Competition of selective catalytic reduction and non selective catalytic reduction over MnOx/TiO2 for NO removal: The relationship between gaseous NO concentration and N2O selectivity

Article abstract of DOI:10.1039/c3cy00648d

In this work, a novel phenomenon was discovered that N₂O selectivity of NO reduction over MnO_x/TiO₂ was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N₂O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley-Rideal mechanism (the reaction of over-activated NH₃ with gaseous NO) and the Langmuir-Hinshelwood mechanism (the reaction of adsorbed NO₃^- with adsorbed NH₃ on the adjacent sites) could contribute to the formation of N₂O. Kinetic study demonstrated that N₂O selectivity would be independent of gaseous NO concentration if NO reduction over MnO _x/TiO₂ mainly followed the Langmuir-Hinshelwood mechanism. If NO reduction over MnO_x/TiO₂ mainly followed the Eley-Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more -NH₂ was used to reduce gaseous NO to form N₂ and the further oxidization of -NH₂ to -NH was restrained, resulting in an obvious decrease of N₂O selectivity. The Eley-Rideal mechanism played an important role in NO reduction over MnO_x/TiO₂, especially at higher temperatures. Therefore, N₂O selectivity of the low temperature SCR reaction over MnO_x/TiO₂ decreased especially at higher temperatures after the increase of gaseous NO concentration.

