Sulfation effect of Ce/TiO2 catalyst for the selective catalytic reduction of NO: X with NH3: Mechanism and kinetic studies

Article abstract of DOI:10.1039/c9ra06985b

Ceria-based catalysts are competitive substitutes for the commercial SCR catalysts due to their high SCR activity and excellent redox performance. For a better understanding of the SO₂ poisoning mechanism over ceria-based catalysts, the sulfation effect of the Ce/TiO₂ catalyst on the SCR activity over a wide reaction temperature range was systematically studied via comprehensive characterizations, in situ DRIFT studies and kinetic studies. The results demonstrated that the NO conversion at 150 °C is significantly inhibited by the formation of cerium sulfites/sulfates due to the inhibited redox properties and excessive adsorption of NH₃, which restrict the dissociation of NH₃ to NH₂, resulting in a much lower reaction rate of E-R reaction over the sulfated Ce/TiO₂ catalyst. With the increase in the reaction temperature, the reaction rate of the E-R reaction significantly increased due to the improved redox properties and weakened adsorption of NH₃. Moreover, the rate of the C-O reaction over the sulfated Ce/TiO₂ catalysts is obviously lower than that of the fresh Ce/TiO₂ catalyst. The promotion of NO conversion over the sulfated catalyst at 330 °C is attributed to both the increase in the reaction rate of E-R reaction and the inhibition of the C-O reaction.

Full text of DOI:10.1039/c9ra06985b

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Sulfation effect of Ce/TiO catalyst for the selective
2
catalytic reduction of NO with NH : mechanism
x
3
Cite this: RSC Adv., 2019, 9, 32110
and kinetic studies†
a
a
b
c
d
e
Wenjie Zhang,
Guofu Liu, Jie Jiang, Yuchen Tan, Qi Wang, Chenghong Gong,
a
f
Dekui Shen * and Chunfei Wu*
Ceria-based catalysts are competitive substitutes for the commercial SCR catalysts due to their high SCR
activity and excellent redox performance. For a better understanding of the SO poisoning mechanism
over ceria-based catalysts, the sulfation effect of the Ce/TiO catalyst on the SCR activity over a wide
2
2
reaction temperature range was systematically studied via comprehensive characterizations, in situ DRIFT
ꢀ
studies and kinetic studies. The results demonstrated that the NO conversion at 150 C is significantly
inhibited by the formation of cerium sulfites/sulfates due to the inhibited redox properties and excessive
adsorption of NH
of E–R reaction over the sulfated Ce/TiO
reaction rate of the E–R reaction significantly increased due to the improved redox properties and
weakened adsorption of NH . Moreover, the rate of the C–O reaction over the sulfated Ce/TiO
3
, which restrict the dissociation of NH
3 2
to NH , resulting in a much lower reaction rate
2
catalyst. With the increase in the reaction temperature, the
Received 2nd September 2019
Accepted 20th September 2019
3
2
2
catalysts is obviously lower than that of the fresh Ce/TiO catalyst. The promotion of NO conversion
DOI: 10.1039/c9ra06985b
ꢀ
over the sulfated catalyst at 330 C is attributed to both the increase in the reaction rate of E–R reaction
and the inhibition of the C–O reaction.
rsc.li/rsc-advances
benign catalysts with a superior low-temperature SCR activity,
1
. Introduction
3
4
5
such as Fe O -based, CeO -based, CuO-based and MnOx-
2
3
2
6
Selective catalytic reduction (SCR) by ammonia has been proven based catalysts.
to be the most efficient technology for removing NOx that
Ceria-based catalysts have been considered as potential
originates from stationary and mobile pollution sources. SCR candidates for the replacement of V –WO /TiO due to their
catalysts are the core of the NH -SCR system, and the high oxygen storage capacity and excellent redox properties. A
1
2
O
5
3
2
3
commercial SCR catalyst based on the VO
5
–WO
3
/TiO
2
catalyst series of ceria-based catalysts, including CeO
–CeO /TiO and CeO –SnO , have shown superior
high operating temperature (>350 C), the toxicity of vanadium middle-temperature SCR activity and N
and the low N
selectivity at high temperatures restrict the use the presence of a low concentration of SO
2 2 2 3
/TiO , CeO –WO ,
has been widely used for the NH
3
ꢀ
-SCR system. However, the Sb–V
O
2
5
2
2
2
x
7–9
selectivity. However,
in the ue gas could
2
2
2
of vanadium-based catalysts on industry boiler systems, which cause an inevitable deactivation effect on NO conversion over
2
is urgently needed for the removal of NO in China. Therefore, ceria-based catalysts. The sulfation of ceria-based catalysts can
x
signicant efforts have been made to develop environmentally slightly promote SCR performance by the enhancement of the
acidity and the reactivity of cerium sulfates on the premise of
10
sufficient redox properties. In most cases, it has been found
that ceria-based catalysts are sensitive to SO2 poisoning and
a good amount of cerium sultes/sulfates are deposited on the
a
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education,
School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu, PR
China. E-mail: 101011398@seu.edu.cn
8,11
surface of catalysts, blocking the catalyst pore channels and
b
Engineering Laboratory of Energy System Process Conversion & Emission Control
cutting off the redox cycle involved in the standard SCR reac-
Technology of Jiangsu Province, School of Energy and Mechanical Engineering,
Nanjing Normal University, Nanjing 210042, PR China
9
tion. A previous investigation by Muhammad et al. simply
c
Nanjing Foreign Language School, Nanjing 210018, Jiangsu, PR China
2 2
studied the sulfation effect of the Ce/TiO catalyst and Ti/CeO
d
College of Metrological Technology and Engineering, China Jiliang University, catalyst on SCR activity, suggesting that the low-temperature
Hangzhou 310018, PR China
SCR activity is much more sensitive to the formation of
cerium sultes/sulfates, while middle and high-temperature
SCR activities are less affected. It was demonstrated that the
sulfation effects of the ceria-based catalyst on NO conversion at
e
Jiangsu Yanxin SCI-TECH Co., Ltd, WuXi 214426, PR China
f
School of Chemistry and Chemical Engineering, Queen's University Belfast, Belfast
BT7 1NN, UK. E-mail: c.wu@qub.ac.uk
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra06985b
different reaction temperatures are different, while the insight
1
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into the sulfation effect on the SCR activity was not revealed. UK) with Al KR radiation (1486.6 eV). The binding energy was cor-
Moreover, the SCR reaction mechanism of the sulfated Ce/TiO2 rected using contaminated carbon (BE ¼ 284.6 eV).
catalyst may differ from that of a fresh catalyst.
2.2.4 H -TPR
analysis.
Hydrogen-temperature
pro-
2
In order to investigate the sulfation effect of the Ce/TiO2 grammed reduction (H -TPR) was performed using a FineSorb
2
catalyst on the SCR activity and the reaction mechanism over 3010D chemisorption analyzer (FINETEC instruments, China).
