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Article

Effect of Tourmaline Addition on the Anti-Poisoning Performance of MnCeOx@TiO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx

by
Zhenzhen Zhao
1,†,
Liyin Wang
2,†,
Xiangqing Lin
1,
Gang Xue
2,*,
Hui Hu
3,
Haibin Ma
4,
Ziyu Wang
1,
Xiaofang Su
3 and
Yanan Gao
3,*
1
School of Advanced Agricultural Science, Weifang University, Weifang 261061, China
2
Institute of Power Source and Ecomaterials Science, Hebei University of Technology, Tianjin 300130, China
3
Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, Hainan University, Haikou 570228, China
4
School of Chemistry, Chemical & Environmental Engineering, Weifang University, Weifang 261061, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Molecules 2024, 29(17), 4079; https://doi.org/10.3390/molecules29174079
Submission received: 3 August 2024 / Revised: 15 August 2024 / Accepted: 17 August 2024 / Published: 28 August 2024
(This article belongs to the Special Issue Chemical Technologies for Environmental Analysis and Pollution Removal, 2nd Edition)
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Abstract

:
In view of the flue gas characteristics of cement kilns in China, the development of low-temperature denitrification catalysts with excellent anti-poisoning performance has important theoretical and practical significance. In this work, a series of MnCeOx@TiO2 and tourmaline-containing MnCeOx@TiO2-T catalysts was prepared using a chemical pre-deposition method. It was found that the MnCeOx@TiO2-T2 catalyst (containing 2% tourmaline) exhibited the best low-temperature NH3-selective catalytic reduction (NH3-SCR) performance, yielding 100% NOx conversion at 110 °C and above. When 100–300 ppm SO2 and 10 vol.% H2O were introduced to the reaction, the NOx conversion of the MnCeOx@TiO2-T2 catalyst was still higher than 90% at 170 °C, indicating good anti-poisoning performance. The addition of appropriate amounts of tourmaline can not only preferably expose the active {001} facets of TiO2 but also introduce the acidic SiO2 and Al2O3 components and increase the content of Mn4+ and Oα on the surface of the catalyst, all of which contribute to the enhancement of reaction activity of NH3-SCR and anti-poisoning performance. However, excess amounts of tourmaline led to the formation of dense surface of catalysts that suppressed the exposure of catalytic active sites, giving rise to the decrease in catalytic activity and anti-poisoning capability. Through an in situ DRIFTS study, it was found that the addition of appropriate amounts of tourmaline increased the number of Brønsted acid sites on the catalyst surface, which suppressed the adsorption of SO2 and thus inhibited the deposition of NH4HSO4 and (NH4)2HSO4 on the surface of the catalyst, thereby improving the NH3-SCR performance and anti-poisoning ability of the catalyst.

1. Introduction

With the development of human society, the composition of the atmosphere is being changed, which has led to serious environmental problems [1,2,3]. Nitrogen oxides are one of the main pollutants in the prevention and control of atmospheric environmental pollution. The emission of NOx in the cement industry has become the third-largest source of pollution after thermal power plants and motor vehicle exhaust. Therefore, there is an urgent need to research and develop efficient strategies to reduce NOx emissions. Compared to the power industry, the flue gas of cement kilns in China is characterized by high oxygen content, high humidity, and low temperature window (80–200 °C). Ammonia-selective catalytic reduction of NOx (NH3-SCR) is considered to be an effective strategy for reducing NOx emissions [4]. Composite metal oxide catalysts such as manganese-based and cerium-based oxides exhibit good denitrification capability in the low-temperature section, but their anti-poisoning (mainly refereeing to SO2 and water vapor) performance is found to be poor [5,6,7]. The deposition of (NH4)2SO4 and NH4HSO4 on the surface of the catalyst blocks the exposure of active sites and water vapor can lead to partially destructing acid sites to decrease the number of available active sites. Therefore, research and exploitation of NH3-SCR catalysts with high activity and selectivity, and good anti-poisoning capability as well as broad range of operating temperature have become the focus of current research in the field.
To date, various strategies have been attempted to enhance the catalytic activity and the anti-poisoning performance of NH3-SCR catalysts. Metal oxides, such as V2O5, Fe3O4, and MnOx and bimetallic compounds, such as MnOx/TiO2 [8], CrOx/TiO2 [9], and CuO/MnOx [10] Zr/Ce phosphates [11], have been found to be highly active and selective catalysts in NH3-SCR. For instance, Tang et al. reported that nearly 100% N2 selectivity was obtained on MnOx-CeO2 catalyst at 100–150 °C [12], but the selectivity was relatively low on MnOx in the same temperature range [13,14]. It has been reported that the strong interaction between CeO2 and SO2 promoted the formation of CeSO4 or Ce2(SO4)3. Thus, using CeO2 as a sacrificial site is effective to reduce the sulfation of the main active phase [15]. Han et al. [16] found that the mesoporous structure and the presence of metal oxides in the catalyst can greatly promote the decomposition of ammonium sulfate deposited on the surface of the catalyst during the SCR reaction. Yu et al. [17] found that the mesoporous TiO2 shell of the TiO2@Fe2O3 monolithic catalyst promoted the decomposition of NH4HSO4. Han et al. [18] successfully coated mesoporous TiO2 layers on carbon-nanotube-supported MnOx and CeOx nanoparticles to obtain a core–shell catalyst. The mesoporous TiO2 sheaths enhanced the acid strength and quantity, thus leading to higher activity and a wider operating temperature window. Additionally, the strong interaction between MnOx, CeOx and the mesoporous TiO2 can inhibit the aggregation of MnOx and CeOx nanoparticles, affording the catalyst with good stability.
Tourmaline shows several unique properties, including far-infrared radiation, spontaneous polarity, and the release of negative ions. Thanks to these properties, tourmaline has been widely used as a function material in the field of catalysis [19,20,21,22]. For instance, Hu et al. [23] found that the presence of electric field and far-infrared radiation of tourmaline can promote the synthesis of Pd nanoparticles with smaller size, weaken the Pd-O bond, and increase molecular vibration and migration rate, thus improving the electrocatalytic performance of Pd for formic acid electrooxidation. Luo et al. [24] used a co-precipitation method to prepare MnOx/TiO2 catalysts with addition of tourmaline. When the tourmaline content was 10%, the conversion of NOx at 200 °C exceeds 97%, which was about 7% higher than that of the undoped counterpart. Due to the permanent electrode of tourmaline, it was reported that tourmaline can effectively promote the dispersion of MnOx/TiO2 catalyst, increasing the number of acidic sites and changed the valence distribution of manganese ions in the product, thereby accelerating the diffusion of manganese ions and leading to accelerated redox reactions. The far-infrared radiation and spontaneous polarity of tourmaline can generate electric fields, which tends to weaken or destroy the hydrogen bonds of water. During the reaction, many hydrogen bonds will be broken and rearranged. Therefore, the water clusters will become smaller, reducing the clustering effect of water molecules caused by hydrogen bonds [25]. Despite these achievements, it remains a great challenge to develop low-temperature SCR catalysts with high catalytic activity SO2/H2O tolerance and stability to eliminate NOx.
Based on the above, we herein prepared a MnOx/CeO2 bimetallic oxide catalyst and dopped mesoporous TiO2 on the surface of the MnOx/CeO2 with the addition of different amounts of tourmaline. The NOx conversion and anti-poisoning performance of the obtained MnCeOx@TiO2 and MnCeOx@TiO2-T were examined. Among all catalysts, MnCeOx@TiO2-T2 exhibited the best catalytic performance. It was found that the addition of appropriate amounts of tourmaline enhanced the catalytic activity and increased the number of Brønsted acid sites on the catalyst surface, and inhibited the deposition of ammonium sulfate, thus improving the SCR reaction and anti-poisoning ability of the catalyst.

