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Article

Ordered Mesoporous MnAlOx Oxides Dominated by Calcination Temperature for the Selective Catalytic Reduction of NOx with NH3 at Low Temperature

1
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(6), 637; https://doi.org/10.3390/catal12060637
Submission received: 11 May 2022 / Revised: 4 June 2022 / Accepted: 6 June 2022 / Published: 10 June 2022
(This article belongs to the Special Issue Frontiers in Catalytic Emission Control)

Abstract

:
Manganese alumina composited oxides (MnAlOx) catalysts with ordered mesoporous structure prepared by evaporation-induced self-assembly (EISA) method was designed for the selective catalytic reduction (SCR) of NOx with NH3 at low temperature. The effect of calcination temperature of MnAlOx catalysts was investigated systematically, and it was correlated with SCR activity. Results showed that with an increase in calcination temperature, the SCR activity of MnAlOx catalysts increased. When the calcination temperature was raised up to 800 °C, the NOx conversion was more than 90% in the operation temperature range of 150~240 °C. Through various characterization analysis, it was found that MnAlOx-800 °C catalysts possessed enhanced redox capacities as the higher content of Mn4+/(Mn3+ + Mn4+). Moreover, the improved redox properties could contribute to a higher NOx adsorption and activation ability, which lead to higher SCR performance of MnAlOx-800 °C catalysts. In situ DRIFTs revealed that the adsorbed NO2 and bidentate nitrate are the reactive intermediate species, and NH3 species bonded to Lewis acid sites taken part in SCR progress. The SCR progress predominantly followed E–R mechanism, while L–H mechanism also takes effect to a certain degree.

1. Introduction

Emission of nitrogen oxides (NOx) from the burning of fossil fuel, including the stationary and mobile source, are regarded as the major source of atmospheric contamination. They have brought about a series of serious healthcare and environmental issues, such as fine particle pollution, acid rain, photochemical smog, and ozone depletion [1,2,3]. Great efforts have been devoted to alleviating the emission of NOx to meet the increasingly strict emission legislation and policies enacted by government. Selective catalytic reduction of NOx with NH3 (NH3-SCR) is well-established as the most effective technology for the abatement of NOx [4].
Many kinds of SCR catalysts have been extensively reported, among which V2O5-WO3/TiO2 catalysts are the most worldwide used and effective commercial catalyst on account of the admirable catalytic activity in a working temperature range of 300~400 °C [5,6]. However, this kind of SCR catalyst has some non-negligible drawbacks in practical application, such as the toxicity of V2O5, narrow working temperature window, the generation of N2O at high temperature, and particularly the poor activity at low temperature [7,8,9]. Simultaneously, the SCR system has always placed the dust precipitation and desulfurization unit upstream, and the catalysts inevitably suffered the serious deactivation from dust accumulation on the catalyst surface and the chemical poisoning of SO2 [10]. Therefore, a tail-end installation and development of an appropriate SCR catalyst with high activity at low working temperature (<250 °C) is significant for industrial applications.
Some transition metal oxides (e.g., MnOx, FeOx, CoOx, CuOx, CeOx) catalysts have been extensively studied in the low temperature NH3-SCR reaction. Among these, manganese-based catalysts demonstrated high SCR catalytic activity since they possess various kinds of labile surface oxygen, which are significant to accomplish the catalytic cycle [11,12]. In order to enhance the acidic/redox properties of pure MnOx catalysts, supported MnOx multiple oxides catalysts are extensively investigated due to the improvement of the dispersion of MnOx, the strong metal–support interaction, and promotion of the electron transfer between active constituents. MnOx supported on Al2O3 [13,14], TiO2 [12,15,16], and carbon materials [17,18,19] for NH3-SCR reaction have been intensively studied. According to the previous investigations, MnOx with amorphous phase demonstrated higher SCR activity at low temperature in comparison with highly crystallized ones [20,21,22]. It is known from previous literatures that the high dispersion of MnOx contributes to generate amorphous phase [20].
However, most of the MnOx/Al2O3 catalysts afore mentioned are prepared by the conventional impregnation method, which cannot guarantee that MnOx dispersed on the surface of the Al2O3 support homogeneously and tend to agglomerate during the process of catalytic reaction. Wu et al. reported MnOx/TiO2 catalysts prepared by three methods for SCR at low temperature [23]. The SCR activity of MnOx/TiO2 increased in the following sequence: coprecipitation < impregnation < sol–gel. MnOx/TiO2 catalysts prepared by sol–gel method demonstrated higher dispersion of active components, lower crystallinity, and stronger interaction than other two catalysts. Compared to the Mn-Fe/TiO2 catalysts prepared by an impregnation method, the catalysts prepared by a deposition–precipitation exhibited higher catalytic activity at low temperature. The deposition–precipitation method facilitated to the amorphous active components, enhanced the surface area, surface labile oxygen, acidity, and acid strength while promoting the reduction of active components [24]. Hence, it is deduced that the preparation method has a terrific effect on the dispersion and structure of the active species, as well as the interaction between the active constituents. Therefore, it is worth exploring more effective preparation methods to enhance the NH3-SCR activity.
Ordered mesoporous alumina (OMA) supported metal oxides have emerged as a new group of functional materials with enhanced catalytic activity and selectivity. In comparison to the wet impregnation of OMA with metal oxide precursors, which often leads to structure blockage and/or damage, the evaporation-induced self-assembly (EISA) synthesis of OMA-supported metal oxides is suitable to achieve high-quality mesostructure that exhibit strong metal–support interactions and retain homogeneous distribution of active sites [25,26]. Hence, we assume that the EISA method might a promising way to obtain high dispersion of MnOx on the surface of Al2O3 and these catalysts would exhibit high SCR activity at low temperature. However, MnOx/Al2O3 catalysts prepared by EISA method for SCR reaction have rarely been investigated. Besides, owning to the SCR reaction being an exothermic process, the fluctuation of heat is bound to have an impact on the structure and reaction activity of the catalyst. Hence, it is very imperative to investigate the influence of the calcination temperature on the catalytic performance of the catalysts.
In this paper, MnOx/Al2O3 catalysts prepared by EISA method with different calcination temperature were applied to the NH3-SCR at low temperature. The SCR performance of the MnAlOx catalysts with different calcination temperature were compared and discussed. To illuminate the outstanding performance of MnAlOx-800 °C, the physico-chemical characters of the MnAlOx catalysts were investigated by multifarious characterization methods. Simultaneously, the reaction mechanism was investigated by using in situ DRIFTs. The main objective of our present work is not only to optimize the calcination temperature of the MnAlOx catalysts prepared by EISA method, but more significantly, it seeks to elucidate the structural–performance correlation of the catalyst.

