Enhanced catalytic performance of V2O5-WO 3/Fe2O3/TiO2 microspheres for selective catalytic reduction of NO by NH3

Article abstract of DOI:10.1039/c2cy20332d

The V₂O₅-WO₃/Fe₂O ₃/TiO₂ microsphere catalysts were prepared by an impregnation method. The catalytic test results showed that the Fe ₂O₃ additives in V₂O

Full text of DOI:10.1039/c2cy20332d

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Enhanced catalytic performance of V2O5–WO3/Fe2O3/
TiO2 microspheres for selective catalytic reduction of
NO by NH3†
Cite this: Catal. Sci. Technol.,
2013, 3, 191
Ruihua Gao,a Dengsong Zhang,*ab Xingang Liu,c Liyi Shi,ab
Phornphimon Maitarad,a Hongrui Li,a Jianping Zhangb and Weiguo Caob
The V2O5–WO3/Fe2O3/TiO2 microsphere catalysts were prepared by an impregnation method. The
catalytic test results showed that the Fe2O3 additives in V2O5–WO3/Fe2O3/TiO2 improved the NO
decomposition in the temperature range of 200–400 1C. The characterization results indicated that the
iron oxides mainly existed in the form of Fe2O3, which was beneficial for the oxidation of NO to NO2.
Meanwhile, the superior catalytic performance of V2O5–WO3/Fe2O3/TiO2 for the selective catalytic
reduction of NO has been attributed to its highly dispersed active species, and lots of surface adsorbed
oxygen. Prominently, the activity of V2O5–WO3/Fe2O3/TiO2 microspheres can be recovered after cutting
off SO2, and thus these combination metal oxide catalysts show not only high catalytic activities but
also good resistance to SO2.
Received 18th May 2012,
Accepted 26th August 2012
DOI: 10.1039/c2cy20332d
www.rsc.org/catalysis
Recently, it has been reported that Fe2O3,4,5 Fe containing
1. Introduction
mixed oxides6–9 and Fe-exchanged materials10–13 exhibit
14
As we know, nitrogen oxides (NOx) emitted from stationary
sources and automobiles are major causes for acid rain, smog,
and ozone depletion. The selective catalytic reduction (SCR) of
NOx to N2 using NH3 as a reductant is now considered to be the
most effective process for the treatment of nitrogen oxides from
stationary sources.1–3 V2O5–WO3 (MoO3)/TiO2 has been widely
used as an industrial catalyst. This kind of catalyst shows high
NOx reduction efficiencies and the high resistance to SO2
poisoning. However, the reaction should be operated at relatively
high temperatures (350–450 1C) and the SCR reactor is located
upstream before the dust removal or desulfurizer device to avoid
reheating of the flue gas. Therefore, the catalysts are easily
poisoned by the high concentration of dust and SO2. Thus, the
design of the highly active catalysts for low-temperature SCR that
is located downstream of the dust removal or desulfurizer device
becomes important.
remarkable activity in SCR of NO by NH3. Liu and He
reported that iron titanate catalysts exhibit high activity at
250–350 1C in the SCR of NOx with NH3. This indicated that
the iron oxides show better activity at lower temperature than
V2O5–WO3/TiO2. It is commonly accepted that the catalytic
behavior of the supported vanadium oxides is sensitive to the
intrinsic nature of the vanadium species, including oxidation
state, coordination circumstance, dispersion, and stability.15–18
The chemical and structural features of the promoters affect
catalytic performance indirectly, by influencing vanadium dis-
persion and stabilization of the vanadium oxide species.
Herein, iron oxides were incorporated into V2O5–WO3/TiO2
catalysts to improve the SCR activity at low temperature. Much
attention has been given to the synthesis of nano-structured
inorganic materials with hierarchical morphologies in the field
of catalysis. For example, Yang et al.19 have reported that the
novel WO3/TiO2 microspheres show good catalytic performance
for the selective oxidation of cyclopentene to glutaraldehyde.
The high dispersion of the tungsten species can be achieved on
the titania spheres. Here we used TiO2 microspheres as support.
