English

• Nitrogen oxides (NO_x) are main air pollutants, and they are detrimental to both environment and human health [1,2]. Abundant amounts of NO_x emitted from diesel vehicles are highly dispersive and hard to control [3,4]. Selective catalytic reduction with ammonia (NH₃-SCR) is one of the most commonly used technologies to abate nitrogen oxide (NO_x) emissions from diesel vehicles [5–8]. Among the various NH₃-SCR catalysts, Cu-based small-pore zeolites have shown extraordinary properties, and Cu-SSZ-13 catalysts are commercially applied in the after-treatment systems of diesel vehicles [9–11].

  Cu-SSZ-39 catalysts possess comparable NH₃-SCR activity to Cu-SSZ-13 and much better hydrothermal stability, and thus have a bright future for application [12–14]. Considering the complex composition of the exhaust gas, SCR catalysts usually suffer from a variety of poisoning conditions, such as sulfur poisoning, phosphorus poisoning, alkali/alkaline earth metal poisoning and hydrocarbon (HCs) poisoning [15–17]. Our group has carried out a series of studies on the deactivation mechanisms of Cu-SSZ-39 catalysts [18–20]. The formation of H₂SO₄ and CuSO₄ species is the main reason causing low-temperature activity loss in Cu-SSZ-39 catalysts [19]. Alkali/alkaline earth metals were found to induce deterioration of the framework and loss of isolated Cu²⁺ ions, and thus led to serious deactivation. The formation of Cu-P species in phosphorus-poisoned Cu-SSZ-39 catalysts was found to cause a decrease in the number of active copper species and resulted in serious low-temperature deactivation, but such inhibition effect could be alleviated after hydrothermal aging treatment [20]. Understanding the poisoning mechanism provides a guide to improving the resistance of catalysts. However, there is still a lack of understanding of the HCs poisoning mechanism over Cu-SSZ-39 catalysts.

  The effects of HCs poisoning on Cu-SSZ-13 catalysts have been widely studied, and C₃H₆ was often chosen as a typical HC compound. Duan et al. revealed that C₃H₆ was competitively adsorbed at the active copper sites, and the redox cycle was impeded in the low-temperature range [21]. A similar phenomenon was observed by Ma et al. in which active copper sites are covered by C₃H₆ moreover, zeolite pores are blocked by carbon deposits. However, the carbon deposits could be burned off at high temperature and catalytic activity was recovered [22]. Zhao et al. further revealed that the carbon deposits are mainly composed of aliphatic and aromatic compounds, which deepened our understanding of the HCs poisoning mechanism [23]. In our recent study, we thoroughly investigated the C₃H₆ poisoning mechanism of the NH₃-SCR reaction over Cu-SSZ-13 with various Cu contents and Si/Al ratios. It was found that besides the accumulation of carbon deposits, C₃H₆ ammoxidation occurred, which caused a lack of NH₃ and also resulted in a decrease in the high-temperature catalytic activity [24].

  In this study, we investigated the effects of C₃H₆ on the catalytic activity of fresh and hydrothermally aged Cu-SSZ-39 catalysts, simulating the short-distance and long-distance driving SCR catalysts. Meanwhile, the influence of Cu content on the C₃H₆ resistance of Cu-SSZ-39 catalysts was discussed, to better understand the C₃H₆ resistance of different Cu species.

  The crystallinity of the catalyst was investigated by XRD, and the results are presented in Fig. S1 (Supporting information). Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh catalysts both showed the typical diffraction peaks of the AEI structure. After hydrothermal aging treatment, little change in crystallinity was observed, and no characteristic peaks of CuO_x clusters were detected. This indicates that the framework of the prepared catalysts is stable and copper species are highly dispersed in both fresh and aged Cu-SSZ-39 catalysts.

  The ²⁷Al NMR and ²⁹Si NMR results of the Cu-SSZ-39 catalysts are shown in Fig. 1. Only a framework Al (FAl) peak at 58 ppm was observed in Fig. 1a, and no extra-framework Al (EFAl) peak at 0 ppm was detected in both fresh and aged Cu-SSZ-39 catalysts [25,26]. Moreover, three peaks at around −111, −105 and −99 ppm were observed in the ²⁹Si NMR profiles (Fig. 1b), and they were assigned to Si(0Al4Si), Si(1Al3Si), and Si(2Al2Si), respectively [25,27]. According to Fig. 1b and Table S1 (Supporting information), a slight increase in Si/Al ratios can be observed in both Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh after hydrothermal aging treatment, indicating that weak dealumination occurred [28,29]. Combined with the results of XRD, it can be seen that the framework of the prepared Cu-SSZ-39 catalysts presented good stability.

