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The Extraordinary Deactivation Offset Effect of Arsenic and Calcium on CeO2-WO3 SCR Catalysts Xiang Li, Xiansheng Li, Tianle Zhu, Yue Peng, Junhua Li, and Jiming Hao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00746 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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Environmental Science & Technology

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The Extraordinary Deactivation Offset Effect of Arsenic and

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Calcium on CeO2−WO3 SCR Catalysts

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Xiang Lia, Xiansheng Lib, Tianle Zhu*a, Yue Pengb, Junhua Li*b and Jiming Haob

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a

School of Space and Environment, Beihang University, Beijing, 100191, PR China.

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b

State Key Joint Laboratory of Environment Simulation and Pollution Control, School

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of Environment, Tsinghua University, Beijing 100084, P. R. China

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*Corresponding author’s E-mail address: [email protected] (T. Zhu), [email protected] (J. Li); Tel.: +86 10 82314215, fax: +86 10 82314215 1

ACS Paragon Plus Environment


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Abstract

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An extraordinary deactivation offset effect of calcium and arsenic on CeO2-WO3

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catalyst had been found for selective catalytic reduction of NO with NH3 (NH3-SCR).

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It was discovered that the maximum NOx conversion of As-Ca poisoned catalyst

12

reached up to 89% at 350°C with the gaseous hourly space velocity of 120,000

13

mL·(g·h)-1. The offset effect mechanisms were explored with respect to the changes of

14

catalyst structure, surface acidity, redox property and reaction route by XRD, XPS,

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H2-TPR, O2-TPD, NH3-TPD and in situ Raman, in situ TG and DRIFTS. The results

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manifested that Lewis acid sites and reducibility originated from CeO2 were obviously

17

recovered, because the strong interaction between cerium and arsenic was weakened

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when Ca and As coexisted. Meanwhile, the CaWO4 phase generated on Ca poisoned

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catalyst almost disappeared after As doping together, which made for Brønsted acid

20

sites reformation on catalyst surface. Furthermore, surface Ce4+ proportion and oxygen

21

defect sites amount were also restored for two-component poisoned catalyst, which

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favored NH3 activation and further reaction. Finally, the reasons for the gap of catalytic

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performance between fresh and As-Ca poisoned catalyst were also proposed as follows:

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(1) surface area decrease; (2) crystalline WO3 particles generation; (3) oxygen defect

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sites irreversible loss.

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Keywords: NH3-SCR; offset effect; deactivation; acid sites; surface oxygen species

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Graphical Abstract

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30 31

3



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1. Introduction

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As important precursors of photo-chemical smog and PM2.5 pollution, nitrogen

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oxides (NOx) emitted from power plants and industrial kilns have been suffering the

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attention of the public opinion in recent years. Selective catalytic reduction (SCR) with

36

NH3 has always been considered as the most pervasive and efficient method for NOx

37

emissions control.1-3 However, conventional V2O5-WO3(MoO3)/TiO2 catalysts have

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some serious problems in practical applications, such as the physiological toxicity of

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vanadium pentoxide, poor alkaline metal tolerance, narrow operating temperature

40

window and high SO2 oxidation rate at high temperature. Therefore, it is of great

41

significance to develop non-toxicity, high-activity, low-cost and long-life SCR catalyst.

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Recently, CeO2 has been widely chosen as promising candidates to substitute harmful

43

active ingredient V2O5 in the NH3-SCR reaction, because of its nontoxicity, no

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secondary pollution, high BET surface area and remarkable oxygen storage capacity

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(OSC).4-7 Among the Ce-based SCR catalysts, CeO2-WO3 catalysts present the most

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excellent catalytic performance, wide operating temperature range (250-400 °C) and

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outstanding H2O and SO2 resistance.8, 9

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Heavy metal arsenic in the fly ash from wet-bottom furnaces is a serious toxicant for

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SCR catalysts. In our previous research, when As2O3 content reaches to 2.9 wt%, the

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NOx conversion of commercial catalyst drops to less than 40% in the temperature range

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of 250-500°C.10 Although MoO3 shows more remarkable arsenic deactivation

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resistance than WO3, but the resistance effect is still relatively limited. E. Hums 4


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investigates the exposure time of a V2O5-MoO3-TiO2 catalysts to a slag tap furnace

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influence on the relative activity(kt/k0) change with various arsenic content in the flue

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gas. It is found that the relative activity dropped to less than 0.4 with the exposure time

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being up to 15000h, when arsenic concentration in the flue gas is 500µg/Nm3.11 The

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situation of CeO2-based catalyst is not much better, since both the NOx conversions of

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As induced CeWAlO3 and Ce-W/Ti catalysts was less than 50% in the temperature

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window from 150 to 500 °C.12,

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deactivation effect on SCR catalyst is mainly reflected in three aspects: (1) it can

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interact with active sites vanadium or cerium to depress NH3 adsorption and activation

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on the surface Lewis acid sites; (2) it enhances the NH3 oxidization ability and the

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byproducts N2O production because of more oxygen species formation on the catalyst

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surface. (3) it blocks up the pore channel and decreases the surface area of catalyst.14-16

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Among these three factor, the first one is the major reason for activity loss. Therefore,

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the key to reduce the As deactivation is to unleash the active ingredients from the strong

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interaction with arsenic.

