Promoting Simultaneous Desulfurization and Denitrification

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Kinetics, Catalysis, and Reaction Engineering

Promoting Simultaneous Desulfurization and Denitrification Performance of Al2O3@TiO2 Core-shell Structure Adsorbents by Enhancing Oxidation Performance: Modification by Rare Earth Elements (La, Ce, and Y), Reaction Temperature and Oxygen Concentration Honghong Yi, Kun Yang, Xiaolong Tang, Shunzheng Zhao, Fengyu Gao, Yonghai Huang, Yiran Shi, Xizhou Xie, and Runcao Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b06051 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Industrial & Engineering Chemistry Research

Promoting Simultaneous Desulfurization and Denitrification Performance of Al2O3@TiO2 Core-shell Structure Adsorbents by Enhancing Oxidation Performance: Modification by Rare Earth Elements (La, Ce, and Y), Reaction Temperature and Oxygen Concentration Honghong Yi1, 2, Kun Yang1, Xiaolong Tang1, 2, Shunzheng Zhao1, 2, Fengyu Gao1, 2, Yonghai Huang1, Yiran Shi1, Xizhou Xie1 and Runcao Zhang1 1College

of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083,

People’s Republic of China; 2Beijing

Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing, China

Abstract: The simultaneous adsorption capacity of SO2 and NO on the Al2O3@TiO2 core-shell structure adsorbent was improved by improving oxidation performance of the materials. The effects of rare earth elements (La, Ce, and Y) modification, reaction temperature and oxygen concentration on oxidation performance were studied. The results showed that rare earth elements modification could promote the oxidation performance of adsorbents, increase the specific surface area and reduce the pore diameter of the adsorbents. The addition of rare earth elements was positive for reducing the competitive adsorption between SO2 and NO. The decrease of pore diameter would increase the resistance of NO and SO2 penetrating shell structure, which could enhance the oxidation and extend the adsorption time of NO and SO2 on the shell material. And the increase of oxidation performance could decrease the concentration of SO2 on the surface of core material, because more SO2 was oxidized to SO3, and SO3 was more easily adsorbed on the shell material. The best reaction temperature was 100 ℃, and the adsorption effect kept increasing with the increase of oxygen concentration until the oxygen concentration reached 5%, which led to a gentle increasing trend. Key words: Core-shell structure, Simultaneous desulfurization and denitrification, Oxidation, Adsorption



Corresponding Author: Xiaolong Tang.

E-mail: [email protected]

ACS Paragon Plus Environment

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

1. Introduction Sulfur dioxide (SO2) and nitrogen oxides (NOx) in flue gas are directly discharged into the atmosphere without treatment could cause a serious environmental problem, such as haze, photochemical smog and acid rain [1, 2]. Separate control of SO2 and NO, which may result in high cost, has been extensively studied and applied in practical application [3-6]. Simultaneous desulfurization and denitrification had attracted more attention for saving space and reducing costs [7-9]. However, the competitive adsorption was found between SO2 and NO, the presence of SO2 could significantly reduce the adsorption of NO on traditional adsorbents [9-12]. Haneda et al. prepared Ga2O3-Al2O3 with different methods, and the effect of SO2 on the removal of NO was studied, the results found that the negative effect of SO2 on NO removal contributed by the poisoning of NOx adsorption sites [13]. In Hao’s study, Ha-Na was dropped to prepare a complex absorbent solution, which was used to simultaneous removal SO2 and NO, the results showed that the absorbent had an excellent SO2 adsorption performance, and the removal efficiency could reach 98.6%, but the removal efficiency of NO was only 1.0% [14]. In our previous study[15], Al2O3@TiO2 was designed, prepared, and used to simultaneous desulfurization and denitrification, the SO2 and NO adsorption capacity was significantly increased, while the SO2 removal efficiency was high when NO breakthrough. So, the NO adsorption capacity had potential for improvement and need to be improved. It had been reported that NO2 and SO3 were more easily adsorbed than NO and SO2 [16, 17], so increased the oxidation performance of adsorbent could improve simultaneous desulfurization and denitrification performance[14, 18]. Rare earth element modification could improve the oxidation performance of the catalyst surface [19-21]. What is more, reaction temperature and oxygen content played important roles in the catalytic reaction process [22]. In this paper, rare earth element loading Al2O3@TiO2 core-shell structure adsorbent were prepared and used to improve the performance of simultaneous desulfurization and denitrification. The best rare earth element and load capacity were selected, the oxidation reaction process occurring on the surface of the adsorbents was described, and the effects of reaction temperature and oxygen content on the SO2 and NO removal efficiencies were studied.


