Study on NH 3 -SCR Activity and HCl/H 2 O Tolerance of Titanate-Nanotube-Supported MnO x -CeO 2 Catalyst at Low Temperature

: Manganese oxide-cerium oxide supported on titanate nanotubes (i.e., MnCe/TiNTs) were prepared and their catalytic activities towards NH 3 -SCR of NO were tested. The results indicated that the MnCe/TiNT catalyst can achieve a high NO removal efficiency above 95% within the temperature range of 150–350 ◦ C. Even after exposure to a HCl-containing atmosphere for 2 h, the NO removal efficiency of the MnCe/TiNT catalyst maintains at approximately 90% at 150 ◦ C. This is attributed to the large specific surface area as well as the unique hollow tubular structure of TiNTs that exposes more Ce atoms, which preferentially react with HCl and thus protect the active Mn atoms. Moreover, the abundant OH groups on TiNTs serve as Brønsted acid sites and provide H protons to expel Cl atom from the catalyst surface. The irreversible deactivation caused by HCl can be alleviated by H 2 O. That is because the dissociated adsorption of H 2 O on TiNTs forms additional OH groups and relieves HCl poisoning.


Introduction
Incineration has been widely applied to dispose of municipal solid waste in China since it can reduce the volume of solid waste and allow energy recovery.Nitrogen oxide (NO x ) emissions from waste incinerators are comparable to those from fossil fuel combustion [1].Selective catalytic reduction (SCR) with NH 3 has been proven to be an efficient postcombustion deNO x technology and has been applied in some waste incinerators.Although the monolith V 2 O 5 -WO 3 /TiO 2 catalyst is the most commonly used SCR catalyst, several critical issues still need to be solved.Firstly, the high and narrow active temperature window (300-400 • C) of the V 2 O 5 -WO 3 /TiO 2 [2,3] demands additional device and extra energy to reheat the flue gas behind baghouse filter (150-170 • C) so that high NO conversion can be guaranteed [4][5][6]; secondly, the poor resistance of SCR catalysts to complicated flue gas compositions (e.g., SO x , HCl and alkali metals) causes the inevitable deactivation of catalysts and increases the catalyst replacement cost; thirdly, vanadium has biological toxicity, and the final disposal of the discarded V 2 O 5 -WO 3 /TiO 2 catalyst still remains an urgent problem.Therefore, an environmentally friendly and non-vanadium-based catalyst with excellent low-temperature activity and strong resistance needs to be developed.
Manganese oxides (MnO x ) are the most promising alternative to VO x due to its excellent low-temperature activity towards NH 3 -SCR of NO x [7].However, MnO x -based catalysts are susceptible to chlorinated contaminants generated from the incineration of various chlorine-containing wastes [8].As reported, chlorine in wastes mainly transforms into HCl, and its concentration in raw flue gas ranges between 200 and 1500 ppm [9].Although this value can be effectively controlled with alkaline reagents (Ca(OH) 2 or NaHCO 3 ) to meet the China emission limit of 60 mg/m 3 (GB18485-2014), the residual HCl in flue gas still leads to the deactivation of MnO x catalysts [10,11].Cerium oxide (CeO 2 ) is often chosen as a promoter to enhance the low temperature activity as well as the anti-poisoning property of MnO x [12,13].Our previous studies [14,15] found that the combination of CeO 2 with MnO x forms MnCeO x solid solution and accelerates the redox cycle of the catalyst [16].Moreover, the preferential reaction of CeO 2 with poisonous substances protects the active MnO x species from being poisoned [13].
Catalyst support also plays an important role in the optimization and regulation of catalyst performance.Compared with other morphologies of TiO 2 (e.g., nanoparticle, nanowire, nanorod, etc.), titanium dioxide nanotubes (TiNTs) with larger specific surface areas (approximately 400 m 2 /g) and unique hollow tubular structures have been shown to be effective in promoting low-temperature activity and enhancing the resistance property of catalysts [16][17][18].In our previous study [19], we prepared a TiNT-supported MnO x -CeO 2 composite and found that MnCe/TiNT catalyst exhibited higher catalytic activity within a wider temperature range of 100-400 • C and stronger resistance to SO 2 and H 2 O. Lower levels of the harmful byproduct N 2 O and higher N 2 selectivity were achieved using the TiNT-supported MnO x -CeO 2 catalyst.
In this paper, a new preparation process was employed to further improve the catalytic activity and HCl/H 2 O tolerance of the MnCe/TiNT catalyst.The sol-gel method was first used to synthesize a TiO 2 -supported MnO x -CeO 2 composite (denoted as MnCe/TiO 2 ) to enhance the interaction between active components and support.Then, the prepared MnCe/TiO 2 sample was subsequently transformed into the nanotube-like MnCe/TiNT catalyst using hydrothermal treatment.After that, the NH 3 -SCR activity and HCl/H 2 O tolerance of the prepared MnCe/TiNT catalysts were evaluated.Furthermore, catalytic characterizations were also conducted to clarify the promotion mechanism of TiNTs on the performance of the catalyst.

