Passive Ammonia-SCR Catalyst System for NO_X Abatement from Lean-Burn Gasoline Engines: NH₃ formation over TWC

This dissertation summarizes experimental and computational observations from investigations of a selective catalytic reduction (SCR) system for reducing nitrogen oxides (NO_X) in lean gasoline engine exhaust based on utilizing ammonia (NH₃) generated by a three-way catalyst (TWC) during brief periods of fuel-rich engine operation. NH₃ released from the TWC is stored and available to reduce NO_X on a downstream SCR catalyst during subsequent periods of lean engine operation. The experimental results include high-speed measurements of transient NH₃ formation on the TWC monolith catalysts, as the catalysts were exposed to lean gasoline engine exhaust from a commercial engine. In addition to the experimental investigations, dynamic computational simulations of NH₃ generation on the TWC catalyst were implemented to provide more detailed information about NH₃ generation on TWCs based on available reaction kinetic mechanisms. Based on the experimental and computational results, estimates of the potential fuel efficiency gains and emissions relevant to simulated drive cycles indicate that passive SCR can potentially achieve significant fuel efficiency benefits while still meeting regulated NO_X emissions limits for vehicles powered by lean gasoline engines. However, optimal performance of the system will most likely require development of emission control methods that include accurate models for SCR catalyst NH₃ storage and reaction under realistic drive-cycle transients.