Effect of water vapor on sulfur poisoning of MnO_x–CeO₂/Al₂O₃ catalyst for diesel soot oxidation

Abstract

MnO_x–CeO₂ mixed oxide supported on γ-Al₂O₃ was sulfated in dry and wet atmospheres to explore the effect of water during sulfur poisoning. The fresh and poisoned catalysts were characterized by Brunauer–Emmett–Teller (BET), thermo-gravimetric analysis (TGA), NH₃ temperature-programmed desorption (NH₃-TPD), infrared spectroscopy (IR), H₂ temperature-programmed reduction (H₂-TPR), NO temperature-programmed oxidation (NO-TPO) and soot temperature-programmed oxidation (soot-TPO). The results show that water hinders sulfate deposition on the catalyst. The wet sulfated catalyst with abundant surface hydroxyl groups has higher surface acidity than the dry sulfated one. Although the presence of water does not prevent the deprivation of redox properties, the generated surface acid sites may promote the NO₂ utilization efficiency and deep oxidation of soot by O₂, resulting in less deactivation of the wet sulfated catalyst.

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1. Introduction

Soot is an undesirable byproduct in the exhausts of diesel engines.^1,2 Several techniques have been applied in the elimination of soot particulates. Trapping on diesel particulate filters (DPFs) followed by catalytic oxidation is one of the efficient routes to soot elimination.^3,4 Platinum is an important ingredient of commercial soot oxidation catalysts. But the high cost limits the application of platinum catalysts. The applications of ceria based mixed oxide catalysts for soot oxidation, such as Co–Ce,⁵ Mn–Ce,⁶ Cu–Ce⁷ and Sm–Ce,⁸ have been well reported in recent articles. Our previous studies show that MnO_x–CeO₂/Al₂O₃ is an alternative promising candidate to commercial soot oxidation catalysts.^9,10 It shows high soot oxidation activity in both O₂ and NO + O₂ due to the superior redox properties and high NO oxidation ability with the synergetic effect between MnO_x and CeO₂. Besides, the alumina supported MnO_x–CeO₂ catalyst shows a higher thermal stability than the unsupported mixed oxides because of the diffusion barrier effect of Al₂O₃.^9–11 However, the poor tolerance to sulfur dioxide may retard its application. Although sulfur poisoning is no longer a major threat to soot oxidation catalysts as the sulfur component in diesel is being limited more and more strictly, some undeveloped zones are still lacking access to low sulfur diesel fuels.¹² Thus, the sulfur poisoning performance and mechanism of the mixed oxides catalyst are still worth studying.

As water vapor is one of the major components of exhaust gas (≈10%) from diesel engines,¹³ the catalyst tolerance to sulfur poisoning under wet atmosphere have been widely evaluated. A few of studies has focused on SO₂ and H₂O in reactant gas. Oi-Uchisawa et al. studied the effect of NO, H₂O and SO₂ during the soot oxidation catalyzed by Pt/SiO₂ and draw the conclusion that SO₃ (or H₂SO₄) produced from SO₂ has a role as a catalyst that accelerates the carbon oxidation by NO₂ in the presence of H₂O, and H₂O is indispensable for this promoted oxidation process, possibly as a reactant for the hydrolysis of partially oxidized species.¹⁴ Hernández-Giménez et al. provided a descriptive discussion of CO₂, H₂O and SO₂ effect on CeZr and CeZrNd catalysts during reaction and paid special attention to the soot combustion mechanism. They concluded that the inhibiting effect follows the order of SO₂ > H₂O > CO₂.¹⁵ However, the corresponding structural variations of the catalysts were not taken into account. On the other hand, some literatures studied the effects of water and sulfur dioxide treatment on catalyst separately but the synergistic effect was not considered. For example, Peralta et al. studied water and sulfur dioxide on Ba,K/CeO₂ respectively, but did not involve the effect of water vapor and SO₂ pumped in together.¹⁶ Tikhomirov et al. studied wet sulfated MnO_x–CeO₂ mixed oxides but the water effect on sulfur poisoning was not analyzed.¹⁷

In our previous studies, severe sulfur poisoning under dry condition was observed for MnO_x–CeO₂/Al₂O₃ catalyst, although the activity could be partially recovered after regeneration in oxidative or reducing atmosphere.^18–20 In this study, the mixed oxides catalyst was sulfated in dry and wet atmospheres, respectively. The amounts and types of the deposited sulfates were analyzed and the soot oxidation activities of the catalysts were measured. Water effect on sulfur poisoning were explored based on the catalyst redox property and surface acidity.

