H-SAPO-34 was prepared by a hydrothermal method. The mixture solution of Orthophosphoric acid (H₃PO₄, 85 wt%), morpholine (MOR, 99 wt%), aluminium (boehmite, 75 wt%) and silica (silica sol, 30 wt%) was stirred for 5 h at a temperature of 38˚C. The mixture was put in autoclave at 200˚C for 48 h under autogenous pressure for crystallization process. The product was filtered, washed, dried at 110˚C overnight and the dried product was calcined in the air at 550˚C for 4 h. H-SAPO-34 was ion-exchanged with NH 4 + ion by using NH₄NO₃ (95 wt%). Then, the solid was filtered, washed with deionized water and the obtained sample NH 4 + -SAPO-34 was dried at 110˚C overnight before the ammonium-exchange process was repeated for a total of two exchanges. The Cu-SAPO-34 zeolite was also prepared via wet ion exchange method by using an aqueous solution of copper sulfate (2%) [29][30]. The mixture of copper sulfate aqueous solution (0.1M) and the NH 4 + -SAPO-34 was magnetically stirred for 12 h at 70˚C. The product was washed with deionized water and dried at 110˚C overnight. After the product was dried, it was calcined in the air at 550˚C for 4 h.

The Cu/ZSM-5 catalyst was prepared using a commercial H-ZSM-5 (SiO₂/Al₂O₃ = 25) by the improved incipient wetness impregnation method, as previously reported [31]. It was performed by mixing the H-ZSM-5 powers with the required amount of Cu (NO₃)₂ solution at ambient temperature. The mixture was stirred and then ultrasonically treated for 1 h. The prepared sample was dried at 100˚C for 12 h, calcined at 500˚C for 2 h. After calcination, certain amount of water was added to the as-prepared powders, and then all these were mixed to form some well-proportioned slurries. A cordierite (cylinder, diameter: 11 mm, length: 22 mm, bulk: 2.1 cm³, 400 cell·cm⁻²) was coated by dipping it into the slurries [32].

The monolithic SCR catalyst was used to evaluate the catalytic activity. Firstly, a certain amount of powder was added into deionized water to form slurry until the channel was filled with liquid. And then, the cordierite (cylinder, diameter: 11 mm, bulk: 2.1 cm³, length: 22 mm, 400 cell·cm⁻²), was put in the as-prepared slurry until the channel was filled with liquid. The immersion times and concentration of slurry were controlled strictly, in order to deposit a similar amount of catalyst powder on each cordierite. Finally, the activity of the monolithic SCR catalysts was tested in fixed-flow reactor (VDRT-200SCR) produced by Quzhou Vodo instrument Co. Ltd., China. The temperature was varied from 225˚C to 550˚C. The reaction gas mixture was typical: 1000 ppm NO, 1100 ppm NH₃, 5% O₂, 10% water vapor and N₂ as the carrier gas. The water vapor was generated by passing N₂ through a heated gas-wash bottle containing deionized water. The total flow rate was 1000 L·min⁻¹ and thus a GHSV was obtained 30,000 h⁻¹. The NO gas concentration in an outlet stream was monitored by an exhaust analyzer. An NH₃ trap containing phosphoric acid solution was installed before the input of the exhaust analyzer to avoid errors caused by the reduction of NH₃. The NO conversion of catalyst was evaluated as follows [33].

[NO] = ([NO]_in − [NO]_out) × 100/[NO]_in

The interactions between propene or SO₂ and NO_x on the catalyst surface were investigated [34]. At a steady state, a gas mixture containing 50 ppm SO₂ or 50 ppm propene, 1000 ppm NO, 1100 ppm NH₃, 5% O₂, 10% water vapor and N₂ was introduced into the reactor [35]. The total flow rate was 1000 L·min⁻¹ and thus a GHSV was obtained 30,000 h⁻¹.

The hydrothermal aging was performed in a simulated diesel engine exhaust gas composed of 10% H₂O in air with a flow rate of 1000 cm³/min for 12 h at 650˚C, 700˚C and 750˚C, respectively.

