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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

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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.

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