2. Results and Discussion

2.1. NOx Conversion and N2 Selectivity

Figure 1 shows that the NH3-SCR performances over MnAlOx-T catalysts calcined at different temperatures. As shown in Figure 1, the NOx conversion of MnAlOx-T catalysts increased with raising the reaction temperature in the temperature range of 90~240 °C. It could be observed that the MnAlOx-450 °C and MnAlOx-550 °C catalysts exhibited a similar catalytic activity throughout the whole temperature range. In contrast, the MnAlOx-800 °C shows exceptional activity, and the NOx conversion was more than 90% in the range of 150~240 °C.
As shown in Figure 1B, the N2 selectivity of MnAlOx-T catalysts decreased with raising the reaction temperature due to the unselective oxidation of NH3 by O2 into N2O. It is noted that there is only a few ppm of N2O detected over MnAlOx-T (T = 450, 550, 700 °C) catalysts in the temperature range of 90~240 °C, which makes a major contribution to the excellent N2 selectivity with more than 96% even at 240 °C. The N2 selectivity of MnAlOx-800 °C decreases remarkably with the increase reaction temperature, and MnAlOx-800 °C demonstrates an excellent N2 selectivity with more than 82%, even at 240 °C.

2.2. Textural and Structural Properties

2.2.1. BET

First, we determined the calcination temperature of MnAlOx samples on the basis of TG-DTG analysis (Supporting Information in Figure S1). To investigate the texture and porosity of mesoporous MnAlOx-T catalysts, we characterized the catalysts using N2 adsorption/desorption isotherm analysis. The N2 adsorption/desorption isotherms and the corresponding pore size distribution (PSD) of mesoporous MnAlOx catalysts are shown in Figure 2. It is suggested that the adsorption/desorption isotherms of MnAlOx-T catalysts all yield type IV isotherms, suggesting the presence of mesopores according to the definition of IUPAC [27]. All the isotherms were type H1-shaped hysteresis loops in the relative pressure from 0.5 to 0.95, indicating that these structures highly ordered cylindrical mesoporous channels, which were in accordant with TEM results. In the Type H1 hysteresis loops, the two branches are almost vertical and nearly parallel over an appreciable range of gas uptake. Type H1 is often associated with porous materials known, from other evidence, to consist of agglomerates or compacts of approximately uniform spheres in properly regular array, and hence to have narrow distribution of pore size. The steepness of the capillary condensation step indicates uniformity of mesopores. The corresponding PSD curves in Figure 2B are in accordance with the adsorption isotherms in Figure 2A. As depicted in Figure 2B, the PSD curves of these mesoporous material were mainly focused on 8.0 nm, indicating that the PSD of all the samples was uniform relatively. Meanwhile, narrow pore size distribution is maintained though the calcination temperature increases.
The structural parameters derived from these isotherms are summarized in Table 1. As shown in Table 1, it was found that the surface area of mesoporous MnAlOx-450 °C was 247.7 m2/g, which was larger comparing to these of mesoporous MnAlOx-550 °C (221.3 m2/g), MnAlOx-700 °C (226.5 m2/g), and MnAlOx-800 °C (196.2 m2/g) catalysts. Although the surface area of MnAlOx catalysts prepared at a low calcination temperature is higher than that prepared at a high calcination temperature, the SCR activity over later is still higher than that over the former (Figure 1), which signifies that the SCR activity over these catalysts is not only related with the surface area, but also related with some other structural or redox properties, which will be discussed later in this paper.

