J. Phys. Chem. 1996, 100, 9421-9428

9421

Catalysis on Ruthenium Clusters Supported on CeO2 or Ni-Doped CeO2: Adsorption
Behavior of H2 and Ammonia Synthesis
Yasuo Izumi,* Yasuhiro Iwata, 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: September 6, 1995; In Final Form: March 14, 1996X

Catalysis on Ru clusters supported on CeO2 or Ni-doped CeO2 was investigated. Ru3(CO)12 was reactedwith CeO2, followed by heating in vacuum at 673 (i) or 813 K (ii) and then in H2 at 588-1073 K (TH2).
Activities of both catalysts had the TH2 dependence, which has a maximum at TH2 ) 873 K for ammonia
synthesis. The rates on i were faster than those on ii by a factor of 2.0-1.1 in the range of TH2 ) 588-973
K. On a sample in which Ru3(CO)12 was supported on previously reduced Ni/CeO2 (in H2 at 773 K) iii, thehighest synthesis rate was 1.5 × 10-3 mol h-1 gcat-1 at TH2 ) 588 K on iii. The activity order iii > i > ii canbe understood in terms of two factors: (A) reduction extent of support and (B) number of active Ru sites.
The two factors conflicted with each other when the treatment temperature in H2 increased. By heating thesamples in H2 up to 873 K to satisfy factor A, the aggregation of Ru clusters for i or physical blocking of
surface Ru sites by CeO2-x for ii occurred: factor B was not satisfied. The two factors should be optimized
in catalyst iii, where the support cerium oxide was thoroughly reduced through the doped Ni. On reduced
Ni/CeO2, the Ru cluster implantation can be done at low temperature (588 K). Obtained values of rRu-Ru at
2.62 Å (N ) 7.1) and rRu-O(s) (O(s) is the oxygen atom at surface) at 2.12 Å (N ) 1.2) by EXAFS for
Ru3-Ni/CeO2 suggested a flat Ru cluster model comprised of several Ru atoms on reduced Ni/CeO2-x surface.
The H(a)/Rutotal ratio exceeded unity for catalysts i and iii, suggesting new H adsorption sites. The temperatureprogrammed desorption for hydrogen (simultaneous desorption of HD and D2 for iii at 330-430 K suggested
that the H at the new site and H on Ru surface were exchangeable above 330 K. The “reservoir” effect of
the new site for H on catalysis is discussed in relation to new kinetic design of hydrogenation catalyst.

Introduction
Recently, we studied hydrogen adsorption and ammonia
synthesis on Ru cluster catalysts by means of EXAFS (extended
X-ray absorption fine structure) and FTIR (Fourier transform
infrared spectroscopy).1-3 Several adsorption sites of H weredetected by FTIR.1 The in situ structure change of Ru clusters
was observed by the interaction with H2 by EXAFS.2 The
promotion of ammonia synthesis was suggested by elongating
the Ru-Ru bonding distance to facilitate the dissociative
adsorption of N2.2,3
The H adsorption was examined for Rh/TiO24 and Ru/SiO25
by solid-state 1H-NMR. Two peaks were observed for Rh/TiO2
when heated in H2 above 573 K. The R peak around δ ) -130
ppm was assigned as H atom on Rh particles on the basis of
PH2 dependence and spin-lattice relaxation time T1. The peakintensity decreased when the sample was heated in H2 above
723 K in relation to the SMSI (strong metal-support interaction)effect. A part of β peak around 0 ppm was implied as “weakly
bound H” on Rh particles present at relatively higher PH2 (>2kPa) compared to R peak H. Two peaks were observed for
Ru/SiO2 around -60 (R) and -30 ppm (β) in H2.5 The R andβ peaks were assigned as H on Ru particles and “weakly boundH” on Ru particles, respectively, on the basis of heat ofadsorption, T1, and H/Ru(s) ratio. The β peak was only observedat PH2 > 13 kPa. The weakly bound H was implied to have aninteraction with lower coordination Ru sites because the β peak
population increased as the Ru particle size decreased.
CeO2 is an effective promoter or support of Ru catalysts, andthe importance of metal-support interaction is often implied* To whom correspondence should be addressed. E-mail:yizumi@chemenv.titech.ac.jp.
X Abstract published in AdVance ACS Abstracts, May 15, 1996.

S0022-3654(95)02602-5 CCC: $12.00

when reduced at high temperatures. In this paper, we reportpromoted catalysis over Ru clusters supported on CeO2 or on
Ni-doped CeO2 as a first objective. The support was reduced
at high temperatures (∼873 K) in H2 (CeO2) or at moderatetemperatures (623-773 K) by spiltover H atoms from doped
Ni (Ni/CeO2). The major control factors of ammonia synthesis
should be the number of active sites, the nature of the Ru/support
interface, and the electronic effect of support.3 The catalyst
preparation procedure, reacting the Ru3(CO)12 with previously
reduced Ni-doped CeO2, optimized these control factors.
The second objective is the clarification of the promoted
reaction mechanism. The hydrogen uptake exceeded unity for
Ru clusters on CeO2 or Ni-doped CeO2. We propose thecondensation of H atoms at the interface between Ru clusters
and the reduced CeO2-x surface. The “reservoir” effect of theinterface site on catalysis is discussed.
Experimental Section
Cerium oxide was prepared from a 0.2 mol L-1 Ce(NO3)3‚
6H2O (Wako, 99.9%) solution. The 5% ammonia was added
until the pH of solution was 10. The obtained Ce(OH)4 wasfiltered and repeatedly washed. Yellow powder of CeO2 (BET
surface area 57 m2 g-1) was obtained by heating Ce(OH)4 inair (4 h), evacuating (1 h) at 773 K, and then treating in O2 at
290 K. Ru3(CO)12 (1) was interacted with CeO2 in distilled
THF (tetrahydrofuran) (Wako, Special Grade) at 290 K for 2
h, with the subsequent removal of THF in vacuum (Ru3/CeO2).
The obtained CeO2 was impregnated with Ni(NO3)2‚6H2O(Wako, 99.9%) to give 0.25 wt % Ni and treated in H2 (12 h)
at 773 K, followed by a procedure similar to the case of Ru3/
CeO2 (Ru3-Ni/CeO2). The loading of Ru was 1.7 wt %. Allprocedures of sample preparation and transfer were performed© 1996 American Chemical Society

