The superior performance of CoMnOx catalyst with ball-flowerlike structure for low-temperature selective catalytic reduction of NOx by NH3

Chemical Engineering Journal 381 (2020) 122753

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The superior performance of CoMnOx catalyst with ball-flowerlike structure for low-temperature selective catalytic reduction of NOx by NH3

T

⁎

Zhong-yi Wanga,b, Rui-tang Guoa,b,c, , Xu Shia,b, Xing-yu Liua,b, Hao Qina,b, Yuan-zhen Liua,b, ⁎ Chao-peng Duana,b, De-yu Guoa,b, Wei-guo Pana,b, a

School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai 200090, PR China c Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

ball-flowerlike structure of • The CoMnO -BF catalyst could remarkably x

• •

enhance its low-temperature SCR activity. More ad-NH3 and ad-NOx species with higher reactivity are available on the surface of CoMnOx-BF catalyst. The redox cycle “Mn4+ + Co2+ ↔ Mn3+ + Co3+” greatly promotes the low-temperature SCR reaction over CoMnOx-BF catalyst.

A R T I C LE I N FO

A B S T R A C T

Keywords: SCR Low temperature CoMnOx Ball-flowerlike structure

A novel ball-flowerlike catalyst (CoMnOx-BF) was developed by hydrothermal method for the selective catalytic reduction (SCR) of NOx by NH3. Besides, a counterpart sample (CoMnOx) was prepared by coprecipitation way for comparison. The activity test results demonstrated that CoMnOx-BF catalyst exhibited admirable SCR performance and N2 selectivity in a broad temperature range of 150–350 °C, along with an excellent resistance to SO2 and high durability. Based on characterization experiments, it could be found that the superior surface area resulted from suppressed crystallization, the enrichment of surface labile oxygen and Mn4+, the stronger surface acidity and redox ability were all beneficial to the NH3-SCR performance of CoMnOx-BF catalyst. Additionally, the enhanced NO oxidation and more activated ad-reactants on catalyst surface could also conduce to the outstanding SCR activity of CoMnOx-BF at low temperature.

1. Introduction Among the most noxious air pollutants, nitrogen oxides (NOx) are usually emitted from both fixed sources (power plants, etc.) and mobile emissions (diesel vehicles, etc.), and they would easily give rise to the formation of photochemical smog, acid precipitation, ozone destruction, and greenhouse phenomenon [1–3]. As the most efficient ⁎

technology of NOx removal, selective catalytic reduction (SCR) with NH3 has been applied into the industrial field diffusely [4,5]. Nevertheless, the widely used V2O5-WO3(MoO3)/TiO2 products still have some considerable drawbacks including the poisonousness of V to environment and human health, narrow operating temperature range (300–400 °C), and the excessive conversion of SO2 into SO3 [6–9]. Consequently, it has caused extensive concern to develop an innovative

Corresponding authors at: School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China. E-mail addresses: [email protected] (R.-t. Guo), [email protected] (W.-g. Pan).

https://doi.org/10.1016/j.cej.2019.122753 Received 21 April 2019; Received in revised form 26 August 2019; Accepted 6 September 2019 Available online 07 September 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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structures of the catalysts were detected through an X-ray diffraction (XRD) measurement (Bruker D8 Advance powder diffractometer with CuKα radiation, λ = 0.154056 nm). The elementary states on samples were investigated by X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (Thermo ESCALAB 250, USA). The temperature programmed reduction of H2 (H2-TPR) experiments were conducted on a Quantachrome Autosorb-iQ-C chemisorption facility in 10% H2/Ar (30 mL/min) at a constant heating rate of 10 °C/min by using 0.05 g sample. Temperature-programmed desorption of NH3 (NH3-TPD) tests were also performed on the same instrument under 5% NH3/He atmosphere by using 0.1 g sample. The in situ DRIFT spectra were obtained on a FTIR spectrometer (Nicolet iS50). The spectra of catalysts were acquired by accumulating 100 scans and the tests were achieved based on the following gas conditions: 300 mL/min overall gas flowrate, 600 ppm NH3 or/and 600 ppm NO + 5% O2, and balance Ar.

