Low-Temperature NH₃-SCR Performance and In Situ DRIFTS Study on Zeolite X-Supported Different Crystal Phases of MnO₂ Catalysts

Abstract

In this study, a series of zeolite-X-supported different crystal phases of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂, and σ-MnO₂) catalysts were prepared via a solid-state diffusion method and high-heat treatment method to explore their low-temperature NH₃-SCR performance. All of the catalysts featured typical octahedral zeolite X structures and manganese dioxides species of various crystal types dispersed across the support surface. Throughout the entire temperature range of the reaction, γ-MnO₂/X catalyst had the highest NO conversion. Additionally, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts had nearly 100% of N₂ selectivity, whereas the α-MnO₂/X catalyst had the lowest N₂ selectivity (about 90%) below 125 °C. Moreover, the γ-MnO₂/X catalyst demonstrated superior acidity capacity and reduction ability compared with the other three catalysts. All the catalysts contained the essential intermediates NH₂NO and NH₄NO₃ species, which are essential to the SCR reaction. More acid sites and nitrate species existed on the γ-MnO₂/X catalyst than on the other catalysts, thereby boosting the SCR reaction.

1. Introduction

Nitrogen oxides (NO_x), which are emitted by the natural gas and the combustion of fossil fuels, are one of the primary air pollutants responsible for grave environmental issues, such as acid rain, ozone depletion, and photochemical smog [1,2,3]. As the most effective method for reducing NO_x, selective catalytic reduction via NH₃ technology (NH₃-SCR) has been applied extensively [4]. Due to the exceptional catalytic activity, the commercial V₂O₅-WO₃/TiO₂ (VWT) catalyst is widely utilized [5]. Regrettably, the VWT catalyst has some flaws, such as a high reacted temperature of 300–400 °C and the environmental hazard of vanadium, which prevents it from being used optimally with low-temperature flue gas [6]. Consequently, the search for more active, efficient, and eco-friendly low-temperature SCR catalysts is required.

Generally, rare earth and transition metal oxides are used as the active component for catalysts; among them, Mn-based catalysts have exhibited ideal low-temperature SCR activity due to various chemical valence states of Mn element (Mn²⁺, Mn³⁺, Mn⁴⁺) [7,8]. Our previous work investigated the SCR performance and N₂O pathways in the SCR process over manganese oxides with different valence states (MnO₂, Mn₂O₃, Mn₃O₄), and discovered that MnO₂ demonstrated the highest SCR activity at low temperature [9]. Furthermore, the MnO₂ four common crystal phases and structures are α-MnO₂, β-MnO₂, γ-MnO_2, and σ-MnO₂ [10]. In particular, pure Mn-based catalysts generated an amount of byproduct of N₂O within the SCR reaction and reduced the N₂ selectivity of the catalysts, which affected the catalytic activity. Then, the N₂O formation for different crystal phases of manganese oxide (α-MnO₂, β-MnO₂, γ-MnO₂ and σ-MnO₂) catalysts during a low-temperature NH₃-SCR reaction was investigated in our study using DRIFT measurement, which generated a higher content of N₂O even at a low temperature [11]. Guo et al. [12] investigated the atomic-level mechanism of N₂O formation on the α-MnO₂ catalyst via DFT calculations and thermodynamics/kinetic analysis and pointed out that the (310) plane of α-MnO₂ owned the highest N₂ selectivity by avoiding NH₃ dissociation. Some studies have attempted to enhance the catalyst activity and N₂ selectivity at the same times. Gan et al. [13] prepared the α-MnO₂ @CeO₂ catalyst with the core shell-like structure and found that the CeO₂ hall could improve the N₂ selectivity of the catalysts. However, there is few research compared with the N₂ selectivity enhancement of α-MnO₂, β-MnO₂, γ-MnO_2, and σ-MnO₂ catalysts, and the previous prepared methods for the catalysts are complex. Thereby, it is vital to find a new way to improve the catalytic activity of the different crystal phases of MnO₂.

