Catalytic Reduction Of Nitrogen Oxides Research Articles

Optimization of isolated copper species on the NH3-SCR performance over Cu/SSZ-39 modified by ammonia water

To further improve the activity of Selective Catalytic Reduction of nitrogen oxide (NOx) with Ammonia (NH3-SCR) and the hydrothermal stability at 900 °C on Cu-based small-pore zeolite catalyst, the interaction between ammonia water and Cu2+ is used to regulate the interaction between Cu2+ and Si-O-Al structures of the zeolite, and affect the migration of Cu2+ in the channels of zeolite. The characterization results show that the microchemical environment of isolated Cu2+ species is optimized, and more isolated Cu2+ species can stably exist over the surface of Cu/SSZ-39, exhibiting a strong redox property. Besides, the ability of acid sites to adsorb and activate NH3 is also enhanced, and the amount of H2O adsorbed in the pores of zeolite is reduced due to the stable surface isolated Cu2+ species, which is conducive to the stability of the catalyst after hydrothermal aging at 900 °C. When the mole ratio of ammonia and copper is 3, the NH3-SCR performance and the hydrothermal stability at 900 °C are both optimized on the modified catalyst. The temperature window that the NOx conversion is higher than 90% (ΔT90) on the unmodified catalyst is 200–475 °C, but the ΔT90 on the modified catalyst is broadened to 178–544 °C. More importantly, after hydrothermal aging at 900 °C, the ΔT90 on the modified catalyst is shifted to the higher temperature by 48 °C, with the maximum NOx conversion of 96%. This work would provide a new strategy for further optimizing Cu/SSZ-39 catalyst for deNOx emitted from diesel engines to meet higher emission standards in future.

Two easy methods to increase SO2-resistance ability of Mn-based catalyst

Manganese-based catalysts show high activities in selective catalytic reduction of nitrogen oxide (NO), but easily deactivated by sulfur dioxide. In this work, operation sequence and rare-earth-element doping were investigated to increase the anti-SO2 ability of Fe-doped manganese oxide (FM300). On one hand, when the catalyst pre-adsorbed NH3, SO2 only decreased the NO conversion from 100 % to 75 % at 200 °C. Otherwise, the conversion was decreased to 30 %. In-situ Diffuse Reflection Fourier Transform Infrared Spectroscopy proved that NH3 competed with SO2 and protected part of the Brønsted acidic sites. On the other hand, rare-earth-element doping increased the long-time anti-SO2 performance of FM300 from 150 min to 300 min (Sm-FM300) and > 700 min (Ho-FM300). Differential Scanning Calorimeter analysis indicated that Ho optimized ferromagnetism of FM300, which was in favor of NH3 adsorption. The main result of this work will help to increase applicability of the manganese-based catalyst.

Superior room-temperature efficacy of selective catalytic reduction of NOx by CO utilizing metal-modified Fe2O3 catalysts

Fe2O3-based catalysts demonstrate significant potential for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) by CO. However, their effectiveness is limited at lower temperatures, impeding widespread industrial adoption. To overcome this, we optimize the Fe2O3-based catalyst component through 3 wt% cobalt (Co) loading, which improves the synergistic removal of CO and NOx pollutants. Our developed catalyst, Co-Fe2O3 on an amorphous SiO2 substrate, exhibits superior room-temperature activity in CO-SCR, achieving CO and NOx conversion ratios as well as N2 selectivity above 80 % at 25 °C, and over 93 % at 100–500 °C. The enhancement in catalyst performance is elucidated through density functional theory calculations and experimental investigations, revealing the creation of heteronuclear active sites via metal modification, and consequent improvements in redox characteristics and acidity. Our findings offer a robust pathway towards the industrial implementation of ammonia-free-SCR technology with synergistic removal of CO and NOx and provide insights for designing superior heterogeneous catalysts.

