in-situ IR, H2-D2 exchange reactions,reaction orders, and WD isotope effects.

+

Introduction
Compared to the simple structure and properties of the H2
molecule, many unknown factors in the reaction mechanismsof hydrogenations on heterogeneous metal catalysts remain.
There have been many studies on hydrogenations over heterogeneous catalysts on the molecular level for unsupported Ru
and supported Ru c a t a l y s t ~ . ~ . The~ - ~ ~reasons forcomplexity of hydrogenation are the weaker adsorption of H
than the other small molecules,'I2 the spillover from metal
particle to upp port,^^'^ H-induced structure changes of active~ i t e ,etc.
~.~
We reported the [Ru~@-O,,,),](0,,,,oxygen atom at surface)clusters on MgO, K+/MgO, and Cs+/MgO stabilized by nitrido
nitrogen in the [Ru~]framework as catalysts for hydrogenationin the previous paper.I3 The number x varied according to thechange of Ru wt %, 5-7 at Ru 0.5 wt % and 3 at Ru 2.5
wt %, and the [Ru~N]began to aggregate to larger particles at
Ru 4 wt % on the basis of the coordination number NR,,-o,,(rR,,-O,, = 2.09-2.20 A) and NR"-R,, (Q,,-R,, = 2.62-2.65 A)
by EXAFS. The dependence on Ru loading was interpreted as
the difference of adsorption site of the [RugN(CO)i6]- cluster
on a MgO surface (MgO preheated at 773 K). The [Ru6N]
cluster was suggested to be attached to a lower coordinationsite of the MgO surface at Ru
0.5 wt %, compared to theinteraction with relatively flat MgO planes at Ru 2.5 wt %.
The supported [Ru6N] clusters on MgO, K+/MgO, and Cs'l
MgO had common structures with regard to cluster framework
structure judging from the N R , , - R ~andobtained by
EXAFS, but ltot with regard to Ru-Os,, bondings (metahupport
interface).
H-induced structure changes were observed as the changesof ~ - R - R , , (0.03-0.08 8,) for [Ru6N]/MgO and [RusN]-Cs+/

-

-

-

-

-

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

0022-365419512099-10346$09.00/0

MgO by the adsorption/desorption of hydrogen. The hexamer[Ru&-OsU)3-4] on MgO without nitrido nitrogen prepared fromthe [RU&(CO)i&k]- cluster showed no H-induced change of(2.63 A) in H2. Negligible structure changes wereobserved by the adsorption of molecular N2 on [RusN]/MgO
or [Ru6N]-Cs+/MgO. The study on the changes of catalytic
performance should be useful for the better understanding ofhydrogenation when the adsorption sites of [Ru6N] on MgO
were varied at different Ru wt % and when supported clusters
showed H-induced structure changes of the cluster framework[Ru~N]. The importance of hydrogen to the structure of thecluster and catalysis was reported in several catalyst systems.
The palladium clusters Pdl3(CO), or PQ(CO), were suggested
to be bonded to protons in the zeolite and formed an electrondeficient species [Pd,-H,y+. I4,l5 These catalysts were usedin COM2 reactioni6 and the hydrogenolysis of methylcyclopentenel' or neopentene.'* The effects of H2 treatment on the
structures of supported iridium clusters on MgO,I9 A1203,20or
platinum clusters in zeolites2I were reported. Two kinds ofmetal-Os,, (oxygen atom at surface) bondings were reported:
2.55-2.77 8, after reductions at lower temperatures (-573 K)
and 2.19-2.24 8, after reductions at higher temperatures (-773
K). From the viewpoint of catalysis, the relations between
cluster structure change and the hydrogenations have been
investigated, and promoted reaction mechanisms on cluster sites
were proposed for h o m o g e n e ~ u s ~and~ - heterogeneous~~
catal y s t ~ .The
~ ~ structural activations were reported for CH bondsof alkenes over [Rhz], [Ru~],or [Os31 clusters,22 carbon
monoxide over a [ R u ~ cluster23]
or [ R u ~ complex,24]
and theOH bond of ethanol over [Mo02@-0)~1dSi02or [Nb0@-0)3121
si0z.25
In this paper, we report the promoted ammonia synthesis on
supported [Ru~N]clusters and the contributions of nuclearityof clusters, cluster support interface, structure change of [ R u a ]cluster framework, and chemical effect of nitrido nitrogen to
catalysis. Six-Ru-atom clusters without an interstitial atom
0 1995 American Chemical Society

Catalysis on Supported [ R u a ] Clusters[Ru6(CO)l8l2-, the carbido cluster [Ru&(C0)16Me]-, andconventional impregnated Ru catalysts were employed ascomparisons.
Experimental Section
Supported clusters were prepared in a manner similar to thatin a previous paper.I3 Briefly, [PPN]+[RU&(CO)I~]-(1) (PPN
= N(PPh3)2) ([PPN]+Cl-, Aldrich Chem Co., 97%; NaN3, Wako
Pure Chemical Ind., >90%;Ru3(CO)lz, Soekawa) was supported
on MgO, K+/MgO, Csf/MgO, or A1203 by reaction at 290 K(1 h) in purified THF (tetrahydrofuran, Wako, Special Grade)in Ar (99.99%). MgO was prepared from Mg(OH)2 (Wako,
99.99%) by heating at 773 K (2 h) in vacuum. A water solutionof Cs2CO3 (Wako, >95%) or K2CO3 (Wako, >99.5%) was
impregnated onto MgO, followed by treatments in 0 2 and thenin H2 at 773 K. A1203 (aerosil C) was treated at 623 K (2 h)in vacuum ([Ru6N]/oxide). [PPN]+~[RU~(CO)I~]~(2) or[NM~~CH~P~]+[RU&(CO)I&le]- (3) ([NMe3CH2PhIfC1-, Wako;
CH31, Wako, Special Grade) was supported on MgO in THF
solution in a manner similar to the case of 1 ([Ru6]/MgO and[Ru&]/MgO). As pretreatment, incipient supported clusters
were heated in vacuum at 813 K except for [Rua]-Cs+/MgO
and [Ru6N]-K+/MgO at 673 K, followed by treatment in H2
for 1 h ( T H=
~ 588-773 K). Thus-prepared samples are denoted
as clusterloxide-xH (x, temperatureK in H2). The conventional
Ru/MgO, Ru-CsfNgO (Cs+/Ru = 2.0 in atomic ratio), and
Ru/A1203 catalysts were prepared from a solution of Ru(N0)(NO& (N. E. Chemcat, Ru 5 g L-I). They were heated in 0 2
and then in H2 at 773 K before used as catalysts. The Ru
loadings were 2.5 wt % on MgO, K+/MgO, Cs+/MgO and 1.6
wt % on A1203 except for experiments of weight percent
dependence for MgO-supported clusters (0.48-3.9 wt %). The
descriptions without special notation for catalysts on MgO were
for samples at 2.5 wt % Ru.
The ammonia synthesis reactions were canied out under 101
kPa of reaction gas ( P N J P H=~ 1/31 at 588 K in a flow system(flow rate 60 cm3 min-I). Produced ammonia was analyzed
by the decrease of electron conductivity (1.7-0.8 mS cm-I) of
the HzSO4 solution (0.004-0.002 N). The linearity of thecorrelation between the produced NH3 amount and the decreaseof electron conductivity was checked before observation.26The
reaction temperature was chosen to be relatively low (588 K)
to realize differential (not near equilibrium) working conditions
in kinetic measurements based on the thermodynamics; that is,
the equilibrium value for ammonia synthesis was calculated fromthe Gibbs free energy to be 1.7 and 0.45% of NH3 at 588 and
673 K, respectively, compared to the highest values of Nz-H~
conversion in the case of the most active [Ru6N]-Cs+/MgO(0.046 and 0.44% at 588 and 673 K, respectively, 0.070 g ofcluster catalyst used). The rates of H2-D2 exchange reaction
were monitored in a closed circulating system (dead volume
100 cm3) connected with a mass spectrometer (ANELVA
NAG515) with 6.7 kPa of H2 and 6.7 kPa of DZat 273 K. The
same reactions were also carried out in the presence of 4.5 kPa
of N2. The reverse reactions of the N2-Hz reaction, thedecomposition reaction of ammonia, were carried out in theclosed circulating system with 1.3 kPa of NH3 and 76.0 kPa of
H2 at 588 K. Produced N2 was detected by GC (Shimadzu
GC8A) with a Unibeads C (GL Science) column at 433 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 (PF) in the National
Laboratory for High Energy Physics (Proposal No. 93G010)
between June 1993 and November 1994. The sample wastransferred to an EXAFS Pyrex cell with Kapton films on bothsides 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

