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Aerosol and Air Quality Research, 18: 655–670, 2018 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2017.04.0148 In Situ FT-IR and DFT Study of the Synergistic Effects of Cerium Presence in the Framework and the Surface in NH3-SCR Yinming Fan1, Wei Ling1, Lifu Dong1, Shihui Li1, Chenglong Yu1, Bichun Huang1,2*, Hongxia Xi3 1 School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, China 2 Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, China 3 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, China ABSTRACT

Mn-Ce/CeAPSO-34 was prepared, in which manganese and cerium were supported on the surface through using the Ethanol dispersion method, while cerium was incorporated in the SAPO-34 framework by a one-step hydrothermal method. Based on our previous study, a strong synergistic effect of cerium presented in the framework and the surface was existing in Mn-Ce/CeAPSO-34 catalyst, which showed outstanding SO2 tolerance and H2O resistance in the low-temperature NH3-SCR. In situ FT-IR and DFT calculations were used to investigate the synergistic effects. Based on the characterization results of in situ FT-IR study, it was found that more amount of nitrate species and NH3 species adsorbed on the surface of Mn-Ce/CeAPSO-34, while less the amount of sulfate species deposited during reaction process, which in the presence of SO2. Meanwhile, DFT calculations revealed that Ce site supported on the surface, which neighbored by Ce site in the framework more were capable of reacting with NO and NH3. Keywords: Mn-Ce/CeAPSO-34; NH3-SCR; Synergistic effect; SO2 tolerance. INTRODUCTION

Selective catalytic reduction of NO with ammonia (NH3-SCR), as an effective and stable technology, is widely used to eliminate the nitrogen oxides from stationary sources (Bosch, 1988; Nova et al., 2006). While catalysts are the key factor for the SCR technology, which directly determinate the denitrification efficiency. However, the most commercial used catalyst, V2O5-WO3(MoO3)/TiO2, with a relatively narrow and high temperature window of 300–400°C, which need the SCR system be placed upstream of the electrostatic precipitant and desulfurizer to avoid reheating the gas. But the problem is that the catalyst have a short lifetime and easy to be deactivated in the presence of SO2 and H2O (Forzatti et al., 1996; Fuet al., 2014). Hence, there is a great motivation to develop new catalysts to apply in relatively low operating temperatures and resistant to SO2 and H2O. Many kind of of metal oxides (such as Fe, Cu, Mn, Ce) supported on that * Corresponding author.

Tel.: +86 20 39380519 E-mail address: [email protected]

different carriers have been found as efficient catalysts at low temperature (Wang et al., 2012; Xue et al., 2013; Qu et al., 2015; Wang et al., 2017). In particular, Mn-based catalysts (such as nano-MnOx, MnOx/TiO2, MnOx/MWCNTs, MnOx/Graphene, MnOx/ZSM-5 and MnOx/USY) exhibited a relatively higher activity at low temperature (Qi et al., 2003; Wu et al., 2007; Jiang et al., 2009; Fang et al., 2013; Lou et al., 2014; Jiao et al., 2015; Andreoli et al., 2015). However, the vulnerability of these catalysts suffering from the deactivation by SO2 and H2O, which inhibited them for industrial application. Generally speaking, SO2 deactivates the catalysts through the formation of metal sulfites/sulfates, or the deposition of ammonium sulfates that can generate pore plugging and blockage of active sites (Wu et al., 2009; Jin et al., 2010a; Zhang et al., 2014). And H2O inhibited the activity of catalysts by the competitive adsorption with NH3 (Busca et al., 1998; Thirupathi et al., 2011).

