Selective Catalytic Reduction (SCR) catalysts used in Lean
NOx 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 deNOx 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
deNOx 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 NOx reduction capabilities of the catalyst. A
degradation mechanism is proposed to account for the decrease in
NOx 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:NOx ratio favors NO, but can increase
NOx conversion when the NO:NOx 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.