X-ray diffraction measurements were conducted on multi-sample Philips X’Pert diffractometer with a Cu Kα detector, and the diffraction angle ranged from 5˚ to 50˚ at a scanning speed of 5˚/min. The surface area and N₂ physisorption isotherm (77.3 K) were determined using Micrometrics ASAP 2020M surface area and porosity analyzer from the linear portion of the BET plot by measuring the N₂ isotherm of the samples. A UV-vis-DRS absorbance spectrum was obtained at a room temperature on a Shimadzu spectrophotometer. The temperature-programed desorption of hydrogen (H₂-TPR) was conducted on Micrometrics AutoChem 2920II chemisorption analyzer. After being pre-treated in Ar at 550˚C for 1 h, the sample was cooled down to 40˚C. Finally, the sample was heated to 1000˚C in a flow of 10% H₂/Ar at a ramp rate of 10˚C/min. The liquid nitrogen cold trap was used to remove the interference of H₂O. The temperature-programed desorption of ammonia (NH₃-TPD) was also carried out on Micrometrics AutoChem 2920II chemisorption analyzer. 50 mg of the sample was first pre-treated in Ar at 550˚C for 1 h to remove the impurities, and then cooled down to 100˚C. The sample was saturated with 3000 ppm NH₃/Ar at 100˚C for 45 min. After that, the sample was purged with Ar for 1 h to remove the physical adsorption NH₃ and then heated in Ar from 100˚C to 600˚C at ramp rate of 10˚C/min. The outlet NH₃ was monitored by the thermal conductivity detector. Thermal analysis was performed in a TGA/DSC system. The samples were heated in the flowing 20/min Air as protective gas from 50˚C to 700˚C at ramp rate of 10˚C/min [36]. 50 ppm of SO₂ or propene was introduced into the reactor for 1 h, then the weight signals were recorded.

NH₃-SCR performance of Cu-SAPO-34 and Cu-ZSM-5, along with their hydrothermal aged catalysts, were examined between 200˚C and 550˚C. As shown in Figure 1, an excellent NO reduction activity was obtained from fresh Cu-ZSM-5 and Cu-SAPO-34 catalysts. While the fresh Cu-ZSM-5 catalyst presents an inferior NO conversion in the range of lower and higher temperatures, it exhibits a superior NO conversion between 250˚C and 350˚C. After hydrothermal aged at 650˚C for 12 h, the catalytic activities of Cu-SAPO-34 catalyst is well maintained and only seen a slight decrease at lower temperatures (<250˚C) and higher temperatures (>500˚C). However, with further increasing the aging temperature to 750˚C, as presented in Figure 1(a), shows a slight decline of NO conversion, corresponding to a 10% reduction in the catalytic activity of Cu-SAPO-34 aged (750˚C) compared with the fresh catalyst. Figure 1(b) shows that, after aging at 650˚C for 12 h, the catalytic activities of Cu-ZSM-5 catalyst are maintained at higher temperature (>400˚C) and significantly dropping at temperatures < 400˚C. After increasing the aging temperature to 700˚C, the catalytic activities of Cu-ZSM-5 almost remain the same, at the temperature > 450˚C and then they further decrease at temperature < 450˚C. Interestingly, with further increasing the aging temperature to 750˚C, the NO conversions are well-maintained at temperatures > 500˚C, and further dropping at temperature < 500˚C. For instance, the NO conversion of Cu-ZSM-5 aged (750˚C) drop from 90% to 20% at 500˚C and 200˚C, respectively. Following the hydrothermal treatment at 750˚C for 12 h, a slight loss of NO_x reduction activity is observed in Cu-SAPO-34 at low and high temperatures, and a total loss is observed for Cu-ZSM-5 in a part (temperature < 500˚C) as shown in Figure 1. Ja et al. [1] found similar results, they reported that the total loss of activity of Cu-ZSM-5, in part, could be detected owing to the increased N₂O formation following hydrothermal aging.