Concerning the resistance to SO2 and H2O, it is proved to be an effective method that adding the extra elements (Cu, Sn, Ti, Fe, Zr, Ce) as modified material to the SCR catalysts (Shen et al., 2010; Chang et al., 2013; Cao et al., 2014; Guo et al., 2014; Pan et al., 2014; Zhang et al., 2015; Du et al., 2016; Niu et al., 2016; Zhang et al., 2016). Among them, Ce was widely used for it’s better redox ability, high

Fan et al., Aerosol and Air Quality Research, 18: 655–670, 2018 656

oxygen storage capacity and environmental friendliness. Jin et al. (2010b) found that NO conversion was higher over Mn-Ce/TiO2 catalyst than over Mn/TiO2 sample in the presence of SO2. Adding of Ce metal greatly inhibited the deposition of ammonium sulfates at the surface of catalysts and decreased the formation of metal sulfites/sulfates. Shu et al. (2014) indicated that Ce were sulfated preferentially over Ce-Fe/TiO2 in the presence of SO2, and then the quantities of surface active oxygen species and hydroxyls were increased, which supplied more acid sites. It is the actual reason for the strong SO2 resistance of Ce-Fe/TiO2

catalyst. Ce doping were more effective than other metal oxides to promote the catalytic activity under SO2 atmosphere to a certain extent, but what the most serious problem was that the Ce-modified catalysts would suffering severe deactivation by SO2 permanently at low temperature. Wherefore, the resistance to SO2 and H2O over Ce-modified catalysts need be improved more.

In our previous study (Fan et al., 2017), it was found that a strong synergistic effect was existing between cerium oxides in the framework and the surface of Mn/SAPO-34, which were beneficial to improve the resistance to SO2 and H2O in the low-temperature NH3-SCR. Cerium was incorporated in the SAPO-34 framework by a one-step hydrothermal method and supported on the surface by Ethanol dispersion method, respectively. We found that Ce could prefer to react with SO2 then protect the manganese active sites. Meanwhile, the synergistic effects of cerium existed in the framework and the surface not a simple overlap of single factors but a complex phenomenon. In this work, to investigate the mechanism of the synergistic effects of cerium exist in the framework and the surface, the in situ Fourier translation infrared spectrum (FT-IR) was used to study the behavior of the formed active intermediates and surface species over the catalysts in the presence of SO2 at low temperature NH3-SCR. Meanwhile, DFT calculations were used to research the synergistic effect existing between the cerium in the framework and the surface. EXPERIMENTAL Catalyst Preparation

CeAPSO-34 were synthesized by a one-step hydrothermal method. Diisopropylamine (DIPA, 99 wt%, Aladdin) was used as a template, cerium acetate hydrate (99.9 wt%, Aladdin) as the metal source, phosphoric acid (85 wt%, Aldrich), pseudoboehmite (68 wt%, PetroChina, China) and colloidal silica (30 wt%, Aldrich) as the phosphorus source, aluminum source and silicon source, respectively. The molar ratio used was 1.0P2O5:0.8Al2O3:0.2SiO2:3.0DIPA: 0.3Ce:50H2O in the crystallization solution. The synthesis steps were as follows: cerium acetate hydrate was added to a select quantity of distilled water and fully stirred until the cerium acetate hydrate was completely dissolved. Pseudoboehmite was then added to form a homogeneous gel by stirring. Subsequently, colloidal silica was added to the gel with stirring. Finally, the diisopropylamine was slowly added into the gel mixture and stirred. The resulting gel was transferred to a 100 mL autoclave with a Teflonliner

and statically crystallized for 48 h at 200°C. The crystalline products was filtered and washed with distilled water and dried at 110°C overnight. Then, the samples were calcined at 550°C for 6 h to remove the template. For comparison, SAPO-34 was also prepared according to the above-mentioned methods.

Mn/CeAPSO-34 and Mn-Ce/CeAPSO-34 were prepared successfully by the Ethanol dispersion method in which manganese nitrate (50 wt% in H2O, Aladdin) and cerium nitrate (99.5 wt%, Aladdin) used as precursor, respectively. The detailed process was described in previous study (Yang et al., 2016). For comparison, Mn/SAPO-34 and Mn-Ce/SAPO-34 were also prepared according to the above-mentioned methods. In Situ FT-IR Experiments