  Figure 1

  

  Figure 1.  (a) ²⁷Al and (b) ²⁹Si NMR spectra of Cu-SSZ-39 catalysts with different copper contents before and after hydrothermal aging treatment. Si/Al ratio was calculated from areas of deconvoluted ²⁹Si NMR peaks.

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  Copper species, as the active centers in Cu-SSZ-39 catalysts, play an important role in the NH₃-SCR reaction. Therefore, to better understand the effect of C₃H₆ on different Cu species in Cu-SSZ-39 catalysts, H₂-TPR experiments were conducted on both fresh and aged samples, and the results are shown in Fig. 2. The peak B at about 280 ℃ was attributed to the reduction of Cu²⁺ to Cu⁺ in the D6R, and the peaks at temperatures above 500 ℃ (peak C and peak D) corresponded to the reduction of Cu⁺ [30,31]. Only Cu²⁺ and Cu⁺ species existed in the Cu_1.9-SSZ-39-Fresh sample. However, a small peak emerged at around 180 ℃ (peak A) in Cu_3.4-SSZ-39-Fresh, assigned to CuO_x species [32]. Moreover, more CuO_x species were formed in Cu_1.9-SSZ-39-HTA and Cu_3.4-SSZ-39-HTA.

  Figure 2

  

  Figure 2.  H₂-TPR profiles of Cu-SSZ-39 catalysts with different copper contents before and after hydrothermal aging treatment.

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  To evaluate the amount of divalent Cu ions in the catalysts, EPR experiments were carried out, with the results depicted in Fig. S2 (Supporting information). Only one type of hexacoordinated Cu²⁺ species (g_‖ = 2.41, A_‖ = 133 G) was observed in both the fresh and aged catalysts, assigned to Cu²⁺ species [13,33]. Moreover, the normalized Cu²⁺ concentration was calculated and the data are presented in Fig. S2b. As can be observed, both Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh faced a decrease in the number of Cu²⁺ ions after hydrothermal aging treatment, which indicated that a large amount of Cu²⁺ transformed to EPR-inactive copper species (such as CuO_x).

  To sum up, Cu²⁺ species dominated in both the Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh catalysts. After hydrothermal aging treatment, part of the Cu²⁺ species transformed to CuO_x species, reducing the concentration of EPR-active Cu²⁺ species.

  The influence of C₃H₆ on the fresh and aged Cu-SSZ-39 catalysts with different Cu contents was evaluated, and the results are presented in Fig. 3. After hydrothermal aging treatment, the low-temperature catalytic activity of Cu_1.9-SSZ-39-HTA significantly increased, while the high-temperature activity slightly decreased. However, the Cu_3.4-SSZ-39-HTA sample did not achieve an observable increase in low-temperature catalytic activity, while the high-temperature catalytic activity declined, which is in accordance with our previous studies [13,20,32]. After the introduction of C₃H₆, the catalytic activity above 250 ℃ of Cu_1.9-SSZ-39-Fresh decreased significantly, with NO_x conversion around 60% at 550 ℃. However, the negative affect of C₃H₆ on catalytic activity was alleviated in Cu_1.9-SSZ-39-HTA, and an increase in NO_x conversion was observed above 450 ℃. Meanwhile, the inhibition of catalytic activity in Cu_3.4-SSZ-39-Fresh caused by C₃H₆ was weaker than that of Cu_1.9-SSZ-39-Fresh, and the NO_x conversion of Cu_3.4-SSZ-39-Fresh also began to increase above 400 ℃. Moreover, the effect of C₃H₆ on catalytic activity was even weaker in Cu_3.4-SSZ-39-HTA. The NO_x conversion of Cu_3.4-SSZ-39-HTA-500 ppm C₃H₆ was even higher than that of Cu_3.4-SSZ-39-HTA above 450 ℃.