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2. Experimental Section 2.1 Synthesis of adsorbents The adsorbents were prepared by sol-gel method, 10mL tetrabutyl titanate, 30mL anhydrous ethanol and 5mL glacial acetic acid were mixed to prepare solution A, 20mL ethanol, 2mL deionized water and a certain amount of R(NO3)x·6H2O (R=La, Ce or Y) were mixed to prepare solution B. solution A and solution B were mixed to form a sol after their pH were adjusted to 4. Then 2.89g activated Al2O3 was dispersed in to the sol (the mole ratio of Al2O3 and TiO2 was 1:1). Aging 12 hours after gel formation, the products was dried 12 hours at 110℃ and calcined 6 hours at 500℃. Rare earth element loaded Al2O3@TiO2 core–shell structure adsorbents were defined as Al2O3@TiO2-R x%, R=La, Ce or Y, and x% was the mass fraction of rare earth element.

2.2 Characterization A multifunctional X-ray diffractometer was used to perform the X-ray powder diffraction (XRD) (D/MAX-2200, Rigaku Corporation, Japan) from 2θ= 10° to 90° with the rate of 10°/min by using Cu Kα radiation (λ=0.15406nm). N2 adsorption-desorption isotherm was measured by Autosorb-IC automatic specific surface area and pore size distribution analyzer at 77K after the samples were degassed for 2h at 578K. The X-ray photoelectron spectroscopy (XPS) was got by performing a VG Scientific ESCALab220i-XL electron spectrometer from 1100 to 0 eV with a dwell time of 100 ms and steps of 1 eV, and the Al Kα radiation as excitation source was operated at 300 W.

2.3 Adsorption performance evaluation In this work, N2 (99.999%), O2 (99.9%), NO (1%) and SO2 (1%), controlled by Mass flow meter at 200 mL/min with an accuracy of ± 0.01mL/min, were mixed in mixing tank to form the simulated flue gas, the concentration of O2, NO and SO2 were 5%, 300 ppm and 500 ppm, and balance with N2. The gaseous hourly space velocity (GHSV) was controlled to 24000 h-1. The adsorption process occurred in a fixed bed reactor, and the reaction temperature was kept at 100℃ by a heating furnace with an accuracy of ± 0.1℃. The concentration of NO and SO2 were determined by a gas analysis (Kane KM9106, U.K.) with an accuracy of ± 1 ppm. The simple schematic of adsorption performance evaluation was shown in Figure 1.



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Figure 1 Schematic diagram of adsorption performance evaluation

The SO2 and NOx removal efficiency was calculated by equation 1. Considering that NOx is faster to breakthrough than SO2, the NO removal efficiency of 0.2 was defined as the adsorption breakthrough point. The adsorption capacity of SO2 and NO at the breakthrough point could be obtained by equation 2. Removal Efficiency (%) =

(𝐶0 ― 𝐶i) 𝐶0

1

𝑡

𝑆=

10 ―6𝑞∫0𝑚(𝐶0 ― 𝐶𝑖)𝑑𝑡 22.4𝐺

2

Where C0 and Ci are the import and export concentration of SO2 or NOx (ppm), S is the adsorption capacity of SO2 and NO at the breakthrough point (mmol/g), q is the flow of simulated flue gas (mL/min), tm is the time at the adsorption breakthrough point. G is the adsorbent dosage (g).

