SCR Architectures for Low N
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.