Low temperature selective catalytic reduction (SCR) of NO with NH3 over Fe-Mn based catalysts

Article abstract of DOI:10.1039/b111382h

Fe-Mn based transition metal oxides (Fe-Mn, Fe-Mn-Zr and Fe-Mn-Ti) show nearly 100% NO conversion at 100-180°C for selective catalytic reduction of NO with NH₃ under the applied conditions with a space velocity of 15 000 h^-l.

Full text of DOI:10.1039/b111382h

Low temperature selective catalytic reduction (SCR) of NO with NH3
over Fe–Mn based catalysts
Richard Q. Long,a Ralph T. Yanga and Ramsay Changb
a Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA.
E-mail: yang@umich.edu
b Air Pollution Control, Power Generation, Electric Power Research Institute, Palo Alto, CA 94303-0813,
USA
Received (in Cambridge, UK) 13th December 2001, Accepted 21st January 2002
First published as an Advance Article on the web 4th February 2002
Fe–Mn based transition metal oxides (Fe–Mn, Fe–Mn–Zr
and Fe–Mn–Ti) show nearly 100% NO conversion at
100–180 °C for selective catalytic reduction of NO with NH3
under the applied conditions with a space velocity of 15 000
h21 (ambient conditions). The premixed gases (1.01% NO/He,
1.00% NH3/He and 0.99% SO2/He) were supplied by Mathe-
son. Water vapor was generated by passing He through a gas-
wash bottle containing deionized water. The tubings of the
reactor system were heat-traced to prevent formation and
deposition of ammonium sulfate/bisulfate and ammonium
nitrate. The NO and NO2 concentrations were continually
monitored by a chemiluminescent NO/NOx analyzer (Model
42C, Thermo Environmental Instruments Inc.). To avoid errors
caused by the oxidation of ammonia in the converter of the NO/
NOx analyzer, an ammonia trap containing phosphoric acid
solution was installed before the sample inlet to the chem-
iluminescent analyzer. The products were analyzed by a gas
chromatograph (Shimadzu, 14A) at 50 °C with 5A molecular
sieve column for N2 and Porapak Q column for N2O. All the
data were obtained after 60–200 min when the SCR reaction
reached a steady state. The SCR activities of Fe–Mn, Fe–Mn–Zr
and Fe–Mn–Ti oxides at 80–180 °C are shown in Fig. 1. These
mixed oxides showed excellent activities at low temperatures.
At 80 °C, 68–74% NO conversions were obtained at a space
velocity of 15 000 h21. With increasing temperature, NO
conversion increased significantly and reached nearly 100% at
above 120 °C. The products were N2 and H2O only and N2O
formation was not observed by GC. However, on pure MnOx,
we observed 8–35% N2O selectivity (with 72–93% NO
conversion) at 80–180 °C under the same reaction conditions.
The SCR activity at low temperatures decreased in the
following order: Fe–Mn > Fe–Mn–Zr > Fe–Mn–Ti. As
mentioned above, the Fe–Mn catalyst has a much lower surface
area than Fe–Mn–Zr and Fe–Mn–Ti oxides. Hence it seems that
the SCR activity does not have a correlation with the surface
area.
h21
.
Nitrogen oxides (NO, NO2 and N2O) have been a major source
of air pollution. They contribute to photochemical smog, acid
rain, ozone depletion and the greenhouse effect. An efficient
technology for removing NO from power plants is selective
catalytic reduction (SCR) with ammonia (4 NO + 4 NH3 + O2 =
4 N2 + 6 H2O). The industrial operations are carried out on
V2O5 + WO3(MoO3)/TiO2 catalysts at 350–400 °C.1–3 How-
ever, the high concentration of ash (e.g., K2O, CaO and As2O3)
in the flue gas reduces their performance and longevity,
although they are resistant to SO2.2,4 To solve this problem, one
attractive option is to place the SCR unit downstream of the
desulfurizer and electrostatic precipitators where SO2 and ash
have been removed. Since the temperature in the downstream is
typically below 200 °C, this makes it necessary to develop low
temperature SCR catalysts to avoid reheating of the flue gas and
thus decrease the capital cost.
