Conversion of NO to N2 in continuous microwave discharge

Article abstract of DOI:10.1246/cl.2000.916

The direct conversion of NO to N₂ by a continuous microwave stable discharge at atmospheric pressure is reported, and the conversion of NO to N₂ is up to 60% or more in the presence of excess O₂ or O₂ and H₂O. The effect of the power of microwave on the conversion of NO is observed, and novel results are obtained.

Full text of DOI:10.1246/cl.2000.916

916
Chemistry Letters 2000
Conversion of NO to N2 in Continuous Microwave Discharge
Junwang Tang, Tao Zhang,* Dongbai Liang, Xiaoying Sun, and Liwu Lin
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
PO Box 110, Dalian 116023, P. R. China
(Received May 1, 2000; CL-000419)
The direct conversion of NO to N2 by a continuous
microwave stable discharge at atmospheric pressure is reported,
and the conversion of NO to N2 is up to 60% or more in the
presence of excess O2 or O2 and H2O. The effect of the power
of microwave on the conversion of NO is observed, and novel
results are obtained.
NOx pollution has become one of the most serious environ-
mental problems, and many research groups are attracted to this
topic. Although considerable progress has been achieved in the
reduction of NOx emissions, the release of NOx into the atmos-
phere from the combustion of fossil fuels in both stationary and
mobile sources continues to be a major environmental problem.
Among the many abatement processes developed, one of them
is the study on the removal of NOx from combustion gases
using a pulsed streamer corona discharge (PSCD), which uti-
lizes a high-voltage electrical discharge.1–4 Using the PSCD
method, in the presence of excess O2, NO was mainly convert-
ed to NO2,2,5 or NO3,4,6 then the NOx was subsequently
absorbed by NH3 or other additives.2 The problems existing in
this process are high energy consumption and secondary pollu-
tion. In order to adapt to the higher requirement of environ-
mental protection in the future, new methods of reducing NO
have been explored. In the paper, it probably is the first time to
report the new method, which is the direct conversion of NO to
N2 by a continuous microwave stable discharge (CMD) at
atmospheric pressure without the need of any additives.
to NO2 increased. The total conversion of NO also decreased,
which can be attributed to the effect of excess O2. In group 3,
namely the wet gas, which consisted of NO (2000 ppm), O2
(2%), H2O (5%), and He, compared with group 2, the conver-
sion of NO to N2 decreased a little, but that to NO2 increased
remarkably. About 60% of NO was converted to N2, and the
total conversion of NO was higher than 80%. This suggested
that the addition of H2O promoted the conversion of NO to
NO2, but did not lower the production of N2 in the CMD.
The microwave reaction system consisted of a microwave
generator, a rectangular waveguide, a circulator, a resonant cav-
ity and a plunger. The microwave energy was supplied by a
200 W, 2.45 GHz microwave generator and the effective
microwave power for the experiments ranged from 5 to 60 W.
For the experiments, a special reactor was used that enabled the
CMD to be carried out at atmospheric pressure. A quartz tube
of 8mm i. d. was aligned vertically at the center of the single
mode resonant cavity, so that the discharge region seated in the
microwave field of maximum intensity. In this paper, the resi-
dence time of the flowing gas in the CMD region was about 1
second, which is much shorter than that reported by other
papers,1 The concentration of NO was also much higher, up to
2000 ppm. The gas composition (NO, NO2, N2 and so on) was
determined by an on-line NOx-analyser (FGA 4000/4005) and
Gas Chromatograph (GC-8800, 13X and PQ columns).
Table 1 shows the conversion of NO under different condi-
tions, in which the gas flow-rate is invariable, namely, 60
mL/min. In group 1, where there were only NO (2000 ppm)
and the balance gas (He), the conversion of NO to N2 was up to
88.03%, and the total conversion of NO was 96.1%. In group
2, namely the dry gas, which consisted of O2 (2%), NO (2000
ppm) and He, the conversion of NO to N2 decreased, while that
In order to study the effect of the power of the CMD on the
conversion of NO, another experiment was made under the con-
dition of group 2 at atmospheric pressure, and the results are
shown in Figure 1. The total conversion of NO increased
slightly with the increasing of the power, while the conversion
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
917
of NO to N2 increased greatly, and that to NO2 decreased
remarkably. When the power was increased further, all conver-
sions were leveled to the constant values. This suggests that
increasing the power of the CMD is beneficial for the conver-
sion of NO to N2.
The authors thank Prof. Can Li, director of State Key
Laboratory of Catalysis, Dalian Institute of Chemical Physics,
for helpful discussions.
References
Although the reaction mechanism in the CMD is still not
clear, it is speculated as follows. The difference of energy
between the electrons and the heavy ions is large, and the aver-
age temperature of the reacting gas is comparatively low in the
CMD.7 At the same time, the life time of the electrons are long
in the CMD.7 By these electrons with high energy and long life
time, it is prompted the decomposing of NO to a number of
active N radicals. The formation of the N radicals can induce,
and sometimes accelerate the reaction between N and N, or N
and NO, which is easier in lower average temperature of the
reacting gas,1,3,8 so the conversion of NO to N2 can be very
high in the CMD, especially when there is no O2 present. It is
interesting to note that the decomposition of NO to N2 is not
weakened with the presence of H2O. On the other hand, NO2
formation is strengthened by the existence of H2O.
1
2
3
4
5
6
G. Sathiamoorthy, S. Kalyana, W. C. Finney, and B. R.
Locke, Ind. Eng. Chem. Res., 38, 1844 (1999).
Y. S. Mok and I. S. Nam, J. Chem. Eng. Jpn., 31, 391
(1998).
A. Harano, Y. Imaizumi, and M. Sadakata, J. Chem. Eng.
Jpn., 31, 694 (1998).
K. Takaki, M. A. Jani, and T. Fujiwara, IEEE Trans.
Plasma Sci., 27, 1137 (1999).
H. Miessner, R. Rudolph, and K. P. Francke, Chem.
Commun., 1998, 2725.
K. Urashima, J. S. Chang, J. Y. Park, D. C. Lee, A.
Chakrabarti, and T. Ito, IEEE Trans. Ind. Appl., 34, 934
(1998).
H. Zhao, in “Plasma Chemistry and Processing,” ed. by H.
Zhao, USTC (University of Science and Technology of
China) Press, Hefei (1993), pp. 181–186.
H. Namba, Y. Aoki, O. Tokunaga, R. Suzuki, and S. Aoki,
Chem. Lett., 1988,1465.
7
8
So far as we know, this work represents the first observa-
tion of the conversion of NO in the CMD at atmospheric pres-
sure. The present finding also reveals the difference between
the CMD and other discharge methods.4–6