Commercial three way catalysts have limited capacity towards reducing NOx in the presence of excessive oxygen. This prevents lean-burn combustion concepts from meeting legislative emission standards. A solution towards decreasing NOx emissions in the presence of excess air is the use of a passive-SCR system. Under rich conditions ammonia is formed over an ammonia formation catalyst, the ammonia is stored in the SCR and in its turn reacts with the NOx under lean engine conditions.
Here up-scaled Pt/Al2O3 and Pd/Al2O3 catalysts as well as a commercially Pd-Rh based three-way catalyst (TWC) are evaluated using both engine and further lab-scale tests.
The purpose of these tests is to compare the ammonia production for the various catalysts under various lambda values and temperatures by means of engine and lab scale tests. The Pd/Al2O3 showed little sensitivity to temperature both under engine and lab scale experiments. The Pt/Al2O3 was affected to a large extend by temperature for both test methods. The TWC showed stable production during the engine measurements while under lab tests an increased temperature resulted in a lower ammonia yield.
Differences between the engine and lab scale tests are mainly due to catalyst temperatures, space velocity, CO poisoning and uncertainties in the composition of the engine's exhaust gas. Both Pt/Al2O3 and Pd/Al2O3 form ammonia although the former generate higher amounts at high temperature but are believed to suffer from CO poisoning at low temperatures.