JPH04354519A - 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 engines, etc., and simultaneously of effectively removing NOx. CONSTITUTION:A catalyst layer consisting of a uniform mixture 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 and a ceramic powder carrier is installed on a heat resistant porous filter to form an exhaust gas purifying material. The catalyst is 1 to 20 pts.wt. per 100 pts.wt. of the filter and the ceramic powder carrier is 1 to 10 pts.wt.

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 devised a method for dissolving an alkali metal element, a specific transition metal element, and a rare earth metal element on a heat-resistant porous filter. If you use a purifying material that has a catalyst layer formed by uniformly mixing the catalyst and the ceramic powder carrier,
We have completed the present invention by discovering that particulates and NOx can be efficiently removed at the same time even in exhaust gas where the oxygen concentration varies greatly, and that the removal ability lasts for a long time.

That is, the exhaust gas purifying material of the present invention has (a) an alkali metal element and (
b) A catalyst layer made of a homogeneous mixture of a catalyst made of Co element and/or Mn element, (c) V element, and (d) rare earth element, and a ceramic powder carrier,
1 to 20 parts of the catalyst per 100 parts by weight of the filter
The amount of the ceramic powder carrier is 1 to 10 parts by weight.

[0010] Furthermore, the exhaust gas purification method of the present invention uses the exhaust gas purification material to oxidize particulates in the exhaust gas with a catalyst supported on the filter, and at the same time reduce nitrogen oxides using the particulates as a reducing agent. It is characterized by

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 cesium is used, many unburned HCs, including saturated hydrocarbons such as propane, also react 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
) It is preferable to use cerium, lanthanum, neodymium, etc. as the rare earth metal, 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] The above-mentioned catalyst is uniformly mixed with ceramic powder serving as a carrier, and then supported on a thermoporous filter. That is, a catalyst layer made of a uniform mixture of the above catalyst and ceramic powder is formed on the heat-resistant porous filter. This catalyst layer can be formed, for example, by a wash coating method, a sol-gel method, etc., which will be described later. According to such a method, the catalyst layer can be made into a porous layer, and the contact area with exhaust gas becomes large.

As the ceramic powder serving as the carrier, porous ceramics with a large surface area such as titania, alumina, zirconia, silica, titania-alumina, alumina-zirconia, alumina-silica, titania-silica, and titania-zirconia may be used. Can be done. Preferably titania is used.

The amount of catalyst supported is 1 to 20 parts by weight per 100 parts by weight of the filter. If the amount of catalyst is less than 1 part by weight, it becomes difficult to simultaneously remove particulates and NOx. On the other hand, even if more than 20 parts by weight of catalyst is supported, the exhaust gas purification ability is not significantly improved, so the upper limit is set to 20 parts by weight. The preferred amount of catalyst is 5 to 15 parts by weight.

The amount of ceramic powder serving as a catalyst carrier is 1 to 10 parts by weight per 100 parts by weight of the filter. If the amount of ceramic powder is less than the above-mentioned lower limit, it is not preferable because a sufficient amount of catalyst cannot be supported. On the other hand, if the amount of ceramic powder exceeds the above upper limit, the pressure loss in the purifying material will become too large. The preferred amount of ceramic powder is 2 to 6 parts by weight.

[0021] In forming the catalyst carrier layer (porous ceramic layer) by the wash coating method, the catalyst is impregnated in the ceramic powder such as titania mentioned above in advance, and then the catalyst is impregnated in the slurry of the ceramic powder containing the catalytically active species. This can be done by soaking the filter in water.

[0022] Furthermore, the formation of the carrier layer by the sol-gel method is carried out as follows. Organic salts of metals that form the ceramic layer that serves as the catalyst carrier layer (for example, the ceramic layer is made of TiO2
In the case of Ti alkoxide (specifically, Ti (
A coating solution is prepared by adding an acid such as CH3 COOH, HNO3, HCl, etc., and an aqueous solution of a salt of a catalytically active metal species to an alcoholic solution of O-isoC3 H7 )4 etc.). Next, after immersing the filter in this coating liquid, a film of colloidal particles is formed on the filter by contact with water vapor or the like, and then dried and fired to form a porous ceramic layer containing a catalyst on 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.

