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Applied Catalysis A: General 327 (2007) 261–269
www.elsevier.com/locate/apcata
Manganese oxide catalysts for NOx reduction with NH3
at low temperatures
Min Kang a, Eun Duck Park b, Ji Man Kim c, Jae Eui Yie d,*
b
a
R&D Center, EnD Solutions Co. Ltd., 223-235, Seoknam-Dong, Seo-Gu, Incheon 405-220, Republic of Korea
Department of Chemical Engineering, Division of Chemical Engineering and Materials Engineering, Ajou University, Wonchun-Dong,
Yeongtong-Gu, Suwon 443-749, Republic of Korea
c
Functional Materials Laboratory, Department of Chemistry and Sungkyunkwan Advanced Institute of Nano Technology,
Sungkyunkwan University, Cheoncheon-Dong, Jangan-Gu, Suwon 440-746, Republic of Korea
d
Catalyst and Surface Laboratory, Department of Applied Chemistry, Ajou University, Wonchun-Dong,
Yeongtong-Gu, Suwon 443-749, Republic of Korea
Received 20 December 2006; received in revised form 15 May 2007; accepted 17 May 2007
Available online 21 May 2007
Abstract
Manganese oxide catalysts prepared by a precipitation method with various precipitants were investigated for the low temperature selective
catalytic reduction (SCR) of NOx with NH3 in the presence of excess O2. Various characterization methods such as N2 adsorption, X-ray diffraction
(XRD), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and X-ray absorption near edge structure (XANES) were
conducted to probe the physical and chemical properties of MnOx catalysts. The active MnOx catalysts, precipitated with sodium carbonate and
calcined in air at moderate temperatures such as 523 K and 623 K, have the high surface area, the abundant Mn4+ species, and the high
concentration of surface oxygen on the surface. The amorphous Mn3O4 and Mn2O3 were mainly present in this active catalyst. The carbonate
species appeared to help adsorb NH3 on the catalyst surface, which resulted in the high catalytic activity at low temperatures.
# 2007 Elsevier B.V. All rights reserved.
Keywords: NO reduction; Manganese oxides; NH3-SCR; Precipitation method; Calcinations
1. Introduction
The emission control of nitrogen oxides (NO, NO2 and N2O)
from various combustion processes has been a major
environmental concern related to the air quality because
nitrogen oxides have been reported as one of the most serious
pollutants causing acid rain along with sulfur dioxide and play a
major role in the photochemical chain reaction leading to the
formation of photochemical smog. The primary nitrogen oxides
(NOx) produced from combustion processes with fossil fuel are
nitric oxide (NO) and nitrogen dioxide (NO2), which are 90–
95% of the total NOx emission from automotive exhausts and
stationary sources such as thermoelectric power plant and
incinerator [1].
* Corresponding author. Tel.: +82 31 219 2513; fax: +82 31 219 2394.
E-mail address: [email protected] (J.E. Yie).
0926-860X/$ – see front matter # 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2007.05.024
The removal of nitrogen oxides from stationary or mobile
sources has become an important issue and a variety of
reduction methods to minimize the emission of NOx such as a
combustion control and a post-combustion control technology
have been developed. Among the proposed post-combustion
methods for NOx removal, the technologies using the catalysts
are known as one of most efficient methods in terms of
relatively low cost and high efficiency. Especially, the selective
catalytic reduction (SCR) of nitrogen oxide has been generally
recognized as the most effective and widely commercialized
removal technology of NOx emitted from the stationary sources
[1–3].
In order to convert NO contained in the flue gas into N2, the
reducing agent must be employed. NH3, CO, H2 and a variety of
hydrocarbons such as methane, propylene, and ethane have
been employed as reductants for NO removal reaction [4].
Although a number of reducing agents can be utilized in SCR,
ammonia is the most effective and widely commercialized,
which is called NH3-SCR, for stationary sources such as power
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M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
plants and nitric acid plants [5]. The selective catalytic
reduction (SCR) is a process in which a reducing agent, e.g.
NH3, reacts selectively with NOx to produce N2 without
consuming an excess O2.
In the past few decades, the backbone of SCR technology is
the development of SCR catalysts such as noble metals [6],
supported metal oxides [7], zeolites [8] and others [1,9,10].
Among them, vanadia supported on titania is known to be the
most effective and widely used commercial SCR catalyst due to
its high activity and durability to sulfur compounds. Because
this catalyst exhibits high conversions only in the temperature
range of 573–673 K, the SCR should be applied before units for
particle removal and desulphurization where the gas temperature decreases [11]. However, when the flue gas has high
concentrations of particles and other contaminants which are
deleterious for the catalyst, proper units should be located at the
upstream of the catalyst bed to resolve above problems, which
causes the decrease of the exit gas temperature. Therefore, there
is a great interest in the development of SCR catalysts active at
low temperatures (<573 K).
