Catalytic Performance Of Manganese-based NO_x Storage-reduction Catalysts

Lean burn combustion technology is the effective measures to reduce vehicle emissions and fuel consumption.Compared with the traditional engine,lean burn engine normally can save 15-20 % of the fuel.Carbon monoxide,hydrocarbons and greenhouse gas CO₂ in the exhaust gas can also be greatly reduced.However,under lean burn condition,the excess oxygen inevitably caused the high concentration of NO_x.The NO_x removal efficiency of the existing three-way catalysts is very low.NO_x storage reduction（NSR）technology is one of the most effective measures to eliminate NO_x emission.A series of noble metal-free Mn-K₂CO₃/γ-Al₂O₃ catalysts with different Mn loadings for NSR were prepared via dry impregnation.The effects of manganese loadings on the structure and catalytic activity of the catalysts were investigated,and the effects of H₂ O and CO₂ on the NO_x storage capacities were also investigated.The techniques,including XRD,EXAFS,H₂-TPR and CO₂-TPD,were used to characterize the structure of the catalysts.The results showed that manganese was mainly in the forms of MnO₂.With the increase of Mn loading,the redox ability of the catalysts to NO_x was enhanced.However,with the increase of Mn loading,the specific surface area of the catalysts decreased,resulting in the decrease of the K₂CO₃ distribution,and the surface K₂CO₃ gradually turned into bulk K₂CO₃.The NO_x storage capacity（NSC）was affected by the two factors,namely catalyst redox ability and K₂CO₃ dispersion.When the mass ratio of Mn/Al₂O₃ is 0.10,the catalyst has the largest NSC（1.30 mmol/g）.After 10 cycles under lean-burn/fuel-rich conditions,the NO_x reduction efficiency reached as high as 99 %.The NSC of the catalyst decreased when H₂ O and CO₂ were added to the reaction gas,and the effect of CO₂ was larger than H₂ O.A series of noble metal-free Mn-K-CHT catalysts for NSR were prepared using hydrotalcite-derived mixed oxide MgAlOx（denoted as CHT）as support and K₂CO₃,CH3 COOK or K3C6H5O7 as the precursor of potassium,respectively.The effects of different precursor of potassium on the structure and catalytic activity of the catalysts were investigated.The catalysts were characterized by XRD,FT-IR,CO₂-TPD,EXAFS,H₂-TPR and in situ DRIFTS.The results indicate that all precursors of potassium turn into K2 O,-OK groups,surface K₂CO₃ and bulk or bulk-like K₂CO₃ eventually.The different precursors of potassium can influence the existing state of potassium and also influence the oxidation-reduction ability of the catalysts.MnO₂ is the main active species for NO_x storage and reduction.The catalyst with the higher the relative proportion of MnO₂ had better activity.The activity test experimental results show that the NSCs of Mn-CH3COOK-CHT and Mn-K3C6H5O7-CHT are 1.33 and 1.58 mmol/g,respectively,which are larger than Mn-K₂CO₃-CHT（1.16 mmol/g）.After 10 cycles,all of the NO_x reduction percentages（NRP）of the catalysts are very high.However,after 20 cycles,the advantage of Mn-CH3COOK-CHT and Mn-K3C6H5O7-CHT is significant,and the NO_x reduction percentage of Mn-CH3COOK-CHT and Mn-K3C6H5O7-CHT are 88.1% and 92.3%,respectively,which are obviously higher than Mn-K₂CO₃-CHT（81.6%）.The experimental results show that CH3 COOK or K3C6H5O7 as a precursor of potassium is better than K₂CO₃.