  Figure 3

  

  Figure 3.  Effect of C₃H₆ on the activity of Cu-SSZ-39 catalysts with low-copper-loading (a) and high-copper-loading (b) before and after hydrothermal aging. Gas conditions: [NO] = 500 ppm, [NH₃] = 500 ± 5 ppm, [O₂] = [H₂O] = 5 vol%, [C₃H₆] = 500 ppm (when used), balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  To better understand the poisoning mechanism of C₃H₆ on Cu-SSZ-39 catalysts, NH₃ conversion, C₃H₆ conversion and the concentration of HCN during NH₃-SCR activity tests are depicted in Fig. 4. The shapes of the NH₃ conversion curves of Cu_1.9-SSZ-39-Fresh and Cu_1.9-SSZ-39-HTA are similar to their NO_x conversion curves, while the NH₃ conversion of Cu_1.9-SSZ-39-HTA was slightly higher than NO_x conversion over 450 ℃ (Figs. 4a and b). Considering the formation of CuO_x species in Cu_1.9-SSZ-39-HTA (Fig. 2), the decline in NO_x conversion at high temperature was probably due to non-selective oxidation of NH₃. In the presence of C₃H₆, the NH₃ conversion of Cu_1.9-SSZ-39-Fresh and Cu_1.9-SSZ-39-HTA was clearly higher than NO_x conversion, especially over 350 ℃. This result indicates that NH₃ was consumed by other reactions, and not enough NH₃ participated in the reduction of NO_x. Meanwhile, the C₃H₆ conversion of Cu_1.9-SSZ-39-Fresh and Cu_1.9-SSZ-39-HTA during NH₃-SCR tests is presented in Fig. 4c. The ignition temperature of C₃H₆ in Cu_1.9-SSZ-39-Fresh and Cu_1.9-SSZ-39-HTA is around 300–350 ℃, and as the conversion of C₃H₆ increased, HCN began to form, as presented in Fig. 4d. Therefore, it can be inferred that the NH₃ reacted with C₃H₆ over 300 ℃, forming HCN according to Eq. 1 [17,34], and the amount of NH₃ was insufficient for the NH₃-SCR reaction. It is worth mentioning that Cu_1.9-SSZ-39-HTA possessed higher C₃H₆ conversion than Cu_1.9-SSZ-39-Fresh, while more HCN formed for Cu_1.9-SSZ-39-Fresh, indicating that part of the C₃H₆ was consumed by other reactions for Cu_1.9-SSZ-39-HTA. Furthermore, the concentration of HCN over Cu_1.9-SSZ-39-HTA began to decrease over 450 ℃ (Fig. 4d), accompanied by an increase in NO_x conversion (Fig. 3a). This indicates that the reaction between C₃H₆ and NH₃ was hindered in Cu_1.9-SSZ-39-HTA over 450 ℃, so that sufficient NH₃ was available to participate in the NH₃-SCR reaction.

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  Figure 4

  

  Figure 4.  The NH₃ conversion of Cu-SSZ-39 catalysts with low copper (a) and high copper (b) before and after hydrothermal aging, C₃H₆ conversion (c) and HCN concentration (d). Gas conditions: [NO] = 500 ppm, [NH₃] = 500 ± 5 ppm, [O₂] = [H₂O] = 5 vol%, [C₃H₆] = 500 ppm (when used), balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  The NH₃ conversion of Cu_3.4-SSZ-39-Fresh and Cu_3.4-SSZ-39-HTA is illustrated in Fig. 4b. The NH₃ conversion of Cu_3.4-SSZ-39-Fresh was similar to its NO_x conversion, however, Cu_3.4-SSZ-39-HTA possessed higher NH₃ conversion than NO_x conversion over 350 ℃, owing to the existence of CuO_x species. When C₃H₆ was introduced, the NH₃ conversion of both Cu_3.4-SSZ-39-Fresh and Cu_3.4-SSZ-39-HTA was higher than NO_x conversion over 300 ℃. From 300 ℃ to 400 ℃, C₃H₆ conversion rose dramatically meanwhile, the HCN concentration increased and NO_x conversion decreased. In this temperature range NH₃ reacted with C₃H₆ therefore, the NH₃-SCR reaction was hindered. When the temperature rose over 450 ℃, the HCN concentration decreased and NO_x conversion recovered, indicating that C₃H₆ was consumed by other reactions and more NH₃ participated in the NH₃-SCR reaction.

