Unexpected Low Temperature Activity with Low N2 O Emission of Stabilized Al-rich Zeolite Beta for Selective Catalytic Reduction of NOx

The low temperature activity of Fe-loaded zeolites as selective catalytic reduction of NOx by NH3 (NH3 -SCR) catalysts is a critical drawback for practical application. Here, we found unexpected improvement of low temperature activity by our proposed post-synthetic treatment. An Al-rich zeolite beta (Si/Al=5) is employed as the catalyst support, and the parent sample is dealuminated for higher hydrothermal stability, followed by the liquid-mediated stabilization treatment and impregnation. It is found that stabilized samples feature excellent low temperature activity and high N2 selectivity even for a long-term operation, along with the ability to maintain high NOx conversion after aging. The improved SCR activity should be attributed to abundant acid sites in Al-rich framework and better stabilization of monomeric iron species after the stabilization treatment. Furthermore, the low yield of side product N2 O is probably due to the absence of the generation of NH4 NO3 during NH3 -SCR catalyzed by Fe-loaded zeolites.

Direct observation of Cu in high-silica chabazite zeolite by electron ptychography using Wigner distribution deconvolution

Direct observation of Cu in Cu-chabazite (CHA) zeolite has been achieved by electron ptychography using the Wigner distribution deconvolution. The imaging properties of ptychographically reconstructed images were evaluated by comparing the intensities of six-membered-ring columns of the zeolite with and without Cu using simulated ptychography images. It was concluded that although false contrast may appear at Cu-free columns for some acquisition conditions, ptychography can discriminate columns with and without Cu. Experimental observation of CHA with and without Cu was performed. Images obtained from the Cu-containing sample showed contrast at the six-membered-rings, while no contrast was observed for the Cu-free sample. The results show that ptychography is a promising technique for visualizing the atomic structures of beam-sensitive materials.

Recent progress in performance optimization of Cu-SSZ-13 catalyst for selective catalytic reduction of NO x

Nitrogen oxides (NO x ), which are the major gaseous pollutants emitted by mobile sources, especially diesel engines, contribute to many environmental issues and harm human health. Selective catalytic reduction of NO x with NH3 (NH3-SCR) is proved to be one of the most efficient techniques for reducing NO x emission. Recently, Cu-SSZ-13 catalyst has been recognized as a promising candidate for NH3-SCR catalyst for reducing diesel engine NO x emissions due to its wide active temperature window and excellent hydrothermal stability. Despite being commercialized as an advanced selective catalytic reduction catalyst, Cu-SSZ-13 catalyst still confronts the challenges of low-temperature activity and hydrothermal aging to meet the increasing demands on catalytic performance and lifetime. Therefore, numerous studies have been dedicated to the improvement of NH3-SCR performance for Cu-SSZ-13 catalyst. In this review, the recent progress in NH3-SCR performance optimization of Cu-SSZ-13 catalysts is summarized following three aspects: 1) modifying the Cu active sites; 2) introducing the heteroatoms or metal oxides; 3) regulating the morphology. Meanwhile, future perspectives and opportunities of Cu-SSZ-13 catalysts in reducing diesel engine NO x emissions are discussed.

A review on the characterization of metal active sites over Cu-based and Fe-based zeolites for NH3-SCR

Cu-based and Fe-based zeolites are promising catalysts for NH3-SCR due to their high catalytic activity, wide temperature window and good hydrothermal stability, while the detailed investigation of NH3-SCR mechanism should be based on the accurate determination of active metal sites. This review systematically summarizes the qualitative and quantitative determination of metal active sites in Cu-based or Fe-based zeolites for NH3-SCR reactions based on advanced characterization methods such as UV-vis absorption (UV-vis), temperature-programmed reduction with H2 (H2-TPR), X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure spectroscopy (XAFS), Infrared spectroscopy (IR), Electron paramagnetic resonance (EPR), Mössbauer spectroscopy and DFT calculations. The application and limitations of different characterization methods are also discussed to provide insights for further study of the NH3-SCR reaction mechanism over metal-based zeolites.

