Performances of SCR of NO of impregnated Catalysts
Supported on Active Carbon Fibers
Gui Keting Wu Haimiao Wang Xiaobo
(Key Laboatoy of Enegy Themal Convesion and Contol of Ministy of Education, School of Enegy and
Envionment, Southeast Univesity, Nanjing, 210096)
Abstract: Selective catalytic reduction (SCR) is a widely used method in the emission control of NOx from combustion
processes. It has high removal efficiency, and consumes less reducing agent. But in the process of SCR of NOx,
catalyst is expensive, the high temperature of catalyst possessing active needs reheating flue gas, and wasted catalyst
induces secondary pollution. Therefore, it is significant to develop new efficient and low-temperature catalysts. Hence,
the new impregnated catalyst was prepared through impregnating Fe, Cr, Cu, Mn and other transition metal
components on the support active carbon fibers. The influences of different parameters, such as reaction components,
impregnating quantity, reaction temperature and, oxygen concentration et.al, on the efficiency of SCR of NO were
experimentally studied. The results show: The activity of Mn is superior to Cu, Cr and Fe, and is better to promote the
efficiency of SCR of NO. The efficiency of SCR of NO of compound-component catalyst is higher than that of
single-component. When support of active carbon fibers is impregnated 8% of Mn, Cu, Cr, Fe, the efficiency of SCR of
NO of catalyst is the best compared with other quantity of impregnated metal, and the catalyst 8Mn-8Fe/AC is one of
the best kinds of catalysts with stable performance and high efficiency of SCR of NO in all impregnated catalysts
prepared by this paper.
Keywords:
active carbon fibers, impregnated catalyst, selective catalytic reduction (SCR)
1. Introduction
The emission control of NOx from combustion processes has become one of the most important
methods to protect environment. This is because NOx in air may cause acid rain, photochemical smog,
ozone depletion and greenhouse effects [1]. In recent years, many methods have been developed to
reduce the emission of NOx. Selective catalytic reduction (SCR) of NOx with NH3 has been proven as
an effective method among them and employed in the removal of NOx from stationary sources
because of its low cost and high efficiency [2-3]. However, the current commercialized catalysts for
SCR of NOx, such as V2O5/TiO2 or V2O5–WO3/TiO2, must be operated within a narrow temperature
range of 300-400℃[2-4]. But the temperature of flue gas undergoing the desulphurization and particle
removal is much lower than this one. In the traditional power plant, hence, the flue gas reheating is
needed and it would consume energy. Therefore, it is very interesting to develop highly active
catalysts for low temperature SCR of NOx and such a catalyst would be active at low temperature [3].
Since the catalysts produced by impregnating transition metals on support material can be worked on
low-temperature SCR of NOx with ammonia, a number of impregnated catalysts have been developed.
Besides impregnating metals, the support material is also important to promote the activity of the
catalysts. Since titania, alumina and active carbons have large specific surface area and pore volumes,
they are selected as the support materials for impregnated catalysts. Tang et al.[4] reported that more
than 90% NOx conversion could be achieved over the Mn-based monolithic catalysts prepared by
impregnation method at low-temperature. Yoshikawaa et al.[5] reported their research results of
catalytic reduction of NO with NH3 over active carbon fibers impregnated with metal oxides and
found Mn2O3 showed the best activity among these metal oxides at low temperature. Blanco et al.[6]
investigated the activity of the copper-nickel and vanadium oxides supported on titania and alumina
and high NOx conversions were obtained. Qi and Yang achieved over 90% NO conversion at 120℃
on the Fe–Mn/TiO2 catalysts prepared by impregnation method [7]. Among these impregnated catalysts
mentioned above, Fe–Mn-based transition metal oxides are considered as one of the most active
catalysts for low-temperature SCR of NOx with ammonia [7]. However, much work has been focused
on iron and manganese oxides supported on titania or other metal materials [8-14], few studies have
been done for the low-temperature SCR of NOx with ammonia by the impregnated catalysts supported
on active carbon fibers.
In this work, therefore, active carbon fibers were chosen as the catalyst support due to its high
specific area and its chemical stability. Fe–Mn-based catalysts were prepared by impregnation with
certain aqueous solutions of nitrate (Fe, Mn) and were applied to low temperature SCR of NO with
NH3. The effects of catalysts prepared by impregnating different metal oxides on NO conversions at
the temperature range of 90–300℃ were studied. Fe-Mn based catalysts showed good activity for
SCR of NO with NH3 at low temperature.