2. Results and Discussion

2.1. NOx Reduction and SO2 Tolerance

Figure 1a shows the effect of tourmaline addition on the denitration performance of MnCeOx@TiO2 catalyst. It can be seen that after adding tourmaline, the MnCeOx@TiO2-T2 catalyst exhibited the best denitration performance compared to other catalysts at a temperature ranging from 80 to 110 °C. The MnCeOx@TiO2-T2 catalyst showed a 75% NOx conversion at 80 °C, which is higher than that of the MnCeOx@TiO2 catalyst (65%) at the same temperature. A 100% NOx conversion was achieved by MnCeOx@TiO2-T1 and MnCeOx@TiO2-T2 in the temperature range of 110~200 °C. However, when the tourmaline content is 3% and 4%, the NOx conversion of MnCeOx@TiO2-T3 and MnCeOx@TiO2-T4 is lower than that of MnCeOx@TiO2-T2 at temperature less than 110 °C, indicating that the amount of added tourmaline affected the denitration activity of the catalyst.
Considering that MnCeOx@TiO2-T2 exhibited the best catalytic activity, the catalyst was mainly investigated in the following research. The anti-poisoning performance of MnCeOx@TiO2 and MnCeOx@TiO2-T2 was examined at 170 °C, and the results are shown in Figure 1b. For MnCeOx@TiO2, when 100 ppm SO2 was introduced, the NOx conversion decreased from 100% to 93% and then remained stable for 12 hours. As the SO2 concentration increased to 200 ppm, the NOx conversion decreased to 85% and remained stable for 12 h. As the SO2 concentration further increased to 300 ppm, the NOx conversion dropped to 78%, and remained stable for another 12 h. After 10 vol.% H2O was introduced, the NOx conversion further dropped to 70%. In contrast, after 100–200 ppm of SO2 was introduced into the feed, the NOx conversion of MnCeOx@TiO2-T2 catalyst was still 100% and remained unchanged and stable for 24 h. As the SO2 concentration increased to 300 ppm, the NOx conversion of MnCeOx@TiO2-T2 decreased slightly from 100% to 95%. After 10 vol.% H2O was further introduced, the NOx conversion decreased to about 90%. The decrease in NOx conversion can be attributed to the sulfation of metal oxides and the deposition of ammonium sulfate. After cutting off H2O and SO2, the NOx conversion of MnCeOx@TiO2 and MnCeOx@TiO2-T2 rebounded to 72% and 93%, respectively, and remained stable for 12 h. These results show that the presence of sulfur species on the surface of catalysts seriously affected their catalyst performance, and the poisoning of the catalysts was substantially irreversible. Moreover, it can also be seen that the MnCeOx@TiO2-T2 catalyst exhibited better anti-poisoning performance than the MnCeOx@TiO2 catalyst. A comparison with the previous reports was listed in Table S1.