3. Results and Discussion 3.1 Characterization of the adsorbents The XRD patterns of adsorbents were shown in Figure 2. It can be seen that the typical peaks assigned to anatase phase of TiO2 (PDF 21-1272) were observed over all absorbents. And the diffraction peak of Ce oxides was not obtained, it could due to that Ce oxides were highly dispersed on the surface of TiO2[23, 24]. For Al2O3@TiO2-La 1.0%, Al2O3@TiO2-Ce 1.0% and Al2O3@TiO2Y 1.0%, the diffraction peaks of γ-Al2O3 were obtained, it may due to that the crystal form of TiO2 became worse after the addition of rare earth elements, which made the typical peaks of anatase broader. So, the small typical peaks of γ-Al2O3 became obvious and could be found in the figure.


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The intensities of the peaks ascribed to TiO2 decreased, suggesting that the addition of rare earth element could also exert a strong inhibiting effect on the crystallization of the TiO2 phase[24]. As shown in Figure 2b, it could be seen that the more Ce was added, the weaker of anatase diffraction peak was, and the stronger of γ-Al2O3 diffraction peaks was.

a

b

Anatase γ-Al2O3

Amatase -Al2O3

Al2O3@TiO2-Ce 3.0%

Intensity (a.u.)

Intensity (a.u.)

Al2O3@TiO2-Ye 1.0% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2-La 1.0%

Al2O3@TiO2-Ce 2.2%

Al2O3@TiO2

Al2O3@TiO2-Ce 1.0%

TiO2 Al2O3@TiO2

Al2O3 10

20

30

40

50

60

70

80

90

10

20

30

40

2θ (degree)

50

60

70

80

90

2θ (degree)

Figure 2 The XRD patterns of adsorbents

N2 adsorption-desorption isotherms was used to characterize the pore size and specific surface area of adsorbents, and the effect of earth element addition on pore structure were confirmed. As shown in Figure 3, all of the adsorbents exhibited a typical type-V isotherm with obvious type H5 hysteresis loop, indicating the formation of mesoporous structures in all of the adsorbents (Figure 4). As listed in Table 1, the addition of rare earth element could promote the surface area of adsorbents, because the aggregation of the particles formed the secondary particle piled pore[23]. But the pore volume and diameter decreased after rare earth element introduced, which may be attributed to the pore was blocked by rare earth element.

0.0

0.2

0.4 0.6 Relative Pressure (P/P0)

Al2O3@TiO2-Ce 3.0% Al2O3@TiO2-Ce 2.2% Al2O3@TiO2-Ce 1.6% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2-Ce 0.3% Al2O3@TiO2-Ce

Volume @ STP (cc/mm/g)

Al2O3@TiO2-La 1.0% Al2O3@TiO2-Y 1.0% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2

Volume @ STP (cc/mm/g)

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


0.8

1.0

0.0

0.2

0.4

0.6

Relative Pressure (P/P0)


0.8

1.0


Figure 3 N2 adsorption-desorption isotherm of adsorbents Table 1 Specific surface areas and average pore diameter of adsorbents* Adsorbent

Surface area (m2·g-1)

Pore volume (cm3·g-1)

Pore diameter (nm)

Al2O3@TiO2

111.26

0.64

10.90

Al2O3@TiO2-La 1.0%

201.25

0.43

4.90

Al2O3@TiO2-Ce 1.0%

185.13

0.38

3.82

Al2O3@TiO2-Y 1.0%

192.21

0.42

3.41

Al2O3@TiO2-Ce 0.3%

166.12

0.36

4.89

Al2O3@TiO2-Ce 1.6%

181.85

0.37

4.90

Al2O3@TiO2-Ce 2.2%

183.61

0.36

4.79

Al2O3@TiO2-Ce 3.0%

185.28

0.38

4.71

*

Surface area, pore volume and pore diameter were obtained were calculated by BET method

0.10

Al2O3@TiO2 Al2O3@TiO2-La 1.0% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2-Y 1.0%

