Fe2O3 nanoparticles anchored in situ on carbon nanotubes via an ethanol-thermal strategy for the selective catalytic reduction of NO with NH3

Article abstract of DOI:10.1039/c4cy00789a

Fe₂O₃ nanoparticles were anchored in situ on carbon nanotubes (CNTs) via an ethanol-thermal route, for the selective catalytic reduction (SCR) of NO with NH₃. The structure and surface characteristics of the obtained catalysts were measured by transmission electron microscopy, X-ray diffraction, N₂ adsorption-desorption isotherms, Raman, X-ray photoelectron spectroscopy, H₂-temperature programmed reduction, and NH₃-temperature programmed desorption. Compared with catalysts prepared via impregnation or co-precipitation methods, the synthesized catalyst showed better catalytic activity and a more extensive operating-temperature window. The TEM and XRD results suggested that the iron species was uniformly anchored on the surface of the CNTs. The Raman and XPS results indicated that the catalyst has a relatively higher number of defects, a higher atomic concentration of Fe present on the surface of the CNTs and a higher content of chemisorbed oxygen species. The H₂-TPR and NH₃-TPD results demonstrated that the catalyst possesses a more powerful reducibility and stronger acid strength than the other two catalysts. Based on the above-mentioned physicochemical properties, the obtained catalyst showed an excellent performance in the SCR of NO to N₂ with NH₃. Additionally, the catalyst also presented outstanding stability, H₂O resistance and SO₂ tolerance.

Full text of DOI:10.1039/c4cy00789a

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Fe O nanoparticles anchored in situ on carbon
2
3
nanotubes via an ethanol-thermal strategy for the
selective catalytic reduction of NO with NH3†
Cite this: DOI: 10.1039/c4cy00789a
Jin Han, Dengsong Zhang,* Phornphimon Maitarad, Liyi Shi, Sixiang Cai, Hongrui Li,
Lei Huang and Jianping Zhang
Fe
the selective catalytic reduction (SCR) of NO with NH
obtained catalysts were measured by transmission electron microscopy, X-ray diffraction, N
desorption isotherms, Raman, X-ray photoelectron spectroscopy, H -temperature programmed reduction,
and NH -temperature programmed desorption. Compared with catalysts prepared via impregnation or
2
O
3
nanoparticles were anchored in situ on carbon nanotubes (CNTs) via an ethanol-thermal route, for
. The structure and surface characteristics of the
adsorption–
3
2
2
3
co-precipitation methods, the synthesized catalyst showed better catalytic activity and a more extensive
operating-temperature window. The TEM and XRD results suggested that the iron species was uniformly
anchored on the surface of the CNTs. The Raman and XPS results indicated that the catalyst has a rela-
tively higher number of defects, a higher atomic concentration of Fe present on the surface of the CNTs
and a higher content of chemisorbed oxygen species. The H -TPR and NH -TPD results demonstrated
Received 16th June 2014,
Accepted 29th August 2014
2
3
that the catalyst possesses a more powerful reducibility and stronger acid strength than the other
two catalysts. Based on the above-mentioned physicochemical properties, the obtained catalyst showed
DOI: 10.1039/c4cy00789a
an excellent performance in the SCR of NO to N
2 3
with NH . Additionally, the catalyst also presented
www.rsc.org/catalysis
outstanding stability, H O resistance and SO tolerance.
2
2
So it is meaningful to design and develop a suitable non-
vanadium catalyst with a relatively low working temperature
and a great resistance to SO toxicity.
2
1
. Introduction
1
1,14,15
Nitrogen oxides (NO ), which arise from the combustion of
x
fossil fuels in industrial processes, can result in environmen-
tal contamination, such as acid rain, photochemical smog
Iron oxide, as an ordinary transition metal oxide, has been
widely investigated either as an active ingredient or promoter
for DeNOx catalysts, due to its inherently environmentally
friendly character, its prominent thermal stability and its
1
–4
and air haze.
employed to remedy these environmental problems.
Nowadays, certain processes have been
4
–10
1
6
Among these methods, selective catalytic reduction (SCR)
technology is a state-of-the-art process which, through a
reductant-NH selective chemical reaction with NO , forms
outstanding SO
2
2 3
resistance; for example, as Fe O ,
1
1,17–22
23,24
25
26
Fe/ZSM-5,
Fe/HBEA,
Au/Fe O , Fe O –TiO , and
2 3 2 3 2
2
7
Fe O –CeO /TiO . The catalytic performance and mecha-
3
x
2
3
2
2
5
,6
the environmental products N
2
and H
2
O. The commercial
3
nism of Fe zeolites in NH -SCR have been systematically
1
1,17–24
catalyst V O –WO IJMoO )/TiO has been widely used for NO
x
studied,
additive agents.
