Effect of Mn addition on the low-temperature NH3 -selective catalytic reduction of NO x over Fe2 O3 /activated coke catalysts: Experiment and mechanism

Tungsten modified natural limonite catalyst for efficient low-temperature selective catalytic reduction of NO removal with NH3: preparation and characterization

With natural limonite as the precursor and an ammonium tungstate hydrate as modification, the W/limonite composite catalysts were synthesized by the impregnation method. Their structures and properties were systematically characterized and analyzed; the denitrification activity and resistance to water and sulfur on catalysts were investigated. The results indicated that the W/limonite composite with W/Fe mass ratio of 9% and calcination temperature of 300 °C had highly catalytic activity, enhanced resistance to sulfur and water. The NO conversion efficiency was maintained over 85% with NO initial concentration of 500 ppm, the gas hourly space velocity (GHSV) of 36,000 h^-1, and reaction temperature of 100 °C, while it was greater than 98% with addition of 200 ppm SO₂ and 3 vol. % H₂O at the reaction temperature of 250 °C. The superior performance was mainly ascribed to the formation of W-OH species and W = O species with wide dispersion on the surface of goethite or in Fe₂O₃ lattice defects, to generate more acidic hydroxyl groups and more oxygen defects and strong acidity Brønsted for the SCR reaction.

Low-temperature selective catalytic reduction of NO with NH3 over an FeO x /β-MnO2 composite

A series of Fe-modified β-MnO₂ (FeO _x /β-MnO₂) composite catalysts were prepared by an impregnation method with β-MnO₂ and ferro nitrate as raw materials. The structures and properties of the composites were systematically characterized and analyzed by X-ray diffraction, N₂ adsorption-desorption, high-resolution electron microscopy, temperature-programmed reduction of H₂, temperature-programmed desorption of NH₃, and FTIR infrared spectroscopy. The deNO _x activity, water resistance, and sulfur resistance of the composite catalysts were evaluated in a thermally fixed catalytic reaction system. The results indicated that the FeO _x /β-MnO₂ composite (Fe/Mn molar ratio of 0.3 and calcination temperature of 450 °C) had higher catalytic activity and a wider reaction temperature window compared with β-MnO₂. The water resistance and sulfur resistance of the catalyst were enhanced. It reached 100% NO conversion efficiency with an initial NO concentration of 500 ppm, a gas hourly space velocity of 45 000 h^-1, and a reaction temperature of 175-325 °C. The appropriate Fe/Mn molar ratio sample had a synergistic effect, affecting the morphology, redox properties, and acidic sites, and helped to improve the low-temperature NH₃-SCR activity of the composite catalyst.

CO + NH3 coupling denitration at low temperatures over manganese/activated carbon catalysts

To explore the mechanism of low-temperature carbon monoxide and ammonia (CO + NH₃) coupling denitration of manganese/activated carbon (Mn/AC) catalysts, Mn/AC series catalysts were prepared using the impregnation method with AC activated by nitric acid as a precursor and manganese nitrate as a precursor. We characterized the surface morphology, pore structure, active component phase, functional group, and active component valence change law of the Mn/AC catalyst. The denitration rate order with different Mn loadings is 7Mn/AC > 9Mn/AC > 5Mn/AC. When the Mn loading was 7%, the catalyst's surface was smooth, with a good pore structure and uniform surface distribution of metal particles. These features increased the reacting gas's contact area, improving the denitration rate. The reason for this was oxygen chemisorption on the catalyst's surface. The Mn⁴⁺ and the number of oxygen-containing functional groups on the catalyst surface increase after Mn loading increases; this provides more active sites for denitration and promotes the reaction's conversion to fast selective catalytic reduction. The low-temperature CO + NH₃ coupling denitration of Mn/AC catalysts conforms to the Langmuir-Hinshelwood mechanism when the temperature is lower than 230 °C and the Eley-Rideal mechanism when the temperature is higher than 230 °C. The research results can provide new ideas for low-temperature flue gas denitration.

Effect of pre-oxidation process on V2O5/AC catalyst for the selective catalytic reduction of NOx with NH3

Activated coke-based catalysts have attracted extensive attention in denitration by selective catalytic reduction by NH₃ (NH₃-SCR), due to their excellent catalytic performance at low temperature. In the paper, the V₂O₅/AC catalyst was prepared by the impregnation method to investigate the effect of pre-oxidation process on its NH₃-SCR activity. Activity test results show that the V₂O₅/AC catalyst with 4-h pre-oxidation exhibits the best NO_x removal efficiency, which reaches the NO_x conversion is over 75% in the range of 200-240 °C and exhibits an excellent resistance to SO₂ and H₂O. Characterization results demonstrate that the V⁴⁺ was oxidized by oxygen molecule during pre-oxidation process, which contributes to the formation of V⁵⁺ ions and surface-active oxygen species. The surface-active oxygen species are conducive to promoting the "fast SCR" reaction; thus, the pre-oxidized process can contribute to the superior NH₃-SCR performance for V₂O₅/AC catalyst at low temperature.

Understand the role of redox property and NO adsorption over MnFeOx for NH3-SCR

Widening the operation temperature window of selective catalytic reduction NO by NH3 (NH3-SCR) is a challenge to meet the increasingly stringent emission control regulations of NOx. Hence, MnFeOx with different...

Promotional Effect of Manganese on Selective Catalytic Reduction of NO by CO in the Presence of Excess O2 over M@La–Fe/AC (M = Mn, Ce) Catalyst

The catalytic performance of a series of La-Fe/AC catalysts was studied for the selective catalytic reduction (SCR) of NO by CO. With the increase in La content, the Fe2+/Fe3+ ratio and amount of surface oxygen vacancies (SOV) in the catalysts increased; thus the catalytic activity improved. Incorporating the promoters to La3-Fe1/active carbon (AC) catalyst could affect the catalyst activity by changing the electronic structure. The increase in Fe2+/Fe3+ ratio after the promoter addition is possibly due to the extra synergistic interaction of M (Mn and Ce) and Fe through the redox equilibrium of M3+ + Fe3+ ↔ M4+ + Fe2+. This phenomenon could have improved the redox cycle, enhanced the SOV formation, facilitated NO decomposition, and accelerated the CO-SCR process. The presence of O2 enhanced the formation of the C(O) complex and improved the activation of the metal site. Mn@La3-Fe1/AC catalyst revealed an excellent NO conversion of 93.8% at 400 °C in the presence of 10% oxygen. The high catalytic performance of MnOx and double exchange behavior of Mn3+ and Mn4+ can increase the number of SOV and improve the catalytic redox properties.