The Role of Lewis and Br?nsted Acid Sites in NO Reduction with NH3 on Sulfur Modified TiO2-Supported V2O5 Catalyst

Article abstract of DOI:10.1134/S003602441713026X

V₂O₅/S-doped TiO₂ was prepared by the sol-gel and impregnation methods. The adsorption of NO, NH₃, and O₂ over the catalyst was studied by in situ DRIFTS spectroscopy to elucidate the reaction mechani

Full text of DOI:10.1134/S003602441713026X

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 13, pp. 2489–2494. © Pleiades Publishing, Ltd., 2017.
CHEMICAL KINETICS
AND CATALYSIS
The Role of Lewis and Brønsted Acid Sites in NO Reduction
with NH3 on Sulfur Modified TiO2-Supported V2O5 Catalyst1
Wei Zhaoa,*, Shengping Doua, Qin Zhongb,**, Licheng Wua, Qian Wanga, and Aijian Wanga
aSchool of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013 P.R. China
b School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094 P.R. China
*e-mail: wzhao@ujs.edu.cn
**e-mail: zq304@mail.njust.edu.cn
Received July 2, 2016
Abstract—V2O5/S-doped TiO2 was prepared by the sol-gel and impregnation methods. The adsorption of
NO, NH3, and O2 over the catalyst was studied by in situ DRIFTS spectroscopy to elucidate the reaction
mechanism of the low-temperature selective catalytic reduction of NO with NH3. Exposing the catalyst to O2
and NO, three types of nitrates species appeared on the surface. The introduction of S to TiO2 could generate
large amounts of acid sites for ammonia adsorption on the catalyst, which was believed to be an important
role in the SCR reaction and hereby improved the catalytic activity. The results indicated two possible SCR
reaction pathways for catalyst. One was that NO was absorbed to form nitrite species, which could react with
NH3 on Lewis acid sites, producing N2 and H2O. Another way was that NH3 was adsorbed, then reacted with
gas phase NO (E–R) and nitrite intermediates on the surface (L–H).
Keywords: S-doping, NH3–SCR, reaction mechanism
DOI: 10.1134/S003602441713026X
INTRODUCTION
In our previous work, S-doping TiO2 support was
applied in V2O5-based catalyst to enhance the catalytic
activity for NO reduction with ammonia and the
V2O5/S-doped TiO2 catalyst was found to perform
superior catalytic activity at low temperature com-
pared with undoped V2O5/TiO2 catalyst [6]. There-
fore, we were inspired to investigate the relationships
between surface properties and NH3–SCR behaviors
of the catalyst. In this paper, we aim to study the reac-
tion intermediates produced from the adsorption of
NH3 and NO on the catalyst during the SCR reaction,
and provide interesting information about the reaction
mechanism for the SCR of NO with NH3 at low tem-
perature by in situ DRIFTS.
Nitrogen oxides (NOx) from stationary power plant
and vehicular remain a major source for air pollution,
which are responsible for acid rain and town smog [1].
Selective catalytic reduction (SCR) of NOx with NH3
is an effective method for removing NOx. In situ
DRIFTS technique is a suitable method to character-
ize reacted catalysts, for investigating relationships
between the surface structure and the reaction gas
under reaction conditions.
To understand the reaction mechanisms and the
role of the catalysts, extensive studies of the in-situ
DRIFT over the catalysts using reaction gas as probe
molecules have been reported in the past. Extensive
studies of the in situ DRFTIR have been reported in
the past. Xu et al. [2] studied the reaction mechanism
of NO oxidation on Mn–Co/Ti catalyst by DRFTIR.
Zhou et al. [3] proposed two possible reaction path-
ways of NO reduction by NH3 over Fe–Ce–
Mn/ZSM-5 catalysts by in situ DRIFTS. Rodrigues
et al. [4] reported the aromatization reaction scheme
for Ga/HZSM5 catalyst using in situ DRIFTS
method. Jin et al. [5] proposed a reaction mechanism
on Mn–Ce/TiO2 and Mn–Ce/Al2O3 catalysts, and
found that the Brønsted acid sites played an important
role in SCR according to in-situ DRIFTS analysis.
EXPERIMENTAL
Sample Preparation
S-Doped TiO2 support was prepared by the sol–gel
method. 0.064 mol of tetrabutyl tianate (C16H36O4Ti)
was added to 0.128 mol of acetylacetone (C5H8O2)
under continuous stirring. Then the solution was
diluted with 50 mL ethanolic solution of thiocarbam-
ide (CH4N2S). Undoped support was obtained with-
out addition of CH4N2S, which was denoted as TiS0.