3
NH -SCR of NO is always downstream of the desulfurizer and
and ferric oxides have also been applied as
2
5
3
3
2
4
–6
13,14,27
elimination.
unsolved problems, such as low N
atures, and the volatility and toxicity of V O .
However, there still remain some inevitable
However, as the application of the
2
selectivity at high temper-
5
,11–13
What's
more, the oxidation of SO2 to SO3 could cause etching of
electrostatic precipitator, it is still necessary to improve the
low temperature activity of ferric oxide catalysts.
2
5
5
,11,12
the equipment and blocking of the pores of the catalyst.
Recently, it was demonstrated that carbon nanotubes
(
CNTs) possess the feature of outstanding adsorption of
2
8–31
ammonia, nitric oxides and other gaseous substances.
Besides, CNTs could decompose and directly reduce NO,
and improve the SO resistance with a decrease in the decom-
position temperature of sulfates and bisulfates.
CNTs have attracted great attention for the NH
NO.
Research Center of Nano Science and Technology, Shanghai University, Shanghai
00444, China. E-mail: dszhang@shu.edu.cn; Fax: +86 021 66136079;
Tel: +86 021 66136081
Electronic supplementary information (ESI) available: TEM images of Fe
3
2,33
2
2
†
2
O
3
/
34,35
Hence,
CNTs-IM, Fe
adsorption–desorption isotherm, H
vs. temperature over the Fe /TiO
0.1039/c4cy00789a
2
O
3
/CNTs-CP, Fe
2
O
3
/TiO
2
; XRD patterns of the samples; nitrogen
3
-SCR of
2 3
-TPR, NH -TPD profiles and NO conversion
3
6–39
It is considered that a good dispersion of active
2
O
3
2
, and Raman spectrum of CNTs. See DOI:
1
ingredients on the support could favor activity during the
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−
1
in dilute HNO (6 mol L ) under stirring for 6 h at 120 °C,
3
to remove metal species and impurities that existed in the
surface of the raw CNTs; they were then washed with an
excess of deionized water until the pH was neutral, and were
dried at 100 °C in oven overnight.
In a typical synthesis, 0.23 g of pretreated CNTs, 0.03 g of
ironIJIII) nitrate nonahydrate and 80 mL of ethanol were
mixed with subsequent ultrasonic treatment at room temper-
ature for 0.5 h. Then the mixture was poured into a stainless
steel autoclave with PTFE lining (100 mL) and maintained at
120 °C for 24 h. After the autoclave was cooled to an ambient
temperature, the suspension was filtered and washed, and
Scheme 1 Schematic illustration of the in situ synthetic process for
the Fe O /CNTs-ET catalyst.
2 3
then dried at 100 °C for 12 h. Finally, it was calcined in a N
stream at 450 °C for 4 h, with a heating rate of 2 °C min
The obtained catalyst was denoted as Fe /CNTs-ET.
2
−
1
.
2 3
O
For comparison, the catalysts were also separately pre-
pared by impregnation and co-precipitation methods. In the
impregnation procedure, 0.69 g of pretreated CNTs were
added to a solution of ironIJIII) nitrate nonahydrate (0.09 g)
4
0,41
NH -SCR reaction,
and thus it is significant to realize
3
active species with a smaller size that are highly dispersed on
the support. Normally, CNT-carried catalysts are synthesized
by an impregnation method
However, the active nanoparticles can't be uniformly dis-
persed on the surface of the CNTs when using these routes.
Therefore, there still remains a great challenge to uniformly
disperse the active species on the CNTs. Recently, we accom-
plished in situ the production of MnO –CeO nanoparticles
supported on CNTs, via a surfactant assisted reflux route
and a pyridine-thermal route, and found that they all
displayed enhanced NH -SCR activity and improved resis-
tance to SO and H O when compared with the same catalyst
2 2
3
6,42
26,35,43,44
or sol–gel method.
and H
2
O (80 mL) with stirring until the solution dried at
stream at 450 °C for
80 °C, and was then calcined in a N
4 h, with a heating rate of 2 °C min . The catalyst synthe-
sized by this method was denoted as Fe /CNTs-IM. In the
2
−
1
2
O
3
co-precipitation procedure, 0.23 g of pretreated CNTs, 0.03 g
of ironIJIII) nitrate nonahydrate, 0.5 g of urea and 80 mL
of H O were mixed in a beaker with subsequent ultrasonic
2
treatment for 0.5 h. Then the mixture was transferred into
a stainless steel autoclave with PTFE lining (100 mL) and
maintained at 120 °C for 24 h. After the autoclave was cooled
to room temperature, the suspension was filtered and
washed with an excess of water, and then dried at 100 °C for
x
x
4
1
4
5
3
synthesized by an impregnation method.