2. Experimental
2.1 Catalysts preparation
The catalysts used in the experiment are prepared by an impregnation method. The procedures of
the catalysts preparation are as follows. About 20g active carbon fibers with the diameters of
0.25-0.50 mm are placed in a beaker with some water, and appropriate aqueous solution of manganese
nitrate (Mn(NO3)2.4H2O) is added to the beaker. The catalysts supported on the active carbon fibers
are prepared by impregnating metal manganese with an aqueous solution of manganese nitrate. A
solution of ammonia (1:1, v/v) is gradually with stirring added to the mixture until the pH of the
solution reached 8, than the solution will be maintained about 48h in the circumstance. Subsequently,
the impregnated catalyst is first dried in air at 150℃ for 24 h, followed by calcination in air at 450℃
for 5 h. So that, the catalyst of 8Mn/AC is prepared. Changing the composition of impregnation metals,
other impregnated catalysts supported on the active carbon fibers, as shown in Tab.1, are obtained.
Tab.1 Experimental conditions of SCR of NO of impregnated Catalysts supported on Active Carbon Fibers
Compositions of
2Fe
2Cr
2Cu
2Mn
8Fe-2Cr
8Fe-2Cu
8Fe-2Mn
loaded metals
5Fe
5Cr
5Cu
5Mn
8Fe-5Cr
8Fe-5Cu
8Fe-5Mn
8Fe
8Cr
8Cu
8Mn
8Fe-8Cr
8Fe-8Cu
8Fe-8Mn
10Fe
10Cr
10Cu
10Mn
8Fe-10Cr
8Fe-10Cu
8Fe-10Mn
Diameter of
0.25-0.50
catalysts (mm)
Reaction
90、120、150、180、210、240、270、300
temperatures (℃)
2.2 Catalytic activity measurement
The experimental setup for the measurement of SCR of NO activity of catalysts is shown in Fig.1.
It basically consists of following four units: a simulated flue gas system, a reactor, an electric heating
system and an online flue gas analysis system. The reaction conditions are as follows: 500ppm NO,
500ppm NH3, 3% O2, balanced gas N2, 1500 ml/min total flow rate. The simulated flue gas from the
gas tank is premixed in the glass chamber (5) and then heated in Pre-heater (6) by the electric heating
system. The NH3 is fed directly into the reactor by the mixing chamber (7). All the data are obtained
when the SCR reaction reached steady state, and the components of the outlet gases are measured
online by an online flue gas analyzer (11) (rbr ecom-J2KN, Germany). The other experimental
conditions are shown in Tab.1.
11
N2
1
2
3
10
9
8
NO
7
NH3
4
O2
6
5
Fig.1 Schematic of the experimental apparatus for NO removal
(1) Filter; (2) Buffer tank; (3) Mass flow rate controller (MFC); (4) Valve; (5) glass chamber; (6) Pre-heater ;
(7)mixing chamber; (8) Attemperator; (9) Reactor; (10) Heater; (11).Gas analyzer
3. Results and discussion
3.1 Effects of impregnated composition of catalysts on the efficiency of SCR of NO
Four kinds of elements, Fe, Cr, Cu, and Mn, were used as the impregnated compositions supported
on the active carbon fibers to form four kinds of new catalysts. The experimental results of
efficiencies of SCR of NO with these four catalysts are shown in Fig.2.
It can be seen from Fig.2 that the efficiencies of SCR of NO of catalysts increase gradually to an
extreme point with the rising of temperature, and than decrease. The efficiencies of SCR of NO of
catalysts are different with the impregnating compositions. Fig.2 shows that the efficiency of SCR of
NO of catalyst 8Mn/AC obtains its maximum value, 95%, at 210℃. This is obviously higher than the
efficiencies of catalysts 8Cu/AC, 8Cr/AC and 8Fe/AC at this temperature. It can be conclude,
therefore, that the activity of element Mn is superior to that of Cu, Cr and Fe for the impregnated
catalysts with 8% impregnating quantity on the active carbon fibers.
NO Conversion（%）
100
90
80
70
60
50
40
30
20
10
0
8Mn/AC
8Cu/AC
8Cr/AC
8Fe/AC
80 110 140 170 200 230 260 290 320
Temperature（℃）
Fig. 2. Effects of different active components on NO conversion
[NO] = [NH3] = 500ppm, [O2] = 3%, balance N2, total flow rate 1500 ml/min, catalyst 12mL.