2.2. Research on Crystal Structure and Morphology

The crystal structures of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts before and after poisoning are shown in Figure 2. It can be seen that both catalysts exhibited characteristic peaks of anatase TiO2 (PDF#21-1272) [26] and CeO2 (PDF#43-1002) [27]. The peaks at 25.3, 37.8, and 48.1° are due to the (101), (004), and (200) facets of TiO2, respectively, and the peak at 28.3° can be attributed to the (111) facet of CeO2. No characteristic peaks of MnOx were observed in either powder X-ray diffraction (PXRD) pattern, suggesting that amorphous MnOx was highly dispersed within the catalysts, which is favorable for improving their catalytic performance [28,29]. After the addition of 2% tourmaline, the intensity ratio of I(004)/I(200) of TiO2 in MnCeOx@TiO2-T2 pattern was slower than that in MnCeOx@TiO2 because MnCeOx@TiO2-T2 demonstrated a decrease in the (004) diffraction intensity and an increasement in the (200) intensity compared with that of MnCeOx@TiO2, which showed that MnCeOx@TiO2-T2 possesses more exposed {001} facets [30], which can increase the concentration of surface adsorbed oxygen of the catalyst and increase the surface acidity, giving rise to better NOx conversion [31]. After the SO2 poisoning, the CeO2 diffraction peaks of MnCeOx@TiO2 catalyst were weakened. The decreased diffraction peak intensity of CeO2 could be due to the formation of CeSO4. As mentioned above, the CeO2 acts as a sacrificial site that can reduce the sulfation of the main active phase [15]. In contrast, the diffraction peaks of MnCeOx@TiO2-T2 catalyst were not obviously changed, suggesting that the addition of an appropriate amount of tourmaline can suppress the adsorption of SO2, thereby demonstrating the outstanding sulfur resistance of the catalyst [32].
The scanning electron microscopy (SEM) images of MnCeOx@TiO2 with different amounts of tourmaline are shown in Figure 3. It can be seen that as the added amount of tourmaline increases, the pores on the surface of spherical catalysts disappear gradually and the MnCeOx@TiO2 particles become more and more dense, which is consistent with the PXRD results that showed enhanced crystallinity of both TiO2 and CeO2 (Figure S1). This is actually unfavorable for the NH3-SCR. Since the spontaneous polarization characteristics of tourmaline would change the crystal growth during the catalyst preparation process, the morphology of the catalyst was changed with the addition of different amounts of tourmaline, which further affects the denitration performance of the catalyst. The catalytic experiments showed that when 3% or 4% tourmaline was added, the catalytic performance of MnCeOx@TiO2-T3 and MnCeOx@TiO2-T4 was decreased, which may be attributed to the dense surface of the catalyst that blocks the exposure of active sites and suppresses the mass transfer and diffusion of substance.
Figure 4a,b show the energy-dispersive spectrometer (EDS) element mapping of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts. It is evident that that the particle size is about 1.5 um, and TiO2 crystals are uniformly dopped on the spherical particle surface. In addition, the Mn, Ce, and O elements were also uniformly dispersed within the catalyst. Additionally, the uniform distribution of Al and Si elements was found on the MnCeOx@TiO2-T2 catalyst, which confirms the existence of tourmaline in the catalyst (the component of tourmaline is listed in Table S2). The SiO2 component of tourmaline can decrease the thermal stability of NH4HSO4 and thus promote its decomposition on the surface of catalyst [33]. Additionally, both SiO2 and Al2O3 components of tourmaline can enhance the acidity of the catalyst, weakening the adsorption of SO2 and thus improving the SO2 poisoning resistance of the catalyst [34]. Figure 4c,d showed the transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) image of MnCeOx@TiO2-T2 catalyst, respectively. The different lattice fringes were measured to be 0.312 nm and 0.352 nm, which are well matched with the (111) crystal plane of CeO2 [35] and the (101) crystal plane of TiO2 [36], respectively.

2.3. Specific Surface Area and Surface Element Analysis

In order to explore the effect of tourmaline addition on the specific surface area and pore of the catalyst, the N2 adsorption–desorption measurement was performed on the MnCeOx@TiO2 and MnCeOx@TiO2-T catalysts (Figure 5 and Table 1). All catalysts exhibited type-IV adsorption isotherms but with different types of hysteresis loops. The MnCeOx@TiO2, MnCeOx@TiO2-T1, and MnCeOx@TiO2-T2 catalysts correspond to the H3 type hysteresis loops [37], while the MnCeOx@TiO2-T3 and MnCeOx@TiO2-T4 catalysts follow the H4 type of hysteresis loops, both of which reflect the irregular porous structure of the catalysts. Figure 5b shows the pore size distributions of various catalysts. The pore diameters of the five catalysts are in the range of 6–13 nm, indicating that the catalysts are all mesoporous. It can be seen from Table 1 that with the increase in tourmaline content, the specific surface area of the catalyst decreases from 84.00 to 42.15 m2 g–1 gradually, which accords with the SEM result that the catalyst particles become more and more dense. However, this result is different from the reports published previously [23,24,25]. It is accepted that the micro-electric field of tourmaline would decrease the particle size of catalysts, presenting their agglomeration and thus slightly increasing the specific surface area of catalysts. In our case, this unusual phenomenon could be attributed to the blocking of the pores of MnCeOx@TiO2 by the impregnated species. Although the specific surface area of the catalyst was decreased, the catalytic activity of MnCeOx@TiO2-T2 was relatively higher. This indicates that the addition of appropriate amounts of tourmaline changed the crystal growth and preferentially exposed the active {001} facets of TiO2, which enhanced the catalytic activity. Additionally, the addition of tourmaline introduced the acidic SiO2 and Al2O3 components into the catalyst, which can improve SO2 poisoning resistance. We consider that these advantageous factors contribute to the improvement of the catalytic performance.
Figure 6 shows the X-ray photoelectron spectroscopy (XPS) spectra of Mn 2p, Ce 3d, O 1s, and S 2p of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts before and after catalyst poisoning. Table 2 summarizes the surface atomic concentration and relative content of Mn, Ce, O and S determined by XPS spectra. As shown in Figure 6a, the Mn 2p region is composed of spin–orbit doublets. The binding energy of Mn 2p1/2 appears about 653.7 eV, and the binding energy of Mn 2p3/2 corresponds to Mn2+ (641.3–641.5 eV), mixed valence states of Mn3+ (642.3–642.8 eV), and Mn4+ (644.6–643.6 eV) [38]. The relative concentration of Mn4+ on the surface of different tourmaline-added catalysts is different. The highest Mn4+ content was observed for MnCeOx@TiO2-T2. The redox performance of SCR is related to the surface concentration of Mn4+. The higher the relative concentration of Mn4+, the more favorable the low-temperature SCR reaction [39]. This may be another reason that MnCeOx@TiO2-T2 exhibited the best catalytic activity. Meanwhile, it can be found from Figure 6a that the binding energy of Mn in MnCeOx@TiO2-S catalyst is higher than that of the fresh catalyst, while the binding energy of Mn in MnCeOx@TiO2-T2-S catalyst does not change significantly compared with the fresh catalyst. This result reveals that the Mn atoms on the surface of MnCeOx@TiO2 bonded with electronegative S in addition to elemental O. As shown in Table 2, we can see that when the MnCeOx@TiO2 was poisoned by SO2, the content of Mn4+ on the surface of the catalyst decreased from 14.92% to 12.22%, while under SO2 atmosphere, the Mn4+ content of MnCeOx@TiO2-T2 decreased from 16.93% to 15.42%, a slighter decrease compared to MnCeOx@TiO2, indicating that the MnCeOx@TiO2 bonded with more SO2 than MnCeOx@TiO2-T2. That is, MnCeOx@TiO2-T2 has better SO2-tolerance performance.
The XPS spectra of Ce 3d in catalysts can be fitted into eight peaks, as shown in Figure 6b. Ce3+ is shown by the subbands labeled u′ and v′, and Ce4+ is shown by the subbands labeled u, u″, u‴, v, v″, and v‴ [40]. The slight shift in the Ce 3d spectra to higher binding energies was observed in MnCeOx@TiO2 when the catalyst was introduced to SO2, suggesting that the density of electron clouds of Ce in MnCeOx@TiO2 was decreased due to oxidation by O2. However, no obvious change in binding energies of Ce 3d was found for MnCeOx@TiO2-T2, revealing that MnCeOx@TiO2-T2 had outstanding SO2-tolerance performance [41]. Figure 6c shows the O 1s spectra of MnCeOx@TiO2 and MnCeOx@TiO2-T2 before and after SO2 poisoning. The O 1s spectra can be divided into two peaks. The band at high binding energy (531.5 eV) is attributed to chemically adsorbed oxygen, denoted by Oα. The low binding energy band (529.8 eV) corresponds to lattice oxygen, denoted as Oβ [42]. NH3-SCR is a gas–solid reaction, and the surface-active oxygen plays a key role in catalyzing NH3-SCR reaction. Due to its higher mobility, the performance of chemically adsorbed oxygen in the surface oxidation reaction is better than that of lattice oxygen [43]. It can be seen in Table 2 that the relative content of Oα in the MnCeOx@TiO2-T2 catalyst is 26.16%, which is slightly higher than that of other catalysts, indicating that the introduction of a small amount of tourmaline into MnCeOx@TiO2 increased the Oα content. We can also see from Table 2 that the Oα content of the poisoned MnCeOx@TiO2-T2-S catalyst is slightly reduced to 25.98%, which is much higher than the Oα content (20.38%) of the MnCeOx@TiO2-S catalyst. It is well known that Oα species are beneficial to the oxidation of NO from NO to NO2, which promotes the NH3-SCR reaction through the “fast SCR” approach [44,45]. Therefore, it is proved that the addition of appropriate amount of tourmaline can significantly improve the catalytic activity and anti-poisoning performance of the catalyst. Figure 6d shows the S 2p spectra of MnCeOx@TiO2-S and MnCeOx@TiO2-T2-S. Two peaks at 169.8 eV and 168.5 eV were observed, which are attributed to HSO4− and SO42− [46,47], respectively, indicating the presence of sulfate and bisulfate on the catalyst surface, and the catalysts were sulfated to generate ammonium salt, manganese salt and cerium salt. Compared to the MnCeOx@TiO2-S catalyst, the content of HSO4− and SO42− on the surface of the MnCeOx@TiO2-T2-S catalyst is much less, indicating that the MnCeOx@TiO2-T2 catalyst is not easy to be sulfated and effectively inhibits the deposition of NH4HSO4 on the surface of the catalyst. As shown in Table 2, the relative concentration of Mn4+ and Ce3+ ions on the surface of the poisoned catalyst decreased, while the relative concentration of Mn2+ and Ce4+ ions increased and meanwhile, the binding energy of Mn2+ and Ce4+ increased, both of which indicate that MnSO4 and Ce(SO4)2 were formed, which led to the deactivation of catalysts [48,49,50].