It is common knowledge that zeolite-supported catalysts have been used extensively in NH₃-SCR condition owing to their porous structure and high specific surface area [14,15,16,17,18]. Our previous study showed that zeolite X supported and doped with Mn or Ce oxides catalysts had excellent low-temperature SCR activity [19]. Resultantly, in this study, zeolite-X-supported different crystal phases of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO_2, and σ-MnO₂) catalysts were synthesized and their SCR performance was measured, and then the SCR reaction process of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts was investigated via in situ DRIFTS study and their possible mechanism was pointed out.

2. Results

2.1. Microtopography of Catalysts

The microtopography of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts was measured by SEM, and the surface element distribution was observed by EDS mapping (Figure 1). It was found that all of the catalysts possessed the regular octahedral zeolite X structures [8], and manganese dioxide species with different crystal types distributed on the support surface [11], indicating that the catalysts were synthesized successfully. According to the EDS mapping results, some Mn elements aggregated together in α-MnO₂/X and σ-MnO₂/X, whereas Mn element on β-MnO₂/X and γ-MnO₂/X distributed uniformly, which may impact the different SCR activity of the catalysts. From the XRD pattern in Figure 1d, all the catalysts showed the characterized peaks of zeolite X, and no peaks belonged to manganese species.

2.2. Catalytic Performance

NH₃-SCR activity of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts were measured in the temperature range of 50–250 °C, and the results are depicted in Figure 2. NO conversion of all the catalysts increased with increasing temperature, as shown in Figure 2a. The γ-MnO₂/X catalyst had the highest NO conversion across the entire reaction temperature range, and the NO conversion was nearly 35% at 50 °C and then increased to 100% at 200 °C. α-MnO₂/X and σ-MnO₂/X catalysts exhibited nearly the same NO conversion from 100–250 °C, which was both 100% at 250 °C. While β-MnO₂/X had the lowest NO conversion rate from 75 °C to 225 °C, it also reached 100% at 250 °C. According to Figure 2b, N₂ selectivity of all the catalysts declined with increasing temperature. The β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts displayed nearly 100% N₂ selectivity below 125 °C then decreased gradually under the higher temperature range, and the σ-MnO₂/X catalyst showed the poorest N₂ selectivity at higher temperature (over 175 °C). Meanwhile, the α-MnO₂/X catalyst had the lowest N₂ selectivity (about 90%) at 50–125 °C. From Figure 2c, it could be found that N₂O concentration of all the catalysts increased with increasing temperature. In addition, the α-MnO₂/X catalyst had the highest N₂O concentration during 50–150 °C, and the σ-MnO₂/X catalyst had the highest N₂O concentration above 175 °C, while β-MnO₂/X had the lowest N₂O concentration over the whole reaction temperature range. Combined with pure manganese dioxides with different crystal types [11], the support of zeolite X could enhance the N₂ selectivity of the catalysts and reduce the generation of N₂O byproduct.

2.3. Surface Acidity and Reducibility

Not only did the structure of the catalysts affect its ability to act as a catalyst, but its chemical properties were also important for the SCR reaction. The surface acidity of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts was studied using NH₃-TPD, and the results are displayed in Figure 3. There were several peaks at approximately 100–600 °C; among them, the peak at approximately 100 °C belonged to the weak acid sites for NH₃ physical adsorption, the peaks at 150–400 °C related to the medium acid sites for NH₃ chemical adsorption, and the peaks over 400 °C were assigned to NH₃ adsorption on the strong acid sites [8,20,21]. The α-MnO₂/X catalyst possessed the highest peak of weak acid sites, and the γ-MnO₂/X catalyst had the highest peak of medium acid sites. It was observed that α-MnO₂/X, β-MnO₂/X, and γ-MnO₂/X catalysts had stronger surface acidity than that of the σ-MnO₂/X catalyst, which was correlated with the SCR activity results, indicating that the surface acidity played a significant role in SCR reaction. Furthermore, the peaks of acid sites for the γ-MnO₂/X catalyst shifted to a lower temperature. It could be found that the γ-MnO₂/X catalyst showed better acidity capacity than that of other catalysts, which might be one of reasons for its enhanced catalytic activity at lower temperatures.

