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• The NH 3 -SCR mechanisms (standard SCR and fast SCR) are elaborately reviewed. • The sulfur/alkali metal/phosphorus poisoning mechanisms are comprehensive reviewed. • The hydrothermal deactivation mechanisms of Cu/CHA are comprehensive reviewed. • The poisoning-resistant and protective strategies of Cu/CHA are proposed. • Ultrafast and continuous flow synthesis of Cu/CHA zeolites are reviewed. Copper-exchanged SSZ-13 and SAPO-34 (Cu/CHA) zeolites catalysts have been commercially utilized for selective catalytic reduction (SCR) of NO x by NH 3. Despite the significant advances in both fundamental research and industrial application have been made, the limited catalytic activity at low exhaust temperatures, the challenges of sulfur/alkali (alkaline) metal/phosphorus poisoning and hydrothermal deactivation have spurred the development of new generation NH 3 -SCR catalysts. This paper reviews the recent progress for Cu/CHA in regard to zeolite synthesis, the latest SCR reaction mechanisms, the poisoning and hydrothermal deactivation mechanisms, more importantly, corresponding poisoning-resistant and protective strategies are also proposed, some issues that still need to be addressed are also highlighted. We hope to construct a linkage between next generation of catalysts and real-world applicable demands to meet the escalating emission regulations in the near future. [ABSTRACT FROM AUTHOR]

The impact of SO 2 poisoning on Cu-SAPO-18 physicochemical properties and its NH 3 -SCR reaction mechanism was investigated. After exposing Cu-SAPO-18 to SO 2 +O 2 for 40 min, the sulfite and sulfate species were detected on Cu-SAPO-18 surface, which caused a small decrease in BET surface area and micropore volume, the increase in NH 3 storage and the deactivation of some isolated Cu2+ sites. These could be interpreted by the slight micropore blocking, interaction between sulfates and NH 3 forming new NH 4 + species and the formation of Cu-sulfate, respectively. Additionally, it was observed that isolated Cu2+ ions in pear-shaped cavity of AEI structure, which are beneficial to good low-temperature deNO x activity, were more severely poisoned by SO 2 than Cu2+ in the double 6-membered ring (D6R). Accordingly, the NH 3 -SCR activity below 350 °C was restrained by SO 2. Moreover, the in-situ DRIFTS results indicate that Cu-SAPO-18 catalyzed NH 3 -SCR reaction predominantly proceeds through Langmuir−Hinshelwood (L−H) mechanism at low temperature. SO 2 poisoning inhibited the adsorption of NO + O 2 and NH 3 on Cu2+ sites, thereby decreasing the formation of Cu−NO x and Cu−NH 3 intermediates, evidently, the L−H mechanism was suppressed by SO 2. Interestingly, the deactivated Cu-SAPO-18 could be regenerated under SCR gas at 550 °C and resumed at least most NH 3 -SCR activity. Image 1 • After exposing Cu-SAPO-18 to SO 2 +O 2 , the sulfites and sulfates were detected. • The SO 2 poisoning caused the increase of NH 3 storage and the deactivation of Cu2+. • The low-temperature NH 3 -SCR activity over sulfated Cu-SAPO-18 was inhibited. • The NH 3 -SCR over fresh and sulfated Cu-SAPO-18 mainly follows the L−H mechanism. • The deactivated Cu-SAPO-18 could be regenerated under SCR gas at 550 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe-13 with a Si/Al ratio of 6.9 exhibited optimal NH 3 -SCR activity. • Fe-13 with a Si/Al ratio of 17.5 displayed excellent hydrothermal stability. • Above 200 °C, Fe-13(6.9) had a higher reaction rate than the other two samples. • Above 200 °C, Fe-13(17.5)-800 demonstrated the fastest reaction rate. • The Si/Al ratio has an impact on the NH 3 -SCR reaction mechanism of Fe-13. Fe-SSZ-13 (Fe-13) is an effectual catalyst for NO x abatement with extensive application potential in the selective reduction of NO x by NH 3 (NH 3 -SCR). It is necessary to further research and expand the fundamental knowledge of Fe-13 catalysts. In this research, Fe-13 zeolites possessing Si/Al ratios of 6.9, 12.0, and 17.5 were prepared. Fe-13(6.9) showed excellent catalytic activity because it had a larger number of isolated Fe3+ ions and well-dispersed Fe species. Upon hydrothermal aging, the Fe-13(6.9) catalyst diminished significantly in catalytic activity. Among the three, Fe-13(17.5) exhibited the best hydrothermal stability. It is because Fe-13, possessing a higher Si/Al ratio, had fewer Si OH Al bonds but more Si(0Al) structures. Thus, Fe-13(17.5) suffered less structural collapse during hydrothermal aging, leading to a smaller decrease in active isolated Fe3+ ions and acid sites, as well as a lower aggregation of Fe species. Moreover, above 200 °C, Fe-13(6.9) exhibited a higher reaction rate compared to Fe-13(12.0) and Fe-13(17.5), and the SCR reaction route of Fe-13(6.9) was distinctly different from that of the other two catalysts. [ABSTRACT FROM AUTHOR]

