Catalytic ozonation of NO with low concentration ozone over recycled

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Kinetics, Catalysis, and Reaction Engineering

Catalytic ozonation of NO with low concentration ozone over recycled SAPO-34 supported iron oxide Bing Liu, Xiaochen Xu, Lifen Liu, Wenchen Dai, Hongbin Jiang, and Fenglin Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04941 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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Industrial & Engineering Chemistry Research

Catalytic ozonation of NO with low concentration ozone over recycled SAPO-34 supported iron oxide Bing Liu, Xiaochen Xu*, Lifen Liu, Wenchen Dai, Hongbin Jiang, Fenglin Yang Key Laboratory of Industrial Ecology and Environmental Engineering, MOE, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China

Corresponding author. Tel.: +86 411 84706328; fax: +86 411 84706328. E-mail address: [email protected]. (X. Xu).

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A series of SAPO-34-supported iron oxide (FeOx/SAPO-34) catalysts were prepared by a two-step liquid ion-exchange method. The effects of the exchange time and iron nitrate concentration on the structure and properties of the catalysts were investigated. The results showed that the FeOx/SAPO-34 catalysts significantly enhanced the ozonation of NO. The 0.05-FeOx/SAPO-34-15 catalyst prepared by mixing NH4+/SAPO-34 with 0.05 M iron nitrate solution for 15 h showed the most effective NO oxidation. Oxygen vacancies acted as the active sites for the catalytic oxidation of NO. The electron paramagnetic resonance results showed that the hydroxyl and superoxide radicals generated from ozone and the oxygen vacancies on the surface of the catalysts promoted the oxidation of NO. The 0.05-FeOx/SAPO-34-15 catalyst has a certain stability when the volume fraction of water vapor was in the range of 08%.

Keywords: FeOx/SAPO-34 catalysts; NO oxidation rate; oxygen vacancies; superoxide radicals; hydroxyl radicals.

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1. Introduction Nitrogen oxides (NOx) are toxic air pollutants and cause acid rain, photochemical smog, and ozone destruction. Among the multifarious methods employed to reduce NOx emissions, selective catalytic reduction (SCR) is the most widely adopted approach. However, since the optimum temperature range for NO reduction is 250‒450 °C1-3, this method suffers from temperature constraints and limited efficiency. The flue gas temperature for a large-scale coal-fired boiler is about 120‒150 °C (after dust removal), which is significantly lower than the active temperature range for industrial SCR. Hence, in order to meet the reaction temperature requirement of the SCR process, the flue gas temperature should be increased. This can be achieved by using a large amount of power. Hence, it is imperative to develop low-temperature methods for NOx removal. The low-temperature oxidation for NOx control method involves the oxidation of insoluble NO and NO2 to N2O5 (a high solubility species of NOx) by adding ozone to realize the effective removal of NOx using wet scrubbers4. However, this method requires large amounts of ozone, and hence is costly. The use catalysts can reduce the amount of ozone required for the oxidation of NO and NO2. Therefore, the catalytic ozonation of NOx (with various catalysts) has been extensively investigated. Most of the catalytic ozonation systems reported till date involve hydroxyl radicals, which improve the oxidation efficiency of the systems5-8. For example, Ding et al. investigated the ozonation properties of fluoride-doped nanostructured cerium and titanium-mixed oxides and found that oxygen vacancies play an important role in the generation of hydroxyl radicals9. Zhao et al. used copper ferrite (CuFe2O4) for the ozonation of NOx and found that an increase in the density of surface hydroxyl groups and oxygen

