Abundant Oxygen Vacancies Induced by the Mechanochemical Process Boost the Low-Temperature Catalytic Performance of MnO₂ in NH₃-SCR

Abstract

Manganese oxides (MnO_x) have attracted particular attention in the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) because of their excellent low-temperature activity. Herein, we prepared a highly efficient MnO₂ (MnO₂-M) catalyst through a facile ball milling-assisted redox strategy. MnO₂-M shows a 90% NO_x conversion in a wide operating temperature window of 75–200 °C under a gas hourly space velocity of 40,000 h⁻¹, which is much more active than the MnO₂ catalyst prepared by the redox method without the ball-milling process. Moreover, MnO₂-M exhibits better H₂O and SO₂ resistance. The enhanced catalytic properties of MnO₂-M originated from the high surface area, abundant oxygen vacancies, more acid sites, and higher Mn⁴⁺ content induced by the ball-milling process. In situ DRIFTS studies probed the reaction intermediates, and the SCR reaction was deduced to proceed via the typical Eley–Rideal mechanism. This work provides a facile method to enhance the catalytic performance of Mn-based catalysts for low-temperature denitrification and deep insights into the NH₃-SCR reaction process.

1. Introduction

One of the most significant factors causing environmental pollution is NO_x emitted by burning fossil fuels, leading to a series of environmental concerns such as ozone depletion, acid rain, and photochemical smog [1,2]. At present, selective catalytic reduction by NH₃ (NH₃-SCR) is one of the effective technologies to reduce NO_x [3,4,5], typically using the V₂O₅-WO₃/TiO₂ and V₂O₅-MoO₃/TiO₂ catalysts [6,7]. However, the V-based catalysts have some disadvantages, e.g., the narrow working temperature window and biological toxicity. Additionally, their operating temperature is required to be above 300 ℃, which is energy-consuming [8,9]. Therefore, it is essential to develop novel non-V-based SCR catalysts with high activity at low temperatures [10,11,12].

Mn-based catalysts have been widely investigated for low-temperature NH₃-SCR due to their high catalytic activity, good redox properties, relatively low toxicities, and low cost [13,14]. In the case of pure MnO_x catalysts, their different valence states typically affect the catalytic activity. Yang et al. [15] found that MnO₂ exhibited higher NO conversion at 75–250 °C than Mn₂O₃ and Mn₃O₄ because of the higher valence state of Mn in MnO₂. In addition, the crystallinity and surface area of MnO_x can also affect its catalytic performance. Tang et al. [16] found that amorphous MnO_x with a higher specific area had higher activity than highly crystallized MnO_x. Moreover, the NH₃-SCR reaction involves the participation of reactive oxygen species, e.g., NO oxidation to NO₂. Oxygen vacancies (OVs) are also helpful in improving the catalytic activity of MnO_x [17]. For example, Liu et al. [18] reported that mesoporous α-MnO₂ nanosheets with more OVs reached 100% NO conversion under a gas hourly space velocity (GHSV) of 700,000 h⁻¹ at 100 °C.

Several strategies have been developed to generate OVs in metal oxides. For example, Chen et al. [19] employed aqueous NaBH₄ to reduce Co₃O_4, and OVs were generated during the surface reduction process. Sun et al. [20] found that the post-acid-etching process effectively made OVs in MnO_x. Hilpert et al. [21] prepared LaCrO_3-δ with many OVs by exposing LaCrO₃ to oxidizing and reducing atmospheres under 900–1100 °C operating conditions. Yim et al. [22] used ion sputtering and annealing reduction method to obtain rutile TiO₂ with high OVs concentration. Although these approaches have proven effective, they are relatively complicated and costly. Increasing the OVs concentration of MnO_x through simple and economical alternative routes to improve its catalytic performance is essential but challenging.

