Promotional effect of doping SnO2 into TiO2 over a CeO2/TiO2 catalyst for selective catalytic reduction of NO by NH3

Article abstract of DOI:10.1039/c4cy01412j

Ti_xSn_1-xO₂ was prepared by a co-precipitation method, and a series of CeO₂/Ti_xSn_1-xO₂ samples were prepared to investigate the effect of doping SnO₂ into TiO₂ for selective catalytic reduction of NO by NH₃. The results of catalytic tests suggested that the catalyst with the optimal molar ratio (Ti:Sn = 1:1) exhibited the best catalytic performance. Moreover, the NO removal efficiency of CeO₂/Ti_0.5Sn_0.5O₂ was higher than that of CeO₂/TiO₂. The obtained samples were characterized by BET, XRD, H₂-TPR, XPS, NH₃-TPD and in situ DRIFT. The results revealed that the introduction of SnO₂ resulted in the formation of rutile-type Ti_0.5Sn_0.5O₂ solid solution with larger specific surface area and better thermal stability. The interactions between CeO₂ and the Ti_0.5Sn_0.5O₂ support could improve the redox performance of the catalyst, which was beneficial to the enhancement of catalytic activity at low temperature. Furthermore, doping SnO₂ enhanced the surface acid sites and weakened the adsorption of nitrates, which played an important role in the catalytic reaction process. Finally, in situ DRIFT demonstrated that the competition adsorption happened between bridging nitrates and NH₃ gas and the selective catalytic reduction of NO by NH₃ proceeded mainly via the Eley-Rideal mechanism over Ce/TiO₂ and Ce/Ti_0.5Sn_0.5O₂.

Full text of DOI:10.1039/c4cy01412j

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Graphical Abstract
The promotional effect of SnO2 in Ce/TiO2 catalyst for NH3–SCR reaction.

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ARTICLE TYPE
Promotional effect of doping SnO2 into TiO2 over CeO2/TiO2 catalyst for
the selective catalytic reduction of NO by NH3
Lei Zhanga, Lulu Lia, Yuan Caoa, Yan Xionga, Shiguo Wua, Jingfang Sunb, Changjin Tanga*, Fei Gaob,
Lin Donga,b*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Abstract: TixSn1–xO2 was prepared by coꢀprecipitation method, and a series of CeO2/TixSn1–xO2 samples
were prepared to investigate the effect of doping SnO2 into TiO2 for the selective catalytic reduction of
NO by NH3. The results of catalytic tests suggested that the catalyst with the optimal mole ratio
10 (Ti:Sn=1:1) exhibited the best catalytic performances. Moreover, the NO removal efficiency of
CeO2/Ti0.5Sn0.5O2 was higher than that of CeO2/TiO2. The obtained samples were characterized by BET,
XRD, H2–TPR, XPS, NH3–TPD and in situ DRIFT. The results revealed that the introduction of SnO2
resulted in the formation of rutile style Ti0.5Sn0.5O2 solid solution with larger specific surface area and
better thermal stability. The interactions between CeO2 and Ti0.5Sn0.5O2 support could improve the redox
15 performance of catalyst, which was beneficial to enhance catalytic activity at low temperature.
Furthermore, doping SnO2 enhanced surface acid sites and weakened the adsorption of nitrates, which
played an important role in the process of catalytic reaction. Finally, in situ DRIFT demonstrated that the
competition adsorption happened between bridging nitrates and gas NH3 and the selective catalytic
reduction of NO by NH3 proceeded mainly via the Eley–Rideal mechanism over Ce/TiO2 and
20 Ce/Ti0.5Sn0.5O2.
mainly focus on transition metal and rare earth metal catalysts
mainly including Fe, Cu, Mn, Ce based catalysts. In addition, the
45 structures and components of catalysts usually influence the
NH3–SCR operation temperature window and the resistance of
1. Introduction
8
H2O and/or SO2.7, It is worth noting that CeO2–TiO2 based
Nitrogen oxides (NOx) are responsible for acid rain,
photochemical smog and ozone depletion threatening ecological
25 environment and human health.1 Stringent environmental
legislation has been made across the world to regulate the
emission standard of NOx from stationary sources (such as coal–
fired power plants) and mobile sources (such as vehicle exhaust).