Full text of DOI:10.1039/c3cy00648d

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Competition of selective catalytic reduction and
non selective catalytic reduction over MnOx/TiO2
for NO removal: the relationship between gaseous
NO concentration and N2O selectivity†
Cite this: Catal. Sci. Technol., 2014,
4, 224
Shijian Yang,ab Yuwu Fu,a Yong Liao,a Shangchao Xiong,a Zan Qu,c Naiqiang Yanc
b
*
and Junhua Li
In this work, a novel phenomenon was discovered that N2O selectivity of NO reduction over MnOx/TiO2 was
related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause
higher N2O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley–Rideal
mechanism (the reaction of over-activated NH3 with gaseous NO) and the Langmuir–Hinshelwood mecha-
−
nism (the reaction of adsorbed NO3 with adsorbed NH3 on the adjacent sites) could contribute to the
formation of N2O. Kinetic study demonstrated that N2O selectivity would be independent of gaseous NO
concentration if NO reduction over MnOx/TiO2 mainly followed the Langmuir–Hinshelwood mechanism.
If NO reduction over MnOx/TiO2 mainly followed the Eley–Rideal mechanism, there was competition
between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR)
reaction. As gaseous NO concentration increased, more –NH2 was used to reduce gaseous NO to form
N2 and the further oxidization of –NH2 to –NH was restrained, resulting in an obvious decrease of N2O
selectivity. The Eley–Rideal mechanism played an important role in NO reduction over MnOx/TiO2,
especially at higher temperatures. Therefore, N2O selectivity of the low temperature SCR reaction over
MnOx/TiO2 decreased especially at higher temperatures after the increase of gaseous NO concentration.
Received 30th August 2013,
Accepted 8th October 2013
DOI: 10.1039/c3cy00648d
www.rsc.org/catalysis
fired power plants for several decades.2 The temperature
window of V2O5–WO3(MoO3)/TiO2 is about 300–400 °C, so the
1. Introduction
Nitrogen oxides (NO and NO2), emitted from automobiles
and stationary sources, greatly contribute to the formation of
smog, acid rain and ozone.1 Selective catalytic reduction
(SCR) of NO with NH3 has been an efficient technique for the
control of NOx emission from coal fired power plants and
automobiles.2 The standard SCR process is based on the
following reaction between NH3 and NO:3
SCR unit is located upstream of the desulfurizer and electro-
static precipitator in order to avoid reheating of the flue gas.4
However, retrofitting the SCR devices into existing systems is
difficult because the space and access in many power plants
are extremely limited.5 Therefore, there has been strong
demand in developing highly active SCR catalysts at low tem-
peratures, which will be placed downstream of the electro-
static precipitator and desulfurizer.6 Mn based catalysts, for
example MnOx–CeO2,7,8 MnO2/TiO2,9–13 MnOx–CeO2/TiO2
14
4NO þ 4NH3 þ O2 → 4N2 þ 6H2O
ð1Þ
15
and Fe2O3–MnO2/TiO2, show excellent low temperature SCR
V2O5–WO3(MoO3)/TiO2 has been widely used as a SCR
catalyst to control the emission of NO from stationary coal
activity among the first row transition metal based cata-
lysts.16–20 However, some N2O would form during the low
temperature SCR reaction over Mn based catalysts.6,7,21 The
non selective catalytic reduction (NSCR) reaction is based on
the following reaction between NH3 and NO:22
a School of Environmental and Biological Engineering, Nanjing University of
Science and Technology, Nanjing, 210094, PR China.
E-mail: yangshijiangsq@163.com
b State Key Joint Laboratory of Environment Simulation and Pollution Control
(SKLESPC), School of Environment, Tsinghua University, Beijing, 100084,
PR China. E-mail: lijunhua@tsinghua.edu.cn; Fax: +86 10 62771093;
Tel: +86 10 62771093
4NH3 þ 4NO þ 3O2 → 4N2O þ 6H2O
ð2Þ
N2O is now considered as a pollutant due to its green-
house effect and its depletion of the ozone layer.22–25 How-
ever, only a little work focused on the mechanism of N2O
formation during the low temperature SCR over Mn based
c School of Environmental Science and Engineering, Shanghai Jiao Tong
University, Shanghai, 200240, PR China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00648d
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catalyst.21 Some groups have demonstrated that one of the
two N atoms in N2O originates from NH3 and the other from
NO under the SCR conditions.3,22 However, there is no agree-
ment on the mechanism of N2O formation: by (1) reaction of
gaseous NO with over-activated NH3 (–NH) to N2O (i.e. the
2[N2O]out
[NH3]in [NOx ]in [NH3 ]out [NOx ]out
N2O selectivity 
ð6Þ
where, [NH3]in and [NOx]in were the concentrations of NH3
and NOx (including NO and NO2) in the inlet, and [NH3]out
−
,
Eley–Rideal mechanism), or (2) adsorption of NO3 on the
[NOx]out and [N2O]out were the concentrations of NH3, NOx
(including NO and NO2) and N2O in the outlet.
adjacent sites of adsorbed NH3, followed by reaction to an
activated transition state (i.e. NH4NO3) and then decomposi-
tion to N2O (i.e. the Langmuir–Hinshelwood mechanism).3
Recently, a novel phenomenon has been found that N2O selec-
tivity of the low temperature SCR reaction over MnOx/TiO2
was related to the concentration of gaseous NO. N2O selectiv-
ity obviously decreased after the increase of gaseous NO con-
centration in the inlet. Herein, the mechanism of N2O
formation during the low temperature SCR reaction over
MnOx/TiO2 was studied, and the effect of NO concentration
on N2O selectivity was investigated.
2.3 In situ DRIFTS study
In situ DRIFT spectra were recorded on a Fourier transform
infrared spectrometer (FTIR, Nicolet NEXUS 870) equipped
with a smart collector and an MCT detector cooled by liquid
N2.26 The catalyst was finely ground and placed in a ceramic
crucible and manually pressed. The FTIR spectra were recorded
by accumulating 100 scans with a resolution of 4 cm−1
.
3. Results
3.1 SCR performance of MnOx/TiO2 for the low temperature
2. Experimental
2.1 Catalyst preparation
SCR reaction
MnOx/TiO2 (Mn loading was 5 wt%) was prepared by the
impregnation method using Degussa TiO2 P25 as support
