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Article

High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release

by
Chenchen Cui
1,
Junwei Ma
2,
Zhongpeng Wang
1,*,
Wei Liu
1,
Wenxu Liu
1 and
Liguo Wang
1,*
1
School of Water Conservation and Environment, University of Jinan, 336 West Road of Nanxinzhuang, Jinan 250022, China
2
Environmental Protection Monitoring Station of Anqiu, Shongdong Province, Middle Section of Gongnong Road, Anqiu 262100, China
*
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(8), 677; https://doi.org/10.3390/catal9080677
Submission received: 19 July 2019 / Revised: 6 August 2019 / Accepted: 7 August 2019 / Published: 9 August 2019
(This article belongs to the Special Issue Catalysis for the Removal of Gas-Phase Pollutants)
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Abstract

:
NOx storage-reduction (NSR) is a potential approach for the effective removal of NOx under the lean conditions in lean-burn engines. Herein, manganese-doped mixed oxides (Mn/MgAlOx) with high performance for low temperature NOx storage and release were derived from hydrotalcites precursors prepared by a facile coprecipitation method. The catalysts were characterized by X-ray diffraction (XRD), SEM, N2 adsorption-desorption, H2-TPR, FT-IR, and X-ray photoelectron spectroscopy (XPS) techniques. The Mn-doped MgAlOx catalysts exhibited high NOx storage capacity (NSC) at low temperature range (150–300 °C), which was related to their increased surface area, improved reducibility and higher surface Mn3+ content. The largest NSC measured, 426 μmol/g, was observed for NOx adsorption at 200 °C on Mn15 catalyst (the sample containing 15 wt% of Mn). The in situ DRIFTS spectra of NOx adsorption proved that the Mn-doped hydrotalcite catalysts are preferred for low temperature NOx storage and release due to their ability to store NOx mainly in the form of thermally labile nitrites. NSR cycling tests revealed the NOx removal rate of Mn15 sample can reach above 70% within the wide temperature range of 150–250 °C. Besides, the influence of CO2, soot, H2O and SO2 on NOx storage performance of Mn15 catalyst was also studied. In all, owning to their excellent NOx storage capacity, NSR cycling performance, and resistance to CO2, soot, SO2 and H2O, the Mn-doped MgAlOx NSR catalysts have broad application prospects in NOx control at low temperatures.

1. Introduction

The Lean-burn engines (diesel and lean-burn gasoline engines) are gradually being promoted and applied due to their high fuel economy and low pollution emissions. Traditional three-way catalysts (TWC), however, have low NOx removal efficiency under lean-burn conditions, which are not satisfied with the strict emission standards. NOx storage and reduction (NSR) technology is a potential method to effectively solve the problem of vehicle exhaust pollution [1,2,3]. NSR operates in a cyclic pattern with alternating lean-burn and rich-burn periods. Under lean-burn conditions, NO is oxidized to NO2 by the active component of the catalysts and stored in the form of nitrite or nitrate. While in the very short rich-burn conditions, stored NOx can be released and reduced, at the same time, the catalysts are regenerated [4,5]. A typical NSR catalyst is primarily composed of active component (generally a noble metal), storage component (an alkali metal or alkaline earth metal) and supports having large specific surface area. Presently, Pt/BaO/Al2O3 system is regarded as a model NSR catalyst and has been extensively studied [6,7,8,9]. Nevertheless, this catalyst system also has some disadvantages, such as the inferior resistance to SO2, CO2 and water vapor, as well as thermal degradation. Many strategies have been reported to solve these problems, for example, looking for the best Pt/Ba ratio [10,11], using MgO or CaO instead of BaO [12], adding various metal components [13,14,15], or applying hydrotalcite-like materials and their derivatives, et al. Furthermore, it is worth noting that although Pt/BaO/Al2O3 catalyst exhibits excellent activity at higher temperatures (> 300 °C) owing to the powerful basicity of BaO, it has insufficient NOx storage capability at low temperatures. It is reported that the typical engine-out temperatures are about 150–350 °C [2]. Therefore, how to exploit the NSR catalysts with good low temperature activity is an urgent problem to be solved in this field.
Mn-based mixed oxides have been thoroughly studied at length and proved to exhibit good low-temperature activity for selective catalytic reduction (SCR), NO oxidation and simultaneous soot-NOx removal. Wu et al. [16] reported MnOx/TiO2 mixed oxides for low-temperature NH3-SCR. They found that the conversion rate of NOx can reach nearly 100% at 120–220 °C after adding Ce, and doping V to MnOx/TiO2 catalysts can efficiently increase the selectivity of nitrogen. Tian et al. [17] prepared three different morphologies MnO2 using a hydrothermal method and observed that MnO2 nanorods exhibited the highest activity for reduction of NOx with conversion efficiencies of above 90% in the range of 250–300 °C. Qi et al. [18] investigated the Mn-Ce mixed oxides for NO oxidation and found that the incorporation of ZrO2 into MnOx-CeO2 can improve the NO oxidation activity. Additionally, Mn has been proved to be an effective dopant for Ba-based NOx storage and reduction catalysts due to its higher NO oxidizability [19,20,21]. Li et al. [22] studied the hydrotalcite-based MnxMg3−xAlO catalysts and revealed that the Mn4+ ions are the most active species for soot combustion and simultaneous soot-NOx removal.
Mn-based mixed oxides also possess excellent NOx adsorption performance at low temperatures. Guo et al. [23] investigated MnOx catalysts with different calcination temperatures for NO adsorption and noticed that the catalyst calcined at 550 °C containing a single α-Mn2O3 phase exerted the best NO adsorbability. Machida et al. [24,25] observed that Mn-Ce oxides have superior NOx removal at temperatures lower than 150 °C. Huang et al. [26] studied a sequence of Fe-Mn-based metal oxides for NOx adsorption at 25 °C and pointed out that they were efficient catalysts for NOx removal. Sun et al. [27] reported that Mn-Sn-Ce-O catalysts with different Sn/(Mn + Sn + Ce) molar ratios showed higher NOx storage capacity at 100 °C, with Mn0.4Sn0.5Ce0.1 showing the highest NSC of 565.9 μmol/g. Recently, Guo et al. [28] systematically studied the NOx storage and release in the alternative lean-burn/rich-burn atmospheres over Mn2O3, Mn3O4 and MnO2 catalysts at low temperatures (≤ 200 °C) and found that Mn2O3 showed high NO oxidizability and MnO2 had the biggest NOx storage capacity. They suggested that the Mn3+ ions and surface-active oxygen species are the key to high NO oxidation activity. Zeng et al. [29] found that MnOx-CeO2 catalysts exhibited stronger ability to adsorb NOx and resistance to SO2 poisoning than that of MnOx-TiO2 catalysts, which was attributed that MnOx-CeO2 catalysts possess more Mn3+ and surface adsorbed oxygen.
Recently, well-dispersed metal oxides derived from hydrotalcite-like compounds (also called layered double hydroxides, LDH) have aroused wide attention from researchers due to their special layer structure, exchangeability of anions, stronger basicity, memory effect and superior thermostability [30,31,32], wherefore they are attractive active environmental catalysts and promising substitution of the commercial NSR catalysts. Whereas, similar to Pt/BaO/Al2O3 NSR catalysts, Mg/Al hydrotalcite derived oxides also have inferior performance in NOx storage and release at low temperatures. Extensive researches have shown that the incorporation of active component (e.g., Cu [33], Fe [34], Co [35], Ag [36]) into hydrotalcite precursors can effectively enhance the redox properties and NSR catalytic properties of the oxides’ catalysts.
In consideration of excellent NO oxidation activity of Mn-based oxides and potential NOx storage performance of hydrotalcites catalysts, a series of Mn-doped MgAlOx catalysts derived from hydrotalcites were prepared as alternative NSR catalysts and their low-temperature performance have been studied in the present work. The Mn-doped oxides were synthesized using an LDH approach using a cetyltrimethyl ammonium bromide (CTAB)-assisted co-precipitation method, which can obtain large surface area, high metal dispersion and thermal stability. The molar ratio of divalent and trivalent metal ions in raw materials was controlled to 3:1 in order to obtain LDH precursors with high crystallinity [31]. Then we can effectively utilize the dispersing effect of LDH structure on active metal components. Specific studies were implemented to characterize the NOx storage and release performance of samples. The surface species formed during NOx adsorption were investigated by in situ DRIFTS measurements. Finally, the effects of CO2, soot, H2O and SO2 on NOx storage performance of Mn15 catalyst were also studied.

