NOx Release Characteristics of Lean NOx Traps During Rich Purges 2003-01-1159
This paper summarizes results from a large study on the release of NOx from a lean NOx trap during rich purges. Under certain purge conditions, some NOx trap formulations have the propensity to release some of the NOx stored during previous lean operation without reducing it. This purge NOx release was examined for different NOx trap formulations. The purge NOx release was evaluated for one of the formulations as a function of several variables, including the aging condition of the trap, the trap temperature, the trap volume, the purge A/F ratio, the purge flow rate, and the amount of NOx stored. The effect of hot lean pretreatments on the purge NOx release was studied. In addition, the effect of the rhodium level on the purge NOx release was examined. Mechanisms for the NOx release are proposed that are consistent with the observed data.
The results indicate that the purge NOx release is very low for thermally aged traps and is primarily a concern for fresh or stabilized traps. The release of NOx is a very strong function of temperature and increases as the oxygen storage capacity (OSC) of the trap increases. The NOx release can be minimized by using shorter lean periods (i.e., less NOx storage) and by performing very rich purges under high flow conditions. Larger trap volumes help to lower the NOx release at 400°C and below; the data suggest that some of the NOx released from the front of the trap is readsorbed and converted on subsequent sections of the trap. Higher loadings of rhodium help decrease the NOx release at low temperatures (e.g., 250°C). Hot lean pretreatments of even short duration increase the NOx release during the subsequent storage and purge cycle, presumably due to oxidation of the precious metal.
At temperatures of 350°C and above, it is proposed that a major cause of NOx release is due to the reaction between the reductants (i.e., CO, HC, and H2) and oxygen from the oxygen storage components in the washcoat. The resulting exotherm raises the local temperature of the washcoat, including the NOx storage sites nearby. If the temperature before the purge is higher than the peak storage temperature of the trap (i.e., in the range of decreasing NOx capacity) and the amount of NOx stored is near the maximum capacity at that temperature, then the exotherm causes NOx to be released in order to bring the amount of NOx storage back to the maximum level that can exist at the higher temperature. Similarly, the exotherm from reducing some of the stored NOx can cause NOx that is still stored to be released, particularly for large amounts of NOx storage.
Another source of NOx release occurs at temperatures above 500°C because, as the front of the trap is being purged, the rear part of the trap is exposed to stoichiometric conditions with very low levels of oxygen and reductants. The adsorbed nitrates become unstable in the absence of oxygen, and at these high temperatures, the rate of nitrate decomposition becomes rapid enough to result in additional NOx release. Due to the low levels of reductants, the released NOx escapes from the trap without being reduced.