Fe-Exchanged Small-Pore Zeolites as Ammonia Selective Catalytic Reduction (NH3-SCR) Catalysts

Mechanisms for deNOx and deN2O Processes on FAU Zeolite with a Bimetallic Cu-Fe Dimer in the Presence of a Hydroxyl Group-DFT Theoretical Calculations

In this paper, a detailed mechanism is discussed for two processes: deNOx and deN2O. An FAU catalyst was used for the reaction with Cu-Fe bimetallic adsorbates represented by a dimer with bridged oxygen. Partial hydration of the metal centres in the dimer was considered. Ab initio calculations based on the density functional theory were used. The electron parameters of the structures obtained were also analysed. Visualisation of the orbitals of selected structures and their interpretations are presented. The presented research allowed a closer look at the mechanisms of processes that are very common in the automotive and chemical industries. Based on theoretical modelling, it was possible to propose the most efficient catalyst that could find potential application in industry-this is the FAU catalyst with a Cu-O-Fe bimetallic dimer with a hydrated copper centre. The essential result of our research is the improvement in the energetics of the reaction mechanism by the presence of an OH group, which will influence the way NO and NH3 molecules react with each other in the deNOx process depending on the industrial conditions of the process. Our theoretical results suggest also how to proceed with the dosage of NO and N2O during the industrial process to increase the desired reaction effect.

Structure Features and Physicochemical Performances of Fe-Contained Clinoptilolites Obtained via the Aqueous Exchange of the Balanced Cations and Isomorphs Substitution of the Heulandite Skeletons for Electrocatalytic Activity of Oxygen Evolution Reaction and Adsorptive Performance of CO2

Fe(III)-modified clinoptilolites (Fe-CPs) were prepared by hydrothermal treatment. The collapse of the heulandite skeletons was avoided by adjusting the pH value using HCl solution, showing the maximum relative crystallinity of the Fe-CPs at an optimal pH of 1.3. The competitive exchange performances between Fe³⁺ ions and H⁺ with Na⁺ (and K⁺) suggested that the exchange sites were more easily occupied by H⁺. Various characterizations verified that the hydrothermal treatments had a strong influence on the dispersion and morphology of the isolated and clustered Fe species. The high catalytic activity of the oxygen evolution reaction indicated the insertion of Fe³⁺ into the skeletons and the occurrences of isomorphic substitution. The fractal evolutions revealed that hydrothermal treatments with the increase of Fe content strongly affected the morphologies of Fe species with rough and disordered surfaces. Meanwhile, the Fe(III)-modified performances of the CPs were systematically investigated, showing that the maximum Fe-exchange capacity was up to 10.6 mg/g. Their thermodynamic parameters and kinetic performances suggested that the Fe(III)-modified procedures belonged to spontaneous, endothermic, and entropy-increasing behaviors. Finally, their adsorption capacities of CO₂ at 273 and 298 K were preliminarily evaluated, showing high CO₂ adsorption capacity (up to 1.67 mmol/g at 273 K).

Influence of the Valence of Iron on the NO Reduction by CO over Cu-Fe-Mordenite

A comprehensive study of the catalytic properties of the copper-iron binary system supported on mordenite, depending on the iron valence—CuFe2MOR and CuFe3MOR—was carried out, and redox ability has been considered as a decisive factor in determining catalytic efficiency. Acidity was studied by TPD-NH3, DRIFT-OH, and DRT methods. The total acidity of both samples was high. The Brönsted acidity is similar for both bimetallic samples and is explained by the acidity of zeolite; Lewis acidity varies greatly and depends on the exchange cations. A screening DRIFT study of CO and NO has shown redox capacity and demonstrated a potential for using these materials as catalysts for ambient protection. CuFe2MOR demonstrated stable Cu and Fe species, while CuFe3MOR showed redox dynamic species. As expected, CuFe3MOR displayed higher catalytic performance in NO reduction via CO oxidation, because of the easily reduced intermediate NO-complex adsorbed on the metallic Cu and Fe sites, which were observed through in situ DRIFT study.

Influence of Components Deposition Order on Silver Species Formation in Bimetallic Ag-Fe System Supported on Mordenite

In the present work, various experimental and theoretical methods were combined to study in detail the modifying effect of differences in the order of deposition of components on the state of silver in bimetallic iron–silver samples based on mordenite. In each of the silver-containing samples, the formation of large (≥2 nm in diameter) varieties of silver was observed, which differed from the varieties in the other samples, and in varying degrees. The formation of large Ag NPs on the outer surface of mordenite is explained by the redox interaction of Ag+-Fe2+ and the selectivity of ion exchange. The local surrounding of Ag in the studied samples is different: for AgMOR—monatomic species dominate, FeAgMOR—silver dimers and AgFeMOR—metal particles. In all investigated samples, the partially charged intra-channel Agnδ+ clusters (~0.7 nm in size) were formed due to partial Ag+ reduction and subsequent Ag0 agglomeration into the mordenite channel. Most of the silver in the bulk of the zeolite is represented in the cationic state attached to the mordenite framework by differently coordinated electrostatic forces, which can be Ag-O, Ag-Si or Ag-Al, with variations in interatomic distances and do not depend on the order of metal deposition. In addition, the arrangement of the cations in the side pockets means that the transport channels of mordenite are free, which is favorable for the application of the materials under study in catalysis and adsorption.