13

It is generally acknowledged that the arsenic

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Since low-rank coals with high calcium contents have been widely used for power

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generation, alkaline-earth metal calcium deactivation for SCR catalyst is another

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unavoidable problem. However, its influence on SCR catalyst has been proven less

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serious than that of K, Na and As.17 Yang et al. believes that CaO could promote the

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NH3 oxidation reaction. Their DRIFTS results indicate that CaO could activate

73

ammonia into the -NH species, which further react with surface oxygen to produce 5



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NO.18 Furthermore, CeO2-WO3 catalyst show higher calcium resistance than V2O5–

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WO3/TiO2. It is found that its highest NO conversion is still up to 85% at 350 °C

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despite 5wt% CaO doping, while only less than 20% conversion for V2O5–WO3/TiO2

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catalyst.19 The Ca resistance on CeO2-WO3 catalyst is primarily because amorphous

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WOx species on the catalyst surface transform into crystalline CaWO4 preferentially,

79

which protects CeO2 structure from deactivation. Meanwhile, CaWO4 structure can

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still provide parts of Brønsted acid sites for SCR reaction. Therefore, CeO2-WO3

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catalyst is a more appropriate choice for NOx emission reduction under the usual

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calcic flue gas condition. Generally, calcium and arsenic often coexist in fly ash from

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coal-fired plants. Whether there is an enhancement or offset deactivation effect on

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CeO2-WO3 catalyst remains unknown.

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In this study, fresh, single component poisoned and two component deactivated

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CeO2-WO3 catalysts are prepared and compared to analyze the synergistic effect of

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arsenic and calcium on catalysts. The effects of two poisons on the catalyst textures,

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surface acidity and reducibility are investigated using XRD, XPS, H2-TPR, O2-TPD,

89

NH3-TPD, in situ Raman, in situ TG and in situ DRIFTS. Finally, their synergistic

90

effect mechanism is also proposed and discussed in the last section.

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2. Experiments and Methods

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Catalyst Preparation and Deactivation. The CeO2–WO3 (CW) catalysts were

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prepared using co-precipitation method with ammonium tungstate and cerium nitrate

94

as precursors and ammonium hydroxide as the precipitant. Typically, the desired 6


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amount of two precursors with the 3:2 molar ratio of Ce to W were dissolved firstly in

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the DI water. Then 25% NH3·H2O was added dropwise the Ce-W solution with

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vigorous stirring to keep the pH value at 11. After continuous stirred and washed by DI

98

water, the precipitates were filter, collected and dried overnight at 100°C. The obtained

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samples were calcined at 500 °C for 4 h in air with a heating rate of 2 °C min−1. The Ca

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(Ca: 3.3 wt%) and As poisoned samples (As: 3.5 wt%) with same poison weight

101

proportion calculated by CaO and As2O3 were obtained by volumetric immersion

102

method in Ca(NO3)2 and As2O3 solutions respectively. Then the mixtures were directly

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dried at 100 °C for 10 h and then calcined at 500 °C for 3 h. The mixed As-Ca poisoned

104

samples were obtained by deactivating them first with arsenic and subsequently with

105

calcium, followed by drying and calcination. The Ca, As and mixed As-Ca poisoned

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sample were denoted as CWC, CWA and MAC respectively.

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Activity Measurements. SCR activity tests of four samples were performed using a

108

fixed-bed quartz flow reactor (Inner diameter = 6 mm) and 100 mg of the catalyst with

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40-60 meshes. SCR activity measurements were carried out in the temperature range of

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150-450 °C. The feed gas mixture was consist of 500 ppm NO, 500 ppm NH3, 5% H2O

111

(if used) and 3% O2 and N2 as the balance gas. The total flow rate was controlled to 200

112

mL·min-1, corresponding to a gas hourly space velocity (GHSV) of 120,000 mL·(g·h)-1.

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The gases concentrations of NO, NO2, NH3, H2O and N2O were simultaneously

114

monitored by an infrared gas detector (Gasmet FTIR DX-4000). The NOx conversion,

115

N2 selectivity and reaction rate constants of four catalysts were calculated with 7



116

reference to the following equation 1:

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NO x Conversion=

[NO x ]inlet − [NO x ]outlet × 100% [NO x ]inlet

  2[N2O]outlet N2 Selectivity = 1 −  ×100%  [NOx ]inlet + [NH3 ]inlet − [NOx ]outlet − [NH3 ]outlet  k =−

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F ln(1 − x) W

(1)

(2)

(3)

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Where [NOx]inlet and [NH3]inlet were the concentrations of gaseous NOx and NH3 in

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the inlet; [N2O]outlet, [NOx]outlet and [NH3]outlet were the concentration of gaseous N2O,

122

NOx and NH3 in the outlet, respectively. In addition, F represented the total flow rate

123

(cm3s−1), W was the catalyst weight (g) and x was the NOx conversion at different

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temperatures.

