JPH0857314A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE: To prevent the poisoning of an NOx absorbent by sulfur while ensuring the initial rate of removal of NOx and prevent the deterioration of NOx removing performance after repeated use CONSTITUTION: An oxide selected from silica, titania and zirconia is carried on an alumina carrier by impregnating and firing alkoxide of Si, Ti or Zr and an NOx absorbent selected from alkali metals, alkaline earth metals and rare earth elements and a catalytic noble metal are further carried on the carrier to obtain the objective catalyst. The characteristics of the selected oxide and alumina are exhibited and the poisoning of the NOx absorbent by sulfur is prevented.

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 more particularly to a catalyst capable of efficiently purifying nitrogen oxides (NOx) even in lean exhaust gas.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as a catalyst for purifying exhaust gas of an automobile.
As such a catalyst, for example, a catalyst in which a supporting layer made of γ-alumina is formed on a heat resistant carrier such as cordierite and a catalytic precious metal such as Pt, Pd, Rh is supported on the supporting layer is widely known. There is.

By the way, the purification performance of such an exhaust gas purification catalyst greatly differs depending on the air-fuel ratio (A / F) of the engine. That is, on the lean side where the air-fuel ratio is large, that is, where the fuel concentration is lean, the amount of oxygen in the exhaust gas is large, and the oxidation reaction for purifying CO and HC is active, but N
The reduction reaction for purifying Ox becomes inactive. On the contrary, on the rich side where the air-fuel ratio is small, that is, where the fuel concentration is high, the amount of oxygen in the exhaust gas is small, and the oxidation reaction becomes inactive but the reduction reaction becomes active.

On the other hand, when driving an automobile, acceleration and deceleration are frequently performed in urban areas, and the air-fuel ratio frequently changes within the range from near stoichiometric (theoretical air-fuel ratio) to the rich state. In order to meet the demand for low fuel consumption in such traveling, it is necessary to operate on the lean side to supply an air-fuel mixture with excess oxygen as much as possible. Therefore, it is desired to develop a catalyst that can sufficiently purify NOx even on the lean side.

Therefore, the applicant of the present application has previously proposed an exhaust gas purifying catalyst in which an alkaline earth metal and Pt are carried on a porous carrier such as alumina (Japanese Patent Laid-Open No. 5-317652).
issue). According to this catalyst, NOx is adsorbed on the alkaline earth metal and is reacted with a reducing gas such as HC to be purified, so that the lean side is also excellent in NOx purification performance.

In the catalyst disclosed in Japanese Unexamined Patent Publication No. 5-317652, for example, barium is supported on a carrier as a single oxide, which reacts with NOx to produce barium nitrate (Ba (N
It is considered that NOx is adsorbed by producing O ₃ ) ₂ ).

[0007]

However, in the exhaust gas, sulfur (S) contained in the fuel is burned to generate S.
Ox is contained, and it is oxidized to SO ₃ by the catalytic metal in the oxygen excess atmosphere. It was also clarified that sulfuric acid is easily converted to sulfuric acid by the water vapor contained in the exhaust gas, which reacts with barium to generate barium sulfite or barium sulfate, which causes the barium to be poisoned and deteriorated. In addition, the porous carrier such as alumina is SOx.
Since it has a property of easily adsorbing sulfur, there is a problem that the sulfur poisoning is promoted.

When barium becomes a sulfite or a sulfate as described above, NOx can no longer be adsorbed, and as a result, the catalyst has a problem that the NOx purification performance after durability is deteriorated. Further, since titania hardly absorbs SOx, it was remembered to use a titania carrier instead of an alumina carrier, and an experiment was conducted. As a result, it became clear that SOx was not easily adsorbed by titania and flowed as it was, and only SOx that was in direct contact with the catalytic noble metal was oxidized, so that the degree of poisoning was small. However, it became clear that the titania carrier has a fatal defect that the initial activity is low and the NOx purification performance after the endurance remains low.

The present invention has been made in view of such circumstances, and it is possible to secure the initial NOx purification rate while maintaining NOx.
The purpose of the present invention is to prevent sulfur poisoning of the absorbent material and prevent deterioration of NOx purification performance after endurance.

[0010]

The exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems is an oxidation selected from silica, titania and zirconia supported on an alumina carrier by impregnating a metal alkoxide and then calcining. And an NOx absorbent selected from an alkali metal, an alkaline earth metal and a rare earth element, and a catalytic noble metal.

