Effect of Sulfur Poisoning During Worldwide Harmonized Light Vehicles Test Cycle on NOx Reduction Performance and Active Sites of Selective Catalytic Reduction Filter

Abstract

Selective catalytic reduction filter (SDPF) technology constitutes a critical methodology for controlling nitrogen oxide (NOx) and particulate matter emissions from light-duty diesel vehicles. A series of SDPFs with different sulfur poisoning times and concentrations were prepared using the worldwide harmonized light vehicles test cycle (WLTC). Bench testing revealed that sulfur poisoning diminished the catalyst’s NH₃ storage capacity, impaired the transient NOx reduction efficiency, and induced premature ammonia leakage. After multiple sulfur poisoning incidents, the NOx reduction performance stabilized. Higher SO₂ concentrations accelerated catalyst deactivation and hastened the attainment of this equilibrium state. The characterization results for the catalyst indicate that the catalyst accumulated the same sulfur content after tail gas poisoning with different sulfur concentrations and that sulfur existed in the form of SO₄²⁻. The sulfur species in low-sulfur-poisoning-concentration catalysts mainly included sulfur ammonia and sulfur copper species, while high-sulfur-poisoning-concentration catalysts contained a higher proportion of sulfur copper species. Neither species type significantly altered the zeolite coating’s crystalline structure. Sulfur ammonia species could easily lead to a significant decrease in the specific surface area of the catalyst, which could be decomposed at 500 °C to achieve NOx reduction performance regeneration. In contrast, sulfur copper species required higher decomposition temperatures (600 °C), achieving only partial regeneration.

1. Introduction

Cu-SSZ-13 has emerged as the preferred catalyst for SCR due to its excellent selective catalytic reduction of NOx with NH₃ (NH₃-SCR), low-temperature/high-temperature hydrothermal performance, and anti-poisoning performance [1,2,3,4,5]. With stricter regulations, the sulfur content in fuel has also decreased, but there is a risk of sulfur poisoning and failure during the long-term use of Cu-SSZ-13.

The formation of distinct sulfur poisoning species on Cu-SSZ-13 is contingent upon the sulfation atmosphere, leading to disparate failure mechanisms. Research has shown that Cu-SSZ-13 exhibits a significant number of sulfur copper species after poisoning in SO₂ and O₂ atmospheres at 250 °C [6]. Increasing the H₂O in the atmosphere did not increase the types of sulfur species but significantly increased the formation of sulfur copper species. Research suggests that in anhydrous conditions, SOx can react with mono-aluminum ZCuOH (Cu ions form [Cu(OH)]⁺ and occupy one -Al-O(H)-Si-site), whereas under hydrous conditions, SOx can react with double aluminum Z₂Cu (Cu ions occupy two -Al-O(H)-Si-sites) and water to produce more copper sulfide [7,8]. The presence of H₂O may also weaken the adsorption of intermediate SO₃ species on the zeolite framework, promote the surface diffusion of SO₃, and increase the rate of sulfate formation [9]. When SO₃ is present in an SO₂ atmosphere, sulfates are more easily formed [10,11]. Notably, the inclusion of NH₃ leads to substantial sulfur ammonium species formation, concurrently increasing the sulfur copper species content, potentially due to the thermal conversion of some ammonium sulfates during heating [12]. Molokov et al. [13] found that ammonia and sulfur atmospheres can easily lead to the formation of [CuII₂(NH₃) ₄O₂]²⁺ from CuII, NH₃, and oxygen outside the skeleton, while SO₂ can easily complex with [CuII₂ (NH₃)₄O₂]²⁺, resulting in a significant decrease in the low-temperature NH₃-SCR performance. There was no significant difference in the types and contents of sulfur poisoning species due to the presence of NO and NO₂ in the reaction atmosphere [12]. Research has found that in addition to sulfur ammonia and sulfur copper species, sulfur poisoning species also exist in other forms of sulfur binding. Under an SO₂ + O₂ poisoning atmosphere, the SO₂-TPD response peak at ~540 °C was attributed to weakly bound sulfur species [12] or chemisorbed SO₂ species [14], while other scholars have attributed it to H₂SO₄ species [6,9]. In addition, sulfur poisoning species also include CuSO₄ and Al₂SO₄ species, which require higher decomposition temperatures [9,15]. In contrast, Al₂SO₄ exhibits a higher decomposition temperature.

The temperature during sulfur poisoning also has a significant impact on the type and content of sulfur poisoning species. Therefore, catalysts exhibit different failure mechanisms under sulfur poisoning at different temperatures. Cheng et al. [16] observed that under a standard SCR sulfur atmosphere, sulfur poisoning at 400 °C inhibits the formation of sulfur ammonia species, resulting in a lower cumulative sulfur species content compared to that at 200 °C [12]. Mandal et al. [17] reported that sulfur poisoning at 400 °C is more likely to lead to a decrease in the low-temperature performance of NH₃-SCR. They classified this as an active copper species that easily forms copper dimers at high temperatures. Compared to monomeric copper, copper dimers are more likely to form stable Cu₂SO₄ complexes. After sulfur poisoning, the catalyst will further promote the conversion of sulfur ammonia species to form sulfur copper species during heat treatment regeneration in an aqueous and O₂ atmosphere [12]. Compared to fresh samples, Cu-SSZ-13 samples aged at 800 °C formed lower levels of sulfur ammonia species after sulfur poisoning, while the copper sulfate species increased instead. This may have been related to changes in the acidic sites and skeletal stability of the copper species after aging [12]. Shan et al. [18] found that under high-temperature sulfur poisoning conditions, the catalyst did not form sulfate species in a hydrothermal aging atmosphere containing sulfur at 750 °C. The decrease in the catalytic performance was due to the dealumination of the molecular sieve skeleton under an acidic sulfur aging atmosphere.

