New insights into the role of WO3 in improved activity and ammonium

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Kinetics, Catalysis, and Reaction Engineering

New insights into the role of WO3 in improved activity and ammonium bisulfate resistance for NO reduction with NH3 over V-W/Ce/Ti catalyst Chenxu Li, Meiqing Shen, Jianqiang Wang, Jun Wang, and Yanping Zhai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01031 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Industrial & Engineering Chemistry Research

New insights into the role of WO3 in improved activity and ammonium bisulfate resistance for NO reduction with NH3 over V-W/Ce/Ti catalyst

Chenxu Lia, Meiqing Shen*a,b,c, Jianqiang Wanga, Jun Wanga and Yanping Zhai*d

a

Key Laboratory for Green Chemical Technology of Ministry of Education,

School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China b

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin

300072, PR China c

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China

d

China Huadian Science and Technology Institute, Beijing 100070, PR China

* Corresponding author: Meiqing Shen, Yanping Zhai

Postal address: School of Chemical Engineering and Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China China Huadian Science and Technology Institute, Beijing 100070, China Email: [email protected]; [email protected] Tel./Fax: (+86) 22-27892301; (+86)-10-59216261 1

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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 The W-modified V-W/Ce/Ti-x% catalysts are prepared to investigate their SCR activity. The V-W/Ce/Ti-5% with 5% WO3 loading shows the superior NH3-SCR activity and higher ammonium bisulfate (ABS) resistance at 280 ˚C. 5% WO3 doping can induce the isolated tetrahedrally coordinated vanadium species to transform into the oligomeric ones and strongly combine with neighboring V and Ce via the synergetic interaction (V4+ + Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+), which favors the electron conduction among these active sites and significantly enhances the NOx conversion rate. Moreover, WO3 doping suppresses the formation of ABS and metal sulfate species over active sites by lowering the alkalinity of V-W/Ce/Ti-5%. The synergetic interaction through electron transfer can lower thermal stability of ABS species and incur NH3-SCR reaction between NO+O2 and ABS to completely prohibit the formation of ABS due to the faster de-NOx conversion rate with 5% WO3 loading.

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1.

Introduction

The ammonia selective catalytic reduction (SCR) is an efficient technology for abating nitrogen oxides from the exhausting gas1. V/WTi catalysts have been industrialized for de-NOx from the coal power plants. However, some SO2 species in the exhaust gas are usually oxidized by the NH3-SCR catalysts to produce SO3 species. Meanwhile, the surplus NH3 species can combine with SO3 species to produce ammonium (bi)sulfate species with the presence of H2O, when the operation temperature is low (< 290 ˚C). The formation of abundant ABS species can further accumulate and deactivate the SCR catalysts by plugging the pore and embedding vanadia species2. To avoid these issues, the operation temperature must be controlled above 290 ˚C. However, the de-NOx system may pause when the operation temperature lowers 290 ˚C under low load operation. To tackle the issues, there is a very urgent need to design a superior NH3-SCR catalyst with higher ABS resistance below 290 ˚C. Many researches were reported to enhance the NH3-SCR performance and SO2 resistance of SCR catalysts, recently. Doping other metallic oxides into SCR catalysts is an effective approach, such as WO3 promoter. The addition of WO3 can improve the low-temperature catalytic performance and widen operating temperature window by promoting the redox capability of active vanadia sites and providing more Brønsted acid sites 3. In addition, the SO2 tolerance can be enhanced due to the suppressed SO2 oxidation activity 4. Liu

5

found that the WO3-doped Fe2O3 catalyst possessed the

superior catalytic activity and SO2 resistance because of the synergetic influences of the both WO3 and Fe2O3. Hong 6 reported that 5 wt % W adding to Mn/Ce/Ti sharply 3



increased the amount of Mn4+ and Ce3+, which accounted for the higher SCR activity and SO2 tolerance. Nevertheless, few researches illustrated the promotion of WO3 adding on ammonium bisulfate resistance. Gao7 found that the introduction of WO3 promoted the thermal decomposition and SCR activity of NH4HSO4 deposited on Vbased catalyst by wet impregnation method. However, the behavior of impregnated ABS species is not similar to the as-formed ABS on catalysts under in situ conditions. Thereby, these studies were failing to represent the NH3-SCR reaction process of catalysts under in situ conditions with SO2. Besides, no researches emphasized the role of WO3 doping on the ABS resistance under in situ conditions with 1000 ppm SO2 below 290 °C. Recently, we have developed a superior Ce-doped V/WTi catalyst with an excellent NH3-SCR performance and ABS resistance in the presence of 1000 ppm SO2 at 280 °C 8

. Our previous research illustrated that the synergetic interaction between V and Ce

species contributed to the higher SCR activity and ABS tolerance. But, it is still unclear about the impact of WO3 on the catalytic performance of SCR catalysts with 1000 ppm SO2. Besides, many well used supports usually contain 8-10 wt% WO3 and the price of W resources is much higher compared to ten years ago

9,10

. To further explore the

promotion of WO3 doping into V-W/Ce/Ti catalysts and reduce the WO3 content to attain a more cost-effective SCR catalyst, the WO3 modified catalysts are developed to test their SCR activity. We find that the V-W/Ce/Ti-5% with 5% WO3 loading shows a higher NOx conversion and SO2 tolerance. Therefore, this research emphasizes on the role of WO3 doping into SCR catalysts on the structure-performance and further 4


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unravels the excellent ABS resistance of V-W/Ce/Ti-5% with 5% WO3 loading via in situ FTIR technology. More importantly, we also present the mechanism of the role of WO3 on the improved activity and ABS resistance for NO reduction with NH3 of VW/Ce/Ti-5%. This research is also expected to contribute to developing superior SCR catalysts with excellent ABS resistance in the low temperature range to meet the applied condition.