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vacancies was critical for the production of hydroxyl radicals and the enhancement of the ozonation efficiency10. Recently, several studies have demonstrated that catalysts (such TiO2 or MnOx loaded spherical alumina catalyst) promote the ozone oxidation of NOx to N2O511, 12. NO2 is oxidized to N2O5 at O3/NO molar ratios higher than 1. The use of a catalyst can reduce the NO2 concentration, reaction time, and amount of ozone required for the oxidation. However, the deep ozone oxidation of NO requires a large amount of ozone, thus producing residual ozone. There have been very few reports on the catalytic oxidation of NO using low ozone concentrations (i.e., at O3/NO molar ratios < 1). Catalysts can promote NO oxidation even at low ozone concentrations. In addition, by using catalysts, desulfurization and denitrificaion can be achieved simultaneously along with NO pre-oxidation. In this study, silico-alumino-phosphate-34 (SAPO-34) molecular sieves were produced using SAPO-34 crystallization mother liquor. These SAPO-34 molecular sieves were used as the support for iron oxide catalysts, which were used for oxidizing NO. The silicon-aluminum ratio of the raw material was higher than that of the product. SAPO-34 zeolite catalysts can be synthesized by adding aluminum, phosphorus, and silicon to SAPO-34 mother liquor templates13. This approach is not only environmentally friendly but is also effective for making the full use of SAPO-34 mother liquor. Due to its micro pore structure, high hydrothermal stability and as well as superior activity over a wide temperature range, Cu/SAPO-34 has received substantial attention for eliminating NOx pollution by selective catalytic reduction (SCR)14, 15. However, the study of application aspects on catalytic ozonation of SAPO34 has been reported rarely. Catalytic ozonation utilizes catalysts to improve the decomposition of ozone and enhanced the generation of free radicals. Iron-containing 4


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catalysts have received increasing attention since they are environment-friendly, inexpensive, and chemically active materials16, 17. What’s more, it has been determined recently that Fe can help the catalysts resist water in ozone decomposition18, 19. In this study, we prepared a series of FeOx/SAPO-34 catalysts for ozone oxidation (at low ozone concentrations) of NO. We also investigated the effect of the preparation conditions on the catalytic ozonation of NO. Preparation conditions such as the ionexchange period and ferric nitrate solution concentration were optimized. The effect of the H2O volume fraction on the rate of NO oxidation was also investigated. The physicochemical properties and NO oxidation efficiency of the catalysts were investigated using X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) measurements,

scanning

electron

microscopy

(SEM),

X-ray

photoelectron

spectroscopy (XPS), Inductively Coupled Plasma (ICP), temperature-programmed desorption of NO (NO-TPD), and electron paramagnetic resonance (EPR). We also investigated the NO oxidation mechanism of the catalysts.

2. Materials and methods 2.1. Preparation of the SAPO-34 support SAPO-34 molecular sieves were hydrothermally synthesized using triethylamine (TEA), a silica solution, and SAPO-34 crystallization mother liquor. The crystallization mother liquor was obtained from a Dalian company producing methanol-to-olefin catalysts. The chemical composition of the mother liquor is given in Table 1. A certain amount of silica solution was added to the mother liquor. The resulting gel was stirred in a stainless steel autoclave and TEA was added to it. The Al2O3:P2O5:SiO2:TEA molar ratio of the resulting mixture was 1:1:0.16:3. A certain amount of silica sol (27 wt.% SiO2) was added to the SAPO-34 mother liquor, and the mixture was stirred for 5 min to form a gel. A desired amount of TEA was then added to this gel. Finally, the gel was 5



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sealed in the autoclave and was heated at 200 °C for 6 h. It should be noted that the crystallization time required in this study was shorter than that required by traditional methods (12‒72 h)20, 21. Residual SAPO-34 zeolites in mother liquor can serve as seeds, and hence can reduce the synthesis time22-24. After the crystallization, the product was separated from the mother liquor by centrifugation, washed with water until the pH became ~7, dried at 105 °C overnight, and calcined in air at 550 °C for 4 h to remove the organic template. Table 1 Chemical composition of the mother liquor. composition

mol %

Al2O3

1

SiO2

0.07

P2O5

1

TEA

2.2

H2O

93.8

Residual SAPO-34 and other substances

1.0 (wt.%)

2.2. Preparation of the FeOx/SAPO-34 catalysts A series of FeOx/SAPO-34 catalysts with different ion-exchange times were prepared by a two-step liquid ion-exchange method2. First, NH4+/SAPO-34 was prepared by ion exchange (with NH4+) by suspending 10 g of the as-synthesized SAPO34 in 100 mL of a 27 wt.% ammonium nitrate (Alfa Aesar, >95%) solution at 80 °C for 3 h. After the ion exchange, the sample was filtered and rinsed with deionized water. NH4+/SAPO-34 was then dried at 105 °C for 16 h before repeating the ammonium exchange process for twice. Subsequently, Fe ion-exchange was performed by mixing 10 g of NH4+/SAPO-34 with 200 mL of 0.03 M Fe(NO3)3 at 80 °C under continuous stirring for different durations (12, 15, 20, and 24 h). The obtained powders were then filtered and washed thoroughly with distilled water, dried at 105 °C for 12 h, and 6