During an operation of diesel engine, the catalyst is exposed to a hydrocarbon coking and a hostile environmental where SO₂ is an important contaminant generated by burning sulfurous species in fuels [9]. Figure 1(c) and Figure 1(d)

Figure 1. NH₃-SCR performance of the fresh, aged and poisoned: (a) Cu-SAPO-34; (b) Cu-ZSM-5; (c) Poisoned Cu-SAPO-34; (d) Poisoned Cu-ZSM-5 catalysts.

show the NO conversion with propene or SO₂ over Cu-ZSM-5 and Cu-SAPO-34 catalysts. After propene and SO₂ poisoning, the Cu-SAPO-34 show that the NO conversions are well-maintained at a temperature above 350˚C, but the NO conversion of Cu-SAPO-4 with propene slightly decreases, and that of Cu-SAPO-34 with SO₂ significantly decreases at temperature below 350˚C. However, the NO conversion of Cu-ZSM-5 in the presence of propene or SO₂ is maintained at temperature above 350˚C and then dramatically dropping at a temperature below 350˚C.

In order to better understand the changes in the NO_x reduction activity induced by hydrothermal aging, the changes in the morphology and the structure of the zeolites catalysts were examined by using XRD and N₂-desorption techniques. The XRD patterns show that the fresh Cu-ZSM-5 has MFI structure and the fresh SAPO-34 has the typical CHA structure as shown in Figure 2. After hydrothermal aging, no detectable CuO crystallites are observed in any of two catalysts [37], probably due to the proper dispersion of the Cu species or the CuO

Figure 2. XRD patterns of the fresh, aged and poisoned: (a) Cu-SAPO-34, (b) Cu-ZSM-5, (c) poisoned Cu-SAPO-34 and (d) poisoned Cu-ZSM-5 catalysts.

particles are too small to be detected. In addition, the XRD patterns of fresh and aged samples show no changes for Cu-SAPO-34, which indicates that the zeolite structure remains largely intact during the hydrothermal aging [1][37]. However, little changes are observed for Cu-ZSM-5, suggesting that its structure also remains less affected during the hydrothermal aging [38]. Based on the relative crystallinity, the BET surface area and micropore volume results show that the hydrothermal have no significant effect on the crystal structure or textural properties of Cu-SAPO34 catalyst.

Even for the Cu-SAPO-34-750 sample, its relative crystallinity remained almost 95%. Compared with Cu-ZSM-5, the surface area and micropore volume of Cu-ZSM-5 Aging (750˚C) drop from 342 to 280 m²·g⁻¹ and 0.16 to 0.08 cm³·g⁻¹, respectively (Table 1). These results in good agreement with the early report, which indicates that the Cu-ZSM-5 catalyst displays good hydrothermal stability, however, it is remarkable that Cu-SAPO-34 displays a better hydrothermal stability. After propene and SO₂ poisoning, the physical properties were investigated by using the XRD, and the results are shown in Figure 1(c) and Figure 1(d).

Table 1. Physicochemical properties of CU-SAPO-34 and Cu-ZSM-5 before and after hydrothermal aging at 750˚C.

For Cu-ZSM-5 catalysts, the results show that all the XRD patterns and their intensities are well-preserved after SO₂ and propene poisoning. Similar results are obtained between Cu-SAPO-34 catalyst and its propene or SO₂ poisoning form. These results confirm that the crystalline structure of the sample is well-preserved after hydrothermal treatment, propene and SO₂ poisoning.

