Aiming to develop low cost and environmentally friendly SCR catalysts, various transition metals and rare earth elements have been used and combined to prepare polymetallic oxide catalysts in recent years, which generally exhibit excellent SCR performance. Here four typical metal elements (Mn, Ce, W and Ti) have been selected to prepare polymetallic oxides (WMnCeTiOx) catalysts, and the effects of preparation methods, such as coprecipitation (CP), sol-gel (SG), and deposition precipitation (DP), on the performance of selective catalytic reduction of NO with NH3 at 160~360 ºC have been investigated systematically. Some key techniques of XRD, BET, XPS, H2-TPR, NH3-TPD and in-situ DRIFTS were used to characterize the physicochemical properties of the prepared WMnCeTiOx catalysts. The SCR activity measurement results showed that NH3-SCR activities of WMnCeTiOx catalysts prepared through coprecipitation and deposition precipitation methods exhibited much better NO removal performance in the temperature range of 160~360 ºC than that prepared via sol-gel method. More importantly, WMnCeTiOx catalysts prepared through coprecipitation method exhibited superior SO2 and H2O tolerance compared with those prepared via deposition precipitation and sol-gel methods. It demonstrated that the preparation process imposed a crucial impact on the physicochemical properties of the prepared WMnCeTiOx catalysts, thus influencing on their SCR performance. According to the characterization results, the better performance of WMnCeTiOx(CP) catalyst could be ascribed to its higher concentrations of Mn4+ and Ce3+, larger surface area and highly-dispersed active species on catalyst surface. In addition, compared with WMnCeTiOx catalysts prepared by deposition precipitation and sol-gel methods, WMnCeTiOx(CP) catalyst possessed much more surface acid sites, stronger reduction capacity and better adsorption capacity for NH3 and NO, which might also play a key role in promoting its SCR performance.
Research Article
Characterization of W-Mn-Ce-Ti Oxides Catalysts Prepared by Different Methods for Selective Reduction of NO with NH3
https://doi.org/10.21203/rs.3.rs-658018/v1
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Selective Catalytic Reduction
Polymetallic Oxides Catalyst
WMnCeTiOx
Preparation Method
in-situ DRIFTS
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WMnCeTiOx catalysts were prepared by coprecipitation, deposition precipitation and sol-gel .
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The catalyst prepared by coprecipitation showed excellent NH3-SCR activity and much better SO2 tolerance.
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The catalyst prepared by coprecipitation possessed much more surface acid sites, stronger reduction capacity and better adsorption capacity for NH3 and NO.
At present, nitrogen oxide (NOx) emitted from stationary and mobile sources are still one of main pollutants to the atmosphere, which is related with a series of environmental problems, such as acid rain, photochemical smog, green-house effect, ozone depletion, haze and so on (Bosch. 1988; Castro et al. 2001). Selective catalytic reduction (SCR) with NH3 is the most widely-used technology to control NOx emission due to its high removal efficiency and low running cost (Park et al. 2013; Cimino et al. 2016). As the core of NH3-SCR system, the commercially available catalyst is usually V2O5-WO3(MoO3)/TiO2 (Pei et al. 2018; Wu et al. 2020). It exhibits desirable performance in eliminating NOx with excellent thermal stability, but its narrow working temperature range (300 ~ 400°C), poor low temperature activity and toxicity of vanadium species are still critical challenges, which hinders their applications especially in low-temperature flue gas cases (Yan et al. 2017; Li et al. 2016). Thus, it is very imperative to develop novel vanadium-free NH3-SCR catalysts with a wide working temperature window.
