Direct synthesis of V-W-Ti nanoparticle catalysts for selective catalytic reduction of NO with NH3

Article abstract of DOI:10.1039/c5ra05978j

A series of V-W-Ti nanoparticle catalysts with variable V doping amounts were directly synthesized by the sol-gel method, and their catalytic performances were tested for the selective catalytic reduction of NO with ammonia. The catalysts were characterized by means of XRD, Raman, BET, TEM, SEM, NH₃-TPD, H₂-TPR and XPS. SCR kinetic studies were conducted to understand the mechanistic features of V-W-Ti catalysts. It was found that the V_0.02W_0.04Ti catalyst exhibited the highest NO conversion and the lowest apparent activation energy. The characterization results showed that V was incorporated into the TiO₂ framework and the redox cycle of V⁴⁺ + Ti⁴⁺ → V⁵⁺ + Ti³⁺ existed over the V-W-Ti catalysts. A high concentration of reducible and distorted V species could account for the excellent NH₃-SCR catalytic performance of the V_0.02W_0.04Ti catalyst. In situ FT-IR spectroscopy was performed to investigate the mechanism of the NH₃-SCR reaction over the V_0.02W_0.04Ti catalyst. Experimental results showed that both Lewis and Bronsted acid sites over the V_0.02W_0.04Ti catalyst were involved in the NH₃-SCR reaction. The adsorption of nitrate species was significantly limited and the adsorbed NO₂ gaseous molecules were easily formed over the V_0.02W_0.04Ti catalyst, which resulted in the high catalytic activity at low temperature. This journal is

Full text of DOI:10.1039/c5ra05978j

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Direct synthesis of V–W–Ti nanoparticle catalysts
for selective catalytic reduction of NO with NH3
Cite this: RSC Adv., 2015, 5, 45172
Kai Cheng, Jian Liu,* Zhen Zhao, Yuechang Wei, Guiyuan Jiang and Aijun Duan
A series of V–W–Ti nanoparticle catalysts with variable V doping amounts were directly synthesized by the
sol–gel method, and their catalytic performances were tested for the selective catalytic reduction of
NO with ammonia. The catalysts were characterized by means of XRD, Raman, BET, TEM, SEM, NH
TPD, H -TPR and XPS. SCR kinetic studies were conducted to understand the mechanistic features of
V–W–Ti catalysts. It was found that the V0.02 0.04Ti catalyst exhibited the highest NO conversion and
the lowest apparent activation energy. The characterization results showed that V was incorporated
3
-
2
W
4
+
4+
5+
3+
into the TiO
catalysts. A high concentration of reducible and distorted V species could account for the excellent
NH -SCR catalytic performance of the V0.02 0.04Ti catalyst. In situ FT-IR spectroscopy was performed
to investigate the mechanism of the NH -SCR reaction over the V0.02 0.04Ti catalyst. Experimental
results showed that both Lewis and Brønsted acid sites over the V0.02 0.04Ti catalyst were involved in
the NH -SCR reaction. The adsorption of nitrate species was significantly limited and the adsorbed NO
2
framework and the redox cycle of V + Ti / V + Ti existed over the V–W–Ti
3
W
3
W
Received 3rd April 2015
Accepted 15th May 2015
W
3
2
DOI: 10.1039/c5ra05978j
gaseous molecules were easily formed over the V0.02
catalytic activity at low temperature.
W0.04Ti catalyst, which resulted in the high
www.rsc.org/advances
1
0
2 5
isolated polymer. Finally, crystalline V O is present when
vanadium loading amount is beyond the theoretical mono-
layer coverage.
1
. Introduction
Over the past decades, nitrogen oxides (NO , including NO, NO
x
2
and N O) from stationary and mobile sources have remained as
Most V-based catalysts were prepared by solution
2
1
1–13
major air pollutants. NO
on the human respiratory system, due to the formation of acid at high vanadium loading, and amorphous V species would
x
emissions have direct negative effects impregnation method,
and the dispersion would decline
1,2
rain, photochemical smog and ozone depletion. In recent transform to crystalline one. Thus, the limit of the dispersion
years, the NO
emission standard is becoming more and more would lead to the decrease of NH -SCR activity. In this work,
stringent worldwide. Therefore, it is necessary to develop sol–gel method is used to introduce vanadium active sites
advanced technology and seek better catalysts to control NOx into the framework of TiO supports and improve its catalytic
emissions.
activity. The catalysts, which exhibited highly dispersed
x
3
2
The selective catalytic reduction (SCR) of NO by NH in the active species and in which the active metals were existed in a
3
presence of excess oxygen is one of the best world technologies tetrahedral coordination structure, have higher activity than
to remove NO
The commonly used commercial catalysts for NH
based on V –WO /TiO
-based catalysts owing to its highly component. Many reports are focused on the effects of W in
catalytic activity and sulfur tolerance under actual ue-gas V-based catalysts. Shan et al. and Peng et al. reported that
conditions. However, these kinds of catalysts are only effi- the addition of WO can provide much more Brønsted acidic
sites compared with other metal oxides, which is thought to
Generally, titanium dioxide (TiO ) supporters and vana- be the reason why the activity are higher than that of other
x
from stationary sources and diesel engines. those prepared by an impregnation or precipitation method.
-SCR are Moreover, TiO support is modied by doping another
3
2
2
O
5
3
2
1
4
15
3
ꢀ
3–5
cient in a narrow temperature window (300–400 C).