The selective catalytic reduction of NOx by NH3 (NH3–SCR) is a
30 kind of high efficient deNOx technology for the emission
reduction of NOx from coal–fired power plants.2 The industrial
catalysts for NH3–SCR process are based on TiO2–supported
V2O5–WO3 and/or V2O5–MoO3 oxides for their high NOx
removal efficiency at 300–400 ºC and excellent resistance to SO2
35 poisoning. However, V–W(Mo)/TiO2 catalyst will be gradually
replaced for some practical problems: the toxicity of active
component V2O5, narrow operation temperature window and poor
N2 selectivity at higher temperature.3–5 In addition, anatase TiO2
as catalyst carrier is easy to suffer from the crystal transformation
40 to form rutile TiO2 when temperature is over 600 ºC, which
results in the decrease of surface area and catalytic activity.6, 7
Nowadays, researches about non–vanadium–SCR catalysts
catalysts may be a potential replacement of V2O5/WO3–TiO2
catalyst for its excellent NO conversion, high N2 selectivity at
50 medium–high
characteristic.9–11
temperature
and
environmentally–benign
SnO2 is a wide band gap n–type semiconducting metal oxide12, 13
and can form oxygen vacancies,14, 15 which is widely used in gas
sensors, transparent conductors and catalysis. SnO2 has a rutile–
55 style structure (P42/mnm) as same as rutile TiO2. It is reported
that SnO2 can be doped into TiO2 to form rutile TixSn1–xO2 solid
solution with high surface area and excellent thermal stability.16,
17
Meanwhile, our previous studies18–20 suggest that supported
Ti0.5Sn0.5O2 solid solution based catalysts showed excellent NO
60 removal efficiency over both three–way catalysts (TWC) and
NH3–SCR catalysts. Recently, Chang et.al21 reported that adding
SnO2 to MnOx–CeO2 could improve the catalytic activity at low
temperature and SO2 resistance in NH3–SCR reaction.
In this work, we attempt to substitute Ti0.5Sn0.5O2 solid solution
65 for TiO2 over CeO2/TiO2 to prepare CeO2/Ti0.5Sn0.5O2 by an
impregnation method. Catalytic performance evaluation suggests
that CeO2/Ti0.5Sn0.5O2 displayed higher NO removal rate than
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CeO2/TiO2 within the whole temperature range. Therefore, we
values with accuracy at ± 0.1 eV.
focus on investigating the promotional effect of doping SnO2 into
TiO2 on NH3–SCR performances over CeO2/TiO2. To elucidate
H2–temperature programmed reduction (H2–TPR) was carried out
in a quartz U–tube reactor connected to a TCD with H2–Ar
the functions of Ti0.5Sn0.5O2 in NH3–SCR reaction, textural 60 mixture (7% H2 by volume) as reductant. 50 mg of the sample
5
structure, surface acidity, redox performance and surface
component of samples have been explored by BET, XRD, in situ
DRIFT, H2–TPR, NH3–TPD and XPS.
was used for each measurement. Before switching to the H2–Ar
stream, the sample was preꢀtreated in a N2 stream at 200 ºC for 1
h. TPR started from room temperature to 850 ºC at a rate of 10
ºC·min–1.
65 The in situ DRIFT experiments were performed on a Nicolet
Nexus 5700 FTIR spectrometer by using a diffuse reflectance
attachment (HARRICK) equipped with a reaction cell (ZnSe
windows). The number of scans was 32 at a resolution of 4 cm–1
and the spectra were presented as KubelkaꢀMunk function
2. Experimental
2.1. Catalyst preparation
10 A series of TixSn1–xO2 solid solution (x is molar ratio) were
prepared by a coꢀprecipitation method. An ethanol and water
mixed solution containing required amounts of TiCl4 and SnCl4 70 referred to background spectra of sample recorded in N2. The
was precipitated by ammonium hydroxide under vigorous stirring
(pH ≈ 9.0). The precipitate was washed with deionized water
15 until no Cl– could be detected by 1.0 mol·L–1 AgNO3 solution,
then dried at 110 ºC for 12 h and calcined at 550 ºC for 4 h in
flowing air.
Both anatase TiO2 (denoted as TiO2–A) and rutile TiO2 (denoted
as TiO2–R) were prepared by TiCl4 dropwise into ethanol in an
20 iceꢀwater bath under stirring. The differences were that TiO2–A
was obtained by adding appropriate amount of NH3·H2O until the
powder sample was used (ca. 25 mg) and preꢀtreated in N2 at 400
ºC for 1 h prior to adsorption experiments. The reaction
conditions as follows: 500 ppm NH3, 500ppm NO, 5 vol.% O2,
N2 balance and 100 ml·min–1 flow rate. For each sample, the
75 NH3/(NO + O2) were adsorbed for 45 min to steady state at
required temperature. Afterwards, (1) the sample was purged by
N2 for 10 min, then spectra were recorded in pure N2 stream at a
desired temperature by the heating rate of 10 ºC·min–1; (2) the
sample was purged by N2 for 10 min, then passing (NO +
pH value was 9.0 while TiO2–R was obtained by adding 80 O2)/NH3 into the gas chamber to react with the pre–adsorbed
appropriate amount of H2O (pH < 1.0). Furthermore, SnO2 was
prepared by the same procedure as anatase TiO2 and SnCl4 was
25 used as the precursor. The precipitate was washed with distilled
water to be free of chloride, then dried at 110 ºC for 12 h and
calcined at 500 ºC for 4 h in flowing air.
A series of yCeO2/TixSn1–xO2 (denoted as yCe/TixSn1–x, and “y”
is the mass fraction of CeO2 in sample) and CeO2/TiO2 (using
30 anatase TiO2 as support, denoted as Ce/Ti) were prepared by
incipient wetness impregnation with the solution of
NH3/NOx species.