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Catalyst Characterization. The N2 adsorption-desorption isotherm, BET surface

126

area and pore volume of the catalysts were performed at 77 K with a physisorption

127

apparatus (Micromeritics, ASAP2020, USA). XRD patterns were carried out between

128

10° and 85° at a step length of 5°·min-1 on X-ray diffractometer (Rigaku,

129

D/max-2200/PC, Japan) with a Cu Kα radiation source (λ = 0.15405 nm). Temperature

130

programmed reduction of H2 (H2-TPR) and temperature programmed desorption of O2

131

(O2-TPD) experiments were operated on a chemisorption instrument (Micromeritics,

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ChemiSorb 2920 TPx, USA) from 50 °C to 900 °C and from 50 °C to 700 °C under a 5%

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H2/N2 and O2/N2 gas flow (50 mL·min−1) with a heating rate of 10°C·min−1 respectively.

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Before each temperature programmed test, 100 mg samples were pretreated at 300 °C

135

for 60 min under Helium. X-ray photoelectron spectroscopy (XPS) was executed using 8


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an electron spectrometer (VG Scientific, ESCALab220i-XL, UK) with 300 W Al Kα

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radiations. All the binding energies were calibrated by the C 1s line of 284.8 eV.

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In situ Raman spectra were obtained by a Raman microscope (Renishaw, InVia

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Reflex, UK) equipped with a cooled charge-coupled device (CCD) array detector, an in

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situ reaction cell and a high-grade Leica microscope (L50×). The 532 nm line of the

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laser was chosen for recording each Raman spectra. In sit thermogravimetric (TG)

142

experiments were carried out on an integrated thermal analyser (Mettler Toledo,

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TGA/DSC, CH). In situ IR spectra were obtained on a Fourier transform infrared

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spectrometer (Thermo Fisher Scientific, Nicolet 6700, USA) equipped with an MCT/A

145

detector and an in situ cell with ZnSe window. The spectra were recorded by

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accumulating 32 scans at a resolution of 4 cm-1. Prior to each in situ experiment, the

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catalyst was pretreated at 450 °C for 1 h under flowing He (100 mL min-1) then cooled

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down to room temperature. The gas used in the in situ experiment included 500 ppm

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NH3/He, 500 ppm NO/He, 5% O2/He and He.

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3. Results and Discussion

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3.1 Performances and Textural Characteristics. Figure 1(a) shows the NOx

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conversion over the fresh and three kinds of poisoned samples at 150-450 °C under a

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high GHSV of 120,000 mL·g-1·h-1. Both CW and CWC catalysts present over 90% NOx

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above 250°C, indicating excellent calcium resistance performance over CeO2–WO3

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catalyst, which is in agreement with our previous results.19 The As poisoned CWA

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catalyst exhibits the lowest activities with 60% conversion at 350°C among the four 9



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catalysts. While the NOx conversion of CMA catalyst reaches significantly up to 89% at

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350°C. The offset deactivation effect on MAC catalyst can also be found in the

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presence of 5% H2O (figure S1). On the other hand, the N2 selectivity of CWA at 450°C

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is obviously less than other catalysts (figure 1(b)), implying that byproduct N2O

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formation is accelerated by As. However, with the addition of calcium, N2 selectivity of

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As poisoned catalyst is also promoted to the level of fresh catalyst. Additionally, it

163

should be pointed out that the N2O production promotion behavior caused by As

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addition are not observed at below 400°C. Physical deactivation may be a significant

165

reason. The results of the BET surface area and pore volume on fresh and poisoned

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catalysts are provided in table 1. It can be seen that the channel plugging effect by As

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(CWA:SBET=25.7 m2/g; Vpore=0.08 cm3/g) can be obviously alleviated by Ca

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introduction (MAC:SBET=41.7 m2/g; Vpore=0.14 cm3/g). In order to avoid the physical

169

poisoning effect, the reaction rate constants (200 °C) were normalized by the surface

170

area and pore volume respectively. The lower constants of CWA catalyst (0.09

171

mmol·m−2·s−1 and 0.03 mmol·cm−3·s−1) can also be improved dramatically after Ca

172

addition (MAC: 0.24 mmol·m−2·s−1 and 0.07 mmol·cm−3·s−1). These implies that the

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serious As chemical deactivation can be alleviated by Ca as well as physical blockage

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of active sites.

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Figure S2 shows the XRD patterns of fresh and poisoned samples. The cubic

176

fluorite-type phase of CeO2 (JCPDS 41-1431) can be clearly observed for all samples.

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The XRD spectra of CWC show typical diffraction peaks for CaWO4 scheelite phase, 10


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indicating that amorphous tungsten oxides are crystallized by Ca. However, the peaks

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for CaWO4 phase almost diminish for MAC catalyst, implying that the internal

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interactions between Ca and W may be replaced by two poisons reciprocity. This

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interactions between poisons can also be proved by the results of CeO2 lattice constant

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change in table 1. The increase of the lattice constant can be seen after Ca or As

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introduction on the CW catalyst, but it is rehabilitated when two poisons coexists.

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Furthermore, the crystallite size (d) of CeO2 was also estimated from the reflection of

185

the CeO2 cubic phase using the Scherrer equation.20 In addition, fresh CW, CWA and

186

MAC have a similar crystallite size of about 17.4nm, but it increases significantly to

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18.2nm in the case of the CWA catalysts. Since no other peaks can be associated with

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As2O5 and As2O3, the CeO2 particles surface may be rough and covered with

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amorphous arsenic species, which is in an agreement with the results of reduced BET

190

surface.