The redox capacity of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts was detected by H₂-TPR, and the results are depicted in Figure 4. There were several peaks existing at 200–500 °C for all the catalysts. For the α-MnO₂/X catalyst, there were two peaks at 376 °C and 420 °C, corresponding to the reduction of Mn₂O₃ to Mn₃O₄ and Mn₃O₄ to MnO, respectively [9,22]. As for the β-MnO₂/X catalyst, two peaks appeared at 369 °C and 430 °C, also related to Mn₂O₃ to Mn₃O₄ and Mn₃O₄ to MnO, respectively [9,22]. The two abroad peaks on the γ-MnO₂/X catalyst could be fitted into three peaks at 281 °C, 327 °C, and 426 °C, assigned to MnO₂ to Mn₂O₃, Mn₂O₃ to Mn₃O₄, and Mn₃O₄ to MnO, respectively [9,19,22]. It could be found that the reduction peaks of the γ-MnO₂/X catalyst had lower temperature than that of the other three catalysts, illustrating that the γ-MnO₂/X catalyst was more reducible. There were also three fitting peaks at 330 °C, 371 °C, and 431 °C on the σ-MnO₂/X catalyst, attributed to MnO₂ to Mn₂O₃, Mn₂O₃ to Mn₃O₄, and Mn₃O₄ to MnO, respectively.

2.4. In Situ DRIFTS Studies on the Catalysts

In order to further investigate the reaction of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts, the in situ DRIFTS studies were measured. Combined with the SCR activity of the catalysts, the reaction temperature was 200 °C, and the in situ DRIFTS spectra of only NH₃ adsorption, reactions between NO+O₂ and pre-adsorbed NH₃ species, NO+O₂ adsorption, reactions between NH₃ and pre-adsorbed NO+O₂ species, and reactions between NH₃+NO+O₂ were tested.

2.4.1. Reactions between NO+O₂ and Pre-Adsorbed NH₃ Species on the Catalysts

In order to research the nature of adsorbed NH₃ species and some potential intermediates, in situ DRIFTS spectra of only NH₃ adsorption at 200 °C on α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts were collected. From Figure 5(a1), there were several bands at 1000–2000 cm⁻¹ on the α-MnO₂/X catalyst, which were assigned to Brønsted acid sites (1640 cm⁻¹ and 1480 cm⁻¹), Lewis acid sites (1610 cm⁻¹ and 1193 cm⁻¹), and -NH₂ species (1520 cm⁻¹) [23,24,25]. It could be found that the bands’ intensity of 1640 cm⁻¹ and 1193 cm⁻¹ increased at first, but declined from 40 min, while the bands’ intensity of Brønsted acid sites (1480 cm⁻¹), Lewis acid sites (1610 cm⁻¹), and -NH₂ species (1520 cm⁻¹) increased with the increasing time of NH₃ introducing. For the β-MnO₂/X catalyst (in Figure 5(b1)), the bands of -NH₂ species, Brønsted acid sites, and Lewis acid sites could be observed at 1520 cm⁻¹, 1480 cm⁻¹, and 1213 cm⁻¹, respectively [26,27,28], and the bands intensity of -NH₂ species and Brønsted acid sites became strengthened with increasing time. As for the γ-MnO₂/X catalyst (in Figure 5(c1)), it also could be found that the bands of Brønsted acid sites (1640 cm⁻¹ and 1480 cm⁻¹), Lewis acid sites (1610 cm⁻¹ and 1193 cm⁻¹), and -NH₂ species (1520 cm⁻¹) existed. The bands of Brønsted acid sites (1640 cm⁻¹ and 1480 cm⁻¹), Lewis acid sites (1610 cm⁻¹ and 1193 cm⁻¹), and -NH₂ species (1520 cm⁻¹) were also detected on the σ-MnO₂/X catalyst (in Figure 5(d1)) [23,24,25,26,27,28]. It could be found that the bands’ intensity of -NH₂ species on β-MnO₂/X was stronger than that of other catalysts, which might speed up the combination -NH₂ species with NO species and reduce the byproduct of N₂O generation. Moreover, the types of acid bands on the γ-MnO₂/X catalyst were more than those of other catalysts, which facilitated the SCR reaction of the catalyst.

