Chemical Reaction Studies in CH 4 /Ar and CH 4 /N 2 Gas Mixtures of a Dielectric Barrier Discharge Abhijit Majumdar, Ju 1 rgen F. Behnke, and Rainer Hippler* Institut fu ¨r Physik, Ernst-Moritz-Arndt-UniVersita ¨t Greifswald, Domstrasse 10a, 17489 Greifswald, Germany Konstantin Matyash and Ralf Schneider Max-Planck-Institut fu ¨r Plasmaphysik, EURATOM Association, Wendelsteinstrae 1, 17491 Greifswald, Germany ReceiVed: June 30, 2005; In Final Form: August 22, 2005 Chemical reactions in a dielectric barrier discharge at medium pressure of 250-300 mbar have been studied in CH 4 /Ar and CH 4 /N 2 gas mixtures by means of mass spectrometry. The main reaction scheme is production of H 2 by fragmentation of CH 4 , but also production of higher order hydrocarbon molecules such as C n H m with n up to 9 including formation of different functional CN groups is observed. Formation of C 2 H 2 ,C 2 H 4 , and C 2 H 6 molecules has been investigated in some detail. Significant differences are noted in comparison to a theoretical estimate. I. Introduction Atmospheric pressure dielectric barrier discharges (DBD) are of great interest for application in, e.g., gas chemistry, steriliza- tion, surface activation, and modification or thin film deposi- tion. 1-5 The development of a new process based on this discharge needs a clear understanding of plasma and discharge physics and chemistry. At the present time much attention is paid to the chemical processes in barrier discharge plasma in various gas mixtures, since the understanding of these processes is necessary for the development of industrial reactors. 6-8 We have chosen a gas mixture of CH 4 /N 2 with a gas ratio of 1:2 to investigate physical properties and chemical efficiency of barrier discharges. Nitrogen and methane are abundantly available in the Earth’s and particularly in Titan’s atmosphere and thus play an important role in atmospheric plasma chemistry. 9 Further- more, Ar- and N 2 -containing plasmas are more stable to operate than a pure CH 4 plasma. Hence, a mixture of CH 4 and N 2 or Ar offers stable plasma conditions. Industrial applications of CH 4 /N 2 and CH 4 /Ar gas mixtures under consideration here are production of hydrogen and of higher order hydrocarbon molecules having applications in polymer industries and fun- damental plasma chemistry. Moreover, such composition can result in unexpected changes in the physical properties of the discharge itself and can turn into a strong influence on the formation of plasma chemical products. 10 The aim of this work is to study discharge properties of CH 4 /N 2 and CH 4 /Ar gas mixtures in a high voltage dielectric barrier discharge (DBD) medium, the influence of the plasma on organic gases, and interpretation of the experimental results. II. Experiment Section The experimental set up is shown in Figure 1. The plasma chamber is made of stainless steel. The inner dimensions of the chamber are height 12.3 cm, length 18.0 cm, and width 15.0 cm, yielding a chamber volume of 3.32 dm 3 . The two electrodes are made from Ag plates with a length of 8.3 cm, width 3.3 cm, and thickness 0.15 cm. Both Ag electrodes are covered by dielectrics: the upper (powered) electrode is covered with aluminum oxide (ǫ ≈ 10); the lower (grounded) electrode with a glass plate (ǫ ) 3.8). Both electrodes are separated by 0.15 cm from each other. The upper electrode is connected to a home-built high voltage power supply, while the lower electrode is grounded. The chamber is pumped by a membrane pump down to about 10 mbar. Pressure inside the plasma chamber was controlled by two gas flow controllers for methane and nitrogen and by an adjustable needle valve between the chamber and the membrane pump. The experiments were performed with the chamber filled at a pressure of 250-300 mbar and with pump and gas flow shut off. The high voltage power supply consists of a frequency generator delivering a sinusoidal output that is fed into an audio amplifier. The amplifier can be operated at up to 500 W; its output is fed into a spark plug transformer. Experiments were performed at 10.5 kV (peak-to-peak) and at 5.5 kHz. The electrical power under these conditions was 5 W. It was measured by placing a probe capacitor (10 nF) between the lower electrode and ground and measuring the collected charge together with the applied voltage as function of time, as described by Wagner et al. and Sonnenfeld et al. 5,11 Gas composition of stable reaction products only was detected by a mass spectrometer (Balzers QMS 200). It is pumped by a turbomolecular pump (Pfeiffer TSU 062H) to a base pressure of about 1 × 10 -8 mbar increasing to about 10 -6 mbar during the experiment. A capillary tube of length 103 cm and inner diameter 0.01 cm connects the mass spectrometer with the plasma chamber. A pressure of 10 -2 mbar at the entrance to the mass spectrometer is maintained during the experiments with the help of a second turbomolecular pump (Balzers 071P). Figure 2 shows two typical mass spectra in the range of mass numbers up to m/z ) 140 that were obtained after the chamber has been filled with 250 mbar of a CH 4 /N 2 gas mixture (mixing ratio 1:2). Figure 2a represents the initial gas composition * To whom correspondence should be addressed. E-mail: hippler@ physik.uni-greifswald.de. 9371 J. Phys. Chem. A 2005, 109, 9371-9377 10.1021/jp053588a CCC: $30.25 © 2005 American Chemical Society Published on Web 09/28/2005