Selective catalytic reduction of NOx by NH₃ at low temperature over manganese oxide catalysts supported on titanate nanotubes

We report the NO conversion with NH₃ at low temperature over MnO_x-supported on titanate nanotubes. The effect of SO₂ and water on catalytic activity was also analyzed. For the catalytic activity tests, three catalysts with 1, 3, and 5 wt.% of MnO_x were synthesized. A high NO conversion (92%) at 180 °C is reported for the 1Mn/NTiO₂ catalyst. The further addition of MnO_x to the support improves the catalytic activity but NO conversion (95%) was shifted to a higher temperature (from 180 to 220 °C). However, the presence of SO₂ (50 ppm) and water (5 vol.%) diminishes the NO conversion up to 60%, which remains after 300 min. We found that addition of MnO_x increases the Lewis acid sites concentration of the nanotubes. As the Lewis acid sites increase (from 1.86 to 3.56 µmol m⁻²), the Mn⁴⁺/Mn³⁺ ratio on the surface of the nanotubes decreases (from 4 to 1.6), which indicates that the surface of the catalysts is deficient in electrons. We concluded that a high Lewis acid sites concentration and a low Mn⁴⁺/Mn³⁺ ratio, hence a surface deficient in electrons, improves the catalytic activity to remove NO on the Mn/NTiO₂ catalysts.