Mechanistic Insight into the Promotion of the Low-Temperature NH3-SCR Activity over NiMnFeOx LDO Catalysts: A Combined Experimental and DFT Study

Mn-based catalysts have attracted much attention in the field of the low-temperature NH3 selective catalytic reduction (NH3-SCR) of NO. However, their poor SO2 resistance, low N2 selectivity, and narrow operation window limit the industrial application of Mn-based oxide catalysts. In this work, NiMnFeOx catalysts were prepared by the layered double hydroxide (LDH)-derived oxide method, and the optimized Ni0.5Mn0.5Fe0.5Ox catalyst had the best denitration activity, excellent N2 selectivity, a wider active temperature range (100-250 °C), higher thermal stability, and better H2O and/or SO2 resistance. A transient reaction revealed that Ni0.5Mn0.5Fe0.5Ox inhibited the NH3 + O2 + NOx pathway to generate N2O, which may be the main reason for its improved N2 selectivity. Combining experimental measurements and density functional theory (DFT) calculations, we elucidated at the atomic level that sulfated NiMnFeOx (111) induces the adjustment of the acidity/basicity of up and down spins and the ligand field reconfiguration of the Mn sites, which improves the overall reactivity of NiMnFeOx catalysts. This work provides atomic-level insights into the promotion of NH3-SCR activity by NiMnFeOx composite oxides, which are important for the practical design of future low-temperature SCR technologies.

Mechanistic Insight into the Promotion of the Low-Temperature NH3-Selective Catalytic Reduction Activity over MnxCe1-xOy Catalysts: A Combined Experimental and Density Functional Theory Study

CeO₂ has attracted much attention in the field of selective catalytic reduction of NO with NH₃ (NH₃-SCR). However, poor low-temperature activity and a narrow operation window restrict the industrial application of Ce-based oxide catalysts. Herein, the low-temperature NH₃-SCR activity of Ce-based oxide catalysts was dramatically improved by Mn doping, and the mechanism was elucidated at the atomic level by experimental measurements and density functional theory calculations. We found that the addition of Mn significantly promoted the formation of surface oxygen vacancies. The oxygen vacancies easily captured O₂ in air and formed active oxygen species (superoxide and peroxide) on the surface. The surface active oxygen species efficiently oxidized NO into NO₂ and then facilitated the "fast SCR" reaction. This study provides atomic-level insights into the promotion of the NH₃-SCR activity over Mn-Ce composite oxides and is beneficial for the development of low-temperature Ce-based catalysts.

Recent advances and perspectives in the resistance of SO2 and H2O of cerium-based catalysts for NOx selective catalytic reduction with ammonia

This review emphasizes the aspects related to cerium-based catalysts at different levels: metal modification, preparation methods, structures, and reaction mechanisms.

A plasma thermal slag-derived from hazardous waste has a born hydrothermal stability

Hydrothermal instability restricts performances of silica-based catalysts, which have wide applications in both industry and environment. For the first time, plasma-thermal slag was revealed to be a catalyst with a born hydrothermal stability in selective catalytic reduction of nitric oxide. The slag catalyst removed 98.5 % of NO with a high N₂ selectivity (> 95 %) at 200 °C. After a hydrothermal treatment at 900 °C, the activity of the slag only decreased to 84.0 %. According to characterizations of XRD, HTREM, XPS, and EPR, active metals existed in coordination states in the slag at first. Under hydrothermal conditions, these species transformed to short-range single crystals, which were hindered from sintering by surrounded Si-O bands. At the same time, in-situ DRIFT indicated that more Brønsted and Lewis acid sites were formed. Hence, enough active sties were reserved for effective catalytic reduction of nitric oxide. The main result of this work helps us to understand hydrothermal stability of a catalyst. What's more, the high-value-added utilization of plasma-thermal slag is in favor of the development of hazardous-waste treatment.

A Study on Mn-Fe Catalysts Supported on Coal Fly Ash for Low-Temperature Selective Catalytic Reduction of NOX in Flue Gas