2. Results and Discussions

2.1. XRD Analysis

The XRD patterns of the Mn/Mg/Al hydrotalcites precursors are presented in Figure 1a. The chemical composition of the synthesized precursors, the lattice parameters, as well as mean crystallite size are displayed in Table 1. For all the precursors, the characteristic diffraction peaks at 11°, 23°, 34°, 38°, 46°, 61°, and 63° corresponding to hydrotalcite-like compound (JCPDS 22-0700) were observed, which can be assigned to the (003), (006), (009), (015), (018), (110) and (113) crystal planes, respectively. The addition of Mn to Mg/Al hydrotalcites led to the appearance of weak characteristic diffraction peaks corresponding to MnCO3 at 2θ = 24.2°, 32°, 42.5° and 53°. Furthermore, the peak of crystal plane (003) shifted to higher 2θ angle with the increase of Mn contents, which was ascribed to lattice contraction due to the isomorphous substitution of Mg2+ by manganese cations with smaller ionic radius. A decrement of lattice parameter a was observed in Mn-based samples, implying part of Mn dopant may incorporate into the Mg/Al hydrotalcite structure. Besides, the parameter c decreased when dopant Mn was introduced, which was due to the revised electrostatic interactions between the layer and the interlayer network [37]. As shown in Table 1, the mean crystallite size of the Mn/Mg/Al hydrotalcite varied in the range of 9–19 nm, with the largest size of 18.8 nm for Mn10-HT sample.
After calcination at 600 °C, the hydrotalcite structure was completely decomposed and new oxide derivatives were formed (Figure 1b). The diffraction patterns of Mn0 sample showed three well-defined peaks centered at 35°, 43° and 62°, which belonged to the MgO phase (JCPDS 45-0946). However, the peaks of MgO phase became gradually weaker with increasing Mn content. At the same time, new diffraction peaks at 18°, 33°, 36.5°, 38.5°, 45°, 52°, 54°, 58.5°, 60 °, 61° and 65° can be observed after Mn doping, which can be attributed to Mn3O4 phase (JCPDS 80-0382) and/or MgMn2O4 spinel phase (JCPDS 23-0392). Because the peaks position of Mn3O4 and MgMn2O4 spinel are very similar, it is difficult to differentiate them from the XRD results.

2.2. SEM Analysis

The morphology of the calcined oxides catalysts was studied by SEM as demonstrated in Figure 2. After calcination at 600 °C, the hydrotalcite-like layer structure was collapsed and small particles of mixed oxides were formed. The particles were agglomerated together, which may be related to the high temperature calcination. Mn0 sample exhibited a worm-like structure with plentiful intergranular pores. No distinct morphological changes can be observed on Mn5 sample. However, Mn15 catalyst showed a spherical shape nanoclusters structure. Small pores formed by the accumulation of elementary particles were observed on all oxides, which was in favor of the gaseous reaction in the catalytic process [38].