2. Experimental sections

2.1 Synthesis catalysts

Several V-W/Ce/Ti-x% catalysts with 1wt % V2O5 and x wt% WO3 were obtained using the wet impregnation method. The detail process of the synthesis catalysts was shown in supporting information and the weight percentage value of WO3 loading is x and the molar ratio value of Ce/V is 2.

2.2 Catalysts characterization

UV-visible (DRUV-vis), H2-TPR, XPS, Temperature-programmed decomposition (TPDC) experiment and in-situ FTIR techniques were used to characterize the obtained samples. The details of all the mentioned catalysts characterization are presented in Supporting Information.

5



2.3 NH3-SCR performance test with SO2

The details of the SCR activity measurements and kinetic analysis of catalysts under the standard NH3-SCR condition is also provided in supporting information. Meanwhile the influence of WO3 doping on NH3-SCR activity and ABS resistance is tested under the reaction gas of 500 ppm NO, 425 ppm NH3, 5% H2O, 5% O2, 1000 ppm SO2 and the balance of N2 (500 ml/min) with a GHSV of 18,000 h-1. To remove any impurities adsorbed on catalysts and obtain a clean catalytic surface, all the samples (0.2 g) are pre-treated at 500 ˚C within 30 min under 500 ml min-1 flow rate of N2 and then the NOx conversion is measured at 280 ˚C for various time.

3. Results

3.1 Activity study

Figure 1. The NOx conversion of the samples at 280 °C.

The NH3-SCR catalytic activity presents an obviously decrease with the introduction of 1000 ppm SO2. The de-NOx efficiency of V-W/Ce/Ti-0% is sharply 6


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reduced from 61 % to 45 % within 4 h (Figure 1). However, the NOx conversion only slightly decreases from 75 % to 62 % over V-W/Ce/Ti-3% under the same SCR condition after 4 h. With 5% WO3 loading, the SCR activity maintains 85% with the addition of 1000 ppm SO2 for 4 h. This result clearly demonstrates that V-W/Ce/Ti-5% catalyst possesses better SO2 resistance than V-W/Ce/Ti-0% catalysts. As shown in Figure S1, the addition of WO3 also results in the higher NOx conversion at low temperature and broadens temperature window. Combined with the above results, it can be concluded that 5% WO3 loading contributes to the superior low-temperature SCR activity of V-W/Ce/Ti-x% catalysts with a significant enhancement of ABS-resistant. These results also imply that the doping of 5% WO3 species interacting with V and Ce may lead to the higher NOx conversion with 1000 ppm SO2.

3.2 Characterization

3.2.1 UV-VIS

Figure 2. DRUV-VIS profile for the obtained catalysts.

Since the TiO2 support also absorbs light in the UV region of 200-400 nm, the TiO2 7



support is used as a reference material for the obtained catalysts. There are two obvious peaks at 318 and 383 nm appear over V-W/Ce/Ti-0% (Figure 2). The former band represents the charge transfer transitions derived from the monomeric vanadia species, while the later one is ascribed to the oligomeric tetrahedrally coordinated V5+ species11. With doping 3% WO3, the band at 318 nm decreases in intensity, indicating fewer isolated tetrahedrally coordinated V5+ species. In contrast, the band at 383 nm increases in intensity and moves to higher wavelength. For V-W/Ce/Ti-5% and V-W/Ce/Ti-7%, the band at 318 nm ascribed to the isolated tetrahedrally coordinated V5+ species completely disappears and the another one centered at 383 nm shifts to 385 nm. As the higher wavelength implying the higher coordination and degree of polymeric vanadia species 12, the band of oligomeric tetrahedrally coordinated V5+ species increasing from 383 nm to 385 nm with WO3 loading increasing from 0% to 5% indicates more oligomeric vanadia specie. So, it could be speculated that WO3 adding can occupy some loading sites on the surface of support and results in less ones for vanadia species. Therefore, 5% WO3 doping can enforce the closely proximate V species to aggregate and transform the isolated tetrahedrally coordinated vanadia species into the polymeric ones13. Accordingly, it is reasonably supposed that the doping of WO3 induces the isolated tetrahedrally coordinated V5+ species to transform into the oligomeric ones.

8


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3.2.2 H2-TPR

Figure 3. Influences of WO3 adding on the H2-TPR over V-W/Ce/Ti-x%. Table 1. H2-TPR measurements. V

Ce

W

μmol g-1

Catalysts

monomeric

oligomeric

V-W/Ce/Ti-0%

5.4

35.8

36.9

-

V-W/Ce/Ti-3%

3.1

38.3

43.6

24.3

V-W/Ce/Ti-5%

-

43.1

49.5

43.6

V-W/Ce/Ti-7%

-

43.2

51.2

66.2

Ce3+-O-Ce4+

In Figure 3, the V-W/Ce/Ti-0% sample presents three reduction peaks below 600 °C. As the redox ability increasing with the degree of polymeric vanadia species 14, the former hydrogen consumption perk at 245 represents the reduction of monomeric V oxides, while the later peak at 394 °C is due to the oligomeric ones 15. Meanwhile the surface oxygen of the nonstoichiometric ceria (Ce3+-O-Ce4+) shows one hydrogen consumption peak at 489 °C

16

. The new reduction peak of W6+ species over V-

W/Ce/Ti-3% catalyst appears at 553 °C 17. The reduction temperature of both V and Ce

