Trends in NOx and NH3 emissions caused by three-way catalysts

•Three-way catalysts (TWC) are used in spark-ignition engines to meet exhaust regulations.•NH3 emissions are generated in TWCs during the NOx reduction.•In Euro 7, NH3 has been designated as a new regulated substance.•NH3 formation is greatly influenced by catalyst activation. Emission regulations have been made more strengthened both regulated (NOx) and unregulated emissions (NH3, N2O, and CH4) from internal combustion engines. Delays in supply of batteries and semiconductors has slowed the commercialization of electric and hydrogen electric vehicles, necessitating further research on internal combustion engines for the time being. Gasoline and liquefied petroleum gas (LPG) vehicles equipped with spark-ignition (SI) engines use three-way catalysts (TWC) to meet exhaust regulations; however, recent studies have shown that TWC can cause NH3 emissions. This study analyzed the NOx and NH3 emission characteristics of LPG engines owing to catalyst activation, aging and capacity. A basic study of TWC was conducted using engine experiments, and real vehicle characteristics were analyzed using a worldwide harmonized light vehicle test procedure (WLTP) and real driving emissions (RDE). As a result, it was confirmed that there is a trade-off relationship between NOx reduction and the generation of NH3 in the three-way catalyst system. a decrease in ambient temperature delays catalyst activation, resulting in reduced NH3 emissions. Aged catalysts exhibit an increase in NH3 due to accumulated ammonia within the catalyst. Moreover, higher catalyst capacity is associated with a more substantial reduction in NOx, contributing to an increase in NH3 generation.