Catalyst formulation variations also engender differences in the sulfur resistance and failure mechanisms. Jiang et al. [19] demonstrated that Cu-SSZ-13 with high copper loading is more prone to CuSO₄ formation, resulting in more significant performance degradation after sulfur poisoning. This propensity is linked to a higher copper content resulting in more bridge oxygen atoms in copper metal oxides, which can easily oxidize SO₂ and then combine with different sites to form more sulfate species. Abasabadi et al. [20] found that SO₂ competes with NO and NH₃ to complex [CuII₂(NH₃)₄O₂]²⁺, leading to a decrease in the SCR activity. And catalysts with different silicon aluminum ratios exhibit the same sulfur poisoning failure mechanism. Su et al. [9] believe that compared to Cu-SAPO-34, Cu-SSZ-13 has stronger oxidizing properties and can promote the oxidation of SO₂ to SO₃, leading to the formation of more sulfates. However, the sulfate species on Cu-SSZ-13 are mainly H₂SO₄, which can be decomposed at lower temperatures. Al₂(SO₄)₃, which is difficult to decompose, forms on Cu-SAPO-34, making regeneration even more challenging. Shen et al. [21] compared the sulfur regeneration performance of Cu-SAPO-34 and Cu-SSZ-13 and found that the performance could not be fully restored after Cu/SSZ-13 regeneration. They believed that regeneration temperatures above 700 °C could easily lead to the dealumination of the Cu-SSZ-13 framework, preventing copper species from migrating back to the active site.

Despite the inherent limitations in its sulfur resistance, Cu-SSZ-13 remains the industrially preferred SCR material. Currently, the industry mainly conducts research on sulfur poisoning by stabilizing the temperature [3,19,20,22,23]. However, practical applications involve transient fluctuations in the temperature, space velocity, and reaction atmosphere, potentially altering the Cu-SSZ-13 sulfur poisoning mechanism. Furthermore, Cu-SSZ-13 is widely used in selective catalytic reduction filter (SDPF) for light vehicles. There is also a lack of research on the sulfur poisoning failure mechanism in SDPF during the transient worldwide harmonized light vehicles test cycle (WLTC) reaction process. Addressing this gap, this article conducted WLTC sulfur poisoning research on an SDPF catalyst coated with Cu-SSZ-13 on a bench. We performed a comparative study on the emission performance under different sulfur poisoning frequency and poisoning concentration operating conditions. A CS analyzer, thermogravimetric analysis/derivative thermogravimetric analysis (TG/DTG), temperature-programmed desorption (TPD), temperature-programmed reduction with H₂ (H₂-TPR), and X-ray photoelectron spectroscopy (XPS) were used to analyze the sulfur poisoning concentration and sulfur species in the catalyst. We used ammonia temperature programmed desorption (NH₃-TPD) for acid site analysis, displayed the change in the specific surface area after catalyst poisoning using the Brunauer–Emmett–Teller (BET) method, and performed in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) research to elucidate the impact of the sulfur species on the nitrate and NH₃ adsorption. Based on these comprehensive findings, a plausible sulfur poisoning and regeneration pathway for Cu-SSZ-13 is proposed.

2. Results

2.1. Research on Sulfur Poisoning and Regeneration Performance

We performed WLTC tests on 5 ppm, 30 ppm, 300 ppm SO₂, and 600 ppm SO₂ sulfur poisoning samples and 600 ppm SO₂ sulfur poisoning regeneration samples on an engine bench. The outcomes (Figure 1) demonstrate that initial exposure to 5 ppm SO₂ reduced the NOx conversion efficiency by approximately 3%. After the second sulfur poisoning incident, the NOx conversion efficiency further decreased by about 12%, followed by an additional ~8% reduction after the third exposure. As the sulfation frequency increased to 11 cycles, the degradation in the NOx conversion efficiency plateaued. This indicates that under specific poisoning conditions, sulfur poisoning may not initially have had a significant impact on the catalyst, potentially due to sufficient active site availability, resulting in less significant effects after poisoning; as the number of poisoning incidents increased, the active species of the catalyst became covered or the active species became inactive, resulting in a significant decrease in activity. After further increasing the frequency of sulfur poisoning, the number of active sites that could bind to SO₂ decreased, resulting in no significant degradation of the performance after poisoning. Elevating the SO₂ concentration to 30 ppm induced rapid deactivation after the first exposure, with the degradation only stabilizing after five cycles. Further increasing the SO₂ to 300 ppm and 600 ppm, the NOx conversion efficiency showed that higher concentrations of SO₂ led to the faster sulfur poisoning and deactivation of the catalyst, and after two sulfur poisoning cycles, the NOx conversion efficiency of the catalyst reached a performance equilibrium due to the poisoning. The regeneration (600 °C, 30 min) of the catalyst poisoned at 600 ppm partially restored the NOx conversion efficiency to 54.68%.

To analyze the impact of sulfur poisoning on the transient performance, a transient reaction process data analysis was conducted, as shown in Figures S1 and S2. According to Figure S1a, relative to the fresh sample, the NOx emissions increased between 845–1840 s after the first WLTC sulfur poisoning incident, which occurred over a temperature range of 200–375 °C. As the number of sulfur poisoning times increased, the impact on the NOx emissions under transient conditions increased, with a year-on-year increase in the NOx emissions, starting at 240 s under WLTC conditions, corresponding to a temperature range of 176–375 °C, and a further expansion of the range within which the performance was impacted. After seven cycles of poisoning, NOx emissions did not have an earlier impact, but the transient NOx concentration emissions significantly increased. Compared to the seventh sulfur poisoning incident and the eleventh sulfur poisoning incident, the NOx emissions still began to have an impact at 240 s, with only a slight increase in the NOx emissions. This indicates that under the current sulfur poisoning conditions, the catalyst had reached the maximum limit of sulfur poisoning. The effect of different sulfur concentrations on the transient emissions of NOx/NH₃ after the first WLTC sulfur poisoning incident affecting the catalyst is shown in Figure S1b. As the concentration of the sulfur poisoning increased, the NOx emission concentration gradually increased throughout the entire operating condition, and the impact of the NOx emissions also gradually increased. The regeneration of the component poisoned at 600 ppm slightly reduced the transient NOx concentration but failed to restore it to a fresh state (Figure S1c).

The ammonia slip during the reaction process can further illustrate the impact of sulfur poisoning on the catalytic performance. The instantaneous emission curve of NH₃ under the WLTC is shown in Figure S2. Under WLTC conditions, the fresh sample only exhibited an NH₃ slip during the heating and cooling intervals at 1100–1460 s and 1548–1840 s. When the catalyst was poisoned with sulfur and the number of sulfur poisoning incidents increased, NH₃ leakage occurred earlier, there was an increased NH₃ slip magnitude, and the concentration of the ammonia slip even far exceeded the limit of the analyzer. The regeneration of the component poisoned at 600 ppm slightly reduced the transient NH₃ slip but could not restore the fresh-state performance.

