JPH04354518A - Material and method for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To provide a material and a method for purifying exhaust gas capable of effectively burning and removing particulates contained in exhaust gas with much change in oxygen concentration, such as that discharged from diesel, etc., and simultaneously of effectively removing NOx. CONSTITUTION:An exhaust gas purifying material consists of a catalyst consisting of (a) an alkali metal element, (b) Co element and/or Mn element, (c) V element and (d) a rare earth element which is supported by a porous ceramic layer provided on a heat resistant porous filter and deposited on it.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to an exhaust gas purifying material and an exhaust gas purifying method using this exhaust gas purifying material, and more specifically to nitrogen oxides (
The present invention relates to an exhaust gas purification material that can simultaneously remove NOx (hereinafter referred to as NOx) and particulate carbon substances (hereinafter referred to as particulates), and an exhaust gas purification method using the exhaust gas purification material.

0002

[Prior art and problems to be solved by the invention] In recent years,
Particulate carbon substances (mainly composed of solid carbon particles, called particulates), NOx, etc. in exhaust gas emitted from diesel engines and the like have become a problem as harmful to environmental health. In particular, particulates have an average particle size of 0.1 to 1 μm and are easily suspended in the air, so they are easily taken into the human body through breathing, and recent clinical test results indicate that they also contain carcinogenic substances. This has been confirmed.

[0003] As methods for removing particulates, the following two methods are being considered. One method is to capture particulates by filtering exhaust gas using a heat-resistant filter, and when the resulting pressure loss increases, the filter is regenerated by burning the captured particulates with a burner, electric heater, etc. It is. Heat-resistant filters that can be used include honeycomb ceramic filters, foam ceramic filters with a three-dimensional network structure, steel wool, wire mesh, and the like. The other method is to filter particulates and self-combust them using the action of a catalyst supported on a filter as described above.

In the former case, the higher the particulate removal effect, the faster the rise in pressure loss, the higher the frequency of regeneration, the higher the reliability of regeneration is required, and it is considered to be economically disadvantageous. On the other hand, the latter method is considered to be a much better method if there is a catalyst that can maintain catalytic activity under the exhaust gas emission conditions (gas composition and temperature) of a diesel engine.

However, since diesel engines use light oil as fuel, the exhaust gas contains a large amount of SO2, and the oxygen concentration in the exhaust gas varies over a wide range of 2 to 20% depending on the operating conditions of the diesel engine. Under such exhaust gas conditions, a method for regenerating an exhaust gas purification filter that satisfactorily ignites and burns the accumulated particulates and does not cause secondary pollution has not yet been established. For example, catalysts that have been proposed to purify particulates in exhaust gas from diesel engines include those based on noble metals and base metals. Hydrocarbon (hereinafter referred to as HC)
Although it has high oxidizing properties such as (referred to as ), it is easy to convert SO2 present in exhaust gas into SO3, and there is a high possibility that it will cause secondary pollution. It also has the disadvantage that the soot in the particulates deteriorates the combustion characteristics. On the other hand, base metal catalysts are effective as particulate purification catalysts, but have problems in terms of durability.

[0006] Furthermore, most of the exhaust gas purification catalysts and purification materials that have been proposed so far have focused on lowering the ignition temperature of particulates, and the oxygen concentration in the exhaust gas is generally high or the oxygen concentration is low. In exhaust gas from diesel engines, etc., which fluctuates greatly, the N contained in it
Ox removal remains an open question.

[0007] Therefore, an object of the present invention is to provide an exhaust gas purifying material that can efficiently burn and remove particulates contained in exhaust gas with large changes in oxygen concentration, such as those emitted by diesel engines, and at the same time, can also effectively remove NOx. and to provide an exhaust gas purification method.