NH3–temperature
programmed
desorption
(NH3–TPD)
experiments were carried out on a multifunction chemisorption
analyzer with a quartz U–tube reactor, detected by a thermal
85 conductivity detector (TCD). About 0.1 g of the sample was pre–
treated by high purified N2 (40 ml·min–1) at 450 ºC for 1 h. After
pre–treatment, the sample was saturated with 1 vol.% NH3 (10
ml·min–1) at 100 ºC for 1 h and subsequently flushed with high
purified N2 at the same temperature for 1 h to remove gaseous
Ce(NO3)3·6H2O. The samples were vigorously stirred for 1 h and 90 and weakly adsorbed NH3, then the sample was heated to 600 ºC
then evaporated at 100 ºC until achieving a paste, which was
dried at 110 ºC overnight and then calcined in a muffle stove at
35 500 ºC for 4 h in flowing air. For yCe/Ti0.5Sn0.5, the CeO2 loading
was 5%, 10%, 15% and 20 wt.% respectively. For Ce/Ti and
Ce/Sn samples, the CeO2 loading was fixed at 10 wt.%.
at a rate of 10 ºC·min–1 in the flowing high purified N2 (40
ml·min–1).
2.3. Catalytic performance test
Catalytic reaction was performed in a fixed–bed quartz reactor
95 tube. The reaction condition were: 500 ppm NO, 500 ppm NH3, 5
vol.% O2, 100 ppm SO2 (when used), 5 vol.% H2O (when used),
balance N2 and the gas flow rate of 100ml·min–1, GHSV was
90,000 h–1. Prior to catalytic test, the sample was pre–treated in
N2 at 200 ºC for 1 h. The effluent gases including NO, NH3, NO2,
2.2. Catalyst characterization
Xꢀray powder diffraction (XRD) patterns were collected using a
40 Philips X’pert Pro diffractometer with Niꢀfiltered CuKα1
radiation (0.15408 nm). The X–ray tube was operated at 40 kV
and 40 mA. Average crystallite size was calculated by Debye– 100 and N2O were continuously analysed by an online Nicolet IS10
Scherrer equation.22
spectrometer equipped with a gas cell. IR Data were collected at
required temperature with keeping reaction for 15 min to reach a
steady state. The NO conversion and N2 selectivity were
calculated by the following equations:
Specific surface areas of samples were measured by nitrogen
45 adsorption at 77 K on a Micrometrics ASAP–2020 analyzer by
the Brunauer–Emmet–Teller (BET) method. Before each
adsorption measurement, about 0.1 g of the sample was degassed
in a N2/He mixture at 300 ºC for 4 h.
X–ray photoelectron spectroscopy (XPS) experiments were
50 performed on a PHI 5000 Versa Probe high performance electron
spectrometer, using monochromatic Al Kα radiation (1486.6 eV)
operating at an accelerating power of 15 kW. Before the
measurement, the sample was outgassed at room temperature in a
UHV chamber (< 5×10–7 Pa). The sample charging effects were
55 compensated by calibrating all binding energies (BE) with the
adventitious C 1s peak at 284.6 eV. This reference gave BE
[NO]in – [NO]out
105 NO conversion (%)
N2 selectivity (%)
=
×100
[NO]in
[NO]in –[NO]out +[NH3 ]in –[NH3 ]out –[NO2 ]out – 2[N2O]out
=
×100
[NO]in –[NO]out +[NH3 ]in –[NH3 ]out
3. Results and discussion
2
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3.1. Catalytic performances evaluation
conducted in the range from 150 to 450 ºC. As shown in Fig. 2,
the NO conversion of 10Ce/TiSn was higher than that of Ce/Ti at
the same reaction temperature. It seemed that NH3–SCR
50 proceeded more easily via the “fast SCR” reaction route over
10Ce/TiSn. Possible reasons for improving the “fast SCR”
reaction by doping SnO2 would be discussed combined with the
results of XPS and H2–TPR.
3.1.1. Effect of doping SnO2 on catalytic activity over Ce/Ti
Firstly, the influence of Ti/Sn mole ratio on NO conversion over
Ce/TixSn1–x samples was investigated. Herein, the loading of
CeO2 was fixed at 10 wt.%. As shown in Fig. S1, the order of
5
catalytic activity followed Ce/Ti0.5Sn0.5
> Ce/Ti0.75Sn0.25 >
Ce/Ti0.9Sn0.1 > Ce/Ti0.95Sn0.05 > Ce/Ti0.25Sn0.75. The results of
catalytic activities indicated that Ce/TixSn1–x samples exhibited
the optimal NO conversion when the molar ratio of Ti/Sn was
10 1:1. So a series of yCe/Ti0.5Sn0.5 samples were prepared, and
comparative studies between Ce/Ti0.5Sn0.5 (Ce/TiSn for short) and
Ce/Ti samples were conducted to elucidate the effect of doping
SnO2 on NH3–SCR performances.