2. Experimental

2.1. Catalyst preparation

MnO_x–CeO₂/Al₂O₃ mixed oxides were prepared with a sol–gel method which was reported previously,²⁰ with a Mn : Ce molar ratio of 15 : 85 and a mass ratio of (Mn₃O₄ + CeO₂) : Al₂O₃ = 1 : 2. As proved in our previous work,¹⁰ the manganese species existed mainly in the form of Mn₃O₄ in the mixed oxides. The as-received powders were grinded for 10 min and were labeled as MCA-F.

Sulfur poisoning processes were treated with SO₂ as sulfur species in automobile exhaust gas is almost exclusively SO₂.²¹ Dry sulfated catalyst (MCA-DS) was obtained by flushing 100 ppm SO₂/10% O₂/N₂ through 1 g MCA-F catalyst (50–100 mesh) at 350 °C. The sulfation poisoning lasted for 3.5 h until a significant deactivation was observed. During the process, the SO₂ concentration was monitored by an emission analyzer (ecom EN-2F, RBR, Germany). Wet sulfated catalyst (MCA-WS) was obtained by a similar process except 10% H₂O was added in the inlet gas. For reference, a sulfate-impregnated sample (MCA-IS) was prepared by impregnating H₂SO₄ solution on MCA-F with a theoretical content of SO₃ at 6 wt%, followed by drying at 110 °C overnight and calcination at 500 °C for 2 h.

2.2. Catalyst characterizations

Thermo-gravimetric analysis (TGA) was conducted by a thermos gravimetric analyzer (Mettler Toledo TGA/DSC 1). Approximately 20 mg sample was added into an alumina pan and each test was preformed from room temperature (RT) to 1000 °C under a flow rate of 50 mL min⁻¹ of N₂.

Specific surface areas (S_BET) of the samples were measured by means of the N₂ adsorption at −196 °C (F-Sorb 3400, Gold APP Instrument). The samples were degassed at 220 °C for 2 h before the measurements to remove adsorbed water and other atmospheric contaminants.

Infrared spectra (IR) on the samples were recorded on a FTIR spectrometer (Thermo Nicolet iS50) equipped with a MCT detector. The spectra was taken under RT. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with ammonia desorption were also taken in the same apparatus from 50 to 350 °C.

NH₃ temperature-programmed desorption (NH₃-TPD) tests were performed. The sample powders were firstly treated in N₂ at 500 °C for 30 min. Then the samples were exposed in 1000 ppm of NH₃/N₂ at RT. After N₂ flushing for 30 min, the reactor were ramped from RT to 600 °C at a heating rate of 10 °C min⁻¹ under N₂ stream and the outlet NH₃ concentration was analyzed by an infrared spectrometer (MKS 2030).

H₂ temperature-programmed reduction (TPR) was carried out in a fixed bed reactor. 50 mg sample was pretreated under He flow at 400 °C for 30 min. The reduction test was performed in 10% H₂/Ar (50 mL min⁻¹) from room temperature (RT) to 900 °C at a heating rate of 10 °C min⁻¹ and H₂ consumption was recorded using a thermal conductivity detector (TCD).

2.3. Activity measurements

10 mg soot (Printex-U, Degussa AG) and 100 mg catalyst powders were mixed by a spatula for 2 min so the soot and the catalyst was in the state of “loose contact”.²² The mixture was diluted with 300 mg inert silica to avoid pressure drop and reaction runaway. Temperature-programmed oxidation (soot-TPO) were performed from RT to 650 °C at a heating rate of 10 °C min⁻¹. The inlet gas mixture was 1000 ppm NO/10% O₂/N₂ or 10% O₂/N₂ by 500 mL min⁻¹. Although the concentration of NO in exhaust from advance diesel engines is generally lower than 1000 ppm, this concentration is also widely found and applied to evaluate the catalyst activities.^23,24 The outlet CO₂, CO, NO and NO₂ concentrations were monitored by an infrared spectrometer (Thermo Nicolet iS10).

NO temperature-programmed oxidation (NO-TPO) tests were carried out in the same apparatus in soot-TPO tests. 100 mg catalyst powders were mixed with 300 mg silica and the inlet gas mixture was 1000 ppm NO/10% O₂/N₂.