environmental-friendly SCR catalyst with superior performance during the past years. In recent years, various transition metal oxides (FeOx, CuOx and MnOx, etc.) have been confirmed to be effective for NH3-SCR [10–13]. Nevertheless, when the low temperature (< 250 °C) catalytic property and environmentally friendly peculiarity are taken into consideration, Mn-based catalyst usually exhibits great excellence, as depicted by many previous studies [14–16]. Although the weak SO2 tolerance and relatively poor N2 selectivity of simplex MnOx would limit its potential application, plenty of former researches have proved that bimetallic Mn-based catalysts (CeMnOx, FeMnOx, CoMnOx, EuMnOx etc.) could get over these disadvantages and even enhance their SCR activity effectively [17–20]. Among them, Mn-Co catalysts have been researched widely on account of their high low-temperature NH3-SCR activity and outstanding SO2 resistance owing to the intense synergistic effect between the Co and Mn species [21]. Although some insightful works have been carried out to explain the effect of various factors on potential mechanism and catalytic performance, optimization of the catalyst formulation still continues. Recently, increasing studies showed that the structures of catalysts played a crucial role in catalytic behavior [22,23]. Generally, the catalysts with hierarchical structure could provide large specific area, low length diffusion of reactant gases and efficient mass transport channels, and all characters mentioned above are indisputably beneficial for catalytic property [24]. Therefore, it is necessary to further investigate the catalyst formulation by not only variation of ingredients but also appropriate structural optimization for NH3-SCR reaction. In present study, a new form of CoMnOx catalyst like ball-flower (CoMnOx-BF) was synthesized, which presented the superior low-temperature SCR performance, great SO2 resistance and high durability. By right of some characterization experiments, the relationship between surface physicochemical properties and SCR activities of CoMnOx-BF and CoMnOx would be further investigated.

2.3. Catalytic activity test The SCR activity evaluation was conducted on a stationary quartz reactor (0.80 cm diameter) in which 0.556 cm3 of samples (ground to 60–100 mesh) was used. The simulated fume gas was consisted of 600 ppm NO, 5% O2, 600 ppm NH3, 5% H2O and Ar as the balance gas. The overall flow rate was 1000 mL/min and the related gas hourly space velocity (GHSV) was 108, 000 h−1. The Nicolet iS 50 FTIR spectrometer was employed to detect the ingredients of the outlet gas (including NO, NO2, N2O and NH3). Furthermore, the NOx removal efficiency and N2 selectivity were calculated by the equations as follows:

NOx conversion =

[NOx ]in − [NOx ]out × 100% [NOx ]in

(1)

2[N2 O]out ⎞ × 100% N2 selectivity = ⎛1 − [NOx ]in + [NH3]in − [NOx ]out − [NH3]out ⎠ ⎝ (2) ⎜

2. Experimental section

⎟

Besides, the NO oxidation activity over each catalyst was also tested under the similar conditions. However, NH3 was not introduced into the inlet gas flow in this part and the relevant formula for activity calculation is as follows:

