The nitrogen oxides (hereinafter referred to as “NOx”) contained in the exhaust gas emanating from aninternal combustion engine of an automobile, a boiler, an industrial plant, etc. form the cause for air pollution and acid precipitation. It is urgently necessary for the exhaust gas to be purged of the NOx.
In the case of the exhaust gas from the gasoline engine of an automobile, for example, a method has been heretofore known which comprises treating the exhaust gas with the so-called three-way catalyst using platinum, for example, and consequently purging this exhaust gas of not only hydrocarbons (hereinafter referred to as “HC”) and carbon monoxide (hereinafter referred to as “CO”) but also NOx. This method is unusually effective where the air-fuel ratio (hereinafter referred to as “A/F”) falls in the proximity of the stoichiometry (A/F=14.7).
In recent years, however, the development of the lean-burn engine has been under way with a view to minimizing the fuel cost and curtailing the amount of carbon dioxide to be discharged. The engine of this kind, however, does not permit easy expulsion of the NOx with an ordinary three-way catalyst because it has an increased A/F (hereinafter referred to as “oxygen-excess atmosphere”) and the exhaust gas has an oxygen concentration exceeding the amount of oxygen necessary for perfect combustion of such unburned components as HC and CO.
In contrast, in the case of the diesel engine, the exhaust gas constitutes an oxygen-excess atmosphere. For the exhaust gas from such a fixed source of generation as a boiler, it is known to purge the exhaust gas of the NOx by a method of using such a reducing agent as ammonia, hydrogen, or carbon monoxide.
These methods, however, necessitate a separate device to be used for the addition of the reducing agent and a special device for recovering and treating the unaltered portion of the reducing agent and, consequently, entail complication and expansion of the system as a whole. The reducing agent such as ammonia, hydrogen, or carbon monoxide which is highly toxic and hazardous cannot be easily mounted in such a source for generation of motion as an automobile and can be applied thereto only with difficulty by reason of safety.
Recently, with a view to avoiding the problems mentioned above, various catalysts having iridium as the catalytically active species have been suggested for the catalysis of the expulsion of the NOx from an oxygen-excess atmosphere and have been partly utilized to practice.
The exhaust gas from the lean-burn engine is not always in an oxygen-excess atmosphere but is caused to assume a various atmosphere by the driving condition of the relevant automobile. Under the working environment of this nature, the catalyst which has iridium as the catalytically active species is liable to be degraded by an elevated temperature. Particularly, under the condition of exposing the surface of the catalyst to a high-temperature oxidizing atmosphere as when the automobile during driving at a high speed is decelerated so much as to entail a cut in the fuel, the fact that the degradation of the catalyst rapidly proceeds poses a problem.
As typical examples of the catalyst having iridium as the catalytically active species, catalysts having iridium deposited on such a refractory inorganic oxide as alumina (JP-B-56-54173, JP-B-57-13328, Japanese Patent No. 2618316, and JP-A-10-94730), catalysts having iridium deposited on support material of such substances as zeolite and crystalline silicate (JP-A-6-296870, JP-A-7-80315, and JP-A-7-88378), and catalysts having iridium deposited on support material of such substances as metallic carbides and metallic nitrides (JP-A-6-31173, JP-7-31884, JP-A-7-246337, JP-A-8-33845, and JP-A-8-71422) may be cited.
The working examples reported in these official gazettes, however, pay no consideration to the problem mentioned above. None of them depicts the test of a catalyst for durability under a high-temperature oxidizing atmosphere.
The desirability of developing a NOx expelling catalyst which efficiently decomposes and expels the NOx from the exhaust gas in an oxygen-excess atmosphere, excels in resistance to heat and in durability under a high-temperature oxidizing atmosphere, and in the presence of moisture and SOx, and moreover manifests a catalytic activity in a wide range of temperature has been finding enthusiastic recognition.