Catalytic reduction of NOₓ with NH₃ over different-shaped MnO₂ at low temperature

MnO₂ nanotubes, nanorods, and nanoparticles were prepared using a hydrothermal method, after which the different activities for selective catalytic reduction (SCR) of nitrogen oxides (NOₓ) were compared. MnO₂ nanorods performed the highest activity for reduction of NOₓ under a gas hourly space velocity of 36,000h⁻¹ with conversion efficiencies of above 90% between 250 and 300°C; it also had the highest removal efficiency of 98.2% at 300°C. From the analysis of X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, and temperature-programmed reduction, we can ascribe the high activity of MnO₂ nanorods to low crystallinity, more lattice oxygen, high reducibility, and a large number of strong acid sites. The apparent activation energy of the SCR reaction on the surface of nanorods was calculated to be 20.9kJ/mol, which favored the reaction better than the other catalysts.