JPS61149222A - Purification filter for collecting fine particle - Google Patents

Abstract

PURPOSE:To effectively regenerate a filter by enhancing the combustion rate of a particulate at low temp., by supporting a catalyst consisting of palladium or rhodium and alkali metal oxide by a ceramic filter substrate. CONSTITUTION:A filter substrate is formed by using ceramics such as cordierite, mullite or spinel. 27-35/cm<2> of exhaust gas passages are pref. formed and a support layer of almina is pref. formed to the surface of said substrate. A catalyst based on palladium or rhodium and alkali metal oxide is supported by said support layer. This catalyst is constituted of 0.1-5g of palladium, 0.1-5g of rhodium and 5-25g of alkali metal oxide per 1l of the apparent volume of the filter substrate.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is a particulate collector that purifies the exhaust gas by collecting particulates (fine particles mainly composed of carbon) contained in the exhaust gas of a diesel engine, etc. The present invention relates to a particulate collection and purification filter, and more particularly to a particulate collection and purification filter that burns and removes particulates by ignition for reuse.

[Prior Art] As particulate collection and purification filters that collect particulates and purify exhaust gas, conventionally, ceramic honeycomb structures (honeycomb filters) or three-dimensional mesh filters, which have excellent particulate collection properties, have been used. Foam filters and the like having a structure are known. In these particulate filters, as the mileage of the vehicle increases, particulates accumulate in the exhaust passage and the filter becomes clogged, resulting in a gradual increase in pressure loss and a decrease in engine output. . However, since most of these particulates are carbon particles, they can be burned and removed by heating to a high temperature, thereby making it possible to regenerate the particulate collection and purification filter.

Therefore, in order to regenerate this particulate collection and purification filter, methods have been developed in which particulates are burned using an external ignition means such as a burner or an electric heater. In such a method of combusting particulates, it is effective to support a catalyst on the filter base of the particulate collection and purification filter in order to more reliably combust the particulates.

Examples of this catalyst include the palladium catalyst and rhodium catalyst conventionally disclosed in JP-A-55-24597, the noble metal-chromium catalyst disclosed in JP-A-57-24.640, and the noble metal-chromium catalyst disclosed in JP-A-57-24.640. Copper, nickel, manganese, vanadium catalysts, etc. disclosed in Japanese Patent Application Laid-open No. 1091364 and Japanese Patent Application Laid-open No. 109139/1982 are known.

[Problems to be solved by the invention] Although the above-mentioned catalyst improves the ignitability of particulates, since particulates inherently have a property of being difficult to twist, they do not necessarily burn sufficiently, resulting in unburned particulates. Something happened. Therefore, pressure loss tended to gradually increase. The present invention is intended to solve this problem.

[Means for Solving the Problems] The particulate collection and purification filter of the present invention consists of a filter base made of ceramics and a catalyst supported on the surface of the filter base, and is capable of removing particles contained in engine exhaust gas. In the particulate collection and purification filter that collects curates, the catalyst contains at least one of palladium and rhodium and an oxide of at least one metal selected from alkali metals as main components. It is characterized by being configured as follows.

The filter base of the particulate collection and purification filter of the present invention has an exhaust gas passage through which exhaust gas passes, and can be made of ceramics such as cordierite, mullite, and spinel, which are resistant to thermal shock, as in the past. It is preferable that 27 to 35 exhaust gas passages are formed per square centimeter. The shape of the filter base includes an exhaust gas passage for introducing exhaust gas that is open in a checkered pattern at one end and closed at the other end, and an exhaust gas passage for introducing exhaust gas that is opened in a checkered pattern at the other end and closed at the other end. There are honeycomb-shaped ones that have exhaust gas passages for gas discharge adjacent to each other at different intervals, and foam-shaped ones that have a three-dimensional network structure. Either of these can be used, but it is not limited to these. . The foam-shaped filter base can be obtained by attaching a ceramic slurry to a flexible polyurethane foam and firing it to remove carbonization of the flexible polyurethane foam.

It is desirable that a porous ceramic support layer be formed on the surface of the filter base. This support layer is intended to improve the collection performance or prevent the collected particulates from re-dispersing, and can be selected from various materials such as alumina, titania, magnesia, and silica, but in particular, γ-alumina can be used. It is desirable to create one.

In the particulate collection and purification filter of the present invention, a catalyst is supported directly on the filter base or on a porous ceramic support layer provided on the surface of the filter base. The most distinctive feature of the present invention lies in this catalyst.

