Challenges and opportunities for manganese oxides in low-temperature selective catalytic reduction of NOx with NH3: H2O resistance ability

Interfacial Co-O-Cu bonds prompt electrochemical nitrate reduction to ammonia in neutral electrolyte

In this work, the formed interfacial Co-O-Cu bonds in Co-doped Cu(OH)2 (Co2-Cu(OH)2) sufficiently expose active sites and improve the reaction kinetics. As a result, the optimal Co2-Cu(OH)2 provides an amazing faradaic efficiency (91.6%), high selectivity (93.2%) and robust stability toward the NO3RR.

Effect of Metal Complexing on Mn–Fe/TS-1 Catalysts for Selective Catalytic Reduction of NO with NH3

TS-1 zeolite with desirable pore structure, an abundance of acidic sites, and good thermal stability promising as a support for the selective catalytic reduction of NO with NH3 (NH3-SCR). Herein, a series of Mn–Fe/TS-1 catalysts have been synthesized, adopting tetraethylenepentamine (TEPA) as a metal complexing agent using the one-pot hydrothermal method. The introduced TEPA can not only increase the loading of active components but also prompts the formation of a hierarchical structure through decreasing the size of TS-1 nanocrystals to produce intercrystalline mesopores during the hydrothermal crystallization process. The optimized Mn–Fe/TS-1(R-2) catalyst shows remarkable NH3-SCR performance. Moreover, it exhibits excellent resistance to H2O and SO2 at low temperatures. The characterization results indicate that Mn–Fe/TS-1(R-2) possesses abundant surface Mn4+ and Fe2+ and chemisorbed oxygen, strong reducibility, and a high Brønsted acid amount. For comparison, Mn–Fe/TiO2 displays a narrower active temperature window due to its poor thermostability.

Ce(SO4)2/α-Fe2O3 selective catalytic reduction of NOx with NH3: preparation, characterization, and performance

To achieve a low-cost, high-activity denitrification catalyst with excellent water and sulfur resistance, goethite and Ce(SO₄)₂·4H₂O were used to prepare Ce(SO₄)₂/α-Fe₂O₃ composite catalyst by the impregnation way and investigated the effect of Ce(SO₄)₂ on the properties of goethite. Ce(SO₄)₂/α-Fe₂O₃ with various preparation conditions for denitration was systematically discussed, and its structure and properties were characterized by XRD, BET, TEM, XPS, H₂-TPR, and NH₃-TPD methods. The results showed that Ce(SO₄)₂/α-Fe₂O₃ over the Ce/Fe molar ratio of 0.02 and calcination temperature of 350 ℃ had excellent catalytic activity, resistance to sulfur, and water properties and stability. When NO_x initial concentration was 500 ppm, gas hourly space velocity was 36,000 h^-1 and its reaction temperature was 300 ℃; the NO_x conversion efficiency was maintained at over 95% along with 300 ppm SO₂ and nearly 100% couple with 10% H₂O. Its superior performance was mainly attributed to the enhancement of the surface adsorbed oxygen and acidity of α-Fe₂O₃ by cerium sulfate. The multiple advantages of Ce_0.02/α-Fe₂O₃(350) made it feasible for practical engineering application.

The promoting/inhibiting effect of water vapor on the selective catalytic reduction of NOx

In the field of nitrogen oxides (NO_x) abatement, developing selective catalytic reduction (SCR) catalysts that can operate stably in the practical conditions remains a big challenge because of the complexity and uncertainty of actual flue gas emissions. As water vapor is unavoidable in the actual flue gas, it is indispensable to explore its effect on the performance of SCR catalysts. Many studies have proved that the effects of H₂O on de-NO_x activity of SCR catalysts were indeed observed during SCR reactions operated under wet conditions. Whether the effect is promotive or inhibitory depends on the reaction conditions, catalyst types and reducing agents used in SCR reaction. This review focuses on the effect of H₂O on SCR catalysts and SCR reaction, including promoting effect, inhibiting effect, as well as the effecting mechanism. Besides, various strategies for developing a water-resistant SCR catalyst are also included. We hope that this work can give a more comprehensive insight into the effects of H₂O on SCR catalysts and help with the rational design of water-resistant SCR catalysts for further practical application in NO_x abatement field.