2
1
6
dium(V) are used as active catalytic materials. With the metal oxides. Cristallo et al. revealed that the suitable
increasing of vanadium loading on titania surface, vanadium amount of WO can stabilize the anatase phase of TiO and
3
2
species are transformed from tetrahedral VO
partly distorted dimeric or oligomeric species,
(ref. 6 and 7) to preserve the surface characteristic of the catalysts. Recently,
4
17
8
,9
then to Kompio et al. pointed a new view that the promotional effect
of tungsten may originate from the neighboring vanadium
oxide species or from the disruption of surface vanadium
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, 18# oxide aggregating, which is benecial to the SCR
Fuxue Road, Chang Ping District, Beijing 102249, China. E-mail: liujian@cup.edu.
performance.
cn; Fax: +86-10-69724721; Tel: +86-10-89732326
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In present work, we synthesized a series of V–W–Ti nano- 84.0 eV with respect to the Fermi level. Binding energies of Ti
particle catalysts with variable V doping amounts by the sol–gel 2p, Ce 3d, O 1s were calibrated using the C 1s peak (BE ¼
method. Their catalytic performances were tested for selective 284.8 eV) as standard.
catalytic reduction of NO with ammonia and the activation
energy was calculated to describe the activity of catalysts. temperature (Renishaw, UK). A laser radiating at 532 nm was
NH -SCR mechanism of these catalysts was also investigated used as the excitation source, and the laser power of below
systematically.
The laser Raman experiments were performed at room
3
0.07 mW was applied at the sample. Before measurements, the
samples were well ground and mounted into a spinning holder
to avoid thermal damage during the scanning.
2. Experimental
Temperature programmed reduction with H2 (H -TPR)
2
2
.1. Catalyst preparation
experiments were performed on the Quantachrome Instru-
ments of autosorb iQ. 100 mg sample was pretreated under Ar
V–W–Ti nanoparticle catalysts were prepared by the sol–gel
method. In a typical synthesis, 34 mL of deionized water, 34 mL
of anhydrous ethanol and 6.8 mL of nitric acid were mixed in
the break (denoted as solution A), and then a certain amount of
ammonium metavanadate and ammonium tungstate were
dissolved in solution A (denoted as solution B). The solution of
butyl titanate (34 mL) and anhydrous ethanol (136 mL) were
added dropwisely to solution B under vigorous stirring to carry
out hydrolysis. Aer continuously strirring for 3 h, the yellowish
ꢀ
by calcination at 300 C for 1 h and subsequently cooled to
ꢀ
ꢁ1
3
0 C. Aerwards, 10% H
2
/Ar ow (60 mL min ) was passed
over the catalyst bed while the temperature was ramped from
ꢀ
ꢀ
ꢁ1
3
0 to 800 C at a heating rate of 10 C min
.
Temperature-programmed desorption of ammonia (NH
3
-TPD)
was performed in a conventional ow apparatus. Each sample
ꢀ
was pretreated with N at 500 C for 1 h, then saturated with high-
2
ꢀ
purity anhydrous ammonia at 100 C for 1 h and subsequently
ushed at the same temperature for 1 h to remove physisorbed
transparent sol was yielded. Subsequently, the sol was dried at
ꢀ
ammonium. Finally, the TPD operation was carried out from
8
4
0 C for 24 h to form xerogel. Aer being crushed and sieved to
0–60 mesh, the xerogel was calcinated at 500 C for 5 h in air.
ꢀ
ꢀ
ꢁ1
ꢀ
100 to 700 C at a heating rate of 10 C min . The amount of NH
desorbed was monitored by a thermal conductivity detector
TCD).
In situ DRIFTS spectra were recorded using a thermo Nicolet
3
For the sake of the brevity, the catalysts were denoted as
VxW0.04Ti. Where, x and 0.04 represented the molar ratio of V/Ti
and W/Ti, respectively, and x ¼ 0, 0.004, 0.012, 0.02, 0.028,
(
IS50 spectrometer, which was equipped with a high tempera-
ture environmental cell tted with ZnSe window and an MCT
0.036.
detector cooled with liquid N
2
. The catalyst was loaded in the
2
.2. Catalyst characterization
ꢀ
Harrick IR cell and heated to 400 C under N
2
at a total ow rate
3
ꢁ1
The crystal structures of the fresh catalysts were determined of 50 cm min for 60 min to remove adsorbed impurities.
with a powder X-ray diffractometer, using Cu Ka (l ¼ 0.1542 nm) A background spectrum was collected under a owing N
radiation combined with a nickel lter operating at 40 kV and atmosphere and was subtracted from the sample spectra. The
0 Ma (Shimadzu, Japan). The diffractometer data were recor- DRIFTS spectra were recorded by accumulating 32 scans with a
2
1
ꢀ
ꢀ
ꢀ
ꢁ1
ꢁ1
ded for 2q values from 10 to 80 at a scanning rate of 4 min
.
resolution of 4 cm .
The patterns were compared with ICDD les for phase
identication.
2.3. Catalytic performance test
The Brunauer–Emmett–Teller (BET) surface areas were
measured by N2 adsorption and desorption (Quantachrome The activities of various catalysts for the NH -SCR of NO were
3
ꢀ
Instruments, USA). The samples were degassed at 200 C for 12 h. carried out in a xed bed quartz reactor (i.d. 6 mm). The feed
The TEM images were carried out using a JEILJEM 2100 gas mixture consisted of 1000 ppm of NO, 1000 ppm of NH ,
3
ꢁ
1
electron microscope equipped with a eld emission source at an 3 vol% of O . A total ow rate of 500 mL min was maintained
2
accelerating voltage of 200 kV. Drops of the suspension were for all experiments with N as the balance gas. The concentra-
2
applied, and aer drying, the ne particles were well dispersed tion of NO in the inlet and outlet gas was measured by a ue gas
on a copper grid coated with carbon.
analyzer (Model-4000VM, SIGNAL International Ltd., UK).