This catalyst is composed mainly of at least one of palladium and rhodium and an oxide of at least one metal selected from alkali metals. The alkali metal is generally lithium (L l )
, sodium (Na), potassium (K), rubidium (
Rb), cesium (C5), and francium (Fr) are used. As an oxide of alkali metal, lithium oxide (
L120), sodium oxide (Nano>, potassium oxide (K2O), rubidium oxide (Rb20), cesium oxide (C5tO), etc. are used.The alkali metal oxide may be one of the above (two or more types). It can also be used in combination.In the present invention, 0.1 to 5 g of palladium, 0
．． It is preferable to consist of 1 to 5 g of rhodium and 5 to 25 degrees of alkali metal oxide. Note that the apparent volume means the total volume including the volumes of the introduction passage and the discharge passage in the case of a honeycomb filter, and the total volume including the internal space of the three-dimensional network structure in the case of a foam filter. Among these, palladium is mainly used to improve ignitability.
Rhodium is mainly used for the purpose of suppressing sulfate formation and improving durability, and exhibits the best performance when supported within the above range. Further, within the above range, palladium and rhodium can be supported at a ratio of 1:1. however,
Considering the price, the proportion of expensive rhodium can be reduced. In general, if the amount of palladium exceeds 10 times the amount of rhodium, the amount of sulfate produced tends to increase, and if the amount of palladium is less than the amount of rhodium, the ignitability becomes inferior, which is not preferable. In some cases, beryllium oxide (BeO), magnesium oxide (M(10), calcium oxide (CaO), strontium oxide (SrO), barium oxide (BaO)
) and other alkaline earth metal oxides can also be used in combination.

Next, a typical manufacturing method of the particulate collection and purification filter of the present invention will be described. First, a filter base such as a honeycomb filter or a foam filter is formed as in the conventional method. At this time,
Preferably, a porous ceramic support layer is formed on the surface of the filter base.

Then, the filter base is immersed in a solution containing palladium and rhodium chlorides, etc., and a solution containing the alkali metal ions, thereby impregnating the surface of the filter base with the solution. Thereafter, the filter base is fired to convert the metal into a metal oxide and support the catalyst. Note that the firing temperature is usually 300 to 1000°C. Here, the amount of catalyst supported can also be changed by changing the concentration of the solution. Note that palladium, rhodium, and alkali metals may be dissolved or dispersed in a solution, and are not limited to chlorides and nitrates. Further, various solvents such as water, alcohol, acetone, etc. can be selected as the solvent for dispersing the metal.

The present invention will be specifically explained with reference to each example below, in which the filter substrate is immersed in a solution in which each catalyst compound is mixed and the catalyst is immersed once.

(Example) A cordierite honeycomb structure having a large number of passages parallel to the axial direction was used as a filter base. This filter base has the passage open at both ends and has a diameter of 3.
0 mm, the length is 50 mm, and the number of exhaust gas passages is 31/am'.

Next, as a first step, 100 parts by weight of γ-alumina powder,
The filter base was immersed in a slurry consisting of 100 parts by weight of alumina sol, 6 parts by weight of aluminum nitrate, and 50 parts by weight of distilled water, pulled up, blown off excess droplets, dried at 120°C for 3 hours, and further It was fired at 700° C. for 2 hours to form a porous ceramic support layer made of γ-alumina on the wall forming the exhaust gas passage of the filter base.

Next, in the second step, an aqueous solution containing 0.04% by weight of palladium chloride (PdC1z) is used, and in this aqueous solution, γ
- The filter base on which the alumina support layer was formed was immersed for 2 hours to adsorb palladium, reduced with an aqueous boron hydride solution, and then washed with water. As a result, 1 (1) amount of palladium was supported per 1 liter of the apparent volume of the filter base.

Furthermore, the filter base supporting palladium was immersed in an aqueous solution containing 0.04% by weight of rhodium chloride (Rhc13) for 2 hours, reduced with an aqueous sodium borohydride solution, and then washed with water. It was further dried at 120° C. for 2 hours, thereby supporting 1 g of rho 10 =dium per 1 liter of apparent volume of the filter substrate.

Next, in the third step, the filter base supporting palladium and rhodium is immersed in a solution containing lithium ions, specifically, a lithium nitrate aqueous solution for 1 minute, and after blowing off excess droplets, the temperature is raised to about 120°C. The particulate collection and purification filter (
Samples a to e) were prepared. Note that the amount of catalyst supported in samples a to e is shown in Table 1.

(Example 2) In Example 2, the same filter base as in Example 1 was used,
The same first and second steps as in Example 1 were performed. The filter base that has undergone the second step is immersed in a solution containing sodium ions, specifically a sodium nitrate aqueous solution, for 1 minute, and after blowing off excess droplets, the filter base is heated at approximately 120°C. It was dried for 3 hours and fired at about 600°C for about 2R, thereby supporting sodium oxide (NazO) on the filter base to form the particulate collection and purification filter of Example 2. It was designated as sample f. The amount of catalyst supported on sample f is shown in Table 1.

