Catalytic Reduction Of Nitrogen Oxides Research Articles

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.

Revealing the Dynamic Behavior of Active Sites on Acid-Functionalized CeO2 Catalysts for NOx Reduction.

Unraveling the dynamics of the active sites upon CeO2-based catalysts in selective catalytic reduction of nitrogen oxides by ammonia (NH3-SCR) is challenging. In this work, we prepared tungsten-acidified and sulfated CeO2 catalysts and used operando spectroscopy to reveal the dynamics of acid sites and redox sites on catalysts during NH3-SCR reaction. We found that both Lewis and Brønsted acid sites are needed to participate in the catalytic reaction. Notably, Brønsted acid sites are the main active sites after a tungsten-acidified or sulfated treatment, and the change of Brønsted acid sites significantly affects the NOx removal. Moreover, acid functionalization promotes the cerium species cycle between Ce4+ and Ce3+ for the NOx reduction. This work is critical to deeply understanding the natural properties of active sites, and it also provides new insights into the mechanism for NH3-SCR over CeO2-based catalysts.

Recent progress in novel zeolite catalysts for selective catalytic reduction of nitrogen oxides

Selective catalytic reduction (SCR) using zeolite catalysts is a key technology for the abatement of nitrogen oxides (NOx) from lean-burn heavy-duty vehicles. With the increasing stringency of emission regulations, researchers are now devoting more efforts to enhancing the efficiency and durability of zeolite catalysts. This paper intends to summarize the very recent (mainly in the last five years) advancements in the development and application of novel zeolites as SCR catalysts. Specifically, we first provide a detailed overview of the synthesis, structure-performance relationship, and potential application of small-pore zeolites with different topologies (CHA, AEI, LTA, AFX, SFW, KFI, ERI/EAB, LEV and RHO) as catalysts in SCR with NH3 as the reductant. Then, advances in the post-synthetic modifications of small-pore zeolites for NH3-SCR are reviewed, followed by an introduction of Cu-free small-pore zeolites as NH3-SCR catalysts. We also summarize the recent mechanistic understandings of NH3-SCR chemistry over small-pore zeolite catalysts, mainly over Cu-SSZ-13, using in situ or operando spectroscopy. The application of zeolites as catalysts in SCR with hydrocarbons (e.g., CH4, CH3OH) or H2 as the reductant is also briefly discussed. Finally, we discuss the potential of several novel small-pore zeolites, such as Cu-SSZ-39, for real application in NH3-SCR and the development of new SCR systems with minimized formation of byproducts, such as the greenhouse gas N2O and the toxic HCN.

Recognizing the formation of single-atom vanadium by enthalpy and its advantage in selective catalytic reduction of nitrogen oxide

Developing a more effective catalyst for the environmental pollutant is always the pursuit of worldwide researchers. In previous reports, most works were focused on prepared catalysts, and more efforts should explore process from a precursor to the formed catalyst. In this work, V supported TiO2 catalysts were synthesized with different V loadings, and compared in selective catalytic reduction of nitrogen oxide. For the first time, differential scanning calorimetry was used to analyze the forming enthalpy for these catalysts. As a result, turnover frequency (TOF) was decreased from 0.05 to 0.001 s−1 when the loading was increased from 0.05 to 15 mol%. At the same time, the forming enthalpy was decreased from 16.53 to 1.80 KJ·mol−1. Spherical Aberration Corrected Transmission Electron Microscope recognized both single-atom and cluster V even at the lowest loading. What’s more, density functional theory revealed that single-atom V had a lower dehydrogenation energy (1.088–1.152 eV) than cluster V during the catalysis (1.834–1.850 eV). According to these evidences, single-atom V had bigger formation enthalpy, lower dehydrogenation energy, and larger TOF than cluster V. These results help to optimize the preparation process for an effective catalyst, which is in favor of hazardous-pollutant treatment.

A method for calculating catalytic-site amount and judging antagonistic/synergetic effect in multi-metallic catalyst

Bimetallic doping is widely utilized to increase the catalytic activity of TiO2-supported catalyst in selective catalytic reduction of nitrogen oxide (SCR). However, a “bimetallic enhancement” is hard to judge since the ratio of active sites, which actually devotes to the catalytic Turnover frequency (TOF, s−1), is difficult to obtain. In this work, Mn, Fe, V, and their pairs were loaded on TiO2, and applied in SCR. Diffuse Reflection Fourier Transform Infrared Spectrometer established a method to evaluate the ratio of working catalytic sites. TOF of these sites were calculated. As a result, single-metallic Mn and V resulted in an average TOF of 0.17 s−1 and 0.11 s−1, respectively, bigger than that (0.10 s−1) of bimetallic MnV. With the increase of loading amount, single-atom cation was agglomerated to form cation pair, and then cluster. Among these three species, cation pair had the biggest TOF, indicating a synergetic effect. Mn obtained the best TOF of 0.18 s−1 at a loading of 1 mmol. One cation species accelerated agglomeration and crystal transformation of the other cation, thus decreasing the TOF and exhibiting an antagonistic effect. These results are in favor of mechanism understanding and catalyst optimization.