The surface morphology of the catalyst was observed by eld Meanwhile, the concentration of NH , NO, NO and N O were
3
2
2
emission scanning electron microscopy (FESEM) on a Quanta measured by a FTIR spectrometer (Nicolet IS50). All catalysts
00F instruments using accelerating voltages of 5 kV, in were kept on stream at each temperature for 1/2 h. The NO
2
combination with an EDAX genesis 4000 energy-dispersive X-ray conversion was dened as follow:
spectrometer (EDX). Samples for SEM were dusted on an
adhesive conductive carbon belt attached to a copper disk and
were coated with 10 nm Au prior to measurement.
NOðinletÞ ꢁ NOðoutletÞ
NOðinletÞ
NO conversion ð%Þ ¼
ꢂ 100%;
(1)
X-ray photoelectron spectroscopy (XPS) spectra were recor- where NO(inlet) represents the NO concentration in the inlet
ded with a standard AlK source (1486.6 eV) working at 350 W (ppm), NO(outlet) represents the NO concentration in the outlet
(
XSAM800, Kratos Analytical company, UK). The working pres- (ppm).
ꢁ7
sure was less than 2 ꢂ 10 Pa. The spectrometer was calibrated
The N
2
selectivity in SCR reaction was calculated from
by assuming the binding energy (BE) of the Au 4f7/2 line to lie at eqn (2).
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2
½N
2
ꢃout
below 25% and in the absence of inter-particle mass transfer
limitations, NO conversion is expected to increase n-fold as the
space velocity decreases n-fold. Based on this criterion, NOx
N
2
selectivity ð%Þ ¼ 2
½N
ꢂ 100%
2
ꢃout þ 2½N
2
Oꢃ þ ½NO ꢃout
out
2
(
2)
conversion ratio (a ratio of NO conversion at different space
x
ꢁ
1
velocity to that at GHSV ¼ 400 000 h ) is shown in Fig. 2b. In
this investigation, for catalysts, interparticle mass-transfer
3
3
. Results and discussion
ꢁ1
limitation can be ignored under GHSV $ 400 000 h and
.1. NH
3
-SCR performance
ꢀ
temperature #175 C conditions.
NH
3
-SCR activities of V–W–Ti catalysts were evaluated as a
3
The NH -SCR on the V based catalysts are generally consid-
20
function of temperature and the results are presented in Fig. 1a. ered to be a rst-order reaction with respect to NO. Assuming
V–W–Ti catalysts showed relatively high activity, while W–Ti the diffusion-limitation free, the effective rst-order rate
catalyst exhibited nearly no activity in the temperature range of constant is related to NO conversion (X) by
ꢀ
2
00–400 C. This result suggests that vanadium oxide species be
F
o
k ¼ ꢁ
lnð1 ꢁ XÞ
(3)
(4)
the main activity phase of V–W–Ti catalysts. The V doping at an
appropriate amount enhanced NO conversion and widened the
active temperature range. With the increasing of V doping
molar amount from 0 to 0.02, the SCR activity showed a
monotonic increase. Further increasement of the vanadium
doping amount to 0.036 led to the decrease of the activity. For
V0.02W0.04Ti catalyst, NO conversion was obtained at 90% in a
½
NOꢃ Wcat
o
ꢀ
ꢁ
ꢁ
E
a
k ¼ A exp
RT
3
ꢁ1 ꢁ1
where k is the reaction constant (cm g
NO feed rate (mol s ); [NO] is the molar NO concentration at
o
the inlet (mol cm ); Wcat is the catalyst weight (g); X is the
conversion of NO (%); A is the pre-exponential factor; E is the
s
); F is the molar
o
ꢁ
1
ꢁ
3
ꢀ
wide temperature range (200–400 C) under a GHSV of
ꢁ
1
a
5
0 000 h . N
2
selectivities of all samples were above 95%
ꢁ
1
ꢀ
apparent activation energy (kJ mol ); R is the gas constant
irrespective of the catalytic activity below 300 C (Fig. 1b). The
ꢁ1
ꢁ1
ꢀ
(8.3145 J mol
K ); and T is the temperature (K).
Fig. 2c shows the Arrhenius plots based on the reaction
between 100 and 175 C. The A and E are determined from
these plots based on eqn (4) and given in Table 1. E values of
all samples are above 40 kJ mol
NH -SCR process at low temperature was controlled by chem-
ical reaction control. E values gradually decrease with the
increasing of vanadium molar contents from 0.004 to 0.02.
Further increasing vanadium content led to values
N
2
selectivity showed a decrease above 300 C with the
increasing of vanadium content, which was mainly due to the
occurrence of unselective catalytic oxidation of NH
0.04Ti catalyst is an optimal NH -SCR catalyst at low
temperature.
ꢀ
a
3
.
a
V
0.02
W
3
ꢁ
1
21
,
which indicated that the
3
a
3
.2. Kinetics analysis of V–W–Ti catalysts on NH
reactivity
3
-SCR
E
a
To understand the intrinsic effect of vanadium doping on the increasement.
activity of V–W–Ti catalysts, the catalytic activity data are
quantitatively analyzed using the macro-kinetic approach. To
3
.3.
2
N physisorption results
avoid the diffusive effect during the reaction, the inuence of
different space velocity on the NH -SCR activity of V0.02
3
W0.04Ti Table 2 displays the textural and structural properties of V–W–Ti
catalyst has been studied. As shown in Fig. 2a, the NO conver- catalysts. The surface area and pore volume showed a monotonic
sion decreased at whole temperature window as the GHSV decrease with the increasing of V doping amount and
ꢁ1
increased from 50 000 to 400 000 h , which is due to the more
0.036
V W0.04Ti catalyst was of the lowest surface area. All V–W–Ti
contact time for reactant gas to react on the catalysts. According catalysts exhibited pore sizes in the range of 10–30 nm, indi-
18,19
to the previous study,
it was reasonable to assume this cating that this series of catalysts prepared by the sol–gel
reactor as a differential reactor when the NO conversion is method were present as mesoporous materials. Although the
Fig. 1 NO conversion (a) and N
2
selectivity (b) for V–W–Ti catalysts as a function of reaction temperatures. Reaction conditions: [NO] ¼ [NH ] ¼
3
ꢁ1
ꢁ1
1
000 ppm, [O
2
] ¼ 3%, balance N
2
, total gas flow rate 500 mL min , and GHSV ¼ 50 000 h
.
