JPS5820307B2 - Catalyst for vehicle exhaust gas purification - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to reduce nitrogen oxides (NOx) and hydrocarbons (HC) in exhaust gas emitted from vehicle engines, etc., especially automobile engines.
) and carbon monoxide (CO) efficiently.

In recent years, NO emitted from internal combustion engines such as automobiles
BACKGROUND ART Harmful gases such as x, HC, and CO pollute the atmosphere and cause photochemical smog, etc., and have a large impact on the human body, so reducing harmful exhaust gases is an urgent issue.

Various countermeasures have been proposed for this purpose, such as improving the engine of automobiles to purify the exhaust gas;
There is a method that attempts to purify exhaust gas using a catalyst.

Exhaust gas from automobiles is generated by combustion of a mixture of gasoline and air in the combustion chamber of an engine, and the relationship between air firing ratio and exhaust characteristics at this time is as shown in the attached drawings.

As is clear from the drawing, the generation of HC and CO and NOx
There is a unique relationship between the production of HC and CO.
When combustion is performed in such a way that the concentration of NOx decreases, the concentration of NOx increases, which is a contradictory phenomenon, and it is extremely difficult to simultaneously reduce the three harmful components.

The current flow of catalyst development under these conditions is commonly referred to as a dual-bed catalyst system.

The main method is a two-stage treatment method that combines a reduction catalyst and an oxidation catalyst.

According to this method, the exhaust gas is first reduced to NOx due to oxygen deficiency in the reduction catalyst, then air is supplied, and then HC and CO are oxidized in the oxidation catalyst bed.

In particular, reduction catalysts have extremely high operating temperatures of 600 to 900°C, which poses durability problems, and in some cases, a large amount of ammonia (NH3) is produced, and after this NH3 passes through the oxidation catalyst, it is reoxidized to NOx. This includes the problem of being

On the other hand, until now, when there is a lack of oxygen in the exhaust gas, N
Catalysts have been developed that are highly efficient in purifying Ox and are effective in removing HC and CO when there is sufficient oxygen in the exhaust gas.

However, conventional catalysts have the disadvantage that they are inefficient outside of the above conditions; to remove NOx, the amount of oxygen in the exhaust gas is relatively large; to remove HC and co, the amount of oxygen in the exhaust gas is relatively high. It is not possible to remove exhaust gas with a single catalyst without a catalyst 6j that can efficiently remove NOx, HC, and CO in a state where the ratio of oxidizing substances and reducing substances in the exhaust gas is almost equal. Since this was not the case, a dual-bed catalyst system was used as described above.

In view of the current situation, the present inventors conducted various studies to detoxify exhaust gas by simultaneously performing oxidation and reduction using a single catalyst bed. The present inventors have discovered that NOx, HC and CO can be effectively rendered harmless at the same time by using a catalyst consisting of the following, and have achieved the present invention.

The catalyst of the present invention has platinum, rhodium, and cerium supported on a ceramic refractory carrier that is stable in oxidizing, neutral, and reducing atmospheres at high temperatures as described above. As for the quality, γ-alumina is suitable, and its shape includes pellets, honeycomb, etc.

There are various methods for supporting platinum, rhodium, and cerium on pellets or honeycomb-shaped carriers, and one example will be described below.

As a method for producing a pellet catalyst, first, a predetermined amount of a catalyst reagent is dissolved in water in an amount twice the amount of pellets.

The pellet carrier is immersed in this solution for about 1 hour at room temperature or heated to a temperature of about 50°C.

After removing the support from the solution and drying it, it was heated for 5 minutes in a hydrogen atmosphere.
Bake at 50°C for 3 hours.

For the honeycomb catalyst, first, an alumina coating composition consisting of 3 parts by weight of alumina sol containing 30% alumina, 1 part by weight of γ-alumina, and a small amount of other silicate is placed in a ball mill, and the solids in the composition are heated at room temperature for 2 hours. crush things.

Next, the coating liquid obtained by the above operation is poured onto a honeycomb carrier made of alumina, cochillerite, etc., dried at room temperature for about one day, and then fired at 650° C. for 3 hours.

Such a honeycomb carrier is impregnated with a solution in which a predetermined amount of catalytic metal is dissolved, and if necessary, exposed to a hydrogen sulfide gas stream and then dried at room temperature for 24 hours.

After this treatment, the carrier to which the catalytic metal is attached is calcined in hydrogen at a temperature of 550° C. for 3 hours to obtain a finished catalyst.

Next, the present invention will be explained with reference to Examples and Test Examples.

