Plasmacatalytic low-temperature conversion of NO(x) to N2 by ammonium-loaded zeolites in a dielectric barrier discharge

Article abstract of DOI:10.1039/a807546h

The direct application of a silent electrical discharge on an ammonium-loaded zeolite as catalyst to remove NO in excess oxygen results in a synergetic improvement of NO(x) abatement at temperatures below 373 K.

Full text of DOI:10.1039/a807546h

Plasmacatalytic low-temperature conversion of NO to N by
x
2
ammonium-loaded zeolites in a dielectric barrier discharge
a
a
b
H. Miessner,* R. Rudolph and K.-P. Francke
a
b
Institut für Umwelttechnologien GmbH and Gesellschaft zur Förderung der naturwissenschaftlich-technischen
Forschung e.V., Rudower Chaussee 5, D-12489 Berlin, Germany. E-mail: info@iut-berlin.com
Received (in Cambridge, UK) 28th September 1998, Accepted 13th November 1998
The direct application of a silent electrical discharge on an
ammonium-loaded zeolite as catalyst to remove NO in
concentration (the NO is converted to NO
2
vide supra), the
behaviour in the presence of NH -mordenite is just the opposite:
4
excess oxygen results in a synergetic improvement of NO
abatement at temperatures below 373 K.
x
The conversion strongly increases with the oxygen content.
3
This is in line with the proposal of the intermediate oxidation
2
of NO to NO as the rate limiting step in the reaction
The selective catalytic reduction (SCR) of NO using the
addition of reductants like NH , urea, hydrocarbons, alcohols,
etc. has been studied intensively as a potential method to
mechanism. The electric discharge obviously promotes this
intermediate NO-oxidation, resulting in an enhanced overall
reaction rate.
To analyse the influence of the electric discharge in more
detail, the local position of the catalyst was varied in order to
have the discharge in front of, behind and directly on the
catalyst bed. Fig. 3 shows the effect of these combinations on
3
1
remove NO
x
from exhaust gases with excess oxygen. Depend-
ing on the type of catalysts and the reductant used, SCR is
effective in a temperature range between 450 and 900 K.
In a recent publication, Richter et al.2 have described a
,3
+
x 2 4
catalytic low-temperature conversion of NO to N using NH
ions fixed in zeolites as reductant. NO is converted in these
systems to N in excess oxygen even at temperatures as low as
73 K. The intermediate oxidation of NO to NO is believed to
2
3
2
be the key step limiting the reaction rate of the reaction
sequence. As we will show here, the abatement of NO in oxygen
excess can be significantly enhanced further by applying a silent
electrical discharge immediately on the ammonium-loaded
zeolite as catalysts.
For the experiments a reactor was used that enables a
dielectric barrier discharge (DBD) directly on the catalyst bed.
A glass tube as dielectric is surrounded by a copper grid as
ground electrode and contains an inner electrode, which
consists of a rod with equidistant steel discs leaving a gap to the
glass tube of 0.5 mm. Between these discs the catalyst can be
positioned. As catalyst, a mordenite with a Si:Al ratio of 11 was
used in the ammonium form. The mordenite has been shown to
x
be effective in the low temperature conversion of NO , but also
other zeolites (e.g. Y, ZSM-5) could be used.2 The electric
,3
discharge was initiated by a high-voltage pulse generator with
2
1
0 kV peak voltage, 20 ms rise time and a repetition rate up to
x 2 2
Fig. 1 NO concentration after conversion of 500 ppm NO in N –O (5
15 Hz. The gas composition (NO, NO , N O, HNO ) was
2
2
x
vol%) with (- : NO, 8 : NO ) and without (5 : NO, 2 : NO ) NH -
2
2
4
2
1
determined by an FTIR spectrometer equipped with a long-path
20 m) gas cell (Perkin Elmer) and a NO -analyser (ECO-
mordenite as catalyst at 343 K and GSHV = 3000 h
.
(
x
Physics). The energy deposited into the discharge was deter-
mined by integrating the voltage–charge traces monitored with
a digitising oscilloscope (Tektronics TDS 520 C).
Fig. 1 shows the dependence of the conversion of 500 ppm
NO in N
without the catalyst. Without the NH
removed by reactions with the active species from the electric
discharge, but at the same time NO is formed and the overall
NO concentration remains nearly constant. Using the catalyst
2
–O
2
(5 vol%) on the applied energy both with and
4
-mordenite, NO is partly
2
x
alone, part of the NO is removed already without an electric
discharge. This is due to the reaction described by Richter et al.3
and also by adsorption phenomena at 343 K. At 373 K ca. 400
ppm and at 423 K 450 ppm NO remain in the gas phase without
electric discharge. Applying additionally the DBD, a complete
conversion of NO without the formation of a significant amount
2
of NO was observed. For comparison it should be noted that
2
3
the energy input of 20 W h m is equivalent to an adiabatic
temperature increase of ca. 70 K.
The conversion of NO to N
concentration in the gas. Fig. 2 shows this dependence again
with and without NH -mordenite as catalyst. Whereas the
conversion without catalyst declines with increasing oxygen
2
strongly depends on the oxygen
at DBD (25 W h m23) without
2
Fig. 2 Conversion of 500 ppm NO into N
(5) and with (-) NH -mordenite as catalyst at 343 K and GSHV = 3000
4
4
2
1
h
.
Chem. Commun., 1998, 2725–2726
2725