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Fig. 2 (a) NO conversion versus reaction temperature for NH
3
-SCR over V0.02
W
0.04Ti sample at various space velocities from 50 000 to
ꢁ1
ꢁ1
4
00 000 h ; (b) NO conversion ratios with respect to that at GHSV ¼ 400 000 h as a function of reaction temperature, using data from (a);
(
c) Arrhenius plots of intrinsic reaction rate constants of the catalysts.
BET surface area of V0.02
W
0.04Ti catalyst was lower than that of Table 2 The textural and structural properties of V–W–Ti catalysts
V
0.004
W
0.04Ti catalyst, it exhibited higher NH -SCR activity. This
3
a
b
S
BET
V
mic
Average pore
diameter (nm)
result indicates that the BET surface area did not play a key role
in the SCR reaction.
2
ꢁ1
3
ꢁ1
Catalysts
(m g
)
(cm g
)
W
0.04Ti
60.3
57.2
49.5
45.4
39.5
25.9
1.477
1.366
1.016
0.866
0.617
0.477
17.25
16.80
18.14
12.72
21.79
29.00
V
0.004
W0.04Ti
3.4. XRD results
V0.012W0.04Ti
V
V
V
0.02
W0.04Ti
To investigate the crystal structure of the active phase in the
different catalysts, XRD patterns of the catalysts are shown in
Fig. 3. For all catalysts, anatase phase (PDF-ICDD21-1272) was
the main phase, and only a little rutile phase (PDF-ICDD21-
0.028
W
W
0.04Ti
0.04Ti
0.036
a
b
Calculated by BET method. Calculated by t-plot method.
1
0
276) appeared. With the increasing of V molar amount from
to 0.036, XRD patterns of V–W–Ti catalysts showed no
monomeric or polymeric vanadium oxide species for SCR
reaction. The negligible effect of increasing V amount above
signicant variation. It is worthy to note that the peak ascribed
to crystalline V O and WO3 phase was absent in the XRD
patterns even if V molar ratio increased from 0.004 to 0.02. It
suggested that V and W be well dispersed. However, for
V
2
5
2
2
0
.02 on the SCR activity is related to this change.
The average crystalline size was determined according to the
diffraction peak indexed to the crystal plane by using the
0.028
W
0.04Ti and V0.036
W
0.04Ti catalysts, a V
2
O
5
peak appeared
23
ꢀ
Scherrer formula: D ¼ 0.89l/(b cos q), where l is the wave-
length of radiation; b is the corrected peak width at half-
maximum intensity (FWHM in radians); and q is the peak
at ꢄ20.2 , which indicated that vanadium oxide species trans-
forms from monomeric or polymeric state to a bulk phase via
2 5
crystallization. Bulk (i.e., crystallized) V O is more inactive than
a 3
Table 1 Kinetic parameter (A and E ) for NH -SCR reaction over the catalysts
Sample
V
0.004
W
0.04Ti
V
0.012
W
0.04Ti
V
0.02
W
0.04Ti
6
V
0.028
W
0.04Ti
6
V
0.036
W
0.04Ti
ꢁ1
E
A
a
(kJ mol
)
51.5
1.8 ꢂ 10
43.2
1.2 ꢂ 10
42.4
1.6 ꢂ 10
44.1
2.2 ꢂ 10
47.4
6 ꢂ 10
7
6
6
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Fig. 3 X-ray diffraction patterns of V–W–Ti catalysts.
Fig. 4 Laser Raman spectra of V–W–Ti catalysts.
position of the main reection. The crystalline sizes of V–W–Ti
catalysts were in the range of 10–14 nm in Table 3, which
revealed that these catalysts prepared by the sol–gel method
possess nanometer particle sizes. In addition, the diffraction
peaks of V–W–Ti catalysts slightly shied to low-angle direction
compared with W–Ti catalyst, which also suggested that vana-
V
0.036
W0.04Ti samples were abundant, the Raman spectra
ꢁ
1
showed a sharp and intense peak at 993 cm assigned to a trace
of crystalline V . It was consistent with the XRD results.
2 5
O
dium ions be incorporated into the lattice of TiO to form
uniform solid solution.
2
3.6. TEM results
24
Fig. 5 shows the HAADF-TEM images of V0.02W0.04Ti catalyst
together with the elemental mapping images for Ti, V, and W in
the same region. In the HAADF-STEM image, small particles in
3.5. Raman results
To further investigate the structures of the vanadium and the bright contrast were observed on the V0.02W0.04Ti catalyst.
tungsten species, the Raman spectra are displayed in Fig. 4. Two From the EDX mapping images, it is conrmed that tungsten
ꢁ1
peaks at 956 and 802 cm , which can be assigned to W]O elements were not aggregated, which correlates with the lack of
stretching mode and the polymeric tungsten oxide species, WO3 peak in the XRD pattern. As V-L (0.511 keV) and Ti-L
25,26
respectively can be detected in W0.04Ti sample.
