Effect of post-treatment on the selective catalytic reduction of NO with NH3 over Mn3O4

Materials Chemistry and Physics 237 (2019) 121845

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Effect of post-treatment on the selective catalytic reduction of NO with NH3 over Mn3O4 Song Shu a, Jiaxiu Guo a, b, *, Jing Li a, Ningjie Fang a, Jianjun Li a, b, Shandong Yuan b, c, ** a

College of Architecture and Environment, Sichuan University, Chengdu, 610065, Sichuan, China National Engineering Technology Research Center for Flue Gas Desulfurization, Chengdu, 610065, Sichuan, China c Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu, 610065, Sichuan, China b

H I G H L I G H T S

� Mn3O4 nanocrystals were successfully prepared by rapid precipitation method at room temperature. � When temperature increases to 400 � C, main exposed crystal faces change from (101) to (200) and (112). � When the post‒treatment temperature increases to 500 � C, Mn2O3 is dissociated from Mn3O4. � Mn3O4_400 has low reduction peaks and similar reduction properties after multi‒reaction of NH3‒SCR. � Mn3O4_400 has good resistance of SO2 and can generate MnSO4.�H2O in the presence of H2O and SO2. A R T I C L E I N F O

A B S T R A C T

Keywords: Mn3O4 Calcination Crystal phase Crystal face NH3‒ SCR

The Mn3O4 nanocrystals were successfully prepared by rapid precipitation method at room temperature from MnSO4, H2SO4 and NH3�H2O, and the influences of post‒treatment temperatures at 200‒500 � C on texture, morphology, crystal face, redox and the selective catalytic reduction of NO with NH3 were investigated. The results show that a single tetragonal phase of Mn3O4 nanocrystals is obtained after dryness and it has a BET surface area of 112 m2/g and exposes the (101) plane. When the post‒treatment temperature increases from 200 to 400 � C, both crystal phase and crystallite sizes of Mn3O4 are almost unchanged, but their main exposed crystal faces change from (101) to (200) and (112). When the post‒treatment temperature increases to 500 � C, Mn2O3 is dissociated from Mn3O4. Mn3O4_300 possesses a developed pore structure with a SBET of 201 m2/g and a pore volume of 0.41 cm3/g. Mn3O4_400 has low reduction peaks at 210 and 467 � C and similar reduction properties after multi‒reaction of NH3‒SCR, but the peak temperature shifts to higher temperature compared to the fresh sample. When the post‒treated sample is applied to the NH3‒SCR reaction, Mn3O4_400 exhibits excellent NO removal efficiency at low temperatures, which may be related to the surface oxidation of Mn3O4 to promote the formation of NO2. Mn3O4_400 has good resistance of SO2 and has 92% removal efficiency of NO when only SO2 is induced in the simulated gas, but NO removal efficiency decreases to 63% in the presence of H2O and SO2 due to the generation of MnSO4�H2O.

1. Introduction MnOx is considered as an effective catalyst for the reduction of NO with NH3 [1,2], and Mn3O4 nanocrystals can be synthesized by several techniques, such as high temperature calcination [3,4], sol­ vothermal/hydrothermal method [5], one-step synthesis and ultrasonic irradiation in alcohol-water mixture [6,7]. Yu [8] et al. have successfully synthesized various shapes of Mn3O4 nanocrystals in the presence of

organic solvents and surfactants. However, organometallic compounds are expensive and difficult to handle. Simple synthetic methods with inexpensive and non-toxic precursors under mild and friendly synthesis conditions are highly desirable for large scale synthesis of nanocrystals. Therefore, there is a challenge in obtaining a Mn3O4 nanocrystal having a uniform morphology and a nanometer size by a simple method using an inorganic reactant. For nanocrystals, both microstructure and texture properties play an

