Sn-doped rutile TiO2 for vanadyl catalysts: Improvements on activity and stability in SCR reaction

Journal Pre-proof Sn-doped rutile TiO2 for vanadyl catalysts: Improvements on activity and stability in SCR reaction Wenzhe Si (Conceptualization) (Validation) (Investigation) (Writing original draft) (Writing - review and editing) (Visualization) (Supervision), Haiyan Liu (Investigation) (Writing - original draft), Tao Yan (Visualization), Hui Wang (Investigation), Chi Fan (Visualization), Shangchao Xiong (Writing - review and editing), Ziqi Zhao (Investigation), Yue Peng (Writing - review and editing), Jianjun Chen (Project administration) (Funding acquisition) (Conceptualization) (Writing - review and editing), Junhua Li (Project administration) (Funding acquisition)

PII:

S0926-3373(20)30212-5

DOI:

https://doi.org/10.1016/j.apcatb.2020.118797

Reference:

APCATB 118797

To appear in:

Applied Catalysis B: Environmental

Received Date:

13 November 2019

Revised Date:

10 February 2020

Accepted Date:

20 February 2020

Please cite this article as: Si W, Liu H, Yan T, Wang H, Fan C, Xiong S, Zhao Z, Peng Y, Chen J, Li J, Sn-doped rutile TiO2 for vanadyl catalysts: Improvements on activity and stability in SCR reaction, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118797

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Sn-doped rutile TiO2 for vanadyl catalysts: Improvements on activity and stability in SCR reaction Wenzhe Si1,2, Haiyan Liu1, Tao Yan1, Hui Wang1,3, Chi Fan1, Shangchao Xiong1, Ziqi Zhao1, Yue Peng1, Jianjun Chen1*, Junhua Li1

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1. State Key Joint Laboratory of Environment Simulation and Pollution Control, National Engineering Laboratory for Multi Flue Gas Pollution Control Technology and Equipment, School of Environment, Tsinghua University, Beijing 100084, PR China 2. Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, 15213, USA 3. School of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng, 224051, P. R. China.

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Graphical Abstract

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Highlights 

Rutile TiO2 was used as support for preparing vanadyl SCR catalysts

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The catalyst showed better SCR performance, H2O and SO2 resistance after Sn doping

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Sn doping increased the redox ability and the surface acidity of V2O5/ TiO2

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Sn doping inhibited aggregation and crystal transformation of TiO2 during aging

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Abstract: Sn-doped rutile TiO2 supports (SnxTi1-xO2, X=0.1, 0.2, 0.5) were designed and V2O5/SnxTi1-xO2 catalysts were synthesized by incipient wetness impregnation. The V2O5/Sn0.2Ti0.8O2 exhibited higher low-temperature SCR activity and better H2O and SO2 resistance than the traditional V2O5/TiO2 (anatase) catalyst. In situ DRIFTs, XPS, H2-TPR and NH3/NO+O2-TPD results showed that Sn doping improved the redox property, NH3/NO adsorption and the amount of surface chemisorbed oxygen. The

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kinetic study indicated that Sn doping had little effect on SCR reaction pathways, but promoted SCR reaction, mainly through Eley-Rideal mechanism. More importantly,

the V2O5/Sn0.2Ti0.8O2 showed better thermal stability than V2O5/TiO2. For V2O5/TiO2,

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the aggregation and sintering of the catalyst occurred, and the surface area decreased

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significantly after thermal aging. The monomeric vanadyl species partially transferred to crystalline V2O5. But for V2O5/Sn0.2Ti0.8O2, the surface area did not decrease as much

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polymeric vanadyl species.

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as that for V2O5/TiO2, and the monomeric vanadyl species partially transferred to the

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Keywords: vanadyl catalysts, NH3-SCR, thermal aging, Sn doping, rutile structure

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1. Introduction

The emission of nitrogen oxide (NOx) has become a notable problem, due to its

adverse effects on the environment, such as photochemical smog, acid rain, and haze [1, 2]. Selective catalytic reduction of NOx with NH3 (NH3-SCR) is one of the most effective technologies for the elimination of NOx from stationary and mobile sources 2 / 33

[3-6]. W/Mo-promoted V2O5/TiO2 has been widely used as commercial NH3-SCR catalysts for stationary sources owing to its high activity and good SO2 resistance. However, the temperature of flue gas may suddenly rise to an extreme high value by thermal shock in the practical applications, leading to the deactivation of V2O5/TiO2 [7, 8]. Hence, it is essential to improve the thermal stability of V2O5/TiO2 catalysts to avoid the deactivation by thermal shock.

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Most existing studies attributed the thermal deactivation of V2O5/TiO2 to two aspects. One is the reduction of redox property and surface acidity. The phase transition of TiO2 from anatase to rutile (ATR) takes place at high temperature, which brings

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about the sintering of TiO2 supports [9]. As a result, the surface area of V2O5/TiO2

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catalysts decrease significantly and the amount of active sites for redox property and surface acidity is reduced accordingly. Another is the change of dispersion and state of

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V2O5 during ATR. The active monomeric vanadyl species prefer to exist on the surface

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of anatase TiO2 [10]. Such species tend to form crystalline V2O5 at high temperature, which is less active and can accelerate the oxidation of NH3 [11, 12]. Recently, the large

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decrease of surface area during ATR has been proven to increase the density of vanadyl species, which facilitates the formation of crystalline V2O5 [11, 13]. Therefore, the fatal

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reason for the deactivation of V2O5/TiO2 catalysts can ascribe to the great loss of surface area of the catalysts during ATR. The key to improve the thermal stability of V2O5/TiO2 is to prevent the large decline of surface area at high temperature. Several approaches have been taken to optimize the thermal stability of V2O5/TiO2 catalysts. One is to elevate ATR temperature to maintain the stability of V2O5/TiO2 in 3 / 33

a wider temperature range. Bucharsky et al. found that ATR temperature depended strongly on the concentration of V2O5. This temperature could increase by lessening V2O5 content [14]. However, the deactivation of V2O5/TiO2 still took place above ATR temperature. A more effective way is to take rutile TiO2 as supports directly in order to block the way to ATR. But the rutile TiO2 synthesized by conventional methods usually possess low surface area [15, 16], which hinders its application as SCR catalyst support.

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Sn is a well-known ATR promoter and can be doped into TiO2 by a coprecipitation method. The Sn can be inserted to the TiO2 crystal lattice to form rutile SnxTi1−xO2 solid solution, which owns larger surface area than that of common rutile

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TiO2 [17]. In addition, SnO2 possesses abundant surface oxygen species and Lewis

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acidic sites, which benefit for NH3-SCR reaction [18]. Herein, SnxTi1−xO2 supports were synthesized by doping Sn into rutile TiO2 using a co-precipitation method. And

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then vanadyl catalysts were prepared on SnxTi1−xO2 by incipient wetness impregnation

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[19]. It is the first time to systematically study the influence of Sn doping on redox property, surface acidity, and H2O/SO2 resistance over V2O5/SnxTi1-xO2. It was found

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that V2O5/SnxTi1-xO2 catalysts showed better SCR activity and H2O/SO2 resistance than V2O5/TiO2. In addition, thermal aging was adopted to simulate the process of thermal

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shock to investigate the stability of the catalysts. The results indicated that doping Sn into TiO2 supports could improve the stability of the catalysts. At last, V2O5/TiO2 and V2O5/SnxTi1-xO2 catalysts were compared on the structures, morphologies, surface areas and the states of vanadyl species after thermal shock. The outcomes in this study may provide a practical solution for the deactivation of V2O5/TiO2 catalysts by thermal 4 / 33

shock in NH3-SCR reactions.

