22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Methane activation in a microwave plasma reactor
T. Minea1, D.C.M. van den Bekerom1, N. den Harder1, M.F. Graswinckel1, P.W.C. Groen1, W.A. Bongers1,
M.C.M. van de Sanden1, J. van de Loosdrecht2, L. Lefferts3 and G.J. van Rooij1
1
Dutch Institute for Fundamental Energy Research (DIFFER), MaSF Solar Fuels, De Zaale 20, 5612 AJ Eindhoven,
the Netherlands
2
Sasol, Group Technology, 1 Klasie Havenga road, Sasolburg 1947, South Africa
3
University of Twente, MESA+, Faculty of Science and Technology, 7500 AE Enschede, the Netherlands
Abstract: Dissociation of methane into free radicals is studied in a microwave plasma
reactor. Emphasis is on far from equilibrium conditions, where T gas < T vib < T e to optimize
vibrational excitation of methane as the main dissociation channel. Rates of various
reaction mechanisms are studied within a Boltzmann solver in order to estimate the
importance of candidate reaction channels and to optimize radical densities to the formation
of C 2 hydrocarbons and higher. A microwave test reactor was commissioned, and
operating parameters were characterized by FTIR spectroscopy.
Keywords: methane activation, non-equilibrium plasma, FTIR spectroscopy
1. Introduction
The direct conversion of CH 4 into C 2 or higher
hydrocarbons is still high on the industrial wish-list, but
it remains a major scientific and technological challenge.
The relevance of the problem has been very high and is
even continuously increasing, given the fact that natural
gas is becoming more and more important as compared to
mineral oil; the shale-gas revolution recently strengthened
this trend even further. The bottleneck is the dissociation
of the stable CH 4 molecule. In this work we assess the
potential of microwave plasma reactors for this purpose.
The rationale behind the microwave plasma approach is
the potential of realizing high energy efficiencies by
enhancing vibrational excitation and therewith lowering
the energy barrier for dissociation. Indeed, the fractional
power dissipated from the electrons into the molecular
degrees of freedom of CH 4 is close to unity towards the
lower values of the reduced electric field in the plasma, as
shown in Figure 1. However, the average electron energy
in the plasma is not a free parameter but mainly set by the
ionization energy of the discharge gas molecules (and the
ionization loss rate) via the particle balance. In this study
we yield insight in the main plasma and dissociation
dynamics using a Boltzmann solver to take into account
the non-Maxwellian energy distribution of the plasma
electrons in the oscillating microwave field. Furthermore,
we report on first experiments of an in house built test
reactor.
The MW generator supplies up to 1 kW continuous
power at 2.45 GHz operating frequency. A three stub
tuner is used for impedance matching of the waveguide
with the plasma for maximizing the absorbed input
power. At elevated pressures (>100 mbar), a stainless
steel nozzle was inserted for electric field enhancement to
promote breakdown. Input power and methane flow rates
were mutually adjusted to correspond to specific energy
inputs E v (electric power absorbed in the plasma divided
by flow rate) in the range 0.5-3.5 eV/molecule. This
parameter range is thought to be specific for optimal
process stimulation by vibrational excitation [1].
2. Experimental setup
Steady state microwave discharges are generated in 20
and 30 mm inner diameter tubular quartz reactors of 20
cm length, inserted through the wide faces of a
rectangular waveguide in a position that corresponds to a
maximum of the electric field in the center of the tube, as
shown in Figure 2.
The set-up was attached to a Fourier transform infrared
spectrometer (FTIR Varian Cary 670) for in situ
identification of the products in the reactor. The IR beam
is absorbed over the entire path length of 20 cm in the
reactor quartz tube, thus covering the cold gas injection
region, the active plasma zone and the downstream
region.
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Fig. 1. Fractional power dissipated by electrons into
molecular degrees of freedom of methane as function
of the electron energy [2]
1
Fig. 2. Schematic drawing of the experimental set-up.
