Excellent NH3-SCR Research Articles

Novel framework-confined Cu@ 13X catalyst with excellent resistance to toxic SO2 as an eco-friendly substitute for hazardous V-based NH3-SCR catalyst

V2O5-WO3/TiO2 (VWTi) catalyst has long been utilized in fixed source flue gases to purify harmful NO gas. However, VWTi is classified as a hazardous material because of its harm to human health and environment. To address this issue, a low-cost green Cu-based zeolite catalyst (Cu@13X) with a wide temperature range was synthesized using an in-situ hydrothermal method. This method is intended to control the adsorption capacity of toxic SO2 by regulating the location of Cu species. UV-Vis DRS and EXAFS analyses revealed that a significant amount of Cu was encapsulated within the 12-membered ring pores of the 13X molecular sieve. SO2-TPD and DFT calculations further indicated that Cu@ 13X exhibits reduced SO2 chemical adsorption capacity. Consequently, this green catalyst demonstrated superior catalytic performance, maintaining superior NOx conversion for over 70 h in the mixture gas containing 250 ppm SO2; while the Cu/13X catalyst lacked NH3-SCR catalytic activity under the same conditions. Moreover, Cu@ 13X catalyst also has excellent NH3-SCR catalytic performance in low temperature flue gas below 250 °C, which is significantly better than the hazardous VWTi catalyst. Characterization techniques such as NH3-TPD, H2-TPR, and XPS confirmed that the Cu@ 13X catalyst possesses excellent acidity, robust redox capabilities, and abundant surface defects. In-situ DRIFT spectra further illustrated that the NH3-SCR reaction on the catalyst surface adheres to the Eley-Rideal mechanism both before and after the introduction of toxic SO2. This study offers a fresh perspective on the development of framework-confined NH3-SCR catalysts and elucidates the mechanism behind the sulfur resistance of these catalysts. And the green Cu@ 13X catalyst is anticipated to serve as an effective alternative to the hazardous VWTi catalyst.

Promoting metal oxides–zeolite electron interaction on MnCeOx/HY catalyst for boosting nitrogen oxides reduction

The MnOx-CeO2 metal oxides are considered as promising alternative catalysts for selective catalytic reduction with NH3 (NH3-SCR) to remove NOx due to its excellent low temperature performance. However, their strong oxidative ability can lead to NH3 over-oxidation, narrowing the active temperature range and reducing N2 selectivity. Moreover, their weak surface acidity hampers medium to high-temperature activity and alkali metal resistance. Hence, in this study, MnCeOx metal oxides were coupled with HY zeolite to create MnCeOx/HY, demonstrating excellent NH3-SCR performance. The high dispersion of metal oxides on the zeolite surface and their close integration promoted strong electron interaction, effectively reducing oxygen vacancies and surface adsorbed oxygen concentrations. Consequently, the oxidative ability of active metal oxides was appropriately weakened, suppressing undesirable side reactions. MnCeOx/HY also notably suppressed NO adsorption and nitrate formation, promoting the catalytic reaction solely through the E-R mechanism and enhancing N2 selectivity. The abundant strong acid sites on the zeolite surface facilitates NH3 adsorption at moderate to high temperatures, notably expanding the active temperature window. Furthermore, the acid sites of HY zeolite serve as sacrificial sites, preferentially reacting with alkali metals, thus exhibiting excellent resistance to alkali metal poisoning on MnCeOx/HY. Combining with the DFT results, the structure-activity relationships in this study also reveal the importance of the effective synergy between acid sites and redox sites for optimal catalytic performance, offering valuable insights into the development of highly active and alkali-resistant denitrification catalysts.