A number of catalysts consisted of various transition metal
(V, Cr, Mn, Fe, Co, Ni and Cu) oxides on different commercial
supports such as silica and alumina have been studied. Among
these catalysts, manganese oxides such as MnOx/Al2O3 [12],
MnOx/NaY [13], MnOx/USY [14] and MnOx/TiO2 [15,16]
have attracted much interests due to their high catalytic
activities. These catalysts were prepared by the solution
impregnation method on supports using manganese nitrate or
acetate. In case of unsupported metal oxides, only limited
works have been reported because unsupported MnOx catalysts
suffer from very low surface areas [17]. Some additives such as
citric acid were added in the preparation step to increase the
surface area as well as the catalytic activity [18].
Recently, we found that MnOx catalysts prepared by a
simple precipitation method with sodium carbonate showed the
high catalytic activity for low temperature NH3-SCR [19]. This
catalyst also appeared to be stable in the presence of excess
water vapor. In the present study, we investigated the effect of
preparation methods including kinds of precipitants and
calcinations temperatures of the MnOx catalysts on their
structural features and catalytic performance for selective
catalytic reduction of NOx with ammonia.
2. Experimental
2.1. The preparation of catalysts
The various kinds of manganese oxides were prepared by a
precipitation method with different precipitants such as
ammonium carbonate (AC), potassium carbonate (PC), sodium
carbonate (SC), ammonium hydroxide (AH), potassium
hydroxide (PH) and sodium hydroxide (SH). Each catalyst is
denoted as MnOx-AC, MnOx-PC, MnOx-SC, MnOx-AH,
MnOx-PH and MnOx-SH, respectively. 0.5 M ammonium
carbonate ((NH4)2CO3, DAEJUNG, 99.5%) aqueous solution,
0.5 M potassium carbonate (K2CO3, DAEJUNG, 99.5%)
aqueous solution, 0.5 M sodium carbonate (Na2CO3, SHINYO,
99.5%) aqueous solution, ammonium hydroxide (NH4OH,
DAEJUNG, 25.0–28.0%) solution, potassium hydroxide
(KOH, DAEJUNG, 99%) or sodium hydroxide (NaOH,
SAMCHUN, 99%) was continuously added to 500 ml of
0.5 M manganese nitrate (Mn(NO3)2xH2O, Aldrich, 98.0+%)
aqueous solution until the pH of the solution reached 8. The
resulting precipitate was aged at 298 K for 1 h, filtered, and
washed several times with distilled water. The cake was dried in
air at 393 K for 12 h and calcined in static air at different
temperatures such as 523 K, 623 K, 723 K and 823 K.
2.2. Catalyst characterization
X-ray diffraction (XRD) patterns were recorded on a Rigaku
D/MAC-III using Cu Ka radiation (l = 0.15406 nm), operated
at 50 kV and 30 mA (1.5 kW). BET surface areas were
calculated from N2 adsorption data that were obtained using
Autosorb-1 apparatus (Quantachrome) at liquid N2 temperature. Before the measurement, the sample was degassed for 12 h
at 150 8C. The amount of adsorbed NH3 and NO was measured
at 300 K by a pulse adsorption method using helium as a carrier
gas. X-ray photoelectron spectroscopy (XPS) data were
obtained with an Mg Ka (1253.6 eV) X-ray source using
ESCA2000 (VG Microtech) instrument. The binding energy of
C 1s (284.7 eV) was used as an internal standard. The thermal
gravimetric analysis (TGA) was carried out using a PerkinElmer Series 7 thermal analysis system under a flow of dry air.
The temperature was raised from room temperature to 1173 K
using a linear programmer at a heating rate of 40 K min1. The
X-ray absorption near edge structure (XANES) spectra were
taken in a transmission mode for the K-edge of Mn at beamline
3C1 of the pohang light source (PAL) operating at 2.5 GeV with
ca. 100–150 mA of stored current. The detector gas was N2 for
the incident beam and the transmitted beam. In addition to
catalyst samples, XAFS data were also obtained for Mn foil,
MnCO3, MnO, Mn2O3, Mn3O4, and MnO2 as references. They
were analyzed by using ATHENA [20].
2.3. Activity measurements
Catalytic activities were measured over a fixed bed of
catalysts in a tubular flow reactor of 8 mm i.d. Reactant gases
were fed to the reactor by means of electronic mass flow
controller (MKS).
