Selective catalytic oxidation of ammonia over MnO: X-TiO2 mixed oxides

Article abstract of DOI:10.1039/c6ra20999h

The selective catalytic oxidation of ammonia to nitrogen (NH₃-SCO) was investigated over MnO_x-TiO₂. The physicochemical properties of MnO_x-TiO₂ were characterized by XRD, O₂-TPD, NH₃

Full text of DOI:10.1039/c6ra20999h

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Selective catalytic oxidation of ammonia over MnO -TiO mixed
x
2
oxides
Dongdong Song, Xunzhe Shao, Menglong Yuan, Li Wang*, Wangcheng Zhan, YangLong
Guo,Yun Guo*, Guanzhong Lu
Abstract: The selective catalytic oxidation of ammonia to nitrogen (NH
investigated over MnO ꢀTiO . The physicochemical properties of MnO
characterized by XRD, O ꢀTPD, NH ꢀTPD, H ꢀTPR and XPS, and reaction
3
ꢀSCO) was
x
2
x 2
ꢀTiO were
2
3
2
mechanism was studied by in situ DRIFTS. The addition of Mn into TiO accelerated
2
the support phase transformation and the formation of MnꢀOꢀTi bond.
MnO
x
(0.25)ꢀTiO
2
behaved the best performance in NH
3
3
ꢀSCO, NH complete
o
o
o
conversion was obtained at 200 C with the temperature window (180 Cꢀ300 C) for N
2
yield >80%. The formation of MnꢀOꢀTi provided abundant oxygen vacancy which
promoted the absorption and dissociation of molecule oxygen to form active oxygen
species. The finely dispersed MnO
x
species on support favored NH
3
adsorption. N
2
O
produced in the whole temperature range while NO
x
only at high temperature ( >
o
2
50 C). N
2
O was from the combination of two HNO species at low temperature while
and nitrate/nitrite species reaction at high temperature.
Keywords: NH ꢀSCO, TiO , Mn O , mechanism, low temperature
from NH
3
3
2
2
3
Laboratory for Research Institute of Industrial Catalysis, East China University of Science and
Technology, Shanghai, China
E Mail: wangli@ecust.edu.cn, yunguo@ecust.edu.cn
Fax: +86 21 6425ꢀ3703; Tel: +86 21 6425ꢀ3703

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1
. Introduction
With the increase of automobiles, NO as one of the exhaust of vehicles lead to
x
the atmosphere worsen in the case of photochemical smog, acid rain. How to
efficiently remove NO become urgent for environment protection. Selective catalytic
x
reduction (SCR) of NO with NH is a mature technology for reducing NO emission.
x
3
x
In a typical SCR, the main reaction proceeds in the following way:
NH + 4NO + O → 4N + 6H
In order to efficiently remove NO, the over stoichiometric NH
3
results in the NH slip at low temperature. The concentration of NH slip of SCR units
4
3
2
2
2
O
3
in application
3
has been regulated to less than 10 ppm in China. Catalytic oxidation, with low level of
energy consumption and high purification efficiency, is a promising technology for
NH
3
2
emission control. Due to the variant products in NH
3
oxidation, N
2 x
, NO (NO,
NO
) , N
2
O, the selective catalytic oxidation (SCO) of NH
3
to N is a potential
2
technology to control NH
3
slip without secondary pollutants .
Supported noble metal and transition metal oxides work well in NH
3
ꢀSCO . The
o
1
2
higher NH
3
2
conversion less than 180 C could be obtained on Ag/Al
2
O
3
, CuO/RuO
2
,
3
4
Pt/CuO/Al
3 2
O , Ir , but the N selectivity need further improved. Compared with the
5
6
7
8
2 3 2 5
supported noble catalysts, transition metal oxides catalysts, CuO , Fe O , NiO ,V O ,
o
exhibit higher N
2
selectivity at rather high temperature (300ꢀ400 C). Exploring the
activity of metal oxides at low temperature makes it possible for practical application
in future.
4+
3+
9
The presence of Mn /Mn type redox couples , the tunnel structure and active
1
0
lattice oxygen species contribute to the excellent performance of manganese oxides
11
12
in oxidation. Due to excellent SO
usually used as the support. So MnO
temperature Selective Catalytic Reduction of NH
2
tolerance and oxygen sensitivity , TiO
2
is
x
/TiO catalysts have been studied at low
2
1
3
3
3
(NH ꢀSCR) .
Several mechanisms of the SCO process have been suggested. Three major
reaction pathways have been proposed. The imido mechanism suggests that the
14
intermediate species are the imido (NH) and nitrosyl (HNO) species , and N
2
comes
from their reaction. The hydrazine (N ) mechanism proposes that N from two
2
H
4
2 4
H