0.004
V W0.04Ti (0.452 keV) peaks in EDS spectrum completely overlap, they
catalyst showed a broad Raman band ranged from 950 to 990 were not distinguished by current EDS spectroscopy. TiO grain
2
ꢁ
1
cm , which was attributed to the overlap of symmetrical V]O was made of nanoparticles in the size range of 15–30 nm, as
27
and W]O stretching mode under monolayer coverage. Since shown in Fig. 5b and c. In addition, the microstructure of the
the Raman cross section of two-dimensional V]O species is V0.02W0.04Ti catalyst was also investigated by HRTEM in Fig. 5d
almost four times that of two-dimensional W]O species over and e. Two phases with the interplanar spacing of 0.32 and
28,29
titania surface,
the Raman band of tungsten species were 0.35 nm were shown in the images, which was corresponding to
likely to be overshadowed by the strong Raman band of vana- the interplanar distance of rutile phase (110) plane and anatase
dium species. Therefore, the peak shi in this range mainly phase (101) plane, respectively. No individual particles of V O5
2
resulted from the structure transformation of vanadium species. and WO were seen on the surface of the catalyst. This result
3
With the increasing of vanadium content to 0.02, the Raman was in agreement with the XRD determination results.
ꢁ1
band at 977 cm attributing to two-dimensional surface vana-
dium species became stronger. This result indicated that the
partly distorted vanadium species. For V0.028W0.04Ti and
3
.7. SEM results
The SEM image in Fig. 6 shows that V0.02
W0.04Ti catalyst is
composed of the homogeneity and monodispersity of spherical-
shaped nanoparticles. The EDS results from different regions
Table
3 XRD parameters and average crystal sizes of V–W–Ti
(Point 1 and Point 2) give similar compositions. The molar
catalysts
ratios of V/Ti and W/Ti are 0.020–0.019 and 0.035–0.036 for
V0.02W0.04Ti catalyst, respectively, which are in good agreement
with the initial nominal amounts.
ꢀ
ꢀ
Catalysts
Crystal face
l/nm
2q/( )
b/( )
D/nm
W0.04Ti
101
101
101
101
101
101
0.1542
0.1542
0.1542
0.1542
0.1542
0.1542
25.35
0.557
0.797
0.64
0.607
0.646
0.695
14.9
10.3
12.9
13.6
12.8
11.8
V
V
V
V
V
0.004
W
W
0.04Ti
0.04Ti
25.309
25.147
25.217
25.146
25.157
0.012
3.8. XPS results
0.02
W0.04Ti
0.028
W
0.04Ti
0.04Ti
V
x
W
0.04Ti catalysts are excellent catalysts for NH -SCR. The XPS
3
0.036
W
was carried out to investigate the chemical compositions and
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Fig. 5 HAADF-STEM images and EDS elemental mapping (a) of V0.02
W
0.04Ti catalyst; and TEM images of V0.02
W
0.04Ti catalyst with (b and c) low
resolution, and (d and e) high resolution.
5
+
4+ 32,33
the valence states of the various species of the catalysts. Ti 2p 2p3/2 binding energies of V and V .
It could be seen that
XPS studies of V–W–Ti catalysts are shown in Fig. 7a. The peaks there is a little blue shi for V 2p3/2
.
of the samples appearing at around 458.7 and 464.5 eV could be
4
+
30,31
attributed to Ti 2p3/2 and Ti 2p1/2 of Ti , respectively.
V–W–Ti catalysts, Ti 2p position was shied toward lower BE by
.5 eV with increasing vanadium amount from 0 to 0.036, In the H -TPR proles of Fig. 8, it can be seen that all catalysts
For
3
2
.9. H -TPR results
0
2
4
+
3+
indicating that some Ti could be reduced to Ti . As shown in gave a single reduction peak at the temperature ranging from
Fig. 7b, the peaks at 516.8 and 515.6 eV could be ascribed to the 420 to 600 C. This peak can be assigned to the coreduction of
ꢀ
Fig. 6 SEM images and EDS analyses for V0.02W0.04Ti catalyst.
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Fig. 7 XPS spectra of (a) Ti2p and (b) V2p for various catalysts.
this peak could be ascribed to the weak acidic sites, which is
corresponding to physisorbed ammonia and partially ionic
+
35
NH
4
bound to Brønsted acid sites. A shoulder peak in the
ꢀ
ranged 250–430
0.012
C was detected on V W0.04Ti and
V
0.02
W0.04Ti catalysts. This peak was ascribed to the desorption
36
3
of coordinated NH bound Lewis acid sites. Indeed, the
ammonia adsorption ability of the catalyst is generally consid-
ered as one of the most important criteria for the screening of
catalytic systems for the NH -SCR. As was shown in Fig. 9,
3
V
0.02
W0.04Ti catalyst showed predominantly more desorbed
ammonia. It seems that vanadium species create new acidic
sites and a progressive increasement of the surface acidity
owing to the addition of vanadium. Thus, the improvement in
ammonia adsorption over V0.02W0.04Ti catalyst is believed to be
signicantly benecial to the NO reduction by ammonia at the
x
wide temperature range.
Fig. 8
2
H -TPR profiles of V–W–Ti catalysts.
3
.11. In situ DRIFTS studies
.11.1 Adsorption of NH
spectra of NH adsorption over V0.02
temperatures. When NH was introduced into the DRIFTS cell
at room temperature, several vibration bands can be detected in
3
3
. Fig. 10 shows the in situ DRIFT
0.04Ti catalyst at different
13
W(VI) to W(IV) and V(V) to V(III). With the increasing of V doping
amount from 0 to 0.02, the temperature of the reduction peak
3
W
3
ꢀ
showed a monotonic decrease from 593 to 427 C. Further
increasement of V doping molar amount to 0.036 led to the
ꢁ1
ꢁ1
the range of 1000–4000 cm . The bands at 1632 and 1228 cm
higher peak temperature increase. V0.02
W0.04Ti catalyst showed
can be assigned to the asymmetric and symmetric bending
the lowest reduction temperature, thus it exhibited the highest
vibrations of NH bond in NH coordinately linked to Lewis acid
3
34
reducibility. Sorrentino et al. reported that catalysts with good
reducibility in the H -TPR results had good catalytic activity
towards the SCR process for DeNO . The high reducibility of
0.04Ti catalyst was contributed to NH -SCR of NO.