The reactant gas typically consisted of 500 ppm NO,
500 ppm NH3, 5 vol.% O2, and N2 or He. The NOx
concentration in the inlet and outlet gas was analyzed by
means of a NO/NO2 combustion gas analyzer (Euroton). The
N2 and N2O in the effluent were separated at 353 K with
HeySep D column and their concentrations were analyzed with
a thermal conductive detector (TCD) in a gas chromatography
(Hewlett-Packard, HP 5890). From the concentration of the
gases at steady state both the conversion and the selectivity are
calculated according to the following formula [12,17]:
NO conversionð%Þ ¼
½NOin ½NOout
100
½NO in
(1)
M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
N2 selectivityð%Þ ¼
½N2 out
100
½N2 out þ ½N2 Oout
(2)
The subscripts in and out indicated the inlet concentration and
outlet concentration at steady state, respectively.
3. Results and discussion
3.1. The effect of kinds of precipitant
Fig. 1 compared the catalytic activities of the MnOx
catalysts precipitated with different kinds of precipitant in
terms of NOx conversion and N2 selectivity as a function of
reaction temperature. The NOx conversion was affected
noticeably by kinds of its precipitant especially at low
temperatures. The catalysts precipitated with precipitants
containing carbonate as a anion showed the higher NOx
conversion than those prepared with alkali or ammonium
hydroxides. The former also exhibited the better N2 selectivity
Fig. 1. NOx conversions and N2 selectivities at different reaction temperatures
over various MnOx catalysts prepared by different precipitants. All catalysts
were calcined in air at 623 K. Reactants: 500 ppm NO, 500 ppm NH3 and
5 vol.% O2 in N2. The gas hourly space velocity (GHSV) was 50,000 h1.
263
Table 1
N2 adsorption data of the MnOx catalysts precipitated with a different precipitant such as (NH4)2CO3 (AC), K2CO3 (PC), Na2CO3 (SC), NH4OH (AH),
KOH (PH), and NaOH (SH)
Catalystsa
SBET (m2/g)
VP (cm3/g)
MnOx-AC (623)
MnOx-PC (623)
MnOx-SC (623)
MnOx-AH (623)
MnOx-PH (623)
MnOx-SH (623)
132.7
154.3
173.3
25.9
29.1
31.0
0.32
0.38
0.35
0.26
0.29
0.27
a
Catalysts were calcined in air at 623 K before the measurement.
than the latter. For all catalysts, the N2 selectivity decreased
with increasing reaction temperature. The NOx conversion and
the N2 selectivity were also affected by kinds of cations such as
sodium ion, potassium ion, and ammonium ion and decreased
in this order. Therefore, the highest NOx conversion and N2
selectivity was observed over MnOx catalysts precipitated with
sodium carbonate.
Table 1 gives the BET surface area and pore volume of the
various MnOx catalysts prepared with different precipitants.
The surface areas and pore volumes of MnOx catalysts
precipitated with alkali or ammonium hydroxides are much
smaller than those of the MnOx catalysts prepared with
precipitants containing carbonate. The kinds of cations in the
precipitant also appeared to affect the surface area. The BET
surface area decreased in the order when Na+, K+, and NH4+
was utilized as a cation in the precipitant, respectively.
Therefore, the manganese oxide catalyst precipitated with
sodium carbonate had the highest surface area. It is worth
noting that the SCR activity closely correlates with the surface
area. Based on the previous work [19], the additional factors
besides the surface area must affect the SCR activity because
MnOx-SC showed the higher specific activity per surface area
than MnOx-AH.
Fig. 2 shows the XRD patterns of the various MnOx catalysts
just dried at 373 K and calcined in air at 623 K. All the asprepared MnOx-AH, MnOx-PH and MnOx-SH sample give
sharp XRD peaks representing Mn3O4 phase. This phase was
transformed into Mn5O8 phases after heat treatment at 623 K.
For MnOx-AC, MnOx-PC and MnOx-SC catalysts, MnCO3
phase was observed when they were dried at 373 K. However,
when they were calcined in air at 623 K, no noticeable
crystalline phase was observed. This can be interpreted that asprepared MnCO3 was decomposed into the amorphous
manganese oxide phase during calcinations. This partially
decomposed and amorphous structure of the MnOx catalysts
may also be a reason for the excellent catalytic activity at low
temperatures, in addition to the high surface area.