Fig. 1 showed the NO conversion as a function of temperature for
15 NH3–SCR over Ce/TiSn, Ce/Ti, Ce/Sn, TiSn support and CeO2. It
could be seen that Ce/Sn, TiSn solid solution and CeO2 exhibited
much poor catalytic activity. In contrast, Ce/Ti displayed better
catalytic activity with over 90% NO conversion between 300 ºC
and 400 ºC while Ce/TiSn samples had the best catalytic activity
20 with more than 88% NO conversion from 250 ºC to 450 ºC in the
present experimental conditions. The order of catalytic activity
followed Ce/TiSn > Ce/Ti > Ce/Sn > CeO2> TiSn, indicating the
sample with triple components had the best catalytic activity.
Based on the above results, doping SnO2 into TiO2 was beneficial
25 to improve the catalytic activity of Ce/Ti, and the promotional
effect was possible related to the interactions among CeO2, SnO2
and TiO2. It is worth noting that the loading amounts of CeO2 for
Ce/TiSn had little effect on catalytic activity. Thus, it is rational
to choose the 10Ce/TiSn sample as a represent to investigate the
30 promotional effect of SnO2 on CeO2/TiO2. Fig. S2 indicated that
the N2 selectivity of Ce/TiSn and Ce/Ti was more than 95% in the
entire temperatures. In addition, as shown in Fig.S3, when
10Ce/TiSn was calcined at 750 ºC for 4 h, although the catalytic
activity of 10Ce/TiSn decreased, its catalytic activity was much
35 higher than that of Ce/Ti calcined at 750 ºC, which indicated that
Ce/TiSn had better thermal stability than Ce/Ti.
55
Fig. 2 NO conversion in NO oxidation over Ce/Ti and 10Ce/TiSn.
Reaction conditions: 500 ppm NO and 5 vol.% O2.
Fig. 3 5 vol.% H2O and/or 100 ppm SO2 resistance testing over Ce/Ti and
10Ce/TiSn.
60 3.1.3. Effect of H2O and/or SO2 on catalytic activity over
10Ce/TiSn and Ce/Ti
The actual working conditions of coal fired plants usually
contained a certain amount of SO2 and H2O, thus it was necessary
to investigate the influence of SO2 and H2O on SCR activities of
65 catalysts.27 All tests were carried out at 300 ºC. As shown in Fig.
3, 5 vol.% H2O had basically no influence on the activity of
10Ce/TiSn with keeping nearly 100% NO conversion in the
whole testing. When 100 ppm SO2 was injected, the NO
conversion of 10Ce/TiSn slowly decreased with the increase of
70 time and maintained ca. 81% after 24 h. Meanwhile, NH3–SCR in
the presence of 100 ppm SO2 was measured over Ce/Ti as a
comparison, and NO conversion decreased from ca. 94% to ca.
62% after 24 h. Thus, it indicated that doping SnO2 into TiO2
could improve the SO2 resistance of Ce/Ti. When both SO2 and
75 H2O were introduced, the NO conversion of 10Ce/TiSn was
lower than that of only adding SO2 in the whole testing. In terms
of the accelerated deactivation of catalyst influenced by H2O, two
possible reasons could be proposed. On one hand, NH4HSO4
could be more easily formed in the presence of SO2 and H2O than
80 only introducing SO2, which led to the deactivation of catalysts.28
Fig. 1 NO conversion of CeO2, TiSn, Ce/Sn, Ce/Ti, and Ce/TiSn samples.
3.1.2. NO + O2 reaction on 10Ce/TiSn and Ce/Ti
40 As reported elsewhere,23–26 the oxidation of NO to NO2 over
catalysts (such as V2O5–WO3/TiO2 and Fe–zeolite catalysts)
could improve the low temperature NH3–SCR activity by the
“fast SCR” reaction route as following:
NO + NO2 + 2NH3 → 2N2 + 3H2O
45 In order to investigate the influence of doping SnO2 on NH3–SCR
catalytic activities at low temperature, NO oxidation reaction was
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On the other hand, active sites could be occupied by the adsorbed 45 spin–orbit states and those labelled v represented 3d5/2 states. The
H2O as a consequence of the competitive adsorption of NH3 and
H2O, which resulted in the reversible deactivation of catalysts.29
Thus, catalysts were easier to be deactivated by introducing both
SO2 and H2O comparing to only introducing SO2. Interestingly, it
could be seen that NO conversion could recover up to ca. 96%
when cutting off SO2 and H2O over 10Ce/TiSn (the inset in Fig.
3), which indicated that the deactivation in the coexistence of SO2
and H2O was reversible over Ce/TiSn. As reported elsewhere,30, 31
bands labelled as u' and v' were ascribed to the primary
photoemission of Ce3+ and the other six bands labelled as u''' and
v''', u'' and v'', u and v were assigned to Ce4+.33 Ce3+ content could
be estimated by the formula:34
5


100× S u′ +S v′
(
)
( )

%

50
Ce III %=
(
)


S(u)+S v
( )
∑


Table 1 The textural data of supports and catalysts.
10 the NH3–SCR activity could be regenerated automatically after
cutting off SO2 and H2O over Ce0.2W0.2TiOx or Fe–Mn/TiO2
catalysts.