2.2.2. Small-Angle XRD and TEM

Proof of the appearance of ordered mesostructure for the MnAlOx samples is afforded by the small angle XRD patterns (Figure 3) and TEM images (Figure 4). It is well-known that the small angle X-ray diffraction patterns provide information about the possible organization of mesopores [25,28]. As illustrated in Figure 3, very strong diffraction peaks of (110) plane around 0.8° and a weak and overlapped diffraction peaks of (110) and (200) plane around 1.7° for mesoporous MnAlOx were observed, which suggested the formation of ordered mesoporous structure. Therefore, the small-angle XRD patterns indicate the presence of uniform mesoporous.
The morphology and structure of MnAlOx catalysts were investigated by TEM, the results are illustrated in Figure 4. The TEM images of MnAlOx samples show the presence of domains with ordered channel-like mesopores, which is in good agreement with the XRD patterns at small angles. The TEM images also show that the channels are uniform, which corresponds to the pore diameter obtained from the adsorption/desorption isotherms illustrated in Figure 2B. Therefore, on the basis of the BET, small angle XRD, and TEM results, MnAlOx samples with uniform and ordered mesopores were successfully prepared by EISA method.

2.2.3. Wide-Angle XRD and SEM

Wide-angle XRD patterns of mesoporous MnAlOx catalysts results were depicted in Figure 5. The diffraction peaks of all MnAlOx catalysts are a weak and broad peak, indicating a poor crystallinity of MnAlOx catalysts. MnAlOx catalysts calcined at 800 °C begin to display crystallinity, as supported by the powder XRD patterns with distinctive peaks. Calcination at 450 °C gives rise to the mesostructure with amorphous wall, and then the amorphous wall is converted to γ-Al2O3 phase (JCPDS No. 10-0425) after further treatment at a temperature of 800 °C. Furthermore, the diffraction peaks assigned to MnOx cannot be detected, indicating that the active components (MnOx) were evenly distributed on the Al2O3 surface. Furthermore, we characterized the element dispersion of the MnAlOx catalysts via SEM-EDS measurements.
The SEM-EDS measurements were further conducted to determine the elemental constituents of the MnAlOx samples, and the results are illustrated in Figure 6. The homogeneous distribution of the elements within the framework of MnAlOx samples was confirmed by the density of the Mn, O, and Al spots in the elements mapping. The SEM-EDS measurements indicate a homogeneous distribution of Mn, Al, and O species throughout the entire periodic mesostructure (mixed oxides phase) instead of isolated single oxide domains, which is consistent with the wide angle XRD results. Thus, the atomic-level homogeneity of the OMA-supported metal oxides framework was clearly achieved by EISA method.

2.3. Adsorption/Desorption Properties

2.3.1. NH3-TPD

The acidity of SCR catalyst is crucial for the catalytic activity because it is responsible for the adsorption and activation of NH3. Herein, NH3-TPD was performed to investigate the strength of acid sites and the surface acid amount of the catalysts, and the results are demonstrated in Figure 7. All MnAlOx catalysts exhibit two desorption peaks: the low temperature desorption peaks around at 144 °C attributed to the NH3 desorbed by weak acid sites, and the high-temperature desorption peak around at 524 °C assigned to strong acid sites on the catalysts. Because the NH3 molecules coordinated to the Lewis acid sites exhibit higher thermal stability than the NH4+ ions bounded to the Brønsted acid sites, it can be deduced that the low temperature peak is assigned to NH4+ ions bound to the Brønsted acid sites, and the desorption high-temperature peaks are associated with coordinated NH3 molecular originating from the Lewis acid sites [29]. Moreover, it is well known that the area and position of desorption peak are correlated with the acid concentration and acid strength, respectively. In addition, the NH3-TPD profile of MnAlOx catalysts calcined at low temperature reveals the much larger area, indicating the presence of abundant acid sites due to its larger surface area. Although the NH3 adsorption capacity of MnAlOx catalysts prepared at a low calcination temperature is much higher than that prepared at a high calcination temperature, the SCR activity over the latter is still a little higher than that over the former, as shown in Figure 1, which implies that the SCR activity over these catalysts is not only related with the NH3 adsorption capacity, but also related with some other structural or redox properties, which will be discussed in detail later.

2.3.2. NO + O2-TPD

The variation of the NOx concentration during the desorption process as a function of the temperature is illustrated in Figure 8. It is evidenced that there are three groups of NOx desorption peaks detected over MnAlOx serials catalysts. In comparison with the in situ DRIFTs spectra of NO + O2 co-adsorption on MnAlOx catalysts at different temperature (which would be further discussed in Section 2.5), among which the peak at 133 °C is ascribed to the decomposition of the adsorbed NO2, and the peak at 189 °C is assigned to the decomposition of the bridging nitrate, linear nitrite and monodentate nitrite according to the DRIFTs result of NO + O2 co-adsorption. The band located at 331 °C is attributed to the decomposition of the bidentate nitrate, since bidentate nitrate usually shows much higher thermal stability [30,31,32,33].
The normalized integral areas of the NOx desorption bands by the surface area depicted in Table 2. It is noticeable that with the increase of the calcination temperature from 450 to 800 °C, the total desorption amount of NOx over MnAlOx catalysts increases dramatically. Meanwhile, the desorption amount of adsorbed NO2 and bidentate nitrate progressively increase and the desorption amount of monodentate nitrite, linear nitrite, and bridged nitrate progressively decrease as the calcination temperature continues increasing. This result means that the high calcination temperature results in the enhancement of NO oxidation to NO2 and the improvement of the adsorption of NOx as nitrate species. Compared with these MnAlOx catalysts, more NOx-intermediates species on the MnAlOx catalyst surface calcined at high temperatures could participate in the SCR reaction, which is beneficial to promoting the SCR activity.