Abstract To investigate the effect of alkali (K and Na) and alkaline earth (Ca and Mg) metal poisoning and hydrothermal aging on Cu-SAPO-34 catalyst; a series of samples containing various metals (Mg, Ca, Na and K) loadings were prepared by an impregnation method, and the changes of catalytic properties were evaluated before and after hydrothermal aging by various characterization techniques. The results indicated that the surface area, the isolated Cu2+ ions and the acidity of Cu-SAPO-34 catalyst were well-maintained after low alkali and alkaline earth metal contents (0.50 mmol/g cat) incorporation. The results showed that at high content (1.50 mmol/g cat), the great decrease of SCR performance was explained by the destruction of the zeolite structure, the decrease in acidity and the transformation of Cu2+ ions to CuO detected by XRD, NH 3 -TPD and H 2 -TPR techniques, respectively. The NH 3 -SCR performance of the fresh and aged samples was similar after high-temperature (650 °C) hydrothermal aging treatment for 12 h. A slight increase of activity was observed when the samples were hydrothermal aging at 700 and 750 °C for 12 h due to the change of Cu species distribution. The NH 3 -TPD results show that the acidity was not affected by hydrothermal aging treatment. During hydrothermal aging process, CuO species first precipitated on the external surface and then dispersed as isolated Cu2+ ions over Cu-SAPO-34. The Cu-SAPO-34 could withstand the co-effect of chemical poisoning (K, Na and Ca) and hydrothermal aging at 750 °C at the same times. In summary, our results indicated that the pore blocking, disruption of the zeolite framework, reduction of acid sites and active sites were the reasons for the chemical the deactivation of Cu-SAPO-34 catalyst after the introduction of metals. Graphical abstract Unlabelled Image Highlights • Poisoning of Cu-SAPO-34 with different alkali metals were reported. • The Cu-SAPO-34 catalyst shows excellent alkali metal resistance below 0.50 mmol/g cat. • The impregnation of alkali metal induces the transformation of Cu+/Cu2+ into CuO clusters. • The hydrothermal aging has no significant effect on the structure, the isolated Cu2+ ions and the acidity of Cu-SAPO-34. [ABSTRACT FROM AUTHOR]

