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Similar to single-brick SCR architectures, the multi-brick SCR systems described in this paper require urea injection control software that meets the NOx conversion performance target while maintaining the tailpipe NH3 slip below a given threshold, under all driving conditions. The SCR architectures containing a close-coupled SCRoF and underfloor SCR are temperature-wise more favorable than the under-floor location and lead to significant improvement of the global NOx conversion, compared to a single-brick system. But in order to maximize the benefit of close-coupling, the urea injection control must maximize the NH3 stored in the SCRoF. The under-floor SCR catalyst can be used as an NH3 slip buffer, lowering the risk of NH3 slip at the tailpipe with some benefit on the global NOx conversion of the system. With this approach, the urea injection strategy has a limited control on the NH3 coverage of the under-floor SCR catalyst.

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.