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calcined at 550 °C for 4 h. The NH4+/SAPO-34 sample was also calcined at 550 °C in air for 4 h. The structural and catalytic properties of this sample were also evaluated. All the catalysts were pressed, rushed, and then sieved to 8‒12 meshes. The catalysts were denoted as FeOx/SAPO-34-x, where x represents the ion-exchange reaction time. To further investigate the effect of the Fe(NO3)3 concentration, Fe-loaded samples with various Fe(NO3)3 concentrations (0.01, 0.03, 0.05, and 0.1 M) were prepared from NH4+/SAPO-34. The samples were denoted as y-FeOx/SAPO-34, where y represents the Fe(NO3)3 concentration. 2.3. Characterization Various characterization techniques were used to investigate the physical and chemical properties of the prepared samples. The specific surface area (BET) of the samples was estimated from their N2 adsorption/desorption isotherms obtained at the liquid-nitrogen temperature using a Quantachrome SI gas sorption analyzer. The XRD patterns of the samples were obtained using a LabX-6000 X-ray powder diffractometer (Shimadzu) with a Cu Kα radiation source (λ = 0.154056 nm) and a 2θ scan rate of 5°/min over a 2θ range of 5‒90°. ICP-AES (OPTIMA 2000 DV) was used to determine the iron content of the synthesized samples. The SEM images the samples were obtained using a scanning electron microscope (KYKY-2000B) and their composition was investigated by energy dispersive spectroscopy (SEM-EDS). The XPS spectra of the samples were recorded using a photoelectron spectrometer (AMICUS, Shimadzu, Japan) with a standard Al Kα source. The photoluminescence

(PL) spectra of the

catalysts were obtained using w a FL4500 fluorometer spectrophotometer with a Xenon lamp as the excitation source. Ultraviolet and visible diffuse reflectance spectroscopy (UV-Vis DRS) analysis was carried out on a JASCO V-550 spectrometer and the spectra were recorded over the wavelength range of 200‒800 nm. The EPR spectra of 7



the samples were recorded on a Bruker ELEXSYS E500 X-band (~9.8 GHz) spectrometer at ambient temperature. NO-TPD was carried out in a quartz reactor using 0.1 g of the catalysts. Prior to the test, each catalyst was heated in a pure He flow at 573 K for 1 h and was then cooled down to 373 K. Then, the 5% NO/He flow was switched on and the sample was saturated at 373 K for 1 h. The system was then cooled down to 293 K and was purged with He. Finally, desorption was carried out at 773 K (heating rate = 10 °C /min) under the flow of He. 2.4. Catalytic activity The NO oxidation set up used in this study is shown in Figure 1. All the catalytic activity experiments were carried out in this setup. The setup consisted of a gas supply, a reactor, and an analytical system. First, the reaction gas was prepared by mixing NO and N2 in cylinders. Ozone was generated using an ozonator (model 1000BT-12, Shanghai Enaly Mechanical and Electrical Technology Company) by arc discharge under the flow of O2, and the ozone concentration was adjusted by varying the voltage. A certain amount of water was added to the mixed gas using an injection pump. The total flow of all the reactants was fixed at 500 mL/min, and the flow of the inlet ozone was 50 mL/min. The initial gas concentrations used for the tests were NO = 536 mg/m3, O3 = 0.87 mg/L, and H2O = 4%. The reactor used was cylindrical in shape (40 mm inner diameter and 350 mm height) and was made up of stainless steel. The reactor was thermostatted at the desired reaction temperature (40–120 °C) using an oil bath. The catalysts (4 g) were placed in the reactor and the residence time was about 1 s. The concentration of NOx was monitored continuously using a NO-NO2-NOx analyzer (Model 42i-HL). The NO oxidation rate was calculated using the following relation: NO oxidation rate = 100% × (Cin − Cout) / Cin. All the experiments were carried out in triplicate and the mean values were reported. 8


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Figure 1. Experimental setup for NO oxidation by ozone catalytic oxidation.

3. Results and Discussion 3.1. Characterization of the catalyst support The XRD patterns of the as-synthesized SAPO-34 and NH4+/SAPO-34 samples are shown in Figure 2(a). The peaks at 2θ = 9.5°, 16°, and 20.5° could be indexed to SAPO-3425,

26.