ꢀ
a wide reaction temperature range, the SCR activities of the First, the catalyst (60 mg) was pre-treated at 300 C in a ow of
ꢁ
1
ꢀ
fresh Ce/TiO
2
catalyst and sulfated catalyst were studied. The Ar (20 mL min ) for 1 h and cooled to 50 C. Then, the
ꢀ
sulfation effect of the Ce/TiO
physisorption, XPS, H -TPR, NH
Furthermore, kinetic studies involving the E–R mechanism, consumption was monitored by TCD.
L–H mechanism, nonselective catalytic reduction reaction (i.e., 2.2.5 XRF analysis. The S content of the sulfated Ce/TiO2
2
catalyst was fully investigated via temperature was raised linearly to 800 C at the rate of
ꢀ
ꢁ1
ꢁ1
N
2
2
3
-TPD and in situ DRIFT. 10 C min in a ow of 10 vol% H /Ar (20 mL min ). The H
2
2
NSCR reaction) and catalytic oxidation reaction (i.e., the C–O catalyst was determined by the XRF method using an Energy-
reaction) were performed for a clearer understanding of the Dispersive XRF spectrometer (Thermo Scientic, ARL
sulfation effect of the Ce/TiO catalyst on the SCR reaction. It is QUANT'X with an Rh anode, X-rays of 4–50 kV (1 kV step), 1 mm
2
expected that the present study will help in understanding the size beam). A 3.5 mm Si(Li) dried crystal with Peltier cooling
ꢀ
deactivation mechanism of sulfation at different reaction (ꢁ88 C was used as a detector).
temperatures and further provide useful guidance for the
3
2.2.6 NH -TPD analysis. Temperature programmed desorp-
design of catalysts with higher SO resistance.
tion of ammonia (NH -TPD) was performed using a FineSorb
3
2
3
010D chemisorption analyzer (FINETEC instruments, China).
ꢁ1
First, the sample (80 mg) was pre-treated in He (20 mL min ) at
2. Experimental
ꢀ ꢀ
00 C for 1 h. Aer cooling to 80 C, the sample was exposed to
3
ꢁ1
2.1 Catalyst preparation
3 2
a ow of 5 vol% NH /N (20 mL min ) for 30 min, followed by He
purging for another 1 h. Finally, the temperature was raised to
The Ce/TiO
2
catalyst (Ce/Ti mole ratio ¼ 0.25) was prepared by
ꢀ
ꢀ
ꢁ1
650 C in He ow at the rate of 10 C min . The amount of
a single step sol–gel method using butyl titanate (Sichuan
Kelong, China), anhydrous ethanol acetic acid, deionized water,
nitric acid and a certain amount of cerium nitrate. The chem-
icals were mixed under continuous and vigorous stirring at
room temperature. Aer 3 h, a transparent yellowish sol was
formed and was then aged for 12 h, which then became a gel.
ammonia desorbed from the catalysts was monitored using
a thermal conductivity detector (TCD).
2.2.7 In situ DRIFTS analysis. The in situ DRIFT experi-
ments were performed on a NicoletNexus 5700 FTIR spec-
trometer using a diffuse reectance attachment (HARRICK)
equipped with a reaction cell (ZnSe windows). Each spectrum
ꢀ
The derived sample was dried at 90 C for 12 h to form a xerogel,
ꢁ1
ꢀ
was collected by averaging 8 scans with a 4 cm resolution to
ensure high temporal resolution while maintaining a reason-
followed by calcination at 450 C at a heating rate of
ꢀ
ꢁ1
1
0 C min in air for 6 h. Finally, the catalyst was crushed and
able signal to noise ratio. Each sample was pre-treated under
sieved to 40–80 mesh size. The sulfated Ce/TiO catalyst was
2
ꢀ
ꢁ
1
a N atmosphere at 400 C for 1 h and then cooled down to
2
pretreated in a ow of 1000 ppm SO and 3% O (750 mL min
)
2
2
ꢀ
ꢀ
ꢀ
ꢀ
240 C or 150 C. Subsequently, (1) for the adsorption/
desorption of NH , 600 ppm of NH was injected into the IR
cell for 40 min at 150 C until saturation, followed by a N
at 180 C and 240 C for 8 h, respectively, followed by ushing
with N for 1 h to remove the physically adsorbed SO . These
3
3
2
2
ꢀ
ꢀ
ꢀ
2
purge
catalysts were assigned as 180 C-sulfated and 240 C-sulfated
catalysts, respectively.
ꢀ
for 10 min and then heating to 200, 250, 300, 350 and 400 C.
The spectra were recorded based on the Kubelka–Munk func-
tion, with reference to the background spectra recorded in N
the corresponding temperatures. (2) For the reaction of NH
2
at
+
2
.2 Catalyst characterizations
.2.1 XRD analysis. Powder X-ray diffraction measure-
ments of the catalysts were performed using a computerized 600 ppm NO was injected into the IR cell for 40 min until
PANalytical X'Pert Pro diffractometer with Cu Ka (l ¼ 0.15406 saturation, followed by N purging for 10 min and then NH
nm) radiation. were added; the spectra were also recorded. This procedure
3
ꢀ
2
O
2
with pre-adsorbed NO, aer the pretreatment at 400 C,
+
3
2
O
2
2
.2.2 BET analysis. N adsorption–desorption isotherms is similar to the reaction of NO + O
2
with pre-adsorbed NH
3
.
2
ꢀ
were obtained at ꢁ196 C over the whole range of relative
pressures using a JW-BK112 instrument (Beijing JWGB Sci. &
2.3 SCR activity estimation of catalyst samples
Tech. Co. Ltd, China). Prior to N
pre-treated at 150 C for 5 h. Specic areas were computed from The NH
2
adsorption, the catalysts were
ꢀ
3
-SCR reaction and studies of the effect of SO on the
2
adsorption/desorption isotherms using the BET equation. Pore SCR reaction were performed in a xed-bed quartz tube reactor.
volumes and average pore diameters were computed by The reactor consists of a quartz tube with an inner diameter of
applying the BJH method from the adsorption branches of the 16 mm and a thermocouple placed on the outside wall of the
isotherms.