3. Results and discussion

3.1. Deposition of sulfates

TGA curves of the fresh and poisoned catalysts were measured to determine the amounts of the deposited sulfates, and the results are shown in Fig. 1. All the catalysts are dehydrated at 30–200 °C.¹⁸ The mass loss for 200–500 °C was mainly due to O₂ released from the mixed oxides due to the Mn–Ce synergistic effect.²⁵ The decomposition of chemically adsorbed hydroxyl species, nitrates, and organic substrates from the catalyst cannot be excluded, which will be confirmed by the following IR results, although the catalyst was calcined at 500 °C. For the poisoned samples, the decomposition of sulfate occurs mainly at 500–1000 °C.²⁰ The basicity of ceria is much stronger than those of alumina and manganese oxides, and the former oxide can inhibit the formation of manganese sulfates.²⁶ Thus, the dominating sulfate on MCA-DS and MCA-WS is suggested as cerium sulfate. According to our previous study¹⁸ and other report,²⁵ the initial decomposition of Ce(SO₄)₂ is observed at a temperature interval of 200–500 °C via the conversion to an oxysulfate intermediate CeOSO₄. The rapid decrease in mass occurred at a temperature range 500–1000 °C (mainly 680–900 °C) corresponds to the further decomposition to CeO₂, although the possibility of decomposition of MnSO₄ (820–900 °C) and Al₂O(SO₄)₂ (700–900 °C) cannot be excluded.¹⁸ It is noted that the mass loss for MCA-WS at high temperatures consists of two parts. The mass loss at 500–680 °C may be attributed to the decomposition of surface cerium sulfate generated in wet atmosphere, and that at higher temperatures can be due to the decomposition of bulk sulfates. It reflects differences in the thermal stability of surface versus bulk sulfates species.^27,28

Fig. 1 TGA curves of the catalysts in N₂.

The mass loss of the catalysts within the temperature range from 500 to 1000 °C was calculated, and the results are listed in Table 1. The sulfation degree was defined as molar ratio of (cerium which formed sulfate)/(total cerium), in which cerium in sulfates was estimated by the mass loss (SO₃) based on the assumption of the conversion from Ce(SO₄)₂ to Ce₂O₃. The sulfation degree of MCA-DS and MCA-WS was calculated as 24% and 18%, respectively. It suggests that the presence of water vapor in the sulfation atmosphere prevents the catalyst absorbing SO₃ and sulfates deposition. The deposition of sulfates blocks the pore structure and reduces the BET surface area of the catalyst. As listed in the Table 1, MCA-DS with more deposited sulfates has a smaller specific surface area than MCA-WS. The outlet SO₂ concentration was recorded during the sulfation process, and the SO₂ conversion curves as a function of time are shown in Fig. 2. It is seen that the SO₂ oxidation is always close to 100% within the first 100 min. After then, the SO₂ conversion decreases more rapidly under the dry condition, which may be due to the severe loss in redox property of the catalyst with the formation of more sulfates. It is noted, nevertheless, that the gap between the SO₂ conversions under different conditions is not so large to induce the different amounts of sulfates deposited on the catalysts. Thus, the less sulfates on MCA-WS may be more probably caused by the hindering effect of the water adsorbed on the Ce and Mn sites on the SO₃ adsorption and sulfate deposition.

Table 1 Structural features of the catalysts

a Measured at 500–1000 °C by TGA.
Sample	MCA-F	MCA-DS	MCA-WS
Mass loss^a (%)	0.9	6.8	5.3
SBET (m^2 g^−1)	126	83	108

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Fig. 2 SO₂ conversion during the sulfur poisoning process under different conditions.