Karlsson et.al[15] reported the performances of catalystic oxidation NOx of impregnated metal
catalysts supported on Al2O3. It was shown that the catalytic activity of impregnated metal catalyst,
from large to small, in turn was Co2O3, MnO2, Fe2O3 and CuO, and the efficiencies of SCR of NOx of
all impregnated metal catalysts were above 50% in the temperature between 250℃～370℃. Wang
et.al[16] showed that impregnated metal catalysts supported on γ-Al2O3 had effective affects on SCR
of NOx, and the catalytic activity of impregnated metals, from large to small, in turn was Mn, Cr, Co,
Cu, Fe, Ni and Zn. These results are corresponding to our experiment.
3.2 Effects of single or compound impregnated composition of catalysts on the
efficiency of SCR of NO
The experimental results of efficiencies of SCR of NO of single composition impregnated catalyst
8Mn/AC and compound composition impregnated catalyst 8Fe-8Mn/AC are shown in Fig.3. It can be
seen from Fig.3 that the efficiency of SCR of NO of compound composition impregnated catalyst
8Fe-8Mn/AC is superior to that of single composition impregnated catalyst 8Mn/AC.
NO Conversion（%）
100
90
80
70
60
50
40
30
20
10
0
8Fe-8Mn/AC
8Mn/AC
8Fe/AC
80 110 140 170 200 230 260 290 320
Fig. 3. Effects of single or compound of active components on NO conversion.
[NO] = [NH3] = 500ppm, [O2] = 3%, balance N2, total flow rate 1500 ml/min, catalyst 12mL.
Fig.3 shows that the efficiency of SCR of NO of catalyst 8Fe-8Mn/AC is almost not change in
the experimental temperature range 100℃～300℃, and is all higher than 90%. The reason is also can
be seen from Fig.3. Fig.3 presents that the efficiency of SCR of NO of single composition of
impregnated catalyst 8Mn/AC with the temperature and that of 8Fe/AC with the temperature are
complementary each other. The efficiency of SCR of NO of single composition catalyst 8Mn/AC gets
the maximum value at 210℃, and the efficiency of SCR of NO of catalyst 8Fe/AC is low at this
temperature. Furthermore, the efficiency of SCR of NO of 8Mn/AC decreases with the rising of
temperature and obtains its minimum value at 240℃. But the efficiency of SCR of NO of 8Fe/AC
obtains its maximum value at this temperature. Hence, the compound composition impregnated
catalyst 8Fe-8Mn/AC combines the both single composition catalysts, and achieves the high and
stable efficiency of SCR of NO though the whole experimental temperature range.
3.3
Effects of impregnated quantity of catalyst on the efficiency of SCR of NO
In order to study the effects of impregnated quantity of catalyst on the efficiency of SCR of NO,
following percents of metal Mn, 2%, 5%, 8% and 10%, are impregnated on the active carbon fibers,
respectively. The effects of impregnated quantity of catalyst on the efficiency of SCR of NO are
experimental studied, and shown in Fig.4.
NO Conversion（%）
100
90
80
70
60
50
40
30
20
10
0
2Mn/AC
5Mn/AC
8Mn/AC
10Mn/AC
80 110 140 170 200 230 260 290 320
Temperature（℃）
Fig. 4. NO conversion on different Mn/AC catalysts.
[NO] = [NH3] = 500ppm, [O2] = 3%, balance N2, total flow rate 1500 ml/min, catalyst 12mL.
It can be seen from Fig.4 that the efficiency of SCR of NO of catalyst is improved with the
increasing of impregnated metal quantity when the temperature is below 120℃.This tendency will be
changed when the temperature is above 120℃.The order of the efficiency of SCR of NO of catalyst,
from large to small, in turn is 8Mn/AC, 10Mn/AC, 5Mn/AC and 2Mn/AC in the temperature range of
120℃～150℃,and it will be further changed to 8Mn/AC, 5Mn/AC, 10Mn/AC and 2Mn/AC in the
temperature range of 150℃～210℃.Since the efficiency of SCR of NO of catalyst is low when the
temperature below 120℃, the experimental results when the temperature above 120℃ is interesting.
We can see from Fig.4 that when impregnated quantity less than 8%, the efficiency of SCR of NO of
catalyst is improved with the increasing of impregnated metal quantity. However, the efficiency
decreases with the increasing of impregnated quantity when it is larger than 8%. It can be concluded,
therefore, that the optimum impregnated quantity of metal Mn on active carbon fibers is 8% in the
experiments. This is because when the impregnated quantity is less than 8%，the activated sites on the
surface of support material are not saturated by the impregnating metal, and further impregnating
activated metal composition will improve the activation of catalyst. However, after the activated sites
on the surface of support material saturated by the impregnated metal, redundant impregnating metal
composition will cover on the activated sites and obstruct the effects of catalyst. Hence, when the
percent of impregnated quantity is larger than 8%，the efficiency of SCR of NO of catalyst will
decrease. The effects of impregnated quantity of other impregnating metal, such as Cu, Cr and Fe, on
the efficiency of SCR of NO are almost same as the impregnating metal Mn, and get there optimum
percent at 8%.