Designing a Ce/In-CHA OXZEO catalyst for high-efficient selective catalytic reduction of nitrogen oxide with methane

Selective catalytic reduction of nitrogen oxides with methane (CH4-SCR) has emerged as a promising technology for mitigating exhaust emissions from natural gas vehicles, while its insufficient catalytic activity and durability are key barriers to application. In this study, novel Ce/In-CHA OXZEO catalysts were prepared, demonstrating exceptional catalytic performance and stability in the CH4-SCR reaction. Over these OXZEO catalysts, indium species predominantly existed in the form of InO+ and InxOy within the CHA zeolite framework, while CeO2 nanoparticles primarily resided as an external layer. During the CH4-SCR reaction, NO oxidation to form NO2 and nitrate species was triggered by CeO2, which further enhanced the activation of CH4 on InO+ sites, thus boosting the low-temperature CH4-SCR process by facilitating the formation of the key intermediate CH3NO2. The synergy between InO+ and CeO2 underscores the exceptional performance and stability of the Ce/In-CHA with a low SiO2/Al2O3 ratio in reducing NOx and CH4 emissions.

Synergistic catalytic removal of NOx and chlorinated organics through the cooperation of different active sites

The synergistic removal of NOx and chlorinated volatile organic compounds (CVOCs) has become the hot topic in the field of environmental catalysis. However, due to the trade-off effects between catalytic reduction of NOx and catalytic oxidation of CVOCs, it is indispensable to achieve well-matched redox property and acidity. Herein, synergistic catalytic removal of NOx and chlorobenzene (CB, as the model of CVOCs) has been originally demonstrated over a Co-doped SmMn2O5 mullite catalyst. Two kinds of Mn-Mn sites existed in Mn-O-Mn-Mn and Co-O-Mn-Mn sites were constructed, which owned gradient redox ability. It has been demonstrated that the cooperation of different active sites can achieve the balanced redox and acidic property of the SmMn2O5 catalyst. It is interesting that the d band center of Mn-Mn sites in two different sites was decreased by the introduction of Co, which inhibited the nitrate species deposition and significantly improved the N2 selectivity. The Co-O-Mn-Mn sites were beneficial to the oxidation of CB and it cooperates with Mn-O-Mn-Mn to promote the synergistic catalytic performance. This work paves the way for synergistic removal of NOx and CVOCs over cooperative active sites in catalysts.

TEAOH-assisted cost-effective preparation of SSZ-13 using L zeolite as Al source

Developing an economical and high-efficiency synthesis pathway of SSZ-13 zeolite is of great significance for the ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR of NOx) reactions. Herein, SSZ-13 zeolite were synthesized using L zeolite as the aluminum source in the presence of a mixed structure directing agent (SDA) composed of N,N,N, trimethtyl-1-adamantammonium hydroxide (TMAdaOH) and tetraethylammonium hydroxide (TEAOH). This synthesis strategy aims to accelerate the crystallization rate through interzeolite transformation, while less expensive TEAOH can play a cooperative structure-directing role, thus reducing the consumption of expensive TMAdaOH template. The results showed that pure SSZ-13 zeolites with high crystallinity can be obtained from L zeolite within 12 h at 160 ℃ under low TMAdaOH/SiO2 ratio of 0.05 (too low to carry out the synthesis on its own in the current synthesis system) in the presence of TEAOH. Moreover, the crystallization mechanism was also investigated by XRD, FT-IR, SEM and TG-DTG techniques. It was proposed that the crystallization of SSZ-13 occurs through the formation of SSZ-13 nuclei under the direction of TMAdaOH and promotion of TEAOH, subsequently TMAdaOH was incorporated in the framework to further induce the crystal growth of SSZ-13. Especially, the zeolite crystal size can be modulated by the amount of TEAOH added. In addition, After Cu2+ exchange, the obtained Cu-SSZ-13 sample exhibits superior catalysis performance in the NH3-SCR of NOx reaction, and the NOx conversion rate remained over 90 % across a broad range from 200 to 600 °C, demonstrating its significant application potentials in NOx removal from diesel vehicle exhausts.

Knowledge-Driven Experimental Discovery of Ce-Based Metal Oxide Composites for Selective Catalytic Reduction of NOx with NH3 through Interpretable Machine Learning.