Herein, we report a facile ball milling-assisted redox strategy to obtain a high-performing MnO₂ catalyst (MnO₂-M) by introducing abundant OVs. In contrast to the MnO₂ catalyst obtained by the conventional redox method (MnO₂), MnO₂-M exhibits much higher activity in the NH₃-SCR reaction. This is mainly attributed to the abundant OVs, more acid sites, and higher Mn⁴⁺ content induced by the ball-milling process. The corresponding structure-activity relationship and reaction mechanism was elucidated by investigating the catalyst structure in detail. This work demonstrates significant progress for the practical applications of pure MnO_x in NO_x elimination at low temperatures.

2. Results and Discussion

2.1. Characterization of the Catalysts

2.1.1. XRD and PSD Analysis

Figure 1a shows the XRD patterns of the catalysts. In the pattern of MnO₂-M, three prominent diffraction peaks appear at 37.5°, 42.0°, and 57.0°, which can be indexed as the (211), (301), and (431) planes of α-MnO₂ (PDF#44-0141), respectively [23]. Compared with the MnO₂-M catalyst, MnO₂ shows more diffraction peaks. These diffraction peaks match well with the standard pattern of the α-MnO₂ phase (PDF#44-0141), where the stronger diffraction peaks at 2θ = 28.8°, 37.5°, and 42.0° can be ascribed to the (310), (211), and (301) planes of the MnO₂, respectively [24,25]. It should be noted that the XRD pattern of the MnO₂-M catalyst shows significantly broadened diffraction peaks with poor crystallinity, indicating that its crystal growth is suppressed during the ball milling-assisted redox process. Indeed, M-MnO₂ obtained by the ball-milling of MnO₂ for 1 h also presents lower crystallinity and smaller particle size, indicating that the ball-milling process does affect the structure of the catalyst. The crystal sizes of the MnO₂-M catalyst based on the Debye–Scherrer equation calculated from the strongest diffraction peak at 2θ = 37.5° is 4.92 nm, while that at 2θ = 28.8° calculated at 27.17 nm for the MnO₂ catalyst (

Table 1. Physical properties of the catalysts.

Catalyst	SBET i (m2 g−1)	VP ii (cm3 g−1)	Lattice Parameter iii (Å)		Average Crystallite Size iv (nm)
			a = b	c
MnO2-M	177.9	0.335	9.7904	2.8544	4.92
MnO2	66.4	0.149	9.8156	2.8561	27.17

ⁱ Surface area derived from the BET equation. ⁱⁱ Pore volume was obtained from the volume of nitrogen adsorbed at a relative pressure of 0.99. ⁱⁱⁱ XRD results for the MnO₂ phase in the catalysts, calculated by the Bragg equation. ^iv Estimated from the XRD diffraction peak (2θ = 37.5° for MnO₂-M and 28.8° for MnO₂) using the Debye–Scherrer equation.

). The particle size of the catalysts was measured using a laser particle size analyzer, and the results are shown in Figure 1b. The size of MnO₂ is centered at 1–20 µm. However, the MnO₂-M catalyst has different distributions at 0.2–2, 2–6, and 6–20 µm, suggesting that both small and large particles coexist. The mixture of particles with different sizes benefits the formation of inter-particle pores.

2.1.2. N₂ Adsorption Measurements

The BET surface area and total pore volume of the catalysts are listed in Table 1. The MnO₂-M catalyst possesses a larger surface area (177.9 vs. 66.4 m²/g) and pore volume (0.335 vs. 0.149 cm³/g) than the MnO₂ catalyst. The larger surface area may be because the MnO₂-M catalyst contains abundant inter-particle pores, which can also be deduced from the PSD results. To interpret such a difference, the N₂ adsorption–desorption isotherms and pore size distribution of the two catalysts are also shown in Figure 2. The two catalysts display the type IV adsorption isotherms with H3-type hysteresis, indicating the formation of a porous structure [26]. As seen in Figure 2b, the pore size distributions of the two catalysts obtained from nitrogen adsorption are both in the range of 5–30 nm. However, the peak intensity of the pore size distribution in the MnO₂-M catalyst is much higher than in the MnO₂ catalyst, which means that more pores, including large and small sizes, exist in the MnO₂-M catalyst.