2. Experimental 2.1. Catalysts preparations A series of SnxTi1-xO2 supports with different Sn/Ti molar ratios (where x represents the Sn/(Sn+Ti) molar ratio) were synthesized by a co-precipitation method. Firstly, a 25% ammonia solution was slowly added to an aqueous solution containing

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Ti(SO4)2 and SnCl4·5H2O under vigorous stirring until the pH was approximately 10.

And then, the obtained suspension was filtered and washed with deionized water. At

last, the precipitate was dried at 120 °C for 12 h and subsequently calcined at 500 °C

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for 4 h in air. Pure TiO2 was prepared by the same method.

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A series of V2O5/SnxTi1-xO2 and V2O5/TiO2 (the content of V2O5 in the samples were 1 wt.%) were prepared by incipient wetness impregnation using a mixed solution

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of ammonium meta vanadate (NH4VO3) and oxalic acid (H2C2O4·2H2O). After 2 h of

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continuous stirring, the samples were dried at 120 °C overnight and then calcined at 500 °C for 4 h in air.

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A thermal aging treatment was carried out over the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts at 650 °C for 24 h under a flowing air atmosphere with 10

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vol% water vapor to get aged samples. These samples were denoted as V2O5/TiO2-A and V2O5/SnxTi1-xO2-A. 2.2 Catalytic activity measurement The NH3-SCR activity tests were evaluated in a fixed-bed quartz micro-reactor with 120mg catalyst of 40-60 mesh. The reaction feed gas contained 500 ppm NO, 500 5 / 33

ppm NH3, 5% O2, 5% H2O, 100ppm SO2 (only when needed), N2 as balance gas. The flow rate was 300 cm3·min−1, which corresponded with a gas hourly space velocity (GHSV) of 79 000 h−1. The reaction temperature range was from 250 to 450 °C. The concentrations of the gas in the inlet and outlet streams were continually monitored by an FTIR spectrometer (MKS6030HS). The NOx conversion and N2 selectivity were

NOX conversion = N2 selectivity =

[NOX ]in −[NOX ]out [NOX ]in

× 100% (1)

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calculated by the following equations:

[NOX ]in +[NH3 ]in −[NOX ]out −[NH3 ]out −2[N2 O]out [NOX ]in +[NH3 ]in −[NOX ]out −[NH3 ]out

2.3 Turnover frequency (TOF)

× 100% (2)

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The TOF values were calculated by the following equations:

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TOF = rAc /nOF (3)

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(rAc: the conversion of nitrogen oxides per gram catalyst in per second, the unit is mol/g·s；nOF: the amount-of-substance of V atoms in per gram catalysts, the unit is

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mol/g.)

The reaction feed gas contained 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O, N2

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as balance gas. At the same time, NOx conversion was controlled less than 15% by tuning GHSV (i.e. 474 000 - 79 000 h−1), in order to suppress the influence of the inner

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diffusion and external diffusion. 2.4 The reaction kinetics experiment The reaction kinetics experiment was carried out to investigate the contributions of NO reduction over V2O5/TiO2 and V2O5/SnxTi1-xO2 catalysts. Gaseous NH3, O2 and H2O concentrations were remained in 500 ppm, 5% and 5%, respectively, while the NO 6 / 33

concentration increased from 300 ppm to 700 ppm. In order to suppress the influence of the inner diffusion and external diffusion, higher GHSV (i.e. 474 000 - 79 000 h−1) were adopted to control NOx conversion less than 15% [20]. According to previous studies [20-22], the SCR reaction rate (i.e., δSCR) of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts can be described as: δSCR = kL-H + kE-R [NO(g)]. In this equation, δSCR and [NO(g)] represent the N2 formation rate and gaseous NO concentration (ppm),

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respectively. kL-H and kE-R represent the kinetic constant of the SCR reaction through

the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism, respectively.

Therefore, kE-R and kL-H can be obtained after the linear regression of the N2 formation

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rates (Fig. S8, the slope is kE-R and the intercept is kL-H).

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2.5 Catalyst characterizations

NO oxidation, N2 adsorption-desorption isotherms, inductively coupled plasma

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spectroscopy (ICP), powder X-ray diffraction (XRD), H2 temperature programmed

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reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), NH3 temperature programmed desorption (NH3-TPD) and NO+O2 temperature programmed desorption

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(NO+O2-TPD), in situ DRIFTS and Raman were carried out to characterize the catalysts. All the details are described in Supporting Information.

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3. Results and discussion 3.1 Crystal structures of the catalysts

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Fig. 1 XRD patterns of V2O5/TiO2 andV2O5/SnxTi1-xO2 catalysts.

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Powder X-ray diffraction (XRD) was used to investigate the crystal structures of

V2O5/TiO2 and V2O5/SnxTi1-xO2 (shown in Fig. 1). It can be observed that the

V2O5/TiO2 catalyst showed a phase of the anatase TiO2 type (PDF#21-1272), while the

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V2O5/Sn0.1Ti0.9O2 sample exhibited the mixed-phase of the rutile TiO2 type (PDF#21-

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1276) and anatase TiO2 type (PDF#21-1272). A further increase of Sn/Ti ratio promoted the formation of the rutile phase while inhibiting the anatase phase. The peaks

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ascribed to rutile TiO2 (PDF#21-1276) were observed over V2O5/Sn0.2Ti0.8O2 and

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V2O5/Sn0.5Ti0.5O2, and no peaks for anatase phase were observed over these two samples. As for the V2O5/Sn0.5Ti0.5O2 catalyst, the lower-shift peak at 2θ = 27.1°

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caused by the larger ionic radius of Sn (r (Sn4+) = 0.069 nm > r (Ti4+) = 0.0605 nm)) was present, indicating the successful incorporation of Sn4+ into the TiO2 crystal lattice

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to form the SnxTi1-xO2 solid solution of the rutile type [17, 23]. Furthermore, there were no characteristic peaks of crystalline V2O5 over the fresh V2O5/SnxTi1-xO2 and V2O5/TiO2 catalysts, demonstrating the good dispersion of vanadium species on the surface of the supports, which was further confirmed by the Raman results (Fig. S2). 3.2 Textural properties 8 / 33

Table 1 Structure and textural data of V2O5/TiO2 andV2O5/SnxTi1-xO2 catalysts

BET Surface Area (m2/g)

Pore Volume (cm3/g)

V2O5/TiO2

79

0.296

V2O5/Sn0.1Ti0.9O2

44

0.214

V2O5/Sn0.2Ti0.8O2

76

0.267

V2O5/Sn0.5Ti0.5O2

94

0.214

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Samples

The textural parameters of the obtained catalysts, including the BET specific areas

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and pore volumes, are summarized in Table 1. The surface area of V2O5/TiO2 catalyst was 79 m2/g, which was close to the literature [24]. The surface area of

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V2O5/Sn0.1Ti0.9O2 catalyst was approximately 44 m2/g, which was lower than that of

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V2O5/TiO2. With the increase of the Sn/Ti molar ratio, a significant increase of surface area was observed. For the V2O5/Sn0.5Ti0.5O2 sample, the surface area could reach to 94

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m2/g. The results proved that the surface area could be increased with the successful incorporation of Sn4+ into the TiO2 crystal lattice.