9



species over V-W/Ce/Ti-3% is lower than that of V-W/Ce/Ti-0%. As for V-W/Ce/Ti5%, the typical reduction peak of isolated surface monomeric VOx species disappears and the reduction peaks of the oligomeric V oxides and the nonstoichiometric ceria species presents at 380 and 477 °C, respectively. The hydrogen consumption peak of WO3 also moves to lower temperature at 548 °C. In case of V-W/Ce/Ti-7%, the similar results are also observed. These results suggest that the strong interface among the W, V and Ce species improves the redox ability of these species with the WO3 doping. The disappearance of the oligomeric V oxides also indicates that these V species may aggregate and transfer into the oligomeric ones with increasing WO3 content. As listed in Table 1, the peak areas of different kinds of V species over the samples are calculated. As for V-W/Ce/Ti-0%, the dominate peak area of H2 consumption for the oligomeric V species is 35.8 μmol/g with a less H2 consumption of 5.4μmol/g for the isolated ones. With 3% WO3 loading, the H2 consumption of the isolated monomeric VOx species reduces to 3.1 μmol/g and that of the dominantly oligomeric V-O-V oxides species increases to 38.3 μmol/g. With 5% WO3 or even more adding, no H2 consumption of the isolated monomeric VOx species can be detected and the amount of dominantly oligomeric V oxides species gradually increases, which further confirms that the monomeric V oxides species transfer into the oligomeric ones with 5% WO3 doping. Compared with V-W/Ce/Ti-0%, 5% WO3 loading results in more H2 consumption originated from the total V species, meaning higher state of V species. The perk area of Ce3+-O-Ce4+ species increases from 36.9 μmol/g over V-W/Ce/Ti-0% to 49.5 μmol/g over V-W/Ce/Ti-5%, implying more surface oxygen originated from 10


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Ce3+-O-Ce4+. The larger amount of higher state V species and Ce3+-O-Ce4+ species combined with their promotional redox ability strongly indicates the synergistic interaction among the W, V and Ce species. Similarly, Kompio

18

reported that the

formation of V-O-W band attributed to the intimate mixing of both them contributed to the superior activity of V/WTi catalysts, which was also reported by Wang17. Moreover, both Shan19 and Peng20 also presented the existence of W-O-Ce band and the promotional redox ability of both them were due to their synergistic interaction. Therefore, it can be concluded that the WO3 doping may combine with the intimate V and Ce species to form V-O-W and Ce-O-W bands. Furthermore, WO3 loading on the surface of catalyst induces to form more oligomeric vanadia species, which are strongly bound to neighboring ceria via V-O-Ce band

21

to promote the synergistic interaction

between them and consequent more V5+ and Ce3+-O-Ce4+ species with a higher redox capability. As is well known, the WO3 species can play the “chemical ” the role via storing and transferring electrons to the adjacent species and promote their redox properties3. Combined with the above results, the presence of WO3 as the promoter may also store and transfer electrons to accelerate the reaction between Ce and V and further strengthen the synergistic interaction.

11



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3.2.3 XPS

Figure 4. the XPS results over V-W/Ce/Ti-x% catalysts. Table 2. XPS analysis over the V-W/Ce/Ti-x% catalysts. Atomic concentration Catalysts

V5+/(V5++V4+)

Ce3+/(Ce3++Ce4+)

Oα/Oα+Oβ

(%) V-W/Ce/Ti-0%

86.28

21.92

18.45

V-W/Ce/Ti-3%

88.74

26.47

22.72

V-W/Ce/Ti-5%

92.74

30.88

26.08

V-W/Ce/Ti-7%

92.85

32.66

27.29

The XPS results of V 2p (Figure 4a) are deconvoluted to detect the vanadium oxidation states. The profiles of V-W/Ce/Ti-0% presents two bands ascribed to V4+ and 12


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V5+ species(515.6 eV and 517.0 eV) 22, respectively. The bands are observed at 515.8 eV and 517.2 eV with 3% WO3 loading. With 5% WO3 adding, the peaks appear at much higher values at 515.9 eV and 517.3 eV, compared with V-W/Ce/Ti-0%, which means that WO3 adding can alter the chemical environment of V species. As the WO3 content increasing to 7%, the values of these binding energy are the same with that of V-W/Ce/Ti-5%, implying the similar chemical environment of V species. In addition, the surface percentages of V5+/(V5++V4+) is presented in Table 2. The V5+ percentage increases to 88.74% over V-W/Ce/Ti-3% and further reaches up to 92.74% over VW/Ce/Ti-5%, while only 86.28% is obtained over V-W/Ce/Ti-0%. The results indicate that WO3 doping increases the valence state of V species and contributes to more V5+ species23. In the O 1s profile, two obvious bands are observed. The band with lower binding energy (529.7 eV) is attributed to the lattice oxygen (Oβ), while the another one at 531.2 eV is belonged to surface oxygen(Oα)

24

. The percentage of Oα increases with the

amount of WO3. Owing to the greater mobility, the Oα species possesses the higher reactivity for the oxidation reaction in comparison with Oβ species

