Periodic forcing of three-way catalyst with diffusion in the washcoat

Operation of Pt–Rh/Ce/ image-Al2O3 three-way catalytic converter (TWC) with periodic variations of the inlet oxygen concentration is simulated. A lumped mathematical model with complex microkinetic reaction scheme, surface deposition of reaction components and description of diffusion in the porous washcoat structure is employed. The effects of the forcing frequency and amplitude on the light-off and time-averaged conversions of CO, hydrocarbons (HC) and nitrogen oxides NOx are studied. The influence of oxygen storage capacity (OSC) and effective diffusivity in the washcoat are also examined. The results are discussed on the basis of computed spatiotemporal concentration patterns in the catalytic washcoat. Earlier light-off (i.e., at lower temperature) and higher time-averaged conversions are observed for the forced TWC in comparison with the TWC operated without oscillations in inlet O2 concentration. Maximum conversions have been found for the forcing frequencies in the range 1–2 Hz and amplitudes of the oscillations higher than image10% around the stoichiometric value. The frequency 1 Hz corresponds to typical characteristic times for image-sensor control of air/fuel ratio (A/F) in the automobile engines.