Scalable synthesis of activated carbon with superparamagnetic properties

Article abstract of DOI:10.1039/b608231a

Magnetic activated carbons were obtained by in situ formation of coated superparamagnetic iron nanoparticles in the pores of carbons initially impregnated with ferric salt solution. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b608231a

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Scalable synthesis of activated carbon with superparamagnetic
properties{
Manfred Schwickardi, Stefan Olejnik, Elena-Lorena Salabas, Wolfgang Schmidt and Ferdi Sch u¨ th*
Received (in Cambridge, UK) 9th June 2006, Accepted 19th July 2006
First published as an Advance Article on the web 8th August 2006
DOI: 10.1039/b608231a
Magnetic activated carbons were obtained by in situ formation
of coated superparamagnetic iron nanoparticles in the pores of
carbons initially impregnated with ferric salt solution.
activated carbon with well-protected iron particles as the magnetic
component and high specific surface area.
The method consists of a series of simple preparation steps
starting from an activated carbon which is impregnated with
aqueous iron(III) solution, followed by drying and calcination
under inert atmosphere. During the calcination iron particles are
formed in the pores of the carbon, which are then coated with a
carbon layer formed by CVD using benzene as carbon precursor.
In a typical preparation, 20 g of a microporous activated carbon
(Kugelkohle, Bl u¨ cher GmbH) which had been exposed to a second
Activated carbons are widely used as adsorbents, catalyst supports
and as bio-compatible materials. For catalysis, purification of
liquids or medical applications, the separation of the carbons from
the fluid is crucial. The use of magnetic carbons allows separation
by simply applying a magnetic field. For this reason, several
groups started investigating the magnetic properties of carbon
materials. There are two pathways to obtain magnetic carbon. The
first is based on the ferromagnetic properties of pure carbon
activation step in N
specific surface area of 2099 m was impregnated under
3 3
stirring with 16 mL of 2.33 M Fe(NO ) solution. The solution was
2 2
–H O at 900 uC and having an apparent
2 21
g
1
materials. The magnetization of such carbon materials is rather
low, and therefore the alternative, composites of carbon materials
with magnetic particles, seems to be more promising for technical
applications. Deposition of magnetic oxide particles on carbon
completely absorbed by the carbon upon continued intense stirring
for 5 min. After drying at 90 uC, the impregnated carbon was
transferred into a rotary oven (quartz glass tube, see ESI{) and
2
21
nanotubes has been reported as well as formation of magnetic
3
oxides in the pores of activated carbons. However, the magnetic
heated (4.5 uC min ) to a temperature of 700 uC under argon.
After reaching this temperature, rotation of the tube was started
and benzene vapor was flowed through the tube. The activated
carbon material was agitated for 30 min in the rotation tube in the
particles decorating the carbon materials are easily dissolved and/
or washed out by acidic solutions. Protective coatings of the
magnetic particles are a feasible method to overcome this
drawback. By deposition of preformed cobalt nanoparticles on a
mesoporous carbon support followed by carbon coating of the
cobalt particles, it was possible to obtain a chemically stable and
magnetically separable carbon material. Additional deposition of
palladium on the magnetic carbon resulted in a magnetically
2
1
argon–benzene flow (1.87 mmol min ). After cooling to room
temperature, 22.13 g of product were obtained. The apparent
2
specific surface area of the carbon material was reduced to 1642 m
21
g
after this treatment (see ESI). Upon calcination under argon
atmosphere, elemental iron particles are formed in the pores of the
activated carbon as evidenced by the XRD patterns shown in
Fig. 1.
4
separable hydrogenation catalyst. Also the coating of magnetic
cobalt nanoparticles by carbon of different origin is a possibility
for obtaining such materials, which have, however, a rather high
5
metal content. The pathways based on cobalt nanoparticles are
rather complex, since the nanoparticles need to be synthesized first
by liquid phase reactions under a protective atmosphere before
they can be further processed. Magnetic monodispersed carbon
nanoparticles on a relatively large scale or magnetic carbon
nanotubes have been synthesized by pyrolysis of polypyrrole which
6
contains iron oxide. However, the surface areas of these materials
2
were relatively small i.e. 360 m
21
g , since the aim was the
production of carbon nanoparticles, not that of a magnetic
activated carbon. Here, we present an easy and scalable method
for the preparation of bulk quantities of superparamagnetic
Max-Planck-Institut f u¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1,
D-45470, M u¨ lheim an der Ruhr, Germany.
