Applied Catalysis A: General 208 (2001) 193–201
Ammonia synthesis over Ru/C catalysts with different carbon
supports promoted by barium and potassium compounds
Changhai Liang, Zhaobin Wei, Qin Xin, Can Li∗
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, PR China
Received 14 March 2000; received in revised form 19 June 2000; accepted 24 June 2000
Abstract
Ammonia synthesis over ruthenium catalysts supported on different carbon materials using Ba or K compounds as promoters
has been investigated. Ba(NO3 )2 , KOH, and KNO3 are used as the promoter or promoter precursor, and activated carbon
(AC), activated carbon fiber (ACF), and carbon molecular sieve (CMS) are used as the support. The activity measurement for
ammonia synthesis was carried out in a flow micro-reactor under mild conditions: 350–450◦ C and 3.0 MPa. Results show that
KOH promoter was more effective than KNO3 , and that Ba(NO3 )2 was the most effective promoter among the three. The roles
of promoters can be divided into the electronic modification of ruthenium, the neutralization of surface functional groups on
the carbon support and the ruthenium precursor. The catalyst with AC as the support gave the highest ammonia concentration
in the effluent among the supports used, while the catalyst with ACF as the support showed the highest turnover-frequency
(TOF) value. It seems that the larger particles of Ru on the carbon supports are more active for ammonia synthesis in terms
of TOF value. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Ammonia synthesis; Ru catalysts; Carbon supports; Promoters
1. Introduction
The development of ammonia synthesis is considered as a landmark in heterogeneous catalysis.
The prevailing process of ammonia synthesis involves the reaction of gaseous nitrogen and hydrogen on an iron-based catalyst at high temperatures
(400–550◦ C) and high pressures (15–35 MPa) [1].
Ammonia synthesis under such severe reaction conditions is energy-intensive as well as capital-intensive,
although lot of advances in engineering have been
made within the past few decades. Therefore, the catalytic transformation of N2 and H2 into ammonia un∗ Corresponding author. Tel.: +86-411-4671991/728;
fax: +86-411-4694447.
E-mail address: [email protected] (C. Li).
der milder conditions has been a longstanding goal in
catalysis.
In the last decades, a marked improvement in the
activity of catalysts for ammonia synthesis has allowed the process to operate at lower temperature
and lower pressure. In the early 1970s, Ozaki and
co-workers [2] introduced an alkali-metal-promoted
carbon-supported Ru catalyst, which exhibited a
10-fold increase in activity over the conventional
iron-based catalyst under similar conditions. Using
this kind of graphite-supported ruthenium catalyst,
a 600 t NH3 per day plant has begun to produce
ammonia based on the Kellogg advanced ammonia
process [3] in 1992 under milder condition compared
with the plants using iron-based catalysts. Thus, it
is believed that Ru-based catalyst could become the
second-generation catalyst for ammonia synthesis.
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 7 1 3 - 4
194
C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
Because of the advance in the ruthenium-based
catalyst for ammonia synthesis, a number of research
groups have investigated the roles of support and
promoter of the catalyst played in the catalytic reaction. Various supports, such as active carbon and
graphite [2–9], alumina [10–13], carbon-coated alumina [14,15], magnesia [16–19], a series of zeolites
[20–22] and even rare-earth metal oxides [23,24]
were used in Ru-based catalysts for the ammonia synthesis. The promoters used are mainly alkali metal,
alkaline earth metal, rare earth metal and their oxides
or hydroxides. However, it seems that the ruthenium
supported on carbon material with promoters is the
most promising catalyst for commercial applications.
This is due to the many attributes linked with carbon materials, such as specific electronic properties,
variable surface functional groups, and easy metal
recovery by the burning of the support [25]. Previous
studies generally focused on the kinetics of ammonia
synthesis over Ru catalyst supported on oxides and
the effects of promoters on the catalytic performance.
Few detailed scientific data are available yet for the
effects of different promoters and carbon supports on
the catalytic performance of the supported Ru catalyst
under moderate pressure [4–9].