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3.3 Catalytic performance

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Fig. 2 NOx conversion (a), N2 selectivity (b) over V2O5/TiO2 and V2O5/SnxTi1-xO2 catalysts, Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O, balance N2, GHSV = 79,000h−1.

Fig. 2 illustrates the effect of temperature on the NOx conversion and N2 selectivity over the V2O5/TiO2 and V2O5/SnxTi1-xO2 catalysts. It can be clearly observed that the doping of Sn improved the SCR activity of the vanadyl catalysts significantly (Fig.

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2(a)), especially at low temperature (250-350 ºC). The V2O5/Sn0.2Ti0.8O2 catalyst

exhibited the best SCR performance. Although the V2O5/Sn0.2Ti0.8O2 sample possessed a much higher catalytic activity than V2O5/TiO2, the BET surface areas and pore

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volumes were almost the same on the two catalysts, which indicated that BET surface

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area and pore volume had little effect on the promotion of catalytic activity after Sn doping. It was reported that an increase of activity at low temperature generally led to

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the loss of N2 selectivity for SCR catalysts [25]. But the N2 selectivity did not decrease

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after Sn doping in this work (Fig. 2(b)). Both V2O5/TiO2 and V2O5/SnxTi1-xO2 catalysts exhibited outstanding N2 selectivity over the entire temperature range.

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3.4 Turnover frequency (TOF)

Fig. 3 The relative TOF profiles as a function of temperature over the V2O5/TiO2 and V2O5/SnxTi110 / 33

xO2 catalysts.

Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O, balance N2,

GHSV= 474,000 – 79,000 h−1 to control NOx conversion less than 15%.

A relative turnover frequency (TOF) values were used to compare the catalytic velocity of different catalysts. The TOF values of NOx for the V2O5/SnxTi1-xO2 and V2O5/TiO2 catalysts were estimated according to Eq. (3) and the corresponding profiles as a function of temperature are shown in Fig. 3. The TOF values over the V2O5/SnxTi1catalysts were higher than that of the V2O5/TiO2 catalyst in the whole temperature

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xO2

range, demonstrating that per V2O5 as active center over the SnxTi1-xO2 supports reacted

with NH3 and NOx more effectively than that over TiO2 supports. Among these four

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catalysts, the V2O5/Sn0.2Ti0.8O2 sample showed the highest TOF value. The superior

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activity of the V2O5/Sn0.2Ti0.8O2 catalyst may be benefited from satisfactory specific surface area and synergistic interaction between Ti and Sn. In order to exclude the

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influence of surface area for evaluating the role of Sn doping, V2O5/TiO2 and

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V2O5/Sn0.2Ti0.8O2 samples were chosen to be compared on the reducibility and surface acidity in the following sections.

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3.5 Improvement in catalytic activity 3.5.1 Reducibility of the catalysts

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The H2-TPR experiments were conducted to test the reducibility of the samples.

As shown in Fig. 4(a), for the V2O5/TiO2 sample, there were two reduction peaks at 384 and 754 ºC, which were assigned to the reduction of surface vanadium oxide from V5+ to V4+ and the reduction of the remaining vanadium species, respectively [26, 27]. And the reducibility especially at low temperatures was mainly determined by the low11 / 33

temperature peak (384 ºC). For V2O5/Sn0.2Ti0.8O2, there were mainly three peaks located at 195, 507 and 633 ºC. In order to distinguish the ascriptions of these three peaks, a comparison of H2-TPR results between V2O5/Sn0.2Ti0.8O2 catalyst and Sn0.2Ti0.8O2 support was given in Fig. 4(b). It can be seen that the intensity of the reduction peak from 450 to 800 ºC over Sn0.2Ti0.8O2 support was lower than that over the V2O5/Sn0.2Ti0.8O2 catalyst, especially at around 710 ºC. According to the earlier

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literature, the peak over the support at 450-800 ºC was assigned to the stepwise

reduction of Sn oxides from Sn4+ to Sn2+ and Sn2+ to metallic Sn0 [28]. So it could be deduced that the peaks at 633 ºC were assigned to the reduction of both Sn and V

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species. The reduction peak at 754 ºC over V2O5/TiO2 shifted to a lower temperature

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(around 633 ºC) after Sn doping.

Then the same way was used to confirm the ascription of the 195 ºC peak over the

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V2O5/Sn0.2Ti0.8O2 catalyst. The reduction peak at around 195 ºC over Sn0.2Ti0.8O2

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support was assigned as the reduction from surface Sn4+ to Sn2+ (Fig. 4(b)). From Fig. 4(b) (insert) it can be seen that the intensity of the peak at 195 ºC over V2O5/Sn0.2Ti0.8O2

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catalyst was higher than that over Sn0.2Ti0.8O2 support, which indicated that the reduction peak at 195 ºC over V2O5/Sn0.2Ti0.8O2 was assigned as the reduction of V and

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Sn species [29, 30]. The reduction peak at 384 ºC over the V2O5/TiO2 catalyst shifted to a low-temperature location (195 ºC) after Sn doping, which meant that the reduction of V species took place much easier on V2O5/Sn0.2Ti0.8O2 catalysts. To sum up, the lowtemperature reducibility was improved after Sn doping, which may facilitate the SCR activity, especially at low temperature. This result was also corresponded to the 12 / 33

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catalytic performance results (Fig. 2(a)).

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Fig. 4 H2-TPR profiles of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts (a), H2-TPR profiles of

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V2O5/Sn0.2Ti0.8O2 catalyst and Sn0.2Ti0.8O2 support (b), O1s spectra (c) and Ti 2p spectra (d) of V2O5/Sn02.Ti0.8O2 and V2O5/TiO2 catalysts.