2.1. Catalysts preparation In present work, the CoMnOx-BF catalyst was synthesized through hydrothermal method. First, 0.98 g cobalt acetate tetrahydrate [C4H6CoO4·4(H2O)] and 0.49 g manganese acetate tetrahydrate [Mn (CH3COO)2·4H2O] were added into 160 mL of ethylene glycol [(CH2OH)2] containing 2 mL deionized water to obtain a transparent solution under continuous stirring. Then 0.44 g polyvinyl pyrrolidone [(C6H9NO)n] was added into the solution above. The obtained mixture was moved into a Teflon-lined autoclave at 175 °C for 12 h. Next, the precipitate was collected and washed with deionized water for 5–7 times. The as-prepared precursor was dried at 85 °C overnight and next calcined in the air atmosphere at 500 °C for 4 h with a steady heating rate of 2 °C/min. Moreover, a common CoMnOx catalyst with ordinary particle structure was synthesized by coprecipitation method for comparison. Firstly, the desired amounts of cobalt acetate tetrahydrate and manganese acetate tetrahydrate were dissolved in deionized water. Under the continuous stirring, a certain quantity of ammonia [NH3·H2O] was added into the miscible liquor gradually until the pH rose to 11. After 30 min sufficient reaction, the precipitate in solution was washed and filtered by deionized water. After that, the catalyst was dried overnight and then calculated in muffle furnace at 500 °C for 4 h.

NO oxidation =

[NO]in − [NO]out × 100% [NO]in

(3)

3. Results and discussion 3.1. NH3-SCR activity The de-NOx activities of CoMnOx-BF and CoMnOx are presented in Fig. 1(A). For CoMnOx catalyst, a low catalytic activity (< 200 °C) could be detected and its maximum NOx conversion is 89% at 250 °C. Remarkably, the NOx removal efficiency of CoMnOx-BF catalyst is always higher than 95% in a wide temperature range of 150–350 °C, which indicates that the CoMnOx-BF possesses the more excellent SCR activity compared with CoMnOx, especially at low temperature (< 250 °C). However, an evident activity decrease of CoMnOx-BF catalyst can be detected when the temperature exceeds 350 °C, which ought to be ascribed to NH3 oxidation at high temperature [25]. Furthermore, the N2 selectivity results of CoMnOx and CoMnOx-BF are shown in Fig. 1(B) and CoMnOx-BF also exhibits more higher N2 selectivity in the entire reaction temperature range, further demonstrating the excellent SCR performance of CoMnOx-BF catalyst.

2.2. Characterizations The surface characteristic of the obtained samples were detected on a scanning electron microscopy (SEM, Phillips XL-30 FEG/NEW) and a transmission electron microscopy (TEM, JEOL 2100F). The textural properties of catalysts were measured by N2 adsorption and desorption at −196 °C on a Quantachrome Autosorb-iQ-AG apparatus. The crystal

3.2. SO2/H2O tolerance and stability test The high sensitivity to sulfur of Mn-based catalysts is a non-negligible handicap for their industrial application [26]. Hence, the NH32

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Fig. 2. (A) SO2 tolerance and (B) stability tests of CoMnOx and CoMnOx-BF catalysts at 200 °C. Conditions: [NO] = 600 ppm, [NH3] = 600 ppm, [O2] = 5%, [H2O] = 5%, [SO2] = 100 ppm (when used), balanced Ar, GHSV = 108,000 h−1.

Fig. 1. (A) NH3-SCR activities and (B) N2 selectivity of CoMnOx and CoMnOxBF catalysts. Conditions: [NO] = 600 ppm, [NH3] = 600 ppm, [O2] = 5%, [H2O] = 5%, balanced Ar, GHSV = 108, 000 h−1.

agglomerated near-spherical particles with different sizes, while the CoMnOx-BF (Fig. 3(C) and (D)) presents an excellent ball-flower morphology aggregated by many sheets, agreeing well with its correspondingly large BET surface area. Furthermore, the ball-flowerlike structure of CoMnOx-BF catalyst is also conductive to the contact between catalyst surface and reactants.