The titania microspheres have a mesoporous structure with a
narrow pore size distribution in the nanometer range, and are
also thermally stable and have much more surface area. These
novel characteristics suggest that the TiO2 microspheres can be
applied as good candidates of catalyst supports. In addition, the
a Research Center of Nano Science and Technology, Shanghai University,
Shanghai, 200444, China. E-mail: dszhang@shu.edu.cn; Fax: +86-21-66136038;
Tel: +86-21-66134852
b Department of Chemistry, Shanghai University, Shanghai, 200444, China
c Center of Analysis and Measurement, Fudan University, Shanghai, 200433, China
† Electronic supplementary information (ESI) available: The morphology of
1V10W3FeTMS catalysts, and the catalytic performance of samples with various
fabrication parameters. See DOI: 10.1039/c2cy20332d
c
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promotional effect of iron oxides on the V2O5–WO3/TiO2 catalysts N2 (30 mL minÀ1) at 300 1C for 2 h. For NH3-TPD, each sample
for the SCR remains unreported. was saturated with high-purity anhydrous ammonia at 120 1C
Therefore, the aim of the present study was to study for 1 h and subsequently flushed at the same temperature for
the promotional effect of iron oxides on the V2O5–WO3/TiO2 1 h to remove physisorbed ammonium. The NH3-TPD analysis
catalysts for the SCR to attain a catalyst with enhanced activity was carried out at 120–850 1C at a heating rate of 10 1C minÀ1
.
at relatively low temperatures. Fe2O3 was firstly employed to For NOx-TPD, each sample was saturated with high-purity NO
coat TiO2 microspheres (Fe2O3/TiO2 microspheres, denoted and O2 at 100 1C for 1 h and subsequently flushed at the same
FeTMS), and then, we added V, W oxides as additives to get temperature for 1 h to remove physisorbed NOx. The NOx-TPD
V2O5–WO3/Fe2O3/TiO2 microspheres (denoted VWFeTMS) with analysis was carried out at 100–850 1C at a heating rate of 10 1C
improved resistance to SO2 poisoning for NH3-SCR. To gain minÀ1. For H2-TPR, the flowing gas was switched to 5% H2/N2,
insight into the key role of iron species in the VWFeTMS and the sample was heated to 950 1C at a ramping rate of 10 1C
catalysts in the SCR of NO with NH3, VWFeTMS catalysts with minÀ1. The laser Raman experiment was performed using an
different iron loadings were prepared by an impregnation inVia-reflex Renishaw spectrometer equipped with a holo-
method and compared in the SCR of NO with NH3 at different graphic notch filter, a CCD detector and a laser radiating at
reaction temperatures in the range of 200–500 1C.
532 nm. The X-ray photoelectron spectroscopy (XPS) was
recorded on a Perkin–Elmer PHI 5000C ESCA system equipped
with a dual X-ray source, using the MgKa (1253.6 eV) anode and
a hemispherical energy analyzer. The background pressure
during data acquisition was kept below 10À6 Pa. All binding
2. Experimental section
2.1 Catalyst preparation
The FeTMS supports were synthesized as follows. The required energies were calibrated using contaminant carbon (C1s =
amount of iron nitrate was dissolved in water. The TiO2 micro- 284.6 eV) as a reference. The IR spectra were recorded on a
sphere materials, prepared following a procedure reported by Nicolet Nexus 470 Fourier transform infrared spectrometer.
Guo et al.,20 were added into the stirred solution at 60 1C. Then The surface characterisation has been performed using pressed
the excessive water was completely evaporated at 80 1C, and the disks of pure powders (15 mg for 2 cm diameter), activated by
FeTMS was obtained after calcination at 500 1C in air for 3 h. If outgassing (10À5 Torr) at 300 1C into the IR cell. A conventional
the Fe loading was 3 wt%, the support was denoted 3FeTMS. manipulation/outgassing ramp connected to the IR cell
The catalyst was prepared by a conventional incipient wetness was used. The adsorption procedure involves contact of the
impregnation method. The required amount of ammonium activated samples disk with ammonia or NO and O2 mixed gas
vanadate and ammonium tungstate was dissolved in water at RT at a pressure of 15 Torr and outgassing at 25 1C.
acidified by oxalic acid. To this solution, the synthesized FeTMS
was added and then completely evaporated, and followed by
drying and calcination at 550 1C for 6 h under an air atmo-
2.3 Catalytic reaction
The SCR activity measurement was carried out in a fixed-bed
quartz micro-reactor operating in a steady state flow mode. The
reactant gas composition was typically: 550 ppm NO, 550 ppm
NH3, 3% O2, and balance N2. The total flow rate was 500 mL
minÀ1 and thus a gas hourly space velocity (GHSV) of 30 000 hÀ1
was obtained. The temperature was increased from 200 to
500 1C. At each temperature step the data were recorded when
the SCR reaction reached steady state after 15 min. The
concentration of NO in the inlet and outlet gas was measured
by a KM9106 flue gas analyzer. The concentrations of N2O and
sphere. Typically, 1V10W3FeTMS represents the loading of
V2O5 (1 wt%), WO3 (10 wt%), and Fe (3 wt%), respectively.