J. Phys. Chem., Vol. 99,No. 25, 1995 10347(Cry0 System, LTS-21). The procedure of data analysis was
described in the accompanying paper.I3
FTIR spectra of supported Ru clusters were recorded on a
FTIR spectrometer (JASCO FIIR-5OOO) in a quartz IR cell 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 drop of a THF
solution of the Ru clusters. The temperature of the IR cell canbe controlled at 193-295 K by a tube wound around the cell(cooled by liquid nitrogen by using a rotary pump) and at 295823 K by a heating coil wound around the cell. H2 (99.99%)
and N2 (99.99%) gas were purchased from Toyo Sans0 Co.,
Ltd. The impurities (as molecular content) were less than thefollowing values: H20 < 10, NZ < 50, 0 2 < 10, CO < 10,
C02 < 10 ppm, and total hydrocarbons < 10 ppm (as carbon
content) in the HZ gas, and H20 < 10, 0 2 < 2 ppm, and totalhydrocarbons < 1 ppm (as carbon content) in the N2 gas. D2and I5N2 gas were purchased from Syoko Co., Ltd. The
deuterium content in total hydrogen was >99.8% for the D2gas, and impurities (as molecular content) were less than thefollowing values: HD < 4000, H20 < 10, N2 < 50, 0 2 < 10,
CO < 10, C02 < 10 ppm, and total hydrocarbons < 10 ppm(as carbon content). The I5N atom content in total nitrogen
was 99.3% for I5N2gas. The purity was on the same level as
I4N2. The gas for adsorption (H2 and N2) was again purified
by a liquid nitrogen trap before each measurement.
The hydrogen and nitrogen uptake measurements were carriedout in a small space (30 cm3) in a closed circulation system
connected to the manometer. The temperature of the samples
was maintained in liquid N2 (77 K), dry ice
liquid N2acetone (179 K), or ice (273 K). The uptake on Ru at 77-179
K was calculated by subtracting the observed uptake on Rufree oxide from the observed uptake on the Ru-supported
sample. The Ru-free support oxides, MgO, Cs+/MgO, or A1203,
were pretreated in the same conditions as corresponding Rucluster catalysts.

++

Results
Promoted Ammonia Synthesis on Supported [RuaN]
Clusters. We observed ammonia synthesis reactions on supported nitrido ruthenium clusters and related Ru catalysts at 588
K in 25.3 kPa of NZ and 76.0 kPa of H2 (Table 1 (A)). The
observed turnover frequencies (TOFs) were estimated per[Ru6N] (or [Ru~])cluster for supported clusters and per surface
Ru atoms estimated from the Ru dispersion ( m u ) by hydrogen
adsorption at 273 K for conventional Ru/MgO, Ru-Cs+/MgO,
and RdAlz03 (WRu were 0.13, 0.10, and 0.037, respectively).
The TOFs of supported nitrido-Ru clusters on MgO, K+/
MgO, or Cs+/MgO treated in HZ at 588 K (0.11-0.40 min-I)
were superior to the conventional Ru/MgO (0.002 min-I) or
the Ru catalysts in the literature on the TOF or product h-'gCal-'basis. The turnovers of [Ru6N] on MgO, K+/MgO, andCs+/MgO corresponded to 13-48 NH3 per [Ru6N] cluster (inmol) in 2 h, excluding the possibility of unusually high activity
due to hydrogenation of nitrogen atoms native to supported
nitrido clusters.
The TOF on [Ru6N]-Cs+Mg0-588H (0.40 min-I) corresponds to 9.9 x loW4mol-NH3 h-' gCat-', larger than 0.0071
min-' on conventional K+-Ru/Ti02 (7 wt % Ru, K+/Ru =1/10, 101
0.06 min-' on Ru/zeolite KX at 623 K (2 wt% Ru, 101 kPa),28 6.9 x
mol-NH3 h-' gcal-I on
conventional Ru-Cs+/MgO at 588 K (2 wt % Ru, Cs+/Ru =1.0, 80 kPa),26 or others.29 The dependence of activities for[Ru6N]/oxides on the kind of support was Cs+/MgO (0.40
min-I) > K+/MgO (0.19) > MgO (0.11) >> A1203 (0) whenthese cluster catalysts were pretreated in H2 at 588 K. The[Ru6N]/A1203 exhibited no activities after the H2 treatment atIzumi and Aika

10348 J. Phys. Chem., Vol. 99, No. 25, 1995

TABLE 1: Turnover Frequencies (TOFsY (product cluster-' min-l) of Ammonia Synthesis (A), Decomposition (B), Hz-Dz
Exchange (C) Reactions on Supported [R&N(co)&, [RyC(CO)l&le]-, or [R~(CO)~FJ~Clusters, and Conventional Ru
Catalysts
reaction condition
(A)
(C)
(c')
(B)
TreaJK
PNzkPaPH,kPaPDzkPa
PNH,Wa

588
25.3
76.0

273
4.5
6.7
6.7

6.7
6.7

588
76.0
1.3

samplea273

structure formulation

THJK
588
713
588
588
623
588
773
773
588
773

TOF
~

0.11
0.017
0.19
0.40
0.018
0.013
0.002
0.017
0
0.001

160
58

22
21

0.001

110
42
42
0.013

60

0.018

0.016

2.0

In the units (product surface Ru-' min-') for conventional Ru catalysts based on the uptake ( m u ) measurements.