191

In situ Raman spectra of the samples were recorded to investigate the interaction

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between poisons and the active ingredients under dehydration, NH3 adsorption and O2

193

atmosphere at 350 °C. As shown in figure 2, the peaks at around 454 cm-1 is assigned to

194

the F2g vibration of CeO2.21-23 While the peaks at 995 and 958 cm−1 are relative to W=O

195

stretching modes of monotungstate and isopolytungstate species respectively, the peaks

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at 908 and 327 cm−1 are attributed to the [WO4] tetrahedron unit of CaWO4, and the

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peaks at 806 and 697 cm−1 are indicative of Raman-active modes of distorted [WO6]

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units from crystalline WO3 particles.24-26 In spite of deactivation by Ca, the peaks 11



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related to CaWO4 weaken under the NH3 or O2 atmosphere for CWC catalyst, implying

200

that it can also provide parts of acid sites for NH3 adsorption. Compared with fresh

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catalyst, the WOx species of CWA catalyst almost remains unchanged, while the CeO2

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peak shifts toward lower wavenumbers from 456 to 449 cm-1. These results indicate

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that As primarily interacts with the structure of the CeOx species on CW catalyst, but Ca

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prefers to destroy the original structure of WOx species. When two poisons coexist,

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both of the CaWO4 characteristic peaks and the peak shift of CeO2 caused by As almost

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disappears. Instead, crystalline WO3 particles generate and obviously increase after

207

NH3 or O2 introduction, meaning that these WO3 particles can also participate into SCR

208

reaction. Therefore, the antidotal effect of Ca on As deactivation is good for the CeO2

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active sites restoration but is limited to recover original amorphous WOx species.

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The Ce 3d and O 1s XPS spectra of fresh and poisoned catalysts is shown in figure3

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and figure S3. The bands marked by v, v’’, v’’’, u, u’’, and u’’’ can be ascribed to 3d104f0

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state of surface Ce4+, whereas the other sub-peaks labelled v’ and u’ represent 3d104f1

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state of surface Ce3+.20 The surface Ce3+ proportion of CWA (23.1%) is the largest

214

among the four samples. It is suggested that Ce4+ species are crucial to the NH3

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adsorption (Lewis acid sites) and activation for CeO2-based catalyst.27 So the reduction

216

of Ce4+ by As species on the catalyst surface should play an important role for

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deactivation. On the other hand, CWC catalyst shows the lowest surface Ce3+

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concentration (19.3%), which is connected with the surface WOx structure changed

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thereby destroying the interaction between CeOx and WOx. The surface Ce3+ 12


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concentration of MAC catalyst (21.3%) is located between two poisoned catalysts,

221

indicating that the influence of two poisons on Ce4+ concentration can also be

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counteracted. According to the XPS spectra of O 1s, unlike other catalysts, a

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remarkable sub-band centered at approximately 531.8 eV occurs for CWA catalyst,

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which can be assigned to surface chemisorbed oxygen species.28 The higher surface

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chemisorbed oxygen species may be originated from AsOx rather than defected oxygen

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species from CeO2, since more As atoms (11%) exist on the surface at the same time.

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After the Ca addition on the catalyst, the surface concentrations of chemisorbed oxygen

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and As atoms significantly decrease with more Ca atoms forming on the surface (table

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1). These results show that Ca may interact with As species to inhibit arsenic surface

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accumulation, which is helpful for reactants adsorption.

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Additionally, the XPS spectra of As and Ca for CWA, CWC and MAC have also

232

been investigated and shown in figure S4. It can be seen that the peak positions of As 2p

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and Ca 2p for CWA and CWC are obviously lower BE than the MAC catalyst. In

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general, a shift of BE in XPS is usually associated with an electron density and

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oxidation state change. Therefore, it is suggested that As and Ca would obviously

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influence each other's valence state on MAC catalyst, which can also be regarded as a

237

direct evidence for As and Ca interaction existence. In short, the As or Ca deactivation

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effect on physical structure and surface properties can be significantly relieved by

239

mutual effect between poisons generation under coexistent conditions.

240

3.2 Reducibility and Surface Acidity. It is generally considered that redox property 13



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and surface acidity of catalysts are crucial for SCR reaction. Figure 4(a) illustrates the

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H2-TPR profiles of four catalysts. Two peaks are observed for the CW catalyst: the

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main reduction band centered at 599 °C are assigned to the reduction of Ce4+, while a

244

small high-temperature peak located at 801 °C is corresponding to the W6+

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reduction.29-31 The main peak shifts to a higher temperature at 634°C for CWC catalyst,

246

which can be due to surface Ce4+ atoms loss on the basis of XPS result above. For CWA

247

catalyst, a large H2 consumption peak responding to As (III) reduction (720 °C) occurs

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and covers the Ce4+ reduction peak with the combined reduction effect. However, the

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enhancement from As reducibility would not do SCR any favors but make higher N2O

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production at high temperature according to our previous researches.10, 13 A remarkable

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improvement in the redox property of catalyst for MAC happens with the main peak

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shifting to lower temperatures at 680 °C. This result also suggests that the strong

253

interaction between AsOx and CeOx has been weakened after Ca adding, which is in

254

agreement with Raman and XPS result.