The catalysts maintained the spherical shape as shown in
Fig. S1 (ESI†).
2.2 Characterizations
Nitrogen adsorption–desorption isotherms of the samples were
measured at À196 1C using an ASAP 2010Micromeritics, and
the corresponding pore size distribution curves are calculated
from desorption branches by the BJH method. The specific
surface area of the samples was calculated by the Brunauer–
Emmett–Teller (BET) method. The SEM micrographs were
obtained using a JEOLJSM-6700F scanning electron micro-
scope. The samples were deposited on a sample holder with a
piece of adhesive carbon tape and were then sputtered with a
thin film of gold. Powder X-ray diffraction (XRD) was performed
with a 222 Rigaku D/MAX-RB X-ray diffractometer by using Cu
Ka (40 kV, 40 mA) radiation and a secondary beam graphite
monochromator. Ammonia temperature programmed desorption
(NH3-TPD), NOx temperature programmed desorption (NOx-TPD),
and temperature programmed reduction by hydrogen (H2-TPR)
were conducted on TianjinXQ TP-5080 auto-adsorption apparatus,
and the NH3, NOx and H2 were monitored by a TCD. Before TPD
or TPR, each sample was pre-treated with high-purity (99.999%)
Fig. 1 X-ray diffraction patterns of various catalysts.
c
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imply the type IV isotherm with an inflection of nitrogen-
adsorbed volume at P/P0 = 0.45 (type H2 hysteresis loop),
indicating the presence of well-developed mesopores in the
samples. The pore size distribution measurement was evaluated
by the BJH model, as shown in Fig. 2(b); it can be seen that the
TMS and different catalysts afford the relatively narrow pore-size
distribution with their pore diameters centered at approximately
6.0 nm. The BET model was applied to the desorption branch,
resulting in both specific surface areas and pore volumes for
these samples. As listed in Table 1, due to the introduction of
V2O5, nanoparticles within the TMS support are mostly located
on the outer surface or presumably infiltrated in mesopores. The
BJH desorption average pore diameter is opposite to the surface
area. This conclusion can be easily interpreted according to the
pores in TMS with different sizes. When the metal oxides are
deposited on the surface of TMS or in the small pores, there is
the possibility of blocking the Fe2O3 and WO3; the surface areas
and pore volumes correspondingly decrease. It is most likely that
the embedded entrance of small pores would also block the
access to a portion of the smaller pores. Therefore, the more the
blocked small-pores, the higher the desorption average pore
diameter and the lower the BET surface area.
3.1.2 SURFACE ACIDITY AND REDOX PROPERTIES. The TPD of ammonia
is a frequently used method for determining the surface acidity of
solid catalysts as well as acid strength distribution. The NH3-TPD
profiles of the different catalysts are shown in Fig. S2 (ESI†). The
NH3-TPD results of the different catalysts are summarized in
Table 1. Compared with the 1V10WTMS sample, the
1V10W1FeTMS sample has more acidic content and stronger
acidic strength, which should be due to the cooperative effects
between the iron oxide species and VWTMS that may be helpful to
the SCR of NO with NH3. It is noted that the acidic amounts vary
slightly with the change of Fe loadings in the range of 1–5 wt%.
The NOx-TPD results are shown in Fig. 3. All the catalysts
show broad bands between 150 1C and 680 1C. This indicates
that there are not only weakly adsorbed NOx but also nitrate
species which exist in the type of monodentate nitrate, bridging
nitrate, and bidentate nitrate formed on the catalyst surface.22
The sequence of NOx desorption amounts from the catalysts is
1V10W3FeTMS > 1V10W5FeTMS > 1V10W1FeTMS > 1V10WTMS.
