| Lean-burn NOx trap (LNT), also called as NOx storage and reduction (NSR), is recognized as one of the most effective and promising solutions to the NOx released from lean-burn engines. Conventional LNT catalysts often use noble metals such as Pt or Rh as active components, exhibiting high catalyst cost. Meanwhile, aiming at the situation of high-concentration NOx in the exhaust, stand-alone LNT catalyst cannot capture and store NOx completely in the lean period. As a consequence, it is necessary to develop novel and highly efficient noble-metal-free LNT catalyst, or novel coupled catalytic systems such as SCR-LNT system. In this work, ceria supported nonplatinic LNT catalyst using LaCoO3 perovskite as active phase is designed and employed in lean NOx storage and reduction; in addition, a dual-bed SCR-LNT coupled system is also proposed and employed in the elimination of lean-burn NOx with relatively high concentration.A series of ceria supported perovskite-based lean-burn NOx trap (LNT) catalysts 10 wt.% LaCoO3/xK2CO3/CeO2 (denoted as L/xK/C, x=1,3,5,8 wt%) were prepared by successive impregnation. The catalyst L/3K/C exhibits the best performance of NOx storage and reduction, over which a particularly high NOx reduction percentage (97.7%) is achieved in cyclic lean/rich atmospheres. FT-IR and CO2-TPD results demonstrate the diversity of K species including -OK groups, dispersed surface K2CO3 species and bulk or bulk-like K2CO3 species. On L/xK/C NOx is mainly stored as diverse nitrates during NOx adsorption and storage, which indicates the high capability of these catalysts for NOx oxidation.To further increase the BET specific surface area and the reducibility of the support, a series of non-platinic ceria-based solid solution supported LNT catalysts LaCoO3/K2CO3/S (S=Ce0.75Zr0.25O2 or 5%Y/Ce0.75Zr0.25O2) were prepared by using the same successive impregnation method. It is found that the doping of Zr or YZr into CeO2 facilitates the formation of CeZrO binary or YCeZrO ternary solid solutions, increasing the specific surface area of the catalyst and promoting the LaCoO3 dispersion, which is favorable for fast NOx storage. The YCeZrO ternary solid solution supported catalyst containing 5 wt.% K2CO3 exhibits the best performance for NOx storage and reduction at 350℃, showing a very high NOx reduction percentage (98.2%) and an extremely high NOx-to-N2 selectivity (98.8%) in the absence of CO2. After addition of 5 vol.% CO2 in the atmosphere, the NOx reduction percentage during NOx storage and reduction tests can still be maintained at high level (above 90%) in the temperature region of 350-400 ℃. The results of FT-IR, CO2-TPD and in situ DRIFTS indicate the diverse K species and the reaction pathways of NOx storage and reduction.Aiming at the elimination of high-concentration NOx, the dual-bed SCR-LNT coupled system is designed based on the configuration of Cu/Al2O3-Lao.7Sro.3Co03 and Cu/ZSM-5-Lao.7Sro.3Coo.9Pdo.i03. According to the catalytic performance for C3H6-SCR reaction, the optimized Cu loading in Cu/Al2O3 and Cu/ZSM-5 is 5 wt.% and 7.5 wt.%, and the optimal active temperature is 300℃ and 350℃, respectively. Under the cyclic lean/rich atmospheres, stand-alone SCR catalyst can present highly efficient and stable performance of lean-burn NOx elimination, while the dual-bed SCR-LNT coupled system can further promote the capability of lean-burn NOx removal. Comparing with stand-alone LNT catalyst, the dual-bed SCR-LNT coupled system decreases the NOx leak in both the lean period and rich period, thus promoting the total NOx reduction percentage significantly. Simultaneously, the LNT catalyst in the dual-bed SCR-LNT coupled system exhibits high NOx storage efficiency in lean period and higher NOx reduction efficiency in rich period than the stand-alone LNT catalyst, broadening the "temperature window" to some extent. All these reflect the advantages of dual-bed SCR-LNT coupled system. |
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