It is known that manganese oxides (MnOx) contain multivalent Mn species (Mn4+, Mn3+, Mn2+) and abundant labile lattice oxygen. Since MnOx catalyst exhibits good SCR low-temperature activity in NH3-SCR reaction, it is regarded as one of the promising alternatives for vanadium-based catalysts (Liu et al. 2016; Xin et al. 2018). But it is difficult for MnOx catalysts to maintain high denitrification activities in a wide working temperature window, and its N2 selectivity is also very poor especially with the increase of operating temperature (Chen et al. 2017). Up to now, several strategies, such as dispersion on a high-surface-area support (Li. 2013; Sang et al. 2012; Gang et al. 2013), modification by mixing with other metal oxides, and doping with other ions (Zhang et al. 2021; Zhang et al. 2020), have been employed to improve the de-NOx activity of MnOx catalysts. Moreover, rare-earth elements are often used as active catalyst components or catalyst promoters due to their incompletely occupied 4f and empty 5d orbitals (Yang et al. 2002). Among them, Ce has attracted increasing attentions due to its unique redox properties and high oxygen storage capacity, and it is of great potential to combine Ce with Mn to develop a novel SCR catalyst (Kwon et al. 2015). Boningari et al demonstrated that the introduction of ceria in MnOx could improve the acid sites distribution and increase the concentration of acid sites (Boningari et al. 2015). Lin et al. prepared Mn-Ce oxide catalysts by sol-gel method yielded nearly 98% NO removal efficiency below 200 ℃ (Lin et al. 2019). TiO2 is generally used as a support for commercial catalysts (Phil et al. 2008), but it can also play a crucial role in modifying the polyoxides catalysts to improve the comprehensive SCR performance such as N2 selectivity and SO2 resistance in NH3-SCR reactions (Zhang et al. 2019). As to WO3, it has been extensively used as stabilizer and promoter in the traditional V2O5-WO3/TiO2 catalysts during the past decades, and it was found that WO3 could greatly enhance the amount and strength of surface Bronsted acid sites and NO oxidation ability (Ma et al. 2015).
In recent years, more and more researchers around the world devoted to combine various metal oxides to develop a low cost and environmentally friendly SCR catalysts. Some previous work indicated that Ce-Mn-Ti ternary metallic oxides exhibited excellent catalytic performance and became a research hotspot for SCR application. Here W was selected to combine with Mn-Ce-Ti to prepare quaternary metal oxides catalysts aiming to further improve their SCR activity. Considered that, the catalytic activity of W-Mn-Ce-Ti oxides catalysts is also closely related to the dispersion and interaction of active components (Wang et al. 2019). And the catalyst preparation method is the key factor for determining the interaction and dispersion of active components, therefore, it is very meaningful to investigate the effects of preparation methods on the SCR activities of W-Mn-Ce-Ti oxides catalysts, and further explore the relation of structure and activity of WMnCeTiOx catalysts.
In this work, W-Mn-Ce-Ti quaternary metal oxides catalysts have been prepared via three typical methods including coprecipitation (CP), sol-gel (SG), and deposition precipitation (DP). The results indicated that NH3-SCR activities of W-Mn-Ce-Ti catalysts prepared through coprecipitation and deposition precipitation methods exhibited much better NO removal performance in the temperature range of 160 ~ 360 ℃ than that prepared via sol-gel method. More importantly, W-Mn-Ce-Ti catalysts prepared through coprecipitation method exhibited superior SO2 and H2O tolerance compared with those prepared via deposition precipitation and sol-gel methods. XRD, BET, XPS, H2-TPR, NH3-TPD and in-situ DRIFTS were adopted to characterize the physicochemical properties of the prepared W-Mn-Ce-Ti catalysts, and the relevant influencing mechanism and the possible reaction pathways were discussed in detail.
2.1 Catalyst preparation
The W-Mn-Ce-Ti catalysts were prepared using three different methods, namely CP, DP, and SG. TiO(SO4)·xH2SO4·xH2O, TiO2, Mn(NO3)2·4H2O, Ce(NO3)3·6H2O and (NH4)10W12O41~xH2O were used as the precursors for Ti, Mn, Ce and W, respectively. Deionized water was used as solvent. NH4OH (25-28%) and citric acid (C6H8O7, 99.5%) were used as precipitator and complexing agent, respectively. The molar ratio of W, Mn, Ce, and Ti was set at 0.15:0.075:0.3:1.