.2.3 XPS analysis. The content and chemical states of surface catalyst (40–80 mesh) was used in all experiments. The reaction
compositions were determined by X-ray photoelectron spectroscopy conditions were as follows: 1 mL of catalyst, 600 ppm NO,
XPS) using an ESCALAB Mark II spectrometer (Vacuum Generators, 600 ppm NH , 3 vol% O , N as the balance, 1000 ppm SO (if
reactor to control the temperature of the furnace. About 1 mL of
2
(
3
2
2
2
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ꢁ1
ꢀ
required) and a gas hourly space velocity (GHSV) of 40 000 h
.
is higher than 75% in the temperature range of 180–330 C. The
NO , NH and N O in the effluent gas were analyzed using a ue higher NH conversion as compared to NO conversion at each
x
3
2
3
gas analyzer (Testo 350), Nessler's reagent spectrophotometry temperature suggests that a part of the NH was oxidized to NO
3
12
method and a N O analyzer (Medi-Gas G200) when the reac- and N O due to the catalytic oxidation (i.e., C–O reaction) and
2
2
tion system reached a steady state for 30 min. The NO
x
NSCR reaction, contributing to the decrease in the NO conver-
ꢀ
conversion (h), N
2
O selectivity (S) and the pseudo-rst-order rate sion with the increase in the reaction temperature to 330 C. As
constant (s) were calculated using the following formulae:
2
for the sulfated Ce/TiO catalyst (Fig. 1B), a much more severely
ꢀ
ꢁ
inhibited low-temperature SCR activity was observed. The
(1) reduced NO conversions for the sulfated catalyst with respect to
½
NO þ NO
2
ꢂ
out
h ¼ 1 ꢁ
ꢃ 100%
½
NO þ NO
2
ꢂin
fresh Ce/TiO were 75.6%, 66.2%, 45.5%, 36.8% and 26.8% at
2
ꢀ ꢀ ꢀ ꢀ ꢀ
1
50 C, 180 C, 210 C, 240 C, 270 C, respectively, indicating
2
½N
2
Oꢂ
out
S ¼
ꢃ 100% (2)
that the sulfation effect of the Ce/TiO catalyst on NO conver-
2
½
NH
3
ꢂin ꢁ ½NH
3
ꢂ
þ ½NO
x
ꢂin ꢁ ½NO ꢂout
x
out
sion is greatly dependent on the reaction temperature. The
lower the reaction temperature, the greater the negative effects
of sulfation. Moreover, it was noted that the sulfated catalyst
showed higher NO conversion, as compared to the fresh cata-
V
s ¼ ꢁ ꢃ lnð1 ꢁ hÞ
(3)
W
ꢁ
1
where V was the total ow rate (mL s ), and W was the mass of
the catalyst used in the experiment (g).
ꢀ
13
lyst, at 330 C, which was also reported in a previous study; the
fresh Ce/TiO2 showed a decrease in NO conversion with the
ꢀ
increase in reaction temperature from 300 to 330 C. The
2
.4 Experiments for kinetic studies
sulfated Ce/TiO catalyst showed a lower N O selectivity (lower
2
2
ꢀ
than 13%) in the temperature range of 150–330 C as compared
with the fresh Ce/TiO catalyst. This result demonstrates that
the sulfation of the Ce/TiO catalyst signicantly inhibits the
To obtain the kinetic parameters of the SCR reaction, the NSCR
reaction and catalytic oxidation reaction (i.e., the C–O reaction),
and steady-state kinetic studies were performed. The inlets
consisted of 0–600 ppm NO, 600 ppm NH
the balance. A very high GHSV of 600–2400 dm g
total ow rate was 750 mL min and the loading of the cata-
lysts were 33–135 mg) was used to exclude the diffusion limi-
tation (including the internal and external diffusion). The
steady-state kinetic experiments were performed in the
2
2
low-temperature SCR activity while showing a higher SCR
3
and 3% O
2
with N
2
as
ꢀ
3
ꢁ1 ꢁ1
activity at 330 C as compared to the fresh Ce/TiO catalyst by
2
h
(the
ꢁ
1
inhibiting the catalytic oxidation of NH3 to NO. The SCR
ꢀ
performance and N O selectivity of the 240 C-sulfated catalyst
2
ꢀ
are similar to those of the 180 C-sulfated catalyst shown in
Fig. S1.†
ꢀ
temperature range of 150–400 C. The concentrations of outlet
NH
3 2
and N O were obtained via the Nessler's reagent spectro- 3.2 Characterisations of the catalysts before and aer SO2
12
photometry method and N
2
O analyzer (Medi-Gas G200).
sulfation
3.2.1 XRD and BET analysis. XRD patterns of the fresh and
sulfated Ce/TiO catalysts are shown in Fig. S2.† All the catalysts
2
3
. Results and discussion
exhibited characteristic TiO
peaks corresponding to CeO
that the ceria is well dispersed on the catalysts. The intensity of
catalysts. For the fresh the TiO diffraction peaks of sulfated catalysts is stronger than
catalyst, indicating the higher crystallinity
2
-anatase [PDF-ICDD 21-1272]. The
3.1 Catalytic performance
2
could not be detected, indicating
Fig. 1 illustrates the NO conversion, NH
selectivity of fresh and sulfated Ce/TiO
Ce/TiO
3
2
conversion and N O
2
2
2
catalyst (Fig. 1A), it shows excellent SCR activity, which that of fresh Ce/TiO
2
ꢀ
Fig. 1 SCR performance and N
] ¼ 3%, N
2
O selectivity of (A) fresh Ce/TiO
2
and (B) 180 C-sulfated Ce/TiO
2 3
. Reaction conditions: [NH ] ¼ [NO] ¼ 600 ppm,
ꢁ1
[
O
2
2
balance, GHSV ¼ 40 000 h
.
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0
0
4+
0
0
3+
of TiO
2
of sulfated catalysts.
N
2
adsorption–desorption v and u/v) are from Ce and one doublet (u /v ) is from Ce .
0
0
isotherms and pore size distribution of the fresh and sulfated Aer sulfation treatment, the v at 885.9 eV and u at 904.1 eV
0
0
00
catalysts are shown in Fig. S3.† All the isotherm curves are type were revealed, whereas v at 888.9 eV and u at 907.6 eV van-
IV with H2 type hysteresis loops according to the International ished gradually, which was more pronounced with the increase
14
Union of Pure and Applied Chemistry (IUPAC). This indicates in the reaction temperature, suggesting that the electronic
the existence of ink-bottom-type pore structures. Moreover, the conguration of Ce aer SO sulfation is far from IV. As shown
2
3
+
desorption curve is a little steeper than the adsorption curve, in Table 2, aer the sulfation treatment, the Ce /Ce ratio
indicating that the neck size distribution shows a similar trend increased to 30.9% and 26.9%, respectively, indicating that
15
to the main pore cavities of the ink-bottom-type pore. As a part of CeO
2 2 4 3
was sulfated to Ce (SO ) , which was further
shown in Fig. S3(B),† the fresh Ce/TiO2 catalyst is mainly conrmed by the S2p spectra in Fig. 2C. Moreover, the sulfate
3
+
composed of mesopores with the mean pore size of 5.96 nm, ions with respect to the Ce ions are 3.95 and 3.24, respectively,
17
whereas the mean pore size of the sulfated catalysts shied to which is higher than the reported values from 1.3 to 1.6. Thus,
3
+
4+
5
.17 nm. Table 1 shows the structural parameters of the cata- it is reasonable to believe that the Ce sulfates/sultes and Ce
lysts as calculated based on N adsorption. The decreased sulfates/sultes were both formed aer the SO sulfation
surface area suggests that cerium sulfates/sultes deposit on treatment. It can also be seen from Table 2 that the atomic ratio
the surface of the Ce/TiO catalyst and mesopores are blocked to of S on the surface (6.01%) is much higher than that of bulk
form micro pores during the SO sulfation process. sulfates (1.4%), suggesting that SO preferred to react with
2
2
2
2
2
The pseudo-rst-order rate constants of the fresh and surface ceria to form surface cerium sulfates/sultes. Compared
ꢀ
sulfated Ce/TiO catalysts were calculated according to eqn (3) with the 180 C-sulfated catalyst, the lower concentration of
2
and shown in Table 1. The rate constants of the fresh Ce/TiO
surface S species combining with the higher content of bulk S
2
ꢀ
ꢀ
catalyst calculated by mass and the SBET result at 150 C are 6.59 species over the 240 C-sulfated catalyst indicates the formation
3
ꢁ1 ꢁ1
2
ꢁ1
cm g
s
and 0.050 mL m s , respectively, which are much of bulk-like and bulk cerium sulfates/sultes with the increase
ꢀ
18
higher than the rate constants of the sulfated Ce/TiO
With the increase in the reaction temperature to 270 C, the rate
2
catalyst. in sulfation temperature to 240 C.