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15
amide species (NH
mechanisms is NH
iꢀSCR) mechanism focuses on the reaction between NH
Different mechanism varies with the properties of the catalysts.
In this paper, a series of MnO ꢀTiO were prepared by solꢀgel method and the
2 2 2 4
), and N from N H oxidation . The key step of the above
3
dehydrogenation. The internal selective catalytic reduction
1
6
(
3 x
and NO species .
x
2
catalytic performance of NH
3
selective oxidation was investigated. XRD, Raman, N
2
absorptionꢀdesorption, ICP, O
2
ꢀTPD, H ꢀTPR and XPS were used to characterize the
2
physicochemical properties of the samples, and in situ DRIFT was applied to
understand the oxidation reaction process. The correlation between the activity and
physicochemical properties was investigated.
2
. Experimental
2
.1 catalyst preparation
The catalyst was obtained by solꢀgel method. All the chemicals used are from
shanghai Lingfeng chemical reagent Co. Butyl titanate (0.02mol) and diethanol
amine(0.02mol) solution were added into 30ml ethanol denoted as solution A.
2 2
Mn(OAc) ·4H O and acetic acid were dissolved in distilled water to get solution B.
Then solution B was dropwised into solution A under vigorous stirring to form a
brownish red homogeneous sol. After aging from the sol to the gel , the gel was dried
at 353 K for 12h and calcined at 773 K for 4 h in air. The obtained sample was
denoted as MnO
.2 Activity test
Catalytic activity measurements were carried out in a tubular quartz, fixedꢀbed
x 2 2
(z)ꢀTiO , where z represented the weight ratio of Mn/TiO .
2
reactor (4 mm internal diameter). 500ppm ammonia, 5vol% oxygen and helium at
flow rates of 200 ml/min were passing through the catalyst bed (50mg, 40ꢀ60 mesh).
The inlet and outlet gases were analyzed by an Nicolet FTIR 6700 spectrometer
(
Thermo Fisher Scientific, USA) and 42iꢀHL NOꢀNOxꢀchemiluminescence analyzer
Thermo Fisher Scientific, USA). The NH conversion, N , NO , N O yield were
(
3
2
x
2
calculated from the concentrations of the gases according to Eqs. (1) ~(4).

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[
NH
3
]
in −[NH
3]out
NH
3
conversion(%) = (
)×100%
(1)
[
NH
3]in
[
NH
3
]in −[NH
3]
out −[NO]out −[NO
2
]
out − 2[N O]out
2
N
2
yield(%) = (
)×100% (2)
[
NH
3]in
[
NO]out +[NO
2 out
]
NO
x
yield(%) =
×100%
(3)
(4)
[
NH
3]in
2
[N
2
O]out
N
2
O yield(%) =
×100%
in
[
NH
3
]
2
.3 Catalyst characterization
The N
adsorption−desorption isotherms were measured on ASAP2020
2
o
o
(
Micromeritics, USA) at −196 C. All samples were degassed at 180 C before test.
Powder Xꢀray diffraction (XRD) measurements of the catalysts were carried out
on a Bruker/D8 XꢀRay diffractometer (Bruker, Germany) by CuKα radiation
(λ=0.154056nm) at 40 kV and 40 mA. The patterns were taken over the 2θ range from
°
°
°
1
0 to 80 at a scan speed of 6 /min.
Xꢀray Photoelectron Spectroscopy (XPS) analysis was performed on ESCALAB
50Xi (Thermo Fisher Scientific,USA) with a monochromatic Al Kα source (hν =
486.6 eV), operating at 150 W. The binding energy (BE) of adventitious C1s (284.6
2
1
eV) was used as a reference.
2
Hydrogen temperature programmed reduction (H ꢀTPR) experiments were
carried out on a commercial system PX200 (PengXiang, China) equipped with a
thermal conductivity detector (TCD). Ammonia temperature programmed desorption
(
NH
3
ꢀTPD) and Oxygen temperature programmed desorption (O
2
ꢀTPD) were all
performed in the same apparatus. In H
2
ꢀTPR, the samples (50mg) were heated in 5
vol.% H
In NH
2
/N
2
(40 mL/min) from room temperature to 850 °C at the ramp of 10 °C/min.
o
3
ꢀTPD, the samples (50 mg) were preheated in N
2
at 500 C for 1 h before test.
o
After 5%NH
3
adsorption saturation at 30 C, N
2
was introduced to remove
3 2
physicꢀadsorption NH . In O ꢀTPD, the outlet gas was recorded with an online mass
spectrometer apparatus (HIDEN QICꢀ20, England). The samples (100mg) were
o
pretreated in 3%O
2
in He stream (30ml/min) at 450 C for 1h, then cooled down to
ambient temperature in He to get stable MS baseline. In all the experiments the heat
o
o
rate (10 C/min ) was the same from 50 to 800 C.
The Raman spectra were obtained on a Renishaw inVia Reflex (Renishaw,