2.1.3. SEM Observation

Figure 3 depicts the typical SEM images of MnO₂-M and MnO₂. The MnO₂-M catalyst exhibits irregular particles of varying sizes, while the MnO₂ catalyst displays a unique waxberry-like shape with consistent dimensions. Compared with the MnO₂ catalyst containing particles in close contact, the particles in the MnO₂-M catalyst are in loose connection, which is more conducive to forming inter-particle pores. This is also evidenced by the PSD result (Figure 1b) and N₂ adsorption result (Figure 2). In short, inter-particle pores can increase the specific surface area of the catalyst and provide more active sites between the catalyst and gas-phase reactants.

2.1.4. H₂-TPR Analysis

It is well known that the redox property of catalysts is a critical factor for the NH₃-SCR reaction [27]. Therefore, we investigated the reduction behavior of the catalysts by H₂-TPR test, and the results are presented in Figure 4. For the two catalysts, there are two obvious reduction peaks located in the temperature range of 213–400 ℃ (I) and 400–533 ℃ (II), which can be assigned to the successive reduction of MnO₂ → Mn₂O₃/Mn₃O₄ → MnO [28,29]. It should be noted that the reduction of MnO₂-M catalyst starts at a lower temperature but ends up at a higher temperature. In contrast, the two reduction peaks of the MnO₂ catalyst overlap, and the peaks are more concentrated. This difference between the two catalysts may be attributed to their different particle size distributions. Smaller particles in MnO₂-M catalyst are easier to be reduced at a low temperature, while larger particles are more difficult to be reduced because of limited mass transfer. Therefore, the two reduction peaks of the MnO₂-M catalyst have a larger temperature gap due to the varying particle size. By contrast, the uniform particle size in the MnO₂ catalyst leads to a narrow reduction peak. This result indicates that the ball-milling process can create small particles and improve the reduction potentials at low temperature. Furthermore, quantitative analysis data of H₂-TPR are listed in

Table 2. H₂ adsorption amount of the catalysts.

Catalyst	H2 Consumption i (mmol g−1)		Total H2 Consumption i (mmol g−1)	Theoretical H2 Consumption i (mmol g−1)
	I	II
MnO2-M	4.85	3.91	8.76	11.50
MnO2	3.91	2.27	6.18	11.50

ⁱ Calculated based on the H₂-TPR results.

. Although the two catalysts in the H₂-TPR experiments are reduced to green MnO, the experimental H₂ consumption of the catalysts is significantly less than the theoretical consumption (11.5 mmol/g). This may be due to some manganese in the Mn³⁺ oxidation state in catalysts. In addition, the MnO₂-M catalyst shows more H₂ consumption than the MnO₂ catalyst, which indicates that there exists more of an Mn⁴⁺ oxidation state than MnO₂. It has been widely reported that Mn⁴⁺ is the most effective oxidation state for NO_x conversion [30]. Therefore, the MnO₂-M catalyst can be a significant advantage in NH₃-SCR compared with MnO₂.

2.1.5. XPS Analysis

Figure 5 shows the XPS Mn 2p and O 1s spectra of MnO₂-M and MnO₂ catalysts. The Mn 2p_3/2 spectra can be separated into three characteristic peaks attributed to Mn²⁺ (641.0 eV), Mn³⁺ (642. 4 eV), and Mn⁴⁺ (643.5 eV), respectively (Figure 5a), by performing peak-fitting deconvolutions [31,32,33,34]. The Mn⁴⁺/(Mn³⁺ + Mn²⁺) atomic ratio (

Table 3. Surface compositions of the catalysts by XPS analysis.