255

O2-TPD profiles of all the samples are provided in figure 4(b). In general, the

256

stepwise desorption of O2 with the increase temperature follows the order: O2 ad →

257

O2-ad (100-200 °C) → O-ad (300-400 °C) → O2-lattice (>500 °C).32, 33 All the samples

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present two desorption peaks at around 150 °C and 360 °C. The first peak can be

259

assigned to surface oxygen species O2-ad provided by surface oxygen defect sites of

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CeO2, whereas the latter one can be attributed to O-ad species caused by charge

261

imbalance from amorphous WOx. It can be seen that the peak of O2-ad weakens with the 14


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order of CW> CWA> CMC> MAC, indicating both Ca and As introduction can affect

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the oxygen defect sites of CW catalyst. Although larger O2-ad loss is shown on MAC

264

catalyst, its catalytic performance is obviously higher than CWA catalyst. Besides the

265

positive effect of surface acidity on activity improvement, another possible explanation

266

is that O-ad species from amorphous WOx species or lattice O species from CeO2 may

267

have enough energy to participate in SCR reaction, since the SCR reaction temperature

268

is significantly higher than 200°C.34, 35 Furthermore, the changes of O-ad peak area show

269

the different trends depending on the varieties of toxicants: increase by As and decrease

270

by Ca. Based on the XRD and XPS analysis, it is reasonably believed that As species

271

promote more O-ad generation from AsOx species but Ca species destroy the amorphous

272

WOx to result in crystallized CaWO4 with more lattice oxygen formation. Therefore, it

273

is proposed that As and Ca poisoning vary the CW catalyst’s reducibility, which may be

274

an important reason for partial-recovery of SCR activity under the poisons coexistence

275

condition.

276

Figure S5 presents the IR spectra of NH3 adsorption on the four catalysts at 200 °C.

277

The negative band at 3670 and 3626 cm-1 are ascribed to mono-coordinated and

278

bridging –OH respectively.36 The bands at 3100-3400, 1580-1620 and 1160-1180 cm-1

279

represent coordinated NH3 linked to Lewis acid sites.37 While the peaks at 2800-3020,

280

1660-1680 and 1420 cm-1 are attributed to the NH4+ species chemisorbed onto Brønsted

281

acid sites.3, 38 It is seen that Ca poisoning decreases the Brønsted acid sites amount on

282

the CW catalyst, while As deactivation destroys the original Lewis acid sites. Previous 15



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283

researchers found that surface Ce4+ and Wn+ cations mainly provided Lewis acid sites

284

on CW catalyst, and amorphous WOx species were responsible for the Brønsted acid

285

sites.9, 27 On the basis of structural analysis above, the As and Ca deactivation reasons in

286

aspect of surface acidity are directly connected with surface Ce4+ species reduction by

287

AsOx and WOx species transformation into CaWO4 phase respectively.

288

NH3-TPD profiles of four catalysts in the range of 50−600 °C are shown in figure S6.

289

Fresh sample present two main desorption peaks located at 121 °C (weak acid sites) and

290

351 °C (strong acid sites). The total acidity (2.61 mmol/g) and amounts of weak acid

291

sites on CW catalyst are significantly higher than that of CWC and CWA catalyst.

292

While these acid sites loss are markedly improved, at the same time, its total acidity is

293

close to fresh level at 2.57 mmol/g for MAC catalyst. These results indicate that As-Ca

294

coexistence can significantly decrease their independent toxic effect on surface acidity

295

of CeO2-WO3 catalyst. The IR profiles of NH3 desorption were also carried out in the

296

range of 100-350 °C shown in figure 5. It can be seen that the peak position of Lewis

297

acid sites for CWA catalyst (1619 cm-1) is clearly different from CW and CWC catalysts

298

(1580 cm-1). So the Lewis acid sites on CWA may be provided by As species but not by

299

previous Ce4+. For MAC catalyst, the peak at 1619 cm-1 decreases gradually while the

300

one at 1580 cm-1 increases with the elevated temperature. This result implies that Lewis

301

acid sites of MAC catalyst provided by As is not as stable as by Ce4+, i.e., the As

302

influence on Lewis acid sites is weaker and weaker with increasing temperature. To

303

further investigate the thermal stability of two kinds of acid sites, their loss ratio with 16


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temperature by integrated peak areas were also calculated and evaluated. With the

305

addition of As, the decrease of Brønsted acidity on catalysts get more slowly. By

306

contrast, Ca doping almost have no influence on the two kinds of acid sites stability.

307

Unlike Ca, As-OH moieties can also provide additional Brønsted acid sites, which

308

changes their stability of CWA catalyst. But these acid sites given by As are proved to

309

be no use for SCR process based on the previous researches.39 Therefore, the activity

310

recovery for MAC catalyst should be directly related to the restoration of acid sites by

311

Ce4+ and WOx.

