Facile H2O-Contributed O2 Activation Strategy over Mn-Based SCR Catalysts to Counteract SO2 Poisoning

Redox-induced engineering of amorphous/crystalline MnFeOx catalyst enables H2O/SO2-tolerant NOx abatement at ultra-low temperatures

Enhancing resistance to H2O and SO2 poisoning below 150 °C is essential for advancing Mn-based oxide catalysts in ultra-low temperature NH3-SCR of NO. To address this challenge, an amorphous/crystalline MnFey catalyst with engineered Mn-O-Fe interfaces and abundant surface defects was developed using a redox-induced precipitation method. The optimized MnFe0.2 catalyst demonstrates exceptional catalytic performance, achieving over 90 % NO conversion and N2 selectivity across a broad 120-260 °C range under highly humid conditions (15 vol% H2O). Most significantly, MnFe0.2 maintains remarkable stability under high humidity and SO2 at 120 °C for 60 h, vastly outperforming conventionally coprecipitated MnFe0.2(CP), which gradually deactivates. This superior performance is attributed to the uniform elemental distribution in MnFe0.2, which enhances the Mn-O-Fe redox cycle through improved electron transfer. These features promote superior low-temperature reducibility and acidity, enabling effective reactant adsorption and activation. Mechanistic studies further reveal that SO2 exposure deactivates MnFe0.2(CP) by forming ammonium (bi)sulfates and MnSO4, which hinder reactant adsorption and subsequent reactions. In contrast, the engineered Mn-O-Fe interfaces in MnFe0.2 enable Fe species to preferentially interact with SO2, shielding Mn from sulfation and significantly reducing deactivation. This work demonstrates a significant breakthrough in catalyst design for ultra-low temperature NH3-SCR, paving the way for the broader application of Mn-based catalysts in industrial NOx control technologies.

Insights into the Synergistic Promotion Effect of Cu and W Modification on the Catalytic Performance of α-Fe2O3 for NH3-SCR of NOx

A dual-element modification strategy was proposed to promote the catalytic performance of the α-Fe2O3 catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR of NOx) at both low and high temperatures. By optimizing the loading amount of CuO (4 wt %) and WO3 (5 wt %), a wide operating temperature window (150-350 °C) was achieved on the modified α-Fe2O3 catalyst (W5/Cu4/Fe). Further characterizations revealed that the enhanced low-temperature activity could be attributed to the improved redox performance of α-Fe2O3 through CuO modification, while the superior high-temperature activity was primarily ascribed to the enhanced surface acidity induced by WO3 modification. The synergistic effect of CuO and WO3 modifications facilitated the NH3-SCR reaction on the modified α-Fe2O3 catalysts to efficiently proceed through the Eley-Rideal (E-R) mechanism pathway. This provided compelling evidence that the catalytic performance of NH3-SCR catalysts at different temperatures was dominantly governed by distinct factors (e.g., redox property and surface acidity), offering valuable insights for the rational design of robust catalysts for NOx abatement.

NOx Reconstruction Triggered by Zeolite Reverses Alkali Metal Poisoning in NO Selective Catalytic Reduction

Alkali metal poisoning represents a formidable challenge for the effective utilization of ammonia selective catalytic reduction (NH3-SCR) technology. Herein, we propose a versatile strategy to radically circumvent the deactivation effect from alkali metals via regulated activation of surface NOx species. Activity test showed that after physical mixing with a series of zeolites (ZSM-5, MOR, and SAPO-34), the activity of the K-poisoned CeO2-MnOx catalyst achieved complete recovery, and the observed NO conversion was found to be even superior to that of the fresh catalyst. Mechanism analysis from in situ DRIFTS and TPSR revealed that K deposition shut off the transformation of surface NOx from chelating bidentate nitrites to bidentate nitrates (both bridging and chelating bidentate nitrates), leading to catalyst deactivation. Zeolite coupling introduced labile NO+ species, which interacted facilely with the chelating bidentate nitrites to generate chemically reactive bidentate nitrates, enabling a thorough regeneration of NH3-SCR performance. In addition to CeO2-MnOx, this strategy was also found to be valid for a variety of NH3-SCR catalysts, demonstrating great potential in reversing alkali metal poisoning.

