A highly active nano-Ru catalyst supported on novel Mg-Al hydrotalcite precursor for the synthesis of ammonia

Article abstract of DOI:10.1016/j.molcata.2006.08.071

Supported Ru catalysts using MgO, Al₂O₃ and calcined Mg-Al (2:1) hydrotalcite (HT) with alkali promoter (Cs) were prepared by two different methods viz., the conventional impregnation method and polyol reduction method using ethylene glycol as solvent for obtaining nano-Ru for ammonia synthesis under atmospheric pressure. The catalysts were characterized by XRD, BET surface area, TPR and evaluated for the synthesis of ammonia in the temperature range of 523-698 K at atmospheric pressure. CO chemisorption studies were performed to find out Ru dispersion, particle size and active metal area. The following sequence with respect to the %NH₃ (v/v) formation was found: Cs-Ru/HT (ED) > Cs-Ru/HT > Cs-Ru/Al₂O₃ > Cs-Ru/MgO. The nano-Cs-Ru/HT (ED) catalyst prepared by polyol reduction method showed superior activity over the catalyst prepared from conventional impregnation method. The higher activity of the Cs-Ru/HT (ED) catalyst has been attributed to the presence of highly dispersed nano-particles of Ru as observed from CO chemisorption results and the presence of easily reducible Ru species as observed from TPR studies.

Full text of DOI:10.1016/j.molcata.2006.08.071

Journal of Molecular Catalysis A: Chemical 263 (2007) 253–258
A highly active nano-Ru catalyst supported on novel Mg–Al hydrotalcite
precursor for the synthesis of ammonia
P. Seetharamulu, V. Siva Kumar, A.H. Padmasri, B. David Raju, K.S. Rama Rao∗
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 26 July 2006; received in revised form 11 August 2006; accepted 14 August 2006
Available online 26 August 2006
Abstract
Supported Ru catalysts using MgO, Al2O3 and calcined Mg–Al (2:1) hydrotalcite (HT) with alkali promoter (Cs) were prepared by two
different methods viz., the conventional impregnation method and polyol reduction method using ethylene glycol as solvent for obtaining nano-
Ru for ammonia synthesis under atmospheric pressure. The catalysts were characterized by XRD, BET surface area, TPR and evaluated for
the synthesis of ammonia in the temperature range of 523–698 K at atmospheric pressure. CO chemisorption studies were performed to find
out Ru dispersion, particle size and active metal area. The following sequence with respect to the %NH3 (v/v) formation was found: Cs–Ru/HT
(ED) > Cs–Ru/HT > Cs–Ru/Al2O3 > Cs–Ru/MgO. Thenano-Cs–Ru/HT(ED)catalystpreparedbypolyolreductionmethodshowedsuperioractivity
over the catalyst prepared from conventional impregnation method. The higher activity of the Cs–Ru/HT (ED) catalyst has been attributed to the
presence of highly dispersed nano-particles of Ru as observed from CO chemisorption results and the presence of easily reducible Ru species as
observed from TPR studies.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ammonia synthesis; Ru; Mg–Al HT; CO chemisorption; Polyol reduction
1. Introduction
and zeolites [16,17]. It has been observed that the high activ-
ity of the active carbon supported promoted ruthenium catalysts
are attributed to the electron deficient graphite lattice of active
carbon [1]. However ruthenium can catalyze the methanation
of carbon in the typical environment. The MgO support shows
less activity because of strong interaction with chlorine in RuCl3
[12]. The strong chemisorption of product ammonia on the acid
centers of alumina makes it a less attractive support for ruthe-
nium. The other supports show less activity compared to former.
Hydrotalcite-like compounds are a class of precursors useful for
thepreparationofcatalyticallyactiveoxidesshowingbasicprop-
erties. The present work is related to the comparison of activity
studies of ammonia synthesis over Ru supported on mixed oxide
support obtained from Mg–Al HT precursor (with Mg/Al ratio
of 2) with the Ru catalysts supported on simple oxides viz., MgO
and Al2O3 under atmospheric pressure. To get a closer insight
into the role of the supports and aiming at better understand-
ing of the supported Cs–Ru systems the effect of Ru dispersion
with and without Cs has also been investigated. CO chemisorp-
tion technique is used for particle size determination and
dispersion.
