Effect of SO₂ on the Selective Catalytic Reduction of NO_x over V₂O₅-CeO₂/TiO₂-ZrO₂ Catalysts

Abstract

The effect of SO₂ on the selective catalytic reduction of NO_x by NH₃ over V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalysts was studied through catalytic activity tests and various characterization methods, like Brunner−Emmet−Teller (BET) surface measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray fluorescence (XRF), hydrogen temperature-programmed desorption (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS). The results showed that the catalyst exhibited superior SO₂ resistance when the volume fraction of SO₂ was below 0.02%. As the SO₂ concentration further increased, the NO_x conversion exhibited some degree of decline but could restore to the original level when stopping feeding SO₂. The deactivation of the catalyst caused by water in the flue gas was reversible. However, when 10% H₂O was introduced together with 0.06% SO₂, the NO_x conversion was rapidly reduced and became unrecoverable. Characterizations indicated that the specific surface area of the deactivated catalyst was significantly reduced and the redox ability was weakened, which was highly responsible for the decrease of the catalytic activity. XPS results showed that more Ce³⁺ was generated in the case of reacting with SO₂. In situ DRIFTS results confirmed that the adsorption capacity of SO₂ was enhanced obviously in the presence of O₂, while the SO₂ considerably refrained the adsorption of NH₃. The adsorption of NO_x was strengthened by SO₂ to some extent. In addition, NH₃ adsorption was improved after pre-adsorbed by SO₂ + O₂, indicating that the Ce³⁺ and more oxygen vacancy were produced.

1. Introduction

Selective catalytic reduction (SCR) catalysts are commonly severely deactivated by SO₂, which is abundantly present in flue gas. There are two mechanisms that can explain the sulfur poisoning of catalysts. Firstly, SO₂ reacts with NH₃ and vapor in the oxygen atmosphere, producing sulfate species including ammonium sulfate and ammonium bisulfate. These sulfate substances can deposit on the catalytic surface and cause pore plugging. As a result, the specific surface area and pore volume decrease observably, ending up with the catalyst deactivation. Studies have shown that the thermal decomposition temperature range of ammonium sulfate and ammonium bisulfate are 213–308 °C and 308–419 °C [1], respectively. It is still hard for these sulfate species to decompose over a traditional V/TiO₂ catalyst. In the second, SO₂ can react with the active center atoms and produce metal sulfates, which will decrease catalyst activity [2,3]. The former deactivation is reversible and the catalysts can be reactivated by washing and high-temperature processing. However, the later deactivation is irreversible.

The mechanism of catalyst sulfur poisoning has been extensively studied. Wei et al. [4] claimed that SO₂ significantly reduced the adsorption of NH₃ on the Lewis acid sites. At the same time, SO₂ would react with NH₄⁺ to form NH₄HSO₃, thereby reducing NO_x conversion. However, Jiang et al. [5] proposed that SO₂ had little effect on the adsorption of NH₃, and conversely promoted the formation of new Bronsted acid sites. The weakening of NO adsorption on the catalyst surface was the main cause of deactivation. With the deposition of sulfate on the catalyst surface, less NO participated in the SCR reaction, resulting in a decrease in NO_x conversion. Similarly, Liu et al. [6] claimed that SO₂ had a significant inhibitory effect on the reduction of NO_x due to the deposition of sulfate substances. It hindered the adsorption of NO and the production of the intermediate ammonium nitrate, resulting in a decrease in catalyst activity. Moreover, Pan et al. [7] found that the main reason for MnO_x/TiO₂ catalysts deactivation was that the active center atom manganese was sulfated. Besides, the presence of SO₂ caused ammonium sulfate to deposit on the catalyst, decreasing the adsorption of NO. Gu et al. [8] reported that the main reason for the deactivation of Ce/TiO₂ catalyst was that SO₂ could react with the catalyst to form high thermally stable Ce(SO₄)₂ and Ce₂(SO₄)₃, and Xu et al. obtained similar results [9].

