Improved low temperature NH3-SCR performance of FeMnTiOx mixed oxide with CTAB-assisted synthesis

Article abstract of DOI:10.1039/c4cc09719j

FeMnTiO_x mixed oxide is prepared by the CTAB-assisted co-precipitation method, and the transformation of anatase into rutile is inhibited by CTAB to some extent. The catalyst obtained in the present work shows nearly 100% NO conversion at 100-350 °C, more than 80% N₂ selectivity at 75-200 °C, and excellent H₂O durability for the selective catalytic reduction of NO by NH₃ with a space velocity of 30000 mL g^-1 h^-1. This journal is

Full text of DOI:10.1039/c4cc09719j

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Improved low temperature NH -SCR performance
3
of FeMnTiO mixed oxide with CTAB-assisted
x
Cite this:DOI: 10.1039/c4cc09719j
synthesis†
ab
Received 5th December 2014,
Accepted 20th January 2015
c
a
a
a
a
Shiguo Wu, Xiaojiang Yao, Lei Zhang, Yuan Cao, Weixin Zou, Lulu Li,
a
a
d
ad
Kaili Ma, Changjin Tang, Fei Gao‡* and Lin Dong‡*
DOI: 10.1039/c4cc09719j
www.rsc.org/chemcomm
FeMnTiO
x
mixed oxide is prepared by the CTAB-assisted co-precipitation unit can be placed in the downstream of the electrostatic precipitator
method, and the transformation of anatase into rutile is inhibited by and the desulfurizer where SO and dusts have been removed.
2
CTAB to some extent. The catalyst obtained in the present work shows
nearly 100% NO conversion at 100–350 8C, more than 80% N selectivity NH
at 75–200 8C, and excellent H O durability for the selective catalytic to show high NO
reduction of NO by NH with a space velocity of 30 000 mL g
Many investigations have been carried out on low-temperature
-SCR catalysts. Manganese species have been demonstrated
conversion but low N selectivity, poor H
durability and a narrow operating temperature window.
Therefore, many research studies have focused on the modification
) are some of the major pollutants in the of the manganese species. For instance, the performance of
atmospheric environment, which can lead to photochemical smog, (Fe3 Mn ) O , Mn–Ce and Fe0.5Mn0.5TiO mixed oxide catalysts
2
3
2
x
2
2
O
À1 À1
3,5,6
3
h .
Nitrogen oxides (NO
x
Àx
x 1Àa
4
x
1
acid rain, ozone depletion, greenhouse effect, etc. Mitigation of NO
with respect to selectivity, activity and water resistance is improved
x
3,7,8
from the stationary and mobile sources, such as coal-burning power respectively.
However, it is still a challenge to develop low-
-SCR catalysts which have not only remarkable
key subjects in environmental protection. In recent years, many low-temperature activity, but also a wide operating temperature
methods have been used to reduce the emission of NO . Among window and excellent water resistance at low temperature. The
them, the selective catalytic reduction of NO by NH (NH -SCR) is a FeMnTiO catalyst is considered to be a potential candidate for
very effective technology for denitration (deNO ) in flue gas from industrial application. Anatase is considered to be more beneficial
stationary sources. The industrial operations are carried out on for NH -SCR than rutile. However, whether supported catalysts
plants, automotive exhaust emissions, etc., has been one of the temperature NH
3
2
x
3
3
x
3
x
9
3
3
V O –WO (MoO )/TiO catalysts during the NH -SCR process.
prepared by the impregnation method or mixed oxide prepared by
2
5
3
3
2
3
Furthermore, the deNO device is placed in the upstream of the co-precipitation method of FeMnTiO , the transformation of
x
x
the electrostatic precipitator and the desulfurizer, because the anatase into rutile will occur, which is not beneficial for improving
9
high-activity temperature range of the above-mentioned catalysts catalytic activity at low temperature.
is 300–400 1C. However, this arrangement causes many disadvantages,
such as low N selectivity in the high-temperature range and can release cations in aqueous solution. The FeMnTiO
Cationic surfactant cetyltrimethyl ammonium bromide (CTAB)
2
x
mixed
deactivation of catalysts caused by sulfur containing com- oxide catalyst is synthesised by the CTAB-assisted method (denoted
4
pounds and dusts. Therefore, it is of great significance to develop as FMT(S)) in the present work. For comparison, the FeMnTiOx
low-temperature (o200 1C) NH -SCR catalysts, so that the deNO
mixed oxide catalyst is also synthesized without CTAB (denoted as
FMT). The details of the preparation process are described in the
ESI.† The NH -SCR activity and selectivity measurements of FMT
3
and FMT(S) were performed in a fixed-bed quartz reactor, and the
results are shown in Fig. 1.