2.3. N2 Adsorption-Desorption Characterization

The nitrogen adsorption-desorption isotherms and pore size distributions of the catalysts are exhibited in Figure 3. All the hydrotalcites-derived mixed oxides showed IUPAC type IV isotherms, indicating that the hydrotalcite-like samples produced mesoporous mixed oxides after calcined at 600 °C. The H3 type of hysteresis loops indicate a capillary condensation in a high P/P0 range of 0.6–1.0 [39]. The hysteresis loop is usually found in adsorbents containing slit-shaped pores.
As seen from the pore size distribution curves, a large proportion of pores fell in mesopores range (2 nm < rp < 50 nm) with a fraction in macro size, suggesting that the catalysts are hierarchical porous materials. The specific surface areas, pore volume and average pore diameter of samples are presented in Table 2. All the samples showed relatively large surface areas of 100–140 m2·g−1. Mn substitution substantially increased the specific surface areas of the samples. The high surface areas can offer more accessible active sites for NOx adsorption on the catalysts. The averaged pore diameter varied in the range of 14–23 nm, which was beneficial for gas molecule diffusion in the pores during NOx adsorption and desorption process [32].

2.4. XPS and H2-TPR Characterization

X-ray photoelectron spectroscopy (XPS) characterization was implemented on samples in order to define the surface species. The XPS spectra of Mn2p and O1s are showed in Figure 4. As shown in Figure 4a, the Mn 2p XPS spectra of the Mn5, Mn10, Mn15 and Mn20 samples presented two peaks centered at around 642.7 eV and 654.2 eV, corresponding to Mn 2p3/2 and Mn 2p1/2, respectively [40]. The Mn 2p3/2 peak was relatively wide, because manganese existed in different oxidation states on the surface of the catalysts. By deconvoluting, it can be fitted into three peaks located at around 643.9 eV, 642.7 eV and 641.2 eV, corresponding to Mn4+, Mn3+, and Mn2+, respectively [41]. Additionally, surface Mn3+ was reported to perform better catalytic activity than Mn4+ and Mn2+ in the NO oxidation reaction [28,29,42]. In order to define the relative concentration of Mn3+ ions in samples, the molar ratio of Mn3+/(Mn3+ + Mn4+) was calculated from XPS results and summarized in Table 3. As shown, Mn15 and Mn20 samples possessed higher surface Mn3+ content compared to others.
O1s spectra in Figure 4b exhibit two fitted oxygen peaks, which correspond to two types of surface oxygen species. The peak centered at ~531.2 eV can be assigned to lattice oxygen (Olatt) [43], while the peak at ~532.6 eV can be assigned to surface adsorbed oxygen (Oads) [44,45]. We calculated the value of Oads/(Olatt + Oads) to roughly estimate the amount of surface adsorbed oxygen for all samples and the obtained results are illustrated in Table 3. After addition of Mn, the surface adsorbed oxygen content decreased. However, further increase of Mn doping had no significant effect on the surface adsorbed oxygen content. It can be speculated that the surface adsorbed oxygen content was not a decisive factor in the NOx storage capacity.
H2-TPR experiments were performed to study the reducibility of the catalysts. The TPR profiles are displayed in Figure 5. No reduction peak was observed on Mn0 sample. However, after Mn doping, two peaks located at about 250 °C and 460 °C appeared, which can be ascribed to the reduction of Mn3O4 [46]. Based on literature reports, the lower temperature peak was ascribed to the reduction of Mn4+→Mn3+, whereas the higher temperature peak was ascribed to the reduction of Mn3+→Mn2+ [18,47]. Obviously, the higher temperature reduction peak was stronger, which may also include the reduction of Mn3+ in MgMn2O4 spinel. Therefore, the H2-TPR data was basically consistent with the phase composition of the catalysts in XRD analysis. Mn3O4 and MgMn2O4 spinel are the main crystalline phase besides brucite MgO, especially for oxides samples with higher Mn content. As shown in Table 3, relative Mn3+ content estimated from integrated reduction peak areas increased with increasing Mn doping. Mn15 and Mn20 samples had higher Mn3+ content compared to others, which was in keeping with XPS results.
Interestingly, as the Mn content increased, the first reduction peak shifted to lower temperature range, indicating that Mn doping can improve the reducibility of the samples. The H2 consumption of all samples was calculated and the results were listed in Table 3. It can be concluded that with the increase of Mn content, more H2 was consumed. Noticeably, Mn15 catalyst possessed the highest H2 consumption of 34.86 mmol·g−1.