NO Conversion（%）
100
90
80
70
60
50
40
30
20
10
0
8Fe-2Mn/AC
8Fe-5Mn/AC
8Fe-8Mn/AC
8Fe-10Mn/AC
80 110 140 170 200 230 260 290 320
Temperature（℃）
Fig. 5. NO conversion on different Fe-Mn/AC catalysts.
[NO] = [NH3] = 500ppm, [O2] = 3.%, balance N2, total flow rate 1500 ml/min, catalyst 12mL.
Besides the effects of single impregnated quantity Mn of catalyst on the efficiency of SCR of NO
were investigated experimentally, the effects of compound impregnated quantity of Mn and Fe of
catalyst, supported on active carbon fibers, on the efficiency of SCR of NO were also studied. The
experimental results are shown in Fig.5. It can be seen from Fig.5 that except the catalyst
8Fe-2Mn/AC, other three catalysts show high and stable efficiency of SCR of NO, and 8Fe-8Mn/AC
is the best. Its efficiency of SCR of NO is almost higher than 90% in all experimental temperature
range. It can also be seen from Fig.5 that the efficiencies of SCR of NO of four catalysts increase with
the rising of temperature between 90℃～200℃, and get their maximum efficiency, and than decrease
with the temperature. This is because the impregnated composition of catalyst assembles and obstructs
the activated site under the high temperature and affects the efficiencies of SCR of NO of catalysts.
3.4 Effects of oxygen concentration on the efficiency of SCR of NO
The effects of oxygen concentration on the efficiency of SCR of NO were investigated, and the
experimental results are shown in Fig.6.
The comparison of the efficiencies of SCR of NO of catalyst 8Fe-8Mn/AC under the condition of
with and without oxygen is shown in Fig.6. It can be seen that the efficiency of SCR of NO of catalyst
8Fe-8Mn/AC under the condition of 3% oxygen concentration is higher than that of catalyst without
oxygen in all experimental temperature range. This result conforms that the existence of a little
oxygen in the reaction of SCR of NO promotes the efficiency of SCR of NO. The investigation of
Iwamoto et.al[17] showed that the adsorbability of P-type transitional metal-oxide to oxygen was very
high, and the oxygen will be decomposed to some adsorbable matter of O2- and O- et.al. When the
oxygen concentration increases from 0% to 3%, the adsorbable matter of O2- and O- will increase,
enhance the reaction 2NO+O2→2NO2, and promotes the conversion of NO. Hence, the existence of a
little oxygen in the reaction of SCR of NO promotes the efficiency of SCR of NO.
NO Conversion（%）
100
90
80
70
60
50
40
30
20
10
0
3% oxygen
without oxygen
80 110 140 170 200 230 260 290 320
Temperarure（℃）
Fig. 6. Effect of oxygen on NO conversion over 8Fe-8Mn/AC catalysts.
[NO] = [NH3] = 500ppm, [O2] = 3%, balance N2, total flow rate 1500 ml/min, catalyst 12mL.
.
However, after saturation of adsorbable oxygen on the surface of catalyst, redundant adsorbable
oxygen will cover on the activated sites and obstruct the effects of the activation of catalyst. Therefore,
the efficiency of SCR of NO of catalyst 8Fe-8Mn/AC under the condition of 3% oxygen concentration
is higher than that of no oxygen, but further increase oxygen concentration in the reaction of SCR of
NO is not favorable to improve the efficiency of SCR of NO.
4. Conclusion
In this paper, we study the performances of SCR of NO of impregnated catalysts supported on the
active carbon fibers. The following can be concluded from the experiment results:
1. The active composition of impregnated metal makes an important role in the SCR of NO of
catalyst. The activity of metal Mn is superior to Cu, Cr and Fe, and is better to promote the efficiency
of SCR of NO in the experiment.
2. The efficiency of SCR of NO of compound-component catalyst is higher than that of
single-component. The efficiency of SCR of NO of catalyst 8Fe-8Mn/AC is all higher than 90% in the
experimental temperature range 100 ℃ ～ 300 ℃ . This efficiency is superior to that of
single-component impregnated catalyst.
3. The impregnated quantity affects largely to the efficiency of SCR of NO of catalyst. When support
active carbon fibers impregnate 8% of Mn, Cu, Cr, Fe, the efficiency of SCR of NO of catalyst is the
highest. The catalysts that with less or more than this impregnated quantity will obtain lower
efficiency of SCR of NO.
