Science and technology of catalytic diesel particulate filters

Abstract

During the last few decades, concerns have grown on the negative effects that diesel particulate matter has on health. Because of this, particulate emissions were subjected to restrictions and various emission-reduction technologies were developed. It is ironic that some of these technologies led to reductions in the legislated total particulate mass while neglecting the number of particles. Focusing on the mass is not necessarily correct, because it might well be that not the mass but the number of particles and the characteristics of them (size, composition) have a higher impact on health. To eliminate the threat of diesel particulate matter, essentially absolute filtration in combination with the oxidation of all emitted hydrocarbons is what will be required. After two decades of development, the first filters will soon be introduced on a large scale. Many different problems had to be overcome; it was especially important that the filter was robust and its regeneration was controllable. The key technology to controllable regeneration is oxidation catalysis, which is the main area of focus in this review. Catalytic filter regeneration is very complex, something which is apparent in the main aspects of catalysis (i.e., activity, stability, and selectivity). Complications are that the process conditions can be very transient and that the temperatures are usually low. It is shown that the oxidation catalyst cannot be examined isolated from the total system. Within the margins of size restrictions and an engine's service life, essentially all particulate matter should be trapped, the filter should be regenerated safely, no toxic by-products should be formed, and the catalyst should not alter the filtration characteristics, and vice versa. The exhaust conditions of passenger cars are not favorable for continuous regeneration strategies, because the best strategy seems to be periodic regeneration with the aid of a catalyst. This concept is not passive, which makes it complex and expensive. The best technology for filter regeneration with trucks and buses seems to be continuous regeneration. Using the NO _x present in the exhaust gas for soot oxidation amounts to a simple and robust concept. A future limitation might be the minimal required NO _x :soot ratio; it is not sure if this will be met in future engines. Alternatively, a low-temperature catalyst may be developed that does not require NO _x . Developing such an advanced catalytic trap will be one of the major challenges of catalytic filter engineering.