Some transition metal containing catalysts have been investi-
gated for the low temperature SCR reaction, such as amorphous
chromia,5 NiSO4/Al2O3,6 MnOx/Al2O3,7 V2O5/activated car-
bon8 and MnOx/TiO2.9 They showed various SCR activities at
below 200 °C under different conditions. It is known that
ammonium nitrite decomposes quickly into N2 (majority) and
NO (minority) below 100 °C.10 The formation of ammonium
nitrite from NO, O2 and NH3 requires oxidation of NO to NO2
by O2. In our previous work,11,12 we have shown that an
increase in NO oxidation to NO2 on Fe-ZSM-5 will result in a
significant improvement of the SCR activity and, moreover, the
reaction rate of NH3 with NO2 + NO is much higher than that
with NO. Therefore, it is expected that a high activity for NO
conversion to NO2 at low temperatures may facilitate a high
SCR activity because it can enhance the formation of ammo-
nium nitrite. More recently, we found that Fe–Mn based
transition metal oxides were highly efficient sorbents for NO
removal.13 In particular, these sorbents showed very high
activity for NO oxidation to NO2 at room temperature; 63–76%
NO conversions were obtained on Fe–Mn, Fe–Mn–Zr and Fe–
Mn–Ti oxides under the conditions of 500 ppm NO, 10% O2
and GHSV (gas hourly space velocity) = 6000 h21. Therefore,
we investigated these mixed oxides as low temperature SCR
catalysts in this work.
The Fe–Mn based transition metal oxides (Fe–Mn, Fe–Mn–
Zr and Fe–Mn–Ti, with equal mol of metals) were prepared by
the coprecipitation method as described in detail previously.13
The BET surface areas were 54, 178 and 183 m2 g21
,
respectively, for Fe–Mn, Fe–Mn–Zr and Fe–Mn–Ti oxides. The
SCR activity measurements were carried out in a fixed-bed
quartz reactor. The reaction conditions were as follows: 0.5 g
sample (0.4 ml), 1000 ppm NO, 1000 ppm NH3, 2% O2, 2.5%
water vapor (when used), 37.5 or 1000 ppm SO2 (when used),
balance He, 100 ml min21 total flow rate and GHSV = 15 000
Fig. 1 SCR activities on the Fe–Mn based transition metal oxides in the
absence of H2O. Reaction conditions: 0.5 g catalyst, [NO] = [NH3] = 1000
ppm, [O2] = 2%, He = balance, total flow rate = 100 ml min21 and GHSV
= 15 000 h21
.
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Fig. 3 SCR activities on the Fe–Mn based transition metal oxides in the
presence of SO2 + H2O. Reaction conditions: 0.5 g catalyst, [NO] = [NH3]
= 1000 ppm, [O2] = 2%, [SO2] = 37.5 ppm and [H2O] = 2.5% (when
used), He = balance, total flow rate = 100 ml min21 and GHSV = 15 000
h21
.
Fig. 2 SCR activities on the Fe–Mn based transition metal oxides in the
presence of H2O. Reaction conditions: 0.5 g catalyst, [NO] = [NH3] = 1000
ppm, [O2] = 2%, [H2O] = 2.5%, He = balance, total flow rate = 100 ml
In the presence of SO2, SO2 can be oxidized to SO3 by O2. The
SOx compounds (SO2 + SO3) are adsorbed on the transition
metal oxides and they are difficult to desorb at low tem-
peratures. Our FTIR spectrum of the used Fe–Mn–Zr oxides
min21 and GHSV = 15 000 h21
.
showed two strong bands centered at 1402 and 1120 cm21
,
Since the combustion gases contain water vapor, we further
studied the effect of H2O on the SCR activities of the mixed
oxides. As shown in Fig. 2, when 2.5% H2O was added to the
reactants, NO conversion decreased sharply at lower tem-
peratures, e.g., 80 °C. It is likely that H2O occupies some of the
active sites for the SCR reaction. When the reaction temperature
was increased, the H2O inhibition effect became less sig-
nificant. At 160–180 °C, nearly 100% NO conversion was still
obtained on the three catalysts. In addition, the Fe–Mn based
catalysts did not show any decrease in activity after 10 h on
stream at 140 °C in the presence of H2O. Also, the inhibition by
H2O is reversible. When H2O was removed from the reaction
gases, NO conversions were recovered on these catalysts.
