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

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.


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 NO x 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 NO x 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 NO x (NH 3 -SCR) is considered to be an effective strategy for reducing NO x 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.

NO x Reduction and SO 2 Tolerance
Figure 1a shows the effect of tourmaline addition on the denitration performance of MnCeO x @TiO 2 catalyst.It can be seen that after adding tourmaline, the MnCeO x @TiO 2 -T2 catalyst exhibited the best denitration performance compared to other catalysts at a temperature ranging from 80 to 110 • C. The MnCeO x @TiO 2 -T2 catalyst showed a 75% NO x conversion at 80 • C, which is higher than that of the MnCeOx@TiO 2 catalyst (65%) at the same temperature.A 100% NO x conversion was achieved by MnCeO x @TiO 2 -T1 and MnCeO x @TiO 2 -T2 in the temperature range of 110~200 • C.However, when the tourmaline content is 3% and 4%, the NO x conversion of MnCeO x @TiO 2 -T3 and MnCeO x @TiO 2 -T4 is lower than that of MnCeO x @TiO 2 -T2 at temperature less than 110 • C, indicating that the amount of added tourmaline affected the denitration activity of the catalyst.
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.

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 MnCeO x @TiO 2 -T2 exhibited the best catalytic activity, the catalyst was mainly investigated in the following research.The anti-poisoning performance of MnCeO x @TiO 2 and MnCeO x @TiO 2 -T2 was examined at 170 • C, and the results are shown in Figure 1b.For MnCeO x @TiO 2 , when 100 ppm SO 2 was introduced, the NO x conversion decreased from 100% to 93% and then remained stable for 12 hours.As the SO 2 concentration increased to 200 ppm, the NO x conversion decreased to 85% and remained stable for 12 h.As the SO 2 concentration further increased to 300 ppm, the NO x conversion dropped to 78%, and remained stable for another 12 h.After 10 vol.%H 2 O was introduced, the NO x conversion further dropped to 70%.In contrast, after 100-200 ppm of SO 2 was introduced into the feed, the NO x conversion of MnCeO x @TiO 2 -T2 catalyst was still 100% and remained unchanged and stable for 24 h.As the SO 2 concentration increased to 300 ppm, the NO x conversion of MnCeO x @TiO 2 -T2 decreased slightly from 100% to 95%.After 10 vol.%H 2 O was further introduced, the NO x conversion decreased to about 90%.The decrease in NO x conversion can be attributed to the sulfation of metal oxides and the deposition of ammonium sulfate.After cutting off H 2 O and SO 2 , the NO x conversion of MnCeO x @TiO 2 and MnCeO x @TiO 2 -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 MnCeO x @TiO 2 -T2 catalyst exhibited better anti-poisoning performance than the MnCeO x @TiO 2 catalyst.A comparison with the previous reports was listed in Table S1.