After NO+O₂ introducing, in situ DRIFTS spectra of reactions between NO+O₂ and pre-adsorbed NH₃ species on catalysts were obtained. In Figure 5(a2), the bands’ intensity of the acid sites was weak and hardly found when NO+O₂ was introduced on α-MnO₂/X catalysts, and then some bands at 1178 cm⁻¹, 1394 cm⁻¹, 1400 cm⁻¹, 1430 cm⁻¹, 1460 cm⁻¹, and 1660 cm⁻¹ belonging to bridging nitrate species (M-O₂NO), nitro species (M-NO₂), free nitrate species, monodentate nitrate species (M-ONO₂), monodentate nitrite species (M-ONO), and bidentate nitrate species (M-O₂NO) were observed, respectively [29,30,31]. Meanwhile, the bands of NH₂NO and NH₄NO₃ species were found at 1480 cm⁻¹ and 1380 cm⁻¹, respectively, which played an essential role in SCR reaction [8,14]. The bands’ intensity of nitrate species increased with the increasing time of NO+O₂ purging. For the β-MnO₂/X catalyst (Figure 5(b2)), the bands of monodentate nitrite species (M-ONO), monodentate nitrate species (M-ONO₂), nitro species (M-NO₂), trans-N₂O₂²⁻ species, bidentate nitrate species (M-O₂NO), and NH₂NO species were observed; moreover, the bands’ intensity of nitrate species increased. To the γ-MnO₂/X catalyst (in Figure 5(c2)), the bands of the monodentate nitrite species (M-ONO), monodentate nitrate species (M-ONO₂), nitro species (M-NO₂), trans-N₂O₂²⁻ species, bidentate nitrate species (M-O₂NO), and NH₂NO species also could be found, and the bands’ intensity of nitrate were the most strengthened among the four catalysts [9,29,30,31]. There were bands of monodentate nitrite species (M-ONO), bidentate nitrate species (M-O₂NO), nitro species (M-NO₂), trans-N₂O₂²⁻ species on the σ-MnO₂/X catalyst (Figure 5(d2)) when NO+O₂ was introduced, but fewer types of nitrate species generated.

To better determine the intermediates of the SCR reaction over the four catalysts, NH₃, NO, and O₂ were introduced into the reaction container under 200 °C to simulate the SCR reaction process, and in situ DRIFTS spectra of NH₃, NO, and O₂ reaction were measured at the same time (Figure 5(a3,b3,c3,d3)). At 1200–1400 cm⁻¹, all catalysts exhibited bands belonging to bridging nitrate species (M-O₂NO), nitro species (M-NO₂), free nitrate species, monodentate nitrate species (M-ONO₂), monodentate nitrite species (M-ONO), and bidentate nitrate species (M-O₂NO). All the intensities of nitrate species’ bands strengthened when NH₃, NO, and O₂ were introduced, Additionally, it could be discovered that some negative bands appeared when NH₃, NO, and O₂ reacted, such as bridging nitrate species and NH₂NO species, indicating that the SCR reaction occurred on the catalyst surface. At the same time, the N₂ and H₂O were produced, which followed both the Eley–Rideal (E-R) and Langmuir–Hinshelwood (L-H) mechanism [32,33].