2.1.1 Coprecipitation method
In a typical preparation process, 1.030 g (NH4)10W12O41~xH2O, 0.505 g Mn(NO3)2·4H2O, 3.505 g Ce(NO3)3·6H2O, and 5.981 g TiO(SO4)·xH2SO4·xH2O were dissolved in 100 mL deionized water, followed by stirring thoroughly at 60 °C for 4 h. Then ammonia solution was added dropwise until solution pH reached at 10. After aging for 12 h, the synthetic precipitate was filtered. Then, the mixture solution was dried at 110 °C for 5 h and calcined at 500 °C for 3 h. The catalysts were crushed and sieved to 40-60 mesh for activity evaluation. The obtained catalyst was marked as WMCT(CP).
2.1.2 Deposition precipitation method
In a typical preparation process, 1.030 g (NH4)10W12O41~xH2O, 0.505 g Mn(NO3)2·4H2O and 3.505 g Ce(NO3)3·6H2O were dissolved in 100 mL deionized water, followed by stirring thoroughly at 60 °C for 2 h. 2.5 g TiO2 powder (Degussa P25) was added into the mixture solution, followed by stirring thoroughly at 60 °C for 2 h. Then ammonia solution was added dropwise until solution pH reached at 10. After aging for 12 h, the synthetic precipitate was filtered. Then, the mixture solution was dried at 110 °C for 5 h and calcined at 500 °C for 3 h. The catalysts were crushed and sieved to 40–60 mesh for activity evaluation. The obtained catalyst was marked as WMCT(DP).
2.1.3 Sol-gel method
In a typical preparation process, 1.030 g (NH4)10W12O41~xH2O, 0.505 g Mn(NO3)2·4H2O and 3.505 g Ce(NO3)3·6H2O were dissolved in 100 mL deionized water, followed by stirring thoroughly at 60 °C for 2 h. 7.810 g C6H8O7 were dissolved in the mixture solution, followed by stirring for 0.5 h. The pH value of the resulting liquid mixture was maintained at 10, followed by stirring thoroughly at 80 °C for 4h form a transparent sol. After aging for 12 h, the sol was dried at 150 °C for 5 h to form a gel, followed by calcined at 500 °C for 3 h. The catalysts were crushed and sieved to 40–60 mesh for activity evaluation. The obtained catalyst was marked as WMCT(SG).
2.2. SCR activity test
The SCR performance of all prepared catalysts were evaluated in a fixed-bed, stainless steel reactor (inner diameter: 6 mm). A catalyst of 0.5 mL was used for catalytic assessment with a gas hourly space velocity (GHSV) of 30,000 h−1. The feed-gas mixture contained 500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O (if used), 100 ppm SO2 (if used), and N2 as the balance gas. The operating temperature window was 160~360 °C. The concentrations of out gas were monitored continuously by a FTIR spectrometer (iS50, Thermo Fisher Scientific) equipped with a heated low-volume multiple-path gas cell (2 m) and a MCT detector cooled by liquid nitrogen. The NO removal efficiency and N2 selectivity were calculated as follows:
2.3. Catalyst characterization
The surface structure parameters of these catalysts were measured with a physisorption (ASAP 2020 PLUS, Micromeritics) by N2 adsorption at 77 K, using the Brunauer-Emmet-Teller (BET) method to determine their specific surface area. The pore volumes and pore size distributions were calculated from the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) algorithm. The sample was degassed for 4 h at 350 ℃ before each analysis.
Powder XRD patterns were performed on an X-ray diffraction meter (Empyrean, PANalytical) with Cu Kα (40 kV, 30 mA) radiation. The diffractogram was recorded in the 2θ range of 10 to 80 ° with a scan rate of 0.02 ° 2θ s-1.