Fig. 2 (a) Nitrogen adsorption–desorption isotherms and (b) the corresponding
pore size distribution for the samples.
NH3 were measured by a Transmitter IR N2O analyzer and an
IQ350 ammonia analyzer.
3. Results and discussion
3.1 Characteristics of catalysts
3.1.1 PHASE COMPOSITION AND SURFACE AREA. The effect of differ-
ent iron loadings on the catalysts structure was investigated by
XRD techniques. The XRD patterns of VWTMS and VWFeTMS
are shown in Fig. 1. The peaks at 251, 371, 481 and 531 could be
ascribed to the characteristic diffraction peaks of anatase TiO2
(JCPDS #21-1272). Also there are no peaks observed that are
derived from the crystalline V2O5, Fe2O3, or WO3, which
indicates a high dispersion of vanadia phase as typically found
for other oxide-supported vanadia catalysts.15,21
Fig. 2(a) shows the nitrogen adsorption–desorption iso-
therms of the TMS and other different catalysts. All of them
Table 1 Physico-chemical properties of the catalysts
a
b
c
d
SBET
VBJH
DBJH
Td
Acidic amountsd
Samples
(m2 gÀ1
)
(cm3 gÀ1
)
(nm) (1C) (mmol gÀ1
)
TMS
144
80
87
88
60
0.266
0.186
0.173
0.173
0.168
5.2
7.3
6.2
6.1
8.9
—
—
1V10WTMS
1V10W1FeTMS
1V10W3FeTMS
1V10W5FeTMS
187 0.423
194 0.483
185 0.495
179 0.506
a
b
BET surface area of the samples. BJH desorption cumulative volume
c
d
Fig. 3 The NOx-TPD profiles of various catalysts.
of pores. BJH desorption average pore diameter (4V/A). NH3-TPD.
c
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Fig. 4 H2-TPR profiles of various catalysts.
These results mean that the appropriate Fe2O3 species can
improve basic sites over the 1V10WTMS on which the NOx
species were adsorbed. While excess Fe2O3 is not beneficial to
the adsorption of NOx. According to the literature,23 the active
nitrate species could be continuously formed on the fresh
FeTiOx catalyst, which favours the low temperature SCR reac-
tion. Liu et al.24 also reported that more nitrate species on the
catalyst surface could participate in the SCR reaction, which
Fig. 5 Raman spectra of various samples (a) full observation between 100 and
1200 cmÀ1 and (b) deep observation between 700 and 1200 cmÀ1
.
was beneficial to promote the SCR reaction. So the addition of 1200 cmÀ1, so the Raman spectra between 700 and 1200 cmÀ1
Fe2O3 species should be positive to improve the low tempera- were carefully detected in Fig. 5b. It is noted that the band
ture SCR activity of 1V10WTMS.
around 800 cmÀ1 consists of the overlapping contribution of a
The TPR is widely used to study the redox property of metal second-order anatase feature and of the W–O–W stretching of
oxide catalysts. Fig. 4 shows the H2-TPR profiles of VWTMS and octahedrally coordinated W units in an environment similar to
VWFeTMS catalysts. As for 1V10WTMS catalysts the main peak the one of bulk WO3,28 and the intensity of this signal increases
is found at 870 1C and the two shoulders appear at 560 and with the length of the chains. A peak close to 960 cmÀ1 is
715 1C. The two peaks at low temperatures are attributed to the reported for the VQO stretch of vanadyl groups internal to
reduction of highly dispersed vanadium species, while the metavanadate chains.16 From Fig. 5b, the bands of 800 and
presence of a peak at 870 1C can imply the tungsten oxide 960 cmÀ1 are both weaker. Meanwhile, the corresponding XRD
reduction.25,26 The addition of Fe to the VWTMS catalysts patterns did not show any diffraction peaks corresponding to
caused the three reduction peaks to shift to the relative lower crystalline tungsten and vanadia oxides. This indicates that the
temperatures. For the 1V10W3FeTMS catalysts, the three dominant tungsten and vanadia oxides over these catalysts are
reduction peaks shifted to 475, 680, and 850 1C. Liu and He14 monomeric and polymeric oxide species.
