Substitution of WO3 in V2O5/WO 3-TiO2 by Fe2O3 for selective catalytic reduction of NO with NH3

Article abstract of DOI:10.1039/c2cy20383a

To improve N₂ selectivity and lower the cost, WO₃ in V₂O₅/WO₃-TiO₂ was substituted by a low cost composition Fe₂O₃ for selective catalytic reduction (SCR) of NO with NH₃. The SCR reaction over V ₂O₅/Fe₂O₃-TiO₂ mainly followed the Eley-Rideal mechanism (i.e. the reaction between activated ammonia species and gaseous NO). There were two active components on V₂O ₅/WO₃-TiO₂ for the activation of adsorbed NH₃ (i.e. V⁵⁺ and Fe³⁺). The acid sites on V₂O₅/Fe₂O₃-TiO₂ mainly resulted from the support Fe₂O₃-TiO₂, so the adsorbed NH₃ preferred to be activated by Fe³⁺ rather than by V⁵⁺. V⁵⁺ on V₂O₅/Fe ₂O₃-TiO₂ could accelerate the regeneration of Fe³⁺ on Fe₂O₃-TiO₂ due to the rapid electron transfer between V⁵⁺ and Fe²⁺ on the surface, so the activation of adsorbed NH₃ by Fe³⁺ was promoted. As some NH₃ adsorbed on V₂O₅/Fe₂O ₃-TiO₂ was not activated by Fe³⁺, the inactivated NH₃ could then be activated by V⁵⁺ on the surface. As a result, 2% V₂O₅/Fe₂O ₃-TiO₂ showed excellent SCR activity, N₂ selectivity and H₂O/SO₂ durability at 300-450°C. Furthermore, the emission of 2% V₂O₅/Fe₂O ₃-TiO₂ to the fly ash can be prevented by an external magnetic field due to its inherent magnetization. Therefore, 2% V ₂O₅/Fe₂O₃-TiO₂ could be a promising low-cost catalyst in NO emission control. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20383a

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PAPER
Substitution of WO in V O /WO –TiO by Fe O for selective
3
2 5
3
2
2 3
catalytic reduction of NO with NH3
Shijian Yang,abc Chizhong Wang, Lei Ma, Yue Peng, Zan Qu, Naiqiang Yan,
b
b
b
c
c
b
b
b
Jinghuan Chen, Huazhen Chang and Junhua Li*
Received 6th June 2012, Accepted 31st August 2012
DOI: 10.1039/c2cy20383a
2 3 2 5 3 2
To improve N selectivity and lower the cost, WO in V O /WO –TiO was substituted by a low
cost composition Fe O for selective catalytic reduction (SCR) of NO with NH . The SCR reaction
2
3
3
over V O /Fe O –TiO mainly followed the Eley–Rideal mechanism (i.e. the reaction between
2
5
2
3
2
activated ammonia species and gaseous NO). There were two active components on V O /
5
2
5
+
3+
WO
3
–TiO
/Fe
be activated by Fe rather than by V . V on V
2
for the activation of adsorbed NH
3
(i.e. V and Fe ). The acid sites on
–TiO , so the adsorbed NH
/Fe –TiO
due to the rapid electron transfer between V and Fe on
2
V O
5
2
O
3
–TiO mainly resulted from the support Fe
2
2
O
3
2
3
preferred to
3
+
5+
5+
2
O
5
O
2 3
2
could accelerate the
3
+
5+
2+
regeneration of Fe on Fe
2
O
3
–TiO
2
3
+
the surface, so the activation of adsorbed NH
3
by Fe was promoted. As some NH
3
adsorbed on
3+
V O /Fe O –TiO was not activated by Fe , the inactivated NH could then be activated by V
5+
2
5
2
3
2
3
on the surface. As a result, 2% V O /Fe O –TiO showed excellent SCR activity, N selectivity and
2
2
5
2
3
2
H O/SO durability at 300–450 1C. Furthermore, the emission of 2% V O /Fe O –TiO to the fly
2
2
2
5
2
3
2
ash can be prevented by an external magnetic field due to its inherent magnetization. Therefore, 2%
/Fe –TiO could be a promising low-cost catalyst in NO emission control.
V O
2 5
O
2 3
2
1
. Introduction
plants for several decades, some problems still exist, such as
the relatively narrow temperature window of 300–400 1C, the
low N2 selectivity at high temperature, and the toxicity of
2
Nitrogen oxides (NO and NO ), which result from automobile
exhaust gas and industrial combustion of fossil fuels, have
1
been a major contribution to air pollution. They contribute to
7
vanadium pentoxide to the environment. Especially, the price
of W resources is now soaring. Therefore, a more cost-effective
and more environmentally-friendly SCR catalyst with better
photochemical smog, acid rain, ozone depletion and the
greenhouse effect. So far, the most efficient technology for
the removal of nitrogen oxides from coal-fired power plants
N
2
selectivity should be developed.
