JPH0780301A - Exhaust gas purifying catalyst and nitrogen oxide purifying method - Google Patents

Abstract

(57) [Summary] [Structure] An exhaust gas-purifying catalyst characterized by comprising titania containing indium, and purifying nitrogen oxides from combustion exhaust gas containing oxygen by using hydrocarbons in the presence of such a catalyst. A method for purifying nitrogen oxides, which comprises: [Effect] Even if the oxygen concentration is at a practically high level, a sufficient nitrogen oxide purification rate is exhibited, and nitrogen oxides can be efficiently removed from the combustion exhaust gas containing oxygen. Again 100,000 h
^Even at a gas space velocity of ^-1 or higher, a high nitrogen oxide purification rate can be obtained. Further, a catalyst having excellent resistance to sulfur oxides can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst and a method for purifying nitrogen oxides, and more particularly to a method for efficiently purifying nitrogen oxides, which are air pollutants, from combustion exhaust gas containing oxygen. is there.

[0002]

2. Description of the Related Art From the viewpoint of environmental protection, purification of air pollutants is a major social issue. In particular, purification of combustion exhaust gas accompanying the expansion of industrial activities is an urgent issue at present.

Nitrogen oxides contained in combustion exhaust gas discharged from factories, which are fixed sources, and automobiles, which are mobile sources, are gases that are said to be the cause of photochemical smog and are harmful to the human body, especially nitric oxide ( NO) is difficult to purify, and has become the most important subject for consideration.

Some methods for purifying nitrogen oxides in combustion exhaust gas have been proposed so far. For example, a method called a catalytic reduction method is a method of purifying NO into N ₂ and H ₂ O on a catalyst by using a reducing agent such as ammonia or hydrogen. However, since this method uses a dangerous reducing agent, it is necessary to take measures for recovery and leakage of the reducing agent, which is effective for large-scale fixed sources, but is not suitable for mobile sources such as automobiles.

On the other hand, many catalysts have been developed and are generally used for purification of exhaust gas of a gasoline engine whose exhaust gas is a reducing gas. However, these catalysts cannot be used because they cannot purify nitrogen oxides in the presence of oxygen.

By the way, catalytic decomposition of NO, that is, NO
How directly decomposed into N ₂ and O ₂ is the corner with the exhaust gas only leading to the catalyst layer, very simple for use range is wide. Various catalysts have heretofore been found for this as well. Although Pt, Cu, and Co-based catalysts have an effect on NO decomposing activity, they all have a problem that they are poisoned by generated oxygen. Since exhaust gas from a diesel engine and exhaust gas from a lean-burn gasoline engine usually contain oxygen, conventional catalysts cannot handle it, and development of a new method is desired.

Several catalysts have been proposed for such problems. For example, (A) US Pat.
No. 28 and JP-A-63-283727 propose a method of purifying nitrogen oxides in a combustion gas containing oxygen in the presence of hydrocarbons by a zeolite catalyst containing copper, cobalt or the like. On the other hand, (B) JP-A-4-21
Japanese Patent Laid-Open No. 0241 and Japanese Patent Laid-Open No. 4-334526 propose a catalyst in which copper, cobalt, platinum or the like is supported on titania, which is cheaper than zeolite catalyst and is excellent in heat resistance and steam resistance.

[0008]

However, the known zeolite-based catalysts such as (A) above are said to cause structural destruction under combustion exhaust gas conditions, that is, at high temperature and in the presence of a large amount of steam. Have a problem with. Further, in the above (B), the purifying ability of nitrogen oxides is insufficient, and further improvement is required for practical use.

Since the hydrocarbon concentration in automobile exhaust gas does not exist sufficiently for nitrogen oxides, it is necessary to efficiently purify nitrogen oxides with a small amount of hydrocarbons, but these catalysts are hydrocarbons. There has been a problem that when the concentration decreases, the purifying ability of nitrogen oxides decreases. Therefore, in order to put it into practical use, a catalyst having a further improved nitrogen oxide purification capability is required. In addition, it is known that sulfur oxides are present in actual exhaust gas, which poisons the catalyst and greatly reduces the activity. Therefore, for practical use, it is also an important issue that the catalyst has sulfur oxide resistance.

An object of the present invention is to purify nitrogen oxides from combustion exhaust gas containing oxygen with hydrocarbons at a practical level.

[0011]

DISCLOSURE OF THE INVENTION The present invention is to solve the above-mentioned problems, and the present inventors have found that nitrogen oxides in combustion exhaust gas containing oxygen in the presence of a catalyst composed of titania containing indium. It was found that hydrocarbons can be efficiently purified. Further, according to the catalyst of the present invention, 50,000 h ⁻¹ or more, further 10
It was found that even at a high gas hourly space velocity (GHSV) of 10,000 h ^-1 or more, a sufficiently high nitrogen oxide purification capacity is exhibited. Furthermore, it was found that the catalyst of the present invention exhibits a high nitrogen oxide purification ability even in the presence of sulfur oxides.