[0023] Even when the catalyst carrier layer is formed of ceramics other than titania, it 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.

As salts of the catalytically active metal species, as long as they are soluble in water, carbonates, nitrates,
Any type of acetate, hydroxide, etc. can be used. For supporting V, a solution of NH4 VO3 and oxalic acid can be used. Furthermore, an alkali metal and V can be simultaneously supported using an alkali vanadate (for example, potassium vanadate, sodium vanadate, cesium vanadate, etc.).

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.

As mentioned above, the exhaust gas purifying material of the present invention has a catalyst layer on the surface made of a uniform mixture of catalyst and porous ceramic powder, so even if the amount of ceramics serving as a catalyst carrier is small, it can be compared to A large amount of catalyst can be supported, thereby reducing the pressure loss of the filter. That is, a large amount of catalyst can be supported on the filter even with a small amount of ceramics that does not cause a large pressure loss.

[0027]

EXAMPLES The present invention will be explained in more detail with reference to the following specific examples. Examples 1 to 4 A predetermined amount of Ti alkoxide (Ti(O-isoC3
H7) To the alcohol solution of 4 and acetic acid added,
Predetermined concentrations of CoCl2 (Example 1) or manganese acetate (Example 2), La(NO3)3, CsNO3
and a mixed aqueous solution of NH4 VO3 and oxalic acid to prepare a coating liquid.

[0028] Cordierite ceramic foam (apparent volume 0.25 liters, density 0.65 g/ml)
was used and immersed in the above coating solution. This foam was taken out from the coating solution, and water vapor was applied to it to turn the coating solution into a sol and further into a gel. After drying, it was fired at 600° C. for 3 hours to obtain a foam-type filter on which a ceramic layer containing a catalyst was formed. The amount of TiO2 supported on the filter was 3% (weight %, same hereinafter) based on the filter. Also, Co(
The amount of La, Cs, and V (or Mn) was 1.5% (metal base, hereinafter the same) with respect to the filter. (T
iO2: Cs/Co/V/La: Example 1) (TiO2: Cs/Mn/V/La: Example 2)

0
[029] In the same manner as in Example 1, La(NO 3 )
Using Ce(NO3)3 instead of 3, a filter having the following catalytically active metal species was created. (TiO
2: Cs/Co/V/Ce: Example 3) (TiO2: Cs/Mn/V/Ce: Example 4) The amount of TiO2 supported on the filter was 4% with respect to the filter, and Cs , Co (or Mn), V and Ce
The amount supported on the filter was 1.5%.

[0030] Four filters each of Examples 1 to 4 were stacked one on top of the other to obtain a purifying material having an apparent volume of 1 liter.

[0031] Regarding this purifying material, each displacement amount is 50
Using a 0 cc diesel engine, the pressure loss caused by capturing particulates, the temperature when the particulates ignite and burn and the filter is regenerated (the temperature at which the pressure drop decreases), and the conversion of NOx at that time. The rate was calculated. 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 is operated at a rotation speed of 2500 rpm and a load of 80%.
And so. Under these operating conditions, the total amount of HC in the exhaust gas is 90 ppm of all hydrocarbons, 460 ppm of CO, and 460 ppm of NOx.
was 480 ppm, O2 was 10%, and SO2 was 200 ppm. The results are shown in Table 1.

Examples 5 to 8 Potassium titanate, La(NO 3 ) 3 and C
o(NO3) 2 (or manganese acetate) and NH4
A slurry consisting of a mixed aqueous solution of VO3 and oxalic acid was prepared. A filter similar to that in Example 1 was immersed in this slurry, taken out, dried, and then fired at 700° C. for 3 hours to obtain a foam filter on which a ceramic layer containing a catalyst was formed. (TiO2: K/Co/V/La
: Example 5) (TiO2 : K/Mn/V/La : Example 6) The amount of TiO2 for this filter is 3%, K is 3% for the filter, Co (or Mn), and La are 2% for each filter. Further, V was 2%.