9422 J. Phys. Chem., Vol. 100, No. 22, 1996
in argon (99.99%) or in vacuum. Incipient supported clusters
were heated (elevating rate of temperature 4 K min-1) in vacuum
up to 673 or 813 K, followed by the treatment in H2 for 1 h(588-1073 K). The catalysts were denoted as Ru3-Ni/CeO2673ev, for example, where the evacuation temperature wasnoted. Subsequent treatment temperature in H2 (TH2) is notedin each description.
The conventional Ru/CeO2 catalyst was prepared from 1 inair. 1 was interacted with CeO2 and decomposed (the color of
the solution turned from orange to dark yellow) in THF for 12
h (1.0 wt % Ru). After evaporation of the THF, the powder
was heated in O2 (1 h) and then in H2 (1 h) at 773 K before
being used as catalyst. The weight of the catalyst was 0.0150.030 g for Ru3/CeO2, Ru3-Ni/CeO2, and conventional Ru/
CeO2.
The ammonia synthesis reactions were carried out under 101
kPa of reaction gas (PH2/PN2 ) 3.0) at 588 K in a flow system(flow rate 60 cm3 min-1). The produced ammonia was analyzed
by the decrease of electron conductivity (1.7-0.8 mS‚cm-1) inthe H2SO4 solution (<0.004 N). The linearity of the correlation
between the produced amount of NH3 and the decrease ofelectron conductivity was checked. The chosen reaction temperature was relatively low (588 K) to realize differential
working conditions. The catalytic rates were stable in 36 h,
and the rates at 2 h were listed for all the catalysts in this paper.
The rates on Ru3/CeO2-673ev and Ru3-Ni/CeO2-673ev catalysts
were relatively unstable only when treated in H2 at lower
temperatures (588-673 K) before catalysis. They increased in
2 h (within 30%), and a smaller increase was observed during
2-10 h (within 10%).
Hydrogen uptake (ν) measurements were carried out in aclosed circulating system connected to the manometer. The
equilibrium gas pressure (P) was varied in the range 1.3-20
kPa, and the b of adsorption isothermν-1 ) (ab)-1P-1/2 + b-1
was obtained by the extrapolation of obtained data (five to sevenpoints; a is a constant). The samples were kept in ice (273 K)
during measurements. As fast uptake (adsorption on Ru +hydrogen spillover from Ru to support surface) terminated in 1
h followed by linear increase of uptake during 1-10 h (hydrogen
spillover), uptake on Ru (ν) was estimated by subtracting the
H spillover amount during initial 1 h from total (observed)
uptake amount at 1 h. The temperature-programmed desorption(TPD) was monitored in the closed circulating system connectedto the mass. The samples were treated (i) in 76 kPa of H2 (1h) at 293 K, evacuated (0.5 h) at 293 K, and then treated in 76
kPa of D2 (1 h) at 293 K (adsorption A), or (ii) treated in D2 at
293 K (1 h) (adsorption B). The elevation rate of temperature
was 4 K min-1.
The EXAFS spectra of Ru K edge were measured at thebeamline 10B and 6B (2.5 GeV, current 360-260 mA) of the
Photon Factory in the National Laboratory for High Energy
Physics (Proposal No. 93G150). The X-ray radiation was
monochronized through double crystals of Si(311), and thespectra were obtained in transmission mode with ionizationchambers at 100-293 K. The sample was transferred to an
EXAFS Pyrex cell (0.073 < ∆µt < 0.25) with Kapton films.
The analysis was performed by the program EXAFSH written
by Yokoyama, Hamamatsu, and Ohta (the University of Tokyo,
1994).2 The background subtraction was performed by calculating the cubic spline with three blocks, and the obtained functionµt(k) - µ0t(k) was normalized by using Victoreen parameterµ0(λ) ) Cλ3 - Dλ4. The Fourier transform (FT) of the k3weighted EXAFS oscillation was carried out over the range of

Izumi et al.

Figure 1. Dependence of ammonia synthesis rates at 588 K onhydrogen treatment temperature (TH2) over Ru3/CeO2-673ev (b), Ru3/
CeO2-813ev (2), Ru3-Ni/CeO2-673ev (0), and conventional Ru/CeO2(O).

kmin ) 3 and kmax ) 12.6-13.5 Å-1. The Hanning function
was multiplied by k-width of (kmax - kmin)/20 on both ends.
The inverse FT was performed in the r-range of rmin ) 1.041.36 and rmax ) 2.71-2.98 Å multiplied by the Hanning
function with the r-width of 0.1 Å on both ends. The curvefitting analysis was performed in the k-region of 4-12 Å-1 on
the basis of plane wave single scattering theory, using empiricalphase shift and amplitude functions extracted from 1 for Ru-C
bond, Ru powder for Ru-Ru bond, RuO2 for Ru-O and Ru‚‚‚
Ru (next-nearest coordination in RuO2) bonds, and [RuCl2(CO)3]2 for Ru(-C-)O bond. The EXAFS spectra for thesereference compounds were taken at room temperature. The
residual factor (Rf) was calculated by the following equation