Meanwhile, the H consumption of V–W–Ti catalysts were larger
than that of W–Ti catalyst except for V0.004 0.04Ti, as shown in
ꢁ1
site, respectively. And the peaks at 3348, 3256 and 3149 cm
2
can be ascribed to the N–H stretching vibration modes of the
x
ꢁ1
coordinated NH
3
, while the band at 1435 cm is due to the
V
0.02
W
3
2
W
2
Table 4 The max temperature and area of reduction peaks in the H -
Table 4. This indicated that doping V ions into W–Ti catalyst
increased the reduction degree of the catalysts, which was also
correlated to the SCR activity.
TPR
a
ꢀ
Catalysts
T
max ( C)
Peak area
W
0.04Ti
593
521
438
427
524
595
278
203
346
329
597
859
V
V
V
V
V
0.004
W
W
0.04Ti
0.04Ti
3
.10. NH -TPD results
3
0.012
Temperature-programmed desorption of ammonia was carried
out to determine the strength and amount of different acid
0.02
W0.04Ti
0.028
W
W
0.04Ti
0.04Ti
0.036
sites. The NH
3
-TPD patterns of V–W–Ti catalysts are depicted in
ꢀ
a
Fig. 9. All catalysts showed one peak around 150–250 C, and
The max temperature of the reduction peak.
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3
Fig. 9 NH -TPD curves of V–W–Ti catalysts.
Fig. 11 In situ DRIFT spectra of NO + O
catalyst measured at 30–350 C.
2
adsorption on V0.02
W
0.04Ti
ꢀ
ꢁ
1
monodentate nitrite (1443 cm ) and chelating bidentate nitrate
ꢁ
1
(
1540 cm ) appeared, suggesting a transformation from mono-
2ꢁ
dentate and bidentate nitrate to cis-N
and monodentate nitrite over the V0.02
2
O
2
, bridged bidentate
41–44
W0.04Ti catalyst.
With
ꢀ
the increasing of the temperature to 250 C, all bands attributed
to NO adspecies vanished, indicating that the absorbed nitrate
and nitrite species were decomposed or inhibited.
.11.3 Reaction between nitrogen oxides and ammonia
adspecies. Fig. 12 shows the in situ DRIFT spectra of V0.02W0.04Ti
catalyst as a function of time in a ow of NO + O aer the
catalysts was pretreated. The coordinated NH on Lewis
1210 and 1599 cm ) and Brønsted acid site (1428 and
x
3
2
3
ꢁ
1
(
1
NH
1
ꢁ1
658 cm ) were formed on V0.02
. All bands due to ammonia adspecies diminished in
0 min aer V0.02 0.04Ti catalyst was purged by NO + O , and
W0.04Ti catalyst with feeding
Fig. 10 In situ DRIFT spectra of NH
catalyst measured at 30–350 C.
3
adsorption on V0.02W0.04Ti
ꢀ
3
W
2
ꢁ1
subsequently the absorbed NO (1614 cm ) was observed. It
2
could be concluded that NO readily reacted with surface-active
x
4
+
asymmetric bending vibration of NH bond in NH chem-
37,38
isorbed on Brønsted acid site.
These adsorbed NH species
3
decreased slowly with the increasing of the temperature.
However it is noticeable that these vibration bands were not
ꢀ
disappeared completely even at 350 C. The obtained results
suggested that there be both Lewis and Brønsted acid site on the
surface of V0.02W0.04Ti catalyst and these adsorbed NH species
3
be very stable. Previous reports showed Brønsted acid site was
benecial to the adsorption of NH thus improving the low-
3
39,40
temperature activity.
.11.2 Co-adsorption of NO and O
DRIFT spectra of NO + O desorption on V0.02
different temperatures. Several distinct bands appeared at 1241,
3
2
. Fig. 11 shows the in situ
0.04Ti catalyst at
2
W
ꢁ
1
1
292, 1580 and 1608 cm at room temperature, which were
respectively assigned to the asymmetric frequency of gaseous NO
2
ꢁ1
ꢁ1
ꢁ1
molecules (1608 cm ), bidentate nitrate (1580 cm ), mono-
dentate nitrate (1292 cm ), bridged nitrate (1241 cm ). When
the temperature was 150 C, the bands ascribed to monodentate
ꢁ
1
ꢀ
Fig. 12 In situ DRIFT spectra over V0.02W0.04Ti catalyst as a function of
and bridged nitrate vanished. However, the peaks attributed to
time in a flow of NO + O
2
after the catalysts was pre-exposed to a flow
2ꢁ
ꢁ1
ꢁ1
ꢀ
cis-N O
2 2
(1356 cm ), bridged bidentate nitrite (1227 cm ), of NH
3
for the 60 min and followed by N
2
purging for 30 min at 250 C.
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NH species over V0.02W0.04Ti catalyst. It was noticeable that the
Paper
3
bands assigned to NO species were absent and only one band
x
related to NO2 appeared. This result indicated that the
adsorption of nitrate species was inhibited, leading to more
active sites available for the adsorption and activation of NH
Thus a higher catalytic activity was obtained.
3
.
3
.11.4 Reaction between ammonia and adsorbed nitrogen
oxides species. Fig. 13 shows the in situ DRIFT spectra over
0.04Ti catalyst as a function of time in a ow of NH aer
V
0.02
W
3
the catalysts was pre-exposed to a ow of NO + O for the 60 min
2
ꢀ
followed by N purging for 30 min at 250 C. The band for the
2
ꢁ1
absorbed NO (1614 cm ) vanished in 5 min aer NH was
2
3
introduced into the system. Simultaneously, the peaks ascribed
to coordinated NH on Lewis acid sites (3362, 3256, 3165, 1594,
3
ꢁ
1
+
1
1
NO
213 cm ) and NH
654 cm ) appeared. All the results indicated that the absorbed
species could easily react to NH species to form NH NO
2 4 4 2
4
species on Brønsted acid sites (1429,
ꢁ
1
Fig. 14 In situ DRIFT spectra of V0.02
W
0.04Ti catalyst in a flowing of
ꢀ
+
1000 ppm NH + 1000 ppm NO + 3% O at 30–400 C.