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3.5.2 Chemical state of the catalysts and oxidation of NO to NO2 XPS was conducted to reveal the component and chemical state of the elements

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on the surfaces of the catalysts. The XPS spectra of the O 1s and Ti 2p of the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts were collected and shown in Fig. 4(c) and 4(d). The

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XPS results of V 2p and Sn 3d of the catalysts were also collected and shown in Fig. S3. It is found that Sn-doping did not exhibit a significant effect on the valence state of the V ions (Fig. S3(a)). For the XPS spectra of O1s, Oα peaks at 531-533 eV were considered to be surface adsorbed oxygen species and Oβ peaks at 528.5-531eV were ascribed to lattice oxygen 13 / 33

[31]. The ratio of Oα/(Oα+Oβ) was further calculated based on the integral area of Oα and Oβ (Fig. 4(c)). It can be seen that the V2O5/Sn0.2Ti0.8O2 catalyst had a higher Oα concentration than that of V2O5/TiO2 catalysts. This result indicated that the doping of Sn could efficiently increase surface chemisorbed oxygen, which was beneficial for the NH3-SCR of NOx due to the increasing rate of NO to NO2 oxidation [32, 33]. For the XPS spectra of Ti 2p (Fig. 4(d)), there were two photoelectron peaks at

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approximately 458.7 and 464.4 eV, which were attributed to Ti 2p 3/2 and Ti 2p1/2 of

Ti4+ over the V2O5/TiO2 catalyst, respectively [34]. In contrast to the V2O5/TiO2 catalyst, the positions of the Ti 2p peaks slightly shifted to a higher binding energy for the

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V2O5/Sn0.2Ti0.8O2 catalyst, which was caused by the ionization energy of Ti and Sn [35].

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In this work, the charge balance in the original system was affected by doping Sn into the lattice. After doping Sn ions, the excess electrons were released and the Ti ions

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shifted to a higher valence to maintain the charge balance. Ti4+ became more

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electropositive, which could decrease the recombination between free electrons and holes [36]. The titanium species in the V2O5/Sn02Ti0.8O2 catalysts had a lower density

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electron cloud around Ti atoms than the V2O5/TiO2 catalyst, further proving the stronger interaction among the metal oxides [37].

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In previous studies [32, 33, 38], the improvements of catalytic activity are

generally ascribed to the increase of catalytic oxidation ability, especially at low temperatures. The oxidation of NO by O2 was tested to characterize the oxidation properties of the samples. As shown in Fig. S4(a), V2O5/Sn0.2Ti0.8O2 catalyst showed much better NO oxidation activity than the V2O5/TiO2 catalyst over the whole 14 / 33

temperature range. NO conversion showed an increasing trend with the increase of temperature for all catalysts. The higher NO oxidation conversion on V2O5/Sn0.2Ti0.8O2 suggested that moderate Sn doping was helpful for improving the catalyst oxidation ability, further promoting the oxidation of NO to NO2. In addition, this could also significantly increase the low-temperature SCR activity, which was due to the occurrence of “fast SCR” : NO + NO2 + 2NH3 → 2N2 +3H2O [38].

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It is well known that the adsorption of NO over the catalyst surface plays an

important role in the SCR reaction [39]. The results of our NO+O2-TPD experiments

are shown in Fig. S4(b). A wide peak of NOx was desorbed at 150-350ºC on the

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V2O5/TiO2 catalyst, while two peaks of NO2 were desorbed at approximately 180 ºC

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and 390 ºC on the V2O5/Sn0.2Ti0.8O2 catalyst. The area of the NOx-desorbed peak on the V2O5/Sn0.2Ti0.8O2 catalyst was much higher than that on the V2O5/TiO2 catalyst.

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Interestingly, a high NO2 desorption peak was observed on a V2O5/Sn0.2Ti0.8O2 catalyst,

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and the area of the desorbed NO2 peak was much higher than that of the desorbed NO. This result indicated that the introduction of Sn not only increased NO adsorption but

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also promoted the oxidation of NO to NO2, which was in accordance with the results of the NO adsorption DRIFT (Fig. S6). It can be concluded that the improvement in the

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SCR activity of V2O5/Sn0.2Ti0.8O2 can be ascribed to the increase of the oxidation property.

3.5.3 Surface acidity NH3-TPD experiments (Fig. 5(a)) were conducted to exam the surface acidity of the V2O5/Sn0.2Ti0.8O2 and V2O5/TiO2 catalysts. Three main NH3 desorption peaks, 15 / 33

located at 100-200 ºC, 200-400 ºC and 400-600 ºC, were observed. The peaks at around 200 ºC and 400-600 ºC should be assigned respectively to the weakly adsorbed NH3 and strongly adsorbed NH3 species on acid sites [40]. There were only weak acid sites over the V2O5/TiO2 catalyst, but both weak and strong acid sites were found over the V2O5/Sn0.2Ti0.8O2 catalysts. Furthermore, the desorbed NH3 over the V2O5/TiO2 catalyst was much weaker than that over the V2O5/Sn0.2Ti0.8O2 catalyst. It is evident

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that the doping of Sn improved the surface acidity. The enhanced acid capacity enhanced the NH3-SCR activity [41, 42], which was consistent with the activity results

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(Fig. 2(a)).

Fig. 5 NH3-TPD profiles of V2O5/Sn0.2Ti0.8O2 and V2O5/TiO2 catalysts (a) and NH3 adsorption on

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Lewis and Brønsted acid sites over the V2O5/Sn0.2Ti0.8O2 and V2O5/TiO2 catalysts at different temperatures (b).

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To further investigate the properties of the acid sites (Brønsted acid sites and Lewis

acid sites), the DRIFT of NH3 adsorption from 150 to 400 ºC on these two catalysts were tested and the spectra were exhibited in Fig. S5. The assignments of NH3 adsorption bands were intensively studied in the literature [23, 43]. The integrated peak areas in the range of 1097-1304 cm-1 and 1329-1531 cm-1 represented NH3 adsorption 16 / 33

on respectively Lewis and Brønsted acid sites [44], which were integrated and shown in Fig. 5(b). It is noted that both the intensities of the NH3 adsorption on Brønsted and Lewis acid sites on the V2O5/Sn0.2Ti0.8O2 catalyst were much higher than those on the V2O5/TiO2 catalyst, which was consistent with the NH3-TPD results. However, the NH3 adsorption on Brønsted acid sites over the V2O5/Sn0.2Ti0.8O2 catalyst reduced more quickly with the increase of temperature, which illustrated the less thermal stability of

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NH3 adsorption on Brønsted acid sites compared to that on the V2O5/TiO2 catalyst. By contrast, the stability of NH3 adsorption on the Lewis sites of the V2O5/Sn0.2Ti0.8O2 catalyst was much stronger than that of the V2O5/TiO2 catalyst below 250 ºC. In

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summary, the NH3-TPD and in situ DRIFT results indicated that the surface acidities of

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the catalysts were enhanced after doping Sn, which could improve the SCR activity.

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3.5.4 Tolerance to H2O and SO2

Fig. 6 Effects of H2O and SO2 on the activity of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts at

350 ºC. Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O and 100 ppm SO2, balance N2, GHSV = 79,000 h−1 for V2O5/Sn0.2Ti0.8O2 (blue) and GHSV = 26,000 h−1 for V2O5/TiO2 (orange).