2.5. NOx Adsorption and Desorption Behavior

NOx adsorption experiments were implemented at 300 °C to investigate the effect of Mn incorporation on NOx storage. The obtained NOx concentration profiles are showed in Figure S1. At the initial stage of adsorption, no NOx was monitored in the outlet, indicating NOx was all adsorbed on the catalysts surface. Subsequently, with increasing adsorption time, NOx concentrations at the outlet increased continually and then reached to a steady state. This phenomenon suggested that the adsorption process achieved a stable equilibrium. Additionally, in the whole process, small amount of NO2 was detected at the outlet, indicating the oxidation of NO to NO2 on the catalysts. With the increase of Mn content, the NO2 percentage at the outlet after adsorption saturation was 5.7%, 12.5%, 17.5%, 25.0%, 25.2%, respectively, revealing that the dopant Mn can facilitate the oxidation of NO into NO2.
The NOx storage efficiency (NSE) based on the NOx adsorption profiles are summarized in Table 4. At the beginning of storage (less than 1 min), NOx was all stored on the oxides catalysts. However, differences in NSE became increasingly obvious with increasing storage time. Mn doping resulted in higher NSE, for example, an NSE of about 60% was achieved for Mn15 and Mn20 catalysts compared with 39.5% NSE for Mn0 sample after 30 min storage. It is related to the fact that Mn was profitable for the oxidation of NO into NO2, which was stored more effectively.
The NOx desorption profiles of the catalysts after NOx adsorption at 300 °C are displayed in Figure S1. For Mn0 catalyst, the NOx desorption followed a two-step process, corresponding to the desorption of various nitrites and nitrates, respectively. NO was the primary species and less than 20 ppm NO2 over 200–700 °C was produced. After Mn doping, the NOx desorption followed a one-step process, and the peak temperature of desorption shifted to lower temperature, indicating that Mn decreased the stability of the formed nitrite and nitrate species. The NOx storage capacity (NSC) of the catalysts calculated from desorption profiles are displayed in Figure 6. It is noticeable that the NSC significantly increased from 50 μmol/g for Mn0 sample to about 160–200 μmol/g after Mn doping, indicating that Mn played a key role in NOx storage. The improvement can be ascribed to the increased surface area, promoted oxidation of NO into NO2, and high surface adsorbed oxygen species. Notably, Mn15 sample showed the best NOx storage capacity (206 μmol/g) at 300 °C. When the dopant Mn continued to increase, the NOx storage capacity decreased. The decrease in Mn20 indicated that the doping amount of Mn has a threshold value, beyond which it is not conducive to storage.
In the above studies, Mn15 catalyst had superior NOx storage performance, so we further investigated its NOx storage performance at different adsorption temperatures (150, 200, 250, 300 and 350 °C). The experimental results are presented in Figure S2. Generally speaking, NO was completely trapped by the samples at the outset and almost no NOx was monitored at the reactor outlet. With increasing reaction time, NO and a small amount of NO2 gradually increased and finally reached a certain level. When the catalysts reached adsorption saturation at different temperatures, the NO2 concentrations at the outlet were 70 ppm, 106 ppm, 145 ppm, 223 ppm, 375 ppm, and 377 ppm, respectively. This reflected that NO oxidation took place on catalysts during NOx adsorption, and higher adsorption temperatures were profitable to promote the conversion of NO to NO2. The NOx storage efficiency (NSE) of Mn15 sample at different temperatures are summarized in Table 5. NSE of 100% was achieved on all the samples for less than 1 min. Nevertheless, NSE value decreased with increasing reaction time. Within 10 min, over 50% NSE was found on NOx adsorption at 150–350 °C, indicating that Mn15 catalyst can effectively store NOx at low temperature ranges.
Figure S2 shows the desorption progress of NOx on Mn15 catalyst. Apparently, NOx desorption followed a two-step process at the adsorption temperature of 150 °C and 200 °C, corresponding to various types of nitrites and nitrates. However, after the adsorption temperature was raised, the NOx desorption followed a one-step process. Simultaneously, NO was the main desorption species, accompanied by different amounts of NO2 depending on the adsorption temperature. The NSC calculated from the desorption profiles of Mn15 sample at different temperatures are listed in Figure 7. Mn15 catalyst possessed better NOx storage capacity of above 200 μmol/g in the temperature range of 150–300 °C. The highest NSC was observed at 200 °C of 426 μmol/g on Mn15. Nevertheless, when the adsorption temperature continued to increase, the NOx storage capacity decreased. This decrement was probably due to the decomposition of stored nitrite and nitrate by thermodynamic instability. At the same time, we compared other types of oxide catalysts with excellent NOx storage activity reported in recent years. The NOx storage performance of different types of catalysts is summarized in the Table S1. It can be seen that the Mn15 catalyst can still exhibit high NOx storage capacity at low temperatures compared to the bimetallic oxides and La-containing perovskite-type oxide catalysts, even higher than the costly storage of precious metal Pt-based catalysts and the conventional Pt/BaO/Al2O3 catalysts.