Catalytic Performance and Sulfur Dioxide Resistance of One-Pot Synthesized Fe-MCM-22 in Selective Catalytic Reduction of Nitrogen Oxides with Ammonia (NH3-SCR)—The Effect of Iron Content

The catalytic performance of Fe-catalysts in selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR) strongly depends on the nature of iron sites. Therefore, we aimed to prepare and investigate the catalytic potential of Fe-MCM-22 with various Si/Fe molar ratios in NH₃-SCR. The samples were prepared by the one-pot synthesis method to provide high dispersion of iron and reduce the number of synthesis steps. We have found that the sample with the lowest concentration of Fe exhibited the highest catalytic activity of ca. 100% at 175 °C, due to the abundance of well-dispersed isolated iron species. The decrease of Si/Fe limited the formation of microporous structure and resulted in partial amorphization, formation of iron oxide clusters, and emission of N₂O during the catalytic reaction. However, an optimal concentration of Fe_xO_y oligomers contributed to the decomposition of nitrous oxide within 250–400 °C. Moreover, the acidic character of the catalysts was not a key factor determining the high conversion of NO. Additionally, we conducted NH₃-SCR catalytic tests over the samples after poisoning with sulfur dioxide (SO₂). We observed that SO₂ affected the catalytic performance mainly in the low-temperature region, due to the deposition of thermally unstable ammonium sulfates.

Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH3. However, their activity is unexpectedly inhibited in the presence of NO2 at low temperatures. This is strikingly distinct from the NO2-accelerated NOx conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as CuII species (fw-Cu2+ and NH3-solvated CuII with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these CuII sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Brønsted acid sites (BASs) rather than the CuII sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.

Understanding the superior NH3-SCR activity on CHA zeolite synthesized via template-free interzeolite transformation

Synthesizing CHA-type zeolite via template-free interzeolite transformation has been considered as a green manner, but the resultant zeolite exhibits low activity in selective catalytic reduction of NOx by ammonia (NH3-SCR)....

Recent Advances in Catalysis Based on Transition Metals Supported on Zeolites

This article reviews the current state and development of thermal catalytic processes using transition metals (TM) supported on zeolites (TM/Z), as well as the contribution of theoretical studies to understand the details of the catalytic processes. Structural features inherent to zeolites, and their corresponding properties such as ion exchange capacity, stable and very regular microporosity, the ability to create additional mesoporosity, as well as the potential chemical modification of their properties by isomorphic substitution of tetrahedral atoms in the crystal framework, make them unique catalyst carriers. New methods that modify zeolites, including sequential ion exchange, multiple isomorphic substitution, and the creation of hierarchically porous structures both during synthesis and in subsequent stages of post-synthetic processing, continue to be discovered. TM/Z catalysts can be applied to new processes such as CO₂ capture/conversion, methane activation/conversion, selective catalytic NO_x reduction (SCR-deNO_x), catalytic depolymerization, biomass conversion and H₂ production/storage.

High-Throughput NOx Removal by Two-Stage Plasma Honeycomb Monolith Catalyst

A two-stage plasma catalyst system for high-throughput NO_x removal was investigated. Herein, the plasma stage involved the large-volume plasma discharge of humidified gas and was carried out in a sandwich-type honeycomb monolith reactor consisting of a commercial honeycomb catalyst (50 mm high; 93 mm in diameter) located between two parallel perforated disks that formed the electrodes. The results demonstrated that, in the plasma stage, the reduction of NO_x did not occur at room temperature; instead, NO was only oxidized to NO₂ and n-heptane to oxygenated hydrocarbons. The oxidation of NO and n-heptane in the honeycomb plasma discharge state was largely affected by the humidity of the feed gas. Furthermore, the oxidation of NO to NO₂ occurs preferably to that of n-heptane with a tendency of the NO oxidation to decrease with increasing feed gas humidity. The reason is that the generation of O₃ decreases as the amount of water vapor in the feed gas increases. Compared to the catalyst alone, the two-stage plasma catalyst system increased NO_x removal by 29% at a temperature of 200 °C and an energy density of 25 J/L.

Meziane K, Kaddouri A, Nguyen HP, Taoufik M. Well-defined surface tungstenocarbyne complex through the reaction of [W(CtBu)(CH2tBu)3] with CeO2: a highly stable precatalyst for NOx reduction with NH3

A highly-efficient NH₃-SCR single site catalyst W(CtBu)(CH₂tBu)₃/CeO_2–200, was prepared by surface organometallic chemistry approach. This catalyst showed high catalytic activity and stability with a broad operational temperature window.

Catalytic Performances of Cu/MCM-22 Zeolites with Different Cu Loadings in NH3-SCR