Fig. 1 shows that NO conversion is about 82% at 75 1C, and
reaches almost 100% in the range of 100–350 1C on FMT(S).
While the NO conversion is only about 6% at 75 1C, and about
3
x
a
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093, PR China
Changzhou Institute of Engineering Technology, Changzhou 213164, PR China
Research Center of Atmospheric Environment, Key Laboratory of Reservoir Aquatic
Environment, Chongqing Institute of Green and Intelligent Technology,
Chinese Academy of Sciences, Chongqing 400714, PR China
b
c
d
Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis,
Nanjing University, Nanjing 210093, PR China
9
3% in the range of 225–350 1C on FMT. In other words, the
activity of FMT(S) is much better than that of FMT in the low
temperature range (below 200 1C). Furthermore, the N selectivity
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc09719j
Present address: School of Chemistry and Chemical Engineering, Nanjing University,
2
‡
of the two catalysts is almost the same and above 80% from 75 to
200 1C. Apparently, the addition of CTAB is beneficial to improve
Hankou Road 22#, Nanjing, Jiangsu 210093, PR China. E-mail: donglin@nju.edu.cn,
gaofei@nju.edu.cn; Fax: +86 25 83317761; Tel: +86 25 83592290
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in the present work. The possible mechanism is as follows: the
isoelectric point of TiO
2 2 3 2
, Fe O and MnO are 3.9–8.2, 6.5–6.8
and 4–5, respectively, and the mixed oxides of them exhibit an
1
1
isoelectric point value among the corresponding pure oxides;
hence all metal oxide species are negatively charged at pH = 11;
+
the cationic surfactant molecules provide CTA which combines
with negatively charged sites on the metal oxide surface through
electrostatic interaction, as shown in the schematic diagram
(
Fig. 2); MnO
is 535 1C, which is far lower than that of TiO
considered to promote the crystalline transition because of the
2
has a rutile crystal structure, and its melting point
2
Fig. 1 (a) NO conversion and N selectivity (inserted) of FMT and FMT(S) in
2
(1830 1C) and
NH
NH
3
-SCR reaction. (b) NO conversion per unit area of FMT and FMT(S) in
-SCR reaction at 50 1C and 75 1C.
3
1
2
inducement effect. While due to steric hindrance of the long
chain alkane ends and no hydroxyl condensation, CTAB prevents
the occurrence of this process to some extent. As we all know, the
transformation of anatase into rutile will lead to a sharp decline
2
in the area. N -physisorption characterization is employed to
determine the specific surface area, and the results demonstrate
that FMT(S) maintains a large area (Table S1, ESI†). Thus, the
specific surface area may be one of the factors, which leads to
such a significant difference in reactivity. However, NO conver-
sion per unit area of FMT(S) is still higher than that of FMT, and
the corresponding results are presented in Fig. 1(b). This
indicates that there are some other factors that affect the activity
of catalysts in addition to the specific surface area.
2
Fig. 2 (a) The XRD patterns of TiO (anatase and rutile) and mixed oxide
catalysts (FMT and FMT(S)), (b) schematic diagram of the preparation
process of FMT and FMT(S).
