Mn-doped Ti-Fe composite oxide catalysts for efficient low-temperature NOx reduction: Enhancing catalytic performance and sulfur resistance.

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 (Ti0.1Fe0.05Mnx, 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 Ti0.1Fe0.05Mn0.02 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 (Oα) 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 Oα, 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 Ti0.1Fe0.05Mn0.02 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.