SCR activity test of CoMnOx and CoMnOx-BF catalysts was also conducted in the condition of 100 ppm SO2, as displayed in Fig. 2(A). Obviously, the introduction of 100 ppm SO2 into the flue gas leads to an evident activity drop on CoMnOx, which declines from 76.3% to around 63.6% in about 4.5 h. For CoMnOx-BF catalyst, the SO2 resistance is much higher than that of CoMnOx and its SCR activity always keeps more than 88.2% in the presence of SO2 all the time. Besides, the NOx conversion of CoMnOx-BF could recover 93.1% gradually after cutting off the SO2 supply. Moreover, the stabilities of CoMnOx and CoMnOx-BF catalysts were also evaluated, as presented in Fig. 2(B). It is evident that the de-NOx activity of CoMnOx begins to decrease slowly after about 18 h, while the CoMnOx-BF catalyst always keeps a steady and high NOx conversion in 36 h, indicating the excellent durability of CoMnOx-BF. Moreover, H2O resistance test was also carried out (Fig. S1) and 5% water vapor has a little effect on both CoMnOx-BF and CoMnOx catalysts in NH3-SCR reaction.

3.4. BET and XRD The relevant figures of BET analysis including N2 adsorption-desorption and pore size distribution are exhibited in Fig. 4. It is noticeable that both the adsorption isotherms of two catalysts present type-IV curves with a hysteresis loop, indicating the mesopores structure of the samples. The BJH pore size distributions of CoMnOx and CoMnOx-BF catalysts present a narrow pore diameter distribution ranged from 3.4 nm to 4.3 nm. Moreover, the specific surface areas and pore sizes of CoMnOx and CoMnOx-BF are listed in Table 1. In contrast to the ordinary granular CoMnOx catalyst, the ball-flower like structure of CoMnOx-BF could greatly increase the BET surface area (from 27.56 to 40.79 m2/g). The enhancement of the specific surface area could supply numerous active sites for NH3-SCR reaction.

3.3. SEM images and properties The SEM images of CoMnOx and CoMnOx-BF samples are shown in Fig. 3. From Fig. 3(A), it can be observed that CoMnOx is composed of 3

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Fig. 3. SEM images of CoMnOx (A) and CoMnOx-BF (B–D).

Fig. 4. N2 adsorption–desorption isotherms and the pore size distribution curves (inset) of CoMnOx and CoMnOx-BF catalysts.

Fig. 5. XRD patterns of CoMnOx and CoMnOx-BF catalysts.

Table 1 Textural properties of samples.

would also contribute to the increase of its specific surface area.

Samples

BET surface area (m2/g)

Pore volume (cm3/g)

Average pore diameter (nm)

CoMnOx CoMnOx-BF

27.56 40.79

0.072 0.093

3.679 3.825

3.5. TEM analysis Fig. 6(A) and (B) present the TEM images of a single CoMnOx-BF sample, which further makes clear the flaky structure and ball-flower morphology with transparent edge, as that shown in the SEM images above. From the HR-TEM image of Fig. 6(C), the lattice spacings of 0.272 nm and 0.503 nm should be attributed to the (Co, Mn)(Co, Mn)2O4 (1 1 3) and (1 1 1) facets, and that of 0.478 nm is assigned to the (1 1 1) facet of CoMn2O4 species. In addition, the lattice spacing equaling 0.390 nm should be attributed to the (2 1 1) facet of extra Mn2O3 [21]. In order to identify the elemental distribution on the CoMnOx-BF catalyst, the corresponding EDS elemental mapping was also performed, as exhibited in Fig. 6(D). The EDS elemental maps

XRD patterns of CoMnOx and CoMnOx-BF catalysts are presented in Fig. 5. All diffraction peaks in the patterns are attributed to (Co, Mn) (Co, Mn)2O4 (JCPDS 18-0408) and CoMn2O4 (JCPDS 23-1237) [21,27]. Obviously, the peak intensity of CoMnOx-BF is lower than that of CoMnOx catalysts, demonstrating the intense interaction between Co and Mn species in CoMnOx-BF. Hence, the crystallization of MnOx and CoOx species should be suppressed on CoMnOx-BF catalyst, which 4

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Fig. 6. (A) and (B) TEM images, (C) HR-TEM image, (D) EDX elemental mappings of Co, Mn and O for CoMnOx-BF catalysts.