Ru-based catalysts are known to be active for NH3 synthesis
at atmospheric pressure [1–7]. The most stable and commer-
cially available Ru precursor used for ammonia synthesis is
RuCl3 [2,5]. The main advantage of this metal precursor is
its low cost in comparison to other metal precursors, but the
main disadvantage of using RuCl3 is the strong binding of chlo-
rine atom with the metal surface even after reduction. Alkali
metal nitrate used as a promoter is thought to have two effects:
one is to remove chlorine ions from the catalyst and the other
is to donate electrons to ruthenium. The promoting effect of
alkali metal to Ru on NH3 synthesis activity is found to be
inversely proportional to the electro negativity of the alkali metal
in the order of Cs > Ba > K > Na [8]. Many materials have been
investigated as supports for ruthenium catalysts in ammonia syn-
thesis, such as carbon [9,10], MgO [11–13], Al2O3 [14,15,2],
∗
Corresponding author. Fax: +91 40 27160921.
E-mail address: ksramarao@iict.res.in (K.S. Rama Rao).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.08.071

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P. Seetharamulu et al. / Journal of Molecular Catalysis A: Chemical 263 (2007) 253–258
linear heating ramp of 5 K min−1 from 303 to 973 K. The CO
2. Experimental
chemisorption was carried out at 303 K on a homemade pulse
reactor to evaluate the dispersion and metal particle size. In a
typical experiment, about 150 mg of the catalyst sample was
placed in a micro-reactor of 8 mm i.d., and 250 mm long quartz
reactor and the catalyst sample was first reduced under a hydro-
gen flow at 723 K for 2 h, pre-treatment at 723 K for 1 h under
He flow and finally was cooled in He flow up to 303 K. The out-
let of the reactor was connected to a micro-thermal conductivity
detector (TCD) equipped GC-17A (M/S. Shimadzu Instruments,
Japan) through an automatic six-port valve (M/S. Valco Instru-
ments, USA). After cooling, pulses of 10% CO balance He were
injected at room temperature through a 1 ml loop connected to
the six-port valve until no further change in the intensity of the
outlet CO (from GC-software). Assuming CO:Ru stoichiome-
try of 1:1, dispersion, particle size and metal area of Ru were
calculated using Ru metal cross-sectional area as 0.0821 nm2
[20,21].
2.1. Preparation of catalysts
The supports MgO and Al2O3 were prepared by precipitat-
ing 10 wt% aqueous solution of corresponding nitrate precur-
sors with 10 wt% aqueous Na2CO3 solution up to pH 10–11.
The precipitated mass was filtered and washed thoroughly with
deionised water until excess of Na was completely removed.
Then the filtered mass was kept for oven drying at 383 K for
10 h and then calcined at 773 K for 6 h. Mg–Al HT (Mg/Al = 2)
was prepared by Reichle’s method of co-precipitation under
super saturation conditions and calcined at 723 K for 18 h [18].
The catalysts were prepared by impregnating the support with
10 wt% aqueous solution of RuCl3·3H2O (M/S. Loba Chemie)
in a rotary evaporator. After drying in air (373 K, 12 h), sam-
ples were reduced in hydrogen at 723 K for 4 h. These reduced
catalysts were named as Ru/MgO, Ru/Al2O3 and Ru/HT with
Ru/support ratio (by weight) of 1:10. The reduced samples of
supported Ru catalysts were impregnated with aqueous CsNO3
solution and dried at 373 K for 12 h. In all these promoted cata-
lysts the Ru:Cs:support weight ratio was kept at 10:51:100 and
the catalysts after reduction were designated as Cs–Ru/MgO,
Cs–Ru/Al2O3 and Cs–Ru/HT. An alternative method to obtain
HT supported well-dispersed metal particles of Ru catalyst was
prepared by polyol reduction method using ethylene glycol as
solvent as described [19]. The catalysts were designated before
and after the addition of Cs promoter as Ru/HT (ED) and
Cs–Ru/HT (ED), respectively.
