Exhaust gas purifying system and catalyst

Abstract

An exhaust gas purifying system for an automotive internal combustion engine which discharges oxygen-excessive exhaust gas under lean-burn operation. The exhaust gas purifying system comprises a NOx reducing catalyst for reducing NOx in the presence of at least hydrogen serving as reducing agent. The NOx reducing catalyst is disposed in an exhaust gas passageway of the engine. The NOx reducing catalyst includes a porous substrate, and platinum and cesium carried on the porous substrate. A catalyst is disposed in the exhaust gas passageway in order to produce hydrogen so as to enrich the oxygen-excessive exhaust gas with hydrogen before the oxygen-excessive exhaust gas reaches the NOx reducing catalyst. The oxygen-excessive exhaust gas enriched with hydrogen is introduced through the exhaust gas passageway to the NOx reducing catalyst. Here, the oxygen-excessive exhaust gas enriched with hydrogen is brought into contact with the NOx reducing catalyst under a condition in which temperature of at least one of the NOx reducing catalyst and the oxygen-excessive exhaust gas enriched with hydrogen is within a range of from 250 to 600° C.

Description

BACKGROUND OP THE INVENTION[0001] This invention relates to improvements in an exhaust gas purifying system for purifying exhaust gas discharged from an internal combustion engine, a furnace, a burner or the like, and more particularly to the exhaust gas purifying system for effectively reducing nitrogen oxides (NOx) in oxygen-excessive exhaust gas discharged from the engine under lean-burn operation.[0002] Hitherto three-way catalysts for simultaneously oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NOx) have been extensively used as exhaust gas purifying catalysts installed to internal combustion engines of automotive vehicles and the like, However, such catalysts cannot reduce NOx particularly when exhaust gas of the engine is oxygen-excessive (i.e., has an air-fuel ratio leaner than stoichiometric value). In this regard, Japanese Patent No. 2600492 discloses a so-called rich spike process as a method of reducing NOx when exhaust gas of the eng...

AI Technical Summary

Benefits of technology

[0011] Another object of the present invention is to provide an improved exhaust gas purifying system which can effectively remove NOx, HC and CO in the oxygen-excessive exhaust gas under a high temperature condition higher than 250.degree. C. while obtaining a high fuel economy improvement effect.

Problems solved by technology

Hitherto three-way catalysts for simultaneously oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NOx) have been extensively used as exhaust gas purifying catalysts installed to internal combustion engines of automotive vehicles and the like, However, such catalysts cannot reduce NOx particularly when exhaust gas of the engine is oxygen-excessive (i.e., has an air-fuel ratio leaner than stoichiometric value).

That is, the NOx reducing method requires to periodically lower the oxygen concentration in exhaust gas in an engine operating region where exhaust gas takes the oxygen-excessive atmosphere.

This makes it impossible to obtain fuel economy improvement effect due to vehicle running on oxygen-excessive air-fuel mixture having an air-fuel ratio leaner than the stoichiometric value.

Additionally, in the above NOx reducing method, the catalyst contains a large amount of alkali metal and alkaline earth metal for the purpose of trapping NOx in the oxygen-excessive atmosphere of exhaust gas, and therefore a catalytic activity (for oxidation) of platinum and rhodium cannot be sufficiently exhibited when exhaust gas is low in temperature.

However, such the NOx reducing catalyst or the three-way catalyst disposed downstream of the NOx reducing catalyst are located considerably far from the engine, and therefore the temperature of exhaust gas flown to the catalysts are unavoidably lowered, thereby making it impossible to exhibit a sufficient oxidizing performance for HC and CO.

Particularly immediately after engine starting, sufficient oxidation of HC and CO is difficult to be made.

Additionally, when HC and CO components in exhaust gas are increased lowering an oxygen concentration in exhaust gas and reducing NOx as discussed above, it is impossible to make vehicle running under lean-burn operation, thereby exhibiting an insufficient fuel economy improvement effect.

However, according to this technique, hydrocarbons are used as the reducing agent, and therefore chemical reaction rate for reduction of nitrogen oxides is low.

This requires a large volume of the catalyst.

It will be understood that mounting the large volume catalyst on a movable body such as an automotive vehicle is not practical.

However, according to this technique using ammonia as the reducing agent, ammonia itself is noxious, and additionally it is difficult to supply and store ammonia in a small scale facility such as an automotive vehicle.

Therefore, nitrogen oxides cannot be effectively reduced in a temperature range higher than the above temperature range.

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