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.

PTFE-Modified Mn-Co-Based Catalytic Ceramic Filters with H2O Resistance for Low-Temperature NH3-SCR

Mn-Co-based catalysts were loaded to ceramic filters element by impregnation for the coprocessing of dust and nitrogen oxide (NOx) in flue gas. The influence of the Mn/Co ratio and loading on the catalytic performance was investigated. The Mn-Co-based catalytic ceramic filter with a Mn/Co molar ratio of 2/1 can achieve 99% NO conversion by selective catalytic reduction of NOx with NH3 (NH3-SCR) in the temperature range of 100−180 °C, but its resistance to H2O was relatively poor. The filter element was modified by PTFE to improve the H2O resistance. After modification, the catalytic ceramic filter showed superior resistance to H2O and SO2 at low temperatures (100–180 °C) and satisfactory self-cleaning performance.

One-step synthesis by redox co-precipitation method for low-dimensional Me-Mn bi-metal oxides (Me=Co, Ni, Sn) as SCR DeNOx catalysts

In this research, one-step synthesis of redox co-precipitation method (using sodium lauryl sulfate, KMnO₄, and metal precursor) was well applicable in universally preparing low-dimensional Me-MnOx nanosheet catalysts with different metal doping (Me=Co, Ni, or Sn). NH₃-SCR activity was explored to the relationship with structure morphology and physio-chemical properties via the characterization techniques of SEM, XRD, XPS, H₂-TPR, and NH₃-TPD. It was found that Ni-MnOx has a relatively poor activity at low-down temperature but was improved as the reaction temperature rising. Co-MnOx presented a relatively stable catalytic activity of which the NOx conversion rate can be maintained 80~90% in a wide temperature window of 100-250 °C with relatively better N₂ selectivity. Compared with Co- or Ni-modified MnOx, Sn-MnOx catalyst has an excellent low-temperature catalytic activity (93% NOx conversion at 100 °C) that was maintained > 80% before 200 °C but with poor selectivity to N₂. Due to its nanosheet-structured solid solution structure, Sn-MnOx promoted the interaction between MnOx and SnO₂ with the increased contents of adsorbed oxygen and also the numbers of surface Lewis acid sites, which integrally promoted the NH₃-SCR reaction at low temperature and also contributed to an acceptable resistances to water and sulfur. High content of adsorbed oxygen was beneficial to improve the catalytic activity at lower temperatures, while the electron cycle interaction of different metal valence ions will play a more important role with the increase of reaction temperature.

Co- or Ni-modified Sn-MnOx low-dimensional multi-oxides for high-efficient NH3-SCR De-NOx: Performance optimization and reaction mechanism

NH₃-SCR performances were explored to the relationship between structure morphology and physio-chemical properties over low-dimensional ternary Mn-based catalysts prepared by one-step synthesis method. Due to its strong oxidation performance, Sn-MnOx was prone to side reactions between NO, NH₃ and O₂, resulting in the generation of more NO₂ and N₂O, here most of N₂O was driven from the non-selective oxidation of NH₃, while a small part generated from the side reaction between NH₃ and NO₂. Co or Ni doping into Sn-MnOx as solid solution components obviously stronged the electronic interaction for actively mobilization and weakened the oxidation performance for signally reducing the selective tendency of side reactions to N₂O. The optimal modification resulted in improving the surface area and enhancing the strong interaction between polyvalent cations in Co/Ni-Mn-SnO₂ to provide more surface adsorbed oxygen, active sites of Mn³⁺ and Mn⁴⁺, high-content Sn⁴⁺ and plentiful Lewis-acidity for more active intermediates, which significantly broadened the activity window of Sn-MnOx, improved the N₂ selectivity by inhibiting N₂O formation, and also contributed to an acceptable resistances to water and sulfur. At low reaction temperatures, the SCR reactions over three catalysts mainly obeyed the typical Elye-rideal (E-R) routs via the reactions of adsorbed l-NHx (x = 3, 2, 1) and B-NH₄⁺ with the gaseous NO to generate N₂ but also N₂O by-products. Except for the above basic E-R reactions, as increasing the reaction temperature, the main adsorbed NOx-species were bidentate nitrates that were also active in the Langmuir-Hinshelwood reactions with adsorbed l-NHx species over Co/Ni modified Mn-SnO₂ catalyst.