any catalyst, NO removal is limited because of back-reaction (2)
and only a part of NO can be converted to NO (Fig. 3, columns
and 3). In the presence of NH -zeolite, however, NO is
removed from the gas stream by the fast catalytic reaction (3),
2
1
4
2
3
thus preventing the undesirable back conversion (2) of NO
NO.
2
to
NO + O + M ? NO
NO
NO + NO
2
+ M
(1)
(2)
(3)
2
+ O ? NO + O
2
2
+ 2 NH
3
? 2N
2
+ 3H
2
O
The ammonia bonded to the zeolite is consumed during the
reaction with NO , leaving the zeolite in the H-form. To
maintain the catalytic activity, the zeolite has to be reloaded
from time to time with gaseous NH or by ion exchange. Initial
x
3
experiments have demonstrated that this can be done without
loss of activity.
The authors thank M. Richter and R. Eckelt (Berlin) for
Fig. 3 NO
vol%): DBD, without catalyst (1); with NH
behind the catalyst (3); DBD in front of the catalyst (4); DBD over the
x
concentration after conversion of 500 ppm NO in N
2 2
–O (2
4
-mordenite, no DBD (2); DBD
4
helpful discussions and for supplying the sample of the NH -
2
1
catalyst (in situ) (5). Conditions at the catalyst: 343 K, GSHV = 3000 h
,
mordenite. Prof. Dr. J. Leonhardt (Berlin) is acknowledged for
his encouragement of the work and for useful discussions. The
Bundesministerium für Bildung, Wissenschaft, Forschung und
Technologie (BMBF) is acknowledged for financial support
(13N7178).
DBD: ca. 25 W h m2 , NO: hatched, NO
3
: black.
2
the NO
catalyst (column 3) converts a part of the remaining NO to NO
as should be expected (cf. column 2). If the DBD is applied in
front of the catalyst, the NO formed in the electric discharge
cf. column 1) is converted by the catalyst to nitrogen (column
x
concentration. The application of the DBD behind the
2
2
Notes and references
(
4
). Finally, the in situ application of the discharge directly on the
1 For recent reviews, see for example: R. Burch, Pure Appl. Chem., 1996,
68, 377 or A. Fritz and V. Pitchon, Appl. Catal. B: Environ., 1997, 13,
catalyst bed results in a synergetic increase of the NO
conversion to a degree much higher than the sum of the
individual effects (column 5).
This synergy can be explained by the interaction of
discharge-induced plasmachemical reactions with the catalysis
on the zeolite. A silent electrical discharge in an oxygen-
containing gas mixture produces mainly oxygen radicals as
1
.
2
3
4
M. Richter, B. Parlitz, R. Eckelt and R. Fricke, Chem. Commun., 1997,
83.
M. Richter, R. Eckelt, B. Parlitz and R. Fricke, Appl. Catal. B: Environ.,
998, 15, 129.
B. Eliasson, M. Hirth and U. Kogelschatz, J. Phys. D: Appl. Phys., 1987,
0, 1421.
3
1
2
4
active species. Atomic oxygen leads to fast oxidation by the
three body reaction (1). In case of the discharge alone without
Communication 8/07546H
2726
Chem. Commun., 1998, 2725–2726