Research on Crystal Structure and Morphology
The crystal structures of MnCeO x @TiO 2 and MnCeO x @TiO 2 -T2 catalysts before and after poisoning are shown in Figure 2. It can be seen that both catalysts exhibited characteristic peaks of anatase TiO 2 (PDF#21-1272) [26] and CeO 2 (PDF#43-1002) [27].The peaks at 25.3, 37.8, and 48.1 • are due to the (101), (004), and (200) facets of TiO 2 , respectively, and the peak at 28.3 • can be attributed to the (111) facet of CeO 2 .No characteristic peaks of MnO x were observed in either powder X-ray diffraction (PXRD) pattern, suggesting that amorphous MnO x 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 TiO 2 in MnCeO x @TiO 2 -T2 pattern was slower than that in MnCeO x @TiO 2 because MnCeO x @TiO 2 -T2 demonstrated a decrease in the (004) diffraction intensity and an increasement in the (200) intensity compared with that of MnCeO x @TiO 2 , which showed that MnCeO x @TiO 2 -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 NO x conversion [31].After the SO 2 poisoning, the CeO 2 diffraction peaks of MnCeO x @TiO 2 catalyst were weakened.The decreased diffraction peak intensity of CeO 2 could be due to the formation of CeSO 4 .As mentioned above, the CeO 2 acts as a sacrificial site that can reduce the sulfation of the main active phase [15].In contrast, the diffraction peaks of MnCeO x @TiO 2 -T2 catalyst were not obviously changed, suggesting that the addition of an appropriate amount of tourmaline can suppress the adsorption of SO 2 , thereby demonstrating the outstanding sulfur resistance of the catalyst [32].
The scanning electron microscopy (SEM) images of MnCeO x @TiO 2 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 MnCeO x @TiO 2 particles become more and more dense, which is consistent with the PXRD results that showed enhanced crystallinity of both TiO 2 and CeO 2 (Figure S1).This is actually unfavorable for the NH 3 -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 MnCeO x @TiO 2 -T3 and MnCeO x @TiO 2 -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.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 Figure 4a,b show the energy-dispersive spectrometer (EDS) element mapping of MnCeO x @TiO 2 and MnCeO x @TiO 2 -T2 catalysts.It is evident that that the particle size is about 1.5 um, and TiO 2 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 MnCeO x @TiO 2 -T2 catalyst, which confirms the existence of tourmaline in the catalyst (the component of tourmaline is listed in Table S2).The SiO 2 component of tourmaline can decrease the thermal stability of NH 4 HSO 4 and thus promote its decomposition on the surface of catalyst [33].Additionally, both SiO 2 and Al 2 O 3 components of tourmaline can enhance the acidity of the catalyst, weakening the adsorption of SO 2 and thus improving the SO 2 poisoning resistance of the catalyst [34].Figure 4c,d showed the transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) image of MnCeO x @TiO 2 -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 CeO 2 [35] and the (101) crystal plane of TiO 2 [36], respectively.

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 exhib-

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 N 2 adsorption-desorption measurement was performed on the MnCeO x @TiO 2 and MnCeO x @TiO 2 -T catalysts (Figure 5 and Table 1).All catalysts exhibited type-IV adsorption isotherms but with different types of hysteresis loops.The MnCeO x @TiO 2 , MnCeO x @TiO 2 -T1, and MnCeO x @TiO 2 -T2 catalysts correspond to the H3 type hysteresis loops [37], while the MnCeO x @TiO 2 -T3 and MnCeO x @TiO 2 -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 m 2 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 MnCeO x @TiO 2 by the impregnated species.Although the specific surface area of the catalyst was decreased, the catalytic activity of MnCeO x @TiO 2 -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 TiO 2 , which enhanced the catalytic activity.Additionally, the addition of tourmaline introduced the acidic SiO 2 and Al 2 O 3 components into the catalyst, which can improve SO 2 poisoning resistance.We consider that these advantageous factors contribute to the improvement of the catalytic performance. 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.Table 1.The specific surface area, pore volume and average pore diameter of MnCeOx@TiO2 with different additions of tourmaline.

