Superior Fe-ZSM-5 catalyst for selective catalytic reduction of nitric oxide by ammonia

Full text of DOI:10.1021/ja9842262

J. Am. Chem. Soc. 1999, 121, 5595-5596
5595
Superior Fe-ZSM-5 Catalyst for Selective Catalytic
Reduction of Nitric Oxide by Ammonia
Richard Q. Long and Ralph T. Yang*
Department of Chemical Engineering
UniVersity of Michigan
Ann Arbor, Michigan 48109-2136
ReceiVed December 7, 1998
Nitrogen oxides in the exhaust gases from combustion of fossil
fuels remain a major source for air pollution and acid rain. The
current technology for reducing NOx (NO + NO2) emissions from
power plants is selective catalytic reduction (SCR) with ammonia
in the presence of oxygen. For the SCR reaction, V2O5 + WO3
(or MoO3) supported on TiO2 are the commercial catalysts.1 The
mechanism of the reaction on the vanadia catalysts has been
studied extensively, and several different mechanisms have been
proposed.1,2 Ion-exchanged zeolite catalysts have also been
studied, e.g., Fe-Y,3 Cu-ZSM-5,4 and Fe-ZSM-5,5 but the
reported activities were lower than that of the commercial vanadia
catalysts. The SCR technology based on vanadia catalysts is being
used in Europe and Japan and is being quickly adopted in the
US. However, problems associated with vanadia catalysts remain,
e.g., high activity for oxidation of SO2 to SO3, toxicity of vanadia,
and formation of N2O at high temperature. Hence, there are
continuing efforts in developing new catalysts.1 In this paper, we
report a superior Fe-ZSM-5 catalyst that is much more active
than the commercial vanadia catalysts and does not have the
deficiencies that are associated with the vanadia catalysts.
The Fe-ZSM-5 catalysts were prepared by using specific ion-
exchange procedures, and the iron in the catalysts was present in
the form of Fe3+ ions.6 The catalytic activity experiments were
performed at 1 atm with a conventional fixed-bed flow reactor.8
Figure 1. Catalytic activities for NO reduction by ammonia on Fe-ZSM-5
and 4.4%V2O5+8.2%WO3/TiO2 catalysts. Reaction conditions: 50 mg
(0.065 mL) catalyst, 1000 ppm NO, 1000 ppm NH3, 2% O2, balance He,
and GHSV ) 4.6 × 105 1/h (ambient conditions). * denotes addition of
500 ppm SO2 and 5% water vapor. The numbers in the parentheses
following Fe indicate % ion exchanges.
The results on % NO conversion with different catalysts are
compared directly with a commercial-type catalyst8 in Figure 1.
Surprisingly high NO conversions were obtained on the Fe-ZSM-5
catalysts, with a broad temperature window (375-600 °C) under
a high gas hourly space velocity (GHSV) of 4.6 × 105 1/h (Figure
1). At 450-500 °C, NO conversions reached nearly 100%. It
appears that the activities were little influenced by the Fe content
between 1.14 and 3.57 wt % in Fe-ZSM-5. The addition of a
small amount (0.054 wt %) of cerium further increased the
activities (Figure 1). In comparison, the commercial 4.4%V2O5
+ 8.2%WO3/TiO2 catalyst8 showed substantially lower activities
in NO reduction under the same conditions, and the NO
conversions decreased sharply when the temperature was above
450 °C (Figure 1) due to oxidation of ammonia by oxygen.1
However, when the amount of the commercial catalyst was
increased from 50 to 800 mg while keeping all other conditions
identical, the maximum NO conversion was increased from 64.0%
at 400 °C to 99.8% at 375 °C. This indicates that Fe-ZSM-5
catalysts were more than 16 times as active as the commercial
catalyst. In addition, on the Fe-ZSM-5 catalysts, no N2O was
detected in the entire temperature range of 300-600 °C, with
only N2 and H2O as the reaction products. With the (V2O5 +
WO3)/TiO2 catalyst, about 5% of N2O yield was observed in the
reaction at 375 °C, but N2O yield would be decreased when the
feed stream contained water vapor.
(1) (a) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369-532. (b) Busca,
G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal. B 1998, 18, 1-36.
(2) (a) Miyamoto, A.; Kobayashi, K.; Inomata, M.; Murakami, Y. J. Phys.
Chem. 1982, 86, 2945-2950. (b) Janssen, F. J. J. G.; van den Kerkhof, F. M.