The above results indicate that the Fe–Mn based transition
oxides are highly active for the low temperature SCR reaction
and they are resistant to water vapor at 140–180 °C. Their SCR
activities are much higher than those on 20%MnOx/Hombikat
TiO2, the ‘best’ low temperature catalyst reported in ref. 9. For
instance, at 100 °C, 89–98% NO conversions were observed on
the Fe–Mn based catalysts at GHSV = 15 000 h21 (Fig. 1),
whereas only 82% NO conversion was reported on the MnOx/
TiO2 at GHSV = 8000 h21. The superior SCR activity on these
Fe–Mn catalysts is probably related to their high activity for NO
oxidation to NO2 at low temperatures. Since NO2 + NO is very
active in reacting with NH3 adsorbed species at low tem-
peratures,12 the formed NOx can be reduced to nitrogen by NH3.
It is noted that there are still small concentrations of SO2 in
combustion gases even after the desulfurizer (except when
using very clean fuels such as liquefied natural gas). Effects of
SO2 on the SCR activity are also important for the low
temperature SCR catalyst. Our result indicated that, when 1000
ppm SO2 was added to the reaction gas and the space velocity
was kept at 15 000 h21, NO conversions on Fe–Mn–Zr oxides
were decreased significantly to 8–23% at 80–180 °C under
steady state conditions. This is consistent with the previous
observations that MnOx catalysts were deactivated by SO2.7,14
indicating formation of ammonium ions and sulfate species,
respectively.15 The occupation of the active sites by metal
sulfates and ammonium sulfates will decrease the SCR
activity.14 The effects of lower levels of SO2 have also been
studied. The results for 37.5 ppm SO2 plus 2.5% H2O are shown
in Fig. 3. As expected, less deactivation was seen for 37.5 ppm
SO2 compared with that by 1000 ppm SO2. Further work on the
modification of the catalysts is needed to improve sulfur
resistance.
We thank Mr G. Qi for measuring the effects of SO2 at 37.5
ppm. This work was supported by EPRI and NSF under Grant
CTS-0095909.
Notes and references
1 H. Bosch and F. Janssen, Catal. Today, 1988, 2, 369.
2 G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal. B, 1998, 18,
1.
3 P. Forzatti and L. Lietti, Heterog. Chem. Rev., 1996, 3, 33.
4 J. P. Chen, M. A. Buzanowski, R. T. Yang and J. E. Cichanowicz, J. Air
Waste Manage. Assoc., 1990, 40, 1403.
5 E. Curry-Hyde and A. Baiker, Ind. Eng. Chem. Res., 1990, 29, 1985.
6 J. P. Chen, R. T. Yang, M. A. Buzanowski and J. E. Cichanowicz, Ind.
Eng. Chem. Res., 1990, 29, 1431.
7 L. Singoredjo, R. Korver, F. Kapteijn and J. Moulijn, Appl. Catal. B,
1992, 1, 297.
8 Z. P. Zhu, Z. Y. Liu, H. X. Niu and S. J. Liu, J. Catal., 1999, 187,
245.
9 P. G. Smirniotis, D. A. Pena and B. S. Uphade, Angew. Chem., Int. Ed.,
2001, 40, 2479.
10 F. Notoya, C. Su, E. Sasako and S. Nojima, Ind. Eng. Chem. Res., 2001,
40, 3732.
11 R. Q. Long and R. T. Yang, J. Catal., 2000, 190, 22.
12 R. Q. Long and R. T. Yang, J. Catal., 2001, 198, 20.
13 H. Y. Huang and R. T. Yang, Langmuir, 2001, 17, 4997.
14 T. S. Park, S. K. Jeong, S. H. Hong and S. C. Hong, Ind. Eng. Chem.
Res., 2001, 40, 4491.
15 T. Yamaguchi, Appl. Catal., 1990, 61, 1.
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