Catalyst
Specific Surface Area (m 2 g -1 ) Pore Volume (cm 3 g -1 ) Pore Size (nm) Figure 6 shows the X-ray photoelectron spectroscopy (XPS) spectra of Mn 2p, Ce 3d, O 1s, and S 2p of MnCeO x @TiO 2 and MnCeO x @TiO 2 -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 Mn 2+ (641.3-641.5 eV), mixed valence states of Mn 3+ (642.3-642.8eV), and Mn 4+ (644.6-643.6 eV) [38].The relative concentration of Mn 4+ on the surface of different tourmaline-added catalysts is different.The highest Mn 4+ content was observed for MnCeO x @TiO 2 -T2.The redox performance of SCR is related to the surface concentration of Mn 4+ .The higher the relative concentration of Mn 4+ , the more favorable the low-temperature SCR reaction [39].This may be another reason that MnCeO x @TiO 2 -T2 exhibited the best catalytic activity.Meanwhile, it can be found from Figure 6a that the binding energy of Mn in MnCeO x @TiO 2 -S catalyst is higher than that of the fresh catalyst, while the binding energy of Mn in MnCeO x @TiO 2 -T2-S catalyst does not change significantly compared with the fresh catalyst.This result reveals that the Mn atoms on the surface of MnCeO x @TiO 2 bonded with electronegative S in addition to elemental O.As shown in Table 2, we can see that when the MnCeO x @TiO 2 was poisoned by SO 2 , the content of Mn 4+ on the surface of the catalyst decreased from 14.92% to 12.22%, while under SO 2 atmosphere, the Mn 4+ content of MnCeO x @TiO 2 -T2 decreased from 16.93% to 15.42%, a slighter decrease compared to MnCeO x @TiO 2 , indicating that the MnCeO x @TiO 2 bonded with more SO 2 than MnCeO x @TiO 2 -T2.That is, MnCeO x @TiO 2 -T2 has better SO 2 -tolerance performance.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 Mn 4+ on the surface of the catalyst decreased from 14.92% to 12.22%, while under SO2 atmosphere, the Mn 4+ 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.Ce 3+ is shown by the subbands labeled u ′ and v ′ , and Ce 4+ 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 MnCeO x @TiO 2 when the catalyst was introduced to SO 2 , suggesting that the density of electron clouds of Ce in MnCeO x @TiO 2 was decreased due to oxidation by O 2 .However, no obvious change in binding energies of Ce 3d was found for MnCeO x @TiO 2 -T2, revealing that MnCeO x @TiO 2 -T2 had outstanding SO 2 -tolerance performance [41].Figure 6c shows the O 1s spectra of MnCeO x @TiO 2 and MnCeO x @TiO 2 -T2 before and after SO 2 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 O α .The low binding energy band (529.8 eV) corresponds to lattice oxygen, denoted as O β [42].NH 3 -SCR is a gas-solid reaction, and the surface-active oxygen plays a key role in catalyzing NH 3 -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 O α in the MnCeO x @TiO 2 -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 MnCeO x @TiO 2 increased the O α content.We can also see from Table 2 that the O α content of the poisoned MnCeO x @TiO 2 -T2-S catalyst is slightly reduced to 25.98%, which is much higher than the O α content (20.38%) of the MnCeO x @TiO 2 -S catalyst.It is well known that O α species are beneficial to the oxidation of NO from NO to NO 2 , which promotes the NH 3 -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 MnCeO x @TiO 2 -S and MnCeO x @TiO 2 -T2-S.Two peaks at 169.8 eV and 168.5 eV were observed, which are attributed to HSO 4 − and SO 4 2− [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 MnCeO x @TiO 2 -S catalyst, the content of HSO 4 − and SO 4 2− on the surface of the MnCeO x @TiO 2 -T2-S catalyst is much less, indicating that the MnCeO x @TiO 2 -T2 catalyst is not easy to be sulfated and effectively inhibits the deposition of NH 4 HSO 4 on the surface of the catalyst.As shown in Table 2, the relative concentration of Mn 4+ and Ce 3+ ions on the surface of the poisoned catalyst decreased, while the relative concentration of Mn 2+ and Ce 4+ ions increased and meanwhile, the binding energy of Mn 2+ and Ce 4+ increased, both of which indicate that MnSO 4 and Ce(SO 4 ) 2 were formed, which led to the deactivation of catalysts [48][49][50].