and manganese nitrate as precursor. The sample was dried at
110 °C for 12 h, and it was then calcined at 500 °C under air
atmosphere for 3 h.
The performance of MnOx/TiO2 for the low temperature SCR
reaction at 100–250 °C is shown in Fig. 1. Both the ratio of
NO conversion and N2O selectivity increased with the
increase of reaction temperature, which is consistent with
previous research on the low temperature SCR reaction over
2.2 Catalytic test
The reduction of NO was performed on a fixed-bed quartz
tube reactor (6 mm of internal diameter). The catalyst with
40–60 mesh was placed on the quartz wool held in the reac-
tor, which was heated by a vertical electrical furnace. The
total flow rate was 200 mL min−1 (room temperature), and
the mass of catalyst was 200 mg. The corresponding gas
hourly space velocity (GHSV) was 6 × 104 cm3 g−1 h−1 (i.e.
75 000 h−1). The feed contained 500 or 1000 ppm of NO,
500 or 1000 ppm of NH3, 2% of O2, and balance of N2. The
concentrations of NO, NO2, NH3 and N2O in the outlet were
continually monitored by an FTIR spectrometer (MKS Instru-
ments). The ratios of NOx and NH3 conversion, the amount
of N2 formed and N2O selectivity were calculated using the
following equations:
[NOx ]in [NOx ]out
NOx conversion 
ð3Þ
[NOx ]in
[NH3]in [NH3 ]out
NH3 conversion 
ð4Þ
[NH3]in
[NH3]in [NH3]out [NOx ]in [NOx ]out  2[N2O]out
N2 formation 
Fig. 1 SCR performance of MnOx/TiO2: (a) [NO] = [NH3] = 500 ppm;
2
(b) [NO] = [NH3] = 1000 ppm. Reaction conditions: [O2] = 2%, catalyst
mass = 200 mg, total flow rate = 200 mL min−1, GHSV = 60000 cm3 g−1 h−1
.
ð5Þ
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MnOx/TiO2.13,27,28 As the concentrations of NO and NH3 in
the inlet increased from 500 to 1000 ppm, the ratios of NO
and NH3 conversion over MnOx/TiO2 slightly decreased. A
similar result once happened on Mn–Fe spinel.6 However,
N2O selectivity obviously decreased after the increase of the
concentrations of gaseous NO and NH3 especially at higher
temperatures (shown in Fig. 1). It suggests that N2O selectiv-
ity of the low temperature SCR reaction over MnOx/TiO2 was
related to the concentrations of gaseous NO and NH3 in the
inlet. This phenomenon was seldom previously reported in
the literature.
Meanwhile, the formation of N2O over MnOx/TiO2 was
further restrained (shown in Fig. 2b). It suggests that N2O
selectivity over MnOx/TiO2 was related to the concentration of
gaseous NO.
3.3 In situ DRIFTS study
3.3.1 Transient reaction at 150 °C. MnOx/TiO2 was first
treated with 500 ppm of NH3 at 150 °C for 30 min followed
by N2 purged for 5 min. 500 ppm of NO and 2% of O2 were
then introduced into the IR cell (shown in Fig. 3a). After the
adsorption of NH3, five characteristic vibrations at 1680,
1600, 1437, 1206 and 1165 cm−1 appeared on MnOx/TiO2. The
bands at 1600 and 1206 cm−1 were assigned to coordinated
NH3 bound to the Lewis acid sites, and the bands at 1680
3.2 Effect of gaseous NO concentration on N2O selectivity
Fig. 2 shows the effect of gaseous NO concentration on NH3
conversion and N2O formation. As shown in Fig. 2a, little
NH3 can be oxidized over MnOx/TiO2 below 175 °C in the
absence of NO. With the increase of reaction temperature
from 175 to 250 °C, NH3 oxidation was promoted (shown in
Fig. 2a). However, more than 50% of NH3 was oxidized to
N2O at 200–250 °C (shown in Fig. 2b).
As 500 ppm of NO was introduced, NH3 conversion over
MnOx/TiO2 was promoted (shown in Fig. 2a). Meanwhile,
N2O selectivity of MnOx/TiO2 for the SCR reaction was much
less than that for NH3 oxidation at 200–250 °C. As the con-
centration of gaseous NO increased from 500 to 1000 ppm,
NH3 conversion was further promoted (shown in Fig. 2a).
Fig. 3 (a) DRIFT spectra taken at 150 °C upon passing NO + O2 over NH3
presorbed MnO2/TiO2; (b) DRIFT spectra taken at 150 °C upon passing
NH3 over NO + O2 presorbed MnO2/TiO2; (c) DRIFT spectra taken at
150 °C upon passing NH3 + NO + O2 over MnO2/TiO2.
Fig. 2 Effect of NO concentration on: (a) NH3 conversion; (b) N2O
selectivity. Reaction conditions: [O2] = 2%, catalyst mass = 200 mg,
total flow rate = 200 mL min−1, GHSV = 60 000 cm3 g−1 h−1
.
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and 1437 cm−1 were attributed to ionic NH4 bound to the
Brønsted acid sites.8,10 The band at 1165 cm−1 could be
attributed to –NH2, which resulted from the activation of
coordinated NH3 bound to the Lewis acid sites.29 After NO + O2
passed over NH3 pretreated MnOx/TiO2, the bands at 1680,
1600, 1437, 1206 and 1165 cm−1 corresponding to adsorbed
ammonia species gradually diminished. Meanwhile, three
characteristic vibrations at 1609, 1529 and 1280 cm−1
appeared. The band at 1609 cm−1 was assigned to monodentate
nitrite, and the bands at 1529 and 1280 cm−1 were attributed to
bidentate nitrate.30 Moreover, adsorbed H2O, which is the
product of the SCR reaction, appeared at 1630 cm−1. These
bands suggest that adsorbed NH3 can react with gaseous NO
(i.e. the Eley–Rideal mechanism). The concentrations of N2O
and NO in the outlet during the transient reaction were
simultaneously recorded (shown in Fig. 4a). After NO + O2
passed over NH3 pretreated MnOx/TiO2, NO concentration
gradually increased to about 480 ppm. Meanwhile, the
concentration of N2O in the outlet rapidly increased to about
30 ppm, and it then gradually decreased to the background
of N2O in NO (9 ppm) (shown in Fig. 4a). It suggests that the
reaction between adsorbed NH3 and gaseous NO (i.e. the
Eley–Rideal mechanism) at 150 °C can produce N2O.
nitrite (1609 cm−1) and bidentate nitrate (1529 and 1280 cm−1).
After NH3 was introduced into the cell, the bands correspond-
ing to monodentate nitrite (at 1609 cm−1) and bidentate nitrate
(1529 cm−1) firstly shifted to 1600 and 1506 cm−1. Then, the
intensities of the two bands gradually decreased (shown in
Fig. 3b). They suggest that adsorbed NOx can react with
adsorbed NH3 (i.e. the Langmuir–Hinshelwood mechanism).
The concentrations of N2O and NH3 in the outlet during the
transient reaction were simultaneously recorded (shown in