3
2
species. Thus, it may be reasonable to assume that NH NO is
4
2
reaction intermediate on NH -SCR.
3
3
.11.5 In situ DRIFT spectra in a ow of NO + NH + O .
3 2
4
. Discussion
Fig. 14 shows the in situ DRIFT spectra of V0.02
W
0.04Ti catalyst in
at various
temperatures under the steady-state condition. No band
assigned to NO species was detected in the whole region, which
was resulted from the competitive adsorption and following the
reactions among NH . With the increasing of the temperature,
4.1. The relation between the structures of catalysts and
3 2
a owing of 1000 ppm NH + 1000 ppm NO + 3% O
their catalytic performance for NH -SCR
3
x
In the present work, V, W-incorporated TiO2 catalysts with
different molar ratios were synthesized by a direct sol–gel
method. When the vanadium content is no more than 0.02, the
3
4
+
the intensity of the bands assigned to NH species on Brønsted
promoting effect on NH
3
-SCR reaction at the low temperature
ꢁ
1
acid sites (1429, 1654 cm ) decreased faster than that of the
bands ascribed to coordinated NH on Lewis acid sites (3362,
264, 3180, 1594, 1213 cm ). The adsorbed NH species on
Brønsted acid site disappeared completely at 350 C, while the
band at 1203 cm attributed to the symmetric bending vibra-
ꢀ
(below 250 C) can be observed. Several early studies showed
3
that the incorporation of some transition metal ions such as
cerium and ferric into the framework of the support can
promote remarkably the catalytic activity for NH -SCR reac-
ꢁ1
3
3
ꢀ
3
ꢁ
1
45
tion. This kind of framework incorporated catalysts is
tions of N–H bond coordinately linked to Lewis acid site was
still detected even at 400 C. Combined with the results of the
different from supported catalysts. The active metal ions are
fully isolated and dispersed throughout the support. Therefore,
high concentrations of active sites can be achieved in
framework-incorporated catalysts. Thus, this kind of catalyst
exhibits a strong synergetic effect between active species and
support. Similar conclusions have been reached in V–W–Ti
catalysts. The XRD and XPS results demonstrate that V ions are
ꢀ
in situ DRIFT spectra of NH adsorption, Brønsted acid sites was
3
ꢀ
not disappeared at 350 C. These results indicated that the
Brønsted acid site over V0.02
W0.04Ti catalyst may be more active
than Lewis acid site in NH -SCR reaction.
3
incorporated into the framework of TiO . V and W are well
2
dispersed in the framework of TiO
distorted vanadium species are formed in V0.02
Fig. 4). It is helpful to enhance the NH -SCR activity and inhibit
2
support and the partly
W0.04Ti catalyst
(
3
the vanadium loss at the high temperature. Furthermore, the
vanadium oxide species was found to exist as an amorphous
phase when the molar ratio of V/Ti below 0.02. Above monolayer
coverage, bulk V O phase appears. The highly dispersed and
2
5
partly distorted vanadium species are benecial to NH -SCR
3
reaction, and crystalline state V
reaction.
2 5
O is unfavorable for this
The electronic and redox properties of V–W–Ti catalysts are
also important factors for determining the extent of NO
conversion in NH -SCR reaction. The results of XPS show that
3
there is a red shi of the binding energy of Ti 2p with the
increasing of vanadium molar amount from 0.004 to
Fig. 13 In situ DRIFT spectra over V0.02W0.04Ti catalyst as a function of
time in a flow of NH
NO + O
3
after the catalysts was pre-exposed to a flow of 0.036 (Fig. 7). This might be caused due to the difference of
ꢀ
2
for the 60 min followed by N
2
purging for 30 min at 250 C.
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ionization energy of W and V. Ion doping into the lattice would
slightly change the chemical valence of Ti and destroy the
charge balance of the original system. In order to maintain the
charge balance in the crystal lattice, some Ti ions would release
excess electrons and result in some titanium ions into a lower
valence. Hence, the red-shi of Ti 2p XPS peak was observed,
4
+
and Ti became more electropositive, which could decrease
the recombination between free electrons and holes. Impor-
tantly, the cation-anion codoping was successfully achieved by
Fig. 15 Schematic diagram of NH
oxygen over V0.02 0.04Ti catalyst.
3
-SCR in the presence of excess
W
incorporating V and W into TiO . Meanwhile, it could be seen
2
that the peaks of Ti and V 2p3/2 of V–W–Ti catalysts shied in
the reverse direction, which was in accordance with the strong
interaction between V and Ti in V–W–Ti catalysts. These results
manifested that the rich-electron on the surface of the catalyst,
and a higher reduction activity could be expected. The redox
cycle resulted in a decrease in the energy required for the
electron transfer between Ti and V active sites, promoting the
activation of NH3 and NO, and consequently causing an
be an important intermediate species for the NH
3
-SCR reac-
tion. In the present study, the nitrate species were highly
inhibited and the adsorbed NO gaseous molecule was easily
3
6
2
formed over V0.02W0.04Ti catalyst (see Fig. 11 and 12), which
resulted in the NH -SCR performance improvement. It may be
3
5
+
4+
attributed to the well balance V /V present over V0.02W0.04Ti
catalyst, which would promote the adsorption and activation
of gas-phase oxygen. The activated oxygen atom is benecial to
improvement of the NH -SCR activity. V0.02W0.04Ti catalyst
3
4
+
5+
showed a better balance of V and V in the V2p Gaussian-
tted curves than the other samples. Furthermore, the pres-
the formation of NO
are both Lewis and Brønsted acid site over V0.02 0.04Ti catalyst
3
and both types of acid sites are involved in NH -SCR reaction.