G.; Bosch, H.; Ross, J. R. H. ibid. 1987, 91, 5921-5927. (c) Odriozola, J.
A.; Heinemann, H.; Somorjai, G. A.; Garcia de la Banda, J. E.; Pereira, P. J.
Catal 1989, 119, 71-82. (d) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P.
Appl. Catal. 1990, 64, 259-278. (e) Schramlmarth, M.; Wokaun, A.; Baiker,
A. J. Catal. 1990, 124, 86-96. (f) Chen, J. P.; Yang, R. T. ibid. 1990, 124,
411-421. (g) Went, G. T.; Leu, L.-J.; Rosin, R. R.; Bell, A. T. ibid. 1992,
134, 492-505. (h) Ozkan, U. S.; Cai, Y.; Kumthekar, M. W. ibid. 1994, 149,
390-403. (i) Odenbrand, C. U. I.; Bahamonde, A.; Avila, P.; Blanco, J. Appl.
Catal. B 1994, 5, 117-131. (j) Topsøe, N.-Y.; Dumesic, J. A.; Topsøe, H. J.
Catal. 1995, 151, 241-252.
(3) Amiridis, M. D.; Puglisi, F.; Dumesic, J. A.; Millman, W. S.; Topsøe,
N.-Y. J. Catal. 1993, 142, 572-584.
(4) Komatsu, T.; Nunokawa, M.; Moon, S.; Takahara, T.; Namba, S.;
Yashima, T. ibid. 1994, 148, 427-437.
(5) Komatsu, T.; Uddin, M. A.; Yashima, T. In Zeolite: A Refined Tool
for Designing Catalytic Sites; Bonneviot, L.; Kaliaguine, S., Eds.; Elsevier:
Amsterdam, 1995; pp 437-441.
(6) Fe(58)-ZSM-5 was prepared by exchanging 2 g NH4-ZSM-5 (Si/Al )
10) with 200 mL of 0.05 M FeCl2 solution. The number in the parentheses
following Fe indicates % ion exchange determined by neutron activation
analysis. Ce-Fe(42)-ZSM-5 was obtained from 3 g of NH4-ZSM-5 exchanged
with 200 mL of 0.05 M Ce(NO3)3 solution, followed by exchanging with 200
mL of 0.05 M FeCl2 solution. All of the exchange processes were performed
at room temperature for 1 day. Fe(130)-ZSM-5 was obtained from exchanging
2 g of H-ZSM-5 (Si/Al ) 10) with 0.73 g of iron powder mixed in 200 mL
of 0.1 M HCl solution at 50 °C for 10 days in flowing He. H-ZSM-5 was
prepared by calcining NH4-ZSM-5 at 500 °C for 3 h. The obtained solid
samples were first dried at 120 °C in air for 12 h and then calcined at 600 °C
for 6 h. Fe2+ in the zeolite framework was oxidized to Fe3+.7 The Fe contents
were 1.59% (wt) in Fe(58)-ZSM-5 (i.e., 57.7% ion exchange), 1.14% in Ce-
Fe(42)-ZSM-5 (0.054% for Ce, i.e., 42.3% total ion exchange) and 3.57% in
Fe(130)-ZSM-5 (i.e., 130% ion exchange). Besides the Fe-ZSM-5 catalysts,
(4.4%V2O5 + 8.2%WO3)/TiO2 catalyst was also prepared by incipient wetness
impregnation.8 This catalyst had SCR activity and behavior nearly identical
to that of the commercial SCR catalyst supplied by a major catalyst
manufacture.8
(8) Cheng, L. S.; Yang, R. T.; Chen, N. ibid. 1996, 164, 70-81. The typical
reaction conditions were as follows: 50 mg (0.065 mL) catalyst, 1000 ppm
NO, 1000 ppm NH3, 2% O2, 500 ppm SO2 (when used), 5% water vapor
(when used), and balance He. The total flow rate was 500 mL/min (ambient
conditions). The NOx concentration was continuously monitored by a
chemiluminescent NO/NOx analyzer, and the products N2 and N2O were
analyzed simultaneously by a gas chromatograph.
(7) Feng X.; Hall, W. K. J. Catal. 1997, 166, 368-376.