Recent progress of low-temperature selective catalytic reduction of NOx with NH3 over manganese oxide-based catalysts

Selective catalytic reduction with NH3 (NH3−SCR) was the most efficient approach to mitigate the emission of nitrogen oxides (NOx). Although the conventional manganese oxide-based catalyst had gradually become a kind...

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.

The Deactivation Mechanism of the Mo-Ce/Zr-PILC Catalyst Induced by Pb for the Selective Catalytic Reduction of NO with NH3

As a heavy metal, Pb is one component in coal-fired flue gas and is widely considered to have a strong negative effect on catalyst activity in the selective catalytic reduction of NOx by NH3 (NH3-SCR). In this paper, we investigated the deactivation mechanism of the Mo-Ce/Zr-PILC catalyst induced by Pb in detail. We found that NO conversion over the 3Mo4Ce/Zr-PILC catalyst decreased greatly after the addition of Pb. The more severe deactivation induced by Pb was attributed to low surface area, lower amounts of chemisorbed oxygen species and surface Ce3+, and lower redox ability and surface acidity (especially a low number of Brønsted acid sites). Furthermore, the addition of Pb inhibited the formation of highly active intermediate nitrate species generated on the surface of the catalyst, hence decreasing the NH3-SCR activity.

Simulation of Denitrification of Vehicle Exhaust over Cu-CHA Bazite Catalyst for a Monolith Reactor

A CFD model with chemical reaction kinetic and heat and mass transfer for a monolith reactor is established by COMSOL Multiphysics to investigate the influence of different operating conditions and water on denitrification efficiency for Cu-CHA. At the low temperature range, water has little effect on the denitrification efficiency over the Cu-CHA catalyst while NO conversion is increased by about 30% at the medium temperature. The concentration of O2 (CO2) has no significant effect on the performance of Cu-CHA catalyst. The best ratio of NO2 to NOx in feed gases may be 1/2, which improves the denitrification efficiency and the yield of N2 but it produces relatively little N2O. The optimal ammonia-nitrogen ratio is 1.1, where Cu-CHA catalyst has fairly great denitrification efficiency and low NH3 leakage. Increasing inlet flow velocity and cross area of channels have negative effect on NO conversion, while longer channels and thicker substrate have the opposite effect.

Selective Catalytic Reduction of NO by NH3 over Mn–Cu Oxide Catalysts Supported by Highly Porous Silica Gel Powder: Comparative Investigation of Six Different Preparation Methods

In this study, Mn-based catalysts supported by highly porous silica gel powder (SSA up to 470 m2·g−1 and total pore volume up to 0.8 cm3·g−1) were prepared by six different methods in liquid solutions (electroless metal deposition, stepwise addition of a reducing agent, wet impregnation, incipient wetness impregnation, urea hydrolysis, and ammonia evaporation) and tested for selective catalytic reduction of NOx with ammonia (NH3-SCR de-NOx). Prior to the activity test all the catalysts prepared were characterized by ICP-OES, SEM, EDX mapping, XPS, XRD and N2 adsorption techniques to provide the comprehensive information about their composition and morphology, investigate the dispersion of active components on the carrier surface, identify the chemical forms and structural properties of the catalytically active species of the catalysts prepared. The results revealed that all the methods applied for preparation of SCR de-NOx catalysts can ensure the uniform distribution of Mn species on the carrier surface, however as it is typical for preparation techniques in a liquid phase the significant reduction in SSA and pore volume along with increasing the loading was observed. Considering both the physicochemical properties and the catalytic performance of the catalysts the least effective preparation method was shown to be ammonia evaporation, while the most attractive techniques are incipient wetness impregnation and electroless metal deposition.