Mining the scientific literature, combined with data-driven methods, may assist in the identification of optimized catalysts. In this paper, we employed interpretable machine learning to discover ternary metal oxides capable of selective catalytic reduction of nitrogen oxides with ammonia (NH3-SCR). Specifically, we devised a machine learning framework utilizing extreme gradient boosting (XGB), identified for its optimal performance, and SHapley Additive exPlanations (SHAP) to evaluate a curated database of 5654 distinct metal oxide composite catalytic systems containing cerium (Ce) element, with records of catalyst composition and preparation and reaction conditions. By virtual screening, this framework precisely pinpointed a CeO2-MoO3-Fe2O3 catalyst with superior NOx conversion, N2 selectivity, and resistance to H2O and SO2, as confirmed by empirical evaluations. Subsequent characterization affirmed its favorable structural, chemical bulk properties and reaction mechanism. Demonstrating the efficacy of combining knowledge-driven techniques with experimental validation and analysis, our strategy charts a course for analogous catalyst discoveries.

Selective catalytic reduction of NOX with CH4 over In/SSZ-13 zeolites: The enhancement of high-temperature catalytic activity by Ce modification

The selective catalytic reduction of nitrogen oxides by methane (CH4/NOX-SCR) is a potential technique for the abatement of the NOX emissions from lean-burn GDI engines, due to the excellent thermal stability of CH4. This work focuses on the promotion of In/SSZ-13 catalysts by Ce modification for the CH4/NOX-SCR performance at high temperatures. A series of In/SSZ-13 and Ce-In/SSZ-13 catalysts were prepared using the wetness impregnation method. The effects of Ce modification on the catalytic performance were analysed in detail using the scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance spectrum and temperature-programmed desorption/reduction techniques. The results show that the Ce modification significantly enhances the high-temperature activity of the In/SSZ-13 catalysts for the NOX-SCR reaction, especially for the 5 wt% Ce-15 wt% In/SSZ-13 sample which has more than 75% NOX conversion at 390–700 °C. In addition, the Ce modification enhances the water vapor tolerance of the catalyst sample. The Ce modification increases not only the concentration of surface chemisorption oxygen species, but also the relative amount of InO+ species which serve as the main active sites for CH4 activation. Furthermore, the incorporation of Ce into In/SSZ-13 greatly promotes NOX adsorption on the catalyst surface. Most importantly, the Ce modification makes In species well-dispersed on the catalyst surface and thus suppresses the formation of bulk In2O3 species which are more active for the CH4 non-selective oxidation. The combination of all these effects results in the Ce-In/SSZ-13 samples with the superior catalytic activity.

Strong metal oxide-zeolite interactions during selective catalytic reduction of nitrogen oxides

In response to the stricter EU VII emission standards and the "150 ℃ challenge", selective catalytic reduction by ammonia (NH3-SCR) catalysts for motor vehicles are required to achieve high NO conversion below 200 °C. Compounding metal oxides with zeolites is an important strategy to design the low-temperature SCR catalysts. Here, we original prepared Cu-SSZ-13 @ MnGdOx (Cu-Z @ MGO), which achieved over 90% NO conversion and 95% N2 selectivity at 150 ℃. It has been demonstrated that a uniform mesoporous loaded layer of MGO grows on Cu-Z, and a recrystallization zone appears at the MGO-Cu-Z interface. We discover that the excellent low-temperature SCR activity derives from the strong metal oxide-zeolite interaction (SMZI) effects. The SMZI effects cause the anchor and high dispersion of MGO on the surface of Cu-Z. Driven by the SMZI effects, the Mn3+/Mn4+ redox cycle ensures the low and medium temperature-SCR activity and the Cu2+/Cu+ redox cycle guarantees the medium and high temperature-SCR activity. The introduction of MGO improves the reaction activity of -NH2 species adsorbed at Mn sites at 150 ℃, achieving a cycle of reduction and oxidation reactions at low temperatures. This strategy of inducing SMZI effects of metal oxides and zeolites paves a way for development of high-performance catalysts.