2
The Ti2p spectra of the fresh and sulfated Ce/TiO catalyst
ꢀ
2
constants of the sulfated Ce/TiO catalyst calculated by mass are shown in Fig. 2B. The binding energies of the Ti2p photo-
and SBET obviously increased, which narrowed the gap between electron peaks over the fresh Ce/TiO catalyst are about 458.2 eV
2
the rate constants of the fresh and sulfated Ce/TiO catalysts. and 463.9 eV, representing Ti2p3/2 and Ti2p1/2, respectively. It is
2
4
+
These results demonstrate that the decrease in SCR activity at indicated that Ti exists in the form of the Ti oxidation state.
50 C is not attributed to the structure parameters, but due to Aer sulfation treatment, the binding energy of both Ti2p3/2
ꢀ
1
the change in the chemical properties aer sulfation treatment. and Ti2p1/2 shi to higher values. It is implied that the Ti
ꢀ
At 270 C, the inhibition effect of the sulfation treatment on NO
x
species have lower electron density as compared to Ti species in
catalyst, further indicating the stronger
conversion is weakened, suggesting that the low-temperature the fresh Ce/TiO
SCR activity is sensitive to the change in the chemical proper- inductive effect of the S]O bond. Compared with the 180 C-
ties aer sulfation treatment while middle and high- sulfated catalyst, there is a lower binding energy of Ti2p over the
2
19
ꢀ
ꢀ
temperature SCR activity is less affected. More characteriza- 240 C-sulfated catalyst, which is close to that over the fresh Ce/
tions were further performed to elucidate the chemical TiO2 catalyst, further conrming the lower concentration of
properties.
surface S species.
3
.2.2 XPS analysis. The concentration of surface atoms and
Fig. 2C and D show the S2p and O1s spectra of the fresh and
20
2
the oxidation states of surface elements of the fresh and sulfated Ce/TiO catalysts. According to the literature, the S2p
sulfated catalysts are summarized in Fig. 2 and Table 2. Fig. 2A spectra can be split into two doublets of S2p3/2 (168.6 eV) and
displays the Ce3d XPS spectra of the fresh catalyst and the S2p1/2 (169.8 eV), and the spin–orbit splitting of each doublet is
ꢀ
ꢀ
sulfated catalyst at 180 C and 240 C. The deconvolutions of the 1.18 eV. These two doublets can be attributed to hexavalent
16
Ce3d spectra were processed with reference to the literature.
The Ce3d spectra have complex peaks composed of four
doublets of Ce3d5/2/Ce3d3/2, in which three doublets (u /v , u / catalysts are numerically tted into two sub-bands.
sulfur.
The O1s ionization features of the fresh and sulfated Ce/TiO2
0
00 000
00
21–23
These
Table 1 Structural parameters of fresh and sulfated catalysts
a
b
c
d
S
BET
Reaction rate constant
Reaction rate constant
Reaction rate constant
Reaction rate constant
2
ꢁ1
3
ꢁ1 ꢁ1
2
ꢁ1
3
ꢁ1 ꢁ1
2
ꢁ1
Sample
(m g
)
(cm g
s
)
(mL m s
)
(cm g
s
)
(mL m s
)
Fresh Ce/TiO
2
132.9
106.5
108.4
6.59
1.32
1.74
0.050
0.012
0.016
43.8
23.71
26.50
0.329
0.223
0.244
ꢀ
1
80 C-sulfated
ꢀ
2
40 C-sulfated
a
ꢀ
b
ꢀ
c
Calculated by the NO
70 C. Calculated by the NO
x
conversion at 150 C. Calculated by the NO
x
conversion at 150 C and SBET result. Calculated by the NO
x
conversion at
ꢀ
d
ꢀ
2
x
conversion at 270 C and SBET result.
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Fig. 2 XPS results for (A) Ce3d, (B) Ti2p, (C) S2p, and (D) O1s of the fresh and sulfated catalysts.
4
+
2ꢁ
4+
ꢁ
2+
d
3+
3+
include an intense peak located at 529.5 eV and a faint peak at reaction: 2Ce + 2O 4 2Ce + O
2
+ 2e /V
d
4 Ce –V –Ce
5
31.1 eV over the fresh Ce/TiO catalyst, which represent lattice + O . The symbol V represents the defects and oxygen vacancies
2
2
d
2
ꢁ
oxygen (O , denoted as O ) and chemisorbed oxygen or weakly generated from the removal of lattice oxygen due to the
b
2
ꢁ
ꢁ
bonded oxygen species (O
tively. Aer sulfation treatment, the binding energies of O
shi towards higher values (0.3–0.7 eV), which is due to the factors determining the low-temperature activity of SCR. In
stronger affinity of S bonded to the oxygen species. As shown in order to nd out the effects of cerium sulfates/sultes on the
Table 2, the surface concentration of lattice oxygen follows an redox properties of Ce/TiO catalyst, H -TPR experiments were
upward trend from 26.7% to 43.6% and 42.9%, respectively, as conducted and the reduction proles are shown in Fig. 3. The
2
or O , denoted as O
a
2 4 3
), respec- formation of Ce (SO ) .
a
and 3.2.3 -TPR. The redox properties of the catalyst are key
H
2
24
O
b
2
2
3
+
along with the Ce /Ce ratio. Generally, O is generated during fresh Ce/TiO catalyst exhibited one overlapped reduction peak
the transition from Ce to Ce according to the following centered at 465 C (starting from 250 C and ending at 688 C),
a
2
4
+
3+
ꢀ
ꢀ
ꢀ
Table 2 The surface compositions of the samples derived from XPS data
Atomic concentration (at%)
Atomic ratio (%)
3
+
a
Sample (Ce/TiO
2
)
Ce
Ti
O
S
Ce /Ce
O
a
/(O
a
+ O
b
)
Bulk S
Fresh
6.07
4.92
5.07
24.95
19.92
20.20
68.97
69.14
70.31
—
6.01
4.42
25.6
30.9
26.9
26.7
43.6
42.9
0
1.4
1.7
ꢀ
1
2
80 C-sulfated
40 C-sulfated
ꢀ
a
Atomic ratio of S on the bulk of samples from XRF characterization.