25

. So, the larger

amount of surface oxygen (Oα) species with WO3 doping promote the oxidation capability of catalysts and further enhance the low-temperature SCR activity. In Figure 4d, the bands labeled u, u′′ and u′′′ and v, v′′ and v′′′ are ascribed to Ce4+ 3d3/2 and Ce4+ 3d5/226, respectively. Peaks of u′ and v′, corresponding to 3d3/2 and 3d5/2, are related to the Ce3+ 3d final state27. The percentage of ceria species in low valence of V-W/Ce/Ti-5% is 30.88 %, while only 21.92 % is obtained over V-W/Ce/Ti-0%. The 13



ratio of Ce3+ increases with the amount of WO3 increasing from 0 % to 7%. Meanwhile, the bands of W 4f5/2 and 4f7/2 at 37.2 and 35.4 eV related to the W6+ state move to higher binding energy with the content of WO3 increasing from 3 % to 5 % 5, which reveals the stronger interaction among W, Ce and V species. Combined with the higher ratio of Ce3+ species, both the higher binding energy of V species and more V5+ species formation suggests the altering of the chemical environment of V species by redox cycle of V4+ + Ce4+ ↔ V5+ + Ce3+. In addition, more Ce3+ species and the higher binding energy of W species also imply the existence of the redox cycle of W5+ + Ce4+ ↔ W6+ + Ce3+, which further accelerates the electrons transfer among them. Therefore, 5% WO3 doping can strongly combine with the V and Ce species to form a positive interaction, which contributes to the electrons transfer among the active W, V and Ce sites and facilitates the NH3-SCR performance.

3.2.4 In situ FTIR spectroscopy

Figure 5. The variations and the relative reactive rate of the normalized in situ FTIR band intensity (1426 and 1205 cm-1, 1185 cm-1) during the process that samples are adsorbed by NH3 and purged by N2 flowing and then adding NO+O2 with various time at 200 ˚C.

14


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When the samples are saturated with the adsorbed ammonia species, the details are presented in Figure S6. With NO and O2 adding, the peaks of the ammonia linked to Lewis (L) acid sites decrease in intensity. Meanwhile, the peak of H2O at 1616 cm-1 is detected over V-W/Ce/Ti-0%28. These results suggest that these ammonia species bound to acid sites are activated and participate in the SCR reaction. Similarly, when NO and O2 are added, the peaks of the ammonia linked to both the Brønsted (B) and Lewis (L) acid sites present a remarkable decrease in intensity over V-W/Ce/Ti-5%. Meantime, the peak of H2O is also observed. In order to well understand the reactivity of the ammonia linked to both B (1426 cm-1) and L acid sites (1185 cm-1and 1205 cm1

) of the two samples 29, the normalized integral area consumption of the feature peaks

is plotted in Figure 5. As for V-W/Ce/Ti-5%, the adsorbed ammonia species bonded to both B and L acid sites simultaneously disappear within 15 min. However, only after 30 min, the peak of ammonia species on L acid sites (1185 cm-1) is completely vanished over V-W/Ce/Ti-0%. The results suggest that both B and L acid sites are crucial for the catalytic reactivity over V-W/Ce/Ti-5%. More importantly, the adsorbed NH3 species over V-W/Ce/Ti-5% are more reactive than that of V-W/Ce/Ti-0% due to the stronger interaction among the W, V and Ce species (V4+ +Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+). Besides, the WO3 doping can also provide some new B acid sites and contributes to more adsorbed ammonia species, which is in favor of the better NH3SCR performance. In addition, the double redox cycles enhance the electrons transfer among the Ce, V and W active sites to further facilitate the activation of NH3 adsorbed acid sites and nitrates species adsorbed on ceria sites, as a result, the superior low15



temperature catalytic activity over V-W/Ce/Ti-5%.

Figure 6. The profile of V-W/Ce/Ti-0% (a) and V-W/Ce/Ti-5% (b) catalysts saturated with SO2 and O2 at 280 °C and the bands of sulfated samples at 1800-1000 cm-1(c).

Figure 6a presents that several peaks at 1226, 1306 and 1370 cm-1 appear after the adsorption of SO2+O2 and increase in intensity with time, which are assigned to the chelating bidentate SO42- 30. Moreover, a new peak of coordinated sulfate species bound to vanadyl (V=O) species is also detected at 2045 cm-1 over V-W/Ce/Ti-0%, indicating the formation of VOSO4 species 31. The related peak of asymmetric vibration of O=S=O covalent (SO42-) originated from VOSO4 at 1383 cm-1 is overlapped within the stronger peak at 1370 cm-1 and thus no obvious band is observed. For V-W/Ce/Ti-5%, the similar bands at 1368, 1290 and 1190 cm-1 appear after SO2+O2 adsorption for 180 min. These peaks indicate that these sulphate are dominantly adsorbed on Ce32. In Figure 6c, the 16


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negative band at 2045 cm-1 disappears and no peak at 1383 cm-1 is observed, signifying no VOSO4 with 5% WO3 loading. Compared with V-W/Ce/Ti-0%, the bands belonged to the as-formed sulfate over V-W/Ce/Ti-5% shift to low wavenumber and obviously decease in intensity, which indicates the weaker stability and less amount of sulfate species. The WO3 adding may reduce the alkalinity of catalyst and lessen the sulfate species formation. Besides, the introduction of WO3 compresses the vanadia species to tightly connect with the intimate WO3 and ceria species, which strengthens the affinity among them and acts as protective agents for the active vanadia sites. More importantly, the presence of WO3 can also weaken the stability of sulfate species bound to ceria due to the electron transference among the active sites via the double redox cycles, resulting in less amount of weaker sulfate species on V-W/Ce/Ti-5%.

Figure 7. The spectra of NO+O2 adsorption over V-W/Ce/Ti-0% (a) and V-W/Ce/Ti-5% (b) pretreated by SO2 + O2 at 280 °C.