On the basis of the above literatures, it is found that there are still many controversies about the mechanism of catalyst sulfur poisoning. Moreover, there is no report found currently on the sulfur poisoning mechanism of V₂O₅-CeO₂/TiO₂-ZrO₂ catalysts, which were reported to be an excellent SCR catalyst with a wide temperature range [10,11,12]. Our previous studies indicated that the V₂O₅-0.2CeO₂/TiO₂-ZrO₂ catalyst exhibited superior catalytic performance as well as high tolerance of SO₂ and H₂O.In this paper, the catalytic activity of V₂O₅-0.2CeO₂/TiO₂-ZrO₂ in the presence of different SO₂ content was further studied. Various characterization methods such as the Brunner−Emmet−Teller (BET) surface measurement, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray fluorescence (XRF), hydrogen temperature-programmed desorption (H₂-TPR), X-ray photoelectron spectroscopy (XPS) and in situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed to study the poisoning mechanism from a microcosmic aspect.

2. Materials and Methods

2.1. Catalyst Preparation

The Ti–Zr carrier was prepared by a coprecipitation method with the molar ratio of Ti:Zr = 1:1. An equal amount of TiCl₄ solution and ZrOCl₂ 8H₂O were dissolved in deionized water and stirred constantly. With stirring, NH₃·H₂O was slowly added until the pH reached 10. The obtained solution was aged at room temperature for 24 h. Then, the precipitate was washed with deionized water until the supernatant contained no Cl⁻. Finally, the sample was dried at 110 °C for 12 h and then calcined at 450 °C for 4 h in a muffle furnace.

A step-by-step impregnation method was used to prepare V₂O₅-0.2CeO₂/TiO₂–ZrO₂. A certain amount of CeNO₃·6H₂O and Ti–Zr powder were added to deionized water. The obtained suspension was stirred at room temperature for 2 h, followed by stirred at 85 °C for 4 h. After dried at 110 °C for 12 h and calcined in a muffle furnace at 450 °C for 4 h, the Ce/Ti–Zr sample was obtained. Thereafter, the resulting Ce/Ti–Zr was impregnated with NH₄VO₃ solution in the same manner. The obtained sample was denoted as V-0.2Ce/Ti–Zr, where the content of V₂O₅ loading was 1 wt. % and the molar ratio of Ce to Ti–Zr support = 0.2.

For short, the V-0.2Ce/Ti–Zr catalyst after reaction with 0.06% SO₂ for 2 h was denoted as S-V-0.2Ce/Ti–Zr, while the catalyst after reaction with 10 vol% H₂O and 0.06% SO₂ for 2 h was denoted as HS-V-0.2Ce/Ti–Zr, respectively.

2.2. Activity Measurements

The SCR activity measurement of catalysts was carried out in a fixed-bed reactor with the inner diameter of 7 mm. The 0.3 g catalyst (40–60 mesh) was placed in the reactor with a gas hourly space velocity (GHSV) of 20000 h⁻¹. Typically, the total gas flow was 100 mL/ min. The simulated gas was composed of 0.08% NO, 0.08% NH₃, 5% O₂, 5%/10% H₂O (when used), 0.02%/0.04%/0.06% SO₂ (when used) and N₂ as the balanced gas. The NO, NO₂ and NO_x concentration were persistently monitored with a flue gas analyzer (Testo350-XL).

2.3. Catalyst Characterization

The BET was measured using a specific surface area and pore size analyzer V-Sorb 2800P (Beijing Gold APP, Beijing, China). The sample was pretreated under vacuum at 250 °C for 5 h, and the adsorbate was high purity nitrogen.

XRD patterns were carried out for phase analysis via a SmartLab™ X-ray diffractometer (Rigaku, Tokyo, Japan). Cu target acted as the X-ray source.

The morphology of the catalyst was determined by TEM (Thermo Fisher Scientific, Waltham, Massachusetts, America). The catalyst sample was dispersed in an ethanol solution after thoroughly ground, followed by shaken under ultrasonic waves for 15 min.

XRF spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, MA, America) was used to analyze the content of component in the catalyst sample.

