Roles of C₃H₆ in NH₃ generation and NO_x reduction over a Cu-chabazite SCR catalyst under lean/rich cycling conditions

We studied the spatiotemporal profiles of NO_x reduction with NH₃, C₃H₆, or NH₃ + C ₃H₆ over a commercial Cu-chabazite SCR catalyst (washcoated honeycomb monolith) to better understand the effects of C ₃H₆ on NO_x reduction under fast lean/rich cycling conditions relevant to coupled LNT-SCR catalysts. NO_x reduction by NH₃ was very effective with total NH₃ consumption within the first quarter of the catalyst at all temperatures. By contrast, NO_x reduction by C₃H₆ was more gradual along the catalyst length and sensitive to temperature with maximum performance obtained at 300-400 °C. Temperature-programmed desorption performed after cycling experiments with C₃H₆ evidenced the presence of NH₃ and/or NH₃-precursor intermediates on the surface. These surface species were formed as a result of reactions between NO_x and C₃H₆ during the rich as well as lean phases, but started to be used for NO_x reduction only when all the stored hydrocarbons were depleted. When fed together, the contributions of C₃H₆ and NH₃ to the cycle-averaged NO _x conversion were essentially additive. However, temporally resolving N₂ formation using isotopically-labeled NO and a mass spectrometer revealed that C₃H₆ actually inhibited reactions between NO_x and feed NH₃ until well into the subsequent lean phase, i.e., until the stored hydrocarbons were depleted. This study highlights the importance of controlling lean/rich cycling time partitioning to alleviate the impact of the inhibiting effect of C₃H₆ on NH ₃ chemistry and achieve the maximum NO_x reduction possible over the SCR catalyst in coupled LNT-SCR systems.