H2-temperature programmed reduction (H2-TPR) and NH3-temperature programmed desorption (NH3-TPD) experiments were carried out on a chemisorption analyzer (Autochem II 2920, Micromeritics). Prior to each test, the samples were pretreated at 350 ºC in a flow of high purified Helium (50 mL/min) for 1 h. Then the catalysts were cooled down to 50 ℃ in a flow of Helium. Next, the samples were flushed at 100 ℃ for 2 h in pure He stream (50 mL/min) to remove physically absorbed NH3. Finally, TPD profiles were obtained by heating the samples from 100 to 400 ºC with a ramping rate of 10 ℃/min in He stream (50 mL/min).
X-ray photoelectron spectroscopy (XPS) profiles were collected using a surface analysis photoelectron spectrometer (Kratos AXIS Supra, Shimadzu), with Al Kα radiation.
The Fourier Transform InfraRed Spectrometer (FTIR) and the in-situ Diffuse Reflectance Infrared Fourier Transform spectra (DRIFTS) were collected using a Nicolet iS50 spectrometer to detect the functional group and the reaction behavior between adsorbed species, respectively. Prior to each test, the catalysts were pretreated in a flow of N2 at 350 ℃ for 30 min and then cooled to room temperature. Then background spectra recorded in N2 flow were automatically subtracted from the corresponding spectra. The spectral resolution was 4 cm-1 with co-addition 64 scans.
3.1. NH3-SCR performance
NO removal efficiencies of WMnCeTiOx catalysts prepared by various methods were illustrated in Fig. 1(a). The figure showed that the WMnCeTiOx catalysts prepared by the CP and DP methods yielded good catalytic activities over a broad temperature range, and nearly 97 % NO removal efficiencies was achieved in the temperature range 200 ~ 360 ºC. By contrast, the WMCT(SG) catalyst could only achieve similar NO removal efficiency in the temperature range 280~360 ºC. Interestingly, the N2 selectivity of all these catalysts is extremely high (~99 %) in 160~360 ℃ (shown in Fig. 1(b)).
H2O and SO2 are main components of flue gas from stationary and mobile sources. Therefore, it is necessary for any low temperature SCR catalyst with application potential to be simultaneously resist to H2O and SO2 degradation and poison (Guo et al. 2009; Yezerets et al. 2016; Zhang et al. 2019 ).
The effects of 100 ppm SO2 on NO removal efficiencies of the different WMnCeTiOx catalysts at 250 °C were tested and the result was presented in Fig. 2(a). Compared with WMCT(SG) and WMCT(DP) catalysts, WMCT(CP) catalyst displayed much better SO2 resistance performance. With the introduction of SO2 in feed gas, NO removal efficiencies of WMCT(CP) catalyst had a slight drop, and it could still be maintained to 89 % after exposure to 100 ppm SO2 for 5 h.
The resistance of H2O and SO2 over the different WMnCeTiOx catalysts at 250 °C was presented in Fig. 2(b). At first 10 % H2O was introduced into the flue gas, there was a decrease in NO removal efficiency. It indicated that H2O had an inhibiting effect on SCR activity of WMnCeTiOx at low temperature. Moreover, the catalytic activity was completely recovered after removal of water vapor, indicating that the inhibition was reversible. When 200 ppm of SO2 was added to the reaction gas mixtures, a rapid decrease of NO removal efficiencies was observed. When H2O and SO2 were removed from the reaction stream, the activity of WMCT(CP) and WMCT(DP) catalysts was restored. It can be observed that the resistance of H2O and SO2 over the different WMnCeTiOx catalysts decrease in the following order: WMCT(CP) > WMCT(DP) > WMCT(SG).
