22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Iron functionalization of graphene nanoflakes in thermal plasma conditions for
catalyst applications
U. Legrand, N.Y. Mendoza-Gonzalez, J.-L. Meunier and D. Berk
Plasma Processing Laboratory (PPL), McGill University, Chemical Engineering Montreal, Quebec, Canada
Abstract: Graphene nanoflakes (GNFs), a stack of 5 to 20 layers of graphene sheets, are
the product of methane decomposition by a thermal plasma. The GNFs were functionalized
with nitrogen to support atomic iron and create catalytic sites. The possibility to evaporate
iron particle has been proven by modelling. The iron functionalization step is realized
within the reactor, by the partial evaporation of iron powder injected in the argon plasma.
Keywords: graphene nanoflakes, thermal plasma, iron, catalytic sites, evaporation model
Introduction
Decreasing the price of catalyst for polymer membrane
electrolyte fuel cells (PEMFC), and replacing the
platinum based catalyst by a corrosive resistant and nonnoble catalyst are major issues to allow PEMFC to reach
larger markets. The non-noble catalyst which has been
proposed in 2011 by Pristavita [1][2] is based on the
implementation of catalytic sites on well crystallized
carbon matrix, the graphene nanoflakes (GNFs) . The
catalytic sites are formed by an atom of iron linked to
nitrogen atoms, and are inspired by the work of Jasinski,
where he observed a high activity of iron phthalocyanine
in the oxygen reduction reaction. While it has been shown
that nitrogen can be efficiently added to GNFs within the
thermal plasma system resulting in low (~2 at%) and high
(~16 at%) functionalization levels [3], the present study is
focusing on the optimization of the iron functionalization
step within the reactor. With this objective in mind, a
modelling study of iron particle evaporation, and
experimental optimization of the iron particles feeding
within the reactor to get nitrogen and iron functionalized
GNFs (N/Fe-GNFs) are presented.
heat-mass transfer calculations are based on the force
balance on the particle and on the convective heat and
mass transfer from the particle, using the local continuous
phase conditions as the particle moves through the flow.
The following assumptions are considered for the
discrete phase model: particles are spheres and the fluid is
an ideal gas, the gas-particle system is diluted, the
thermophoresis effect is considered as an external force
on the transport of particles. This effect is responsible for
small particles in a temperature gradient being driven
from high to low temperature regions. Finally, gravity and
turbulence have no effect and there is no particle radiation
interaction.
Because in the discrete phase model the particle phase
is regarded as a source of mass, momentum, and energy to
the gaseous phase, the following equations are used to
calculate the set of heat and mass transfer laws involved.
The trajectory of a discrete phase particle is calculated
by integrating the force balance on the particle as shown
in Eq. 1 This force balance equates the particle inertia
with the forces acting on the particle.
(Eq. 1)
Methods
A. Particle evaporation model
In the synthesis of GNF by ICP plasma, solid micron
sized iron particles are injected into the plasma core to be
evaporated. The choice of using iron powders instead of
some liquid-based iron precursors follows a stringent
requirement on the purity of the resulting Fe/N-GNF
structure for fuel cell catalyst applications. The modeling
of the evaporation of the raw iron particles is solved with
a discrete phase model using the PSI-Cell approach [4] in
a Lagrangian frame of reference. The calculation method
implies that raw particles are considered as a second
phase composed of spherical particles dispersed in the
continuous phase. The effects of the particles are
considered negligible, which means that the volume
fraction has not to exceed 10-12%. The particles
trajectories are computed using the equations of the
movement of a rigid particle in a fluid. The trajectory and
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𝑚𝑝
𝑑𝑢𝑝
1 1
= 𝐶𝐷 𝜌 � 𝜋𝑑𝑝2 � �𝑢 − 𝑢𝑝 ��𝑢 − 𝑢𝑝 � + 𝐹𝑡ℎ + 𝑚𝑝 𝐹𝑏 (𝑡)
𝑑𝑑
2 4
where 𝐶𝐷 is the drag coefficient that depends on the
particle form and the relative Reynolds number, 𝑚𝑝 the
particle mass, 𝑢𝑝 the particle velocity, 𝑢 the fluid phase
velocity, 𝜌 the fluid density, 𝑑𝑝 the particle diameter, and
finally 𝐹𝑏 (𝑡) is the Brownian force per unit mass which is
considered important for submicron particles and 𝐹𝑡ℎ the
thermophoretic force. The thermophoresis effect 𝐹𝑡ℎ is
included in the particle force balance according to [5].
The heat transfer to the particle is determined through
an energy balance as shown in Eq. 2.