Unveiling the mechanistic insights into the potassium resistance of 3.5 W-V-1 %K NH3-SCR catalysts: The dual functionality of V-O-W structure as acid and redox sites

The poisoning effect of alkali metals over Vanadium-based catalysts still faced a significant challenge in the selective catalytic reduction of NOx with NH3 (NH3-SCR). In order to improve the alkali (mainly potassium resistance) resistance of the Vanadium-based catalysts, WO3 was introduced into the vanadium oxide catalysts. The obtained 3.5WO3-V2O5(3.5 W-V) catalyst exhibited the excellent low-temperature NH3-SCR performance owing to the formation of V-O-W structures. Noteworthily, when 1 wt% K was introduced into the 3.5 W-V catalyst, vanadium oxide (V-O, V=O) species preferentially bonded to alkali metal K as an acidic sacrifice site, prompting more active V-O-W species to remain. Thus, the 3.5 W-V catalyst also possessed excellent alkali resistance. The NH3-TPD results showed that the 3.5 W-V-1 %K catalyst had a considerable acid site to ensure the adsorption capacity of NH3, while the XPS results showed that the 3.5 W-V-1 %K catalyst had excellent redox performance to ensure the activation capacity of the reactant NH3. Moreover, in-situ DRIFTS transient experiments revealed that the 3.5 W-V and 3.5 W-V-1 %K catalysts were more capable of adsorbing and activating NH3 species to rapidly react with gaseous NO, obeying the Eley-Rideal (E-R) pathway in NH3-SCR reaction. This work provides new insights into poisoning resistant of vanadium-based catalysts.

Facile one-pot synthesis of Cu-SSZ-39 catalysts with excellent catalytic performance in NH3-SCR reaction

Nitrogen oxide (NOx) emission from diesel vehicles has caused severe pollution to the atmospheric environment. Selective catalytic reduction of NOx with NH3 (NH3-SCR) technology has been widely used to remove NOx from diesel vehicle exhaust. Cu-SSZ-39 zeolites have exhibited excellent NH3-SCR catalytic activity and hydrothermal stability, giving them promise for practical application. A facile one-pot synthesis method to prepare Cu-SSZ-39 zeolites was developed, with complete crystallization achieved in 24 h. Moreover, the Cu content could be controlled by adjusting the amount of Cu-TEPA added without the need for complex post-treatment processes. Cu-SSZ-39 catalysts synthesized through the one-pot synthesis method exhibited excellent NH3-SCR activity and hydrothermal stability. The one-pot-synthesized Cu-SSZ-39 catalysts contained in situ generated CuO species, which can act as sacrificial sites to protect Cu2+ under sulfidation conditions. Considering their efficient synthesis process, excellent NH3-SCR activity, hydrothermal stability and sulfur resistance, one-pot-synthesized Cu-SSZ-39 catalysts hold great promise for future applications.

Cu-SAPO-56, a potential catalyst for the selective catalytic reduction of NOx by NH3

A novel Cu-exchanged catalyst, Cu-SAPO-56 prepared by direct ion exchange method, was studied for NH3-SCR reaction. Cu-SAPO-56 exhibits excellent NH3-SCR activity and hydrothermal stability. The NOx conversion of Cu-SAPO-56 maintains more than 90 % at a wide temperature range between 250 °C and 400 °C even after a rigorous steam treatment (800 °C for 16 h or 50 °C for 24 h). The isolated Cu2+ ions located in double 6-ring of AFX structure are determined as the catalytic active sites. Both of Cu content and Si content in Cu-SAPO-56 play critical roles in NH3-SCR activity and hydrothermal stability. The excellent catalytic performance and strong hydrothermal stability exhibits the huge application potential of Cu-SAPO-56 for NH3-SCR reaction.

Rapid synthesis of Cu-SSZ-39 and study of phosphorus poisoning in NH3-SCR

Cu-SSZ-39 has attracted great interest due to its excellent NH3-SCR activity and hydrothermal stability. However, impediments such as high synthesis costs, lengthy preparation duration, and the issue of phosphorus-induced deactivation have hampered its widespread commercialization. Herein, H2O2 was first introduced into the synthesis system dedicated to the rapid synthesis of SSZ-39, and the crystallization time was shortened to 3 d. Various characterization techniques were used to analyze deeply the structure–activity relationship of the resulting Cu-SSZ-39 catalysts before and after phosphorus poisoning. Our findings revealed a pronounced decline in acidic sites and active copper species as Cu-SSZ-39 underwent deeper phosphorus poisoning, leading to the formation of Cu-P species and a consequential reduction in the low-temperature activity of the catalyst. Surprisingly, hydrothermal treatment of the poisoned catalyst allowed the decomposition of Cu-P species to generate active Cu2+ species, alleviating the poisoning effect. It was also shown that phosphorus poisoning did not affect the low-temperature reaction mechanism of Cu-SSZ-39, which still followed the “L-H” reaction pathway. Finally, it was unexpectedly found that the anti-phosphorus poisoning performance of 0.9C-Cu-SSZ-39 significantly improved after mesopores were introduced.