4. The efficiency of SCR of NO of catalyst under the condition of 3% oxygen concentration is higher
than that of catalyst under the condition of without oxygen. The result conforms that the existence of a
little oxygen in the reaction of SCR of NO promotes the efficiency of SCR of NO.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of
China (51276039).
References
[1]Min, K.; Eun, D. P.; Ji, M. K.; Jae, E. Y. Cu–Mn mixed oxides for low temperature NO reduction with NH 3 .
Catalysis Today2006, 111,236-241.
[2]Zhu, Z.P.; Liu, Z.Y.; Niu, H.X.; Liu, S. J. Promoting effect of SO 2 on activated carbon-supported vanadia catalyst
for NO reduction by NH 3 at low temperatures. Journal of Catalysis 1999,187, 245-248.
[3]Qi, G.; Yang,R.T.; Chang, R. Low-temperature SCR of NO with NH 3 over USY-supported manganese
oxide-based catalysts. Catalysis Letters2003, 87, 67-71.
[4]Tang, X. L.; Hao, J. M.; Yi, H. H.; Li, J. H. Low-temperature SCR of NO with NH 3 over AC/C supported
manganese-based monolithic catalysts. Catalysis Today 2007, 126, 406-411.
[5]Yoshikawaa, M.; Yasutakeb, A.; Mochidac. I. Low-temperature selective catalytic reduction of NOx by metal
oxides supported on active carbon fibers. Applied Catalysis A: General 1998,173(2), 239-245.
[6]Blanco, J.; Avila, P.; Suárez, S.;
, J. A.; Knapp.C. Alumina- and titania-based monolithic catalysts for low
temperature selective catalytic reduction of nitrogen oxides. Applied Catalysis B: Environmental 2000, 28,
235-244.
[7]Qi, G.; Yang, R. T. Low-temperature selective catalytic reduction of NO with NH 3 over iron and manganese
oxides supported on titania. Applied Catalysis B: Environmental 2003,44, 217-225.
[8].Panayiota S. Lambrou, Angelos M. Efstathiou. The effects of Fe on the oxygen storage and release properties of
model Pd–Rh/CeO2–Al2O3 three-way catalyst [J]. Journal of Catalysis 240 (2006) 182–193
[9]. Xiwei Xu, Jiang Enchen, Wang Mingfeng, Li Bosong, Zhou Ling. Hydrogen production by catalytic cracking of
rice husk over Fe2O3/γ-Al2O3 catalyst[J]. Renewable Energy 41 (2012) 23-28
[10]. A. Derylo-Marczewska, W. Gac, N. Popivnyak, G. Zukocinski, S. Pasieczna. The influence of preparation method
on the structure and redox properties of mesoporous Mn-MCM-41 materials [J]. Catalysis Today 114 (2006)
293–306
[11] B.R. Strohmeier, D.M. Hercules. Surface spectroscopic characterization of manganese/aluminum oxide catalysts
[J].The Journal of Physical Chemistry, 88 (1984) 4922-4929.
[12] S. V. Tsybulya,G. N. Kryukova,T. A. Kriger, P. G. Tsyrul'nikov. Structural Aspect of a Thermal Activation Effect in
the MnO x /γ-Al2O3 System [J]. Kinetics and Catalysis 2003, 44(2):287-296.
[13]Doggalia, P.; Teraokab, Y.; Mungsea, P.; Shahc, I. K.; Rayalua, S.; Labhsetwara, N. Combustion of volatile organic
compounds over Cu–Mn based mixed oxide type catalysts supported on mesoporous Al2O3, TiO2 and ZrO2[J].
Journal of Molecular Catalysis A: Chemical 2012, 358, 23-30.
[14]Shimokawabe, M.; Asakawa, H.; Takezawa, N. Characterization of copper/zirconia catalysts prepared by an
impregnation method. Applied Catalysis 1990, 59(1), 45-58.
[15] Karlsson H T, Rosenberg H S. Flue gas denitrification: selective catalytic oxidation of NO to NO2[J].Industrial
and Engineering Chemistry: Process Design and Development, 1984, 23(4): 808-814.
[16] Wang Hui, Zhao Xiuge, et.al. The oxidation reaction mechanism of NO on the impregnated metal oxide catalysts
[J].Journal of east china university of science and technology, 2001,27 (1): 6-10.
[17] Iwamoto M, Yoda Y, Yamazoe N, etal. Study of metal oxide catalysts by temperature programmed desorption 4.
Oxygen adsorption on various metal oxides[J]. Journal of Physical Chemistry,1978,82(24):2564.