Fig. 4b). After NH3 was introduced to NO + O2 pretreated
MnOx/TiO2 for 5 min, little NH3 was observed. Meanwhile,
the concentration of N2O in the outlet rapidly increased to
about 13 ppm after the introduction of NH3, and it then
gradually decreased to about 5 ppm in 60 min. It suggests
that the reaction between adsorbed NH3 and adsorbed NOx
(i.e. the Langmuir–Hinshelwood mechanism) can also pro-
duce N2O. Fig. 3b shows that the decrease of the band at
1600 cm−1 corresponding to adsorbed NH4NO2 was much
faster than that at 1506 cm−1 corresponding to adsorbed
NH4NO3. It suggests that the reaction through the nitrite
route was faster than that through the nitrate route, which
was consistent with the result of Mn–Fe spinel.6 Previous
research demonstrated that the product of the nitrite route
was N2, while that of nitrate route was N2O.22
Finally, the IR spectra during the SCR reaction (i.e.
500 ppm of NH3, 500 ppm of NO and 2% of O2 were simulta-
neously introduced) at 150 °C were recorded. As shown in
Fig. 3c, adsorbed H2O (at 1630 cm−1), coordinated NH3 or
adsorbed NH4NO2 (at 1600 cm−1), adsorbed NH4NO3 (at 1566
and 1506 cm−1) and ionic NH4+ (at 1437 cm−1) were all observed.
It suggests that both the Langmuir–Hinshelwood mechanism
and the Eley–Rideal mechanism could happen at 150 °C.
3.3.2 Transient reaction at 250 °C. MnOx/TiO2 was first
treated with 500 ppm of NH3 for 30 min at 250 °C, and
500 ppm of NO and 2% of O2 were then introduced into the
IR cell (shown in Fig. 5a). After the adsorption of NH3,
MnOx/TiO2 was mainly covered by coordinated NH3 bound
to the Lewis acid sites (at 1602 cm−1). After NO + O2 passed
over NH3 pretreated MnOx/TiO2, coordinated NH3 rapidly
diminished, and adsorbed H2O (at 1620 cm−1) appeared.
Then, MnOx/TiO2 was mainly covered by monodentate
nitrite (at 1607 cm−1) and monodentate nitrate (1559 cm−1).30
The concentrations of NH3, N2O, NO and NO2 during the
transient reaction were simultaneously recorded (shown in
Fig. 6a). As NH3 was introduced to MnOx/TiO2, about 90 ppm
of N2O was observed, which resulted from the oxidation of
NH3 by the lattice oxygen of MnOx/TiO2. After NO + O2 passed
over NH3 pretreated MnOx/TiO2 at 250 °C, N2O concentration
rapidly increased from 90 to 160 ppm, and it then decreased
to the background of N2O in NO (9 ppm). The concentration
of N2O from the transient reaction at 250 °C was much higher
than that at 150 °C. It suggests that the formation of N2O
from the Eley–Rideal mechanism was obviously promoted
with the increase of reaction temperature.
+
Then, the reactants were introduced to MnOx/TiO2 in the
reverse order. MnOx/TiO2 was first treated with 500 ppm of
NO and 2% of O2 for 30 min at 150 °C followed by N2 purged
for 5 min. 500 ppm of NH3 was then introduced into the IR
cell (shown in Fig. 3b). After the adsorption of NO + O2 at
150 °C, MnOx/TiO2 was mainly covered by monodentate
Fig. 4 (a) Transient reaction taken at 150 °C upon passing NO + O2
over NH3 presorbed MnO2/TiO2; (b) Transient reaction taken at 150 °C
upon passing NH3 over NO + O2 presorbed MnO2/TiO2.
Then, the reactants were introduced to MnOx/TiO2 in the
reverse order. MnOx/TiO2 was first treated with 500 ppm of
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Fig. 6 (a) Transient reaction taken at 250 °C upon passing NO + O2 over
NH3 presorbed MnO2/TiO2; (b) Transient reaction taken at 250 °C upon
passing NH3 over NO + O2 presorbed MnO2/TiO2.
NH3 oxidation or the Langmuir–Hinshelwood mechanism
was difficult to be differentiated. However, the concentration
of N2O in the first 5 min was slightly higher than the con-
centration of N2O from the oxidation of 500 ppm of NH3.
It suggests that the Langmuir–Hinshelwood mechanism can
also contribute to N2O formation at 250 °C.
Finally, the IR spectra during the SCR reaction (i.e. 500 ppm
of NH3, 500 ppm of NO and 2% of O2 were simultaneously
introduced) at 250 °C were recorded. As shown in Fig. 5c,
only adsorbed H2O (at 1620 cm−1) and coordinated NH3 or
adsorbed NH4NO2 (at 1602 cm−1) can be clearly detected.
However, the band at 1534 cm−1 corresponding to adsorbed
NH4NO3 can not be observed. Fig. 5a shows that the disap-
pearance of NH4NO3 was much slower than that of NH4NO2.
It suggests that NH4NO3 did not form during NO reduction
over MnOx/TiO2 at 250 °C. Therefore, the contribution of
NH4NO3 decomposition to N2O formation can be neglected
and N2O formation at 250 °C mainly resulted from the
Eley–Rideal mechanism.
Fig. 5 (a) DRIFT spectra taken at 250 °C upon passing NO + O2 over NH3
presorbed MnO2/TiO2; (b) DRIFT spectra taken at 250 °C upon passing
NH3 over NO + O2 presorbed MnO2/TiO2; (c) DRIFT spectra taken at
250 °C upon passing NH3 + NO + O2 over MnO2/TiO2.
NO and 2% of O2 for 30 min followed by N2 purged for 5 min
at 250 °C. 500 ppm of NH3 was then introduced into the
IR cell (shown in Fig. 5b). After the adsorption of NO + O2 at
250 °C, MnOx/TiO2 was mainly covered by monodentate
nitrite (1607 cm−1) and monodentate nitrate (1559 cm−1).
After NH3 was introduced into the cell, the band at 1607 cm−1
corresponding to monodentate nitrite (at 1607 cm−1) rapidly
diminished. However, monodentate nitrate (1559 cm−1)
firstly shifted to 1534 cm−1. Then, it gradually diminished.
Finally, MnOx/TiO2 was mainly covered by coordinated NH3
(at 1602 cm−1). The concentrations of N2O and NH3 during
the transient reaction were simultaneously recorded (shown
in Fig. 6b). After NH3 was introduced to NO + O2 pretreated
MnOx/TiO2 for 10 min, NH3 in the outlet was observed.
Meanwhile, the concentration of N2O in the outlet rapidly
increased to about 110 ppm after the introduction of NH3,
and it then gradually decreased to about 90 ppm in 50 min
(shown in Fig. 6b). In this case, the origination of N2O from
4. Discussion
4.1 Mechanism of N2O formation
4.1.1 N2O formation from the Langmuir–Hinshelwood
mechanism. The reduction of NO (including the SCR reaction
and the NSCR reaction) through the Langmuir–Hinshelwood
mechanism can be approximately described as:3,6,22
NH3ðgÞ → NH3ðadÞ
ð7Þ
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NO → NO
ð8Þ
ðgÞ
ðadÞ

