Laboratory Screening of Diesel Oxidation Catalysts and Validation with Vehicle Testing: The Importance of Hydrocarbon Storage

A laboratory flow reactor test has been developed to examine hydrocarbon (HC) storage for diesel catalysts. Light-off testing alone has not been sufficient to rank diesel oxidation catalysts (DOCs) in agreement with vehicle HC conversions over the European driving cycle. HC emissions are important because of Stage II combined HC+NOx standard. During cold start and much of the ECE driving cycle, inlet catalyst temperatures on diesel passenger cars spend much time below 200°C. This is where more than half of the HC mass can be emitted. To be effective, DOCs must achieve sufficiently low HC light-off temperatures, or incorporate materials such as zeolites that trap HC until light-off is achieved.

Consideration of both HC storage and light-off results together improve ranking of DOCs similar to vehicle ranking. Three supplier DOCs have been evaluated. Test conditions that reflect vehicle feed gas, e.g., composition, HC type, and space velocity, are critical for effective flow reactor screening. The HC storage test procedure is similar to flow reactor light-off activity scans, however it was important to have a long chain normal alkane in the feed gas. n-Decane was chosen based on literature reports of HC speciation for diesel passenger car exhaust. High boiling properties of n-decane enable characterization of HC storage. Storage contributions of other HC types also are examined in this study. In addition, CO light-off and SO₂ oxidation activity have been evaluated.