BET surface
area/m2·gꢀ1
Crystallite
size/nm
a=b
/nm
Sample
c/nm
ꢀ
TiO2–A
TiO2–R
54.2
19.5
30.5
86.4
75.6
47.3
22.6
33.9
22.3
10.2
ꢀ
ꢀ
3.2. Structural characterization (XRD and BET)
0.4602 0.2972
0.4692 0.3151
0.4640 0.3058
As shown in Fig. S4, TixSn1ꢀx (x=0–1) samples were rutile–style
15 structure (PDF 77–0450 for pure SnO2 and PDF 78–1510 for pure
TiO2). With the increase of Ti/Sn mole ratio, diffraction peak
SnO2
Ti0.5Sn0.5O2
10Ce/TiSn
gradually shifted to lower angle (the effective ionic radius r
(Sn4+)
= 0.069 nm > r
(Ti4+) = 0.0605 nm) comparing to TiO2–R,
ꢀ
ꢀ
ꢀ
ꢀ
indicating that Sn4+ had been successfully incorporated into the
20 lattice of TiO2 to form TiSn solid solution maintaining the rutile–
style structure, which was in agreement with the literatures.16, 22,
Ce/Ti
ꢀ
32
When CeO2 was supported and the loading amount of CeO2
was less than 10 wt.%, as shown in Fig. 4, only the peaks related
to TiSn support were detected, which indicated that CeO2 was
25 dispersed on the surface of TiSn support or its particle size was
smaller than the detection limit of XRD. When CeO2 loading was
more than 15 wt.%, the peaks at 28.6o and 47.6o ascribed to CeO2
phase were emerged. Combined with the results of catalytic
activity, it could be seen that crystalline CeO2 over Ce/TiSn
30 catalyst had little effect on catalytic performance. In addition,
diffraction peaks of TiSn became significantly broader than those
of pure TiO2 and SnO2, indicating the smaller size of crystallite
for TiSn as summarized in Table 1. Meanwhile, the specific
surface area of TiSn solid solution was much larger than those of
35 pure TiO2 and SnO2, which suggested that the introduction of
SnO2 improved the textural structure of TiO2.
Fig. 5 XPS spectra of Ce 3d, Sn 3d, Ti 2p, O 1s for Ce/Ti and 10Ce/TiSn.
55 As listed in Table 2, the relative content of Ce3+ increased from
17.0% in Ce/Ti to 35.1% in 10Ce/TiSn. Meanwhile, in terms of
Sn 3d, it can be seen that the bands of 10Ce/TiSn shifted to lower
binding energy comparing to these of pure SnO2, indicating that
part of low valence tin species existed over 10Ce/TiSn sample.
60 However, the binding energies of Ti 2p for Ce/Ti and 10Ce/TiSn
samples kept unchanged. Thus, the increase of Ce3+ over
10Ce/TiSn sample could be due to the synergetic interaction of
Fig. 4 XRD results of supports and catalysts.
3.3. Surface component and chemical states (XPS)
40 XPS was performed to illustrate the surface component and
chemical state of elements in Ce/Ti and 10Ce/TiSn samples. The
CeO2 spectrum was composed of two multiplets (v and u) by
Gaussian–Lorentz fitting procedure with the assignment defined
in Fig. 5 of Ce 3d spectra. The bands labelled u stand for 3d3/2
CeO2 and SnO2 via the redox equilibrium of 2Ce4+ + Sn2+
↔2Ce3+ + Sn4+, as reported by the previous studies.35,
In
36
4
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40 3.5.1. NH3 adsorption–desorption
addition, For the XPS spectra of O1s, the band of O' at 531.2 eV
could be assigned to surface adsorbed oxygen species and the
band of O'' at 529.8 eV could be ascribed to lattice oxygen.37
From data listed in Table 2, the relative content of O' over
10Ce/TiSn was larger than that of Ce/Ti sample, which indicated
that more surface adsorbed oxygen species were formed over
10Ce/TiSn sample. Therefore, in NH3–SCR reaction, the
synergetic interaction between CeO2 and SnO2 was beneficial to
activate O2 molecular to react with NO, which could improve the
In order to investigate the differences of NH3 adsorption
behaviours over the samples induced by doping SnO2, the DRIFT
spectra of NH3 adsorption–desorption were recorded from 150 to
350 ºC. NH3 adsorption results of pure TiO2 and TiSn solid
45 solution were respectively shown in Fig. 7a and 7b. The triplet
peaks in the range of 3100ꢀ3400 cm–1 could be assigned to the N–
H stretching vibration of coordinated NH3, the peaks in the range
of 1660–1680 cm–1 and 1490–1450 cm–1 could be ascribed to
5
+
ionic NH4 bounded to Brønsted acid sites (denoted as B acids),
10 “fast–SCR” route over Ce/TiSn.
