| NOX storage and reduction reaction is an effective method for the removal of NOX released from lean-burn engines, and Pt/Ba/Al is the most commonly studied model catalyst for NOX storage and reduction. However, the practical application of the Pt/Ba/Al catalyst is limited by its high cost and poor NOX removal efficiency at low temperatures (≤300℃). Based on such research background, a new way for removal of NOX emissions from lean burn engines, which combined non-thermal plasma and hetergeneous catalysis for NOX storage and reduction, were proposed. During lean phase, nitrogen oxides were stored on the catalyst; while H2-plasma was introduced in the rich phase to assist the catalyst’s regeneration in low temperature. Through such a NOX storage-plasma assisted regeneration cycle, higher NOX conversion could be obtained in the low temperature range (<300℃). The main research results are summarized as follows:(1) In order to study the role of manganese in LNT catalysis, model Pd/Mn/Ba/Al, Pd/Mn/Al and Pd/Ba/Al catalysts were prepared and characterized. Compared to the Pd/Ba/Al reference, Pd/Mn/Ba/Al showed significantly improved NOX conversion under lean-rich cycling conditions as a consequence of its increased activity for NO oxidation and hence, superior NOX storage efficiency. In addition, the presence of Mn greatly lessened the inhibiting effects of H2O and CO2on cycle-averaged NOX conversion, this being due to the facile decomposition of manganese carbonate at low temperatures as evidenced by DRIFTS. Significantly, the Pd/Mn/Ba/Al catalyst displayed comparable activity to a traditional LNT catalyst of the Pt/Ba/Al type, showing the promising prospect of such new type of LNT catalysts.(2) Based on its high NOX storage capacity, a study of the properties of LaMno.9Fe0.1O3in NOX storage-reduction catalysis was undertaken. Though the LaMno.9Fe0.1O3perovskite has high NSC values even at relatively low temperature (≤300℃), it shows much lower activity during lean-rich cycling compared with a traditional Pt/BaO/Al2O3catalyst, indicating that regeneration of stored NOX is the rate limiting step for the perovskite catalyst. By employing an H2-plasma in the rich phase to assist reduction of the stored NOX, the NOX conversion is greatly improved, especially at low temperature. However, the NOX removal efficiency was greatly depressed by the presence of H2O and CO2in the feed. In order to achieve a better NOX removal efficiency during plasma-assisted NOX storage-reduction process even in the presence of H2O and CO2, a Pt/Ba/Al+LaMno.9Fe0.1O3mixing catalyst (PBA+LMF) was prepared by mechanical mixing method. A higher NOX conversion (>80%) could be obtained over the PBA+LMF sample at a wide temperature range (150-350℃). (3) Based on their high NOX storage capacities, a series of M/Ba/Al (M=Mn, Fe, Co, Ni, Cu) catalysts were evaluated in NOX storage-reduction catalysis. Although the M/Ba/Al catalysts exhibited high NSC values, they exhibited much lower activity during lean-rich cycling compared with a traditional Pt/Ba/Al catalyst. By employing an Hb-plasma in the rich phase to assist reduction of the stored NOX, the NOX conversion was greatly improved, especially for Co/Ba/Al catalyst. The Co/Ba/Al catalyst exhibited similar catalytic properties to those of the Pt/Ba/Al catalyst when an H2-plasma was employed in the rich phase to assist reduction of the stored NOX.However, the NOX removal efficiency was greatly depressed by the presence of H2O and CO2in the feed. Notably, the addition of Pd to the Co/Ba/Al sample greatly lessened the inhibiting effects of H2O and CO2on plasma-assisted cycle-averaged NOX conversion, this being due to the excellent NOX storage performance of the Pd/Co/Ba/Al sample even in the presence of H2O and CO2. The results of the present study show that by combining the high NOX storage capacity of Pd/Co/Ba/Al in the lean phase with non-thermal plasma-assisted activation of the reductant in the rich phase, high NOX conversion can be obtained over a broad temperature window of150-350℃. |
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