For the above-mentioned reason, this work has been
carried out to investigate in detail the ammonia synthesis on the ruthenium catalysts supported on different
carbon materials promoted with K and Ba compounds
under moderate pressure.
2. Experimental
2.1. Catalyst preparation
Three kinds of carbon materials: active carbon (AC)
from coconut; carbon molecular sieve (CMS) from
walnut nut; and active carbon fiber (ACF) from coal
pitch, were used as the catalyst supports. The elemental analysis, the specific surface area and electric
resistivity of the three carbon materials are listed in
Table 1.
The carbon supports were impregnated in a rotary
evaporator at room temperature with RuCl3 ·3H2 O in
an aqueous solution. The ruthenium content was set
between 2.0 and 8.0 wt.%. After being dried in air at
120◦ C for 10 h, the samples were reduced in a stream
of hydrogen at 400◦ C for 4 h, followed by cooling in
nitrogen atmosphere to room temperature. The promoters were introduced by the incipient wet impregnation method with aqueous solutions of KOH, KNO3 , or
Ba(NO3 )2 at room temperature. The samples impregnated with the promoter were dried in air at 120◦ C for
10 h. The contents of promoter in the catalysts are expressed as promoter:Ru molar ratio, which was equal
to 6.2 for most samples.
2.2. Characterization of catalysts
The amount of surface Ru atoms on the catalysts
was estimated by hydrogen chemisorption at 35◦ C.
The pre-treatment of catalyst is as follows. The catalyst sample was first reduced under a hydrogen flow at
450◦ C for 2 h, and then evacuated at 400◦ C under high
vacuum for 30 min; finally it was cooled in vacuum to
35◦ C for chemisorption. The total and the reversible
hydrogen adsorption isotherms were measured over
the hydrogen pressure range of 10–200 Torr. The intercepts of the chemisorption isotherm, extrapolated
to zero pressure, were the total and the reversible
amount of hydrogen adsorbed. The irreversible hydrogen uptake was obtained by taking the difference between the values of the total and the reversible amount
of hydrogen uptake. The dispersion of the ruthenium
metal particles was calculated from the chemisorption
Table 1
Element analyses and specific surface areas of carbon materials used as catalyst supports
Supports
Surface areaa (m2 g−1 )
C
H
N
Others (Ash, S, Cl, O)b
Resistivity (m m)
AC
ACF
CMS
1290
844
14
81.79
91.05
93.31
2.07
0.09
0.27
0.75
0.15
n.d.c
15.39
8.71
6.42
∼11
∼5
∼11
a
BET surface area.
By difference.
c Not detected.
b
C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
195
isotherm by assuming that hydrogen dissociatively adsorbed with H:Rusurf ratio as unity. CO chemisorption
was also measured for some samples; the procedure
is analogous to that of H2 chemisorption.
rates of hydrogen and nitrogen after the reaction were
determined by a wet flow meter.
The ammonia concentration (vol.%) is calculated
from the following equation:
2.3. Activity measurement of ammonia synthesis
NH3 concentration (vol.%)
The ammonia synthesis activities of as-prepared
catalysts and a commercially used fused iron catalyst were tested in a stainless steel micro-reactor with
a stoichiometric H2 and N2 mixture (H2 /N2 = 3)
flow at 3.0 MPa. The purity of H2 and N2 gases was
99.99%, and the mixture gas was further purified by
using self-designed guard containers filled with palladium catalyst and molecular sieves. About 0.5 g of the
catalyst sample, with size of 40–60 mesh was used.
The sample in the reactor was activated in the stream
of N2 + 3H2 under 3.0 MPa following a temperature
program, i.e. heating to 450◦ C in 100 min, standing at
450◦ C for 240 min, and then cooling to the reaction
temperature in 30 min.