50 and the peaks in the range of 1610–1580 cm–1 and 1160–1220
cm–1 could be assigned to the coordinated NH3 bounded to Lewis
acid sites (denoted as L acids).44, 45 It could be seen that B acids
totally desorbed after the temperature exceeded 300 ºC and L
acids still exhibited even at 350 ºC, which indicated that L acid
55 sites were stronger acid sites than B acid sites over Ce/Ti and
10Ce/TiSn. Compared with pure TiO2, the peaks of L acids at
1160–1220 cm–1 obviously were widened in the range from 1100
cm–1 to 1300 cm–1 and the intensity of B acids at 1490–1450 cm–1
increased over TiSn solid solution. It was reported that peak of L
60 acid appeared at about 1240–1250 cm–1 for pure SnO246 and Shen
et.al47 also found that a new peak of L acid at 1195 cm–1 appeared
for NH3 adsorption over SnO2/Al2O3 compared with pure Al2O3.
Thus, L acid sites consisted of not only NH3 bounded to TiO2 at
ca. 1180 cm–1 but also NH3 bounded to SnO2 at ca. 1220 cm–1
65 over Ce/TiSn. It could be seen that doping SnO2 into TiO2 could
enhance the sort and amount of acid sites over TiO2. As shown in
Fig. 7c and 7d, Ce/Ti and 10Ce/TiSn respectively exhibited a
similar spectrum as the corresponding support. By carefully
comparing to pure supports, the peaks of NH3 adsorption over
70 Ce/Ti and 10Ce/TiSn shifted to higher wavenumbers, implying
that the adsorption behaviour of NH3 was influenced probably by
the interaction between CeO2 and supports.
3.4. Redox performances (H2–TPR)
In NH3–SCR reaction, the reaction temperature window of NO
removal rate largely depended on the redox properties of catalyst.
Therefore, the influence of doping SnO2 on the reducibility
15 performance of Ce/Ti was investigated by H2–TPR. As shown in
Fig. 6, pure CeO2 displayed two peaks at ca. 500 and 780 ºC,
which could be respectively assigned to surface oxygen and bulk
lattice oxygen reduction.38–40 In terms of pure SnO2, a strong
reduction peak located at 630 ºC could be ascribed to the
20 reduction of Sn4+ to metal Sn.35, 41 In addition, TiSn solid solution
displayed the similar redox behaviour as pure SnO2. The peak α
of Ce/Ti could be ascribed to the dispersed CeO2, and peak β was
due to the reduction of bulk TiO2.42, 43 With regard to Ce/TiSn,
peak α (150–500 ºC) could be ascribed to the reduction of surface
25 species including Ce4+ or/and Sn4+ and peak β (above 500 ºC)
could be due to the reduction of bulk species including bulk TiSn
and CeO2. Compared with Ce/Ti and TiSn samples, peaks α and β
of Ce/TiSn samples shifted to the lower temperature, it could be
explained that the interaction between CeO2 and SnO2 evidenced
30 by XPS could improve the redox properties of Ce/TiSn.
Considering the results of NO + O2 reaction, it could be deduced
that the better redox properties of Ce/TiSn promoted the
occurrence of “fast–SCR” reaction and enhanced the NO removal
rate at low temperature over Ce/TiSn.
35 Table 2 The fitting data of XPS over Ce/Ti and 10Ce/TiSn.
Ce3+/(Ce3++Ce4+)/%
Sample
O'/( O' +O'')/%
23.9
Ce/Ti
17.0
35.1
10Ce/TiSn
39.5
75 Fig. 7 In situ DRIFT spectra of NH3 adsorption–desorption over TiO2 (a),
TiSn (b), Ce/Ti (c) and 10Ce/TiSn (d).
3.5.2. NO + O2 adsorption–desorption
NO + O2 adsorption–desorption spectra were recorded from 150
80 to 400 ºC to investigate the adsorption differences of nitrogen
oxides between Ce/Ti and 10Ce/TiSn samples. It was reported
Fig. 6 H2–TPR results of supports and catalysts.