[ SO2] =100 ppm, 200 ppm, balance with N2, and GHSV = 30,000 h-1)
3.2.XRD
Fig. 3. Showed XRD patterns of the different catalysts. The diffraction peaks of 2θ at 25.3 °, 37.8 °, 48.0 °, 53.9 °, 55.1 °, 62.7 °, 68.8 °, 70.3 ° and 75.0 °could be attributed to anatase TiO2 characteristic peak (PDF#21-1272), and the diffraction peaks of 2θ at 28.6 °, 33.1 °, 47.5 and 56.3° could be attributed to CeO2 characteristic peaks (PDF#34-0394). It could be seen that all catalysts exhibited the anatase TiO2 diffraction peaks, and no obvious diffraction peaks of rutile TiO2 were found. Anatase TiO2 had more abundant active sites than rutile phase, which was beneficial to promote the catalytic performance of NH3-SCR (Liu et al. 2008; Zhao et al. 2020). The diffraction peaks of CeO2 in the WMCT(CP) and WMCT(SG) catalysts were not obvious. It suggested that WMnCeTiOx catalysts prepared by CP and SG led to the amorphous structure or high dispersion of CeO2, which was beneficial to enhance the catalytic. However, the activity of WMCT(DP) was better than WMCT (SG). Therefore, the high dispersion property of the active species was probably not the determining factor for the activity difference of WMnCeTiOx catalysts.
3.3. N2 adsorption/desorption
The N2 adsorption-desorption isotherms of the catalysts were shown in Fig. 4. According to the classification of IUPAC, the absorption isotherms were type IV, indicating that the prepared catalysts belonged to mesoporous materials. The specific surface areas, total pore volumes, and average pore diameters of these three catalysts were summarized in Table 1. Compared with WMCT(DP) and WMCT(SG), WMCT(CP) has higher BET surface area (137.25 m2/g). It is generally believed that higher BET surface area could provide more available active sites on the catalyst surface for the reactants to participate in the reactions, Which may be one of the reasons for facilitating the de-NOx activity of the catalyst (Yan et al. 2018; Chen et al. 2017).
Table1 The BET specific area (SBET), total pore volume (Vtotal) and average pore diameter of all samples
|
Samples |
SBET(m2/g) |
Vtotal(cm3/g) |
Average pore diameter(nm) |
|
WMCT(CP) |
137.25 |
0.25 |
6.92 |
|
WMCT(DP) |
61.79 |
0.353 |
22.42 |
|
WMCT(SG) |
37.99 |
0.091 |
9.09 |
WMCT (DP), WMCT(SG)
3.4. H2-TPR
H2-TPR was used to characterize the redox property of the prepared catalysts, and the results were shown in Fig. 5. All catalysts showed two reduction peaks between 400 to 800 °C, which could be assigned to the reduction of surface manganese, ceria and tungsten species (Aguilera et al. 2011; Fang et al. 2017; Putluru et al. 2015; Wu et al. 2010). In the low-temperature region, the peak at 432 °C in the profile attributed to the reduction of Mn3O4 to MnO (Aguilera et al. 2011; Fang et al. 2017), and the peak at 495 and 519 °C in the profile attributed to the reduction of Ce4+ to Ce3+ (Putluru et al. 2015). In the high temperature region, the peak at 700 to 800 °C in the profile attributed to the reduction of WO3 to WO2 (Wu et al. 2010). Compared with WMCT(SG), WMCT(CP) and WMCT(DP) had a better reducibility at lower temperature, which contributed to promoting the low temperature SCR reaction.
WMCT (DP), WMCT(SG)
3.5. NH3-TPD
NH3-TPD was frequently carried out to investigate the amount and strength of surface acid sites on the catalysts surface. Surface acid sites are crucial to the adsorption of NH3 and the following SCR reaction (Fan et al. 2018). As illustrated in Fig. 6, all catalysts showed similar desorption curves and two desorption peaks: the NH3 desorption peaks centered at low temperature (lower than 300 ℃) assigned to weak acid sites, and the other NH3 desorption peaks centered at high temperature (higher than 300 ℃) originating from strong acid sites. It is well known that the area of desorption peak is proportional to the acid amount. The area of the WMCT(CP) was the largest, indicating that there are the largest amounts of weak adsorption sites on its surface. The more acid sites on the catalyst surface, the more adsorbed ammonia species available for surface reaction, Which would be a obvious benefit to boost of de-NOx efficiency.