After being stirred for 2 h at room temperature, the
solution was heated at 333 K for 4 h in a water bath and
1
The article is published in the original.
2489

2490
WEI ZHAO et al.
1385
1615
1385
1615
1361
(c)
(e)
1361
(b)
(a)
(d)
(c)
(b)
(a)
800
1600
2400
3200
4000
1200
1800
2400
3000
3600
Wavenumber, cm−1
Wavenumber, cm−1
Fig. 2. DRIFTS spectra of the V O /S-doped TiO cata-
2
5
2
lysts under 2000 ppm NO for 1 h at 303 K, and increasing
temperature to 393 (a), 423 (b) 453 (c), 483 (d), and
513 K (e).
Fig. 1. DRIFTS spectra of TiO (a), V O /TiO (b), and
2
2
5
2
V O /S-doped TiO (c) at room temperature.
2
5
2
dried at 393 K for 6 h. Then the dried material was cal- was investigated. Figure 1 showed the DRIFTS data
cined at 773 K for 3 h. The loading of V2O5 on the collected from (a) TiO2, (b) V2O5/TiO2, and (c)
S-doped TiO2 support was performed by impregnation S-doped V2O5/TiO2 at room temperature. The peak at
2800–3717 cm–1 resulted from the consumption of
isolated surface OH groups [7]. The peak located at
2343 cm–1 was assigned to the interference of CO2
from air. Exposed to NO in the absence of O2, it was
scarcely any nitrates species to be found on the surface
of the TiO2, V2O5/TiO2, and S-doped V2O5/TiO2 sam-
ples. This meant that NO was difficult to be adsorbed
on the surface of these catalysts to form nitrates in the
absence of oxygen at room temperature. Increasing
the temperature, there were nitrates species observed
on the surface of the samples, as shown in Fig. 2
(S-doped V2O5/TiO2). The peaks at near 1361 and
method. The catalysts prepared by this process con-
tained 1 wt % V2O5. The required amount of NH4VO3
was dissolved in distilled water at 333 K, and impreg-
nated by contacting the support. The mixture was
refluxed for 4 h at 333 K in a water bath. Then the
obtained mixture was dried at 393 K for 6 h and cal-
cined at 623 K for 4 h.
Diffused Reflectance Infrared Fourier Transform
Spectroscopy (DRIFTS) Experiments
The in-situ DRIFTS experiments were carried out
on a Nicolet IZ10 FTIR spectrometer, equipped with
a liquid-nitrogen-cooled MCT detector. For each
spectrum 32 scans recorded at a resolution of 4 cm–1
were averaged. Prior to each experiment, the samples
were preheated in a flow of 20 mL/min N2 at 300°C
for 60 min and then cooled to the desired temperature.
At each temperature, the background spectrum was
recorded under N2 atmosphere and was subtracted
from the sample spectrum that was obtained at the
same temperature. After cooling to the room tempera-
ture, NH3 (1% in N2), NO (1% in N2), or pure O2
(20 mL/min) was introduced into the cell for 2 h at
303 K to ensure complete saturation of the sample.
FT-IR spectra were collected at temperatures ranging
from 303 to 513 K.
1385 cm–1 could be assigned to M–NO2 nitro com-
pounds [8]. And the peak at 1615 cm–1 was due to NO2
[9]. With increasing temperature, the intensity of this
peak decreased. The peaks at 1839, 1906, 2043, 2204,
and 2272 cm–1 might attribute to nitrate species. This
indicated that the activity of surface oxygen became
more active with increasing temperature.
Nitric Oxide Adsorption with O2
The DRIFTS spectra of the TiO2 (a), V2O5/TiO2
(b), and V2O5/S-doped TiO2 (c) collected after the
adsorption of NO in the presence of O2 were showed in
Fig. 3. The coadsorption of NO and O2 led to the for-
mation of a number of peaks. The peaks appeared
around 1345 cm–1, corresponding to M–NO2 nitro
compounds. However, only monodentate nitrate spe-
cies (1283 and 1358 cm–1) adsorbed on the surface of
the TiO2 (a) support [10–12]. The peak observed at
RESULTS AND DISCUSSION
Nitric Oxide Adsorption in the Absence of O2
The surface of the samples after the adsorption of
nitric oxide (NO, which is the one reactant in SCR)
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THE ROLE OF LEWIS AND BRØNSTED ACID SITES IN NO REDUCTION
2491
1619 cm–1 was assigned to adsorbed NO2 species.