resulting in an increase of SCR activity [35,36]. The XPS spectra of O 1s displayed in Fig. 7(C) could be separated into two peaks: lattice oxygen species (designated as Oα) at about 529.9 eV and surface adsorbed oxygen belonging to hydroxyl-like group or defect oxide, such as O− or O22− (defined as Oβ) at around 531.5 eV [20]. It has been acknowledged that Oβ species are much more activated than Oα ascribing to their high mobility, which is preferable to NO oxidation [37,38]. Therefore, the relative high Oβ/O ratio of CoMnOx-BF (32.95%) is also conductive to the promotion of the lowtemperature NH3-SCR reaction over it.

reveal the uniform distributions of Co, Mn, and O, which is certainly favorable for the reaction in SCR process.

3.6. XPs To deeply investigate the surface elements distribution and oxidation state of catalysts, XPS analysis was carried out and the relevant spectra are shown in Fig. 7. Furthermore, the concentrations of surface elements are also given in Table 2. The spectra of Mn 2p for CoMnOx and CoMnOx-BF are displayed in Fig. 7(A). Obviously, the two primary peaks located at around 653.7 and 642.2 eV should be assigned to Mn 2p1/2 and Mn 2p3/2 separately [28,29]. After a peak fitting deconvolution, two peaks of Mn 2p could be deconvoluted into six peaks attributing to Mn2+ (640.9 and 652.4 eV), Mn3+ (642.3 and 653.7 eV) and Mn4+ (643.8 and 655.5 eV), respectively [25,30]. The related percentages of Mn4+ deduced from peak area is 46.05% for CoMnOx-BF, which is quite higher than that of CoMnOx (25.64%). For NOx removal at low-temperature, Mn4+ species play a major role in the oxidation of NO, which would facilitate the NH3-SCR process via the “fast SCR” route (NO + NO2 + 2NH3 = 2 N2 + 3H2O) [31–33], as reflected by the remarkable SCR performance of CoMnOx-BF catalyst (Fig. 1(A)). In Fig. 7(B) of Co 2p XPS spectra, two peaks at 782.5 eV and 780.4 eV of Co 2p3/2 are ascribed to Co2+ and Co3+and the binding energies at 797.6 eV of Co2+ and 795.5 eV of Co3+ belong to the Co 2p1/2 in the main peak, accompanied with two satellite peaks simultaneously [34]. As displayed in Table 2, the atomic proportion of Co3+/Co in the CoMnOx-BF is 66.91%, which is also much higher compared with CoMnOx (48.18%). It is widely accepted that abundant Co3+ species are beneficial to the redox ability on Co-based samples,

3.7. H2-TPr In order to identify the reduction behaviors of CoMnOx and CoMnOx-BF, H2-TPR tests were performed, as displayed in Fig. 8. Two reduction peaks at 220 °C and 416 °C of CoMnOx-BF should be ascribed to the conversion of Mn4+ → Mn3+ and Mn3+ → Mn2+, respectively [20,30]. And the peaks in the two profiles at 300–336 °C belong to the reduction of Co3+ → Co2+ and Mn2O3 → Mn3O4 [21]. The other peaks at 523–542 °C of the two catalysts are ascribed to Co2+ → Co and Mn3O4 → MnO. Besides, the peaks appearing at high temperature (> 600 °C) could be attributed to the stepwise reduction of CoMn2O4, including the bulk oxygen substances of manganese and cobalt oxides with different valences [19]. Visibly, the reduction peak (150–600 °C) intensity of CoMnOx-BF is much higher than that of CoMnOx, revealing the superior reducibility of CoMnOx-BF catalyst. From Table 3, the H2 consumption of CoMnOx-BF is about 2.5 times higher than that of CoMnOx, demonstrating that more active ingredients are available on the surface of CoMnOx-BF, which is quite preferable to the NH3-SCR reaction. 5

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Fig. 8. H2-TPR profiles of CoMnOx and CoMnOx-BF catalysts. Table 3 H2 consumption of CoMnOx and CoMnOx-BF catalysts.