In this study, WO in V /WO –TiO
low cost composition Fe to improve its N
lower its cost for SCR of NO with NH
Fe –TiO was prepared using a co-precipitation method
at room temperature, and V was supported on Fe
TiO by the conventional impregnation method. Then, V
Fe O –TiO was characterized using nitrogen physisorption,
3
2
O
5
3
2
was substituted by a
selectivity and
. The support
2
has been selective catalytic reduction (SCR) of NO with NH3.
2
O
3
2
The standard SCR process is based on the following reaction
3
between NH and NO:
3
2
O
3
2
2
O
5
2 3
O –
SCR catalyst
4
NO þ 4NH3 þ O2 ƒƒƒƒƒƒƒƒƒ ƒƒ ! 4N2 þ 6H2O
ð1Þ
2
2 5
O /
2
3
2
3
Many metal oxide based systems, such as V O /WO –TiO ,
2 5 3 2
X-ray diffraction (XRD), X-ray photoelectron spectroscopy
XPS), magnetization and H2 temperature-programmed
reduction (TPR). Subsequently, the SCR performance of
/Fe –TiO was estimated using a packed-bed micro-
4
5
6
CeO
2
/WO
3
–TiO
2
, iron titanate and Fe–Ti spinel, have been
/WO –TiO
(
extensively studied. Among these catalysts, V
2
O
5
3
2
has been widely accepted as an industrial catalyst. Although
V O /WO –TiO has been employed as a commercial SCR
V
2
O
5
2
O
3
2
2
5
3
2
reactor. At last, the mechanism of the SCR reaction over
catalyst to control the emission of NO from coal-fired power
x
V O /Fe O –TiO was studied using in situ DRIFTS.
2
5
2
3
2
a
School of Environmental and Biological Engineering, Nanjing
University of Science and Technology, Nanjing, 210094, P. R. China
State Key Joint Laboratory of Environment Simulation and Pollution
b
2
. Experimental
Control (SKLESPC), School of Environment, Tsinghua University,
Beijing, 100084, P. R. China. E-mail: lijunhua@tsinghua.edu.cn;
Fax: +86-10-62771093; Tel: +86-10-62771093
School of Environmental Science and Engineering,
Shanghai Jiao Tong University, Shanghai, 200240, P. R. China
2.1 Catalyst preparation
c
The support Fe O –TiO was prepared using a co-precipitation
2
8
method at room temperature. Suitable amounts of ferrous
3
2
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2
+
sulfate, ferric trichloride, and titanium tetrachloride (Fe
:
N2 selectivity %
3
Fe : Ti
+
4+
= 3 : 2 : 1) were dissolved in a HCl solution.
ð3Þ
This mixture was added to an ammonia solution leading to an
instantaneous precipitation. During the reaction, the system
was continuously stirred at 800 rpm. The particles were then
separated by centrifugation at 4500 rpm for 5 min and washed
with distilled water followed by a new centrifugation. After
2½N Oꢂ þ ½NO ꢂ
2
out
2 out
¼
1 ꢁ
ꢃ 100%
out
½
NOꢂ ꢁ ½NOꢂ þ ½NH3ꢂ ꢁ ½NH3ꢂ
in
out
in
2
.4 In situ DRIFTS study
4
washings, the particles were collected and dried at 105 1C
for 12 h.
/Fe
were prepared by the conventional impregnation method using
NH VO and H O as precursors, and Fe –TiO
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
V
2
O
5
2 3 2 2 5
O –TiO catalysts with 1 wt% and 2 wt% V O
4
3
2
C
2
O
4
ꢀ2H
2
2
O
3
2
N . The diffuse reflectance measurement was carried out in situ
2
as a support. After the impregnation, excess water was removed in
a rotary evaporator at 80 1C. The sample was dried at 105 1C
overnight and then calcined at 500 1C for 3 h under air. Meanwhile,
Fe O –TiO was calcined at 500 1C for 3 h under air as a
in a high temperature cell with a ZnSe window. The FTIR
spectra were recorded by accumulating 100 scans with a
1
ꢁ
resolution of 4 cm
.
2
3
2
comparison. Furthermore, conventional vanadium-based
3
. Results
catalysts (V
2
O
5
/WO
3
–TiO
2
) with 1 wt% V
2
O
5
and 10 wt%
were prepared
VO
O as precursors, and anatase
WO , and 2 wt% V
3
2
O
5
and 10 wt% WO
3
3.1 Characterization
by the conventional impregnation method using NH
NH 41 and H
ꢀ2H
TiO as a support.