0.10

0.06

0.04

0.02

Al2O3@TiO2 Al2O3@TiO2-Ce 0.3% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2-Ce 1.6% Al2O3@TiO2-Ce 2.2% Al2O3@TiO2-Ce 3.0%

0.08

dv(d) (cc/mm/g)

0.08

dv(d) (cc/mm/g)

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

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0.06

0.04

0.02

0.00

0.00 2

4

8

16

32

64

128

256

2

4

8

16

32

64

128

256

Pore Diamerter (nm)

Pore Diamerter (nm)

Figure 4 pore size distribution curves of adsorbents

3.2 Effect of Rare Earth Element on Adsorption Efficiency In order to select the best rare earth element, the adsorbents of Al2O3@TiO2-R 1.0% were prepared and used to simultaneously removal of SO2 and NO, the SO2 and NOx removal efficiency were shown in Figure 5, and the SO2 and NOx adsorption capacity were shown in Table 2.


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b

1.0

SO2 Removal Efficiency

a NOx Removal Efficiency

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


0.8

0.6

Al2O3@TiO2 Al2O3@TiO2-La 1% Al2O3@TiO2-Ce 1% Al2O3@TiO2-Y 1%

0.4

0.2

4

8

12

16

1.0

0.9

0.8

0.7

Al2O3@TiO2 Al2O3@TiO2-La 1% Al2O3@TiO2-Ce 1% Al2O3@TiO2-Y 1%

0.6

0.5

20

24

28

32

36

40

0.4

10

20

30

40

50

t/ min

t/min

Figure 5 Effect of rare earth element on NO and SO2 adsorption efficiency It could be seen from the Figure 5a and Table 2 that the rare earth element loading was positive for removal of NOx, which increased the NOx adsorption capacity, and extended the breakthrough time. The NOx removal efficiency of Al2O3@TiO2 was the worst, the NOx removal efficiency was kept at 97.00% in first 8 min, and the breakthrough point reached at 26.6 min, the adsorption capacity of NOx was 0.1811 mmol/g. The Al2O3@TiO2-Ce 1.0% adsorbent showed the best NOx adsorption performance, and the NOx removal efficiency was kept at 97.67% in first 14 min, then the efficiency began to decline and reached the breakthrough point at 35.67 min, the adsorption capacity of NOx was 0.2290 mmol/g. The high NOx removal efficiency of Al2O3@TiO2-La 1.0% (about 97.33%) and Al2O3@TiO2-Y 1.0% (about 95.33%) adsorbents were maintained for 12 min and 9 min before the efficiency dropping, and reached the breakthrough point at 31.96 min and 29.80 min, respectively. NOx removal performance in the presence of SO2 came in the following order from greatest to least: Al2O3@TiO2-Ce 1.0% > Al2O3@TiO2-La 1.0% > Al2O3@TiO2-Y 1.0% > Al2O3@TiO2. Table 2 Adsorption capacities of samples with different rare earth elementw Adsorption capacity (mmol/g)

Al2O3@ TiO2

Al2O3@ TiO2-La 1.0%

Al2O3@ TiO2-Ce 1.0%

Al2O3@ TiO2-Y 1.0%

NOx

0.1811

0.2091

0.2290

0.1963

SO2

0.3873

0.4270

0.4627

0.4064

The SO2 removal efficiency of adsorbents with different rare earth element doping were shown in figure 5b. In the early stage of the adsorption process, the SO2 removal efficiency of all the adsorbents kept at 100%, and the rare earth element loading was also positive for removal of SO2. Ce and La loaded adsorbents could extend the time of SO2 removal efficiency with 100%. The