NH 3 -SCR Activity of the MnCe/TiNTs and MnCe/TiO 2 Catalysts
The notations and molar ratios of the synthesized catalysts are listed in Table 1.As shown in Figure 1, the Mn 0.15 Ce 0.05 /TiNT catalyst demonstrates the highest η NO in the entire temperature range of 150-400 • C. Its η NO reaches above 98% even when the temperature is as low as 150 • C and can be maintained at approximately 95% in a wider temperature range of 150-350 • C.This indicates that TiNTs can significantly improve the SCR activity and broaden the active temperature window of the catalyst.This can be attributed to the larger specific surface area and unique hollow tubular structure of TiNTs.Both of them improve the adsorption capacity of the catalyst and capture more reactant molecules for NH 3 -SCR reaction.Moreover, TiNTs achieve high dispersion of active components on the inner and outer wall of TiNTs, which enriches the number of active sites on the catalyst surface [20,21].The above reasons explain the higher catalytic activity of MnCe/TiNTs at low temperatures.nanotubes [12].The above factors explain the lower SCR activities of Mn0.15Ce0.10/TiNTsand Mn0.15Ce0.15/TiNTcatalysts at temperatures below 200 °C.Above 250 °C, the SCR activities of these catalysts also drop dramatically.This can be ascribed to the high temperature over oxidation of NH3 induced by excess CeO2.Therefore, the Mn0.15Ce0.05/TiNTcatalyst (subsequently abbreviated as MnCe/TiNTs) with the highest low temperature activity as well as the broadest active temperature window is selected for the following study.In comparison with our previous obtained MnCe/TiNT catalyst [19], which loaded MnO x and CeO 2 onto TiNTs using an impregnation method (denoted as Mn 0.15 Ce 0.05 /TiNTs-IM), the MnCe/TiNT catalyst in this paper exhibits higher catalytic activity especially at low temperature.This is because the sol-gel method was employed rather than the impregnation method to load active components in this work, effectively enhancing the interaction between Mn, Ce, O and Ti atoms, thus improving the NH 3 -SCR activity of the catalyst.

HCl/H2O Tolerance of the MnCe/TiNTs and MnCe/TiO2 Catalysts
An either too high or too low Ce/Ti molar ratio reduces the NH 3 -SCR activity of the MnCe/TiNT catalyst, especially with an elevated Ce/Ti molar ratio.Excessively low CeO 2 content lowers its interaction with MnO x as well as the catalyst carrie [22].However, excessively high CeO 2 content may (1) reduce the exposure of active Mn atoms instead by incorporating most of the Mn atoms into its bulk phase [22]; (2) block the pore structures on the catalyst surface due to its agglomeration [23]; and (3) obstruct the construction of nanotubes [12].The above factors explain the lower SCR activities of Mn 0.15 Ce 0.10 /TiNTs and Mn 0.15 Ce 0.15 /TiNT catalysts at temperatures below 200 • C. Above 250 • C, the SCR activities of these catalysts also drop dramatically.This can be ascribed to the high temperature over oxidation of NH 3 induced by excess CeO 2 .Therefore, the Mn 0.15 Ce 0.05 /TiNT catalyst (subsequently abbreviated as MnCe/TiNTs) with the highest low temperature activity as well as the broadest active temperature window is selected for the following study.

HCl/H 2 O Tolerance of the MnCe/TiNTs and MnCe/TiO 2 Catalysts
According to Figure 2, the MnCe/TiO 2 and MnCe/TiNT catalysts exhibit a high and stable η NO of 98% at 150 • C for 12 h without HCl or H 2 O. Their SCR activities are almost unaffected with 50 ppm HCl.As for 100 ppm and 200 ppm HCl, the η NO of the MnCe/TiO 2 catalyst decreases rapidly to 70% and 45% within 2 h, respectively, while that of the MnCe/TiNT catalyst maintained at 90% and 62%.The stronger HCl tolerance of the MnCe/TiNT catalyst can be attributed to the following reasons: (1) the highly dispersed Ce atoms can react with HCl preferentially, protecting the active Mn atoms [12]; (2) TiNTs mainly exist in the form of H 2 Ti 3 O 7 , which can provide abundant H protons for Cl atoms to leave the catalyst surface in the form of HCl [24].
the MnCe/TiNT catalyst possesses stronger HCl/H2O tolerance; its ηNO can be maintained above 50% after being exposed to 200 ppm HCl and 15 vol.%H2O for 3 h.After removing HCl and H2O, the ηNO of the MnCe/TiO2 catalyst recovers to 75% while that of the MnCe/TiNT catalyst can be restored to 90%.This result indicates that TiNTs as a support can facilitate the removal of HCl from the catalyst surface and alleviate the irreversible deactivation caused by HCl and H2O.