4
3
,
3
.1.1 XRD and BET. XRD patterns of synthesized
(
4
)
10
W
12
O
2
C
2
O
4
2
catalysts are shown in Fig. 1. The characteristic peaks of the
support Fe O –TiO corresponded very well to Fe–Ti spinel,
2
2
3
2
and the characteristic peaks corresponding to rutile and
anatase did not appear. It indicates that Ti could be intro-
9
duced into the spinel structure. Furthermore, electron energy
2
.2 Catalyst characterization
Crystal structure was determined using an X-ray diffraction-
meter (Rigaku, D/max-2200/PC) between 101 and 801 at a step
loss spectroscopy (EELS), X-ray adsorption near edge struc-
ture (XANES) and extended X-ray adsorption fine structure
(EXAFS) also demonstrated that Ti could be incorporated
ꢁ
1
of 71 min operating at 30 kV and 30 mA using Cu Ka
radiation. BET surface area was determined using a nitrogen
adsorption apparatus (Quantachrome, Autosorb-1). The catalyst
1
0
into the spinel structure.
After the loading of V
Fe –TiO
additional reflections that would indicate the presence of other
crystalline vanadium oxides, such as V , VO , V , V
or FeVO , were not present in the diffraction scan. It indicates
that V cations could mainly present as an amorphous phase of
VO , which was well dispersed on Fe –TiO
The BET surface areas of Fe –TiO
Fe –TiO and 2% V /Fe –TiO were 81.4, 73.9 and
70.0 m g , respectively.
was outgassed at 200 1C before BET measurement. H -TPR was
2
O
5 2 5
, the characteristic peaks of V O /
2
recorded on a chemisorption analyzer (Micromeritics, ChemiSorb
2
O
3
2
still corresponded very well to Fe–Ti spinel, and
2
720 TPx) under a 10% hydrogen–90% nitrogen gas flow
3
ꢁ1
ꢁ1
(50 cm min ) at a rate of 10 1C min . Saturation magnetization
O
2
5
2
O
2
4
O
2 3
was determined using a vibrating sample magnetometer (VSM,
Model JDM-13) at room temperature. X-ray photoelectron
spectroscopy (Thermo, ESCALAB 250) was used to determine
4
O
.
2
x
2
3
the Fe 2p, V 2p, Ti 2p and O 1s binding energies with Al Ka (h
486.6 eV) as the excitation source. The C 1s line at 284.6 eV was
taken as a reference for the binding energy calibration.
v
=
2
O
3
2 2 5
, 1% V O /
1
2
O
3
2
ꢁ1
2
O
5
2
O
3
2
2
3
2 5 2 3 2
.1.2 XPS. Surface information of V O /Fe O –TiO was
2
.3 Catalytic test
analyzed by XPS. XPS spectra over the spectral regions of Fe
2p, Ti 2p, O 1s and V 2p were evaluated (shown in Fig. 2).
SCR reaction was performed on a fixed-bed quartz tube
reactor (6 mm of internal diameter) containing 100–200 mg
of the catalyst (40–60 mesh). The typical reactant gas composition
3 2
was as follows: 500 ppm of NO, 500 ppm of NH , 2% of O , 10%
of H O (when used), 200 ppm of SO (when used), and the
2
2
ꢁ
1
balance of N . The total flow rate was 200 mL min , and the gas
2
hourly space velocity (GHSV) varied from about 60 000 to
3
ꢁ1 ꢁ1
1
20 000 cm g
and N O were continually monitored by an FTIR spectrometer
Gasmet FTIR DX4000).
As the SCR reaction reached the steady state, the ratios of
NO conversion and N selectivity were calculated according to
h . The concentrations of NO, NO , NH
2
3
2
(
2
the following equations:
½
NOꢂ ꢁ ½NOꢂ
in
out
Fig. 1 XRD patterns of: (a) Fe
(c) 2% V /Fe –TiO
2 3 2 2 5 2 3 2
O –TiO ; (b) 1% V O /Fe O –TiO ;
NO conversion % ¼
ꢃ 100%
ð2Þ
½
NOꢂ
in
2
O
5
O
2 3
2
.
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2 5 2 3 2
Fig. 2 XPS spectra of V O /Fe O –TiO over the spectral regions of Fe 2p, Ti 2p, O 1s and V 2p.
The Fe peaks of Fe
2
O
3
+
–TiO
2
were assigned to oxidized Fe
type species. The binding energies
centered at about 709.9 eV and 711.0 eV could be assigned to
the spectral regions of Fe 2p, Ti 2p and O 1s (shown in Fig. 2).
The V 2p peak mainly centered at about 516.8 eV, which was
3
species, more likely Fe
5
+ 14
.
assigned to V
3
+
3+
4+
The ratios of Fe , Ti , O and V on Fe O –TiO , 1%
2ꢁ
5+
Fe in the spinel structure, and the binding energy centered
3
2
3
2
+
at about 712.5 eV could be ascribed to Fe
1
bonded with
–TiO were
2 5 2 3 2 2 5 2 3 2
V O /Fe O –TiO and 2% V O /Fe O –TiO collected from
1,12
hydroxyl groups.
