Spatio-temporal features of the sequential NOx storage and reduction and selective catalytic reduction reactor system

Abstract

Combined NOx storage and reduction (NSR) and selective catalytic reduction (SCR) were conducted in a sequential reactor system containing a Pt/Rh/BaO/Al2O3 Lean NOx Trap (LNT) catalyst and Cu-SSZ-13 SCR catalyst. Spatially-resolved mass spectrometry (SpaciMS) was used to construct temporal concentration profiles spanning the two monolith catalysts. The effects of feed gas temperature, gas hourly space velocity (GHSV) and carrier gas water were examined with propylene as the reductant. The working concept of the sequential LNT+SCR is evident in both the transient and cycle-averaged concentration profiles. During the rich phase NH3 is generated in the upstream LNT and trapped in the downstream SCR where it reacts with NOx that slips from the LNT during the subsequent lean phase. The instantaneous profiles provide insight into the storage and reduction dynamics and the mass coupling between the LNT and SCR catalysts. Axial gradients in the NOx storage and release during the lean and rich phases confirm classical LNT cyclic behavior. The spatio-temporal temperature measurements reveal a large exotherm caused by the propylene oxidation, manifested as a propagating temperature front. The cycle-averaged concentration profiles help to pinpoint the LNT length that gives a product mixture having a NH3/NOx ratio approaching unity, the desired stoichiometry for promoting NOx reduction in the SCR. The generation of NH3 and conversion of NOx is enhanced by water, suggesting an important role of the water gas shift chemistry. Propylene consumption and breakthrough from the LNT reveals its role in contributing to the overall NOx reduction. A non-NH3 SCR reaction pathway is identified that has an increasing contribution down the length of Cu-SSZ-13 SCR catalyst. The generation of formaldehyde over the Cu-SSZ-13 SCR catalyst suggests a pathway resulting from breakthrough of propylene from the LNT, followed by its oxidation to acrolein and followed in turn by reverse aldol condensation.