H₂-TPR was carried out in a quartz U-tube reactor connected to a thermal conduction detector (TCD) using an H₂–Ar mixture (10% H₂ by volume) as reductant (Finetec Instruments, Hangzhou, Zhejiang, China). The temperature range during the test was for 25 °C to 800 °C with a heating rate of 10 °C /min.

XPS analysis was performed on a PHI Quantera II system (Ulvac-PHI, Chigasaki, Kanagawa Prefecture, Japan). The binding energies were referenced to the C 1 s line at 284.8 eV from adventitious carbon.

In situ DRIFTS studies were performed on a Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts, MA, USA). The scanning wave number ranged from 400 cm⁻¹ to 4000 cm⁻¹. Before the test, the sample was pretreated with N₂ at 400 °C for 1 h to remove impurities. The background at a certain temperature was collected during the cooling process.

3. Results and Discussion

3.1. Effects of SO₂ and H₂O on Catalyst Activity

Figure 1 showed the catalytic activity results with different concentration SO₂ over the V-0.2Ce/Ti–Zr catalyst at 250 °C. In the absence of SO₂, NO_x conversion of the V-0.2Ce/Ti–Zr catalyst was approximately 90%, indicating that the catalyst had high activity at the low temperature. After 0.02% SO₂ was introduced, NO_x conversion remained stable. However, NO_x conversion decreased rapidly to 71% and 65% at 20 min respectively when 0.04% and 0.06% SO₂ were added. Furthermore, it could be found that the catalytic activity could recover after stopping SO₂. It was inferred that SO₂ and SO₃ reacted with NH₃ in the early stage, leading to the decrease of NH₃ as well as the catalytic performance. As the reaction went on, SO₂ and SO₃ reacted with CeO₂ and produced Ce³⁺ in the presence of excess oxygen, which could strengthen the acid sites on the catalyst and increased NO_x conversion [13].

The effect of H₂O on the catalytic activity of the catalyst was investigated and the results were shown in Figure 2. It was found that the activity of the catalyst was rapidly decreased with the introduction of H₂O. However, after the H₂O was stopped, the activity gradually recovered almost to the original level, thus indicating that the effect of H₂O on the catalyst was reversible.

The activity of the V-0.2Ce/Ti–Zr catalyst was tested at 250 °C in the presence of 10% H₂O and different concentrations of SO₂, and the results were shown in Figure 3. After the introduction of SO₂ and H₂O, NO_x conversion decreased rapidly to less than 30%, which was much lower than that in the presence of SO₂ or H₂O alone. After SO₂ and H₂O were removed, NO_x conversion increased to some extent but could not be restored to the initial level, indicating an irreversible deactivation occurred. Studies showed that SO₂ would react with NH₃ to form ammonium sulfates, and block active sites on the surface of the catalyst.

Furthermore, the performance of the HS-V-0.2Ce/Ti–Zr catalyst at different temperatures was also tested, and the results were shown in Figure 4. It could be found that the mid-temperature (200–300 °C) activity of the HS-V-0.2Ce/Ti–Zr catalyst was significantly decreased, while it at a higher temperature (>300 °C) was obviously enhanced. One literature indicated that the formation of Ce(SO₄)₂ led to a decrease in the activity of the catalyst at moderate temperatures [14]. At the same time, Xu et al. [15] pointed out that the sulphate was produced during the poisoning process, which had a certain activity at high temperatures and could improve the high temperature activity of the catalyst.

3.2. Physico-Chemical Characterization of Catalysts

3.2.1. BET Analysis

The specific surface area of the catalysts before and after being poisoned by SO₂ was illustrated in

Table 1. BET surface area and pore volume of the fresh and poisoned catalyst.

Sample	BET Surface Area (m2/g)	Pore Volume (mL/g)
V-0.2Ce/Ti–Zr	54.45	0.14
S-V-0.2Ce/Ti–Zr	25.07	0.16
HS-V-0.2Ce/Ti–Zr	15.27	0.11

. Compared with the fresh sample, the BET surface area of the S-V-0.2Ce/Ti–Zr catalyst decreased from 54.45 m²/g to 25.07 m²/g, and that of the HS-V-0.2Ce/Ti–Zr catalyst further reduced to 15.27 m²/g. While with comparison to the fresh counterpart, there was no apparent change on the pore volume. The specific surface area of the catalyst was related to the adsorption capacity of NH₃ during the SCR reaction, thereby further affecting the denitrification activity.