Catalyst	Mn4+/(Mn3+ + Mn2+)	Oads/Olatt
MnO2-M	0.81	1.17
MnO2	0.56	0.60

) was calculated from XPS spectra. The relative surface content of Mn⁴⁺/(Mn³⁺ + Mn²⁺) on MnO₂-M (0.81) is much higher than that of MnO₂ (0.56). Therefore, much more Mn⁴⁺ is exposed on the surface of the MnO₂-M catalyst, which is beneficial for NO oxidation to NO₂, leading to high activity in the NH₃-SCR reaction at low temperatures.

Two peaks can be distinguished in the O 1s spectra of the catalysts (Figure 5b). The peak at 529.9 eV is assigned to the lattice oxygen (marked as O_latt), and the peak at 531.3 eV is ascribed to the surface adsorbed oxygen (marked as O_ads, O₂²⁻ or O⁻ belonging to defect-oxide or hydroxyl-like group) [35,36]. It is well known that the O_ads species are more active than the O_latt species due to their higher mobility [37]. From Table 3, the O_ads/O_latt ratio in MnO₂-M (1.17) is much higher than that in MnO₂ (0.60), suggesting that the ball-milling process could produce much more OVs in the MnO₂-M catalyst. The above results show that the MnO₂-M catalyst has a higher Mn⁴⁺/(Mn³⁺ + Mn²⁺) content and more surface oxygen species than the MnO₂ catalyst, which is critical to the NH₃-SCR reaction.

2.1.6. NH₃-TPD Analysis

The surface acidity of the catalyst also plays a vital role in the low-temperature SCR of NO by NH₃. Therefore, NH₃-TPD was carried out to evaluate the acidity of the catalysts, as shown in Figure 6. The detailed ammonia desorption parameters of the catalysts are summarized in

Table 4. Quantitative analysis of NH₃-TPD over the catalysts.

Catalyst	Surface Acidity i (μmol g−1)
MnO2-M	0.43
MnO2	0.32

ⁱ Calculated based on the NH₃-TPD results.

. From Figure 6, both MnO₂-M and MnO₂ catalysts exhibit one broad desorption peak below 300 ℃, which can be attributed to the subsequent desorption of NH₃ coordinated with weak acid sites [36]. As shown in Table 4, the total amount of desorbed NH₃ for MnO₂-M (0.43 μmol g⁻¹) is higher than that of MnO₂ (0.32 μmol g⁻¹), possibly due to the larger specific surface area of the MnO₂-M catalyst. The superiority of acid sites on the MnO₂-M catalyst could promote the adsorption and activation of NH₃, which favors the SCR reaction [38].

2.2. Formation Mechanism of MnO₂-M

Previously, studies reported that MnO₂ with different morphology structures could be obtained by tailoring manganese precursors, such as MnCl₂, MnSO₄⋅H₂O, Mn(NO₃)₂ H₂O, and KMnO₄ [39]. Here, we used Mn(NO₃)₂ solution and KMnO₄ as precursors. Mn precursors underwent an oxidation-reduction reaction (2KMnO₄ + 3Mn(NO₃)₂ + 2H₂O = 5MnO₂ +HNO₃ + 2KNO₃) to generate MnO₂ sediment (Scheme 1). Ball-milling as a mechanical technique is widely used to grind powders into fine particles. When ball-milling was introduced into the catalyst preparation process, these Mn²⁺ and Mn⁷⁺ precursors could quickly react and convert to irregular MnO₂-M particles with uneven particle size. The crystal cannot grow due to inadequate time, evidenced by SEM and PSD results. The irregular particles with variable sizes in MnO₂-M benefit the formation of inter-particle pores, thereby increasing the surface area of the catalyst. The higher the surface area, the more active sites are available for NH₃-SCR reaction. Considering the ball milling method is an environmentally friendly and cost-effective technique, the facile ball milling-assisted synthesized strategy provides the potential for the scaled-up production of MnO_x catalysts in industrial applications.