The Ti peaks of Fe
2
O
3
2
assigned to Ti 2p 1/2 (464.0 eV) and Ti 2p 3/2 (458.3 eV) of
+ 13
2 5 2 3 2
Table 1 Data of atomic ratios on V O /Fe O –TiO collected from
XPS/%
4
Ti
.
2 3 2
The O 1s peaks of Fe O –TiO mainly centered at
about 529.8 eV, as expected for the transition metal oxides.
Another oxygen species centered at about 532.0 eV was also
3+
Fe
4+
2ꢁ
5+
Ti
O
V
8
observed, which was assigned to ꢁOH.
Fe O –TiO
2
28.5
26.0
22.0
9.5
9.7
7.9
62.0
62.6
64.1
—
1.7
6.0
3
2
1
2
% V
% V
2
2
O
O
5
/Fe
2
2
O
O
3
–TiO
–TiO
2
2 5 2 3 2
After the loading of V O on Fe O –TiO , no obvious
5
/Fe
3
2
changes happened to XPS spectra of V O /Fe O –TiO over
2
5
2
3
2
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5
+
3+
XPS spectra are shown in Table 1. The percentage of V on the
surfaces of 1% V /Fe –TiO and 2% V /Fe –TiO
was much more than the content of V in 1% V O /
Fe O –TiO and 2% V O /Fe O –TiO , respectively. It indicates
reduction of Fe
promoted. As a result, the first reduction peak of V
Fe O –TiO obviously shifted to low temperature although
2 3 2
in the octahedral site of Fe O –TiO was
2
O
5
2
O
3
2
2
+
O
5
2
O
3
2
2 5
O /
5
2
5
2
3
2
the amount of V O loaded was very small.
5
2
3
2
2
5
2
3
2
2
5
+
that V could mainly present as an amorphous phase of V O
2
5
3+
3+
2+
4+
0
E = 0.41 V
Febulk
+ ꢄVSurf
- Febulk
+ ꢄVSurf
on Fe O –TiO . As V O was loaded on Fe O –TiO , the
2
2
3
2
2
5
2
3
3+
percent of Fe
(6)
2 5 2 3 2
on V O /Fe O –TiO obviously decreased
5+
(
shown in Table 1). It suggests that the loaded V could mainly
3
+
cover Fe on Fe
2
O
3
2
–TiO .
3.1.4 Magnetization. Vanadium(V) species (e.g., vanadium
pentoxide, V O ) are considered particularly toxic with mutagenic
2
5
3
.1.3 TPR. H
illustrated in Fig. 3. TPR profiles of Fe
two obvious reduction peaks. The first peak centered at about
42 1C corresponded to the reduction of non-stoichiometric
Fe2.5Ti O to stoichiometric Fe2.5Ti0.5O , and the broad
2
-TPR profiles of V
2
O
5
/Fe
2
O
3
–TiO
2
are
effects, respiratory tract toxicity, and possible carcinogenic
1
activity. SCR catalyst can be abraded by the fly ash, and then
7
2
O
3
–TiO
2
showed
1
8
emits to the fly ash. Therefore, the emission of vanadium
based catalyst to the environment during the SCR reaction is a
serious concern.
4
(
)
0.5 1-d
4
4
peak at higher temperature was attributed to the reduction of
As is well known, the support Fe O –TiO is a magnetic
2
3
2
8
19
2 5 2 3 2
material. 2% V O /Fe O –TiO had the super-paramagnetism
Fe2.5Ti0.5
O
4
to Fe and TiO
2
. Our previous study demonstrated
that the reduced Fe corresponding to the first reduction peak
of Fe –TiO mainly located in the octahedral site, which
resulted from the oxidization of Fe in the octahedral site of
3+
with a minimized coercivity and a negligible magnetization hyster-
ꢁ
1
O
2 3
2
esis, and its saturation magnetization was about 43 emu g
2
+
(shown in Fig. 4). The magnetization characteristics ensure that
the emission of 2% V O /Fe O –TiO to the fly ash during the
8
stoichiometric Fe2.5Ti0.5
4
O .
2
5
2
3
2
After the loading of V
2
O
5
, a strong displacement of the first
SCR reaction can be effectively prevented by exposure to an
external magnetic field.
reduction peak to low temperature happened in the TPR
profiles (shown in Fig. 3). It suggests that the oxidization ability
of Fe O –TiO was gradually enhanced with the increase of
3
.2 SCR performance
2 5 3 2
.2.1 SCR activity and selectivity. 1% V O /WO –TiO
2
3
2
V O loading.
