Superior low temperature activity over α-MnO2/β-MnOOH catalyst for selective catalytic reduction of NOx with ammonia

Abstract

Manganese octahedral molecular sieves with an α-MnO₂ crystal structure (OMS-2) and their related materials have attracted significant attention for the selective catalytic reduction of NO_x using NH₃ (NH₃-SCR) at low temperatures. Further lowering their operating temperature should be an effective method to develop an environmentally friendly de-NO_x system; however, their catalytic activity at low temperatures, especially below 100 °C, remains poor. This study describes a post-synthetic approach to develop Mn-based catalysts superior to those in the literature that operate at ultralow temperatures. Post-synthetic planetary ball milling for OMS-2 caused the partial conversion of OMS-2 into β-MnOOH. The obtained nanocomposite catalysts possessed abundant surface oxygen vacancies and strong surface acidity, allowing the milled catalyst to exhibit higher NO conversion at 90 °C (91%) than that in freshly prepared OMS-2 without planetary ball milling (29%). Lowering the operation temperature of OMS-2 catalysts contributed to the suppression of N₂O evolution during NH₃-SCR over manganese-based catalysts, resulting in high N₂ selectivity over the milled OMS-2 catalyst (93%).