Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1997-12-11
2001-04-10
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213700, C423S239100, C502S223000, C502S217000, C502S325000, C502S339000, C502S328000
Reexamination Certificate
active
06214307
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an exhaust-gas-purifying catalyst for reducing hydrocarbons, carbon monoxide and nitrogen oxides, especially nitrogen oxides, in exhaust gas released from combustion engines, such as gasoline engines, diesel engines, boilers and industrial plants, and concerns a process for purifying exhaust gas.
BACKGROUND OF THE INVENTION
Hydrocarbons (hereinafter, referred to as HC), carbon monoxide (hereinafter, referred to as CO) and nitrogen oxides (hereinafter, referred to as NOx) , contained in exhaust gas discharged from internal combustion engines such as cars, boilers and industrial plants, form the main cause of air pollution. Especially, since NOx form the cause of acid rain, the development of a technique for removing NOx from exhaust gas is considered to be an urgent task.
Conventionally, in the case of exhaust gas from gasoline engines, such as those of cars, a method has been known in which exhaust gas is treated by a so-called three way catalytic converter using platinum or other elements so as to remove NOx together with HC and CO. This method is extremely effective when the air-fuel ratio (hereinafter, referred to as A/F) is set in the vicinity of the stoichiometrical air-fuel ratio (A/F=14.6).
In recent years, lean-burn engines have been gained attention as a means for improving fuel economy and for reducing CO
2
. However, such engines have a greater A/F than the stoichiometrical air-fuel ratio, and form an atmosphere in which an excessive amount of oxygen exists in the exhaust gas (hereinafter, referred to as an “oxidizing atmosphere”) Since an excessive amount of oxygen exists as compared with the amount required for completely burning unburned ingredients such as HC and CO in the exhaust gas, it is difficult to remove NOx through reduction by using a normal three way catalytic converter.
Moreover, in the case of Diesel engines whose exhaust gas forms an oxidizing atmosphere, a method is known in which a reducing agent, such as ammonia, hydrogen or carbon monoxide, is used to remove NOx from exhaust gas from a Diesel engine that forms a stationary source of generation, such as a boiler.
In this method, however, an additional device for adding the reducing agent and a special device for recovering and treating the unreacted reducing agent are needed; this makes the entire equipment more complex and bulky, and the resulting problem is that this method is not applicable to engines that form a movable source of generation such as cars.
In order to solve the above-mentioned problems, various catalysts have been proposed for removing NOx in an oxidizing atmosphere.
However, no conventional methods have successfully provided an NOx removing catalyst which can decompose and remove NOx in exhaust gas effectively even in an oxidizing atmosphere, which is superior in heat resistance and durability under high temperatures, and which can exert a catalytic activity in a wide temperature range.
As some of those NOx removing catalysts, for example, aluminosilicate with exchanged transition-metal ions, such as copper ions, (see Japanese Laid-Open Patent Publication No. 125250/1985 (Tokukaisho 60-125250), Japanese Laid-Open Patent Publication No. 100919/1988 (Tokukaisho 63-100919) and the specification of the U.S. Pat. No. 4,297,328) or metallo-aluminosilicate (see Japanese Laid-Open Patent Publications No. 127628/1991 (Tokukaihei 3-127628) and No. 229620/1991 (Tokukaihei 3-229620)), and silico-aluminophosphate (see Japanese Laid-Open Patent Bublication No. 293049/1990 (Tokukaihei 2-293049) have been proposed.
However, these so-called ion-exchange zeolite catalysts require high temperatures to remove NOx and consequently have reduced effects at low temperatures, and are inferior in heat resistance to the point that their NOx decomposing capability is extremely reduced when exposed to high-temperature exhaust gas; accordingly, these catalysts have not been successfully put into practical use.
Moreover, as an NOx removing catalyst for use in an oxidizing atmosphere, a catalyst having iridium deposited on a fire-resisting inorganic oxide such as alumina has been disclosed (see Japanese Examined Patent Publications No. 54173/1981 (Tokukousho 56-54173) and No. 13328/1982 (Tokukousho 57-13328)). However, in the embodiments described in these patent publications, only examples using the oxygen concentration in exhaust gas not more than 3 volume % are shown, and no consideration was given to NOx purifying capability and heat resistance with respect to exhaust gas from diesel engines and lean-burn engines that contains oxygen not less than the above-mentioned amount.
Furthermore, catalysts having iridium deposited on a support such as zeolite and crystalline silicate have been proposed (see Japanese Laid-Open Patent Publications No. 296870/1994 (Tokukaihei 6-296870), No. 80315/1995 (Tokukaihei 7-80315) and No. 88378/1994 (Tokukaihei 7-88378)). However, with respect to conditions of the durability tests for these catalysts, those tests were merely carried out in an reducing atmosphere of exhaust gas, and no consideration was given to durability and heat resistance in an oxidizing atmosphere such as exhaust gas from diesel engines, lean-burn engines and gasoline engines of the fuel-direct-injection type.
Moreover, catalysts having iridium deposited on a support made of a material such as a metallic carbide and a metallic nitride have been proposed (see Japanese Laid-Open Patent Publications No. 31173/1994 (Tokukaihei 6-31173), No. 31884/1995 (Tokukaihei 7-31884), No. 246337/1995 (Tokukaihei 7-246337, No. 33845/1996 (Tokukaihei 8-33845) and No. 71422/1996 (Tokukaihei 8-71422).
However, the examples of the above-mentioned Patent Publications merely show maximum NOx removing rates, and with respect to temperature ranges at which the maximum NOx removing rates are exerted, nothing is clarified except for the case in which light-off characteristics are shown. The light-off characteristic shows the exhaust-gas-purifying characteristic at various exhaust gas temperatures.
Judging from the example having the light-off characteristic, it is the temperature range exceeding 350° C. that the activity for removing NOx appears, and the NOx purifying activity is extremely reduced in the temperature range not more than 350° C.
In addition, another disadvantage of these conventional catalysts is that after having been used for a long time, the temperature at which the NOx purifying activity rises is greatly shifted toward the high-temperature side. Further, since metallic carbides and metallic nitrides are expensive, the cost increases. Consequently, the catalysts, described in the above-mentioned Patent Publications, have merely narrow temperature ranges in the activity for removing NOx, and also have high costs.
Consequently, at present, no conventional methods have successfully provided an NOx removing catalyst which can decompose and remove NOx in exhaust gas effectively even in an oxidizing atmosphere, which is superior in heat resistance and durability under high temperatures, and which can exert a catalytic activity in a wide temperature range at low costs.
Moreover, with respect to car engines, there is a tendency to set the temperature of exhaust gas at a low level with a view to achieving a more efficient combustion and a lower fuel consumption; therefore, there is an increasing demand for a method for reducing HC, CO and NOx from exhaust gas even at lower exhaust gas temperatures.
Furthermore, Laid-Open International Patent Publication No. WO 93/08383 discloses a catalyst which oxidizes and adsorbs NOx in an oxidizing atmosphere, while discharging NOx in a reducing atmosphere, and an exhaust-gas-purifying method using such a catalyst.
In this method, however, sulphur oxides, contained in exhaust gas, are irreversibly adsorbed simultaneously with NOx, and the resulting disadvantage is that the NOx purifying capability deteriorates with time due to the adsorption.
Consequently, at present, the above-mentioned conventional methods have failed t
Gotoh Hideki
Hori Masao
Horiuchi Makoto
Okumura Akihisa
Griffin Steven P.
ICT Co., Ltd.
Ildebrando Christina
Nixon & Vanderhye P.C.
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