E-mail: schueth@kofo.mpg.de; Fax: +49 208 306 2995;
Tel: +49 208 306 2373
{
Electronic supplementary information (ESI) available: Photographs of
Fig. 1 XRD patterns of parent activated carbon (bottom) and the
composite containing Fe particles in activated carbon (top) obtained in the
lab rotary oven. The dashed line indicates the position of Fe reflections.
the laboratory rotary oven and the pilot scale rotary kiln, nitrogen
sorption isotherms of the parent activated carbon and the magnetic
carbon. See DOI: 10.1039/b608231a
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Chem. Commun., 2006, 3987–3989 | 3987

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The iron particles in the carbon material are very resistant
against leaching and dissolution. Upon treatment with 2 M HNO3
solution only 3% of the iron present in a typical material obtained
in the lab rotary oven was dissolved. The iron particles are
obviously well-protected by the carbon shells. A material which
had not been treated with benzene vapor (no protective carbon
shell) lost more than 90% of the iron under the same conditions.
The majority of the iron particles have sizes of less than 10 nm
as evidenced by the broad XRD reflections and the TEM image
shown in Fig. 2 (top). Occasionally also larger particles are
observed.
temperature to 2 K. This implies that by decreasing the
temperature the particle moments are blocked progressively along
their anisotropy directions. After reaching T 5 2 K the
magnetization is recorded on warming and in a weak external
field of 20 Oe. The observed temperature-dependent magnetization
is shown in Fig. 3.
The ZFC curve shows a peak at about 50 K which corresponds
to the blocking temperature of the assemblies of the single-domain
Fe nanoparticles. In principle, particle sizes can be obtained from
the blocking temperature, if the particles are magnetically isolated
and monodisperse. However, this is not the case for our samples
due to the relatively high concentration of iron particles and the
size distribution, and therefore, no correlation of blocking
temperature and the particle sizes determined by TEM was
attempted.
Visualization of the carbon shells of iron particles inside the
activated carbon is difficult because of the more or less identical
absorption contrast of the shell and the carbon host but
occasionally particles are found also on the external surface of
the activated carbon as shown by the high resolution TEM image
The hysteresis loop measured at 5 K (Fig. 4) shows the
ferromagnetic behavior of the material with a saturation
(Fig. 2, bottom). The lattice planes of the iron particles and the
2
1
graphene sheets in the surrounding carbon shells are clearly
discernible.
magnetization of about 18 emu g and a coercivity of about
2
1
2
20 Oe. Remanence magnetization of about 4.3 emu g has been
The small size of the iron particles results in superparamagnetic
properties. In order to identify the transition temperature (blocking
temperature) from the ferromagnetic state to the superparamag-
netic state, the zero-field cooled (ZFC) magnetization as a function
of temperature was measured using a superconducting quantum
interference device magnetometer (SQUID). For the ZFC
experiment, the sample was cooled in zero field from room
measured. The field-cooled hysteresis loop (not shown) was not
shifted along the field axis indicating the absence of an exchange
bias effect. The lack of this exchange coupling implies that the Fe
nanoparticles are not surrounded by an oxide layer or that this
layer is very thin (less than 2 nm).
Fig. 3 Zero field cooled (ZFC) temperature dependent magnetization for
Fe nanoparticles embedded in an activated carbon matrix measured at
20 Oe.
Fig. 2 TEM image of iron nanoparticles in the activated carbon matrix
(top, scale bar 50 nm) and high resolution TEM image of iron particles
Fig. 4 Hysteresis loop at 5 K for Fe nanoparticles embedded in the
with carbon shell (bottom, scale bar 10 nm).
activated carbon matrix. Inset shows the low field region of the hysteresis.