[0011]

In the exhaust gas purifying catalyst of the present invention, an oxide selected from silica, titania and zirconia, which is supported by using a metal alkoxide as a raw material, is supported on an alumina carrier. This oxide has low initial activity when used as a carrier, but in the present invention, since the main component of the carrier is alumina, a sufficiently high NOx purification performance is secured in the initial stage.

Further, these oxides have a property that SOx is hard to be adsorbed, and although the reason is not clear, the alumina carrier itself is hard to adsorb SOx, and sulfur poisoning of the NOx absorbent is prevented. The action of the oxide appears when the oxide is supported by impregnating the alumina carrier with the metal alkoxide, and such a action does not appear only by physically mixing the alumina and the oxide powder.

[0013]

【Example】

Specific Example of the Invention The amount of the oxide selected from silica, titania and zirconia is preferably in the range of 0.01 to 1 mol with respect to 100 g of the alumina carrier. When the amount of the oxide is less than 0.01 mol, sulfur poisoning occurs and the NOx purification performance after endurance deteriorates, and when it exceeds 1 mol, the initial NOx purification performance decreases and the oxidation activity also decreases. As the kind of oxide, one kind of silica, titania and zirconia may be used, or a plurality of kinds may be mixed.

Examples of alkali metals include lithium, sodium, potassium, rubidium, cesium and francium. The alkaline earth metal is an element of Group 2A of the periodic table, and examples thereof include barium, beryllium, magnesium, calcium, and strontium. Further, as rare earth elements, scandium, yttrium, lanthanum,
Examples include cerium, praseodymium, and neodymium.

The amount of the NOx absorbent supported is preferably in the range of 0.05 to 0.5 mol with respect to 100 g of the alumina carrier. N is less than 0.05 mol of NOx absorbent
The Ox purification performance deteriorates, and if it exceeds 0.5 mol, the oxidation activity will decrease. At least one of platinum (Pt), palladium (Pd), and rhodium (Rh) is used as the catalytic noble metal. The amount of platinum or palladium supported is 0.1 to 20.
The range of 0 g is desirable, and the range of 0.3 to 10.0 g is particularly preferable. If the supported amount is less than 0.1 g, the NOx purification performance at the initial stage and after endurance will decrease, and if the supported amount exceeds 20.0 g, the effect will be saturated, and the excessively supported catalytic noble metal cannot be effectively utilized.

The amount of rhodium supported is 100% on the alumina carrier.
The range of 0.001 to 1.0 g is desirable with respect to g,
The range of 0.05 to 0.5 g is particularly preferable. If the supported amount is less than 0.001 g, the NOx purification performance at the initial stage and after the durability deteriorates, and if it exceeds 1.0 g, the effect of platinum or palladium decreases conversely. It is desirable that the amount of rhodium supported be determined relative to the amount of platinum or palladium supported, which is 1 of the total amount of platinum or palladium supported.
It is preferably / 3 or less, and more preferably 1/5 or less.

In the case of producing the exhaust gas purifying catalyst of the present invention, the order of loading the oxide selected from silica, titania and zirconia, the NOx absorbent, and the catalyst noble metal is not particularly limited, but the noble metal is more highly supported on the alumina carrier. For reasons of dispersion, it is desirable to support the catalytic noble metal before the oxide. (Example 1) 600 g of activated alumina powder was immersed in 1 L of an aqueous solution of dinitrodiammine platinum having a predetermined concentration and stirred,
After evaporating to dryness, baking at 250 ° C for 1 hour gives alumina powder 1
A Pt-supported powder in which 2 g of Pt was supported with respect to 00 g was prepared.

610 g of this Pt-supported powder was mixed in 6 L of 2-propanol and stirred at 80 ° C. for 1 hour. While maintaining it at 80 ° C. and stirring, tetraethyl orthosilicate (1.5 mol, 312.5 g) was added dropwise.
Stirring was continued for 2 hours at 0 ° C. Then, the mixture was cooled to room temperature and filtered, and the obtained powder was dried and calcined at 500 ° C. for 1 hour.
As a result, silica was added to 100 g of the alumina powder in an amount of 0.
3 mol of Pt-Si supported powder was obtained.