1385
1619
Compared the results of NO adsorption with O2 and
without O2, it indicated that the adsorption of NO was
influenced by O2. This was reasonable to exist a low
NO removal efficiency during SCR reaction without O2.
1345
1283
(c)
The peak near 2267 cm–1 was assigned to NO+ spe-
cies [13]. The observed peaks of nitrates suggested that
the oxidation of NO to NO2 by gaseous O2 and the for-
(b)
(a)
NO3−
mation of
on the catalysts surface. Compared
with the support, V2O5-loading samples showed much
−
−
stronger
peak, suggesting more adsorbed
NO3
NO3
NO3−
present on the catalyst. The peak intensity of
increased in the following order: V2O5/TiO2
<
800
1600
2400
3200
4000
Wavenumber, cm−1
V2O5/S-doped TiO2. This was ascribed to sulfur dop-
ing which could create a charge imbalance, the vacan-
cies and unsaturated chemical bonds on the catalyst
surface, and then more NO could be oxidized to form
Fig. 3. DRIFTS spectra of adsorbed NO (1000 ppm) in the
presence of O on the (a) TiO , (b) V O /TiO , and (c) S-
2
2
2
5
2
-
doped V O /TiO catalysts at room temperature.
NO2, resulting in more absorbed NO3 produced on
2
5
2
the surface of the doped catalysts (XPS and EPR
results [6]). Compared with the results shown in
Fig. 1, when NO was adsorbed with O2, it could be
seen that there was more NO2 and nitrate species
absorbed on the samples. This intermediate was gen-
erally considered to play an important role in the SCR
reaction.
It was found that the role of the increasing tem-
perature toward the V2O5/S-doped TiO2 catalyst after
NO + O2 adsorption. The intensity of the peaks due to
peak formed at 1646 cm–1 might be the bending modes
of H2O. The peak at around 1371 and 1473 cm–1 was
+
due to
chemisorbed on the Brønsted acid sites.
NH4
The results indicated that there were more Brønsted
acid sites than Lewis acid sites. The acidity was gotten
on the catalyst surface could be conducive to adsorb
NH3. After loaded V2O5 onto the TiO2 (a), the samples
(b, c) showed a peak around at 2043 cm–1 was typical
for V5+ = O sites [16]. This peak indicated that V5+
=
monodentate/bridged nitrates (1351 and 1623 cm–1)
decreased with the increase of temperature, which
suggested that these nitrate species were not stable
under the condition investigated [14]. This instability
suggested that these intermediates enable easy strip-
ping from the surface of catalyst, without covering the
active component.
O groups presented on the catalyst surface were con-
sumed upon reduction with NH3. These peaks were
present around at 2785, 2973, 3205, and 3535 cm–1
due to the interaction between V and NH3. The peak
at 1415 cm–1 was not detected for TiO2 (a), which indi-
cated that the Brønsted acidity was mainly introduced
as a result of V2O5 loading. It is important to note that
the intensity of this peak became stronger via the addi-
tion of S ion. This indicated that S-doping could
increase Brønsted acidity, which was suggested to play
an important role in the low-temperature SCR. Thus,
catalysis activity was improved by doping method.
Ammonia Adsorption
The samples of the TiO2 (a), V2O5/TiO2 (b), and
V2O5/S-doped TiO2 (c) after being treated with NH3
were presented in Fig. 4. After the samples treated with
NH3 at room temperature, the peaks at 1719 and
1415 cm–1 were assigned to symmetric and asymmet-
ric deformation modes of NH3 coordinated on Brøn-
Adsorption of NH3 Followed by NO
Figure 5 shows the DRIFT spectra of the TiO2 (a),
V2O5/TiO2 (b), and V2O5/S-doped TiO2 (c) obtained
from the adsorption of NH3, purged by N2, and then
flowed by NO + O2. The main product on the sample
sted acid sites (
+), respectively. The peaks at 1265
NH4
and 1571 cm–1 was due to the asymmetric bending
vibrations of the N–H bonds in NH3 coordinately
linked to Lewis acid sites. The peaks observed at 2786,
3010, 3200, and 3530 cm–1 were corresponding to the
NH stretching modes [15]. The peak around 3695 cm–1
was attributed to the surface O–H stretching. There
was a distinct phenomenon on the catalyst support
TiO2 (Fig. 4a). A broad peak in the range of 2465–
when introducing NH3 was NH3 and
+. After
NH4
NO + O2 was passed over the NH3-treated samples,
the peak at 1563 cm–1 resulting from the NH3 adsorp-
tion coordinated to Lewis acid sites disappeared upon
introducing NO for all samples. The disappearance of
3725 cm–1 was attributed to any adsorbed water. The this peak strongly suggested that Lewis acid sites were
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2492
WEI ZHAO et al.