2.2. Characterization Analysis

2.2.1. Sulfur Content Analysis

The sulfur content on Cu-13-5, Cu-13-30, Cu-13-300, and Cu-13-600 catalysts was determined to be 0.31%, 0.33%, 0.30%, and 0.32%, respectively; the sulfur content did not differ due to differences in the sulfur poisoning concentration. This was because sulfur mainly had a 1:1 molar coordination ratio with copper. Using the same catalyst formula resulted in the same sulfur content [23]. After the regeneration of sulfur-poisoned samples, the sulfur content was 0.07%, 0.12%, 0.18%, and 0.20%, respectively. This indicates that catalysts are prone to forming unstable sulfur species at low sulfur levels. Brookshear et al. [24] conducted a sulfur saturation poisoning study on a coated Cu-CHA catalyst in an atmosphere of 500 ppm SO₂, 14% O_2, and 5% H₂O. ICP-OES characterization revealed that the accumulated sulfur content of the coated Cu-CHA catalyst was approximately 0.23% and 0.265% at 200 °C and 400 °C, respectively. The sulfur poisoning content was consistent with the sulfur poisoning amount in this study. Brookshear et al. further found that after regeneration at 500 °C, the sulfur content of the sample exposed to sulfur poisoning at 200 °C decreased to 0.22%. After regeneration at 600 °C, the sulfur content of the sample exposed to sulfur poisoning sample at 400 °C was less than 0.05%. In contrast, the sulfur -poisoned and regenerated samples in this study showed a higher sulfur content. This may have been due to the existence of a variable temperature under the transient WLTC poisoning conditions in this study, which facilitated the formation of difficult-to-decompose sulfur copper species.

2.2.2. BET

To investigate the effect of sulfur poisoning on the catalyst’s specific surface area, BET analyses were performed on fresh, poisoned, and regenerated catalysts, along with parallel measurements of a DPF and Cu-SSZ-13 powder. The specific surface areas of the DPF supporter and Cu-SSZ-13 powder were 6.78 m²/g and 701.09 m²/g, respectively. Following the coating of the DPF with Cu-SSZ-13 (Cu-13-F), the specific surface area of the catalyst decreased to 99.59 m²/g. The trend of the changes in the specific surface area after catalyst poisoning and regeneration is shown in Figure S3. The specific surface areas of sulfur-poisoned Cu-13-5, Cu-13-30, and Cu-13-600 samples decreased by 27.92%, 20.27%, and 18.32%, respectively, reaching 71.79 m²/g, 79.40 m²/g, and 81.35 m²/g, respectively. After regeneration at 600 °C, the specific surface area of the catalysts recovered to some extent. The specific surface areas of the Cu-13-5-R, Cu-13-30-R, and Cu-13-600-R catalysts were 89.76 m²/g, 85.94 m²/g, and 83.73 m²/g, respectively, which were 90.13%, 86.29%, and 84.07% of that of the fresh state, respectively. BET research further indicated that low sulfur concentrations favor unstable species formation, leading to catalyst site coverage and a decrease in the specific surface area [24]. A high sulfur content could easily lead to the formation of stable sulfur species, while the impact on the surface area was relatively low. Notably, regeneration at 600 °C proved insufficient for complete sulfur species decomposition.

2.2.3. XRD

Fresh, sulfur-poisoned, and sulfur-regenerated samples were characterized using an X-ray polycrystalline diffractometer (XRD) (Figure S4 and Table S1). After 5 ppm sulfur poisoning, the catalyst still maintained the characteristic peak of Cu-SSZ-13, but the diffraction intensity decreased compared to that of the fresh samples (Table S1). When the sulfur poisoning concentration increased to 600 ppm, the diffraction intensity only slightly decreased. The above explanation shows that sulfur poisoning caused the deformation of the microstructure of the zeolite, but the crystal form of the zeolite was not significantly affected. Heat treatment regeneration could achieve the recovery of the zeolite diffraction intensity to 95.86%.

