Influence of Hydrocarbon Storage on the Durability of SCR Catalysts

Selective catalytic reduction (SCR) is a technology capable of meeting Tier 2 Bin 5 emissions levels of oxides of nitrogen (NO_X) for diesel engines. Base metal zeolite catalysts show the best combination of thermal durability and NO_X 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. The storage of hydrocarbons is primarily a concern at temperatures below around 200°C, and the oxidation occurs at temperatures around 300°C to 350°C. It is shown that improved SCR formulations have lower HC storage and therefore have much less risk of exotherms based on hydrocarbon storage.