FT-IR spectra were conducted on a Nicolet 6700 FTIR spectrometers (2 cm–1 resolution with 100 accumulate scan) equipped with a MCT detector cooled by liquid nitrogen and a transmission reflection accessory with high temperature reaction cell. The catalyst were first mixed with KBr in a ratio of 1/100 by weight and were loaded into an IR cell with CaF2 windows. Prior to each experiment, the sample was pretreated at 350°C in the atmosphere for 60 min to remove any adsorbed species, then cooled down to the reaction temperature of 180°C. The background spectrum was recorded in flowing Ar and automatically subtracted from the sample spectrum during the experiment. The FT-IR experiment included transient response and steady-state response experiments. It must be emphasized that all catalyst samples are pretreated under the same condition in the whole experiment process. Computational Models and Details

All the Calculations were based on DFT and performed by Modeling Dmol3 package (Peng et al., 2015; Wei et al., 2016). SAPO-34 (1 × 1 × 1) model was selected in this study and SAPO-34 (111) face was choose to calculation the adsorption energy. Mn3O4 and CeO2@Ce2O3 crystal molecular were build for active component. All the geometry optimization were performed by local density approximation functional LDA-PWC, and the self-consistent field energy was set to 2.0 × 10−5 Ha. The adsorption energy are calculated as follows: Ead = Egas + Esurface – Egas@surface (1)

where Esurface, Egas and Egas@surface correspond to the energy of surface, an isolated gas molecule and the same molecule adsorbed on the surface, respectively. RESULTS AND DISCUSSION In Situ FT-IR Analysis

The effect of adsorbed NO on SO2 adsorption over Mn/SAPO-34, Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/CeAPSO-34 were investigated. Fig. 1 shows the in situ FT-IR spectra over the four catalysts in flowing NO + O2 at 180°C for 30 min and then purged with Ar for 30 min,


finally introduced with SO2. After exposing Mn/SAPO-34 to NO and O2, the bands corresponded to the surface O-H stretching (3800–3500 cm–1) (Jiang et al., 2010; Wang et al., 2014), nitrosyl NO- (1904 and 1847 cm–1) (Qi et al., 2004a; Zhang et al., 2013a), the gaseous or weakly adsorbed NO2 (1626 cm–1) (Jiang et al., 2010), bridging nitrate (1595 cm–1) and nitrite compounds (1407 cm–1) (Xu et al., 2013; Qiu et al., 2015) were detected. The new bands at 1725 cm–1 (N2O4) (Zhou et al., 2011), 1670 cm–1 (bidentate nitrate) (Meng et al.,

2015), 1530 and 1483 cm–1 (monodentate nitrate) (Wu et al., 2010) and 1456 cm–1 (nitrite compounds) (Jin et al., 2010c) were appeared over Mn/CeAPSO-34 and Mn-Ce/SAPO-34. And for Mn-Ce/CeAPSO-34, the new bands shifted to 1743 and 1697 cm–1 (N2O4) (Zhou et al., 2011) and 1518 cm–1

(monodentate nitrate) (Zhang et al., 2013b), respectively. Band intensity improved gradually with time. Compared to Mn/SAPO-34, the amount of nitrate species is much higher on the surface of the catalysts with Ce modification, indicating

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Fig. 1. In situ FT-IR spectra of (A) Mn/SAPO-34, (B) Mn/CeAPSO-34, (C) Mn-Ce/SAPO-34, (D) Mn-Ce/CeAPSO-34 exposed to NO + O2 at 180°C for 30 min and then purged with Ar for 30 min and finally SO2 was introduced. Experiment conditions: 0.08% NO, 5.0% O2, 0.01% SO2 and a balanced of Ar, total flow rate 100 mL min–1.


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Fig. 1. (continued).

that Ce modification would be beneficial to increase the nitrate species adsorbed on the catalyst surface. Then purged with Ar for 30 min, the initial peaks about nitrate species on all of the catalysts were vanished, indicating that nitrate species weakly adsorbed on catalysts surface. And finally SO2 was introduced, the new bands appeared at 1742, 1699, 1644, 1540, 1509, 1460, 1425, 1375 and 1339 cm–1 on Mn/SAPO-34. Similarly, these new bands were also detected on Mn/CeAPSO-34. The bands at 1742, 1699, 1540, 1425 and 1375 cm–1 associated with surface sulfate species (Jin et al., 2014), 1644 cm–1 ascribed to HSO4