It is well known that the H2O and SO2 in flue gas mainly affect the performance 17 / 33

of the V-based NH3-SCR catalyst [37]. The effects of H2O and SO2 on the activity of the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts at 350 ºC are shown in Fig. 6. We tuned the GHSV for the V2O5/TiO2 to 26,000 h−1 to make the NOx conversion similar for the two samples. It can be observed that there was a slight decrease in NOx conversion over the V2O5/Sn0.2Ti0.8O2 catalyst, which was due to the feeding of H2O and SO2. Then it became stable after about 2 hours. The decrease of NOx conversion after adding H2O

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and SO2 was 5.3%. And NOx conversion was almost restored after excluding H2Oand SO2 from the reactant feed. As for the V2O5/TiO2 catalyst, the NOx conversion decreased (16.5%) after introducing H2O and SO2. The results above suggested that the

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doping of Sn could greatly improve its tolerance to H2O and SO2, which is promising

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for practical applications in NOx emission control.

In situ DRIFT of H2O and SO2 adsorption studies were carried out to investigate

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the reason for the improvement on H2O and SO2 resistance after Sn doping, and the

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results are shown in Fig. S7. The peaks at 1628-1630, 3202-3324 cm-1 were assigned as the δ(O-H) and perturbation of the OH groups from hydrogen bonded molecules

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respectively, which was caused by the H2O physisorption. And the peaks at 1175 and 1270 cm-1 were assigned as the bidentate sulfate or bisulfate on the surface of the

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samples, which were caused by SO2 adsorption. It can be seen that the intensities of peaks for H2O and SO2 adsorption were decreased after Sn doping. This may be the reason for the enhanced H2O and SO2 resistance over the V2O5/Sn0.2Ti0.8O2 catalyst. 3.6 Reaction Mechanism and Kinetics

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Fig. 7 Kinetic constants of the SCR reaction through the Langmuir-Hinshelwood (kL-H) (a) and the Eley-Rideal (kE-R) (b) mechanism over V2O5/Sn0.2Ti0.8O2 and V2O5/TiO2 catalysts at different temperatures.

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The SCR reaction kinetic constants of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts are shown in Fig. 7. The kL-H and kE-R represent the kinetic constants of the SCR

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reaction through the Langmuir-Hinshelwood mechanism and the Eley-Rideal

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mechanism, respectively. Fig. 7 shows that the kL-H of V2O5/Sn0.2Ti0.8O2 and V2O5/TiO2 catalysts were almost the same at 250-350 ºC. And the kL-H value of the

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V2O5/Sn0.2Ti0.8O2 catalyst was much higher than that of the V2O5/TiO2 catalyst when the temperature was above 350 ºC. For the kE-R value, the value of the V2O5/Sn0.2Ti0.8O2

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catalyst was significantly higher than that of the V2O5/TiO2 catalyst over the whole

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temperature range. The results indicated that Sn doping could promote the SCR reaction by the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism. From the in situ DRIFT study, it was demonstrated that both the Langmuir-Hinshelwood and the Eley-Rideal mechanisms contributed to the NO reduction over V2O5/Sn0.2Ti0.8O2 catalyst (Fig. S9 and S10). However, the kE-R value was almost 104 higher than kL-H value over the catalysts, which proved that the Eley-Rideal mechanism played the main 19 / 33

role in this reaction. Therefore, it can be concluded that Sn doping can accelerate the SCR reaction by the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism. But the promotion of SCR reduction by the Eley-Rideal mechanism plays the main role on the improvement of the low temperature SCR activity over V2O5/Sn0.2Ti0.8O2 catalyst. 3.7 Improvement in thermal stability

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3.7.1 Catalytic performance after thermal aging

Fig. 8 NH3-SCR activity (a) and N2 selectivity (b) of V2O5/ TiO2 and V2O5/ Sn0.2Ti0.8O2 before and after the thermal aging, Reaction conditions: 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O,

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balance N2, GHSV = 79,000 h−1. (A-thermal aging)

A thermal aging treatment on the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 was carried out

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to simulate the process of thermal shock in the practical applications. And the

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comparison of SCR activity and N2 selectivity between V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 before and after the thermal aging treatment are shown in Fig. 8. It can be seen that the NOx conversion of the V2O5/TiO2-A catalyst increased at low temperature, compared with the V2O5/TiO2 catalyst (Fig. 8(a)). But the N2 selectivity strongly decreased over the whole temperature range (Fig. 8(b)) along with the generation of a large amount of N2O after thermal aging (Fig. 8 (insert)), which is in 20 / 33

accordance with previous studies [29, 45]. Interestingly, there were almost no changes in the SCR activity and N2 selectivity over the V2O5/Sn0.2Ti0.8O2 catalyst after thermal aging, which suggested the doping of Sn could improve the thermal stability of vanadyl catalysts without sacrificing their catalytic selectivity. 3.7.2 ICP analysis of V2O5 contents ICP is introduced to detect if the V2O5 volatilized after thermal aging, and the V

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contents of the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts before and after thermal

aging are listed in Table 2. Before thermal aging, the V2O5 loadings on the two catalysts were almost the same, both of which were around 0.96 wt.%. After thermal aging, the

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V2O5 contents on the two catalysts were around 1.07 wt.%, which was about 11% higher

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than the fresh counterparts. However, the absolute V2O5 contents on the two aged samples were still almost identical. The almost equal V2O5 contents suggested that the

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activity variations on V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 for both fresh and aged samples were not influenced by the amount of V2O5 loadings.

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Table 2 V2O5 contents, BET Surface Area and Pore Volume of the V2O5/Sn0.2Ti0.8O2, V2O5/TiO2,

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V2O5/TiO2-A, and V2O5/Sn0.2Ti0.8O2-A catalysts. (A: after thermal aging)

Sample

BET Surface

Pore Volume

Area (m2/g)

(cm3/g)

V2O5 (wt.%) 0.96±0.010

78.54

0.2963

V2O5/TiO2-A

1.08±0.005

11.09

0.0434

V2O5/Sn0.2Ti0.8O2

0.96±0.011

75.91

0.2668

V2O5/Sn0.2Ti0.8O2-A

1.07±0.003

38.23

0.2064

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V2O5/TiO2

3.7.3 The textural properties 21 / 33

Table 2 also summarizes the BET specific areas and pore volumes of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts before and after thermal aging. The surface area of V2O5/TiO2 decreased from 78.54 to 11.09 m2/g after aging, while that of V2O5/Sn0.2Ti0.8O2 reduced from 75.91 to 38.23 m2/g. The pore volume of V2O5/TiO2 strongly shrank from 0.2963 to 0.0434 cm3/g after thermal treatment, whereas that of V2O5/Sn0.2Ti0.8O2 catalyst only slightly declined from 0.2268 to 0.2064 cm3/g. The

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morphologies of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 before and after thermal aging were investigated by HRTEM (Fig. 9). Both V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts

exhibited nanoparticles with the sizes around 10-20 nm. The particles sizes of

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V2O5/TiO2 significantly increased to 50-100 nm after thermal aging. But those of

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V2O5/Sn0.2Ti0.8O2 only slightly increased after thermal aging. It can be seen from Fig. 9(b) and 9(d) that the sizes of the particles for V2O5/TiO2-A were much larger than

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those for the V2O5/Sn0.2Ti0.8O2-A. The BET and HRTEM results suggested that thermal

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aging led to the aggregation and sintering of TiO2 support, which decreased the catalyst surface area and pore volume. But the V2O5/Sn0.2Ti0.8O2 catalyst exhibited a much

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better thermal stability after thermal aging treatment.