As we all know, the redox property is an important factor to
the low-temperature catalytic activity of FMT(S), and in order influence NH -SCR reaction, and the TPR results are shown in
3
to clarify the effect of the CTAB-assisted preparation process, Fig. 3(a). The profile of FMT(S) exhibits two main reduction
the physicochemical properties of these catalysts are further peaks at around 346 1C and 438 1C, and the peaks of FMT are
characterized.
about 410 1C and 508 1C. On one hand, starting reduction peak
The XRD patterns in Fig. 2 show that the mixed crystal temperature of FMT(S) is much lower than that of FMT, which
configurations of anatase and rutile were detected on FMT(S), suggests that the redox property of FMT(S) is stronger than that
and only diffraction lines attributed to rutile were detected on of FMT. The reason may be that the interaction between active
FMT. There are no other diffraction lines attributed to iron or components and anatase is stronger than that between active
1
3
manganese species were detected on both samples. Moreover, components and rutile. On the other hand, the peak area of
it can be seen that the rutile diffraction peaks of the two FMT(S) is larger than that of FMT, which may be related to two
samples are slightly shifted to a high angle direction when factors: firstly, FMT(S) possesses more surface reducible species
4
+
compared with pure rutile (inset in Fig. 2), which suggests that of Mn ions, which has been demonstrated by XRD; secondly,
4
+
the metal cations with smaller radii replace Ti in the rutile there are more surface adsorbed oxygen species on FMT(S)
4
+
crystal lattice. It is well known that the ionic radius of Mn
(Fig. S4(b), ESI†). In order to further verify these results,
4
+
(
0.53 Å) is smaller than that of Ti (0.605 Å), while the ionic the activities of FMT(S) and FMT for NO oxidation are also
3
+
2+
3+
2+
radii of Mn (0.645 Å), Mn (0.67 Å), Fe (0.645 Å) and Fe
(
measured in the present work, and the results are shown in
Fig. 3(b), and the NO conversion of FMT(S) is higher than that
of FMT. Koebel et al. demonstrated that the partial conversion
4
+
4+
0.78 Å) are larger than that of Ti . It indicates that some of Ti
4
+
ions have been replaced by Mn in the lattice of rutile.
1
4
Comparing with pure anatase (inserted in Fig. 2), the anatase of NO into NO2 is helpful to ‘‘fast SCR’’. Therefore, it is
diffraction peaks of FMT(S) are not shifted, which suggests that
no foreign cations are incorporated into the lattice of anatase. In
addition, comparing with FMT, the proportion of rutile is so
4
+
small on FMT(S) that less Mn ions are introduced into the
lattice of FMT(S). It is reported that tetravalent manganese
1
0
3
species has good low-temperature activity in NH -SCR reaction,
thus more of that exposed on the surface of the catalyst, which
will be beneficial for NH -SCR reaction.
3
According to XRD results, iron and manganese species can
transform anatase into rutile in the process of preparing FMT,
9
which is consistent with many literature reports. However, the
Fig. 3 (a) H
2
-TPR profiles of FMT and FMT(S). (b) Oxidation activity of NO
on FMT and FMT(S).
transformation of anatase into rutile can be restrained by CTAB to NO
2
by O
2
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reasonable that FMT(S) exhibits excellent reaction activity than
FMT at low temperature in the present work.
X-ray photoelectron spectra (XPS) of Ti 2p and O 1s for the
two catalysts are collected (Fig. S4, ESI†), and the value of the Ti
Surface acid properties of catalysts play an important role in 2p3/2 XPS peak at 457.9 eV on FMT(S) is lower than 458.0 eV on
1
5
NH -SCR reaction. In situ diffuse reflectance infrared Fourier FMT. Literature suggests that the shift of peaks toward lower
3
transform spectra (in situ DRIFTS) of NH adsorption are acquired binding energy is due to the stronger interaction of titanium
3
2
2
at various temperatures to determine the type and quantity of acid with manganese. Hence the interaction on FMT(S) is stronger
sites, and the corresponding results are exhibited in Fig. 4. It is than that on FMT, which is consistent with the conclusion of
À1
À1
widely reported that the bands at about 1600 cm and 1170 cm
TPR. The O 1s spectra after deconvolution are shown in Fig. S4(b)
À1
are attributed to Lewis acid sites, and the bands at 1460 cm and (ESI†). The sub-bands at 529.3–529.5 eV correspond to the lattice
À1
16
2À
1
680 cm are attributed to Brønsted acid sites. The results show oxygen O (denoted as O
a
), and the sub-bands at 531.4 eV
that FMT(S) has more Lewis acid sites and less Brønsted acid sites, correspond to the surface adsorbed oxygen (denoted as O ), such
b
+
À
2À
À
23
in contrast to FMT, which might be related to the more CTA on as O2 , O2 , and O belonging to defect-oxide. Generally, Ob
FMT(S) and the more hydroxyl on FMT in the process of prepara- is more reactive in oxidation reactions due to its higher mobility
2
4
tion. In addition, the TPD results (Fig. S2, ESI†) show that the than O
quantity of Lewis acid sites of FMT(S) is far more than that of FMT. calculated and listed in Table S3 (ESI†), and the ratio is 38.4% on
It is reported that NH preferentially adsorbed on manganese FMT(S) which is larger than that 28.8% on FMT. Combining with
species for the MnO /TiO
manganese species lead to the more Lewis acid sites, which is in than FMT. Kapteijn et al. pointed out that NO easily adsorbed on
a
.