To find out the surface chemical states of MnOx catalysts
prepared with different precipitants, XPS spectra of Mn 2p, O
1s, and C 1s were obtained as shown in Fig. 3. For all samples,
two main peaks due to Mn 2p3/2 and Mn 2p1/2 were observed
from 639 eV to 660 eV. It was reported that 2p3/2 binding
energy of Mn(0) and Mn(IV)O2 was 639.0 eV and 642.1 eV,
respectively [21,22]. Therefore, it is not easy to discern the
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M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
Fig. 2. XRD patterns of MnOx catalysts just dried (A) and calcined in air at 623 K (B) after precipitated with different precipitants: (a) MnOx-AC, (b) MnOx-PC, (c)
MnOx-SC, (d) MnOx-AH, (e) MnOx-PH and (f) MnOx-SH (* MnCO3, ^ Mn3O4 and & Mn5O8).
oxidation states of manganese species without peak deconvolution because there is only 3.1 eV energy difference between
Mn(0) and Mn(IV). The peak position of Mn 2p3/2 for MnOx
catalysts can exclude the possibility that there exist manganese
species whose oxidation state is 0 or +2. Therefore, the
deconvolution of Mn 2p3/2 was conducted based on the
assumption that there are only Mn(III) and Mn(IV) species.
The presence of satellite peak was also considered in the peak
deconvolution. This satellite structure, noticeable on the higher
binding energy side of the Mn 2p core-line in the XPS spectrum
of the manganese compounds, originates from the charge
transfer between outer electron shell of ligand and an unfilled
3d shell of Mn during creation of core-hole in the photoelectron
process [23]. As shown in Fig. 3, the asymmetric peak was
Fig. 3. XPS spectra for (A) Mn 2p, (B) O 1s, and (C) C 1s of the MnOx catalysts: (a) MnOx-AC (623), (b) MnOx-PC (623), (c) MnOx-SC (623), (d) MnOx-AH (623),
(e) MnOx-PH (623) and (f) MnOx-SH (623).
M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
265
Table 2
Mn 2p and O 1s binding energies on MnOx catalysts precipitated with a different precipitant such as (NH4)2CO3 (AC), K2CO3 (PC), Na2CO3 (SC), NH4OH (AH),
KOH (PH), and NaOH (SH)
Catalystsa
Mn
MnOx-AC (623)
MnOx-PC (623)
MnOx-SC (623)
MnOx-AH (623)
MnOx-PH (623)
MnOx-SH (623)
a
Mn4+/Mn3+
Mn 2p (eV)
4+
643.7
643.7
643.7
643.8
643.8
643.7
3+
Mn
641.9
641.9
641.9
641.9
641.9
642.0
0.97
1.00
1.10
0.71
0.63
0.69
O 1s (eV)
Oa/(Oa + Ob)
Oa
Ob
531.3
531.3
531.4
531.7
531.5
531.5
529.6
529.6
529.7
530.1
530.0
529.9
42.6
41.3
52.2
30.4
31.0
28.1
Catalysts were calcined in air at 623 K before the measurement.
observed in XPS spectra of O 1s for all samples. The peak at
529.6–530.0 eV corresponds to lattice oxygen (O2) (hereafter
denote as Ob) whereas the one at 531.3–531.7 eV corresponds
to several O 1s states assigned to the surface-adsorbed oxygen
such as O22 or O, in the form of hydroxyl, OH, and
carbonate, CO32 (hereafter denoted as Oa) [24,25]. XPS
spectra of C 1s were also obtained as shown in Fig. 3. For all
samples, two adjacent peaks were observed from 282 eV to
292 eV. The peak at 285 eV, 286.5 eV, and 289 eV corresponds
to contaminated carbon, CO bond, and surface carbonate
species, respectively. The peak intensity at a higher binding
energy in C 1s XPS spectra for MnOx prepared with
precipitants containing carbonate anions was stronger than
those for others.
Table 2 lists the surface compositions from the XPS spectra
of the MnOx catalysts prepared with different precipitants. The
value of the Mn4+/Mn3+ ratio in the surface layer of MnOx
catalyst was quite high (0.97–1.10) in catalysts precipitated
with carbonate-containing precipitants. Especially, this value
appeared to be the largest for MnOx-SC which showed the
highest catalytic activity for the selective NOx reduction with
NH3 at low temperatures. The relative concentration ratio of
Oa/(Oa+Ob) was also higher in MnOx-SC, MnOx-PC, and
MnOx-AC than that in MnOx-AH, MnOx-PH, and MnOx-SH.