H₂-TPR experiments were performed to examine the effects of hydrothermal aging on the nature of Cu species [37]. Figure 3 represents the H₂-TPR profiles of the fresh and aged catalysts. According to the literature, the H₂-TPR spectra collected over fresh and aged Cu-ZSM-5 zeolites could be divided into two groups based on the reducibility of copper species. According to the early report [37][39], it was suggested that Cu²⁺ is reduced to Cu⁺ at ~ 200˚C, while Cu⁺ ions is reduced to Cu⁰ at 250˚C - 350˚C for Cu-ZSM-5 catalysts. The hydrogen reduction peaks of Cu-ZSM-5 and Cu-ZSM-5 (650) were similar. Increasing the temperature of hydrothermal aging to 750˚C, significant differences in the H₂-TPR patterns were observed over the Cu-ZSM-5 as shown in Figure 3(b). The peak at ~200˚C tends to disappear with a further increase of the hydrothermal aging temperatures, indicating the migration of copper species might have taken place and that a much more stable copper species was formed during the high temperature hydrothermal treatment [4]. In addition, the peak assigned to the reduction of Cu⁺ to Cu⁰ at 250˚C - 350˚C gradually increased with the increase of the hydrothermal aging temperature. Thus, the amount of isolated Cu²⁺ ions in the structure of Cu-ZSM-5 catalyst decreases while the CuO increases after the hydrothermal treatment, which could be attributed to the transformation of isolated Cu²⁺ ions to CuO species. However, the H₂ consumption profiles of Cu-SAPO-34 catalysts could be divided into three peaks (A, B and C) based on the reducibility of Cu ions. According to the reviewed literature, peak A appearing at (200˚C - 300˚C) represents the reduction of isolated Cu²⁺ ions to Cu⁺ ions, peak B appearing at (500˚C - 600˚C) could be assigned to the reduction of CuO to

Figure 3. H₂-TPR profiles of the fresh and aged: (a) Cu-SAPO-34 and (b) Cu-ZSM-5 catalysts.

Cu⁰ and peak C appearing at high temperature (600˚C - 700˚C) being attributed to the reduction of Cu⁺ ions to Cu⁰ [1][3][9][24][30]. Figure 3(a) shows that the reduction peaks are still present and are almost maintained after hydrothermal aging of Cu-SAPO-34 at 650˚C, 700˚C and 750˚C for 12 h. Moreover, the H₂ consumed are quite similar to both fresh and aged samples, suggesting relatively small changes in the distribution of Copper species in their zeolite structures [1].

The UV-vis spectra of fresh and aged Cu-SAPO-34 and Cu-ZSM-5 catalysts are shown in Figure 4. As can be seen, fresh Cu-SAPO-34 and Cu-ZSM-5 exhibited two bands at ~200 and ~210 nm, and at ~800 and 700 nm, respectively. The band located at lower wavelengths can be assigned to a charge transfer (CT) band, while the band at higher wavelengths was attributed to the d-d transitions of isolated Cu²⁺ ions in distorted octahedral surrounding by oxygen in CuO particles [40][41]. After the hydrothermal aging of Cu-SAPO-34 at 650˚C for 12 h, the band ascribed to the isolated Cu/Cu²⁺ ions remained unchanged in its intensity, and its position was slightly shifted to 215 nm due to a strong interaction between copper species and the SAPO-34 support. Up to the aging temperature of 750˚C, that band related to the CuO species gradually disappeared, while the band related to the isolated Cu/Cu²⁺ ions remained high in its intensity. These changes confirm that, during the hydrothermal process of Cu-SAPO-34 catalyst, the copper oxide species on the external surface could disperse as copper ions and migrate to the exchanged sites, rather than sintering to even larger particles on the external surface [41]. In the order hand, the hydrothermal treatment may enhance the performance of NH₃-SCR over Cu-SAPO-34. As for the Cu-ZSM-5, both bands ascribed to isolated Cu/Cu²⁺ ions and the CuO species significantly changed after the hydrothermal treatment at 650˚C during 12 h. The band at 210 nm shows a significant decrease in its intensity, while that of high wavelength shows a significant increase in intensity. This phenomenon becomes more significant with the temperature increase of hydrothermal aging to 700˚C for 12 h.

Figure 4. UV-vis-DRS profiles of the fresh, aged and poisoned: (a) Cu-SAPO-34; (b) Cu-ZSM-5; (c) poisoned Cu-SAPO-34; (d) poisoned Cu-ZSM-5 catalysts.