2
5
3
Because V O was loaded on Fe O –TiO , the first step of
2
5
2
3
2
showed a moderate SCR activity at 200–250 1C, and an
excellent SCR activity at 300–500 1C (shown in Fig. 5a). As
the loading of V O increased from 1% to 2%, the SCR
5
+
V
2
O
5
2 3 2
/Fe O –TiO reduction was the reduction of V on the
surface (reaction (4)), and the next step could be the reduction
2
5
4
+
of V on the surface (reaction (5)).
reaction over V O /WO –TiO was obviously promoted at
2
5
3
2
5
+
4+
3+
+
+
2 5
150–300 1C. However, NO conversion over 2% V O /
2
ꢄVSurf
+ H
2
- 2ꢄVSurf
+ 2H
(4)
(5)
WO
reaction temperature from 350 to 500 1C. N
/WO –TiO gradually decreased above 300 1C, and N
selectivity of 2% V /WO –TiO was much worse than that
of 1% V /WO –TiO (shown in Fig. 5b).
Fe –TiO showed a moderate SCR activity at 200–300 1C,
3
–TiO
2
gradually decreased with the further increase in
4
+
2
ꢄVSurf + H - 2ꢄV
+ 2H
2
selectivity of
2
Surf
V
2
O
5
3
2
2
The electrical property of Fe–Ti spinel is similar to that of
2
O
5
3
2
1
5
magnetite, and its band gap is very small. Hence, it has the
lower resistivity and its conductivity is almost metallic. As a result,
the electron can migrate easily from the bulk to the surface. The
2
O
5
3
2
O
2 3
2
and an excellent SCR activity at 350–450 1C (shown in Fig. 5a).
Meanwhile, little N O can be observed during the SCR reaction
3
+
3+
redox potential of the reduction of Fe by V is 0.41 V, so
2
16
reaction (6) is thermodynamically favorable. Therefore, the
4+
formed V on the surface could be reoxidized to V by the
over Fe O –TiO (shown in Fig. 5b). The SCR activity of
2
3
2
3
+
Fe O –TiO was obviously promoted due to the loading of
2
3
2
3
+
4+
reducible Fe in the bulk. Then, the formed V on the surface
was reduced by gaseous H again. Through the recycle, the
V
2
O
5
, and NO conversion over V
2 5 2 3 2
O /Fe O –TiO increased
2
obviously with the increase of V O loading (shown in Fig. 5a).
5
2
5+
2 5 2 3 2
Although there was a large amount of V on V O /Fe O –TiO
Fig. 3
2 2 3 2 2 5 2 3 2
H -TPR profiles of: (a) Fe O –TiO ; (b) 1% V O /Fe O –TiO ;
(
c) 2% V
2
O
5
/Fe –TiO2.
2
O
3
2 5 2 3
Fig. 4 Magnetization characteristics of 2% V O /Fe O –TiO2.
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Fig. 6 Effect of 10% of H
reaction over: (a) 2% V /Fe
Reaction condition: [NO] = [NH
catalyst mass = 200 mg, total flow rate = 200 mL min
2
O and 200 ppm of SO
–TiO ; (b) 1% V
] = 500 ppm, [O
2
on the SCR
/WO –TiO
2
] = 2 vol%,
2
O
5
2
O
3
2
2
O
5
3
2
.
3
ꢁ1
,
3
ꢁ1 ꢁ1
GHSV=60 000 cm g
h .
Fig. 5 SCR performance of synthesized catalysts: (a) NO conversion;
b) N selectivity. Reaction conditions: [NO] = [NH ] = 500 ppm,
] = 2 vol%, catalyst mass = 100 mg, total flow rate = 200 mL min
(
2
3
ꢁ1
characteristic vibrations mainly centered at 1425 and 1245 cm
ꢁ1
ꢁ
1
[O
2
,
(
shown in Fig. 7a). The band at 1425 cm was assigned to ionic
3
ꢁ1 ꢁ1
GHSV=120 000 cm g
h
.
+
NH4 bound to the Brønsted acid sites, and the band at 1245 cm
ꢁ1
21
was attributed to coordinated NH bound to the Lewis acid sites.
3
(
shown in Table 1), V
2
O
5
/Fe
2
O
3
–TiO
2
showed an excellent N
2
After NO + O /N passed over NH3 pretreated 2% V O /
2
2
2 5
selectivity. N2 selectivity of 2% V O /Fe O –TiO only
2
+
4
2
5
2
3
WO –TiO
3
2
at 250 1C, both ionic NH
and coordinated NH
3
slightly decreased to about 90% as the reaction temperature
increased to 500 1C, which was much better than those of 1%
V O /WO –TiO and 2% V O /WO –TiO (shown in Fig. 5b).
diminished (shown in Fig. 7a). Meanwhile, adsorbed H O,
2
which is a product of the SCR reaction, can be clearly observed
2
5
3
2
2
5
3
2
ꢁ1 22
at 1620 cm
.
They both suggest that adsorbed ammonia
/WO –TiO can react with NO.
/WO –TiO was treated with NO + O
at 250 1C, the bands corresponding to adsorbed NO species
species on 2% V
After 2% V
2
O
5
3
2
3
.2.2 Effect of H
flue gas often lead to the deactivation of the SCR catalyst.