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England) spectrometer equipped with a CCD detector at ambient temperature and
moistureꢀfree conditions. The excitation source was the 514.5 nm line of Ar ion laser.
The laser power was set at 3 mW.
The in situ DRIFTS were performed on a Nicolet 6700 FTꢀIR spectrometer
(Thermo Fisher Scientific,USA) with an MCT detector. In the DRIFT cell with ZnSe
o
windows connected with a gas flow system, the sample was pretreated at 400 C in Ar
for 1 h before each test. The background spectra were recorded at each temperature.
5
00ppm NH
3 2
, 5%O and Ar in balance were used in the experiments at the total flow
rate of 50 ml/min.
3
. Results
.1 NH oxidation over MnO ꢀTiO catalysts
3
3
x
2
3 x 2
The catalytic performance of the SCO of NH on MnO ꢀTiO catalysts at various
temperatures is shown in Fig. 1. Temperature had positive effects on NH
3
conversion,
the higher temperature was, the higher NH
3
conversion would be. The complete NH
3
o
conversion temperature was almost at 400 C, and 80% N
2
yield was in a sharp
o
temperature window from 290ꢀ340 C on TiO
2
. With the introducing of Mn, the low
temperature activity was enhanced and broadened the operation window of high N
2
3
yield accordingly. The optimum Mn loading amount was 25wt%, NH could be
o
completely converted at 200 C and the temperature range of N
2
yield above 80% was
o
in the range of 180ꢀ300 C at the weight space velocity of 240,000 ml/(min.g). At
o
1
50 C, NH
3
conversion on MnO
x 2 2
(0.25)ꢀTiO reached to 78% while on TiO only 10%.
The increase in NH
3
conversion displayed the promotion effects of Mn species.
In order to exclude the effect of surface area on activity, the specific activity are
calculated and shown in Table 1. With the Mn loading increasing from 0.1 to 0.25, the
2
ꢀ6
o
specific activity enhanced from 0.7 to 3.6（mmol/(min.m )×10 ） at 130 C, while the
further increasing of Mn loading, it had less effects on specific activity. Therefore,
there existed an optimum Mn amount (25%) for selective NH
3
oxidation.
The Nꢀcontaining product distribution (N
2
, NO
x
and N O) is also shown in Fig.1
2
o
N
2
O was produced even from low temperature (>100 C) with the yield lower than

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o
1
x
0%; while the appearance of NO were at high temperature (>250 C) with yield even
o
up to 20% at 350 C. The yield of N
2
O decreased with the increase of NO
x
yield.
100
100
8
0
80
60
40
20
0
6
0
40
MnOx(0.1)-TiO2
MnOx(0.2)-TiO2
MnOx(0.25)-TiO2
MnOx(0.3)-TiO2
TiO2
MnOx(0.1)-TiO2
MnOx(0.2)-TiO2
MnOx(0.25)-TiO2
20
MnO
TiO2
(0.3)-TiO
x
2
0
50
100
150
200
250
300
350
50
100
150
200
250
C)
300
350
Temperature(/
)
Temperature(
°
1
00
40
MnOx(0.1)-TiO2
MnOx(0.2)-TiO2
MnOx(0.25)-TiO2
MnOx(0.3)-TiO2
MnOx(0.1)-TiO2
MnOx(0.2)-TiO2
MnOx(0.25)-TiO2
MnOx(0.3)-TiO2
TiO2
8
6
4
2
0
0
0
0
0
3
2
1
0
0
0
0
TiO
2
50
100
150
200
250
300
350
50
100
150
200
250
300
350
Temperature(
°C)
Temperature (℃ )
Fig. 1 The results of activity test over TiO
2
x 2
and MnO (z)-TiO catalysts
3
3
.2 Catalysts characterization
.2.1 BET surface area and XRD
The texture data of the samples is shown in Table 1. All the samples were with
mesoꢀpore structure because of the pore size within 4ꢀ9nm. The surface area of
2
support was only 45m /g, and the surface area of MnO
x
ꢀTiO
2
increased until Mn
2
loading amount up to 25wt%. The highest value of surface area (113 m /g) was
x 2
obtained on MnO (0.2)ꢀTiO for the addition of Mn efficiently suppressed the growth
of TiO crystallite which resulted in the increase of surface areas. With the further
2
increasing loading, Mn led to the decrease of surface area because of the aggregation
of Mn species on support.
x 2
The XRD patterns of MnO ꢀTiO catalysts are presented in Fig.2. It suggested
that the support was in the form of anatase. With the introducing of Mn, the

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diffraction peaks of anatase weakened and the characteristic peaks of rutile gradually
intensified. The support of MnO (0.2)ꢀTiO exhibited the complete rutile phase which
x
2
indicated that Mn loading speeded up the anataseꢀtoꢀrutile (AꢀR) transition. The
°
doping of Mn caused the diffraction peak (2θ=36.2 ) shifted to higher angles
2
compared with TiO , which was attributed to the lattice shrinkage due to the lower
4
+
4+
ionic radius of the Mn (0.060nm) with respect to Ti (0.068nm).
New characteristic peaks assigned to Mn and Mn
was due to the separation of Mn from
2
O
3
3 4
O were detected on
MnO
x
2 x
(0.3)ꢀTiO . The appearance of MnO
2
TiO .
Table 1 The texture data of samples
2
Catalysts
S
BET(m /g)
Pore
Crystallite Mn content
Specific activity
a
b
2)
ꢀ6
size(nm)
size (nm)
(wt.%)
(mmol/(min.m )×10
o
1
30 C
Pure TiO
2
45.1
74.2
113.2
46.6
41.7
8.7
5.0
4.2
6.2
4.8
21.0
8.8
－
8
0.5
0.7
0.8
3.6
3.1
MnO
MnO
MnO
MnO
x
(0.1)ꢀTiO
2
2
x
(0.2)ꢀTiO
10.4
11.7
11.1
18
x
(0.25)ꢀTiO
2
27.8
33.7
x
(0.3)ꢀTiO
2
a
calculated from the Scherer’s equation according to the (110) diffraction peak of TiO
2
b
Mn content of the catalyst determined by ICPꢀAES
∗
∇
♦
•
Mn O3
2
Mn O4
• ♦
3
rutile
anatase
∗
♦
♦
♦
30
35
40
2
θ
(°)
∗
MnOx(0.3)ꢀTiO2
∇∇
∗
•
♦
♦
∇
∇
∗
∇∗
•
∇
♦
∇
♦
♦
♦
♦
MnOx(0.25)ꢀTiO2
MnOx(0.2)ꢀTiO2
∇∇
♦
♦
♦
♦
•∇ ♦∇
♦
♦
♦
•
♦
MnOx(0.1)ꢀTiO2
TiO2
♦
♦
♦
•
♦
♦
•
•
•
•
•
♦
• •
•
10
20
30
40
50
60
70
80
2
θ(degree)
Fig. 2 XRD patterns of MnO
x
(z)-TiO
2
catalysts
3
.2.2 Raman spectroscopy
Raman spectroscopy has become an increasingly valuable tool for the
investigation of the structure variance. The spectra of MnO , Mn , Mn , TiO
and MnO (z)ꢀTiO are comparatively shown in Fig.3. Peaks for MnO were observed
2
2
O
3
3
O
4
2
x
2
2