This indicates that the NH4+/SAPO-34 support was successfully

synthesized and that the NH4NO3 ion-exchange process did not affect the main structure significantly. The SEM image of NH4+/SAPO-34 (Figure 2(b)) shows that this sample crystallized with a cubic morphology. SAPO-34 was highly crystalline and showed no amorphous phase. The physical properties of the as-synthesized SAPO-34, NH4+/SAPO-34, and FeOx/SAPO-34 samples are listed in Table 2. The NH4+/SAPO34 sample showed a large surface area and high porosity, and hence was found to be a suitable catalyst support.

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Figure 2. (a) XRD patterns of the as-synthesized SAPO-34 and NH4+/SAPO-34; (b) SEM image of NH4+/SAPO-34. 3.2. Effect of the ion-exchange time on the performance and properties of the catalysts Among all the MOx/SAPO-34 catalysts (M = Fe, Mn, Co, Ce) compared in Figure S1, FeOx/SAPO-34 showed the strongest catalytic activity over the entire temperature range of 40-120 °C. This indicates that Fe promotes the ozone oxidation of NO significantly. The FeOx/SAPO-34 catalysts were obtained by varying the exchange time (12, 15, 20, and 24 h) during the two-step liquid ion-exchange process. The oxidation rate of NO was measured as a function of the steady temperature (40–120 °C) and the results are shown in Figure 3(a). The NO oxidation rate decreased with an increase in the reaction temperature. This can be attributed to the temperature dependence of the active substances present in the catalyst10. It can be observed that the oxidation rate increased as the exchange time increased from 12 to 15 h. However, a further increase in the exchange time to 24 h resulted in a decrease in the oxidation rate. With an increase in the exchange time, the distribution Fe on the catalyst surface became more homogeneous, which increased the oxidation rate. However, a further increase in the exchange time damaged the crystal structure of the catalysts, which reduced the oxidation rate. Various characterization tools were used to confirm this. 10


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The XRD patterns of the FeOx/SAPO-34-x samples are shown in Figure 3(b). All the peaks could be indexed to SAPO-3426, 27. In the case of the samples ion-exchanged for more than 20 h, the peaks at about 9.5°, 20.5°, and 31° became less intense, indicating that the crystallinity of FeOx/SAPO-34 decreased at higher exchange times. It can be stated that the crystal structure of the catalysts damaged at longer exchange times. This can be attributed to the reduction of the oxidation rate at longer exchange times. Moreover, the peaks corresponding to FeO and Fe2O3 were not observed, indicating that either Fe was well distributed on the surface of SAPO-34 or the concentration of Fe was very low. The BET results of the SAPO-34 and FeOx/SAPO34-x samples are summarized in Table 2. As the ion-exchange time increased from 12 to 15 h, the BET surface area and pore volume of the samples increased to the maximum values. In the case of the FeOx/SAPO-34-15, FeOx/SAPO-34-20, and FeOx/SAPO-3424 samples, the BET surface area and pore volume decreased with an increase in the exchange period. This indicates that the ion-exchange reaction affected the pore structure of the catalysts significantly. This is in consistence with the XRD results. The morphology and size of the synthesized FeOx/SAPO-34 catalysts were examined by SEM. Figure S2 shows the SEM images of the FeOx/SAPO-34-x samples prepared at different exchange times. It can be observed that before the ion-exchange treatment, the catalysts showed a morphology similar to that of the parent zeolites. This suggests that the catalysts showed a cubic crystalline structure with a crystal size of 2–3 µm.

Table 2 BET surface area, pore structure, and Fe loading of the as-synthesized SAPO34 and FeOx/SAPO-34 catalysts. 11



