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Model low temperature NOx adsorbers (LTNA) consisting of Pd on a ceria/zirconia washcoat on monoliths were evaluated for low temperature NOx storage under lean conditions to assess their potential for adsorbing the cold-start NOx emissions on a diesel engine during the period before the urea/SCR system becomes operational. A reactor-based transient test was performed with and without C2H4, CO/H2, and H2O to assess the effects of these species on the NOx storage performance. In the absence of C2H4 or CO/H2, H2O severely suppressed the NOx storage of these model LTNAs at temperatures below 100°C, presumably by blocking the storage sites. When C2H4 was included in the feedgas, H2O still suppressed the NOx storage below 100°C. However, the C2H4 significantly increased the NOx storage efficiency above 100°C, attributable to the formation of alkyl nitrites or alkyl nitrates on the catalyst.

Selective catalytic reduction (SCR) is a technology capable of meeting Tier 2 Bin 5 emissions levels of oxides of nitrogen (NOX) for diesel engines. Base metal zeolite catalysts show the best combination of thermal durability and NOX conversion activity. It is shown in this work that some base metal zeolite catalysts can store high levels of hydrocarbons (HCs). Also, base metal zeolite catalysts can catalyze oxidation of HCs under certain conditions. Oxidation of stored hydrocarbons can lead to permanent catalyst deactivation due to the exotherm generated in the SCR catalyst (over-temperature condition leading to SCR catalyst damage). This paper discusses a laboratory bench test to characterize hydrocarbon storage and burn-off characteristics of several SCR catalyst formulations, as well as engine dynamometer tests showing hydrocarbon storage and exotherm generation.