The Formation of Ammonia in Three-Way Catalysts Fitted to Spark Ignition Engines - Mechanisms and Magnitudes

Exhaust gas aftertreatment systems can, under certain conditions, create undesired chemical species as a result of their elimination reactions. A prime example of this is ammonia (NH₃), which is not formed in the combustion reaction, but which can be formed within a three-way catalyst (TWC) when physicochemical conditions permit. The elimination of NO_x in the TWC thus sometimes comes at the cost of significant emissions of NH₃. Ammonia is a pollutant and a reactive nitrogen compound (RNC) and NH₃ emissions should be analyzed in this context, alongside other RNC species. Examination of the literature on the subject published over the past two decades shows that ammonia, a species which is currently not subject to systematic emissions requirements for road vehicles in any market, is often identified as forming the majority of the RNC emissions under a range of operating conditions. It is expected that upcoming automotive emissions legislation (e.g. the European Union’s Euro 7/VII standards; perhaps also China 7) will set an NH₃ emissions limit, mandating the measurement of NH₃ under laboratory conditions. A pool of European passenger cars was tested on a chassis dynamometer under laboratory conditions, using the WLTP test procedure; ammonia emissions were measured using a dedicated analyzer. Other test procedures were used to generate further data NH₃ formation trends and gain further insight, particularly into the relative proportions of NH₃ and NO_x in the exhaust gas and the impact of cold start and powertrain warmup. Much of the literature focuses on periods of high load as causing significant NH₃ emissions; here it was found that cold start is a very significant contributor. Separation of the effects of powertrain temperature and load confirm that TWC behavior determining NH₃ emissions is non-trivial. As a result, NH₃ emissions can be highly sensitive to test parameters.