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Fig. 9 HRTEM images of the V2O5/TiO2 (a), V2O5/TiO2-A (b), V2O5/Sn0.2Ti0.8O2 (c) and

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3.7.4 Crystal structure of the catalysts

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V2O5/Sn0.2Ti0.8O2-A (d) catalysts

The crystal structures of the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts before and

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after thermal aging were investigated and the results are shown in Fig. 10 (a). Over the V2O5/TiO2-A catalyst, both anatase and rutile phase TiO2 appeared, which suggested

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that ATR took place on the partial of V2O5/TiO2 after thermal aging treatment. There

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was no difference between the V2O5/Sn0.2Ti0.8O2 and V2O5/Sn0.2Ti0.8O2-A catalysts in XRD patterns, indicating that Sn doping suppressed the sintering of TiO2 and improved the structural stability of the V2O5/Sn0.2Ti0.8O2 catalyst. The XRD results are in good accordance with the BET and HRTEM results.

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Fig. 10 XRD patterns of the V2O5/TiO2, V2O5/TiO2-A, V2O5/Sn0.2Ti0.8O2 and V2O5/Sn0.2Ti0.8O2-A catalysts (a) and Raman spectra of the V2O5/TiO2, V2O5/TiO2-A, V2O5/Sn0.2Ti0.8O2 and V2O5/Sn0.2Ti0.8O2-A catalysts (b).

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3.7.5 Raman analysis

It was well established for the vanadyl catalysts that the dispersed vanadyl species

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presented as monomeric and/or polymeric states were served as the active sites [46]. To

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get a deeper insight into the vanadyl species transformation of V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 catalysts during thermal aging, the Raman spectra of the samples

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were collected and shown in Fig. 10 (b). According to the literature, experimentally observed bands at 1027-1033, 970 and 921 cm−1 were assigned to the monomeric

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vanadyl species, crystalline V2O5 and polymeric vanadyl species, respectively [29, 46-

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50]. It can be seen that the V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 both possessed monomeric vanadyl species before thermal aging. After aging the crystalline V2O5 appeared on the V2O5/TiO2-A sample, which indicated that partial monomeric vanadyl species transferred to crystalline V2O5. This may be caused by the great loss of the surface area of V2O5/TiO2 after aging [11, 13]. As we know, the crystalline V2O5 usually shows low catalytic activity and bad N2 selectivity in SCR reaction. But for V2O5/Sn0.2Ti0.8O2-A 24 / 33

sample, polymeric vanadyl species appeared and no crystalline V2O5 was presented, which showed that partial monomeric vanadyl species transferred to polymeric vanadyl species. So the catalytic activity and N2 selectivity stayed stable on the V2O5/Sn0.2Ti0.8O2 after thermal aging. The results are consistent very well with the catalytic performance after thermal aging (Fig. 8). The possible explanation for the change of dispersion and state of V after thermal aging was as follows: For the

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V2O5/Sn0.2Ti0.8O2, the surface area was decreased not so much after aging. The density of vanadyl species was increased and partial monomeric vanadyl species transferred to

polymeric vanadyl species; But for the V2O5/TiO2 sample, the surface area was

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decreased significantly after aging, and the density of vanadyl species was increased a

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lot, which facilitated the formation of crystalline V2O5.

4. Conclusions

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In this study, Sn-doped rutile TiO2 (SnxTi1-xO2, X=0.1, 0.2, 0.5) were designed and

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a series of V2O5/SnxTi1-xO2 catalysts were prepared by incipient wetness impregnation. Sn doping could convert anatase TiO2 to the rutile phase to improve its physical-

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chemical properties. The V2O5/Sn0.2Ti0.8O2 catalyst expressed better NH3-SCR performance than the V2O5/TiO2 catalyst. The improvement of the activity can be

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mainly attributed to the better low-temperature reducibility, higher Oα concentration, and increased surface acidity after Sn doping. In the meanwhile, the V2O5/Sn0.2Ti0.8O2 also showed high tolerance to H2O and SO2, which may be caused by the weak adsorption of H2O and SO2 after Sn doping. The kinetic study indicated that Sn doping had little effect on the SCR reaction pathways, but promoted the SCR reaction, which 25 / 33

mainly followed the Eley-Rideal mechanism. What’s more, a thermal aging treatment on V2O5/TiO2 and V2O5/Sn0.2Ti0.8O2 was carried out to simulate the process of thermal shock in the practical applications. The SCR activity and N2 selectivity on V2O5/TiO2 catalyst decreased a lot at the high temperature range (350 - 450 °C) after thermal aging. But the SCR activity and N2 selectivity on V2O5/Sn0.2Ti0.8O2 stayed almost the same after aging, and were much

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better than those on V2O5/TiO2 catalyst. The difference on the activities over the two

catalysts after aging were ascriced to the follow reasons. For the V2O5/TiO2 sample, partial anatase TiO2 transferred to rutile phase after aging. The aggregation and

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sintering of the catalyst occurred, and the surface area decreased significantly. The

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active monomeric vanadyl species partially transferred to crystalline V2O5. So the catalytic activity and N2 selectivity at high temperature decreased a lot. But for the

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V2O5/Sn0.2Ti0.8O2 sample, no ATR took place after aging. The surface area did not

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decrease so much as that for V2O5/TiO2. The active monomeric vanadyl species partially transferred to the active polymeric vanadyl species. So the catalytic activity

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and N2 selectivity stayed stable over the whole temperature range. This work developed a practicable strategy for Sn-doped V2O5/Sn0.2Ti0.8O2 catalysts with high catalytic

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activity, excellent thermal stability and tolerance for NH3-SCR, which may supply the possibility for industrial applications in SCR processes.

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Credit Author Statement Wenzhe Si: Conceptualization; Validation; Investigation; Writing - Original Draft; Writing - Review & Editing; Visualization; Supervision Haiyan Liu: Investigation; Writing - Original Draft Tao Yan: Visualization Hui Wang: Investigation Chi Fan: Visualization Shangchao Xiong: Writing - Review & Editing

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Ziqi Zhao: Investigation Yue Peng: Writing - Review & Editing

Jianjun Chen: Project administration; Funding acquisition; Conceptualization;

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Writing - Review & Editing

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Junhua Li: Project administration; Funding acquisition

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgments：

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This work was financially supported by the National Natural Science Foundation of China (21876093 and 21777081), the National Key Research and Development Program (2017YFC0210700, 2017YFC0212804 and 2018YFC0213400) and Science Foundation for Young Scientists of Changchun University of science and technology (XQNJJ-2018-05).

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References [1] H.J. Bosch, F, Formation and Control of Nitrogen Oxides, Catal. Today, 2 (1988) 369-532. [2] I.N. Luca Lietti, Pio Forzatti, Selective catalytic reduction (SCR) of NO by NH3 over TiO2supported V2O5–WO3 and V2O5–MoO3 catalysts, Top. Catal., 11 (2000) 111-122. [3] F.L. Zhihua Lian, Wenpo Shan, Hong He, Improvement of Nb Doping on SO2 Resistance of VOx/CeO2 Catalyst for the Selective Catalytic Reduction of NOx with NH3, J. Phys. Chem. C, 121

ro of

(2017) 7803-7809.