2.4.2. Reactions between NH₃ and Pre-Adsorbed NO+O₂ Species on Catalysts

In situ DRIFTS spectra of NO+O₂ adsorption at 200 °C on α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts were detected. Some bands were observed at 1178 cm⁻¹, 1394 cm⁻¹, 1400 cm⁻¹, 1430 cm⁻¹, 1460 cm⁻¹, and 1660 cm⁻¹ over four catalysts (Figure 6(a1,b1,c1,d1)), which were correlated to bridging nitrate species (M-O₂NO), nitro species (M-NO₂), free nitrate species, monodentate nitrate species (M-ONO₂), monodentate nitrite species (M-ONO), trans-N₂O₂²⁻ species, and bidentate nitrate species (M-O₂NO), respectively [9,29,30,31]. The intensity of bands belonged to monodentate nitrite species and bridging nitrate species at 1200–1300 cm⁻¹ over α-MnO₂/X, β-MnO₂/X, and σ-MnO₂/X catalysts strengthened in the first 20 min, and then decreased gradually. Meanwhile, bands of nitro species, trans-N₂O₂²⁻ species, free nitrate specie, and bidentate nitrate species at 1350–1450 cm⁻¹ appeared at approximately 15 min and the intensity of these bands increased with NO+O₂ adsorption. However, for the γ-MnO₂/X catalyst (Figure 6(c1)), the intensity of the bands for nitrate species increased with the introduction of NO+O₂. Consequently, the nitrate species on the γ-MnO₂/X catalyst was more stable than those of the other three catalysts.

After NO+O₂ adsorption for 60 min and N₂ purging for 10 min, NH₃ was introduced and reacted with the pre-adsorbed NO+O₂ species at 200 °C on α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts, and the in situ DRIFTS spectra were collected. There was a broad band at 1350–1450 cm⁻¹ on α-MnO₂/X, β-MnO₂/X, and σ-MnO₂/X catalysts (Figure 6(a2,b2,d2)), which was assigned to nitro species, trans-N₂O₂²⁻ species, free nitrate specie, and bidentate nitrate species [30,31,34]. Meanwhile, the broad band on the γ-MnO₂/X catalyst was at 1200–1450 cm⁻¹, attributed to bridging nitrate species (M-O₂NO), nitro species (M-NO₂), free nitrate species, monodentate nitrate species (M-ONO₂), monodentate nitrite species (M-ONO), trans-N₂O₂²⁻ species, and bidentate nitrate species (M-O₂NO), which indicated that the types of nitrate species on the γ-MnO₂/X catalyst were more than that of the other three catalysts. Moreover, two negative bands could be observed at 1660 and 1483 cm⁻¹ on the four catalysts, owing to the consumption of bridging nitrate species and NH₂NO species. When NH₃, NO, and O₂ were introduced over α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts (Figure 6(a1,b1,c1,d1)), the intensity and position of bands on α-MnO₂/X, β-MnO₂/X, and σ-MnO₂/X catalysts were nearly unchanged. While the intensity of bands varied on the γ-MnO₂/X catalyst, the intensity of some bands at approximately 1300 cm⁻¹ become weak in the first 20 min and then increased gradually over 20 min. Two negative bands of the bridging nitrate species and NH₂NO species were detected, suggesting the effective reaction pathway of the SCR process [35,36].

3. Discussion

All the catalysts had regular octahedral zeolite X structures; α-MnO₂/X and σ-MnO₂/X catalysts exhibited Mn elements aggregation, whereas β-MnO₂/X and γ-MnO₂/X catalysts had uniform Mn element distribution. This suggested that the aggregation of α-MnO₂ or σ-MnO₂ species on the zeolite X support could reduce the active sites for the SCR reaction, resulting in the low SCR activity. For β-MnO₂/X and γ-MnO₂/X catalysts, the distribution of β-MnO₂ and γ-MnO₂ species on the support could provide an abundance of active sites for reacted gas adsorption and the SCR reaction. From the NH₃-TPD results, the α-MnO₂/X catalyst possessed the highest peak of weak acid sites, and the γ-MnO₂/X catalyst had the highest peak of medium acid sites, while the σ-MnO₂/X catalyst had the lower surface acidity, which was associated with the lowest SCR activity of the σ-MnO₂/X catalyst among the four catalysts. All the four catalysts had four peaks from approximately 250 °C to 500 °C, which belonged to MnO₂ to Mn₂O₃, Mn₂O₃ to Mn₃O_4, and Mn₃O₄ to MnO, respectively. It was noted that the reduction peaks of the γ-MnO₂/X catalyst had lower temperature than that of the other three catalysts, illustrating that the γ-MnO₂/X catalyst had a stronger reduction capacity.