3.2. Surface properties of the catalysts

Ex situ IR was applied to characterize the sulfur and hydroxide species on the catalyst surface, and the results are shown in Fig. 3. Bands at 1520, 1340 and 1090 cm⁻¹ are associated with OH, C–H and C–O groups of the incorporated organics, respectively, for the unburnt precursor in MCA-F.^20,33 These species disappeared after sulfation either in dry or wet atmosphere. The band at 990 cm⁻¹ is attributed to Al–O in Al₂O₃ for all the three catalysts.³⁴ The two sulfated samples exhibit some strong bands at around 1205–1195 cm⁻¹, which are mainly assigned to bulk sulfate species.²⁶ Band at 1395 cm⁻¹ for MCA-WS is associated with surface sulfate species.³⁵ This band is not found in the spectrum of MCA-DS, which is in consistent with the TGA analysis that sulfates in MCA-DS are mainly bulk sulfates. Moreover, the band shifts towards lower wavenumbers (from 1205 to 1195 cm⁻¹) when the sulfation atmosphere contains water vapor, demonstrating the transformation of sulfates from bulk to surface species.³⁶ The weak band at 1050 cm⁻¹ can be assigned to OH-bending of bayerite in MCA-F and MCA-WS, which is not found on MCA-DS due to the dehydration under dry condition. It has been reported that sulfate can generate aluminum-bonded hydroxyl on CuO/Al₂O₃ catalyst under water vapor treatment,³⁷ and the appearance of this band on MCA-WS may be related to this mechanism.

Fig. 3 IR spectra of the catalysts (1) MCA-F, (2) MCA-DS and (3) MCA-WS.

Ammonia was employed as probe molecule to determine the type of acid sites in catalysts and the results are shown in Fig. 4. Bands coordinated to adsorbed species are listed in Table 2. As the temperature went up, the peaks attributed to Brønsted and Lewis acid sites in MCA-F decrease in intensity gradually and almost disappear at 250 °C. The peaks attributed to Brønsted acid sites on the two sulfated catalysts disappear at 300 °C and Lewis acid sites even remain a certain intensity at 350 °C. These results indicate that stronger acid sites are formed on the sulfated catalysts compared with the fresh one. It has been reported that Lewis acid sites on sulfate may transfer to Brønsted acid sites in the presence of water.^38,39 However, no obvious differences can be found on the spectra of the two poisoned catalysts in this case.

Fig. 4 DRIFT spectra of the adsorbed NH₃ species on (a) MCA-F, (b) MCA-DS and (c) MCA-WS catalysts from 50 to 350 °C.

Table 2 Band positions and assignments of the ammonia adsorbed IR spectra

Wavenumber^a (cm^−1)	Species	Reference
a The data inside and outside the bracket are for MCA-DS/MCA-WS and MCA-F, respectively.
1590 (1620)	N–H bond in the NH3 linked to Lewis acid sites	29
1417 (1447), 1683 (1690)	NH4^+ chemisorbed onto Brønsted acid sites	30
3402 (3275), 3362 (3182)	νs(NH3) coordinated to Lewis acid sites	31
3464 (3378)	νas(NH3) coordinated to Lewis acid sites	31
1533	Coordinated to amide species (–NH2)	25
1247	NH3 adsorbed on Lewis acid	32

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The sulfated catalysts exhibit similar DRIFT spectra of the adsorbed NH₃ species, implying the same origin of surface acid sites, probably arising from the formation of sulfates. Nevertheless, ammonia is not a reliable probe for the quantitative evaluation of acid solids via FTIR spectroscopy. In order to estimate the surface acidity more quantitatively, NH₃-TPD was performed and the obtained profiles are shown in Fig. 5. The profiles can be deconvoluted into three Gaussian functions associated with weak acidity attributed to acid sites of ceria^40,41 characterized by the peak at around 100 °C, weak acidity attributed to aluminum-bonded hydroxyl⁴² characterized by the peak at around 165 °C, and medium acidity mainly attributed to surface sulfate^28,43,44 characterized by the peak at around 250 °C. With the assumption of one NH₃ molecule absorbing on one acid site,⁴⁵ the amounts of acid sites on the catalysts were calculated. The results in Table 3 reveals that the surface acidity of the catalysts is in the order of MCA-WS > MCA-F > MCA-DS. The formation of cerium sulfates and dehydration of alumina reduce the surface acidity of MCA-DS severely. It is suggested that the formation of sulfates compensates the losses in both Brønsted and Lewis acid sites to some extent. During wet sulfation, the water promotes the hydrolysis of sulfates and the synergistic effect between sulfates and water enhances the surface acidity of MCA-WS greatly. It contributes not only to the medium acidity but also weak acidity.

Fig. 5 NH₃-TPD profiles of (a) MCA-F, (b) MCA-DS and (c) MCA-WS.