The manganese oxide prepared with sodium carbonate which
was the best in NH3-SCR at low temperatures showed the
highest relative concentration ratio of Oa/(Oa+Ob) among all
catalysts. Therefore, the effect of calcinations temperature on
the catalytic activity and structure and electronic state of
catalysts were examined further for MnOx catalysts precipitated with sodium carbonate.
temperatures. Therefore, the MnOx-SC catalysts calcined at
moderate temperature such as 523 or 623 K have the highest
catalytic activity for NOx reduction with NH3. The N2
selectivity gradually decreased with increasing calcination
temperatures. Table 3 shows the BET surface areas, pore
volumes, and average pore diameters of the MnOx-SC catalysts
calcined at different temperatures. The BET surface areas and
3.2. The effect of calcination temperatures
Fig. 4 compares the NOx conversions and N2 selectivity over
MnOx-SC catalysts calcined at different temperatures. The
MnOx-SC catalyst dried at 373 K showed less than 20% NOx
conversion when the reaction temperature was below 400 K.
This catalyst exhibited more than 80% NOx conversion only in
the temperature range from 448 K to 498 K. However, 100%
NOx conversion was obtained from 348 K to 448 K when
MnOx-SC catalyst was calcined at 523 or 623 K. Further
increase in calcinations temperature above 623 K caused
decreasing NOx conversion especially at low reaction
Fig. 4. NOx conversions and N2 selectivities at different reaction temperatures
over various MnOx-SC catalysts calcined at different temperatures. Reactants:
500 ppm NO, 500 ppm NH3 and 5 vol% O2 in N2. The gas hourly space velocity
(GHSV) was 50,000 h1.
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M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
Table 3
Surface area, pore volume and average pore diameter of the MnOx catalysts
precipitated with sodium carbonate (SC) and calcined in air at different
temperatures
Catalysts
MnOx-SC
MnOx-SC
MnOx-SC
MnOx-SC
(523)
(623)
(723)
(823)
SBET (m2/g)
VP (cm3/g)
DP (nm)
178.5
173.3
80.4
22.2
0.35
0.35
0.31
0.07
8.0
10.9
15.5
21.3
the pore volumes of the MnOx-SC catalysts monotonically
decreased with increasing the calcination temperature from
523 K to 823 K. The corresponding average pore diameter
gradually increased from 8.0 nm to 21.3 nm as expected. This
can be due to the sintering to some extent at high calcinations
temperature.
Fig. 5 illustrates the XRD patterns of MnOx catalysts
precipitated with sodium carbonate and calcined at different
temperatures. For the MnOx-SC dried at 373 K, the strong
peaks representing crystalline MnCO3 phase were observed.
However, these strong peaks disappeared completely and very
weak peaks which cannot be assigned to any manganese oxides
were found when MnOx-SC catalysts were calcined at
temperatures from 523 K to 723 K. This can be interpreted
that MnCO3 was transformed into an amorphous phase during
calcinations at this temperature. This partially decomposed and
amorphous structure of the MnOx-SC catalyst may also be a
reason for the high NOx conversion and N2 selectivity at low
temperatures. This was also consistent with the previous
observation that nitrous oxide formation occurred preferentially on crystalline phases [12]. When the MnOx-SC catalyst
was calcined at 823 K, the formation of Mn2O3 crystalline
phase was observed in XRD pattern.
XPS measurements were performed to examine the effect of
calcinations temperatures on the surface electronic state of
MnOx-SC catalysts. Fig. 6 shows the XPS spectra of Mn 2p, O
1s, and C 1s of MnOx-SC catalysts with different calcination
temperatures. For all samples, two main peaks due to Mn 2p1/2
and Mn 2p3/2 were observed from 639 eV to 660 eV, which can
exclude the possibility of the presence of Mn(0) and Mn(II)
species. Therefore, the deconvolution of Mn 2p3/2 was
conducted based on the assumption that there are only Mn(III)
and Mn(IV) species. The presence of satellite peak was also
considered in the peak deconvolution. As shown in Fig. 6, the
asymmetric peak was observed in XPS spectra of O 1s for all
samples. As stated previously, the peak at 530.0 eV and
531.5 eV can be assigned to the lattice oxygen (Ob) and the
Fig. 5. XRD patterns of MnOx-SC catalysts calcined at different temperatures.
surface-adsorbed oxygen (Oa). XPS spectra of C 1s were also
obtained as shown in Fig. 6. For all samples, two adjacent peaks
were observed from 282 eV to 292 eV. The peak at 285 eV,
286.5 eV, and 289 eV corresponds to contaminated carbon, CO
bond, and surface carbonate species, respectively. For MnOxSC catalysts, the peak intensity at a higher binding energy in C
1s XPS spectra decreased with increasing calcinations
temperature.