A series of Mn0.15Fe0.05/fly-ash catalysts have been synthesized by the co-precipitation method using coal fly ash (FA) as the catalyst carrier. The catalyst showed high catalytic activity for low-temperature selective catalytic reduction (LTSCR) of NO with NH3. The catalytic reaction experiments were carried out using a lab-scale fixed-bed reactor. De-NOx experimental results showed the use of optimum weight ratio of Mn/FA and Fe/FA, resulted in high NH3-SCR (selective catalytic reduction) activity with a broad operating temperature range (130–300 °C) under 50000 h−1. Various characterization methods were used to understand the role of the physicochemical structure of the synthesized catalysts on their De-NOx capability. The scanning electron microscopy, physical adsorption-desorption, and X-ray photoelectron spectroscopy showed the interaction among the MnOx, FeOx, and the substrate increased the surface area, the amount of high valence metal state (Mn4+, Mn3+, and Fe3+), and the surface adsorbed oxygen. Hence, redox cycles (Fe3+ + Mn2+ ↔ Mn3+ + Fe2+; Fe2+ + Mn4+ ↔ Mn3+ + Fe3+) were co-promoted over the catalyst. The balance between the adsorption ability of the reactants and the redox ability can promote the excellent NOx conversion ability of the catalyst at low temperatures. Furthermore, NH3/NO temperature-programmed desorption, NH3/NO- thermo gravimetric-mass spectrometry (NH3/NO-TG-MS), and in-situ DRIFTs (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) results showed the Mn0.15Fe0.05/FA has relatively high adsorption capacity and activation capability of reactants (NO, O2, and NH3) at low temperatures. These results also showed that the Langmuir–Hinshelwood (L–H) reaction mechanism is the main reaction mechanism through which NH3-SCR reactions took place. This work is important for synthesizing an efficient and environmentally-friendly catalyst and demonstrates a promising waste-utilization strategy.

Insights into the promotion role of phosphorus doping on carbon as a metal-free catalyst for low-temperature selective catalytic reduction of NO with NH3

The catalytic reduction of NO with NH₃ (NH₃-SCR) on phosphorus-doped carbon aerogels (P-CAs) was studied in the temperature range of 100–200 °C. The P-CAs were prepared by a one-pot sol–gel method by using phosphoric acid as a phosphorus source followed by carbonization at 600–900 °C. A correlation between catalytic activity and surface P content is observed. The P-CA-800_vac sample obtained via carbonization at 800 °C and vacuum treatment at 380 °C shows the highest NO conversion of 45.6–76.8% at 100–200 °C under a gas hourly space velocity of 500 h⁻¹ for the inlet gas mixture of 500 ppm NO, 500 ppm NH₃ and 5.0 vol% O₂. The coexistence of NH₃ and O₂ is essential for the high conversion of NO on the P-CA carbon catalysts, which can decrease the spillover of NO₂ and N₂O. The main Brønsted acid sites derived from P-doping and contributed by the C–OH group at edges of carbon sheets are beneficial for NH₃ adsorption. In addition, the C₃–P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O configuration seems to have the most active sites for favorable adsorption and dissociation of O₂ and facilitates the formation of NO₂. Therefore, the simultaneous presence of acidic groups for NH₃ adsorption and the C₃–PO active sites for NO₂ generation due to the activation of O₂ molecules is likely responsible for the significant increase in the NH₃-SCR activity over the P–CAs. The transformation of C₃–PO to C–O–P functional groups after the reaction is found, which could be assigned to the oxidation of C₃–PO by the dissociated O*, resulting in an apparent decrease of catalytic activity for P-CAs. The C–O–P based functional groups are also active in the NH₃-SCR reaction.

P species can effectively enhance the catalytic activity of carbon aerogels for NO reduction at low temperature.

MnO2–GO-scroll–TiO2–ITQ2 as a low-temperature NH3-SCR catalyst with a wide SO2-tolerance temperature range

GO-scroll–TiO₂–ITQ2 improved the steam-resistance and SO₂-resistance of the low-temperature manganese-based NH₃-SCR catalyst.

Significantly enhanced Pb resistance of a Co-modified Mn–Ce–Ox/TiO2 catalyst for low-temperature NH3-SCR of NOx

Co modification can significantly improve the performance for low-temperature NH₃-SCR of NO_x and the Pb resistance of the Mn–Ce–O_x/TiO₂ catalyst.

MnO2–Graphene-oxide-scroll–TiO2 composite catalyst for low-temperature NH3-SCR of NO with good steam and SO2 resistance obtained by low-temperature carbon-coating and selective atomic layer deposition

Coating GO at low temperature and selectively depositing TiO₂ on oxygen-containing functional groups on GO.

MnOx–CeO2@TiO2 core–shell composites for low temperature SCR of NOx

The MnO_x–CeO₂@TiO₂ catalyst presents excellent NH₃-SCR activity and the TiO₂ shell is responsible for the good SO₂ tolerance.

Doping effect of Sm on the TiO2/CeSmOx catalyst in the NH3-SCR reaction: structure–activity relationship, reaction mechanism and SO2 tolerance

A good balance between the redox properties and surface acidity induces the high activity of the Sm doped TiO₂/CeO₂ catalyst.