Sensitivity of Three-Way Catalyst Light-Off Temperature to Air-Fuel Ratio

The operating conditions of the automotive three-way catalyst (TWC) are characterized by continuous variation of the air-fuel ratio (λ) that determines the composition of the exhaust supplied to the catalyst. It is well known that the ability of the TWC to simultaneously catalyze reduction of NO_x and oxidation of CO and hydrocarbons is sensitive to the air-fuel ratio. In efforts to formulate improved TWCs with greater activity at lower temperatures, the impact of air-fuel ratio on light-off temperature must therefore be considered. This paper reports an investigation of the impact of air-fuel ratio on the temperatures at which representative exhaust species in a simulated exhaust mixture reach 90% conversion (T90) over a family of rhodium-based model catalysts, with focus on the performance of a recently developed catalyst comprising rhodium supported on titania-modified alumina with exceptional light-off performance. Over a range of air-fuel ratios 0.977 < λ < 1.005, the T90 for CO is nearly insensitive to λ; the T90 for NO is constant for λ < 0.995 but increases by > 200 °C between 0.995 < λ < 1.001 and 90% conversion is not achieved at any temperature for λ > 1.001; and the T90s for representative hydrocarbons ethylene, propylene, and propane decrease by more than 100 °C between 0.977 < λ < 0.995, then increase again by 30 °C (propylene) to 180 °C (propane) between 0.998 < λ < 1.001. These dramatic shifts in T90 over a small range of air-fuel ratio are attributed to facile conversion of CO and olefins by oxidation, facile conversion of propane by steam reforming, and inhibition of propane steam reforming by both oxygen and CO. Balancing these factors leads to optimal conversion of all exhaust components at an air-fuel ratio λ ~ 0.995.