Table 4 lists the surface compositions from the XPS spectra
of the MnOx catalysts calcined at different temperatures. The
value of the Mn4+/Mn3+ ratio in the surface layer of MnOx
catalyst was quite high in catalysts calcined at 523 or 623 K
which showed the high catalytic activity for the selective NOx
reduction with NH3 at low temperatures. The relative
concentration ratio of Oa/(Oa+Ob) decreased with increasing
calcinations temperature for manganese oxides catalysts
precipitated with sodium carbonate. The most active manganese oxide prepared with sodium carbonate and calcined at
523 K showed the highest concentration of surface oxygen.
Therefore, the active catalyst has the high Mn4+/Mn3+ ratio and
surface oxygen concentration in the surface layer of MnOx
catalyst.
Table 4
Mn 2p and O 1s binding energies on MnOx-SC catalysts precipitated with sodium carbonate (SC) and calcined in air at different temperatures
Catalysts
Mn
MnOx-SC
MnOx-SC
MnOx-SC
MnOx-SC
Mn4+/Mn3+
Mn 2p (eV)
(523)
(623)
(723)
(823)
4+
643.6
643.7
643.5
643.5
3+
Mn
641.9
641.9
641.9
641.8
1.03
1.10
0.92
0.78
O 1s (eV)
Oa/(Oa + Ob)
Oa
Ob
531.6
531.4
531.5
531.5
529.8
529.7
529.7
529.9
68.7
52.2
42.5
40.2
M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
267
Fig. 6. XPS spectra for (A) Mn 2p, (B) O 1s, and (C) C 1s of the MnOx-SC catalysts calcined in air at different temperatures.
The value of the Mn4+/Mn3+ ratio was previously considered
as a significant parameter characterizing the intrinsic properties
of other metal oxide catalysts. Machocki et al. [22] have
investigated the catalytic combustion of methane over silver
modified manganese–lanthanum oxides and found that the rate
of methane oxidation is a linear function of the Mn4+/Mn3+
surface ratio. Tang et al. [26] also reached the same conclusion
that MnOx–CeO2 catalysts showed higher catalytic activity for
HCHO oxidation with increasing the Mn4+/Mn3+ surface ratio.
Similar result was observed in the present study.
In the general NH3-SCR process, NO is easily converted to
N2 and H2O by NH3:
4NO þ 4NH3 þ O2 ! 4N2 þ 6H2 O
(3)
When both NO and NO2 are present, the following reaction
proceeds much more rapidly than the above reaction (3):
4NH3 þ 2NO þ 2NO2 ! 4N2 þ 6H2 O
(4)
Wallin et al. [27] reported that the presence of NO2 in the gas
mixture enhanced the SCR performance of a catalyst.
Therefore, it can be inferred that the difference in the maximum
NOx conversion between the catalysts is probably due to the
difference in the amount of NO2 produced from the oxidation of
NO by high Mn4+/Mn3+ surface ratio.
The value of Oa/(Oa + Ob) in the surface layer of MnOx
catalyst prepared with precipitants containing carbonate anions
was higher than that of others. The high relative concentration
ratio of Oa/(Oa+Ob) was previously found to be preferable for
SCR reactions over the manganese-containing catalysts.
Kapteijn et al. [17] indicated that the product distribution is
determined by the concentration of reactive oxygen and nitric
oxide, since both affect the relative distribution of the surface
species. A high surface oxygen concentration facilitates the (–
NH2) formation. Kijlstra et al. [10] studied the mechanism of
SCR of NO with ammonia at low temperatures on MnOx/Al2O3
catalyst. They proposed that the reaction starts with the
adsorption of NH3 on a Lewis acid and subsequently transforms
into NH2; the NH2 would then react with gas-phase NO (an ER
mechanism) and nitrite intermediates on the surface (a LH
mechanism). This is supported by our results, which indicated
that the N2 selectivity increased with the relative ratio between
concentration of surface oxygen and that of lattice oxygen.
The thermal gravimetric analysis was conducted to observe
the change in the weight of manganese oxides precipitated with
sodium carbonate with increasing temperature as shown in
Fig. 7. The gradual weight loss occurred from room
temperature to 850 K. This decreased weight (35 wt.%) is
almost same to the calculated value assuming that MnCO3 was
transformed into Mn2O3. Therefore, MnOx-SC catalyst
calcined below 850 K must contain lots of carbon oxide
species (COx). There is no significant weigh loss in the MnOxAH catalyst. It is reasonable that the presence of COx species in
the MnOx-SC catalyst also affects the de-NOx activity at low
temperatures because the residual COx species may act as
acidic sites on the catalyst surface. These acidic sites can help
Fig. 7. Thermal gravimetric analysis of manganese oxides precipitated with
sodium carbonate and dried at 373 K.