 Mn4+  NO(ad)   O2  Mn3+  NO2
ð9Þ
4
3

ð10Þ
2NO(ad)

O2   O2 Mn2NO3
2
NH3(ad)  NO2  H+  NH4NO2  N2  2H2O
NH3(ad)  NO3  H+  NH4NO3  N2O  2H2O

ð11Þ
ð12Þ
ð13Þ
1
4
1
2
 Mn3+

O2  Mn4+

 O2
Reaction 7 is the adsorption of gaseous ammonia on the
acid sites (i.e. Brønsted acid sites and Lewis acid sites) to
+
form adsorbed ammonia species including ionic NH4 and
coordinated NH3. Reaction 8 is the physical adsorption of
gaseous NO on MnOx/TiO2. Then, adsorbed NO is oxidized by
Mn4+ on MnOx/TiO2 to form adsorbed NO2 (i.e. reaction 9).
−
Reaction 10 is the oxidation of adsorbed NO by Mn4+ on
−
−
−
MnOx/TiO2 to NO3 . Subsequently, adsorbed NO2 and NO3
react with adsorbed NH3 species on the adjacent sites to
form NH4NO2 and NH4NO3 (i.e. reactions 11 and 12), respec-
tively. Finally, NH4NO2 and NH4NO3 are decomposed to N2
and N2O, respectively. Reaction 13 is the regeneration of Mn4+
on MnOx/TiO2.
The kinetic equations of the formation of N2 and N2O over
MnOx/TiO2 through the Langmuir–Hinshelwood mechanism
can be approximately described as:
d[N2 ]
d[NH4NO2 ]
ð14Þ
ð15Þ
 
 k1[NH4NO2 ]
 k2[NH4NO3]
dt
dt
d[N2O]
d[NH4NO3]
 