[0008]

[Means for Solving the Problems] In view of the above problems, as a result of intensive research, the present inventor has developed an alkali metal element and a specific transition layer on a porous ceramic layer provided on a heat-resistant porous filter. By using a purifying material that supports a catalyst made of metal elements and rare earth metal elements, particulates and NOx can be efficiently removed even in exhaust gas where the oxygen concentration varies greatly.
The present invention has been completed based on the discovery that it is possible to simultaneously remove and maintain the removal ability for a long time.

That is, the exhaust gas purifying material of the present invention has a porous ceramic layer provided on a heat-resistant porous filter.
It is characterized by supporting a catalyst consisting of (a) an alkali metal element, (b) a Co element and/or a Mn element, (c) a V element, and (d) a rare earth element.

[0010] Furthermore, the exhaust gas purification method of the present invention is characterized in that nitrogen oxides are reduced by using the exhaust gas purifying material and using particulates in the exhaust gas as a reducing agent by a catalyst supported on the filter.

The present invention will be explained in detail below. Since the filter used in the present invention filters high-temperature exhaust gas, a material that is porous and has high heat resistance, particularly thermal shock resistance, is used as the material for forming the filter. Moreover, it is necessary that the pressure loss be within an allowable range while maintaining the necessary particulate collection performance. Examples of such filter forming materials include ceramics such as alumina, silica, titania, zirconia, silica-alumina, alumina-zirconia, alumina-titania, silica-titania, silica-zirconia, titania-zirconia, mullite, and cordierite. It will be done.

[0012] The porosity of the filter is preferably 40 to 80%. Further, as the filter, a well-known honeycomb type or foam type filter can be used. The shape and size of the filter can be varied depending on the purpose. When using a foam type filter, it is generally formed into a cylindrical shape, with a diameter of 30 to 400 mm and a thickness of 50 to 30 mm.
It is preferable to set it to 0 mm. Moreover, if necessary, a plurality of sheets may be laminated.

[0013] The catalyst supported on the above filter is as follows:
(a) Alkali metals (Li, Na, K, Cs, etc.)
(b) Co element and/or Mn element, (c)
V element and (d) rare earth elements (Ce, La, Nd,
Sm, etc.) is used. (a) As the alkali metal, it is particularly preferable to use at least one of sodium, potassium, and cesium. Among them, when using cesium, any unburned HC, including saturated hydrocarbons such as propane, reacts with NOx, resulting in very good NOx removal. This is thought to be because the presence of cesium increases the selectivity of the reaction between NOx and hydrocarbons, suppressing the reaction between oxygen in the exhaust gas and hydrocarbons. Also, (d)
As the rare earth metal, cerium, lanthanum, neodymium, etc. are preferably used, and misch metal, which is a mixture of these rare earth metals, can also be used.

Each of the above catalyst components (a), (b), (
The blending amount of c) and (d) is 10 to 50% by weight when calculated as each metal alone.
, (b) is 15-75%, (c) is 15-75%,
(However, (b) + (c) is 30-90%), (d)
is preferably 10 to 50%. As for alkali metals, if the amount is less than 10% by weight or more than 50% by weight, the particulate and NOx removal properties will deteriorate. In addition, if component (b) is less than 15% by weight, the ignition characteristics of particulates will deteriorate, and if it exceeds 75% by weight, HC
The NOx reduction characteristics due to this decrease. Furthermore, (c)
If the content of the component is less than 15% by weight, the sulfur resistance property will be reduced, and if it exceeds 75% by weight, the simultaneous removal property of NOx and particulates will be reduced. (d) Component (rare earth metal element)
If the amount is less than 10% by weight or more than 50% by weight, the simultaneous removal properties of NOx and particulates will deteriorate. The preferred composition range is 15 to 30% of (a), (
b) is 20-50%, (c) is 20-50% (however, (b) + (c) is 40-80%), (d) is 1
It is 5-30%.