Next, 700 g of this Pt-Si supported powder was immersed in 1 L of an aqueous barium acetate solution having a predetermined concentration and stirred,
After evaporating to dryness, it was baked at 500 ° C. for 1 hour. As a result, 0.3 mol of barium oxide was added to 100 g of alumina powder.
A Pt-Si-Ba-supported powder on which 1 was supported was prepared. 970 g of this Pt-Si-Ba supported powder, 680 g of alumina sol (alumina content 10% by weight), and 290 g of water
Was mixed to prepare a slurry, and a honeycomb carrier base material made of cordierite was dipped in the slurry, pulled up to blow off excess slurry, dried and baked at 500 ° C. for 1 hour to form a coat layer. The coating layer was formed in an amount of 120 g per 1 L of the carrier substrate, and as shown in Table 2, Pt was 2 g, Si (metal conversion) was 0.3 mol, and B was relative to 1 L of the carrier substrate.
A catalyst supporting 0.3 mol of a (metal conversion) was obtained. (Example 2) Instead of tetraethyl orthosilicate, tetraisopropyl titanate 1.5 mol (426.345)
A catalyst of Example 2 was prepared in the same manner as in Example 1 except that g) was added. Titanium is supported as titania, and the supported amount is 0.3 mol of Ti (metal conversion) with respect to 1 L of the carrier substrate. (Example 3) Zirconium tetra n-butoxide 1.5 mol (575.5) instead of tetraethyl orthosilicate
A catalyst of Example 3 was prepared in the same manner as in Example 1 except that 2 g) was added. Zirconium is supported as zirconia, and the supported amount is Zr based on 1 L of the carrier substrate.
(Metal conversion) is 0.3 mol. Comparative Example 1 A catalyst of Comparative Example 1 supporting only Pt and Ba was prepared in the same manner as in Example 1 except that tetraethyl orthosilicate was not added. (Comparative Example 2) A Pt-Ba-supporting powder supporting only Pt and Ba was prepared in the same manner as in Example 1 except that tetraethyl orthosilicate was not added.

Silica powder (SiO ₂ ) 1.5 mol (90.13 g) per 610 g of this Pt-Ba supported powder.
Was physically mixed, and the powder was slurried and coated in the same manner as in Example 1 to prepare a catalyst of Comparative Example 2. Silica is 0. 1 as Si (metal conversion) with respect to 1 L of the carrier substrate.
3 mol is contained. (Comparative Example 3) Instead of silica powder, titania (Ti
O ₂₎ powder except that the mixed 1.5mol (119.85g) and in the same manner as in Comparative Example 2, Ti (in terms of metal relative to the support substrate 1L) of Comparative Example 3 contained 0.3mol catalyst Was prepared. (Comparative Example 4) Instead of silica powder, zirconia (ZrO
₂ ) In the same manner as in Comparative Example 2 except that 1.5 mol (184.83 g) of powder was mixed, the catalyst of Comparative Example 4 containing 0.3 mol of Zr (metal conversion) was added to 1 L of the carrier substrate. Prepared. (Evaluation of Purification Performance) The purification performance of the above catalyst was evaluated under the following conditions.

Emission evaluation The above catalyst is installed in the waste passage of a vehicle equipped with a lean burn engine (1.6 liters), and the vehicle is run in the suburban running mode.
The NOx purification rate was measured, and the results are shown in Table 1. The endurance-treated catalyst is the lean-burn engine (1.6 liters) with the catalyst installed in the exhaust passage, and the engine has an A / F = 18,
It was operated for 50 hours at an inlet gas temperature of 650 degrees.

[0022]

[Table 1]

From Table 1, it can be seen that in the catalysts using the carrier obtained by physically mixing the oxide powder of Comparative Examples 2 to 4 with the alumina powder,
Almost no difference is found in the NOx purification rate with respect to the catalyst containing no oxide of Comparative Example 1. However, it is clear that the catalyst of the example having the oxide impregnated and supported as the metal alkoxide is superior to the comparative example in the initial NOx purification performance and the deterioration of the purification performance after the durability is small.

[0024]

[Effects of the Invention] That is, according to the exhaust gas purifying catalyst of the present invention, the initial NOx purifying performance is maintained at a level equal to or higher than that of the conventional one, and the sulfur poisoning of the NOx absorbent is prevented so that the NOx purifying rate is high even after the endurance. Can be maintained, and NOx in the exhaust gas on the lean side can be efficiently purified.

Claims (1)

[Claims] 1. An oxide selected from silica, titania, and zirconia, which is supported by impregnating a metal alkoxide on an alumina carrier and then calcining the alkali alkoxide,
An exhaust gas purifying catalyst comprising a NOx absorbent selected from alkaline earth metals and rare earth elements and a catalytic precious metal.