Mn-doped Ti-Fe composite oxide catalysts for efficient low-temperature NO_x reduction: Enhancing catalytic performance...

To achieve efficient denitrification performance at low temperatures while maintaining catalyst stability under SO₂/H₂O-rich conditions, a series of Mn-doped Ti-Fe composite oxide catalysts (Ti_0.1Fe_0.05Mn_x, where x = 0-0.03) were synthesized in this study. The ammonia selective catalytic reduction (NH₃-SCR) activity, SO₂ and H₂O resistance, and structure-mechanism relationships of the catalysts were systematically investigated. Catalytic performance tests showed that the optimized catalyst Ti_0.1Fe_0.05Mn_0.02 maintained NO conversion rates above 95 % across a broad temperature range of 150-350 °C and exhibited excellent durability in the presence of 100 ppm SO₂ and 5-10 vol% H₂O. Physicochemical characterizations revealed that Mn incorporation significantly enhanced the specific surface area, improved the pore structure, and enriched the proportion of surface adsorbed oxygen (O_α) and Lewis acid sites. These modifications promoted the adsorption and activation of NH₃ and NO. In situ DRIFTS analysis identified key intermediates, included -NH₂ and NH₂NO, and confirmed the coexistence of the Langmuir-Hinshelwood and Eley-Rideal mechanisms. The coexistence of Langmuir-Hinshelwood and Eley-Rideal mechanisms was identified based on in situ DRIFTS spectra showing sequential and co-adsorbed intermediate species, and further supported by Density functional theory (DFT)-calculated energy barriers of key reaction steps. H₂ temperature-programmed reduction (H₂-TPR) and temperature-programmed desorption (TPD) results indicate that Mn doping significantly enhances the release and migration ability of O_α, which is conducive to the formation of active intermediates and realises an efficient catalytic pathway over a wide temperature range. DFT calculations show that Mn doping reduces the energy barrier of the rate-determining step (∗NH₃ → NH₂ + H) from 1.62 to 1.46 eV, which is evidenced by the enhanced activation of NH₃. Furthermore, Mn sites exhibited weak electronic interactions with SO₂, thereby suppressing deep adsorption and side reactions. In conclusion, the Ti_0.1Fe_0.05Mn_0.02 catalysts have significant advantages in structure modulation, reaction pathway coordination, and resistance to SO₂ and H₂O toxicity, which provide theoretical insights and experimental support for the design of high-performance SCR catalysts with a wide range of temperature applicability and enhanced stability under harsh conditions.