Reduction of Low Temperature Pollutants by Understanding the Exhaust Species Interactions in a Diesel Oxidation Catalysts

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Article by Dr. Athanasios Tsolakis

In 2012, the Intenational Agency for Research on Cancer (IARC) changed the classification of diesel engine exhaust gas from possibly carcinogenic to humans (Group 2A) to carcinogenic to humans (Group 1). This classification justifies the stringent emission legislation imposed on diesel cars, promoting a reduction of air pollution, especially in urban environments, where humans are more exposed to vehicle exhaust gas. To meet these emission limits, vehicles are now widely fitted with different exhaust converters such as the diesel oxidation catalyst (DOC), which promotes carbonaceous species such as CO and unburned hydrocarbon oxidation. These converters are coated with catalytic materials that reduce the required temperatures to allow the reactions to take place within the range of diesel exhaust gas temperature. Nevertheless, the catalytic activity can still remain limited at low exhaust gas temperature associate with engine cold start phase and/or during low load engine conditions. This creates high peak emissions of CO and hydrocarbons  as the catalyst effectiveness in removing pollutants is limited.

The research work presented in this paper concentrates on promoting the DOC’s low temperature activity by understanding the exhaust species interactions within the catalyst. DOC oxidation efficiency can be maintained or even promoted when the reactions inhibiting the catalytic activity are eliminated, especially at low temperatures.

Some of these interactions have previously been studied using synthetic mixtures of gaseous species. Although those synthetic mixtures are necessary for fundamental understandings of the main chemical reactions,  theylack resemblance to real diesel engine exhaust gaseous mixtures conditions and compositions (presence of water, sulphur, numerous and various hydrocarbon species...). In this study, bespoke experimental facilities for catalytic studies that utilise real vehicle exhaust gases was designed and used. Various diesel engine exhaust gas compositions were used from the engine operating at advanced combustion modes and range of diesel fuels, including new generation alternative ones.

The findings of this study provide tools to qualitatively predict the DOC oxidation efficiency based on the exhaust gas composition produced by a genuine diesel engine. This can be valuable during the process of designing fuels and assessing how their pollutant emissions will be efficiently catalytically removed, especially at low temperature conditions. Furthermore, it also gives insight on diesel aftertreatment systems design and strategies aiming to enhance low temperature catalysts activity. Finally, the synergies identified between engine combustion mode , fuels specifications and catalytic activity are valuable in promoting pollutant removal over a wider temperature range, necessary for reaching the near future legislative diesel exhaust emission limits.  

Research by: Isaline Lefort, Dr. Jose Herreros and Dr. Athanasios Tsolakis

This paper is published in: Environ. Sci. Technol., 2014, 48 (4), pp 2361–2367, DOI: 10.1021/es4051499. American Chemical Society Publications. January 201