  It should be noted that HC-SCR is also a way for NO_x removal [35,36], and the reaction between C₃H₆ and NO followed Eq. 2. Therefore, the C₃H₆-SCR reaction was conducted to evaluate its impact on the NH₃-SCR reaction over Cu-SSZ-39 catalysts. Fig. S3a (Supporting information) shows that the NO_x conversion was quite low below 300 ℃ during the C₃H₆-SCR reaction in all the tested Cu-SSZ-39 catalysts. Meanwhile, NO_x conversion began to increase over 300 ℃, together with the formation of a small amount of HCN (Fig. S3b in Supporting information). The formation of HCN may have followed Eq. 3, and the concentration of HCN (Fig. S3b) was lower than that generated in the NH₃-SCR reaction (Fig. 4d). Considering that the inlet concentration of NO was 500 ppm, less than 8% NO_x conversion was obtained for Cu_1.9-SSZ-39-Fresh, and the HCN concentration was over 20 ppm at 400 and 450 ℃. Therefore, over half of the NO_x was consumed following Eq. 3 for Cu_1.9-SSZ-39-Fresh at 400 and 450 ℃. However, for the other catalysts, most of the NO_x was consumed following Eq. 2, and C₃H₆-SCR played an important role in the NO_x conversion over 350 ℃.

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  To further prove the side reaction between NH₃ and C₃H₆ led to a deficiency in the amount of NH₃ available for the NH₃-SCR reaction, NH₃ was introduced at various concentrations during the NH₃-SCR tests. As presented in Figs. 5a and b, both fresh Cu_1.9-SSZ-39 and Cu_3.4-SSZ-39 catalysts faced a decline in NO_x conversion when C₃H₆ was introduced regardless of the NH₃ concentration. When the NH₃ concentration increased from 400 ppm to 500 ppm, the NO_x conversion for both the Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh catalysts increased dramatically. However, the NO_x conversion was only slightly improved when the NH₃ concentration further increased to 600 ppm. Similar phenomenon was observed in the aged catalysts as presented in Figs. 5c and d. When C₃H₆ was introduced, NO_x conversion declined in both Cu_1.9-SSZ-39-HTA and Cu_3.4-SSZ-39-HTA catalysts; with the increase of NH₃ concentration from 400 ppm to 500 ppm NO_x conversion was improved significantly.

  Figure 5

  

  Figure 5.  NO_x conversion of Cu-SSZ-39 catalysts using different NH₃ concentrations: (a) Cu_1.9-SSZ-39-Fresh, (b) Cu_3.4-SSZ-39-Fresh, (c) Cu_1.9-SSZ-39-HTA, (d) Cu_3.4-SSZ-39-HTA. HCN concentration: (e) Cu_1.9-SSZ-39, (f) Cu_3.4-SSZ-39 catalysts. Gas conditions: [NH₃] = 400, 500, 600 ppm, [NO] = 500 ppm, [O₂] = [H₂O] = 5 vol%, [C₃H₆] = 500 ppm, balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  HCN concentration of catalysts was helpful to analyze side reactions and the results were presented in Figs. 5e and f. More HCN was generated when the NH₃ concentration increased, which indicated that the additional NH₃ was depleted by C₃H₆. For the Cu_3.4-SSZ-39 catalyst, NO_x conversion decreased from 300 ℃ to 400 ℃, and further increased from 450 ℃ on. This was in accordance with the turning point of HCN concentration at 400 ℃ as presented in Fig. 5f. This phenomenon indicates that when HCN formed (from C₃H₆ reacting with NH₃), the NH₃-SCR reaction was hindered; when the HCN concentration decreased (C₃H₆ was consumed by other reactions), the NH₃-SCR reaction recovered. The relationship between HCN generation and NO_x conversion indicates that C₃H₆ hindered the NH₃-SCR reaction by depleting NH₃. Cu_1.9-SSZ-39-HTA exhibited similar NO_x conversion and HCN concentration curves to those of Cu_3.4-SSZ-39-Fresh, which was probably due to the composition of Cu species in them being similar (both had a small amount of CuO_x species). However, Cu_3.4-SSZ-39-HTA possessed much more CuO_x species, and the NO_x conversion curve was somewhat different. Besides the turning point at 400 ℃, another turning point at 450 ℃ emerged in the NO_x conversion curve of Cu_3.4-SSZ-39-HTA. As presented in Figs. 5e and f, the least amount of HCN was generated compared with the other tested catalysts. Therefore, the decrease in NO_x conversion over 450° in Cu_3.4-SSZ-39-HTA was probably due to the non-selective oxidation of NH₃.

