The Dynamics of Methane and NO _x Removal by a Three-Way Catalyst: A Transient Response Study

Natural gas-powered engines are widely used due to their low fuel cost and in general their lower emissions than conventional diesel engines. In order to comply with emissions regulations, an aftertreatment system is utilized to treat exhaust from natural gas engines. Stoichiometric burn natural gas engines use three-way catalyst (TWC) technology to simultaneously remove NO_x, CO, and hydrocarbon (HC). Removal of methane, one of the major HC emissions from natural gas engines, is difficult due to its high stability, posing a challenge for existing TWC technologies. In this work, degreened (DG), standard bench cycle (SBC)-aged TWC catalysts and a DG Pd-based oxidation catalyst (OC) were evaluated and compared under a variety of lean/rich gas cycling conditions, simulating stoichiometric natural gas engine emissions. Transient response techniques were applied to reveal the effect of the oxygen storage component on the performance of DG TWC as well as upon SBC aging in comparison to DG OC. It is illustrated that the oxygen storage component of TWC can extend the rich phase’s high conversion efficiency of both CH₄ and NO_x into lean phase for a few seconds during lean/rich cycling. Therefore, TWC displays high CH₄ and NO_x conversion with a wide λ (air-fuel equivalence ratio) operating window. These results are expected to provide general guidance for optimization of stoichiometric burn natural gas engines and their aftertreatment systems.