Temperature-dependent frequency control of TWC operation for efficient CH4 and NOx abatement of stoichiometric natural gas engines

A novel approach to optimize the cold start performance of Pd- and Pt-based three-way catalysts in stoichiometric natural gas engine tailpipes is presented, utilizing temperature-dependent frequency control of lambda switches. In comparison to Pt/Al2O3, Pd/Al2O3 exhibits a lower optimal frequency at the same inlet temperature, which can be attributed to the differing noble metal-specific adsorption-desorption energies of CO and O, which can inhibit the catalytic activity by surface poisoning, lower exothermicity evolving due to its lower catalytic activity and its higher oxygen storage capacity. Catalyst operation at a fixed frequency during light-off resulted in improved pollutant conversion only within a specific temperature range and relevant NH3 slip was observed at elevated temperatures compared to conventional stoichiometric steady-state operation. In contrast, controlling the frequency as a function of the catalyst inlet temperature enabled a substantial decrease of the light-off temperature of CH4 and NOx and an increase in N-2 selectivity.