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25,26
ꢀ
ꢀ
which is very similar to other reports.
The overlapped compared to the 240 C-sulfated catalyst, the 180 C-sulfated
reduction peak can be ascribed to the stepwise reduction of catalyst had more adsorbed NH . This can be attributed to the
3
ꢀ
surface oxygen species. At 250 C, the reduction of lattice oxygen presence of more cerium sulfates/sultes on the surface of the
4
+
4+
of stoichiometric ceria (Ce –O–Ce ) begins. With the increase catalyst.
3
+
in the reduction temperature, the reduction of ceria (Ce –O–
It is believed that the capacity for NH
3
adsorption is related
4
+
3+
3+
Ce ) to ceria (Ce –O–Ce ) begins with the elimination of to the surface area of the catalyst. The enhanced capacity of NH
3
27
ꢀ
lattice oxygen. For the 180 C-sulfated Ce/TiO
2
catalyst, the adsorption and the higher desorption temperature indicate the
reducibility was severely inhibited as compared to the fresh Ce/ changes in the acid sites for NH
3
adsorption aer sulfation
ꢀ
TiO
2
catalyst; the reduction peak at 465 C disappeared and an treatment. In order to further investigate the changes in acid
ꢀ
intense reduction peak located at 645 C was observed, which sites over the fresh catalyst and the sulfated catalysts, in situ
can be attributed to the reduction of surface cerium sulfates/ DRIFTS was performed during the NH adsorption/desorption
3
ꢀ
ꢀ
sultes. This suggests that the lattice oxygen may react with within the temperature range from 150 C to 400 C, shown in
the sulfur to form cerium sulfates/sultes since no reduction Fig. S4.† The main characteristic vibrations of NH
3
adsorption
28
ꢁ1
ꢁ1
peak assigned to lattice oxygen was observed. Aer the sulfa- over the fresh Ce/TiO
tion treatment at 240 C, the reduction peak at 465 C still 1170 cm (Fig. S4(A)†). The band at 1680 cm can be attrib-
2
catalyst are at 1680 cm , 1598 cm , and
ꢀ
ꢀ
ꢁ1
ꢁ1
+
existed but the peak strength decreased with the narrowed uted to the symmetric stretching vibration of NH
4
species on
ꢀ
ꢀ
29,30
ꢁ1
reduction temperature range between 410 C and 520 C. More Bronsted acid sites,
while the band at 1598 cm can be
SO reacted with the deeper interior of ceria to form bulk-like assigned to the formation of nitrate intermediates due to the
2
3
1
and bulk cerium sultes/sulfates with the increase in sulfa- excessive catalytic oxidation of NH by lattice oxygen. The
3
ꢀ
18
ꢁ1
tion temperature to 240 C, which is in line with the XPS band at 1170 cm can be assigned to the symmetric stretching
results shown in Table 2.
vibration of NH
3
1
adsorbed on the Lewis acid sites. The bands at
ꢁ
3.2.4 Surface acidity. Another crucial factor of low- 3100–3400 cm are attributed to the vibrations of N–H bonds,
temperature SCR activity is the surface acidity of the catalyst. and the negative bands due to the O–H vibrations at 3716 and
ꢁ
1
NH
3 3
-TPD and NH -DRIFT have been used to characterize the 3658 cm also showed up. For the sulfated catalysts shown in
ꢁ
1
amount and the strength of the acid sites of the catalysts. The Fig. S4(B),† several bands at 1000–1700 cm
and 3100–
ꢁ
1
ꢁ1
ꢁ1
NH -TPD prole is shown in Fig. 4, where two NH desorption 3700 cm were seen. The band at 1670 cm and 1432 cm
3
3
ꢀ
ꢀ
peaks at 215 C and 590 C can be observed with the increase in could be due to the symmetric and asymmetric stretching
temperature over the fresh Ce/TiO catalyst. The strength of the vibrations of N–H bonds adsorbed on the Bronsted acid
2
3
2,33
acidity is indicated by the desorption temperature. The region sites.
2
These vibrations are related to the adsorbed SO on the
ꢀ
ꢀ
below 200 C is assigned to the weak acidities and above 300 C unsaturated metallic sites, which further provide more sites for
can be assigned to the strong acidities. Accordingly, the fresh the adsorption of H and further form S–OH groups to form the
3
4
ꢁ1
ꢁ1
2
Ce/TiO catalyst has mainly medium-strength acidities. Aer Bronsted acid sites. The bands at 1319 cm , 1262 cm and
ꢁ1
sulfation treatment, three desorption peaks were found; the 1096 cm could be assigned to the vibrations of NH
rst was centered at 242 C and the intensity was further on Lewis acid sites. Two negative bands centered at 3708 cm
strengthened as compared to the fresh Ce/TiO catalyst. The and 3666 cm can be assigned to the consumption of Ce–OH
second peak was centered at 587 C and an additional NH3 due to NH adsorption. The other negative band at 1367–
3
adsorbed
ꢀ
ꢁ1

ꢁ1
2
ꢀ
3
ꢀ
ꢁ1
3
desorption peak centered at 420 C was observed. Moreover, 1351 cm can be attributed to NH adsorption on O]S]O on
Fig. 3
2
H -TPR profiles of the fresh and sulfated catalysts.
3
Fig. 4 NH -TPD profiles of fresh and sulfated catalysts.
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2
ꢁ
35
the surface of SO
4
sites. With the increase in the tempera- and nitrite/nitrate species on the surface of the sulfated catalyst
ꢀ
ture of the IR cell (from 150 to 400 C), the intensity of the cannot coexist. This could be due to the reaction between
negative band at 1367–1351 cm gradually decreased due to adsorbed nitrite/nitrate species with NH to form NH NO and
the desorption of NH from surface SO
ꢁ
1
3
4
3
2
ꢁ
37
sites. It should be then decompose to N and H O, suggesting that the reaction
2 2
3
4
ꢁ1
noted that aer the sulfation treatment, the band at 1598 cm
still exists on the catalyst but the strength of the peak is reaction pathway. Moreover, the negative band at 1361 cm
signicantly reduced, indicating that the excessive catalytic can be attributed to NH adsorption on the S]O of the surface
sites. It is speculated that the NO adsorption on O]S]O
The integration of the peaks at 1300–1150 cm (for Lewis causes the vibrations of S]O and O–N, contributing to the shi
3
between adsorbed NO and gaseous NH can proceed by the L–H
ꢁ
1
3
2
ꢁ
oxidation of NH
3
was inhibited aer the sulfation treatment.
