22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
FT-ICR analysis of toluene removal by non-thermal plasmas
S. Pasquiers1, M. Heninger2, B. Bournonville1, N. Blin-Simiand1, J. Lemaire2, F. Jorand1 and H. Mestdagh2
1
Laboratoire de Physique des Gaz et des Plasmas, CNRS, University of Paris-Sud, FR-91405 Orsay, France
2
Laboratoire de Chimie Physique, CNRS, University of Paris-Sud, FR-91405 Orsay, France
Abstract: The removal of low concentrations of toluene (100 ppm) diluted in dry air was
studied for pulsed dielectric and electro-ceramic barrier discharges. Proton transfer
reaction associated with FTICR mass spectrometry (PTR-FTICR) was used to follow in
real time the concentration of the pollutant at the exit of the discharge reactors, and to
identify by-products coming from the decomposition of the molecule.
Keywords: FT-ICR mass spectrometry, dielectric barrier discharge, toluene
1. Introduction
Researches on the removal of VOCs diluted in
atmospheric gases using non-plasmas are still going on, in
particular through coupling of barrier discharges with
oxidation catalysts [1]. Optimization of the plasma
catalysis coupling can be improved if all by-products
coming from the discharge are known. In this way, the
catalyst can be correctly chosen to perform complete
oxidation of the VOC and its by-products at minimum
expense of energy deposited in the plasma. In the present
work, we used proton transfer reaction associated with
FTICR mass spectrometry (PTR-FTICR) [2] in order to
follow, in real time, the concentration of toluene and its
main by-products at the exit of two different plasma
reactors, without catalysis.
2. Experimental set-up
The first reactor was a dielectric barrier discharge
(DBD) operated in a cylindrical coaxial geometry, the
plasma being created at 1 bar and room temperature in an
alumina tube (i.d. 20 mm, 3 mm thick). The discharge
volume was about 16 cm3. A high voltage pulse was
applied on a central stainless steel threaded rod (2 mm
diameter), the grounded counter-electrode (copper, 5 cm
length) being wrapped around the tube. The plasma
volume of the second reactor was identical, but we used a
tube (i.d. 20.5 mm, 2.5 mm thick) made of a ceramic
matrix composite material from Streamer™ [3]. This
material allowed a significant decrease of the applied
voltage to the discharge reactor [4, 5]. Both the electroceramic barrier discharge (EBD) and the DBD were
driven by HV-pulses produced by the same power supply,
at a repetition frequency up to 1250 Hz.
Voltage and discharge current were monitored using
adapted electrical probes connected to a digital
oscilloscope, from which the energy deposited in the
discharge was deduced. Toluene was either vaporised
through an air flow by making it stream along a rod
located at the centre of a heated tube using of a syringe
pump, or it came from a calibrated air-toluene mixture
(108 ppm, Crystal mixture by Air Liquide); the first
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set-up allowed to produce a slowly varying-in-time
concentration of the pollutant molecule (100 ppm max.).
The FTICR compact mass spectrometer BTrap [6] was
used to detect molecules in the late afterglow at the exit of
reactors. Moreover, ozone concentration was measured
by UV-absorption at 254 nm.
Mass spectrometry is a versatile and sensitive tool for
on line detection of VOCs, provided it is associated with a
soft and selective ionization technique such as controlled
chemical ionization. This term means that the sample
interacts with a unique, well-identified precursor ion
under well-defined pressure and time conditions so that
quantitative information is obtained. Proton transfer
reaction with H 3 O+ as precursor has been used in this
work. Since it is a soft ionization technique resulting in
little or no fragmentation, identification is made directly
from the mass of the observed ions and can benefit from
high mass resolution techniques leading to the molecular
formula of the ions. The gas flow from the exit of
reactors was sampled through a capillary tube followed by
a three-way pulsed valve generating the controlled gas
pulses sent to the mass spectrometer. For each pulse the
mass spectrum was recorded after PTR and the
concentrations of the detected VOCs were derived. The
analyses were performed with a minimum sampling rate
of 3 seconds.
3. Toluene removal by the DBD reactor
As a typical example, in Fig. 1 are plotted toluene and
ozone concentrations measured for the DBD reactor, for
an applied voltage pulse of 26.9 kV at 1250 Hz.