[0015] By using the catalyst having the above structure,
Particulates in exhaust gas can be ignited and burned at a relatively low temperature, and NOx can be effectively removed. In other words, particulates in the exhaust gas coexist with the catalyst element and oxygen in the filter, lowering the ignition temperature and reducing the particulates to 40%.
Burns (oxidizes) below 0°C. At the same time, the particulates act as a reducing agent to reduce NOx, thereby effectively purifying the exhaust gas. In this way, NOx reduction occurs efficiently at relatively low temperatures because of the particulates in the exhaust gas and the catalyst components (a) and (
This is thought to be due to the synergistic effect caused by the simultaneous presence of b), (c) and (d). In the present invention, the above-mentioned elements (b) and (c) are selected as transition elements, and as a result, there is no decrease in durability as seen in conventional Cu-based catalysts, and stable NOx can be produced over a long period of time. can be removed.

[0016] By the way, exhaust gas from diesel engines usually contains 100 to 500 p of unburned hydrocarbons (HC).
pm, but this HC includes propane,
Includes propylene, acetylene, ethylene, etc. In the catalysts proposed so far in which Cu exists alone as a catalytically active species (for example, JP-A-63-100919), unburned HC has unsaturated bonds such as propylene and acetylene. In some cases, purification by reducing NOx using HC as a reducing agent has been effective to some extent, but the reaction activity using saturated hydrocarbons such as propane as a reducing agent is low, and the reduction and removal of NOx cannot be said to be sufficient. . However, by using a catalyst with the composition described above, even saturated hydrocarbons such as propane can be used as a reducing agent to efficiently reduce NOx, resulting in significantly superior NOx emissions compared to conventional catalysts. can be purified.

[0017] In order to support the above-mentioned catalyst on the heat-resistant porous filter, a carrier layer made of ceramics which is more porous and has a larger surface area than the filter is formed on the filter, and then the catalyst is supported on this carrier layer. As the material for forming the carrier layer, it is preferable to use ceramics that are porous and have a large surface area, such as alumina, titania, zirconia, silica, titania-alumina, alumina-zirconia, alumina-silica, titania-silica, and titania-zirconia. .

[0018] The amount of carrier layer is, in the case of ceramic filters, between 3 and 15% by weight of the filter, preferably between 5 and 12%. In addition, catalytically active species ((a) to (d) described above)
) Components) have a total content of 1 to 40% by weight of the carrier layer.
, preferably 5 to 30% by weight.

The carrier layer made of ceramics can be formed by a wash coat method, a sol-gel method, or the like. The wash coat method is a method in which a carrier layer is formed on the filter by immersing the filter in the above-mentioned porous ceramic slurry and drying it.

[0020] Furthermore, the formation of the carrier layer by the sol-gel method is carried out as follows. The organic salt of the metal (e.g. alkoxide) that forms the ceramic for the carrier layer is hydrolyzed, the resulting sol is coated on a filter, a film of colloidal particles is generated by contact with water vapor, etc., and then dried and fired. to form a catalyst carrier layer on the filter. For example, when titania (TiO2) is used as the ceramic for the carrier layer, first a Ti alkoxide (e.g., Ti(O-
CH
3. A coating liquid is produced by adding acids such as COOH, HNO3, and HCl. After immersing the filter in this coating liquid and pulling it out, it is reacted with steam or water to form a gel. Next, when the filter is dried and fired, a titania film is formed on the pore surface of the filter. In the sol-gel method, an acid is added as a catalyst for the hydrolysis reaction during gelation, but the hydrolysis reaction can also be promoted by adding an alkali instead of the acid.

Although titania has been described above as an example of the ceramic for the carrier layer, other ceramics can be similarly supported by the sol-gel method. For example, when forming an alumina carrier layer, aluminum alkoxide is used in the same manner as in the case of titania described above. The same applies when using other porous carriers.