Low temperature reaction mechanism of NH3-SCR reaction on MoOx modified CePO4 catalyst: Combination study on experiments and DFT calculations

Developing CePO4-based NH3-SCR catalysts with high NOx conversion, N2 selectivity, and SO2 tolerance at low temperatures is challenging. Analyzing the atomic-level surface structure of these catalysts is particularly important. Hence, we conducted a combined computational and experimental study to design the surface structure of MoxCe1-xPO4 catalysts, corroborate the reaction performance and structural sensitivity, and investigate the relationship between active sites and structural performance at the atomic level. DFT calculations were used to systematically investigate the NH3 and NO adsorption capacity of the Mo-doped CePO4 solid solution structure model. The rationality of using the Mo-doped CePO4 solid solution structure as a catalyst was further supported by DFT calculation data and experimental results from BET, XRD, Raman, SEM, TEM, HR-TEM, NH3-TPD, H2-TPR, XPS and in situ DRIFTS. The source of the excellent intrinsic NH3-SCR activity of the Mo-doped CePO4 solid solution structure is disclosed. The role of the Mo dopant in the Mo-doped CePO4 solid solution as an SO2 trapping site to protect the catalytic active site from sulfation is discussed from a molecular perspective.

A novel bifunctional Pt/Ce/WZrOx catalyst for efficient selective oxidation of high-concentration NH3

A novel bifunctional catalyst with excellent NH3-SCO and NH3-SCR functions is effective to achieve excellent low-temperature activity and high N2 selectivity simultaneously, especially for removing high-concentration NH3 slipped from NH3-fuel engine exhaust gas. Herein, a small amount of noble metal Pt (0.1 wt%) was introduced in Ce/WZrOx to synthesize a novel bifunctional NH3-SCO catalyst. The experimental results showed that deposition order of Pt and Ce active species supported on WZrOx had a significant influence on NH3-SCO performance. Pt/Ce/WZrOx catalyst prepared through impregnation-precipitation method exhibited a much superior comprehensive NH3 removal performance compared to Pt/Al2O3 catalyst (Pt 0.1 wt%). It achieved an NH3 conversion of 96.4 % and a N2 selectivity of 94.2 % at 310 °C. A series of characterization tests were adopted to compare the physicochemical properties of Pt/Ce/WZrOx and Ce/WZrOx catalysts to reveal the possible functions of Pt and Ce active species in NH3-SCO reactions. The results of H2-TPR implied that there was a strong interaction between the Pt and Ce active species, which greatly enhanced the low-temperature redox property of Pt/Ce/WZrOx catalyst. Besides, electron transfer between Pt0/Ptn+ and Ce3+/Ce4+ cycles facilitated the generation of oxygen vacancies on the Pt/Ce/WZrOx catalyst surface, which played a key role in the reaction for removing high-concentration NH3. In-situ DRIFTS results suggested that NH3-SCO reactions over Pt/Ce/WZrOx catalyst might follow both imide (–NH) and hydrazine (-N2H4) mechanisms.

Sulfur and Water Resistance of Carbon-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NOx: A Review

Low-temperature NH3-SCR is an efficient technology for NOx removal from flue gas. The carbon-based catalyst designed by using porous carbon material with great specific surface area and interconnected pores as the support to load the active components shows excellent NH3-SCR performance and has a broad application prospect. However, overcoming the poor resistance of H2O and SO2 poisoning for carbon-based catalysts remains a great challenge. Notably, reviews on the sulfur and water resistance of carbon-based low-temperature NH3-SCR catalysts have not been previously reported to the best of our knowledge. This review introduces the reaction mechanism of the NH3-SCR process and the poisoning mechanism of SO2 and H2O to carbon-based catalysts. Strategies to improve the SO2 and H2O resistance of carbon-based catalysts in recent years are summarized through the effect of support, modification, structure control, preparation methods and reaction conditions. Perspective for the further development of carbon-based catalysts in NOx low-temperature SCR is proposed. This study provides a new insight and guidance into the design of low-temperature SCR catalysts resistant to SO2 and H2O in the future.