2.3. Activity studies
Activity tests over Cs promoted catalysts were carried out
in a fixed bed glass reactor (i.d. 18 mm and 300 mm long) at
temperatures ranging from 523 to 698 K with a successive rise
of 25 K under atmospheric pressure. The stoichiometric ratio of
N2–H2 mixture was 1:3 and the total flow rate was 10 l/h. The
generators, NG 2081 (M/S. Claind, Italy) and HOGEN GC300
(M/S. Proton Energy Systems, USA) were used for N2 and H2
gases, respectively, with >99.99% purity. Prior to the activity
measurements the catalysts were reduced at 723 K for 4 h. The
complete reduction of the catalyst was confirmed by testing the
vent gas with silver nitrate solution for hydrochloric acid coming
from−Cl− ion and Nessler’s reagent for ammonia coming from
NO3 ion. The ammonia concentration at the outlet mixture
was determined by neutralizing with a known volume of 0.01N
aqueous H2SO4 solution at regular intervals.
2.2. Characterization techniques
BET surface areas of the reduced catalysts of both promoted
and unpromoted were obtained on Autosorb Automated Gas
Sorption System (M/S. Quantachrome, USA) with N2 as adsor-
bate at liquid nitrogen temperature. X-ray powder diffraction
(XRD) patterns of reduced catalysts of both unpromoted and
promoted were recorded on a Rigaku Miniflex (M/S. Rigaku
Corporation, Japan) instrument using Ni filtered Cu K␣ radia-
tion, with a scan speed of 2◦ min−1 and a scan range of 2–80◦
at 30 kV and 15 mA. Temperature programmed reduction (TPR)
profiles of the unpromoted and promoted Ru uncalcined samples
were generated on a home made on-line quartz micro reactor
interfaced to a thermal conductivity detector (TCD) equipped
with a gas chromatograph (M/S. Shimadzu, model: GC-17A,
Japan) and the profiles were recorded using a GC software
Class-GC10. H2/Ar (11 vol.% of H2 and balance Ar) mixture
was used as the reducing gas while the catalyst was heated at a
3. Results and discussion
The results of the BET surface area measurements are sum-
marized in Table 1. It is observed that the surface area of Al2O3
is higher compared to HT and MgO supports. The addition of Ru
to the support leads to decrease in the surface area and addition
of Cs in succession leads to further decrease in the surface area
of the catalyst. In case of MgO supported catalysts, the surface
area decreased drastically by the addition of Ru and Cs in com-
parison to other supported catalysts. This is due to sintering of
Cs in case of pure MgO supported catalyst [11]. The catalysts
Table 1
BET surface area of unpromoted and Cs promoted Ru catalysts on various supports
Catalyst
BET surface
area (m2/g)
Catalyst
BET surface
area (m2/g)
Catalyst
BET surface
area (m2/g)
Catalyst
BET surface
area (m2/g)
MgO
Ru/MgO
Cs–Ru/MgO
147
17
9
Al2O3
Ru/Al2O3
Cs–Ru/Al2O3
198
170
63
HT (Mg–Al)
Ru/HT
Cs–Ru/HT
172
145
30
–
–
158
68
Ru/HT (ED)
Cs–Ru/HT (ED)

P. Seetharamulu et al. / Journal of Molecular Catalysis A: Chemical 263 (2007) 253–258
255
Fig. 3. XRD patterns of Cs promoted reduced Ru catalysts on various supports.
Fig. 1. XRD patterns of MgO, Al2O3 and calcined Mg–Al HT.