b a b
The ratios of O /(O + O ) on the two samples are
3
17
x
2
catalyst; therefore, more surface TPR results, it indicates that FMT(S) more easily adsorbs oxygen
2
5
agreement with BET and XRD results. In order to investigate the the active center was oxidized by O2. Therefore, it is beneficial
effect of Lewis acid sites on NH -SCR reaction at low temperature, for further adsorption of NO and formation of nitrite and nitrate
3
the reaction of in situ DRIFTS between NO and pre-saturated NH + on FMT(S), which may be related to the crystal structure of
3
O
2
is conducted on FMT(S), and the results are exhibited in anatase and rutile in the TiO
Fig. S3(a) (ESI†). After the introduction of NO for 3 min, Lewis acid hedron shows a slight orthorhombic distortion; in anatase, the
sites disappeared, which implies that the coordinate NH on Lewis octahedron is significantly distorted and its symmetry is lower
acid sites is very important for the activity of NH -SCR reaction at than rutile. Accordingly, anatase more easily produces defects
low temperature, coincidentally, many literatures have also and dislocations, and more easily interacts with manganese or
6
octahedron. In rutile, the octa-
3
2
6
3
9
,18
reported similar results.
For NH3 and O2 co-adsorption, iron species. Hence, it is helpful to adsorb oxygen and form
compared with only NH adsorption in Fig. 4(a), the amount nitrite on the surface of catalysts.
3
of Lewis acid increased obviously, which may be related to the
Since the flue gas still contains steam after electrostatic
precipitation and desulfurization under practical conditions,
at a temperature below we further investigated the effect of H O on the performance of
00 1C, and the reaction rate is very quick, which indicates FMT(S). The process and results are shown in Fig. S5 (ESI†),
that nitrite species play an important role in NH -SCR reaction and FMT(S) still exhibits excellent activity and selectivity when
at low-temperature. Thus, in situ DRIFTS of NH adsorption are water is introduced in NH -SCR reaction at low temperature.
carried out on FMT(S) and FMT pre-saturated with NO + O2, In summary, CTAB can suppress the transformation of
and the results are respectively shown in Fig. S3(b) and (c) anatase into rutile during the preparation of FMT(S). According
1
9
oxidative abstraction of hydrogen from absorbed ammonia.
Nitrites are able to react with –NH
2
2
2
2
0
3
3
3
À1
À1
À1
(
ESI†). The bands at 1235 cm , 1550 cm , and 1610 cm are to the BET specific surface area, XRD, TPR, in situ DRIFTS and
À1
À1
À1
attributed to nitrite, and 1276 cm , 1543 cm and 1581 cm
XPS results, due to the presence of anatase, FMT(S) exhibits
The peak intensity of nitrite on greater specific surface area, stronger redox property, more
FMT(S) is stronger than that on FMT, and nitrite disappeared Lewis acid sites and more nitrite species. All of them greatly
quickly and part of nitrate species still existed when NH was enhanced the activity of NH -SCR reaction at low temperature,
3
,7
are attributed to nitrate.
3
3
introduced into a sample cell. It may be considered that and further research need to be done on the mechanism of
ammonium nitrite decomposes quickly into N and H O below CTAB action during the preparation of FMT(S).
2
2
2
1
1
00 1C, hence, the results demonstrate that nitrite species are
We gratefully acknowledge financial support from National
Nature Science Foundation of China (21273110, 21303082),
Jiangsu Province Science and Technology Support Program
more easily formed on FMT(S) than on FMT.
(Industrial, BE2014130), and the Doctoral Fund of Ministry of
Education of China (2013009111005).
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