Therefore, the effect of 10% of H O and 200 ppm of SO on
the SCR activity of 2% V /Fe –TiO was investigated
with a 24 h test. As shown in Fig. 6a, the presence of H O and
SO showed a severe interference with the SCR reaction over
% V O /Fe O –TiO at 150–250 1C. However, the ratio of
2 2 2
O and SO . Water vapor and SO in the
2
0
2
O
5
3
2
2 2
/N
x
2
2
were hardly observed (shown in Fig. 7b). It suggests that
2
O
5
2
O
3
2
gaseous NO can hardly adsorb on 2% V O /WO –TiO to
2
5
3
2
2
form adsorbed NO . After NH /N passed over NO + O
2
x
3
2
2
pretreated 2% V O /WO –TiO , its surface was mainly covered
2
5
3
2
2
+
by ionic NH
4
2
5
2
3
2
bound to the Brønsted acid sites and coordinated
O,
NO conversion over 2% V O /Fe O –TiO in the presence of
2
5
2
3
2
NH bound to the Lewis acid sites. Furthermore, adsorbed H
3
2
H O and SO can reach above 95% at 300–450 1C, and it did
2
2
which is a product of the SCR reaction, can hardly be detected at
ꢁ1
not decrease in the 24 h test. This performance was close to
that of 1% V /WO –TiO (shown in Fig. 6b). It indicates
that 2% V /Fe –TiO had an excellent H O and SO
about 1620 cm . It suggests that the reaction between ammonia
2
O
5
3
2
and adsorbed nitrogen oxide species cannot happen on 2% V
WO –TiO . Therefore, the SCR reaction over V /WO –TiO
mainly followed the Eley–Rideal mechanism.
2 5
O /
2
O
5
2
O
3
2
2
2
3
2
2
O
5
3
2
2
durability in the temperature range of the SCR unit of the
coal-fired power plant.
3
2 3 2 2 3
.3.2 In situ DRIFTS study on Fe O –TiO . Fe O –TiO
+
3
.3 In situ DRIFTS study
.3.1 In situ DRIFTS study on V O /WO –TiO . After
was mainly covered by ionic NH
bound to the Brønsted
4
ꢁ1
3
acid sites (1430 cm ) and coordinated NH bound to the
3
2
5
3
2
ꢁ1
2
% V O /WO –TiO was treated with NH /N at 250 1C, the
Lewis acid sites (1221 and 1606 cm ) after the treatment of
2
5
3
2
3
2
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Fig. 7 (a) In situ DRIFT spectra taken at 250 1C upon passing NO +
over NH presorbed 2% V /WO –TiO ; (b) in situ DRIFT
spectra taken at 250 1C upon passing NH over NO + O presorbed
O
2
3
2
O
5
3
2
3
2
2
% V
2
O
5
/WO
3
–TiO
2
.
NH
3
/N
2
at 250 1C (shown in Fig. 8a). After NO + O
pretreated Fe –TiO at 250 1C, both ionic
and coordinated NH diminished (shown in Fig. 8a). It
suggests that adsorbed ammonia species on Fe –TiO can
react with NO. Meanwhile, three new vibrations at 1614, 1576
2 2
/N
passed over NH
3
2
O
3
2
+
4
NH
3
2
O
3
2
ꢁ
1
and 1551 cm appeared, which were assigned to monodentate
2
2
nitrite and monodentate nitrate respectively.
After the adsorption of NO + O at 250 1C, Fe O –TiO
2
2
2
3
was mainly covered by monodentate nitrite and monodentate
Fig. 8 (a) In situ DRIFT spectra taken at 250 1C upon passing NO +
ꢁ
1
nitrate (1614, 1576, 1551 and 1371 cm ). After NH
passed over NO + O pretreated Fe –TiO at 250 1C, the
bands corresponding to adsorbed NO species disappeared.
3 2
/N
O
2
over NH
3
presorbed 2% Fe
2
O
3
–TiO
2
; (b) in situ DRIFT spectra
2
2
O
3
2
taken at 250 1C upon passing NH
2
over NO + O presorbed
3
x
2 3 2
Fe O –TiO ; (c) in situ DRIFT spectra taken at 250 1C upon passing
NO + O + NH over 2% Fe O –TiO .
2 3 2 3 2
ꢁ1
Meanwhile, the bands at 1606, 1430 and 1221 cm corres-
ponding to adsorbed ammonia species appeared. It suggests
that adsorbed NO
Fe –TiO
At last, the IR spectra during the SCR reaction (i.e. NH3
and NO O2 were simultaneously introduced) over
x
could take part in the SCR reaction over
2 3 2
Therefore, The SCR reaction over Fe O –TiO mainly fol-
2
O
3
2
.
lowed the Eley–Rideal mechanism.
