Impact of Alkali Metals on the Performance and Mechanical Properties of NOx Adsorber Catalysts

Performance of two types of NOx adsorber catalysts, one based on Ba and the other based on Ba with alkali metals, was compared fresh and after thermal aging. Incorporation of sodium(Na), potassium(K) and cesium(Cs) into NOx adsorber washcoat containing barium significantly increases the NOx conversions in the temperature range of 350-600°C over that of the alkali metal free NOx adsorber catalysts. NOx performance benefit and HC performance penalty were observed on both engine dynamometer and vehicle tests for the “Ba+alkali metals” NOx adsorber catalysts. “Ba+alkali metals” NOx adsorber catalysts also demonstrate superior sulfur resistance with better NOx performance after repeated sulfur poisonings and desulfations over the “Ba based” NOx adsorber catalysts. An exhaust system containing an aged closed-coupled Ce free warm up catalyst and an 850°C aged underfloor “Ba+alkali metals” NOx adsorber catalyst is capable of achieving Euro IV emission standard and offering better fuel economy. However, high mobility and high affinity of alkali metals with SiO₂ lead to migration of alkali metals away from the NOx adsorber washcoat and accumulation on cordierite substrates after aging. Mechanical properties of a “Ba+alkali metals” NOx adsorber catalyst are compared with those of a 3-way catalyst after aging. In summary, “Ba+alkali metals” NOx adsorber catalysts with improved mechanical durability are most suitable for broader future applications in Europe.