DEVELOPMENT OF A REACTOR FOR LOW-TEMPERATURE CATALYTIC REDUCTION OF NITROGEN OXIDE EMISSIONS AT NITROGEN-FERTILIZER ENTERPRISES

The article presents the development of a more efficient use of the processes of catalytic reduction of nitrogen oxides of gas emissions for industrial use. The advantages of plate catalyst blocks with a deposited active substance in comparison with industrial granular aluminum-vanadium catalysts are presented. Chemical and kinetic regularities of this process are shown. It is noted that the use of plate deposited catalysts based on noble metals is a more effective means of reducing nitrogen oxides than granular ones due to the presence of low hydraulic resistance and increased activity. On the basis of experimental data of laboratory and pilot studies of the developed catalyst obtained by impregnating the applied metal carrier based on TiO2 with salts of platinum group metals, physicochemical and kinetic calculations of the process of reduction of nitrogen oxides using ammonia were carried out. Graphical dependencies of the activity of the developed catalysts on the process temperature and volume velocity are given. The optimal values of these indicators have been determined. It is shown that the effect of temperature is explained by the growth of the reaction rate constant in proportion to the temperature according to Arrhenius' law, and the effect of the volumetric velocity is associated with a change in the duration of contact of the reacting components with the active surface of the catalyst. The calculation of the Reynolds criterion was carried out, which showed that this process in the conditions of industrial technologies is characteristic of the laminar regime. It is noted that the use of the developed catalyst in production provides an expansion of the temperature range of the emission purification process, it is more suitable for operation in industry in accordance with the determined optimal conditions of its application.

Modified bentonite-derived materials as catalysts for selective catalytic reduction of nitrogen oxides

Modified bentonite-derived materials as catalysts for selective catalytic reduction of nitrogen oxides

Copper Site Motion Promotes Catalytic NOx Reduction under Zeolite Confinement.

Ammonia-mediated selective catalytic reduction (NH3-SCR) is currently the key approach to abate nitrogen oxides (NOx) emitted from heavy-duty lean-burn vehicles. The state-of-art NH3-SCR catalysts, namely, copper ion-exchanged chabazite (Cu-CHA) zeolites, perform rather poorly at low temperatures (below 200 °C) and are thus incapable of eliminating effectively NOx emissions under cold-start conditions. Here, we demonstrate a significant promotion of low-temperature NOx reduction by reinforcing the dynamic motion of zeolite-confined Cu sites during NH3-SCR. Combining complex impedance-based in situ spectroscopy (IS) and extended density-functional tight-binding molecular dynamics simulation, we revealed an environment- and temperature-dependent nature of the dynamic Cu motion within the zeolite lattice. Further coupling in situ IS with infrared spectroscopy allows us to unravel the critical role of monovalent Cu in the overall Cu mobility at a molecular level. Based on these mechanistic understandings, we elicit a boost of NOx reduction below 200 °C by reinforcing the dynamic Cu motion in various Cu-zeolites (Cu-CHA, Cu-ZSM-5, Cu-Beta, etc.) via facile postsynthesis treatments, either in a reductive mixture at low temperatures (below 250 °C) or in a nonoxidative atmosphere at high temperatures (above 450 °C).

Glass-fiber catalysts for selective catalytic reduction of nitrogen oxides by CO and hydrocarbons

Glass-fiber catalysts (GFCs) are investigated in the process of the catalytic reduction of nitrogen oxides by hydrocarbons and carbon monoxide contained in waste and flue gases. Studies show that the platinum-based GFCs prepared by surface high-temperature synthesis approach demonstrates higher activity in the reactions of NOx reduction and CO and CxHy oxidation, in comparison with conventional GFCs and catalytic systems based on oxides of MnCe and FeV. Structured cartridges with a corrugated and flat metal mesh with longitudinal flow direction and satin type GFC were found to provide the highest apparent catalyst activity. The kinetic model for Pt/GFC was proposed based on the experimental data. The best kinetic description is provided under account of formation of nitrite-nitrate surface compounds resulting from NO chemisorption and further interaction of these compounds with CO, as well as under account of simultaneous catalytic and non-catalytic oxidation of propane by oxygen both to CO and CO2. The proposed model may adequately reproduce the non-trivial experimental regularities, such as CO and NO conversion decrease at high temperatures and synergetic effect of oxygen on the rate of NO reduction by CO.