(c)
(c)
1415
1265
2786
3200
1571
1719
3010
1415
3530
1563
(b)
(a)
(b)
(a)
1351
1371
800
1600
2400
3200
4000
800
1600
2400
3200
4000
Wavenumber, cm−1
Wavenumber, cm−1
Fig. 4. DRIFT spectra of the adsorbed species arising from
contact of NH (1000 ppm) with the (a) TiO , (b)
Fig. 5. DRIFT spectra of the adsorbed species arising from
contact of NH (1000 ppm) followed by NO with the TiO
3
2
3
2
V O /TiO , and (c) S-doped V O /TiO catalysts at room
(a), V O /TiO (b), and V O /S-doped TiO (c) catalysts
2 5 2 2 5 2
2
5
2
2
5
2
temperature.
at room temperature.
peak at 1614 cm–1 was corresponded to adsorbed NO2
species, as previously discussed in Fig. 3. After intro-
ducing NH3, the peak at 1344 cm–1 shifted to 1371 cm–1,
the activated sites of ammonia, which could be capa-
ble of participating in low-temperature SCR. The
monodentate nitrate species (1351 cm–1) observed
after NO + O2 adsorption was similar to the results of
the peak at 1425 cm–1 shifted to 1418 cm–1, while the
peak at 1614 cm–1 was red-shifted to 1628 cm–1. The
intensity of the peaks observed near 1371, 1418, and
1628 cm–1 increased with increasing the adsorption
time of NH3. Upon the adsorption of NH3, a new peak
Fig. 3. The peak intensity at 1415 cm–1 for V2O5/S-
doped TiO2 (c) was initially small, but increased upon
the adsorption time of NO [17], which was assigned to
the oxidation products of NO. However, there was no
significant change observed on V2O5/TiO2 (b).
was observed at 1418 cm–1 corresponding to the
adsorbed NH3 at Brønsted acid sites, meanwhile
As the temperature was increased from room tem-
+
perature to 513 K, the peak at 1425 cm–1
(
)
monodentate nitrate species present at 1425 cm–1 dis-
appeared. The peak at 1628 cm–1 rapidly increased
upon the adsorption time of NH3, but after heating to
333 K it disappeared. This peak could be attributed to
a combination of δas(NH3) and the scissoring vibra-
tions of NH2 species [18]. Additionally, new peaks
NH4
decreased slightly and diminished after NH3–NO +
O2 adsorption. This proved that the rate of the reaction
+
of NO with
ions was accelerated with increasing
NH4
temperature, which was consistent with the change of
catalytic activity during observed temperature win-
dow. The peak located at near 1684 cm–1 appeared
and had strong signal intensity with an increase in
temperature, which was attributed to the adsorption of
water, which might be generated by the SCR reaction
between NO and NH3. The intensity of surface OH
peaks increased with increasing temperature. All of the
peaks in the high wavenumber region disappeared
upon reaching 513 K. A broad peak in the range from
2830 to 3445 cm–1 was observed due to the formation
of water at higher temperatures [18].
appeared at 2817, 3026, and 3195 cm–1 upon intro-
ducting NH3. The two peaks at 2817 and 3026 cm–1
NH4+
was attributed to the symmetric stretching of
bound to Brønsted acid sites (υ (
+)), while the
NH4
s
peak at 3195 cm–1 was attributed to υs(NH3) coordi-
nated to Lewis acid sites [19].
The peaks intensity at 1371 and 1418 cm–1
decreased with increasing temperature. The peak at
1418 cm–1 almost disappeared at 513 K. The peaks of
the nitrate species decreased in area and shifted to
slightly higher wavenumbers as increasing the tem-
perature, suggesting that they might be readily avail-
Adsorption of NO + O2 Followed by NH3
Figure 6 showed the DRIFT data collected from
the samples after NO + O2–NH3 adsorption at room able to participate in the SCR reaction. As a result of
temperature increase, the peak at 1654–1796 cm–1
temperature. The initial peaks observed around 1344
and 1425 cm–1 were assigned to monodentate and the appeared, which provided evidence that the SCR
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THE ROLE OF LEWIS AND BRØNSTED ACID SITES IN NO REDUCTION
reaction occurred during the course of these experi-
2493
ments.