The amount of ammonia in the effluent was determined by a chemical titration method. Namely, the
gaseous effluent from the reactor was neutralized by
sulfuric acidic solution containing an indicator (methyl
red). As soon as the neutralizing reaction is over, the
color of the solution turned from red to orange. Thus,
the amount of effluent ammonia can be estimated from
the amount of sulfuric acidic solution used. The flow
=
NH3effluent
× 100%
(H2 + N2 )flow-meter + NH3effluent
3. Results and discussion
3.1. Ammonia synthesis activity of catalysts with
different Ru loadings
Fig. 1 shows a series of ammonia concentrations
in the effluent versus reaction temperature with different Ru loadings of catalyst with the same K:Ru
molar ratio = 6.2 as parameter. For the catalyst
with 2.0 wt.% Ru loading, ammonia concentration
increases as reaction temperature increased. The ammonia concentration in the effluent increases with the
increase of Ru loading, but no longer increases when
the Ru loading is higher than 4.0 wt.% at the reaction
temperature above 375◦ C, due to a thermodynamic
equilibrium limitation.
From data given in Table 2, it is clear that the H2
uptake increases with increasing Ru loading from 2.0
to 8.0 wt.%, while the corresponding dispersion of Ru
Fig. 1. Ammonia concentration in the effluent for the ammonia synthesis on Ru-K/AC catalysts as a function of the ruthenium loading
with the same K:Ru molar ratio.
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C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
Table 2
TOF of ammonia synthesis on Ru-K/AC at 400◦ C and 3.0 MPa, based on chemisorption data
Catalyst
Ru-K/AC
Ru-K/AC
Ru-K/AC
Ru-K/AC
a
Ru loading (wt.%)
2.0
4.0
6.0
8.0
Uptake (␮mol/g)
Ru dispersion (%)
Mean particle size (nm)a
TOF (s−1 )
H2
CO
H2
CO
H2
CO
H2
CO
29.7
42.4
44.8
56.3
116.3
153.6
157.9
103.7
30.1
21.4
15.1
13.4
58.8
38.8
26.3
13.1
3.9
5.9
8.9
10.1
1.7
2.9
4.6
10.3
0.008
0.034
0.037
0.034
0.004
0.017
0.021
0.036
Particle size was estimated from the equations proposed by Borodzinski and Bonarowska [34].
decreases from 30.1 to 13.4 wt.% and the mean particle
size of Ru increases from 3.9 to 10.1 nm. The TOFs
of the catalysts with above 4.0 wt.% Ru are almost
the same (about 0.035 s−1 ). The TOF of the catalyst
with 2.0 wt.% Ru is only 0.008 s−1 and is much lower
than those of the other catalysts. These results suggest
that the ammonia synthesis reaction on Ru-K/carbon
is structurally sensitive.
The CO chemisorption was also measured to confirm the usability of H2 chemisorption, since hydrogen spillover might occur on the catalyst. The results
are given in Table 2. It can be seen that the amount
of CO uptake was higher than that of H2 uptake, especially for the low Ru loading. The differences may
be due to the non-complete removal of H2 on Ru
during the activation process and to the presence of
residual chloride species which poison the metal Ru
surface, with a subsequent decrease of H2 chemisorption capacity. In addition, multiple adsorption of
CO is possible on the low-coordination surface Ru
atoms.
The TOFs based on CO chemisorption increase
from 0.004 to 0.036 s−1 with the increase of Ru
metal size from 1.7 to 10.3 nm. The trend is similar
to that obtained from H2 chemisorption although the
detailed data are different. This further suggests that
the ammonia synthesis reaction on Ru-K/carbon is
structurally sensitive.
Aika and Murata [10] also noted a similar effect of
the particle size on TOF for ammonia synthesis using
promoted Ru/Al2 O3 . There was a trend that the TOF
of larger particles was higher than that for smaller particles, but the effect was not remarkable when compared with the Fe/MgO catalyst [26], where the TOF
increased greatly as the dispersion of Fe decreased.
Cisneros and Lunsford [20] found that the activity was
more than doubled and the TOF was increased by a
factor of 4 upon increasing the Ru particle size by a
factor of 3 in the RuNaX zeolite.
