New carbon nanofiber/graphite felt composite for use as a catalyst support for hydrazine catalytic decomposition

Article abstract of DOI:10.1039/b202032g

Graphite felt supporting 40 nm diameter carbon nanofibers was synthezised and successfully used as a support for a high loaded iridium catalyst (30 wt%) in the decomposition of hydrazine; a strong mechanical resistance and a high thermal conductivity led to a very efficient and stable catalyst as compared to that used industrially, iridium supported on a high surface area alumina.

Full text of DOI:10.1039/b202032g

New carbon nanofiber/graphite felt composite for use as a catalyst
support for hydrazine catalytic decomposition
R. Vieira, C. Pham-Huu,* N. Keller and M. J. Ledoux
Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, UMR 7515 CNRS, ECPM, Université
Louis Pasteur, 25, rue Becquerel, BP 08 Strasbourg Cedex 2, France.
E-mail: cuong.lcmc@ecpm.u-strasbg.fr
Received (in Cambridge, UK) 26th February 2002, Accepted 18th March 2002
First published as an Advance Article on the web 3rd April 2002
Graphite felt supporting 40 nm diameter carbon nanofibers
was synthezised and successfully used as a support for a high
loaded iridium catalyst (30 wt%) in the decomposition of
hydrazine; a strong mechanical resistance and a high
thermal conductivity led to a very efficient and stable
catalyst as compared to that used industrially, iridium
supported on a high surface area alumina.
solvent. The solid was dried overnight at 110 °C and then
calcined in air at 350 °C for 1 h to form the corresponding oxide.
For the carbon nanofiber synthesis, 0.5 g of the graphite felt
supported Ni catalyst was placed inside a quartz tube located in
an electric furnace. After reduction of the material at 400 °C
under hydrogen for 1 h, the hydrogen flow was replaced by a
mixture of ethane and hydrogen with a molar ratio of 1+5 (total
flow rate of 120 ml min21) and the carbon nanofibers were
synthezised by catalytic hydrocarbon decomposition over the
graphite felt supported nickel catalyst at 700 °C for 2 h. Fig.
1(a)–(d) show the structure of the graphite felt supported carbon
nanofiber composite. Fig. 1(b) focuses on a naked part of the
composite, showing the increase in diameter when the starting
graphite microfibers forming the graphite felt were coated, after
growth, by a several micrometer thick layer composed of
entangled carbon nanofibers. It should be noted that very few
microfibers or parts of microfibers remained naked after
reaction. Fig. 1(c) shows the high density of the entangled
synthesized nanofibers on the graphite felt support. Transmis-
sion electron microscopy also revealed the total selectivity
towards nanofiber formation, since no amorphous carbon,
onion-like carbon or nanoparticles were detected. The sup-
ported carbon nanofibers displayed the well known herring-
Direct decomposition of hydrazine (N2H4) in microthrusters
over iridium metal supported on stabilized g-alumina catalyst
(Shell 405, Shell Corporation Co.) is up to now the main way of
providing propulsion power for satellite manoeuvring during
path corrections.1 The hydrazine decomposition pathway can be
described as follows: part of the injected hydrazine is first
catalytically decomposed into ammonia, nitrogen and hydrogen
(eqns. (1) and (2)) over about 10 vol% of the catalyst bed, and
thermal decomposition of the mixture subsequently occurs over
the remainder of the catalyst bed (eqn. (3)).2–4
(1)
(2)
(3)
3 N2H4 ? 4 NH3 + N2
3 N2H4 ? 3 N2 + 6 H2
4 NH3 + N2H4 ? 3 N2 + 8 H2
The extremely high exothermicity of the overall reaction
leads to very hot gases and pressure release and provides the
energy for the satellite propulsion. In addition, the reaction
should take place in the millisecond range and is thus controlled
by heat and mass diffusion processes: both the pressure and
temperature of the catalyst bed instantaneously increase with
hydrazine introduction, from atmospheric pressure and room
temperature to respectively several dozens of atmospheres and
about 800 °C. The catalyst efficiency under these extreme
conditions of use is limited by several critical drawbacks due to
the alumina support, such as a low thermal conductivity limiting
heat diffusion leading to formation of hot spots on the catalyst
surface, attrition problems resulting in the formation of fine
powder and its structural instability at high temperature (g to a
phase transformation).
Carbon nanotubes and nanofibers have recently been re-
ported to be suitable active phase supports for heterogeneous
catalytic reactions, either in liquid or in gas phases.5–7 Such new
catalytic materials displayed unusual behaviours as compared to
generally used supported catalysts, due to their peculiar
interactions and/or their high external surface area. The aim of
the present article is to report the synthesis and use of a new
carbon nanofiber composite material, which combines the
advantages due to carbon nanostructures and the easy handling
of a macrostructured graphite felt and which will be used as a
support for iridium metal for the catalytic decomposition of
hydrazine. The catalytic performances are compared with those
obtained on a commercially available high surface area alumina
supported iridium catalyst (30 wt% and 110 m2 g21).
The macroscopic graphite felt, supplied by Carbone Lorraine
Co., was composed of entangled microfibers with a mean
diameter of 10 mm and lengths up to several dozen millimeters.
It had a non-microporous surface area of 1 m2 g21. The nickel
catalyst deposition (1 wt%) onto this felt was achieved by
incipient impregnation with a nickel nitrate salt using ethanol as
Fig. 1 (a)–(c) SEM images of the graphite felt supported carbon nanofiber
composite. (d) TEM image of the composite supported iridium catalyst,
showing the high dispersion of the iridium metal particles (narrow size
distribution centered around 1.5 nm).
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CHEM. COMMUN., 2002, 954–955
This journal is © The Royal Society of Chemistry 2002

