Decomposition of N2O in a microwave-absorbent assisted discharge of N2 at atmospheric pressure

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Abstract

The decomposition of N2O has been studied in a microwave-absorbent assisted discharge of N2 at atmospheric pressure. Although discharge could not be maintained without using microwave absorbents, stable discharge could be maintained by using two rods type of Zr or W absorbents. The maximum conversion of N2O was ≥97% at a microwave power of 150–200 W and a distance of 10 mm between the rods. The decomposition mechanisms of N2O in the gas phase and on the hot surfaces of absorbents were discussed on the basis of mass spectroscopic data of products and optical spectroscopic analysis of discharge spectra. The best conversion of O atom in N2O into O2 was obtained by using Zr rods at the longest distance of 10 mm between the rods and the highest high N2 flow rate of 1000 sccm.

Introduction

Nitrous oxide (N2O) in the Earth’s atmosphere, about 12% of which arises from combustion processes, is a major contributor to the destruction of the ozone layer and the greenhouse effect due to its long residence time of about 150 years and its relatively large energy absorption capacity per molecule [1]. Removal of N2O has recently been studied using such catalysts as Rh/ZrO2 [2] and also by a microwave discharge in N2O/N2 mixtures at 5–350 Torr (1Torr=133.3 Pa) [3]. We have recently studied the decomposition of N2O by a microwave discharge in N2O/He and N2O/Ar mixtures [4], [5]. Although discharge could be maintained only in the low total-pressure range of 1–80 Torr in N2O/He mixtures, stable microwave discharge could be maintained in the wide total-pressure range of 1–760 Torr in N2O/Ar mixtures at a microwave power of 200 W. The decomposition efficiency of N2O and the formation ratios of products in N2O/Ar mixtures at 760 Torr were measured as a function of the microwave power or the N2O flow ratio in the N2O concentration range of 0.99–4.8% (v/v). A high decomposition efficiency of ≥94% was obtained in N2O/Ar mixtures at 760 Torr.

Our previous studies on N2O abatement by using a microwave discharge have been carried out in such rare gases as Ar or He, because stable discharge can be easily maintained in rare gases. However, the use of expensive rare gas makes the practical application of this N2O removal technique difficult. For the practical application of microwave discharge to a new N2O removal technique, N2O removal methods in N2 must be developed at atmospheric pressure. A disadvantage of microwave discharge is that the generation of stable discharge of N2 is generally difficult at atmospheric pressure at microwave power below 200 W. Wójtowicz et al. [3] studied the decomposition of N2O by a microwave discharge in N2O/N2 mixtures. However, the highest total pressure of N2O/N2 mixtures used in their experiments was 236 Torr at 300 W, which was far below the atmospheric pressure.

In order to ignite and maintain stable microwave discharge at low power under microwave irradiation, various types of microwave absorbents, which emit slow secondary electrons, have been used. It was reported that such microwave absorbents as carbon fibers, Co3O4, SiC, and La0.8Sr0.2CoO3 were effective to initiate and maintain stable discharge at atmospheric pressure [6], [7], [8], [9]. We have recently used one or two pieces of graphite, Zr, or W rods as microwave absorbents in order to start and maintain stable microwave discharge of N2 [10]. These absorbents were found to be useful for the decomposition of NO by a microwave discharge in N2/NO mixtures.

In the present study, N2O removal in N2O/N2 mixtures is studied at atmospheric pressure by using microwave-absorbent assisted discharge under low microwave power below 200 W. The N2O conversion and the formation ratios of products were measured as a function of the distance between the two rods or the N2 flow rate. Emission spectra from discharges of pure N2 or N2O/N2 mixture, generated by using Zr or W rods, were observed in order to obtain information on major reactions in the discharge region. The decomposition mechanism of N2O was discussed by combining mass spectroscopic and optical spectroscopic data with known gas kinetic data.

Section snippets

Experimental

Fig. 1 shows the reaction chamber used for the decomposition of N2O by a microwave discharge in N2O/N2 mixtures. It consists of a quartz discharge tube, where two pieces of rod-form absorbents were placed in the middle of the tube. An Fe rod placed upstream of the absorbent was used to change the distance between two rods using an external magnet. Quartz wool was placed downstream in order to prevent metal-oxide powders entering into the pumping system. The decomposition chamber was

N2O removal by using two pieces of Zr or W rods

Microwave discharge in N2 at a microwave power of 200 W could be maintained only at total pressure below 50 Torr without using microwave absorbents. This upper limit was lower than 80 Torr in He and 760 Torr in Ar [5]. We have recently found that graphite, Zr, and W rods were effective for the generation of microwave discharge in N2 at atmospheric pressure [10]. Therefore, these materials were used as microwave absorbents. In general, microwave discharge becomes unstable when a small amount of such

Conclusion

In conclusion, N2O removal was studied in a microwave-absorbent assisted discharge of N2 at atmospheric pressure. Stable discharges could be maintained by using two pieces of microwave absorbents (W or Zr). The conversion of N2O was ≥97% at a distance of 10 mm between the absorbents and a microwave power of 200 W. It was found that conversion of O atom in N2O to O2 by using Zr was higher than that by using W due to slower surface reactions leading to ZrO2. It was thus concluded that Zr rods were

Acknowledgements

The authors acknowledge financial support from a Grant-in-Aid for Scientific Research No. 13558056 and 15310059 from the Ministry of Education, Science, Sports and Culture, and from Kyushu University Interdisciplinary Programs in Education and Projects in Research Development (2001–2002).

References (29)

  • S. Tanaka et al.

    Catal. Today

    (2000)
  • M.A. Wójtowicz et al.

    J. Hazardous Mater.

    (2000)
  • K. Kiyokawa et al.

    Surf. Coat. Technol.

    (1999)
  • M. Tsuji et al.

    Surf. Coat. Technol.

    (2003)
  • J. Lemaire et al.

    Chem. Phys. Lett.

    (1988)
  • R.E. Dickinson et al.

    Nature

    (1986)
  • M. Tsuji et al.

    Jpn. J. Appl. Phys.

    (2000)
  • M. Tsuji et al.

    Jpn. J. Appl. Phys.

    (2001)
  • Y. Shimizu et al.

    J. Electrochem. Soc.

    (1999)
  • H.S. Roh, Y.K. Park, S.E. Park, Chem. Lett. (2000)...
  • M. Tsuji, K. Nakano, J. Kumagae, T. Tsuji, S.H. Yoon, Y. Korai, I. Mochida, Chem. Lett. (2002)...
  • M. Tsuji et al.

    Bull. Chem. Soc. Jpn.

    (2002)
  • F. Kaufman, in: M.J. Cormier, D.M. Hercules, J. Lee (Eds.), Chemiluminescence and Bioluminescence, Plenum Press, New...
  • H. Okabe, Photochemisty of Small Molecules, Wiley, New York,...
  • Cited by (0)

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