  In addition, to investigate whether carbon deposition contributed to the decline in NO_x conversion activity at high temperatures, steady-state NH₃-SCR testing was carried out. Both fresh and aged Cu-SSZ-39 catalysts with different Cu content were exposed to a NH₃-SCR+C₃H₆ atmosphere at 400 ℃, and the resulting NO_x conversion is presented in Fig. 6. In the first 180 min, the NO_x conversion of all the tested catalysts remained constant without a detectable decline trend, excluding the effect of carbon deposition under the conditions we used. The concentration of input NH₃ was increased from 500 ppm to 600 ppm at the 180th min, and an increase in NO_x conversion could be observed for all tested catalysts. This was in accordance with the result of Fig. 5, which further proved that the depletion of NH₃ was the main reaction for the decline in NO_x conversion.

  Figure 6

  

  Figure 6.  NO_x conversion of Cu-SSZ-39 at 400 ℃. (a) Cu_1.9-SSZ-39-Fresh, (b) Cu_1.9-SSZ-39-HTA, (c) Cu_3.4-SSZ-39-Fresh and (d) Cu_3.4-SSZ-39-HTA. Gas conditions: [NH₃] = 500 ppm, [C₃H₆] = 500 ppm, 600 ppm, [O₂] = [H₂O] = 5 vol%, balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  In order to further investigate the side reaction, NH₃ (+C₃H₆) oxidation experiments were conducted, and results are shown in Fig. S4 (Supporting information). The NH₃ conversion of the fresh and aged Cu_1.9-SSZ-39 catalysts was below 50% under the entire tested temperature range without C₃H₆ introduction (Fig. S4a). When C₃H₆ was introduced, the NH₃ conversion of Cu_1.9-SSZ-39 catalysts began to increase significantly over 400 ℃, and a large amount of HCN species formed for Cu_1.9-SSZ-39-Fresh (Figs. S4a and b). The NH₃ conversion of Cu_3.4-SSZ-39 was not over 50% above 500 ℃ without C₃H₆. When C₃H₆ was introduced, more NH₃ was consumed at 400 and 450 ℃ and large amount of HCN was produced (Figs. S4b and c). However, Cu_3.4-SSZ-39-HTA exhibited higher NH₃ conversion without C₃H₆, and hardly any HCN formed with Cu_3.4-SSZ-39-HTA when C₃H₆ was introduced (Figs. S4b and c). This indicates that NH₃ was not consumed by C₃H₆, and non-selective oxidation of NH₃ was the main reason for the high NH₃ conversion of Cu_3.4-SSZ-39-HTA. Meanwhile, Cu_3.4-SSZ-39-HTA exhibited the highest C₃H₆ conversion among the tested catalysts, indicating that the oxidation of C₃H₆ is also more facile with this catalyst. Moreover, C₃H₆ conversion was presented in Fig. S4d, and C₃H₆ conversion of prepared catalysts followed sequence as: Cu_3.4-SSZ-39-HTA > Cu_3.4-SSZ-39-Fresh > Cu_1.9-SSZ-39-HTA > Cu_1.9-SSZ-39-Fresh. Considering the Cu species in prepared catalysts, it can be deduced that CuO_x species promoted C₃H₆ oxidation.

  As presented in Fig. 3, little effect of C₃H₆ was observed on the catalytic activity of Cu-SSZ-39 catalysts below 250 ℃, and the negative affect on catalytic performance mainly occurred at high temperatures. The diffusion diameter of C₃H₆ molecules is around 4.5 Å, but the pore size of Cu-SSZ-39 zeolites is 3.84 Å × 3.84 Å × 3.64 Å. Therefore, it is difficult for C₃H₆ molecules to enter the pore channels of Cu-SSZ-39 catalysts and effect catalytic activity below 250 ℃ [37,38]. With increasing temperature, the thermal vibrations of the zeolite framework and C₃H₆ molecules were intensified simultaneously [39,40]. Therefore, C₃H₆ molecules could enter the channels of the Cu-SSZ-39 zeolite and affect the catalytic activity of the Cu-SSZ-39 catalysts.