Fig. 7. XPS spectra of (A) Mn 2p, (B) Co 2p, and (C) O 1s spectra of CoMnOx and CoMnOx-BF catalysts.

Samples

H2 consumption (mmol/g)

CoMnOx CoMnOx-BF

18.7 46.3

Fig. 9. NH3-TPD profiles of CoMnOx and CoMnOx-BF catalysts.

samples. From Fig. 9, it can be seen that desorption of NH3 occurrs in a broad temperature range, owing to the presence of adsorbed NH3 with different thermostability. The peaks appearing at low temperatures (133 and 165 °C) ought to be ascribed to adsorbed NH3 and some NH4+ linked to weak Brønsted acid sites and the peaks at 209 and 245 °C are assigned to NH4+ species connected with the strong Brønsted acid sites,

3.8. NH3-TPd The existence of the acid sites on catalyst surface is crucial for NH3 adsorption, which plays a major role in NH3-SCR reaction. Therefore, NH3-TPD analysis was applied to investigate the surface acidity of Table 2 Surface atomic concentrations of Mn, Co, and O species (by XPS). Samples

Mn (at.%)

Co (at.%)

O (at.%)

Mn4+/Mn (%)

Co3+/Co (%)

Oβ/O (%)

CoMnOx CoMnOx-BF

9.48 11.86

17.54 21.72

72.95 66.42

25.64 46.05

48.18 66.91

28.83 32.95

6

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Table 4 Surface acidities of CoMnOx and CoMnOx-BF catalysts. Samples

Surface acidity (mmol/g)

CoMnOx CoMnOx-BF

0.217 0.376

while the peaks at high temperature (321–460 °C) are attributed to NH3 bound to Lewis acid sites [39]. It is commonly recognized that the desorption peak position and area are correlated with the acid strength and amount. Compared with CoMnOx, CoMnOx-BF exhibits a quite larger desorption peak area in the whole temperature range, confirming the existence of more Brønsted and Lewis acid sites on it. As listed in Table 4, the surface acidity of CoMnOx-BF is much stronger than that of CoMnOx, which is critical for the enhancement of NH3-SCR reaction. 3.9. NO oxidation It is generally known that the NO oxidation to NO2 can effectively accelerate the NH3-SCR reaction via the “fast SCR” route [40,41]. Thus, NO oxidation activities over CoMnOx and CoMnOx-BF catalysts were also measured, and the results are displayed in Fig. 10. With the increase of temperature, a convex-parabola trend of NO oxidation is visible on the two samples, which should be owing to the transformation from kinetics-control to thermodynamics-control [42]. The detailed computational procedure for thermodynamic equilibrium in this study is given in Section S1 (Supplementary materials). From Fig. 10, the NO oxidation over CoMnOx-BF is obviously higher than that over CoMnOx, especially below 300 °C, corresponding well with the superior low-temperature SCR activity of CoMnOx-BF catalyst. 3.10. In situ DRIFT spectra 3.10.1. Adsorption of NH3 The NH3 adsorption spectra over CoMnOx and CoMnOx-BF at the different temperature are exhibited in Fig. 11. As illustrated in Fig. 11(A), the bands in CoMnOx spectra at 1611 and 1574 cm−1 belong to coordinated ad-NH3 on Lewis acid sites, while the band at 1352 cm−1 is assigned to asymmetrical bending vibration of NH4+ attached to Brønsted acid sites [19,43]. From the spectra of CoMnOx-BF, the four bands appearing at 1611, 1553, 1295 and 1182 cm−1 should be attributed to the asymmetrical and symmetrical bending vibrations of coordinative NH3 on Lewis acid sites, and the 1392 cm−1 band