Interestingly, further increase of temperature was marked at 750˚C for 12 h, the band related to isolated Cu/Cu²⁺ ions trends to disappear, while that band attributed to CuO species becomes lager. These changes during hydrothermal aging of Cu-ZSM-5 catalyst confirm that, the copper ions are transformed to copper oxide species, which migrate to the external surface. After propene or SO₂ poisoning, it is observed that the bands centered at 200 - 250 nm decrease in the intensity, while those bands centered at 600 - 800 nm increase. These results demonstrated an effect on Cu²⁺ sites upon propene and SO₂ addition, leading to the reduction in the number of isolated Cu²⁺ ions and the formation of Cu_xO_y clusters.

NH₃-TPD is used to determine the strength and the amount of different acid sites [38]. In order to probe the effect of hydrothermal aging on the acidity of Cu-SAPO-34 and Cu-ZSM-5, NH₃-TPD measurement is performed on the fresh and hydrothermally aged catalysts as shown in Figure 5. Both fresh Cu-SAPO-34 and Cu-ZSM-5 catalysts show three NH₃ desorption peaks. Furthermore, a low-temperature peak at around 170˚C and 180˚C was attributed to the weakly adsorbed NH₃ on the weak Lewis acid sites, a medium-temperature peak at 230˚C and 270˚C due to the NH₃ adsorbed on the strong Lewis acid sites was created by the isolated Cu²⁺ ions at around 280˚C and a high-temperature desorption peak at 390˚C and 360˚C was assigned to the NH₃ desorption from the Brønsted acid sites, respectively [33][38][41].

The acid amounts of the fresh and hydrothermally aged catalysts obtained by deconvolution of the NH₃-TPD curves are shown in Table 2. After the hydrothermal treatment at 650˚C for 2 h, all of the acid sites were reduced as compared with the fresh catalyst due to the destruction of the MFI structure and the loss of isolated Cu²⁺ ions. Following the hydrothermal aging at 750˚C, the desorption peaks of ammonia ligands bound to copper cations and the chemical desorption peaks on strong acid sites decreased for Cu-ZSM-5 catalysts. Taking into account the above results, it was obvious that the hydrothermal treatment of Cu-ZSM-5 catalyst would result in a drop in the ammonia ligands bound to copper cations. A similar result was found by Lei et al. [4], and they demonstrated that it might be mainly caused by the migration of copper species and the result in a decrease in the Lewis and Brønsted acidity. However, after hydrothermal treatment of Cu-SAPO-34, all of the acid sites were similar compared to their fresh counterpart. Therefore, the hydrothermal aging showed a slight effect on the isolated Cu²⁺ ions for the Cu-SAPO-34 catalyst. For instance, the 750˚C aging treatment reduces the strong Lewis acidity in the Cu-ZSM-5 catalyst from 0.23 to 0.11 mmol/g, yet the decline in the Cu-SAPO-34 (750) catalyst is negligible. These results indicated that the isolated Cu²⁺ ions were better preserved in the Cu-SAPO-34 (750) catalyst, which is also in good agreement with the UV and H₂-TPR results. In addition, it is widely accepted that the transformation of Cu²⁺ ions to CuO occurs during the hydrothermal process, and the formation of CuO may result in the collapse of zeolite structure of Cu-ZSM-5. Meanwhile, the hydrothermal treatment of Cu-SAPO-34 catalyst improves its NH₃-SCR performance.

Table 2. The adsorbed NH₃ content over the fresh and hydrothermal aged catalysts.

Figure 5. NH₃-TPD profiles of the fresh and aged: (a) Cu-SAPO-34 and (b) Cu-ZSM-5 catalysts.