TEM and SEM Analysis
To clarify the promotion effect of TiNTs on catalytic performance, the MnCe/TiO2 and MnCe/TiNT catalysts treated under different conditions (shown in Table 2) were characterized.The TEM images (Figure 3) demonstrate the unique hollow tubular structure of the MnCe/TiNT catalyst, and its outer diameter, wall thickness and tube lengths are 10-20 nm, 3-4 nm and 300-500 nm, respectively.The surface of the MnCe/TiNT catalyst was found to be smooth, indicating that Mn and Ce atoms enter the TiNTs bulk phase rather than being loaded on the surface.The overlapping of the H2Ti3O7(110) lattice plane (interplanar crystal spacing = 0.36 nm) and Mn2O3(220) lattice plane (interplanar crystal spacing = 0.27 nm) is observed in the TEM image of the MnCe/TiNT catalyst.This observation confirms the close interaction between active components and catalyst support.The MnCe/TiO2 catalyst exists in the form of a granular morphology with particle sizes

Discussion Regarding the Promotion Effect of TiNTs on Catalytic Performance 2.3.1. TEM and SEM Analysis
To clarify the promotion effect of TiNTs on catalytic performance, the MnCe/TiO 2 and MnCe/TiNT catalysts treated under different conditions (shown in Table 2) were characterized.The TEM images (Figure 3) demonstrate the unique hollow tubular structure of the MnCe/TiNT catalyst, and its outer diameter, wall thickness and tube lengths are 10-20 nm, 3-4 nm and 300-500 nm, respectively.The surface of the MnCe/TiNT catalyst was found to be smooth, indicating that Mn and Ce atoms enter the TiNTs bulk phase rather than being loaded on the surface.The overlapping of the H 2 Ti 3 O 7 (110) lattice plane (interplanar crystal spacing = 0.36 nm) and Mn 2 O 3 (220) lattice plane (interplanar crystal spacing = 0.27 nm) is observed in the TEM image of the MnCe/TiNT catalyst.This observation confirms the close interaction between active components and catalyst support.The MnCe/TiO 2 catalyst exists in the form of a granular morphology with particle sizes ranging from 10 to 20 nm.The overlapping of anatase TiO 2 (101) lattice plane (interplanar crystal spacing = 0.35 nm) with the MnO 2 (100) lattice plane (interplanar crystal spacing = 0.24 nm) is also found, indicating the successful loading of MnO x species on TiO 2 .However, CeO 2 crystals are not detected in the TEM images, likely due either to its low content or to its incorporation with the TiNTs bulk phase [19].In the SEM-EDS map, Mn and Ce elements are more highly dispersed on TiNTs surface compared to that on the TiO 2 surface.ranging from 10 to 20 nm.The overlapping of anatase TiO2(101) lattice plane (interplanar crystal spacing = 0.35 nm) with the MnO2(100) lattice plane (interplanar crystal spacing = 0.24 nm) is also found, indicating the successful loading of MnOx species on TiO2.However, CeO2 crystals are not detected in the TEM images, likely due either to its low content or to its incorporation with the TiNTs bulk phase [19].In the SEM-EDS map, Mn and Ce elements are more highly dispersed on TiNTs surface compared to that on the TiO2 surface.

XRD and BET Analysis
According to the XRD patterns in Figure 4, the diffraction peaks corresponding to anatase (PDF#21-1272) rather than rutile TiO2 appear in the MnCe/TiO2 catalyst.The diffraction peaks at 37.1° and 55.1° correspond to the (100) lattice plane of MnO2 (PDF#30-

XRD and BET Analysis
According to the XRD patterns in Figure 4, the diffraction peaks corresponding to anatase (PDF#21-1272) rather than rutile TiO 2 appear in the MnCe/TiO 2 catalyst.The diffraction peaks at 37.1 • and 55.1 • correspond to the (100) lattice plane of MnO 2 (PDF#30-0820) and (440) lattice plane of Mn 2 O 3 (PDF#41-1422), respectively [7].The peaks belonging to the CeO 2 crystal are not detected either due to its low content or the formation of MnCeO x solid solution [12].As for the MnCe/TiO 2 -Cl catalyst, the diffraction peaks assigned to MnO x and TiO 2 become weakened, mainly because the chlorination of the MnCe/TiO 2 catalyst distorts the crystal structures [14].The diffraction peaks of the MnCe/TiO 2 -Cl-H catalyst are not further weakened, implying that the addition of H 2 O barely influences the crystal structure of catalyst.Compared with the MnCe/TiO2 catalyst, the SBET and Vtot of the MnCe/TiNT catalyst increase from 143.5 m 2 /g and 0.28 cm 3 /g to 191.7 m 2 /g and 0.37 cm 3 /g, respectively.However, those of the MnCe/TiO2-Cl catalyst decrease by 45% and 22%, while those of the MnCe/TiNTs-Cl catalyst decrease by only 14%.This further proves that TiNTs can relieve the negative effect of HCl on the catalyst.The SBET value of the MnCe/TiNTs-Cl-H catalyst is slightly larger than that of the MnCe/TiNTs-Cl catalyst, indicating that the presence of H2O can prevent the catalyst from being destructed by HCl to some extent.