3.2. Effects of promoters on the catalytic activity
Fig. 2 gives the catalytic activity of Ru-based catalysts promoted with KOH, KNO3 , and Ba(NO3 )2 . The
activity of the Ru/AC catalyst without promoter was
also measured under the same conditions. This catalyst
shows low activity, much lower than that for the promoted Ru/C catalysts. It can be seen that all the promoters can enhance the ammonia synthesis activity of
Ru/AC catalyst. For potassium promoter, the hydroxide precursor is more effective than the nitrate precursor. Ba(NO3 )2 was the most effective promoter among
the precursors used. The data listed in Table 4 show
the effects of promoters on the Ru dispersion and TOF
of ammonia synthesis. KOH, KNO3 , and Ba(NO3 )2
promoters gave similar dispersion, about 22%, which
was higher than that of unpromoted Ru/AC, probably due to the effects of surface functional groups of
the carbon support itself and of chloride ions from Ru
precursors. The order of TOF for the Ru/AC catalyst
with different promoters is as follows: Ba(NO3 ) >
K(OH) > K(NO3 ).
Fig. 2 shows that the pre-treatment of catalyst in
the nitrogen atmosphere has no effect on the ammonia
synthesis for the Ba(NO3 )2 promoted Ru/AC catalyst.
The activity enhancement for the KNO3 -promoted
Ru/AC catalyst after pre-treatment may be due to the
decomposition of KNO3 under pre-treatment conditions, because the bulk KNO3 can decompose at
400◦ C. Additionally, the N2 treatment might lead to
re-dispersion of Ru on the support. The treatment
with N2 + H2 mixture reduces RuOx to Ru metal
at low temperature, while RuOx may be maintained
to higher temperature in N2 . This little difference
C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
197
Fig. 2. Ammonia concentration in the effluent for the ammonia synthesis on Ru/AC catalysts with different promoters (KOH, KNO3 , and
Ba(NO3 )2 ).
between treatment processes can result in different
states and dispersions of Ru.
Fig. 3 shows the effects of potassium (from KOH)
and barium (from Ba(NO3 )2 ) on the ammonia synthesis activity for the catalysts containing 4.0 wt.% of
ruthenium. For potassium promoter, ammonia concentration in the effluent was increased almost linearly
with the K:Ru ratio up to 6. For barium promoter, ammonia concentration in the effluent is significantly increased when the Ba:Ru molar ratio is increased from
0 to 1.0. Ammonia concentration in the effluent is
only slightly increased when the Ba:Ru molar ratio is
beyond 1.0.
Table 3 shows the effect of promoter:Ru molar ratio
on the TOF under the same reaction conditions and for
the same loading of Ru. It can be seen that promoters
have significant influences on the dispersion of Ru
and the activity of catalyst. The addition of KOH and
Ba(NO3 )2 enhances the dispersion of Ru on the carbon
support and the amount of H2 uptake. The TOFs of
Ru-K/AC catalysts increase from 0.002 to 0.034 s−1
(17 times) as the promoter:Ru molar ratio changes
Fig. 3. Ammonia synthesis activity for Ru/AC catalysts (4.0 wt.% Ru) with different potassium and barium contents.
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C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
Table 3
Effects of promoter on the dispersion and activity of Ru-K(-Ba)/AC catalysts at 400◦ C (4.0 wt.% Ru)
Catalysta
K(Ba):Ru molar ratio
H2 uptake (␮mol/g)
Ru dispersion (%)
TOF (s−1 )
Ru-K/AC
Ru-K/AC
Ru-K/AC
Ru-K/AC
Ru-Ba/AC
Ru-Ba/AC
Ru-Ba/AC
Ru-Ba/AC
0.5
1.0
2.0
6.2
0.5
1.0
2.0
6.2
40.0
33.3
35.9
42.4
43.1
80.6
83.9
43.6
20.2
16.8
18.2
21.4
21.8
40.7
42.4
22.0
0.002
0.004
0.011
0.034
0.022
0.024
0.023
0.041
a
K and Ba in Ru-K/AC and Ru-Ba/AC catalysts represent KOH and Ba(NO3 )2 as promoter precursors, respectively.
from 0.5 to 6.2. For Ru-Ba/AC catalysts, the addition
of a little promoter sharply enhanced TOF of ammonia
synthesis. But it is understood why the Ba compound
is so effective.