2.4. Redox Capability

The H2-TPR diagram of the catalysts is shown in Figure 7. The reduction peaks of the catalysts are superimposed. The H2 consumption peak below 450 °C is attributed to the reduction of MnO2→Mn2O3 and Mn2O3→Mn3O4 and the reduction of surface cerium. At the same time, the peak at about 600 °C is the reduction of bulk cerium. The addition of appropriate amount of tourmaline (1%–3%) caused the H2 consumption peak of the catalyst to shift to low temperature (Figure 7a), indicating that the reduction temperature of H2 was low and the redox performance of catalysts was strong [49]. However, the addition of excess amount of tourmaline (4%) caused an increase in H2 reduction temperature (Figure 7a), which is in accordance with the catalytic performance result. It can be deduced that the addition of appropriate amounts of tourmaline promoted the enhancement of the oxidation ability of Mn species and active components on the catalyst surface and thus improved the low-temperature SCR reaction [50].
The hydrogen-temperature-programmed reduction (H2-TPR) diagram of the SO2 poisoned catalysts is shown in Figure 7b. The reduction peaks of MnCeOx@TiO2-S and MnCeOx@TiO2-T2-S catalysts appeared at 610 and 508 °C, respectively and are attributed to the reduction in MnSO4 [51]. The presence of MnSO4 confirmed the sulfation of active ingredient manganese oxide on the surface of the catalyst. However, compared to the MnCeOx@TiO2-S catalyst, the reduction peak temperature of the MnCeOx@TiO2-T2-S catalyst shifted to low temperature, indicating that the oxidation–reduction performance of the MnCeOx@TiO2-T2 catalyst was better than MnCeOx@TiO2 after SO2 poisoning. The results show that under high SO2 concentration, the reduction ability of the poisoned catalyst is weakened due to the decrease in the relative concentration of Mn4+, which is consistent with the XPS results.

2.5. Study on the Mechanism of Catalyst Anti-Poisoning Reaction

2.5.1. Adsorption of SO2 + O2 on the Catalyst Surface

In order to analyze the adsorption behavior of SO2 on the surface of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts, in situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFT) was carried out, as shown in Figure 8. We introduced 500 ppm SO2 + 5 vol.% O2 gas to the surfaces of the two catalysts at 170 °C. After the introduction of SO2 to MnCeOx@TiO2 for 5 min, several peaks were detected at 1246, 1145, and 1035 cm–1 (Figure 8a). These peaks can be attributed to the sulfate species formed on the catalyst surface [52,53]. It can be thus deduced that the MnCeOx@TiO2 catalyst was strongly sulfated, and the active sites of the SCR reaction were occupied by the sulfate species adsorbed on the catalyst surface, which will affect the adsorption and activation of NH3 and NOx species on the catalyst surface, inhibiting the SCR reaction progress. Figure 8b shows the in situ DRIFTS spectra of the MnCeOx@TiO2-T2 catalyst after adsorbing 500 ppm SO2 + 5 vol.% O2. Only 15 min after the introduction of SO2, the faint infrared peak attributed to the surface sulfate species (1167 cm−1) was detected. With continuous introduction of SO2 for 25 min, the peaks at 1246 and 1167 cm−1 attributable to the surface sulfate species gradually increased. The surface sulfate species of the MnCeOx@TiO2-T2 catalyst are much less than that of the MnCeOx@TiO2 catalyst, and the prolonged time of resistance to SO2 poisoning indicates that the addition of appropriate amount of tourmaline can effectively improve the anti-poisoning performance of the catalyst.