– (Wei

et al., 2016), 1509 and 1460 cm–1 corresponded to SO32–

species (Yang et al., 1998; Watson et al., 2003; Pan et al., 2013), and 1339 cm–1 attributed to SO4

2– (Yang et al., 2013). For Mn-Ce/SAPO-34, the new bands at 1540 and 1460 cm–1 shifted to 1590 (surface sulfate species) and 1475 cm–1 (SO3

2–

species), respectively. While there are only three new bands at 1425 and 1375 cm–1 (surface sulfate species) and 1339 cm–1 (SO4

2–) were observed on Mn-Ce/CeAPSO-34. It was noted that SO2 would be weakly adsorbed on the surface of Mn-Ce/CeAPSO-34, indicating that cerium exist in the framework and the surface of catalysts have a significant

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inhibiting influence on the formation of sulfate species. The effect of adsorbed SO2 on NO adsorption over all of

the catalysts was investigated and the results are shown in Fig. 2. The catalysts were first exposed to SO2 at 180°C for 30 min and then purged with Ar for 30 min, finally NO + O2 was introduced. After exposing Mn/SAPO-34 to SO2, the bands corresponded to the surface O-H stretching (3800–3500 cm–1), surface sulfate species (1742, 1699, 1540, 1425 and 1375 cm–1), SO3

2– species (1509 and 1460 cm–1), HSO4–

(1644 cm–1) and SO42– (1339 cm–1) were observed. Similarly,

these new bands were also detected on Mn/CeAPSO-34 and Mn-Ce/SAPO-34. And for Mn-Ce/CeAPSO-34, the bands at 1590, 1475, 1425, 1375, 1339 cm–1 were found. It was noted that the amount of sulfate species was much lower on Mn-Ce/CeAPSO-34, indicating that cerium exist in the framework and the surface of catalyst would be inhibit the sulfate species adsorbed on catalyst surface. Then purged with Ar for 30 min, the initial peaks about sulfate species on the catalysts remained almost unchanged, indicating that the deposition of sulfate species were stable,

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Fig. 2. In situ FT-IR spectra of (A) Mn/SAPO-34, (B) Mn/CeAPSO-34, (C) Mn-Ce/SAPO-34, (D) Mn-Ce/CeAPSO-34 exposed to SO2 at 180°C for 30 min and then purged with Ar for 30 min and finally NO + O2 was introduced. Experiment conditions: 0.08% NO, 5.0% O2, 0.01% SO2 and a balanced of Ar, total flow rate 100 mL min–1.


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which maybe one of the reasons for the deactivation by SO2 was irreversible. After NO + O2 was introduced, the new bands appeared at 1904, 1847, 1626, 1595 cm–1 on all of the catalysts, which attributed to the adsorbed nitrate species. It was noted that the new peaks at 1725 cm–1

(N2O4) and 1456 cm–1 (nitrite compounds) were observed on Mn-Ce/CeAPSO-34. It is know that NO and SO2 tend to be adsorbed on the same active sites on the surface of catalysts. From these results, the competitive adsorption between NO and SO2 presented on all of the catalysts. Compared to other catalysts, the adsorption ability of SO2

was much lower on Mn-Ce/CeAPSO-34. Meanwhile, more

nitrate species adsorbed on the surface, indicating that NO adsorption over Mn-Ce/CeAPSO-34 was less affected by SO2 adsorbed. The results implied that cerium presented in the framework and the surface of catalyst could inhibit the sulfate species formed and increase the nitrate species adsorbed on catalyst surface.