Rf ) ∫|k3χobs(k) - k3χcalc(k)|2dk/∫|k3χobs(k)|2 dk
The impurities (as molecular content) for the H2 (99.99%, Toyo
Sanso Co., Ltd.) and D2 (99.99%, Syoko Co., Ltd.) gas were
H2O < 10, N2 < 50, O2 < 10, CO < 10, CO2 < 10 ppm, andtotal hydrocarbons < 10 ppm (as carbon content). The
deuterium content in total hydrogen was >99.8% for the D2gas. The gas was introduced to the EXAFS or IR cell after
purification by passage through a liquid nitrogen trap.
The supported Ru clusters were suspended in ethanol(exposed in air during few minutes), and mounted on a TEM(transmission electron microscope) apparatus (JSM-T220, JEOL).
The dispersion (D) of a Ru particle was estimated from theaverage diameter of Ru particles (2d) by TEM by using thefollowing equation assuming the closest packed structure of Ru
surface (Avogadro number NA, molecular weight MW, interatomic distance rRu-Ru, and the density F of Ru).

D)

4πd2/(x3/2)rRu-Ru2(4/3)πd3FNA/MW

)

6.7
d/Å

(1)

Results
Catalysis. When incipient Ru3/CeO2 cluster was heated in
vacuum, the carbonyl ligands were totally desorbed as CO (anda few CO2) until 673 K. Figure 1 shows ammonia synthesis
rates at 588 K on Ru3/CeO2 heated in vacuum at 673 or 813 K.
At TH2 ) 588 K, the rate for Ru3/CeO2-673ev was faster by 1.2
times than that for Ru3/CeO2-813ev (Table 1a,c).
The two cluster catalysts were treated in hydrogen (76 kPa)
between 673 and 1073 K (Figure 1 and Table 1). The rates on
Ru3/CeO2-673ev had a maximum at TH2 ) 873 K. Similarly,
the rate over Ru3/CeO2-813ev reached a maximum at TH2 )873 K. The maximum synthesis rate for Ru3/CeO2-813ev was
63% of the maximum for Ru3/CeO2-673ev (Figure 1), suggest-

Ammonia Synthesis on Ru3-Ni/CeO2

J. Phys. Chem., Vol. 100, No. 22, 1996 9423

TABLE 1: Rates of Ammonia Synthesis on Ru Catalysts
Supported on CeO2acatalysts

TH2/K

rate/10-3
mol h-1 gcat-1

Ru3/CeO2-673ev

588
873
588
873
588
773

0.24
1.2
0.20
0.75
1.5
0.69

entry
a
b
c
d
e
f

Ru3/CeO2-813ev
Ru3-Ni/CeO2-673ev
conventional Ru/CeO2

a Temperature ) 588 K.

Total pressure ) 101 kPa, PN2/PH2 ) 1/3.

TABLE 2: Amounts of Hydrogen Uptake for Ru Catalysts
Supported on CeO2
catalysts

TH2/K

Ru3/CeO2-673ev
Ru3-Ni/CeO2-673ev
conventional Ru/CeO2

588
588
773

H/Rutotala(molar ratio)
1.5
1.2
4.3
4.1
0.5

a
By manometer, at 1 h in H2 (1.3-20 kPa) at 273 K. The amountof H spillover for 1 h was subtracted from the observed total uptake at
1 h.

ing that the heating in severe condition (813 K in vacuum)
induced the aggregation of Ru clusters and reduced the numberof surface active sites. The rates on conventional Ru/CeO2 arealso shown in Figure 1. It produced ammonia at rates similar
to those of Ru3/CeO2-673ev around TH2 ) 673-773 K, but therate began to decrease above TH2 ) 773 K.
The synthesis rates on Ru3-Ni/CeO2-673ev were faster thanthe others in all the range of TH2 (Figure 1). In contrast to theTH2 dependence for other catalysts, the rate on Ru3-Ni/CeO2
monotonously decreased with the increase of TH2. The maximum rate at TH2 ) 588 K (Table 1e) was larger than 9.9 ×
10-4 mol h-1 gcat-1 on [Ru6N]-Cs+/MgO (2.5 wt % Ru, 101kPa)3 or 6.9 × 10-4 mol h-1 gcat-1 on Ru-Cs+/MgO (2 wt %Ru, 80 kPa)6 at 588 K, suggesting super-catalytic activity of
Ru3-Ni/CeO2 which can exceed the present industrial catalysts
under the reaction conditions in this work.
The isotope effects of hydrogen were examined for ammonia
synthesis in N2 + H2 and in N2 + D2. In the case of Ru3/
CeO2-673ev and Ru3-Ni/CeO2-673ev heated in H2 at 588 K,
strong inverse isotope effects (rD2/rH2 ) 2.0-2.2) were observed.
On the other hand, Ru3/CeO2-673ev exhibited no isotope effectwhen heated in H2 at 773 K. Also, conventional Ru/CeO2 had
no isotope effect, similar to other conventional Ru catalysts.3
Hydrogen Uptake and TPD Measurements. Hydrogen
uptake was measured on Ru3/CeO2-673ev and Ru3-Ni/CeO2673ev before and after hydrogen treatment (Table 2). Compared
to 0.5 for conventional Ru/CeO2, the H/Rutotal ratio exceeded
unity for Ru3/CeO2-673ev ()1.5). After in H2 at 588 K, thevalue decreased to 1.2. The H/Rutotal value reached 4.3 for Ru3Ni/CeO2-673ev and slightly decreased to 4.1 by the hydrogen
treatment at 588 K. No hydrogen was adsorbed on metalunsupported cerium oxide heated in vacuum at 673-813 K and/or in H2 at 588-773 K.
The TPD of hydrogen was measured for Ru3/CeO2-673ev,
Ru3-Ni/CeO2-673ev, and conventional Ru/CeO2. Figure 2ashows the TPD spectrum for the Ru3/CeO2-673ev after adsorption A. D2 and HD began to be desorbed around 330 and 450
K, respectively. This difference implied that the adsorption ofhydrogen (H, D) on Ru was weak for Ru3/CeO2-673ev because
adsorbed H in the first adsorption step in H2 should be easily
replaced by D in the second adsorption step in D2. The originof H atoms of desorbed HD and lesser amounts of H2 can be