TABLE 2: Hydrogen Isotope Effects for Ammonia
Synthesis on [R&N(C0)161-/MgO, [R&N(CO)d--CS+/hfgO,
and Conventional Ru/Mgo"
samplestructure formulation[RU6N(c0)16]-/Mgo[RU~N(CO)I~]--CS+/M~O
conv. RulMgO[Ru$rl@-O&][RU6N@-OsU)3]a

TH? r 0 4 r ~ ,
588
588
773

1.4
1.5
1.0

Reaction temperature 588 K. PN? = 25.3, P H (or~ PDJ = 76.0

@a.

I

1

1

2

;

r
4

Ru wt%
Figure 1. Ru wt % dependence of turnover frequencies (productcluster-' min-I) of ammonia synthesis: reaction temperature 588 K ,flow rate 60 cm3 min-], P N=
~ 25.3 kPa, PH?= 76.0 kPa.

588-773 K, although conventional RdA1203 slowly produced
ammonia (0.001 min-I).
The control experiments were performed for the supported
Ru cluster prepared from [ R u ~ ( C O ) I ~which] ~ - did not haveinterstitial nitrogen in the [RQ] framework. The TOF was 0.013
min-I, only 12% of that of the corresponding supported nitrido
cluster (0.1 1 min-l) (Table 1). However, the [ R u ~framework]of [RUs]/MgO was aggregated to larger particles ( N R " - R=
~ 6.2)
by heating in vacuum (813 K), and in H2 (588 K).I3 The [Rug@-osu)3-4] species can be prepared from cluster 3 by heatingin vacuum and in H2 at 623 K.13930The activity was 0.018 min-'(Table I), still smaller than that of [RusN]/MgO. It should be
noted that the activity on [RufllNgO had strong dependence
on the pretreatment temperature in H2 ( T H ~ )The. TOF on[Ru6N]/Mg0-773H (0.017 min-') was only 15% of that on[Ru6N]/Mg0-588H (Table l), whereas the dependence on T H ~was not observed for [Ru6]/MgO or [Ru6@-Osu)3-4] on MgOwhen treated in H2 at 588-773 K.
The dependence of ammonia synthesis rates on Ru wt % wasshown in Figure 1 for [Ru6N]/Mg0-588H catalysts. The TOF
was not varied so much around 1.5-3 wt % Ru (0.10-0.12
min-I). The increase at lower Ru loading (0.48 wt % Ru) by
about 30% and the decrease at higher Ru loading (3.9 wt %)by about 45% were observed.
Inverse Hydrogen Isotope Effects in Ammonia Synthesison Supported [RwjN] Clusters. The H/D isotope effects in
N2-H2 reactions were observed for [RusN]/Mg0-588H, [RM-

Csf/Mg0-588H, and conventional R a g 0 at 101 Wa (N2 25.3,
H2 (or D2) 76.0 Wa) (reaction temperature 588 K). The ratio(m2/m2)was 1.4 for [RusN]/MgO, 1.5 for [Ru,jN]-Cs+/MgO,
and 1.0 (no isotope effect) for conventional R a g 0 (Table
2).
H2-D2 Exchange Reactions in the PresencdAbsence of
N2. H2-D2 exchange reactions were carried out in H2 (6.7 Wa)
D2 (6.7 Wa) or in H2 (6.7 P a ) f D2 (6.7 Wa) NZ (4.5
Wa) at 273 K. The TOFs of H2-D2 exchange reaction in H2
4- D2 were in the order [Ru6N]/Mg0-588H (160 min-') >[Ru&l]-Cs+/Mg0-588H (1 10) > [R&N]/MgO-773H (58)[Ru6]/MgO-588H (42) [ R u ~ C ] / M ~ O -(42)~ ~ >~>H[Ru6N]/
A 1 2 0 3 (2.0) >> conventional Ru/MgO (0.013) (Table 1 (C)). The
TOF on [Ru&l]/MgO or [Rufl]-Cs+/MgO was faster thanaggregated Ru clusters or [Ru@-Osu)3-4] on MgO (without
nitrido N) by 2-4 times, than [Ru6N]/A1203 on the order of 2,
and than conventional Ru/MgO on the order of 3. The TOFs
of H2-D2 exchange reaction in H2 D2 N2 were in the order[Ru&l]-Cs+/Mg0-588H (60 min-I) > [R~~&]/Mg0-588H(22)[Ru&]/Mg0-773H (21) >> conventional Ru/MgO (0.016)(Table 1 (C')). The decreased ratios of the H2-D2 exchange
rate by the addition of N2 were 0.14 for [RusN]/Mg0-588H,
0.55 for [RudV]-Cs+/Mg0-588H, and 0.36 for [RhN]/MgO773H compared to the smaller change for conventional R a g 0(1.2).
Other Kinetic Measurements for Supported [RmlClusters and Conventional Ru/MgO. The pressure dependence
was observed by changing the pressures of NZ and H2
independently ( P N ,= 12.7-25.3 kPa, PH,= 38.0-76.0 Wa).
The apparent reaction orders of the Nz-H~reactions were 0.78
and 0.20 on [Ru6N]/Mg0-588H and 0.89 and 0.18 on [ R u a l Cs+/Mg0-588H for P N and~ P H ~respectively.
,
The decomposition rates of NH3 (1.8 kPa) in H2 (76.0 Wa)
were observed for [RusN]/MgO-588H and [Ru6N]-Cs+/MgO588H at 588 K (Table 1 (B)). This reverse reaction proceeded
slower on the order of 1-2 magnitudes than the forward(synthesis) reaction.