SO
4
ꢁ1
ꢁ
1
2ꢁ
ꢁ1
ꢁ1
acid sites), 1432 cm (for Bronsted acid sites over sulfated in the vibration of SO4 from 1434 cm to 1361 cm . The
catalysts) and 1680 cm (for Bronsted acid sites over fresh Ce/ integration area of the adsorbed NO bands have been calculated
ꢁ
1
TiO catalyst) were calculated. As shown in Table S1,† there are and are shown in Fig. S5.† It can be seen that the reactivity of
2
signicantly more Bronsted acid sites on the sulfated catalysts adsorbed NO over the fresh Ce/TiO
as compared to the fresh Ce/TiO , which are 5.54 vs. 0.62, the nitrite/nitrate formed on the sulfated Ce/TiO
respectively. The negative band of O]S]O provides more reactive during the SCR reaction.
Bronsted acid sites in the sacrice of Lewis acid sites. Thus, the 3.3.2 Reaction between NO + O
2
catalyst was inhibited and
2
2
catalyst was
2
and the pre-adsorbed NH
3
Lewis acid sites of the sulfated catalysts almost disappear, species. The evolution of the DRIFTS spectra for the reaction
which are 0.48 vs. 4.47 with respect to the fresh catalyst, between NO + O and adsorbed NH is shown in Fig. 6. For the
2
3
respectively. Moreover, it can be speculated that for the fresh fresh Ce/TiO catalyst, NH is mainly adsorbed on the Lewis
2
3
ꢀ
ꢁ1
Ce/TiO catalyst, the peak centered at 214 C as shown in Fig. 4 acid sites to form coordinated NH (at 1180 cm ) aer purging
on the Lewis acid with 600 ppm NH
sites and the peak at 598 C could be assigned to the desorption attributed to the formation of nitrate intermediates due to the
2
3
ꢁ
1
could be attributed to the desorption of NH
3
3
for 40 min. The band at 1598 cm is
ꢀ
+
of NH
2
4
on the Bronsted acid sites. Aer sulfation, the peak at excessive catalytic oxidation of NH
3
by lattice oxygen as
desorbed from Lewis acid sites mentioned above. With the introduction of NO + O , both the
on Bronsted
ꢀ
42 C can be assigned to NH
and most NH
3
2
+
+
4
desorbed from Bronsted acid sites. Therefore, it NH
3
adsorbed on the Lewis acid sites and NH
4
ꢀ
is suggested that the peak centered at 430 C could be assigned sites gradually decrease. Aer 60 min, the Ce/TiO catalyst is
2
ꢁ
1
to NH desorbed from strong Bronsted acid sites.
mainly covered by adsorbed NO (at 1619 cm ), bidentate
2
nitrate (at 1554 cm ), monodentate nitrate (at 1369 cm ) and
monodentate nitrito groups (at 1232 cm ). It is suggested that
the reaction between adsorbed NH and gaseous NO over fresh
catalyst follows the E–R reaction pathway. As for the 180
3
ꢁ1
ꢁ1
ꢁ
1
3
.3 Reaction mechanism study by in situ DRIFTS
3
3
.3.1 Reaction between NH3 + O2 and the adsorbed NO Ce/TiO
species. Transient reactions were carried out to investigate the
2
ꢀ
3
C-sulfated catalyst, NH is mainly adsorbed on the Bronsted
+
ꢁ1
ꢁ1
reactivity of the adsorbed reactants in the SCR reaction. The acid sites to form NH
evolution of DRIFTS spectra for the reaction between NH + O
purging with 600 ppm NH
and adsorbed NO are shown in Fig. 5. Aer purging with NO for 1358 cm can be assigned to NH
4
(at 1434 cm and 1697 cm ) aer
for 40 min. The negative band at
adsorption on S]O of the
sites. With the introduction of NO + O , the
3
2
3
ꢁ
1
3
2
ꢁ
35
4
0 min, the fresh Ce/TiO
adsorbed NO
2
ꢁ
catalyst was mainly covered by surface of SO
4
2
1
ꢁ1
ꢁ1
intensity of the negative band at 1358 cm decreased gradually,
and monodentate nitrito (O–N–O–, at 1232 cm ). The bands of indicating that the adsorbed NH
2
(at 1617 cm ), bidentate nitrate (at 1558 cm
)
ꢁ1
2ꢁ
3
on SO
4
is reactive. Aer
NO2 and nitrate/nitrito showed little change with the intro- 60 min, all of the peaks associated with NH
3
bands disappeared
ꢁ
1
ꢁ1
ꢁ1
duction of NH
3
+ O
2
. Aer purging with NH
3
+ O
2
for 4 min, the and new bands at 1613 cm , 1430 cm and 1361 cm were
ꢁ
1
band associated with the monodentate nitrito group dis- observed. The negative band at 1430 cm can be assigned to
ꢁ
1
42ꢁ
sites.
appeared and a new band at 1197 cm was observed, which NO adsorption on S]O of the surface SO
was assigned to the adsorption of NH on Lewis acid sites. It is
The integration areas of the adsorbed NH
suggested that the reactivity of the adsorbed NO
species over calculated and are shown in Fig. 7. It can be seen from Fig. 7A
the fresh Ce/TiO
4
catalyst is restricted, indicating that the L–H that the intensities of the coordinated NH decreased
bands were
3
3
x
+
and NH
catalyst, the bands associated
decreased slowly and still existed aer
for 30 min, suggesting that the E–R
2
3
reaction pathway is inhibited over the fresh Ce/TiO catalyst.
rapidly. For the sulfated Ce/TiO
2
2
ꢀ
+
As for the 180 C-sulfated catalyst, several bands at 1612, with NH
3
and NH
4
ꢁ
1
1434, 1365 and 1288 cm were observed as shown in Fig. 5B. purging with NO + O
2
ꢁ1
These can be assigned to surface nitrate species (1612 cm for reaction pathway between the adsorbed NH
3
and gaseous NO
ꢁ
1
ꢁ
ꢀ
NO
1
2
, 1365 cm for free NO
3
(ref. 36)). The negative band at was inhibited in spite of the enhanced acidity over the 180 C-
ꢁ1
ꢁ1
434 cm and 1288 cm can be assigned to NO adsorption on sulfated catalyst.