312

3.3 In Situ Studies of reaction process. In situ TG experiment was performed to

313

explore the deactivation offset mechanism under the SCR condition at 300°C. As

314

shown in figure 6, when treated with NH3 and follow purged by He, the weight of CW

315

catalyst reduces but that of deactivated catalysts increase. Generally, the mass of

316

catalyst can be raised by NH3 adsorption step (Eq.4) while decreased by NH3 oxidation

317

reaction through surface oxygen species (Eq.5). Since the molecular weight of NH3 is

318

less than H2O, its decrease is explicable after the NH3 introduction. Therefore, the mass

319

augment for CWA and CWC catalyst should be due to surface oxygen species loss

320

caused by As and Ca deactivation thereby inhibiting the step of NH3 oxidation based on

321

the O2-TPD results. After the introduction of NO, all the catalysts weights are dropped

322

significantly except CWC. According to the proposed SCR mechanism before, the NH2

323

intermediate formed by hydrogen abstraction from adsorbed NH3 species (Eq.4) can

324

react with gas phase or weakly adsorbed NO to generate the nitrosamide (NH2NO) 17



Page 18 of 37

325

intermediate which further decomposes into N2 and H2O (Eq.6).34, 40 It is considered

326

that the constant mass for CWC catalyst might be related to the NOx strengthening

327

adsorption effect caused by basic surface Ca species (Eq.7), which could counteract

328

with the mass loss of NH2 intermediate reaction (Eq.6). The DRIFT experiment of

329

NO+O2-TPD has been carried out to further support our standpoint. As shown in the

330

figure S7, CWC catalyst shows the strongest adsorption thermostability of nitrate

331

characteristic peaks at around 1277-1311 and 1507-1560cm-1 among the four catalysts,

332

since peak areas are almost unchanged with the temperature increase up to 350°C.

333

Therefore, it is believed that the NOx adsorption effect has been strengthened by basic

334

surface Ca species. This effect is adverse to the gaseous NO consumption with NH2

335

species in Eq.6, at the same time, the reserved adsorbed NxOy species (formed by Eq.7)

336

on catalyst at high temperature could also offset the mass loss caused by the Eq.6. In

337

addition, it is noted that the change of catalysts weight is not apparent when O2 is

338

purged to system. According to the Mars-van-Krevelen mechanism, the oxygen defect

339

sites on catalyst surface will be further supplemented by gas O2 resulting in the

340

oxidation of reduced Ce3+ to Ce4+ (Eq.8). But this step is usually slow, and the residual

341

adsorbed NH3 or NxOy species probably consume the gaseous O2 reaction and thus lead

342

to the mass reduction. So the slight change of catalyst mass after O2 introduction is

343

observed. Notably, the weight variation trend of MAC catalyst under different

344

atmospheric condition is same with CW catalyst, but on a minor degree. This

345

phenomenon indicates that the original reaction route is not destroyed under the 18


Page 19 of 37


346 347

coexistence condition of Ca and As.

NH g +O − Ce → O − Ce − NH → HO − Ce − NH

(4)

348

4NH g + 6Osurf → 4N g + 6H Og

349

NOg + HO − Ce − NH → H O g + N g + HO − Ce

350 351

NOg + − 1Osurf → N O 4 HO − Ce + O g → 4 O − Ce + 2H Og

(5) (6)

(7) (8)

352

In situ DRIFTS experiments for NO+O2 and pre-adsorbed NH3 species over catalysts

353

were also performed at 300°C and shown in figure 7. The NH3 adsorption bands on

354

Brønsted acid sites of CWC catalyst at 1419 cm-1 and on Lewis acid sites of CWA and

355

MAC catalysts at 1200 cm-1 are significantly weaker than CW catalyst after NH3

356

pretreatment. These results are similar to the IR results of NH3 adsorption at 200°C

357

shown before. When NO + O2 is introduced into the system, the characteristic peaks of

358

NH3 adsorption are gradually disappeared with the NxOy surface species appearance. It

359

can be seen that the disappearance rates of those peaks on MAC catalyst are faster than

360

that on CWA and CWC. Moreover, the NxOy adsorption species on MAC catalyst is

361

closed to fresh CW catalyst at 1606 and 1542 cm-1 attributed to bridging and bidentate

362

nitrate respectively.41 While those on CWC and CWA catalysts are strong adsorbed

363

NO2 at 1625-1618 cm-1 and bidentate nitrite species at 1309-1307 cm-1.42 In general,

364

these species are primarily adsorbed on Ce4+ atoms, so Lewis acid sites for NH3

365

adsorption may be occupied competitively, which brings about the activity loss. Finally,

366

the diminished –OH negative band at 3580-3700 cm-1 on CMA catalyst reappears on 19



Page 20 of 37

367

MAC catalyst after the NH3 introduction. The results above manifest that the inhibiting

368

effect on Lewis acid sites by As and Brønsted acid sites by Ca weaken under the

369

mixture of two poisons condition.