That is, according to the present invention, an exhaust gas-purifying catalyst characterized by comprising titania containing indium and hydrocarbons in the presence of such a catalyst are used to purify nitrogen oxides from combustion exhaust gas containing oxygen. This is a method for purifying nitrogen oxides.

In the present invention, the method of supporting indium on titania is not particularly limited. As the supporting method, an impregnation method in which a carrier is immersed and supported in a solution containing a catalytically active component, a precipitating agent is added to a mixed solution of a carrier component and a catalytically active component, and at the same time a precipitate of both is formed and then calcined After immersing the carrier in the catalytically active component, and then adding a precipitant with stirring to deposit the precipitate of the active component on the carrier, after preliminarily forming the precipitate of the catalytic component, this and the carrier or carrier raw material are mixed. Examples thereof include a kneading method of kneading with a ball mill or a kneader.

The raw material compound of indium used in the present invention can be used in any form as long as it is a water-soluble salt. For example, sulfates, hydrochlorides, nitrates and the like can be mentioned. The indium content of titania used in the present invention is 0.05 to 20% by weight, preferably 0.2.
10 to 10% by weight. As the titania used in the present invention, anatase, brookite and rutile type are known, but they are not particularly limited, and a mixture thereof may be used.
Alternatively, it may be a compound having a composition corresponding to a hydrate of TiO ₂ .nH ₂ O, which is produced and separated in the process of synthesizing anatase and rutile titania. In any case, it is not particularly limited. Also, the titania used is A
l ₂ O ₃ is 5 wt% or less, SiO ₂ is 5 wt% or less, F
The e ₂ O ₃ content may be 2 wt% or less and the Na ₂ O content may be 2 wt% or less. As a method for preparing titania, generally used is a method in which a titanium salt solution is neutralized or thermally hydrolyzed to obtain a titanic acid precipitate, which is filtered, dried and calcined. Examples of the titanium salt include hydrochlorides and sulfates. Instead of the titanium salt solution, titanium ore dissolved in sulfuric acid may be used. Alternatively, there is also a method of hydrolyzing a titanate ester such as titanium alkoxide and dehydrating by heating. In any case, it is not particularly limited. Titania is generally commercially available and they may be used. In addition, in order to increase the acidity of titania, titania is generally treated with sulfuric acid, but in the present invention, titania treated with sulfuric acid may be used.

The catalyst used in the method of the present invention is obtained by introducing indium into titania, and then using, for example, an inorganic oxide such as silica or alumina or clay as a binder to form spherical, columnar,
It may be molded into an appropriate shape such as a honeycomb shape, or may be coated into a molded body such as a honeycomb made of alumina, cordierite or the like. Alternatively, a binder may be added to the titania before the indium is introduced into the titania for molding, and then the indium may be introduced. In any case, it is not particularly limited.

In the present invention, hydrocarbons are used to purify nitrogen oxides from combustion exhaust gas containing oxygen in the presence of an exhaust gas purification catalyst composed of titania containing indium.

The hydrocarbon used in the present invention is a compound composed of carbon and hydrogen, usually so-called olefins, paraffins, cyclic compounds or hydrocarbons containing these compounds. It is preferably volatile and gaseous at the processing temperature of the present invention. More preferably, it is at least one hydrocarbon selected from olefins having 1 to 6 carbon atoms, paraffins, naphthenes and cyclic unsaturated hydrocarbons. Specific examples of preferable hydrocarbons include ethylene, propylene, butylene, pentene, hexene, methane, ethane, propane, butane, pentane, hexane, cyclopropane, cyclobutane, cyclopentane, cyclohexane and cyclohexene. It goes without saying that unburned hydrocarbons contained in the combustion exhaust gas are also preferably used. The hydrocarbons present on the catalyst are in a ratio of 0.2 to 5 in terms of methane with respect to nitrogen oxides contained in the combustion exhaust gas, more preferably 0.4.
It is preferable to exist in a molar ratio of from 4 to 4. If it is less than 0.2 mol ratio, the purification rate of nitrogen oxides will be low, while if it is more than 5 mol ratio, excess hydrocarbon will be present and a new hydrocarbon purification device will be required, which is not preferable.

The combustion exhaust gas in the present invention contains oxygen, and preferably contains 0.1% by volume or more of oxygen. This combustion exhaust gas is emitted from a normal internal combustion engine or boiler. The present invention is particularly effective for exhaust gas containing a large amount of oxygen, such as combustion exhaust gas from a diesel engine or a lean-burn gasoline engine. The oxygen concentration in the combustion exhaust gas of the diesel engine varies depending on the operating conditions, but is typically 5 to 16%, and is 3 to 8% in the lean-burn gasoline engine.