Al2O3@TiO2 also showed the worst SO2 removal efficiency, and the SO2 removal efficiency dropped after 18 min with the fastest decline rate. The SO2 removal efficiency of Al2O3@TiO2-Y 1.0% adsorbent also dropped after 18 min, while the rate of decline was the slowest. Al2O3@TiO2Ce 1.0% showed the best SO2 removal performance and the longest time of SO2 was fully adsorbed (28 min). The SO2 removal performance of Al2O3@TiO2-La 1.0% is second only to Al2O3@TiO2Ce 1.0%, and the 100% removal efficiency maintained for 26 minutes. According to Figure 5 and Table 2, before adsorption penetration, SO2 removal performance came in the following order from greatest to least: Al2O3@TiO2-Ce 1.0% > Al2O3@TiO2-La 1.0% > Al2O3@TiO2-Y 1.0% > Al2O3@TiO2, and the SO2 adsorption capacity were 0. 4627 mmol/g, 0.4270 mmol/g, 0.4064 mmol/g and 0.3837 mmol/g, respectively. The results showed that rare earth elements loading was positive for simultaneous desulfurization and denitrification of Al2O3@TiO2 core-shell structure adsorbents. The promotion effect of rare earth elements on the catalyst was considered to be contributed by the change in the physicochemical property of active sites, and the addition of surface area[25]. The addition of rare earth elements could refine grains and increase oxygen storage capacity, which could improve the oxidizing ability of the adsorbent [26-28]. What is more, SO3 and NO2 were more easily adsorbed than SO2 and NO [16, 17]. The decrease of pore diameter would increase the resistance of NO and SO2 penetrating shell structure, which could increase the oxidation and adsorption time of NO and SO2 on the shell material. The increase of oxidation performance could decrease the concentration of SO2 on the surface of core material, because more SO2 was oxidized to SO3, and SO3 was more easily adsorbed on the shell material. Ce element had the capability to enhance and stabilize the dispersion of TiO2 and Al2O3 [29, 30], and cerium oxides also had an excellent oxygen storage capacity and redox properties for it could produce a lot of labile oxygen vacancies and oxygen species in the process of redox shift between Ce3+ and Ce4+ [31-33]. Therefore, the excellent performance of Ce loading adsorbents was owing to the strongest optimization of Al2O3@TiO2 core-shell structure adsorbents.

3.3 Effect of Ce Amount on Adsorption Efficiency The loading amount was an important factor affecting the adsorption performance, the simultaneous removal SO2 and NO capacity of Al2O3@TiO2 core-shell structure adsorbents with different Ce


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mass fraction were evaluated, and the removal efficiency were shown in Figure 6, adsorption capacity was shown in Table 3. As shown in figure 6a and Table 3, it could be found that the NO adsorption capacity was greatly affected by the loading amount of Ce. With the increasing of the Ce mass fraction from 0.3 to 2.2, the NO adsorption capacity kept a sustained increase. The Al2O3@TiO2-Ce 2.2% showed the best NOx adsorption capacity with the NOx breakthrough adsorption capacity were 0.2587 mmol/g. The result may be attributed to the enhancement of oxidation performance by Ce. However, the NOx adsorption capacity of Al2O3@TiO2-Ce 3.0% was worse than that of Al2O3@TiO2-Ce 2.2%. The order of NOx adsorption capacity over different adsorbents were Al2O3@TiO2-Ce 0.3% < Al2O3@TiO2-Ce 1.0% < Al2O3@TiO2-Ce 3.0% < Al2O3@TiO2-Ce 1.6% < Al2O3@TiO2-Ce 2.2%. In addition, the SO2 adsorption capacity of different adsorbents followed the same order with NOx adsorption capacity, and it is attributed to the adsorbents with a better denitrification efficiency had longer adsorption times. a

1.0

NOx Removal Efficiency

0.8

b

1.0

0.9

0.6

0.4

0.2


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


Al2O3@TiO2-Ce 0.3% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2-Ce 1.6% Al2O3@TiO2-Ce 2.2% Al2O3@TiO2-Ce 3.0% 10

20

0.8

0.7

0.6

Al2O3@TiO2-Ce 0.3% Al2O3@TiO2-Ce 1.0% Al2O3@TiO2-Ce 1.6% Al2O3@TiO2-Ce 2.2% Al2O3@TiO2-Ce 3.0%