2.6. In Situ DRIFTS Spectra

The in situ DRIFTS spectra experiments were carried out at 300 °C on Mn0, Mn5 and Mn15 catalysts to probe the adsorbed species formed during NOx storage. Generally, they were various nitrites and nitrates. The specific species are summarized in Table 6. As can be seen from Figure 8a, when NO + O2 was introduced to Mn0 catalyst, the peak at 1230 cm−1 attributed to bridged bidentate nitrite appeared within 1 min and its intensity increased with increasing adsorption time. The weak peak located at 1310 cm−1 belonged to bridged bidentate nitrate appeared after 5 min. This indicated nitrites were the dominant adsorbed species on Mn0 sample. For Mn5 catalyst, nitrites were formed rapidly in the initial period. Subsequently, the intensity of nitrates increased whilst that of nitrites decreased, indicating that NO adsorbed on Mn5 to form nitrites and then formed NO2 and stable nitrates [48,49,50]. In other words, the storage of NO on Mn5 catalyst followed the “nitrite” pathway. Similar formation and conversion were observed on Mn15 sample. The bands at 1312 cm−1 and 1447 cm−1 due to monodentate nitrate were the primary NOx species stored on Mn15 catalyst. Compared with Mn5 sample, the rate of nitrite transformation on Mn15 catalyst was higher than that on Mn5 catalyst, revealing that Mn content played a favorable role in the oxidation of nitrite.
Figure 9 shows the in situ DRIFTS spectra of Mn15 sample after exposure to NO + O2 at different temperatures for 60 min. The specific species are summarized in Table 7. At 150 °C, the broad peak located at 1220 cm−1 was assigned to bridged bidentate nitrite, corresponding to the adsorption of NO on the weak basic sites [32,51]. The weak peak at 1321 cm−1 was associated with bridged bidentate nitrate. For adsorption at 200 °C, the peaks at 1226 cm−1 assigned to bridged bidentate nitrite appeared in the initial stage and later shifted to 1310 cm−1 and 1480 cm−1 that ascribed to bridged bidentate nitrate and monodentate nitrate, indicating a conversion from nitrite to nitrate on the sample. The species formed at 250 °C and 300 °C were similar to that at 200 °C. Differently, the rate of nitrite oxidation was improved with increasing adsorption temperature, indicating high adsorption temperatures accelerated the transformation of nitrites to nitrates. Besides, the peak intensity of nitrate species increased, suggesting high temperatures were favorable to form more stable nitrates species. For adsorption at 350 °C, only bridged bidentate nitrate (1315 cm−1) and monodentate nitrate (1438 cm−1) were formed and became the major species in the final period, implying the storage of NO on the sample followed the “nitrate” pathway. Besides, their intensities increased with an increment in adsorption time. It is concluded that, within 10 min of adsorption, the NOx adsorbed species on Mn15 catalyst were mainly various nitrites at lower temperatures (<300 °C), while NOx was mainly stored in the form of nitrate at higher temperatures (≥300 °C). Therefore, Mn-doped hydrotalcite catalysts are preferred for low temperature NOx storage and release may because they can store NOx as thermally labile nitrites.

2.7. NOx Lean-Rich Cycling Performance

The lean-rich cycling experiments were also tested on Mn15 sample at different temperatures and the results are displayed in Figure S3. During the lean periods, NO was first oxidized to NO2 and then NO2 was stored in the form of nitrate or nitrite. During rich periods, the stored thermodynamic instability nitrite and nitrate were decomposed and release NOx. Subsequently, the released NOx was reduced by H2. As shown in Figure 10, it can be found that the average removal rate of the Mn15 sample can reach above 70% in the range of 150–250 °C. Nevertheless, with increasing reaction temperature, the average removal rate decreased, indicating that Mn15 sample had better lean-rich cycling performance at lower temperatures than that at higher temperatures. It was in accordance with the isothermal NOx adsorption and desorption results. This may be related to the fact that NOx was mainly stored in the form of labile nitrite at lower temperatures, while NOx was mainly stored in the form of stable nitrate at higher temperatures. It has been known that nitrites were more easily decomposed than nitrates, which was profitable to the regeneration of the catalysts, thus improving the average removal rate of the catalysts.

2.8. The Influence of CO2, Soot, H2O, and SO2

To further investigate the influence of CO2, soot, H2O and SO2 on catalysts, the isothermal NOx storage performance tests were carried out on Mn15 sample at 200 °C. As shown in Figure 11, after the introduction of CO2, soot, H2O and SO2, the NOx storage performance of Mn15 catalyst had different degrees of decrement. Or rather, after adding CO2, the NOx storage capacity of the sample slightly decreased from 426 μmol/g to 409 μmol/g, which was attributed that CO2 competed with NOx for chemical adsorption sites [52]. Similar results were observed on soot-catalyst mixtures. It may be related to the fact that the exothermic combustion of soot destabilized the stored species and the soot combustion produced CO2 would compete with NOx for storage sites on the catalyst.
According to reports [53], the total exhaust usually contains about 2–15% H2O, thus the influence of water vapor on the NOx storage was examined. It can be clearly seen that H2O restrained the NOx storage capacity by close to 42%, which was due to the competitive adsorption between NOx and H2O on the surface of the catalyst [54]. Then SO2 was also taken into consideration, the NSC significantly decreased from 426 μmol/g to 242 μmol/g. This deactivation caused by SO2 could be related to the formation of sulfites and sulfates, which would decrease the basicity of the storage sites. In conclusion, during the NOx storage and release process, Mn15 catalyst exhibited high resistance to CO2 and soot, with a certain sulfur and water resistance.

3. Experimental

3.1. Preparation of Catalysts

The Mn/Mg/Al hydrotalcites precursors were prepared using a cetyltrimethyl ammonium bromide (CTAB)-assisted co-precipitation method [55]. The mixed salt solution is composed of metal nitrates of Mg(NO3)2·6H2O, Al(NO3)3·9H2O and Mn(NO3)2·4H2O with disparate metal molar ratio displayed in Table 1. The mixed basic solution includes 2 M NaOH and 1 M Na2CO3. The two solutions were simultaneously added dropwise into deionized water at constant pH = 10.0 under mechanical stirring. Then, the CTAB (with a 0.1:1 ratio of CTAB to metal ions) solution was added into the mixed solution. The achieved precipitate was suspended and stirred at 60 °C for 18 h. After filtration, it was thoroughly washed with anhydrous ethanol and deionized water until the pH of the filtrate was around 7. The filtered precipitates were dried at 80 °C for 12 h to receive hydrotalcites precursors. In the end, the as-prepared hydrotalcites were calcined at 600 °C in air for 6 h to obtain the mixed oxide catalysts, marked as Mn0, Mn5, Mn10, Mn15 and Mn20, respectively.

