ALnalyse of System Factors Affecting Performance in Lean NOx Catalysis. 2. The Deleterious Role of Parasitic Homogeneous Hydrocarbon Oxidation on the Performance of High Temperature Lean NOx Catalysts

Increasing interest in lean NOx catalysis at temperatures between about 300-550°C has led to development of catalytic materials with thermal durability considerably improved over academic benchmark catalysts such as Cu-ZSM-5. The breaching of thermal durability barriers brings new obstacles into focus. Practical implementation of high temperature HC-based lean NOx catalysis entails delivery of hydrocarbons to the catalyst inlet at high temperatures. We have found initially unexpected, but scientifically precedented, phenomena regarding gas-phase kinetic instability of hydrocarbons in diesel exhaust atmospheres above 300°C. Around 300°C, homogeneous hydrocarbon oxidation can begin to occur. Rates of oxidation decrease between about 350-450°C and then increase again at higher temperatures. Some apparent NOx disappearance that does not correspond to chemical reduction of NOx can also occur homogeneously throughout this temperature range. An analysis of our results, in the context of what is known about hydrocarbon oxidation reaction pathways, yields new constraints on the design of systems that sustain HC-based lean NOx catalysis at temperatures greater than 300°C. These constraints tend to increase fuel economy penalties. Implications for systems design that require either in-cylinder or in-exhaust hydrocarbon supplementation are briefly considered.