WMCT (DP), WMCT(SG)
3.6. XPS
To investigate the surface element concentrations and elementary oxidation states of the prepared catalysts, Ce, Mn, W, O and Ti elements over the three catalysts were measured by XPS (Table 2 and Figure 7), As displayed in Table 2, Surface atomic ratios of Mn 2p, Ce 3d and O1s of the prepared catalysts were obviously different, which is mainly due to the different preparation methods.
As shown in Fig. 7(a), the binding energies of Mn 2p3/2 and Mn 2p1/2 peaks were located at 641.7 and 653.5 eV, corresponding to Mn4+ and Mn3+, respectively (Lei et al. 2014). It is well known that Mn4+ is more active than Mn3+ species in redox reaction, so that a high relative proportion of Mn4+/ (Mn3+ + Mn4+) is beneficial to the low-temperature NH3-SCR activity (Chen et al. 2014; Liu et al. 2017; Leng et al. 2018). It can be seen from Table 2 that the values of Mn4+/ (Mn3+ + Mn4+) decrease in the following order: WMCT(CP) > WMCT(DP) > WMCT(SG). This order was consistent with that of the low-temperature activity. It demonstrated that Mn4+ played a key role in determining the low-temperature activity of the WMnCeTiOx catalysts.
The significant differentials of Ce valence state had a great influence on redox reaction (Yao et al. 2016). Fig. 7(b) showed the XPS spectra of Ce 3d for the different catalysts. Two sets of multipltes related to Ce 3d5/2 and Ce 3d3/2 were labelled as u and v, respectively. The band labeled u′ and v′ can be ascribed to Ce3+, while the peaks labeled u, u′′, u′′′, v, v′′, v′′′ can be assigned to Ce4+ (Romeo et al. 1993; Si et al. 2010). According to literatures (Yao et al. 2014; Zheng et al. 2003), Ce3+ contributes to generate oxygen vacancies and leads to charge imbalance. The highly enriched oxygen vacancies would bring the rising of surface chemisorbed oxygen by supplying gaseous oxygen, which would increase the activity of the catalyst (Liu et al. 2009; Gu et al. 2010). Moreover, the Ce3+/(Ce3+ + Ce4+) ratio of WMnCeTiOx catalysts decreased according to the sequence: WMCT(DP) < WMCT(CP) < WMCT(SG), which was inconsistent with catalytic activity. It suggested that the Ce3+/(Ce3+ + Ce4+) ratio may not be the determinant factor for SCR performance of the prepared for WMnCeTiOx catalysts.
Fig. 7(c) exhibited the O1s spectra of WMnCeTiOx catalysts, which can be fitted into three peaks, lattice oxygen (Oα, 528.6~530.0 eV), surface chemisorbed oxygen species (Oβ, 531.4–532.0 eV), and oxygen containing surface groups (Oγ, 532.6~533.5 eV) like the defect oxide and hydroxyl species (Schindler et al. 2009; Wang et al. 2015). Some previous studies had pointed out that surface chemisorbed labile oxygen (Oβ) was a key factor affecting the SCR reaction of the catalysts because it was of high mobility and played a critical role in oxidation reaction (Wang et al. 2016; Du et al. 2020). The order of Oβ/(Oα + Oβ) is WMCT(CP) > WMCT(DP)>WMCT(SG),
which is consistent with the low-temperature SCR performance, indicating that a higher ratio of Oβ/(Oα + Oβ) was conductive to improve the NO removal efficiency.
