Microwave discharge-assisted catalytic conversion of NO to N2
Junwang Tang, Tao Zhang,* Dongbai Liang, Changhai Xu, Xiaoying Sun and Liwu Lin
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
116023, China. E-mail: taozhang@ms.dicp.ac.cn
Received (in Cambridge, UK) 2nd May 2000, Accepted 14th August 2000
First published as an Advance Article on the web 15th September 2000
By coupling microwave discharge with an Fe/HZSM-5
catalyst, novel effects have been observed for the conversion
of NO to N2 in the presence of excess oxygen with high
efficiency.
reduction of NO with CH4 in excess O2 in the CRM. When CH4
was wholly consumed, the conversion of NO to N2 did not
exceed 5%, which is almost in accordance with the results of
Chen et al.9 The results indicate that the activity of the Fe/
HZSM-5 catalyst for CH4–NO reaction is very low in the
presence of excess O2 in the CRM.
Results obtained over Fe/HZSM-5 in the CAMD process are
presented in Table 2. It can be seen from these results that the
conversion of NO to N2 is increased to 44.5% in the presence of
excess O2, while at the same time the CH4 consumption is
50.5%.
In order to confirm the enhancement of the catalytic activity
by the microwave discharge, another microwave discharge
experiment without any catalyst was conducted and the results
are also given in Table 2. In this experiment, the average
conversion of NO to N2 was 18.2%. It can be noted that even the
sum of the conversion of NO to N2 in the microwave discharge
mode without any catalyst and that in the CRM were still lower
than that of NO to N2 via CAMD.
From the above results it can be seen that, regardless of
temperature, the Fe/HZSM-5 catalyst is inactive in the CRM for
the CH4–NO reaction (Table 1). However, when in the
microwave discharge mode the conversion of NO to N2 on Fe/
HZSM-5 increases from ca. 5 to 44%. Apparently the catalytic
activity of the Fe/HZSM-5 is enhanced remarkably by the
microwave discharge. This increase in catalytic activity cannot
be readily explained by an increase in reaction temperature,
suggesting that microwave discharge can cause non-Arrhenius
effects.
As we know, in the CRM all reactants gain energy by thermal
conduction. In fact, the reaction of CH4 with O2 proceeds more
rapidly than the reaction of CH4 with NO at high temperatures
so that a large portion of CH4 is consumed by O2, resulting in
the restraint of the reaction rate for CH4 with NO. Therefore, the
conversion of NO to N2 in the presence of excess O2 is both very
low and independent of temperature in the CRM. On the other
hand, in the microwave discharge mode a high frequency
electromagnetic field is formed easily in the reactor, and since
the molecules in the gas phase are transparent to the electro-
magnetic field no interaction will take place between the
electromagnetic field and the gaseous molecules. Our tem-
perature-programmed desorption (TPD) experiments on Fe/
HZSM-5 indicated that supported Fe2O3 is a good material for
the adsorption of NOx, which agrees well with the report of Otto
and Shelef.10 Moreover, it is known that the high frequency
The reduction of NO has been investigated for many years.
Especially in the past decade, it has attracted much attention.1
Recently, with the progress of advanced techniques, many
papers have described the application of new methods in
different branches of chemistry.2 Among these techniques,
microwave dielectric heating is a convenient and effective way
of bringing about chemical reactions in the field of catalysis.3
The use of microwave heating to stimulate catalytic reactions
has provided some remarkable results,4–6 Bond et al. and Wan
and co-workers have investigated the effect of microwave
heating on many catalytic reactions.7 The reduction of NO by
CH4 by a microwave heating technique has also been reported
by this laboratory.8 Here, we report on a new process for the
removal of NO by coupling the microwave discharge (not
microwave heating) with the selective catalytic reduction (SCR)
reaction by CH4. We designate this kind of process as catalysis
assisted by microwave discharge (CAMD).
A Fe/HZSM-5 catalyst was employed to demonstrate the
effect of the CAMD of NO–CH4. The Fe/HZSM-5 was
prepared by impregnating HZSM-5 (SiO2/Al2O3 = 25) in an
aqueous Fe(NO3)3 solution (Fe in the catalyst was 10 wt%); all
catalysts prepared were of 1.25–1.60 mm granule size. The flow
rate of the feed, which consisted of 2000 ppm NO, 1600 ppm
CH4 and 2.0% O2 (helium as a balance gas), was 60 ml min21
(GHSV = 3600 h21).
A special quartz tubular reactor (i.d. 10 mm) was aligned
vertically at the center of the microwave cavity. The catalyst bed
was supported on a fused quartz frit of medium porosity. The
gas composition was determined by an on-line NOx-analyser
and gas chromatograph (GC-8800 type, with 13X and PQ
columns).
In this work, Fe/HZSM-5 was used not only as a catalyst, but
also as a discharge igniter, by which discharge was achieved
without any electrode at atmospheric pressure: after the
microwaves were induced in the reactor, red discharge with
weak sounds was observed among the catalyst granules.
Results obtained over the Fe/HZSM-5 in a conventional
reaction mode (CRM) are presented in Table 1. From these data,
it is apparent that the Fe/HZSM-5 was almost inactive for the
Table 1 Activity of Fe/HZSM-5 catalyst in the conventional reaction
modea
Table 2 Performance of the system with Fe/HZSM-5 catalyst and without
a catalyst in the microwave discharge modea
T/°C
NO conversion (%)
CH4 conversion (%)
Microwave
power/W
NO conversion CH4 conversion
300
400
500
600
650
700
740
0.7
4.5
5.0
4.4
3.0
3.3
2.1
2.0
4.5
7.3
Catalyst
(%)
(%)
Fe/HZSM-5
43–54
54–65
65–76
54–65
65–76
40.0
42.0
44.5
18.0
18.4
37.1
47.3
50.5
70.2
74.1
9.0
27.3
81.3
100.0
No catalyst
a Reaction conditions: 2000 ppm NO, 1600 ppm CH4, 2% O2, balance He;
total flow rate 60 ml min21 (GHSV = 3600 h21).
a Reaction conditions: 2000 ppm NO, 1600 ppm CH4, 2% O2, balance He;
total flow rate 60 ml min21 (GHSV = 3600 h21).
DOI: 10.1039/b003499l
Chem. Commun., 2000, 1861–1862
This journal is © The Royal Society of Chemistry 2000
1861
Tang