3.2. Catalysts Characterization

The X-ray diffraction (XRD) patterns were measured on a BRUKER-AXS D8Adance X-ray Diffractometer (Bruker, Berlin, Germany) operating at 40 kV and 30 mA using Cu Ka radiation (λ = 1.5418 Å). The scanning angle (2θ) range was 5–90° with a step size of 0.02° and the scanning speed was 6°/min.
The scanning electron microscopy (SEM) images were observed on a Quanta FEG250 instrument (FEI, Houston, TX, USA) with an accelerating voltage of 10 kV. Prior to each test, catalysts were sprayed with gold for better conductivity.
N2 adsorption/desorption experiments were conducted on a Micromeritics ASAP 2460 surface area analyzer (Micromeritics Instruments, Norcross, GA, USA). The samples were outgassed at 300 °C under vacuum for 6 h before analysis. The specific surface areas of the catalysts were determined according to the BET equation. The desorption curves of the catalyst were analyzed by BJH (Barrett-Joyner-Halenda) method to obtain the mean pore size and pore size distribution.
X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Scientific Escalab 250Xi X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with Al Kα X-ray radiation source. The measured binding energies were adjusted using C1s peak (BE = 284.6 eV) as standard.
H2 temperature-programmed reduction (H2-TPR) measurements were conducted on a quartz reactor. The H2 signal was detected by a thermal conductivity detector (TCD). A 50 mg catalyst (40–80 mesh) was preprocessed at 500 °C for 30 min in a flow of N2 (50 mL/min) and then cooled to room temperature. In the end, the reactor was heated in a flow of 5% H2/N2 (50 mL/min) at a rate of 10 °C/min from room temperature to 900 °C. CuO was used as a calibration reference to quantify H2 consumption.

3.3. NOx Storage and Desorption Measurements

The isothermal NOx storage experiments were implemented in a quartz flow reactor using 50 mg of the catalysts (40–80 mesh). Samples were pretreated at 500 °C for 1 h in N2 and then cooled to the experimental temperature. When the temperature stabilized at the experimental temperature, the flow gas of 1000 ppm NO + 5 vol.% O2 in He at a rate of 100 mL/min was introduced for 1 h for NOx adsorption. Concentrations of NO, NO2 and NOx from the reactor outlet was detected by the chemiluminiscence NOx analyzer (Model 42i-HL, Thermo Fisher Scientific, Waltham, MA, USA.). The NOx storage efficiency (NSE) was calculated by integrating the concentration curves of NOx according to the following Equation (1).
N S E = ( 1 − ∫ 0 t N O x , o u t d t ∫ 0 t N O x , i n d t ) × 100 %
NOx,in is the NOx concentration at the reactor inlet (ppm), NOx,out is the NOx concentration at the reactor outlet (ppm), t is the storage time (s).
After the NOx adsorption, 100 mL/min N2 was introduced to purge the catalysts for 30 min until NOx was not detected. The temperature programmed desorption (TPD) reactions were implemented by heating the catalysts from 100 °C to 700 °C at a heating rate of 10 °C /min in 100 mL/min N2. Concentrations of NO, NO2 and NOx at the reactor outlet were detected. In order to study the influence of CO2, H2O and SO2 on NOx storage, 1200 ppm CO2, 5% water vapor or 50 ppm SO2 were introduced to the input gases. Besides, the influence of soot was also studied. The model soot used in the present study was Printex-U provided by Degussa company. Its average particle size is 25 nm and the specific surface area is 93.5 m2/g. Elemental analysis showed that its carbonaceous nature was 90.43 wt% C, 1.09 wt% H, 0.17 wt% N and 0.51 wt% S. The volatile matter was detected to to be about 5 wt% and desorbed at about 200 °C (by thermogravimetric analysis). The soot-catalyst mixture was mixed in a 1/20 weight ratio and grinded for “loose contact” condition. The NOx storage capacity (NSC) of the catalyst was estimated by integrating the concentration curves of desorbed NOx according to the following Equation (2).
N S C = ( ∫ 0 t N O x , o u t d t ) × F 22.4 × R T × m
NOx,in is the NOx concentration at the reactor inlet (ppm), NOx,out is the NOx concentration at the reactor outlet (ppm), t is the desorption time (s). F is the total flow rate of the inlet gas (mL/min; STP = 0 °C, 1 atm), RT is the heating rate (°C/min), m is the amount of the catalyst (g).
The lean-rich cycling performance of samples were investigated using a quartz flow reactor under cycling lean (120 s, 500 ppm NO and 7.5 vol.% O2 in He) and rich (60 s, 7.5 vol.% H2 in N2) conditions. The overall flow rate of gas was 50 mL/min. The catalyst (50 mg) was pretreated with N2 at 500 °C for 1 h before the tests. The NOx removal performance was monitored as above for about 15 cycles.

3.4. In Situ DRIFTS Experiments

The in situ DRIFTS experiments of NOx storage were conducted on a Thermal Nicolet iS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a high sensitivity of MCT detector and a temperature-controllable diffuse reflection chamber. The catalysts were pretreated at 500 °C in a flow of N2 (100 mL/min) for 30 min so as to eliminate weakly adsorbed species and then cooled to 100 °C. Simultaneously, the spectra of the catalysts in the flow of N2 served as the background was recorded. After the pretreatment, 1000 ppm NO and 5 vol.% O2 in He was introduced at the experimental temperature (the overall flow rate = 100 mL/min), and then the chronological DRIFTS spectra were recorded.