370

The DRIFT experiment on the reaction between pre-adsorbed NOx and NH3 have

371

also been carried out and presented in figure S8 . It can be seen that the peaks attributed

372

to adsorbed NOx species are displaced by the peaks assigned to NH3 adsorption on acid

373

sites for four catalysts. At the same time, the peak at around 1510 cm-1 attributed to the

374

scissoring vibration of NH2 group increases along with the emergence of the negative

375

hydroxyl peak at around 3650cm-1, which indicates that the NH2 intermediate exists

376

and participates into the SCR reaction (Eq.6). However, this phenomenon for CWA

377

catalyst is less significant than the other catalysts, meaning that the SCR reaction is

378

apparently affected by arsenic.

379

3.4 Deactivation Offset Effect Mechanism. Based on the results from XRD, Raman,

380

TPR, TPD and XPS above, the structure−activity relationship and offset effect

381

mechanism of Ca and As on CW catalysts were proposed in figure 8. Ca exhibits a

382

bigger influence on the transformation of amorphous WOx into CaWO4 on CWC

383

catalyst, but As effects the CeO2 structure and decreases the surface Ce4+ atoms

384

concentrations on CWA catalyst. For CeO2–WO3 catalyst, Brønsted acid sites mainly

385

derived from highly dispersed WOx species, while surface Ce4+ species are the main

386

Lewis acid sites.27 Therefore, the CaWO4 phase formation causes the change of

387

Brønsted acid sites on CWC catalyst compared with fresh catalyst as shown in the IR 20


Page 21 of 37


388

spectra of NH3 adsorption. By contrast, the interaction between AsOx and CeOx triggers

389

the reduction of partial Ce4+ atoms, which results in the Lewis acid sites loss. In

390

addition, redox sites of the CW catalyst are also provided by active oxygen species

391

from O−Ce4+ (Eq.2), so the catalytic performance on CWA is worse than CWC. When

392

arsenic and calcium are doped together on the CW catalysts, the CaWO4 phase almost

393

disappears (XRD) and the change of F2g vibration position of CeO2 is recovered

394

(Raman). Meanwhile, surface Ce4+ proportion and oxygen defect sites amount are also

395

restored to a certain degree for MAC catalyst. All these above play important roles on

396

activity promotion under the two poisons coexistence. But there is still a sharp divide

397

on catalytic activity between MAC and fresh CW catalyst. We determine the reasons

398

from three viewpoints: (1) a nonreversible change of the surface area and pore volume

399

on MAC catalyst is observed, which restricts the reactant molecules adsorption; (2)

400

Crystalline WO3 particles with distorted [WO6] units forms, which makes the

401

interaction between Ce and W weakens; (3) surface oxygen species O2-ad and oxygen

402

defect sites of CeO2 decrease, so the NH3 activation step (Eq.2) is inevitably limited.

403

Notes

404 405

The authors declare no competing financial interest.

Acknowledgments

406

This work was financially supported by Beijing Natural Science Foundation

407

(8182033) and National key research and development program (2017TFC0211800

408

and 2016TFC0209200). 21



409 410

Page 22 of 37

Supporting information Associated figures and tables are provided. This information is available free of

411

charge via the Internet at http://pubs.acs.org/.

412

References

413

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414

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Keung, M.; Lievens, J.; Jacobs, P. A., NOx abatement in exhaust from lean-burn

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(3) Topsoe, N. Y.; Topsoe, H.; Dumesic, J. A., Vanadia-titania catalysts for selective

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selective catalytic reduction of NO with NH3. Catal. Sci. Technol. 2016, 6, (5),

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1248-1264.

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(5) Qi, G. S.; Yang, R. T.; Chang, R., MnOx-CeO2 mixed oxides prepared by

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Appl. Catal., B 2004, 51, (2), 93-106. 22


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Oxides for Selective Catalytic Reduction of NO with NH3: Confirmation of Ce–O–Ti

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Active Sites. Environ. Sci. Technol. 2012, 46, (17), 9600-9605.

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Arandiyan, H., CeO2-WO3 Mixed Oxides for the Selective Catalytic Reduction of

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NOx by NH3 Over a Wide Temperature Range. Catal. Lett. 2011, 141, (12),

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activity of MoO3–CeO2 catalysts for NO removal: influences of acidity and

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of arsenious oxide with DeNOx-catalysts: An X-ray absorption and diffuse reflectance

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infrared spectroscopy study. J. Catal. 1991, 129, (1), 168-176.

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Catalysts. J. Phys. Chem. C 2016, 120, (32), 18005-18014.

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ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3.

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(18) Yang, X.; Zhao, B.; Zhuo, Y.; Gao, Y.; Chen, C.; Xu, X., DRIFTS Study of

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Ammonia Activation over CaO and Sulfated CaO for NO Reduction by NH3. Environ.

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70-79.

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promoting role of process on the CeO2–WO3/TiO2 catalyst for NO reduction with NH3

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at low-temperature. J. Colloid Interface Sci. 2015, 448, (0), 417-426.

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(21) Vuurman, M. A.; Wachs, I. E.; Hirt, A. M., Structural determination of

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supported vanadium pentoxide-tungsten trioxide-titania catalysts by in situ Raman

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spectroscopy and x-ray photoelectron spectroscopy. J. Phys. Chem. C 1991, 95, (24),

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9928-9937.