* Corresponding author. College of Architecture and Environment, Sichuan University, Chengdu, 610065, Sichuan, China. ** Corresponding author. Institute of New Energy and Low Carbon Technology, Sichuan University, Chengdu, 610065, Sichuan, China. E-mail addresses: [email protected] (J. Guo), [email protected] (S. Yuan). https://doi.org/10.1016/j.matchemphys.2019.121845 Received 28 November 2018; Received in revised form 12 June 2019; Accepted 9 July 2019 Available online 10 July 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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important role in increasing catalytic activity [9]. Mn3O4 is generally grown perpendicular to the normal direction of the (101) lattice plane and has an undeveloped pore structure [10–13]. Post-treatment at different temperatures can result in changes in the crystalline phase and pore structure [14]. However, there is lack of information on the changes in the surface oxidation state of Mn3O4 at different calcination temperatures. In addition, rapid SCR reaction is believed to increase NO removal efficiency at low temperatures, and Mn3O4 can promote rapid SCR reactions through rapid electron transfer [1,2]. In this paper, Mn3O4 nanocrystals were successfully prepared by simple non-organic method, and the effect of post-treatment tempera­ ture on the surface oxidation state of Mn3O4 crystal was studied. The effects of these changes on the selective catalytic reduction of nitric oxide (NH3-SCR) with ammonia and the resistance of SO2 and H2O have also been studied.

2.3. NH3‒SCR evaluation

2. Experimental

3. Results

2.1. Sample preparation

3.1. XRD

MnSO4, H2SO4 and NH3�H2O were purchased from Kelong Chemical Reagent Factory, Chengdu, China. An appropriate amount of MnSO4 and H2SO4 were dissolved in 2 L distilled water to form a homogenous so­ lution. And then, NH3�H2O was quickly poured into the above solution under strong agitation at room temperature until the pH reached 10. After the reaction finished, the mixture was centrifuged, washed by the distilled water several times until the washing liquid became neutral. The filter cake was dried in a vacuum oven at 60 � C overnight to obtain fresh sample. The obtained sample was named as Mn3O4_60. Subse­ quently, the Mn3O4_60 was solely calcined at 200, 300, 400 and 500 � C in a muffle furnace, and they were labeled as Mn3O4_200, Mn3O4_300, Mn3O4_400 and Mn3O4_500, respectively.

The XRD patterns, as illustrated in Fig. 1(a), can be divided into two groups. Mn3O4_60, Mn3O4_200, Mn3O4_300 and Mn3O4_400 are assigned to the first group, and Mn3O4_500 is another group. For the first group, their XRD patterns are similar, and all the diffraction peaks are indexed to the tetragonal Mn3O4 [3‒9, 11‒17]. It indicates that in our experimental conditions, Mn3O4 nanocrystals with a single tetragonal phase are obtained after only dryness at 60 � C and the products calcined at 200‒400 � C can still obtain Mn3O4 with a single tetragonal phase. For Mn3O4_500, the well-crystallized Mn3O4 is still observed, but the char­ acteristic peaks of Mn2O3 appear at 23.0� , 32.9� , 55.1� and 65.7� [3,13, 18], meaning that the Mn3O4 crystal can partially transform into Mn2O3 after post‒treatment at 500 � C. Remarkably, the diffraction peaks cor­ responding to the (004) and (105) faces show a slight shift, indicating lattice distortion of the Mn3O4 crystal for Mn3O4_60, Mn3O4_200 and Mn3O4_300. The lattice parameters are estimated using the d‒spacing values and the respective parameters also indicate that the distorted Mn3O4 structure can be easily formed and gradually improved according to the standard data of JCPDs card No.24‒0734 (a ¼ b ¼ 0.576, c ¼ 0.947 nm) when the calcination temperature increases. It should be noted in Table 1 that low temperature calcination has little influence on the crystallite size of Mn3O4, but once the temperature increases to 500 � C, Mn3O4 crystallite is ready to grow up from 15.1 to 18.5 nm. Thus, post‒treatment at 500 � C can lead to the transformation of Mn3O4 to Mn2O3, accompanied by the growth of crystallite size. After the SO2 resistance testing, Mn3O4 is still observed and has a crystallite size of 15.4 nm, and no other Mn species such as MnO, MnO2, Mn2O3 or sulfate are detected. It indicates that Mn3O4 does not interact with SO2. In other words, SO2 is only adsorbed on the surface of the catalyst to cover the active sites. After the SO2 and H2O resistance testing, Mn3O4 is obviously observed and small MnSO4�H2O is also detected. This finding indicates that Mn3O4 is reacted with SO2 in the presence of H2O. This is because Mn3O4 can catalyze the oxidation of SO2 to H2SO4 in the presence of water vapor and O2 [19]. If the H2SO4 produced cannot be removed from Mn3O4 in time, it will react with the Mn species to form MnSO4 [20]. Further, MnSO4 easily react with H2O to form MnSO4�H2O. At the same time, it is found that after SO2 with or without H2O resistance testing, the crystallite size of Mn3O4 can still be maintained at 15.6 and 15.4 nm. This finding suggests that temperature is the key factor in Mn3O4 sintering.