In order to further investigate the reason for the significant decrease in the BET specific surface area, the pore size distribution of the catalyst was mapped. As shown in Figure 5, the pore size range of the fresh catalyst was mainly concentrated at 2 nm to 10 nm, indicating that the mesopores contributed the most to the specific surface area of the catalyst. For the catalyst after reaction with O₂, the number of macropores and mesopores (in the range of 5.3 nm to 50 nm) increased, and the increase of pore diameter in HS- V-0.2Ce/Ti–Zr catalyst was more pronounced. It was found that the mesopores (mainly 2 nm to 10 nm) contributed the most to the specific surface area of the catalyst. Therefore, it was presumed that the number of mesopores determined the NH₃-SCR activity of the V-0.2Ce/Ti–Zr catalyst. After the reaction in presence of H₂O and SO₂, mesopores in the range of 5–50 nm contributed greatly to the specific surface area of the catalyst, while the mesopores and micropores with pore diameters less than 5 nm gradually decreased. Based on the above analysis, it was speculated that in the presence of SO₂ and H₂O, (NH₄)₂SO₄ and Ce(SO₄)₂ formed during the reaction were adsorbed on the catalyst, and blocked 2 to 5 nm mesopores and micropores around the active component of the catalyst. These caused a decrease in specific surface area of the catalyst, further inhibiting catalytic activity.

3.2.2. XRD Analysis

To further study the microstructure of the poisoned V-0.2Ce/Ti–Zr, an XRD analysis was performed. As shown in Figure 6, catalysts before and after being poisoned by SO₂ showed similar spectra. According to our previous research [12], these diffraction peaks mainly corresponded to ZrTiO₂, ZrO₂, V₂O₅, CeO₂ and TiO₂. Diffraction peaks of substances such as (NH₄)₂SO₄ or Ce(SO₄)₂ were not detected. The results illustrated that no sulphate with good crystal form was formed, or the amount of sulphate formed on the surface was small and highly dispersed.

3.2.3. TEM and XRF Analysis

TEM was carried out to study the morphology changes of catalysts poisoned by H₂O and SO₂. As shown in Figure 7, the surface of the fresh catalyst was uniform and had a good dispersion. While for the poisoned one, it was clearly observed that the surface was covered with some substances, and the particles were agglomerate.

Element content of the catalyst was tested and the results were shown in

Table 2. XRF results of the catalysts before and after being poisoned by SO₂.

Sample	V	Ce	Ti	Zr	S
V-0.2Ce/Ti–Zr	0.723	12.512	1.2	34.66	0
S-V-0.2Ce/Ti–Zr	0.63	12.365	19.75	26.66	0
HS-V-0.2Ce/Ti–Zr	0.558	12.932	20.46	26.4	1.13

. No S element was detected in the S-V-0.2Ce/Ti–Zr catalyst, while a trace amount of the S element was detected in the HS-V-0.2Ce/Ti–Zr catalyst. The results revealed that when SO₂ was separately introduced, substantially no S element was present on the catalyst surface, or its content was extremely low. However, once H₂O was introduced together, SO₂ was more likely to participate in the reaction, and was present on the surface in the form of ammonium sulfate or Ce(SO₄)₂.

3.2.4. H₂-TPR Analysis

TPR profiles for catalysts before and after poisoned by SO₂ were presented in Figure 8. For the fresh catalyst, five reduction peaks were observed at 343 °C, 418 °C, 507 °C, 580 °C and 732 °C, respectively. Studies have shown that the low temperature reduction of V/TiO₂ catalyst is mainly related to the monomer vanadium or highly dispersed vanadium species [16]. Held et al. proposed that the reduction peak at about 730 °C was mainly related to V₂O₄ [17]. Based upon our previous work, peaks centered at 343 °C and 580 °C were due to the reduction of vanadium from V⁵⁺ to V⁴⁺ and V⁴⁺ to V³⁺, respectively. The reduction peak centered at 418 °C was attributed to the surface (α) reduction processed of CeO₂, and the subsurface layers and deeper regions of the catalyst nanoparticles were reduced at 507 °C.

