Understanding ammonia and nitrous oxide formation in typical three-way catalysis during the catalyst warm-up period

Ammonia (NH₃) and nitrous oxide (N₂O) have been regarded as the typical secondary pollutants emitted from vehicles equipped with a three-way catalyst (TWC). MultiGas FT-IR Analyzer was applied to determine the outlet gas concentrations in the light-off experiments, in order to understand how different reaction conditions and catalyst aging affect the production of these two pollutants. It was found that N₂O formation is favored by the existence of excess oxygen during NO reduction, whereas NH₃ is readily formed within the lack of reactive oxygen species. Interestingly, the reduction of NO by H₂ in presence of excess oxygen can also lead to NH₃ formation when the active metal particles are large enough, which provides the rational explanation why the increased NH₃ was emitted from older gasoline vehicles. The loss of the catalytically active sites and reducibility caused by thermal aging requires longer time to warm-up thereby favors the N₂O and NH₃ formation, which is the major reason for the higher CO, NOx, HC, N₂O and NH₃ emissions from the old gasoline vehicles than that of low-mileage gasoline vehicles.