2.3. Catalytic Performance

Figure 7a shows the NO_x conversion as a function of temperature in the NH₃-SCR reaction on MnO₂-M and MnO₂ catalysts. MnO₂-M exhibits higher activity with NO_x conversion of 68% at 50 °C and 100% at 100 °C. In contrast, only 49.8% NO_x conversion is obtained at 50 °C on MnO_2, and 100% NO_x conversion is not achieved even at 200 °C. In addition, the N₂ selectivity of the catalysts was evaluated, and the results are shown in Figure 7b. Both the catalysts exhibit an acceptable and similar N₂ selectivity below 150 °C.

Based on the results of BET, H₂-TPR, XPS, and NH₃-TPD, the much higher NO_x conversion on MnO₂-M can be attributed to its higher specific surface area, more adsorbed surface oxygen, higher reducibility, and more acid sites. It should be noted that the preparation process of the two catalysts is similar, except that the latter uses ball milling. Therefore, it can be deduced that the ball-milling process may be the key to the optimization of catalyst structure and the improvement in the SCR performance. To verify this, we applied the ball milling method to the MnO₂ catalyst and obtained the M-MnO₂ catalyst. As shown in Figure 8, the M-MnO₂ catalyst presents an NO_x conversion of 55% at 50 °C, 78% at 75 °C, and 97% at 100 °C, suggesting a much better NH₃-SCR performance than the pristine MnO₂.

The resistance to H₂O and SO₂ under reaction conditions is important for low-temperature SCR catalysts. The H₂O and SO₂ tolerance tests of the MnO₂-M and MnO₂ were conducted at 150 °C. Figure 9a shows the H₂O tolerance results of the catalysts. When 10 vol.% H₂O is added to the reaction gas, the NO_x conversion on the MnO₂-M catalyst changes from nearly 100% at 4 h to 85.5% at 14 h, and that of MnO₂ decreases from nearly 100% at 4 h to 76.9% at 14 h, suggesting that the MnO₂-M catalyst shows better resistance to H₂O than MnO₂. When H₂O is cut off, its catalytic activities recover quickly. In particular, the MnO₂-M catalyst can be fully recovered to the original level. Therefore, the activity decreases in the two catalysts caused by H₂O originated from competitive adsorption of H₂O and NH₃/NO_x for active sites, but not the deactivation of active sites. As shown in Figure 9b, further introduction of SO₂ to the reaction gas significantly decreased the initial NO_x conversion over the two catalysts compared to the case of only H₂O. In addition, the NO_x conversions of the two catalysts decrease gradually along with reaction time. However, the NO_x conversion on the MnO₂-M catalyst is still much higher than that on MnO₂. Moreover, when the H₂O and SO₂ are removed from the reaction gas, the NO_x conversions of the MnO₂-M and MnO₂ catalysts cannot recover, which may be due to the formation of sulfating Mn species. Even so, the MnO₂-M catalyst could achieve more than 88.4% NO_x conversion after the process, implying that it has good resistance to H₂O and SO₂.

2.4. In Situ DRIFTS Studies

To obtain insights about the reaction mechanism, in situ DRIFTS studies were conducted. In situ DRIFTS spectra of MnO₂-M and MnO₂ catalysts exposed to NO + O₂ at 50 °C as a function of time are shown in Figure 10. Four main bands over the two catalysts appear after introducing NO + O₂ into the IR cell. For the MnO₂-M catalyst, the band located at 1026 cm⁻¹ is ascribed to monodentate nitrite, while those at 1539 and 1271 cm⁻¹ are assigned to bidentate nitrate. The band at 1630 cm⁻¹ is attributed to the NO₂ ad-species (nitro group or adsorbed NO₂ molecule) [40,41]. The intensity of these peaks becomes stronger with the increase in exposure time. On the MnO₂ catalyst, the bands assigned to monodentate nitrite and the NO₂ ad-species at 1031 and 1620 cm⁻¹ are also detected. Additionally, the bands that are indexed to monodentate nitrates (1313 cm⁻¹) and ionic nitrate (1462 cm⁻¹) appear on the MnO₂ catalyst. However, those bands corresponding to bidentate nitrate disappear. The above results show that more stable nitrate exists, e.g., the bidentate nitrate on the MnO₂-M catalyst, which further proves that the formation of more OVs on the MnO₂-M catalyst promotes NH₃-SCR performance.

