A novel catalyst for hydrazine decomposition: Molybdenum carbide supported on γ-Al2O3

Article abstract of DOI:10.1039/b109400a

An alumina-supported Mo₂C catalyst is found to be as active as a conventionally used Ir/γ-Al₂O₃ catalyst for catalytic decomposition of hydrazine tested in a monopropellant thruster.

Full text of DOI:10.1039/b109400a

A novel catalyst for hydrazine decomposition: molybdenum carbide
supported on g-Al2O3
Xiaowei Chen, Tao Zhang,* Pinliang Ying, Mingyuan Zheng, Weicheng Wu, Liangen Xia, Tao Li,
Xiaodong Wang and Can Li*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O.
Box 110, Dalian 116023, China. E-mail: taozhang@dicp.ac.cn. E-mail: canli@dicp.ac.cn
Received (in Cambridge, UK) 16th October 2001, Accepted 17th December 2001
First published as an Advance Article on the web 16th January 2002
An alumina-supported Mo2C catalyst is found to be as active
as a conventionally used Ir/g-Al2O3 catalyst for catalytic
decomposition of hydrazine tested in a monopropellant
thruster.
a-Mo2C/g-Al2O3 catalyst with a face-centered cubic structure
(fcc) was prepared by carburizing the produced Mo2N/g-Al2O3,
whereas the b-Mo2C/g-Al2O3 catalyst with a hexagonal-close
packed structure (hcp) was synthesized from MoO3/g-Al2O3
directly.10–13 Both of the supported Mo2C/g-Al2O3 catalysts
with different structures were passivated by 1% O2/N2 for 10 h
to form a protective oxide layer on the surface.
The catalytic decomposition of hydrazine has been of interest
for many years since this process has been successfully used in
a monopropellant thruster to control and adjust the orbits and
altitudes of spacecrafts. The traditionally used catalyst for
hydrazine decomposition is a 20–40 wt% Ir/g-Al2O3 catalyst.1-4
The Ir-based catalyst is very expensive because iridium is a rare
and noble metal. Therefore it is highly desirable to develop an
inexpensive, active, stable and readily available catalyst for
hydrazine decomposition.
Transition metal carbides and nitrides have received a great
deal of attention because of their excellent catalytic behavior
resembling group VIII metals in a number of reactions,5,6 such
as hydrogenolysis, hydrogenation, hydrotreating (hydrode-
nitrogenation and hydrodesulfurization), NH3 synthesis and
decomposition. Pure phases of g-Mo2N, W2N, NbN and W2C
have been tested as possible substitutes for the Ir/g-Al2O3
catalyst for hydrazine decomposition in space technology.7,8
Their behaviors were similar to or inferior to that of the Ir/g-
Al2O3 catalyst.
Until now, no studies on the catalytic performance of
molybdenum carbide for hydrazine decomposition have been
reported. Supported Mo2C on alumina catalysts were prepared
in order to obtain highly dispersed molybdenum carbide on the
alumina with a high surface area. Compared with the bulk
molybdenum carbide, alumina-supported molybdenum carbide
has some advantages, such as relatively low Mo loading, large
surface area, strong mechanical strength and easy molding. In
this work, for the first time it has been found that the supported
Mo2C catalysts are very active for hydrazine decomposition.
The theoretical monolayer capacity of the MoO3/g-Al2O3
sample is 0.12 g MoO3/100 m2 of g-Al2O3 surface.9 The
theoretical monolayer coverage of MoO3 on g-Al2O3 with 198
m2 g21 corresponds to a loading of ca. 12.9 wt% Mo. The
MoO3/g-Al2O3 precursor (Mo in the catalyst is 12.9 wt%) was
first prepared by the incipient wetness method using an aqueous
(NH4)6Mo7O24·4H2O solution and g-Al2O3 (SBET = 198 m2
g21, 20–30 mesh), followed by drying at 393 K for 12 h and
calcination at 773 K for 4 h. The Mo2N/g-Al2O3 precursor was
prepared by temperature-programmed reaction (TPR) of MoO3/
g-Al2O3 with ammonia. The temperature was increased from
room temperature to 573 K in 1 h; then from 573 K to 973 K at
a rate of 1 K min21; and then kept at 973 K for 2 h. The nitrided
sample was cooled down to room temperature in flowing
ammonia and passivated in a mixture of 1% O2/N2 so as to avoid
the violent oxidation of the Mo2N/g-Al2O3 precursor. Sup-
ported molybdenum carbides with the same Mo loading but
different structures were prepared by TPR between the
precursors (Mo2N/g-Al2O3 or MoO3/g-Al2O3) and 20% CH4/
H2 (v/v). A four-stage heating ramp was used: the temperature
was first heated from room temperature to 573 K in 0.5 h; then
to 823 K at a rate of 0.5 K min21; further from 823 to 973 K at
a rate of 1 K min21; and finally maintained at 973 K for 1 h. The
The catalytic activity of hydrazine decomposition was
evaluated in an experimental apparatus similar to that used by
Tian et al.14 and Rodrigues.7,8 A monopropellant thruster of 10
Newtons containing a catalyst bed (40 mm long with a diameter
of 16 mm), was placed in a vacuum chamber (evacuated to 0.5
Torr). In this thruster the theoretical chamber pressure (Pc)
produced by hydrazine decomposition was ca. 1.02–1.05 MPa
(at flow rate of hydrazine of 4.41 g s21). Approximately 9 g of
catalyst was used for each batch. The hydrazine injection
pressure (Pi) was controlled by pressurized N2 and was kept
constant (1.5 MPa) during the hydrazine decomposition to
obtain an almost constant hydrazine feeding rate. When the
electromagnetic valve was opened, the hydrazine was pressed
into the catalytic chamber by N2. Almost at the same time, the