Through in situ DRIFTS spectra of NH₃ adsorption, the bands of Brønsted acid sites, Lewis acid sites, and -NH₂ species were observed on α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts, respectively, which followed reaction (1)–(3). When NO+O₂ was introduced, following reaction (4)–(6), especially, the bands intensity of -NH₂ species on β-MnO₂/X was stronger than that of other catalysts, which might have speeded up the combination -NH₂ species with NO species and reduced the byproduct of N₂O generation, shown as reaction (7). Moreover, the varieties of acid bands on the γ-MnO₂/X catalyst were more than those of other catalysts which facilitated the SCR reaction of the catalyst after NO+O₂ addition, and the bands’ intensity of nitrate species on the γ-MnO₂/X catalyst were the most strengthened among the four catalysts. However, the σ-MnO₂/X catalyst produced fewer types of nitrate species when NO+O₂ was added. When NH₃, NO, and O₂ were introduced, all the intensities of nitrate species bands strengthened for all four catalysts. In addition, some negative bands (bridging nitrate species and NH₂NO species) appeared when NH₃, NO, and O₂ reacted, following reaction (8)–(11), which indicated that the SCR reaction occurred on the catalyst surface; the N₂ and H₂O were produced at the same time, following both the Eley-Rideal (E-R) and Langmuir–Hinshelwood (L-H) mechanism. As for NO+O₂ adsorption, bridging nitrate species (M-O₂NO), nitro species (M-NO₂), free nitrate species, monodentate nitrate species (M-ONO₂), monodentate nitrite species (M-ONO), trans-N₂O₂²⁻ species, and bidentate nitrate species (M-O₂NO) were detected on the four catalysts (seen as reaction (4)–(6)); particularly, the nitrate species on the γ-MnO₂/X catalyst was more stable than those of the other three catalysts. When NH₃ was introduced and reacted with pre-adsorbed NO+O₂ species, two negative bands could be observed at 1660 and 1483 cm⁻¹ on the four catalysts, due to the consumption of bridging nitrate species and NH₂NO species, and the types of nitrate species on the γ-MnO₂/X catalyst were more than that of the other three catalysts. Two negative bands of the bridging nitrate species and NH₂NO species were also detected when NH₃, NO, and O₂ were introduced over the γ-MnO₂/X catalyst, suggesting that the effective reaction pathway of the SCR process happened, as shown in reaction (7)–(11).

$${\mathrm{NH}}_3\left(\mathrm{g}\right)\to {\mathrm{NH}}_3\left(\mathrm{a}\right)$$

(1)

$${\mathrm{NH}}_3{\left(\mathrm{g}\right)+\mathrm{H}}^{+}\to {\mathrm{NH}}_4^{+}\left(\mathrm{a}\right)$$

(2)

$${\mathrm{NH}}_3{\left(\mathrm{a}\right)+\mathrm{O}\left(\mathrm{a}\right)/\mathrm{O}}^{2-}\left(\mathrm{a}\right)\to { -\mathrm{NH}}_2\left(\mathrm{a}\right)+-\mathrm{OH}\left(\mathrm{a}\right)$$

(3)

$$\mathrm{NO}\left(\mathrm{g}\right)+\mathrm{O}\left(\mathrm{a}\right)\to { \mathrm{NO}}_2{\left(\mathrm{a}\right)/-\mathrm{NO}}_2\left(\mathrm{a}\right)$$

