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

Abstract

NOx 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 NOx 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 NOx adsorber for lean-burn exhausts even at low operating temperatures.