[Display omitted] • The low/high calcination temperature results in more isolated Fe3+/oligomeric Fe x O y clusters in Fe-SSZ-13. • The low-temperature activity of Fe-SSZ-13 is prompted by active isolated Fe3+. • The high-temperature activity of Fe-SSZ-13 is facilitated by active oligomeric Fe x O y clusters. To shed light on the relationships among catalyst calcination temperature, physicochemical properties and catalytic properties of Fe-SSZ-13, a series of catalysts prepared by aqueous post-synthetic exchange and calcined at different temperatures are characterized and evaluated in NH 3 -SCR reaction. The characterizations and kinetic analysis indicate that the calcination temperature has a great effect on the acidity, Fe species distribution and catalytic properties of the Fe-SSZ-13 catalyst. The rise of calcination temperature decreases the low-temperature activity (< 425℃) and conversely improves the high-temperature activity of Fe-SSZ-13. It could be attributed to the following reasons: under low calcination temperatures, more isolated Fe3+ ions are maintained which can behave as Lewis acid sites and active sites in low reaction temperatures. Simultaneously, stronger surface acidity helps to alleviate NH 3 inhibition effect at low temperatures, resulting in good low-temperature activity. On the other hand, the high calcination temperature brings about slight aggregation of isolated Fe3+ to form dimeric/oligomeric Fe clusters, leading more superior high-temperature activity. On the whole, the major active Fe species as well as the contribution of acid sites for low/high temperature activity are different. It could be concluded that the isolated Fe3+ ions and oligomeric Fe clusters are the active sites for low/high SCR reaction temperature, respectively. And zeolite acidity plays a more important role in low-temperature SCR reaction catalyzed by Fe-SSZ-13. [ABSTRACT FROM AUTHOR]

The Cu-SAPO-18 catalyst is efficient for the selective reduction of NO x with NH 3 reaction. Nonetheless, enhancement of the hydrothermal stability of the Cu-SAPO-18 catalyst is needed to satisfy the stringent emission regulations. Sm with partially occupied 4 f orbitals and unoccupied 5d orbitals exhibits excellent physical and chemical properties and is often used as a catalyst promoter in catalytic reactions. In this study, we introduced Sm to Cu-SAPO-18 by a hydrothermal method, and when the added amount of Sm was 0.78 wt%, there was a significant improvement in hydrothermal stability. After Sm introduction, there was a decrease in Si(4Si, 0Al) amount and in unstable Cu(OH)+-Z concentration, thus inhibiting the formation of CuO species during hydrothermal aging. Simultaneously, the Sm replaced the hydrogens of a certain amount of Si–OH–Al bonds, stabilizing the Si–O–Al bonds and alleviating the Cu-SAPO-18 dealumination. Besides, after hydrothermal aging, there was an increase of isolated Cu2+ content in Sm 0.78 /Cu-SAPO-18, consequently promoting the adsorption of NH 3 and NO x species and enhancing the Langmuir–Hinshelwood reaction rate. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • The moisture stability was boosted by REM incorporation, AM exchange and pre-adsorbed NH 3. • The protective mechanism differences were systematically and explicitly revealed. • Better water-resistant Cu-SAPO-34 was obtained by the optimized combination strategy. The low-temperature hydrothermal instability of Cu-SAPO-34 is an unresolved issue, thus jeopardizing its commercial application in NH 3 -SCR due to severe structural damage and loss of activity. Herein, we reported that efficient strategies i.e., rare earth metal (REM, denotes Ce, La, and Sm) incorporation, alkali metal (AM, denotes Na, K, and Cs) exchange, and NH 3 pre-adsorption were utilized to boost the moisture stability of Cu-SAPO-34 through various protective mechanisms. Our extensive characterizations revealed that AM ions readily exchanged with the susceptive Si–O(H)–Al bonds and ZCu(OH) sites, and at the same time left a majority of undamaged Z 2 Cu sites providing optimal protective effect in the entire temperature window studied here. On the other hand, REM co-cations only guarded Si–O(H)–Al bonds and Z 2 Cu sites but not the ZCu(OH) sites, this resulted in a poor protective effect in the low-temperature region. By contrast, chemisorbed NH 3 only partially prevented Si–O(H)–Al and ZCu(OH) sites from hydrolysis, explaining why this protective effect was superior to REM in the low-temperature region but inferior to REM in the high-temperature region. These different protective mechanisms were explicitly revealed and were used in combination for the first time showing that the hydrothermal stability of Cu-SAPO-34 could be tuned precisely, the mechanistic guidance given here will surely be useful in developing more durable SCR catalysts. [ABSTRACT FROM AUTHOR]