[4] X.G. Xuesen Du , Yincheng Fu , Feng Gao , Zhongyang Luo, Kefa Cen, The co-effect of Sb and

Nb on the SCR performance of the V2O5/TiO2 catalyst, J. Colloid Interf. Sci., 368 (2012) 406–412.

-p

[5] Z. Hao, Z. Shen, Y. Li, H. Wang, L. Zheng, R. Wang, G. Liu, S. Zhan, The Role of Alkali Metal

re

in α-MnO2 Catalyzed Ammonia-Selective Catalysis, Angew. Chem. Int. Ed., 58 (2019) 6351-6356. [6] L. Kang, L. Han, J. He, H. Li, T. Yan, G. Chen, J. Zhang, L. Shi, D. Zhang, Improved NO x

lP

Reduction in the Presence of SO2 by Using Fe2O3-Promoted Halloysite-Supported CeO2–WO3

na

Catalysts, Environ. Sci. Technol., 53 (2019) 938-945.

[7] J. Li, H. Chang, L. Ma, J. Hao, R.T. Yang, Low-temperature selective catalytic reduction of NOx

ur

with NH3 over metal oxide and zeolite catalysts—A review, Cataly. Today, 175 (2011) 147-156. [8] C. Chen, Y. Cao, S. Liu, J. Chen, W. Jia, Review on the latest developments in modified

Jo

vanadium-titanium-based SCR catalysts, Chinese J. Catal., 39 (2018) 1347-1365. [9] X. Li, D. Yao, F. Wu, X. Wang, L. Wei, B. Liu, New Findings in Hydrothermal Deactivation Research on the Vanadia-Selective Catalytic Reduction Catalyst, ACS Omega, 4 (2019) 5088-5097. [10] H. Fu, Z. Duan, G. Henkelman, Computational Study of Structure and Reactivity of Oligomeric Vanadia Clusters Supported on Anatase and Rutile TiO2 Surfaces, J Phy. Chem. C, 119 (2015) 28 / 33

15160-15167. [11] J.W. Choung, I.-S. Nam, S.-W. Ham, Effect of promoters including tungsten and barium on the thermal stability of V2O5/sulfated TiO2 catalyst for NO reduction by NH3, Catal. Today, 111 (2006) 242-247. [12] B.M. Reddy, I. Ganesh, E.P. Reddy, Study of Dispersion and Thermal Stability of V2O5/TiO2−SiO2 Catalysts by XPS and Other Techniques, J. Phy. Chem. B, 101 (1997) 1769-1774.

ro of

[13] Y. Xi, N. Ottinger, Z. Liu, Effect of Hydrothermal Aging on the Catalytic Performance and Morphology of a Vanadia SCR Catalyst, SAE Technical Papers, 2 (2013).

[14] E.C. Bucharsky, G. Schell, R. Oberacker, M.J. Hoffmann, Anatase–rutile transformation in

-p

TiO2–V2O5 catalyst coatings for ceramic foams, J. Eur. Ceram. Soc., 29 (2009) 1955-1961.

re

[15] D. Wang, J. Huang, F. Liu, X. Xu, X. Fang, J. Liu, Y. Xie, X. Wang, Rutile RuO2 dispersion on

lP

rutile and anatase TiO2 supports: The effects of support crystalline phase structure on the dispersion behaviors of the supported metal oxides, Catal. Today, 339 (2020) 220-232.

na

[16] X. Yao, R. Zhao, L. Chen, J. Du, C. Tao, F. Yang, L. Dong, Selective catalytic reduction of NOx by NH3 over CeO2 supported on TiO2: Comparison of anatase, brookite, and rutile, Appl. Catal., B,

ur

208 (2017) 82-93.

[17] R.S. Shailendra K. Kulshreshtha, Vasanth Sudarsan, Non-random distribution of cations in Sn1 ≤ x ≤ 1.0): a 119Sn MAS NMR study, J. Mater. Chem., 11 (2001) 930-935

Jo

− xTixO2(0.0

[18] J. Zhang, Y. Liu, Y. Sun, H. Peng, X. Xu, X. Fang, W. Liu, J. Liu, X. Wang, Tetragonal Rutile SnO2 Solid Solutions for NOx-SCR by NH3: Tailoring the Surface Mobile Oxygen and Acidic Sites by Lattice Doping, Ind. Eng. Chem. Res., 57 (2018) 10315-10326. [19] L. Dong, C. Sun, C. Tang, B. Zhang, J. Zhu, B. Liu, F. Gao, Y. Hu, L. Dong, Y. Chen, 29 / 33

Investigations of surface VOx species and their contributions to activities of VOx/Ti0.5Sn0.5O2 catalysts toward selective catalytic reduction of NO by NH3, Appl. Catal., A, 431-432 (2012) 126136. [20] S. Xiong, X. Xiao, Y. Liao, H. Dang, W. Shan, S. Yang, Global Kinetic Study of NO Reduction by NH3 over V2O5–WO3/TiO2: Relationship between the SCR Performance and the Key Factors, Ind. Eng. Chem. Res., 54 (2015) 11011-11023.

ro of

[21] S. Yang, F. Qi, S. Xiong, H. Dang, Y. Liao, P.K. Wong, J. Li, MnOx supported on Fe–Ti spinel: A novel Mn based low temperature SCR catalyst with a high N2 selectivity, Appl. Catal., B, 181 (2016) 570-580.

-p

[22] N. Usberti, M. Jablonska, M.D. Blasi, P. Forzatti, L. Lietti, A. Beretta, Design of a “high-

re

efficiency” NH3-SCR reactor for stationary applications. A kinetic study of NH3 oxidation and NH3

lP

-SCR over V-based catalysts, Appl. Catal., B, 179 (2015) 185-195. [23] L. Zhang, L. Li, Y. Cao, Y. Xiong, S. Wu, J. Sun, C. Tang, F. Gao, L. Dong, Promotional effect

na

of doping SnO2 into TiO2 over a CeO2/TiO2 catalyst for selective catalytic reduction of NO by NH3, Catal. Sci. Technol., 5 (2015) 2188-2196.

ur

[24] S. Youn, S. Jeong, D.H. Kim, Effect of oxidation states of vanadium precursor solution in V2O5/TiO2 catalysts for low temperature NH3 selective catalytic reduction, Catal. Today, 232 (2014)

Jo

185-191.