Unique κ-Ce2Zr2O8 Superstructure Promoting the NOx Adsorption-Selective Catalytic Reduction (AdSCR) Performance of the WO3/CeZrOx Catalyst.

Selective catalytic reduction of NOx by NH3 (NH3-SCR) for diesel emission control at low temperatures is still a great challenge due to the limit of the urea injection threshold and inferior SCR activity of state-of-the-art catalyst systems below 200 °C. Fabricating bifunctional catalysts with both low temperature NOx adsorption-storage capacity and medium-high temperature NOx reduction activity is an effective strategy to solve the issues mentioned above but is rarely investigated. Herein, the WO3/Ce0.68Zr0.32Ox (W/CZ) catalyst containing the κ-Ce2Zr2O8 pyrochlore structure was successfully developed by a simple H2 reduction method, not only showing superior NOx adsorption-storage ability below 180 °C but also exhibiting excellent NH3-SCR activity above 180 °C. The presence of the pyrochlore structure effectively increased the oxygen vacancies on the κ-Ce2Zr2O8-containing W/CZ catalyst with enhanced redox property, which significantly promoted the NOx adsorption-storage as active nitrate species below 180 °C. Upon NH3 introduction above 180 °C, the κ-Ce2Zr2O8-containing W/CZ catalyst showed greatly improved NOx reduction performance, suggesting that the pyrochlore structure played a vital role in improving the NOx adsorption-selective catalytic reduction (AdSCR) performance. This work provides a new perspective for designing bifunctional CeZrOx-based catalysts to efficiently control the NOx emissions from diesel engines during the cold-start process.

Improving low-temperature NH3-SCR denitration activity and resistance to H2O and SO2 over Nb-modified NbaSn0.3CeOx catalysts by enhancing acidity to compensate for its weak oxidation ability

In this paper, a series of Nb-modified NbaSn0.3CeOx (a = 0.2, 0.4, 0.5) catalysts for denitrification were prepared using a facile co-precipitation method. In order to explore the various individual properties of the samples, the catalysts were characterized by XRD, Raman, N2 adsorption and desorption, TEM, NH3-TPD, H2-TPR, XPS and in-situ DRIFTS tests to understand the influences of the Nb introduction on the NH3-SCR reaction. Among these catalysts, the Nb0.4Sn0.3CeOx catalyst exhibit excellent NH3-SCR performance in the temperature range of 155–450 °C with more than 90 % NO conversion, 100 % N2 selectivity, and possess good resistance to H2O and SO2. Compared with the Sn0.3CeOx catalyst, we believe that the improvement of the low-temperature NH3-SCR performance of the Nb0.4Sn0.3CeOx catalysts is mainly due to the change in the amount of acid, special for the increase in B acid content, which compensates for the effect of the weaken oxidizing ability on the low-temperature activity. Beyond that, the good H2O and SO2 resistance at low temperature after the introduction of Nb into Nb0.4Sn0.3CeOx catalysts was analyzed by using in-situ DRIFTS characterization in detail, and the results indicate that the incorporation of Nb can inhibit the oxidation of SO2 to form sulfate on the catalysts, and prevent the competing adsorption of H2O and NOx, but the adsorption of NH3 is not affected. This work proves that the acidity regulation can improve low-temperature SCR activity by the synergistic effect between oxidation capacity and acidity.

Enhanced low-temperature performance of Al-rich Cu-SSZ-13 by Ce modification upon hydrothermal aging for NH3-SCR

