Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2000-06-30
2004-02-03
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C502S244000, C502S245000, C502S331000, C502S345000
Reexamination Certificate
active
06685899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst and method for purifying exhaust gas from vehicle engines, and more particularly, to a catalyst and method for purifying exhaust gas from diesel engines. The catalyst and method of the invention provide increased efficiency for purifying both nitrogen oxide and soot particles (particulates) from the exhaust emissions of the engine.
2. Description of the Related Art
Across the world, there is an increasing interest in preserving the environment, along with other environmental concerns. In particular, air pollution rather than water and soil pollution is caused primarily by combusters, (e.g., combustion engines), and air pollution is seriously affected by the structure of the exhaust system of the combuster, the operating principles thereof, and weather conditions. Motor vehicle combustion engines are typical combusters that cause a considerable amount of air pollution.
Air pollution produced by vehicles is quite substantial, given the fact that vehicles emit pollutants wherever they go, and the use of vehicles sharply increases with improvements in living conditions. For this reason, various regulatory agencies have imposed restrictions on the exhaust emissions from vehicles. As a result of efforts made to comply with and exceed these restrictions, development and use of a three-way catalyst and a lean burnt catalyst has achieved almost complete removal of carbon hydroxide, carbon monoxide, and nitrogen oxide from the exhaust emissions of gasoline engines.
The problems associated with diesel engines, however, are different from the problems associated with gasoline engines. In addition, the use of diesel engines has greatly increased due to the high combustion efficiency of diesel and its low cost, when compared to gasoline. Due to the combustion principle of diesel engines which bum diesel under high-pressure, and in an oxygen-rich atmosphere, diesel engines emit solid and liquid composite pollutants such as soot particles (particulates), nitrogen oxides, soluble organic substances, sulfides, etc. In particular, particulates containing a carcinogenic substance such as a multinucleate aromatic substance are considered to be the most harmful exhaust emissions, and they are emitted in the form of undesirable visible smog. For this reason, there has been significant research into development of an exhaust gas purification system for diesel engines, which has been focused primarily on the development of a catalyst that is useful for removing such particulates.
The carbonic substances contained in particulates typically are burnt by an oxidation catalyst, and then purified in the form of carbon dioxide. The temperature of diesel engines on starting, however, is room temperature, and while running, increases to 450° C. Thus, unless the oxidation temperature of the oxidation catalyst is low enough, almost all particulates that are emitted at room temperature may be discharged in the air. Oxygen catalysts having a lower activation temperature range are effective in reducing particulates.
Sulfur contained in diesel typically is emitted in the form of sulfur dioxide via combustion, and then it is oxidized into sulfur trioxide by a catalyst. Sulfur trioxide is converted into sulfuric acid by combination with moisture, which causes acid rain damage. In addition, sulfur trioxide itself serves as crystal nuclei, so that it facilitates the generation of particulates. The generation of sulfur trioxide is considered to be a factor that adversely affects the emissions purification catalysts, that deteriorates the performance of an exhaust gas post-treatment apparatus, and that increases the exhaust pressure by sticking to the apparatus. Accordingly, when developing a catalyst for oxidizing particulates of diesel engine emissions, the composition of the catalyst should be carefully considered so that it exhibits oxidation activity with respect to particulates at a temperature as low as possible, and that is stable in a sulfur dioxide atmosphere without causing oxidation of sulfur dioxide into sulfur trioxide.
Catalysts useful for purifying exhaust gases from vehicles usually are comprised of a carrier and a main catalyst. Typical examples of the carrier, which has its inherent activity and is a decisive factor in determining the characteristics of the purification catalyst, include alumina, titanium dioxide, zirconium dioxide, silicon dioxide, and the like. In addition, a precious metal, a transition metal, a rare earth metal, an alkali metal or an alkali earth metal can be added as the main catalyst. Alumina, although recognized as a stable carrier for gasoline engines, has a disadvantage when used for diesel engines in that it adsorbs sulfur dioxide at low temperatures and emits sulfur trioxide at high temperatures (e.g., of 350° C. or more) via oxidation. This oxidation increases particulates in the exhaust emissions and reduces the activity and durability of the catalyst.
Titanium dioxide and zirconium dioxide, which are used alone or in a mixture, adsorb only a small amount of sulfur dioxide and produce only a small amount of sulfate, but exhibit a sharp reduction in their specific surface area at high temperatures. These oxides therefore cannot sufficiently exert their functions as a carrier. In addition, titanium dioxide and zirconium dioxide lower the activity of precious metals and transition metals, and in turn deteriorate the catalyst. Silicon dioxide has a strong resistance against the adverse effects of both sulfur dioxide and water, but due to its low activity, a large amount of catalyst needs to be impregnated therewith.
Catalysts useful for purifying exhaust gases from vehicles typically are comprised of precious metals. Platinum (Pt) and palladium (Pd), which are typical precious metals used in a three way catalyst for gasoline engines, are known as effective catalysts due to their considerably high purification activity with respect to nitrogen oxides, in addition to hydrocarbons and carbon monoxide. Accordingly, Pt and Pd have also been used widely for purification of the exhaust gas from diesel engines.
Although Pt has an advantage of exhibiting good purification activity for nitrogen oxide in diesel engines operating under an oxygen-rich atmosphere, it has a disadvantage in that it facilitates oxidation of sulfur dioxide in an oxygen-rich atmosphere. Pt also serves as crystal nuclei for particulates, thereby increasing the amount of particulates in the exhaust. Adding vanadium oxides has been proposed to account for this problem, due to their ability to suppress the oxidizing power of sulfur dioxide. However, vanadium oxides reduce the oxidation activity for pollutants including particulates, hydrocarbons, and carbon monoxide, along with the oxidizing power of sulfur dioxide, thereby lowering the durability of the catalyst.
While Pd has an advantage in that it facilitates the oxidation activity for sulfur dioxide at fairly high temperatures, for example, at least 450° C., it has a low oxidation activity for pollutants at low temperatures and a reduced durability at low temperatures.
In terms of cost and limited reserves of precious metals, there is a need for new substitutes for precious metals. However, since a main catalyst component capable of satisfactorily substituting for a precious metal has not yet been found, the amount of the precious metal used has been reduced with the aid of co-catalysts such as transition metals, rare earth metals, and oxides of these metals. However, these co-catalysts have a low initial activity, and are adversely affected by sulfur dioxide and water, which results in reduced durability.
SUMMARY OF THE INVENTION
There exists a need to develop a catalyst and method for purifying exhaust gas from vehicle engines that provides improved purification efficiency with respect to other exhaust substances including soot particulates. There also exists a need to develop a catalyst and method for purifying exhaust gas from vehicle engines, preferably diesel
Lee & Sterba, P.C.
Samsung Electro-Mechanics Co. Ltd.
Silverman Stanley S.
Strickland Jonas N.
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