Catalytic Performance Of SmMn₂O₅ Mullite For Diesel Exhaust Purification And Methane Combustion

Lean-burn diesel engines have attracted much attention due to their higher fuel effi-ciency.However,high air/fuel ratio with oxygen rich atmosphere leads to high content of nitrodren oxides?NO_x?and soot in exhaust emission,which is difficult to remove.As for NO_x elimination,there are various technologies including SCR?Selective Catalytic Reduc-tion?,NSR?NO_x Storage Reduction?and LNT?Lean NO_x Trap?etc.,and NO oxidation al-ways plays a key role in those technologies;while DPF?Diesel Particle Filter?is mostly applied for soot purification through catalytic combustion.Though the natural gas-diesel dual fuel vehicle can partly decrease the emission of NO_x and soot,the unburned methane in exhaust would contribute to the greenhouse effect,and its global warming potential is approximately 21 times higher than that of carbon dioxide.The low temperature catalytic combustion has been considered as an effective measure to dispose lean methane for pollu-tion abatement.Recently,the mullite oxides have attracted much attention due to their high oxidative catalytic activity,strong thermal stability and low cost.Therefore,the manga-nese-based mullite SmMn₂O₅ was chosen and applied for catalyzing NO oxidation,soot combustion and methane combustion reactions.The catalytic activity was ehhanced through La partial substitution,morphology control and Co₃O₄ deposition,respectively.Various physico-chemical characterizations were conducted to investigate the influence of constitution,structure on catalytic performance.Furthermore,the reaction mechanism of the catalysts was also studied.A series of La_xSm_1-xMn₂O_??x=0,0.1,0.3,0.5?catalysts were synthesized through a co-precipitation method,the catalytic activity of NO oxidation was enhanced with La sub-stitution,and the maximum activity was achieved at x=0.3.XRD and HRTEM results re-vealed the formation of multiphase oxide as well as the interface structure between mullite?SmMn₂O₅?phase and Mn-rich perovskite(La_0.96MnO_3.05)phase.The La addition not only affected the amount of surface adsorbed oxygen and surface Mn⁴⁺ions,but also improved the reducibility of surface adsorbed oxygen.The NO oxidation performance was enhanced by promoting the decomposition of nitrate/nitrite species and desorption of NO₂.Based on the in situ DRIFTS results,mono-,bi-dentate and bridged nitrates formed on the surface were determined to be primary reaction intermediates,the reaction pathways were proposed as well.A series of mullite SmMn₂O₅ oxides were prepared by the citric acid?CA?,hydro-thermal?HT?,co-precipitation?CP?,and combustion of ethylene glycol and methanol solu-tions?EG&M?methods and tested for NO_x-assisted soot combustion.SmMn₂O₅-EG&M catalyst exhibited the overall highest soot combustion catalytic activity and lowest activa-tion energy of 65kJ/mol.It obtained a distinct morphology with slabs separated by inter-connected macropores allowing the soot particles to transfer easily throuth the structure and leading to more“catalyst/soot/gas”active sites.Moreover,the mobility and reducibility of surface adsorbed oxygen was improved for the EG&M sample as well and lowered the ig-nition temperature for soot combustion.The soot combustion was greatly accelerated by the NO₂-assisted mechanism.A series of Co₃O₄/SmMn₂O₅ oxides with different Co content were prepared through deposition-precipitation method and applied for methane combustion.The catalytic activity got dramatically enhanced for Co₃O₄/SmMn₂O₅ with Co/SmMn₂O₅ theritical weight ratio of 50 wt.%compared with SmMn₂O₅ mullite,achieving a comparable activity with Co₃O₄.The catalytic stability got improved compared with Co₃O₄ and Co₃O₄/?-Al₂O₃ with same Co content,showing better catalytic performance than Co₃O₄ after recycle tests,high tem-perature aging and long-term experiments.Deposition of Co₃O₄ induced more surface ad-sorbed oxygen formation,facilitated the activation and reduction and enhanced the activity of surface lattice oxygen.Thermal stable SmMn₂O₅ dispersed Co₃O₄ and partially inhabited the particle aggregation at high temperature,resulting in enhanced thermal resistance for the composite cataltyst.