4. Conclusions

The performance of Mn-doped hydrotalcites derived oxides (Mn/MgAlOx) were investigated for low temperature NOx storage and reduction in this paper. The incorporation of Mn into hydrotalcites changed the crystal phases, structural, redox and surface properties of the oxide catalysts, with Mn3O4 and MgMn2O4 spinel as the main crystalline phase besides brucite MgO. Mn doping resulted in an evident increment in catalysts surface area and pore size, and the redox behavior was also improved. The XPS results indicated that Mn15 and Mn20 catalysts possessed more Mn3+, which played a favorable role in NO oxidation. Mn doped MgAlOx catalysts exhibited high NOx storage capacity at low temperatures (150–300 °C). It is related to its greater surface area, improved reducibility and higher surface Mn3+ content. The largest NSC measured, 426 μmol/g, was observed for NOx adsorption at 200 °C on the sample containing 15 wt% of Mn (Mn15). NOx was stored in the form of nitrites on oxides at lower temperature (<300 °C), while nitrates at higher temperatures. The Mn-doped oxides catalysts are preferred for low temperature NOx storage and release due to its ability to store NOx as thermally labile nitrites. During the lean-rich cycling tests, the average NOx removal rate can reach above 70% after Mn doping. Mn15 catalyst displayed high resistance to CO2 and soot, with a certain sulfur and water resistance. In short, these Mn-doped hydrotalcite catalysts exhibit excellent performance in low temperatures NOx storage and release and can be used as alternative NSR catalysts.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/8/677/s1, Figure S1: NOx adsorption and desorption profiles of the samples: (a)Mn0, (b)Mn5, (c)Mn10, (d) Mn15, (e) Mn20 catalysts at 300 °C. (Reaction conditions: 1000 ppm NO, 5 vol.% O2, balanced with He, 100 mL/min; STP = 0 °C, 1 atm), Figure S2: NOx adsorption and desorption profiles of Mn15 catalyst at different temperatures (a)150 °C, (b)200 °C, (c)250 °C, (d)300 °C, (e)350 °C (Reaction conditions: 1000 ppm NO, 5 vol.% O2, balanced with He, 100 mL/min; STP = 0 °C, 1 atm), Figure S3: lean- rich cycling performance of Mn15 sample at different temperatures (a)150 °C, (b)200 °C, (c)250 °C, (d)300 °C, (e)350 °C (lean condition: 500 ppm NO, 7.5 vol.% O2, balanced with He, 50 mL/min; rich condition: 5 vol.% H2, balanced with N2, 50 mL/min; STP = 0 °C, 1 atm), Table S1: Summary of catalysts NOx storage capacity and reaction conditions.

Author Contributions

Conceptualization, C.C. and Z.W.; methodology, J.M. and W.L. (Wei Liu); software, W.L. (Wenxu Liu); validation, Z.W., J.M. and L.W.; formal analysis, C.C., J.M. and W.L. (Wei Liu); investigation, C.C.; resources, L.W.; data curation, C.C.; writing—original draft preparation, C.C.; writing—review and editing, Z.W.; visualization, C.C.; supervision, Z.W. and L.W.; project administration, Z.W.; funding acquisition, Z.W. and L.W.