The effect of adsorbed NH3 on SO2 over all of the catalysts was investigated and the results are shown in Fig. 3. The catalysts were first exposed to NH3 at 180°C for 30 min and then purged with Ar for 30 min, finally SO2 + O2 was introduced. After injecting NH3, several bands could be attributed to the surface O-H stretching (3800–3500 cm–1),


NH4+ species formed on Brønsted acid sites(1690, 1664,

1652, 1623, 1478, 1456 and 1402 cm–1) (Jin et al., 1986; Jiang et al., 2010; Liu et al., 2010), coordinated NH3 on Lewis acid sites (1278, 1248, 1120, 1077 and 1040 cm–1 ) (Sun et al., 2009; Jiang et al., 2014; Li et al., 2016) and -NH2 species (1588, 1558 and 1538 cm–1) (Sun et al., 2009; Qi et al., 2004b; Ettireddy et al., 2012) were observed over all of the catalysts (Ma et al., 2014; Ding et al., 2015; Zhu

et al., 2015). Compared with Mn/SAPO-34, the band at 3331 cm–1 was detected in the NH stretching region on the catalysts with Ce modification (Sun et al., 2009). It is noted that the amount of NH3 species is much higher on the catalysts with Ce modification. The results suggest that Ce modification would be beneficial to the improved NH3 adsorption on catalyst surface. Their intensities improved with exposure time. Then purged with Ar for 30 min, the

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Fig. 3. In situ FT-IR spectra of (A) Mn/SAPO-34, (B) Mn/CeAPSO-34, (C) Mn-Ce/SAPO-34, (D) Mn-Ce/CeAPSO-34 exposed to NH3 at 180°C for 30 min and then purged with Ar for 30 min and finally SO2 + O2 was introduced. Experiment conditions: 0.08% NO, 5.0% O2, 0.01% SO2 and a balanced of Ar, total flow rate 100 mL min–1.


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intensity of the initial peaks was declined. After SO2 + O2 was introduced, the new bands appeared at 1742, 1699, 1644, 1540, 1509, 1460, 1425, 1375 and 1339 cm–1 on Mn/SAPO-34, Mn/CeAPSO-34 and Mn-Ce/SAPO-34, which attributed to the adsorbed sulfate species, indicating the same sulfate species were formed. While there are only two new bands at 1375 cm–1 (surface sulfate species) and 1339 cm–1 (SO4

2–) were observed on Mn-Ce/CeAPSO-34, indicating that less the amount of sulfate species adsorbed and SO2 would be weakly adsorbed or the deposition rate of sulfate is slower

compared with other catalysts. The effect of adsorbed NO + NH3 + O2 on SO2 adsorption

on all of the catalysts was investigated and the results are shown in Fig. 4. The catalysts were first exposed to NO + NH3 + O2 at 180°C for 30 min and then purged with Ar for 30 min, finally SO2 was introduced. After injecting NO + NH3 + O2, the new bands could be assigned to nitrosyl NO- (1904 and 1847 cm–1), N2O4 (1754 cm–1), bidentate nitrate (1670 cm–1), the gaseous or weakly adsorbed NO2 (1626 cm–1), bridging nitrate (1595 cm-1), monodentate nitrate


(1483 cm–1), nitrite compounds (1456 cm–1), NH4+ species

formed on Brønsted acid sites (1402 cm–1) and coordinated NH3 on Lewis acid sites (1120, 1077 and 1040 cm–1) were observed on all of the catalysts. For Mn-Ce/CeAPSO-34, another new bands at 1530 cm–1 (monodentate nitrate) and 1248 cm–1 (coordinated NH3 on Lewis acid sites) were detected. Then purged with Ar for 30 min, the bands at 1904, 1847, 1120, 1077 and 1040 cm–1 were vanished. After SO2 was introduced, the new bands attributed to the adsorbed

sulfate species appeared at 1742, 1699, 1644, 1540, 1509, 1460, 1375 and 1339 cm–1 on Mn/SAPO-34. Similarly, these new bands were also detected on Mn/CeAPSO-34 and Mn-Ce/SAPO-34. While there are only two new bands at 1375 cm–1 surface sulfate species) and 1339 cm–1 (SO4

2–) were observed on Mn-Ce/CeAPSO-34. The results implied that SO2 would be weakly adsorbed on the surface of Mn-Ce/CeAPSO-34 after the catalyst in flowing NO + NH3 + O2 then purged with Ar.