Sample

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SBET (m2/g）

SAPO-34

Fe Loading (wt.%) -

395.296

Pore volume (cm3/g) 0.090

NH4+/SAPO-34

-

378.976

0.071

FeOx/SAPO-34-12

1.16

394.739

0.111

FeOx/SAPO-34-15

1.22

422.611

0.177

FeOx/SAPO-34-20

1.05

408.895

0.126

FeOx/SAPO-34-24

0.92

312.804

0.076

0.01-FeOx/SAPO-34

1.02

416.314

0.168

0.03-FeOx/SAPO-34

1.22

422.611

0.177

0.05-FeOx/SAPO-34

2.49

425.092

0.215

0.1-FeOx/SAPO-34

2.30

416.723

0.211

XPS measurements were carried out to examine the Fe oxidation states and composition of the catalysts. The survey spectrum (Figure S3(a)) clearly indicated the presence of Al, Si, P, Fe, and O. As shown in Figure 3(c), all the catalysts showed two peaks at 711 ± 0.5 and 725 ± 0.5 eV attributing to the ionization of Fe 2p3/2 and Fe 2p1/2 electrons28, 29. The Fe 2p3/2 peak could be deconvoluted into two peaks at 710 and 712 eV corresponding to Fe2+ and Fe3+, respectively30, 31. The Fe2+/Fe3+ molar ratios of the catalysts are given in Table 3. The FeOx/SAPO-34-15 catalyst showed the highest Fe2+/Fe3+ molar ratio (0.63). FeOx/SAPO-34-24 on the other hand, showed a Fe2+/Fe3+ ratio of only 0.43. This suggests that in the case of the FeOx/SAPO-34-15 catalyst, the partial transformation of Fe (III) to Fe (II) was more. It is known that the generation of oxygen vacancies in FeOx is strongly affected by the presence of Fe2+. The change in the oxidation state from Fe3+ to Fe2+ indicates that oxygen vacancies were present in the crystal lattice. The O 1s spectra shown in Figure S3(b) could be deconvoluted into three distinct peaks corresponding to lattice oxygen Oɑ (at 531.5 eV), chemisorbed oxygen Oβ (at 532.5 eV), and surface oxygen of the hydroxyl species Oγ (at 533 eV). The integrated peak area ratios are given in Table 332, 33. Among all the FeOx/SAPO12


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34 catalysts, the FeOx/SAPO-34-15 catalyst showed the highest Oβ/Oɑ ratio. It has been reported previously that the number of Oβ molecules increases with an increase in the number of oxygen vacancies33, 34. We believe that this might be the main reason for the excellent catalytic activity of FeOx/SAPO-34-15. The Fe 2p3/2 and Fe 2p1/2 bands of the fresh 0.05-FeOx/SAPO-34-15 catalyst and the 0.05-FeOx/SAPO-34-15 catalyst after reacting 25 h (reaction temperature, 80 °C; H2O, 8%) are shown in Figure 3(d). It is worthy of note that the Fe 2p3/2 peak binding energy for the catalyst after 25 h are shifted apparently to higher binding energy compared to the fresh catalyst. The higher bingding energy means that some of Fe2+ was oxidized to Fe3+ during a long reaction35. This could also be a strong evidence for the decrease in the oxygen vacancies36.

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Figure 3. (a) Effect of the exchange time on the performance of the catalysts; (b) Powder XRD patterns of the FeOx/SAPO-34-x catalysts; (c) Fe 2p XPS spectra of the FeOx/SAPO-34-x samples; (d) Fe 2p XPS spectra of the fresh 0.05-FeOx/SAPO-34-15 and 0.05-FeOx/SAPO-34-15 catalyst after reacting 25 h. Table 3 Fen+ content, Fe valence-state ratios, and oxygen species distribution of the obtained samples. Sample

FeOx/SAPO-34-12 FeOx/SAPO-34-15 FeOx/SAPO-34-20 FeOx/SAPO-34-24 0.01-FeOx/SAPO-34

Fe 2p3/2 Fe2+ (%) 37.7 4 38.5 0 36.5 8 29.9 4 26.9 7

O 1s

Fe3+ (%) 62.26

Fe2+/Fe3+

61.50

0.63

63.42

0.58

70.06

0.43

73.03

0.37

0.61

Oα (%) 31.4 5 29.5 0 30

Oβ Oγ (%) (%) 39.14 29.41

Oβ/Oα

40.55 29.95

1.37

39.78 30.22

1.33

33.8 5 31.9 5

38.58 27.57

1.14

40.23 27.82

1.26

14


1.24

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0.03-FeOx/SAPO-34

38.5 0 0.05-FeOx/SAPO-34 40.8 1 0.1-FeOx/SAPO-34 39.5 6 3.3. Effect of the Fe(NO3)3