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ꢀ1
ꢀ1
at 760, 664, 578 and 535 cm as well as Mn
2
O
3
at 700, 646, 308 and 197 cm ,
ꢀ1
Mn O at 640, 360 and 300cm . The support was characterized by anatase (bands at
3
4
ꢀ1
ꢀ1 17
1
43, 193 ,396, 512 and 638 cm ) and rutile (band at 446 cm ) , which corresponded
to the phase transition of the support in XRD results.
In the typical sample of MnTiO , the characterized peaks could be observed at
3
−
1
6
84, 611, 465, 360, 336, 264, 236, 202cm separately, and the intensive Raman band
−1
18
at 684cm is regarded as the typical feature for MnTiO
3
.The strongest Raman mode
ꢀ1
around 684cm is assigned to the highest frequency vibrational mode of MnO
6
ꢀ1
octahedral, while the weak phonon modes below 300cm arises from lattice
1
9
vibrations .
With the addition of Mn, the characteristic peak of MnTiO
appeared. The peak intensified with the increasing of Mn loading amount up to
5wt%; and the further increase in Mn loading, such as 30wt%, led to the peak at 684
ꢀ1
3
at 684cm was
2
ꢀ
1
cm broadened which was due to the overlapping with MnO
fundamentals, absorption at 684, 610, 460, 360, 335, 262, 234, 202cm , were clearly
observed on MnO (0.25)ꢀTiO which indicated the formation of MnTiO . When taken
the sensitivity of Raman in the vibration of chemical bonding into account, it could
understand why MnTiO species could not be detected by XRD. It was known from
Raman results that the formation of MnꢀOꢀTi supported the strongest interaction
between TiO and Mn on MnO (0.25)ꢀTiO
x
. Eight Raman active
ꢀ1
x
2
3
3
2
x
2
.

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3
97
08
MnO (0.3)ꢀTiO
x 2
1
6
84
610
MnO (0.25)ꢀTiO
335
463
x
2
234
360
2
02 262
MnO (0.2)ꢀTiO
x
2
MnO (0.1)ꢀTiO
x
2
143
638
5
12
396
193
446
TiO2
634
Mn O
3
4
3
00
3
60
35
664
5
578
760
MnO2
Mn O
197
308
700
646
2
3
500
1000
ꢀ1
Raman shift(cm )
Fig. 3 Raman spectra of MnO , TiO and MnO (z)ꢀTiO
2
x
2
x
3
.2.3 H
2
ꢀTPR
ꢀTemperature programmed reduction (H
oxidation states of the samples. The H ꢀTPR profiles of the catalysts and the H
consumption amount are presented on Fig.4 and Table 2.
H
2
2
ꢀTPR) was used to investigate the
2
2
MnOx(0.3)ꢀTiO2
b
c
e
e
e
a
MnOx(0.25)ꢀTiO2
MnOx(0.2)ꢀTiO2
MnOx(0.1)ꢀTiO2
d
d
TiO2
2
00
400
600
800
Temperature(℃
)
Fig.4
2 x 2
H -TPR profiles of MnO (z)-TiO catalyst