Herein, we have rationally designed and prepared for the
first time, highly dispersed Fe
an ethanol-thermal route. As illustrated in Scheme 1, Fe
nanoparticles were anchored in situ on the CNTs by an
2
O
3
nanoparticles on CNTs via
2
12 h. Finally, it was calcined in a N stream at 450 °C for 4 h,
with a heating rate of 2 °C min . The obtained catalyst was
−
1
2
O
3
denoted as Fe O /CNTs-CP. The Fe O supported on TiO was
2
3
2
3
2
3
+
ethanol-thermal strategy. Firstly, the Fe interacts with the
hydroxyl and carboxyl groups on the surface of the pretreated
CNTs due to the electrostatic effect. Secondly, the ethanol
synthesized by an impregnation method with the iron con-
tent unchanged, and was denoted as Fe O /TiO .
2 3 2
3
+
may bond with the Fe through the strongly electronegative
3
+
2.2 Characterization
end to afford the Fe electric neutrality. After the solvothermal
process and calcination, the Fe O nanoparticles were anchored
The morphologies and surface structures of the catalysts were
observed using a transmission electron microscope (TEM,
JEOL JEM-200CX) and a field emission high resolution trans-
mission electron microscope (HRTEM, JEOL JEM-2100F). The
energy dispersive X-ray spectroscopy (EDS) was carried out on
a TEM (JEOL JEM-2100F). Powder X-ray diffraction (XRD) was
performed with a Rigaku D/MAX-2200 X-ray diffractometer,
using Cu Kα (40 kV, 40 mA) radiation and a secondary beam
graphite monochromater. Nitrogen adsorption–desorption
isotherms of the samples were taken at 77 K using an ASAP
2
3
in situ on the CNTs. In this synthetic process, the steric
3
+
hindrance of ethanol can effectively separate each of the Fe
particles, and thus highly dispersed Fe O nanoparticles were
2
3
formed in situ on the CNTs. The obtained catalysts were sys-
tematically characterized, and their NH -SCR activities, stabili-
3
ties, H O resistances and SO tolerances were also investigated.
2
2
2
. Experimental
2
.1 Catalyst preparation
2
020 volumetric adsorption analyzer. Before the measure-
The multiwall carbon nanotubes (CNTs) were furnished by
Qinhuangdao Tai Chi Ring Nano-products Co. Ltd (China).
All other chemicals were purchased from the Sinopharm
Chemical Regent Company and were used without any fur-
ther purification. The raw CNTs were 1–10 μm in length and
ments, all the samples were degassed overnight at 573 K in a
vacuum line. The specific surface areas and the pore volumes
of the samples were calculated by the Brunauer–Emmett–
Teller (BET) method and the pore size distributions were
derived from the adsorption branches of the isotherms, using
the Barrett–Joyner–Halenda (BJH) model. The Raman spectra
1
0–30 nm in diameter. The CNTs were pretreated by refluxing
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were recorded on a spectrometer (JY H800UV) equipped with
an optical microscope at room temperature. X-Ray photoelec-
tron spectroscopy (XPS) was recorded on a Perkin-Elmer
PHI-5000C ESCA system equipped with a dual X-ray source,
using an Mg Kα (1253.6 eV) anode and a hemispherical
energy analyzer. The background pressure during data
−
6
acquisition was kept below 10 Pa. The XPS species of the
elements in these catalysts were recorded and fitted by
XPS PEAK 4.1 software, which had been calibrated against
the standardized C 1s peak at 284.6 eV. Temperature-
programmed reduction by hydrogen (H -TPR) was obtained
2
on a Tianjin XQ TP5080 auto-adsorption apparatus. 50 mg of
the catalyst was outgassed at 300 °C under a N atmosphere.
2
After cooling to room temperature under a N atmosphere,
2
the flowing gas was switched to 5% H
was heated to 840 °C at a heating rate of 10 °C min . The H
2 2
/N and the sample
−
1
2
consumption was monitored by a thermal conductivity detec-
tor (TCD). Temperature-programmed desorption experiments
Fig. 1 (a, b) TEM images of Fe
Fe
distribution of ferric oxide nanoparticles of Fe O /CNTs-ET.
