Phenomenology of NOx Adsorber Catalysts

Lean phase operation of lean NOx adsorbers or lean NOx traps (LNTs) is analyzed using data from highly controlled isothermal bench-flow experiments on Pt/K/Al2O3 catalysts. Simulated diesel exhaust was used for inflow in the experiments. Pt promotes the oxidation of NO, the primary component of engine-out NOx, to NO2 which is more easily adsorbed and stored as KNO3. While the presence of Pt was found to be essential for capturing the NOx from the exhaust, it was seen that the NOx storage efficiency did not increase indefinitely with increasing Pt loading. Pt loading at a given temperature of operation is optimal when the rate of NO oxidation to NO2 is nearly equal to the NO2 adsorption rate at the sorbent sites. The NOx storage efficiency seems to be kinetically limited at lower temperatures and equilibrium limited at higher temperatures. When the lean/NOx capture operation was started from a fully regenerated condition, the NOx storage efficiency under most conditions was found to be independent of the reductant mixture used for regenerating the catalyst. The experiments also indicate the possibility of a significant NOx reduction reaction even under the highly oxidizing conditions. This reduction reaction seems to be of fractional order and is more significant at lower temperatures. More detailed analysis of the outflow from the reactor using Fourier-Transform-Infrared (FTIR) spectroscopy is being planned to identify the final product of this reduction reaction.