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2
A weak peak was observed on the reduction of TiO in the temperature range of
o
5
00ꢀ800 C with 0.04mmol/g total H
2
consumption. With the presence of Mn, a peak
o
with shoulder at 100ꢀ450 C was observed and intensified with the increase of Mn
concentration. Combined with the ICP and the integral H consumption, it was known
that the chemical value of Mn was around +3 which indicated that MnO was mainly
in the form of Mn . By deꢀconvoluting the main reduction peak, three peaks were
2
x
2
O
3
20
observed which presented the reduction of different species. Ettireddy assigned the
o
peak at 215 C to the reduction of anatase TiO
2
closely interacted with MnO
concentration. Although XRD results
presented the support AꢀR transformation with the increase of Mn amount, the H
x
, and the
peak area correlated with the anatase TiO
2
2
consumption amount of peak a still kept increasing shown in Table 2. So it excluded
the possibility that peak a was from the support reduction, and it should be assigned to
the reduction of Mn
2 3 2 2 3
O closed to TiO . The reduction of bulk Mn O took place
through a distinct twoꢀstep process: the first step involved the reduction of Mn
2
3
O to
2
1
3 4 3 4 2
Mn O ,and the second step represented the reduction of Mn O to MnO . The H
consumption ratio confirmed the dominant Mn
were denoted to the reduction of bulk Mn
assigned to the reduction of Mn
when the introducing of Mn loading up to 30wt%.
2
O
3
species, so the peak of b and c
o
2
O
3
. The shoulder peak e at 400ꢀ550 C was
3
4 3 4
O , for XRD also confirmed the existence of Mn O
Table 2 The data obtained from H
2 2
-TPR , O -TPD and XPS
3
+
b
H
2
consumption (mmol/g)
Mn
O /
Surface atmo ration
of Mn/Ti
S
a
Catalysts
(O
α
β
+O )
total
a
b
c
d
e
L S
/Mntotalt (O +O )
TiO
(0.1)ꢀTiO
(0.2)ꢀTiO
2
0.04
0.75
2.11
3.13
3.20
－
0.1
0.3
－
－
0.04
－
－
－
－
－
0.16
0.19
0.25
0.3
－
0.3
0.7
1.2
1.7
MnO
MnO
x
2
2
0.33 0.27
0.99 0.75
1
0.58
0.59
0.61
0.56
x
－
0.05
0.27
0.5
2.9
5.9
2.6
MnO (0.25)ꢀTiO
0.43 1.42
0.34 1.38 0.98
ꢀTPD
b: Obtained from XPS
1.0
－
x
2
MnO
x
(0.3)ꢀTiO
2
－
0.27
a: Obtained from O
2
3
.2.4 O
2
ꢀTPD
O
2
ꢀTPD technique is useful to study oxygen species of metal oxides,

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2
characterizing the type of oxygen species and their properties. The O ꢀTPD results are
shown in the Fig.5. Three desorption peaks were observed in the temperature range of
o
o
o
o
o
o
2
00 Cꢀ300 C, 400 Cꢀ650 C and 650 Cꢀ800 C respectively. The weak peak (O
α
) at
weakly bound to the
) at 400 Cꢀ600 C with shoulder denoted to
o
o
ꢀ
2
00 Cꢀ300 C is related to desorption of superoxide ion O
2
o
o
surface of Mn
2
O
3
. The broad peak (O
β
2−
2
−
desorption of peroxide ion O
/O bound to oxygen vacancies. The highꢀtemperature
o
o
2−
peak at 600 Cꢀ800 C is assigned to the desorption of lattice oxygen ion O in
22, 23
Mn
2
O
3
.
Compared with the support, the intensity of the peaks (O
amount of Mn up to 25%, and the decrease in intensity was also observed on
MnO (0.3)ꢀTiO . The increased area of O implied that the strong interaction between
TiO and MnO which led to the enhancement in lattice defects and oxygen vacancies
and promoted the adsorption/mobility/desorption of O
TiꢀOꢀMn or MnTiO detected by Raman also confirmed the strong interaction
α β
, O ) increased with the
x
2
β
2
x
2
−
−
2
/O . The formation of
3
between TiO
2
x
and MnO . Meanwhile, the peak at 200ꢀ300 °C attributed to the
ꢀ
desorption of O
2
becomes a little stronger, which was consistent with the XPS results
(0.25)ꢀTiO
that more surface oxygen species existed on the surface of MnO
x
2
.
2
−
The temperatures of the O desorption peaks increased as Mnꢀsubstitution
amounts increased. It demonstrated that the interaction of MnꢀO bond were
2−
strengthened so the desorption of O became difficult, which was in an agreement
with the result of H ꢀTPR.
2
Oα
MnOx(0.3)ꢀTiO
2
MnOx(0.25)ꢀTiO
MnOx(0.2)ꢀTiO
MnOx(0.1)ꢀTiO
TiO
2
2
2
2
1
00
200
300
400
500
Temperature(℃
)
MnOx(0.3)ꢀTiO2
MnOx(0.25)ꢀTiO2
MnOx(0.2)ꢀTiO2
MnOx(0.1)ꢀTiO2
TiO2
1
00
200
300
400
500
600
700
800
Temperature(℃
)
Fig.5
2 x 2
O -TPD profiles of MnO (z)-TiO catalysts