NH3-TPD and H2-TPR Analysis
NH3-SCR reaction begins with the adsorption of NH3 onto the surface acid sites, followed by the redox reaction between the adsorbed NH3 with the gaseous NO or adsorbed NO [27].Therefore, surface acidity is one of the crucial factors that determines the catalytic activity.Studies [12,28] have reported that the desorption peak at approximately 200 °C refers to the desorption of NH4 + adsorbed on Brønsted acid sites (labeled as B acid sites), while the peak increases at approximately 400 °C due to the desorption of NH3 coordinated on Lewis acidic sites (labeled as L acid sites).As shown in Figure 5a, the total number of surface acid sites on the fresh MnCe/TiNT catalyst is larger than that on the MnCe/TiO2 catalyst.This could be attributed, on one hand, to the TiNTs being able to expose more active metal atoms to catalyst surface and increase the surface L acid sites [29]; on the other hand, TiNTs mainly exists in the form of H2Ti3O7, which introduces abundant OH groups serving as B acid sites.
The number of surface acid sites on the MnCe/TiO2-Cl catalyst is significantly reduced due to the adsorption of HCl onto the uncoordinated metal sites (M-O-M+HCl→M-OH+M-Cl) [30] as well as the interaction between HCl and surface OH groups (M- As for the MnCe/TiNT catalyst, three main diffraction peaks at 24.5 • , 27.5 • and 48.5 • correspond to (200), ( 110) and (020) lattice planes of H 2 Ti 3 O 7 (PDF#47-0561), respectively [26].However, the intensities of these diffraction peaks are rather weak, possibly resulting from the incorporation of Mn and Ce atoms into its bulk phase, which distorts the crystal structure of TiNTs.The diffraction peak corresponding to CeCl 3 appears in the XRD pattern of the MnCe/TiNTs-Cl catalyst, possibly because the highly dispersed Ce atoms react with HCl.Meanwhile, more reflections belonging to H 2 Ti 3 O 7 appear at the positions of 50-80 • .This phenomenon can be attributed to the interaction between Cl atoms and TiNTs, which alters the crystal structure of TiNTs [24].As for the MnCe/TiNTs-Cl-H catalyst, the intensity of the diffraction peak corresponding to CeCl 3 is significantly weakened, confirming that the presence of H 2 O can alleviate the catalytic deactivation induced by HCl [25].
Compared with the MnCe/TiO 2 catalyst, the S BET and V tot of the MnCe/TiNT catalyst increase from 143.5 m 2 /g and 0.28 cm 3 /g to 191.7 m 2 /g and 0.37 cm 3 /g, respectively.However, those of the MnCe/TiO 2 -Cl catalyst decrease by 45% and 22%, while those of the MnCe/TiNTs-Cl catalyst decrease by only 14%.This further proves that TiNTs can relieve the negative effect of HCl on the catalyst.The S BET value of the MnCe/TiNTs-Cl-H catalyst is slightly larger than that of the MnCe/TiNTs-Cl catalyst, indicating that the presence of H 2 O can prevent the catalyst from being destructed by HCl to some extent.

NH 3 -TPD and H 2 -TPR Analysis
NH 3 -SCR reaction begins with the adsorption of NH 3 onto the surface acid sites, followed by the redox reaction between the adsorbed NH 3 with the gaseous NO or adsorbed NO [27].Therefore, surface acidity is one of the crucial factors that determines the catalytic activity.Studies [12,28] have reported that the desorption peak at approximately 200 • C refers to the desorption of NH 4 + adsorbed on Brønsted acid sites (labeled as B acid sites), while the peak increases at approximately 400 • C due to the desorption of NH 3 coordinated on Lewis acidic sites (labeled as L acid sites).As shown in Figure 5a, the total number of surface acid sites on the fresh MnCe/TiNT catalyst is larger than that on the MnCe/TiO 2 catalyst.This could be attributed, on one hand, to the TiNTs being able to expose more active metal atoms to catalyst surface and increase the surface L acid sites [29]; on the other hand, TiNTs mainly exists in the form of H 2 Ti 3 O 7 , which introduces abundant OH groups serving as B acid sites.
Catalysts 2024, 14, x FOR PEER REVIEW 8 of 14 observed for the fresh catalyst, suggesting that the presence of H2O can improve the surface acidity, alleviating HCl poisoning of the catalyst to some extent.As shown in Figure 5b, the reduction peaks of the MnCe/TiNT catalyst are shifted to the higher temperature region in comparison with the MnCe/TiO2 catalyst.This is possibly because the partial surface reactive oxygen species enter the nanotube and bond more tightly to the inner walls of TiNTs, which makes it more difficult for them to be offered to oxidize H2.A similar result was observed in our previous study [19].TiNTs can effectively regulate the redox ability of the catalyst, which contributes to a reduction in the excessive oxidation of NH3 and broadens the active temperature window of the MnCe/TiNT catalyst.
As for the MnCe/TiO2-Cl catalyst, the total area of the reduction peaks decreases and their positions are shifted to higher temperatures significantly more compared to the fresh one.This can be ascribed to the anchor of the chlorine atoms on the surface Ov, which hinders the redox cycle of the catalyst [32,33].As for the MnCe/TiNTs-Cl catalyst, the positions remain unchanged, and their areas decline slightly.This explains the stronger resistance of the MnCe/TiNT catalyst to HCl.Furthermore, the reduction peaks of the MnCe/TiO2-Cl-H and MnCe/TiNTs-Cl-H catalysts change less.This phenomenon confirms that the chlorination of the surface reactive components can be effectively alleviated in the presence of H2O, especially in the case of the MnCe/TiNT catalyst.