2.5.2. Adsorption of NH3 + SO2 on the Catalyst Surface

As shown in Figure 9a, when the MnCeOx@TiO2 catalyst was exposed to NH3 for 30 min, new adsorption bands appeared at 1615, 1453 and 1329 cm–1. The bands at 1615 and 1170 cm−1 are attributable to the coordinated NH3 at the Lewis acid site [54,55], and the band at 1453 cm−1 is attributable to the NH4+ symmetric bending vibration at the Brønsted acid site [56]. It can be found that after adding 100 ppm SO2 for 5 min, the adsorption peak intensity corresponding to the coordinated NH3 on the Lewis acid site and the NH4+ formed on the Brønsted acid site has increased, but due to the continuous introduction of SO2, after 10 min, the peak intensity at 1615 and 1453 cm−1 gradually weakened. However, two adsorption bands appeared at 1261 and 1225 cm−1 after the introduction of SO2, which was attributed to SO2 adsorption. The results show that SO2 and NH3 are competitively adsorbed on the surface of MnCeOx@TiO2 catalyst. The addition of SO2 occupied some adsorption sites of NH3 on the catalyst surface, thereby inhibiting the adsorption and activation of NH3.
Figure 9b shows the in situ DRIFTS spectra of the MnCeOx@TiO2-T2 catalyst upon exposure to the NH3 + SO2 atmosphere. Thirty minutes after the introduction of only NH3, several obvious adsorption bands were detected at 1657, 1609, 1458, 1402, 1359, and 1245 cm−1. The bands at 1609 and 1245 cm–1 are assigned to the coordinated NH3 on the Lewis acid site, and the band at 1359 cm–1 is attributed to the amino compound (-NH2) on the catalyst surface [57]. The peaks around 1657, 1458, and 1402 cm−1 are attributed to the symmetrical bending vibration of the coordinated NH4+ adsorbed on the Brønsted acid site by NH3. In the presence of 100 ppm SO2, the intensity of the adsorption zone is basically unchanged. The addition of SO2 caused the occupation of part of the catalytic adsorption sites on the surface of the MnCeOx@TiO2 catalyst by SO2, thereby inhibiting the adsorption and activation of NH3. In contrast, the intensity of the adsorption peak of NH3 species adsorbed on the MnCeOx@TiO2-T2 catalyst did not decrease. The results show that the NH3 adsorption capacity of the MnCeOx@TiO2-T2 catalyst is stronger than that of the MnCeOx@TiO2 catalyst. In this case, SO2 did not significantly inhibit the adsorption of NH3. This confirms that the addition of appropriate amounts of tourmaline effectively prevents the adsorption of SO2 on the surface of the catalyst. The added tourmaline can increase the number of Brønsted acid sites. The strong electronic interactions between HSO4− and Brønsted acid sites of mesoporous TiO2 damaged the bond between NH4+ and HSO4−, thus promoting the decomposition of NH4HSO4 and improving the catalytic activity and the anti-SO2 performance.

2.5.3. Adsorption of NO + O2 + SO2 on the Catalyst Surface

The effect of SO2 on the adsorption of NO + O2 on the MnCeOx@TiO2 catalyst was also studied by in situ DRIFTS, and the results are shown in Figure 10a. The peak at 1765 cm−1 is attributed to the N2O4 species [58], while the peak at 1416 cm−1 is the infrared peak of nitrate, and the peak at 1346 cm−1 is attributed to bidentate nitrate [59]. When the catalyst was exposed to 100 ppm SO2, the peaks at 1416 and 1346 cm−1 disappeared and a new peak appeared at 1365 cm−1, which can be attributed to the asymmetric tensile vibration of O=S=O in the sulfate component on the catalyst surface [60]. These results indicate that SO2 and NO have competitive adsorption on the catalyst surface and that the adsorption capacity of SO2 on the catalyst surface is significantly stronger than that of NO. The effect of SO2 on the adsorption of NO + O2 on the MnCeOx@TiO2-T2 catalyst is shown in Figure 10b. After the exposure to NO + O2 for 30 min, the peak at 1765 cm−1 is attributed to the N2O4 species [58], the peak at 1628 cm−1 is attributed to bridging nitrate [61], the peak at 1593 cm−1 is the infrared signal of nitrite [62], the peak at 1426 cm−1 is due to the linear nitrite species, and the peaks at 1381 and 1323 cm−1 are due to the infrared signals of bidentate nitrate [63]. There is also a competitive adsorption of NO and SO2 on the surface of the MnCeOx@TiO2-T2 catalyst. It should be pointed out that the bridging nitrate (1628 cm−1) and nitrite (1593 cm−1) peaks disappeared when SO2 was introduced for 20 min, while the bands of other nitrate species still exist stably. However, the peak of the nitrate species of MnCeOx@TiO2 catalyst disappeared after the introduction of SO2 for 5 min, suggesting that the adsorption capacity of MnCeOx@TiO2-T2 catalyst for NO is also stronger than that of the MnCeOx@TiO2 catalyst.