The DBD was working for about 1 hour. In such
conditions the energy release in the discharge volume lead
to an increase of the gas temperature, and the deposited
energy per pulse continuously increased from 3.5 mJ (at
the very beginning of the experiment, time 24 min in
Fig. 1) up to 11.8 mJ (at the end). As a consequence the
ozone concentration continuously decreased, and it is
interesting to note that it is also the case for the toluene
concentration.
1
and for a HV-pulse of 16.6 kV at 500 Hz. The ozone
concentration is also given, divided by 10. For these
conditions, the energy per pulse was low and it increased
very slightly during time from 3.35 mJ up to 3.50 mJ, so
that there was no increase of the gas temperature which
could affect the ozone concentration.
Fig. 1. Toluene and ozone concentrations at the exit of
the DBD reactor.
Initial pollutant concentration:
108 ppm. Other parameters: 26.9 kV, 1250 Hz, 1 l/min
NTP.
Formaldehyde was detected as a major by-product
coming from the decomposition of toluene. Fig. 2
presents the CH 2 O concentration (in arbitrary unit)
measured simultaneously to the toluene concentration for
4 different values of the HV-pulse frequency (given on
the diagram), all other parameters being equal with
respect to Fig. 1; the discharge was each time switched
off before changing the frequency value.
Fig. 2. Formaldehyde concentration at the exit of the
DBD reactor. Same parameters as those of Fig. 1, except
the HV-pulse frequency.
For an increasing frequency from 500 up to 1000 Hz,
the maximum formaldehyde concentration increased, but
this maximum is lower at 1250 Hz with respect to
1000 Hz. Moreover, the concentration decreased during
time for 1250 Hz, unlike for the other values. Thus
CH 2 O was also efficiently decomposed in such
conditions.
4. Toluene removal by the EBD reactor
In Fig.3 is plotted the toluene concentration at the exit
of the electro-ceramic barrier discharge for a slowly
time-varying inlet concentration, between 20 and 50 ppm,
2
Fig. 3. Toluene and ozone concentrations at the exit of
the EBD reactor for a varying initial pollutant
concentration. 16.15 kV, 500 Hz, 2 l/min NTP.
The effect of the discharge is to decrease the toluene
concentration by a percentage depending on the inlet
value: 56.8% at 44 ppm, 67.0% at 37 ppm, 77.8% at
27 ppm.
Moreover, higher is the inlet toluene
concentration, lower is the ozone one. This can be
interpreted as a consequence of the oxygen atoms
consumption by the pollutant molecule and its byproducts, in particular formaldehyde.
Fig. 3. emphasizes the interest in the FT-ICR real time
monitoring of molecule concentrations for the
understanding of VOC removal by non-thermal plasmas.
5. Acknowledgments
The authors thank the French agency ADEME for
financial support (Grant No 11-81-C0090), and are
grateful to the company AlyXan (J. Leprovost) for the use
of the BTrap device in the course of the SPECPLAS
project, with financial assistance from CNRS (MI –
Instrumentation aux limites).
6. References
[1] A. Vandenbroucke, R. Morent, N. De Geyter and C.
Leys. J. Hazard. Mat., 195, 30 (2011)
[2] A. Chiper, N. Blin-Simiand, M. Heninger,
H. Mestdagh, P. Boissel, F. Jorand, J. Lemaire,
J. Leprovost, S. Pasquiers, G. Popa and C. Postel.
J. Phys. Chem. A, 114, 397 (2010)
[3] B. Drazenovic. Semiconductor Ceramic. Patent
WO2010015789A3 (2010)
[4] H. Nizard, P. Jeanney, S. Bentaleb, B. Drazenovic,
P. Tardiveau and S. Pasquiers. IEEE Trans. Plasma
Sci., 39, 2188 (2011)
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[5]
[6]
S. Pasquiers, B. Drazenovic, N. Blin-Simiand,
B. Bournonville, P. Jeanney, F. Jorand and
Y. Jorand.
in: ESCAMPIG XXI (Greisfwald,
Germany) (July 2014)
M. Heninger, L. Clochard, H. Mestdagh, P. Boissel
and J. Lemaire. Spectra Anal., 248, 44 (2006)
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