(Eq. 2)
𝑚𝑝 𝐶𝑝
𝑑𝑇𝑝
𝑑𝑚𝑝
= ℎ𝑐 𝜋𝑑𝑝2 �𝑇 − 𝑇𝑝 � +
ℎ
𝑑𝑑
𝑑𝑑 𝐿
1
where 𝐶𝑝 is the particle heat capacity (J/kg K), 𝑇𝑝 the
particle temperature (K), ℎ𝑐 the convective heat transfer
coefficient (W/m2 K), and T the temperature of
𝑑𝑚
𝑝
continuous phase (K). The factor
is the rate of
𝑑𝑑
evaporation (kg/s) and ℎ𝐿 is the latent heat. The rate of
vaporization is governed by concentration gradients, with
the flux of droplet vapour into the gas phase related to the
gradient of the vapour concentration between the droplet
surface and the bulk gas.
For the discrete phase model, the iron raw particles are
specified as having a logarithmic distribution with
diameters summarized in Table 1. Also, conditions like
position, velocity, size and initial temperature of
individual particles as shown in Table 1.
Table 1: Properties of iron in solid and gaseous states,
and properties of the injected particles in the reactor.
Properties of iron particles
Density [kg/m3]
7874
.
Cp [J/mol K]
25.10
Thermal
conductivity
80.4
[W/m.K]
Latent heat [kJ/kg]
272
Boiling point [K]
3134
Volatile component
100
fraction [%]
1728K: 1 Pa
2346K: 1kPa
Saturation vapour
1890K: 10 Pa
2679K: 10kPa
pressure
2091K: 100 Pa 3132K: 100kPa
Properties of iron gas
1000K: 22.5
4000K: 29.7
Cp [J/mol.K]
2000K: 23.2
5000K: 34.0
3000K: 26.2
6000K: 39.9
Thermal
conductivity
79.45
[W/m.K]
Molecular weight
55.845
Injection properties for the discrete phase model
Temperature [K]
300
Mass flow rate
0.006
[g/min]
Max. Diameter [m]
3x10-4
Min. Diameter [m]
1.1x10-5
B. Experimental procedure
GNFs are grown following Pristavita et al. procedure
[1], with methane as a carbon source that is decomposed
within an argon thermal plasma. GNFs are formed as
small nuclei through homogeneous nucleation, and
essentially grow laterally in a sheet-like geometry within
2
a very narrow growth window in the plasma decay zone
[2][6]. The nanoparticles are deposited on the walls and
the end plate downstream of the reactor. It is to be noted
the experimental reactor is totally axisymmetric with
respect to the flow pattern, including for the exit flow that
extends radially from the endplate. This is providing a
very good match with the modelled geometry, and an
experimental flow pattern having a true stagnation point
flow geometry. A small amount of nitrogen is added
during the growth, leading to nitrogen content up to 2 at%
on the surface of the GNFs.
As indicated above, iron is added in the form of iron
powders using a PFV100-VM-NO powder feeder from
Tekna. A mixture of two iron powders is fed in the
reactor. The first has a size distribution between 10 and
300 µm, while the second powder shows a smaller
distribution between 1 and 10 µm. It has been observed
that the smallest particles were difficult to feed
continuously, while the biggest particles were difficult to
evaporate. Mixing the two powders results in taking
advantage of both aspects, i.e. a continuous feeding for a
good rate of evaporation. The iron particles are carried by
an argon flow of 5 slpm.
The iron powder is added simultaneously with the
methane and the nitrogen in order to process the growth
of the GNFs, the nitrogen functionalization and the iron
implementation in the same time. An equilibrium
thermodynamic evaluation using the software Factsage
shows that iron vapours are present along with the
growing GNFs and atomic nitrogen in the temperature
range 3000-5000 K, indicating that it is thermodynamically possible to produce the catalyst in only one step.
Scanning Electron Microscopy (SEM) was made using
a FEI Inspect F-50 FE-SEM. X-ray Photoelectron
Spectroscopy (XPS) were obtained using a VG Scientific
ESCALAB MK II operating at a pressure of 10-9 torr, and
using an aluminium x-ray source.
Results and discussion
A. Modeling results
The evaporation of iron particles is first analyzed using
the present model. A very low amount of iron particles is
injected in the ICP torch (0.006 g/min) in the presence of
argon plasma at 55.2 kPa and 10 kW effective power. The
resulting temperature (K) and mass fraction or iron
vapour (kg fe /kg Ar ) profiles can be observed in Figure 1.