2
O
3
/CNTs-ET; (c) HRTEM image of
2
3 2 3
O /CNTs-ET; (d) EDS spectrum of Fe O /CNTs-ET; (inset) size
on ammonia (NH -TPD) were conducted on a Tianjin XQ
3
2
3
TP5080 auto-adsorption apparatus. Before the TPD, each sam-
−1
ple was pretreated with high-purity (99.999%) He (35 mL min )
at 300 °C for 0.5 h, then saturated with high-purity
anhydrous ammonia at 100 °C for 1 h and subsequently
flushed at the same temperature for 1 h to remove physically-
adsorbed ammonium. Finally, the TPD experiment was
carried out from 100 °C to 525 °C at a heating rate of
surface of the CNTs. As can be seen in the Fig. 1 inset, it
was observed that the sizes of the ferric oxide nanoparticles
on the CNTs were in the range of 2.5–8.0 nm and that the
average size of the nanoparticles was 4.8 nm. However, for
Fe
(Fig. S1a, ESI†); and for Fe
Fe O nanoparticles displayed a distinct agglomeration
2
O
3
/CNTs-IM, the particles aggregated to some extent
−
1
1
0 °C min . The amount of NH
3
that was desorbed was
2
O
3
/CNTs-CP and Fe /TiO
2
O
3
2
,
monitored by a TCD.
2
3
(
Fig. S1b and c, ESI†). This indicates that the active compo-
2
.3 Catalytic tests
nents are uniformly anchored on the surface of the CNTs by
the ethanol-thermal method. Moreover, for Fe O /CNT-ET,
the XRD peaks assigned to Fe O were negligible, which
2 3
2
3
3
The NH -selective catalytic reduction activity tests were
completed in a fixed-bed quartz reactor using 0.2 g catalyst
40–60 mesh). The gas mixture was composed of 500 ppm
NO, 500 ppm NH , 3 vol.% O , N balance, 200 ppm SO
when used) and 4 vol.% H O (when used). The gas hourly
space velocity (GHSV) was about 18 000 h while the total
suggests a good dispersion of Fe species (Fig. S2, ESI†). It
(
was confirmed that the Fe O nanoparticles can be highly
2
3
3
2
2
2
dispersed on CNTs by the ethanol-thermal method.
(
2
−
1
The specific surface areas, pore volumes and pore sizes of
−
1
the catalysts were analyzed using N adsorption–desorption
2
flow rate of the feed gas was approximately 220 mL min
.
2 3
isotherms. As displayed in Fig. 2, the Fe O nanoparticles
The reaction temperature was from 150 °C to 400 °C. All
the concentrations of the feed gas and the tail gases were
detected by a KM9106 flue gas analyzer when the reaction
reached 15 min.
3
. Results & discussion
3
.1 Characteristics of the catalysts
The microstructures of the catalysts and the size distribution
of ferric oxide nanoparticles supported on the surface of
the CNTs were investigated using a TEM. As shown in
Fig. 1a and b, the TEM images show that Fe O /CNTs-ET
2
3
exhibited a homogeneous dispersion of ferric oxide nano-
particles supported on the surface of the CNTs, and that the
particle size was uniform. The HRTEM image in Fig. 1c
shows that the inter-planar distance was 0.23 nm, which is
related to the (222) plane of Fe
2 3
O . Fig. 1d presents the EDS
spectrum of Fe O /CNTs which confirms the presence of iron
elements, suggesting that the Fe species are supported on the
2
3
Fig.
Fe
2 3
2 N2 adsorption–desorption isotherms of Fe O /CNTs-ET,
2
O
3
/CNTs-IM and Fe
2
O
3
/CNTs-CP.
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supported on CNTs by three different methods imply the IV
isotherm with the relative pressure P/P = 0.15, indicating the
o
existence of a mesoporous structure, and Fe
sents a mesoporous structure due to the stack of particles
Fig. S3, ESI†). In addition, the specific surface areas, pore
2 3 2
O /TiO also pre-
(
volumes and pore sizes of the catalysts are summarized in
Table 1. As shown directly in Table 1, there is no obvious dif-
ference between the three catalysts. Moreover, the BET sur-
2
−1
face area of the Fe
2 3
O /CNTs catalyst was about 145 m g ,
which is higher than that of Fe O /TiO .
2
3
2
Raman spectroscopy is a powerful tool in probing the
lattice vibrational states and dynamics of CNTs, under the
variation of the layer number with respect to the CNT surface
4
6–49
plane.
The Raman spectra of Fe O /CNTs prepared by
2 3
the three routes are displayed in Fig. 3. The G-band, at
Fig.
3
Raman spectra of Fe
2
O
3
/CNTs-ET, Fe
2
O
3
/CNTs-IM and
−
1
around 1580 cm , is associated with an E mode of graphite
2
Fe O /CNTs-CP.
2
g
3
2
and is related to the vibration of sp bonded carbon atoms
in a 2-dimensional hexagonal lattice. The D-band, centred
−
1
around 1360 cm , corresponds to the vibrations of carbon
atoms with dangling bonds in plane terminations of disor-
the above results, it is reasonable that Fe
display the best capacity for NH
the three catalysts.
2
O
3
/CNTs-ET could
5
0–52
dered graphite or glassy carbon.