2.5.4. Adsorption of NH3 on the Catalyst Surface before and after Poisoning

The in situ DRIFTS spectra of NH3 adsorbed on the surface of MnCeOx@TiO2 and MnCeOx@TiO2-2 catalyst before and after poisoning for different time are shown in Figure 11. Compared to the non-poisoned catalyst, the symmetrical and asymmetrical stretching vibration peaks (3320, 3134, and 3025 cm−1) of the N-H bond in the coordination state of NH3 did not change significantly [64]. The peak of NH3 coordinated on the Lewis acid (1605 and 1245 cm−1) and the coordinated NH4+ peaks (1403 and 1453 cm−1) attributable to the Brønsted acid site of the poisoned MnCeOx@TiO2 catalyst were significantly weakened. The 1355 cm−1 peak attributable to the oxidized and deformed intermediate generated by the adsorption of NH3 on the Lewis acid site migrates to 1369 cm−1 [65], and a new weak peak appears at 1335 cm–1, which is attributed to the adsorption coordination of the NH3 peak on the Lewis acid. This is because the presence of SO2 and H2O will weaken the Brønsted acid sites on the MnCeOx@TiO2 catalyst, thereby weakening the adsorption of NH3.
The in situ DRIFTS spectra of the MnCeOx@TiO2-T2 catalyst before and after the poisoning of NH3 adsorption for different time are shown in Figure 11c,d. Compared tothe non-poisoned catalyst, the peak of NH3 coordinated on the Lewis acid (1605 and 1245 cm−1) of the poisoned MnCeOx@TiO2-T2 catalyst are significantly weakened. The peak at 1355 cm−1, which is attributed to the oxidation and deformation intermediates generated by the adsorption of NH3 on the Lewis acid site, shifts to 1369 cm–1. A new peak around 1205 cm−1 appeared, which is attributable to the peak of NH3 adsorbed on the Lewis acid, while the peak attributed to the Brønsted acid site has no obvious change, which means that in the presence of SO2 and H2O, the MnCeOx@TiO2-T2 catalyst inhibits the loss of Brønsted acid sites and effectively inhibits the bonding of NH4+ and HSO4−, and meanwhile promotes the decomposition of NH4HSO4, thus improving the anti-poisoning performance of the catalyst.

2.5.5. Adsorption of NO + O2 on the Catalyst Surface before and after Poisoning

The in situ DRIFTS spectra of NO + O2 adsorbed on the MnCeOx@TiO2 catalyst before and after poisoning are shown in Figure 12a,b. The peak at 1765 cm−1 is attributed to the N2O4 species, the peak at 1628 cm−1 is attributed to bridged nitrate, 1593 cm−1 is the infrared absorption peak of nitrite, 1416 cm−1 is the infrared peak of nitrate, and the peak at 1346 cm−1 is attributed to bidentate nitrate. Compared to the non-poisoned catalyst, the poisoned MnCeOx@TiO2 catalyst bridges the peaks of nitrate (1628 cm−1) and nitrite (1593 cm−1) only after 15 min, which is due to the fact that after H2O and SO2 poisoning, ammonium sulfate was generated that cover the active sites on the catalyst surface, thereby inhibiting the adsorption of NO on the catalyst surface. The peak at 1382 cm−1 is due to the gas adsorbed on the surface of the catalyst, SO2 and H2O are poisoned to form an asymmetric tensile vibration of O=S=O in the surface sulfate composition, NO at O=S=O after adsorption, O=S=O was shielded, so a negative peak was generated at 1382 cm–1 [66,67]. The in situ DRIFTS spectra of NO + O2 adsorbed on the MnCeOx@TiO2-T2 catalyst before and after poisoning are shown in Figure 12c,d. The bidentate nitrate peak at 1323 cm−1 disappeared on the poisoned catalyst, and the peak bridging the nitrate (1628 cm−1) and the nitrite peak (1593 cm−1) also disappeared after 15 min, and a new bidentate nitrate peak appeared at 1343 cm−1, which may be due to the MnCeOx@TiO2-T2 catalyst undergoing H2O and after SO2 is poisoned, the produced sulfate or ammonium sulfate will have new adsorption sites for NO. It shows that even if the MnCeOx@TiO2-T2 catalyst is affected by the poisoning of SO2 and H2O, the adsorption of NO at some active sites is inhibited, but the MnCeOx@TiO2-T2 catalyst can still generate new adsorption sites for NO and formed bidentate nitrate at this site.

2.5.6. Anti-Poisoning Mechanism of MnCeOx@TiO2-T2 Catalyst

The schematic diagram of the anti-poisoning mechanism of the MnCeOx@TiO2-T2 catalyst is shown in Figure 13. The addition of appropriate amounts of tourmaline effectively exposes the active {001} facets and meanwhile introduces the acidic SiO2 and Al2O3 components into the catalyst that can improve the catalytic activity and enhance the anti-poisoning performance. In addition, the addition of tourmaline reduces the adsorption of SO2 on the catalyst surface and the competitive adsorption of NH3 and NO by SO2. Compared to the MnCeOx@TiO2 catalyst, the MnCeOx@TiO2-T2 catalyst has stronger NH3 and NO adsorption capacity. Additionally, MnCeOx@TiO2-T2 exhibits more Brønsted acid sites and thus weakens the SO2 adsorption. As a result, MnCeOx@TiO2-T2 efficiently inhibited the deposition of NH4HSO4 and (NH4)2HSO4 on the surface of the catalyst, thereby improving the NH3-SCR performance and anti-poisoning ability of the catalyst.

3. Experimental Section

3.1. Preparation of Catalysts

Manganese acetate and cerium nitrate (in the molar ratio 4:6) were added to a mixture of ethylene glycol (30 mL) and isopropanol (30 mL). The mixture was ultrasonically stirred until dissolved. The solution was transferred in a 100 mL hydrothermal kettle (lined with polytetrafluoroethylene) and heated at 180 °C for 24 h. The solid was filtered and washed with absolute ethanol and deionized water three times. After drying at 80 °C for 6 h, the product was obtained and designated as MnCeOx. After that, 0.52 g of MnCeOx was dissolved in 200 mL of absolute ethanol through ultrasonic treatment for 30 min. 0.52 g of urea was then added to the above solution under continuous sonication for another 30 min. 1.5 mL of butyl titanate was dispersed in 20 mL of ethanol and the resulting dispersion was added dropwise to the above solution. The mixture was stirred at 45 °C for 24 h. The obtained precipitate was filtrated and washed with deionized water several times and then dried at 80 °C for 12 h. Finally, the sample was calcined in a muffle furnace at 500 °C for 4 h (heating rate 1 °C min−1) to obtain MnCeOx@TiO2. To investigate the effect of tourmaline on the denitration performance, a suspension of butyl titanate (1.5 mL) and a certain amount of tourmaline in 20 mL of ethanol was used instead, and the other procedure was the same, with the preparation of MnCeOx@TiO2. The mass ratios of tourmaline and MnCeOx@TiO2 are 1%, 2%, 3%, and 4%, respectively, and the tourmaline-doped MnCeOx@TiO2 is denoted as MnCeOx@TiO2-T1, MnCeOx@TiO2-T2, MnCeOx@TiO2-T3, and MnCeOx@TiO2-T4, respectively. After SO2 and H2O poisoning, MnCeOx@TiO2 and MnCeOx@TiO2-T catalysts are denoted as MnCeOx@TiO2-S and MnCeOx@TiO2-T-S, respectively.