(d)
2817
3026
1418
1628
Reaction Mechanism
(c)
1371
When only NO was adsorbed on the samples, only
little nitrate species were observed in the absence of
oxygen at room temperature in this study. But in the
presence of oxygen, three species formed on the sur-
face of the samples except TiO2. Monodentate nitrates
formed on the surface of all the samples, and the rela-
tive strengths decrease in the following order: TiO2 <
V2O5/TiO2 < V2O5/S-doped TiO2. Additionally,
bidentate nitrates were only observed over V2O5-load-
ing samples.
(b)
(a)
3195
800
1600
2400
3200
4000
Wavenumber, cm−1
After the introduction of NH3, four species were
observed on the samples. First, the species was the
Fig. 6. DRIFT spectra of the adsorbed species arising from
contact of NO (2000 ppm) in the presence of O followed
NH4+
symmetric and asymmetric deformation of
2
coordinated to Brønsted acid sites, which were
observed over all of the samples. Second, the asym-
metric deformation of NH3 (δas(NH3)) coordinated to
Lewis acid sites were also observed over all of the sam-
ples studied. The relative strength of both Brønsted
and Lewis acid sites increased after V2O5-loading in
the following order: V2O5/S-doped TiO2 > V2O5/TiO2.
This result was indicated that the acid sites of the cat-
alysts increased by the addition of the impurities.
When NO and O2 was introduced to the samples
with preadsorbed NH3, nitrates species could form on
the surface of the samples. Increasing the tempera-
ture, some NH3 species disappeared. This phenome-
non could occur due to the fact that the bond between
V and NH3 species was weaker than the bond between
V and NO species. The generation of water provided
evidence that the SCR reaction occurred during the
adsorption of NH3 followed by NO. It was noted that
by NH with the (a) TiO , (b) NH with TiO ,
3
2
3
2
(c) V O /TiO , and (d) V O /S-doped TiO catalysts at
2
5
2
2
5
2
room temperature.
tion did proceed. Therefore, the reaction might occur
between the coordinated NH3 and NO in gas phase,
+
and started with the adsorbed NH and
to react
NH4
3
with nitrite intermediates on the surface of the cata-
lysts (the L–H hypothesis).
All of above information has been used to propose
a reaction mechanism for the SCR of NO with NH3 at
low temperature for V2O5 supported on S doped TiO2,
which have been shown to have high activity compared
with supported on undoped support.
CONCLUSIONS
the peak (1607 cm–1 for V2O5/S-doped TiO2) assigned
to Lewis acid sites disappeared upon the introduction
of NO and O2, indicating that it was this type of NH3
that participated in the SCR reaction. But this species
on the surface of V2O5/TiO2 was present after the
introduction of NO + O2 at the room temperature.
Meanwhile, the peaks of V2O5/S-doped TiO2
Nitric oxide and ammonia were adsorbed or coad-
sorbed onto the surfaces of TiO2, V2O5/TiO2, and
V2O5/S-doped TiO2. Ammonia is adsorbed on the
+
Brønsted or Lewis acid sites to form
or coordi-
NH4
nated NH3 in the SCR reaction, and then, gaseous or
+
adsorbed nitric oxides react with
or coordinated
NH4
NH4+
(1481 cm–1), which were attributed to
species
NH3 to form N2 and H2O. S-Doping leads to
improved Brønsted and Lewis acidity of catalyst. This
improved Brønsted acid sites introduced facilitated
the adsorption of ammonia instead of ammonia oxi-
dation. Lewis acid sites facilitated the low-tempera-
ture (<573 K). Therefore, the NH3–SCR activity of
the S-doped catalyst was improved.
bound to Brønsted acid sites, decreased quickly and
disappeared finally.
The reaction intermediates changes due to revers-
ing the order of reaction gases introduced to the sam-
ples were studied. The peaks due to NO complexes
could also be observed with the existence of coordi-
nated NH3 at room temperature. The coexistence
indicated that there should not be rapid reaction
between them at room temperature. Increasing the
temperature, the nitrates or amide species could be
consumed and the peaks assigned to H2O were
ACKNOWLEDGMENTS
This research was supported by National Key
Technology Support Program (2015BAD21B06), the
detected, which strongly suggested that the SCR reac- National Natural Science Foundation of China
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2494
WEI ZHAO et al.
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