Metal ion-modulated synthesis of γ-MnO2 nanosheet for catalytic oxidative degradation of clomiprazole

Two-dimensional non-layered oxide nanosheets exhibit exceptional catalytic properties, offering significant potential for environmental applications. In this study, we report the development of a novel Fe-doped γ-MnO2 material with a hierarchical microsphere morphology, achieved through a metal ion regulation strategy. Unlike conventional sea urchin-like γ-MnO2, Fe doping induced a transformation to a two-dimensional non-layered structure composed of nanosheets, significantly increasing the specific surface area and exposing more active sites. The Fe-doped γ-MnO2 catalysts were evaluated for the degradation of chlorimiprazole (CBZ), a persistent pollutant, using a sulfate radical-based advanced oxidation process. Among the synthesized catalysts, NF-0.25Fe exhibited superior performance, achieving 93% CBZ removal within 16 min under near-neutral conditions. This exceptional activity was attributed to the optimized morphology, higher low-valence Mn content, and enhanced surface-active oxygen species. Systematic investigations revealed that the catalyst dosage, PMS concentration, and pH critically influenced the catalytic efficiency. This work demonstrates the potential of metal ion modulation in tailoring the structural and catalytic properties of transition metal oxides. The insights gained here provide a robust foundation for designing advanced nanomaterials for environmental remediation and other catalytic applications.

SO2-Promoted Molecular Oxygen Activation and NO Oxidation on γ-MnO2

The activation of molecular oxygen on transition metal oxide surfaces plays a crucial role in atmospheric chemistry and heterogeneous catalysis; however, understanding the intricate mechanisms remains a challenge. In this study, we elucidate for the first time the role of sulfur dioxide (SO2) in enhancing oxygen activation on manganese oxide (γ-MnO2) surface, thereby facilitating the oxidation of nitric oxide (NO) to nitrogen dioxide (NO2). The theoretical calculation results further demonstrate that the adsorbed SO2 and the formed sulfate can promote the adsorption of O2 and the reactivity of surface reactive oxygen species toward NO oxidation on γ-MnO2 (110) surface, respectively. During SO2 with O2 reaction, the formation of surface sulfate and surface-active oxygen atoms serves as new active sites to facilitate the oxidation of NO to NO2. Comparative analysis of the energy profiles reveals that NO oxidation with SO2 can release more heat energy than that without SO2, indicating enhanced thermodynamic accessibility in the presence of SO2. Therefore, a novel mechanism for the sulfate-mediated activation of O2 is proposed, which sheds light on the synergistic effects of multiple pollutants in heterogeneous reactions and is significant for understanding NO oxidation both in the atmosphere and in exhaust treatment.

Modulating NH3 oxidation and inhibiting sulfate deposition to improve NH3-SCR denitration performance by controlling Mn/Nb ratio over MnaNbTi2Ox (a = 0.6-0.9) catalysts

The MnaNbTi2Ox (a = 0.6-0.9) catalysts for NH3 selective catalytic reduction denitration were prepared using the co-precipitation method. Among them, Mn0.7NbTi2Ox exhibits well low-temperature catalytic performance, wide activity temperature range (180-480 ℃), and worthy resistance to SO2 even with H2O. XRD was used to investigate the structure of MnaNbTi2Ox, in which Mn and Nb oxides highly dispersed in the MnaNbTi2Ox catalysts and Nb can dope into the crystal lattice of TiO2. XRF and XPS results show Nb can affect the transfer of electrons to Mn4+, and changing the Mn/Nb ratio can regulate the Mn4+ content on the MnaNbTi2Ox catalysts. H2-TPR, NH3 and NO oxidation results verify that Nb inhibits the oxidation capacity of MnaNbTi2Ox, and altering the Mn/Nb ratio can get appropriate oxidation property, which facilitates low-temperature NH3 activation and limit non-selective oxidation for NH3. In-situ DRIFTS results show Nb-OH bonds can provide new Brønsted acid sites, and both Lewis and Brønsted acid sites are active. Furthermore, Nb addition prevents sulphate deposition on the catalyst. The effect of Mn/Nb on catalytic performance, N2O formation and inhibition, SO2 poisoning, SO2 effect on NH3-SCR, and the enhancement of SO2 tolerance are also analyzed.

Facet-Dependent Photocatalytic CO2 Reduction on TiO2 Crystals in the Presence of SO2: Role of Surface Hydroxyl

Photocatalytic CO2 reduction to solar energy makes great sense to mitigate the greenhouse effect caused by CO2, and great efforts have been made to promote CO2 conversion efficiency. However, the effect of impurities in the photoreduction of CO2 has received relatively little attention. Here, the different CO2 photoreduction behaviors of TiO2 exposed (101) facet (TiO2-101) and (001) facet (TiO2-001) with an SO2 impurity were investigated. On TiO2-101, SO2 accelerates the deactivation of the catalyst for CO2 photoreduction activity, since SO2 mainly binds to the OHb site of TiO2-101. This site is highly susceptible to the transfer of photogenerated holes, resulting in the rapid generation of SO42-, which occupies the active site and poisons the catalyst. For TiO2-001, SO2 has relatively little negative effect on stability, as SO2 mainly binds to the weak binding site (OHt) of TiO2-001, preventing it from being oxidized to SO42-, which alleviates catalyst deactivation and ensures the continuity of CO2 reduction. This paper provides further insights into the role of SO2 in CO2 photoreduction over distinct TiO2 facets and reveals the importance of facet engineering in the photocatalytic reduction of CO2 from industrial flue gas.