2.4. Redox Capability

According to SCR reaction mechanism [2], the excellent reducibility and strong acidity are two crucial factors for rendering a catalyst with an admirable SCR performance at a broad temperature range, which are responsible for the adsorption/activation of NH3 and NOx. The redox behaviors of MnAlOx catalysts were investigated simultaneously by H2-TPR, XPS, and NO oxidation experiments.

2.4.1. H2-TPR

To discuss the redox behavior of catalysts in this work, the H2-TPR profiles of different catalysts are shown in Figure 9. Previous studies have demonstrated that the H2-TPR profile of pristine MnOx exhibits three reduction peaks at approximately 367, 455, and 517 °C, ascribed to the stepwise reduction of MnO2 to Mn2O3, Mn2O3 to Mn3O4, Mn3O4 to MnO, respectively [34]. All of the H2-TPR profiles of the MnAlOx catalysts present two distinct H2 consumption peaks around 345 and 441 °C, which were attributed to the reduction of MnO2 to Mn2O3, then Mn2O3 to Mn3O4 with the increased temperature. Compared to the area of the two peaks, it is deduced that the peak ascribed to the reduction of MnO2 to Mn2O3 has a larger area, indicating that MnO2 is the predominant phase in the catalytic formulations in comparison with Mn2O3.
The H2 consumption of MnAlOx catalysts calcined at different temperature is quantified and listed in Table 3. Note that the H2 consumption of MnAlOx catalysts increased with the increased calcination temperature, indicated that the average oxidation valence states of MnAlOx-800 °C were higher than that of MnAlOx catalysts calcined at other temperature. This result suggested that the redox capacity of the MnAlOx serial catalysts enhanced in the following sequence: MnAlOx-800 °C >MnAlOx-700 °C > MnAlOx-550 °C > MnAlOx-450 °C. The higher calcination temperature of MnAlOx catalysts can enhance the redox capacity, which appears to be responsible for the higher SCR activity of MnAlOx-800 °C catalysts.

2.4.2. XPS

In order to better understand the Mn species atomic concentration and chemical state on the catalyst surface, XPS analysis of MnAlOx catalyst with different calcination temperature was carried out. The deconvoluted peaks of Mn 2p are displayed in Figure 10 and the relative concentration ratios of different Mn oxidation states are summarized in Table 3. As shown in Figure 10, two prime peaks assigned to Mn 2p3/2 and Mn 2p1/2 of the MnAlOx catalysts centered at 642.5 eV and 654.0 eV. In order to determine the oxidation state of Mn and the relative ratios of Mnn+, the Mn 2p3/2 spectra of catalysts are fitted into three characteristic peaks, which corresponded to Mn3+ (641.3 eV) and Mn4+ (642.1 eV), respectively [34,35,36,37,38,39]. The final peak at 644.5 eV corresponds to the satellite peak of manganese [34,37,38,39,40,41].
From Table 3, it can be seen that the molar concentration of Mn on the surface of MnAlOx-800 °C (6.033%) is much higher than that of MnAlOx-700 °C (3.78%), MnAlOx-550 °C (3.49%), and MnAlOx-450 °C (2.227%). The relative surface content of Mn4+ fraction, namely, the molar ratio of Mn4+/(Mn4+ + Mn3+), gradually increase from 49.31% for MnAlOx-450 °C to 51.36% for MnAlOx-800 °C with the increase calcination temperature. It is clear that much more Mn4+ species are exposed on the surface of MnAlOx-800 °C. It has been demonstrated that the Mn4+ species and their redox cycle might be beneficial for high activity in the NH3-SCR reaction at low temperature, attributed to the enhancement of NO oxidation to NO2 [30,35,42,43] and the subsequent facilitation of the fast-SCR reaction [30,44]. On the basis of the XPS analysis, it can be concluded that the excellent catalytic activity of MnAlOx-800 °C could be attributed to the higher content of active Mn4+. Meanwhile, the XPS data are consistent with the H2-TPR analysis, and MnAlOx catalysts calcined at higher temperature are beneficial to the formation of MnO2 phase.

2.4.3. NO Oxidation

It is reported that the enhancement of NO oxidation to NO2 could significantly promote the catalytic activity of SCR catalysts at low temperature due to the occurrence of the fast SCR reaction: 2NH3 + NO + NO2  2N2 + 3H2O [37,45]. Although it is mentioned that the fast SCR reaction can promote the denitration activity, this conclusion is only based on speculation. In order to more accurately determine whether a fast SCR occurred in the reaction, we further studied the process of NO oxidation to NO2 and the results are clearly illustrated in Figure 11. With the increasing calcination temperature, the NO oxidation to NO2 shows an obvious increase. This activity might be related to the fact that the catalyst prepared at higher calcination temperatures could provide a much more Mn4+, which is beneficial to the NO oxidation reaction. Furthermore, the stronger basicity of the higher calcination temperature catalysts, which is beneficial to the adsorption of NOx, is also the cause of the enhanced NO oxidation ability [46].