Fig. 11. DRIFT spectra of NH3 adsorption over: (A) CoMnOx; (B) CoMnOx-BF at different temperature.

should be related to NH4+ on Brønsted acid sites [44,45]. For both two catalysts, it is evident that the band intensities reduce gradually with temperature, and the bands at 1352 and 1392 cm−1 nearly disappear at 300 °C, reflecting the relatively lower thermostability of Brønsted acid sites. However, most other bands are still present when the temperature exceeds 300 °C, which demonstrates that the ad-NH3 species on Lewis acid sites are more stable. Besides, the band intensities in Fig. 11(B) are much higher than that in Fig. 11(A), indicating that more ad-NH3 species are available over CoMnOx-BF, as also proven by the NH3-TPD results. 3.10.2. Co-adsorption of NO + O2 The NO + O2 adsorption on CoMnOx and CoMnOx-BF was also tested by DRIFT experiments and the obtained spectra are presented in Fig. 12. As illustrated in Fig. 12(A) and (B), various adsorbed NOx species can be seen on CoMnOx and CoMnOx-BF, which contains adsorbed NO2 (1625 cm−1), monodentate nitrate (1533, 1518, 1342 and 1281 cm−1), bidentate nitrate (1551 cm−1) and bridging nitrate species (1245 cm−1) [46–48]. It is noteworthy that the bands at 1551 and 1245 cm−1 assigned to bidentate nitrate and bridged nitrate still can be detected at 300 °C on account of their higher thermostability. By

Fig. 10. NO oxidation over CoMnOx and CoMnOx-BF catalysts. 7

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Fig. 12. DRIFT spectra of NO + O2 adsorption over: (A) CoMnOx; (B) CoMnOxBF at different temperature.

Fig. 13. DRIFT spectra of the reaction between NOx and pre-adsorbed NH3 species over: (A) CoMnOx; (B) CoMnOx-BF at 200 °C.

contrast, the bands ascribed to monodentate nitrate with lower thermostability almost disappear at 250 °C. And the higher band intensities in Fig. 12(B) means the improved NOx adsorption on CoMnOx-BF. In addition, more ad-NO2 species could also be observed on CoMnOx-BF, as reflected by the increased band intensity at 1625 cm−1, which is consistent with the NO oxidation results.

3.10.4. Reaction between NH3 and the pre-adsorbed NOx species In other aspect, the catalysts were studied in analogous experimental conditions as depicted above. Differently, the catalyst was first exposed to 600 ppm of NO + 5% O2/Ar for 0.5 h at 200 °C, then it was purged by Ar for 10 min. After that, 600 ppm of NH3 was introduced. As displayed in Fig. 14, several bands (1625, 1551, 1533, 1342, 1281 and 1245 cm−1) of ad-NOx species could be observed after the treatment with NO + O2. Visibly, it took nearly 5 min for consuming all ad-NOx species on CoMnOx after the introduction of NH3, and some bands attributed to ad-NH3 species began to appear later. From the CoMnOx-BF spectra (Fig. 14(B)), similar phenomenon of pre-adsorbed NOx species reappeared, while all ad-NOx species on it vanished immediately within 2 min after NH3 introduction. Hence, the reactivity of ad-NOx species on CoMnOx-BF is also much higher than that on CoMnOx, which is also in line with the excellent SCR activity of CoMnOx-BF catalyst.