[25] F. Liu, H. He, Y. Ding, C. Zhang, Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3, Appl. Catal., B, 93 (2009) 194-204. [26] C. Wang, S. Yang, H. Chang, Y. Peng, J. Li, Dispersion of tungsten oxide on SCR performance 30 / 33

of V2O5WO3/TiO2: Acidity, surface species and catalytic activity, Chem. Eng. J., 225 (2013) 520527. [27] X. Zhao, L. Huang, H. Li, H. Hu, J. Han, L. Shi, D. Zhang, Highly dispersed V 2O5/TiO2 modified with transition metals (Cu, Fe, Mn, Co) as efficient catalysts for the selective reduction of NO with NH3, Chinese J. Catal., 36 (2015) 1886-1899. [28] X. Yao, Y. Xiong, W. Zou, L. Zhang, S. Wu, X. Dong, F. Gao, Y. Deng, C. Tang, Z. Chen, L.

ro of

Dong, Y. Chen, Correlation between the physicochemical properties and catalytic performances of CexSn1–xO2 mixed oxides for NO reduction by CO, Appl. Catal., B, 144 (2014) 152-165.

[29] H. Chen, Y. Xia, R. Fang, H. Huang, Y. Gan, C. Liang, J. Zhang, W. Zhang, X. Liu, The effects

-p

of tungsten and hydrothermal aging in promoting NH3-SCR activity on V2O5/WO3-TiO2 catalysts,

re

Appl. Surf. Sci., 459 (2018) 639-646.

lP

[30] V.V. Kaichev, Y.A. Chesalov, A.A. Saraev, A.Y. Klyushin, A. Knop-Gericke, T.V. Andrushkevich, V.I. Bukhtiyarov, Redox mechanism for selective oxidation of ethanol over

na

monolayer V2O5/TiO2 catalysts, J. Catal., 338 (2016) 82-93. [31] J.Z. Zhiming Liu, Junhua Li, Lingling Ma, Seong Ihl Woo, , Novel Mn-Ce-Ti mixed-oxide

ur

catalyst for the selective catalytic reduction of NOx with NH3, ACS Appl. Mater. Inter., 6 (2014) 14500-14508.

Jo

[32] J.H. Liang Chen, LI, Maofa Ge, DRIFT Study on Cerium-Tungsten/Titiania Catalyst for Selective Catalytic Reduction of NOx with NH3, Environ. Sci. Technol., 44 (2010). [33] Z. Ma, X. Wu, Z. Si, D. Weng, J. Ma, T. Xu, Impacts of niobia loading on active sites and surface acidity in NbO /CeO2–ZrO2 NH3–SCR catalysts, Appl. Catal., B, 179 (2015) 380-394. [34] L. Li, C.-y. Liu, Y. Liu, Study on activities of vanadium (IV/V) doped TiO2(R) nanorods 31 / 33

induced by UV and visible light, Mater. Chem, Phys., 113 (2009) 551-557. [35] S.M. Lee, H.H. Lee, S.C. Hong, Influence of calcination temperature on Ce/TiO2 catalysis of selective catalytic oxidation of NH3 to N2, Appl. Catal., A, 470 (2014) 189-198. [36] S. Liao, H. Donggen, D. Yu, Y. Su, G. Yuan, Preparation and characterization of ZnO/TiO 2, SO42−/ZnO/TiO2 photocatalyst and their photocatalysis, J. Photoch. Photobio., A, 168 (2004) 7-13. [37] S. Zhang, H. Li, Q. Zhong, Promotional effect of F-doped V2O5–WO3/TiO2 catalyst for NH3-

ro of

SCR of NO at low-temperature, Appl. Catal., A, 435-436 (2012) 156-162.

[38] D. Meng, W. Zhan, Y. Guo, Y. Guo, L. Wang, G. Lu, A Highly Effective Catalyst of Sm-MnOx

Performance, ACS Catal., 5 (2015) 5973-5983.

-p

for the NH3-SCR of NOx at Low Temperature: Promotional Role of Sm and Its Catalytic

re

[39] J. Liu, X. Li, Q. Zhao, J. Ke, H. Xiao, X. Lv, S. Liu, M. Tadé, S. Wang, Mechanistic

lP

investigation of the enhanced NH3-SCR on cobalt-decorated Ce-Ti mixed oxide: In situ FTIR analysis for structure-activity correlation, Appl. Catal., B, 200 (2017) 297-308.

na

[40] S.S.R. Putluru, L. Schill, A. Godiksen, R. Poreddy, S. Mossin, A.D. Jensen, R. Fehrmann, Promoted V2O5/TiO2 catalysts for selective catalytic reduction of NO with NH3 at low temperatures,

ur

Appl. Catal., B, 183 (2016) 282-290.

[41] W. Shan, Y. Geng, X. Chen, N. Huang, F. Liu, S. Yang, A highly efficient CeWOx catalyst for

Jo

the selective catalytic reduction of NOx with NH3, Catal. Sci. Technol., 6 (2016) 1195-1200. [42] X. Li, K. Li, Y. Peng, X. Li, Y. Zhang, D. Wang, J. Chen, J. Li, Interaction of phosphorus with a FeTiOx catalyst for selective catalytic reduction of NOx with NH3: Influence on surface acidity and SCR mechanism, Chem. Eng. J., 347 (2018) 173-183. [43] G. Qi, R.T. Yang, R. Chang, MnOx-CeO2 mixed oxides prepared by co-precipitation for 32 / 33

selective catalytic reduction of NO with NH3 at low temperatures, Appl. Catal., B, 51 (2004) 93106. [44] Y. Peng, K. Li, J. Li, Identification of the active sites on CeO2–WO3 catalysts for SCR of NOx with NH3: An in situ IR and Raman spectroscopy study, Appl. Catal., B, 140-141 (2013) 483-492. [45] M. Casanova, K. Schermanz, J. Llorca, A. Trovarelli, Improved high temperature stability of NH3-SCR catalysts based on rare earth vanadates supported on TiO2WO3SiO2, Catal. Today, 184

ro of

(2012) 227-236.

[46] G. He, Z. Lian, Y. Yu, Y. Yang, K. Liu, X. Shi, Z. Yan, W. Shan, H. He, Polymeric vanadyl

species determine the low-temperature activity of V-based catalysts for the SCR of NOx with NH3,

-p

Sci. Adv., 4 (2018) 4637-4644.

re

[47] A. Christodoulakis, M. Machli, A.A. Lemonidou, S. Boghosian, Molecular structure and

lP

reactivity of vanadia-based catalysts for propane oxidative dehydrogenation studied by in situ Raman spectroscopy and catalytic activity measurements, J. Catal., 222 (2004) 293-306.

na

[48] S. Besselmann, E. Löffler, M. Muhler, On the role of monomeric vanadyl species in toluene adsorption and oxidation on V2O5/TiO2 catalysts: a Raman and in situ DRIFTS study, J. Mol. Catal.,

ur

A, 162 (2000) 401-411.

[49] J.-K. Lai, I.E. Wachs, A Perspective on the Selective Catalytic Reduction (SCR) of NO with

Jo

NH3 by Supported V2O5–WO3/TiO2 Catalysts, ACS Catal., 8 (2018) 6537-6551. [50] Y. He, M.E. Ford, M. Zhu, Q. Liu, U. Tumuluri, Z. Wu, I.E. Wachs, Influence of catalyst synthesis method on selective catalytic reduction (SCR) of NO by NH3 with V2O5-WO3/TiO2 catalysts, Appl. Catal., B, 193 (2016) 141-150.

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