+

+

-

-

-

+ +

Catalysis on Supported [ R u a ] Clusters

J. Phys. Chem., Vol. 99, No. 25, 1995 10349

Wavenumberlcm 'Figure 3. IR spectra of (a) [Ru$J]/A1203 in H2 (23.3 Wa), (b) [ R u a ] /Wavenumber/cm'

Figure 2. IR spectra of (a) [Ru6N]/MgO in HZ(23.3 H a ) , (b) [Ru&l]/
MgO in DZ(23.3 Wa), (c) MgO in H2 (23.3 E a ) , (d) [Ru6N]NgO in
I4N2(23.3 Wa), (e) [Ru6N]/MgO in 15Nz (23.3 H a ) , and (f) MgO in
I4N2(23.3 Wa). Each background spectrum was for the sample before
the introduction of gas. Observed temperature = 193 K. [Ru6N]/MgO
was evacuated at 813 K and in HZ at 588 K. MgO was evacuated at
813 K.

TABLE 3: Amount of Gas Uptake on the Supported
TRuNCOhl- Clusters
number of H(a) or
W a ) per tRu6Nl
sample[RusNINgO[Ru6N]-Cs+/MgO[h6N]/A1203

TadsorptidK

uptake"IR intensityb
uptake"uptake"uptake"IR intensityb

179
193
77
179
179
193

H(a)
4.0
3.1
5.2

0
0

NAa)
1.1
0.7
1.9'

By manometer, in H2 (76 Wa) or N2 (25 P a ) . The uptake on Ru
was calculated by subtracting the observed uptake on MgO, Cs'NgO,
or A1203 from the observed uptake on [Ru$rl]/MgO, [Ru$ri]-Cs+/MgO,
or [Ru6N]/A1203, respectively. Calculated from the IR peak area in
Hz (23.3 kPa) compared to YR"-H for H(a) on a conventional R u N g O
sample. 1.9/2 per [ R u ~ ] .

IR Spectra for Supported [RuaN] Clusters in H2 or in N2.
I . [Ru&]/MgO. Figure 2a shows the IR spectrum observed
at 193 K for 30 min in H2 (23.3 kPa) for [Rua]/Mg0-588H(also in the case of the following IR spectra, the background
spectrum was for the sample before introduction of gas). A
strong peak at 1944 cm-I in Figure 2a was shifted to 1374 cm-'by evacuating the HZ and introducing D2 (23.3 P a ) (Figure2b). In contrast to these peaks on [Ru6N]/MgO, no peak was
observed on MgO in HZ (23.3 kPa) (Figure 2c). The ratio of
the wavenumber in DZ and in HZ (0.707) on [Ru6N]/MgO was
similar to the theoretical ratio Y R ~ - ~ Y R ~ (0.71- H 1). The amountof H(a) was estimated to be 3.1 per [RuaN] cluster (Table 3)on the basis of the intensity of the Y R ~ - Hpeak compared t o thecorresponding peak of H(a) on conventional Ru/MgO, assuming
the absorption coefficients were the same between the two. The
molar ratio W[Ru6N] was 4.0 by uptake measurement at 179
K (Table 3).
I4N2(23.3 kPa) was introduced to [Rua]/Mg0-588H at 193
K. The IR spectrum after 30 min in NZis shown in Figure 2d.
Two peaks at 1605(w, sh) and 1517(m) cm-I were observed inaddition to strong peaks at 1664 and 1308 cm-I. In thecorresponding spectrum in I5N2(23.3 kPa) (Figure 2e), the peakat 1517 cm-' was shifted to 1459 cm-', while the other three

A1203 in I4N2(23.3 Wa), (c) [Ru&J]/A1203 in l5N2(23.3 P a ) , and (d)A1203 in l4N2 (23.3 Wa). Each background spectrum was for the
sample before the introduction of gas. Observed temperature = 193
K. [Ru6N]/A1203 was evacuated at 813 K and in H2 at 588 K. A1203
was evacuated at 813 K.

peaks remained at the same wavenumbers. The ratio (0.962)
for the peak at 1517 (1459) cm-' was in accordance with thetheoretical ratio wN-15N/wN-'4N (0.966). No peak was observed
around 1450-1520 cm-I when 23.3 kPa of l4Nzwas introduced
to MgO (Figure 20. Hence, the peak at 1517 (1459) cm-' canbe assigned to Nz(a) on [Ru6N(p-OSu)3].The other three peaks
remained at the same positions in Figure 2d-f and appeared
only on MgO. Therefore, they should be due to the interactionsof NZ with the MgO surface or a trace amount of impurity gas
adsorbed on MgO such as a trace amount of water orhydrocarbons. The amount of Nz(a) was estimated to be nearly
one per [Ru~N]cluster by the uptake measurement at 179 K in
Table 3.
The IR study on [Ru6N]-Cs+/MgO in HZ or NZ wasexperimentally difficult because of the strong absorption by theCs+/MgO support (some part of impregnated Cs2CO3 remained
undecomposed) and the difficulty to regulate an adequate
amount of cluster 1 solution dropped onto the Cs+/MgO disk
in the IR cell.
2. [Ru&]/Al203. The IR observations in HZand in N2 werealso performed for [Ru6N]/A1203-588H. The IR spectrum at
193 K for 30 min in HZ (23.3 kPa) had no peak (Figure 3a),indicating Hz was not adsorbed on [Ru6N]/A1~03at 193 K. The
spectrum for 30 min in I4Nz(23.3 Wa) had four peaks (Figure3b), and two among the four shifted in I5N2(Figure 3c) (from1661 to 1602 cm-', from 1444 to 1390 cm-I). Therefore, thetwo peaks at 1661 and 1444 cm-I can be assigned to Nz(a).
The ratios (0.963-0.964) were in accordance with the theoreticalratio vi5N-1sN/v14N-14N(0.966). When 23.3 kPa of I4N2 wasintroduced on AlzO3, two peaks were observed, at 1822 and1207 cm-'. These two peaks were not shifted in Figure 3b-d
and appeared only on AlzO3, probably due to the interactionsof N2 with the A1203 surface or trace amount of impurity gas(water or hydrocarbon) adsorbed on the A 1 2 0 3 surface. The
molar ratio N2(a)/[Ru3(p-Osu)3] was nearly 1 by the uptake
measurement at 179 K (Table 3).
3. In-Situ IR Spectra in N2
302 at 588 K . In-situ IR
spectra were observed for [Ru6N]/MgO in Nz (16 P a ) and D2(48 Wa) at 588 K. In the spectrum for 60 min (background
spectrum was for the sample before the introduction of reaction
gas at 588 K), no absorptions were observed in the YN-D, YN-N,
and Y R ~ - D regions except for the growth of YO-D at 2756(s)and 2661(m, br) cm-I (exchange of hydroxyl on MgO). Afterthe evacuation of reaction gas at 588 K, 8.7 Wa of ND3 was+Izumi and Aika

10350 J. Phys. Chem., Vol. 99, No. 25, 1995
introduced to this sample at 588 K. In the spectrum for 60
min in ND3, two new weak peaks were observed at 2540(vw)and 2362(vw) cm-I.

