There are concerns about emissions of particulate matter (PM), commonly referred to as soot, from internal combustion engines and especially from Diesel engines in automotive applications that operate in populated urban areas. The main concerns are associated with potential health effects, and most recently with very tiny particles having sizes in the nanometer range. Nanoparticles around 100 nm in size are often referred to as an accumulation mode, and the very tiny particles around 10 nm as a nucleation mode. Nanoparticles when inhaled can penetrate deeply into the lungs and from there can easily enter the blood stream and thence travel to all of the organs of the body where they can cause a variety of problems. There is also evidence that nanoparticles can translocate along nerves from the olfactory glands directly into the brain of animals. Because of these concerns the maximum amount of tailpipe particulate emissions from Diesel powered passenger cars and heavy duty vehicles is limited by legislation that has decreased over recent years in step with concern about environmental impact. Until recently these emissions limits were expressed in grams and the current European passenger car limit of 5 mg/km (Euro 5) demands the fitment of exhaust gas filters to achieve such low levels.
Diesel particulate filters (DPFs) have been fabricated using a variety of materials including sintered metal, ceramic or metal fibres etc, with the most common type in actual mass production being the wall-flow kind made from porous ceramic material fabricated in the form of a monolithic array of many small channels running along the length of the body. Alternate channels are plugged at one end so the exhaust gas is forced through the porous ceramic channel walls that prevent most of the particulate from passing through so only filtered gas enters the environment. Ceramic wall-flow filters in commercial production include those made from cordierite, various forms of silicon carbide and aluminium titanate. The actual shape and dimensions of practical filters on vehicles as well as properties such as the channel wall thickness and its porosity etc. depend on the application concerned.
The average dimensions of the pores in the filter channel walls of a ceramic wall-flow filter through which the gas passes are typically in the range 10 to 50 μm and usually about 20 μm. In marked contrast, the size of most Diesel particulate matter from a modern passenger car high speed Diesel engine is very much smaller, e.g. 10 to 200 nm, so they ought to be able to pass through the filter unheeded, and this is indeed what happens when exhaust gas passes through a clean filter for the first time. However, some PM is retained within the pore structure in the filter walls and this gradually builds up until the pores are bridged over by a network of PM and this PM network then enables the easy formation of a cake of particulate on the internal walls of the filter channels. The particulate cake is an excellent filter medium and its presence affords very high filtration efficiency.
Periodically it is necessary to remove trapped PM from a filter to prevent the build-up of excessive backpressure that is detrimental to engine performance and can cause poor fuel economy. So in Diesel applications, retained PM is removed from the filter by burning it in air in a process during which the amount of air available and the amount of excess fuel used to achieve the high temperature needed to ignite the retained PM are very carefully controlled. Towards the end of this process that is usually called regeneration, the removal of the last remaining particulate in the filter leads to a marked decrease in filtration efficiency and release of a burst of many small particles into the environment. Thus, filters have low filtration efficiency when they are first used and subsequently after each regeneration event and also during the latter part of each regeneration process.
Previously the legislative particulate emissions limits were on a weight basis and so were biased towards the larger heavier particles. Now a particle number measurement is being introduced, which for diesel passenger cars is 6.0×1011 for new models from 1 Sep. 2011 (Euro 5b limit values) and the same for Euro 6b limit values (implementation date to be confirmed), and this has a bias towards the smaller, environmentally more dangerous particles. A further practical reason for this change is because the mass of particles permitted has been progressively lowered and is now at a level where determining them by weighting very small masses is practically difficult. With the introduction of particle number legislation it is very important to maintain filtration efficiency at all times—emissions during regeneration and immediately afterwards can be a very significant contribution to what is permitted overall and as a result present Diesel filtration systems in particular are not adequate to meet the new legislative requirements.
There is a related problem with filters for gasoline spark ignition engines that operate at much higher temperatures than those prevailing in the exhaust gas of Diesel engines, and especially those of Diesel passenger cars. Direct injection gasoline engines are particularly prone to forming relatively high levels of exhaust particulate matter. Here the temperature can be so high particulate matter is burnt soon after it is retained in the filter so a significant amount of particulate cake is never formed in the filter and high filtration efficiency is never achieved.
It is known to catalyse filters for particular applications. For example, U.S. Pat. No. 4,477,417 (the entire contents of which are incorporated herein by reference) discloses a catalyst for reducing the ignition temperature of diesel soot.
There is therefore a major requirement for a means of improving the filtration efficiency of filters without causing additional backpressure to the filtration system.
EP 2158956 (the entire contents of which is incorporated herein by reference) discloses a honeycomb filter of the wall-flow type and a surface layer provided only on an inflow side partition wall or both the inflow side and outflow side partition wall. The document discloses specifically two honeycomb filter embodiments and five honeycomb filter manufacturing methods. The surface layer in the first or second honeycomb filter preferably carries fine particles of one or both of platinum and palladium and complies with the following conditions: (1) the peak pore diameter of the surface layer is equal to or smaller than the average pore diameter of the partition wall base material, and the porosity of the surface layer is larger than that of the partition wall base material; (2) the surface layer has a peak pore diameter of 0.3 μm or more and less than 20 μm and a porosity of 60% or more and less than 95% (measurement method is mercury porosimetry); (3) the thickness L1 of the surface layer is 0.5% or more and less than 30% of the thickness L2 of the partition walls; (4) the mass of the surface layer per filtration area is 0.01 mg/cm2 or more and less than 6 mg/cm2; and (5) the partition wall base material has an average pore diameter of 10 μm or more and less than 60 μm and a porosity of 40% or more and less than 65%. The five honeycomb filter manufacturing methods comprise preparing a slurry comprising at least one fibrous material and applying the slurry to the honeycomb filter substrate by an atomisation process using e.g. a needle-like atomiser.
Society of Automotive Engineers (SAE) Technical Paper 2008-01-0621 from the 2008 World Congress held in Detroit, Mich. Apr. 14-17, 2008 by the authors of EP 2158956 describes using a surface layer of CeO2-based material having 300 nm particle size at a loading of 15 g/l (no precious metal) on a silicon carbide diesel particulate filter.