3wt.% Cu/SSZ-13 (Cu3/Z) and Ce modified 3wt.% Cu/SSZ-13 (Ce-Cu3/Z) catalysts were prepared and then hydrothermally aged. The Ce-modified Ce-Cu3/Z catalyst revealed excellent NH3-SCR activity and hydrothermal stability. Combined with a series of characterizations, it was proved that the introduction of Ce could inhibit the dealumination, and prevent the aggregation of Cu ions to form CuOx and CuAl2O4 species upon hydrothermal aging, thus improving the hydrothermal stability of Ce-Cu3/Z. In addition, Ce could interact with Cu species to enhance the reducibility of catalyst, and the interaction between Ce and Cu species was strengthened after hydrothermal aging. Also, the presence of Ce species could enhance the NH3 and NOx adsorption capacity of the catalyst, resulting in higher NH3-SCR activity of Ce-Cu3/Z after hydrothermaly aged at 700 °C. In situ DRIFTS results showed that the NH3-SCR reaction followed both E-R and L-H mechanisms on the Cu3/Z-700 and Ce-Cu3/Z-700 catalysts at 150 °C. Ce doping could promote the formation of L-NH3 (NH3 species coordinated to Lewis acid sites) and nitrate species on the Ce-Cu3/Z-700 catalyst, as well as the involvement of L-NH3 in the reaction, which play a crucial role as the main active intermediates at low temperature.

Excellent NH3-SCR activity and significant sulfur resistance over novel core–shell Cu-SSZ-13@meso-CeO2 catalyst

A core–shell Cu-SSZ-13@meso-CeO2 catalyst with both microporous and mesoporous pores was successfully synthesized using a self-assembly technique in this research. The catalyst was tested via NH3–SCR of NOx removal from diesel exhausts. The Cu-SSZ-13@meso-CeO2 catalyst was found to have significantly improved activity at high temperatures, wider temperature range, improved hydrothermal stability, and anti-sulfur performance. After the introduction of meso-CeO2, a significant synergistic effect between the Cu2+/Ce4+ ions on the core–shell layer was observed, and the amounts of Cu2+–2Z species and acidic sites increased, which may be conducive to the superior catalytic performance of Cu-SSZ-13@meso-CeO2 contrasted to Cu-SSZ-13. Subsequent to hydrothermal treatment, Cu-SSZ-13@meso-CeO2 maintains a superior CHA-type structure, additional L-acidic sites, and isolated Cu2+ ions while being protected by a meso-CeO2 shell. In addition, in the presence of SO2, the meso-CeO2 shell in Cu-SSZ-13@meso-CeO2 reacts with SO2 to produce cerium sulfate, which serves as a protective component for the copper species on Cu-SSZ-13. Moreover, the reaction mechanism studies have indicated that NO reduction primarily adheres to the E–R mechanism on Cu-SSZ-13, whereas both L–H and E–R mechanisms contribute to Cu-SSZ-13@meso-CeO2. SO2 has a less significant inhibitive effect on the adsorption of NH3 over Cu-SSZ-13@meso-CeO2 compared with that over Cu-SSZ-13. The inhibitive influence of SO2 on Cu-SSZ-13@meso-CeO2 is primarily owing to the inhibitory adsorption of NO and the reduction of the L–H reaction path, with little effect on the E–R reaction mechanism, which is significantly affected in Cu-SSZ-13. The Cu-SSZ-13@meso-CeO2 catalyst demonstrated exceptional NH3–SCR performance, a broad temperature range, and super hydrothermal stability as well as anti-sulfur performance, indicating great application prospects for NOx removal from diesel vehicle exhausts.

Comparison of precursors for the synthesis of Cu-SSZ-39 zeolite catalysts for NH3-SCR reaction

Cu-SSZ-39 zeolites are noted for excellent NH3-SCR activity and hydrothermal stability for applications on diesel vehicles. In this study, Cu-SSZ-39 zeolites were prepared using four precursors. The results showed that the Cu-SSZ-39 prepared using ZSM-5 as precursor (Cu2.5-SSZ-39-Z) has high atomic utilization and the largest particle size, comparing with those using Y, beta and colloidal silica. The Si/Al ratio of Cu2.5-SSZ-39-Z is 9.0, and over 32% of Cu ions are Cu(OH)+ species. Cu2.5-SSZ-39-Z exhibited the best low-temperature NH3-SCR activity and hydrothermal stability. Over 90% of NOx conversion was achieved in the temperature range of 200–550 oC; meanwhile, over 90% NOx conversion could be obtained between 225 oC and 450 oC after hydrothermal aging at 850 oC for 16 h or 900 oC for 5 h. The excellent activity and hydrothermal stability make ZSM-5 the preferred precursor for industrial preparation of Cu-SSZ-39 zeolite.