3
988 | Chem. Commun., 2006, 3987–3989
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In order to prove the scalability of the preparation method,
magnetic carbons were also synthesized on a larger scale in a pilot
plant-size rotary kiln (see ESI{). For this preparation, 125 g
3
21
of activated carbon (P-43 171, Adsor-Tech, 0.598 cm
2
g ,
21
1
3 3
207 m g ) was impregnated with 59.8 mL of 2 M Fe(NO )
solution and fed into the continuously operated rotary kiln. The
impregnated 0.3–0.5 mm activated carbon beads are successively
transferred through a drying zone, a decomposition zone, a
calcination zone and a cooling zone. In order to prevent working
Fig. 5 Interaction of the magnetic carbon with a permanent magnet.
with larger amounts of benzene, a nitrogen–toluene stream (N
2
21
flow 50 L h ) was passed at 700 uC in concurrent flow over the
carbon material in the kiln. In this way, a magnetic carbon
containing about 5 wt% of iron was obtained. The toluene acts in
the same way as benzene. Using this pilot scale kiln, a productivity
It should be noted that the iron–carbon composites also
contain a small fraction of iron carbide as evidenced by XPS
spectroscopy. This minor by-phase is probably formed on the
surface of the iron particles during the calcination under protective
atmosphere.
21
of about 25 g h of magnetic carbon could be achieved. However,
if working with higher loading as in the experiment described, the
fraction of ferromagnetic domains increases.
Fig. 5 shows the magnetic carbon sedimented in a solution (left)
and attracted by a permanent magnet. After removing the
permanent magnet, the carbon is easily redispersed in the solution
because of the superparamagnetic properties of the iron particles.
Their size in the range of a few nanometres prevents ferromagnet-
ism because the size of the magnetic domains (,10 nm) falls below
its critical value. At this size, the thermal energy at room
temperature is sufficient to change the direction of magnetization
of the entire crystallite at room temperature. In presence of a field,
the magnetic momentum of the complete crystallite is aligned with
the external field. After removal of the external magnetic field, the
alignment of the magnetic momentum is lost, allowing redispersion
of the particles. It should be noted that a small residual
ferromagnetism at room temperature was observed which can be
attributed to the small fraction of larger iron particles in the
material. This, however, did not negatively affect the redispersi-
bility of the materials.
The materials obtained via the pathway described in this
manuscript are essentially superparamagnetic, have high surface
areas and high porosities, they are bio-compatible, and stable
under alkaline as well as under acidic conditions. Last but not
least, upscaling of the preparation process is easily possible. This
makes these magnetic carbons very promising candidates for larger
scale applications in catalysis (e.g. magnetically separable catalyst
supports) and/or separation technology (e.g. magnetically separ-
able adsorber).
Notes and references
1
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L. C. A. Oliveira, R. V. R. A. Rios, J. D. Fabris, V. Garg, K. Sapag and
R. M. Lago, Carbon, 2002, 40, 2177–2183; Z. Sun, Z. Liu, Y. Wang,
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2
The preparation of magnetic carbon as described above thus
results in a material with excellent magnetic properties for various
applications. The particles only aggregate in a magnetic field and
redispersion occurs if the external magnetic field is switched off. In
contrast to materials from other reports where the magnetic
properties are due to metal oxide particles, here metallic iron
nanoparticles are causing the magnetism. Also in contrast to most
reports, a protective carbon shell prevents dissolution and/or
leaching of the metal nanoparticles.
3
4
A. B. Fuertes and P. Tartaj, Chem. Mater., 2006, 18, 1675–1679.
A.-H. Lu, W. Schmidt, N. Matoussevitch, H. B o¨ nnemann, B. Spliethoff,
B. Tesche, E. Bill, W. Kiefer and F. Sch u¨ th, Angew. Chem., Int. Ed., 2004,
43, 4303–4306.
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A.-H. Lu, W.-C. Li, N. Matoussevitch, B. Spliethoff, H. Boennemann
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J. Jang and H. Yoon, Small, 2005, 1, 1195–1199; J. Jang and H. Yoon,
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This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 3987–3989 | 3989