Selective catalytic reduction of NO by ammonia over Fe-ZSM-5 catalysts

Article abstract of DOI:10.1039/a807490i

In the selective catalytic reduction (SCR) of NO by ammonia, over-exchanged Fe-ZSM-5 prepared by sublimation of FeCl₃ into H-ZSM-5, shows superior catalytic activity and stability in a wide temperature range; its activity is promoted by the presence of water in the feed while SO₂ is a weak poison at low but a promoter at high temperatures; its remarkable durability towards H₂O and SO₂ makes this zeolite catalyst a potential choice in Denox applications for stationary sources and heavy Diesel engines with ammonia or urea reductants.

Full text of DOI:10.1039/a807490i

Selective catalytic reduction of NO by ammonia over Fe-ZSM-5 catalysts
Ai-Zeng Ma† and W. Grünert*
Lehrstuhl für Technische Chemie, Ruhr-Universität Bochum, D-44 780 Bochum, Germany.
E-mail: w.gruenert@techem.ruhr-uni-bochum.de
Received (in Bath, UK) 21st September 1998, Accepted 9th November 1998
In the selective catalytic reduction (SCR) of NO by
ammonia, over-exchanged Fe-ZSM-5 prepared by sublima-
tion of FeCl
and stability in a wide temperature range; its activity is
promoted by the presence of water in the feed while SO is a
weak poison at low but a promoter at high temperatures; its
remarkable durability towards H O and SO makes this
3
into H-ZSM-5, shows superior catalytic activity
2
2
2
zeolite catalyst a potential choice in Denox applications for
stationary sources and heavy Diesel engines with ammonia
or urea reductants.
In the selective catalytic reduction (SCR) of nitrogen oxides,
metal-exchanged MFI-type zeolites, in particular Cu-ZSM-5,
have received much attention due to their ability to catalyze the
Fig. 1 Selective catalytic reduction of NO by NH
ZSM-5 and comparison with over-exchanged Cu-ZSM-5. Dry condition:
3
with over-exchanged Fe-
SCR not only by hydrocarbons,1 but also by ammonia. The
insufficient stability of these catalysts has so far prevented their
practical application in flue-gas abatement devices. Faced with
the tail-gases of combustion engines containing high amounts of
–3
4
21
1
000 ppm NO, 1000 ppm NH
Fe-ZSM-5-14-300, (2, 5) Cu-ZSM-5-14-220, filled symbols, NO conver-
sion, open symbols, NH conversion.
3 2
, 2% O , balance He; 304 000 h , (:, Ω)
3
2
water, and often also SO , they suffer structural changes leading
to deactivation. For the use of hydrocarbon reductants, water is
also known as a reversible poison while it has been reported to
promote the SCR activity with the reductant ammonia.
3
Fig. 1 shows the conversions of NO and NH at different
4
reaction temperatures over Fe-ZSM-5-14-300 and Cu-ZSM-
5-14-220. Cu-ZSM-5 is very active for the SCR with ammonia,
and the conversions obtained with our preparation are at all
temperatures well above those reported in the literature for a
In view of these problems, the recent discovery of the
remarkable stability of over-exchanged Fe-ZSM-5 in the SCR
5
of NO with isobutane has been considered a breakthrough in
4
flue-gas catalysis research. Further progress has to cope with a
somewhat lower space velocity. Fe-ZSM-5-14-300 provides
6
reproducibility problem in the preparation of these materials
the same conversions as Cu-ZSM-5-14-220 proving inferior
only in the low-temperature region (@573 K). Over this
catalyst, the temperature window for high NO conversion is
significantly wider than over the Cu catalyst: NO conversions
> 75% can be held between 573 and 823 K. The ammonia
conversion is close to the NO conversion below 423 K, but is
100% at all temperatures above, which makes ammonia slip
rather unlikely with such catalysts. Remarkably, the only
and with the fact that the reductant isobutane that provides the
highest reaction rates with these catalysts is impractical for any
commercial purpose. We report here a superior catalytic
performance of over-exchanged Fe-ZSM-5 in the SCR of
nitrogen oxides with ammonia. The catalyst provides excellent
activity and selectivity over a wide temperature range. It is
promoted by water and, in certain temperature ranges, also by
SO
2
(with or without water) and does not suffer any deactiva-
2 2
oxidation product is nitrogen, neither NO nor N O formation
tion from these feed components on a time scale of 10–15 h.
The Fe-ZSM-5 was prepared on the basis of a Na-ZSM-5 (Si/
Al ≈ 14, Chemiewerk Bad Köstritz, Germany). Iron was
introduced by a modified sublimation technique derived from a
were detected in the whole temperature range. With Cu-ZSM-
5-14-220, ammonia oxidation becomes significant already at
523 K, and since the NO conversion decreases steadily with
increasing temperature, the copper catalyst is inferior in the
ammonia utilization at almost all temperatures.
3
route described in ref. 7 (sublimation of FeCl vapour into the
H-form of the zeolite under vacuum). Subsequently, the catalyst
was calcined in air at 823 K without any washing step.
Elemental analysis showed that this preparation provides an Fe/
Al ratio of 1 : 1, which is a formal exchange degree of 300%.
The sample will be, therefore, denoted as Fe-ZSM-5-14-300
indicating the Si/Al ratio and the formal exchange degree by the
last two figures. A Cu-ZSM-5-14-220 prepared by the conven-
8
tional ion-exchange method described by Iwamoto et al. was
also obtained from the same Na-ZSM-5 for use as a reference
material.
The SCR of NO by ammonia was carried out in a
microcatalytic flow reactor. A calibrated mass spectrometer and
a combined non-dispersive IR/UV detector were employed to
analyse the reaction product. The feed gas contained 1000 ppm
NO, 1000 ppm NH
.5% H O and/or 200 ppm SO
Under standard reaction conditions, 260 ml min
mixture were fed over 35 mg of catalyst, which results in a
3
, and 2% O
2
(balance He) and was loaded by
2
2
2
in the poisoning experiments.
Fig. 2 Influence of water and sulfur dioxide on the NO conversion over Fe-
ZSM-5-14-300. Space velocity, 304 000 h , dry condition: 1000 ppm NO,
2
1
of this
21
1000 ppm NH , 2% O , balance He. Moist condition: as dry, with 2.5% H O
3
2
2
2
1
space velocity of 304 000 h
.
2 2
added, ‘+SO ’, with 200 ppm SO added.
Chem. Commun., 1999, 71–72
71

