Deactivation mechanisms influencing the performance and durability of exhaust aftertreatment systems

Doctoral thesis, 2020

As countermeasure to air pollution and global warming, more stringent environmental policy measures will probably come into force with the future emission standards, which will require important developments for gasoline and diesel vehicles to meet the emerging criteria. Efficient catalytic formulations and advanced strategies for exhaust aftertreatment systems will be therefore demanded to provide both high catalytic activity and stability, minimizing the environmental impact by automobiles.

The vehicular transport sector has the highest share of nitrogen oxides (NOx) emissions, mostly addressed for diesel vehicles. Lean NOx trap (LNT) represents a simple and cost-efficient solution for the abatement of NOx emissions from lean-burn diesel engines. Although LNTs have been commercialized for some applications, the durability of LNT catalysts still remains problematic; sulfur poisoning and thermal aging are the major causes of deactivation. Part of this thesis explored the deactivation mechanisms affecting the performance and durability of commercial LNTs through engine bench, vehicle chassis dynamometer and flow reactor experiments. Likewise, this work intended to establish a proper correlation between vehicle-aged and rapid-aged LNT catalysts, which is critical for cost effectiveness when evaluating new catalyst formulations. Particular attention was also given to the role of modern LNT materials on the sulfur poisoning and regeneration characteristics due to the relevance of this deactivation mechanism on the LNT performance and durability.

Furthermore, gasoline vehicles equipped with direct injected gasoline (GDI) engines can produce important amounts of particulate matter. Therefore, many automotive manufacturers are now equipping gasoline powered vehicles with a gasoline particle filter (GPF) in their exhaust system to have a reliable PM and PN reduction and comply with current and future emission limits. Probably the most economical way to introduce this technology into a gasoline exhaust system, that already contains one or more TWCs, is to combine TWC + GPF in one single device, referred to as coated gasoline particulate filter (cGPF). In this thesis, the conversion efficiency and durability of model cGPF catalysts were also studied under different feed temperatures and multicomponent feed gas mixtures. The samples consisted of soot-free and real soot-loaded GPFs coated with TWC material. The experimental results were then used to develop a global kinetic model able to capture the CO, ethylene and toluene conversion behavior both in soot-free and soot-loaded cGPFs.