Justin Ura

NOx Release Characteristics of Lean NOx Traps During Rich Purges

This paper summarizes results from a large study on the release of NO x from a lean NO x trap during rich purges. Under certain purge conditions, some NO x trap formulations have the propensity to release some of the NO x stored during previous lean operation without reducing it. This purge NO x release was examined for different NO x trap formulations. The purge NO x 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 NO x 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 NO x release was examined. Mechanisms for the NO x release are proposed that are consistent with the observed data. The results indicate that the purge NO x release is very low for thermally aged traps and is primarily a concern for fresh or stabilized traps. The release of NO x is a very strong function of temperature and increases as the oxygen storage capacity (OSC) of the trap increases. The NO x release can be minimized by using shorter lean periods (i.e., less NO x storage) and by performing very rich purges under high flow conditions. Larger trap volumes help to lower the NO x release at 400°C and below; the data suggest that some of the NO x released from the front of the trap is readsorbed and converted on subsequent sections of the trap. Higher loadings of rhodium help decrease the NO x release at low temperatures (e.g., 250°C). Hot lean pretreatments of even short duration increase the NO x 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 NO x release is due to the reaction between the reductants (i.e., CO, HC, and H 2 ) and oxygen from the oxygen storage components in the washcoat. The resulting exotherm raises the local temperature of the washcoat, including the NO x 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 NO x capacity) and the amount of NO x stored is near the maximum capacity at that temperature, then the exotherm causes NO x to be released in order to bring the amount of NO x storage back to the maximum level that can exist at the higher temperature. Similarly, the exotherm from reducing some of the stored NO x can cause NO x that is still stored to be released, particularly for large amounts of NO x storage. Another source of NO x 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 NO x escapes from the trap without being reduced.

Nitrous oxide emissions from a medium-duty diesel truck exhaust system

Starting in 2010, medium-duty diesel trucks in the USA were introduced with aftertreatment systems that contained precious metal oxidation catalysts, soot filters, and selective catalytic reduction (SCR) systems for control of nitrogen oxides (NO, NO2). While modern diesel aftertreatment systems have high performance for meeting hydrocarbons, carbon monoxide (CO), NOx, and particulate matter, there is some concern over emission of nitrous oxide (N2O) that can be formed within the exhaust system. N2O has an atmospheric lifetime of approximately 114 years and is 298 times more effective than CO2 at trapping heat in the atmosphere. In this study, the sources of N2O were compared in the laboratory flow reactor and at the system level on diesel trucks. The interactions of HC with NOx on the DOC and NOx with NH3 within the SCR catalyst were the predominant mechanisms for N2O formation. The composite N2O mass emission was calculated to be approximately 43 mg/mi, resulting in an equivalent CO2 penalty of about 2%, similar to the 1% to 3% penalty estimated for the global light-duty vehicle fleet.

The Effects of Aging Temperature and PGM Loading on the NOx Storage Capacity of a Lean NOx Trap

A laboratory aging study was performed on samples of a lean NO x trap with platinum group metal (PGM) loadings of 0.53, 1.06, 2.12, and 3.18 g/liter. The LNT samples were aged at inlet temperatures of 650°C, 750°C, 800°C, and 850°C behind samples of a three-way catalyst that were aged on a pulse-flame combustion reactor with a Ford-proprietary durability schedule representing 80,000 km of customer use. For all aging temperatures, higher PGM loadings were beneficial for low temperature NO x performance, attributable to an increase in the oxidation of NO to NO 2. Conversely, lower PGM loadings were beneficial for high temperature NO x performance after aging at 650°C and 750°C, as higher loadings promoted the decomposition of the nitrates during lean operation and thereby decreased the NO x storage capability at high temperatures. Also, higher PGM loadings increased the OSC of the trap and thereby increased the purge requirements. After aging at 800°C and 850°C, higher PGM loadings were beneficial for NO x performance up to 450°C. The samples with low PGM loadings that were aged at 650°C had similar NO x and HC performance as the samples with high PGM loadings that were aged at 850°C. The laboratory results were confirmed with full-size converters aged on an engine-dynamometer with the Ford-proprietary durability schedule, as a NO x trap aged behind an exhaust gas heat exchanger had much better NO x performance than a trap aged without the heat exchanger. These results suggest that an exhaust gas heat exchanger could allow reductions in the PGM loading of the LNT while maintaining good NO x performance after thermal aging.

Laboratory Study of Lean NOx Trap Desulfation Strategies

Desulfation characteristics of several model and fully-formulated monolithic lean NO x trap materials were studied in a laboratory flow reactor employing a chemical ionization mass spectrometer. For all samples, desulfation at elevated temperatures under reducing conditions resulted in appearance of sulfur dioxide (SO2) followed by carbonyl sulfide (COS) and hydrogen sulfide (H 2 S). The data appear consistent with a desulfation mechanism involving elimination of SO 2 from stored sulfates under reducing conditions, followed by reaction of the SO 2 with CO and H 2 to produce COS and H 2 S, respectively. Based on these observations, several cyclic and multistage desulfation strategies were devised which greatly decreased H 2 S emissions while achieving relatively rapid and complete sulfur removal.