The Effect of Iron Content on the Ammonia Selective Catalytic Reduction Reaction (NH3-SCR) Catalytic Performance of FeOx/SAPO-34

Iron-based catalysts are regarded as promising candidates for the ammonia selective catalytic reduction reaction (NH₃-SCR) which show good catalytic activity at medium and high temperatures, whereas SAPO-34 molecular sieves have a micro-pore structure and are ideal catalyst carriers. In this paper, four FeO_x/SAPO-34 molecular sieve catalysts with different iron contents (Fe = 1%, 2%, 3%, 4%) were prepared using an impregnation method. The effect of iron content on the surface properties and catalytic activity was investigated by a series of characterization techniques including XRD, SEM, BET, XPS, H₂-TPR and NH₃-TPD. Iron species in the FeO_x/SAPO-34 catalysts exist in the form of isolated iron ions or well-dispersed small crystals and iron oxide species clusters. With the addition of iron content, the integrity of CHA (chabazite) zeolite structure remained, but the crystallinity was affected. The FeO_x/SAPO-34 catalyst with 3% Fe loading showed a relatively flat surface with no large-diameter particles and strong oxidation-reduction ability. Meanwhile, more acidic sites are exposed, which accelerated the process of catalytic reaction. Thus, the FeO_x/SAPO-34 catalyst with 3% Fe showed the best NO conversion performance among the four catalysts prepared and maintained more than 90% NO conversion efficiency in a wide temperature range from 310 °C to 450 °C.

Pinpointing basic sites formed upon incorporation of iron in hierarchical SAPO-11 using catalytic model reactions

By utilizing previously established catalytic model reactions, a method for probing the topological location of transition metal sites incorporated in hierarchical silicoaluminophosphates (SAPOs) is presented. For the first time, iron(III)-incorporated hierarchical SAPO-11 (FeCTAB-11) was prepared and thoroughly characterized with conventional iron(III)-incorporated SAPO-11 (FeSAPO-11) as a reference. Initially, inductively coupled plasma - mass spectrometry (ICP-MS) indicated that the FeSAPOs contained similar amounts of metal (∼2.0 wt%), while N₂-physisorption confirmed the bimodal porosity of the hierarchical FeCTAB-11. Furthermore, X-ray absorption spectroscopy (XAS) revealed that iron(III) was isomorphously incorporated into both SAPO-11 samples, whereas CO₂-temperature programmed desorption (TPD) revealed the first reported presence of strong basic sites in the vicinity of a transition metal incorporated into a SAPO framework. The location of the basic sites, and thus the incorporated iron, was subsequently probed by studying the products of the base-catalyzed vapor phase isomerization of cyclohexanone oxime (Beckmann rearrangement, BMR) model reaction. Through an increased lifetime for the base-catalyzed production of aniline, the incorporated iron for FeCTAB-11 was found to be located in highly accessible mesopores, whereas the conventional FeSAPO-11 had incorporated iron located in its micropores. Lastly, the methanol-to-hydrocarbons (MTH) model reaction showed that both FeSAPOs only had Brønsted acid sites in the micropores of the structures. This was used to verify the pore connectivity of the hierarchical FeCTAB-11 by utilizing the base-catalyzed BMR mechanism's dependency on acid sites.

Boosting SO2-Resistant NOx Reduction by Modulating Electronic Interaction of Short-Range Fe-O Coordination over Fe2O3/TiO2 Catalysts

SO₂-resistant selective catalytic reduction (SCR) of NO_x remains a grand challenge for eliminating NO_x generated from stationary combustion processes. Herein, SO₂-resistant NO_x reduction has been boosted by modulating electronic interaction of short-range Fe-O coordination over Fe₂O₃/TiO₂ catalysts. We report a remarkable SO₂-tolerant Fe₂O₃/TiO₂ catalyst using sulfur-doped TiO₂ as the support. Via an array of spectroscopic and microscopic characterizations and DFT theoretical calculations, the active form of the dopant is demonstrated as SO₄^2- residing at subsurface TiO₆ locations. Sulfur doping exerts strong electronic perturbation to TiO₂, causing a net charge transfer from Fe₂O₃ to TiO₂ via increased short-range Fe-O coordination. This electronic effect simultaneously weakens charge transfer from Fe₂O₃ to SO₂ and enhances that from NO/NH₃ to Fe₂O₃, resulting in a remarkable "killing two birds with one stone" scenario, that is, improving NO/NH₃ adsorption that benefits SCR reaction and inhibiting SO₂ poisoning that benefits catalyst long-term stability.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

How changes in Cu-SSZ-13 catalytic oxidation activity via mild hydrothermal aging influence sulfur poisoning extents

Sulfur poisoning of Cu-SSZ-13 is a function of the catalyst's oxidation ability.

Enhanced hydrothermal stability and SO2-tolerance of Cu–Fe modified AEI zeolite catalysts in NH3-SCR of NOx

Cu/Fe-SSZ-39 zeolites with AEI structure show significant enhancements on the NH3-SCR reactivity, hydrothermal stability and SO2-tolerance, which makes them very promising in deNOx applications.

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.

Modified Layered Silicas as Catalysts for Conversion of Nitrogen Pollutants in Flue Gases—A Review

This paper is focused on the recent achievements in the studies of modified layered zeolites and cationic layered clay minerals. These materials are very promising catalysts in green chemistry processes, such as selective catalytic reduction of NOx with ammonia (NH3-SCR) and selective catalytic oxidation of ammonia to dinitrogen (NH3-SCO). Special attention is paid to the roles of the micro- and mesoporous structures of the catalytic materials, the type and location of deposited transition metals, as well as surface acidity in the design of effective catalysts for the NH3-SCR and NH3-SCO processes. The majority of the presented analysis is based on the authors’ research.

Selective Catalytic Reduction: From Basic Science to deNOx Applications

The combustion of coal, petroleum and biofuel for energy generation has resulted in unprecedented benefits to the prosperity of our modern society and will continue to do so in the future [...]

One-pot synthesis of FeCu–SSZ-13 using Cu–TEPA as the template by adding iron complexes

Hydrolysis of cyanide and “sustained-release” of iron during hydrothermal synthesis.