3.5. Surface molecular adsorption (in situ DRIFT)
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that IR active modes of nitrates adsorbed on metal oxides are
followed by N2 purging, and then NO and O2 were introduced and
the DRIFT spectra were collected as a function of time. The
results of Ce/Ti sample were shown in Fig. 9a. The peaks at 1601
located in 1650–1500 cm–1 and 1300–1200 cm–1.48 As shown in
Fig. 8a for Ce/Ti sample, different kinds of surface nitrate species
formed during the process of NO + O2 desorption. According to 45 and 1178 cm–1 due to L acids and the peaks of N–H stretching
5
the change regulation of peak intensity and precious studies, the
peaks at 1616, 1579 and 1230 cm–1 could be attributed to bridging
nitrates (including bridging monodentate and bridging bidentate
nitrates as shown in Fig. S5), the peaks at 1554 and 1245 cm–1
vibration in the range of 3100–3400 cm–1 decreased with the
increase of time and disappeared after 20 min. With the
consumption of adsorbed NH3, the peak at 1621 cm–1 due to
bridging nitrates and 1558 cm–1 due to bidentate nitrates
could be ascribed to chelating bidentate nitrates, the peaks at 50 appeared. It implied that NH3 coordinated with L acid sites was
10 1510 and 1286 cm–1 could be assigned to monodentate nitrates,
and the peak at 1351 cm–1 could be due to free nitrate ions.48
Raising the temperature from 150 to 400 ºC, the intensity of
monodentate nitrates continuously decreased and totally
the active species and the reaction between pre–adsorption NH3
and NO + O2 proceeded by Eley–Rideal (E–R) mechanism. As
shown in Fig. 9b, the adsorption behaviours of 10Ce/TiSn were
similar to those of Ce/Ti sample. The peaks of L acids at 1602
eliminated at 350 ºC. Similarly, the peaks of bidentate nitrates 55 and 1188 cm–1 gradually disappeared accompanying with the
15 also decreased but still existed at 400 ºC. However, the intensity
of bridging nitrates increased firstly and then decreased. All the
results indicate that the transformation from monodentate nitrates
to bridging nitrates occurred during the process of thermal
appearance of nitrate species, which indicates that E–R
mechanism could proceed over 10Ce/TiSn sample. As reported
49
elsewhere,44,
NH3–SCR process mainly followed E–R
mechanism over tungsten or manganese modified Ce/TiO2 based
desorption, and the order of thermal stability for these nitrate 60 catalysts, and the reaction between adsorbed NH3 species and NO
20 species followed bridging nitrates
>
bidentate nitrates
>
gas was a key intermediate reaction. In addition, it is worth
noting that the peaks of N–H stretching vibration in 3100–3400
cm–1 and the peak at 1188 cm–1 due to L acid mostly disappeared
after 15min, and the consumption rate of NH3 over 10Ce/TiSn
monodentate nitrates. In terms of 10Ce/TiSn sample shown in
Fig. 8b, the peaks at 1612, 1575 and 1232 cm–1, 1545 and 1250
cm–1, 1510 and 1280 cm–1 could be respectively assigned to
bridging nitrates, chelating bidentate nitrates and monodentate 65 was faster than that of Ce/Ti, indicating that doping SnO2 into
25 nitrates. The intensity of all peaks uniformly decreased with the
increase of temperature and completely disappeared at 400 ºC,
which indicated that nitrate species over 10Ce/TiSn were
continuously desorbing and the desorption of nitrate species over
10Ce/TiSn was easier than that of Ce/Ti. Our previous study
30 suggested that the adsorption of NOx species on the surface of
SnO2 was considerably weak.35 Based on the present results, it
could be seen that doping SnO2 resulted in weakening the
adsorption ability of nitrate species over Ce/TiSn.
TiO2 promoted the activation of NH3.
Fig. 10 In situ DRIFT spectra of NH3 reaction with pre–adsorbed NO +
O2 over Ce/Ti (a) and 10Ce/TiSn (b) at 250 ºC.
70 3.5.4. NH3 adsorption after pre–adsorption NO + O2
NO and O2 were pre–adsorbed over Ce/Ti and 10Ce/TiSn
samples for 1 h followed by N2 purging, NH3 was introduced and
the DRIFT spectra were recorded as a function of time. In terms
of Ce/Ti shown in Fig. 10a, bridging nitrates at 1609, 1578 and
75 1238 cm–1 and chelating bidentate nitrates at 1561 and 1257 cm–1
formed. After injecting NH3 for 10 min, L acids at 1600 and 1182
cm–1 formed with N–H stretching peaks in the range of 3100–
3400 cm–1. In the meantime, the peaks at 1506 and 1288 cm–1 due
to monodentate nitrates formed and the intensity of bidentate
80 nitrates at 1561 and 1257 cm–1 increased with the intensity of
bridging nitrates at 1609, 1578 and 1238 cm–1 decreasing. NO +
O2 adsorption–desorption has demonstrated that the order of
thermal stability followed bridging nitrates > bidentate nitrates >
monodentate nitrates over Ce/Ti sample. Therefore, the
85 possibility that the disappearance of bridging nitrates was caused
by thermal desorption could be ruled out. In addition, surface
adsorption was saturated over the samples for 1 h NO+O2 preꢀ
adsorption judged from the DRIFT intensity of nitrate species.
Thus, it could be deduced that competition adsorption happened
90 between bridging nitrates and gas NH3. Combined with DRIFT
results, it could be proposed that one NH3 molecular could snatch
35
Fig. 8 In situ DRIFT spectra of NO + O2 adsorption–desorption over
Ce/Ti (a) and 10Ce/TiSn (b).
Fig. 9 In situ DRIFT spectra of NO + O2 reaction with pre–adsorbed NH3
over Ce/Ti (a) and 10Ce/TiSn (b) at 250 ºC.
40 3.5.3. NO + O2 adsorption after pre–adsorption NH3
NH3 was pre–adsorbed over Ce/Ti and 10Ce/TiSn samples for 1 h
6
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an adsorption site from bridging nitrates to form one L acid, and 50 indicated that doping SnO2 into TiO2 could enhance the amount
then bridging nitrates transformed to monodentate and chelating
bidentate nitrates over Ce/Ti as shown in Fig. S5. With regard to
10Ce/TiSn (in Fig. 10b), NH3 adsorption resulted in the
disappearance of bridging nitrates at 1603, 1571 and 1231 cm–1
after 10 min. Meanwhile the broad and asymmetric peaks at 1545
and 1255 cm–1 including in monodentate and chelating bidentate
nitrates increased. Thus, it could be seen that competition
adsorption as shown in Fig. S5 also happened over 10Ce/TiSn
of acid sites over Ce/Ti sample.
