Fast cycling NOx storage and reduction: Modeling and analysis of reaction pathways, transport and reductant effects

Abstract

NOx Storage and Reduction (NSR) conducted at higher cycling frequency enables enhanced conversion of NOx emitted by lean burn gasoline and diesel vehicles. Ting et al. (2018) reported data using a Pt/Rh/BaO/CeO2/Al2O3 showing that stored NOx utilization is primarily responsible for enhanced NOx conversion during fast cycling. In this study a low-dimensional, two-phase monolith reactor model comprising a network of reactions with global kinetics using C3H6 as the reductant is developed. Kinetic parameters not available from the literature are estimated through a fit of NSR with conventional cycling and anaerobic rich feed devoid of CO2 and H2O, while the model is validated for NSR spanning a range of cycle times for a rich feed containing CO2 H2O, and O2. The model predicts most of the data features, including transient and cycle-averaged species concentrations and temperature. The model elucidates differences between C3H6 and H2 during both conventional and fast cycling. For example, the higher NOx conversion obtained with C3H6 during conventional cycling is a result of a higher temperature rise with H2 which adversely impacts NOx storage, while the higher conversion obtained during fast cycling is attributed to the faster diffusional flux of H2, which results in a larger fraction of NOx released during the rich feed (“NOx puff”). The reaction network includes a two-step HC-intermediate pathway involving vinyl isocyanate (C2H3NCO) as the proposed surface intermediate. This pathway predicts the observed double peaks of N2, while requiring substantially more C3H6 to reduce stored NOx to N2. As a result, the vinyl isocyanate pathway is shown to be less efficient than the conventional NSR pathway.