In the actual diesel vehicle after-treatment systems, Cu-SAPO-34 selective catalytic reduction (SCR) of NO x catalyst simultaneously experiences the failure of sodium (Na) poisoning, low-temperature hydrothermal (LTH) deactivation and SO 2 poisoning. This work investigated that the Na-loaded Cu-SAPO-34 catalysts were severally subjected to LTH treatment and sulfur poisoning for the first time. Our characterizations revealed that Na played entirely different roles in above two scenarios. In the LTH process, Na readily exchanges the protons of Si-OH-Al moieties on the zeolite framework, which inhibits the hydrolysis of the framework and further restrains the formation of SCR-inactive species. However, for SO 2 poisoning, the presence of Na accelerates the deactivation of Cu-SAPO-34 because more stable sulfate species are formed on the catalyst, which severely block the pores and reduce the active sites. It is expected that these results could assist the design of SCR catalysts to meet ultra-low NO x emission. [Display omitted] • The co-effects of Na poisoning and moisture on Cu-SAPO-34 were studied. • The influences of Na poisoning and S poisoning on Cu-SAPO-34 were investigated. • Hydrothermal stability of Cu-SAPO-34 enhanced by introducing Na+ species. • The activity of Na impregnated Cu-SAPO-34 reduced dramatically after S poisoned. [ABSTRACT FROM AUTHOR]

[Display omitted] • The solid-state ion-exchange technique achieved promoted Cu ion-exchange level. • The NH 4 - or H-forms zeolites led to various Cu ion-exchange levels. • The formation of Cu(NH 3) x 2+ was a critical factor for high Cu2+ dispersion. • The SCR reaction mechanisms were investigated by in situ DRIFTS. In this paper, Cu-exchanged SSZ-13, SAPO-18, and SAPO-34 were synthesized via a novel green and sustainable solid-state ion-exchange (SSIE) route with excellent NH 3 -SCR activities, which could simplify the preparation process and reduce the wastewater production in comparison to conventional aqueous ion-exchange route. The results also proved that Cu-exchanged zeolites synthesized by SSIE technology presented higher Cu ion-exchange (IE) levels and better SCR activities than those prepared via the traditional incipient impregnation route. In the case of SSIE, the usage of NH 4 -form substrates could achieve more Cu2+ ions and fewer CuO x than those H-form substrates since the formation of Cu(NH 3) x 2+ intermediate could promote the Cu2+ mobility to facilitate Cu2+ occupying IE sites, which was favorable for the SCR reaction. The combined DFT data confirmed the SSIE reaction energy barriers between Cu(NO 3) 2 and the NH 4 -form substrates were almost identical, whereas the higher reaction energy was required for H-form substrates. In-situ DRIFTS results manifested the labile species like NH 4 +, –NH 2 , NO 2 –, NO 3 –, NO 2 , etc were the reaction intermediates over the catalysts prepared from NH 4 -form substrates. The Langmuir-Hinshelwood ("L-H") and Eley-Rideal ("E-R") mechanisms contributed collectively over both Cu-SSZ-13 and Cu-SAPO-18 in the SCR reaction, however, the "L-H" mechanism dominated over Cu-SAPO-34. [ABSTRACT FROM AUTHOR]