3.10.3. Reaction between NOx and the pre-adsorbed NH3 species In this section, the catalysts were pretreated with 600 ppm NH3 for 0.5 h, followed by Ar purging for 10 min. After that, 600 ppm NO + 5% O2/Ar was passed and the temperature was maintained at 200 °C. As exhibited in Fig. 13, all bands (1611, 1572, 1553, 1392, 1354, 1295 and 1182 cm−1) of ad-NH3 species appeared on CoMnOx and CoMnOx-BF after the exposure to NH3 gas flow. From Fig. 13(A), the bands of adNH3 of CoMnOx vanished after passing NO + O2 for 10 min and several bands of ad-NOx (1624, 1536, 1342 and 1278 cm−1) gradually appeared, confirming that all adsorbed NH3 species on CoMnOx involved in the SCR reaction. For CoMnOx-BF, all bands ascribed to the ad-NH3 vanished rapidly after introducing NO + O2 for 2 min and the corresponding bands belonging to ad-NOx were observed, indicating the improved reactivity of ad-NH3 on CoMnOx-BF catalyst.

3.11. Redox cycle of NH3-SCR reaction The redox ability is a key factor controlling NH3-SCR reaction [49–51]. Based on XPS and H2-TPR analysis, the redox couples of 8

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species [53]. In addition, Liu et al. indicated that the addition of Mn on FeTiOx catalysts would contribute to the formation of Brønsted acid sites, suggesting that Mn active sites were mainly answerable to the formation of NH4+ species [54]. Consequently, we proposed the activation of NH3 and NOx in redox cycle over CoMnOx and CoMnOx-BF catalyst, as illustrated in Fig. 15. The ad-NO2 could react with NH4+ to form the NH4NO2, which would react further to generate NH2NO [55]. After that, the NH2NO species could decompose to yield N2 and H2O. Besides, other adsorbed nitrates could also involve in the SCR process with the ad-NH4+ and ad-NH3 via the Langmuir-Hinshelwood route at low temperature, as reflected by Fig. 14. Based on the discussion above, the redox cycle (Mn4+ + Co2+ ↔ Mn3+ + Co3+) plays a pivotal role in the SCR reaction over CoMnOx-BF catalyst, which could promote the adsorption and activation of NH3 and NOx species, as depicted in the DRIFT results above, conducing to an increased SCR performance of CoMnOx-BF at low temperature accordingly. 4. Conclusions In this study, a novel ball-flowerlike catalyst, defined as CoMnOxBF, was synthesized for the low-temperature SCR reaction with NH3. The obtained CoMnOx-BF catalyst showed the outstanding low-temperature NH3-SCR activity (more than 95% in 150–350 °C), a high N2 selectivity (nearly 98% in 100–350 °C), and superior sulfur-tolerance and stability compared with the counterpart sample (CoMnOx) without ball-flowerlike structure. The feature of ball-flowerlike structure of CoMnOx-BF catalyst supplies a large surface area and abundant active sites, contributing to the enhancement of SCR activity. Furthermore, the enhanced SCR performance of CoMnOx-BF catalyst should also be attributed to the intense interaction between Mn and Co, which would suppress the crystallization and greatly improve its reducibility, accompanied with more surface Mn4+ and chemisorbed oxygen. The characteristics summarized above are all beneficial to the “fast SCR” process. Significantly, the DRIFT analysis indicates that more ad-NH3 and ad-NOx species with much higher reactivity attaches to the surface of CoMnOx-BF, which could further account for its superior low-temperature NH3-SCR activity. Acknowledgment This work was economically supported by the National Key R&D Program of China (2018YFB0605002).

Fig. 14. DRIFT spectra of the reaction between NH3 and pre-adsorbed NOx species over: (A) CoMnOx; (B) CoMnOx-BF at 200 °C.

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Fig. 15. Redox cycle for the activation of NO and NH3 on catalyst.

Co3+/Co2+ and Mn4+/Mn3+ exist over CoMnOx and CoMnOx-BF catalysts and the redox cycle (Mn4+ + Co2+ ↔ Mn3+ + Co3+) should be available for them at low temperature. The Co2+ → Co3+ conversion is correlated to the generation of numerous single-electron-trapped oxygen vacancies, which could improve the activation and adsorption of gas-phase oxygen [52]. It is well known that the activated oxygen atom plays a key role for the formation of ad-NO2 and other nitrate 9

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