Investigation of Nitric
Dioxide-Suppressing Diesel Particulate
Filter Catalytic Coating, 03-R9715
Principal Investigator
Magdi K. Khair
Inclusive Dates: 05/21/07 – Current
Background - Diesel particulate filters (DPF) are chiefly ceramic porous devices placed in the path of exhaust from diesel engines. They are designed to trap particulate matter from the exhaust and collect it within their volume. Allowing this process to continue without other measures leads to increased exhaust back pressure, thus reducing engine efficiency and performance. Therefore, it is necessary to dispose of the collected particulate matter in a safe way. The process of disposing of particulate matter collected in DPFs is referred to as regeneration. DPF regeneration is performed in a number of ways, including electric heating of the exhaust, using heat from a burner to incinerate particulate, and mechanically removing particulate from the DPF. Most of the regeneration methods depend on thermal means and the excess oxygen present in the exhaust of a diesel engine.
One unique method of DPF regeneration is based on the finding that nitric dioxide (NO2) can assist in reducing carbonaceous material stored in the DPF at a much lower temperature than oxygen-assisted regeneration. To achieve this regeneration, a diesel oxidation catalyst is placed upstream of the DPF for the purpose of forming generous amounts of NO2. The latter would then react with carbon collected in the DPF as particulate matter and produce a combination of CO2 and CO. While the NO2-assisted regeneration may be good for engine performance and fuel economy, it may not be preferred for its potential impact on human health. NO2 is a very reactive gas, and research in Europe, especially at the University of Rouen in France, has disclosed a close relationship between particulate matter toxicity and the NO2 content in the exhaust. These findings prompted the California Air Resources Board to regulate NO2 in diesel exhaust when using DPF retrofit equipment.
Approach - SwRI is researching the effect of different DPF coatings on NO2 formation and investigating the effect on a long list of unregulated emissions. The project involves using three DPFs with different levels of catalytic coatings. In an effort to market this capability, SwRI approached several catalyst manufacturers and conducted detailed technical discussions with one European catalyst manufacturer. These discussions resulted in the development of an NO2-suppressing washcoat formulation designed to allow for good trap regeneration without the accompanying NO2 emissions.
Accomplishments - The first half of the project is complete and involved the performance evaluation of the DPF with the NO2-suppressing coating versus three other DPFs. Indeed the candidate DPF showed its superior quality in suppressing the formation of NO2 relative to other DPF coating formulations. A coating formulation using 50 g/ft3 platinum coating proved extremely detrimental to suppressing NO2 formation even though it is very efficient for DPF regeneration. An interim report was published and discussed with the European catalyst supplier. The supplier is now interested in combining this DPF with its NO2 suppressing quality and another catalytic layer to reduce NOx through selective catalytic reduction. This will be a separate project funded by the catalyst company as a continuation of this project.