The selective catalytic reduction of NO with NH3 (NH3‒SCR) over all samples was conducted in a fixed‒bed quartz reactor (i.d. 8 mm) from 80 to 400 � C. The reaction temperature was measured by thermocouple inserted directly into the catalyst bed. The typical feeding gases were composed of 500 ppm NO, 500 ppm NH3, 5 vol% O2, SO2 (when used), H2O (when used) and N2 as balance. The gas flow rate was controlled by mass flow controllers before entering the gas mixer. The total flow rate was 500 mL/min, corresponding to 50000 h 1 of gas hourly space ve­ locity (GHSV). The gas concentrations in the outlet stream were online monitored by a flue gas analyzer (Gasboard‒3000, Wuhan Quartet Optoelectronics Technology Co., Ltd, China). The NO removal efficiency is calculated as follows: NO removal efficiency (%) ¼ ([NO]in ‒ [NO]out)/[NO]in � 100.

2.2. Sample characterization X‒ray powder diffraction patterns were acquired on an X’pert Pro MPD powder diffractometer (PANAlytical Company, Holland) equipped with a nickel‒filtered Cu‒K (0.15418 nm) radiation source and a scin­ tillation counter detector. The analyses were performed for values of 2θ between 10� and 80� , and crystalline phases were identified by matching with the International Centre for Diffraction Data Powder Diffraction File (ICDD‒PDF). Raman spectra were conducted at room temperature on a Horiba Jobin‒Yvon HR 800 Raman spectrometer. The line at 632 nm of Arþ ion (Spectra Physics) laser was used as an excitation source. The wave number values from the spectra were collected from 100 to 1200 cm 1. Temperature‒programmed reduction (TPR) of H2 was carried out on a TP‒5076 apparatus (Tianjin Xianquan Co. Ltd., China) with a thermal conductivity detector (TCD). The samples were purged by high pure He with a flow rate of 30 ml/min at its calcined temperature for 1 h and then cooled to 40 � C. After the gas was switched to 5 vol% H2‒N2 and the baseline was straight, the sample was heated from 40 to 800 � C with a heating rate of 10 � C/min. High‒resolution transmission electron microphotographs (HRTEM) were obtained in a JEM 2100F instrument, operating at 200 kV. N2 adsorption‒desorption isotherms were obtained using an AUTOSORB‒IQ (Quantachrome Instruments, USA) apparatus at 196 � C. Prior to measurement, the samples were degassed at its treated temperature (60, 200, 300, 400 and 500 � C) for 5 h under vacuum. The surface area (SBET) was calculated from the adsorption isotherm data using the Brunauer‒Emmett‒Teller (BET) equation at a relative pressure (P/P0) between 0.05 and 0.35. The total pore volume was assessed from the volume of N2 at P/P0 ¼ 0.95, and the pore‒size distribution was determined from the N2 adsorption‒desorption isotherm using the Barrett‒Joyner‒Halenda (BJH) formula.