Figure 2. TPD spectra for hydrogen on Ru3/CeO2-673ev (a), conventional Ru/CeO2 (b), Ru3-Ni/CeO2-673ev (c, d). Sample was in H2(76 kPa) at 293 K, evacuated at 293 K and then in D2 (76 kPa) at 293
K (adsorption A) for parts a-c. Sample was in D2 (76 kPa) at 293 K(adsorption B) for part d. (s) D2; (-‚-) HD; (- - -) H2. The heating
rate was 4 K min-1.

ascribed to inversely spiltover H from cerium oxide surface to
Ru clusters because the H(a) on cerium oxide (as hydroxyl, etc.)
was desorbed only as H2O in the absence of Ru clusters.
The order of beginning temperature of desorption (D2 < HD< H2) was the same for conventional Ru/CeO2 (Figure 2b) after
adsorption A. Compared to Figure 2a, the adsorption of D
should be weaker on Ru particles of conventional Ru/CeO2 tomake the first maximum of D2 desorption as low as around390 K. The higher temperature peak of D2 (around 690 K)
may originate from inversely spiltover D by the recombination
on Ru. Again after adsorption A, TPD was measured for Ru3Ni/CeO2-673ev. D2 and HD began to be desorbed at the same
temperature (around 330 K) (Figure 2c). The temperature where
H2 began to be desorbed (around 430 K) was similar to thosefor HD in Figure 2a,b (around 450 K).
TPD was observed also for Ru3-Ni/CeO2-673ev after
adsorption B. Around 310-450 K, only D2 was desorbed(Figure 2d). HD and H2 began to be desorbed around 450 K
similar to HD in Figure 2a,b or H2 in Figure 2c. The inversehydrogen spillover (from support to Ru) should occur above
450 K also for Figure 2c,d compared to the HD desorption in
Figure 2a,b. The ratio of total desorbed hydrogen amount in
Figure 2 (1.2:0.70:4.0:3.8) corresponded to the uptake ratio in
Table 2 (Ru3/CeO2-673ev:conventional Ru/CeO2:Ru3-Ni/CeO2673ev ) 1.2:0.5:4.1) except for the relatively higher value for
Figure 2b probably because the amount of spiltover hydrogen
onto support was also included for TPD observation.
Ru Cluster Structures. 1. EXAFS. a. Incipient Cluster.
When 1 was reacted with CeO2 in THF, the yellow solution
turned transparent in 3 min, suggesting fast reaction between 1
and CeO2. In fact, the EXAFS drastically changed on supporting on CeO2 from EXAFS for 1. The data for incipient Ru3/
CeO2 were well fitted with three waves (Ru-C, Ru(-C-)O,
Ru-O) with Rf ) 1.2% (Table 3a). Alternative fitting withfour waves (three waves and Ru-Ru) did not improve the fitting(Rf ) 2.0%), indicating the cleavage of Ru-Ru bonds of 1 upon
the reaction with CeO2.
b. Ru3/CeO2. Incipient Ru3/CeO2 was heated at 673 or 813
K in vacuum. The FT of the Ru K-edge EXAFS is shown in9424 J. Phys. Chem., Vol. 100, No. 22, 1996

Izumi et al.

TABLE 3: Results of Curve-Fitting Analysis of Ru K-Edge EXAFS Spectra for Ru3/CeO2-673ev, Ru3/CeO2-813ev, andRu3-Ni/CeO2-673ev before/after Hydrogen Treatment at 588 K
entry
a
b
c
d
e
f

sampleincip Ru3/CeO2
Ru3/CeO2-673ev
Ru3/CeO2-813ev
Ru3-Ni/CeO2-673ev

hydrogentreatment at 588 K
beforebeforeafter
before
after
after

N

Ru-C
r/Å

3.1

Ru-Ru
N
r/Å

1.95

Ru(-C-)-O
N
r/Å
3.0

4.4
5.4

2.62
2.62

7.0
7.1

2.61
2.62

3.00

N

Ru-O
r/Å

2.0
1.5
0.6
4.7
1.1
1.2

2.10
2.04
2.14
2.02
2.08
2.12

Ru‚‚‚Ru
N
r/Å

6.2

3.37

Rf/%
1.2
4.7
2.4
2.1
0.6
1.7

Figure 3. Ru K-edge EXAFS spectra observed at 293 K for Ru3/CeO2-673 ev. (a) Raw spectrum, (b) k3-weighted EXAFS oscillation, (c) its
associated Fourier transform, and (d) curve-fitting analysis. (s) Observed; (- - -) calculated.