2.2.4. TG

Figure 2 shows the TG, DTG (a), and mass loss plots (b) of fresh, sulfurized, and regenerated samples within different temperature ranges. From Figure (a), it can be seen that the fresh catalyst sample displayed a significant mass loss at <300 °C, with a DTG peak at ~100 °C attributed to the desorption of water in the zeolite. When the catalyst sulfur was poisoned, in addition to the thermogravimetric loss below 300 °C, new DTG peaks were observed at ~425 °C, ~540 °C, ~750 °C, and~985 °C. According to literature reports, NH₄SO₄ undergoes a two-step decomposition process, first decomposing into NH₄⁺ and NH₄HSO₄ at ~310 °C, then further decomposing into NH₃, SO₂, O₂, and H₂O at ~420 °C [12,25,26]. Therefore, the decomposition at ~310 °C can be attributed to NH₄SO₄ species, while the decomposition at ~420 °C can be attributed to NH₄HSO₄ species. Yu et al. [27] attributed the peak at ~310 °C to NH₄HSO₄ decomposition and the decomposition at ~420 °C to NH₄SO₄ species. Regardless of the species attribution analysis, the decomposition peak below 450 °C corresponded to the decomposition of sulfur ammonia species, with sulfur existing as SO₄²⁻. Jangjou et al. [23] found that SO₂ readily combines with ZCuOH to form CuHSO₃ species, which decompose below 580 °C. In the presence of O₂, CuHSO₃ can further oxidize to form CuHSO₄ species, requiring higher temperatures to decompose (e.g., 750 °C). Su et al. [9] found that Cu-CHA forms H₂SO₄ after sulfur poisoning, while CuSO₄ and Al₂(SO₄) species were analyzed using SO₂-TPD, with decomposition temperatures of 545 °C, 632 °C, and 772 °C, respectively. Shen et al. [28] found that the DTG of CuSO₄·5H₂O demonstrated mass loss at the same temperature (~700 °C). Therefore, the DTG peak at ~540 °C was attributed to the decomposition of H₂SO₄ and CuHSO₃. The ~750 °C DTG peak was attributed to the decomposition of CuHSO₄, CuSO₄, and Al₂(SO₄) species. Through the analysis of the sulfur species attribution mentioned above, it can be concluded that there were significant differences in the sulfur species deposited by SDPF catalysts after poisoning with different sulfur concentrations. The Cu-13-5 sample exhibited three DTG peaks at ~420 °C, ~540 °C, and ~720 °C, with the highest mass loss observed at ~420 °C. This indicates that under low sulfur poisoning, the catalyst was mainly poisoned by sulfur ammonia species, followed by H₂SO₄ and CuHSO₃ species at ~540 °C, and minimal numbers of CuHSO₄, CuSO₄, and Al₂(SO₄)₃ species. As the sulfur concentration gradually increased to 30 ppm and 600 ppm, the DTG peak gradually decreased at ~420 °C. The DTG peaks gradually increased at ~540 °C and ~750 °C, indicating that an increase in the sulfur concentration made it easier to form stable sulfur species. In addition, the regenerated catalyst showed a new DTG peak at ~985 °C, potentially related to the accelerated collapse of the molecular sieve skeleton caused by the decomposition of sulfur copper species or sulfur poisoning. After the regeneration of the sample poisoned with 600 ppm sulfur, the sulfur ammonia species decomposed nearly completely below 550 °C, leaving only a small DTG peak at ~610 °C, while the sulfur species’ DTG peak significantly increased at ~750 °C. This indicates that after thermal regeneration, the sulfur ammonia species decomposed completely, while the CuHSO₃ species further oxidized to form more stable CuHSO₄ and CuSO₄ species. The peak at ~985 °C shifted towards that of lower-temperature products, possibly reducing the stability of the skeleton during the heat treatment process and accelerating the dealumination and decomposition of the skeleton. According to the DTG diagram, the evolution of the thermogravimetric loss could be divided into three intervals: (1) the low-temperature interval (300–485 °C) dominated by sulfur ammonia species; (2) the medium-temperature interval (485–660 °C) dominated by H₂SO₄ and CuHSO₃ species; and (3) the high-temperature interval (660–1000 °C) dominated by CuHSO₄, CuSO₄, and Al₂(SO₄) species. The semi-quantitative analysis results regarding the sulfur species in the corresponding intervals are shown in Figure 2b. According to Figure 2b, under the three temperature conditions, the mass of the catalyst poisoned by the sulfur species was not affected by the sulfur poisoning concentration, and the sulfur poisoning content was between 1.0 and 1.14%. Under low-concentration sulfur poisoning at 5 ppm (Cu-13-5), the mass loss in the low-, medium-, and high-temperature ranges was 0.50%, 0.38%, and 0.15%, respectively. As the sulfur concentration increased to 30 ppm (Cu-13-30), the mass loss in the three sulfur temperature intervals was 0.28%, 0.47%, and 0.29%, respectively, showing a decrease in sulfur ammonia species and an increase in copper sulfur and other species. Further increasing the sulfur concentration to 600 ppm (Cu-13-600) resulted in mass losses of 0.26%, 0.47%, and 0.41% in the three sulfur temperature intervals, respectively. The mass loss remained relatively constant at low and medium temperatures, while the mass loss at high temperatures increased further, indicating that a high sulfur content may be more likely to cause skeleton collapse. After the regeneration of the sulfur-poisoned catalyst, the mass loss in the low- and middle-temperature regions decreased to 0.18% and 0.18%, respectively, while the mass loss in the high-temperature region increased to 0.64%. This indicates that while regeneration can decompose sulfur ammonia species and convert copper sulfur species, it simultaneously reduces the stability of the skeleton.

2.2.5. TPD

TPD

Using a chemical adsorption instrument, a temperature-programmed experiment was conducted directly on sulfur -poisoned samples to analyze the differences in the sulfur species among the sulfur-poisoned samples, as shown in Figure S5. As shown in Figure S4, the sample that had not undergone sulfur poisoning exhibited a TCD response peak centered at ~330 °C, which may have been due to the response peaks generated by the adsorption of impurities by the molecular sieve and the decomposition of the skeleton material. After being poisoned by 5 ppm sulfur, the peak at ~330 °C shifted to ~310 °C. This peak may have been related to the decomposition of sulfur ammonia species, and a new desorption peak appeared at ~480 °C. Also considering the decomposition temperatures of sulfur ammonia and the less stable H₂SO₄ and CuHSO₃ in the TG analysis, the ~485 °C peak can be attributed to the TCD response peak generated by the decomposition of sulfur ammonia species and H₂SO₄ and CuHSO₃ species. When the sulfur poisoning concentration increased to 30 ppm, the TCD response peak shifted towards higher temperatures, which may have been caused by inter species interactions on the catalyst, leading to the high-temperature shift in the decomposition peak. A new, corresponding TCD peak appeared with a center peak position of about ~520 °C, which could be attributed to H₂SO₄ and CuHSO₃ species. This indicates that an increase in the sulfur poisoning concentration made it easier to form unstable copper sulfur species, consistent with the TG results. The Cu-13-30 sample exhibited a trace desorption strength at ~660 °C. When the sulfur concentration increased to 600 ppm, neither the peak temperature nor the intensity changed significantly, possibly because the sulfur poisoning concentration difference was too small for TCD detection. After the catalyst was regenerated at 600 °C, the peaks at ~485 °C, ~520 °C, and ~660 °C disappeared, and the TCD response peak became nearly identical to that of the fresh sample. According to the TG results, the regenerated sample still showed weight loss above 580 °C, which may have been due to a decrease in the sulfur species content and the low response of residual species formed by decomposition, resulting in there being no significant TCD response peak above 580 °C.