A review of Mn-based catalysts for low-temperature NH3-SCR: NOx removal and H2O/SO2 resistance

The development of high-efficiency catalysts is the key to the low-temperature NH₃-SCR technology. The introduction of SO₂ and H₂O will lead to poisoning and deactivation of the catalysts, which severely limits the development and application of NH₃-SCR technology. This review introduces the necessity of NO_x removal, explains the mechanisms of H₂O and SO₂ poisoning on NH₃-SCR catalysts, highlights the Mn-based catalysts of different active metals and supports and their resistance to H₂O and SO₂, and analyses the relationship between metal modification, selection of support and preparation method, morphology and structure design and SO₂/H₂O resistance. Given the current problems, this review points out the future research focus of Mn-based catalysts and also puts forward corresponding countermeasures to solve the existing problems.

Silver Single-Atom Catalyst for Efficient Electrochemical CO2 Reduction Synthesized from Thermal Transformation and Surface Reconstruction

We report an Ag₁ single-atom catalyst (Ag₁ /MnO₂ ), which was synthesized from thermal transformation of Ag nanoparticles (NPs) and surface reconstruction of MnO₂ . The evolution process of Ag NPs to single atoms is firstly revealed by various techniques, including in situ ETEM, in situ XRD and DFT calculations. The temperature-induced surface reconstruction process from the MnO₂ (211) to (310) lattice plane is critical to firmly confine the existing surface of Ag single atoms; that is, the thermal treatment and surface reconstruction of MnO₂ is the driving force for the formation of single Ag atoms. The as-obtained Ag₁ /MnO₂ achieved 95.7 % Faradic efficiency at -0.85 V vs. RHE, and coupled with long-term stability for electrochemical CO₂ reduction reaction (CO₂ RR). DFT calculations indicated single Ag sites possessed high electronic density close to Fermi Level and could act exclusively as the active sites in the CO₂ RR. As a result, the Ag₁ /MnO₂ catalyst demonstrated remarkable performance for the CO₂ RR, far surpassing the conventional Ag nanosized catalyst (Ag_NP /MnO₂ ) and other reported Ag-based catalysts.

Improved SO2 Tolerance of Cu-SAPO-18 by Ce-Doping in the Selective Catalytic Reduction of NO with NH3

The Ce-Cu-SAPO-18 catalysts were prepared using the ion exchange method. The impact of sulfur dioxide on catalytic performance of Ce-Cu-SAPO-18 for the selective catalytic reduction (SCR) of NO with NH3 was examined. Detailed characterization of the fresh and sulfur-poisoning Cu-SAPO-18 and Ce-Cu-SAPO-18 samples was conducted. XRD and BET results show that SO2 treatment of the Ce-doped Cu-SAPO-18 (Ce-Cu-SAPO-18-S) sample did not induce a remarkable change in structure, as compared with that of the fresh counterpart. According to in situ DRIFT, H2-TPR, SEM, and EDS results, it is found that the sulfation species attached preferentially to the cerium species, rather than the isolated Cu2+ species. In particular, the TG/DSC results confirm that the sulfate species on the Ce-Cu-SAPO-18-S sample was easier to decompose than that on the Cu-SAPO-18-S sample. The catalytic active sites of Ce-Cu-SAPO-18 were less influenced after SO2 treatment, as demonstrated by the TPR and XPS results. All of the above results show that the Ce-Cu-SAPO-18 sample exhibited better sulfur-resistant performance than the Cu-SAPO-18 sample.

Spinel-structured Mn–Ni nanosheets for NH3-SCR of NO with good H2O and SO2 resistance at low temperature

High specific surface area, more NH₃ adsorption ability and efficient electronic interaction over Mn–Ni spinel nanosheet leaded to good SCR activity, and Ni-outside with active Mn-inner spinel configuration and nanosheet morphology relieved SO₂-poisoning.