(4)

$${\mathrm{NO}}_2{\left(\mathrm{a}\right)+\mathrm{O}}^{2-}\left(\mathrm{a}\right) \to { -\mathrm{NO}}_3\left(\mathrm{a}\right)$$

(5)

$${-\mathrm{NO}}_3\left(\mathrm{a}\right)+\mathrm{NO} \left(\mathrm{g}\right)\to {\mathrm{NO}}_2{\left(\mathrm{a}\right)+\mathrm{NO}}_2^{-}\left(\mathrm{a}\right)$$

(6)

$${-\mathrm{NH}}_2\left(\mathrm{a}\right)+\mathrm{NO}\left(\mathrm{g}\right)\to {\mathrm{NH}}_2\mathrm{NO}\left(\mathrm{a}\right)\to {\mathrm{N}}_2{\left(\mathrm{g}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$$

(7)

$${\mathrm{NH}}_4^{+}{\left(\mathrm{a}\right)+\mathrm{NO}}_2^{-}\left(\mathrm{ads}\right) \to {\mathrm{NH}}_4{\mathrm{NO}}_2\left(\mathrm{a}\right) \to {\mathrm{N}}_2{\left(\mathrm{g}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$$

(8)

$${\mathrm{NH}}_4{\mathrm{NO}}_3\left(\mathrm{a}\right)+\mathrm{NO}\left(\mathrm{a}\right)\to {\mathrm{NH}}_4{\mathrm{NO}}_2{\left(\mathrm{a}\right)+\mathrm{NO}}_2\left(\mathrm{g}\right)$$

(9)

$${\mathrm{NH}}_4^{+}{\left(\mathrm{a}\right)+\mathrm{NO}}_3^{-}\left(\mathrm{a}\right)\to {\mathrm{NH}}_4{\mathrm{NO}}_3\left(\mathrm{a}\right)\to {\mathrm{N}}_2{\mathrm{O}\left(\mathrm{g}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$$

(10)

$${-\mathrm{NH}}_2\left(\mathrm{a}\right)+\mathrm{NO}\left(\mathrm{g}\right)\to {\mathrm{NH}}_2\mathrm{NO}\left(\mathrm{a}\right)\to {\mathrm{N}}_2{\left(\mathrm{g}\right)+\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$$

(11)

4. Materials and Methods

4.1. Catalyst Preparation

The different crystal phases of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂, and σ-MnO₂) were prepared by a redox hydrothermal method as in our previous studies [11,22]. A certain amount of KMnO₄ and MnSO₄·H₂O, MnSO₄·H₂O and (NH₄)₂S₂O₈, MnSO₄·H₂O and (NH₄)₂S₂O₈, and MnSO₄·H₂O and KMnO₄ were mixed in 72 mL H₂O, respectively, and then transferred to Teflon-lined stainless-steel autoclaves under 160 °C for 12 h, 160 °C for 12 h, 90 °C for 24 h, and 180 °C for 24 h, respectively, and finally washed by deionized water to obtain α-MnO₂, β-MnO₂, γ-MnO₂, and σ-MnO₂, respectively.

Different crystal phases of MnO₂/X catalysts were synthesized via a solid-state diffusion method and high-heat treatment. A certain amount of α-MnO₂, β-MnO₂, γ-MnO₂, and σ-MnO₂, and 1 g of zeolite X and 10 mL of anhydrous ethanol were mixed in an agate quartz mortar. After ultrasonic grinding until the anhydrous ethanol volatilized, the mixture dried at 80 °C for 12 h and then calcined at 300 °C for 2 h in a muffle furnace, finally obtained α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts, respectively, and the synthesis procedures and experimental steps of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts are shown in Figure 7.