NH₃-TPD

The ammonia storage capacity of the catalyst samples showed a significant correlation with the NH₃-SCR performance, as analyzed using a chemical adsorption instrument. As shown in Figure 3, the Cu-13-F sample exhibited three desorption peaks, with the α peak indicating weak Lewis acid sites and the β peak indicating strong Lewis acid sites; the γ peak represented the Bronsted acid site [29]. The intensity of the α and β peaks was related to the active Cu²⁺ species. When the catalyst was sulfur-poisoned at 5 ppm, it exhibited four response peaks: α and β remained, the γ peak nearly disappeared, and new NH₃ desorption peaks (δ and ε) emerged at 400–450 °C and 580–730 °C. Shen et al. [21] analyzed the NH₃-TPD of Cu-SAPO-34 after sulfur poisoning and found an NH₃ desorption peak at 360 °C, attributed to the decomposition of sulfur ammonia species. Thus, the intensity of the new β peak can be attributed to the decomposition of sulfur ammonia species, the δ peak may have arisen from the formation of H₂SO₄ and CuHSO₃ species through decomposition, and the ε peak may have been related to the ammonia adsorption of sulfur copper species or the self-decomposition of the sulfur copper species. As the sulfur poisoning concentration increased to 30 ppm, a new ζ peak appeared at 750–900 °C, which may have been related to the adsorption of ammonia and self-decomposition of CuHSO₄ and CuSO₄. Under 600 ppm sulfur poisoning, the number of peaks, peak temperature, and peak shape were consistent with those of the 30 ppm sample. Only the ε and ζ peaks interacted and fused into one peak, indicating that more sulfur copper species were generated under a high sulfur content. The sulfur copper species interacted and fused to form a fresh response peak. After the regeneration of the 600 ppm catalyst, the NH₃-TPD peak was similar to that of the 5 ppm sulfur catalyst, with only a slight decrease in the peak intensity. A quantitative analysis of the response peak of the catalyst sample is shown in Figure 3b. The ammonia desorption amounts of the fresh, 5 ppm, 30 ppm, 600 ppm, and 600 ppm sulfur poisoning samples were 0.41 mmol/g, 0.87 mmol/g, 1.10 mmol/g, 1.14 mmol/g, and 0.62 mmol/g, respectively. As the concentration of the sulfur poisoning increased, the amount of ammonia desorbed by α and β ammonia first increased and then decreased. Firstly, there was an increase in the partial decomposition of sulfur ammonia species that may have been unstable at low concentrations. Further reductions may have been due to the formation of sulfur copper species, which reduced the ammonia adsorption on Cu²⁺. The disappearance of the γ desorption peak after sulfur poisoning may have been due to sulfonamide species covering the Brønsted acid sites or the formation of H₂SO₄ species due to the binding of the Brønsted acid sites with SO₃ [9]. With an increase in the sulfur poisoning concentration, the desorption amount indicated by the δ peaks related to CuHSO₃ and H₂SO₄ increased, indicating that the formation of copper sulfur species and zeolite skeleton poisoning deactivation were more likely under a high sulfur concentration. Compared with the Cu-13-F sample, the desorption amount indicated by the α and β peaks of the regenerated catalyst sample was significantly reduced. Compared with the Cu-13-600 sample, the ammonia desorption amount indicated by the ε + ζ peak of the regenerated sample was significantly reduced. In summary, the regeneration at 600 °C effectively decomposed most of the sulfur ammonia species and partially restored the sulfur copper species but inevitably reduced the number of active Cu²⁺ sites.

2.2.6. H₂-TPR

H₂-TPR analysis was conducted to investigate alterations in the copper speciation following the sulfur poisoning and regeneration treatments. To eliminate the effect of interference from sulfur ammonia species decomposition on the TCD signals, the sulfur-poisoned sample was maintained at 500 °C for 1 h before testing. The test results are shown in Figure 4. The fresh sample exhibited four reduction peaks, with the ~243 °C peak attributed to the reduction of Cu(OH)⁺ to Cu⁺ on the eight-membered ring inside the CHA cage, the ~315 °C peak attributed to the reduction of CuO nanoparticles to Cu, the ~410 °C peak attributed to the reduction of Cu²⁺ to Cu⁺ on the six-membered ring, and the ~600 °C peak attributed to the reduction of Cu⁺ [29,30,31]. When the catalyst was sulfur-poisoned at 5 ppm, two significant reduction peaks were observed at~330 °C and ~540 °C, and a small reduction peak was observed at ~760 °C. According to reports, SO₂ species are easily adsorbed on CuOx nano aggregates, leading to the formation of sulfur copper species [32,33,34]. Shen et al. studied Cu/SSZ-13 after sulfur poisoning and attributed the H₂-TPR reduction peak at ~540 °C to CuSO₄ species reduction and the peak at~760 °C to Cu⁺ reduction [28]. Jangjou et al. [23] found that sulfur species combine with ammonia to form sulfur ammonia species and deposit on Z₂Cu, while ZCuOH is more prone to combining with sulfur species to form sulfur copper species. A 500 °C heat treatment can achieve the regeneration of sulfur ammonia species [9,14,28]. The catalyst had the ability to maintain a certain catalytic performance even after sulfur poisoning and the catalyst pretreatment step at 500 °C before testing. Therefore, the reduction peak at ~325 °C can be attributed to the Z₂Cu species reduction peak, the ~540 °C peak to the sulfur copper species reduction peak, and the ~765 °C peak to Cu⁺ reduction. Compared to the fresh samples, Cu-13-5 sulfur poisoning samples formed a large number of sulfur copper species, and the Z₂Cu reduction peak shifted towards lower temperatures, possibly due to differences in the coordination environment around the Cu²⁺ species. Further increasing the concentration of the sulfur poisoning did not significantly change the temperature of the catalyst reduction peak; only the intensity of the reduction peak changed. After catalyst regeneration, a new reduction peak appeared at ~265 °C, the reduction peak at ~540 °C shifted to ~475 °C, and a significant reduction peak appeared at ~780 °C. Based on the significant improvement trend of the catalyst regeneration performance, the ~265 °C reduction peak can be attributed to the ZCuOH formed after the regeneration of CuSO₄ species, ~475 °C to the reduction of sulfur copper species, and ~780 °C to the reduction of Cu⁺. The significant increase in the Cu⁺ reduction peak indicates irreversible effects on the zeolite skeleton after regeneration. The ability of the copper species and molecular sieve skeleton to bind together weakened, making it easier to achieve reduction, which was consistent with the results of the TG analysis. A semi-quantitative analysis of the copper content in the catalyst samples before and after sulfur poisoning and regeneration is shown in Figure 4b. The copper species in the fresh catalysts mainly existed in the form of Cu²⁺, accompanied by a small amount of CuOx species. After catalyst sulfur poisoning, ZCuOH and CuO disappeared, the Z₂Cu species slightly decreased, and a large number of sulfur copper species were formed. As the concentration of the sulfur poisoning increased, the content of Z₂Cu species further decreased, indicating that under extreme sulfur concentrations, Z₂Cu also coordinated with SO₂, leading to a decrease in the Z₂Cu content. After the regeneration of the 600 ppm sulfur poisoning sample, there was no significant change in the peak area of the Z₂Cu species, and the ZCuOH peak was partially restored, while the intensity of the reduction peak of the sulfur copper species was almost halved. The above results further confirm that sulfur ammonia species adhere to Z₂Cu species, and regeneration at 600 °C can cause the decomposition or transformation of ZCuOH-coordinated CuHSO₃ species, which is consistent with the research results of Jangjou et al. [23].