  The effects of C₃H₆ on the catalytic activity of Cu-SSZ-39 catalysts with different Cu loadings varied significantly. As presented in Fig. 3, NO_x conversion was inhibited by C₃H₆ for both Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh above 250 ℃. However, the activity loss was much more severe for Cu_1.9-SSZ-39-Fresh, and an increase in NO_x conversion was even observed for Cu_3.4-SSZ-39-Fresh over 400 ℃. H₂-TPR results (Fig. 2) revealed that CuO_x species emerged in Cu_3.4-SSZ-39-Fresh, which may explain the difference between Cu_1.9-SSZ-39-Fresh and Cu_3.4-SSZ-39-Fresh. As presented in Figs. 4c and d, C₃H₆ conversion was higher in Cu_3.4-SSZ-39-Fresh with less HCN formation, indicating that oxidation of C₃H₆ occurred more easily when Cu loading was high. Moreover, we have concluded that the reaction between C₃H₆ and NH₃ was the main reason for catalytic activity loss at high temperature. Cu_3.4-SSZ-39-Fresh, with more CuO_x, can more easily oxidize C₃H₆ therefore, more NH₃ can participate in the NH₃-SCR reaction, and it possessed better catalytic performance than Cu_1.9-SSZ-39-Fresh.

  Hydrothermal aging treatment also affected the C₃H₆ resistance of Cu-SSZ-39 catalysts. As presented in Fig. 3, C₃H₆ had less effect on the catalytic activity of the aged Cu-SSZ-39 catalysts than the fresh ones. Meanwhile, higher C₃H₆ conversion and lower HCN production was achieved by the aged Cu-SSZ-39 catalysts (Figs. 4c and d). Similar to Cu_3.4-SSZ-39-Fresh, both Cu_1.9-SSZ-39-HTA and Cu_3.4-SSZ-39-HTA possess CuO_x species, as shown in Fig. 2. Meanwhile, a turning point of NO_x conversion emerged for Cu_3.4-SSZ-39-Fresh, Cu_1.9-SSZ-39-HTA and Cu_3.4-SSZ-39-HTA around 400 ℃ (Fig. 3). As we discussed above, CuO_x species are active for C₃H₆ oxidation over 400 ℃, which alleviates the reaction between C₃H₆ and NH₃ [41]. Therefore, NH₃ is released to participate in the NH₃-SCR reaction, resulting in an increase in NO_x conversion. When Cu-SSZ-39 catalysts are hydrothermally aged, abundant CuO_x species form therefore, catalytic activity recovers. It is worth mentioning that a further decrease in NO_x conversion was observed in Cu_3.4-SSZ-39-HTA above 500 ℃ (Fig. 3b). This is because the presence of excessive CuO_x species could also increase the oxidation of NH₃ (Fig. S4b), which again resulted in a lack of NH₃ [42,43].

  It is interesting that Cu-SSZ-13 catalysts behaved differently when subjected to C₃H₆ poisoning. Studies revealed a decrease in low-temperature catalytic activity when C₃H₆ was introduced to Cu-SSZ-13, and the blocking of pores by carbon deposits was the main reason for activity loss [21–23,44]. In our recent study, we found that besides the propylene ammoxidation reaction, carbon deposits were also responsible for the activity loss of Cu-SSZ-13. Meanwhile, carbon can be deposited from 250 ℃ to 500 ℃ [24]. However, the decline in NO_x conversion for Cu-SSZ-39 was not due to carbon deposits in this study. As presented in Fig. 6, no decline in NO_x conversion was observed with the introduction of C₃H₆ for any of the Cu-SSZ-39 catalysts within 180 min. Comparatively, small-pore zeolites possess better HC resistance than medium- or large-pore zeolites, because HC molecules are too large to enter the zeolite pores. Moreover, compared with Cu-SSZ-13, the channels of Cu-SSZ-39 are more tortuous, therefore C₃H₆ molecules have more difficulty entering the channels of Cu-SSZ-39 under the conditions we carried out, avoiding carbon deposition. Furthermore, the CuO_x species in Cu-SSZ-39 catalysts possessed better redox ability than Cu²⁺ species [32]. Therefore, CuO_x species in Cu-SSZ-39 can help to oxidize C₃H₆, preserving NH₃ for the NH₃-SCR reaction.