It can be seen that the promoters can change the
amount of H2 adsorption for the low Ru/promoter,
indicating that the addition of promoter can increase
the dispersion of Ru metal. But the further addition
of promoter gives a low amount of H2 uptake. The
effects of oxide promoters on carbon supported Ni
and Co catalysts showed that the oxide promoter was
mostly concentrated on the pore mouths of carbon
supports [27,28]. For our case, some of the promoter
may cover Ru surface and/or block the pores of the
carbon support also.
Additionally, the activity of the catalyst is low when
the amount of promoter is small, because these promoters are consumed to neutralize the surface anions
from carbon support and Ru precursors. The amount
of K consumption may be larger than that of Ba because of the different chemical valence of Ba and K
compounds.
The surface of activated carbon, 1290 m2 g−1 ,
could accommodate 38 mmol g−1 of Ba2+ or K+
ions when Ba2+ or K+ ions are closely packed on
the surface. The surface quantity of Ba2+ or K+ promoter with promoter:Ru = 6.2 is about 2.5 mmol g−1 ,
which is far below the value of 38 mmol g−1 . The
fact that the promoted Ru/C catalysts show high activity for ammonia synthesis may result from the
electronic-conductivity property of carbon materials, which makes it possible to transfer electrons of
promoters to Ru metal through the carbon support
because most promoter and Ru metal particles are not
in contact with each other. This also explains why
catalysts with graphite as support show even higher
activity because graphite has high electron conductivity. The catalyst with the activated carbon support is
quite different from that with the Al2 O3 , SiO2 supports
because the oxide supports are insulators while the
activated carbon materials are conductors.
3.3. Activity of Ru/C catalyst with different carbon
supports
Fig. 4 illustrates a comparison of ammonia concentration in the effluent for the Ru catalysts with
three different carbon supports. Active carbon from
coconut nut is the most effective support among the
three supports. The low ammonia concentration in the
effluent for the catalyst supported on carbon molecular sieve is due to the low dispersion of Ru because of the low specific surface area of the carbon
molecular sieve. The catalyst supported on the active carbon fiber shows a moderate activity, but its
TOF is much higher than those of the other two supports (as seen in Table 4). The order of the TOFs is
ACF > AC > CMS. This fact can be explained by
the following factors. Firstly, the carbon materials are
quite different if precursors or preparation conditions
were different. The three carbon materials (AC, CMS,
and ACF) are different in the amount of ash, O-, Nand S-containing and the surface properties (Table 1).
Zhong and Aika [30,31] have recently found that electronegative impurities, such as S, O, Cl, N, might inhibit the nitrogen activation; these impurities may be
eliminated through a hydrogen treatment at 900◦ C.
Secondly, the pore structure and specific surface area
of the carbon supports can affect the dispersion of
Ru metal particles and of promoters. From Table 4,
C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
199
Fig. 4. A comparison of ammonia synthesis activity for Ru/C catalysts with three kinds of carbon supports: AC, CMS and ACF.
it can be seen that the dispersion of Ru has big differences (AC > ACF > CMS). The interaction between promoters and Ru metal particles is varied with
the dispersion of Ru metal particles. Finally, the different degree of graphitization of the carbon materials results in the different electrical conductivity, because the electrical conductivity of the carbon supports strongly depends on the extent of their graphitization. The carbon materials become more ordered
from CMS, AC to ACF according to their carbonization temperatures, their precursors and their electrical conductivity in Table 1. This is in agreement with
the TOF value of Ru catalysts with three carbon supports (see Table 4). Kowalczyk et al. [32] also reported that the high temperature graphitization treatment of carbon results in the high ammonia synthesis activity of Ru-based catalyst, but they suggested
that the modification of structure and texture of the
carbon supports play a significant role in catalytic
performance.