3.2. Catalytic Performance Tests

The NH3-SCR performance testing was carried out in a fixed-bed quartz reactor (inner diameter of 8 mm). The typical reaction condition is as follows: [NO] = [NH3] = 500 ppm, [O2] = 5 vol.%, and N2 balance; 0.5 mL of catalyst (40–60 mesh) was used for the SCR activity study. The total flow rate of feed gases was 100 mL min−1, and the GHSV was 10,000 h−1. KM940 flue (UK, KANE) gas analyzer was used to analyze the feed gases and the effluent streams. The reaction temperature ranged from 100 to 450 °C. The NOx conversion was calculated according to Equation (1):
x = [ N O x ] i n − [ N O x ] o u t [ N O x ] i n × 100 %
where [NOx]in and [NOx]out are the inlet and outlet concentration at steady-state, respectively.

4. Conclusions

In summary, a series of MnOx/CeO2 bimetallic oxide catalysts were synthesized, and mesoporous TiO2 was doped on the surface of the MnOx/CeO2 with the addition of different amounts of tourmaline. The effect of tourmaline on the NH3-SCR catalytic performance of MnCeOx@TiO2 was intensively investigated. The catalytic experiments reveal that the addition of appropriate amounts of tourmaline (2 wt%) can improve the low-temperature denitration performance of the catalyst. The MnCeOx@TiO2-T2 catalyst exhibited the most excellent NH3-SCR activity, and the NOx conversion is maintained at 100% in the temperature window of 110–200 °C. After the exposure to 300 ppm SO2 and 10 vol.% H2O at 170 °C, the denitration activity of the catalyst can still be maintained above 90%. Although the specific surface area of catalysts was decreased with the addition of tourmaline, the active {001} facets of TiO2 were more exposed and the content of Mn4+ and Oα on the surface of the catalyst was increased. In addition, the addition of tourmaline introduced the acidic SiO2 and Al2O3 components that is favorable for the improvement of catalytic activity. The in situ DRIFTS measurement indicated that the addition of appropriate amounts of tourmaline increased the number of Brønsted acid sites on the catalyst surface and weakened the adsorption of SO2, suppressing the deposition of ammonium sulfate on the surface of the catalyst, resulting in the enhanced NH3-SCR performance and anti-poisoning ability of the catalyst.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29174079/s1, Figure S1: PXRD patterns of MnCeOx@TiO2 with the addition of different amounts of tourmaline. Table S1: Comparison of the catalytic performance with previous work [14,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84]. Table S2: Main composition of tourmaline (wt. %).