61.50

0.63

29.5 40.55 29.95 1.37 0 59.19 0.69 28.8 40.90 30.26 1.42 4 60.44 0.65 29.2 40.88 29.92 1.40 0 solution concentration on the performance and

properties of the catalysts In order to investigate the effect of the Fe(NO3)3 solution concentration on the performance of the FeOx/SAPO-34-15 catalyst, various Fe(NO3)3 concentrations were used. Figure 4(a) shows the NO oxidation behavior of the FeOx/SAPO-34 catalysts synthesized at various concentrations. The NO oxidation rate increased when the Fe(NO3)3 solution concentration was increased up to 0.05 M, indicating that this catalyst showed the optimal catalytic activity. However, an opposite trend was observed when the concentration was further increased to 0.1 M. This can be attributed to the excess Fe3+ didn’t substitute NH4+ and they were washed thoroughly with deionized water. In other words, excessive Fe(NO3)3 could not increase the amount of Fe. Further investigation was carried out to confirm this. The Fe content of each y-FeOx/SAPO-34 sample was determined by ICP-AES (Table 2). When the Fe(NO3)3 concentration was increased to 2.49 wt.%, the iron content of the catalysts first increased and then decreased to 2.30 wt.%. This indicates that the maximum Fe loading was achieved at the Fe(NO3)3 concentration of 2.49 wt.%. At the Fe(NO3)3 concentration of 0.1 M, the excess iron was washed away with the filtrate. The structure of the y-FeOx/SAPO-34 catalysts was observed by XRD. As shown in Figure S4, all the samples showed peaks corresponding to SAPO-34, indicating that the samples were crystalline. Figure 5 shows the SEM images of the FeOx/SAPO-34 catalysts with various Fe(NO3)3 concentrations. For these catalysts, many large particles (0.1–1 µm) were clearly observed on the external surface of the 15



SAPO-34 support. The number of these particles increased with an increase in the Fe(NO3)3 concentration. In the case of the FeOx/SAPO-34 catalyst with 0.05 M Fe(NO3)3, the Fe content of the large particles was 1.46 wt.%, while that of the cubic crystal was 0.42 wt.% (Figure S5). Therefore, it can be stated that the external surface particles were mainly composed of Fe-containing species. In order to investigate the NO adsorption properties of the prepared catalysts, NO-TPD experiments were carried out. Figure 4(b) shows the NO-TPD profiles of the NH4+/SAPO-34 and y-FeOx/SAPO-34 catalysts. Both the samples showed two main desorption peaks: one at around 120 °C and the other at around 400 °C. This indicates that NO adsorption occurred at different sites on NH4+/SAPO-34 and FeOx/SAPO-34. The first peak, at around 120 °C can be attributed to the physical desorption of NOx and the decomposition of the chemical desorbed of NOx37. The peaks obtained at temperatures higher than 300 °C can be attributed to the multiple desorption of bidentate and bridging nitrates38, 39. Furthermore, the introduction of FeOx into the SAPO-34 catalysts resulted in enhanced NO adsorption. The 0.05-FeOx/SAPO-34 showed a remarkably larger peak area than the other y-FeOx/SAPO-34 catalysts especially at low temperatures, indicating that the NO adsorption capacity of this catalyst was larger than that of the other catalysts. In addition, the absorbed species could be easily desorbed in the case of this catalyst. The NO oxidation rate depends on the weakly adsorbed NO, which is a prerequisite of catalytic oxidation of NO40-42. The optimum concentration of Fe(NO3)3 facilitated the enrichment and high dispersion of FeOx on the surface of NH4+/SAPO-34. Therefore, the 0.05-FeOx/SAPO-34 catalyst consisted of a large number of active sites, which contributed to its higher NO adsorption capacity. The high resolution Fe 2p and O 1s spectra of the catalysts are shown in Figure S6(a) and S6(b), respectively. It can be clearly observed that the O 1s binding energy 16


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changed from 532.6 to 532.3 eV with an increase in the Fe content, indicating that the accumulation of oxygen vacancies increased the density of the electron cloud. The 0.05FeOx/SAPO-34 catalyst showed the highest Fe2+/Fe3+ (0.69) and Oβ/Oɑ (1.42) molar ratios (Table 3). The XPS results showed that the optimum concentration of Fe(NO3)3 resulted in the formation of a large number oxygen vacancies on the surface of FeOx/SAPO-34. To further examine the presence and concentration of the oxygen vacancies, EPR, which is a sensitive method to investigate the behavior of oxygen defects, was carried out. As can be seen from Figure 4(c), the spectra of all the catalysts showed two distinct signals at effective g values of 4.3 and 2.0. In zeolites, the line at g = 4.3 corresponds to the Fe3+ sites at tetrahedral framework positions43. The broad signal at g = 2.0 on the other hand corresponds to cluster-shaped Fe oxides44. As can be seen from Figure 4(c), the intensity of the signal at g = 2.0 increased with an increase in the iron content, and the 0.05-FeOx/SAPO-34 catalyst showed the highest intensity. This is consistent with the ICP results. A detailed analysis revealed the presence of a paramagnetic center in the EPR signal (see Figure 4(d)). As reported previously, g = 2.004 corresponded to the electrons trapped in the oxygen vacancies45, 46. All the catalysts showed a resonance signal at g = 2.004 corresponding to the unpaired electrons trapped in the surface oxygen vacancies. With an increase in the Fe contents, the intensity of the EPR signals increased. The 0.05-FeOx/SAPO-34 catalyst showed the strongest EPR signal. This is consistent with the XPS results.