Figure 3c for the sample heated at 673 K. The main peakaround 2.4 Å (phase shift uncorrected) can be ascribed to RuRu bonding. The shoulder on the lower-distance side in Figure
3c can be Ru-O(s) bonding.2,7,8 In fact, the EXAFS was wellfitted with two waves Ru-Ru and Ru-O(s) (Figure 3d). The
obtained rRu-Ru ()2.62 Å in Table 3b) meant the metallic
bonding compared to clusterlike distance 2.85 Å (average) for
1.9 The NRu-Ru ()4.4 in Table 3b) suggested the Ru cluster
size of less than 10 Å.
The k3χ oscillation in the EXAFS for Ru3/CeO2-813ev was
slower than that in Figure 3b. The peak around 1.6 Å (phaseshift uncorrected) was strongest in its FT. The curve-fitting
analysis was performed with two waves Ru-O and Ru‚‚‚Ru(second coordination in RuO2 bulk). The best fit result was
rRu-O ) 2.02 Å (N ) 4.7) and rRu‚‚‚Ru ) 3.37 Å (N ) 6.2)
(Table 3d) (rRu-O ) 1.98 Å (N ) 6) and rRu‚‚‚Ru ) 3.55 Å (N) 8) for bulk RuO2). The smaller value for rRu‚‚‚Ru may be
because the formed [RuOx] particles on cerium oxide were too

small to constitute regular crystals. Alternative fitting with two
waves Ru-O and Ru-Ru (Ru metal) did not fit in the entire
wavenumber region, excluding the possibility that the second
shell around 2.5 Å was Ru-Ru bonding.
The Ru3/CeO2-673ev was put in H2 at 588 K. The Ru K-edge
EXAFS was fitted with Ru-Ru (r ) 2.62 Å) and Ru-O(s) (r) 2.14 Å) (Table 3c). The value of NRu-Ru ) 5.4 suggested
that the cluster size was still smaller than 10 Å. NRu-O(s)decreased by 0.9 and rRu-O(s) increased by 0.10 Å by heatingin H2 at 588 K.
Similarly, the EXAFS was observed for Ru3/CeO2-813ev
heated in H2 at 588-773 K (Table 3e). These data were wellfitted with two waves Ru-Ru and Ru-O(s), demonstrating thetransformation of [RuOx] into Ru metal clusters at lower than
588 K in H2. With the increase of TH2 from 588 to 773 K, thestructural data did not change very much (rRu-Ru ) 2.62 ( 0.01
Å, NRu-Ru ) 7.0 ( 0.6, rRu-O(s) ) 2.08 ( 0.01 Å) except for
the gradual increase of NRu-O(s) (1.1 f 2.1).

Ammonia Synthesis on Ru3-Ni/CeO2

J. Phys. Chem., Vol. 100, No. 22, 1996 9425

Figure 4. Ru K-edge EXAFS spectra observed at 100 K for Ru3/CeO2-673ev in H2 (53 kPa). The captions for parts a-d are the same as thosein Figure 3.

c. Ru3-Ni/CeO2. For Ru3-Ni/CeO2-673ev heated in H2 at
588 K, the curve-fitting result (Table 3f) was similar to thosefor Ru3/CeO2-673ev and Ru3/CeO2-813ev heated in H2 at 588
K (Table 3c,e). NRu-Ru ()7.1) was a little larger than thosefor corresponding samples heated in H2 at 588 K (5.4-7.0).
d. The Interaction of Ru3/CeO2-673eV with H2. In the Ru
K-edge EXAFS in H2 (53 kPa), the shoulder of Ru-O(s)bonding became weaker compared to the main peak of Ru-Ru(Figure 4c) than in the case in vacuum (Figure 3c). The curvefitting result indicated the decrease of NRu-O(s) from 1.5 to 0.6
by hydrogen adsorption. The distances rRu-Ru and rRu-O(s)increased by 0.03-0.09 Å in H2 (Table 4a,b).
2. TEM. No Ru particles were observed for Ru3/CeO2-673ev
in H2 at 588 K by TEM. Considering the lower limit ofdetection (about 12 Å), this does not contradict results with the
NRu-Ru ) 5.4 by EXAFS (Table 3c). By heating the sample in
H2 at 773 K, the average Ru particle size was 16 Å. Realaverage size may be smaller because the smaller (<∼12 Å)
particles must not be detected. The NRu-Ru of 6.9 by EXAFS
for the sample (TH2 ) 773 K) corresponds to Ru particle sizeof ca. 14 Å with the assumption of complete sphere shape.
In the TEM image for Ru3-Ni/CeO2-673ev heated in H2 at
588 K, black particles with an average particle size of 70 Å
were observed. The hydrogen uptake measurement for Ni/CeO2
heated in H2 at 773 K resulted in H/Rutotal ) 0.10. The NRu-Ru()7.1, Table 3f) by EXAFS corresponds to particles of 1015 Å. Hence, it is reasonable to think that the detected particles
by TEM consisted of Ni. The dispersion of Ru was calculated

to be 0.55 from an average particle size of 24 Å for conventional
Ru/CeO2 by TEM (eq 1), consistent with hydrogen uptake data
(H/Rutotal ) 0.50) in Table 2.
Discussion
In THF at 290 K, cluster 1 was rapidly reacted with CeO2.
The EXAFS analysis (NRu-C ) 3.1, NRu(-C-)O ) 3.0, and NRu-O) 2.0 in Table 3a) and the TPD for incipient Ru3/CeO2 (9.3
CO desorbed per 1 Ru3) suggested the formation of monomeric[Ru(CO)3(µ-O(s)2] species (Figure 5a). 1 should be reacted with
Ni/CeO2 similarly. Besides the cleavage of Ru-Ru bonds by
the coordination of O(s) to [Ru3] cluster in the case of cerium
oxide, the higher oxidation ability of cerium oxide surface wasalso evidenced as the formation of [RuOx] (x ∼ 2) particles(Figure 5d) by heating [Ru(CO)3(µ-O(s))2] on cerium oxide at
813 K in vacuum (Table 3d). By heating at 673 K in vacuum,
the incipient [Ru(CO)3(µ-O(s))2] transferred to Ru clusters ofless than 10 Å (Figure 5b) on the basis of the NRu-Ru of 4.4(Table 3b).
Reduction of Ru3/CeO2 and Catalysis. According to theincrease of TH2, ammonia synthesis rates increased up to at TH2) 773-873 K and decreased at higher TH2 than 873 K on Ru3/
CeO2-673ev, Ru3/CeO2-813ev, and conventional Ru/CeO2 (Figure 1). The corresponding curve-fitting results for EXAFS arelisted in Table 3. With the increase of TH2 from 588 to 873 K,
NRu-Ru increased from 5.4 (Table 3c) to 9.3 for Ru3/CeO2-673ev,
while it remained almost unchanged around 7.0 ( 0.6 for Ru3/
CeO2-813ev (Table 3e).