The influence of post-synthesis treatment with MnSO 4 , (NH 4) 2 SO 4 and (NH 4) 2 S 2 O 8 blended solution on NH 3 -SCR activity and hydrothermal stability of Cu-SAPO-18 (Cu-18) was investigated. Via adjusting the mixed solution concentration, the optimal Mn/Cu-18-Ⅲ with higher deNO x activity and hydrothermal stability was acquired. Based on the optimal Mn/Cu-18-Ⅲ, diverse metal cations such as La3+, Ce3+, Sm3+ or Zr4+ were simultaneously doped into Cu-18 during Mn2+ and NH 4 + blended solution treatment to further promote its catalytic performance before and after hydrothermal treatment. The doping with Sm could improve the NH 3 activity and hydrothermal stability of Mn/Cu-18-Ⅲ to the greatest extent. The elevation of acid sites content and the disappearance of CuO species in Mn/Cu-18-Ⅲ caused the enhancement of its NH 3 -SCR activity. More Cu2+ and acid sites in SmMn/Cu-18-Ⅲ were responsible for its higher NH 3 -SCR activity than Mn/Cu-18-Ⅲ. The decline of vulnerable Brønsted acid sites by substituting Mn2+ or Sm3+ for H+ could restrain the hydrolysis of Cu-18 and enhance the hydrothermal stability of Cu-18 during hydrothermal treatment. The mechanism analysis demonstrates that the fresh and aged Cu-18, Mn/Cu-18-Ⅲ and SmMn/Cu-18-Ⅲ follow both Eley–Rideal (E–R) and Langmuir–Hinshelwood (L–H) mechanism in NH 3 -SCR reaction, with the L–H mechanism being more dominated. [Display omitted] • Cu-18 treated with Mn2+ and NH 4 + solution enhanced its deNO x activity and stability. • Mn/Cu-18-Ⅲ showed optimal NO conversion and hydrothermal stability after treatment. • Doping with Sm improved the NH 3 -SCR activity and hydrothermal stability of Mn/Cu-18-Ⅲ. • DRIFT results confirmed the L–H mechanism is more dominated than the E–R mechanism. • The L–H reaction rate of SmMn/Cu-18-Ⅲ was faster than that of Cu-18. [ABSTRACT FROM AUTHOR]

Cu-based SAPO-18, SAPO-18/34 intergrowth and SAPO-34 zeolites were applied for the selective catalytic reduction of NO x by ammonia (NH 3 -SCR) catalysts. Comprehensive characterization results revealed that the SAPO-18/34 with higher amount and strength of acid sites could facilitate the generation of more isolated copper ions (Cu2+ and Cu+) and suppress the formation of CuO x , which might account for the fact that intergrowth crystal structure of Cu-SAPO-18/34 exhibited higher fresh NH 3 -SCR activity, more robust hydrothermal durability and better SO 2 -resistance ability than that Cu-SAPO-18 and Cu-SAPO-34. In situ DRIFTS results provided the formation of reaction intermediates, such as -NH 2 , NH 4 +, NO 3 -, NO 2 -, etc. Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction mechanisms were both involved in Cu-based SAPO-18 and SAPO-18/34 intergrowth zeolites, but the L-H mechanism dominated the NH 3 -SCR reaction, in addition, Cu-SAPO-34 only followed "L-H" mechanism. [Display omitted] • Controlled synthesis of intergrowth AEI/CHA, pure AEI and CHA zeolites. • Template composition affected the size, acidity, SCR activity of Cu-based zeolites. • Cu-SAPO-18/34 showed enhanced SCR activity, durability and SO 2 -resistance ability. • The SCR reaction mechanisms were investigated by in situ DRIFTS. [ABSTRACT FROM AUTHOR]

• Zr/Fe-SSZ-13 is prepared by in situ introduction of Fe and Zr via one-pot method. • Zr favors Fe dispersion and inhibits Fe agglomeration during hydrothermal aging. • New acid sites and more isolated Fe ions are created after Zr addition. • The intermidates of SCR reaction over Fe-SSZ-13 are increased by Zr addition. • The hydrothermal stability of Fe-SSZ-13 is improved as high as 30 % after Zr in situ doping. Fe-SSZ-13 has been proven to be a highly efficient catalyst for the ammonia selective catalytic reduction (NH 3 -SCR) of NO x , especially at medium and high reaction temperatures. Nevertheless, the hydrothermal stability of Fe-SSZ-13 catalyst is inferior, limiting its application in vehicle exhaust purification. Impressively, it is found that the hydrothermal stability of Fe-SSZ-13 can be remarkably enhanced (as high as 30 % improvement at 300 °C) by in situ introduction of Zr and Fe into initial gels via one-pot synthesis in this work. A small amount of Zr addition (0.31 wt.%) is beneficial for improving the relative crystallinity of Fe-SSZ-13. The characterization results (obtained by 27Al NMR, NH 3 -TPD, UV–vis, XPS and in situ DRIFTS) revealed that Zr doping by a one-pot method would not only enhance the dispersion of active isolated Fe ions but also inhibit the migration and agglomeration of isolated Fe ions during hydrothermal aging, improving the stability of the catalyst. Additionally, the introduction of Zr generates new hydroxy groups and Lewis acid sites, facilitating the formation of reaction intermediates, such as -NH 2 , -HNO 2 , -NO 2 , etc., and promoting the SCR reaction especially over the aged catalyst. [ABSTRACT FROM AUTHOR]