As shown in Figure 7a, the bottom spectrums of all the samples are obtained in a flow of SO2+O2 after 3h. After NO+O2 adding, several weak peaks at 1613 and 1330 cm-1 ascribed to the molecularly adsorbed NO2 33and cis-N2O2- species 34 are observed over V-W/Ce/Ti-0%. Besides, several negative bands at 1383, 1371 and 1362 cm-1 are 17



observed with time prolonging. The phenomenon can be explained that the adsorbed nitrates can displace these as-formed sulphate species bound to V, Ti 31 and Ce 32species, which also suggest the same adsorption sites on catalyst for both the nitrate and sulphate species. Figure 7b shows that two negative bands at 1423 and 1292 cm-1 are detected with adding NO+O2, which is due to the displacement of the sulphate adsorbed on catalyst by NOx species34. In addition, those peaks of adsorbed nitrates at 1613, 1378 and 1198 cm-1 appear with 5% WO3 loading.35 Both the negative bands attributed to the displacement of sulfate species and the bands of the adsorbed nitrate species over VW/Ce/Ti-5% are much stronger in intensity than that of V-W/Ce/Ti-0%, which indicates that the sulfates species are less stability and the as-formed nitrate species are stable enough to competitively inhibit the adsorption of SO2. Therefore, the adding of NO+O2 can result in less sulfate adsorbed on catalyst due to the weaker stability of those sulfate species with 5% WO3 doping.

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Figure 8. The spectra of V-W/Ce/Ti-0% (a) and V-W/Ce/Ti-5% (b) are pre-adsorbed by NH3 for 90 min and then adding SO2+O2 for another 6 h at 280 °C combined with band at 1426 cm-1 (c) over two samples pretreated at different condition.

After V-W/Ce/Ti-0% is pretreated by NH3 for 90 min, the adsorbed NH3 species are observed in Figure S8. When SO2 +O2 is added, several bands at 1426, 1383, 1334 and 1302 cm-1 appear at first 30 min (Figure 8a), which are ascribed to NH4+ adsorbed on B acid sites, VOSO4

31

, weakly adsorbed SO2 in form of SO32-30and the chelating

bidentate SO42- species, respectively. The new bands at 1266 and 1183 attributed to HSO4- species are detected with time and the band at 1426 cm-1 increase in intensity, indicating the formation of ABS species36. Thus, the bands at 1255, 1145 and 1080 cm1

for the sulfate species are speculated to be HSO4- of ABS

37

and their intensities

increase with time, which implies more ABS agglomeration and blocking on the surface of catalyst. Moreover, the band at 1383 cm-1 also manifests the formation of VOSO4 and some ABS species depositing on active V species. Figure.S8b illustrates that adsorbed ammonia species are also observed after 90 min. With the addition of SO2+O2, several peaks at 1426, 1260 and 1185 cm-1 appear and increase in intensity with time, which also suggests the formation of ABS species. However, no peaks of VOSO4 19



species are observed 31, revealing that the ABS species primarily form and agglomerate on TiO2 support or ceria species. As shown in Figure S9, the intensity of bands is much weaker with 5% WO3 doping. Especially, the V-W/Ce/Ti-0% possesses the obviously stronger intensity of the band at 1426 cm-1 in comparison with V-W/Ce/Ti-5%, implying less ABS with 5% WO3 loading. Thus, the presence of WO3 can inhibit the formation of ABS species and protect vanadia species by avoiding the formation of VOSO4 species, benefiting for ABS resistance over V-W/Ce/Ti-5%.

Figure 9. The variations and the relative reactive rate of the normalized in situ FTIR band intensity (1426 cm-1) during the process that samples are pre-sulfated and then adding NO+O2 with various time at 280 ˚C.

All the samples are firstly sulfated by NH3+SO2+O2. Afterward, the NO+O2 is switched into the reaction system. As for V-W/Ce/Ti-0%, the addition of NO+O2 results in an obvious intensity weakening of adsorbed ammonia species (Figure S10). The band of the coordinated ammonia species at 1601cm-1

29

thoroughly vanishes after 14 min.

Accordingly, the band of the adsorbed ammonia species at 1426 cm-1 mainly derived from as-formed ABS disappears after 22 min. The longer time for the complete disappearance of the band at 1426 cm-1 takes, the lower reactivity of this kind of 20


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ammonia species dominatingly originated from ABS is. The bands at 1426, 1266 and 1183 cm-1 corresponding to as-formed ABS species also become weaker with time. Some new bands of sulfate species (1370, 1306 and 1180 cm-1) appear at the same time. Similar with the results of V-W-Ce/Ti-0%, the intensity of band at 1426 cm-1 also presents a slower decrease than that of the band at 1601 cm-1 with 5% WO3 adding. With NO+O2 introducing, several new peaks at 1367 and 1180 cm-1 are also detected after 30 min. The intensities of those bands become much weaker than that of VW/Ce/Ti-0%, meaning less sulfate species and thus fewer ABS formation in the presence of WO3. Figure 9 presents the relative reactive rate of the normalized in situ FTIR band intensity (1426 cm-1) during the reaction between NO+O2 and as-formed ABS species of the obtained samples at 280 ˚C. It is obvious that the as-formed ABS over the two samples possess different catalytic reactivity. The ammonia species of ammonium bisulfate with superior reactivity over V-W/Ce/Ti-5% is thoroughly consumed within 12 min. However, the peak of those ABS species with poor reactivity over V-W/Ce/Ti-0% thoroughly disappears within 22 min. The results suggest that the addition of 5% WO3 leads to the higher catalytic reactivity of the as-formed ABS in comparison with V-W/Ce/Ti-0%, which may be attributed to the weaker stability of asformed ABS and the superior NOx conversion rate in the presence of 5% WO3.

21



Figure 10. The spectra of NH3-SCR reaction over V-W/Ce/Ti-0% (a) and V-W/Ce/Ti-5% (b) with or without SO2 at 280 °C.