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Fig. 4. In situ FT-IR spectra of (A) Mn/SAPO-34, (B) Mn/CeAPSO-34, (C) Mn-Ce/SAPO-34, (D) Mn-Ce/CeAPSO-34 exposed to NO + NH3 + O2 at 180°C for 30 min and then purged with Ar for 30 min and finally SO2 was introduced. Experiment conditions: 0.08% NO, 5.0% O2, 0.01% SO2 and a balanced of Ar, total flow rate 100 mL min–1.


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The formation of surface species on all of the catalysts under an atmosphere of NO NH3, O2 and SO2 mixture was also investigated and the results are shown in Fig. 5. After all the reaction gases were supplied, the bands could be assigned to the surface O-H stretching (3800–3500 cm–1), NH stretching (3331 cm–1), nitrosyl NO- (1904 and 1847 cm–1), weakly adsorbed sulfate species (1742 and 1540 cm–1), bidentate nitrate (1670 cm–1), the gaseous or weakly adsorbed NO2 (1626 cm–1), bridging nitrate (1595 cm–1), nitrite compounds (1456 cm–1), NH4

+ species formed on Brønsted acid sites (1402 cm–1) and coordinated NH3 on

Lewis acid sites (1120, 1077 and 1040 cm–1) were observed on Mn/SAPO-34. Band intensity improved gradually with time. It was noted that several nitrate species were first detected after injection of all the reaction gases for about 5 min, indicating that the rate of the NOx adsorption was much faster than the process of ammonia adsorption. Compared with Mn/SAPO-34, the amount of nitrate species was higher that N2O4 (1754, 1725 cm–1) and monodentate nitrate (1483 cm–1) were detected. Meanwhile, the amount of NH3 species was almost the same on Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/CeAPSO-34. While the sulfate


species was not found on the catalysts with Ce modification. It is known that the NH3-SCR reaction occurs following

two pathways: L-H mechanism (between NO2 and NH4+)

and E-R mechanism (between gas phase NO and coordinated NH3). In this work, the mechanism of NH3-SCR of NOx with SO2 on the catalysts has been discussed at 180°C. After all the reaction gases participated in the SCR reaction, NH4

+ species formed on Brønsted acid sites, coordinated NH3 on Lewis acid sites, nitrosyl NO- species and the gaseous or

weakly adsorbed NO2 were appeared on all of the catalysts. Hence, the reaction both takes place via L-H mechanism and E-R mechanism. The reaction could take place as follows: NH3(g) → NH3 (a) (2) NH3 (g) + H+ → NH4

+ (a) (3) O2 (g) → 2O (a) (4)

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Fig. 5. In situ FT-IR spectra of (A) Mn/SAPO-34, (B) Mn/CeAPSO-34, (C) Mn-Ce/SAPO-34, (D) Mn-Ce/CeAPSO-34 exposed to NO + NH3 + O2 + SO2 at 180°C for 60 min. Experiment conditions: 0.08% NO, 5.0% O2, 0.01% SO2 and a balanced of Ar, total flow rate 100 mL min–1.


4000 3500 3000 2000 1500 1000

3331

1725O-H

175460 min

30 min

20 min

10 min

5 min

0 min

Kub

elka

-Mun

k

Wavenumbers(cm-1)

1904 1847

1670

16261595

10401077

1120

(C)1402

14561483

4000 3500 3000 2000 1500 1000

3331

1725

1754

Kub

elka

-Mun

k

Wavenumbers(cm-1)

19041847

1670

16261595 10401077

1120140214561483

0 min

5 min

10 min

20 min

30 min

60 minO-H

(D)


NH3 (a) + O (a) → OH (a) + NH2(a) (5) NH3 (a) + O2– → OH (a) + NH2 (a) (6) NO (g) + e → NO-(a) (7) NO-(a) + O2 (g) → NO2(a)/NO2

–/NO3– (8)

2NO2 (a) → N2O4 (a) (9) NH2 (a) + NO2

– → NH2NO2 (a) → … → H2O + N2 (g) +

O (a) (10) NH2 (a) + NO3

– → NH2NO3 (a) → … → H2O + N2 (g) + 2O (a) (11) 2NH2(a) + N2O4 (a) → 2NH2NO2(a) → …→ H2O + N2(g) + O(a) (12) NH4