Author Contributions

Conceptualization, Z.Z.; methodology, Z.Z. and L.W.; software, H.H. and L.W.; validation, H.M. and Z.W.; resources, Y.G. and G.X.; data curation, Z.Z., L.W., and X.L.; writing—original draft preparation, Z.Z.; writing—review and editing, Y.G. and G.X.; supervision, Y.G. and G.X.; project administration, Y.G.; funding acquisition, Y.G. and X.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Key Research and Development Project of Hainan Province, China (ZDYF2024GXJS005), the Major Science and Technology Plan of Hainan Province, China (ZDKJ202016), the National Natural Science Foundation of China (22105053), and the Natural Science Foundation of Hainan Province, China (521QN209).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that most of the data supporting the findings of this study are available within the article and its supplementary material. Raw data are available from the corresponding author (Y.G.) on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 29 04079 g001
Figure 1. (a) Reversion of NOx catalyzed by MnCeOx@TiO2 with different amounts of tourmaline. (b) Anti-poisoning performance of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts investigated at 170 °C.
Figure 1. (a) Reversion of NOx catalyzed by MnCeOx@TiO2 with different amounts of tourmaline. (b) Anti-poisoning performance of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts investigated at 170 °C.
Molecules 29 04079 g001
Molecules 29 04079 g002
Figure 2. PXRD patterns of (a) MnCeOx@TiO2 and (b) MnCeOx@TiO2-T2 catalysts before and after SO2 poisoning.
Figure 2. PXRD patterns of (a) MnCeOx@TiO2 and (b) MnCeOx@TiO2-T2 catalysts before and after SO2 poisoning.
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Molecules 29 04079 g003
Figure 3. SEM images of MnCeOx@TiO2 catalysts with different amounts of tourmaline (a) MnCeOx@TiO2; (b) MnCeOx@TiO2-T1; (c) MnCeOx@TiO2-T2; (d) MnCeOx@TiO2-T3; and (e) MnCeOx@TiO2-T4.
Figure 3. SEM images of MnCeOx@TiO2 catalysts with different amounts of tourmaline (a) MnCeOx@TiO2; (b) MnCeOx@TiO2-T1; (c) MnCeOx@TiO2-T2; (d) MnCeOx@TiO2-T3; and (e) MnCeOx@TiO2-T4.
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Molecules 29 04079 g004
Figure 4. (a) EDS image of MnCeOx@TiO2 catalyst; (b) EDS image of MnCeOx@TiO2-T2 catalyst; (c) TEM; and (d) HRTEM images of MnCeOx@TiO2-T2 catalyst.
Figure 4. (a) EDS image of MnCeOx@TiO2 catalyst; (b) EDS image of MnCeOx@TiO2-T2 catalyst; (c) TEM; and (d) HRTEM images of MnCeOx@TiO2-T2 catalyst.
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Molecules 29 04079 g005
Figure 5. Adsorption isotherms of MnCeOx@TiO2 with different additions of tourmaline (a) and BJH pore size distribution curve (b).
Figure 5. Adsorption isotherms of MnCeOx@TiO2 with different additions of tourmaline (a) and BJH pore size distribution curve (b).
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Molecules 29 04079 g006
Figure 6. XPS diagrams of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts before and after poisoning (a) Mn2p3/2; (b) Ce3d; (c) O1s; and (d) S2p.
Figure 6. XPS diagrams of MnCeOx@TiO2 and MnCeOx@TiO2-T2 catalysts before and after poisoning (a) Mn2p3/2; (b) Ce3d; (c) O1s; and (d) S2p.
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Molecules 29 04079 g007
Figure 7. (a) H2-TPR diagrams of MnCeOx@TiO2 with different additions of tourmaline. (b) H2-TPR diagram of MnCeOx@TiO2-S and MnCeOx@TiO2-T2-S catalysts after poisoning.
Figure 7. (a) H2-TPR diagrams of MnCeOx@TiO2 with different additions of tourmaline. (b) H2-TPR diagram of MnCeOx@TiO2-S and MnCeOx@TiO2-T2-S catalysts after poisoning.
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Molecules 29 04079 g008
Figure 8. In situ DRIFTS spectra of SO2+O2 adsorption on the surface of catalyst (a) MnCeOx@TiO2; (b) MnCeOx@TiO2-T2.
Figure 8. In situ DRIFTS spectra of SO2+O2 adsorption on the surface of catalyst (a) MnCeOx@TiO2; (b) MnCeOx@TiO2-T2.
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Molecules 29 04079 g009
Figure 9. In situ DRIFTS spectra of (a) MnCeOx@TiO2 and (b) MnCeOx@TiO2-T2 upon exposure to the NH3 + SO2.
Figure 9. In situ DRIFTS spectra of (a) MnCeOx@TiO2 and (b) MnCeOx@TiO2-T2 upon exposure to the NH3 + SO2.
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Molecules 29 04079 g010
Figure 10. Effect of SO2 on the adsorption of NO + O2 on the surface of (a) MnCeOx@TiO2 and (b) MnCeOx@TiO2-T2 by in situ DRIFTS spectra.
Figure 10. Effect of SO2 on the adsorption of NO + O2 on the surface of (a) MnCeOx@TiO2 and (b) MnCeOx@TiO2-T2 by in situ DRIFTS spectra.
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Molecules 29 04079 g011
Figure 11. Adsorption of NH3 on the surface of MnCeOx@TiO2 catalyst (a) before poisoning; (b) after poisoning. Adsorption of NH3 on the surface of MnCeOx@TiO2-T2 catalyst (c) before poisoning; (d) after poisoning.
Figure 11. Adsorption of NH3 on the surface of MnCeOx@TiO2 catalyst (a) before poisoning; (b) after poisoning. Adsorption of NH3 on the surface of MnCeOx@TiO2-T2 catalyst (c) before poisoning; (d) after poisoning.
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Molecules 29 04079 g012
Figure 12. Adsorption of NO + O2 on the surface of MnCeOx@TiO2 catalyst (a) before poisoning; (b) after poisoning. Adsorption of NO + O2 on the surface of MnCeOx@TiO2-T2catalyst (c) before poisoning; (d) after poisoning.
Figure 12. Adsorption of NO + O2 on the surface of MnCeOx@TiO2 catalyst (a) before poisoning; (b) after poisoning. Adsorption of NO + O2 on the surface of MnCeOx@TiO2-T2catalyst (c) before poisoning; (d) after poisoning.
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Molecules 29 04079 g013
Figure 13. Schematic diagram of the anti-poisoning mechanism of MnCeOx@TiO2-T2 catalyst.
Figure 13. Schematic diagram of the anti-poisoning mechanism of MnCeOx@TiO2-T2 catalyst.
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Table 1. The specific surface area, pore volume and average pore diameter of MnCeOx@TiO2 with different additions of tourmaline.
Table 1. The specific surface area, pore volume and average pore diameter of MnCeOx@TiO2 with different additions of tourmaline.
CatalystSpecific Surface Area (m2 g–1)Pore Volume
(cm3 g–1)		Pore Size (nm)
MnCeOx@TiO284.000.146.70
MnCeOx@TiO2-T181.550.167.73
MnCeOx@TiO2-T266.770.148.17
MnCeOx@TiO2-T356.660.149.64
MnCeOx@TiO2-T442.150.1512.81
Table 2. The content and relative concentration of surface elements before and after poisoning of MnCeOx@TiO2 and MnCeOx@TiO2-T2.
Table 2. The content and relative concentration of surface elements before and after poisoning of MnCeOx@TiO2 and MnCeOx@TiO2-T2.
CatalystSurface Atom Concentration (%)Relative Concentration (%)
MnCeTiOSMn4+Mn2+Ce3+Oα
MnCeOx@TiO22.322.8017.0451.27-7.4624.3713.9323.07
MnCeOx@TiO2-S1.352.3915.8554.955.256.1126.7311.2820.38
MnCeOx@TiO2-T22.262.9618.0653.12-8.4627.1814.2526.16
MnCeOx@TiO2-T2-S2.202.8918.7654.011.187.7127.8111.5225.98
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Zhao, Z.; Wang, L.; Lin, X.; Xue, G.; Hu, H.; Ma, H.; Wang, Z.; Su, X.; Gao, Y. Effect of Tourmaline Addition on the Anti-Poisoning Performance of MnCeOx@TiO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx. Molecules 2024, 29, 4079. https://doi.org/10.3390/molecules29174079

AMA Style

Zhao Z, Wang L, Lin X, Xue G, Hu H, Ma H, Wang Z, Su X, Gao Y. Effect of Tourmaline Addition on the Anti-Poisoning Performance of MnCeOx@TiO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx. Molecules. 2024; 29(17):4079. https://doi.org/10.3390/molecules29174079

Chicago/Turabian Style

Zhao, Zhenzhen, Liyin Wang, Xiangqing Lin, Gang Xue, Hui Hu, Haibin Ma, Ziyu Wang, Xiaofang Su, and Yanan Gao. 2024. "Effect of Tourmaline Addition on the Anti-Poisoning Performance of MnCeOx@TiO2 Catalyst for Low-Temperature Selective Catalytic Reduction of NOx" Molecules 29, no. 17: 4079. https://doi.org/10.3390/molecules29174079

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