NO adsorption behaviors of the MnOx catalysts in lean-burn atmospheres

NO(x) emission control of lean-burn engines is one of the great challenges in the world. Herein, the MnOx model catalysts with the different calcination temperatures were synthesized to investigate their NO adsorbability for lean-burn exhausts. The transformation from (β-)MnO₂ to (α-)Mn₂O₃ following the increased calcination temperatures was evidenced from the viewpoint of the local atomic level. Among these samples, the one calcined at 550 °C containing the single α-Mn₂O₃ phase displayed the best NO adsorbability: NO was mainly adsorbed in the forms of NO/nitrites and NO₂/nitrates at the low and high temperatures, respectively; the NO oxidation ability displayed the volcano-shape following the increased operating temperatures, and reached the maximum, i.e. 92.4% of the NO-to-NO₂ conversion, at 250 °C. Moreover, this sample presented the efficiently reversible NO adsorption/desorption performance in alternative lean-burn/fuel-rich atmospheres, due to the weakly bonded NO(x) on it. The superficial oxygen species plays a critical role for the NO oxidation over α-Mn₂O₃. The consumed superficial oxygen could be further compensated by the gaseous and lattice oxygen therein. Our findings show that the α-Mn₂O₃ material is a promising NO(x) adsorber for lean-burn exhausts even at low operating temperatures.