Cs–Ru/MgO catalyst. This means an anion exchange between
RuCl3 and CsNO3 has occurred. In case of MgO supported
catalyst no CsCl peak is observed but MgCl2 is observed in unre-
duced unpromoted catalyst. After reduction, MgCl2 can diffuse
over the surface, although it is not volatile [17]. The intensity of
CsCl peak is low in Cs–Ru/HT (ED) catalyst compared to other
catalysts although it is prepared from polyol reduction method
as some amount of RuCl3 is left unreduced, and after addition of
Ru/HT (ED) and Cs–Ru/HT (ED) prepared by polyol reduc-
tion method show high surface areas in comparison to catalysts
prepared by impregnation method.
The XRD pattern of calcined MgO, Al2O3 and HT sup-
ports are portrayed in Fig. 1. It is clearly observed from the
figure that hydrotalcite structure [d = 7.84, 3.90, 2.57, ICDD no.
22-700] is regained by exposing to air after calcination [22].
The calcined samples of magnesia and alumina showed peri-
clase [d = 2.11, 1.49, 1.20, ICDD no. 04-0829] and ␥-Al2O3
phases [d = 1.4, 1.99, 2.46, ICDD no. 16-0394], respectively.
Figs. 2 and 3 manifest the XRD patterns of reduced sam-
ples of both unpromoted and Cs promoted catalysts, respec-
tively. Both unpromoted and promoted catalysts show the pres-
ence of Ru0 phase [ICDD no. 06-0663] and the intensity of
the peaks in unpromoted catalysts increases in the order of
Ru/HT (ED) < Ru/HT < Ru/Al2O3 < Ru/MgO which indicates
that MgO supported catalyst contains more crystalline Ru. The
intensity of diffused peaks of Ru in the HT samples indi-
cate the presence of highly dispersed/smaller particles of Ru
which are further confirmed to be in nano-range from the
CO-chemisorption results. The X-ray diffraction analysis of
reduced promoted samples gave signals due to CsCl except in
−
CsNO3 the Cl− ion exchanged to NO3 to form CsCl. Table 2
brings out the physical characteristics of Ru catalysts viz., dis-
persion, surface metal area and particle size. It is obvious from
the data that the dispersion of Ru is high in the case of HT
supported catalyst prepared from polyol method and low in the
case of MgO supported catalyst. It is also clear that the Ru dis-
persion in Al2O3 supported catalysts decreased drastically by
the addition of Cs promoter this is because of strong interaction
between basic promoter and the acidic support. The presence
of Cl− in case of MgO supported catalysts is known to cat-
alyze brucite–periclase transformation [23,24]. As a result, Ru
particles are decorated with MgO leading to lower values of Ru
dispersion. In recent studies of Ru/graphite systems [25] demon-
strate that at low metal surface concentration the particles have a
Table 2
Dispersion, metal surface area and particle size of Ru in unpromoted and pro-
moted Ru catalysts
Catalyst
CO uptake
Dispersion
(%)a
Metal area
(m2/g)b
Particle size
(nm)c
(␮mol g−1
)
Ru/MgO
Ru/Al2O3
Ru/HT
Ru/HT (ED)
Cs–Ru/MgO
Cs–Ru/Al2O3
Cs–Ru/HT
30.1
101
76.0
116.0
4.4
21
68
52
78
4
27
39
48
1.52
3.75
5.0
31.8
12.9
9.7
8.5
143
22.8
15.8
13.0
5.7
0.21
1.32
1.90
2.31
26.7
38.5
Cs–Ru/HT (ED) 46.8
a
Calculated as CO uptake (␮mol g−1) × 100/total Ru (␮mol g−1).
b
Metal area is calculated as metal cross-sectional area × no. of Ru atoms on
surface metal. Cross-sectional area of Ru is 0.0821 nm2.
Particle size (nm) is calculated as 6000/[metal area (m2 g Ru−1)ρ]. Ru den-
c
sity (ρ) = 12.4 g cm−3
.
Fig. 2. XRD patterns of unpromoted reduced Ru catalysts on various supports.

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P. Seetharamulu et al. / Journal of Molecular Catalysis A: Chemical 263 (2007) 253–258
Fig. 4. TPR patterns of unpromoted Ru catalysts on various supports.