dt
dt
Fig. 7 Concentrations of NOx (a), NH3 (b), N2 formed (c) and N2O (d) in
the outlet of the reactor. Reaction conditions: [O2] = 2%, catalyst mass =
where, k1, k2, [NH4NO2] and [NH4NO3] are the decomposition
rate constants of NH4NO2 and NH4NO3, and the concentra-
tions of NH4NO2 and NH4NO3 on MnOx/TiO2, respectively.
[NH4NO2] and [NH4NO3] are mainly related to the
concentrations of NO adsorbed ([NO(ad)]) and Mn4+ on
MnOx/TiO2 (the deduction is shown in the ESI†). The GHSV
used in this work was quite high and there were generally
large amounts of NOx and NH3 in the outlet (shown in
Fig. 7a and b), so MnOx/TiO2 was almost saturated with the
adsorption of NO and NH3. Furthermore, there is generally
agreement that the SCR reaction starts with the adsorption
of NH3, which is very strong compared to the adsorption of
200 mg, total flow rate = 200 mL min−1, GHSV= 60 000 cm3 g−1 h−1
.
NO + O2 and the products.3 Thus, the increase of gaseous
NO concentration from 500 to 1000 ppm could not break
the adsorption equilibrium of NH3 and NO. It suggests that
[NO(ad)] and [NH3(ad)] would not vary after increasing the
concentrations of gaseous NO and NH3. As a result, the
concentrations of NH4NO2 and NH4NO3 on MnOx/TiO2
were independent of the concentrations of gaseous NO
and NH3.
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4.1.2 N2O formation from the Eley–Rideal mechanism.
According to eqn (20)–(22), the variation of –NH2 concen-
The reduction of NO (including the SCR reaction and the
NSCR reaction) through the Eley–Rideal mechanism can be
approximately described as:3,6,22
tration on MnOx/TiO2 can be described as:
d[NH2 ]

 k5[NH2 ][NO(g) ]  k4[NH2 ][Mn4+
 k3[NH3(ad) ][Mn4+
]
dt
ð24Þ
]
NH3ðgÞ → NH3ðadÞ
ð7Þ
As the reaction reached the steady state, –NH2 concentra-
tion on MnOx/TiO2 would not vary. Therefore,
NH3ðadÞ þ ≡Mn4þ → –NH2 þ ≡Mn3þ þ Hþ
–NH2 þ ≡Mn4þ → –NH þ ≡Mn3þ þ Hþ
ð16Þ
d[NH2 ]
ð25Þ

 0
ð17Þ
ð18Þ
dt
Thus,
–NH2 þ NO → N2 þ H2O
ðgÞ
k3[NH3(ad) ][Mn4+
k5[NO(g) ]  k4[Mn4+
]
[NH2 ] 
ð26Þ
]
–NH þ NO → N2O þ Hþ
ð19Þ
ð13Þ
ðgÞ
Then, the formation of N2 (eqn (22)) can be transformed as:
1
4
1
2
 Mn3+

O2  Mn4+

 O2
k3[NH3(ad) ][Mn4+
k5[NO(g) ]  k4[Mn4+
]
]
k3[NH3(ad) ][Mn4+
]
d[N2 ]
 k5[NO(g)
]
 k5
[Mn4+
]
]
dt
k5  k
4 [NO(g)
Reaction 16 is the activation of adsorbed ammonia species
by Mn4+ on MnOx/TiO2 to form amide species (–NH2). –NH2
on MnOx/TiO2 can be further oxidized to –NH (i.e. reaction
17). Then, gaseous NO was reduced by –NH2 and –NH on the
surface to form N2 and N2O (i.e. reactions 18 and 19),
respectively.
ð27Þ
According to eqn (21) and (23), the variation of –NH
concentration on MnOx/TiO2 can be described as:
The kinetic equation of reaction 16 can be described as:
d[NH]
 k4[NH2 ][Mn4+ ]  k6[NH][NO(g)
]
ð28Þ
dt
d[NH3(ad)
]
d[NH2 ]
dt
 k3[NH3(ad) ][Mn4+
]
ð20Þ
 
dt
As the reaction reached the steady state, –NH concentra-
tion on MnOx/TiO2 would not vary. Therefore,
where k3 and [–NH2] are the kinetic constant of reaction 16
and the concentration of –NH2 on MnOx/TiO2, respectively.
The kinetic equation of reaction 17 can be described as:
d[NH]
ð29Þ
 0
dt
d[NH]
dt
d[NH2 ]
dt
 k4[NH2 ][Mn4+
]
ð21Þ
 
Thus,
where k4 and [–NH] are the kinetic constant of reaction 17
and the concentration of –NH on MnOx/TiO2.
The kinetic equations of reactions 18 and 19 can be
described as:
k3[NH3(ad) ][Mn4+
k5[NO(g) ]  k4[Mn4+ ] k6[NO(g)
]
k4[NH2 ][Mn4+
]
k4[Mn4+
]
]
[NH] 

i
k6[NO(g)
]
ð30Þ
Then, the formation of N2O (eqn (23)) can be transformed as:
d[NO(g)
]
d[N2 ]
d[NH2 ]
dt
ð22Þ
ð23Þ
 