4.2. Catalytic Activity Tests

NH₃-SCR catalytic performance of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts were measured by a catalytic testing system under the following conditions: 0.5 mL of sample (40–60 mesh), 500 ppm of NH₃, 500 ppm of NO, 11% O₂, GHSV of 36,000 h⁻¹, and N₂ as balance. The concentration of outlet gases was tested by the flue gas analyzer (Thermo Scientific, Antaris IGS). NO conversion and N₂ selectivity of all catalysts were calculated as Equations (12) and (13).

$$\mathrm{NO} \mathrm{conversion} (\%)=(1-\frac{{\left[\mathrm{NO}\right]}_{\mathrm{out}}}{{\left[\mathrm{NO}\right]}_{\mathrm{in}}})\times 100\%$$

(12)

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

(13)

4.3. Catalysts Characterization

Scanning electron microscope (SEM) tests were used to investigate the surface microstructure of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts via the Thermo Scientific Quattro SEM. The distribution of surface elements was measured by energy dispersive X-ray spectrometry (EDS) mapping.

NH₃-TPD measurements were used to investigate the surface acidity of all catalysts on a ChemBET Pulsar TPR/TPD. For each test, 0.15 g of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts were carried into the U-shaped quartz tube, pretreated under helium atmosphere for 5 min, then heated at 300 °C for 30 min, followed by cooling to 60 °C, and purged by helium atmosphere for 60 min. After that, the sample was exposed under NH₃/Ar atmosphere for 15 min, and then heated to 650 °C at a constant heating rate of 5 °C/min. At the same time, TPR analysis was performed and the TCD signal was recorded by the detector. H₂-TPR was conducted, being similar to that of NH₃-TPD. Firstly, the sample was exposed under H₂/Ar atmosphere for 15 min, and then heated to 800 °C at a constant heating rate of 5 °C/min.

In situ DRIFTS tests were measured on a Thermo Fisher Nicolet iS50 spectrometer. A certain weight of α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts (80–100 mesh) was carried into solid reaction in situ cell, respectively, and all catalysts were pretreated at 300 °C for 30 min under N₂ atmosphere (100 mL/min of flow rate). After cooling to 200 °C, the background spectrum was collected. Then, α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts were exposed under the gas atmosphere in turn: 500 ppm of NH₃ for 60 min, N₂ atmosphere for 10 min, 500 ppm of NO₂, 11 vol% O₂, and N₂ for 60 min, and 500 ppm of NH₃, 500 ppm of NO₂, 11 vol% O₂, and N₂ for 40 min, and finally flushed under N₂ gas for 60 min, respectively. At the same time, the spectrum of all the catalysts were recorded by the instrument. All the catalysts were also exposed under the gas atmosphere in turn: 500 ppm of NO₂ and 11 vol% O₂ for 60 min, N₂ atmosphere for 10 min, 500 ppm of NH₃ for 60 min, 500 ppm of NO₂ and 11 vol% O₂ for 40 min, finally flushed under N₂ gas for 60 min, respectively, and the related spectra were collected.

5. Conclusions

In this study, a series of zeolite-X-supported different crystal phases of MnO₂ (α-MnO₂, β-MnO₂, γ-MnO₂, and σ-MnO₂) catalysts were synthesized using a solid-state diffusion method and their low-temperature NH₃-SCR performance was evaluated. The results showed that the α-MnO₂/X, β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts had the regular octahedral zeolite X structures and different crystal types of MnO₂ on the supported surface. Furthermore, the γ-MnO₂/X catalyst had the highest NO conversion over the whole reaction temperature range. β-MnO₂/X, γ-MnO₂/X, and σ-MnO₂/X catalysts had nearly 100% of N₂ selectivity, while the α-MnO₂/X catalyst had the lowest N₂ selectivity (approximately 90%) below 125 °C. Furthermore, the γ-MnO₂/X catalyst had better acidity capacity and was more reducible than the other three catalysts via NH₃-TPD and H₂-TPR tests. From the in situ DRIFTS study, all the catalysts both followed E-R and L-H mechanism, and the acid sites and nitrate species on the γ-MnO₂/X catalyst were more than that of the other three catalysts, which enhanced the SCR reaction.