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Figure 4. (a) Effect of the Fe(NO3)3 concentration on the performance of the catalysts; (b) NO-TPD profiles of the NH4+/SAPO-34 and y-FeOx/SAPO-34 samples; (c) EPR spectra of the y-FeOx/SAPO-34 catalysts; (d) EPR spectra of the oxygen vacancies of the y-FeOx/SAPO-34 catalysts.

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Figure 5. SEM images of the y-FeOx/SAPO-34 catalysts (a) 0.01-FeOx/SAPO-34, (b) 0.03-FeOx/SAPO-34, (c) 0.05-FeOx/SAPO-34, and (d) 0.1-FeOx/SAPO-34. 3.4. Effect of water vapor on the NO ozonation and stability of the catalysts There are two main effects of water vapor on the catalytic ozonation reaction: one is the formation of hydroxyl radicals which are generated by the interplay of ozone and water vapor on the catalyst surface, promoting the oxidation of NO8, and the other is the formation of a liquid film by the accumulation of water molecules, preventing the contact between ozone and the surface active sites, and thus decreasing the catalytic activity47, 48. The stability of the 0.05-FeOx/SAPO-34 sample was investigated under 0– 8% of water volume fraction and the result is shown in Figure 6. The experiments were carried out for almost 25 hours. The catalytic activity did not change significantly without water vapor. The addition of water vapor accelerated NO oxidation in 20 hours, indicating that the catalytic activity of the FeOx/SAPO-34 samples could be enhanced by some adding species such as ∙OH radicals with a much higher oxidizing ability than O3. However, a slight decrease in the catalytic activity was found within 20 to 25 hours 19



when the volume fraction of water vapor was 4%. The catalytic activity did not change significantly with an increase in the water vapor content (4% to 8%). The oxidation rate has fallen by about 4.5% after 25 hours of continuous reaction, suggesting that water vapor has a negative influence on catalytic performance with the increase of reacting time. It might be because the formation of a liquid film by the accumulation of water molecules with the prolongation of reacting time, which prevented the contact between ozone and oxygen vacancies49. After reacting 25 hours, 0.05-FeOx/SAPO-34 still maintained about 36% efficiency. In summary, the catalyst has a certain stability when the volume fraction of water vapor was in the range of 0-8%.

Figure 6. Catalyst stability for NO oxidation at 0, 4, and 8 vol.% H2O at 80 °C. 3.5. Reaction kinetics Figure 7 shows the NOx removal efficiency of the catalyst as a function of time. The results showed that NH4+/SAPO-34 and 0.05-FeOx/SAPO-34-15 showed poor adsorption effect and the NOx removal was mainly carried out by the catalytic ozonation reaction. In the catalytic ozonation process, In order to investigate the effect of time on NO oxidation, it is important to study its reaction kinetics. In order to define the oxidation kinetics of NO, the kinetic parameters for the oxidation process were studied. The zero order, pseudo first order, and pseudo second order equations were used to fit 20


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the kinetic data50-52. The fitting results of the kinetics data for NO oxidation of ozone, NH4+/SAPO-34, and FeOx/SAPO-34 are shown in Table 4. For these reaction systems, the correlation coefficient (R2) values obtained from pseudo first order kinetics were higher than those obtained from zero order kinetics and pseudo second order kinetics. Therefore, pseudo first order kinetics could reasonably describe the NO catalytic ozonation. Table 4 Parameters of zero order, pseudo-first order, and pseudo-second order reaction kinetic equations of NO catalytic ozonation under different conditions. Catalyst Sole O3