3. Reaction mechanism
The rate limiting step in formation of higher
hydrocarbons is given by the rate of the first C-H bond
cleavage of the methane molecule [3]. Thus, it is
insightful to put a lower limit on the CH 3 densities that
can be obtained in a MW plasma reactor. We assume that
the main production channel of the methyl radicals is by
electron impact dissociation (other reactions such as
𝐶𝐻4 + 𝐻 → 𝐶𝐻3 + 𝐻2 can become important once
enough 𝐻 is produced and kept within the reactive core) :
(R.1)
𝐶𝐻4 + 𝑒 → 𝐶𝐻3 + 𝐻 + 𝑒
(R.2)
𝐶𝐻4 + 𝑒 → 𝐶𝐻2 + 𝐻2 + 𝑒
(R.3)
𝐶𝐻4 + 𝑒 → 𝐶𝐶 + 𝐻 + 𝐻2 + 𝑒.
For electrons with energies higher than 14 eV direct
decomposition can occur:
(R.4)
𝐶𝐻4 + 𝑒 → 𝐶(𝑠) + 2𝐻2 + 𝑒
to produce solid carbon (soot) and hydrogen. Soot
formation happens also in the absence of higher energetic
electrons when the plasma is in contact with the very hot
quartz tube ( glow temperature >1000 K ). From the
discussion on the reaction mechanism of the nonoxidative conversion of methane, in the work of Liu et
al.[4], it is concluded that the main channel for electron
impact dissociation is given by the reaction R.1.
In a methane plasma with an average electron energy in
the order of few eV it is obviously more favourable to
break C-H bonds that have a binding energy of 4 eV than
to provide the 13 eV required for ionization. Nevertheless,
for increased degrees of ionization of the methane plasma,
i.e. 𝑛𝑒 = 𝑛𝐶𝐶4+ , the dissociative recombination of methane
ions may also become a significant process for methyl
radical production:
(R.5)
𝐶𝐻4+ + 𝑒 → 𝐶𝐻3𝑟,𝑣 + 𝐻 ∗ .
Thus, one can calculate the fluxes of methyl radicals
obtained via electron impact dissociation and dissociative
recombination
(E.1)
Γ𝐶𝐶3 = 𝑛0 𝑛𝑒 𝑉𝑘𝐼𝐼 + 𝑛𝑒2 𝑉𝑘𝐷𝐷
where n 0 and n e are neutral and electron densities, V is
the volume of the reactor and k ID and k DR are electron
impact dissociation and dissociative recombination rates,
respectively. From equation E.1 methyl densities
according to the reactor flow geometry can be obtained.
Figure 3 shows a change in regime for the production of
methyl radicals: at approximately 3 eV the electron
impact dissociation channel becomes important as well
along the dissociation recombination one.
2
In order to achieve high energy efficiency the main
pathway for radical formation would be via vibrationally
activated methane. These vibrationally excited molecules
not only have a lower dissociation energy, but also an
increased reactivity with catalytic surfaces. Then, the
coupling to higher hydrocarbons would go via radical
chemistry and/or dehydrogenation steps of the already
formed molecules that are favoured by the strong VT nonequilibrium conditions.
Fig. 3. Back of an envelope estimation of CH 3
densities produced in a MW plasma reactor, with
n 0 =1023 m-3 and an ionization degree of 105
corresponding up to 1 kW input power.
In the remaining of this section, simple particle and
power balance allows us to estimate the operational
plasma parameter space versus the input power. The
average electron energy in a discharge is set by the
requirement that on average an electron should be able to
replace itself by an ionization event before it escapes to
the wall or it recombines with a positively charged heavy
particle in the volume. In other words: the production by
ionization must be equal to the losses, or, the electron
temperature is determined by the particle balance. The
electron density is determined by the requirement that the
number of collisions between electrons and heavy
particles is sufficient to transfer the power injected into
the discharge. In other words, the electron density is
determined by the power balance.
Let us assume that diffusion to the wall and volume
recombination are the main ionization losses.
Furthermore, we evaluate only ionization and vibrational
excitation in the power balance of the electrons. With
these simplifications, we estimate the particle and power
balance and evaluate plasma density and temperature as a
function of the input power. Figure 4 shows the results for
a cylindrical plasma column with a radius of 2 cm and a
gas density of 1023 m-3. It is seen that in order to maintain
ionization, a minimum average electron energy of ~4.5
eV is required at input powers smaller than 1 kW, i.e.
relevant for the present work.