The left side of the figure corresponds to the temperature
profiles, where we can observe that the highest
temperature is 10500 K located within the torch zone and
which is actually the plasma core. This temperature
further decreases once the plasma gas enters the reactor
zone due to cooled walls. Relatively smooth temperatures
gradients are observed which are characteristic of the
present conical reactor design. The isotherm line
corresponding to 1811 K is depicted in the figure as an
indication of the melting temperature of iron. The mass
fraction of iron vapour is observed on the right side of the
Figure 1. The range of mass fraction of iron vapour from
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1x10-6 to 5x10-4 clearly depicts the species diffusion in the
reactor. The iron vapour created in the torch is further
transported by diffusion with the highest amount at the
exit of the torch and along the axis region of the reactor.
The largest volume of the reactor is showing a mass
fraction of 1x10-6, this being sufficient to promote the iron
functionalization in the zone of graphene growth (35005000 K). Since the melting point of iron particles is 1811
K, we expect to have the formation of iron particles at the
bottom end of the reactor.
the analysed sample does not interfere with the
determination of the iron bonded to nitrogen; the iron
from iron particles being mainly in oxidized form while
the binding energy for Fe-N is different than Fe-O. The
composition of the overall sample is given in Table 2.
Fig. 2. SEM picture of Fe particles fed within the reactor.
Fig. 1. Distribution of a) temperature and b) mass
fraction of iron vapour inside the conical reactor, with a
plasma power of 10 kW and a pressure of 55.2 kPa.
The formation of these particles should however be
minimum since the amount of iron precursor is very low.
Iron particles are not desirable for the process but are not
problematic as such, these can also be removed in a
subsequent step.
B. Structure and composition of N/Fe-GNFs
The iron particles to be injected during the growth of
N/Fe-GNFs have been collected at the exit of the powder
feeder without plasma heating, and observed by SEM
(Figure 2). The smallest particles with a diameter less
than 10 µm are agglomerated around biggest particles,
suggesting large particles are efficiently carrying the
smallest one.
A SEM image of the N/Fe-GNFs produced is shown in
Figure 3. The GNFs look homogeneous, and it can be
noted that no amorphous carbon or polymer are present in
the sample. It has been observed that some partially
evaporated particles remain in the N/Fe-GNFs samples
collected.
XPS was used to determine the atomic composition of
the N/Fe-GNFs surface. The presence of iron particles in
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Fig. 3. SEM picture of N/Fe-GNFs.
Table 2: Atomic composition at the surface of the N/FeGNFs.
Element
At%
C
94.5
O
3.6
N
0.4
Fe
1.5
A closer look at the iron peak is made through high
resolution analysis and deconvolution using GaussianLorentzian curve fitting to determine the different states
of this element (Figure 5). Most of the iron is bonded with
oxygen, corresponding to the oxide layer at the surface of
the partially evaporated iron particles. This oxidation
readily occurs upon removal of the samples from the
reactor. A small percentage of the whole measured iron
3
(13.3%) has a lower binding energy (~707.9 eV) which
corresponds to Fe-N binding. There is hence some
probability that atomic iron has effectively been
implemented on the surface of the GNFs through nitrogen
coordination, a requirement for catalytically active sites
distributed at the atomic level.
Fig. 4. XPS survey scan of N/Fe-GNFs.
Fig. 5. High resolution and deconvolution of iron peak of
the N/Fe-GNFs.
4
Conclusion
The modeling results showed that a sufficient amount
of iron vapour is available in the graphene nucleation
zone within the reactor. Equilibrium thermodynamic
evaluations indicate the synthesis of graphene and their
functionalization with iron can be made in the same onestep process. Experimentally, a fraction of the iron vapour
has been implemented on GNFs through Fe-N bonds.
Further work needs to be done to improve the evaporation
of the iron particles, and increase the amount of iron able
to bond to GNFs. For the non-noble metal catalyst
applications, measurements of the activity and stability
over the time need to be performed on this N/Fe-GNF
structure.
Acknowledgements
We acknowledge Dr. J. Lefebvre (Ecole Polytechnique
de Montreal) for her help on the XPS analysis. We
acknowledge the funding contributions from the McGill
Engineering Doctoral Award (MEDA), the Fonds de
Recherche Nature et Technologie du Quebec (FRNTQ)
and from Natural Science and Engineering Research
Council (NSERC) of Canada.
References
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267-279, 2010.
[2] R. Pristavita et al., Plasma Chem Plasma Process, 31:
393-403, 2011.
[3] D. Binny et al., IEEE Nano, 2012.
[4] P. Proulx et al., Plasma Chem. Plasma Proc. Vol. 7,
29-53, 1987.
[5] L. Talbot et al., J. Fluid. Mech. Vol. 101 (4), p. 737758, 1980.
[6] J.-L. Meunier et al., Plasma Chem Plasma Process
34 : 505-521, 2014.
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