2.5. In Situ DRIFTs

2.5.1. In Situ DRIFTs of NOx/NH3 Adsorption

To investigate the adsorption behaviors of the NH3/NOx on the surface of the catalysts, the in situ DRIFTs experiments of NH3/NOx adsorption over the MnAlOx catalysts with different calcination temperatures are carried out, and the results are clearly presented in Figure 12 and Figure 13.
Figure 12 displays the in situ DRIFTs spectra of MnAlOx serial catalysts after the adsorption of NH3 at different temperature. Several bands attributed to the intermediate species of NH3 appear at 1686, 1659, 1513, 1439, 1360, and 1242 cm−1. The bands at 1686 and 1439 cm−1 correspond to the symmetric and asymmetric deformation vibrations of NH4+ ionic (δs(NH4+) and δas(NH4+)) bound to the Brønsted acid sites [31,38,41,47,48,49]. The bands at 1659, 1360 and 1224 cm−1 are result of symmetric deformation vibration of NH3s(NH3)) coordinative bound to Lewis acid sites [30,38,50,51]. The bands at 1512 cm−1 can be assigned to the characteristic bands of amide (-NH2) species [41,48,52,53]. In the high wavenumber region, the broad bands at 3000~3600 cm−1 can be ascribed to the symmetric and asymmetric stretching vibration of NH3s(NH3) and νas(NH3)) coordinated to Lewis acid sites [50]. The bands at 3750 cm−1 could be assigned to the O-H stretching vibration modes of the surface acidic hydroxyl.
With the increase of the calcination temperature of MnAlOx catalysts, the number of peaks corresponding to adsorbed NH3 increase, signifying that MnAlOx catalysts calcined at higher temperature could more easily activated NH3. As the desorption temperature increase, the intensity of all bands corresponding to adsorbed NH3 species decrease until disappear during the temperature range of 50~300 °C.
The NO + O2 adsorption over MnAlOx catalysts at different temperature was investigated by in situ DRIFTs and the obtained spectra are shown in Figure 13, the catalysts surface is mainly covered by various types of nitrites and nitrates. The bands at 1656 cm−1 are assigned to the adsorbed NO2 molecules [38,49,52]. The bands at 1511 and 1290 cm−1 are ascribed to chelating bidentate nitrate [49]. The bands at 1238 cm−1 could be assigned to bridged nitrate due to the disproportionation of NO [48]. The bands at 1408 and 1347 cm−1 could be ascribed to the monodentate nitrite. The bands at 1484 cm−1 could be assigned to the linear nitrite [36,38,54]. The bands at 3000~3700 cm−1 are ascribed to the stretching interaction between the surface basic hydroxyls and NOx [47].
With the increase of the calcination temperature of MnAlOx catalysts, the number and intensity of peaks corresponding to adsorbed NH3 increase, signifying that MnAlOx catalysts calcined at higher temperature could more easily adsorbed and activated NOx. Adsorbed NO2, bridged nitrate, linear nitrite, and monodentate nitrite completely vanished with the temperature up to 250 °C, and bidentate nitrates with higher thermal stability still existed as the temperature increased up to 350 °C. It is worthy noting that the band around 1486 and 1346 cm−1 increased at first, and afterwards decreased with the increase of temperature. With an increase in temperature to 200 °C, the bands around 1486 and 1346 cm−1 shifted into new bands around 1550 and 1302 cm−1, respectively. These phenomena imply that NOx species could be coordinated to the surface of catalysts in the form of unstable NOx firstly, for instance, monodentate and linear nitrite. Thereafter, they were gradually oxidized into bidentate nitrates with higher thermostability, among which the active centers can be liberated. Simultaneously, NO can coordinate the released active centers to generate bidentate nitrates, giving rise to the obvious enhance in the absorbed peaks which is consistent with bidentate nitrate. Afterwards they were increasingly oxidized into bidentate nitrates, and thus lead to the distinct improvement in the absorbed peaks of bidentate nitrate. Furthermore, MnAlOx catalysts calcined at higher temperature could enhance the transformation from unstable nitrites to bidentate nitrates.

2.5.2. Adsorption of NH3 Followed by Introduction of NO + O2

In order to investigate the reaction pathways of NH3 and NO reactants on MnAlOx-800 °C catalysts, in situ DRIFTs spectra of the reaction between NO + O2 and pre-adsorbed NH3 at 210 °C as a function of time are recorded. As shown in Figure 14, the bands at 1238 cm−1 ascribed to δs(NH3) coordinated to Lewis acid sites disappeared gradually with an increase in the exposure time of NO + O2, which indicates that coordinated NH3 have taken part in the SCR process. Meanwhile, the bands at 3000~3600 cm−1 assigned to νs(NH3) and νas(NH3) adsorbed on Lewis acid sites were gradually weakened. In addition, the bands 1662, 1550, 1473, and 1290 cm−1 assigned to multifarious nitrites and nitrates emerged and strengthened with increase the exposure time of NO + O2. The Eley–Rideal (E–R) mechanism is generally considered to involve reaction between adsorbed A and gas phase B. On the basis of the above discussion, the adsorbed NH3 species were capable of reacting with gaseous NO following E–R mechanism.