2.2.7. UV–Vis Spectra

UV-Vis spectroscopy was employed to identify distinct copper species within the samples, as presented in Figure S6. The sample spectrum of Cu-SSZ-13 (Cu-13-P) exhibited two different absorption bands, with maxima at approximately 210 and 860 nm. The former was attributed to the d-d transition of Cu²⁺ ions in octahedral coordination, while the absorption band at 210 nm was attributed to a ligand–metal charge transfer (LMCT), O→Cu, caused by isolated Cu²⁺ [12,20]. The DPF sample displayed three absorption bands near 204, 325, and 700 nm. The Cu-13-F sample combined the absorption bands of the Cu-13-P and DPF samples. Upon sulfur poisoning at 5 ppm, the intensity of the absorption band centered at 860 nm diminished significantly, and the intensity of the band with a center position of 210 nm decreased. Concurrently, an absorption peak emerged within the 250–300 nm range for the copper sulfate species. Compared to the Cu-13-5 sample, the Cu-13-600 sample demonstrated a further reduction in the intensity of the 210 nm band, while the 250–300 nm absorption peak of the copper sulfate species increased. The regeneration of the Cu-13-600 sample resulted in an enhancement of the 210 nm band’s intensity, a reduction in the 250–300 nm band’s intensity, and a slight recovery of the 860 nm band’s intensity. These UV-Vis findings are consistent with the H₂-TPR results.

2.2.8. XPS

Further analysis of the sulfur poisoning species’ attribution was conducted using XPS (as shown in Figure 5). The fresh sample exhibited a broad background peak (166–178 eV), which corresponded to the surface plasmon emission of Si2s caused by its emission [35]. When the catalyst was sulfur-poisoned, it could be classified as a sulfur-metal binding species in the binding energy range of 166–172 eV [36]. The Cu-13-5 sulfur poisoning sample exhibited a spectral peak at a binding energy of approximately 168.4 eV, which could be attributed to the SO₄²⁻ species. A small SO₃²⁻ reduction peak was suspected to appear at ~166.6 eV. Wijayanti et al. [12] used XPS to study the species after sulfur poisoning of SO₂ + O₂, SO₂ + O₂ + NH₃, 200 °C SO₂ + StdSCR, and 400 °C SO₂ + StdSCR. The study found that a very small peak was observed at ~166.6 eV under various atmospheric conditions. Yong et al. [26] exposed a Cu-SSZ-13 catalyst to NO, NH₃, and O₂ atmospheres before switching to 0.01% (vol) SO₂ treatment, finding a prominent XPS peak at ~166.6 eV in all the catalysts. Based on a comprehensive analysis, it was speculated that the ~166.6 eV peak in the Cu-13-5 sample was caused by equipment shaking or other factors. Therefore, after sulfur poisoning saturation, sulfur existed on the catalyst in the form of SO₄²⁻. When the sulfur poisoning concentration increased to 30 ppm, the binding energy of SO₄²⁻ increased from 168.4 eV to 168.6 eV; further increasing it to 600 ppm resulted in the SO₄²⁻ peak shifting to ~168.8 eV. Following the regeneration of the 600 ppm sulfur poisoning catalyst, no notable change in the binding energy was detected. Wijayanti et al. [12] found that the binding energy of SO₄²⁻ under a SO₂ + O₂ + H₂O atmosphere was 169.7 eV, and the binding energy of sulfur poisoning species on the catalyst under a SO₂ + O₂ + H₂O + NH₃ or SO₂ + standard SCR reaction atmosphere was 169.1 eV. This indicates that the differences in the SO₄²⁻ species in the catalyst will further affect the binding energy, and the formation of sulfur ammonia species can easily cause the XPS peak to shift towards a lower binding energy. Therefore, the peak position of SO₄²⁻ reflects that catalysts subjected to different sulfur poisoning concentrations formed different sulfur poisoning species. At low concentrations, the catalysts were more likely to form sulfur ammonia species. As the concentration of toxic sulfur increased, the species of sulfur and copper also increased. Under high-concentration sulfur poisoning at 600 ppm, the catalyst mainly formed sulfur copper species. Post-regeneration, despite the partial decomposition of the sulfur ammonia species, the SO₄²⁻ binding energy remained largely unaffected. This also indirectly indicates that under a high sulfur concentration, catalyst-poisoning sulfur species exist more often in the form of sulfur copper.

2.2.9. In Situ DRIFTS

NO + O₂ Adsorption

The in situ DRIFTS spectra of fresh catalysts, catalysts subjected to sulfur poisoning at different concentrations, and regenerated catalysts are shown in Figure 6a. The fresh Cu-SSZ-13 powder sample exhibited four nitrogen oxide adsorption species in the spectral range of 1450–1700 cm⁻¹. The nitrogen oxide adsorption species at wavenumbers of ~1635 cm⁻¹, ~1626 cm⁻¹, ~1598–1572 cm⁻¹, and ~1554 cm⁻¹ belonged to nitrite, bridging nitrate, bidentate nitrate, and monodentate nitrate, respectively [37,38]. When Cu-SSZ-13 coated the DPF supporter, the fresh, poisoned, and regenerated samples showed no characteristic peaks except for an adsorption peak attributed to monodentate nitrate, which may have been due to a weak carrier signal or low molecular sieve content, resulting in the signal peak being covered by other impurity peaks. The peak areas resulting from Gaussian deconvolution integration for each sample are shown in Figure S7a. The peak fitting results indicate that as the concentration of the sulfur poisoning increased, the content of nitrogen oxide-adsorbed species decreased. After regeneration, the adsorption amount only reached 93.7% of that of the fresh samples. This also indicates that regeneration at 600 °C can achieve the regeneration of most of the active sites but not complete regeneration.