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3
.2.5 NH
3
ꢀTPD
ꢀTPD profiles of catalysts are shown in Fig.6A. The desorbed
The NH
3
temperature and desorbed ammonia amount directly relate to the kind and amount of
acid sites on the sample. The lower temperature desorption meant the weaker
interaction between NH
3
and acid sites and vice versa. A NH
desorption at weak acid sites
desorption temperature at weak acid sites
3
desorption peak was
o
found at 100ꢀ500 C which was related to the NH
3
2 3
Compared with the TiO , the NH
shifted to lower temperature. The presence of Mn promoted the amount of acid sites
increase which suggested that Mn was the source of acid. The maximum acid sites
x 2
seemed to be obtained on MnO (0.2)ꢀTiO , while the further increasing of Mn led to
the decrease of acid sites amount which was attributed to the aggregation of
manganese oxide.
By excluding the effect of surface area, the acid density is calculated and the
results are shown in the Fig.6B. Unlike the results in Fig.6A , the presence of Mn had
no obvious effects on acid density until Mn loading was up to 0.25. With the further
increase of Mn loading up to 0.3, it led to the decrease of acid density on catalyst. The
highest acid density was obtained on MnO
x
(0.25)ꢀTiO
2
which favored NH
3
adsorption
and activation.
4
.0
100
(
A)
(B)
T=165℃
MnOx(0.3)ꢀTiO2
3.5
8
6
4
2
0
0
0
0
0
3
2
2
.0
.5
.0
MnOx(0.25)ꢀTiO2
MnOx(0.2)ꢀTiO2
MnOx(0.1)ꢀTiO2
TiO2
1.5
1.0
0.5
0.0
0
10%
20%
25%
30%
1
00
200
300
400
500
the content of manganese
Temperature(/
)
3 x 2
Fig.6 NH -TPD profiles (A) and acid destiny (B) of MnO (z)-TiO catalysts
3
.2.6 Xꢀray photoelectron spectroscopy
MnO ꢀTiO catalysts were investigated by XPS to understand the surface atomic
x
2
concentration and oxidation states of Mn on catalysts. The XPS spectra of O1s, Mn2p

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and Ti 2p3/2 are shown in Fig. 7(A)ꢀ7(C), meanwhile the molar ratio are summarized
in Table 2.
As shown in Fig.7(A), two peaks can be distinguished in the XPS O1s spectra.
2
−
The peak at 529.5 eV was assigned to the lattice oxygen O (O
L
), and the peak at
ꢀ
2−
2
−
5
31.5 eV was denoted to the surface oxygen (Os), such as O
2
, O
and O ，belonging
2
4
to defect oxide and hydroxylꢀlike groups . The O
6%, with the introducing of Mn, the O /(O +O ) ratios of the catalysts are increased
to 30%. It indicated that Mn promoted the amount of surface oxygen on the catalyst.
The O /(O +O ) ratio on MnOx(0.25)ꢀTiO reached to 30% which was 2 times higher
S L S 2
/(O +O ) ratios of TiO was only
1
S
L
S
S
L
S
2
2
than that on TiO .
4+
MnOx(0.3)-TiO2
Mn 2P 1/2
Mn 2P 3/2
(C)
MnOx(0.3)-TiO2
4+
Ti 2p
(A)
3/2
(B)
3+
Ti 2p
Mn
OL
MnO (0.3)ꢀTiO
x
2
1/2
4+
Mn
OS
2+
Mn
MnO (0.25)ꢀTiO
x
2
MnOx(0.25)-TiO2
MnOx(0.25)-TiO2
MnOx(0.2)-TiO2
MnO (0.2)ꢀTiO
x
2
MnOx(0.2)-TiO2
MnOx(0.1)-TiO2
MnO (0.1)ꢀTiO
x
2
MnOx(0.1)-TiO2
TiO2
TiO2
466
464
462
460
458
456
454
534
532
530
528
526
655
650
645
640
635
Binding Energy/(eV)
Binding Energy/(eV)
Binding Energy/(eV)
Fig.7 XPS spectra of (A)O 1s (B)Ti 2p (C)Mn 2p
Fig.7(C) shows the XPS spectra of Mn 2p in the MnO
x 2
ꢀTiO samples. The Mn
2
p peaks could be deꢀconvoluted to three peaks, the peak at 643.0ꢀ644.0, 641.0ꢀ641.9
4
+
3+
2+
25
and 640.2ꢀ640.7eV corresponded with Mn ,Mn and Mn , respectively . It was
3
+
interesting to note that Mn was the dominant species in the samples with small part
4
+
2+
3+
of Mn and Mn species. The ratio of Mn /Mntotal increased with the addition of Mn
4+
x 2
and reached to the maximum on MnO (0.25)ꢀTiO . The Mn species was in the form
2
+
of MnꢀOꢀTi confirmed by Raman; while the Mn species originated from Mn O
3
4
detected by XRD. With the increase of Mn loading, the surface atomic ratio of Mn/Ti

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increased accordingly which suggested the enrichment of Mn species on the catalyst
surface. Mn 2p3/2 binding energy slightly shifted to high binding energy (641.2eV to
6
41.7eV) because the Mn species showed deviation of electron cloud by interacting
26
with titanium species, leading to an enhancement in oxidative ability .
According to the law of conservation of charge, a binding energy increase of one
element must be accompanied by a binding energy decrease of another element in an
isolated system. The shift of Ti 2p3/2 (458.3 eV to 457.8eV) and O1s (529.5 eV to
5
29.2 eV) also confirmed the existence of interaction between Mn and Ti.
3
3
.3 Studies of reaction mechanism
.3.1 DRIFT of NH adsorption
3
0
.005
2213 2240
3
50℃
00℃
3
1
178 1603
250℃
3
227
3377
3134
1
694
200℃
1440
160℃
1
40℃
00℃
1
1000
1500
2000
2500
ꢀ13000
3500
4000
Wavenumber(cm )
Fig.8 in situ DRIFT spectra of catalyst treated with NH
3
at various temperature
NH absorption measurements were performed in a feed of 500ppm NH in Ar at
3
3
ꢀ1
various temperature and the spectra are shown in Fig.8. The bands at 1178,1603cm
27
are ascribed to the coordinated NH
3
on Lewis acid sites while the bands of 1440,
ꢀ1
4+
28
1
694cm are denoted to NH on Bronsted acid sites . In the high frequency region,
ꢀ1
bands at 3134,3227 and 3377cm are attributed to the NH stretching coordinate to
29
ꢀ1
NH
3
from Lewis acid sites . The band at 1630cm is ascribed to the OꢀH stretching
30
vibration modes due to the interaction of surface hydroxyls with NH
3
and the one at
ꢀ1
3
600cm is due to the hydroxyl consumption through interaction with NH to form
3
+
31
NH
4
.