0.5 30

40

10

t/min

20

30

40

50

t/min

Figure 6 Effect of Ce amount on NO and SO2 adsorption efficiency Table 3 Adsorption capacities of samples with different Ce amount Adsorption capacity

Al2O3@ TiO2-Ce

Al2O3@ TiO2-Ce

Al2O3@ TiO2-Ce

Al2O3@ TiO2-Ce

Al2O3@ TiO2-Y

(mmol/g)

0.3%

1.0%

1.6%

2.2%

3.0%

NOx

0.2069

0.2290

0.2430

0.2587

0.2386

SO2

0.4129

0.4627

0.4877

0.5025

0.4741

To further investigate the effect of Ce loading amount on the adsorption capacity, the XPS of adsorbents was carried out to clarify the chemical state of elements on the surface of the adsorbents with different Ce loading amount. Figure 7a exhibits the XPS spectra of the Ce 3d. According to the report previously[24, 34], the 3d level spectra of Ce could be ascribed to Ce3+ and Ce4+ bands,



which revealed the coexistence of Ce3+ and Ce4+ on adsorbents surface. The amount of Ce3+ was more than Ce4+, and the atomic ratios of Ce3+ (C3+/(C3++ C4+)) in Al2O3@TiO2-Ce 2.2% was the largest (Table 4). As shown in Figure 7b, the O 1s bands in all of the adsorbents could be fitted into adsorbed oxygen (Oa) and lattice oxygen (Ol) peaks[23, 35], and the adsorbed oxygen was predominant. Similarly, the atomic ratios of Oa (Oa / (Oa + Ol)) value of Al2O3@TiO2-Ce 2.2% was the largest.

a

Ce3+ Ce4+

Ce3+

Ce4+

b

Al2O3@TiO2-Ce 1.0%

Intensity (a.u.)

Al2O3@TiO2-Ce 2.2%

Al2O3@TiO2-Ce 2.2%

Al2O3@TiO2-Ce 3.0% 910

Ol

Oa

Al2O3@TiO2-Ce 1.0%

Intensity (a.u.)

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

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Al2O3@TiO2-Ce 3.0%

900

890

880

537

534

531

528

525

Binding energy (ev)

Binding energy (ev)

Figure 7 XPS spectra for (a) O 1s and (b) Ce 3d of different adsorbents Table 4 XPS data of different adsorbents Adsorbents

Ce3+ / (Ce3+ + Ce4+)

Oa / (Oa + Ol)

Al2O3@TiO2-Ce 1.0%

66.09%

68.97%

Al2O3@TiO2-Ce 2.2%

72.16%

77.14%

Al2O3@TiO2-Ce 3.0%

68.49%

71.79%

The order of C3+/(C3++ C4+), Oa/(Oa + Ol) and adsorption capacity over the adsorbents were the same, so the loading amount of Ce affected the adsorption capacity by affecting the atomic ratios of Ce3+ and Oa. In the process of conversion between Ce4+ and Ce3+, some lattice oxygen lost when Ce4+ converted to Ce3+ by getting electrons. And the oxygen vacancies appeared in the places where lattice oxygen wass missing. Therefore, more Ce3+ would produce more oxygen vacancies[36], and the enrichment of Ce3+ could lead to more unbalanced charges and chemical bonds, which was beneficial to the activation of adsorbed oxygen[37, 38].


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Fresh Transmitance (a.u.)