Redox Capability
The H 2 -TPR diagram of the catalysts is shown in Figure 7.The reduction peaks of the catalysts are superimposed.The H 2 consumption peak below 450 • C is attributed to the reduction of MnO 2 →Mn 2 O 3 and Mn 2 O 3 →Mn 3 O 4 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 H 2 consumption peak of the catalyst to shift to low temperature (Figure 7a), indicating that the reduction temperature of H 2 was low and the redox performance of catalysts was strong [49].However, the addition of excess amount of tourmaline (4%) caused an increase in H 2 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].
Molecules 2024, 29, x FOR PEER REVIEW 11 of 22 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 The hydrogen-temperature-programmed reduction (H 2 -TPR) diagram of the SO 2 poisoned catalysts is shown in Figure 7b.The reduction peaks of MnCeO x @TiO 2 -S and MnCeO x @TiO 2 -T2-S catalysts appeared at 610 and 508 • C, respectively and are attributed to the reduction in MnSO 4 [51].The presence of MnSO 4 confirmed the sulfation of active ingredient manganese oxide on the surface of the catalyst.However, compared to the MnCeO x @TiO 2 -S catalyst, the reduction peak temperature of the MnCeO x @TiO 2 -T2-S catalyst shifted to low temperature, indicating that the oxidation-reduction performance of the MnCeO x @TiO 2 -T2 catalyst was better than MnCeO x @TiO 2 after SO 2 poisoning.The results show that under high SO 2 concentration, the reduction ability of the poisoned catalyst is weakened due to the decrease in the relative concentration of Mn 4+ , which is consistent with the XPS results.In order to analyze the adsorption behavior of SO 2 on the surface of MnCeO x @TiO 2 and MnCeO x @TiO 2 -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 SO 2 + 5 vol.%O 2 gas to the surfaces of the two catalysts at 170 • C.After the introduction of SO 2 to MnCeO x @TiO 2 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 MnCeO x @TiO 2 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 NH 3 and NO x species on the catalyst surface, inhibiting the SCR reaction progress.Figure 8b shows the in situ DRIFTS spectra of the MnCeO x @TiO 2 -T2 catalyst after adsorbing 500 ppm SO 2 + 5 vol.%O 2 .Only 15 min after the introduction of SO 2 , the faint infrared peak attributed to the surface sulfate species (1167 cm −1 ) was detected.With continuous introduction of SO 2 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 MnCeO x @TiO 2 -T2 catalyst are much less than that of the MnCeO x @TiO 2 catalyst, and the prolonged time of resistance to SO 2 poisoning indicates that the addition of appropriate amount of tourmaline can effectively improve the anti-poisoning performance of the catalyst.
Molecules 2024, 29, x FOR PEER REVIEW 12 of 22 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 Mn 4+ , which is consistent with the XPS results.

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.

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 (b) MnCeO x @TiO 2 -T2.

Adsorption of NH 3 + SO 2 on the Catalyst Surface
As shown in Figure 9a, when the MnCeO x @TiO 2 catalyst was exposed to NH 3 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 NH 3 at the Lewis acid site [54,55], and the band at 1453 cm −1 is attributable to the NH 4 + symmetric bending vibration at the Brønsted acid site [56].It can be found that after adding 100 ppm SO 2 for 5 min, the adsorption peak intensity corresponding to the coordinated NH 3 on the Lewis acid site and the NH 4 + formed on the Brønsted acid site has increased, but due to the continuous introduction of SO 2 , 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 SO 2 , which was attributed to SO 2 adsorption.The results show that SO 2 and NH 3 are competitively adsorbed on the surface of MnCeO x @TiO 2 catalyst.The addition of SO 2 occupied some adsorption sites of NH 3 on the catalyst surface, thereby inhibiting the adsorption and activation of NH 3 .
Molecules 2024, 29, x FOR PEER REVIEW 13 of 22 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.