Table 2 Surface atomic ratios of Mn 2p, Ce 3d and O1s of the different catalysts
|
Catalyst |
Surface atomic ratio (%) |
|||
|
Mn4+/(Mn4+ + Mn3+) |
Ce3+/(Ce3+ + Ce4+) |
Oβ/(Oβ+Oα) |
||
|
WMCT(CP) |
40.0 |
42.4 |
92.5 |
|
|
WMCT(DP) |
31.7 |
28.4 |
92.1 |
|
|
WMCT(SG) |
18.1 |
65.1 |
69.4 |
|
3.7 In-situ DRIFTS
In order to investigate the mechanism and the effect of SO2 in the SCR reaction with WMnCeTiOx catalysts prepared by different methods, we conducted in-situ DRIFTS experiments. Fig. 8 and 9 showed the in-situ DRIFTS spectra of the WMnCeTiOx catalysts prepared by different methods at 250 °C, and spectra from the reactions of the adsorbed NH3 with NO + O2 and the adsorbed NO + O2 with NH3 as well as the reaction under an atmosphere containing SO2 after the SCR are included.
Fig. 8 (a), (c), (e) showed the in-situ DRIFTS spectra of the reaction between the NH3 species and NO + O2 at 250 °C on the prepared catalysts. After the catalysts absorbed NH3 for 30 min, several bands were observed in the ranges of 1000-1750 and 3000-4000 cm-1 on WMnCeTiOx catalysts. the bands (1201, 1435, 3355, 3260 and 3160 cm-1) were assigned to coordinate NH3 on Lewis acid sites, and the band (1675 cm-1) was assigned to NH4+ species on Brønsted acid sites (Chen et al. 2017; Wu et al. 2007; Zhang et al. 2015; Mu et al. 2018; Zhang et al. 2020). After NO+O2 was introduced, , we found that the intensity of the bands corresponding to the NH3 groups on the Lewis acid sites deceased obviously, and the intensity of the bands assigned to NH4+ groups on the Brønsted acid sites changed a little. This result indicated that the NH3 groups adsorbed on the Lewis acid sites were more active than the NH4+ groups on the Brønsted acid sites. As compared to WMCT(DP) and WMCT(SG) catalysts, WMCT(CP) catalyst had more NH3 groups on the Lewis acid sites, which was beneficial to the activity in the SCR reaction. As the reaction progressed, the band at 1590 cm-1 was overridden by the bands assigned to bidentate nitrate (1574 cm-1) (Li et al. 2017). It can be observed that new bands ascribed to bidentate nitrate (1559 cm-1), free nitrate ions (1379,1362 cm-1), monodentate nitrate (1374 cm-1) appeared on the catalysts surface and they were enhanced with the proceeding of reaction (Gao et al. 2018; Zeng et al. 2017; Yang et al. 2020) . It implied that the NH3-SCR reaction over the WMnCeTiOx catalysts followed an Eley-Ridel (E-R) mechanism.
Fig. 8 (b), (d), (f) showed the in-situ DRIFTS spectra of the reaction between the NO species and NH3 at 250 °C on the prepared catalysts. After the catalysts absorbed NO + O2 for 30 min, bands located at 1362, 1370, 1380, 1559 and 1580 cm-1 were detected, which were ascribed to bidentate nitrate (1559, 1580 cm-1), free nitrate ions (1380, 1362 cm-1), monodentate nitrate (1370 cm-1). With the introduction of NH3, the bands assigned to bidentate nitrate (1580 cm-1), free nitrate ions (1380 cm-1) on WMnCeTiOx catalysts exhibited an obvious decrease in intensity. We concluded that bidentate nitrate and free nitrate ions were active compounds and were involved in the reaction. WMCT(CP) and WMCT(DP) catalysts had more bidentate nitrate and free nitrate ions, which was beneficial to the activity in the SCR reaction. As the reaction progressed, while coordinate NH3 species on Lewis acid sites (1201, 3156, 3267 and 3356 cm-1) and NH4+ species (1721,1675 and 1430 cm-1) on Brønsted acid sites began to appear.