+
3.3.3 In situ DRIFTS study on V O /Fe O –TiO . After
2
2
5
2
3
Fe O –TiO at 250 1C were recorded. As shown in Fig. 8c,
2% V O /Fe O –TiO was treated with NH /N at 250 1C,
2
3
2
2
2
5
2
3
2
3
2
+
+
Fe
2
O
3
–TiO
was mainly covered by ionic NH
4
bound to the
bound to
the Lewis acid sites (1221 and 1606 cm ), and the characteristic
vibrations corresponding to adsorbed NO species were hardly
observed. It suggests that the adsorption of NO + O on
Fe –TiO could not happen in the presence of NH . There
is general agreement that the SCR reaction starts with
the adsorption of NH , which is very strong compared to
V
2
O
5
/Fe
2
O
3
–TiO
2
was mainly covered by ionic NH
ꢁ1
4
bound
ꢁ
1
Brønsted acid sites (1430 cm ) and coordinated NH
3
to the Brønsted acid sites (1430 cm ) and coordinated NH
ꢁ1
3
ꢁ
1
bound to the Lewis acid sites (1221 and 1606 cm ). The
x
characteristic vibration corresponding to NH
3
adsorbed on
–TiO
2
V
2
O
5
/Fe –TiO was the same as that on Fe
2
2
O
3
2
O
3
2
2
O
3
2
3
(shown in Fig. 8a and 9a), and it was quite different from that
on V O /WO –TiO (shown in Fig. 7a and 9a). It suggests that
2
5
3
2
3
the acid sites (i.e. Brønsted acid sites and Lewis acid sites) on
2
3
the adsorption of NO + O2 and the reaction products.
V O /Fe O –TiO mainly resulted from the support Fe O –TiO ,
2
2
5
2
3
2
3
2
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3
+
2+
+
NH3(ad) + ꢄFeSurf
- ꢁNH
2
+ ꢄFeSurf
+ H
(8)
(9)
ꢁ
NH + NO -N + H O
2 (g) 2 2
2
+
1
3+
1
ꢄFeSurf + O - ꢄFe
+ ꢄO
(10)
2
4
Surf
2
Meanwhile, the SCR reaction over V O /WO –TiO can be
2
5
3
2
2
4
approximately described as follows:
NH3(g) " NH3(ad)
(7)
5
+
4+
Surf
+
NH3(ad) + ꢄVSurf - ꢁNH + ꢄV
+ H (11)
NH + NO - N + H O (9)
2 (g) 2 2
2
ꢁ
4
+
1
5+
1
ꢄVSurf
+ O
2
- ꢄVSurf
+ ꢄO
(12)
4
2
Reaction (7) was 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
NH . Reactions (8) and (11) were the activation of adsorbed
3
3+
5+
ammonia species by Fe and V on the surface to form amide
species (ꢁNH ), respectively. Then, gaseous NO was reduced by
NH on the surface to form N and H O (reaction (9)). Reactions
10) and (12) were the reoxidization of formed Fe and V
respectively. Therefore, the SCR reaction over V /Fe –TiO
can be approximately described using reactions (7–12) and both
2
–
2
2
2
2+
4+
,
(
2
O
5
2
O
3
2
5+
V
3+
and Fe could be the active components for the activation
of adsorbed NH3.
Fig. 9 (a) in situ DRIFT spectra taken at 250 1C upon passing NO +
over NH presorbed 2% V /Fe –TiO ; (b) in situ DRIFT
spectra taken at 250 1C upon passing NH over NO + O presorbed
% V /Fe –TiO
In situ DRIFTS study demonstrated that the acid sites on
V O /Fe O –TiO mainly resulted from Fe O –TiO , and
O
2
3
2
O
5
2
O
3
2
2
5
2
3
2
2
3
2
3
2
gaseous NH mainly adsorbed onto Fe O –TiO . Therefore,
3
2
3
2
2
2
O
5
2
O
3
2
.
the adsorbed NH
3
activated by Fe
3
+
on V
2
O
5
/Fe
2
O
3
–TiO
5+
2
preferred to be
rather than V
2+
2 5 2 3 2
on V O /Fe O –TiO .
5+
and the effect of V
approximately neglected. After NO + O
pretreated 2% V /Fe –TiO at 250 1C, both ionic NH
2
O
5
loading on the acid sites can be
Moreover, the oxidization of Fe by V is thermodynami-
2
/N passed over NH
2
3
cally favorable because the redox potential of reaction (13) is
3+
+
4
O
2 5
2
O
3
2
0
.23 V. Therefore, the regeneration of Fe
2 3 2
on Fe O –TiO
and coordinated NH diminished (shown in Fig. 9a). Meanwhile,
3
could be accelerated due to the rapid electron transfer between
16
adsorbed H O, which is a product of the SCR reaction, was
2
5+
2+
and Fe on V
V
2
O
5
/Fe
2
O
3 2
–TiO .
