DIESEL EMISSION CONTROL: CATALYTIC FILTERS FOR PARTICULATE REMOVAL

Debora Fino
Department of Materials Science and Chemical Engineering, Politecnico di Torino

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     Last modified: April 3, 2006

Abstract
The European Diesel engine industry represents a vital sector across the Continent, with more than two million direct work positions and a turnover of over 400 billion Euro. Diesel engines provide large paybacks to society since they are extensively used to transport goods, services and people. They are the power source behind commercial transport, being employed in trucks, buses, trains and ships, as well as off-road industrial vehicles such as excavation machinery and mining equipment. The Diesel engine is being increasingly used to power passenger cars and vans, particularly in Europe where the current penetration of the Diesel engine into the light-duty market is around 30-35%, and is predicted to grow to 50% by 2010.
In recent years increasing attention has been paid to the emissions from Diesel engines which, like gasoline engine emissions, include carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx). Diesel engines also produce significant levels of Particulate Matter (PM), which consists mostly of carbonaceous soot and a Volatile Organic Fraction (OF) of hydrocarbons that have condensed on the soot.
Meeting the emission levels imposed for NOx and particulate matter (PM) by the legislation (Euro IV in 2005 and, in the 2008 perspective, Euro V) requires the development of a number of critical technologies, so as to fulfill these very stringent emission limits (e.g. 0.025 g/km for PM). These limits cannot be accomplished by only either engine modifications [1] or fuel pre-treatments [2] or more simply by a better tuning of the combustion process [3]: a convenient way of treating diesel off-gases has thus to be worked out. The control of emissions from Diesel engines presents many challenges for the engine control and catalyst system development communities. This paper provides a survey of the advancement status of R&D in the field of diesel particulate traps for both passenger cars and heavy-duty vehicles. Special emphasis is given to the combined use of traps and catalysts for regeneration purposes via catalytic combustion of the collected soot in the traps [4-6]. Issues like trap materials selection (cordierite, SiC, FeCrAlloy, …), catalyst development (fuel additives, catalyst coatings, precious metals...), catalytic vs. non-catalytic trap performance are addressed, providing a critical analysis on the techno-economical feasibility of the various systems currently being developed by several car-components suppliers and catalyst manufacturers. Specific highlights of the research in catalytic materials developed at Politecnico di Torino in the framework of European projects will also be provided. In order to enhance the soot-catalyst contact conditions, several kinds of nanostructured catalysts have been developed based on different key reaction pathways: oxygen spillover oxide (e.g. Cs4V2O7, [7]), mobile catalysts based on alkali vanadates (e.g. Cs2O·V2O5, [8]), layered perovskites for the combined removal of particulate and NOx (La2-xKxCu1-yVyO4 [9]), precious metals enabling the NO oxidation to NO2 followed by reaction of this latter with particulate [10], alkaline or heavy metal oxides [11], alkali-metal substituted perovskites capable of delivering alpha-oxygen species (La0.8Cr0.9Li0.1O3 [12]). An overview of these different approaches to soot oxidation will be provided pointing the way towards possible synergetic effects in multi-component catalysts.
References
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