2.5.3. Adsorption of NO + O2 Followed by Introduction of NH3

The reaction between NH3 and pre-adsorbed NO + O2 at 210 °C is also performed on MnAlOx-800 °C catalysts by in situ DRIFTs. As shown in Figure 15, the intensity of bands at 1535 and 1292 cm−1 assigned to bidentate nitrate progressively decreased to some extent with an increase in the exposure time in 500 ppm of NH3, indicating that bidentate nitrates were the reactive species for NH3-SCR reaction. Meanwhile, the bands at 1627 cm−1 attributed to adsorbed NO2 progressively disappeared with increasing the exposure time of NH3, suggesting that the adsorbed NO2 were the reactive species. In addition, new bands at 3328 cm−1 corresponding to the stretching vibration of NH3 adsorbed on Lewis acid sites were detected and strengthened with increasing the exposure time of NH3. The Langmuir–Hinshelwood (L–H) mechanism is universally considered as the adsorbed A reacted with adsorbed B. Therefore, the adsorbed NOx species were capable of reacting with coordinated NH3 following L–H mechanism. Furthermore, the MnAlOx-800 °C catalysts could enhance the transformation of nitrites to nitrates, which are the reactive intermediates and could react with NH3, thus the MnAlOx-800 °C catalysts have a higher SCR activity than that of the other MnAlOx catalysts.
Hence, it could be deduced that the NH3-SCR performance on the MnAlOx-800 °C samples follows both L–H and E–R mechanism. Compared to the reaction rate of adsorbed NH3 with the pre-adsorbed NOx over the MnAlOx-800 °C catalysts, the reaction rate of gas phase NO with the pre-adsorbed NH3 was rapid according to the above discussion. Hence, the E–R mechanism conducts a more dominant role.

3. Experimental

3.1. Preparation of Catalyst

Manganese alumina oxides were synthesized using a modified procedure according to those reported by Jaroniec, Yuan, and their co-workers [55,56]. The details of the synthesis processes are described in the Supporting Information (S1.1 Synthesis of Catalysts).

3.2. Characterization of the Catalysts

Thermogravimetry/differential thermogravimetry (TG/DTG) measurement was performed on a NETZSCH STA409PC (Netzsch, Selb, Germany) thermogravimetric analyzer. The TG/DTG profiles were recorded in flowing air up to 800 °C (5 °C/min heating rate).
N2 adsorption isotherms were performed at −196 °C on Quantachrome Autosorb volumetric analyzer (Quantachrome, Boynton Beach, FL, USA). Prior to N2 adsorption, the samples were degassed at 300 °C for 6 h under vacuum.
X-ray diffraction (XRD) was performed using D8 ADVANCE A25 (Bruker Co., Billerica, MA, USA) diffractometer with Ni-filtered Cu Kα radiation in the 2θ range from 0.4° to 5° (small angle) and from 10° to 80° (wide angle).
Energy dispersive spectroscopy (EDS) was conducted on JSM-7001F (Hitachi Limited, Tokyo, Japan) field emission scanning electron microscopy. HRTEM images was performed on a JEOL JEM 2100 transmission electron microscope(JEOL, Ltd., Tokyo, Japan).
NH3-TPD experiments were conducted on Chemstar TPx (Quantachrome, Boynton Beach, FL, USA) apparatus. Prior to the NH3 adsorption, the samples (100 mg) were pretreated under He flow (30 mL/min) at 350 °C for 1 h and then cooled down to 50 °C in a flow of He. Next, the catalysts were saturated with a flow of 10 vol.% NH3/He at 50 °C for 1 h, followed by He purging for 0.5 h. After that, the furnace temperature was raised to 800 °C (10 °C/min heating rate) under He flow.
NO + O2-TPD experiments were performed on a self-made apparatus. Prior to NO + O2-TPD experiments, the samples (0.5 g) were pretreated under an N2 stream (200 mL/min) at 350 °C for 1 h and then the reactor temperature was cooled to 50 °C. After that, the samples were exposed to 1000 ppm NO + 6.5% O2 for 1 h, followed by N2 purging for 0.5 h. Finally, the samples were heated to 600 °C (10 °C/min ramping rate) with flowing N2 (200 mL/min). The inlet and outlet gas concentrations of NOx were monitored by a portable FTIR gas analyzer (Gasmet Instruments DX-4000, Vantaa, Finland).
H2-TPR experiments were performed on a Quantachrome ChemStar TPx apparatus. 100 mg of catalyst was pretreated with flowing Ar (30 mL/min) at 350 °C for 1 h. Then, the samples cooled down to 50 °C, the flowing Ar was replaced by a flow of 10.0% H2/Ar (30 mL/min), and the furnace temperature was raised to 900 °C with a heating rate of 10 °C/min.
X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos Axis Ultra DLD (Japan) multifunctional photoelectron spectrometer with Al-Kα radiation (12 kV × 15 mA, hν = 1486.6 eV) under ultrahigh vacuum. The element binding energies were calibrated by C 1s (284.8 eV).
NO oxidation experiments were performed on a custom-made apparatus. The catalysts (0.5 g) were exposed to 500 ppm NO + 6.5 vol.% O2/N2 (200 mL/min). The inlet and outlet gas concentration of NOx was detected by a FTIR gas analyzer (Gasmet Instruments DX-4000, Vantaa, Finland).
The in situ DRIFTs measurements were performed on an FTIR spectrometer (Bruker Tensor 27, Germany) equipped with an MCT detector and a Harrick temperature controller. The spectra were recorded in the range of 4000~1000 cm−1 at a resolution of 8 cm−1 with 100 scans in Kubelka–Munk mode. Details are given in Supporting Information (S1.3 Characterization of Catalysts).