Acidic TiNb2O7 nanospheres supported with CeO2 to enhance NH3-SCR denitrification activity and resistance to H2O and SO2: Synergistic effect of CeO2 and TiNb2O7

TiNb2O7 material can be selected as the good support for NH3-SCR catalysts because of its shear structure and surface acidity. Increasing the acidity of catalyst is the key to achieving excellent NH3-SCR performance. In this work, m%CeO2/TiNb2O7 (m = 1, 2, 5, 8 and 10) catalysts were prepared by adjusting the Ce amount supported on TiNb2O7 with good acidity. Among them, 8%CeO2/TiNb2O7 exhibited good activity, N2 selectivity, and excellent SO2/H2O resistance. A series of analytical techniques were employed to examine the relationship among catalyst structure, surface properties and performance. The results show that the short-range ordered Cen+–O2--Nbn+ and Cen+–O2--Tin+ species from interactions between CeO2 and acidic support TiNb2O7 result in improved redox ability and acidity (especially Brønsted acids). The influence of SO2/H2O on SCR reaction and the reaction mechanism were researched using in-situ DRIFTS. The results indicate that SO2/H2O can promote NH3 adsorption and slightly inhibit NOx adsorption. The SCR reaction over 8%CeO2/TiNb2O7 mainly follows the E-R mechanism within the activation window, thus, the influence of SO2 and H2O on the reaction pathway is low, which probably contributes to the better tolerance against SO2 and H2O of the catalyst.

A superior CeO2 @TiO2 catalyst with high dispersion of CeO2 for selective catalytic reduction of NOx with NH3

NOx emissions have raised many environmental problems and posed a serious threat to human health. Development of excellent catalysts for selective catalytic reduction of NOx by NH3 (NH3-SCR) is desirable. In this study, by ingeniously encapsulating Ce ions into MIL-125 channels, CeO2 @TiO2 catalysts were prepared by annealing the Ce/MIL-125 precursor. Due to the porous framework structure of the MIL-125, a large amount of Ce ions were adsorbed into the pores, thus achieving high dispersion of CeO2 in the composites. All the CeO2 @TiO2 catalysts exhibit a NO conversion of nearly 100% at 350 ℃ and a wide temperature range larger than 200 ℃. The CeO2 @TiO2 (2/3) catalyst shows high specific surface area and porous structure. Meanwhile, a large amount of Ce3+ sites, enriched surface oxygen species, outstanding adsorption of NOx species and high surface acidity for CeO2 @TiO2 (2/3) catalyst account for the excellent NH3-SCR properties with a wide temperature range (200 ℃ ∼ 450 ℃) and excellent H2O and SO2 resistance.

Identifying the promotional mechanism for the activity and SO2 resistance of Ce-doped K-OMS-2 for the low-temperature SCR of NO with NH3

Mn-based catalysts for the low-temperature selective catalytic reduction (SCR) of NOx by NH3 can be strongly deactivated due to the generation of MnSO4 derived from upstream SO2 and H2O. In this study, a catalyst architecture based on a Ce-doped manganese oxide octahedral molecular sieve (Ce-OMS-2) was successfully synthesized to promote NH3-SCR activity and enhance SO2 tolerance at low temperatures. We investigated the role of Ce and its specific octahedral molecular sieve structure on the anti-SO2 poisoning effect of Ce-OMS-2. Maintaining the manganese oxide octahedral molecular sieve structure with polyvalent manganese species and active oxygen species and the charge transfer between Mn and Ce via the Ce-O-Mn bond in the Ce-OMS-2 catalyst are found to be the key factors for the realization of excellent NH3-SCR activity and SO2 tolerance. In-depth in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) show that the number of Brønsted acid sites in Ce-OMS-2 can also favor SO2 resistance. This study provides new insight into the mechanism for de-NOx activity and SO2 tolerance on Ce-OMS-2 catalyst and is beneficial for the design of anti-SO2 NH3-SCR catalysts.

