The Chemistry and Control of H ₂ S Emissions in Three-Way Catalysts

Recent reports of significant H₂S emissions from single-bed three-way catalyst equipped vehicles have prompted vehicle and laboratory studies of the mechanism of H₂S formation over three-way catalysts. Vehicle studies have shown the occurrence of short duration H₂S emissions significantly higher than fuel sulfur inlet levels under certain transient conditions. These emissions cannot be explained by the previously reported steady-state conversion of SO₂ to H₂S. Laboratory flow reactor studies have identified a second mechanism for H₂S formation involving the rapid reductive release of sulfur stored on the catalyst surface. This mechanism can account for H₂S emission levels which can significantly exceed inlet fuel sulfur levels for short durations. Results are presented to define factors which affect H₂S formation under these two mechanisms and to determine methods for controlling these emissions under vehicle operation.