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M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
Fig. 8. Mn K-edge XANES spectra of Mn reference samples and MnOx-SC catalysts calcined at different temperatures. The fitted spectra were plotted in dotted line.
the basic reductant, NH3, adsorb on the surface at low
temperatures, and therefore the reductant may be enriched
compared with the surface of MnOx-AH catalyst. This was also
supported by the fact that 0.207 mmol NH3/gcat and
0.011 mmol NH3/gcat could be adsorbed on MnOx-SC and
MnOx-AH, respectively. This difference is much more than
expected even if MnOx-SC has seven times larger surface area
than that of MnOx-AH. The amount of chemisorbed NO on
MnOx-SC and MnOx-AH at room temperature was 0.50 mmol
NO/gcat and 0.12 mmol NO/gcat, respectively. This can be
mainly due to difference in surface area between two catalysts.
To determine the structural and electronic information of
MnOx catalysts which were determined to be amorphous from
XRD data, Mn K-edge XANES spectra were obtained. Fig. 8
presents Mn K-edge XANES spectra of MnOx-SC catalysts
calcined at different temperatures with Mn reference compounds such as Mn foil, MnCO3, MnO, Mn3O4, Mn2O3, and
MnO2. Easily discernible features of XANES spectra were
observed for Mn reference samples and the edge energy shifted
toward a higher energy with an increase in the oxidation state of
Mn from metallic Mn foil to Mn(IV)O2. Mn K-edge XANES
spectra of as-prepared MnOx-SC catalyst was similar to that of
MnCO3, which is consistent with the fact that crystalline
MnCO3 phase was observed from XRD. MnOx-SC catalysts
calcined at 523 K, 623 K and 723 K appeared to be different
compared with any XANES spectra of Mn reference samples
measured. However, Mn K-edge XANES spectra of MnOx-SC
calcined at 823 K was similar to that of Mn2O3, which is
consistent with the fact that crystalline Mn2O3 phase was
observed from XRD.
To determine the quantitative amount of each manganese
oxides, a linear XANES fitting was conducted for MnOx-SC
catalysts calcined at different temperatures as shown in Table 5.
Different weight fractions of manganese oxides appeared to be
present in these catalysts. The manganese carbonate was
determined to be mainly present with a small amount of
Mn2O3 in MnOx-SC just dried at 373 K. When MnOx-SC was
calcined at 523 K, the weight fraction of MnCO3 decreased
noticeably and those of Mn3O4 and Mn2O3 increased. When this
catalyst was calcined at 623 or 723 K, most of MnCO3 must be
transformed into Mn2O3. Because Mn2O3 was determined to be
mainly present with a small amount of Mn3O4 in MnOx-SC
calcined at 823 K, some of Mn3O4 appeared to be further
transformed into Mn2O3. Although no noticeable difference in
the composition of manganese oxides for MnOx-SC calcined at
623 K compared with that of MnOx-SC calcined at 723 K from
XANES fitting results, the former was superior to the latter in the
catalytic activity at low temperatures. This indicates that the kind
of manganese oxide as well as its crystallinity should be related to
the catalytic activity in NH3-SCR at low temperatures.
Table 5
The linear combination fitting result for MnOx catalysts precipitated with
sodium carbonate (SC) and calcined in air at different temperatures
Catalysts
E0/(eV)
Standard
sample
Weight
fraction
R-factora
MnOx-SC (373)
6548.733
MnCO3
Mn2O3
0.966
0.034
0.15801
MnOx-SC (523)
6548.719
MnCO3
Mn3O4
Mn2O3
MnO2
0.424
0.279
0.286
0.011
0.02717
MnOx-SC (623)
6548.719
MnCO3
Mn3O4
Mn2O3
MnO2
0.135
0.280
0.510
0.075
0.52299
MnOx-SC (723)
6548.719
MnCO3
Mn3O4
Mn2O3
MnO2
0.128
0.314
0.487
0.071
0.12677
MnOx-SC (823)
6553.719
MnCO3
Mn3O4
Mn2O3
0.012
0.106
0.882
0.15782
a
R-factor is defined as follows. R = sum((data-fit)^2)/sum(data^2)).
M. Kang et al. / Applied Catalysis A: General 327 (2007) 261–269
[4]
[5]
[6]
[7]
The formation of ammonium nitrate on the catalyst surface
during the course of the reaction has been claimed to be a main
problem for low temperature NH3-SCR performance [28]. The
temperature programmed desorption (TPD) was conducted
from 373 K to 1073 K over the catalyst after a reaction and its
effluent gas was analyzed with a mass spectrometer (PFEIFFER Vacuum Quadstar). During TPD experiment, no NH3
species was detected. This can exclude the formation of
ammonium salts during a reaction.
