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

Abstract

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.