TG Analysis
As reported, mass loss at 30-200 °C can be attributed to the evaporation of physically adsorbed H2O, and that within 200-600 °C is assigned to the decomposition of the residual The number of surface acid sites on the MnCe/TiO 2 -Cl catalyst is significantly reduced due to the adsorption of HCl onto the uncoordinated metal sites (M-O-M+HCl→M-OH+M-Cl) [30] as well as the interaction between HCl and surface OH groups (M-OH+HCl→M-Cl+H 2 O) [31].As for the MnCe/TiNT catalyst, the surface acidity is almost maintained after treatment with HCl.This is mainly because (1) the highly dispersed Ce atoms on the TiNTs surface can preferentially react with HCl and protect the surface active MnO x and (2) the abundant OH groups on TiNTs surface provide more H protons to expel the adsorbed Cl atoms.However, shifts in the NH 3 desorption peaks for the MnCe/TiNTs-Cl catalyst are observed due to the reaction of HCl with surface OH group and active components, which changes the surface structure of the catalyst.The total number of surface acid sites on the MnCe/TiNTs-Cl-H catalyst was found to be even higher than that observed for the fresh catalyst, suggesting that the presence of H 2 O can improve the surface acidity, alleviating HCl poisoning of the catalyst to some extent.
As shown in Figure 5b, the reduction peaks of the MnCe/TiNT catalyst are shifted to the higher temperature region in comparison with the MnCe/TiO 2 catalyst.This is possibly because the partial surface reactive oxygen species enter the nanotube and bond more tightly to the inner walls of TiNTs, which makes it more difficult for them to be offered to oxidize H 2 .A similar result was observed in our previous study [19].TiNTs can effectively regulate the redox ability of the catalyst, which contributes to a reduction in the excessive oxidation of NH 3 and broadens the active temperature window of the MnCe/TiNT catalyst.
As for the MnCe/TiO 2 -Cl catalyst, the total area of the reduction peaks decreases and their positions are shifted to higher temperatures significantly more compared to the fresh one.This can be ascribed to the anchor of the chlorine atoms on the surface O v , which hinders the redox cycle of the catalyst [32,33].As for the MnCe/TiNTs-Cl catalyst, the positions remain unchanged, and their areas decline slightly.This explains the stronger resistance of the MnCe/TiNT catalyst to HCl.Furthermore, the reduction peaks of the MnCe/TiO 2 -Cl-H and MnCe/TiNTs-Cl-H catalysts change less.This phenomenon confirms that the chlorination of the surface reactive components can be effectively alleviated in the presence of H 2 O, especially in the case of the MnCe/TiNT catalyst.

TG Analysis
As reported, mass loss at 30-200 • C can be attributed to the evaporation of physically adsorbed H 2 O, and that within 200-600 • C is assigned to the decomposition of the residual organic compounds on the catalyst surface [34].Additionally, the mass loss above 600 • C occurs due to the decomposition of active components on the catalyst [35].The MnCe/TiNT catalyst shows larger mass loss below 200 • C than the MnCe/TiO 2 catalyst (Figure 6), possibly because the greater hydrophilicity of TiNTs allows the adsorption of more H 2 O molecules.
Catalysts 2024, 14, x FOR PEER REVIEW 9 of 14 the desorption of the weakly adsorbed HCl [36]; the latter can be ascribed to the volatilization and decomposition of metal chlorides on the catalyst surface, which is produced from the chlorination of the catalyst [37].The mass loss of the MnCe/TiNTs-Cl catalyst above 600 °C is clearly lower than that of the MnCe/TiO2-Cl catalyst.This confirms that TiNTs can hinder the chlorination of catalyst induced by HCl.The MnCe/TiO2-Cl-H and MnCe/TiNTs-Cl-H catalysts exhibited significant mass loss below 600 °C, while that above 600 °C was barely observed.This further suggests that the presence of H2O can alleviate the chlorination of catalysts.