2
ꢁ
the S]O of surface SO
4
3
sites. With the introduction of NH +
2
O , the intensity of the negative bands gradually decreased due
2
ꢁ
3.4 Kinetic studies
that the adsorbed NO on SO4 is reactive. It can be seen that During the SCR reaction, the nonselective catalytic reduction
the intensities of NO and nitrates obviously decreased with the reaction (NSCR reaction) and the catalytic oxidation of NH to
introduction of NH NO (i.e., C–O reaction) may happen simultaneously. Therefore,
to the desorption of NO from surface SO
sites, indicating
4
2
ꢁ
2
3
+
3
2 3 4
+ O . Moreover, the adsorbed NH /NH
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ꢀ ꢀ
3 2 2
Fig. 5 In situ DRIFTS of the reaction between NH + O and pre-adsorbed NO at 240 C over (A) fresh Ce/TiO , and (B) 180 C-sulfated catalysts.
in order to further identify the contribution of the SCR reaction
following the L–H reaction pathway and the E–R reaction NO
Both the SCR reaction and NSCR reaction contribute to the
conversion, while the C–O reaction contributes to the
. Thus, the NO conversion rate can be
x
pathway, NSCR reaction and the C–O reaction over the fresh and formation of NO
sulfated Ce/TiO catalyst, the steady-state kinetic studies were described as
performed as shown in Fig. S6 and S7.† It can be seen that the
NH conversion rate (dNH3), NO conversion rate (dNO) and N
x
2
dNO ¼ dSCR + dNSCR ꢁ dC–O
(5)
3
x
2
O
formation rate (dN2O) over the fresh and the sulfated catalysts
increased with the increase in NO concentration. The rate of the
SCR reaction (dSCR) also increased and exhibited a linear rela-
tionship with the NO concentration (the correlation coefficients
are higher than 0.99), indicating that the reaction order of the
SCR reaction in relation to the gaseous NO concentration is 1.
Moreover, the rate of the C–O reaction (dC–O) decreased with the
increase in gaseous NO concentration, indicating that the
reaction order of the C–O reaction in relation to the gaseous NO
concentration is less than zero.
Therefore, the rates of SCR and C–O reaction can be
described as
d
NH3 þ dNO
d
SCR
¼
¼
ꢁ dNSCR
(6)
(7)
2
dNH3 ꢁ dNO
d
CꢁO
2
The kinetic models of the L–H reaction, E–R reaction, NSCR
catalyst and
catalyst are shown in ESI.† All the reaction
paths have been widely recognized and are suitable for the
The SCR reaction, C–O reaction and NSCR reaction all reaction and C–O reaction over the fresh Ce/TiO
contribute to NH conversion. Thus, the NH
conversion rate the sulfated Ce/TiO
can be described as
2
3
3
2
38
ceria-based catalyst. According to eqn (10S) and (11S),† the
dNH3 ¼ dSCR + dC–O + dNSCR
(4)
ꢀ ꢀ
3 2
and the pre-adsorbed NH at 240 C over (A) fresh Ce/TiO , (B) 180 C sulfated catalyst.
Fig. 6 In situ DRIFTS of the reaction between NO + O
2
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ꢀ
Fig. 7 Integration area from in situ DRIFTS of the reaction between NO + O
2
and pre-adsorbed NH
3
over (A) fresh Ce/TiO
2
, (B) 180 C sulfated
catalyst.
rate of the SCR reaction over the ceria-based catalyst can be with the increase in reaction temperature as shown in Table 3.
described as
The in situ DRIFTS results in Fig. 6 demonstrate that the reac-
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢀ
tivity of NO adsorbed on the fresh Ce/TiO
2
catalyst at 240 C is
d½N
2
ꢂ
d½N
dt
2
ꢂ
d
SCR
¼
¼
þ
restricted, while the reaction between the adsorbed NO and
gaseous NH over the sulfated Ce/TiO catalyst can proceed by
dt
EꢁR
LꢁH
Â
Ã
Â
Ã
3
2
3þ
k
3
NOðgÞ ½NH
Â
2
ꢂ þ k10 Ce ꢁ O ꢁ NO ꢁ NH
3
the L–H reaction pathway due to the formation of NH NO and
4
2
its decomposition to N and H O. Thus, the NH NO formed on
Ã
2
2
4
2
¼
k
EꢁR NOðgÞ þ kLꢁH
(8)
the sulfated Ce/TiO
fresh Ce/TiO
2
catalysts was much more than that on the
catalyst at 240 C, contributing to the higher
ꢀ
2
4 2 2
concentration of the surface NH NO over the sulfated Ce/TiO
The rate of the NSCR reaction over the ceria-based catalyst
can be described as
catalyst. Therefore, the kL–H value of the sulfated Ce/TiO
2
cata-
ꢀ
lyst at 250 C is higher than that of the fresh Ce/TiO
2
catalyst.
d½N
2
Oꢂ
k
4
k
5
½NH
2
ꢂ Â
ÃÂ
Ã
4
þ
With the increase in the reaction temperature, the chemical
d
NSCR
¼
¼ k
NOðgÞ Ce ]O
dt
5
½NOꢂ þ k
6
4
bonds between NO and the fresh Ce/TiO catalyst may be acti-
2
(
9) vated, facilitating the formation of NH
TiO
higher than that of the sulfated Ce/TiO
4
NO
2
over the fresh Ce/
catalyst is
k
4
½NH
2
ꢂ½Ce þ]Oꢂ
¼
catalyst. Thus, the kL–H value of the fresh Ce/TiO
2
k
6
2
1
þ k
Â
Ã
catalyst.
5
NOðgÞ
2
According to eqn (16S),† the kE–R value is dependent on k
and the concentration of surface NH . According to eqn (12S),†
the concentration of surface NH is mainly related to the
3
2
The rate of C–O reaction over the Ce/TiO
2
catalyst can be
2
described as
4+
oxidation of surface Ce , k and the concentration of adsorbed
2
d½NOꢂ
dt
k
6
k
4
½NH
2
ꢂ
Â
Ã
NH . With the increase in the reaction temperature, the oxida-
tion of Ce and k2 increased while the adsorption of NH3
decreased. Thus, the concentration of surface NH may show
2
3
4
Ce ]O
þ
d
CꢁO
¼
¼
Â
Ã
4+
k NO
5
ðgÞ
þ k
6
4
þ
(10)
k
4
½NH
2
ꢂ½Ce ]Oꢂ
a peak value with the increase in reaction temperature,
¼
Â
Ã
k
5
NOðgÞ
k
ꢀ
contributing to the peak value of kE–R at 350 C as shown in
þ 1
39
6
Table 3, which is in line with previous study. For the sulfated
Ce/TiO
at middle temperatures than that at low temperatures as shown
in Fig. 4. Thus, the concentration of surface NH may increase
2
catalyst, the promotion of acidity is more pronounced
It can be seen from Fig. S6(D) and S7(D)† that the rate of the
SCR reaction calculated according to eqn (6) is linear with the
gaseous NO concentration (the correlation coefficients are
higher than 0.99), which is in line with eqn (8). The parameters
of kL–H and kE–R of the SCR reaction were calculated by the linear
2
further, contributing to the increased k
with the increase in
E–R
ꢀ
the reaction temperature to 400 C.