reported that the Fe3+ species have been totally converted into
The XPS investigation of binding energies and intensities of
Fe2+ until 500 1C, and then partial Fe2+ species have been the surface elements provides information on the chemical
converted into metallic Fe. So the peaks of lower temperatures states and relative quantities of the outermost surface com-
were attributed to the reduction of V2O5 and Fe2O3. If over- pounds. From the V 2p XPS spectra of the VWTMS and
laying the lines of the H2-TPR, the peak areas of the VWFeTMS VWFeTMS samples given in Fig. 6a, we can distinguish vana-
catalysts are far greater than that of VWTMS. Among all the dium oxide species in different chemical states from the posi-
catalysts, the 1V10W3FeTMS catalysts have the largest area. tion of the V 2p3/2 level, and the peak of V 2p1/2 (B524.7 eV) was
These results indicate that the integration of Fe with the very weak and hindered by O 1s satellites (521.1 eV). As shown
VWTMS catalyst can enhance the oxidative ability of the in Fig. 6a, the XPS spectra of the VWTMS and VWFeTMS were
VWFeTMS catalyst.
too weak for ready analysis indicating that the vanadia is highly
3.1.3 SURFACE COMPOSITION AND CHEMISORPTION PROPERTIES. The dispersed on the TMS support. The V 2p3/2 of the VWTMS and
normalized Raman spectra of the catalysts are shown in Fig. 5a VWFeTMS is between 514.0 and 517.5 eV which indicates that
and b. The bands at 639, 515, 396, 195 and 143 cmÀ1 are due to vanadium species in the VWTMS and VWFeTMS catalyst are in
the five Raman active fundamentals of anatase TiO2.27 In mixed oxidation states of V5+ and V4+ 29
.
Fig. 5a, due to the stronger peaks of anatase TiO2, the peaks
Two peaks ascribed to Fe 2p3/2 (710.0–711.4 eV) and Fe 2p1/2
of V, W and Fe oxides are hard to observe between 200 and (724.4–725.0 eV) showed up in Fig. 6b. The binding energies of
c
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Table 2 The quantitative results of the mole ratio of different atoms by XPS and
peak-fitting results of O1s spectra of different samples
Surface compositiona/mol% BE (eV)
b
Catalysts
W
Fe
—
Ti
Oa
Ob
Oa
1V10WTMS
2.39
(1.21)
22.04
(32.08)
20.09
(31.17)
18.47
(30.81)
16.72
531.2 529.5 31.1
531.5 529.6 40.7
530.9 529.6 45.6
531.4 529.5 37.3
1V10W1FeTMS 2.46
(1.22)
1V10W3FeTMS 6.02
(1.23)
1V10W5FeTMS 2.61
(1.25)
0.30
(0.51)
0.89
(1.54)
1.52
(2.59)
(30.43)
a
b
The average value is provided in the brackets. Oa(%) = SO /SO + SO .
b
a
a
The quantitative peak-fitting results of O1s can be seen in
Table 2. Usually, the surface oxygen Oa is more reactive in
oxidation reactions due to its high mobility than lattice oxygen
Ob,33 which is beneficial to the NO oxidation to NO2 in the SCR
reaction. Herein the relative concentration ratios of Oa/Oa + Ob
over all the catalysts are calculated and listed in Table 2. The
Oa/Oa + Ob ratio of 1V10W3FeTMS catalyst is much higher than
that of the other catalysts, indicating the presence of more
abundant surface oxygen. This might be beneficial for the
enhancement of NO oxidation and thus the SCR activity.14,34
Table 2 gives the quantitative results of the mole ratios of
different atoms by XPS. The XPS spectra of V 2p3/2 over the
VWTMS and VWFeTMS samples were too weak for quantitative
analysis. It is interesting to note that the surface W content is
much higher than the mean values for all catalysts. It is also
clear that the surface Fe content is lower than the average
values for all catalysts. These XPS results show that surface
enrichment of WO3 is obviously observed for all of the catalysts,
suggesting that WO3 is dispersed on the surface of the TMS and
the Fe2O3 is mainly dispersed in the pores of the TMS. According
to the comparison of the surface molar ratio of Fe with that of
the mean value as added, it can be concluded that the increase of
this ratio with the increasing weight percentage of iron is linear.
This suggests that the dispersion of the surface iron species is
uniform with increasing iron loading.
Fig. 7a shows the in situ IR results of 1V10WTMS and
1V10W3FeTMS catalysts during 500 ppm NH3 adsorption.