The I
D
/I
G
(the ratio of
3
-SCR performance among
the intensities of the D and G bands) values over Fe O /
2
3
CNTs-ET, Fe
2
O
3
/CNTs-IM, Fe
2
O
3
/CNTs-CP and CNTs were
The XPS spectra of O 1s for the three catalysts are
presented in Fig. 4B. All three catalysts show two overlapping
peaks of the O 1s. The overlapping peaks of O 1s at a binding
energy of 529.8 eV could be assigned to lattice oxygen
around 1.02, 0.99, 0.89 and 0.81 (Fig. 3 and S4, ESI†), respec-
tively. Compared with the value of ID/IG over CNTs, the value
over Fe
2 3
O /CNTs-ET is larger than the others, which suggest
that more defects were created by introducing the active
(donated as O
binding energies of about 531.7 eV could attributed to
chemisorbed oxygen (donated as O ), which could belong to
β
), and the overlapping peaks of O 1s at
5
2
species. Furthermore, the increased ratio of I /I indicates
D
G
5
3
an increase of surface defects in the Fe
should be favorable to the NH -SCR of NO. The Raman
results of Fe O /CNTs and CNTs demonstrate that the struc-
2
O
3
/CNTs-ET, which
α
2
7,56–58
3
defect oxide or hydroxyl-like groups.
rizes the binding energies of O 1s and the atomic ratios of
/IJO + O ) for the three catalysts. As seen clearly in Fig. 4B
and Table 2, Fe /CNTs-ET had a higher ratio of O /IJO + O
than Fe O /CNTs-IM and Fe O /CNTs-CP, mainly on account
Table 2 summa-
2
3
ture of multi-wall CNTs can be retained after being loaded
O
α
α
β
5
0
with Fe
2
O
3
nanoparticles.
2
O
3
α
α
β
)
In order to acquire information on the atomic concentra-
tions and element chemical states of each catalyst's surface,
the XPS spectra of the Fe 2p and O 1s in these catalysts were
recorded. Fig. 4A shows the Fe peaks assigned to Fe species
2
3
2 3
of the increase in chemisorbed oxygen species on the catalyst
2
7
surface by the iron species. The surface chemisorbed
oxygen Oα has been reported to be the most active oxygen,
and is highly active in the oxidation reaction due to its higher
mobility. Besides, the high relative concentration ratio of
O /IJO + O ) on the surface of the catalyst could be correlated
(
Fe 2p3/2 and Fe 2p1/2). The Fe peaks of Fe
2 3
O /CNTs were
3
+
assigned to oxidized Fe species, most likely Fe
species.
and 711.2 eV may be assigned to Fe in the spinel structure,
and the binding energy centered at about 712.4 eV may be
ascribed to Fe bonded with a hydroxyl group. It is inter-
esting that the surface atomic concentration of Fe over
type
The binding energies centered at about 709.8 eV
1
3,54
α
α
β
3
+
59
3 2 3
with the high NH -SCR activity. The loading of Fe O nano-
particles on the CNTs by the ethanol-thermal route could
create more vacancies on the catalyst surface, and lead to the
increase of chemisorbed oxygen on the surface. It is reported
3
+
55
Fe
2
O
3
/CNTs-ET is higher than those of Fe
2
O
3
/CNTs-IM and
2 3
that the NO would enhance the NH -SCR reaction by partici-
pating in the “fast SCR” reaction route on the catalysts. It
1
5
Fe O /CNTs-CP, as shown in Table 2. It has been demon-
strated that accessible Fe can participate in the reversible
redox cycle, which is beneficial for SCR activity. Based on
2
3
3
+
has been demonstrated that the chemisorbed oxygen plays
1
3
59
an important role in oxidation reactions. Therefore, the O
α
Table 1 BET surface areas and pore characterization of catalysts
2 3
Fe O /CNTs-ET
2 3
Fe O /CNTs-IM
2 3
Fe O /CNTs-CP
Fe O /TiO
2 3 2
2
−1
BET surface area (m g
)
147
0.41
95
144
0.48
110
146
0.43
101
55
0.33
199
3
−1
Pore volume (cm g
)
Pore size (Å)
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Fig. 5
CNTs-IM and (c) Fe
H
2
-TPR profiles of the catalysts: (a) Fe
/CNTs-CP.
