Calvin Thomas

Date of Award

Fall 2018

Document Type

Open Access Dissertation


Chemical Engineering

First Advisor

Jochen Lauterbach


Conventional gasoline-powered engines operate with a stoichiometric air-fuel ratio (AFR), but greater fuel economy is achieved in lean burn engines by increasing the AFR. The primary drawback to these engines is the nitrogen oxide (NOX) emissions, which cannot be reduced over a conventional three-way catalyst (TWC). Passive selective catalytic reduction (SCR) is a promising approach for the control of NOX emissions in lean burn systems. By periodically decreasing the AFR to fuel-rich levels, ammonia (NH3) can be produced over a TWC and stored on a downstream SCR catalyst for the reduction of NOX during normal lean operation. While passive SCR has shown promise, work to date has not sufficiently examined the effects of physical or chemical degradation on the TWC in a passive SCR system or how these factors are affected by the transient nature of passive SCR. Furthermore, many questions about the formulation of the TWC have yet to be answered.

This work is focused on answering these questions to better understand NH3 production over TWCs and the degradation mechanisms of passive SCR. Using commercially formulated TWCs, the effects of hydrothermal aging and sulfur dioxide (SO2) are examined, while model catalysts are synthesized to investigate the roles of ceria on the catalyst. Hydrothermal aging at 920°C for 100 hours results in significant noble metal sintering as well as degradation of both oxygen storage and NOX storage components. Long term exposure to 2 ppm SO2 results in significant deactivation of catalyst activity, which is attributed primarily to the deactivation of water gas shift and steam reformation reactions. These sulfur effects can be partially mitigated by lowering the AFR during rich operation. Finally, in the investigation of ceria, it was found that low levels of ceria promote water gas shift, but elevated levels result in inhibition of catalyst reduction.