SCR Architectures for Low N ₂ O Emissions 2015-01-1030

The high global warming potential of nitrous oxide (N₂O) led to its inclusion in the list of regulated greenhouse gas (GHG) pollutants [1, 2]. The mitigation of N₂O on aftertreatment catalysts was shown to be ineffective as its formation and decomposition temperatures do not overlap. Therefore, the root causes for N₂O formation were investigated to enable the catalyst architectures and controls development for minimizing its formation. In a typical heavy-duty diesel exhaust aftertreatment system based on selective catalytic reduction of NOx by ammonia derived from urea (SCR), the main contributors to tailpipe N₂O are expected to be the undesired reaction between NOx and NH₃ over SCR catalyst and NH₃ slip in to ammonia slip catalyst (ASC), part of which gets oxidized to N₂O.

Based on the empirical measurements and reaction engineering principles it was established that under process conditions, the NOx and NH₃ concentration profiles in the catalyst exponentially decline along the axial direction due to their consumption by SCR reactions. This also entails the decrease in the evolution of N₂O, an SCR byproduct, along the axial length of the catalyst. In this study, we show that the part of the SCR catalyst facing the exhaust gas where majority of NOx conversion and N₂O formation occurs, when replaced with a catalyst that makes less N₂O, for example V-SCR or Fe-SCR, can result in lower tailpipe N₂O without compromising the NOx conversion. When SCR catalyst facing the exhaust gas has lower NH₃ storage, such architectures also lead to lower NH₃ slip into ASC during low to high temperature transients. Several SCR architectures based on the above principles, i.e., combination of catalysts with inherently low N₂O formation and low NH₃ storage functions with catalysts having high NOx conversion ability, that have the potential to decrease N₂O make will be discussed.