  In this study, the effect of C₃H₆ on Cu-SSZ-39 catalysts was investigated. It turned out the propylene ammonia oxidation reaction at high temperatures resulted in the consumption of NH₃, therefore, affecting NH₃-SCR activity. CuO_x species in high Cu loaded or hydrothermally aged Cu-SSZ-39 catalysts facilitated the oxidation of C₃H₆, preserving NH₃ for SCR reaction. As a result, catalyst with higher Cu content performed better with C₃H₆ existing, meanwhile, long-distance driving (hydrothermal aging process) cannot result in the decline of HC resistance in Cu-SSZ-39 catalysts. For one thing, Cu-SSZ-39 catalysts are known for excellent hydrothermal stability, and the aged catalysts even presented better HCs resistance, which proved that Cu-SSZ-39 is capable of application. For another, according to the results of this study, Cu-SSZ-39 catalysts of high Cu content with a little CuO_x species should be used considering SCR activity and HCs resistance. The research on the HCs resistance of Cu-SSZ-39 catalysts proved that Cu-SSZ-39 catalysts are capable of application in diesel vehicles. The application of Cu-SSZ-39 catalysts can help to further improving atmospheric environment.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Jinpeng Du: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing, Funding acquisition. Junlin Chen: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft. Yulong Shan: Conceptualization, Data curation, Resources, Supervision, Writing – review & editing, Funding acquisition. Tongliang Zhang: Investigation, Validation. Yu Sun: Data curation, Investigation, Validation. Zhongqi Liu: Data curation, Formal analysis, Validation. Xiaoyan Shi: Data curation, Formal analysis. Wenpo Shan: Data curation, Validation. Yunbo Yu: Data curation, Funding acquisition, Resources, Supervision, Validation, Writing – review & editing. Hong He: Project administration, Resources, Validation.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (Nos. 52200136, 52270112). National Energy-Saving and Low-Carbon Materials Production and Application Demonstration Platform Program (No. TC220H06N). Young Elite Scientists Sponsorship Program by CAST (No. 2022QNRC001).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110019.

  
  
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  Figure 1  (a) ²⁷Al and (b) ²⁹Si NMR spectra of Cu-SSZ-39 catalysts with different copper contents before and after hydrothermal aging treatment. Si/Al ratio was calculated from areas of deconvoluted ²⁹Si NMR peaks.

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  Figure 2  H₂-TPR profiles of Cu-SSZ-39 catalysts with different copper contents before and after hydrothermal aging treatment.

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  Figure 3  Effect of C₃H₆ on the activity of Cu-SSZ-39 catalysts with low-copper-loading (a) and high-copper-loading (b) before and after hydrothermal aging. Gas conditions: [NO] = 500 ppm, [NH₃] = 500 ± 5 ppm, [O₂] = [H₂O] = 5 vol%, [C₃H₆] = 500 ppm (when used), balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  Figure 4  The NH₃ conversion of Cu-SSZ-39 catalysts with low copper (a) and high copper (b) before and after hydrothermal aging, C₃H₆ conversion (c) and HCN concentration (d). Gas conditions: [NO] = 500 ppm, [NH₃] = 500 ± 5 ppm, [O₂] = [H₂O] = 5 vol%, [C₃H₆] = 500 ppm (when used), balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  Figure 5  NO_x conversion of Cu-SSZ-39 catalysts using different NH₃ concentrations: (a) Cu_1.9-SSZ-39-Fresh, (b) Cu_3.4-SSZ-39-Fresh, (c) Cu_1.9-SSZ-39-HTA, (d) Cu_3.4-SSZ-39-HTA. HCN concentration: (e) Cu_1.9-SSZ-39, (f) Cu_3.4-SSZ-39 catalysts. Gas conditions: [NH₃] = 400, 500, 600 ppm, [NO] = 500 ppm, [O₂] = [H₂O] = 5 vol%, [C₃H₆] = 500 ppm, balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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  Figure 6  NO_x conversion of Cu-SSZ-39 at 400 ℃. (a) Cu_1.9-SSZ-39-Fresh, (b) Cu_1.9-SSZ-39-HTA, (c) Cu_3.4-SSZ-39-Fresh and (d) Cu_3.4-SSZ-39-HTA. Gas conditions: [NH₃] = 500 ppm, [C₃H₆] = 500 ppm, 600 ppm, [O₂] = [H₂O] = 5 vol%, balanced by N₂, total gas flow: 500 mL/min, GHSV = 200,000 h⁻¹.

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