To investigate the nature of the adsorbed NH₃ species and potential intermediates, in situ DRIFTS spectra of the two catalysts exposed in the NH₃ atmosphere at 50 °C were collected, as shown in Figure 10c,d. The bands in 3450–3000 cm⁻¹ can be assigned to N-H stretching vibration of ordinated NH₃ adsorbed on Lewis sites [42,43]. The corresponding bending vibrations are observed at 1170/1178 cm⁻¹ [42,43,44]. With the increasing adsorption time, the bands of the two catalysts increase until the adsorption is saturated. For the MnO₂-M catalyst, the band at 1597 cm⁻¹ is assigned to NH₃ coordinately linked to the Lewis acid sites, while the band at 1388 cm⁻¹ can be ascribed to monodentate nitrites. In Figure 10d, these bands show a little shift over the MnO₂ catalyst, e.g., from 1597 cm⁻¹ and 1388 cm⁻¹ to 1608 cm⁻¹ and 1398 cm⁻¹, respectively. Compared with the MnO₂ catalyst, a new band at 1641 cm⁻¹ corresponding to bridging nitrates appears. This indicates that partial NH₃ at Lewis acid sites can be oxidized to bidentate nitrate and monodentate nitrites. The MnO₂-M catalyst possesses stronger acid intensity to absorb the ammonia species, further explaining the higher SCR activity of the MnO₂-M catalyst.

To further investigate the SCR reaction mechanism over the two catalysts, in situ DRIFTS spectra in a flow of NO + O₂ + NH₃ were conducted at different temperatures. As shown in Figure 11, raising the temperature decreased the intensity of all the bands. For the MnO₂-M catalyst at 50 ℃, the band at 1196 cm⁻¹ is attributed to N-H bending vibrations of NH₃ adsorbed on Lewis sites. The bands at 1319, 1602, and 3200–3400 cm⁻¹ are attributed to ordinated NH₃ adsorbed on Lewis sites. And the band at 1319 cm⁻¹ shifts to lower wavenumbers with the increasing temperature, eventually reaching 1278 cm⁻¹ at 300 ℃. The bands at 1394 cm⁻¹ were attributed to monodentate nitrate [40,45]. In situ DRIFTS spectra of the MnO₂ catalyst, as shown in Figure 11c, are similar to those obtained over the MnO₂-M catalyst. However, the intensities of the bands on MnO₂ are much lower than that on the MnO₂-M catalyst, possibly due to the less active sites in the MnO₂ catalyst.

The above results suggest that the adsorption of NH₃ is dominated under the SCR reaction conditions for the two catalysts. The possible reaction mechanism over MnO₂-M at low temperature can be described in Scheme 2. The NO molecules in the gas phase are adsorbed on the catalyst surface and react with O₂ with OVs’ assistance to form NO₂. Due to Lewis acidity, the NH₃ molecules are adsorbed onto the MnO₂-M surface and form NH₃(ad). Then, the formed NH₃(ad) can further react with NO_x to generate N₂ and H₂O. Therefore, the SCR reaction on MnO₂-M proceeds via the typical Eley–Rideal (ER) mechanism.

