The Effects of CO, H ₂ , and C ₃ H ₆ on the SCR Reactions of an Fe Zeolite SCR Catalyst 2013-01-1062

Selective Catalytic Reduction (SCR) catalysts used in Lean NO_x Trap (LNT) - SCR exhaust aftertreatment systems typically encounter alternating oxidizing and reducing environments. Reducing conditions occur when diesel fuel is injected upstream of a reformer catalyst, generating high concentrations of hydrogen (H₂), carbon monoxide (CO), and hydrocarbons to deNO_x the LNT. In this study, the functionality of an iron (Fe) zeolite SCR catalyst is explored with a bench top reactor during steady-state and cyclic transient SCR operation.

Experiments to characterize the effect of an LNT deNO_x event on SCR operation show that adding H₂ or CO only slightly changes SCR behavior with the primary contribution being an enhancement of nitrogen dioxide (NO₂) decomposition into nitric oxide (NO). Exposure of the catalyst to C₃H₆ (a surrogate for an actual exhaust HC mixture) leads to a significant decrease in NO_x reduction capabilities of the catalyst. A degradation mechanism is proposed to account for the decrease in NO_x conversion efficiency, highlighted by reactions between NO₂ and C₃H₆ to make NO at a rate of similar order of magnitude as the Fast SCR reaction. This inhibits SCR reactions when the NO:NO_x ratio favors NO, but can increase NO_x conversion when the NO:NO_x ratio favors NO₂.

Ammonia (NH₃) storage is only marginally affected by the presence of H₂, CO, or C₃H₆; but significant amounts of C₃H₆ can be stored on the catalyst. Further observation reveals that the oxidation effects of C₃H₆ are non-negligible and C₃H₆ strongly influences the oxidation of NH₃. The degradation mechanism includes seven proposed reactions to model the experimental results of adding H₂, CO, and C₃H₆ to the SCR feed gas during steady-state and transient operation.