NH₃ + O₂ Adsorption

Figure 6b shows the NH₃ + O₂ saturated DRIFTS spectra (850–1000 cm⁻¹) of the fresh, sulfurized, and regenerated Cu-SSZ-13 samples. All the samples contained two Cu²⁺ active sites, namely Z₂Cu at ~950 cm⁻¹ and ZCuOH at ~900 cm⁻¹ [6]. After sulfur poisoning, the intensity of all the peaks decreased significantly. Moreover, the spectra show an increasing trend for the sulfur poisoning concentration and a decreasing trend for the peak intensity. Further integral under the curve analysis was performed on the peak areas of Z₂Cu and ZCuOH (as shown in Figure S7b). The peak areas of Z₂Cu in the Cu-13-5, Cu-12-30, and Cu-13-600 sulfur-poisoned samples were 85.98%, 78.52%, and 74.82% of those in the fresh samples, respectively. Meanwhile, the peak areas of ZCuOH in the Cu-13-5, Cu-12-30, and Cu-13-600 sulfur-poisoned samples were 80.38%, 73.45%, and 68.17% of those in the fresh samples, respectively. In contrast, the peak of ZCuOH decreased more significantly compared to that of Z₂Cu, indicating that ZCuOH is more reactive and binds more easily with sulfur species. After the regeneration of the Cu-13-600 sample, the peak areas of ZCuOH, Z₂Cu, and the total Cu²⁺ were restored to 84.56%, 89.80%, and 88.47% of those in the fresh sample, respectively. In contrast, the recovery of Z₂Cu was more significant, and Cu²⁺ could not be completely restored, which was consistent with the H₂-TPR results and performance under WLTC conditions.

2.3. Possible Sulfur Poisoning Reaction Pathway

The above characterization studies indicate that after sulfur poisoning under WLTC conditions, the catalyst sulfur poisoning species were mainly NH₄SO₄, H₂SO₄, and +6 valent sulfur copper species, including CuHSO₄ and CuSO₄. A difference in the sulfur concentration led to differences in the content of various sulfur species. The copper species on the Cu-13-F catalyst mainly existed as Z₂Cu and CuOx, with a small amount of copper species present as ZCuOH. Some -Si-O(H)-Al-sites were retained without exchange. In a tail gas atmosphere containing SO₂, O₂, and NH₃, SO₂ was first adsorbed on the bridging oxygen atoms of CuOx and subsequently oxidized to SO₃ (Table S2, Reaction R-1). The adsorbed SO₃ diffused and reacted with NH₃ adsorbed on Cu²⁺ to produce NH₄HSO₄ (Reaction R-2), and the reduced oxidation sites were re-oxidized by oxygen [9]. NH₄HSO₄ could further react with NH₃ to generate NH₄SO₄ (Reaction R-3), and NH₄HSO₄ and NH₄SO₄ species coordinated with Z₂Cu and covered the catalyst pores, causing pore blockages. SO₂ easily formed H₂SO₄ species with O₂ and H₂O (Reaction R-4) and was mainly enriched at Bronsted acid sites [19]. Some of the SO₂ species reacted with Z₂Cu and ZCuOH sites to form ZCuHSO₃ species (Reactions R-5 and R-6) and further oxidizes ZCuHSO₃ to form ZCuHSO₄ species (Reaction R-7) during the heating process under WLTC conditions [23]. Additionally, some SO₃ species directly reacted with Z₂Cu and ZCuOH to form ZCuHSO₄ species (Reactions R-8 and R-9). The formation of CuSO₄ may occur through multiple pathways. ZCuHSO₃ species can undergo a one-step transformation into CuSO₄ species under a heated oxygen atmosphere (Reaction R-10), or ZCuOH can directly react with SO₂ and O₂ to generate CuSO₄ (Reaction R-11) [23,38]. CuOx may also undergo multi-step oxidation with SO₂ to form CuSO₄ (Reactions R-12 and R-13) [32,34]. After the catalyst was regenerated at 600 °C, (NH₄)₂SO₄ underwent two-step decomposition to achieve regeneration (reverse processes of Reactions R-3 and R-2), H₂SO₄ species decomposed and regenerated (reverse process of Reaction R-4), and ZCuHSO₄ and CuSO₄ species partially decomposed to form CuO species, which further underwent solid-phase ion exchange to form Cu²⁺, achieving catalyst regeneration.

3. Experimental Section

3.1. Catalyst Preparation

The Cu-SSZ-13 used in this study was prepared by an ion exchange using H-SSZ-13, which was obtained from CP Energy Material (Dalian) Co., Ltd., Dalian, China. First, so that H-SSZ-13 (Si/Al = 17) underwent copper nitrate exchange, we stirred 1000 g zeolite into a solution of 4000 mL of 0.3 mol/L Cu(NO₃)₂ at 80 °C for 2 h. The zeolite was then filtered and washed several times with deionized water until it became neutral. Thereafter, the copper exchange was repeated one time, and the resulting Cu-SSZ-13 was dried at 120 °C for 12 h and calcined at 650 °C for 3 h. The final target Cu loading of Cu-SSZ-13 was 3.20 wt% (labeled Cu-13-P). The Cu-SSZ-13 powder was added to a deionized water solution containing the binder. After stirring and mixing evenly, we ground the slurry to a particle size of D90 = 5.0 μm. We weighed a certain amount of the Cu-SSZ-13 zeolite slurry, used it to coat the upper face of the DPF carrier, and applied a certain degree of vacuum to the lower face of the DPF carrier so that the slurry entered the carrier pores. After coating, the DPF carrier was dried at 120 °C for 8 h and calcined at 500 °C for 3 h. The obtained SDPF catalyst was labeled as Cu-13-F.

