An exhaust gas emitted from diesel engines contains PM (particulate matter) based on carbonaceous soot and SOF (soluble organic fraction) of high-boiling-point hydrocarbons. When such exhaust gas is released into the air, it may adversely affect human beings and the environment. For this reason, a PM-capturing ceramic honeycomb filter, which may be called “honeycomb filter” in short, has been disposed in an exhaust pipe connected to a diesel engine. One example of honeycomb filters for purifying an exhaust gas by removing particulate matter is shown in FIGS. 2(a) and 2(b). The honeycomb filter 10 comprises a ceramic honeycomb structure comprising porous cell walls 2 defining large numbers of outlet-side-sealed flow paths 3 and inlet-side-sealed flow paths 4 and an outer peripheral wall 1, and upstream-side plugs 6a and downstream-side plugs 6c alternately sealing the outlet-side-sealed flow paths 3 and the inlet-side-sealed flow paths 4 at an exhaust-gas-inlet-side end 8 and an exhaust-gas-outlet-side end 9 in a checkerboard pattern. The honeycomb filter is disposed in a metal container (not shown), with its outer peripheral wall 1 gripped by a holding member (not shown) constituted by a metal mesh, a ceramic mat, etc. such that the honeycomb filter used is stationary.
In the honeycomb filter 10, an exhaust gas is cleaned as follows. As shown by the dotted arrow, the exhaust gas flows into the outlet-side-sealed flow paths 3 opening at the exhaust-gas-inlet-side end 8. While it passes through the cell walls 2, specifically through penetrating holes constituted by communicating pores on and in the cell walls 2, PM contained in the exhaust gas is captured. The cleaned exhaust gas is discharged from the inlet-side-sealed flow paths 4 opening at the exhaust-gas-outlet-side end 9 into the atmosphere.
As PM continues to be captured by the cell walls 2, the penetrating holes on and in the cell walls are clogged by PM, resulting in pressure loss increase when the exhaust gas passes through the honeycomb filter. Accordingly, it is necessary to burn PM before the pressure loss reaches the predetermined level to regenerate the honeycomb filter. However, as high exhaust gas temperatures as burning PM are less obtained in a usual diesel engine operation. Accordingly, with oxidation catalysts comprising, for instance, platinum-group metals and rare earth oxides such as cerium oxide, etc. carried by alumina, a high-specific-surface-area material, integrally supported on the cell walls 2 or in the pores, catalyst-carrying honeycomb filters capable of accelerating an oxidation reaction to burn PM even at low exhaust gas temperatures have been provided for practical use. Because such catalyst-carrying honeycomb filters accelerate the burning of PM with catalysts even at relatively low exhaust gas temperatures, continuous regeneration (continuous burning and removal of PM) can be conducted in a usual diesel engine operation.
In an operation state where exhaust gas temperatures are so low that the above continuous regeneration cannot be conducted (catalysts are not activated), PM is accumulated on the cell wall surfaces and in the penetrating holes in the cell walls, resulting in the increased pressure loss of the honeycomb filter. In such a case, PM is burned with a heating means such as a heater, etc., or by adding an unburned fuel to the exhaust gas, such that the forced regeneration of the filter is conducted.
FIG. 3 shows the change of the pressure loss of a honeycomb filter with time from the start of capturing PM to the forced regeneration. The pressure loss, which is P0 at the start of capturing PM, increases as the amount of PM accumulated increases with time, and reaches a predetermined level P1, at which the forced regeneration is conducted. Because the forced regeneration consumes energy regardless of whether a heater, etc. are used or an unburned fuel is added to the exhaust gas, the time period T from the start of capturing PM to the forced regeneration is preferably as long as possible. Although the extension of the time period T can be achieved by reducing the pressure loss of the honeycomb filter by increasing the volume and average diameter of pores in the cell walls, such method causes decrease in initial PM-capturing efficiency during the period A in the figure.
To solve the above problem, JP 2006-685 A discloses a honeycomb structure in which ceramic particles are attached to cell wall surfaces to clog large open pores. It describes that this honeycomb structure less suffers capturing efficiency decrease, particularly PM-capturing efficiency decrease in an early stage, even when the volume and average diameter of pores in the cell walls are increased. As a similar technology, JP 2004-74002 A discloses a honeycomb filter having particles coated only on part of cell wall surfaces including pore openings and nearby regions to increase capturing efficiency with reduced pressure loss.
Because pores opening at the cell wall surfaces are clogged by ceramic particles in the honeycomb structure of JP 2006-685 A and the honeycomb filter of JP 2004-74002 A, PM does not easily enter pores in the cell walls but are captured on the cell wall surfaces, so that small initial PM-capturing efficiency decrease is expected. However, when PM does not easily enter pores in the cell walls, it is less brought into contact with catalysts in the pores, resulting in insufficient effect of accelerating the burning of PM. As a result, continuous regeneration is not conducted even at exhaust gas temperatures at which the catalysts are activated, so that pressure loss increases in a short period of time by PM accumulated on the cell wall surfaces, frequently needing the forced regeneration.
JP 2005-296935 A discloses an exhaust gas filter having connected agglomerates of fine particles with small gaps in pores or on the surface to have high PM-capturing efficiency in an early stage with small pressure loss. Paying attention to the fact that PM layers accumulated on the cell walls of conventional honeycomb filters are useful as filters having low pressure loss and high capturing efficiency, the exhaust gas filter described in JP 2005-296935 A was obtained by forming fine pore structures in place of the PM layers, and is considered effective for high initial PM-capturing efficiency and small pressure loss. It describes that the fine pore structures should be as thin as 3.5 μm or less to reduce pressure loss, but it is extremely difficult to form fine pore structures as thin as 3.5 μm or less with substantially spherical fine particles. Although the fine pore structures can be formed by using fibrous fine particles, the fibrous fine particles are harmful to humans, thus difficult to handle in the production process.
JP 2007-130629 A discloses an exhaust-gas-cleaning, porous filter having inner surfaces of pores coated with a heat-resistant material to form three-dimensional, cross-linked structures in the pores, thereby exhibiting a high PM-capturing ratio even without PM layers. However, because the coating of the heat-resistant material is conducted by dipping, pores opening at the cell wall surfaces are clogged to have smaller diameters, and heat-resistant particles are attached to inner surfaces of pores in the cell walls to reduce pore diameters, resulting in pressure loss increase.