tting of Fig. S6(D) and S7(D),† and the results are shown in 3.5 The mechanism of the SCR performance of the fresh Ce/
Table 3.
2 2
TiO catalyst is better than that of sulfated Ce/TiO
According to eqn (8), kL–H is dependent on k10 and the
concentration of surface NH NO . k10 increases rapidly with the
increase in the reaction temperature and thus, kL–H increased
Table 3 shows that the rate constant of the E–R reaction (kE–R) is
much higher than that of the L–H reaction (kL–H), suggesting
that the E–R reaction mechanism dominates in the standard
4
2
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2 2
Table 3 Kinetic parameters of the SCR reaction over fresh Ce/TiO and sulfated Ce/TiO
dSCR ¼ kE–R[NO(g)] + kL–H
ꢀ
6
2
Temperature/ C
k
L–H
k
E–R/10
R
d
C–O([NO] ¼ 600 ppm)
Ce/TiO
2
150
3.61
0.008
0.059
0.165
0.238
0.443
0.436
0.003
0.022
0.043
0.153
0.414
0.539
0.995
0.996
0.998
0.999
0.999
0.999
0.997
0.998
0.999
0.999
0.999
0.999
1.31
4.72
19.90
30.45
62.69
191.95
0.19
3.33
2
2
3
3
4
00
50
00
50
00
3.71
4.672
20.05
89.16
188.76
0.151
0.53
14.84
18.22
87.53
133.91
Sulfated Ce/TiO
2
150
2
2
3
3
4
00
50
00
50
00
15.49
27.41
50.6
128.25
SCR reaction. According to kinetic studies, the contribution of obviously greater than that over the fresh Ce/TiO2 catalyst.
the C–O reaction and the NSCR reaction at low temperature can Moreover, the weaker binding of NH with the sulfated Ce/TiO2
3
be neglected. Therefore, the severely inhibited low-temperature catalyst promoted the reactivity of the adsorbed NH
NO conversion of the sulfated Ce/TiO catalyst is due to the gaseous NO and the redox properties of the sulfated Ce/TiO
inhibited reaction rate of the SCR reaction, more precisely, the catalyst was signicantly enhanced, suggesting that more
reaction rate of the E–R reaction. According to eqn (12S),† the adsorbed NH dissociated into NH according to reaction (2S).†
concentration of NH is dependent on k , the concentration of Thus, the reaction rate of the E–R reaction pathway over the
Ce ]O and the adsorbed NH . The H -TPR results suggest that sulfated Ce/TiO catalyst was close to that over the fresh Ce/TiO2
3
with
2
2
3
2
2
2
4
+
3
2
2
the reducibility of the sulfated Ce/TiO was severely inhibited, catalyst. Moreover, the rate of the C–O reaction of the sulfated
2
indicating that the rate constant of reaction (2S)† (k ) over the Ce/TiO catalyst was obviously lower than that of the fresh Ce/
2
2
sulfated Ce/TiO
Ce/TiO
the surface Ce ]O over the sulfated Ce/TiO
over the fresh Ce/TiO catalyst. NH
concentration of NH adsorbed on the sulfated Ce/TiO
2
catalyst is much lower than that over the fresh TiO
catalyst. The XPS results show that the concentration of conversion at 330 C over the sulfated catalyst was higher than
2
catalyst, shown in Fig. S6(E) and S7(E).† Therefore, NO
ꢀ
2
4
+
2
is less than that that over the fresh Ce/TiO
2
catalyst. The promotion of NO
ꢀ
2
3
-TPD suggests that the conversion over the sulfated catalyst at 330 C, as shown in
3
2
catalyst Fig. 1, is attributed to both the increase in the reaction rate of
is higher as compared to the fresh Ce/TiO catalyst, which is the E–R reaction and the inhibition of the C–O reaction.
2
approximately 1.18 times that of the fresh Ce/TiO2 catalyst
according to the NH -DRIFT results (Table S1†). Thus, the
3
4
2
catalyst is much
. Conclusions
concentration of NH
lower than that over the fresh Ce/TiO
the k [NH ] over sulfated Ce/TiO is much lower than that over
fresh Ce/TiO catalyst, contributing to the much lower reaction
2
over the sulfated Ce/TiO
2
catalyst and, therefore,
The sulfation effect of the Ce/TiO catalyst on NO conversion
2
3
2
2
was found to be dependent on the reaction temperature.
Signicantly inhibited low-temperature SCR activity and
enhanced SCR activity at higher temperature were found.
Comprehensive characterizations, in situ DRIFT studies and
kinetic studies were performed to investigate the mechanisms
of these inhibiting and enhancing effects. It can be concluded
that the different sulfation effects of the Ce/TiO on SCR activity
2
at different reaction temperatures can be attributed to the
different reaction rates of the E–R reaction (kE–R). At 150 C, the
contribution of the C–O reaction and the NSCR reaction can be
neglected. The severely inhibited low-temperature SCR activity
is due to the inhibited redox properties and excessive adsorp-
tion of NH
contributing to the much lower reaction rate of the E–R reaction
over the sulfated Ce/TiO catalyst. With the increase in the
reaction temperature, the reaction rate of the E–R reaction (kE–R
signicantly increased due to the improved redox properties
2
rate of the E–R reaction (reaction (3S)†). It is suggested that
reaction (2S)† is the rate-determining step in the SCR reaction
following the E–R reaction pathway. In conclusion, the reason
for the inhibited low-temperature performance with enhanced
NH
demonstrate that the sulfated Ce/TiO
more strongly acidic sites as compared to the fresh Ce/TiO
catalyst. The strong binding of NH to the sulfated Ce/TiO
3
adsorption is as follows. Firstly, the NH
3
-TPD results
2
catalyst has signicantly
ꢀ
2
2
3
catalyst restricts its reactivity with gaseous NO, which is hard to
overcome for reaction (2S)† to proceed. Secondly, the formation
4
+
of cerium sultes/sulfates inhibited the reduction rate of Ce to
3 3 2
, which restrict the dissociation of NH to NH ,
3
+
Ce aer the sulfation treatment, resulting in the inhibition of
the reaction rate of reaction (2S)† (k ).
With the increase in the reaction temperature to 350 C,
although the adsorbed NH desorbed from the surface of the
fresh and sulfated Ce/TiO catalyst, the adsorption of NH over
the sulfated Ce/TiO catalyst at middle temperatures was
2
2
ꢀ
)
3
2
3
and the weakened binding of NH
catalyst. Moreover, the C–O reaction rate of the sulfated Ce/TiO
3
with the sulfated Ce/TiO
2
2
2
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the Policy leading Program (Inter-
national Science and Technology Cooperation) Science and
Technology Project of Jiangsu Province [grant number
BZ2017014].
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