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


Used

1380

3000

2500

2000

1108

1500

1022

1000

500

Wavenumber (cm-1)

Figure 8 FTIR spectra of Al2O3@TiO2-Ce 2.2% adsorbent

In order to confirm the adsorption product, the FTIR was used to characterize the fresh and used Al2O3@TiO2-Ce 2.2% adsorbents, the FITR spectra were shown Figure 8. There were two peaks at 1022 and 1108 cm-1, which were assigned to symmetric stretching vibration and antisymmetric stretching vibration of SO42-. The peak at 1380 cm-1 were ascribed to the stretching vibration of NO3-. And SO42- and NO3- were considered as the chemical adsorption products of SO2 and NO. According to the above experimental results, the adsorption process was considered as follows. When the simulated smoke reaches the surface of the adsorbents, the O2 was adsorbed and activated by oxygen vacancies, which was formed in the process of Ce4+ converted to Ce3+. Then SO2 and NO were oxidized to SO3 and NO2 by activated oxygen. Most of SO3 and some NO2 were chemical adsorbed by shell structure, and Most of NO2 and small amount of SO2 were chemical adsorbed by core structure, the adsorption products were SO42- and NO3-. The reaction process can be described by the following equation: 2𝐶𝑒𝑂2 + 2e ↔ 𝐶𝑒2𝑂3 + 2𝑂 + 𝑉 𝑂2 + 𝑉→2𝑂· 𝑆𝑂2 + 𝑂·→ 𝑆𝑂3 𝑁𝑂 + 𝑂·→ 𝑁𝑂2 𝑆𝑂3 + 𝑎𝑏𝑡→ 𝑆𝑂42 ― 𝑁𝑂2 + 𝑎𝑏𝑡→ 𝑁𝑂3 ― Where V is the oxygen vacancies, 𝑂· is activated oxygen, and abt is adsorbent.



3.4 Effect of Reaction Temperature on Adsorption Efficiency The reaction temperature is an important parameter in the adsorption process and has a great influence on the adsorption effect. In general, high temperature could promote the chemical processed but is negative for adsorption. In this part, the effect of reaction temperature on SO2 and NO adsorption capacity over Al2O3@TiO2-Ce 2.2% was studied, and the results were shown in Figure 9 and Table 5.

1.0

0.8

b

0.6

0.4

0.2

1.0

0.9



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50℃ 100℃ 150℃

0.8

0.7

0.6

50℃ 100℃ 150℃

0.5 10

20

30

40

10

t/min

20

30

40

50

t/min

Figure 9 Effect of adsorption temperature on NO and SO2 adsorption efficiency

As can be seen in Figure 9a, the NOx adsorption efficiency and breakthrough adsorption capacity at different temperatures was significantly different. The best adsorption capacity was not at the highest reaction temperature, and the best adsorption temperature is 100 ℃. It could be attributed to the chemical adsorption of NO and the physical adsorption of O2 on the surface of the adsorbent. At high temperature (150 ℃), there was no enough activated oxygen to oxidize NO to NO2 for the adsorption amount of O2 was reduced, so excessive temperature was negative for chemical adsorption of NO. The adsorbent surface could adsorb more oxygen at low temperature (50 ℃), but low temperature is not conducive to chemical processes. And the NO adsorption capacity at 50 ℃ was better than that at 150 ℃, which may due to the contribution of physical adsorption at 50 ℃. Table 5 Adsorption capacities of samples with different reaction temperature Adsorption capacity

50℃

100℃

150℃

NOx

0.1619

0.2587

0.1055

SO2

0.4446

0.5025

0.3432

(mmol/g)

For the removal efficiency of SO2 (Figure 9b), the best adsorption temperature was 50 ℃, and the removal efficiency at 100 ℃ was the worst. SO2 could also be chemically adsorbed by being


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converted to SO32- when the adsorbed oxygen was not enough, so the SO2 removal efficiency at 150 ℃ was better than that at 100 ℃. And the SO2 removal efficiency at 50 ℃ was attributed to the contribution of physical adsorption and chemisorption. What is more, it could also be a reason for this result that SO2 got more adsorption sites when the NO adsorption capacity was poor. However, due to the influence of adsorption breakthrough time, the order of SO2 breakthrough adsorption capacity with different reaction temperature were 100℃ > 50℃ > 150℃.