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The intensity of Lewis acid sites turned to be strong when the temperature
o
o
o
increased from 50 C to 140 C, while the further elevating temperature up to 300 C
caused the disappearance of the bands. By comparing the band intensities of Lewis
acid sites and Bronsted acid sites, it was found that ammonia mainly existed on Lewis
o
acid sites and kept stable even at 250 C.
o
o
When the temperature increased from 100 C to 140 C, two new weak bands at
ꢀ1
1
456,1510cm were observed which are ascribed to ꢀNH deformation models of
32
33
imido =NꢀH species and amide(ꢀNH
2
) , respectively. The appearance of these
intermediates indicated absorbed NH
hydrogen.
3
could be activated through the abstraction of
ꢀ1
Meanwhile, two weak bands at 2213, 2240cm were observed which can be
34
assigned to NꢀN stretching modes of absorbed N O from the oxidation of ammonia .
2
The appearance of N
lattice oxygen.
2
O in the absence of O
2
3
demonstrated the oxidation of NH by
3
.3.2 The interaction of NH with O
3
2
1612
0.004
2
213 2240 350℃
3
2
00℃
50℃
00℃
60℃
40℃
1
832
2
1
1
3242
3
141
3376
1
00℃
1000
1500
2000
2500
31 000
3500
4000
ꢀ
Wavenumber(cm )
3 2
Fig.9 in situ DRIFT spectra of catalyst treated with NH +O at various temperature
The sample behavior in NH
Fig.9. By comparing with the intensity of absorbed NH
Bronsted acid sites with or without O , the weaker intensities of these bands were
ascribed to the reaction between oxygen and absorbed NH
The presence of NH and O also led to the arising of bridge nitrite (1234 cm )
3 2
and O was evaluated and the spectra are shown in
3
on Lewis acid sites and
2
3
.
ꢀ1
3
2

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ꢀ1 35
o
36,37
ꢀ1
and monodentate nitrate (1277 cm ) at 100 C and bidentate nitrate
(1540 cm )
o
ꢀ1
38
even higher than 250 C. A weak band at 1832cm assigned to nitrosyl (ꢀHNO)
species was observed which was from the interaction between ꢀNH and oxygen atom.
Combined with the appearance of nitrite and nitrate species, it proposed that nitrosyl
(ꢀHNO) could be further oxidized to nitrite and nitrate.
ꢀ1
2
The gas phase N O (2213, 2240cm ) was found in the whole temperature range,
ꢀ1
39
while the band at 1612cm ascribed to the weak absorbed gas NO
2
was only
o
observed at 250 C. The activity test results also confirmed that the product
distribution of Nꢀcontaining species depended on the temperature. There are many
researches on the form of Nꢀcontaining in the exhaust. It is usually considered that the
4
0
2
N was from the reaction between NH and HNO . There are two main ways to get
4
1
N O, one is from the interaction between two HNO species , and the other way is by
2
4
2
x
the reaction between nitrate species and ammonia . NO is usually considered as the
43
decomposition product of absorbed nitrate/nitrite species .
1
276
1544
1612
0.005
1
234
O +20min
2
O +18min
2
2240
2211
O +15min
2
O +10min
2
O +8min
2
1
190
1603
O +5min
2
O +1min
2
pretreat with NH3
1000
1500
2000
2500
3000
3500
4000
ꢀ1
Wavenumber(cm )
o
Fig.10 in situ DRIFT spectra of catalyst exposed to O
2
after NH
3
absorption at 230 C
o
The interaction of O
2
with the in situꢀformed NH
3
species at 230 C was
investigated, and the experiment was conducted as follow: prior to the introduction of
o
O
2
, the catalyst was exposed to 500ppm NH
3
at 230 C for 1 h, and then followed by
purging with Ar for 30 min. The in situ DRIFT spectra of the catalyst were recorded
with the time, and the results are shown in Fig.10.
The intensities of Lewis acid site and Bronsted acid site decreased even after