• Fe-zeolite catalyst with superiority in high-temperature NH 3 -SCR performance is an important class of material for NO reduction in diesel vehicle. • The highlights and challenges about the application of Fe-zeolite SCR catalysts and the origins of some peculiar properties that differ from Cu-zeolite are reviewed. • Some feasible strategies in tailoring/designing Fe-zeolite catalyst to improve activity, stability and toxic tolerance are discussed. • The construction methods and potential advantages of (Fe + Cu)-zeolites for SCR application is addressed. • The reaction mechanisms of NH 3 -SCR reaction and Fe species nature over Fe-zeolites are concluded. Selective catalytic reduction of NO x with NH 3 over zeolite catalysts is recognized as the most practical technology used in mobile exhaust gases to transform NO x into N 2. Despite a comprehensive structural-activity insight into Cu-zeolite SCR catalyst, Fe-zeolite, which is also popular in the automotive catalysis community, is necessary to be thoroughly investigated on micro-mechanism. This review summarizes the highlights and challenges in the application of Fe-zeolite, and simultaneously analyzes the reasons of some peculiar properties, such as the NH 3 inhibition effect at low temperatures, the low N 2 O production and the sensitivity to NO 2 /NO x ratio in the feed. In addition, some feasible strategies are classified and summarized subsequently to provide suggestions for the improvement of low-temperature activity, the enhancement of resistance to hydrothermal aging and sulfur poisoning, the construction of controlled structure and the combination of Fe- and Cu- zeolites. Finally, the reaction mechanisms of NH 3 -SCR over Fe-zeolites are concluded. After establishing a comprehensive overview of Fe-zeolite SCR catalysts, some directions that remain to be thoroughly explored on Fe-zeolite are put forward to assist the future catalyst design and investigation of NH 3 -SCR. [ABSTRACT FROM AUTHOR]

• The NH 3 -SCR activity of metal doped OMS-2 is higher than that of undoped K-OMS-2. • The NH 3 -SCR activity over M-OMS-2 increased with the Brønsted acid sites. • Na-OMS-2 showed optimal NO conversion due to the presence of more Brønsted acid sites. • The Brønsted acid sites and lattice oxygen play important role in NH 3 -SCR reaction. Different metal cations (Li+, Na+, Mg2+, Ca2+, Al3+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, and Ag+) were doped into the OMS-2 material via "one-pot" refluxing approach to investigate the influence of metal cations doping on the structure of OMS-2 and on the activity of selective catalytic reduction (SCR) of NO x with NH 3. The catalytic performance over the doped M-OMS-2 catalysts increased with the Brønsted acid sites. Na-OMS-2 displayed the best deNO x performance, which is primarily due to the higher surface area, reducibility and Brønsted acid sites. Kinetic results demonstrate that the activation energy (E a) of Na-OMS-2 catalyzed NH 3 -SCR reaction is evidently lower than that of the undoped K-OMS-2 catalyst owing to a larger number of NH 3 adsorbed on Brønsted acid sites and more facile activation of NH 3. Meanwhile, the Na+ loading of Na-OMS-2 is further optimized for the NH 3 -SCR reaction, the results indicate that the Na-OMS-2 with Na/(Na + Mn) ratio of 0.03 exhibits the best activity because of the higher reducibility and more acid sites. A feasible NH 3 -SCR mechanism is proposed to highlight the importance of the Brønsted acid sites and the lattice oxygen. [ABSTRACT FROM AUTHOR]