Physicochemical properties and NH<sub>3</sub>-SCR catalytic performance of intercalated layered aluminosilicates

The representative of natural layered clays, bentonite, was modified according to two routes and tested as a new catalyst for selective catalytic reduction of nitrogen oxides with ammonia (NH<sub>3</sub>-SCR). The natural acid-activated clay was ion-exchanged with Na<sup>+</sup> or remained in H-form and pillared with metal oxides. In order to limit the number of synthesis steps, iron as an active phase was introduced simultaneously with Al<sub>2</sub>O<sub>3</sub> during the intercalation procedure. Additionally, the samples were doped with 0.5 wt% of copper to promote low-temperature activity. It was found that the performed modifications resulted in disorganization of the ordered layered arrangement of bentonite. Nevertheless, acid activation and pillaring improved structural and textural parameters. The results of catalytic tests indicated that the samples containing Fe<sub>2</sub>O<sub>3</sub> pillars promoted with Cu exhibited the highest NO conversion of 85% at 250 °C (H-Bent-AlFe-Cu) and 75% at 300 °C (Na-Bent-AlFe-Cu). What is important, activity of the protonated samples in the high-temperature region was noticeably affected by the side reaction of ammonia oxidation, correlated with the production of NO and resulting in N<sub>2</sub>O emission during the process comparing to Na-Bentonite catalysts.

On the monolayer dispersion behavior of Co3O4 on HZSM-5 support: designing applicable catalysts for selective catalytic reduction of nitrogen oxides by ammonia

On the monolayer dispersion behavior of Co3O4 on HZSM-5 support: designing applicable catalysts for selective catalytic reduction of nitrogen oxides by ammonia

Promoted NH3-SCR activity and hydrothermal stability of Cu-SSZ-50 catalyst synthesized by one-pot method

Nitrogen oxide (NOx) is one of the most critical contaminants in the air, and the control of NOx emission from diesel vehicles is very important. Cu-based small-pore zeolites have already been applied for NOx abatement on diesel vehicles. Among the small-pore zeolites, Cu-SSZ-50 catalysts with good NH3-SCR catalytic activity were believed to have potential for application. In this study, a one-pot synthesis method for Cu-SSZ-50 catalysts was developed for the first time, using the co-templates of Cu-TEPA and 2,6-dimethyl-N-methylpyridinium hydroxide. In this synthesis method, Cu-SSZ-50 with various Cu contents can be obtained by adjusting the amount of Cu-TEPA without the need for a further after-treatment process. The addition of Cu-TEPA affected the framework atoms and Cu species, and a lower Si/Al ratio and more SCR active Cu species were obtained. The synthesized catalyst with a Cu/Al ratio of 0.40 exhibited over 90% NOx conversion between 200 °C and 450 °C for the selective catalytic reduction of NOx with NH3 (NH3-SCR). Meanwhile, over 80% NOx conversion could be obtained from 250 °C to 450 °C after hydrothermal aging at 750 °C for 16 h. In addition, both L-H and E-R mechanisms were proven to exist for the one-pot-synthesized Cu-SSZ-50 by in situ DRIFTS experiments. The simple synthesis procedure, excellent catalytic activity and hydrothermal stability brighten the prospects for the application of Cu-SSZ-50.

A Research of Selective Catalytic Reduction of NOx with NH3 by Using Novel Catalysts

Nitrogen oxides are one of the most important industrial pollutants, which have many harmful effects on the human body and the environment. In this paper, the formation mechanism of nitrogen oxides was studied, and the selective catalytic reduction of nitrogen oxides was carried out by selective catalytic reduction technology. This paper also reviews the low-temperature denitration performance of molecular sieve catalysts, manganese oxide and ceria-based catalysts, and expounds on the influence of catalysts on denitration activity under different active components. It is found that ceria-based catalytic materials have better structural properties and catalytic performance, the methods to improve the performance of ceria-based catalysts are discussed, and the future development of ceria-based catalysts has been prospected.