To reveal the ABS resistance mechanism of V-W/Ce/Ti-5% at 280 °C, the spectra of NH3-SCR process over both samples without and with SO2 are recorded. Figure S11 presents that the dominantly adsorbed ammonia with a weak band of the bridging bidentate nitrates at 1542 cm-1 21 are observed over V-W/Ce/Ti-0% exposed to the reaction gas containing the NH3+ NO+O2 for 60 min. These obvious bands deprived from the adsorbed reactants imply an inferior NH3-SCR activity on V-W/Ce/Ti-0%. As illustrated in Figure 10, the introduction of 1000 ppm SO2 results in the appearance of several peaks at 1183, 1266 and 1302 cm-1 and strengthens the intensity of these bands, revealing the generation of sulphate. More importantly, the peak at 1426 cm-1 obviously raises with time, suggesting fast generation of ABS. However, the peaks of ammonia and sulphate specie are lower in intensity than that of Figure 8a, implying that the NH3SCR process has a negative effect on the formation of ABS. The peak of VOSO4 species at 1383 cm-1 also appears, locking and poisoning the vanadia species. So, the poor SCR reactivity of V-W/Ce/Ti-0% leads to the generation of ABS and VOSO4 species during the process of NH3-SCR reaction with SO2 at 280 °C. Moreover, more as-formed ABS 22


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species aggregating and depositing on catalyst further deactivate the catalyst, which is well coincident with the results of Catalytic Activity (Figure 1). The Figure.10b also shows that several weak bands of adsorbed NH3 species are detected and the band of H2O at 1616 cm-1 is obvious within 1 h, which indicates the higher NH3-SCR activity. After adding SO2, some weak bands of sulphate bound to Ce species at 1147 and 1356 cm-1 are observed. The peak of adsorbed ammonia species (1426 cm-1) maintains the same strength without the appearance of peak at 1383 cm-1 after 90 min. The results illustrate that no ABS and VOSO4 can be observed on V-W/Ce/Ti-5% during the NH3SCR process with the adding of SO2. So, 5% WO3 doping can also protect V species from being deactivated by SO2. Compared with V-W/Ce/Ti-0%, the bands of sulfate species are much weaker and still unchanged within 90 min, revealing that the adsorption of SO2 is restrained in the condition of NH3-SCR reaction and fewer weakly adsorbed sulfate species with 5% WO3 doping. In addition, 5% WO3 loading leads to the higher SCR catalytic activity and no ABS generation due to the inferior reactivity of the side reaction between NH3 and SO2 species. Thereby, it can be concluded that the V-W/Ce/Ti-5% still possesses the higher NOx conversion after 1000 ppm SO2 introducing under NH3-SCR reaction condition.

23



Figure 11. The variations and the relative decomposition rate of ABS calculated by the Differential Intensity of 1426 cm-1 over sulphated V-W/Ce/Ti-0% (a) and V-W/Ce/Ti-5% (b).

As shown in Figure S12, the peaks corresponding to ABS could be detected over pretreated samples. The band at 1302 cm-1 belonged to bidentate SO42- shift towards higher wavenumbers and the bands at 1426, 1266 and 1183 cm-1 ascribed to ABS species decrease in intensity with temperature rising, which is owing to the decomposition of as-formed ABS on V-W/Ce/Ti-0% 38. Further increasing temperature, the bands of S-containing group of ABS disappear and transform into sulfate species. The similar decomposition behavior is also observed with 5% WO3 loading (Figure 11b). The variations in the relative concentrations of NH4+ derived form as-formed ABS and the relative decomposition rate of ABS calculated by the Differential Intensity of 1426 cm-1 during the heat process are illustrated in Figure 11. The peak at 350 °C is observed over V-W/Ce/Ti-0% in the Differential Intensity profile, while the peak of VW/Ce/Ti-5% movers to low temperature at 335 °C, indicating the lower decomposition temperature of as-formed ABS. Therefore, the presence of WO3 weaken the thermal stability of as-formed ABS and accelerate their decomposition over V-W/Ce/Ti-5%.

24


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3.2.5 Thermal stability of as-formed ABS on catalysts

Figure 12. NH3 and SO2 profiles of V-W/Ce/Ti-0% (a) and V-W/Ce/Ti-5% (b) pretreated by NH3+SO2+O2 for 6 h (V-W/Ce/Ti-0%-S and V-W/Ce/Ti-5%-S) and fresh samples pre-adsorption by NH3 (V-W/Ce/Ti-0%-F and V-W/Ce/Ti-5%-F) at 280 °C and then heated in the flow of N2 at rate of 10 °C/min. Table 3. Quantities of the different surface species on the various catalysts. NH3a

NH3b

SO2a

NH3 amount c

Samples (μmol gcat-1) V-W/Ce/Ti-0%

81.62

10.54

20.16

71.08

V-W/Ce/Ti-5%

55.46

19.59

14.11

35.87

a Amount estimated from the TPDC processes over V-W/Ce/Ti-0%-S and V-W/Ce/Ti-5%-S. b Amount estimated from the NH3-TPD processes over V-W/Ce/Ti-0%-F and V-W/Ce/Ti-5%-F. c Amount originating from the ABS formed over sulfated samples.