TABLE 4: IR Absorption Peaks of €€(a)on [Ru6N(CO)dClusters Supported on MgO, and IR and Raman Peaks of
Related Systems
vlcm-'

Discussion
The active structures of [ R W on MgO, K+/MgO, Cs+/MgO,
and A1203 were summarized in Figure 7 in the previous paper.I3
The [RU6N(C0)16]- 1 was reacted with MgO and transformed
to [Ru6N@-Osu)3](0,,,oxygen atom at surface) by the evacuation at 813 K (decarbonylated). The structure was stable anddid not change by subsequent treatment in H2 at 588 K ortreatment in N2 and HZ at 588 K. Also on Cs+/MgO and K+/
MgO, cluster 1 was transformed to [Ru6N@-Osu)31 by theevacuation at 673 K and hydrogen treatment at 588 K. In
contrast, cluster 1 was cleaved to the trimer [Ru3@~-0~~)31
on
A1203by the evacuation at 813 K, and the species were very
stable by TH*= 588-773 K. Cluster 3 was transformed to a
non-nitrido hexamer [Ru6@-Osu)3-4] on MgO in vacuum andin H2 at 623 K. Cluster 2 without interstitial nitrogen readily
aggregated to larger Ru particles on MgO during evacuation at
813 K and/or subsequent HZtreatment at 588 K ( N R ~ -=R5.9~
6.2).
The TOFs of ammonia synthesis at 588 K were in thefollowing order: [RugN@-Osu)3]on Cs+/MgO (0.40 min-I) >[Ru6N@-Osu)3]on K+/MgO (0.19) > [Ru6N@-Osu)31on MgO(0.11) > [Ru6@-Osu)3-4] on MgO (0.018) > [Ru3@2-Osu)31on
A1203 (0) (Table 1). Aggregated Ru particles from clusters 1
or 2 ( N R , - R ~= 6.2-6.6) or conventional Ru catalysts on MgOor on A1203 exhibited lower activities (0.001-0.017 min-I),
but still better than the supported triangle cluster on A1203 (noactivity) (Table 1). Taking into account each active structure,these TOF data are discussed with regard to three control factors
over the catalysis: (1) nuclearity of the Ru cluster, (2) cluster/
support interface, and (3) nitrido nitrogen.
Nuclearity of the Ruthenium Cluster and Catalysis. We
can compare the effect of nuclearity on catalysis for four samples
on MgO which were free from nitrido N in the cluster structure,[Ru&]/Mg0-623H, [R~]/Mg0-588H,[R~&J]/Mg0-773H,andconventional R m g O (Table 1). Supported Ru clusters with
six Ru atoms without nitrido N (0.018 min-I) were more active
than the aggregated Ru clusters (0.013-0.017 min-I) and much
more active than the conventional one (0.002 min-I) (Table1). The sample [Ru$J]/Mg0-773H, originally prepared from
nitrido cluster 1, must have lost nitrido N during the hydrogen
treatment above 600 K (Figure 5 in the accompanying paper)I3
and aggregated ( N R , - R ~= 6.6). The activity was nearly equal
to that of [Ru6]/Mg0-588Hprepared from [Rus(CO)~sl~(0.013
min-I). No activity on triangle Ru cluster [Ru3@2-Osu)3]on
A1203 should be mainly due to the nature of A1203 support.
Conventional RdA12O3 could not activate N2 very well and gave
low ammonia synthesis activity (0.001 min-I). However, it isinteresting to point out that [Ru3@2-Osu)3]on A1203 was notable to dissociate H2 on the basis of IR in H2 (no adsorption ofH) (Figure 3a) and the low exchange rate of Hz/Dz (2.0 min-I)(Table 1). The poorer ability to activate hydrogen must be
another reason for the low TOF in ammonia synthesis. Thus,
we cannot discuss the case of the three-Ru-atom ensemble inthe same context as hexamer and larger particles. Roughly
speaking, the smaller the nuclearity of the cluster, the higher
the activity at least up to the size of the six-Ru-atom ensemble.
The cluster nuclearity should be related to the support effectbecause the smaller cluster was more strongly affected by the
support. In fact, the TOF on supported [RugN] was dependent
on the Ru wt % (Figure 1) and the kind of support (Table 1).
Hence, the discussion should be extended to include the cluster/
support interface.

on-top
HdWC0)41980
H2Ru(C0)2(PPh3)2 1878, 1823
Fe-Al203-KzO
1954, 1902,

bridging

3-foldmethod
ref
IR
31
IR
31
Raman 32

1870

Fe, Co, Ni, Rh, Pd, 1940-1850
IRor Ir on ,41203
RulMgO
1880, 1801, 1550, 1330 1120,933 IR
1717
1781