bone structure, sometimes connecting in the middle of the
fibers, according to the results reported in the literature for
nickel-based catalysts.8,9 These carbon nanofibers exhibited a
mean diameter of 40 nm and lengths up to several micrometers.
The iridium loaded catalyst (30 wt% of metal) was prepared by
incipient wetness impregnation of the carbon composite felt
with an iridium H2IrCl6 salt. After drying, oxidation in air at 350
°C for 2 h and final reduction in flowing hydrogen at 400 °C for
2 h, the catalyst exhibited a high metal dispersion onto the
carbon nanofiber surface, with a narrow particle size distribu-
tion centered around 2 nm diameter (99% of the iridium
particles with diameter lower than 5 nm), similar to that
observed on the alumina supported catalyst (Fig. 1(d)). Such a
high dispersion was attributed to the strong interaction between
the iridium particles and the prismatic planes exposed by the
carbon nanofibers and made of hydrophylic oxygenated surface
groups.9 These groups also allow an easy anchorage of the
supported phases on the carbon support, as compared to what is
observed with the less reactive basal planes of graphite or
carbon nanotubes. The carbon nanofiber synthesis led to a large
increase in the surface area of the material, from 1 up to 95 m2
g21. The surface area was lower than that of pure carbon
nanofibers,9 because of the remaining low surface area graphite,
located in the core of the composite. Such an increase could be
explained by the presence of graphite prismatic edges, which
could act as adsoprtion sites and also to the relatively low
crystallinity along the nanofiber axis as compared to more
ordered carbon nanotubes. The composite displayed a high
mechanical resistance, neither soot formation nor nanofiber loss
have ever been observed after sonication treatments. Such a
high strength is required to resist the pressure shock inside the
decomposition chamber.
The decomposition of hydrazine was carried out in a stainless
steel reactor (13 mm i.d. and 30 mm length) using 100 mg of
catalyst. 0.34 mL of liquid hydrazine ( > 99.99 vol%) was
injected at atmospheric pressure into the decomposition cham-
ber through an electronic valve with a short injection time (0.6
s), and the pressure increase inside the chamber was monitored
by a Keller gauge, allowing pressure measurements within 0.1
s.
The high performance of the carbon composite felt as
compared to the commercial alumina-based catalyst is reported
in Fig. 2. The use of the carbon composite felt supported iridium
as a catalyst led to a pressure increase from atmospheric
pressure up to 15 bar whereas the peak pressure generated on the
alumina-based catalyst was limited to 5 bar. It cannot be
excluded that part of the injected hydrazine was trapped inside
the micropores of alumina containing no iridium particles, and
thus could not be decomposed. The absence of any detrimental
microporosity in the carbon composite supported iridium
catalyst is therefore of great interest. The strong interaction
between the exposed prismatic planes of the carbon nanofibers
and the iridium particles could provide particles exposing
peculiar faces, and/or also developing specific electronic and
adsorption properties, similar to those reported by other groups
to explain very unusual catalytic behaviours.8 The high
performance for the catalytic decomposition of hydrazine over
the iridium/carbon composite catalyst was also attributed to the
extremely high external surface area of the composite support,
which induces a high dispersion of the active phase. This high
external surface area was observed because of the entangled
carbon nanofiber structure.