hydrazine was vaporized and decomposed by the catalyst bed,
generating the gases of nitrogen, hydrogen and ammonia. The
initial temperature in the catalyst bed was 373 K. The
continuous feeding of hydrazine lasted 30 s, and then the
catalyst bed was allowed to cool to 373 K before the next 30 s
test began. Chamber pressure (Pc), catalyst bed temperature (Tc)
and ignition delay (t0) were recorded with a frequency of 1 kHz
and calculated automatically by computer. The performance of
hydrazine decomposition over the Mo2C/g-Al2O3 catalysts with
different structures and commercial 31.6 wt% Ir/g-Al2O3
catalysts were compared.
The Mo2C/g-Al2O3 catalysts (12.9 wt% Mo) prepared by
different methods both show broad XRD peaks which make it
difficult to distinguish the Mo2C phase from g-Al2O3. However,
the bulk materials synthesized under similar preparation
conditions give strong and sharp XRD peaks due to a-Mo2C
(fcc) and b-Mo2C (hcp) phases. These results suggest that the a-
Mo2C and b-Mo2C particles are well dispersed on g-Al2O3
under these preparation conditions.
Hydrazine decomposition is a volume expansion and exo-
thermic reaction. When hydrazine was fed into the thruster and
contacted with the catalyst, it was decomposed immediately.
Consequently, the chamber pressure (Pc) increases quickly, and
then reaches a maximum value. Fig. 1 compares the catalytic
behaviors of the commercial 31.6 wt% Ir/g-Al2O3, a-Mo2C/g-
Al2O3 and b-Mo2C/g-Al2O3 catalysts in the first 30 s con-
tinuous feeding of hydrazine. It can be seen that Pc attains a
steady value within a short time. The higher the catalyst activity,
the more rapid the increase of the chamber pressure. These
results indicate that both the a-Mo2C/g-Al2O3 and b-Mo2C/g-
Al2O3 catalysts are effective for the catalytic decomposition of
hydrazine. Furthermore, there is no obvious difference between
the stable-state chamber pressures of the Mo2C/g-Al2O3
catalysts with different structures and the Ir/g-Al2O3 catalyst
under the same reaction conditions. The steady Pc produced by
hydrazine decomposition is close to or greater than the
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depends mainly upon the intrinsic activity of the catalyst at the
initial time. The smaller the values of t0, the higher the initial
activity of the catalyst. The t0 value of the Ir/g-Al2O3 catalyst is
the shortest with an average of 28 ms, whereas the t0 values of
the a-Mo2C/g-Al2O3 and b-Mo2C/g-Al2O3 catalysts are longer
than that of the Ir-based catalyst with the average of 56 and 46
ms, respectively. The results indicate that the initial activity of
the Ir/g-Al2O3 catalyst is higher than that of the Mo2C/g-Al2O3
catalysts, but is within the same order of magnitude. The
difference in the t0 value of these two Mo2C/g-Al2O3 catalysts
of the same chemical composition is small and is probably due
to their different phases.
The chamber pressure (Pc) and catalyst bed temperature (Tc)
at the steady states remains almost unchanged for six tests of the
30 s continuous feeding of hydrazine. This indicates that these
two Mo2C/g-Al2O3 catalysts are stable for hydrazine decom-
position.
The ammonia component of the resultant mixture produced
by hydrazine decomposition may further dissociate into H2 and
N2 at elevated temperatures. The ammonia decomposition is
endothermic and the hydrazine decomposition is exothermic.
Hence the catalyst bed temperature (Tc) is the parameter of the
ammonia and hydrazine conversion. As shown in Fig. 1, Tc
increases when hydrazine decomposes, and rises to a stable
level finally. The Tc of the Ir/g-Al2O3 catalyst is lower than
those of the Mo2C/g-Al2O3 catalysts with different phases. The
results suggest that the ammonia conversion over the Ir/g-Al2O3
catalyst is higher than that of the Mo2C/g-Al2O3 catalysts.
In conclusion, alumina-supported molybdenum carbides
have been applied to hydrazine decomposition for the first time.
The Mo2C/g-Al2O3 catalysts exhibit excellent catalytic per-
formances for hydrazine decomposition. They show compara-
ble activities to that of the Ir/g-Al2O3 catalyst. Alumina-
supported molybdenum carbide catalysts are therefore
promising substitutes for the Ir-based catalyst for hydrazine
decomposition in space technology because of their low cost,
high activity, strong mechanical strength and easy prepara-
tion.
Notes and references
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theoretical Pc. The activities of supported molybdenum carbide
catalysts for hydrazine decomposition are comparable to that of
the Ir/g-Al2O3 catalyst.
Table 1 summarizes the experimental results of steady
chamber pressure (Pc), catalyst bed temperature (Tc) and
ignition delay (t0). The parameter t0 is the time when the
pressure of the chamber reaches 10% of the stable-state pressure
of the chamber. It reflects the initial activity of the catalyst and
Table 1 Performance of the catalysts with a continuous feeding of hydrazine
for 30 s in a thruster of 10 N. The initial temperature in the catalyst bed was
373 K
Hydrazine
flow
rate/g s21
Average
t0/ms
Pc/
MPa
Catalyst
Tc/K
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a-Mo2C/g-Al2O3 (fcc)
b-Mo2C/g-Al2O3 (hcp)
Ir/g-Al2O3
4.70
4.42
4.63
56
46
28
1171
1123
1071
1.08
1.04
1.07
CHEM. COMMUN., 2002, 288–289
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