Depending on the metal oxide surface, the adsorption of NH3
can occur through a hydrogen bond, coordination to an electron-
Fig. 6 XPS spectra of V 2p (a) and Fe 2p (b) and O 1s (c) over different catalysts.
these two peaks correspond well with characteristic Fe3+ deficient metal atom (Lewis acid site), dissociation of NH3 with
according to the literatures.7,30 According to works by Santhosh formation of NH2 or NH group, and the formation of NH4 at
+
34–36
Kumar et al.,31 for SCR, the concentration of accessible Fe3+ Bronsted acid sites.
Fig. 7a indicates that 1V10WTMS and
¨
that can participate in a reversible redox cycle should be 1V10W3FeTMS have similar adsorption sites. The features cen-
maximized, whereas the formation of FexOy clusters must be tered at 1200 and 1603 cmÀ1 were attributed to the symmetric
completely or largely avoided, indicating that Fe3+ may play a and asymmetric deformation vibration of ammonia coordinated
major role in the catalytic cycle. The O 1s bands of different to Lewis acid sites. The bands at 1647 and 1450 cmÀ1 were
+
34
¨
samples in Fig. 6c were deconvoluted by the curve-fitting proce- correlated to NH4 ions formed on Bronsted acidic sites. As
dure. The sub-bands at lower binding energy (530.2–530.5 eV) compared with the 1V10WTMS catalyst, the Lewis acid sites
corresponded to the lattice oxygen O2À (denoted Ob), and the (1200 cmÀ1) of 1V10W3FeTMS were weaker, but the Bronsted
¨
sub-bands at higher binding energy (531.6–532.1 eV) corre- acid sites (1450 cmÀ1) were obviously similar. According to the
sponded to the surface adsorbed oxygen (denoted Oa), such as NH3-SCR mechanism over vanadia–titania catalyst by Topsøe,37
2À
O2 or OÀ belonging to defect oxide or hydroxyl-like group.32 both types of acid sites were involved in NH3 adsorption.
c
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Fig. 7 In situ IR results of (a) comparison of NH3 on different catalysts at 25 1C; (b) NH3 on 1V10W3FeTMS from 25 to 250 1C; (C) comparison of NO + O2 adsorption on
different catalysts at 25 1C; (d) NO + O2 adsorption on 1V10W3FeTMS from 25 to 250 1C.
The in situ IR results of NH3 on 1V10W3FeTMS from 25 to were more important than the bridging nitrate species for the
250 1C were shown in Fig. 7b. With an increase of temperature, NH3-SCR process.
the bands of 1200, 1603, and 1647 cmÀ1 all shifted to the higher
wave bands. The intensity of the band at 1450 cmÀ1 decreased
3.2 Catalytic activity
more rapidly than that at 1200 cmÀ1. This indicated that the The effects of various Fe loadings from 1 to 5% on the
¨
Lewis acid sites were much stable than the Bronsted acid sites performance of the 1V10WFeTMS catalyst are shown in Fig. 8.
over the 1V10W3FeTMS catalysts, which indicated that the Fig. 8a indicates that NO conversion below 350 1C was sharply
¨
Bronsted acid sites were more important than the Lewis acid improved with the increase of Fe loading. At 250 1C, the NO
sites at the low temperature NH3-SCR process. The comparison conversion was generally improved from 52.3 to 81.3% with an
of NO plus O2 adsorption on different catalysts was presented increase of Fe loading from 0 to 5%. The catalyst with a 3 wt%
in Fig. 7c. According to Liu et al.,24 the bands at 1612 and Fe loading had the widest temperature window and the best
1243 cmÀ1 are attributed to bridging nitrate species, whereas activity. It is worth noting that the 1V10W3FeTMS catalyst
the band at 1581 cmÀ1 is attributed to bidentate nitrate species. showed almost the same activity as the 1V10WTMS catalyst at
The bands at 1543 and 1296 cmÀ1 can be assigned to mono- higher temperature. This indicates that Fe could especially
dentate nitrate. As compared with the 1V10WTMS catalyst, the increase the activity of the SCR at low temperature. While the
bands of bridging nitrate species (1612 cmÀ1
)
on 1V10W1FeTMS catalyst and 1V10W5FeTMS catalyst showed
1V10W3FeTMS were weaker, but the bands of monodentate lower activity than 1V10W3FeTMS, it may be attributed to the
nitrate (1296 cmÀ1) were obviously stronger. The M–O– presence of less surface oxygen on the two catalysts than that
NO2(MQFe) species could rapidly react with adjacent adsorbed on 1V10W3FeTMS. As seen in Fig. 8b, N2 selectivity over the
+
NH4 or NH3 to produce more reactive intermediates M–O– 1V10WFeTMS catalyst at any temperature is higher than 90%.
