The invention relates to a particulate filter coating used in the treatment of exhaust or waste gases, as well as to the particulate filter itself. The invention also relates to a method for manufacturing and using said particulate filter.
During the years 2005-2012, the exhaust gas emission standards for diesel cars shall become so stringent that it becomes necessary to use particulate filters in order to follow the emission standards for particulate matter (PM). With diesel cars, the most difficult standards to follow are PM and NOx emission standards, but carbon monoxide and hydrocarbon emissions can be effectively eliminated by oxidation catalysts. As regards NOx emissions, the chosen strategy in the first stages has been one where it would not be necessary to use after-treatment techniques, because these tend to increase the general fuel consumption, or they require separate components in the car. For reducing NOx, there can be used motor technical methods (combustion temperature, cylinder conditions, EGR) which, however, have an effect that increases PM and HC emissions. Therefore the use of diesel particulate filters (DPF) is necessary in these targets. The average, concentration-based particle size obtained from modern diesel motors is less than 100 nm. There has recently been a widespread discussion of the hazardous nature of PM emissions, because small particles are conducted to the lungs, and very small particles may end up in the human system. The particles contain heavy hydrocarbons, soot (CHx) that is classified as carcinogenic, as well as various inorganic compounds.
DPF:s are typically made of cordierite, SiC or sintered metal, and the aim is to realize DPF structures where the increase of counterpressure and the PM storage capacity are optimized. Regular commercial DPF:s have a cellular structure, where the gas flow must penetrate a porous wall (wall-flow DPF). The cellular system contains open channels (hydraulic diameter roughly 1-2 mm), half of which are closed at the inlet end and half at the outlet end, with the purpose to force the flow through the porous wall. Generally the share of the pores in the walls is roughly 40-60%, and the average pore size is roughly 10-40 μm, in which case the counterpressure remains low, but with this structure, reasonably small particles can be filtered at an efficiency of over 90%. In some cases, particularly in retrofit targets, there also are used DPF:s operated by a so-called deep filtration principle, and with these filters, the filtering efficiency remains, owing to larger pore sizes, lower than with cellular-type DPF:s, because the particles are partly dispersed in the filter material. This kind of DPF:s are made, among others, of ceramic or metallic fibers, foams and other porous materials.
By using a DPF, it is possible to remove the hydrocarbon-bearing volatile fraction (VOF=volatile organic fraction or SOF=soluble organic fraction) of the PM:s by an efficient oxidation catalyst. The removal of the carbon fraction requires thermal or catalytic combustion, for which there are developed various different regeneration methods for maintaining the counterpressure of the filter reasonably low and for preventing it from blocking by PM. Generally the PM soot is burned thermally by additional heat. The combustion of soot is started, when the temperature reaches the level of over 550-600° C. in the exhaust gas containing excessive oxygen. The additional heat is generated for example by burners, additional fuel supply, electric resistors or by some other way by supplying additional energy in the DPF or in the exhaust gas before the DPF. Soot can be oxidized by an intensive combustion reaction with oxygen at a temperature higher than 550° C., or slowly at lower temperatures (250-350° C.) by NO2. In both reactions, it is possible to use catalysts. Catalytic soot combustion by oxygen has been widely studied for several decades, and the typical catalysts are compounds forming so-called molten salts, containing, among others, vanadium, Cu, K, Cs and perovskite compounds (Fino 2003). In active regeneration, additional heat is fed in the DPF, but in passive filtering, the purpose is to oxidize the particles continuously by means of NO2. A developed CRT method (Continuous Regenerating Trap) includes a Pt-bearing oxidation catalyst, and thereafter an uncoated or catalyst-coated DPF (EP 341,832). The NO2 created in the oxidation catalyst oxidizes soot at reasonably low temperatures (>250-300° C.), when the oxidation catalyst is sufficiently efficient. The problems in the passive method are connected to situations where the creation of NO2 is not sufficient, for example in congested urban driving, and the method requires a fuel with a very low sulfur content (S<10 ppm) for minimizing the creation of sulfate in the efficient and expensive Pt-bearing oxidation catalyst. The blocking of a DPF cannot be accepted in any situation, because this will interrupt the driving. Consequently, most current systems include active regeneration, which principle has already been applied for several decades. In 1979. Virk and Alperstain introduced a system provided with an oxidation catalyst and a DPF, in which system active regeneration is achieved by fuel supply. The use of modern adjusting technique together with motor control enables a reasonably accurate, active regeneration by means of a DPF.
DPF coating has already been used for over 25 years, and in the DPF, there have been added for example soot combustion or oxidation catalysts (Ernest 1980 and Enga 1981). With Pt-containing DPF:s, there are achieved systems where soot combustion temperature is lowered near to 300° C. (EP 0,160,482 and U.S. Pat. No. 4,510,265).
Usually the catalyst is added to a DPF by normal slurries used in catalyst coating (colloidal suspensions) or from aqueous solutions, and the most common coatings have been alumina-, zirconia- or silica-based (www.dieselnet.com/, Catalyzed diesel filters, 15.3.2006). Into the suspension, there are added catalyst raw materials from powders in particles with a regular size of roughly 1-40 μm. Pt/ceria-zirconia type coatings are also developed for DPF (Grimston 2002). Adsorption with aqueous solutions has several drawbacks, such as a low quantity of catalyst during each addition, and the difficulty to create coatings for a large surface area. With coatings used in cellular catalysts, the coating is normally obtained only on the channel walls, but less in the pores. The quantity of slurry coating must be kept low, in order to avoid raising the counterpressure excessively. If only the channel walls at the inlet end are coated, the catalyst approaches soot, but the total quantity of catalyst remains low. By coating with slurries, there also is obtained catalyst separately on both sides of the DPF. It has been suggested that the particle sizes in the slurries must be as small as possible, in order to make the stabilizing agent particles enter the pore surface. The size of DPF pores is within the range 10-20 μm. However, this kind of pore size filters nearly all of the slurry particles onto the channel walls, because in practice the minimum particle size in slurries is normally roughly 1 μm. The degree of filtering is high even for stabilizing agent particles of 100-500 nm, in similar fashion as for PM particles in practical usage conditions in a gas phase.
Most catalyst reactions in exhaust gas targets are kinetically restricted, and the use of a high Pt charge results in a very high Pt density, if the stabilizing agent quantity is low, in which case the expensive precious metal cannot be utilized properly. It has been suggested that catalytically active components can be added directly in the cordierite, but catalytic activity (NO/HC/CO oxidation) cannot be on the same level as the on the surface of a stabilizing agent with a large surface area. With respect to activity, the ceramic surface as such is not a good adhesion substrate for precious metals. The surface area of normal alumina-based stabilizers is over 200 m2/g, whereas the material surface area of thermally burnt DPF material is below 5 m2/g, typically even clearly below 1 m2/g. It has also been suggested that on the surface of SiC, there is made a preliminary layer, to which the coating is attached (NoTox Corporation 1997). It has been suggested that larger pores are used in a DPF to be catalyst coated, so that it can be coated by the catalyst.
Irrespective of the regeneration of carbon, unburnt ashes are accumulated in the DPF, and the quantity of said ashes must be taken into account as regards measures, lubricant recommendations and possible maintenance operations.