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(22) Madia, G.; Elsener, M.; Koebel, M.; Raimondi, F.; Wokaun, A., Thermal

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stability of vanadia-tungsta-titania catalysts in the SCR process. Appl. Catal., B 2002,

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39, (2), 181-190.

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(23) Bond, G. C.; Tahir, S. F., Vanadium oxide monolayer catalysts Preparation, characterization and catalytic activity. Appl. Catal. 1991, 71, (1), 1-31.

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(24) Picquart, M.; Castro-Garcia, S.; Livage, J.; Julien, C.; Haro-Poniatowski, E.,

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Structural studies during gelation of WO3 investigated by in-situ Raman spectroscopy.

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J. sol-gel sci. technol. 2000, 18, (3), 199-206.

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(25) Ross-Medgaarden, E. I.; Wachs, I. E., Structural determination of bulk and

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surface tungsten oxides with UV-vis diffuse reflectance spectroscopy and Raman

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spectroscopy. J. Phys. Chem. C 2007, 111, (41), 15089-15099.

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structure–surface acidity relationships of model-supported tungsten oxide catalysts. J. 25



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for SCR of NOx with NH3: An in situ IR and Raman spectroscopy study. Appl. Catal.,

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McDonald, C. A.; Smirniotis, P. G., Influence of elevated surface texture hydrated

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oxygen-rich conditions. J. Catal. 2015, 325, 145-155.

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Vanadia and tungsta grafted on TiO2: influence of the grafting sequence on structural

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and chemical properties. Appl. Catal., A 2000, 198, (1–2), 155-169.

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(31) Engweiler, J.; Harf, J.; Baiker, A., WOx/TiO2 catalysts prepared by grafting of

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tungsten alkoxides: Morphological properties and catalytic behavior in the selective

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reduction of NO by NH3. J. Catal. 1996, 159, (2), 259-269.

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(32) Wang, B.; Chi, C.; Xu, M.; Wang, C.; Meng, D., Plasma-catalytic removal of

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toluene over CeO2-MnOx catalysts in an atmosphere dielectric barrier discharge.

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Catalytic oxidation of vinyl chloride emissions over Co-Ce composite oxide catalysts. 26


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Chem. Eng. J. 2017, 315, 392-402.

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Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium-Based

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529

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534

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FT-IR study. Catal. Today 1996, 28, (4), 373-380. (41) Hadjiivanov, K. I., Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev. 2000, 42, (1-2), 71-144.

538

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539

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540

deposit for reforming CH4 with CO2. J. Chem. Technol. Biot. 2014, 89, (3), 372-381.

541 542 543

28


Page 29 of 37


Table 1. Physical structures and surface properties of four catalysts.

544

Sample

545 546 547 548 549 550

BET (m2/g)

Pore volume (cm3/g)

Lattice

k200°C

k'200°C

(mmol/m2·s)

(mmol/cm3·s)

constant (Å)

Crystallite sizea (nm)

Position of CeO2 F2gb (cm-1)

CW

63.9

0.12

0.49

0.26

5.407

17.5

456

Cac

Asc

Cad

Asd

Aciditye

(%)

(%)

(wt%)

(wt%)

(mmol/g)

—

—

—

—

2.61

CWA

25.7

0.08

0.09

0.03

5.410

18.2

449

—

11

—

2.3

2.49

CWC

54.1

0.13

0.29

0.12

5.417

17.4

454

2.93

—

2.25

—

1.65

MAC

41.7

0.14

0.24

0.07

5.406

17.3

456

3.2

4.5

1.92

2.07

2.57

a

Calculated by the CeO2 (200) plane. Obtained by Raman. c Atomic percent calculated by XPS. d Mass percent calculated by ICP e Calculated by NH3-TPD. b

551

29



Page 30 of 37

552 553

Figure 1. NOx conversion (a) and N2O production (b) of four catalysts. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 3%, total flow

554

rate = 200 mL/min, GHSV = 120,000 mL/(g·h).

555 556

30


Page 31 of 37


557 558

Figure 2. In situ Raman patterns of four catalysts at 350 °C under different conditions: (a) Dehydration, (b) NH3 atmosphere and (c) O2

559

atmosphere.

31



Page 32 of 37

560

561 562

Figure 3. Ce 3d XPS spectra of four kinds of catalysts.

32


Page 33 of 37


563

564 565

Figure 4. H2-TPR (a) and O2-TPD (b) curves of four catalysts.

566 567

33



Page 34 of 37

568 569

Figure 5. NH3 desorption IR spectra and acid sites loss ratio of four catalysts in the temperature range of 100 to 350 °C.

34


Page 35 of 37


570 571

Figure 6. In situ TG experiment over four catalysts under different gas conditions.

35



Page 36 of 37

572 573

Figure 7. In situ DRIFT spectra of CW (a), CWC (b), CWA (c) and MAC (d) catalysts at 300 °C under the atmosphere that the dehydrated

574

catalyst is first treated by NH3, then NO + O2 is added. 36


Page 37 of 37

575 576


Figure 8. Schematic of the structure−activity relationship and offset effect mechanism of arsenic-calcium deactivation on CeO2-WO3 catalyst.

37


Extraordinary Deactivation Offset Effect of Arsenic ... - ACS Publications

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