9426 J. Phys. Chem., Vol. 100, No. 22, 1996

Izumi et al.

Figure 5. Active structures of supported clusters on CeO2.

The maximum in Figure 1 can be explained by the balanceof two factors. Factor A is the extent of reduction of CeO2.
The surface part of CeO2 is reduced to transfer to Ce2O3 by theremoval of surface or lattice oxygen. More reduced CeO2-x at
higher TH2 was able to donate the negative charge to Ru clusters
more effectively, and to facilitate the dissociative adsorptionof N2 on Ru.3 Factor B is the number of surface Ru active
sites. The number of Ru(s) decreased by the aggregation of
Ru cluster as indicated by the increase of NRu-Ru according to
the increase of TH2 from 588 to 873 K for Ru3/CeO2-673ev(Figure 5b,c). In the case of Ru3/CeO2-813ev, NRu-O(s) gradually
increased from 1.1 at TH2 ) 588 K to 2.1 at TH2 ) 773 K,
keeping NRu-Ru almost unchanged (Table 3e). Hence, thenumber of Ru(s) decreased by the growth of CeO2-x at theinterface by the increase of TH2 (Figure 5d-f). The reductionof support was facilitated by the spiltover H from Ru clusters,
and formed CeO2-x at the interface may further react withinterface Ru atoms to physically cover the Ru particle surface(SMSI).
On the basis of the difference of NRu-O(s) (>2.1 and 0.3,
respectively), the extent of Ru site blocking by CeO2-x was the
major reason for lower maximum rate for Ru3/CeO2-813ev thanthat for Ru3/CeO2-673ev (Table 1). The optimum TH2 was lower
by about 100 K for conventional Ru/CeO2 than the other two.
This difference may be because both Ru particle growth andthe blocking of Ru site by CeO2-x inhibited catalysis inconventional Ru/CeO2.
Reduction of Ru3-Ni/CeO2 and Catalysis. In the catalyst
preparation, the amount of hydrogen consumption was 5.04 ×
10-4 mol of H2 gcat-1 for Ni/CeO2 in H2 at 773 K for 12 h:
17.4% of incipient CeO2 was converted to Ce2O3 (CeO2 f
CeO1.91). The stoichiometry CeO1.90 was reported by thethermogravimetry after heating CeO2 powder in H2 at 773 K.10
The particles around 70 Å were detected by TEM, and H/Ni
uptake measurement (0.10) also supported the data. On theother hand, the Ru cluster size of Ru3-Ni/CeO2-673ev in H2
at 588 K was estimated to be 10-15 Å on the basis of NRu-Ru) 7.1 (Table 3f). As the Ru-O(s) bonds were observed by
EXAFS (Table 3f), the Ni and Ru clusters should be attachedon the cerium oxide surface separately. Geometrically, it is
not likely that the Ru clusters bonded to the larger Ni particles
also coordinate to O(s) of cerium oxide with average NRu-O(s)of 1.2. The EXAFS curve-fitting analysis in Table 3f without
Ru-Ni bonds should be valid.

As expected from higher reduction extent of support Ni/CeO2,
the ammonia synthesis rates were best on Ru3-Ni/CeO2-673ev(Figure 1). The number of surface Ru sites should havedecreased by the growth of Ru particles at higher TH2 (factorB), resulting in the rate decrease, while factor A was kept
unchanged because the reduction temperature in catalyst preparation for Ni/CeO2 was higher ()773 K).
Hydrogen Adsorption on Ru3/CeO2 and Ru3-Ni/CeO2.
The hydrogen uptake measurements (Table 2) and TPD
observations for hydrogen (Figure 2) indicated that the H/Rutotal
exceeded 1 for Ru3/CeO2-673ev and Ru3-Ni/CeO2-673ev.
Excessive amounts of hydrogen adsorption are reported for Ru/
SiO2 (H/Ru(s) ) 3.3-5.6),5 Rh/Al2O3 (H/Rh(s) ) 0.70-1.39),11
Rh/CeO2 (H/Rhtotal ) 0.75-3.97),12 or Pt/zeolite (H/Rutotal )1.4-1.7).13
The TPD were observed for samples after adsorption A(Figure 2a-c) and adsorption B (Figure 2d). In the range of
300-450 K (region I), D2 was exclusively desorbed in Figure
2a,b,d. In these three spectra, the incorporation of H (as HD
for Figure 2a,b,d and as H2 for Figure 2d) began at ∼450 K, ingood coincidence. It should be noted that only Figure 2c had
HD desorption in region I and that the total desorption amount(H2 + HD + D2) was larger in Figure 2c,d than in Figure 2a,bby the factor of 3.2-5.7 (the y-axes in Figure 2 are common
on the basis of per gram of catalyst). Hence, we believe that
the Ru3-Ni/CeO2 had a different kind of adsorption site for H,
compared to the Ru3/CeO2 or conventional Ru/CeO2.
The H incorporation in region II (450-750 K) seems due toinverse hydrogen spillover, similar to H spillover initiated at∼413 K on Rh/TiO2.4 The inversely spiltover H should havereacted with D on Ru sites to form HD (Figure 2a,b). However,Ru3-Ni/CeO2 had again different desorption character. Namely,a significant amount of H2 was also desorbed in region II of
Figure 2c besides HD. The total desorption curve of HD + D2
in Figure 2c was similar to that of D2 in Figure 2d.
Taking these facts into account, we propose an interface
adsorption site for H in the case of Ru3-Ni/CeO2. The H on
this new site was not replaced by D even in D2 (second
adsorption step before TPD for Figure 2c), different from
replaceable H on ordinary Ru sites where only D2 was desorbed
as in region I of Figure 2a or Figure 2b. This new site for H
on Ru3-Ni/CeO2 should be different from ordinary Ru sites(D2 in region I) or cerium oxide surface sites (H as HD or H2
in region II). The H of HD in region I in Figure 2c can beAmmonia Synthesis on Ru3-Ni/CeO2

