Research On The Active Species On MnO_x-CeO₂ Oxides For NO Oxidation And The Thermal Stability

NSR?NO_x Storage and Reduction?and SCR?Selective Catalytic Reduction?,have been developed to convert NO_x to harmless N₂.In both techniques,NO to NO₂oxidation reaction is a critical step.Pt-based catalysts have long been used for NO oxidation to NO₂ due to the excellent catalytic oxidation activity.However,its use is limited to the high cost of noble metals.Thus,the inexpensive NO oxidation catalysts with high catalytic performance are required to develop to substitute noble metal catalysts.In recent years,MnO_x-CeO₂ oxides have attracted growing attentions in oxidizing NO to NO₂.This paper is focus on the active species over MnO_x-CeO₂oxides for NO oxidation.The MnO_x-CeO₂ oxides with different aging treatment were prepared by the impregnation method.XRD,BET,and SEM were conducted to characterize the physical and textural properties of the catalysts.XPS and H₂-TPR were performed to obtain the chemical states and redox properties.The active manganese species over MnO_x-CeO₂ oxides for NO oxidation were investigated.In addition,the thermal stability of the catalysts was inspected.The results demonstrate that there are two kinds of manganese species exist in MnO_x-CeO₂ oxides:the Mn ions incorporated into the ceria lattice with high valence,and the manganese oxide dispersed on the surface of ceria with low valence.According to the kinetics study,more than one kind of manganese species is active for NO oxidation.The manganese oxides with high valence have the best activity.The ceria with more small pores and surface active species are beneficial to the form of high valence manganese ions.The results indicate that the physical and chemical properties change a lot after one hour aging treatment.After six hours aging,the rate of change in the structure and properties slows down.The aging of catalysts causes the sintering of the catalysts.Meanwhile,the amount of high valence manganese ions and active oxygen decreases,which may account for the poor redox properties and decreased NO oxidation activity.