Fig. 5. TPR patterns of Cs promoted Ru catalysts on various supports.
round shape and at high Ru surface concentration Ru forms flat
particles and the latter one is more active in nitrogen desorption.
The TPR profiles of unpromoted and promoted Ru catalysts
are displayed in Figs. 4 and 5, respectively. Fig. 4 indicates the
presence of two major peaks with Tmax varying in the range
of 423–453 and 573 K. The HT supported samples showed the
presence of low temperature peak at a Tmax of 423 and 453 K
over Ru/HT (ED) and Ru/HT, respectively. However, a very
small shoulder at a Tmax > 473 K is observed with Ru/MgO and
Ru/Al2O3 catalysts and the high temperature is observed at a
Tmax of 573 K over all the unpromoted Ru catalysts. The pres-
ence of two different peaks clearly indicates the reduction of
surface or more dispersed RuCl3 and the bulk RuCl3 over all
the catalysts. The presence of low temperature peak in HT sam-
ples shows the ease of reducibility of these catalysts with Ru
being in more dispersed form or in the form of smaller parti-
cles as compared to MgO and Al2O3 samples. From Fig. 5 it is
clearly observed that the MgO supported Cs promoted Ru cata-
lyst showed Tmax at higher temperature and the Al2O3 supported
catalyst showed Tmax at lower temperature compared to other
catalysts. The higher reducibility of Ru precursor over Al2O3
and HT as compared to MgO can be attributed to the higher dis-
persion of Ru over high surface area supports of Al2O3 and HT.
MgO possesses a very low surface area and hence the dispersion
of Ru is also found to be much lower. This can be further sup-
ported from the formation of bigger particles of Ru as observed
from CO-chemisorption results. In fact the particle size of Ru is
the highest over MgO support. This is further complicated with
the brucite–periclase transformation due to the presence of Cl−
in MgO supported catalyst. The Ru precursors due to the cover-
age with MgO phase might not be accessible for the reduction.
The Tmax for unsupported RuCl3 catalyst is reported at 433 K
with another broad shoulder besides main peak, which has been
attributed to the reduction of ruthenium oxide [26]. An interest-
ing point observed in case of HT supported catalysts is that the
catalyst prepared by conventional impregnation method showed

P. Seetharamulu et al. / Journal of Molecular Catalysis A: Chemical 263 (2007) 253–258
257
value is much lower though it has considerable number of Ru
on surface. The acidic character of Al2O3 may be the reason for
lower TOF value towards NH3 formation. Thus Mg–Al HT sup-
ported system can be considered as a better one, which shows
a considerably good TOF value with higher number of Ru on
the surface compared to the other two supported catalysts. The
catalystpreparedbypolyolreductionmethodusingethylenegly-
col as solvent, i.e. Ru–Cs/HT (ED) shows a two-fold increase
in the TOF value compared to Ru–Cs/HT and higher rate of
NH3 formation at 598 K. At 623 K Ru–Cs/HT (ED) shows an
equal rate of ammonia formation as on HT supported catalyst
prepared by impregnation method. The structure sensitivity of
ammonia synthesis on ruthenium is ascribed to the presence
of the so-called B5 sites, which are believed to be extremely
active for ammonia synthesis and the number of active sites
is correlated with the number of surface atoms. Larger parti-
cles are observed to have shown a discrepancy of active B5
sites [28]. It is observed that on HT support the metal parti-
cle size is comparably lower hence availability of the number
of active B5 sites is higher. In the case of Cs–Ru/HT (ED)
catalyst the dispersion is still high and particle size is compa-
rably much lower over the other catalysts thus showing highest
activity among all the catalysts studied. An additional advan-
tage observed with Cs–Ru/HT (ED) is its higher activity at a
lower temperature of 598 K as compared to other catalysts. It
is observed that the major characteristics of a support for high
catalytic activity is mainly related to the basicity [5], less inter-
action with Cl atom in RuCl3 [2,11], and high surface area
of the support. The more basic is the support; higher is the
activity of the catalyst for ammonia synthesis, because basic
supports donate electrons to Ru surface atoms, which promote
the dissociation of dinitrogen. The lower activity for ammo-
nia synthesis on MgO supported catalyst is due to low surface
area and low dispersion before and after promoter addition and
strong interaction with Cl atom. On other hand although Al2O3
supported catalysts have comparable surface area and disper-
sion before addition of promoter, an intense decrease of these
is observed after addition of promoter. Due to acidic nature of
Al2O3, which has strong interaction with ammonia probe, it
shows less activity. The calcined hydrotalcite supported cata-
lysts are having high basic nature, high surface area and high
dispersion and also not having any strong interactions with Cl
atom and hence the superior activity shown by HT supported
catalysts.