3.2. Raman The Raman spectra of all samples are shown in Fig. 2 and give three scattering peaks between 100 and 1200 cm 1, a sharp peak and two small peaks. The peaks at 655, 310 and 366 cm 1 are attributed to the tetragonal Mn3O4 [11, 12, 16, 21‒23], while the one presented on 2

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244

400

105

312 303 321

220

004

211

101

(b)

112 200 103

Materials Chemistry and Physics 237 (2019) 121845

MnSO4 H2O

Mn3O4 Mn3O4_400

Intensity (a.u.)

after SO2+H2O

Mn3O4_400

after SO2

fresh Mn3O4_400

10

20

30

40

50

60

70

80

2 Theta Fig. 1. Powder X‒ray diffraction patterns: (a) fresh samples; (b) selected sample after resistance testing.

reduction from Mn3O4 to MnO [3,18]. This also verifies that the pre­ pared Mn3O4_60 has only Mn3O4 nanocrystals. When the sample is post‒ treated at different temperature, the number of reduction peaks increase obviously, and the dominant one appearing at around 470 � C with a wide temperature range is undoubtedly the reduction of Mn3O4 nano­ crystals to MnO. Only Mn3O4_500 gives a distinct reduction peak at 370 � C, which coincides with the reduction of Mn2O3 crystals to Mn3O4 [18,25]. Reduction components appearing below 300 � C show some differences with different calcination temperatures, and these species are inferred to be associated with Mn ions in an overall oxidation state [18,26], or they are due to the reduction of oxygen adsorbed on the Mn3O4. A reasonable explanation is that the exposed crystal faces are different, and the oxygen vacancies are different. This vacancy is easy to adsorb oxygen. The adsorbed oxygen can be activated and transferred as following: O2(gas) ↔ O2(ads) ↔ O2 (ads) ↔ 2O (ads) ↔ 2O2 (ads) ↔ 2O2 (lattice). The study has shown that O2 (ads) and O (ads) are formed below 300 � C while the adsorbed surface lattice oxygen (O2 (ads)) appears above 300 � C [27]. According to the differences of the reduction peaks and the results of XRD and Raman, the prepared Mn3O4_60 is speculated to undergo a gradual oxidation from the surface to the subsurface, and further partially turn into Mn2O3 crystal when the calcination temperature increases to 500 � C because of the oxygen migration. This shows the difference in the reduction peak. Overall, Mn3O4_400 shows the best oxidation performance because it has a peak at 210 � C and a big peak at 467 � C, which may be beneficial to the catalytic reaction. The increase of chemisorbed oxygen was beneficial to

Table 1 Lattice parameters (nm) of all samples. Samples

Phase

a¼b

c

Crystallite sizes

Mn3O4_60 Mn3O4_200 Mn3O4_300 Mn3O4_400 Mn3O4_500

Mn3O4 Mn3O4 Mn3O4 Mn3O4 Mn3O4 Mn2O3

0.577 0.577 0.577 0.578 0.578 0.940

0.941 0.940 0.941 0.944 0.945

15.1 15.4 15.2 15.5 18.5

Fig. 2. Raman spectra of all fresh samples.

Mn3O4_500 at 645 cm 1 is more like an overlapping peak of Mn2O3 and Mn3O4 [23,24]. These Raman data are consistent with the XRD results, confirming only Mn3O4_500 to be the Mn2O3 phase. The transformation of Mn3O4 to Mn2O3 is closely related to the oxygen transfer on the surface, which is as follows: O2 (gas) ↔ O2 (ads) ↔ O2 (ads) ↔ 2O (ads) ↔ 2O2 (ads) ↔ 2O2 (lattice). When Mn3O4 is calcined at 500 � C, the O2 adsorbed on Mn3O4 can be activated to O2 (lattice). Thus, Mn2O3 crystal can be formed. Therefore, the post‒treatment temperature is crucial for the phase change of the prepared Mn3O4. 3.3. H2‒TPR As seen in Fig. 3, Mn3O4_60 shows a single peak in the range of 350–540 � C with centered at 478 � C, which is due to a one‒step