[0022] After forming a porous carrier layer made of ceramics on the filter by the above-mentioned wash coat method or sol-gel method, next, catalytically active species such as carbonate, nitrate, acetate, hydroxide, etc. The carrier layer is impregnated with an aqueous solution of , dried and calcined again to support the catalyst. In addition, as salts of catalytically active metal species, as long as they are soluble in water,
As mentioned above, any type of salt such as carbonate, nitrate, acetate, hydroxide, etc. can be used. For supporting V, a solution of NH4VO3 and oxalic acid can be used. Furthermore, an alkali metal and V can be simultaneously supported using an alkali vanadate.

[0023] In the method of the present invention, NOx in the exhaust gas is reduced and removed by HC using the above-mentioned exhaust gas purifying material. Since HC in the exhaust gas mainly consists of methane, ethylene, acetylene, etc., the NOx reduction temperature is 200°C. ~5
00°C, preferably 250-450°C. If the reaction temperature is too high, combustion of HC itself will occur, reducing the NOx reduction properties. In the present invention, if there is little HC in the exhaust gas that acts as a reducing agent, it is also possible to forcibly introduce HC such as propane or propylene in an amount necessary and sufficient to reduce NOx.

[0024]

EXAMPLES The present invention will be explained in more detail with reference to the following specific examples. Example 1 Cordierite ceramic foam (apparent volume 2
The filter was coated with 10% (weight %, same hereinafter) of TiO2 by wash coating method. To the obtained filter, C
oCl2, La(NO3)3 and CsNO3
Using an aqueous solution of Co, La and Cs, respectively
It was impregnated with 2.5% O2 (metal base, same hereinafter), dried, and fired at 700°C. Next, the filter obtained above was immersed in a mixed aqueous solution of NH4 VO3 and oxalic acid to impregnate it with 2.5% V. After drying this filter, it was fired again at 700°C for 3 hours to obtain a purifying material.

[0025] Regarding this purifying material, the displacement is 510 cc.
Using a diesel engine, the temperature at which particulates ignite and burn and the filter is regenerated (temperature at which pressure loss decreases) and the NOx conversion rate at that time were determined. The regeneration temperature and NOx conversion rate are the same at the beginning of the use of the purifying material (when a decrease in pressure loss is observed for the first time) and at the time when the pressure drop decreases after 10 hours of using the purifying material. It was calculated at two points. The engine was operated at a rotational speed of 1500 rpm and a load of 90%. Under these operating conditions, the total amount of HC in the exhaust gas is 90pp of all hydrocarbons.
m, CO is 460 ppm, NOx is 480 ppm
, O2 10%, and SO2 200 ppm
Met. The results are shown in Table 1.

Examples 2 to 8 A foam filter was coated with 10% TiO2 in the same manner as in Example 1, and CoCl2, Ce(NO3)3
The TiO2 layer was impregnated with 2.5% each of Co, Ce and Cs using aqueous solutions of Co, Ce and CsNO3, and dried and fired in the same manner as in Example 1. Next, 2.5% of V was supported by the same method as in Example 1, and Co, Cs, Ce
A purifying material supporting a catalyst consisting of and V was obtained (Example 2
).

Similarly, CoCl2, La(NO3)
3, and each aqueous solution of KCl and NH4 VO3
Using an aqueous solution, Co, La, K and V were each added to TiO
A purifying material with a loading of 2.5% on 2 was obtained (Example 3).

Furthermore, CoCl2, Ce(NO3)
3. Using KCl and NH4VO3 aqueous solution,
A purifying material carrying 2.5% each of Co, Ce, K and V was obtained (Example 4).

Furthermore, in the same manner as in Example 1 or 2, using manganese acetate instead of CoCl2,
A purifying material having the following catalytically active species was prepared. The amount of each catalytically active species is 2.5% of each metal component.
%. Example 5: Cs/Mn/V/La Example 6: Cs/Mn/V/Ce Example 7: K/Mn/V/La Example 8: K/Mn/V/Ce

For each of the obtained purifying materials, the regeneration temperature and NOx purification rate were determined in the same manner as in Example 1. The results are shown in Table 1.