XPS Analysis
The XPS spectra of the catalysts are shown in Figure 7.In the Mn 2p spectra, the peaks located at binding energies of 652.2 eV (or 640.7 eV), 653.5 eV (or 642.1 eV), and 655.3 eV (or 645.5 eV) correspond to Mn 2+ , Mn 3+ , and Mn 4+ respectively.As for the Ce 3d spectra, the peaks at 904.7 eV and 885.7 eV can be assigned to Ce 3+ , with the rest belonging to Ce 4+ Error!Reference source not found.. A higher Mn 4+ content indicates a better redox ability of the catalyst, and Ce 3+ content is regarded as an indicator of surface oxygen vacancy (Ov) [20].As shown in Table 3, the MnCe/TiNT catalyst exhibits higher Mn 4+ (30.8%) and Ce 3+ (27.3%) contents, while those of the MnCe/TiO2 catalyst are merely 26.0% and 21.7%, respectively.This is one of the main explanations for the higher catalytic activity of the MnCe/TiNT catalyst.In comparison with the fresh MnCe/TiO 2 catalyst, more mass loss is observed below 200 • C and above 600 • C for the MnCe/TiO 2 -Cl catalyst.The former can be attributed to the desorption of the weakly adsorbed HCl [36]; the latter can be ascribed to the volatilization and decomposition of metal chlorides on the catalyst surface, which is produced from the chlorination of the catalyst [37].The mass loss of the MnCe/TiNTs-Cl catalyst above 600 • C is clearly lower than that of the MnCe/TiO 2 -Cl catalyst.This confirms that TiNTs can hinder the chlorination of catalyst induced by HCl.The MnCe/TiO 2 -Cl-H and MnCe/TiNTs-Cl-H catalysts exhibited significant mass loss below 600 • C, while that above 600 • C was barely observed.This further suggests that the presence of H 2 O can alleviate the chlorination of catalysts.

XPS Analysis
The XPS spectra of the catalysts are shown in Figure 7.In the Mn 2p spectra, the peaks located at binding energies of 652.2 eV (or 640.7 eV), 653.5 eV (or 642.1 eV), and 655.3 eV (or 645.5 eV) correspond to Mn 2+ , Mn 3+ , and Mn 4+ respectively.As for the Ce 3d spectra, the peaks at 904.7 eV and 885.7 eV can be assigned to Ce 3+ , with the rest belonging to Ce 4+ [12].A higher Mn 4+ content indicates a better redox ability of the catalyst, and Ce 3+ content is regarded as an indicator of surface oxygen vacancy (O v ) [20].As shown in Table 3, the MnCe/TiNT catalyst exhibits higher Mn 4+ (30.8%) and Ce 3+ (27.3%) contents, while those of the MnCe/TiO 2 catalyst are merely 26.0% and 21.7%, respectively.This is one of the main explanations for the higher catalytic activity of the MnCe/TiNT catalyst.
Therefore, the Oα/(Oα+Oβ) ratio is an important indicator of the catalytic activity [19].Compared with the MnCe/TiO2 catalyst, the MnCe/TiNT catalyst has a higher Oα/(Oα+Oβ) value and the position of its Oβ peak shifts to higher binding energy.Both of these factors contribute to the larger oxygen storage capacity as well as the higher surface oxygen mobility of the catalyst [19].The Oα/(Oα+Oβ) ratio of the MnCe/TiO2-Cl catalyst is significantly lower than that of the fresh one.As for the MnCe/TiNTs-Cl catalyst, a lower reduction in the Oα/(Oα+Oβ) ratio is observed, mainly because the abundant OH groups on the TiNTs surface provide H + protons to expel Cl atoms in the form of HCl.The Oα/(Oα+Oβ) ratios of the MnCe/TiO2-Cl-H and MnCe/TiNTs-Cl-H catalysts are larger than those of the MnCe/TiO2-Cl and MnCe/TiNTs-Cl catalysts.This agrees well with the above conclusion that the presence of H2O mitigates the chlorination of the catalyst.
The contents of Cl atoms adsorbed on the catalyst surface are determined from XPS spectra, and the results are also listed in Table 3.Compared to the MnCe/TiO2-Cl catalyst, the MnCe/TiNTs-Cl catalyst shows a significantly lower surface Cl content.This further supports the conclusion that TiNTs can reduce the adsorption of HCl on the catalyst surface.Moreover, the MnCe/TiO2-Cl-H and MnCe/TiNTs-Cl-H catalysts show lower Cl content than the MnCe/TiO2-Cl and MnCe/TiNTs-Cl catalysts.This also confirms that the presence of H2O can relieve the deposition of Cl atoms on the catalyst surface.

Catalyst Preparation and Characterization
MnCe/TiNT catalysts were synthesized following two steps.Firstly, the MnCe/TiO2 sample was prepared using the sol-gel method [7] using manganese nitrate, cerium ni-  The Mn 4+ and Ce 3+ contents on the MnCe/TiO 2 catalyst decrease to 21.8% and 16.2%, respectively, resulting from the adsorption of HCl, which inhibits the redox cycle of the catalyst.The MnCe/TiNTs-Cl catalyst exhibits a smaller decrease in both Mn 4+ (28.9%) and Ce 3+ (24.0%) content, confirming that TiNTs can protect the surface-active sites and enhance the resistance of the catalyst to HCl.The Mn 4+ (29.9%) and Ce 3+ (25.5%) contents of the MnCe/TiNTs-Cl-H catalyst are almost equal to that of the fresh catalyst, further confirming that the presence of H 2 O can protect the catalyst from being poisoned by HCl.
The O 1s spectra show two peaks at 531.5 eV and 530.0 eV that correspond to the surface chemisorbed oxygen (O α ) and surface reactive lattice oxygen (O β ), respectively [38].O α is more readily involved in the redox reactions due to its strong mobility [39].Therefore, the O α /(O α +O β ) ratio is an important indicator of the catalytic activity [19].Compared with the MnCe/TiO 2 catalyst, the MnCe/TiNT catalyst has a higher O α /(O α +O β ) value and the position of its O β peak shifts to higher binding energy.Both of these factors contribute to the larger oxygen storage capacity as well as the higher surface oxygen mobility of the catalyst [19].The O α /(O α +O β ) ratio of the MnCe/TiO 2 -Cl catalyst is significantly lower than that of the fresh one.As for the MnCe/TiNTs-Cl catalyst, a lower reduction in the O α /(O α +O β ) ratio is observed, mainly because the abundant OH groups on the TiNTs surface provide H + protons to expel Cl atoms in the form of HCl.The O α /(O α +O β ) ratios of the MnCe/TiO 2 -Cl-H and MnCe/TiNTs-Cl-H catalysts are larger than those of the MnCe/TiO 2 -Cl and MnCe/TiNTs-Cl catalysts.This agrees well with the above conclusion that the presence of H 2 O mitigates the chlorination of the catalyst.
The contents of Cl atoms adsorbed on the catalyst surface are determined from XPS spectra, and the results are also listed in Table 3.Compared to the MnCe/TiO 2 -Cl catalyst, the MnCe/TiNTs-Cl catalyst shows a significantly lower surface Cl content.This further supports the conclusion that TiNTs can reduce the adsorption of HCl on the catalyst surface.Moreover, the MnCe/TiO 2 -Cl-H and MnCe/TiNTs-Cl-H catalysts show lower Cl content than the MnCe/TiO 2 -Cl and MnCe/TiNTs-Cl catalysts.This also confirms that the presence of H 2 O can relieve the deposition of Cl atoms on the catalyst surface.

Catalyst Preparation and Characterization
MnCe/TiNT catalysts were synthesized following two steps.Firstly, the MnCe/TiO 2 sample was prepared using the sol-gel method [7] using manganese nitrate, cerium nitrate and tetrabutyl titanate as precursors.Secondly, the prepared MnCe/TiO 2 sample was mixed with 10 mol/L NaOH solution and then transferred to a high-pressure hydrothermal reactor.After reacting at 150 • C for 24 h, the mixture was cooled and washed with deionized water.Subsequently, the obtained solid was aged in 0.1 mol/L HCl solution for 12 h, followed by washing with deionized water to achieve a neutral pH.Then, the sample was dried at 100 • C for 12 h and crushed to 40-60 mesh.Finally, the sample was calcined in air at 400 • C for 2.5 h to obtain the MnCe/TiNT catalyst.For comparison, the catalytic activity of the MnCe/TiO 2 sample obtained in the first step was also tested.
Several characterizations including TEM, SEM-EDS, XRD, N 2 adsorption-desorption measurements with BET analysis, EPR, NH 3 -TPD, H 2 -TPR, XPS, and TG/DTA were employed.The XRD patterns of the samples were measured using an XRD-6100 powder diffractometer (Shimadzu, Tokyo, Japan) with CuKα radiation (λ = 0.15406 nm).The catalyst samples were scanned over the range of 10−80 • at a rate of 2 • /min.SEM-EDS (MIRA3&Aztec X-Max, FEI, Hillsboro, OR, USA) was carried out at an operating voltage of 0.2 kV-30 kV.The elemental distribution of catalyst was obtained using EDS energy spectroscopy scanning.The morphologies of samples were examined using TEM (JEM-2010, Tokyo, Japan).The specific surface area and pore size distribution of the catalysts were determined at 77 K using an N 2 adsorption instrument equipped with an automated BET and pore analyzer (ASAP2020, Mike Instrument Co. Atlanta, GA, USA).Prior to the measurement, the samples were degassed in vacuum at 573 K for 3 h.XPS with Al Ka X-rays (Thermo ESCALAB 250Xi, Thermo Fisher Scientific, Wilmington, MA, USA) was employed to measure the surface atomic states of the catalysts.All the binding energies in XPS were calibrated using C1 s (284.8 eV) as a reference.H 2 -TPR (Micromeritics Auto chem II 2920, Mike Instrument Co. Atlanta, GA, USA) was employed to measure the reducibility of catalysts, respectively.Prior to H 2 -TPR, the catalyst samples (ca.70 mg) were pretreated in He at 300 • C for 2 h followed by cooling down to 50 • C.After that, the H 2 -TPR was implemented from 100 to 700 • C at a heating rate of 10 • C/min in 10% H 2 /Ar (50 mL/min).Prior to NH 3 -TPD, the catalyst samples (ca.70 mg) were pretreated in He at 300 • C for 2 h followed by cooling down to 50 • C.After that, the NH 3 -TPD was implemented from 100 to 700 • C at a heating rate of 10 • C/min in 10% NH 3 /Ar (50 mL/min) and the NH 3 consumption was monitored using a thermal conductivity detector (TCD).TG/DTA was performed on a STA-449F3 thermal analyzer at a heating rate of 10 • C/min from room temperature to 600 • C under an air atmosphere.

Catalytic Activity Measurement
The activity evaluation of the catalysts was carried out using a quartz tube fixed-bed reactor (inner diameter × length = 10 mm × 500 mm), in which approximately 2.5 g (~2.0 mL) of powder catalyst within 40-60 mesh was loaded with the aid of a quartz screen.The catalyst was heated to the desired temperature (150-400 • C) using a tubular furnace equipped with an electric heating device.The simulated flue gas consisted of NO (500 ppm), NH 3 (500 ppm), O 2 (11 vol.%),HCl (50-200 ppm, when used), H 2 O (5-15 vol.%, when used) and N 2 (as a balance).The total gas flow was maintained at 1000 mL/min using a mass flow controller, corresponding to a gas hourly space velocity (GHSV) of 30,000 h −1 .Repeat experiments were conducted to ensure the reliability of the data.The inlet (C in ) and outlet (C out ) concentrations of NO were monitored using a flue gas analyzer (Delta 2000CD-IV, MRU, Heilbronn, Germany).The NO conversion efficiency (η NO , %) was calculated as follows:

Conclusions
In this paper, the MnCe/TiNT catalyst was prepared from the MnCe/TiO 2 catalyst using a hydrothermal method.The MnCe/TiNT catalyst with molar ratios of Mn/Ce/ Ti = 0.15/0.05/1exhibited optimal NH 3 -SCR performance (above 95%) within a wide temperature window of 150-350 • C. In addition, the MnCe/TiNT catalyst demonstrated strong resistance to HCl.Its high catalytic activity as well as excellent HCl tolerance at low temperature can be attributed to several factors: (1) the large specific surface area of the TiNTs achieves high dispersion of active components on its inner and outer walls, creating more active sites for NH 3 -SCR reaction and exposing more Ce atoms to preferential reaction with HCl; (2) TiNTs primarily exist in the form of H 2 Ti 3 O 7 and can provide abundant OH groups to serve as B acid sites; (3) the OH groups on the TiNTs surface can also provide sufficient H + protons and drive the adsorbed Cl atoms away from the catalyst surface in the form of HCl.The presence of H 2 O reduces the NO x removal efficiency, while the irreversible deactivation caused by HCl can be effectively alleviated with the addition of H 2 O, especially for the MnCe/TiNT catalyst.Therefore, TiNTs represent an effective support to enhance the low-temperature deNO x activity as well as the resistance of the MnO x -CeO 2 composite to HCl and H 2 O and have the potential to be further improved in the future study.

Figure 2 .
Figure 2. The HCl and H 2 O tolerances of the MnCe/TiNTs and MnCe/TiO 2 catalysts.The addition of H 2 O further reduces the η NO of the MnCe/TiO 2 and MnCe/TiNT catalysts at 150 • C.This is because H 2 O competes with the reactants for surface adsorption sites and hinders the NH 3 -SCR reaction [25].In comparison with the MnCe/TiO 2 catalyst, the MnCe/TiNT catalyst possesses stronger HCl/H 2 O tolerance; its η NO can be maintained above 50% after being exposed to 200 ppm HCl and 15 vol.%H 2 O for 3 h.After removing HCl and H 2 O, the η NO of the MnCe/TiO 2 catalyst recovers to 75% while that of the MnCe/TiNT catalyst can be restored to 90%.This result indicates that TiNTs as a support can facilitate the removal of HCl from the catalyst surface and alleviate the irreversible deactivation caused by HCl and H 2 O.

Figure 4 .
Figure 4. XRD patterns of the fresh and the treated MnCe/TiNTs and MnCe/TiO2 catalysts.

Figure 4 .
Figure 4. XRD patterns of the fresh and the treated MnCe/TiNTs and MnCe/TiO 2 catalysts.

Table 1 .
The notations and molar ratios of the MnCe/TiO 2 and MnCe/TiNT catalysts.

Table 2 .
The treatment conditions and structural information of the MnCe/TiO 2 and MnCe/TiNT catalysts.
a S BET , V tot and D ave represent BET specific surface area, total pore volume and average pore diameter of catalysts determined via N 2 adsorption-desorption.

Table 2 .
The treatment conditions and structural information of the MnCe/TiO2 and MnCe/TiNT catalysts.
a SBET, Vtot and Dave represent BET specific surface area, total pore volume and average pore diameter of catalysts determined via N2 adsorption-desorption.Catalysts 2024, 14, x FOR PEER REVIEW 6 of 14

Table 3 .
Surface element content and metal valence state distribution of the fresh and treated MnCe/TiNTs and MnCe/TiO2 catalysts.

Table 3 .
Surface element content and metal valence state distribution of the fresh and treated MnCe/TiNTs and MnCe/TiO 2 catalysts.