aqueous exchange with subsequent precipitation provided NO
conversions below 50%, with considerable N O formation at
lower temperatures, at a space velocity of only 180 000 h
2
2
1
.
Notably, in the SCR of NO by isobutane, our Fe-ZSM-5-14-300
sample could not compete with Cu-ZSM-5-14-220, and the NO
conversions achieved were much lower than those reported by
9
5
Feng and Hall. In addition, the air calcination step in the
catalyst preparation (vide supra), which proved deleterious for
9
6
the SCR with isobutane in accordance with the literature, had
a slightly deactivating effect only in the low-temperature range
9
but did not did not affect the peak conversion with ammonia.
This implies that the active sites required by the two reactions
may be different.
In conclusion, over-exchanged Fe-ZSM-5 prepared by a
sublimation technique shows highly promising catalytic per-
formance in the SCR of NO by ammonia. Studies concerning
the structure of the active sites in this catalyst and about its
Fig. 3 Durability of Fe-ZSM-5-14-300 under the influence of water and
SO : dry condition; moist condition; moist condition + SO as in Fig. 2.
2
2
The impact of H
2 2
O and/or SO on the NO conversion over
Fe-ZSM-5-14-300 is shown in Fig. 2. It is evident that SO
2
2
behaviour with respect to the oxidation of SO are under way.
We acknowledge financial support from the Research
Institute of Petroleum Processing, Beijing, China
exerts a poisoning influence at low temperature both in dry and
in moist feed. Remarkably, this effect is reversed at higher
temperature: Above 700 K, the NO conversion is always higher
2
with SO present. On the other hand, water promotes the
Note and references
† On leave from Research Institute of Petroleum Processing, Beijing
100083, China.
activity at all temperatures over our Fe-ZSM-5, which is similar
to the behaviour found with Cu-ZSM-5.4 This is most
favourable at the upper end of the temperature range where an
,9
2
1
NO conversion of 85% (at ca. 300 000 h ) may be held up to
a reaction temperature of 850 K in a moist feed stream.
The durability of Fe-ZSM-5-14-300 in various feed mixtures
was tested at 840 K (Fig. 3). Under dry conditions, a slight
increase of the NO conversion (by ca. 3% in 15 h) can be noted.
1
2
W. Held, A. König, T. Richter and L. Puppe, SAE-Pap., 1990,
00496.
M. Iwamoto, H. Yahiro, S. Shundo, Y. Yu-u and N. Mizuno, Shokubai,
990, 32, 430.
9
1
3
4
M. Shelef, Chem. Rev., 1995, 95, 209.
J. A. Sullivan, J. Cunningham, M. A. Morris and K. Keneavey, Appl.
Catal. B, 1995, 7, 137.
Upon addition of 2.5% H
the NO conversion of ca. 10% is achieved. Addition of SO
2
O to the feed, a sudden increment in
into
2
the wet feed does not lead to further changes. No deactivation of
the catalyst is observed either in 15 h time-on-stream in moist
5 X. Feng and W. K. Hall, Catal. Lett., 1996, 41, 45.
6 W. K. Hall, X. Feng, J. Dumesic and R. Watwe, Catal. Lett., 1998, 52,
1
3.
2 2
feed or under the simultaneous action of H O and SO . It should
7
8
H.-Y. Chen and W. M. H. Sachtler, Catal. Today, 1998, 42, 73.
M. Iwamoto, H. Yahiro, Y. Mine and S. Kagawa, Chem. Lett., 1989,
be noted that such conditions would cause severe deactivation
with the Cu-ZSM-5 system.3
2
13.
The preparation of the Fe-ZSM-5 catalyst is critical also for
its application to the SCR with ammonia. An Fe-ZSM-5
catalysts obtained by conventional aqueous exchange, which
resulted in a formal exchange degree of < 100%, was inferior to
9
A.-Z. Ma, F. Heinrich and W. Grünert, to be published.
1
0 A. V. Salker, B. Maurer and W. Weisweiler, Chem.-Ing.-Technol.,
998, 70, 566.
1
9
10
Fe-ZSM-5-14-300. In a recent paper, Fe-ZSM-5 prepared by
Communication 8/07490I
72
Chem. Commun., 1999, 71–72