Fig. 3. H2‒TPR patterns of all fresh samples. 3

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SCR reaction. In Fig. 4(a), it is easy to find that the low‒temperature reduction peaks of the reacted sample have changed significantly compared to those of fresh sample. After the first catalytic reaction, Mn3O4_200, Mn3O4_300 and Mn3O4_400 show a new small reduction peak at 224 � C, while the original peaks at 208 � C and 239 � C disappear. This reduction peak is still due to the consumption of oxygen species adsorbed on catalysts. One possible reason is that in the NH3‒SCR, the surface‒ adsorbed oxygen species participate in the oxidation of NO, while O2 in gas phase is adsorbed and activated on the catalyst surface to change the redox performance of the catalyst. For the reacted Mn3O4_400 and Mn3O4_500, the reduction peak of Mn3O4 to MnO slightly shifts to high temperature. These changes in H2‒TPR results before and after the first NH3‒SCR reaction demonstrate that the high temperature reaction af­ fects the overall structure of the catalyst, while the low temperature reaction only consumes surface adsorbed oxygen species and has no significant effect on the Mn3O4 crystal. The results in Fig. 4(b) further confirm this point. Obviously, there are three peaks for all used samples, and the reduction peaks of Mn3O4_400 shift to the high temperature region after several catalytic reactions. In addition, the peaks below 250 � C due to the reduction of O2 (ads) and O (ads) and in the range of 250–330 � C due to the reduction of O (ads) and O2 (ads) are observed. The big peak in the range of 340–510 � C shows that only Mn3O4 exists in catalyst after several catalytic reactions. These findings suggest that in NO removal process, O2 is adsorbed and activated on Mn3O4 to take part in the NO removal reaction. More importantly, when the number of reaction cycles increases, the change tendency of oxidation performance of Mn3O4_400 is consistent with that of NO removal efficiency below 200 � C (below NH3‒SCR), which indicates that redox performance may be the key factor to control its SCR activity at low temperature because of oxygen activation.

400 � C would not cause severe agglomeration of Mn3O4 nanocrystals. According to the particle size distribution, it is found that the average particle size of Mn3O4_500 is about 20 nm, which is slightly larger than the average particle diameter of the sample treated at a low tempera­ ture. In addition, HRTEM images are used to explore microstructure evolution and show that the separation distance between the lattice layers is consistent with the lattice parameters of the Mn3O4 haus­ mannite phase. The interplanar spacing is specifically indexed for (101) (0.506, 0.504, 0.508 or 0.490 nm) [5,10,11,15,17,25], (200) (0.292 or 0.287 nm) [4,6,28], (211) (0.254 nm) [29], and (112) (0.319 or 0.309 nm) [10,25,29] faces of tetragonal Mn3O4. Moreover, it is found that with the increase of calcination temperature, the (101) plane gradually disappears, while (200) and (112) are exposed. This reveals that the post treatment can change the exposed crystal faces from (101) to (200) and (112), and calcination at 500 � C can change the crystal phase of the Mn oxides. The crystal models of the primitive Mn3O4 and their faces of (101), (200) and (112) before and after calcination are depicted in Fig. 5(b). Studies have shown that the long bonds of Mn3þ‒O are easily broken at moderate temperatures [2], followed by surface remodeling and facet transformation to maintain a stable crystal struc­ ture. During high temperature calcination, the crystal surface of (101) tends to transform into (200) and (112), accompanied by surface oxygen loss and surface reconstruction. For the (200) and (112) faces, both Mn and O are exposed to surfaces with unsaturation, where O2 (gas) can be easily adsorbed and activated to form O2 (ads) and O (ads), and further converted into lattice oxygen as the calcination temperature increases. When Mn3O4 is combined with lattice oxygen, Mn2O3 is formed (in Fig. 1(a)). For Mn3O4_500, it is found that Mn2O3 is dissociated from Mn3O4 due to oxygen immigration.

