The Performance Of NO_x Storage And Reduction Over Manganese Oxides Based Catalysts At Low Temperatures

NOxemission control of lean-burn engines is one of the great challenges in theworld. Herein, the MnOxmodel catalysts with the different calcination temperatureswere synthesized to investigate their NO adsorbability for lean-burn exhausts. Thetransformation from (β-)MnO2to (α-)Mn2O3following the increased calcinationtemperatures was evidenced from the viewpoint of the local atomic level. Amongthese samples, the one calcined at550C containing the single α-Mn2O3phasedisplayed the best NO adsorbability: NO was mainly adsorbed in the forms ofNO/nitrites and NO2/nitrates at the low and high temperatures, respectively; the NOoxidation ability displayed the volcano-shape following the increased operatingtemperatures, and reached the maximum, i.e.92.4%of the NO-to-NO2conversion, at250C. Moreover, this sample presented the efficiently reversible NO adsorption/desorption performance in alternative lean-burn/fuel-rich atmospheres, due to theweakly bonded NOxon it. The superficial oxygen species plays a critical role for theNO oxidation over α-Mn2O3. The consumed superficial oxygen could be furthercompensated by the gaseous and lattice oxygen therein. The process for the NOadsorption and oxidization appeared to follow the Langmuir-Hinshelwood and/orEley-Rideal mechanism. Compared with the Mn2O3sample, the MnO2sampleshowed large NOxstorage capacity at low temperatures. It also displayed weakly NOcatalytic oxidation, which the NO adsorbed species were mainly stored as nitrite. TheMn3O4sample displayed higher NO catalytic oxidation ability. However, it showedworse NSR ability due to the unstable nitrate species.In order to improve the NSR performance, we prepared the noble catalysts usingmechanical mixing method. We studied the effect of noble metals on the catalyticactivities of catalysts and found that the Pd and Rh had strong interaction with MnOx.During the pre-reduction process, it easily leaded to the change of Mn2O3phase toMn3O4phase, which decreased the NSR performance of the catalysts. The Pt catalystdisplayed the better redox ability and performed higher NSR activity, because of thesynergistic effect between Pt and MnOx. Moreover, the NOxconversion of the1.5Pt-Al-Mn catalyst could reach100%, and the N2selectivity was also higher, i.e.98.6%when used H2as reducing agent. When C3H6was used as the reducing agent,the NOxconversion reached98.4%, but the N2selectivity was only53.2%. The bestloading of Pt over the catalysts was1.5wt.%. Furthermore, we also investigated the NSR mechanisms of the1.5Pt-Al-Mncatalyst. Our findings show that the superficial oxygen species plays a critical role forthe NO oxidation. The active sites of oxidation reaction were Mn3+and Pt species,while the active site of reduction reaction was Pt species.