2 3 2 3
O /CNTs-ET, (b) Fe O /
2
O
3
can be seen clearly that the area of the reduction peak over
Fe O /CNTs-ET is the largest among the three catalysts. The
2
3
H
2
2 3
consumption values of all the three Fe O /CNTs are given
2 3
in Table 3. As shown in Table 3, Fe O /CNTs-ET exhibited the
largest H consumption among the three catalysts. In addi-
2
tion, the content of the reductive species was related to the
degree of dispersion, which suggests that Fe
were highly dispersed on the CNTs by the ethanol-thermal
method. Thus, the Fe /CNTs-ET can offer more active spe-
2 3
O nanoparticles
Fig. 4 (A) Fe 2p spectra and (B) O 1s spectra of the catalysts:
2
O
3
(
2 3 2 3 2 3
a) Fe O /CNTs-ET, (b) Fe O /CNTs-IM and (c) Fe O /CNTs-CP.
cies, which is in accordance with the XPS results. Since more
reductive species could enhance the NH -SCR reaction, the
3
2 3
Fe O /CNTs synthesized by the ethanol-thermal route expose
plays a crucial role in the NH
process for promoting the oxidation of NO to NO2. Thus,
the Fe /CNTs-ET, which possess the highest O content,
3
-selective catalytic reduction
more reductive species, which should be beneficial to low-
6
0
temperature NH -SCR performance.
3
2
O
3
α
3
The NH -TPD technique has been extensively used to
could be beneficial for improving the low-temperature
characterize the adsorption of ammonia on the surface
active sites of catalysts, which plays a significant role in the
NH -SCR catalytic activity, as expected.
3
H
2
-TPR measurements were extensively performed to
NH
3
-SCR reaction. To investigate the surface acid amount
-TPD was performed. Fig. 6
investigate the reducibility of the iron species in the different
and strength of the catalysts, NH
3
catalysts. Fig. 5 presents the H -TPR profiles of the different
samples. In the temperature range 100–840 °C, all three
shows the temperature-programmed desorption after ammo-
nia adsorption over the three catalysts. It is clear that all
three catalysts present three desorption peaks at desorption
temperatures in the range 100–525 °C. The peaks around
150–160 °C were caused by the desorption of weak acid sites;
the peaks around 200–220 °C were caused by the medium
acid sites; and the peaks around 390–420 °C were caused by
2
catalysts presented three distinct H
2
consumption peaks,
which were assigned to the three-stepwise reduction of
2
5
Fe
the H
to Fe (Fig. S5a, ESI†). Comparing the Fe O /CNTs made
O
2 3
3 4 3 4 2 3 2
→ Fe O , Fe O → FeO, and FeO → Fe. For Fe O /TiO ,
2
-TPR profile showed the sequential reduction of Fe
2 3
O
2
3
3
5
by each method, the corresponding reduction peaks of
Fe /CNTs-ET shifted to a relatively low temperature. It
should be pointed out that the area of the reduction peak
has a direct relationship with the consumed content of H . It
the strong acid sites. It is generally accepted that Lewis
acids attributed to coordinated NH molecules are more ther-
2
O
3
3
+
mally stable than Brønsted acid sites attributed to NH , so it
4
2
can be inferred that the desorption peak at a low temperature
Table 2 The binding energies and surface atomic concentrations of the catalysts
Surface atomic concentration/% Binding energy/eV
The relative concentration ratios/%
β
/IJO + O /IJO + O )
Catalysts
C
Fe
O
O
β
O
α
O
β
α
β
)
O
α
α
Fe
2
Fe
2
Fe
2
3
O /CNTs-ET
3
O /CNTs-IM
3
O /CNTs-CP
92.93
92.01
92.19
0.43
0.42
0.23
7.64
7.57
7.59
529.8
529.8
529.8
531.7
531.7
531.7
34.45
42.41
47.81
65.55
57.59
52.19
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Table 3
2
H consumptions of the catalysts
Catalysts
Fe /CNTs-ET
0.072
2
O
3
Fe
2
O
3
/CNTs-IM
Fe
2
O
3
/CNTs-CP
−1
Consumed hydrogen (mmol g
)
0.056
0.034
Fig.
6
NH
3
-TPD profiles of the catalysts: (a) Fe
/CNTs-CP.
2
O
3
/CNTs-ET,
Fig. 7 NO conversion vs. temperature over the catalysts. Reaction
conditions: [NO] = [NH ] = 500 ppm, [O ] = 3 vol.%, N balance, and
(
b) Fe
2
O
3
/CNTs-IM and (c) Fe
2
O
3
3
2
2
−
1
GHSV = 18 000 h
.
is assigned to physisorbed NH
high temperature is assigned to chemisorbed NH3.
3
, and the desorption peak at
2
6,59
It was
remarkable influence on the NO reduction efficiency over
different catalysts. A negligible NO reduction efficiency
demonstrated that the position and area of the desorption
peaks are correlated with the acid strength and acid amount,
was noticed for Fe
400 °C, whereas highly dispersed Fe
fabricated by the ethanol-thermal route displayed a favorable
NH -SCR performance. The light-off temperature (at which
the conversion of NO reaches 50%, T50) of Fe /CNTs-ET
was 220 °C, and 96% NO conversion was achieved at 320 °C.