It could be found that the redox ability of the S-V-0.2Ce/Ti–Zr catalyst was significantly weakened, according to the fact that several peaks disappeared (343 °C, 418 °C and 580 °C). After reaction with SO₂ and H₂O, the redox ability of the HS-V-0.2Ce/Ti–Zr catalyst was further attenuated, and only one reduction peak appeared at 458 °C was detected. Peaks centered at 495 °C and 458 °C were due to the reduction of Ce⁴⁺, while the peak at 653 °C was attributed to the reduction of V⁵⁺ to V³⁺ and the reduction of bulk phase Ce⁴⁺. It could be found that the disappearance of reduction peaks related with vanadium was an important cause of catalytic deactivation. For S-V-0.2Ce/Ti–Zr sample, the α-reduction peak of Ce⁴⁺ disappeared, which should be related to the reduction of Ce⁴⁺ to Ce³⁺. It was reported that the concentration of Ce³⁺ was used to assess the amount of oxygen formed. The oxygen vacancies were driven by the transition from Ce⁴⁺ to Ce³⁺, accelerating the transport of active oxygen species and facilitating the reaction vacancies, which could explain the phenomenon that the activity of the catalyst decreased first and then gradually increased. After being poisoned by SO₂ and H₂O, the intensity of the β reduction peak (458 °C) was significantly weakened, and the reduction peak of the vanadium oxide completely disappeared. It was speculated that the formed ammonium hydrogen sulfate was deposited on the surface of the catalyst or more stable Ce₂(SO₄)₃ was formed, which hindered the conversion of vanadium active species and led to a reduction of the redox ability.

3.2.5. XPS Analysis

Figure 9a showed the XPS results of Ce 3d. Peaks u′′′, u′′, u and v′′′, v′′, v can be corresponded to Ce⁴⁺, while peaks u′ and v′ were assigned to Ce³⁺, respectively. It can be seen from Figure 9a that Ce in the catalyst mainly existed in the Ce⁴⁺ valence state. As shown in

Table 3. Peak area ratio of XPS.

	Area Ratio
Sample	$\frac{\mathit{C}{\mathbf{e}}^{3+}}{\mathit{C}{\mathbf{e}}^{3+}+\mathit{C}{\mathbf{e}}^{4+}}$	$\frac{{\mathit{O}}_{\mathit{\beta }}}{{\mathit{O}}_{\mathit{\beta }}+{\mathit{O}}_{\mathit{\alpha }}}$
V-0.2Ce/Ti–Zr	24.94	28.14
S-V-0.2Ce/Ti–Zr	25.15	20.71
HS-V-0.2Ce/Ti–Zr	24.93	18.67

, after reacting with 0.06% SO₂ for 2 h, the ratio of Ce³⁺/ (Ce⁴⁺+Ce³⁺) in S-V-0.2Ce/Ti–Zr catalysts was increased to 25.15% compared with that in V-0.2Ce/Ti–Zr (24.94%), indicating more Ce³⁺ was generated. However, in the presence of SO₂ and H₂O, the ratio of Ce³⁺/(Ce⁴⁺ + Ce³⁺) was reduced again to 24.93%. Hence, it was deduced that the introduction of SO₂ promoted the conversion of Ce⁴⁺ and Ce³⁺, while the formed ammonium hydrogen sulfate on the surface of the catalyst could block the path of mutual conversion between Ce⁴⁺ and Ce³⁺.

As shown in Figure 9b, the three peaks in O 1 s can be assigned to oxygen vacancy, adsorbed oxygen and oxygen lattice from higher binding energy to lower binding energy. The adsorbed oxygen and oxygen lattice were marked as Oβ and Oα, respectively [18,19]. The oxygen vacancy was formed due to the evolution of the oxygen lattice [20].