 
 
 k5[NH2 ][NO(g) ]
dt
dt
k3[NH3(ad) ][Mn4+
k5[NO(g) ]  k4[Mn4+ ] k6[NO(g)
]
d[N2O]
k4[Mn4+
]
]
 k6[NO(g)
]
i
d[NO(g)
]
d[N2O]
d[NH]
dt
 
 k6[NH][NO(g) ]
ð31Þ
k3[NH3(ad) ][Mn4+
k5[NO(g) ]  k4[Mn4+
]
dt
dt
dt

(k4[Mn4+ ])
]
where, k5, k6 and [NO(g)] are the kinetic constants of reac-
tions 18 and 19, and the concentration of gaseous NO,
respectively.
Taking account of the contributions of both the Langmuir–
Hinshelwood mechanism and the Eley–Rideal mechanism,
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the formation of N2 and N2O can be approximately described
as follows:
amounts of N2 and N2O formed over the whole catalyst bed
should be described as follows:
k3[NH3(ad) ][Mn4+
]
d[N2 ]
k3[NH3(ad) ][Mn4+
]
CN
t k5
dt
 k1[NH4NO2 ]  k5
[Mn4+
4 [NO(g)
]
]
 k [NH NO ]t  0
ð32Þ
ð33Þ
dt
1
4
2
[Mn4+
]
2
ð36Þ
k5  k
k5  k
4 [NO(g)
]
t
k3[NH3(ad) ][Mn4+
k5[NO(g) ]  k4[Mn4+
]
]
d[N2O]
 k2[NH4NO3] 
(k4[Mn4+ ])
k3[NH3(ad) ][Mn4+
]
0 k5[NO(g) ]t  k4[Mn4+
dt
t
CN
(k4[Mn4+ ])dt
 k [NH NO ]t  
O
2
4
3
2
]
The concentrations of NH4NO2 and NH4NO3 on MnOx/TiO2
were independent of gaseous NO and NH3, so the contribution
of the Langmuir–Hinshelwood mechanism to the formation of
N2 and N2O would not vary after the increase of gaseous NO
concentration. However, the contribution of the Eley–Rideal
mechanism to N2 formation would increase after the increase
of gaseous NO concentration (hinted by eqn (32)). Meanwhile,
the contribution of the Eley–Rideal mechanism to N2O forma-
tion would decrease after the increase of gaseous NO concen-
tration (hinted by eqn (33)).
ð37Þ
where t is the time how long gaseous NO passed through the
catalyst column, which is inversely proportional to the GHSV.
Fig. 7a shows that the concentrations of gaseous NO in
the outlet of the reaction with 1000 ppm of NO and 500 ppm
of NH3 were much higher than that with 500 ppm of NO and
500 ppm of NH3. Meanwhile, the concentration of gaseous
NO in the inlet of the reaction with 1000 ppm of NO and
500 ppm of NH3 were twice that with 500 ppm of NO and
500 ppm of NH3. They suggest that the concentrations of
gaseous NO at each section of the catalyst bed during the reac-
tion with 1000 ppm of NO and 500 ppm of NH3 were all higher
than those with 500 ppm of NO and 500 ppm of NH3. Hinted
by eqn (36), the amount of N2 formed during the reaction with
1000 ppm of NO and 500 ppm of NH3 was higher than that with
500 ppm of NO and 500 ppm of NH3, which was demonstrated
in Fig. 7c. Hinted by eqn (37), the amount of N2O formed during
the reaction with 1000 ppm of NO and 500 ppm of NH3 was less
than that with 500 ppm of NO and 500 ppm of NH3, which was
demonstrated in Fig. 7d. As a result, N2O selectivity of the reac-
tion with 1000 ppm of NO and 500 ppm of NH3 was less than
that with 500 ppm of NO and 500 ppm of NH3 (shown in Fig. 2b).
The concentrations of gaseous NO in the outlet and in the
inlet during the reaction with 1000 ppm of NO and 1000 ppm
of NH3 were both close to those with 1000 ppm of NO and
500 ppm of NH3 below 200 °C (shown in Fig. 7a). It suggests
that the concentrations of gaseous NO at each section of the
catalyst bed during the reaction with 1000 ppm of NO and
1000 ppm of NH3 were all close to those with 1000 ppm of NO
and 500 ppm of NH3 below 200 °C. Therefore, the amounts of
N2 and N2O formed during the reaction with 1000 ppm of NO
and 1000 ppm of NH3 were close to those with 1000 ppm of
NO and 500 ppm of NH3 below 200 °C, which is demonstrated
in Fig. 7c and d. As a result, N2O selectivity of the reaction
with 1000 ppm of NO and 1000 ppm of NH3 was close to that
with 1000 ppm of NO and 500 ppm of NH3 below 200 °C
(shown in Fig. 2b). However, little NH3 can be observed in the
outlet of the reaction with 1000 ppm of NO and 500 ppm of
NH3 above 200 °C (shown in Fig. 7b). It suggests that some
catalyst in the bottom of the catalyst bed did not take part in
the reaction because gaseous NH3 had been completely con-
sumed. As the concentration of gaseous NH3 increased from
500 to 1000 ppm, a large amount of NH3 can be observed in
the outlet above 200 °C (shown in Fig. 7b). Therefore, most of
If the reduction of NO mainly followed the Langmuir–
Hinshelwood mechanism, N2O selectivity can be described as:
C
k2[NH4NO3]
N2
O
N2O selectivity 