The Fe /CNTs-ET showed the best low-temperature NO
conversion among the three catalysts and displayed excellent
NH -SCR activity over a broad temperature window, varying
from 250 °C to 400 °C, with NO conversions of more than
80%. It was noted that the Fe /CNTs-ET also showed a
better performance at low temperatures than Fe /TiO
2
O
3
/CNTs-CP until the temperature reached
4
1
respectively. The desorption peaks of Fe O /CNTs made by
O
2 3
nanoparticles on CNTs
2
3
different methods locate at similar temperatures, indicating
that the acid sites are similar among them.
3
It is clear that the amounts of adsorbed NH for Fe O /
O
2 3
3
2
3
CNTs-ET and Fe O /CNTs-IM are greater than that of Fe O /
2
3
2
3
CNTs-CP. However, it is notable that Fe
2
O
3
/CNTs-ET shows a
2 3
O
higher amount of Lewis acid sites than the other two Fe O /
2
3
CNTs catalysts. For Fe O /TiO , the intensity of the desorp-
3
2
3
2
tion peaks for adsorbed NH
3
is very weak and mainly con-
centrates at low temperature (Fig. S5b, ESI†). It has been
O
2 3
demonstrated that the CNTs themselves present both weak
2
O
3
2
2
8,29
acid sites and strong acid sites as a catalyst support.
(Fig. S6, ESI†). The differences between the three catalysts
could be due to the different physical chemical characteriza-
tions of them. The results of the TEM characterization
Additionally, the results of the NH -TPD imply that the syn-
3
thesis route has a crucial effect on the amount of acidic sites
on the catalysts. As has been reported, the highly dispersed
active species on the support could offer a large number of
and XRD demonstrated that the Fe
uniformly anchored on the surface of the CNTs for Fe
CNTs-ET, which could be beneficial for outstanding NH -SCR
activity. Based on the results of the Raman spectroscopy and
XPS for Fe /CNTs-ET, it is clear that more defect Fe atoms
2 3
O nanoparticles were
O /
3
2
2
8
acidic sites and result in the best catalytic activity. There-
fore, the greater amount of the strongest acid sites over
Fe O /CNTs-ET could be attributed to the highly dispersed
3
O
2 3
2
3
Fe O nanoparticles and the CNTs themselves.
exist on the catalyst surface, which indicates that more active
sites are provided for the SCR reaction. Additionally, the high
content of chemisorbed oxygen would enhance the low-
2
3
3
.2 Catalytic performance
temperature NH -SCR reduction activity by accelerating the
3
Fig. 7 shows the NO conversion curves of the catalysts
with reaction temperatures ranging from 150 °C to 400 °C. It
can clearly be observed that the reaction temperature has a
oxidation of NO to NO
2
during the “fast SCR” reaction. The
results of the H -TPR indicated that the reducibility of Fe O /
2
2 3
CNTs-ET was the strongest among the three catalysts, which
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could promote the NH -SCR reaction. Therefore, the robust
interactions between ferric oxide and CNTs could cause the
the active sites of the catalyst surface could lead to the inhibi-
tion of the H O. This combined with the NH -TPD results
3
2
3
improvement in the NH
3
-SCR activity of Fe
2
O
3
/CNTs-ET.
(Fig. 6) suggests that the Fe
2
O
3
/CNTs-ET could preferentially
Assessing the change in catalytic activity with different
times on stream is a significant way to estimate catalysts'
properties. Fig. 8A displays the catalytic activity of the differ-
ent catalysts with different times on stream at 320 °C. All the
catalysts remained unchanged and kept their highest conver-
adsorb NH rather than H O in the feed gas, using the total
3 2
amount of acid sites over Fe O /CNTs-ET, leading to an excel-
2
3
1
6
lent H
2
O resistance ability. The above results suggest that
Fe O /CNTs-ET could be an appropriate candidate for H O
2
3
2
resistance. Fig. 8C shows the effect of SO on the NO conver-
2
sion values during the whole test period, and Fe
2
O
3
/CNTs-ET
sion of the catalysts at the typical temperature of 320 °C. The
−
1
showed the highest activity. It was noted that oxidation of
the carbon nanotubes didn't occur at this temperature during
the SCR process, which has also been confirmed in our previ-
total flow rate of the feed gas was 220 mL min . As shown in
Fig. 8C, the NO conversion of Fe O /CNTs-ET was 97% in the
2
3
2 2
absence of SO , which decreased to 91% when SO (200 ppm)
4
1,61
ous work.