NO2[NH4+]2 or M–O–NO2[NH3]2, which could further react with This indicates that the Fe-doped catalyst displays excellent N2
gaseous NO to form N2 and H2O.24 The in situ IR results of NO selectivity.
plus O2 adsorptions on 1V10W3FeTMS from 25 to 250 1C were
The SCR activities of the different catalysts indicated that Fe,
shown in Fig 7d. With an increase of temperature, the intensity V, and W elements supported on TiO2 had a positive synergetic
of 1612 and 1243 cmÀ1 decreased rapidly, while the 1543 cmÀ1 interaction and accelerated the SCR reaction (Fig. S3, ESI†). The
band still remained. This indicated that monodentate nitrates performance of 1V10W5FeTMS was better than that of 1%
c
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Fig. 9 (a) The effect of SO2 on NO conversion over the 1V10W3FeTMS,
1V10WTMS, and 3FeTMS catalysts. (b) The effect of SO2 and H2O on NO
conversion over the 1V10W3FeTMS, 1V10WTMS catalysts. 500 mL minÀ1 total
rate, 550 ppm NO, 500 ppm NH3, 3% O2, 10% H2O (when used), 100 ppm SO2
Fig. 8 (a) NO conversion against temperature under SCR conditions over different
samples, (b) N2 selectivity against temperature under SCR conditions over
different samples. 500 mL minÀ1 total rate, 550 ppm NO, 500 ppm NH3, 3% O2,
(when used) and N2 balance, and 3FeTMS 320 1C, GHSV = 30 000 hÀ1
.
and N2 balance, GHSV = 30 000 hÀ1
.
V2O5–10% WO3/5% Fe–TiO2 (common material) (shown in in Fig. 9b. When 100 ppm SO2 and 10% H2O were added to the
Fig. S4, ESI†). This phenomenon shows that the mesoporous reactants, the NO conversion over the 1V10W3FeTMS catalyst
spheres are more favorable for the title reaction. The catalyst rapidly decreased from 97% to 64% in 1 h, and then nearly
exhibited its best performance at a calcination temperature of stabilized. After turning off SO2 and H2O, the activity restored
550 1C (Fig. S5, ESI†). The NO conversion and selectivity of N2 to 75%. The NO conversion over the 1V10WTMS catalyst
over the 1V10W3FeTMS catalyst still kept above 80% and 92% decreases from 80% to 53% in 1 h, and then nearly kept. After
after the second cycle (Fig. S6, ESI†). This finding clearly removing SO2 and H2O, the activity restored to 61%. For the
indicates that the 1V10W3FeTMS catalyst shows good stability 3FeTMS catalyst, the NO conversion reduces from 94% to 62%
which makes possible its further application as a good catalyst. in 1 h, and then nearly stabilizes. After shutting off SO2 and
The effect of SO2 on NO conversion over the 1V10W3FeTMS, H2O, the activity recovered to 67%. These results indicate that
1V10WTMS and 3FeTMS catalysts was shown in Fig. 9a. When the resistance to the mixture of H2O and SO2 over the
100 ppm SO2 was added to the reaction gases, the NO conver- 1V10W3FeTMS catalyst is stronger than the 3FeTMS catalyst.
sion over the 1V10W3FeTMS decreased from 97% to 75% in This indicates that the addition of V2O5 and WO3 oxide could
1 h, and then nearly stabilized. After turning off SO2, the activity increase 3FeTMS catalyst resistance to SO2 and H2O. The
restored to 95%. The NO conversion over the 1V10WTMS decreased activity is mainly caused by the tightly adsorbed
decreased from 80% to 61% in 1 h after SO2 was added to SO2, the formed sulfates, and the other species on the surface
the reaction, and then nearly stabilized. After removing SO2, the occupy the active sites, which decrease the SCR activity.
activity recovered to 71.8%. While the NO conversion over the
For the SCR of NOx with NH3, two reaction routes have been
3FeTMS decreased from 94% to 70% in 1 h, and then nearly proposed: the ‘‘standard’’ reaction occurring between NH3 with
kept. When shutting off SO2, the activity restored to 76.6%. NO, and the ‘‘fast’’ SCR reaction where both NO and NO2 are
This indicates that the combination effect of iron oxide and involved. The ‘‘fast’’ path has an important contribution,
V2O5–WO3/TiO2 could enhance the 1V10W3FeTMS catalyst particularly at low temperature. So the generation of NO2 from
resistance to SO2.