The unexpectedly large TOF of the ACF may be a
result of several contributions: the larger Ru particles,
the high electric conductivity of the activated carbon
fiber and the special structure of ACF [29]. The electric conductivity of graphite may be the main reason
for the high TOF because ACF has better graphitization. In summary, purity and electronic properties
of carbon with a suitable surface area are the key
factors influencing the catalytic performance of the
catalyst.
3.4. A comparison between promoted Ru/carbon
catalyst and a commercial fused-iron catalyst
The ammonia concentration in the effluent for the
ammonia synthesis on Ru-K/AC catalyst is slightly
Table 4
Effects of carbon materials used as catalyst supports on the dispersion of Ru and activity of Ru/C catalysts (3.0 MPa, 400◦ C)
Catalystsa
Promoter
H2 uptake (␮mol/g)
Ru dispersion (%)
TOF (s−1 )
Ru-Ba/AC
Ru-K/AC
Ru-KN/AC
Ru-Ba/CMS
Ru-K/CMS
Ru-KN/CMS
Ru-Ba/ACF
Ba(NO3 )2
KOH
KNO3
Ba(NO3 )2
KOH
KNO3
Ba(NO3 )2
43.57
42.38
46.77
0.94
1.65
6.12
7.44
22.0
21.4
23.65
0.48
0.83
3.09
3.76
0.041
0.034
0.024
0.030
0.012
0.003
0.089
a
Promoter:Ru = 6.2.
200
C. Liang et al. / Applied Catalysis A: General 208 (2001) 193–201
higher than the commercial iron catalyst under the
reaction conditions (3.0 MPa, 400◦ C and 3000 h−1 ).
However, the ammonia concentration in the effluent
for Ru(4)-Ba/AC catalyst is about twice as active as
the commercial iron catalyst, which was in rough
agreement with the result of Forni et al. [9]. Based on
TOF values, the TOFs of the Ru-K/AC and Ru-Ba/AC
catalysts at 400◦ C and 3.0 MPa are more than three
to four times as high as that of the commercial iron
catalyst (0.011 s−1 ) at 400◦ C and 6.0 MPa. The TOF
for the Ru-Ba/ACF catalyst at 400◦ C and 3.0 MPa
is almost eight times higher than the TOF for iron
catalyst at 400◦ C and 6.0 MPa. Aika et al. [4] found
that the ammonia synthesis rate for K-Ru/C catalyst
could be raised to about 10 times that of conventional doubly promoted iron catalyst at 250◦ C and
600 Torr. Kowalczyk et al. [33] reported that TOFs
of ammonia synthesis based on H2 chemisorption
(0.13 s−1 ) were about an order of magnitude higher
than that for the fused iron catalyst (0.011 s−1 ) at
400◦ C and 6.0 MPa. The results further confirm that
the Ru-Ba/ACF can have the activity about an order
of magnitude higher than that of commercial iron
catalyst.
4. Summary
The Ba and K promoters change Ru dispersion,
modify electron properties on Ru surface, and neutralize the surface groups of carbon supports; as a
result, they significantly enhance the ammonia synthesis activity of Ru/C catalysts. The Ba-promoted
Ru/AC catalyst gives the highest ammonia concentration, 5.7 vol.%, because Ru metal is well-dispersed
on AC support, while Ru-Ba/ACF gives the highest
TOF value 0.089 s−1 . This is due to the high purity
and electronic conductivity of ACF. The low activity
on the CMS support can be attributed to the low surface area and the lower graphitization. Therefore, the
carbon supports with high purity, high electronic conductivity and high surface area favor the high activity of Ru-based catalysts for ammonia synthesis. The
ammonia synthesis on Ru/C catalyst is structure sensitive and exhibits higher activity, but the TOF becomes constant when Ru particle size is beyond about
5 nm.
Acknowledgements
This work was partly supported by the National
Natural Science Foundation of China (NNSFC) for
distinguished young scholars (Grant No. 29625305).
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