Sensitivity of Alkane Light-off to Variations in Lambda in Model TWCs
Andrew (Bean) Getsoian, Ford Motor Company
Light-off curves of exhaust components at different air-fuel ratios (lambda) are vital inputs in the development of models and control strategies for three way catalysts. In the present study, light-off curves were collected on a library of single component model catalysts (Rh or Pd on different supports) as a function of lambda. In a model exhaust blend containing O2, CO, H2, NO, C2H4, C3H6, C3H8, CO2, and H2O, the light-off curves for CO and C3H6 showed relatively little sensitivity to lambda. Light-off curves for C2H4 were qualitatively different in rich and lean feeds, but T50 and T90 varied by only 20-30°C across the range examined. By contrast, C3H8 light-off curves were exquisitely sensitive to air-fuel ratio: the temperature at which 90% conversion of C3H8 was achieved decreased by 150 – 200°C between lambda=0.973 and lambda=0.998, with decreases of up to 60°C just between lambda=0.995 and lambda=0.997 observed on some catalysts. Crossing from rich to lean increased T90 for C3H8 by 200°C – 400°C or more between lambda=0.997 and lambda=1.001. On some catalysts, a similar sensitivity to lambda was observed for T90 for NO conversion, whereas T50s for NO conversion were almost unaffected by lambda. These findings are rationalized as resulting from changes in most abundant surface intermediate in response to changes in lambda and temperature, leading to changes in reaction mechanism for alkane oxidation. Also highlighted in this talk is performance of a recently developed model TWC capable of exceptional low temperature light-off performance, including T90s for propane below 250°C after 50h of 4-mode aging at 850-950°C.