3.4. HRTEM

The N2 adsorption‒desorption isotherms and pore size distributions of all samples are illustrated in Fig. 6, and the BET surface area (SBET) and pore volume are listed in Table 2. According to the IUPAC classifi­ cation, the shapes of all isotherms are closely resembled with type IV and an obvious hysteresis loop is observed, which is characteristic of mes­ oporous materials [30]. The pore size distributions in Fig. 6(b) further indicate that all samples are composed of mesopores and give two pore size distribution peaks. One pore size distribution peak is at 2 nm, and the other dominated is concentrated on 22 nm. Comparison with the fresh Mn3O4_60, the calcined sample has more pores with a diameter of about 22 nm. When the calcination temperature is 200 � C, Mn3O4_200 has broad and short peak in the range of 10–32 nm and most of pore size is below 22 nm Mn3O4_300 has a narrow and high peak, indicating that the pore size distribution is mainly concentrated at 21.7 nm. For Mn3O4_400, a broad and short peak is observed and most of the pore size is above 22 nm. For Mn3O4_500, its pore size range directly shifts to 22‒ 45 nm and peak height is centered at 27 nm, indicating that calcination at 500 � C is a node with a large pore size distribution. Meanwhile, Mn3O4_60 has a surface area of 112 m2/g and a pore volume of 0.31 cm3/g, which shows a porous structure formed after dryness at 60 � C. As the calcination temperature increases from 200 to 500 � C, the surface areas and pore volumes increase to 201 m2/g and 0.41 cm3/g and then decrease to 120 m2/g and 0.22 cm3/g. All of these changes in texture performance demonstrate that appropriate post‒treatment temperature is beneficial to the formation of a well‒developed porous structure through the evaporation of water and the accumulation of micropores. When the calcination temperature is higher than 300 � C, the collapse of pores, companied with the phase change, results in the deterioration of the texture properties of Mn3O4_500.

3.5. N2 adsorption/desorption

Typical TEM and HRTEM images of the as‒synthesized samples are depicted in Fig. 5(a). For fresh Mn3O4_60, multi‒morphologies of tet­ ragon and sphere with edge lengths below 20 nm are observed, indi­ cating that the prepared samples are nanoparticles. Compared with Mn3O4_60, no significant changes in the morphology and size are observed except for Mn3O4_500. This means that calcination below

3.6. NH3‒SCR evaluation

Fig. 4. (a) H2‒TPR patterns of all samples before and after the first NH3‒SCR reaction (The black is pre-reaction and color represents the reaction after); (b) H2‒TPR patterns of Mn3O4_400 after continuous NH3‒SCR reaction.

In the presented work, we further verified the activity of as‒prepared samples for the selective catalytic reduction of NO with NH3. Since the 4

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Fig. 5. (a) TEM and HRTEM images; (b) crystal model of Mn3O4 and the crystal planes of (101), (200) and (112).

post‒treatment temperature influences the exposed crystal plane and the redox properties of Mn3O4, the evaluation temperature does not exceed the post‒treatment temperature to describe its catalytic activity. The curves of relationship between NO removal efficiency and the re­ action temperature of all fresh samples are illustrated in Fig. 7(a). It is found that except for Mn3O4_500, other samples have a similar NO removal efficiency at 50 � C. Obviously, below 130 � C, the removal effi­ ciency of NO from high to low is as follows: Mn3O4_400 > Mn3O4_300 > Mn3O4_200 > Mn3O4_500. When the reac­ tion temperature increases, the NO removal efficiency increases, but the increase rate of Mn3O4_500 is higher than others. At 180 � C, Mn3O4_400 achieves 100% NO removal efficiency, while Mn3O4_200 is obviously worst than Mn3O4_500 and Mn3O4_300. Except for Mn3O4_200, other samples achieve 100% NO removal efficiency in the range of 200–300 � C due to the similar redox property in the range of 200–300 � C. However, once the reaction temperature is above 300 � C, the NO removal ability starts to decrease, and the descent rate of Mn3O4_500 is lower than that of Mn3O4_400. This is because the former has a different type of Mn

oxide than the latter. Therefore, the post‒treatment temperature has a significant effect on the NH3‒SCR activity of the post‒treated Mn3O4. In addition, it is noteworthy the results of several runs of NH3‒SCR shown in Fig. 7(b). Obviously, as the number of cycles increases, the NO removal efficiency of Mn3O4_400 below 200 � C continues to decrease. For the second evaluation, the NO removal efficiency of Mn3O4_400_C1 still reaches about 100% in the range of 200–250 � C, indicating a decrease in activity. Next, the sample is subjected to a third NH3‒SCR evaluation, and it is found that the NO removal efficiency of Mn3O4_400_C2 still reaches about 100% in the range of 200–300 � C, which is close to the activity of fresh sample. For the fourth evaluation, Mn3O4_400_C3 achieves 100% NO removal efficiency in the range of 250–320 � C. It exhibits good high temperature activity compared to fresh samples. In addition, its SCR activity decreases first and then in­ creases to near fresh sample with the increase of cycles when reaction temperature is higher than 300 � C. These findings indicate that Mn3O4 post‒treated at 400 � C has excellent NH3‒SCR activity for a long time. This means that there is a clear structure‒activity relationship among 5