3. Experiment

3.1. Chemical Reagents

Manganese nitrate solution (Mn(NO₃)₂, 50% in H₂O) was purchased from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China and potassium permanganate (KMnO₄) was purchased from Beijing Chemical Works Co., Ltd., Beijing, China. All the chemicals were analytical grade and used as received without further treatment. Deionized water was used in all experiments.

3.2. Preparation of Catalysts

The Mn-based catalyst was prepared by the ball milling-assisted redox method. Scheme 3 depicts the synthesis procedure of the catalyst. In brief, 3.64 g of KMnO₄ and 6.17 g of Mn(NO₃)₂ were added to an agate jar (100 mL) along with some zirconia balls (5 mm diameter). Here, the ball to powder mass ratio was 10:1. The agate jar was placed in a high-speed ball milling apparatus (Planetary Ball Mill QM-3SP04, Nanjing Chishun Sclence And Technology Development Co., Ltd, Nanjing, China), and the reactants were ball-milling for 1 h at a frequency of 580 r/min. The resulting solid was washed several times with deionized water, dried overnight at 105 °C, and calcined at 400 °C for 3 h to obtain the Mn-based catalyst. The prepared catalyst was denoted as MnO₂-M.

For comparison, the MnO₂ catalyst was prepared without ball milling. First, 3.64 g of KMnO₄ was dissolved in 100 mL of deionized water to obtain solution A. Next, 6.17 g of Mn(NO₃)₂ was dissolved in 100 mL of deionized water to obtain solution B. Subsequently, solution B was added dropwise into solution A under stirring at room temperature, and the obtained mixture was stirred for 4 h to form a slurry. Then, the slurry was filtered and washed several times with deionized water. The filter cake was dried overnight at 105 °C and calcined at 400 °C for 3 h. The obtained catalyst was named MnO₂. For comparison with MnO₂ and MnO₂-M, part of the synthesized MnO₂ catalyst was further ball-milled for 1 h, obtaining the reference catalyst named M-MnO₂.

3.3. Characterization

X-ray diffraction (XRD) patterns were recorded using a Cu Kα radiation (λ = 1.5148 Å) range from 10.0 to 90.0° (X’Pert PRO MPD, PANalytical, Almelo, the Netherlands). The scanning electron micrograph (SEM) images were obtained at 5.0 kV (Regulus 8100, JEOL, Tokyo, Japan). The particle size distribution (PSD) was measured using a laser particle size analyzer (BT-9300Z, Bettersize Instruments Ltd., Dandong, China). The N₂ adsorption-desorption isotherms were measured on a surface area and pore size analyzer (NOVA 3200e, Quantachrome, FL, USA). The specific surface areas of the catalysts were determined by using the Brunauer–Emmett–Teller (BET) method. The catalysts were degassed at 200 °C for 4 h and analyzed at −196 °C. The pore volume and pore size distribution were measured from the desorption branch using the Barrett–Joyner–Halenda (BJH) method.

H₂ temperature-programmed reduction (H₂-TPR) experiments were carried out on an automated chemisorption analyzer (ChemBET pulsar TPR/TPD, Quantachrome, FL, USA). 0.05 g of catalyst was loaded into a quartz U-tube, then the catalyst was degassed at 150 °C for 0.5 h under helium. When the temperature was cooled to 50 °C, the gas was changed to 10% H₂/Ar. Finally, the catalyst was heated from 50 to 1000 °C at 10 °C min⁻¹ under a H₂/Ar flow (30 mL min⁻¹). X-ray photoelectron spectra (XPS) were collected to analyze the surface chemical compositions of the catalysts (Model VG ESCALAB 250 spectrometer, Thermo Electron, London, UK). Temperature-programmed desorption of ammonia (NH₃-TPD) experiments were carried out on a FT-IR instrument (Nicolet 380, Thermo Fisher Scientific, Waltham, MA, USA). Before the experiment, 100 mg of catalyst was treated in 5% O₂/N₂ at 300 °C for 30 min. After cooling to 50 °C in the same gas mixture, the catalyst was exposed to 500 ppm NH₃/N₂ for 30 min and then flushed in N₂ gas flow for 30 min. The NH₃ desorption was recorded in N₂ while heating to 500 °C at a rate of 10 °C/min. In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of the catalyst was carried out with an FTIR (Nicolet iS50, Thermo Fisher Scientific, Waltham, MA, USA) equipped with an in situ diffuse reflection chamber and MCT detector. The catalysts were pretreated at 300 °C in a 5% O₂/N₂ atmosphere for 30 min and then gradually cooled to 50 °C. Then, the catalysts were purged with the reaction atmosphere, and the spectra were collected as a function of time.

