Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1997-08-28
1999-11-16
Langel, Wayne
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
502 66, 502 74, 4232135, B01D 5356, B01J 29072
Patent
active
059852259
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a catalyst for reducing nitrogen oxides in exhaust gas, and a process for said reduction. More particularly, the invention relates to a catalyst for reducing nitrogen oxides by hydrocarbons in exhaust gas containing an excessive amount of oxygen, and to a process for reducing nitrogen oxides in exhaust gas containing hydrocarbons and an excessive amount of oxygen.
BACKGROUND ART
Ammonia denitration has been used to remove nitrogen oxides (hereinafter referred to as "NOx") from gas containing oxygen exceeding in quantity the oxidizing capacity of the reductive gas, particularly exhaust gas from combustion at a higher air/fuel ratio than stoichiometric value. However, this process cannot be applied to small combustors, for the following two reasons. First, it requires an ammonia source. Secondly, "slip" of ammonia in excessive quantity can cause a secondary environmental pollution problem. Recently it was found, as disclosed in Japanese Patent Laid Open Sho 63-100919, that NOx can be selectively reduced by hydrocarbons on a zeolite catalyst ion-exchanged with Cu or the like metal.
However, if hydrocarbons having four or fewer carbons are used as a reducing agent, the above-mentioned catalyst provides low selectivity (molar ratio of hydrocarbons used in NOx reduction to total hydrocarbons consumed) and therefore low NOx conversion, in the presence of the water vapor always contained in ordinary exhaust gas.
Armor, et al. reported (Applied Catalysis B: Environmental, vol. 1, p. L31) that NOx can be selectively reduced by methane on Co ion-exchanged ZSM-5 (MFI zeolite). It is known, however, that the catalytic activity is also decreased in the presence of water vapor, so that the catalyst does not have a sufficient activity for practical use. Therefore, a catalyst which is effective even in the presence of water vapor is awaited.
As a solution to the above-mentioned conventional problems, Italian Patent Application No. MI93A2337 discloses a NOx reduction process which uses a BEA zeolite ion-exchanged with cobalt (Co-BEA).
The Co-BEA offers substantial improvement in catalytic activity and durability at low temperature in actual exhaust gas conditions (in the presence of water vapor etc.). However, if the concentrations of NOx and reductant hydrocarbon are low, the reaction rate decreases on this catalyst, and a sufficient NOx conversion for practical use cannot be achieved. Therefore, more active catalyst has been desired
BRIEF DESCRIPTION OF THE DRAWINGS
[FIG. 1] Raman spectra of Co.sub.3 O.sub.4 and Co-loaded zeolites.
PROBLEMS TO BE SOLVED BY THE INVENTION
It is assumed that the catalytic activity of cobalt-loaded zeolite can be enhanced by increased loading of cobalt, which constitutes the active sites of the catalyst. However, if cobalt is loaded excessively, they will aggregate in the form of oxides, etc., clogging micropores in the zeolite and decreasing the catalytic activity. It has therefore been considered difficult to improve catalytic activity by increasing cobalt content while maintaining dispersion of cobalt.
To solve the above-mentioned problem, an object of the present invention is to provide a NOx reduction catalyst loaded with highly dispersed cobalt to secure high NOx conversion and high durability, even under low NOx concentration, in exhaust gas containing water vapor and relatively light hydrocarbons, such as exhaust gas for natural gas combustion. Another object of the invention is to provide a NOx reduction process using the above-mentioned catalyst.
Means to Solve the Problems
Through intensive study, the inventors found that catalysts which are not active despite high cobalt loading show the Raman spectral band that is assigned to cobalt oxide (Co.sub.3 O.sub.4) (FIG. 1 (1)) at around 680 cm.sup.-1 in the Raman shift (FIG. 1 (4)). However, in X-ray diffraction conventionally used for catalyst analysis, these catalysts show no diffraction line assignable to cobalt oxide. This indicates that NOx reduction activity is dimi
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Bellussi Giuseppe
Ohtsuka Hirofumi
Okada Osamu
Sabatino Luigina Maria Flora
Tabata Takeshi
DiMauro Peter
Eniricerche S.p.A.
Langel Wayne
Osaka Gas Company Limited
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