3.5 Effect of Oxygen Concentration on Adsorption Efficiency The adsorbed oxygen on adsorbents surface played an important role in the oxidation of SO2 and NO, so the effect of oxygen concentration on adsorption efficiency was worth studying. The Al2O3@TiO2-Ce 2.2 was selected to simultaneous removal of SO2 and NO with different oxygen concentration at 100 ℃, the results was shown in Figure 10 and Table 6.

1.0

b

0% 3% 5% 10%

0.8

0.6

0.4

0.2

5

10

15

20

25

30

35

1.0

0% 3% 5% 10%

0.9



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


0.8 0.7 0.6 0.5 0.4

40

10

20

t/min

30

40

50

t/min

Figure 10 Effect of oxygen concentration on NO and SO2 adsorption efficiency

As shown in Figure 10a, the NOx removal efficiency and adsorption capacity increased with the increasing of the oxygen concentration, so the efficiency detected without oxygen was the worst, and the NOx removal efficiency with the oxygen concentration of 10 was the best. Within the oxygen concentration range from 0 to 5%, the effect of oxygen concentration on NOx removal efficiency was very significant. It is because more activated oxygen was needed in this range. At high oxygen concentration (5 to 10%), the adsorbed oxygen on the surface of the adsorbent was close to saturation, so the effect of oxygen concentration on the NOx removal efficiency was not obvious. Table 6 Adsorption capacities of samples with different oxygen concentration Adsorption capacity (mmol/g) NOx

0

3%

5%

10%

0.1366

0.1974

0.2587

0.2621



SO2

0.3844

0.4082

0.5025

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0.4978

As seen in Figure 10b, the SO2 removal efficiency was quite high without introducing oxygen. Both the adsorption sites and SO2 oxidation would affect the SO2 removal efficiency. When the oxygen concentration was 3%, oxidation process had a certain promoting effect on SO2 adsorption, but more adsorption sites were occupied by NOx resulting in reducing the SO2 removal efficiency. In the range of 3% to 5%, the promotion of SO2 removal efficiency increased with the increasing of oxygen concentration. When the oxygen concentration reached a certain value, the promotion effect on SO2 removal efficiency reached its limit, and more oxygen would cause the decrease of SO2 removal because of the occupation of adsorption sites. So, the order of the oxygen concentration of the SO2 removal efficiency was 5% > 10% > 3%. However, the order of SO2 breakthrough adsorption capacity with different reaction temperature were 5% > 10% > 3% > 0, because of the difference of adsorption breakthrough time.

4. Conclusions In this work, the simultaneous adsorption capacity of SO2 and NO on the Al2O3@TiO2 core-shell structure adsorbent was improved by improving oxidation performance. Rare earth modification, reaction temperature and oxygen concentration could affect the adsorption efficiency by affecting oxidation performance. Rare earth modification can increase the specific surface area and decreased the pore diameter of adsorbents. The decrease of pore diameter would increase the resistance of NO and SO2 penetrating shell structure, which can increase the oxidation and adsorption time of NO and SO2 on the shell material. The increase of oxidation performance could decrease the concentration of SO2 on the surface of core material, because more SO2 was oxidized to SO3 and SO3 was more easily adsorbed on the shell material. The Al2O3@TiO2-Ce 2.2% adsorbent had the best simultaneously desulfurization and denitrification performance, for it had the most mole ratio of Ce3+ and adsorbed oxygen. Oxygen vacancies were generated with generating Ce3+, then oxygen was adsorbed and active by oxygen vacancies. Furthermore, these activated oxygens increased the adsorption capacity by increasing the oxidation performance. The reaction process in this system is chemisorption, and the production of adsorption were SO42- and NO3-. The adsorption capacity increased with the increase of oxygen concentration, and the enhancement was not obvious when oxygen concentration was higher than 5%. The best reaction temperature was 100 ℃ for the


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chemical adsorption of NO and the physical adsorption of O2 on the surface of the adsorbent. ACKNOWLEDGMENTS

The work was supported by the National Natural Science Foundation of China (21577006) and National Key R&D Program of China (2017YFC0210303).

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Promoting Simultaneous Desulfurization and Denitrification

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