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Transcript of 3P10-48
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Chemistry of Methane-Nitrogen in a dielectric barrier discharge at
atmospheric pressure
G. Scarduelli1, P. Franceschi1, G. Guella1, G. Dilecce2,3, S. De Benedictis3 and P. Tosi1
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1P Department of Physics, University of Trento, via Sommarive 14, 38050 Povo, Trento, Italy
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2P Istituto di Fotonica e Nanotecnologie - CNR, sede di Trento, via alla Cascata 56/C, 38050 Povo, Trento, Italy
3Istituto di Metodologie Inorganiche e dei Plasmi - CNR, sede di Bari, via Orabona 4, 70126 Bari, Italy
Methane is the principal component of natural gas, coal-bed methane and associated gas. The
valorization of methane into higher hydrocarbons and oxygenates has been desired for several
decades. Moreover the direct conversion of methane into more valuable chemicals and versatile
fuels constitutes an important task. In this contribution we present a preliminary investigation of
the liquid deposit obtained treating a methane-nitrogen mixture in a dielectric barrier discharge
working at atmospheric pressure. The deposit was analyzed by means of various techniques
(Nuclear Magnetic Resonance, Fourier Transform Infrared Spectroscopy and Matrix-Assisted
Laser Desorption/Ionisation -Time Of Flight Mass Spectrometry), in order to explain its polymeric
nature. Nevertheless its composition is at present not completely identified.
1. Introduction
Methane is the main component of gaseous/solid
fossil fuel resources and constitutes one of the
largest organic carbon reserves.
The direct conversion of CH4 into more valuable
chemicals is very attractive [1, 2]. However, there is
still no practical technology for such conversion [3].
Both thermal and nonthermal plasmas have been
widely used for CH4 conversion. However
nonthermal plasmas are the choice for chemical
synthesis of higher hydrocarbons and oxygenatessince the electron temperature is high (up to 105 K),
while the gas remains at room temperature.
Most of products produced using corona,
microwave and radio frequency discharges are small
molecules, like ethylene, acetylene, hydrogen and
carbon monoxide. Products can be more complex
under the condition of dielectric barrier discharge
including light hydrocarbons, liquid fuels, alcohols
and acids [4]. The distribution of products depends
on the different parameters, such as dielectric
material, length, distance of discharge gap andelectrode configuration.
In this contribution we present a preliminary
investigation of CH4-N2 mixtures treated in a
dielectric barrier discharge (DBD) working at
atmospheric pressure. In particular, we have
observed the deposition of a viscous liquid polymer.
2. Experimental
2. 1. Dielectric Barrier Discharge
The methane-nitrogen mixtures are treated in a
dielectric barrier discharge (DBD) working at
atmospheric pressure. Different amounts of CH4
were admitted in the plasma reactor and mixed with
N2.
Our DBD consists of a quartz tube ( = 10 mm)
surrounded by a metal outer electrode (a spiral
electrode). The internal electrode is a metallic rod
embedded in a glass tube ( = 4 mm). The length of
the plasma system is about 5 cm. Electrodes are
powered at a frequency variable between 6 kHz and
15 kHz by a variable high voltage transformer. The
energy transferred to the plasma is evaluated by
measuring both the applied voltage (with an HVprobe) and the charge (with a HV capacitor in series
with the discharge) by the standard Manleys
charge-voltage plot method [5]. The average power
transmitted is ~10 W working at 16 kV with a
frequency of 10 kHz. The reactor is vacuum sealed
to allow control of the gas sample under treatment.
2. 2. NMR
Nuclear Magnetic Resonance (NMR) spectra were
taken at 298.2 K by an Avance 400 Bruker
spectrometer operating at 400.13 MHz for1
H andequipped with a 5 mm inverse broad band (BBI)
probe. Data were collected with XWINNMR
(version 1.3) and processed using both Bruker
TOPSPIN (version 1.3) and MestReC data
processing software.
2. 3. FT-IR
Fourier Transform Infrared (FT-IR) Spectra were
acquired by a spectrometer FT-IR EQUINOX 55
Bruker Optics capable of 0.2 cm-1 maximum
resolution, equipped with a DTGS detector and a
Kbr beam splitter. Acquisitions have been
performed with a resolution of 4 cm-1 by a Silver
28 ICPIG, July 15-20, 2007, Prague, Czech Republicth Topic number: 10
1001
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Gate Reflection Attenuated Total Reflection (ATR)
System.
2. 4. MALDI-TOF MS
Matrix-Assisted Laser Desorption/Ionisation -
Time Of Flight (MALDI-TOF) spectra were takenby an Ultraflex II (Bruker Daltonics) spectrometer
operating in reflectron mode with the following
experimental parameters: delay time 0 ns, IS1 25 V,
IS2 21.65 V, IS lens 10.5 V, laser energy (N 2, 337
nm) 72.3 J.
3. Preliminary results
During DBD plasma treatment we have observed
the formation of a liquid deposit on the surface of
the plasma reactor. This material has been dissolved
in a suitable solvent for NMR, FTIR and MALDI-
TOF analysis.MALDI-TOF spectra (Fig. 1) of the deposit
show, in the 200-1500 m/z region, the typical peak
distribution of a polymeric compound. The repeated
clusters of peaks present a constant mass difference,
that is evaluated in 14 Da, as shown in Fig. 1. This
value corresponds to a CH2 group.
The polymeric nature of the deposit is confirmed
also by broad signals in the 1H-NMR spectrum of
the liquid deposit taken in acetone-d6, as shown in
Fig. 2.
In order to obtain further structural informationon the chemical functions present in the deposit, FT-
IR measurements were also carried out. IR spectrum
(Fig. 3) indicates the presence of CN groups
possibly due to nitrogen containing impurities, and
confirms the nature of polymer-like branched chain
structure.
Fig.1: MALDI-TOF spectra of the deposit obtained by
DBD plasma on methane/N2 reacting system: in (a) we
report the spectrum of the deposit dissolved in acetone,
while in (b) we present the spectrum of the solution
diluted 10 times.
Fig.2: 1H-NMR spectrum of the liquid deposit taken in
acetone-d6.
Fig.3: FT-IR spectrum of the deposit obtained by DBD
plasma on methane/N2 reacting system compared to FT-IR
spectrum of the deposit obtained by DBD plasma on
methane/Ar reacting system.
Observations of the time evolution of the
CN(B2+X2+) violet system emission during the
DBD plasma treatment of a CH4-N2 mixture are
presented in [6].
4. References
[1] J.H. Lunsford, Catal. Today 63 (2000) 165.[2] J.M. Fox, Catal. Rev., Sci. Eng. 35 (1993)
169.
[3] Y. Xu, X. Bao and L. Lin, J. Catal. 216
(2003) 386.
[4] Y. Zhang, Y. Li, Y. Wang, C. Liu and B.
Eliasson, Fuel Process. Technol. 83 (2003) 101.
[5] T. Manley, Trans. Electrochem. Soc. 84
(1945) 83.
[6] G. Scarduelli, P. Franceschi, G. Dilecce, S.
De Benedictis and P. Tosi, ICPIG 2007, this book.
28 ICPIG, July 15-20, 2007, Prague, Czech Republicth
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