Several peaks of SO2 and NH3 desorption upon TPDC of the sulfated catalysts are shown in Figure 12. As for V-W/Ce/Ti-0%-S, the TPDC curves presents a maximum NH3 desorption at 350 °C with a major peak of SO2 at 423 °C. The desorption temperature of NH3 is lower than the NH3 desorption temperature of V-W/Ce/Ti-0%-F at 380 °C and the total amount of NH3 species increases from 10.54 μmol gcat-1 to 81.62 25



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μmol gcat-1, which confirms the ABS species formation on V-W/Ce/Ti-0% during sulfation process. The amount of NH3 originated from ABS is calculated by subtracting the total NH3 storage capacity of V-W/Ce/Ti-0%-F from that of V-W/Ce/Ti-0%-S, which are listed in Table 3. As for V-W/Ce/Ti-5%-S, both the desorption peaks of NH3 and SO2 shift to lower temperature at 333 °C and 410 °C, which indicates the weaker thermal stability of ABS species. The amount of NH3 derived from the as-formed ABS species is 35.87 μmol gcat-1, which is much less than the 71.08 μmol gcat-1 of V-W/Ce/Ti0%. Therefore, it can be concluded that 5% WO3 loading results in less ABS on catalyst. More importantly, 5% WO3 adding can also obviously weaken their stability over VW/Ce/Ti-5%. 3.3 Kinetic analysis

Figure 13. The influences of WO3 doping on the kinetic analysis (a) and the rateNO for

NH3-SCR reaction over obtained samples (b). Table 4. Apparent activation energy (Ea). Catalysts

V-W/Ce/Ti-0%

V-W/Ce/Ti-x% (x= 3, 5, 7)

Ea

55.0±0.5 26


49.0±0.5

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(kJ mol-1)

The Arrhenius plots over the samples are presented in Figure 13 and the corresponding energy barrier (Ea) are listed in Table 4. Compared with the V-W/Ce/Ti0% (Ea=55.0±0.5 kJ mol-1), the addition of WO3 results in an obvious decrease of Ea (49.0±0.5 kJ mol-1). Accordingly, the WO3 loading leads to the sharply increasing of the intrinsic reaction rate for NH3-SCR. Especially, the V-W/Ce/Ti-5% with 5% WO3 loading shows triple higher rateNO in comparison with V-W/Ce/Ti-0%, which is benefit for the superior SCR activity and ABS tolerance. Therefore, 5% WO3 adding can obviously improve the NH3-SCR performance by decreasing the Ea and increasing the NOx conversion rate.

4. Discussion

4.1 Influence of WO3 doping on SCR activity.

The results of this research illustrate V-W/Ce/Ti-5% presents a superior NOx conversion compared with V-W/Ce/Ti-0% with 1000 ppm SO2. UV-vis results reveal that 5% WO3 doping can induce the isolated tetrahedrally coordinated vanadium species to transform into the oligomeric ones due to less amounts of available sites on TiO2 support, as illustrated in scheme 1. In comparison with the former, the oligomeric ones have tenfold higher SCR activity 18, thereby benefiting for the excellent NOx conversion over V-W/Ce/Ti-5% at 280 °C. Meanwhile, H2-TPR and XPS illustrates that 5% WO3 loading can enforce the neighboring oligomeric V oxides species bound to the intimate 27



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Ce via V-O-Ce band to generate larger amount of V5+ and Ce3+ species. More Ce3+ and W6+ species also indicate that the WO3 species strongly connect with the affinitive ceria species via W-O-Ce band, which can further strength the interaction among the W, V and Ce species to promote their redox capability. Consequently, three redox couples of V5+/V4+, Ce4+/Ce3+ and W6+/W5+ appear and the double redox cycles (V4+ + Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+) exist over V-W/Ce/Ti-5%. The electron transfer among the active V, Ce and W sites via the double redox couples can be facilitated as following9, 39: 𝑉 5+ − 𝑂2− − 𝐶𝑒 4+ + e ↔ 𝑉 5+ − 𝑂2− − 𝐶𝑒 3+ ↔ 𝑉 4+ − 𝑂2− − 𝐶𝑒 4+ ↔ 𝑉 5+ − 𝑂2− − 𝐶𝑒 4+ + e

(redox I)

𝑊 6+ − 𝑂2− − 𝐶𝑒 4+ + e ↔ 𝑊 6+ − 𝑂2− − 𝐶𝑒 3+ ↔ 𝑊 5+ − 𝑂2− − 𝐶𝑒 4+ ↔ 𝑊 6+ − 𝑂2− − 𝐶𝑒 4+ + e

(redox II)

The double redox cycles can contribute to the electron conduction among the active V, Ce and W sites and favor to activate the adsorbed reagent species for NH3-SCR reaction. NH3-TPD results reveal that WO3 loading can provide more adsorbed ammonia specie and thereby contribute to the phenomenal NOx conversion of V-W/Ce/Ti-5% with 5% WO3 at low temperature. What’s more, these adsorbed ammonia species (on both B and L sites) of V-W/Ce/Ti-5% possess the higher NH3-SCR activity than that of V-W/Ce/Ti-0% attributed to the existing of double redox cycles (V4+ + Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+). The double redox cycles can contribute to the electron transfer from the adsorbed ammonia species to the V and W acid sites to facilitate the activation of those ammonia species bound to the acid sites with 5% WO3 28


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doping. On account of the higher efficiency of electron transport among the active V, Ce and W sites through the double redox cycles, the reduced state vanadium and tungsten species are easily re-oxidized by the immediate vicinity of ceria via the electron conduction, which results in the faster replenishment of active sites and rapidly achieves the closed reaction cycle. The higher efficiency of electron transport also favors to reduce the apparent activation energy and contributes to the superior de-NOx efficiency of V-W/Ce/Ti-5% with the higher intrinsic reaction rate. Moreover, 5% WO3 doping induces to produce a larger amount of Ce3+ attributed to the electron conduction and results in a great deal of highly active surface oxygen(Oα), which promotes to activate the nitrate species adsorbed on ceria to further benefit for the SCR activity over V-W/Ce/Ti-5%. As illustrated in scheme 1, we suppose that the higher efficiency of electron transfer among the active V, Ce and W sites via the double redox cycles is responsible for the excellent catalytic reactivity with the 5% WO3 doping.