3.3. Catalytic Activity Measurement

The catalytic activities of MnAlOx for NH3-SCR in excess oxygen were evaluated under atmospheric pressure in a fixed-bed continuous-flow quartz microreactor (inner diameter 12 mm). The details of the activity testing process are presented in the Supporting Information (S1.3 Catalytic Activity Measurement). The NOx conversion and N2 selectivity were calculated on the flowing equations [57].
NO x   conversion ( % ) = ( 1 [ NO x ] out [ NO x ] in )   ×   100 %
N 2   selectivity ( % ) = ( 1 2   ×   [ N 2 O ] out [ NO x ] in + [ NH 3 ] in [ NO x ] out [ NH 3 ] out )   ×   100 %
where the subscript indicated the inlet and outlet concentration at steady-state, respectively.

4. Conclusions

In summary, a series of mesoporous amorphous MnAlOx catalysts for NH3-SCR at low temperature was successfully prepared. Among them, the MnAlOx-800 °C catalysts exhibit the best SCR performance. The NOx conversion of MnAlOx-800 °C catalysts could maintain above 90% at 150~240 °C and the N2 selectivity of MnAlOx-800 °C catalysts could maintain 80% up to 240 °C.
MnAlOx catalysts calcined at different temperature with uniform and ordered mesoporous structure confirmed by means of N2-adsorption/desorption isotherms, small angle XRD, and TEM. Meanwhile, the active components MnOx could atomic-level homogeneity distribute on the surface of MnAlOx catalysts. The MnAlOx-800 °C catalysts possess improved redox properties than other MnAlOx catalysts, which is assigned to the higher content of Mn4+/(Mn4+ + Mn3+). In comparison with other MnAlOx catalysts, the MnAlOx-800 °C catalyst has a higher adsorption ability for NOx due to enhanced redox properties. The high adsorption ability for NOx and improved redox properties of MnAlOx-800 °C catalysts are beneficial for formation of nitrate/nitrite species on the catalyst surface to results in high SCR activity. Furthermore, the results of in situ DRIFTs indicated that adsorbed NO2 and bidentate nitrate are the reactive intermediate species, and NH3 species coordinated to Lewis acid sites, taking part in SCR reaction. The SCR performance followed L–H and E–R mechanism concurrently, while E–R mechanism plays a significant role.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12060637/s1, Figure S1. TG-DTG curves for manganese alumina samples.

Author Contributions

Methodology, Q.H.; validation, Q.H. and Y.L.; writing—original draft preparation, Q.H.; writing—review and editing, Y.L., Y.H., X.H. and Z.H.; supervision, Y.H.; project administration, Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA29020501), the National Natural Science Foundation of China (Grants 21902173 and 21978314).