NO is a key factor.38 The conversion of NO to NO2 over the
The effect of SO2 and H2O on NO conversion over the different catalysts was measured and listed in Fig. S7 (ESI†). It
1V10W3FeTMS, 1V10WTMSand 3FeTMS catalysts was shown is obvious that the introduction of Fe additive to the catalyst
c
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4. Conclusion
The catalytic activity of 1V10WTMS was greatly enhanced by the
addition of 3 wt% of Fe in the temperature range of 200–500 1C.
V2O5 and WO3 could enhance the 3FeTMS catalyst resistance to
SO2. The XRD results indicate that the active components of V
and W oxides are well-dispersed. The Raman spectra indicate
that the dominant vanadia and tungsten oxides over these
catalysts are the monomeric and oligomeric tetrahedrally coor-
dinated vanadia and tungsten species. NOx-TPD results show
that the appropriate Fe2O3 species can improve basic sites over
the 1V10WTMS on which the NOx species were adsorbed. The
H2-TPR indicated that the addition of Fe over the VWTMS
catalyst enhanced the oxidative ability of the VWFeTMS
catalyst. The Oa/Oa + Ob ratio of the 1V10W3FeTMS catalyst is
much higher than that of the other catalysts resulting in the
enhancement of NO oxidation and thus the SCR activity. More-
over the IR-NO + O2 results show the monodentate nitrate
species are essential for the SCR reaction.
Scheme 1 A redox catalytic cycle of the low temperature SCR reaction over
VWFeTMS catalysts.
increases the oxidation activity of NO to NO2. It has been
accepted that NO is more readily reduced to N2 along with a
portion of NO2 than single NO by NH3.39,40 The introduction of
iron oxide increases the oxidation rate of NO to NO2, which
increases the SCR activity.
Acknowledgements
The authors acknowledge the support of National Natural
Science Foundation of China (51108258), Science and Technol-
ogy Commission of Shanghai Municipality (10540500100 &
11nm0502200), Research Fund for the innovation Program of
Shanghai University (A.10040711003), Shanghai Municipal
Education Commission (B.37040711001), and Key Subject of
Shanghai Municipal Education Commission (J50102). The
authors would like to thank Prof. Y. L. Chu and Prof. W. J. Yu
from analysis and test center of Shanghai University for help
with the SEM and TEM measurements.
3.3 Correlation between catalytic activity and structures
It is confirmed that 1V10W3FeTMS exhibits the best activity
among all the employed catalysts in the SCR reaction. It is
demonstrated that the active components of vanadium oxide
and tungsten oxide were well-dispersed on the surface of the
iron oxide coated TiO2 microspheres, and the integration of
Fe3+ with the VWTMS catalyst can enhance the oxidative ability
of the VWFeTMS catalyst. It is clear that the dominant tungsten
and vanadia oxides over these catalysts are monomeric and low
polymeric oxide species. Based on the characterization results
above, the surface oxygen Oa is more reactive in oxidation
reactions due to its high mobility than lattice oxygen Ob, which
is beneficial to the NO oxidation to NO2 in the SCR reaction.
Herein, the Oa/Oa + Ob ratio of the 1V10W3FeTMS catalyst is
much higher than that of the other catalysts, indicating the
presence of more abundant surface oxygen. We proposed
electron transfer between V and Fe in Scheme 1. The V and
Fe species are interconnected in the form of V–O–Fe through
oxygen bridges, facilitating electron transfer. This is beneficial
for the enhancement of NO oxidation and thus the SCR activity.
This agrees with the experiment of the conversion of NO to NO2
over 1V10W3FeTMS catalysts. The NOx-TPD results show that
the appropriate Fe2O3 species can improve basic sites over the
1V10WTMS on which the NOx species were adsorbed and
the active nitrate species could be easily formed resulting in the
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