As is well-known, some residual H O and SO
was introduced to the feed gas. With the continual addition
of 200 ppm SO , the NO conversion of the Fe O /CNTs-ET
2
2
still exist in the exhaust gas after the desulphurization pro-
2
2
3
cess, which can poison and devitalize the catalysts for the
remained stable. In contrast, the addition of SO
2
to the reac-
3
NH -SCR reaction. Thus, it is worth noting the NO conver-
tion conditions induced a significant decrease in the NO con-
version of the Fe O /CNTs-IM and Fe O /CNTs-CP, by 22%
3
sions of the catalysts in the presence of H O and SO . The
2
2
2
3
2
3
influence of H
2
O on the NH
3
-SCR activity of the Fe
2
O
3
and 30% respectively. After cutting off the SO
2
supply to the
supported on CNTs by the three different methods, as a func-
feed gas, the NO conversion of Fe O /CNTs-ET was restored
2
3
tion of time at 320 °C, was investigated, as shown in Fig. 8B.
to 92% without any fluctuations during the test periods.
However, for Fe /CNTs-IM and Fe /CNTs-CP, once the
SO2 stream was stopped, NO conversion was restored to a
certain extent, and finally returned to 77% and 46%, respec-
When introducing 4 vol.% H
version of all catalysts remained unchanged during the whole
test period whether or not H O was present. It was confirmed
that the competitive adsorption between H O and NH on
2
O to the feed gas, the NO con-
2
O
3
2 3
O
2
2
3
2 3
tively. The above results indicate that the Fe O /CNTs-ET
Fig. 8 (A) NO conversion vs. time on stream for the catalysts; (B) NO conversion vs. time on stream for the catalysts in the presence of H
C) NO conversion vs. time on stream for the catalysts in the presence of SO ; (D) NO conversion vs. time on stream for the catalysts in the
presence of both H O and SO . Reaction conditions: [NO] = [NH ] = 500 ppm, [O ] = 3 vol.%, [SO ] = 200 ppm (when used), [H O] = 4 vol.%
when used), N balance, T = 320 °C and GHSV = 18 000 h
2
O;
(
2
2
2
3
2
2
2
−
1
(
2
.
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shows the best resistance to SO among the three catalysts. It
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is well known that the SO stream can poison and deactivate
2
the catalyst with the formation and deposition of sulfates
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3
3
4,35
reduction.
At the same time, NH reacts with SO instead
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3
2
of reacting with NO, which could decrease the reaction
between NH and NO. Compared with Fe O /CNTs-IM and
3
2
3
Fe O /CNTs-CP, Fe O /CNTs-ET presents a better resistance
2
3
2
3
to SO
2 3
. The results of NH -SCR activity under the reaction
conditions in the presence of both H O and SO over the cat-
2
2
alysts were also investigated, as shown in Fig. 8D. As could
clearly be seen, the activity of Fe /CNTs-ET only decreased
by about 6% after introducing 4 vol.% H O and 200 ppm SO
2
O
3
2
2
together, but the volume raised to 92% when the mixture of
O and SO was eliminated. In contrast, the NO conversions
of Fe O /CNTs-IM and Fe O /CNTs-CP radically decreased, by
H
2
2
10 D. H. Lee, K. T. Kim, H. S. Kang, Y. H. Song and J. E. Park,
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2 3
1
8% and 26%, respectively, in the mixture of 4 vol.% H O
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2
and 200 ppm SO
2
; they then recovered to 75% and 57%,
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2
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O /CNTs-ET possesses a good capacity
for resistance to water and sulfur dioxide.
2
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4
. Conclusions
In summary, Fe O nanoparticles anchored on CNTs were
2
3
prepared in situ, via an ethanol-thermal route, for the selec-
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sented a better NH -SCR performance, higher H O-resistance
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3
2 3
O
3
2
and greater SO tolerance than the catalysts synthesized by
2
impregnation or co-precipitation routes. Additionally, it
presented a better NH -SCR activity than Fe O /TiO . The out-
3
2
3
2
standing NH -SCR activity of the Fe O /CNT-ET could be
3
2
3
attributed to the high dispersion of Fe
2
O
3
nanoparticles on
the CNTs, the greatest amount of defects on the surface, the
robust reducibility, the stronger acid strength and the larger
amount of chemisorbed oxygen. According to these excellent
properties, the Fe O /CNT-ET catalyst might be a promising
2
3
candidate for the NH
3
-SCR of NO with NH
3
.
22 T. Zhang, J. Liu, D. X. Wang, Z. Zhao, Y. C. Wei, K. Cheng,
G. Y. Jiang and A. J. Duan, Appl. Catal., B, 2014, 148–149,
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The authors acknowledge the support of the National Natural
Science Foundation of China (51108258) and the Shanghai
Municipal Education Commission (14ZZ097). We thank
Prof. W. J. Yu and Prof. B. Lu from the Analysis and Test
Centre of SHU for help with the TEM and Raman measure-
ments. The authors would like to thank Dr. K. Zhang from
the Analysis and Test Centre of NERCN for help with the
HRTEM measurements.
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