Investigation On The Role Of Mn Species And CeO₂ Support For NO Oxidation Reaction Over MnO_x-CeO₂ Catalysts

NO oxidation to NO₂ is a critical reaction step for two typical diesel NOx emission after-treatment techniques,NSR?NO_x Storage and Reduction?and SCR?Selective Catalytic Reduction?.Though Pt-based catalysts have excellent catalytic performance and have been used for NO oxidation,its practical application is limited to the high cost of noble metals.Hence,it is required to develop high performance,low cost and durable NO oxidation catalysts.MnO_x-CeO₂ catalystshave attracted growing attentions in oxidizing NO to NO₂ due to its strong oxidizing property and the low cost.The two sections ofthis study focus on the active Mn species over MnO_x-CeO₂ oxides for NO oxidation and the main contribution of CeO₂support.Low Mn doping amounts?mole ratio?5%?of the MnO_x-CeO₂ samples are designed to simplify the interaction between MnO_x and CeO₂.Several techniques including XRD,BET and XPS were used to study the structure features and redox properties.It is concluded that trace amount of manganese tends to enrich on the surface of CeO₂ as higher oxidation states.When the amount gets larger,manganese is introduced into CeO₂ lattice as Mn²⁺.According to H₂-TPR results,the redox properties of surfaceand sub-surface oxygen on CeO₂ have been enhanced by Mn doping,promoting NO oxidation reaction activity on MnO_x-CeO₂ mixed oxides.In kinetic study,TOF is calculated with H₂ consumption form active Mn⁴⁺/Mn³⁺to Mn₃O₄ by H₂-TPR.The result shows that Mn⁴⁺/Mn³⁺is the active species for NO oxidation reaction over MnO_x-CeO₂ mixed oxides.MnO_x-CeO₂ mixed oxides with various crystallization treatments are designed to make differences in strcture features and redox properties of CeO₂ support.Crystallization treatment leads to larger specific surface area,smaller crystal size and weaker lattice contraction of CeO₂.CeO₂ with the least crystallization treatment contains more Ce³⁺and active oxygen species on the surface,which contributes to stronger interaction between Mn and Ce.According to H₂-TPR,the redox properties have been enhanced by Mn doping.Mn species remains higher oxidation states by oxygen transport from CeO₂,promoting NO oxidation reaction activity.And also,CeO₂ provides active oxygen species for NO oxidation reaction.However,there is trade-off relationship between activity and stability,which may limit its practical application.