J. Phys. Chem., Vol. 100, No. 22, 1996 9427

TABLE 4: Results of Curve-Fitting Analysis of Ru K-Edge EXAFS Spectra for Ru3/CeO2-673ev in Vacuum or in H2 (53 kPa)
Ru-Ru
entry
a
b
c
d

TH2/K

ambient gasa588

vacuumH2bvacuuma
H2a

Ru-O

N

r/Å

∆(σ2)/10-3 Å2Nr/Å∆(σ2)/10-3 Å2Rf/%

4.4 ((0.7)
4.5 ((0.6)
5.4
5.5

2.62 ((0.02)
2.65 ((0.02)
2.62
2.65

2.8
0.45
2.9
0.8

1.5 ((0.4)
0.6 ((0.3)
0.6
0.7

2.04 ((0.03)
2.13 ((0.02)
2.14
2.14

5.6
-0.6
-5.6
-4.7

4.7 ((2.4)
3.8 ((1.9)
2.4
2.3

a Observed at 290 K. b Observed at 100 K.

understood to be derived from this new site, in equilibrium with
D(a) on ordinary Ru sites at >∼330 K on the basis of thecoincidence of the beginning temperatures of HD and D2
desorption in Figure 2c (∼330 K). The reason why H2 was
not formed in region I of Figure 2c besides HD and D2 isunclear. The very fast recombination of H from the proposed
new site with D(a) on Ru sites near the interface may be thereason.
Figures 4 and 5 show the structure change for Ru clusters of
Ru3/CeO2-673ev by interaction with H2. The bonding distancesrRu-Ru (2.62 Å) and rRu-O(s) (2.04 Å) in vacuum were enlarged
by 0.03 and 0.09 Å, respectively, in H2 (Table 4a,b). The
relaxation induced by H adsorption was also observed for Ru3/
CeO2-673ev heated in H2 at 588 K. rRu-O(s) was alreadyenlarged to 2.14 Å by heating in H2 at 588 K (Table 4c), andit did not show further change by hydrogen adsorption (Table
4d). The rRu-Ru changed from 2.62 to 2.65 Å by H adsorption,same as the change by the H adsorption on Ru3/CeO2-673ev(Table 4a,b). The relaxation of metal cluster by hydrogen
adsorption was reported for supported Ru catalysts (∆rRu-Ru )0.03-0.08 Å and ∆rRu-O(s) ) 0.03-0.04 Å)2 and supported Pt
catalysts (∆rPt-Pt ) 0.12-0.19 Å).14
Adsorption Sites of Excessive Hydrogen. 1. Weakly Bound
Hydrogen. Weakly bound H was proposed on metal catalysts
by using NMR: part of a peak around 0 ppm on Rh/TiO24 andpeak β around -30 ppm on Ru/SiO2.5 In comparison between
two Ru/SiO2 samples (Ru(s)/Rutotal ) 0.29 and 0.19), “weakly
bound H” was implied to have an interaction with lower
coordination Ru sites because the β peak population increased
as the Ru particle size decreased. The dispersions of Ru in
this study were nearly 100% except for conventional Ru/CeO2.
“Weakly bound H” on lower coordination Ru atoms or hydrogen
overlayer was possible as a new site for H discussed above
because the largest average Ru particles (24 Å by TEM) for
conventional Ru/CeO2 are still smaller than those for Ru/SiO2
in NMR study.
2. Hydrogen Absorption in Ru. Another possibility is the
subsurface hydrogen incorporation.15,16 On the basis of the
NRu-Ru ) 4.4 by EXAFS for Ru3/CeO2-673ev (Table 3b), a
monolayer-like flat Ru cluster (packed cluster of 4 atoms × 4
atoms has an average NRu-Ru of 4.3) or octahedral 6-Ru (or alittle larger) cluster (NRu-Ru of 4) is postulated on supports. In
these plausible small clusters, almost no space for hydrogen
incorporation remains. The larger NRu-O(s) ()1.5) for Ru3/CeO2673ev (Table 3b) suggests a flat structure of Ru clusters (Figure5b) rather than a spherical structure. NRu-O(s) was 1.2 for Ru3Ni/CeO2-673ev (Table 3f), also suggesting flat Ru morphology.
H incorporated in Ru bulk sounds very unlikely as a new sitefor H.
3. Interface Site for Hydrogen. The H uptake on Ru3-Ni/
CeO2-673ev was 4.3 H per total Ru atoms (Table 2), andsubsequent TPD had concomitant HD and H2 desorption below
450 K (Figure 2). These facts suggest the new site for H isclosely related to the reduction of cerium oxide support.
Adsorbed hydrogen was monitored by FTIR; however, we could
not discriminate between “weakly bound H” and “interface siteH” (see the Appendix).