NH4+/SAPO34

FeOx/SAPO34

Kinetics equation

R2

C0 - C = 12.068 x

Rate Constant/K 12.068

0.8508

- ln (C / C0) = 0.0492 x - 0.075

0.0492

0.9556

1/C - 1/C0 = 8E-05 x

8E-05

0.8261

C0 - C = 14.755 x

14.755

0.9406

- ln (C / C0) = 0.0509 x 0.0472 1/C - 1/C0 = 0.0001 x

0.0509

0.9556

0.0001

0.9069

C0 - C = 24.194 x

24.194

0.9766

- ln (C / C0) = 0.0811 x 0.0404 1/C - 1/C0 = 0.0002 x

0.0811

0.9775

0.0002

0.9636

Figure 7. Effect of time on the NOx removal efficiency of the catalysts. 21



3.6. The catalytic ozonation of NO The results discussed thus far, clearly indicate the presence of oxygen vacancies in the catalysts. Other characterization methods such as UV-Vis and PL were also used to investigate the properties of the 0.05-FeOx/SAPO-34 catalysts (Figure S7(a) and S7(b)). Oxygen vacancies act as reaction sites in catalysts and facilitate the absorption of various gases onto their surface, thus enhancing their activity. It has been reported that oxygen vacancies enhance the adsorption of ozone on the surface of catalysts53 and act as adsorption and reaction sites for ozone decomposition. Recent studies have shown that higher oxidation activities can be achieved with the help of oxygen vacancies, which generate superoxide species54. Therefore, we can assume that oxygen vacancies improved the catalytic efficiency of FeOx/SAPO-34 by producing superoxide species. Furthermore, since the simulate gas consisted of 4% water vapor, it is possible that ozone and water vapor produced hydroxyl radicals on the surface of the catalyst, which promoted the oxidation of NO together with superoxide radicals. To further examine the presence of superoxide and hydroxyl radicals, we used the EPR DMPO spin-trap technique. The test was performed by mixing 0.8 g of 0.05FeOx/SAPO-34 in 10 mL solution of methanol and 1 mM DMPO at 80 °C under continuous aeration for 10 min. The initial N2 and O3 flow rates were 90 and 10 mL/min, respectively. The peak characteristic of superoxide radicals could be observed (Figure 8(a))55. Similarly, we replaced methanol with ultra-pure water to obtain the DMPO-·OH peaks of the catalyst under the same experimental conditions. As shown in Figure 8(b), the system generated a stable quarter EPR signal, confirming the presence of hydroxyl radicals56, 57. From this discussion, it is clear that Fe doping of SAPO-34 resulted in the generation of oxygen vacancies, which in turn generated free radicals. Such radicals have greater oxidizing efficiency than ozone. Therefore, it can 22


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be concluded that superoxide and hydroxyl radicals realize the catalytic oxidation of NO.

Figure 8. (a) EPR spectra of superoxide radicals; (b) Typical DMPO-OH signal obtained with 1mM DMPO.

4. Conclusions In summary, the catalytic activity of the FeOx/SAPO-34 catalysts prepared for the oxidation of NO at low ozone concentrations was evaluated. The SAPO-34 molecular sieve was produced using the SAPO-34 crystallization mother liquor. The synthesis method used in this study is environmentally friendly. The use of the FeOx/SAPO-34 catalysts enhanced the NO oxidation rate. The catalysts obtained by mixing NH4+/SAPO-34 with 0.05 M Fe(NO3)3 at 80 °C for 15 h showed the highest NO oxidation rate. The iron content of this catalyst was also the highest, as shown by the ICP-AES, XPS, and EPR results. The EPR results showed that the concentration of oxygen vacancies in 0.05-FeOx/SAPO-34 was the highest. Because of these oxygen vacancies, O3 served as an oxygen supplier in the formation of superoxide and hydroxyl radicals on the catalyst surface.

Acknowledgment

23



Sincere thanks are extended to the Natural Science Foundation of China (NO. 21277020) for its financial contributions.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The comparison of different catalysts; SEM images and XPS spectra of FeOx/SAPO-34-x catalysts; XRD patters of y-FeOx/SAPO-34 catalysts; SEM image and EDS pattern of 0.05-FeOx/SAPO-34; the Fe 2p XPS spectra, the O 1s XPS spectra, UV-vis adsorption spectra and PL spectra of y-FeOx/SAPO-34 catalysts.

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Catalytic ozonation of NO with low concentration ozone over recycled

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