3.4. Catalytic Measurement

The NH₃-SCR activity reaction was carried out in a fixed-bed reactor operating at atmospheric pressure. Before each test, 1.5 mL of catalyst was pretreated with N₂ gas flow in the reactor at 300 °C for 1 h. The inlet gas comprised NO (500 ppm), NH₃ (500 ppm), O₂ (5%), and N₂ (the balance). The GHSV was 40,000 h⁻¹. A FT-IR spectrometer monitored the gaseous species (NO, NH₃, NO₂, and N₂O) in the exhaust (Nicolet 380, Thermo Fisher Scientific, Waltham, MA, USA).

The NH₃-SCR stability reaction was carried out in a fixed-bed reactor operating at atmospheric pressure. Before each test, 1.5 mL of catalyst was pretreated with N₂ gas flow in the reactor at 300 °C for 1 h. The inlet gas comprised NO (500 ppm), NH₃ (500 ppm), O₂ (5%), SO₂ (50 ppm, in use), H₂O (10 vol.%, in use), and N₂ (the balance). The GHSV was 40,000 h⁻¹. The NO_x concentrations in the exhaust were analyzed by a multiple gas analyzer (Testo 350, Testo SE & Co. KGaA, Baden-Württemberg, Germany). The NO_x conversion and N₂ selectivity were calculated as follows:

$${\mathrm{NO}}_x\mathrm{conversion}(\%)=\frac{[{\mathrm{NO}}_x{]}_{\mathrm{in}}-[{\mathrm{NO}}_x{]}_{\mathrm{out}}-2\times [{\mathrm{N}}_2\mathrm{O}{]}_{\mathrm{out}}}{[{\mathrm{NO}}_x{]}_{\mathrm{in}}}\times 100$$

(1)

$${\mathrm{N}}_2\mathrm{selectivity}(\%)=(1-\frac{2\times [{\mathrm{N}}_2\mathrm{O}{]}_{\mathrm{out}}}{[{\mathrm{NH}}_3{]}_{\mathrm{in}}-[{\mathrm{NH}}_3{]}_{\mathrm{out}}+[{\mathrm{NO}}_x{]}_{\mathrm{in}}-[{\mathrm{NO}}_x{]}_{\mathrm{out}}})\times 100$$

(2)

The “in” and “out” subscripts represent the inlet and outlet gas concentrations in the steady state, respectively.

4. Conclusions

The ball milling-assisted redox method was successfully applied to prepare the MnO₂-M catalyst with a high specific surface area. Compared with the MnO₂ catalyst obtained by the redox method, the MnO₂-M catalyst exhibits a higher catalytic activity and better resistance against H₂O and SO₂ in NH₃-SCR reaction at low temperatures. Based on characterization analysis, more OVs can be manufactured, which is beneficial for NO oxidation and promotes the NH₃-SCR reaction. Moreover, the MnO₂-M catalyst has more acid sites, higher Mn⁴⁺ content, surface adsorbed oxygen, and active sites. All these features can contribute to the high NH₃-SCR performance of the MnO₂-M catalyst. In situ DRIFTS studies probed the reaction intermediates and predicted that the adsorption of NH₃ is dominated under the SCR reaction via the typical Eley–Rideal mechanism.