Ru-Cs'iMgO
H~Ru~(CO)IZ
H~RU~(C~)I~[RU6N(c0)16]-/
MgO

1944

33
34

1540, 1407 1233,940 IR
34
1585, 1290
Raman 35
708,660, IR
36
652

IR

this
work

Cluster/Support Interface and Catalysis. The Ru wt %dependence of TOF for ammonia synthesis was shown in Figure
1. The TOF exhibited only a slight change around 1.5-3 wt% Ru, but the TOF increased at 0.48 wt % Ru and decreased
at 3.9 wt %. These changes corresponded well to the structurechanges of the cluster and cluster/support interface. The
aggregation to larger Ru particles was observed by EXAFS( N R ~ -=R5.0)
~ at 3.9 wt % Ru,I3 and this decrease tendency of
TOF was in accordance with the change of TOF for [Ru6N]/
MgO (2.5 wt % Ru) (Table 1) at different reduction temperatures( N R " - Rwas
~ 4.1 at TH*= 588 K and 6.6 at T H =~ 773 K).I3
The N R ~ - o ,was, observed to increase from 0.5 to 1.2 when theRu wt % decreased from 2.5 to 0.48, keeping the N R ~ - R ~unchanged (4.0 f O.l).I3 The increase of TOF at 0.48 wt %Ru must be closely related to the cluster/support interface(number and nature of Ru-Os, bondings) because the cluster
framework structure [ R u a ] was kept unchanged.
Nitrido Nitrogen and Catalysis. 1 . H-induced Structure
Change for Supported [ R u a ] . The TOF on the ruthenium
hexamer with nitrido N was larger by 6.1 times than that without
nitrido N on MgO (Table 1). In EXAFS measurements,
expansion/contraction of the [Ru~N]framework was observed
as a change O f rRu-Ru = 2.62
2.65 8, on MgO and 2.63
2.71 8, on Cs+/MgO by the adsorption/desorption of hydrogen.I3
In the IR observations, the adsorbed H showed peaks at 1944
cm-' in H2 and at 1374 cm-' in D2 on [Ru6N@-Osu)3]on MgO(Figure 2). Reported vibrational frequencies are listed in Table
4 for the stretching mode Y R ~ - H of on-top, bridging, and 3-fold
H(a) on metal clusters/complexes and supported catalysts. The
on-top H(a)'s on organometallic Ru complexes or conventional
metal (Fe, Co, Ni, Ru, Rh, Pd, Ir) catalysts were in the range
1980-1717 cm-1.31-34 The bridging and 3-fold H(a)'s on Ru
carbonyl clusters have two vibrational modes, vasand Y,, in therange 1585-1290 cm-' 35 and 708-652 cm-1,36respectively(Table 4). The amount of H(a) per [Ru6N@-Osu)3] wasestimated to be three to four on [RusN]/MgO and five to six on[RugN]-Cs+/MgO on the basis of the intensity of the Y R ~ - H
peak and the uptake measurements at 77-193 K (Table 3).
There is a tendency for hydrogen to be adsorbed at on-top sites
on metal particles of conventional metal catalysts and at bridging
or 3-fold sites on organometallic carbonyl Ru clusters in Table
4. However, the hydrogen on organometallic carbonyl clusters
is located in the small free space between the bulky carbonyl
ligands. Our [RugN@-Osu)3]had metallic mu-^, (2.62-2.63
8,)13 shortened from a cluster-like 2.88 8, (determined by
EXAFS) of unsupported cluster 1, and all the carbonyl ligandsof cluster 1 desorbed by heating in vacuum. In this context,
adsorption of H on [RugN@-Osu)3]as on-top does not contradict
with previous reports in Table 4. On the basis of EXAFS and
IR data, this reversible change induced by the adsorption/
desorption of hydrogen is shown in Scheme la,b. The changes

Catalysis on Supported [ R u a ] Clusters

SCHEME 1: Proposed Reaction Mechanism ofAmmonia Synthesis on Supported [ R W ] Clusters( Y z-N&[RyN( Y -0)pl-CdlMpO(contracted fnunmork)(rh.Ru = 2.65A)ofwere 1.1% for [RusN]/MgO and 3.0% for[Ru6N]-cS+/Mgo.
2. N2 Adsorption on Supported [ R u a ] . The adsorptions ofmolecular nitrogen were observed on [Rua]/Mg0-588H and[Ru&i]/Al203-588H by IR. The structures of each cluster
sample were determined as [RugN@-Osu)31and [Ru3@2-Osu)31
by EXAFS.I3 For the MgO-supported system, the peak at 1517
cm-l in I4N2(1459 cm-' in I5N2)(Figure 3) can be assigned
as N2(a) on [Ru6N@-Osu)3],judging from the accordance of theisotope shift of the wavenumber with the theoretical value(0.966) and the absence of these peaks for the MgO sample.
We believe that the vibration of lying-down N2 was possible todetect on the dispersed [Ru6N] cluster, not directly adsorbedon the rather flat MgO surface. In the case of [Ru6N]/A1203588H, two IR peaks were observed, at 1661 and 1444 cm-'
(1602 and 1390 cm-' for 15N2), ascribable to N2(a) on [Ru3@2-osu)3](Figure 3), judging from the isotope shift and theabsence of these peaks for the Ai203 disk.
On the Fe( 111) surface, adsorbed I5N2 at lower than 80 K
showed an absorption peak at 2100 cm-I, and adsorbed 15N2at
100 K showed an absorption peak at 1490 cm-' (Table 5).3337
The two kinds of N2(a) were assigned as on-top and bridging
N2(a), respectively, by X-ray and ultraviolet photoelectron
spectroscopy observations. It should be noted that some partof this bridging N2(a) on Fe( 111) was transformed to atomic
nitrogen on the Fe(ll1) surface, and the rates of ammonia
synthesis at 673 K were in the order Fe( 111) =- Fe(211) =- Fe(100) =- Fe(210) > Fe(l10),43 suggesting that the lower
coordination Fe face was favorable to ammonia synthesis.
Compared with vibrational data in Table 5, peaks on [ R u a ] /MgO at 1517 cm-' can be assigned as bridging N2(a) on [ R u a ( D - O ~ ~ )It~ was] . suggested that one or two bridging N2(a) werecoordinated to the [Ru6N@-Os,)3] cluster on the basis of the
N R ~ - N(0.4-0.7) for N2(a) on [Rua]/MgO and [Ru6N]-Cs+/
MgO observed by EXAFSI3 and uptake measurements of N2(a) on [Rua@-Osu)3](0.7-1.1) in Table 3. Similarly, about
one bridging N2(a) was coordinated to [Ru3@2-Osu)3]on A1203
on the basis of Table 3. Upon the adsorption of N2 on [ R u a @-Osu)3], negligible changes of bond distances for Ru-Ru and
Ru-Os, were observed on MgO and C S + / M ~ O . ' ~3. Reaction Mechanism of Ammonia Synthesis. While no
IR peaks were observed in the wavenumber region of VN-D for[Ru6N]/MgO in N2
3D2 at 588 K, two new peaks wereobserved, at 2540 and 2362 cm-I, in ND3 at 588 K. Comparedto ammonia vibrational data (3444(E), 3337(A1), 1627(E), 950(AI) for NH3 and 2564, 2420, 1191, 748 cm-I for ND3)@ andreported data for NH2(a) (3380, 3290, 1610 cm-I) and NH(a)(3200 cm-I) on Fe/Si02$5 these two peaks can be assigned as+