4.2 Influence of WO3 loading on ABS resistance of V-W/Ce/Ti-5%

Scheme 1. Proposed structure-activity relationship with the higher ABS resistance after 5% WO3 29



loading.

It can be observed that 5% WO3 adding results in less adsorbed acid SO2 species by lessening the alkalinity of V-W/Ce/Ti-5%. Besides, the thermal stability of adsorbed SO2 species decreases because of the formation of the double redox couples, which further restrains the adsorption capacity of SO2 with the addition of NO+O2. Two above points result in less amount of weakly adsorbed SO2 and thereby much fewer ammonium bisulfate species with 5% WO3 loading. The WO3 adding also contributes to the larger amount of Ce3+ species by captured electrons from the adjacent V and W, which can deviate the electrons originated from Ce toward those depositing ammonium bisulfate species to weaken their stability over V-W/Ce/Ti-5%. Consequently, the asformed ammonium bisulfate species of V-W/Ce/Ti-5% with 5% WO3 loading possess the lower thermal stability than that of V-W/Ce/Ti-0%, which is further confirmed by the TPDC and the in situ FTIR-TPD. More importantly, the less stable ammonium bisulfate species can react with NO+O2 and participate in the SCR reaction by offering NH4+ species, which can break the equilibrium of ammonium bisulfate generation and further facilitate their decomposing with the doping of 5% WO3. Compared with VW/Ce/Ti-0%, the V-W/Ce/Ti-5% with 5% WO3 loading presents the higher activity of these ammonium bisulfate species due to their weaker stability and the superior deNOx reaction rate at 280 °C. In addition, the 5% WO3 doping can also protect V species from being deactivated via avoiding the formation of VOSO4 over V-W/Ce/Ti-5% under the reaction condition containing 1000 ppm SO2 at 280 ˚C. As the sulfates species are less stability and the as-formed nitrate species are stable enough to competitively inhibit the adsorption of SO2, the 5% WO3 loading can still restrain sulphate bound to 30


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Ce sites and prevent the ceria from being deactivated under NH3-SCR reaction with SO2 at 280 ˚C. Besides, less ABS generate during the NH3-SCR process with 1000 ppm SO2 over V-W/Ce/Ti-5% because that the ammonia species in the reaction gas can be competitively consumed by NO+O2. Furthermore, 5% WO3 doping contributes to the outstanding de-NOx efficiency rate because of double redox cycles (V4+ + Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+) and leads to all the ammonia specie completely consumed by NH3-SCR reaction, as a result, almost no ammonia species are available to react with sulphate to form ABS. The rapider the de-NOx efficiency rate is, the fewer ammonium bisulfate species generate. Therefore, the proposed structure-activity relationship with the higher ABS resistance after 5% WO3 loading are presented in scheme 1. It can be concluded that 5% WO3 doping suppresses the formation of ammonium bisulfate (ABS) and metal sulfate species over active sites. The electron conduction via the double redox cycles lowers thermal stability of ammonium bisulfate species and incurs NH3-SCR process between NO+O2 and ABS to completely inhibit the formation of ABS over V-W/Ce/Ti-5%. So, V-W/Ce/Ti-5% shows the superior NOx conversion and ammonium bisulfate tolerance at 280 ˚C. Based on above discussions, our research not only unravels the mechanism of the enhancement of 5% WO3 doping on the de-NOx catalytic activity with the higher ammonium bisulfate resistance, but also contributes to developing a more cost-effective SCR catalyst with lower WO3 content to meet the applied condition for coal power plants.

31



5. Conclusions

A series of W-modified catalysts are prepared and their de-NOx conversions are fully illustrated. Among all the obtained catalyst, V-W/Ce/Ti-5% with 5% WO3 loading shows the superior NH3-SCR activity and higher ABS resistance at 280 ˚C. The characterization results reveal that 5% WO3 adding can induce the isolated tetrahedrally coordinated vanadium species to transform into the oligomeric ones and strongly combine with neighboring V and Ce via the synergetic interaction (V4+ + Ce4+ ↔ V5+ + Ce3+ and W5+ + Ce4+ ↔ W6+ + Ce3+), which favors the electron conduction among these active sites. The higher efficiency of electron conduction facilitates to activate the adsorbed reagent, which contributes to the faster de-NOx conversion rate. Besides, 5% WO3 loading results in less amount of weakly adsorbed SO2 species by altering the alkalinity of V-W/Ce/Ti-5%, which not only suppresses the formation of ABS and metal sulfate species over active sites but also weakens the thermal stability of both them. The synergetic interaction through electron transfer can lower thermal stability of ABS species and incur NH3-SCR reaction between NO+O2 and ABS to completely prohibit the formation of ABS due to the faster de-NOx conversion rate with 5% WO3 loading. Supporting information Catalyst preparation and characterization; NH3-SCR performance (Figure S1); XRD (Figure S2) and BET (Table S1); NH3 and NO+O2-TPD (Figure S3 and Figure S5); In situ FTIR profiles (Figure S4 and Figures S6-S12)

32


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Acknowledgements

This research is financial supported by National key R&D program (2017YFC0211302) and China Huadian Science and Technology Institute (CHDI.KJ-20). Besides, first author also appreciates his pretty girlfriend (Yahuang Han) for her many valuable comments to promote the quality of the manuscript.

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New insights into the role of WO3 in improved activity and ammonium

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