Advanced Catalyst Solutions for Hydrocarbon Emissions Control During Rich Operation of Lean NOx Trap Systems 2009-01-0282

The operation of NOx Adsorber catalysts (NAC), also often referred to as Lean NOx Trap catalysts or NOx Storage-reduction catalysts, entails frequent periodic NOx regeneration events. These are accomplished by creating a net reducing, fuel-rich environment in the exhaust. The reduction of hydrocarbon emissions which occur during such fuel-rich events is challenging, due to the oxygen-deficient environment. In order to overcome this limitation, two possibilities exist: (i) oxygen can be stored during lean phase, to be used for hydrocarbon slip oxidation in the subsequent rich phase, or (ii) unreacted hydrocarbons can be trapped during the rich phase and oxidized during the following lean phase. In this work, two groups of catalytic solutions were developed and evaluated for hydrocarbon emission control based on these approaches: an Oxygen Storage Compound (OSC) based catalyst and zeolite-based hydrocarbon trap catalyst.

The experimental evaluation of these two approaches included several stages. The initial concept demonstration was focused on comparing powder samples against a reference oxidation catalyst, using a simplified laboratory screening protocol. Following that, a series of monolith-coated samples with various levels of precious metal loading were studied in-depth using a set of cyclic and steady-state bench-reactor performance tests. These tests were targeted at characterizing the hydrocarbon slip reduction performance, as well as understanding the details of their behavior, in order to guide their potential practical application. Finally, the developed catalyst solutions were tested on engine, including a more detailed study of the precious metal loading level impact on the hydrocarbon slip control.

Both the OSC-based and zeolite-based catalyst formulations demonstrated excellent rich hydrocarbon slip control performance. The OSC-based catalysts performed better at higher temperatures, due to increased dynamic availability of stored oxygen; on the other hand, zeolite-based catalysts performance was less dependent on the temperature, due to efficient hydrocarbon trapping. The resulting understanding enabled the development of advanced hydrocarbon slip control system with reduced precious metal loading.