J. Phys. Chem., Vol. 99, No. 25, 1995 10351
ND2(a) on [RugN@-Osu)3]. It is interesting that NH2(a) formation was reported on the stepped (coordinatively less saturated)
Ru(l,l,lO) surface, but not on the flat Ru(001)compared to our case of unsaturated supported clusters [Ru~N@-Osu)3]. NH3 was molecularly adsorbed on Ru(001) and Ru(l,l,lO) surfaces at 120 K. By the subsequent heating to 300
K, the NH3(a) was transformed to NH2(a) on the Ru(l,l,lO)
surface, as c o n f i i e d by the binding energies of N 1s (398.1
eV) in X P S and of valence level (7.4, -10 eV) in U P S , although
only the desorption of NH3 was observed on Ru(001) surface.46
In other words, higher ability to activate the NH, species was
suggested on stepped (unsaturated) Ru( l , l , 10) surfaces than flat(001) surfaces.
Inverse Dfl2 isotope effects were observed for [Ru$J]/MgO
and [RusN]-Cs+/MgO ( I - D J ~=H1.4~
1 3 , but no isotope effectwas observed for conventional Ru/MgO (Table 2). The totalammonia synthesis reaction rate can be formulated on the basisof the Langmuir equation and classified according to the main
surface species during catalysis (whose residence times on the
Ru cluster surface are longer than the other species) on the Ru
surface in the following reaction conditions: (i) NH,(a) (x =0, 1, 2, 3) was the main surface species, and (ii) H(a) was themain surface ~pecies.4~+ K1PNH3/PH;(3-x)'2}2 (0r = kPN1/(l + K"2PHz1'2)2(ii)
The in-situ IR observation in N2 + 3D2 showed no peaks ofr = kPNd{1adsorbates (or intermediate species), while two peaks corresponding to ND2(a) were observed in ND3 at 588 K. Hence,
we can exclude the possibility that NH2(a) or NH3(a) was themain surface species during the synthesis reaction. The
equilibrium constant K in the above equation is for the stepN(a) (3/2)H2 N H 3 in the case where N(a) was the main
surface species, and the ratio of K D , / K Hwas
~ calculated on thebasis of the vibrational partition function48with vibrational data
for ammonia (listed above) and the hydrogen molecule (VH-H= 4161, VD-D = 2993 cm-I.@+

KDdKH,= exp[{-z(VN-D(ND3) - VN-H(NH3)) +3/2(~D-, - VH-H)}h/2kT] = 12 (1)In the case where NH(a) was the main surface species, theequilibrium constant in question is for the step NH(a) H2 z
NH3, and the ratio was calculated similarly.

+

eXP[{-x(VN-D(m3)VN-H(NH))- VN-H(NH3))(VD-D+ ('N-dm)-- V,-~)}h/2kn = 8.2 (2)The value of K in the case where H(a) was the main surface
species is for the step H2 2H(a), and the ratio was calculatedwith V R ~ - H = 1944, V R ~ - D = 1374 cm-' (Figure 2), VH-H, andVD-D.

KDdKH, = ~xP[{-~(vR,-D - vRu-H) +(VD-D - V ~ - ~ ) ) h / 2 k f=
l 0.97 (3)Therefore, the observed inverse isotope effects for [Ru$J]/MgO
and [Rua]-Cs+/MgO can be explained when the dissociatedN(a) or NH(a) was the main surface species on Ru atoms of[Rua@-Osu)3]clusters with the K values 8-12 in eq 1 or 2.
In contrast, no isotope effect for conventional R a g 0 was
consistent with almost the same value of K as unity (0.97) in10352 J. Phys. Chem., Vol. 99, No. 25, I995

Izumi and Aika

TABLE 5: IR Absorption Peaks of N2 Adsorbed on [Rm(CO)1& Clusters Supported on MgO and A1203, and IR and EELS(Electron Energy Loss Spectroscopy) Peaks of Related Systems
vlcm-I

on-top
CoH(Nz)(PPhs)s
RUH~(N~)(PP~)~
RdA1203
Ru-Cs+/A1203
Ru/MgO
Ru -Csc/MgO

R~C~(PM~~P~)~-N~-MOC~~(OM~)

bridging

method

1282

IR
IR
IR
IR
IR
IR
IR
IR

38
39
40
40
34
34
41
42

EELS
EELS

37
43
this work
this work

2085
2147
2268,2214
2095
2154
2120,2020
1660

013-N2)[(r5:r5-ClaH8)(r-Cp)2Ti21'[(r1:r5-C5~)-ref(r-C~)~Ti21~[(r-Cp)2(~6H1~~3)Til~6H1~~~
2100"

Fe(ll1)
K-doped Fe( 111)
[Ru$J(C0)16I-/MgO[RU$J(C0)161-/A1203

1490"
1370"
1517
1661, 1444IRIR

For I5N2.

TABLE 6: IR Absorption Peaks of CO Adsorbed on [R&N(CO)&, [R%C(CO)l&iel-, and [Rus(CO)l~l~Clusters Supported
on MgO and Conventional Ru/MgO Catalysts
Ru(NO)(NO&
2.5
2059w(sh)2045w(sh)1992s1978w(sh)1965s

eq 3. In the IR
no adsorption of N2 was reported onconventional Ru/MgO (183 K) or Ru-Cs+/MgO (293 K) in
N2 H2 (8.2 Wa) due to the stronger occupation of surface
Ru sites by H(a). The N2 adsorption (ON,) was only 8.0 and
1.3% on H-preadsorbed conventional Ru/MgO (183 K) and RuCsf/MgO (293 K) in N2 (1.5 Wa) compared to a OH of 65 and50%, respectively, although the values might be lower atelevated temperature during catalysis. Thus, main surface
species on conventional R a g 0 or Ru-Cs+/MgO should be
H(a), which had no H2/D2 isotope effect (Table 2).
The proposed reaction mechanism on supported [Rua(pcluster is illustrated in Scheme 1 based on in-situ
EXAFS,I3 IR for adsorbed H and N2, the N2 pressure dependence (0.84 f 0.06), and inverse isotope effects of WD on
ammonia synthesis. We propose two possibilities of reaction
cycles according to the order of adsorptions of H2 and N2.
Hydrogen adsorption was in very fast equilibrium compared tosteady state ammonia synthesis on the basis of the higher HID2 exchange rates, even in the presence of N2 on supported[R~6N@-0,,)3] clusters (Table 1). The equilibrium step (species(a)(b)) should be accompanied by cluster expansion/
contraction observed as the change of R,,-R~(2.63 z= 2.71 8,
on Cs+/MgO and 2.62 t 2.65 8, on MgO) by the adsorptiod
desorption by means of EXAFS.I3 The N2 can be adsorbed on
species (b) as p2-bridging style on [RugN(u-Osu)3]to be species(c) on the basis of the wavenumber of Nz(a) in IR spectra (Figure2d,e) and EXAFS in N2,I3 keeping the expanded cluster
framework, although the concentration might be low at elevatedtemperature during catalysis. The main surface species during
the catalysis should be N(a) or NH(a) on the basis of the inverse
H2/D2 isotope effects (Table 2) and calculations of equilibrium
constants in N2 3H2 or in N2 3D2 at 588 K (eqs 1 and 2).
Compared to the relatively stable species (d), progressive stepsof hydrogenation of NHZ(a) and desorption of NH3(a) shouldbe very fast, similar to the equilibrium of H2 adsorption between
species (a) and (b), and contracted species (a) was reproduced(route 1).
The second possible route is through the adsorption of N2
on [Ru6N(u-0,,)3], keeping the contracted framework, as in