As can be seen in Table 3, the ratio of Oβ/(Oβ + Oα) was reduced significantly in the S-V-0.2Ce/Ti–Zr catalysts and even worse in the HS-V-0.2Ce/Ti–Zr catalysts. The existence of Oβ could promote the oxidation of NO to NO₂, which can explain the decrease in activity of the catalysts poisoned by SO₂.

3.3. In Situ DRIFTS Study

3.3.1. Sulfur Dioxide Adsorption.

Figure 10 showed the DRIFTS spectra of the V-0.2Ce/Ti–Zr catalyst in the flow of 0.06% SO₂ at 50 °C and then purged by N₂ with increasing temperatures from 50 °C to 400 °C. The peak at 1633 cm⁻¹ was linked to H₂O vibration produced by the reaction of SO₂ and hydroxyl on the catalytic surface [8]. With the addition of SO₂ at 50 °C, peaks at 1376 cm⁻¹, 1338 cm⁻¹, 1265 cm⁻¹, 1097 cm⁻¹ and 1049 cm⁻¹ were detected. Based on the research of Peak et al. [21], the triply degenerate asymmetric stretching ν3 band were accessible to FTIR investigation, and would split into three bands when the bidentate sulfate complex was formed. Therefore, it was deduced that the bands at 1265 cm⁻¹, 1097 cm⁻¹ and 1049 cm⁻¹ were attributed to bidentate sulfate on V-0.2Ce/Ti–Zr. The band at 1338 cm^–1 was assigned to the adsorbed SO₂, which mainly exists in the form of SO₃²⁻. The band at 1376 cm⁻¹ might be due to the asymmetric vibration of O = S = O covalent groups (SO₄^2–). When the temperature reached 100 °C, the adsorption peak at 1376 cm⁻¹ disappeared [22,23].

Figure 11a shows the in situ DRIFTS spectra of the V-0.2Ce/Ti–Zr catalyst in the flow of 0.06% SO₂ at 250 °C and then purged with N₂. After adding SO₂ for 5 min, bands at 1363 cm⁻¹, 1344 cm⁻¹, 1295 cm⁻¹, 1083 cm⁻¹ and 1047 cm⁻¹ were detected with increasing intensity. The bands at 1295 cm⁻¹, 1107 cm⁻¹ and 1050 cm⁻¹ were contributed to the bidentate sulfate on V-0.2Ce/Ti–Zr while the band at 1344 cm^–1 was assigned to the adsorbed SO₂ (SO₃^2–). The band at 1363 cm⁻¹ could be attributed to the asymmetric vibration of O = S = O covalent groups (SO₄^2–). It had been found that the O = S = O asymmetric covalent groups arose from the VOSO₄ adsorption peak that appeared at 1383 cm⁻¹ [24,25]. Hence, we deduced the reaction between the adsorbed SO₂ with V₂O₅ in the catalyst to form the VOSO₄ intermediate.

Figure 11b shows the DRIFTS spectra of the V-0.2Ce/Ti–Zr catalyst in the flow of O₂ + SO₂ at 250 °C and then purged with N₂. In general, catalysts exhibited similar peaks, while the intensity of SO₂ adsorption was enhanced significantly in the presence of O₂.

3.3.2. Effect of SO₂ on NH₃ Adsorption

Figure 12 showed the NH₃ adsorption results on V-0.2Ce/Ti–Zr in the presence of SO₂. It could be found that original adsorption capacity of NH₃ was very weak, and only the band at 1182 cm⁻¹ was observed [26]. Then NH₃ was switched off and SO₂ + O₂ was introduced. The NH₃ adsorption peak was replaced by SO₄²⁻ adsorption peak quickly. Bands at 1278 cm⁻¹, 1178 cm⁻¹ and 1050 cm⁻¹ were assigned to SO₄²⁻ three-fold degeneracy asymmetric stretching vibration v3 and the band at 1340 cm⁻¹ was assigned to the adsorbed SO₂ while the band at 1373 cm⁻¹ could be attributed to the asymmetric vibration of O = S = O covalent groups (SO₄^2–).

