NH₃ formation over a lean NO_X trap (LNT) system: Effects of lean/rich cycle timing and temperature

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• 作者：Christopher D. DiGiulio ; Josh A. Pihl ; Jae-Soon Choi ; James E. Parks II ; Michael J. Lance ; Todd J. Toops ; Michael D. Amiridis
• 关键词：NO reduction ; Lean NOX trap (LNT) ; Selective catalytic reduction (SCR) ; LNT-SCR ; NH3 formation
• 刊名：Applied Catalysis B: Environmental
• 出版年：5 April, 2014
• 年：2014
• 卷：147
• 期：Complete
• 页码：698-710
• 全文大小：2928 K

文摘

A commercial lean NO_X trap (LNT) catalyst containing Pt, Pd, Rh, BaO, and CeO₂ was evaluated in this investigation. The effects of lean/rich cycle timing on the NO_X, CO and C₃H₆ conversions and on the NH₃ and N₂O selectivities were considered. Two distinct lean/rich cycling regimes were identified. At low temperatures, NO_X release and reduction were relatively slow processes. As a result, a longer, lower-concentration rich dose favored increased cycle averaged NO_X conversions. For example, extending the rich period from 5 to 15 s at 250 掳C, while holding the overall reductant dose constant, resulted in an increase in cycle averaged NO_X conversion from 59 to 87%. At high temperatures, the opposite was found to be true. Above 450 掳C, NO_X release and reduction occurred very rapidly and shorter, higher concentration rich doses yielded significantly higher NO_X conversions. For example, extending the rich period from 5 to 15 s at 500 掳C, while holding the overall rich dose constant, resulted in a decrease in the cycle averaged NO_X conversion from 76 to 54%. The selectivities to NH₃ and N₂O were found to be primarily a function of temperature, with both being higher at lower temperatures. The effect of cycle timing and reductant concentration were of secondary importance. In contrast, NH₃ and N₂O yields were significantly affected by the cycle timing since they depend on the NO_X conversion. Therefore, any combination of changes in the lean/rich timing protocol or reductant concentrations that resulted in increased NO_X conversion also resulted in increased NH₃ and N₂O yields for a given temperature. Thus, concerted control of NH₃ generation by varying the lean/rich cycle timing was demonstrated for this LNT catalyst. At both lower and higher temperatures, variations in the rich cycle duration resulted in NH₃/NO_X ratios that could extend the region of operation for a close-coupled LNT-SCR system, with the greatest effect observed at temperatures between 250 and 450 掳C. However, N₂O yield also increased with NH₃ yield under the same conditions.