In the same manner as in Example 5 or 6, La(N
A filter having the following catalytically active metal species was created using Ce(NO3)3 instead of O3)3. (TiO2: K/Co/V/Ce: Example 7) (TiO2: K/Mn/V/Ce: Example 8) In this filter, the amount of TiO2 supported was 3% of the filter, and K is 3% for the filter and C
o (or Mn), La are each 2% for the filter
Met. Further, V was 2%.

[0034] Four of the filters obtained above were stacked each as a purifying material in the same manner as in Example 1, and the pressure loss, temperature at that time, and NOx were measured in the same manner as in Example 1.
The conversion rate was measured. Both results are shown in Table 1. Comparative Examples 1 to 8 For comparison, a TiO2 layer containing the catalytically active metal described below was formed on the same cordierite foam filter as in Example 1 in the same manner as in each of the above-described Examples. The amount of TiO2 layer was 10% for both filters and the amount of each metal was 2.5% each. Comparative example 1: Cs/Co/La Comparative example 2: Cs/Mn/La Comparative example 3: Cs/Co/Ce Comparative example 4: Cs/Mn/Ce Comparative example 5: K/Co/La Comparative example 6: K /Mn/La Comparative Example 7: K/Co/Ce Comparative Example 8: K/Mn/Ce As in Example 1, the pressure loss, regeneration temperature, and NOx removal rate were determined for the obtained purifying material. The results are shown in Table 1.

[0035]
Table 1
Early (1)
10 hours later (2)
Pressure loss Regeneration NOx
Regeneration NOx
Example No. (mmHg)
Temperature (3) Purification rate (4) Temperature (
3) Purification rate (4) Example 1 5
0 390 18
390 18 Example 2
52 400 17
390 17 Example 3
70 400 13
400 13 Example 4
70 405 14
405 14 Example 5 68 405 12
405 12 Example 6 67 405 1
2 405 12
Example 7 70 410
12 410 12
Example 8 71 410
10 410 1
0 Comparative example 1 95 390
18 430
15 Comparative example 2 96 39
5 16 430
12 Comparative example 3 100 4
10 15 435
12 Comparative Example 4 102
412 14 435
10 Comparative example 5 100
415 13 440
10 Comparative Example 6 98
415 12 4
45 10 Comparative example 7 9
5 415 13
445 10 Comparative example 8
96 415 13
445 10 Table 1 Note (1)
: The point at which the first decrease in pressure drop 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 purifying material of this example provides a small pressure loss. Furthermore, even after 10 hours have passed since the start of use, the NOx purification rate is higher than that of the comparative example. Furthermore, the regeneration temperature of the filter at that time (
The temperature at which particulates are ignited and burned was also low, indicating that the exhaust gas was effectively purified.

[0037]

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.

The purifying material of the present invention supports a relatively large amount of catalyst on the filter even if the amount of ceramic used as a carrier is small, so the pressure loss is small and particulates and NOx can be effectively removed simultaneously. can be done.

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] [Claim 1] 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, on a heat-resistant porous filter; A catalyst layer made of a homogeneous mixture with a ceramic powder carrier is provided, and the catalyst is 1 to 20 parts by weight and the ceramic powder carrier is 1 to 10 parts by weight per 100 parts by weight of the filter. Exhaust gas purification material. 2. The exhaust gas purifying material according to claim 1, wherein the ceramic powder carrier is TiO2. 3. A method for purifying exhaust gas using the exhaust gas purifying material according to claim 1 or 2, wherein particulates in the exhaust gas are oxidized by the catalyst supported on the filter and at the same time, the particulates are oxidized. An exhaust gas purification method characterized by reducing nitrogen oxides as a reducing agent.