Performance Demonstration of a Precious Metal Lean NOx Catalysts in Native Diesel Exhaust

The pathbreaking papers by Held and Iwamoto has led to a reinvigorated search for lean NO_x catalysts worldwide. Extensive effort is currently being expended to develop precious metal lean NO_x catalysts, with application intended for diesel engines and low-temperature lean-burn gasoline engines. Here we concentrate on the properties of such a catalyst, referred to as LNX3. The NO_x reduction selectivity of LNX3 is examined with model reducing agents. Hydrocarbons are more effective at reducing NO_x than CO or H₂. Next it is shown that excessively high HC/NO_x ratios can degrade the performance of noble metal lean NO_x catalysts. This is because the exotherm from the hydrocarbon oxidation can increase the temperature in a significant portion of the catalyst bed beyond the effective temperature window wherein the catalyst is able to reduce NO_x. Depending on the concentration of NO_x, the need for adequate HC/NO_x ratios in order to perform the reduction will have to be balanced with this thermal consideration. Next we demonstrate the performance of LNX3 during hot-start ECE+EUDC tests on a European light-duty diesel vehicle. Although overall performance in native exhaust is inadequate, NO_x conversions as high as 34% are observed when the HC/NO_x ratio was in excess of 2.0. The catalyst also demonstrates good control of CO, HC, and the soluble organic fraction of the particulate on these hot-start tests. These results demonstrate the feasibility of obtaining effective “four-way” performance from such a Lean NO_x catalyst in Diesel exhaust, provided the overall system is optimized to meet catalytic requirements.