[10]
4. Conclusions
[11]
The manganese oxides catalyst prepared by a precipitation
method using sodium carbonate and calcined at moderate
temperatures such as 523 K and 623 K showed the high NOx
conversion and N2 selectivity for low temperature selective
catalytic reduction of NOx with NH3. This active MnOx
catalyst has the high surface area, the abundant Mn4+ species,
and the rich concentration of surface oxygen on the surface.
The amorphous Mn3O4 and Mn2O3 were determined to be
mainly present. The residual carbonate species on this catalyst
also help NH3 adsorb on the surface, which resulted in the high
catalytic activity at low temperatures.
Acknowledgements
The financial support by the Research Initiation Program at
Ajou University (20041340) was appreciated by one of authors,
Eun Duck Park. Experiments at PLS were supported in part by
MOST and POSTECH.
References
[8]
[9]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[1] H. Bosch, F.J.I.G. Janssen, Catal. Today 2 (1988) 369–379.
[2] J.N. Armor, Appl. Catal. B 1 (1992) 221–256.
[3] H.S. Rosenberg, L.M. Curran, A.U. Slack, J. Ando, J.H. Oxley, EPRIRP925, October.1978.
[27]
[28]
269
H. Hamada, Catal. Today 22 (1994) 21–40.
J.M. Garcia-Cortes, Appl. Catal. B: Environ. 30 (2001) 399–408.
G.A. Papapolymerou, L.D. Schmidt, Langmuir 1 (1985) 488–495.
Y. Murakami, M. Inomata, K. Mori, K. Suzuki, A. Miyamoto, T. Hattori,
in: G. Poncelet, P. Grange, P.A. Jacobs (Eds.), Preparation of Catalysis III,
Elsevier, Amsterdam, 1983, pp. 531–534.
J.R. Kiovsky, P.B. Koradia, C.T. Lim, Ind. Eng. Chem. Prod. Res. Dev. 19
(1980) 218–225.
G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B: Environ. 18 (1998)
1–36.
W.S. Kijlstra, J.C.M.L. Daamen, J.M. van de Graff, B. van der Linden,
E.K. Poels, A. Bliek, Appl. Catal. B: Environ. 7 (1996) 337–357.
J. Muniz, G. Marban, A.B. Fuertes, Appl. Catal. B: Environ. 27 (2000) 27–
36.
L. Singoredjo, R. Korver, F. Kapteijn, J. Moulijn, Appl. Catal. B: Environ.
1 (1992) 297–316.
U. Bentrup, A. Bruckner, M. Richter, R. Fricke, Appl. Catal. B: Environ.
32 (2001) 229–241.
G. Qi, R.T. Yang, R. Chang, Catal. Lett. 87 (2003) 67–71.
P.G. Smirniotis, D.A. P na, B.S. Uphade, Angew. Chem. Int. Ed. 40 (2001)
2479–2482.
G. Qi, R.T. Yang, Appl. Catal. B: Environ. 44 (2003) 217–225.
F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Appl. Catal. B:
Environ. 3 (1994) 173–189.
G. Qi, R.T. Yang, Chem. Commun. (2003) 848–849.
M. Kang, T.H. Yeon, E.D. Park, J.E. Yie, J.M. Kim, Catal. Lett. 106 (2006)
77–80.
B. Ravel, M. Newville, J. Synchrotron Radiat. 12 (2005) 537–541.
S. Ponce, M.A. Pena, J.L.G. Fierro, Appl. Catal. B 24 (2000) 193–
205.
A. Machocki, T. Ioannides, B. Stasinska, W. Gac, G. Avgouropoulos, D.
Delimaris, W. Grzegorczyk, S. Pasieczna, J. Catal. 227 (2004) 282–
296.
R.J. Iwanowski, M.H. Heinonen, W. Paszkowicz, R. Minikaev, T. Story, B.
Witkowska, Appl. Surf. Sci. 252 (2006) 3632–3641.
J.P. Holgado, G. Munuera, J.P. Espinós, A.R. González-Elipe, Appl. Surf.
Sci. 158 (2000) 164–171.
R. Larachi, J. Pierre, A. Adnot, A. Bernis, Appl. Surf. Sci. 195 (2002) 236–
250.
X. Tang, Y. Li, X. Huang, Y. Xu, H. Zhu, J. Wang, W. Shen, Appl. Catal. B:
Environ. 62 (2006) 265–273.
M. Wallin, C. Karlsson, M. Skoglundh, A. Palmqvist, J. Catal. 218 (2003)
354–364.
M. Koebel, M. Elsener, G. Madia, Ind. Eng. Chem. Res. 40 (2001) 52–59.