Acknowledgments

The authors gratefully acknowledge the financial support by the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA29020501), the National Natural Science Foundation of China (Grants 21902173 and 21978314).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) The NOx conversion and (B) N2 selectivity of MnAlOx catalysts.
Figure 1. (A) The NOx conversion and (B) N2 selectivity of MnAlOx catalysts.
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Figure 2. (A) N2 adsorption/desorption isotherm and (B) the corresponding pore size distribution for mesoporous MnAlOx catalysts with different calcined temperature.
Figure 2. (A) N2 adsorption/desorption isotherm and (B) the corresponding pore size distribution for mesoporous MnAlOx catalysts with different calcined temperature.
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Figure 3. Small-angle patterns of mesoporous MnAlOx-T catalysts.
Figure 3. Small-angle patterns of mesoporous MnAlOx-T catalysts.
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Figure 4. TEM images of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C.
Figure 4. TEM images of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C.
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Figure 5. Wide-angle XRD patterns of mesoporous MnAlOx catalysts.
Figure 5. Wide-angle XRD patterns of mesoporous MnAlOx catalysts.
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Figure 6. SEM-EDS images of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C.
Figure 6. SEM-EDS images of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C.
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Figure 7. NH3-TPD profiles of the MnAlOx catalysts.
Figure 7. NH3-TPD profiles of the MnAlOx catalysts.
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Figure 8. NO + O2-TPD profiles of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C.
Figure 8. NO + O2-TPD profiles of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C.
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Figure 9. H2-TPR profiles of different catalysts.
Figure 9. H2-TPR profiles of different catalysts.
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Figure 10. XPS spectra of Mn 2p over different catalysts.
Figure 10. XPS spectra of Mn 2p over different catalysts.
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Figure 11. NO oxidation to NO2 over the MnAlOx catalysts with different calcination temperatures. Reaction conditions: [NO] = 500 ppm, [O2] = 6.5 vol.%, N2 as balance gas, total gas flow rate 400 mL/min, and GHSV = 12,000 h−1.
Figure 11. NO oxidation to NO2 over the MnAlOx catalysts with different calcination temperatures. Reaction conditions: [NO] = 500 ppm, [O2] = 6.5 vol.%, N2 as balance gas, total gas flow rate 400 mL/min, and GHSV = 12,000 h−1.
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Figure 12. In situ DFIRTs of spectra of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C catalysts under atmosphere of 500 ppm of NH3/N2 (100 mL/min) at 50, 100, 150, 200, 250, 300, and 350 °C.
Figure 12. In situ DFIRTs of spectra of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C catalysts under atmosphere of 500 ppm of NH3/N2 (100 mL/min) at 50, 100, 150, 200, 250, 300, and 350 °C.
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Figure 13. In situ DFIRTs of spectra of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C catalysts under atmosphere of 500 ppm of NO + 6.5 vol. %/N2 (100 mL/min) at 50, 100, 150, 200, 250, 300, and 350 °C.
Figure 13. In situ DFIRTs of spectra of (A) MnAlOx-450 °C, (B) MnAlOx-550 °C, (C) MnAlOx-700 °C, (D) MnAlOx-800 °C catalysts under atmosphere of 500 ppm of NO + 6.5 vol. %/N2 (100 mL/min) at 50, 100, 150, 200, 250, 300, and 350 °C.
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Figure 14. In situ DRIFTs spectra of the MnAlOx catalyst under an atmosphere of 500 ppm NO + 6.5 vol. %O2/N2 at different times after adsorption of 500 ppm of NH3 at 210 °C and flowing of N2 for 30 min.
Figure 14. In situ DRIFTs spectra of the MnAlOx catalyst under an atmosphere of 500 ppm NO + 6.5 vol. %O2/N2 at different times after adsorption of 500 ppm of NH3 at 210 °C and flowing of N2 for 30 min.
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Figure 15. In situ DRIFTs spectra of the MnAlOx catalyst under an atmosphere of 500 ppm NH3 at different times after adsorption of 500 ppm NO + 6.5 vol. %O2/N2 of at 210 °C and flowing of N2 for 30 min.
Figure 15. In situ DRIFTs spectra of the MnAlOx catalyst under an atmosphere of 500 ppm NH3 at different times after adsorption of 500 ppm NO + 6.5 vol. %O2/N2 of at 210 °C and flowing of N2 for 30 min.
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Table 1. BET surface area and NH3 storage of MnAlOx-T catalysts.
Table 1. BET surface area and NH3 storage of MnAlOx-T catalysts.
SamplesSBETVporeDiameterNH3 Storage
m2/gcm3/gnmmmol/g
MnAlOx-450 °C247.70.66110.6831.692
MnAlOx-550 °C221.30.62111.2221.470
MnAlOx-700 °C226.50.58410.3120.994
MnAlOx-800 °C196.20.50110.2200.795
Table 2. Capacities of MnAlOx catalysts for NOx adsorption.
Table 2. Capacities of MnAlOx catalysts for NOx adsorption.
SamplesNOx Storage
mmol/g
Peak ⅠPeak ⅡPeak Ⅲ
Adsorbed NO2Monodentate Nitrite
Linear Nitrite
Bridged Nitrate
Bidentate Nitrate
MnAlOx-450 °C0.13720.01380.09110.0323
MnAlOx-550 °C0.13660.03270.06660.0373
MnAlOx-700 °C0.14140.04780.05030.0434
MnAlOx-800 °C0.16230.05880.04940.0541
Table 3. H2 consumption amount, surface atomic concentrations of Mn, O, Al, and the relative concentration ratios.
Table 3. H2 consumption amount, surface atomic concentrations of Mn, O, Al, and the relative concentration ratios.
CatalystsH2 Consumption Amount
mmol/g
Surface Atom Concentrations (%)Mn4+/Mnn+
MnOAl
MnAlOx-450 °C0.1142.22746.2751.5149.31
MnAlOx-550 °C0.1323.4939.6256.8949.20
MnAlOx-700 °C0.1703.7851.1545.0749.03
MnAlOx-800 °C0.2006.03345.2648.7151.36
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Hou, Q.; Liu, Y.; Hou, Y.; Han, X.; Huang, Z. Ordered Mesoporous MnAlOx Oxides Dominated by Calcination Temperature for the Selective Catalytic Reduction of NOx with NH3 at Low Temperature. Catalysts 2022, 12, 637. https://doi.org/10.3390/catal12060637

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Hou Q, Liu Y, Hou Y, Han X, Huang Z. Ordered Mesoporous MnAlOx Oxides Dominated by Calcination Temperature for the Selective Catalytic Reduction of NOx with NH3 at Low Temperature. Catalysts. 2022; 12(6):637. https://doi.org/10.3390/catal12060637

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Hou, Qixiong, Yongjin Liu, Yaqin Hou, Xiaojin Han, and Zhanggen Huang. 2022. "Ordered Mesoporous MnAlOx Oxides Dominated by Calcination Temperature for the Selective Catalytic Reduction of NOx with NH3 at Low Temperature" Catalysts 12, no. 6: 637. https://doi.org/10.3390/catal12060637

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