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activity. Generally, in actual flue gas, after desulfurization, there is still SO2 with low concentrations and some water vapor in the flue gas. SO2 could react with manganese to disturb the reaction circulation or combine with NH3 to decrease the reduction of NO. As shown in Fig. 8, when only 100 or 200 ppm SO2 is induced into the simulated gas, the NO removal efficiency of Mn3O4_400 gradually decreases to about 92% with the reaction time increasing. When SO2 is shut off, the removal efficiency of NO doesn’t return to the initial value and maintains at about 92%. This finding indicates that SO2 could be adsorbed on Mn3O4 to cover some active sites. It results in the deactivation of Mn3O4_400. When only 5% H2O is induced into the simulated gas, the NO removal efficiency of Mn3O4_400 gradually decreases to 90% with the reaction time increasing. It may be that H2O may prevent the diffusion of NO, NH3 and O2 due to the deposition on the catalyst surface or the reducing agent decreases due to the reaction of NH3 with H2O. However, when H2O is shut off, the removal efficiency of NO returns to the initial value, indi­ cating that the activity decrease caused by H2O is temporary. When 100 ppm SO2 and 6% H2O is simultaneously introduced into the simu­ lated gas, the decreased removal efficiency of NO is very fast in the range of 60–600 min. With the increase of reaction time, its decreased rate becomes slow, and the NO removal efficiency decreases to 63% at 720 min and maintains this value for the rest of the time. It suggests that Mn3O4_400 is extremely easy to deactivate in the presence of SO2 and H2O.

Fig. 6. (a) N2 adsorption‒desorption isotherms and (b) pore size distributions. Table 2 The BET surface area (SBET) and pore volume of all samples. Samples

SBET (m2/g)

Pore volume (cm3/g)

Mn3O4_60 Mn3O4_200 Mn3O4_300 Mn3O4_400 Mn3O4_500

112 141 201 145 120

0.31 0.33 0.41 0.36 0.22

4. Discussion In the NH3‒SCR reaction, the oxidation of NO is very important. The formation of intermediates with the reducing agent NH3 is an important step [33]. The pore structure provides a convenient chance for gas adsorption and diffusion, and the abundant mesopores provide channels for the reaction gases entering into the inner surface of catalysts to arrive the active sites. Larger specific surfaces can provide more adsorption sites. When they can meet the gas diffusion, the microenvironment of the active site plays a key role in the catalytic reaction. Some chemi­ sorptions and chemical reactions will occur at the active sites, including the breaking and reorganization of reactive molecular bonds. For the post‒treated Mn3O4 sample, it is found that they have similar textures except Mn3O4_300 and different exposed crystal faces and similar redox except Mn3O4_60 and Mn3O4_500. Crystal structure is closely related to its redox properties, and the crystal face directly determines the surface atomic configuration, which influence the adsorption and activation of oxygen. Thus, it leads to the difference in adsorption and oxidation of NO, as shown in Fig. 9. We found that Mn3O4_400 shown the best NO removal ability at low temperature, which may be due to both (200) and (112) faces caused by calcination to promote the “fast SCR” reaction. On (200) and (112) faces, both Mn and O are exposed to the surface with unsaturation, which promotes the adsorption and activation of O2. As shown in Figs. 3 and 4, post‒treated Mn3O4 has some active oxygen species such as O2 (ads) and O (ads). These oxygen species can accel­ erate the formation of NO2, which is a key step to improve the removal efficiency of NO at low temperature [30, 34‒37]. This is because NO can be more easily reduced by the following reaction when there is enough NO2: 2NH3 þ NO þ NO2→2N2 þ 3H2O [38]. When reaction temperature increases, more NO2 is formed and further combines with active O to form catalyst–NO3 [39]. The adsorbed nitric oxides and ammonia spe­ cies participate in the NH3‒SCR reaction as intermediates, such as NH4NO3. This NH4NO3 is further decomposed into N2 and H2O. Thus, NO can be removed in the middle temperature range. Here, electron transfer can be formed between Mn3þ and Mn2þ in Mn3O4 and Mn3þ is dominant. The study has shown that Mn3þ is one of adsorption sites for NO [40]. Mn2þ is easy to adsorb and activates oxygen, which can accelerate the oxidation of NO. At the same time, when reaction tem­ perature continues to increase, especially more than 300 � C, NH3 oxidation is more likely to occur, which is partial hindered through ‘‘Fast