+ (a) + N2O4 (a) → NH4N2O4 (a) → …→ H2O + N2 (g) + O (a) (13)


NH4+ (a) + NO2

– → NH4NO2 (a) → …→ H2O + N2 (g) (14) NH4

+ (a) + NO3– → NH4NO3 (a) → …→ H2O + N2 (g) +

O (a) (15)

In our previous study (Fan et al., 2017), Mn-Ce/CeAPSO-34 catalyst show outstanding SO2 tolerance. It is known that SO2 deactivated the catalysts by the deposition of ammonium sulfates or forming metal sulfites/sulfates lead to irreversible loss of active sites. The in situ FT-IR spectrum show that more nitrate species and NH3 species formed on catalyst surface with Ce modification. And in the presence of SO2, less the amount of sulfate species adsorbed on Mn-Ce/CeAPSO-34, indicating the amount of the deposition of ammonium sulfates or forming metal sulfites/sulfates was much lower. So NO conversion is still at higher level. DFT Study

The model of Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/CeAPSO-34 were established to research the synergistic effect present between the cerium in the framework and the surface. The Ce cation in the framework was labeled as Ce1 and on the surface neighbored by Ce site existed in the framework was labeled as Ce2. The adsorption energy of NO and NH3 molecule adsorbed on the surface of the catalysts were analyzed and the results shown in Table 1, and the optimized structures of NO and

NH3 adsorption over the catalysts was shown in Fig. 6. It was noted that the Ead(NO) and Ead(NH3) of Ce site supported on the surface are greater than Ce site existed in the framework, respectively. Meanwhile, the cerium incorporated in the framework and supported on the surface of the catalysts simultaneously, the Ead(NO) and Ead(NH3) of Ce sites are improved. Moreover, the Ead(NO) and Ead(NH3) of Ce site supported on the surface neighbored by Ce site existed in the framework are greater than other Ce sites. We know that the positive value of Ead reveals a strong interaction between adsorption surface and gas. Therefore, these results implied that Ce site supported on the surface neighbored by Ce site existed in the framework are more capable of reacting with NO and NH3, indicating a synergistic effect existing between the cerium in the framework and the surface. CONCLUSIONS

The synergistic effects of cerium existing in the framework and the surface of Mn-/SAPO-34 for low temperature NH3-SCR was investigated by in situ FT-IR and DFT calculations. The in situ FT-IR spectrum revealed that the strong synergistic effect would be beneficial to promote the adsorbtion of more nitrate species and NH3 species adsorbed on the surface of Mn-Ce/CeAPSO-34, and inhibit the deposition of sulfate species, which in the presence of SO2. Meanwhile, the theoretical results of DFT

Table 1. Adsorption energies of NO and NH3 on catalysts.

Sample Ead(NO) (eV) Ead(NH3) (eV)

Ce1 Ce2 Ce1 Ce2 Mn/CeAPSO-34 0.43 -- 0.67 -- Mn-Ce/SAPO-34 -- 0.52 -- 0.79 Mn-Ce/CeAPSO-34 0.61 0.75 0.94 1.08

Fig. 6. Optimized structures of NO and NH3 adsorptions on Mn/CeAPSO-34 (a, e), Mn-Ce/SAPO-34 (b, f) and Mn-Ce/CeAPSO-34 (c, d, g, h). The red balls are oxygen, purple balls are manganese, white balls are cerium, yellow balls are silicon, pink balls are aluminum, light pink balls are phosphorus.


calculations illumnate that the cerium incorporated in the framework and supported on the surface of the catalysts simultaneously were more capable of reacting with NO and NH3, enhancing the catalyst activity of low temperature NH3-SCR. ACKNOWLEDGMENT

The research was financially supported by the National Natural Science Foundation of China (NSFC-51478191) and Science and Technology Project of Guangdong Province (2014A020216003). REFERENCES Andreoli, S., Deorsola, F.A., Galletti, C. and Pirone, R.

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Received for review, May 9, 2017 Revised, July 31, 2017

Accepted, August 1, 2017

in situ ft-ir and dft study of the synergistic effects of ...a strong synergistic effect was...

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