The nitrogen oxide purification temperature according to the present invention is a catalyst layer temperature of preferably 200 to 800 ° C, more preferably 250 to 600 ° C. If this purification temperature is low, the purification of nitrogen oxides will be insufficient, and if the purification temperature is too high, the coexisting hydrocarbons will burn and the purification of nitrogen oxides will decrease, which is not preferable.

In the conventional method, when the combustion exhaust gas treatment rate per catalyst volume, that is, the gas space velocity is increased, for example, 50,000 h ⁻¹ or more, and further 100,000 h ⁻¹ or more, the nitrogen oxide purifying ability is lowered. However, some mobile sources such as automobiles were far from the practical level. However, according to the method of the present invention, sufficient nitrogen oxide purifying ability is exhibited even at a high gas space velocity such as automobile exhaust gas and in the presence of sulfur oxides.

It is needless to say that the hydrocarbon contained in the combustion exhaust gas can be used to carry out the removal method of the present invention, but in order to increase the hydrocarbon concentration in the combustion exhaust gas, the existing fuel as hydrocarbon is used. It is possible to use a part of fuel oil such as light oil or gasoline in a tank and directly add it to the catalyst layer provided on the exhaust gas outlet side through a bypass line without sending it to the engine. A reforming unit may be provided, and a part of fuel oil such as light oil or gasoline may be subjected to a reforming treatment or the like and then added to the catalyst layer.

Further, in the case of a diesel engine, it is possible to change the explosive combustion conditions and increase the hydrocarbon concentration in the exhaust gas by delaying the fuel injection timing in the engine. Further, in order to maintain the temperature of the catalyst layer in an appropriate range, combustion exhaust gas that has been brought to a predetermined temperature by a cooler or the like may be introduced, and if the combustion exhaust gas does not reach the predetermined temperature, the catalyst layer may be heated. Good.

[0023]

EXAMPLES The present invention will be described below with reference to examples.

Example 1 (Catalyst preparation) 60 g of titanium tetraisopropoxide was diluted with 75 ml of isopropanol and then gradually poured into 1 liter of distilled water with sufficient stirring. The mixture was stirred as it was at room temperature for 1 hour and then filtered. Then, distilled water 5
Wash 3 times with 00 ml, then dry at 110 ° C overnight, 500
It was calcined at 4 ° C. for 4 hours to obtain anatase type titania. A 5.0 ml aqueous solution containing 0.92 g of indium sulfate nonahydrate was added dropwise to 10.0 g of this anatase-type titania through a buret. It was dried at 110 ° C. overnight. The amount of indium supported was 3.0% by weight as metal.

Comparative Example 1 The same titania as in Example 1 (10.0 g) was added to cobalt nitrate.6.
A 5.0 ml aqueous solution containing 1.48 g of the hydrate was added dropwise from a buret. It was dried at 110 ° C. overnight. The supported amount of cobalt was 3.0% by weight as metal.

Comparative Example 2 A 5.0 ml aqueous solution containing 0.09 g of tetraammine platinum chloride monohydrate was added dropwise to 10.0 g of the same titania as in Example 1 from a buret. It was dried at 110 ° C. overnight. The amount of platinum supported was 0.5% by weight as metal.

Example 2 (Catalyst evaluation) Using the catalyst obtained in Example 1, the removal performance of nitric oxide was examined under the reaction conditions shown in Table 1. The conversion rate of NO was calculated from the conversion rate of NO to N ₂ . The results are shown in Table 2.

Example 3 The results of examining the influence of the reaction temperature in the same manner as in Example 2 are shown in Table 3.

Comparative Examples 3 and 4 Using the catalysts obtained in Comparative Examples 1 and 2, the removal performance of nitric oxide was examined under the reaction conditions shown in Table 1. The conversion rate of NO was calculated from the conversion rate of NO to N ₂ . The results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

As is clear from the results shown in Table 2, when a catalyst characterized by comprising titania containing indium is used, a trace amount of hydrocarbons are used to efficiently generate nitrogen oxides from combustion exhaust gas containing oxygen. I found that I can purify. It was also found that the sulfur poison, which is a catalyst poisoning component, works effectively even if it is contained in the reaction gas.

[0034]

EFFECTS OF THE INVENTION According to the present invention, a sufficient nitrogen oxide purification rate is exhibited even when the oxygen concentration is at a high level of practical use, and nitrogen oxides can be efficiently removed from combustion exhaust gas containing oxygen. Also, 100,000 gas space velocity in h ^-1 or more (G
Even with HSV), a high nitrogen oxide purification rate can be obtained. Further, a catalyst having excellent resistance to sulfur oxides can be provided.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01J 27/053 ZAB A 9342-4G 27/13 ZAB A 9342-4G

Claims (2)

[Claims] 1. An exhaust gas purifying catalyst comprising titania containing indium. 2. A method for purifying nitrogen oxides, which comprises purifying nitrogen oxides from a combustion exhaust gas containing oxygen using hydrocarbons in the presence of the catalyst according to claim 1.