Abstract: Selective catalytic reduction (SCR) is a key technology for industrial flue gas denitrification. However, traditional SCR catalysts suffer from low efficiency below 200 ℃. Mn-based catalysts exhibit high catalytic performance and significant application potential at low temperatures; however, systematic studies on monolithic Mn-based catalysts remain limited. In this study, Sm–Mn/Ti-y (y = 3, 1.5, 1) monolithic honeycomb catalysts with varying active component loads were prepared by co-precipitation combined with mixing-extrusion molding. Honeycomb catalysts with no surface cracks, good smoothness, and excellent molding abilities were obtained. Performance test results demonstrate that the Sm–Mn/Ti-1.5 catalyst exhibits over 90% NO conversion and 70% N₂ selectivity between 100‒180 ℃. Characterization and testing techniques, including XRD, FESEM, N₂ adsorption-desorption, XPS, H₂-TPR, and NH₃-TPD, were conducted to explore the effect of the Sm–Mn/Ti ratio on surface phase, structure, species distribution, redox capacity, and adsorption capacity of the catalyst. XRD results indicate that only the TiO₂ phase is present, with no other phases detected, indicating that Sm and Mn are uniformly dispersed on the catalyst surface without forming long-range ordered lattice. SEM characterization shows that the catalysts consist of colonies and nanoparticles on their surfaces. A highly dispersed elemental distribution and fine surface structure are beneficial for improving catalytic performance. N₂ adsorption-desorption tests confirm that the catalysts possess mesoporous structures. XPS, H₂-TPR and NH₃-TPD results reveal that the Sm–Mn/Ti-1.5 catalyst has the highest atomic percentage of Mn⁴⁺ in Mn (56.5%) and the greatest amount of weak acid site amount (280.7 μmol·g⁻¹) among the tested samples. The elevated Mn⁴⁺ concentration enhances the catalytic activity by facilitating the activation of more reactive species, while the increased number of weak acid sites provides sufficient adsorption sites to promote NO conversion and reduce N₂O formation. Notably, XPS results also show that the Sm–Mn/Ti-1.5 catalyst exhibits a lower atomic percentage of Sm³⁺ in Sm (x(Sm³⁺/Sm)=24.1%) compared to Sm–Mn/Ti-3, and a lower atomic percentage of O_α in O (x(O_α/O)=25.1%) compared to Sm–Mn/Ti-1. This effectively regulates the dual redox cycles of Mn⁴⁺ + Sm²⁺ ↔ Mn³⁺ + Sm³⁺, sustaining high activity while mitigating losses in N₂ selectivity. Sm–Mn/Ti-3 achieves high NO conversion but exhibits lower N₂ selectivity owing to excessive x(Sm³⁺/Sm) and reduced surface acidity. Although Sm–Mn/Ti-1 has the highest value of x(O_α/O), its low NO conversion is attributed to the smaller amounts of Mn⁴⁺ species and weak acid sites. In summary, the synergistic optimization of oxidation ability and acidic sites of the Sm–Mn/Ti catalyst was achieved by regulating the loading of active components, resulting in the Sm–Mn/Ti-1.5 catalyst that balances the high NO conversion rate and good N₂ selectivity. To verify the SO₂ poisoning resistance of the Sm–Mn/Ti-1.5 catalyst, the NH₃-SCR experiments were carried out in a SO₂ atmosphere at 120 ℃. Results show that the high NO conversion rate and N₂ selectivity were maintained for 12 h without significant decrease, demonstrating–excellent SO₂ resistance. This study achieved the formation and optimization of active components in Mn-based catalysts, offering a new strategy for developing efficient low-temperature SCR catalysts.