Low-silica Cu-CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance.

Cu-exchanged low-silica CHA zeolites (Si/Al ≤ 4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3-SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low-silica CHA zeolite (CHA-SPAEI, Si/Al = 3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of -Al-O-P(Si)-O-Al-O- tetrahedra as seed. The proportion of Al pairs in CHA-SPAEI is 78%, which is much higher than that in the conventional low-silica CHA (CHA-LS, 52%). After hydrothermal ageing at 800 °C for 6 h, Cu-exchanged CHA-SPAEI shows NO conversion above 90% within 225-500 °C under a gas hourly space velocity of 200,000 h-1, which is much better than that of Cu-exchanged CHA-LS. The spatial close proximity of Al pairs in CHA-SPAEI is confirmed by the 27Al double-quantum single-quantum two-dimensional NMR analyses. The strict -P(Si)-O-Al-O-P(Si)-O- sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the -Al-O-Si-O-Al-O- sequence via a transcription process. The utilization of aluminophosphate-based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.

Low‐silica Cu‐CHA Zeolite Enriched with Al Pairs Transcribed from Silicoaluminophosphate Seed: Synthesis and Ammonia Selective Catalytic Reduction Performance

AbstractCu‐exchanged low‐silica CHA zeolites (Si/Al≤4) synthesized without organic templates are promising candidate catalysts for ammonia selective catalytic reduction of nitrogen oxides (NH3‐SCR), but their practical application is restricted due to the low hydrothermal stability. Here, inspired by the transcription from duplex DNA to RNA, we synthesized Al pairs enriched low‐silica CHA zeolite (CHA‐SPAEI, Si/Al=3.7) by using silicoaluminophosphate (SAPO) featured by strict alternation of −Al−O−P(Si)−O−Al−O− tetrahedra as seed. The proportion of Al pairs in CHA−SPAEI is 78 %, which is much higher than that in the conventional low‐silica CHA (CHA‐LS, 52 %). After hydrothermal ageing at 800 °C for 6 h, Cu‐exchanged CHA‐SPAEI shows NO conversion above 90 % within 225–500 °C under a gas hourly space velocity of 200,000 h−1, which is much better than that of Cu‐exchanged CHA‐LS. The spatial close proximity of Al pairs in CHA‐SPAEI is confirmed by the 27Al double‐quantum single‐quantum two‐dimensional NMR analyses. The strict −P(Si)−O−Al−O−P(Si)−O‐ sequence in the fragments from the dissolution of SAPO seed promotes the Al pairs with the −Al−O−Si−O−Al−O− sequence via a transcription process. The utilization of aluminophosphate‐based zeolites as seeds opens up a new avenue for the regulation of the Al distribution in zeolites.

Water-tolerant and anti-dust CeCo-MnO2 membrane catalysts for low temperature selective catalytic reduction of nitrogen oxides

The present work successfully proposes a domain knowledge–guided Machine Learning (ML) strategy, which successes the development of a water-tolerant anti-dust catalyst for Low-Temperature (LT) Selective Catalytic Reduction (SCR) of nitrogen oxides (NOx) from catalyst discovery to industrial deployment. The discovered catalyst is able to convert 99 % of NOx at 150 °C in the standard waste gas, even in the waste gas containing 7 vol% of water vapors, the efficiency is still retained at 97 %. The superior LT activity and water-tolerance are attributed to abundant surface-active oxygen, Brønsted acid and microcellular structure. The SCR reaction mainly follows the Eley-Rideal (E-R) pathway driven by Brønsted acid and the Langmuir-Hinshelwood (L-H) pathway maintained by abundant reactive oxygen species in moisture waste gases. And then, catalyst is synthesized on a polyphenylene sulfide filter to render the membrane configuration in order to have the anti-dust ability. The final membrane catalyst has the capacity of converting 95 % NOx in the waste gas containing 7 vol% of moisture at 150 °C and the dedusting, and denitrification ability. The domain knowledge–guided Machine Learning (ML) strategy paves a wide avenue for the whole chain development from the data-driven discovery to the industrial deployment associated with mechanism exploration.