Fig. 7. The curves of relationship between NO removal efficiency and reaction temperature: (a) fresh samples; (b) cycle runs of Mn3O4_400.

the properties of the catalyst, the reaction temperature and the NO removal efficiency. Obviously, as shown in Table 3, the NH3-SCR ac­ tivity of the pure Mn3O4 catalyst is different, which may be due to the difference in the preparation method of the catalyst and testing condi­ tions. Manganese oxides contain various unstable oxygen and oxidative states such as Mn2þ, Mn3þ and Mn4þ. In the low temperature SCR re­ action, the different valence states of manganese play different roles. Mn3O4 contains Mn2þ and Mn3þ and exhibits different NH3-SCR Table 3 NO conversion (%) at 150 � C over all prepared Mn3O4. NO conversion (%) at 150 � C

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this article Reference[31] Reference[32] Reference[18]

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Materials Chemistry and Physics 237 (2019) 121845

Fig. 8. The curves of relationship between NO removal efficiency and reaction time at 150 � C for Mn3O4_400.

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Fig. 9. Schematic diagram of fast SCR reaction.

SCR” reaction. A good SCR activity at high temperature is observed on the Mn3O4_500 sample, which is due to more Mn3þ. When SO2 is induced into the reactive gas, it can be adsorbed and oxidized on Mn3O4, which prevents adsorption and oxidation of NO. In addition, some NH3 can combine with the adsorbed SO2, resulting in a reduction of reducing agent. They lead to a decrease in NO removal ability. When only H2O is introduced into the simulated flue gas, it can be adsorbed on the catalyst surface and competes with NH3, causing a decrease in NO removal ability. When H2O and SO2 are simultaneously added into the simulated flue gas, the formation of MnSO4 is the main reason for the decline of NH3‒SCR activity. It can prevent the contact of NO, O2 and NH3 with the catalyst. At the same time, NH3 is easy to react with H2SO4 to form (NH4)2SO4 and loses the role of reducing agent. The above two factors lead to a sharp decline in NH3‒SCR activity. 5. Conclusion The Mn3O4 nanocrystals with single (101) plane were prepared by a simple precipitation method at room temperature. The well crystallized Mn3O4 is still the only phase with stable morphology and crystallinity when the calcination temperature is in the range of 200–400 � C, while the exposed faces of Mn3O4 change from (101) to (112) and (200). Mn2O3 is dissociated from Mn3O4 after calcination at 500 � C. On (200) and (112) faces, both Mn and O are exposed to the surface with unsa­ turation, which improves the adsorption and activation of O2 and pro­ motes the “fast SCR” reaction. Redox properties have a greater impact on the NO removal efficiency of the calcined Mn3O4 catalysts than texture properties. Mn3O4_400 exhibits the highest NO removal efficiency should be attributed to more O which promotes the formation of NO2 at low temperature. Mn3O4_400 shows good resistance of SO2 when only SO2 is induced into the simulated gas because SO2 can only adsorb on Mn3O4 to decrease the activity of NH3‒SCR, while the formation of MnSO4 is the main reason for the decline of NH3‒SCR activity when H2O and SO2 are simultaneously added into the simulated flue gas. Acknowledgements This work is financially supported by the National Natural Science 7

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