Research progress of Mn-based low-temperature SCR denitrification catalysts

Selective catalytic reduction (SCR) is a efficiently nitrogen oxides removal technology from stationary source flue gases. Catalysts are key component in the technology, but currently face problems including poor low-temperature activity, narrow temperature windows, low selectivity, and susceptibility to water passivation and sulphur dioxide poisoning. To develop high-efficiency low-temperature denitrification activity catalyst, manganese-based catalysts have become a focal point of research globally for low-temperature SCR denitrification catalysts. This article investigates the denitrification efficiency of unsupported manganese-based catalysts, exploring the influence of oxidation valence, preparation method, crystallinity, crystal form, and morphology structure. It examines the catalytic performance of binary and multicomponent unsupported manganese-based catalysts, focusing on the use of transition metals and rare earth metals to modify manganese oxide. Furthermore, the synergistic effect of supported manganese-based catalysts is studied, considering metal oxides, molecular sieves, carbon materials, and other materials (composite carriers and inorganic non-metallic minerals) as supports. The reaction mechanism of low-temperature denitrification by manganese-based catalysts and the mechanism of sulphur dioxide/water poisoning are analysed in detail, and the development of practical and efficient manganese-based catalysts is considered.

Water-Driven Surface Lattice Oxygen Activation in MnO2 for Promoted Low-Temperature NH3-SCR

Water is ubiquitous in various heterogeneous catalytic reactions, where it can be easily adsorbed, chemically dissociated, and diffused on catalyst surfaces, inevitably influencing the catalytic process. However, the specific role of water in these reactions remains unclear. In this study, we innovatively propose that H2O-driven surface lattice oxygen activation in γ-MnO2 significantly enhances low-temperature NH3-SCR. The proton from water dissociation activates the surface lattice oxygen in γ-MnO2, giving rise to a doubling of catalytic activity (achieving 90% NO conversion at 100 °C) and remarkable stability. Comprehensive in situ characterizations and calculations reveal that spontaneous proton diffusion to the surface lattice oxygen reduces the orbital overlap between the protonated oxygen atom and its neighboring Mn atom. Consequently, the Mn-O bond is weakened and the surface lattice oxygen is effectively activated to provide excess oxygen vacancies available for converting O2 into O2-. Therefore, the redox property of Mn-H is improved, leading to enhanced NH3 oxidation-dehydrogenation and NO oxidation processes, which are crucial for low-temperature NH3-SCR. This work provides a deeper understanding and fresh perspectives on the water promotion mechanism in low-temperature NOx elimination.

Efficient Catalytic Elimination of Chlorobenzene Based on the Water Vapor-Promoting Effect within Mn-Based Catalysts: Activity Enhancement and Polychlorinated Byproduct Inhibition

Achieving no or low polychlorinated byproduct selectivity is essential for the chlorinated volatile organic compounds (CVOCs) degradation, and the positive roles of water vapor may contribute to this goal. Herein, the oxidation behaviors of chlorobenzene over typical Mn-based catalysts (MnO2 and acid-modified MnO2) under dry and humid conditions were fully explored. The results showed that the presence of water vapor significantly facilitates the deep mineralization of chlorobenzene and restrains the formation of Cl2 and dichlorobenzene. This remarkable water vapor-promoting effect was conferred by the MnO2 substrate, which could suitably synergize with the postconstructed acidic sites, leading to good activity, stability, and desirable product distribution of acid-modified MnO2 catalysts under humid conditions. A series of experiments including isotope-traced (D2O and H218O) CB-TPO provided complete insights into the direct involvement of water molecules in chlorobenzene oxidation reaction and attributed the root cause of the water vapor-promoting effect to the proton-rich environment and highly reactive water-source oxygen species rather than to the commonly assumed cleaning effect or hydrogen proton transfer processes (generation of active OOH). This work demonstrates the application potential of Mn-based catalysts in CVOCs elimination under practical application conditions (containing water vapor) and provides the guidance for the development of superior industrial catalysts.