The standard SCR reaction activities were performed over the monolith-supported Cu-ZSM-5 and Cu-SAPO-34 samples from 200˚C - 550˚C. According to the above-given results, it has been proved that the catalytic activity is maintained at higher temperature and significantly dropping at a temperature between 200˚C and 300˚C after propene and SO₂ exposing. Similar results were reported by Li Zhang et al. [42], in which the SO₂ significantly decreases the catalytic activity at low temperature (130˚C - 300˚C). Wenkang et al. [43] also reported that a significant decrease of the activity was detected for the poisoned catalyst at 130˚C - 350˚C and more pronounced degradation was found in the mid-temperature range (250˚C - 350˚C). In order to further investigate the effects of sulfur and hydrocarbon poisoning on the catalytic activities, Cu-ZSM-5 and Cu-SAPO-34 catalysts were exposed to propene and SO₂. The effects of SO₂ and the propene on Cu-ZSM-5 and Cu-SAPO-34 at various temperatures 250˚C, 350˚C and 450˚C are shown in Figure 6. The results show that at 450˚C, NO conversion decreases from 98% to 97% and from 90% to 85% for Cu-SAPO-34 and Cu-ZSM-5 catalysts, respectively. After a removal of SO₂, the activity almost recovered to the same value as it was at the beginning, at higher temperatures as shown in Figure 6(a) and Figure 6(b). In addition, at 350˚C, NO conversion decreases from 96% to 88% and from 94% to 73% for Cu-SAPO-34 and Cu-ZSM-5 catalysts, respectively, while at 250˚C with significant decreases from 92% to 70% and 93% to 65%. Meanwhile, the catalytic activity could recover to some extent at 250˚C and 350˚C. Figure 6(c) and Figure 6(d) show the results of propene effect on the catalytic activities of Cu-SAPO-34 and Cu-ZSM-5 catalysts, respectively. The results show that the NO conversions at 250˚C, 350˚C and 450˚C decrease were in existence when the Cu-SAPO-34 and Cu-ZSM-5 were exposed to propene. The NO conversion of Cu-SAPO-34 catalyst decreases from 98% to 97% at 450˚C, 96% to 93% at 350˚C and 93% to 80% at 250˚C, respectively. Thus, the catalytic activity of Cu-SAPO-34 in the presence of propene at 350˚C and 450˚C exhibits a slight change compared with that of 250˚C. However, the

Figure 6. The deactivation at various temperature of: (a) Cu-SAPO-34 with propene; (b) Cu-ZSM-34 with propene; (c) Cu-SAPO-34 with SO₂; ZSM-5 with SO₂ catalysts. Reaction conditions: 1.25 g catalyst, 50 ppm SO₂ or 50 ppm propene, 1000 ppm NO, 1100 ppm NH₃, 5% O₂, 10% water vapor and N₂.

NO conversion of ZSM-5 decreases from 89% to 82% at 450˚C, 98% to 75% at 350˚C and 93% to 50% at 250˚C, respectively. After an injection of propene into Cu-ZSM-5 catalyst, the results show that the catalytic activity drops by propene at 350˚C and 450˚C. However, the catalytic activity of Cu-ZSM-5 significantly decreases at low temperature (250˚C). After the propene was removal, the catalytic activity could recover to some extent at 250˚C, 350˚C and 450˚C.

TG was conducted to further investigate the changes in the zeolite structure after being exposed to the propene or SO₂ at 250˚C as shown in Figure 7. The results show a slight drop and a significant drop in TG curves at a temperature below 200˚C for Cu-SAPO-34 and Cu-ZSM-5 catalysts, respectively, which were attributed to the weight loss. Similar results were found by Qing et al. [9], the weight loss was attributed to the removal of deposited carbonaceous on the catalyst. It was obvious that coke formation in Cu-ZSM-5 was larger than in Cu-SAPO-34 catalyst.

Figure 7. TG curves of poisoned: (a) Cu-SAPO-34 and (b) Cu-ZSM-5 catalysts.