ꢁ
1 22
clearly observed at 1620 cm
ammonia species on 2% V
After 2% V /Fe –TiO
50 1C, the bands corresponding to adsorbed nitrite and nitrate
cannot be observed (shown in Fig. 9b). It indicates that the active
sites on Fe –TiO for the adsorption of gaseous NO were
covered by the loaded V . Hence, the adsorption of gaseous
NO on Fe –TiO was restrained due to the loading of V
After NH passed over NO + O pretreated 2% V
Fe O –TiO , its surface was mainly covered by ionic NH
.
They both suggest that adsorbed
/Fe –TiO can react with NO.
was treated with NO + O /N at
2+
5+
3+
4+
0
E = 0.23 V
2
O
5
2
O
3
2
ꢄFeSurf + ꢄVSurf - ꢄFeSurf + ꢄVSurf
O
5
2
O
3
2
2
2
(13)
2
2
At 150–300 1C, a large amount of NO cannot be reduced
over Fe –TiO (shown in Fig. 5a). It indicates that adsorbed
NH cannot be completely activated by Fe on Fe
at 150–300 1C. The loading of V accelerated the regeneration
through reaction (13), which promoted
2
O
3
2
2
O
3
2
3
+
3
2 3 2
O –TiO
2 5
O
2 5
O
2
O
3
2
2 5
O .
3+
of Fe on Fe
the activation of adsorbed NH
Furthermore, NH adsorbed on V O /Fe O –TiO , which was
2 3 2
O –TiO
3
/N
2
2
2 5
O /
+
3
+
3
by Fe
2 3 2
on Fe O –TiO .
2
3
2
4
3
2
5
2
3
2
bound to the Brønsted acid sites and coordinated NH bound
3
3+
5+
not activated by Fe , could then be activated by V on the
surface. As a result, the SCR activity of Fe O –TiO improved
to the Lewis acid sites. Furthermore, adsorbed H O, which is a
2
2
3
2
product of the SCR reaction, can hardly be detected at about
ꢁ
1
due to the loading of V O . However, the oxidization ability of
2 5
1
620 cm . It suggests that the reaction between adsorbed
nitrogen oxide species and ammonia cannot happen on 2%
/Fe –TiO . Therefore, the SCR reaction over V
–TiO mainly followed the Eley–Rideal mechanism.
3
+
5+
Fe is much less than that of V , so the activation of NH
3
by
Fe was much slower than that by V . As a result, the SCR
activity of 2% V /Fe –TiO was much less than that of 2%
/WO –TiO at 150–300 1C.
Some adsorbed NH could be over-oxidized by V cations
on the surface to form ꢁNH or ꢁN above 300 1C. Then, ꢁNH
3+
5+
V O
2 5
2
O
3
2
2 5
O /
O
2 5
O
2 3
2
Fe O
2 3
2
V
2
O
5
3
2
5
+
3
4
. Discussion
2
5
or ꢁN reacted with gaseous NO to form N
2 2
O. Therefore, N
The SCR reaction over Fe O –TiO can be approximately
2
2
3
6
selectivity of V O /WO –TiO obviously decreased above
2
5
3
2
described as follows:
3
00 1C (shown in Fig. 5b). If a large amount of NH adsorbed
3
5
+
NH3(g) " NH3(ad)
This journal is c The Royal Society of Chemistry 2012
(7)
on V O /Fe O –TiO was activated by V
3
above 300 1C,
2
5
2
2
Catal. Sci. Technol.

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some N
/Fe
adsorbed on V O /Fe O –TiO was mainly activated by Fe
2
O should form. However, little N
2
O formed over
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2
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–TiO (shown in Fig. 5b). It suggests that NH
2
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Fe –TiO even if V
of adsorbed NH . Because gaseous NH mainly adsorbed on
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Fe
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on V O /Fe O –TiO . Meanwhile,
2
2
5
2
3
3
the ratios of Fe to V on 1% and 2% V O /Fe O –TiO
+
5+
2
5
2
3
2
were 15.3 and 3.7, respectively. Therefore, reaction (8) pre-
dominated over the activation of adsorbed NH during the
SCR reaction over V /Fe –TiO at 350–500 1C. As a
result, V /Fe –TiO showed an excellent N selectivity at
50–500 1C. Because the regeneration of Fe on Fe –TiO
was accelerated due to the loading of V , the SCR activity
of V /Fe –TiO was generally better than that of
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. Conclusion
2
% V
and H
% V
external magnetic field due to its inherent magnetization. Therefore,
% V O /Fe O –TiO could be a promising low-cost SCR catalyst
2
O
5
/Fe
O/SO
/Fe
2
O
3
–TiO
durability at 300–450 1C. Meanwhile, the emission of
–TiO to the fly ash can be prevented by an
2 2
showed excellent SCR activity, N selectivity
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This study was financially supported by the National Natural
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National High-Tech Research and Development (863) Program of
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Catal. Sci. Technol.
This journal is c The Royal Society of Chemistry 2012