In Figure 13, the catalysts were pretreated by 0.06% SO₂ + O₂ for 30 min, and then exposed to 800 ppm NH₃, followed by treating with SO₂ and O₂ again. After adding NH₃, several bands at 1660 cm⁻¹, 1600 cm⁻¹, 1440 cm⁻¹ and 1220 cm⁻¹ were detected. The band at 1600 cm⁻¹ and 1220 cm⁻¹ was associated with the asymmetric and symmetric deformation vibration of NH₃ adsorbed on Lewis acid sites, while the band at 1660 cm⁻¹ and 1440 cm⁻¹ were due to the asymmetric and symmetric deformation of NH₄⁺ bound to Brönsted acid sites [26,27]. Compared with the results of Figure 9, it could be found that the intensity of bands due to adsorbed NH₃ was significantly increased after being pretreated by SO₂ and O₂. It was deduced that the presence of SO₂ and O₂ could promote the transformation from Ce⁴⁺ to Ce³⁺ (the formation of Ce₂(SO₄)₃), resulting in the enhancement of the NH₃ adsorption ability. Furthermore, it was observed that the intensity of the NH₃ adsorption peaks did not change significantly after the introduction of SO₂ and O₂ again.

3.3.3. Effect of SO₂ on NO Adsorption

The competitive adsorption behavior of SO₂ with NO₂ in the presence of O₂ was investigated, and in situ DRIFTS results were shown in Figure 14. As shown in Figure 15, bands appeared at 1375 cm⁻¹ and 1315 cm⁻¹ were due to cis-N₂O₂²⁻, and it could be found that the intensity of the bands increased gradually and the purge of N₂ had little influence on the adsorption bands. Band at 1190 cm⁻¹ were attributed to the bridging nitrates [28]. After 10 min, SO₄²⁻ adsorption peak appeared at 1046 cm⁻¹. As the reaction went on, it divided into two adsorption peaks (1049 cm⁻¹ and 1035 cm⁻¹). Along with the purge of N₂, band at 1095 cm⁻¹ assigned to bidentate sulfate was observed. Furthermore, weak SO₄²⁻ adsorption band was detected at 1135 cm⁻¹.

In Figure 15, the catalyst was first treated by NO + O₂ for 60 min until the adsorption was saturated and 0.06% SO₂ was added into the cell, and then SO₂ were introduced into the in-situ cell for various times, followed by N₂ purging. As shown in Figure 15, bands due to the cis-N₂O₂²⁻ were also detected at 1375 cm⁻¹ and 1315 cm⁻¹. Band at 1188 cm⁻¹ was assigned to the adsorbed NO_x species. Nitrate species adsorption peak appeared at 1144 cm⁻¹ and 1067 cm⁻¹ after NO + O₂ pre-adsorption. Once feeding SO₂, the intensity of the bands at 1375 cm⁻¹, 1315 cm⁻¹, 1188 cm⁻¹ and 1144 cm⁻¹ were strengthened and the purge of N₂ had little influence on the adsorption bands, while the band at 1067 cm⁻¹ gradually disappeared. The SO₄²⁻ adsorption peak appeared at 1046 cm⁻¹.

Based on the analysis above, it could be concluded that the presence of SO₂ could promote NO adsorption on the catalyst surface. In addition, our previous research has a certified catalyst that mainly follows the Eley–Rideal (E-R) mechanism, and NO adsorption on the catalyst surface restrains the SCR reaction [12]. Thus, it can explain the decrease of catalyst activity when SO₂ was injected.

3.3.4. Effect of SO₂ on NH₃ + NO + O₂ Co-Adsorption

As shown in Figure 16, in the presence of NH₃ + NO + O₂, only the band at 1205 cm⁻¹ due to adsorbed NH₃ was observed. After 0.06% SO₂ was injected, adsorption intensity of bands due to adsorbed NH₃ decreased rapidly. These results indicated that the presence of SO₂ significantly weakened the NH₃ adsorption, which was consistent with the result of Figure 12. After the introduction of SO₂ for 20 min, bands at 1373 cm⁻¹, 1280 cm⁻¹, 1095 cm⁻¹ and 1043 cm⁻¹ were detected. The band at 1373 cm⁻¹ could be attributed to the asymmetric vibration of O = S = O covalent groups and bands at 1280 cm⁻¹, 1095 cm⁻¹ and 1043 cm⁻¹ were assigned to SO₄²⁻ three-fold degeneracy asymmetric stretching vibration v3. In our previous study [12], the SCR reaction over V-0.2Ce/Ti–Zr obeyed the E-R mechanism. Hence, as an important reaction substance, the weakening of NH₃ adsorption would greatly inhibit the SCR reaction.

