Engineering 3D structure Mn/YTiOx nanotube catalyst with an efficient H2O and SO2 tolerance for low-temperature selective catalytic reduction of NO with NH3

Abstract

TiO₂ with a 3D structure is considered to be a promising support for Mn-based catalysts for the NH₃-SCR reaction, but it is still insufficient to solve problems such as poor N₂ selectivity and tolerance of H₂O/SO₂ at low temperature. In this work, a novel 3D-structured Mn/YTiO_x nanotube catalyst was designed and the role of Y on the catalytic performance was investigated for the NH₃-SCR reaction at low temperature. The results indicated that the Y-doped TiO_x gradually transformed from nanotubes to nanosheets with the increase in Y doping, leading to a reduction in specific surface area and Brønsted acid sites. An appropriate amount of Y doping could distinctly improve the dispersion of MnO_x and increase the concentration of surface Mn⁴⁺, Lewis acid sites and chemisorbed oxygen of catalysts, which was beneficial to the low-temperature NH₃-SCR reaction, while excessive Y doping could cause a sharp decrease in specific surface area and Lewis acid sites. Therefore, Mn/YTiO_x catalysts exhibited a volcano-type tendency in NO conversion with an increase in Y doping, and the highest activity was obtained at 3% doping, showing more than 90% NO conversion and N₂ selectivity in a wide temperature window from 120 to 320 °C. The N₂ selectivity and H₂O/SO₂ resistance of the catalysts was also enhanced with the increase in Y doping mainly due to the increased chemisorbed oxygen and electron transfer between Y and Mn. An in situ DRIFTS study demonstrated that Lewis acid sites played a more important role in the reaction than Brønsted acid sites, and the coordinated NH₃ absorbed on Lewis acid sites, –NH₂, monodentate nitrate and free nitrate ions were the main reactive intermediate species in the NH₃-SCR reaction over an Mn/3%YTiO_x catalyst. Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) reaction mechanisms co-existed in the NH₃-SCR reaction, but the L–H reaction mechanism predominated.