5
10 sample. It was worth noting that the competition adsorption over
Ce/Ti and 10Ce/TiSn was beneficial to form L acids, which
guaranteed the successful proceeding of NH3–SCR reaction by
E–R mechanism. After injecting NH3 for 10 min, although L
acids at 1597 and 1189 cm–1 formed, the intensities of both nitrate
15 species and NH3 bounded to L acids kept unchanged over Ce/Ti
and 10Ce/TiSn samples, indicating the adsorbed nitrate species
could not react with the adsorbed NH3 species. Therefore, NH3–
SCR reaction could not proceed by Langmuir–Hinshelwood (H–
L) mechanism over Ce/Ti and 10Ce/TiSn.
20 From the above results of in situ DRIFT, doping SnO2 influenced
the adsorption behaviors of both ammonia and nitrate species.
Comparing to Ce/Ti sample, the adsorption behaviors over
Ce/TiSn sample were characterized as following: 1. increasing
the sorts of L acid sites; 2. weakening the adsorption of nitrates;
25 3. improving the activation of NH3. In situ DRIFT results
demonstrated that NH3–SCR reaction proceeded by E–R
mechanism over the studied samples, since the adsorption of
nitrate species was weakened over Ce/TiSn by doping SnO2, L
acid sites were not easily blocked by nitrate species and could be
30 highly effective to activate NH3.
Fig. 11 NH3–TPD results of supports and catalysts.
4. Conclusions
55 Based on the all above results and discussion, Ce/TiSn sample
exhibited better catalytic performances than Ce/Ti sample by
doping SnO2 into TiO2, the reasons as following:
1. Rutile Ti0.5Sn0.5O2 solid solution prepared by co–precipitation
method displayed more excellent texture structure than pure
60 anatase or rutile TiO2, including larger specific surface area and
better thermal stability in NH3–SCR reaction.
2. H2–TPR and XPS suggested that doping SnO2 into TiO2
improved the redox ability of Ce/TiO2 and enhanced the
interaction between CeO2 and TiSn support, which contributed to
65 the “fast–SCR” route and promoted the deNOx activity at low
temperature.
3. In situ DRIFT results suggested that competition adsorption
happened between bridging nitrates and gas NH3 and NH3–SCR
reaction proceeded by the Eley–Rideal mechanism over Ce/Ti
70 and Ce/TiSn. Doping SnO2 not only improved L acid sites to
activate NH3 but also weakened the adsorption ability of nitrates,
which was conductive to push forward NH3–SCR reaction by
Eley–Rideal mechanism. In addition, NH3–TPD demonstrated
that the amount of acid sites increased by doping SnO2.
Table 3 The amount of surface acids over supports and catalysts.
Weak acids
Sα/a.u.
Strong acids
Sβ/a.u.
Total acids
Sα+ Sβ/a.u.
Sample
TiO2–A
TiSn
6630
7889
3756
5922
14038
20224
9885
20668
28113
13461
20519
Ce/Ti
10Ce/TiSn
14597
75 Acknowledgements
The financial supports of the National Natural Science
Foundation of China (Nos. 21303082, 21273110), Doctoral Fund
of Ministry of Education of China (No. 2013009111005) and
Sinopec Shanghai Research Institute of Petrochemical
80 Technology are gratefully acknowledged.
3.6. Surface acidities (NH3–TPD)
NH3–TPD was carried out to further investigate the effect of
35 doping SnO2 on acid properties over the samples. As shown in
Fig. 11, all the TPD curves could be fitted into peak α and peak β
indicating two different kinds of acid species. NH3 adsorption
DRIFT had displayed that B acids totally desorbed at 300 ºC and
L acids still exhibited even at 350 ºC, thus peak α for weak acid
40 could be ascribed to B acid and peak β for strong acid could be
assigned to L acid. As listed in Table 3, the amount of L acid was
higher than that of B acid for all the samples. Meanwhile, the
total acid amount of TiSn solid solution was larger than that of
pure TiO2, which indicated that surface acid sites of TiO2 were
45 added by doping SnO2 due to the increase of specific surface area
and the new added acid sites on SnO2. When loading CeO2 on
TiO2 and TiSn solid solution, the total acid amount of Ce/Ti and
Ce/TiSn decreased compared with pure supports because of the
Notes and references
aKey Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry
and Chemical Engineering, Nanjing University, Nanjing, 210093, P. R.
China
85 bJiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern
Analysis, Nanjing University, Nanjing 210093, PR China
Corresponding author:
Fax: +86 25 83317761; Phone: +86 25 83592290
Address: Hankou Road 22#, Nanjing, Jiangsu province, P. R. China,
90 210093
E-mail address: donglin@nju.edu.cn (L. Dong); tangcj@nju.edu.cn (C. J.
Tang)
decrease of specific surface area. Thus, NH3–TPD results
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