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 Figure 9b shows the in situ DRIFTS spectra of the MnCeO x @TiO 2 -T2 catalyst upon exposure to the NH 3 + SO 2 atmosphere.Thirty minutes after the introduction of only NH 3 , 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 NH 3 on the Lewis acid site, and the band at 1359 cm -1 is attributed to the amino compound (-NH 2 ) on the catalyst surface [57].The peaks around 1657, 1458, and 1402 cm −1 are attributed to the symmetrical bending vibration of the coordinated NH 4 + adsorbed on the Brønsted acid site by NH 3 .In the presence of 100 ppm SO 2 , the intensity of the adsorption zone is basically unchanged.The addition of SO 2 caused the occupation of part of the catalytic adsorption sites on the surface of the MnCeO x @TiO 2 catalyst by SO 2 , thereby inhibiting the adsorption and activation of NH 3 .In contrast, the intensity of the adsorption peak of NH 3 species adsorbed on the MnCeO x @TiO 2 -T2 catalyst did not decrease.The results show that the NH 3 adsorption capacity of the MnCeO x @TiO 2 -T2 catalyst is stronger than that of the MnCeO x @TiO 2 catalyst.In this case, SO 2 did not significantly inhibit the adsorption of NH 3 .This confirms that the addition of appropriate amounts of tourmaline effectively prevents the adsorption of SO 2 on the surface of the catalyst.The added tourmaline can increase the number of Brønsted acid sites.The strong electronic interactions between HSO 4 − and Brønsted acid sites of mesoporous TiO 2 damaged the bond between NH 4 + and HSO 4 − , thus promoting the decomposition of NH 4 HSO 4 and improving the catalytic activity and the anti-SO 2 performance.

Adsorption of NO + O 2 + SO 2 on the Catalyst Surface
The effect of SO 2 on the adsorption of NO + O 2 on the MnCeO x @TiO 2 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 N 2 O 4 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 SO 2 , 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 SO 2 and NO have competitive adsorption on the catalyst surface and that the adsorption capacity of SO 2 on the catalyst surface is significantly stronger than that of NO.The effect of SO 2 on the adsorption of NO + O 2 on the MnCeO x @TiO 2 -T2 catalyst is shown in Figure 10b.After the exposure to NO + O 2 for 30 min, the peak at 1765 cm −1 is attributed to the N 2 O 4 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 SO 2 on the surface of the MnCeO x @TiO 2 -T2 catalyst.It should be pointed out that the bridging nitrate (1628 cm −1 ) and nitrite (1593 cm −1 ) peaks disappeared when SO 2 was introduced for 20 min, while the bands of other nitrate species still exist stably.However, the peak of the nitrate species of MnCeO x @TiO 2 catalyst disappeared after the introduction of SO 2 for 5 min, suggesting that the adsorption capacity of MnCeO x @TiO 2 -T2 catalyst for NO is also stronger than that of the MnCeO x @TiO 2 catalyst.
Molecules 2024, 29, x FOR PEER REVIEW 14 of 22 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.
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.

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 NH 3 adsorbed on the surface of MnCeO x @TiO 2 and MnCeO x @TiO 2 -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 NH 3 did not change significantly [64].The peak of NH 3 coordinated on the Lewis acid (1605 and 1245 cm −1 ) and the coordinated NH 4 + peaks (1403 and 1453 cm −1 ) attributable to the Brønsted acid site of the poisoned MnCeO x @TiO 2 catalyst were significantly weakened.The 1355 cm −1 peak attributable to the oxidized and deformed intermediate generated by the adsorption of NH 3 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 NH 3 peak on the Lewis acid.This is because the presence of SO 2 and H 2 O will weaken the Brønsted acid sites on the MnCeO x @TiO 2 catalyst, thereby weakening the adsorption of NH 3 .
The in situ DRIFTS spectra of the MnCeO x @TiO 2 -T2 catalyst before and after the poisoning of NH 3 adsorption for different time are shown in Figure 11c,d.Compared tothe non-poisoned catalyst, the peak of NH 3 coordinated on the Lewis acid (1605 and 1245 cm −1 ) of the poisoned MnCeO x @TiO 2 -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 NH 3 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 NH 3 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 SO 2 and H 2 O, the MnCeO x @TiO 2 -T2 catalyst inhibits the loss of Brønsted acid sites and effectively inhibits the bonding of NH 4 + and HSO 4 − , and meanwhile promotes the decomposition of NH 4 HSO 4 , thus improving the anti-poisoning performance of the catalyst.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.

