The effects of regeneration conditions on NOX and NH3 release from NOX storage/reduction catalysts

A standard protocol developed by the Cross-Cut Lean Exhaust Emissions Reduction Simulations (CLEERS) group was used to investigate the evolution of N-byproduct species and the release of unreduced NOX from a commercial NOX storage/reduction (NSR) catalyst. NH3 was readily formed at temperatures below 375 °C, and the onset of its formation was typically observed coincident with reductant breakthrough. N2O was also observed at these lower test temperatures. The rate of NOX release, as both NO and NO2, increased with increasing temperature due to decreasing nitrate stability. Reduction of NOX necessarily involved the presence of reductant, which was also used to titrate oxygen species from oxygen-storage components such as ceria. Changes in the release of unreduced NOX from the catalyst as a function of temperature were directly attributable to the temperature dependencies of nitrate stability and decomposition, NOX diffusion to the precious metal sites, the rate of the NOX reduction reaction and the rate of reduction of these oxygen-storage components. Furthermore, by accounting for the amount of reductant needed for titration of the oxygen-storage components and the amount of NOX trapped, mass balance calculations were performed and used to estimate the amounts of residual nitrates on the catalyst surface after regeneration. These calculations indicate that only at the lower temperatures were the regenerations not effective enough to remove all the trapped NOX.