Table 3 Amounts of surface acid sites on the catalysts by NH₃-TPD

Catalyst	NH3 desorption (mmol g^−1)
	Total	90–105 °C	160–170 °C	245–255 °C
MCA-F	0.35	0.09	0.19	0.06
MCA-DS	0.27	0.05	0.12	0.10
MCA-WS	0.52	0.15	0.16	0.22

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H₂-TPR profiles of the catalysts are shown in Fig. 6 to characterize the redox property of the catalysts. MCA-F exhibits three overlapped peaks at 155, 285 and 385 °C which may be ascribed to the reduction of highly dispersed MnO_x clusters, larger MnO₂/Mn₂O₃ crystallites and CeO₂ promoted by the adjacent MnO_x species, respectively.^9,20 Another individual peak centered at 770 °C is ascribed to the reduction of CeO₂ unpromoted by MnO_x. This is confirmed by the distinct peak at 725 °C in the profile of pure CeO₂. For both of the poisoned catalyst, the low-temperature reduction peaks disappear almost completely. Instead, an intensive peak is found at 542 °C for MCA-DS, which is ascribed partially to the reduction of sulfates as indicated by the reduction behavior of the reference sample MCA-IS. It is difficult to distinguish the specific sulfates because both cerium sulfates and manganese sulfates decompose at 480–600 °C.^28,46 The profile of pure Ce(SO₄)₂ is also acquired with an intensive peak at 563 °C. This peak shifts to lower temperatures for the sulfated catalysts, indicating the promoted reducibility of sulfates by the residual active metal oxides in the sulfated catalysts. This peak shifts to 521 °C for MCA-WS compared with MCA-DS. It has been reported that bulk sulfate species are less reducible than surface sulfates.²⁸ Thus, the shift should be associated with the formation of more surface sulfates on MCA-WS than on MCA-DS, which is in agreement with the TGA and IR results. A shoulder peak is observed at around 450 and 485 °C on MCA-WS and MCA-IS, respectively. It is ascribed to the reduction of the unsulfated ceria¹⁶ and manganese oxide, indicating the preservation of oxidation ability of the wet and impregnated sulfated catalyst.

Fig. 6 H₂-TPR profiles of the catalysts and reference sample.

3.3. Catalytic activities for NO and soot oxidation

NO₂ derived from NO oxidation plays an important role in soot combustion in the presence of NO + O₂.⁴⁷ Thus, NO oxidation ability is a key feature of catalyst. Fig. 7 shows the NO-TPO profiles of the fresh and poisoned catalysts. The fresh catalyst has a much higher activity for NO oxidation, while both the poisoned catalyst are severely deactivated. It is noted that the poisoned catalysts show similar NO oxidation activities although MCA-WS exhibits some superior redox property than MCA-DS. It has been reported that the rate-determining step of NO oxidation is the decomposition of nitrates catalyzed by manganese oxides.⁴⁸ After sulfation, the NO₂ and nitrates formation are both inhibited,⁴⁹ and the NO oxidation activity is does not appear to be affected by the different surface acidity.⁵⁰ In this sense, water does not prevent the deprivation of NO oxidation activity of the sulfated catalyst.

Fig. 7 NO-TPO profiles of the catalysts. Reactant gas: 1000 ppm of NO/10% O₂/N₂.

Soot-TPO tests were conducted in the presence of NO + O₂ to measure the activities of the catalysts for soot oxidation. The profiles of CO_x and NO₂ during the soot-TPO are shown in Fig. 8. The temperature corresponding to 50% conversation of soot was denoted as the light-off temperature (T₅₀), and the CO₂/(CO + CO₂) ratio was defined as the selectivity to CO₂.⁵¹ The results are listed in Table 4. The fresh catalyst has a satisfactory catalytic performance compared to the commercial platinum catalyst (T₅₀ ≈ 440 °C).⁵² However, the mixed oxide catalyst shows a poor resistance to sulfur dioxide. Both of the poisoned catalysts have low selectivity to CO₂. The T₅₀ of MCA-WS shifts to higher temperatures by 54 °C, while the increase in the light-off temperature of MCA-DS is almost doubled (96 °C). The water reduces the deactivation of the catalyst to a great degree. However, they show similar NO oxidation activities in Fig. 7. Thus, the different NO_x-assisted soot oxidation activities between the two poisoned catalysts cannot be well explained by their NO oxidation abilities. In our previous studies, we reported the preferential NO₂ absorption on soot rather than on catalyst driven by surface acidity over the sulfated Pt/Al₂O₃ (ref. 53) and Pt/H-ZSM5 catalysts.^54,55 This mechanism may also exist over the sulfated mixed oxide catalysts. According to Table 3, the amount of total surface acidity follows the order of MCA-WS > MCA-F > MCA-DS. The stronger surface acidity may promote the NO₂ transfer from catalyst to soot and hereby the NO₂ utilization efficiency on MCA-WS compared with MCA-DS, although the further experimental evidences remain to be proceeded.