10.1021/ja9842262 CCC: $18.00 © 1999 American Chemical Society
Published on Web 05/27/1999

5596 J. Am. Chem. Soc., Vol. 121, No. 23, 1999
Communications to the Editor
When H2O and SO2 were added to the reactants, the SCR
activity increased on Ce-Fe(42)-ZSM-5 at temperature above 350
°C, and the temperature window was widened (Figure 1). The
Ce-Fe-ZSM-5 was also stable in the SCR reaction. During a
run of 60 h on stream at 375 °C, under the conditions of 100 mg
sample, 1000 ppm NO, 1000 ppm NH3, 2% O2, 5% H2O, and
500 ppm SO2, NO conversion was only changed from 99.9 to
99.0%. However, the activity on Fe(58)-ZSM-5 decreased from
99.5 to 80.0% under the same conditions. This indicates that
cerium also plays a stabilization role for Fe-ZSM-5. The prepara-
tion of even more stable catalyst is in progress. Activities for
SO2 oxidation were measured by collecting SO3 from the products
as BaSO4 precipitates.8 The Ce-Fe-ZSM-5 showed a much lower
activity for SO2 oxidation to SO3 (0.7 vs 3.8% for (V2O5 + WO3)/
TiO2) at 375 °C under the same reaction conditions.
In another experiment, NH3 was first adsorbed on the Fe-ZSM-5
to form NH4+, and the sample was subsequently exposed to (NO
+
+ O2)/He at 300 °C. The NH4 bands vanished gradually in 5
min. This indicates that NH4+ reacted with NO/NO2 to form N2,
as identified by gas chromatograph. When the Fe-ZSM-5 was
exposed to the reactant mixture (NO + NH3 + O2)/He at 200-
450 °C, only adsorbed NH4+ ions were detected. The above results
taken together indicate the following scheme for NO reduction
+
by NH3 on the Fe-ZSM-5 catalyst. NH3 is first activated to NH4
on Brønsted acid sites of the zeolite and then weakly adsorbed,
and gaseous NO and NO2 (resulting from NO oxidation by O2)
-
+
and NO3 species react with NH4 to form N2 and H2O. The
+
reaction between NOx adspecies and NH4 is much faster than
that between NO and O2.
In conclusion, the Fe-ZSM-5 catalyst shows remarkably high
activities for the NH3 SCR reaction, which is more than 67 times
as active as the Fe-ZSM-5 (Si/Al ) 29) reported by Komatsu et
al.,5 judging by the first-order rate constants. In their work, an
Fe-ZSM-5 catalyst was prepared by exchanging ZSM-5 with
FeCl3 solution and thus resulted in a low Fe content (0.28 wt %)
in the sample. Also, a higher Si/Al ratio in their catalyst led to a
weaker acidity. Both of these would result in a lower activity in
their sample. The high activity of the Fe-ZSM-5 catalyst achieved
in this work may be attributed to its strong Brønsted acidity (thus,
The mechanism of NO reduction by NH3 on the Fe-ZSM-5
catalyst was studied by in situ FT-IR. NH4+ ions (bands at 3351-
2730, 2154, 1944, and 1466 cm-1 9) were observed on the NH3-
adsorbed Fe(58)-ZSM-5 at 100 °C. An increase in temperature
resulted in desorption of NH3 from the sample. However, some
+
NH4 ions still remained on the Fe-ZSM-5 catalyst at 450 °C.
This suggests that strong Brønsted acid sites existed on the sample.
In a separate experiment, when the Fe-ZSM-5 was exposed to
1000 ppm NO and 2% O2 at 200-400 °C, three bands at 1880,
1624, and 1580 cm-1 were detected, which can be assigned to
weakly adsorbed NO, NO2, and NO3- species, respectively.10 This
suggests that NO was oxidized to NO2 by O2. After the sample
was purged with He for 5 min at 400 °C, all of the bands vanished.
+
a high concentration of NH4 ions) and a high activity for
oxidation of NO to NO2. It is known that both of them are
important for the SCR reaction on zeolite-type catalysts.9b
Acknowledgment. We gratefully acknowledge Kent Zammit of EPRI
for discussion and John Armor of Air Products for providing the NH4-
ZSM-5 sample. U.S. and foreign patents on the catalysts are pending.
(9) (a) Teunissen, E. H.; van Santen, R. A.; Jansen, A. P. J.; van Duijneveldt,
F. B. J. Phys. Chem. 1993, 97, 203-210. (b) Eng J.; Bartholomew, C. H. J.
Catal. 1997, 171, 27-44.
(10) Laane J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465-513.
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