ð34Þ
CN O  CN
k1[NH4NO2 ]  k2[NH4NO3]
2
2
The concentrations of NH4NO2 and NH4NO3 did not
change after the increase of gaseous NO and NH3, so N2O
selectivity was independent of gaseous NO concentration.
If the reduction of NO mainly followed the Eley–Rideal
mechanism, N2O selectivity can be described as:
k4[Mn4+
k5[NO(g) ]  k4[Mn4+
]
C
N2
O
N2O selectivity 

ð35Þ
CN O  CN
]
2
2
Eqn (35) suggests that N2O selectivity would decrease after
the increase of gaseous NO concentration.
There is generally agreement that the Langmuir–Hinshelwood
mechanism plays an important role on the SCR reaction at
lower temperatures.3,6 Meanwhile, the Eley–Rideal mechanism
can also contribute to NO reduction at lower temperatures.
The Eley–Rideal mechanism was obviously promoted with the
increase of reaction temperature and it predominated over the
SCR reaction at higher temperatures.3,6
4.2 Effect of gaseous NO concentration in the inlet on N2O
selectivity
If the concentrations of gaseous NO and NH3 were sufficiently
high, the whole catalyst bed was saturated with the adsorption
of gaseous NO and NH3. Thus, [NH3(ad)], [NH4NO2] and
[NH4NO2] can be regarded as constants on the whole catalyst
bed. However, the concentration of gaseous NO at the bottom
of the catalyst bed was much less than that at the top of the
catalyst bed due to the reduction of NO. Therefore, the
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the catalyst took part in the reaction with 1000 ppm of NO
and 1000 ppm of NH3 above 200 °C. They suggest that t of
the reaction with 1000 ppm of NO and 1000 ppm of NH3
was higher than that with 1000 ppm of NO and 500 ppm
of NH3 above 200 °C. Hinted by eqn (36) and (37), the
amounts of N2 and N2O formed during the reaction with
1000 ppm of NO and 1000 ppm of NH3 above 200 °C were
both much higher than that with 1000 ppm of NO and
500 ppm of NH3 (shown in Fig. 7c and d). Gaseous NO con-
centration at the top of the catalyst bed was much higher
than that at the bottom of the catalyst bed due to NO reduc-
tion. It suggests that N2O selectivity at the top of the catalyst
bed was much less than that at the bottom of the catalyst
bed. As a result, N2O selectivity of the reaction with 1000 ppm
of NO and 1000 ppm of NH3 above 200 °C were higher than
that with 1000 ppm of NO and 500 ppm of NH3, which was
demonstrated in Fig. 2b.
Fig. 7a shows that the concentrations of gaseous NO at
each section of the catalyst column during the reaction with
1000 ppm of NO and 1000 ppm of NH3 were all higher than
those with 500 ppm of NO and 500 ppm of NH3. Hinted by
eqn (35), N2O selectivity of the reaction with 1000 ppm of NO
and 1000 ppm of NH3 was less than that with 500 ppm of NO
and 500 ppm of NH3 (shown in Fig. 2b).
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N2O selectivity of the low temperature SCR reaction over
MnOx/TiO2 was related to gaseous NO concentration in the
flue gas. The lower concentration of gaseous NO in the flue
gas would cause the higher N2O selectivity. If the concentra-
tion of gaseous NO in the flue gas is very low, low tempera-
ture SCR of NO with MnOx/TiO2 as the catalyst could not be
the right choice for the control of NO emission due to the
lower N2 selectivity. Furthermore, N2O selectivity at the bot-
tom of the catalyst bed was much higher than that at the top
of the catalyst column due to the lower gaseous NO concen-
tration. Therefore, the decrease of GHSV to excessively pursue
the removal efficiency of NO will cause lower N2 selectivity.
Acknowledgements
This study was financially supported by the National Natural
Science Fund of China (Grant No. 21207067 and 41372044),
Environmental scientific research of Jiangsu Province (2012026),
the Fundamental Research Funds for the Central Universities
(grant no. 30920130111023), the Zijin Intelligent Program, Nanjing
University of Science and Technology (grant no. 2013–0106), and
special fund of State Key Joint Laboratory of Environment
Simulation and Pollution Control.
Notes and references
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232 | Catal. Sci. Technol., 2014, 4, 224–232
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