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introducing of O
exposing to 10min, the nitrate species (1234, 1276, 1544cm ) appeared. The changes
in first 5 min was ascribed to the reaction between absorbed NH and O , and the
arising of nitrate species showed that absorbed NH could be further oxidized to
nitrate species. Meanwhile, the bands (2211, 2240cm ) assigned to N O were
2
for 1 min; and it became obvious after 5 min. With the further
ꢀ1
3
2
3
ꢀ1
2
observed after introducing of O
2
for 1 min and then decreased with the further
exposing time. With the nitrate species appeared, the weak absorbed gas NO
2
species
ꢀ1
(
1612cm ) were observed.
3
3 2
.3.3 The interaction of NH and O with formed nitrate species
1
547 1575
T=160℃
T=250℃
1
178
0.01
0.001
1
602
3141
2240
2211
3242
NH +O2 60min
NH +O 75min
3376
3
3
2
NH +O 60min
NH +O 50min
3 2
3
2
NH +O2 40min
3
NH +O 50min
3
2
1
547
1
575
NH +O2 30min
1
234 1477
3
1613
NH +O 40min
3 2
1
605
NH +O2 20min
3
NH +O 30min
3
2
NH +O2 15min
3
NH +O 20min
3
2
NH +O2 10min
3
NH +O 10min
3
2
1270
NH +O 5min
NH +O2 5min
3
2
3
pretreat with NO
pretreat with NO
1
000
1500
2000
2500
3000
3500
4000
1000
1500
2000
2500
Wavenumber(cm )
3000
3500
4000
ꢀ1
ꢀ1
Wavenumber(cm )
o
Fig.11 in situ DRIFT spectra of catalyst exposed to NH
3
+O
2
after NO absorption at 160 C
o
or 250 C
The interaction of NH
investigated to understand the reaction mechanism, and the experiment was conducted
as follow: prior to the introduction of NH and O , the catalyst was exposed to
00ppm NO at 160 C or 250 C for 1 h , and then followed by purging Ar for 30 mins.
3
with in situ formed nitrate species over the catalysts was
3
2
o
o
5
The in situ DRIFT spectra of the catalyst were recorded with the time, and the results
are shown in Fig.11.
ꢀ1
ꢀ1
Bridge nitrite (1234 cm ) and monodentate nitrate (1270 cm ), the bidentate
ꢀ1
nitrate (1547 cm ) were observed after NO pretreatment which presented NO could
o
be oxidized to nitrite and nitrate by lattice oxygen at 160 and 250 C.
o
After the presence of NH
3
and O
2
for 10 min at 160 C, the bands of absorbed

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ꢀ1
NH
exposing time. Meanwhile, the intensity of the nitrite and nitrate species kept stable
with the presence of NH and O
3
on Lewis acid (1178, 1602cm ) appeared and the intensity enhanced with the
3
2
.
o
Unlike the experiment conducted at 160 C,obvious disappearance of nitrite and
o
nitrate species was observed after the adding of NH and O for 20 min at 250 C, and
3
2
ꢀ1
the appearance of N
suggested that N O was the reaction product of NH
Based on the above results, NH was mainly adsorbed on Lewis acid site (Fig.8)
and activated through the hydrogen abstraction. The intermediates ( ꢀNH , ꢀNH) was
further oxidized to HNO species, and two HNO species can convert to N O at low
can be directly oxidized to nitrite/nitrate
2
O (2211, 2240cm ) after 15 min can be found. The phenomena
2
3
and nitrite/nitrate species.
3
2
2
temperature(Fig.9).Meanwhile adsorbed NH
species (Fig.10).
3
The performance of nitrite and nitrate species depended on the temperature
o
(
Fig.11). With the nitrite and nitrate species disappeared ,N
2
O was observed at 250 C
which suggested that N O was from the reaction between NH and nitrite/nitrate
2
3
species. Combined with the in situ DRIFT results, the reaction mechanism of NH
3
selective oxidation is shown in Fig.12.
3 x 2
Fig.12 The reaction mechanism of NH selective oxidation on MnO -TiO
4
. Conclusions
The introducing of Mn can effectively promote the NH
The finely dispersed MnO on support can increase the surface acid density,
especially weak acid density, which benefited NH absorption. Meanwhile, the
formation of MnꢀOꢀTi provided abundant oxygen vacancy that promoted the
absorption and dissociation of oxygen species. MnO (0.25)ꢀTiO behaved the best
3
2
selective oxidation to N .
x
3
x
2

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performance in NH
3
2
ꢀSCO. The temperature window for N yield higher than 80% was
o
in the range of 180ꢀ300 C. N O was produced in the whole temperature window while
2
o
the exhibition of NO
Adsorbed NH
dehydrogenization and HNO species by further oxidation; meanwhile it also can be
oxidized to nitrite and nitrate species deposited on the surface. N was from the
reaction between HNO and NH species. N O was from the combination of two HNO
x
was mainly at higher temperature (>250 C).
3
2
on the Lewis acid sites can convert to NH and NH species by
2
2
species at low temperature, while from the reaction between adsorbed NH
nitrite/nitrate species at high temperature.
3
and
Acknowledgements
This work was financially supported by the National Basic Research Program of
China (2013CB933200), National High Technology Research and Development
Program of China (2015AA034603), National Natural Science Foundation of China
(
21333003, 21577034), Fundamental Research Funds for the Central Universities
WJ1514020)
(
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Graphical abstract
The formation of Mn-O-Ti and high dispensal of Mn O promoted
2
3
oxygen activation and NH adsorption, and the formation of N O depends
3
2
on temperature