Transient kinetic analysis of passive SCR systems for NH3 abatement from natural gas fueled heavy duty engines over dual-layer ASC catalysts: An experimental and modelling study

Air-to-fuel ratios fluctuating around the stoichiometric value enable the best CH4, NOX and CO abatement performance of the three-way catalytic converters implemented in NGVs after-treatment systems; however, this leads to undesired NH3 production and potential slip under rich combustion conditions. An additional Passive SCR catalytic device placed downstream of the TWC, based on a dual-layer SCR+PGM configuration, could represent the solution. Through both transient experimental tests and modelling analysis, we demonstrate that effective NH3 abatement is achieved at low temperatures by its adsorption onto the SCR catalyst during the rich phases, and its further consumption by NO+O2 under lean conditions. Differently, at high temperatures NH3 is efficiently and selectively converted to nitrogen by direct oxidation through a redox reaction mechanism. The developed model can predict the performances of both fresh and aged catalysts under the transient rich-lean cycling experimental conditions investigated.