Insights into Synergy of Copper and Acid Sites for Selective Catalytic Reduction of NO with Ammonia over Zeolite Catalysts

Herein, we report the function of copper sites in Cu-SSZ-13, Cu-ZSM-5 and Cu-Beta catalysts with the same Si/Al ratio (14) and Cu/Al ratio (0.4) on selective catalytic reduction of NO with NH3 (NH3-SCR) and reveal the relationship between active sites (Cu sites, acid sites) and catalytic activity. The results show that the amount of isolated Cu2+ ions in the catalysts directly determines the formation of strong Lewis acid sites and reaction intermediate NO3− ions, thus affecting the low-temperature SCR performance, while the amount of highly stable Cu+ ions and Brønsted acid sites is related to the high-temperature SCR performance of the catalysts. Consequently, it contains enough isolated Cu2+ ions, highly stable Cu+ ions and Brønsted acid sites, which endows Cu-SSZ-13 with excellent NH3-SCR activity.

Insight into the enhanced catalytic activity and H2O/SO2 resistance of MnFeOx/Defect-Engineered TiO2 for low-temperature selective catalytic reduction of NO with NH3

A series of Mn-Fe mixed oxide (MnFeOx) supported on high-surface-area TiO2 (Tiov) containing engineered Ti3+/oxygen vacancies was demonstrated for NH3-SCR of NO. The best catalyst MnFe/Tiov(0.05)-1 exhibited excellent NH3-SCR activity (>99 % conversion) over 60–260 °C and superior resistance to high H2O content (20 vol%), with NO conversion exceeding 90 % at temperatures lower than 150 °C. Furthermore, MnFe/Tiov(0.05)-1 exhibited outstanding durability over 70 h of sustained operation in the co-presence of H2O and SO2, whereas catalytic deactivation was observed for the conventional TiO2-supported catalyst (MnFe/Ti). The combined results of various characterization techniques confirmed that the interactions between the MnFeOx and Tiov supports were strengthened. The defect-rich surface of the Tiov support promoted the dispersion of MnFeOx and facilitated the redox cycle Mn4+ + Ti3+ ↔ Mn3+ + Ti4+, thus facilitating the adsorption and activation of NH3 and NO molecules. Moreover, the in situ diffuse reflectance infrared Fourier transform results indicated that H2O strongly suppressed the adsorption/activation of NH3 and NOx and the reactions between NH3 and NOx through the Langmuir–Hinshelwood pathway on MnFe/Ti, while such adverse effects were significantly reduced when using MnFe/Tiov(0.05)-1, in which the Eley–Rideal mechanism was dominant.

Application progress of small-pore zeolites in purifying NOx from motor vehicle exhaust

• Small-pore SSZ-13 and SSZ-39 zeolites have unique advantages in purifying NOx. • Pd/zeolites as PNAs present the superior low-temperature NOx adsorption activity. • Non-precious metal Co/zeolite is an ideal substitute for Pd/PNAs to reduce the cost. • Cu/zeolites possess the excellent high-temperature NH3-SCR catalytic activity. • Fe/Mn/Ce doped Cu/zeolites can broaden the NH3-SCR active temperature window. Lean-burn mode results in the high NOx emission of diesel vehicles despite its high fuel economy. NOx can be effectively removed by the SCR unit during normal operation, however, elimination during the cold start period needs to be addressed soon. Passive NOx adsorber (PNA) was proposed to adsorb and store NOx emitted during the cold start period. With the increase of exhaust gas temperature during normal operation, the adsorbed NOx will be quickly released and efficiently converted to N 2 when flowing through the downstream SCR catalyst. After years’ investigation, small-pore zeolites such as CHA and AEI have been found to possess superior hydrothermal stability and NOx selectivity compared with medium and large pore zeolites, and the former’s eight-membered ring structure effectively exclude large hydrocarbons from entering the zeolite channels, making them play a prominent role in NOx purification via both PNA and SCR units. SSZ-13 and SSZ-39 zeolites, which belong to CHA and AEI topologies respectively, has been confirmed to exhibit excellent low-temperature NOx adsorption performance and high-temperature SCR denitrification performance. In the field of purifying NOx from the diesel vehicle exhaust, the review simultaneously focused on the application of SSZ-13 and SSZ-39 zeolites in NOx purification from two aspects of PNA and SCR in recent decades. In addition, the unique advantages of their modified form of loading Pd/Co as PNAs, or loading Cu doped with Fe/Mn/Ce as SCR catalysts were systematically clarified, in order to provide some advanced guidance for the researchers in the purification of NOx from the diesel engine exhaust.