This study was conducted in order to assess the behaviors of Cu-SAPO-34 and ZSM-5 catalysts after hydrothermal aging. Concretely, we tested the catalytic activity of both commercial Cu-SAPO-34 and Cu-ZSM-5 catalysts aged at 650˚C, 700˚C and 750˚C for 12 h, and compared the changes in their microstructure properties, acidy and copper species. The activity test results indicated that Cu-SAPO-34 was much more robust than Cu-ZSM-5 at the temperatures < 450˚C and was resistant to harsh hydrothermal treatment. Cu-SAPO-34 (700) and Cu-SAPO-34 (750) showed a slight decrease in activity at low and high temperatures, while Cu-ZSM-5 (700) and Cu-ZSM-5 (750) showed very low NO conversion at the temperatures < 450˚C. Lei et al. [41] reported that after hydrothermal treatment at 700˚C for 48 h, the Cu-exchanged SAPO-34 showed no deterioration in SCR activity, and no essential changes in the structure were observed. However, the SCR activity of the aged Cu-SAPO-34 was found to be improved. We speculated that the slight decrease in activity of Cu-SAPO-34 could be explained by the differences in hydrothermal aging treatment operating conditions. In addition, Ja et al. [1] reported that after hydrothermal treatment at 800˚C for 16 h, Cu-ZSM-5 was found losing its NH₃-SCR activity. Following the hydrothermal aging at 750˚C for 12 h, significant differences in the H₂-TPR and UV patterns were observed for Cu-ZSM-5catalysts. Considering these observed changes, it seems like some Cu²⁺ ions in aged Cu-ZSM-5 are still located in ion exchange position within the zeolite, which are responsible for the high NO_x reduction activity at higher temperatures. Recently, Ja et al. [7] reported that the new Cu/AlO_x structures that form upon hydrothermal aging and which exhibit strong interactions between Al ions and paramagnetic Cu are, at least partially, responsible for the maintenance of higher temperature NH₃-SCR activity. The NH₃-TPD results described above show that the total acidity in Cu-ZSM-5 catalyst significant decreases with the temperature increase of hydrothermal treatment, while that of Cu-SAPO-34 catalyst show a slight change. Following the hydrothermal aging of Cu-ZSM-5 at 750˚C for 12 h, dramatic decreases in the Brønsted acidity and strong Lewis acidity created by Cu²⁺ ions are also observed, while a small change is observed in Cu-SAPO-34.

Additionally, the Cu-ZSM-5 and the Cu-SAPO-34 catalysts have shown distinct responses to propene and sulfur dioxide poisoning. The NO conversions of Cu-ZSM-5 and Cu-SAPO-34 catalysts are affected after exposed to propene and SO₂. The above results show that after an injection of propene into Cu-ZSM-5 and Cu-SAPO-34 catalysts, the catalytic activity of the cu-ZSM-5 catalyst significantly drops at low temperature compared to the Cu-SAPO-34 catalysts. The catalytic activity of Cu-ZSM-5 significantly decreases at temperature below 250˚C compared to Cu-SAPO-34 catalyst. However, at high temperature the catalytic activities of both Cu-ZSM-5 and Cu-SAPO-34 catalysts show a slight changes after SO₂ addition. After the propene and the SO₂ removal, the catalytic activities of Cu-ZSM-5 and Cu-SAPO-34 catalysts could recover to some extent at 250˚C, 350˚C and 450˚C, and 250˚C, respectively. However, the catalytic activity of Cu-SAPO-34 catalyst almost recovered to the same value as it was at the beginning, at higher temperatures. These XRD results confirm that the crystalline structures of the Cu-ZSM-5 and the Cu-SAPO-34 catalysts are well-preserved after propene and SO₂ poisoning. Nevertheless, the TG results show a weight loss which could be attributed to the removal of deposited carbonaceous on the catalyst, however, the coke formation in Cu-ZSM-5 was larger than in Cu-SAPO-34 catalyst. Furthermore, the UV-vis-DRS results demonstrated an effect on Cu²⁺ sites upon propene and SO₂ addition, leading to the reduction in the number of isolated Cu²⁺ ions and the formation of Cu_xO_y clusters.

Therefore, the Cu-ZSM-5 catalyst, without sufficient amounts of Brønsted acid sites, exhibits a poor NH₃-SCR performance because the Brønsted acid sites in Cu-CHA catalysts which act as reservoir providing some reactive NH₃ molecules to Cu sites play a beneficial role in NH₃-SCR reaction. We can conclude that the actively isolated Cu²⁺ ions content, the distribution of Cu species and the zeolite acidity, which are related to the catalytic activity and hydrothermal durability of both Cu-SAPO-34 as well as ZSM-5 catalysts are all found to be influenced by the hydrothermal treatment.