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 The in situ DRIFTS spectra of NO + O 2 adsorbed on the MnCeO x @TiO 2 catalyst before and after poisoning are shown in Figure 12a,b.The peak at 1765 cm −1 is attributed to the N 2 O 4 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 MnCeO x @TiO 2 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 H 2 O and SO 2 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, SO 2 and H 2 O 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 + O 2 adsorbed on the MnCeO x @TiO 2 -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 MnCeO x @TiO 2 -T2 catalyst undergoing H 2 O and after SO 2 is poisoned, the produced sulfate or ammonium sulfate will have new adsorption sites for NO.It shows that even if the MnCeO x @TiO 2 -T2 catalyst is affected by the poisoning of SO 2 and H 2 O, the adsorption of NO at some active sites is inhibited, but the MnCeO x @TiO 2 -T2 catalyst can still generate new adsorption sites for NO and formed bidentate nitrate at this site.
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.The schematic diagram of the anti-poisoning mechanism of the MnCeO x @TiO 2 -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 SiO 2 and Al 2 O 3 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 SO 2 on the catalyst surface and the competitive adsorption of NH 3 and NO by SO 2 .
Compared to the MnCeO x @TiO 2 catalyst, the MnCeO x @TiO 2 -T2 catalyst has stronger NH 3 and NO adsorption capacity.Additionally, MnCeO x @TiO 2 -T2 exhibits more Brønsted acid sites and thus weakens the SO 2 adsorption.As a result, MnCeO x @TiO 2 -T2 efficiently inhibited the deposition of NH 4 HSO 4 and (NH 4 ) 2 HSO 4 on the surface of the catalyst, thereby improving the NH 3 -SCR performance and anti-poisoning ability of the catalyst.
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 antipoisoning 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.

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

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 MnCeO x .After that, 0.52 g of MnCeO x 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 MnCeO x @TiO 2 .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 MnCeO x @TiO 2 .The mass ratios of tourmaline and MnCeO x @TiO 2 are 1%, 2%, 3%, and 4%, respectively, and the tourmaline-doped MnCeO x @TiO 2 is denoted as MnCeO x @TiO 2 -T1, MnCeO x @TiO 2 -T2, MnCeO x @TiO 2 -T3, and MnCeO x @TiO 2 -T4, respectively.After SO 2 and H 2 O poisoning, MnCeO x @TiO 2 and MnCeO x @TiO 2 -T catalysts are denoted as MnCeO x @TiO 2 -S and MnCeO x @TiO 2 -T-S, respectively.

Figure 5 .
Figure 5. Adsorption isotherms of MnCeOx@TiO2 with different additions of tourmaline (a) and BJH pore size distribution curve (b).

Figure 5 .
Figure 5. Adsorption isotherms of MnCeO x @TiO 2 with different additions of tourmaline (a) and BJH pore size distribution curve (b).

2. 5 .
Study on the Mechanism of Catalyst Anti-Poisoning Reaction 2.5.1.Adsorption of SO 2 + O 2 on the Catalyst Surface

Figure 10 .
Figure 10.Effect of SO 2 on the adsorption of NO + O 2 on the surface of (a) MnCeO x @TiO 2 and (b) MnCeO x @TiO 2 -T2 by in situ DRIFTS spectra.2.5.4.Adsorption of NH 3 on the Catalyst Surface before and after Poisoning

Figure 11 .
Figure 11.Adsorption of NH 3 on the surface of MnCeO x @TiO 2 catalyst (a) before poisoning; (b) after poisoning.Adsorption of NH 3 on the surface of MnCeO x @TiO 2 -T2 catalyst (c) before poisoning; (d) after poisoning.2.5.5.Adsorption of NO + O 2 on the Catalyst Surface before and after Poisoning

Table 2 .
The content and relative concentration of surface elements before and after poisoning of MnCeO x @TiO 2 and MnCeO x @TiO 2 -T2.