3.4. Possible SO₂ Reaction Mechanism Over The Catalyst

The adsorption ability of SO₂ had something with its concentration. There was no adsorption when SO₂ concentration was low, which explained the stable activity of catalysts at low SO₂ concentration. With the increase of the SO₂ concentration, SO₂ and SO₄²⁻ adsorption appeared. SO₂ was adsorbed on the surface in the form of SO₃²⁻. The consumption of surface OH species meant that SO₂ was able to react with surface hydroxyl groups to form adsorbed H₂O. It was speculated that in the presence of O₂, SO₂ would react with NH₃ to form (NH₄)₂SO₄, which could deposit on the surface of the catalyst and reduce the catalyst activity. The sulfate products could combine with Ce to form more stable Ce(SO₄)₂. The reaction might be proposed as follows:

SO₂(g) + O²⁻(a) →SO₃²⁻(a),

(1)

SO₂(g) + 2OH⁻(a) →SO₃²⁻ (a) + H₂O,

(2)

O₂(g) →2[O](a),

(3)

SO₃²⁻ (a) + [O](a) →SO₄²⁻ (a)

(4)

SO₄²⁻ (a) + 2NH₄⁺(a)→(NH₄)₂SO₄

(5)

Ce⁴⁺ + SO₄²⁻ →Ce(SO₄)₂

(6)

According to the in-situ DRIFTS results and other study [19], SO₂(SO₃^2–) could react with V⁵⁺–OH to form VOSO₄ intermediate.

SO₃²⁻ (a) + V⁵⁺-OH(a) →VOSO₄

(7)

With the time passing, absorbed SO₂ (SO₃²⁻) contacted with the active component Ce and caused the transformation from Ce⁴⁺ to Ce³⁺, increasing the oxygen vacancy and enhancing the NH₃ adsorption ability, which can explain the recovery of activity. The presence of O₂ promoted the redox of SO₂ and formation of sulfate Ce₂(SO₄)₃.

3SO₂ + 2CeO₂ + O₂ →Ce₂(SO₄)₃

(8)

4. Conclusions

Catalyst activity test results showed that low concentration SO₂ (<0.02%) had little influence on catalyst activity. With increasing SO₂ concentration, NO_x conversion of the catalyst gradually decreased but could restore to the original level when stopping feeding SO₂. A reversible deactivation would occur in the presence of H₂O. However, in the presence of SO₂ and H₂O, the catalyst was irreversibly deactivated, which was worse than that of adding SO₂ or H₂O alone.

The characterization results showed that the BET specific surface area of the catalysts poisoned by SO₂ were reduced, and the redox capacity were significantly weakened. The sulfuration of the active component Ce and the deposition of ammonium sulfate might be the causes of the deactivation. XPS analysis showed that the presence of SO₂ promoted the generation of Ce³⁺, which probably promoted the SCR reaction.

In situ DRIFTS analysis indicated that the adsorption capacity of SO₂ was enhanced in the presence of O₂. The presence of SO₂ would inhibit the adsorption of NH₃. At the same time, the NO adsorption ability of the catalyst was enhanced to some extent by SO₂. Moreover, the NH₃ adsorption ability of the catalyst with pre-adsorption of SO₂ + O₂ was markedly enhanced, indicating that when SO₂ + O₂ existed, more Ce³⁺ and oxygen vacancy were produced. These conclusions might contribute to a better understanding of the SO₂ poisoning mechanism over the V₂O₅–CeO₂/TiO₂–ZrO₂ catalyst.