The NH₃-SCR activities and hydrothermal stabilities of five xCu/MCM-22 zeolites with different Cu loadings (x = 2–10 wt%) prepared by incipient wetness impregnation method were systematically investigated. The physicochemical properties of xCu/MCM-22 zeolites were analyzed by XRD, nitrogen physisorption, ICP-AES, SEM, NH₃-TPD, UV-vis, H₂-TPR and XPS experiments. The Cu species existing in xCu/MCM-22 are mainly isolated Cu²⁺, CuO_x and unreducible copper species. The concentrations of both isolated Cu²⁺ and CuO_x species in xCu/MCM-22 increase with Cu contents, but the increment of CuO_x species is more distinct, especially in high Cu loadings (>4 wt%). NH₃-SCR experimental results demonstrated that the activity of xCu/MCM-22 is sensitive to Cu content at low Cu loadings (≤4 wt%). When the Cu loading exceeds 4 wt%, the NH₃-SCR activity of xCu/MCM-22 is irrelevant to Cu content due to the severe pore blockage effects caused by aggregated CuO_x species. Among the five xCu/MCM-22 zeolites, 4Cu/MCM-22 with moderate Cu content has the best NH₃-SCR performance, which displays higher than 80% NO_x conversions in a wide temperature window (160–430 °C). Furthermore, the hydrothermal aging experiments (xCu/MCM-22 was treated at 750 °C for 10 h under 10% water vapor atmosphere) illustrated that all the xCu/MCM-22 zeolites exhibit high hydrothermal stability in NH₃-SCR reactions.

Rational Design of a Novel Catalyst Cu-SAPO-42 for NH3 -SCR Reaction

Cu-exchanged LTA-type aluminosilicate catalyst has been considered as an efficient catalyst for the selective catalytic reduction of NO_x with ammonia (NH₃ -SCR). However, expensive organic structure-directing agents (OSDAs) and the corrosive fluoride medium are inevitably used to synthesize LTA-type molecular sieve (high-silica LTA-type aluminosilicate and its analogue LTA-type silicoaluminophosphate SAPO-42). Herein, a series of cheap and commercialized OSDAs, which are successfully applied for the targeted synthesis of SAPO-42 in the fluoride-free system, are identified by a novel RSS (refine, summarize, and search) approach. Furthermore, Cu-SAPO-42 catalysts are utilized for NH₃ -SCR. Among these catalysts, Cu-SAPO-42 prepared with 2-(butylamino)ethanol (BAEA) as OSDA demonstrates the excellent activity even after hydrothermal aging at 800 °C for 16 h, which shows much better hydrothermal stability than the commercialized Cu-SAPO-34 catalyst with comparable Si and Cu contents. Electron paramagnetic resonance (EPR) spectroscopy and Rietveld refinement are performed to identify the locations of active Cu²⁺ ions. It turns out that the active Cu²⁺ ions are distributed near the center of single 6-rings of the lta cage.

3D-3D topotactic transformation in aluminophosphate molecular sieves and its implication in new zeolite structure generation

Zeolites have unique pore structures of molecular dimensions and tunable compositions, making them ideal for shape selective catalysis and separation. However, targeted synthesis of zeolites with new pore structures and compositions remains a key challenge. Here, we propose an approach based on a unique 3D-3D topotactic transformation, which takes advantage of weak bonding in zeolites. This is inspired by the structure transformation of PST-5, a new aluminophosphate molecular sieve, to PST-6 by calcination. The structure of nano-sized PST-5 crystals is determined by 3D electron diffraction. We find that the 3D-3D topotactic transformation involves two types of building units where penta- or hexa-coordinated Al is present. We apply this approach to several other zeolite systems and predict a series of new zeolite structures that would be synthetically feasible. This method provides a concept for the synthesis of targeted zeolites, especially those which may not be feasible by conventional methods.

Excellent performance of one-pot synthesized Fe-containing MCM-22 zeolites for the selective catalytic reduction of NOx with NH3

One-pot synthesized OP-Fe/M22 zeolites exhibit excellent performances in NH₃-SCR reactions.

Plasma-Assisted Selective Catalytic Reduction for Low-Temperature Removal of NOx and Soot Simulant

The challenge that needs to be overcome regarding the removal of nitrogen oxides (NOx) and soot from exhaust gases is the low activity of the selective catalytic reduction of NOx at temperatures fluctuating from 150 to 350 °C. The primary goal of this work was to enhance the conversion of NOx and soot simulant by employing a Ag/α-Al2O3 catalyst coupled with dielectric barrier discharge plasma. The results demonstrated that the use of a plasma-catalyst process at low operating temperatures increased the removal of both NOx and naphthalene (soot simulant). Moreover, the soot simulant functioned as a reducing agent for NOx removal, but with low NOx conversion. The high efficiency of NOx removal required the addition of hydrocarbon fuel. In summary, the combined use of the catalyst and plasma (specific input energy, SIE ≥ 60 J/L) solved the poor removal of NOx and soot at low operating temperatures or during temperature fluctuations in the range of 150–350 °C. Specifically, highly efficient naphthalene removal was achieved with low-temperature adsorption on the catalyst followed by the complete decomposition by the plasma-catalyst at 350 °C and SIE of 90 J/L.