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/YTiOx 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 TiOx 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 MnOx 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/YTiOx 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%YTiOx 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.