Fig. 6. Effect of reaction temperature on the steady-state concentrations of
ammonia over Cs promoted supported Ru catalysts.
Tmax at higher temperature which is nearer to Tmax of MgO sup-
ported catalyst, but the catalyst prepared by polyol reduction
method showed Tmax at lower temperature which is closer to the
Tmax of Al2O3 supported one. This is due to high surface area
and high dispersion of Cs–Ru/Al2O3 and Cs–Ru/HT (ED).
Fig. 6 shows the effect of reaction temperature on ammonia
synthesis activities obtained under steady-state conditions over
all the Cs promoted catalysts. Steady-state ammonia concentra-
tion and rate of formation of ammonia over all the catalysts at
598 K are shown in Table 3. The rate of NH3 formation is much
higher on Cs–Ru/HT (ED) catalyst. The higher Ru dispersion is
responsible for the high activity. From Fig. 6, ammonia synthe-
sis activities of the catalysts increase with rise in temperature
and reach a maximum value at 623 K above that they decline.
Among the three supported catalysts the HT supported cata-
lysts shows highest activity. From Table 3, it is clear that the
rate of formation of ammonia is more on HT supported cat-
alysts compared to MgO and Al2O3 supported catalysts. The
TOF value, as observed from Table 3, is higher for MgO sup-
ported catalyst compared to other catalysts, even though the
number of Ru atoms on surface is very low. The higher TOF
on MgO supported catalyst is indeed due to the basic char-
acter of MgO. However the presence of Cl− and thereby the
negative aspect of brucite–periclase transformation masks the
beneficial role of MgO. The beneficial role of MgO is reported
in the literature when organic precursor salts of Mg, Ru and
Cs are employed [27]. In Al2O3 supported catalyst the TOF
Table 3
Ammonia synthesis yields and TOF of cesium promoted catalysts
Catalystsa
Temperature (K)
Steady-state NH3
concentration [%, v/v]
NH3 rateb
[cm3 h−1 g Ru−1
TOFc ×104 at a reaction
]
temperature (K)
Cs–Ru/MgO
Cs–Ru/Al2O3
Cs–Ru/HT
598
598
598
598
0.098
0.182
0.287
0.7
32
59
93
61.41
18.75
20.50
41.4
Cs–Ru/HT (ED)
226
a
Weight ratio of Ru:Cs:support = 10:51:100.
Calculated from steady-state NH3 concentrations [%, v/v] obtained by 5 g catalyst.
TOF calculated as [NH3 formed s−1]/[Ru].
b
c

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promising support for the ammonia synthesis. It is highly basic,
thermally stable, and is resistant to Cl− when compared to MgO
support. HT supported catalyst is unveiling highest TOF values
with paramount number of Ru surface atoms. Ru with Cs pro-
moter over novel Mg–Al HT support is found to be very efficient
for the synthesis of ammonia at atmospheric pressure compared
to Cs–Ru catalysts on MgO and Al2O3 supports and the cata-
lyst prepared by polyol reduction method is showing 10% more
activity compared to impregnated catalyst at a lower tempera-
ture of 598 K. The higher activity of the Cs–Ru/HT (ED) catalyst
has been attributed to the presence of easily reducible Ru species
and nano-particles of Ru in highly dispersed form over calcined
Mg–Al HT support.
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