Example 1 0.9 g each of platinum and rhodium as metals in 21 water
Each chlorine compound (namely, chloroplatinic acid, rhodium chloride, and cerium chloride) was dissolved so that 0.2 g of cerium was present as a metal, and the pellets 11 made of γ-alumina were further immersed at room temperature for 1 hour to impregnate.

The post-treated support was dried and heated at 550°C in a hydrogen atmosphere.
It was calcined for 13 hours and used as a catalyst.

Example 2 Platinum and rhodium are added as metals to water 21, respectively: o, B,
Each chlorine compound (namely, chloroplatinic acid, rhodium chloride, and cerium chloride) was dissolved so that 0.4 g of cerium was present as a metal, and the pellets made of γ-alumina were further immersed at room temperature for 1 hour to impregnate.

The post-treated support was dried and incubated at 550°C in a hydrogen atmosphere for 3
It was fired for a period of time and used as a catalyst.

Example 3 0.6 g each of platinum and rhodium as metals in water 21
Each chlorine compound (i.e., chloroplatinic acid, rhodium chloride, and cerium chloride) was dissolved so that 0.8 g of cerium was present as a metal, and the pellets made of γ-alumina were further immersed at room temperature for 1 hour to impregnate.

The post-treated support was dried and incubated at 550°C in a hydrogen atmosphere for 3
It was fired for a period of time and used as a catalyst.

Example 4 0.2 g each of platinum and rhodium as metals in water 21,
Each chlorine compound (i.e., chloroplatinic acid, rhodium chloride, and cerium chloride) was dissolved so that 1.6 g of cerium was present as metal, and the pellets, preferably made of γ-alumina, were immersed for 1 hour at room temperature to impregnate.

The post-treated support was dried and incubated at 550°C in a hydrogen atmosphere for 3
It was fired for a period of time and used as a catalyst.

Test Example 1 The catalysts of Examples 1 to 4 above were each filled into an automobile exhaust gas purification reactor, and their performance was evaluated under the following conditions.

Exhaust gas from an engine with a displacement of 2,000 cc was introduced into the reactor at a space velocity of 30,000 per hour.

The exhaust gas temperature at the catalyst inlet was 450° C., and the components of the exhaust gas were HC 1500 ppm COO, 7% NO 500 ppm CO2 14% H2O, 5% 0.62 to 0.72% N2 balance.

In addition, in the exhaust gas composition, the ratio of oxidizing substances to reducing substances is in the range of 0.95 to 1.05.

The removal rate of each component evaluated under the above conditions is shown in the table below.

In the above examples, pellets were used as the carrier, but honeycomb also exhibits similar performance.

In addition, in the evaluation of the above examples, the ratio of oxidizing substances to reducing substances in the exhaust gas was set in the range of 0.95 to 1.05, but the ratio of reducing substances, that is, carbon monoxide, hydrogen, and hydrocarbons, was In large quantities, nitrogen oxides (NOx)
The removal of hydrocarbons and carbon monoxide becomes more efficient when oxidizing substances, ie, oxygen (0□) and nitrogen monoxide (NO) are present in large amounts.

Therefore, the catalyst of the present invention exhibits high performance even when used exclusively for removing nitrogen oxides (NOx).

Comparative Example 1 A catalyst was prepared in the same manner as in Example 1 except that water 21 containing only 1.0 g of platinum and rhodium each was prepared without adding cerium.

Comparative Example 2 A catalyst was prepared in the same manner as in Example 1 except that water 21 containing 1.8 g of platinum and 0.2 g of cerium was prepared without adding rhodium.

Comparative Example 3 A catalyst was prepared in the same manner as in Example 1 except that water 21 containing 1.8 g of rhodium and 0.2 g of cerium was prepared without adding platinum.

Test Example 2 The three catalysts of Comparative Examples 1 to 3 above were evaluated under the same conditions as Test Example 1.

The results obtained are shown below. In this way, the catalyst of the example is platinum-
It can be seen that since rhodium-cerium was used as an active ingredient, the performance was clearly superior to that of the catalyst of the comparative example.

As described above, the catalyst of the present invention converts nitrogen oxides, carbon monoxide, and hydrocarbons with high efficiency and renders them harmless, and has a remarkable effect as a catalyst for purifying vehicle exhaust gas.

[Brief explanation of drawings]

The attached drawing shows the air-fuel ratio, carbon monoxide (CO), and hydrocarbon (
HC) and nitrogen oxides (NOx).

Claims (1)

[Claims] 1. A catalyst for purifying vehicle exhaust gas, characterized in that a composition comprising platinum, rhodium, and cerium is supported on a refractory carrier.