Relevance to Catalysis. The reaction mechanism of ammonia synthesis on Ru catalysts was investigated.1,3,17 No N2
adsorption was reported on Ru/MgO (183 K) or Ru-Cs+/MgO(293 K) in N2 + H2. The adsorbed amount of N2 per Ru(s)decreased drastically when the adsorbed H increased for RuCs+/MgO at 293 K. These results demonstrated preferable
adsorption of H on surface Ru sites rather than N2.1 In fact,
PH2 pressure dependence of ammonia synthesis rate was -0.5
to +0.5 for Ru catalysts compared to ∼1.5 for Fe catalysts,3,17
demonstrating that adsorbed H retarded catalysis by the strong
blocking of Ru active sites not to afford enough ensemble sites
for N2 dissociative adsorption for Ru catalysts.
In the TPD for Ru3-Ni/CeO2-673ev (Figure 2c), the ratio of
HD and D2 desorption was about 2:1 (D/H ∼ 2) in region I, in
fast equilibrium of hydrogen between the “new site” (interface
H site or weakly bound H site) and normal Ru surface sites. In
region II of Figure 2c, the peak ratio of HD/H2 was about 1:1.
The stronger intensity of H2 peak than HD peak in Figure 2ccompared to in Figure 2a or Figure 2b suggests that the H2 was
not only native to H(a) on cerium oxide but was also native tothe new site H. The new site H was still exchangeable with
D(a) on the Ru cluster surface in region II. As the ammonia
synthesis temperature (588 K) was within region II, about half
or an equivalent amount of new site hydrogen may be in
equilibrium with hydrogen on surface Ru active sites on thebasis of the D/H ratios. The contribution of the new H site canbe one important factor for catalysis as a “reservoir” of reactant.
Conclusions
The Ru cluster supported on reduced Ni/CeO2 was a good
catalyst for ammonia synthesis. The Ru cluster size was lessthan 10 Å and within 10-15 Å for Ru3/CeO2 and Ru3-Ni/
CeO2, respectively. The Ru-O(s) bondings were observed at
2.02-2.14 Å for these catalysts. The flat Ru clusters weresuggested as active sites. The amounts of hydrogen uptake
exceeded unity for Ru3/CeO2 and Ru3-Ni/CeO2, and H on the
new site was proposed in relation to an excess amount of H
uptake and subsequent TPD for H(a). The interface between
Ru clusters and the reduced CeO2-x surface is one possible newsite.
Acknowledgment. This research was supported by a Grantin-Aid for Scientific Research (No. 05225206) from the Ministryof Education, Science, and Culture.
Appendix
FTIR. FTIR spectra of Ru3-Ni/CeO2 and Ni/CeO2 wererecorded on a FTIR spectrometer (JASCO, Valor III, resolution
1 cm-1).3 CeO2 with higher surface area (165 m2 g-1, Anan
Kasei Co.) was used. Five peaks were observed at 3687m,
2206w, 2087w, 1947vw, and 1895w cm-1 for Ru3-Ni/CeO2673ev in H2 at 203 K for 30 min (Figure A1 in the supporting
information). Compared to the spectrum in H2 for 5 min, threepeaks at 3687, 1947, and 1895 cm-1 grew stronger as the time
passed in H2. Three peaks around 3687, 2206, and 2087 cm-1
were also seen for Ni/CeO2. Ru3-Ni/CeO2-673ev was evacu-

9428 J. Phys. Chem., Vol. 100, No. 22, 1996
ated and in D2 for 30 min. A new peak appeared at 2675 cm-1,
and two peaks at 1947 and 1895 cm-1 disappeared.
The peak at 3687 cm-1 can be assigned to hydroxyl on thecerium oxide surface because it shifted to 2675 cm-1 in D2.
The two peaks at 2206 and 2087 cm-1 may arise from electronictransitions for Ce3+.18 The remaining two peaks at 1947 and1895 cm-1 may be assigned to H on (or near) Ru clusters basedon Table 3 of ref 3 or Table 1(a) of ref 1. The proposed newsite (see Discussion) for H may be at the interface between Ru
clusters and cerium oxide surface as detected around 3687 cm-1
in FT-IR, probably overlapping ordinary surface hydroxyl
groups.
X-ray Photoelectron Spectroscopy (XPS). XPS was measured by ESCALAB 202I (VG). The catalyst sample (powder)
was sealed off from the glass vacuum system in a glass ampuleand transferred inside the ESCALAB without contact with air.
The peak of C 1s was taken as the standard of binding energy()284.5 eV). In Table A1 (see the supporting information)compared to the Ru 3p3/2 peak for neutral Ru powder at 462.0
eV, the Ru 3p3/2 peak was at lower binding energy by 0.7 eV
for Ru3/CeO2-673ev in H2 at 588 K and by 1.3 eV for Ru3Ni/CeO2-673ev in H2 at 588 K. It should be noted thatconventional Ru/CeO2 had a main neutral peak at 462.0 eV anda weaker shoulder peak at 461.1 eV.
Supporting Information Available: Table A1 lists XPS
binding energies for Ru catalysts, and Figure A1 shows the FTIR
spectra of Ru3-Ni/CeO2-673ev and Ni/CeO2 in H2 (2 pages).
Ordering information is given on any current masthead page.

Izumi et al.
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