The importance of assessing the average electron
energy with a Boltzmann solver, i.e. taking into account
the non-Maxwellian nature of the electron energy
distribution function, is also illustrated in Figure 4 by
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including the electron temperature behaviour for an ideal
Maxwellian distribution, where the average electron
3
energy is taken as 𝑇𝑒 . The latter gives approximately a
2
factor of three difference as compared to the Boltzmann
solver results [2].
Furthermore, it is observed that increasing input power
also increases the electron energy. This is caused by the
density dependence of the dissociative recombination
term, as shown in Figure 3 . This rate is only slowly
varying with the electron temperature. For stimulating the
vibrational channel and avoiding transferring the input
power towards excitation processes and gas heating, a
plasma with mean energy up to 3 eV would be most
favourable, as shown in Figure 4.
pentad regions of methane with and without plasma are
shown in Figure 5. The contribution of the neutral gas is
subtracted from the plasma spectra as background. Higher
gas temperatures can be concluded from the number of
rotational peaks in the plasma together with formation of
broad absorption peaks at around 1370 cm-1 and 3010 cm1
that are attributed to cluster formation. Due to the
measurements along the z axis of the quartz cylinder, at
least three different regions of absorption are present: cold
injected gas, plasma region and the hot plasma after
glow–gas mix region. This overlapping parts of
absorption do not allow for a more clear plasma effect
observation, especially when most of the absorption is
due to the cold gas region. In addition, in the overview
FTIR spectra none of the C 2 hydrocarbons were seen at
this point. Quantification of gas temperatures and rovibrational excitation is currently ongoing and will be
discussed.
Fig. 4. Electron density and average kinetic energy
as function of power input in the microwave plasma
reactor.
4. Experimental Results
In a first series of experiments, we established the
parameter regime with minimal soot formation in order to
achieve stable plasma discharges. Table 1 shows a
parameter space in which soot was formed. Temperatures
in excess of 500 oC were measured within the immediate
vicinity of the waveguide cavity, in a close region where
soot was forming and plasma was still present, via a
thermocouple attached to the outside of the quartz tube.
The exact mechanism of soot formation was not studied
within the scope of the present study. A detailed
investigation can be found in the work of Heintze et al.[5]
for the case of a pulsed microwave reactor.
Table 1. Characteristic parameter space of the methane plasma discharge
Pressure
(mbar)
Energy input
(eV/mol.)
Soot formation
Setup
1
1.4-3.5
Yes (outside cavity)
20 mm tube
1
3.8
Yes (inside cavity)
20 mm tube
15
0.5
Yes (inside cavity)
30 mm tube+nozzle
54
0.5
Yes (outside cavity)
30 mm tube+nozzle
In the parameter space where soot was not visually
observed, FT-IR spectra with a resolution of 0.1 cm-1 and
128 averaged scans, have been taken of both neutral gas
and plasma for comparison and product identification. A
typical FT-IR absorption spectra of the (a) dyad and (b)
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Fig. 5. Example of an absorbance spectrum. The blue line
FTIR absorption spectra of the (a) dyad and (b) pentad
regions of methane with and without plasma. The plasma
spectra are y-shifted by 0.2. The reactor pressure was
0.75 mbar at an energy input of E v =1.3 eV/mol that
corresponds to 200 W microwave power and 2 slm flow.
5. Conclusions
Methane discharges running in a microwave plasma
reactor have been studied both numerically and
experimentally via in situ absorption spectroscopy.
Electron energy distribution function calculations result in
average elecctron energy of 4-5 eV, significantly higher
than estimated on basis of Maxwellian distribution. Soot
formation is encountered at pressures of tens of mbar
and/or at higher energy input of few eV/mol. and limits
3
the discharge duration. FT-IR measurements are ongoing
for
species
determination
and
gas
heating
characterization.
6. References
[1] A. Fridman, Cambridge university press (2008)
[2] G.J.M Hagelaar, L.C. Pitchford, Plasma Sci Sources
and Tech 14 (2005).
[3] M. Capitelli, E. Molinari, Topics in Current chemistry,
Volume 90 (1980).
[4] S.Y. Liu, et al. The Journal of Physical Chemistry
C (2014).
[5] M. Heintze, et al. Journal of applied physics (2002).
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