+

+

+

2045w(sh)1989s1933w(sh)1795w(br)2069w(sh)2006s1925w(sh)2010s(br)species (b'). The structure was based on the EXAFS measurement in N2 ( I R ~ - R ~= 2.65, mU-o,, = 2.07, R,,-N = 2.02 &.I3
Subsequently, the H2 can be dissociatively adsorbed to be
species (d). The cluster should be expanded by the adsorptionof H, and the expansion can promote the dissociation of N2(a)to N(a) or NH(a) by elongating the Ru-Ru distance (route 2).
In these two possible routes (Scheme l), the structural contribution was suggested for the promotions of hydrogen adsorption(route 1) or nitrogen dissociation (route 2). The supported [Rug(p-Osu)3-4] did not show H-induced structure change, keeping
Q,,-R~ = 2.63 8, unchanged (N = 4.0 f 0.3).13
The expansiodcontraction of the cluster framework was also
reported by the CO adsorptioddesorption for [Ru~C],and it
became possible when catalysts were in CO H2 at elevatedtemperature (473-523 K), as observed by the change of EXAFS( h m u - ~ , and, ) the CO uptake.30 In the case of [RusC]/MgO,
the mu+,,changed reversibly between 2.63 and 2.87 8, and CO(a)/[Ru&] (molar ratio) was 2.9 at 290 K and 10.7 at 523 K.
The temperature range where the cluster expansiodcontraction
was observed coincided with the catalytic reaction temperature,
but the reported expansiodcontraction did not necessarily
correspond to elementary steps in overall catalysis because theadsorption of CO should be very strong even around 473-523
K, and the [Ru~C]framework may always be expanded by CO(a) in reaction conditions of CO/H2. At least it was valid thatthe catalysis was promoted by the elongated Ru-Ru which wascovered by CO(a) and by facilitating the dissociative adsorptionof H2. In this paper, the [Ru~N]clusters also exhibited
framework expansiodcontraction by the adsorptioddesorptionof H, but the temperature range did not correspond to thereaction temperature of catalysis. We would like to point out
the possibility that the expansiodcontraction of the [Ru~N]
framework was incorporated as an elementary step of the overall
reaction in Scheme 1, similar to the promoted reactions in refs
22-25, because the population of species (b), (c), or (b') should
be low at elevated temperatures during catalysis. The strong
coordination of N2 (as p2-style) was suggested by EXAFS and
IR on [Ru6N@-Os,)3], but only on-top @ I )N2(a) was observedon conventional Ru/MgO and Ru-Cs+/MgO (Ru particle size+

Catalysis on Supported [ R u a ] Clusters-

30 A) (Table 5).34 The promoted catalysis was possible when
NZ was strongly adsorbed as p2-style. This difference may be
the main factor which made smaller clusters (n = 6) morefavorable to ammonia synthesis.
4. Chemical Effect of Nitrido Nitrogen. It is impossible todistinguish the structural and chemical effects of nitrido nitrogenbecause structural effects (cluster expansiodcontraction andchange of coordination style of N2(a)) were only observed in
the system with nitrido nitrogen, i.e. supported [Ru$J@-OSu)3]
clusters on MgO and Cs+/Mgo. Prepared supported [RU6@Osu)3-4] did not have nitrido nitrogen nor H-induced cluster
expansiodcontraction.
A general comparison of the carbonyl frequencies is listed
in Table 6 for MgO-supported Ru clusters (pretreated in thesame procedure as for ammonia synthesis) in CO. The
stretching vibration wavenumber YCO for [Ru$J]/MgO (2.5 wt%) was similar to [ R U ~ C ] / M ~andO ~smaller~
than that for
aggregated [Ru&klgO ( N R ~ -=R 6.2)~ or conventional R a g 0
catalyst by 14- 18 cm-'. The difference became larger in thecase of [Ru6N]/MgO at 0.06 wt % Ru (41-45 cm-I). On thebasis of the EXAFS analysis in the previous paper,I3 cluster 1
always maintained the [Ru6N] unit upon supporting ( T H<~ 623K), contrary to the change of the number x (change of adsorption
site on MgO). Thus, although there are several factors for the
IR wavenumber shifts, these red-shifts of YCO for [Ru6N] and[Ru~C]on MgO may be ascribed to total electron donation by
nitrido N49and by support MgO, which were larger than generalvco shifts induced by the change of Ru particle size on support.5o
The promotion of N2 dissociation by electron donors to metal
active sites is widely k n o ~ n . ~ ~ 3 ~ ~ ~ ~ ~Conclusions
1. Supported [Ru6N@-OSu)3]clusters on MgO, K+/MgO, or
Csf/MgO were found to be active for ammonia synthesis.
2. The TOF (per cluster) of ammonia synthesis was larger
by -30% at 0.5 wt % Ru than that at 2.5 wt % for [Ru$J]/
MgO. Cluster/support interface should be the major factor on
the basis of the NR"-o,, of 1.2 (0.5 wt %) and 0.5 (2.5 wt %)below the common [Ru$J] species.
3. During ammonia synthesis on supported [Ru6N@-Osu)3]
clusters, N(a) or NH(a) was suggested to be the main surface
species on the basis of IR and H/D isotope effects (m2/m2=
1.4- 1.3, compared to conventional Ru catalysts on whose Ru
surface H(a) should predominantly adsorb.
4. New promoted catalysis was suggested on [Ru$J] clusters.
Compared to the strong adsorption of hydrogen for conventional
Ru catalysts on which no H/D isotope effect was observed, thestructural promotion of adsorption of H2 and/or dissociation of
NZ was suggested. The p2-style adsorption of N2 on [Ru~N@-o&] may be related to promoted catalysis compared to ontop N2(a) on conventional Ru catalysts.
5. Chemical promotion was also implied as the electron
donation by nitrido nitrogen.
References and Notes(1) Hydrogen Effects in Catalysis, Fundamentals and Practical Applications; Paal, Z . , Menon, P. G..
Eds.; Marcel Dekker: New York, 1988.
(2) Christmann, K. Surf: Sei. Rep. 1988, 9, 1.
(3) Ertl, G. In Catalytic Ammonia Synthesis, Fundamentals andPractice; Jennings, J. R., Ed.; Plenum Press: New York, 1991; p 109.
(4) Held, G.; Pfniir, H.; Menzel, D. Su$. Sci. 1992, 271, 21.
( 5 ) Solokowski, M.; Koch, T.; Pfnur, H. SurJ Sei. 1991, 243, 261.
(6) Feulner, P.; Menzel, D. Sur$ Sci. 1985, 154, 465.

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Catal. 1994, 147, 455.
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