(Example 3) In Example 2, the same filter base as in Example 1 was used,
The same first and second steps as in Example 1 were performed. Then, the filter base that has undergone the second step is placed in a solution containing potassium (K) ions, specifically, in a potassium nitrate aqueous solution.
Soak for 1 minute and blow off excess droplets. After that, about 1
It was dried at 20'C for 3 hours and fired at about 600°C for about 2 hours, thereby supporting potassium oxide (K2O) on the filter base to form the particulate collection and purification filter of Example 3. It was designated as sample Q. Note that the amount of catalyst supported in Sample 9 is shown in Table 1.

(Comparative Example) In a comparative example, a particle collection and purification filter was formed using the same filter base as in Example 1 and the same first and second steps as in Example 1, and this was used as a sample. Therefore, the sample catalyst does not contain alkali metal oxides.

(Evaluation) Samples a-e1 of the particle collection and purification filter of Example 1 above
Sample f1 of the particle collection and purification filter of Example 2 Example 3
Sample 9 of the particle collection and purification filter, a comparative example, was installed in the exhaust system of a 2200 cc displacement chamber type diesel engine, and the engine rotation speed was 200 rpm1t.
- The filter was operated for 3 hours under the condition of 3 kQ.m, and about 0.59 particulates were collected in the filter. Next, the filter in which the Putty Table 2 hydrate was collected was placed at filter set position 1 of the experimental apparatus shown in FIG.

This experimental device consists of connected reaction tubes 4 and 5, an electric furnace 6 surrounding the reaction tube 4, a power source 7 that supplies electricity to the heater 2, and a temperature recorder 1 that measures the temperature of the filter with a thermocouple 11.
2, a temperature recorder 15 that measures the temperature of the filter using thermal lightning pairs 13 and 14, a gas rectifier 3 in the reaction tube 4, a gas mixer 8, and a rectifier 9. Then, while heating the particle collection and purification filter with the heater 2, a mixed gas of nitrogen and oxygen is introduced into the reaction tubes 4 and 5.
This burned particulates adhering to the filter base, and measured the relationship between heating temperature and particulate combustion rate. The measurement results are shown in Table 2. The combustion rate of particulates was calculated by the formula (Wl-W2 > /WI X 100) where Wl is the weight of particulates before measurement, W2 is the weight of particulates after measurement.

As is clear from Table 2, the combustion rate of particulates is improved in the particulate collection and purification filters of Samples a to g, which support an alkali metal oxide, compared to the comparative example. In particular, when the filter heating temperature was 500°C, the combustion rate of sample C1 sample d1 sample e1 sample f1 sample q according to the example was all as high as 90% or more, but the combustion rate of the sample sample of the comparative example was was as low as 85%. Similarly, when the filter heating temperature is 475°C, sample C1 sample d1 trial e1
The combustion rates of samples f and g were both high at 81% or higher, but the combustion rate of the comparative sample was as low as 74%. This result shows that it is preferable to support 50 or more alkali metal oxides per liter of apparent volume of the filter base. Alternatively, in order to maintain the same combustion rate, it is sufficient to heat to a lower temperature than before.

For example, in the sample sample of the comparative example, to achieve a combustion rate of 85%,
It was necessary to heat the filter to 00°C. However, for samples c and d1e of the example, it is sufficient to heat them to a temperature lower than 500°C. Therefore, damage to the filter base due to thermal shock can be suppressed to that extent.

It is presumed that the above-mentioned effects were obtained because the alkali metal oxide has the effect of propagating the combustion of particulates.

[Effects of the Invention] In the particulate collection and purification filter of the present invention, the combustion rate of particulates is improved compared to the conventional example.

Therefore, the regeneration process of the particulate collection and purification filter can be carried out effectively. Moreover, even in order to obtain the same combustion rate, it is sufficient to heat the filter to a lower temperature than in the past, and therefore damage to the filter base due to thermal shock can be suppressed to that extent.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an experimental apparatus used when testing the relationship between particulate combustion rate and heating temperature. Patent applicant: Toyota Motor Corporation Agent
Patent Attorney Hirodo Okawa 1st Patent Attorney Shudou Fujitani 1st Patent Attorney Akio Maruyama

Claims (4)

[Claims] (1) In a particulate collection and purification filter that consists of a filter base made of ceramics and a catalyst supported on the surface of the filter base, and that collects particulates contained in engine exhaust gas, the catalyst is made of palladium. , at least one kind of rhodium, and an oxide of at least one kind of metal selected from alkali metals as main components. (2) The catalyst is composed of 0.1 to 5 g of palladium, 0.1 to 5 g of rhodium, and 5 to 25 g of an alkali metal oxide per 1 liter of apparent volume of the filter base. A particulate collection and purification filter according to claim 1. (3) The particulate collecting and purifying filter according to claim 1, wherein the surface of the filter base is provided with a porous ceramic support layer. (4) The particulate collection and purification filter according to claim 3, wherein the porous ceramic coating layer is made of γ-alumina.