3.2. Catalyst Sulfur Poisoning During WLTC and Emission Testing

We selected an engine operating point to achieve a pre-catalyst exhaust temperature of 200 °C and an exhaust flow rate of 100 kg/h. We introduced pure SO₂ gas into the front end of the SDPF. We adjusted the mass flow rate to achieve the target SO₂ concentrations (5, 30, 300, and 600 ppm). We performed the sulfur poisoning of the SDPF catalyst by running the WLTC multiple times. The WLTC emission test was conducted in accordance with Appendix C of GB 18352.6-2016, “Emission Limits and Measurement Methods for Light-Duty Vehicles (China 6)”, using a hot-start emission test. The poisoned catalyst sample was named Cu-13-X-Y, where X represents the SO₂ concentration and Y represents the poisoning frequency. When the catalyst performance stabilized after multiple WLTC runs, the catalyst was named Cu-13-X. The regenerated sample after heat treatment at 600 °C for 30 min was labeled Cu-13-X-R.

3.3. Characterization

The specific surface area of the catalyst samples was determined by means of nitrogen adsorption–desorption at the temperature of liquid N₂ (77 K) using an Autosorb-IQ2 apparatus (Quantachrome Instruments, Boynton Beach, FL, USA). The specific surface area was evaluated using the Brunauer–Emmett–Teller (BET) method.

The crystal structure of the catalyst was characterized on a TTRIII-18KW target X-ray polycrystalline diffractometer (XRD) produced by Rigaku, Japan. The instrumental testing conditions were as follows: the radiation source was a Cu K α source (λ = 1.5406 Å), the X-ray tube voltage was 40 kV, the tube current was 80 mA, the angle scanning range was 2θ = 5–70°, the scanning step size was 0.02°, and the scanning speed was 10°/min.

Thermogravimetric analysis (TG/DTG) was performed on a NETZSCH STA 449 F3 Jupiter (Selb, Germany) in an air flow (60 mL·min⁻¹) with an increase from 50 to 1000 °C at a heating rate of 10 °C·min⁻¹.

The sulfur content was measured using a LECO CS844 carbon sulfur analyzer. For testing, the prepared sample was placed into an electric arc furnace, with the flux evenly distributed on the top, and high-purity oxygen was used to assist combustion.

Temperature-programmed reduction with H₂ (H₂-TPR) was performed using Micromeritics CHEMBET PULSAR equipment. The reducibility of the samples and the states of the active sites were investigated. Approximately 50 mg of each sample was placed in a U-shaped tube and sealed with quartz wool. The sample was heated to 500 °C for 1 h using a heating furnace to remove the adsorbed sulfur and ammonia species. A thermal conductivity detector was used to record the H₂ signal (TCD signal). Subsequently, an automatic cooling fan system was activated to cool the system to 150 °C at a cooling rate of 20 °C/min. After stabilization for 5 min under a He flow, the gas was switched to 10% H₂/Ar mixed gas (flow rate: 75 mL/min) for 60 min, after which the temperature was increased to 820 °C at a heating rate of 10 °C/min.

The ammonium temperature-programmed desorption (NH₃-TPD) experiment and the temperature-programmed desorption (TPD) experiment were performed using a CHEMBET3000 (Quantachrome Instruments, Boynton Beach, FL, USA) with a thermal conductivity detector. Before the NH₃-TPD experiments, the samples (100 mg) were pretreated at 250 °C for 30 min in a flow of 10% O₂/N₂ and then cooled to 150 °C. Next, a NH₃ (10.0 vol%) flow was introduced into the system for 1 h, and the system was purged using a pure He flow for another hour to remove the physically adsorbed ammonia. The reactor temperature was ramped up to 820 °C at a heating rate of 10 °C/min. For the TPD measurements, the sample was heated directly to 800 °C in a He atmosphere without pretreatment.

The properties of the different copper species in the samples were examined by UV–visible spectroscopy (U-4100 instrument). The reflectance was measured at a data interval of 1 nm and a scanning rate of 300 nm/min.

X-ray photoelectron spectroscopy (XPS) was performed with a PHI-1600 ESCA SYSTEM spectrometer using Mg Ka as the X-ray source (1253.6 eV) under a residual pressure of 5 × 10⁻⁶ Pa. The binding energy error was ±0.2 eV, calibrated using C 1s at 284.6 eV as the reference.

In situ DRIFTS experiments were performed using an FTIR spectrometer (Thermo NICOLET 6700) equipped with a Praying Mantis DRIFTS cell and an MCT detector. Approximately 20 mg of the catalyst powder was placed in the DRIFTS cell and pretreated at 180 °C for 30 min under a N₂ flow. The in situ adsorption DRIFTS experiments were conducted after the sample had been exposed to a 500 ppm NH3 or NO + 5% O₂/N₂ flow of 200 mL/min until saturation. The spectra were recorded by accumulating 64 scans over the range of 4000–650 cm⁻¹, and the resolution was 4 cm⁻¹.

4. Conclusions

This investigation employed an engine bench to subject a Cu-SSZ-13-coated SDPF catalyst to sulfur poisoning under transient WLTC conditions. The analysis demonstrated that the NH₃-SCR performance of the SDPF catalyst deteriorated significantly with an increasing sulfation frequency and SO₂ concentration, particularly within the 200–375 °C range under WLTC conditions. This degradation stemmed from the coordination of active Cu²⁺ species with sulfur entities, forming +6 valent sulfur ammonium and sulfur copper species. The sulfur ammonia species occluded the catalyst surface, reducing the specific surface area and hindering the accessibility of Brønsted acid sites and active sites, thereby inhibiting reactant adsorption. The sulfur copper species stably bound to Cu²⁺ sites, reducing the amount of active Cu²⁺. After stable sulfur poisoning at varying concentrations, the catalysts exhibited nearly identical sulfur accumulation levels, attributed to the 1:1 molar coordination between sulfur and copper in both sulfur ammonia and sulfur copper species. Consequently, under the same catalyst formulation, the accumulated sulfur content reached an equivalent equilibrium after stable sulfur poisoning, regardless of the concentration. Notably, low-concentration sulfur poisoning tended to generate renewable, decomposable sulfur ammonia species, whereas high-concentration sulfur poisoning favored the formation of more stable sulfur copper species. Regeneration at 600 °C only partially restored the catalytic performance. Thus, the primary mechanisms of catalyst deactivation involve the depletion of acidic sites and the reduction of active copper species, providing critical insights for optimizing sulfur resistance in catalyst design.