Fig. 8 The outlet CO_x and NO₂ concentration profiles during the soot-TPO over the catalysts. Reactant gas: 1000 ppm of NO/10% O₂/N₂. Loose catalyst-soot contact.

Table 4 Soot oxidation activities of the catalysts

Catalyst	Reaction in NO + O2		Reaction in O2
	T50 (°C)	SCO2	T50 (°C)	SCO2
MCA-F	426	97	573	99
MCA-DS	518	75	575	83
MCA-WS	480	71	562	64
Uncatalyzed	610	45	612	47

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Soot oxidation mechanism with the assistance of NO_x involves roughly the soot ignition by NO₂ at low temperatures and soot oxidation by O₂ at high temperatures.¹⁷ It is noted in Fig. 8 that the produced NO₂ is almost completely consumed by reacting with soot over the poisoned catalysts, while excess NO₂ remain for the fresh catalyst. Therefore, the influence of O₂-assisted soot oxidation ability cannot be excluded when considering the NO_x-assisted soot oxidation activities of the poisoned catalysts. The soot-TPO tests in O₂ were performed and the results are shown in Fig. 9 and Table 4. It is generally known that the soot oxidation activity in O₂ is generally determined by active oxygen on ceria-based oxide catalysts.^56,57 MCA-DS exhibits a similar T₅₀ to MCA-F but a poor selectivity. More interestingly, MCA-WS shows even higher activity in O₂ than MCA-F. The repeated experiments show that the error range is around ±2 °C for the T₅₀. The difference of T₅₀ between MCA-F and MCA-WS is 11 °C, which cannot be merely contributed by the experimental scatter. Thus, the higher soot oxidation activity of this catalyst cannot be ascribed to the deprivation of redox property.

Fig. 9 CO_x profiles of soot-TPO over the catalysts. Reactant gas: 10% O₂/N₂. Loose catalyst-soot contact.

The soot oxidation occurs generally at the temperatures above 300 °C. According to Table 3, the amount of medium acidity follows the order of MCA-WS > MCA-DS > MCA-F. It has been reported that the oxidation of surface oxygenated complexes (SOCs) such as carboxyl esters and acid anhydrides may be more difficult than the formation of SOCs. The hydrolysis and decarboxylation of these intermediates can be catalyzed by acid sites especially a strong acid like H₂SO₄.^14,17,58,59 However, the product of this acid-assisted SOC decomposition appears to be CO, resulting in low selectivity to CO₂ for soot oxidation. These are in good consistent with the low T₅₀ and low S_CO2 for the O₂-assisted soot oxidation over MCA-WS with more stable acid sites.⁵⁵

Although the wet sulfated catalyst maintains the activity to some degree, the robustness of the MCA oxide catalyst towards sulfur poisoning still need to be improved. Our previous work found that tungsten modification on Pt/Al₂O₃ catalyst can decrease the deposition of sulfate during sulfur poisoning.⁶⁰ Flouty et al. also found that molybdenum impregnation on ceria can show a better resistance to sulfur poisoning for soot oxidation catalysis.⁶¹ Thus, the acid additive may be a promising way to improve the sulfur tolerance of the mixed oxide catalyst.

4. Conclusions

Water vapor in the sulfation atmosphere can hinder the deposition of sulfates over MnO_x-CeO₂/Al₂O₃ catalyst, although it cannot depress the catalytic oxidation of SO₂ to SO₃. More importantly, the hydrolysis of sulfates in H₂O increases the surface acidity significantly. These medium acid sites are confirmed to catalyze the oxidation of soot by O₂, which, as well as the higher NO₂ utilization efficiency driven by total surface acidity, may explain the relative higher activity of the wet sulfated catalyst in NO_x-assisted soot oxidation compared with the dry sulfated one.

Acknowledgements

The authors would like to acknowledge Project 2015AA034603 by the. Moreover, we would also thank the financial support from the Key Laboratory of Advanced Materials of Ministry of Education.

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