The NH3-SCR reaction over the WMnCeTiOx catalysts followed an Langmuir-Hinshelwood (L-H) mechanism.
To elucidate the mechanism of sulfur poisoning on the SCR reaction, we conducted the reaction under an atmosphere containing 100 ppm SO2 after conducting the SCR under standard conditions at 250 °C. Fig. 9 (a), (c), (e) showed the in-situ DRIFTS spectra of the reaction between the NH3 species and SO2 at 250 °C on the prepared catalyst. All catalysts were then exposed to both NH3 and SO2 for 30 min. Compared with only NH3 exposure, the intensity of bands corresponding to NH3 on Lewis sites (3355, 3260 and 3160 cm-1) and Brønsted sites (1675 cm−1) slightly decreased, suggesting that SO2 caused the number of acid sites to decrease. However, there was less reduction in the acid sites on WMCT(CP) catalyst. The band at 1590 cm-1 corresponds to NH3 species on Lewis acid sites shifted to 1580 and 1542 cm-1. The bands at 1340 and 1374 cm-1 which ascribed to sulfates appeared, and they also enhanced with the increase of SO2 injection (Chen et al. 2010; Yan et al. 2018).
Fig. 9 (b), (d), (f) showed the in-situ DRIFTS spectra of the reaction between the NO species and SO2 at 250 °C on the prepared catalysts. All catalysts were then exposed to both NO+O2 and SO2 for 30 min. When SO2 was introduced, a new band (1351 cm-1) ascribed to sulfates appeared and the bands corresponding to bidentate nitrate (1557, 1545 cm-1), monodentate nitrate (1370 cm-1) were decreased in intensity. It indicated that SO2 could restrain the adsorption between NOx species on WMnCeTiOx catalysts which inhibited the SCR reaction under L-H mechanism, which might be the main reason for the low NO removal efficiency in the presence of SO2. However, the bands assigned to bidentate nitrate (1545 cm-1) on WMCT(CP) catalyst exhibited a slowly decrease in intensity. This data demonstrated that the WMCT(CP) catalyst has better resistance to SO2 than WMCT(DP) and WMCT(SG) catalysts.
In this paper, WMnCeTiOx catalysts were prepared by CP, DP and SG methods. Different preparation methods lead to different surface structural characteristics, dispersion of active components, chemical states of the surface species. More importantly, the reduction capacity and the number of acid sites of WMnCeTiOx catalysts varied with the preparation process, which also would impose a great impact on the absorption and activation of NO and NH3 on the surface of WMnCeTiOx catalysts. These factors made WMnCeTiOx catalysts show an obvious difference in catalytic activity.
The result indicated that the catalysts prepared by the CP method had better SCR activity and SO2 resistance than that prepared by the DP and SG methods. The WMnCeTiOx catalyst prepared by CP method has the largest specific surface area and highly-dispersed active species. From the result of in-situ DRIFTS, WMCT(CP) catalyst had more NH3 groups on the Lewis acid sites and bidentate nitrate and free nitrate ions, which was beneficial to the activity in the SCR reaction. It was revealed that the NH3-SCR reaction over the prepared WMCT(CP), WMCT(DP) and WMCT(SG) obeyed both L-H and E-R mechanisms.
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Authors' contributions
Xiaodi Li: Conceptualization, Investigation, Writing - Original draft. Zhitao Han: Conceptualization, Validation, Supervision, Project administration, Writing - Review & Editing. Qingdong Shi: Formal analysis. Xitian Wu, Yu gao: Methodology. Chenglong Li, gang liu: Data curation.
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Competing interests
The authors declare that they have no competing interest.
Funding
This work was supported by National Natural Science Foundation of China (51779024), Natural Science Foundation of Liaoning Province of China (2020MS130), and Fundamental Research Funds for the Central Universities (3132019330).
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