Comparative Examples 1 to 8 Using the same cordierite foam filter as in Example 1, 10% TiO2 was supported on the filter by the wash coating method. For the obtained filter,
A purification material supporting the following metal species was prepared using La(NO3)3, CoCl2, manganese acetate, CsNO3, or KCl aqueous solution. The method of supporting each metal was the same as in Example 1, and the amount of each metal supported was 2.5% (for metal components). Comparative example 1:
Cs/Co/La Comparative example 2: Cs/Co/Ce Comparative example 3: K/Co/La Comparative example 4: K/Co/Ce Comparative example 5: Cs/Mn/La Comparative example 6: Cs/Mn/Ce Comparative example 7: K/Mn/La Comparative example 8: K/Mn/Ce

[0032] Regarding the obtained purifying material, the regeneration temperature and NOx removal rate were determined in the same manner as in Example 1. The results are shown in Table 1.

[0033]
Table 1
Early (1)
After 10 hours (2) Example No. Regeneration temperature (3) NOx purification rate (4) Regeneration temperature (3) NOx purification rate (4) Example 1 3
90 15
390 18 Example 2
395 12
395 15 Example 3 410 10
410 14
Example 4 410 10
410 14
Example 5 410
14 410
18 Example 6 415
12 415
16 Example 7 42
0 10
420 14 Example 8
420 10
420 14 Comparative example 1 340 25
420 12
Comparative example 2 360 20
430 12
Comparative example 3 370
18 440
10 Comparative Example 4 370
18 450
10 Comparative Example 5 370
24 4
20 12 Comparative example 6
375 20
430 12 Comparative example 7 380 18
440 10
Comparative example 8 380 18
450 10
Table 1 Note (1): The point at which the first decrease in pressure loss was observed. (2): When the pressure loss decreases 10 hours after gas starts passing through the filter. (3) : Unit is °C (4) : NO in exhaust gas before and after passing through the filter
NOx purification rate (%) calculated from x amount

As is clear from Table 1, the exhaust gas purification material of this example has a higher NOx purification rate than that of the comparative example even after 10 hours from the start of use. It can also be seen that the filter regeneration temperature (the temperature at which particulates are ignited and burned) at that time was low, indicating that the exhaust gas was well purified.

[0035]

As explained above, by using the exhaust gas purifying material of the present invention, both particulates and nitrogen oxides in the exhaust gas can be effectively removed. Further, even relatively low-temperature exhaust gas from a diesel engine or the like can be efficiently purified.

In the purifying material of the present invention, a porous ceramic layer is first formed on a filter by a wash coat method or a sol-gel method, and a catalyst is supported on the ceramic layer. The catalyst is supported at a uniform concentration, resulting in good exhaust gas purification performance.

Furthermore, since the purifying material of the present invention contains Co and/or Mn and V as transition metals in the catalyst, it can be used for a long period of time without deteriorating its purifying properties even in exhaust gas with a high SO2 concentration. It is possible to perform good simultaneous removal of NOx and particulates over the entire range. The purifying material of the present invention is suitable for purifying exhaust gas from diesel engines and the like.

Claims (3)

[Claims] 1. An exhaust gas purifying material comprising a catalyst supported on a porous ceramic layer provided on a heat-resistant porous filter, wherein the catalyst comprises (a) an alkali metal element; and (b)
) Co element and/or Mn element; (c) V element;
(d) An exhaust gas purification material characterized by comprising a rare earth element. 2. The exhaust gas purifying material according to claim 1, wherein the ceramic layer is made of TiO2. 3. A method for purifying exhaust gas using the exhaust gas purifying material according to claim 1 or 2, wherein nitrogen oxides are reduced using particulates in the exhaust gas as a reducing agent by a catalyst supported on the filter. An exhaust gas purification method characterized by: