SCR Of NO Research Articles

Construction of silicon-rich hollow nanotubes embedded in hierarchical SSZ-13 and enhanced SCR high-temperature activity

The meso-channel structure of the SSZ-13 molecular sieve can both slightly speed up the diffusion of reactants or products and lower the chance of pore blockage. Micro-mesoporous SSZ-13 was constructed using hydroxylated carbon nanotubes (CNTs) as the hard template. CNTs not only introduced silica-rich and aluminum-poor hollow nanotubes to SSZ-13, but also shortened the crystallization period of SSZ-13 by no less than 24 h. SSZ-13–0.04 underwent a “crystallization-dissolution-re-crystallization” process during the 24th and 96th hours of crystallization. Compared with the conventional SSZ-13, the SSZ-13 constructed with adamantane-carbon nanotubes as the double template showed better adsorption capacity for NO, stronger acidity, and higher SCR high-temperature activity.

Exploring the Synthesis of Mesoporous In/H-β-NC Catalysts by Desilication Using CTAB Combined with NaOH and Its Application in CH4–SCR of NO

Exploring the Synthesis of Mesoporous In/H-β-NC Catalysts by Desilication Using CTAB Combined with NaOH and Its Application in CH₄–SCR of NO

Role of Vanadium in Thermal and Hydrothermal Aging of a Commercial V2O5-WO3/TiO2 Monolith for Selective Catalytic Reduction of NOx: A Case Study

In recent years, increased attention to air pollutants such as NOx has led the scientific community to focus meaningfully on developing strategies for NOx reduction. Selective catalytic reduction of NOx by ammonia (NO SCR by NH3) is currently the main method to remove NOx from diesel engine exhaust emissions. The catalysts with typical V2O5-WO3/TiO2 (VWTi) composition are widely used in NH3-SCR for their high NOx conversion activity, low cost, and robustness, especially concerning sulfur poisoning. However, in real diesel engine working conditions, the thermal and hydrothermal aging of catalysts can occur after several hours of operation at high temperature, affecting the catalytic performance. In this study, the stability of a commercial VWTi monolith, self-supported and containing glass fibers and bentonite in its matrix, was investigated as a case study. In laboratory conditions, NO SCR tests were performed for 50 h in the range of 150 to 350 °C. Subsequently, the VWTi monolith was thermally and hydrothermally aged at 600 °C for 6 h. The thermal aging increased the NOx conversion, especially at low temperature (<250 °C), while the hydrothermal aging did not affect the SCR. The differences in NOx conversion before and after aging were associated with the change in vanadium and tungsten oxide surface coverage and with the reduction in the surface area of catalysts. In order to correlate the change in SCR activity with the modifications occurring after aging processes, the monolithic samples were characterized by several techniques, namely XRD, SSA and pore analysis, TPR, XPS, Raman, TGA and SEM/EDX.

Significant enhanced SO2 resistance of Ce-doped OMS-2 catalyst by building the CeO2 shell for the low-temperature SCR of NO with NH3

Developing highly SO2-resistance catalysts for the NOx reduction at low temperatures is a significant challenge. In this study, we fabricate a catalyst of Ce-doped manganese oxide octahedral molecular sieve by building the CeO2 shell (Ce-OMS-2@CeO2) to solve this problem. This catalyst showed high selective catalytic reduction with NH3 (NH3-SCR) activity and enhanced resistance to SO2-induced deactivation. Ce-OMS-2@CeO2 exhibited outstanding NH3-SCR activity and anti-sulfur performance at 200 °C. The mesoporous CeO2 shells exerted no influence on the transport of reactants to catalytic active sites. The NH3-SCR on Ce-OMS-2@CeO2 catalyst occurred via the Langmuir-Hinshelwood mechanism with adsorption of NH3 and NO on different sites, thus preventing competing adsorption. The CeO2 shell bound SO2 strongly in priority, thus protecting the Ce-OMS-2 core from SO2 poisoning. Moreover, a dynamic balance was established between NH4HSO4 formation and decomposition over our Ce-OMS-2@CeO2 catalyst at low temperatures, which also hindered the SO2 poisoning in NH3-SCR reaction.

Elucidating the role of the state of Pd in the H2-SCR of NOx by operando XANES and DRIFTS

Characterization of a Pd/V2O5/TiO2 catalyst by operando XAS and DRIFTS links the Pd state to reactive intermediates and resulting products for the selective catalytic reduction of NOx by H2.

Revealing the influencing mechanism of CO2 on SCR of NOx with NH3 over FeW mixed oxide catalyst: interactions of carbonate-Fe2O3–FeWO4

The effect of CO2, a major component of the exhaust gas, on the elimination of NOx over FeW oxide catalyst was studied.

Efficient MnFe/Al2O3 catalyst for NH3-SCR of NO at low temperature: the influence of strong interactions between active components and the carrier

Supported MnFeOx catalysts have been prepared using Al2O3 as the substrate and were investigated for NH3-SCR.

Design Strategy of the MnOx Catalyst for SCR of NO with NH3: Mechanism of Lead Poisoning and Improvement Method.

The conventional Mn-based catalysts suffer from lead toxicity and require other transition-metal oxides to enhance their resistance in the selective catalytic reduction of NOx with ammonia (NH3-SCR). Herein, we found that the incorporation of inert silica into pure MnOx effectively improved the Pb resistance. The NOx conversion of the MnOx-SiO2-Pb catalyst was nearly 55% higher than that of the MnOx-Pb catalyst, exhibiting enhanced activity at lower temperatures (150-225 °C). To reveal the essential roles at the molecular level, the types and numbers of surface acidity, nitrate species, and catalytic cycle were established through experimental analysis and theoretical calculations of catalysts. The presence of PbCl2 occupied the active Mn sites, resulting in an obvious decline in the Brønsted acid sites (B-NH4+) and the oxidation performance, and the NH3-SCR cycle was energetically less favorable on the MnOx-Pb catalyst. Conversely, SiO2 played a crucial role in preserving the activity of Mn sites on the MnOx-SiO2-Pb catalyst by preferentially bonding with PbCl2, generating more active intermediates. Significantly, this work provided mechanistic insights into the role of SiO2 in regulating the surface acidity, oxidation performance, and stability of active Mn sites, which is helpful for the design of Mn-based catalysts with high Pb resistance for the NH3-SCR reaction.

Enhanced Catalytic Activity of Pd Supported on TiO2 Nanowire for the H2-SCR of NOx in the Presence of Oxygen

Enhanced Catalytic Activity of Pd Supported on TiO₂ Nanowire for the H₂-SCR of NO_<i>x</i> in the Presence of Oxygen

Superior catalytic performance of Zr-incorporated MnCu/SBA-15 catalyst for low-temperature NH3-SCR of NO: Effect of support

High surface area mesoporous solid support has attracted much interest in the development of cost-effective catalysts for NOx abatement at low temperatures. SBA-15, a high surface area mesoporous material, was employed in the preparation of Al, Ti, and Zr incorporated SBA-15 through a direct synthesis approach under long hydrothermal reaction conditions (100 °C for 72 h). Heteroatom incorporation facilitated change in the morphology and textural properties of the material and was applied as a catalytic supports in order to facilitate the development of a MnCu bimetallic catalyst by a facile conventional co-impregnation method. At temperatures as low as 260 °C, the catalyst that was developed displayed superior deNOx activity for SCR of NO with NH3. There have been various physicochemical studies undertaken on the catalysts that were prepared, such as XRD, XPS, BET, HR-SEM, and HR-TEM. The deNOx optimization and deNOx activity over the catalysts are discussed. The exceptional deNOx activity at low temperatures may be attributed to the rod-like morphology, extensive oxygen vacancies, and high ratio of Mn3+ + Mn4+/Mn on 10Mn4Cu/Zr-SBA-15 catalyst. Furthermore, the catalyst also displayed a synergetic interaction between the Mn and the Zr species on its surface, which contributed to the catalyst's superior catalytic recycle ability and its long-term stability on stream stability. A useful approach of preparation is provided in order to accomplish the industrial use of a bimetallic catalyst consisting of manganese and copper at a low cost.

Design of Ca-type todorokite catalysts with highly active for the selective reduction of NOx by NH3 at low temperatures

Ca-type todorokite catalysts were designed and prepared by a simple redox method and applied to the selective reduction of NOx by NH3 (NH3-SCR) for the first time. Compared with the Na-type manjiroite prepared by the same method, the todorokite catalysts with different Mn/Ca ratios showed greatly improved catalytic activity for NOx reduction. Among them, Mn8Ca4 catalyst exhibited the best NH3-SCR performance, achieving 90% NOx conversion within temperature range of 70–275°C and having a high sulphur resistance. Compared to the Na-type manjiroite sample, Ca-type todorokite catalysts possessed an increased size of tunnel, resulting in a larger specific surface area. As increased the amounts of Ca doping, the Na content in Ca-type todorokite catalysts significantly decreased, providing larger amounts of Brønsted acid sites for NH3 adsorption to produce NH4+. The NH4+ species were highly active for reaction with NO + O2, playing a determining role in NH3-SCR process at low temperatures. Meanwhile, larger amounts of surface adsorbed oxygen contained over the Ca-doping samples than that over Na-type manjiroite, promoting the oxidation of NO and fast SCR processes. Over the Ca-type todorokite catalysts, furthermore, nitrates produced during the flow of NO + O2, were more active for reaction with NH3 than that over Na-type manjiroite, benefiting the occurrence of NH3-SCR process. This study provides novel insights into the design of NH3-SCR catalysts with high performance.

Modulating active oxygen species on α-MnO2 with K and Pb for SCR of NO at low temperatures

In this study, a completely new mechanism for the effect of K and Pb on α-MnO2 was proposed. The findings provide valuable insights on the roles of active oxygen species and oxygen vacancies in the low-temperature SCR process.

In situ synthesis of low-valence MnO/Mn3O4 catalyst via carbon-acid hydrothermal strategy for NO removal

A rational design of high dispersion MnO/Mn 3 O 4 catalysts based on carbon-acid hydrothermal strategy was proposed for selective catalytic reduction of NO at low temperature. The ultra-high denitration rate can be attributed to the abundant Lewis acid sites and reactive oxygen species, and the reaction followed Langmuir-Hinshelwood reaction route. • High dispersed MnO/Mn 3 O 4 catalyst was prepared by carbon-acid hydrothermal method. • Catalyst achieved exceeding 98 % NO removal in the temperature window of 80–200 °C. • Rich Lewis acid sites and reactive oxygen species contributed to high SCR activity. • In-situ DRIFTS proved the SCR reaction process followed Langmuir-Hinshelwood route. A rational design of high dispersion MnO/Mn 3 O 4 catalysts based on carbon-acid hydrothermal strategy was proposed for selective catalytic reduction of NO at low temperature. The reaction mechanisms of low valence manganese coordination and catalytic path were revealed through active site analysis and in-situ DRIFTS characterization. The results showed that reasonable low valence manganese nanofibers cooperated to derive hierarchical mesoporous structure and abundant surface active oxygen. The chemisorbed oxygen specie cooperated with Lewis acid sites accelerated the production of NO 2 in the catalytic process and realized high-efficiency NO removal at ultra-low temperature. Such a facile preparation method of low valence manganese nanofibers structure widens the way for NH 3 -SCR of NO in industrial practice.

Monomeric Fe Species in Square Planar Geometry Active for Low Temperature NH3-SCR of NO

Monomeric Fe species in zeolites are considered to be the active sites for the low temperature activity toward ammonia-assisted selective catalytic reduction of nitrogen oxides (NH3-SCR) in exhaust gases. Herein, we report on a preparation method to synthesize single-site Fe/ZSM-5 by combination of dealumination with the use of a bulky iron complex to introduce Fe. Transient in situ XAS experiments at the Fe K-edge under dynamic NH3-SCR reaction conditions demonstrated the involvement of all iron atoms in the redox cycle and, with that, suggest that Fe species in the studied Fe/ZSM-5 catalyst are monomers. Simulation of experimental X-ray absorption data revealed that the Fe species in the sample obtained by this approach adopt a square planar geometry, which changes into fivefold coordination at low temperature by strongly binding either NO or NH3 as the ligand. The presence of monomeric Fe species in this Fe/ZSM-5 conveys molecular level insights into the temperature-dependent NH3-SCR activity and might prove useful in the study of other reactions over monomeric Fe species, such as methane partial oxidation or dehydroaromatization reactions.

Synthesis of Cu-SAPO-RHO Catalysts by Template-Assisted Ion Incorporation and Their Catalytic Performance for NH3-SCR of NOx

Synthesis of Cu-SAPO-RHO Catalysts by Template-Assisted Ion Incorporation and Their Catalytic Performance for NH₃-SCR of NO_<i>x</i>

Poisoning mechanism of KCl, K2O and SO2 on Mn-Ce/CuX catalyst for low-temperature SCR of NO with NH3

In this study, a series of K species or/and SO2 poisoned Mn-Ce doped CuX (MCCX) catalysts were prepared from waste blast furnace slag (BFS), and the influence mechanism of K species or/and SO2 poisoning on the catalysts in low-temperature NH3-SCR process was investigated. The results demonstrated that MCCX-KCl catalyst had poorer NO conversion than MCCX-K2O below 200 °C, and the co-existence of K species and SO2 considerably inhibited NO conversion. Aggregation of the Cu active component might also be facilitated by co-poisoning with K species and SO2. Furthermore, K species and SO2 co-poisoned catalysts showed fewer isolated Cu2+ species and Mn4+ species than that of K-poisoned catalysts, indicating a decrease in active sites after the addition of SO2. The co-effect of K2O/KCl and SO2 species impaired the redox capacity and decreased the surface acidity than K-poisoned catalysts to a greater extent. Nevertheless, K-poisoning had little effect on the surface intermediates during SCR reaction process, and K-poisoned catalysts followed both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanism. However, MCCX-KCl-SO2 and MCCX-K2O-SO2 catalysts contained fewer nitrate species, indicating that the co-effect of K species and SO2 impeded the reaction between NO + O2 species and pre-NH3 species, thereby resulting in poor low-temperature NH3-SCR activity.

Insights into the highly activity CuMgFe oxides for the selective catalytic reduction of NO by CO: Structure-activity relationships and K/SO2 poisoning mechanism

The selective catalytic reduction of NO by CO is a promising technology for the simultaneous removal of both NO and CO pollution. Hence, a series of CuMgFe composite metal oxides with flower-like morphology was prepared by co-precipitation method for SCR of NO by CO. CuMgFe oxide calcined at 500 °C (CuMgFe-500) exhibited excellent low temperature activity. NO conversion reaches 100% at 225 °C, and it exhibits superior anti-H2O/SO2 ability (250 °C). The excellent activity of CuMgFe-500 was associated with its abundance of surface oxygen vacancies and excellent redox performance. The reaction mechanism of CuMgFe-500 CO catalytic reduction of NO was investigated by in situ DRIFTS. The results show that monodentate nitrate and labile carbonate intermediate species are generated on the catalyst surface. Its reaction follows the E-R mechanism and the L-H mechanism. The introduction of K reduced the surface oxygen adsorption and redox properties of CuMgFe-500 catalyst, which were the main reason for the poor low-temperature activity of K-poisoned catalyst. In situ DRIFTS analysis showed that SO2 hardly inhibits NO adsorption, which were the main reason for the excellent sulfur resistance of CuMgFe-500. This work provides new insights into the development and design of low-temperature CO-SCR catalysts.

Cover Feature: Role of V and W Sites in V2O5−WO3/TiO2 Catalysts and Effect of Formaldehyde during NH3−SCR of NOx (ChemCatChem 15/2022)

The Cover Feature illustrates the effect of different sites in V2O5-WO3/TiO2 catalysts on the conversion of NOx and HCHO in vehicle exhaust gas. In their Research Article, A. Brückner and co-workers show that two vanadyl sites coexist in those catalysts. Only the highly redox-active V-O-W moiety can convert NOx completely to eco-friendly N2 and H2O. Unfortunately, HCHO is transformed to toxic HCN on both sites. More information can be found in the Research Article by A. Brückner and co-workers.

Role of V and W Sites in V2O5−WO3/TiO2 Catalysts and Effect of Formaldehyde during NH3−SCR of NOx

AbstractWO3/TiO2, V2O5/TiO2 and V2O5−WO3/TiO2 catalysts (1 wt % V, 6 wt % W) were tested in NH3‐SCR of NOx with/without formaldehyde and water and analyzed by in situ FTIR and operando EPR spectroscopy. Activity decreased in the order V−W−Ti&gt;V−Ti≫W−Ti. Two VO3+/VO2+ sites with different redox behavior exist in V‐containing catalysts. Sites I were less active with restricted accessibility by reactants while activity is mainly driven by undercoordinated sites II exposed on the surface. In V−W−Ti, sites II can form W6+−O(H)−V5+ bridges, accelerating surface acidity and V reduction rate. HCHO lowers NOx conversion and N2 selectivity by reacting with NH3 to formamide that decomposes to HCN. Formation of formamide is promoted by high surface acidity and redox activity. Water lowers NH3 conversion by creating Brønsted sites that adsorb NH3 and suppress oxidation to N2O, thereby improving N2 selectivity.

Promotional effect for SCR of NO with CO over MnO -doped Fe3O4 nanoparticles derived from metal-organic frameworks

Abstract MnOx-Fe3O4 nanomaterials were fabricated by using the innovative scheme of pyrolyzing manganese-doped iron-based metal organic framework in inert atmosphere and exhibited extraordinary performance of NO reduction by CO (CO-SCR). Multi-technology characterizations were conducted to ascertain the properties of fabricated materials (e.g., TGA, XRD, SEM, FT-IR, XPS, BET, H2-TPR and O2-TPD). Moreover, the interaction between reactants and catalysts was ascertained by in situ FT-IR. Experimental results demonstrated that Mn was an ideal promoter for iron oxides, resulting in decrease of crystallite size, improve reducibility property, enhance the mobility and the amount of lattice O2– species, as well as strength the adsorption ability of active NO and CO to form multiple species (e.g., nitrate and carbonate). The unprecedented enhancement of CO-SCR activity over Mn-Fe nanomaterials follows the Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) reaction pathway.