2
. From Fig. 11 and 13, it is seen that there

4
+
W
ence of more V in the V0.02W0.04Ti catalyst suggests that more
4
+
active sites (i.e., redox site V –OH) be present. Based on the
4
+
4+
5+
3+
It indicated that surface acidity plays an important role in the
adsorption and activation of NH , and thus affects the
occurrence of SCR reaction. Based the above results and
analyses, a cycle of the catalytic active site and NH -SCR
above results, the redox cycle of V + Ti / V + Ti may be
4
2,46
3
established, which is in agreement with the literature.
The
4
+
5+
presence of mixed V
and V
enhanced the electrical
47
3
conductivity. V0.02W0.04Ti catalyst is even more reducible than
reaction mechanism is proposed over framework-
incorporation V–W–Ti oxide catalysts. As shown in Fig. 15,
the other catalyst, as shown in H -TPR results. Thus, it exhibits
2
the highest catalytic activity.
+
NO
2
could react with NH
4
to NH
NO species. Then the NH
and H O. The whole process follows
the L–H mechanism. On the other hand, the adsorbed NH on
Lewis acid sites would react with gas-phase NO (Eley–Rideal
mechanism) to form N . The catalytic active site over
0.04Ti catalyst consists of three steps. Firstly, gas-phase
were absorbed on the Brønsted (V–OH) and Lewis (V]O)
acid sites. Secondly, these adsorbed NH species reacted with
4
NO
2
species, which reacted
further to yield the NH
decomposed to form N
2
2
NO
4.2. The kinetic investigation and NH
3
-SCR mechanism on
2
2
V–W–Ti catalysts
3
Fig. 2 shows the Arrhenius plots based on the reaction rate data
ꢀ
2
between 100 and 175 C. The A and E
a
were determined from
0.04Ti catalyst, A and E
were the highest comparing to the other catalysts. There were
V
0.02
W
these plots given in Table 1. On V0.004
W
a
NH
3
ꢁ
1
3
+
small difference for E (42.4–44.1 kJ mol ) and A (1.2–2.2 ꢂ
a
5
6
absorbed NO or gas-phase NO and V ]O were reduced to
V –OH. Finally, V –OH were re-oxidized to V ]O by O
2
1
0 ) of V0.012–0.028W0.04Ti, and V0.02W0.04Ti catalyst exhibited the
4
+
4+
5+
ꢁ
1
and
2
a
lowest E value of 42.4 kJ mol . It indicated that less energy was
the catalyst was regenerated. Therefore, the redox couples of
needed for activation in the reaction. Therefore, the catalytic
activity was signicantly improved and NO conversion would
increase. Further increasing V doping molar amount to 0.036,
4
+
5+
V /V play an important role for the SCR reaction to proceed.
4
+
4+
5+
3+
The existence of redox cycle (V + Ti / V + Ti ) can
4
+
5+
contribute to the redox process of V /V and an improvement
of the NH -SCR activity.
a
A and E values increased again. This phenomenon could be
3
explained by the change of V active sites. When the vanadium
doping amount is low, V active species may be not sufficient.
Then the vanadium species reach the highest dispersity with the
increasing of V doping molar amount to 0.028. Further
5
. Conclusion
increasing vanadium molar amount to 0.036 leads to the Novel framework-incorporation V–W–Ti nanoparticle catalysts
formation of crystalline V , which is harmful to the activity of with different vanadium contents for the selective catalytic
NH -SCR reaction. These results conrm the inuence of reduction of NO have been developed by direct sol–gel method.
vanadium oxide structure on SCR activity.
2 5
O
3
x
V
0.02
W
0.04Ti catalyst exhibited the high NO conversion (>90%)
ꢀ
The inactive nitrate species are oen thought to be ther- in the wide temperature range (200–400 C) and the lowest E
a
mally stable and might be accumulated with the increasing of value.
4
8
ꢁ
reaction time. These NO3 species covered the surface active
sites to prohibit the adsorption of NH , thus showing harmful TiO
effects for NH -SCR reaction. However, NO is considered to of V/Ti is #0.02. V0.02
Vanadium was found to incorporate into the framework of
2
and existed as an amorphous phase when the molar ratio
3
3
2
W0.04Ti catalyst with the best vanadium
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dispersion exhibited the best catalytic performance for the NH
SCR reaction. With the increasing of vanadium doping amount,
3
-
17 P. G. W. A. Kompio, A. Br u¨ ckner, F. Hipler, G. Auer, E. L ¨o ffler
and W. Gr u¨ nert, J. Catal., 2012, 286, 237.
vanadium species were transformed from partly distorted 18 H. Y. Huang, R. Q. Long and R. T. Yang, Appl. Catal., A, 2002,
4
+
4+
vanadium species to crystalline V O . The redox cycle (V + Ti
235, 241.
2
5
5
+
3+
5+ 4+
/
V
+ Ti ) and the well balance V /V play key roles for the 19 F. Gao, E. D. Walter, E. M. Karp, J. Luo, R. G. Tonkyn,
highly catalytic deNO
x
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J. H. Kwak, J. Szanyi and C. H. F. Peden, J. Catal., 2013,
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The reaction mechanism for NH
3
-SCR over V0.02W0.04Ti
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3
the one hand, the catalyst prevented the formation of stable
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (21376261 and 21173270), 863
Program (2015AA034603 and 2013AA065302), the Beijing
Natural Science Foundation (2142027), and the China Univer-
sity of Doctoral Selective Fund (20130007110007).
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