Supported?Pt,MO_x?-K₂CO₃ Based Lean NO_x Trap Catalysts:Catalytic Performance And Reaction Mechanism

Lean-burn NO_x trap?LNT?is recognized as one of the most promising solution to lean NO_x emission.In present work,a series of LNT catalysts with potassium carbonates as storage components exhibits large NSC and effectively prevent NO_x escaping during lean-burn condition.In addition,a series of novel and highly efficient non-platinic LNT catalysts with the active components Pt completely replaced by transitional metal oxides are designed and explored.A series of LNT catalysts Pt-K₂CO₃/K₂Ti₈O₁₇ with different weight loading of K₂CO₃ were synthesized by successive impregnation.The catalyst with 25% K₂CO₃ loading exhibits the largest NSC?2.68 mmol/g?and the highest NO_x reduction percentage?NRP: 99.2%?.HR-TEM,FT-IR and TPD results indicate that the K species exist as-OK groups,K₂O,surface K₂CO₃ and bulk or bulk-like K₂CO₃;surface K₂CO₃ species are identified as the most favorable species to NO_x storage.A carbonate-involved NO_x storage/reduction mechanism is revealed based on in situ DRIFTS results.The effect of support calcinations temperature on the activity of Pt-K₂CO₃/K₂Ti₈O₁₇ is investigated.The results of NO_x storage and reduction show that the catalyst with its support calcined at 350 ? possesses the largest NSC?0.68 mmol/g?and the highest NRP?99.1%?.The results of XRD and TPD indicate that the calcination over 650? can cause the decomposition of K₂Ti₈O₁₇ to K₂Ti₆O₁₃ and TiO₂,making the specific surface areas decrease remarkably and the amount of bulk or bulk-like potassium carbonate increases obviously.Thus,the NSC decreases with the elevating of support calcinations temperature.Manganese oxides,which are very active for the oxidation of NO to NO₂,were selected to completely replace Pt to decrease the cost of LNT catalysts and a non-platinic LNT catalyst MnO_x-K₂CO₃/K₂Ti₈O₁₇ was prepared.The optimal mass ratio of Mn/K₂Ti₈O₁₇ is 10% and the optimal K₂CO₃ loading is 25% by weight.The catalyst with excellent activity exhibits the largest NSC?3.21 mmol/g?,the highest NRP?98.5%?and an ultralow selectivity of NO_x to N₂O?0.3%?.Except for K₂O,-OK groups,surface K₂CO₃ and bulk or bulk-like K₂CO₃,an unknown titanate phase with a K/Ti atomic ratio higher than 2/8 is identified,which is also active and regenerative for NO_x storage and reduction.Nanobelt morphology of support is well maintained in the catalysts in fresh state or after used in NSR tests and MnO₂,Mn₂O₃ and Mn₃O₄ are coexisting in the catalysts,little change has taken place for the Mn species before and after NO_x storage or cyclic NO_x storage and reduction,which indicates the high stability of catalyst.The NO_x storage and reduction performance over the LNT catalysts MOx-K₂CO₃/K₂Ti₈O₁₇?M = Ce,Fe,Cu,Co?completely using base metal oxides as active phases is investigated.The Co-based catalyst exhibits the largest NSC?3.32 mmol/g?,the highest NRP?99.0%?and an ultralow selectivity of NO_x to N₂O?0.3%?.The excellent performance of this catalyst results from its largest amount of surface active oxygen species as revealed by XPS,O₂^- TPD and NO-TPD.Based upon the in situ DRIFTS results,the predominant NO_x storage species is revealed as bidentate nitrites formed via multiple kinetic pathways.Co-K based inexpensive LNT catalysts Co-K₂CO₃/MOx?M = Ce,Zr,Ti,Al?with different metal oxides as support were developed.The catalyst Co-K₂CO₃/CeO₂ displays the largest NSC?2.88 mmol/g?and the highest NRP?98.9%?.The excellent performance of this catalyst results from the catalytic synergistic effect between cobalt and cerium oxides,which promotes the oxidation of NO to NO₂ up to 62%.In situ DRIFTS indicates that pure support could provide some storage capacity,however,its contribution can be neglected;the main NO_x storage route is via the potassium sites as bidentate nitrites;gas oxygen is the dominant active oxidative species;the most favorable temperature for NO_x storage in the form of nitrate is around 350 ?.