Funding

This research was funded by the National Natural Science Foundation of China (No. 21777055), Shandong Provincial Natural Science Foundation (No.ZR2017BB004), Shandong Province Key Research and Development Plan (No. 2017GGX202004, 2019GSF109116), and Shandong Province Major Science and Technology Innovation Project (No. 2017CXGC1004). The APC was funded by the National Natural Science Foundation of China (No. 21777055).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 09 00677 g001 550
Figure 1. X-ray diffraction (XRD) patterns of hydrotalcite precursors (a) and calcined samples (b).
Figure 1. X-ray diffraction (XRD) patterns of hydrotalcite precursors (a) and calcined samples (b).
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Catalysts 09 00677 g002a 550Catalysts 09 00677 g002b 550
Figure 2. SEM images of Mn0 (a,b), Mn5 (c,d) and Mn15 (e,f).
Figure 2. SEM images of Mn0 (a,b), Mn5 (c,d) and Mn15 (e,f).
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Catalysts 09 00677 g003a 550Catalysts 09 00677 g003b 550
Figure 3. N2 adsorption/desorption isotherms of Mn-doped MgAlOx mixed oxides. (a) Mn0; (b) Mn5; (c) Mn10; (d) Mn15; (e) Mn20.
Figure 3. N2 adsorption/desorption isotherms of Mn-doped MgAlOx mixed oxides. (a) Mn0; (b) Mn5; (c) Mn10; (d) Mn15; (e) Mn20.
Catalysts 09 00677 g003aCatalysts 09 00677 g003b
Catalysts 09 00677 g004 550
Figure 4. X-ray photoelectron spectroscopy (XPS) spectra for Mn2p (a) and O1s (b) of the catalysts.
Figure 4. X-ray photoelectron spectroscopy (XPS) spectra for Mn2p (a) and O1s (b) of the catalysts.
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Catalysts 09 00677 g005 550
Figure 5. H2-TPR profiles of the samples.
Figure 5. H2-TPR profiles of the samples.
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Figure 6. NOx storage capacity of the catalysts at 300 °C.
Figure 6. NOx storage capacity of the catalysts at 300 °C.
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Figure 7. NOx storage capacity of Mn15 sample at different temperatures.
Figure 7. NOx storage capacity of Mn15 sample at different temperatures.
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Catalysts 09 00677 g008 550
Figure 8. In situ DRIFTS spectra of NOx adsorption at 300 °C on Mn0 (a), Mn5 (b) and Mn15 (c).
Figure 8. In situ DRIFTS spectra of NOx adsorption at 300 °C on Mn0 (a), Mn5 (b) and Mn15 (c).
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Catalysts 09 00677 g009a 550Catalysts 09 00677 g009b 550
Figure 9. In situ DRIFTS spectra of NOx adsorption on Mn15 sample at 150 °C (a), 200 °C (b), 250 °C (c), 300 °C (d), 350 °C (e).
Figure 9. In situ DRIFTS spectra of NOx adsorption on Mn15 sample at 150 °C (a), 200 °C (b), 250 °C (c), 300 °C (d), 350 °C (e).
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Figure 10. Average NOx removal rate of Mn15 sample at different temperatures during lean-rich cycling experiments.
Figure 10. Average NOx removal rate of Mn15 sample at different temperatures during lean-rich cycling experiments.
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Figure 11. The influence of CO2, soot, H2O and SO2 on the NOx storage capacity at 200 °C.
Figure 11. The influence of CO2, soot, H2O and SO2 on the NOx storage capacity at 200 °C.
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Table
Table 1. Chemical compositions and lattice parameters of the synthesized hydrotalcites.
Table 1. Chemical compositions and lattice parameters of the synthesized hydrotalcites.
SampleMetal Molar Ratio in Mixed Salt Solution2θ (°) aLattice ParameterXs (nm) b
a (Å)c (Å)
Mn0-HTMg:Al = 75:2511.323.10123.43510.5
Mn5-HTMn:Mg:Al = 5:70:2511.353.09923.4319.8
Mn10-HTMn:Mg:Al = 10:65:2511.383.09923.43018.8
Mn15-HTMn:Mg:Al = 15:60:2511.513.09823.42914.4
Mn20-HTMn:Mg:Al = 20:55:2511.573.10023.43110.1
a 003 crystal face. b Mean crystallite size calculated from the 003 and 110 crystal face according to Debye-Scherrer equation.
Table
Table 2. Textural properties of the oxide catalysts.
Table 2. Textural properties of the oxide catalysts.
SampleSBET a (m2·g−1)VP b (cm3·g−1)DP c (nm)
Mn01080.6716.9
Mn51440.6814.3
Mn101360.5516.0
Mn151330.8322.6
Mn201390.8219.6
a The BET specific surface area. b Total pore volume. c Average pore diameter.
Table
Table 3. X-ray photoelectron spectroscopy (XPS) and H2-TPR results of samples.
Table 3. X-ray photoelectron spectroscopy (XPS) and H2-TPR results of samples.
SampleMn3+/(Mn3+ + Mn4+) (%)Oads/(Olatt + Oads) (%)H2 Consumption a (mmol·g−1)Mn3+% b
Mn0-820-
Mn547656.5169
Mn10515710.3276
Mn15676534.8682
Mn20626421.3478
a The total H2 consumption of the reduction peaks from 100 to 800 °C. b Relative Mn3+ content estimated from integrated reduction peak areas.
Table
Table 4. NOx storage efficiency of the catalysts at 300 °C.
Table 4. NOx storage efficiency of the catalysts at 300 °C.
Time (min)NOx Storage Efficiency (%)
Mn0Mn5Mn10Mn15Mn20
1100100100100100
562.869.279.987.291.3
1039.546.455.160.763.4
3018.923.126.028.428.9
6012.916.117.319.219.4
Table
Table 5. NOx storage efficiency of Mn15 sample at different temperatures.
Table 5. NOx storage efficiency of Mn15 sample at different temperatures.
Time (min)NOx Storage Efficiency (%)
150 °C200 °C250 °C300 °C350 °C
1100100100100100
596.188.380.587.293.4
1069.157.552.360.764.2
3032.524.224.328.430.3
6022.914.215.019.221.3
Table
Table 6. Characteristic vibrations of adsorbed NOx species on Mn0, Mn5, Mn15 catalysts.
Table 6. Characteristic vibrations of adsorbed NOx species on Mn0, Mn5, Mn15 catalysts.
NOx SpeciesStructureIR Bands/cm−1
Mn0Mn5Mn15
Bridged bidentate nitrite Catalysts 09 00677 i00112301226
Bridged bidentate nitrate Catalysts 09 00677 i00213101310
1480
Monodenate nitrate Catalysts 09 00677 i003 1312
1447
Chelating bidentate nitrate Catalysts 09 00677 i004 1503
Table
Table 7. Characteristic vibrations of adsorbed NOx species on Mn15 catalyst.
Table 7. Characteristic vibrations of adsorbed NOx species on Mn15 catalyst.
NOx SpeciesStructureIR Bands/cm−1
150 °C200 °C250 °C300 °C350 °C
Bridged bidentate nitrite Catalysts 09 00677 i0011220123012371237
Bridged bidentate nitrate Catalysts 09 00677 i00213211300131213121315
Monodentate nitrate Catalysts 09 00677 i003 144514451438
Chelating bidentate nitrate Catalysts 09 00677 i004 1503

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MDPI and ACS Style

Cui, C.; Ma, J.; Wang, Z.; Liu, W.; Liu, W.; Wang, L. High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release. Catalysts 2019, 9, 677. https://doi.org/10.3390/catal9080677

AMA Style

Cui C, Ma J, Wang Z, Liu W, Liu W, Wang L. High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release. Catalysts. 2019; 9(8):677. https://doi.org/10.3390/catal9080677

Chicago/Turabian Style

Cui, Chenchen, Junwei Ma, Zhongpeng Wang, Wei Liu, Wenxu Liu, and Liguo Wang. 2019. "High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release" Catalysts 9, no. 8: 677. https://doi.org/10.3390/catal9080677

APA Style

Cui, C., Ma, J., Wang, Z., Liu, W., Liu, W., & Wang, L. (2019). High Performance of Mn-Doped MgAlOx Mixed Oxides for Low Temperature NOx Storage and Release. Catalysts, 9(8), 677. https://doi.org/10.3390/catal9080677

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