It is worth noting that formation of fine powder occured when
using the alumina-based catalyst which, after performing
several successive catalytic shoots, led to a complete destruc-
tion of the material, probably due to the sudden pressure
increase inside the pore network. No structural modification
could be observed on the spent carbon composite as compared
to the fresh material. This mechanical strength was assigned to
the high mechanical stability provided by the graphite felt,
which limited friction phenomena between the carbon nano-
fibers during the high pressure increase. The strong elasticity of
the graphite felt support probably contributed to absorb the
energy during the pressure shock inside the decomposition
chamber. The high thermal conductivity of the carbon compos-
ite also led to a rapid and homogeneous dispersion of the heat
across the overall material, whereas hot spots should occur on
the insulator alumina-based catalyst. This probably contributed
to the fragilization of the alumina catalyst. In addition, the
iridium particles completely sintered after a series of runs on the
alumina-based catalyst, giving the surface a molten aspect,
whereas, only a slight sintering of the iridium particles, i.e. 3 nm
instead of 2 nm after a dozen of hydrazine decomposition
cycles, was observed on the carbon composite and the
homogeneous dispersion of the metal phase was left quite
unchanged. The pressure generation during the catalytic
decomposition of hydrazine remained almost unchanged after a
dozen injections evidencing the high mechanical stability of the
composite-based catalyst. A more erratic behaviour in the
catalytic decomposition of hydrazine has been observed on the
alumina-based catalyst, probably due to the beginning of the
catalyst body breaking as reported above.
In conclusion, a pre-shaped carbon nanofiber supported
iridium catalyst can be prepared, conserving the macrostructural
and mechanical flexibility properties of the graphite felt
precursor. The graphite felt supported carbon nanofiber com-
posite can be efficiently used as a catalyst support for the
decomposition of hydrazine with higher performance as
compared to the industrial catalyst, due to the high external
surface area, providing a close contact between the reactant and
the active phase. This new catalyst is resistant to the great
increase in pressure which occurs inside the decomposition
chamber. The macroscopic design of such a composite could
open new routes for feasible industrial uses of carbon
nanostructures, which remained problematic up to now, due to
the powder form of the primary structure.
Notes and references
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2 T. W. Price and D. D. Evans, The status of monopropellant hydrazine
technology, JPL-32-1227, Jet Propulsion Laboratory, Passadena, CA.
3 E. Schmidt, Hydrazine and its derivatives. Preparation, Properties,
Applications, John Wiley, New York, 1984.
4 G. Schulz-Ekloff and H. G. Deppner, Chem. Eng. Technol., 1989, 12,
426.
5 C. Park and R. T. K. Baker, J. Phys. Chem. B, 1999, 103, 2453.
6 K. P. de Jong and J. W. Geus, Catal. Rev.-Sci. Eng., 2000, 42, 481.
7 C. Pham-Huu, N. Keller, L. J. Charbonnière, R. Ziessel and M. J. Ledoux,
Chem. Commun., 2000, 19, 1871.
8 N. M. Rodriguez, J. Mater. Res., 1993, 8, 3233.
9 C. Pham-Huu, N. Keller, V. V. Roddatis, G. Mestl, R. Schlögl and M. J.
Ledoux, Phys. Chem. Chem. Phys., 2002, 4, 514.
Fig. 2 Performance obtained on the graphite felt supported carbon nanofiber
composite catalyst for hydrazine decomposition as compared to the high
specific surface area alumina-based catalyst (HSS alumina).
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