Solvent-free elaboration of Ni-doped MnOx catalysts with high performance for NH3-SCR in low and medium temperature zones

A Review of Mn-Based Catalysts for Abating NOx and CO in Low-Temperature Flue Gas: Performance and Mechanisms

Mn-based catalysts have attracted significant attention in the field of catalytic research, particularly in NOx catalytic reductions and CO catalytic oxidation, owing to their good catalytic activity at low temperatures. In this review, we summarize the recent progress of Mn-based catalysts for the removal of NOx and CO. The effects of crystallinity, valence states, morphology, and active component dispersion on the catalytic performance of Mn-based catalysts are thoroughly reviewed. This review delves into the reaction mechanisms of Mn-based catalysts for NOx reduction, CO oxidation, and the simultaneous removal of NOx and CO. Finally, according to the catalytic performance of Mn-based catalysts and the challenges faced, a possible perspective and direction for Mn-based catalysts for abating NOx and CO is proposed. And we expect that this review can serve as a reference for the catalytic treatment of NOx and CO in future studies and applications.

Unraveling the structure and role of Mn and Ce for NOx reduction in application-relevant catalysts

Mn-based oxides are promising for the selective catalytic reduction (SCR) of NOx with NH₃ at temperatures below 200 °C. There is a general agreement that combining Mn with another metal oxide, such as CeOx improves catalytic activity. However, to date, there is an unsettling debate on the effect of Ce. To solve this, here we have systematically investigated a large number of catalysts. Our results show that, at low-temperature, the intrinsic SCR activity of the Mn active sites is not positively affected by Ce species in intimate contact. To confirm our findings, activities reported in literature were surface-area normalized and the analysis do not support an increase in activity by Ce addition. Therefore, we can unequivocally conclude that the beneficial effect of Ce is textural. Besides, addition of Ce suppresses second-step oxidation reactions and thus N₂O formation by structurally diluting MnOx. Therefore, Ce is still an interesting catalyst additive.

Effect of Indium Addition on the Low-Temperature Selective Catalytic Reduction of NO x by NH3 over MnCeO x Catalysts: The Promotion Effect and Mechanism

A MnCeInO _x catalyst was prepared by a coprecipitation method for denitrification of NH₃-SCR (selective catalytic reduction). The catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffractometry, scanning electron microscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller analysis, H₂ temperature-programmed reduction, and NH₃ temperature-programmed desorption. The NH₃-SCR activity and H₂O and SO₂ resistance of the catalysts were evaluated. The test results showed that the SCR and water resistance and sulfur resistance were good in the range of 125-225 °C. The calcination temperature of the Mn₆Ce_0.3In_0.7O _x catalyst preparation was studied. The crystallization of the Mn₆Ce_0.3In_0.7O _x catalyst was poor when calcined at 300 °C; however, the crystallization is excessive at a 500 °C calcination temperature. The influence of space velocity on the performance of the catalyst is great at 100-225 °C. FTIR test results showed that indium distribution on the surface of the catalyst reduced the content of sulfate on the surface, protected the acidic site of MnCe, and improved the sulfur resistance of the catalyst. The excellent performance of the Mn₆Ce_0.3In_0.7O _x catalyst may be due to its high content of Mn⁴⁺, surface adsorbed oxygen species, high specific surface area, redox sites and acid sites on the surface, high turnover frequency, and low apparent activation energy.

Converting Poisonous Sulfate Species to an Active Promoter on TiO2 Predecorated MnOx Catalysts for the NH3-SCR Reaction

MnO_x-based catalysts possess excellent low-temperature NH₃ selective catalytic reduction (NH₃-SCR) activity, but the poor SO₂/sulfate poisoning resistance and the narrow active-temperature window limit their application for NO_x removal. Herein, TiO₂ nanoparticles and sulfate were successively introduced into MnO_x-based catalysts to modulate the NH₃-SCR activity, and the active-temperature window (NO conversion above 80%, T₈₀) was significantly broadened to 100-350 °C (SO₄^2--TiO₂@MnO_x) compared to that of the pristine MnO_x catalyst (ca. T₈₀: 100-268 °C). Combined with advanced characterizations and control experiments, it was clearly shown that the poisonous effects of sulfate on the MnO_x catalyst could be efficiently inhibited in the presence of TiO₂ species due to the interaction between sulfate and TiO₂ to form a solid superacid (SO₄^2--TiO₂) species as NH₃ adsorption sites for the low-temperature process. Furthermore, such solid superacid (SO₄^2--TiO₂) species could weaken the redox ability to inhibit the excessive oxidation of NH₃ and thus enhance the high-temperature activity significantly. This work not only puts forward the TiO₂ predecoration strategy that converts sulfate to a promoter to broaden the active temperature window but also experimentally proves that the requirement of redox ability and acidity in the MnO_x-based NH₃-SCR catalyst was dependent on the reaction temperature range.